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.

Is iron unique in promoting electrical conductivity in MOFs?

Identifying the metal ions that optimize charge transport and charge density in metal–organic frameworks is critical for systematic improvements in the electrical conductivity in these materials.

Identifying the metal ions that optimize charge transport and charge density in metal–organic frameworks is critical for systematic improvements in the electrical conductivity in these materials. In this work, we measure the electrical conductivity and activation energy for twenty different MOFs pertaining to four distinct structural families: M₂(DOBDC)(DMF)₂ (M = Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Zn²⁺); H₄DOBDC = 2,5-dihydroxybenzene-1,4-dicarboxylic acid; DMF = N,N-dimethylformamide), M₂(DSBDC)(DMF)₂ (M = Mn²⁺, Fe²⁺; H₄DSBDC = 2,5-disulfhydrylbenzene-1,4-dicarboxylic acid), M₂Cl₂(BTDD)(DMF)₂ (M = Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺; H₂BTDD = bis(1H-1,2,3-triazolo[4,5-b],[4′,5′-i]dibenzo[1,4]dioxin), and M(1,2,3-triazolate)₂ (M = Mg²⁺, Mn²⁺, Fe²⁺, Co²⁺, Cu²⁺, Zn²⁺, Cd²⁺). This comprehensive study allows us to single-out iron as the metal ion that leads to the best electrical properties. The iron-based MOFs exhibit at least five orders of magnitude higher electrical conductivity and significantly smaller charge activation energies across all different MOF families studied here and stand out materials made from all other metal ions considered here. We attribute the unique electrical properties of iron-based MOFs to the high-energy valence electrons of Fe²⁺ and the Fe^3+/2+ mixed valency. These results reveal that incorporating Fe²⁺ in the charge transport pathways of MOFs and introducing mixed valency are valuable strategies for improving electrical conductivity in this important class of porous materials.

The impact of solvent relative permittivity on the dimerisation of organic molecules well below their solubility limits: examples from brewed coffee and beyond

The formation of aqueous intermolecular dimers is governed by both the nature and strength of the intermolecular interactions and the entropy of dissolution. The former interaction energies are determined by the polarity of the solvent and the functionality of the solute. Using quantum chemical methods, we probe the energetics of dimer formation of representative compounds found in coffee well below their solubility limits. We find that with the exclusion of entropy, the dimer formation is thermodynamically unfavorable with negligible dependence on the dielectric medium.

Designing porous electronic thin-film devices: band offsets and heteroepitaxy

Increasing numbers of electrically active porous framework materials are being reported, with conductivities that make them attractive for technological applications. As design strategies for efficient carrier transport emerge, the next challenge is to incorporate the materials into a functioning device. In thin-film devices interface effects are of critical importance to overall function. In this article we present a method to identify compatible materials combinations to achieve mechanically robust, electronically optimal pairings. The computational screening is based on a two-step procedure: (i) matching of lattice constants to ensure interfaces with minimal epitaxial strain and therefore maximal mechanical and chemical stability; (ii) matching of absolute electron energies to construct energy-band-alignment diagrams, which can be used to screen for particular electronic applications. We apply the methodology to search for zeolitic imidazolate framework (ZIF) type materials that are compatible with native metal electrodes. The procedure allows us to predict simple routes for electrochemical deposition of ZIFs for application as conductive porous electrodes.

Realistic Surface Descriptions of Heterometallic Interfaces: The Case of TiWC Coated in Noble Metals

Noble metal-coated core-shell nanoparticles have been applied to a suite of catalytic applications, with the aim of decreasing the noble metal loading while ideally improving their performance. The chemistry and therefore activity at the surface of these materials are intimately related to the accurate description of the core-shell interface. Using density functional theory, we developed a procedure to obtain realistic surface topology descriptions at the heterometallic junction. This procedure was applied to a topical series of catalysts: Ti_0.1W_0.9C coated in atomically thin monolayers of noble metals. Our quantum chemical calculations provide access to both relevant surface descriptions of these materials and also rationalize several experimental observations. Our general procedure paves the way for the rationalization and prediction of next-generation heterometallic catalysts.

Tracking a Common Surface-Bound Intermediate during CO2-to-Fuels Catalysis

Rational design of selective CO2-to-fuels electrocatalysts requires direct knowledge of the electrode surface structure during turnover. Metallic Cu is the most versatile CO2-to-fuels catalyst, capable of generating a wide array of value-added products, including methane, ethylene, and ethanol. All of these products are postulated to form via a common surface-bound CO intermediate. Therefore, the kinetics and thermodynamics of CO adsorption to Cu play a central role in determining fuel-formation selectivity and efficiency, highlighting the need for direct observation of CO surface binding equilibria under catalytic conditions. Here, we synthesize nanostructured Cu films adhered to IR-transparent Si prisms, and we find that these Cu surfaces enhance IR absorption of bound molecules. Using these films as electrodes, we examine Cu-catalyzed CO2 reduction in situ via IR spectroelectrochemistry. We observe that Cu surfaces bind electrogenerated CO, derived from CO2, beginning at -0.60 V vs RHE with increasing surface population at more negative potentials. Adsorbed CO is in dynamic equilibrium with dissolved (13)CO and exchanges rapidly under catalytic conditions. The CO adsorption profiles are pH independent, but adsorbed CO species undergo a reversible transformation on the surface in modestly alkaline electrolytes. These studies establish the potential, concentration, and pH dependencies of the CO surface population on Cu, which serve to maintain a pool of this vital intermediate primed for further reduction to higher order fuel products.

The effect of bean origin and temperature on grinding roasted coffee

Coffee is prepared by the extraction of a complex array of organic molecules from the roasted bean, which has been ground into fine particulates. The extraction depends on temperature, water chemistry and also the accessible surface area of the coffee. Here we investigate whether variations in the production processes of single origin coffee beans affects the particle size distribution upon grinding. We find that the particle size distribution is independent of the bean origin and processing method. Furthermore, we elucidate the influence of bean temperature on particle size distribution, concluding that grinding cold results in a narrower particle size distribution, and reduced mean particle size. We anticipate these results will influence the production of coffee industrially, as well as contribute to how we store and use coffee daily.

Electronic origins of photocatalytic activity in d0 metal organic frameworks

Metal-organic frameworks (MOFs) containing d⁰ metals such as NH₂-MIL-125(Ti), NH₂-UiO-66(Zr) and NH₂-UiO-66(Hf) are among the most studied MOFs for photocatalytic applications. Despite structural similarities, we demonstrate that the electronic properties of these MOFs are markedly different. As revealed by quantum chemistry, EPR measurements and transient absorption spectroscopy, the highest occupied and lowest unoccupied orbitals of NH₂-MIL-125(Ti) promote a long lived ligand-to-metal charge transfer upon photoexcitation, making this material suitable for photocatalytic applications. In contrast, in case of UiO materials, the d-orbitals of Zr and Hf, are too low in binding energy and thus cannot overlap with the π* orbital of the ligand, making both frontier orbitals localized at the organic linker. This electronic reconfiguration results in short exciton lifetimes and diminishes photocatalytic performance. These results highlight the importance of orbital contributions at the band edges and delineate future directions in the development of photo-active hybrid solids.

A Simple and Non-Destructive Method for Assessing the Incorporation of Bipyridine Dicarboxylates as Linkers within Metal-Organic Frameworks

As a novel avenue for applications, metal-organic frameworks (MOFs) are increasingly used for heterogenizing catalytic molecular species as linkers into their crystalline framework. These multifunctional compounds can be accessed with mixed linkers synthesis or postsynthetic-exchange strategies. Major limitations still reside in their challenging characterization; in particular, to provide evidence of the genuine incorporation of the functionalized linkers into the framework and their quantification. Herein, we demonstrate that a combination of computational chemistry, spectroscopy and X-ray diffraction allows access to a non-destructive analysis of mixed-linker UiO-67-type materials featuring biphenyl- and bipyridine-dicarboxylates. Our UV/Vis-based methodology has been further applied to characterize a series of Rh-functionalized UiO-67-type catalysts. The proposed approach allows a recurrent key issue in the characterization of similar supported organometallic systems to be solved.

Role of entropic effects in controlling the polymorphism in formate ABX3 metal-organic frameworks

Polymorphism in formate-based dense metal-organic frameworks with the general formula ABX3 is predicted by quantum chemical calculations and confirmed experimentally. In particular [NH3NH2]Zn(HCOO)3 crystallizes in two different polymorphs, a perovskite-like framework and a chiral structure with hexagonal channels. A detailed thermodynamic analysis reveals that both structures are very close in free energy and that entropy driven effects are responsible for stabilizing the channel structure.

Magnetic coupling in a hybrid Mn(ii) acetylene dicarboxylate

The design of ligands that mediate through-bond long range super-exchange in metal–organic hybrid materials would expand chemical space beyond the commonly observed short range, low temperature magnetic ordering.

Chemical principles underpinning the performance of the metal-organic framework HKUST-1

A common feature of multi-functional metal-organic frameworks is a metal dimer in the form of a paddlewheel, as found in the structure of Cu3(btc)2 (HKUST-1). The HKUST-1 framework demonstrates exceptional gas storage, sensing and separation, catalytic activity and, in recent studies, unprecedented ionic and electrical conductivity. These results are a promising step towards the real-world application of metal-organic materials. In this perspective, we discuss progress in the understanding of the electronic, magnetic and physical properties of HKUST-1, representative of the larger family of Cu···Cu containing metal-organic frameworks. We highlight the chemical interactions that give rise to its favourable properties, and which make this material well suited to a range of technological applications. From this analysis, we postulate key design principles for tailoring novel high-performance hybrid frameworks.

Catalytic Amine Oxidation under Ambient Aerobic Conditions: Mimicry of Monoamine Oxidase B

The flavoenzyme monoamine oxidase (MAO) regulates mammalian behavioral patterns by modulating neurotransmitters such as adrenaline and serotonin. The mechanistic basis which underpins this enzyme is far from agreed upon. Reported herein is that the combination of a synthetic flavin and alloxan generates a catalyst system which facilitates biomimetic amine oxidation. Mechanistic and electron paramagnetic (EPR) spectroscopic data supports the conclusion that the reaction proceeds through a radical manifold. This data provides the first example of a biorelevant synthetic model for monoamine oxidase B activity.

Nanocrystals of Cesium Lead Halide Perovskites (CsPbX₃, X = Cl, Br, and I): Novel Optoelectronic Materials Showing Bright Emission with Wide Color Gamut

Metal halides perovskites, such as hybrid organic-inorganic CH3NH3PbI3, are newcomer optoelectronic materials that have attracted enormous attention as solution-deposited absorbing layers in solar cells with power conversion efficiencies reaching 20%. Herein we demonstrate a new avenue for halide perovskites by designing highly luminescent perovskite-based colloidal quantum dot materials. We have synthesized monodisperse colloidal nanocubes (4-15 nm edge lengths) of fully inorganic cesium lead halide perovskites (CsPbX3, X = Cl, Br, and I or mixed halide systems Cl/Br and Br/I) using inexpensive commercial precursors. Through compositional modulations and quantum size-effects, the bandgap energies and emission spectra are readily tunable over the entire visible spectral region of 410-700 nm. The photoluminescence of CsPbX3 nanocrystals is characterized by narrow emission line-widths of 12-42 nm, wide color gamut covering up to 140% of the NTSC color standard, high quantum yields of up to 90%, and radiative lifetimes in the range of 1-29 ns. The compelling combination of enhanced optical properties and chemical robustness makes CsPbX3 nanocrystals appealing for optoelectronic applications, particularly for blue and green spectral regions (410-530 nm), where typical metal chalcogenide-based quantum dots suffer from photodegradation.

Photocatalytic carbon dioxide reduction with rhodium-based catalysts in solution and heterogenized within metal-organic frameworks

The first photosensitization of a rhodium-based catalytic system for CO2 reduction is reported, with formate as the sole carbon-containing product. Formate has wide industrial applications and is seen as valuable within fuel cell technologies as well as an interesting H2 -storage compound. Heterogenization of molecular rhodium catalysts is accomplished via the synthesis, post-synthetic linker exchange, and characterization of a new metal-organic framework (MOF) Cp*Rh@UiO-67. While the catalytic activities of the homogeneous and heterogeneous systems are found to be comparable, the MOF-based system is more stable and selective. Furthermore it can be recycled without loss of activity. For formate production, an optimal catalyst loading of ∼10 % molar Rh incorporation is determined. Increased incorporation of rhodium catalyst favors thermal decomposition of formate into H2 . There is no precedent for a MOF catalyzing the latter reaction so far.