Abstract
Oxidative coupling of carbon monoxide (CO) with methyl nitrite (MN) to dimethyl oxalate (DMO) on the Pd/α-Al2O3 catalyst is a vital step for ethylene glycol (EG) and glycolic acid (GA) production from syngas. In this work, we revisited the reaction mechanism and kinetics by including the side reactions to dimethyl carbonate (DMC) and methyl formate (MF). Based on the Langmuir–Hinshelwood (LH) mechanisms, kinetic models were proposed, coupling pathways occurring between two Pd-COOCH3 (Pathway 1) or between Pd-CO and Pd-COOCH3 (Pathway 2). Among them, the LH mechanism with dissociation of MN based on Pathway 1 illuminated the overall reaction best. In addition, the coupling reactions of adsorbed CH3OCO governed the reaction pathways for DMO, and the hydrogenation of CH3OCO for MF. The model reveals that the strongly competitive adsorptions of both CO and NO at high concentrations will suppress the reaction. Overall, the followed kinetic analysis accurately predicted and explained the experimental data. The resulted best-fit model applies to reactor simulation, with satisfactory agreement with three pilot-scale fixed-bed reactors disclosed in the literature by adjusting only the activity coefficient. Therefore, this comprehensive kinetic modeling can be a solid foundation for reactor design and retrofit and operation optimization.
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