Organo-sulfonic acids doped polyaniline – based solid acid catalysts for the formation of bio-esters in transesterification and esterification reactions

In the present work catalytic reactivity of polyaniline protonated with organo-sulfonic acids, namely methanosulfonic (MSA-Pani), camphorosulfonic (CSA-Pani) and lignosulfonic (LG-Pani) acids was evaluated for the transesterification of triglycerides (triacetin, castor oil) and esterification of fatty acid (ricinoleic) with methanol. Their reactivity was also compared with previously studied alkyl-free polyaniline-sulfate (S-Pani) catalyst. The textural and acid properties of the catalysts before and after catalytic tests were characterized by BET, FT-IR, XRD, SEM, thermogravimetric and ammonia sorption techniques. It was observed that acid characteristics, namely concentration and the strength of acid sites as well as chemical properties of the surface e.g. the type of organo-sulfonic acid played an important role in catalytic performance of studied samples. In all tested reactions, the MSA- and CSA-doped polyanilines exhibited the highest initial activity referred to the acid site capacity. However, too weak acid sites in these catalysts resulted in their partial blockage during methanolysis of triglycerides. This was not observed in the esterification reaction. Among all studied samples, the LG-Pani (commercial reagent) and MSA-Pani catalysts seem to be the most promising owing to their high activity and good stability in reaction mixture, in particular during methanolysis of triglycerides. In contrast to the MSA-Pani catalyst (thermally stable up to 260 °C), lower stability of LG-Pani catalyst (up to ca. 120 °C) may limit its application.