Preparation, application, and optimization of Zn/Al complex oxides for biodiesel production under sub-critical conditions

Biodiesel produced by transesterification is a promising green fuel in the future. A new heterogeneous catalyst, Zn/Al complex oxide, was prepared for biodiesel production. The results showed that the catalyst derived from a hydrotalcite-like precursor with a zinc/aluminum atom ratio of 3.74:1 and calcined at 450 °C gave the highest conversion of 84.25%. Analysis of XRD, XPS, FI-IF, TG-DTA, BET and alkalinity tests demonstrated that it is the unique structure of hydrotalcite-like compound precursor that gave the catalyst a high alkalinity greater than 11.1. The optimal reaction condition for Zn/Al complex oxide was under methanol sub-critical condition: 200 °C, 2.5 MPa, 1.4% (wt) catalyst dosage, and 24:1 methanol to oil ratio. Under these conditions, the conversion reached 84.25% after 90 min, which was better than Mg/Al complex oxides. The excellent tolerance to water and free fatty acid was exhibited when the oil feed had fewer than 6% FFA or 10% water content with a conversion greater than 80%.