Green chemistry perspectives of methane conversion via oxidative methylation of aromatics over zeolite catalysts

Abstract

Methane gas is known to be the most destructive greenhouse gas. The current world reserves of natural gas, which contains mainly methane, are also still underutilized due to high transportation costs. Thus, considerable interest is presently shown in conversion of methane to transportable liquid fuels and chemicals of importance to the petrochemical industry. The catalytic methylation of aromatics is one possible new route for converting methane to more valuable higher hydrocarbons. This paper provides a general overview of the recent work that we and other researchers have done on the utilisation of methane for catalytic methylation of aromatic compounds and for direct coal liquefaction for the production of liquid hydrocarbons. In particular, the paper presents a detailed description of more recent substantial experimental evidence that we have provided for the requirement of oxygen as a stoichiometry reactant for benzene methylation with methane over moderately acidic zeolite catalysts. The reaction, which has been termed “oxidative methylation”, was thus postulated to involve a two-step mechanism involving intermediate methanol formation by methane partial oxidation, followed by benzene methylation with methanol in the second step. However, strongly acidic zeolites can cause cracking of benzene to yield methylated products in the absence of oxygen. The participation of methane and oxygen, and the effective use of zeolite catalysts in this methylation reaction definitely have some positive green chemistry implications. Thus, the results of these previous studies are also discussed in this review in light of the principles and tools of green chemistry. Various metrics were used to evaluate the greenness, cost-effectiveness, and material and energy efficiency of the oxidative methylation reaction.