Methane-to-Methanol: Activity Descriptors in Copper-Exchanged Zeolites for the Rational Design of Materials

The direct conversion of methane to methanol in a stepwise process over copper-exchanged zeolites is currently one of the most-studied reactions. Despite these studies and the partial identification of the active species and reaction mechanism, it has not been possible to design the best material and determine the ideal process conditions for industrial implementation. This study shows how to design a significantly better performing material and the reaction parameters to enhance the methanol yield. This led to the highest methanol productivity to date, using a material that until recently was considered to be inactive, opening an original direction for the design of active materials. It is shown that the zeolite framework affects the structure and redox properties of copper species, formed during ion exchange and treatment procedures. In turn, these Cu II -oxo species demonstrate different redox properties, which correlate with the methanol productivity and selectivity: high temperature of copper reduction leads to low activity in the reaction with methane, while low reduction temperature results in overoxidation of methanol to carbon monoxide and formate species. This relationship enables the prediction of the optimal operation temperature of copper-exchanged zeolite, as illustrated for CuFAU, which had been considered to be inactive in methane conversion to methanol. Its multicyclic operation in the isothermal regime at 633 K and ambient pressure gave a stable methanol yield of 90 μmol/g and 92% selectivity, while an increase in the copper loading and methane pressure resulted in the unprecedented yield of 360 μmol/g.