Perspectives in Molecular Sieve Science - ACS Publications

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5

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Studies of Molecular Sieve Catalysis by Diffuse Reflectance IR Spectroscopy 1

V. A. Maroni, K. A. Martin , and S. A. Johnson Materials Science Division, Argonne National Laboratory, Argonne, IL 60439

Diffuse reflectance infrared Fourier transform spectroscopy is being used to investigate the interaction of hydrocarbons with catalytically active zeolites, such as H-ZSM-5. The experiments are carried out in a controlled environment (temperature, gas composition) diffuse reflectance cell. The reactivity of alcohols, light olefins, and ethers on ZSM-5- and offretite-type zeolites has been probed by observing the changes in intensity of the framework O-H stretching vibrations as a function of the type of reactant, temperature, and product mix (determined by gas chromatography/mass spectrometry). C-H stretching vibrations of organic fragments retained in the zeolite channels and pores are also observed in these experiments, and they too are correlated with reactant, temperature, and product mix. T h i s paper p r e s e n t s examples o f t h e a p p l i c a t i o n o f d i f f u s e r e f l e c t a n c e i n f r a r e d F o u r i e r t r a n s f o r m (DRIFT) s p e c t r o s c o p y t o t h e s t u d y o f a c i d s i t e c h e m i s t r y and c a t a l y s i s by p r o d u c t - s e l e c t i v e m o l e c u l a r s i e v e m a t e r i a l s . The most a t t r a c t i v e f e a t u r e s o f t h e DRIFT method a r e t h a t ( i ) uncompacted, h i g h - s u r f a c e - a r e a powders c a n be s t u d i e d i n c o n t r o l l e d c h e m i c a l environments and o v e r a wide range o f tempera t u r e s ; ( i i ) r e a c t a n t s c a n be p a s s e d t h r o u g h a bed o f t h e powdered sample and i n f r a r e d s p e c t r a r e c o r d e d s i m u l t a n e o u s l y a t t e m p e r a t u r e s i n e x c e s s o f 500°C w i t h h i g h s p e c i e s d e t e c t i o n s e n s i t i v i t y ; and ( i i i ) t h e sample c a n be r e p e a t e d l y r e g e n e r a t e d and exposed t o r e a c t a n t s w i t h o u t b e i n g removed from t h e d i f f u s e r e f l e c t a n c e (DR) c e l l assembly. The DRIFT method has p r o v e n t o be e x t r e m e l y u s e f u l f o r s t u d i e s o f t h e 0-H, 0-D, C-H, and C-D s t r e t c h i n g r e g i o n ( n o m i n a l l y 2000 t o 4000 cm~l) o f m o l e c u l a r s i e v e m a t e r i a l s ( 1 - 3 ) . Kazansky and c o w o r k e r s (4-6) have employed t h e DRIFT t e c h n i q u e t o i n v e s t i g a t e t h e c o m b i n a t i o n and o v e r t o n e s t r u c t u r e o f t h e 0-H and 0-D band systems 1

Current address: Institute of Gas Technology, Chicago, IL 60616 0097-6156/88/0368-0085$06.00/0 © 1988 American Chemical Society

Flank and Whyte; Perspectives in Molecular Sieve Science ACS Symposium Series; American Chemical Society: Washington, DC, 1988.

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o f z e o l i t e s i n the n e a r - i n f r a r e d r e g i o n . However, i n the a p p l i c a t i o n o f DRIFT s p e c t r o s c o p y t o the s p e c t r a l r e g i o n below 2000 cm~l, problems w i t h anomalous s p e c u l a r r e f l e c t a n c e and o t h e r l i g h t s c a t t e r i n g phenomena are o f t e n e n c o u n t e r e d when s t u d y i n g m o l e c u l a r s i e v e m a t e r i a l s (2,3). W h i l e t h e s e e f f e c t s can be overcome i n most c a s e s by 10- t o 1 0 0 - f o l d d i l u t i o n o f the sample i n a powder m a t r i x o f a m a t e r i a l h a v i n g v e r y low absorbance over the s p e c t r a l range o f i n t e r e s t , s u c h d i l u t i o n l e a d s t o a s i g n i f i c a n t r e d u c t i o n i n the i n t e n s i t y o f s p e c t r a l f e a t u r e s o r i g i n a t i n g from t h e sample. Also, f o r t h e t y p e s o f i n v e s t i g a t i o n s d e s c r i b e d i n t h i s p a p e r , the d i l u e n t chosen must be one t h a t does not ( i ) i o n exchange or r e a c t w i t h the m o l e c u l a r s i e v e m a t e r i a l ( a t or below ~ 6 0 0 ° C ) , ( i i ) r e a c t d i r e c t l y w i t h t h e h y d r o c a r b o n s f e d t o the d i f f u s e r e f l e c t a n c e c e l l , o r ( i i i ) have c a t a l y t i c a c t i v i t y o f i t s own under the r e a c t i o n c o n d i t i o n s and at t h e t e m p e r a t u r e s o f i n t e r e s t . The r e s u l t s and d i s c u s s i o n w h i c h f o l l o w show how DRIFT s p e c t r o s c o p y i n a c o n t r o l l e d environment c e l l can be used t o probe the a c t i v e s i t e s i n s y n t h e t i c z e o l i t e c a t a l y s t s , the n a t u r e and temperat u r e dependence o f the o r g a n i c s p e c i e s ( r e f e r r e d t o h e r e i n as f r a g ments) bound w i t h i n the z e o l i t e framework d u r i n g and a f t e r exposure o f the z e o l i t e t o l i g h t h y d r o c a r b o n s , and, i n c o n j u n c t i o n w i t h gas chromatography/mass s p e c t r o m e t r y (GC/MS), the r a t e , p r o d u c t s p e c i a t i o n , and mechanism of the z e o l i t e - i n d u c e d r e f o r m i n g r e a c t i o n s . The a u t h o r s r e c o g n i z e t h a t some o f the r e s u l t s p r e s e n t e d i n t h i s paper p a r a l l e l p u b l i s h e d f i n d i n g s based on o t h e r t y p e s o f i n f r a r e d s p e c t r o s c o p i c t e c h n i q u e s , but f e e l t h a t t h e i r p r e s e n t a t i o n i n t h e c o n t e x t o f t h i s paper s e r v e s t o e s t a b l i s h the c r e d i b i l i t y o f t h e DRIFT/GC/MS p r o c e d u r e i n advance o f the p u b l i c a t i o n o f s i m i l a r s t u d i e s c u r r e n t l y under way on o t h e r m o l e c u l a r s i e v e systems t h a t have not been i n v e s t i g a t e d i n d e t a i l by s u c h methods. E x p e r i m e n t a l Methods The a l c o h o l s used i n t h i s work were o b t a i n e d from A l d r i c h C h e m i c a l Co. (99+% g r a d e ) . The e t h y l e n e and d i m e t h y l e t h e r were Matheson CP. grade. The H-ZSM-5, the H - o f f r e t i t e , and t h e i r a l u m i n o f e r r i s i l i c a t e a n a l o g s were s y n t h e s i z e d by s t a n d a r d methods t h a t a r e d e s c r i b e d i n the l i t e r a t u r e (7-11). The sodium s u b s t i t u t e d forms o f t h e s e same z e o l i t e s were p r e p a r e d by r e p e a t e d washing o f the p r o t o n a t e d form o f the z e o l i t e i n 1 M N a C l . The a p p a r a t u s used f o r t h e DRIFT s t u d i e s c o n s i s t s o f a c o n t r o l l e d environment d i f f u s e r e f l e c t a n c e c e l l ( H a r r i c k S c i e n t i f i c C o r p . , Model HVC-DRP) i n t e r f a c e d w i t h o p t i c s ( H a r r i c k S c i e n t i f i c Corp., Model DRA-SID) t h a t a l l o w a d a p t a t i o n t o a BOMEM D3A FTIR s p e c t r o meter. A s k e t c h o f t h e d i f f u s e r e f l e c t a n c e (DR) c e l l i s shown i n F i g u r e 1. A powdered specimen (8 t o 10 mg) o f t h e z e o l i t e i s p l a c e d on t h e s c r e e n i n the sample cup at the top o f the sample p o s t (see F i g u r e 1). To i n t r o d u c e a l i g h t h y d r o c a r b o n r e a c t a n t w h i c h i s a l i q u i d a t room t e m p e r a t u r e , a s t r e a m o f dry n i t r o g e n i s b u b b l e d t h r o u g h a g l a s s t r a p c o n t a i n i n g the l i q u i d and t h e n p a s s e d on t o t h e DR c e l l . Gaseous r e a c t a n t s ( e . g . , e t h y l e n e , d i m e t h y l e t h e r ) cont a i n e d i n t h e i r a s - r e c e i v e d l e c t u r e b o t t l e s are fed d i r e c t l y i n t o t h e DR c e l l t h r o u g h a b l e e d v a l v e . The r e a c t a n t s t r e a m f l o w s i n t o the c e l l a t i t s base (gas e n t r a n c e p o r t ) , upwards a l o n g the h e a t e d sample p o s t , t h r o u g h the powdered sample, and i n t o an a n n u l a r s p a c e


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F i g u r e 1. C o n t r o l l e d environment d i f f u s e r e f l e c t a n c e ( H a r r i c k S c i e n t i f i c Corp., Model HVC-DRP).

cell


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below t h e s c r e e n w h i c h i s c o n n e c t e d ( v i a a s m a l l d i a m e t e r tube) t o the gas e x i t p o r t . The thermocouple b u i l t i n t o t h e H a r r i c k Model HVC-DRP c e l l i s a t t a c h e d t o t h e o u t s i d e s u r f a c e o f t h e sample p o s t a t a p o i n t below the sample cup (as shown i n F i g u r e 1 ) . I n o r d e r t o g e t a more a c c u r a t e r e a d i n g o f t h e a c t u a l sample t e m p e r a t u r e , an a u x i l i a r y t h e r m o c o u p l e ( n o t shown i n F i g u r e 1) was e p o x i e d i n t o a f i t t i n g t h a t a t t a c h e d t o a t h i r d p o r t i n t h e base o f t h e c e l l . The s e c t i o n o f t h i s a u x i l i a r y ( i r o n / c o n s t a n t a n ) thermocouple i n s i d e t h e c e l l was s t r a p p e d t o t h e gas e x i t tube t h a t runs from t h e upper p o r t i o n o f the sample p o s t t o t h e gas e x i t p o r t , a f t e r w h i c h t h e t i p end was bent i n t o a s m a l l s e m i c i r c l e so t h a t t h e t i p o f t h e t h e r m o c o u p l e c o u l d be p o s i t i o n e d a t t h e s u r f a c e o f t h e powdered specimen i n t h e sample cup. The b u i l t - i n thermocouple was used as t h e c o n t r o l t h e r m o c o u p l e , b u t i t s t e m p e r a t u r e s e t t i n g was a d j u s t e d t o g i v e t h e d e s i r e d t e m p e r a t u r e a t t h e sample s u r f a c e as measured by t h e a u x i l i a r y thermocouple. A l l t e m p e r a t u r e s r e p o r t e d i n t h i s paper were t h o s e measured by t h e a u x i l i a r y t h e r m o c o u p l e . The DRIFT s p e c t r a c o n s i s t o f 500 co-added scans t a k e n a t 4 cm"* resolution. Each sample spectrum i s r a t i o e d a g a i n s t t h e DRIFT s p e c t r u m o f powdered KBr (under d r y N2 f l u s h ) r e c o r d e d i n t h e DR c e l l a t a t e m p e r a t u r e near t h a t a t which t h e sample spectrum was taken. T h i s p r o c e d u r e i s b e n e f i c i a l i n r e d u c i n g t h e t h e r m a l background enhancement due t o i n f r a r e d e m i s s i o n , w h i c h becomes p r o g r e s s i v e l y more t r o u b l e s o m e as t h e t e m p e r a t u r e i s i n c r e a s e d . These e f f e c t s a r e , however, l e s s s e v e r e i n t h e 2000 t o 4000 cm"* r e g i o n t h a n a t f r e q u e n c i e s below 2000 cm"^ where t h e r m a l e m i s s i o n c o n s t i t u t e s a s i g n i f i c a n t l y g r e a t e r f r a c t i o n o f t h e t o t a l i n f r a r e d power reaching the detector. The major s p e c t r a l f e a t u r e s i n F i g u r e s 2 t h r o u g h 5, i . e . , t h e 0-H and C-H s t r e t c h i n g bands a r e t y p i c a l l y 0.1 t o 0.2 absorbance u n i t s ( p e a k - t o - b a s e l i n e ) . This s e n s i t i v i t y i s a c h i e v e d f o r a l o a d i n g o f ~10 mg o f z e o l i t e packed i n t o a c i r c u l a r w e l l ~3 mm i n d i a m e t e r and 2 mm deep. In a t y p i c a l e x p e r i m e n t , t h e z e o l i t e sample i s f i r s t h e a t e d s l o w l y i n f l o w i n g d r y a i r t o 525+25*C where i t i s h e l d f o r 20 t o 30 m i n u t e s ; t h e a i r f l u s h i s t h e n s w i t c h e d t o a d r y N2 f l u s h t o purge the system o f O2. D u r i n g t h i s t r e a t m e n t , h e r e a f t e r c a l l e d " c o n d i i t i o n i n g , " a l l adsorbed water and o r g a n i c m a t t e r ( e . g . , from p r i o r e x p o s u r e s t o l i g h t h y d r o c a r b o n s ) a r e removed, l e a v i n g t h e z e o l i t e p o r e s and c h a n n e l s f r e e o f p h y s i - and chemi-sorbed s p e c i e s . The z e o l i t e i s t h e n c o o l e d t o t h e d e s i r e d t e m p e r a t u r e f o r t h e next exposure e x p e r i m e n t , a DRIFT spectrum i s t a k e n , and t h e r e a c t a n t i s introduced. The r a t e o f d e l i v e r y o f a l i q u i d r e a c t a n t c a n be cont r o l l e d by a d j u s t i n g t h e N2 f l o w r a t e and/or t h e t e m p e r a t u r e o f t h e g l a s s t r a p ; b l e e d v a l v e adjustments a r e made i n t h e c a s e o f gaseous reactants. A f t e r a 5 t o 15 minute exposure o f t h e c a t a l y s t sample t o t h e r e a c t a n t stream, t h e f l o w o f r e a c t a n t i s t e r m i n a t e d , t h e DR c e l l i s purged w i t h d r y N2 f o r s e v e r a l m i n u t e s , and a second DRIFT spectrum i s taken. I n e x p e r i m e n t s where p r o d u c t s p e c i a t i o n i s s t u d i e d , t h e r e a c t a n t - p l u s - p r o d u c t s t r e a m coming o u t o f t h e gas e x i t p o r t i s passed through a g l a s s c a t c h bulb which i s designed t o c o l l e c t specimens f o r subsequent a n a l y s i s by gas chromatography/mass s p e c t r o m e t r y ( u s i n g a F i n n e g a n Model 4021C GS/MS s y s t e m ) .


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R e s u l t s and D i s c u s s i o n One o f t h e most i n t e r e s t i n g and i n f o r m a t i v e f e a t u r e s o f t h e i n f r a r e d s p e c t r a o f p r o t o n a t e d ( o r a c i d ) form m o l e c u l a r s i e v e c a t a l y s t s i s t h e c h a r a c t e r o f t h e 0-H s t r e t c h i n g v i b r a t i o n s . F o r example, i t i s u s u a l l y t h e case t h a t t h e a c i d form o f a m o l e c u l a r s i e v e m a t e r i a l i s t h e most a c t i v e c a t a l y t i c a l l y ; f o r z e o l i t e s t h i s a c i d i t y i s p r i m a r i l y B r o n s t e d i n n a t u r e and c a n have a f a i r l y wide a c i d e f f e c t i v e n e s s range. F o r a g i v e n z e o l i t e framework s t r u c t u r e , t h e B r o n s t e d a c i d i t y c a n be c o r r e l a t e d w i t h t h e f r e q u e n c y o f t h e 0-H s t r e t c h i n g v i b r a t i o n — t h e lower t h e f r e q u e n c y t h e more w e a k l y bound (more a c i d i c ) the proton. Two t y p e s o f 0-H v i b r a t i o n a l s i t e s a r e n o r m a l l y d e t e c t e d by i n f r a r e d s p e c t r o s c o p y , t h o s e a s s o c i a t e d w i t h h y d r o x y l s conn e c t e d o n l y t o s i l i c o n atoms and t h o s e a s s o c i a t e d w i t h h y d r o x y l s c o n n e c t e d t o aluminum atoms. The former t y p e (SiOH) a r e found m a i n l y on t h e o u t e r p e r i p h e r y o f z e o l i t e c r y s t a l l i t e s and i n e s s e n c e t e r m i n a t e t h e d i s r u p t e d framework s t r u c t u r e at t h e s u r f a c e . The l a t t e r t y p e , t h o s e i n v o l v i n g aluminum, a r e f o r t h e most p a r t l o c a t e d i n t r a s t r u c t u r a l l y at silicon-oxygen-aluminum b r i d g e s (Si-OH-Al), where t h e a t t a c h e d p r o t o n b a l a n c e s t h e n e g a t i v e charge a s s o c i a t e d w i t h the aluminate subunit. DRIFT s p e c t r o s c o p y i s e s p e c i a l l y u s e f u l f o r p r o b i n g t h e a c i d c h a r a c t e r o f z e o l i t e 0-H v i b r a t i o n s . T h i s u t i l i t y c a n be i l l u s t r a t e d by c o m p a r i s o n o f r e s u l t s f o r t h r e e z e o l i t e systems h a v i n g t h e f o l l o w i n g framework s t r u c t u r e s and m e t a l atom s t o i c h i o m e t r i e s : H-ZSM-5 w i t h S i / A l Z I O and Fe = 0, AFS-H-ZSM-5 w i t h S i / F e ^ 4 3 and A l ^ O , and H - o f f r e t i t e w i t h Si/A1^4. DRIFT s p e c t r a o f t h e s e t h r e e z e o l i t e s i n t h e 0-H s t r e t c h i n g r e g i o n ( F i g u r e 2) d e m o n s t r a t e how h y d r o x y l s i t e c h a r a c t e r i s a f f e c t e d by m e t a l atom s u b s t i t u t i o n i n a f i x e d framework s t r u c t u r e (H-ZSM-5 v s . AFS-H-ZSM-5) and by v a r i a t i o n of a l u m i n o s i l i c a t e framework (H-ZSM-5 v s . H - o f f r e t i t e ) . It i s well e s t a b l i s h e d (5,6,10,12-17) t h a t t h e band a t "3745 cm"" i n e a c h s p e c trum ( F i g u r e 2) i s t h e 0-H s t r e t c h i n g mode o f t h e t e r m i n a l s i l a n o l (SiOH) s i t e s on t h e o u t e r s u r f a c e s o f t h e z e o l i t e c r y s t a l l i t e s . The f r e q u e n c y o f t h e s i l a n o l 0-H s t r e t c h shows l i t t l e s e n s i t i v i t y t o z e o l i t e s t r u c t u r e and c o m p o s i t i o n when i t i s measured on t h e same i n s t r u m e n t u s i n g t h e same s a m p l i n g t e c h n i q u e . W i t h t h e DRIFT method a p p l i e d i n t h i s work, i t i n v a r i a b l y appears a t 3745 + 5 cm" for a wide range o f z e o l i t e s and r e l a t e d s i l i c a t e s . 1

1

The r e m a i n i n g 0-H bands i n F i g u r e 2 l i e a t f r e q u e n c i e s below t h e s i l a n o l s t r e t c h and have been a t t r i b u t e d t o t h e framework h y d r o x y l groups t h a t e x h i b i t B r o n s t e d a c i d i t y . The p r o m i n e n t bands at 3610 cm" f o r H-ZSM-5 and 3630 cm" f o r t h e AFS-H-ZSM-5 a r i s e from t h e 0-H s t r e t c h o f a p a r t i c u l a r l y a c t i v e M-OH-Si h y d r o x y l s i t e (M = A l and Fe, r e s p e c t i v e l y ) . The s h i f t t o h i g h e r f r e q u e n c y when t h e A l ( I I I ) i s r e p l a c e d by F e ( I I I ) r e f l e c t s t h e r e d u c e d B r o n s t e d a c i d i t y o f AFS-H-ZSM-5 compared t o t h a t o f H-ZSM-5 ( 1 0 ) . 1

1

1

The weak 0-H band a t 3660 cm" i n t h e H-ZSM-5 s p e c t r u m i s s e e n w i t h v a r y i n g i n t e n s i t y i n most p r e p a r a t i o n s o f t h i s s y n t h e t i c zeol i t e (1,5,6,12,17). I t seems t o g a i n i n t e n s i t y w i t h i n c r e a s i n g s i l i c o n - t o - a l u m i n u m r a t i o i n t h e framework (5,12) and s e v e r a l e x p l a n a t i o n s have been o f f e r e d c o n c e r n i n g i t s o r i g i n . Sayed e t a l . (12) s u g g e s t t h a t t h i s mode i s due t o w a t e r - p e r t u r b e d n e s t s o f s i l a n o l s i t e s , w h i l e Kazansky e t a l . (5) c l a i m t h a t i t s h o u l d be a s c r i b e d t o 0-H v i b r a t i o n s o f nonframework a l u m i n a t e s p e c i e s .



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F i g u r e 2. DRIFT s p e c t r a (3000 -• 4000 c m ) (b) AFS-H-ZSM-5, and ( c ) H - o f f r e t i t e .

o f (a) H-ZSM-5,


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A n o t h e r p o s s i b i l i t y ( r e l a t e d t o t h e one p r o p o s e d by Kazansky) i s t h a t t h e 3660 cm~l band a r i s e s from s m a l l amounts o f amorphous o r p o o r l y c r y s t a l l i z e d phases c o n t a i n i n g t e r m i n a l -A10H s i t e s . The 0-H s p e c t r u m o f H - o f f r e t i t e ( F i g u r e 2) i s dominated by t h e a b s o r p t i o n e n v e l o p e between 3500 and 3700 cm"^ w h i c h appears t o c o n s i s t o f a t l e a s t t h r e e i n d i v i d u a l components--3550, 3610, and 3660 cm~l, i n good a c c o r d w i t h t h e f i n d i n g s o f M i r o d a t o s e t a l . (18). The much g r e a t e r combined i n t e n s i t y o f t h e s e bands v e r s u s t h a t o f t h e s i l a n o l s t r e t c h a t 3745 cm'-'- must be due i n p a r t t o t h e low s i l i c o n - t o - a l u m i n u m r a t i o f o r t h e H - o f f r e t i t e (~4). However, when making s u c h comparisons i t i s i m p o r t a n t t o keep i n mind t h a t the s i l a n o l peak i n t e n s i t y can be as much a f u n c t i o n o f t h e s u r f a c e a r e a - t o - v o l u m e r a t i o o f t h e z e o l i t e c r y s t a l l i t e s as i t i s o f t h e silicon-to-aluminum r a t i o (13). The DRIFT method can a l s o be employed i n c o n j u n c t i o n w i t h gas chromatography/mass s p e c t r o m e t r y (GC/MS) t o c o r r e l a t e a c i d s i t e occupancy and o r g a n i c fragment s t r u c t u r e w i t h t h e r a t e o f r e a c t i o n and t h e s p e c i a t i o n o f p r o d u c t s . T h i s t y p e o f a p p l i c a t i o n i s i l l u s t r a t e d i n F i g u r e 3 f o r t h e r e a c t i o n o f a 50/50 m i x t u r e o f CH3OH and CH3OD on t h e f e r r i s i l i c a t e (AFS) form o f ZSM-5 w i t h S i / F e = 43 and F e / A l >>1. The f i g u r e i n c l u d e s r e s u l t s f o r t h e a c i d form o f t h e f e r r i s i l i c a t e z e o l i t e (AFS-H-ZSM-5) a t 200 and 350°C and f o r t h e s o d i u m - s u b s t i t u t e d form (AFS-Na-ZSM-5) a t 350°C. DRIFT s p e c t r a o f the r e s p e c t i v e forms o f t h e AFS z e o l i t e r e c o r d e d a f t e r 15 m i n u t e s o f e x p o s u r e t o CH3OH/CH3OD f o l l o w e d by a f i v e - m i n u t e purge w i t h N2 a r e shown on t h e r i g h t - h a n d s i d e o f the f i g u r e . The c o r r e s p o n d i n g r e c o n s t r u c t e d i o n chromatograms (RICs) from GC/MS a n a l y s e s o f t h e gas f l u s h e d from t h e D R - c e l l j u s t p r i o r t o t h e r e c o r d i n g o f each DRIFT s p e c t r u m o f t h e exposed z e o l i t e sample a r e p r e s e n t e d t o t h e l e f t o f t h e c o r r e s p o n d i n g spectrum. GC/MS and DRIFT r e s u l t s o b t a i n e d a t 200°C and 350°C f o r t h e i n a c t i v e AFS-Na-ZSM-5 a r e n e a r l y the same i n appearance and hence o n l y t h e 350*C s e t i s p r e s e n t e d i n F i g u r e 3. A l s o , s i n c e t h e s p e c t r a l p r o f i l e s i n t h e 0-H and 0-D s t r e t c h i n g r e g i o n s a r e i d e n t i c a l i n each DRIFT spectrum, o n l y t h e 0-D r e g i o n i s shown i n F i g u r e 3. At 200*C, t h e RIC f o r t h e a c t i v e AFS-H-ZSM-5 i n d i c a t e s a p r o d u c t mix c o n s i s t i n g o f a wide range o f p a r a f f i n i c , o l e f i n i c , and a r o m a t i c compounds h a v i n g f o u r (C4) t o t w e l v e (C12) c a r b o n atoms. At 350°C t h e p r o d u c t RIC i s reduced t o t h a t o f a m i x t u r e o f a l k y l s u b s t i t u t e d benzenes. RICs o f the f l u s h gas t a k e n f o l l o w i n g t h e methanol/AFS-Na-ZSM-5 exposure show l i t t l e e v i d e n c e o f C4 o r g r e a t e r p r o d u c t s a t e i t h e r 200 o r 350*C. The c o r r e s p o n d i n g DRIFT s p e c t r a c o n t a i n t h e SiOH and SiOD v i b r a t i o n s f o r b o t h z e o l i t e s a t each r e a c t i o n t e m p e r a t u r e , but t h e 0-H v i b r a t i o n s o f t h e Fe-OH-Si and Fe-OD-Si s i t e s o n l y appear f o r t h e a c t i v e (AFS-H-ZSM-5) form o f t h e z e o l i t e a t 350*C. A n o t h e r key f e a t u r e o f t h e DRIFT s p e c t r a i n F i g u r e 3 i s t h a t r e s t r u c t u r i n g o f t h e C-H s t r e t c h i n g band accompani e s t h e t e m p e r a t u r e i n c r e a s e f o r AFS-H-ZSM-5, but t h e r e i s v e r y l i t t l e e v i d e n c e o f r e s t r u c t u r i n g i n t h e C-H r e g i o n when t h e AFS-NaZSM-5 r e a c t i o n t e m p e r a t u r e i s i n c r e a s e d from 200 t o 350°C. The r e l a t i v e number o f c o u n t s o b t a i n e d from t h e i n t e g r a t e d RICs f o r each experiment r e p r e s e n t e d i n F i g u r e 3 were >10^ f o r AFS-H-ZSM5 a t 350°C, >10 f o r AFS-H-ZSM-5 a t 200*C, n=l 3 n

n=2

n=4

AFS-Na-ZSM-5 at 350 C

400 600 800 Time (Channel Number)

PQ

Pi O

' 2675 cm i -l - 2755 cm

Figure 3. DRIFT and GC/MS study of the reaction of CH3OH/CH3OD on AFS-H-ZSM-5 and AFS-Na-ZSM-5. Numbers i n parentheses on each RIC indicate the t o t a l number of counts i n the C4 to C^2 range.


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93

forms o f t h e AFS m a t e r i a l ( w i t h t h e r e s i d u a l a c t i v i t y o f t h e sodium form most p r o b a b l y a r e s u l t o f i n c o m p l e t e exchange o f t h e p r o t o n s ) . They a l s o h i g h l i g h t t h e c l e a r c o r r e l a t i o n between C-H band s t r u c t u r e and r e a c t i v i t y . The DRIFT t e c h n i q u e t h a t i s most d e f i n i t i v e f o r e l u c i d a t i o n o f t h e i n t r a f r a m e w o r k i n t e r a c t i o n s and s p e c i e s t h a t accompany a d s o r p t i o n and r e a c t i o n o f l i g h t h y d r o c a r b o n s on shape s e l e c t i v e m o l e c u l a r s i e v e c a t a l y s t s i s one based on t h e a p p l i c a t i o n o f s p e c t r a l s u b t r a c t i o n p r o c e d u r e s t o t h e DRIFT s p e c t r a r e c o r d e d p r i o r t o and f o l l o w i n g exposure o f t h e z e o l i t e sample t o a r e a c t a n t . T y p i c a l l y , the spectrum r e c o r d e d p r i o r t o exposure i s s u b t r a c t e d from t h e p o s t - e x p o s u r e spectrum. The r e s u l t i n g d i f f e r e n c e s p e c t r u m p r o v i d e s a s i g n a t u r e o f t h e changes i n p r o t o n a t e d s i t e occupancy and g i v e s e v i d e n c e o f t h e f o r m a t i o n o f o r g a n i c fragments t h a t tend t o remain i n t h e z e o l i t e a t moderate t e m p e r a t u r e s even a f t e r p u r g i n g o f t h e r e a c t a n t . Results f o r e x p o s u r e s o f H-ZSM-5 t o i s o p r o p a n o l , n - b u t a n o l , and n - p e n t a n o l , which i l l u s t r a t e the e f f e c t s t y p i c a l l y observed i n these s p e c t r a l d i f f e r e n c i n g e x p e r i m e n t s , a r e shown i n F i g u r e 4. These s p e c t r a c o v e r t h e s p e c t r a l r e g i o n o f t h e 0-H and C-H s t r e t c h i n g v i b r a t i o n s . They show t h a t t h e s i l a n o l (3745 cm"^) and a l u m i n o l (3610 cm~l) s i t e o c c u p a n c i e s a r e reduced i n t h e p r e s e n c e o f t h e " t r a p p e d " o r g a n i c f r a g m e n t s , t h a t a l l 0-H s i t e s tend t o r e c o v e r t h e i r o c c u p a n c i e s as t h e t r a p p e d fragments d e s o r b a t h i g h e r t e m p e r a t u r e s , and t h a t t h e h i g h l y a c i d i c 3610 cm"! s i t e i s t h e l a s t one t o r e c o v e r i t s i n i t i a l p r o t o n occupancy. F o r r e a c t i o n s i n v o l v i n g l i g h t a l c o h o l s and o l e f i n s on H-ZSM-5, t h e t e m p e r a t u r e a t w h i c h fragment r e t e n t i o n and p r o t o n a t e d s i t e occupancy r e a c h z e r o (as d e t e r m i n e d by DRIFT d i f f e r ence s p e c t r a ) i s g e n e r a l l y around 400°C. E x p e r i m e n t s o f t h e type d e s c r i b e d above and i l l u s t r a t e d i n F i g u r e 4 have been c a r r i e d o u t f o r a s e r i e s o f z e o l i t e s w h i c h i n c l u d e d H-ZSM-5, H - o f f r e t i t e , and t h e i r i r o n s u b s t i t u t e d a n a l o g s , u s i n g l i g h t a l c o h o l s , d i m e t h y l e t h e r , and e t h y l e n e as r e a c t a n t s . From one z e o l i t e t o a n o t h e r , o n l y s u b t l e d i f f e r e n c e s a r e seen i n t h e shape and i n t e n s i t y o f t h e C-H band e n v e l o p e as a f u n c t i o n o f r e a c t a n t and t e m p e r a t u r e . The r e s p o n s e p a t t e r n d e p i c t e d i n F i g u r e 4 -a double-peaked C-H band e n v e l o p e a t lower t e m p e r a t u r e s ( u s u a l l y up t o ~200°C) w h i c h broadens t o a s i n g l e band a t h i g h e r t e m p e r a t u r e s and e v e n t u a l l y d i s a p p e a r s ( c i r c a 400*0) -- i s t y p i c a l o f what i s o b s e r v e d i n e v e r y c a s e i n v e s t i g a t e d where t h e r e i s p a r a l l e l i n g e v i dence (from downstream GC/MS) t h a t t h e r e a c t a n t i s b e i n g c o n v e r t e d i n t o higher molecular weight products. G r e a t e r d i f f e r e n c e s a r e seen i n t h e r e s p o n s e o f t h e framework 0-H s t r e t c h i n g v i b r a t i o n s t o v a r i o u s t y p e s o f r e a c t a n t s and t o t h e p r e s e n c e o f w a t e r i n t h e incoming r e a c t a n t stream. Some o f t h e s e r e s p o n s e s a r e shown i n F i g u r e 5. Dry o l e f i n s exposed t o c o n d i t i o n e d H-ZSM-5 and i t s i r o n - s u b s t i t u t e d a n a l o g l e a v e a fragment t h a t d e p l e t e s t h e p r o t o n s a t t h e a l u m i n o l ( s i t e ) b u t does n o t s i g n i f i c a n t l y a l t e r t h e p r o t o n occupancy o f t h e s i l a n o l s i t e . This e f f e c t , e x e m p l i f i e d by s p e c t r a a and b o f F i g u r e 5, i s i n t e r p r e t e d as e v i dence t h a t t h e r e t a i n e d fragment i s a c a r b o c a t i o n w h i c h has assumed t h e c h a r g e b a l a n c i n g r o l e f o r m e r l y p l a y e d by t h e a l u m i n o l p r o t o n . The C-H band s t r u c t u r e o f t h i s c a r b o c a t i o n f i t s t h e p a t t e r n o f many l i g h t p a r a f i n s ; and t h e l a c k o f any s i g n i f i c a n t i n f r a r e d a b s o r p t i o n i n t h e 3000 t o 3200 cm" r e g i o n p r e c l u d e s t h e p r e s e n c e o f s i g n i f i c a n t amounts o f o l e f i n i c o r a r o m a t i c e n t i t i e s . 1



F i g u r e 4. DRIFT spectrum o f c o n d i t i o n e d H-ZSM-5 a t 523 K (Curve a) and d i f f e r e n c e s p e c t r a o f H-ZSM-5 t a k e n a f t e r e x p o s u r e t o i s o p r o p a n o l , (b) and ( c ) ; n - b u t a n o l , (d) and ( e ) ; and n - p e n t a n o l , ( f ) , ( g ) , and (h) a t v a r i o u s t e m p e r a t u r e s .


MARONI ET AL.


F i g u r e 5. DRIFT d i f f e r e n c e s p e c t r a f o r C2H4 on H-ZSM-5 a t (a) 373 K and (b) 473 K; DRIFT s p e c t r a o f H - o f f r e t i t e ( c ) b e f o r e and (d) a f t e r exposure t o C2H4 a t 573 K; (e) = (d) - ( c ) ; ( f ) DRIFT d i f f e r e n c e s p e c t r u m f o r ( ^ 3 ) 2 0 on H - o f f r e t i t e a t 573 K.



96

DRIFT s p e c t r a o f H-ZSM-5, H - o f f r e t i t e , and t h e AFS a n a l o g o f H-ZSM-5 t a k e n a f t e r exposure t o l i g h t a l c o h o l s o r d i m e t h y l e t h e r r e v e a l r e t a i n e d o r g a n i c fragments w i t h C-H s t r u c t u r e s v e r y much l i k e t h o s e o b s e r v e d f o l l o w i n g exposure t o e t h y l e n e . These s p e c t r a a l s o show d e p l e t i o n o f t h e a l u m i n o l p r o t o n s (3610 cm~l) b u t o f t e n g i v e e v i d e n c e o f d e p l e t i o n o f t h e s i l a n o l s i t e (3745 cm"^) as w e l l . This e f f e c t , s e e n i n F i g u r e 4 and i n s p e c t r a e and f o f F i g u r e 5, has been t i e d t o t h e p r e s e n c e o f t r a c e s o f water i n t h e i n c o m i n g r e a c t a n t s t r e a m and i n t h e case o f H-ZSM-5 i s more p r e v a l e n t a t temperat u r e s below 150°C, t h e t e m p e r a t u r e a t w h i c h water appears t o f r e e l y d e s o r b e d from H-ZSM-5 t h a t i s a c t i v e l y b e i n g exposed t o a l i g h t a l c o h o l , o l e f i n , or ether. The r e s u l t s i n F i g u r e 5 f o r t h e C 2 H 4 / H - o f f r e t i t e exposure ( s p e c t r a c and d and t h e i r d i f f e r e n c e c u r v e , s p e c t r u m e) a r e p a r t i c u l a r l y i n t e r e s t i n g because t h e n e g a t i v e peak s t r u c t u r e i n t h e 3600 t o 3750 cm~l r e g i o n i s v e r y c l o s e l y matched w i t h t h e 0-H r e g i o n o f H-ZSM-5 ( s e e s p e c t r u m a i n F i g u r e 2) b u t s t r i k i n g l y d i f f e r e n t from t h a t o f s p e c t r u m c i n F i g u r e 5. Much o f t h e i n t e n s i t y i n t h e 0-H r e g i o n o f spectrum c (Figure 5 ) , i . e . , that remaining i n spectrum d ( F i g u r e 5 ) , must, t h e r e f o r e , be due t o p r o t o n a t e d s i t e s t h a t a r e e i t h e r not a c c e s s i b l e t o the reactant or not exchangeable. This b e h a v i o r i s i d e n t i c a l t o t h a t o b s e r v e d by M i r o d a t o s e t a l . (.18) and i s n i c e l y summarized i n T a b l e 1 o f t h e i r p a p e r . The e s s e n c e o f t h e i r e x p l a n a t i o n i s t h a t t h e 3745 cm~l band i s a c c e s s i b l e , i . e . , t o exchange w i t h l a b i l e p r o t o n s , b u t n o n a c i d i c ( j u s t l i k e i n H-ZSM-5); t h e 3660 cm~l band i s n o t a c c e s s i b l e t o h y d r o c a r b o n s ( p o s s i b l y l o c a t e d i n t h e h e x a g o n a l p r i s m s ) ; t h e 3610 cm"^ band i s b o t h a c c e s s i b l e and a c i d i c ( j u s t l i k e t h e 3610 c m band o f H-ZSM-5); and t h e 3550 cm~l band i s a c i d i c b u t l a r g e l y i n a c c e s s i b l e ( p o s s i b l y l o c a t e d i n the c a n c r i n i t e cages). - 1

Concluding

Remarks

The r e s u l t s p r e s e n t e d i n t h i s paper show t h a t d i f f u s e r e f l e c t a n c e i n f r a r e d F o u r i e r t r a n s f o r m s p e c t r o s c o p y , used i n c o n j u n c t i o n w i t h c o n t r o l l e d environment t e c h n i q u e s and gas chromatography/mass s p e c t r o m e t r y , c a n be a p o w e r f u l t o o l f o r t h e s t u d y o f c a t a l y s i s by p r o d u c t (shape) s e l e c t i v e m o l e c u l a r s i e v e m a t e r i a l s . By u t i l i z i n g s p e c t r a l d i f f e r e n c i n g techniques i t i s p o s s i b l e t o t r a c k the v a r i a t i o n s i n p r o t o n a t e d s i t e occupancy and t h e f o r m a t i o n o f s t a b l e o r g a n i c s p e c i e s t h a t o c c u r d u r i n g exposure o f m o l e c u l a r s i e v e catalysts t o organic reactants. Acknowledgments The a u t h o r s a r e i n d e b t e d t o L. E. I t o n f o r s u p p l y i n g t h e z e o l i t e samples used i n t h i s r e s e a r c h and f o r many i n f o r m a t i v e d i s c u s s i o n s of t h e r e s u l t s . The a s s i s t a n c e o f R. J . Wingender w i t h t h e GC/MS a n a l y s e s i s a l s o g r a t e f u l l y acknowledged. T h i s work was supported by t h e U.S. Department o f Energy, O f f i c e o f B a s i c Energy S c i e n c e s , D i v i s i o n o f M a t e r i a l s S c i e n c e s , under c o n t r a c t W-31-109-Eng-38.


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Literature Cited 1. Klier, K. In Vibrational Spectroscopies for Adsorbed Species; Bell, A. T., Hair, M. L., Eds.; ACS Symposium Series No. 137; American Chemical Society: Washington, DC, 1980, pp 141-162. 2. Kubelkova, L., Hoser, H., Riva, A., Trifiro, F. ZEOLITES 1983, 3, 244. 3. Chalmers, J. M., Mackenzie, M. W. Applied Spectroscopy 1985, 39, 634. 4. Kustov, L. M., Borovkov, V. Y., Kazansky, V. B. J. Catal. 1981, 72, 149. 5. Kazansky, V. B., Kustov, L. M., Borovkov, V. Yu. ZEOLITES 1983, 3, 77. 6. Kazansky, V. B., Minachev, K. M., Nefedov, B. K., Borovkov, V. Y., Kondrat'ev, D. A.,Chukin, G. K., Kustov, L. M., Bondarenko, T. N., Konoval'chikov, L. D. Kinetics and Catalysis 1983, 24, 572. 7. Argauer, R. J . , Landolt, G. R. U.S. Patent 3 702 886, 1972. 8. Derouane, E. G., Janos, B. N., Dejaifve, P., Nagy, J. B., van Hooff, J. H. C., Spekman, B. P., Vedrine, J. C., Naccache, C. J. Catal. 1978, 53, 40. 9. Barrer, R. M., Harding, D. A. Sep. Sci. 1974, 9, 195; Aiello, R., Barrer, R. M. J. Chem. Soc. (A) 1970, 1470. 10. Chu, C. T.-W., Chang, C. D. J. Phys. Chem. 1985, 89, 1569. 11. Iton, L. E., Beal, R. B., Hodul, D. T. J. Molecular Catal. 1983, 21, 151. 12. Sayed, M. B., Kydd, R. A., Cooney, R. P. J. Catal. 1984, 88, 137. 13. Qin, G., Zheng, L., Xie, Y., Wu, C. J. Catal. 1985, 95, 609. 14. Nunan, J . , Cronin, J . , Cunningham, J. J. Catal. 1984, 87, 77. 15. Ono, Y., Mori, T. J. Chem. Soc. Faraday Trans. I 1981, 77, 2209. 16. Grady, M. C., Gorte, R. J. J. Phys. Chem. 1985, 89, 1305. 17. Woolery, G. L., Alemany, L. B., Dessau, R. M., Chester, A. W. ZEOLITES 1986, 6, 14. 18. Mirodatos, C., Abou-Kais, A., Vedrine, J. C., Barthomeuf, D. J. Chem. Soc. Faraday Trans. I 1978, 74, 1786. RECEIVED February 4, 1988


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