Isothermal cyclic conversion of methane to methanol using copper-exchanged ZSM-5 zeolite materials under mild conditions

Abstract

Direct conversion of methane to methanol (DMTM) has become a particularly attractive route for the functionalisation of natural gas. Here the proven capability of copper-exchanged ZSM-5 zeolites to carry out DMTM at mild conditions has been extended by also demonstrating its capability to perform repeated reactions in a concept known as cycling. A series of five copper-exchanged ZSM-5 zeolites with different Si:Al ratio were prepared via aqueous ion exchange. The materials characterisation was carried out using a combination of transmission electron microscopy (TEM), X-ray diffraction (XRD), nitrogen adsorption isotherms at −196 °C (BET and BJH methods), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) and in-situ ultraviolet-visible spectroscopy (UV–vis) techniques. Under mild isothermal conditions (air activation; T = 200 °C; P = 1 atm), methanol productions of 2.8 ± 0.1 μmol·gcat−1, 26.6 ± 0.1 μmol·gcat−1, 12.5 ± 0.1 μmol·gcat−1, 10.8 ± 0.1 μmol·gcat−1 and 3.2 ± 0.1 μmol·gcat−1 were achieved for copper-exchanged ZSM-5 containing 20:1, 30:1, 50:1, 80:1, and 200-400:1 Si:Al ratio, respectively. Comparing the results of cycling Cu-ZSM-5 (Si:Al = 200-400:1) with those obtained for Cu-ZSM-5 (Si:Al = 30:1) has led to the conclusion that hydrophobicity plays a decisive role in the capability to cycle, with materials containing a higher Si:Al being better suited to cycling. A total of five repeated cycles were achieved using Cu-ZSM-5 (Si:Al = 200-400:1). The methanol production was higher, the lower the Si:Al ratio with an optimal Si:Al = 30:1. However, TEM analysis suggests that precipitation of copper nanoparticles on the catalyst support structure could account for the reduced activity found within the material containing the lowest Si:Al. in situ UV-vis spectroscopy characterisation of the copper-exchanged ZSM-5 zeolite materials under similar DMTM reaction conditions suggested that active copper complexes were being created and then destroyed during the DMTM reaction. A speculative discussion of the copper complexes present within these materials has also been provided.