## 4 Role of Amine Type in CO2 Separation Performance
### Gas Permeation Properties
This implies that the hindered amines showed a greater advantage in facilitating CO\({}_{2}\) transport through amine-functionalized silica-based membranes (based on single gas permeation test), consistent with the results observed (based on binary gas separation test) by Prof. [PERSON]'s group (see Figure 2) [18; 19; 20]. Therefore, these results suggested that the amine type could play a coincidental role in CO\({}_{2}\) transport behaviors for either single or binary/mixed gas systems, given that the differences between single and mixed gas separation performance can be generally observed due to the competitive adsorption/interaction. This consistent tendency can probably be ascribed to the intermediate-to-low CO\({}_{2}\) binding energy of hindered amines that enables both fast adsorption on and fast diffusion/desorption through/from these membranes of CO\({}_{2}\) molecules, as illustrated in one of our previous studies (see Figure 17) [21]. Generally, all the gases could permeate across membranes via less-interactive diffusion while sorptive gases (CO\({}_{2}\), etc.) permeate the amine-functionalized membranes via interactive diffusion in addition to less-interactive diffusion. Different amine types could lead to different CO\({}_{2}\) binding energies and in turn to different sorption types. The CO\({}_{2}\) binding energy of the active carrier sites probably plays an important role in CO\({}_{2}\) transport efficiency considering the contribution of interactive diffusion. Owing to the formation of a stable carbamate or to strong CO\({}_{2}\)-amine interactions with high CO\({}_{2}\) binding energy, a dual-mode sorption always occurs, and the effective CO\({}_{2}\) transport through membranes with unhindered amines  
Figure 15: Kinetic diameter dependence of gas permeance for QA-Si derived membranes fired at different temperatures and performance comparison with TA-Si derived membranes at both (**a**) high and (**b**) low temperatures. Gas permeation test of QA-180 fabricated with a calcination temperature of 180 \({}^{\circ}\)C was performed at 180 \({}^{\circ}\)C and 200 \({}^{\circ}\)C for the remaining three membranes. Reproduced from ref. [23] with permission. Copyright (2017), John Wiley and Sons. (strong-affinity) may be restricted. On the other hand, by employing sterically hindered amines, reversible CO\({}_{2}\) adsorption/desorption behavior in the xerogel powders can be observed and a good balance between favorable adsorption and prohibited diffusion/desorption of CO\({}_{2}\) is thus expected in relevant membranes. As a result, more effective CO\({}_{2}\) transport through these membranes could be accomplished with an intermediate-to-low level of CO\({}_{2}\) binding energy. However, it is worth to note that in some cases enhanced affinity to CO\({}_{2}\) of a membrane was proposed to facilitate more favorable CO\({}_{2}\) sorption so as to limit access by other gases, resulting in a higher CO\({}_{2}\)/gas selectivity in a mixed gas system [43]. Indeed, in a certain range, a higher CO\({}_{2}\) binding energy would improve CO\({}_{2}\)/gas sorption selectivity and lead to a high separation performance. Nevertheless, this tendency generally occurs on the conditions that the increases in CO\({}_{2}\) binding energy (or adsorption heat) could not largely affect the reversible adsorption properties, since too sufficiently high CO\({}_{2}\) binding energy may result in irreversible CO\({}_{2}\) adsorption that cannot generate flux.