Background of CO2 capture using porous silica materials

https://doi.org/10.1039/C8TA06224B

“The CO₂ adsorption of immobilised amine adsorbents follows similar reaction mechanisms to those in the aqueous amine absorption process, which involves the reversible formation of ammonium carbamate under anhydrous flue gas conditions and ammonium bicarbonate under wet conditions. Two typical preparation methodologies are used, namely surface grafting and direct impregnation. In general, amine-grafted silica sorbents benefit from improved amine efficiency and thermal stability but usually suffer from low amine loadings due to the limited density of accessible surface silanol groups, giving rise to CO₂ adsorption capacities being generally well below 3 mmol CO₂ per g sorbent (hereinafter abbreviated to mmol g⁻¹) or 13.2 wt%. The highest adsorption capacity of ca. 4 mmol g⁻¹ (17.6 wt%) was obtained by Drese et al.²⁴ at 75 °C in 10% CO₂/N₂ for a hyperbranched aminosilica with a grafted amine loading of 9.78 mmol amine per g SBA-15, whereas three-dimensional macro-porous silica with a grafted amine density of 10.98 mmol N per g could only achieve a maximum CO₂ capacity of 2.76 mmol g⁻¹ under similar conditions.²⁵ In comparison, impregnation as a more convenient and less corrosive preparation methodology can achieve much larger amine loading capacity but the produced sorbents may suffer from the potential evaporation loss of the impregnated amines, an issue that can be improved by using higher boiling amines.²⁶ Builes and Vega²⁷ compared the performance of immobilised amines prepared from different methodologies and found that compared to grafting, impregnation persistently gives rise to significantly higher adsorption capacities particularly at low CO₂ partial pressures, due to the mobility of the functionalised amine chains that leads to the formation of microporosity-like micro-cavities inside the bulk phase of the impregnated amines. In contrast, similar structures cannot be formed within the densely grafted amine groups and this can limit the potential of achieving higher adsorption capacities by increasing the grafting density.

Numerous investigations have shown that for both impregnation and grafting, the performance of supported amine sorbents is not only determined by the chemical nature of the amines but also by the type of porous solid support, with mesoporous silica materials such as MCM-41, MCM-42, MCM-48, SBA-15, SBA-16 and hexagonal mesoporous silica (HMS) being the most widely used solid supports because of their tuneable structures and re-addressable surface chemistries. Son et al.²⁸ studied the CO₂ adsorptive properties of different mesoporous silica materials at the same maximum PEI loading of 50 wt% and found that dictated by the average pore diameter of the solid support, the CO₂ uptake of PEI-modified silica sorbent follows the order of KIT-6 > SBA-16 ≈ SBA-15 > MCM-48 > MCM-41, with the capacities varying from 2.5 mmol g⁻¹ for MCM-41 with a 1-dimensional (1D) pore diameter of 2.8 nm to 3.1 mmol g⁻¹ for KIT-6 with a 3D pore diameter of 6.5 nm. Over recent years, many efforts have been made to expand the pore diameter or increase the pore volume of mesostructured silica supports to increase the amine loading and hence CO₂ uptake capacity. It has been found that regardless the preparation techniques employed, the immobilised amine sorbents prepared using pore-expanded MCM-41,^29–32 MCM-48,²⁹ SBA-15 ^29–32 and HMS^33,34 all showed considerably higher CO₂ adsorption capacities than their pore-unexpanded counterparts. In addition to the pore size and volume, the length of pore channels of support materials also appears to play a vital role in determining the CO₂ adsorption capacity and kinetics of amine-modified silica adsorbents due to the mass transfer limitations within extended meso-channel networks. Studies found that pore-expanded mesoporous silica supports with shorter meso-channels can outperform those with longer channels by a factor of 2.7 and 1.45 for PEI-impregnated MCM-41 and SBA-15 in pure CO₂ at 50 °C ³⁰ and a factor of 2.2 for amine-grafted SBA-15 at 75 °C in 15% CO₂/N₂,³⁵ respectively. However, despite the efforts in recent years, very few of the reported amine-modified conventional mesoporous silica materials have CO₂ capture capacities greater than 3 mmol g⁻¹ or 13.2 wt% in 15% CO₂/N₂ at the desirable adsorption temperature range of 50–75 °C, due to amine loading limitations. Previous studies have demonstrated that for post-combustion capture, solid amine adsorbents with a CO₂ capture capacity of ca. 3 mmol g⁻¹ for 50 wt% PEI loading are equivalent to 20 wt% aqueous MEA solvent when 90% of the CO₂ in typical flue gas streams was captured.^36,37 This suggests that more effective solid supports need to be explored to further improve the adsorption performance of immobilised amine adsorbents.”