Synthesis, characterization, and catalytic performance of sulfonated polystyrene–divinylbenzene resin for biodiesel production via oleic-acid esterification

Abstract

The high content of free fatty acids (FFA) in vegetable oils, such as oleic acid, requires an effective acid catalyst for the esterification process in biodiesel production. This investigation endeavors to synthesize sulfonated polystyrene-divinylbenzene resin (PS-DVB-S) as a heterogeneous catalyst through the polymerization of styrene and divinylbenzene, succeeded by sulfonation to yield solid acid catalyst properties. Characterization was conducted via Fourier Transform Infrared Spectroscopy (FTIR), which confirmed the successful sulfonation through the detection of the – SO₃H group absorption band within the range of 1018–1021 cm⁻¹, in contrast, thermogravimetric analysis (TGA) indicates that increasing the concentration of sulfonic acid groups reduces the thermal stability of the resin at elevated temperatures. The catalytic activity assessment in the oleic acid esterification reaction effectively diminished the acid number of oleic acid from 199.71 mg KOH/g to 20.78 mg KOH/g when utilizing the PS-DVB-S/D catalyst, achieving the highest FFA conversion rate of 89.59% and a biodiesel yield of up to 86.41%. The quality of the biodiesel produced was also commendable, exhibiting a Fatty Acid Methyl Ester (FAME) content of 94.99% alongside a minimal glycerol residue of 0.1728%, thereby approaching the quality benchmarks established for commercial biodiesel. The results of this study indicate that PS-DVB-S resin has the potential to be an effective, stable, and economical alternative heterogeneous catalyst for biodiesel production from vegetable oils.