The term post-combustion capture refers to separation of CO₂ from flue gas at the exit of the combustion process. The established technique at present is to scrub the flue gas with an amine solution (alkanolamines). The amine-CO₂ complex formed is decomposed by heat to release high purity CO₂ and the regenerated amine is recycled to the scrubber. Post-combustion capture is applicable to the thermal power plants. Additional measures, such as desulphurization are considered to prevent the impurities in the flue gas from contaminating the CO₂ capture solvent. The large volumes of gas requiring large-scale equipment and high capital costs, and the amount of additional energy needed to operate the process are the main challenges for the post-combustion capture approach.

The combustion of coal in air generates flue gas with a relatively low CO₂ concentration (15 - 16%), while the bulk of the effluent is composed of N2 and other minor components, such as H₂O, O₂, CO, NOx, and SOx. The gas stream is released at a total pressure of approximately 1 bar. Since SOx removal would precede CO₂ capture, the flue gas would be expected to enter the CO₂ scrubber at temperatures between 40 and 60 ⁰C. An ideal adsorbent for capturing CO₂ from post-combustion flue gas would exhibit a high selectivity for CO₂ over the other flue gas components, high gravimetric and volumetric CO₂ adsorption capacities, minimal energy penalty for regeneration, long-term stability under the operating conditions, and rapid diffusion of the gas through the adsorbent material. The preparation of next-generation metal-organic frameworks that satisfy all of these requirements is currently a difficult synthetic challenge.

Metal-Organic Frameworks for CO₂/N₂ Separation

The lower-pressure (<1.2 bar) CO₂ uptake capacities for all metal-organic frameworks are relatively low. However owing to their high surface areas many of the frameworks exhibit large CO₂ adsorption capacities at pressures of 1 bar or above. However, these compounds are generally not well suited for post-combustion capture, since the adsorption capacity at lower pressures is a more relevant consideration due to the low partial pressure of CO₂.

The materials offering the highest selectivity are generally those bearing functionalized pore surfaces. Surface functionalities that interact strongly with CO2 frequently increase adsorbent capacity at low pressures. Ideally, for high adsorption selectivity, the CO₂ adsorption should be maximized. For metal-organic frameworks with strongly polarizing sites, the selectivity values underestimate the true adsorptive selectivity of the adsorbent.

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