J. Phys. Chem. 1987, 91, 2682-2684
Mechanrstk Studies on the Oxldative Coupling of Methane Jay A. Labinger*'. and Kevin C. OttIb Atlantic Richfield Company, Los Angeles, California (Received: January 15, 1987)
Methane reacts with a mixed Mn-Mg oxide above 800 OC to give higher hydrocarbons as well as C02. The selectivity to hydrocarbons decreases with increasing conversion. At low conversion selectivity increases with higher pressure, whereas at high conversion it decreases. Kinetics studies implicate a Rideal-type mechanism wherein methyl radicals are produced at the oxide surface and most of the subsequent hydrocarbon-forming chemistry takes place in the gas phase. A computer model quantitatively accounts for the dependence of selectivity on conversion and pressure.
Introduction The conversion of methane to higher value or more easily transportable substances is a goal of considerable scientific and practical interest. As methane is less reactive than virtually any substance that might be made from it, achievement of significant conversion (Le., more than a few percent) while maintaining high selectivity to desired products will be difficult. Among a number of approaches to this problem that have attracted attention,2 the oxidative coupling of methane to higher hydrocarbons has met with some recent s u c ~ e s s . ~In+ ~particular, reaction of methane with several reducible metal oxidess at temperatures around 800-850 OC can yield hydrocarbons (primarily ethane and ethylene, with lesser amounts of heavier products, the latter consisting mostly of propene, butadiene, cyclopentadiene, benzene, and toluene) with selectivity 75% or higher (selectivity being defined as total moles of carbon appearing in hydrocarbon products divided by moles of methane reacted; nonselective products are CO, C 0 2 , and coke) a t methane conversions at or above 25%.3c Most previously reported systems, however, exhibit substantially lower activity and ~electivity.~ In order to understand and optimize catalyst performance, we must answer questions such as: to what extent are waste products formed directly from methane, as opposed to formation by subsequent oxidation of desired products (which ones)? How does the mechanism of the reaction influence the dependence of performance on parameters such as temperature and pressure, and what (if any) limits does it place upon the best potential performance? We here report kinetic studies on one catalyst system which begin to show in detail how oxidative coupling operates and what limitations are likely.
Experimental Section The catalyst system utilized was prepared elsewhere,3c by impregnating MgO (Dart Industries) with a solution of NaMn04, (1) (a) Address correspondance to this author at California Institute of Technology, Division of Chemistry and Chemical Engineering, 127-72, Pasadena, CA 91 125. (b) Los Alamos National Laboratory, Los Alamos, NM
87545. (2) Pitchai, R.;Klier, K. C a d . Rev.-Sci. Eng. 1986, 28, 13-88. (3) (a) Sofranko, J. A.; Leonard, J. J.; Jones, C. A. J. Cutal. 1987, 103, 302-10. (b) Jones, C. A,; Leonard, J. J.; Sofranko, J. A. Ibid. 31 1-19. (c) Jones, C. A.; Sofranko, J. A. US. Patent 454761 1. (4) (a) Keller, G. E.;Bhasin, M. M. J. Card. 1982, 73,s 1 9 . (b) Hinsen, W.; Baerns, M.Chem.-Zrg. 1983, 107, 223-6. (c) Ito, T.; Lunsford, J. H. Nature (London) 1985, 314, 721-2. (d) Ito, T.; Wang, J.-X.; Lin, C.-H.; Lunsford, J. H. J. Am. Chem. Soc. 1985, 107, 5062-68. (e) Lin, C.-H.; Campbell, K. D.; Wang, J.-X.; Lunsford, J. H. J. Phys. Chem. 1986, 90, 534-7 ( f ) Otsuka, K.; Yokoyama, S.; Morikawa, A. Chem. Lett. 1985, 319-22. (g) Otsuka,K.; Jinna, K.; Morikawa, A. Chem. Lett. 1985,499-500. (h) Otsuka, K.;Liu, Q.;Morikawa, A. J. Chem. Soc., Chem. Commun. 1986, 586-8. (i) Ali Amesh, I. T.; Amenomiya, Y. J. Phys. Chem. 1986, 90, 4785-9. Imai. H.; Tagawa, T. J. Chem. Soc., Chem. Commun. 1986,52-3. (k) Aika, K.-I.; Moriyama, T.; Takasaki, N.; Iwamatsu, E. J . Chem. Sor., Chem. Commun. 1986, 1210-1. (5) Such reactions have been camed out in two different ways: by reacting
u)
methane alone with a reducible oxide in its oxidized form, which is subsequently regenerated by air oxidation; or by coreacting methane and air in the true catalytic mode. Our studies have been restricted to the former, where possible complications arising from rate-limiting reoxidation steps and/or gas-phase reactions of O2 are avoided.
0022-365418712091-2682$01 SO10
drying, and calcining. A catalyst bed containing 1-2 g of material was prepared in a reactor constructed from high-purity '/,-in. alumina tubing and rod. The reactor was designed to minimize dead volume and to avoid any contact of hot gases with metallic surfaces. Reactor temperatures of 750-850 OC were maintained with a tube furnace and monitored at two or three different points in the bed by alumina-sheathed thermocouples; gradients were less than I O C . Reactant gas flows were maintained and measured by Linde mass flow controllers and were varied to give a range of residence times from 50 ms to 2 s. Methane partial pressures were varied from 0.1 to 3.5 atm, by diluting with N2 for subatmospheric pressures and by a backpressure regulator above 1 atm. Heavier reactant gases were all studied at low partial pressures, using commercially supplied mixtures with Ar or with methane. Prior to each kinetic run the catalyst was oxidized with air followed by brief purging with inert gas. Reactant flow was initiated, and product gases were bled to a UTI lOOC quadrupole mass spectrometer interfaced to a Hewlett-Packard Model 21 7 microcomputer. Intensities for the following ions were recorded and stored a t 2-s intervals: m / e 2 (H2), 15 (CH,), 26 (C2H4), 30 (C2H6), 42 (C3H6), 44 (C02),54 (C.tH,J, 78 (Cd-M, and 92 (C,H,). From calibration using analyzed gas mixtures and the known residence time, these data were converted to concentrations and thence to rates by software written for the HP 217 computer. Concentrations were periodically checked by gas chromatography and agreed within experimental uncertainty. The oxidation state of the catalyst was calculated continuously by summing up the oxygen equivalents required to account for the observed gaseous products and subtracting from the initial capacity. Conversion of methane or other reactant and selectivity to the various products were calculated from concentration data and displayed as a function of oxidation state at the end of the run. All comparisons of rates or selectivities between different runs refer to a constant oxidation level, 90%.
Results and Discussion Kinetics data were collected for reaction of methane over the wide range of operating parameters indicated in the Experimental Section, as well as for ethane, ethylene, and propene over smaller ranges. The results can be summarized as follows: The rate of methane conversion exhibits Arrhenius temperature dependence, with E, = 58 kcallmol, and saturation kinetics with regard to methane pressure (Le., a rate law of the form rate = aP/(P b))? All subsequent comparisons refer to data at a single temperature, 825 O C . At very low conversions, C2H6 and C 0 2 are the only significant products. Selectivity to the former increases at higher pressures (Figure 1). With increasing conversion, ethylene and heavier products appear; overall selectivity for hydrocarbons decreases. Moreover, at higher conversions the pressure dependence reverses: selectivity decreases at higher pressure.
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(6) The mechanistic interpretation of this pressure dependence will be discussed elsewhere: Labinger, J. A,; Ott, K. C.; Mehta, S.; Rockstad, H. K.; Zoumalan, S. J. Chem. Soc., Chem. Commun., in press.
0 1987 American Chemical Society
The Journal of Physical Chemistry, Vol. 91, No. 11, 1987 2683
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