Low-pressure CO2 hydrogenation coupled with toluene methylation to para-xylene using atomic Pd-doped ZnZrOx–HZSM-5

Abstract

Selective methylation of aromatics using CO2/H2 presents a promising avenue for producing high-value-added chemicals with high selectivity. Herein, we introduced atomically dispersed Pd species into ZnZrOx solid solution and combined with HZSM-5 to create a bifunctional catalyst, applying to selectively synthesize para-xylene through toluene methylation using CO2/H2 at low pressure. Remarkably, 0.1 wt% Pd in PdZnZrOx–HZSM-5 afforded the selectivity of xylene in CO-free products and para-xylene in xylene to 90.0% and 85.6% at 0.5 MPa, respectively. Spectroscopic characterizations revealed that atomically dispersed Pd species enhance the dissociation of adsorbed hydrogen and facilitate the creation of oxygen vacancies, benefiting the CO2 hydrogenation to CHxO intermediates. Density functional theory calculations suggested that Pd-doped ZnZrOx reduces the energy barrier for hydrogenating H2COOH* to H2CO*, aiding to form CH3O* intermediate for toluene methylation. This work provides insights into the design of catalysts for the selective methylation of aromatics using CO2/H2 at low pressure.