The combination of well-defined molecular cavities and chemical functionality makes crystalline porous solids attractive for a great number of technological applications, from catalysis to gas separation. However, in contrast to other widely applied synthetic solids such as polymers, the lack of processability of crystalline extended solids hampers their application. In this work, we demonstrate that highly crystalline porous solids, metal-organic frameworks, can be made solution processable via outer surface functionalization using N-heterocyclic carbene ligands. Selective outer surface functionalization of relatively large nanoparticles (250 nm) of the well-known metal organic framework ZIF-67 allows for the stabilization of processable dispersions exhibiting permanent porosity. The resulting type III porous liquids can either be directly deployed as liquid adsorbents or be co-processed with state-of-the-art polymers to yield highly loaded mixed matrix membranes with excellent mechanical properties and an outstanding performance in the challenging separation of propylene from propane. We anticipate that this approach can be extended to other metal-organic frameworks, and for other applications.
Metal–organic
frameworks (MOFs) with an exceptionally large
pore volume and inner surface area are perfect materials for loading
with intelligent guest molecules. First, an ultrathin 200 nm high-flux
UiO-67 layer deposited on a porous α-Al2O3 support by solvothermal growth has been developed. This neat UiO-67
membrane is then used as a host material for light-responsive guest
molecules. Azobenzene (AZB) is loaded in the pores of the UiO-67 membrane.
From adsorption measurements, we determined that the pores of UiO-67
are completely filled with AZB and, thereby, steric hindrance inhibits
any optical switching. After in situ thermally controlled
desorption of AZB from the membrane, AZB can be switched and gas permeation
changes are observed, yielding an uncomplicated and effective smart
material with remote controllable gas permeation. The switching of
AZB in solution and inside the host could be demonstrated by ultraviolet–visible
spectroscopy. Tracking the completely reversible control over the
permeance of CO2 and the H2/CO2 separation
through the AZB-loaded UiO-67 layer is possible by in situ irradiation and permeation. Mechanistic investigations show that
a light-induced gate opening and closing takes place. A remote controllable
host–guest, ultrathin smart MOF membrane is developed, characterized,
and applied to switch the gas composition by external stimuli.
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