## 1 Introduction
Theoretically, this phenomenon could offer innovations for the design and development of amine-containing CO2 separation membranes that integrate CO2 adsorption, diffusion, and desorption processes into a thin membrane layer. Indeed, Prof. [PERSON]'s group developed and studied a series of CO2-selective polymeric membranes containing sterically hindered polyamines [18; 19; 20]. They found that the steric hindrance effect of polyamines could significantly promote CO2 transport performance in both CO2 permeability and CO2/gas (gas = H2, N2) selectivity, and this trend was more distinct in the case of moderately hindered polyamines (see Figure 2) [18]. Unfortunately, only a very limited number of systematic studies providing simultaneous CO2 adsorption and diffusion/desorption kinetic properties have been devoted to amine-functionalized, silica-based membranes. Given this, in our earlier studies, we developed a series of amine-containing organosilica membranes including unhindered (PA-Si, SA-Si) and sterically hindered (TA-Si, BTPP) amines, as well as a quaternary ammonium salt (QA-Si), and investigated the effect that amine type exerted on the CO2 adsorption/desorption properties of solid powders and the related membrane performance (see Figure 3) [21; 22; 23; 24]. It seems really important to more systematically compare the CO2 separation performance of amine-functionalized silica-based membranes, despite many review papers for CO2 separation membranes in the literature, which mostly review hydrocarbon-based membranes [4; 25]. In this review, we briefly summarize studies on amine-functionalized silica/organosilica membranes and pay special attention to the role of amine type in CO2 separation performance. Synthesis approaches, characterizations of the xerogel powders in terms of CO2 adsorption/desorption properties, and comparison of CO2 transport behaviors within related membranes are included. Furthermore, future prospects and possible directions for the design and fabrication of amine-containing membranes for CO2 separation are also presented, and this portion is not limited to silica-based membranes. or heterocyclic. The diverse structures of amines confer basicity and a reaction activity that varies with acidic molecules (e.g., CO\({}_{2}\), H\({}_{2}\)S). Generally, there are three factors that influence amine basicity: alkyl group substitution (including solvation effects and steric hindrance), resonance effect, and hybridization effect. The interlaced influences complicate the actual situation and make it more difficult to predict the down-to-earth basicity (or even the basicity order), which, however, can be reflected by the actual measurement of _pKa_. Consequently, the normal aliphatic amines of primary, secondary and tertiary types are all of approximately equal basicity, which is generally stronger than aromatic amines. Notwithstanding, it is the steric hindrance effect rather than amine basicity that plays a key role in impacting their reaction activities with CO\({}_{2}\). Usually, a normal aliphatic primary or secondary amine can form a zwitterion upon encountering a CO\({}_{2}\) molecule (Equation (1)) and can then proceed with the formation of a fairly stable carbamate complex with another amine group (Equation (2)). Under humid conditions, this carbamate complex can further react with a water molecule and form a bicarbonate ion and a free amine group (Equation (3)) [26]. \[\text{RR}^{*}\text{NH}+\text{CO}_{2}\leftrightarrow\text{RR}^{*}\text{NH}^{+} \text{COO}^{-} \tag{1}\]  
\[\text{RR}^{*}\text{NH}^{+}\text{COO}^{-}+\text{RR}^{*}\text{NH}\leftrightarrow \text{RR}^{*}\text{NCOO}^{-}+\text{RR}^{*}\text{NH}_{2}^{+} \tag{2}\]  
\[\text{RR}^{*}\text{NCOO}^{-}+\text{H}_{2}\text{O}\leftrightarrow\text{RR}^{*} \text{NH}+\text{HCO}_{3}^{-} \tag{3}\]  
In Equations (1) to (3), R\({}^{*}\) represents a H atom for a primary amine or another alkyl group. Equation (1) is a nucleophilic addition reaction, which is the rate-limiting process regardless of whether water is involved.