Structure-activity relationships of Fe-Co/K-Al2O3 catalysts calcined at different temperatures for CO2 hydrogenation to light olefins

Fe-Co/K-Al2O3 catalyst has been reported to be active and selective for olefin production from CO2 hydrogenation. However, the effect of calcination temperature on the physicochemical properties of Fe-Co/K-Al2O3 catalysts as well as their catalytic activity in CO2 hydrogenation to olefins has not yet been addressed. Here, we show that the calcination temperature (400-800 °C) has significant impacts on metal oxides crystallite size, on the interaction between Fe2O3 and other metal oxides and on the transformation of potassium phases, which affect CO2 conversion, product selectivity as well as yield of olefins. The Fe-Co/K-Al2O3 catalyst calcined at 400 °C achieves the highest CO2 conversion, hydrocarbons selectivity and olefins yield of 49.0%, 90.6% and 18.1%, respectively. The CO2 conversion and hydrocarbons selectivity decrease with ascending calcination temperature which is ascribed to the increase of metal oxides crystallite size, inducing stronger interactions between Fe2O3 and other metal oxides. Unlike the CO2 conversion and hydrocarbons selectivity, the olefins to paraffins (O/P) ratio followed a volcano-shaped trend as the function of calcination temperature with a maximum at 7.6 over the Fe-Co/K-Al2O3 catalyst calcined at 700 °C. This volcanic trend is attributed to the interplay among the positive effect of high calcination temperature on the complete decomposition of KNO3 to K2O, a stronger interaction between Fe2O3 and K2O and the formation of KAlO2 phase, favorably suppressing the hydrogenation of olefins, and the negative effect of high calcination temperature on the increase of the particle size of the active phase and lower dispersion of mixed metal oxides due to a drastic decrease in BET surface area.