## 4 Role of Amine Type in CO2 Separation Performance
### Reaction Activities of Amines  
Acid catalysts (HCl, etc.) are generally required during the preparation of _polymer_ sols to perform hydrolysis for membrane preparation via a sol-gel route. In addition, sometimes, an excessive amount of acid should be used in some cases to avoid the gelation of sols due to the basic nature of amine groups. Thus, after the membrane formation, the reactivation of these temporarily deactivated amine groups should be taken into consideration. Generally, either calcination at sufficiently high temperatures or vacuum calcination has been adopted to remove acidic molecules used for sol preparation. However, different alkylamines vary in reaction activities with acidic molecules, which may depend on the degree of basicity as well as on steric hindrance. Figure 8 shows the temperature dependence of HCl removal for two different amine-functionalized organosilica materials in one of our earlier studies [21]. APTES, with unhindered primary amine groups, demonstrated greater difficulties for the removal of the stable HCl that existed at temperatures as high as 200 \({}^{\circ}\)C. By contrast, in the case of BTPP with pyrimidine (hindered amine) groups, HCl had started to diminish at the first measured temperature, which was as low as 50 \({}^{\circ}\)C. Hence, a temperature of higher than 200 \({}^{\circ}\)C should be used to reactivate the amine groups, particularly for unhindered amines. Note that calcination under vacuum could to some extent lower the temperature employed. On the other hand, this result indicated that unhindered amines and sterically hindered amines showed very different reaction/interaction activities with acidic molecules. Furthermore, the different reaction activities may play an important role in CO\({}_{2}\) transport behaviors across the relevant membranes. In another study [34], we also investigated the HCl removal among three amine-functionalized organosilica materials (PA-Si, SA-Si, and TA-Si shown in Figure 3) via EDS measurement. Figure 9 shows the HCl removal of these xerogel powders as a function of post-heat-treatment. After post-heat-treatment at 250 \({}^{\circ}\)C, the Cl signals almost disappeared for all three samples, but the removal rate was variable due to the differences in reaction activities with HCl (see Table 4). Specifically, HCl can be much easier to remove from xerogel powders in the case of TA-Si with tertiary amines compared with the other two samples with primary (PA-Si) or secondary (SA-Si) amines.  
Figure 8: Temperature dependence of residual HCl in APTES and BTPP xerogel powders measured via EDS. The mole ratios of HCl/NH\({}_{2}\) = 1 for APTES and HCl/pyridine = 1 for BTPP were employed in the preparation of _polymer_ sols. Prior to EDS measurement, APTES and BTPP xerogel powders prepared by drying the solvent of the sols at \(\sim\)50 \({}^{\circ}\)C, were fired under N\({}_{2}\) for 30 min at 50, 100, 150, 200, 250, 300, and 250 \({}^{\circ}\)C. The molar compositions of the elements of Cl and Si were recorded and then the ratios of Cl/NH\({}_{2}\) for APTES and Cl/pyrimidine for BTPP were calculated by assuming molar ratios of Si over either NH\({}_{2}\) or pyrimidine groups that were the same as those of the associated precursors. Reproduced from ref. [21] with permission. Copyright (2017), Elsevier.