Selective Oxidation of Methane to Methanol over Cu- And Fe-Exchanged Zeolites: The Effect of Si/Al Molar Ratio

A series of Cu-, Fe- And Fe/Cu-containing zeolite (ZSM-5, beta, Y) catalysts were prepared to investigate the effect of zeolite's physicochemical properties on the total oxygenates production and MeOH selectivity from the partial methane oxidation using HO as oxidizing agent. The NH-TPD studies have shown that the zeolite type and Si/Al molar ratio are correlated with the acid sites strength and concentration. The latter surface property was proved to have a strong influence on the oxygenate productivity. In particular, a significant increase of the methanol production was observed when lowering Si/Al ratio in the ZSM-5, Fe/ZSM-5, Cu/ZSM-5 and Cu-Fe/ZSM-5 catalysts. This can be explained by the increased amount of Brønsted acid sites capable of accommodating the active catalyst (Fe species). The Fe-only ZSM-5 catalysts exhibited the highest catalytic activity (total oxygenated products) with HCOOH being the major product, whereas the presence of only Cu was found to suppress the production of MeOOH and HCOOH. On contrary, the deposition of both Fe and Cu results to a switch in selectivity and the target product, MeOH, was observed in ~80 % selectivity. In the case of Cu-only ZSM-5 catalysts, a similar activity to methanol was observed regardless the copper source and synthesis method. The activity/selectivity findings of the present study confirm and complement the conclusions of the previous work by Hammond et al. (ACS Catal 3:689, 2013; ACS Catal 3:1835, 2013; Angew Chem Int Ed 51:5129, 2012; Chem Eur J 18:15735, 2012) over the well-studied Cu-Fe-zeolite system, providing also complete material balance based on both gas and liquid reaction products.