A recent feature paper (Fischer and Forgan, 2015, DOI: 10.1039/c4cc04458d) investigates how surface modifications might be used to tune bulk properties in a variety of popular MOFs and ZIFs. Three types of modifications were examined: 1) use of coordination modulation in synthesis to stabilize and controlling size/growth of MOF crystals, 2) post-synthetic modifications, and 3) epitaxial growth of material.
It was found that adding a modulator during MOF synthesis is an effective way to tune crystal size distribution, growth rate, and growth direction, but functionality is usually confined to the surface and not the bulk material. Zr carboxylate MOFs (e.g. UiO-66) are an exception; they have highly coordinated frameworks and missing linker defects – where fragments of a modulator ligand are incorporated in lieu of linker units – can be induced during synthesis, increasing pore volume and surface area. Post-synthetic modifications likewise do not permeate into the bulk, but surface functionalization can render a MOF more water-stable. Epitaxial growth offers the most potential to deliberately control the bulk structure as seen in previous MOF-5/IRMOF-3 core-shell synthesis. Careful design of surface chemistry could facilitate self-assembly of hybrid structures, or layer-by-layer deposited MOF thin films.
The EFRC is already familiar with and studying various methods of functionalizing MOF/ZIF surfaces for increased water stability, but there may be other simple post-synthesis modification that can enhance chemical selectivity of certain acid gases. Designing hybrid MOFs is also of interest to the EFRC, and tuning surface chemistry could be a way to efficiently propagate an intergrowth defect through a bulk material.
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