Use of Unmodified Coffee Husk Biochar and Ashes as Heterogeneous Catalysts in Biodiesel Synthesis

Biochar and ashes derived from coffee husk were used as heterogeneous catalysts in the synthesis of biodiesel through a transesterification reaction. The catalysts were obtained from the carbonization, activation, and combustion of raw coffee husks. Their properties were characterized by solid-state 13C and 31P nuclear magnetic resonance (NMR) spectroscopies and X-ray diffraction, among other techniques. The results evidenced the presence of different inorganic compounds (mostly K- and Ca-containing phases) mixed with the turbostratic structure (in the case of the biochar samples). A biodiesel conversion of 66% (evaluated by 1H NMR analysis of the liquid reaction products) was found for the biochar sample prepared at 700 °C; the activated biochar catalyst prepared at the same temperature showed a higher biodiesel conversion (74%), which can be attributed to its superior specific surface area. The best catalytic efficiency (biodiesel conversion of 93%) was observed for the coffee husk ashes, which is consistent with the higher contents of Ca and K salts in the ashes in comparison with the biochar samples. Reuse tests conducted with the ashes samples showed an efficiency reduction after the second cycle (from around 90% in the first two cycles to 8% in the third cycle), due to the partial removal of active phases (mostly K-containing salts) within the reaction medium. The presented results show that coffee husks are a cheap and environmentally viable source for the production of materials of interest in heterogeneous catalysis, with no need for chemical modification to achieve good efficiency.