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Co-Mo-O-Ti, A New Catalyst Effective in Butane Oxidation to Maleic Anhydride
M. Che and G.C. Bond (Editors), Adsorption and Catalysis on Oxide Surfaces 0 1985 Elsevier Sciencp Publishers B.V.. Amsterdam - Printed in T h e Netherlands
Co-Mo-0-Ti,
345
A NEW CATALYST EFFECTIVE I N BUTANE OXIDATIOP! TO MALEIC AP!HYDRIDE
J.S. JUNG, E. BORDES and P . COURTINE D e p a r t e m e n t de G e n i e C h i m i q u e , U n i v e r s i t e de T e c h n o l o g i e de Compiegne, B.P.
233,
60206 Compiegne Cedex ( F r a n c e )
ABSTRACT By a p p l y i n g t h e c r i t e r i a a l r e a d y drawn f r o m numerous examples o f m u l t i c o m p o n e n t c a t a l y s t s o f m i l d o x i d a t i o n i n v o l v i n g i n t e r f a c i a l e f f e c t s , we have f o u n d t h a t CoMo04/Mo03-Ti02 c o u l d b e a s e l e c t i v e c a t a l y s t i n t h e r e a c t i o n b u t a n e m a l e i c a n h y d r i d e . The m o s t e f f e c t i v e c o m p o s i t i o n i s Co/Mo/Ti = 1/21,5/42.5, for w h i c h a s e l e c t i v i t y o f 46.4 mol. YC i s o b t a i n e d a t 53 m o l . ‘i c o n v e r s i o n o f b u t a n e . The p o l y m o r p h i c t r a n s i t i o n s CoMo04 ( b ) + ( a ) a n d a n a t a s e + r u t i l e a r e a c c e l e r a t e d ( o r d e l a y e d ) a c c o r d i n g t o t h e phases f a c e t o f a c e . I t i s c o n c l u d e d t h a t a t t h e s t e a d y s t a t e i n t e r f a c i a l e f f e c t s t a k e p l a c e between t h e c l e a v a g e p l a n e s ( 1 1 0 ) CoMoO,(b), (OIO), ( 1 0 0 ) MOO, and (OIO), ( 0 1 1 ) T i O z a n a t a s e .
INTRODUCTION When a c a t a l y t i c phase a l o n e i s n o t a b l e b y i t s e l f t o g i v e s u b s t a n t i a l y i e l d s o f a g i v e n p r o d u c t i n m i l d o x i d a t i o n c a t a l y s i s , dopes, o r a c t i v e s u p p o r t ( T i D z f o r e x a m p l e ) , o r o t h e r a c t i v e phases ( m u l t i c o m p o n e n t n o l y b d a t e s ) a r e g e n e r a l l y added t o i m p r o v e t h e a c t i v i t y and s e l e c t i v i t y . The known c r i t e r i a o f s e l e c t i v i t y a r e o f course involved i n these multiphasic c a t a l y s t s , b u t the decisive p a r t comes f r o m t h e i n t e r f a c i a l phenomena o c c u r i n g between t h e s e phases ( r e f . 1 ) . L o o k i n g f o r a c a t a l y s t o t h e r t h a n (VO)2P2D7 i n t h e o x i d a t i o n o f b u t a n e t o m a l e i c a n h y d r i d e , we have c h o s e n t h e s y s t e m CoMo04-Mo03-Ti02a n a t a s e f o r t w o m a i n r e a s o n s : ( i ) CoMo04 i s known t o b e a c t i v e a n d s e l e c t i v e i n t h e o x i d a t i v e dehyd r o g e n a t i o n o f b u t a n e i n t o b u t e n e and b u t a d i e n e ( r e f . 2 , 3 )
and Moo3 s u p p o r t e d b y
TiOz anatase i s a c t i v e f o r t h e t r a n s f o r m a t i o n o f butene (ref.4,5) (ref.6)
and b u t a d i e n e
i n t o m a l e i c a n h y d r i d e , ( i i ) a l l y 1 s p e c i e s c a n be formed on CoMo04 w i t h o u t
desorption (ref.7),
whereas t h e s e s p e c i e s a r e t r a n s f o r m e d i n t o o x y a e n a t e d com-
p l e x e s o n MOO, w h i c h a r e t h e n d e s o r b e d ( r e f . 8 ) . On t h e o t h e r hand, we have shown r e c e n t l y t h a t such o x i d e s o r o x y s a l t s b e l o n g t o a c l a s s i f i c a t i o n o f m i l d o x i d a t i o n c a t a l y s t s we s e t u p , t h e i r thermodynamic and s t r u c t u r a l p r o p e r t i e s a l l o w i n g i n t e r f a c i a l e f f e c t s which a r e evidenced b y coherent boundaries (ref.1).
A f t e r t h e c a l c u l a t i o n of t h e c r y s t a l l o g r a p h i c mis-
f i t s between each component o f t h e s y s t e m CoMoOh-MoO3-Ti02 showing t h a t c o h e r e n t b o u n d a r i e s c o u l d e x i s t between t h e c l e a v a g e p l a n e s , we h a v e u n d e r t a k e n t h e c a t a l y t i c s t u d y o f t h e r e a c t i o n b u t a n e t o m a l e i c a n h y d r i d e o n t h i s system.
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EXPERIMENTAL T h e preparation of the c a t a l y s t i s c a r r i e d out in two steps : the "support"
Mo03-TiC12 being f i r s t obtained, i s then added as a s o l i d powdered phase t o a solution of cobalt n i t r a t e and ammonium heptamolybdate ; the s l u r r y i s heated a t 80°C ( 2 h r s ) , b r o u g h t t o dryness with s t i r r i n g , a n d the s o l i d residue i s c a l c i ned in a i r a t 550°C ( 1 6 h r s ) . Different preparations of the support have been used : ( i ) Ti02 anatase ( T h a n n e t Mulhouse) ( 7 m'g-l) i s suspended i n ammonium heptamolybdate s o l u t i o n , the r e s u l t a n t s l u r r y heated a t 80°C ( 2 h r s ) , brought t o dryness with s t i r r i n g a n d dried in a n oven a t 120°C ( 5 h r s ) , ( i i ) TiC14 i s hydrolyzed by water and ammonia a t O'C, yielding a p r e c i p i t a t e of titanium hydroxide which i s f i l t e r e d , washed, and added t o molybdenum solution as above ; such c a t a l y s t s will be specified a s Mo03-TiOe(H) f u r t h e r in the t e x t ; ( i i i ) s o l u t i o n s of titanium and molybdenum t a r t r a t e s of known composition a r e prepared according t o ref.9 and mixed in t h e desired r a t i o . This solution i s evaporated under vacuum, yielding a vitreous black precursor which i s dried a t 120°C i n an oven and calcined a t 500°C in a i r ( 8 h r s ) , r e f e r r e d a s MoCl3-TiO2(T). The c a t a l y t i c runs have been performed in a conventional fixed-bed r e a c t o r , with chromatographic analysis of t h e feed and products ( r e f . 5 ) , in t h e temperat u r e range 390 t o 490°C, f o r various contact times ( 1 t o 9 s e c . ) and 1-2 % butane i n the a i r feed. The s t a i n l e s s - s t e e l r e a c t o r contained 30 ml of broken pieces of supported c a t a l y s t . The main part of t h e e f f l u e n t passed t h r o u g h water bubblers and was d i r e c t e d t o two chromatographs f o r a n a l y s i s of permanent gases and hydrocarbons. A branching s i t u a t e d in f r o n t of t h e bubblers led t h e e f f l u e n t towards a t h i r d chromatograph t o analyze the oxygenated compounds (mainly a c e t i c and a c r y l i c a c i d s , and maleic anhydride). X-ray d i f f r a c t i o n (diffractometer. C u Ka), infrared spectroscopy (KBr d i s c s ) and e l e c t r o n microscopy ( 1 0 0 kV) have been systematically performed on pure phases and c a t a l y s t s . As f a r a s possible the samples were n o t ground (except in IR s t u d i e s ) a n d simply displayed on t h e sample holder ( X R D ) or dispersed by u l t r a s o n i c waves (electron microscopy). The r e a c t i v i t y of t h e c a t a l y s t s under a i r o r nitrogen flow (pOn = atm) has been examined by thermomicrogravimetric analysis (TGA) and semimicro DTA ( c r u c i b l e content 6 u l ) . RESULTS
Catalytic results Various compositions concerning Mo/Ti in t h e support, Co/Mo i n t h e a c t i v e phase and t h e r a t i o a c t i v e phase/support have been prepared, in order t o f i n d the more e f f e c t i v e composition i n each kind of preparation modes. The c a t a l y t i c r e s u l t s ( s e e Fig. 1 and 2 ) show respectively t h a t t h e highest y i e l d s in maleic
341
anhydride are obtained f o r 30 pol.' w i t h 10 w t
Mo0,in 7 0 m o l .
o f a c t i v e Co-Mo-0 phase on 9Gwt
TiO, and Co/Mo
-
1.0 t o 1.9,
M o O j - T i 0 2 . The same b e h a v i o u r i s
o b s e r v e d w i t h t h e samples o f s u p p o r t ( H I o r ( T ) p r e p a r e d b y d i f f e r e n t ways. I n t h e same c o n d i t i o n s , CoMoO, a l o n e y i e l d s 5 m o l . ,
o f m a l e i c a n h y d r i d e and 1.8 %
o f a c r y l i c a c i d a t 2 1 . 6 '' c o n v e , - s i o n o f b u t a n e , t h e m a i n p r o d u c t bein:
Ti02
mol%
-
butadiene
f
100 Moo3
f f
F i g . 1. A c t i v i t y and s e l e c t i v i t y o f 10 wt% CoMoOt+/9O % Moos-Ti02 as a f u n c t i o n o f t h e c o m p o s i t i o n o f t h e s u p p o r t (490"C, T = 9 sec., 1.5 % CQHlo . Legend : b l a c k c i r c l e s : c o n v e r s i o n ; b l a c k s q u a r e : s e l e c t i v i t y m a l e i c anh. ; open c i r c l e s : y i e l d C O + C02 ; open squares : y i e l d m a l e i c anh. ; open t r i a n g l e y i e l d a c r y l i c acid.
-
-Co
I Mo
F i g . 2. A c t i v i t y and s e l e c t i v i t y o f Co-Mo-0 o n s u p p o r t 3 0 mol.% Moo3 on TiOn, as a f u n c t i o n o f t h e c o m p o s i t i o n o f t h e a c t i v e phase. Same l e g e n d a s above.
348
Since t h e s u r f a c e area o f t h e s u p p o r t s
(H) o r (T) i s improved as compared t o
t h e above c a t a l y s t ( r e s p e c t i v e l y 18 and 28 m ' g - l ) , duced f r o m 9 t o 1 sec.,
t h e c o n t a c t t i m e can be r e -
and t h e t e m p e r a t u r e f r o m 490 t o 420°C as a r e s u l t o f
g r e a t e r a c t i v i t y ( T a b l e 1 ) . The b e s t s e l e c t i v i t y i s o b t a i n e d f o r C O M O O L , / M O O ~ T i 0 2 ( H ) , whereas s u p p o r t ( T ) g i v e s an h i g h e r a c t i v i t y and t h e r e f o r e a h i g h e r y i e l d i n m a l e i c anhydride. I n a l l cases ( e x c e p t f o r CoMoO, a l o n e ) n e i t h e r b u t a d i e n e n o r f u r a n were observed on t h e chromatograms, and o n l y few amount of acet i c and a c r y l i c a c i d s a r e desorbed as oxyaenated compounds. TABLE 1 E f f e c t o f t h e p r e p a r a t i o n o f t h e s u p p o r t on c a t a l y t i c p r o p e r t i e s
Kind o f support
TiOE a
Temp.
T
Conv.
"C
sec.
mol. %
49 0
9
51.9
Sel ec.
Y i e l d s , mol.? CO
t
CO
25.8
MAA
AA
21.2
2.2
40.8
3.0 0.5
40.2
TiC2(H)
420
1
53.0
27.1
24.8
Ti02(T)
420
1
70.0
40.2
28.0
MAA 7;
46.4
a T i 0 2 7 m'9-l washed w i t h b o i l i n g w a t e r ( 1 h r ) , f i l t r a t e d , d r i e d a t 120°C i n an oven (20 h r s ) and t h e n added t o molybdenum s o l u t i o n . S t r u c t u r a l p r o p e r t i e s and r e a c t i v i t y The comparison o f XRD p a t t e r n s o f f r e s h and used c a t a l y s t s w i t h t h o s e o f p u r e CoMoO,(a),
( b ) , Moo3 and TiOn anatase a l l o w s t o know what phases a r e p r e s e n t and
i f t h e i r t e x t u r e i s m o d i f i e d as compared t o p u r e compounds ( T a b l e 2) TABLE 2 Phases d e t e c t e d i n f r e s h and used c a t a l y s t s s i t i o n CoMoO ( b ) -+ ( a ) (DTA).
Mo 0 3
Ca t a 1y s t s +++
+
+
+++
CoM004/M003
+
CoMo04/MOO 3 - T i 0 2
+
-
Co/Mo
=
1
(CoMoO,)
Co/Mo = 1.9 (COMOOs + C O s O s )
CoMoO, / T i
(XRD), and t e m p e r a t u r e o f t h e t r a n -
02
CoMoOs/Ti 02 (H)
Ti02 anatase
Temp. (b) + ( a ) 490
+++ +++
+++ +++ ~
507
++++ ++++ ++++
520-530 450
500 ~
The number o f + r e f l e c t s a p p r o x i m a t e l y t h e r e l a t i v e i n t e n s i t i e s o f t h e main l i n e s o f t h e d i f f e r e n t phases.
349
The predominant form of CoMoO, when alone i s respectively ( b ) a n d ( a ) f o r Co/Mo = 1 . 0 a n d 1 . 9 together withsomeCo,O, in the l a s t case, a s observed by other workers ( r e f . l G , l l ) . The l i n e s of M O O ? of ( O k O ) type a r e more intense in t h e c a t a l y s t s t h a n the ( O 2 1 ) , ( 0 6 1 ) l i n e s which a r e usually higher in pure MOO,. T h i s means t h a t the layered t e x t u r e of MOO, along ( O k O ) planes i s improved by the preparation. No modification of TiOzpattern has been observed. Infrared spectra a r e i n accordance with these r e s u l t s . The e f f e c t of the support on t h e temperature of the polymorphic t r a n s i t i o n CoMoOt,(b) + ( a ) has been examined by D T A . The small endothermic peak corresponding t o t h i s t r a n s i t i o n appears a t 490°C f o r pure CoMoO, prepared in a s i m i l a r way as t h e c a t a l y s t s , while i t has been observed a t 430 ( r e f . 3 ) a n d 407°C ( r e f . 2 ) according t o t h e preparation a n d the c r y s t a l l i n i t y of t h e sample. I t can be seen on Table 2 t h a t Moos alone o r with TiOn delays t h e transformation while Ti02 alone seems t o favourize i t . The same kind of study has been carried o u t f o r the t r a n s i t i o n a n a t a s e - r u t i l e which cannot be observed by DTA because i t i s t o o sluggish. The c a t a l y s t s have been heated in a N Z stream a t high temperature (600 t o 860°C) a n d the XRD pat-
t e r n s taken on samples cooled under N Z . No reduction of Moo3 occurred under these conditions a s v e r i f i e d by TGA. However, t h e pattern of r u t i l e was observed in the case of Mo03/TiOn and CoMoO,+/TiOz,respectively heated a t 700°C ( 9 h r s ) and 820°C ( 2 h r s ) . DISCUSSION
The necessary conditions t o observe i n t e r f a c i a l e f f e c t s between two o r more phases have been recently established and v e r i f i e d on various examples of mild oxidation c a t a l y s t s ( r e f . 1 ) . These phases belong t o two s t r u c t u r a l f a m i l i e s roughly defined as ReOs-like oxides and oxysalts AnBOk which contain molecular oxygenated complexes, and have p a r t i c u l a r thermodynamic and s t r u c t u r a l propert i e s allowing the formation o f coherent boundaries. On both s i d e s of these boundaries, the a c t i v e ions a r e in an excited s t a t e as compared t o t h e bulk ions in t h e l a t t i c e ; moreover, since t h e frameworks a r e - a t l e a s t l o c a l l y closely r e l a t e d , microdomains of one phase i n s i d e t h e o t h e r e x i s t involving
-
.
A t t h e steady s t a t e t h i s excess energy i s released by t h e l a t t i c e s towards t h e gaseous r e a c t a n t , whereas in a simple reduction process i t would be compensated by mechanical relaxation (e.g. C.S. planes). In multicomponent c a t a l y s t s any t r a n s f e r due to a redox mechanism, from s t r a i n a n d internal energies (ref.12)
one phase t o another, involving various species such a s oxygen, e l e c t r o n s , . . can proceed only i f these coherent boundaries e x i s t ( r e f . 1 ) .
.
I n t e r f a c i a l phenomena can be t h e o r e t i c a l l y expected when the crystallographic m i s f i t s along a given d i r e c t i o n common t o t h e cleavage planes (which a r e s t a t i s -
350
t i c a l l y f o u n d i n g r e a t number a t t h e s u r f a c e o f t h e c a t a l y s t ) have l o w v a l u e s . The Table 3 p r e s e n t s such values computed f r o m t h e c e l l parameters of CoMoOl,(b), PloOs and TiOn anatase.
Coincident planes w i t h (110) CoMo01,
Equivalent c e l l parameters
Crystallog. misfits %
C e l l parameters a b
/ (100) Moos
C COMOOk/ZC
2~ CoMoOb/b Moos
MOO?
4.7 10.7
9.666
/ ( 0 1 0 ) Moo3
c CoMoOs/2a Moo3 C COMOO~/~C MOO3
2.2 4.7
3.962
/ ( 0 0 1 ) Ti02
c CoMoOb/2a Ti02 c CoMoO4/2b Ti02
2.4
2.4
3.785
/ (010) Ti02
c CoMoOslPa T i 0 2
2.4
(crystallographic m i s f i t
t
=
8.854 ( COHOO~ (b) 13.855
(i) C
7.755 3.606
(MOOS)
3.785 (Ti021
9.514
c o ~ o ~ ~ 'M ' O o 3~ o, ~f ocr example)
C o n s i d e r i n g these l o w v a l u e s l e d us t o u n d e r t a k e t h e c a t a l y t i c s t u d y o f t h e o x i d a t i o n o f n-butane, and we supposed i t c o u l d proceed b y a r a k e mechanism
butane-butene-butadiene-furan-maleic
a n h y d r i d e . By means o f i n t e r f a c i a l e f f e c t s
between CoMo04 and Mo03/TiOz , t h i s c a t a l y s t would be a b l e t o p r e v e n t t h e desorp t i o n o f b u t a d i e n e and c o n s e q u e n t l y t o a l l o w i t s o x i d a t i o n i n t o m a l e i c a n h y d r i d e . The r e s u l t s exposed i n t h e f i r s t p a r t o f t h i s paper g i v e evidence f o r t h e v a l i d i t y o f t h e s e views. Moreover, t h e s o l i d - s o l i d t r a n s f o r m a t i o n s we s t u d i e d a r e i n t e r p r e t a b l e t o o by t h e e x i s t e n c e o f c o h e r e n t boundaries and c o n s t i t u t e a n add i t i o n a l though i n d i r e c t p r o o f . The DTA experiments show indeed t h a t t h e t r a n s i t i o n (b)
-f
( a ) o f CoMoOl, i s l e s s delayed when T i 0 2 i n s t e a d o f MOO, i s p r e s e n t .
I t has a l r e a d y been shown by one o f us t h a t t h i s endothermic t r a n s i t i o n c o n s i s t s
i n t h e m o d i f i c a t i o n o f t h e c o n f i g u r a t i o n o f Mo atoms, f r o m o c t a h e d r a l t o t e t r a h e d r a l (ref.2,lO).
T h i s m o d i f i c a t i o n happens b y u s i n g t h e C2/m symmetry which i s
common t o b o t h ( b ) and (a) forms, and i t i s n o t e w o r t h y t h a t t h e same elements C2/m o f TiO2 a r e n o t f a r f r o m c o i n c i d e n c e ( s e e F i g . 3 ) ; on t h e c o n t r a r y i n
MOO3
t h e C2 a x i s does n o t e x i s t , and t h i s o x i d e h i n d e r s t h e t r a n s f o r m a t i o n . Assuming t h a t t h i s t r a n s f o r m a t i o n b e g i n s t o t a k e p l a c e near t h e i n t e r f a c e boundaries, t h e s e c o n s i d e r a t i o n s c o u l d e x p l a i n t h e succession o f t h e t r a n s i t i o n temperatures
407°C
<
(CoMoO, p u r e and w e l l - c r y s t a l l i z e d )
<
430°C (CoMoOc
-
Ti02 )
500°C (CoMoOs
-
Moos)
As a l r e a d y observed i n t h e case o f V 2 0 5 - T i 0 2 ( r e f . 1 3 ) showing t h a t t h e r e d u c t i o n o f V205 i s accompanied by t h e t r a n s f o r m a t i o n anatase
-+
rutile, this transition
occurs a l s o i n t h e cases o f CoMo04 and Moosbut n o t a t t h e same temperature.
351
\ proj. along a
proj,along c
F i g . 3. R e l a t i o n s h i p s between C2/m symmetries a t t h e i n t e r f a c e ( t r a c e ) (110) CoMoO, and (011) anatase. T h e r e f o r e t h e s t r a i n s due t o coherent boundaries between CoP’l004 and T i 0 2 h e l p t o i n i t i a t e t h e t o p o t a c t i c t r a n s i t i o n a n a t a s e - r u t i l e a t a l o w e r temperature (820°C) t h a n when anatase i s a l o n e (950°C). The examination o f t h e frameworks which correspond t o t h e v a r i o u s p l a n e s i n v o l v e d i n Table 3 a l l o w s t o draw them i n c o i n c i d e n c e . F o r t h e sake o f c l a r i t y , o n l y t h e planes (110) CoMoO, and (010) Moo3 have been d i s p l a y e d o v e r t h e rows o f (011) T i 0 2 which i s one of t h e n a t u r a l cleavage planes (see F i g . 4 ) .
I n a per-
p e n d i c u l a r d i r e c t i o n t o t h e s e p l a n e s o f CoM004 and No03, a n o t h e r f i t i s observed
a l o n g t h e d i r e c t i o n s 1001 1 common t o b o t h phases (see F i g . 5 ) . A c c o r d i n g t o these models and i n v i e w of t h e p r e c e d i n g experiments, i t would be b e t t e r t o c o n s i d e r t h e multicomponent c a t a l y s t as made up o f t h e two a c t i v e phases, Co!1004 and MOO3 between which c o h e r e n t boundaries e x i s t , d i s p l a y e d on TiOzanatase which i s t h e n a l l o w e d t o e x e r t i t s e f f e c t on each a c t i v e phase. Such a model a l l o w s t o s p e c i f y t h e i n f l u e n c e o f t h e polymorphic t r a n s i t i o n (b)
+
( a ) on t h e c a t a l y t i c p r o p e r t i e s of CoMoO,
. It
i s well-known indeed t h a t
t h e m o b i l i t y of t h e oxygen atoms i n t h i s molybdate i s weak (ref.3,7,11),
but i f
t h e t r a n s i t i o n occurs i n t h e c a t a l y t i c c o n d i t i o n s t h e m e t a s t a b i l i t y o f t h e l a t t i c e can be s u f f i c i e n t l y h i g h . The low Tamman t e m p e r a t u r e which r e s u l t s o f t h i s t r a n s i t i o n i s one o f t h e c o n d i t i o n s t o observe t h e f o r m a t i o n o f c o h e r e n t bound a r i e s ( r e f . 1 ) . Consequently,
if T i 0 2 anatase i s added as a s u p p o r t t o
COMOO4,
t h e m e t a s t a b i l i t y o f t h e l a t t i c e and t h e n t h e m o b i l i t y o f oxygen a r e increased. T h i s can be c o r r e l a t e d w i t h t h e r e s u l t s shown on F i g . 1, which i n d i c a t e a l o w
352
Moo3
COMOO~
(010)
(110)
[OOl] i [ O O l ]
Moo3
I
COMOO~
F i g . 4. Projections o f the crystallographic f i t s between (110) CofloOb(b), (010) No03 and ( 0 1 1 ) Ti02 .
M 003 Fig. 5. Projection o f t h e i n t e r f a c e between ( 0 1 0 ) 1400, a n d ( 1 1 0 ) ColloOs(b). a c t i v i t y ( 2 6 mol.% conversion) b u t a f a i r l y good s e l e c t i v i t y in maleic anhydride (40 mol.%).
However the action o f Moo3 i s not so obvious, although i t could a d j u s t the oxygen mobility t o the r i g h t value t o get rnaleic anhydride. Another argument would c o n s i s t t o examine the e f f e c t due t o t h e position o f the symmetry elements a t t h e surface on t h e rake mechanism. The Fig. 6 shows how the C 2 a x i s and mirror rn a r e displayed r e l a t i v e l y t o the surface ( 1 1 0 ) of CoMoOk; i t must be
3 53
k e p t i n mind t h a t t h e symmetry a t t h e s u r f a c e can be d e r i v e d from t h e normal symmetry i n t h e b u l k , as e x t e n s i v e l y used i n t h e f a c t o r group t h e o r y . Since t h e experiments made by o t h e r a u t h o r s have shown t h a t t h e a d s o r p t i o n of butane proceeds on a c o b a l t atom ( r e f . l 4 ) ,
i t can be assumed t h a t t h e f o u r carbon
atoms of butane (symmetry C I ) , c i s - b u t e n e (CZv), and c i s - b u t a d i e n e (C2v) l i e i n t h e plane
oh c o r r e s p o n d i n g t o t h e m i r r o r
.
The presence o f t h e C2 a x i s (which
makes an a n g l e o f 45" w i t h t h e (110) p l a n e and i s p e r p e n d i c u l a r t o t h e m i r r o r ) a l l o w s t h e c i s - b u t a d i e n e t o t a k e t h e t r a n s - c o n f i g u r a t i o n : t h e n s t a b l e gaseous t r a n s - b u t a d i e n e i s a b l e t o desorb.
F i g . 6. Model o f a d s o r p t i o n o f butane y i e l d i n g butene and b u t a d i e n e on a c o b a l t atom o f t h e cleavage p l a n e ( 1 1 0 ) . The symmetry elements o f t h e s e molecules a r e i n c o i n c i d e n c e w i t h t h o s e o f CoMoO,(b). T h i s would n o t be t h e case when Moo, i s p r e s e n t n e a r CoMoO,
: a t t h e coherent
boundaries, t h e a d j a c e n t Mo atoms b e l o n g i n g t o MOO, do n o t show t h e C2 a x i s which i s n o t a symmetry element o f MOO,
.
Consequently, t h e c i s - b u t a d i e n e would n o t
desorb and would be f u r t h e r o x i d i z e d t o m a l e i c a n h y d r i d e (C2v) by t a k i n g t h r e e n e i g h b o u r i n g oxygens. The d e s o r p t i o n o f m a l e i c a n h y d r i d e i s a l l o w e d by t h e h i g h m o b i l i t y of t h e oxygens o f MOO, on t h e c o n t r a r y t o CoMoO,,
as e x p l a i n e d by o t h e r
a u t h o r s ( r e f . 14). TO conclude, t h e e l e c t r o n i c p r o p e r t i e s o f t h e c o n s t i t u t i v e phases s h o u l d be c o n s i d e r e d , even i f t h e i r i n f l u e n c e i s n o t y e t c o n t r o l l a b l e . The e x i s t e n c e o f c o h e r e n t boundaries between CoMoO,
( o r MOO,) and TiOn used i n excess as an a c t i v e
s u p p o r t a l l o w s t h e p o t e n t i a l b a r r i e r t o be lowered f o r any charge t r a n s f e r (ref.1).
I n t h e s e c o n d i t i o n s , TiOn which i s a n - t y p e semiconductor would c o n t r i -
b u t e t o d i m i n i s h t h e r e d u c i b i l i t y o f MOO, ( n - t y p e ) , and on t h e c o n t r a r y t o
354
enhance t h e m o b i l i t y o f t h e oxygen atoms o f CoMoO,
(p-type).
I n o t h e r words,
a c c o r d i n g t o t h e e l e c t r o n i c p r o p e r t i e s o f t h e a c t i v e phases, CoMoO., and Moo3, t h e s u p p o r t T i 0 2 a l l o w s t o o p t i m i z e t h e i r a b i l i t y t o r e l e a s e oxygen s u f f i c i e n t l y r e a d i l y ( a c t i v i t y ) and t o c o n t r o l t h e i r b i n d i n g e n e r g i e s ( s e l e c t i v i t y ) . REFERENCES P. C o u r t i n e , Amer.Chem.Soc.Symp., 1982, L a s Vegas ; Amer.Chem.Soc.Series, submitted f o r publication. P.P. Cord, P. C o u r t i n e , G. P a n n e t i e r , J. G u i l l e r m e t , S p e c t r o c h i m . A c t a , 28A
(1972) 1601.
10 11 12 13 14
K . B o u t r y , J.C. Daumas, K . M o n t a r n a l , B u l l . S o c . C h i m . F r . , 1968, 4050. M. A i , Bull.Chem.Soc.Jap., 50 (1977) 3 5 5 ; 49 ( 1 9 7 6 ) 1328. E. B o r d e s a n d P. C o u r t i n e , J. C a t a l . , 57 ( 1 9 7 9 ) 236 ; E . Bordes, These,1973, Compi Pgne ( F r a n c e ) . M. A k i m o t o and E. E c h i g o y a , Bull.Chem.Soc.Jap., 48 (1975) 3518. B. Grzvbowska. J. Haber and J . Janas. J. C a t a l . . 49 ( 1 9 7 7 ) 150. V.A. Dbroshenko e t a l . , Z h . P r i k l . K h i m . , 5 5 ( 1 9 8 2 ) 180. B.Vanhove, S.R. Op, A. Fernandez, M. B l a n c h a r d , J. C a t a l . , 57 ( 1 9 7 0 ) 253. P. C o u r t i n e and J.C. Daumas, C . R . A c a d . S c i . P a r i s , 268 ( 1 9 6 9 ) 1568. R. B o u t r y , J.C. Daumas, R. Montarna1,P. C o u r t i n e , G. P a n n e t i e r , B u l l . S o c . Chim.Fr., 1 9 6 8 , 4811. A.R. Ubbelohde, J.Chim.Phys., 62 (1966) 3 3 . A. V e j u x and P . C o u r t i n e , J . S o l i d St.Chem., 23 ( 1 9 7 8 ) 93. J . Haber, M. Sochacka, B. Grzybowska, A. G o l e b i e w s k i , J . M o l . C a t a l . , 1 (1975)
35.
Co-Mo-0-Ti,
345
A NEW CATALYST EFFECTIVE I N BUTANE OXIDATIOP! TO MALEIC AP!HYDRIDE
J.S. JUNG, E. BORDES and P . COURTINE D e p a r t e m e n t de G e n i e C h i m i q u e , U n i v e r s i t e de T e c h n o l o g i e de Compiegne, B.P.
233,
60206 Compiegne Cedex ( F r a n c e )
ABSTRACT By a p p l y i n g t h e c r i t e r i a a l r e a d y drawn f r o m numerous examples o f m u l t i c o m p o n e n t c a t a l y s t s o f m i l d o x i d a t i o n i n v o l v i n g i n t e r f a c i a l e f f e c t s , we have f o u n d t h a t CoMo04/Mo03-Ti02 c o u l d b e a s e l e c t i v e c a t a l y s t i n t h e r e a c t i o n b u t a n e m a l e i c a n h y d r i d e . The m o s t e f f e c t i v e c o m p o s i t i o n i s Co/Mo/Ti = 1/21,5/42.5, for w h i c h a s e l e c t i v i t y o f 46.4 mol. YC i s o b t a i n e d a t 53 m o l . ‘i c o n v e r s i o n o f b u t a n e . The p o l y m o r p h i c t r a n s i t i o n s CoMo04 ( b ) + ( a ) a n d a n a t a s e + r u t i l e a r e a c c e l e r a t e d ( o r d e l a y e d ) a c c o r d i n g t o t h e phases f a c e t o f a c e . I t i s c o n c l u d e d t h a t a t t h e s t e a d y s t a t e i n t e r f a c i a l e f f e c t s t a k e p l a c e between t h e c l e a v a g e p l a n e s ( 1 1 0 ) CoMoO,(b), (OIO), ( 1 0 0 ) MOO, and (OIO), ( 0 1 1 ) T i O z a n a t a s e .
INTRODUCTION When a c a t a l y t i c phase a l o n e i s n o t a b l e b y i t s e l f t o g i v e s u b s t a n t i a l y i e l d s o f a g i v e n p r o d u c t i n m i l d o x i d a t i o n c a t a l y s i s , dopes, o r a c t i v e s u p p o r t ( T i D z f o r e x a m p l e ) , o r o t h e r a c t i v e phases ( m u l t i c o m p o n e n t n o l y b d a t e s ) a r e g e n e r a l l y added t o i m p r o v e t h e a c t i v i t y and s e l e c t i v i t y . The known c r i t e r i a o f s e l e c t i v i t y a r e o f course involved i n these multiphasic c a t a l y s t s , b u t the decisive p a r t comes f r o m t h e i n t e r f a c i a l phenomena o c c u r i n g between t h e s e phases ( r e f . 1 ) . L o o k i n g f o r a c a t a l y s t o t h e r t h a n (VO)2P2D7 i n t h e o x i d a t i o n o f b u t a n e t o m a l e i c a n h y d r i d e , we have c h o s e n t h e s y s t e m CoMo04-Mo03-Ti02a n a t a s e f o r t w o m a i n r e a s o n s : ( i ) CoMo04 i s known t o b e a c t i v e a n d s e l e c t i v e i n t h e o x i d a t i v e dehyd r o g e n a t i o n o f b u t a n e i n t o b u t e n e and b u t a d i e n e ( r e f . 2 , 3 )
and Moo3 s u p p o r t e d b y
TiOz anatase i s a c t i v e f o r t h e t r a n s f o r m a t i o n o f butene (ref.4,5) (ref.6)
and b u t a d i e n e
i n t o m a l e i c a n h y d r i d e , ( i i ) a l l y 1 s p e c i e s c a n be formed on CoMo04 w i t h o u t
desorption (ref.7),
whereas t h e s e s p e c i e s a r e t r a n s f o r m e d i n t o o x y a e n a t e d com-
p l e x e s o n MOO, w h i c h a r e t h e n d e s o r b e d ( r e f . 8 ) . On t h e o t h e r hand, we have shown r e c e n t l y t h a t such o x i d e s o r o x y s a l t s b e l o n g t o a c l a s s i f i c a t i o n o f m i l d o x i d a t i o n c a t a l y s t s we s e t u p , t h e i r thermodynamic and s t r u c t u r a l p r o p e r t i e s a l l o w i n g i n t e r f a c i a l e f f e c t s which a r e evidenced b y coherent boundaries (ref.1).
A f t e r t h e c a l c u l a t i o n of t h e c r y s t a l l o g r a p h i c mis-
f i t s between each component o f t h e s y s t e m CoMoOh-MoO3-Ti02 showing t h a t c o h e r e n t b o u n d a r i e s c o u l d e x i s t between t h e c l e a v a g e p l a n e s , we h a v e u n d e r t a k e n t h e c a t a l y t i c s t u d y o f t h e r e a c t i o n b u t a n e t o m a l e i c a n h y d r i d e o n t h i s system.
346
EXPERIMENTAL T h e preparation of the c a t a l y s t i s c a r r i e d out in two steps : the "support"
Mo03-TiC12 being f i r s t obtained, i s then added as a s o l i d powdered phase t o a solution of cobalt n i t r a t e and ammonium heptamolybdate ; the s l u r r y i s heated a t 80°C ( 2 h r s ) , b r o u g h t t o dryness with s t i r r i n g , a n d the s o l i d residue i s c a l c i ned in a i r a t 550°C ( 1 6 h r s ) . Different preparations of the support have been used : ( i ) Ti02 anatase ( T h a n n e t Mulhouse) ( 7 m'g-l) i s suspended i n ammonium heptamolybdate s o l u t i o n , the r e s u l t a n t s l u r r y heated a t 80°C ( 2 h r s ) , brought t o dryness with s t i r r i n g a n d dried in a n oven a t 120°C ( 5 h r s ) , ( i i ) TiC14 i s hydrolyzed by water and ammonia a t O'C, yielding a p r e c i p i t a t e of titanium hydroxide which i s f i l t e r e d , washed, and added t o molybdenum solution as above ; such c a t a l y s t s will be specified a s Mo03-TiOe(H) f u r t h e r in the t e x t ; ( i i i ) s o l u t i o n s of titanium and molybdenum t a r t r a t e s of known composition a r e prepared according t o ref.9 and mixed in t h e desired r a t i o . This solution i s evaporated under vacuum, yielding a vitreous black precursor which i s dried a t 120°C i n an oven and calcined a t 500°C in a i r ( 8 h r s ) , r e f e r r e d a s MoCl3-TiO2(T). The c a t a l y t i c runs have been performed in a conventional fixed-bed r e a c t o r , with chromatographic analysis of t h e feed and products ( r e f . 5 ) , in t h e temperat u r e range 390 t o 490°C, f o r various contact times ( 1 t o 9 s e c . ) and 1-2 % butane i n the a i r feed. The s t a i n l e s s - s t e e l r e a c t o r contained 30 ml of broken pieces of supported c a t a l y s t . The main part of t h e e f f l u e n t passed t h r o u g h water bubblers and was d i r e c t e d t o two chromatographs f o r a n a l y s i s of permanent gases and hydrocarbons. A branching s i t u a t e d in f r o n t of t h e bubblers led t h e e f f l u e n t towards a t h i r d chromatograph t o analyze the oxygenated compounds (mainly a c e t i c and a c r y l i c a c i d s , and maleic anhydride). X-ray d i f f r a c t i o n (diffractometer. C u Ka), infrared spectroscopy (KBr d i s c s ) and e l e c t r o n microscopy ( 1 0 0 kV) have been systematically performed on pure phases and c a t a l y s t s . As f a r a s possible the samples were n o t ground (except in IR s t u d i e s ) a n d simply displayed on t h e sample holder ( X R D ) or dispersed by u l t r a s o n i c waves (electron microscopy). The r e a c t i v i t y of t h e c a t a l y s t s under a i r o r nitrogen flow (pOn = atm) has been examined by thermomicrogravimetric analysis (TGA) and semimicro DTA ( c r u c i b l e content 6 u l ) . RESULTS
Catalytic results Various compositions concerning Mo/Ti in t h e support, Co/Mo i n t h e a c t i v e phase and t h e r a t i o a c t i v e phase/support have been prepared, in order t o f i n d the more e f f e c t i v e composition i n each kind of preparation modes. The c a t a l y t i c r e s u l t s ( s e e Fig. 1 and 2 ) show respectively t h a t t h e highest y i e l d s in maleic
341
anhydride are obtained f o r 30 pol.' w i t h 10 w t
Mo0,in 7 0 m o l .
o f a c t i v e Co-Mo-0 phase on 9Gwt
TiO, and Co/Mo
-
1.0 t o 1.9,
M o O j - T i 0 2 . The same b e h a v i o u r i s
o b s e r v e d w i t h t h e samples o f s u p p o r t ( H I o r ( T ) p r e p a r e d b y d i f f e r e n t ways. I n t h e same c o n d i t i o n s , CoMoO, a l o n e y i e l d s 5 m o l . ,
o f m a l e i c a n h y d r i d e and 1.8 %
o f a c r y l i c a c i d a t 2 1 . 6 '' c o n v e , - s i o n o f b u t a n e , t h e m a i n p r o d u c t bein:
Ti02
mol%
-
butadiene
f
100 Moo3
f f
F i g . 1. A c t i v i t y and s e l e c t i v i t y o f 10 wt% CoMoOt+/9O % Moos-Ti02 as a f u n c t i o n o f t h e c o m p o s i t i o n o f t h e s u p p o r t (490"C, T = 9 sec., 1.5 % CQHlo . Legend : b l a c k c i r c l e s : c o n v e r s i o n ; b l a c k s q u a r e : s e l e c t i v i t y m a l e i c anh. ; open c i r c l e s : y i e l d C O + C02 ; open squares : y i e l d m a l e i c anh. ; open t r i a n g l e y i e l d a c r y l i c acid.
-
-Co
I Mo
F i g . 2. A c t i v i t y and s e l e c t i v i t y o f Co-Mo-0 o n s u p p o r t 3 0 mol.% Moo3 on TiOn, as a f u n c t i o n o f t h e c o m p o s i t i o n o f t h e a c t i v e phase. Same l e g e n d a s above.
348
Since t h e s u r f a c e area o f t h e s u p p o r t s
(H) o r (T) i s improved as compared t o
t h e above c a t a l y s t ( r e s p e c t i v e l y 18 and 28 m ' g - l ) , duced f r o m 9 t o 1 sec.,
t h e c o n t a c t t i m e can be r e -
and t h e t e m p e r a t u r e f r o m 490 t o 420°C as a r e s u l t o f
g r e a t e r a c t i v i t y ( T a b l e 1 ) . The b e s t s e l e c t i v i t y i s o b t a i n e d f o r C O M O O L , / M O O ~ T i 0 2 ( H ) , whereas s u p p o r t ( T ) g i v e s an h i g h e r a c t i v i t y and t h e r e f o r e a h i g h e r y i e l d i n m a l e i c anhydride. I n a l l cases ( e x c e p t f o r CoMoO, a l o n e ) n e i t h e r b u t a d i e n e n o r f u r a n were observed on t h e chromatograms, and o n l y few amount of acet i c and a c r y l i c a c i d s a r e desorbed as oxyaenated compounds. TABLE 1 E f f e c t o f t h e p r e p a r a t i o n o f t h e s u p p o r t on c a t a l y t i c p r o p e r t i e s
Kind o f support
TiOE a
Temp.
T
Conv.
"C
sec.
mol. %
49 0
9
51.9
Sel ec.
Y i e l d s , mol.? CO
t
CO
25.8
MAA
AA
21.2
2.2
40.8
3.0 0.5
40.2
TiC2(H)
420
1
53.0
27.1
24.8
Ti02(T)
420
1
70.0
40.2
28.0
MAA 7;
46.4
a T i 0 2 7 m'9-l washed w i t h b o i l i n g w a t e r ( 1 h r ) , f i l t r a t e d , d r i e d a t 120°C i n an oven (20 h r s ) and t h e n added t o molybdenum s o l u t i o n . S t r u c t u r a l p r o p e r t i e s and r e a c t i v i t y The comparison o f XRD p a t t e r n s o f f r e s h and used c a t a l y s t s w i t h t h o s e o f p u r e CoMoO,(a),
( b ) , Moo3 and TiOn anatase a l l o w s t o know what phases a r e p r e s e n t and
i f t h e i r t e x t u r e i s m o d i f i e d as compared t o p u r e compounds ( T a b l e 2) TABLE 2 Phases d e t e c t e d i n f r e s h and used c a t a l y s t s s i t i o n CoMoO ( b ) -+ ( a ) (DTA).
Mo 0 3
Ca t a 1y s t s +++
+
+
+++
CoM004/M003
+
CoMo04/MOO 3 - T i 0 2
+
-
Co/Mo
=
1
(CoMoO,)
Co/Mo = 1.9 (COMOOs + C O s O s )
CoMoO, / T i
(XRD), and t e m p e r a t u r e o f t h e t r a n -
02
CoMoOs/Ti 02 (H)
Ti02 anatase
Temp. (b) + ( a ) 490
+++ +++
+++ +++ ~
507
++++ ++++ ++++
520-530 450
500 ~
The number o f + r e f l e c t s a p p r o x i m a t e l y t h e r e l a t i v e i n t e n s i t i e s o f t h e main l i n e s o f t h e d i f f e r e n t phases.
349
The predominant form of CoMoO, when alone i s respectively ( b ) a n d ( a ) f o r Co/Mo = 1 . 0 a n d 1 . 9 together withsomeCo,O, in the l a s t case, a s observed by other workers ( r e f . l G , l l ) . The l i n e s of M O O ? of ( O k O ) type a r e more intense in t h e c a t a l y s t s t h a n the ( O 2 1 ) , ( 0 6 1 ) l i n e s which a r e usually higher in pure MOO,. T h i s means t h a t the layered t e x t u r e of MOO, along ( O k O ) planes i s improved by the preparation. No modification of TiOzpattern has been observed. Infrared spectra a r e i n accordance with these r e s u l t s . The e f f e c t of the support on t h e temperature of the polymorphic t r a n s i t i o n CoMoOt,(b) + ( a ) has been examined by D T A . The small endothermic peak corresponding t o t h i s t r a n s i t i o n appears a t 490°C f o r pure CoMoO, prepared in a s i m i l a r way as t h e c a t a l y s t s , while i t has been observed a t 430 ( r e f . 3 ) a n d 407°C ( r e f . 2 ) according t o t h e preparation a n d the c r y s t a l l i n i t y of t h e sample. I t can be seen on Table 2 t h a t Moos alone o r with TiOn delays t h e transformation while Ti02 alone seems t o favourize i t . The same kind of study has been carried o u t f o r the t r a n s i t i o n a n a t a s e - r u t i l e which cannot be observed by DTA because i t i s t o o sluggish. The c a t a l y s t s have been heated in a N Z stream a t high temperature (600 t o 860°C) a n d the XRD pat-
t e r n s taken on samples cooled under N Z . No reduction of Moo3 occurred under these conditions a s v e r i f i e d by TGA. However, t h e pattern of r u t i l e was observed in the case of Mo03/TiOn and CoMoO,+/TiOz,respectively heated a t 700°C ( 9 h r s ) and 820°C ( 2 h r s ) . DISCUSSION
The necessary conditions t o observe i n t e r f a c i a l e f f e c t s between two o r more phases have been recently established and v e r i f i e d on various examples of mild oxidation c a t a l y s t s ( r e f . 1 ) . These phases belong t o two s t r u c t u r a l f a m i l i e s roughly defined as ReOs-like oxides and oxysalts AnBOk which contain molecular oxygenated complexes, and have p a r t i c u l a r thermodynamic and s t r u c t u r a l propert i e s allowing the formation o f coherent boundaries. On both s i d e s of these boundaries, the a c t i v e ions a r e in an excited s t a t e as compared t o t h e bulk ions in t h e l a t t i c e ; moreover, since t h e frameworks a r e - a t l e a s t l o c a l l y closely r e l a t e d , microdomains of one phase i n s i d e t h e o t h e r e x i s t involving
-
.
A t t h e steady s t a t e t h i s excess energy i s released by t h e l a t t i c e s towards t h e gaseous r e a c t a n t , whereas in a simple reduction process i t would be compensated by mechanical relaxation (e.g. C.S. planes). In multicomponent c a t a l y s t s any t r a n s f e r due to a redox mechanism, from s t r a i n a n d internal energies (ref.12)
one phase t o another, involving various species such a s oxygen, e l e c t r o n s , . . can proceed only i f these coherent boundaries e x i s t ( r e f . 1 ) .
.
I n t e r f a c i a l phenomena can be t h e o r e t i c a l l y expected when the crystallographic m i s f i t s along a given d i r e c t i o n common t o t h e cleavage planes (which a r e s t a t i s -
350
t i c a l l y f o u n d i n g r e a t number a t t h e s u r f a c e o f t h e c a t a l y s t ) have l o w v a l u e s . The Table 3 p r e s e n t s such values computed f r o m t h e c e l l parameters of CoMoOl,(b), PloOs and TiOn anatase.
Coincident planes w i t h (110) CoMo01,
Equivalent c e l l parameters
Crystallog. misfits %
C e l l parameters a b
/ (100) Moos
C COMOOk/ZC
2~ CoMoOb/b Moos
MOO?
4.7 10.7
9.666
/ ( 0 1 0 ) Moo3
c CoMoOs/2a Moo3 C COMOO~/~C MOO3
2.2 4.7
3.962
/ ( 0 0 1 ) Ti02
c CoMoOb/2a Ti02 c CoMoO4/2b Ti02
2.4
2.4
3.785
/ (010) Ti02
c CoMoOslPa T i 0 2
2.4
(crystallographic m i s f i t
t
=
8.854 ( COHOO~ (b) 13.855
(i) C
7.755 3.606
(MOOS)
3.785 (Ti021
9.514
c o ~ o ~ ~ 'M ' O o 3~ o, ~f ocr example)
C o n s i d e r i n g these l o w v a l u e s l e d us t o u n d e r t a k e t h e c a t a l y t i c s t u d y o f t h e o x i d a t i o n o f n-butane, and we supposed i t c o u l d proceed b y a r a k e mechanism
butane-butene-butadiene-furan-maleic
a n h y d r i d e . By means o f i n t e r f a c i a l e f f e c t s
between CoMo04 and Mo03/TiOz , t h i s c a t a l y s t would be a b l e t o p r e v e n t t h e desorp t i o n o f b u t a d i e n e and c o n s e q u e n t l y t o a l l o w i t s o x i d a t i o n i n t o m a l e i c a n h y d r i d e . The r e s u l t s exposed i n t h e f i r s t p a r t o f t h i s paper g i v e evidence f o r t h e v a l i d i t y o f t h e s e views. Moreover, t h e s o l i d - s o l i d t r a n s f o r m a t i o n s we s t u d i e d a r e i n t e r p r e t a b l e t o o by t h e e x i s t e n c e o f c o h e r e n t boundaries and c o n s t i t u t e a n add i t i o n a l though i n d i r e c t p r o o f . The DTA experiments show indeed t h a t t h e t r a n s i t i o n (b)
-f
( a ) o f CoMoOl, i s l e s s delayed when T i 0 2 i n s t e a d o f MOO, i s p r e s e n t .
I t has a l r e a d y been shown by one o f us t h a t t h i s endothermic t r a n s i t i o n c o n s i s t s
i n t h e m o d i f i c a t i o n o f t h e c o n f i g u r a t i o n o f Mo atoms, f r o m o c t a h e d r a l t o t e t r a h e d r a l (ref.2,lO).
T h i s m o d i f i c a t i o n happens b y u s i n g t h e C2/m symmetry which i s
common t o b o t h ( b ) and (a) forms, and i t i s n o t e w o r t h y t h a t t h e same elements C2/m o f TiO2 a r e n o t f a r f r o m c o i n c i d e n c e ( s e e F i g . 3 ) ; on t h e c o n t r a r y i n
MOO3
t h e C2 a x i s does n o t e x i s t , and t h i s o x i d e h i n d e r s t h e t r a n s f o r m a t i o n . Assuming t h a t t h i s t r a n s f o r m a t i o n b e g i n s t o t a k e p l a c e near t h e i n t e r f a c e boundaries, t h e s e c o n s i d e r a t i o n s c o u l d e x p l a i n t h e succession o f t h e t r a n s i t i o n temperatures
407°C
<
(CoMoO, p u r e and w e l l - c r y s t a l l i z e d )
<
430°C (CoMoOc
-
Ti02 )
500°C (CoMoOs
-
Moos)
As a l r e a d y observed i n t h e case o f V 2 0 5 - T i 0 2 ( r e f . 1 3 ) showing t h a t t h e r e d u c t i o n o f V205 i s accompanied by t h e t r a n s f o r m a t i o n anatase
-+
rutile, this transition
occurs a l s o i n t h e cases o f CoMo04 and Moosbut n o t a t t h e same temperature.
351
\ proj. along a
proj,along c
F i g . 3. R e l a t i o n s h i p s between C2/m symmetries a t t h e i n t e r f a c e ( t r a c e ) (110) CoMoO, and (011) anatase. T h e r e f o r e t h e s t r a i n s due t o coherent boundaries between CoP’l004 and T i 0 2 h e l p t o i n i t i a t e t h e t o p o t a c t i c t r a n s i t i o n a n a t a s e - r u t i l e a t a l o w e r temperature (820°C) t h a n when anatase i s a l o n e (950°C). The examination o f t h e frameworks which correspond t o t h e v a r i o u s p l a n e s i n v o l v e d i n Table 3 a l l o w s t o draw them i n c o i n c i d e n c e . F o r t h e sake o f c l a r i t y , o n l y t h e planes (110) CoMoO, and (010) Moo3 have been d i s p l a y e d o v e r t h e rows o f (011) T i 0 2 which i s one of t h e n a t u r a l cleavage planes (see F i g . 4 ) .
I n a per-
p e n d i c u l a r d i r e c t i o n t o t h e s e p l a n e s o f CoM004 and No03, a n o t h e r f i t i s observed
a l o n g t h e d i r e c t i o n s 1001 1 common t o b o t h phases (see F i g . 5 ) . A c c o r d i n g t o these models and i n v i e w of t h e p r e c e d i n g experiments, i t would be b e t t e r t o c o n s i d e r t h e multicomponent c a t a l y s t as made up o f t h e two a c t i v e phases, Co!1004 and MOO3 between which c o h e r e n t boundaries e x i s t , d i s p l a y e d on TiOzanatase which i s t h e n a l l o w e d t o e x e r t i t s e f f e c t on each a c t i v e phase. Such a model a l l o w s t o s p e c i f y t h e i n f l u e n c e o f t h e polymorphic t r a n s i t i o n (b)
+
( a ) on t h e c a t a l y t i c p r o p e r t i e s of CoMoO,
. It
i s well-known indeed t h a t
t h e m o b i l i t y of t h e oxygen atoms i n t h i s molybdate i s weak (ref.3,7,11),
but i f
t h e t r a n s i t i o n occurs i n t h e c a t a l y t i c c o n d i t i o n s t h e m e t a s t a b i l i t y o f t h e l a t t i c e can be s u f f i c i e n t l y h i g h . The low Tamman t e m p e r a t u r e which r e s u l t s o f t h i s t r a n s i t i o n i s one o f t h e c o n d i t i o n s t o observe t h e f o r m a t i o n o f c o h e r e n t bound a r i e s ( r e f . 1 ) . Consequently,
if T i 0 2 anatase i s added as a s u p p o r t t o
COMOO4,
t h e m e t a s t a b i l i t y o f t h e l a t t i c e and t h e n t h e m o b i l i t y o f oxygen a r e increased. T h i s can be c o r r e l a t e d w i t h t h e r e s u l t s shown on F i g . 1, which i n d i c a t e a l o w
352
Moo3
COMOO~
(010)
(110)
[OOl] i [ O O l ]
Moo3
I
COMOO~
F i g . 4. Projections o f the crystallographic f i t s between (110) CofloOb(b), (010) No03 and ( 0 1 1 ) Ti02 .
M 003 Fig. 5. Projection o f t h e i n t e r f a c e between ( 0 1 0 ) 1400, a n d ( 1 1 0 ) ColloOs(b). a c t i v i t y ( 2 6 mol.% conversion) b u t a f a i r l y good s e l e c t i v i t y in maleic anhydride (40 mol.%).
However the action o f Moo3 i s not so obvious, although i t could a d j u s t the oxygen mobility t o the r i g h t value t o get rnaleic anhydride. Another argument would c o n s i s t t o examine the e f f e c t due t o t h e position o f the symmetry elements a t t h e surface on t h e rake mechanism. The Fig. 6 shows how the C 2 a x i s and mirror rn a r e displayed r e l a t i v e l y t o the surface ( 1 1 0 ) of CoMoOk; i t must be
3 53
k e p t i n mind t h a t t h e symmetry a t t h e s u r f a c e can be d e r i v e d from t h e normal symmetry i n t h e b u l k , as e x t e n s i v e l y used i n t h e f a c t o r group t h e o r y . Since t h e experiments made by o t h e r a u t h o r s have shown t h a t t h e a d s o r p t i o n of butane proceeds on a c o b a l t atom ( r e f . l 4 ) ,
i t can be assumed t h a t t h e f o u r carbon
atoms of butane (symmetry C I ) , c i s - b u t e n e (CZv), and c i s - b u t a d i e n e (C2v) l i e i n t h e plane
oh c o r r e s p o n d i n g t o t h e m i r r o r
.
The presence o f t h e C2 a x i s (which
makes an a n g l e o f 45" w i t h t h e (110) p l a n e and i s p e r p e n d i c u l a r t o t h e m i r r o r ) a l l o w s t h e c i s - b u t a d i e n e t o t a k e t h e t r a n s - c o n f i g u r a t i o n : t h e n s t a b l e gaseous t r a n s - b u t a d i e n e i s a b l e t o desorb.
F i g . 6. Model o f a d s o r p t i o n o f butane y i e l d i n g butene and b u t a d i e n e on a c o b a l t atom o f t h e cleavage p l a n e ( 1 1 0 ) . The symmetry elements o f t h e s e molecules a r e i n c o i n c i d e n c e w i t h t h o s e o f CoMoO,(b). T h i s would n o t be t h e case when Moo, i s p r e s e n t n e a r CoMoO,
: a t t h e coherent
boundaries, t h e a d j a c e n t Mo atoms b e l o n g i n g t o MOO, do n o t show t h e C2 a x i s which i s n o t a symmetry element o f MOO,
.
Consequently, t h e c i s - b u t a d i e n e would n o t
desorb and would be f u r t h e r o x i d i z e d t o m a l e i c a n h y d r i d e (C2v) by t a k i n g t h r e e n e i g h b o u r i n g oxygens. The d e s o r p t i o n o f m a l e i c a n h y d r i d e i s a l l o w e d by t h e h i g h m o b i l i t y of t h e oxygens o f MOO, on t h e c o n t r a r y t o CoMoO,,
as e x p l a i n e d by o t h e r
a u t h o r s ( r e f . 14). TO conclude, t h e e l e c t r o n i c p r o p e r t i e s o f t h e c o n s t i t u t i v e phases s h o u l d be c o n s i d e r e d , even i f t h e i r i n f l u e n c e i s n o t y e t c o n t r o l l a b l e . The e x i s t e n c e o f c o h e r e n t boundaries between CoMoO,
( o r MOO,) and TiOn used i n excess as an a c t i v e
s u p p o r t a l l o w s t h e p o t e n t i a l b a r r i e r t o be lowered f o r any charge t r a n s f e r (ref.1).
I n t h e s e c o n d i t i o n s , TiOn which i s a n - t y p e semiconductor would c o n t r i -
b u t e t o d i m i n i s h t h e r e d u c i b i l i t y o f MOO, ( n - t y p e ) , and on t h e c o n t r a r y t o
354
enhance t h e m o b i l i t y o f t h e oxygen atoms o f CoMoO,
(p-type).
I n o t h e r words,
a c c o r d i n g t o t h e e l e c t r o n i c p r o p e r t i e s o f t h e a c t i v e phases, CoMoO., and Moo3, t h e s u p p o r t T i 0 2 a l l o w s t o o p t i m i z e t h e i r a b i l i t y t o r e l e a s e oxygen s u f f i c i e n t l y r e a d i l y ( a c t i v i t y ) and t o c o n t r o l t h e i r b i n d i n g e n e r g i e s ( s e l e c t i v i t y ) . REFERENCES P. C o u r t i n e , Amer.Chem.Soc.Symp., 1982, L a s Vegas ; Amer.Chem.Soc.Series, submitted f o r publication. P.P. Cord, P. C o u r t i n e , G. P a n n e t i e r , J. G u i l l e r m e t , S p e c t r o c h i m . A c t a , 28A
(1972) 1601.
10 11 12 13 14
K . B o u t r y , J.C. Daumas, K . M o n t a r n a l , B u l l . S o c . C h i m . F r . , 1968, 4050. M. A i , Bull.Chem.Soc.Jap., 50 (1977) 3 5 5 ; 49 ( 1 9 7 6 ) 1328. E. B o r d e s a n d P. C o u r t i n e , J. C a t a l . , 57 ( 1 9 7 9 ) 236 ; E . Bordes, These,1973, Compi Pgne ( F r a n c e ) . M. A k i m o t o and E. E c h i g o y a , Bull.Chem.Soc.Jap., 48 (1975) 3518. B. Grzvbowska. J. Haber and J . Janas. J. C a t a l . . 49 ( 1 9 7 7 ) 150. V.A. Dbroshenko e t a l . , Z h . P r i k l . K h i m . , 5 5 ( 1 9 8 2 ) 180. B.Vanhove, S.R. Op, A. Fernandez, M. B l a n c h a r d , J. C a t a l . , 57 ( 1 9 7 0 ) 253. P. C o u r t i n e and J.C. Daumas, C . R . A c a d . S c i . P a r i s , 268 ( 1 9 6 9 ) 1568. R. B o u t r y , J.C. Daumas, R. Montarna1,P. C o u r t i n e , G. P a n n e t i e r , B u l l . S o c . Chim.Fr., 1 9 6 8 , 4811. A.R. Ubbelohde, J.Chim.Phys., 62 (1966) 3 3 . A. V e j u x and P . C o u r t i n e , J . S o l i d St.Chem., 23 ( 1 9 7 8 ) 93. J . Haber, M. Sochacka, B. Grzybowska, A. G o l e b i e w s k i , J . M o l . C a t a l . , 1 (1975)
35.