Ultrathin Metal–Organic Framework Nanosheets as a Gutter Layer for Flexible Composite Gas Separation Membranes

Ultrathin metal-organic framework (MOF) nanosheets show great potential in various separation applications. In this study, MOF nanosheets are incorporated as a gutter layer in high-performance, flexible thin-film composite membranes (TFCMs) for CO2 separation. Ultrathin MOF nanosheets (∼3-4 nm) were prepared via a surfactant-assisted method and subsequently coated onto a flexible porous support by vacuum filtration. This produced an ultrathin (∼25 nm), extremely flat MOF layer, which serves as a highly permeable gutter with reduced gas resistance when compared with conventional polydimethylsiloxane gutter layers. Subsequent spin-coating of the ultrathin MOF gutter layer with a polymeric selective layer (Polyactive) afforded a TFCM exhibiting the best CO2 separation performance yet reported for a flexible composite membrane (CO2 permeance of ∼2100 GPU with a CO2/N2 ideal selectivity of ∼30). Several unique MOF nanosheets were examined as gutter layers, each differing with regard to structure and thickness (∼10 and ∼80 nm), with results indicating that flexibility in the ultrathin MOF layer is critical for optimized membrane performance. The inclusion of ultrathin MOF nanosheets into next-generation TFCMs has the potential for major improvements in gas separation performance over current composite membrane designs.