3. CO2 Adsorption Using Mesoporous Silica Materials (Physisorbents)
3.1. Mesoporous Silica Materials
Mesoporous silica materials are used for various applications, including catalysis and wastewater treatment
[44]. 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
[45]. The first synthesized mesoporous silica material was M41S in the 1990s
[46]. 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
[47].
In 1990, Mobil Oil Corporation discovered molecular sieves of the M41S family consisting of silicate/aluminosilicate
[48]. 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)
[48]. Moreover, the structural ordering of these M41S family materials can be changed with increasing hydrothermal synthesis temperature and time
[48]. These M41S molecular sieves are mainly applied in catalysis
[49], adsorption
[48], and controlled release of drugs
[50]. 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
[48][51][52].
Santa Barbara Amorphous family (SBA) first prepared silica-based materials with well-ordered mesoporous in 1998
[48]. This 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 a cubic cage)
[48][53]. These nanostructured mesoporous materials comprise a silica-based framework with uniform and well-ordered mesopores, large pores, thick and porous walls, high surface area, and high thermal stability
[52][54]. The 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 commonly utilized as adsorbents
[52], catalysts or catalytic
[55], and drug deliveries
[56].
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
[57]. 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
[52].
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
[58]. KIT-5-based nanostructured mesoporous materials have well-ordered 3D cage-like mesopores in a face-centered close-packed cubic lattice architecture
[52]. In addition, KIT-6 shows 3D mesoporous amorphous walls with large pore size, uniform pore distribution, high surface area, and thermal stability
[52].
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
[52].
3.2. Synthesis Procedures of Mesoporous Silica
Initially, Stöber et al.
[59] 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
[60]. 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
[22]. Moreover, the particle size of Stöber silica depends on the concentration of the aqueous ammonia solution and water in the ethanol reaction
[22].
Many experimental factors control hydrolysis, silica condensation rate, assembly kinetics, nucleation, and growth rates
[48][61]. 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
[61].
Sakamoto et al.
[62] prepared silica nanoparticles (NPs) via the evaporation and self-assembly 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.
[63] 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.
[64].
In thiAs
study, as tthe 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 [65]. 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
[66][67].
The hard-templating method involves nano-casting using pre-synthesized mesoporous solids
[68]. Hard templating is a facile synthesis method for fabricating porous materials with a stable porous structure. The structure replication is very straightforward
[66]. 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
[66]. However, the method is costly and time-consuming. Moreover, the mesoporous parameters, such as mesostructure and pore sizes, are difficult to change
[67].
In contrast, soft templating methods use cationic and anionic surfactants or block copolymers as templates
[61]. 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
[22]. 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
[68].
The soft templating method mainly depends on the self-assembly of the surfactant
[66]. 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)
[66].
Figure 3 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
[22]. In this process, surfactant plays a significant role in defining the pore size and volume of silica
[22]. 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
[22][69][70].
Figure 4 shows the schematic diagram for synthesizing mesoporous silica using block copolymer. As can be seen from
Figure 4, 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
[71]. 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.
Figure 3. Mechanism for the synthesis of mesoporous silica in the presence of a cationic surfactant.
Figure 4. Mechanism for the synthesis of mesoporous silica using block copolymer.
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
[11]. 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
[22]. Wu et al.
[62] and Hao et al.
[71] 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
[72].
Recently, Singh and Polshettiwar
[73] 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.
[74] 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.
3.3. Importance of Micro-Porosity and CO2 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
[48]. Apart from that, the pore volume and pore size distribution are determined using the Barrett–Joyner–Halenda (BJH) method
[48].
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
[75][76][77].
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 silica-supported 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)
[78]. 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
[78].
Lashaki and Sayari
[79] 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 intra-wall 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
[79].