Magnetic Resonance in Colloid and Interface Science - American

11 Characterization of the Small-Port Mordenite Adsorption Sites by Carbon-13 NMR

Downloaded by WEIZMANN INST OF SCIENCE on May 26, 2018 | https://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0034.ch011

M. D. SEFCIK, JACOB SCHAEFER, and Ε. O. STEJSKAL Monsanto Company, St. Louis, Mo. 63166

Complete characterization of adsorbate-adsorbent systems require the application of diverse analytical procedures. Adsorbents are usually characterized by what might be called steady state measurements (such as XRD, adsorption isotherms, chemical composition, e t c . ) . These experiments determine macroscopic or static properties of the adsorbent. Adsorbates, on the other hand, are routinely analyzed in terms of their high frequency spectra. Infrared, Raman and ultraviolet spectroscopies measure vibrations and electronic transitions which occur in the frequency range of 10 12 to 10 14 Hertz. What is lacking from a complete picture of the adsorbate-adsorbent system is an analysis of the low-frequency dynamic properties of the system; that i s , a description of any orientational influence the adsorbent may have on the adsorbate. Nuclear magnetic resonance has been applied to this problem with some success. Resing and coworkers (1) have used proton-NMR relaxation properties to study diffusion coefficients, jump times and the temperature of onset of molecular rotation in the zeolite system. Kaplan, Resing and Waugh reported the f i r s t carbon-13 NMR spectra of benzene adsorbed on charcoal and s i l i c a gel and demonstrated that the chemical shift aniso tropy could be interpreted in terms of the molecular rotation and reorientation (2). High resolution carbon-13 NMR has also been recently used to determine the extent of interactions between olefins and zeolytic cations (3,4). We have found carbon-13 NMR to be particularly useful in obtaining information about the motions of molecules in the range of 102 to 106 Hertz. Analysis of the NMR lineshape produced by these slow moving molecules can provide information about the adsorbate rotational axis and the geometry of the adsorption site. 109 Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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We w i l l p r e s e n t h e r e t h e r e s u l t s o f a s t u d y o f C O , C 0 , COS a n d C S a d s o r b e d on s m a l l - p o r t N a - and c a t i o n exchanged mordenites u s i n g *C NMR, w h i c h l e a d t o a description of adsorption sites i n this particular adsorbate-adsorbent system. 2

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Lineshape A n a l y s i s There are three sources of l i n e broadening i n the NMR e x p e r i m e n t w h i c h l i m i t t h e a p p l i c a b i l i t y o f t h i s technique i n studying adsorbent-adsorbate interactions. The f i r s t i s d i p o l e - d i p o l e i n t e r a c t i o n s . If a molecule c o n t a i n s two n u c l e i w i t h m a g n e t i c moments, t h e n t h e e x t e n t o f the magnetic i n t e r a c t i o n between those n u c l e i depends on t h e i r s p a t i a l s e p a r a t i o n , r e l a t i v e orientat i o n a n d t h e s i z e o f t h e m a g n e t i c moments ( 5 ) . These i n t e r a c t i o n s may b e b e t w e e n s i m i l a r n u c l e i T h o m o nuclear broadening) or u n l i k e n u c l e i (heteronuclear b r o a d e n i n g ) a n d may b e e i t h e r i n t e r - o r i n t r a m o l e c u l a r . In l i q u i d s these i n t e r a c t i o n s are u s u a l l y averaged to zero by the t u m b l i n g motions o f the m o l e c u l e s , b u t t h i s m i g h t n o t be t h e c a s e f o r a d s o r b e d m o l e c u l e s . Homonuclear d i p o l a r b r o a d e n i n g i s a s e r i o u s problem i n p r o t o n NMR d u e t o t h e h i g h n a t u r a l a b u n d a n c e a n d h e n c e r e l a t i v e l y s h o r t i n t e r n u c l e a r d i s t a n c e between p r o t o n s . M u l t i p l e p u l s e experiments have been used to e l i m i n a t e h o m o n u c l e a r i n t e r a c t i o n s i n p r o t o n NMR b u t w i l l n o t b e d e s c r i b e d h e r e (a d i s c u s s i o n o f t h i s t e c h n i q u e c a n b e f o u n d i n r e f . 6). By u s i n g r a r e - s p i n NMR ( C, N, S i , e t c . ) , homonuclear i n t e r a c t i o n s are greatly reduced, while s t a t i c heteronuclear dipolar broadening c a n be e l i m i n a t e d by d o u b l e r e s o n a n c e t e c h n i q u e s (7). N e i t h e r l i n e - n a r r o w i n g t e c h n i q u e removes d i p o l a r broadening associated with intermediate-frequency m o t i o n s c h a r a c t e r i z e d by c o r r e l a t i o n f r e q u e n c i e s on the order of 10 Hz. 1

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A second s o u r c e o f unwanted l i n e b r o a d e n i n g a r i s e s from paramagnetic i m p u r i t i e s i n the sample. Paramagnetic broadening i s s i m i l a r to the d i p o l e - d i p o l e i n t e r a c t i o n s d i s c u s s e d above e x c e p t t h a t i t i n v o l v e s an e l e c t r o n d i p o l e a n d c a n n o t be e l i m i n a t e d b y instrumental procedures. Since the e l e c t r o n d i p o l e i s 657 t i m e s s t r o n g e r t h a n t h e *H n u c l e a r d i p o l e i t is e a s y t o see t h a t v e r y low c o n c e n t r a t i o n s o f p a r a m a g n e t i c i m p u r i t i e s c a n d r a s t i c a l l y a f f e c t t h e NMR lineshape. The e f f e c t o f p a r a m a g n e t i c i m p u r i t i e s i n z e o l i t e s on m o l e c u l a r r e l a x a t i o n has been d i s c u s s e d i n d e t a i l by R e s i n g ( £ ) and w i l l n o t be p r e s e n t e d h e r e . In p r a c t i c e , p a r a m a g n e t i c i m p u r i t i e s as g r e a t as 200 ppm i n m o l e c u l a r s i e v e a d s o r b e n t s a r e t o l e r a b l e since

Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.


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Small-Port Mordenite Adsorption

many o f the paramagnetic c e n t e r s a r e a p p a r e n t l y t r a p p e d away from t h e a d s o r p t i o n s i t e s . While b o t h d i p o l a r and paramagnetic i n t e r a c t i o n s u s u a l l y g i v e r i s e t o symmetric b r o a d e n i n g o f t h e n u c l e a r magnetic resonance, m o l e c u l a r motions which a r e s l o w e r than about 10** H e r t z a l l o w a t h i r d major s o u r c e o f l i n e b r o a d e n i n g , a d i s p e r s i o n o f resonance frequencies. The c h e m i c a l s h i f t o r r e s o n a n t Larmor frequency of a p a r t i c u l a r nucleus i s p r o p o r t i o n a l t o t h e l o c a l magnetic f i e l d a t t h a t n u c l e u s . The l o c a l magnetic f i e l d i s , i n the absence o f d i p o l a r i n t e r a c t i o n s , dependent on t h e s t r e n g t h o f the a p p l i e d magnetic f i e l d , H , and t h e v e r y s m a l l f i e l d s g e n e r a t e d by e l e c t r o n s moving about the n u c l e u s . In a m o l e c u l e t h e r e i s f r e q u e n t l y an a n i s o t r o p i c d i s t r i b u t i o n o f e l e c t r o n s about the n u c l e u s c a u s i n g a d i r e c t i o n a l dependence i n the c h e m i c a l s h i f t . Theoretical reso nance d i s p e r s i o n s a r i s i n g from c h e m i c a l s h i f t a n i s o tropics f o r n u c l e i i n randomly o r i e n t e d m o l e c u l e s such as e n c o u n t e r e d here may be c a l c u l a t e d f o r v a r i o u s n u c l e a r s i t e symmetries (9) . A c u b i c n u c l e a r s i t e symmetry r e s u l t s i n o n l y one v a l u e f o r the c h e m i c a l s h i f t , a d e l t a f u n c t i o n shown i n F i g u r e 1 a t σ^. (The h i g h f r e q u e n c y m o l e c u l a r motions e n c o u n t e r e d i n l i q u i d s and gases e f f e c t i v e l y i n c r e a s e the n u c l e a r s i t e symmetry o f the m o l e c u l e s t o c u b i c r e s u l t i n g i n t h e narrow l i n e s c h a r a c t e r i s t i c o f h i g h r e s o l u t i o n NMR). I t i s w o r t h w h i l e t o note a t t h i s p o i n t t h a t any i n t e r a c t i o n between the a d s o r b a t e and t h e a d s o r b e n t which changes the e l e c t r o n d e n s i t y near t h e o b s e r v e d n u c l e u s w i l l r e s u l t i n a chemical s h i f t , thus p r o v i d i n g a mechanism f o r d i s t i n g u i s h i n g chemi- from p h y s i s o r p t i o n . M o l e c u l e s w i t h a x i a l symmetry have two p r i n c i p l e v a l u e s o f t h e c h e m i c a l s h i f t , one p e r p e n d i c u l a r t o the symmetry a x i s and one p a r a l l e l t o i t . Figure 1 r e p r e s e n t s the c h e m i c a l s h i f t d i s p e r s i o n o f a C0 molecule. The l i n e a r C0 m o l e c u l e has t h r e e p o s s i b l e o r i e n t a t i o n s w i t h r e s p e c t t o the a p p l i e d magnetic f i e l d , two o f which have i d e n t i c a l c h e m i c a l s h i f t s . The s p e c t r a l d i s p e r s i o n which a r i s e s from a c o l l e c t i o n o f randomly o r i e n t e d C0 molecules, as i n a f r o z e n s o l i d , i s a broad l i n e w i t h t h e doubly degenerate c h e m i c a l s h i f t s (and hence g r e a t e r i n t e n s i t y ) on the downf i e l d (10) s i d e , as i l l u s t r a t e d by the dashed c u r v e i n F i g u r e TT I f i n s t e a d o f a f r o z e n s o l i d t h e C0 molecules f i n d themselves i n an environment where t h e y can execute a n i s o t r o p i c r o t a t i o n , a very d i f f e r e n t chemical s h i f t d i s p e r s i o n i s observed. As shown by t h e drawing i n F i g u r e 1, r o t a t i o n about one o f the C axes Q

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Figure 1. Some theoretical spectra observed in the NMR of adsorbed carbon dioxide: a random array of nonrotating CO molecules produces a chemical shift dispersion (dashed curve) with two principal values for the chemical shift tensors (σ parallel to the molecular axis and σ ,σ perpendicular to it). With rotation about the x-axis the C0 molecule experiences anisotropic rota tion. This rotation leaves σ unchanged but averages σ and a to < σ > (solid curve). The areas beneath the two curves are not shown to scale. Isotropic rotation of the CO molecule averages the chemical shift dispersion to its isotropic value, σ< (delta function, broken line). g

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p e r p e n d i c u l a r to the molecular a x i s leaves the chemical s h i f t f o r one o r i e n t a t i o n u n c h a n g e d , b u t a v e r a g e s t h e o t h e r two p r o d u c i n g a n a r r o w e r s p e c t r a l d i s p e r s i o n w i t h t h e d e g e n e r a t e c h e m i c a l s h i f t s o n t h e up f i e l d side (solid curve). w i t h r o t a t i o n about the other C s y m m e t r y a x i s , t h e CO m o l e c u l e a s s u m e s i s o t r o p i c o r " f r e e " r o t a t i o n and the c h e m i c a l s h i f t d i s p e r s i o n c o l l a p s e s to the d e l t a f u n c t i o n , as mentioned e a r l i e r . The f o u r t h c h a r a c t e r i s t i c l i n e s h a p e w h i c h c a n be e n c o u n t e r e d i n t h e NMR o f s o l i d s i s o n e i n w h i c h a l l three o r i e n t a t i o n s of a molecule w i t h r e s p e c t to the a p p l i e d magnetic f i e l d have unique c h e m i c a l s h i f t s . T h i s a r i s e s from m o l e c u l e s w h i c h have lower than a x i a l symmetry (nonsymmetric i n t h e m o l e c u l a r c o o r d i n a t e s y s t e m ) o r f r o m a x i a l l y s y m m e t r i c m o l e c u l a r whose a n i s o t r o p i c r o t a t i o n i s not s u f f i c i e n t to average the p r i n c i p l e c h e m i c a l s h i f t v a l u e s (σ and σ in Figure 1). These s p e c t r a l d i s p e r s i o n s a r é r c h a r a c t e r i z e d by t h e i r " t e n t " shape h a v i n g the g r e a t e s t i n t e n s i t y b e t w e e n t h e two c h e m i c a l s h i f t e x t r e m e s . In the f o l l o w i n g d i s c u s s i o n o f the e x p e r i m e n t a l r e s u l t s we h a v e f o u n d i t u s e f u l t o r e f e r t o t h e various chemical s h i f t dispersion spectra acronymically. There are four general lineshapes encountered i n t h e NMR o f s o l i d s ; S y m m e t r i c , A x i a l l y s y m m e t r i c w i t h t h e d e g e n e r a c y on the L e f t , A x i a l l y s y m m e t r i c w i t h t h e d e g e n e r a c y on the R i g h t , and t h e " t e n t " shaped N o n Symmetric chemical s h i f t d i s p e r s i o n . T h u s , S , A L , AR a n d NS d e s c r i b e t h e s p e c t r a l s h a p e . In a d d i t i o n , the s p e c t r a l d i s p e r s i o n s may b e a s s o c i a t e d w i t h m o l e c u l e s which are n o n r o t a t i n g (NR), a n i s o t r o p i c a l l y r o t a t i n g (AR) o r i s o t r o p i c a l l y r o t a t i n g ( I R ) . A combination o f b o t h d e s c r i p t o r s fonts the complete acronym f o r the s h a p e a n d o r i g i n o f t h e NMR s p e c t r a . For example, the s p e c t r a d i s c u s s e d i n F i g u r e 1 may b e r e f e r r e d t o a s S ( I R ) , A L ( N R ) a n d AR(AR) s p e c t r a respectively. Since determination of the molecular motions depends on t h e c a r e f u l a n a l y s i s o f t h e b r o a d s p e c t r a l d i s p e r s i o n s a r i s i n g from molecular chemical s h i f t anisotropy, e v e r y e f f o r t s h o u l d b e made t o e l i m i n a t e unwanted s o u r c e s o f l i n e b r o a d e n i n g and s p e c t r a l overl a p f r o m m o l e c u l e s w h i c h have more t h a n one r e s o n a n c e . A s m e n t i o n e d p r e v i o u s l y , r a r e - s p i n NMR h a s t h e a d v a n tage of m i n i m i z i n g homonuclear d i p o l a r i n t e r a c t i o n s , a n d t h e u s e o f i s o t o p i c a l l y e n r i c h e d s a m p l e s may s i m p l i f y t h e s p e c t r a t o t h a t o f o n l y one n u c l e a r resonance. As i n the work p r e s e n t e d h e r e , the c a r b o n - 1 3 NMR o f c a r b o n - c o n t a i n i n g i n o r g a n i c g a s e s a c t u a l l y p r o v i d e the most s t r a i g h t f o r w a r d and a c c e s s i b l e experiment since only single resonance


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e x p e r i m e n t s n e e d t o be p e r f o r m e d . Investigations of a d s o r b e d o r g a n i c v a p o r s and l i q u i d s r e q u i r e C n u c l e a r magnetic double resonance techniques to e l i m i n a t e heteronuclear dipolar interactions. 1 3


Experimental

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The s o d i u m m o r d e n i t e m o l e c u l a r s i e v e s were p r e p a r e d from h i g h p u r i t y r e a g e n t s by L e o n a r d B. Sand a t W o r c e s t e r P o l y t e c h n i c I n s t i t u t e and c o n t a i n e d 1 0 - 1 7 0 ppm i r o n . The x - r a y d i f f r a c t i o n p a t t e r n s i n d i c a t e d t h e s a m p l e s t o b e 90-95% m o r d e n i t e w i t h t h e r e m a i n d e r as a n a l c i m e . The m o r d e n i t e s were o f t h e small-port variety? t h e y a d s o r b e d 15% b y w e i g h t o f C 0 a t 1 a t m . a n d 0.1% b y w e i g h t o f b e n z e n e a t 72 mm a n d room t e m p e r a t u r e . I o n - e x c h a n g e d m o r d e n i t e s were p r e p a r e d by the c o n v e n t i o n a l p r o c e d u r e from 1 molar salt solutions. A p p r o x i m a t e l y 1/2 g r a m s a m p l e s o f t h e m o l e c u l a r s i e v e w e r e p l a c e d i n 10 mm O . D . NMR t u b e s and d r i e d vacuo u n d e r a programmed t e m p e r a t u r e r i s e to 3 0 0 ° C . ( T h e " a m m o n i u m e x c h a n g e d s i e v e was shown t o l o s e ammonia o n l y a b o v e 4 1 5 ° C b y D T A . ) After cooling, t h e s i e v e s w e r e a l l o w e d t o a d s o r b 90% i s o t o p i c a l l y enriched C gases to the d e s i r e d l e v e l . The l o a d i n g l e v e l was d e f i n e d a s a w e i g h t p e r c e n t o f t h e capacity o f t h e s i e v e a t room t e m p e r a t u r e and 1 atm. (300 mmHg for C S ) . The t u b e s were s e a l e d w i t h a T e f l o n p l u g and V i t o n o - r i n g w i t h o u t e x p o s u r e t o t h e atmosphere and were u s e d r e p r o d u c i b l y o v e r s e v e r a l months. 2

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The F o u r i e r t r a n s f o r m C-NMR s p e c t r a were o b t a i n ed on a B r u k e r s p e c t r o m e t e r , e q u i p p e d w i t h a b r o a d b a n d r e c e i v e r and q u a d r a t u r e d e t e c t o r (11), and o p e r a t i n g a t 22.6 MHz, w i t h f i e l d s t a b i l i z a t i o n p r o v i d e d by an external time-share F l o c k (11). The e x p e r i m e n t s d e s c r i b e d h e r e can be p e r f o r m e c T o n any c o m m e r c i a l i n s t r u m e n t , w h i c h i s e q u i p p e d w i t h an e x t e r n a l field l o c k , and which i s f r e e from b a s e l i n e a r t i f a c t s (12). 1 3

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The C-NMR s p e c t r a of i s o t o p i c a l l y enriched C0 a d s o r b e d i n v a r y i n g amounts on t h e N a * m o r d e n i t e a r e shown i n F i g u r e 2 . Three d i f f e r e n t s p e c t r a l lineshapes appear i n t h i s s e r i e s i n d i c a t i n g t h a t the a d s o r b e d CO^ e x i s t s i n a t l e a s t t h r e e d i s t i n c t s t a t e s (or a d i s t r i b u t i o n o f s t a t e s ) , p r e s u m a b l y d i f f e r i n g i n t h e i r r o t a t i o n a l freedom. A n a l y s i s of the resonance lineshape f o r each of these s t a t e s should provide i n f o r m a t i o n about the l o c a l s i t e geometry which i n f l u e n c e s C0 adsorption. 1 3

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A t the h i g h e s t l o a d i n g l e v e l (upper spectrum i n F i g u r e 2) a n a r r o w , r e l a t i v e l y weak s y m m e t r i c l i n e appears s u p e r i m p o s e d upon the b r o a d c h e m i c a l s h i f t dispersion. We b e l i e v e t h i s n a r r o w l i n e i s d u e t o C 0 m o l e c u l e s which are f r e e l y r o t a t i n g and t r a n s l a t i n g i n the large channel areas of the mordenite. (The l a r g e c h a n n e l i s c o m p r i s e d o f 12-member r i n g s p e r p e n d i c u l a r to the c r y s t a l l o g r a p h i c c - a x i s . ) This conclusion is s u b s t a n t i a t e d b y . t h e o b s e r v a t i o n of narrow l i n e resonances f o r C0 a d s o r b e d i n t h e z e o l i t e s Y (13) and L i n w h i c h t h e m i n i m u m i n t e r n a l p o r e d i m e n s i o n s Π-3 a n d 7.1 Â, r e s p e c t i v e l y ) are g r e a t e r t h a n the van der Waals l e n g t h o f the C0 m o l e c u l e (5.1 A ) . The i n t e n s i t y o f t h e s e narrow symmetric l i n e s (which r e f l e c t the a f f i n i t y of C0 f o r t h e s e s i t e s ) i n c r e a s e s as t h e p o r e d i mensions d e c r e a s e to match the d i m e n s i o n s o f the C 0 . These r e s u l t s imply t h a t i n t e r c a l a t e d m a t e r i a l or r a n domly l o c a t e d c a t i o n s (14) f r e q u e n t l y reduce the large c h a n n e l s i z e to l e s s t h a n t h e l e n g t h o f t h e 0 0 m o l e c u l e . The second s p e c t r a l l i n e s h a p e w h i c h i s p r e s e n t i n F i g u r e 2 i s b e s t seen at low l o a d i n g l e v e l s . At the 40% l e v e l o n l y a b r o a d c h e m i c a l s h i f t d i s p e r s i o n o f type AL(NR)(see text) i s observed, i n d i c a t i n g t h a t the adsorbed C 0 molecules are not allowed to r o t a t e i n their absorption sites. M e i e r (15), in his definitive c r y s t a l s t r u c t u r e a n a l y s i s of the Na -mordenite, identi f i e d a s e r i e s of s m a l l e r channels opening i n the b d i r e c t i o n w h i c h a r e c i r c u m s c r i b e d b y 8-member r i n g s h a v i n g a f r e e a p e r t u r e o f 2.9 χ 5.7 Â. The s m a l l c h a n n e l s do n o t , h o w e v e r , i n t e r c o n n e c t w i t h t h e n e x t main c h a n n e l but r a t h e r branch through d i s t o r t e d 8-member r i n g s t o w a r d s s i m i l a r a r e a s i n t h e a d j o i n i n g large channel. T h e s e d i s t o r t e d 8-member r i n g s , w h i c h have a f r e e a p e r t u r e of o n l y 2.8 Â, i s o l a t e the main c h a n n e l s , l e a v i n g t h e m l i n e d w i t h two r o w s o f s i d e pockets. Meier found t h a t four sodium c a t i o n s per u n i t c e l l r e s i d e d i n the c e n t e r of these d i s t o r t e d 8-member r i n g s (the l o c a t i o n o f t h e o t h e r f o u r c o u l d n o t be determined). When t h e s o d i u m c a t i o n s a r e e x c h a n g e d for l a r g e r c a t i o n s s u c h a s Cs+ o r NH+ t h e s e r e m a i n i n t h e s i d e - p o c k e t s , b e i n g u n a b l e t o assume a p o s i t i o n i n t h e d i s t o r t e d 8-member r i n g (16).


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To d e t e r m i n e w h e t h e r t h e a d s o r p t i o n s i t e s r e s p o n s i b l e f o r the n o n r o t a t i n g C0 m o l e c u l e s were the s i d e - p o c k e t s , we e x a m i n e d t h e e f f e c t o f c a t i o n e x c h a n g e on t h e C NMR o f a d s o r b e d C 0 . When t h e s o d i u m c a t i o n s were e x c h a n g e d , f o r m i n g the C s - and N H m o r d e n i t e t h e r e was n o e v i d e n c e o f n o n r o t a t i n g C 0 at any l o a d i n g l e v e l . Thus, our r e s u l t s are consistent w i t h the s i d e - p o c k e t s b e i n g the f i r s t a d s o r p t i o n s i t e s 2

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RESONANCE

C02 adsorbed ort

,S


Na^ "small port" morcknitt

-

lOOpprrv

Figure 2. Carbon-13 NMR spectra of CO adsorbed on small-port Na*-mordenite as a function of loading level t


11.

SEFCIK

E TA L .

Small-Port

Mordenite

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to be f i l l e d i n t h e N a i m o r d e n i t e and a l s o p r o v i d i n g the most s e v e r e r o t a t i o n a l h i n d e r a n c e . T h i s phenomena was f i r s t r e c o g n i z e d b y B a r r e r a n d P e t e r s o n (17) i n t h e i r study o f i n t e r c r y s t a l l i n e a d s o r p t i o n by s y n t h e t i c mordenites. They reasoned t h a t the f i r s t m o l e c u l e s adsorbed should p r e f e r e n t i a l l y occupy the s i d e - p o c k e t s since these s i t e s o f f e r e d the highest coordination between the adsorbed m o l e c u l e and t h e a n i o n i c oxygens. We h a v e o b s e r v e d t h a t C O , C 0 a n d COS a r e r e a d i l y adsorbed i n the side-pockets of the Naimordenite but e x c l u d e d from t h e s e s i t e s i n t h e C s - and NH+-mordenites. The resonance a r i s i n g from m o l e c u l e s i n t h e s i d e - p o c k e t s i t e s i s narrowed at h i g h e r l o a d i n g l e v e l by c o l l i s i o n i n d u c e d r o t a t i o n . The b r o a d r e s o n a n c e p r o d u c e d b y C 0 a t h i g h l o a d i n g l e v e l s i s o f t h e t y p e AR(AR). Since C 0 has a x i a l s y m m e t r y , a r e s o n a n c e o f t h i s shape must b e due t o anisotropic rotation. The a n i s o t r o p i c r o t a t i o n o f m o l e c u l e s r e s p o n s i b l e f o r t h e AR(AR) s p e c t r a i s b e s t c h a r a c t e r i z e d as a r o t a t i o n i n a s i n g l e p l a n e w i t h a f r e q u e n c y g r e a t e r t h a n 10** H e r t z . Anisotropic r o t a t i o n a l b e h a v i o r o f adsorbed s p e c i e s on m o r d e n i t e s has been suggested i n t h e l i t e r a t u r e . Gabuda and c o - w o r k e r s (18,19) have i d e n t i f i e d t h r e e t y p e s o f z e o l i t e w a t e r b a s e d o n t h e p r o t o n -NMR l i n e w i d t h : (1) r i g i d l y b o u n d , (2) a n i s o t r o p i c a l l y m o b i l e , a n d (3) i s o t r o p i c a l l y m o b i l e . Water a d s o r b e d on m o r d e n i t e and o t h e r c h a n n e l s i e v e s s u c h as c h a b a z i t e , huelandite a n d l a u m o n t i t e was f o u n d t o b e a n i s o t r o p i c a l l y m o b i l e . The m o t i o n a l b e h a v i o r o f c a r b o n d i o x i d e a d s o r b e d on s y n t h e t i c mordenite has been s t u d i e d by T a k a i s h i , e t a l . (20). These authors argued t h a t the s t r o n g e l e c t r i c q u a d r a p o l e moment o f C 0 i n t e r a c t s w i t h t h e c r y s t a l f i e l d i n the channel s i e v e , causing s e r i o u s l y hindered rotation. Based on s t a t i s t i c a l mechanics, T a k a i s h i f e l t t h a t at low coverage adsorbed C0 would b e r i g i d l y h e l d b e l o w 5 0 ° C a n d become a n a n i s o t r o p i c r o t o r above t h a t t e m p e r a t u r e . Our r e s u l t s , at high coverage, i n d i c a t e t h a t C0 d i s p l a y s a n i s o t r o p i c r o t a t i o n even a t room t e m p e r a t u r e . E v i d e n c e w i l l be o f f e r e d below t o suggest t h a t s t e r i c r a t h e r than electronic e f f e c t s are responsible f o r t h i s behavior.


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The a d s o r p t i o n s i t e s i n t h e m o r d e n i t e w h i c h permit only a n i s o t r o p i c motion of adsorbed C 0 are not located near the z e o l i t i c c a t i o n s . When s y n t h e t i c N a - m o r d e n i t e was c o n v e r t e d t o t h e C s a n d N H ^ f o r m s , t h e AR(AR) s p e c t r a r e m a i n e d u n c h a n g e d , i n d i c a t i n g t h a t these l a r g e r cations d i d not i n h i b i t the a n i s o t r o p i c r o t a t i o n of the adsorbed gases. We c o n c l u d e , theref o r e , t h a t t h e a d s o r b e d C 0 w h i c h p r o d u c e s t h e AR(AR) 2

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must be l o c a t e d i n the o c c l u d e d a r e a s o f t h e m o r d e n i t e main c h a n n e l . O v e r t h e r a n g e o f c o v e r a g e s t u d i e d h e r e we h a v e found evidence f o r n o n r o t a t i n g , a n i s o t r o p i c a l l y rotati n g and i s o t r o p i c a l l y r o t a t i n g adsorbed s p e c i e s a t room t e m p e r a t u r e . The s e r i e s o f s p e c t r a i n F i g u r e 2 suggest the f o l l o w i n g sequence o f e v e n t s : at the l o w e s t c o v e r a g e CO m o l e c u l e s a r e preferentially adsorbed i n the s i d e - p o c k e t s which l i n e the main c h a n n e l and are h e l d w i t h l i t t l e r o t a t i o n a l freedom. When a l l o f t h e s i d e - p o c k e t s i t e s a r e f i l l e d , adsorpt i o n occurs i n the occluded or smallest s e c t i o n s of the main c h a n n e l . I n i t i a l l y , the molecules which occupy these s i t e s are r e l a t i v e l y immobile but, w i t h increasing concentration, intermolecular collisions induce r o t a t i o n i n a s i n g l e plane of the small channel where t h e c o o r d i n a t i o n number w i t h r e s p e c t t o a n i o n i c o x y g e n s i s somewhat l e s s t h a n i n t h e side-pockets. F i n a l l y , as t h e s e s i t e s a r e f i l l e d , a d s o r p t i o n b e g i n s i n §ome l a r g e p o r e a r e a s o f t h e m o r d e n i t e , presumably the unblocked r e g i o n s o f the main c h a n n e l . The a f f i n i t y of the C0 for these large channel s i t e s i s low, c o n s i s t e n t w i t h t h e low c o o r d i n a t i o n number and relatively unrestricted rotational behavior. T h e ^ C - N M R s p e c t r a o f C S a n d COS a d s o r b e d o n t h e N a - m o r d e n i t e a r e shown i n F i g u r e 3, a g a i n a s a function of loading Jevel. Due t o t h e l a r g e r c r i t i c a l d i a m e t e r o f C S ( 3 . 6 A) o n e w o u l d n o t e x p e c t a d s o r p t i o n to occur i n the s i d e - p o c k e t s , a n d i n d e e d , t h e r e was n o A L (NR) t y p e c h e m i c a l s h i f t d i s p e r s i o n i n d i c a t i v e o f t h i s s i t e , even at the lowest d e t e c t a b l e l o a d i n g levels. T h i s c o n c l u s i o n i s f u r t h e r s u p p o r t e d by experiments w i t h multicomponent systems. When t h e N a + - m o r d e n i t e was l o a d e d t o t h e 40% l e v e l w i t h C0 # and t h e n a l l o w e d t o a d s o r b CS to i t s capacity, the C-NMR spectrum (which d e t e c t s o n l y the C0 ) i n d i c a t e d t h a t none o f t h e C 0 had been e x c l u d e d from the s i d e - p o c k e t s . In a c o n t r o l experiment, the a d d i t i o n of C0 t o a sample p a r t i a l l y f i l l e d w i t h ^0 r e s u l t e d i n complete scrambling o f the l a b e l ; the NMR s p e c t r u m was s i m i l a r t o t h a t o b t a i n e d w i t h f u l l y loaded ^0 , albeit less intense. A t a l l l o a d i n g l e v e l s t h e NMR s p e c t r a o f C S a d s o r b e d on t h e m o r d e n i t e e x h i b i t t h e AR(AR) spectral l i n e s h a p e i n d i c a t i v e o f an a n i s o t r o p i c a l l y rotating a x i a l l y symmetric m o l e c u l e . As i n the c a s e o f C O 2 , the a d s o r p t i o n s i t e which permits t h i s r o t a t i o n is b e l i e v e d t o be t h e o c c l u d e d a r e a s o f t h e m a i n c h a n n e l . I t i s i n t e r e s t i n g t o n o t e t h a t t h e NMR s p e c t r a o f C S e x h i b i t much more a r e a ( i . e . , c o n c e n t r a t i o n ) i n t h e 2

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narrow symmetric l i n e than was o b s e r v e d f o r C 0 o r , as can be seen i n F i g u r e 3, COS. The i n c r e a s e d i n t e n s i t y of t h e narrow l i n e component on g o i n g from CO t o COS to CS suggests the l a r g e r molecules p r e f e r e n t i a l l y adsorb i n t h e s e l a r g e volume s i t e s . The i n c r e a s e d a f f i n i t y o f COS and CS i n t h e s e a r e a s may r e f l e c t e i t h e r a h i g h e r c o o r d i n a t i o n w i t h the z e o l i t i c oxygens or the g r e a t e r p o l a r i z a b i l i t y o f the a d s o r b a t e s (21). The NMR s p e c t r a f o r adsorbed c a r b o n y l s u l f i d e , shown i n F i g u r e 3, have the g r e a t e s t i n t e n s i t y t o the l e f t o f the spectrum a t a l l l o a d i n g l e v e l s . A t low coverage the c h e m i c a l s h i f t d i s p e r s i o n i s o f the t y p e AL(NR), s i m i l a r t o t h a t o b s e r v e d f o r C0 a t low coverage. To e s t a b l i s h the a d s o r p t i o n s i t e r e s p o n s i b l e f o r t h i s spectrum we a g a i n t u r n t o r e s u l t s o f e x p e r i ments on multicomponent systems. When t h e N a mordenite was f i l l e d t o the 40% l e v e l w i t h C0 ( s u f f i c i e n t t o f i l l the s i d e - p o c k e t s ) , and then exposed to an atmosphere o f COS, the r e s u l t i n g spectrum i n d i c a t e d t h a t the l a b e l e d carbon d i o x i d e had been e x c l u d e d from the s i d e - p o c k e t s and f o r c e d i n t o the l e s s f a v o r a b l e a d s o r p t i o n s i t e s i n the l a r g e c h a n n e l area. A p p a r e n t l y , the s m a l l end o f the c a r b o n y l s u l f i d e i s a b l e t o f i t i n t o the s i d e - p o c k e t s where i t i s p r e f e r e n t i a l l y adsorbed i n c o m p e t i t i o n w i t h C 0 . At h i g h e r l o a d i n g l e v e l s the NMR o f COS does n o t s h i f t t o the AR(AR)type s p e c t r a n o t e d f o r C 0 and C S . S i n c e the c a r b o n y l s u l f i d e i s i n t e r m e d i a t e i n s i z e between CO, and CS we would have e x p e c t e d a d s o r p t i o n i n the o c c l u d e d a r e a s o f the l a r g e c h a n n e l which l e a d s to a n i s o t r o p i c r o t a t i o n . To u n d e r s t a n d t h i s apparent anomaly i t i s n e c e s s a r y t o r e f e r t o the i l l u s t r a t i o n s of the c h e m i c a l s h i f t d i s p e r s i o n i n F i g u r e 1. For the l i n e a r symmetric m o l e c u l e CO ( p o i n t group D. ) a r o t a t i o n o f 90° i s s u f f i c i e n t t o average the C h e m i c a l s h i f t t e n s o r s and produce the AR(AR) t y p e spectrum. C a r b o n y l s u l f i d e , which i s an asymmetric l i n e a r m o l e c u l e ( p o i n t group C ï r e q u i r e s a f u l l 180° r o t a t i o n t o produce the same r e s u l t . Based on t h e i r van der Waals r a d i i , C 0 , CO§ and CS have m o l e c u l a r l e n g t h s o f 5.1, 6.0 §nd 6.6 A and c r i t i c a l d i a m e t e r s of 2.8, 3.6 and 3.6 A, r e s p e c t i v e l y . The main c h a n n e l a p e r a t u r e o f t h e N a - m o r d e n i t e , which i s d e f i n e d by 12-member r i n g s p e r p e n d i c u l a r t o the c - a x i s , measures 6.7 χ 7.0 Â {15) . The s m a l l - p o r t mordenite, however, does n o t adsorb m o l e c u l e s w i t h c r i t i c a l d i a m e t e r s l a r g e r than 4 Â . Our r e s u l t s i n d i c a t e t h a t CS can a p p a r e n t l y execute a t l e a s t 90° r o t a t i o n s i n the main c h a n n e l o f t h e s m a l l - p o r t m o r d e n i t e , b u t t h a t COS can not r o t a t e a f u l l 180°. T h i s s u g g e s t s t h a t the 2


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CSi Figure 3.

Carbon-13 N M R specfra o/ CS

S

(left) and COS (right)

adsorbed on small-port Ή a'-mordenite as a function of loading level

!5

C0 adsorbed, ort "smalt por t " mordauxc

Figure 4. Carbon-13 NMR spectra of CO adsorbed on small-port Νa*-mordenite as a function of loading level

NaT" mordcniXfc


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e f f e c t i v e c h a n n e l dimensions are reduced t o a p p r o x i mately 4 χ 5.5 Â by o c c l u s i o n s . T h i s c h a n n e l s i z e f o r the s m a l l - p o r t mordenite i s a l s o s u p p o r t e d by s t u d i e s w i t h carbon monoxide. The C-NMR s p e c t r a o f CO adsorbed on N a - m o r d e n i t e are shown i n F i g u r e 4. A t a l l l o a d i n g l e v e l s the s p e c t r a l d i s p e r s i o n remains o f the type A L ( N R ) . Carbon monox i d e a b s o r p t i o n i n the s i d e - p o c k e t s was a g a i n v e r i f i e d by examining the s p e c t r a o f CO when i t was adsorbed on the i o n - e x c h a n g e d m o r d e n i t e s . With C s - and N H ^ mordenite the C - s p e c t r a was r e l a t i v e l y n a r r o w , symmetric, and weak w i t h an i n t e n s i t y o n l y s l i g h t l y g r e a t e r than observed f o r gaseous CO a t one atmosphere. The absence o f any i n d i c a t i o n o f main c h a n n e l a d s o r p t i o n on the i o n - e x c h a n g e d s i e v e s was unexpected. Since C0 i s r e a d i l y adsorbed i n t o t h e s e a r e a s we c o n c l u d e t h a t the m o l e c u l a r dimensions o f carbon monoxide a r e s u f f i c i e n t l y s m a l l e r than the s m a l l - p o r t c h a n n e l s i z e so t h a t no s i g n i f i c a n t a d s o r p t i o n o c c u r s . This i s c o n s i s t e n t w i t h the reduced c a p a c i t y o f the N a mordenite f o r CO compared t o CO . The r e s u l t s p r e s e n t e d here demonstrate the i n f o r m a t i o n which can be o b t a i n e d c o n c e r n i n g the dynamic s t a t e o f adsorbed s p e c i e s by r a r e - s p i n NMR. By j u d i c i o u s c h o i c e o f p r o b i n g g a s e s , t h i s t e c h n i q u e not o n l y a l l o w s one t o view each adsorbed s t a t e e s s e n t i a l l y independent o f the o t h e r s but i s a l s o u s e f u l i n d e f i n i n g the g e o m e t r i c a l c o n s t r a i n t s o f the v a r i o u s a d s o r p tion sites. Other NMR p a r a m e t e r s , such as r e l a x a t i o n r a t e s o r temperature dependence o f the s p e c t r a l l i n e shape, may be u s e f u l f o r d e t e r m i n i n g d i f f u s i o n coefficients, exchange r a t e s , jumping mechanisms and the e n e r g e t i c s o f each a d s o r b a t e - a d s o r b e n t i n t e r a c t i o n . 1


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Acknowledgment The a u t h o r s w i s h t o thank Leonard B . Sand f o r h i s a s s i s t a n c e i n p r e p a r i n g the s y n t h e t i c mordenites used i n t h i s study and f o r h i s v a l u a b l e d i s c u s s i o n and criticism.

Literature Cited 1. 2.

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M A G N E T I C

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Magnetic Resonance in Colloid and Interface Science - American

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