Carbon Capture Using Porous Silica Materials

1. Introduction

With exponential growth of industrialization, global warming and climate change have become a global concern and drawn much attention in recent decades [1]. As a solution for these key issues, research activities are developed for capturing carbon dioxide (CO₂), the major greenhouse gas. However, atmospheric CO₂ 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 of CO₂ concertation results climate change such as rise of 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 types of CO₂ stationary emission sources are power plants, refineries, chemical and petrochemical, iron and steel, gas processing, and cement industries. Due to the growth of industrialization and population, the emission of CO₂ has been drastically increased to approach an irreversible climate change level. To tackle these issues, the international communities led by the United Nation reached a landmark international accord, the Paris Agreement, that was adopted by nearly every nation in 2015 to address climate change and its related issues. Countries around the globe made their “nationally determined contributions” of greenhouse gas reduction and plans to pursue their domestic measures. Different approaches are considered and employed in different countries in order to reduce CO₂ emissions and those are indicated in Table 1.

Each of the above-mentioned approaches possesses pros and cons. Among these approaches, the CO₂ capture and storage (CSS) can be used to reduce CO₂ emissions by 85-90% from large emission sources [4]. CCS includes different CO₂ capture, separation, transport, storage technologies, and chemical conversion which are discussed in detail below.

2. CO₂ Capture

2.1. CO₂ capture technologies

Capture and sequestration of CO₂ (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 CO₂ capture, compression, transport, and storage, therefore, developing an efficient and economically feasible technology for the capture and sequestration of CO₂ produced by anthropogenic emissions is critically important. CO₂ 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 CO₂ from the emission gases of a combustion system [7,8,9,10,11]. Firstly, flue gas (mainly consists of CO₂, H₂O and N₂) passes through denitrification and desulphurization treatments. As the next step, the flue gas is fed to an absorber which contains solvent. Herein, CO₂ regeneration occurs. Then the CO₂-rich absorbent is sent to a CO₂-stripper unit to release the CO₂ gas. Also, CO₂-lean absorbent is sent back to the CO₂-absorber unit [1]. Next, the captured CO₂ is then compressed into supercritical fluid and then transported [1] as shown in Figure 1.

Pre-combustion capture is a technology where CO₂ is captured before the combustion 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 fertilizers and natural gas [13,14].

In oxyfuel combustion, the carbon-based fuel consumes in re-circulated flue gas and oxygen (O₂) stream. CSS capture technology is considered expensive due to the high cost of O₂ separation and production. However, the capture and separation of CO₂ 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 CO₂ capture compared to other available methods [1]. Post-combustion capture uses different methods for gas separation, and 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 also depicts the efficiency, advantages and disadvantages of the different types of post-combustion capture technologies.

Absorption process mainly uses liquids to capture CO_2. During adsorption, once CO₂ is separated from the gas, the sorbent should be regenerated by using a stripper, heating, or depressurization. Also, this method is considered as the most established process for CO₂ separation [21]. In general, adsorbents can be divided into two types, namely, chemical and physical adsorbents (see Table 2 for details).

2.2. Criteria for Selecting CO₂ Sorbent Material

Certain economical and technical properties are required in order to select the best solid adsorbent candidate for a particular CO₂ capture application. These criteria are listed and described below.

• Adsorption capacity for CO₂:

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. Besides, it causes to reduce the sorbent quantity, and it’s 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 CO₂ working capacity should be in the range of 2-4 mmol/g of the sorbent [25].

• Selectivity for CO₂:

The adsorption selectivity or selectivity of CO₂ is explained as the sorption uptake ratio of a target gas species compared to another type (as example N₂) 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 CO₂ influences transportation and sequestration and, therefore, this criterion plays an important role in CO₂ sequestration [24].

• Adsorption and desorption kinetics:

It is necessary to have fast adsorption/desorption kinetics for CO₂ and it controls the cycle time of a fixed-bed adsorption system. Fast kinetics results in a sharp CO₂ breakthrough curve in which effluent CO₂ 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 CO₂ 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 CO₂ 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 CO₂ capture process cost-effective [24].

• Chemical stability/tolerance towards impurities:

Solid CO₂ 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 CO₂ and the solid adsorbent [24]. Considering regeneration, physisorption is mostly favored as compared with chemisorption since later 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].

2.3. 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 equation 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.

$$\mathrm{C}\mathrm{O}₂+2\mathrm{R}₁\mathrm{N}\mathrm{H}₂\stackrel{}{\leftrightarrow }\mathrm{R}₁\mathrm{N}\mathrm{H}₃⁺+\mathrm{R}₁\mathrm{N}\mathrm{H}\mathrm{C}\mathrm{O}\mathrm{O}⁻$$

(1)

$$\mathrm{C}\mathrm{O}₂+2\mathrm{R}₁\mathrm{R}₂\mathrm{N}\mathrm{H}₂\stackrel{}{\leftrightarrow }\mathrm{R}₁\mathrm{R}₂\mathrm{N}\mathrm{H}⁺+\mathrm{R}₁\mathrm{R}₂\mathrm{N}\mathrm{C}\mathrm{O}\mathrm{O}⁻$$

$$\mathrm{C}\mathrm{O}₂+2\mathrm{R}₁\mathrm{R}₂\mathrm{R}₃\mathrm{N}+\mathrm{H}₂\mathrm{O}\stackrel{}{\leftrightarrow }\mathrm{R}₁\mathrm{R}₂\mathrm{N}\mathrm{H}⁺+\mathrm{H}\mathrm{C}\mathrm{O}₃⁻$$

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 absorption. The absorption reaction started with the reversible reactions between MEA and CO₂ to form carbamate at low CO₂ loading, followed by the CO₂ hydration to form HCO₃⁻/CO₃²⁻ under high CO₂ loading, and accompanied by the hydrolysis of carbamate.

Table 3. Comparison between different liquid amines.

Criteria	Alkanolamines			Sterically hindered Amines
	Primary	Secondary	Tertiary
Examples	Monoethanolamine (MEA)	Diethanolamine (DEA)	N-methyldiethanolamine (MEDA)	2-amino-2-methyl-1- propanol (AMP)
Structure
CO2 loading at 59.85 °C (mol CO2/mol amine)	0.426 (MEA 30 wt %) [33]	0.404 (DEA 30 wt %) [33]	0.141 (TEA 30 wt %) [33]	0.466 (AMP 30 wt %) [33]
Regeneration efficiency (%) at 90 °C	75.5 [34]	84.89 [34]	95.09 [34]
Advantages	● Inexpensive solvent ● Reversible absorption ● High selectively (between acid and other gases) ● Reacts with CO2 more rapidly [33]	● Inexpensive solvent ● Reversible absorption ● High selectively (between acid and other gases) ● Reacts with CO2 more rapidly [33]	● Inexpensive solvent ● Reversible absorption ● High selectively (between acid and other gases) ● High CO2 absorption capacity ● Requires low regeneration energy [33]	● High CO2 absorption capacity ● Requires low regeneration energy [34]
Disadvantages	● Lower CO2 absorption capacity ● Requires high regeneration energy ● Oxidative degradation occurs in the presence of other gas components ● Corrosive ● High capital costs [33]	● Lower CO2 absorption capacity ● Requires high regeneration energy ● Oxidative degradation occurs in the presence of other gas components ● Corrosive ● High capital costs [33]	● Reaction rate with CO2 is low compared to MEA and DEA ● Corrosive ● High capital costs [33]	● Low reaction rate [34]

Table 3. Comparison between different liquid amines.

Criteria	Alkanolamines			Sterically hindered Amines
	Primary	Secondary	Tertiary
Examples	Monoethanolamine (MEA)	Diethanolamine (DEA)	N-methyldiethanolamine (MEDA)	2-amino-2-methyl-1- propanol (AMP)
Structure
CO2 loading at 59.85 °C (mol CO2/mol amine)	0.426 (MEA 30 wt %) [33]	0.404 (DEA 30 wt %) [33]	0.141 (TEA 30 wt %) [33]	0.466 (AMP 30 wt %) [33]
Regeneration efficiency (%) at 90 °C	75.5 [34]	84.89 [34]	95.09 [34]
Advantages	● Inexpensive solvent ● Reversible absorption ● High selectively (between acid and other gases) ● Reacts with CO2 more rapidly [33]	● Inexpensive solvent ● Reversible absorption ● High selectively (between acid and other gases) ● Reacts with CO2 more rapidly [33]	● Inexpensive solvent ● Reversible absorption ● High selectively (between acid and other gases) ● High CO2 absorption capacity ● Requires low regeneration energy [33]	● High CO2 absorption capacity ● Requires low regeneration energy [34]
Disadvantages	● Lower CO2 absorption capacity ● Requires high regeneration energy ● Oxidative degradation occurs in the presence of other gas components ● Corrosive ● High capital costs [33]	● Lower CO2 absorption capacity ● Requires high regeneration energy ● Oxidative degradation occurs in the presence of other gas components ● Corrosive ● High capital costs [33]	● Reaction rate with CO2 is low compared to MEA and DEA ● Corrosive ● High capital costs [33]	● Low reaction rate [34]

The mechanism of CO₂ capture into MEA solution with different CO₂ loadings is shown in Figure 2.

According to 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].

MEA is considered as a well-established solvent to separate CO₂ because it can be regenerated easily [35]. On the other hand, Rinprasertmeechai et al. [33] reported the order of CO₂ absorption capacity of the different alkanolamines as MEA > DEA > TEA. Moreover, they have further reported the regeneration ability of the amines in the following order: TEA > > DEA > MEA. According to Table 3, it is clearly shown that, MEA gave the lowest regeneration efficiency of 75.75 % in the first cycle, whereas, TEA offered the highest regeneration performance of 95.09 % [33]. As shown in Table 3, MEA reacts with CO₂ more rapidly compared to MEDA due to the formation of carbamate but MEDA has a higher CO₂ absorption capacity and requires lower energy to regenerate CO₂ [36]. However, Wang et al. [37] found that, when MEA and MEDA are mixed with the appropriate ratio, the energy consumption for CO₂ regeneration is reduced significantly.

Sterically hindered amines are based on primary or secondary amines, with alkyl groups attached to the amino group, which is inhibited from reacting with CO₂ through the effect of steric hindrance [28]. One example of sterically hindered amines is 2-amino-2-methyl-1- propanol (AMP) which reduces the stability of the formed carbamate due to the formation of weak bonds, promoting hydrolysis to form bicarbonate and reducing regeneration energy, but it also releases free amine molecules to react with CO₂, thus increasing absorption capacity as shown in Table 3. Dave et al. [38] compared the CO₂ absorption of different amine solvents such as MEA, MEDA and NH₃ at various concentrations. They showed that 30wt% AMP-based process has the lowest overall energy requirement over other solvents such as 30% MEA, 30% MEDA, 2.5% NH₃, and 5% NH_3.

Recently, ionic liquids (IL) have gained attention due to their properties such as very low vapor pressure, thermal stability, and non-toxicity [39]. The widely used ILs are bis(trifluoromethylsulfonyl)imide (TF₂N), tetrafluoroborate (BF₄) and hexafluorophosphate (PF₆). The main drawbacks of the ILs are high viscosity and they reduce to their blended solutions containing IL and alkanolamines. However, due to the reduction of high viscosity, some rewards are gained such as, increase of energy efficiency, absorption rate, and CO₂ absorption capacity [40,41].

2.4. Comparison between major non-carbonaceous solid sorbents for CO₂ capture and importance of silica materials

Because of disadvantages present in the aqueous amine absorption processes including low contact area between gas and liquid, low CO₂ loading, and absorbent corrosion, as an alternative option, solid adsorption has achieved concern of researchers and industries in recent years [42,43]. A variety of solid adsorbents have been proposed according to their structures and compositions, adsorption mechanisms, and regeneration [43]. Although solid sorbents are inexpensive, they reduce heat capacity, show fast kinetics, increase CO₂ adsorption capacity and selectivity, and exhibit high thermal, chemical, and mechanical stability [43].

However, it is important to improve the CO₂ selectivity and adsorption capacity of a solid sorbent when targeting large volumes of combustion emissions. Recently, most of the commercially available adsorbents are activated carbons, silica, zeolites, hollow fibers, and alumina [6]. These materials show different pore structures, specific surface areas, and surface functional groups, and thus their application fields are highly specific. Table 4 tabulates some typical non-carbonaceous dry adsorbents used for CO₂ capture.

As mentioned earlier, carbonaceous adsorbents such as activated carbon have been widely used for CO₂ capture due to their wide availability, low cost, and high thermal stability. However, weak CO₂ 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, much researchs have focused on non-carbonaceous materials such as mesoporous silica, zeolites, etc. due to their advantages shown in Table 4.

Zeolites are aluminosilicates with a particularly ordered 3-dimensional (3D) microporous structures which have high crystallinity and surface area [44]. The adsorption efficiencies of zeolites are largely affected by their size, charge density, and chemical composition of cations in their porous structures [37]. Literature has reported the change of CO₂ adsorption of zeolites by altering their composition as Si/Al ratio [45] and the exchange with alkali and alkaline-earth cations in the structure of zeolites. However, zeolites present several drawbacks such as relatively low CO₂/N₂ selectivity and high hydrophilicity [46]. Apart from the above, zeolites show reduced CO₂ adsorption capacity when CO₂/N₂ mixtures contain moisture and zeolites require high temperatures (> 300 °C) for regeneration [47].

Recently, metal organic frameworks (MOF) have attracted much attention owing to their unique properties such as controllable pore structure and high surface area [48]. However, the MOFs show decreased adsorption capacities when exposed to gas mixtures [46]. Moreover, previous reports indicate that MOFs are promising materials for CO₂ capture in laboratory settings; however, further research is required to confirm their practical applicability [49]. Furthermore, water vapor negatively affects the application of these sorbents. Also, water vapor is adsorbed competitively on physisorbents, and thus decreasing their CO₂ 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 Microporous: M41S, Santa Barbara Amorphous type material (SBA-n), anionic surfactant- template mesoporous silica (AMS) [44]. However, the CO₂ adsorption capacities of them observed at atmospheric pressure are not high. Therefore, many studies based on the synthesis and modification of silica materials containing nano-porous and mesoporous materials for efficient CO₂ capture have been recently reported [51,52].

Several reviews have recently focused on the potential applications of porous silica materials as CO₂ adsorbents. Reddy et al. [53] reported the CO₂ adsorption based on porous materials and in this review, it has mainly reported about MOFs, clay-based adsorbents, porous carbon-based materials and polymer-based adsorbents. However, this recently published review did not mention porous silica materials. Liu et al. [54] also discussed about different porous materials (including pours silica) for post-combustion CO₂ capture but there is limited literature reported about synthesis methods. Therefore, there is a need to identify the best performing porous silica materials and synthesis methods among the thousands of synthesized silica-based materials. Therefore, this review is mainly aimed to discuss the adsorption of CO₂ gas onto different types of porous silica materials and functionalized silica materials. In addition, overview of synthesis processes and comparison between the adsorption capacities are also deeply discussed. Finally, the technical challenges and the future research directions of the porous silica materials for CO₂ adsorption are also presented in this review.

3. CO₂ separation methods

There are two general mechanisms involved in the CO₂ capturing using solid sorbents which are known as chemisorption and physisorption. Table 5 represents the comparison between chemisorption and physisorption. However, the difference between the two mechanisms is interactions between the gas molecules and the sorbent surface in adsorption which is shown in Figure 3.

The process of CO₂ capturing using solid adsorbent involves in selective separation [24]. The important parameters for sloid sorbents are surface tension, pore size, temperature and pressure [24,60]. Adsorption process involves a repeated cycle of adsorption and desorption which is also known as regeneration. The different types of adsorption are shown in Figure 4 such as (i) Pressure Swing Adsorption (PSA) (ii) Temperature Swing Adsorption (TSA) (iii) Electric Swing Adsorption (ESA) and (iv) Vacuum Swing Adsorption (VSA).

Adsorption and desorption occur at low temperature range(40-120 ^oC) and high temperature range (120-360 ^oC) in the TSA process, respectively [3]. VSA process involves CO₂ uptake at high pressure and ESA conducts by performing the adsorption–desorption 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 [60].

In contrast, microwave-swing adsorption (MWSA) process has gained much attention in terms of energy management. The main advantage of MWSA is, unlike conventional heating where solids heat through conduction and convection, microwaves are able to provide energy to materials in a direct transfer of the energy to the adsorbent without being absorbed by the adsorbent [62].

4. CO₂ adsorption using mesoporous silica materials (Physisorbents)

4.1. Description about mesoporous silica materials

The mesoporous silica materials are used for a wide variety of applications. Apart from the CO₂ adsorption, those materials are used in various fields such as catalytic and wastewater treatment applications [63]. Mesoporous silica has unique properties such as uniformity of pore distribution (with size between 0.7 and 50 nm), high surface area, (around 1000 m²/g) and good thermal stability [64]. The very first synthesized mesoporous silica material is M41S, in the 1990s [65]. However, 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, KIT-5, etc. with a variety of pore geometries like cubic, and hexagonal, and morphologies like rods, spheres, and discs [66].

In 1990, Mobil Oil Corporation discovered molecular sieves of M41S family consisting of silicate/aluminosilicate [67]. In general, these materials are prepared via the sol-gel method. Three well-defined structural arrangements were identified after studying the effect of surfactant concentration and those are: hexagonal structure (MCM-41), cubic structure (MCM-48), and lamellar structure (MCM-50). Therefore, these materials (M41S family) exhibit mesoporous arrays with amorphous walls of about 10 Å (1 nm) [67]. Moreover, the structural ordering of these M41S family materials can be changed with increasing hydrothermal synthesis temperature and time [67]. These, M41S molecular sieves have attracted great attention in different applications such as catalysis area [68], adsorption [67] and controlled release of drugs [69]. The main advantage of this group is their unique chemical structure consisting with high density of functional silanol groups (Si–OH), pore size, and shape can be modeled during the synthesis process and internal surface can be easily modified with organic and/or inorganic groups [67,70,71].

Santa Barbara Amorphous family (SBA) prepared silica-based materials with well-ordered mesoporous firstly, in 1998 [67]. The material group consists of SBA-2 (hexagonal close-packed array), SBA-12 (three-dimensional hexagonal network), SBA-14 (cubic structure), SBA-15 (two-dimensional hexagonal), and SBA-16 (structured in cubic cage) [67,72]. These nanostructured mesoporous materials are composed of silica-based framework with uniform and well-ordered mesopores, large pores, thick porous walls, high surface area, and high thermal stability [71,73]. Most widely investigated members of the SBA-n family in the literature are SBA-15 and SBA-16. The SBA-15- and SBA-16-based mesoporous arrays are widely utilized as adsorbents [71], catalysts or catalytic [74] and drug deliveries [75].

The Fudan University synthesized mesoporous materials family (FDU-n)-based mesoporous silica arrays have 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 [76]. FDU-1-based mesoporous materials have a 3D face-centered cubic (FCC) structure with large cage-like mesopores while FDU-2 mesoporous array possesses a mesostructured FCC unit cell and well-ordered 3D architecture [71].

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 [77]. KIT-5-based nanostructured mesoporous materials have a well-ordered 3D cage-like mesopores in a face centered close-packed cubic lattice architecture [71]. In addition, KIT-6 shows a 3D mesoporous amorphous walls with large pore size, uniform pore distribution, high surface area, and thermal stability [71].

Besides, 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, catalyze, drug release adsorption, sensors, matrix solid phase dispersion (MSPD) and/or solid-phase extraction [71].

4.2. Synthesis procedures

Initially, Stöber et al. [78] discovered an effective method for the synthesis of 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 [79]. In this reaction, TEOS undergoes a hydrolysis in ethanol/ammonia solution. As a result, it produces silanol monomer (-Si-OH) with the epoxy groups (-Si-OEt) as shown in equation 4. As the next step, as shown equation 5, silanol groups undergoes condensation to produce branched siloxane clusters, which causes to initiate the nucleation and growth of silica particles. Simultaneously, silanol monomers may react with the unhydrolyzed TEOS via the condensation (equation 6) and also participate in the nucleation and growth of silica particles [30]. Moreover, particle size of Stöber silica depends on the concentration of the aqueous ammonia solution and water in ethanol ration [30].

$$\mathrm{S}\mathrm{i}\left(\mathrm{O}\mathrm{E}\mathrm{t}\right)₄+\mathrm{X}\mathrm{H}₂\mathrm{O}\stackrel{\mathrm{H}\mathrm{y}\mathrm{d}\mathrm{r}\mathrm{o}\mathrm{l}\mathrm{y}\mathrm{s}\mathrm{i}\mathrm{s}}{\to }\mathrm{S}\mathrm{i}\mathrm{O}\left(\mathrm{O}\mathrm{E}\mathrm{t}\right)₄₋\mathrm{ₓ}\left(\mathrm{O}\mathrm{H}\right)\mathrm{ₓ}+\mathrm{X}\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}$$

(4)

$$\mathrm{S}\mathrm{i}\mathrm{O}\left(\mathrm{O}\mathrm{E}\mathrm{t}\right)₄₋\mathrm{ₓ}\left(\mathrm{O}\mathrm{H}\right)\mathrm{ₓ}\stackrel{\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}{\to }\left(\mathrm{O}\mathrm{E}\mathrm{t}\right)₄₋₂\mathrm{ₓ}\left(\mathrm{O}\mathrm{H}\right)₂\mathrm{ₓ}₋₂+\mathrm{H}₂\mathrm{O}$$

(5)

$$\mathrm{S}\mathrm{i}\left(\mathrm{O}\mathrm{E}\mathrm{t}\right)₄+\mathrm{S}\mathrm{i}\mathrm{O}\left(\mathrm{O}\mathrm{E}\mathrm{t}\right)₄₋\mathrm{ₓ}\left(\mathrm{O}\mathrm{H}\right)\mathrm{ₓ}\stackrel{\mathrm{C}\mathrm{o}\mathrm{n}\mathrm{d}\mathrm{e}\mathrm{n}\mathrm{s}\mathrm{t}\mathrm{a}\mathrm{t}\mathrm{i}\mathrm{o}\mathrm{n}}{\to }\left(\mathrm{O}\mathrm{E}\mathrm{t}\right)₇₋\mathrm{ₓ}\left(\mathrm{O}\mathrm{H}\right)\mathrm{ₓ}₋₁+\mathrm{E}\mathrm{t}\mathrm{O}\mathrm{H}$$

(6)

Many experimental factors (such as pH) control the interactions, silica condensation rate, the assembly kinetics and also the nucleation and growth rates [67,80]. The pH is an important 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 very slow near the neutral conditions [80].

Sakamoto et al. [81] prepared silica nano particles (NPs) via the evaporation and self-assembly of silicate and quaternarytrialkylmethylammonium as a surfactant. This study shows that size of NPs is dependent on the ratio between the surfactant and silica precursor. Apart from that, Sihler et al. [82] used dye-stabilized emulsion to synthesize SiO₂ NPs. Moreover, this synthesis method provides silica capsules and sub particles with exact size control. Monodispersed colloidal silica NPs (diameter of 15-25 nm) were prepared by Murray et al. [83]. In this study, as the silica source, octadecyltrimethoxysilane (OTMS) was used.

In order to increase the pore volume and loading capacity of prepared hollow mesoporous SiO₂, researchers have used simple synthesis methods called soft and hard templating methods [84]. Template synthesis of mesoporous materials typically enroll in mainly three steps: template preparation, template directed synthesis of the target materials (sol gel, precipitation, hydrothermal synthesis) and template removal [85].

The hard-templating method involves in nano-casting which uses pre-synthesized mesoporous solids [87]. Hard templating is a facile synthesis method for the fabrication of porous materials with a stable porous structure. The structure replication is very straightforward [85]. This approach utilizes porous ‘‘hard templates’’ (usually 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 in-situ. The template is finally removed to yield the desired mesoporous material as a negative structural replica of the hard template [85]. However, the method is costly and a time-consuming method. Moreover, the mesoporous parameters such as meso structure and the pore sizes are difficult to change [86].

In contrast, soft templating method, cationic and anionic surfactants or block co-polymers were used as templates [80]. During the synthesis, surfactant or block co-polymers is used as a soft template. Also, the increase of surfactant micelle concentration causes the formation of large assembly or self-assembly of 3D mesoporous [30]. Different 3D micelle structures can be obtained by varying solvent, ratio between the aqueous and non-aqueous and the combination between the co-solvents. Moreover, without any phase separation, silica source interacts with structure directing agent (SDA) because interactions between ions or charged molecules play an important role in the formation of well-defined porous nanostructures [87].

The soft templating method mainly depends on the self-assembly of the surfactant [85]. 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 co-polymers by low-temperature calcination (up to 600 °C) or by different washing techniques (extraction) [85]. 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 major role such as defining the pore size and pore volume of silica [30]. Cationic surfactant forms micelle structures with water and which leads to arrange 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, surfactant is removed via calcination or extraction and it resulted in porous silica [30,88,89].

According to Figure 6, Titania-incorporated organosilica-mesostructures (Ti-MO) were synthesized via condensation method using silica precursors ([3-(trimethoxysilyl) propyl] isocyanurate and tetraethylorthosilicate) and titanium precursor (titaniumisopropoxide) in the presence of the triblock copolymer, Pluronic P123. The method consists of removing template mainly using two independent steps (i) extraction with a 95% ethanol solution (ii) calcination of sample at 350 ^◦C. This method changes the adsorption and enhanced the structural properties such as specific surface area, micro-porosity, and pore volume.

The synthesis of MCM-41 and SBA-15 are preformed 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]. The detailed description about the mechanism was reported by Wu et al. [79] and Hao et al. [90]. Paneka and co-workers have reported the synthesis of MCM-41 from fly ash using a hydrothermal process. However, synthesis of MCM-41 shows reduced BET surface area and increased pore volume, and pore size [91].

Recently, Singh and Polshettiwar [92] reported the synthesis of silica nano-sheets using ammonium hydroxide. They have developed a method to synthesize silica nano-sheets 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 shows 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.

4.3. Importance of micro-porosity and CO₂ adsorption capacity of the different mesoporous silica materials

The textural properties such as surface area, pore diameter and volume of mesoporous materials are usually determined by the study of nitrogen adsorption-desorption isotherms. The specific surface area is calculated using the volume adsorbed at different relative pressure data by the Brunauer-Emmett-Tellet (BET) method [67]. Apart from that, the pore volume and pore size distribution are usually determined using the Barrett-Joyner-Halenda (BJH) method [67].

However, the textural properties are important parameters when considering CO₂ adsorption using physisorbent materials. However, microporosity of the physisorbents plays a major role in CO₂ gas adsorption because it involves in diffusion of CO₂ gas molecules with sorbent [94]. Table 7 represents the textural properties and CO₂ absorption capacity recorded for different types of ordered mesoporous silica materials studied.

MCM-41 has high porosity and ordered hexagonal pore structure arrangement. However, it showed low CO₂ adsorption capacity of 0.63 mmol/g at 25 °C and 1 bar (see Table 7). This behavior may be due to the weak chemical interactions (physical interaction) between MCM-41 and CO₂ gas molecules because the existing hydroxyl groups are not capable of forming interactions or lack/ missing of adsorption sites for CO₂ [96]. Apart from the above, Son et al. [96] prepared the KIT-6, SBA-15, SBA-16, MCM-48 and MCM-41 and their textural properties of the materials are tabulated in Table 7. Pore size of mesoporous materials are varied in the descending order of KIT-6 > SBA-15 > SBA-16 > MCM-48 > MCM-41. According to the aforementioned data, KIT-6 exhibits 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 and allow higher loadings inside silica particles than other silica supported materials. Besides, Zeleˇnák 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) [97]. During their studies, amine functionalization was investigated with the effect of pore size and pore architecture on CO₂ sorption. According to the reported data, SBA-15 showed the highest CO₂ adsorption of 1.5 mmol/g. This may be due to the highest amine surface density in SBA-15 [97].

Lashaki and Sayari [98] also investigated the impact of the support pore structure on the CO₂ adsorption performance of SBA-15 silica. In this study, SBA-15 silica supports are used to obtain different pore sizes and intra-wall pore volumes. These materials were functionalized further with triamine through dry and wet grafting. CO₂ sorption measurements showed the positive impact of support with large pore size and high intra wall pore volume on adsorptive properties, with the former being dominant. Large pore volume influenced to load more amine groups, CO₂ uptakes and CO₂/N₂ ratios and faster kinetics. When the intra-wall pore volume decreased by 53%, it caused a reduction of CO₂ uptake capacity up to 63% and CO₂/ N₂ ratios up to 62% and slower adsorption kinetics. Also, it was inferred that, large pore size and/or high intra-wall pore volume of the support improved the adsorptive properties via enhanced amine accessibility [98].

Table 6. The textural properties and CO₂ absorption capacity of different types of ordered mesoporous silica materials.

Types of mesoporous silica	Structure	Silica Source	Surfactant/ Block co-polymer	BET Specific surface area (m2/g)	Pore volume (cm3/g)	Pore size (nm)	Adsorption capacity (mmol/g)	Adsorption Conditions		Ref.
								Temp. (°C)	Pressure (bar)
KIT-5	3D-cubic	TEOS	Pluronic P123	711	1.05	8.04	0.48	30	1	[99]
KIT-6	3D-cubic	TEOS	Pluronic P123	895	1.22	6.0	-	-	-	[96]
MCM – 41	Hexagonal	Na2SiO3	CTAB	994	1.00	3.03	0.63	25	1	[95]
		Na2SiO3	CTAB	993	1.00	3.1	0.63	25	1	[100]
		Na2SiO3	CTAB	980	0.92	4.08				[92]
MCM 48	Cubic	SiO2	CTAB	1287	1.1	3.5		25	1	[101]
SBA-15	2D hexagonal	TEOS	P123	1254	2.44	11.4	-	-	-	[102]
SBA-16	Cubic cage	TEOS	Pluronic F127	736	0.75	4.1	-	-	-	[96]
SNS		TEOS	Pluronic F127	394	0.10	21.1	2.06	25	1	[103]
SNT		TEOS	Pluronic F127	319	0.07	26.0	2.46	25	1	[103]

where CTAB: cetyltrimethylammoniumbromide and hexadecyltrimethylammoniumbromide, F127: tri-block copolymer F127, Na₂SiO_3: sodium silicate, P123: triblock copolymer (Pluronic P123), SiO_2: silica, SNS: silica nano spheres, SNT: silica nano tube, TEOS: tetraethyl orthosilicate.

Table 6. The textural properties and CO₂ absorption capacity of different types of ordered mesoporous silica materials.

Types of mesoporous silica	Structure	Silica Source	Surfactant/ Block co-polymer	BET Specific surface area (m2/g)	Pore volume (cm3/g)	Pore size (nm)	Adsorption capacity (mmol/g)	Adsorption Conditions		Ref.
								Temp. (°C)	Pressure (bar)
KIT-5	3D-cubic	TEOS	Pluronic P123	711	1.05	8.04	0.48	30	1	[99]
KIT-6	3D-cubic	TEOS	Pluronic P123	895	1.22	6.0	-	-	-	[96]
MCM – 41	Hexagonal	Na2SiO3	CTAB	994	1.00	3.03	0.63	25	1	[95]
		Na2SiO3	CTAB	993	1.00	3.1	0.63	25	1	[100]
		Na2SiO3	CTAB	980	0.92	4.08				[92]
MCM 48	Cubic	SiO2	CTAB	1287	1.1	3.5		25	1	[101]
SBA-15	2D hexagonal	TEOS	P123	1254	2.44	11.4	-	-	-	[102]
SBA-16	Cubic cage	TEOS	Pluronic F127	736	0.75	4.1	-	-	-	[96]
SNS		TEOS	Pluronic F127	394	0.10	21.1	2.06	25	1	[103]
SNT		TEOS	Pluronic F127	319	0.07	26.0	2.46	25	1	[103]

where CTAB: cetyltrimethylammoniumbromide and hexadecyltrimethylammoniumbromide, F127: tri-block copolymer F127, Na₂SiO_3: sodium silicate, P123: triblock copolymer (Pluronic P123), SiO_2: silica, SNS: silica nano spheres, SNT: silica nano tube, TEOS: tetraethyl orthosilicate.

Table 7. CO₂ adsorption capacities and structural properties of amine functionalized silica-based adsorbents.

Silica-based sorbent	Amine types	CO2 adsorption performance capacity (mmol/g)	Conditions		BET Specific surface area (m2/g)	Pore volume (cm3/g)	Pore size (nm)	Preparation methods	Ref
			Temperature (°C)	Pressure (bar)
DWSNT	-	0.1	25		83	0.58		Immobilization	[126]
DWSNT	APTMS	1.0	25		112	0.72		Immobilization	[126]
DWSNT	MAPTMS	1.5	25		114	0.79		Immobilization	[126]
DWSNT	DEAPTMS	1.8	25		68.9	0.49		Immobilization	[126]
DWSNT	AEAPTMS	2.25	25		60.9	0.45		Immobilization	[126]
HAS	Aziridines	3.25	25		71	5	0.15		[127]
HPS	PEI	2.44	75	1	0.5	0.009		Impregnation	[128]
HVMCM-41	PEHA	4.07	105	1				Impregnation	[125]
KIT-6	PEHA	4.48	105	1				Impregnation	[125]
MCM-41	EDA	1.19	35					Impregnation	[129]
MCM-41	DETA	1.43	35					Impregnation	[129]
MCM-41	TEPA	1.96	35					Impregnation	[129]
MCM-41	PEHA	2.34	35					Impregnation	[129]
MCM-41	MEA (3%)	11.39	25		426	0.42	3.12	Impregnation	[130]
MCM-41	PEI	0.39	40	0.15	443	0.340	2.95	Impregnation	[49]
MCM-41	PEI	0.22	75	1	590	1.4	13.6	Impregnation	[122]
MCM-41	PEIAziridine	0.98	75	1				In-situ grafted polymerization	[131]
MCM-41	APTS	94	25	1	10	0.01		Grafting	[116]
MCM-41	APTS	2.48	20	1	17	0.04	20.1	Grafting	[133]
MCM-41	PEHA	4.5	105	1				Impregnation	[122]
MCM-41	MEA	0.89	25	1	19	0.82		Impregnation	[100]
MCM-41	DEA	0.80	25	1	13	0.07		Impregnation	[100]
MCM-41	TEA	0.63	25	1	213	0.17		Impregnation	[100]
MCM-41	Branched PEI	1.08	100	1	6	0	-	Impregnation	[95]
MCM-41	Branched PEI	0.79	100	1	12	0.04	-	Impregnation	[95]
MCM-41	Branched PEI – (30 wt%)	0.70	100	1	80	0.14	-	Impregnation	[95]
MCM-41	Branched PEI	28	100	1	104	0.12	2.05	Impregnation	[95]
MCM-41	Branched PEI	17.5	100	1	291	0.17	2.05	Impregnation	[95]
MCM-41	TEPA	1.24	25	1	11	0.05	1.8	Impregnation	[134]
MCM-48	APTES	0.62	25	1.01	1072	0.52	2.9	Grafting	[101]
MCM-48	TRI	0.46	25	1.01	698	0.39	2.6	Grafting	[101]
MCM-48	TRI	0.44	25	1.01	463	0.23	2.5	Grafting	[101]
MsiNTs	PEI	2.75	92		52.4	0.17	12.4	Impregnation	[135]
OMS	PEI	1.4	25		352	0.79		Grafting	[122]
SAB-15	PEHA	4.0	105	1				Impregnation	[125]
SBA-15	PEI	0.65	25		683	1.19	8.5	Impregnation	[124]
SBA-15	PEI/Zr4	1.34	25		642	1.08	8.6	Impregnation	[124]
SBA-15	PEI/Zr7	1.56	25		674	1.23	9.5	Impregnation	[124]
SBA-15	PEI/Zr14	1.41	25		601	0.69	7.0	Impregnation	[124]
SBA-15	PEI/Ti1.4	0.24	25		510	0.39	4.4	Impregnation	[124]
SBA-15	NH2OH	1.65	25	1	435.6	0.54	6.85	Grafting	[136]
SBA-15	APTMS	1.46	25	0.15	82	0.16	5	Grafting	[137]
SBA-15	TEPA	2.45	70		5	0.03		Grafting	[102]
SBA-15	AMP	1.79	70		372	0.21		Grafting	[122]
SBA-15(0.2µm)	PEI	5.84	100	1	590	1.44	13.6	Impregnation	[122]
SBA-15 (1.5µm)	PEI	-	100	1	746	0.80	7.2	Impregnation	[122]
SBA-15 (25µm)	PEI	5.81	100	1	580	0.95	10.5	Impregnation	[122]
SiO2	APTES	4.3	30		67	0.51		In-situ polymerization	[29]
SiO2	AEAPTMS	5.7	30		45	0.37		In-situ polymerization	[29]
SiO2	TRI	5.6	30		25	0.22		In-situ polymerization	[29]
SiO2	APTES	0.5	30		216	1.11		Grafting	[29]
SiO2	AEAPTMS	0.3	30		206	1.10		Grafting	[29]
SiO2	TRI	0.8	30		172	0.99		Grafting	[29]
SMCM-41	MEA	10.40	25		405	0.39	3.01	Impregnation	[130]
SBA-15	TEPA	4.5	75	1	121.1	0.327		Impregnation	[138]
MPSM	TEA	4.27	75	1	34	0.08	9.5	Impregnation	[50]
MCM-41	TRI	1.74	25	0.05	678.3	1.47		Grafting	[139]
MCM-41	APTES	1.20	30	1	1045.21	2.59	30	Grafting	[140]
MCM-41	PEI	0.98	30	1	6.6	0.01	0.8	Grafting	[141]
MCM-41	PEI	4.68	45	1	894	1.28	5.1	Grafting	[118]
MCM-41	PEI	2.92	50	0.1	508	0.98	2.54	Impregnation	[142]
MCM-41	TEPA	2.25	50	0.1	431	0.83	2.21	Impregnation	[142]
MCM-41- KOH	PEI-	3.38	50	0.1	391	1.08	2.33	Impregnation	[142]
MCM-41- Ca(OH)2	PEI-	3.81	50	0.1	411	1.12	2.50	Impregnation	[142]
MCM-41- CsOH	PEI-	5.02	50	0.1	306	0.91	2.14	Impregnation	[142]
MCM-41- KOH	TEPA-	3.93	50	0.1	322	0.97	2.15	Impregnation	[142]
MCM-41- Ca(OH)2	TEPA-	3.76	50	0.1	405	0.94	2.31	Impregnation	[142]
PET- CsOH	TEPA-	5.42	50	0.1	293	0.97	2.61	Impregnation	[142]
MCM 48	PEI	1.09	80	0.24	79.3	0.02	1.68	Impregnation	[143]
MCM-41	PEI	1.23	80	0.24	59.1	0.02	1.80	Impregnation	[143]
SBA-15	PEI	1.07	80	0.24	62.1	0.01	5.2	Impregnation	[143]
SBA-15	PEI	1.77	0	1	783	0.03	7.0	Impregnation	[144]
SBA-15	PEI	1.26	45	0.15	399	0.79	8.2	Impregnation	[145]
MCM 41	PEI	3.53	25	1	24	0.012		Impregnation	[146]
MCM 41	APTS	2.41	25	1	736	0.37		Grafting	[146]
SBA-15	PEI	1.84	25	1.2	195	0.39	7.0	Grafting	[147]
SBA-15- APES		1.78	25	1.2	190	0.37	7.2	Grafting	[147]
SBA-15- APES	PEI	1.54	25	1.2	24	0.21	2.7	Grafting	[147]
OMS	PEI	2.43	25	1.2	167	0.33	7.6	Grafting	[147]
OMS- APES		3.03	25	1.2	180	0.37	7.2	Grafting	[147]
OMS- APES	PEI	1.18	25	1.2	39	0.18	2.3	Grafting	[147]
OMS- NCC	Amidoxime	5.54	120	1	315	0.69	9.3		[148]
MPS-MCC*		2.41	120		302	0.44	7.0		[149]
MPS-MCC**		3.85	120		285	0.40	6.7		[149]
OMS- MgO		4.71	120	1	261	0.48	7.25		[150]
OMS-CaO		3.85	120	1	163	0.25	6.76		[150]
SiO2- Al2O3	APTS	2.64	25	1	740	1.24	5.1	Grafting	[151]
SiO2- Al(NO3)3	APTS	0.78	25	1	319	0.63	2.9	Grafting	[151]
OMS-Ti		0.81	25	1	487				[90]
MsiNTs	APTES	2.87	25	1.2	293	0.79	22	Grafting	[103]
SNS	APTES	2.13	25	1.2	210	0.31	19.6	Grafting	[103]
Al(NO3)3	AP	0.98	25	1	359	0.62	10.0		[152]
OMS-Al-Zr		2.60	60	1	441	0.61	6.9		[153]

Where, ** MCC- mesoporous silica with amidoxime functionalities, *MCC-mesoporous silica with cyanopropyl groups, APTMS: 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane, AEAPTMS: [3-(2-aminoethyl) aminopropyl]trimethoxysilane, AMP: 2-amino-2-methyl-1-propanol, AP: 3-aminopropyltriethoxysilane, APTMS: (3-aminopropyl) trimethoxysilane, APTS: 3-aminopropyltrimethoxysilane, DEA: diethanolamine, DEAPTMS: [3-(diethylamino) propyl]trimethoxysilane, DETA: diethylenetriamine, DWSNT: double-walled silica nano tube, EDA: ethylenediamine, HPS: Hierarchically porous silica, MAPTMS: [3-(methylamino) propyl]trimethoxysilane, MCC: microcrystalline cellulose, MEA: monoethanolamine, MPSM: monodispersed porous silica microspheres, MsiNTs: mesoporous silica nanotubes, NCC: nanocrystalline cellulose, OMS: ordered mesoporous organosilica, OMS: Oxide-templated silica monoliths, PEHA: pentaethylenehexamine, PEI: polyethylenimine, SNS: silica nano spheres,TEA: triethanolamine, TEPA: tetraethylenepentamine, TRI: 3-[2-(2-Aminoethylamino)ethylamino]propyltrimethoxysilane.

Table 7. CO₂ adsorption capacities and structural properties of amine functionalized silica-based adsorbents.

Silica-based sorbent	Amine types	CO2 adsorption performance capacity (mmol/g)	Conditions		BET Specific surface area (m2/g)	Pore volume (cm3/g)	Pore size (nm)	Preparation methods	Ref
			Temperature (°C)	Pressure (bar)
DWSNT	-	0.1	25		83	0.58		Immobilization	[126]
DWSNT	APTMS	1.0	25		112	0.72		Immobilization	[126]
DWSNT	MAPTMS	1.5	25		114	0.79		Immobilization	[126]
DWSNT	DEAPTMS	1.8	25		68.9	0.49		Immobilization	[126]
DWSNT	AEAPTMS	2.25	25		60.9	0.45		Immobilization	[126]
HAS	Aziridines	3.25	25		71	5	0.15		[127]
HPS	PEI	2.44	75	1	0.5	0.009		Impregnation	[128]
HVMCM-41	PEHA	4.07	105	1				Impregnation	[125]
KIT-6	PEHA	4.48	105	1				Impregnation	[125]
MCM-41	EDA	1.19	35					Impregnation	[129]
MCM-41	DETA	1.43	35					Impregnation	[129]
MCM-41	TEPA	1.96	35					Impregnation	[129]
MCM-41	PEHA	2.34	35					Impregnation	[129]
MCM-41	MEA (3%)	11.39	25		426	0.42	3.12	Impregnation	[130]
MCM-41	PEI	0.39	40	0.15	443	0.340	2.95	Impregnation	[49]
MCM-41	PEI	0.22	75	1	590	1.4	13.6	Impregnation	[122]
MCM-41	PEIAziridine	0.98	75	1				In-situ grafted polymerization	[131]
MCM-41	APTS	94	25	1	10	0.01		Grafting	[116]
MCM-41	APTS	2.48	20	1	17	0.04	20.1	Grafting	[133]
MCM-41	PEHA	4.5	105	1				Impregnation	[122]
MCM-41	MEA	0.89	25	1	19	0.82		Impregnation	[100]
MCM-41	DEA	0.80	25	1	13	0.07		Impregnation	[100]
MCM-41	TEA	0.63	25	1	213	0.17		Impregnation	[100]
MCM-41	Branched PEI	1.08	100	1	6	0	-	Impregnation	[95]
MCM-41	Branched PEI	0.79	100	1	12	0.04	-	Impregnation	[95]
MCM-41	Branched PEI – (30 wt%)	0.70	100	1	80	0.14	-	Impregnation	[95]
MCM-41	Branched PEI	28	100	1	104	0.12	2.05	Impregnation	[95]
MCM-41	Branched PEI	17.5	100	1	291	0.17	2.05	Impregnation	[95]
MCM-41	TEPA	1.24	25	1	11	0.05	1.8	Impregnation	[134]
MCM-48	APTES	0.62	25	1.01	1072	0.52	2.9	Grafting	[101]
MCM-48	TRI	0.46	25	1.01	698	0.39	2.6	Grafting	[101]
MCM-48	TRI	0.44	25	1.01	463	0.23	2.5	Grafting	[101]
MsiNTs	PEI	2.75	92		52.4	0.17	12.4	Impregnation	[135]
OMS	PEI	1.4	25		352	0.79		Grafting	[122]
SAB-15	PEHA	4.0	105	1				Impregnation	[125]
SBA-15	PEI	0.65	25		683	1.19	8.5	Impregnation	[124]
SBA-15	PEI/Zr4	1.34	25		642	1.08	8.6	Impregnation	[124]
SBA-15	PEI/Zr7	1.56	25		674	1.23	9.5	Impregnation	[124]
SBA-15	PEI/Zr14	1.41	25		601	0.69	7.0	Impregnation	[124]
SBA-15	PEI/Ti1.4	0.24	25		510	0.39	4.4	Impregnation	[124]
SBA-15	NH2OH	1.65	25	1	435.6	0.54	6.85	Grafting	[136]
SBA-15	APTMS	1.46	25	0.15	82	0.16	5	Grafting	[137]
SBA-15	TEPA	2.45	70		5	0.03		Grafting	[102]
SBA-15	AMP	1.79	70		372	0.21		Grafting	[122]
SBA-15(0.2µm)	PEI	5.84	100	1	590	1.44	13.6	Impregnation	[122]
SBA-15 (1.5µm)	PEI	-	100	1	746	0.80	7.2	Impregnation	[122]
SBA-15 (25µm)	PEI	5.81	100	1	580	0.95	10.5	Impregnation	[122]
SiO2	APTES	4.3	30		67	0.51		In-situ polymerization	[29]
SiO2	AEAPTMS	5.7	30		45	0.37		In-situ polymerization	[29]
SiO2	TRI	5.6	30		25	0.22		In-situ polymerization	[29]
SiO2	APTES	0.5	30		216	1.11		Grafting	[29]
SiO2	AEAPTMS	0.3	30		206	1.10		Grafting	[29]
SiO2	TRI	0.8	30		172	0.99		Grafting	[29]
SMCM-41	MEA	10.40	25		405	0.39	3.01	Impregnation	[130]
SBA-15	TEPA	4.5	75	1	121.1	0.327		Impregnation	[138]
MPSM	TEA	4.27	75	1	34	0.08	9.5	Impregnation	[50]
MCM-41	TRI	1.74	25	0.05	678.3	1.47		Grafting	[139]
MCM-41	APTES	1.20	30	1	1045.21	2.59	30	Grafting	[140]
MCM-41	PEI	0.98	30	1	6.6	0.01	0.8	Grafting	[141]
MCM-41	PEI	4.68	45	1	894	1.28	5.1	Grafting	[118]
MCM-41	PEI	2.92	50	0.1	508	0.98	2.54	Impregnation	[142]
MCM-41	TEPA	2.25	50	0.1	431	0.83	2.21	Impregnation	[142]
MCM-41- KOH	PEI-	3.38	50	0.1	391	1.08	2.33	Impregnation	[142]
MCM-41- Ca(OH)2	PEI-	3.81	50	0.1	411	1.12	2.50	Impregnation	[142]
MCM-41- CsOH	PEI-	5.02	50	0.1	306	0.91	2.14	Impregnation	[142]
MCM-41- KOH	TEPA-	3.93	50	0.1	322	0.97	2.15	Impregnation	[142]
MCM-41- Ca(OH)2	TEPA-	3.76	50	0.1	405	0.94	2.31	Impregnation	[142]
PET- CsOH	TEPA-	5.42	50	0.1	293	0.97	2.61	Impregnation	[142]
MCM 48	PEI	1.09	80	0.24	79.3	0.02	1.68	Impregnation	[143]
MCM-41	PEI	1.23	80	0.24	59.1	0.02	1.80	Impregnation	[143]
SBA-15	PEI	1.07	80	0.24	62.1	0.01	5.2	Impregnation	[143]
SBA-15	PEI	1.77	0	1	783	0.03	7.0	Impregnation	[144]
SBA-15	PEI	1.26	45	0.15	399	0.79	8.2	Impregnation	[145]
MCM 41	PEI	3.53	25	1	24	0.012		Impregnation	[146]
MCM 41	APTS	2.41	25	1	736	0.37		Grafting	[146]
SBA-15	PEI	1.84	25	1.2	195	0.39	7.0	Grafting	[147]
SBA-15- APES		1.78	25	1.2	190	0.37	7.2	Grafting	[147]
SBA-15- APES	PEI	1.54	25	1.2	24	0.21	2.7	Grafting	[147]
OMS	PEI	2.43	25	1.2	167	0.33	7.6	Grafting	[147]
OMS- APES		3.03	25	1.2	180	0.37	7.2	Grafting	[147]
OMS- APES	PEI	1.18	25	1.2	39	0.18	2.3	Grafting	[147]
OMS- NCC	Amidoxime	5.54	120	1	315	0.69	9.3		[148]
MPS-MCC*		2.41	120		302	0.44	7.0		[149]
MPS-MCC**		3.85	120		285	0.40	6.7		[149]
OMS- MgO		4.71	120	1	261	0.48	7.25		[150]
OMS-CaO		3.85	120	1	163	0.25	6.76		[150]
SiO2- Al2O3	APTS	2.64	25	1	740	1.24	5.1	Grafting	[151]
SiO2- Al(NO3)3	APTS	0.78	25	1	319	0.63	2.9	Grafting	[151]
OMS-Ti		0.81	25	1	487				[90]
MsiNTs	APTES	2.87	25	1.2	293	0.79	22	Grafting	[103]
SNS	APTES	2.13	25	1.2	210	0.31	19.6	Grafting	[103]
Al(NO3)3	AP	0.98	25	1	359	0.62	10.0		[152]
OMS-Al-Zr		2.60	60	1	441	0.61	6.9		[153]

Where, ** MCC- mesoporous silica with amidoxime functionalities, *MCC-mesoporous silica with cyanopropyl groups, APTMS: 3-[2-(2-aminoethylamino)ethylamino]propyltrimethoxysilane, AEAPTMS: [3-(2-aminoethyl) aminopropyl]trimethoxysilane, AMP: 2-amino-2-methyl-1-propanol, AP: 3-aminopropyltriethoxysilane, APTMS: (3-aminopropyl) trimethoxysilane, APTS: 3-aminopropyltrimethoxysilane, DEA: diethanolamine, DEAPTMS: [3-(diethylamino) propyl]trimethoxysilane, DETA: diethylenetriamine, DWSNT: double-walled silica nano tube, EDA: ethylenediamine, HPS: Hierarchically porous silica, MAPTMS: [3-(methylamino) propyl]trimethoxysilane, MCC: microcrystalline cellulose, MEA: monoethanolamine, MPSM: monodispersed porous silica microspheres, MsiNTs: mesoporous silica nanotubes, NCC: nanocrystalline cellulose, OMS: ordered mesoporous organosilica, OMS: Oxide-templated silica monoliths, PEHA: pentaethylenehexamine, PEI: polyethylenimine, SNS: silica nano spheres,TEA: triethanolamine, TEPA: tetraethylenepentamine, TRI: 3-[2-(2-Aminoethylamino)ethylamino]propyltrimethoxysilane.

5. Chemisorbents (amine functionalized Si-based materials) – application at low and high temperature CO₂ sorption

In the physisorption process, CO₂ molecules attach to the pore walls through the weak Van der Waals and pole–pole interactions [104]. However, the unmatched pore size of the mesoporous silica and small dimeter of CO₂ gas molecule causes low CO₂ adsorption capacities. The heat of adsorption of physisorption process ranges from -25 to -40 kJ/mol [105], which is approximately closer to the heat of sublimation [106]. Recently, it has been reported about the mesoporous silica materials with improved CO₂ sorption capacity with amine functionalization [107]. However, the adsorption capacity of CO₂ depends on the nature of the amine groups and the spacing between the amino silanes [108]. Figure 7 represents the different types of amino silanes- and polymer containing amino groups used during the functionalization of mesoporous silicas for enhanced adsorption or separation.

5.1. Synthesis procedures

Recently, the improvement of amine-based adsorbents was conducted by three approaches: the preparation of supports with high amine loadings, use of amine with high nitrogen content and use of effective methods for amine introduction [44]. Synthesis methods of amine functionalized silica materials have three main pathways including impregnation, grafting, and in situ polymerization as shown in Figure 8.

In impregnation, amines are physically trapped in the pores of silica materials as shown in Figure 8. Moreover, performance of amine-silica adsorbents is influenced by the pore structure of silicas. However, Chen et al. [109] and Chen et al. [110] have reported that the CO₂ adsorption capacities follow the order of decreasing pore diameter. Moreover, surfactant, surface functional groups, amine types and heteroatom incorporation affect the impregnation process [54]. In this method, the amine loading is affected by the total pore volume of the silica materials and the density of the amine. Moreover, if the amount of amine exceeds the capacity of the support, the amine species will agglomerate. The main advantage of this method is its simplicity and involvement of its mild synthesis procedure. Further, large amount of amine species is cooperated with mesoporous silica due to the large pore volume of the porous silica materials [111].

Grafting occurs between an aminosilane with silica to introduce amine groups as shown in Figure 8 where the amines are grafted on the silica surface via covalent bonds [112]. Mainly, three methods are used for the grafting of amine onto a silica support which are, post-synthesis grafting, direct synthesis by co-condensation (one pot synthesis), and anionic template synthesis [113]. In a typical process, silica is dispersed in a solvent and then amino silanes were 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 [111]. In-situ polymerization is another promising method for functionalizing porous silica such as hyperbranched aminosilica (HAS). This category of supported sorbents can be considered as a hybrid of the grafting and impregnation [114].

In contrast, toxic solvents (toluene) are used for grafting process. Therefore, because of the simplicity, low cost, environmental friendliness, and convenience for large scale production, the impregnating technique is widely used [116]. However, in order to overcome the challenges caused by grafting, researchers have recently investigated the aminosilane gas-phase grafting and supercritical fluid impregnation [117].

Supercritical fluid impregnation is considered to be one of the most effective, simple and reproducible methods for producing homogeneous, covalently bonded and high-density silane [117]. López-Aranguren et al. [117] synthesized functionalized silica via supercritical CO₂ grafting of aminosilanes. For this study, silica gels (4.1 and 8.8 nm pore diameter), mesoporous silica MCM-41 (3.8 nm pore diameter) and mono- and di-aminotrialkoxysilane were chosen.

The method called double functionalization of mesoporous materials is also widely used in recent years. Several groups have prepared amine–silica composites using a double-functionalization method [118,119,120]. In those studies, impregnation and grafting were used to improve the CO₂ uptake [118].

5.2. Comparison of adsorption capacities

Nigar et al. [101] have synthesized the ordered mesoporous (MCM-48) silica with different silane molecules including 3-triethoxysilylpropylamine, 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 of the surface area and the pore volume compared to the non-functionalized MCM-48 (1287 m²/g and 1.1 cm²/g) (see Table 7). Most importantly, it is clearly seen that, with the increment of number amine groups in silane molecules caused a decrease of CO₂ absorption capacity governed through chemisorption [101].

Moreover, Park et al. [29] synthesized functionalized silica using the silane molecules as similar to the studies conducted by Niger et al. [101]. However, they have compared in-situ polymerization and grafting. According to the data reported here, (see Table 7) it is shown that, sorbent prepared through in-situ polymerization show enhanced CO₂ adsorption capacity. Ahmed et al. [95] reported a detailed study about functionalization of mesoporous Si-MCM-41 with different loadings of PEI. According to that, with the increment of PEI loading, the CO₂ adsorption capacity increased (see Table 7). This behavior is suggested as the branched PEI exhibits high density of amino groups as potential CO₂-affinity sites and the hierarchical mesoporous structure of Si-MCM-41 makes these sites accessible by improving the dispersion of PEI [121].

Gargiulo and co-workers reported the effect of temperature on CO₂ adsorption capacity on SBA- 15 and PEI. CO₂ adsorption were evaluated at 25, 40, 55, and 75 °C temperatures [122]. According to the experimental data, a significant dependence of the CO₂ adsorption capacity on temperature was observed (Table 7). The effect of pore dimension on CO₂ adsorption over amine-modified mesoporous silicas were reported by Heydari-Gorji et al. [102]. 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 which are caused by enhanced amine accessibility inside the pores. Heydari-Gorji and Sayari, [123] investigated the PEI impregnation for CO₂ removal applications. It demonstrated that PEI functionalized silica materials were thermally stable at mild temperatures. On the contrary, the adsorbent was stable after long-term exposure to gas mixtures (CO₂ and O₂) due to the presence of CO₂. Amine groups are protected from oxygen attack, because of rapid formation of carbamate and bicarbonate structures. Kuwahara et al. [124] synthesized poly(ethyleneimine)/silica composite adsorbents by incorporating zirconium (Zr) into the silica support. According to the results observed, Zr sites show an increase in the CO₂ adsorbent capacity (see table 7), regenerability, and stability.

Apart from that, Kishor and Ghoshal [125] investigated the effect of the structural parameters of the silicas and amine-functionalized silicas such as pore size, pore-volume, and surface area on the CO₂ sorption capacity. For this study, various silicas such as KIT-6, MCM-41, SBA-15, and HV-MCM-41 were used. Wet impregnation method was used by them to prepare the pentaethylenehexamine (PEHA) functionalized silicas. The CO₂ 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 CO₂ capture capacity of 4.48 mmol/g of CO₂ 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.

In contrast, Sim and co-workers [147] have studied the CO₂ 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 sample (see Table 7) showed the highest CO₂ adsorption capacity, selectivity and reproducibility. Another silica composite was prepared by Dassanayake et al. [148] using nano-crystaline cellulose. According to the reported data, nano-crystalline cellulose mesoporous silica showed high CO₂ absorption capacity (see Table 7), recyclability and thermal stability. Authors further reported that, synthesis process is relatively cheap and simple compared to other composites studied. Gunathilake et al., [149] have prepared the microcrystalline cellulose (MCC) mesoporous silica composites. Prepared composites-based sorbent materials showed biocompatibility, biodegradability, non-toxicity, cyclic stability, and thermal and mechanical stability. Herein, two MCC-mesoporous silica composites were prepared as follows; MCC-mesoporous silica with cyanopropyl groups were first synthesized and then cyanopropyl groups were converted into the amidoxime functionalities. CO₂ adsorption was evaluated at 25 and 120 ^oC. According to the results, MCC- mesoporous silica with amidoxime functionalities exhibited the highest absorption capacity (see Table 7) at 120 ^oC due to the oxime and amine groups in amidoxime functionality and hydroxyl groups in MCC which serve as active sites while assuring highest CO₂ adsorption.

On the other hand, Rao et al. [146] determined the effect of impregnation and grafting of the amine-functionalized MCM-41. The results exhibited that (Table 7) grafted sorbents showed highest thermal stability than the impregnation ones. They concluded that, adsorbents modified by impregnation showed higher amine-loading efficiencies and thus higher CO₂ adsorption capacities whereas those prepared by the 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 CO₂ absorption capacity [142]. The results showed that all the three kinds of inorganic alkali-containing adsorbents exhibit higher CO₂ 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-CO₂ and the adsorbent surface due to the increase of hydroxyl groups. Also, they reported that CO₂ adsorption capacities have a linear dependence relationship with the amounts of alkali adsorbents. Apart from that, Gunathilake and Jaroniec [150] reported the incorporation of magnesium oxide and calcium oxide into mesoporous silica surface (OMS) and applied those materials for CO₂ sorption at ambient and elevated temperatures. The materials were synthesized using sol−gel method. However, the resulted composite sorbents performed relatively high adsorption capacities (see Table 7). It suggested that MgO and CaO caused an increase of physisorption of CO₂ via microspores plus chemisorption via hydroxyl groups. Those synthesized CaO-SiO₂ and MgO-SiO₂ composites showed high surface area, basic surface properties and high thermal and chemical stability.

Alumina materials also possess high surface area, porosity, and thermal and mechanical stability. Therefore, recently, researchers have used the amine-grafted mesoporous silica and impregnated alumina as solid sorbents for CO₂ capture [151]. Alumina-based materials for CO₂ capture includes basic Al₂O₃, amine-impregnated oramine-modified mesoporous Al₂O₃ and Al₂O₃–organosilica [151]. Gunathilake et al. [151] synthesized Al₂O₃–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)). In this study, two alumina precursors aluminum nitrate nanahydrate and aluminum isopropoxide were used whereas grafting of amine groups was performed using 3-aminopropyltriethoxysilane (APTS). According to the results obtained of this study, SiO₂- Al₂O₃ showed the highest absorption capacity (Table 7) and the adsorption properties of the materials depend on the surface area of the sample, alumina precursor, and structure and functionality of the organosilica bridging group. Besides, Choi et al. [154] used epoxy-functionalized PEI for the synthesis of CO₂ sorbents. According to the reported data, epoxy-functionalized PEI exhibited a CO₂ capacity of 2.2 mmol/g at 120 °C and 100% regeneration capability at similar temperature. This can be attributed to the heat resistant properties of epoxy butane which enhanced the CO₂ capture capacity and thermal stability of the silica-epoxy-PEI sorbent.

However, according to the reported data by Hu et al. [155], Li₄SiO₄ exhibited attractive prospects for CO₂ capture and the main advantage is high CO₂ sorption capacity (theoretical sorption capacity of 0.367 g CO₂/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 [155].

5.3. Sorbent selectivity, regeneration and stability in the cyclic CO₂ adsorption –desorption runs

During industrial applications, high adsorption capacity along with good regenerability of the sorbents in the cyclic adsorption–desorption process is of vital importance [119]. The practical application of an adsorbent requires high sorption capacity, easy regeneration, stability in normal atmosphere, as well as stable performance during cyclic use for a long-term operation.

For instance, Ahmed et al. [95] reported a detailed study about functionalization of mesoporous MCM-41 with different loadings of polyethylenimine (PEI). In this study, the selectivity measurement was conducted for CO₂ over N₂ and H₂. According to the results obtained, the adsorption capacities of N₂ and H₂ on 50 wt% PEI-Si-MCM- 41 were obtained as 3.89 mg/g and 6.51 mg/g, respectively (see Table 8). Table 8 represents the summary of gas selectivity values of previous studies done for porous SiO₂.

Wang et al. [156] prepared SBA-15 using two template removal methods which are, silica: ethanol extraction and conventional high-temperature calcination. Then, the resulted silicas were subjected to amine (3-aminopropyl) grafting and studied for their CO₂ adsorption properties. The aim of this study was to increase the surface silanol density, and hence the grafted amine loading leads directly to increased CO₂ adsorption capacity and CO₂/N₂ selectivity. According to the reported data, CO₂/N₂ selectivity changed from 46 to 13 (see Table 8) and these results ensured that solvent extraction also leads to an enhancement in CO₂/N₂ selectivity. Also, the authors performed a test to measure the stability of amine-SBA-15 (solvent extracted). According to the results, after each adsorption step, amine-SBA-15 (solvent extracted) was regenerated under a vacuum.

In industrial applications of adsorbents, it is important to remain stable during cyclic operations. This section summarizes the reported studies on sorbent regeneration and stability in cyclic CO₂ adsorption –desorption runs by amine–silica composites and reported data is 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 a temperature of 50 ∼120 °C in N₂, He, or Ar flow. As depicted in Table 9, the amine-impregnated silica composites show loss of CO₂ capture capacity in the cyclic CO₂ sorption –desorption runs, due to amine leaching from the silica surface and degradation [112]. 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 [111].

Therefore, Guo et al. [128] have 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 CO₂ partial pressure was maintained at 1 bar. According to the data, adsorption capacities are similar in eight adsorption/desorption cycles, showing that the aforementioned sorbents have good stability and regenerability.

Wang et al. [119] investigated the regenerability of the amine modified MCM-41 (MCM-41-TEPA and MCM-41-AMP). Authors have 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 [125] 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. According to the results, PEHA impregnated KIT-6 showed sorption capacities of 4.0 and 4.3 mol CO₂/ kg at 90 and 105 °C at 1 bar even after 6 months. Also, the sorption performance of adsorbent was tested for ten consecutive adsorption/desorption cycles. The sorption capacity of the sorbent decreased by less than 4% in 90−105 °C at 1 bar without any structural degradation. Moreover, the results exhibited that, PEHA impregnated KIT-6 have better sorption performance than those of earlier reported adsorbents, except for silica aerogel.

Liu and co-workers carried out regeneration test for zeolite-mesoporous silica-supported-amine hybrids sorbent [162]. According to the reported data, after 10 cycles, the adsorption capacity demonstrated almost no change, therefore, the sample performed a very stable cyclic adsorption-desorption performance. In contrast, López-Aranguren et al. [131] examined the regeneration of CO₂ from branched PEI - mesoporous silica.

Table 9. Summary of stability of silica-based adsorbent studied in past performance capacity.

Synthesis method	Type of silica-based sorbent	Amine type	Regeneration condition		Stability performance		References
			Temperature (°C)	Types of gas flow	No. of cycles (cyclic runs)	Capacity loss (%)
Impregnated	MCM-41	PEHA	100	N2	15	Less than 1	[161]
	MCM-41	TEPA +AMP	100	N2 for 60 min	15	4.32	[119]
	SBA-15	PEI-linear	100	Ar	12	13.5	[162]
	SBA-15	Acrylonitrile-modified TEPA	100	N2	12	1.1	[163]
	HMS	PEI-linear	75	N2 for 100 min	4	1.6	[164]
	MCF	PEI- branched	115	Ar for 20 min	10	32	[165]
	MCF	PEI	100	H2	10	5	[166]
	MCF	Guanidinylated poly(allylamine)	120	He	5	17	[52]
	Fumed silica	PEI-linear	55	N2 for 15min	180	Stable	[167]
	MCM-41	TEPA	100	N2	10	3.43	[168]
	Silica fume	Diisopropanolamine	50	N2	10	7	[169]
	Nano- SiO2	PEI- branched	120	N2	30	10.5	[170]
	Nano- SiO2	PEI- branched	120	N2	30	19.4	[171]
	Mesoporous-SiO2	APTS	120	Air for 30 min	11	4.3	[172]
	Porous SiO2	PEI	100	N2 for 30 min	20	5	[173]
	Silica aerogel	TEPA	75	Ar for 20 min	10	3.9	[174]
	Porous SiO2	TEPA	75	He for 20 min	10	2	[175]
	SNT	PEI	110	N2 for 40 min	10	3.3	[134]
	KCC-1- SiO2	TEPA	110	N2	21	1.2	[176]
	Mesoporous multilamellar SiO2	PEI	110	N2	10	3.7	[177]
	Silica aerogel	TEPA	80	Ar for 30 min	100	12	[176]
	Mesoporous SiO2	DEA	90	N2	10	12	[172]
Grafting	SBA-15	AP	90	Vacuum	10	1	[178]
	SBA-15	DEAPTMS	120	N2 for 10 min	100	7.2	[179]
	MCM-48	2-[2-(3-trimethoxysilyl propylamino) ethylamino] ethylamine	-	N2	20	Stable	[100]
	KIT-6	APTES	120	He	10	Stable	[99]
	MCF	TRI	150	N2 for 30 min	5	1.9	[180]
	HMS	APTS	110	N2 for 180 min	3	Less than 1	[181]
	MCM-41	APTS	105	N2 for 90 min	10	Stable	[117]

Table 9. Summary of stability of silica-based adsorbent studied in past performance capacity.

Synthesis method	Type of silica-based sorbent	Amine type	Regeneration condition		Stability performance		References
			Temperature (°C)	Types of gas flow	No. of cycles (cyclic runs)	Capacity loss (%)
Impregnated	MCM-41	PEHA	100	N2	15	Less than 1	[161]
	MCM-41	TEPA +AMP	100	N2 for 60 min	15	4.32	[119]
	SBA-15	PEI-linear	100	Ar	12	13.5	[162]
	SBA-15	Acrylonitrile-modified TEPA	100	N2	12	1.1	[163]
	HMS	PEI-linear	75	N2 for 100 min	4	1.6	[164]
	MCF	PEI- branched	115	Ar for 20 min	10	32	[165]
	MCF	PEI	100	H2	10	5	[166]
	MCF	Guanidinylated poly(allylamine)	120	He	5	17	[52]
	Fumed silica	PEI-linear	55	N2 for 15min	180	Stable	[167]
	MCM-41	TEPA	100	N2	10	3.43	[168]
	Silica fume	Diisopropanolamine	50	N2	10	7	[169]
	Nano- SiO2	PEI- branched	120	N2	30	10.5	[170]
	Nano- SiO2	PEI- branched	120	N2	30	19.4	[171]
	Mesoporous-SiO2	APTS	120	Air for 30 min	11	4.3	[172]
	Porous SiO2	PEI	100	N2 for 30 min	20	5	[173]
	Silica aerogel	TEPA	75	Ar for 20 min	10	3.9	[174]
	Porous SiO2	TEPA	75	He for 20 min	10	2	[175]
	SNT	PEI	110	N2 for 40 min	10	3.3	[134]
	KCC-1- SiO2	TEPA	110	N2	21	1.2	[176]
	Mesoporous multilamellar SiO2	PEI	110	N2	10	3.7	[177]
	Silica aerogel	TEPA	80	Ar for 30 min	100	12	[176]
	Mesoporous SiO2	DEA	90	N2	10	12	[172]
Grafting	SBA-15	AP	90	Vacuum	10	1	[178]
	SBA-15	DEAPTMS	120	N2 for 10 min	100	7.2	[179]
	MCM-48	2-[2-(3-trimethoxysilyl propylamino) ethylamino] ethylamine	-	N2	20	Stable	[100]
	KIT-6	APTES	120	He	10	Stable	[99]
	MCF	TRI	150	N2 for 30 min	5	1.9	[180]
	HMS	APTS	110	N2 for 180 min	3	Less than 1	[181]
	MCM-41	APTS	105	N2 for 90 min	10	Stable	[117]

In this study, CO₂ adsorption/desorption cycles showed that the uptake measured in the first cycle was successfully maintained even after 20 cycles. Zhang et al. [177] examined the stability of the adsorbents based on linear PEI supported on silica. According to the reported data, the adsorbent maintained its adsorption capacity, but the adsorption capacity reduced by approximately 5.6% when the temperature was increased to 100 °C, which was attributed to amine leaching. Furthermore, Subagyono et al. [165] found that the branched PEI containing adsorbent showed a decrease in CO₂ adsorption –desorption capacity during cycling which is attributed to the by-product formation.

6. Technical Challenges and Future Trends

Facing financial and technical challenges are the main barriers for CO₂ commercial utilization. From technical aspects, technologies are not sufficient for all large point sources and also, largescale development should consider regarding the eco-friendliness. One of the major challenges associated with CCS is moving CO₂ storage to remote sites using CO₂ pipeline networks from the location it is captured to the storage site. These pipelines are costly, difficult to obtain permits and cause lot of environmental problems. As a suggestion, industries can be moved to remote storage sites.

There are several studies on the requirements and a working definition for carbon dioxide capture (CCS). Overall, it is required to develop some advanced physical adsorbents which have high CO₂ selectivity and gas uptake. Stability (over 1000 cycles), CO₂ affinity, scalability, reusability, resistance against surface erosion and additionally required energy are the major concerns in carbon capture. The sorbent cost is the most significant part of an air capture system; however, it is difficult to estimate the cost 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 CO₂ revenue collected during its lifetime. Therefore, it is necessary for a sorbent to possess constant stability and performance for its lifetime [181].

The other main challenges associated with sorbents are stability, kinetics, and sorbent capacity. However, thermodynamically, many sorbents are strong enough to capture CO₂ from ambient air and allowed for easy regeneration. Despite the reported data, further studies on stability, kinetics and capacity are still needed to be improved in SiO₂-based adsorbents. Another factor is sorbent loading and unloading cycles since it is an important factor for reducing the cost. Also, kinetics is affected by binding energies and also by diffusion into porous materials and by the geometry of sorbent materials. According to the researchers, many sorbents require longer sorption times. Therefore, improved kinetics can lower the cost. High adsorption capacity can reduce the cost of CO₂ capture by reducing the amount of sorbent required. Physisorbents that selectively separate CO₂ from gaseous mixtures formed a revolution in CCS since it requires less energy for recycling, with enhanced CO₂ capacity. It is evident that a new generation of physisorbent materials is urgently required to address the carbon capture applications.

As a solution, amine-based sorbents are widely used during CCS. However, amine sorbent depends on the molecular weight of the sorbent and 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 of cation distances can be used under different humidity conditions. The potential of solid sorbents to remove CO₂ from flue gas is huge, compared to conventional liquid amine processes in terms of regeneration energy and significant cost reduction. However, as discussed previously, solid sorbents also have limitations and challenges to be addressed and solved before these can be deployed commercially in post-combustion CO₂ capture.

There is limited literature on CO₂ capture behavior of the silica-based materials synthesis using different low-cost silica sources such as rice husk. These sources lead to the reduction of production cost. Nevertheless, novel silica-based materials such as lithium orthosilicate (Li₄SiO₄), silica nano tubes, silica nano spheres, silica-based composites and silica aero gels lead to high CO₂ gas capture at elevated temperatures. Therefore, researchers should have focused on different silica-based materials for CO₂ capture.

Besides, currently, the majority of researchers have used sol gel, perception and hydrothermal process for synthesis of silica-based sorbent. However, apart from the aforementioned methods, microwave treatment can be used which is a cost effective and time serving method. Moreover, different surfactants can be used for the preparation of silica with different pore sizes and morphologies. On the contrary, another issue raised with silica-based sorbent is the lack of literature on kinetic studies regarding the CO₂ adsorption and the removal of CO₂ from the natural gas sources at different temperatures. Moreover, silica-based sorbents need to be used with simulation work with the process design which might lead to successful deployment in industrial applications.

7. Conclusions

Here, a review on capture and the separation of CO₂ gas using different technologies is presented. Though chemical absorption is an energy intensive process, owing to the huge amount of relapsed CO₂ in power plants, chemical absorption is definitely more suitable than physical absorption to achieve a better CO₂ capture. Thus, due to the simplicity and lower regeneration energy requirements, sorption has been used a lot in the post-combustion CO₂ capture applications. This review paper is mainly focused on SiO₂-based adsorbents used in this process. Apart from that, amine-functionalized SiO₂ possesses higher CO₂ selectivity over other gases and high CO₂ adsorption capacities which make them ideal candidates for CO₂ capture. Moreover, some of the experimental studies on these sorbents have been reported, compared and discussed. However, the performance of currently available amine functionalized SiO₂ should be further developed and improved in terms of stability, gas selectivity and resistivity to thermal degradation. Furthermore, the review pointed out some of the barriers and opportunities for CO₂ capture technology. More effective materials for CO₂ capture and storage are required to minimize the emissions of greenhouse gases.