Asymmetric mixed-matrix membranes incorporated with nitrogen-doped graphene nanosheets for highly selective gas separation

Conventional mixed-matrix membranes (MMMs) possess a dense structure with a filler material uniformly dispersed within the polymer matrix to engineer the transport properties and achieve enhanced gas separation performances with respect to pure polymeric membranes. However, a dense membrane structure increases the transport resistance and undesirably requires a high filler loading to see a substantial enhancement in separation performances. To address this problem, we fabricated asymmetric MMMs that have a thin-selective layer of around 0.5 μm and a low loading (0.03–0.10 wt%) of nitrogen-doped graphene (N-G) nanosheets. The presence of the nitrogen- and oxygen-containing functional groups on the N-G nanosheets ensured good compatibility between filler and polymer matrix, which resulted in strong polymer/filler interfacial adhesions. The N-G nanosheets were also found to be capable of migrating to the top of the membranes during phase inversion. Hence, despite the low filler loadings used in this work, the capacity of the dense selective layers was greatly enhanced. Based on our experimental results, 0.07 wt% and 0.10 wt% loading of N-G nanosheets can improve both O2/N2 (126.9%) and CO2/N2 (45.8%) selectivities with respect to pure polymeric membranes, resulting in O2/N2 separation performance surpassing the Robeson upper bound. [Display omitted] •Asymmetric mixed matrix membranes (MMMs) comprising nitrogen-doped graphene (N-G) were fabricated.•Asymmetric MMMs have a thin-selective layer and a very low lading of N-G (0.03–0.10 wt%).•Most N-G fillers were migrated to the dense (top) layer to reduce interface energy during the phase inversion.•The O2/N2 and CO2/N2 selectivities of MMMs increased by 126.9% and 45.8%, respectively.•The selectivity improvement is due to the formation of more selective tortuous pathway by N-G fillers.