## 2 Crosslinking of PEI and Reaction Mechanisms
Researchers have designed different strategies to capture CO\({}_{2}\) from its source [40], of which one of them is the absorption CO\({}_{2}\) using aqueous amines, such as monoethanolamine (MEA), diethanolamine (DEA), and piperazine (PZ) [44; 45], where the CO\({}_{2}\) reacts with the adsorbing amine. This method, however, is known to form corrosive fumes and the regeneration process is energy-demanding. As alternative to the use of aqueous amines, amine-impregnated solid adsorbents were studied for capturing CO\({}_{2}\) from plants. However, with this method, the formation of urea during the regeneration process and leaching of the amine molecules are some of the issues observed [46]. With the objective of overcoming the abovementioned issues associated with aqueous amines, crosslinked materials are used as amine-based adsorbents. [PERSON] et al. evaluated an epoxidecrosslinker (BDDE and ECH) (Figure 2) as a crosslinker for PEI on silica to enhance the physical and chemical stability of PEI-impregnated adsorbents [37]. To understand the extent of crosslinking and to propose the structures of the resulting crosslinked materials, \({}^{13}\)C NMR was employed. The \({}^{13}\)C NMR spectra of the BDDE- and ECH-crosslinked PEI showed distinct and sharp peaks in the aliphatic region (appearing below 60 ppm). The peak positions also have a similar chemical shift position. Both materials have also given weak signals at around 70 and 71 ppm attributed to the OH-containing carbon atoms. The ratios of the primary, secondary, and tertiary amines in the crosslinked materials were also calculated. The result showed that the pristine PEI (before crosslinking) showed 36.6%, 34.5% and 29.1% of primary amines, secondary amines, and tertiary amines, respectively. The BDDE-crosslinked material, on the other hand, showed 33.5% primary amines, 36.2% secondary amines, and 30.3% tertiary amines. A total of 34.1% primary amines, 35.8% secondary amines, and 30.1% tertiary amines were found in ECH-crosslinked PEI. The ratio of the primary, secondary, and tertiary amines in the three materials clearly show the degree of the crosslinking reactions. In more detail, the primary amine concentration in the pristine PEI is higher than in the two crosslinked materials. The secondary amine and tertiary amine concentrations have increased in both cases, which indicates the crosslinking of the materials. A higher conversion rate of primary amines to secondary amines (8.6%) compared to a lower conversion of secondary amines to tertiary amines (3.3%) was observed in the BDDE-crosslinked material. The corresponding values for the ECH-crosslinked material were 6.8% (conversion of primary amines to secondary amines) and 2.8% (conversion rate of secondary amines to tertiary amines). These conversion values indicate that both crosslink react with primary amines better than they do with secondary amines. When the two crosslinkers are compared, BDDE is more reactive with primary amines than ECH. It is noted that the chemical shift of the carbons in pristine PEI were assigned as follows. The carbon that has a tertiary amine at the \(\alpha\)-position and a primary amine at the \(\beta\)-position has the highest chemical shift in the spectrum (at \(>\)55 ppm). The next carbon with a chemical shift to a lower ppm is the one that has a tertiary amine at the \(\alpha\)-position and a secondary amine at the \(\beta\)-position (\(\sim\)53 ppm). The last carbon, which appears up field (\(\sim\)35 ppm) in the spectrum, was assigned to the carbon in PEI that has a primary amine at the \(\alpha\)-position and a tertiary amine at the \(\beta\)-position. Such an analysis was applied to the \({}^{13}\)C NMR spectra of the crosslinked materials, and the structures were proposed, as shown in Figure 4.  
The adsorption capacity of the materials for CO\({}_{2}\) was investigated, and it was found that BDDE was higher than the ECH-crosslinked material.