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A High Resolution Silicon-29 N M R and Neutron Powder Diffraction Study of Na-A Zeolite: Loewenstein's Rule Vindicated J. M. BENNETT and C. S. BLACKWELL Union Carbide Corporation, Tarrytown, NY 10591 D. E. COX Brookhaven National Laboratory, Physics Department, Upton, NY 11973
The structure of dehydrated Na-A zeolite has been re-examined by magic angle spinning Si-NMR and neutron powder diffraction techniques. Thisisin light of recent proposals by B u r s i l l etal.,that the Si:Al ordering is different from the simple alternating scheme generally accepted, and violates Loewenstein's rule. The Si-NMR chemical shifts observed for Na-A zeolite and a series of nitrogeneous-A zeolites with varying Si:Al contents demonstrate conclusively that in the former each S i is surrounded by four A l and no S i near neighbors. This 4:0 ordering scheme is consistent with previous X-ray structure determinations and does not violate Loewenstein's rule. Neutron diffraction data confirm the results of B u r s i l l et a l . that Na-A zeolite has rhombohedral symmetry at 4.5K and 296K. However, there is a transition to the cubic symmetry at about 335K. Structural analyses at selected temperatures between 4.5K and 603K were carried out by Rietveld refinement of the neutron data. Above the transition, the results are in good agreement with recent X-ray single crystal refinements by Pluth and Smith. Below the transition, constrained refinement strongly indicates that ordering of Na atoms in partially occupied sites takes place, with a consequent lowering of the symmetry to at least R3c. There is no necessity for the 3:1 ordering scheme proposed by B u r s i l l et a l . to be invoked to account for this lowering of symmetry. 29
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0097-6156/83/0218-0143$06.00/0 © 1983 American Chemical Society Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 24, 2018 | https://pubs.acs.org Publication Date: May 17, 1983 | doi: 10.1021/bk-1983-0218.ch009
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Since the original determination of the crystal structure of Linde zeolite 4A (_1), the refinement of the structure of the hydrated variety in space group Fm3c with a unit c e l l edge of 24.6A instead of space group Pm3m with a pseudo-cell edge of 12.3A, (2) showed that the S i and Al alternate in the framework. Further accurate single crystal X-ray determinations of the structures of potassium and sodium zeolites by Pluth and Smith, (3» 4) contained alternating tetrahedra with mean tetrahedral cation-oxygen distances of about I.60X and 1.738, consistent with occupation by S i and A l respectively. In a series of recent papers the structure of zeolite A has been re-examined with particluar reference to the S i and A l ordering, and a different arrangment has been proposed,(5, j?,) based mainly upon results from three different techniques, electron diffraction, C5, 7) Si-NMR(5, 7_ 8) and neutron diffraction, (5, 9) andisat variance with Loewenstein s rule.(10) Magic angle spinning (MAS) Si-NMR is a valuable tool for revealing details of the structural environment in s i l i c a t e s , aluminosilicates and zeolites(11, 12) and can give valuable information about the Si:Al distribution in any zeolite framework. I t is essential i f the data are to be used for evaluation of Si:Al ordering that the Si-NMR lines be quantitative such that their respective areas are proportional to the number of S i atoms of each type. A correct assignment of the identity of each line to i t s S i type is also crucial. In the case of Linde A zeolite, which has a chemical Si:Al ratio of unity, i t was originally concluded from Si-NMR results that A1-0-A1 linkages are present, (_7, 8)inviolation of Loewenstein*s rule. This finding was based upon the observation of a S i chemical s h i f t of -88.9 ppm which was assigned to an Si:Al ordering scheme of 3:1. A similar conclusion was reached on the basis of neutron powder diffraction experiments, which revealed a small rhombohedral distortion at 290 K and 5 K, and were interpreted in terms of a 3:1 ordering scheme with R3 symmetry. (5) This rather unexpected result stimulated a number of other investigations, including the present one, with the conclusion that the 4:0 ordering scheme is correct.(13) In addition, the NMR and neutron diffraction data reported in reference(5) have been re-interpreted in favor of 4:0 rather than 3:1 order,(14, 15) and i t is concluded that the rhombohedral distrotion observed in the neutron pattern is strongly dependent upon the identity of the exchangeable cations. However, in an independent neutron study of Na-A zeolite(16), Adams and Haselden reported no evidence for such a distortion, and conclude that the symmetry must depend subtly upon the method of preparation. The present work was undertaken to investigate the structure of Na-A zeolite in more detail, particularly with respect to the 29
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Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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Loewenstein's Rule Vindicated
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S i : A l o r d e r i n g . Si-NMR experiments have been c a r r i e d out on a s e r i e s o f nitrogeneous-A z e o l i t e s and show c l e a r l y t h a t the -88.9 ppm s h i f t in Na-A z e o l i t e should be assigned t o S i surrounded by f o u r A l near neighbors. I n a d d i t i o n , neutron powder d i f f r a c t i o n data have been c o l l e c t e d as a f u n c t i o n o f temperature between 4.5 and 600 K, and these are a l s o c o n s i s t e n t w i t h the 4:0 o r d e r i n g scheme. S t r u c t u r a l a n a l y s i s by the R i e t v e l d refinement technique (17) r e v e a l s t h a t the rhombohedral symmetry is a consequence of o r d e r i n g o f the Na i o n s in 8-rings a t a temperature of about 335K. Experimental 29
NMR Experiments Si-NMR s p e c t r a were obtained on a Bruker CXP-200 s o l i d s t a t e h i g h Dower and h i g h r e s o l u t i o n spectrometer equipped w i t h the Bruker * C-CPMAS accessory. Chemical s h i f t s are r e p o r t e d r e l a t i v e t o t e t r a m e t h y l s i l a n e (TMS), t o an accuracy b e t t e r than can be j u s t i f i e d from the l i n e - w i d t h s (about 2.5 ppm) in these m a t e r i a l s . The sample s p i n n i n g r a t e was u s u a l l y between 3 and 4 kHz, and no s p i n n i n g s i d e bands were observed. A t y p i c a l spectrum was acquired w i t h Bloch decays e x c i t e d by 4 usee pulses separated by 10 second recovery d e l a y s , and the data should g i v e a reasonably q u a n t i t a t i v e estimate of the S i content. Previous work on other types o f z e o l i t e s has demonstrated the importance o f checking f o r complete r e l a x a t i o n i f the s p e c t r a are t o be used f o r q u a n t i t a t i v e s t u d i e s . The samples used were a standard Na-A z e o l i t e and f i v e nitrogeneous types of z e o l i t e A, o r N-A.(18) The N-A z e o l i t e s are s i l i c e o u s analogues o f z e o l i t e A, synthesized w i t h t e t r a m e t h y l ammonium c a t i o n . The S i : A l r a t i o v a r i e d from 0.94 (NaA) t o 3.54 f o r the most s i l i c e o u s N-A sample.t The r a t i o s were determined by wet chemical a n a l y s i s , and the s t r u c t u r e type and absence of i m p u r i t y phases were confirmed by X-ray powder d i f f r a c t i o n techniques. A d s o r p t i o n measurements (oxygen, -183 C) showed a z e o l i t e A content o f g r e a t e r than 90%. 3
Neutron D i f f r a c t i o n Experiments The sample o f Na-A z e o l i t e used in the neutron d i f f r a c t i o n s t u d i e s was prepared by the standard technique and had a S i : A l r a t i o s l i g h t l y g r e a t e r than u n i t y (1.06), corresponding t o the composition Nag.9781^^3^0.97 O4. The sample was dehydrated by s l o w l y heating in a i r in a shallow d i s h up t o 300°C, maintained a t 300°C f o r 16 hours, r a p i d l y t r a n s f e r r e d to a d e s i c c a t o r and allowed t o c o o l . This procedure does not y i e l d a completely anhydrous m a t e r i a l . The sample was f i n a l l y loaded i n t o a c y l i n d r i c a l aluminum h o l d e r under an atmosphere o f dry helium. tThese samples were f u r n i s h e d by Miss E.M. Flanigen's Group, M o l e c u l a r Sieve Department, Engineering Products D i v i s i o n , Union Carbide C o r p o r a t i o n , Tarrytown, New York 10591
Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.
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Data were collected with 2. 385& neutrons from the (002) reflection of a pyrolytic graphite monochromator and an analyzing crystal at two different settings, at various times over a period of several months. The analyzing crystal yields a significant improvement in peak-to-background ratios. In both cases a pyrol y t i c graphite f i l t e r was used to remove higher order wavelengths. The angular step interval was 0.05°. Downloaded by UNIV OF MASSACHUSETTS AMHERST on May 24, 2018 | https://pubs.acs.org Publication Date: May 17, 1983 | doi: 10.1021/bk-1983-0218.ch009
Results and Discussion 29
Si-NMR. samples are:
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The Si-NMR chemical shifts observed for various Table I
A 0.94
Si:Al ratio from chemical analysis 29
Sample No. B C 1.32 1.54
D 1.79
E 2.73
F 3.54
Si-NMR Chemical Shifts (6)
Assignments 1
Sioif * Q**(4:0) Q (3:1)
