Alkaline earth metal oxides supported on WO3@MCM-41; bifunctional catalysts for biodiesel production from corn and waste cooking oil

Abstract

The novelty of this work lies in the development of a bifunctional acid-base catalyst comprising tungsten oxide (WO₃) supported on mesoporous MCM-41 and impregnated with alkaline earth metal oxides (Mg, Ca, Sr, Ba), which enables simultaneous esterification and transesterification for biodiesel production. The acidic and basic properties of the catalysts were quantitatively assessed through potentiometric titration, confirming the presence of both acid and basic sites, which are essential for the concurrent esterification of free fatty acids (FFAs) and transesterification of triglycerides. This bifunctionality is particularly advantageous for feedstocks with high FFA content, such as waste cooking oil, addressing a key challenge in biodiesel synthesis. The SrO-based catalyst demonstrated exceptional performance, achieving yields of 96 % for corn oil and 93 % for waste cooking oil, with minimal yield differences of 3 % attributed to the latter's impurities and higher FFA content. The study also provides detailed optimization of reaction conditions, revealing that waste cooking oil requires slightly harsher conditions (100 °C, 6 h, 20 mg catalyst) compared to corn oil (80 °C, 5 h, 10 mg catalyst), which is consistent with its complex composition. Comprehensive characterization (PXRD, SEM-EDX, BET/BJH) and validation through ¹H NMR spectroscopy (yields of 92 % and 91 % for corn and waste cooking oil, respectively) confirm the reliability of the results. Moreover, the biodiesel's physicochemical characteristics were examined thoroughly and contrasted with the ASTM biodiesel requirements. The outcomes demonstrated that the generated biodiesel's characteristics met the requirements of international standards.