이산화탄소(CO2)로부터의 화학 합성

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Carbon dioxide is a colourless gas present in the atmosphere at approximately 0.04 percent by volume, or around 420 parts per million. The molecule contains three atoms — one carbon bonded to two oxygens in a linear, symmetrical arrangement, which is why it has no net dipole moment despite the polarity of each individual bond.

It arises from three broad sources. Aerobic organisms produce it during respiration, and it is also generated by the microbial breakdown of organic matter. Combustion of fossil fuels — coal, oil, gasoline, and natural gas — produces the bulk of anthropogenic emissions. And it is released as a process emission in industrial operations such as cement and lime manufacture, where carbonates are decomposed thermally.

Rather than being vented, CO₂ can be used as a chemical feedstock. It is already consumed industrially at scale in urea production, and a range of catalytic routes convert it into fuels, solvents, polymers, and intermediates. This article sets out those routes with their reaction chemistry.

Carbon dioxide utilisation methods

Several classes of technology convert carbon dioxide into valuable chemical compounds. Each differs in the energy input required, the products accessible, the reaction rate, and the maturity of the technology.

Thermocatalytic conversion

Heat and pressure over a solid catalyst drive CO₂ and hydrogen to products such as methanol, methane, dimethyl ether, and longer-chain hydrocarbons. This is the most industrially established class, since it uses reactor designs, catalysts, and engineering practice already familiar from conventional syngas chemistry.

Routes to hydrocarbons typically proceed through carbon monoxide, produced by the reverse water-gas shift reaction, which is then converted by Fischer-Tropsch synthesis.

Direct electrochemical reduction

Applying a potential across an electrode in contact with CO₂ drives its reduction to products including carbon monoxide, formate, methanol, methane, ethylene, and ethanol. Which product forms depends principally on the catalyst material and the applied potential — silver and gold favour CO, tin and bismuth favour formate, and copper is the only metal that produces multi-carbon products in significant quantity.

Powered by renewable electricity, this route avoids the emissions associated with fossil-derived hydrogen. Its current limitations are the ones that keep it from commercial scale: low current density and therefore low throughput per unit of electrode area, competition from the hydrogen evolution reaction which lowers faradaic efficiency, catalyst and membrane durability, and the cost of separating dilute products from the electrolyte.

Photocatalytic reduction

A semiconductor photocatalyst — commonly titanium dioxide, often doped with transition metals or coupled with other oxides to extend absorption into the visible range — absorbs light and generates electron-hole pairs. The electrons reduce CO₂ at the catalyst surface, while the holes oxidise water. Reported products include carbon monoxide, formate, methanol, and methane.

Microbial and enzymatic conversion

Certain microorganisms fix CO₂ directly into organic products. Acetogenic bacteria such as Clostridium species convert CO₂ and hydrogen, or syngas, into acetate and ethanol. Methanogenic archaea produce methane. Engineered strains and cyanobacteria have been developed to produce a range of target metabolites.

Enzymatic routes use isolated enzymes rather than whole cells — carbonic anhydrase to accelerate CO₂ hydration, or formate dehydrogenase to reduce CO₂ to formate. These operate under mild conditions with high selectivity, but enzyme stability, cofactor regeneration, and cost remain the barriers to scale.

How the methods compare

Conversion routes at a glance

Examples of chemical synthesis from carbon dioxide

1. 1 Methanol synthesis. CO₂ is hydrogenated over a copper-zinc oxide-alumina catalyst. Water is a co-product and must be removed to drive conversion.
   CO₂ hydrogenation to methanol CO₂ + 3H₂ ⇌ CH₃OH + H₂O

   Cu/ZnO/Al₂O₃ catalyst, approximately 200–300 °C and 50–100 bar. Exothermic and equilibrium-limited, so unreacted gas is recycled.

2. 2 Methanation (the Sabatier reaction). A distinct reaction producing methane rather than methanol, used in power-to-gas systems to make synthetic natural gas.
   Sabatier reaction CO₂ + 4H₂ → CH₄ + 2H₂O

   Ni or Ru catalyst, approximately 300–400 °C. Strongly exothermic; heat removal governs reactor design.

3. 3 Formic acid production. Obtained by catalytic hydrogenation of CO₂, usually in the presence of a base to shift the equilibrium, or electrochemically on tin, bismuth, or lead cathodes.
   Formic acid CO₂ + H₂ ⇌ HCOOH CO₂ + 2H⁺ + 2e⁻ → HCOOH   (electrochemical)

   Homogeneous Ru or Ir catalysts with an amine base; the free acid is recovered from the resulting formate salt.

4. 4 Oxalate formation. Produced by electrochemical reductive coupling of two CO₂ molecules in aprotic solvent, forming the carbon-carbon bond directly.
   Reductive coupling to oxalate 2CO₂ + 2e⁻ → C₂O₄²⁻

   Aprotic electrolyte; the oxalate is subsequently isolated as its alkali or alkaline earth salt.

5. 5 Urea synthesis. The largest existing industrial use of CO₂ as a feedstock, consuming CO₂ from the ammonia plant itself. It proceeds through ammonium carbamate.
   Bosch-Meiser urea process 2NH₃ + CO₂ → NH₂COONH₄ NH₂COONH₄ → (NH₂)₂CO + H₂O Overall:   2NH₃ + CO₂ → (NH₂)₂CO + H₂O

   Approximately 150–200 °C and 150–250 bar. The first step is exothermic, the second endothermic.

6. 6 Carbonic acid and carbonates. CO₂ dissolves in water to form carbonic acid, which dissociates. This equilibrium underlies carbonated beverages, ocean acidification, and mineral carbonation.
   Aqueous carbonate equilibrium CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ ⇌ 2H⁺ + CO₃²⁻
7. 7 Dimethyl ether (DME). Made by dehydrating methanol, or directly from CO₂ and hydrogen over a bifunctional catalyst combining methanol synthesis and dehydration functions.
   Methanol dehydration and the direct route 2CH₃OH → CH₃OCH₃ + H₂O 2CO₂ + 6H₂ → CH₃OCH₃ + 3H₂O

   Dehydration over γ-alumina or a zeolite at approximately 250–400 °C; the direct route runs at similar conditions to methanol synthesis.

8. 8 Cyclic and polymeric carbonates. CO₂ reacts with epoxides to give cyclic carbonates, or copolymerises with them to give polycarbonates. This is the principal route by which CO₂ is incorporated directly into a polymer backbone, and it is commercially operating.
   Epoxide routes CO₂ + C₃H₆O → C₄H₆O₃   (propylene oxide → propylene carbonate) n CO₂ + n C₃H₆O → [–OCH(CH₃)CH₂OC(O)–]_n

   Zinc, cobalt, or chromium complexes as catalyst; mild temperatures and moderate CO₂ pressure. Up to roughly 40 percent of the polymer mass can be CO₂-derived.

9. 9 Dimethyl carbonate (DMC). Directly from methanol and CO₂, giving a route that avoids phosgene entirely. Equilibrium is unfavourable, so the water produced must be removed continuously.
   Direct DMC synthesis 2CH₃OH + CO₂ ⇌ (CH₃O)₂CO + H₂O

   Ceria or zirconia-based catalysts with a dehydrating agent. DMC is used as a solvent, a methylating and carbonylating agent, an electrolyte component in lithium-ion batteries, and a precursor to polycarbonate.

10. 10 Salicylic acid (Kolbe-Schmitt reaction). A long-established industrial carboxylation using CO₂, and the route to the precursor of aspirin.
    Kolbe-Schmitt carboxylation C₆H₅ONa + CO₂ → C₆H₄(OH)COONa

    Sodium phenoxide under approximately 100 bar CO₂ at 125 °C; acidification liberates salicylic acid.

11. 11 Mineral carbonation. CO₂ reacts with alkaline earth oxides and silicates to form stable carbonates — the only utilisation route in which the carbon is permanently bound.
    Carbonation reactions CaO + CO₂ → CaCO₃ Mg₂SiO₄ + 2CO₂ → 2MgCO₃ + SiO₂

    Exothermic and thermodynamically favourable; the challenge is reaction rate, addressed by grinding, elevated temperature and pressure, or aqueous routes.