Mixed matrix membranes incorporating two-dimensional ZIF-8 nanosheets for enhanced CO₂/N₂ separation

In this report, a few unit-cell thick ZIF-8 nanosheets with (002) basal planes exposed with 4-membered windows were synthesized by using sodium dodecyl sulfate, an anionic surfactant as a structure directing agent, which were used as fillers to fabricate mixed matrix membranes (MMMs) for CO₂/N₂ separation. The 2D ZIF-8 fillers contributed to not only decreasing transport resistance due to highly porous nature but also strengthening CO₂/N₂ diffusive separation of the MMMs due to keen molecular sieving through 4-membered windows, smaller than conventional 6-membered windows of ZIF-8. Aligned filler dispersion in out-of-plane direction was maintained up to 10 wt% filler loading and the MMM with 10 wt% filler loading showed optimal performance of CO₂ permeability of ∼350 Barrer and CO₂/N₂ selectivity of ∼94, outperforming most of the reported 2D nanosheet-based MMMs in addition to exceeding the 2019 upper bound. To check commercial viability, thin film nanocomposite (TFN) membranes composed of a skin layer of ∼780 nm including 10 wt% filler loading were prepared. The TFN membranes exhibited CO₂ permeance of ∼710 GPU and CO₂/N₂ selectivity of ∼77, which surpasses most of state-of-the-art polymeric thin film composite (TFC) and TFN membranes especially in terms of selectivity.