Copper-zeolites Prepared by Solid-state Ion Exchange-Characterization and Evaluation for the Direct Conversion of Methane to Methanol

Direct conversion of methane to methanol (MTM) over Cu-zeolites is a so-called "dream reaction " for the chemical industry. There is still a lot that can be done in order to optimize the reaction by e.g. achieving a deeper understanding of the reaction mechanism and the nature of the Cu-sites. In this study, we investigated a solid-state ion exchange method to incorporate Cu-I ions into zeolites (MOR, BEA, ZSM-5 and FAU), as a more scalable technique. The solid-state ion exchange led to a Cu/Al ration of about 0.8, however with a heterogeneous distribution of Cu. Regardless, Fourier transform-infrared spectroscopy still revealed that most Bronsted acid sites were exchanged in all four samples. Further, CH4-temperature programmed reaction experiments showed that some Cu-sites formed were reactive towards CH4, with Cu-I-MOR and Cu-I-FAU having the largest CH4 consumption. Ultimately, the Cu-I-zeolites were tested in the MTM reaction and proved capable of producing methanol, even without the presence of Bronsted sites. A MOR with lower Cu/Al ratio (0.30) was also tested for comparison, and as this sample obtained a much higher productivity than the Cu-I-MOR with high Cu-loading (0.10 vs. 0.03 mol(MeOH)/mol(Cu)), it was demonstrated that some fine-tuning is necessary to obtain the active Cu sites for methane activation.