The Catalytic Oxidative Coupling of Methane

Abstract

One of the great challenges in the field of heterogeneous catalysis is the conversion of methane to more useful chemicals and fuels. A chemical of particular importance is ethene, which can be obtained by the oxidative coupling of methane. In this reaction CH4 is first oxidatively converted into C2H6, and then into C2H4. The fundamental aspects of the problem involve both a heterogeneous component, which includes the activation of CH4 on a metal oxide surface, and a homogeneous gas‐phase component, which includes free‐radical chemistry. Ethane is produced mainly by the coupling of the surface‐generated CH 3• radicals in the gas phase. The yield of C2H4 and C2H6 is limited by secondary reactions of CH 3• radicals with the surface and by the further oxidation of C2H4, both on the catalyst surface and in the gas phase. Currently, the best catalysts provide 20% CH4 conversion with 80% combined C2H4 and C2H6 selectivity in a single pass through the reactor. Less is known about the nature of the active centers than about the reaction mechanism; however, reactive oxygen ions are apparently required for the activation of CH4 on certain catalysts. There is spectroscopic evidence for surface O− or O 22− ions. In addition to the oxidative coupling of CH4, cross‐coupling reactions, such as between methane and toluene to produce styrene, have been investigated. Many of the same catalysts are effective, and the cross‐coupling reaction also appears to involve surface‐generated radicals. Although a technological process has not been developed, extensive research has resulted in a reasonable understanding of the elementary reactions that occur during the oxidative coupling of methane.

Conversion of Methane to Benzene over Transition Metal Ion ZSM-5 Zeolites

“…The catalytic conversion of methane to desired commodity chemicals is a challenging approach for the utilization of natural gas resources (1,2). Several recent studies have demonstrated that methane can be converted to benzene in the absence of an oxidant, such as oxygen, and the main literature results are summarized in Table 1 (3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14)(15).…”

“…However, some active TMI's, in particular Mo, show little or no activity in other characterization studies (3,4). Thus, it seems 1 On leave from the Centrum voor Oppervlaktechemie en Katalyse, Departement Interfasechemie, K. U. Leuven, Kardinaal Mercierlaan 92, 3001 Heverlee, Belgium. 2 To whom correspondence should be addressed.…”

Fuel-rich methane oxidation in a high-pressure flow reactor studied by optical-fiber laser-induced fluorescence, multi-species sampling profile measurements and detailed kinetic simulations

“…It is enclosed by an electrical furnace (not shown in the drawing), the temperature of which is monitored using a thermocouple. The gas flow is regulated by calibrated mass-flow controllers for CH 4 , O 2 , and Ar/He mixture (8 vol-% He in Ar, serving as internal standard and carrier gas for the analytics).…”

“…Another reason for using Ar as 50th species was that all numerical rounding errors as well as errors from the mixture averaged diffusion treatment were lumped in the Ar mass fraction. The process was repeated for CH 4 /O 2 feed ratios ranging from 4, 8 and 16, temperatures at 700 and 1100 K, and pressures of 1 and 8 bar. An alternative approach is to use the absolute flux through each reaction, rather than its sensitivity coefficient.…”

“…It has been suggested in the literature that the OCM reaction proceeds via a homogeneous/heterogeneous coupled mechanism [3,4]. According to this model, methane is first activated on the catalyst, and the resulting methyl radical desorbs.…”