This was attributed to low of lignin conversion and the large number of unit Alternative renewable production routes to adipic acid have been reported, including synthesis from HMF, where FDCA was obtained via the oxidation of HMF and subsequently deoxygenated to form adipic acid (Scheme 7); 92 and from succinic acid, which was hydrogenated into 1,4-butanediol and then converted into adipic acid through carboxylation with carbon monoxide (Scheme 8). Adipic acid have also been produced directly from glucose by a variety of metabolic pathways. The various preparation methods were summarized and compared for sustainability and cost competitiveness in a review by Rios et al. 96 Unlike the above-mentioned monomers, bio-based succinic acid (SA) has only been produced directly via fermentation of carbohydrates or biomass feedstocks. Wild bacteria and fungi as well as metabolically engineered strains have been employed, and the fermentation of a great variety of substrates, including glucose, glycerol, bagasse, wood hydrolysates, food wastes, etc., has been studied. Again, crop residues and food wastes are environmentally preferable but their sustainability advantages are undermined by challenges in the efficient utilization of their carbohydrate components.