Preparation of Halloysite-Based Hollow Tubular ZSM-5 and Its Catalytic Performance for CO2 Hydrogenation to p-Xylene

Abstract

The development of the ZSM-5 zeolite with natural minerals is a promising strategy. In this study, natural halloysite was used as a silicon source and aluminum source, as well as a template for the expansion of mesopores and a skeletal template to prepare halloysite-based hollow tubular ZSM-5 zeolites by extending the b-axis and exposing more zigzag pore characteristics. In combination with the steam-assisted crystallization method, the outer surface of ZSM-5 zeolite is covered with high silicon or all silicon; thereby, aluminum is mainly concentrated inside ZSM-5 zeolite. This results in an encapsulated structure that passivates the acid site on the outer surface of ZSM-5 zeolite and realizes the regulation of aluminum distribution in ZSM-5 zeolite. The effect of the alkali environment on the morphology and properties of ZSM-5 zeolite during the preparation process was emphasized. The results show that low alkalinity leads to a decrease in the solubility of silicon and aluminum sources, which limits the formation of precursor solutions and thus affects the growth of the ZSM-5 zeolite. However, high alkalinity will lead to crystal defects, which will affect the structural stability and catalytic performance of the zeolite. ZnZrOx metal oxide was used as the bridge of CO₂ hydrogenation catalytic system, and ZnZrOx/halloysite-based hollow tubular ZSM-5 tandem catalyst was constructed. The para-xylene selectivity was 74.5% under reaction conditions of 320 °C, 3.0 MPa, flow rate of 2400 mL·g^–1·h^–1, and H₂/CO₂ molar ratio of 3:1. This provides a sustainable development strategy for the targeted conversion of CO₂, the high-value utilization of halloysite, and green chemistry.