The role of surface Zn2+ ions in the transesterification of vegetable oils over ZnO supported on Al2O3 and Fe2O3

Abstract

Samples of ZnO–Al₂O₃ and ZnO–Fe₂O₃ 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, ZnAl₂O₄ and ZnFe₂O₄. The catalytic activity of the supported spinel samples, and samples of stoichiometric ZnAl₂O₄ 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.