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 (CLAl₂O₃) 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).