A series of commercially available (and low-cost) silica gels were amine-grafted under dry conditions using N 1 -(3-trimethoxysilylpropyl)diethylenetriamine (i.e., "triamine"). CO 2 capture performance, pore properties, and amine loading were investigated, and the results showed that the silica gel with the largest pores (150A) was the most suitable for further enhancement. Amine loading needed to be increased in order to further enhance the CO 2 adsorption capacity. The addition of water during the grafting process was implemented on the 150A silica gel. The wet-grafted 150A silica gel exhibited a 2.3 mmol/g adsorption capacity at 75 °C and 1 bar, which is comparable to the best literature reported values that used similar synthesis methods. The wetgrafted 150A exhibited good cycle stability as well as fast CO 2 uptake rates. The fixed-bed breakthrough capacity for air capture (ambient air with 415 ppm of CO 2 at 25 °C) under dry (0.773 mmol/g) and wet (1.098 mmol/g) conditions are among the highest CO 2 capacities for air capture. The capacity of the low-surface area silica gel (150A, S.A. = 309 m 2 /g) rivaled those of the best of the SBA and MCM type materials, which was unexpected. These results indicate that wet-grafted silica gel 150A is a promising and low-cost sorbent for direct capture of CO 2 from ambient air and flue gas applications.

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Conventional and pore expanded SBA-15 were amine-grafted under dry and wet conditions using N 1-(3-trimethoxysilylpropyl)­diethylenetriamine. The effects of pore properties and gas hourly space velocity (GHSV) as well as the CO2 and H2S capture performance were investigated, and the results indicated that pore expanded SBA-15 displayed the best performance. Clumping and agglomeration of SBA-15 were observed for conventional SBA-15 under wet grafting conditions due to interparticle polymerization of amines after pores are completely filled. This phenomenon was not observed for pore expanded SBA-15, resulting in viable adsorbents with greater amounts of grafted amines. Pore expanded SBA-15 exhibited the highest CO2 capacity (3.27 mmol/g), which to the best of our knowledge is the largest for amine-grafted adsorbents. It also exhibited high amine efficiency (0.39 mol CO2/mol N) and faster uptake rates compared to conventional SBA-15 due to enhanced amine accessibility. For direct air capture, higher GHSV values result in lower breakthrough CO2 capacities and the breakthrough CO2 capacity of wet grafted pore expanded SBA-15 is more dependent on GHSV than that of dry grafted pore expanded SBA-15 due to diffusion resistance. Last, conventional SBA-15 displayed a marginally lower H2S adsorption capacity compared to pore expanded SBA-15, suggesting that diffusion resistance does not play a significance role during H2S adsorption. These results suggest the consideration of wet grafted pore expanded mesoporous siliceous supports for the design of promising adsorbents for the capture of CO2 and the removal of H2S from natural gas.

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