## 4 Role of Amine Type in CO2 Separation Performance
### CO\({}_{2}\) Adsorption/Desorption Behaviors  
As mentioned above, CO\({}_{2}\) adsorption/desorption behaviors are important factors that probably largely affect their transport behaviors across amine-containing membranes. Therefore, this review summarizes CO\({}_{2}\) adsorption/desorption properties including both static (measured by adsorption isotherms) and kinetic (measured by TG/DTA method) evaluations for the amine-functionalized organosilica materials studied in our previous studies [13; 34]. Figure 10 shows the CO\({}_{2}\) adsorption/desorption isotherms at 35 \({}^{\circ}\)C for four amine-functionalized organosilica xerogel powders with unhindered amines (left figure), as well as for tertiary and sterically hindered amines (right figure). It should be apparent that the xerogel powders with unhindered amines demonstrated irreversible Langmuir-dominant adsorption/desorption isotherms with hysteresis loops probably due to the formation of stable carbamates after CO\({}_{2}\) adsorption, whereas tertiary and sterically hindered amines showed almost reversible Henry-dominant adsorption/desorption isotherms. Generally, CO\({}_{2}\) adsorption behaviors on amine-functionalized solid materials can be described by a dual-mode sorption model as a combination of Langmuir-type and Henry-type sorption as discussed in our earlier studies [23]. Note that SA-Si displayed a much higher adsorption capacity than the other three samples, which were comparable, presumably due to a somewhat stronger basicity that could improve the reaction capacity [10; 37; 38]. In addition to adsorption/desorption isotherms, the values of the isosteric adsorption heat of CO\({}_{2}\) (\(Q_{Si}\)), which can also be regarded as CO\({}_{2}\) binding energy, for these xerogel powders were also calculated using _Clausius-Clapeyron_ equation, as summarized in Figure 11. Consistent with their adsorption type, xerogel powders with unhindered amines presented impressively higher levels of adsorption heat at lower levels of CO\({}_{2}\) uptake by comparison with hindered amines. However, their adsorption heat decreased rapidly as the CO\({}_{2}\) uptake increased before reaching a 30% level of CO\({}_{2}\) saturated sorption, which suggested the presence \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multirow{2}{*}{**Material**} & \multicolumn{3}{c}{**Post-Heat-Treatment Temperature [\({}^{\circ}\)C]**} \\ \cline{3-5}  & & **150** & **200** & **250** \\ \hline \multirow{2}{*}{PA-Si} & Cl (at. \%) & 3.37 & 3.50 & 0.79 \\  & Cl/Si (molar ratio) a  & 0.26 & 0.28 & 0.048 \\ \hline \multirow{2}{*}{SA-Si} & Cl (at. \%) & 3.31 & 3.34 & 0.63 \\  & Cl/Si (molar ratio) & 0.25 & 0.26 & 0.046 \\ \hline \multirow{2}{*}{TA-Si} & Cl (at. \%) & 4.8 & 4.3 & 0.14 \\  & Cl/Si (molar ratio) & 0.49 & 0.42 & 0.017 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Residual Cl content in amine-silica xerogel powders as a function of post-heat-treatment temperature (N\({}_{2}\) atmosphere, 30 min). Reproduced from ref. [22] with permission. Copyright (2017), _Elsevier_. Figure 9: EDS profiles of amine-silica xerogel powders as a function of post-heat-treatment temperature (N\({}_{2}\) atmosphere, 30 min). Reproduced from ref. [22] with permission. Copyright (2017), _Elsevier_. of heterogeneous surfaces containing alkylamine moieties and Si-O-Si (or Si-OH) networks. On the contrary, xerogel powders with hindered amines demonstrated intermediate-to-low values for \(Q_{st}\) throughout the adsorption process. Generally, \(Q_{st}\) also can be regarded as CO\({}_{2}\) binding energy, and higher values equate to greater difficulties for CO\({}_{2}\) desorption. It is interesting that the kinetic CO\({}_{2}\) adsorption/desorption results measured by TG/DTA methods verified the different difficulties among these xerogel powders, as shown in Figure 12. It seems that all samples demonstrated a fairly fast adsorption process regardless of amine type but demonstrated significant differences in the CO\({}_{2}\) desorption processes.