Biodiesel production from high acid value oils with a highly active and stable bifunctional magnetic acid

Biodiesel production was catalyzed by a novel magnetic carbonaceous acid (Zr-CMC-SO 3 H@3Fe-C 400 ) with both Brønsted and Lewis sites synthesized by a four-step method: (i) metal (Fe) ion chelation, (ii) calcination, (iii) metal (Zr) ion chelation and embedding, and (iv) sulfonation. It catalyzed the esterification of oleic acid with 97% biodiesel yield, transesterification of high acid value (AV) soybean oil with 95% biodiesel yield, and pretreatment of Jatropha oil with AV reduced from 17.2 to 0.7   mg KOH/g. Biodiesel yields (>90%) at 90   °C for 4   h reaction time were obtained for ten cycles by easy magnetic separation which showed potential practical applications in the field of green production. The synthesized catalyst was characterized with elemental analysis, XRD, ICP-OES, FT-IR, BET, VSM, SEM-EDX, HRTEM, TG-DSC and Boehm titration.