Synthesis of Poly(Lactic Acid) from Ammonium Lactate Fermentation Broth of Food Waste

In this study, ammonium lactate from food waste fermentation broth was used as raw material to synthesize butyl lactate with butanol catalyzed by SnCl₂-modified cation exchange resin (Sn-CER) followed by direct synthesis of lactide and final polymerization to obtain poly(lactic acid) (PLA). A new Lewis acidic center was formed in the Sn-CER by complexation reaction and the maximum loading of Sn was 31%. The Lewis acid center was converted to the Bronsted acid center by combining with water during the catalytic process, which facilitated the deammoniation and esterification reactions. The conversion yield of butyl lactate reached 87.7% at an esterification time of 6 h, an alcohol/ammonium lactate molar ratio of 3:1, an initial concentration of 30 wt% of ammonium lactate and a catalyst loading of 1.5 wt%. The synthesized butyl lactide was converted into lactide in the presence of stannous octanoate, and the yield reached 61.6% under optimal conditions (polymerization temperature of 160 °C, polymerization time of 7 h, and catalyst loading of 1.0 wt%). PLA with a molecular weight of 8.05 × 10⁴ was synthesized by the polymerization reaction of lactide at 160 °C for 5 h. Therefore, it is feasible to synthesize PLA from food waste, which is an effective way to resource the waste.