Recent progress in metal-organic frameworks (MOFs) for CO2 capture at different pressures

Global climate change ensued by the rise in atmospheric CO2 levels is one of the greatest challenges our planet is facing today. This worldwide distress demands technologies that can contribute to our society toward “negative carbon emissions”. Carbon capture and storage (CCS) technologies are in important role for capturing CO2 from existing emission sources, such as industrial and energy production point sources, before new more prominent modifications to the energy infrastructure can be implemented. Recently, alongside point source capture, direct air capture (DAC) processes have emerged as highly sought-after technologies that are able to capture CO2 from the ambient air. Alongside the traditional inorganic adsorbents, a new class of solid porous adsorbents, called as metal-organic frameworks (MOFs) have emerged in recent years also, as a group of potentially very efficient materials to capture CO2. The promising results of MOF-based adsorbents have already achieved great interest and have contributed to their ever-accelerating research to develop new and even better adsorbents for both point source and DAC recovery technologies. This review highlights the research that has been focused on utilizing MOFs in the carbon capture processes, particularly targeting materials applicable to low CO2 partial pressures but also capturing processes in pure CO2 (1 bar) will be reviewed, because it is a widely used test condition for characterizing sorption properties of MOF adsorbents. Herein, we outline four major approaches, through which the CO2 adsorption capacity and selectivity can be boosted, including targeted modifications of the metal centers, pore size control, proper selection and substitution of linker units, and functionalization of MOFs by amines. The mechanisms of the sorption event are also reviewed from the perspective of both physisorption and chemisorption phenomena. At the end of the review, we briefly examine the variables related to the coordination of technical-economical, process-technical, and physicochemical properties of adsorbents, which both researchers and engineers should consider when developing new adsorbents and recovery processes, with emphasis on material processing, capture capacity, selectivity, regeneration cyclicity, and cost.