Production of adipic acid from sugar beet residue via combined biological and chemical catalysis

Adipic acid is an important industrial dicarboxylic acid that is mainly used as a precursor to nylon. Currently, commercial adipic acid is primarily produced from petrochemical reactions that are highly dependent on benzene. In view of the associated environmental and health safety issues, biotechnological production of adipic acid provides a promising alternative. Here we demonstrate a new process to convert an inexpensive renewable feedstock, sugar beet residue, to adipic acid by combining a biocatalytic step involving an engineered Escherichia coli strain and a chemical catalytic step involving a rhenium catalyst. Sugar beet residue is an abundant pectin-rich biomass, and the structural monomer of pectin is D-galactuornic acid that can be converted to mucic acid by uronate dehydrogenase. In the engineered E. coli strain, we not only overexpressed the Agrobacterium tumefaciens uronate dehydrogenase gene, udh, but also removed the endogenous uronoate isomerase gene, uxaC, and D-galactarate dehydrogenase gene, garD, to reduce intermediate and product utilization by the host cell. The resulting strain E. coli BL21(DE3)ΔuxaCΔgarD/UDH could convert pure D-galacturonic acid to mucic acid with nearly 100% conversion. Sugar beet residue was treated with commercial enzymes to release carbohydrate monomers, D-galactuornic acid, glucose and L-arabinose, and the broth was directly used as substrate for whole cell based biocatalytic reaction. From this hydrolyzed substrate, an 86% conversion yield was achieved for mucic acid production. The bioproduced mucic acid was then converted to adipic acid using the oxorhenium-complex-catalyzed deoxydehydration reaction and subsequent Pt/C-catalyzed transfer hydrogenation with high yield.