Enhancing low-temperature methane conversion on Zn/ZSM-5 in the presence of methanol by regulating the methanol-to-aromatics reaction pathway†
The co-reaction method provides an efficient strategy for methane conversion under mild conditions, which is of urgent importance for direct valorization of natural gas to liquid hydrocarbons. However, a vague mechanism and the neglect of methanol conversion make methane conversion difficult to achieve. In this work, the methane conversion pathway and catalysis mechanism are investigated by temperature programmed surface reaction-mass spectrometry (TPSR-MS) and reaction pathway regulation, and methane conversion can be achieved at the mild conditions of 300 °C and 1 atm by co-reaction with methanol using the Zn/ZSM-5 catalyst. It is observed that methane participates in an auto-catalysis process in which the resultant/intermediates, formed from the “aromatics-based cycle” of the methanol-to-aromatic (MTA) reaction, perform a critical function in place of methanol in the methane conversion. In an opposite way, C4–C6 alkene and/or alcohol additives suppress the cycle even at low addition amounts. Subsequent kinetic experiments demonstrate that the activation barrier for methane conversion can be significantly reduced by the addition of a small amount of aromatics. Furthermore, an effective increase in methane conversion (from 7.3% to 14.1%) and aromatic concentration (from 57.5% to 74.3% in C%) can be achieved by co-feeding 3 mol% p-xylene. This work provides a new strategy to promote methane conversion under mild conditions and to understand the role of reaction intermediates in methane conversion for catalyst design.