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.

Recent Progress in Direct Conversion of Methane to Methanol Over Copper-Exchanged Zeolites

“…Cu-mordenite and Cu-omega catalysts prepared from their H + -forms produced a substantial amount of methanol even at 200°C both applied for oxygen activation and methane reaction. It was also reported that the Cu-mordenite prepared by liquid phase ion-exchange with its H + -form precursor showed the highest methanol yield compared to the other cation precursors (Dyballa et al, 2019). Furthermore, they suggested an optimum stoichiometry between Si, Al, and Cu of Cu-mordenite (i.e., Si/Al = 7 and Cu/Al = 0.18), which exhibited reproducible methanol productivity up to 169 μmol gcat-1.…”

“…More recently, much higher methanol yields were achieved, for example, ca. 125 and 169 μmol gcat-1 using Cu-SSZ-13 (CHA) and Cu-mordenite, respectively (Pappas et al, 2017, 2018; Dyballa et al, 2019). The methanol selectivity close to 100% was attained as a result of inert gas treatment between the oxygen activation and the methane reaction to prevent the excess oxidation of generated methanol in an O 2 -free atmosphere.…”

Comparing the Nature of Active Sites in Cu-loaded SAPO-34 and SSZ-13 for the Direct Conversion of Methane to Methanol

“…Last, methanol is extracted by sending steam through the reactor isothermally, or by using a polar liquid solvent at room temperature (RT). The highest productivity achieved at atmospheric pressure thus far is 0.47 mol MeOH /mol Cu , for a Cu-MOR zeolite [18,19]. If a dinuclear active site is assumed, the theoretical limit is 0.5.…”

Effect of aluminum and sodium on the sorption of water and methanol in microporous MFI-type zeolites and mesoporous SBA-15 materials

“…Because of the raising industrial application of the methanol to hydrocarbon conversion on acidic zeolite catalysts (Olsbye et al 2012;Tian et al 2015), which is accompanied by the formation of water, it is also interesting to clarify the adsorption properties of methanol and to compare them with those of water. Another application is the desorption of surface-bound methanol from zeolites in the presence of water, for example after methane oxidation to methanol (Dyballa et al 2019a, b;Pappas et al 2017;Ravi et al 2017). The presence of bulk species and the interaction sites of water and methanol are also of interest for improving the stability of zeolites and related materials of different pore size, as the interaction sites like silanols can react with water at higher temperatures and damage the framework (Kalantzopoulos et al 2018(Kalantzopoulos et al , 2020Prodinger et al 2016).…”