Catalytic esterification of fatty acids using solid acid catalysts generated from biochar and activated carbon

Reusable, solid acid carbon supported catalysts were generated from biomass by pyrolysis (400–500 °C) to generate a soft to hard carbon backbone (i.e., biochar) for addition of acidic functional groups. Acid catalysts were synthesized by sulfonating the biochar and wood derived activated carbon using concentrated H2SO4 at 100, 150 and 200 °C (12 h) and gaseous SO3 (23 °C). Attenuated Total Reflectance, sulfur, and NH3-TPD analysis of the sulfonated carbons indicated the presence of single bondSO3H groups on the 100 °C sulfonated biochar and activated carbon (AC), with higher active site densities (SO3H density) for the SO3 sulfonated material. The sulfonated carbons were tested for their ability to esterify free fatty acids with methanol in blends with vegetable oil and animal fat (5–15 wt.% FFA). Esterification of the fatty acids was typically complete (∼90–100% conversion) within 30–60 min at 55–60 °C (large methanol excess), but decreased with lower methanol to oil ratios using the biochar catalysts (e.g., 70%, 6 h, 20:1). Solid acid catalysts derived from wood based activated carbon had significantly higher activity compared to the biochar derived catalysts (e.g., 97%, 6 h, 6:1). Of the synthesized biochar catalysts, 400 °C pyrolyzedpine chip biochar, sulfonated at 100 °C, resulted in the highest reaction rate and lowest reduction in conversion (or deactivation) when reused multiple times. Drying the biochar catalysts for 1 h at 125 °C between uses maintained esterification activity, allowing the catalysts to be reused up to 7 cycles. For the SO3 sulfonated AC catalyst, such a regeneration step was not required, as the fractional conversion of palmitic and stearic acid (5% FFA, 10:1, 3 h) remained >90% after 6 cycles.