Hydrogenation of CO2 to p-xylene over ZnZrOx/hollow tubular HZSM-5 tandem catalyst

Abstract

The conversion of CO₂ into specific aromatics by modulating the morphology of zeolites is a promising strategy. HZSM-5 zeolite with hollow tubular morphology is reported. The morphology of zeolite was precisely controlled, and the acid sites on its outer surface were passivated by steam-assisted crystallization method, so that the zeolite exhibits higher aromatic selectivity than sheet HZSM-5 zeolite and greater p-xylene selectivity than chain HZSM-5 zeolite. The tandem catalyst was formed by combining hollow tubular HZSM-5 zeolites with ZnZrO_x metal oxides. The para-selectivity of p-xylene reached 76.2% at reaction temperature of 320 °C, pressure of 3.0 MPa, and a flow rate of 2400 mL g⁻¹ h⁻¹ with an H₂/CO₂ molar ratio of 3/1. Further research indicates that the high selectivity of p-xylene is due to the pore structure of hollow tubular HZSM-5 zeolite, which is conducive to the formation of p-xylene. Moreover, the passivation of the acid site located on the outer surface of zeolite effectively prevents the isomerization of p-xylene. The reaction mechanism of CO₂ hydrogenation over the tandem catalyst was investigated using in-situ diffuse reflectance Fourier transform infrared spectroscopy and density functional theory. The results showed that the CO₂ to p-xylene followed a methanol-mediated route over ZnZrO_x/hollow tubular HZSM-5 tandem catalysts. In addition, the catalyst showed no significant deactivation in the 100 h stability test. This present study provides an effective strategy for the design of catalysts aimed at selectively preparing aromatics through CO₂ hydrogenation.