Oxidation of ethane to ethanol by N2O in a metal-organic framework with coordinatively unsaturated iron(II) sites

Selective Methane Oxidation to Methanol on Cu-Oxo Dimers Stabilized by Zirconia Nodes of an NU-1000 Metal-Organic Framework

Mononuclear and dinuclear copper species were synthesized at the nodes of an NU-1000 metal-organic framework (MOF) via cation exchange and subsequent oxidation at 200 °C in oxygen. Copper-exchanged MOFs are active for selectively converting methane to methanol at 150-200 °C. At 150 °C and 1 bar methane, approximately a third of the copper centers are involved in converting methane to methanol. Methanol productivity increased by 3-4-fold and selectivity increased from 70% to 90% by increasing the methane pressure from 1 to 40 bar. Density functional theory showed that reaction pathways on various copper sites are able to convert methane to methanol, the copper oxyl sites with much lower free energies of activation. Combining studies of the stoichiometric activity with characterization by in situ X-ray absorption spectroscopy and density functional theory, we conclude that dehydrated dinuclear copper oxyl sites formed after activation at 200 °C are responsible for the activity.

A new iron-based metal-organic framework with enhancing catalysis activity for benzene hydroxylation

A new Fe-based metal-organic framework (MOF), termed Fe-TBAPy Fe2(OH)2(TBAPy)·4.4H2O, was solvothermally synthesized. Structural analysis revealed that Fe-TBAPy is built from [Fe(OH)(CO2)2]∞ rod-shaped SBUs (SBUs = secondary building units) and 1,3,6,8-tetrakis(p-benzoate)pyrene (TBAPy4-) linker to form the frz topological structure highlighted by 7 Å channels and 3.4 Å narrow pores sandwiching between the pyrene cores of TBAPy4-. Consequently, Fe-TBAPy was used as a recyclable heterogeneous catalyst for benzene hydroxylation. Remarkably, the catalysis reaction resulted in high phenol yield and selectivity of 64.5% and 92.9%, respectively, which are higher than that of the other Fe-based MOFs and comparable with those of the best-performing heterogeneous catalysts for benzene hydroxylation. This finding demonstrated the potential for the design of MOFs with enhancing catalysis activity for benzene hydroxylation.

Computational Approach to Molecular Catalysis by 3d Transition Metals: Challenges and Opportunities

Computational chemistry provides a versatile toolbox for studying mechanistic details of catalytic reactions and holds promise to deliver practical strategies to enable the rational in silico catalyst design. The versatile reactivity and nontrivial electronic structure effects, common for systems based on 3d transition metals, introduce additional complexity that may represent a particular challenge to the standard computational strategies. In this review, we discuss the challenges and capabilities of modern electronic structure methods for studying the reaction mechanisms promoted by 3d transition metal molecular catalysts. Particular focus will be placed on the ways of addressing the multiconfigurational problem in electronic structure calculations and the role of expert bias in the practical utilization of the available methods. The development of density functionals designed to address transition metals is also discussed. Special emphasis is placed on the methods that account for solvation effects and the multicomponent nature of practical catalytic systems. This is followed by an overview of recent computational studies addressing the mechanistic complexity of catalytic processes by molecular catalysts based on 3d metals. Cases that involve noninnocent ligands, multicomponent reaction systems, metal-ligand and metal-metal cooperativity, as well as modeling complex catalytic systems such as metal-organic frameworks are presented. Conventionally, computational studies on catalytic mechanisms are heavily dependent on the chemical intuition and expert input of the researcher. Recent developments in advanced automated methods for reaction path analysis hold promise for eliminating such human-bias from computational catalysis studies. A brief overview of these approaches is presented in the final section of the review. The paper is closed with general concluding remarks.

Heme and Nonheme High-Valent Iron and Manganese Oxo Cores in Biological and Abiological Oxidation Reactions

Utilization of O₂ as an abundant and environmentally benign oxidant is of great interest in the design of bioinspired synthetic catalytic oxidation systems. Metalloenzymes activate O₂ by employing earth-abundant metals and exhibit diverse reactivities in oxidation reactions, including epoxidation of olefins, functionalization of alkane C-H bonds, arene hydroxylation, and syn-dihydroxylation of arenes. Metal-oxo species are proposed as reactive intermediates in these reactions. A number of biomimetic metal-oxo complexes have been synthesized in recent years by activating O₂ or using artificial oxidants at iron and manganese centers supported on heme or nonheme-type ligand environments. Detailed reactivity studies together with spectroscopy and theory have helped us understand how the reactivities of these metal-oxygen intermediates are controlled by the electronic and steric properties of the metal centers. These studies have provided important insights into biological reactions, which have contributed to the design of biologically inspired oxidation catalysts containing earth-abundant metals like iron and manganese. In this Outlook article, we survey a few examples of these advances with particular emphasis in each case on the interplay of catalyst design and our understanding of metalloenzyme structure and function.

Room-Temperature Activation of the C-H Bond in Methane over Terminal ZnII-Oxyl Species in an MFI Zeolite: A Combined Spectroscopic and Computational Study of the Reactive Frontier Molecular Orbitals and Their Origins

Oxygenase reactivity toward selective partial oxidation of CH₄ to CH₃OH requires an atomic oxygen-radical bound to metal (M-O^•: oxyl intermediate) that is capable of abstracting an H atom from the significantly strong C-H bond in CH₄. Because such a reaction is frequently observed in metal-doped zeolites, it has been recognized that the zeolite provides an environment that stabilizes the M-O^• intermediate. However, no experimental data of M-O^• have so far been discovered in the zeolite; thus, little is known about the correlation among the state of M-O^•, its reactivity for CH₄, and the nature of the zeolite environment. Here, we report a combined spectroscopic and computational study of the room-temperature activation of CH₄ over Zn^II-O^• in the MFI zeolite. One Zn^II-O^• species does perform H-abstraction from CH₄ at room temperature. The resultant CH₃^• species reacts with the other Zn^II-O^• site to form the Zn^II-OCH₃ species. The H₂O-assisted extraction of surface methoxide yields 29 μmol g^-1 of CH₃OH with a 94% selectivity. The quantum mechanics (QM)/molecular mechanics (MM) calculation determined the central step as the oxyl-mediated hydrogen atom transfer which requires an activation energy of only 10 kJ mol^-1. On the basis of the findings in gas-phase experiments regarding the CH₄ activation by the free [M-O^•]⁺ species, the remarkable H-abstraction reactivity of the Zn^II-O^• species in zeolites was totally rationalized. Additionally, the experimentally validated QM/MM calculation revealed that the zeolite lattice has potential as the ligand to enhance the polarization of the M-O^• bond and thereby enables to create effectively the highly reactive M-O^• bond required for low-temperature activation of CH₄. The present study proposes that tuning of the polarization effect of the anchoring site over heterogeneous catalysts is the valuable way to create the oxyl-based functionality on the heterogeneous catalyst.

Structural variation of transition metal–organic frameworks using deep eutectic solvents with different hydrogen bond donors

Seven transition metal–organic frameworks with structures ranging from one-dimensional chains to three-dimensional networks have been synthesized in deep eutectic solvents.

The enzyme-like catalytic hydrogen abstraction reaction mechanisms of cyclic hydrocarbons with magnesium-diluted Fe-MOF-74

Enzymatic heme and non-heme Fe(iv)–O species usually play an important role in hydrogen abstraction of biocatalytic reactions, yet duplicating the reactivity in biomimicry remains a great challenge. Based on Xiao et al.'s experimental work [Nat. Chem., 2014, 6(7), 590], we theoretically found that in the presence of the oxidant N₂O, the enzyme-like metal organic framework, i.e., magnesium-diluted Fe-MOF-74 [Fe/(Mg)-MOF-74] can activate the C–H bonds of 1,4-cyclohexadiene (CHD) into benzene with a two-step hydrogen abstraction mechanism based on the density functional theory (DFT) level. It is shown that the first transition state about the cleavage of the N–O bond of N₂O to form the Fe(iv)–O species is the rate-determining step with activation enthalpy of 19.4 kcal mol⁻¹ and the complete reaction is exothermic by 62.8 kcal mol⁻¹ on quintet rather than on triplet PES. In addition, we proposed a rebound mechanism of cyclic cyclohexane (CHA) hydroxylation to cyclohexanol which has not been studied experimentally. Note that the activation enthalpies on the first hydrogen abstraction for both cyclic CHD and cyclohexane are just 8.1 and 3.5 kcal mol⁻¹, respectively, which are less than that of 13.9 kcal mol⁻¹ for chained ethane. Most importantly, for the hydrogen abstraction of methane catalyzed by M/(Mg)-MOF-74 (M = Cu, Ni, Fe, and Co), we found that the activation enthalpies versus the C–H bond length of methane of TSs, NPA charge of the reacting oxyl atom have linear relationships with different slopes, i.e., shorter C–H bond and less absolute value of NPA charge of oxyl atom are associated with lower activation enthalpy; while for the activation of methane, ethane, propane and CHD catalyzed by Fe/(Mg)-MOF-74, there also exists positive correlations between activation enthalpies, bond dissociation energies (BDEs) and C–H bond lengths in TSs, respectively. We hope the present theoretical study may provide the guideline to predict the performance of MOFs in C–H bond activation reactions.

The enzyme-like catalytic hydrogen abstraction reaction of cyclic hydrocarbons.

Metal-Organic Framework (MOF)-Based Materials as Heterogeneous Catalysts for C-H Bond Activation

Converting light hydrocarbons such as methane, ethane, propane, and cyclohexane into value-added chemicals and fuel products by means of direct C-H functionalization is an attractive method in the petrochemical industry. As they emerge as a relatively new class of porous solid materials, metal-organic frameworks (MOFs) are appealing as single-site heterogeneous catalysts or catalytic supports for C-H bond activation. In contrast to the traditional microporous and mesoporous materials, MOFs feature high porosity, functional tunability, and molecular-level characterization for the study of structure-property relationships. These virtues make MOFs ideal platforms to develop catalysts for C-H activation with high catalytic activity, selectivity, and recyclability under relatively mild reaction conditions. This review highlights the research aimed at the implementation of MOFs as single-site heterogeneous catalysts for C-H bond activation. It provides insight into the rational design and synthesis of three types of stable MOF catalysts for C-H bond activation, that is, i) metal nodes as catalytic sites, ii) the incorporation of catalytic sites into organic struts, and iii) the incorporation of catalytically active guest species into pores of MOFs. Here, the rational design and synthesis of MOF catalysts that lead to the distinct catalytic property for C-H bond activation are discussed along with the post-synthesis of MOFs, intriguing functions with MOF catalysts, and microenvironments that lead to the distinct catalytic properties of MOF catalysts.

MIL-100Cr with open Cr sites for a record N2O capture

Nitrous oxide (N2O) is considered as the third most important greenhouse gas after carbon dioxide and methane and needs to be removed from air. Herein, we reported the metal-organic framework MIL-100Cr with open Cr sites for record N2O capture capacities of 5.78 mmol g-1 at 298 K and 8.25 mmol g-1 at 273 K, respectively. DFT calculations showed that the static binding energy of N2O on the open-Cr site is notably higher than that of N2, 72.5 kJ mol-1vs. 51.6 kJ mol-1, which enforces MIL-100Cr to exhibit extremely high N2O/N2 ideal adsorbed solution theory (IAST) gas separation selectivity up to 1000.

Lewis Acid-Mediated One-Electron Reduction of Nitrous Oxide

The one-electron reduction of nitrous oxide (N₂ O) was achieved using strong Lewis acids E(C₆ F₅ )₃ (E=B or Al) in combination with metallocenes. In the case of B(C₆ F₅ )₃ , electron transfer to N₂ O required a powerful reducing agent such as Cp*₂ Co (Cp*=pentamethylcyclopentadienyl). In the presence of Al(C₆ F₅ )₃ , on the other hand, the reactions could be performed with weaker reducing agents such as Cp*₂ Fe or Cp₂ Fe (Cp=cyclopentadienyl). The Lewis acid-mediated electron transfer from the metallocene to N₂ O resulted in cleavage of the N-O bond, generating N₂ and the oxyl radical anion [OE(C₆ F₅ )₃ ]^⋅- . The latter is highly reactive and engages in C-H activation reactions. It was possible to trap the radical by addition of the Gomberg dimer, which acts as a source of the trityl radical.

Well-Defined Rhodium-Gallium Catalytic Sites in a Metal-Organic Framework: Promoter-Controlled Selectivity in Alkyne Semihydrogenation to E-Alkenes

Promoters are ubiquitous in industrial heterogeneous catalysts. The wider roles of promoters in accelerating catalysis and/or controlling selectivity are, however, not well understood. A model system has been developed where a heterobimetallic active site comprising an active metal (Rh) and a promoter ion (Ga) is preassembled and delivered onto a metal-organic framework (MOF) support, NU-1000. The Rh-Ga sites in NU-1000 selectively catalyze the hydrogenation of acyclic alkynes to E-alkenes. The overall stereoselectivity is complementary to the well-known Lindlar's catalyst, which generates Z-alkenes. The role of the Ga in promoting this unusual selectivity is evidenced by the lack of semihydrogenation selectivity when Ga is absent and only Rh is present in the active site.

Ethane/ethylene separation in a metal-organic framework with iron-peroxo sites

The separation of ethane from its corresponding ethylene is an important, challenging, and energy-intensive process in the chemical industry. Here we report a microporous metal-organic framework, iron(III) peroxide 2,5-dioxido-1,4-benzenedicarboxylate [Fe2(O2)(dobdc) (dobdc4−: 2,5-dioxido-1,4-benzenedicarboxylate)], with iron (Fe)–peroxo sites for the preferential binding of ethane over ethylene and thus highly selective separation of C2H6/C2H4. Neutron powder diffraction studies and theoretical calculations demonstrate the key role of Fe-peroxo sites for the recognition of ethane. The high performance of Fe2(O2)(dobdc) for the ethane/ethylene separation has been validated by gas sorption isotherms, ideal adsorbed solution theory calculations, and simulated and experimental breakthrough curves. Through a fixed-bed column packed with this porous material, polymer-grade ethylene (99.99% pure) can be straightforwardly produced from ethane/ethylene mixtures during the first adsorption cycle, demonstrating the potential of Fe2(O2)(dobdc) for this important industrial separation with a low energy cost under ambient conditions.

Two-Dimensional Metal-Organic Layers on Carbon Nanotubes to Overcome Conductivity Constraint in Electrocatalysis

Application of metal-organic frameworks (MOFs) in electrocatalysis is of great interest, but is limited by low electrical conductivities of most MOFs. To overcome this limitation, we constructed a two-dimensional version of MOF-metal-organic layer (MOL) on conductive multiwalled carbon nanotubes (CNTs) via facile solvothermal synthesis. The redox-active MOLs supported on the CNT efficiently catalyze the electrochemical oxidation of alcohols to aldehydes and ketones. Interestingly, this CNT/MOL assembly also endowed the selectivity for primary versus secondary alcohols via well-designed interfacial interactions. This work opens doors toward a variety of designer electrocatalysts built from functional MOFs.

Lewis basicity generated by localised charge imbalance in noble metal nanoparticle-embedded defective metal-organic frameworks

Interactions between metal nanoparticles (NPs) and metal–organic frameworks (MOFs) in their composite forms have proven to exhibit beneficial properties, such as enhanced catalytic performance through synergistic effects. Herein, we show that Lewis basic sites can be created within an anionic defective MOF by engineering the electronic state of the pendant carboxylate groups situated at the defect sites. This is achieved from the concerted interactions between the pendant carboxylate groups, embedded Pd NPs and charge-balancing cations (Mⁿ⁺ = Ce³⁺, Co²⁺, Ni²⁺, Cu²⁺, Mg²⁺, Li⁺, Na⁺ or K⁺). This work is the first example of generating a new collective property, i.e. Lewis basicity, in metal-carboxylate MOFs. Importantly, the choice of Mⁿ⁺, used during cation exchange, acts as a convenient parameter to tune the Lewis basicity of the MOF-based nanocomposites. It also provides a facile way to incorporate active metal sites and basic sites within carboxylate-based MOFs to engineer multifunctional nanocatalysts.

Interactions between metal nanoparticles (NPs) and metal–organic frameworks (MOFs) in their composite forms have proven to exhibit beneficial properties. Here the authors present a unique approach to immobilise Pd NPs and, more importantly, to generate tunable basic sites within an anionic defective MOF.

Interfacial coupling between noble metal nanoparticles and metal-organic frameworks for enhanced catalytic activity

Metal-organic frameworks (MOFs) have great potential to become innovative heterogeneous supports for immobilizing catalytically active noble metal nanoparticles (NPs). However, unlike metal oxide supports, the interfacial interactions between noble metal NPs and MOFs are currently neglected, thus dramatically diminishing the advantage of MOFs as supports. Herein, ZIFs(Co/Zn)@M (M = Pd, Pt or Au) nanocomposites with well-defined interfaces are synthesized and used as catalysts in gas-phase CO oxidation and liquid-phase C6H5CHO oxidation. Notably, in both reactions, ZIF-67(Co)@M samples exhibit better catalytic activity than ZIF-8(Zn)@M samples, and moreover, the catalytic activity of ZIFs@Pd is higher than that of ZIFs@Pt and ZIFs@Au samples. Experimental and theoretical results reveal that the enhanced catalytic activity originates from the interfacial electron transfer from ZIFs to noble metal NPs as well as the coupling between d band of noble metal in NPs and metal node in ZIFs.

Enhancement of CO2 binding and mechanical properties upon diamine functionalization of M2(dobpdc) metal-organic frameworks

The family of diamine-appended metal-organic frameworks exemplified by compounds of the type mmen-M2(dobpdc) (mmen = N,N'-dimethylethylenediamine; M = Mg, Mn, Fe, Co, Zn; dobpdc4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) are adsorbents with significant potential for carbon capture, due to their high working capacities and strong selectivity for CO2 that stem from a cooperative adsorption mechanism. Herein, we use first-principles density functional theory (DFT) calculations to quantitatively investigate the role of mmen ligands in dictating the framework properties. Our van der Waals-corrected DFT calculations indicate that electrostatic interactions between ammonium carbamate units significantly enhance the CO2 binding strength relative to the unfunctionalized frameworks. Additionally, our computed energetics show that mmen-M2(dobpdc) materials can selectively adsorb CO2 under humid conditions, in agreement with experimental observations. The calculations further predict an increase of 112% and 124% in the orientationally-averaged Young's modulus E and shear modulus G, respectively, for mmen-Zn2(dobpdc) compared to Zn2(dobpdc), revealing a dramatic enhancement of mechanical properties associated with diamine functionalization. Taken together, our calculations demonstrate how functionalization with mmen ligands can enhance framework gas adsorption and mechanical properties.

The role of reticular chemistry in the design of CO2 reduction catalysts

The problem with current state-of-the-art catalysts for CO₂ photo- or electroreduction is rooted in the notion that no single system can independently control, and thus optimize, the interplay between activity, selectivity and efficiency. At its core, reticular chemistry is recognized for its ability to control, with atomic precision, the chemical and structural features (activity and selectivity) as well as the output optoelectronic properties (efficiency) of porous, crystalline materials. The molecular building blocks that are in a reticular chemist's toolbox are chosen in such a way that the structures are rationally designed, framework chemistry is performed to integrate catalytically active components, and the manner in which these building blocks are connected endows the material with the desired optoelectronic properties. The fact that these aspects can be fine-tuned independently lends credence to the prospect of reticular chemistry contributing to the design of next-generation CO₂ reduction catalysts.

Metal-organic and covalent organic frameworks as single-site catalysts

Heterogeneous single-site catalysts consist of isolated, well-defined, active sites that are spatially separated in a given solid and, ideally, structurally identical. In this review, the potential of metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) as platforms for the development of heterogeneous single-site catalysts is reviewed thoroughly. In the first part of this article, synthetic strategies and progress in the implementation of such sites in these two classes of materials are discussed. Because these solids are excellent playgrounds to allow a better understanding of catalytic functions, we highlight the most important recent advances in the modelling and spectroscopic characterization of single-site catalysts based on these materials. Finally, we discuss the potential of MOFs as materials in which several single-site catalytic functions can be combined within one framework along with their potential as powerful enzyme-mimicking materials. The review is wrapped up with our personal vision on future research directions.

Separation of Xylene Isomers through Multiple Metal Site Interactions in Metal-Organic Frameworks

Purification of the C8 alkylaromatics o-xylene, m-xylene, p-xylene, and ethylbenzene remains among the most challenging industrial separations, due to the similar shapes, boiling points, and polarities of these molecules. Herein, we report the evaluation of the metal-organic frameworks Co2(dobdc) (dobdc4- = 2,5-dioxido-1,4-benzenedicarboxylate) and Co2( m-dobdc) ( m-dobdc4- = 4,6-dioxido-1,3-benzenedicarboxylate) for the separation of xylene isomers using single-component adsorption isotherms and multicomponent breakthrough measurements. Remarkably, Co2(dobdc) distinguishes among all four molecules, with binding affinities that follow the trend o-xylene > ethylbenzene > m-xylene > p-xylene. Multicomponent liquid-phase adsorption measurements further demonstrate that Co2(dobdc) maintains this selectivity over a wide range of concentrations. Structural characterization by single-crystal X-ray diffraction reveals that both frameworks facilitate the separation through the extent of interaction between each C8 guest molecule with two adjacent cobalt(II) centers, as well as the ability of each isomer to pack within the framework pores. Moreover, counter to the presumed rigidity of the M2(dobdc) structure, Co2(dobdc) exhibits an unexpected structural distortion in the presence of either o-xylene or ethylbenzene that enables the accommodation of additional guest molecules.

Structural and dynamic studies of substrate binding in porous metal-organic frameworks

Porous metal-organic frameworks (MOFs) are the subject of considerable research interest because of their high porosity and capability of specific binding to small molecules, thus underpinning a wide range of materials functions such as gas adsorption, separation, drug delivery, catalysis, and sensing. MOFs, constructed by the designed assembly of metal ions and functional organic linkers, are an emerging class of porous materials with extended porous structures containing periodic binding sites. MOFs thus provide a new platform for the study of the chemistry and reactivity of small molecules in confined pores using advanced diffraction and spectroscopic techniques. In this review, we focus on recent progress in experimental investigations on the crystallographic, dynamic and kinetic aspects of substrate binding within porous MOFs. In particular, we focus on studies on host-guest interactions involving open metal sites or pendant functional groups in the pore as the primary binding sites for guest molecules.