Effect of ZSM-5@Silicalite-1 zeolites prepared by solid phase epitaxial growth method on CO₂ hydrogenation and toluene alkylation reactions

CO₂ hydrogenation to synthesize high value-added aromatics is of great significance to alleviate the energy climate problem caused by CO₂ emission. It is generally believed that the reaction course of CO₂ hydrogenation of toluene coupled with alkylation to produce xylenes is as follows: firstly, CO₂ reacts with H₂ to produce methanol intermediates, and then the methanol intermediates react with toluene on zeolite catalysts to produce para-xylene (PX) by alkylation. According to the reaction pathway, it is necessary to construct a bifunctional catalyst with synergistic matching of the two process conditions to simultaneously realize the hydrogenation of CO₂ to methanol intermediate and the alkylation of the intermediate and toluene to generate para-xylene. The ZnZrO_x/ZSM-5 catalytic system, in which the ZnZrO_x has strong thermal stability and CO₂ activation ability, and the ZSM-5 has a good morphology selectivity for PX, is considered to be a promising CO₂ hydrogenated toluene coupled alkylation catalyst. However, this system still suffers from low PX selectivity, mainly due to the presence of non-selective acidic sites on the outer surface of the zeolite or near the pore orifice, which leads to the generation of side reactions, such as deep methylation and toluene isomerization, and reduces the selectivity. In this paper, ZSM-5@Silicalite-1 zeolites were prepared by epitaxial growth of Silicalite-1 on the surface of ZSM-5 using solid-phase synthesis. At the same time, the highly active oxide ZnZrO_x was prepared and physically mixed with ZSM-5@Silicalite-1 to form a ZnZrO_x/ZSM-5@Silicalite-1 bifunctional catalyst to study the catalytic performance of CO₂ hydrogenation coupled with toluene alkylation. Compared with the ZnZrO_x/ZSM-5 catalyst, the modified zeolite improved the para-xylene (PX) selectivity. The effect of crystallization conditions (silicon source, crystallization process, and number of crystallizations) on the epitaxial growth of Silicalite-1 from ZSM-5 was investigated, as well as the effect of the thickness of the Silicalite-1 passivation layer on the performance of the reaction between carbon dioxide hydrogenation and toluene alkylation. The ZZO/1:3.5Z5-Na-SiO₂ catalyst showed a toluene conversion of 12.0%, a xylene selectivity of 77.4%, and a PX selectivity of 73.4% in xylene under 400 ℃ and 3 MPa reaction conditions. The structure and acid properties of the zeolites were investigated in detail by SEM, XRD, N₂ adsorption-desorption, XPS, NH₃-TPD and Py-FTIR characterization, and the results show that the selectivity of para-xylene (PX) can be effectively improved by solid-phase epitaxial growth to extend the pore channels of ZSM-5, increase the diffusion resistance of m-xylene (MX) and o-xylene (OX), and passivate the acidity of the outer surface at the same time. This method abandons the disadvantage of previous modification of molecular sieves by blocking the pores to narrow the orifice, and upgrades the product selectivity while ensuring the catalyst activity.