CO2 capture (including direct air capture) and natural gas desulfurization of amine-grafted hierarchical bimodal silica

Abstract

Amine grafting under hydrous conditions is emerging as a promising solution to developing amine grafted adsorbents with high amine loading and superior capture performance. In this study, a hierarchical bimodal mesoporous silica (HBS) that can be easily and cost-effectively synthesized was identified. Subsequently, HBS was amine grafted in anhydrous and hydrous conditions. The resulting material and its adsorptive performance were investigated. Hierarchical bimodal silica supports allow aminosilanes to more easily access the deep channels of the pores resulting in greater amine dispersion than SBA-15 type supports. The CO2 adsorption capacity of amine grafted hierarchical bimodal silica adsorbents and SBA-15 type adsorbents were measured at 25, 75, 90, 100 °C. The CO2 adsorption capacity and amine efficiency (reached 0.41 mol CO2/mol N) were significantly higher than that of SBA-15 type adsorbents. This is because HBS possesses two series of mesopores as opposed to one which promotes the diffusion of CO2 and allows for greater amine dispersion and consequently greater amine utilization. Wet grafted HBS displayed excellent cyclic stability after approximately 10 adsorption-desorption cycles. Under dry conditions and using ambient air containing 415 ppm of CO2 at 25 °C, the fixed bed breakthrough capacity of WG-HBS-0.6 was 1.04 mmol/g. To the best of our knowledge, the CO2 adsorption capacity and the direct air capture capacity exceed reported literature values for amine-grafted silica adsorbents. At similar amine loadings the H2S adsorption capacity of wet grafted hierarchical bimodal silica was similar to SBA-15 type adsorbents suggesting thermodynamics as opposed to pore structure plays a greater role during the H2S adsorption process. From the obtained results, hierarchical bimodal silicas are promising adsorbents for the capture of CO2 from ambient air and industrial gas streams and are also suitable for the removal of H2S.