Hypercrosslinked polystyrene sulphonic acid catalysts for the esterification of free fatty acids in biodiesel synthesis

New sulphonic acid catalysts supported on hypercrosslinked polystyrene have been studied in the esterification of oleic acid with methanol and in the rearrangement of α-pinene to camphene and limonenes. The catalysts have been characterised in terms of specific surface areas and porosities, affinities for water and for cylcohexane vapours, and both concentrations and strengths of acid sites. They have been compared with conventional macroporous polystyrene sulphonic acids (Amberlysts 15 and 35) and SAC-13, a composite between Nafion and silica. The results show that the hypercrosslinked polystyrene sulphonic acids, despite exhibiting relatively low concentrations of acid sites and acid site strengths below those of Amberlysts 15 and 35, are very much more catalytically active than conventional resins in reactions such as the esterification in which high acid site strengths are not required. It is thought that this is due to the highly accessible acid sites throughout the catalyst particles. Reusability studies are reported and it appears that the temperature at which the catalyst is used is important in controlling and minimising catalyst deactivation.

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► Hypercrosslinked polystyrene sulphonic acid resins are active solid acid catalysts.
► Highly accessible acid sites on hypercrosslinked polystyrene sulphonic acid resins.
► New solid acid catalysts for free fatty acid esterification in biodiesel synthesis.