Beyond biodegradation: upcycling of polylactic acid plastic waste into amino acids via cascade catalysis under mild conditions

Abstract

Biodegradable plastics, represented by polylactic acid (PLA), are considered suitable alternatives to non-degradable commodity plastics; however, their degradation through microbial decomposition is seen as a waste of carbon resources and inevitably brings about CO₂ emission. Alternatively, waste PLA plastics can be utilized as feedstocks for the sustainable production of value-added chemicals, which not only avoids a carbon footprint but also realizes upcycling of plastic waste. Here, we show a cascade system that combines alkali-induced depolymerization, pulsed electrooxidation, and electrochemical reductive amination to produce alanine with an overall yield of 69% under near ambient conditions. Specifically, we first implement an alkali catalyst in an aqueous solution to depolymerize PLA into lactate. The obtained PLA hydrolysate is directly utilized as both feedstock and electrolyte for pyruvate production over a nickel hydroxide-supported Pd electrocatalyst (Pd/Ni(OH)₂) via a pulsed potential, which is followed by the transformation of pyruvate into alanine using a TiO₂ catalyst. This cascade process for alanine production benefits from high efficiencies maintained in three consecutive processes, mild operation conditions, and the use of cost-effective feedstocks (plastic wastes), without suffering harsh conditions and expensive feedstock consumption required by conventional biological and chemical approaches for alanine synthesis, respectively.