CO2-oxidative ethane dehydrogenation over highly efficient carbon-resistant Fe-catalysts

Abstract

Ethylene is a very important raw material for chemical industry, which can be produced from the underutilized ethane, originating from shale gas, through the oxidative ethane dehydrogenation (EDH) using CO2 as a mild oxidant. Two 5 wt%Fe-based catalysts, supported on mixed MgO-ZrO2 and NiO-MgO-ZrO2 oxides, prepared by auto-combustion sol-gel method, were investigated for CO2-EDH. The crystallographic structure of the catalysts was examined by X-ray diffraction (XRD), while the surface composition and oxidation state were identified using X-ray photoelectron spectroscopy (XPS). The effect of catalyst pre-treatment, reduction by H2, on catalytic activity, selectivity and carbon-resistance was evaluated for the studied samples.Both catalysts provided high ethylene selectivity and activity during CO2-oxidative dehydrogenation of C2H6 at 873 K, atmospheric pressure and equimolar CO2/C2H6 ratio. The addition of NiO in the support material had a promoting effect, since it resulted in a catalyst with a twofold higher activity. Similarly, the selectivity towards C2H4 remained at high levels, above 70%, slightly increased, as it was exemplified during an iso-conversion experiment. The best candidate lost 25% of its initial activity during the first 3 h of a stability test, while it remained unchanged from 3 to 17 h. Deposited carbon was negligible, whereas sintering was identified as the main deactivation reason. Systematic XPS analysis revealed that deposition of Fe on NiO-MgO-ZrO2 support significantly affected the local environment of Ni, contributing to the enhanced catalytic performance. Increased Fe2+ concentration was found for the “used” Fe-catalyst supported on NiO-MgO-ZrO2, compared to the “as-prepared”, implying partial reduction of Fe during CO2-EDH reaction, as it was also observed by HRTEM.