Temperature-dependant active sites for methane continuous conversion to methanol over Cu-zeolite catalysts using water as the oxidant

Abstract

Methane-selective continuous conversion to methanol is highly challenged by the high CH dissociation energy and the facile over-oxidation of methane. In this work, we reported continuous methane conversion to methanol over Cu-zeolites using water as the oxidant at different reaction temperatures (200–450 °C) and found that the space–time yield (STY) of methanol depended on the reaction temperature, with a high methanol STY of 189.9 μmol/gcat./h (578.7 mmol/molCu/h) at the selectivity of 91 % on Cu-SSZ-13 being achieved at the optimal temperature of 350 °C. Increasing the reaction temperature from 200 to 350 °C resulted in the increase of the kinetic orders of methane and water for catalytic oxidation of methane. An important reason was indicated that higher reaction temperatures not only enhanced a high methane conversion rate but also triggered the reactivity of more Cu species on Cu-zeolites based on the quantification analysis of the available active sites and in-situ spectroscopic study. It was revealed that the CuxOy clusters (e.g., [Cu2(μ-O)]2+) as the active sites exhibited the reactivity at 200 °C, while the Cu(II)–OH species on 8-membered rings and the isolated Cu2+ ions on 6-membered rings presented the reactivity at higher temperatures (≥300 °C). This could be attributed to higher activation energies of Cu(II)–OH species and isolated Cu(II) sites. Besides, isotope labeling experiments indicated that the combination of methyl and hydrogen dissociated from water and methanol reforming resulted in a low methanol yield at high temperatures (≥350 °C).