Redox Behavior of Zeolite Aluminosilicates and the Nature of the Sites

46 Redox Behavior of Zeolite Aluminosilicates and the Nature of the Sites Responsible for the Electron-Transfer Activity

Downloaded by UNIV OF PITTSBURGH on July 3, 2013 | http://pubs.acs.org Publication Date: June 1, 1973 | doi: 10.1021/ba-1973-0121.ch046

B. D. FLOCKHART, M. C. MEGARRY, and R. C. PINK Department of Chemistry, The Queen's University, Belfast, BT9 5AG, Northern Ireland

An ESR study of the redox properties of hydrogen Y zeolites has given information about the nature of sites responsible for oxidizing and reducing activities. Both electron-transfer activities depend almost linearly on the aluminum content of the zeolite to the point where the crystalline structure begins to collapse. Strong interaction between the two types of site is shown by enhancement (up to tenfold) of the reducing power of zeolite samples when certain electron-donor molecules are adsorbed on the surface. As the aluminum-silicon ratio decreases, the enhancing effect remains unchanged, indicating an interaction between oxidizing sites associated with a single reducing center rather than between separated sites in the zeolitic framework. The results imply that a range of sites of varying electron-donor power exists on the hydrogen Y-zeolite surface.

' " p h e surface of a Y zeolite, w h e n s u i t a b l y a c t i v a t e d , m a y possess b o t h o x i d i z i n g a n d r e d u c i n g p r o p e r t i e s ; h y d r o c a r b o n molecules s u c h as perylene are r e a d i l y c o n v e r t e d at t h e surface i n t o t h e c o r r e s p o n d i n g c a t i o n radicals whereas electron acceptors l i k e the nitrobenzenes are c o n v e r t e d i n t o the a n i o n - r a d i c a l f o r m (1). I n a l u m i n a , w h i c h has s i m i l a r redox properties, the o x i d i z i n g a n d r e d u c i n g a c t i v i t i e s are t o some degree m u t u a l l y dependent (2). T h e p r i n c i p a l objects of t h e present i n v e s t i g a t i o n w ere t o see w h e t h e r t h i s also is t r u e for t h e zeolite surface a n d t o s t u d y t h e dependence of the redox properties o n t h e a l u m i n u m - s i l i c o n r a t i o i n t h e zeolite. A

r

Experimental L i n d e N a - f o r m Y zeolite (wt % , d r y b a s i s : N a 0 , 12.9; A 1 0 , 2 3 . 1 ; S i 0 , 64.0) was c o n v e r t e d i n t o t h e a m m o n i u m f o r m u s i n g t h e m e t h o d d e 2

2

509 In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.

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s c r i b e d b y L u n s f o r d (3). W i t h r e p e a t e d exchange a t 80° C , 9 0 % of t h e s o d i u m ions were r e p l a c e d b y a m m o n i u m . A l u m i n u m - d e f i c i e n t zeolites were o b t a i n e d b y K e r r ' s m e t h o d (4). R e f l u x i n g a s l u r r y of t h e 9 0 % N H Y for 6 h o u r s w i t h different a m o u n t s of e t h y l e n e d i a m i n e t e t r a a c e t i c a c i d g a v e a r a n g e of N H - Y zeolites w i t h 0 - 9 4 % of t h e a l u m i n u m r e m o v e d . T w o procedures were used t o a c t i v a t e the c a t a l y s t samples. I n t h e first, t h e s a m p l e was h e a t e d i n a n electric muffle furnace i n flowing o x y g e n a t 600° C for 30 m i n a n d t h e n i n a i r a t t h e same t e m p e r a t u r e for a f u r t h e r 4 h o u r s . T h e s a m p l e w a s a l l o w e d t o cool a t 1 0 ~ m m H g o v e r p h o s p h o r i c oxide for 30 m i n , a n d was s u b s e q u e n t l y h a n d l e d u n d e r a n a t m o s p h e r e of d r y n i t r o g e n . I n the second procedure t h e sample was p r e t r e a t e d i n flow i n g o x y g e n for 1 h r a t 600° C i n t h e muffle furnace, a n d i t was t h e n t r a n s f e r r e d t o a q u a r t z vessel a t t a c h e d t o a v a c u u m l i n e . D i m e n s i o n s of the q u a r t z vessel were s u c h as t o p e r m i t a c t i v a t i o n i n a s h a l l o w b e d ; m a x i m u m d e p t h of t h e sample d i d n o t exceed 3 - 4 m m . T h e sample was h e a t e d i n o x y g e n (20 c m H g ) for 30 m i n a t 600° C , a n d t h e n outgassed a t t h i s t e m p e r a t u r e for 16 h o u r s a t 1 0 ~ m m H g or better. A f t e r cooling, t h e s a m p l e w a s t r a n s f e r r e d a n d h a n d l e d u n d e r a n atmosphere of d r y n i t r o g e n . T h i s procedure was designed t o a v o i d the f o r m a t i o n of t h e " u l t r a s t a b l e " f o r m of t h e zeolite (δ). T e t r a c y a n o e t h y l e n e ( T C N E ) , 1,3,5-trinitrobenzene ( T N B ) , ra-dinitrobenzene, nitrobenzene, n a p h t h a l e n e , a n d anthracene were a l l l a b o r a t o r y c h e m i c a l grade reagents a n d were p u r i f i e d b y s t a n d a r d procedures. Pery lene f r o m R u t g e r s w e r k e - A k t i e n g e s e l l s c h a f t was used as received. Benzene ( A R ) was stored over a c t i v e s i l i c a - a l u m i n a a n d filtered before use. S o l u t i o n s of t h e adsorbates i n benzene were 1 0 M for T C N E a n d T N B a n d 5 X 1 0 ~ M for n a p h t h a l e n e , anthracene, a n d perylene. T h e electron s p i n resonance ( E S R ) measurements were m a d e b y t h e m e t h o d a l r e a d y described (1). 4

4


2

5

_ 1

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Results W h e n T N B was adsorbed at r o o m t e m p e r a t u r e f r o m s o l u t i o n i n b e n zene o n zeolite 9 0 % exchanged w i t h a m m o n i u m i o n a n d h e a t e d at t e m peratures a b o v e ~ 4 0 0 ° C , t h e c a t a l y s t i m m e d i a t e l y developed a y e l l o w i s h b r o w n surface c o l o r a t i o n a n d gave a three-fine E S R s p e c t r u m c h a r a c t e r istic of the T N B a n i o n r a d i c a l adsorbed o n a l u m i n a a n d a m o r p h o u s a l u m i n o s i l i c a t e surfaces (6). F i g u r e 1 shows the v a r i a t i o n of T N B r a d i c a l c o n c e n t r a t i o n w i t h a c t i v a t i o n t e m p e r a t u r e of t h e z e o l i t e ; t h e m a x i m u m s p i n c o n c e n t r a t i o n occurs at ~ 5 7 5 ° C . These concentrations were o b t a i n e d after the c a t a l y s t h a d been i n c o n t a c t w i t h t h e T N B s o l u t i o n for 3 d a y s , a n d the c a t a l y s t + s o l u t i o n were t h e n i r r a d i a t e d w i t h l i g h t f r o m a low-pressure m e r c u r y l a m p for 1 h o u r (1). P e r y l e n e adsorbed o n zeolite samples s i m i l a r l y a c t i v a t e d gave t h e n i n e - l i n e s p e c t r u m a t t r i b u t e d t o t h e c o r r e s p o n d i n g c a t i o n r a d i c a l , the m a x i m u m r a d i c a l c o n c e n t r a t i o n o c c u r r i n g at ^ 6 2 5 ° C . These c o n c e n t r a t i o n s were measured after the c a t a l y s t h a d been i n c o n t a c t w i t h the perylene s o l u t i o n for 7 d a y s . I r r a d i a t i o n w i t h l i g h t f r o m t h e low-pressure m e r c u r y l a m p h a d n o effect o n t h e c a t i o n r a d i c a l c o n c e n t r a t i o n (1). F u r t h e r s t u d y of the redox properties of t h e

In Molecular Sieves; Meier, W., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1973.


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Activation Temperature, °C

Figure 1. Radical-forming activity of 90% exchanged Y zeolite as a function of activation temperature with TNB (•) and perylene (O) as adsorbates

zeolite was m a d e o n samples h e a t e d t o 600° C (subsequently referred t o as decationated Y ) . A d s o r p t i o n of anthracene ( F i g u r e 2a), perylene ( F i g u r e 2 b ) , a n d n a p h thalene (not shown) o n d e c a t i o n a t e d Y p r e v i o u s l y s a t u r a t e d w i t h T N B gave superimposed E S R s p e c t r a of t h e T N B a n i o n r a d i c a l a n d t h e h y d r o c a r b o n c a t i o n r a d i c a l . These s p e c t r a were o b t a i n e d irrespective of t h e order of a d d i t i o n of t h e adsorbate molecules. N o E S R a b s o r p t i o n was detected, even a t m a x i m u m spectrometer s e n s i t i v i t y , w h e n benzene s o l u t i o n s of t h e a r o m a t i c h y d r o c a r b o n a n d T N B were m i x e d , b u t t h e a d d i t i o n of a c t i v e zeolite i m m e d i a t e l y p r o d u c e d t h e superimposed s p e c t r a of F i g u r e s 2 a a n d 2b. N o a t t e m p t was m a d e t o exclude m o l e c u l a r o x y g e n f r o m a n y of these systems. I n t h e presence of anthracene a n d n a p h t h a l e n e t h e t r i p l e t a t t r i b u t e d t o t h e a n i o n r a d i c a l r e m a i n e d w e l l resolved, e v e n at h i g h c a t i o n - r a d i c a l c o n c e n t r a t i o n s . O n t h e other h a n d , w i t h c o n c e n t r a t i o n s of the p e r y l e n i u m i o n greater t h a n a b o u t one-quarter of t h e m a x i m u m o b t a i n a b l e o n the surface, t h e outer features of t h e T N B s p e c t r u m were b a r e l y detectable (see b e l o w ) . S u b s t i t u t i o n of ra-dinitrobenzene o r m o n o n i t r o b e n z e n e for T N B gave b r o a d l y s i m i l a r results. S u p e r i m p o s e d s p e c t r a were also o b t a i n e d for t e t r a c y a n o e t h y l e n e + anthracene a n d t e t r a c y a n o ethylene + p e r y l e n e ; w i t h these systems o v e r l a p of t h e s p e c t r a w a s a l m o s t complete. A d s o r p t i o n o f benzene o n d e c a t i o n a t e d Y g a v e a singlet. A l t h o u g h t h e a d d i t i o n of T N B t o t h i s s y s t e m p r o d u c e d t h e expected t r i p l e t ,



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Figure 2. ESR spectra (first derivative) at 20° C: (a) TNB + anthracene on decationated Y, (b) TNB + perylene on decationated 7 , (c) outer feature of spectrum a as the cation-radical concentration is increased from left to right t h e a m p l i t u d e of the c e n t r a l feature was m u c h larger t h a n t h a t o b s e r v e d w h e n T N B a n d benzene were a d d e d s i m u l t a n e o u s l y t o a n a c t i v e c a t a l y s t sample. Since o v e r l a p of t h e s p e c t r a of t h e T N B a n i o n r a d i c a l a n d the a n t h r a cene c a t i o n r a d i c a l is v i r t u a l l y confined t o the c e n t r a l feature of t h e a n i o n s p e c t r u m , o b s e r v a t i o n of t h e i n t e n s i t y of one of t h e o u t e r features p e r m i t s separate assessment of the a n i o n - r a d i c a l c o n c e n t r a t i o n ( F i g u r e 2c). As i n a p r e v i o u s i n v e s t i g a t i o n (2) a q u a n t i t a t i v e s t u d y of the enhancement of t h e i o n - r a d i c a l s p e c t r u m i n the presence of coadsorbate was therefore possible b y u s i n g a c a l i b r a t i o n c u r v e i n w h i c h the i n t e n s i t y of the o u t e r l i n e of t h e T N B s p e c t r u m w a s p l o t t e d against t h e d o u b l y i n t e g r a t e d area of t h e whole of the T N B s p e c t r u m i n a separate series of experiments. F i g u r e 3 shows the effect of a d d e d anthracene a n d perylene o n the surface c o n c e n t r a t i o n of T N B a n i o n r a d i c a l s . A t e n f o l d increase i n the T N B r a d i c a l c o n c e n t r a t i o n w a s observed i n the presence of either h y d r o c a r b o n . A d d i t i o n of n a p h t h a l e n e , o n the other h a n d , p r o d u c e d no enhancement of t h e T N B a n i o n - r a d i c a l c o n c e n t r a t i o n .


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R e m o v a l of the solvent f r o m systems i n w h i c h t h e a n i o n - r a d i c a l c o n c e n t r a t i o n h a d been increased b y t h e c o a d s o r p t i o n of anthracene or perylene caused n o change i n the n u m b e r of u n p a i r e d spins. I n c a t a l y s t samples where the T N B r a d i c a l c o n c e n t r a t i o n h a d n o t been i n i t i a l l y increased b y u l t r a v i o l e t i r r a d i a t i o n or b y w a r m i n g the c a t a l y s t + T N B s o l u t i o n a b o v e r o o m t e m p e r a t u r e (6), a 12-fold increase i n the a n i o n - r a d i c a l c o n c e n t r a -


t i o n w a s o b t a i n e d i n t h e presence of a d s o r b e d a n t h r a c e n e or p e r y l e n e . F o r samples of d e c a t i o n a t e d Y f o r m e d b y d e h y d r o x y l a t i o n in vacuo, a sevenf o l d enhancement was observed. T h e enhancement effect r e m a i n e d s u b s t a n t i a l l y u n c h a n g e d as t h e a l u m i n u m - s i l i c o n r a t i o i n the zeolite was reduced—e.g., a zeolite sample w i t h a n a l u m i n u m c o n t e n t o n l y one-half t h a t of the o r i g i n a l Y zeolite h a d its r e d u c i n g a c t i v i t y enhanced t o a l m o s t the same extent (eightfold) as t h a t of the Y zeolite (tenfold) w h e n anthracene or perylene was a d s o r b e d o n t h e surface. T h e u n p a i r e d s p i n c o n c e n t r a t i o n s at s a t u r a t i o n , h o w e v e r , were s i g n i f i c a n t l y different. B o t h t h e r e d u c i n g a n d o x i d i z i n g a c t i v i t i e s of t h e zeolite fell i n a n a p p r o x i m a t e l y l i n e a r m a n n e r as t h e a l u m i n u m c o n t e n t was decreased t o ^ 2 0 a l u m i n u m a t o m s p e r u n i t c e l l ( F i g u r e 4). X - r a y e x a m i n a t i o n showed t h a t at a r o u n d t h i s c o m p o s i t i o n t h e c r y s t a l l i n e s t r u c t u r e of the zeolite began t o collapse. N o reinforcement of the anthracene c a t i o n - r a d i c a l s i g n a l c o u l d be detected w h e n T N B was c o a d s o r b e d o n t h e d e c a t i o n a t e d Y surface.

Figure 3.

Effect of cation-radical concentration on TNB anion-radical concentration: anthracene (O), perylene (•)



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Ο

20

40

60

Al atoms per unit cell

Figure 4· Radical-forming activity as a function of the aluminum content of the zeolite with anthracene (O), TCNE (•), and TNB (Δ) as adsorbates F i g u r e s 5 a , 5c, a n d 5e d e m o n s t r a t e t h e effect o n t h e T N B s p e c t r u m of a n increase i n t h e c o n c e n t r a t i o n of adsorbed perylene c a t i o n r a d i c a l s ( F i g u r e 5c, ~ 4 Χ 1 0 c a t i o n r a d i c a l s p e r g r a m ; F i g u r e 5e, ~ 1 Χ 1 0 ) . T h e outer features of t h e a n i o n - r a d i c a l s p e c t r u m b e c o m e m u c h less e v i d e n t as t h e perylene r a d i c a l c o n c e n t r a t i o n is increased. T h i s effect is more p r o n o u n c e d a t l o w t e m p e r a t u r e s (Figures 5 d a n d 5f). A l t h o u g h t h e s p e c t r u m for T N B adsorbed alone o n d e c a t i o n a t e d Y is less w e l l resolved a t t h e l o w e r t e m p e r a t u r e ( F i g u r e 5 b ) , t h e outer features are s t i l l c l e a r l y d i s c e r n i b l e ; w i t h t h i s s y s t e m s a t u r a t i o n b r o a d e n i n g accounts for t h e loss of r e s o l u t i o n . 1 8

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T h e E S R s p e c t r u m observed w h e n T N B w a s adsorbed o n m e t a l exchanged Y zeolite w a s essentially a singlet ( C o - , N i - , L a - e x c h a n g e d ) or a singlet w i t h a d d i t i o n a l hyperfine s t r u c t u r e ( C a exchanged). These systems are b e i n g f u r t h e r i n v e s t i g a t e d . Discussion S i n c e m a x i m u m r e d u c i n g a n d o x i d i z i n g power i n t h e zeolite requires a c t i v a t i o n temperatures a r o u n d 6 0 0 ° C , d e h y d r o x y l a t i o n is necessary for t h e f o r m a t i o n of t h e a c t i v e centers. E l e c t r o p o s i t i v e a n d electronegative sites p r o d u c e d as s h o w n b e l o w m a y be responsible. O n t h i s basis t h e H 0 Ό


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f i n d i n g t h a t t h e redox a c t i v i t y decreases as t h e a l u m i n u m c o n t e n t is l o w e r e d is n o t u n e x p e c t e d . T h e f a c t t h a t t h e decrease obeys a linear r e l a t i o n s h i p ( F i g u r e 4), however, shows t h a t t h e a c t i v i t y is associated w i t h a v e r y l o c a l i z e d e n v i r o n m e n t , a n d is n o t a c o o p e r a t i v e effect i n v o l v i n g a n u m b e r of a l u m i n u m centers.


T h e s t r i k i n g increase i n r e d u c i n g p o w e r p r o d u c e d b y t h e a d d i t i o n of e l e c t r o n donors t o t h e s y s t e m i n d i c a t e s the existence of a s t r o n g i n t e r a c t i o n between t h e t w o t y p e s of site a n d means, i n effect, t h a t closely a d j a c e n t a l u m i n u m centers m u s t be i n v o l v e d , at least i n t h e cases where

Figure 5. ESR spectra of TNB on decationated Y: (a) al 20°C, (b) at -186°C. ESR spectra of TNB + perylene (~4 Χ 10 cation radicals per gram): (c) at 20°C, (d) at -186°C. ESR spectra of TNB + perylene (~1 Χ 10 cation radicals per gram: (e) at 20°C (J) at -186°C 18

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enhancement occurs. O n e a d d i t i o n a l T N B a n i o n r a d i c a l is p r o d u c e d for a b o u t e v e r y three a n t h r a c e n e o r perylene c a t i o n r a d i c a l s adsorbed o n t h e surface ( T a b l e I ) . T h i s c o u l d i m p l y t h a t a p p r o x i m a t e l y one i n three of t h e redox sites is e n v i r o n m e n t a l l y a n d energetically f a v o r a b l e for e n h a n c e m e n t t o occur, o r a l t e r n a t i v e l y t h a t enhancement i n v o l v e s a cooperative effect i n w h i c h as m a n y as three n e i g h b o r i n g o x i d i z i n g sites i n t e r a c t w i t h one r e d u c i n g center. T h e finding t h a t t h e e n h a n c i n g effect r e m a i n s s u b s t a n t i a l l y u n c h a n g e d e v e n i n s t r o n g l y a l u m i n u m - d e f i c i e n t zeolites suggests t h a t the former is more p r o b a b l e .


Table I.

Ion-Radical Concentrations on Decationated Y Mean Number of Cation Maximum Radicals to Generate One Cation-Radical Additional Concentration radicals per gram of TNB Anion Ionization Radical catalyst Adsorbate Potential, ev Perylene Anthracene Naphthalene Benzene • Ref. Ref. * Ref.

6

6.83» 7.23° 8. Ρ 9.25'

5 1.2 6 1

Χ Χ Χ X

10 10 10 10

19

19 17

17

2.7 2.7

— —

7. 8. 9.

C l e a r l y , f r o m t h e n u m b e r of r a d i c a l s f o r m e d a t t h e surface either f r o m electron acceptors o r e l e c t r o n donors, o n l y a s m a l l p r o p o r t i o n of t h e possible sites, of t h e t y p e s h o w n i n t h e scheme above, are a c t i v e redox centers. T h i s m a y be p a r t l y due t o t h e i n a c c e s s i b i l i t y of some of t h e sites (1 ). How ever, t h e f a c t t h a t s u c h a large enhancement (up t o tenfold) of the a n i o n r a d i c a l c o n c e n t r a t i o n c a n be o b t a i n e d b y the f o r m a t i o n o n t h e surface of a r a d i c a l of opposite sign shows t h a t i n a c c e s s i b i l i t y is n o t the sole e x p l a n a t i o n . I t seems c e r t a i n t h a t a range of sites w i t h v a r y i n g electron-transfer p o w e r m u s t exist o n t h e surface, a n d t h a t o n l y a s m a l l p r o p o r t i o n of these are sufficiently p o w e r f u l t o reduce, say, T N B or t o o x i d i z e anthracene or p e r y lene. S i n c e these h y d r o c a r b o n s are e q u a l l y effective i n e n h a n c i n g t h e a n i o n - r a d i c a l c o n c e n t r a t i o n u p t o a c a t i o n - r a d i c a l c o n c e n t r a t i o n of ^ 6 X 1 0 spins per g r a m a n d a b o v e t h i s c o n c e n t r a t i o n h a v e n o f u r t h e r e n h a n c i n g effect ( F i g u r e 3), the observed t e n f o l d enhancement p r o b a b l y reflects t h e n u m b e r of electron-donor centers m a r g i n a l l y insufficient t o c o n v e r t a d sorbed T N B i n t o the a n i o n - r a d i c a l f o r m . These are t h e sites t h a t c a n be reinforced t o t h e p o i n t a t w h i c h electron transfer occurs b y t h e a d s o r p t i o n of anthracene or perylene molecules o n n e i g h b o r i n g o x i d i z i n g centers. 18



46.

FLOCKHART ET AL.


517

Naphthalene, with an ionization potential higher than that of anthra cene or perylene, produces a much lower radical concentration in the zeolite (Table I), and appears to have no observable enhancing effect on the for mation of anion radicals. Probably only certain sites of high energy are involved in this oxidation, and these sites may not be of the type in which interaction with an adjacent reducing center is possible. Examination of the superimposed spectra of the T N B anion radical and the perylene cation radical reveals that at high perylene concentrations broadening of the T N B spectrum occurs (Figures 5c and 5e), and to this extent the calculated data for the perylenium ion shown in Figure 3 are in error. The error, however, is such that the anion-radical enhancement is greater rather than less than that shown in this figure. Some broadening is observed in the T N B spectrum at —186°C (Figure 5b), and this can be attributed to saturation broadening. The broadening of the T N B spec trum that occurs at room temperature in the presence of perylene possibly arises from a restriction in motion of the T N B radical in the surface com plex caused by adsorbed perylenium cations. Noticeably, no such broad ening occurs with the smaller anthracene radical. If this is the explana tion of the room-temperature broadening of the T N B spectrum, it gives further evidence that the interacting redox sites must be close. Literature Cited 1. 2. 3. 4. 5. 6.

Flockhart, B. D., McLoughlin, L., Pink, R. C., J. Catal. (1972) 25, 305. Flockhart, B. D., Leith, I. R., Pink, R. C., J. Catal. (1967) 9, 45. Lunsford, J. H., J. Phys. Chem. (1968) 72, 4163. Kerr, G. T., J. Phys. Chem. (1968) 72, 2594. Kerr, G. T., J. Catal. (1969) 15, 200. Flockhart, B. D., Leith, I. R., Pink, R. C., Trans. Faraday Soc. (1970) 66, 469. 7. Hedges, R. M., Matsen, F. Α., J. Chem. Phys. (1958) 28, 950. 8. Hammond, V. J., Price, W. C., Teegan, J. P., Walsh, A. D., Discuss. Faraday Soc. (1950) 9, 52. 9. Wilkinson, P. G., Can. J. Phys. (1956) 34, 596. RECEIVED December 5, 1972


Redox Behavior of Zeolite Aluminosilicates and the Nature of the Sites

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