Advances in The Structural Analysis of Zeolites, Zeolitic Precursors and Their Analogues

P.A. Jacobs and R.A. van Santen (Editors). Zeolites: Focfs, Fikyires, F u w e 0 1989 Elscvicr Scicncc Publishers B.V.. Amsterdam - Printed in The Netherlands

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ADVANCES I N THE STRUCTURAL ANALYSIS OF ZEOLITES, ZEOLITIC PRECURSORS AND T H E I R ANALOGUES

J.M.

THOMAS

Davy F a r a d a y R e s e a r c h L a b o r a t o r y , The Royal I n s t i t u t i o n , 2 1 A l b e m a r l e S t r e e t , London W1X 4 B S ( T h e U n i t e d Kingdom)

ABSTRACT The a c h i e v e m e n t and c o n t i n u i n g v a l u e of h i g h r e s o l u t i o n e l e c t r o n m i c r o s c o p y , e l e c t r o n d i f f r a c t i o n , SEM, n e u t r o n s c a t t e r i n g , s y n c h r o t r o n r a d i a t i o n and s o l i d - s t a t e NMR a s s t r u c t u r e - e l u c i d a t i n g t e c h n i q u e s a r e summarized. I n a s s e s s i n g t h e p o t e n t i a l r o l e o f new m e t h o d s o f a n a l y s i s e m p h a s i s i s p l a c e d upon e l e c t r o n - e n e r g y - l o s s s p e c t r o s c o p y , R u t h e r f o r d s c a t t e r i n g , c h e m i c a l l y - s e n s i t i v e s t r u c t u r e ( e l e c t r o n microscopic) imaging, m a g n e t i c ( e s p e c i a l l y d o u b l e r o t a t i o n ) r e s o n a n c e s p e c t r o s c o p y and f u r t h e r u s e s o f s y n c h r o t r o n r a d i a t i o n and n e u t r o n s . The a d v a n t a g e s o f p r o b i n g r e a d i l y p r e p a r e d , s i n g l e - c r y s t a l s p e c i m e n s o f model g u e s t - h o s t s y s t e m s t h a t s i m u l a t e Lhe b e h a v i o u r of z e o l i t e s a r e a l s o b r i e f l y d e s c r i b e d . INTRODUCTION

I t would he a m i s t a k e i f , i n t h i s s u r v e y , w e were t o r e s t r i c t o u r a t t e n t i o n t o q u e s t i o n s p o s e d o n l y by t h o s e p r a c t i t i o n e r s i n t e r e s t e d i n t h e u s e o f z e o l i t e s a s c a t a l y s t s , a d s o r b e n t s and i o n - e x c h a n g e r s .

A v o r t e x of a c t i v i t y has

a l r e a d y commenced c e n t r e d on t h e e x p l o i t a t i o n o f z e o l i t e s a n d o t h e r s o l i d m o l e c u l a r s i e v e s as m i c r o e l e c t r o d e s , n o v e l e l e c t r o c h e m i c a l b a t t e r i e s , s o l a r e n e r g y c o n v e r t e r s , a s w e l l a s quantum d o t a n d quantum w e l l d e v i c e s . a d d i t i o n , t h e somewhat a r c a n e areas o f o p t o - ,

photo-

In

and m o l e c u l a r - e l e c t r o n i c s

have, i n r e c e n t y e a r s , s p i l l e d over I n t o z e o l i t e s c i e n c e and technology. I n c r e a s i n g l y , t h e r e f o r e , o n e i s c a l l e d upon t o improve e x i s t i n g , a n d t o d e v e l o p new, s t r u c t u r a l p r o b e s s o t h a t t h e y may c o p e w i t h n a n o s c a l e , a l o n g w i t h macro amounts o f z e o l i t e : w e must p e r f e c t s t r u c t u r a l p r o c e d u r e s f o r c o p i n g w i t h f e m t o g r a m s o r less a t t h e one e x t r e m e , a n d w i t h gram o r k i l o g r a m q u a n t i t i e s a t the other. We s h a l l f i r s t assess t h e v i a b i l i t y a n d r e l a t i v e power o f t h o s e major s t r u c t u r a l t e c h n i q u e s t h a t h a v e e n t e r e d t h e a r e n a of z e o l i t e s c i e n c e o n l y w i t h i n t h e l a s t d e c a d e o r so.

(None o f t h e t e c h n i q u e s t h a t a r e c o n s i d e r e d

under t h i s heading w e r e , f o r example, d i s c u s s e d i n t h e s e v e r a l hundred p a p e r s p r e s e n t e d a t t h e F i f t h I n t e r n a t i o n a l Z e o l i t e Conference h e l d i n Naples i n

1980).

S e c o n d l y , w e s h a l l f o c u s on p r o m i s i n g new b u t n o t , as y e t , f u l l y

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t e s t e d , t e c h n i q u e s l i k e l y t o be of v a l u e t o t h e ever-expanding communtty of t h o s e s c i e n t i s t s i n t e r e s t e d in t h e s t r u c t u r e of n o v e l z e o l i t i c s o l i d s .

Lastly,

w e d w e l l b r i e f l y on some s p e c i a l s y s t e m s in which h o s t o r g a n i c frameworks a r e

c o n s i d e r e d a s models t h a t s i m u l a t e z e o l i t i c s t r u c t u r e s , and where a c o m b i n a t i o n of modern e x p e r i m e n t a l methods of s t r u c t u r a l a n a l y s i s a l o n g w i t h e a s e of p r e p a r a t i o n has proved e f f e c t i v e in d t s e n t a n g l t n g t h e m o t i o n a l and r e a c t i o n dynamics of o r g a n i c g u e s t s p e c i e s . KEY, STRUCTURAL TECHNIQUES OF THE 1980s: AN ASSESSMENT

Of t h e v a r i o u s e x p e r i m e n t a l t o o l s .deployed o n l y meagrely i f a t a l l by z e o l i t e s c i e n t i s t s p r t o r t o 1980, the key o n e s a r e h i g h - r e s o l u t i o n e l e c t r o n microscopy (HREM), n e u t r o n s c a t t e r i n g , s o l t d - s t a t e NMR and X-ray s c a t t e r i n g of synckotron radiation. Htgh r e s o l u t i o n e l e c t r o n microscopy It was my l o n g - s t a n d t n g i n t e r e s t ( r e f s . 1-3) tn t h e power of HREM as a

s t r u c t u r a l t o o l i n t n o r g a n i c s o l i d - s t a t e c h e m i s t r y and i n t h e s t u d y o f heterogeneous c a t a l y s t s t h a t f t r s t ( r e f s . 4 , s ) l e d me i n t o z e o l i t e s .

For a

long time i t was thought t h a t t h e beam i n s t a b t l i t y of z e o l i t e s i n a n e l e c t r o n mtcroscope would be a s e r i o u s drawback f o r t h e d t r e c t , r e a l - s p a c e s t u d y of

Fig. l a

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Fig. 1. (a) Early (1980) HRE micrograph of a doubly twinned zeolite-Y crystal viewed along [llo]. Only the main channels, separated by 17.4 8, are visible. ( b ) Present-day micrographs, like this one of ZSM-5 viewed along [OlO], reveal much greater detail. The nature of the subsidiary channels circumjacent to the main ones (e5 . 5 8, diameter) are sufficiently well resolved (see computed image at left) that the local symmetry of the projected structure may be evaluated. their projected structures. This is no longer felt to be the case. The essence of HREM is to "match" computed and observed images, under a variety of well-defined instrumental conditions (sample thlckness, electron wavelength, the spherical aberration coefficient, Cs, of the lens, etc (refs. 6,7)).

But nowadays the reverse is possible.

From a series o.f observed HREM

images one may infer the details of the structure that gives rise to them. This has been rendered posible not only by a logical developmeat of the intrinsic power of the technique since 1980, but by a concomitant series of technical advances

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higher accelerating voltages of commercially availahle microscopes,

better Cs values, improved vacua (and therefore lifetime of specimen), along with more widespread acquaintance with computations of image contrast and the procedures for recording optical dtffractograms (ref. 8). There has been a progressive tmprovement (ref. 9) in the quality of HREM images of zeolites availahle in the past eight years (see Fig. 1 ) .

But even

when, some y e a r s a g o , r e s o l u t i o n was r e l a t i v e l y p o o r , many new i n s i g h t s i n t o t h e s t r u c t u r e of z e o l i t e s were a f f o r d e d hy HREM. (i)

These i n c l u d e d t h e following.

Evidence f o r b o t h i s o l a t e d ( n o n - r e c u r r e n t ) and r e g u l a r ( d i f f e r e n t k i n d s

of r e c u r r e n t ) twinning i n f a u j a s i t i c z e o l i t e s .

This, i n t u r n , has l e d t o a

c l a r i f i c a t i o n ( r e f s . 10-15) of t h e n a t u r e of z e o l i t e s CSZ-1, ZSM-3,

CSZ-3,

ZSM-2,

ZSM-20 as w e l l as t o a n a p p r e c i a t i o n t h a t between t h e end members

f a u j a s i t e (FAU) and B r e c k - s t r u c t u r e - s i x

( B S S ) , t h e r e may n o t he a s h a r p

d i s t i n c t i o n between polymorphism and polytypism.

(The s i t u a t i o n is a k i n t o

t h a t which e x i s t s w i t h diamond and l o n s d a l e i t e o r w i t h z i n c h l e n d e and w u r z i t e , where, r e s p e c t i v e l y , as w i t h FAU and BSS, t h e r e i s ABCA...

and ABAB...

stacking

of t h e l a y e r s .)

(ii)

D i r e c t proof ( r e f s . 11 and 16-17) of t h e ' i n v e r s i o n '

'mirror-symmetry'

( i ) and

( a ) r e l a t i o n s h i p s between s u c c e s s i v e (100) s h e e t s i n ZSM-5

(MFI) and ZSM-11 (MXL), and t h e o c c u r r e n c e of b o t h non-random and random sequences of s t a c k i n g of t h e componenent ( 1 0 0 ) s h e e t s . ( i i i ) D i r e c t proof of t h e o c c u r r e n c e o f i n t e r g r o w t h s t r u c t u r e s of a r e g u l a r and non-regular k i n d i n t h e ABC-6 f a m i l y of z e o l i t e s ; a n d , i n p a r t i c u l a r , t h e i d e n t i f i c a t i o n of t h i n s l i v e r s of g u e s t ( o f t e n of u n i t - c e l l

t h i c k n e s s ) housed

within another z e o l i t i c host (eg. e r i o n i t e o r s o d a l i t e s l i v e r s within a predominantly o f f r e t i t i c matrix).

Barrer and o t h e r s ( r e f s . 1 8 , 1 9 ) had e a r l i e r

shown t h a t z e o l i t e G , f o r example, is composed of i n t e r g r o w t h s of chahazite-like

ABC-6 z e o l i t e s ; l i k e w i s e z e o l i t e T was, e a r l y o n , deduced t o he

i n t e r g r o w n o f f r e t i t e and e r i o n i t e . d i r e c t l y r e s o l v e ( r e f s . 20-22)

The g r e a t power of HREM i s t h a t i t can

t h e p r e c i s e n a t u r e o f t h e s e (see F i g . 2 ) and

o t h e r ( a s y e t unimagined) i n t e r g r o w t h s .

And w i t h t h e new a t t a i n m e n t s ( r e f . 9 )

of h i g h r e s o l u t i o n e l e c t r o n microscopes o p e r a t i n g a t p r o g r e s s i v e l y h i g h e r

a c c e l e r a t i n g v o l a t a g e s , t h i s a s p e c t of HREM s t u d i e s i s l i k e l y t o he ever-more powerful f o r s t r u c t u r a l a n a l y s i s .

A u t o e p i t a x y ( r e f s . 5 and 1 2 ) , i n which, f o r

example, a c r y s t a l of z e o l i t e - Y grows in an o r i e n t a t e d f a s h i o n on one of t h e high-symmetry p l a n e s of a n o t h e r c r y s t a l of z e o l i t e - Y i s r e v e a l e d by a comhination of HREM and o p t i c a l d i f f r a c t i o n . (iv)

Discovery ( r e f s . 2 3 , 2 4 ) of u n u s u a l r o t a t i o n a l h o u n d a r i e s i n z e o l i t e s w i t h

n o n - i n t e r s e c t i n g one-dimensional c h a n n e l s as i n z e o l i t e - L .

These b o u n d a r i e s ,

which g i v e r i s e t o a c o i n c i d e n c e l a t t i c e , a r e p e r p e n d i c u l a r t o t h e c h a n n e l directions.

T h e i r p r e s e n c e d i m i n i s h e s t h e d i f f u s i v i t y of g a s e o u s r e a c t a n t s and

-..

products i n t h e z e o l i t e . HREM has c o n t r i b u t e d t o t h e s t r u c t u r a l a n a l y s i s of z e o l i t i c m a t e r i a l s i n a t l e a s t two o t h e r ways. m e t a l l i c (E

F i r s t by r e v e a l i n g t h e n a t u r e and d i s t r i b u t i o n of minute

10 fi d i a m e t e r ) p a r t i c l e s of n o b l e metal c a t a l y s t s , s u c h as P t

and Pd w i t h i n t h e p o r e s of z e o l i t e h o s t s ( r e f s . 25.26);

a n d , s e c o n d , by paving

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F i g . 2 . Various t y p e s of i n t e r g r o w t h s o c c u r , o f t e n a t t h e s u b - u n i t - c e l l l e v e l , i n t h e A B C - 6 family of z e o l i t e s . HREM e n a b l e s t h e l o c a l s t r u c t u r e t o b e 'read o f f ' .

More w i l l d o u b t l e s s he

t h e way t o t h e d i s c o v e r y of new z e o l i t e s t r u c t u r e s .

But i t i s worth emphasizing

heard a t t h i s c o n f e r e n c e c o n c e r n i n g hoth of t h e s e . h e r e t h a t t h e s t r u c t u r e s of theta-one 29-30)

and o f ECR-1

( r e f s . 28-29),

of z e o l i t e b e t a ( r e f s .

were a l l a r r i v e d a t t h r o u g h t h e p i v o t a l use o f HREM.

p a r t i c u l a r l y i n t e r e s t i n g t h a t t h e d i s c o v e r y o f ECR-1, structural principle

-

sub-unit-cell

I t is

v i a HREM, uncovered a

i n t e r g r o w t h s of m o r d e n i t e and m a z z i t e

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t h a t was not p r e s e n t i n any of t h e enumerations of t h e o r e t i c a l l y p o s s i h l e open networks f o r z e o l i t e s and o t h e r molecular s i e v e s .

T h i s i l l u s t r a t e s t h e need t o

incorporate 'recurrent intergrowths', i n the sense h i t h e r t o elaborated ( r e f . 3 3 ) , i n t o s t r a t e g i e s f o r t h e g e o m e t r i c c o n s t r u c t i o n of new t y p e s of open networks.

The e x i s t e n c e of ECR-1

s u g g e s t s t h e e x i s t e n c e of o t h e r , c l o s e l y

r e l a t e d , h u t h i t h e r t o u n d i s c o v e r e d , z e o l i t i c s t r u c t u r e s s u c h a s DF-1, l a t t i c e energy of which i s e s t i m a t e d t o he comparable t o t h a t of ECR-1

the (ref.

34).

H R E M has proved i n v a l u a h l e a s a means of probing t h e s t r u c t u r e of z e o l i t e s a s h o s t s i n opto- and molecular e l e c t r o n i c m a t e r i a l s of t h e k i n d t h a t w i l l d o u b t l e s s he u t i l i z e d i n f u t u r e new d e v i c e s and p r o c e s s e s .

F i g . 3 shows t h e

way i n which segments of s e l e n i u m are d i s t r i h u t e d w i t h i n t h e p o r e s of m o r d e n i t e ( r e f s . 35,36).

I t has a l s o a f f o r d e d i n c o n t r o v e r t i b l e e v i d e n c e f o r t h e

u l t r a m i c r o c r y s t a l l i n i t y of z e o l i t i c p r e p a r a t i o n s which, on t h e b a s i s of X-ray powder d i f f r a c t o g r a m s , seemed t o be amorphous.

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F i g . 3 . HRE m i c r o g r a p h of m o r d e n i t e c o n t a i n i n g i n c o r p o r a t e d S e . The l a r g e w h i t e d o t s , more c l e a r l y v i s i b l e i n t h e i n s e t , a r e t h e main c h a n n e l s i n t h e 13.5 A. projeccion. The s h o r t e s t d i s t a n c e s e p a r a t i n g t h e m a i n c h a n n e l s i s Dark c o n t r a s t s i g n i f i e s a r e g i o n w h e r e Se i s p r e s e n t a s g u e s t . T a h l e 1 summarizes t h e c o n t r i b u t i o n s made by HKEM t o t h e s t r u c t u r a l a n a l y s i s of zeolites, Electron d i f f r a c t i o n (ED) The e l e c t r o n f l u x e s r e q u i r e d f o r r e c o r d i n g d i f f r a c t i o n p a t t e r n s i n a n e l e c t r o n m i c r o s c o p e a r e s o much s m a l l e r t h a n t h o s e n e e d e d f o r HREM,

t h a t i t is

i n v a r i a b l y possihle to determine u n i t c e l l dimensions (admittedly with l e s s a c c u r a c y t h a n w i t h X-rays)

a n d o f t e n t h e s p a c e - g r o u p s of a n y z e o l i t - l c m a t e r i a l

h y s e l e c t e d a r e a d i f f r a c t i o n e m p l o y i n g many d i f f e r e n t zone axes.

Patterns are

r e c o r d e d at d i f f e r e n t v a l u e s o f specimen t i l t and t h i c k n e s s s o a s t o minimize t h e chance o f e r r o n e o u s d e d u c t i o n of s p a c e g r o u p , which i s always a p o s s i h i l i t y when m u l t i p l e s c a t t e r t n g i s d o m i n a n t .

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Table 1:

Contributions made by HREM to the StructuralAnalysis of Zeolites

1. Identifying and Characterizing Isolated and Recurrent Intergrowths in:

ZSM-5 and ZSM-11; Theta- 1 and ZSM-23; Members of the ABC-6 family; FAU-family, hence clarifying nature of CSZ-1, CSZ-3 and ZSM-2, -3, -20 and BSS.

2 . Identifying Planar Faults and Coincidence Lattices in Zeolite L. 3. Discovery of Auto-epitaxy in FAU-family and of Overgrowths of FAU on LTA and vice versa.

4. Establishing that Many X-ray Amorphous Zeolites (LTA andLTL) are Crystalline. 5 . Demonstrating that Uptake of Some Guest Species (e.g. Selenium in Mordenite) is

Spatially Non-uniform.

6. Locating and Establishing Distribution of certain Cations and Metal Clusters in Intrazeolitic Cavities. 7 . Establishing Structure of New Zeolites (Theta-1; ECR-1 and Beta). ED i s a t i t s b e s t i n t h e s t r u c t u r a l a n a l y s t s o f z e o l t t e s , as w i t h o t h e r ( n o t a b l y o r g a n t c ( r e f s . 37,38))

m a t e r i a l s w h e r e wtde v a r t e t t e s o f s t r u c t u r a l

s u b t l e t i e s a r e p o s s i b l e , when u s e d i n c o m h t n a t t o n w i t h o t h e r t e c h n i q u e s . Knowing t h e s t r u c t u r a l m o t i f ( d e d u c e d from a c o m h t n a t i o n o f g a s a d s o r p t i o n a n d s o l i d - s t a t e NMR s t u d t e s

-

see b e l o w ) , t h e s t z e o f t h e u n i t - c e l l a n d a knowledge

of t h e s p a c e g r o u p c a n l e a d t o a f i n a l s t r u c t u r e f o r t h e z e o l i t e . S c a n n i n g e l e c t r o n m t c r o s c o p y (SEM) A l t h o u g h c a p a h l e o f d e t e r m i n i n g o n l y t h e morphology o f a s p e c t m e n a n d n o t i t s t n t e r n a l s t r u c t u r e , SEM i s a n e x t r e m e l y t m p o r t a n t t o o l i n s t r u c t u r a l

assessment.

Because X-rays a r e e m i t t e d ( a n d r e a d t l y d e t e c t e d hy

energy-dtspersive

a n a l y z e r s ) d u r i n g t h e c o u r s e of SEM ( o r m i c r o p r o b e ) s t u d y ,

t h e c o m b i n a t i o n of morphology a n d a s s o c t a t e d c h e m i c a l c o m p o s l t t o n c o n s t i t u t e s

a n a l m o s t t n f a l l i h l e g u t d e as t o w h e t h e r a g t v e n s y n t h e s t s h a s y i e l d e d o n e o r more d i s t i n c t c r y s t a l l o g r a p h i c a n d / o r amorphous p h a s e s .

W t t h t h e a d v e n t of

w i n d o w l e s s , o r thin-window X-ray d e t e c t o r s , SEM a n a l y s t s has t u r n e d o u t t o he v e r y u s e f u l ( r e f s . 39-41)

i n t h e s t u d y of ALPOs, SAPOs and MeASPOs ( s e e F i g .

4 ) , j u s t a s I t h a s been i n t n v e s t t g a t i o n s o f new f a m i l i e s o f warm s u p e r c o n d u c t o r s , t h e s t r u c t u r a l e l u c i d a t i o n o f which p o s e s q u e s t i o n s s i m i l a r t o those encountered i n z e o l i t e s c i e n c e . Two o t h e r i m p o r t a n t a s p e c t s o f SEM a r e t h a t I t r e a d t l y t d e n t i f t e s a u t o e p i t a x y o r t h e e p i t a x i a l g r o w t h of one z e o l i t e on a n o t h e r ( r e f . 4 2 ) , and e n a h l e s t h e s p a t i a l u n i f o r m i t y of t h e e l e m e n t a l c o m p o s t t t o n w t t h t n a g t v e n

10 z e o l i t e o r ( p r e c u r s o r ) s o l t d i f i e d g e l t o he d e t e r m i n e d .

65

75

85

95

105

115

mm

F i g . 4 . T y p i c a l a n a l y t i c a l r e s u l t s i n t h e microprobe a n a l y s i s o f a t e m p l a t e c o n t a i n i n g SAPO. (Yan Xu t o be published).

u,

Neutron s c a t t e r i n g Whereas t h e i n e l a s t i c and q u a s i - e l a s t i c s c a t t e r i n g of n e u t r o n s was used i n t h e 1970s f o r t h e s t u d y of adsorbed s p e c i e s i n z e o l i t t c h o s t s , t h e R i e t v e l d method of powder p r o f i l e a n a l y s i s ( r e f s . 43-48) f i r s t came i n t o prominence i n r e f i n i n g s t r u c t u r a l p a r a m e t e r s i n t h e e a r l y 1980s.

One of t h e e a r l i e s t

a p p l i c a t i o n s ( r e f . 45) t o z e o l i t e s f o c u s s e d on a n e x a m i n a t i o n of S i , A 1 o r d e r i n g i n t h e framework of Tl+-exchanged zeolite-A.

Direct conf i r m a t i o n

t h a t t h e r e was s t r i c t a l t e r n a t i o n of t h e S i 4 + and A13+

ions i n the

t e t r a h e d r a l s i t e s was o h t a i n e d , and hond-length,

bond-angle

d a t a were

comparable w i t h t h o s e o b t a i n e d from s i n g l e - c r y s t a l X-ray a n a l y s i s .

Two o t h e r t y p e s of s t r u c t u r a l i n f o r m a t i o n d e r i v e d from R i e t v e l d p r o f i l e a n a l y s i s of pure z e o l i t e s are noteworthy.

F i r s t , there is t h e d i r e c t

d e t e r m i n a t i o n ( r e f . 4 6 ) of t h e a t o m i c c o o r d i n a t e s of t h e c a t a l y t i c a l l y a c t i v e s i t e , where a d e t a c h a b l e p r o t o n s i t s ( a t low t e m p e r a t u r e ) , i n La3+-exchanged z e o l i t e Y. zeolite-L,

Second, i n a range of g a l l o - z e o l i t i c

s t r u c t u r e s , e s p e c i a l l y of

Newsam ( r e f . 49) and o t h e r s ( r e f . 5 0 ) have shown t h a t t h e r e i s f a r

l e s s tendency f o r Ga3+ and Si4+ i o n s t o t a k e up p r e f e r r e d s i t i n g i n t h e t e t r a h e d r a l s u h - l a t t i c e t h a n f o r A13+

and Si4+ i n t h e same z e o l i t i c

A t p r e s e n t , t h e r e is no s a t i s f a c t o r y t h e o r e t i c a l e x p l a n a t i o n f o r

structures.

t h i s preference. N e u t r o n - e l a s t i c s c a t t e r i n g has heen i n v a l u a h l e i n d e t e r m i n i n g t h e s i t i n g of guest species catalysts.

-

model r e a c t a n t s , p r o d u c t s o r p o i s o n s

-

w-ithin z e o l i t i c

Especially useful data i n t h i s regard a r e a v a i l a b l e pertaining t o

such s p e c i e s a s xenon ( r e f s . 50,511, p y r i d i n e ( r e f . 4 8 ) and benzene ( r e f s . 52,53).

I n view of t h e s t r o n g s c a t t e r i n g of n e u t r o n s by l i g h t e l e m e n t s ,

n e u t r o n d i f f r a c t i o n i s p r e f e r r e d , o t h e r t h i n g s heing e q u a l , r a t h e r t h a n X-ray d i f f r a c t i o n , as a t e c h n i q u e f o r d e t e r m i n i n g t h e s t r u c t u r e of z e o l i t e - g u e s t complexes.

Neutron i n e l a s t i c s c a t t e r i n g is a l s o a promising t o o l f o r s u c h

s t u d i e s and i s w e l l s u i t e d f o r i d e n t i f y i n g t h e n a t u r e of sorbed m o l e c u l e s , such a s H 2 and simple hydrocarbons from t h e i r r o t a t i o n a l spectrum ( r e f . 5 4 ) . For z e o l i t e s r e p l e t e w i t h non-random i n t e r g r o w t h s , n e u t r o n d i f f r a c t i o n s t u d i e s of powdered specimens a r e n o t l i k e l y t o prove a s p r o f i t a b l e a s HREM. They a r e c e r t a i n l y not l i k e l y t o y i e l d t o t h e i n g e n i o u s a h i n i t i o methods of powder p r o f i l e a n a l y s i s ( r e f . 5 5 ) which have r e c e n t l y proved s o s u c c e s s f u l f o r non-zeolitic

solids.

There a r e o t h e r , more unusual and a s y e t i n c o m p l e t e l y e x p l o r e d methods of s t r u c t u r a l a n a l y s i s u s i n g n e u t r o n s which are d i s c u s s e d below.

It must n o t he

f o r g o t t e n , however, t h a t 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 ( r e f . 5 6 ) i s nowadays a r e l i a b l e method, though not s u i t e d t o ( s p a t i a l l y ) micro- o r nano-scale a n a l y s i s , f o r d e t e r m i n i n g Si/A1 r a t i o s . Synchrotron r a d i a t i o n

To d a t e , more p r o g r e s s has been a c h i e v e d , u s i n g X-ray a h s o r p t i o n , w i t h i n o r g a n i c s o l i d s of a n o n - z e o l i t i c

t h a n of z e o l i t i c c h a r a c t e r .

EXAFS and

XANES, a s w e l l a s pre-edge s t r u c t u r e has proved of immense v a l u e , f o r example, i n t h e s t u d y of i n o r g a n i c and o r g a n o m e t a l l i c compounds c o n t a i n i n g vanadium ( r e f . 57) ( s e e Fig. 5).

No s u b s t a n t i a l r e p o r t devoted t o t h e d e d u c t i o n of

framework s t r u c t u r e o r topology has a p p e a r e d , a l t h o u g h i t has y i e l d e d u s e f u l i n s i g h t s i n t o t h e c o n f i g u r a t i o n of v a r i o u s t r a n s i t i o n metal c a t i o n ( r e f . 5 8 ) , i n c l u d i n g more r e c e n t l y Rh-exchanged,

z e o l i t e s ( r e f . 59).

One may e x p e c t more

work u s i n g EXAFS t o he done on m e t a l c l u s t e r s e n c a p s u l a t e d w i t h i n z e o l i t e s . d a t e i t has n o t been easy t o r e c o r d , 0-, A l ahsorption spectroscopy.

and Si-core-edge

To

f e a t u r e s i n X-ray

Recent r e s u l t s , as y e t p r e l i m i n a r y , l e a v e l i t t l e

doubt t h a t XANES i s a u s e f u l means of e x p l o r i n g t h e chemical environment of A 1 i o n s both p r i o r t o and f o l l o w t n g p a r t i a l d e a l u m i n a t i o n .

EXAFS h a s r e c e n t l y

( r e f . 60) been put t o good u s e i n d e t e r m i n i n g t h e environment of Cd2+ i o n s , a s w e l l a s t h e dimensions of CdS c l u s t e r s e n t r a p p e d w i t h i n z e o l i t e c a g e s . Some of t h e g r e a t e s t o p p o r t u n i t i e s o f f e r e d by s y n c h r o t r o n r a d i a t i o n l i e i n t h e domain of X-ray d i f f r a c t i o n ( r e f s . 6 1 , 6 2 ) .

Advantage can he t a k e n of t h e

i n t e n s e f l u x of X-rays and of smoothly a d j u s t a b l e wave-length f o r t h e s t u d y of minute s i n g l e c r y s t a l s of z e o l i t e .

Such work i s e x p e r i m e n t a l l y v e r y demanding

because of t h e e x t r a o r d i n a r y minuteness ( a s s i n g l e c r y s t a l s ) of t h e z e o l i t e , t h e s t r u c t u r e of which has t o he s o l v e d hy a combination o f f o u r - c i r c l e

12

30 28

2G

> 01

16

0

2o

L

3

4-

Fig. 5. Pre-edge and near-edge X-ray absorption spectra reveal a great deal about the oxidation state of a transition metal, as this illustration, from the work of Wong shows.

m

Y0,

c

12

.-c 0

n L

0

vl

n m

8

.e 0

c

.-0

c .-

vl

G

0

n

0 Vanadium

oxidation

State

dlffractometry and hlgh-flux synchrotron radlatlon.

That It

Is,

under certain

favourable clrcumstances, worthwhile to persevere with this approach has heen demonstrated by Elsenberger et a1 (ref. 6 3 ) for a small specimen (800 urn3) of cancrinlte. To our knowledge no determination of the structure of a new synthetic zeollte has yet been accompllshed, hut conspicuous success has heen achieved by McCusker (ref. 6 4 ) in solving the structure of the related clathrasll slgma-2.

The hlgh-resolution data on slgma-2 ( S 1 6 4 0 1 2 8 . 4 C l o ~ 1 , ~ )

were collected at the NSLS, Brookhaven, and then processed through

a

series o f

computer programs assembled and modlfled by McCusker.

The symmetry tnformation, the unlt-cell dlmensions, and the data (up to 28 of 73O)

were

Input to Pawley's program (ref. 65) ALLHKL, modlfled to Include a pseudo-voigt

13 peak-shape function (V =aL+(l-K)G,

where V, L and G are pseudo-Voigt,

Lorentian and Gaussian functions, respectively, and

u

is a variable).

The

pattern decompositton with ALLHKL is thought (ref. 6 4 ) to he the most critical step in the entire procedure. The structure of sigma-2 (Fig. 6 ) is quite novel: neither of the two cages which make up the repeat units has been encountered hitherto. is roughly spherical with a free diameter of 7.5

d

and

7

The large cage

point symmetry.

are four of these cages and eight small ones in the unit cell.

There

As tn other

(ref. 6 6 ) synthesized clathrastls utilizing 1-aminoadamantane as template, the entrapped organic guest is disordered.

Fig. 6 . The framework structure of Sigma-2, the structure of which was determined, ab initio, by McCusker from synchrotron powder diffraction data. The vertices correspond to Si atoms; 0 atoms have been omitted for clarity.

Recent developments (refs. 67-69) with non-zeolitic soltds show that

ah

initio crystal-structure determination can in principle be carried out from the high-resolution diffractometric data that synchrotron sources are now capable of yielding. And a great deal of high preciston data can he transformed, by expert (ref. 70) Rietveld reflnement of pentasil structures. With the advent of area detectors and other technical Improvements one may envisage that many new zeolite, porosil, clathrasil and other solid molecular sieves will succumb to structure determination using the Laue (micro) single crystal technique.

The feasiblity of such an approach was demonstrated (ref.

71) some time ago for the close-packed mineral berlinite ( A l P 0 4 ) , and it has been pursued with conspicuous success recently in organometallic (ref. 72) and enzyme crystallography (ref. 73). Solid-state NMR Although the dynamics of reorientation, diffusion and ion-migration within

14

zeolitic and related soltds have long been tnvestigated from line-narrowing, sptn-lattice relaxattons and other measurements (refs. 74-75), tt was not until Ltppmaa and Engelhardt (ref. 76) and thetr coworkers in the early 1980s showed the mertt of magtc-angle-sptnntng 29St NMR that soltd-state measurements began (refs. 77-81) to he applied (wtth frenetic tntenstty) to the study of zeolttes. By any standards, the outcome has been dramatic and clarifying revtews in refs. 82-88.

-

see

Vartable-angle-sptnntng NMR spectra of quadrupolar

nuclet in zeolites, espectally of 27Al, began to gain popularity in 1982 (ref. 89), when it was shown that octahedral and tetrahedral coordtnatton could be readily disttnguished.

Stnce that time, thanks to a series of multtnuclear

NMR studies of the local envtronments of certain nuclei ('H, 14N, 15N, 170, "F,

7Lt, "B,

13C,

23Na, 27Al, 29St, 31P, 51V, 69Ga, 71Ga, '19Sn, 12'Xe

and

133Cs) that can be accommodated into the structures of zeolttes and gels, etther as framework components, exchangeable cations and guests, o r probes withtn tntracrystalltne cavtttes, a great deal of worthwhile structural tnformatton has been retrieved (see Table 2). been done as yet ustng 2H soltd-state NMR.

Surpristngly, ltttle work has Multiple-pulse, zero-fteld,

two-dimenstonal quadrupolar nutatton, multiple-quantum and other methods of recovertng htgh-resolution, multtnuclear NMR spectra of soltds have been used to good effect, and further refinements and technical achievements are continually being regtstered.

This is espectally the case in regard to double

rotation NMR, the stgntftcance of whtch is descrthed brtefly below. The chemical shtft of a particular nucleus in a trapped (or template) molecule can be a useful tndtcator of cavtty and channel dimension.

For

example, there I s a correlatton between the 13C-chemtcal shtft in the tetramethyl ammontum (TMA) ton and the average diameter of a trapping site for TMA encapsulated in various zeolttes during synthests (ref. 90).

Stmtlar, hut

more complex, correlations have been noted (refs. 91,92) wtth 12'Xe.

The

advantage of Xe (over TMA) is that it can he introduced tnto a large number of structures (if necessary at high temperature and pressure (ref. 93)), whereas the Il3C method' is effective only for 'as synthesized' zeolites and precursors.

12'Xe

chemical shtfts are very senstttve to other factors apart

from cavity dtmenston; and Fratssard (refs. 92,93) has made imaginative use of thts nuclear probe to tnvesttgate the size, and extent of coverage, of minute parttcles of noble metal catalysts housed wtthtn a zeoltttc matrix.

29St and 27Al soltd-state (sptnntng sample) NMR are the most widely, now almost routtnely, used methods for zeolltes, and so also are 27Al and 31P for the ALPOs.

If structural tnterest Is focussed matnly on the framework

there ts generally no need to employ cross-polartzatton. When tnterest centres on the stttng and envtronment of hydrocarbons, template bases and on other

15

Table 2:

ContributionsMade by SolidState NMR: A Selection

1. Establishes the coordination and local atomic environment (in crystalline and precursor

gels) of Si and Al in zeolites and of A], P and Si in ALPOs and SAPOs.

2. In certain structures, number of distinct tetrahedral sites and framework composition may be determined from the Z9Si spectrum of the dealuminated parent.

3. Enables T-6-T angles to be deduced from 29Siand

chemical shifts.

4. Quantitative monitoring of de- and re-alumination, and de- and re-galliation. 5 . Charts: Structural and dynamical changes in framework caused by sorption or temperature variation; Thermally-stimulated migration of extra-framework cations; and re-distribution of framework atoms.

6. Pinpoints (via multinuclear NMR) Bronsted acid sites; in favourable circumstances yields bond (OH) distances.

7. Identifies adsorbed intermediates in catalytic conversions (see separate table). 8. Determination of size of sorbed clusters (atom counting from correlated spins).

9. Determination of size of cages and metal clusters. 10. Demonstrates Occurrence of Al-0-A1 linkages in certain zeolitic solids.

i n c a r c e r a t e d o r g a n i c s p e c i e s , f u r t h e r i n s i g h t i s gained by u s i n g c r o s s polarization.

Of l a t e remarkable d e t a i l s p e r t a i n i n g t o t h e n a t u r e of

adsorbed i n t e r m e d i a t e s i n t h e c o n v e r s i o n of methanol t o p e t r o l on ZSM-5 c a t a l y s t s have been achieved ( r e f s . 94-95), u s i n g a 13C e n r i c h e d r e a c t a n t and a special c e l l suited for i n s i t u investigation.

Dozens of a l i p h a t i c and

a r o m a t i c i n t e r m e d i a t e s , some of which are enumerated i n T a b l e 3, have been identified.

But i t i s n o t y e t c l e a r which of t h e s e are s p e c t a t o r s and which

a r e t h e a c t o r s i n t h e p r o c e s s of o v e r a l l c o n v e r s i o n .

It i s c l e a r , however,

t h a t unexpectedly bulky i n t e r m e d i a t e s as w e l l as b y p r o d u c t s s u c h as CO a r e g e n e r a t e d a t t h e i n t e r n a l s u r f a c e of t h e c a t a l y s t , and t h a t some of t h e s e would

be i n c a p a b l e of d i f f u s i n g o u t i n t a c t t o t h e e x t e r n a l s u r f a c e .

Table 3: Some of the Adsorbed Intermediates in the ZSM-5 Catalyzed Conversion of Methanol to Petrol at 3700C

ALKANES: Methane, ethane, cyclopropane, propane, n-butane, 1-butane, n-pentane, neopentane, n-hexane, 3-methyl pentane, 2,3-dimethylbutane and n-heptane. AROMATICS: Benzene, toluene; Q-, a-and p-xylene; 1,2,3-trimethylbenzene; 1,2,4-trimethylbenzene; 1,3,5-trimethylbenzene; 1,2,4,5-tetramethylbenzene;1,2,3,5tetramethylbenzene; 1,2,3,4-tetramethylbenzene; pentamethylbenzene and hexamethylbenze

NEW METHODS Recently new vartants of more tradittonal techntques have proved valuahle as structural probes for the analysts of zeoltttc matertals.

Dtffuse reflectance

Fourier transform tnfra red spectroscopy (DRIFT) and Raman spectroscopy are two such (complementary) techniques; electron spin resonance (and especially electron spin echo modulatton spectroscopy) and Mossbauer spectroscopy are others.

But there have also been qutte new developments which have not yet

reached full maturtty.

Some of these (Table 4 ) are expertmentally demanding;

others rest more on conceptual rather than technical advance. Electron-based methods There are four promtsing techniques In this category: electron-energy-loss spectroscopy (EELS), Rutherford hack-scattering studtes, chemically-sensitive structure imaging and atomtc force mtcroscopy.

(t) EELS. Most electron mtcroscopes can he readtly fttted with electron spectrometers s o that It is a relatively stmple matter to record the EEL spectrum during the course of other studtes entailtng dtffractton, imagtng or X-ray emtsston spectroscopy.

It has long been recogntzed (ref. 97) that EELS

is a conventent, often superior, method of quantttatively determtntng ltght

elements ( 2

<

10) in an electron mtcroscope, there being dtfftculttes in

satisfactorily detecting the soft X-rays emttted by these elements.

It ts less

wtdely apprectated (ref. 98) that fine structure in the EELS spectrum, eg. of

K- or L-shell excttatton can yteld valuable tnformatton about the coordtnatton of the element responsible for the electron energy l o s s .

Instrumental

improvements have, of late, heen considerahle; and now that parallel detectton

17

Table 4:

Potential New Techniquesfor Structural Analysis

1. Electron based Electron-Energy-Loss Spectroscopy (microscopy) (EELS) High-angle Rutherford Scattering and Chemically-sensitive Structure Imaging (STEM) Atomic Force Microscopy ( A M ) 2. Magnetic Resonance (Double Rotation NMR)

3. Neutron-based: Incoherent Inelastic Scattering (IINS) In situ methods: INS Epithermal Resonance Broadening 4. Synchrotron Radiation and Laboratory X-rays: X-ray Absorption High Resolution Powder Diffraction Micro (single) Crystal Diffraction (including Laue and Time-resolved) I n situ Studies (Low and High Temperatures) systems are increasingly avatlahle, it has become much easter to record EELS data on l e s s than femtogram amounts of sample in an electron microscope. Computattonal procedures are also avatlable (refs. 99-101) to evaluate what the EELS should look like for a given coordtnatton of a non-metal around a central metallic cation such as Be2+, B3+,

A13+ etc.

Typical results are shown

t n Fig. 7. The key points to note here are that the sample does not have to he

crystalltne, and that very htgh spatial resolution i s achievable

(cf our

study (ref. 102) of semi-tnsulattng polycrystalltne silicon electronic devices) with this technique. Httherto, there has been no technique for determtntng the electrontc environment, atomtc coordinatton and elemental composition at the nanoscale. EELS goes some way towards repatring this omission. (ii) Atomtc force microscopy ( A F M ) (ref. 103).

Unlike scanning tunnelling

microscopy, which requires electrtcally conducting samples, and which therefore is lfkely to be 111 suited for the structural study of most zeoltttc materials, AFM, since tt can cope with insulating matertals may well yield penetrating tnsights tnto the surface topography of cleaved or freshly grown zeolites. Topographical studtes, in general, reveal a great deal ahout the structure and chemistry of the underlying solid, as I s evtdent from earlier-work (ref. 104) on layered minerals.

Bearing in mtnd the recent results of Vaughan

(ref. 105) on the propensity for a few monomolecular layers of one zeolite to

I

I

200

.

.

5

220

210

Fig. 7 . Typical EELS data showing how the state of coordination of light atoms (Be(a), B(b) and Al(c)) may be determined from the structure of the 'core-loss' peak. The lower curves i n (a) and ( b ) are computed results, top ones experimental. In (c) the computed curves show that tetrahedrally and octahedrally coordinated A1 yield very different EELSs, and this is borne out experimentally (not shown).

I

230

ENERtY I eV

80

100 ENERGY / cV

I20

grow coherently on top of another (as In ECR-l), new Insights Into novel zeolltlc structures may emerge from thls route of enqulry. (Ill) Hlgh-angle Rutherford scatterlng.

Shortly after Its Introductlon In

the early 1970s, STEM became popular as a means of tdentlfylng metalllc catalysts of colloidal dlmenslon such as Pd supported on carbon by so-called Z-contrast, whlch entails recording (usually ratlolng) both elastlc and inelastically scattered electrons (ref. 106).

Recently (ref. 107) a method has

been devised of determlnlng the sizes of mlnute particles even to the extent of countlng the number of atoms present in the ultrafine particle.

This I s

achieved by electron-scatterlng Into a high-angle annuler detector s o as to avoid lntenslty modulations arlslng from Bragg reflectlons.

The signal I s

mostly high-angle dlffuse scatterlng, and I s proportlonal to the number of atoms probed by the beam, weighted by thelr Individual scatterlng cross sectlons.

Scatterlng strengths of lndlvldual clusters are computed from

dlgltlzed high-angle annular detector Images; and when data are plotted as (Image area) ' I 2 versus (Inten~lty)~'~they lle close to a straight line. Such plots provide callberatlon of the lntenslty Increment per atom, without the necesslty of external callbratlon. three atoms of Pt (supported on A1203),

As thls technlque can detect a s few a s It should prove Invaluable for the

future characterlzatlon of bifunctional zeolitic catalysts. (lv) Chemically-sensitive structure imagine

Conventlonal HREM is based

on phase contrast In which the diffracted beams emerglng from the sample are

19 recombined on t h e viewing s c r e e n of t h e microscope.

The r e s u l t i n g c o n t r a s t i s

governed by t h e r e l a t i v e phases of t h e d i f f r a c t e d beams.

Whilst i t i s

sometimes p o s s i b l e t o i d e n t i f y ( r e f . 108) t h e s p a t i a l d i s t r i b u t i o n of heavy atoms i n a q u a s i - c r y s t a l l i n e z e o l i t i c s o l i d u s i n g c o n v e n t i o n a l HREM, i t would he much e a s i e r t o ' r e a d o f f ' t h e d i s t r i b u t i o n of chemical e l e m e n t s i n r e a l - s p a c e e l e c t r o n microscope images, i f a t e c h n i q u e based on atomic-number ( 2 ) c o n t r a s t were evolved.

The STEM a p p r o a c h of Treacy and Howle ( r e f . 106)

d e s c r i b e d above o f f e r s j u s t such a p r o s p e c t .

R e c e n t l y Pennycook and Boatner

( r e f . 109) have demonstrated how heavy-atom p l a n e s , i e . t h o s e t h a t c o n t a i n rare-earths

such as E r ( 2 = 6 8 ) , s t a n d o u t more c l e a r l y i n STEM images t h a n

t h o s e c o n t a i n i n g l i g h t e r e l e m e n t s such as Y (X = 3 9 ) ( F i g . 8 ) . development should now make i t e a s i e r t o pin-point

This important

t h e d i s t r i b u t i o n of

exchangeable c a t i o n s by d i r e c t imaging.

I"

1

Cul

I Ba

F i g . 8. C a l c u l a t e d image i n t e n s i t y a c r o s s two u n i t ~ c e l l s of ( a ) Y B a 2 C ~ 3 0 7 - and (b) ErBa2Cu307-,. The difference i n intensity a t the Y ( E r ) planes (borne out e x p e r i m e n t a l l y by t h e work of Pennycook and B o a t n e r ) shows t h e v a l u e o f a h i g h angle detector for chemically-sensitive imaging by STEM.

, 39Y

cu2 I cu2 I Y Ba

El

Cul

I cu2 Ba

I cu2 I CI Y Ba Er

Magnetic r e s o n a n c e The s t r u c t u r a l s t u d y of z e o l i t e s h a s been advanced by t h e a p p l i c a t i o n of MASNMR, as t h e c o n t e n t s of Table 2 t e s t i f y .

Magic-angle-spinning,

s h a r p e n s t h e resonance peaks o n l y f o r s p i n 1/2 n u c l e i . quadrupolar nuclei ( r e . with s p i n

>

however,

There are many more

1 / 2 ) t h a n non-quadrupolar o n e s ; and a s

t h e s e i n c l u d e 1 7 0 , 23Na, and 2 7 A l it i s c l e a r l y d e s i r a b l e t o e x p l o r e e x p e r i m e n t a l means of s h a r p e n i n g t h e peaks a r i s i n g from q u a d r u p o l a r n u c l e i . U n t i l very r e c e n t l y , t h e s t r a t e g y h a s been t o r e c o r d NMR s p e c t r a under v a r i a b l e a n g l e s of s p i n n i n g i n t h e maximum p o s s i b l e magnetic f i e l d s , t h e magnitude of t h e second-order q u a d r u p o l a r broadening b e i n g i n v e r s e l y p r o p o r t i o n a l t o t h e

20

/i MAS

arm

m

urn

nm

0

-am

-4m

-MM

-em

Frequency (Hertz) Fig. 9. (a) In double-rotation NMR, the solid sample is rotated at an angle to the axis of theholder which is itself spun at an angle, the magic angle (54.74") Together these The angle between R1and R2 is 30.56'. to the applied field Bo. rotations remove dipolar and quadrupolar line broadening. (b) "he NMR spectrum o f 23Na in a polycrystalline mixture of sodium oxalate and sodium sulphate from: (i) a static sample; (ii) a magic-angle-spinning experiment; and (iii) a double-rotation experiment. The line width is reduced from 12,000 to 4000 to 140 Hz respectively. fteld strength. This approach has borne frutt, stnce, wtth zeolite omega (ref.

110) for example, two disttnct tetrahedral sltes are revealed by 27Al soltd-state spectra at ftelds of 11.744 tesla but not at 7.946 or 9.359 tesla. Double rotatton NMR, recently introduced (refs. 111-1131, whtch uses two magic angles, the customary one of 54.74'

and a new one 30.56',

approprtate to

the quadrupolar nuclet eltmtnates essenttally all the broadentng of the resonance. The powdered soltd rotates tnstde a holder whtch stmultaneously rotates on a second axts, a i r jets and air hearings hetng used to achieve rotation speeds of up to 6 KHz (Ftg. 9 ) .

The Berkeley-Tallinn team responsible

for thts work has demonstrated that the technique works for 23Na nuclet, two well-resolved signals (wtdth 75 Hz) betng obtatned from a mtxture of the

21 s u l p h a t e and o x a l a t e of sodium.

The c h a l n s l l l c a t e m l n e r a l d l o p s l d e ,

CaMg(S103)2, ( I s o t o p l c a l l y e n r i c h e d ) y i e l d s t h r e e d l s t l n c t 170 s i g n a l s . The s t a g e I s t h e r e f o r e set f o r e x c l t l n g developments I n t h e e l u c i d a t l o n of I n z e o l l t e s r h o , Y and f e r r l e r l t e t h e r e are known t o b e ,

zeollte structures.

r e s p e c t i v e l y , two, f o u r and e l g h t c r y s t a l l o g r a p h i c a l l y d l s t i n c t oxygen s i t e s . I f t h e s e can be r e s o l v e d by d o u b l e - r o t a t l o n NMR, we may e x p e c t even g r e a t e r p r o g r e s s s t h a n h i t h e r t o i n a s s e s s t n g t h e l o c a l a t o m i c environments of polycrystalllne zeolites. Neutron-based methods Apart from t h e now r a t h e r s t a n d a r d ways ( R l e t v e l d powder a n a l y s l s , e t c . )

In

whlch n e u t r o n s c a t t e r l n g I s c u r r e n t l y a p p l i e d t o z e o l t t e s c l e n c e , one p e r c e l v e s t h a t t h e r e are many o t h e r powerful uses t o which n u e t r o n s may be p u t .

The

supreme advantage t h a t n e u t r o n s p o s s e s s o v e r a l l o t h e r p r o h l n g r a d l a t l o n I s t h a t they r e a d l l y p e n e t r a t e walls of s t a l n l e s s s t e e l , s u c h as are r e q u i r e d f o r high temperature condltlons.

-

h l g h p r e s s u r e f u r n a c e s t o house c a t a l y s t s under o p e r a t i n g

Owing t o t h e fundamental c h a r a c t e r of n e u t r o n s c a t t e r l n g l l g h t

atoms, such a s 'H,

2H, '*C e t c . e x e r t a much more s l g n i f l c a n t i n f l u e n c e

upon t h e primary beam t h a n does Fe, N I o r C r .

Neutrons a r e , t h e r e f o r e , w e l l

s u i t e d f o r I n s i t u s t u d l e s of z e o l l t l c c a t a l y s t s . U n f o r t u n a t e l y , v e r y few s t u d l e s of z e o l i t e s under o p e r a t i n g c o n d l t l o n s t a k l n g advantage o f t h l s f a c t have been c a r r i e d o u t .

The k i n d of I n f o r m a t i o n

r e t r l e v e d u s i n g n e u t r o n s from model h y d r o d e s u l p h u r l z a t l o n c a t a l y s t s ( r e f s . 114,115) ought t o he f e a s l h l e f o r z e o l i t e s ; and d o u b t l e s s e f f o r t s I n t h l s d l r e c t l o n (dependent on a c c e s s t o I n t e n s e n e u t r o n s o u r c e s ! ) w t l l be pursued I n the near future. But t h e r e has r e c e n t l y been a n e l e g a n t u s e of n e u t r o n s t o monitor t h e p r e c i s e t e m p e r a t u r e of a s u p p o r t e d c a t a l y s t under I n s l t u c o n d l t l o n s .

The

p r i n c i p l e s of t h e new method were t e s t e d on a 14 p e r c e n t P t on s a m a r l a catalyst.

There are I n d i c a t i o n s t h a t t h l s t e c h n i q u e I s a p p l i c a b l e t o many

other c a t a l y t i c s o l l d s Including z e o l l t e s . I n e s s e n c e e p l t h e r m a l n e u t r o n s ( 1 < E <100eV) are used f o r t h l s I n g e n i o u s , n o n - l n t r u s l v e method of r e c o r d l n g t e m p e r a t u r e of c a t a l y s t s under r e a l i s t i c operating condltions.

The t e c h n l q u e was conceived by Fowler ( r e f . 116) who

t e s t e d I t a t t h e p u l s e d n e u t r o n s o u r c e i n Los Alamos I n 1984.

When c e r t a l n

nuclel (eg. P t ) a r e I r r a d i a t e d with eplthermal neutrons, resonance absorption occurs a t energies correspondlng t o nuclear t r a n s l t l o n s .

Resonance b r o a d e n l n g

a r i s e s from Doppler e f f e c t s owlng t o t h e t h e r m a l motion of t h e n u c l e i .

With

a p p r o p r l a t e c a l l b r a t l o n , t h e resonance width may be used t o p r o v l d e a measure of t e m p e r a t u r e .

S l n c e each n u c l e u s p o s s e s s e s a unlque r e s o n a n c e s p e c t r u m

22

d

TIME pS

u,

F i g . 1 0 . P o r t i o n of a t i m e - o f - f l i g h t n e u t r o n r e s o n a n c e s p e c t r u m , measured i n t r a n s m i s s i o n by F r o s t of a Pt/Sm203 c a t a l y s t a t 25'C. The P t and Sm r e s o n a n c e s a t 11.9eV ( c e n t r e d a t 2 4 7 ~ s )and 3.4eV ( 4 6 6 ~ s )r e s p e c t i v e l y were used f o r subsequent i n s i t u t e m p e r a t u r e d e t e r m i n a t i o n o f t h e n o b l e m e t a l and i t s s u p p o r t . ( F i g . l o ) , i t i s p o s s i b l e s i m u l t a n e o u s l y t o measure t h e t e m p e r a t u r e of two o r more components of t h e s o l i d under i n v e s t i g a t i o n .

a1 ( r e f .

In a recent study, Frost

117) examined, under i n s i t u c o n d i t i o n s , t h e Pt/Sm203 c a t a l y s t ,

and f o c u s s e d on t h e 11.9 and 3.4 eV r e s o n a n c e s of P t and Sm r e s p e c t i v e l y . T y p i c a l t e m p e r a t u r e s o b t a i n e d f o r t h e metal and s u p p o r t were 4 2 0 t 6 C and 4 2 4 + 2 C r e s p e c t i v e l y , compared t o a thermocouple r e a d i n g of 4 2 4 ' 1

C.

This

t e c h n i q u e s h o u l d he p a r t i c u l a r l y a t t r a c t i v e i n t h e s t u d y of new z e o l i t i c cracking c a t a l y s t s . Many more a p p l i c a t i o n s of n e u t r o n s c a t t e r i n g a r e t o he e x p e c t e d i n z e o l i t e s c i e n c e , from t h e f u r t h e r d e t e r m i n a t i o n of g u e s t s p e c i e s ( r e f s . 48 and 118) by p r o f i l e r e f i n e m e n t , t o m o t i o n a l s t u d i e s of occluded o r g a n i c s by i n c o h e r e n t i n e l a s t l c s c a t t e r l n g ( r e f . 119) t o t h e r o l e of t e m p l a t e s i n g o v e r n i n g n u c l e a t i o n and growth ( r e f .

120).

Use of s y n c h r o t r o n and c o n v e n t i o n a l X-ray s o u r c e s N o t w i t h s t a n d i n g t h e c o n s i d e r a b l e v a r i e t y of u s e s t o which s y n c h r o t r o n r a d i a t i o n , a s a s t e p p i n g s t o n e f o r s t r u c t u r a l a n a l y s i s , h a s a l r e a d y been p u t

see e a r l i e r s e c t i o n s powerful s o u r c e .

-

-

t h e r e i s s t i l l room f o r f u r t h e r e x p l o i t a t i o n of t h i s

One f o r e s e e s t h a t , in t h e s t u d y of b i n a r y compounds of

s u h - c o l l o i d a l dimension ( r e f . 60) and c o l l o i d a l metals ( r e f . 121) b u r i e d i n s i d e z e o l i t i c h o s t s , EXAFS and XANES are l i k e l y t o f i g u r e p r o m i n e n t l y .

But

t e c h n i c a l advances ought soon t o permit more r o u t i n e use of m i c r o ( s i n g l e ) c r y s t a l d i f f r a c t t o n ; and even h i g h e r r e s o l u t i o n powder d i f f r a c t i o n .

Cox

a 1 ( r e f . 122) have r e c e n t l y used s y n c h r o t r o n d a t a t o y l e l d t h e f i r s t d e t a i l e d r e f i n e m e n t of t h e i d e a l i z e d s t r u c t u r e of ZSM-11 d e r i v e d e l e v e n y e a r s ago by

23 dtstance least-squares modelling methods.

In addition, the use of anomalous

scattertng, carried out by tuning the X-ray wavelength on either side of a critical value for the atom under investtgation, ts already feasible, as has heen demonstrated by Ltang et a1 (ref. 123) for metallic catalysts supported on alumtna.

Anomalous scattertng, whtch has already heen used effectively by

protein crystallographers (ref. 124), to tackle the phase problem, has been elegantly harnessed (ref. 125) to ascertain how different tontc spectes tn Zr0.81Y0.1901.90

may occupy crystallographtcally equivalent sites.

High-resolution synchrotron X-ray powder dtffractograms were first reported in 1983 (ref. 126).

The continuing tmprovement In resolutton will, as is

evtdent from the foregoing, ensure extensive appltcatton of this technique as key review articles ( s e e refs. 127-129) and Ftgure 11 testify.

Higher

instrumental resolution obviously factlttates phase tdenttficatton. Single crystal Laue dtffractton explotts concurrently both the full X-ray spectral dtstrthutton and tts tntenstty.

The vartous planes of the fixed

crystal dtffract those beams from the tnctdent 'white' source for whtch Bragg's law is obeyed in that orientatton.

Structure reftnements and structure

solutions based on reflection intenstties measured tn Laue geometry have already been demonstrated.

Indeed Hajdu et a1 (refs. 130,131) have recorded

stngle Laue exposures from enzyme crystals, and followed enzyme catalysis on very raptd time-scales. 2400

1200

0 !O

Fig. 11. Sections of the powder diffraction profile o f the product of hydrothermal crystallization from a gel consisting of Cs20.Sn02.Si02 and water. The inset, which shows a higher resolution scan of a small portion of two theta, implies that at least two different phases, one giving sharper diffraction peaks than the other, are present (after Newsam).

24 MODEL SYSTEMS

A s w e l l as d e v e l o p i n g new methods of s t r u c t u r a l a n a l y s i s I t I s p r o f i t a h l e

a l s o t o e x t e n d e x l s t i n g ones e s p e c i a l l y s p e c t r o s c o p i c t e c h n i q u e s s u c h a s electron-spln-echo

( r e f . 132) vlsihle-u.v.-

and Raman measurements.

and photoluminescence ( r e f . 133)

But sometlmes even g r e a t e r i n s i g h t i n t o t h e b e h a v l o u r

of o r g a n l c g u e s t molecules i n s i d e z e o l i t i c c a g e s may he g l e a n e d from s t u d i e s of model h o s t s such a s t h o s e t h a t occur I n t h e u r e a and t h i o u r e a c h a n n e l complexes There a r e v e r y many a d v a n t a g e s i n u s l n g u r e a and t h i o u r e a a s model z e o l l t e s c o n t a l n l n g one-dimensional c h a n n e l s ( a s i n t h e t a - o n e and L) ( r e f . 1 3 4 ) .

First

It i s r e l a t i v e l y e a s y t o grow s i n g l e c r y s t a l s of t h e g u e s t - h o s t complex; and v e r y l o n g m o l e c u l e s ( e g . t h e h i g h e r a l k a n e s ) may he r e a d i l y I n s e r t e d , d u r l n g growth, i n t o t h e c h a n n e l s .

Second, s i n g l e - c r y s t a l X-ray s t u d i e s on such

s y s t e m s p r o v l d e l n f o r m a t l o n n o t y e t a t t a i n a b l e w i t h z e o l i t l c h o s t s , I n t o which,

i n any c a s e , I t I s n o t e a s y t o i n s e r t a d e q u a t e amounts of long-chaln g u e s t s . 2H NMR s p e c t r o s c o p y may he conducted on e l t h e r powdered o r s i n g l e - c r y s t a l

g u e s t - h o s t complexes; and a number of s u b t l e c h e m i c a l o p e r a t l o n s and t e s t s may be c a r r i e d o u t . Thus, a f t e r p r e p a r i n g s i n g l e - c r y s t a l s of u r e a - d l a c y l p e r o x i d e s , u.v.-Induced p h o t o l y s l s can l e a d t o i n s l t u p r o d u c t i o n of f r e e r a d i c a l s , t h e f a t e of which may he f o l l o w e d , a t any d e s i r e d t e m p e r a t u r e from l i q u l d He upwards, by r e c o r d l n g t h e magnitude of t h e z e r o - f i e l d

s p l i t t i n g (ZFS) In t h e ESR s p e c t r u m

of t h e c r y s t a l o r i e n t e d such t h a t t h e c h a n n e l a x i s i s p a r a l l e l t o t h e a p p l l e d magnetlc f l e l d .

Using t h l s t e c h n i q u e , i t h a s been found ( r e f . 135) t h a t t h e

a l k y l r a d i c a l s I n t h e p a i r s produced from undecanoyl peroxide-urea s e p a r a t e d by more t h a n 8 n-d34-hexadecane

are

a t t e m p e r a t u r e s as low a s 20 K ( F i g . 1 2 ) .

With

i n i t s u r e a I n c l u s i o n complex, 2H NMR s p e c t r o s c o p y may he

c o n v e n i e n t l y measured ( r e f s . 135,136) o v e r a range of t e m p e r a t u r e .

It

t r a n s p i r e s t h a t , a t room t e m p e r a t u r e , t h e r e I s motion w i t h l n i t s l o n g a x i s w l t h torsional l l h r a t i o n (of

t 2 5 ) a b o u t t h e p e n u l t l m a t e C-C

bond and r a p i d

There I s a n a b r u p t change

r o t a t i o n of t h e CD3 group about i t s symmetry a x i s .

i n t h e m o t l o n a l freedom of t h e hexadecane below t h e p h a s e - t r a n s i t l o n a t 147 K; hut t h e r e is s t i l l some motion below t h l s t e m p e r a t u r e . Another merlt of model s y s t e m s of t h i s k l n d i s t h a t t h e I n t r o d u c t i o n of bulky s i d e g r o u p s ( e g . a B r atom i n p l a c e of H i n s e r t e d a l o n g t h e backbone o f t h e g u e s t ) b r i n g s I n t o p l a y a w e l l - d e f i n e d h t n d r a n c e t o l l h r a t i o n a l and d i f f u s i v e motion w l t h t n t h e c h a n n e l s of t h e h o s t .

This a f f o r d s f u r t h e r

q u a n t l t a t i v e i n s i g h t i n t o t h e dynamics of g u e s t - h o s t

i n t e r a c t i o n , and h e l p s t o

h u l l d up a f u l l e r p i c t u r e of t h e s u b t l e t i e s of m o l e c u l a r r e c o g n l t l o n and m i g r a t i o n i n c o n s t r a i n e d environments whlch are a t t h e v e r y h e a r t of z e o l i t e science.

25

I

F i g . 1 2 . Long-chain of a u r e a h o s t , and be followed by z e r o m o t i o n a l freedom of ( r e f . 135, 1 3 6 ) .

a c y l p e r o x i d e s can be accommodated i n s i d e t h e c h a n n e l s t h e f a t e of t h e f r e e r a d i c a l s g e n e r a t e d on p h o t o l y s i s may *H s o l i d - s t a t e NMR r e v e a l s t h e e x t e n t o f f i e l d ESR. long-chain a l k a n e s such as n-dg4-hexadecane ( s e e t e x t )

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