On How Copper Mordenite Properties Govern the Framework Stability and Activity in the Methane-to-Methanol Conversion

Abstract

Herein we investigated the activity of copper mordenites in the methane-to-methanol conversion and the material de- and realumination. From four parent materials, a library of copper mordenites was synthesized by liquid- and solid-state ion-exchange techniques. Two key properties govern the activity of these materials in the methane conversion: the parent counterion and the copper ion-exchange procedure. H-form parents result in more active materials. The optimum stoichiometry between silicon, aluminum, and copper leads to a methanol productivity of up to 169 μmol/g. This equals a stoichiometry of up to 0.47 methanol molecules formed per copper atom. The methanol productivity is constant over up to three cycles. The stability of the mordenite framework was monitored by SEM, EDX, 27Al, and 29Si MAS NMR spectroscopy. No detectable copper nanoparticles formed. However, a dealumination of the mordenite framework and the formation of extra-framework aluminum (EFAl) species in quantities of up to 12% were observed on H-form copper mordenites. The dealumination is weak or completely inhibited if counterions like Na+ or Cu2+ are present. These ions stabilize the framework aluminum during the reaction steps and upon heat treatments. Notably, the most active materials have significant EFAl contents present.