## 1 Introduction  
The ever-increasing rate of greenhouse gas emissions in the atmosphere, particularly CO\({}_{2}\) generated from the combustion of fossil fuels, has led to serious global climate concerns. Therefore, efficient CO\({}_{2}\) capture technologies such as absorption/adsorption of CO\({}_{2}\) sorbents, cryogenic separation, and membrane separation, have been developed experimentally and industrially over the past few decades. Among them, membrane technology is attracting growing attention owing to its inherent advantages that include energy-efficiency, ease of operation and scale-up, and a small footprint [1; 2; 3; 4; 5]. As one of the most important members of the membrane family, organosilica membranes have been applied to various membrane processes with superior separation performance and a high tolerance to harsh conditions. Typical organosilica membranes developed for CO\({}_{2}\)/N\({}_{2}\) separation are either microporous with rigid pores or nonporous with \"free-volume\" pores. Separations are therefore achieved mainly by the effect of molecular sieving along with partial contributions from selective surface diffusion through rigid pores and/or solution-diffusion within a nonporous structure. The selectivity is always less arresting due to the comparable kinetic diameters of CO\({}_{2}\) (3.3 A) and N\({}_{2}\) (3.64 A) and due as well to the difficulty in precisely controlling the pore (rigid or free-volume type) sizes. Given this, various amine-functionalized silica-based membranes have been developed to accomplish CO\({}_{2}\) separation from gas mixtures with potentially enhanced efficiency [6; 7; 8; 9; 10; 11; 12; 13]. These membranes are expected to show great potential in simultaneously promoting CO\({}_{2}\) permeance and permselectivity due to the reversible reactions between CO\({}_{2}\) and amine groups. Thus far, however, only a limited number of membranes have offered appealing CO2 separation performance with both high CO2 permeance and CO2/gas permselectivity irrespective of many reports of amine-functionalized silica-based membranes. Instead, in some cases even reverse CO2/N2 selectivity under a dry state has been observed [13]. Whereas, in some cases, other types of membranes with only CO2-philic groups rather than chemical reactions, such as poly(ethylene oxide)-based membranes, have resulted in attractive CO2 separation performance [2]. Hence, further fundamental studies in terms of the effect of amine-CO2 chemical interactions on CO2 transport behaviors through amine-functionalized silica-based membranes are needed to direct the fabrication of high-performance CO2 separation membranes. In general, how fast and selectively the species to be separated can enter into, transport through, and exit from the membrane govern the membrane separation properties in terms of both flux and selectivity. Therefore, the reaction activities of amine groups (or membrane affinity) should be regarded in a broad sense, comprising not only the favorable and rapid reaction/interaction with CO2 molecules but also the promoted/inhibited motion of CO2 within amine-containing membranes. Consequently, to push the CO2 separation performance (or transport efficiency) of these membranes to the limit, the first idea to keep in mind in the design process should be to choose an amine-containing material with a convenient affinity for CO2 molecules. In fact, the effects that amine type (primary, secondary, and tertiary), basicity (or _pKa_), and steric hindrance can exert on CO2 sorption performance in terms of absorption/adsorption capacity and rate, as well as energy cost during the CO2 regeneration/release process, have been extensively studied in the field of amine-based CO2 sorbents (solid adsorbents or aqueous alkylamine solutions) [14; 15; 16; 17]. It is common knowledge that tertiary and/or sterically hindered amines in some cases could exceed the trade-off between the heat of reaction and the CO2 absorption rate (see Figure 1), which thus enables energy-saving CO2 sorption and release processes.