Sulfonated TiO2 quantum dots enabled constructing of bicarbonate highways in quaternary ammonium poly (ether ether ketone) membranes for efficient CO2 separation

Polyelectrolyte-based mixed matrix membranes have achieved great success as ion exchange membranes, of which the fast ion transport (especially anion transport) is being realized available to facilitate CO(2 )transport. In this study, quaternary ammonium poly (ether ether ketone) was synthesized to provide a polymer matrix for anion transport, and nanoscale sulfonated TiO2 quantum dots (S-TiQDs) were designed as a functional filler with ionic groups. The incorporation of S-TiQDs at optimal loading (3 wt%) was found to induce the formation of more segregated and ordered ionic cluster domains. These domains were envisaged to pave bicarbonate highways (affording a 3-fold increase of bicarbonate diffusivity) within the membranes, conferring substantial enhancement of CO2 permeability and CO2/N-2 selectivity (up to 979 Barrer and 70.8, respectively). The role of STiQDs as structure modifier was elucidated by combining a series of characterization data and the water retention properties. In addition, thanks to the cation exchange capacity of sulfonate groups on S-TiQDs, the Cl- type membranes (without alkaline treatment) showed comparable separation performance to the corresponding OH- type ones (with alkaline treatment), even higher at elevated operating pressure. Based on these findings, two representative modes of bicarbonate-facilitated CO2 transport were proposed and discussed.