A review on adsorption isotherms and kinetics of CO2 and various adsorbent pairs suitable for carbon capture and green refrigeration applications

In this state-of-the-art review, various adsorbents (i.e., Metal-Organic Framework (MOF), Activated Carbons and their composites, Carbon Molecular Sieve (CMS) and zeolites) for CO2 adsorption both at low and high-pressure applications (i.e., pre- and post-combustion CO2 capture, adsorption heat pumps employing CO2 as refrigerant) are explored. The most suitable candidate, i.e., the various grades of activated carbons (ACs), is identified based on their equilibrium uptake (isotherm data), rate of adsorption (kinetic data), isosteric heat of adsorption and cost. The study presents a comprehensive review on the basis of various models of adsorption isotherms and kinetics, their merits and demerits, and their applicability, especially in the context of CO2-adsorbent pairs. The literature shows that the activated carbons with high surface area, pore volume and better pore network results in higher equilibrium uptake and faster kinetics. A comparative analysis presented in the review work highlights that high-grade activated carbons having higher absolute uptake, also result in higher net uptake, i.e., the deciding factor for selecting the adsorbents for adsorption-based refrigeration and heat transformation applications. The comparative study clearly shows that most of the MOFs with high surface area outperform the best-activated carbons in equilibrium CO2 uptake. However, their high heat of adsorption, slower kinetics and significantly high cost comes in their way of commercialization for high/low-pressure CO2 adsorption applications. One of the notable observations of the review work is that adsorbents that perform better in low-pressure applications may not be a handsome candidate for high-pressure applications, as both mechanisms are different. Various isotherm models are compared based on the R-2 value of the fitted data. The comparison clearly demonstrates that some of the models, i.e., (Langmuir, Freundlich), give better predictions at low-pressure conditions while some (Toth, D-A, Modified D-A, and D-R) give a better prediction for high-pressure adsorption. Some isotherm models take care of the surface heterogeneity, hence suitable for AC-CO2 pair. Most importantly, the compiled data for the adsorption isotherms and kinetics will be useful for further analysis and design of adsorption systems and selection of adsorbents, especially for CO2 adsorption systems suitable for green refrigeration/heat pump and carbon capture application.