Sustainable Hybrid Route to Renewable Methacrylic Acid via Biomass-Derived Citramalate

The combined chemical technologies of microbial fermentation and thermal catalysis provide a hybrid process for sustainable manufacturing of biorenewable sugar-derived monomers for plastics. In this work, methacrylic acid (MAA), a target molecule for the polymer industry, was produced from biomass-derived glucose through the intermediate molecule, citramalic acid. The biosynthetic pathway engineered in E. coli produced citramalic acid intermediate with a high yield (91% of theoretical maximum) from glucose by overexpressing citramalate synthase, removing downstream degradation enzyme 3-isopropylmalate dehydratase, and optimizing the fermentation medium. Thermal heterogeneous catalysis converted the citramalate intermediate to MAA via decarboxylation and dehydration. A selectivity of ∼71% for the production of MAA and its intermediate α-hydroxybutyric acid was achieved at a temperature of 250 °C and an acidity of 1.0 mol acid/mol citramalate. An alumina catalyst was found to enhance the selectivity to MAA in a single reactor pass from 45.6% in the absence of catalyst to 63.2%. This limited selectivity to MAA was attributed to equilibrium between MAA and α-hydroxybutyric acid, but the overall process selectivity to MAA was shown to be higher upon separation and recycle of reaction intermediates. The highest overall glucose-to-MMA yield was 0.65 mol MAA per mole of glucose. A process flow diagram was proposed of the hybrid route for the conversion of glucose to the final end product, methacrylic acid, for poly(methyl methacrylate) (PMMA).