Excellent catalytic performance of Cu+ modified HZSM-5 to produce para-xylene via the toluene methylation reaction: High catalytic active site combining with channel shape selectivity

Abstract

The toluene methylation reaction via methanol is an important process to produce high-value para-xylene (PX), it has always been our goal to explore a catalyst with high activity and selectivity for PX. Cu+ is used to modify HZSM-5 (BAS/Cu+-HZSM-5) zeolite, and the microscopic reasons for the excellent catalytic performance to produce PX on BAS/Cu+-HZSM-5 are investigated according to the intrinsic reaction and molecular diffusion. The intrinsic activation energies of methanol dissociation and xylene formation on BAS/Cu+-HZSM-5 are obtained by the density functional theory (DFT). Calculations indicate that the energy required for methanol dissociation is 109.7 kJ mol−1, which is lower than that on pure HZSM-5. In addition, PX, meta-xylene (MX) and ortho-xylene (OX) formed with little activation energies of 12.7, 11.8 and 14.2 kJ mol−1, respectively. Therefore, the formation of these xylenes on BAS/Cu+-HZSM-5 has high activity and strong competition. On the other hand, the self-diffusion coefficients of three xylenes on BAS/Cu+-HZSM-5 are analyzed by molecular dynamics (MD) simulation. And the self-diffusion coefficient of PX is found to be much larger than that of MX and OX in all cases, which indicates that the channel characteristics of BAS/Cu+-HZSM-5 are beneficial to the diffusion and separation of PX. Therefore, it can be concluded that BAS/Cu+-HZSM-5 has better catalytic performance and high PX selectivity for the toluene methylation reaction via methanol.