Evolution of active sites during selective oxidation of methane to methanol over Cu-CHA and Cu-MOR zeolites as monitored by operando XAS

Abstract

The local structural and electronic properties of Cu sites during the direct conversion of methane to methanol with molecular oxygen was followed by X-ray absorption spectroscopy (XAS) on a set of Cu-zeolites with different topology (large pore MOR and small pore CHA frameworks) and similar chemical composition (Si/Al ∼ 11 and 12, respectively). Two low loading (LL) and high loading (HL) samples were prepared by ion exchange, and their methanol productivity measured in laboratory fixed bed reactor tests, resulting in selectivity higher than 85%. Both Cu-MOR samples outperformed Cu-CHA ones, resulting in 0.28 and 0.26 molCH3OH/molCu for the LL (Cu/Al = 0.13) and HL (Cu/Al = 0.36) samples, vs 0.10 and 0.17 molCH3OH/molCu for LL and HL Cu-CHA, respectively (Cu/Al = 0.16 and 0.49). The evolution of Cu oxidation state and local coordination environment was followed in all the reaction steps (O2 activation, flush with He, CH4 dosage and subsequent CH3OH desorption by applying a wet stream) both during temperature changes (ramp in O2 up to 500 °C, cooling to 200 °C for subsequent steps, etc.) and in steady-state conditions. Strong similarities were observed in the local structure of Cu sites in all samples, which showed similar redox dynamics during O2 activation and subsequent cooling to 200 °C. Even though only qualitative considerations can be made on the overall spectral evolution, XANES indicates that not all the Cu(I) formed during CH4 loading belongs to active sites.