Introduction of mesoporous silica materials for CO2 capture

https://doi.org/10.1093/ce/zkac020

“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%) [52, 53]. 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].”