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Applications of High-Resolution C-NMR and Magic-Angle Spinning NMR to Reactions on Zeolites and Oxides Downloaded by CHINESE UNIV OF HONG KONG on March 23, 2016 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch007
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ERIC G. DEROUANE and JANOS B. NAGY Facultés Universitaires de Namur, Laboratoire de Catalyse, Rue de Bruxelles 61, B-5000 Namur, Belgium 31
Conventional high resolution C-NMR is shown to pro vide quantitative information on the conformation of 1-and 2-butene molecules adsorbed on zeolites and on mixed tin-antimony oxides. The kinetics of isomerization of 1-butene into the two 2-butene isomers is rea dily determined and enables the proposition of a reac tion mechanism involving an intermediate cyclic complex. Rate constants can be related to the acid-base properties of the mixed oxide surfaces. An in-situ characterization of reaction products is readily achieved in the conver sion of methanol, ethanol and ethylene on the highly aci dic and shape selective ZSM-5 zeolite. In addition, C isotopic labeling proves to be a powerful technique to dis criminate between possible reaction pathways of ethylene. High resolution magic angle spinning C-NMR appears as a superior technique to investigate the nature of carbon-con taining residues in which molecular motion is highly redu ced. There exists a relationship between the nature of the se species and the acidic and molecular shape selective properties of the zeolites which are considered (ZSM-5 and mordenite). This technique allows a distinction between hydrocarbon molecules which are trapped inside the zeoli te framework as a result of pore plugging and strongly chemisorbed or surface alkoxide species. Large molecules or ions can also be trapped inside the zeolitic framework during cristallization. Tetrapropylam monium ions in ZSM-5 zeolite and tetrabutylammonium ions in ZSM-11 zeolite (intact in their respective frameworks) occupy the channel intersections and their alkyl chain ex tend in the linear and zig-zag channels in ZSM-5 zeolite or in the perpendicular linear channels of ZSM-11 zeolite. 13
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Current address: Central Research Division, Mobil Technical Center, Princeton, NJ 08540.
0097-6156/84/0248-0101$07.00/0 © 1984 American Chemical Society Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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High r e s o l u t i o n C-NMR s p e c t r a of systems p r e s e n t i n g a c a t a l y t i c i n t e r e s t can be obtained i n two d i f f e r e n t modes. The f i r s t one, which i s the approach used over the l a s t ten y e a r s , c o n s i s t s i n a s t r a i g h t f o r w a r d a p p l i c a t i o n of conventional high r e s o l u t i o n F o u r i e r transform ^c-NMR to s t u d i e s of r e a c t i n g adsorbates, l o o k i n g i n t o product formation and r e a c t i o n k i n e t i c s . A s a t i s f a c t o r y r e s o l u t i o n i s achieved i n t h i s case by c o n s i d e r i n g a s t a t i s t i c a l number of r e a c t a n t monolayers such that the molecules w i l l r e t a i n s u f f i c i e n t m o b i l i t y i n the adsorbed phase. The second one, more recent, i s r e f e r r e d to as high r e s o l u t i o n magic angle spinning (HRMAS), that i s a r o t a t i o n , at high frequency (ca. 3kHz) and at a given and f i x e d angle, of the sample placed i n the e x t e r n a l magnetic f i e l d . This procedure averages out the d i p o l a r i n t e r a c t i o n b e t ween n u c l e i and leads to sharp NMR s p e c t r a from s o l i d samples i n which molecular motion i s reduced. The use of both techniques i s i l l u s t r a t e d i n the present work. ^ The f i r s t mode of the high r e s o l u t i o n C-NMR of adsorbed mol e c u l e s was r e c e n t l y reviewed (^-3) and the NMR parameters were thoroughly discussed. In t h i s work we emphasize the study of the s t a t e of adsorbed molecules, t h e i r m o b i l i t y on the s u r f a c e , the i d e n t i f i c a t i o n of the surface a c t i v e s i t e s i n presence of adsorbed molecules and f i n a l l y the study of c a t a l y t i c t r a n s f o r m a t i o n s . As an i l l u s t r a t i o n we report the study of 1- and 2-butene molecules adsorbed on z e o l i t e s and on mixed tin-antimony oxides (4,5). Another a p p l i c a t i o n of t h i s technique c o n s i s t s i n the i n - s i t u i d e n t i f i c a t i o n of products when a complex r e a c t i o n such as the convers i o n of methanol, of ethanol (6,7) or of ethylene (8) i s run on a h i g h l y a c i d i c and s h a p e - s e l e c t i v e z e o l i t e . When the conversion of methanol-ethylene mixtures (9) i s considered, isotopic labeling proves to be a powerful technique to d i s c r i m i n a t e between the poss i b l e r e a c t i o n pathways of ethylene. HRMAS !3c -NMR i s a promising method to i n v e s t i g a t e the nature of carbon-containing residues trapped i n z e o l i t i c s t r u c t u r e s when running hydrocarbons and oxygenates conversion r e a c t i o n s (10). These deposits can be heavy molecular weight molecules or groups attached to the z e o l i t e s u r f a c e . Large molecules can a l s o be trapped i n s i d e the z e o l i t e framework during c r i s t a l l i z a t i o n . Examples are found i n the syntheses of z e o l i t e s ZSM-5 and ZSM-11 i n which tetrapropy1ammonium and tetrabutylammonium c a t i o n s have been shown to act as templates (11). High r e s o l u t i o n s o l i d s t a t e magic angle s p i n n i n g ^ C-NMR enables one to show that these o c c l u ded molecules are i n t a c t i n t h e i r r e s p e c t i v e frameworks and provides i n f o r m a t i o n on t h e i r c o n f i g u r a t i o n s . Future a p p l i c a t i o n s w i l l a l s o be considered, i n p a r t i c u l a r w i t h respect to the use of these techniques combined w i t h the NMR of other n u c l e i (Na, A l , S i ) f o r the i n v e s t i g a t i o n of z e o l i t e synt h e s i s mechanisms (12-14).
Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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Experimental M a t e r i a l s NaGeX z e o l i t e was k i n d l y s u p p l i e d by Dr. G. Poncelet (Université Catholique de Louvain) and the mixed tin-antimony oxide c a t a l y s t s (SnSbO) by I.C.I. L t d . The H-Z i s the a c i d i f i e d form o f commercially a v a i l a b l e Norton mordenite. The ZSM-5 and ZSM-11 z e o l i t e s were synthesized f o l l o w i n g the patent l i t e r a t u r e (15,16). 1-Butene (Prochem) was a n a t u r a l abundance compound, w h i l e methanol (95 % C , B r i t i s h Oxygen Corporation (B.O.C.)), ethanol (95 % C , B.O.C.) and ethylene (^90 % C , Prochem) were C-enriched compounds. F o r the l a t t e r a 30 % v/v d i l u t i o n was r e a l i z e d p r i o r to adsorption. A l l the c a t a l y s t s (ca.0.6 g) were p r o g r e s s i v e l y dehydrated and a c t i v a t e d at temperatures 573-673 Κ and a t a f i n a l pressure of 2.10""^Torr. The a d s o r p t i o n f o r a l l the r e a c t a n t s was made at room temperature. The mixed tin-antimony oxide samples were r a p i d l y cooled down t o 77 Κ i n order t o avoid i s o m e r i z a t i o n of 1-butene following adsorption. The k i n e t i c measurements were performed by successive h e a t i n g c y c l e s a t the r e a c t i o n temperature, the ^C-NMR s p e c t r a being r e corded a t a lower temperature where the r e a c t i o n was quenched. 1 3
Downloaded by CHINESE UNIV OF HONG KONG on March 23, 2016 | http://pubs.acs.org Publication Date: April 5, 1984 | doi: 10.1021/bk-1984-0248.ch007
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13 -NMR measurements Conventional high r e s o l u t i o n 13 -NMR s p e c t r a were recorded on a Bruker WP-60 spectrometer working i n the Fou r i e r transform mode, u s i n g a ca.ir/6 pulse l e n g t h under proton broad band decoupling. The gated sequence 4s-ls-0.1 s was used t o record the n u c l e a r Overhauser e f f e c t (NOE) - suppressed decoupled s p e c t r a i n order to o b t a i n q u a n t i t a t i v e values f o r the r e l a t i v e i n t e n s i t i e s . The chemical s h i f t s were determined u s i n g benzene as an e x t e r n a l r e f e r e n c e . T i measurements were performed at 250, 275 and 300 Κ by i n v e r s i o n - r e c o v e r y (π-τ-π/2-5Τι) sequences on a JE0L-FX-100 and a Bruker WP-80 spectrometers. On t h i s l a t t e r the " r e p e t i t i v e f r e quency s h i f t " method of Brevard e t a l . (18) was used, where two systematic i n s t r u m e n t a l e r r o r s ( d r i f t , round o f f e r r o r s i n FT pro cessing...) are u n i f o r m l y d i s t r i b u t e d through a l l data p o i n t s . The NOE measurements are r e p r o d u c i b l e w i t h i n 10-20 %, w h i l e the ave rage standard e r r o r on the T j values i s of about 5 %. High r e s o l u t i o n magic angle s p i n n i n g cross p o l a r i z a t i o n (CP/ MAS) C-NMR s p e c t r a were recorded a t room temperature on a Bruker CXP-200 spectrometer. The C (50.3 MHz) and H (200.0 MHz) r f f i e l d s are 39.0 G and 9.8 G r e s p e c t i v e l y s a t i s f y i n g the HartmannHahn c o n d i t i o n . The CP-MAS s p e c t r a were obtained u s i n g a s i n g l e contact sequence (17). A contact time of 5.0 ms and a r e c y c l e time of 4.0 s were used i n these experiments. Α π/2 ^H pulse i s f i r s t a p p l i e d , f o l l o w e d by a π/2 phase s h i f t , a f t e r which the ^H-l^C c r o s s - p o l a r i z a t i o n i s allowed and ^H decoupling i s main t a i n e d d u r i n g data a c q u i s i t i o n (4 Κ data p o i n t s ) . D e l r i n or p o l y methylmethacrylate r o t o r s were span a t 3.1 kHz at the magic angle. One thousand s p e c t r a were accumulated p r i o r to the F o u r i e r transformation. C
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Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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Results and D i s c u s s i o n State of the adsorbed molecules and nature of the a c t i v e s i t e s The state of the adsorbed molecules w i l l be c h a r a c t e r i z e d by the usual NMR parameters, i . e . chemical s h i f t , l i n e w i d t h and r e l a x a t i o n times. The chemical s h i f t s of l i q u i d , gaseous and adsorbed 1-butene and trans 2-butene are reported i n Table I . The volume magnetic s u s c e p t i b i l i t y c o r r e c t i o n s are u s u a l l y made f o l l o w i n g the procedure of F r a i s s a r d et a l . (19) where the chemical s h i f t s of the adsorbed species are e x t r a p o l a t e d to zero surface coverage. The chemical s h i f t c o r r e c t i o n s were computed to be -0.5 ppm f o r NaGeX and NaY, taking y = -0.626 χ 10~ f o r C H and χ = -0.594 χ 10" f o r ethanol (21) ( χ = 0.39 χ 10~ f o r NaGeX and NaY z e o l i t e s (20)). The values f o r the z e o l i t e s were determined by the Faraday method and are i n good agreement with t h e o r e t i c a l c a l c u l a t i o n s using i o n i c s u s c e p t i b i l i t i e s f o r i s o l a ted s o l i d components (20). A fundamental question concerns the state of the adsorbed gas, namely whether i t i s c l o s e r to the gaseous or the l i q u i d s t a t e . At 301 K, the solvent s h i f t i s mainly observed on the t e r minal carbon atoms which are more exposed to i n t e r m o l e c u l a r i n t e r a c t i o n s (22). The carbon C] and C4 of 1-butene experience a small low f i e l d s h i f t with respect to the gas, the Cg carbon a small high f i e l d s h i f t , while the methinic C2 carbon atom i s much more i n f l u e n c e d than the other carbon atoms (low f i e l d s h i f t ) suggesting a s p e c i f i c i n t e r a c t i o n at t h i s s i t e of the molecule. In trans 2-butene, the chemical s h i f t v a r i a t i o n s are almost i d e n t i c a l , which could correspond to a general i n t e r m o l e c u l a r i n t e r a c t i o n . However, i f the comparison i s made w i t h l i q u i d trans 2butene, s p e c i f i c i n t e r a c t i o n s are c l e a r l y shown again by the greater change i n the methinic C carbon atom. From our experimental r e s u l t s and d i f f e r e n t models used i n t h e o r e t i c a l c a l c u l a t i o n s using e i t h e r CND0/2 (23-25, 37>38) and PCIL0 methods (26,27), or the e l e c t r i c f i e l d e f f e c t by IND0 f i n i te p e r t u r b a t i o n theory (28), the f o l l o w i n g models can be suppo sed : 6
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Whyte et al.; Catalytic Materials: Relationship Between Structure and Reactivity ACS Symposium Series; American Chemical Society: Washington, DC, 1984.
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Reactions on Zeolites and Oxides
DEROUANE AND NAGY
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