Tuning the Surface Chemistry of Metal Organic Framework Nodes: Proton Topology of the Metal-Oxide-Like Zr₆ Nodes of UiO-66 and NU-1000

Abstract

Some metal organic frameworks (MOFs) incorporate nodes that are nanoscale metal oxides, and the hydroxy-containing functional groups on them provide opportunities for introducing catalytic sites with precisely defined structures. Investigations have been done to understand the structures of these groups on nodes and node vacancies, because, in prospect, atomic-scale modulation of the composition, areal density, and/or siting of the groups would open up possibilities for exquisite tuning of the siting and performance of subsequently anchored catalytic units (e.g., single metal ions, pairs of metal ions, or well defined metal-ion-containing clusters). We have combined infrared (IR) spectroscopy and density functional theory (DFT) to demonstrate tuning of these sites, namely, hydrogen-bonded OH/OH2 groups on the Zr6 nodes of the MOFs UiO-66 and NU-1000 via the intermediacy of node methoxy (or ethoxy) groups formed from methanol (or ethanol). Methoxy (or ethoxy) groups on node vacancy sites are converted to a structure incorporating one vacant Zr site and one terminal OH group per face by reaction with water. Our results highlight how the combination of DFT and IR spectroscopy facilitate the determination of the identity and chemistry of the functional groups on MOF node vacancies and defect sites. INTRODUCTIONThe nodes of some metal-organic frameworks (MOFs) are essentially small, uniform metal oxide clusters, exemplified by the Zr6 nodes of

Creation of Active Sites in MOF‐808(Zr) by a Facile Route for Oxidative Desulfurization of Model Diesel Oil

“…This means that two coordination vacancies on each Zr 6 node would be left and form defect sites if one formate ligand is removed (Figure S1), which could be active sites for catalytic reactions . However, current methods need to take complicated procedures and long time (several days) to reach the goal . Thus, a simple and efficient method to remove the formate ligands in the structure of MOF‐808(Zr) is highly desired.…”

Influence of a Confined Methanol Solvent on the Reactivity of Active Sites in UiO‐66

“…When atmospheric water is present, the Zr‐brick becomes hydrated: water molecules physisorb on the coordinatively unsaturated metal centers. One of the water molecules then dissociates into a hydroxyl group adsorbed on the Zr‐atom and a proton is adsorbed on the adjacent μ 3 ‐oxygen, to give a more stable structure, as discussed in literature . The unit cell of this hydrated UiO‐66 material with two missing terephthalate linkers is displayed in Figure .…”

“…These species introduce additional Brønsted sites which can play a role in reactions catalyzed by the material. A combined experimental‐theoretical work demonstrated that the topology of the OH/OH 2 defect coordinating species on the Zr 6 node‐face can be tuned with intermediate steps in which methanol replaces water on the Zr sites. Insight in the nature of active sites and the methanol substitution process was obtained, indicating that the thermodynamically most favorable structure was obtained with a node‐face containing a methoxide and an open Zr metal site .…”

“…A combined experimental‐theoretical work demonstrated that the topology of the OH/OH 2 defect coordinating species on the Zr 6 node‐face can be tuned with intermediate steps in which methanol replaces water on the Zr sites. Insight in the nature of active sites and the methanol substitution process was obtained, indicating that the thermodynamically most favorable structure was obtained with a node‐face containing a methoxide and an open Zr metal site . Further understanding of the broad variety of defect structures in the UiO‐66 material was obtained by several recent theoretical works, investigating possible structures where up to three defect coordinating molecules are involved.…”

Modulator‐Controlled Synthesis of Microporous STA‐26, an Interpenetrated 8,3‐Connected Zirconium MOF with thethe‐iTopology, and its Reversible Lattice Shift

“…This is in accordance with the TGA of STA‐26(Zr). The picture is similar to that observed for NU‐1200 and is attributed to the loss of water and hydroxyl groups from the Zr 6 O 8 clusters …”