NANOPARTICLES IN ZEOLITE SYNTHESIS

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de

Rector Magnificus, prof.dr. R.A. van Santen, voor een commissie aangewezen door het College voor

Promoties in het openbaar te verdedigen op dinsdag 18 maart 2003 om 16.00 uur

door

Christophe Jean-Marie Yves Houssin

geboren te Villedieu-les-Poêles, Frankrijk

Dit proefschrift is goedgekeurd door de promotoren: prof.dr. R.A. van Santen en prof.dr. J.A. Martens Copromotor: dr. B.L. Mojet CIP-DATA LIBRARY TECHNISCHE UNIVERSITEIT EINDHOVEN Houssin, Christophe J.Y. Nanoparticles in zeolite synthesis / by Christophe J.Y. Houssin. – Eindhoven : Technische Universiteit Eindhoven, 2003. Proefschrift. – ISBN 90–386–2874–9 NUR 913 Trefwoorden: poreuze materialen ; synthese / zeolieten ; ZSM-5 / nanostructuren / silicalieten / zelforganisatie / kristallisatie / röntgenverstrooiing ; SAXS / kernspinresonantie ; NMR Subject headings: porous materials ; synthesis / zeolites ; ZSM-5 / nanoparticles / silicalites / self-assembly / crystallization / X-ray scattering ; SAXS / nuclear magnetic resonance ; NMR © 2003 by Christophe J.Y. Houssin Printed by Universiteitsdrukkerij Technische Universiteit Eindhoven. The work described in this thesis has been carried out at the Schuit Institute of Catalysis, Laboratory of Inorganic Chemistry and Catalysis, Eindhoven University of Technology, The Netherlands. Financial support was provided by NRSC-Catalysis, NWO and the Spinoza fund.

A mes parents,

Contents Chapter 1 Introduction to zeolites and the scope of this thesis 1 Chapter 2 Zeolite nanoslabs: a combined SAXS and TEM study 17 Chapter 3 A 29Si and 27Al NMR study of MFI precursors 31

Chapter 4 In situ SAXS/USAXS investigation on aluminum incorporation 59

in the synthesis of colloidal TPA-ZSM-5 Chapter 5 Zeolite nanoslabs: building blocks for innovative porous materials 85 Summary 97 Samenvatting 99 Résumé 101 Acknowledgments 103 Curriculum Vitae 105

1

1

Introduction to zeolites

and the scope of this thesis

1.1 Background

Despite being discovered as “boiling stones” more than 250 years ago1, zeolites have

received considerable attention only in the last past decades and have today turned into

essential commercial materials.2,3 This is due to their exceptional ability to adsorb large

amounts of water and other molecules in their micropores.

Zeolites are crystalline aluminosilicates with a 3-dimensional, open anion framework

consisting of oxygen-sharing TO4 tetrahedra, where T is Si or Al. Their framework structure

contains interconnected voids that are filled with adsorbed molecules or cations. Zeolite

micropore channels have very well-defined diameters so that bulky molecules will be

excluded from the internal surface. The general empirical formula is:

Mx/m . AlxSi2-xO4 . n H2O

where m is the valence of cations M, n the water content and 0 ≤ x ≤ 1. The flexibility of the

zeolite Si-O-Si bond explains the fact that more than 200 structures have been determined.

Indeed, there is little energetic difference (10-12 kJ/mol) between these remarkable porous

silicates and higher density phases such as quartz. Several properties account for their

2

commercial use: they are strong adsorbents, they show a very high selectivity and they are

excellent solid acid catalysts. Today, synthetic zeolites are employed in a wide range of

industries.4 For example, they are used in the separation of gaseous or liquid compounds.

Detergents represent the highest market by volume because of their high ion-exchange

capacities. However, the high-added value commercial applications of zeolites are

encountered in the petroleum industry. Fluid catalytic cracking (FCC), main process in the

transformation of crude oil into gasoline, mainly uses zeolite Y as catalyst.

1.2 Structure

The fascinating properties of zeolitic materials essentially originate from their

structures. The lack of proper identification techniques hindered longtime the determination

of structures, explaining the slow progress made in the century following their discovery. The

invention of X-ray diffraction at the beginning of the 20th century to probe the structural

properties of materials initiated systematic studies on zeolite identification. A zeolite

topology concept was introduced. It corresponds to the connectivity of the tetrahedra of the

framework through line segments and nodes and represents the highest possible symmetry.

The structure commission of the International Zeolite Association (IZA) provides up to date

classification by framework type. As of November 2002, 136 framework types have been

accepted by this commission and are available on the internet site of the IZA5 or in the Atlas

of Zeolite Framework Types.6

The large majority of zeolite structures are constructed by repeating so-called

secondary building units (SBUs). There are presently 19 SBUs.6 Another way to classify

zeolites is to take into account their pore openings and the dimensionality of their channels.

Thus, one distinguishes small pore zeolites (eight-membered-ring pores), medium pore

zeolites formed by ten-membered rings and large pore zeolites with twelve-membered-ring

pores. Recently an extra-large pore zeolite category has been added.7,8 This classification

simplifies comparisons in terms of adsorptive, molecular sieving and catalytic properties.

Two important and industrially relevant structures are depicted in figure 1.

3

Figure 1. Two examples of zeolite framework and pore system: MFI and MOR topologies.

1.3 Zeolites and Catalysis

Background

When an aluminum atom substitutes a silicon atom, the +III valence of aluminum

introduces a net negative charge in the framework. Cations are then required to preserve

neutrality. Not only this presence of cations allows zeolite crystals to be used in ion-exchange

processes but it creates an acid site if protons act as counterions. However, it would be too

simple to represent acid sites in zeolites by a simple proton like classical homogeneous acids.

Figure 2 shows a typical zeolite acid site. It consists of a hydroxyl group bridging a silicon

atom and an aluminum atom corresponding to a strong Brönsted site and oxobridges

exhibiting Lewis base properties. The acidity of zeolites is very strong, about 1000 stronger

than that of amorphous aluminosilicates. In catalytic applications, high-silica zeolites are

preferred because of the thermal stability of their framework (crucial for regeneration cycles)

4

and high dispersion of acid sites. Moreover, low aluminum content ensures high acidity for

each proton.

Al

H

Figure 2: Active site in zeolites.

Apart from these acidic properties, zeolites are shape selective regarding molecular

adsorption.4 This is due to their pore system that can be one, two or three-dimensional and

contain pores of different sizes which are in the order of molecular dimensions (from about

0.3 to 1.2 nm). The void dimensionality and very high internal surface area (>500 m2/g) are

responsible for catalytic shape selectivity of zeolites. Derouane et al. proposed in the 1980s

the so-called confinement effect in order to describe interactions of adsorbed molecules

within the curved surface of channels and cages of zeolites.9,10 It is obvious that sorbate-

framework interactions and the local framework topology around the active site largely

influence the reactivity of molecules.

Shape selectivity

Shape selectivity was first observed by Weisz and Frilette in 1960.11 Since then this

phenomenon has been thoroughly documented in literature.12,13,14,15,16 Reactions within

zeolites can be inhibited if there is no matching between certain molecules and a sterically

confined environment allowing conversion of reactants. The unique one, two or three-

dimensional pore system of zeolites enables shape selective catalysis. There is a consensus on

the different mechanisms of molecular shape selectivity in zeolite technology:

Reactant selectivity: This occurs when some molecules preferentially enter the zeolite

pore mouth whereas others are rejected because they are too large with respect to the

pore openings. Once a reactant has adsorbed in the zeolite channels, it must diffuse

towards active sites where reactions can occur. This is where the two following

selectivity phenomena can occur.

5

Product selectivity: This occurs when some reaction products or intermediates formed

within the pores are too bulky to diffuse out. They are either converted to smaller molecules

or deactivate the catalyst by blocking of the pores. After reaction has occurred, products must

diffuse away from the micropores which results in a kind of molecular traffic control.

Transition state selectivity: The distinction between transition state selectivity and

product selectivity is not always obvious. It takes place when the transition state cannot be

accommodated in the space available in the intra crystalline volume. A way to differentiate

product and transition state selectivity is to vary the crystal size because only product

selectivity depends on crystal size, whereas transition state shape selectivity does not.

region forshape-selective catalysis

layer for pore mouthcatalysis

dd

cb bbaa

example: ZSM-22example: ZSM-5example: Erionite

tubular pore structureintersecting tube structurecage and narrowwindow structure

interrupted channelinterrupted intersectionhalf cavity with large aperture

Figure 3: Shape-selective environments in different structure types: (a) large molecules have access

to the interrupted cavities and channel intersections of the layer for pore mouth catalysis; (b)

molecules are plugged into the pore apertures; (c) molecules are converted in multiple pore mouths

according to key-lock catalysis; (d) molecules are converted in the intracrystalline shape-selective

environment (after ref. 18).

Martens et al. prompted the existence of pore mouth and key-lock catalysis which

consists of specific adsorption in the pore mouth at the crystal boundaries.17,18 This explains

why long n-alkanes isomerize selectively on a 10-ring bifunctional zeolites (Pt/H-ZSM-22).

This phenomenon differs from the former selectivities discussed above in that reactants do

not undergo bulk adsorption but pore mouth catalysis. Shape selectivity in pore mouth and

key-lock catalysis is illustrated in figure 3.

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Industrial applications

Since their successful introduction as commercial molecular sieves in 1954, synthetic

zeolites have grown to an estimate $1.6-1.7 billion industry.19 Detergents represent the largest

volume. LTA-type zeolites substitute phosphate compounds in the water softening process in

laundry. The largest market value for zeolites is in refinery catalysis. FCC (Fluid Catalytic

Cracking) catalysts account for more than 95% of zeolite catalyst consumption and consist of

various forms of zeolite Y. MFI-type zeolites are the second most used catalyst, primarily

because they are added to FCC catalysts for octane number enhancement. Zeolites are also

employed in the drying and purification of natural gas, separation of paraffins and

desulfurization processes. Despite being in a relatively early state of development, zeolites

are also used in fine chemicals production such as oxidation and acylation. The main

applications of zeolites are summarized in Table 1.

Process catalyst products

Catalytic cracking Re-Y, US-Y

ZSM-5

Gasoline, fuels

Hydrocracking Y, Mordenite

+ Mo, W, Ni

Kerosene, diesel, Benzene

Alkylation of aromatics ZSM-5, Mordenite p-xylene, ethyl-benzene, styrene

Hydroisomerization Mordenite + Pt, Pd i-pentane, i-hexane

Xylene isomerization ZSM-5 p-xylene

Catalytic dewaxing Mordenite, ZSM-5

+ Ni, noble metals

Improvement of cold flow

properties

Transalkylation Mordenite Xylenes, cumene

MTBE ZSM-5 Aromatics, paraffins

MTG Ga-ZSM-5 Aromatics

Table 1: Main commercial applications involving zeolites.

Zeolite science appears to be a mature science and is still a very dynamic field.

Discoveries of new zeolites continuously open new areas of development. New trends at the

beginning of this century include environmental applications such as De-NOx catalysis and

hydrocarbon storage in vehicles powered with diesel or gasoline engines, and

7

biopharmaceutical applications. Zeolites can also be used in the nuclear industry for

radioactive waste storage. Applications of zeolite material science still play an important role

in many areas of technology.

1.4 Synthesis

Background

Natural zeolites are found in volcanic or metamorphic rocks and their growth involves

geological conditions (low temperature and pressure, low pH (8-9)) and time scale (thousands

of years). Early efforts have been made by Saint Claire de Ville in 1862 to synthesize

zeolites.20 The absence of reliable characterization methods made it impossible to verify that

zeolites were indeed fabricated. The first precise confirmation of zeolite synthesis can be

traced in 1948 when Barrer reported the synthesis of an analogue of mordenite.21 At the same

time Milton and Beck succeeded in synthesizing other zeolite types using lower temperatures

(≈100 °C) and a higher alkalinity.22,23 It led to the discovery of one of the most commercially

successful zeolites which has no natural counterpart, Linde A (LTA). Since then many new

zeolite framework types have been attained thanks to important efforts by oil companies. In

the early 1960s Barrer and Denny were the first to replace inorganic bases in the synthesis

mixture with organic molecules.24 The use of quaternary ammonium salts resulted in an

increase in the Si/Al ratio and the discovery of ZSM-5, being the most important new

structure.25 The quest for higher Si/Al ratios ended in 1978 when Flanigen et al. reported the

synthesis of silicalite-126 which is the all-silica counterpart of ZSM-5. This material shows

remarkable properties because of its hydrophobic and organophilic character. A new class of

materials analogous to zeolites was introduced in the 1980s: microporous

aluminophosphates.27,28 Nevertheless, poor thermal and hydrothermal stability of their metal

substituted analogues hindered their commercial applications. The most noteworthy advance

in crystalline microporous solids has recently been the synthesis of extra large pore zeolites

with more than 12-ring apertures.7,8,29,30

Zeolite synthesis has been extensively reviewed in several books and literature on this

subject is abundant.31,32,33,34 The synthesis of zeolites is carried out under hydrothermal

conditions. An aluminate solution and a silicate solution are mixed together in an alkaline

medium to form a milky gel or in some instances, clear solutions. Various cations or anions

8

can be added to the synthesis mixture. Synthesis proceeds at elevated temperatures (60-200

°C) where crystals form through a nucleation step. The following sections give a general

overview on the parameters governing zeolite synthesis. Emphasis will be given to structure

direction by organic molecules. A schematic representation of zeolite formation process is

given in figure 4.

Basic reactants

Si, AlSolventMineralizer

SDA

Amorphous gelSmall oligomers

precursors

nucleation

Zeolite crystals

crystalgrowth

Basic reactants

Si, AlSolventMineralizer

SDA

Amorphous gelSmall oligomers

precursors

nucleation

Zeolite crystals

crystalgrowth

Figure 4: Simplified zeolite synthesis scheme. SDA stands for structure-directing agent.

a) Molar composition

Although this is not an independent parameter, every zeolite has a specific molar

composition range often represented graphically in a ternary compositional phase diagram

(Na2O, Al2O3 and SiO2). On the other hand, each structure will also impose constraints on the

amount of Al it can incorporate. High-silica molecular sieves such as ZSM-5 can be

synthesized over a wide range of Si/Al ratios (Si/Al from 7 to infinity35).

b) Mineralizer

A mineralizer is a species which enables the formation of a more stable solid phase from

a less stable solid phase via dissolution and crystallization. Supersaturation can be reached by

9

dissolution and these soluble species are then available for nucleation and crystal growth. In

most cases, hydroxyl ions act as mineralizing agents. Indeed, OH- increases the solubility of

silica by depolymerizing amorphous silica particles. Oligomeric species are then present in

solution. Condensation of specific aluminosilicate species, facilitated by the presence of OH-,

occurs and leads to the appearance of the first crystals. In general high pH values increase

crystal growth rates and shorten the nucleation period. Hydroxyl ion concentration can also

influence crystal morphology, crystal yield and final zeolite structure.

Fluoride ions have been used as mineralizers. Silicalite-1 was the first zeolite synthesized

from acidic F- medium. Fluoride anions act similarly to hydroxyl ions without contributing

directly to the pH of the system. Nucleation and crystal growth rates are generally slowed

down resulting in large and high quality crystals. The fluoride ion synthesis route has mostly

been applied in the area of aluminophosphates mainly because it has led to the discovery of

novel aluminophosphates and isomorphously substituted versions that cannot be obtained at

high pH.

c) Inorganic cations

Inorganic cations have been regarded as an important parameter influencing the structure

formed. They are involved in structure direction, solid yield, crystal morphology and purity.

Most of the synthetic analogues of natural zeolites were obtained using alkali and alkaline

earth metal cations. Nucleation and crystal growth can be optimized by the right choice of

inorganic cations.

d) Temperature

Temperature can alter the zeolite structure as well as the induction period and crystal

growth kinetics. The activation energies of zeolite synthesis are quite significant.

e) Silica and alumina sources

Nucleation and growth kinetics can depend on the dissolution of the solid reagents and

formation of aluminosilicates precursors. Kühl found that crystallization of some structures

was dependent on the degree of prepolymerization of the silica source.36 Mintova and

Valtchev recently investigated the colloidal distribution of silicalite-1 synthesis mixtures

containing different silica sources.37 Impurities in silica or alumina sources are likely to

influence crystallization kinetics and framework composition.

10

Structure direction in zeolite synthesis

Structure direction occurs when inorganic or organic molecules are used to direct the

crystallization towards a specific zeolite structure. Structure-directing agents, currently called

templates, are generally: (1) charged molecules which are mostly cations. Inorganic cations

such as Na+, K+, Li+, Ca2+ are frequently used. Organic molecules that are used are usually

tetraalkylammonium, dialkyl and trialkyl amines or phosphonium compounds. (2) neutral

molecules. Water actually plays an important role in the structure direction encountered in

zeolite synthesis. Water molecules act as void fillers in order to stabilize the porous oxide

framework. Interactions of water molecules with cations are part of the template effect and

therefore are of crucial importance. Other molecules include amines, ethers or alcohols. (3)

ionic pairs: Salts (NaCl, KCl, KBr) are occluded into the zeolite framework as guest

molecules, stabilizing the zeolite framework.

The structure-directing role of templates in zeolites can be understood from the

analogy with the formation of clathrasils.38 Guest molecules with size and shape close to

those of the cages were found to accelerate the crystallization of the same product. The

structure of clathrasils is rather independent of the chemical nature of guest molecules.

Structure-direction agents in zeolite synthesis are mostly investigated in high-silica

zeolites.39 In those systems, there is indeed a limited number of variables since low

concentration of alkali metal ions and low Si/Al ratios are used. Moreover, the porous silicon

dioxide framework is mainly uncharged. Defect sites are required to balance the charge of the

cationic structure-directing agent. Interactions between organic molecules and silica are

mostly the Van der Waals forces.

ZSM-5 is one the most important commercial zeolites and has been widely studied.33

This is a very versatile zeolite since it can be made from various organic or inorganic agents

over a wide range of synthesis conditions. Tetrapropylammonium (TPA) has been regarded

as the most efficient template in high-silica ZSM-5 synthesis, thereby appearing as a true

example of structure direction. It indeed enhances nucleation rate and accelerates

crystallization.40 Moreover TPA molecules are located at the channel intersections with their

propyl arms extending into the linear and zig-zag channels. They are tightly encapsulated so

that calcination is required to obtain their removal. This tight entrapment suggests that TPA

molecules are actively involved in the nucleation period and crystal growth. Burkett and

Davis, using 1H-29Si CP, investigated relationships between TPA and silicate species.41,42,43

11

Evidences of close interactions between TPA cations and silicate species have been observed

well before the formation of long-range crystalline structure. They proposed that the key

steps for structure direction were the formation of inorganic-organic composite entities which

may be the precursors of units participating in nucleation and crystal growth. No such

interactions were observed when a molecule lacking of structure-directing properties (i.e.

TMA) was used, suggesting that these intermolecular contacts are specific. At the same time,

Dokter et al. have observed primary silica particles in the range of 1-20 nm by means of

small-angle X-ray/neutron scattering (SAXS/SANS).44 They suggested that these colloidal

particles underwent several aggregation and densification steps leading to the formation of

the first crystals.

Watson et al. have also identified well-defined nanoprecursors in silicalite-1 starting

mixtures.45 Particles with a radius of gyration of 2.8 nm were detected and a cylindrical

model provided the best fit to experimental scattering curves.46 Most importantly, SANS

contrast variation experiments showed that these nanoparticles have a scattering density

almost identical to that of TPA-containing silicalite-1 crystals, suggesting that TPA cations

are occluded in the nanoprecursors.

Hydrophobic hydration

Overlap of hydrophobic

hydration spheres Primary units

2.8 nm

Aggregation

Crystal growth

Nucleation

≈1 nm

5-10 nm

5-10 nm10 nm - microns

N+

HH

H

H HH

HH

H

HH

OO

O

O

HHO

O

Si

OO

O

SiOO

O

Si

Si

Si

SiSi

SiH

H H

HH

H

O

OO

Water solvent

TPA

Hydrophobic hydration

Overlap of hydrophobic

hydration spheres Primary units

2.8 nm

Aggregation

Crystal growth

Nucleation

≈1 nm

5-10 nm

5-10 nm10 nm - microns

N+

HH

H

H HH

HH

H

HH

OO

O

O

HHO

O

Si

OO

O

SiOO

O

Si

Si

Si

SiSi

SiH

H H

HH

H

O

OO

N+

HH

H

H HH

HH

H

HH

OO

O

O

HHO

O

Si

OO

O

SiOO

O

Si

Si

Si

SiSi

SiH

H H

HH

H

O

OO

Water solvent

TPA

Figure 5. Scheme for the crystallization mechanism of Si-TPA-MFI (after ref. 48).

12

De Moor et al., using a combination of in situ SAXS, USAXS and WAXS (ultra-

small and wide-angle X-ray scattering), recently found three particle populations during

TPA-mediated silicalite-1 crystallization. Primary units with a size of 2.8 nm, their

aggregates (≈10 nm) and the growing crystals were identified and their consumption

monitored through the whole course of crystallization.47,48 By varying the alkalinity, it was

found that 2.8 nm particles were always present unlike their aggregates of which the

formation depends on the Si/OH ratio.49 However, the formation of the aggregates appeared

to be an essential step in the nucleation process since it enhances the nucleation rate. It was

suggested that crystal growth probably occurs via the addition of the 2.8 nm primary units to

the growing crystals. Moreover, the same size for primary units were observed using three

other structure-directing agents (a dimer of TPA, a trimer of TPA and trimethylene-bis(N-

hexyl, N-methyl-piperidium)).50 Nanoscale precursors were also found in the crystallization

of other zeolites such as Si-BEA, Si-MTW from gelating systems.51 A mechanism was then

proposed for organic-mediated zeolite synthesis and is depicted in figure 5.

In a series of recent papers,52,53,54,55 Kirschhock et al. described the molecular picture

of the formation of silicalite-1 from TEOS (tetraethylorthosilicate) and an aqueous TPAOH

solution. The first step is the formation of a tetracyclic undecamer at the interface of TEOS

and the aqueous TPAOH solution. An aggregation mechanism then occurs in which three

tetracyclic undecamers form larger units denoted “precursor units”. This particular oligomer

exhibits a characteristic length of 1.35 nm in SAXS. Upon addition of water, stable and well-

defined subcolloidal particles (3.6 nm as characteristic length) were observed. A model

consisting of twelve “precursor units” coupled three by four, having dimensions of 4 × 4 ×

1.3 nm and already containing the MFI topology was proposed. Heating this suspension of

MFI nanoslabs led to the formation of intermediates and eventually to silicalite-1 colloidal

crystals via an aggregation mechanism.55

1.5 Scope of the thesis

The ultimate goal in fundamental research in zeolite synthesis is to arrive at tailor-

made zeolite catalysts with suited pore size, dimensionality and chemical composition for a

given process or reaction. Zeolite synthesis has been mainly an empirical field in which a

large number of experiments were based on the variation of the basic parameters. Even

13

though high-throughput synthesis methods are now being developed56,57,58, the understanding

of the concept of structure direction in microporous materials remains of fundamental

importance for the design of new molecular sieves.39

Over the past decade, new insights into this problem have been brought in.

Experimentally, the use of in situ techniques applied during zeolite synthesis has been

developed. Recent reports agree that subcolloidal particles play an important role during the

nucleation and crystallization of zeolites. The driving force that has prompted this thesis is to

investigate the formation, the nature, the role and the transformations of nanometer-scale

embryonic species during the crystallization of silicalite-1 and ZSM-5. Particularly, this work

aimed at finding unifying concepts regarding the above-mentioned studies using silicic acid

and TEOS as silica sources. TEM, in situ (U)SAXS and NMR were mainly employed to

achieve this goal. This research project involved a cooperation with the group of Prof. Johan

Martens from the Center for Surface Chemistry and Catalysis, KU Leuven, Belgium.

The occurrence of silicalite-1 precursors in clear solutions is investigated in chapter 2.

Special attention has been given to the size of nanoprecursors encountered in several

representative syntheses. The techniques involved in this investigation comprise small-angle

X-ray scattering and transmission electron microscopy.

Chapter 3 is mainly devoted to an extensive NMR study of MFI precursors. 29Si

NMR, SAXS and USAXS were combined to study in situ transformations occurring during

the depolymerization-oligomerization of silicic acid in an aqueous concentrated

tetrapropylammonium hydroxide solution. The results are discussed in terms of intermediate

silicates formed in the oligomerization process. In a second part, specific NMR techniques

were applied to study aluminum incorporation in both the precursors and the final ZSM-5

crystals. The determination of distances between structure-directing agents and silica at early

stages of the synthesis was done with NMR as well.

An introduction in the theory of SAXS and the combined SAXS/WAXS and Bonse-

Hart setup of the ID02 beamline at ESRF is presented in chapter 4. The combination of in situ

SAXS/USAXS allowed us to study the influence of Al framework substitution during the

whole course of ZSM-5 crystallization, from the formation of specific precursors to the

crystal growth.

The possible use of MFI precursors in material synthesis is illustrated in chapter 5.

The method proposed consists of a hierarchical templating scheme in which zeolite building

units assemble into mesostructures via a secondary cooperative templating mechanism.

14

References 1 Cronstedt, A. F. translated by: Schlenker, J. L.; Kühl, G. H., In: Proceedings of the 9th International Zeolite Conference, Montreal 1992, Ed. Von Ballmoos, R.; Higgens, J. B.; Treacy, M. M. J. Butterworth-Heinemann, 1993, 3. 2 Tanabe, K.; Hölderich, W. F. Appl. Catal. 1999, 181, 399. 3 Thomas, J. M.; Thomas, W. J. Principles and Practice of Heterogeneous Catalysis, VCH, Weinheim, New York, 1997. 4 Van Bekkum, H.; Flanigen, E. M.; Jacobs, P. A.; Jansen, J. C. Introduction to zeolite science and practice, 2nd Edition, Stud. Surf. Sci. Catal.; Elsevier, 2001. 5 http://www.iza-online.org 6 Baerlocher, C.; Meier, W. M.; Olson, D. H. Atlas of zeolite framework type, Fifth Revised Edition, Elsevier, 2001. 7 Freyhardt, C. C.; Tsapatsis, M.; Lobo, R. F.; Balkus Jr, K. J.; Davis, M. E. Nature, 1996, 331, 295. 8 Yoshikawa, M.; Wagner, P.; Lovallo, M.; Tsuji, K.; Takewaki, T.; Chen, C-Y.; Beck, L. W.; Jones, C.; Tsapatsis, M.; Zones, S. I.; Davis M. E. J. Phys. Chem. B 1998, 102, 7139. 9 Derouane, E. G., Nagy, J. B. Chem. Phys. Letters 1987, 137, 341. 10 Derouane, E. G.; Lucas, A. A.; André, J. M. Chem. Phys. Letters 1987, 137 336. 11 Weisz, P. B.; Frilette, V. J.; Maatman, R. W.; Mower, E. B. J. Phys. Chem. 1960, 64, 382. 12 Csicsery, S. M. ACS Monograph, 1976, 171, 680. 13 Weisz, P. B. Pure Appl. Chem. 1980, 52, 2091. 14 Csicsery, S. M. Zeolites, 1984, 4, 202. 15 Dwyer, J. Chem. Ind. 1984, 7, 229. 16 Weitkamp, J.; Ernst, S. Catal. Today, 1994, 19, 107. 17 Martens, J. A.; Souverijns, W.; Verrelst, W.; Parton, R.; Froment, G.; Jacobs, P. A. Angew. Chem. Int. Ed. 1995, 35, 2528. 18 Goossens, A. M.; Vanbutsele, G.; Martens, J. A. In Fundamentals of Adsorption 6, F. Meunier, Elsevier, Paris, 1998, 31. 19 Smart, M.; Esker, T.; Leder, A.; Sakota, K. Chemical Economics Handbook, SRI International, 1999, 599. 20 Sainte-Claire-Deville, M. H. Compt. Rend. 1862, 54, 324. 21 Barrer, R. M. J. Chem. Soc. 1948, 2158. 22 Milton, R. M. US Patent 2,882,243, 1959. 23 Milton, R. M. US Patent 3,008,803, 1961. 24 Barrer, R. M.; Denny, P. J. J. Chem. Soc. 1961, 971-982. 25 Argauer, R. J.; Landolt, G. R. US Patent 3,702,886 1972. 26 Grose, R. W.; Flanigen, E. M. US Patent 4,061,724 1977. 27 Wilson, S. T.; Lok, B. M.; Flanigen, E. M. US Patent 4,310,440 1982. 28 Wilson, S. T.; Lok, B. M.; Flanigen, E. M. J. Am. Chem. Soc. 1982, 104, 1146. 29 Zhou, Y.; Zhu, H.; Chen, Z.; Chen, M.; Xu, Y.; Zhang, H.; Zhao, D. Angew. Chem. Int. Ed. 2001, 40, 2166. 30 Lin, C. H.; Wang, S. L.; Lii, K. H. J. Am. Chem. Soc. 2001, 123, 4649. 31 Breck, D. W. Zeolite molecular sieves, John Wiley, New York, 1974. 32 Barrer, R. M. Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982. 33 Jacobs, P. A.; Martens, J. A. Synthesis of high-silica aluminosilicate zeolites, Studies in Surface Science and Catalysis Series; Elsevier Science, New York, 1987, Vol. 33. 34 Szoztak, R. Molecular sieves, Blackie Academic & Professional, 1998.

15

35 Verduijn, J. P.; Martens, L. R. M.; Martens, J. A. US Patent 5,783,321 1995. 36 Kühl, G. H. In 2nd International Conference on Molecular sieve Zeolite, American Chemical Society, Washington, DC, 59, 1970. 37 Mintova, S., Valtchev, V. Microp. Mesop. Mat. 2002, 55, 171. 38 Gies, H.; Marler, B. Zeolites 1992, 12, 42. 39 Lobo, R. F.; Zones, S. I.; Davis, M. E. J. Inclusion Phenom. Mol. Recognit. Chem. 1995, 21, 47. 40 Van Santen, R. A.; Keijsper, J.; Ooms, G.; Kortbeek, A. G. T. G. In New developments in Zeolite Science and Technology, Elsevier, Amsterdam, 169, 1986. 41 Burkett, S. L.; Davis, M. E. J. Phys. Chem. 1994, 98, 4647. 42 Burkett, S. L.; Davis, M. E. Chem. Mater. 1995, 7, 920. 43 Burkett, S. L.; Davis, M. E. Chem. Mater. 1995, 7, 1453. 44 Dokter, W. H.; van Garderen, H. F.; Beelen, T. P. M.; van Santen, R. A.; Bras, W. Angew. Chem. Int. Ed. Engl. 1995, 34, 73. 45 Watson, J. N.; Iton, L. E.; Keir, R. I.; Thomas, J. C.; Dowling, T. L.; White, J. W. J. Phys. Chem. B 1997, 101, 10094-10104. 46 Watson, J. N.; Brown, A. S.; Iton L. E.; White, J. W. J. Phys. Chem. B 1998, 94, 2181. 47 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanshek, B. U.; Diat, O.; van Santen, R. A. J. Phys. Chem. B 1997, 101, 11077-11086. 48 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanschek, B. U.; Beck, L. W.; Wagner, P.; Davis, M. E.; van Santen, R. A. Chem.-Eur. J. 1999, 5, 2083-2088. 49 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A. J. Phys. Chem. B 1999, 103, 1639-1650. 50 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Beck, L. W.; Davis, M. E. J. Phys. Chem. B 2000, 104, 7600-7611. 51 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Tsuji, K.; Davis, M. E. Chem. Mat. 1999, 11, 36-43. 52 Ravishankar, R.; Kirschhock, C. E. A.; Knops-Gerrits, P-P.; Feijen, E. J. P.; Grobet, P. J.;

Vanoppen, P.; De Schryver, F. C.; Miehe, G.; Fuess, H.; Schoeman, B. J.; Jacobs, P. A.; Martens J. A J. Phys. Chem. B 1999, 103, 4960. 53 Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965. 54 Kirschhock, C. E. A.; Ravishankar, R.; van Looveren, L.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4972. 55 Kirschhock, C. E. A.; Ravishankar, R.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 11021-11027. 56 Akporiaye, D.; Dahl, I. M.; Karlsson, A.; Wendelbo, R. Angew. Chem. Int. Ed. 1998, 37, 609. 57 Holmgren, J.; Bem, D.; Bricker, M.; Gillespie, R.; Lewis, G.; Akporiaye, D.; Dahl, I.; Karlsson, A.; Plassen, M.; Wendelbo, R. Zeolites and Mesoporous Materials at the Dawn of the 21st Century, Stud. Surf. Sci. Catal. 2001, 135, 461. 58 Klein, J.; Lehmann, C. W.; Schmidt, H-W.; Maier, W. F. Angew. Chem. Int. Ed. 1998, 37, 3369.

16

17

2

Zeolite nanoslabs: a combined SAXS and TEM study*

The formation and growth of crystal nuclei of TPA-silicalite-1 (aluminum free ZSM-5) from clear solutions using TEOS and silicic acid as silica sources were studied with small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Information was obtained on the size and shape of nanoscopic precursor particles of silicalite-1. TEM provided a direct observation of slab-like embryonic particles. The combined SAXS and TEM data can be interpreted assuming the presence of nanoslabs of 4 × 2 × 1.3 nm up to 8 × 8 × 1.3 nm depending on the synthesis conditions. Starting solutions have been studied varying the silica source and the cation content (sodium, potassium and TPA). In each case, well-defined particle sizes are observed. Although the nanoparticles differ in size, their shape is very similar and these data strengthen our hypothesis that TPA-silicalite-1 formation is a nanoblock-based aggregation mechanism rather than growth via monomer addition.

* Reproduced in part from: Houssin, C. J. Y.; Mojet, B. L.; Kirschhock, C. E. A.; Buschmann, V.; Jacobs, P. A.; Martens, J. A.; van Santen, R. A. Stud. Surf. Sci. Catal. 2001, 135, 135. Kirschhock, C. E. A.; Buschmann, V.; Kremer, S.; Ravishankar, R.; Houssin, C. J. Y.; Mojet, B. L.; Grobet, P. J.; van Santen, R. A.; Jacobs, P. A.; Martens, J. A. Angew. Chem. Int. Ed. 2001, 40, 2637.

18

2.1 Introduction

Monitoring the early stages of zeolite synthesis still remains a challenge.1 It has long

been known that cationic species (Na+, Al3+ or organic molecules) directly influence the

resulting multi-dimensional crystal lattice.2,3,4, Strong indications of interactions between the

organic molecules and the silicate species in the synthesis mixtures have been observed.5

Consequently, nanoscale species are present well before the formation of long-range order.

But the nature and extent of the interactions between the organic and inorganic components

are not very well understood.6,7 Analytical techniques that can be used to follow the

crystallization include IR,8 Raman,9 DLS,10,11,12 NMR spectroscopy5,8,13 and small-angle

scattering (SAS).14,15,16 SAXS (small-angle X-ray scattering) is a powerful technique which

can provide in situ information on particle size, shape and aggregation in the size range from

1 nm to >100 nm. Moreover, silicate intermediates are so fragile that only non-invasive in

situ measurements will give valuable results. Consequently, in situ time-resolved SAXS

experiments are appropriate to probe zeolite synthesis. Nanoscopic species (3-4 nm) were

recently observed in the formation of the pure silica MFI in presence of TPA from a clear

solution.14 They have been proposed to play a key role in the nucleation and crystal growth.17

The detailed structure of these nanoparticles is beyond the scope of this chapter but particles

prepared from TEOS (tetraethylorthosilicate) and TPA (tetrapropylammonium) have been

extensively investigated by 29Si NMR.8 So far, no comparative studies were conducted on the

TEOS-based and silicic acid-based Si-TPA-silicalite-1 crystallization system. TEOS is an

organic component whereas silicic acid belongs to inorganic silica sources, sometimes used

by industry for zeolite synthesis. The presence of sodium is also important since it is believed

to enhance the dissolution of amorphous silica in alkaline solutions. In fact, sodium facilitates

the transport of hydroxyl anions towards the silica surface. Moreover TPA molecules tend to

strongly adsorb to the silica surface. In this study, the effect of those factors has been

investigated in terms of size of the particles formed in the synthesis mixtures for silicalite-1.

19

2.2 Experimental section

Details of the synthesis procedures are:

- TEOS and TPA system: The synthesis is adapted from Ravishankar et al.18 9 g of TEOS

(Acros, 98%) was added dropwise to a 40% aqueous solution of TPAOH (Alfa) under

vigorous stirring. 9 g of distilled water was then added dropwise after 30 min and the

resulting mixture was stirred continuously for 12h to ensure complete hydrolysis of the silica

source.

- Silicic acid and TPAOH system: The recipe used is based on a patent of Exxon

Chemicals.19 0.411 g of NaOH was dissolved in 15 g of 20% TPAOH in water (Merck),

followed by a spoonwise addition of 4.05 g of silicic acid (Baker, 10.2% H2O). The milky

dispersion was boiled under stirring for 10 min to obtain a clear solution. The mixture was

rapidly cooled down to room temperature in a water bath. Distilled water was added for the

correction for loss of water during boiling. The resulting clear solution was then filtered

through a 0.45 µm syringe filter. The sample was then ready for measurement and never aged

more than 1h. For some solutions NaOH was replaced by an equivalent amount of TPAOH or

KOH. In the latter cases, the dissolution of silicic acid appeared to be more difficult, but 10

min boiling ensured complete dissolution of the silica.

SAXS measurements were performed at the Dutch-Belgian beamline (DUBBLE) at

the ESRF (France) using high-brilliance synchrotron radiation. To perform in situ reactions,

rotating cells were used in which liquid samples could be heated. SAXS patterns were

recorded on the synthesis mixtures described in table 1. All samples were clear and

background solutions were measured for data correction.

Transmission electron microscopy was performed at the Materials Science

Department, Darmstadt University of Technology, Germany. The observations were made

with a Philips CM200 TEM (200 kV, point resolution 0.19 nm) and a JEOL JEM3010 (300

kV, point resolution 0.17 nm) instruments equipped with a Gatan GIF200 electron energy

loss spectrometer. Measurements were performed with all necessary precautions, using a

defocused electron beam, rather low magnification and a video camera connected to the TEM

camera to monitor any possible change in time.

20

S1 S2 S3 S4 TPAOH × × × × TEOS × Silicic acid × × × NaOH × KOH ×

Table 1. Composition of the different starting solutions.

2.3. Results and Discussion SAXS

Figure 1 shows the SAXS patterns of the solutions S1 and S3 (Figure 1a) and S2 and

S4 (Figure 1b). At room temperature several peak maxima can be observed. These starting

solutions all lead to silicalite-1 formation upon heating. They contain a well-defined particle

population with a different size depending on the composition.

The effect of the silica source on the formation of subcolloidal particles is illustrated

in figure 1a. TEOS is an organic and monomeric source of silica whereas silicic acid is

polymeric and can provide monomers, dimers and oligomers. Hydrolysis of TEOS is almost

completed after a few hours stirring at RT but boiling 10 min is needed to dissolve the silicic

acid. TPA molecules interact with silica at a liquid-liquid interface and may function as a

structure-directing agent at very early stages8 when using TEOS, but they strongly adsorb on

the solid surface in the case of silicic acid. This phenomenon influences the releasing of

silicate anions by hydroxide ions from the solid surface. Iler has proposed a mechanism for

dissolution of silica in alkaline aqueous solution in which hydroxyl ions act as catalysts.20

Figure 1a shows two different maxima corresponding to two different characteristic

lengths. Obviously, the use of silicic acid leads to smaller particles than TEOS (3.6 nm and

2.1 nm for S1 and S3 respectively). These results clearly show that two distinct but well-

defined particle populations are formed, depending on the silica source.

21

0

0.1

0.2

0.3

0.5 1.5 2.5

S1

I (a.u.)

S3

Solutions d (nm ) S1 3.6Solutions d (nm)

Figure 1a. SAXS patterns recorded at RT of twoThe scattering vector q is related to the chasubcolloidal particles by the equation: q=2π/d.

0

0.1

0.2

0.3

0.5 1.5 2.5

I (a.u.)

S2

S4

Figure 1b. SAXS patterns recorded at RT of start

Figure 1b shows that the use of sodium o

significantly the size of the particles when using

characteristic length obtained (around 3 nm) is diffe

exclusively used as cation. In fact, alkali ions adsor

have a hydration sphere smaller than TPA, facilitati

surface and giving rise to a different silica dissol

counter ions of the negatively charged surface of t

formation and stability. These two effects may exp

3.5 4.5

q (1/nm )

S3 2.2

S1 3.6 S3 2.1

different starting solutions.racteristic length d of the

3.5 4.5

q (1/nm )

Solutions d (nm) S2 2.9 S4 3

ing solutions using two different alkali ions.

r potassium cations does not influence

silicic acid as silica source. But the

rent from the one found when TPA was

b less strongly to the silica surface and

ng the transport of hydroxide ions to the

ution rate. Alkali ions may also act as

he nanoparticles, then influencing their

lain the size difference. In conclusion,

22

there is experimental evidence that the size of the nanoparticles formed during the early

stages of the silicalite-1 synthesis depends not only on the silica source but also on the type of

cations present next to TPA.

-1

-0.8

-0.6

-0.4

-0.2

0

0.2

0.4

-0.6 -0.4 -0.2 0 0.2 0.4

S1

S2

log I (a.u.)

log q (1/nm)

slope -2.2

Figure 2. SAXS patterns after 8 h at 100°C for S1 (125°C for S2).

In an earlier work on the S1 system,8 a model for these nanoparticles was presented in

which the MFI structure is already developed. The particles are slab like with dimensions of 4

× 4 × 1.3 nm. Assuming that the particles observed in the present study are also slab like, the

SAXS signal would correspond to a characteristic length d of 3.7 nm, which is close to the

SAXS experimental results (3.6 nm). The 3 nm dimension in S4 exactly corresponds to a

slab-like particle of 4 × 2 × 1.3 nm which is half of size of the first nanoslabs (S1). Sodium is

shown to influence interactions of these nanoblocks because of its electrostatic charge. SAXS

experiments were done on these nanoparticles (see chapter 4) and it appeared that the Porod

region of the scattering intensity exhibits a slope close to -2. Since the particles formed in S1

are larger, we expect a slope close to -2, which is the value for an infinite sheet. But for the

smallest species (S2 and S3), the Porod slope may be higher and modelisation should be

performed to determine an accurate theoretical slope for a slab of limited size.

Figure 2 shows plots after 8 hours heating of S1 and S2 samples. In the case of S1, the

disappearance of the small particles in favor of the formation of larger entities, not yet

exhibiting Bragg reflections, is observed. These larger nanoparticles are designated as

intermediates. Those intermediates have a Porod slope of –2.2 characteristic of tablet-like

particles. For S2, the increase of intensity after 8h at low q values is due to the scattering of

colloidal crystals already formed. Nanoblocks of dimensions 4 × 2 × 1.3 nm are still present

in the synthesis mixture.

23

In situ temperature dependent dissolution of the silica source in the preparation of the

solution S3 was investigated (Figure 3). SAXS data showed that no particles were formed

after 30 min at 60 °C. It was noted that nanoparticles are growing from 2 to 2.1 nm when

heating few minutes at 100 °C. This growth until an optimum size of 2.1 nm is probably due

to an increasing amount of the soluble silica available with temperature but it indicates that a

very specific size is favoured.

0

0.02

0.04

0.06

0.08

1.5 2 2.5 3 3.5 4 4.5 5

I (a.u.)

q (1/nm)RT

60°C

boiled 10 min.

100°C

Figure 3. In situ time and temperature dependent scattering intensity of Si-TPA-silicic acid

mixture. This solution was first heated 30 min at 60 °C and then to 100 °C. Two spectra were

recorded at 100°C after 3 and 9 min heating.

Transmission electron microscopy

The purpose of the TEM study was to reveal the nature of the particles with

equivalent characteristic length of 2.8-4.3 nm from SAXS and previously detected in MFI-

type zeolite syntheses with DLS, SAXS and neutron scattering.11,12,14,15 Atomic force

microscopy (AFM) on an evaporated suspension of TEOS – TPAOH – H2O revealed a

stepped surface having a characteristic step height of 1.2 ±0.3 nm.21 A detailed structure was

proposed from a 29Si NMR study of the TPA-mediated polycondensation sequence of silicate

monomer using TEOS as silica source.8 29Si NMR spectra provided evidence for the

occurrence of a 33-Si-atom precursor having a characteristic length of 1.35 nm. This

particular specimen is believed to have a size of 1.3 × 1.3 × 1 nm. Addition of water induced

a three by four assembly of this precursor resulting in a very stable suspension of

24

nanoparticles exhibiting a characteristic length of 3.6 nm (Figure 1a). This solution was

spread on a grid for TEM investigations. A large number of nanosized particles was observed

(Figures 4a and 4b). The nanoparticles could survive only few seconds in the electron beam.

From figure 4b, the average in-plane dimensions of individual nanoparticles was estimated to

be 4 × 4 nm, with a thickness of about 1 nm as showed by the dimensions of standing

particles. Occasionally, larger blocks were detected (Figure 4d) which had in-plane

dimensions corresponding to multiples of 4 nm, suggesting they formed through sidewise

aggregation of single nanoslabs observed in figures 4a and 4b. TEM investigations on the

colloidal suspension from room temperature digestion of silicic acid in an aqueous solution of

TPA and sodium hydroxide (solution S2) revealed the presence of rectangular nanoparticles

with in-plane dimensions of 4 × 2 nm (Figure 4c).

Figure 4. TEM images of nanoslabs. a) and b) prepared from TEOS – TPAOH – H2O (solution S1).

c) obtained from silicic acid, TPAOH and NaOH (solution S2). d) occasionally observed larger block

in solution S1.

25

SAXS patterns of solutions S1 and S2 with theoretical scattering functions for nanoslabs

measuring 4 × 4 × 1.3 nm and 4 × 2 × 1.3 nm are displayed in figure 5. Experimental curves

are in excellent agreement with simulated patterns. Consequently the introduction of the in-

plane dimensions derived from the TEM investigations combined with SAXS observations

revealed a thickness of 1.3 nm for the two nanoparticle populations.

Figure 5. SAXS curves of a) solution S1 and b) solution S2. Position and shape agree with simulated

patterns (gray lines) of uniform slab-like entities of dimensions 4 × 4 × 1.3 nm (S1) and 4 × 2 × 1.3

nm (S2).

Formation of TPA-silica composite nanoparticles from the system TEOS – TPAOH –

H2O involves a polymerization process driven by close interactions between TPA cations and

silicate species that shield the propyl groups of TPA cations from the aqueous solution. TPA

molecules are located at the liquid–liquid interface and it has been suggested that the

hydrolysis of TEOS and the structure direction effect take place simultaneously.8 This

complicated process leads to the formation of a peculiar tetracyclic undecamer and eventually

to the occurrence of a 33-Si-atom precursor depicted in figure 6b. A comparative 29Si NMR

study recorded at an early stage of silicic acid digestion and TEOS polymerization in

presence of TPAOH showed that both systems probably proceed under the same silica

polymerization sequence (Figure 6).

26

Figure 6. (a) 29Si NMR spectrum at an early stage of silicic acid digestion in aqueous TPAOH.

Chemical shifts of the 33-Si-atom precursor taken from ref. 8 are indicated in gray lines. (b) For

comparison, the 29Si NMR spectrum of the same precursor in the TEOS system.

In the case of the silicic acid system (solution S2), the signals were rather weak and

significantly broadened (Figure 6a) compared to those in the spectrum of the 33-Si-atom

precursor found in the TEOS system (Figure 6b). This could be due to the fact that silicates

were not solution-borne but still adsorbed on remaining silica. 29Si NMR lines were indeed

not present after removal of the solid phase over a filter with a pore size of 100 nm.

Figure 7. SAXS curves of a) solution S1 prepared with an excess of TPAOH and b) solution S3. The

gray curve shows the simulated pattern for a slab with dimensions of 2.7 × 1 × 1.3 nm.

27

Additional evidence for formation of specific zeolite nanoslabs by self-assembly of the

33-Si-atom precursor irrespective of the silica source are provided from experiments using

higher TPAOH concentrations. Figure 7 displays SAXS experimental and simulated patterns

for solutions prepared from TEOS (solution S1 prepared with an excess of TPAOH) and

silicic acid (replacement of NaOH by TPAOH in S2 i.e. solution S3). The simulated curve

corresponds to a nanoslab with dimensions of 2.7 × 1 × 1.3 nm which matches the size of a

double 33-Si-atom precursor. The asymmetric character of the experimental curves in figures

5 and 7 and their good agreement with slab simulated patterns show that these particles are

not spherical.

Fusion of nanoslabs is hindered at room temperature due to a steric barrier created by

an excess of TPA molecules which are adsorbed on the nanoslabs surface.22 At 100 °C, larger

nanoslabs as identified in figure 2 form by sidewise fusion and lead ultimately to silicalite-1

colloidal crystals. The self-assembly of the 33-Si-atom precursor into different well-defined

and discrete nanoslabs depending on synthesis conditions such as silica source and sodium

content is depicted in figure 8.

S3, S4 S2

S1Aggregate upon heating

Figure 8. Proposed schematic structures for the MFI-type zeosil nanoslabs. The 33-Si-atom precursor

can self-assembly to form stable discrete and organic-inorganic hybrid nanoslabs with dimensions

depending on synthesis conditions.

28

2.4 Conclusion

In this comparative study of the early steps of Si-TPA-silicalite-1 synthesis, the nature of

silica source and inorganic cations were varied. SAXS and TEM revealed to be very powerful

techniques to probe subcolloidal particles formation. It has been shown that, first, these

particles leading to the same zeolite have a well-defined size and, second, they appear to be

tablet-like particles although they differ in size. Based on these results it is suggested that

silicalite-1 crystals form in an organic-mediated synthesis as follows: very well-defined

nanoslabs, which sizes are determined by the silica source and cations, are formed at RT due

to the interactions between the organic template and silicates species. Upon heating,

energetically favorable aggregation occurs and crystal growth is then accomplished by

eliminating water molecules at the interface and oriented attachment.

These peculiar nanoslabs can be used as versatile building units for the synthesis of

innovative porous materials. Synthesis of monoliths, liquid crystals, films or mesoporous

materials are among the potential applications of zeosil nanoslabs.

References

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29

15 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanschek, B. U.; Beck, L. W.; Wagner, P.; Davis, M. E.; van Santen, R. A. Chem.-Eur. J. 1999, 5, 2083. 16 Watson, J. N.; Brown, A. S.; Iton L. E; White, J. W. J. Phys. Chem. B 1998, 94, 2181. 17 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A. J. Phys. Chem. B 1999, 103, 1639. 18 Ravishankar, R.; Kirschhock, C. E. A.; Schoeman, B. J.; De Vos, D.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. In Proceedings of the 12th international zeolite conference, Materials Research Society, 1999, 1825. 19 Verduijn, J. P. Exxon Patent, PCT/EP92/02386, 1992. 20 Iler, R. K. The chemistry of silica, John Wiley and Sons, 1979. 21 Ravishankar, R.; Kirschhock, C. E. A.; Knops-Gerrits, P-P.; Feijen, E. J. P.; Grobet, P. J.;

Vanoppen, P.; De Schryver, F. C.; Miehe, G.; Fuess, H.; Schoeman, B. J.; Jacobs, P. A.; Martens J. A. J. Phys. Chem. B 1999, 103, 4960. 22 Kirschhock, C. E. A.; Ravishankar, R.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 11021-11027.

30

31

3

A 29Si and 27Al NMR study of MFI precursors*

This chapter examines the huge applications of NMR spectroscopy for probing different features of the structure-direction effect of organic molecules encountered in zeolite synthesis. Silicic acid powder dissolution in a concentrated tetrapropylammonium hydroxide aqueous solution was first followed by 29Si NMR. X-ray scattering was used to follow processes at a colloidal level. The appearance of very well-defined colloidal particles was linked to a specific intermediate already observed in systems using an organic and monomeric silica source. Based on these results, we propose a mechanism describing the TPA-mediated self-assembly of silicalite-1 from silicic acid powder as silica source. The development of new high-resolution methods in NMR offers many possibilities for studying supramolecular assemblies involved in zeolite synthesis. After a classical 27Al NMR study on the incorporation of aluminum in silicalite-1 nanoprecursors, rotational-echo double resonance was applied to measure inorganic-organic interactions in the TPA-mediated synthesis of silicalite-1. Multiquantum 27Al MAS NMR was used in an attempt to estimate the broadening effect due to quadrupolar interaction. An attempt was made to probe the aluminum framework incorporation in the nanoblock-based MFI zeolite synthesis.

* Reproduced in part from: C. J. Y. Houssin; C. E. A. Kirschhock; P. C. M. M. Magusin; B. L. Mojet; P. J. Grobet; P. A. Jacobs; J. A. Martens; R. A. van Santen ’Combined in situ 29Si NMR and Small-Angle X-ray scattering study of precursors in the MFI zeolite formation from silicic acid in TPAOH solutions’ submitted.

32

3.1 Combined in situ 29Si NMR and Small-Angle X-ray scattering study of precursors in the MFI zeolite formation from silicic acid in TPAOH solutions

3.1.1 Introduction Zeolite materials are among the most widely used catalysts in the chemical industry

while being among the least well understood. They are crystalline aluminosilicate molecular

sieves and account for one of the most studied inorganic material family in the last decades.1

Starting back in the 1950s, they have progressively replaced amorphous silica-alumina based

catalysts or other acids like clays in the petrochemical industry for processing of crude oil to

fuels and basic chemicals.2 Zeolite catalysts exhibit unique catalytic activity and selectivity

which originate from their well-defined microporosity.3

The flexibility of the Si-O-Si angle and the small energetic difference between various porous

silica materials explain the ever-growing number of known zeolite structures.4 Despite that

more than 200 different topologies have been found, mainly four zeolite frameworks (LTA,

FAU, MOR and MFI topologies) are involved in industrial applications ranging from

heterogeneous catalysts to adsorbents and ion exchangers.2 Macroscopic properties such as

crystal shape, size, polydispersity and framework composition can be achieved using the right

synthesis procedures, thus avoiding expensive post synthesis modification procedures.

Consequently, the rising number of applications as well as the possibility of discovering new

structures require a better comprehension of their synthesis. The fundamental understanding

of the mechanism that directs the assembly of these unique microporous materials is still

limited because zeolite synthesis processes are difficult to study. First, one of the most

important parameters in zeolite synthesis is certainly the composition of the starting mixtures

from which the crystals will grow. It generally consists of a complicated two-phase mixture

of water, a silica source, alkali cations and organics. Even the impurities as well as

differences among batches of silica sources sometimes have a large impact not only on the

kinetics but also on the nature of the crystalline phase obtained. Secondly, the interactions of

33

silicate species with cations present in precursor solutions remain unclear. Indeed, there has

been a lot of speculation on the role of organic molecules, alkali and alkaline-earth cations.5

Therefore, the prediction of the formation of a given topology based on the presence of a

specific guest molecule is not yet possible. Lastly, prenucleation, nucleation and crystal

growth involve very fragile colloidal species so that non-invasive techniques are best suited

for investigating zeolite synthesis. Only a few methods including NMR, Raman spectroscopy

or light and X-ray scattering can probe such transformations under synthesis conditions.

Chemical analysis through trimethylsilylation and GC analysis has extensively been used in

the past but this technique is too invasive regarding the fragile intermediates that are involved

in zeolite synthesis.

The ability of organic templates to organize SiO4 tetrahedra into very well-defined 3-

dimensional structures has long been recognized, notably for the formation of clathrasils.6

The introduction of organic molecules as guest species in the synthesis of zeolite molecular

sieves led to the discovery of the majority of the synthetic zeolites and contributed to achieve

materials with higher Si/Al ratios. In some instances, there is a strong dependency of the

zeolite topology obtained and the geometry of organic molecules and the zeolite topology.

Therefore, a lot of academic efforts have been devoted to study the nature of interactions

between organic molecules and inorganic precursors.7,8,9,10 There is no chemical bond

formation between template molecules and silicate. This supramolecular assembly process

arises from weak interactions such as van der Waals forces leading to intermediate species

that are not likely to withstand ex situ characterizations, involving isolation or chemical and

physical interventions. Consequently, the system necessitates in situ experiments.

In situ 29Si NMR has proven to be a powerful, non-destructive method for studying

silicate anions in solution.11,12 Actually, 23 oligomeric silicate structures were detected using

both 1D and 2D FT NMR spectroscopy during the 1980s, in silicate solutions containing

alkali and tetraalkylammonium cations.11,12 Recently, Kinrade et al. highlighted the strong

influence of tetraalkylammonium ions on silicate polycondensation.13 They concluded that

tetraalkylammonium ions directed the oligomerization of silicate anions towards the

prismatic hexamer, and mainly to the cubic octamer as final stable structures. Those species

having the tetraalkylammonium cations outside do not exhibit Si-Si exchanges with the

solution such as typically observed in alkali metal ion silicate solutions. The quest for stable

silicate oligomers is going on and has led to the recent discovery of three novel species.14

ZSM-5 is a synthetic high-silica zeolite and has the MFI topology characterized by a

3-dimensional pore system. Its remarkable catalytic and sorption properties are exploited in

34

commercial applications. ZSM-5 synthesis and its crystallization using various template

molecules has been reviewed.15 Notably, this is a very versatile zeolite since it can be

synthesized from a large variety of inorganic and organic agents some of which having hardly

any geometrical fit with the host framework. However, tetrapropylammonium (TPA) is

regarded as the strongest structure-directing organic cation for the MFI topology since it

enhances nucleation rate, fits very well in the channel intersections and allows ZSM-5 to be

synthesized over a wide range of synthesis conditions. The formation of peculiar silicate

oligomers during the hydrolysis of tetraethylorthosilicate (TEOS) in aqueous TPAOH was

recently observed using 29Si NMR.16,17 Two particular new species were encountered as

intermediates in the TPA-mediated self-assembly of colloidal silicalite-1, the all-silica

counterpart of ZSM-5. The first species reported was an undecamer having three five-rings

on a four ring that could polycondensate to form a 33-Si-atom precursor showing MFI

topology. It has been shown that this undecamer could also transform into a pentacyclic

dodecamer by addition of one silicate unit17. The observation that colloidal silicalite-1

crystals do not exhibit systematically missing T-sites led to the conclusion that most

undecamers were converted to dodecamers which are coupled into 36-Si-atom precursors.

After addition of water to the solution, the dominant species was found to be silicalite-1

nanoslabs, built from 12 precursors and having dimensions of 4 × 4 × 1.3 nm as confirmed

with TEM and SAXS.18 A model was then proposed in which silicalite-1 crystallization takes

place via nanoslab aggregation mechanism.19

This aggregation pathway leading to silicalite-1 formation was observed with TEOS

as silica source. In the present study the conversion of a polymeric silica source (silicic acid)

in concentrated TPAOH aqueous solutions will be presented. It was already known that at

room temperature the combination of silicic acid-TPAOH-H2O leads to the spontaneous

formation of subcolloidal particles resembling nanoslabs.20 Nanosized particles were also

observed by De Moor et al. using a combination of SAXS/USAXS/WAXS and a particular

system involving sodium cations besides TPA cations.21,22,23,24,25 The goal of the present work

was to investigate the polymerization process of silicic acid in highly concentrated TPAOH

solutions as a continuation of the recent study on the TEOS-TPAOH-H2O system. 29Si NMR

was used to determine the different silicate species present in solution. Subcolloidal and

colloidal particle populations were investigated using in situ synchrotron SAXS and USAXS

techniques. A reaction pathway for the TPA-mediated formation of silicalite-1 from silicic

acid is proposed.

35

3.1.2 Experimental section

Zeolite synthesis. The silicalite-1 synthesis recipe used is based on a patent of Exxon

Chemicals.26 4.37 g of distilled water was added to 12.71 g of 40% TPAOH in water (Fluka),

followed by a spoonwise addition of 4.05 g of silicic acid (Baker, 10.2% H2O). The molar

composition of the synthesis mixture was 4.41 TPAOH / 10 SiO2 / 117 H2O. The milky

dispersion was stirred 24h to obtain a clear solution and transferred into the sample cell.

Synthesis temperature was 125 °C. Crystallinity was verified by XRD.

NMR. 29Si liquid NMR spectra were recorded on a Bruker DMX-500 (11.7T) at 0°C

operating at 99.36 MHz and using a 7 mm probe head at a spin rate of 500 Hz. In a typical

measurement 512 spectra were accumulated with a pulse length of 3 µs (45° pulse) and a

repetition time of 20s. Chemical shifts were referenced internally to the silicate monomer

resonance.

SAXS and USAXS. The combined SAXS and USAXS experiments were performed on the

high-brilliance beamline ID02A at the European Synchrotron Radiation Facilities, Grenoble,

France.27 This beamline uses a highly monochromatic beam with very low divergence and

small cross section. The SAXS setup consists of a pinhole camera with a beam stop located in

front of an image intensified 2-D CCD camera. Sample to detector distances of 1.5, 2.5 and 6

m were used. Conversion of detector pixels (CCD camera) to the scattering vector q (nm-1)

was performed with the help of a lupolen polyethylene (BASF) sample. The X-ray

wavelength was 0.99 Å. An incident X-ray beam with a cross section of 0.2 X 0.2 mm2 was

used. The high brilliance allowed us to record a SAXS pattern in less than 1s. Data were

corrected for detector response and background using a water reference sample at the same

temperature.

USAXS patterns were recorded using a Bonse-Hart type of X-ray Camera.28,29 The

available range of scattering vector q was 0.001-0.14 (nm-1) with q = (4π/λ)sin2θ and λ = 1.0

Å. A first crystal analyzer [Si(220)] was employed to scan the angle, then the X-ray beam

was collimated in the vertical direction with a second analyzer crystal [Si(111)]. The detector

was a high-dynamic range (≈10 7 counts/sec) avalanche photodiode. The beam size at the

sample was about 1 X 2 mm2. Several scans (4-5) over successive 2θ ranges with sufficient

overlap were recorded with different degrees of attenuation of the incident X-ray beam, so

that the intensities on the detector were in the linear range. For each sample, a rocking curve

spectrum was systematically measured, which consists of the scattering produced by the

36

setup, background scattering and the small intensity fluctuations of the beam. Due to the

strong scattering of silica, the contribution of water scattering was negligible.24 A complete

spectrum could be recorded in 15 min.

Sample Cell. An electrically heated brass holder containing a rotating circular sample cell,

designed in our group25, allowed us to perform in situ measurements. The sample cell rotates

at an approximate rate of 2 rpm thus keeping the solution homogeneous, preventing the

precipitation of zeolite crystals and reducing the sample exposure to a small spot. The sample

was inserted between two mica windows (Attwaters and Sons) separated by a PTFE spacer

(0.5 mm thick). Heating of the sample from RT to the reaction temperature (125 °C) requires

only 2 minutes.

3.1.3 Results and discussion

Initially, we performed 29Si NMR measurements on a system used formerly in our

group24 with the following composition: 10 SiO2 : 1.22 (TPA)O2 : 0.848 Na2O : 117 H2O. It

turned out that, even at low temperatures, silicic acid dissolution was too fast to be followed

with 29Si NMR. When TPA molecules replaced sodium cations, silicic acid dissolution rate

decreased dramatically. This enabled us to follow the entire process using natural abundance 29Si NMR. Differences in amorphous silica powder dissolution rate encountered in alkaline

solutions are due to many factors.30 However, at high pH, this process obeys to a single

mechanism in which hydroxyl ions act as catalysts to release monomeric silicate anions Qo.

Negatively charged surface silanols, neutralized by a cation shell, are attacked by hydroxyl

ions, which then increase the coordination number of silicon atoms. As a consequence, the

underlying siloxane bridges are weakened and silicate monomers can be released in solution.

Wijnen et al. ascribed rate differences to diffusion effects in the cation shell surrounding

amorphous silica.31 This implies that, in our case, TPA cations induced a displacement of

sodium cations from the silicic acid surface and caused an inhibition effect due to a slower

diffusion of hydroxyl ions. TPA cations, more hydrophobic than alkali metal cations, adsorb

very strongly to the silicic acid surface.

Natural abundance 29Si NMR of TPA silicate solutions are displayed in figure 1 at

several times after initial mixing of silicic acid powder and concentrated aqueous TPAOH

solutions. Dissolution of amorphous silica in aqueous alkaline solutions results in a gradual

37

increase in monomer concentration, which leads to oligomerization and further

polycondensation. The hydroxyl ion catalyzes the dissolution process which releases

Si(OH)4. Generally, high-resolution 29Si NMR spectra display individual resonance peaks

that can be grouped into bands corresponding to structurally different sites of the 29Si atom in

the oligomer. In this study, we used the Qn notation for the Si coordination as introduced by

Engelhardt.32 Q designs a silicon atom bonded to 4 oxygen atoms forming a tetrahedron and

the superscript n represents the number of siloxane (Si-O-Si) bridges at the specific 29Si atom

under study. For example Q0 denotes the monomeric anion SiO4- (protonation is not

considered by this notation), Q1 end groups with one siloxane bridge. Q2 silicons with two

siloxane bridges, and so on. Many configurations for Q1…Q4 sites are possible depending on

cyclisation and branching of the silicate skeleton. Even though resonance frequency shifts

arise from the particular environment of the 29Si atoms (connectivity, Si-O-Si angle, Si-O

bond length), the chemical shift of a given Si atom also depends on the pH of the medium,

silica concentration and cation type. Therefore the assignment of signals to silicate structures

must be carried out with extreme care. The procedures are even a matter of debate.

-45-40-35-30-25-20-15-10-505

1h

8h

12h20

11h

15h20

9h

7h

23h

ppm

Q0 Q1Q2cyc Q3

cycQ2 Q3

Figure 1. 29Si NMR of aqueous silicate solutions at different times after initial mixing of silicic acid

powder and a concentrated aqueous TPAOH solution. Q2cyc and Q3

cycl design Q2 and Q3 Si atoms in

cyclic positions respectively.

38

0

300

600

900

1200

1500

1800

0 5 10 15 20 25

Q0

Q2

Q1

Q3

Dissolution time (h)

Inte

grat

ed p

eak

area

Figure 2. Integrated peak area of the Qn resonance bands against dissolution time. Data from for the

spectra shown in figure 1. No Q4 were directly detected with NMR.

The evolution of the integrated peak area of the different resonance bands assigned to

Q0, Q1, Q2 and Q3 is shown in figure 2. It is evident that silica first dissolves to yield the

monomer ions (Q0) until a maximum concentration is reached. The monomer oligomerizes

rapidly to form the dimer with two Q1 silicons and higher molecular weight silicates with Q2

and Q3 silicons. A 29Si NMR spectrum of silicates obtained after 15 min of mixing silicic acid

in a concentrated aqueous solution of NaOH is shown in figure 3. Base concentrations are

similar to those with TPAOH (Figure 1). It is clear that the initial rate of silicic acid

dissolution is much higher compared to the rate observed using TPAOH. In contrast to

spectra of TPA silicate aqueous solutions, broad signals are observed corresponding to Q1,

Q2, Q3 and Q4 unspecified environments. With NaOH, dissolution and condensation of

silicates is much more rapid than with TPAOH.

-45-40-35-30-25-20-15-10-505ppm

Q0 Q1

Q2

Q3

Q4

Figure 3. 29Si NMR spectrum of an aqueous silicate solution 15 min after initial mixing of silicic acid

and a concentrated NaOH solution.

39

Structural information from chemical shift

Three selected 29Si NMR spectra of the silicic acid – TPAOH – H2O system after 7, 8

and 9h of mixing time are reported in figure 4. The silica solution after 1h of dissolution

contains the monomer, the dimer (δ = −8.6 ppm) and the cyclic trimer (δ = −10.1 ppm). After

a considerable time, non-cyclic Q2 species emerge with chemical shifts between δ = −16 and

δ = –18 ppm. The lines at δ = −9.9, −16.5 and −17.3 ppm are assigned to the bicyclic

pentamer. The linear trimer is also present. Most importantly, an additional line is found at δ

= −17.1 ppm. It is attributed to the prismatic hexamer. Indeed, this species tends to be favored

and is very stable in tetraalkylammonium solutions.11,33 But the most informative spectrum is

the one after 8h of mixing (Figure 4b). Besides the five species encountered in figure 4a (the

linear trimer at δ = –8.1 ppm is no longer present) additional lines are observed. The

formation of the tricyclic hexamer (structure IX in ref. 11c) is obvious regarding the

appearance of one Q2 line at δ = –16.2 ppm and three Q3 lines (δ = –16.7, –18 and –25 ppm).

The double bridged cyclic tetramer is found at a very small concentration as shown by its

above-noise but very typical signals at δ = –14.7 and –21.4 ppm. Three signals at δ = –17.6, –

24.8 and –25.2 ppm remain to be assigned. Based on the list of previously observed silicate

oligomers11 very few species can give rise to these resonances. The line at δ = –17.6 ppm

could be attributed to the Q3 site of the hexacyclic octamer (structure XVI in ref. 11c) but this

silicate would give rise to a relatively intense line at –20.5 ppm, which is not observed here.

The tricyclic octamer (structure 16 in ref 11d) and the pentacyclic nonamer (structure 13 in

ref. 11d) are also candidates. However, the absence of resonance lines at around δ = –15.7

ppm makes their presence unlikely. Using 29Si NMR, Kirschhock et al. evidenced the

existence of a new intermediate in the formation of silicalite-1 showing the structure-

directing effect of TPA cations in the system TEOS − TPAOH − H2O.17 This pentacyclic

dodecamer exhibits one 29Si NMR line in the Q2 range and two peaks in the Q3 range at δ = –

89.9, –97.2 and –98.2 (δ referred to external TMS or at δ = –18.5, –25.8 and –26.8 ppm

assuming that the monomer signal is located at δ = –71.4 ppm), respectively. This group of

peaks seems to be shifted a little upfield in the present investigation. In the original work of

Kirschhock et al., the signal at δ = –89.3 ppm (–17.9 ppm referred to Q0 of figure 3 of ref. 17)

can be assigned to the prismatic hexamer.34 In the present study this species gives rise to a

line at –17.1 ppm. This 0.8 ppm shift can possibly be attributed to the use of a different

chemical shift reference since unfortunately monomers were not present in the TEOS –

40

TPAOH – H2O system. The 29Si NMR signals were then referred to external TMS. Assuming

a systematic shift of 0.8 ppm, we expect the pentacyclic dodecamer to give rise to three

peaks, one at δ = –17.7 ppm and two at around –25 ppm, in good agreement with the present

values of –17.6 ppm, –24.8 and –25.2 ppm. The pentacyclic dodecamer is most likely formed

in the system under study. The TEOS and the silicic acid systems result in the growth of the

MFI zeolite structure upon heating. Strong similarities were previously observed on the

colloidal scale in terms of precursor particles irrespective of the use of silicic acid or TEOS as

silica source.18

-26-24-22-20-18-16-14-12-10-8

(a)

(b)

(c)

ppm

Figure 4. 29Si NMR spectra of dissolution of silicic acid in a concentrated aqueous TPA solution after

different reaction times: a) 7h, b) 8h and c) 9h.

Figure 5 shows 29Si NMR spectra during the process of silicic acid digestion by TPAOH

between 11h and 23h of mixing time. It is clear that two additional peaks appear in the Q3

region and that the monomer and dimer are absent. The first additional line is located at δ = –

25.9 ppm which corresponds to a species containing equivalent Q3 centers. Literature data

indicate the tetrahedral tetramer as the unique Q3 symmetric silicate in this region.11 The

signal that appears the latest in the NMR spectra is located at δ = –26.9 ppm. Consulting the

chemical shift data from literature, this peak can be assigned to several completely symmetric

41

silicate anions. The double four-rings (cubic octamers), double five-rings and double six-

rings are expected to give rise to a signal in this region. Knight et al. in their study of aqueous

TMA silicate solutions,13 evidenced the preferential formation of cubic octamer anions.

Indeed, TMA is well known to direct the synthesis of even-membered-ring zeolites.1

However double four-ring silicates have not been observed in TPAOH or TBAOH

solutions.11,33 Only one case is known on the existence of this species in TPAOH solution but

it was achieved under addition of a large amount of dimethylsulfoxide as co-solvent and low

alkalinity.13 Double six(or higher)-ring silicates have never been observed and seem unlikely

to be stable in solution.35 Moreover, the fact that double five-rings have only been observed

in ZSM-5 forming mixtures36 led us to the assignment of the –26.9 ppm line to double five-

rings. The assignment of 29Si NMR signals and comparison with literature data.11,12 is given

in figure 6.

-30-28-26-24-22-20-18-16-14-12-10-8ppm

(a)

(b)

(c)

(d)

Figure 5. 29Si NMR spectra of dissolution of silicic acid in a concentrated aqueous TPA solution at

different reaction times: a) 11h, b) 12h20, c) 15h20 and d) 23h. The arrows refer to the resonances of

the pentacyclic dodecamer discussed in the text.

42

Silicate species Experimental chemical shift

Literature

Monomer

Dimer

Trimer

Cyclic trimer

Bicyclic pentamer

Tricyclic hexamer

Prismatic hexamer

Doubly bridged cyclic tetramer

Double five-ring

Tetrahedral tetramer

Pentacyclic dodecamer

0

8.6 8.6

Q QQ Q

1 1

2 2 8.2 8.2 16.8 16.9

10.2 10.1

Q QQ Q

2 2

3 3 9.9 9.8 16.4, 17.2 16.3, 17.1

Q QQ Q

2 2

3 3 16.2 16.2 16.7, 18, 25 16.8, 17.9, 25

17.1 17.2

Q QQ Q

2 2

3 3 14.7 14.6 21.4 21.4

26 25.5

27 26.9

Q QQ Q

2 2

3 3 17.6 17.7 24.8, 25.2 25, 25.8

11,12

Figure 6. Assignment of 29Si liquid NMR resonances relative to the monomer which lies at δ = –71.4

ppm (referenced to tetramethylsilane) and comparison with literature data11,12 (negative signs of

chemical shifts are omitted).

Prolonged polymerization of aqueous silicate solutions leads to the formation of colloidal

structures besides oligomeric silicates.30 This is evident in figure 5 from the broad features

that progressively appear in the Q3 and Q4 ranges. Due to spectral overlap it becomes

increasingly difficult to get comparable chemical information from the 29Si NMR spectra for

silicates larger than 12 T atoms. A different spectroscopic technique was used to study the

ongoing polycondensation during silicic acid digestion: Small-angle X-ray scattering

(SAXS).

X-ray scattering

USAXS and SAXS allowed to probe in situ formation and consumption of

(sub)colloidal particles as well as silicalite-1 crystal growth from initial mixing of silicic acid

and TPAOH to complete crystallization upon heating. Figure 7A displays small-angle X-ray

43

scattering data obtained from an aqueous TPAOH – silicic acid solution after 23h of mixing

time at room temperature. The size of (sub)colloidal particles under study is reflected in the

characteristic length d which is related to the scattering vector q according to q = 2π/d. A

pronounced hump is observed at q = 2.95 (1/nm) corresponding to a discrete particle

population which we will denote as primary particles. Only one particle population was

found. Their estimated characteristic length, determined from the maximum position of the

hump, is ca. 2.1 nm. The scattering intensity of the 2.1 nm particles is plotted against the

mixing times in figure 7B. It was then possible to correlate the formation of subcolloidal

particles to dissolution time of silicic acid in TPAOH and 29Si NMR spectra shown in figures

4 and 5. Clearly no primary particles were forming during the first 10h. Increasing scattering

intensity is found from 11h on due to the progressive formation of this unique particle

population (Figure 7B).

0

0.01

0.02

0.03

0.5 1 1.5 2 2.5 3 3.5 4

2.95 nm-1I (a.u.)

q (1/nm)

A

0.01

0.02

0.03

5 9 13 17 21 25

I (a.u)

dissolution time (h)

B

Figure7. A) SAXS pattern of a solution of silicic acid and TPAOH after 23h of mixing time.

B) Time dependent scattering intensity at a fixed angle (q = 2.95 nm-1) corresponding with a d value

of 2.1 nm (primary particles). Scattering intensity from nanoparticles was not clearly detected before

10h in B.

SAXS spectra did not significantly evolve after 23h of mixing time. Interestingly, the

formation of nanoparticles sets in right after the appearance of 29Si NMR signals at around δ

= –25 ppm (Figure 5). The tetrahedral tetramer and double five-rings were formed when a

non-negligible amount of primary particles was already present. Consequently, these species

are not involved in the formation of primary particles. Their 29Si NMR peak intensity

increased at the expense of that of the prismatic hexamer. Kirschhock et al. proposed that

three pentacyclic dodecamers obtained in the TEOS – TPAOH – H2O system rapidly

condensed into a specific silicalite-1 precursor.17 This particular precursor has a d value of

44

1.35 nm. Its estimated size is 1.3 × 1.3 × 1 nm. Aging of the same solution led to the

dominant presence of dimers with a d value of 2.2 nm. Upon dilution with water, very stable

nanoslabs were obtained having dimensions of 4 × 4 × 1.3 nm according to TEM, SAXS and

AFM18 by coupling of 3 × 4 precursors. Based on the observation of pentacyclic dodecamer

with 29Si NMR and particles corresponding to double precursors with SAXS, we conclude

that the same molecular steps of Si polymerization from 12 to 72 Si atom species are

encountered in both the TEOS – TPAOH – H2O and the silicic acid – TPAOH – H2O

systems. For as yet unknown reasons, the present experimental conditions render the double

precursors (or primary particles) very stable.

0

100

200

300

400

500

600

700

0 100 200 300 400 500 600 700 800time (min.)

I (a.u.)

2.1 nm

crystals

Figure 8. Evolution of scattering intensity of primary particles and growing crystals during the

synthesis of silicalite-1 from a mixture of silicic acid and aqueous TPAOH at 125 °C. Evolutions of

the scattering intensity for primary particles and crystals were followed respectively by SAXS and

USAXS.

After heating at 125 °C the scattering intensity of the primary particles increases only slightly

(Figure 8). This observation suggests that most of the silicic acid powder has been dissolved

at room temperature. The onset of crystallization was determined with in situ USAXS

measurements as reported previously.21 From the X-ray scattering data, the presence of two

interrelated particle populations was evident: primary particles and growing crystals. The

scattering of the primary particles is almost constant until the formation of the first silicalite-1

crystals (Figure 8). Such behaviour has already been observed in similar syntheses.24 In a

mixed system with TPA and sodium cations, De Moor et al. observed three particle

populations: primary units (2.8 nm), their aggregates (≈10 nm) and growing crystals. By

45

varying the synthesis conditions they were able to clarify the role of each of the particles. The

formation of the 10 nm aggregates depends on the alkalinity of the synthesis mixture but the

2.8 primary units were always present. For high alkalinity such as Si/OH = 2.42 (same as the

present study), no aggregates (10 nm) were obtained whereas low alkalinity appeared to favor

their formation. The data strongly suggested that the growth units were the 2.8 nm

nanoparticles while their aggregates would only act as reservoirs. In the present study, i.e. in

the absence of Na+ cations, the formation of smaller nanoparticles (2.1 nm) is favoured. The

crystal growth rate of silicalite-1 was 0.93 nm/min. De Moor et al.21 recorded a rate of 1.2

nm/min, showing that Na+ cations accelerate the crystal growth. The final crystals were found

to be larger in the present study, suggesting that less nuclei were formed. These observations

suggest that TPA molecules hinder the formation of viable nuclei by keeping away primary

particles from each other, whereas the smaller Na+ counterions favor aggregation of particles

and act as bridging ions.

Proposed mechanism

As shown in figure 4, monomer anions oligomerize very slowly to form dimers,

trimers and cyclic trimers. From the moment the first Q3 Si atoms in the δ = –25 and –28 ppm

range (assigned to pentacyclic dodecamers) are present in solution, the dissolution rate

increases. We attribute this phenomenon to the formation of pentacyclic dodecamers around

TPA cations adsorbed on the silica surface. The formation of 2.1 nm particles follows rapidly

the dodecamer formation. The silica surface becomes more hydrophilic and transport of

hydroxyl ions is facilitated. Appearance of double five-rings seems to arise from the

prismatic hexamer by addition of two Si atoms. In the early 1980s, occurrence of double five-

rings in ZSM-5 synthesis mixtures has led to a model in which they were regarded as zeolite

precursors since MFI framework can be built starting from double five-ring silicates.36

However, based on trimethylsilylation methods and using silicic acid as silica source, further

studies have failed to correlate their enhanced presence with faster nucleation.37 It must also

be noted that those authors37 claimed that a large amount of silica was present as polymeric

silicates that could not be detected by neither chemical trapping nor 29Si NMR. Their

following statement “the presence of zeolite precursor species other than DnR silicates in this

(colloidal) range can not be excluded” is particularly relevant to the present data. The

increasing scattering intensity of the 2.1 nm primary particles and 29Si NMR reveal that silica

is mainly present in very well-defined and stable subcolloidal particles that likely have a

46

silica connectivity related with the MFI topology. The strong decrease of the scattered

intensity from the primary particles with the onset of the crystallization process suggests that

crystal growth occurs via integration of 2.1 nm primary particles or secondary particles

obtained by their aggregation at the crystal surface. A proposed zeolite assembly process

from the dissolution of nutrients to crystal growth is depicted in figure 9. The present study

did not allow to specify the steps from the precursor dimers (2.1 nm), possibly via tablets, to

the final crystals.19

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO O-

SiOO

SiOO

O-

O

O

SiOOO

SiOOO

O-

SiOO

SiOO O-

SiO

OSi

OO O -

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO

O-

O

OH-

OH- Crystal growth

Nucleation

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO

O-

SiOO

SiOO O-

SiOO

SiOO

O-

O

a

b

c

d

e

Figure 9. Schematic representation of the proposed mechanism of synthesis of silicalite-1 crystals

from silicic acid powder and a TPAOH solution: a) silica surface with adsorbed TPA. b) pentacyclic

dodecamer. c) double precursor (2.1 nm). d) tablet (observed in the TEOS – TPAOH – H2O system)

but not in the present study (or in non-measurable amount). e) silicalite-1 crystal.

3.1.4 Conclusion

29Si NMR and (ultra)small-angle X-ray scattering were combined to study

transformations taking place in silicalite-1 zeolite synthesis. 29Si NMR allowed the

47

identification of silicate oligomers during the slow dissolution of silicic acid in a concentrated

TPAOH solution. These processes on a molecular scale could be related with events

occurring at a colloidal scale. Followed by the appearance of 2.1 nm primary particles

(monitored by SAXS) and assigned in chapter 2 as nanoslabs, the key steps in the process are

the formation of their probable precursors (pentacyclic dodecamers observed in 29Si NMR

data). These processes accelerate the silica dissolution rate in which zeolite precursors form

at the surface of the amorphous silica and are released upon formation of the nanoslabs. The

quest for zeolite precursors has long been investigated at a molecular level by the means of

NMR. The relevant stable zeolite nanoprecursors are situated at the transition from the

molecular to the subcolloidal scale, as suspected by Keijsper et al. in the early 1980s.37 We

have also shown that double five-rings are observed much later after the formation of the 2.1

nm precursors, suggesting that they are not related to zeolite crystallization. These results

together with recent studies confirm the existence of a common mechanism in which organic-

mediated silicalite-1 crystallization occurs irrespective of the silica source.

3.2 Applications of specific NMR techniques to the study

of nanoslab formation

3.2.1 Introduction

The work reported thus far shows that the mechanisms of zeolite formation are very

complicated. Two hypotheses have originally been proposed in literature: zeolite

crystallization occurs by solid-solid transformation38,39 or via soluble aluminate precursors.40

The first hypothesis is no longer favored. There is now a general agreement that zeolites

crystallize in solution from soluble species even in semi-dry conditions. This has prompted

considerable attention on the study and identification of aluminosilicate anions in alkaline

solutions.41,42,43 As shown in the first section of this chapter, 29Si NMR turns out to be a

powerful tool for probing silicates at early stages of zeolite formation even when starting

from solid silica sources. In the present section, we extended these investigations to include

other NMR methods.

48

As a convenient technique for the evidence of the formation of aluminosilicates, 27Al

NMR spectroscopy was used to follow the evolution of the aluminum connectivity during the

formation of ZSM-5 precursors and the whole course of ZSM-5 crystallization. Based on the

results of the first part of this chapter and the work of Kirschhock et al.16, close contacts

between protons of TPA and silicon atoms are expected irrespective of the silica source. In

some instances, interatomic distances can directly be obtained using a recent and specific

method, rotational-echo double resonance (REDOR).44,45 We were then able to compare the

S-HTPA distance in silicalite-1 nanoslabs spontaneously formed at room temperature with that

encountered in the final TPA-silicalite-1 crystals. Lastly we carried out 27Al multiquantum

MAS NMR experiments on crystals obtained from the TEOS − Al − TPAOH − H2O system

in order to address the issue of random or non-random Al framework incorporation.

3.2.2 Experimental section

Zeolite synthesis. Aluminum containing template solution was first prepared as follows:

aluminum metal powder was dissolved overnight in a concentrated solution TPAOH (Alfa,

40%) and then filtered through a syringe filter (Whatman 0.45 µm).

TEOS or silicic acid was gradually added to the template solution under vigorous stirring at

room temperature. Each step represented an addition of 10% of the final amount of silica. A

20 min delay was applied between each addition in order to allow 27Al NMR measurement.

The final molar compositions were (TPA2O)(SiO2)3.75(Al2O3)0.0375(H2O)30(EtOH)15 and

(TPA2O)(SiO2)5(Al2O3)0.05(H2O)57 for the TEOS and the silicic acid systems, respectively.

SAXS revealed the formation of nanoparticles of dimensions 3.7 nm and 2.1 nm for the

TEOS system and the silicic acid system, respectively. Nanoblock suspensions for REDOR

experiments were prepared as follows: an ion-exchange procedure using Ag2O was

performed on an solution of TPABr in D2O in order to obtained a 37.5% TPAOD solution in

D2O. TEOS and D2O were then added as described in chapter 2.

NMR. 27Al NMR spectra were recorded on a Bruker DMX-500 (11.7T) at 130.3 MHz using

a 4 mm probe head at a spin rate of 1.7 kHz. In a typical measurement 1000 spectra were

accumulated with a pulse length of 3 µs and a repetition time of 1 s. Chemical shifts are

referred to external aqueous AlCl3. 2D 27Al MQ MAS NMR experiments were performed in

the same spectrometer at a spinning rate of 12.5 kHz and a 4 mm probe head, with an

excitation pulse of 10 µs, a conversion pulse of 1.2 µs (both for a rf field strength of 124

49

kHz) and a zero quantum filter consisting of a delay of 20 µs and a 10 µs pulse (for a rf field

strength of 10 kHz). For each t1 900 scans were accumulated, and t1 was incremented 256

times.

3.2.3 Results and discussion 27Al NMR

A series of 27Al NMR spectra of aluminosilicate solutions in which the SiO2

concentration was gradually increased is shown in figures 10 and 11, corresponding to the

TEOS and silicic acid systems, respectively. It is evident that Al connectivity is greatly

affected by the Si/Al ratio. The spectra without silica showed a typical line at δ = 78.9 ppm,

characteristic of the monomeric ion [Al(OH)4]-.

In figure 10, the distinction between Al(0Si) and Al(1Si) may not be straightforward. The

peak assigned to [Al(OH)4]- is known to broaden and shift downfield in presence of

silicates.46 This behaviour was interpreted in terms of loose interactions with silicates. The

same authors identified Al(4Si) between δ = 57 and δ = 58 ppm in very few solutions. The

broad line observed at around δ = 58 ppm (40% silica added) shifting downfield to 55 ppm

(final composition) represents obviously aluminum atoms fully reacted with silicates

(Al(4Si)). The other bands located upfield could then be determined.

40455055606570758085

Q1

Q2

Q3

Q0

Q4

0%

10%

20%

30%

40%

60%100%

%of Si addedfinal ratio : Si/Al=50

ppm

Figure 10. 27Al NMR spectra of the progressive addition of TEOS in an aluminum containing

solution of TPAOH. The relative decrease of intensity at 60% and 100% is related to the fact that

species are more condensed.

50

The broad lines appearing at δ = 73, 71, 65 and 58 ppm are assigned to Al(1Si), Al(2Si),

Al(3Si) and Al(4Si), respectively (nSi refers to the number of siloxane bonds). For relatively

low silica concentrations, three peaks (Al(1Si), Al(2Si) and Al(3Si)) are observed. Those

peaks do not shift. The connectivity of aluminum increases with silica concentration,

suggesting that [Al(OH)4]- or small aluminosilicates appear to complex preferentially with

large silicate species. An explanation could be that highly negatively charged

aluminosilicates react preferentially with silicates having low average negative charge per

silicon, thus highly condensed oligomers such as the pentacyclic dodecamer (See 3.1). The

progressive shifting of the Al(4Si) broad signal is probably due to the ongoing

polymerization process through formation of the 33-Si-atoms precursor and its aggregates

which lead to the presence of very well-defined nanoslabs.47 More importantly, it should be

noted that, from the position of the 27Al NMR signal, the coordination of aluminum atoms

with four silicon atoms (via oxygen bridges) did not change through nucleation and

crystallization of ZSM-5 as shown in figure 12. Since the nanoslabs are the dominant and the

most condensed species after hydrolysis of TEOS, all the aluminum atoms are incorporated in

them and stay there. It was proposed that crystallization occurs via nanoslabs aggregation.19

A different crystallization mechanism like monomer or small oligomers addition is then

excluded.

2030405060708090 ppm

10%

20%30%

40%

50%

60%

100%

%of Si addedfinal ratio : Si/Al=50

Q4

Q3

Q2

Q1

Figure 11. 27Al NMR spectra of the progressive addition of silicic acid in an aluminum containing

solution of TPAOH.

51

Aluminum coordination changes upon addition of silicic acid to an aluminum containing

TPAOH solution are shown in figure 11. The same trends as in the TEOS system are

observed. The final position of the chemical shift for the tetrahedrally coordinated Al atoms

lies at δ = 58.7 ppm. This higher value is probably due to the smaller size of the precursors

(2.1 nm). Unfortunately, no in situ 27Al NMR study of the crystallization of ZSM-5 using

silicic acid has been performed yet as a confirmation of the results shown in figure 12.

01020304050607080

1h

2h

5h

8.5 h

ppm

Figure 12. 27Al NMR spectra of an Al-containing nanoslabs solution (Si/Al=50) heated at 100 °C for

different reaction times. The zeolite crystallization took place in the NMR probe head using a

pressurizable rotor.

REDOR experiments

A solid-state NMR technique was used to investigate the close relationships between

template and silica. As an indication of particular interactions, the distances between silicon

atoms and protons of template molecules were estimated in both the nanoslabs made from the

TEOS system and the silicalite-1 crystals. One of the experimental solid-state NMR

techniques widely used for distance investigation is the so-called REDOR technique.48,49 A

brief description of this experiment is presented below.

Under fast magic angle spinning, usually applied in solid-state NMR, the magnetic

dipole coupling between observed nuclei I (in our case 29Si) and nearest nuclei P (for

example, 1H) is suppressed. This suppression allows a significant improvement of NMR

spectrum resolution and consequently more unambiguous chemical information. But

52

parameters of dipole-dipole coupling also carry some important information about the

investigated system such as distances between interacting spins. Rotational-echo double-

resonance technique (REDOR) reintroduces partially this coupling. In this type of experiment

P (in our case 1H) spins undergo additional inverted pulses in the middle of the sample

rotation period. As a result, complete averaging of dipole-dipole coupling does not appear.

Analysis of REDOR results is based on comparison of spin-echo intensities with and

without refocused 1H pulses. With refocused 1H pulses, this spin-echo intensity is suppressed

due to dipole-dipole coupling. The so-called REDOR fraction (∆S), equal to the normalized

difference between the spin-echo intensity without refocusing pulses (S0) and that with ones

(∆S=(S-S0)/S0)) is used as a measure of this phenomenon.

In the case of isolated I-P pair the REDOR fraction obeys to the equation:

∫ ∫−=− ππ

τπ

2

0

2/

00

0 αββsin)βcosβsinαsin24cos(211 ddDn

SSS

r

where n is the number of rotor cycles in pulse NMR sequence, τr is the sample rotation

period, the angles α and β define the position of internuclear I-P vector with respect to the

sample frame and D is the dipolar coupling constant. Integration provides averaging of all

possible I-P vector positions in a disordered powder sample.

Distance measurement is based on the fact that the dipole constant depends on internuclear

distance rIP:

πµ

πγγ

420

3IP

PI

rD h=

where γI,P is the gyromagnetic ratio for spins I and P; µ0 the universal magnetic constant.

In the case of multiple spins IPn, the situation becomes much more complicated.50 A lot of

additional parameters as I-P-I angles, number of interacting spins, possible spin motion and

so on should be included, which makes distance measurements ambiguous. For this reason,

we used a simplified model in which the REDOR fraction could be described by a simple

monoexponential curve:

)exp(10

0reffnD

SSS τ−=

−

where Deff is a constant, proportional to the dipole constant D mentioned above. This

simplification is possible if we assume that the IPn system is connected with a “spin

temperature bath”. Under dipole coupling, refocusing this bath removes continuously the

53

magnetization from IPn system. This case can be described in a manner similar to the simple

nuclear spin relaxation phenomena that has monoexponential behavior.

Comparison between constant Deff, obtained from a sample with unknown Si-H distances,

and D0 from the reference compound with well-defined distance r0, makes possible to

calculate “effective” Si-H distance in sample under investigation:

0

303

DrD

r effeff =

As reference sample we used 1,3,5,7,9,11,13,15-Octakis

(dimethylsilyloxy)pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane with a well-defined Si-H

distance in the dimethylsilyl group, viz. 1.5 Å (figure 13).

Figure 13. reference sample for REDOR (29Si-1H) measurements: 1,3,5,7,9,11,13,15-Octakis (dimethylsilyloxy)pentacyclo[9.5.1.13,9.15,15.17,13]octasiloxane.

An example of a REDOR curve for silicalite-1 crystals with template molecules inside is

shown in figure 14.

0.00 0.20 0.40 0.60 0.80

1.00 1.20

0 10 20 30 40 50 60

number of rotor cycles

REDOR fraction

Figure 14. Typical REDOR (29Si-1H) curve measured for Silicalite-1 crystals.

In order to exclude any possible influence of protons from non-template molecules,

the precursor solution was prepared in D2O and evaporation of ethanol molecules was

54

performed. Three main peaks can be assigned to the methyl groups and the methylene groups

of TPA by 1H NMR (not shown). The precursor solution was quickly frozen in order to use

the REDOR technique. The formation of solid frozen state was confirmed by observation of

broadening in 1H NMR spectra. All the REDOR studies were carried out at 200 K. Q4 and Q3 29Si peaks in CP-MAS spectrum of the frozen sample have an overall higher intensity that in

the one-pulse excitation spectrum of the liquid sample (Figure 15). This means that 1H-29Si

cross-polarization for silicon atoms in these positions are more effective. Cross-polarization

efficiency depends on Si-H proximity as well as some other factors. The larger intensities in

Q3 and Q4 positions can indicate spatial proximity of protons and silicon atoms in these

positions. Results of effective distance measurements by different REDOR procedures are

shown in table 1:

Figure 15. a) 29Si NMR precursor solution from TEOS – TPAOD – D2O at 288 K, 500 Hz, MAS b) 29Si-1H CP-MAS spectrum of the same solution at 200 K, 2.5 kHz.

Sample Technique Effective Si- HTPA distance (Å)

Silicalite-1 crystals REDOR, 2,5kHz MAS 3.39

Silicalite-1 crystals CP-REDOR, 2,5kHz MAS 3.67

Frozen nanoslab solution CP-REDOR, 3kHz MAS 3.44

Table1. Effective Si-HTPA distance determined from REDOR experiments.

These results evidence close contacts between TPA molecules and silica at the early

stages of zeolite formation. Values for the silicalite-1 crystals are in good agreement with

crystallographic data.51 These interactions, detected prior to the development of long-range

order, are similar to those encountered by TPA cations encapsulated in the silicate-1 crystals.

55

This supports a model in which structure direction involves the preorganization of colloidal

entities resembling the zeolite topology. This is the first direct measurement of interatomic

distances in embryonic species present in zeolite synthesis. Burkett et al. observed van der

Waals interactions based on the efficiency of 1H-29Si CP MAS NMR of freeze-dried samples

obtained at different intervals during the synthesis of ZSM-5.8 The present method does not

require invasive sample treatment and provides a direct interatomic distance.

27Al MQ MAS

Distribution of aluminum in the framework of ZSM-5 is of prime importance. The

occurrence and spatial ordering of Al atoms in the zeolite framework control the properties of

the catalytically active sites. Very similar scattering powers of Si and Al for X-rays and small

crystallite size hinder the efficiency of synchrotron X-ray diffraction. The primary

information available from 27Al MAS NMR is related to the coordination of aluminum.

Generally tetrahedral lattice aluminum in zeolites gives only one signal in the range of 50-65

ppm. It is very difficult to distinguish between two non-equivalent framework Al sites. The

reason for this difficulty is the overlap of lines induced by the second-order broadening and

line shift of 27Al resonance due to quadrupolar interactions. A new 2D multiquantum MAS

(MQ MAS) NMR technique has been developed recently.52 It is then possible to separate the

contributions from the chemical shift (related to the crystallographic site) and quadrupolar

broadening. Han et al. were able to demonstrate the preference of Al atoms for particular T-

sites in as-made and calcined ZSM-5 using 2D 27Al MQ MAS NMR.53 This preference was

observed for a wide range of Si/Al ratios (from 250 to 14). They suggested that Al atoms are

distributed over the 12 possible T-sites with chemical shifts ranging from 51.7 to 56.6 ppm.

We applied the same method for samples prepared from the TEOS – Al – TPAOH –

H2O system having Si/Al ratios of 150, 100 and 50. The 2D 27Al MQ MAS NMR spectrum

of the Si/Al = 50 sample is depicted in figure 15. We were unable to distinguish two or more

crystallographic distinct Al sites for the three different Al contents. Consequently, it is not

possible to determine whether aluminum substitution is random or spatially ordered in the

present case. This study was driven with the hope of resolving a specific T-site for Al

substitution. We would expect that there is a particular pathway of Al incorporation due to

the nanoblock preparation. Nanoslabs indeed exhibit particular Q2, Q3 and Q4 sites but only

Al(4Si) are detected.

56

Figure 16. 2D 27Al MQ MAS NMR of calcined ZSM-5 with Si/Al = 50 made from the TEOS – Al –

TPAOH – H2O system.

3.2.4 Conclusion

Additional NMR experiments gave us some more insight on the formation of MFI

nanoslabs. 27Al NMR revealed the progressive increase of aluminum connectivity with

addition of silica in Al doped TPAOH silica. The exclusive presence of Al (4Si) atoms in

zeolites synthesis starting mixtures irrespective of the silica source makes these systems very

particular. Former studies on the liquid phase of zeolite synthesis did not exhibit such

features.54 The growth of silicalite-1 crystals was followed by in situ 27Al NMR. Results

support a nanoblock-based growth mechanism as indicated by the unique coordination of Al

atoms. Nanoblocks are formed by favorable van der Waals interactions resulting in a

structure direction effect of TPA cations towards silica. These close contacts were directly

detected using the REDOR technique by direct measurement of the Si-HTPA distance, which

appears to be similar to that found in as-made silicalite-1 crystals. 2D 27Al MQ NMR did not

univocally determine the crystallographic position of Al framework substitution via

nanoblock-mediated crystallization. Study of other heteroatoms (Ti or Fe) incorporation

could be more informative since other techniques can be applied.

57

References 1 Szoztak, R. Molecular sieves; Blackie Academic & Professional, 1998. 2 Venuto, P. B. Microporous Mater. 1994, 2, 297-411. 3 Van Bekkum, H.; Flanigen, E. M.; Jacobs, P. A.; Jansen, J. C. Introduction to zeolite science and practice, 2nd Edition, Stud. Surf. Sci. Catal.; Elsevier, 2001. 4 Baerlocher, C.; Meier, W. M.; Olson, D. H. Atlas of zeolite framework type, Fifth Revised Edition, Elsevier, 2001. 5 Lok, B. M.; Cannan, T. R.; Messina, C. A. Zeolites 1983, 3, 282. 6 Gies, H.; Marler, B. Zeolites 1992, 12, 42. 7 Davis, M. E.; Zones, S. I. In Synthesis of Porous Materials: zeolites, clays and nanostructures; Marcel Dekker, New York, 1997, 1-34. 8 Burkett, S. L.; Davis, M. E. J. Phys. Chem. 1994, 98, 4467-4653. 9 Gougeon, R.; Delmotte, L.; Le Nouen, D.; Gabelica, Z. Microp. Mesop. Mat. 1998, 26, 43. 10 Watson, J. N.; Iton, L. E.; Keir, R. I.; Thomas, J. C.; Dowling, T. L.; White, J. W. J. Phys. Chem. B 1997, 101, 10094. 11 a) Harris, R. K.; Knight, C. T. G. J. Mol. Struct. 1982, 78, 273. (b) Harris, R. K.; Knight, C. T. G. J. Chem. Soc., Faraday Trans. 2 1983, 79, 1525. (c) Harris, R. K.; Knight, C. T. G. J. Chem. Soc., Faraday Trans. 2 1983, 79, 1539. (d) Knight, C. T. G. J. Chem. Soc., Dalton Trans. 1988, 1457. 12 (a) Hoebbel, D.; Garzo, G.; Engelhardt, G.; Ebert, R.; Lippmaa, E. T.; Alla, M. Z. Anorg. Allg. Chem. 1980, 465, 15. (b) Hoebbel, D.; Garzo, G.; Englehardt, G.; Vargha, A. Z. Anorg. Allg. Chem. 1982, 494, 31. (c) Hoebbel, D.; Vargha, A.; Engelhardt, G.; Usjszaszy, K. Z. Anorg. Allg. Chem. 1984, 509, 85. (d) Engelhardt, G.; Michel, D. High-Resolution Solid-State NMR of Silicates and Zeolites, John Wiley & Sons, New York, 1987. 13 (a) Kinrade, S. D.; Knight, C. T. G.; Pole, D. L.; Syvitski, R. T. Inorg. Chem. 1998, 37, 4272. (b) Kinrade, S. D.; Knight, C. T. G.; Pole, D. L.; Syvitski, R. T. Inorg. Chem. 1998, 37, 4278. 14 (a) Harris, R. K.; Parkinson, J.; Samadi-Maybodi, A. J. Chem. Soc. Dalton Trans. 1997, 2553. (b) Kinrade, S. D.; Donovan, J. C. H.; Schach, A. S.; Knight, C. T. G. J. Chem. Soc. Dalton Trans. 2002, 1250-1252. 15 Jacobs, P. A.; Martens, J. A. Synthesis of high-silica aluminosilicate zeolites, Studies in Surface Science and Catalysis Series; Elsevier Science, New York, 1987, Vol. 33. 16 Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965-4971. 17 Kirschhock, C. E. A.; Kremer, S. P. B; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 2002, 106, 4897-4900. 18 Kirschhock, C. E. A.; Buschmann, V.; Kremer, S.; Ravishankar, R.; Houssin, C. J. Y.; Mojet, B. L.; Grobet, P. J.; van Santen, R. A.; Jacobs, P. A.; Martens, J. A. Angew. Chem. Int. Ed. 2001, 40, 2637. 19 Kirschhock, C. E. A.; Ravishankar, R.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 11021-11027. 20 Houssin, C. J. Y.; Mojet, B. L.; Kirschhock, C. E. A.; Buschmann, V.; Jacobs, P. A.; Martens, J. A.; van Santen, R. A. Zeolites and Mesoporous Materials at the Dawn of the 21st Century, Stud. Surf. Sci. Catal. 2001, 135, 135. 21 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanshek, B. U.; Diat, O.; van Santen, R. A. J. Phys. Chem. B 1997, 101, 11077-11086. 22 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanschek, B. U.; Beck, L. W.; Wagner, P.; Davis, M. E.; van Santen, R. A. Chem.-Eur. J. 1999, 5, 2083-2088.

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23 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Tsuji, K.; Davis, M. E. Chem. Mat. 1999, 11, 36-43. 24 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A. J. Phys. Chem. B 1999, 103, 1639-1650. 25 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Beck, L. W.; Davis, M. E. J. Phys. Chem. B 2000, 104, 7600-7611. 26 Verduijn, J. P. Exxon Patent, PCT/EP92/02386, 1992. 27 Narayanan, T.; Diat, O.; Bösecke, P. Nucl. Instrum. Methods Phys. Res. A 2001, 467-468, 1005-1009. 28 Bonse, U.; Hart, M. In Small-angle X-ray scattering; Gordon and Breach, New York, 1967, 121. 29 Diat, O.; Bösecke, P.; Lambard, J.; De Moor, P-P. E. A. J. Appl. Crystallogr. 1997, 30, 862. 30 Iler, R. K. The chemistry of silica, John Wiley and Sons, 1979. 31 Wijnen, P. W. J. G.; Beelen, T. P. M.; de Haan J. W.; Rummens, C. P. J.; van de Ven, L. J. M.; van Santen, R. A. J. Non-Cryst. Solids 1989, 109, 85. 32 Engelhardt, G.; Jancke, H.; Mage, M.; Pehk, T.; Lippma, E. J. Organometal. Chem. 1971, 28, 293. 33 Mortlock, R. F.; Bell, A. T.; Radke, C. J. J. Phys. Chem. 1991, 95, 372. 34 Knight, C. T. G.; Kinrade, S. D. J. Phys. Chem. B 2002, 106, 3329-3332. 35 Knight, C. T. G.; Kinrade, S. D. Inorg. Chem. 1988, 27, 4253. 36 Boxhoorn, G.; Sudmeijer, O.; van Kasteren, P. H. G. J. Chem. Soc. Chem. Comm. 1983, 1426. 37 Keijsper, J. J.; Post, M. F. In Zeolite synthesis; Ocelli, M. L., Robson, H. E., Eds; ACS Symposium Series 398, American Chemical Society: Washington, D.C., 1989, 28. 38 Flanigen, E. M. Adv. Chem. Ser. 1973, 121, 119. 39 Derouane, E. G.; Detremmerie, S.; Gabelica, Z.; Blom, N. Appl. Catal. 1981, 101, 101. 40 Barrer, R. M. Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982. 41 Mortlock, R. F.; Bell, A. T.; Radke, C. J. J. Phys. Chem. B 1991, 95, 7847. 42 Fahlke, B.; Mueller, D., Wieker, W. Z. Anorg. Chem. 1988, 562, 141. 43 Kinrade, S. D.; Swaddle, T. W. Inorg. Chem. 1989, 28, 1952. 44 Gullion, T.; Schaeffer, J. Adv. Magn. Reson. 1989, 13, 57. 45 Gullion, T.; Schaeffer, J. J. Magn. Reson. 1989, 81, 196. 46 Harvey, G.; Glasser, L. S. In Zeolite synthesis; Ocelli, M. L., Robson, H. E., Eds; ACS Symposium Series 398, American Chemical Society: Washington, D.C., 1989, 49. 47 Kirschhock, C. E. A.; Ravishankar, R.; van Looveren, L.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4972. 48 Gullion, T.; Schaeffer, J. Adv. Magn. Reson. 1989, 13, 57. 49 Gullion, T.; Schaeffer, J. J. Magn. Reson. 1989, 81, 196. 50 Goetz, J. M.; Schaefer, J. J. Magn. Reson. 1997, 127, 147. 51 Van Koningsveld, H.; van Bekkum, H.; Jansen, J. C. Acta Crystallog., Sect. B 1987, B43, 127. 52 Frydman, L.; Hardwood, J. S. J. Am. Chem. Soc. 1995, 117, 5367. 53 Han, O. H.; Kim, C., Hong, S. B. Angew. Chem. Int. Ed. 2002, 41, 469. 54 Swaddle, T. W.; Salerno, J.; Tregloan, P. A. Chem. Soc. Rev. 1994, 23, 319.

59

4

In situ SAXS/USAXS investigation on aluminum incorporation in the synthesis of colloidal TPA-ZSM-5

*

The formation of nanoparticles and growth of TPA-ZSM-5 crystals from clear homogeneous solutions were monitored by use of X-ray scattering techniques. An introduction to small-angle X-ray scattering is given since the understanding of scattering patterns is not always straightforward. The equipment used at the European Synchrotron Radiation Facility (ESRF) will be presented as well as the specially designed sample cell allowing to perform in situ measurements. SAXS and USAXS experiments using synchrotron radiation showed the influence of aluminum on the crystal growth rate and on the size of the aluminosilicate nanoparticles spontaneously formed at RT due to the interactions between silica and the organic template. Increasing aluminum content results in slightly smaller nanoprecursors and a slower crystal growth. The final size of the crystals was found to increase with higher aluminum content indicating that less viable nuclei are formed. The results presented here are consistent with the proposed mechanism for organic-mediated crystallization of low aluminum containing or all-silica MFI, stating that crystal growth is a nanoblock-based aggregation mechanism rather than monomer addition. The data show that aluminum is incorporated in the nanoparticles together with an effect on the nucleation and crystal growth rate. The moderate incorporation of aluminum, however, does not change the pathway of zeolite crystallization.

* Reproduced in part from: C. J. Y. Houssin; P. J. Kooyman; B. L. Mojet; R. A. van Santen ‘In situ SAXS/USAXS investigation on aluminum incorporation in the synthesis of colloidal TPA-ZSM-5’ submitted.

60

4.1 Introduction Zeolites are crystalline aluminosilicates of natural or synthetic origin with many bulk

industrial applications.1 Some of the applications of these microporous solids are related to

their very high surface area, their catalytic sites, their multidimensional pore channels and

their pore size uniformity.2 Although natural zeolites are available, commercial zeolites are

almost exclusively synthetic because a very high degree of purity and uniform particle size

distribution can be obtained from inexpensive materials. Traditionally, zeolite chemists have

focused on the relations between the starting mixtures (Si/Al ratio, alkalinity, organic

molecules) and final product in terms of crystallite size and topology. Consequently, very

little is understood on the mechanism by which these complicated macroscale objects self-

assemble. The possible discovery of new zeolite topologies and the elucidation of the self-

assembly process of microporous materials drives research towards understanding the

assembly mechanism.3 Investigation of the structure-directing effect that controls the

ordering of silica on the nanometer and micrometer scales is also of more general interest.

Biomimicking the process of diatom formation could provide the production of innovative

and advanced materials.4,5

The preparation of high-silica zeolites generally starts from alkaline mixtures containing

silica and alumina sources, structure-directing agents and water.6 Only a few examples of

non-aqueous systems have been reported.7,8 The reactions are sometimes carried out at

atmospheric pressure but more often under hydrothermal conditions. Lately, many efforts

have been undertaken to understand the role of organic structure-directing agents. It has long

been accepted that size, geometry and charge distribution influence the final structure of the

zeolite.9 A lot of efforts have been devoted recently to the study of the interaction between

organic molecules and silicate species.10,11 No covalent bonds are formed but van der Waals

interactions govern the preassembly process. Thus, in the case of tetraalkylammonium ions

these relatively weak interactions are significant enough to favour the formation of specific

silicate species depending on the alkyl chain length. It is generally agreed that structure-

directing agents spontaneously form well-defined oligomeric species in aqueous silicate

solutions at room temperature.12 Consequently, the question arises regarding the relationship

between the ability of organic structure-directing agent to organize the silica into organic-

61

inorganic composite species at the early stages of the synthesis and the final structure of the

zeolite.

Zeolite ZSM-5 is one of the most frequently used microporous crystalline solids in

chemical industry for its unique catalytic properties and separation applications. Furthermore,

silicalite-1, or aluminum free ZSM-5, can easily be synthesized with tetrapropylammonium

(denoted TPA) as structure-directing agent. This all-silica zeolite was the subject of

considerable academic research towards the understanding of organic mediated zeolite

formation, mainly because TPA is believed to be a very efficient structure-directing agent.

Recently, the group of Martens et al. have attempted to follow the polycondensation of silica

in a concentrated TPAOH solution using 29Si NMR.13,14 They suggested that a precursor

species in which the MFI topology is already fully developed was spontaneously formed and

then aggregated to result in stable subcolloidal particles denoted nanoslabs.

In addition to seeking insight into these interactions on a molecular level, information is

also needed on a larger scale when long-range order and crystalloids arise. Crystals can be

detected by traditional X-ray diffraction whereas nucleation is less suitable to be followed by

typical spectroscopic techniques as it involves nanosized species. Therefore there is a gap of

spectroscopic techniques between formation of oligomeric species and crystal growth.

There has been a rapid pace of progress probing nanoscopic species during the all-silica

MFI zeolite synthesis.15 To accomplish this goal, a combination of in situ small-, ultra-small-

and wide-angle X-ray scattering (respectively SAXS, USAXS and WAXS) allowed our

group to probe zeolite synthesis over a large length scale, from a few nanometers to

micrometer dimensions.16,17,18 As mentioned earlier, the weak interactions between structure-

directing agent and silicates result in very fragile intermediates. We demonstrated that this

combination of techniques appeared to be very reliable to follow the formation and

consumption of nanoparticles during the self-assembly of organic mediated zeolite synthesis.

De Moor et al. have observed nanoparticles for the silicalite-1 synthesis, which have been

proposed to play a key role in the nucleation and crystallization.19 Subcolloidal 2.8 nm

primary units are omnipresent in the synthesis mixtures while their aggregation depends on

the alkalinity. The formation of the aggregates is an essential step in the nucleation process.

In each of the above-mentioned cases, silicalite-1 was the zeolite studied but it should be

kept in mind that the large majority of applicable zeolites contain heteroatoms, aluminum

being the most important one. Substitution of Al for Si (as T atoms) results in the formation

of a negatively charged framework that tends to coordinate more strongly to cations. T-O

bond lengths and T-O-T angles are believed to influence the formation, size, charge and

62

colloidal properties of the nanoparticles. The main purpose of the present work is to study the

formation and consumption of these nanoparticles in situ during ZSM-5 synthesis by adding

aluminum to two different representative all-silica synthesis mixtures that formerly have been

studied extensively. The effect of the incorporation of aluminum on the nucleation and crystal

growth are compared to the aluminum-free systems. Despite the great deal of work on the

subject of silicalite-1 synthesis, the present study proposes for the first time in situ

investigation of the influence of heteroatom framework substitution on different length

scales, from the formation of zeolite nanoprecursors to the growth and shape of the final

crystals. Moreover, this study provides more insight into the effect of variation of the silica

source on processes from the formation of colloidal particles to the ultimate size of ZSM-5

crystals. Results are discussed regarding the growing evidence that synthesis of organic

mediated zeolites occurs via self-assembly of specific nanoparticles.

4.2 Small-angle X-ray scattering and synchrotron radiation

The purpose of this section is to present a short introduction of some principles of

small-angle X-ray scattering and a description of an experimental setup used in synchrotron

radiation. The scattering of X-rays is generally used to describe structural ordering of

materials. The interaction of X-rays with matter is basically influenced by the electron-

density distribution and its efficiency increases linearly with the atomic number. In typical X-

ray diffraction experiments, the range over which a certain ordering can be probed is usually

of the order of the wavelength λ of the incident X-ray radiation. Diffraction angles at which

positive interferences occur follow the well-known Bragg relation:

nλ = 2d sin(θ)

Wide-angle X-ray diffraction typically involves angles from 5° to 90°. If some

structural information is needed at larger d-spacings, either the wavelength of the incident

beam should be increased or one has to investigate the system at lower angles. Laboratory

SAXS equipments generally uses the CuKα wavelength (1.54 Å), which would give an angle

of 0.88° for probing distances of 5 nm. Moreover, the scattering efficiency decreases with

longer wavelength. The general scattering of two points is depicted in figure 1.

63

2dsin(θ)

2θ

d

2θ

incident beam

2θ

Ds1

2

Figure 1. Scattering by two point centers.

The vector q is defined as q = (2π/λ) (s2-s1) (bold type indicates vectors) in which the unit

vectors s1 and s2 define the direction of incident and scattered X-rays respectively. The

magnitude of the vector q is directly related to the scattering angle θ:

q = (2π/λ) sin(θ).

The incoherent or Compton scattering (scattering arising from electron transitions in the

irradiated atoms) can be neglected at small-angles.

The main difference between classical XRD and SAXS is that wide-angle scattering is

aimed at studying periodic arrangement of identical scattering centers of particles (usually in

all three dimensions) whereas in small-angle scattering these particles are not ordered

periodically and, in the present experimental conditions, are embedded in a water matrix.

When elementary particles are uniform, a reasonable approximation for the measured

intensity I(q) can be written as20,21:

I(q) = (N/V) P(q).S(q)

where N/V is the number density of particles or individual scatterers in the sample. The form

factor P(q) is related to the geometry of the individual particles. The structure factor S(q)

reflects the spatial distribution and correlation of the scattering particles in the matrix.22 The

general mathematical expression of the form factor using the Debye approximation is20:

64

∫

∫= L

L

ee

drrrg

drrqr

qrrgNIqP

0

2

0

2

2

)(

)sin()()(

where Ie is the scattered intensity per electron, Ne the number of electrons per scatterer and

g(r) the electron density correlation function. This general equation is particularly complex

and analytical solutions have been found for only a limited number of morphologies.23 For

example, in the case of spherical particles of size r0 and electron density ρ in a surrounding

medium of electron density ρ0, the form factor can be described as:

2

30

00020

2

)()cos()sin(

3)()(

−−=

qrqrqrqr

VqP ρρ

The expression for P(q) at very small angles (qr0« 1) compared to the reciprocal value of the

size of the particles (r0) is given by the approximation of Guinier:

)5

exp()()(2

02

20

2 rqVqP −−= ρρ

At large scattering vectors (qr0 » 1), the form factor can be approximated by Porod’s law:

40

20

2

)(1)(

29)(

qrVqP ρρ −=

P(q) decreases then as q-4, which makes log I versus log q-4 plots very convenient.

The quantity S(q) relates to the contribution due to the structural arrangement of particles. In

many experiments, the concentration is such that this contribution cannot be neglected. For

very dilute solutions or synthesis, S(q) is a constant. The mathematical expression of S(q) is

obtained from the pair correlation function g(r) (representing the chance to find another

particle within a distance r) via a Fourier transform21:

65

[ ]∫∞

−+=0

2 )sin(1)(41)( drqr

qrrrgVNqS π

When scattering particles form aggregates with mass fractal properties, S(q) can be evaluated

as:

[ ])()1(sin)11(

)1()(

11)( 1

2/)1(22

0

ξ

ξ

qtgD

q

DDqr

qS fD

ffDf

f

−

−−

+

−Γ+=

where Γ(x) is the gamma function, ξ is the size of the aggregates and Df is the fractal

dimensionality (Df<3). At large scattering angles (r0<q-1<ξ), P(q) ≈ 1 and I(q) ≈ S(q); the

expression of I(q) simplifies as: fDqqI −=)(

Df can then be informative about the way aggregation or transformations occur in the

structure formed.24,25

Surface roughness can also be investigated using SAXS. It has been demonstrated26,27 that

particles with surface fractal properties show a decay of: )6()( sDqqI −−=

where Ds is the surface fractal dimensions. Typically Ds falls in a range between 2 and 3 so

that the power law exponent lies between –3 and –4. Consequently the magnitude of the

power law exponent indicates if the scatterer is a mass fractal or a surface fractal object.

The relation between the scattering pattern from particles with both mass fractal and surface

fractal properties on different length scales is shown schematically in figure 2. For very low q

values, incoherent scattering or bigger structures are observed. At larger q values, the

scattering of the aggregates is dominated by the structure factor S(q), describing the spatial

arrangement of the primary particles inside the aggregates. The crossover point between

scattering at large d-spacings (low q) and the region dominated by the structure factor gives

the size of the aggregates. At even smaller length scales, the scattering pattern is determined

by the form factor of the primary particles. A crossover between the regions where P(q) and

S(q) dominate the scattering intensity is observed from which the size of the primary particles

66

can be measured. In the following sections, we will also use the term ‘characteristic length’

as a general expression which does not need any assumption on the shape of the particles.

1/ξ log q1/r0

log I

-Df

D -6s

S(q)

P(q)

r0

ξ

Figure 2. Schematic representation of an aggregate of size ξ with mass fractal properties which is

built of primary particles of size r0 and the corresponding X-ray scattering pattern.28

SAXS and USAXS high-brilliance beamline at the ESRF

The high-brilliance beamline ID02 was primarily intended to SAXS experiments

using a highly monochromatic beam with very low divergence and small cross-section

(typically 100 µm to 300 µm).29 The beamline is now designed for time-resolved

simultaneous SAXS/WAXS; high-resolution USAXS is also available. ID02 is mainly

dedicated to the study of soft condensed matter. The optics are optimized for experiments

using a fixed wavelength at around 1 Å but a range between 0.73 Å and 1.55 Å is accessible.

There are two experimental hutches devoted to SAXS/WAXS and USAXS respectively. A

general layout of the beamline is depicted in figure 3. Figure 4 shows the motorized table

available in the SAXS/WAXS experimental hutch.

67

Figure 3. Schematic representation of the combined SAXS/WAXS and USAXS setup at beamline

ID02 of the European Synchrotron Radiation Facility, Grenoble, France. The sample position is fixed

and the SAXS detector can be moved automatically in a vacuum tube from 0.75 m to 10 m from the

sample.

Figure 4. Motorized table available in the SAXS/WAXS hutch where three sample cells (see

experimental section) were inserted and could be translated in any direction.

68

The use of high-brilliance radiation allows users to probe microstructures and dynamics of

soft matter and liquid systems from 1 nm to a few microns, and down to a millisecond time

range. A high-resolution Bonse-Hart camera30 is installed in the second experimental hutch of

the beamline (Figure 5). More details about this setup are given in the experimental section of

this chapter.

Figure 5. Bonse-Hart setup at the high-brilliance beamline ID02 at ESRF.

4.3 Experimental Section

Zeolite synthesis. Aluminum containing template solution was first prepared as follows:

aluminum metal powder was dissolved overnight in a concentrated solution of TPAOH (Alfa,

40%) and then filtered through a syringe filter (Whatman 0.45 µm). 27Al NMR of the

resulting clear solution indicates that tetrahedral Al(OH)4- is the only aluminate species

present in the solution.

TEOS system: 9 g of TEOS (Acros, 98%) was added dropwise to a 40% aqueous solution

of TPAOH containing the appropriate amount (for Si/Al= 150, 100 and 50) of the above-

mentioned Al(OH)4- solution under vigorous stirring. After 30 min, 9 g of distilled water was

added dropwise and the resultant mixture was stirred continuously for 12 h to ensure

complete hydrolysis of the silica source.

Silicic acid system: The recipe used is based on a patent of Exxon Chemicals previously

employed in our group.31 0.411 g of NaOH was dissolved in 15 g of 20% TPAOH, already

containing Al(OH)4- as mentioned above, followed by a spoonwise addition of 4.05 g of

silicic acid (Baker, 10.2% H2O). The milky suspension was boiled under stirring for 10 min

to obtain a clear solution. The mixture was rapidly cooled down to room temperature in a

water bath. Distilled water was added for the correction for loss of water during boiling. The

69

resulting clear solution was filtered through a 0.45 µm syringe filter. The sample was then

ready for measurement and never aged more than 1h at room temperature before heating to

the reaction temperature. The reaction temperature was 125 °C.

Sample Cell. An electrically heated brass holder containing a rotating circular sample cell,

designed in our group, allowed us to perform in situ measurements.18 The sample cell rotates

at an approximate rate of 2 rpm thus keeping the solution homogeneous, preventing the

precipitation of zeolite crystals and reducing the sample exposure to a small spot. The sample

was inserted between two mica windows (Attwaters and Sons) separated by a PTFE spacer

(0.5 mm thick). Heating of the sample from RT to the reaction temperature (125 °C) requires

only 2 minutes.

SAXS and USAXS. The combined SAXS and USAXS experiments were performed on the

high-brilliance beamline ID02A at the European Synchrotron Radiation Facilities, Grenoble,

France.29 This beamline uses a highly monochromatic beam with very low divergence and

small cross section. The SAXS setup consists of a pinhole camera with a beam stop located in

front of an image-intensified 2-D CCD camera. Sample-to-detector distances of 1.5, 2.5 and 6

m were used. Conversion of detector pixels (CCD camera) to the scattering vector q (nm-1)

was performed with the help of a lupolen polyethylene (BASF) sample. The X-ray

wavelength was 0.99 Å and an incident X-ray beam with a cross section of 0.2X0.2 mm2 was

used. The high brilliance allowed us to record a SAXS pattern in less than 1s. Data were

corrected for detector response and background using a water reference sample at the

corresponding temperature.

USAXS patterns were recorded using a Bonse-Hart type of X-ray Camera.30,32 The

available range of scattering vector q was 0.001-0.14 (nm-1) with q = (4π/λ)sin2θ and λ = 1.0

Å. A first crystal analyzer [Si(220)] was employed to scan the angle, then the X-ray beam

was collimated in the vertical direction with a second analyzer crystal [Si(111)]. The detector

was a high-dynamic range (≈107 counts/sec) avalanche photodiode. The beam size at the

sample was about 1X2 mm2. Several scans (4-5) over successive 2θ ranges with sufficient

overlap were recorded with different degrees of attenuation of the incident X-ray beam, so

that the intensities on the detector were in the linear range. For each sample, a rocking curve

spectrum was systematically measured, which consists of the scattering produced by the

setup, background scattering and the small intensity fluctuations of the beam. Due to the

strong scattering of silica, the contribution of water scattering was negligible.18 A complete

spectrum could be recorded in 15 min.

70

Figure 5. Rotating electrically heated sample cell used for X-rayscattering experiments. Right: close-up on the rotation gear.

NMR. 27Al NMR spectra were recorded on a Bruker DMX-500 (11.7T) at 130.3 MHz

using a 4 mm probe head at a spin rate of 1.7 kHz. In a typical measurement, 3000 spectra

were accumulated with a pulse length of 3µs and a repetition time of 1s. Chemical shifts are

referred to external aqueous AlCl3.

Electron microscopy. High-resolution transmission electron microscopy (HRTEM) was

performed using a Philips CM30UT electron microscope with a field emission gun as the

source of electrons operated at 300 kV. Samples were mounted on a Quantifoil microgrid

carbon polymer supported on a copper grid by placing a few droplets of a suspension of

crystals in water on the grid, followed by drying at ambient conditions.

4.4 Results

In order to check whether aluminum was incorporated in the zeolite framework, several

experiments have been performed. First, figure 7A shows the 27Al MAS NMR spectrum

recorded for the starting synthesis mixture using TEOS as silica source and a Si/Al ratio of

50. This spectrum shows the exclusive presence of tetrahedrally coordinated aluminum in the

71

clear solution. The lineshape and relatively low intensity observed are typical for

aluminosilicate solutions.33 Figure 7B displays the 27Al MAS NMR spectrum of the calcined

solid obtained from the hydrothermal treatment at 125 °C for 24h of the solution mentioned

above. It shows that aluminum is mostly tetrahedrally coordinated and is part of the zeolite

framework. Secondly, isopropylamine decomposition showed that Brønsted acidity resulting

from the aluminum incorporation was indeed present in the sample. Crystallinity was verified

using XRD and all the samples exhibited the MFI topology. These features were found in

every aluminum-containing sample in this present study. We can then assume that the main

phase obtained was zeolite ZSM-5 for every sample.

-101030507090

54 ppm

ppm

A

-101030507090

53.7 ppm

ppm

B

Figure 7. 27Al NMR spectra of A) starting synthesis mixture containing colloidal nanoparticles with

Si/Al = 50, B) calcined zeolite crystals with Si/Al = 50. Some octahedral aluminum (chemical shift ~0

ppm) has formed during calcination.

Formation of nanoparticles at RT.

It has been recognized that dissolution of silica in concentrated solution of TPAOH leads to

the spontaneous formation of well-defined nanoparticles.12,16,34 Figure 8 shows the small-

angle X-ray scattering curves obtained from two TPAOH aqueous solutions after addition

and dissolution of silica. Those samples do not contain aluminum but are representative for

those with aluminum that will be described and discussed in more detail below. The features

in the scattering curves are clear since only one type of particles is present in every solution.

However the particle populations are different for the different solutions. When using TEOS

as silica source, the precursor solution exhibits a maximum at log q = 0.26 nm-1,

corresponding to primary particles having a characteristic length of approximately 3.6 nm (d

72

= characteristic length and q = 2π/d). Dissolution of silicic acid displays a maximum at log q

= 0.37 nm-1, arising from smaller particles of 2.8 nm as characteristic length. More

interestingly, the slope of the log q vs log I representation of the scattering data from these

solutions provides information on the morphology of the particles. A slope of –2.2 is

observed in the case of TEOS whereas the slope is –2.6 for the colloidal solution born from

silicic acid. Scattering patterns for the synthesis mixtures with three different Si/Al ratios and

using TEOS as silica source are depicted in figure 9. Again, each of these solutions gave rise

to scattering curves showing only one maximum corresponding to a unique particle size

population. Nevertheless, the three maxima are not located at the same position. The q value

of the maximum (at around 1.7 nm-1) increases as the aluminum content increases. Moreover,

the overall intensity decreases at high Al content.

-1.6

-1.3

-1

-0.7

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

TEOS

Silicic acid

slope -2.6

slope -2.2

log q (1/nm)

log I (a.u.)

Figure 8. SAXS pattern of the nanoparticles obtained after silica dissolution using silicic acid and

TEOS as silica sources.

0.02

0.025

0.03

0.035

0.04

0.2 0.7 1.2 1.7 2.2

Si/Al=100

Si/Al= inf

Si/Al=150

I (a.u.)

q (1/nm)

inf. 1.69 3.72150 1.74 3.61100 1.78 3.53

Si/Al q(1/nm) size (nm)

Figure 9. SAXS patterns on the RT dissolution of Al-TPA-TEOS-H2O system using various Si/Al

ratios: infinite, 150 and 100.

73

Zeolite synthesis.

All synthesis mixtures prepared in the present study have been heated at 125 °C in the

rotating cells and lead ultimately to ZSM-5 as checked by XRD. The crystallization of Si-

TPA-MFI from clear aqueous solutions with varying Al content was studied in situ with

SAXS and USAXS. For the SAXS experiments two sample to detector distances have been

chosen to reveal the formation and consumption of colloidal particles in a range from 2 nm to

50 nm. To show the influence of Al content on the formation and consumption of colloidal

particles, figure 10 displays SAXS patterns of three synthesis mixtures using TEOS as silica

source after 25 minutes of heating at 125 °C. One can clearly see in figure 11 that the

scattering at q = 1.7 nm-1 has almost disappeared in favour of a pronounced shoulder at

around 0.6 nm-1. The size of the particles giving rise to this intensity is approximately 10 nm

and they are assumed to be aggregates of the previously described smaller particles formed at

RT. The effect of the Al content on the formation of these aggregates results in a change of

the crossover position between the Porod regime and the structure factor region. This

crossover value is larger at higher Al content as shown in figure 10. Those results are

consistent with the fact that aluminum tends to form slightly smaller particles at RT, directly

resulting in smaller aggregates.

0.15

0.2

0.25

0.3

0.1 0.3 0.5 0.7 0.9

q (1/nm)

I (a.u.)

Si/Al= inf.

Si/Al= 150

Si/Al= 100

Si/Al crossover (q) size (nm) inf. 150 100

0.56 11.20.60 10.50.63 10

Figure 10. SAXS patterns after 25 min heating at 125 °C of the precursors solution (using TEOS as

silica source). The crossover between the regions where the structure factor and the form factor

dominate is graphically determined as shown by the dashed lines. The crossover position gives the

characteristic length of the precursors.

74

Figure 11. Time-resolved SAXS pattern for the heating of a solution of precursors with Si/Al=100.

Figure 12 shows the crystal growth during two series of synthesis using two different

sources of silica in which the Si/Al ratio was varied from 50 to 150. For both series of

syntheses a linear growth of the average crystal size with the reaction time was obtained.

From the observed USAXS patterns it is clear that the crystal growth rate strongly depends

on the Si/Al ratio. The crystal growth rate decreases with increasing aluminum content. In the

case of TEOS the linear growth rate with reaction time was found to be 1.33, 1.27 and 0.40

nm/min for Si/Al = 150, 100 and 50 respectively. Similar trends were observed when using

silicic acid as silica source. However it is also worth noting that the relative decrease of

crystal growth with respect to the same aluminum content is not similar from one silica

source to another.

Crystal shape and size polydispersity

Figure 13 displays the high-resolution electron microscopy images of the products obtained

after the complete crystallization at 125 °C of high silica TPA mediated MFI synthesis

mixtures having various Al contents and using TEOS as silica source. For Si/Al values of 150

and infinite the crystals are spherical, slightly elliptical and do not show intergrowth.

Moreover the crystal surface appears to be rather smooth and regular. Higher aluminum

contents lead to different crystal morphologies as shown in the case of crystals grown from

75

synthesis mixtures having Si/Al ratios of 100 and 50. Indeed, even though the overall shape is

still spherical, the particles are irregularly shaped and seem to be built up from small

crystallites. The size of most of these particles is larger than that of the low aluminum content

crystals and the size distribution is rather polydisperse. Some crystals smaller than 50 nm can

be seen in Figure 13c. Even though figure 13c displays both large crystals and assemblies of

smaller crystallites, only the latter were observed for the highest Si/Al ratio as shown in

figure 13d. Figure 14 shows log I.q4 vs log q plots derived from USAXS data of the synthesis

with TEOS and Si/Al = 150, 100 and 50 at reaction times when crystals have obviously been

formed.

0

50

100

150

200

250

0 300 600 900 1200

Series3

Series2

Series1

Si/Al rate (nm/min.)

time (min.)

size (nm)

150

100

50

1.33

1.27

0.40

A

0

100

200

300

400

0 300 600 900 1200

Series1

Series2

Series3

Si/Al rate (nm/min.)

size (nm)

time (min.)

0.93

0.71

0.26

inf.

150

100

B

Figure 12. Mean crystal diameter as determined from USAXS patterns for ZSM-5 syntheses using

TEOS (A) and silicic acid (B) as silica source with variable Si/Al ratios.

Morphology of the crystals may be derived from the X-ray scattering of their surface at low

angles. Indeed, the scattering patterns resemble the general scattering features of spheres and

76

certainly not of the typical elongated prismatic MFI shape.2 These features were already

observed by de Moor et al. in the synthesis of silicalite-1 from clear solutions using silicic

acid.16 The presence of wiggles suggests that a good monodipersity in size has been obtained

and it was thus possible to follow the growth of zeolite crystals by means of USAXS. The

size of the growing crystals can be determined by fitting calculated patterns resulting from

particles of a certain size to the measured data. This has been extensively discussed in a

previous paper from our group.16 Basically, the main conclusion was that the most accurate

method is to determine the position of the first maxima and minima.

Figure 13. High-resolution TEM micrographs of Si-TPA-MFI crystals for various Si/Al ratios: a) ∞,

b) 150, c) 100 and d) 50.

77

Crystal aggregation. For long reaction times, aggregation of crystals was found using

USAXS (not shown). This results in an increase in intensity at very low q and the estimated

size of those aggregates would be several µm. Nevertheless no crystal aggregates were

observed by SEM after washing and drying of the product.

4.5 Discussion

The time-resolved SAXS scattering data, presented above, give information on the

formation, size, morphology and consumption of colloidal particles in the synthesis mixtures

with a Si/Al ratio varying from infinite to 150 during the crystallization of ZSM-5. USAXS

results allow us to follow the crystal growth and the average size of the growing crystals.

Formation of precursor solution

A first population of particles was observed upon dissolution of silica in concentrated Al

containing TPAOH aqueous solutions. Obviously, the size of the nanoparticles strongly

depends on the source of silica. These differences may be attributed to a different dissolution

mechanism and rate. TEOS is an organic and monomeric source of silica whereas silicic acid

is an inorganic and polymeric form of silica. Moreover the interface where the formation of

these nanoparticles is supposed to occur is different (liquid-liquid and liquid-solid for TEOS

and silicic acid respectively). The eventual presence of alkali ions in the silicic acid case is

also an important factor. Chapter 2 showed that the particle size was 2.1 nm when sodium

ions were substituted by TPA cations12 while particles of 2.8 nm were obtained with both

cations present in the starting solutions. Even though these three clear solutions -TEOS

system, silicic acid system with and without sodium ions- lead to the same zeolite structure,

the nanoparticles or primary units obtained after silica dissolution differ in size. In spite of the

fact that the same size (2.8 nm) was found when using four different templates for the

synthesis of silicalite-117,19, the present study shows that the size of the primary units is not

specific of a certain zeolite topology but rather depends on the synthesis conditions like silica

source and cations content. Consequently, aluminum is also expected to induce some changes

in the colloidal particle formation and evolution.

The nanoparticles formed in the presence of different aluminum contents using TEOS as

silica source have roughly the same characteristic length (ca. 3.7 nm) and the corresponding

scattering patterns show strong similarities to those obtained in the aluminum free system.

78

However some features are different. First the size of the particles slightly decreases with

increasing aluminum content. We found several characteristic lengths with the aluminum

loading: 3.72, 3.61 and 3.53 nm for Si/Al = infinite, 150 and 100 respectively. There are two

possible reasons for this moderately smaller primary unit size. First, the incorporation of an

aluminum atom in the framework of a nanoparticle introduces a negative charge. Therefore it

is possible that the TPA molecules adsorb more strongly to the hydrophobic surface of the

nanoparticles. Second, since TPA is occluded in the nanoparticles a negative charge is

required to compensate the positive charge of the tetraalkylammonium cation. The presence

of aluminum circumvents or reduces the occurrence of a framework defect. Consequently this

effect induces less distortion of the primary unit internal structure thus giving rise to particles

with a slightly different size.

SAXS also provides information on the morphology of the nanoparticles. However,

deriving a particle shape from X-ray scattering data remains a tricky and delicate exercise. In

the present case the high silica concentration can affect the scattering curve because of non-

negligible interparticle interferences. Consequently any attempt to derive morphological

information from SAXS patterns must be combined with other techniques. TEM was

performed on those precursor solutions as shown in chapter 2 and it appeared that the

particles were tablet like with dimensions of 4 × 4 nm and 4 × 2 nm respectively for the

TEOS and silicic acid cases.34 In the present study the slope close to –2 in the log I vs log q

plot indicates flat nanoparticles and the trends are similar when adding aluminum since the

scattering curve shape does not change significantly. Based on NMR experiments, for the

TEOS case the primary units were proposed to consist of tablets of 4 × 4 × 1.3 nm

dimensions. Combined with these NMR results and TEM studies the scattering curves of

such particles fit very well with experimental data. Moreover spherical particles would give

rise to symmetrical scattering patterns, which are not observed in figure 8. In the log I vs log

q representation an infinite flat sheet will give a slope of -2. Here slopes of -2.2 and -2.6 were

obtained for the TEOS and silicic acid recipes, respectively. This is in agreement with the

theoretical model since a tablet of dimension of 4 × 4 × 1.3 nm (use of TEOS) is not an

infinite sheet and a block of dimensions 4 × 2 × 1.3 nm (use of silicic acid) is closer to a cube

(slope -4) but still exhibits enough characteristic flatness to give a slope close to that of a flat

object.

79

Heating of precursors solutions: zeolite synthesis

In all cases, heating of the starting solutions leads to the specific formation of colloidal

particles with a characteristic length of 10-11 nm. Those are thought to be aggregates of the

nanoparticles formed spontaneously at RT.15 This assumption is strengthened by the fact that

the smaller nanoparticles formed with a higher aluminum content leads to smaller aggregates.

As mentioned in an earlier work on the all silica synthesis,12 the slope of the scattering curve

of those aggregates is approximately –2 (log I vs log q plot) suggesting again that they are

tablet like. Nevertheless it was not possible to derive reliable information on the precise

population after few hours at 125 °C since at that time the system contains nuclei, growing

crystals and most probably still some primary units (their scattering can be overshadowed by

that from their aggregates). From figure 11 it is not obvious that the primary units are still

present when the synthesis temperature is reached. Actually after 1 min of heating in the

rotating cell (approximately 60 °C) there is a spontaneous formation of aggregates

independent of the Al content. Aggregation is thermally activated and the presence of

aggregates seems to be crucial since de Moor et al. showed that they correlate with a faster

crystallization.18 Notably the temperature at which they form in a quantitative way

corresponds to the temperature at which we can obtain colloidal ZSM-5 crystals on a

reasonable time scale. Schoeman et al. have indeed found that colloidal ZSM-5 crystals can

be synthesized at 60 °C under the same synthesis conditions, but below this temperature no

crystalline material could be obtained on an acceptable time scale.35 This again illustrates the

importance of these aggregates in the crystallization process also when aluminum is absent.

Two attempts using the Derjaguin-Landau-Verwey-Overbeek (DLVO) theory have been

recently described in the literature36,37 to judge the possibility for primary units to form

aggregates and act as building blocks. These studies came up with opposite conclusions.

Schoeman stated that crystal growth occurs via addition of low molecular weight entities,

most likely the silicate monomer36 while Nikolakis et al. supported a ~3 nm subcolloidal

particle aggregation mechanism.37 But none of them assumed that the primary units were

nanoblocks since they used hard spheres as precursor particles. Indeed the interactions

between the nanoparticles, assuming that they are slab like, strongly influence their colloidal

and electrostatic properties. Kirschhock et al. performed DLVO calculations38,39 based on the

TEOS system, which consists of nanoslabs having dimensions of 4 × 4 × 1.3 nm. It was

found that further aggregation at RT was energetically unfavourable. However, upon heating

of the solution at 100 °C, the potential energy of the slab-like intermediates (similar to those

80

observed in figure 10) as a function of particle distance displays a minimum at about 0.75

nm. Thus intermediates remain physically trapped next to each other. They have time to align

for subsequent fusing.

-6.5

-6.2

-5.9

-5.6

-1.7 -1.4 -1.1 -0.8

log (I.q4)

log q

470 min.

450 min.350 min.

315 min.285 min.

A

-6.8

-6.5

-6.2

-5.9

-1.7 -1.4 -1.1 -0.8

log (I.q4)

log q

690 min.

546 min.

450 min.

410 min.

360 min.

B

-7.2

-6.9

-6.6

-6.3

-1.8 -1.5 -1.2 -0.9

1215 min.

1145 min.

1030 min.

1910 min.

880 min.

log (I.q4)

log q

C

Figure 14. I.q4 vs q data plot for the 3 different Si/Al ratios using TEOS as silica source: A) 150 B)

100 C) 50.

Crystal growth rate

The crystal growth rate for TPA-ZSM-5 synthesis using two different silica sources was

found to be strongly dependent on aluminum content. In agreement with the DLS results of

Schoeman et al.40 we found that the crystal growth rate decreased with increasing aluminum

concentration. Moderate addition of aluminum (lower than Si/Al = 50) did not affect the

crystal growth rate in either case but at relatively low Si/Al ratios the growth was

significantly slower. For the TEOS case, the significant decrease occurred at Si/Al = 50 (0.40

nm/min), 1.33 and 0.40 nm/min being the values for Si/Al = 150 and 100. Nevertheless, if

silicic acid was used as silica source, this decrease was already observed at Si/Al = 100 (0.26

nm/min) whereas lower aluminum loading gave higher rates (0.71 and 0.93 nm/min for Si/Al

81

= 150 and infinite, the latter being slightly lower consistent with the results of de Moor et

al.19).

The changes in crystal growth rates can be explained by the effects of the incorporation of

aluminum atoms into the nanoblocks. A relative change of the charge of the nanoblocks

could have a strong effect on the colloidal properties of those nanoprecursors. In a model

where crystal growth occurs by nanoblock aggregation, the potential energy of the precursors

would be greatly modified by the incorporation of one or several aluminum atoms into these

nanoblocks. Moreover, precursors which are formed with silicic acid are more sensitive since

they are smaller. This could explain why the silicic acid series is dramatically slowed down at

a lower aluminum content than the syntheses using TEOS.

Crystals shape and polydispersity

The shape of crystals is an important feature which can influence fundamental properties of

catalysts such as diffusion, stability and mechanical strength. High-resolution transmission

electron microscopy (HRTEM) allowed us to investigate the shape of the ZSM-5 crystals

after crystallization was completed. Despite the fact that the crystals do not show the

characteristic elongated prismatic form of ZSM-5, this absence is a typical characteristic of

small crystals grown from clear solutions via a fast growth. Not only TEM but also log Iq4 vs

log q can give us information on the shape and size polydispersity of those colloidal crystals.

Indeed, de Moor et al. showed that more pronounced first maxima and minima in the log Iq4

vs log q plot suggest a higher degree of monodispersity.16 This statement was based on

calculated patterns using perfect spheres as a model. Deviation from theoretical curves may

arise from irregularly shaped crystals. From figure 14 showing log I.q4 vs log q plots of

growing ZSM-5 crystals with different Si/Al ratios, it is clear that a lower aluminum content

leads to more pronounced first maxima and minima in the form factor patterns. The

observation using TEM gives us two possible explanations for this trend. First the form factor

is influenced by the higher polydispersity when the aluminum content is increased. Secondly,

the particles are irregularly shaped and built up of small crystallites for Si/Al ratios of 100

and 50, but their overall shape is still close to that of spherical particles. Consequently the

first maxima and minima are not very much affected but the form factor oscillations decay

faster. The size polydispersity for high Al contents also reveals a less homogeneous

distribution of viable nuclei in the induction period and non-simultaneous events as

nucleation and crystal growth proceed.

82

4.6 Conclusion

Hydrolysis of TEOS in aluminum containing TPAOH solutions leads to the

spontaneous formation of discrete colloidal particles with a size slightly dependent on Si/Al

ratio similar to the nanoslabs observed in chapter 2 with pure silica syntheses. Aggregation of

these primary units is also not affected by the incorporation of aluminum and is similar to the

all silica synthesis previously studied. However, the higher aluminum containing synthesis

mixtures exhibit slower nucleation kinetics. Additionally, USAXS experiments allowed us to

show that aluminum influences the colloidal ZSM-5 crystal growth rate, size and

morphology. Two different silica sources have been used and these show similar trends.

Aluminum has a strong influence on the addition of primary units to the growing crystals. We

observed the final size of the crystals to increase with increasing aluminum content, which

could be related to a lower amount of viable nuclei formed. Monitoring in situ the evolution

of aluminum containing zeolite precursors through a large length scale has made it possible to

establish directly its influence on nucleation and crystal growth. Additionally, aluminum has

a striking influence on the morphology of ZSM-5 crystals, pointing out its important role

during nucleation and crystallization. These results are in agreement with a common

crystallization pathway in the TPA mediated synthesis of high silica ZSM-5 based on a

nanoblocks aggregation mechanism even when aluminum is present in the synthesis mixture.

Although this paper focuses on aluminum influence on the formation of colloidal ZSM-5

crystals, it will be important to examine other heteroatoms (such as titanium or iron)

incorporation in these remarkable zeolites nanoprecursors.

References 1 Szoztak, R. Molecular sieves; Blackie Academic & Professional, 1998. 2 Van Bekkum, H.; Flanigen, E. M.; Jacobs, P. A.; Jansen, J. C. Introduction to zeolite science and practice, 2nd Edition, Stud. Surf. Sci. Catal.; Elsevier, 2001. 3 Davis, M. E.; Zones, S. I. In Synthesis of Porous Materials: zeolites, clays and nanostructures; Marcel Dekker, New York, 1997, 1-34. 4 Noll, F.; Sumper, M.; Hampp, N. Nano Lett. 2002, 2, 91-95. 5 Vrieling, E. G.; Beelen, T. P. M.; van Santen, R. A.; Gieskes, W. W. C. Prog. Ind. Microbiol. 1999, 35, 39-51.

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6 Thompson, R. W. In Molecular sieves, Science and technology, Springer, 1998, 1, 1-33. 7 Huo, Q.; Feng, S.; Xu, R. J. Chem. Soc., Chem. Commun. 1988, 22, 1486-1487. 8 Kuperman, A.; Nadimi, S.; Oliver, S.; Ozin, G. A.; Garces, J. M.; Olken, M. M. Nature 1993, 365, 239-42 9 Gies, H.; Marler, B. Zeolites 1992, 12, 42-9. 10 Burkett, S. L.; Davis, M. E. J. Phys. Chem. 1994, 98, 4467-4653. 11 Watson, J. N.; Iton, L. E.; Keir, R. I.; Thomas, J. C., Dowling, T. L.; White, J. W. J. Phys. Chem. B 1997, 101, 10094-10104. 12 Houssin, C. J. Y.; Mojet, B. L.; Kirschhock, C. E. A.; Buschmann, V.; Jacobs, P. A.; Martens, J. A.; van Santen, R. A. Zeolites and Mesoporous Materials at the Dawn of the 21st Century, Stud. Surf. Sci. Catal. 2001, 135, 135. 13 Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965-4971. 14 Kirschhock, C. E. A.; Ravishankar, R.; Van Looveren, L.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4972-4978. 15 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanschek, B. U.; Beck, L. W.; Wagner, P.; Davis, M. E.; van Santen, R. A. Chem.-Eur. J. 1999, 5, 2083-2088. 16 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanshek, B. U.; Diat, O.; van Santen, R. A. J. Phys. Chem. B 1997, 101, 11077-11086. 17 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Tsuji, K.; Davis, M. E. Chem. Mat. 1999, 11(1), 36-43. 18 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A. J. Phys. Chem. B 1999, 103, 1639-1650. 19 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Beck, L. W.; Davis, M. E. J. Phys. Chem. B 2000, 104, 7600-7611. 20 Guinier, A.; Fournet, G. Small-angle Scattering of X-Rays, Wiley, New York, Chapman Hall, London, 1955. 21 Teixeira, J. In Stanley, H. E.; Ostrowsky, N. On Growth and Form: Fractal and non-fractal Patterns in Physics, NATO-ASI Series E, 100, Martinus Nijhoff Publishers, Dordrecht, 1986, 145-162. 22 Glatter, O.; Kratky, O. Small-angle X-Ray Scattering, Academic Press, 1982. 23 Pedersen, J.S. Adv. Coll. Interf. Sci. 1997, 70, 171-210. 24 Meakin, P. In Stanley, H. E.; Ostrowsky, N. On Growth and Form: Fractal and non-fractal Patterns in Physics, NATO-ASI Series E, 100, Martinus Nijhoff Publishers, Dordrecht, 1986, 111-135. 25 Olivi-Tran, N.; Thouy, R.; Jullien, R. J. Phys. I France 1996, 6, 557-574. 26 Bale, H. D.; Schmidt, P. W. Phys. Rev. Lett. 1984, 53, 596-599. 27 Schmidt, P. W. J. Appl. Cryst. 1991, 24, 414-435. 28 Dokter, W. H. PhD thesis, Transformations in silica gels and zeolite precursors, Eindhoven University of Technology, 1994. 29 Narayanan, T.; Diat, O.; Bösecke, P. Nucl. Instrum. Methods Phys. Res. A 2001, 467-468, 1005-1009. 30 Bonse, U.; Hart, M. Small-angle X-ray scattering; Gordon and Breach, New York, 1967, 121. 31 Verduijn, J. P. Exxon Patent, PCT/EP92/02386, 1992. 32 Diat, O.; Bösecke, P.; Lambard, J.; De Moor, P-P. E. A. J. Appl. Crystallogr. 1997, 30, 862. 33 Kinrade, S. D.; Swaddle, T. W. Inorg. Chem. 1989, 28, 1952-1954.

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34 Kirschhock, C. E. A.; Buschmann, V.; Kremer, S.; Ravishankar, R.; Houssin, C. J. Y.; Mojet, B. L.; Grobet, P. J.; van Santen, R. A.; Jacobs, P. A.; Martens, J. A. Angew. Chem. Int. Ed. 2001, 40, 2637. 35 Persson, A. E.; Schoeman, B. J.; Sterte, J.; Otterstedt, J-E. Zeolites 1994, 14, 557. 36 Schoeman, B. J. Microp. Mesop. Mater. 1998, 22, 9-22. 37 Nikolakis, V.; Kokkoli, E.; Tirrel, M.; Tsapatsis, M.; Vlachos, D. G. Chem. Mater. 2000, 12, 845-853. 38 Kirschhock, C. E. A.; Ravishankar, R.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 11021-11027. 39 Kirschhock, C. E. A.; Ravishankar, R.; Truyens, K.; Verspeurt, F.; Jacobs, P. A.; Martens, J. A. Stud. Surf. Sci. Catal. 2000, 123, 293. 40 Persson, A. E.; Schoeman, B. J.; Sterte, J.; Otterstedt, J-E. Zeolites 1995, 15, 611-619.

85

5

Zeolite nanoslabs: building blocks

for innovative porous materials*

Zeolite nanoparticles containing tetrapropylammonium (TPA) as primary template are assembled into remarkable mesostructures via a secondary cooperative templating mechanism. These crystal-like silicate structures display a uniform morphology. Their spontaneous formation at room temperature shows the possibility of these zeolite nanoblocks becoming units for a new and innovative class of materials.

* Reproduced in part from: C. J. Y. Houssin; B. L. Mojet; R. A. van Santen ‘Remarkable Assembly of Zeolite Nanoprecursors into Crystal-like Mesostructures’ submitted.

86

5.1 Introduction

Mesoporous silicas have emerged as potentially powerful solids for oil refining, fine

chemicals synthesis and separation.1,2 Since their discovery in 19923, hexagonal mesoporous

aluminosilicate sieves have been the subject of considerable academic research.4 The

formation of these ordered mesophases is the result of the interactions between a surfactant

solution and soluble silica. A judicious choice of surfactant and/or cosurfactants allowed

tailoring the architecture and the pore size (2-50 nm). Despite overcoming the pore diameter

restriction of zeolites (≤ 1 nm), these materials are not crystalline i.e. the silica is locally not

very well organized and is closer to amorphous silica. Consequently, these materials cannot

match the high hydrothermal stability and acidity found in zeolites, their crystalline

microporous analogues. In view of these features, a very competitive research activity in this

area is currently directed towards the design of hydrothermally stable and catalytically active

mesoporous silica-based materials. Several attempts such as post synthesis procedures5,

silylation6, thicker walls syntheses7 have been made to improve these materials but both

acidity and stability still remain inferior to zeolites.

Because these limitations are due to the amorphous character of the walls, one might

expect a considerable improvement by rendering the structure of these walls close to those of

zeolites. Recently, two methods have been employed in that regard. The first procedure

involves the transformation of the walls of a conventional hexagonal aluminosilicate into

crystalline areas.8 The second approach consists of assembling zeolites seeds into hexagonal

mesostructures. Both methods have given a remarkable increase in stability and acidity.9,10

De Moor et al. observed the presence of nanoscopic species (few nm) in the reaction

mixture of the pure silica ZSM-5 organic mediated synthesis over the last few years.11,12,13

These nanoparticles, arising from the interactions between the structure-directing agent

(tetrapropylammonium denoted TPA) and the silica, have been proposed to play a key role in

the nucleation and crystal growth.14 Based on NMR experiments from such synthesis

mixtures a model has been presented by Kirschhock et al. in which the MFI structure is

already fully developed.15 Moreover we have extensively studied these nanoparticles in clear

solutions using small-angle X-ray scattering.16 It has been shown that these particles leading

to the same zeolite topology have a well-defined and optimum size depending on the silica

87

source and the cation content. These results combined with TEM showed the RT spontaneous

formation of MFI zeosil nanoslabs with dimensions of 2.7 × 1 × 1.3, 4 × 2 × 1.3 and 4 × 4 ×

1.3 nm (Chapter 2). A model was then proposed in which zeolites growth occurs via nanoslab

aggregation.17

Here, we propose a new method to prepare silica-based materials with both micro and

mesoporosity. The new route consists of organizing nanoparticles that are the primary

building blocks of zeolites with the MFI topology under the influence of a secondary

template that is typically used to synthesize mesoporous silica materials denoted HMS in the

literature.18 Our results indeed showed that a remarkable organization of these nanoparticles

using a larger surfactant has been achieved without destroying them and this self-organization

has been performed at room temperature. These silicate mesostructures were assembled by

adding slowly at RT a MFI nanoslabs solution (leading to silicalite-1 upon heating) to a

mixture of hexadecylamine in ethanol and water. The sample was maintained under gentle

stirring for 2 days. The as-synthesized product was characterized using SEM, XRD, TGA and

nitrogen adsorption.

5.2 Short review on recent advances on synthesis of M41S

mesostructures

The M41S family of mesoporous materials contains several unique structures that can

be indexed to a hexagonal network (MCM-41, SBA-2 and SBA-3), cubic structure (MCM-

48, SBA-1) and lamellar structure (MCM-50).19 The common features of all those

mesoporous materials is that they exhibit a narrow pore size distribution analogous to

crystalline microporous materials, but within the pore dimension of 1.5-20 nm. The initial

member of this family, MCM-41, was first synthesized as aluminosilicate in alkaline media

using a cationic alkyltrimethylammonium surfactant system.3

A better understanding of the synthesis mechanisms is a key point to the rational

design of mesoporous materials. Unlike individual molecule structure-directing agents

encountered in zeolite synthesis, strong organic intermolecular interactions are responsible

for determining the resulting inorganic framework of M41S materials. The original proposed

mechanistic pathways involved liquid crystal templating (LCT).3 However, this simplified

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model is unlikely to take place since most of M41S syntheses are not performed in the LCT

concentration regime. Few years after the MCM-41 discovery, Stucky et al. proposed a

cooperative self-assembly pathway.20 This mechanism involves silicate-covered cylindrical

micelles that act as template source. Those rods would first be disordered and polymerization

of silica would lead to the cooperative self-assembly of the final mesostructures. These

concepts highlighted the importance of the interactions between silica and surfactants. This is

believed to be the key factor in the control of the M41S mesostructures. Recently several in

situ investigations have been carried out in order to follow the entire process of MCM-41

related material synthesis.21,22,23,24 Frasch et al. studied the changes of the properties of the

micelles that may be induced by silicate species during the formation of mesoporous

hexagonal silica.24 These results led to a model described in figure 1. The key step is

considered to be the formation of silica pre-polymers. The growing of theses pre-polymeric

species takes place in a cooperative manner with a supply of surfactants. Further

polymerization and organization of the silica-micellar entities occur during the aging

sequence leading ultimately to M41S mesostructures.

Figure 1. Proposed mechanism of mesoporous

silica formation (adapted from ref. 24)

89

Originally the M41S family was synthesized using cationic surfactants. These procedures

have been extended to non-ionic surfactants like polyoxyethylene alkyl ethers or amines

(figure 2A). Particularly, the so-called HMS materials were obtained by precipitation at

neutral pH of TEOS and primary long alkyl chain amines.18 It has been postulated that the

formation of the HMS silicas occurs through H-bonding interactions between precursor

silanols and the lone electron pairs of the surfactant head groups as depicted in figure 2B.

Figure 1. Schematic representation of A) Neutral silica-surfactant interactions (SoIo). S represents the

surfactant and I, the inorganic framework. Triangles are solvent molecules. Dashed lines correspond

to H-bonding forces. B) Templating mechanism of formation of HMS mesoporous molecular sieves.

5.3 Experimental section

The nanoparticles are prepared as described in literature using TEOS and TPAOH.15 A

typical preparation is as follows: 10.37 g of TEOS (Aldrich) is mixed with 5 g TPAOH 40%

in water (Alfa) and 5.37 g of distilled water under vigorous stirring. This solution was then

stirred overnight to ensure hydrolysis of TEOS. This solution is slowly added to a surfactant

solution containing 3.24 g of hexadecylamine (Aldrich), 20.7 g of ethanol and 18.2 g of

water. The resulting mixture appeared to be milky but turned into a gel after 20h of gentle

stirring at room temperature. The obtained product was recovered by filtration, washed with

deionized water and ethanol (80% water w/w) and air dried at 60 °C overnight.

Powder X-ray diffraction patterns were recorded on a Rigaku diffractometer using CuKα

radiation. N2 adsorption-desorption isotherms were measured at –196 °C on a Micromeritics

90

Tristar sorptometer. Before measurements, samples were outgassed at 200 °C during 5h. The

pore size distribution was calculated from the desorption branch of the isotherms using the

Barret-Joyner-Halenda (BJH) method.

Scanning electron microscopy (SEM) micrographs were taken with a field emission

gun (FEG) XL30 instrument. IR spectra were recorded using a Nicolet 360FT-IR

spectrometer. Before measurements samples were prepared in the form of thin KBr pellets.

High-resolution transmission electron microscopy (HRTEM) was performed using a

Philips CM30UT electron microscope with a field emission gun as the source of electrons

operated at 300 kV. Samples were mounted on a Quantifoil microgrid carbon polymer

supported on a copper grid by placing a few droplets of a suspension of mesostructured silica

in water on the grid, followed by drying at ambient conditions.

5.4 Results and discussion

The SEM pictures of the as synthesized product are shown in figures 3A and 3B. The

most striking feature is the observation of flat crystal-like structures of a few µm long and

200 nm thick whereas classical mesoporous silicas are known to be amorphous. Only heating

at a minimum temperature of 100 °C for 2 days can lead to the self-assembly of the

nanoblocks to form silicalite-1 crystals. Importantly, the sharp edges and the shape of the

structures remind of ZSM-5 crystals.

The XRD pattern of the as-synthesized sample shown in figure 4 does not show any

Bragg reflection related to a crystalline phase. Nevertheless the XRD spectrum exhibits a

strong d100 reflection corresponding to a d-spacing of 6.5 nm accompanied by higher order

Bragg reflections confirming the relative good long-range order observed with SEM. These

reflections do not correspond to any of the three architectures (hexagonal, cubic or lamellar)

found for the M41S family of aluminosilicates. The HMS silica molecular sieves actually

have not been indexed precisely. 18

91

Figure 3.

A) and B) SEM micrographs of

the as-synthesized silica

mesostructure at different

magnifications.

Figure 4. XRD diffraction of the as-synthesized sample.

To test whether the MFI structure was retained IR experiments on the as-synthesized

product were performed (Figure 5). We indeed found a band between 560 and 570 cm-1

showing the presence of building units containing five-membered rings which is indicative of

the MFI structure. Thermogravimetric analysis (TGA) of the sample exhibited a weight loss

92

of 60% which was attributed to the removal the templates. The most interesting fact is that

both templates are still encapsulated in the material, with ratio close to those in the starting

mixture. However the temperature corresponding to TPA loss (280 °C) is lower to the one

found for of silicalite-1 (320 °C), suggesting that TPA is less tightly held in the present case.

Figure 5: IR spectrum of the as-made silica mesostructure.

Figure 6. TEM image of the as made mesostrucutre.

TEM revealed a layered structure (Figure 6) but the fragility of those remarkable

assemblies makes them very sensitive to the electron beam. We then suspect that TEM

measurements destroyed the mesostructures formed despite the use of lower beam intensity.

93

Figure 7. SEM micrograph of the calcined silica.

0

1

2

3

4

5

6

7

1 3 5 7 9 11 13 15

Pore Diameter (nm)

Pore

Vol

ume

(cm

3 /g)

Figure 8. Pore size distribution of the calcined silica estimated by BJH model.

So far, no efficient calcination route has been found in order to retain this uncommon

supramolecular assembly of zeolite precursors as shown in figure 3. Nevertheless, we

performed N2 adsorption on the product obtained after a normal calcination step used for

mesoporous aluminosilicates. A large specific surface area of 800 m2/g and a pore size of 3.8

nm (Figure 8) were observed. Pinnavaia et al.18 found a pore size of 2.5 nm and a slightly

94

smaller specific surface area using hexadecylamine and TEOS as starting materials. The

collapse of this uncommon mesostructure may be due to the fact that the assembly consists of

loosely bonded nanoblocks all the more that the interactions between hexadecylamine and

silica result from a neutral templating route18 thus preventing the use of elevated

temperatures. The next hurdle will be to strengthen these unique architectures and make them

catalytically active by incorporating heteroatoms.

5.5 Conclusion

In conclusion, highly organized and unprecedented assemblies of zeolite

nanoprecursors were obtained through the use of a secondary neutral template. IR showed

that the zeolite structure was kept during the assembly even though XRD could not detect any

long-range orientation of the building blocks. The present study illustrates that a controlled

choice of surfactants and zeolite precursors may be a general way for the synthesis of

innovative silicate mesostructures.

References 1 Corma, A. Top. Catal. 1997, 4, 249. 2 Schüth, F. Zeolites and Mesoporous Materials at the Dawn of the 21st Century, Stud. Surf. Sci. Catal., 2001, 135, 1-12. 3 Kresge, C. T.; Leonowicz, M. E.; Roth, W. J.; Vartuli, J. C.; Beck, J. S. Nature 1992, 359, 710. 4 Trong On, D.; Desplantier-Giscard, D.; Danumah, C.; Kaliaguine, S. Appl. Catal. A 2001, 222, 299. 5 Mokaya, R. Angew. Chem. Int. Ed. 1999, 38, 2930. 6 Zhao, X. S.; Lu, G. Q. J. Phys. Chem. B 1998, 102, 1556. 7 Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Frederikson, G. H.; Chmelka, B. F.; Stucky, G. D. Science 1998, 279, 548. 8 Trong On, D.; Kaliaguine, S. Angew. Chem. Int. Ed. 2002, 41, 1036-1040. 9 Liu, Y.; Zang, W.; Pinnavaia, T. Angew. Chem. Int. Ed. 2001, 40, 1255. 10 Liu, Y.; Pinnavaia, T. Chem. Mater. 2001, 14, 3. 11 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanshek, B. U.; Diat, O.; van Santen, R. A. J. Phys. Chem. B 1997, 101, 11077-11086. 12 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A. J. Phys. Chem. B 1999, 103, 1639-1650.

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13 De Moor, P-P. E. A.; Beelen, T. P. M.; van Santen, R. A.; Beck, L. W.; Davis, M. E. J. Phys. Chem. B 2000, 104, 7600-7611. 14 De Moor, P-P. E. A.; Beelen, T. P. M.; Komanschek, B. U.; Beck, L. W.; Wagner, P.; Davis, M. E.; van Santen, R. A. Chem.-Eur. J. 1999, 5, 2083-2088. 15 Kirschhock, C. E. A.; Ravishankar, R.; Verspeurt, F.; Grobet, P. J.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 4965-4971. 16 Houssin, C. J. Y.; Mojet, B. L.; Kirschhock, C. E. A.; Buschmann, V.; Jacobs, P. A.; Martens, J. A.; van Santen, R. A. Zeolites and Mesoporous Materials at the Dawn of the 21st Century, Stud. Surf. Sci. Catal. 2001, 135, 135. 17 Kirschhock, C. E. A.; Ravishankar, R.; Jacobs, P. A.; Martens, J. A. J. Phys. Chem. B 1999, 103, 11021-11027. 18 Tanev, P. T.; Pinnavaia, T. Chem. Mater. 1996, 8, 2068. 19 Biz, S.; Occelli, M. L. Catal. Rev. – Sci. Eng. 1998, 40, 329. 20 Firouzi, A.; Kumar, D.; Bull, L.; Besier, T.; Sieger, P.; Huo, Q.; Walker, S. A.; Zasadzinski, J. A.; Glinka, C.; Nicol, J.; Margolese, D.; Stucky, G. D.; Chmelka, B. F. Science 1995, 267, 1138. 21 Zhang, J.; Luz, Z.; Godfarb, D. J. Phys. Chem. B 1997, 101, 7087. 22 Zhang, J.; Luz, Z.; Zimmermann, H.; Godfarb, D. J. Phys. Chem. B 2000, 104, 279. 23 Agreen, P.; Linden, M.; Rosenholm, J. B.; Schwarzenbacher, R.; Kriechbaum, M.; Amenitsch, H.; Laggner, P.; Blanchard, J.; Schuth, F. J. Phys. Chem. B 1999, 103, 5943. 24 Frasch, J.; Liebeau, B.; Soulard, M.; Patarin, J.; Zana, R. Langmuir 2000, 16, 9049.

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Summary

The study of the early stages of the crystallization of silica-based porous materials is still a very arduous task. The mechanisms by which guest molecules are enclathrated in an organic-inorganic crystalline structure remained very unclear for a long time. This issue also applies for many systems and for a large range of pore sizes. Huge academic efforts have been devoted to the study of the crystallization of zeolites, M41S mesostructures and diatoms, having well-defined pore sizes of 0.6-2 nm, 2-50 nm and 50 nm-1µm respectively. The main question is: How sophisticated structures self-assemble from atoms to organic-inorganic macroscale objects? Although the answer would probably not be the same over all length scales, the ultimate goal of such an understanding is the rational design of porous materials. Chiral porous solids may even be envisioned. In this thesis, the self-assembly process of tetrapropylammonium-mediated MFI zeolites is discussed. As mentioned above, techniques covering a wide range of lengthscales are prerequisites for investigating the whole course of the crystallization process. In situ and non-invasive methods are also required due to the fragileness of the hybrid intermediates. NMR and X-ray scattering have mainly been employed to fulfill these needs. Chapter 2 provides a comparative study of the early steps of silicalite-1 synthesis varying parameters such as silica source and cations content. SAXS and TEM were applied to characterize subcolloidal particles formed during the mixing of TPA and silica. For the first time, it has been shown that these particles have optimum size and they appear to be slab like irrespective of the silica source. NMR showed that they are likely to contain the MFI topology. The combination of NMR and X-ray scattering revealed to be very efficient in probing the early stages of silicalite-1 formation (Chapter 3). Processes on a molecular scale (29Si NMR) could be related to events occurring on a colloidal scale (SAXS) during the dissolution of silicic acid powder in concentrated TPAOH solutions. Besides classical oligomeric silicates encountered in basic aqueous solutions, 29Si NMR allowed the detection of a specific oligomer which assembles into well-defined subcolloidal MFI precursors having dimensions of 2.7 × 1 × 1.3 nm. Applications of new NMR techniques also gave more insight into the nanoparticles observed in ZSM-5 synthesis. The incorporation of aluminum could be followed using 27Al NMR. It appeared that aluminum preferentially migrates to Q4 positions in the nanoslabs. An in situ 27Al NMR synthesis of ZSM-5 showed that aluminum is tetrahedrally coordinated throughout the crystallization, discarding the possibility of crystal growth via monomer or small oligomer addition. Interactions between TPA molecules and silica could be determined by direct distance measurements. It has been shown that the early contacts are similar to those observed in the final crystals where TPA is tightly encapsulated within the porous framework. Combined in-situ synchrotron SAXS/USAXS allowed the study of aluminum incorporation during the complete course of ZSM-5 crystallization (Chapter 4). Hydrolysis of TEOS in aluminum containing TPAOH solution led to the spontaneous formation of discrete colloidal particles with a size slightly dependent on Si/Al ratios. However, this addition did not change the pathways of crystallization. Incorporation of aluminum slows down the nucleation rate and crystal growth. Morphologies were greatly influenced by the Al content.

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The present identification of very stable and well-defined nanoparticles can be applied to the synthesis of innovative porous materials (Chapter 5). A new method to prepare silica-based materials via organization of zeolite nanoparticles under the influence of a secondary template has been proposed. The driving force of these efforts was to design silica porous materials exhibiting both micro and mesoporosity. Although removal of template molecules led to the collapse of the structure, high surface area silicas were obtained and the as-synthesized mesostructures are promising.

In conclusion, very well-defined and discrete nanoscale MFI precursors have been identified resulting from the mixing of a organic or inorganic silica source and TPA cations. They show typical dimensions of 2.7 × 1 × 1.3, 4 × 2 × 1.3 and 4 × 4 × 1.3 nm. NMR and X-rays scattering studies support a growth mechanism in which adding units are these unique nanoslabs. We do not discard other mechanism in zeolite crystallization, but this pathway may be general for organic-mediated zeolite synthesis. MFI nanoprecursors can also be applied for the synthesis of hierarchical porous materials.

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Samenvatting De bestudering van de vroege kristallisatie stadia van poreuze materialen gebaseerd op silica is nog steeds een erg ingewikkelde taak. De mechanismen waarbij gastmoleculen opgenomen worden in een organisch-anorganische structuur blijven onduidelijk. Dit is ook van toepassing op vele andere systemen met een breed bereik van poriegroottes. Er is enorm veel onderzoeks geleverd om de kristallisatie van zeolieten, M41S mesostructuren en diatomen met goed gedefinieerde poriegroottes van respectievelijk 0.6-2 nm, 2-50 nm en 50 nm-1 µm te bestuderen. De belangrijkste vraag daarbij is hoe deze geavanceerde structuren door de zelforganisatie van atomen tot organisch-anorganische composietmaterialen op macroschaal gevormd worden. Hoewel het antwoord daarop niet eenduidig zal zijn over de gehele lengteschaal, is het uiteindelijke doel van zulk begrip het rationeel kunnen ontwerpen van poreuze materialen. Zelfs chirale poreuze materialen kunnen worden overwogen. In dit proefschrift wordt het zelforganisatiemechanisme van zeoliet met de MFI structuur met behulp van tetrapropylammonium als organisch structuur-bepalend molecuul besproken. Zoals eerder genoemd is het voor de bestudering van het gehele kristallisatieproces noodzakelijk om technieken te gebruiken die een brede lengteschaal kunnen bedekken. In situ en niet-aantastende methodes zijn ook noodzakelijk wegens de breekbaarheid van de hybride intermediaire structuren. NMR (Nuclear Magnetic Resonance) en Röntgendiffractie zijn hoofdzakelijk voor deze doelen gebruikt. Hoofdstuk 2 beschrijft een vergelijkende studie van de eerste stappen in de silicaliet-1 synthese, waarbij parameters als silica-bron en kationengehalte worden gevarieerd. SAXS (Small-Angle X-ray Scattering) en TEM (Transmission Electron Microscopy) werden toegepast om subcolloïdale deeltjes, gevormd tijdens het mengen van TPA en silica, te karakteriseren. Het is voor het eerst aangetoond dat deze deeltjes een optimum grootte bezitten. Ongeacht de silica bron blijken deze deeltjes plaatvormig zijn. Met behulp van NMR is aangetoond dat het waarschijnlijk is dat de deeltjes, MFI topologie bezitten. Het is verder gebleken dat de combinatie van NMR en Röntgendiffractie erg efficiënt is om de vroege stadia van silicaliet-1 formatie te onderzoeken (Hoofdstuk 3). Tijdens de oplossing van siliciumzuur poeder in geconcentreerde TPAOH oplossingen konden processen op moleculaire schaal (29Si NMR) worden gerelateerd aan gebeurtenissen op colloidale schaal (SAXS). Naast de klassieke oligomerische silicaten in basische waterige oplossingen, kon met behulp van 29Si NMR een specifiek oligomeer ontdekt worden dat zich organiseert in goed gedefineerde subcolloïdale MFI-precursors met dimensies van 1.7 x 1 x 1.3 nm. Toepassingen van nieuwe NMR technieken hebben ook meer inzicht gegeven in de nanodeeltjes die kunnen worden waargenomen bij de ZSM-5 synthese. De incorporatie van aluminium kon worden bestudeerd door het gebruik van 27Al NMR. Het bleek dat aluminium bij voorkeur naar Q4 posities in de nanoplaatjes migreert. De in situ 27Al NMR synthese van ZSM-5 liet zien dat aluminium tetraëdrisch gecoördineerd is tijdens de kristallisatie, hetgeen de mogelijkheid van kristalgroei door monomeer- of kleine oligomeer-additie uitsluit. Interacties tussen TPA moleculen en silica konden worden vastgesteld door middel van directe-afstandsmetingen. Het is aangetoond dat de vroege contacten vergelijkbaar zijn met de waargenomen contacten in de eindkristallen, waarbij TPA strak is opgenomen in het poreuze geraamte. Gecombineerde in situ synchrotron SAXS/USAXS heeft het bestuderen van de aluminium incorporatie tijdens de gehele ZSM-5 kristallisatie mogelijk gemaakt (Hoofdstuk

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4). Hydrolyse van TPAOH in een aluminiumhoudende TPAOH oplossing leidde tot de spontane formatie van discrete colloïdale deeltjes met een grootte die enigszins afhankelijk is van Si/Al verhoudingen. Echter, deze toevoeging heeft de manier van kristallisatie niet veranderd. De incorporatie van aluminium vertraagt de nucleatiesnelheid en de kristalgroei. Het is gebleken dat het Al gehalte een grote invloed heeft op de morfologie. De identificatie van zeer stabiele en goed gedefinieerde nanodeeltjes kan worden toegepast op de synthese van innovatieve poreuze materialen (Hoofdstuk 5). Een nieuwe methode is voorgesteld om silica-gebaseerde materialen te bereiden via de organisatie van zeoliet-nanodeeltjes onder de invloed van een tweede template. De drijfkracht van deze inspanningen was om poreuze silica materialen te ontwerpen die zowel micro- als mesoporositeit vertonen. Hoewel de verwijdering van de template moleculen tot een ineenstorting van de structuur leidde, zijn silica’s met hoge specifieke oppervlakken verkregen. De aldus verkregen meso-structuren zijn veelbelovend. Samenvattend zijn zeer goed gedefinieerde en discrete nanometer schaal MFI precursoren geïdentificeerd, voortkomend uit het mengen van een organische of anorganische silica bron met TPA kationen. Typische dimensies van de betreffende deeltjes zijn 2.7 x 1 x 1.3, 4 x 2 x 1.3 en 4 x 4 x 1.3 nm. NMR en Röntgendiffractie ondersteunen een groeimechanisme waarbij de samenstellende deeltjes de unieke nanoplaatjes zijn. We sluiten geen ander mechanisme voor zeolietkristallisatie uit, maar deze route zou algemeen kunnen gelden voor zeolietsynthese met behulp van organische structuur-bepalende moleculen. MFI nanoprecursors kunnen ook worden toegepast voor de synthese van nieuwe materialen waarvan de porositeit hiërarchisch is.

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Résumé

L’étude des premières étapes de la cristallisation des solides poreux à base de silice n’est pas une tâche aisée. Les mécanismes par lesquels les molécules sont piégées dans une matrice organique-inorganique et cristalline n’ont pas encore été clarifiés. Cela concerne également de nombreux systèmes dont la taille des pores est très variée. D’énormes efforts académiques ont été entrepris concernant l’étude de la cristallisation des zéolithes, des solides mésoporeux et des diatomes ayant une taille de pores variant entre 0.6-2 nm, 2-50 nm et 50 nm-1 µm respectivement. La question majeure est de savoir comment ces structures très compliquées s’assemblent à partir d’entités de taille moléculaire en des composites organiques-inorganiques beaucoup plus grand (plusieurs microns). Même si la réponse n’est probablement la même pour chaque échelle, l’objectif est la fabrication sur mesure de matériaux poreux. La synthèse de matériaux poreux et chiraux peut être envisagée. Cette thèse approfondit la compréhension du mécanisme de synthèse de zéolithes par des molécules organiques. Pour les raisons décrites ci-dessus, des techniques couvrant une large échelle sont préférables pour suivre complètement la cristallisation des zéolithes. Des méthodes in situ et non destructives sont également requises à cause de la fragilité des intermédiaires. La RMN (Résonance Magnétique Nucléaire) et la diffusion des rayons X aux petits angles (SAXS) ont été principalement employées dans ce but. Le chapitre 2 présente une étude comparative des premières étapes de la synthèses de la silicalite-1 en variant des paramètres comme la source de silice ou la nature des cations. SAXS et TEM (microscopie électronique à transmission) ont été employées pour caractériser les particules colloïdales formées lors du contact entre les molécules de TPA et la silice. Pour la première fois, il a été montré que ces particules ont une taille optimum et qu’elles sont plates, indépendamment de la source de silice. L’étude RMN a montré qu’elles contiennent très probablement la topologie MFI. La combinaison de la RMN et de la diffusion des rayons X aux petits angles s’est révélée être très efficace pour étudier les premières étapes de la formation de la silicalite-1 (Chapitre 3). Les processus à l’échelle moléculaire (RMN 29Si) ont pu être reliés aux transformations se déroulant au niveau colloïdal (SAXS) pendant la dissolution de l’acide silicique dans des solutions concentrées de TPAOH. En dehors des oligomères classiques de la chimie de la silice, la RMN du silicium a permis la détection d’un oligomère spécifique qui s’assemble en des précurseurs colloïdaux de la silicalite-1 et dont les dimensions sont: 2.7 × 1 × 1.3 nm. L’application de nouvelles techniques RMN a également clarifié la nature des particules colloïdales observées dans la synthèse de la zéolithe ZSM-5. L’incorporation de l’aluminium a pu être suivie à l’aide de la RMN 27Al. Il s’est avéré que l’aluminium se fixe préférentiellement aux positions Q4 dans les nanoparticules MFI. Une étude in situ RMN 27Al de la synthèse de la zéolithe ZSM-5 a prouvé que l’aluminium a un degré de coordination 4 pendant toute la cristallisation, rendant improbable l’hypothèse d’une croissance des cristaux par addition de monomère ou d’oligomères. Les interactions entre les molécules de TPA et la silice peuvent être directement déterminées en mesurant la distance qui les sépare. Les résultats montrent que ces contacts sont similaires à ceux rencontrés dans les cristaux obtenus où les molécules de TPA sont fermement incorporées dans la structure poreuse. La combinaison des techniques synchrotron telles que SAXS et USAXS ont permis l’étude de l’incorporation de l’aluminum pendant toute la cristallisation de la zeolite ZSM-5

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(Chapitre 4). L’hydrolyse du TEOS dans une solution contenant TPAOH conduit spontanément à la formation d’entités colloïdales discrètes dont la taille dépend légèrement du rapport Si/Al. Cependant, cette addition ne change pas le mécanisme de la cristallisation. L’incorporation de l’aluminium ralentit la nucléation et la croissance des cristaux. De plus, le taux d’aluminium influence d’une façon très marquée la morphologie des cristaux obtenus. La présente identification de nanoparticules très stables et bien définies peut avoir des applications dans la synthèse de nouveaux matériaux (Chapitre 5). Une nouvelle méthode pour préparer des matériaux à base de silice en organisant ces nanoparticules à l’aide d’un agent directeur de synthèse secondaire a été proposée. Le but de ces recherches est la fabrication de matériaux à la fois microporeux et mésoporeux. Même si la calcination des agents directeurs de synthèse a conduit à la destruction de la structure, des silices avec une grande surface intérieure ont été obtenues et les matériaux non calcinés restent prometteurs. En conclusion, des nanoparticules bien définies et ayant la topologie MFI ont été identifiées. Elles sont formées par la dissolution de silice dans une solution aqueuse de TPA. Leurs dimensions sont discrètes: 2.7 × 1 × 1.3, 4 × 2 × 1.3 et 4 × 4 × 1.3 nm. La RMN et la diffusion des rayons X aux petits angles confirment l’existence d’un mécanisme de cristallisation dans lequel les unités intervenant dans la croissance des cristaux sont ces uniques entités colloïdales. Nous n’excluons pas un autre mécanisme pour la cristallisation des zéolithes mais il est fort probable qu’il s’applique pour la synthèse des zéolithes en présence de molécules organiques comme agents directeurs de synthèse. Les précurseurs identifiés dans cette étude peuvent être utilisés pour la synthèse de nouveaux matériaux dont la porosité serait hiérarchique.

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Acknowledgments

I would like to express my deepest gratitude to Prof. Rutger van Santen for giving me the opportunity to work in his research group. Rutger, thank you very much for the many interesting discussions, your continuous support and the freedom you gave me throughout this work.

Barbara Mojet is acknowledged for her guidance and for providing me with her extensive knowledge in the field of synchrotron radiation, which is the main technique used in this thesis. Barbara, thank you for teaching me all the tricks of synchrotron measurement and data treatment.

I am very grateful to Prof. Johan Martens for welcoming me in his group in Leuven. Johan, without your contribution and the stimulating discussions we had, a lot of the results described in this thesis would not have been obtained. I am also indebted to Christine Kirschhock for her significant involvement in this project and the beautiful pictures of zeosil nanoslabs (see cover). I also thank Prof. Piet Grobet for his guidance during NMR measurements.

I thank Prof. Rob van Veen and Prof. Wilfried Mortier for the time they spent in reading the thesis and for their helpful suggestions. I also thank Prof. Hans Niemantsverdriet, Prof. Jean-Pierre Gilson and Prof. Bert de With for being part of the thesis committee. Jean-Pierre, you were the first who taught me what zeolites and their use were. Thank you for the interesting and inspiring courses, I hope you can see that your teaching efforts were not in vain.

I acknowledge Véronique Buschmann and Patricia Kooymann for their efforts to get beautiful TEM pictures.

I would like to express my gratitude to the “NMR boys”, Pieter Magusin, Eugène van Oers, Yannick Millot and Vadim Zorin for their patience and their professionalism. It turned out that NMR was a powerful complementary technique to SAXS.

Most of the X-ray scattering experiments were performed at the European Synchrotron Radiation Facility (ESRF), France. I am very grateful to Theyencheri Narayanan and Pierre Panine (ID02) and Prof. Wim Bras and Igor Dolbnya (DUBBLE).

I thank Theo Beelen, Dick Lieftink and Engel Vrieling for their assistance during synchrotron measurements and the many discussions on silica chemistry and diatoms.

I thank the current and former members of the department for their welcoming attitude and the nice atmosphere they create in the SKA group.

A number of them I want to mention specifically:

Alina K.: best competitor tied with Mayela G. for seats next to the coffee machine; elected Miss SKA metal catalysis group in 1999 and 2001, second in 2000 and 2002.

Arian O.: best continental driver in England but not the best driver in continental Europe. The first statement might be a consequence of the second. Thanks for all the discussions on zeolite precursors, your many ideas on this topic and the scientific trip to England.

Chrétien H.: best SKA Francophile.

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Davy N. and Qianyao S.: synchrotron teammates. The easiest way to convince people to come with you for synchrotron measurements is to promise to show them “Grenoble by night”, which turns out to be: change cell 3, put in cell 1, move rotor x +25 etc etc….

Emiel H.: one of the top candidates for the casting of Men in Black III. Thanks for the interest you showed in my project. I really appreciated your curiosity, questions and help in zeolite synthesis.

Eric Z.: aka the “SKA stuntman”, has taken a decisive turn during his Ph.D. thesis.

(the) French Connection: I could not help but I sometimes had to speak my native language. In that regard, I met several compatriots. Many thanks to Alison, Coralie, Nathalie (my employment advisor), Hélène, Abdel and Xavier. Special thanks go to (newly Ir) Luc v. R. for his supporting and positive attitude. I could not convince him to become an AIO but he made an effort in accepting a TWAIO position. For the own safety of the author, Rafael S. was not included in this section.

Joyce O.: the nicest smile of the Faculty of Chemistry, your secretary office should keep some top-secret files because I often saw a bodyguard there.

Mayela G.: best competitor tied with Alina K. for seats next to the coffee machine; elected Miss SKA metal catalysis group in 2000 and 2002, second in 1999 and 2001.

Marco H.: Another top candidate for the casting of Men in Black III. I am very grateful to you for the time you spent helping me with X-ray measurements. Unfortunately, our dream, a rotating anode equipment, has not come true yet despite your efforts.

Rafael S.: unique member of the Alsacian Connection.

Rob H.: see Joyce O.

Ruben v. D.: alternatively triathlete, SKA borrel organizer, cyclist, SKA activity organizer, chemistry journal coverboy, SKA colloquium organizer, bike repairman…occasionally busy with homogeneous catalysis.

Tiny V.: aka “de SKA voetbaldagblad”; unique specimen “uit De Peel”, once believed that The Netherlands would be “WK kampioenen”. Second best SKA Francophile.

Zhu Q.: funniest chinese person I have ever met. Thanks for your help during these 4 years as officemate. I learnt a lot about Chinese culture and…habits. Zhu, je bent een gezellige chinees. Finally, I would like to thank my family for their warm encouragement and permanent support during all these four years.

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Curriculum Vitae

Christophe Houssin was born on the 27th of April 1973 in Villedieu-les-Poêles

(Normandy, France). He got his baccalauréat diploma at Lycée Notre Dame, Avranches in

1991. After three years of preparatory courses at Lycée Victor Hugo in Caen, he entered the

engineering school ENSI-ISMRA Caen in 1994. He graduated as engineer in catalysis,

materials and organic chemistry in 1997. His graduation project on FCC catalysts was

performed at Shell Research in Amsterdam. He obtained a DEA (equivalent to a M.Sc.) in

organic chemistry from Caen University in 1997. He achieved his military duty in 1998 in the

marine infantry. On December 1st 1998, he started his Ph.D. research under supervision of

Prof. Rutger van Santen in the laboratory of Inorganic Chemistry and Catalysis, Eindhoven.

His research project dealt with the understanding of the organic-mediated synthesis of

silicalite-1 and ZSM-5 zeolites. These investigations involved an eight-month stay in the

group of Prof. Johan Martens at the Leuven Catholic University, Belgium. The most

important results of this work are described in this thesis.