6558
J. Phys. Chem. 1991,95,6558-6563
Kinetlc Studles of the Catalytic Oxidation of Methane. 1. Methyl Radical Production on 1% Sr/La,O,
Y.Feng, J. Niiranen, and D. Cutman* Department of Chemistry, Catholic University of America, Washington, D.C. 20064 (Received: December 28, 1990; In Final Form: March 6, 1991)
The kinetics of methyl radical production on 1% Sr/Laz03has been studied by using a heatable tubular flow reactor, containing the catalyst, coupled to a photoionization mass spectrometer. The experimental technique, which permits direct detection of stable species and free radicals and measurementsof their absolute concentrations along the reactor,is described. Experiments were conducted using conditions which isolated the initial CH3(g) production step. Absolute rates of CH3(g) production were determined from measurements of CH3(g) concentration profiles along the reactor. Rates were measured as a function of T (982-1 138 K), [CH,(g)], and [Oz(g)]. The information obtained was used to deduce the rate law for methyl radical production and to identify the most likely kinetic mechanism of this process. The body of data obtained and the rate law indicate the establishment and maintenance of an equilibrium between Oz(g) and surface-bound oxygen atoms (endothermic dissociative adsorption). CH3(g) is formed in a rate-determining step between CH4(g) and a surface-bound oxygen atom. The temperature dependence of the constants in the rate law indicates that AHo = 44 f 10 kcal mol-’ for the endothermic adsorption (Oz(g) 2(-O(surface)), and the activation energy for the gassurface reaction responsible for the production of CH3(g) is 21 f 5 kcal mol-] (CH4(g) + -O(surface) CH,(g) + HO(surface)). Heterogeneous loss of CH,(g) did not occur to an observable degree under the conditions of these experiments.
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Introduction The diminishing supply of petroleum and the existence of abundant reserves of natural gas are renewing interest in converting methane to higher hydrocarbons (particularly C2 compounds such as ethylene), compounds which are convenient starting materials for the production of chemicals and synthetic fuels.’+ Heterogeneous oxidative coupling, i.e., the formation of C2 and higher hydrocarbons from methane, involving metal oxide catalysts appears particularly promising for this purpose. Several reviews have appeared on the subject.3d Considerable research is in progress directed at obtaining a basic understanding of the homogeneous/heterogeneous chemical kinetia occumng during the oxidative coupling of methane/natural gas to form higher hydrocarbons. The goal of these studies is to obtain new fundamental knowledge of the mechanisms of these processes in order to guide the development of new technologies which will increase conversion efficiencies to levels that will make the processes of true commercial importance. Currently singlepass hydrocarbon yields (on a C atom basis) appear to approach a limit of 20-25%.’*7*8 Classical studies, ones that involve the determination of endproduct yields (product selectivity) from steady-state flow reactors as a function of reactor and flow conditions, have been extremely useful for screening catalytic materials to determine their activity and selectivity.”’ However, the information obtained, such as the many relationships between the product selectivities and the reactor conditions, is not a sensitive indicator of the chemical kinetic mechanisms of these complex processes.I2 (1) Jones, C. A.; Leonard, J. J.; Sofranko, J. A. Energy Fuels 1987,1, 12. (2) Hoebink, G . B. J. Ethylene from Natural Gas, Proven and New Technologies. Laboratory of Chemical Process Technology, Eindhoven University of Technology, The Netherlands, 1990. (3) (a) Hutchings, G.J.; Scurrell, M. S.;Woodhouse, J. R. Chem. SOC. Reo. 1989, 18,251. (b) Hutchings, G . J.; Woodhouse, J. R.; Scurrell, M. S. J . Chem. Soc., Faraday Trans. I 1989,85, 2507. (4) Lee, J. S.; Oyama. S.T. Caral. Rev. Sci. Eng. 1988, 30, 249. (5) Lunsford, J. H. Caral. Today 1990, 6, 235. (6) Ross, J. A.; Bakker, A. G.; Bosh. H.; van Ammen, J . G.; Ross, J. R. H. Caral. Today 1987, I , 133. (7) McCarty, J. G.; McEwen, A. B.; Quinlan, M. A. Paper presented at the International Gas Research Conference, Tokyo, November 1989. (8) Preuss, V.;Baerns, M. B. Chem. Eng. Techno/. 1987, 10, 297. (9) Keller, G . E.; Bhasin, M. M. J . Carol. 1982, 73, 9. (10) DeBoy, J. M.; Hicks, R. F. Ind. Eng. Chem. Res. 1988, 27, 1577. (1 1) Burch, R.; Squire, G. D.; Tsang, S.C. Appl. Carol. 1988, 43, 105. (12) For example, see the discussion in the sequence of three letters in J . Caral.: (a) Hatano, M.;Hinson, P. G.; Vines, K. S.;Lunsford, J. H. J. Coral. 1990, 124, 557. (b) Yates, D. J. C.; Zlotin, N . E. J . Carol. 1990, 124, 562. (c) Kalenik, Z.; Wolf, E. E. J . Coral. 1990, 124, 566.
0022-36S4/91/2095-6558$02.50/0
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New studies, still based on end-product analyses but involving pulsed or stepped introduction of reactants followed by temporally resolved measurements of both reactants and stable products, are providing a temporal resolution of these catalytic processes that is useful for obtaining a better understanding of the nature and role of the active sites on the catalysts (surface vs lattice) and the roles of these sites in the overall methane coupling and oxidative portions of the proce~ses.’~*’~ In addition, when isotopically labeled reactants are used in pulsed reactant substitution experiments, additional basic information is obtained, particularly on the time scale and the nature of equilibration between gas and surfacebound species.’”I6 Important information on the mechanism of formation of stable end products has also been derived from the use of methane and deuterated methane (alternately or together) in oxidative methane coupling experiments.”J* Today it is widely believed that the gateway to C2products in all methane coupling schemes is the homogeneous recombination of methyl radicals to produce CzH6 (the radicals being produced heterogeneously on the catalyst and released into the gas phase):’s4
Reaction 1 is a symbolic expression for the heterogeneous production of CH3 sustained by the presence of oxygen. The mechanism for this process is the topic of this investigation. (Hereafter, reference to species in the gas phase will not include the phase designation except for emphasis.) There is significant uncertainty regarding the efficiency of this overall scheme, particularly regarding the importance of parallel oxidative processes which bypass or interfere with this essentially direct conversion of CH4 to CzH6.3*4Such competing processes include direct heterogeneous oxidation of methane to oxygencontaining products and the heterogeneous oxidation of CH3. Questions about the fate of C2H6also remain. The disagreements (13) Lane, G. S.;Miro, E.; Wolf, E. E. J . Catal. 1989. 119, 161. (14) Miro, E.; Santamaria, J.; Wolf, E. E. J . Carol. (a) 1990, 124, 451; (b) 1990, 124,465. (15) Peil, K. P.; Goodwin, J . G.;Marcelin, G.J . Phys. Chem. 1989, 93, 5977. (16) Ekstrom, A. E.; Lapszewicz, J. A. J . Cham. SOC.,Chem. Commun. 1988, 797. (17) Nelson, P. F.; Lukey, C. A.; Cant, N . W. J . Phys. Chem. 1988, 92, 6176. (18) Cant, N . W.; Lukey, C. A,; Nelson, P. F.; Tyler, R. J. J . Chem. Soc., Chem. Commun. 1988. 766.
0 1991 American Chemical Society
Catalytic Oxidation of Methane
The Journal of Physical Chemistry, Vol. 95, No. 17, 1991 6559
‘1 Gas Inlets
Figure 1. Drawing of experimental facility showing heatable tubular reactor (containing a catalyst-coated semicylinder) coupled to a photoionization mass spectrometer.
and lack of knowledge result from the fact that the basic gas/ surface kinetic steps involved in the formation and destruction of C2 compounds have never been isolated before for direct investigation and conclusions regarding their behavior and importance are still based largely on indirect evidence. It is clear that major advances in our understanding of methane conversion processes can be derived from experiments in which the actual gas-phase reaction intermediates which are produced in these processes (particularlythe free-radical intermediates, such as CH3) are detected and monitored. Several groups have now detected CH3 at the exit of catalyst beds used for the oxidative coupling of methane. The pioneering studies are those of Lundsford and co-workers who developed and used a complex but effective matrix isolation electron spin resonance technique (MIESR)lSZ1to obtain relative measurements of CH3 concentrations (as well as semiquantitative determinations of absolute concentration^).^^^ These studies provided, among other things, direct proof that gas-phase methyl radicals are produced on the catalysts used and that the recombination of these radicals constitutes a major source of C2H6 in the systems studied. More recently a laser-based diagnostic method (resonance-enhanced multiphoton ionization) has been used to make measurements of relative CH3 concentrations also at the exit of reactor beds.25*26 Assuming that these measurements were of the steady-state CH3 concentrations established by reactions 1 and 2, these observations have provided the apparent activation energy of reaction 1 for different methane coupling catalysts, 1% Sr/La203,25CaO/ A1203,25and Li/Mg0.26 In order to obtain new basic kinetic information on the elementary kinetic processes occurring during methane-coupling processes, we have developed and are now using a new experimental technique for obtaining quantitative measurements of concentrations of reactants, products, stable intermediates, and, most importantly, of free radical intermediates directly along the catalyst bed. The technique involves the use of a catalyst-coated tubular reactor coupled to a photoionization mass spectrometer. In this paper, we describe the experimental technique and discuss (19) Driscoll, D. J.; Campbell, K. D.; Lunsford, J. H. Adu. Curd 1987, 35, 139. (20) Martir, W.; Lunsford, J. H. J . Am. Chem. Soc. 1981, 103, 3728. (21) Driscoll, D. J.; Martir, W.; Wang, J.-X.; Lunsford, J. H. J. Am. Chem. Soc. 1985,107,58. (22) Ito, T.; Wang, J.-X.; Lin, C.-H.; Lunsford, J. H. J . Am. Chem. Soc. 1985, 107, 5062. (23) Zhang, H.-S.; Wang, J.-X.; Driscoll, D. J.; Lunsford, J. H. J . Curul. 1988,112,366. (24) Campbell, K. D.; Zhang, H.-S.;Lunsford, J. H. J. Phys. Chem. 1988, 92, 750. (25) Gulcicek, E. E.; Colson, S. D.; Pfeffcrle, L. D. J. Phys. Chem. 1990, 94, 7069. (26) Lee, S.-P.; Yu, T.; Lin, M. C. Int. J. Chem. Kine?. 1990, 22, 945.
its first use, characterizing reaction 1, the CH3 production step. This step was isolated for quantitative study by using reaction conditions in which CH3 concentrationswere kept extremely low, so low that the consumption of CH3 by recombination, reaction 2, always had a negligible rate. In part 2, the following paper,” additional sets of experiments are described in which different reactant concentrations were employed, ones which increased the rate of CH3 production by up to 2 orders of magnitude. Under these conditions, CH3 concentrations reached much higher values, and CH3 recombination became an integral part of the overall reaction mechanism. The approach of CH3 to its steady-state concentration was observed as was the accompanying production of C2H6 (and trace amounts of C2H4). Parts 1 and 2 provide the first detailed quantitative characterization of the kinetics of conversion of CH4 to C2H6 over a methane-coupling catalyst..
Experimental Section The basic experimental facility, which consists of a heatable tubular flow reactor coupled to a photoionization mass spectrometer, has been described.28 While the apparatus was developed in our laboratory for investigations of the kinetics of homogeneous gas-phase reactions of polyatomic free radicals, relatively minor modifications converted it into a facility which can be used to isolate and study the chemical kinetics of gassurface processes. The modified system is shown in Figure 1. The gas flowing through the 2.20 cm i.d. tubular reactor (at 3.0 m s-l in all these experiments) contained CH4, 02,and N2 (as well as trace amounts of NO) in varying amounts. Either C& or O2was flowed through a 0.6 cm 0.d. movable injector. Temporal resolution is obtained by moving the injector which varies the reaction time, the time between gas mixing and gas sampling. A fixed gas density was used in all these experiments, 6.0 X 10l6 molecules ~ m - ~(The . total pressure was 6.1-7.1 Torr depending on the temperature.) At this low density, and at the temperatures of these experiments, the mean gas diffusion times across the reactor are short, typically
