Morphology–acidity synergy in ZSM-5 for selective para-xylene synthesis via CO2/H2-mediated toluene methylation

CO2/H2-mediated toluene methylation over metal oxide–zeolite bifunctional catalyst offers a sustainable route for para-xylene (PX) synthesis, yet how zeolite morphology and acidity synergistically govern this coupled process remains unclear. Herein, we report a systematic approach to improve PX yield through the morphological tailoring and elemental doping of ZSM-5 (Z5) zeolites. A series of Z5 with varied morphologies–sheet, hollow, nano, and spheroidal–were synthesized to optimize product diffusion, while Zn, P, and Si dopants were introduced to modulate the distribution and strength of acid sites. The optimized PdZnZrOx–Si/P/sheet-ZnZ5 catalyst achieved 87.0% of PX selectivity with 92.5% of xylene in CO-free products (360 °C, 0.5 MPa, and gas hourly space velocity (GHSV) = 18,000 mL g−1 h−1). Mechanistic studies reveal that methyleneoxy (CH2O*) and/or methoxy (CH3O*) species, originating from formate (HCOO*) species, act as the plausible reactive intermediates for methylation reaction, and that the synergy between short b-axis diffusion paths and tailored acidity effectively suppresses PX isomerization, overcoming the thermodynamic limitations of xylene isomer distributions. This work establishes an apparent structure–diffusion–acidity relationship and provides a rational design strategy for kinetically controlling para-selectivity in tandem catalytic systems beyond thermodynamic constraints.