(446g) Unraveling and Tuning Surface and Catalytic Chemistry of Zr6O8 Nodes in Metal Organic Frameworks

The nodes of some metal-organic frameworks (MOFs) closely resemble small pieces of metal oxides, exemplified by the [Zr₆O₄(OH)₄]¹²⁺ nodes of UiO-66, UiO-67, and NU-1000. Defects including missing linkers or structural vacancies on these nodes are sites on which many acid-base-catalyzed reactions occur, exemplified by alcohol dehydration. These sites are also anchoring sites for metal complexes. As these MOFs are synthesized, various ligands form on the nodes, and their identification has taken years to resolve. The resolution has emerged from characterization of the MOFs by infrared and nuclear magnetic resonance spectroscopies. The initially present ligands were determined to be formate, when HCl or formic acid is used as a modulator in the synthesis, and acetate, when acetic acid is used as a modulator. These ligands can be replaced by alkoxy ligands by treatment of the MOF with an alcohol, and the alkoxy ligands can be converted to hydroxy when the MOF is further treated with water. The nodes were tested as catalysts for ethanol dehydration, which takes place selectively to make diethyl ether but not ethylene at 473-523 K. Calculations by density functional theory have shown that the key to the selective catalysis is the breaking of node-linker bonds or the unintended adjacency of open sites resulting from the synthesis to allow catalytically fruitful bonding of reactant ethanol to neighboring sites on the nodes and thereby facilitate the bimolecular ether formation reaction through an S_N2 mechanism. At the catalyst functions, ethanol facilitates the breaking of node-linker bonds by formation of esters that lead to the destruction of the MOFs. Thus, the choice of the linkers and the synthesis conditions is motivated by the goal of optimizing the activity and stability of the MOF as a catalyst. We address this issue and further the issue of how the linkers affect not only the stability but also the reactivity and catalytic activity of the MOF nodes. Because the linkers are ligands bonded to the nodes, they affect the electronic properties of the nodes. We used µ₃-OH groups on the nodes as sensitive indicators of the electronic properties of the node, characterizing them by infrared spectroscopy and catalyst performance. Thus, we present the first experimental evidence showing how linker ligands affect the catalytic properties of these nodes.