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

Abstract

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.