## 3 Applications of Crosslinked PEI
### Pristine and Crosslinked PEI in CO\({}_{2}\) Adsorption  
Extensive scientific proof indicates that the world has seen an increase in CO\({}_{2}\) emissions from fossil fuel, which have caused the global temperature to rise. In the fight against global warming, reducing the production and emission of CO\({}_{2}\) to the environment has been the focus of researchers. In this quest, the development and use of CO\({}_{2}\) capture systems are worth mentioning. Nitrogen-rich solid adsorbents are widely used to capture CO\({}_{2}\). To enhance CO\({}_{2}\) absorption, researchers have explored modified PEI as a viable approach. PEI, when chemically modified, demonstrates an improved capability to capture and bind CO\({}_{2}\), thereby offering a promising method for mitigating CO\({}_{2}\) impacts. PEI was crosslinked with ECH in a one-step reaction to yield PEI hydrogel beads (PEI HBs) with an increased surface area. The material showed increased performance in capturing CO\({}_{2}\) compared to that of liquid PEI. Moreover, the crosslinked polymer showed excellent thermal stability and issues with solvent loss were avoided [58]. It is noted that a dry crosslinked PEI has very little tendency to capture CO\({}_{2}\) due to the closed structure and hence low diffusibility of the CO\({}_{2}\) gas. However, with the addition of water, the CO\({}_{2}\) uptake showed an improvement, with uptake reaching to 60.2 mg g\({}^{-1}\) at a 70% water weight ratio. The improvement of the CO\({}_{2}\) capture capacity of the material with the addition of water was attributed to the swelling of the crosslinked material and hence allowing the CO\({}_{2}\) gas to diffuse through the material, and the presence of water might have allowed the tertiary amines to play a role in capturing the CO\({}_{2}\) gas [99]. The formation of carbamic acid and bicarbonate was detected in IR study in the wet crosslinked PEI, which is a proof of the capture of CO\({}_{2}\) gas. Microwave heating in a method called \"temperature swing\" (the CO\({}_{2}\)-loaded crosslinked PEI is heated with a microwave followed by addition of water and then cooling) regenerated the used crosslinked PEI successfully. Heating the sample in a convection oven at 165 \({}^{\circ}\)C for 45 min has also achieved the desorption of the CO\({}_{2}\). The material was recycled for about 10 times and the adsorption capacity of the material remained stable, indicating the material's recyclability. PEI crosslinked with triglycidylI trimethylolpropane ether (TTE) also showed a CO\({}_{2}\) uptake efficiency of approximately 50 mg g\({}^{-1}\) of sorbent [100]. PEI dispersed in mesoporous silica and mesoporous silica (without PEI) were compared for their CO\({}_{2}\) adsorption capacities from a mixture of gases consisting of CO\({}_{2}\), N\({}_{2}\), O\({}_{2}\), CO, H\({}_{2}\)O, and NO\({}_{x}\) (a flue gas) generated from a natural gas-fired boiler. The PEI-loaded mesoporous silica showed a higher adsorption capacity and higher selectivity for CO\({}_{2}\) than ordinary sorbents. The mesoporous silica alone (MCM-41) (without PEI) was found to have an adsorption capacity of 8.6 mg g\({}^{-1}\), while the PEI-containing silica (MCM-41-PEI-50) showed a CO\({}_{2}\) adsorption capacity of 112 mg g\({}^{-1}\), clearly showing the positive role of PEI. The lower adsorption capacity of the silica (without PEI) was due to a weak chemical interaction between the adsorbent and CO\({}_{2}\)[101]. Pure PEI has shown an adsorption capacity of 109 mg/g, as reported in another study [102]. In another study, a PEI-loaded porous mesoporous molecular sieve (SBA-15) was used for CO\({}_{2}\) sorption in a temperature- and time-dependent study [103]. An adsorption study at higher temperature (75 \({}^{\circ}\)C) improved the adsorption of CO\({}_{2}\), which could be due to the flexibility of the adsorbent material and better diffusibility of the CO\({}_{2}\) gas.