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/N-2 selectivity, activation energy for permeation (E-p) of CO2 [E-p(CO2)], and differences in E-p between CO2 and N-2 [E-p(CO2)-E-p(N-2)]. 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.