Thermodynamic and Mechanistic Analyses of Direct CO₂ Methylation with Toluene to para-Xylene

Direct CO₂ methylation with toluene, as one of the CO₂ hydrogenation technologies, exhibits great potential for the CO₂ utilization to produce the valuable para-xylene (PX), but the tandem catalysis remains a challenge for low conversion and selectivity due to the competitive side reactions. The thermodynamic analyses and the comparation with two series of catalytic results of direct CO₂ methylation are conducted to investigate the product distribution and possible mechanism in adjusting the feasibility of higher conversion and selectivity. Based on the Gibbs energy minimization method, the optimal thermodynamic conditions for direct CO₂ methylation are 360-420 °C, 3 MPa with middle CO₂/C₇H₈ ratio (1:1 to 1:4) and high H₂ feed (CO₂/H₂ = 1:3 to 1:6). As a tandem process, the toluene feed would break the thermodynamic limit and has the higher potential of >60% CO₂ conversion than that of CO₂ hydrogenation without toluene. The direct CO₂ methylation route also has advantages over the methanol route with a good prospect for >90% PX selectivity in its isomers due to the dynamic effect of selective catalysis. These thermodynamic and mechanistic analyses would promote the optimal design of bifunctional catalysts for CO₂ conversion and product selectivity from the view of reaction pathways of the complex system.