Biodiesel production catalyzed by highly acidic carbonaceous catalysts synthesized via carbonizing lignin in sub- and super-critical ethanol

•Lignin chars are obtained by carbonization in super- and sub-critical ethanol.•Sulphonated chars as catalysts with high acid content of 5.05–5.35 mmol H+/g.•Biodiesel yield of 95.5% is achieved from oleic acid with 5 cycles (> 81%).•Biodiesel yield of 90.9% from Jatropha oil with AV of 17.2 mg KOH/g.•Biodiesel yield of 93.2% from blended soybean oil with AV of 22.1 mg KOH/g.

Carbonaceous acids were synthesized from dealkaline lignin via solvothermal carbonization, pyrolysis and sulfonation. Carbonization of lignin in sub- and super-critical ethanol provided good surface properties with abundant functional groups (2.81 and 1.35 mmol [H+]/g) for the subsequent sulfonation to result in high active and stable catalysts for biodiesel production. Both catalysts (E-P400-2-SO3H and E-260-20-SO3H carbonized in sub- and super-critical ethanol) had high acid content of 5.35 and 5.05 (mmol [H+]/g), respectively. Highest biodiesel yields of 95.4% and 95.5% were obtained from the esterification of oleic acid at 80 °C with E-260-20-SO3H and E-P400-2-SO3H, respectively, with 5 cycles for E-260-20-SO3H (≥81.9% yield) and 3 cycles for E-P400-2-SO3H (≥84.6% yield). Both catalysts also applied to produce biodiesel from actual plant oils at 220 °C. High biodiesel yields of 90.9% and 93.2% were obtained from Jatropha and blended soybean oils with high acid value of 17.2 and 22.1 mg KOH/g, respectively.

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