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Microporous and Mesoporous Materials 83 (2005) 277–282

Synthesis and characterization of mesoporousaluminosilicate molecular sieve from K-feldspar

Shiding Miao a, Zhimin Liu a,*, Hongwen Ma b, Buxing Han a,Jimin Du a, Zhenyu Sun a, Zhenjiang Miao a

a CAS Key Laboratory of Colloid, Interfacial and Chemical Thermodynamics, Institute of Chemistry,

Chinese Academy of Sciences, Beijing 100080, Chinab National Laboratory of Mineral Materials, China University of Geosciences, Beijing 100083, China

Received 7 April 2005; received in revised form 4 May 2005; accepted 5 May 2005

Available online 16 June 2005

Abstract

Mesoporous aluminosilicate molecular sieves have been synthesized using K-feldspar, a natural ore rich in silicon and aluminum

sources, as starting material. In this method, the mixture of K-feldspar and K2CO3 with mass ratio of 2:3 was first calcined at 880 �Cfor 1.2 h. The calcined materials were then dissolved in water together with cetyltrimethylammonium bromide as structure-directing

agent, resulting in mesoporous molecular sieves after heating the solution at 130 �C for 60 h. Scanning electron microscopy and trans-mission electron microscopy observations indicated that the resulting materials were spherical particles with size of about 100 nm. The

mesoporous structure of the as-synthesized materials was confirmed by low angle X-ray diffraction and nitrogen sorption analysis.

The BET surface area of the as-prepared material after calcined at 550 �C was 507 m2 g�1 and the pore volume was 0.854 cm3 g�1.27Al MAS NMR analysis showed that nearly all of aluminium is incorporated into the framework of the mesoporous material with

4-coordinated state. The acidity of the product was analyzed by FTIR spectroscopy of pyridine-adsorbed product. The product was

treated in water at 100 �C for 360 h, and its mesoporous structure was still intact, suggesting its high hydrothermal stability.

� 2005 Elsevier Inc. All rights reserved.

Keywords: Mesoporous material; Aluminosilicate; Molecular sieves; K-feldspar

1. Introduction

Since the discovery of the mesoporous M41S materi-

als in 1992 [1], the ordered mesoporous materials have

attracted much attention. Among the mesoporous mate-

rials, hexagonally ordered silica MCM-41, characterizedwith pore size between 2 and 10 nm with a narrow pore

size distribution, has been investigated extensively. How-

ever, due to the amorphous nature of their walls, silica

MCM-41 materials have relatively low acidity and

hydrothermal stability, which severely hinders their

industrial applications in catalytic reactions [2]. The

1387-1811/$ - see front matter � 2005 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2005.05.006

* Corresponding author. Tel./fax: +86 1062562821.

E-mail address: liuzm@iccas.ac.cn (Z. Liu).

incorporation of Al (called Al-MCM-41) and transition

metal elements within the silica framework has been

implemented, which improved the hydrothermal stability

and acidity to some extent [3]. Al-MCM-41 with tetrahe-

dral coordinated Al into the silica framework can gener-

ate Bronsted acid sites, which has been the focus of muchrecent research. A number of papers concerning the syn-

thesis and characterization of Al-MCM-41 mesoporous

materials have been published [4], and tetraethoxysilane,

aluminum isopropoxide, sodium aluminate, and other

aluminosilicates were generally used as Si, Al sources.

Many natural ores are rich in silicon and aluminum,

which are very cheap, and can be easily obtained. Obvi-

ously, using these ores as raw materials to synthesize Alsubstituted molecular sieves is of great importance.

278 S. Miao et al. / Microporous and Mesoporous Materials 83 (2005) 277–282

However, study on the synthesis of mesoporous molecu-

lar sieves containing Al and Si oxides using natural ores

as aluminum and silicon sources is scarce. Recently,

Kang et al. [5] synthesized Al-MCM-41 using metakaolin

as aluminum source and water glass as silicon source,

and Liu et al. [6] used NaY and kaolin as a starting mate-rial to synthesize kaolin/NaY/MCM-41 composites,

which exhibits good hydrothermal stability.

In this work, we report the synthesis of mesoporous

molecular sieves using K-feldspar as silicon and alumi-

num sources, in which silicon and aluminum atoms are

mixed uniformly in atomic scale. In this method, K-feld-

spar ore was first calcined with K2CO3 at high tempera-

ture to break down the framework of aluminosilicate inthe K-feldspar sample, generating water-soluble silicon

and aluminum sources. Mesoporous materials were then

prepared under basic condition using cetyltrimethylam-

monium bromide (C16TMAB) as the template. The

as-synthesized material was characterized by different

techniques, such as X-ray diffraction (XRD), scanning

electron microscopy (SEM) and transmission electron

microscopy (TEM), 27Al NMR, inferred spectroscopy(IR), and nitrogen sorption. The hydrothermal stability

of resulting mesoporous molecular sieves was also stud-

ied via heating the synthesized sample in water at 100 �Cfor 360 h.

2. Experimental

2.1. Materials

K2CO3, C16TMABr and other chemicals were sup-

plied by Beijing Chemical Reagent Center, which were

used as received. K-feldspar ore used in this work was

collected from Henan Qianhe Gold Mine in mid-China.

The chemical composition of the K-feldspar sample is

listed in Table 1. The main components of K-feldsparsample are silicon and aluminium, with less amount of

other elements including Fe, Mn, Ca, Na, K, P etc.,

and the main minerals in the K-feldspar determined by

XRD were microcline (KAlSi3O8), quartz (SiO2) and

illite (K{Al2[(Si3Al)O10](OH)2}), whose contents were

91.10 wt%, 7.52 wt% and 2.38 wt%, respectively, calcu-

lated by LINPRO method [7].

2.2. Procedures to synthesize mesoporous molecular sieves

Before synthesis of molecular sieves, the mixture of

K-feldspar and K2CO3 with mass ratio of 2:3 were

Table 1

Chemical composition of K-feldspar (wt%)

Sample SiO2 TiO2 Al2O3 Fe2O3 FeO MnO MgO

HN-1 65.00 0.22 16.38 0.66 0.48 0.03 0.00

milled to such a grain size that more than 95% particles

were 120 minus mesh, which was calcined in the temper-

ature range of 860–880 �C for some time to form soluble

potassium compounds.

In a typical experiment to synthesize molecular sieves,

2.3 g of the calcined materials were dissolved in distilledwater (20 mL), and 10 mL of C16TMABr aqueous solu-

tion containing 1.12 g C16TMABr was added into the

solution. After stirring for 1 h, the pH of the solution

was adjusted to 10.5 with HBr (20%), resulting in a gel

under continuous stirring. The gel was then transferred

into a 100 mL PTFE-coated stainless steel autoclave

after stirred for another 1 h at room temperature. The

autoclave was sealed and placed in an oven of 130 �C.After heating for 60 h, the autoclave was cooled to room

temperature naturally, and the resulting solid product

was filtered out and then redispersed into 1 M NH4NO3solution to exchange K+/Na+ with NHþ

4 . The obtained

slurry was filtered again, and washed with deionized

water. Some of the as-prepared sample was dried at

105 �C for 6 h in a vacuum, which is named as Sample

A, and the other was calcined at 550 �C for 6 h, termedas Sample B. Part of Sample B was heated in water in a

sealed autoclave at 100 �C for 360 h, termed as Sample

C after dried at 105 �C for 6 h in a vacuum.

2.3. Characterization

Thermogravimetric-differential thermal analysis (TG-

DTA) of the mixture of K-feldspar and K2CO3 withmass ratio of 2:3 was carried out in a Rheometric scien-

tific (NETZSCH4) thermobalance. The XRD patterns

of the as-prepared samples were collected on an X-ray

diffracometer (X�PERT SW) operated at 40 kV and

10 mA with nickel filtered CuKa radiation (k =1.54060 A). The morphologies of the samples were

observed by means of SEM (JEOL, JSM-6700F) and

TEM (JEOL, JEM-2010) equipped with an energy dis-persive X-ray spectrometer (EDS) at an accelerating

voltage of 200 kV, respectively. The nitrogen sorption

analysis was performed on an ASAP-2405N instrument

at liquid nitrogen temperature. Prior to the adsorption,

the sample was degassed at 300 �C for 12 h at 10�4 Torr.FT-IR spectra of samples were recorded with a TEN-

SOR27 (BRUKER) spectrometer in KBr pellet. 27Al

MAS NMR measurements were collected on a VAR-IAN UNITYINOVA 300M spectrometer with pulse width

of 0.3 ls, recycle delay time of 1 s and spinning speed of7 kHz, using 1.0 M Al(NO3)3 aqueous solution as the

reference solutions.

CaO Na2O K2O P2O5 H2O+ H2O

� Total

0.86 0.89 13.83 0.12 1.23 0.17 99.87

S. Miao et al. / Microporous and Mesoporous Materials 83 (2005) 277–282 279

3. Results and discussion

3.1. Calcination of K-feldspar ore

In order to break down the aluminosilicate in the

framework, the K-feldspar needs to be calcined at hightemperatures. Before calcination, a TG-DTA analysis

was performed, and the weight loss vs temperature plot

is shown in Fig. 1. From the figure, a weight loss of

3.0 wt% appeared in the temperature range of 80–

110 �C, which corresponded to the desorption and re-moval of the water in the mixture, and a second weight

loss occurred between 840 and 880 �C, which implied

that the reactions between K-feldspar and K2CO3 tookplace.

Based on the TG-DTA analysis results, the mixtures

of K-feldspar and K2CO3 with K-feldspar/K2CO3 mass

ratio of 2:3 was calcined at 800, 840, 880 �C, respec-tively, and the calcined materials were determined by

XRD. From the XRD patterns (not shown), it was

found a new phase K3Al3O6 (JCPDS: 27-1336) formed

when the mixture was calcined at 840 �C, suggestingthe reactions between K-feldspar and K2CO3 occurred,

while when the mixture was calcined at 880 �C, KAl-Si3O8 disappeared and most of the peaks in the XRD

pattern are assigned to K3Al3O6, indicating the comple-

tion of the reactions. Due to the amorphous nature,

no characteristic peaks for K2SiO3 could be observed

on the XRD patterns. From the XRD results, we can

deduce that the following representative reactionsoccurred during the calcination process.

KAlSi3O8 ðmicroclineÞ þ 3K2CO3

! 3K2SiO3 þ 1=3 K3Al3O6 þ 3CO2 " ð1ÞKfAl2½ðSi3AlÞO10ðOHÞ2g ðilliteÞ þ 4K2CO3

! 3K2SiO3 þK3Al3O6 þ CO2 " þH2O " ð2ÞSiO2 ðquartzÞ þK2CO3 ! K2SiO3 þ CO2 " ð3Þ

0 200 400 600 800 1000 1200

70

75

80

85

90

95

100

DSC

/(mW

/mg)

DSC

TG

Res

idue

Mas

s/(wt%

)

Temperature (°C)

0

2

4

6

8

10

86.81

849.9

Fig. 1. TG/DSC curve of the K-feldspar and K2CO3 system.

The K2SiO3 and K3Al3O6 formed during the calcina-

tion process may be used as the precursors of molecular

sieves. To prepare the precursors, the mixture of K-feld-

spar and K2CO3 with the K2CO3/K-feldspar mass ratio

of 1.51 was calcined at 880 �C for 1.2 h, and 99.4 wt% of

the K-feldspar were turned into soluble inorganic salts,which were mainly composed of K2SiO3 and K3Al3O6with the K2SiO3/K3Al3O6 mass ratio of about 1.8, esti-

mated from their amount in the K-feldspar. Directed

by C16TMAB as the structure-inducing reagent, meso-

porous materials were produced using the above cal-

cined materials as precursors in basic aqueous

solution. The resulting material was characterized by

the following techniques.

3.2. Nitrogen sorption analysis

The nitrogen sorption analysis was performed for

Sample B, and Fig. 2 shows its nitrogen adsorption–

desorption isotherms and pore size distribution. As dis-

played in Fig. 2a, the sample exhibits two hysteresis

loops, one of which is in the relatively low p/p0 rangeof 0.20–0.40, indicating the presence of framework-con-

fined mesopores, and the other larger hysteresis loop is

encountered in the higher p/p0 range from 0.40 to 1.0,

which is believed to be attributive to the mesoporous

structures. The pore size distribution of the sample,

shown in Fig. 2b, illustrates the existence of a step-

wise-distributed pore structure with an average pore

diameter of 5.59 nm. The nitrogen analysis indicatesthat less ordered mesoporous molecular sieves were pro-

duced. From the t-plot, the BET surface area of the as-

prepared material is 507 m2 g�1 and the pore volume is

0.854 cm3 g�1.

3.3. XRD analysis

The powder XRD patterns of the as-synthesizedSamples A, B, C, are shown in Fig. 3. A peak in the

2h range of 1.80–2.20� appears on each XRD pattern,

suggesting the mesoporous structure of the as-prepared

materials. Compared to Sample A, the diffraction peak

intensities of Samples B and C are weaker, and peak

shapes become wider, demonstrating that the calcined

and hydrothermally treated samples shrink to some ex-

tents, however, Samples B and C still remain the meso-porous structures. This indicates that the mesoporous

materials prepared in this work have relatively good

thermal and hydrothermal stability.

For each sample three diffraction peaks appear at

36.64�, 38.28�, 44.56� on the XRD patterns, and the d-

values were similar to those reported by Inagaki et al.

[8]. Therefore, the diffraction peaks may be attributed

to the crystalline features of the walls of the resultingmesoporous materials, which was also verified by ED

diffraction during the TEM observation for Sample B

Fig. 2. (a) N2 sorption isotherms and (b) corresponding pore size distribution of Sample B.

36 39 42 45 48

Inte

nsity

(a.u

.)

c

b

a

0 8

Inte

nsity

(a.u

.)

c

b

a

2 4 6

Fig. 3. XRD patterns of Samples A (a), B (b) and C (c) in the low

angle range (left) and wide angle range (right) regions.

280 S. Miao et al. / Microporous and Mesoporous Materials 83 (2005) 277–282

(see the inset of Fig. 4b). The intensity and shape of the

peaks for the three samples are similar, which indicates

that heating at high temperature of 550 �C and hydro-

thermally treated in boiled water for long time cannot

destroy the crystalline structure of the mesoporousmaterial, further confirming good thermal and hydro-

thermal stability of the as-prepared materials. In gen-

eral, although the incorporation of Al into the

framework of the silica MCM-41 molecular sieves can

Fig. 4. SEM image of Sample B (a) and TEM i

improve their hydrothermal stability to some extent,

Al-MCM-41 prepared via method reported by Kresge[1] has relatively poor hydrothermal stability. However,

the mesoporous molecular sieves prepared in this work

still keep their mesoporous structure after treated in

boiled water for 360 h. This may result from its special

microstructures, which will be discussed in the following

sections.

3.4. Morphology

The morphologies of the resulting materials were

observed by SEM and TEM, and some SEM and

TEM images are shown in Fig. 4. From the SEM and

TEM observation, the as-prepared samples are spherical

particles with size of about 100 nm. Relatively regular

arrays of the mesopores in the samples can also be ob-

served clearly (Fig. 4b, c) with pore size of 5–6 nm,which is consistent with the XRD results calculated by

Bragg�s law. The size and microstructure of Sample Care similar to those of Sample B, showing that hydro-

thermal treatment cannot destroy the microstructures

of as-prepared sample. This provides further evidence

about good hydrothermal stability of the as-prepared

mages of Sample B (b) and Sample C (c).

-200 -100 0 100 200 300

61.030

ppm

Fig. 6. 27Al MAS NMR spectrum of Sample B.

S. Miao et al. / Microporous and Mesoporous Materials 83 (2005) 277–282 281

mesoporous materials. The corresponding electron dif-

fraction (inset of Fig. 4b) appears as a few spots,

suggesting that the wall of the sample is partially

crystalline.

3.5. IR analysis

Fig. 5a shows IR spectra of Samples A and B. On each

IR spectrum curve, the vibrational bands at 1237, 1084,

801, 460 cm�1 are attributed to the characteristic of silica

framework in MCM-41 [8]. Sample A exhibits absorp-

tion bands around 2921 and 2851 cm�1 assigning to

C–H vibrations of the surfactant molecule, while they

disappear in Sample B because of the removal of surfac-tant. The broad bands around 3500 cm�1 may be attrib-

uted to surface silanols and adsorbed water molecules,

while deformational vibrations of the adsorbed water

molecules cause the absorption bands at 1633 cm�1,

which was also observed by Selvaraj et al. [9]. These IR

spectra demonstrate structural transformations with sig-

nificant vibration bands at 1071, 968, 801 and 458 cm�1

for Al-MCM-41. The vibration peak at 968 cm�1 is as-signed to the Al incorporation into the framework of

the mesoporous silica materials. The samples prepared

in this work also exhibit distinguishable bands at 610,

480, and 440 cm�1, which are similar to those of zeolite

L [10], assigned to characteristic of 5-ring and 6-ring

T–O–T. This suggests that the as-made samples in this

work possess features of zeolite structure, which can

explain its crystalline characteristics and high hydrother-mal stability.

To evaluate the acidity of the as-prepared samples,

Sample B was evacuated at 150 �C for 5 h, followed by

exposure to pyridine at the same temperature for

30 min. The sample was then evacuated at this tempera-

ture under vacuum (10�5 Torr) for 1 h to remove phys-

ically adsorbed pyridine. Finally, IR spectrum of

pyridine-adsorbed on Sample B was recorded at roomtemperature, which is shown in Fig. 4b in the range of

Fig. 5. (a) IR spectra of Samples A (Spectrum 1) and B (Spectrum 2); (b) IR

(Spectrum 2).

1400–1750 cm�1. Compared with that of Sample B, IR

spectrum of pyridine-adsorbed on Sample B exhibits

four bands at 1447, 1490, 1550 and 1596 cm�1, respec-

tively. The bands at 1447 and 1596 cm�1 are attributed

to Lewis acid sites, and that at 1550 cm�1 is assigned

to Bronsted acid centers, while the band at 1490 cm�1

is ascribed to a combinational signal associated with

both Lewis and Bronsted acid sites [13]. The results indi-

cates that the sample prepared in this work has good

acidity, which may be used as acidic catalysts for some

reactions.

3.6. 27Al NMR analysis

Fig. 6 shows the solid 27Al NMR spectrum of Sam-

ple B. Obviously, only a strong and sharp signal ap-

pears at 61.03 ppm, assigning to tetrahedrally

coordinated framework aluminum (Td–Al), and there

is no other signal ascribed to other Al states (such as

6-coordination Al at �10 ppm, octahedrally coordi-

nated non-framework aluminum at 0 ppm, non-frame-

work Td–Al at 30–40 ppm, etc) in the product,implying that the aluminum was incorporated mainly

spectra of Sample B (Spectrum 1) and pyridine adsorbed on Sample B

282 S. Miao et al. / Microporous and Mesoporous Materials 83 (2005) 277–282

with tetrahedral coordination in the molecular sieves

framework. The 27Al chemical shift of the as-made sam-

ple is similar to the values observed for most zeolites

(59–65 ppm) [11], and much higher than that of most

previously reported mesostructured aluminosilicates

(51–56 ppm) [12]. Liu et al. [3] synthesized aluminosili-cate mesoporous molecular sieves derived from zeolite

type Y seeds, which exhibited a chemical shift at

60 ppm and high steam-stability. The mesoporous

material prepared in this work probably has a similar

microstructure to zeolite, which results in high hydro-

thermal stability.

4. Conclusion

Mesoporous aluminosilicate molecular sieves with

crystalline framework have been successfully prepared

by hydrothermal synthesis using K-feldpar as silica

and aluminium sources simultaneously, and C16TMABr

as the template. SEM and TEM observations indicate

that the resulting materials are spherical particles withsize of about 100 nm. The BET surface area and the pore

volume of the as-prepared materials are 507 m2 g�1 and

0.854 cm3 g�1, respectively. Nearly all of the aluminium

is incorporated into the framework of the mesoporous

material with 4-coordinated state, and the materials

show acidity and high hydrothermal stability, which

may be used as acidic catalysts for some reactions. The

method to synthesize mesoporous aluminosilicatesdeveloped in this work is advantageous for large-scale

industrial production because the raw material K-feld-

spar is abundant and very cheap.

Acknowledgement

Financial support from the National Natural Science

Foundation of China (No. 20374057, 50472096) is grate-

fully acknowledged.

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