## 4 Role of Amine Type in CO2 Separation Performance
### Gas Permeation Properties
Generally, the diffusivity of a penetrant is mainly decided by the relative difference between its kinetic diameter and the membrane pore size and/or the segment mobility of the membrane matrix, which demonstrates the ever-increasing nature of the function of temperature (\(E_{d}>0\)). Whereas solubility is greatly influenced by the condensability (critical temperature) of the penetrant and by the affinity/interactions that the membrane matrix has with the penetrant, there often is also a decrease in the function of temperature (\(\Delta H_{s}<0\)) [41; 42]. Hence, He, H\({}_{2}\), N\({}_{2}\), and CH\({}_{4}\), which have no special interactions with amine groups, showed a significant level of activated diffusion (\(E_{p}>0\)), suggesting that temperature has a greater effect on diffusivity than it does on solubility. However, for CO\({}_{2}\) that has strong interactions with amine groups as well as higher solubility in an organic-rich phase, the temperature dependence of permeance differed notably among these four amine-functionalized membranes. The details of these differences are hereinafter compared according to the activation energy for the permeation of CO\({}_{2}\) [\(E_{p}(CO_{2})\)]. Figure 13: Kinetic diameter dependence of gas permeance (left: actual permeance; right: normalized permeance) operated at 200 \({}^{\circ}\)C for amine-functionalized organosilica membranes. The gas permeances were normalized based on their He permeances as standards. Results were reorganized from refs. [21; 22]. In addition to amine type, microporosity is another important factor impacting permeation performance of CO2 as well as that of other gases. In one of our previous studies [23], we used a similar method to develop a microporosity-enhanced, amine-functionalized, organosilica membrane via the thermally induced local liberation effect of QA-Si. In that study, we observed a dealkylation reaction of quaternary ammonium groups at \(\sim\)230 \({}^{\circ}\)C in polycondensed QA-Si derived membranes with the liberation of CH3 Cl molecules. The membranes after liberation of CH3 Cl via either direct calcination at 250 \({}^{\circ}\)C (QA-250) or post-heat treatment (QA-180-250) (see PALS data in Figure 7) demonstrated significantly improved gas permeation performance compared with the membrane fabricated from TA-Si without a liberation effect (see Figure 15). In the case of the liberation of CH3 Cl, a quaternary ammonium chloride was transformed to a tertiary amine group, in which the amine accessibility was improved as a result of the creation of microporosity around the tertiary amine groups. Figure 14: Temperature dependence of gas permeance for amine-functionalized organosilica membranes: (**a**) PA-Si; (**b**) SA-Si; (**c**) TA-Si; and (**d**) BTPP. Results were reorganized from refs. [21; 22]. Figure 16 compares the trade-off between CO\({}_{2}\) permeance and CO\({}_{2}\)/N\({}_{2}\) permselectivity of the amine-functionalized organosilica membranes developed by our group. The membranes functionalized with hindered amines tended to show superior CO\({}_{2}\) separation compared with those with unhindered amines as well as quaternary ammonium. In addition, the membranes with hindered amines demonstrated a greater ability to surpass the Robeson upper bound generally seen in polymeric membranes. Herein, the Robeson upper bound was plotted in the term of permeance rather than permeability by assuming a membrane thickness of 500 nm, due in large part to the difficulties in accurate determination of the thickness of very thin organosilica selective layer with interpenetrations into a nanoporous sublayer and due in part to the typical thickness of organosilica membranes that are estimated to range from 200 to 600 nm, as discussed elsewhere [23; 24].