Abstract
Conventionally, the virtue of porosity is only given to porous solids. Metal Organic Frameworks (MOFs), carbon materials, or zeolites are some examples. However, processing these solids is not a straightforward task. Here, we discuss how to endow porous solids (MOFs) with liquid phase processability. More specifically, we show that surface modification of MOF crystals can lead to the formation of porous liquids (PLs) that can be further processed in the liquid phase. For instance, when placed in mesitylene, ZIF-67 predictably sediments. In contrast, with the adequate surface modification, stable dispersion of ZIF-67 can be achieved. Our proposed surface modification is facile and rapid. N-Heterocyclic carbenes are chosen as modifying agents as they are similar to imidazole linkers present on ZIFs. A simple stirring of a MOF and carbene mixture results in a modified solid. The morphology and textural properties of the modified MOF do not change from the ones of its parent. Since the porosity in solution remains unoccupied, the obtained stable colloids behave as porous liquids. Research into porous liquids is an emerging field that has already shown great promise in gases storage. Our breakthrough experiments show that these particular PLs have large potential for the separation of CO2/CH4 mixtures.
The surface functionalized ZIF-67 could also be coprocessed with polymers to yield highly loaded Mixed-Matrix Membranes (MMMs) that cannot be achieved with a pristine MOF. Dispersions of functionalized ZIF-67 were blended with 6FDA-DAM and other homemade polymers in a shape of MMMs. While MMMs based on a pure MOF maintain good physical resistance at low loadings, increasing the concentration of MOF results in brittle composites. In contrast, MMMs made from functionalized ZIF retain good mechanical strength even at ca. 47.5 wt % loadings. Such high loading was possible to achieve due the better dispersion of the MOF particles during MMM fabrication and to the better affinity of the modified MOF with the polymer. The results obtained for this MMM are among the best MMMs ever reported for high challenging C3H6/C3H8 separation.
The method is not limited to ZIF-67 but can be applied to a large body of MOFs that are constructed from imidazole-based linkers, as shown in this Account. The factors that determine whether a PL is formed include, but are not limited to, surface to volume ratio, framework particle size, and topology. On top of that, we propose potential strategies for the expansion of this method by carefully choosing surface modifiers that will suit other families of MOFs.