Regioselective Atomic Layer Deposition in Metal–Organic Frameworks Directed by Dispersion Interactions

Reversible Low-Temperature Metal Node Distortion during Atomic Layer Deposition of Al2O3 and TiO2 on UiO-66-NH2 Metal-Organic Framework Crystal Surfaces

Metal-organic frameworks (MOFs) are chemically functionalized micro- and mesoporous materials with high surface areas and are attractive for multiple applications including filtration, gas storage, and catalysis. Postsynthetic modification (PSM), via solution or vapor-based techniques, is a way to impart additional complexity and functionality into these materials. There is a desire to shift toward vapor-phase methods in order to ensure more controlled modification and more efficient reagent and solvent removal from the modified MOF material. In this work we explore how the metal precursors titanium tetrachloride (TiCl₄) and trimethylaluminum (TMA), commonly used in atomic layer deposition, react with UiO-66-NH₂ MOF. Using in situ quartz crystal microbalance (QCM) and Fourier transform infrared spectroscopy (FTIR) at 150 and 250 °C, we find that the ALD precursors react with μ₃-OH hydroxyl and μ₃-O bridging oxygen groups on Zr₆ nodes, as well as oxygen from carboxylate linker groups. The reactions occur predominantly at the crystal surface at μ₃-OH hydroxyl sites, with TiCl₄ exhibiting greater diffusion into the MOF subsurface. FTIR analysis suggests that, at 150 °C, both TiCl₄ and TMA reversibly dehydroxylate the hydroxylated UiO-66-NH₂, which is accompanied by distortion of the zirconium metal clusters. Finally, we show that TiCl₄ is able to react with the dehydroxylated UiO-66-NH₂ structure, suggesting that TiCl₄ is also able to react directly with the bridging oxygens in the metal clusters or carboxylate groups on the organic ligand. A better understanding of chemical and thermally driven MOF dehydroxylation reactions can be important for improved postsynthetic modification of MOFs.

Grand Challenges and Future Opportunities for Metal-Organic Frameworks

Metal-organic frameworks (MOFs) allow compositional and structural diversity beyond conventional solid-state materials. Continued interest in the field is justified by potential applications of exceptional breadth, ranging from gas storage and separation, which takes advantage of the inherent pores and their volume, to electronic applications, which requires precise control of electronic structure. In this Outlook we present some of the pertinent challenges that MOFs face in their conventional implementations, as well as opportunities in less traditional areas. Here the aim is to discuss select design concepts and future research goals that emphasize nuances relevant to this class of materials as a whole. Particular emphasis is placed on synthetic aspects, as they influence the potential for MOFs in gas separation, electrical conductivity, and catalytic applications.

Size Effect of the Active Sites in UiO-66-Supported Nickel Catalysts Synthesized via Atomic Layer Deposition for Ethylene Hydrogenation

Ni(II) ions have been deposited on the Zr6 nodes of a metal-organic framework (MOF), UiO-66, via an ALD-like process (ALD = atomic layer deposition). By varying the number of ALD cycles, three Ni-decorated UiO-66 materials were synthesized. A suite of physical methods has been used to characterize these materials, indicating structural and high-surface-area features of the parent MOF are retained. Elemental analysis via X-ray photoelectron spectroscopy (XPS) indicates that the anchored Ni ions are mainly on surface and near-surface MOF defect sites. Upon activation, all three materials are catalytic for ethylene hydrogenation, but their catalytic activities significantly vary, with the largest clusters displaying the highest per-nickel-atom activity. The study highlights the ease and effectiveness ALD in MOFs (AIM) for synthesizing, specifically, UiO-66-supported NiyOx catalysts.

Postsynthetic Tuning of Metal-Organic Frameworks for Targeted Applications

Metal-organic frameworks (MOFs) are periodic, hybrid, atomically well-defined porous materials that typically form by self-assembly and consist of inorganic nodes (metal ions or clusters) and multitopic organic linkers. MOFs as a whole offer many intriguing properties, including ultrahigh porosity, tunable chemical functionality, and low density. These properties point to numerous potential applications, including gas storage, chemical separations, catalysis, light harvesting, and chemical sensing, to name a few. Reticular chemistry, or the linking of molecular building blocks into predetermined network structures, has been employed to synthesize thousands of MOFs. Given the vast library of candidate nodes and linkers, the number of potentially synthetically accessible MOFs is enormous. Nevertheless, a powerful complementary approach to obtain specific structures with desired chemical functionality is to modify known MOFs after synthesis. This approach is particularly useful when incorporation of particular chemical functionalities via direct synthesis is challenging or impossible. The challenges may stem from limited stability or solubility of precursors, unwanted secondary reactivity of precursors, or incompatibility of functional groups with the conditions needed for direct synthesis. MOFs can be postsynthetically modified by replacing the metal nodes and/or organic linkers or via functionalization of the metal nodes and/or organic linkers. Here we describe some of our efforts toward the development and application of postsynthetic strategies for imparting desired chemical functionalities in MOFs of known topology. The techniques include methods for functionalizing MOF nodes, i.e., solvent-assisted ligand incorporation (SALI) and atomic layer deposition in MOFs (AIM) as well as a method to replace structural linkers, termed solvent-assisted linker exchange (SALE), also known as postsynthethic exchange (PSE). For each functionalization strategy, we first describe its chemical basis along with the requirements for its successful implementation. We then present a small number of examples, with an emphasis on those that (a) convey the underlying concepts and/or (b) lead to functional structures (e.g., catalysts) that would be difficult or impossible to access via direct routes. The examples, however, are only illustrative, and a significant body of work exists from both our lab and others, especially for the SALE/PSE strategy. We refer readers to the papers cited and to the references therein. More exciting, in our view, will be new examples and new applications of the functionalization strategies-especially applications made possible by creatively combining the strategies. Underexplored (again, in our view) are implementations that impart electrical conductivity, enable increasingly selective chemical sensing, or facilitate cascade catalysis. It will be interesting to see where these strategies and others take this compelling field over the next few years.

Metal-Organic Framework Supported Cobalt Catalysts for the Oxidative Dehydrogenation of Propane at Low Temperature

Zr-based metal-organic frameworks (MOFs) have been shown to be excellent catalyst supports in heterogeneous catalysis due to their exceptional stability. Additionally, their crystalline nature affords the opportunity for molecular level characterization of both the support and the catalytically active site, facilitating mechanistic investigations of the catalytic process. We describe herein the installation of Co(II) ions to the Zr₆ nodes of the mesoporous MOF, NU-1000, via two distinct routes, namely, solvothermal deposition in a MOF (SIM) and atomic layer deposition in a MOF (AIM), denoted as Co-SIM+NU-1000 and Co-AIM+NU-1000, respectively. The location of the deposited Co species in the two materials is determined via difference envelope density (DED) analysis. Upon activation in a flow of O₂ at 230 °C, both materials catalyze the oxidative dehydrogenation (ODH) of propane to propene under mild conditions. Catalytic activity as well as propene selectivity of these two catalysts, however, is different under the same experimental conditions due to differences in the Co species generated in these two materials upon activation as observed by in situ X-ray absorption spectroscopy. A potential reaction mechanism for the propane ODH process catalyzed by Co-SIM+NU-1000 is proposed, yielding a low activation energy barrier which is in accord with the observed catalytic activity at low temperature.

Addressing the characterisation challenge to understand catalysis in MOFs: the case of nanoscale Cu supported in NU-1000

We explore the dynamic structure and reactivity of Cu species supported on NU-1000. By combining pair distribution function (PDF) analysis and difference envelope density (DED) analysis of in situ synchrotron-based X-ray scattering data, we simultaneously probe the local structure of supported Cu-species, their distribution within NU-1000 and distortions of the NU-1000 lattice under conditions relevant to catalysis and catalyst activation. These analyses show that atomic layer deposition (ALD) of Cu in NU-1000 (Cu-AIM) leads to the formation of Cu-oxo clusters within the small pores that connect the triangular and hexagonal channels. Exposure of Cu-AIM to a reducing atmosphere at 200 °C produces metallic Cu⁰ of two distinct particle sizes: ∼4 nm nanoparticles and small sub-nanometer clusters. The size of these nanoparticles appears to be constrained by NU-1000 pore dimensions, with evidence of the sub-nanometer clusters being bound within the triangular channels flanked by pyrene rings. This supported Cu⁰–NU-1000 system is catalytically active for gas-phase ethylene hydrogenation. Exposure of the catalyst to oxidative atmosphere re-oxidises the Cu species to a Cu₂O cuprite phase. The dynamic restructuring of the system in different chemical environments underscores the importance of probing these systems in situ.

Assembly of dicobalt and cobalt–aluminum oxide clusters on metal–organic framework and nanocast silica supports

NU-1000, a mesoporous metal–organic framework (MOF) featuring hexazirconium oxide nodes and 3 nm wide channels, was infiltrated with a reactive dicobalt complex to install dicobalt active sites onto the MOF nodes. The anchoring of the dicobalt complex onto NU-1000 occurred with a nearly ideal stoichiometry of one bimetallic complex per node and with the cobalt evenly distributed throughout the MOF particle. To access thermally robust multimetallic sites on an all-inorganic support, the modified NU-1000 materials containing either the dicobalt complex, or an analogous cobalt–aluminum species, were nanocast with silica. The resulting materials feature Co₂or Co–Al bimetallated hexazirconium oxide clusters within a silica matrix. The cobalt-containing materials are competent catalysts for the selective oxidation of benzyl alcohol to benzaldehyde. Catalytic activity depends on the number of cobalt ions per node, but does not vary significantly between the NU-1000 and silica supports. Hence, the multimetallic oxide clusters remain site-isolated and substrate-accessible within the nanocast materials.