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Transient response studies of isobutane oxidative dehydrogenation over molybdenum catalysts
Studies in Surface Science and Catalysis 130 A. Corma, F.V. Melo, S. Mendioroz and J.L.G. Fierro (Editors) 9 2000 Elsevier Science B.V. All fights reserved.
1901
T r a n s i e n t r e s p o n s e s t u d i e s of i s o b u t a n e o x i d a t i v e d e h y d r o g e n a t i o n over m o l y b d e n u m c a t a l y s t s N. V. Nekrasova), N. A. Gaidai a), Yu. A. Agafonova), S. L. Kipermana), V. Cortes Corberfin h) and M. F. Portela c) a) N. D. Zelinsky Institute of Organic Chemistry, R.A.S., 47 Leninskii Prospekt, 117913 Moscow, Russia b) Instituto. de Catfilisis y Petroleoquimica, CSIC, Campus UAM Cantoblanco, 28049 Madrid, Spain c) GRECAT, Instituto Superior Tecnico, Av, Rovisco Pais, 1096 Lisbon, Portugal Kinetics and mechanism of isobutane oxidative dehydrogenation were studied over cobalt and nickel molybdate catalysts. The data obtained in nonstationary and stationary regimes showed that kinetics and mechanism are the same over both catalysts. Isobutene and carbon oxides are primary reaction products. Lattice oxygen takes part in dehydrogenation reactions. Carbon oxides formation proceed by the interaction with adsorbed oxygen. Cobalt molybdate is more active and selective catalyst for isobutane oxidative dehydrogenation. It was shown t h a t nickel molybdate catalyst is stable only at high oxygen concentration while cobalt molybdate catalyst can work at lower oxygen concentrations. I. I N T R O D U C T I O N Many studies are carried out in the last years on the light alkane oxidative dehydrogenation (ODH) as ODH is an attractive way for olefin production owing to the absence of thermodynamic limitations. Most of the studies are devoted to developing new catalysts and much less attention was paid to the kinetics and, especially, the dynamics of the process. It is known that the study of the dynamic response can get valuable information about reaction mechanisms [1-3]. The works devoted to studies of isobutane ODH mechanism are practically absent. TAP reactor was used for investigation of propane ODH over V-Mg-O catalyst [4]. It was shown t h a t partial and total oxidation of propane occur on the same surface site but involve different forms of reactive oxygen: nucleophilic lattice oxygen takes part in the propane partial oxidation to propene, while adsorbed electrophilic oxygen is involved in the total oxidation process of propane. The transient response method was used in the study of the ammoxidation of propene and propane over an Sb-V-oxide [5]. It was shown there t h a t ammoxidation of
1902
propane to acrylonitrile occurs t h r o u g h the O D H of p r o p a n e a n d the reaction scheme w a s proposed for this process. However, the t r a n s i e n t r e s p o n s e s were obtained only for the r e p l a c e m e n t of inert gas by reaction m i x t u r e s . In this s t u d y we have i n v e s t i g a t e d t r a n s i e n t processes d u r i n g the a t t a i n m e n t of a s t e a d y s t a t e u n d e r different conditions to obtain the detailed i n f o r m a t i o n for m e c h a n i s m of isobutane ODH over m o l y b d a t e c a t a l y s t s (C00.95M004 a n d aNiMoO~) in a wide i n t e r v a l of process p a r a m e t e r s . Besides, the reaction kinetics of this process was studied u n d e r s t a t i o n a r y conditions over these c a t a l y s t s . 2. E X P E R I M E N T A L
The e x p e r i m e n t s u n d e r n o n s t a t i o n a r y conditions were carried out in special g r a d i e n t l e s s reactors of small volume, connected to a time-of-flight massspectrometer. T r a n s i e n t response m e t h o d [1-3] w a s applied in this case. The relaxation curves describing a t r a n s i t i o n of the s y s t e m to a new s t e a d y s t a t e were obtained after an j u m p w i s e change in the corresponding r e a c t a n t concentrations. A vertical line m e a n s a change of reaction conditions; the r e l a x a t i o n curve is r e l a t e d to the last change of these ones. Since the composition of the reaction m i x t u r e s leaving the reactor was a n a l y z e d every 1 s, the response curves for the time scale indicated in the figures i l l u s t r a t e n e a r l y continuous processes. Initial m i x t u r e s of specified composition such as (O~+C4Hlo+N2), (N2+C4H10), (O~+Ne) were p r e p a r e d by mixing the flows of the corresponding gases, purified and dried prior to the experiments. The residence time, defined as the ratio of the volume of the r e a c t i n g s y s t e m to the flow rate, did not exceed 2 s. It w a s considered at the t r e a t m e n t of the e x p e r i m e n t a l data. For all e x p e r i m e n t s the r e l a x a t i o n time w a s lower t h a n the t u r n o v e r time. This allows to characterize the observed r e l a x a t i o n s as intrinsic ones (i.e., those d e t e r m i n e d only by the reaction m e c h a n i s m ) [6, 7]. The m/e ratios employed were as follows: 15 (methane), 18 (water), 32 (oxygen), 43 (isobutane), 44 (carbon dioxide), 56 (isobutene). Kinetic e x p e r i m e n t s u n d e r s t e a d y - s t a t e conditions were carried out in g r a d i e n t l e s s units at a t m o s p h e r i c pressure. B l a n k r u n s (with a n d w i t h o u t quartz) showed t h a t an exit of the reaction into the volume as well as the homogeneous reaction can be neglected u n d e r e x p e r i m e n t a l conditions. The m a i n reaction products were isobutene, carbon oxides, low hydrocarbons a n d w a t e r . The i n t e r v a l of process p a r a m e t e r s was c h a n g e d in the following ranges: t e m p e r a t u r e 470 - 560 ~ P,:so~,,t,,,: = 0.08 - 0.50, Po.~ = 0.032 - 0.18 a t m . Total conversion (x) and isobutene selectivity varied in the i n t e r v a l s 0.02 - 0.26 a n d 0.70 - 0.95 on CoMoO4. Analogous p a r a m e t e r s on NiMoO4 were 0.05 - 0.20 and 0.50 - 0.80, respectively. 3. R E S U L T S A N D D I S C U S S I O N F i g . l a i l l u s t r a t e s the response w h e n the reaction m i x t u r e w a s i n t r o d u c e d onto CoMoOl a n d NiMoO~l c a t a l y s t s p r e v i o u s l y t r e a t e d by air. The r e l a x a t i o n
1903 0,3
0,5
a 0,4
-~
0,3
:~ g
0,2
~
0,2
0,1
0,1 0,0
JF
l
T
,
0
10
20
30
'
40 0
0
5
1
0
10
I
20
0
10
20
S
II
Fig.1. The change of isobutene concentration with the time (s) in the responses" (O2+N2)/(O2+CnHlo) (a) and (O2+C4Hlo)/(O2+N2) (10) over: I) CoMoO_l, 490 ~
Poe / Pb aH,o
0.2; II) NiMoO4, 520 oC
No2 " / Pb 4H,0
= 1.0.
curve has a delay, which can be attributed to some time necessary for isobutane adsorption, and a maximum. This m a x i m u m in the concentration of isobutene at the initial stage of the process is due to the participation of the highest amounts of lattice oxygen at the beginning of the process. Thus, this type of oxygen takes part in isobutene formation. The form of relaxation curves in this case is the same on both catalysts, but for the reverse response (O2+C..,Hlo)/(O2+N2) (see Ib, IIb) the relaxation curve has a m a x i m u m over NiMoOl at a variance of the response over CoMoO4. This can be connected with a higher heterogeneity of Ni-catalysts as compared with Cocatalysts. This heterogeneity of NiMoO4 is further d e m o n s t r a t e d in the response (N2+C4H10)/(O2+C4Hlo) (curves not shown). The change of isobutene concentration is monotonous in the response (N2+C~,H10)/(O2+C4Hlo) over CoMoO4 while it shows a m a x i m u m over NiMoO4. These two relaxation curves have no delay in these responses as in this case no time is needed for the isobutane adsorption. The relaxation curve of isobutene formation observed in the response O2/N2/(O2+C4Hlo) over both catalysts is monotonous after the intermediate blowing off by nitrogen during 1 minute. It means t h a t concentration of reactive lattice oxygen is decreased during the nitrogen t r e a t m e n t owing to a decrease of adsorbed oxygen concentration. The form of relaxation curves in the tests where the introduction of one reactant is shifted from one to the other (e.g. in the response (O2+N2)/(N2+C4Hlo) or the reverse one) confirms the participation of lattice and adsorbed oxygen in the dehydrogenation reaction and CO2 formation, respectively. The relaxation curves of CO2 formation in the responses (O2+N2)/(O2+C4Hlo)
1904 2
0,8 a
a
b
1,5
0,6
5 a/ 1
C~ o 0,4
r
0,5
0,2
Y
0
35
s I
II
Fig.2. The change of carbon dioxide concentration with time (s) in the responses: (O2+N2)/(O2+C~H10) (a) and (O2+C4Hlo)/(O2+N2) (b) over: "("H,o - 0.2; II) NiMoO4, 520 oC, " " -1.0. I) CoMoO,, 512 oC, P ~
po/PcH,o
over CoMoO.~ and NiMoO ~are p r e s e n t e d in Fig.2. The characteristic curves of the CO2 concentration in the responses (O2+N2)/(O2+C4Hlo) had a m a x i m u m over CoMoO~ and reached the s t a t i o n a r y level monotonously over NiMoO4. The absence of a m a x i m u m over Ni-catalyst is connected with the excess of oxygen in the reaction mixture (a fivefold higher concentration t h a n in the case of Cocatalyst). In the reverse responses (O2+C4Hlo)/(O2+N2) both curves have a m a x i m u m which is much higher in the case of NiMoO~. It can be explained by hydrocarbon reduction and coke formation on catalyst surface, m u c h higher on Ni-catalysts. The oxygen introduction results in coke burning. This m a x i m u m became much lower with a higher oxygen concentration in the reaction m i x t u r e (P"02/ P"isobutane : 2 . 0 ) . Therefore, an excess of oxygen is n e c e s s a r y for keeping stable the performance of Ni-catalysts in ODH of isobutane. Similar results were obtained in [8] for ODH of propane over NiMoO4 The concentration of CO2 in the response (O2+C.IH10)/(O2+N2) (b) is higher t h a n the one in s t a t i o n a r y state (a). It m e a n s t h a t the oxygen concentration in the reaction m i x t u r e (O2+C4H10) is mostly consumed in filling up the oxygen vacancies or in w a t e r formation but not in coke burning. Therefore the filling up the oxygen vacancies proceeds more quickly t h a n coke burning. A complex s t r a t e g y [9-12] to obtain justified models u n d e r s t a t i o n a r y conditions and to prove their adequacy was used in this investigation. We applied the methods of previous analysis to t r e a t the kinetic d a t a obtained: the dependence of reaction rates on dilution [13], a change of process selectivity in dependence on operation conditions [14]. It was shown t h a t overall order in the n u m e r a t o r in kinetic equation for carbon dioxide formation is higher t h a n the power of the
1905
denominator and the order of the n u m e r a t o r in the kinetic equation for light hydrocarbon formation is equal to the power of the denominator. The following kinetic equations describing different routes of the process over both the catalysts under investigation were found: 1) isobutane formation:
Pc4H,oPo2 r, - k, 1-"o.~ + k, t"-'C4H,o
(1)
2) carbon dioxide formation: 0.5
(k,, PC4H,o +k',, PC4H~+k'i, P c o ) Po~ 1" tl =
NO.5
02 + k~
PC4H8
(2)
3) carbon monoxide formation:
r., - k,.
PC4H~P"o~
o~ Po2 +k2 PC4H~
(3)
4) cracking products formation:
r , . - k , . D..~
PC4H,o
~0~ +k~ PC4H~
(4)
The kinetic data are in accordance with the reaction mechanism proposed in the nonstationary investigation: isobutene formation is characterized by redox mechanism (isobutane reduces the catalyst surface which can be oxidized by oxygen), COz is formed from isobutane, isobutene and CO, formation of CO proceeds mainly from isobutene and cracking products are formed from isobutane. Adsorbed oxygen takes part in carbon oxides formation. The observed rate of isobutene formation (rj) taking into account its transformation into carbon oxides can be described by the following equation:
[(k'.+k,.)Pc4HJPo~ F/- r / -
_o
5
Po~ +k2 PC~H~
(5)
The equations (1)-(5) described all the transformations observed in this system. The comparison of isobutene formation rates over the two catalysts shows that it is 1.5 - 2.0 times higher over CoMoO4. This catalyst is more selective for isobutene formation and it needs lower oxygen concentration to keep up a stable performance t h a n NiMoO4, which needs a large excess of air to be stable.
1906 4. C O N C L U S I O N The kinetic data of processes taking place under stationary and unstationary conditions over CoMoO4 and NiMoO4 show that the kinetics and the mechanism for ODH of isobutane are the same over both catalysts. The following mechanism was proposed: isobutene formation proceeded according to the redox mechanism, CO2 is formed from isobutane, isobutene and CO; formation of CO proceeds mainly from isobutene; cracking products are formed from isobutane. Adsorbed oxygen takes part in carbon oxides formation. However, both catalysts differ in their catalytic performance and stability. CoMoO~ is a more active and selective catalyst for ODH of isobutane than NiMoO4. Furthermore, NiMoO.l needs a large excess of oxygen to keep its catalytic activity stable, while CoMoO4 can operate at much lower oxygen-tohydrocarbon ratios (air/isobutane ratio = 2) with no change in stability. As a consequence, productivity of CoMoO~ catalyst can be larger than that of NiMoO4 at equal total space velocity. ACKNOWLEDGEMENTS The authors acknowledge INTAS (N 96-1117) for financial support. REFERENCES
1. C. O. Bennet, Cat. Rev.- Sci. Eng., 13 (1976) 121. 2. H. Kobayashi and M. Kobayashi, Cat. Rev. - Sci. Eng., 10 (1974) 139. 3. M. Kobayashi, Chem. Eng. Sci., 37 (1982) 393. 4. A. Pantazidis, S. A. Bucholz, H. W. Zanthoff, Y. Schuurman and C.Mirodatos, Catal. Today. 40 (1997) 207. 5. R. Nilsson and A. Anderson, Ind. Eng. Chem. Res., 36 (1997) 5209. 6. M. I. Temkin, Kinet. Katal., 17 (1976) 1095. 7. F. S. Shub, A. G. Ziskin, M. G. Slin'ko and, M. I.Temkin, Kinet. Katal., 20 (1979) 334. 8. R. Rosso, A. Kaddouri, R. Anouchinskym, C. Mazzocchia, P. Gronch and P. Centola, J. Mol. Catal., 135 (1998)181. 9. S.L. Kiperman, Uspekhi Khim. (russ.), 47 (1978) 3. 10. S. L. Kiperman, D. M. Shopov, A. Andreev, N. E. Zlotina and B. S. Gudkov, Izv. Khim. Bull. of Chemistry, Bulg. Akad. Sci, 4 (1971) 237. 11. S. L. Kiperman, Chem. Eng .Commun., 100 (1991) 3. 12. S. L. Kiperman, Kinet. Catal. (English Edn.), 36 (1995) 7. 13. N. I. Koltsov and S. L. Kiperman, I. Res. Inst. Catalysis, Hokkaido Univ., 26 (1978) 85. 14. N. I. Koltsov and S. L. Kiperman, Teor. Exp. Khim. (russ.), 13 (1977) 630.
1901
T r a n s i e n t r e s p o n s e s t u d i e s of i s o b u t a n e o x i d a t i v e d e h y d r o g e n a t i o n over m o l y b d e n u m c a t a l y s t s N. V. Nekrasova), N. A. Gaidai a), Yu. A. Agafonova), S. L. Kipermana), V. Cortes Corberfin h) and M. F. Portela c) a) N. D. Zelinsky Institute of Organic Chemistry, R.A.S., 47 Leninskii Prospekt, 117913 Moscow, Russia b) Instituto. de Catfilisis y Petroleoquimica, CSIC, Campus UAM Cantoblanco, 28049 Madrid, Spain c) GRECAT, Instituto Superior Tecnico, Av, Rovisco Pais, 1096 Lisbon, Portugal Kinetics and mechanism of isobutane oxidative dehydrogenation were studied over cobalt and nickel molybdate catalysts. The data obtained in nonstationary and stationary regimes showed that kinetics and mechanism are the same over both catalysts. Isobutene and carbon oxides are primary reaction products. Lattice oxygen takes part in dehydrogenation reactions. Carbon oxides formation proceed by the interaction with adsorbed oxygen. Cobalt molybdate is more active and selective catalyst for isobutane oxidative dehydrogenation. It was shown t h a t nickel molybdate catalyst is stable only at high oxygen concentration while cobalt molybdate catalyst can work at lower oxygen concentrations. I. I N T R O D U C T I O N Many studies are carried out in the last years on the light alkane oxidative dehydrogenation (ODH) as ODH is an attractive way for olefin production owing to the absence of thermodynamic limitations. Most of the studies are devoted to developing new catalysts and much less attention was paid to the kinetics and, especially, the dynamics of the process. It is known that the study of the dynamic response can get valuable information about reaction mechanisms [1-3]. The works devoted to studies of isobutane ODH mechanism are practically absent. TAP reactor was used for investigation of propane ODH over V-Mg-O catalyst [4]. It was shown t h a t partial and total oxidation of propane occur on the same surface site but involve different forms of reactive oxygen: nucleophilic lattice oxygen takes part in the propane partial oxidation to propene, while adsorbed electrophilic oxygen is involved in the total oxidation process of propane. The transient response method was used in the study of the ammoxidation of propene and propane over an Sb-V-oxide [5]. It was shown there t h a t ammoxidation of
1902
propane to acrylonitrile occurs t h r o u g h the O D H of p r o p a n e a n d the reaction scheme w a s proposed for this process. However, the t r a n s i e n t r e s p o n s e s were obtained only for the r e p l a c e m e n t of inert gas by reaction m i x t u r e s . In this s t u d y we have i n v e s t i g a t e d t r a n s i e n t processes d u r i n g the a t t a i n m e n t of a s t e a d y s t a t e u n d e r different conditions to obtain the detailed i n f o r m a t i o n for m e c h a n i s m of isobutane ODH over m o l y b d a t e c a t a l y s t s (C00.95M004 a n d aNiMoO~) in a wide i n t e r v a l of process p a r a m e t e r s . Besides, the reaction kinetics of this process was studied u n d e r s t a t i o n a r y conditions over these c a t a l y s t s . 2. E X P E R I M E N T A L
The e x p e r i m e n t s u n d e r n o n s t a t i o n a r y conditions were carried out in special g r a d i e n t l e s s reactors of small volume, connected to a time-of-flight massspectrometer. T r a n s i e n t response m e t h o d [1-3] w a s applied in this case. The relaxation curves describing a t r a n s i t i o n of the s y s t e m to a new s t e a d y s t a t e were obtained after an j u m p w i s e change in the corresponding r e a c t a n t concentrations. A vertical line m e a n s a change of reaction conditions; the r e l a x a t i o n curve is r e l a t e d to the last change of these ones. Since the composition of the reaction m i x t u r e s leaving the reactor was a n a l y z e d every 1 s, the response curves for the time scale indicated in the figures i l l u s t r a t e n e a r l y continuous processes. Initial m i x t u r e s of specified composition such as (O~+C4Hlo+N2), (N2+C4H10), (O~+Ne) were p r e p a r e d by mixing the flows of the corresponding gases, purified and dried prior to the experiments. The residence time, defined as the ratio of the volume of the r e a c t i n g s y s t e m to the flow rate, did not exceed 2 s. It w a s considered at the t r e a t m e n t of the e x p e r i m e n t a l data. For all e x p e r i m e n t s the r e l a x a t i o n time w a s lower t h a n the t u r n o v e r time. This allows to characterize the observed r e l a x a t i o n s as intrinsic ones (i.e., those d e t e r m i n e d only by the reaction m e c h a n i s m ) [6, 7]. The m/e ratios employed were as follows: 15 (methane), 18 (water), 32 (oxygen), 43 (isobutane), 44 (carbon dioxide), 56 (isobutene). Kinetic e x p e r i m e n t s u n d e r s t e a d y - s t a t e conditions were carried out in g r a d i e n t l e s s units at a t m o s p h e r i c pressure. B l a n k r u n s (with a n d w i t h o u t quartz) showed t h a t an exit of the reaction into the volume as well as the homogeneous reaction can be neglected u n d e r e x p e r i m e n t a l conditions. The m a i n reaction products were isobutene, carbon oxides, low hydrocarbons a n d w a t e r . The i n t e r v a l of process p a r a m e t e r s was c h a n g e d in the following ranges: t e m p e r a t u r e 470 - 560 ~ P,:so~,,t,,,: = 0.08 - 0.50, Po.~ = 0.032 - 0.18 a t m . Total conversion (x) and isobutene selectivity varied in the i n t e r v a l s 0.02 - 0.26 a n d 0.70 - 0.95 on CoMoO4. Analogous p a r a m e t e r s on NiMoO4 were 0.05 - 0.20 and 0.50 - 0.80, respectively. 3. R E S U L T S A N D D I S C U S S I O N F i g . l a i l l u s t r a t e s the response w h e n the reaction m i x t u r e w a s i n t r o d u c e d onto CoMoOl a n d NiMoO~l c a t a l y s t s p r e v i o u s l y t r e a t e d by air. The r e l a x a t i o n
1903 0,3
0,5
a 0,4
-~
0,3
:~ g
0,2
~
0,2
0,1
0,1 0,0
JF
l
T
,
0
10
20
30
'
40 0
0
5
1
0
10
I
20
0
10
20
S
II
Fig.1. The change of isobutene concentration with the time (s) in the responses" (O2+N2)/(O2+CnHlo) (a) and (O2+C4Hlo)/(O2+N2) (10) over: I) CoMoO_l, 490 ~
Poe / Pb aH,o
0.2; II) NiMoO4, 520 oC
No2 " / Pb 4H,0
= 1.0.
curve has a delay, which can be attributed to some time necessary for isobutane adsorption, and a maximum. This m a x i m u m in the concentration of isobutene at the initial stage of the process is due to the participation of the highest amounts of lattice oxygen at the beginning of the process. Thus, this type of oxygen takes part in isobutene formation. The form of relaxation curves in this case is the same on both catalysts, but for the reverse response (O2+C..,Hlo)/(O2+N2) (see Ib, IIb) the relaxation curve has a m a x i m u m over NiMoOl at a variance of the response over CoMoO4. This can be connected with a higher heterogeneity of Ni-catalysts as compared with Cocatalysts. This heterogeneity of NiMoO4 is further d e m o n s t r a t e d in the response (N2+C4H10)/(O2+C4Hlo) (curves not shown). The change of isobutene concentration is monotonous in the response (N2+C~,H10)/(O2+C4Hlo) over CoMoO4 while it shows a m a x i m u m over NiMoO4. These two relaxation curves have no delay in these responses as in this case no time is needed for the isobutane adsorption. The relaxation curve of isobutene formation observed in the response O2/N2/(O2+C4Hlo) over both catalysts is monotonous after the intermediate blowing off by nitrogen during 1 minute. It means t h a t concentration of reactive lattice oxygen is decreased during the nitrogen t r e a t m e n t owing to a decrease of adsorbed oxygen concentration. The form of relaxation curves in the tests where the introduction of one reactant is shifted from one to the other (e.g. in the response (O2+N2)/(N2+C4Hlo) or the reverse one) confirms the participation of lattice and adsorbed oxygen in the dehydrogenation reaction and CO2 formation, respectively. The relaxation curves of CO2 formation in the responses (O2+N2)/(O2+C4Hlo)
1904 2
0,8 a
a
b
1,5
0,6
5 a/ 1
C~ o 0,4
r
0,5
0,2
Y
0
35
s I
II
Fig.2. The change of carbon dioxide concentration with time (s) in the responses: (O2+N2)/(O2+C~H10) (a) and (O2+C4Hlo)/(O2+N2) (b) over: "("H,o - 0.2; II) NiMoO4, 520 oC, " " -1.0. I) CoMoO,, 512 oC, P ~
po/PcH,o
over CoMoO.~ and NiMoO ~are p r e s e n t e d in Fig.2. The characteristic curves of the CO2 concentration in the responses (O2+N2)/(O2+C4Hlo) had a m a x i m u m over CoMoO~ and reached the s t a t i o n a r y level monotonously over NiMoO4. The absence of a m a x i m u m over Ni-catalyst is connected with the excess of oxygen in the reaction mixture (a fivefold higher concentration t h a n in the case of Cocatalyst). In the reverse responses (O2+C4Hlo)/(O2+N2) both curves have a m a x i m u m which is much higher in the case of NiMoO~. It can be explained by hydrocarbon reduction and coke formation on catalyst surface, m u c h higher on Ni-catalysts. The oxygen introduction results in coke burning. This m a x i m u m became much lower with a higher oxygen concentration in the reaction m i x t u r e (P"02/ P"isobutane : 2 . 0 ) . Therefore, an excess of oxygen is n e c e s s a r y for keeping stable the performance of Ni-catalysts in ODH of isobutane. Similar results were obtained in [8] for ODH of propane over NiMoO4 The concentration of CO2 in the response (O2+C.IH10)/(O2+N2) (b) is higher t h a n the one in s t a t i o n a r y state (a). It m e a n s t h a t the oxygen concentration in the reaction m i x t u r e (O2+C4H10) is mostly consumed in filling up the oxygen vacancies or in w a t e r formation but not in coke burning. Therefore the filling up the oxygen vacancies proceeds more quickly t h a n coke burning. A complex s t r a t e g y [9-12] to obtain justified models u n d e r s t a t i o n a r y conditions and to prove their adequacy was used in this investigation. We applied the methods of previous analysis to t r e a t the kinetic d a t a obtained: the dependence of reaction rates on dilution [13], a change of process selectivity in dependence on operation conditions [14]. It was shown t h a t overall order in the n u m e r a t o r in kinetic equation for carbon dioxide formation is higher t h a n the power of the
1905
denominator and the order of the n u m e r a t o r in the kinetic equation for light hydrocarbon formation is equal to the power of the denominator. The following kinetic equations describing different routes of the process over both the catalysts under investigation were found: 1) isobutane formation:
Pc4H,oPo2 r, - k, 1-"o.~ + k, t"-'C4H,o
(1)
2) carbon dioxide formation: 0.5
(k,, PC4H,o +k',, PC4H~+k'i, P c o ) Po~ 1" tl =
NO.5
02 + k~
PC4H8
(2)
3) carbon monoxide formation:
r., - k,.
PC4H~P"o~
o~ Po2 +k2 PC4H~
(3)
4) cracking products formation:
r , . - k , . D..~
PC4H,o
~0~ +k~ PC4H~
(4)
The kinetic data are in accordance with the reaction mechanism proposed in the nonstationary investigation: isobutene formation is characterized by redox mechanism (isobutane reduces the catalyst surface which can be oxidized by oxygen), COz is formed from isobutane, isobutene and CO, formation of CO proceeds mainly from isobutene and cracking products are formed from isobutane. Adsorbed oxygen takes part in carbon oxides formation. The observed rate of isobutene formation (rj) taking into account its transformation into carbon oxides can be described by the following equation:
[(k'.+k,.)Pc4HJPo~ F/- r / -
_o
5
Po~ +k2 PC~H~
(5)
The equations (1)-(5) described all the transformations observed in this system. The comparison of isobutene formation rates over the two catalysts shows that it is 1.5 - 2.0 times higher over CoMoO4. This catalyst is more selective for isobutene formation and it needs lower oxygen concentration to keep up a stable performance t h a n NiMoO4, which needs a large excess of air to be stable.
1906 4. C O N C L U S I O N The kinetic data of processes taking place under stationary and unstationary conditions over CoMoO4 and NiMoO4 show that the kinetics and the mechanism for ODH of isobutane are the same over both catalysts. The following mechanism was proposed: isobutene formation proceeded according to the redox mechanism, CO2 is formed from isobutane, isobutene and CO; formation of CO proceeds mainly from isobutene; cracking products are formed from isobutane. Adsorbed oxygen takes part in carbon oxides formation. However, both catalysts differ in their catalytic performance and stability. CoMoO~ is a more active and selective catalyst for ODH of isobutane than NiMoO4. Furthermore, NiMoO.l needs a large excess of oxygen to keep its catalytic activity stable, while CoMoO4 can operate at much lower oxygen-tohydrocarbon ratios (air/isobutane ratio = 2) with no change in stability. As a consequence, productivity of CoMoO~ catalyst can be larger than that of NiMoO4 at equal total space velocity. ACKNOWLEDGEMENTS The authors acknowledge INTAS (N 96-1117) for financial support. REFERENCES
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