Effect of Calcination Temperatures on Surface Properties of Spinel ZnAl(2)O(4) Prepared via the Polymeric Citrate Complex Method—Catalytic Performance in Glycerolysis of Urea

In this study, we investigated urea glycerolysis over ZnAl(2)O(4) 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(3+) and Zn(2+) cations led to the formation of a disordered bulk ZnAl(2)O(4) phase, different calcination temperatures strongly influenced the surface properties of the ZnAl(2)O(4) catalysts, including oxygen vacancy. The increase in the calcination temperature from 550 °C to 650 °C decreased the inversion parameter of the ZnAl(2)O(4) structure (from 0.365 to 0.222 for AlO(4) and 0.409 to 0.358 for ZnO(6)). The disordered ZnAl(2)O(4) structure led to a decrease in the surface acidity. The ZnAl(2)O(4)-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 ZnAl(2)O(4)-550 than ZnAl(2)O(4)-650. Consequently, the ZnAl(2)O(4)-550 catalyst outperformed the ZnAl(2)O(4)-650 catalyst in terms of glycerol conversion (72%), glycerol carbonate yield (33%), and byproduct formation.