Pore size effects on physicochemical properties of Fe-Co/K-Al2O3 catalysts and their catalytic activity in CO2 hydrogenation to light olefins

Abstract

In this work, the hydrogenation of CO2 to light olefins has been studied over the Fe-Co/K-Al2O3 catalysts, while focusing on the impact by the pore sizes of Al2O3 supports including 6.2 nm (S-Al2O3), 49.7 nm (M-Al2O3) and 152.3 nm (L-Al2O3) on the structure and catalytic performance. The characterization results demonstrate that the pore sizes of the Al2O3 supports play a vital role on the crystallite size of Fe2O3, the reducibility of Fe2O3 and the adsorption-desorption of CO2 and H2. The catalyst with the smallest pore size (CS-Al2O3) allows the formation of a small Fe2O3 crystallite size due to pore confinement effects, yielding a low active component (Fe) after reduction at 400 °C for 5 h. The catalysts with the larger pore sizes of 49.7 nm (CM-Al2O3) and 152.3 nm (CL-Al2O3) provide the larger Fe2O3 crystallite sizes which require a longer reduction time for enhancing degree of reduction, resulting in a high metallic Fe content, leading to a high CO2 conversion and a high selectivity toward hydrocarbon. Eliminating diffusion limitation by increasing the pore sizes of Al2O3 supports can suppress the hydrogenation of olefins to paraffins and thus the largest pore catalyst (CL-Al2O3) gives the highest olefins to paraffins ratio of 6.82. Nevertheless, the CL-Al2O3 also favors the formation of C5+ hydrocarbon. Therefore, the highest light olefins yield (14.38%) is achieved over the catalyst with appropriated pore size (CM-Al2O3).