Oxidative conversion of methane/natural gas into higher hydrocarbons Vasant R. Choudhary  and Balu S. Uphade Chemical Engineering Division, National Chemical Laboratory, Pune – 411 008, India Oxidative coupling of methane (OCM) to ethylene and ethane is a futuristic process of great practical importance for the effective utilization of methane/natural gas. A brief summary of the work carried out at National Chemical Laboratory (Pune) for the development of catalyst and catalytic process for OCM, particularly addressing the major issues/limitations of the OCM process and efforts made to overcome the problems is presented. This review particularly covers the development of a number of highly active/selective and stable unsupported and supported (using commercial catalyst carriers) catalysts for the OCM process and also the process of improvements/modifications to overcome most of the limitations of OCM. KEY WORDS: oxidative coupling of methane; natural gas; ethane; ethylene; C 2þ hydrocarbons. 1. Introduction Methane is the main constituent of natural gas (NG), biogas and coal-bed gas. The reserves of natural gas are comparable to that of oil and more and more natural gas is being discovered than oil. Moreover, there are huge untapped resources of methane, like methane gas hydrate, lying deep on the seabeds. A huge quantity of methane is produced during the oil production as an associated gas. Methane in appreciable quantities is also produced in petroleum refining and petrochemical processes. Because of the predicted decline in oil production in the near future, methane has been considered as a source of energy and organic carbon. However, since natural gas is available/ produced mostly in remote areas, its transportation is uneconomical or even impossible. Because of the high transportation costs, most of the natural gas/methane produced in the remote places is let out and/or flared. However, both methane and carbon dioxide are green- house gases, responsible for global warming. Hence, the presently practiced letting out and/or flaring of the methane produced will not be allowed in the near future. The transportation and environmental problems and the increasing energy cost have led to extensive worldwide efforts since the last two decades for directly converting methane/natural gas into less volatile (easily transporta- ble) value-added products, such as ethylene (a feedstock for petrochemicals), aromatics and liquid hydrocarbon fuels, involving oxidative or nonoxidative activation of methane. A number of reviews are available for both the oxidative [1–14] and nonoxidative [15,16] direct conver- sion of methane to higher hydrocarbons. Since the pioneering work of Keller and Bhasin [17] (which was based on the use of reducible metal oxide catalysts) and Ito and Lunsford [18] (which was based on the use of nonreducible metal oxide catalyst; Li- MgO), the oxidative coupling of methane (OCM) to ethane and ethylene has been widely investigated. In the OCM process (at about 800  C), the following selective and nonselective reactions occur simultaneously. 2CH 4 þ 0:5O 2 ! C 2 H 6 þ H 2 O þ 174:2 kJ=mol ðC 2 H 6 Þ ð1Þ C 2 H 6 þ 0:5O 2 ! C 2 H 4 þ H 2 O þ 103:9 kJ=mol ðC 2 H 4 Þ ð2Þ C 2 H 6 ! C 2 H 4 þ H 2 ÿ 114:6 kJ=mol ðC 2 H 4 Þ ð3Þ CH 4 þ 2O 2 ! CO 2 þ 2H 2 O þ 801:6 kJ=mol ðCO 2 Þ ð4Þ CH 4 þ 1:5O 2 ! CO þ 2H 2 O þ 519:1 kJ=mol ðCOÞ ð5Þ C 2 H 6 ; C 2 H 4 and H 2 ! CO; CO 2 and water þ large amount of heat ð6Þ According to the accepted mechanism, the oxidative methane activation in the catalytic OCM process involves an abstraction of H-atom from methane, leading to the formation of methyl radicals on the catalyst surface; the two desorbed methyl radicals are coupled in the gas phase to form ethane molecule [14,19,20]. In spite of the extensive work done on OCM, the OCM process has not yet been commercialized because of the following important limitations of this process: (1) It is a high-temperature process, generally operated at 750–1000  C, and hence requires a catalyst having high thermal and hydrothermal stability. The evaporation of volatile/low-melting catalytically active catalyst component(s) and/or the chemical  To whom correspondence should be addressed. E-mail: vrc@ems.ncl.res.in or vrc@che.ncl.res.in Catalysis Surveys from Asia, Vol. 8, No. 1, February (# 2004) 15 1384-6574/04/0200–0015/0 # 2004 Plenum Publishing Corporation