Tuning the Basic Properties of ZnAl Hydrotalcites Modified with Ce Applied to Transesterification of Soybean Oil

Abstract

The basicity of ZnAl hydrotalcite type materials was modified through the incorporation of Ce into their structure and used as heterogeneous catalysts in the transesterification of soybean oil. A series of novel ZnAl–Ce(X) catalysts was synthesized by the co-precipitation method varying the loading of Ce (X = Ce/Al molar ratio). The larger ionic radius of Ce3+ (1.01 Å) compared with Al3+ (0.53 Å) and Zn2+ (0.72 Å), hinders the appropriate incorporation of Ce3+ in the brucite-like structure, causing a small segregation of Ce3+ as CeO2. However, the concentration of basic sites (acid–basic Lewis pairs) and specific surface area of the ZnAl–Ce(0.0) sample were enhanced with the incorporation of Ce, improving its catalytic activity. The most appropriate Ce loading was obtained with a Ce/Al molar ratio of 0.03, which was attributed to its large surface area, along with a higher amount of acid–basic Lewis pairs related to the presence of aluminum in pentahedral coordination. Thus, the total amount of basic sites (−OH and M–O−) of ZnAl–Ce(0.03) was triplicated, leading to a FAME yield of 80% with this catalyst, which represents an increase of 11.3% when compared with the ZnAl hydrotalcite without Ce.