Fabrication and CO₂ permeation properties of amine-silica membranes using a variety of amine types

We developed a series of molecule-scale hybrid amine-silica membranes synthesized from organoalkoxysilane precursors of 3-(triethoxysilyl)propan-1-amine (PA-Si) and 3-(triethoxysilyl)-N-methylpropan-1-amine (SA-Si), and made these functional using either unhindered amines or a 3-(triethoxysilyl)-N,N-dimethylpropan-1-amine (TA-Si) that is sterically hindered. CO₂ adsorption-desorption measurements of amine-silica powdered xerogels were conducted to observe the effect of amine type on CO₂ adsorption and desorption/diffusion properties. The results revealed that TA-Si xerogel powders demonstrated faster kinetics for both adsorption and desorption processes compared with those of the PA-Si and SA-Si samples due to the steric hindrance effect of the amine, which reduced the CO₂ binding energy and thereby boosted both the forward and reverse reaction rates of CO₂-amine. In single-gas permeation performances, all the membranes exhibited excellent molecular sieving at higher temperatures and all the gases considered, except CO₂, tended to permeate the membranes via activated diffusion. The effect of amine type on CO₂ separation performance was compared using CO₂ permeance, CO₂/N₂ selectivity, activation energy for permeation (E_p) of CO₂ [E_p(CO₂)], and differences in E_p between CO₂ and N₂ [E_p(CO₂)-E_p(N₂)]. The TA-Si membrane demonstrated superior CO₂ separation performance, and achieved the highest values for both CO₂ permeance and selectivity. A relatively smaller difference in CO₂ separation performance was observed between PA-Si and SA-Si membranes despite some differences in basicity. This suggests that, rather than the basicity, it was the steric hindrance effect that played the greatest role in CO₂ transport performance across amine-silica membranes.