Preliminary Investigation of Organocatalyst Activity Based on <i>C</i>-Arylcalix[4]-2-Methylresorcinarene Sulfonic Acid Materials for Biodiesel Production

At present, biodiesel remains an untapped fuel source while crude oil reserves are being depleted. Extensive efforts are necessary to achieve efficient biodiesel production, but catalytic activity in methyl palmitate production remains unsatisfactory. In this study, we report the synthesis and organocatalyst activity of C-arylcalix[4]-2-methylresorcinarene sulfonic acids for biodiesel production. The presence of a methyl group on an aromatic ring is expected to disturb the intramolecular hydrogen bonds; thus, each hydroxyl group could be utilized for the catalytic process. C-arylcalix[4]-2-methylresorcinarene sulfonic acids were prepared from cyclocondensation and sulfonation reactions yielding compounds 1-6 at rates of 66.7%-86.6%. By evaluating the organocatalytic activity of the C-arylcalix[4]-2-methylresorcinarene sulfonic acids, we found that 4 mol% of compound 5 exhibited the highest yield (97.8%) of methyl palmitate at 338 K for 6-h reaction time, which is remarkable. These findings show that C-arylcalix[4]-2-methylresorcinarene sulfonic acids are promising heterogeneous catalyst materials for biodiesel production.