Amines are incorporated into various membranes to improve their CO2 separation performance. With amine-containing polymeric membranes, gas transport properties are often enhanced under humidity, where CO2 migrates through the membranes in the form of bicarbonate ions. Piperazine (Pz) and its derivatives are known to catalyze the conversion of CO2 to bicarbonate ions and have been used in liquid amine scrubbing technology. Piperazines were immobilized in poly(vinyl alcohol) (PVA), and the resulting polymeric membranes showed high CO2 separation performance over H2 and CH4. The gas transport properties were dependent on the chemical structure of the amines. In particular, 3-(1-piperazinyl)-1,2-propanediol (PzPD)-containing polymeric membranes gave excellent CO2 separation performance, and the CO2 permeability and CO2 selectivity over CH4 were 1060 Barrer and 370, respectively, at 50 °C and 90% relative humidity with a transmembrane CO2 pressure of 11 kPa. The interaction between PzPD and CO2 was quantitatively studied by inverse-gate decoupling 13C NMR spectroscopy. CO2 interacted with the secondary amino group on the Pz ring to form a carbamate, which was readily hydrolyzed to produce bicarbonate ions. The hydroxyl group on the C2 carbon of PzPD facilitated the interaction between CO2 and the amine through hydrogen bonding, resulting in enhanced diffusivity of CO2 in the membranes.
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