CO₂ hydrogenation toward toluene methylation for selective para-xylene synthesis.

Abstract

The selective production of high-value para-xylene (PX) through CO₂ hydrogenation-coupled toluene methylation represents a promising strategy for sustainable carbon utilization. Herein, we report a ZnZrO_x & HMCM-22 tandem catalyst that integrates CO₂ hydrogenation with shape-selective toluene methylation. The optimized catalyst achieves 91% xylene selectivity with 70.4% PX dominance at 10.3% toluene conversion. Controlled tetraethyl orthosilicate (TEOS) deposition passivates Brønsted acid sites in HMCM-22 supercages, suppressing toluene disproportionation while preserving methylation activity in sinusoidal channels. Mechanistic studies reveal that formaldehyde species, generated on ZnZrO_x and transferred to HMCM-22, act as kinetically favored methylating intermediates, enabling direct coupling between CO₂ hydrogenation and toluene methylation. This pathway outperforms conventional methanol-mediated routes in both activity and selectivity. Our work establishes a dual-optimization strategy-zeolite microenvironment engineering and reactive intermediate control-offering mechanistic insights and design principles for high-performance tandem catalysis in carbon recycling aromatic synthesis.