Elucidating and Tuning Catalytic Sites on Zirconium- and Aluminum-Containing Nodes of Stable Metal–Organic Frameworks

Abstract

Conspectus Metal–organic frameworks (MOFs) are a huge, rapidly growing class of crystalline, porous materials that consist of inorganic nodes linked by organic struts. Offering the advantages of thermal stability combined with high densities of accessible reactive sites, some MOFs are good candidate materials for applications in catalysis and separations. Such MOFs include those with nodes that are metal oxide clusters (e.g., Zr6O8, Hf6O8, and Zr12O22) and long rods (e.g., [Al­(OH)] n ). These nanostructured metal oxides are often compared with bulk metal oxides, but they are in essence different because their structures are not the same and because the MOFs have a high degree of uniformity, offering the prospect of a deep understanding of reactivity that is barely attainable for most bulk metal oxides because of their surface heterogeneity. This prospect is being realized as it has become evident that adventitious components on MOF node surfaces, besides the linkers, are crucial. These ligands arise from modulators, solvents, or products of solvent decomposition in MOF synthesis solutions, and because they are minor components that are often irregularly placed on defects, they may not show up in X-ray diffraction (XRD) crystal structures. Hydroxyl groups on the nodes (like those on bulk metal oxides) are regarded as native functional groups arising from solvent water, but they may barely be present initially, with common ligands instead being formate and acetate formed from modulators formic acid and acetic acid. (Formate also arises from the decomposition of dimethylformamide (DMF) solvent.) Replacement and control of the node ligands is facilitated by postsynthesis reactions (e.g., with alcohols or aqueous HCl/H2SO4 solutions) or as a result of high-temperature decomposition. In catalysis, adventitious node ligands can be (a) reaction inhibitors that block active sites on the nodes (e.g., formate blocking Zr, Hf, or Al Lewis acid sites); (b) reaction intermediates (e.g., ethoxy in ethanol dehydration); or (c) active sites themselves (e.g., terminal OH groups in tert-butyl alcohol (TBA) dehydration). Surprisingly, in view of the catalytic importance of such ligands on bulk metal oxides, their subtle chemistry on MOF nodes is only recently being determined. We describe (1) methods for identifying and quantifying node ligands (especially by IR spectroscopy and by 1H NMR spectroscopy of MOFs digested in NaOH/D2O solutions); (2) node ligand surface chemistry expressed as reaction networks; (3) catalysis, with mechanisms and energetics determined by density functional theory (DFT) and spectroscopy; and (4) MOF unzipping by reactions of linker carboxylate ligands with reactants such as alcohols that break node-linker bonds, a cause of catalyst deactivation and also an indicator of node-linker bond strength and MOF stability.

Coordinatively Unsaturated Hf-MOF-808 Prepared via Hydrothermal Synthesis as a Bifunctional Catalyst for the Tandem N-Alkylation of Amines with Benzyl Alcohol

“…Recently, we have determined that the Hf-MOF-808 synthesized via the hydrothermal method contains a much higher concentration of defective −OH sites than the same sample prepared via the solvothermal approach, which undoubtedly plays an important role in the final activity of this material for specific catalytic reactions. , Thus, the different catalytic behaviors observed between the MOF-808 samples prepared via modulated hydrothermal and solvothermal approaches could be tentatively assigned to the fact that the defective −OH species would facilitate the hydride formation due to the proton uptake of the alcohol group (step 1 in Scheme ). To determine the relative concentration of −OH sites, FTIR spectroscopic studies of CO as the probe molecule have been carried out.…”

“…The control of the amount of coordinatively unsaturated open metal sites in MOFs has been demonstrated crucial in rationalizing the nature of the catalytically active sites, allowing fine-tuning the Lewis/Brønsted acid properties of the resultant MOFs. , Different strategies have been described in the literature to adequately generate these defective sites, such as the use of modulators (i.e., monocarboxylic or inorganic acids) during the solvothermal synthesis, , or postsynthetic modifications through acid/base treatments . In fact, the amount of defective −OH groups on the metallic cluster of Hf-containing MOFs can be modulated by carrying out their syntheses via solvothermal or hydrothermal conditions …”

Characterization of a Metal–Organic Framework Zr6O8 Node-Supported Atomically Dispersed Iridium Catalyst for Ethylene Hydrogenation by X-ray Absorption Near-Edge Structure and Infrared Spectroscopies

“…Recent work is helping unravel the ligand-node chemistry, the complexity of which has been underappreciated. 14 The iridium species characterized by the IR and XANES data were synthesized to incorporate CO ligands, chosen because the IR bands of metal carbonyls are especially informativethat is, the ν CO values characterizing the atomically dispersed iridium are sensitive to the electronic state of the iridiumcontrolled by its surroundings. 28 The basis for interpreting the catalytic data is limited because the ligands on iridium in the functioning state are not determined by the data characterizing the iridium gem-dicarbonyls because these are poisoned catalysts.…”

“…The node ligand chemistry is subtle and only recently being elucidated and controlled; the initial node ligands are often derived from modulators (e.g., acetate from acetic acid and formate from formic acid), and they typically are bonded at node vacancy sites and defect sites where linkers are missing. 14 Most representations of MOF structures are based on XRD crystallography and surface area measurements and lack representations of the nonlinker ligands on the nodes. 14 There is a need for sensitive methods to characterize the effects of the various ligands on node-anchored metals.…”

“…14 Most representations of MOF structures are based on XRD crystallography and surface area measurements and lack representations of the nonlinker ligands on the nodes. 14 There is a need for sensitive methods to characterize the effects of the various ligands on node-anchored metals. We reasoned that high-energy-resolution fluorescence detection Xray absorption near-edge structure (HERFD XANES) spectroscopy would be a valuable technique, although there are still only a few data illustrating its use for characterization of atomically dispersed catalysts.…”