Succinate, itaconate, FDCA and BDO

These four molecules are usually named in one breath as bio-based platform monomers. The list is convenient and misleading: their chemistry differs and, more to the point, so did their outcomes. They are worth taking one at a time, each with its result attached.

Succinic acid: the best stoichiometry, the worst outcome

Succinate is the one molecule in this cluster whose pathway fixes carbon dioxide rather than losing it. The reductive branch of the TCA cycle carboxylates phosphoenolpyruvate to oxaloacetate, and one mole of CO2 ends up in the product. The theoretical yield therefore exceeds unity by mass — around 1.1 g per gram of glucose, unusual for an organic acid. Titres above 100 g/L have been published for several hosts, from Actinobacillus succinogenes to engineered yeasts able to work at low pH.

The commercial ventures of the 2010s nonetheless closed one after another. The reason was not the fermentation. Succinic acid is a small diacid, fully water-soluble and non-volatile; recovering it means crystallisation after acidification, or a salt route, or electrodialysis, and every scheme adds more cost than the sugar itself. Meanwhile the market proved smaller than promised: succinate was offered as a replacement for adipic acid and maleic anhydride, and those buyers wanted a price, not a provenance. This is the canonical illustration of the cluster’s thesis — working chemistry does not rescue you from separation cost.

BDO: the one that arrived

1,4-butanediol is made by direct fermentation over a non-natural pathway assembled from enzymes of different origins: succinyl-CoA is reduced to 4-hydroxybutyrate, which is activated and reduced twice more to the diol. The route works, has been licensed, and runs at a commercial plant. What matters is that BDO is a neutral alcohol, not an acid: no neutralisation, no salt, no gypsum, and the product can be distilled. Set against succinate the conclusion is almost literal — same organism class, same sugar, different product class, different recovery economics.

Itaconate: a niche, not a platform

Itaconic acid is made by Aspergillus terreus through decarboxylation of cis-aconitate, a TCA-cycle intermediate. Decarboxylation means carbon lost: the ceiling is one mole of itaconate per mole of glucose, about 0.72 g/g. Titres on the order of a hundred grams per litre are achievable. The product goes into latex co-polymers and specialty resins; it never displaced acrylic acid at volume, and it remains an honest niche.

FDCA: the bottleneck is not the fermentation

2,5-furandicarboxylic acid is interesting as a replacement for terephthalic acid: polyethylene furanoate has a higher glass transition temperature and better barrier properties than PET. But it is not fermented. It comes from hydroxymethylfurfural, which comes from the triple dehydration of fructose — and that is where the problem sits. HMF is unstable; in hot acidic water it rehydrates to levulinic and formic acids and condenses into insoluble humins, so the yield on fructose is always below stoichiometric and falls as concentration rises. The subsequent oxidation of both side groups to the diacid runs over gold or platinum catalysts, or enzymatically with oxidases, and selectivity there is achievable. The losses at the HMF step are what still decide the economics.