Preparation of meso-structured silica–calcium mixed oxide (MSCMO) catalyst for converting Vietnamese rubber seed oil to biodiesel

Abstract

This report covered some new contributions in catalyst preparation and characterization. Meso-structured silica–calcium mixed oxide catalyst possessed both acidic and basic sites was synthesized through co-condensation method in alkaline environment using tetraethylorthosilicate, CaO, and cetyltrimethylammoniumbromide. The co-condensation process was established at 90 °C for 24 h obtaining white-gel precipitate which was dried at 120 °C followed by calcination at 550 °C for 5 h. The as-synthesized catalyst was used in conversion of rich free fatty acid rubber seed oil (22 %wt) in Vietnam to fatty acid methyl esters (FAMEs) in mild conditions such as temperature of 120 °C, time of 4 h, catalyst dosage of 3 %wt, methanol/oil mass ratio of 2.5/1 and agitating speed of 550 rpm achieving the reaction yield of 95.4 %. The catalyst were characterized by various techniques such as X-ray diffraction, transmission electron spectroscopy, Nitrogen Adsorption–Desorption Analysis (BET), temperature programmed desorption (NH₃ and CO₂-TPD). Especially, X-ray absorption spectroscopies was applied to explain the occurrence of acid and base sites on catalysts surface. The analysis showed the sixfold coordinated calcium sites characterizing for the mixed oxide structure of CaO–SiO₂. The results helped to simulate the bonding structure around the Ca sites indicating the electrostatic charge differences along the Ca–O–Si connections and the ability for occurring the defect sites containing the O²⁻ moieties corresponding to the acidity and basicity of the catalysts respectively. Gas chromatography–mass spectroscopy was also used to determine the composition of the FAMEs showing high purity of these products.