C-Arylcalix[4]pyrogallolarene Sulfonic Acid: A Novel and Efficient Organocatalyst Material for Biodiesel Production

The synthesis of C-arylcalix[4]pyrogallolarene sulfonic acid derivatives and their application as an organocatalyst material for biodiesel production were investigated. The C-arylcalix[4]pyrogallolarene derivatives were prepared in high yields through a two-step reaction: condensation between pyrogallol and aromatic aldehydes (i.e. benzaldehyde, 4-hydroxy-3-methoxybenzaldehyde and 4-ethoxy-3-methoxy benzaldehyde) and then followed by a sulfonation reaction with sulfuric acid. They were evaluated as the organocatalyst for the esterification reaction of palmitic acid with methanol as a representation of biodiesel production. Using 4 mol% of C-4-hydroxy-3-methoxyphenylcalix[4]pyrogallolarene sulfonic acid, methyl palmitate was generated in up to 91.9% yield after 4 h at 65 °C, making it similarly efficient to sulfuric acid that is used as the catalyst in the conventional reaction under similar conditions. However, different from sulfuric acid, the C-4-hydroxy-3-methoxyphenylcalix[4]pyrogallolarene sulfonic acid could be recovered by using a simple filtration technique making it better than sulfuric acid from industrial and environmental points of view. These results demonstrate that the C-arylcalix[4]pyrogallolarene sulfonic acid derivatives are a novel and potential organocatalyst for methyl palmitate biodiesel production.