Analytical STEM Investigation of the Post-Synthetic Modification (PMS) of Metal-Organic Frameworks (MOFs): Metal- and Ligand-Exchange in UiO-66

Abstract

Post-synthetic modification (PSM) of metal-organic frameworks (MOFs) is a promising route to create novel, and diverse porous nanostructures with tailored functionality for applications from gas storage and separation to catalysis [1]. Recently, post-synthetic exchange (PSE) of metal ions in the secondary-building units (SBUs) of MOFs has been demonstrated in the UiO-66 system, whereby Hf or Ti ions are exchanged for the initial Zr ions in the SBUs, while appearing to retain structural integrity and porosity [2]. Metal ion PSM (transmetalation) has been observed for both solvent-mediated solid-solid reactions and solid-liquid conditions in "robust, inert" MOFs, such as the UiO-66 system [1,3]. This has been proposed as a route to achieve stable MOF structures with specific metal nodes yet to be directly produced using conventional synthesis (e.g. TiUiO-66). Despite the interest in PSM, very little is understood about the mechanisms of the exchange process that seem to enable the robust conversion of one metal-containing crystalline MOF nanostructure to an equallystable and -crystalline MOF that contains new metal nodes. Furthermore, though exciting property enhancements have been measured and reported for various transmetalated MOFs, which supports the widelyaccepted concept of metal-exchange in these systems, the true extent of the exchange of metal ions on the nanoscale within a single MOF particle has not been accurately analyzed and quantified.Here, were use (scanning) transmission electron microscopy (STEM) energy dispersive X-ray (EDX) spectroscopy and electron energy loss spectroscopy (EELS) elemental mapping in combination with atomic resolution (S)TEM imaging to probe, map, and quantify the distribution of metal species in MOF nanocrystals in the UiO-66 family of MOFs [4]. Using these nanoscale characterization techniques, we explore PSM "metal exchange" in (Zr)UiO-66 in both Hf-salt and Ti-salt solutions, and in (Hf)UiO-66 in Zr-salt solutions. For all the PMS (metal exchange) conditions we investigate, true replacement of the original metal by the exogenous metal in the MOF nanocrystal does not occur as previously reported. Instead, The EDX elemental mapping data indicates (Figure 1) that essentially none of the original metal has been removed from the nanocrystals, and a surface layer of the exogenous metal is present. Using STEM EELS mapping (Figure 2), we find that the exogenous metal surface-coating is not an epitaxial MOF structure, but rather a metal-oxide layer (lacking carbon and oxygen-rich). We also investigate PSM "ligand exchange" in the (Zr)UiO-66 MOF (I-bdc and Brbdc ligand) using analytical STEM where we find that genuine exchange is operative and appears to be uniform into the bulk of the nanocrystals for both ligand-exchange species we investigated.Without understanding the fundamental processes that underlie MOF PSM through nanoscale spectroscopic characterization, the MOF field will struggle to interpret any promising results of property enhancements when PSM has been employed, or t...

Hypsochromically‐shifted Emission of Metal‐organic Frameworks Generated through Post‐synthetic Ligand Reduction

“…[14] While most MTV-MOFs are synthesized by pre-mixing metals and ligands, post-synthetic modification is an attractive strategy for forming MTV-MOFs. [15] Post-synthetic modification of MOFs allows for forming MOFs that cannot be synthesized otherwise. [16] Depending on the orientation and functionality of the ligands in MTV-MOFs, novel properties can emerge.…”

“…While most MTV‐MOFs are synthesized by pre‐mixing metals and ligands, post‐synthetic modification is an attractive strategy for forming MTV‐MOFs [15] . Post‐synthetic modification of MOFs allows for forming MOFs that cannot be synthesized otherwise [16] .…”

Molybdenum Sulfide within a Metal–Organic Framework for Photocatalytic Hydrogen Evolution from Water

“…Subsequent exposure to water vapor or H 2 S converts the attached species to oxyor mixed oxy-, sulfur-ligated complexes. 14,18,[29][30][31] If the MOF nodes are viewed as tiny fragments of metal-oxides, the MOF-supported catalysts obtained by the aforementioned methods can be viewed as structurally well-defined, functional models of known or not-yetdiscovered heterogeneous chemical catalysts or electrocatalysts. Envisioning a metal-chalcogenide-based photocatalyst, we selected a particular MOF, NU-1000, as a candidate support.…”