Magic-Angle Spinning NMR (MASNMR) - American Chemical Society

10

Downloaded by UNIV OF TENNESSEE KNOXVILLE on May 19, 2016 | http://pubs.acs.org Publication Date: May 17, 1983 | doi: 10.1021/bk-1983-0218.ch010

New Approaches to the Structural Characterization of Zeolites: Magic-Angle Spinning N M R (MASNMR) J. M . THOMAS, J. KLINOWSKY, S. RAMADAS, and M . W. ANDERSON University of Cambridge, Department of Physical Chemistry, Cambridge CB2 1EP, England C. A. F Y F E and G . C. GOBBI University of Guelph, College of Physical Science, Guelph-Waterloo Centre for Graduate WorkinChemistry, Guelph, Ontario N1G 2W1, Canada

The advantages and successes achieved by the application of Si and Al high-resolution solid state NMR to the study of zeolites are briefly reviewed. In particular the ease with six-fold coordination of either Si or Al is distinguished from tetrahedrally coordinated analogues is illustrated. Si MASNMR can readily distinguish crystallographically distinct Si(OSi) groups in highly siliceous solids such as silicalite; it enables the framework Si/Al ratio to be quantitatively and conveniently determined; it yields the number of neighbouring Al atoms bound to a central S i , via oxygen, and this, in turn, throws light on Si,Al ordering. Specific ordering schemes can thus be proposed for zeolites X and Y covering a range of Si/Al ratios. Si MASNMR measurements on zeolite ZK-4 confirm that the Si spectrum of zeolite A points to there being s t r i c t alternation of SiO and AlO tetrahedra in this framework when the Si/Al ratio is unity. Al MASNMR shows that the Al present in silicalite is bonded tetrahedrally and possesses an isotropic chemical shift close to that of Al in ZSM-5. A few results, and the structural conclusions to which they lead of Cs-, Ga- and C- MASNMR are also presented. 29

27

29

4

29

29

4

4

27

133

71

17

Physicists have long been aware of the power and usefulness of high-resolution solid-state NMR. Even prior to the development of cross-polarization, in the early to mid-seventies, it was clear that, by recording the nuclear magnetic resonance spectrum when the sample was spun at the magic angle (5** M ) to the magnetic f i e l d , chemical shifts could be readily identified 1

0097-6156/83/0218-0159$06.50/0 © 1983 American Chemical Society Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

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(1) using sol id samples. This meant that structural information, akin to that routinely inferred from solution ( H and 13c NMR) measurements by organic chemists, could now be derived from spectra obtained with polycrystal1ine and non-crystal 1ine specimens . The chemi cal shi ft values are characteristic of crystal lographically and chemically distinct envi ronments. An admi rable summary of the advances achieved in the study of non-zeolitic sol ids using MASNMR, with nuclei other than °Si and 7 A 1 , has been given recently by Andrew ( 2 ) , the discoverer of the technique. On the basis of the pioneering work of Lippmaa et aj[ (3) on 9 s i MASNMR, and that of Engelhardt et al_ (k) on 9 $ i NMR of si 1icate solutions, i t became apparent that *9$i chemical shifts are sensitive to the substituents present in the second coordination sphere. Correlations between chemical shi ft and structure were established, and these were soon uti1ized (5~7) for the structural studies of both soluble and insoluble si 1icates and aluminosi1icates. A wide variety of zeoli t i c sol ids has by now been investigated (6-18) using ^ S i , 7A1 and several other nuclei. The degree of structural insight achievable with MASNMR is exceptional, since i t probes the immediate envi ronment of a particular nucleus (provided i t has a f i n i t e magnetogyric ratio) more or less i rrespective of whether the sample i tself is crystal 1ine or amorphous. A measure of the practical value of this approach'is reflected by the fact that most of the major commercial manufacturers and users of aluminosi1icates have recently undertaken MASNMR studies for research and development purposes (12, 19). The scepticism expressed in some quarters as to the validity of the approach seems to have evaporated, and the techniques of *9Si and'A1 MASNMR have now gained general acceptance. ~q From the outset, i t was clear that Si MASNMR resonances were, to some degree, structure-sensitive. The correlation charts of Lippmaa et al (5, 9) and our own (7, 10) show a range of values at which a ^3si nucleus in a particular envi ronemnt (Si(0A1)^_ (OSi) with n = 0, 1, 2, 3, or k) was expected to resonate: ?his point emerges clearly from Table 1. In this respect MASNMR is similar to IR, UV and Raman and solution NMR spectroscopies for which there are comparable charts that are constructed, as in 9 s i MASNMR, sol id-state spectroscopy, largely on the basis of accumulated practical experience, rather than on theoretical calculations. q Although good MASNMR " $ i spectra may be obtained at ('H) frequencies of 90, 200, 250 and 300 MHz, we have found (17) the advantages that accrue from uti1izing the highest fields to be most valuable. Improvements in sensitivity alone justify the recording of spectra under these condi tions. 1


2

2

2

2

2

2

2

2

?

Studies of Faujasitic Zeoli tes (Zeolites X and Y) The general usefulness and novelty of

S i , and, to a lesser

Stucky and Dwyer; Intrazeolite Chemistry ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

THOMAS

ET AL.

Magic-Angle Spinning NMR TABLE 1

29 Ranges o f

^ S i MASNMR C h e m i c a l

silicates

Shifts

Primary Coordination

Notation

R e s u l t s o f the Tal1in-Berlin Group » 3


for Zeolitic

Alumino

(TMS a s r e f e r e n c e )

b

R e s u l t s o f the Guelph-Cambridge Group » » c

d

e

Al 0

AlOSiOAl

Si(4A1)

-83

to

- 8 7

-80

to

-90.5

Si(3A1)

-88

to

- 9 4

-88

to

- 9 7

Si(2A1)

-92

to

- 9 9

-93

t o -102

-97

to

-105

-97.5

to

-103

to

-101.5

to

0

Al

Al 0

AlOSiOAl 0

Si

Al 0

SiOSiOAl 0

Si

Al 0

SiOSiOSi

s r d AI

)

-107

0

Si

Si 0

SiOSiOSi

Si(0A1)

-111*

-116.5

0

Si

E. Lippmaa, A. Magi, G. Samoson, G. E n g e l h a r d t , A.-R.Grimmer, J . Amer. Chem. S o c , 1 0 2 , 4 8 8 9 ( 1 9 8 0 ) . G. Engelhardt, U. Lohse, E. Lippmaa, M. Tarmak, M. Magi, Z. Anorg. A l l g . Chem., 4 8 2 , 4 9 ( 1 9 8 1 ) . c

J . K l i n o w s k i , J . M. Thomas, C. A. Fyfe and J . S. Hartman, J . Phys. Chem., 8 5 , 2 5 9 0 ( 1 9 8 1 ) .

^ J . M. Thomas, C. A. F y f e , S. Ramdas, J . K l i n o w s k i and G. C. Gobbi, J . Phys. Chem., 8 6 , 3 0 6 1 ( 1 9 8 2 ) . e

C. A. F y f e , J . M. Thomas, J . K l i n o w s k i and G. C. Gobbi, Angewandte Chemie ( i n p r e s s ) .


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2

degree, 7A1, MASNMR in the study of zeoli tes is perhaps best i1lustrated by ci t ing the results obtained with and conclus ions drawn from faujasitic zeolites (8, 9, 11, 17). When we embarked on these studies they were intended to serve both as a test of the validity of the MASNMR approach and as a vehicle for probing hitherto uncharted depths in the structural and analytical chemistry of zeolites. Our studies proceeded independently of, and essentially simultaneously with, those of Engelhardt et a l . Our results concur in al1 major respects: they also agree in most principal respects with those of subsequent studies (12). The advantages in selecting faujasitic zeolites merit enumeration here. (i)

F i r s t , the Si/Al ratio may be altered (by suitable choice of conditions of growth) in smal1 increments between the extrema of 1.0 and 3-0 while the framework structure — the topology and geometry of the T-0 network (T = Si/Al) in tetrahedral coordination — remains unchanged, This enables the spectral characteristics to be monitored as a function of Si/Al ratio, thereby permitting a test of the sensitivity of a given 9 s i envi ronmental resonance upon compos i t ion. 11 al so throws 1ight on the question of Si,A1 ordering (see below) as this is reflected in the intensity ratios of the respective Si(nAl) peaks (n = 1, . . . k being the number of Al's tetrahedrally 1 inked, via oxygen, to a central S i ) . 2

(ii)

Second, the enigmatic, and catalytically important process of ultrastabi1ization in which zeolite Y is converted (20) by suitable heat treatment, to an ultrastable form, may be directly investigated.

(i i i)

Thi rd, the consequences of vapour phase dealumination, effected by using gaseous SiCl^, and to a less effective degree by gaseous PCI^, may be explored.

(iv)

Simple, but useful, point-charge calculations of electrostatic repulsions (between " A l * ions wi thin the aluminosi1icate framework) are readily carried out (11) with the faujasite structure since, in this structure, unlike those of many others, zeolites and porotectos i l i c a t e s , there is only one type of tetrahedral s i t e . + M

(v)

Zeol i t i c gallosi1icates possessing the faujasi te (as well as the sodalite and analcite) structure may be readily prepared so that, in principle, by recording the appropriate 9 s i , ? A 1 and ?'Ga MASNMR spectra information may be obtained pertaining to Si,A1 and Si,Ga ordering schemes. 2

2

Some of our results have been presented and interpreted elsewhere others are s t i l l in the course of assessment. A few



10.

THOMAS

ET AL.

163

Magic-Angle Spinning NMR

t y p i c a l s p e c t r a , a l o n g w i t h c o m p u t e r - s i m u l a t e d a n a l o g u e s (based on G a u s s i a n peak s h a p e s ) a r e shown in F i g u r e 1, f r o m w h i c h i t is s e e n t h a t t h e r e is a g r a d u a l e v o l u t i o n in t h e p a t t e r n o f t h e p e a k s . The 4:0 peak ( i . e . S i ( O A l ) , o t h e r w i s e d e s i g n a t e d S I ( 4 A 1 ) ) d o m i n a t e s a t low S i / A l ; i n d e e d a t S i / A l = 1.0, t h e r e is b u t a s i n g l e peak, c e n t r e d a t -84.0 p p m w . r . t . t e t r a m e t h y l s i l a n e (TMS). These f a c t s d i s c l o s e t h a t t h e r e is s t r i c t S i , A l a l t e r n a t i o n in z e o l i t e X o f c o m p o s i t i o n 1.0 a n d , t h a t L o e w e n s t e i n ' s r u l e (21) w h i c h f o r b i d s t h e o c c u r r e n c e o f A1-0-A1 l i n k a g e s in t e t r a h e d r a l l y bonded a l u m i n o s i 1 i c a t e s is o b e y e d . Many o t h e r c o n c l u s i o n s a l s o stand out. They a r e a s f o l l o w s : 29 (a) Whenever two o r more r e s o l v a b l e S i ( n A l ) peaks o c c u r in S i MASNMR s p e c t r a ( w i t h n_ ? z e r o ) , i t is p o s s i b l e t o compute q u a n t i t a t i v e l y f r o m t h e peak i n t e n s i t i e s t h e S i / A l r a t i o o f t h e a l u m i n o s i 1 i c a t e ( i . e . o f t h e t e t r a h e d r a l l y bonded f r a m e w o r k ) p r o v i d e d t h a t L o e w e n s t e i n ' s r u l e is o b e y e d . As t h i s r u l e is o b e y e d , i t f o l l o w s t h a t e a c h S i - O - A l l i n k a g e in a S i ( n A l T s t r u c t u r a l u n i t s is e q u i v a l e n t t o A l / 4 atoms so t h a t t h e S i / A l r a t i o in t h e t e t r a h e d r a l l y bonded a n i o n i c framework is g i v e n b y : k (Si/Al)

=

0 Q

2 9

Si

MASNMR

I

I S

n=0

- / a i \ / £

c

(

« ^

A 1

> /n-0

0.25 I . / a H ' ^ ° C

S

(

i \

A

29 w h e r e ^51 ( A l ) * ^ i n t e n s i t y f o r t h e peak in w h i c h a c e r f t r a l S i atom is s u r r o u n d e d by nAl and (4-rO S i atoms v i a o x y g e n l i n k a g e s . T h i s e q u a t i o n is e x c e p t i o n a l l y u s e f u l and r e l i a b l e ( 9 , 1 1 ) . Inter a l i a , i t enables a d i r e c t d e t e r m i n a t i o n o f t h e framework S i / A l r a t i o , unencumbered by u n c e r t a i n t i e s i n t r o d u c e d ( a s t h e y i n v a r i a b l y a r e in w e t c h e m i c a l and o t h e r n o n - d i s c r i m i n a t o r y techniques) by t h e p r e s e n c e o f e i t h e r o c c l u d e d a l u m i n o u s c a t i o n s , d e t r i t a l s i l i c a and o t h e r i m p u r i t i e s s u c h a s s i l i c a - a l u m i n a gels o r c r i s t o b a l i t e . N e i t h e r X-ray f l u o r e s c e n c e nor n e u t r o n a c t i v a t i o n a n a l y s i s , b e c a u s e e a c h is g l o b a l , in t h e s e n s e t h a t i t encompasses a l l t h e atoms p r e s e n t i r r e s p e c t i v e of t h e i r s t a t e o f chemical combination, possesses the d i s 2 c r i m i n a t o r y a n a l y t i c a l power o f 9si MASNMR. I n f o r m a t i o n a b o u t S i , A l o r d e r i n g in t h e a l u m i n o s ? 1 i c a t e f r a m e w o r k , emerges f r o m t h e q u a n t i t a t i v e v a l u e s o f t h e S i ( n A l ) i n t e n s i t i e s ( s u c h a s t h o s e shown in F i g u r e 1 ) . I t t r a n s p i r e s t h a t t h e t e n d e n c y t o o r d e r is l e s s p r o n o u n c e d t h e higher the value o f the S i / A l r a t i o . By m a t c h i n g t h e comp u t e d i n t e n s i t i e s f o r a p o s s i b l e o r d e r i n g scheme w i t h t h o s e o b s e r v e d t a k i n g due r e c o g n i t i o n o f o t h e r c o n s t r a i n t s , s u c h as u n i t c e l l r e p e a t d i s t a n c e s and c r y s t a l l o g r a p h i c s p a c e g r o u p , we may a r r i v e a t p l a u s i b l e o r d e r i n g schemes f o r a given Si/Al r a t i o . T h u s , w h i l e b o t h o r d e r i n g schemes a r e s t a n c

s

o r

t

n

e

n

(b)


INTRAZEOLITE

CHEMISTRY


Si/A)

-80

-90

-100

-110

Figure 1. A selection

of the high resolution

2

^ S i MASNMR s p e c t r a

o f s y n t h e t i c NaX and NaY a t 79-80 MHz.

The l e f t - h a n d

columns show t h e o b s e r v e d s p e c t r a , t h o s e on t h e r i g h t a r e t h e c o m p u t e r - s i m u l a t e d a n a l o g u e s , wi t h t h e d e c o n v o l u t i o n drawn as d o t t e d 1ines. S i ( n A l ) peaks a r e i d e n t i f i e d by t h e £ above t h e peak.



10.

THOMAS

ET AL.

165


compatible with S i MASNMR s p e c t r a , we f a v o u r t h e model shown in F i g u r e 2 ( a ) o v e r t h a t in F i g u r e 2 ( b ) , a s t h e f o r m e r r e t a i n s t r a n s l a t i o n a l p e r i o d i c t y between s o d a l i t e c a g e s A and C o r B a n d D w h e r e a s t h e l a t t e r does n o t . As t h e S i / A l r a t i o i n c r e a s e s f u r t h e r t h e t e n d e n c y f o r o r d e r i n g o f t h e S i and A l in t h e framework d i m i n i s h e s p r o gressively. T h e r e a r e even i n d i c a t i o n s t h a t , beyond S i / A l o f c a 2.0, a random model ( s u b j e c t t o t h e p r o v i s o o f L o e w e n s t e i n ' s r u l e ) is a p p l i c a b l e . In t h i s c o n n e c t i o n i t is n o t e w o r t h y t h a t , in a sample o f 1 7 - y e a r o l d s y n t h e t i c f a u j a s i t e w h i c h had c r y s t a l l i z e d o u t a t room t e r m p e r a t u r e f r o m a n u t r i e n t a l u m i n o s i 1 i c a t e g e l p r e p a r e d by Dr. C. V. M c D a n i e l o f t h e W. R. G r a c e S c o . , no s t r o n g p r e f e r e n c e for a well-defined, Si/Al ratio (representing 'integral' v a l u e s o f S i and A l r a t i o s in t h e u n i t c e l l was o b s e r v e d (Figure 3 ) . (c)

MASNMR s p e c t r a y i e l d i n f o r m a t i o n p e r t a i n i n g t o t h e c o o r d i n a t i o n o f A l in z e o l i t e s s i n c e t h e s h i f t s corresponding t o 4- a n d 6- c o o r d i n a t i o n a r e w i d e l y s e p a r a t e d f r o m o n e a n o t h e r ( s e e F i g u r e 4 ) . T h i s f a c t is t u r n e d t o good a d v a n t a g e in m o n i t o r i n g t h e p r o g r e s s o f d e a l u m i n a t i o n o f z e o l i t e s e i t h e r by e x p o s u r e t o S i C l , v a p o u r ( 2 2 , 23) o r by h y d r o t h e r m a l t r e a t m e n t . I d e n t i f y i n g t h e c o o r d i n a t i o n o f A l in t h i s way is a much more s e n s i t i v e t e c h n i q u e t h a n o t h e r a v a i l a b l e methods s u c h a s t h o s e b a s e d on s h i f t s in A1K3 X - r a y e m i s s i o n l i n e s (24) o r on XPS measurements ( 2 5 ) .

(d)

Our knowledge o f t h e a t o m i c r e a r r a n g e m e n t s t h a t t a k e p l a c e during theprocess o f u l t r a s t a b i 1 i z a t i o n o f z e o l i t e Y (see refs 1 4 , 20) h a s been g r e a t l y e n l a r g e d by t h e a p p l i c a t i o n 2 o f 9si and *7A1 MASNMR. F i g . 5 shows how i t h a s p r o v e d p o s s i b l e t o m o n i t o r t h e changes in t h e e n v i r o n m e n t o f S i and t h e c o o r d i n a t i o n o f A l d u r i n g t h e c o u r s e o f d e a l u m i n a tion. S i m i l a r , b u t l e s s d r a m a t i c changes have been o b s e r v e d in s t u d i e s o f t h e d e a l u m i n a t i o n o f m o r d e n i t e by e x p o s u r e t o S i C l ^ ( 2 6 ) . The b u i l d up o f a s i x - c o o r d i n a t e d a l u m i n i u m is s e e n c l e a r l y when m o r d e n i t e is s u b j e c t e d t o p r o l o n g e d e x p o s u r e t o S i C I . a t h i g h t e m p e r a t u r e . L i k e w i s e , when P C I v a p o u r is used (27) in p l a c e o f S i C l ^ ( f o r t h e d e a l u m i n a t i o n o f z e o l i t e s Y) we s e e c l e a r e v i d e n c e , f r o m ^7/\j MASNMR, o f t h e removal o f a l u m i n i u m f r o m t h e framework and a b u i l d - u p o f s i x c o o r d i n a t e d a l u m i n i u m . The r e s u l t i n g p h o s p h o r u s r i c h f a u j a s i t i c z e o l i t e is, however, r a t h e r l e s s o r d e r e d t h a n t h e samples c o r r e s p o n d i n g l y d e a l u m i n a t e d by S i C I . ( F i g . 6). * S t u d i e s o f Z e o l i t e s A and ZK-4 2 9

The e a r l i e s t i n d i c a t i o n s ( 5 , 6 , 2 8 ) f r o m S i MASNMR w e r e t h a t , c o n t r a r y t o what had emerged f r o m X - r a y s t u d i e s , z e o l i t e A



166

INTRAZEOLITE

CHEMISTRY

Figure 2. S c h e m a t i c i l l u s t r a t i o n o f two p o s s i b l e S i , A 1 o r d e r i n g schemes f o r z e o l i t e X w i t h S i / A l = 1.4, b o t h c o m p a t i b l e w i t h t h e o b s e r v e d ^ S i MASNMR s p e c t r u m . Based on t h e o b s e r v e d i n t e n s i t i e s o f S i ( n A l ) peaks and o t h e r a r g u ments — s e e t e x t — we f a v o u r t h e f i r s t o f t h e s e two alternatives. 2



THOMAS ET AL.

167


A

I I

} I

-80

I

-90

I

-100

I

-110

Figure 3. 29 S i MASNMR s p e c t r u m o f a s a m p l e o f s y n t h e t i c Na-Y w h i c h had c r y s t a l l i z e d s l o w l y ( o v e r a p e r i o d o f 17 y e a r s ) a t room t e m p e r a t u r e . U s i n g t h e e q u a t i o n g i v e n in t h e t e x t we c a l c u l a t e t h e ( S i / A l ) framework t o be 2.28. T h i s does not c o r r e s p o n d t o a unique c o m p o s i t i o n , s o t h a t t h e framework must c o n s i s t o f p a t c h e s o r domains o f d i f f e r e n t l o c a l c o m p o s i t i o n ( e . g . o f S i / A l = 16/8, 17/7, 18/6 e t c . ) Detailed e l e c t r o n microscopic analyses (using microd i f f r a c t i o n and m i c r o a n a l y s i s ) may c l a r i f y t h e n a t u r e o f t h e S i , A l d i s o r d e r . I t is c l e a r , however, f r o m r e s u l t s o b t a i n e d t o d a t e t h a t t h e r e is no s t r o n g t e n d e n c y f o r S i , A 1 o r d e r i n g o t h e r w i s e a r a t i o o f 2.28 w o u l d n o t have been o b t a i n e d .


168

INTRAZEOLITE

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Na[AICI ]


4

ppm from TMS

ppm from TMS

Figure 4. D e a l u m i n a t i o n o f z e o l i t e NaY u s i n g S i C l , v a p o u r s t u d i e d by A 1 MASNMR a t 104.22 MHz. (a) P a r e n t NaY z e o l i t e . (b) D e a l u m i n a t e d m a t e r i a l b e f o r e w a s h i n g . (c) A f t e r w a s h i n g wi t h d i 1 u t e a c i d . (d) A f t e r r e p e a t e d w a s h i n g . Note t h a t t h e a l u m i n i u m j e t t i s o n e d f r o m t h e z e o l i t i c framework is f i r s t o f a l 1 bound t e t r a h e d r a l l y (as Na A l C l ^ ) b u t a f t e r w a s h i n g a d o p t s o c t a h e d r a l coordination. Note a l s o t h a t , as e x p e c t e d f r o m e l e c t r o n e u t r a l i t y c o n s i d e r a t i o n s , t h e o c t a h e d r a l peak i n t e n s i t y is c l o s e t o a t h i r d o f t h e t e t r a h e d r a l peak i n t e n s i t y . 7


10.

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ET

AL.

169


Si(2Al) Si(IAI)


Si(3Ai;

100

ppm from TMS

0

ppm from [AMH^II*

Figure 5. 2 9

2 7

High-resolution S i (79-80 MHz) and A 1 (104.22 MHz) MASNMR s t u d i e s o f u l t r a s t a b i 1 i z a t i o n o f z e o l i t e Y. (a) P a r e n t NH^-Na-Y z e o l i t e . (b)

A f t e r c a l c i n i n g in a i r f o r 1 h r a t 400°C. Note changes in relative intensities. Si(nAl) A f t e r h e a t i n g t o 700 C f o r 1 h r in t h e p r e s e n c e to steam. A f t e r repeated i o n exchange, h e a t i n g and prolonged leaching w i t h n i t r i c acid. This m a t e r i a l is e v i d e n t l y v e r y c r y s t a l l i n e ( s e e r e f (14) f o r a f u l l e r d i s c u s s i o n ) . L

(c) (d)



170

150

INTRAZEOLITE

100

50

0

-50 3

ppm from [AI(HO)]* a

6

-100 -60

-80

-100 m , r o m

PP

™

CHEMISTRY

-120

-140

s

Figure 6. (a)

2 7

(b)

2

(c)

3 1

(d)

A l MASNMR o f PCI - t r e a t e d Na-Y. The ' o c t a h e d r a l peak a t c a 0.0 ppm s i g n i f i e s t h a t d e a l u m i n a t i o n is e f f e c t e d by PCI 9 s i MASNMR o f P C l ^ - t r e a t e d Na-Y, washed w i t h DMSO. The b r o a d e n e d l i n e i m p l i e s some l o s s o f crystallinity. P MASNMR o f Na-Y a f t e r e x p o s u r e t o P C I (without washing).

3

a t 200°C

P MASNMR o f Na-Y a f t e r e x p o s u r e t o P C I , a t 200°C and a f t e r w a s h i ng wi t h w a t e r .

3 1


1


10.

THOMAS

ET AL.

111


had a n o r d e r i n g scheme in w h i c h S i ( 3 A l ) p r e d o m i n a t e d . Subsequent work (10, 16) has shown t h a t t h e s i n g l e peak in t h e s o l i d - s t a t e spectrum o f z e o l i t e A a r i s e s from t h e S i ( 4 A l ) n o t t h e S i ( 3 A l ) environment. One p r o o f t h a t t h i s is s o comes f r o m measurements on a s e r i e s o f s a m p l e s o f z e o l i t e ZK-4 where t h e a l u m i n o s i 1 i c a t e framework t o p o l o g y is e x a c t l y t h e same a s i t is in z e o l i t e A, b u t t h e S i / A l r a t i o may v a r y f r o m 1.0 t o 2.7J u s t as t h e e v o l u t i o n o f t h e s p e c t r a in f a u j a s i t i c z e o l i t e s ( s e e F i g . 1) e n a b l e d us t o p i n p o i n t t h e peaks c o r r e s p o n d i n g t o t h e f i v e d i s t i n c t e n v i r o n m e n t s ( s e e T a b l e 1), s o a l s o do o u r e x p e r i m e n t s w i t h ZK-4 l e a d t o s i m i l a r c o n c l u s i o n s . Our r e s u l t s ( F i g . 7) l e a v e l i t t l e doubt t h a t t h e S i ( 4 A l ) peak in t h i s f a m i l y o f z e o l i t e s is c e n t r e d a t -88.6 ppm w . r . t . TMS. A r e s o n a n c e a t t h i s v a l u e o f t h e c h e m i c a l s h i f t in f a u j a s i t i c and most o t h e r z e o l i t i c s t r u c t u r e s corresponds t o a Si(3A1) environment. 2q A d d i t i o n a l c o n f i r m a t i o n t h a t t h e peak a t -88.5+1.5 in t h e S i r e s o n a n c e f o r v a r i o u s c a t i o n - e x c h a n g e d forms o f z e o l i t e - A (Na+, K+, A g , T1 + , C a , M g , B a , La3+) is c o r r e c t l y a s s i g n e d t o S i ( 4 A l ) comes a l s o f r o m o u r NMR measurements o f ^ O - e x c h a n g e d Na -A. As o n l y o n e e n v i r o n m e n t is d e t e c t e d (29) f o r t h e ^ 0 i t must f o l l o w t h a t t h e r e a r e no s i g n i f i c a n t q u a n t i t i e s o f A1-0-A1 o r S i - O - S i bonds in z e o l i t e - A . A l - 0 - S i bonds c l e a r l y p r e d o m i n a t e . Neutron d i f f r a c t i o n ( R i e t v e l d refinement) a l s o c o n f i r m s t h e 4:0 o r d e r i n g in z e o l i t e A (30, 31). The r e a s o n why z e o l i t e A h a s a r e s o n a n c e c o r r e s p o n d i n g t o S i ( 4 A l ) a t -88.5+1.5 stems f r o m t h e u n i q u e f e a t u r e o f t h e s t r u c t u r e : s t r a i n e d d o u b l e four-membered r i n g s , w i t h T-O-T a n g l e s o f 129°, 1 5 2 ° , 152° and 177°. We b e l i e v e t h a t t h e p r e s e n c e o f l i n e a r o r n e a r l y l i n e a r T-O-T l i n k a g e s m o d i f i e s t h e b o n d i n g c h a r a c t e r i s t i c f o r a c e n t r a l S i atom in a S i ( 0 A 1 ) ( O S I ) . unit (10, 3 2 ) . I t is v e r y 1 i k e l y t h a t t h e r e ar^e o t h e r z e o l i t e s in w h i c h t h e c h e m i c a l s h i f t c o r r e s p o n d i n g t o t h e S i ( O A l ) ^ g r o u p o c c u r s c l o s e t o -90 ppm. When Na-A is h e a t e d t o beyond 675°C a marked s h i f t is n o t e d in t h e S i ( 4 A l ) peak. ( A n o t h e r , r e l a t i v e l y m i n o r peak a l s o d e v e l o p s ) - s e e F i g . 8. S e p a r a t e d i f f r a c t i o n e x p e r i m e n t s i n d i c a t e t h a t Na-A is c o n v e r t e d p r i n c i p a l l y t o c a r n e g i e i t e , a c l o s e r packed s t r u c t u r e . When Cs-A is h e a t e d in t h i s f a s h i o n i t , t o o , shows a marked s h i f t in t h e ^ S i r e s o n a n c e . ^33cs MASNMR measurements i n d i c a t e t h a t t h e r e a r e s i g n i f i c a n t changes in c a t i o n e n v i r o n m e n t b r o u g h t a b o u t by h e a t i n g : t h e main ^33cs peak a t -89-3 (ppm r e l a t i v e t o aqueous Cs^CO-) in t h e a s p r e p a r e d Cs-A s o l i d changes t o -137 ppm a f t e r h e a t i n g t o 700 C and t o -221.5 a f t e r a f u r t h e r 8 h r a t 800°C. C o n c e i v a b l y t h e ' • b u r i a l " o f C s c a t i o n s by h e a t - t r e a t m e n t o f C s - e x c h a n g e d z e o l i t e A p r o c e e d s in b r o a d l y t h e same f a s h i o n a s t h e b u r i a l o f C s i o n s in p h i 1 l i p s i t e w h i c h , on h e a t i n g t o v e r y h i g h t e m p e r a t u r e s , t a k e s up a C s - f e l d s p a r s t r u c t u r e . +

2 +

2 +

2 +

+

n

2

+

+

+



172

INTRAZEOLITE CHEMISTRY

SK3AI)

-80

-100 ppm

Figure

from

-120 TMS

7.

S i MASNMR a t 79.8 MHz o f z e o l i t e ZK-4 s h o w i n g f i v e peaks a t t h e c h e m i c a l s h i f t v a l u e s i n d i c a t e d c o r r e s p o n d i n g t o t h e s i l i c o n e n v i r o n m e n t s in t h e f i g u r e . Compare F i g . 1 ( f o r z e o l i t e s X and Y) where t h e S i ( 3 A 1 ) peak has a c h e m i c a l s h i f t a t t h e v a l u e c o r r e s p o n d i n g t o Si(4A1) here.

2 9



10.

I 80

THOMAS

ET

I -90

AL.


I -100

I -80

I -90

Figure 8. 29 S i MASNMR s p e c t r a o f z e o l i t e Na-A a f t e r h e a t - t r e a t m e n t a t (a) 450°C and (b) 675°C. in (b) t h e z e o l i t e A has been c o n v e r t e d t o c a r n e g i e i t e (as i d e n t i f i e d by d i f f r a c t i o n measurements). The S i ( 4 A 1 ) resonance has s h i f t e d s i g n i f i c a n t l y d u r i n g t h i s c o n v e r s i o n and a n o t h e r r e s o n a n c e has a l s o a p p e a r e d .


I -100

174

INTRAZEOLITE

CHEMISTRY

Zeolitic Ga11osi1icates 11 is r e l a t i v e l y e a s y t o r e p l a c e A l i o n s in z e o l i t i c framework by G a 3 . Kuhl e t aj_ (33) showed t h a t d i s c o n t i n u i t i e s in u n i t e e l 1 v e r s u s S i / G a p l o t s w e r e more d i s t i n c t t h a n c o r r e s p o n d i n g ones in u n i t c e l l v e r s u s S i / A l p l o t s f o r t h e a n a l o g o u s s t r u c t u r e s ( z e o l i t e s X and Y in p a r t i c u l a r ) . T h i s is c l e a r l y i n d i c a t i v e o f t h e presence o f Si,Ga o r d e r i n g . T h i s is one r e a s o n why MASNMR measurements on g a l l o s i 1 i c a t e s p o s s e s s i n g open f r a m e w o r k s a r e t i m e 1 y . A n o t h e r is bound up w i t h t h e g e n e r a l u n d e r s t a n d i n g o f t h e f a c t o r s r e s p o n s i b l e f o r the observed magnitude o f 29si MASNMR c h e m i c a l s h i f t s . As may be s e e n f r o m F i g u r e 9 ( a ) wel1-resolved peaks a r e o b t a i n e d wi t h Ga-Y, b u t t h e * ' A 1 s p e c t r u m ( F i g . 9 ( b ) ) , o f t h e r e s i d u a l A l , in 1 i n e w i t h o t h e r 27A1 MASNMR peaks (3*0 is, in r e l a t i v e t e r m s , l e s s i n f o r m a t i v e . L i k e w i s e , g a l l o s o d a l i t e ( F i g . 9 ( c ) ) shows a r i c h , i n f o r m a t i v e 9 s i s p e c t r u m , b u t t h e G a MASNMR s p e c t r u m ( F i g . 9 ( d ) ) is r a t h e r too broad f o r ready i n t e r p r e t a t i o n . We a r e c u r r e n t l y engaged in e x p l o r i n g f u r t h e r t h e s t r u c t u r a l p r o p e r t i e s o f g a l l o z e o l i t i c s t r u c t u r e s u s i n g m u l t i - n u c l e a r MASNMR s p e c t r o s c o p y . In F i g u r e 9 ( c ) t h e f i r s t peak ( a t - 6 4 . 8 ppm) a r i s e s f r o m an impuri t y . The most p r o m i n e n t peak a t - 7 5 . 5 c o r r e s p o n d s t o t h e S i ( O G a ) ^ g r o u p , t h a t a t -81.8 t o S i ( O G a L ( O S i ) , a n d s o o n . The c o r r e c t n e s s o f t h i s a s s i g n m e n t is p r o v e n by t h e f a c t t h a t t h e S i / G a r a t i o , c a l c u l a t e d a s d e s c r i b e d e a r l i e r f r o m MASNMR i n t e n s i t i e s , is f o u n d t o be 1.23, in e x c e l l e n t agreement wi t h t h e c h e m i c a l l y d e t e r m i n e d r a t i o ( a l s o 1.23). ( T h e r e was no d e t e c t a b l e A l in t h i s sample o f g a l l o s o d a l i t e , k i n d l y p r o v i d e d by D r . M. B a r l o w ) . 3

+


+

2

7 1

Six-coordinated Si 1 icon In a few n a t u r a l l y o c c u r r i n g s i 1 i c o n - c o n t a i n i n g s t r u c t u r e s i t is b e l i e v e d t h a t t h e r e is s i x - f o l d c o o r d i n a t i o n o f S i . S t i s h o v i t e (a h i g h p r e s s u r e f o r m o f S i 0 « ) is o n e s u c h m a t e r i a l , thaumas i t e is a n o t h e r . Us i ng s a m p l e s o f s t i s h o v i t e r e t r i e v e d f r o m M e t e o r C r a t e r , A r i z o n a , we have f o u n d (35) t h a t t h e 29$ j MASNMR r e s o n a n c e is -191.3+0.2 ppm f r o m TMS ( F i g u r e 10). F o r c o m p a r i s o n q u a r t z h a s a r e s o n a n c e a t -107-4 ppm a n d l o w c r i s t o b a l i t e a t -109-9 ppm. R e c e n t l y Lippmaa e t al_ (36) have r e p o r t e d a ™ S i r e s o n a n c e a t -179.5 ppm f o r t h a u m a s i t e . C l e a r l y a b r o a d r a n g e o f s t r u c t u r e - s e n s i t i ve 29$; MASNMR c h e m i c a l s h i f t s e x i s t s f o r 6 - c o o r d i n a t e d , a s f o r 4 - c o o r d i n a t e d s i 1 i c o n ( s e e work o n s i 1 i c a l i t e r e p o r t e d b e l o w ) . H i g h p r e s s u r e is used i n c r e a s i n g l y as a means o f s y n t h e s i z i n g n o v e l s i 1 i c a t e s — t h e p r o d u c t i o n o f MgSiO- a s a p e r o v s k ? t e f r o m c l i n o e n s t a t i t e o r f o s t e r i t e is o n e r e c e n t n o t e w o r t h y e x a m p l e — and i t is 1 i k e l y t h a t , in t h e n e a r f u t u r e , a much w i d e r range o f s o l i d s c o n t a i n i n g 6 - c o o r d i n a t e d s i 1 i c o n w i 1 1 become a v a i l a b l e . In due c o u r s e , knowing t h e range o f chemi c a l s h i f t s t a k e n up by S i ( O S 1 ) ^ g r o u p s in s i i g h t l y d i f f e r e n t e n v i r o n m e n t s , °Si MASNMR s p e c t r o s c o p y w?11 s e r v e a s a 2



THOMAS

ET AL.

175


300

200 100 ppm (w.r.t. Ga(N0 ) ) 3

3

Figure 9. 29 2 7 ' MASNMR s p e c t r u m o f a s a m p l e o f Ga-Y. s

(a) (b)

A l MASNMR s p e c t r u m o f t h e r e s i d u a l

A l in t h e same

Ga-Y sample a sin( a ) . (c)

2

^ S i MASNMR s p e c t r u m o f a s a m p l e o f g a l l o s o d a l i t e . 2 7

(d)

(No A 1 MASNMR s i g n a l was d e t e c t a b l e w i t h t h i s sample). 71 Ga MASNMR s p e c t r u m o f t h e same g a l l o s o d a l i t e asin( c ) .



INTRAZEOLITE

CHEMISTRY

Figure 10. 29 S i MASNMR s p e c t r u m o f s t i s h o v i t e in w h i c h t h e s i l i c o n is in 6 - f o l d c o o r d i n a t i o n . Compare t h e p o s i t i o n o f t h e S i ( O S i ) , resonance seen h e r e w i t h t h e range o f v a l u e s f o u n d f o r S I ( O S I ) ^ in s i l i c a l i t e ( F i g . 1 1 ) .


10.

THOMAS

111


ET AL.

u s e f u l s t r u c t u r a l p r o b e f o r b o t h c r y s t a l l i n e (and q u a s i c r y s t a l l i n e ) s i l i c e o u s m a t e r i a l s , j u s t as f o r s i l i c a l i t e d i s c u s s e d below.


Detecting C r y s t a l l o g r a p h i c a l l y D i s t i n c t Tetrahedral Sites in S i l i c a l i t e : D e t e c t i o n o f A l u m i n i u m in T e t r a h e d r a l Coordination In many o f t h e u n c a l c i n e d s a m p l e s o f s i l i c a l i t e , k i n d l y p r o v i d e d by D r s . B. Nay and M. B a r l o w o f B.P. S u n b u r y , h i g h r e s o l u t i o n 9 S i MASNMR o f e x c e p t i o n a l q u a l i t y have been o b t a i n e d ( w i t h o u t r e s o l u t i o n e n h a n c e m e n t ) — s e e F i g u r e 11. A l l t h e peaks and s h o u l d e r s in t h i s s p e c t r u m c o r r e s p o n d t o d i s t i n c l y d i f f e r e n t S i ( O S i ) . e n v i r o n m e n t s . To a s c e r t a i n how many n o n - e q u i v a l e n t t e t r a h e d r a l l y bonded S i atoms t h e r e a r e in t h i s p o r o t e c t o s i 1 i c a t e we u s e t h e i n t e n s i t i e s o f t h e w e l 1 - r e s o l v e d l o w e s t and h i g h e s t f i e l d s i g n a l s as a b a s e u n i t f o r d e c o n v o l u t i o n . It transpires t h a t t h e r e a r e 2k n o n - e q u i v a l e n t S i ( O S i ) ^ e n v i r o n m e n t s in t h e repeat u n i t o f t h e s t r u c t u r e . Crystallographic determinations o f t h e s i l i c a l i t e s t r u c t u r e need t o t a k e a c c o u n t o f t h i s f a c t . A s p a c e g r o u p s u c h a s Pnma, f a v o u r e d in some c i r c l e s , is n o t c o m p a t i b l e w i t h t h e MASNMR s p e c t r u m . The o r i g i n a l d e s c r i p t i o n o f s i l i c a l i t e c l a i m s t h a t no a l u m i n i u m e x i s t s in t h e framework s t r u c t u r e . Our ? A 1 MASNMR s p e c t r a a r e i n c o n s i s t e n t w i t h such a c l a i m . 7/\] NMR s p e c t r a f o r b o t h a s t a t i o n a r y and a m a g i c - a n g l e - s p u n sample y i e l d a d o u b l e t . M o r e o v e r t h e p o s i t i o n o f t h e s e two peaks a r e c l o s e t o t h e p o s i t i o n o f t h e ? A 1 r e s o n a n c e o f numerous s a m p l e s o f ZSM-5 and w e l l removed f r o m t h e r e s o n a n c e o f C a g ( A l

Magic-Angle Spinning NMR (MASNMR) - American Chemical Society

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