Mechanism of the Catalytic Conversion of Methanol to Hydrocarbons

The discovery of the dual aromatic- and olefin-based catalytic cycles in methanol-to-hydrocarbons (MTH) catalysis on acid zeolites has given a new context for rationalizing structure function relationships for this complex chemistry. This perspective examines six major chemistries involved in the hydrocarbon pool mechanism for MTH-olefin methylation, olefin cracking, hydrogen transfer, cyclization, aromatic methylation, and aromatic dealkylation-with a focus on what is known about the rate and mechanism of these chemistries. The current mechanistic understanding of MTH limits structure function relationships to the effect of the zeolite framework on the identity of the hydrocarbon pool and the resulting product selectivity. We emphasize the need for assessing the consequences of zeolite structure in MTH in terms of experimentally measured rates and activation barriers for individual reaction steps and in terms of speciation preferences within the dual olefin-and aromatic-catalytic cycles to alter their relative propagation. In the absence of individual reaction rates, we propose using ethene/isobutane selectivity as a measure to describe the relative rates of propagation for the aromatic- and olefin-based cycles.