3 Correlation Between Spectroscopic Measurements and Catalytic Behavior of Selective Oxidation Catalysts J. F. Brazdil, M. Mehicic, L. C. Glaeser, M. A. S. Hazle, and R. K. Grasselli Sohio Research Center, The Standard Oil Company (Ohio), Cleveland, OH 44128 Direct observations of heterogeneous catalytic processes on a molecular level only became possible with the advent of new spectroscopic techniques and in particular, by utilizing several techniques in concert on a given catalytic process. Combining such spectroscopic investigations with concomitant kinetic investigations provides new insights into the solid state and surface mechanisms of selective olefin oxidation catalysis by mixed metal oxides. The application of multiple techniques including Raman, infrared, XRD, neutron diffraction, and XPS has provided direct evidence of several key aspects of the propylene to acrylonitrile ammoxidation mechanism over bismuth molybdate based catalysts. The mechanistic insights include: 1) direct spectroscopic evidence for solid state disproportionation of the metastable Bi2Mo2O9 phase under redox conditions, 2) identification of key catalytic phases in bismuth-iron molybdate systems, and 3) direct identification of catalytic function of active oxide ions in bismuth molybdate selective olefin oxidation catalysts. Much o f the p i o n e e r i n g work w h i c h l e d t o the d i s c o v e r y o f e f f i c i e n t c a t a l y s t s f o r modern i n d u s t r i a l c a t a l y t i c processes was performed a t a time when advanced a n a l y t i c a l i n s t r u m e n t a t i o n was not a v a i l a b l e . Insights into catalytic phenomena were a c h i e v e d through gas adsorption, molecular r e a c t i o n probes, and m a c r o s c o p i c kinetic measurements. Although Sabatier postulated the existence of u n s t a b l e r e a c t i o n i n t e r m e d i a t e s a t the t u r n o f t h i s c e n t u r y , i t was not u n t i l the 1950 s t h a t such s p e c i e s were a c t u a l l y observed on solid s u r f a c e s by E i s c h e n s and co-workers(I) using infrared spectroscopy. Today, s c i e n t i s t s have t h e l u x u r y of u s i n g a multitude of s o p h i s t i c a t e d surface a n a l y t i c a l t e c h n i q u e s t o study c a t a l y t i c phenomena on a m o l e c u l a r level. Nevertheless, k i n e t i c measurements u s i n g c h e m i c a l l y s p e c i f i c probe molecules are s t i l l the f
0097-6156/85/0288-0026$06.00/0 © 1985 American Chemical Society
3.
B R A Z D I L ET A L .
27
Selective Oxidation Catalysts
main s t a y of m e c h a n i s t i c i n v e s t i g a t i o n s of heterogeneous c a t a l y t i c processes. I t i s , however, becoming i n c r e a s i n g l y apparent t h a t a combination o f k i n e t i c and s p e c t r o s c o p i c approaches can maximize t h e u s e f u l i n f o r m a t i o n about t h e s u r f a c e and b u l k p r o p e r t i e s o f s o l i d catalysts. I n t h e case of s e l e c t i v e o x i d a t i o n c a t a l y s i s , t h e use of s p e c t r o s c o p y has p r o v i d e d critical i n f o r m a t i o n about s u r f a c e and s o l i d s t a t e mechanisms. As i s w e l l known02), some of t h e most e f f e c t i v e c a t a l y s t s f o r s e l e c t i v e o x i d a t i o n o f o l e f i n s a r e those based on bismuth molybdates. The i n d u s t r i a l s i g n i f i c a n c e o f these c a t a l y s t s stems from t h e i r unique a b i l i t y t o o x i d i z e propylene and ammonia t o a c r y l o n i t r i l e a t h i g h s e l e c t i v i t y . S e v e r a l key f e a t u r e s of t h e s u r f a c e mechanism of t h i s c a t a l y t i c process have r e c e n t l y been d e s c r i b e d ( 3 - 4 ) . However, an u n d e r s t a n d i n g o f t h e s o l i d s t a t e t r a n s f o r m a t i o n s w h i c h occur on t h e c a t a l y s t s u r f a c e o r w i t h i n t h e c a t a l y s t b u l k under r e a c t i o n c o n d i t i o n s can o n l y be deduced i n d i r e c t l y by t r a d i t i o n a l probe m o l e c u l e approaches. Direct i n s i g h t s i n t o c a t a l y s t dynamics r e q u i r e t h e use o f t e c h n i q u e s w h i c h can probe t h e s o l i d d i r e c t l y , p r e f e r a b l y under r e a c t i o n c o n d i t i o n s . We have, t h e r e f o r e , examined s e v e r a l c a t a l y t i c a l l y i m p o r t a n t s u r f a c e and s o l i d s t a t e processes o f bismuth molybdate based c a t a l y s t s u s i n g m u l t i p l e s p e c t r o s c o p i c techniques i n c l u d i n g Raman and i n f r a r e d spectroscopies, x-ray and n e u t r o n d i f f r a c t i o n , and p h o t o e l e c t r o n spectroscopy. Experimental Catalyst Preparation Bi„MoO^ was prepared by t h e method of Védrine e t a l (5)· Bi^iMoO^)^ and B i . j F e M o 0 ^ were prepared by c o p r e c i p i t a t i o n o f bismuth n i t r a t e , f e r r i c n i t r a t e and ammonium molybdate. A f t e r drying, the c a t a l y s t s were h e a t - t r e a t e d a t 290° and 425°C f o r 3 hours each. F i n a l c a l c i n a t i o n a t 500°C was 2 hours f o r BiJfoO,. and 3 hours f o r Bi (Mo0 ) and 610°C f o r 12 hours f o r B i F e M o 0 . C h a r a c t e r i z a t i o n by b o t h x-ray d i f f r a c t i o n and Raman s p e c t r o s c o p y i n s u r e d t h a t t h e c a t a l y s t s were t h e pure s i n g l e phases d e s i r e d . 2
2
4
2
3
3
2
1 2
I n - S i t u Raman E x p e r i m e n t s I n - s i t u Raman experiments were performed on a Spex 1401 double monochrometer Raman s p e c t r o m e t e r , u s i n g a S p e c t r a - P h y s i c s Model 165 argon i o n l a s e r w i t h an e x c i t i n g w a v e l e n g t h of 5145 A. The i n - s i t u Raman c e l l c o n s i s t s of a q u a r t z tube s i t u a t e d i n a temperature c o n t r o l l e d h e a t i n g b l o c k . The Raman s p e c t r a were c o l l e c t e d i n t h e 180° b a c k s c a t t e r i n g mode. R e d u c t i o n of t h e c a t a l y s t s was a c h i e v e d by p a s s i n g t h e r e a c t a n t gas over t h e c a t a l y s t a t e l e v a t e d t e m p e r a t u r e s . The time was a d j u s t e d t o r e a c h t h e same degree o f r e d u c t i o n i n each c a s e , and t h e degree of r e d u c t i o n was checked i n d e p e n d e n t l y by 0« t i t r a t i o n . R e o x i d a t i o n of t h e c a t a l y s t was a c c o m p l i s h e d by f i r s t f l u s h i n g t h e system w i t h h e l i u m , then i n t r o d u c i n g oxygen-16 o r oxygen-18.
CATALYST C H A R A C T E R I Z A T I O N SCIENCE
28 K i n e t i c Experiments
A p u l s e r e a c t o r system s i m i l a r t o t h a t d e s c r i b e d by B r a z d i l , e t a l ( 6 ) was used t o o b t a i n t h e k i n e t i c d a t a . The r e a c t o r was a s t a i n l e s s - s t e e l U-tube, composed of a 1/8" χ 6" preheat zone and a 3/^" χ 6" r e a c t o r zone w i t h a maximum c a t a l y s t volume o f about 5.0 cm · The r e a c t o r was immersed i n a temperature c o n t r o l l e d m o l t e n s a l t bath. R e s u l t s and D i s c u s s i o n Redox P r o c e s s e s and S o l i d S t a t e T r a n s f o r m a t i o n s i n Bismuth Molybdates S e l e c t i v e o x i d a t i o n and ammoxidation of p r o p y l e n e over bismuth molybdate c a t a l y s t s occur by a redox mechanism whereby l a t t i c e oxygen ( o r i s o e l e c t r o n i c NH) i s inserted i n t o an a l l y l i c i n t e r m e d i a t e , formed v i a a-H a b s t r a c t i o n from t h e o l e f i n . The r e s u l t i n g anion vacancies are eventually f i l l e d by l a t t i c e oxygen w h i c h o r i g i n a t e s from gaseous oxygen d i s s o c i a t i v e l y chemisorbed a t s u r f a c e s i t e s w h i c h a r e s p a t i a l l y and s t r u c t u r a l l y d i s t i n c t from t h e s i t e s of o l e f i n oxidation. Mechanistic d e t a i l s about t h e reoxidation step are d i f f i c u l t t o o b t a i n s i n c e i t i s u s u a l l y much more r a p i d than t h e o x i d a t i o n o f t h e o l e f i n . T h e r e f o r e , t r a n s i e n t k i n e t i c techniques are required. Using the pulse microreactor method(6), t h e g e n e r a l rate e x p r e s s i o n f o r r e o x i d a t i o n o f bismuth nfolybdate c a t a l y s t s was found t o be: -d[0
e 5
ν
]/dt - k [0 ] = k [0J° [0 ] i app ν reox 2 ν A
where [0 ] i s t h e c o n c e n t r a t i o n of a n i o n v a c a n c i e s i n t h e c a t a l y s t a f t e r r e d u c t i o n w i t h p r o p y l e n e , [0^] i s t h e gas phase c o n c e n t r a t i o n of oxygen, ^ i s t h e p u l s e t i m e , and k^ and k a r e t h e apparent and a c t u a l r e o x i d a t i o n r a t e constantsf^respectively· Examination of t h e temperature dependence of t h e r e o x i d a t i o n r a t e c o n s t a n t s i n d i c a t e s t h e presence of two r e o x i d a t i o n p r o c e s s e s (Table I ) : a low a c t i v a t i o n energy s u r f a c e reoxidation process and a higher a c t i v a t i o n energy process i n v o l v i n g r e o x i d a t i o n o f a n i o n v a c a n c i e s i n the bulk. ο
Table I . A c t i v a t i o n Energies f o r Reoxidation Degree of I n i t i a l R e d u c t i o n [[0] χ 10^/m] 0.2 1.4 0.1 0.3 1.5 0.2 1.3 q
Catalyst Bi Mo~0 o
2
l o
6
1 2
Bi Mo 0 * o
1
o
1
Bi Mo0. o
2
6
Q
χ
(320° t o 460°C) A c t i v a t i o n Energy (kcal/mole) 1.3 25.9 8.1 9.6 25.8 1.2 7.9
29
Selective Oxidation Catalysts
B R A ^ D I L ET A L .
3.
As T a b l e I shows, t h e Bi2Mo20g c a t a l y s t e x h i b i t s unique redox behavior. S p e c i f i c a l l y , the s i m i l a r i t y i n the a c t i v a t i o n energies f o r the s l i g h t l y reduced Bi2Mo2ÛQ and t h e more d e e p l y reduced ^±2^o0^ suggests t h a t s i m i l a r s o l i d s t a t e phases a r e r e s p o n s i b l e f o r r e o x i d a t i o n i n both i n s t a n c e s . X-ray d i f f r a c t i o n of t h e catalysts a f t e r reduction and a f t e r r e o x i d a t i o n show o n l y t h e presence of t h e pure s i n g l e phase m a t e r i a l s . I n order t o i d e n t i f y the s u r f a c e phase c o m p o s i t i o n of the Bi2Mo20^ under redox conditions, the c a t a l y s t was examined using in-situ Raman spectroscopy. I n - s i t u Raman e x a m i n a t i o n of Bi2M©20^ under c y c l i c redox conditions gives e v i d e n c e of s u r f a c e r e s t r u c t u r i n g as shown i n F i g u r e 1. I t i s c l e a r from t h e s p e c t r a that a redox-induced disproportionation occurs. Subjecting t h e Bi2M©20^ phase t o r e d u c t i o n and r e o x i d a t i o n r e s u l t s i n i t s d i s p r o p o r t i o n a t i o n i n t o BÎ2Mo 0^2 and ^IJioO^. The d i s p r o p o r t i o n a t i o n may proceed by t h e f o l l o w i n g mechanism. 3
R
Bi Mo 0
9
Bi Mo 0
9
2
2
2
2
Bi Mo0 _ 2
Bi
Mo
6
e
d
u
c
t
l
o
n
» Βί Μοθ _ 2
+ Mo0 x
B i
R
e
o
x
i
d
a
t
2
°ï
+
+ (x+y)[0] + M o 0 _
χ
3
y
M o
• 2 3°12-y
3 = ?
+ x/2 0
2 3°12-y
6
R
i
o
%
Bi Mo0 2
eoxidation
6
, ^ ^ ^
Based on t h e k i n e t i c r e s u l t s ( T a b l e I ) , i t appears t h a t B i 2 M o 0 i s e n r i c h e d on t h e s u r f a c e while 2 °3°12 ^ P ^ s u b s u r f a c e l a y e r s of t h e r e c o n s t r u c t e d m a t e r i a l . M a i n t a i n i n g t h e sample a t e l e v a t e d temperature r e s u l t s i n f u r t h e r d e c o m p o s i t i o n u n t i l o n l y Bi2Mo0^ and Bi2Mo 0^2 phases a r e observed. Therefore, although Bi Mo«0 i s m e t a - s t a b l e up t o about 500°C(7), i t i s u n s t a b l e i n tfie presence of the B i M o 0 and *2**°3^12 P * h n u c l e a t i o n c e n t e r s and a c c e l e r a t e t h e d i s p r o p o r t i o n a t i o n . S i m i l a r experiments w i t h Bi^MoO. and B i 2 M o 0 2 r e v e a l t h a t they m a i n t a i n t h e i r s t r u c t u r a l i n t e g r i t y under redox conditions. S i m i l a r d i s p r o p o r t i o n a t i o n i s l i k e l y t o occur during c a t a l y t i c h y d r o c a r b o n o x i d a t i o n s i n c e t h e Βΐ2Μθ2θ^ c a t a l y s t i s s u b j e c t e d t o c o n t i n u o u s redox c y c l i n g under such c o n d i t i o n s . T h e r e f o r e , any k i n e t i c or c a t a l y t i c information about Bi2Mo20 c o m b i n a t i o n w i t h Bi^Ho^0.^ /3-FeMoO^ and a s m a l l amount o f Bi^MoO^. The key f e a t u r e s o f t h e Raman s p e c t r a o f t h e a c t i v a t e d c a t a l y s t a r e summarized i n T a b l e I I I . i
n
3
9
Table I I .
Catalytic Activity f o r P r o p y l e n e Ammoxidation B i s m u t h - I r o n Molybdate
Catalyst
A c r y l o n i t r i l e YieldT%7
Bi FeMo 0 After restructuring 3
2
1 2
A c r y l o n i t r i l e S e l e c t i v i t y (%)"
50.9
78.4
73.0
83.3
a)
R e a c t i o n t e m p e r a t u r e : 430*C Feed R a t i o s : C ^ / N ^ A ^ / ^ l / l .2/1.9/12.3
b)
Reaction induced r e s t r u c t u r i n g
Table I I I .
Raman Data F o r B i F e M o 0 3
2
1 2
From M i c r o r e a c t o r
Band P o s i t i o n s (cm )
vs s m
Over
Assignments
800 vs
y-Bi Mo0
872 s
Bi FeMo 0
902 m
a-Bi (Mo0 )
932 m
0-FeMoO
2
3
2
2
6
1 2
4
3
4
very strong strong medium
Comparison o f t h e k i n e t i c and s p e c t r o s c o p i c r e s u l t s i n d i c a t e s that the c a t a l y t i c a l l y active phase f o r p r o p y l e n e ammoxidation i s the predominant bismuth c o n t a i n i n g phase, namely B i 2 M o 0 2 >
