Ionic liquid-decorated nanocages for cooperative CO2 transport in mixed matrix membranes

Abstract An ionic liquid-decorated layered double hydroxide nanocage ([Hmim][NTf2]@LDHN) was designed and incorporated into Pebax matrix to fabricate high-performance MMMs for CO2 separation. The [Hmim][NTf2] on the external surface, interlamellar spacing, and internal hollow core of LDHN plays cooperative roles with LDHN for improving CO2 permselectivity. First, the [Hmim][NTf2] on the external surface of LDHN absorbs more CO2 molecules than CH4, improving CO2/CH4 selectivity. Then, the captured CO2 permeates into LDHN shells and are dissolved by [Hmim][NTf2] in the interlamellar spacing of LDNH. The dissolved CO2 combines quickly with CO32− carriers existed in LDHN itself, accelerating CO2 transport. Third, the [Hmim][NTf2] encapsulated in the internal hollow core of the LDHN has an ultrahigh CO2 capacity, which increases both CO2 permeability and selectivity. The cooperation of [Hmim][NTf2] and LDHN significantly enhances CO2 separation performance. In particular, under humidified conditions the Pebax/[Hmim][NTf2]@LDHN-6 MMM presents optimal separation performance with a CO2 permeability of 644 ± 25 Barrer and a CO2/CH4 separation factor of 34 ± 0.53, which transcends the upper bound relationship developed by Robeson (2008). The nanofillers combining ILs with nanocages exhibit a promising prospect to fabricate high-performance MMMs for gas separations.