A green sulfonated carbon-based catalyst derived from coffee residue for esterification

A green sulfonated carbon-based catalyst was successfully synthesized through sulfonation of incompletely carbonized coffee residue (SCAC catalyst). The sulfonation temperature was investigated and the catalytic activity was tested via esterification of caprylic acid. SCAC-200, the highest caprylic acid conversion at 4 h (71.5%) and initial TOF based on SO3H acid sites, was synthesized under a carbonization temperature of 600 °C for 4 h and sulfonation temperature of 200 °C for 18 h. Sulfonation temperature plays a dominant role in determining the SO3H site density of SCAC catalysts because side reactions (condensation/oxidation/dehydrogenation) take place at high sulfonation temperature as indicated by oxygen-to-carbon surface ratio. The activities of the SCAC catalysts were also substantially greater than that of Amberlyst-15. A high catalytic activity and catalyst stability for esterification of HCp were achieved in the SCAC catalysts with high surface area and by balancing strong (SO3H) and weak (carboxylic and phenolic) acid site densities. Additionally, this catalyst could be regenerated to obtain essentially its initial catalytic activity by MeOH washing. Therefore, the sulfonated coffee residue derived catalyst is promising, economic eco-friendly and potentially substituted for homogeneous H2SO4 catalyst for esterification in industries in the near future.