Znd. Eng. Chem. Res. 1996,34, 474-482
474
Catalytic Oxidation of Hydrocarbons and Alcohols by Carbon Dioxide on Oxide Catalysts? 0. v. Krylov* N . N . Semenov Institute of Chemical Physics, Russian Academy of Sciences, ul. Kosygina, 4, Moscow, 11 7977, GSP-1, Russia
A. Kh. Mamedov and S. R. Mirzabekova Y u . G. Mamedaliev Institute of Petrochemical Processes, Azerbaijan Academy of Sciences, ul. Telnova, 30, Baku, 370604, Azerbaijan
Reactions of oxidative transformation of organic compounds of different classes (alkanes, alkenes, and alcohols) with a nontraditional oxidant, carbon dioxide, were studied on oxide catalysts Fe-0, Cr-0, Mn-0 and on multicomponent systems based on manganese oxide. The supported manganese oxide catalysts are active, selective, and stable in conversion of the CHI C02 mixture into synthesis gas and in oxidative dehydrogenation of c2-c7 hydrocarbons and the lower alcohols. Unlike metal catalysts manganese oxide based catalysts do not form a carbon layer during the reaction.
+
The great interest displayed lately in heterogeneous catalytic reactions of carbon dioxide is caused by two reasons: (1)the necessity t o fight the greenhouse effect and 2) the exhaust of carbon raw material sources. In 1992 143 countries signed an agreement in Rio de Janeiro on the control of the global increase of temperature in the atmosphere and on the necessity to decrease COZcontent in exhaust gases. The increase of the COz content in the atmosphere may create an irreversible change in the Earth’s climate in the middle of the 21st century. Independently of the greenhouse effect C02 can be considered as a very important raw material for the chemical industry in the future. The sources of coal, petroleum, and natural gas are being quickly exhausted. The total amount of carbon contained in CO2 in the atmosphere and in the hydrosphere is by 1%orders of magnitude more than that in coal, petroleum, and gas, while in the lithosphere it is by about 2 orders of magnitude higher. The main obstacle to the wide use of C02 is its low concentration in the atmosphere, comprising only 0.03%. In this respect the use of C 0 2 from exhaust gases of industry and transport is more attractive. However, currently mainly C02 prepared by decomposition of solid carbonates is used in industry. At the present time only three large-scale processes utilizing C02 have found industrial use: the production of calcined soda by the Solvay method, the production of urea (carbamide),and the production of salicylic acid. At the same time scientific investigations show that carbon dioxide can be involved in numerous reactions and can serve as an initial substance for the synthesis of many useful chemical products. Many papers have been devoted to the introduction of COZinto complexes of transition metals with organic ligands; see reviews by Keim (1983), Aresta and Forti (19861, and Braunstein et al. (1988). Decomposition of these complexes gives organic acids, esters, lactons, etc. Bonding of C02 molecules as ligands in transition metal complexes is also known (Breslow et al., 1981; Fachinetti et al., 1978). The synthesis of organic carbonPaper presented at the US-Russia Workshop on Environmental Catalysis, Wilmington, DE, January 14-16, 1994. +
0888-588519512634-0474$09.00/0
ates from C 0 2 is a quickly developing field of C02 chemical use. However, the mentioned reactions cannot solve the problem of COz emission decrease and cannot claim t o be a large-scale consumer of COZ as raw material. In these cases a large number of another, rather expensive, component is necessary for involving COZin the reaction. Redox reactions with the participation of C 0 2 as an oxidant are more representative in this respect for largescale C02 use. Inexpensive reducers, mainly hydrocarbons, can be involved in the reaction with COS. These hydrocarbons are contained in the natural sources of raw materials (petroleum, natural gas). The best reducing agent for C02 is, of course, hydrogen. As was recently shown (Rozovskiiand Lin, 1990),the synthesis of methanol from CO and HZproceeds not by their direct interaction, but by preliminary transformation of CO into COZ CO
+ H20 = C 0 2 + H2
(1)
and after that by the interaction of COz with H2:
C02
+ 3Hz = CH,OH + H20
(2)
Direct utilization of reaction 2 now meets with difficulties because a part of the expensive hydrogen is transformed into inexpensive water. For this reason hydrocarbons are more attractive. We have studied (Krylov et al., 1993;Mamedov, 1991; Mamedov et al., 1990a,b, 1992; Mirzabekova et al., 1992a-e, 1993a-d) new reactions of C1-C7 alkane reduction with COZto olefins and synthesis gas. It was found that manganese-containing catalysts are effective both for methane transformation into synthesis gas and for Cz-C7 alkane dehydrogenation into lower olefins, H2, and CO. These systems were found to be effective also for C02 interaction with organic compounds of other classes, for example, ethylene transformation into butadiene Mirzabekova et al., 1994a,b), the reaction of methanol with CO2 for the formation of formaldehyde (Mirzabekova et al., 1993b), and the reaction of ethanol with COZfor butadiene formation (Mirzabekova et al., 1994b). The use of manganese-containing catalysts in these reactions provides high conversions both of the
0 1995 American Chemical Society
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 476 oxidized substance and of C02 with a stable activity level during a long period without coke accumulation. Many aspects of the reaction mechanism of organic substances with COZ are yet unclear. This paper represents a review of our work and contains a discussion of the general characteristics of these reactions over manganese-containing catalysts of different compositions and of the role of manganese oxide in these reactions.
+
Experimental Section The study of catalytic activity was performed in a flow system with a silica reactor 7 mm in diameter. The length of the catalyst layer was 7-8 cm; the amount of the catalyst was 4 cm3,the grains were 2-4 mm in size; the space velocity was changed from 900 to 12 000 h-l. The pulse method was also used; gaseous mixtures were analyzed by a gas chromatograph. All the catalysts were prepared by impregnation of silica, alumina, or zeolite with nitrates of different metals, followed by drying a t 100-140 "C over 4 h and heating at 800-900 "C. The details of the experimental procedure are given in papers by Krylov et al. (19931, Mamedov (19911, Mamedov et al. (1990a,b, 19921, and Mirzabekova et al. (1992a-e, 1993a-d, 1994a,b). Most of the catalysts contain manganese and chromium oxides supported on alumina or silica. The X-ray diffraction study of initially prepared Mn-Cr/Al-0 catalysts shows the existence of MnO, y-AlzO3 phases and MnAl204 spinel phase formed by interaction of MnO with Al2O3. The absence of chromium-containing phase is probably due to its high dispersity. During the catalytic reaction the total intensity of these phases is not changed but an enlargement of A1203 particles is observed (from 20-30 to 70-80 nm). The dimension of A1203 particles is higher than that of MnAl204 phase. In the case of Mn-Cr-O/SiOn catalyst the X-ray diffraction study did not show a direct interaction of manganese and chromium oxides with Si02; Mn304 and Mn2O3 phases are observed before the reaction and MnO and MnC03 are observed during the reaction. The catalysts prepared on the basis of both A1203 and Si02 do not have strongly different surface areas (34 and 69 m2/g,respectively). This shows that the differences in catalytic properties of these systems are not due to surface area influence. The study of the X-ray structure of the catalyst in situ is described in the paper of Mamedov et al. (1990b).
Results and Discussion Methane Conversion. Methane oxidation by carbon dioxide is a practically important process for obtaining synthesis gas: CH4
SPARG process, in which sulfur vapors are added to the reaction gases. The sulfur passivates nickel and prevents coking (Rostrup-Nielsen, 1988). In order to find the most effective catalyst and to elucidate the characteristics of catalyst selection, we carried out a study of the effect of modifying additives on the catalyst (Mamedov, 1991; Mamedov et al., 1990a,b). Modification of nickel by different additives, such as Cu, Cr, Fe, Mn oxides, etc. changes its properties in CHI CO2 conversion. The study of C 0 2 pulse interaction with methane catalysts coked in methane showed that the best effect was obtained when manganese was used as the modifying additive. Upon increase of Mn content the degree of CO2 conversion increases and the coking decreases. With increase of Mn content in the reduced Ni catalyst the CHI conversion during CH4 transformation (in the absence of CO2) was found to decrease. Therefore, modification of the Ni catalysts by Mn allows to change the reactivity of the C02 CH4 mixture. Further investigation showed that the catalyst can function without any metallic component Mamedov, 1991; Mamedov et al., 1990a,b, 1992, Mirzabekova et al., 1992b,c). The catalyst of the composition 5% Ca12%MnO/A1203was the best one. After initial treating of the catalyst by COS at 850 "C, ethane and ethylene are produced on it in non-stationary-state conditions at the same temperature:
+ CO, = 2H2 + 2CO
(3)
This reaction is known to proceed on Ni and other metal catalysts (Ashcroft et al., 1991, 1993; Rostrup-Nielsen and Hansen, 1993; Erdohelyi et al., 1993). It also proceeds as a side reaction in methane steam reforming. The main difficulty met with when this reaction is used for the practical process of synthesis gas production is coking of the presently utilized nickel catalyst. Different methods are used for the commercial realization of synthesis gas production from the CH4 C02 mixture. The Calcor process of natural or petroleum gas conversion with C02 is known. Topsoe developed a
+
+
+ C 0 2 = C2H6 + CO + H,O C2HG + C02 = C2H4 + CO + H2O
2CH4
(4)
(5)
In this connection it is worthwhile to mention the papers of Aika and Nishiyama (1988) and Nishiyama and Aika (19911, who studied oxide catalysts for methane oxidative coupling. It turned out that the PbO-MgO catalysts with moderate basicity showed a higher C2hydrocarbon yield in the CH4 C02 0 2 mixture than in the CH4 0 2 mixture. Experiments with 13C02and 13CD4showed that all the CO was formed from COZonly but not as a result of subsequent hydrocarbon oxidation in the absence of CH4. The C02 was completely nonreactive on the catalyst. On the surface of MgO only reaction 3 of synthesis gas formation was observed. In our experiments on Ca-Mn-O/AlnOs, in the initial non-stationary-state period the C2 yield was 9% at 850 "C. It increased up to 13% with decrease of the pretreatment temperature. When the initial oxides Mn304 and Mn2O3 are reduced, the formation of C2H6 and CzH4 decreases and only synthesis gas in stationary state conditions is formed; that is, reaction 3 takes place. An X-ray difiaction (XRD)study shows that oxidized Mn phases Mn& and Mn2O3 promote total methane oxidation (in non-steady-stateconditions);reduced phases MnO and MnC03 assist methane conversion into synthesis gas; intermediate phases, where 0- stabilization is possible, promote methane oxidative coupling. The process on the 5% Ca-12% Mn-O/Al203 catalyst proceeds in stationary state conditions without loss of activity (Table 1). A comparison of H2 formation rates in steady state conditions (3 x mol/(g-h))and during the reduction of the catalyst by methane (1.7 x mol/(gh)) shows that they are close to each other. A study of the characteristics of the catalyst reoxidation by COBshowed that the rate of C02 interaction with reduced surface is
+
+
+
476 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 Table 1. Conversion of Methane with C02 in Stationary State Conditions on the Catalyst: 5%Ca- 12%Mn-Ol A l 2 0 3 , Space Velocity 900 h-' init composition of conversion, H2 mixture products, % % selectivity, T, "C CH* C02 CO Hz C02 CHI CH4 COz % 870 890 920 930
43 49 47 43
57 51 53 57
49.7 56.4 55.6 56.8
30.6 14.7 37.4 3.5 40.3 3.6 42.4 2.4
5.1 2.3 0.6
83.1 91.8 97.8 100
62.7 89.1 89.3 94.6
76.5 80.1 84.1 84.6
higher than the rate of reduction. Thus, unlike metallic catalysts (Ashcroft et al., 1991, 1993; Rostrup-Nielsen and Hansen, 1993; Erdohelyi et al., 1993), methane activation is the rate-controlling step on MnO-containing catalysts but not the CO2 activation. The reaction does not seem t o proceed by a cyclic redox mechanism. The stationary MnO phase is not oxidized and is not reduced during catalysis, but prevents coking. The most probable scheme of CH4 conversion with C02 is the following one: stepl:
CH4+MnO-MnO***C+2H2
step 2:
CO,
step 3: step 4: step 5:
+ MnO - MnCO, MnCO, + MnO C - 2Mn0 + 2CO MnCO, + H2 - MnO + CO + H20 MnO C + H20- MnO + CO + H2 (6) ***
***
Kinetic equations for the rate of H2 formation follow from scheme 6:
1 l+--
PCOZ
k2k3 P C O ,
+ k&O,
where ~ c Hpco, ~ ,and pcoz are partial pressures of CHI, CO, are C02, respectively, and kl, k2, and k3 are rate constants for the reactions from scheme 6. Expression 7 is conformed by experimental data and is different from the kinetic equations for metallic catalysts (Bodrov and Apelbaum, 1967). The 1.5%K-5.5% Cr-17% Mn-O/SiOn catalyst shows very different properties in methane conversion with C02. CO prevails in the reaction products, in conformity with reaction 8. CH,
+ 3co2 - 4CO + 2H20
(8)
The process on K-Cr-Mn-O/SiOz proceeds probably by a cyclic redox mechanism as distinct from the process on Ca-Mn-O/AlzOs. The oxygen amount of the K-CrMn-O/SiOz catalyst participated in such cyclic process was considerably (8times) less than in a similar process with oxygen as an oxidant. The 1%La-13% Na-17% Mn-O/SiOz catalyst was very selective in Cz-hydrocarbon formation in nonsteady-state regime (Mirzabekova et al., 1993~).Almost 100%CZhydrocarbons were formed after C02 pretreatment and subsequent CH4 introduction. At the same time no CO is formed after C02 introduction. Thus, the process does not proceed by a cyclic redox mechanism
Table 2. Conversion of Ethane with C02 at 800 "C, V = 3600 h-', COCzfIe = 1.5:l (v/v) conversion, % catalyst 8% MnOISiOz 13% MnO/SiOz 17% MnO/SiOz 25% MnO/SiOz 17% MnOlAl203
C2H6
COZ
CzH4 selectivity, %
CzH4 yield, %
65.3 69.3 73.1 75.0 78.4
42.3 46.0 49.0 46.2 50.3
52.5 58.4 61.0 60.2 46.6
34.3 40.5 44.5 45.2 35.5
and the C2 hydrocarbons are formed by CH4 and C2H6 interaction with carbonates at the surface
+ 2CH4 - C2H6+ CO + H20 + 02C0,2- + C2H6 - C2H, + CO + H 2 0 + 02-
C0:-
c0:-
F't
CO,
+ 02-
(9)
The reaction on La-Na-Mn-O/SiOz was studied at 800 "C, that is, at a higher temperature than the temperature of MnC03 decomposition. However, the presence of an alkali metal, sodium, allows the support of a definite concentration of carbonate during the reaction. The selectivity of another catalyst studied by us, LaCu-O/Si02, in the transformation into CZhydrocarbons was lower. In the non-steady-state-regime CO is formed upon COZinteraction with the catalyst and C2 hydrocarbons are formed after subsequent methane interaction with the catalyst. In this case the cyclic redox mechanism is more probable. Synthesis gas, but not ethane and ethylene, is formed on both catalysts in the stationary conditions. Ethane Conversion. During ethane conversion with C02 a process without coking also takes on manganese-containing catalysts (Mamedov, 1991; Mamedov et al., 1990a,b; 1992; Mirzabekova et al., 1992b,c). Oxidative dehydrogenation of ethane, reaction 5, proceeds here. The side reactions are C2H6
+ c02 - 2 c 0 + 3H2 + c
Table 2 presents data on ethane conversion on the catalysts with different manganese concentration and on some more complex systems. It is seen that the C2H4 selectivity increases with increase of the Mn concentration. 17%MnO/SiO2 was the most effective catalyst among the studied supported manganese oxide systems. A similar catalyst supported on A 1 2 0 3 displayed much less selectivity. On the nickel catalyst the reaction proceeds nonselectively in accordance with reactions 10 with the formation of CO, H2, and coke. The stable activity of manganese systems and the absence of coke formation in the presence of carbon dioxide served as a basis for the choice of manganese oxide as the main component of the catalyst for selective conversion with carbon dioxide. Since ethane conversion to ethylene is a dehydrogenation reaction, we studied the effect as modifying additions to manganese oxides of a series of oxides which are known as dehydrogenation catalysts. The
Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995 477 following systems were treated: MnO-FezOa, MnOZnO, MnO-SbzO,, MnO-Vz05, and MnO-Cr203. The most active and selective catalyst for CzH4 production was found t o be 5.5% Cr-17% Mn-O/SiOz. Further increase of selectivity is observed after additional modification of this system by K. For example, on introduction of 1.5% potassium into the 5.5% Cr-17% Mn-O/SiOz catalyst leads t o selectivity increase from 65.4% up to 76.8% at 830 "C. A study of ethane and COZconversion when they were taken in different proportions showed that CZH4 selectivity at first increased with increase of the COdCzH6 ratio and then reached a constant value at COdCzH6 > 2.5. This effect is probably due to the increase of the oxidation state of the surface, leading to a decrease of the deep cracking of ethane. Such a feature in selectivity change is significantly different from the specific features of the ethane oxidative dehydrogenation in the presence of 0 2 when olefin selectivity decreases with increase of poz. Reaction 5 can also be carried out in a cycling redox regime, where CZH6 is transformed into C2H4 upon catalyst reduction and COZis transformed into CO upon catalyst reoxidation. Taking into account that dehydrogenation and deep transformation of ethane proceed on different sites (Mn and Cr sites, respectively), the scheme of ethane conversions can be presented in the following form (Mamedov et al., 1990b, 1992): C2Hs
7 ;"
The rate of the catalyst reduction by ethane (0.6 x 10-3 mol/(gh) is close to that of the catalyst reoxidation mol/(gh). by COZ(0.7 x A study of phase composition of the K-Cr-MnO/SiOz catalyst in situ (Mamedov et al., 1990b) showed that the observed initial MnsO4 phase was reduced with the formation of a new phase, MnO. This study in reaction conditions was performed in an X-ray chambercatalytic reactor specially constructed by us (Shiryaev et al., 1984). This reactor is less than 1cm3by volume, has a beryllium window, and allows performance measurements at temperatures up to 700 "C. The dispersity of the manganese-containing phase increased during the reaction. The mechanism of ethane dehydrogenation by COZ on the Mn-Cr-O/SiOz catalyst can be presented by the following scheme: step 1: step 2: step 3:
step 4: step 5:
-C2H4
2
-
(11)
CO,products of cracking
step 6:
+ Cr3+02-- CzH5+ Cr2+OHC,H, + Cr3+02-- CzH4 + Cr2+0HC2H6 + MnCO, C2H4 + MnO + CO + H,O 2 Cr2+0H-- Cr2+02-Cr2++ H,O Cr2+02-Cr2++ MnCO, 2Cr3'02- + MnO + CO MnO + CO, - MnCO, (13) CzH6
Thermal dehydrogenation can also be observed
The direction of ethane transformation is determined by the ratio of oxidized and reduced sites (ZO/Z); its increase leads to increase of Cz& selectivity. As ethane conversion and CO accumulation in the reaction mixture increase, the surface of the catalyst is reduced by CO and the reaction switches from route I t o route 11. In the absence of chromium, both oxidized (ZO) and reduced (Z) sites can be attributed to manganese oxides: for example, ZO = Mn3+ in Mnz03 or Mn304 and Z = Mn2+ in MnO or MnC03. However, in the mixed Cr-Mn-0 catalyst chromium oxides are the main phases participating in the redox process: ZO = C 9 + and Z = Cr2+. The main role of manganese oxides is the formation of intermediate MnC03 which decomposes upon redox interaction with the hydrocarbon. Potassium probably increases the stability of this intermediate carbonate. The rate of ethane dehydrogenation by carbon dioxide is described by the equation
step 7:
+
+
-
CzH4
+ H,
and side reactions, decreasing the selectivity step 8:
step 9:
-
CzH6 + Cr2+ Cr2+ Cr2+
CH,
CH,
+ Cr3+02Cr2+
+ [(6- x)/21Hz + CO + (x/2)Hz
Scheme 13 explains the higher yields of CO as compared to the reaction with CH4, where the following CO/H:! ratios are obtained: 5-8 at 830 "C and 1.5-2 at 700 "C. This means that the role of thermal dehydrogenation, step 7 in scheme 13, is relatively small. Activation of the manganese system by chromium and the distinctly observed synergism are probably due t o the fact that manganese oxide (MnO)is a mobile phase and takes part in reoxidation in the oxygen transfer from COZ: MnO.
where k,, and kred are rate constants of catalyst oxidation and reduction. Equation 12 explains the observed decrease of selectivity with conversion increase: an inhibition by COYa product of reaction 5, takes place. This redox equation differs from the corresponding redox equation for alkane oxidation by oxygen, where the denominator is represented by the sum k o g o z k r e ~ c ,(Krylov, ~, 1993). In the case of the C2Hs COz reaction no inhibiting action of CZH6 was observed.
C&t6
+co Cr2+O2-CrZ+O2-2
.
-
MnCO, * Cr2+O2-Cr2+OZMnO CO 02-Cr3+02-Cr3+02-(14)
+
+
The redox mechanism of manganese carbonate decomposition seems to play an important role here. Ethane and C02 conversion on the Cr-O/SiOa catalyst not containing manganese is very small. Temperature programmed desorption (TPD) data for COz desorbed from the Mn-O/SiOz catalyst pretreated
478 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995
by CO2 testifies in favor of scheme 14. Desorption maxima at 335,425, and 750 "C which can be explained by inhomogeneity of acceptor sites were observed. Activation energies for CO2 thermodesorption are close to the values observed for oxygen during 0 2 thermodesorption from the same catalyst. This can point to a similarity of sites for C02 and 0 2 activation and to the existence of general principles of reactions on Mn catalysts in oxidative transformations with 0 2 and with c02. Only one high-temperature peak (at 750 "C) is observed in the TPD spectrum of the Cr-Mn-O/SiOz catalyst after C 0 2 adsorption. Such a change afier transition from Cr-0 to Cr-Mn-0 is probably due to Cr2+ reoxidation by the intermediate Mn-containing phase which accepts C02. The observed (Mirzabekova et al., 1992b,c)decrease of the amount of adsorbed C02 after prereduction by H2 shows that C02 adsorption sites are regular lattice surface 02-ions of a basic nature. The sites adsorbing C02 at high temperature (strong sites) are probably less reactive in Cr2+reoxidation. Thus, the site basicity determines the stability of carbonate structure and, as consequence, the degree of redox site reoxidation. Basic sites of moderate basicity are necessary for C02 acceptance and Cr2+reoxidation. Such sites are located at the surface of moderately basic MnO. The interphase boundary between MnO and CrO makes the reoxidation of CrO easier. The oxygen capacity of the Cr-0 system without Mn after reoxidation by C 0 2 was found to be essentially less as compared to Cr-Mn-0. The Cr-Mn-0 composition can be considered as bifunctional: on the one hand, it provides C02 activation on basic sites; on the other it performs hydrocarbon transformation with redox site participation. The sites adsorbing oxygen and creating nucleophilic oxygen ions are sites of C 0 2 transformation into an activated state, with the formation of mobile oxygen particles. They can be transferred (through the interphase boundary) from manganese carbonate to redox sites of chromium by a spillover mechanism. Propane Conversion. Three reactions proceed during propane conversion with COz, each of them to a different extent on different catalysis: propane dehydrogenation by C02 to propylene (151, selective decomposition with C2H4 formation (16), and deep conversion into CO and H2 (17).
+ co + H2O 2C3H8+ 2C02 - 3C2H4+ 2CO + 2H20 C3H8 + 3co2 6CO + 4H2 C3H8 -k
(202
+
C3H6
+
(15) (16) (17)
Manganese oxide catalysts were in this case also the most active and stable ones. The COSconversion in the equimolar C3H8 C 0 2 mixture on the nickel catalyst was at 750 "C as high as 80-85%. However, propane transforms into CO and H2 and the process is characterized by high coking. The best catalyst for selective propane conversion with C02 is also 1.5% K-5.5% Cr-17% Mn-O/SiOs. This catalyst is also effective for ethane transformation. The yield of C2-C3 olefins at 830 "C 3600 h-l and COz/ C3H8 = 1.5 is 73% at 96% propane conversion and the ratio CO/(C2H4 + C3H6) = 1.4. At 890 "C the ratio CO/ (C2H4 + C3H6) increases up to 4. The amount of hydrogen in the reaction products is 5-8 times less than that of CO. With increase of conversion of considerable
+
olefin selectivity decrease is observed. Such a characteristic is explained by a higher degree of hydrocarbon Conversion and by obtaining a more reduced mixture than in the case of ethane (Mamedov, 1991; Mamedov et al., 1992). The general trends of propane conversion with carbon dioxide are similar to those observed for ethane transformation; see scheme 13. This is probably due to the similarity of the primary activation of the C-H bond in ethane and propane transformation on the partially oxidized Cr-Mn systems. After catalyst reduction by propane the rate of CZC3 olefins formation decreases. The rate of C2H4 formation on the catalyst preoxidized by C02 is 2-3 times higher than on the reduced surface. However, preoxidation of the catalyst by air results mainly in the formation of total oxidation products-CO and ( 2 0 2 . This is due probably to the formation of higher chromium oxides (up to Cr6+)afier catalyst oxidation by air instead of Cr203 formation after C02 treatment. Mn-containing phase (MnO, according t o XRD data in situ) takes part in oxygen transfer and helps to maintain chromium in a higher oxidized state (Cr3+). The amount of coke deposits after steady-state reaction (C3H$C02 = 1:1.2) does not exceed 3.6% (with respect to the mass of the catalyst). This coke after C3& treatment exists in two forms according to electronmicroscopic data: filament-shaped carbon and small scales. However, after C3H8 COZ treatment no filament-shaped coke is formed. The reason for this is the absence of metallic inclusions in the catalyst (CrO or MnO) and the quick removal of the metal nuclei of the carbon phase by the Boudouard reaction
+
CO,
+ c * -2 c o
(18)
The rate of selective transformation of C3Hs with CO2 into ethylene is described by the equation
which is very similar to eq 12. The rate of the catalyst reduction by C3H8 is 17 times less than the rate of reduction by CO. Conversion of n-Butane. The interaction of C02 with n-butane also proceeds on manganese-containing oxide catalysts. n-Butane conversion of 8 1 4 4 % and C2-C4 olefin production with the selectivity 76-78% was observed on the Cr-Mn-La-V-O/AlsOs catalysts at 630-650 "C and with n-C4H$C02 = 1-1.2. The oxidative cracking (reactions 20-22) prevails over the
+ CO, = X 2 H 4+ CO + H 2 0 C4Hl0+ CO, = C3H6+ 2CO + 2H20 C4H1o + C02 = C2H4 + CH4 + 2CO + H2 C4H,,
(20) (21) (22)
oxidative dehydrogenation
+
+
C4HIo C 0 2 = C4H8 CO and thermal dehydrogenation
+ H,O
(23)
Ind. Eng. Chem. Res., Vol. 34, No. 2,1995 479 The selectivity into synthesis gas was 22-24% and CO and H2 are obtained in equal amounts, which differs from the C02 reactions with ethane and propane. The direct cracking C4H10
- C3H6
CH4
(25)
seems to constitute a considerable part of the process. The high content of C02 in the products confirms this conclusion. However, it is not very clear why such a process in the absence of C02 takes place in to a very small degree. Conversion of Isobutane. In the absence of C02 the degree of C4Hlo dehydrogenations into C4H8 is 19% at 665 "C, but C02 is not involved in the reaction. Manganese oxide containing catalysts are active for oxidative dehydrogenation by C 0 2 (23) (Mirzabekova et al., 1993a-d). On the K-Cr-Mn-O/SiOz catalyst 3035% conversion of i-C4H10 and 5 8 % CO2 conversion were observed a t 670 "C; on the K-Cr-Mn-O/Al203 catalyst i-C4H8 conversion was 50% and CO2 conversion was 9-10%. The most effective catalyst was found to be Cr-Mn-O/AlzOa without potassium: &C4H10conversion was 61-66%, C02 conversion was 20-29.5%, and i-C4H8 selectivity was 78-81%. CO and Ha were formed in equal amounts. This is different from the ethane and propane conversion where the COM2 ratio was 7-8. Pretreatment of the catalyst by hydrogen in reaction conditions in contrast to ethane and propane conversion does not lead to a decrease of the isobutylene formation selectivity. The catalyst pretreatment by CO leads to characteristics of isobutane conversion which are different from those observed for ethane and propane: the conversion of isobutane and the selectivity here decrease. However, during the reaction an increase of the catalyst activity and selectivity takes place. The interaction of C02 with isobutane was studied also in the regime of separate interaction with the catalyst. The interaction of isobutane with the surface at 670 "C in the absence of C02 gives i-c4H8, C3H6, CH4, CO, and H2. The rate of CO formation upon i-C4H10 interaction with the surface is substantially (12-30 times) less than the rate of CO formation in the steadystate conditions. Only 3-4% is formed in the steadystate conditions as a result of regeneration of the coked catalyst by reaction 18. A study of the H2 C02 reaction on the same catalyst showed that the rate of CO formation is close to the rate of CO formation in steady-state conditions of the C4H10 C02 reaction. The rate of CO formation during CO2 interaction with the reduced Cr-Mn-O/SiOz catalyst
+
+
KO,]
+ [ 1 - co + Oads
(26)
is also considerably less (10 times) than the rate of CO formation in the steady-state conditions. Thus the main route of the CO formation is the reverse water gas shift (28) while the interaction of isobutane with C02 proceeds mainly by two consecutive reactions
H2
+ C 0 2- H 2 0 + CO
(27)
(28)
The hydrogen, formed upon isobutane dehydrogenation, is not oxidized completely: only about half of the hydrogen participates in the subsequent CO2 reduction in accordance with the equilibrium of reaction 28 at 660
"C. This results in the formation of a reaction mixture containing i-C4H8, CO, and H2 as the main products. The side product here is propylene; the selectivity of its formation is about 10%. The kinetic equation of the process has the form (29), similar to the Mars-Van Krevelen equation. It differs
from eqs 12 and 19 for ethane and propane reactions with CO2. Thus the catalyst reduction by CO formed during the reaction is less significant than the catalyst reduction by isobutane. A change of kinetics upon transition from C2H6 and C3H8 to i-CQH10 is probably due to the lesser energy of the C-H bond in isobutane; this allows splitting of this bond with participation of the less oxidized or carbonate sites. The parallel formation of isobutylene and synthesis gas (CO H2) upon isobutane interaction with C02 allows proposal of a new process for the production of an important high-octane gasoline fuel, MTBE (methyl tert-butyl ether) by isobutane conversion with COS.
+
i-C4HIO + CO2
4 7)C2H*
CO
+
MTBE
H2
(30)
CH30H
Reaction I is endothermic (AH = 37 kcal/mol), and reaction I1 is exothermic (AH = -22 kcaymol). A combination of these reactions will allow a decrease of the energetic expenditure for reaction I by a factor of 2. Conversion of n-Heptane. Heavy alkanes are subjected mainly t o oxidative cracking upon interaction with C02 over manganese oxide catalysts (Mirzabekova et al., 1993d). The process can be directed toward the formation of light olefins, light alkanes, and isoalkanes without accumulation of coke at the surface. The reaction n-CTHls C02 proceeds on the catalysts K-Mn-O/A1203, La-Mn-O/AlzOs, and La-Mn-O/ NaY a t 540-670 "C. Oxidative cracking with the formation of H2, CO, CH4, C2H4, C3H6, and C4Hs is observed. On the La-Mn-O/NaY catalyst 100% nheptane conversion into the cracking products is observed at 670 "C with small C02 conversion (4.2%)and with low H2 yield and COM2 = 10. Over La-Mn-O/ A1203 and K-Mn-O/Al203 CO and H2 are formed in equal amounts; n-heptane conversion on La-Mn-O/ A1203 was 86.4% a t 670 "C and C02 conversion was 30.2%. Thus, La-Mn-O/AlzOa and K-Mn-O/Al203 were more active in CO2 activation. The rate of CO formation in the reaction Ha C02 was somewhat lower than the rate of CO formation in the reaction C7H16 C02. The rate of CO formation in the steady-state conditions at 700 "C was 14 times higher than in conditions of the catalyst reoxidation. This means that the oxidation of the catalyst
+
+
+
+ C 0 2t MnCO, 3MnC03 t Mn304+ CO + C 0 2 MnO
(31)
is not the general route of CO formation in the steadystate conditions. The process of CO formation takes place by interaction of H2 with surface carbonates:
480 Ind. Eng. Chem. Res., Vol. 34, No. 2, 1995
H,
+ MnCO, - CO + H,O + MnO CO + H,O * CO, + H,
were present in the products. Equations for their formation can be presented in the form
Another source of CO formation is reaction 18, i.e., the interaction of COZwith carbon-rich deposits. This provides selective proceeding of the oxidative (carbon dioxide) cracking with a stable activity. Thus, the oxidative cracking of n-heptane upon interaction with COZ can be described by the general equations
H,
+ CO, t H,O + CO c + co, * 2 c o
+ CO, - 2CO + (x/2)Hz CH, - CH4 + C* + - 2)H,
(32)
CH,
(X
(34)
Carbonate fragments at the surface take part also in the reaction. At low temperatures (400-600 "C) oxidation of CH, by COZcorresponds to the scheme
CH,
+ ~ 0 , ' -- CH,O + co + 02CH,O - CO + (x/2)H,
(35)
At high temperature (600-750 "C) CO is formed mainly from carbon:
co,2- + c * (33)
The mechanism of C4-C7 alkane transformation seems to be different from the case of C1-C3 alkanes. The general route in the case of CI-C~alkanes involves dissociation of hydrocarbons and subsequent oxidation of the carbon fragments; oxidative dehydrogenation of ethane and propane proceeds also partially. The catalysts are in a more reduced state, and activation of the hydrocarbon is the rate-controlling step. In the case of C4-C7 alkanes the first step of the process is direct dehydrogenation of alkanes. Activation of COz, but not the activation of hydrocarbon, is the ratecontrolling step. The hydrogen formed interacts with COZand shifts the equilibrium of the dehydrogenation reaction. Coke Gasification. Manganese was also shown to play a big role in the interaction of C02 with CH,, i.e., during the gasification of surface carbon fragments and regeneration of the catalyst (Mirzabekova et al., 1992a, 1993a). Interaction of COz with zeolite-containing catalyst KNCU-B, coked in the cracking of vacuum gas oil, showed COZconversion not more than 1%at 680 "C and COZspace velocity 900 h-l. The rate of CO formation was not high and did not change during 100 min. A considerable increase of the rate of COZinteraction with carbon fragments was observed on a catalyst modified by manganese acetate (5% Mn) after coking. The average rate of regeneration of the catalyst modified with Mn was 15 times higher than that of the nonmodified catalyst. When surface carbon reacts with COz, the formation of CO is accompanied by the formation of methane with a rate that is 1.3 times higher. An increase of the rate of CO formation is also observed when the coked catalyst is modified by chromium. However, the relationship between the rates of CHI and CO formation is substantially different on the catalysts modified with Mn and with Cr. A high CO/ CHI ratio equal t o 4-5 is characteristic for the Crcontaining catalyst, but on the catalyst modified by Mn the rates of CH4 and CO formation are the same. Modifying additives can be introduced into the catalyst in the process of its preparation before its use in cracking and regeneration. Regeneration of zeolitecontaining catalyst upon interaction with COS starts from 400 "C after introduction of 5% La203 into the catalyst. A study of the coked catalyst temperature programmed heating in COz showed that CO, CH4, and Hz
-
2co
+ 0,-
(36)
With x = 2 the stoichiometric equation will be 2CH,
+ CO,
CH4
+ 2CO
(37)
That is, both the methanation and the Boudouard reactions proceed upon oxidation of CH, fragments by
coz.
Thus the effective uptake of COZby manganese or lanthanum oxides quarantees a high rate of CH, fragment oxidation on the cracking catalyst in their presence. Ethylene Conversion. In the absence of a catalyst ethylene does not react with COZ. At 750-820 "C some conversion of C2H4 (up to 12%) was observed with the formation of CH4, C3H6, C4H6, and traces of C4H8. On nickel catalysts mainly synthesis gas is formed C2H4
+ 2C0, = 4CO + 2H2
(38)
Use of manganese and chromium oxide containing catalysts (Mirzabekova et al., 1994a) shows that ethylene is transformed to butadiene and propylene; CO concentration is lower. At 850 "C the 17% MnO/SiOz catalyst exhibited the following properties: CZH4 conversion of 22.3%, c02 conversion of 11.6%, C4H6 selectivity of 25.0%, and C3H6 selectivity of 18.0%. Although the free energy change for the reaction 2C,H4
+ (20, = C4H6 + co + H,
(39)
is positive, nevertheless CH4, C3H8, and C4H8 are also formed. It seems that more complex thermodynamic relations for the total process description should be found. Treatment of the MnO/SiOz catalyst by COZleads to a decrease of C4H6 and C3H6 selectivities, and treatment by Hz causes an increase in selectivity. This trend differs from the corresponding trends for alkanes. The process takes place on the catalyst in the reduced state. Mirzabekova (1993a) proposed that the interaction of C2H4 with the reduced catalyst leads to its metathesis 2C,H4
+ 22 - 2C,H4Z
+
C,H,'
+ C2H,' + 22
whereas the further transformation of the free radicals gives coupling products: C4H8, C4H6, and C3H6. Addition of the metathesis catalysts WO3 and Cr2O3 t o manganese oxides increases their activity and selectivity. The 2% WO-5.5 % CrO/SiOz catalyst showed the highest conversion and C4H6 selectivity and did not coke during catalysis. This showed at 820 "C C2H4 conversion of 30.5%, COZ conversion of 13.7%, C4Hs
Table 3. Methanol Transformation with COz on Catalysts of Different Compositions (z = 1 s, CHsOHC02 = 1:l)
catalyst (supportedonSiO2) 5.5% Cr-17% Mn-0 5.5% Fe-17% Mn-0 10% Mo-17% Mn-0 10%Mo-5.5% Cr17% Mn-0
CHzO conversion' % selectivity, yield, T,"C CH30H COz % % 580 3.8 2.1 60 2.3 650 11.1 6.0 50 5.6 700 16.0 8.7 42 6.7 580 8.5 3.9 53 4.5 650 26.4 12.6 42 11.1 700 32.0 15.4 40 12.8 580 6.5 3.2 95 6.2 650 87 21.9 25.2 12.0 580 10.4 5.1 95 9.9 650 700
30.2 36.8
14.6 16.2
91 89
27.5 32.7
selectivity of 50.2%, and C3H6 selectivity of 30.6%;C4H8, C2H6, CO, and CH4 were also formed. The rate of C02 transformation was 2-3 times less than the rate of C2H4 conversion. A part of COZis spent in the reaction with coke. A study of the phase composition of Cr-W-O/SiOz during the reaction showed that Wos disappeared and most of Cr2.03 was reduced. The kinetic equation for butadiene accumulation in the reaction is described by
where k4 is the rate constant of the homogeneous pyrolysis of ethylene. The second term here is 5-6 times less than the first one. Equation 40 is very different from eq 7, 12, 19, and 29 for alkanes. This difference is possibly due to the Lewis acidity of the W-Cr-0 catalysts, which results in less accepting ability to accept C02. On the K-Cr-MnO/SiOz catalyst, probably on its oxidized form, the reaction of ethylene with COZat 750800 "C results in the formation of CO and H2 as the main products and no butadiene is formed. The possibility of ethylene transformation into butadiene allows putting forward a new scheme of its production from natural gas without the use of crude petroleum:
CH4
- C2H4
C4H6
Methanol Conversion. The operation of manganese oxide catalysts by the redox mechanism was also shown in the oxidation of organic compounds of other classes, in particular of alcohols (Mirzabekova et al., 1993b, 199413). CH30H
+ @02 - CH,O + CO + H 2 0
(41)
Table 3 presents data on methanol dehydrogenation on catalysts of different compositions. In the steady-state conditions in addition t o reaction 41 reactions 42-44 also proceed. CH30H
-
+ 2C0,
CH30H
-
CH30H
3CO
CO
(42)
+ H2
(43)
+ 3H2
(44)
CH20
-
+ 2H20
Conclusion We have shown here the importance of adsorption and activation of COZand the conjugation of its reduction with conversion of c 1 - C ~ hydrocarbons and alcohols. Mn-containing catalysts are the most effective ones in these reactions. There are yet many unclear problems in the mechanism and kinetics of these reactions. However, we can indicate some general conclusions concerning the choice of the catalysts. The indispensable condition is the selection of a system which adsorbs and activates COZ. The acidic properties of C02 necessitate the choice of a catalyst
482 Ind.Eng. Chem. Res., Vol. 34,No. 2, 1995
with basic properties. However, alkali metal and alkaline earth oxides are ineffective because of strong carbonate formation. Oxides of a moderate basicity are necessary, and moreover, they must participate in redox process with C 0 2 reduction. Manganese oxides satisfy this requirement. Redox decomposition of manganese carbonate proceeds by the equation
3MnC0,
- Mn,O, +
2C02
+ CO
Modification of the Mn-containing catalyst by other oxides (for example, by K, Na, Cr, and La) influences both its acceptor function and the degree of surface oxidation. It controls the mechanisms of hydrocarbons or alcohol transformation. Rare earth oxides have properties similar to those of manganese oxides: lanthanum, praseodymium, and cerium oxides can be reduced and oxidized; they can also form carbonates of moderate stability. We have shown that catalysts containing lanthanum can form a catalyst for C02 interaction with hydrocarbons and alcohols. Bernal et al. (1990) showed that La203 is a catalyst of ethane oxidative dehydrogenation by C02.
Literature Cited Aika, K. I. Simultaneous Process of Oxidative Coupling of Methane and Reverse Shift of COz. Proceedings. Ninth International Congress on Catalysis, Calgary, Canada, 1988; The Chemical Institute of Canada: Ottawa, 1988; 2, 907-914. Aresta, M., Forti, G., Eds. Carbon Dioxide a Source of Carbon. Biochemical and Chemical Use;NATO AS1 Series, Series C; D. Reidel Publ. Co.: Dordrecht, Holland, 1986; 421 pp. Ashcroft, A. T.; Cheetham, A. K.; Green, M. L. H.; Vernon, F. D. F. Partial Oxidation of Methane to Synthesis Gas Using Carbon Dioxide. Nature 1991,352, 225-226. Ashcroft, A. T.; Cheetham, A. K.; Jones, R. H.; Natarajan, S.; Thomas, J. M. An in situ Energy Dispersive X-ray Diffraction Study of Natural Gas Conversion by C02 Reforming. J . Phys. Chem. 1993,97,3355-3358. Bernal, S.; Martin, G. A.; Morl, P.; Perrichon, V. Oxidative Dehydrogenation of Ethane over Lanthana. Actual Nature of the Active Phase. Catal. Lett. 1990, 6, 231-238. Bodrov, I. M.; Apelbaum, L. 0. Kinetics of Methane Interaction with Carbon Dioxide at Nickel Surface. Kinet. Katal. 1967,8, 379-385. Braunstein, P.; Matt, D.; Nobel, D. Reaction of Carbon Dioxide with Carbon-Carbon Bond Formation Catalyzed by TransitionMetal Complexes. Chem. Rev. 1988, 88, 747-764. Breslow, G. S.; Hitchcock, P. B.; Lapfert, M. F. A Novel Carbon Dioxide Complex: Synthesis and Crystal Structure of Nb (qCsH4Me)z (CH2SiMe3)(q2-CO).J . Chem. SOC.,Chem. Commun. 1981,21, 1145-1146. Erdohelyi, A,; Czerenyi, J.; Solymosi, F. Activation of CHI and its Reaction with C02 over Supported Rh Catalyst. J . Catal. 1993, 141, 287-299. Fachinetti, G.; Floriani, C.; Zanazzi, P. F. Bifunctional Activation of Carbon Dioxide Synthesis and Structure of a Reversible COz Carrier. J . Am. Chem. SOC.1978,100, 7405-7407. Keim, W., Ed. Catalysis in C1 Chemistry; D. Reidel Publ. Co.: Dordrecht, Holland, 1983. Krylov, 0. V. Catalytic Reactions of Partial Methane Oxidation. Catal. Today 1993,18, 209-302. Krylov, 0. V.; Mamedov, A. Kh.; Mirzabekova, S. R. Catalytic Reduction of Carbon Dioxide by Hydrocarbons and Other Organic Compounds. Proceedings. Second World Congress “New Developments in Selective Oxidation”, Benalmadena, Spain; 1993; preprint 42. Mamedov, A. Kh. Oxidative Transformation of C1-Cs-alkaneswith Oxygen and Carbon Dioxide on Manganese Oxides Modified Catalysts. D.Sci. Dissertation, Institute of Petrochemical Processes, Azerbaijan Academy of Sciences, Baku, AzerbAjan, 1991. Mamedov, A. Kh.; Mirzabekova, S. R.; Nuriev, Sh. A.; Shiryaev, P. A.; Shashkin, D. P. Methane Conversion and its Interaction with Carbon Dioxide over Manganese Catalysts. Neftekhimia 1990a, 30, 30-34.
Mamedov, A. Kh.; Shiryaev, P. A.; Shashkin, D. P.; Krylov, 0. V. Selective Dehydrogenation of Ethane by Carbon Dioxide over Fe-Mn-oxide Catalyst. An in situ Study of Catalyst Phase Composition and Structure. Proceedings. First International Congress “New Developments in Selective Oxidation”, Rimini, Italy; 1989; Elsevier: Amsterdam, 1990b; pp 477-482. Mamedov, A. Kh.; Mirzabekova, S. R.; Aliev, V. S.; Krylov, 0. V. Some Peculiarities of Kinetics of C1C3 Alkanes Conversion with Carbon Dioxide on Manganese-containing Catalysts. Kinet. Katal. 1992, 33, 591-596. Mirzabekova, S. R.; Farkhadova, G. T.; Mamedov, A. Kh.; Rustamov, M. I. Some Peculiarities in Regeneration of Zeolitecontaining Catalysts by Carbon Dioxide. React. Kinet. Catal. Lett. 1992a, 48, 225-231. Mirzabekova, S. R.; Mamedov, A. Kh.; Aliev, V. S.; Krylov, 0. V. Some Peculiarities of Kinetics of C1-C3 Alkanes Conversion with Carbon Dioxide on Manganese Containing Catalysts. Kinet. Katal. 1992b, 33, 591-596. Mirzabekova, S. R.; Mamedov, A. Kh.; Aliev, V. S.; Krylov, 0. V. Conversion of Cl-Cz Alkanes over Manganese Catalysts; Reoxidized by Carbon Dioxide and Oxygen. React. Kinet. Catal. Lett. 1 9 9 2 ~47, , 159-166. Mirzabekova, S. R.; Mamedov, A. Kh.; Krylov, 0. V. About a Possibility of Isobutane Dehydrogenation by Carbon Dioxide over Manganese-containing Catalysts. Izv. RAN, Ser. Khim. 1992d, 5, 1230-1232. Mirzabekova, S. R.; Shachtachtinskaya, A. T.; Mamedov, A. Kh.; Aliev, V. S. Thermodesorption of CO and 0 2 from Manganesecontaining Catalysts. Kinet. Katal. 1992e, 33, 472-475. Mirzabekova, S. R.; Farkhadova, G. T.; Mamedov, A. Kh.; Rustamov, M. I. Regularities in Coke Oxidation by Carbon Dioxide at the Surface of Zeolite-containing Cracking Catalyst. Kinet. Katal. 1993a, 34, 936-938. Mirzabekova, S. R.; Mamedov, A. Kh.; Aliev, V. S. Kinetics and Mechanism of Oxidative Dehydrogenation of Methanol into Formaldehyde by Carbon Dioxide over Cr-Mo-MdSiOz Catalyst,. Kinet. Katal. 1993b, 34, 1043-1048. Mirzabekova, S. R.; Mamedov, A. Kh.; Aliev, V. S. Regularities of Modified Manganese and Magnesium Oxides Reoxidation by Carbon Dioxide. Neffekhimia 1993c, 33,30-34. Mirzabekova, S. R.; Mamedov, A. Kh.; Krylov, 0. V. Catalytic Reaction of Heptane with Carbon Dioxide as a Model of Carbon Dioxide Cracking of Hydrocarbons. Kinet. Katal. 1993d, 34, 184. Mirzabekova, S. R.; Mamedov, A. Kh.; Aliev, V. S. Regularities of Ethylene Conversion by Carbon Dioxide into C3-C4 Unsaturated Hydrocarbons over Manganese and Chromium Oxides. React. Kinet. Catal. Lett. 1994a, in press. Mirzabekova, S. R.; Mamedov, A. Kh.; Krylov, 0. V. A New Approach to Lebedev Reaction: Oxidative Transformation of Ethanol with Carbon Dioxide on the CuO-MnO/AlzO3Catalyst. Izv. RAN,Ser. Khim. 1994b, in press. Nishiyama, T.; Aika, K. I. Mechanism of the Oxidative Coupling of Methane Using COz as an Oxidant over PbO-MgO. J . Catal. 1991,122, 346-361. Rostrup-Nielsen, J. R. Production of Synthesis Gas. Catal. Today 1993,18,305-324. Rostrup-Nielsen, J. R.; Hansen, J. H. B. COz-reforming over Transition Metals. J. Catal. 1993, 144, 38-49. Rozovskii, A. Ya.; Lin, G. Ya. Theoretical Bases of Methanol Synthesis (in Russian); Khimia: Moscow, 1990; 266 pp. Shiryaev, P. A.; Shashkin, D. P.; Zurmuhtashvili, M. Sh.; Margolis, L. Ya.; Krylov, 0. V. X-ray Study of Complex Oxide Catalysts during Catalytic Oxidation of Propylene. Kinet. Katal. 1984, 25, 1164-1168. Vinogradova, 0. M.; Keier, N. P.; Roginsky, S. Z. Study of the Mechanism of Lebedev Butadiene Synthesis with Use of 14C. Roginsky, S. Z., Ed.; Problemy Kinetiki i Kataliza; Nauka: MOSCOW, 1957; Vol. 9, pp 175-186.
Received for review April 7, 1994 Revised manuscript received September 14, 1994 Accepted October 13, 1994@ IE940230K Abstract published in Advance A C S Abstracts, January 1, 1995. @
