Toluene methylation with syngas to para-xylene by bifunctional ZnZrOx-HZSM-5 catalysts

Toluene methylation with methanol on H-ZSM-5 (Z5) zeolite for the directional transformation of toluene to xylene has been industrialized. However, great challenges remain because of the high energy barrier of methanol deprotonation to the methoxy group, the side reaction of methanol to olefins, coke formation, and the deactivation of zeolites. Herein, we report the toluene methylation coupled with CO hydrogenation to showcase an enhancement in para-xylene (PX) selectivity by employing a bifunctional catalyst composed of ZnZrOx (ZZO) and modified Z5. The results showed that a PX selectivity of up to 81.8% in xylene and xylene selectivity of 64.8% in hydrocarbons at 10.3% toluene conversion can be realized over the bifunctional catalyst on a fixed-bed reactor. The selectivity of gaseous hydrocarbons decreased to 10.9%, and approximately half of that was observed in methanol reagent route where the PX selectivity in xylene was 38.8%. We observed that the acid strength, the quantity ratio of Brönsted and Lewis acid sites, and the pore size of zeolites were essential for the PX selectivity. The investigation of the H2/D2 kinetic isotope effect revealed that the newborn methyl group in xylene resulted from the hydrogenation of CO rather than toluene disproportionation. Furthermore, the catalyst showed no evident deactivation within the 100 h stability test. The findings offer a promising route for the production of value-added PX with high selectivity via toluene methylation coupled with syngas conversion.