Platform chemicals
Bio-based acrylic acid
Why the biological route goes through 3-hydroxypropionic acid, what its theoretical yield on glucose is, and why the dehydration step and product toxicity set the limit rather than the microbiology.
Acrylic acid is a three-carbon monomer carrying a double bond and a carboxyl group, and it goes into superabsorbents, paint dispersions and adhesives. The petrochemical route is simple and mature: propylene is oxidised in two stages via acrolein over mixed-oxide catalysts at high selectivity. What blocks a biological replacement is not a missing enzyme but the properties of the molecule itself.
Why it is not fermented directly
Acrylic acid is an activated electrophile that adds thiols by Michael addition — meaning it attacks protein cysteines and glutathione. In the undissociated form it also crosses the membrane and collapses the proton gradient. A living cell tolerates it at fractions of a gram per litre, two to three orders of magnitude below what makes recovery affordable. The second problem is chemical: acrylic acid polymerises spontaneously, and commercial product always ships with an inhibitor. A fermenter full of warm aqueous solution is not where you want that molecule to accumulate.
Hence the detour. The microbe makes a stable, non-toxic, saturated precursor and chemistry installs the double bond afterwards. The precursor is 3-hydroxypropionic acid (3-HP), dehydrated to acrylic acid.
The carbon account
Several pathways reach 3-HP. The malonyl-CoA route starts from acetyl-CoA: pyruvate dehydrogenase loses a CO2, acetyl-CoA carboxylase puts the same carbon back, and malonyl-CoA is reduced to 3-HP by a two-step reductase. On paper the carbon closes — one glucose gives two 3-HP, a theoretical mass yield near 1 g/g — but the route demands more reducing equivalents than glycolysis supplies, and the cell pays for them by oxidising extra substrate. The alternatives carry their own costs: the β-alanine route through aspartate needs a transamination, and the glycerol route depends on a coenzyme-B12-dependent dehydratase and on glycerol as the feedstock. Published 3-HP titres sit in the tens of grams per litre, the best approaching a hundred — an industrial order of magnitude, but visibly short of the ceiling. The gap is carbon diverted to biomass, intermediates leaking out, and product inhibition.
Where the money is actually lost
Two places. First, the dehydration. Thermodynamically it is easy, but hot aqueous 3-HP decarboxylates and forms by-products, and acrylic acid polymerises on the catalyst and plugs its pores. Selectivity there is the whole engineering problem. Second, recovering 3-HP from dilute broth: it is a small oxygenated acid, fully water-miscible and non-volatile, so it can be neither distilled off nor simply extracted. Neutralising gives a salt from which the acid must be liberated again — either with acid, producing a sulfate waste stream, or by electrodialysis, at an energy cost.
The bio-acrylate actually on the market today sidesteps this chain altogether: bioethanol-derived ethylene is esterified with conventional acrylic acid, so the renewable claim covers the alcohol half of the ester, not the monomer. That is an honest but partial substitution, and the distinction is worth holding when reading a label.