Disordered structure of ZnAl2O4 phase and the formation of a Zn NCO complex in ZnAl mixed oxide catalysts for glycerol carbonylation with urea

Abstract

The effect of a disordered ZnAl2O4 spinel structure on glycerol carbonylation with urea was investigated with pure ZnAl2O4 (c-ZnAl2O4) and ZnAl mixed oxide (c-ZnAlO) prepared by a citrate complex method, and ZnAl physically-mixed oxide (p-ZnAlO). During catalysts preparation, a disordered bulk ZnAl2O4 phase generated disordered sites on the surface: the Al3+ cations substituting for Zn2+ cations at the tetrahedral sites, and the surface oxygen vacancy corresponding to the Zn2+ cations substituting for Al3+ cations at the octahedral sites. The disordered surface sites increased in order of p-ZnAlO < c-ZnAlO < c-ZnAl2O4, which was proportional to the surface acidity. c-ZnAlO exhibited the best reaction performance due to the existence of a solid zinc isocyanate (Zn NCO) complex on the disordered sites. Here, we proposed that the solid Zn NCO complex preferentially generated glycerol carbonate (GC), while the liquid Zn NCO complex produced both GC and zinc glycerolate.