Yaqi Qu,Mengying He,Hualiang An,Xinqiang Zhao,Yanji Wang
Selective hydrogenation of dimethyl toluene-2,4-dicarbamate (TDC) to methyl cyclohexyl-2,4-dicarbamate (HTDC) is an essential intermediate step in the new green route for synthesizing methylcyclohexyl-2,4-diisocyanate (HTDI). Herein, we investigated the synthesis of HTDC from TDC by a solvent-free, mild and safe catalytic transfer hydrogenation (CTH) process using isopropanol as H-donor. Firstly, a bimetallic catalyst In 2 O 3 @RhPt with a high metal dispersion was prepared by one-pot method. The characterization results demonstrated that Rh and Pt atoms formed alloy structure and their interaction led to an electron-rich Pt and an electron-deficient Rh. What’s more, Pt could also stabilize the surface of In 2 O 3 . Then the Rh/Pt ratio was optimized and In 2 O 3 @Rh 49 Pt 51 catalyst was found to show the best catalytic performance: about 99% of TDC conversion and 100% of HTDC selectivity at 80 ℃ and ambient pressure. It was proved by the designed experiments that isopropanol dehydrogenation was the dominant step and the active hydrogen species existed in the form of radicals in the CTH process of TDC to HTDC. Additionally, the synergistic effect in the RhPt alloy boosted the catalytic performance. Finally, the CTH reaction pathway and mechanism was elucidated based on the experimental and DFT calculation results.
Shutong Pang,Hualiang An,Guirong Wang,Xinqiang Zhao,Yanji Wang
Methylcyclohexyl-2,4-diisocyanate (HTDI) is an upgraded product of toluene-2,4-diisocyanate. HTDI-derived polyurethane products show excellent properties such as nonyellowing degeneration, light stability, and weather resistance. The cleavage of dimethyl methylcyclohexyl-2,4-dicarbamate (HTDC) is a crucial step for phosgene-free synthesis of HTDI. In this work, the catalytic performance of ZnO nanoparticles (NPs) with different particle sizes was evaluated for the cleavage of HTDC, and the result showed that the selectivity of HTDI up to 81.3% was achieved with a 94.2% conversion of HTDC over a 19.1 nm ZnO catalyst. It was found from the results of experiments and characterizations that the decrease in ZnO NPs particle size led to the following circumstances: (1) the specific surface area increased, (2) the crystal preferred orientation shifted from the (100) plane to the (002) plane, and (3) the oxygen vacancy concentration increased. The first-principles calculation results showed that two electrons left from the formation of an oxygen vacancy on the ZnO surface could transfer to the neighboring Zn site. Thus, the electron-withdrawing ability of the Zn site is changed and the structure of the ZnO surface is rearranged, which facilitates the inhibition of side reactions. Therefore, changing the concentration of oxygen vacancies and then adjusting the electron-withdrawing ability of Zn sites on the ZnO surface are an effective way to achieve highly selective synthesis of HTDI by the cleavage of HTDC.
Yaqi Qu,Xiang Li,Hualiang An,Xinqiang Zhao,Yanji Wang
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-Al 2 O 3 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-Al 2 O 3 could be repetitively used for 4 times without a significant loss of its catalytic activity. TEM, XRD, N 2 adsorption-desorption, H 2 -TPR, NH 3 /CO 2 -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 Rh n + ( 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 Rh n + species. This is conducive to activating the benzene ring of TDC and thereby improving the yield of HTDC.
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