Biocatalytic conversion

Biocatalytic conversion uses enzymes or whole organisms to reduce CO2 instead of a metal catalyst at temperature. The attraction is the opposite of the thermochemical case: reactions run at ambient temperature and pressure in water, and selectivity comes from the enzyme's active site rather than from process control, so a single product can be obtained where a metal catalyst would give a distribution.

Three quite different things travel under this heading, and they have different maturity levels. Isolated enzymes reducing CO2 in a reactor; synthetic multi-enzyme cycles assembled outside any cell; and whole-cell gas fermentation, which is the only one operating at industrial scale.

Isolated enzymes

The enzymatic route to reduced carbon runs through dehydrogenases in sequence: formate dehydrogenase reduces CO2 to formate, formaldehyde dehydrogenase takes it to formaldehyde, and alcohol dehydrogenase to methanol [1]. Each step is specific and mild, which is the appeal.

The problem is what drives it. Each reduction consumes a nicotinamide cofactor, and the cofactor is expensive enough that consuming it stoichiometrically destroys the economics of the process. Cofactor regeneration, and the search for cofactor-free alternatives, is therefore not a refinement of enzymatic CO2 reduction but its central unsolved problem, alongside enzyme immobilisation for stability and reuse, and the low solubility of CO2 in water [1].

Synthetic pathways

Rather than borrowing a natural CO2-fixing pathway, it is possible to design one. The CETCH cycle is a reaction network of 17 enzymes drawn from nine organisms spanning all three domains of life, assembled by metabolic retrosynthesis and then refined by enzyme engineering, that fixes CO2 continuously in vitro at about 5 nanomoles of CO2 per minute per milligram of protein. It is a seventh route alongside the six CO2 fixation pathways that evolution produced [2].

That is a genuine achievement in design. It is also worth reading the rate carefully: nanomoles per minute per milligram of protein is a laboratory quantity, and the distance from there to a process is the substance of the field's remaining work rather than a detail.

Gas fermentation, which actually runs

The one biocatalytic route operating at industrial scale converts waste gases rather than pure CO2. Engineered Clostridium autoethanogenum has been used to produce acetone and isopropanol from industrial emissions and syngas at continuous rates up to about 3 grams per litre per hour with roughly 90 percent selectivity, at industrial pilot scale, with life-cycle analysis confirming a negative carbon footprint for the products [3].

Two things about that result deserve emphasis rather than a footnote. The productivity is orders of magnitude above the isolated-enzyme systems, which is what a whole organism regenerating its own cofactors buys you. And the feedstock is waste gas and syngas, which is carbon monoxide rich, not a stream of CO2. A process that consumes CO is doing something related to but distinct from reducing CO2, and the distinction matters when the same result is cited as evidence for CO2 conversion generally.

Open challenges

• Cofactor economics. Stoichiometric consumption of nicotinamide cofactors is the binding constraint on cell-free enzymatic routes, and regeneration or cofactor-free chemistry is the active research front [1].
• Mass transfer. CO2 is poorly soluble in water, and every aqueous biocatalytic system is limited by getting gas to the catalyst [1].
• Rate. Synthetic in vitro cycles operate at rates far below anything process-relevant, and closing that gap is a different problem from demonstrating the chemistry works [2].
• Feedstock honesty. The industrially demonstrated case runs on CO-rich waste gas [3], which is a real and useful thing to do with a real waste stream, and is not the same claim as fixing CO2.
• Products return the carbon. As with every conversion route, chemical and fuel products may reduce emissions by displacing fossil-derived molecules but have limited potential for CO2 removal [4].