Quaternary ammonium membrane materials for CO₂ separation

Quaternary ammonium compounds are known to have extremely high affinity to carbon dioxide. Two new highly hydrophilic compounds containing one or two quaternary ammonium moieties attached to the reactive trimethoxy silane were synthesized and studied for physico-chemical and gas transport properties in the dry and wet environments. Both compounds in the form of blends with PEBAX^® MH 1657 showed high CO₂ solubility coefficients and high CO₂/N₂ (up to 1500) and CO₂/H₂ (up to 1350) solubility selectivity. The dry gas measurements revealed the presence of specific interaction with CO₂ resulting in strong binding of the gas molecule to the polymer active center resulting in increased solubility and decreased diffusion coefficients. At low temperatures CO₂ was irreversibly sorbed in the quaternary ammonium compound and was released only at temperatures higher than 60°C. For the sample modified by co-hydrolysis with TEOS the temperature of 60°C was found to be an additional transition point giving for H₂, N₂, O₂ and CO₂ break on the solubility coefficient Arrhenius plots. Measurements carried out with humid feed revealed up to 35-fold increase of gas permeability without CO₂/N₂ and CO₂/H₂ ideal selectivity loss compared to the dry gas measurements.