Performance and deactivation mechanism of a carbon-based solid acid catalyst in the esterification of soybean saponin acid oil

In this study, a bamboo charcoal-based solid acid catalyst (BC) was generated by one-step carbonization and sulfonation at 150 °C for 4 h in the presence of sulfuric acid. The BC was characterized by Fourier transform infrared spectrometry, elemental analysis, X-ray powder diffraction, and Brunauer-Emmet-Teller specific surface area analysis. The BC had a specific surface area of 4.54 m2/g, pore volume of 0.0142 cm3/g, and acid density of 1.476 mmol/g. Subsequently, the catalytic activity and deactivation mechanism of the BC in the esterification of soybean saponin acid oil (SAO) and methanol were investigated. The results showed that the conversion yield of esterification reached 98.28% under optimal reaction conditions (i.e., catalyst addition of 10 wt%, methanol/oil molar ratio of 10:1, temperature of 65 °C, and time of 8 h). The conversion yield decreased as reuse batches increased, and was below 80% after three batches. The loss of sulfonic acid groups from the BC occurred mainly in the first esterification batch, caused by residual sulfuric acid leaching and chemical derivatization of sulfonic acid groups. In the second to fourth esterification batches, chemical derivatization was the main reason for deactivation, where the generation of sulfonate esters and exchange of metal cations with hydrogen ions accounted for more than 75% of the chemical derivatization loss. Therefore, without impurity purification of SAO, reducing the amount of sulfonate ester generated on the catalyst was found to be essential for increasing the reuse performance of the BC and reducing the cost of biodiesel production.