Oxygen-Vacancy-Containing ZnO Nanoparticles for the\nCatalytic Cleavage of Dimethyl Methylcyclohexyl-2,4-Dicarbamate to\nthe Corresponding Diisocyanate: Implication for the Preparation of\nPhosgene-Free Polyurethanes

Methylcyclohexyl-2,4-diisocyanate\n(HTDI) is an upgraded product\nof toluene-2,4-diisocyanate. HTDI-derived polyurethane products show\nexcellent properties such as nonyellowing degeneration, light stability,\nand weather resistance. The cleavage of dimethyl methylcyclohexyl-2,4-dicarbamate\n(HTDC) is a crucial step for phosgene-free synthesis of HTDI. In this\nwork, the catalytic performance of ZnO nanoparticles (NPs) with different\nparticle sizes was evaluated for the cleavage of HTDC, and the result\nshowed that the selectivity of HTDI up to 81.3% was achieved with\na 94.2% conversion of HTDC over a 19.1 nm ZnO catalyst. It was found\nfrom the results of experiments and characterizations that the decrease\nin ZnO NPs particle size led to the following circumstances: (1) the\nspecific surface area increased, (2) the crystal preferred orientation\nshifted from the (100) plane to the (002) plane, and (3) the oxygen\nvacancy concentration increased. The first-principles calculation\nresults showed that two electrons left from the formation of an oxygen\nvacancy on the ZnO surface could transfer to the neighboring Zn site.\nThus, the electron-withdrawing ability of the Zn site is changed and\nthe structure of the ZnO surface is rearranged, which facilitates\nthe inhibition of side reactions. Therefore, changing the concentration\nof oxygen vacancies and then adjusting the electron-withdrawing ability\nof Zn sites on the ZnO surface are an effective way to achieve highly\nselective synthesis of HTDI by the cleavage of HTDC.