Reaction Mechanisms and Catalytic Performance of CO2 to Aromatics Over M(ZnO, Ga2O3, In2O3)-UiO-66 Catalysts Without Zeolite

Tandem catalysts comprising HZSM-5 zeolite and metallic oxides are common choices for the hydrogenation of CO2 to aromatics, a process of high strategic value in view of carbon capture, utilization, and storage. Nevertheless, due to time-consuming procedures for the preparation of HZSM-5 and the frequent inactivation of reactions resulting from carbon deposition, lately, researchers have focused their efforts on the modification of HZSM-5. Meanwhile, it is still a huge challenge to find novel catalysts, free from HZSM-5, for successfully hydrogenating CO2 to an aromatic moiety. In this work, tandem catalysts based on metal–organic frameworks (MOFs) and metal oxides were demonstrated to have high selectivity to prepare aromatics from CO2 without using HZSM-5, which has introduced a new way for developing catalysts for facile hydrogenation of CO2 to aromatics. The tandem catalysts were extensively studied for their physicochemical attributes by X-ray diffraction (XRD), scanning electron microscopy (SEM), NH3 temperature-programmed desorption (NH3-TPD), pyridine infrared radiation (Py-IR), H2 temperature-programmed reduction (H2-TPR), CO2 temperature-programmed desorption (CO2-TPD), X-ray photoelectron spectroscopy (XPS), and N2 isothermal adsorption–desorption methods. The effects of the amount and type of metal oxides as well as the influence of reaction conditions on the performance of the catalyst were also examined. The experimental outcome demonstrated that under optimum reaction conditions, the CO2 conversion rate of the 8% ZnO-UiO-66 catalyst was 25.0%, with a selectivity of 76.5% for benzene, toluene, and xylene (BTX). Meanwhile, the side reactions can also be effectively inhibited, and a high selectivity (67.4%) of aromatics can be achieved under atmospheric pressure. The adsorption properties of the ZnO-UiO-66 catalyst for H2 and CO2 were also studied by density functional theory (DFT) and in situ DRIFTS. In addition, the mechanism of the reaction that converts CO2 to aromatics via hydrogenation is also discussed. The attainment of the energy barrier and transition state for the hydrogenation of CO2 to aromatics was achieved through a minimal energy search of the transition state of LST/QST.