M A T E R I A L S R E S E A R C H S O C I E T Y S Y M P O S I U M P R O C E E D I N G S V O L U M E 233

Synthesis/Characterization and Novel Applications of Molecular Sieve Materials

Symposium held May 1-3, 1991, Anaheim, California, U .S .A .

E D I T O R S :

Robert L. Bedard UOP, Tarry town, New York, U.S.A.

Thomas Bein Purdue University, West Lafayette, Indiana, U.S.A.

Mark E. Davis California Institute of Technology, Pasadena, California, U.S.A.

Juan Garces Dow Chemical Company, Midland, Michigan, U.S.A.

Victor A. Maroni Argonne National Laboratory, Argonne, Illinois, U.S.A.

Galen D. Stucky University of California at Santa Barbara, Santa Barbara, California, U.S.A.

I m I r I ^ I m a t e r i a l s R E S E A R C H S O C I E T Y

| 1 V 1 | 1 \ [ Q [ Pi t t sburgh Pennsylvania

1991

Contents

PREFACE MATERIALS RESEARCH SOCIETY SYMPOSIUM PROCEEDINGS

PART I : MOLECULAR SIEVE CATALYSIS

•CRACKING AND AROMATIΖΑΤΙΟΝ OF BUTANES OVER ZSM-5 ZEOLITES Yoshio Ono, Kunihiko Kanae, Kazuaki Osako, and Katsumi Nakashiro

•PREPARATION, CHARACTERIZATION AND ACTIVITY OF MOLECULAR SIEVE MATERIAL AS BASE CATALYSTS

Avelino Corma •NOVEL AND PROMISING APPLICATIONS OF MOLECULAR SIEVES IN ORGANIC SYNTHESIS AND THEIR CONTRIBUTION TO ENVIRONMENTAL PROTECTION

Wolfgang F. Hoelderich SELECTIVE OXIDATION OF HYDROCARBONS BY COBALT-SUBSTITUTED ALUMINOPHOSPHATE MOLECULAR SIEVES

V i c t o r A. Maroni and Lennox Ε. Iton

PART I I : SYNTHESIS OF ZEOLITES, AlP0 4s, AND LAYERED MOLECULAR SIEVES

•DIRECTING EFFECT OF FLUORIDE IN THE SYNTHESIS OF MOLECULAR SIEVES WITH NEW CHARACTERISTICS AND OF THE FIRST TWENTY-MEMBERED RING MICROPOROUS SOLID

Henri K e s s l e r

COBALT AS A PROBE ION IN THE SYNTHESIS OF CoAPO-5 M.G. Uytterhoeven, W.A. Van Keyenberg, and R. A. Schoonheydt

•SYSTEMATIC PREPARATION OF POLYOXOMETALATE PILLARED LAYERED DOUBLE HYDROXIDES VIA DIRECT AQUEOUS REACTION

Jiandang Wang, Ying Tian, Ren-Chain Wang, Jorge L. Colon, and Abraham C l e a r f i e l d

SYNTHESIS AND CHARACTERIZATION OF PILLARED ACID-ACTIVATED MONTMORILLONITES

Robert Mokaya, William Jones, Mavis E. Whittle, and Mary E. Davies

PREPARATION OF MONTMORILLONITE-p-AMINOAZOBENZENE INTERCALATION COMPOUNDS AND THEIR PHOTOCHEMICAL BEHAVIOR

Makoto Ogawa, Keiko F u j i i , Kazuyuki Kuroda, and Chuzo Kato

• I n v i t e d Paper

ZIRCONIUM ORGANIC DIPHOSPHATE-DIPHOSPHONATES WITH TILTED RIGID PILLARS

G. A l b e r t i , U. Costantino, R. Vivani, and P. Zappelli

PREPARATION OF ZIRCONIUM DIPHOSPHONATE-PHOSPHITES WITH A NARROW DISTRIBUTION OF MESOPORES

G. A l b e r t i , U. Costantino, R. Vivani, and P. Zappelli

PART I I I : NOVEL OPTICAL, ELECTRICAL, AND SENSOR MATERIALS

•DOPING AND BAND-GAP ENGINEERING OF AN INTRAZEOLITE TUNGSTEN(VI) OXIDE SUPRALATTICE

Geoffrey A. Ozin, Andrzej Malek, Richard Prokopowicz, Peter M. Macdonald, Saim Özkar, Karin Moller, and Thomas Bein

•PHOTOCHEMISTRY AND PHOTOPHYSICS OF MATERIALS IN ZEOLITES Kai-Kong I u , Xinsheng L i u , and J . Kerry Thomas

GROWTH, SIZE AND PHOTOACTIVITY OF CdS, ZnS, OR PbS PARTICLES IN NaX ZEOLITE

M. Wark, G. Schulz-Ekloff, N.I. Jaeger, and W. Lutz

P b l 2 CONFINED IN THE SPACES OF LTA ZEOLITES O. Terasaki, Z.K. Tang, Y. Nozue, and T. Goto

•ZEOLITIC MATERIALS AS ORGANIZING MEDIA FOR SEMI­CONDUCTOR-BASED ARTIFICIAL PHOTOSYNTHETIC SYSTEMS

Yeong I I Kim, Richard L. Ri l e y , Munir J . Huq, Samer Salim, Angie N. Le, and Thomas E. Mallouk

•MOLECULAR DESIGN OF ALUMINOSILICATE THIN FILM DEVICES Kenneth E. Creasy, Ying Ping Deng, Jongman Park, E r i c V.R. Borgstedt, Shawn P. Davis, Steven L. Suib, and Brenda R. Shaw

PHOTOREDUCTION OF METHYLVIOLOGEN IN THE INTERLAYERS OF SOME LAYERED TITANATES AND NIOBATES

Teruyuki Nakato, Yoshiyuki Sugahara, Kazuyuki Kuroda, and Chuzo Kato

ZEOLITE CRYSTAL LAYERS COUPLED TO PIEZOELECTRIC SENSORS: MOLECULAR RECOGNITION DEVICES

Yongan Yan and Thomas Bein

PART IV: SYNTHESIS DIRECTED TO SPECIAL APPLICATIONS

•INTERCALATION OF LAYERED V 20 5 XEROGEL WITH POLYMERS M.G. Kanatzidis, C.-G. Wu, Y.-J. L i u , D.C. DeGroot, J.L. Schindler, Η.Ο. Marcy, and CR. Kannewurf

• I n v i t e d Paper

vi

SURFACE CHEMISTRY OF HETEROBIMETALLIC Ge-M (M = Mo,W) COMPLEXES IN ZEOLITE Y

Aticha Borvornwattananont, Karin MoHer, and Thomas Bein

FORMATION AND CHARACTERIZATION OF SULFIDE CLUSTERS IN MOLECULAR SIEVE ZEOLITE MATERIALS

Markus Winterer, Lennox E. Iton, Stanley A. Johnson, and Vi c t o r A. Maroni

REDUCTION OF Cu 2 + AND N i 2 + IN ZEOLITE Y USING POLYOL PROCESS: A NEW APPROACH IN THE PREPARATION OF METAL SUPPORTED CATALYSTS

P.B. Malla, P. Ravindranathan, S. Komarneni, E. Breval, and R. Roy

DESIGNING ZEOLITES AS NOVEL PRECURSORS TO ELECTRONIC CERAMICS

David R. Corbin, John B. Par i s e , Uma Chowdhry, and M.A. Subramanian

NANOSCALE ENGINEERED CERAMICS FROM ZEOLITES: CREATING THE IDEAL PRECURSOR FOR HIGH-QUALITY CORDIERITE

Robert L. Bedard and Edith M. Flanigen

THE APPLICATION OF MOLECULAR SIEVES AS MAGNETIC RESONANCE IMAGING CONTRAST AGENTS

Kenneth J . Balkus, J r . , Iwona Bresinska, Stanislaw Kowalak, and Stuart W. Young

PREPARATION OF CERAMIC MOLECULAR SIEVE MEMBRANES E.R. Geus, J . Schoonman, and H. van Bekkum

SYNTHESIS AND WATER SORPTION PROPERTIES OF ALUMINO-PHOSPHATE (A1P0 4) AND SILICOALUMINOPHOSPHATE (SAPO) MOLECULAR SIEVES

P.B. Malla and S. Komarneni

PART V: NOVEL SYNTHETIC AND CHARACTERIZATION METHODS

*THE PHYSICOCHEMICAL PROPERTIES OF VPI-7: A MICROPOROUS ZINCOSILICATE WITH THREE-MEMBERED RINGS

Michael J . Annen, Mark E. Davis, John B. Higgins, and John L. Schlenker

*THE HYDROTHERMAL SYNTHESIS OF LAYERED MOLYBDENUM PHOSPHATES WITH OCTAHEDRAL-TETRAHEDRAL LAYERS

Robert C. Haushalter and Linda A. Mundi

FLEXIBILITY OF THE ZEOLITE RHO FRAMEWORK: THE REDISTRIBUTION OF EXTRA FRAMEWORK CATIONS AS A FUNCTION OF TEMPERATURE

John B. Pa r i s e , Xing L i u , David R. Corbin, and Glover A. Jones

• I n v i t e d Paper

vii

LOW TEMPERATURE STUDIES OF RARE GASES ADSORBED ON MOLECULAR SIEVE MATERIALS

Graham F. McCann, I . Gameson, W.J. Stead, T. Rayment, P.J. Bar r i e , and J . Klinowski

1 2 9XE NMR AS A PROBE OF MICROPOROUS MATERIALS John A. Ripmeester and Christopher I . R a t c l i f f e

ATOMIC FORCE MICROSCOPY OF MICRON SIZE SILICALITE CRYSTALS

P. Rasch, W.M. Heckl, H.W. Deckman, and W. Häberle

STRUCTURAL CHARACTERIZATION OF ZEOLITES USING RAMAN SCATTERING

H.W. Deckman, J.A. Creighton, R.G. Buckley, and J.M. Newsam

AUTHOR INDEX

SUBJECT INDEX

viii

109

DOPING AND BAND-GAP ENGINEERING OF AN INTRAZEOLITE TUNGSTEN(VI) OXIDE SUPRALATTICE

Geoffrey A. Ozin", Andrzej Malek", Richard Prokopowicz", Peter M. Macdonald", Saim özkarb, Karin Moller0 and Thomas Beinc. *Lash Miller Chemistry Laboratories, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 1A1 bChemistry Department, Middle East Technical University, Ankara, Turkey, department of Chemistry, University of New Mexico, Albuquerque, NM, 87131, USA

ABSTRACT

New results are presented concerning the topotactic self-assembly, n-type doping and band-gap engineering of an intrazeolite tungsten(VI) oxide supralattice n(W03)-Na56Y, where 0 < η < 32, built-up of single size and shape (W0 3) 2

dimers. In particular it has been found that the oxygen content of these dimers can be quantitatively adjusted by means of a thermal vacuum induced reversible reductive-elimination oxidative-addition of dioxygen. This provides access to new n(W03.x)-Na56Y materials (0 < χ ^ 1.0) in which the oxygen content, structural properties and electronic architecture of the dimers are changed. In this way one can precisely control the oxidation state, degree of η-doping and band-filling of a tungsten(VI) oxide supralattice through an approach which can be considered akin to, but distinct in detail to, that found in the Magneli crystallographic shear phases of non-stoichiometric bulk W0 3. x. Another discovery concerns the ability to alter local electrostatic fields experienced by the tungsten(VI) oxide moieties housed in the 13Ä supercages of 16(W03)-M36Y, by varying the ionic potential of the constituent supercage M + cations across the alkali metal series. This method provides the first opportunity to fine-tune the band-gap of a tungsten(VI) oxide supralattice. Α miniband electronic description is advanced as a qualitative first attempt to understand the origin of the above effects. The implications of these discoveries are that cluster size, composition and intrinsic electrostatic field effects can be used to "chemically manipulate" (engineer) the doping and band architecture of intrazeolite supralattices of possible interest in quantum electronics and nonlinear optics.

Introduction

The photo-oxidation of α-cage encapsulated n{W(C0)6}-Na5 6Y, provides a mild, clean and quantitative synthetic pathway to single size and shape tungsten(VI) oxide clusters n(W03)-Na36Y according to the reaction [1]:

hv n{W(C0)6}-Na J 6Y + 9n/202 > n(W03)-Na56Y + 6nC02

A combination of spectroscopy, diffraction and microscopy probes, including EXAFS structure analysis [2] demonstrates that over the entire W(CO)6 loading

Mat. Res. Soc. Symp. Proc. Vol. 233. ©1991 Materials Research Society

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range 0 < η < 32, achieved via the sequential saturation impregnation-photooxidation process:

16{W(CO)6}-Na56Y 8{W(CO)6}, 16(W03)-NaJ6Y 4{W(CO)6} ( 24(W03)-NaJ6Y

I I

l{W(CO) 6}, 3fj(W03)-Na36Y

— > 16(W03)-Na56Y — > 24(W03)-Na56Y

- > 28(W03)-Na56Y ι t

. . . > . . . > — > 32(W03)-Na36Y

only the population of the (W0 3) 2 dimer building block grows within the zeolite Y host lattice to form a 8(W03)2-Na36Y supralattice of dimers at half-loading η=16, and eventually to a 8{(W0 3) 2} 2-Na 3 6Y supralattice of dimers-of-dimers (rather than (W0 3) 4 tetramers) at saturation-filling η=32, as illustrated in Figure 1.

1. Chem-X space filling model for two (W0 3) 2 dimers anchored to the 4Na+ site I I cations in the α-cage of Na36Y, (2c).

In this summary paper we report for the first time simple chemical means of injecting variable numbers of electrons into, and adjusting the magnitude of the electrostatic fields experienced by the tungsten(VI) oxide moieties in zeolite Y.

Results And Discussion

A) Doping

Temperature programmed thermal treatment of n(W03)-Na36Y (0 < η ^ 32) shows that dioxygen loss begins around 200°C to yield beautiful royal blue (0 < η < 8) or metallic blue-grey (8 ̂ η ^ 32) or metallic grey (n * 32) materials with W:0 = 1:2.5 at 300°C. Further dioxygen evolution continues between 300-400°C to eventually yield homogeneous white materials with W:0 = 1:2. This

111

process can be quantitatively reversed in 0 2 at 300°C, recreating the original material n(W03)-Na56Y but by-passing the intermediate phase n(W02 5)-Na5 6Y. These observations provide compelling evidence for the reductive-elimination oxidative-addition sequence of reactions:

400°C ->n(W03.x)-Na56Y ^ n(W02)-Na56Y

V A C ; - 300°C; 0 2 ·

A combination of PXRD, 2 9Si/ 2 7Al/ 2 3Na MAS-NMR, Raman, Na+ cation Far-IR and vOH a Mid-IR experiments performed on a range of n(W03_x)-Na56Y samples demonstrate that just like the parent material n(W03)-Na56Y, the degree of crystallinity and integrity of the framework of the zeolite Y host material is maintained, little alteration in the host unit cell dimension is observed (around 24.69 A for all samples), no bulk W0 3 . x oxides are detected, and the W0 3 . x guests remain internally confined and homogeneously dispersed throughout the α-cages of the host.

The optical reflectance spectra provide some interesting structure-electronic clues about the constitution of these materials. From inspection of some representative data for n(W03.x)-Na56Y shown in Figure 2,

300°C n(W03)-Na56Y

Τ V A C ι

niWO^J-NajgY

' J IVCT(LF)

2^(0)

ROYAL BLUE

n(W03)-No56Y

400 600 800 (n m)

(b)

n(W0 3 ) -Na 5 6 Y

i M V ^ 2 p | T i 0 )

WHITE

BAND GAP EXCIIATION

.n(W02)-Na56Y

niWOy.J-NageY

w V 2nx

electrons

METALLIC BLUE-GREY

niWOs-J-NaseY

,n(W03)-N056Y

2 0 0 4 0 0 600 800 (η m)

2. UV-visible reflectance spectra of n(W03.x)-NaJ6Y for η < 8 and η > 8, where χ = 0, 0.5, 1.

one spots two informative effects. An intense broad "blue" visible-band and a

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red-shifted uv-band is observed on passing from n(W03)-Na56Y to n(W02 3 ) -Na5 6Y. This blue band broadens and the uv-band red shifts on passing from "isolated" (n r< 8) to "coupled" (8 ^ η < 32) W0 2 3 materials. The blue band disappears and the uv-band-shifts even further on passing from n(W02 3)-Na36Y to n(W02)-Na56Y. The uv-band red-shifts again on passing from "isolated" (n 8) to "coupled" (8 ^ η 32) W0 2 materials, Figure 2.

Recall (1) that the parents n(W03)-Na36Y display an 0"π(2ρπ) --> WVI(5d) absorption edge around E g = 3.1-3.5 eV (computer fit, Kubelka Münk, a d a=K(E-Eg)^) which is assigned to an interstate (HOMO-LUMO) LMCT excitation for the "isolated" (W0 3) 2 dimers (n <: 8), Figure 3 and an interminiband (MVB-MCB) transition for the "coupled" (W0 3) 2 dimers (8 ^ η ^ 32), Figure 3.

eVN-CH r ; ; r - ZWK^M MINI-VB i(W-O)

TETRAHEDRAL ISOLATED DJH (W0,)2

WQ, BASIC (WOj) 2 D I M E R BUILDING BLOCK D I M E R SUPRALATTICE

3. Evolution of a qualitative miniband electronic scheme for a (W0 3) 2 dimer superalattice, 16(W03)-Na56Y.

Some of the xenon lamp and laser induced emission/excitation spectra with time resolution obtained for n(W03)-Na56Y are depicted in Figure 4. The observation of considerable structure under the absorption-edge (450, 400, 374 nm excitation spectra), short (j4S, 450-300 nm) and long (ms, 680-500 nm) lifetime emissions, alerts one to the existence of complex coupling architecture in the MCB, possible "excitonic" emissions close to the miniband-gap energy, as well as "forbidden or trap" emissive states (Figure 4).

With the above information, the "blue" band in n(W02 3)-Na36Y (η V 8) can be assigned to either a W(IV) — W(VI) intervalence-charge-transfer (IVCT) or a W(V) — W(V) ligand-field (LF) transition while the uv-band is ascribed to either a Ο(-Π) - W(VI) or O(-II) - W(V) LMCT transition (3) (Figure 2). Where the clusters in n(W02 3)-Na3 6Y first begin to couple (through space or zeolite framework) these IVCT (LF) and LMCT bands appear to broaden and/or red-shift. The IVCT (LF) band disappears in n(W02)-Na56Y (η ^ 8) with concomitant red-shifting of the Ο(-Π) - W(IV) LMCT band. Similar shifting and broadening effects occur in n(W02)-Na36Y at η ¥ 8. Note that all n(W03.x)-Na36Y samples

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(300-77K) are EPR süent but all yield high quality 2 9Si/ 2 7Al/ 2 3Na MAS-NMR spectra (see later).

L A S E R F U S E PULSE

2.0 2 Λ 3J0 3.5 4.0 4.5 eV _ l I I J I I 6 2 5 5 0 0 416 3 5 4 312 2 7 6 NM

4. Selected photophysical properties of 16(W03)-Na36Y

Adsorption-induced 23Na MAS-NMR chemical shifts and Far-IR Na +

translatory mode frequency shifts provide direct and complimentary evidence for the anchoring of W0 3 . x moieties to α-cage Na+ cations in all three samples n(W03.x)-Na56Y (x = 0, 0.5, 1). Representative 23Na MAS-NMR data which illustrate this point are shown in Figure 5.

In fully dehydrated Na36Y one observes an intense asymmetric 23Na resonance at -12 ppm which arises from a convolution of signals due to Na+ cations mainly in sites I I , I , Γ (2b). The most prominent narrow component centred at -12 ppm is ascribed to Naj+ (six coordinate, hexagonal prism, essentially Oh symmetry, smallest second order quadrupole broadening). All other hexagonal six-ring N a + n , i (a> ß-v*%s respectively) have lower symmetries C 3 v, are expected to exhibit more pronounced second order quadrupole broadening, and appear to fall within the high-field shoulder (-22 and -38 ppm) of the main 23Na resonance, Figure 5. In 16(W03)-NaJ6Y, the (W0 3) 2 dimer perturbs one of the components of the Na56Y high field shoulder around -22 ppm with a corresponding increase in its intensity (Figure 5). Similar effects exist in the 23Na MAS-NMR spectra of both 16(W02 3)-Na36Y and 16(W02)-Na56Y, where one notes that the perturbed 23Na resonance is broader and less well pronounced in 16(W02 3)-Na36Y compared to 16(W02)-Na36Y and 16(W03)-Na36Y (Figure 5). Adsorption induced charge transfer to the terminal tungsten dioxo group of the imbibed W0 3 . x moieties from the Na +

n cations in Na36Y will serve to (a) decrease the shielding (b) lower the

114

second order quadrupole broadening (c) reduce electric field gradients and (d) cut-down or eliminate site-exchange processes of those Na+ cations accessible to the act of anchoring. The primary effects of anchoring are then expected to be a shift of the α-cage 2 3 Na n NMR resonance to lower fields with a concomitant enhancement of its intensity with increasing symmetry around the Na +

n site, Figure 5. This appears to describe the overall state-of-affairs for the series 16(W03.x)-Na56Y where in essence one "discovers" Na +

n through its selective anchoring and "locking" onto the WO^ moieties, Figure 5.

5. 2 3Na MAS-NMR data for A) Na36Y B) 16(W03)-Na56Y C) 16(W02 5)-Na J 6Y and D) 16(W02)-Na36Y

2 3Na DOR-NMR studies are underway to explore these systems more thoroughly

EXAFS structure analysis has been successfully applied to the precursor 8{W(CO)6}-Na36Y, photo-oxidation products 16(W03)-Na56Y, 28(W03)-Na36Y, 32(W03)-Na36Y and thermal reductive-elimination products 16(W02 3)-Na36Y, 32(W02 3)-Na3 6Y, loiWO^-Na^Y, 28(W02)-Na36Y and 32(W02)-Na56Y (2c, 3). A summary of these experiments is shown in Figure 6. For the precursor, the EXAFS data confirm that the W(CO)6 guest maintains its structural integrity with virtually no observable perturbations of the skeletal W-C=0, R = 2.06 A (2.058 A ) , N c = 6.5(6.0), and ligand W-C=Ü, R = 3.21 A (3.206 A ) , N 0 = 6.8(6.0) bonds compared to those found for the same molecule in the free state (numbers in parenthesis). Estimates of uncertainty in reported R and Ν values are ± 0.02 A and ±20% respectively. The EXAFS structure analysis results for the photo-oxidation products 16(W03)-Na56Y, 28(W03)-Na36Y and 32(W03)-Na36Y are very similar, and display the presence of two short terminal W=0 bonds (1.75 -1.78 A) and two long bridging W-0 bonds (1.94 - 1.96 A ) , together with a short distance to a second W (3.24 - 3.31 A ) . This bond-length and coordination number information for η = 16, 28 and 32 samples is interpreted in terms of the

[4].

115

formation of a "single kind" of tungsten(VI) trioxide dimer unit (W0 3) 2 , most likely interacting with Na +

n cations (Far-IR/NMR) as shown in Figure 6. All the data for 16(W03)-Na56Y support the contention of a uniform array of single-size and shape (W0 3) 2 dimers housed in the 13 Ä supercages of the zeolite Y host. The sequential addition of W0 3 units to the 16(W03)-Na56Y sample to eventually form "fully" filled 32(W03)-Na36Y appears to increase the (W0 3) 2 dimer population, causing a build-up of α-cage dimers-of-dimers {(W0 3 ) 2 } 2 (Figure 1),

6. Summary of structures and anchoring schemes for α-cage encapsulated precursor 16{W(CO)6}-Na36Y as well as its photo-oxidation product 16(W03)-Na56Y and corresponding reductive-elimination, oxidation-addition products 16(W03.x)-Na36Y.

rather than further cluster growth to trimers (W0 3) 3 and/or tetramers (W0 3) 4 . The presently observed EXAFS data set for the 16(W02 3)-Na36Y sample was

not of sufficient quality (5) to permit reliable determination of the oxygen distances or coordination numbers. However, it is valid to make the qualitative statement that W-W backscattering was observed, hence leading to the conclusion that the tungsten oxide moiety in this case is at least dimeric, like that found in 16(W03)-Na36Y. Interestingly the EXAFS data of the 32(W02 3)-Na3 6Y sample are very similar to those of its parent 32(W03)-Na56Y showing two kinds of short/long W=0/W-0 (1.75 - 1.95 A) bond, but now signals a short distance to three other W(3.30 A ) . The bond lengths and coordination numbers for both types of oxygen and tungsten, are fully consistent with a "tetrameric" structure wherein the four tungsten atoms lie at the apices of a tetrahedron. The EPR silence of the 16(W02 3)-Na56Y dimer implies the existence of either a spin-paired singlet or a rapidly relaxing triplet electronic configuration for the two possible dimer formulations, namely either single valent ZONa...O2W(V)0u-O)W(V)O2...NaOZ or mixed valence ZONa...02W(IV)(/u-0)W(VI)02...NaOZ. The ability to collect

l6(W02)-N<J5eY l6(W02.3)-Na s eY

116

high quality 9Si, 2 9 A I , Na MAS-NMR spectra for this sample favours the former spun paired diamagnetic assignment. Much more work, in particular XPS, is needed to clarify this fascinating point.

The EXAFS data of the 16(WO^-Na56Y material are quite distinct to that of the other two samples, indicating only one W-O shell with R = 1 . 8 1 A N C = 4.1 which is very similar to that of the NajWC^ reference compound. There is no evidence of W-W backscattering in the 16(W02)-Na56Y material. The EXAFS results for 28(W02)-Na36Y and 32(W02)-Na56Y are essentially identical to those of 16(W02)-Na36Y. The best model for these intrazeolite W0 2 materials is a 4-ring oxygen framework coordination site to the tungsten(IV) centre of a low spin monomeric W0 2 moiety, of which the terminal W=0 oxygens interact with Na +

n

as indicated in Figure 6. This is the best way to reconcile the oxygen coordination number, tungsten(IV) oxidation state and Na+ anchoring interaction clues for n(W02)-Na36Y (where η = 16, 28, 32) obtained from EXAFS, UV-VIS, FAR-IR, EPR and MAS-NMR spectroscopies.

B) Band-Gap Engineering

The feasibility of fine-tuning the band-gap of the 16(W03)-M36Y supralattice is illustrated in Figure 7. On moving from Μ = Η to Cs at the two extremes one observes a blue-shift of the absorption edge (computer fit, Kubelka Münk, a** = K(E-Eg)}/t) of about 0.75 eV. Eg values estimated in this way are H(3.10 eV), Li(3.53 eV), Na(3.33 eV), K(3.74 eV), Rb(3.85 eV), Cs(3.85 eV). Assuming the (W0 3) 2 dimer structure is maintained across the series Μ = Η, Li , Na, Κ, Rb, Cs, one can envisage that enhanced local electrostatic fields arising from the presence of M +

n cations and experienced by each (W0 3) 2 dimer will serve to (i) deplete the (W0 3) 2 of valence electron density (ii) weaken skeletal W-0 bonds, (iii) destabilize the 0' π(2ρπ) and stabilize the WVI(5d) MVB and MCB respectively, with a concomitant decrease in the band-gap (LMCT) energy, Figure 7. Because of the variation in population of, and distance between M +

n cations, one does not expect the cation dependence of the miniband gap energy to be a straightforward function of ionic potential across the Μ = Η, Li , Na, Κ, Rb, Cs series. Thus amonotonicity in E g for Li 3 6 Y and Na36Y might relate to the known desire of L i +

n to reside in the plane of oxygen 6-rings. Spatial constraints for Rb56Y and Cs36Y (90% M +

n , M +

m ) could be the cause of their similar Eg values. The "special" position of H 3 6Y in this series is probably related to the "dual" involvement of hydrogen-bonding (ZOH to terminal W=0) and protonation (ZOH* to bridging W-O) found for (W0 3) 2 in 16(W03)-H36Y, (1). Rietveld PXRD structure refinements and EXAFS structure analyses are currently underway for the entire n(W03)-M3 6Y series in order to check the validity of the preliminary results and ideas presented above.

Summary

A simple chemical means of injecting electrons and holes into n(W0 3 x)-Na 3 6Y has been achieved (0 χ ^ 1). The method is applicable to isolated (W0 3) 2

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dimers, coupled (W0 3) 2 dimers and a supralattice of coupled {(W0 3 ) 2 } 2 dimer s-of-dimers imbibed in the α-cage of the zeolite Y host lattice. An illustration of this concept is shown in Figure 8.

7. Experimental uv-visible absorption edges of a 16(W03)-M56Y supralattice, where Μ = Η, Na, Κ, Rb together with the best computer fit to the absorption index of an allowed-direct band gap transition, a0* = K(E - Eg)^. Inserts qualitatively illustrate the MVB/MCB energy levels for 16(W03)-M 5 6 Y.

D I M E R S 0 < η t 16

R O Y A L - B L U E

16 < η t 3 2

B L U E - G R E Y

D I M E R S O F

D I M E R S

η = 3 2

METALLIC GREY

8. Intracavity and intercavity connectivity (coupling) in n(W03_x)-Na56Y for χ < 0.5.

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Through alterations of the local electrostatic fields experienced by the (W0 3) 2

dimers, achieved by varying the ionic potential of constituent supercage cations, one is provided with a novel means of fine-tuning the miniband-gap of a tungsten(VI) oxide supralattice. Therefore cluster size, composition and intrinsic electrostatic field effects can be used to "chemically manipulate" (engineer) the doping and band architecture of organized assemblies of intrazeolite (W0 3) 2

dimer s.

Acknowledgements

We wish to acknowledge (GAO, PMM) the Natural Sciences and Engineering Research Council (NSERC) of Canada's Operating and Strategic Grants Programmes for generous financial support of this work. SO expresses his gratitude to the Middle East Technical University for granting him an extended leave of absence to conduct his research at the University of Toronto. K.M. and T.B. acknowledge partial funding for this work from the donors of the Petroleum Research Fund administered by the American Chemical Society. The operational funds for NSLS beamline X-11A are provided by DOE Grant Number DE-AS0580ER10742 (KM, TB) and NSERC (GAO). Supplies of high quality zeolites from Dr. Edith Flanigen at Union Carbide, Tarryton, N.Y. are gratefully appreciated. Dr. Heinz Robota of Allied Signal is acknowledged for his assistance with the collection and analysis of EXAFS data. The assistance of Dr. Andrew Holmes and Mr. Alex Kuperman with various aspects of this project are also most deeply appreciated.

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