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Catalyst for Propene or Isobutene Oxidation to α, β−-Unsaturated Aldehydes and Acids
Preparation of Catalysts, edited by B. Delmon, P.A. Jacobs and G . Poncelet o 1976, Elsevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
CATALYST FOR PROPENE OR ISOBUTENE OXIDATION TO -UNSATURATED ALDEHYDES AND ACIDS
a , B-
0.V.ISAEV and L.Ya.WGOLIS I n s t i t u t e of Chemical Physics, Academy of Sciences o f the USSR, Moscow SUMMARY A technique of preparing t h e Co6Mo12Bi0 .5Fe0
,5-1 .oOx
c a t a l y s t for propene and isobutene oxidation t o unsaturated aldehydes and a c i d s i s proposed, I t c o n s i s t s i n production of mixed he teropolycompounds and t h e i r subsequent decomposition. The c a t a l y s t s t r u c t u r e was s t u d i e d by X-ray, I R , W , ESCA and MGssbauer spectroscopy. C a t a l y t i c o x i d a t i o n of o l e f i n s and unsaturated aldehydes i s an important engineering problem. Many c a t a l y s t s , mostly containing c o b a l t molybdate, have been proposed accordingly. Oxide compounds of tellurium, a r s e n i c , antimony, e t c . have been incorporated i n t o the c a t a l y s t i n o r d e r t o i n c r e a s e the formation s e l e c t i v i t y of unsaturated acids. However, these compounds a r e t o x i c , a r e r e a d i l y reduced by hydrocarbons and become v o l a t i l e . This imposes a 1-2 vol.% decrease i n o l e f i n concentration, thus decreasing the c a t a l y s t e f f e c t i v i t y , and, moreover, the l o s s o f these compounds has t o be compensated for. M a n y attempts of c o b a l t molybdate modification by metal oxides y i e l d i n g no v o l a t i l e s p e c i e s i n the course of r e a c t i o n have been made [I] b u t papers on modification of composite Consequently, the c r e a t i o n oxide systems are sparse [2-4] o f s c i e n t i f i c grounds f o r p r e p a r a t i o n o f e f f e c t multicomponent c a t a l y s t s f o r o l e f i n conversion t o unsaturated aldehydes and a c i d s i s a very urgent problem. The a c t i o n of added s p e c i e s on the c a t a l y t i c a c t i v i t y of c o b a l t molybdate can be t r e a t e d i n terms of t h r e e models: (I) formation of s o l i d s o l u t i o n s upon i n c o r p o r a t i o n of subs t i t u t i o n of a d d i t i v e s , w i t h r e l e v a n t changes i n the c o b a l t t o molybdenum i o n r a t i o ; (2) a l t e r a t i o n o f e l e c t r o n t r a n s i -
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t i o n s by e f f e c t of a d d i t i v e s on t h e c r y s t a l energy spectrum (varying of forbidden band width, appearance of new l e v e l s ) , an (3) appearance of a d d i t i o n a l a c t i v e s i t e s capable o f adaorblng the r e a c t a n t s and c o n t r i b u t i n g t o c a t a l y s i s . In accordance with these models and experimental results [s] bismuth and i r o n were chosen as modifying components of the cobalt-molybdenum c a t a l y s t . Comparison of the various techniques for c o b a l t molybd a t e modification showed t h a t the paths of o l e f i n oxidation t o aldehydes and acids were d i f f e r e n t depending on whether the additives were incorporated onto the surface o r i n the bulk of the c a t a l y s t . Uniform d i s t r i b u t i o n o f c a t i o n s i n the multicomponent c a t a l y s t and a prefixed s t r u c t u r e a r e s t r i n gent requirements here. Consequently, i t i s very d i f f i c u l t t o f i n d the optimal means f o r preparation of such a c a t a l y t i c eystem Usually c o b a l t molybdate i s obtained by c o p r e c i p i t a t i o n , pouring together ammonia paramolybdate and c o b a l t n i t r a t e i n such a way t h a t the r a t i o Co/Mo be unity. The pH value i s maintained w i t h i n 5 t o 7 and i s c o n t r o l l e d by a d d i t i o n of an aqueous ammonia solution. The r a t i o of CX- t o B -modifications of c o b a l t molybdate, the s p e c i f i c surface of samples, and their c a t a l y t i c a c t i v i t y i n propene oxidation t o a c r y l i c a c i d depend on pH of the medium i n which CoMo04 is precipitated[6]. This technique i s inconvenient for o b t a i n i n g c o b a l t molybdate modified by bismuth and i r o n , s i n c e when CoMo04 i s p r e c i p i t a t e d , the molybdates of these metals f a l l o u t simultaneously and independently. Another technique of preparing a multicomponent c a t a l y s t containing c o b a l t , molybdenum, bismuth, i r o n , e t c .ions [7] c o n s i s t s i n a d d i t i o n of powdered molybdic a c i d t o a h o t aqueous s o l u t i o n of c o b a l t bismuth, i r o n , e t c . n i t r a t e s and subsequent a d d i t i o n of an ammonia s o l u t i o n d i l u t e d t o pH = 5. The residue together w i t h the s o l u t i o n of unreacted s a l t s is d r i e d and ca1cined.W spectroscopic, X-ray, and ESCA s t u d i e s revealed a laminar s t r u c t u r e w i t h c o b a l t and i r o n molybdates covered by a t h i n bismuth molybdate f i l m . "he k i n e t i c charact e r i s t i c s of propene oxidation over these c a t a l y s t s have n o t been reported.
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A p a t e n t [gldescribea the p r e p a r a t i o n of a mdticomponent c a t a l y s t containing c o b a l t , molybdenum, bismuth, and i r o n oxides. It c o n s i s t s i n c o p r e c i p i t a t i o n of molybdates, pouring together cobaltous n i t r a t e , bismuth, and i r o n Bolutions w i t h an ammonia paramolybdate s o l u t i o n . The a c r y l i c a c i d y i e l d over such propene o x i d a t i o n c a t a l y s t s was no higher t h a n 10%. For a more uniform d i s t r i b u t i o n of bismuth and i r o n i n the c o b a l t molybdate s t r u c t u r e we prepared t h e c a t a l y s t via formation of heteropolycompounds. Diverse compositions and s t r u c t u r e s of such compounds w i t h Co2+, Co3+, Bi3+, Fe3+ as c e n t r a l atoms have been r e p o r t e d [lO,ll]. Mixed heteropolycompounds, i n which the molybdenum i o n s i n the anion atruct u r e s of 12- or 9-he teropolymolybdates were p a r t i a l l y replaced by Co2+, Co3+, Fe3+, e t c . ions have been described [11J Co2+, Fe3+ e t c . can r e p r e s e n t the c e n t r a l atoms i n these mixed he teropolyanions I n order t o o b t a i n such compounds, the pH value of the s o l u t i o n a f t e r intermixing of molybdenum, c o b a l t , bismuth, and i r o n salts must be w i t h i n 1 t o 4 [lo] A t l o w pH ( 1 - 2 ) t h e 12-heteropolyanions are formed pref e r e n t i a l l y ['lo] I n t h i s case one atom of some element involved i n the heteropolyanion s t r u c t u r e comes t o twelve atoms o f molybdenum. The higher is the charge of the i o n , the strongor i s the complex anion, This suggests t h a t Bi3+ and Fe3+ would be the p r e f e r e n t i a l c e n t r a l atoms i n t h e heteropolyanion s t r u c t u r e , and could a l s o s u b s t i t u t e the molybdenum i o n i n the heteropolyanion. The Go2+ i o n s form the salt o f the r e l e v a n t heteropolyacid. I n t h i s case the sum of bismuth and i r o n i o n s must n o t exceed 1 per 72 rpolybdenum atoms, and the 2. Moreover, a d d i t i o n atomic r a t i o Mo/Co must be w i t h i n 1.5 of excess molybdenum t o s t o i c h i o m e t r i c c o b a l t molybdate inc r e a s e s the a c r y l i c a c i d y i e l d i n propene oxidation [6,12] Consequently, i n preparing the composite oxide c a t a l y s t f o r o l e f i n oxidation the r a t i o Mo/Co must be increased t o 2.Then the empirical formula of the c a t a l y s t w i l l be:
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Co6M012Bi0 .5Fe0 .5-1 ,OoXm
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The c a t a l y s t was prepared by f i n e grinding of paramolybdate or ammonia molybdate, molybdic a c i d , and molybdenum trioxide, and adding an aqueous s o l u t i o n of c o b a l t , bismuth, and iron n i t r a t e s i n the course of intermixing. I n o r d e r t o
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prevent bismuth n i t r a t e hydrolysis and t o o b t a i n an a c i d medium favouring the formation of heteropolycompounds, conc e n t r a t e d excess n i t r i c acid was added i n an amount such that pH be no higher than 1-2. The mixture was c a r e f u l l y s t i r r e d , d r i e d and calcined a t 500°C. The c a t a l y s t obtained was powdered, passed through a 0.1 0.29 mm sieve, and t a b l e t t e d . The crushing s t r e n g t h of the t a b l e t s i s 160-170 kg/cm 2 The spec i f i c surface a r e a of the c a t a l y s t determined from low-temperature argon adsorption i s 4-5 m 2/g. The c a t a l y s t was t e s t e d i n a g r a d i e n t l e s s flow r e a c t o r a t W 0 C and c o n t a c t time 3.6 sec. It was found t h a t i n a mixture containing 5-7 vol.75 of propene, 6345% of a i r , and 30% o f water vapour, the propene conversion amounted t o 98-9996, a t a c r y l i c a c i d and a c r o l e i n y i e l d s 30-32 and 50-552, respectively. The a c e t i c a c i d y i e l d was no higher than 2 4 % . Under the same conditions a t 380°C o l e f i n conversion i n the oxidation of a mixture containing 5 ~01.75of isobutene w a s 95%-, a t methacrylic a c i d and methacrolein y i e l d s 17-19 and 55-60%, r e spec t i v e l y The c a t a l y s t was t e s t e d i n a r e a c t o r w i t h 25 tubes 20 mm d. and 900 mm long. The temperature was 40042O0C, the c o n t a c t t i m e 3.6 sec. The propane-propene f r a c t i o n tested contained 50-55 v01.75 of propene, 32-49 summary vol.% o f ethane and ethylene, 0.1 0.2 summary v01.Z of butane and butenes, no more than 0.001% of hydrogen sulphide, and 40-1000 mg/m3 mercaptenes. 'Phe r e a c t a n t mixture contained 6.2 6.5 v01.P of propene, 4 4.2 vol.% of propane, 10-11 vol.% of oxygen, 37-41 vol.% o f nitrogen, 36-39 vol.2 of water vapour ( t a b l e I).
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Table 1 Time Dependence of Activity and S e l e c t i v i t y of a Propene Oxidation C a t a l p t i n a Reactor a t #O-410°C
........................................................... 100 hr
2500 hr
4200
hr
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The k i n e t i c s o f propene o x i d a t i o n over t h i s c a t a l p t ha8 been s t u d i e d [13j Equations for the s t e p s of t h i s process have been derived. Rate c o n s t a n t s and a c t i v a t i o n e n e r g i e s f o r elementary r e a c t i o n 8 have been obtained ( t a b l e 2). Table 2 Kine t i c Charao t e r i s t i c s of the Propene @-xidation Process (41OoC)
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-+ a c r y l i c acid Propene Propene 3 acrolein 3 s i d e products Propene acrylic acid Acrolein -P Acrolein 3 s i d e products Acrylic -e. s i d e products acid
0 -3 1 .o
19 I0
0.3
13
0 e06 0.02
16
0.03
14
9
The r a t i o o f r a t e c o n s t a n t s f o r propene oxidation t o a c r y l i c a c i d and a c r o l e i n oxidation t o a c q l i c a c i d is seen t o be 5, 5.e. a c r y l i c a c i d i s formed mostly l e a v i n g a c r o l e i n a s i d e , v i a o t h e r surface compounds. Isobutene oxidation k i n e t i c s over t h e same c a t a l y s t was s t u d i e d f o r comparison. The k i n e t i c r e g u l a r i t i e s were c l o s e , b u t the conversion of o l e f i n s and the r a t e s of formation of c e r t a i n products from propene appeared t o be higher, and the r e l e v a n t a c t i v a t i o n energies lower than f o r isobutene oxidat i o n Since the isobutene a c t i v i t y i s higher than t h a t of propene, the controversy seems t o a r i s e from the r e l a t i v e l y s t r o n g adsorption of metacrolein, a r e a c t i o n product g r e a t l y hindering isobutene oxidation. The c a t a l y s t s t r u c t u r e w a s s t u d i e d by X-ray, I R , W spectroscopy, MGssbauer spectroscopy and the ESCA technique. X-ray study of a c o b a l t molybdenum c a t a l y s t and CoMo04 samples modified w i t h bismuth and i r o n i o n s showed t h a t these s p e c i e s a l t e r the r a t i o of ci t o B c o b a l t molybdate phases and t h e r e appear l i n e s assinged t o molybdenum trioxj.de. I n the presence o f i r o n i o n s the 8-phase is formed preferen-
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I
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1
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I
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4
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2
.I. x-Ray diagram o f c o b a l t molybdenum c a t a l y s t s . 1 ground CogMo12Bi0.5Fe0,750X. 2-same c a t a l y s t , ground C O & ~ B ~ ~ , ~ O 4 X . same c a t a l y s t , initial. 3 same c a t a l y s t , i n i t i a l . 5 ground Co6MoI2OX. 6 initial
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t i a l l y . It w i l l be seen from the x-ray diagram that s e v e r a l a d d i t i o n a l l i n e s t h a t can be i n t e r p r e t e d as due t o bismuth molybdate (Bi2O3.3MoO3) appear i n the presence of bismuth ions. To v e r i f y the formation of t h i s compound the 6-phase of C0M004 had t o be removed, s i n c e the most i n t e n s i v e l i n e s coincided w i t h s i m i l a r l i n e s f o r bismuth molybaate. With t h i s a i m the c a t a l y s t w a s ground i n a b a l l g r i n d e r , as t h i s induces a t r a n s i t i o n of the c o b a l t molybdenum B -phase t o a -phase [I41 !The diagram shows that the B -phase l i n e s disappeared, and t h e a-phase l i n e s became s t r o n g e r . The l i n e s due t o bismuth molybdate p e r s i s t e d , t h u s confirming t h e formation of t h i s compound. The same width o f c o b a l t molybdate and bismuth molybdate l i n e s i n d i c a t e s t o c l o s e d i s p e r s i t y of these systems. I R spectra show i n t e n s i v e bands: narrow a t 997 and 950 cm”, broad around 870 ern-', and a broad composite l i n e a t 600-700 cm-l i n the range of l a t t i c e deformation v i b r a t i o n s , as well as a weak band a t 820 cm-’. The c a t a l y s t spectrum
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can be i n t e r p r e t e d as the sum o f the MOO3 spectrum (997, 870 cm”, and weak band a t 820 cm-’) and o f the c o b a l t molybd a t e spectrum (9% cm”’) coinciding w i t h that o f a -CoMoOo. I r o n molybdate bands a r e absent. Small amounts of bismuth molybdate cannot be d e t e c t e d by t h i s technique, as t h e main c o b a l t molybdate and bismuth bands overlap, Absence of the c o b a l t molybdate pphase i s connected w i t h the technique of sample p r e p a r a t i o n f o r I R study (grinding i n the b a l l g r i n d e r and subsequent t a b l e t t i n g w i t h po tassium bromide).
Fig.2.
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W s p e c t r a of c o b a l t molybdenum c a t a l y s t s I Colyl00~; 2 C O ~ M O ~ 3~ O ~ ;
4
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- Co6Moq2Feo.750Xi
5
Co6*012Bi0.5Fe0.750X’
of Fig.2 r e p r e s e n t s W s p e c t r a of CoMoO,, (Mo/Co = I), a sample containing excess molyDaenum (Mo/Co = 2 ) , and o f c o b a l t molybdate modified w i t h bismuth and i r o n ions. A l l s p e c t r a d i s p l a y bands a t 13.500 cm-’ and two i n t e n s i v e narrow bands around 17.000 and 19.000 crn-’, The s p e c t r a coincide w i t h those of c o b a l t molybdate (Co/Mo = 1) b u t t h a t of the c a t a l y s t containing i r o n i o n s shows a lower r e l a t i v e i n t e n s i t y of the 17.000 cm-’ band. This i s p o s s i b l e only i n case the i r o n i o n e n t e r s the c o b a l t molybdate l a t t i c e s u b s t i t u t i n g t h e Co2+ ion i n octahedrons, and takes p a r t i n e l e c t r o n i n t e r a c t i o n s with
184
t h e c o b a l t ion. The presence of bismuth i o n s i n c o b a l t molybdate has no e f f e c t on the s p e c t r a obtained. It was found by the MGssbauer technique t h a t Fe2+ i o n s appeared i n t h e c a t a l y s t i n the course of c a t a l y s i s , and disappeared after removal of the reactant mixture. Upon ads o r p t i o n of propene Mo6+ reduced t o Mo5+ and a charge transf e r i n the system involving Fe3+ w i t h subsequent formation o f bound Fe2+ was observed. This i s evidence t h a t the c a t a l y s t iron ions contribute t o e l e c t r o n t r a n s i t i o n s . It was found by ESCA t h a t i n c o r p o r a t i o n of modifying Cu, V , Fe i n t o the surface-adjacent bed changed the r a t i o of c o b a l t t o molybdenum atoms a t the c a t a l y e t surface. 'Jlhe same was observed when bismuth i o n s were incroporated i n t o the bulk of the cobalt-molybdenum c a t a l y s t ( t a b l e 3 ) . Table 3 Binding Energies and the Ratio Co/Mo a t the Surface of Cobalt-Molybdenum C a t a l y s t s
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Catalyst
Binding e n e r g i e s , eV
co
MO
781.3 781.4 781.6 781.4
233.2 233.3 233.2 233.4 233.1
Ratio C o/Mo c ondi tional
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Uni t B 3d3/2 2p3/2 ............................................................
'Jo-MO ( I : ? ) (1 :2) 06Mo12Bi0.5OX Co6Mo12Fe0, 75OX Co6M012Bi0.'jFeO .75OX
CO-MO
781.1
0.50 0 -23
0.18 0.22
0.17
I t will be seen from t a b l e 3 t h a t bismuth a l t e r s the Co/Mo r a t i o , b u t has no e f f e c t on t h e e l e c t r o n binding energies on 3d and 3p l e v e l s of molybdenum and c o b a l t , r e s pectively. Cobalt, molybdenum, bismuth, and i r o n i o n s were found a t the surface of the modified c o b a l t molybdenum catalyst. Comparison of X-ray r e s u l t s w i t h those obtained by I R and UV spectroscopy shows t h a t the technique described above and provides a c a t a l y s t c o n s i s t i n g of c o b a l t molybdate ( a
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B -phases, i n which the c o b a l t i o n s a r e p a r t i a l l y r e p l a c e d by i r o n i o n s ) , Moo3, and bismuth molybdate d i s t r i b u t e d among these compounds. C o r r e l a t i o n of the c a t a l y s t s t r u c t u r e w i t h the r e s u l t s obtained by ESCA and Y b b a u e r technique shows that the modifying s p e c i e s a l t e r the r a t i o of Co t o Mo a t the s u r f a c e , p a r t i c i p a t e i n e l e c t r o n t r a n s i t i o n s d u r i n g cat a l y s i s , and c r e a t e new a c t i v e s i t e s a t t h e s u r f a c e . Thus, the method proposed permits i n c r e a s i n g the conversion of o l e f i n s t o unsaturated a c i d e and aldehyders. The a u t h o r s wish t o thank K.N .Spiridonov, D.P.Shashkin, M.Ya.Kushnerev, Yu.G.Borod’ko, and Xu.M.Shulga for study of the c a t a l y s t s t r u c t u r e . REFERENCES
I . K.I.Grobova,
V.A.Selyakova, Zhur .Vsesoyuzn.Khim.Obshchest v a , 14(1%9)281; I.K.Kolchin, Khiin.Prom., No.4(1973)249. 2. L.Ya.Margolis, Cat.Rev., 8(2) (1973)241. 3. Ph.A.Batist, J.E.Bowers, G.C,A.Shuit, J.Cat., 25(1972)1, 4. O.V.Isaev, L.Ya.Margolis, M.Ya.Kushnerev, Zhur.Fiz.Khim., 47(1973)2122 5. L.Ya.Margolis, O.V.Krylov, O.V.Isaev, Proceedings o f the 5-th I n t e r n a t i o n a l Congress on C a t a l y s i s (Miami Beach, 1972) vol. 2 Amsterdam, E l s e v i e r , 1973, p.1039. 6. T.G.Alkhazov, K.Yu.Adzhamov, N.Kh.Allakhverdova, Kin.i. K a t a l i z , 15(1974)1492. 7. M.W.J.Wolfs, Ph.A.Batist, J.Cat., 32(1974)25. 8. J.M.Matsuura, M.W.J.Wolfs, J.Cat., 73(1975)174. 9. French P a t e n t 1.604.942, I X C07c. lO.G.A.Tsigdinos, Clymax Xolybd.Bul1. Cd.b-12a (1969). ll.L.P.Kazanskii, E.A.Torchenkova, V.I.Spitsyn, Usp.Khim., 43 (1974) 1137 ?2.Y.Moro-oka, S.Tan, A.Ozaki, J.Cat., 12(1968)291. 13.0 .M.Vinogradova, G.P.Vytnov, I .V .Luiksaar, 0 .V.Al ‘ t s h u l e r , K i n . i , K a t a l i z , 16(1975)671. 14.L.M.Plyasova, V.I.Zharov, T.N.Kustova, L.G.Karakchiev, M ,M Andrushkevich, I zv .Akad .Nauk SSSR 9 (1 973) 519. 15.Yu.V .Maksimov, I .P.Suzdalev, V .I .Go1 ‘ d a n s k i i 0 .V .Krylov, L .Ya .Margolis A .E .Nechi t a i l o , Do k l Akad .Nauk SSSR 221 (1975) 880.
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CATALYST FOR PROPENE OR ISOBUTENE OXIDATION TO -UNSATURATED ALDEHYDES AND ACIDS
a , B-
0.V.ISAEV and L.Ya.WGOLIS I n s t i t u t e of Chemical Physics, Academy of Sciences o f the USSR, Moscow SUMMARY A technique of preparing t h e Co6Mo12Bi0 .5Fe0
,5-1 .oOx
c a t a l y s t for propene and isobutene oxidation t o unsaturated aldehydes and a c i d s i s proposed, I t c o n s i s t s i n production of mixed he teropolycompounds and t h e i r subsequent decomposition. The c a t a l y s t s t r u c t u r e was s t u d i e d by X-ray, I R , W , ESCA and MGssbauer spectroscopy. C a t a l y t i c o x i d a t i o n of o l e f i n s and unsaturated aldehydes i s an important engineering problem. Many c a t a l y s t s , mostly containing c o b a l t molybdate, have been proposed accordingly. Oxide compounds of tellurium, a r s e n i c , antimony, e t c . have been incorporated i n t o the c a t a l y s t i n o r d e r t o i n c r e a s e the formation s e l e c t i v i t y of unsaturated acids. However, these compounds a r e t o x i c , a r e r e a d i l y reduced by hydrocarbons and become v o l a t i l e . This imposes a 1-2 vol.% decrease i n o l e f i n concentration, thus decreasing the c a t a l y s t e f f e c t i v i t y , and, moreover, the l o s s o f these compounds has t o be compensated for. M a n y attempts of c o b a l t molybdate modification by metal oxides y i e l d i n g no v o l a t i l e s p e c i e s i n the course of r e a c t i o n have been made [I] b u t papers on modification of composite Consequently, the c r e a t i o n oxide systems are sparse [2-4] o f s c i e n t i f i c grounds f o r p r e p a r a t i o n o f e f f e c t multicomponent c a t a l y s t s f o r o l e f i n conversion t o unsaturated aldehydes and a c i d s i s a very urgent problem. The a c t i o n of added s p e c i e s on the c a t a l y t i c a c t i v i t y of c o b a l t molybdate can be t r e a t e d i n terms of t h r e e models: (I) formation of s o l i d s o l u t i o n s upon i n c o r p o r a t i o n of subs t i t u t i o n of a d d i t i v e s , w i t h r e l e v a n t changes i n the c o b a l t t o molybdenum i o n r a t i o ; (2) a l t e r a t i o n o f e l e c t r o n t r a n s i -
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178
t i o n s by e f f e c t of a d d i t i v e s on t h e c r y s t a l energy spectrum (varying of forbidden band width, appearance of new l e v e l s ) , an (3) appearance of a d d i t i o n a l a c t i v e s i t e s capable o f adaorblng the r e a c t a n t s and c o n t r i b u t i n g t o c a t a l y s i s . In accordance with these models and experimental results [s] bismuth and i r o n were chosen as modifying components of the cobalt-molybdenum c a t a l y s t . Comparison of the various techniques for c o b a l t molybd a t e modification showed t h a t the paths of o l e f i n oxidation t o aldehydes and acids were d i f f e r e n t depending on whether the additives were incorporated onto the surface o r i n the bulk of the c a t a l y s t . Uniform d i s t r i b u t i o n o f c a t i o n s i n the multicomponent c a t a l y s t and a prefixed s t r u c t u r e a r e s t r i n gent requirements here. Consequently, i t i s very d i f f i c u l t t o f i n d the optimal means f o r preparation of such a c a t a l y t i c eystem Usually c o b a l t molybdate i s obtained by c o p r e c i p i t a t i o n , pouring together ammonia paramolybdate and c o b a l t n i t r a t e i n such a way t h a t the r a t i o Co/Mo be unity. The pH value i s maintained w i t h i n 5 t o 7 and i s c o n t r o l l e d by a d d i t i o n of an aqueous ammonia solution. The r a t i o of CX- t o B -modifications of c o b a l t molybdate, the s p e c i f i c surface of samples, and their c a t a l y t i c a c t i v i t y i n propene oxidation t o a c r y l i c a c i d depend on pH of the medium i n which CoMo04 is precipitated[6]. This technique i s inconvenient for o b t a i n i n g c o b a l t molybdate modified by bismuth and i r o n , s i n c e when CoMo04 i s p r e c i p i t a t e d , the molybdates of these metals f a l l o u t simultaneously and independently. Another technique of preparing a multicomponent c a t a l y s t containing c o b a l t , molybdenum, bismuth, i r o n , e t c .ions [7] c o n s i s t s i n a d d i t i o n of powdered molybdic a c i d t o a h o t aqueous s o l u t i o n of c o b a l t bismuth, i r o n , e t c . n i t r a t e s and subsequent a d d i t i o n of an ammonia s o l u t i o n d i l u t e d t o pH = 5. The residue together w i t h the s o l u t i o n of unreacted s a l t s is d r i e d and ca1cined.W spectroscopic, X-ray, and ESCA s t u d i e s revealed a laminar s t r u c t u r e w i t h c o b a l t and i r o n molybdates covered by a t h i n bismuth molybdate f i l m . "he k i n e t i c charact e r i s t i c s of propene oxidation over these c a t a l y s t s have n o t been reported.
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179
A p a t e n t [gldescribea the p r e p a r a t i o n of a mdticomponent c a t a l y s t containing c o b a l t , molybdenum, bismuth, and i r o n oxides. It c o n s i s t s i n c o p r e c i p i t a t i o n of molybdates, pouring together cobaltous n i t r a t e , bismuth, and i r o n Bolutions w i t h an ammonia paramolybdate s o l u t i o n . The a c r y l i c a c i d y i e l d over such propene o x i d a t i o n c a t a l y s t s was no higher t h a n 10%. For a more uniform d i s t r i b u t i o n of bismuth and i r o n i n the c o b a l t molybdate s t r u c t u r e we prepared t h e c a t a l y s t via formation of heteropolycompounds. Diverse compositions and s t r u c t u r e s of such compounds w i t h Co2+, Co3+, Bi3+, Fe3+ as c e n t r a l atoms have been r e p o r t e d [lO,ll]. Mixed heteropolycompounds, i n which the molybdenum i o n s i n the anion atruct u r e s of 12- or 9-he teropolymolybdates were p a r t i a l l y replaced by Co2+, Co3+, Fe3+, e t c . ions have been described [11J Co2+, Fe3+ e t c . can r e p r e s e n t the c e n t r a l atoms i n these mixed he teropolyanions I n order t o o b t a i n such compounds, the pH value of the s o l u t i o n a f t e r intermixing of molybdenum, c o b a l t , bismuth, and i r o n salts must be w i t h i n 1 t o 4 [lo] A t l o w pH ( 1 - 2 ) t h e 12-heteropolyanions are formed pref e r e n t i a l l y ['lo] I n t h i s case one atom of some element involved i n the heteropolyanion s t r u c t u r e comes t o twelve atoms o f molybdenum. The higher is the charge of the i o n , the strongor i s the complex anion, This suggests t h a t Bi3+ and Fe3+ would be the p r e f e r e n t i a l c e n t r a l atoms i n t h e heteropolyanion s t r u c t u r e , and could a l s o s u b s t i t u t e the molybdenum i o n i n the heteropolyanion. The Go2+ i o n s form the salt o f the r e l e v a n t heteropolyacid. I n t h i s case the sum of bismuth and i r o n i o n s must n o t exceed 1 per 72 rpolybdenum atoms, and the 2. Moreover, a d d i t i o n atomic r a t i o Mo/Co must be w i t h i n 1.5 of excess molybdenum t o s t o i c h i o m e t r i c c o b a l t molybdate inc r e a s e s the a c r y l i c a c i d y i e l d i n propene oxidation [6,12] Consequently, i n preparing the composite oxide c a t a l y s t f o r o l e f i n oxidation the r a t i o Mo/Co must be increased t o 2.Then the empirical formula of the c a t a l y s t w i l l be:
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-
Co6M012Bi0 .5Fe0 .5-1 ,OoXm
.
The c a t a l y s t was prepared by f i n e grinding of paramolybdate or ammonia molybdate, molybdic a c i d , and molybdenum trioxide, and adding an aqueous s o l u t i o n of c o b a l t , bismuth, and iron n i t r a t e s i n the course of intermixing. I n o r d e r t o
180
prevent bismuth n i t r a t e hydrolysis and t o o b t a i n an a c i d medium favouring the formation of heteropolycompounds, conc e n t r a t e d excess n i t r i c acid was added i n an amount such that pH be no higher than 1-2. The mixture was c a r e f u l l y s t i r r e d , d r i e d and calcined a t 500°C. The c a t a l y s t obtained was powdered, passed through a 0.1 0.29 mm sieve, and t a b l e t t e d . The crushing s t r e n g t h of the t a b l e t s i s 160-170 kg/cm 2 The spec i f i c surface a r e a of the c a t a l y s t determined from low-temperature argon adsorption i s 4-5 m 2/g. The c a t a l y s t was t e s t e d i n a g r a d i e n t l e s s flow r e a c t o r a t W 0 C and c o n t a c t time 3.6 sec. It was found t h a t i n a mixture containing 5-7 vol.75 of propene, 6345% of a i r , and 30% o f water vapour, the propene conversion amounted t o 98-9996, a t a c r y l i c a c i d and a c r o l e i n y i e l d s 30-32 and 50-552, respectively. The a c e t i c a c i d y i e l d was no higher than 2 4 % . Under the same conditions a t 380°C o l e f i n conversion i n the oxidation of a mixture containing 5 ~01.75of isobutene w a s 95%-, a t methacrylic a c i d and methacrolein y i e l d s 17-19 and 55-60%, r e spec t i v e l y The c a t a l y s t was t e s t e d i n a r e a c t o r w i t h 25 tubes 20 mm d. and 900 mm long. The temperature was 40042O0C, the c o n t a c t t i m e 3.6 sec. The propane-propene f r a c t i o n tested contained 50-55 v01.75 of propene, 32-49 summary vol.% o f ethane and ethylene, 0.1 0.2 summary v01.Z of butane and butenes, no more than 0.001% of hydrogen sulphide, and 40-1000 mg/m3 mercaptenes. 'Phe r e a c t a n t mixture contained 6.2 6.5 v01.P of propene, 4 4.2 vol.% of propane, 10-11 vol.% of oxygen, 37-41 vol.% o f nitrogen, 36-39 vol.2 of water vapour ( t a b l e I).
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Table 1 Time Dependence of Activity and S e l e c t i v i t y of a Propene Oxidation C a t a l p t i n a Reactor a t #O-410°C
........................................................... 100 hr
2500 hr
4200
hr
181
The k i n e t i c s o f propene o x i d a t i o n over t h i s c a t a l p t ha8 been s t u d i e d [13j Equations for the s t e p s of t h i s process have been derived. Rate c o n s t a n t s and a c t i v a t i o n e n e r g i e s f o r elementary r e a c t i o n 8 have been obtained ( t a b l e 2). Table 2 Kine t i c Charao t e r i s t i c s of the Propene @-xidation Process (41OoC)
.
-+ a c r y l i c acid Propene Propene 3 acrolein 3 s i d e products Propene acrylic acid Acrolein -P Acrolein 3 s i d e products Acrylic -e. s i d e products acid
0 -3 1 .o
19 I0
0.3
13
0 e06 0.02
16
0.03
14
9
The r a t i o o f r a t e c o n s t a n t s f o r propene oxidation t o a c r y l i c a c i d and a c r o l e i n oxidation t o a c q l i c a c i d is seen t o be 5, 5.e. a c r y l i c a c i d i s formed mostly l e a v i n g a c r o l e i n a s i d e , v i a o t h e r surface compounds. Isobutene oxidation k i n e t i c s over t h e same c a t a l y s t was s t u d i e d f o r comparison. The k i n e t i c r e g u l a r i t i e s were c l o s e , b u t the conversion of o l e f i n s and the r a t e s of formation of c e r t a i n products from propene appeared t o be higher, and the r e l e v a n t a c t i v a t i o n energies lower than f o r isobutene oxidat i o n Since the isobutene a c t i v i t y i s higher than t h a t of propene, the controversy seems t o a r i s e from the r e l a t i v e l y s t r o n g adsorption of metacrolein, a r e a c t i o n product g r e a t l y hindering isobutene oxidation. The c a t a l y s t s t r u c t u r e w a s s t u d i e d by X-ray, I R , W spectroscopy, MGssbauer spectroscopy and the ESCA technique. X-ray study of a c o b a l t molybdenum c a t a l y s t and CoMo04 samples modified w i t h bismuth and i r o n i o n s showed t h a t these s p e c i e s a l t e r the r a t i o of ci t o B c o b a l t molybdate phases and t h e r e appear l i n e s assinged t o molybdenum trioxj.de. I n the presence o f i r o n i o n s the 8-phase is formed preferen-
182
I
I
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1
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I
I .
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1.
4
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2
.I. x-Ray diagram o f c o b a l t molybdenum c a t a l y s t s . 1 ground CogMo12Bi0.5Fe0,750X. 2-same c a t a l y s t , ground C O & ~ B ~ ~ , ~ O 4 X . same c a t a l y s t , initial. 3 same c a t a l y s t , i n i t i a l . 5 ground Co6MoI2OX. 6 initial
. -
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t i a l l y . It w i l l be seen from the x-ray diagram that s e v e r a l a d d i t i o n a l l i n e s t h a t can be i n t e r p r e t e d as due t o bismuth molybdate (Bi2O3.3MoO3) appear i n the presence of bismuth ions. To v e r i f y the formation of t h i s compound the 6-phase of C0M004 had t o be removed, s i n c e the most i n t e n s i v e l i n e s coincided w i t h s i m i l a r l i n e s f o r bismuth molybaate. With t h i s a i m the c a t a l y s t w a s ground i n a b a l l g r i n d e r , as t h i s induces a t r a n s i t i o n of the c o b a l t molybdenum B -phase t o a -phase [I41 !The diagram shows that the B -phase l i n e s disappeared, and t h e a-phase l i n e s became s t r o n g e r . The l i n e s due t o bismuth molybdate p e r s i s t e d , t h u s confirming t h e formation of t h i s compound. The same width o f c o b a l t molybdate and bismuth molybdate l i n e s i n d i c a t e s t o c l o s e d i s p e r s i t y of these systems. I R spectra show i n t e n s i v e bands: narrow a t 997 and 950 cm”, broad around 870 ern-', and a broad composite l i n e a t 600-700 cm-l i n the range of l a t t i c e deformation v i b r a t i o n s , as well as a weak band a t 820 cm-’. The c a t a l y s t spectrum
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183
can be i n t e r p r e t e d as the sum o f the MOO3 spectrum (997, 870 cm”, and weak band a t 820 cm-’) and o f the c o b a l t molybd a t e spectrum (9% cm”’) coinciding w i t h that o f a -CoMoOo. I r o n molybdate bands a r e absent. Small amounts of bismuth molybdate cannot be d e t e c t e d by t h i s technique, as t h e main c o b a l t molybdate and bismuth bands overlap, Absence of the c o b a l t molybdate pphase i s connected w i t h the technique of sample p r e p a r a t i o n f o r I R study (grinding i n the b a l l g r i n d e r and subsequent t a b l e t t i n g w i t h po tassium bromide).
Fig.2.
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W s p e c t r a of c o b a l t molybdenum c a t a l y s t s I Colyl00~; 2 C O ~ M O ~ 3~ O ~ ;
4
-
- Co6Moq2Feo.750Xi
5
Co6*012Bi0.5Fe0.750X’
of Fig.2 r e p r e s e n t s W s p e c t r a of CoMoO,, (Mo/Co = I), a sample containing excess molyDaenum (Mo/Co = 2 ) , and o f c o b a l t molybdate modified w i t h bismuth and i r o n ions. A l l s p e c t r a d i s p l a y bands a t 13.500 cm-’ and two i n t e n s i v e narrow bands around 17.000 and 19.000 crn-’, The s p e c t r a coincide w i t h those of c o b a l t molybdate (Co/Mo = 1) b u t t h a t of the c a t a l y s t containing i r o n i o n s shows a lower r e l a t i v e i n t e n s i t y of the 17.000 cm-’ band. This i s p o s s i b l e only i n case the i r o n i o n e n t e r s the c o b a l t molybdate l a t t i c e s u b s t i t u t i n g t h e Co2+ ion i n octahedrons, and takes p a r t i n e l e c t r o n i n t e r a c t i o n s with
184
t h e c o b a l t ion. The presence of bismuth i o n s i n c o b a l t molybdate has no e f f e c t on the s p e c t r a obtained. It was found by the MGssbauer technique t h a t Fe2+ i o n s appeared i n t h e c a t a l y s t i n the course of c a t a l y s i s , and disappeared after removal of the reactant mixture. Upon ads o r p t i o n of propene Mo6+ reduced t o Mo5+ and a charge transf e r i n the system involving Fe3+ w i t h subsequent formation o f bound Fe2+ was observed. This i s evidence t h a t the c a t a l y s t iron ions contribute t o e l e c t r o n t r a n s i t i o n s . It was found by ESCA t h a t i n c o r p o r a t i o n of modifying Cu, V , Fe i n t o the surface-adjacent bed changed the r a t i o of c o b a l t t o molybdenum atoms a t the c a t a l y e t surface. 'Jlhe same was observed when bismuth i o n s were incroporated i n t o the bulk of the cobalt-molybdenum c a t a l y s t ( t a b l e 3 ) . Table 3 Binding Energies and the Ratio Co/Mo a t the Surface of Cobalt-Molybdenum C a t a l y s t s
...........................................................
Catalyst
Binding e n e r g i e s , eV
co
MO
781.3 781.4 781.6 781.4
233.2 233.3 233.2 233.4 233.1
Ratio C o/Mo c ondi tional
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Uni t B 3d3/2 2p3/2 ............................................................
'Jo-MO ( I : ? ) (1 :2) 06Mo12Bi0.5OX Co6Mo12Fe0, 75OX Co6M012Bi0.'jFeO .75OX
CO-MO
781.1
0.50 0 -23
0.18 0.22
0.17
I t will be seen from t a b l e 3 t h a t bismuth a l t e r s the Co/Mo r a t i o , b u t has no e f f e c t on t h e e l e c t r o n binding energies on 3d and 3p l e v e l s of molybdenum and c o b a l t , r e s pectively. Cobalt, molybdenum, bismuth, and i r o n i o n s were found a t the surface of the modified c o b a l t molybdenum catalyst. Comparison of X-ray r e s u l t s w i t h those obtained by I R and UV spectroscopy shows t h a t the technique described above and provides a c a t a l y s t c o n s i s t i n g of c o b a l t molybdate ( a
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B -phases, i n which the c o b a l t i o n s a r e p a r t i a l l y r e p l a c e d by i r o n i o n s ) , Moo3, and bismuth molybdate d i s t r i b u t e d among these compounds. C o r r e l a t i o n of the c a t a l y s t s t r u c t u r e w i t h the r e s u l t s obtained by ESCA and Y b b a u e r technique shows that the modifying s p e c i e s a l t e r the r a t i o of Co t o Mo a t the s u r f a c e , p a r t i c i p a t e i n e l e c t r o n t r a n s i t i o n s d u r i n g cat a l y s i s , and c r e a t e new a c t i v e s i t e s a t t h e s u r f a c e . Thus, the method proposed permits i n c r e a s i n g the conversion of o l e f i n s t o unsaturated a c i d e and aldehyders. The a u t h o r s wish t o thank K.N .Spiridonov, D.P.Shashkin, M.Ya.Kushnerev, Yu.G.Borod’ko, and Xu.M.Shulga for study of the c a t a l y s t s t r u c t u r e . REFERENCES
I . K.I.Grobova,
V.A.Selyakova, Zhur .Vsesoyuzn.Khim.Obshchest v a , 14(1%9)281; I.K.Kolchin, Khiin.Prom., No.4(1973)249. 2. L.Ya.Margolis, Cat.Rev., 8(2) (1973)241. 3. Ph.A.Batist, J.E.Bowers, G.C,A.Shuit, J.Cat., 25(1972)1, 4. O.V.Isaev, L.Ya.Margolis, M.Ya.Kushnerev, Zhur.Fiz.Khim., 47(1973)2122 5. L.Ya.Margolis, O.V.Krylov, O.V.Isaev, Proceedings o f the 5-th I n t e r n a t i o n a l Congress on C a t a l y s i s (Miami Beach, 1972) vol. 2 Amsterdam, E l s e v i e r , 1973, p.1039. 6. T.G.Alkhazov, K.Yu.Adzhamov, N.Kh.Allakhverdova, Kin.i. K a t a l i z , 15(1974)1492. 7. M.W.J.Wolfs, Ph.A.Batist, J.Cat., 32(1974)25. 8. J.M.Matsuura, M.W.J.Wolfs, J.Cat., 73(1975)174. 9. French P a t e n t 1.604.942, I X C07c. lO.G.A.Tsigdinos, Clymax Xolybd.Bul1. Cd.b-12a (1969). ll.L.P.Kazanskii, E.A.Torchenkova, V.I.Spitsyn, Usp.Khim., 43 (1974) 1137 ?2.Y.Moro-oka, S.Tan, A.Ozaki, J.Cat., 12(1968)291. 13.0 .M.Vinogradova, G.P.Vytnov, I .V .Luiksaar, 0 .V.Al ‘ t s h u l e r , K i n . i , K a t a l i z , 16(1975)671. 14.L.M.Plyasova, V.I.Zharov, T.N.Kustova, L.G.Karakchiev, M ,M Andrushkevich, I zv .Akad .Nauk SSSR 9 (1 973) 519. 15.Yu.V .Maksimov, I .P.Suzdalev, V .I .Go1 ‘ d a n s k i i 0 .V .Krylov, L .Ya .Margolis A .E .Nechi t a i l o , Do k l Akad .Nauk SSSR 221 (1975) 880.
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