Selective CO₂ adsorption over functionalized Zr-based metal organic framework under atmospheric or lower pressure: Contribution of functional groups to adsorption

Selective capture of CO₂ from offgas is important to mitigate the global warming; and metal organic frameworks (MOFs) have been attractive in the capture because of huge porosity, ready functionalization and so on. In this study, a stable Zr-based MOF, MOF-808, was modified with ethylenediaminetetraacetic acid (EDTA) and further reacted with ethylenediamine (ED); and finally reduced with lithium aluminum hydride (LAH) to introduce several functional groups (FGs) onto the MOF. Moreover, the MOFs were applied in CO₂ adsorption under low pressure. The efficiency of MOF-808 in CO₂ capture was improved with EDTA loading; however, interestingly, further reaction of MOF-808-EDTA with ED causes a very much decrease in the efficiency. Importantly, the reduction of MOF-808-EDTA-ED with LAH (for MOF-808-EDTA-ED-R) leads to a remarkable increase in the performance of the MOF, for high CO₂ adsorption capacity, CO₂/N₂ selectivity and isosteric heat of adsorption. For example, MOF-808, MOF-808-EDTA, MOF-808-EDTA-ED and MOF-808-EDTA-ED-R showed CO₂/N₂ IAST selectivity (from CO₂/N₂ = 15/75) of 40, 48, 19 and 197, respectively, under 298 K and 1 atm. This unusual observation could be explained with the contribution of FGs and porosity. Or, amides in cyclic rings might be formed during reaction with ED; and the MOF with amides was poor in CO₂ capture partly due to decreased porosity of the MOF; however, can be very effective in adsorption, after further reduction of amides to amines. This work shows the importance of modifications or FGs on MOFs in CO₂ adsorption, or a simple reduction can increase the adsorption selectivity as much as 10 times, which might be helpful to mitigate the global warming.