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 E260-20-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 degrees C with E-260-20-SO3H and E-P400-2-SO3H, respectively, with 5 cycles for E-260-20SO(3)H (>= 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 degrees 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. (C) 2016 Elsevier B.V. All rights reserved.