Suppression of Methane Formation by Regulating the Hydrogenation Capability of Metal Oxides in Alkylation of Benzene with Syngas

In alkylation of benzene with syngas, the inevitable converting of syngas into methane, as a side reaction, severely hinders the effective utilization of carbon monoxide (CO), which is a major challenge to circumvent. In this work, a series of bifunctional catalysts composed of various zinc/zirconium/cerium metal oxides and HZSM-5 zeolite were prepared in order to explore the key factor of methane formation and achieve high CO efficiency. The results show that methane selectivity mostly depends on the hydrogenation capability, which can be regulated via altering compositions of the metal oxides. Combined with density functional theory calculation and characterizations, it is found that the hydrogenation capability of metal oxide is related to the energy barrier of hydrogen (H2) heterolysis. Compared to CeO2, the addition of zinc favors a lower energy barrier and a stronger ability toward H2 heterolysis, while the addition of zirconium makes this ability weaker. Too strong heterolysis capability consequently leads to excessive hydrogenation, which further causes high methane selectivity; however, too weak heterolysis capability will lead to low catalytic activity. Hence, proper hydrogenation capability is an important element for low methane selectivity and high catalytic activity. Our findings reveal the formation mechanism of methane and provide a new strategy to reduce methane content.