Chemicals from CO2

Using CO2 as a chemical feedstock is the oldest form of carbon utilization and the one with genuine industrial precedent. Urea synthesis has consumed CO2 at industrial scale for a century, and salicylic acid and cyclic carbonates have their own established routes. The modern interest is in extending that from a handful of specialities to the bulk of the chemical industry.

This page is about the products and what they are worth. The catalysis behind them is covered under conversion.

Storage time is the thing that separates good routes from bad

The chemical industry makes molecules with very different lifetimes, and for carbon accounting that difference is the whole story. Urea is the largest existing CO2 consumer and releases its carbon within months of being spread on a field. A polycarbonate in a building component holds it for decades.

That distinction runs straight through the utilisation literature. Chemical and fuel pathways may reduce CO2 emissions but have limited potential for CO2 removal, precisely because most of what the chemical industry makes is consumed, burned or degraded on a short timescale [1]. A CO2-derived chemical is worth making because it displaces a fossil-derived one, not because it stores carbon, unless it is one of the few that does.

Polymers are where the durable case sits

CO2 copolymers are the most developed of the long-lived products. Copolymerising CO2 with epoxides gives polycarbonates and polyether carbonate polyols, the latter now made at industrial scale for polyurethane foams, and the field has extended to a range of copolymer architectures for varied applications [2].

The proportion of CO2 in these materials is worth being clear about. CO2 is a comonomer, typically a minority of the polymer mass, so a tonne of CO2-based polyol does not correspond to a tonne of stored CO2. The benefit is partly stored carbon and partly the displacement of petrochemical feedstock, and the two should be counted separately.

The scale question, answered

The most useful assessment of how far this can go models the chemical sector covering about 75 percent of its current greenhouse gas emissions. It finds that carbon capture and utilisation has the technical potential to lead to a carbon-neutral chemical industry and to decouple chemical production from fossil resources, with annual emissions reductions of up to 3.5 gigatonnes of CO2 equivalent in 2030 [3].

The condition attached to that number is the finding. Achieving it means converting on the order of 3.7 gigatonnes of CO2 a year into methanol, methane and other chemicals, and that requires more than 18.1 petawatt-hours of low-carbon electricity, roughly 55 percent of projected global electricity production in 2030 [3]. The emission reductions only exist if that electricity exists.

This reframes the subject usefully. CO2-to-chemicals is not primarily a catalysis problem or a market problem. It is a claim on the low-carbon electricity supply, competing against every other use of it.

Open challenges

• Electricity, not chemistry, is the constraint. A carbon-neutral chemical industry via utilisation implies an electricity demand comparable to a large fraction of global generation [3].
• Counting stored carbon honestly. Product lifetime determines whether a chemical stores carbon or merely delays its release, and most products are short-lived [1].
• CO2 content versus product mass. In copolymers the CO2 is a fraction of the material, so tonnes of product and tonnes of CO2 are different quantities [2].
• Life-cycle boundaries. Whether a CO2-derived chemical beats its petrochemical equivalent depends on the electricity mix, the CO2 source and the system boundary chosen, which is why catalysis and life-cycle assessment have to be read together [4].