Investigating time-dependent Zn species over Zn-based catalysts in glycerol carbonylation with urea and their roles in the reaction mechanism

In this work, two different Zn-based catalysts - ZnO, and ZnAl mixed oxide (ZnAlO) - were employed to investigate Zn-phase-dependent catalysis in glycerol carbonylation with urea as a function of reaction times. ZnAlO catalyst exhibited higher selectivity and yield of glycerol carbonate (GC) over a wide range of glycerol conversion than the ZnO catalyst. The time-dependent Zn species and reaction intermediates were observed in the solid and liquid phases at various reaction times through FTIR and XRD measurements in order to understand Zn-containing intermediates and corresponding reaction routes over each catalyst. The low GC selectivity in the reaction over the ZnO catalyst was closely connected to the formation of zinc glycerolate (ZnGly) in the solid phase. For the ZnO catalyst, ZnGly was formed in the solid phase even at an initial reaction time by the reaction between Zn NCO complex and glycerol, resulting in the loss of GC selectivity. Alternatively, over a ZnAlO catalyst, the formation of the Zn isocyanate (NCO) complex was dominant up to 2 h of reaction time in both the liquid and solid phases. After 2 h of reaction time, ZnGly was observed in the spent ZnAlO catalyst along with decreasing GC selectivity. The relative formation rates of Zn-containing reaction intermediates (ZnGly and Zn NCO complex) over the ZnAlO catalyst were affected by the Zn phases over the solid catalysts and the ratio of urea to glycerol in the liquid phase during the reaction time.