Oxygen-Vacancy-Containing ZnO Nanoparticles for the Catalytic Cleavage of Dimethyl Methylcyclohexyl-2,4-Dicarbamate to the Corresponding Diisocyanate: Implication for the Preparation of Phosgene-Free Polyurethanes
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.
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