Continuously operated liquid-phase methanol synthesis uncovering the de-/activation pathways of a molecular manganese catalyst system

Abstract

Homogeneously catalyzed synthesis of methanol from synthesis gas is achieved under continuous operation with product separation and catalyst recycling via simple flash distillation. A high-boiling alcohol is used as the solvent and stationary phase to immobilize the molecular manganese complex catalyzing the alcohol-assisted methanol synthesis. Detailed studies on the robustness of the system revealed high stability of the organometallic framework of the catalyst and identified the base co-catalyst as the limiting factor. Etherification of the alcoholate base through reaction with the formate ester intermediates was found to lead to its chemical depletion. This results in the accumulation of a formate complex as a dormant species that hinders catalytic turnover. This mechanistic insight into the fundamental (de-)activation phenomena aided the development of an optimal base-dosing strategy and temperature optimization as key levers to achieve long-term stable catalytic performance. Stable catalyst activity was retained over 50 h of operation, resulting in a total turnover number (TTON) of 16 190 mol MeOH per mol Mn.