A Review of CO2 Adsorption From Ambient Air (Direct Air Capture)

Direct Air Capture (DAC) has risen in the past decade as a promising choice to remove CO2 directly from ambient air. Although there are many technologies like pre-combustion, post-combustion and oxy- combustion to mitigate the emissions of CO2, there is still a need for a better or a complementary technology in controlling these emissions. Even though, there are many options for adsorbents that can be used, metal- organic frameworks (MOFs), which are hybrid materials with a crystalline structure, tunable pores, and high surface area, have risen as a promising candidate. Functionalizing MOFs with amine groups will greatly enhance their capacity towards CO2, like mmen-M2 (dobpdc) (M=Mg, Fe, Mn, Zn, Co). These adsorbents show S-shaped adsorption isotherms and have a very high affinity to CO2 under atmospheric conditions, due to the cooperative insertion of CO2 into mmen-M2 (dobpdc) that leads to their unique adsorption isotherms. The energy required to regenerate these materials using small heat variations makes a better choice than aqueous amines. This mechanism has been studied by using X-Ray Diffraction (XRD) and spectroscopy experiments. Furthermore, a thermodynamic study has been reviewed to understand the mechanism through which CO2 is inserted into the diamine bond. These materials are a promising choice for the removal of CO2 directly from the atmosphere and require more future research.