Sulfonated activated carbon acid (AC-SO₃H) can catalyze both esterification and transesterification to produce biodiesel from oils with high acid value (AV) without pretreatment. Because, activated carbon has properties like its surface oxides, reducibility, and stability in both acidic and basic media, as well as its structural resemblance to graphite , fullerenes and nanotubes to support -SO₃H group well. However, the separation of AC-SO₃H catalyst needs filtration or centrifugation that is energy and time consuming. So, many magnetic carbonaceous acids were successfully prepared that are easily separated by a magnet for cellulose hydrolysis, fructose dehydration and hydrolysis of polysaccharides. However, these catalysts have low acid content (e.g., 1.3, 1.95 and 0.38 mmol/g) for effective biodiesel production, some have low magnetism.

Prof. FANG Zhen and his team of Xishuangbanna Tropical Botanical Garden (XTBG) conducted a study aiming to synthesize magnetic carbonaceous acids with high acidity and strong magnetism for biodiesel production from oils with high acid value. A novel route with five steps by double hydrothermal precipitation and pyrolysis, as well as sulfonation was used for the catalyst synthesis:

(i and ii) magnetic core → (iii) magnetic carbon → (iv) carbonized magnetic carbon → (v) magnetic carbonaceous acid (catalyst).

It was found that pyrolysis temperature at 600 °C led to excellent structure to produce catalyst with high acid density (2.79 mmol/g) and strong magnetism (14.4 Am²/kg). The catalyst presents high active, stable and recoverable in the production of Jatropha biodiesel from crude oil with high AV (17.2 mg KOH/g) with high yields for 3 cycles (90.5%, 91.8%, 90.3%), slight reduction in total acid density (2.43 vs. 2.79 mmol/g) and high catalyst recovery rate of 96.3%. It was also found that ultrasound can resist free fatty acids. High biodiesel yield (90.7%) was still achieved from high AV oil (4.8) at low US energy density (0.1 W/mL) with Na₂SiO₃ catalyst.

The study entitled “Biodiesel production direct from high acid value oil with a novel magnetic carbonaceous acid” has been published online in Applied Energy.

 Biodiesel production and catalyst separation in reaction quartz cup: (a) before and (b) after reaction

(Image by ZHANG Fan)