Orienting of metal-organic framework nanosheets into continuous membranes for fast hydrogen permeation
The extremely thin, porous and crystalline nature endow metal-organic framework nanosheets (MOFNs) to be emerging materials for membrane assembly. However, their reliable manufacturing, especially for the continuous growth in controllable direction, remains a great challenge. Herein, we firstly orient a MOFN (NTU-83, a layered framework) either into vertical or horizontal alignments via one-step bottom-up synthesis on the surface-functionalized α-Al 2 O 3 supports, on which the amide groups promote vertical assembly via the formed COOH⋯NH 2 hydrogen bond , while exposed oxygen sites facilitate horizontal growth by Cu⋯O/OH interaction. The H 2 permeance of the vertical aligned MOFN membrane (termed VAM) is 2100 ± 7 GPU at 100 °C, and it sharply increases to 8000 ± 40 GPU in horizontal aligned membrane (termed HAM), together with high-steady operation and nearly same H 2 /CO 2 selectivity (varied in range of 6–7). This anti-trade-off performance should be attributed to the relatively short (500 nm) diffusion pathway and nanosized pore (5.4 × 8.2 Å 2 ) of the horizontal-stacked layers when compare with the slightly bent and long (12 μm) interlayer channel (5.1 Å) that connected by eutectic junction points in VAM. Additionally, the universality of this surface functionalization for directional MOF growth was confirmed by producing another set of horizontal and vertical MOFN membranes with 3D structure. Given the rich building blocks of MOFs and various planting groups on the supports, the presented surface functionalization strategy can be extended to the directional assembly of other MOFNs on different supports into highly-efficient membranes.
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