The role of surface Zn²⁺ ions in the transesterification of vegetable oils over ZnO supported on Al<inf>2</inf>O<inf>3</inf> and Fe <inf>2</inf>O<inf>3</inf>

Samples of ZnO-Al2O3 and ZnO-Fe2O 3 with different loadings of ZnO (5-20 wt%) were prepared by impregnation of the supports (γ-Al2O3 and α-Fe2O3) with ZnNO3 and calcination at 873 K. XRD studies of the calcined samples revealed that the ZnO had reacted with the support to form the corresponding spinels, ZnAl2O 4 and ZnFe2O4. The catalytic activity of the supported spinel samples, and samples of stoichiometric ZnAl2O 4 and ZnFe2O4 prepared by co-precipitation were examined for the transesterification of sunflower oil, waste cooking oil and Jatropha oil. Linear relationships between the spinel content, estimated by XRD, and surface Zn concentration estimated by XPS and transesterification activity of the samples were obtained. XVB (X-ray valence band) studies provided evidence to suggest that the Zn 3d electrons may have played a major part in the electronic excitation of the spinels (ZnAl2O4 and ZnFe2O4) and, therefore, their catalytic activity. © 2014 The Royal Society of Chemistry.