Probing into the factors influencing the catalytic performance of ZnO and its catalytic mechanism in dimethyl methylcyclohexyl-2,4-dicarbamate pyrolysis

The pyrolysis of dimethyl methylcyclohexyl-2,4-dicarbamate (HTDC) is one of the most important steps in the highly selective synthesis of methylcyclohexyl-2,4-diisocyanate (HTDI) through the non-phosgene route. In this work, we found that two commercial ZnO samples showed a distinct catalytic performance in HTDC pyrolysis reaction. With the aid of catalyst characterizations of XRD, SEM, TEM, N₂ adsorption-desorption, XPS, TG/DSC, etc, we are convinced that particle size, specific surface area, crystal plane exposed, and oxygen vacancy concentration are responsible for the catalytic performance of ZnO sample. On this basis, ZnO-catalyzed HTDC pyrolysis reaction process was elucidated by the analyses from both in-situ FTIR and the first-principles calculations. The above investigations demonstrated that employing the ZnO catalyst with suitable oxygen vacancy concentration and achieving a rapid separation of HTDI from the reaction system in the reaction process are the keys to attaining highly selective synthesis of HTDI. The pyrolysis of HTDC proceeds by a predominant adsorption on the ZnO surface through the hydrogen atom of the amino group and the oxygen atom of the methoxy group rather than the carbonyl oxygen. And what’s more, the methyl carbamate group in the para-position is pyrolyzed prefercially compared with that in the ortho-position due to the effect of the steric hindrance of methy group. This work paved a new way to highly selective pyrolysis of HTDC to HTDI.