Environmental, green and energy chemistry

CO₂ conversion, carbon capture

How CO₂ is separated from point sources and air, stored or mineralised, and converted into fuels and materials, with thermodynamic, process and lifecycle limits.

1 Lower secondary 2 Upper secondary 3 Undergraduate 4 Advanced 5 Research

IntuitionCarbon capture and conversion: a managed carbon loop

CO₂ is the fully oxidised “ash” of carbon combustion. Separating it before release can reduce emissions, while converting it into fuels or materials stores carbon only temporarily unless the product remains durable or the carbon is later captured again.

Follow flue gas through the absorber, where circulating amine binds CO₂; the rich solution is heated in the stripper, releasing concentrated CO₂ and returning lean solvent. Adjust the flow rate and compare the process streams.

SchoolSchool chemistry: capture and release

The limewater test illustrates reversible capture: carbon dioxide turns calcium hydroxide solution cloudy as solid calcium carbonate forms. Industrial capture uses selective solvents or solids rather than simply bubbling gas through limewater.

\ce{CO2 + Ca(OH)2 -> CaCO3(s) + H2O}

Definition: CCS, CCU and CDR

CCS captures carbon dioxide and stores it; CCU uses it in products; carbon dioxide removal (CDR) takes net CO₂ from the atmosphere and stores it durably. A point-source capture plant is not automatically CDR: its net effect depends on source, energy, leakage and product lifetime.

Capture routes depend on gas composition and process integration
Route	Feed and principle	Main constraint
Post-combustion	Dilute flue gas; solvent absorption	Steam for regeneration; corrosion
Pre-combustion	Shifted syngas at elevated pressure	Fuel processing and integration
Oxy-fuel	Fuel burned in oxygen/recycled flue gas	Air-separation energy
Direct air capture	Ambient air, about 420 ppm CO₂	Very dilute feed and energy supply

Example: CO₂ from burning carbon

How much CO₂ forms from 12.0 kg of carbon, assuming complete combustion? Use $M(\ce{C})=12.01$ and $M(\ce{CO2})=44.01$ g mol⁻¹.

Solution

The reaction is $\ce{C + O2 -> CO2}$ . The mole ratio is 1:1, so $m_{CO2}=12.0(44.01/12.01)=44.0$ kg (rounded). Capturing that amount from air at 420 ppm would require contacting an enormous air volume; actual DAC uses sorbent contactors and regenerates them rather than treating the ideal minimum as a plant design.

UndergraduateCapture thermodynamics and solvent chemistry

A common post-combustion benchmark is 30 wt% aqueous monoethanolamine (MEA). Primary amines form carbamate through a zwitterionic pathway; the idealized net stoichiometry consumes two amine molecules per CO₂. Real solvents also contain bicarbonate and loaded species depend on water content, temperature and CO₂ pressure. Regeneration duty for conventional MEA is often around 3.5–4 GJ per tonne CO₂, mostly low-pressure steam; advanced piperazine blends and chilled-ammonia cycles seek lower duty or different operating windows.

\ce{CO2 + 2RNH2 <=> RNHCOO^- + RNH3^+}

Representative capture media and limitations
Medium	Mechanism / indicative heat	Trade-off
Aqueous MEA	Reactive absorption; 3.5–4 GJ/t regeneration	Mature, but corrosion and steam use
Zeolite 13X	Physisorption; strong low-pressure uptake	Water competes strongly
Mg-MOF-74	Open metal sites; CO₂ adsorption	Moisture stability and shaping matter
Amine-functionalized silica	Chemisorption; works at dilute CO₂	Oxidative ageing and regeneration
CaO, calcium looping	Carbonation releases ≈178 kJ/mol	Calcination heat and sorbent sintering

\ce{CaO(s) + CO2(g) <=> CaCO3(s)} \qquad \Delta H^\circ \approx -178\ \mathrm{kJ\,mol^{-1}}

UndergraduateEquilibria, separation work and storage

Henry’s law approximates dilute physical dissolution, $c=k_Hp$ (for CO₂ in water near 25 °C, $k_H\approx0.034$ mol L⁻¹ atm⁻¹). A simple Langmuir model, $q=q_{max}bP/(1+bP)$ , describes saturable adsorption on equivalent sites. Neither model alone captures reactive amine speciation, pore heterogeneity or mass-transfer limits.

c=k_Hp \qquad q=q_{\max}\frac{bP}{1+bP}

For an ideal dilute feed of mole fraction $y$ , the reversible minimum work to extract one mole of CO₂ against an ideal separated product is $W_{min}=RT\ln(1/y)$ . At 298 K this is 5.26 kJ mol⁻¹ CO₂ for 12% flue gas and 19.27 kJ mol⁻¹ for air at 420 ppm, about 0.12 and 0.44 GJ/t, respectively. These are ideal thermodynamic lower bounds, not full-plant energy predictions; real systems also pay for incomplete recovery, pressure drop, heat, compression and auxiliaries.

W_{\min}=RT\ln\!\left(\frac{1}{y}\right)

Example: Ideal separation-work comparison

At 298 K compare the reversible minimum work per mole of captured CO₂ from a 12% stream and air at 420 ppm.

Solution

Use $RT=8.314\times298=2.479$ kJ mol⁻¹. For $y=0.12$ , $W=2.479\ln(8.33)=5.26$ kJ/mol; for $y=0.000420$ , $W=2.479\ln(2381)=19.27$ kJ/mol. The air case is about 3.7 times larger per mole, despite both minima being far below real process energy.

Example: Reboiler heat for a 1 Mt/yr plant

A plant emits 1.0 Mt CO₂ per year and captures 90% with a solvent needing 3.7 GJ per tonne CO₂. Estimate the average thermal duty for 8000 operating hours per year.

Solution

Captured CO₂ is $0.90\times1.0\times10^{6}=9.0\times10^{5}$ t/yr, so heat is $9.0\times10^{5}\times3.7=3.3\times10^{6}$ GJ/yr. Dividing by 8000 h gives about 416 GJ/h, i.e. $416\times10^{9}/3600\approx116$ MW thermal. This is steam that would otherwise raise power, and CO₂ compression adds electricity on top.

Pre-combustion capture can follow gasification and water–gas shift in an integrated gasification combined-cycle (IGCC) scheme, yielding pressurised CO₂-rich syngas. Sorption-enhanced shift removes CO₂ as it forms to move equilibrium toward hydrogen. Oxy-fuel combustion uses oxygen and recycled flue gas, producing an exhaust rich in CO₂ and water after condensation, at the cost of air-separation energy. Membranes offer compact separations but face permeability–selectivity trade-offs and often need staged compression or hybridization.

AdvancedDirect air capture and durable storage

At roughly 420 ppm, direct air capture (DAC) must move much more air per tonne than a flue-gas unit. Solid amine contactors and alkaline-liquid cycles are distinct approaches. Carbon Engineering’s KOH/Ca(OH)₂ loop absorbs CO₂ into carbonate, precipitates CaCO₃, then calcines it to release concentrated CO₂ and regenerate CaO; high-temperature heat and equipment integration matter. Climeworks uses modular solid-sorbent contactors. Reported total energy depends on system boundary; a broad order-of-magnitude range is about 5–10 GJ/t CO₂, with heat and electricity reported separately where possible.

Storage can inject compressed CO₂ into deep saline aquifers under a competent caprock, with site-specific monitoring of pressure and leakage. Mineralisation reacts CO₂ with calcium- or magnesium-rich rocks; CarbFix in basalt demonstrates rapid carbonate formation at its Iceland site, but water demand, pumping and local geology govern transferability. Utilisation products range from short-lived fuels to durable mineral or polymer products, so lifecycle accounting must include eventual release.

AdvancedConversion chemistry and process limits

CO₂ is thermodynamically stable: conversion to reduced products requires energy and a reductant, commonly low-carbon hydrogen or renewable electricity. Sabatier methanation is exothermic and uses Ni catalysts; methanol synthesis over Cu/ZnO/Al₂O₃ or In₂O₃-based catalysts couples hydrogenation with water formation and competes with reverse water–gas shift. Heat removal, pressure, equilibrium and catalyst lifetime determine practical operation.

\ce{CO2 + 4H2 -> CH4 + 2H2O} \qquad \Delta H^\circ \approx -165\ \mathrm{kJ\,mol^{-1}}

\ce{CO2 + 3H2 -> CH3OH + H2O} \qquad \Delta H^\circ \approx -49.5\ \mathrm{kJ\,mol^{-1}}

Example: Hydrogen demand for methanating one tonne of CO₂

Estimate the stoichiometric H₂ feed for methanating 1.00 t CO₂, assuming complete conversion.

Solution

One mole CO₂ needs four moles H₂. Thus $m_{H2}=1000(4\times2.016/44.01)=183.2$ kg H₂ (about 184 kg). Producing it at 52 kWh/kg needs about 9.5 MWh electricity, before capture, compression and reactor heat; renewable power and lifecycle boundaries determine the climate result.

Electrochemical reduction can produce CO on Ag or Au, formate on Sn or Bi, and multi-carbon products such as ethylene or ethanol on Cu. Report Faradaic efficiency (fraction of charge to a product), current density, cell voltage, carbon balance and product separation together. Gas-diffusion electrodes and flow cells address CO₂ transport; membrane-electrode assemblies can intensify devices, but alkaline operation consumes CO₂ as carbonate and carbonate crossover lowers carbon efficiency. Photochemical reduction seeks light-driven charge separation and selective catalysis, but efficiency, stability and scale remain research challenges.

\ce{CO2 + 2H+ + 2e- -> CO + H2O}

Approximate equilibrium potentials for CO₂ reduction at pH 7 (V vs SHE)
Product	E° (V)
$\ce{CO}$	−0.52
$\ce{HCOO-}$	−0.43
$\ce{CH3OH}$	−0.40
$\ce{CH4}$	−0.24
$\ce{C2H4}$	−0.34
$\ce{C2H5OH}$	−0.33
$\ce{H2}$ (HER)	−0.41

The equilibrium potentials of all these reactions lie within a few hundred millivolts of hydrogen evolution, so selectivity is set mainly by kinetics and by how strongly a metal binds intermediates such as adsorbed CO, not by thermodynamics alone. Cu is unusual because it binds CO neither too weakly nor too strongly and can couple C–C bonds.

\mathrm{FE}_i=\frac{z_i\,n_i\,F}{Q}

Example: Faradaic efficiency and energy efficiency

A CO₂ electrolyser passes 100 mA for 1.00 h and makes CO with Faradaic efficiency 90%. (a) How many mmol and mL of CO form (25 °C, 1 bar, 24.5 L/mol)? (b) With $E^\circ\approx1.33$ V for $\ce{CO2 -> CO + 1/2 O2}$ and a cell voltage of 3.0 V, what is the electrical energy efficiency toward CO?

Solution

(a) $Q=0.100\times3600=360$ C. Charge to CO: $0.90\times360/96485=3.36$ mmol of electrons, and CO needs 2 e⁻, so $n_{CO}=1.68$ mmol, i.e. $1.68\times10^{-3}\times24.5=0.041$ L = 41 mL. (b) $\eta_E=\mathrm{FE}\,E^\circ/E_{cell}=0.90\times1.33/3.0\approx0.40$ , about 40%.

In neutral or alkaline electrolytes, CO₂ reacts with the hydroxide made at the cathode to form carbonate. For a 2-electron product such as CO, each CO made uses one more CO₂ as carbonate, so at most about 50% of the CO₂ fed can leave as CO; for ethylene the ceiling is about 25%. Acidic electrolytes, bipolar membranes or carbonate-regeneration schemes are being explored to escape this limit.

Mineral carbonation stores carbon as stable carbonates; accelerated routes use alkaline wastes or reactive silicates, with grinding, heat and transport costs. CO₂ can also copolymerize with epoxides to form polycarbonates or polyols, and Lewis-pair catalysts can activate both partners. Such products may displace fossil feedstock, but only a lifecycle analysis can show net benefit; many polymers ultimately release CO₂ at end of life.

ResearchFrontier: net climate value, not just capture rate

References

Amine scrubbing for CO2 capture · G. T. Rochelle, 2009
What would it take for renewably powered electrosynthesis to displace petrochemical processes? · P. De Luna, C. Hahn, D. Higgins, S. A. Jaffer, T. F. Jaramillo, E. H. Sargent, 2019
A Process for Capturing CO2 from the Atmosphere · D. W. Keith, G. Holmes, D. St. Angelo, K. Heidel, 2018
Rapid carbon mineralization for permanent disposal of anthropogenic carbon dioxide emissions · J. M. Matter et al., 2016
IPCC Special Report on Carbon Dioxide Capture and Storage · B. Metz, O. Davidson, H. de Coninck, M. Loos, L. Meyer (eds.), 2005