Introduction of mesoporous silica materials for CO2 capture

“Mesoporous silica is a popular material in CO2 capture applications due to its characteristic ordered pore structure system, high specific surface area and well-defined pore size distribution with a larger pore diameter than zeolite [50]. It can also be synthesized in various morphologies such as spheres, rods, discs, powders, etc. [51]. The diversity of mesoporous silica that is commercially applicable includes Mobil Crystalline Material or MCM (e.g. MCM-41, MCM-48), Santa Barbara Amorphous or SBA (e.g. SBA-12, SBA-15 and SBA-16) and KIT (e.g. KIT-5 and KIT-6) series [50]. They are different in wall thickness, pore size and shape. MCM-41 has a 2D highly uniform hexagonal phase with ordered cylindrical mesopores with a tunable pore size in the range of 15–100 Å, large pore volume (0.7 cm3/g), high surface area (>700 m2/g) and a high number of silanol groups (~40–60%) [5253]. The preparation of MCM-41 is simpler than the preparation method of other mesoporous silicas, such as KIT and SBA [54]. In addition, this mesoporous silica is widely used and commercially applicable in various applications. Therefore, it would have benefits in cost and be economical for use in CO2 capture applications as reported in the literature. Zhang et al. prepared a novel double-functionalized mesoporous silica by grafting 3-aminopropyltriethoxysilane and impregnating 70 wt% tetraethylenepentamine on SBA-15. Combining both modification methods led to a high CO2 adsorption capacity of 5.69 mmol/g at 75°C using 20 vol% of CO2. Moreover, its adsorption capability was only reduced by 6.1% after 25 adsorption/desorption cycles [55]. Yan et al. prepared supported HMS-4 mesoporous multimodal pore silica impregnated with TEPA for use as an adsorbent in CO2 capture applications. 75%TEPA on HMS-4 with 54 m2/g of the surface area and 0.05 cm3/g of pore volume showed an adsorption capacity of 4.55 and 6.04 mmolCO2/g at 30°C and 90°C using 15% CO2. They also investigated the regeneration heat of adsorbents at 100°C [56]. In the work of Belmabkhout et al., MCM-41 showed higher volumetric CO2 uptake than activated carbons and 13X zeolite at 25 bar and ambient temperature using a high pure CO2 feed gas [51]. In addition, its surface is also easy for modification by the amine functional group to accomplish high CO2 adsorption capacity. Several studies on CO2 adsorption using amine-modified MCM-41 have been reported in the literature. Wang et al. prepared polyethylene polyamine (PEPA)-loaded MCM-41 and dispersed with methoxypolyethylene glycol (MPEG). The study found that 50wt%PEPA and 5%MPEG-dispersed MCM-41 showed a high adsorption capacity of 2.39 mmol/g with a rapid breakthrough adsorption [57]. Loganathan and Ghoshal prepared mono- and triamine-tethered pore-expanded MCM-41 for use as an adsorbent for CO2 capture and, from the study, it was found that mono- and triamine-tethered pore-expanded MCM-41 displayed 1.2 and 2.1 mmolCO2/g of adsorption capacity, respectively, under a flue gas condition of 0.2 bar and 75°C, indicating that the amine-tethered MCM-41-30 adsorbents were suitable and adequate for CO2 capture [58]. Mukherjee et al. studied the behaviour of different amines impregnated on MCM-41 for post-combustion CO2 capture. Monoethanolamine (MEA), N-(2-aminoethyl) ethanolamine (AEEA) and benzylamine (BZA) were selected to impregnate on the surface of MCM-41. The 40%AEEA, 40%BZA and 50%MEA-impregnated MCM-41 showed different adsorption abilities of 2.34, 0.908 and 1.47 mmolCO2/g, respectively [59]. Liu et al. studied the CO2 adsorption performance of different amine-based MCM-41 adsorbents and the results suggested that amines with a shorter chain exhibited better adsorption/desorption performance due to better amine dispersion and a lower diffusion barrier [60].”

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