## 4 Role of Amine Type in CO2 Separation Performance
### Thermal Stability and Texture Microstructure  
Generally, amorphous silica-based membranes should be fabricated at a calcination temperature that is sufficiently high to provide a well-crosslinked silica-based network for precise separation. Consequently, to prepare amine-functionalized silica-based membranes via a sol-gel route a moderate calcination temperature should be considered to avoid the thermal degradation of amine groups. Typically, there are two types of alkylamine degradation: thermal and oxidative [35]. Hence, to retain the utmost level of amine groups in the amine-functionalized membranes, both a sufficiently \"safe\" calcination temperature and an inert calcination atmosphere (e.g., He, N2) must be considered. The thermal stability of alkylamine groups contained in silica-based matrix usually can be determined via methods such as Fourier transform infrared (FTIR) spectroscopy, energy dispersive spectroscopy (EDS), and X-Ray photoelectron spectroscopy (XPS) analysis, while the amine density, if necessary, can be quantitatively assessed via XPS analysis. To rationally investigate the role of amine type on CO2 transport behaviors, first, the availability and density of the amine types should be at comparable levels, since this plays a considerable role in CO2 transport behavior. In our earlier studies [22] comparing CO2 separation performance among three amine-functionalized organosilica membranes (PA-Si, SA-Si, and TA-Si, see Figure 3 for their chemical structures), we evaluated the amine state and density for these amine-containing solid powders, as shown in Figure 5 and Table 3. Generally, both free amine groups (C-NH4, \(x\) = 0-2) and protonated amine groups (C-N+H4, \(x\) = 1-3) induced by surface Si-OH groups or  
Figure 4: Schemes of (**a**) CO2 separation through a membrane and (**b**) the factors impacting CO2 transport behaviors including adsorption on the membrane surface, diffusion across the membrane thickness, and desorption from the membrane permeate surface. a small amount of adsorbed CO\({}_{2}\) molecules could be observed for samples using different amine types (see Figure 5). In addition, all samples demonstrated comparable levels in amine density that were also comparable to their expected values as shown in Table 3. This indicated that the amine groups survived under the adopted calcination conditions (at 250 \({}^{\circ}\)C under a N\({}_{2}\) atmosphere) which enabled investigation of the effects amine type on membrane performance. In addition, it is worth to note that in the practical applications the activity and stability of amine groups can also be evidently affected by the presence of toxic components such as H\({}_{2}\)S and water steam. These factors, unfortunately, have not always been involved in investigations of the operation stabilities for amine-containing membranes, and, therefore, relevant systematic studies are anticipated. The physical microstructure of these amine-functionalized silica-based materials is also a very important factor, because this decides CO\({}_{2}\) transport behavior. Gas (N\({}_{2}\), CO\({}_{2}\) etc.) adsorption measurement is a typical approach for the determination of the texture microstructure of micro-to-mesoporous materials, which includes silica-based materials. Following the incorporation of alkylamine moieties, however, a possible dual flexible-rigid network can be formed, and therefore an apparently nonporous structure was observed, as evidenced by the N\({}_{2}\) adsorption/desorption isotherms operated at 77 K. This phenomenon was also generally observed in the cases of organic-rich organosilica membranes, as reported elsewhere [21; 24; 36]. Figure 6 shows the N\({}_{2}\) adsorption/desorption isotherms of several amine-functionalized organosilica materials in our earlier studies. The negligible N\({}_{2}\) adsorption amount (<0.2 cm\({}^{3}\)[STP]/g) for all the powders indicates the formation of a dual flexible-rigid network, in which \"free-volume pores\" were created instead of rigid pores.