Optimal Polyethyleneimine Molecular Weight and Arrangement for Modification of γ-Cyclodextrin Metal Organic Frameworks (γ-CD-MOFs) for Post-Combustion CO₂ Capture

Modified γ-cyclodextrin metal organic frameworks (γ-CD-MOFs) are a promising eco-friendly sorbents for post-combustion CO2 capture. Polyethyleneimine (PEI) has been widely used for modifying MOFs to enhance their CO2 sorption capacity and selectivity through the introduction of CO2 selective amine groups. The main objective of this study was to determine the optimal PEI molecular weight and arrangement (linear or branched) to enhance γ-CD-MOF’s CO2 sorption capacity. γ-CD-MOFs were impregnated with linear as well as branched PEI with molecular weights of 600, 1200 or 10,000. The CO2 sorption capacity of the PEI-impregnated γ-CD-MOFs was determined using a quartz crystal microbalance assembly at CO2 partial pressures from 0.35–1.0 atm. Impregnation with 600 g/mole branched PEI achieved the highest CO2 sorption capacity of 0.9 mmole/g CO2 at 1 atm, followed by the linear PEI (0.12 mmol/g). Modification with the other branched PEI molecular weights did not achieve detectable CO2 sorption, likely because of pore blockage with the relatively larger PEI molecular weights, as demonstrated by molecular docking simulations. Furthermore, the control γ-CD-MOFs did not sorb CO2, likely because of the lower attraction forces between CO2 and the large pore volume of the unmodified MOFs