Concept of rice husk biorefining for levulinic acid production integrating three steps: Multi-response optimization, new perceptions and limitations

Valuable intermediates for solvents, biofuels and biopolymer precursors are produced from levulinic acid (LA). In this study, rice husk (RH) was fractionated using a three-step biorefining approach for LA production: (1) acid pretreatment, (2) alkaline pretreatment and (3) catalytic depolymerization of cellulose. In third step, the H2SO4-acid concentration (A(d)), reaction temperature (T-d) and reaction time (t(d)) on the LA concentration (C-LA), selectivity (S-LA) and yield percentage based on the theoretical value (Y-t,Y-LA), were investigated. Optimal values for C-LA of 27.2 g/L, S-LA of 62.2% and Y-t,Y-LA of 60.6 mol% were obtained at A(d) of 5.0% w/v, T-d of 175 degrees C and t(d) of 75 min. A scenario based on 100 kg of dry RH produced 11.8 kg of LA with a yield of 42.9 mol% based on the theoretical values of the cellulose in the RH, as well as 12.5 kg of xylose, 12 kg of lignin and 2.9 kg of formic acid. Furthermore, the potential route to LA production with the requirement to remove the ash and with a low production of carbonaceous species (similar to 2.5 kg) was highlighted in this study. These results offer new insight into selective conversion of RH to bio-based products.