Functionalized hydrochar-based catalysts for biodiesel production via oil transesterification: Optimum preparation conditions and performance assessment

Abstract

Hydrothermal carbonization of coconut endocarp (Cocos nucifera) and its activation with KOH has been analyzed for the preparation of catalysts to obtain biodiesel. A Taguchi L9 experimental design was utilized to enhance the performance of these catalysts where the effect of biomass/H2O loading, KOH concentration, hydrothermal carbonization time and temperature on the formation of fatty acid methyl esters (FAME) was assessed. Results showed that the raw hydrochars lacked catalytic properties for oil transesterification (FAME yield of 1.63%). KOH activation improved the catalytic properties thus leading to FAME yield from 59.8 to 98.7%. The statistical analysis of Taguchi experimental design via the Signal-to-Noise (S/N) ratio allowed to determine the conditions to prepare the best catalyst to obtain biodiesel via the transesterification of safflower oil with methanol. This catalyst showed 99% FAME yield. Specific reaction rates of oil transesterification were also calculated with the pseudo-second order model at 50–70 °C, which ranged from 0.0887 ± 0.0035 to 0.2492 ± 0.0099 L/mol min, respectively. This transesterification was endothermic with an activation energy of 47.9 ± 1.916 kJ/mol. Two routes were evaluated and compared to regenerate the spent catalyst. Finally, the physicochemical characterization of biomass precursor, hydrochars and catalysts was carried out with XRD, FTIR, XRF, SEM and BET analysis. This study showed that the hydrothermal carbonization is an environmentally friendly and low energy intensive route to obtain solid catalysts from lignocellulosic biomass for biodiesel production.