Revisiting Catalyst Structure and Mechanism in Methanol Synthesis

Abstract

Methanol synthesis is a mature industrial process, where industrial Cu/ZnO/Al 2 O 3 catalyst is commonly used. Howerver, function of the active catalyst component and reaction mechanism is still vague. In this contribution, the function of each catalyst component including the synergy between Cu and ZnO, reaction mechanism including carbon source of methanol, key intermediates and elementary steps, and kinetics of methanol synthesis and impurities formation are discussed based on the latest literature survey. 1 Introduction Global demand for methanol is expected to increase from 60.7 million metric tons (MMT) in 2013 to more than 109 MMT in 2023, with an average annual growth rate of 6 percent (according to research from IHS Markit). As one of the most versatile chemical compounds, methanol is used in numerous applications, including for fuel, solvent, antifreeze and for making biodiesel fuels. Methanol production, however, is currently limited to only a few large-scale plants around the world, resulting in high transportation costs for users who are not located within close proximity to a production facility. Primus Green Energy is a green energy company converting natural gas into syngas through methane reforming and then into methanol. Primus' gas-to-methanol (GTM) STG+™ System can produce Grade A/AA methanol and methanol meeting IMPCA specification onsite in any location where the feed gas is available or where methanol is in strong demand.[1]. Methanol synthesis is a key step in utilizing natural gas into valuable chemicals and polymers. Methanol was identified as the major product in hydrogenation of CO over mixed Cr 2 O 3 and ZnO by BASF in 1923 [2]. ICI (now Johnson Matthey) discovered a preparation method of Cu/ZnO/Al 2 O 3 [3] to overcome the intrinsic instability of Cu-based catalysts in 1966, which is still the most popular catalyst in industrial practices for the established methanol synthesis process from syngas. However, the actual function of each catalyst component and reaction mechanism is still not clear. This mini-review article summarizes state-of-the-art research including function of each component in an industrial Cu/ZnO/Al 2 O 3 catalyst, their synergistic effects, and the mechanism and kinetics of methanol synthesis. The mechanism and kinetics of impurities formation are also discussed. Understanding those topics is important for methanol catalyst developers to locate their focuses on chasing a better catalyst which is viable at lower operating temperatures and pressures with less impurity formation. In turn, industry methanol producers benefit better plant economics in commercializing methanol synthesis or other processes where methanol is as an intermediate product.