Critical factors for levulinic acid production from starch-rich food waste: solvent effects, reaction pressure, and phase separation

A considerable amount of food waste generated globally could be upcycled to synthesise platform chemicals to enhance environmental sustainability and realise a circular economy. This study investigates the catalytic production of the vital platform molecule levulinic acid (LA) from bread waste, a typical stream of starch-rich food waste generated worldwide. Gamma-valerolactone (GVL), isopropanol (IPA), and propylene carbonate (PC) were evaluated as bio-derived and CO2-derived green co-solvents for LA synthesis. In-vessel pressure generated in PC/H2O (1 : 1) solvent was conducive to rapid LA production from bread waste compared to GVL/H2O and IPA/H2O. In PC/H2O, 72 mol% total soluble product yield was observed quickly within 1 min in moderate reaction conditions (130 °C, 0.5 M H2SO4), whereas ∼15–20 mol% of LA could be obtained when the reaction was prolonged for 10–20 min at 130 °C. The yield of LA could be significantly enhanced in GVL/H2O through phase separation using NaCl (30 wt%(aq)). LA yield increased up to a maximum of ∼2.5 times in the biphasic system (28 mol%, 150 °C, 15 min) (representing a theoretical yield of 66%) in GVL/H2O (1 : 1) compared to the monophasic system (∼11 mol%) under the same reaction conditions. The partition coefficient for LA achieved was 4.2 in the GVL/H2O (1 : 1) biphasic medium, indicating that the system was efficient for simultaneous production and extraction of LA. Biphasic GVL/H2O facilitated selective LA production, which could be optimised by tuning the reaction conditions. These new insights can foster the development of high-performance LA production and sustainable biorefinery.