Effect of Calcination Temperatures on Surface Properties of Spinel ZnAlO Prepared via the Polymeric Citrate Complex Method-Catalytic Performance in Glycerolysis of Urea.

In this study, we investigated urea glycerolysis over ZnAlO catalysts that were prepared by using a citrate complex method and the influence of calcination temperatures on the surface properties of the prepared catalysts by varying the calcination temperature from 550 °C to 850 °C. As the reciprocal substitution between Al and Zn cations led to the formation of a disordered bulk ZnAlO phase, different calcination temperatures strongly influenced the surface properties of the ZnAlO catalysts, including oxygen vacancy. The increase in the calcination temperature from 550 °C to 650 °C decreased the inversion parameter of the ZnAlO structure (from 0.365 to 0.222 for AlO and 0.409 to 0.358 for ZnO). The disordered ZnAlO structure led to a decrease in the surface acidity. The ZnAlO-550 catalyst had a large specific surface area, along with highly disordered surface sites, which increased surface acidity, resulting in a stronger interaction of the Zn NCO complex on its surface and an improvement in catalytic performance. Fourier transform infrared and thermogravimetric analysis results of the spent catalysts demonstrated the formation of a greater amount of a solid Zn NCO complex over ZnAlO-550 than ZnAlO-650. Consequently, the ZnAlO-550 catalyst outperformed the ZnAlO-650 catalyst in terms of glycerol conversion (72%), glycerol carbonate yield (33%), and byproduct formation.