Stepwise conversion of methane to methanol over Cu-mordenite prepared by supercritical and aqueous ion exchange routes and quantification of active Cu species by H₂-TPR

Copper-exchanged mordenite prepared by supercritical ion exchange (SCIE) and aqueous ion exchange (AIE) were investigated in stepwise conversion of methane to methanol. Increasing the oxygen activation temperature and methane reaction time enhances the methanol yield of copper-exchanged mordenite prepared by SCIE (CuMORS). The reducibility of Cu-MORS was compared with those of Cu-MORA prepared by aqueous ion exchange (AIE) using H-2-TPR. It was demonstrated for the first time that deconvoluted H2-TPR profile coupled with effects of Cu loading and oxygen activation temperature on methanol yield data can be used to distinguish the active Cu sites from inactive ones based on their reduction temperature. The copper species responsible for methane activation were found to be reduced below 150 C by H-2 in both Cu-MORS and Cu-MORA. From the stoichiometry of the reaction of H-2 with Cu2+ species, the average number of copper atoms of active sites were calculated as 2.07 and 2.80 for Cu-MORS and Cu-MORA, respectively. Differences in structure of copper species caused by the synthesis routes were also detected by in-situ FTIR upon NO adsorption indicating a higher susceptibility of CuMORS towards autoreduction. The results demonstrated the potential of TPR based methods to identify copper active sites and suggested the importance of site selective ion exchange in order to controllably synthesize active Cu species in zeolites.