Most metal-organic frameworks (MOFs) possess anisotropic properties, the full exploitation of which necessitates a general strategy for the controllable orientation of such MOF crystals. Current methods largely rely upon layer-by-layer MOF epitaxy or tuning of MOF crystal growth on appropriate substrates, yielding MOFs with fixed crystal orientations. Here, the dynamic magnetic alignment of different MOF crystals (NH -MIL-53(Al) and NU-1000) is shown. The MOFs were magnetized by electrostatic adsorption of iron oxide nanoparticles, dispersed in curable polymer resins (Formlabs 1+ clear resin/ Sylgard 184), magnetically oriented, and fixed by resin curing. The importance of crystal orientation on MOF functionality was demonstrated whereby magnetically aligned NU-1000/Sylgard 184 composite was excited with linearly polarized 405 nm light, affording an anisotropic fluorescence response dependent on the polarization angle of the excitation beam relative to NU-1000 crystal orientation.
Lining up like soldiers – dynamic control over the orientation of free‐standing MOF crystals in different fluid matrices has been achieved through magnetic alignment. Magnetically aligned NU‐1000 crystals were shown to display an anisotropic optical response to linearly polarized excitation, underscoring the importance of controlling MOF crystal orientation. For more information, see the Communication by J. M. Chin et al. on page 15578 ff.
Metal–organic frameworks (MOFs)
can be advantageous over
traditional materials in applications such as reaction catalysts,
gas desulfurization and carbon dioxide capture. Building on MOFs diverse
range of applications they are suited to act as separation media within
a pressure swing adsorption (PSA) nitrogen generator. The properties
of some MOFs; large surface area, high O2/N2 selectivity, large micropore volume and tunable pore size make MOFs
ideal for this separation technique. These criteria, along with cost
of synthesis and relative abundance in large quantities, were used
to identify three viable materials within the MOF family: MIL-101
(Cr), ZIF-8, and UiO-66 (Zr). The material properties of the three
MOFs are given, with a comparison made to the most commonly used separation
mediacarbon molecular sieve (CMS). Proposals are then presented
for the potential ways MOF media could be used in a PSA nitrogen generator;
either as a direct swap for CMS or utilizing the properties of MOFs
to rearrange the generation system. The merits and limitations of
MOFs when combined with binding material are analyzed in forms, such
as mixed-matrix membranes (MMMs), powder, and monoliths. Nonseparation
performance requirements of a viable PSA nitrogen generation MOFs
are discussed, focusing on lifetime, robustness, and resistance to
contaminants. The gaps in readiness before a MOF can be used within
a manufactured PSA generator are identified with production repeatability,
lack of testing within a generator, and uncertainty around the best
MOF-based material structure forming the chief barriers to adoption.
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