Ruthenium Complexation in an Aluminium Metal-Organic Framework and Its Application in Alcohol Oxidation Catalysis

Photochargeable Mn-Based Metal-Organic Framework and Decoupled Photocatalysis

Designing multifunctional materials that mimic the light-dark decoupling of natural photosynthesis is a key challenge in the field of energy conversion. Herein, we introduce MnBr-253, a precious metal-free metal-organic framework (MOF) built on Al nodes, bipyridine linkers and MnBr(CO)3(bipyridine) complexes. Upon irradiation, MnBr-253 colloids demonstrate an electron photocharging capacity of ~42 C ⋅ g-1 MOF, with state-of-the-art photocharging rate (1.28 C ⋅ s-1 ⋅ g-1 MOF) and incident photon-to-electron conversion efficiency of ~9.4 % at 450 nm. Spectroscopic and computational studies support effective electron accumulation at the Mn complex while high porosity and Mn loading account for the notable electron storage performance. The charged MnBr-253 powders were successfully applied for hydrogen evolution under dark conditions thus emulating the light-decoupled reactivity of photosynthesis.

Decoupled Solar Energy Storage and Dark Photocatalysis in a 3D Metal-Organic Framework

Materials enabling solar energy conversion and long-term storage for readily available electrical and chemical energy are key for off-grid energy distribution. Herein, the specific confinement of a rhenium coordination complex in a metal-organic framework (MOF) unlocks a unique electron accumulating property under visible-light irradiation. About 15 C g_MOF ^-1 of electric charges can be concentrated and stored for over four weeks without loss. Decoupled, on-demand discharge for electrochemical reactions and H₂ evolution catalysis is shown and light-driven recharging can be conducted for >10 cycles with ≈90% of the initial charging capacity retained. Experimental investigations and theoretical calculations link electron trapping to MOF-induced geometry constraints as well as the coordination environment of the Re-center, highlighting the key role of MOF confinement on molecular guests. This study serves as the seminal report on 3D porous colloids achieving photoaccumulation of long-lived electrons, unlocking dark photocatalysis, and a path toward solar capacitor and solar battery systems.

Near-infrared light photocatalysis enabled by a ruthenium complex-integrated metal–organic framework via two-photon absorption

Photocatalysis under UV/visible light irradiation has emerged as one of the green methodologies for solar energy utilization and organic synthesis. These photocatalytic processes are typically initiated by one-photon-absorbing metal complexes or organic dyes. Nevertheless, the intrinsic restrictions of UV/visible light irradiation, such as shallow penetration in reaction solutions, competing absorption by substrates, and limited coverage of the solar spectrum, call for the development of innovative photocatalysts functioning under longer wavelength irradiation. Herein, we report a ruthenium complex containing a metal-organic framework, MOF-Ru1, which can drive diverse organic reactions under 740 nm light irradiation following the two-photon absorption (TPA) process. Various organic transformations such as energy transfer, reductive, oxidative, and redox neutral reactions were realized using this heterogeneous hybrid photocatalyst. Overall, MOF-Ru1 represents an intriguing TPA photocatalyst active under near-infrared light irradiation, paving a way for the efficient utilization of low-energy light and convenient photocatalyst recycling because of phase separation.

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Ru complexes with π-conjugation ligands show two-photon absorption of NIR photons

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Hybrid MOF-Ru has NIR light-driven photocatalytic performance with recyclability

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A variety of organic reactions were photocatalyzed by MOF-Ru under 740 nm irradiation

Post-synthetic modifications (PSM) on metal-organic frameworks (MOFs) for visible-light-initiated photocatalysis

The utilization of green and sustainable solar energy via photocatalysis is regarded as a promising strategy to tackle the ever-increasing energy shortage and environmental deterioration. In addition to traditional semiconductor-based photocatalysts, metal-organic frameworks (MOFs), a class of crystalline micro-mesoporous hybrid materials constructed from metal or metal nodes interconnected with multi-dentate organic linkers, are emerging as a new type of photocatalytic material. Post-synthetic modifications (PSM) on MOFs, in which chemical transformations or exchanges are made on pre-synthesized MOF materials, are found to be a powerful strategy for fabricating photoactive MOFs based on already existing MOFs. In this frontier article, different PSM strategies for the development of photoactive MOFs, including coordination on unsaturated metal sites, metalation on open coordinated sites, covalent modifications on ligands, ligand exchange, metal exchange and cavity encapsulation, have been summarized. Our views on the challenges and the direction in developing photocatalytic MOFs by PSM are also addressed. We hope that this frontier article can provide some guidance for rational designing of highly efficient MOF-based photocatalysts via PSM strategies and to stimulate more research interest to be devoted to this promising yet largely unexplored field.

Structure evolution of single-site Pt in a metal-organic framework

Heterogeneous single-metal-site catalyst or single-atom catalyst research has grown rapidly due to the accessibility of modern characterization techniques that can provide invaluable information at the atomic-scale. Herein, we study the structural evolution of isolated single Pt sites incorporated in a metal-organic framework containing bipyridine functional groups using in situ diffuse reflectance infrared Fourier transform spectroscopy with CO as the probe molecule. The structure and electronic properties of the isolated Pt sites are further corroborated by x-ray photoelectron spectroscopy and aberration-corrected scanning transmission electron microscopy. We find the prerequisite of high temperature He treatment for Pt activation and CO insertion and inquire into the structural transformation of Pt site process by dynamic nuclear polarization-enhanced solid-state nuclear magnetic resonance spectroscopy.

Metal-Organic Framework-Based Catalysts with Single Metal Sites

Metal-organic frameworks (MOFs) are a class of distinctive porous crystalline materials constructed by metal ions/clusters and organic linkers. Owing to their structural diversity, functional adjustability, and high surface area, different types of MOF-based single metal sites are well exploited, including coordinately unsaturated metal sites from metal nodes and metallolinkers, as well as active metal species immobilized to MOFs. Furthermore, controllable thermal transformation of MOFs can upgrade them to nanomaterials functionalized with active single-atom catalysts (SACs). These unique features of MOFs and their derivatives enable them to serve as a highly versatile platform for catalysis, which has actually been becoming a rapidly developing interdisciplinary research area. In this review, we overview the recent developments of catalysis at single metal sites in MOF-based materials with emphasis on their structures and applications for thermocatalysis, electrocatalysis, and photocatalysis. We also compare the results and summarize the major insights gained from the works in this review, providing the challenges and prospects in this emerging field.

Boosting Aerobic Oxidation of Alcohols via Synergistic Effect between TEMPO and a Composite Fe3O4/Cu-BDC/GO Nanocatalyst

Fabrication of a nanocomposite catalyst via a novel and efficient strategy remains a challenge; Fe₃O₄ nanoparticles anchored on graphene oxide (GO) sheet-supported metal-organic frameworks (MOFs). In this study, the physicochemical properties of the ensuing Fe₃O₄/Cu-BDC/GO are investigated using Fourier transform infrared spectrum, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray detector, and atomic absorption spectroscopy. The salient features of the nanocomposite such as Cu-MOF, synergistic effect with GO sheets, and magnetic separation characteristics make it an excellent ternary heterostructure for aerobic oxidation of alcohols. The proposed nanocatalyst and co-catalyst 2,2,6,6-tetramethylpiperidine-N-oxyl substantially enhance the catalytic performance for the aerobic oxidation under very mild and sustainable reaction conditions. The heterogeneity of Fe₃O₄/Cu-BDC/GO composite catalyst is affirmed with the added advantage that the initial activity is well maintained even after seven cycles.

Highly efficient recovery of ruthenium from integrated circuit (IC) manufacturing wastewater by Al reduction and cementation

Ruthenium (Ru) is a rare-earth metal, which is employed widely in metal-processing industries. This study recovered Ru from the wastewater of an IC foundry by cementation using metallic aluminum (Al) powder as the sacrificial agent. Ru ions were efficiently reduced to the metal and coagulated with the derived aluminum hydroxide flocs. Experimental parameters included the particle size of Al, molar ratio of Al to Ru, initial Ru concentration and operation temperature. The recovery rate reached 99% under these conditions: particle size Al powder = 88–128 μm, Al/Ru molar ratio = 2.0, initial Ru = 200 mg L⁻¹, temperature = 338.16 K, reaction time = 120 min, stirring speed = 150 rpm. The cemented Ru over Al powder was spherical with a rough surface. Kinetic modelling suggested that the diffusion of Ru through the ash layer of Al powder controlled the reaction rate with an activation energy of 40.75 kJ mol⁻¹. A brief cost analysis demonstrated that the cementation of Ru yielded a profit of $0.180 per 0.1 m³-wastewater.

Ruthenium (Ru) is a rare-earth metal, which is employed widely in metal-processing industries.

Different functional group modified zirconium frameworks for the photocatalytic reduction of carbon dioxide

UiO-68-PSMs of UiO-68-F, UiO-68-CH₃ and UiO-68-OCH₃ achieved by post-synthetic modification were found to show different activity for photocatalytic CO₂ reduction.

Insights into Functionalization of Metal-Organic Frameworks Using In Situ NMR Spectroscopy

Postsynthetic reactions of metal-organic frameworks (MOFs) are versatile tools for producing functional materials, but the methods of evaluating these reactions are cumbersome and destructive. Here we demonstrate and validate the use of in situ NMR spectroscopy of species in the liquid state to examine solvent-assisted ligand exchange (SALE) and postsynthetic modification (PSM) reactions of metal-organic frameworks. This technique allows functionalization to be monitored over time without decomposing the product for analysis, which simplifies reaction screening. In the case of SALE, both the added ligand and the ligand leaving the framework can be observed. We demonstrate this in situ method by examining SALE and PSM reactions of the robust zirconium MOF UiO-67 as well as SALE with the aluminum MOF DUT-5. In situ NMR spectroscopy provided insights into the reactions studied, and we expect that future studies using this method will permit the examination of a variety of MOF–solute reactions.

Site Isolation in Metal-Organic Frameworks Enables Novel Transition Metal Catalysis

Comprising periodically repeating inorganic nodes and organic linkers, metal-organic frameworks (MOFs) represent a novel class of porous molecular solids with well-defined pores and channels. Over the past two decades, a large array of organic linkers have been combined with many inorganic nodes to afford a vast library of MOFs. The synthetic tunability of MOFs distinguishes them from traditional porous inorganic materials and has allowed the rational design of many interesting properties, such as porosity, chirality, and chemical functionality, for potential applications in diverse areas including gas storage and separation, catalysis, light harvesting, chiral separation, and chemical sensing. In particular, the molecular functionality and intrinsic porosity of MOFs have rendered them attractive candidates as porous single-site solid catalysts for a large number of organic transformations. MOF catalysts offer several advantages over their homogeneous counterparts, including enhanced stability, recyclability and reusability, and facile removal of the toxic catalyst components from the organic products. Additionally, the highly ordered nature of MOFs leads to the generation of single-site solid catalysts, allowing for precise characterization of the catalytic sites through X-ray diffraction, X-ray absorption, and other spectroscopic interrogations and facilitating the elucidation of reaction mechanisms. Thus, MOF catalysis represents a fertile research area that is expected to witness continued growth in the foreseeable future. In this Account, we present our recent research progress in developing ligand-supported single-site MOF catalysts for challenging organic reactions. We present two complementary approaches to the design of ligand-supported MOF catalysts: direct incorporation of prefunctionalized organic linkers into MOFs and postsynthetic functionalization of orthogonal secondary functional groups of the organic linkers in MOFs. Monophosphine-, bipyridine-, β-diketimine-, and salicylaldimine-based ligands have been used to support both precious (Pd, Pt, Ir, Ru) and earth-abundant (Cu, Co, Fe) metals for a number of interesting catalytic reactions. The resulting MOF catalysts feature stable low-coordination species with minimum steric bulk around the active site-a feat that remains a challenge for homogeneous catalysts. For each ligand, we describe types of reactions catalyzed by the MOF in comparison with its homogeneous counterpart. In all cases, MOF catalysts outperformed their homogeneous counterparts in terms of catalyst stability, catalytic activity, and recyclability and reusability. Interestingly, several bipyridine- and salicylaldimine-ligated earth-abundant-metal-based MOF catalysts do not have homogeneous counterparts because the molecular compounds disproportionate or oligomerize to form inactive species in solution. This Account not only presents several interesting designs of ligand-supported single-site MOF catalysts but also provides illustrative examples of how site isolation in MOF catalysts shuts down deactivation pathways experienced by homogeneous systems. With precise knowledge of MOF structures and catalytically active sites, we envision the development of practically useful MOF catalysts comprising tailor-made building blocks that rationally optimize catalytic activities and selectivities.

Metal-Organic Frameworks as Platforms for Catalytic Applications

Metal-organic frameworks (MOFs), also called porous coordination polymers, represent a class of crystalline porous materials built from organic linkers and metal ions/clusters. The unique features of MOFs, including structural diversity and tailorability as well as high surface area, etc., enable them to be a highly versatile platform for potential applications in many fields. Herein, an overview of recent developments achieved in MOF catalysis, including heterogeneous catalysis, photocatalysis, and eletrocatalysis over MOFs and MOF-based materials, is provided. The active sites involved in the catalysts are particularly emphasized. The challenges, future trends, and prospects associated with MOFs and their related materials for catalysis are also discussed.

Tuneable nature of metal organic frameworks as heterogeneous solid catalysts for alcohol oxidation

This Feature article describes on the recent developments in the use of metal organic frameworks as heterogeneous solid catalysts for the selective alcohol oxidation by either tuning the actives sites around the metal centre, or anchoring them on the ligands or using the pores to embed metal nanoparticles inside.

DNP-Enhanced Ultrawideline Solid-State NMR Spectroscopy: Studies of Platinum in Metal-Organic Frameworks

Ultrawideline dynamic nuclear polarization (DNP)-enhanced (195)Pt solid-state NMR (SSNMR) spectroscopy and theoretical calculations are used to determine the coordination of atomic Pt species supported within the pores of metal-organic frameworks (MOFs). The (195)Pt SSNMR spectra, with breadths reaching 10 000 ppm, were obtained by combining DNP with broadbanded cross-polarization and CPMG acquisition. Although the DNP enhancements in static samples are lower than those typically observed under magic-angle spinning conditions, the presented measurements would be very challenging using the conventional SSNMR methods. The DNP-enhanced ultrawideline NMR spectra served to separate signals from cis- and trans-coordinated atomic Pt(2+) species supported on the UiO-66-NH2 MOF. Additionally, the data revealed a dominance of kinetic effects in the formation of Pt(2+) complexes and the thermodynamic effects in their reduction to nanoparticles. A single cis-coordinated Pt(2+) complex was confirmed in MOF-253.

Synergistic oxygen atom transfer by ruthenium complexes with non-redox metal ions

Non-redox metal ions can affect the reactivity of active redox metal ions in versatile biological and heterogeneous oxidation processes; however, the intrinsic roles of these non-redox ions still remain elusive. This work demonstrates the first example of the use of non-redox metal ions as Lewis acids to sharply improve the catalytic oxygen atom transfer efficiency of a ruthenium complex bearing the classic 2,2'-bipyridine ligand. In the absence of Lewis acid, the oxidation of ruthenium(ii) complex by PhI(OAc)2 generates the Ru(iv)[double bond, length as m-dash]O species, which is very sluggish for olefin epoxidation. When Ru(bpy)2Cl2 was tested as a catalyst alone, only 21.2% of cyclooctene was converted, and the yield of 1,2-epoxycyclooctane was only 6.7%. As evidenced by electronic absorption spectra and EPR studies, both the oxidation of Ru(ii) by PhI(OAc)2 and the reduction of Ru(iv)[double bond, length as m-dash]O by olefin are kinetically slow. However, adding non-redox metal ions such as Al(iii) can sharply improve the oxygen transfer efficiency of the catalyst to 100% conversion with 89.9% yield of epoxide under identical conditions. Through various spectroscopic characterizations, an adduct of Ru(iv)[double bond, length as m-dash]O with Al(iii), Ru(iv)[double bond, length as m-dash]O/Al(iii), was proposed to serve as the active species for epoxidation, which in turn generated a Ru(iii)-O-Ru(iii) dimer as the reduced form. In particular, both the oxygen transfer from Ru(iv)[double bond, length as m-dash]O/Al(iii) to olefin and the oxidation of Ru(iii)-O-Ru(iii) back to the active Ru(iv)[double bond, length as m-dash]O/Al(iii) species in the catalytic cycle can be remarkably accelerated by adding a non-redox metal, such as Al(iii). These results have important implications for the role played by non-redox metal ions in catalytic oxidation at redox metal centers as well as for the understanding of the redox mechanism of ruthenium catalysts in the oxygen atom transfer reaction.

A dual-emitting 4d-4f nanocrystalline metal-organic framework as a self-calibrating luminescent sensor for indoor formaldehyde pollution

A dual-emissive 4d-4f Ag(i)-Eu(iii) functionalized MOF nanocomposite was fabricated and utilized as a self-calibrating luminescent nanoprobe for detecting indoor formaldehyde (FA). The implantation of Ag(+) ions can tune the dual-emissive characters of the material. FA can interact with the Ag(+) ions and induce opposite luminescence behaviors of the two emitters in the singular molecular material, thus realizing its recognition. This nanoprobe for FA exhibits many merits, such as excellent selectivity, high sensitivity with a detection limit of 51 ppb, fast response, room-temperature testing ability, easy preparation and low cost. This is the first example of a MOF-implicated self-calibrated sensor for indoor FA detection.

Metal-Organic Frameworks as Catalysts for Oxidation Reactions

This Concept is aimed at describing the current state of the art in metal-organic frameworks (MOFs) as heterogeneous catalysts for liquid-phase oxidations, focusing on three important substrates, namely, alkenes, alkanes and alcohols. Emphases are on the nature of active sites that have been incorporated within MOFs and on future targets to be set in this area. Thus, selective alkene epoxidation with peroxides or oxygen catalyzed by constitutional metal nodes of MOFs as active sites are still to be developed. Moreover, no noble metal-free MOF has been reported to date that can act as a general catalyst for the aerobic oxidation of primary and secondary aliphatic alcohols. In contrast, in the case of alkanes, a target should be to tune the polarity of MOF internal pores to control the outcome of the autooxidation process, resulting in the selective formation of alcohol/ketone mixtures at high conversion.

A computational study of CH4 storage in porous framework materials with metalated linkers: connecting the atomistic character of CH4 binding sites to usable capacity

Open-metal sites are shown to significantly increase the CH₄ storage capacity of porous materials. It is shown that the capacity is not determined solely by their CH₄ affinity, but also by their geometry as well as by guest molecules.

To store natural gas (NG) inexpensively at adequate densities for use as a fuel in the transportation sector, new porous materials are being developed. This work uses computational methods to explore strategies for improving the usable methane storage capacity of adsorbents, including metal–organic frameworks (MOFs), that feature open-metal sites incorporated into their structure by postsynthetic modification. The adsorption of CH₄ on several open-metal sites is studied by calculating geometries and adsorption energies and analyzing the relevant interaction factors. Approximate site-specific adsorption isotherms are obtained, and the open-metal site contribution to the overall CH₄ usable capacity is evaluated. It is found that sufficient ionic character is required, as exemplified by the strong CH₄ affinities of 2,2′-bipyridine-CaCl₂ and Mg, Ca-catecholate. In addition, it is found that the capacity of a single metal site depends not only on its affinity but also on its geometry, where trigonal or “bent” low-coordinate exposed sites can accommodate three or four methane molecules, as exemplified by Ca-decorated nitrilotriacetic acid. The effect of residual solvent molecules at the open-metal site is also explored, with some positive conclusions. Not only can residual solvent stabilize the open-metal site, surprisingly, solvent molecules do not necessarily reduce CH₄ affinity, but can contribute to increased usable capacity by modifying adsorption interactions.

Simultaneous determination of indoor ammonia pollution and its biological metabolite in the human body with a recyclable nanocrystalline lanthanide-functionalized MOF

A Eu(3+) post-functionalized metal-organic framework of nanosized Ga(OH)bpydc(Eu(3+)@Ga(OH)bpydc, 1a) with intense luminescence is synthesized and characterized. Luminescence measurements reveal that 1a can detect ammonia gas selectively and sensitively among various indoor air pollutants. 1a can simultaneously determine a biological ammonia metabolite (urinary urea) in the human body, which is a rare example of a luminescent sensor that can monitor pollutants in the environment and also detect their biological markers. Furthermore, 1a exhibits appealing features including high selectivity and sensitivity, fast response, simple and quick regeneration, and excellent recyclability.

MOF-253-Pd(OAc)2: a recyclable MOF for transition-metal catalysis in water

We report palladium(ii)-functionalized MOF-253 (MOF-253-Pd(OAc)₂) as a recyclable catalyst to form all-carbon quaternary centers via conjugate additions of arylboronic acids to β,β-disubstituted enones in aqueous media.

Novel access to carbonyl and acetylated compounds: the role of the tetra-n-butylammonium bromide/sodium nitrite catalyst

A novel aerobic oxidation of alcohols without the use of any oxidants was developed.

Lanthanides post-functionalized indium metal–organic frameworks (MOFs) for luminescence tuning, polymer film preparation and near-UV white LED assembly

In-MOF–Eu polymer thin films and a white light LED are prepared. The surface of the LED bulb is covered by the polymer and it can emit dazzling white light under a certain excitation wavelength.

Effect of the functionalisation route on a Zr-MOF with an Ir-NHC complex for catalysis

A new iridium N-heterocyclic carbene (NHC) metallolinker has been synthesised and introduced into a metal-organic framework (MOF), for the first time, via two different routes: direct synthesis and postsynthetic exchange (PSE). The two materials were compared in terms of the Ir loading and distribution using X-ray energy dispersive spectroscopy (EDS), the local Ir structure using X-ray absorption spectroscopy (XAS) and the catalytic activity. The materials showed good activity and recyclability as catalysts for the isomerisation of an allylic alcohol.

A robust, catalytic metal-organic framework with open 2,2'-bipyridine sites

We report two synthetic approaches to prepare a highly crystalline Zr(IV)-based metal-organic framework (MOF) containing open 2,2'-bipyridine (bpy) chelating sites. The resulting UiO-67-bpydc readily forms complexes with PdCl2 to produce a MOF that exhibits efficient and recyclable catalytic activity for the Suzuki-Miyaura cross-coupling reaction.