Sulfonated activated carbons as potential catalysts for biolubricant synthesis

[Display omitted] •Sulfonated activated carbons were prepared by aromatic electrophilic substitution.•The effect of sulfonic groups on esterification with long-chain alcohols was evaluated.•The modified activated carbons were assessed by XPS, FTIR, N2 isotherms, TGA and SEM.•Esterification of oleic acid with long-chain alcohols were monitored by 1H NMR.•Conversion and selectivity were improved when using the sulfonated activated carbons. In this study, sulfonated activated carbons have been prepared, under different conditions, with the purpose of evaluating the effect of the nature and amount of sulfonic surface groups on the esterification reaction of free fatty acids (FFA) with different long-chain alcohols. The synthesized catalysts were characterized using different techniques and 1H NMR was used for monitoring the reaction products. The modifications of the surface functionalities were assessed by X-ray Photoelectron Spectroscopy (XPS) and Thermogravimetric analysis (TGA), while changes in the porous network and morphology of the samples were evaluated before and after the treatment of the original activated carbon sample. XPS results showed the presence of two types of sulfur, one from thiophenic sulfur (present on all materials, including the unmodified sample), and the other from sulfonic groups (SO3H), at 168 eV (present only in the modified samples). These catalysts were applied in the esterification reaction and presented excellent catalytic performances, while the original activated carbon exhibited conversions similar to reactions without any catalyst. On the other hand, the conversion of fatty acids when using the modified carbons improves significantly with values up to ∼100 % to mono alcohols and 70 % to trimethylolpropane.