Piperazine-immobilized polymeric membranes for CO₂ capture: mechanism of preferential CO₂ permeation

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.