Abstract
Direct CO(2) methylation with toluene, as one of the CO(2) hydrogenation technologies, exhibits great potential for the CO(2) 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 CO(2) 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 CO(2) methylation are 360-420 °C, 3 MPa with middle CO(2)/C(7)H(8) ratio (1:1 to 1:4) and high H(2) feed (CO(2)/H(2) = 1:3 to 1:6). As a tandem process, the toluene feed would break the thermodynamic limit and has the higher potential of >60% CO(2) conversion than that of CO(2) hydrogenation without toluene. The direct CO(2) 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 CO(2) conversion and product selectivity from the view of reaction pathways of the complex system.