Rh/γ-Al2O3 catalyzed selective hydrogenation of dimethyl toluene-2,4-dicarbamate: mechanism and kinetics

The selective hydrogenation of dimethyl toluene-2,4-dicarbamate (TDC) to methyl cyclohexyl-2,4-dicarbamate (HTDC) is an essential step for the green synthesis of methylcyclohexane-2,4-diisocyanate (HTDI). However, an unclear reaction mechanism and a lack of its reaction kinetics hamper the commercial development of the hydrogenation process. In this work, we investigated the selective hydrogenation of TDC over Rh/γ-Al₂O₃ catalyst and focused on the reaction mechanism and kinetics. Firstly, the adsorption behavior of the reaction components on the surface of Rh/γ-Al₂O₃ catalyst was studied using in situ DRIFTS technique. It was revealed that TDC and hydrogen are competitively adsorbed on the Rh surface but HTDC is adsorbed much more weakly compared with TDC. On this basis, the kinetics of TDC selective hydrogenation were probed based on the Langmuir-Hinshelwood mechanism. After optimizing kinetic parameters via a hybrid algorithm and selecting the optimal model through statistical discrimination, its consistency was confirmed with DRIFTS analysis results. The surface reaction involving the addition of the first hydrogen atom to the benzene ring was identified as the rate-determining step. Based on these, the kinetics of TDC selective hydrogenation catalyzed by Rh/γ-Al₂O₃ were established with an activation energy of 48.1 kJ·mol⁻¹. This work will provide a technical reference for the development of green route to HTDI synthesis, especially the selective hydrogenation of TDC to HTDC process.