Selective hydrogenation of dimethyl toluene-2,4-dicarbamate over supported Rh-based catalysts: Effect of support properties

The selective hydrogenation of dimethyl toluene-2,4-dicarbamate (TDC) to methyl cyclohexyl-2,4-dicarbamate (also called hydrogenated TDC, HTDC) is an essential process for non-phosgene synthesis of methylcyclohexane-2,4-diisocyanate. Herein, we prepared a series of supported Rh-based catalysts by the excessive impregnation method and investigated their catalytic performance for the selective hydrogenation of TDC. The emphasis was put on the influence of support properties on the catalytic performance. Among the prepared catalysts, Rh/γ-Al₂O₃ performed the best: a HTDC yield of 88.4% was achieved with a 100% conversion of TDC under the conditions of 100 °C, 3 MPa and 1 h. Furthermore, Rh/γ-Al₂O₃ could be repetitively used for 4 times without a significant loss of its catalytic activity. TEM, XRD, N₂ adsorption-desorption, H₂-TPR, NH₃/CO₂-TPD, XPS and ICP characterizations were employed to distinguish the properties of the prepared catalysts and the results were correlated with their catalytic performance. It is indicated that the yield of HTDC shows a positive relevance with the percentage of moderate-to-strong acid sites and the content of Rhⁿ⁺ (n ≥ 3) in the catalysts. High values of the percentage and the content can promote a strong interaction between Rh nanoparticles and the supports, facilitating both the transfer of electrons from Rh to the support and the formation of Rhⁿ⁺ species. This is conducive to activating the benzene ring of TDC and thereby improving the yield of HTDC.