Carbon Capture Using Porous Silica Materials

As the primary greenhouse gas, CO2 emission has noticeably increased over the past decades resulting in global warming and climate change. Surprisingly, anthropogenic activities have increased atmospheric CO2 by 50% in less than 200 years, causing more frequent and severe rainfall, snowstorms, flash floods, droughts, heat waves, and rising sea levels in recent times. Hence, reducing the excess CO2 in the atmosphere is imperative to keep the global average temperature rise below 2 °C. Among many CO2 mitigation approaches, CO2 capture using porous materials is considered one of the most promising technologies. Porous solid materials such as carbons, silica, zeolites, hollow fibers, and alumina have been widely investigated in CO2 capture technologies. Interestingly, porous silica-based materials have recently emerged as excellent candidates for CO2 capture technologies due to their unique properties, including high surface area, pore volume, easy surface functionalization, excellent thermal, and mechanical stability, and low cost. Therefore, this review comprehensively covers major CO2 capture processes and their pros and cons, selecting a suitable sorbent, use of liquid amines, and highlights the recent progress of various porous silica materials, including amine-functionalized silica, their reaction mechanisms and synthesis processes. Moreover, CO2 adsorption capacities, gas selectivity, reusability, current challenges, and future directions of porous silica materials have also been discussed.


Introduction
With the exponential growth of industrialization, global warming and climate change have become worldwide concerns and have attracted much attention in recent decades [1]. Furthermore, human activities have significantly contributed to the increased levels of CO 2 in the atmosphere. For example, atmospheric CO 2 measured at NOAA's Mauna Loa Atmospheric Baseline Observatory peaked for 2021 at a monthly average of 419 parts per million (ppm), and it is reported as the highest level since accurate measurements began 63 years ago [2].
The increase in CO 2 concentration leads to the rise in global temperature and sea levels, alternative of rainfall patterns, extinction of species, natural disasters such as severe weather events, ranging from flash floods, hurricanes, freezing winters, severe droughts, heat waves, urban smog, and cold streaks [3].
The main CO 2 stationary emission sources are power plants, refineries, chemical and petrochemical, iron and steel, gas processing, and cement industries. More irreversible and adverse environmental impacts should be expected if atmospheric carbon dioxide continues to rise. Therefore, the international communities led by the United Nations reached a landmark global accord, the Paris Agreement, adopted by 196 nations in 2015 to address climate change and related issues. Moreover, countries around the globe made their "nationally determined contributions (NDCs)" of greenhouse gas reduction. Different approaches employed in different countries to reduce CO 2 emissions are shown in Table 1. Table 1 also summarizes the major advantages and disadvantages of each approach. Table 1. Different approaches used in different countries in order to reduce the CO 2 emissions [4].

Type of Approach Details
Improve energy efficiency and promote energy conservation Among these approaches, the CO 2 capture and storage (CSS) can reduce CO 2 emissions by 85-90% from large emission sources [4]. CCS includes different CO 2 capture, separation, transport, storage technologies, and chemical conversion, which are discussed in detail below.

CO 2 Capture Technologies
Capture and sequestration of CO 2 (CCS) from aforementioned stationary emission sources has been identified as a paramount option for the issues of global warming and climate change. CCS includes four primary steps known as CO 2 capture, compression, transport, and storage, therefore, developing an efficient and economically feasible technology for the capture and sequestration of CO 2 produced by anthropogenic emissions is critically important. CO 2 capture is the central part of the CCS technology process and gained around 70-80% of the total expensive. However, CSS methods can be classified as, for example, (i) Post-combustion (ii) Pre-combustion, and (iii) Oxy-fuel combustion (Oxygen-fired combustion) [5,6].
In post-combustion capture technology, it collects and separates the CO 2 from the emission gases of a combustion system [7][8][9][10][11]. Firstly, flue gas (mainly consists of CO 2 , H 2 O, and N 2 ) passes through denitrification and desulphurization treatments. As the next step, the flue gas is fed to an absorber which contains solvent. Herein, CO 2 regeneration occurs. Then the CO 2 -rich absorbent is sent to a CO 2 -stripper unit to release the CO 2 gas. Moreover, CO 2 -lean absorbent is sent back to the CO 2 -absorber unit [1]. Next, the captured CO 2 is then compressed into supercritical fluid and then transported [1] as shown in Figure 1.
is then compressed into supercritical fluid and then transported [1] as shown in Figure  Pre-combustion capture is a technology where CO2 is captured before the combustio process and CO2 is generated as an intermediate co-product of conversion process [12 The pre-combustion technologies are mainly used in power plants, production of fertili ers and natural gas [13,14]. In oxyfuel combustion, the carbon-based fuel consumes in re-circulated flue gas an oxygen (O2) stream. CSS capture technology is considered expensive due to the high co of O2 separation and production. However, the capture and separation of CO2 are reaso ably easy compared to other methods and is considered as an energy-saving method [15 Among the currently available technologies, post-combustion capture has grabbe much attention because it can be easily accomplished, applicable for large scale-pow plants, easily managed and required short time for CO2 capture compared to other ava able methods [1]. Post-combustion capture uses different methods for gas separation, an collects CO2 by adsorption/desorption, as shown in Table 2, including absorption [6,16 adsorption [6,17], membrane-based technologies [18,19], and cryogenics [20]. Table 2 al depicts the efficiency, advantages, and disadvantages of the different types of post-com bustion capture technologies.
Absorption process mainly uses liquids to capture CO2. During adsorption, once CO is separated from the gas, the sorbent should be regenerated by using a stripper, heatin or depressurization. Moreover, this method is considered as the most established proce for CO2 separation [21]. In general, adsorbents can be divided into two types, namel chemical and physical adsorbents (see Table 2 for details). Pre-combustion capture is a technology where CO 2 is captured before the combustion process and CO 2 is generated as an intermediate co-product of conversion process [12]. The pre-combustion technologies are mainly used in power plants, production of fertilizers and natural gas [13,14].
In oxyfuel combustion, the carbon-based fuel consumes in re-circulated flue gas and oxygen (O 2 ) stream. CSS capture technology is considered expensive due to the high cost of O 2 separation and production. However, the capture and separation of CO 2 are reasonably easy compared to other methods and is considered as an energy-saving method [15].
Among the currently available technologies, post-combustion capture has grabbed much attention because it can be easily accomplished, applicable for large scale-power plants, easily managed and required short time for CO 2 capture compared to other available methods [1]. Post-combustion capture uses different methods for gas separation, and collects CO 2 by adsorption/desorption, as shown in Table 2, including absorption [6,16], adsorption [6,17], membrane-based technologies [18,19], and cryogenics [20]. Table 2 also depicts the efficiency, advantages, and disadvantages of the different types of postcombustion capture technologies.
Absorption process mainly uses liquids to capture CO 2 . During adsorption, once CO 2 is separated from the gas, the sorbent should be regenerated by using a stripper, heating, or depressurization. Moreover, this method is considered as the most established process for CO 2 separation [21]. In general, adsorbents can be divided into two types, namely, chemical and physical adsorbents (see Table 2 for details).

Criteria for Selecting CO 2 Sorbent Material
Certain economical and technical properties are required in order to select the best solid adsorbent candidate for a particular CO 2 capture application. These criteria are listed and described below.

•
Adsorption capacity for CO 2 : The equilibrium adsorption capacity of a sorbent material is represented by its equilibrium adsorption isotherm. The adsorption capacity is an important parameter when considering the cost. Moreover, which causes reduction in the sorbent quantity, and in the size of the adsorption column. However, to enhance the adsorption capacity of solid sorbents, functionalization has been carried out with existing monoethanolamine (MEA) [24]. The CO 2 working capacity should be in the range of 2-4 mmol/g of the sorbent [25].

•
Selectivity for CO 2 : The adsorption selectivity or selectivity of CO 2 is explained as the sorption uptake ratio of a target gas species compared to another type (as example N 2 ) contained in a gaseous mixture under given operation conditions. Therefore, it depends on the purity of the adsorbed gas in the effluent [21]. However, the purity of CO 2 influences transportation and sequestration and, therefore, this criterion plays an important role in CO 2 sequestration [24].

•
Adsorption and desorption kinetics: It is necessary to have fast adsorption/desorption kinetics for CO 2 and it controls the cycle time of a fixed-bed adsorption system. Fast kinetics results in a sharp CO 2 breakthrough curve in which effluent CO 2 concentration changes are measured as a function of time, while slow kinetics provides a distended breakthrough curve. However, both fast and slow adsorption and desorption kinetics impact on the amount of sorbent required. In functionalized solid sorbents, the overall kinetics of CO 2 adsorption mainly depend on the functional groups present, as well as the mass transfer or diffusional resistance of the gas phase through the sorbent structures. The porous support structures of functionalized solid sorbents also can be tailored to minimize the diffusional resistance. The faster an adsorbent can adsorb CO 2 and be desorbed, the less of it will be needed to capture a given volume of flue gas [24].

•
Mechanical strength of sorbent particles: The sorbent must show the stable microstructure and morphological structure in adsorption and regeneration steps. Mainly disintegration of the sorbent particles occurs due to the high volumetric flow rate of flue gas, vibration, and temperature. Apart from that, this could also happen due to abrasion or crushing. Therefore, a sufficient mechanical strength of a sorbent particles is required to keep CO 2 capture process cost-effective [24].

•
Chemical stability/tolerance towards impurities: Solid CO 2 capture sorbents such as amine-functionalized sorbents should be stable in an oxidizing environment of flue gas and should be resistant to common flue gas contaminants [24].

•
Regeneration of sorbents: The regeneration of the sorbent is energy saving and is one of the most important parameters required for improving energy efficiency [26]. Regeneration can be achieved through the adjustment of the thermodynamics of the interaction between CO 2 and the solid adsorbent [24]. Considering regeneration, physisorption is mostly favored over chemisorption since the latter involves high energy consumption for regeneration.

•
Sorbent costs: The production cost is the main key point when considering industrial applications at reasonable gas selectivity and adsorption performance [24].

Liquid Amine for CO 2 Capture
Development of solvents for CO 2 chemical absorption is a major area of research [27]. The ideal solvent should have a high CO 2 absorption capacity and react rapidly and reversibly with CO 2 with minimal heat requirement. The solvent should exhibit the following properties such as stability in oxidative and thermal environment, low vapor pressure, toxicity, flammability, and reasonable production cost [27].
Recently, a most promising CO 2 capture method with chemical absorption is by using liquid amine which can be divided mainly into two groups known as simple alkanolamines and sterically hindered amines [28]. Examples for simple alkanolamines are monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA) [29,30]. Furthermore, alkanolamines are the most widely used sorbents for CO 2 capture. The structures of alkanolamines include primary, secondary, ternary amines containing at least one hydroxyl (-OH) group and amine group-(N-R) as shown in Table 3.
However, these different amine classes have different reaction kinetics with CO 2 , CO 2 absorption capacity and equilibria, stability, and corrosion [28]. Advantages and disadvantages among the alkanolamines are shown in Table 3. As shown in Equations (1) and (2) below, both primary and secondary amines react with CO 2 to form a carbamate and protonated amine, consuming approximately two moles of amine per mole of CO 2 according to the zwitterion mechanism [31]. According to Equation (3), tertiary amines react with CO 2 gas molecules in the presence of H 2 O while forming bicarbonates.
(where R 1 , R 2 , and R 3 are aryl/alkyl groups). However, García-Abuín et al. [32] observed that MEA produced a mixture of carbamate and bicarbonate as the main reaction products during CO 2 absorption. The reaction starts with the reversible reactions between MEA and CO 2 to form carbamate at low CO 2 loading, followed by the CO 2 hydration to form HCO 3 − /CO 3 2− under high CO 2 loading, and accompanied by the hydrolysis of carbamate. The reaction mechanism of CO 2 capture into MEA solution with different CO 2 loadings is shown in Figure 2.

Liquid Amine for CO2 Capture
Development of solvents for CO2 chemical absorption is a major area of research [27]. The ideal solvent should have a high CO2 absorption capacity and react rapidly and reversibly with CO2 with minimal heat requirement. The solvent should exhibit the following properties such as stability in oxidative and thermal environment, low vapor pressure, toxicity, flammability, and reasonable production cost [27].
Recently, a most promising CO2 capture method with chemical absorption is by using liquid amine which can be divided mainly into two groups known as simple alkanolamines and sterically hindered amines [28]. Examples for simple alkanolamines are monoethanolamine (MEA), diethanolamine (DEA), and triethanolamine (TEA) [29,30]. Furthermore, alkanolamines are the most widely used sorbents for CO2 capture. The structures of alkanolamines include primary, secondary, ternary amines containing at least one hydroxyl (-OH) group and amine group-(N-R) as shown in Table 3.
However, these different amine classes have different reaction kinetics with CO2, CO2 absorption capacity and equilibria, stability, and corrosion [28]. Advantages and disadvantages among the alkanolamines are shown in Table 3. As shown in Equations (1) and (2) below, both primary and secondary amines react with CO2 to form a carbamate and protonated amine, consuming approximately two moles of amine per mole of CO2 according to the zwitterion mechanism [31]. According to Equation (3), tertiary amines react with CO2 gas molecules in the presence of H2O while forming bicarbonates.

CO₂ + 2R₁NH₂ ↔ R₁NH₃⁺ + R₁NHCOO⁻
(1) (where R1, R2, and R3 are aryl/alkyl groups). However, García-Abuín et al. [32] observed that MEA produced a mixture of carbamate and bicarbonate as the main reaction products during CO2 absorption. The reaction starts with the reversible reactions between MEA and CO2 to form carbamate at low CO2 loading, followed by the CO2 hydration to form HCO3 − /CO3 2− under high CO2 loading, and accompanied by the hydrolysis of carbamate. The reaction mechanism of CO2 capture into MEA solution with different CO2 loadings is shown in Figure 2.         Table 3, there are three categories of alkanolamines that show increased capital costs due to requirement of specialized and expensive materials for construction [28]. On the contrary, degradation of alkanolamine causes operational, and environmental problems including high amount of absorbent required, corrosion of equipment, and demanding of energy [24].
Among three different alkanolamines, MEA is commonly considered as a well-established solvent to separate CO 2 because it can be regenerated easily [35]. On the other hand, Rinprasertmeechai et al. reported the order of CO 2 absorption capacity of the different alkanolamines as MEA > DEA > TEA (see Table 3) [33]. Moreover, they have further showed the regeneration ability of the amines in the following order: MEEA > > DEA > MEA. According to Table 3, MEA exhibits high CO 2 adsorption capacity as it reacts more rapidly with CO 2 compared to MEDA by forming carbamates. However, MEDA shows high regeneration efficiency and requires lower energy [36]. Moreover, Wang et al. found that, when MEA and MEDA are mixed with the appropriate ratio, the energy consumption for CO 2 regeneration is reduced significantly [37].
Sterically hindered amines are based on primary or secondary amines with bulky alkyl groups, which is inhibited from reacting with CO 2 through the effect of steric hindrance [28]. One example of sterically hindered amines is 2-amino-2-methyl-1-propanol (AMP). Steric factor reduces the stability of the formed carbamate due to the weak interaction between the CO 2 molecule and the NH 2 group, promoting fast hydrolysis to form bicarbonate and reducing regeneration energy. Due to the immediate regeneration process of AMP, the NH 2 group can react with CO 2 molecules over and over, increasing CO 2 adsorption (see Table 3). Moreover, Dave et al. [38] compared the CO 2 absorption of different liquid amine classes and showed a lower regeneration energy requirement for 30 wt% AMP over 30% MEA, 30% MEDA, 2.5% NH 3 , and 5% NH 3 [38].

Comparison between Major Non-Carbonaceous Solid Sorbents for CO 2 Capture and Importance of Silica Materials
Due to the low contact area between gas and liquid, low CO 2 loading, and absorbent corrosion associated with liquid amine-based sorbents, solid sorbents for CO 2 capture have attracted significant attention in recent years [42,43]. Various solid adsorbents have been proposed according to their structures and compositions, adsorption mechanisms, and regeneration process [43]. Many solid sorbents are cheap and readily available and show low heat capacities, fast adsorption kinetics, high CO 2 adsorption capacities and selectivity, and high thermal, chemical, and mechanical stabilities [43].
Commercially available solid adsorbents for CO 2 capture include carbonaceous materials such as activated carbons, nanofibrillated cellulose (CFCs), carbon nanotubes (CNTs), and non-carbonaceous materials, including silica, zeolites, hollow fibers, and alumina [6]. These materials show different surface morphologies, pore structures, specific surface areas, and functional groups.
Carbonaceous adsorbents are widely used for CO 2 capture due to their relative abundance, low cost, renewability, and high thermal stability. However, the weak CO 2 adsorption capacities of carbonaceous materials at 50-120 • C make it challenging to use in industrial CO 2 capture [44]. Therefore, much research focus has been given to non-carbonaceous materials. Table 4 tabulates commonly tested non-carbonaceous solid adsorbents for CO 2 capture and their advantages and setbacks. High cost for regeneration • Complicated process [6] As mentioned earlier, carbonaceous adsorbents such as activated carbon have been widely used for CO 2 capture due to their wide availability, low cost, and high thermal stability. However, weak CO 2 adsorption of carbonaceous materials in the range of 50-120 • C leads to high sensitivity in temperature and relatively low selectivity in operation [44]. Therefore, many research works have focused on non-carbonaceous materials such as mesoporous silica, and zeolites due to their advantages, as shown in Table 4.
Zeolites are aluminosilicates with ordered three-dimensional (3D) microporous structures with high crystallinity and surface area [44]. The adsorption efficiencies of zeolites are primarily affected by their size, charge density, and chemical composition of cations in their porous structures [37]. It has been reported that the CO 2 adsorption of zeolites increases as the Si/Al ratio increases and is exchanged with alkali and alkaline-earth cations in the structure of zeolites [45]. However, zeolites present several drawbacks, such as relatively low CO 2 /N 2 selectivity and high hydrophilicity [46]. Apart from the above, zeolites show reduced CO 2 adsorption capacity when CO 2 /N 2 mixtures contain moisture, and zeolites require high temperatures (>300 • C) for regeneration [47].
Recently, metal-organic frameworks (MOFs) have gained much attention owing to their unique properties, such as tunable pore structure and high surface area [48]. However, when exposed to gas mixtures, the MOFs show decreased adsorption capacities [46]. Moreover, previous reports indicate that MOFs are promising materials for CO 2 capture in laboratory settings; however, further research is required to confirm their practical applicability [49]. Water vapor also negatively affects the application of these sorbents by competing and adsorbing them onto physisorbents, thus decreasing their CO 2 adsorption capacity [50].
Ordered mesoporous silica materials are good candidates because of their high surface area, high pore volume, tunable pore size, and good thermal and mechanical stability. So far, mesoporous silica includes the families of MCM (Mobil Company Matter: M41S, Santa Barbara Amorphous type material (SBA-n), anionic surfactant-template mesoporous silica (AMS) [44]. However, the CO 2 adsorption capacities of them observed at atmospheric pressure are not high. Therefore, many studies have been recently reported on the functionalized mesoporous and nanoporous silica for efficient CO 2 capture [51,52].
Several reviews have recently focused on the potential applications of porous silica materials as CO 2 adsorbents. Reddy et al. [53] reported CO 2 adsorption based on porous materials of MOFs, clay-based adsorbents, porous carbon-based materials, and polymerbased adsorbents. Liu et al. [54] also discussed different porous materials, including silica, for post-combustion CO 2 capture [54]. However, more information on silica-based sorbents and their synthesis methods still needs to be available. Therefore, this review mainly discusses CO 2 capture onto different porous and functionalized silica materials. In addition, an overview of synthesis processes and a comparison between the adsorption capacities are also profoundly discussed. Finally, the technical challenges and the future research directions of the porous silica materials for CO 2 adsorption are also presented in this review.

CO 2 Capture Methods
Two general mechanisms are involved in CO 2 capturing using solid sorbents: chemisorption and physisorption. Table 5 represents the major comparison between chemisorption and physisorption. However, the two mechanisms differ in the interactions between the gas molecules and the sorbent surface. During chemisorption, gas molecules are chemically bonded to the surface, whereas in physisorption, there is no chemical binding of the gas molecules to the surface, see Figure 3.
Several reviews have recently focused on the potential applications of porous silica materials as CO2 adsorbents. Reddy et al. [53] reported CO2 adsorption based on porous materials of MOFs, clay-based adsorbents, porous carbon-based materials, and polymerbased adsorbents. Liu et al. [54] also discussed different porous materials, including silica, for post-combustion CO2 capture [54]. However, more information on silica-based sorbents and their synthesis methods still needs to be available. Therefore, this review mainly discusses CO2 capture onto different porous and functionalized silica materials. In addition, an overview of synthesis processes and a comparison between the adsorption capacities are also profoundly discussed. Finally, the technical challenges and the future research directions of the porous silica materials for CO2 adsorption are also presented in this review.

CO2 Capture Methods
Two general mechanisms are involved in CO2 capturing using solid sorbents: chemisorption and physisorption. Table 5 represents the major comparison between chemisorption and physisorption. However, the two mechanisms differ in the interactions between the gas molecules and the sorbent surface. During chemisorption, gas molecules are chemically bonded to the surface, whereas in physisorption, there is no chemical binding of the gas molecules to the surface, see Figure 3.    CO 2 capturing using solid adsorbent is a selective separation [24]. The critical parameters for solid sorbents are surface tension, pore size, temperature, and pressure [24,59]. The adsorption process involves repeated cycles of adsorption and desorption, also known as regeneration. The four main adsorption processes are: (i) Pressure Swing Adsorption (PSA), (ii) Temperature Swing Adsorption (TSA), (iii) Electric Swing Adsorption (ESA), and (iv) Vacuum Swing Adsorption (VSA). Figure 4 shows the four different adsorption processes and their unique characteristics.
In the PSA process, adsorption happens at low pressure, and desorption occurs at high pressure. The adsorption of the TSA process occurs in the temperature range of 40-120 • C and the desorption process in the temperature range of 120-360 • C, respectively [3]. The VSA process involves CO 2 uptake at ambient pressure, then swings to a vacuum condition to regenerate the adsorbent. The ESA process conducts the adsorption-desorption process by changing the electrical supply [3]. Activated carbons, MOF, zeolites, activated alumina, and silica gel are mainly used sorbents in TSA and PSA processes, while ESA is considered less costly compared to those of both TSA and VSA [59].
The microwave-swing adsorption (MWSA) is another adsorption process that has recently received considerable attention due to its efficient energy management. Unlike in conventional heating, where solids heat through conduction and convection, the MWSA process can transfer energy directly to the adsorbate without transferring the heat to both the adsorbate and adsorbent [11,60]. high pressure. The adsorption of the TSA process occurs in the temperature range of 40-120 °C and the desorption process in the temperature range of 120-360 °C, respectively [3]. The VSA process involves CO2 uptake at ambient pressure, then swings to a vacuum condition to regenerate the adsorbent. The ESA process conducts the adsorption-desorption process by changing the electrical supply [3]. Activated carbons, MOF, zeolites, activated alumina, and silica gel are mainly used sorbents in TSA and PSA processes, while ESA is considered less costly compared to those of both TSA and VSA [59]. The microwave-swing adsorption (MWSA) is another adsorption process that has recently received considerable attention due to its efficient energy management. Unlike in conventional heating, where solids heat through conduction and convection, the MWSA process can transfer energy directly to the adsorbate without transferring the heat to both the adsorbate and adsorbent [11,60].

Mesoporous Silica Materials
Mesoporous silica materials are used for various applications, including catalysis and wastewater treatment [61]. Mesoporous silica has unique properties such as uniformity of pore distribution (with size between 0.7 and 50 nm), high surface area (around 1000 m 2 /g), and good thermal stability [62]. The first synthesized mesoporous silica material was M41S in the 1990s [63]. However, the development of surfactants and synthesis protocols have been able to prepare many types of mesoporous silicas such as MCM-41, SBA-15,

Mesoporous Silica Materials
Mesoporous silica materials are used for various applications, including catalysis and wastewater treatment [61]. Mesoporous silica has unique properties such as uniformity of pore distribution (with size between 0.7 and 50 nm), high surface area (around 1000 m 2 /g), and good thermal stability [62]. The first synthesized mesoporous silica material was M41S in the 1990s [63]. However, the development of surfactants and synthesis protocols have been able to prepare many types of mesoporous silicas such as MCM-41, SBA-15, SBA-16, FDU-2, MCM-50, and KIT-5 with a diverse range of pore geometries such as cubic, and hexagonal, and morphologies such as rods, spheres, and discs [64].
In 1990, Mobil Oil Corporation discovered molecular sieves of the M41S family consisting of silicate/aluminosilicate [65]. Typically, these materials are prepared via the sol-gel method. Three well-defined structural arrangements have been identified after studying the effect of surfactant concentration, and those are hexagonal (MCM-41), cubic (MCM-48), and lamellar (MCM-50) structures. Therefore, these materials (M41S family) exhibit mesoporous arrays with amorphous walls of about 10 Å (1 nm) [65]. Moreover, the structural ordering of these M41S family materials can be changed with increasing hydrothermal synthesis temperature and time [65]. These M41S molecular sieves are mainly applied in catalysis [66], adsorption [65], and controlled release of drugs [67]. The main advantage of this mesoporous silica is its unique chemical structure consisting of the high density of functional silanol groups (Si-OH), pore size and shape can be molded during the synthesis process, and the internal surface can be easily modified with organic and inorganic groups [65,68,69].
The Fudan University synthesized mesoporous materials family (FDU-n)-based mesoporous silica arrays with well-ordered mesostructures and pore arrangements, high surface area, large and uniform distribution of pore diameter, amorphous pore-wall structures, and thermal and mechanical stability [74]. FDU-1-based mesoporous materials have a 3D facecentered cubic (FCC) structure with large cage-like mesopores, while FDU-2 mesoporous array possesses a mesostructured FCC unit cell and well-ordered 3D architecture [69].
On the contrary, the mesoporous material series of the KIT-n family, where n = 1, 5, or 6, are mainly represented by the KIT-1, KIT-5, and KIT-6. However, KIT-1-based mesoporous silicas exhibit a 3D architecture in a disordered framework with high surface area, large pore volume and pore diameter, and thermal and hydrothermal stability [75]. KIT-5-based nanostructured mesoporous materials have well-ordered 3D cage-like mesopores in a face-centered close-packed cubic lattice architecture [69]. In addition, KIT-6 shows 3D mesoporous amorphous walls with large pore size, uniform pore distribution, high surface area, and thermal stability [69].
Moreover, mesoporous silica materials of the M41S, SBA-n, FDU-n, and KIT-n families are used in a wide range of applications such as separation, catalysis, drug release adsorption, sensors, matrix solid-phase dispersion (MSPD) and solid-phase extraction [69].

Synthesis Procedures of Mesoporous Silica
Initially, Stöber et al. [76] discovered an effective method for synthesizing monodispersed silica particles. This process consists of hydrolysis of tetraethyl orthosilicate (TEOS) using ammonia as a catalyst in water and ethanol solution. This method leads to the synthesis of silica particles [77]. In this reaction, TEOS undergoes hydrolysis in an ethanol/ammonia solution. As a result, it produces silanol monomer (-Si-OH) with the epoxy groups (-Si-OEt), as shown in Equation (4). Then silanol groups undergo condensation to produce branched siloxane clusters, which causes to initiate the nucleation and growth of silica particles, see Equation (5). Simultaneously, silanol monomers react with the unhydrolyzed TEOS via condensation (see Equation (6)) and participate in the nucleation and growth of silica particles [30]. Moreover, the particle size of Stöber silica depends on the concentration of the aqueous ammonia solution and water in the ethanol reaction [30].
Many experimental factors control hydrolysis, silica condensation rate, assembly kinetics, nucleation, and growth rates [65,78]. The pH is an essential factor that influences the charges of silica species. Rates of hydrolysis of silane and condensation of the siloxane bond depend strongly on the charge states. Hydrolysis of the Si-OR bond in silanes could be catalyzed by acid and base conditions, but its rate is prolonged near the neutral conditions [78].
Sakamoto et al. [79] prepared silica nanoparticles (NPs) via the evaporation and selfassembly of silicate and quaternarytrialkylmethylammonium as a surfactant. This study shows that the size of NPs depends on the ratio between the surfactant and silica precursor. Apart from that, Sihler et al. [80] used dye-stabilized emulsion to synthesize SiO 2 NPs. Moreover, this synthesis method provides silica capsules and sub-particles with precise size control. Monodispersed colloidal silica NPs (diameter of 15-25 nm) were prepared by Murray et al. [81]. In this study, as the silica source, octadecyltrimethoxysilane (OTMS) was used.
Simple synthesis methods called soft and hard templating are also applied to increase the pore volume and loading capacity of prepared hollow mesoporous SiO 2 [82]. Template synthesis of mesoporous materials typically enrolls in three steps: template preparation, template-directed synthesis of the target materials using sol-gel, precipitation, hydrothermal synthesis, and template removal [83,84].
The hard-templating method involves nano-casting using pre-synthesized mesoporous solids [85]. Hard templating is a facile synthesis method for fabricating porous materials with a stable porous structure. The structure replication is very straightforward [83]. This approach utilizes porous hard templates such as mesoporous silica. The pores of these templates are impregnated with a precursor compound for the desired product, which is then thermally converted into the product. The template is finally removed to yield the desired mesoporous material as a negative structural replica of the hard template [83]. However, the method is costly and time-consuming. Moreover, the mesoporous parameters, such as mesostructure and pore sizes, are difficult to change [84].
In contrast, soft templating methods use cationic and anionic surfactants or block copolymers as templates [78]. During the synthesis, surfactant or block copolymers are used as a soft template. Moreover, the increase in surfactant micelle concentration causes the formation of a large assembly or self-assembly of 3D mesoporous [30]. Different 3D micelle structures can be obtained by varying the solvent ratio between the aqueous and non-aqueous and adding co-solvents. Moreover, the silica source interacts with the structure-directing agent (SDA) without any phase separation. The interactions between ions or charged molecules are vital in forming well-defined porous nanostructures [85].
The soft templating method mainly depends on the self-assembly of the surfactant [83]. The process is based on the interactions between inorganics. The mesoporous structure of the final material is obtained after the removal of the pore-templating surfactant or block copolymers by low-temperature calcination (up to 600 • C) or by different washing techniques (extraction) [83]. Figure 5 represents the synthesis mechanism of mesoporous silica in the presence of a cationic surfactant. The synthesis process of mesoporous silica is carried out using TEOS as the silica source [30]. In this process, surfactant plays a significant role in defining the pore size and volume of silica [30]. Cationic surfactant forms micelle structures with water, which arranges the cationic "heads" of the surfactant molecules to the outer side. It resulted in the hydrophobic "tails" collected in the center of each micelle. As the next step, silica molecules cover the micelle surface. Finally, the surfactant is removed via calcination or extraction, and it results in porous silica [30,86,87]. Figure 6 shows the schematic diagram for synthesizing mesoporous silica using block copolymer. As can be seen from Figure 6, titania-incorporated organosilica-mesostructures (Ti-MO) are synthesized via condensation method using silica precursors ( [3-(trimethoxysilyl) propyl] isocyanurate and tetraethylorthosilicate) and titanium precursor (titanium isopropoxide) in the presence of the triblock copolymer, Pluronic P123 [88]. This method consists of template removal using two independent steps (i) extraction with a 95% ethanol solution and (ii) calcination of the sample at 350 • C. This method improves the adsorption capacity and enhances the structural properties such as specific surface area, micro-porosity, and pore volume.
The synthesis of MCM-41 and SBA-15 is performed using cetrimoniumbromide (CTAB) and Pluronic P123 surfactant. The CTAB is an ionic surfactant and acts as stearidonic acid (SDA) and which causes the formation of a hexagonal array of mesostructured composites [12]. However, as the final step, surfactants are removed by heating in air at high temperatures or by solvent extraction to obtain MCM-41 and SBA-15 [30]. Wu et al. [79] and Hao et al. [88] reported a detailed description of the mechanism. Paneka and coworkers have reported the synthesis of MCM-41 from fly ash using a hydrothermal process. However, the synthesis of MCM-41 shows reduced BET surface area, increased pore volume, and pore size [89]. tanium isopropoxide) in the presence of the triblock copolymer, Pluronic P123 [88]. method consists of template removal using two independent steps (i) extraction w 95% ethanol solution and (ii) calcination of the sample at 350 °C. This method impr the adsorption capacity and enhances the structural properties such as specific sur area, micro-porosity, and pore volume.  The synthesis of MCM-41 and SBA-15 is performed using cetrimoniumbrom (CTAB) and Pluronic P123 surfactant. The CTAB is an ionic surfactant and acts as ridonic acid (SDA) and which causes the formation of a hexagonal array of mesost tured composites [12]. However, as the final step, surfactants are removed by heatin air at high temperatures or by solvent extraction to obtain MCM-41 and SBA-15 [30]. et al. [79] and Hao et al. [88] reported a detailed description of the mechanism. Pan and co-workers have reported the synthesis of MCM-41 from fly ash using a hydrother process. However, the synthesis of MCM-41 shows reduced BET surface area, incre pore volume, and pore size [89]. copolymer. As can be seen from Figure 6, titania-incorporated organosilica-mesostruc-tures (Ti-MO) are synthesized via condensation method using silica precursors ([3-(trimethoxysilyl) propyl] isocyanurate and tetraethylorthosilicate) and titanium precursor (titanium isopropoxide) in the presence of the triblock copolymer, Pluronic P123 [88]. This method consists of template removal using two independent steps (i) extraction with a 95% ethanol solution and (ii) calcination of the sample at 350 °C. This method improves the adsorption capacity and enhances the structural properties such as specific surface area, micro-porosity, and pore volume.  The synthesis of MCM-41 and SBA-15 is performed using cetrimoniumbromide (CTAB) and Pluronic P123 surfactant. The CTAB is an ionic surfactant and acts as stearidonic acid (SDA) and which causes the formation of a hexagonal array of mesostructured composites [12]. However, as the final step, surfactants are removed by heating in air at high temperatures or by solvent extraction to obtain MCM-41 and SBA-15 [30]. Wu et al. [79] and Hao et al. [88] reported a detailed description of the mechanism. Paneka and co-workers have reported the synthesis of MCM-41 from fly ash using a hydrothermal process. However, the synthesis of MCM-41 shows reduced BET surface area, increased pore volume, and pore size [89]. Recently, Singh and Polshettiwar [90] reported the synthesis of silica nano-sheets using ammonium hydroxide. They have developed a method to synthesize silica nanosheets using lamellar micelles as soft templates in a water-cyclohexane solvent mixture. Zhang et al. [19] also reported the large-scale synthesis of mesoporous silica nanoparticles. Reported data show that various morphologies and particle sizes have been obtained during the synthesis. For synthesis process, the reaction occurred at atmospheric pressure with a sol-gel technique using CTAB as a template.

Importance of Micro-Porosity and CO 2 Adsorption Capacity of Mesoporous Silica Materials
The textural properties, including surface area, pore diameter and volume of mesoporous materials, are usually measured by studying nitrogen adsorption-desorption isotherms. The specific surface area is calculated using the volume adsorbed at different relative pressure data by the Brunauer-Emmett-Teller (BET) method [65]. Apart from that, the pore volume and pore size distribution are determined using the Barrett-Joyner-Halenda (BJH) method [65].
Furthermore, the textural properties are important parameters when considering CO 2 adsorption using physisorbents. Moreover, microporosity plays a major role in CO 2 gas adsorption because it involves the diffusion of CO 2 molecules into the physisorbent [91][92][93]. Table 6 represents the textural properties and CO 2 absorption capacity recorded for different ordered mesoporous silica materials studied.
MCM-41 has high porosity and an ordered hexagonal pore structure arrangement. However, it showed a low CO 2 adsorption capacity of 0.63 mmol/g at 25 • C and 1 bar (see Table 6). This behavior may be due to the weak interactions between the hydroxyl groups of MCM-41 and CO 2 molecules [93]. Son Table 6 [94]. The pore size of mesoporous materials varied in the descending order of KIT-6 > SBA-15 > SBA-16 > MCM-48 > MCM-41. The KIT-6 exhibited the largest pore volume among the other sorbents. These combined features of large pore size and large pore volume would enable KIT-6 to better accommodate the bulky polyethyleneimine (PEI) with little hindrance, allowing higher loadings inside silica particles than other silicasupported materials. Moreover, Zelěnák and co-workers prepared three mesoporous silica materials with different pore sizes (3.3 nm MCM-41; 3.8 nm SBA-12; 7.1 nm SBA-15) [95]. During their studies, amine functionalization was investigated with the effect of pore size and architecture on CO 2 sorption. According to the data, SBA-15 showed the highest CO 2 adsorption of 1.5 mmol/g due to the highest amine surface density in SBA-15 [95].
Lashaki and Sayari [96] also investigated the impact of the support pore structure on the CO 2 adsorption performance of SBA-15 silica. In this study, SBA-15 silica supports were used to obtain different pore sizes and intra-wall pore volumes. These materials were functionalized further with triamine through dry and wet grafting. CO 2 sorption measurements showed the positive impact of support with large pore size and high intrawall pore volume on adsorptive properties, with the former being dominant. Large pore volume influenced the load of more amine groups, CO 2 uptakes, and CO 2 /N 2 ratios and faster kinetics. When the intra-wall pore volume decreased by 53%, it caused a reduction in CO 2 uptake capacity by up to 63% and CO 2 /N 2 ratios by up to 62% and slower adsorption kinetics. Moreover, it was inferred that large pore size and high intra-wall pore volume of the support improved the adsorptive properties via enhanced amine accessibility [96].

Chemisorbents (Amine Functionalized Si-Based Materials)-Application at Low and High Temperature CO 2 Sorption
In physisorption, CO 2 molecules attach to the pore walls through weak Van der Waals and pole-pole interactions [102]. However, the unmatched pore size of the mesoporous silica and the small diameter of the CO 2 gas molecule causes low CO 2 adsorption capacities. The heat of adsorption of the physisorption process ranges from −25 to −40 kJ/mol [103], which is approximately closer to the heat of sublimation [104]. Recently, it has been reported about mesoporous silica materials with improved CO 2 sorption capacity with amine functionalization [105]. Hence, the adsorption capacity of CO 2 depends on the nature of the amine groups and the spacing between the amino silanes [106]. Figure 7 represents the different types of amino silanes and polymer-containing amino groups used during the functionalization of mesoporous silica for enhanced adsorption or separation.

Chemisorbents (Amine Functionalized Si-Based Materials)-Application at Low and High Temperature CO2 Sorption
In physisorption, CO2 molecules attach to the pore walls through weak Van der Waals and pole-pole interactions [102]. However, the unmatched pore size of the mesoporous silica and the small diameter of the CO2 gas molecule causes low CO2 adsorption capacities. The heat of adsorption of the physisorption process ranges from −25 to −40 kJ/mol [103], which is approximately closer to the heat of sublimation [104]. Recently, it has been reported about mesoporous silica materials with improved CO2 sorption capacity with amine functionalization [105]. Hence, the adsorption capacity of CO2 depends on the nature of the amine groups and the spacing between the amino silanes [106]. Figure 7 represents the different types of amino silanes and polymer-containing amino groups used during the functionalization of mesoporous silica for enhanced adsorption or separation.

Synthesis of Amine-Functionalized Silica
Amine-based adsorbents are generally synthesized using three approaches: the selection of solid scaffolds with high amine loading ability, use of amines with high nitrogen content, and use of effective methods for introducing amine groups [44]. Synthesis methods of amine-functionalized silica materials include three main pathways: impregnation, grafting, and in-situ polymerization. Figure 8 shows the three different synthesis processes of amine-functionalized silica materials.
In impregnation, amines are physically trapped in the pores of silica materials. Moreover, the performance of amine-silica adsorbents is influenced by the pore structure of silica. For example, Chen et al. [107,108] reported that the CO2 adsorption capacity decreases as the pore diameter decreases. Moreover, surfactants, surface functional groups, amine types and heteroatom incorporation affect the impregnation process [54]. In this method, the amine loading is also influenced by the total pore volume of the silica materials and the amine density.
Moreover, if the amount of amine exceeds the capacity of the support, the amine species agglomerate on the support. The main advantage of this method is the simplicity and easy synthesis procedure. Further, many amine species can be incorporated with mesoporous silica due to the large pore volume of the porous silica materials [109].
Grafting occurs between an aminosilane and silica, as shown in Figure 8, where amine groups are grafted on the silica surface via covalent bonds [110]. Mainly, three methods are used for grafting amine onto silica support: post-synthesis grafting, direct synthesis by co-condensation (one-pot synthesis), and anionic template synthesis [111]. In a typical process, silica is dispersed in a solvent, amino silanes are added, and the mixture is heated under reflux. However, the amount of amine incorporated is related to the number of hydroxyl groups on the silica surface [109]. In-situ polymerization is another promising method for functionalizing porous silica, such as hyperbranched aminosilica (HAS). This category of supported sorbents can be considered a hybrid of grafting and impregnation [112].

Synthesis of Amine-Functionalized Silica
Amine-based adsorbents are generally synthesized using three approaches: the selection of solid scaffolds with high amine loading ability, use of amines with high nitrogen content, and use of effective methods for introducing amine groups [44]. Synthesis methods of amine-functionalized silica materials include three main pathways: impregnation, grafting, and in-situ polymerization. Figure 8 shows the three different synthesis processes of amine-functionalized silica materials.
In impregnation, amines are physically trapped in the pores of silica materials. Moreover, the performance of amine-silica adsorbents is influenced by the pore structure of silica. For example, Chen et al. [107,108] reported that the CO 2 adsorption capacity decreases as the pore diameter decreases. Moreover, surfactants, surface functional groups, amine types and heteroatom incorporation affect the impregnation process [54]. In this method, the amine loading is also influenced by the total pore volume of the silica materials and the amine density.
Moreover, if the amount of amine exceeds the capacity of the support, the amine species agglomerate on the support. The main advantage of this method is the simplicity and easy synthesis procedure. Further, many amine species can be incorporated with mesoporous silica due to the large pore volume of the porous silica materials [109].
Grafting occurs between an aminosilane and silica, as shown in Figure 8, where amine groups are grafted on the silica surface via covalent bonds [110]. Mainly, three methods are used for grafting amine onto silica support: post-synthesis grafting, direct synthesis by cocondensation (one-pot synthesis), and anionic template synthesis [111]. In a typical process, silica is dispersed in a solvent, amino silanes are added, and the mixture is heated under reflux. However, the amount of amine incorporated is related to the number of hydroxyl groups on the silica surface [109]. In-situ polymerization is another promising method for functionalizing porous silica, such as hyperbranched aminosilica (HAS). This category of supported sorbents can be considered a hybrid of grafting and impregnation [112]. Solvents, including toluene are also used for grafting. Moreover, the impregnating technique is widely employed because of its simplicity, low cost, environmental friendliness, and convenience for large-scale production [114]. However, to overcome the challenges caused by grafting, researchers have recently investigated aminosilane gas-phase grafting and supercritical fluid impregnation [115].
The double-functionalization method of mesoporous materials is also widely used in recent years. Several studies prepared amine-silica composites using the double-functionalization method [116][117][118]. Those studies employed impregnation and grafting to improve CO2 uptake [116].

Comparison of Adsorption Capacities of Silica-Based Sorbents
Nigar et al. [99] synthesized the ordered mesoporous (MCM-48) silica with different silane molecules, including 3triethoxysilylpropylamine, 3-(2-aminoethylamino) propyl] trimethoxysilane and 2-[2-(3-trimethoxysilylpropylamino)ethylamino]ethylamine. Herein, silane groups were covalently bound with the silica groups, as shown in Figure 9. The functionalization caused the reduction in the surface area and the pore volume compared to the non-functionalized MCM-48 (1287 m 2 /g and 1.1 cm 2 /g) (see Table 7). Most importantly, it is seen that the increment of the number of amine groups in silane molecules leads to a decrease in CO2 absorption capacity governed via chemisorption [99].
Moreover, Park et al. [29] synthesized functionalized silica using silane molecules, similar to the study conducted by Niger et al. [99]. However, they compared in-situ polymerization and grafting. According to the data (see Table 7), the sorbent prepared Solvents, including toluene are also used for grafting. Moreover, the impregnating technique is widely employed because of its simplicity, low cost, environmental friendliness, and convenience for large-scale production [114]. However, to overcome the challenges caused by grafting, researchers have recently investigated aminosilane gas-phase grafting and supercritical fluid impregnation [115].
The double-functionalization method of mesoporous materials is also widely used in recent years. Several studies prepared amine-silica composites using the doublefunctionalization method [116][117][118]. Those studies employed impregnation and grafting to improve CO 2 uptake [116].

Comparison of Adsorption Capacities of Silica-Based Sorbents
Nigar et al. [99] synthesized the ordered mesoporous (MCM-48) silica with different silane molecules, including 3triethoxysilylpropylamine, 3-(2-aminoethylamino) propyl] trimethoxysilane and 2-[2-(3-trimethoxysilylpropylamino)ethylamino]ethylamine. Here-in, silane groups were covalently bound with the silica groups, as shown in Figure 9. The functionalization caused the reduction in the surface area and the pore volume compared to the non-functionalized MCM-48 (1287 m 2 /g and 1.1 cm 2 /g) (see Table 7). Most importantly, it is seen that the increment of the number of amine groups in silane molecules leads to a decrease in CO 2 absorption capacity governed via chemisorption [99].
Moreover, Park et al. [29] synthesized functionalized silica using silane molecules, similar to the study conducted by Niger et al. [99]. However, they compared in-situ polymerization and grafting. According to the data (see Table 7), the sorbent prepared through in-situ polymerization shows enhanced CO 2 adsorption capacity. Ahmed et al. [93] reported a detailed study about the functionalization of mesoporous Si-MCM-41 with different loadings of PEI. According to their work, with increasing PEI loading, the CO 2 adsorption capacity also increased (see Table 7). They mentioned that the enhanced adsorption is due to branched PEI with many amino groups, providing potential sites for CO 2 molecules. Moreover, the hierarchical mesoporous structure of Si-MCM-41 made these sites accessible to CO 2 by improving the dispersion of PEI [119].
Gargiulo and co-workers investigated the effect of temperature on CO 2 adsorption capacity on SBA-15 and PEI. CO 2 adsorption was evaluated at 25, 40, 55, and 75 • C temperatures [120]. The experimental data showed a significant dependence of the CO 2 adsorption capacity on temperature ( Table 7). The effect of pore dimension on CO 2 adsorption over amine-modified mesoporous silicas was reported by Heydari-Gorji et al. [100]. The pore lengths of the silica supports were 25, 1.5, and 0.2 µm. It showed that the small pore size of silica materials exhibited the highest adsorption capacities due to the enhanced amine accessibility inside the pores. Heydari-Gorji and Sayari [121] showed PEI impregnation for CO 2 removal applications. They demonstrated that PEI-functionalized silica materials were thermally stable at mild temperatures. Kuwahara et al. [122] synthesized poly(ethyleneimine)/silica composite adsorbents by incorporating zirconium (Zr) into the silica support. The authors observed Zr sites with increased CO 2 adsorbent capacity (see Table 7), regeneration, and stability.
Apart from that, Kishor and Ghoshal [123] investigated the effects of the structural parameters such as pore size, pore volume, and surface area of the silicas and aminefunctionalized silica on the CO 2 sorption capacity. The authors used various silica materials such as KIT-6, MCM-41, SBA-15, and HV-MCM-41. The wet impregnation method was employed to prepare the pentaethylenehexamine (PEHA) functionalized silica. The CO 2 capture capacities of the amine-functionalized silicas were measured at 105 • C and 1 bar pressure conditions (see Table 7). The KIT-6 showed the highest CO 2 capture capacity of 4.48 mmol/g of CO 2 at 105 • C and 1 bar pressure) among all the sorbents investigated (MCM-41 < HVMCM-41 < SBA-15 < KIT-6). Furthermore, KIT-6 showed enhanced amine density distribution due to large pore volume. All the other silica sorbents remained stable up to ten adsorption-desorption cycles.        Sim and co-workers [145] studied the CO 2 absorption capacity of the silica-based composites papered using SBA-15 and organosilica as silica precursors and N-[3-(trimethoxysilyl)propyl]ethylenediamine as an aminosilane precursor. Herein, PEI was grafted to the silica composites. Results exhibited that organosilica composites (see Table 7) showed the highest CO 2 adsorption capacity, selectivity, and reproducibility. Another silica composite was prepared by Dassanayake et al. [146] using nanocrystalline cellulose (NCC) and reported that their NCC/mesoporous silica composite showed high CO 2 absorption capacity (see Table 7), recyclability and thermal stability. Gunathilake et al. [147] synthesized microcrystalline cellulose (MCC) mesoporous silica composites using two MCC-mesoporous silica composites: MCCmesoporous silica with cyanopropyl groups and MCC mesoporous silica amidoxime groups. CO 2 adsorption was evaluated at 25 and 120 • C. According to the results, MCC-mesoporous silica with amidoxime functionalities exhibited the highest absorption capacity (see Table 7) at 120 • C due to the oxime and amine groups in amidoxime and hydroxyl groups in MCC which serve as active sites.
Rao et al. [144] determined the effect of impregnation and grafting of the aminefunctionalized MCM-41. The results showed (see Table 7) grafted sorbents with higher thermal stability than the impregnation ones. They concluded that adsorbents modified by impregnation exhibited higher amine-loading efficiencies and, thus, higher CO 2 adsorption capacities, whereas those prepared by grafting had better thermal and cyclic stability.
Moreover, Tang and co-workers have investigated the effect of inorganic alkalis such as (KOH, Ca(OH) 2 and CsOH) on the CO 2 absorption capacity [140]. The results showed that all three kinds of inorganic alkali-containing adsorbents exhibited higher CO 2 adsorption capacities than tetraethylenepentamine (TEPA) and PEI-modified samples (see Table 7). This may be due to the introduction of inorganic alkali, which changes the chemical adsorption mechanism between adsorbate-CO 2 and the adsorbent surface due to more hydroxyl groups. Moreover, they reported that CO 2 adsorption capacities have a linear dependency with the amounts of alkali adsorbents. Apart from that, Gunathilake and Jaroniec [148] reported the incorporation of magnesium oxide (MgO) and calcium oxide (CaO) into mesoporous silica surface (OMS) and applied those materials for CO 2 sorption at ambient and elevated temperatures. The materials were synthesized using the sol-gel method. However, composite sorbents performed relatively high adsorption capacities (see Table 7). It suggested that MgO and CaO enhanced CO 2 adsorption via physisorption and chemisorption. Those synthesized CaO-SiO 2 and MgO-SiO 2 composites possessed high surface area, surface properties and thermal and chemical stability.
Alumina materials also possess high surface area, porosity, and thermal and mechanical stability. Therefore, researchers have recently used amine-grafted mesoporous silica and impregnated alumina as solid sorbents for CO 2 capture [149]. Alumina-based materials for CO 2 capture include basic Al 2 O 3 , amine-impregnated or amine-modified mesoporous Al 2 O 3 and Al 2 O 3 -organosilica [149]. Gunathilake et al. [149] synthesized Al 2 O 3 -organosilica by introducing three different silica precursors such as tris [3-(trimethoxysilyl)propyl] isocyanurate (ICS), 1,4-bis(triethoxysilyl)benzene (BTEB), and bis(triethoxysilyl)ethane (BTEE)). This study used two alumina precursors, aluminum nitrate nanahydrate and aluminum isopropoxide, whereas grafting of amine groups was performed using 3-aminopropyltriethoxysilane (APTS). SiO 2 -Al 2 O 3 showed the highest absorption capacity (Table 7), and the adsorption properties of the materials were dependent on the surface area of the sample, alumina precursor, and structure and functionality of the organosilica bridging group. Moreover, Choi et al. [152] used epoxy-functionalized PEI to synthesize CO 2 sorbents. According to the reported data, epoxy-functionalized PEI exhibited a CO 2 capacity of 2.2 mmol/g at 120 • C and 100% regeneration capability at similar temperatures. This can be attributed to the heat-resistant properties of epoxy butane, which enhanced the CO 2 capture capacity and thermal stability of the silica-epoxy-PEI sorbent.
However, according to the reported data by Hu et al. [153], Li 4 SiO 4 exhibited attractive prospects for CO 2 capture. The main advantage of this material was the high CO 2 sorption capacity (theoretical sorption capacity of 0.367 g CO 2 /g sorbent) and lower regeneration temperature (<750 • C) in comparison with other reported materials such as CaO, which requires a regeneration temperature of over 900 • C [153].

Sorbent Selectivity, Regeneration, and Stability in the Cyclic CO 2 Adsorption-Desorption
During industrial applications, high adsorption capacity along with good regenerability of the sorbents in the cyclic adsorption-desorption process is vital [117]. The practical application of an adsorbent requires high sorption capacity, easy regeneration, stability in normal atmospheric conditions, and stable performance during cyclic use for longterm operation.
For instance, Ahmed et al. [93] reported a detailed study about the functionalization of mesoporous MCM-41 with different loadings of polyethylenimine (PEI). In this study, the selectivity measurement was conducted for CO 2 over N 2 and H 2 and the adsorption capacities of N 2 and H 2 on 50 wt% PEI-Si-MCM-41 were 3.89 mg/g and 6.51 mg/g, respectively (see Table 8). Table 8 summarizes the gas selectivity values of previous studies performed for porous SiO 2 . Wang et al. [154] prepared SBA-15 using silica-ethanol extraction and conventional high-temperature calcination template removal methods. Then, the silica was subjected to amine (3-aminopropyl) grafting and studied for its CO 2 adsorption properties. This study aimed to increase the surface silanol density by grafting amine groups, increasing CO 2 adsorption capacity and CO 2 /N 2 selectivity. According to the reported data, CO 2 /N 2 selectivity changed from 46 to 13 (see Table 8), and these results ensured that solvent extraction also enhanced CO 2 /N 2 selectivity. Moreover, the authors performed a test to measure the stability of amine-SBA-15 (solvent extracted). According to the results, amine-SBA-15 (solvent extracted) was regenerated under a vacuum after each adsorption step.
In industrial applications of adsorbents, it is essential to remain stable during cyclic operations. This section summarizes the previous studies on sorbent regeneration and stability in cyclic CO 2 adsorption-desorption by amine-silica composites, and the reported data are tabulated in Table 9. The regeneration of the amine-impregnated and grafted silica composites was mainly conducted by pressure and temperature swing adsorptions. Typically, the sorbent was regenerated at 50~120 • C in N 2 , He, or Ar flow. As depicted in Table 9, the amine-impregnated silica composites show a loss of CO 2 capture capacity in the cyclic CO 2 adsorption-desorption due to amine leaching from the silica surface and degradation [110]. Amine leaching is closely related to the amine types introduced and the operation temperature, while the degradation of amine is related to the operation temperature and gas atmosphere [109].  Guo et al. [128] conducted the adsorption/desorption cycles for hierarchically porous silica (HPS) grafted PEI at 75 • C. In this experiment, the modelled flue gas flow rate was maintained at 70 mL/min, and the CO 2 partial pressure was held at 1 bar. According to the data, adsorption capacities are similar in eight adsorption/desorption cycles, showing that the aforementioned sorbents with good stability and regenerability.
Wang et al. [117] investigated the regenerability of the amine-modified MCM-41 (MCM-41-TEPA and MCM-41-AMP). The authors conducted fifteen cycles to verify the regenerability. According to the reported data, after fifteen cycles, the adsorption capacity decreased from 3.01 mmol/g to 2.88 mmol/g, and it was shown that both sorbents showed good regenerability. This may be due to the hydrogen-bonding interactions among TEPA, AMP and MCM-41, TEPA.
Kishor and Ghoshal [123] measured the stability of the pentaethylenehexamine (PEHA) impregnated KIT-6. The sorbent was aged for 6 months, and its adsorption performance was explored at 90-105 • C. The results showed that PEHA-impregnated KIT-6 had 4.0 and 4.3 mol CO 2 /kg sorption capacities at 90 and 105 • C at 1 bar even after 6 months. Moreover, the sorption performance of the adsorbent was tested for ten consecutive adsorption/desorption cycles. The sorption capacity of the sorbent decreased by less than 4% at 90-105 • C at 1 bar without any structural degradation. Moreover, the results exhibited that PEHA-impregnated KIT-6 had better sorption performance than those of earlier reported adsorbents, except for silica aerogel.
Liu and co-workers performed a regeneration test for zeolite-mesoporous silicasupported-amine hybrids sorbent [160]. Their data showed that, after 10 cycles, the adsorption capacity remained unchanged. Therefore, the sample performed a very stable cyclic adsorption-desorption performance. In contrast, López-Aranguren et al. [129] examined the regeneration of CO 2 from branched PEI-mesoporous silica. In this study, CO 2 adsorption-desorption cycles showed that the uptake measured in the first cycle was successfully maintained even after 20 cycles. Zhang et al. [174] examined the stability of the adsorbents based on linear PEI supported on silica. According to the reported data, the adsorbent maintained its adsorption capacity. Still, the adsorption capacity was reduced by approximately 5.6% when the temperature was increased to 100 • C, which was attributed to amine leaching. Furthermore, Subagyono et al. [162] found that the branched PEI-containing adsorbent decreased CO 2 adsorption-desorption capacity during cycling, attributed to the by-product formation.

Technical Challenges and Future Trends
Financial, technical, and environmental concerns are the main barriers to CCS technologies. For instance, one major challenge with CCS is moving CO 2 captured to remote storage sites using pipelines, as laying these pipelines is costly and associated with numerous environmental issues.
Several studies reported the requirements and a working definition for carbon dioxide capture (CCS). Advanced physical adsorbents must be developed with high CO 2 selectivity and gas uptake. Stability (over 1000 cycles), CO 2 affinity, scalability, reusability, resistance against surface erosion, and high energy requirement are the major concerns in CO 2 capture technologies. The sorbent cost is the most significant part of an air capture system; however, it is difficult to estimate the price of a particular sorbent in lab-scale experiments. According to the reported data, the value of a kilogram of sorbent is equal to the net present value of the CO 2 revenue collected during its lifetime. Therefore, a sorbent must possess constant stability and performance for its lifetime [178,179].
The other main challenges associated with sorbents are stability, kinetics, and sorbent capacity. However, many sorbents are thermodynamically strong enough to capture CO 2 from ambient air and allow for easy regeneration. Despite the reported data, further studies on stability, kinetics and capacity still need to be improved in SiO 2 -based adsorbents. Another factor is sorbent loading and unloading cycles, which are essential for reducing costs. Moreover, adsorption kinetics is affected by binding energies, diffusion into porous materials, and the geometry of sorbent materials and many sorbents require longer sorption times. Therefore, improved kinetics can lower the cost. High adsorption capacity can reduce the cost of CO 2 capture by reducing the amount of sorbent required. Physisorbents that selectively separate CO 2 from gaseous mixtures formed a revolution in CCS since it requires less energy for recycling, with enhanced CO 2 capacity.
Amine-based sorbents are widely used in CCS technologies. However, amine sorbent depends on the molecular weight of the sorbent and the pore sizes of the sorbent. To improve the capacity of moisture-swing sorbents, the ion exchange resins can be prepared with a higher charge density, and materials with different cation distances can be used under different humidity conditions. The potential of solid sorbents to remove CO 2 from flue gas is enormous compared to conventional liquid amine processes in terms of regeneration energy and significant cost reduction. However, as discussed previously, solid sorbents have limitations and challenges to address before being deployed commercially in post-combustion CO 2 capture.
There is limited literature available on CO 2 capture using low-cost silica-based materials such as rice husks. These sources lead to the reduction in production costs. Nevertheless, novel silica-based materials such as lithium orthosilicate (Li 4 SiO 4 ), silica nanotubes, silica nanospheres, silica-based composites, and silica aero gels give rise to high CO 2 capture at elevated temperatures.
Moreover, most studies have used sol-gel and hydrothermal processes to synthesize silica-based sorbent. However, apart from the aforementioned methods, microwave treatment can also be used, which is cost-effective and timeserving. Moreover, different surfactants can prepare silica with varying pore sizes and morphologies. Another area for improvement with silica-based sorbent is the need for more literature on kinetic data at different adsorption temperatures, which are helpful in industrial implementations.

Summary
CO 2 capture by porous SiO 2 materials, their reaction mechanisms and synthesis processes were extensively discussed in this review. Chemical absorption of CO 2 is more suitable than physical absorption owing to high adsorption capacity, relatively easy synthesis routes, and lower regeneration energy requirements. Among many chemisorbents, SiO 2 -based adsorbents, including amine-functionalized SiO 2 , possess higher CO 2 selectivity and adsorption capacities, making them ideal candidates for CO 2 capture. However, the performance of currently available amine-functionalized SiO 2 needs to be further developed and improved in terms of stability, gas selectivity and resistivity to thermal degradation. Furthermore, the review highlighted major financial, technical, and environmental barriers and prospects associated with porous silica-based materials during the industrial scale-up process.

Conflicts of Interest:
The authors declare no conflict of interest.

Abbreviations and Acronyms
Abbreviation Definition CO 2 Carbon dioxide Li 4