Chapter 4
Silicoaluminophosphate Molecular Sieves X-ray Photoelectron and Solid-State NMR Spectroscopic Results Compared
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David F. Cox and Mark E. Davis Department of Chemical Engineering, Virginia Polytechnic Institute and State University, Blacksburg, VA 24061
The local environments of T-atoms in SAPO materi als were examined using solid-state NMR, a bulk probe, and XPS, a surface sensitive probe. T-atom 2 p binding energies in XPS were found to vary in a predictable fashion with changes in NMR chemical shifts. The comparison demonstrates that XPS is sensitive to variations in the second coordination sphere for T-atoms in SAPO molecular sieves. XPS was also found to give a reasonable, quantitative measure of superficial (surface) Τ-atom fractions thus providing information about elemental homogeneity by comparison to bulk chem ical analysis. S i l i c o a l u m i n o p h o s p h a t e s (SAPO's) (1) a r e m o l e c u l a r s i e v e s w h i c h c o n t a i n t e t r a h e d r a o f oxygen s u r r o u n d e d s i l i c o n , aluminum, a n d phosphorus. These m i c r o p o r o u s s o l i d s n o t o n l y e x h i b i t p r o p e r t i e s c h a r a c t e r i s t i c o f z e o l i t e s b u t a l s o show u n u s u a l p h y s i o c h e m i c a l t r a i t s a s c r i b a b l e t o t h e i r unique chemical compositions (1,2). We have been i n v e s t i g a t i n g t h e n a t u r e o f t h e s e n o v e l s o l i d s i n o r d e r t o a s c e r t a i n i n f o r m a t i o n w h i c h may l e a d t o new c a t a l y t i c applications. Two i m p o r t a n t q u e s t i o n s t o be a d d r e s s e d a r e : ( i ) what a r e t h e l o c a l Τ-atom ( t e t r a h e d r a l atom) arrangements ( s i l i c o n , aluminum, and p h o s p h o r u s ) , and ( i i ) what i s t h e e l e m e n t a l homogeneity? Q u e s t i o n s ( i ) h a s been a d d r e s s e d m a i n l y t h r o u g h t h e u s e o f s o l i d - s t a t e NMR w h i l e t h e answer t o q u e s t i o n ( i i ) i s most a p p r o p r i a t e l y o b t a i n e d from s u r f a c e t o b u l k e l e m e n t a l analyses. The purpose o f o u r work i s t o show t h a t t h e l o c a l environments o f S i , A l , and Ρ i n SAPO's c a n be p r o b e d v i a s o l i d - s t a t e NMR and XPS and t h a t t h e i r r e s u l t s g i v e a c o n s i s t e n t p i c t u r e o f t h e second c o o r d i n a t i o n s h e l l f o r a l l t h e T-atoms. A l s o , o f s e c o n d a r y importance, we w i l l i l l u s t r a t e t h a t XPS c a n be u s e f u l i n a s c e r t a i n i n g whether e l e m e n t a l homogeneity i s a c h i e v e d i n SAPO m o l e c u l a r s i e v e s .
0097-6156/90/0437-0038$06.00/0 © 1990 American Chemical Society In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
4. COX AND DAVIS
39
SUicoaluminophosphate Molecular Sieves
Experimental Section The samples u s e d i n t h i s s t u d y a r e t h o s e t h a t have b e e n d e s c r i b e d p r e v i o u s l y (3-5). Magic a n g l e s p i n n i n g ^ S i , 27 &\ j 31p s p e c t r a were r e c o r d e d on a v a r i e t y o f s p e c t r o m e t e r s . The d a t a r e p o r t e d h e r e a r e from r e s u l t s o b t a i n e d a t 200 MHz p r o t o n . Thus, t h e S i , A1 and P f r e q u e n c i e s a r e 39.7 MHz, 52.15 MHz and 81.0 MHz, respectively. Chemical s h i f t s f o r aluminum a r e r e p o r t e d r e l a t i v e t o Αΐ(Νθ3)β i n aqueous s o l u t i o n a t i n f i n i t e d i l u t i o n and a r e n o t c o r r e c t e d f o r s e c o n d - o r d e r quadrupole e f f e c t s . The phosphorus and s i l i c o n c h e m i c a l s h i f t s a r e r e p o r t e d r e l a t i v e t o 85 wt% H3PO4 and Me^Si, r e s p e c t i v e l y . B u l k c h e m i c a l a n a l y s e s o f t h e s o l i d was p e r f o r m e d b y G a l b r a i t h L a b o r a t o r i e s , K n o x v i l l e , TN. C h e m i c a l a n a l y s i s o f t h e s u p e r f i c i a l r e g i o n s o f t h e samples was p e r f o r m e d b y X - r a y p h o t o e l e c t r o n s p e c t r o s c o p y (XPS) w i t h a P e r k i n - E l m e r P h i 5300 ESCA, employing Mg Κ α x - r a y s . Binding e n e r g i e s a r e r e f e r e n c e d t o t h e g o l d 4 f 7/2 t r a n s i t i o n a t 83.8 eV ( g o l d was s p u t t e r e d onto a s m a l l s p o t o f t h e sample p r i o r t o analysis). a
n
(
2 9
2 7
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3 1
R e s u l t s and D i s c u s s i o n B u l k and S u r f a c e C o m p o s i t i o n s . The c h e m i c a l c o m p o s i t i o n s o f t h e m o l e c u l a r s i e v e s used i n t h i s s t u d y a r e g i v e n i n T a b l e I i n terms o f t e t r a h e d r a l atom (T-atom) f r a c t i o n s , and a r e grouped a c c o r d i n g to s t r u c t u r e type. The b u l k c o m p o s i t i o n s o f AIPO4-5, AIPO4-2O and VPI-5 show t h e i d e a l 1:1 r a t i o o f A l and Ρ c h a r a c t e r i s t i c o f aluminophosphate m o l e c u l a r s i e v e s . The SAPO m a t e r i a l s have frameworks c o n s i s t i n g o f S i , A l and Ρ T-atoms. The two SAPO-20 m a t e r i a l s have v e r y d i f f e r e n t b u l k concentrations. SAPO-20A has a c o m p o s i t i o n c o n s i s t e n t w i t h a " s i l i c o n - s u b s t i t u t e d aluminophosphate", w h i l e t h e h i g h b u l k c o n c e n t r a t i o n o f S i i n SAPO-20B i s more c o n s i s t e n t w i t h a "phosporous-substituted aluminosilicate". The o t h e r SAPO samples a l l have s u f f i c i e n t l y low c o n c e n t r a t i o n s o f S i t o be c o n s i d e r e d as s i l i c o n - s u b s t i t u e d ALPO^'s. S u p e r f i c i a l (surface) compositions a r e a l s o l i s t e d i n Table I as d e t e r m i n e d b y XPS w i t h atomic s e n s i t i v i t y f a c t o r s c h a r a c t e r i s t i c o f t h e e l e c t r o n energy a n a l y z e r u s e d f o r t h e s e studies. The s u p e r f i c i a l c o m p o s i t i o n s agree r e a s o n a b l y w e l l w i t h the b u l k v a l u e s f o r a l l samples w i t h t h e e x c e p t i o n s o f SAPO-5 and the two Si-VPI-5 samples which show a s i g n i f i c a n t e n r i c h m e n t o f S i a t t h e s u r f a c e . Note t h a t samples which were examined w i t h and w i t h o u t d e p o s i t e d g o l d ( t h e r e f e r e n c e f o r t h e b i n d i n g energy s c a l e ) showed l i t t l e d i f f e r e n c e i n measured c o m p o s i t i o n s w i t h XPS. The g e n e r a l l y good agreement between b u l k and s u p e r f i c i a l c o m p o s i t i o n s f o r homogeneous samples p r o v i d e s some a s s u r a n c e t h a t XPS, d e s p i t e i t s s u r f a c e s e n s i t i v i t y , samples a s u f f i c i e n t p o r t i o n o f t h e framework s t r u c t u r e t o g i v e r e a s o n a b l e T-atom c o m p o s i t i o n s . The c o m p o s i t i o n d e t e r m i n a t i o n s a r e a i d e d b y t h e f o r t u i t o u s o c c u r r e n c e o f t h e 2p p h o t o e l e c t r o n peaks w i t h i n a f a i r l y narrow range o f v a l u e s a t low b i n d i n g e n e r g i e s ( S i 2p, A l 2p and Ρ 2p a t
In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
40
NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS TABLE I .
S u p e r f i c i a l and B u l k C o m p o s i t i o n s
Sample
SAPO-37
B u l k Comp.
Si
AlP0 -5
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A1
1 2
Al
4
SAPO-5
0
Si
AIPO4-20
0 0 8
SAPO-20A
Si
0 1 3
SAPO-20B
Si
0 5 0
VPI-5
5
0
A1
Al
0
P
5
0
0
Si
3 8
P
0
4
Si
0
P .
4 7
0
0
5 1
3 8
P
0
A1
4 0
0
4
9
Q
5 4
P
33
0
P
0.52 0.48
20
A1 .51 0.29 0
Al
A1 . P .12
Al
1 3
P
Q
5
0 m 5
A1 .
0
Q
5
P
Sieves
S u p e r f i c i a l Comp.
Al 0
4 7
0m 5
P
of Molecular
Si Si
Q
0m 5 2
1 0
P.ο .
A1 . 0
5 5
Al
4 8
P .35 0
P
0.41 0.44 0.15
A1
Si-VPI-5A
Si
0 0 6
A1
0
4 9
P
0
45
Si
Q
Si-VPI-5B
Si
0 0 9
A1
0
4 8
P
0
43
Si
0
1 ?
0 > 5 4
P
A1 . 0
0 >
5 4
4
6
P . 0
2 9
P
3 1
A1 . 7 0.32 0
3
a p p r o x i m a t e l y 102, 75 and 135 eV, r e s p e c t i v e l y ) . These low b i n d i n g e n e r g i e s c o r r e s p o n d to h i g h v a l u e s o f p h o t o e l e c t r o n k i n e t i c energy and r e p r e s e n t the l e a s t s u r f a c e - s e n s i t i v e p o r t i o n o f the XPS spectrum. The s i m i l a r i t i e s i n p h o t o e l e c t r o n k i n e t i c e n e r g i e s g i v e s r i s e to s i m i l a r i n e l a s t i c mean-free p a t h s . Thus, the s i g n a l a t t e n u a t i o n w i t h r e s p e c t t o depth i n the sample i s e s s e n t i a l l y c o n s t a n t f o r a l l t h r e e t y p e s o f T-atoms. The agreement between XPS and b u l k a n a l y s i s f o r the m a j o r i t y o f the samples s u p p o r t s the c o n c l u s i o n t h a t s i g n i f i c a n t v a r i a t i o n s between the measured b u l k and s u p e r f i c i a l c o m p o s i t i o n s a r e i n d e e d due t o inhomogeneity i n the sample. The c o m p o s i t i o n s measured by XPS i n t h e s e c a s e s a l s o g i v e r e l a t i v e T-atom f r a c t i o n s which agree w i t h known mechanisms o f s i l i c o n s u b s t i t u t i o n i n aluminophosphate m a t e r i a l s (see d i s c u s s i o n b e l o w ) . The SAPO-5 c o m p o s i t i o n d a t a i n T a b l e I i n d i c a t e an enrichment o f s i l i c o n a t the s u r f a c e , and the c o m p o s i t i o n s u g g e s t s s i l i c o n s u b s t i t u t e s p r i m a r i l y f o r phosphorus. The two Si-VPI-5 samples p r e p a r e d by d i f f e r e n t s y n t h e t i c r o u t e s b o t h show an enrichment o f s i l i c o n a t the s u r f a c e , b u t i n SÎ-VPI-5A ( s y n t h e s i z e d from a s i n g l e phase s y n t h e s i s g e l ( 5 ) ) the s u b s t i t u t i o n appears to be p r i m a r i l y f o r phosphorus. In c o n t r a s t , s u r f a c e s i l i c o n i n Si-VPI-5B appears t o s u b s t i t u t e f o r aluminum-phosphorus p a i r s . The two phase s y n t h e s i s u s e d t o p r e p a r e SÎ-VPI-5B (5) appears t o y i e l d a s i g n i f i c a n t l y l e s s homogeneous c r y s t a l w i t h more a s i l i c o n - r i c h s u r f a c e than the c o r r e s p o n d i n g s i n g l e phase s y n t h e s i s f o r SÎ-VPI-5A.
In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
4. COX AND DAVIS
Silicoaluminophosphate Molecidar Sieves
41
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XPS-NMR Comparisons. W h i l e the p h y s i c s a s s o c i a t e d w i t h NMR and XPS measurements a r e v e r y d i f f e r e n t , b o t h t e c h n i q u e s a r e s e n s i t i v e t o changes i n the l o c a l environment and e l e c t r o n i c s t r u c t u r e o f the i n d i v i d u a l a t o m i c s p e c i e s . The o b s e r v a t i o n o f a c o r r e l a t i o n between b u l k T-atom e n v i r o n m e n t s p r o b e d by NMR and s u r f a c e T-atom e n v i r o n m e n t s p r o b e d by XPS would i n d i c a t e t h a t XPS t r u l y p r o v i d e s i n f o r m a t i o n about the framework s t r u c t u r e and n o t j u s t a d e f e c t i v e r e g i o n a t the s u r f a c e o f the c r y s t a l s . Phosphorus. P r e v i o u s **lp s o l i d - s t a t e NMR s t u d i e s o f SAPO m a t e r i a l s have d e t e c t e d o n l y a s i n g l e t y p e o f l o c a l e n v i r o n m e n t f o r phosphorus T-atoms: f o u r aluminum s e c o n d n e a r e s t n e i g h b o r s as i n a p u r e a l u m i n o p h o s p h a t e (ALPO^) m a t e r i a l ( 3 , 4 ) . This e n v i r o n m e n t i s i l l u s t r a t e d i n F i g u r e 1. The NMR c h e m i c a l s h i f t s from t h i s environment t y p i c a l l y range from -25 t o -35 ppm. F i g u r e 2 compares the Ρ 2p b i n d i n g e n e r g i e s from XPS w i t h the ^ I p c h e m i c a l s h i f t s from s o l i d - s t a t e NMR. Chemical s h i f t s are r e p o r t e d as s i n g l e p o i n t s e x c e p t f o r the VPI-5 m a t e r i a l s w h i c h show two ^Ip NMR peaks a s s o c i a t e d w i t h d i f f e r e n t c r y s t a l l o g r a p h i c l o c a t i o n s i n the VPI-5 t o p o l o g y ( 6 ) . The two d i f f e r e n t c h e m i c a l s h i f t v a l u e s f o r the VPI-5 m a t e r i a l s a r e j o i n e d by v e r t i c a l l i n e s i n F i g u r e 2, b u t each i s c o n s i s t e n t w i t h the l o c a l e n v i r o n m e n t i l l u s t r a t e d i n F i g u r e 1. The XPS b i n d i n g e n e r g i e s v a r y o v e r a range o f o n l y 0.3 eV. S i n c e the t y p i c a l a c c u r a c i e s f o r d e t e r m i n i n g b i n d i n g e n e r g i e s a r e ± 0.1 eV, the s p r e a d i n Ρ 2p b i n d i n g e n e r g i e s i s e s s e n t i a l l y t h a t e x p e c t e d from e x p e r i m e n t a l uncertainty. A l s o , the narrow range o f b i n d i n g e n e r g i e s f o r Ρ 2p core l e v e l s i n s i m i l a r l o c a l environments but d i f f e r e n t m o l e c u l a r s i e v e s d e m o n s t r a t e s t h a t the b i n d i n g e n e r g y s c a l e r e f e r e n c e d t o Au 4 f η / 2 r e p r o d u c i b l y a c c o u n t s f o r d i f f e r e n c e s i n the e x t e n t o f c h a r g i n g f o r the d i f f e r e n t samples. The XPS r e s u l t s a r e i n agreement w i t h NMR i n t h a t no s i g n i f i c a n t v a r i a t i o n s i n the b i n d i n g e n e r g i e s a r e measured f o r phosphorous T-atoms w h i c h a r e c o n f i n e d t o a s i n g l e t y p e o f l o c a l (2nd c o o r d i n a t i o n s p h e r e ) environment. These r e s u l t s agree w i t h a p r e v i o u s XPS s t u d y by S u i b e t a l . o f oxygen p o l a r i z a b i l i t i e s i n SAPO's ( 7 ) . Those s t u d i e s f o u n d no v a r i a t i o n i n the p o l a r i z a b i l i t y o f oxygen i o n s s u r r o u n d i n g P^ s i t e s ( i . e . , no l o c a l e l e c t r o n i c v a r i a t i o n s ) as a f u n c t i o n o f s i l i c o n incorporation (7). +
2 7
Aluminum. Previous A 1 NMR s t u d i e s have d e m o n s t r a t e d f o u r p o s s i b l e l o c a l e n v i r o n m e n t s f o r A l i n SAPO m a t e r i a l s ( 3 , 4 ) . These e n v i r o n m e n t s a r e i l l u s t r a t e d i n F i g u r e 3, and may be c l a s s i f i e d as e i t h e r phosphorous r i c h ( i . e . , A L P O ^ - l i k e ) w i t h a c h e m i c a l s h i f t r a n g i n g from 30 t o 40 ppm, or s i l i c o n r i c h ( i . e . , z e o l i t e - l i k e ) w i t h a c h e m i c a l s h i f t g r e a t e r t h a n 48 ppm. Both types o f e n v i r o n m e n t s a r e c h a r a c t e r i s t i c o f a s u b s t i t u t i o n mechanism i n v o l v i n g s i l i c o n s u b s t i t u t i o n f o r phosphorus. A fifth p o s s i b i l i t y f o r an A l environment i n v o l v e s two S i and two Ρ s e c o n d nearest neighbors. However, no s u c h e n v i r o n m e n t has y e t b e e n i d e n t i f i e d by NMR, e i t h e r because the A l c h e m i c a l s h i f t i s s i m i l a r to t h a t f o r the s i l i c o n - o r p h o s p o r o u s - r i c h e n v i r o n m e n t s , o r b e c a u s e m a t e r i a l s w i t h an a p p r o p r i a t e l e v e l o f S i t o g i v e r i s e t o
In Novel Materials in Heterogeneous Catalysis; Baker, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1990.
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NOVEL MATERIALS IN HETEROGENEOUS CATALYSIS
AI
δ - -25 to-35 ppm F i g u r e 1. Phosphorus s o l i d - s t a t e NMR.
1
1
1
1
e n v i r o n m e n t s d e t e r m i n e d f r o m 31p
1
1
1
1
1
1
1
1
1
1
1
1
Phosphorus
25 SÎ-VPI-5B
VPI -5
Τ SAPO-5
Τ
I
·
• AIPO4-5
30
Si-VPI-5^
• SAP0-20A 1
^ · #AIP0 -20
