## 3 Applications of Crosslinked PEI
### Pristine and Crosslinked PEI in CO\({}_{2}\) Adsorption
Linear and branched PEI were crosslinked with divinyl sulfone to yield rigid and insoluble structures that were studied for CO\({}_{2}\) gas adsorption. The best performance was an adsorption capacity of 4.34 mmol g\({}^{-1}\) at 1 bar of CO\({}_{2}\) from the linear microgel, which is an environmentally friendly gel that functions as a well-performing dry CO\({}_{2}\) adsorbent [26]. [PERSON] et al. modified a molecular sieve (MCM-41) using PEI to prepare a MCM-41-PEI adsorbent for CO\({}_{2}\). It was found that PEI loading of 50% (by mass) in MCM-41-PEI showed the highest CO\({}_{2}\) adsorption capacity of 246 mg g\({}^{-1}\) (based on PEI), 30 times higher than that of MCM-41 and 2.3 times higher compared to pristinePEI (109 mg g\({}^{-1}\)) [104]. According to [PERSON] et al., PEI mixed with ethanolamine (MEA) and impregnated with FNG-II silica (FS) adsorbents were good CO\({}_{2}\) adsorbents from a simulated biogas. A composite material made up of FS, 10% PEI, and 10% MEA exhibited an attractive CO\({}_{2}\) adsorption capacity of ca. 64.68 mg g\({}^{-1}\), which was increased by 81 % in comparison to FS-20% PEI. The material has also shown a higher thermal stability [105]. A commercial polymethacrylate resin with a bead size of 0.44 mm diameter was impregnated with PEI and used to adsorb CO\({}_{2}\) from a biogas [106]. The surface area (m\({}^{2}\)/g) and pore volume (m\({}^{3}\)/g) of the resulting material were measured at different loadings of PEI. The surface area dropped from 587 m\({}^{2}\)/g (without PEI) to 14 m\({}^{2}\)/g (with 50% of PEI). Similarly the pore volume decreased from 1.45 cm\({}^{3}\)/g to 0.26 cm\({}^{3}\)/g. Interestingly, both parameters got smaller with an increase in the loading of PEI, with the smallest value recorded being with 50% of PEI loading. The decreases in the parameters indicate the successful loading of the branched PEI into the support pores, reducing the average pore volume of the sorbent with increasing amine loading. The CO\({}_{2}\) gas is expected to undergo an acid-base reaction with the basic amino group of the PEI during the adsorption process. The CO\({}_{2}\) adsorption capacity of the untreated polymethacrylate was found to be negligible, while the highest was 2.7 mmol/g for 30 PEI-HP2 MGL. An IR analysis has shown that the adsorption of CO\({}_{2}\) on the adsorbent is due to the formation of zwitterion intermediate between the CO\({}_{2}\) and the basic amine group of the PEI and finally due to the formation of ammonium-carbamate ion pairs (Scheme 14). An ice templating method was developed to prepare self-supported crosslinked branched PEI materials for CO\({}_{2}\) adsorption from simulated blue gas [50]. The ice templating method requires a thorough mixing of the PEI with the crosslinker (poly(ethylene glycol) diglycidylel) in vortex mixer (500 rpm) for 5 sec followed by keeping the mixture in a freezer (\(-10\)\({}^{\circ}\)C) for two days or putting the vial containing the mixture in liquid nitrogen for instant freezing. The frozen materials are then thawed at room temperature and the sample is collected and washed. The schematic synthesis of the self-supported PEI adsorbent is shown in Scheme 15. The process yields a porous structure of a crosslinked PEI. The sorbent capacity of the prepared materials was measured to be from 0.32 to 2.81 mol CO\({}_{2}\)/kg. At 65% relative humidity, the adsorption capacity was even measured to be higher (5.5 mol CO\({}_{2}\)/kg). The material has also shown fast adsorption kinetics, reaching 80% of its maximum capacity in only 18 min. The prepared adsorbent also presents stable recyclability over 50 dry adsorption-desorption cycles with only a minor loss of CO\({}_{2}\) capacity (4%). The synthesis of the material is also claimed to be economically feasible and it can likely be easily molded into any shape. Therefore, the material is considered a promising new material for post-combustion CO\({}_{2}\) capture.