Bridging Homogeneous and Heterogeneous Catalysis: Phosphine-Functionalized Metal-Organic Frameworks

Phosphine-functionalized metal-organic frameworks (P-MOFs) as an emerging class of coordination polymers, have provided novel opportunities for the development of heterogeneous catalysts. Yet, compared with the ubiquitous phosphine systems in homogeneous catalysis, heterogenization of phosphines in MOFs is still at its early stage. In this Minireview, we summarize the synthetic strategies, characterization and catalytic reactions based on the P-MOFs reported in literature. In particular, various catalytic reactions are discussed in detail in terms of phosphine ligand structure-function relationship, including the potential obstacles for future development. Finally, we discuss the possible solutions, including new types of reactions and techniques as the perspectives for the development of P-MOF catalysts, highlighting the opportunities and challenges.

Recent developments in MIL-101 metal organic framework for heterogeneous catalysis

Metal organic frameworks (MOFs) are currently gaining considerable attention as heterogeneous catalysts. Since the functionality of the framework and the pore size of the MOFs can be adjusted over a wide range for various catalytic reactions, the usage of these materials as solid catalysts is attractive. One of the preferred catalytic mesoMOFs is MIL-101 (MIL: Material of Institute Lavoisier) family which has been mainly investigated. The large surface area, high pore volumes, and acceptable solvent/thermal stability (MIL-101(Cr) up to 300 °C) have led the MIL-101 family to be considered an ideal and widespread MOF for use as a great heterogeneous catalyst or solid support for a variety of reactions. The objective of this review is to present recent research on the use of the MIL-101 family for heterogeneous catalysis in gas and liquid phase reactions.

Introducing High Density of Very Active Sites and Stepwise Postmodification for Tailoring the Porosity of Highly Demanding Cr3+-Based Metal-Organic Frameworks

Chromium(III)-based metal-organic frameworks (Cr-MOFs) are highly robust and porous and have been very attractive in a wide range of investigations. However, the harsh direct synthetic conditions not only impede the synthesis of new Cr-MOFs but also restrict the introduction of functional groups into them. Postsynthetic modification has somewhat alleviated such difficulties; nevertheless, it still suffered from procedures that are tedious and conditions that are not mild, which often result in low concentration of the functional groups introduced. To overcome these shortcomings, here, in this paper, we supplied a new route and prepared a benzyl alcohol functionalized Cr-SXU-2 from the judiciously designed benzyl alcohol functionalized Fe-SXU-2 through solvent-assisted metal metathesis strategy. The functionalized Cr-SXU-2 shows well-preserved crystallinity, porosity, and high chemical stability. The benzyl alcohol group can be converted into a very active benzyl bromide group in an almost quantitative yield and thus for the first time produce the benzyl bromide functionalized MOF, Cr-SXU-2-Br, in which the -Br group can be exchanged by a nucleophilic group. As a proof of concept, -N₃ was introduced and transformed into other active sites via "click reaction" to further tailor the interior of Cr-SXU-2. All these functionalized Cr-MOFs showed improved adsorption performance in contrast to the nonfunctionalized one. This step-by-step postmodification process not only diversifies the functionalization of robust MOFs but also opens a new route to employ many different functional groups in the demanding highly stable Cr-MOF platforms.

Copper-free Sonogashira cross-coupling reactions: an overview

The Sonogashira reaction is a cross-coupling reaction of a vinyl or aryl halide with a terminal alkyne to form a C-C bond. In its original form, the Sonogashira reaction is performed with a palladium species as a catalyst while co-catalyzed by a copper species and a phosphine or amine. The reaction is conducted under mild conditions, i.e., room temperature, aqueous solutions, and the presence of mild bases. Undeniably, the Sonogashira reaction is among the most competent and efficient reactions widely used in organic synthesis. This named reaction has proved useful in many organic synthesis areas, including the synthesis of pharmaceuticals, heterocycles, natural products, organic compounds, complex molecules having biological activities, nanomaterials, and many more materials that we use in our daily lives. The presence of transition metals as a catalyst was indeed essential in the Sonogashira reaction. However, recently, the reaction has been successfully conducted without copper as a co-catalyst and phosphines or amines as bases. In this critical review, we have focused on developments in the Sonogashira reaction successfully performed in the absence of copper complexes, phosphines or amines, which could be of particular advantage in implementing green chemistry principles and making the reactions more achievable from an economic viewpoint.

High-Performance CO2-Selective Hybrid Membranes by Exploiting MOF-Breathing Effects

Conventional CO₂ separation in the petrochemical industry via cryogenic distillation or amine-based absorber-stripper units is energy-intensive and environmentally unfriendly. Membrane-based gas separation technology, in contrast, has contributed significantly to the development of energy-efficient systems for processes such as natural gas purification. The implementation of commercial polymeric membranes in gas separation processes is restricted by their permeability-selectivity trade-off and by their insufficient thermal and chemical stability. Herein, we present the fabrication of a Matrimid-based membrane loaded with a breathing metal-organic framework (MOF) (NH₂-MIL-53(Al)) which is capable of separating binary CO₂/CH₄ gas mixtures with high selectivities without sacrificing much of its CO₂ permeabilities. NH₂-MIL-53(Al) crystals were embedded in a polyimide (PI) matrix, and the mixed-matrix membranes (MMMs) were treated at elevated temperatures (up to 350 °C) in air to trigger PI cross-linking and to create PI-MOF bonds at the interface to effectively seal the grain boundary. Most importantly, the MOF transitions from its narrow-pore form to its large-pore form during this treatment, which allows the PI chains to partly penetrate the pores and cross-link with the amino functions at the pore mouth of the NH₂-MIL-53(Al) and stabilizes the open-pore form of NH₂-MIL-53(Al). This cross-linked MMM, with MOF pore entrances was made more selective by the anchored PI-chains and achieves outstanding CO₂/CH₄ selectivities. This approach provides significant advancement toward the design of selective MMMs with enhanced thermal and chemical stabilities which could also be applicable for other potential applications, such as separation of hydrocarbons (olefin/paraffin or isomers), pervaporation, and solvent-resistant nanofiltration.

Suspending Ion Electrocatalysts in Charged Metal-Organic Frameworks to Improve the Conductivity and Selectivity in Electroorganic Synthesis

Electroorganic synthesis is an environmentally friendly alternative to traditional synthetic methods; however, the application of this strategy is heavily hindered by low product selectivity. Metal-organic frameworks (MOFs) exhibit high selectivity in numerous catalytic reactions; however, poor conductivity heavily limits the application of MOFs in electroorganic synthesis. To realize the electrocatalytic application of MOFs in selective electroorganic synthesis, a practically applicable strategy by suspending ion electrocatalysts in charged MOFs is herein reported. This approach could markedly improve the product selectivity in electroorganic synthesis. In the electrocatalytic oxidative self-coupling of benzylamine experiments, the imine product selectivity is markedly improved from 61.3 to 94.9 %, when the MOF-based electrocatalyst is used instead of the corresponding homogeneous electrocatalyst under the identical conditions. Therefore, this work opens a new route to improve the product selectivity in electroorganic synthesis.

Light-driven hydrogen evolution catalyzed by a cobaloxime catalyst incorporated in a MIL-101(Cr) metal-organic framework

A cobaloxime H₂ evolution catalyst with a hydroxo-functionalized pyridine ligand, Co(dmgH)₂(4-HEP)Cl [dmgH = dimethylglyoxime, 4-HEP = 4-(2-hydroxyethyl)pyridine] was immobilized on a chromium terephthalate metal-organic framework (MOF), MIL-101(Cr), to construct a MOF-catalyst hybrid which displays good photocatalytic H₂ evolution activity. The longevity of the cobaloxime catalyst is increased by MOF incorporation, but limited by the stability of the cobalt-pyridine bond under turnover conditions.

Highly conductive and robust composite anion exchange membranes by incorporating quaternized MIL-101(Cr)

With well-defined channels and tunable functionality, metal-organic frameworks (MOFs) have inspired the design of a new class of ion-conductive compounds. In contrast to the extensive studies on proton-conductive MOFs and related membranes attractive for fuel cells, rare reports focus on MOFs in preparation of anion exchange membranes. In this study, chloromethylated MIL-101(Cr) was prepared and incorporated into chloromethylated poly (ether ether ketone) (PEEK) as a multifunctional filler to prepare imidazolium PEEK/imidazolium MIL-101(Cr) (ImPEEK/ImMIL-101(Cr)) anion exchange membrane after synchronous quaternization. The successful synthesis and chloromethylation of MIL-101(Cr) were verified by transmission electron microscopy, X-ray diffraction and Fourier transform infrared spectroscopy while the enhanced performance of composite membranes in hydroxide conductivity, mechanical strength and dimensional stability were evaluated by alternating-current impedance, electronic stretching machine and measurement of swelling ratio. Specifically, incorporating 5.0wt% ImMIL-101(Cr) afforded a 71.4% increase in hydroxide conductivity at 20°C, 100% RH. Besides, the composite membranes exhibited enhanced dimensional stability and mechanical strength due to the rigid framework of ImMIL-101(Cr). At room temperature and the ImMIL-101(Cr) content of 10wt%, the swelling ratio of the ImPEEK/ImMIL-101(Cr) was 70.04% lower while the tensile strength was 47.5% higher than that of the pure membrane.

Phosphine and phosphine oxide groups in metal-organic frameworks detected by P K-edge XAS

Phosphine metal-organic frameworks (P-MOFs) are crystalline porous coordination polymers that contain phosphorus functional groups within their pores. We present the use of X-ray absorption spectroscopy (XAS) at the P K-edge to determine the phosphine to phosphine oxide ratio in two P-MOFs with MIL-101 topology. The phosphorus oxidation state is of particular interest as it strongly influences the coordination affinity of these materials for transition metals. This method can determine the oxidation state of phosphorus even when the material contains paramagnetic nuclei, differently from NMR spectroscopy. We observed that phosphine in LSK-15 accounts for 72 ± 4% of the total phosphorus groups and that LSK-12 contains only phosphine oxide.

Post‐synthetic Modification of MOFs

Post‐synthetic modification is increasingly recognised as an important and versatile tool in the preparation of functionalised metal organic frameworks (MOFs). The process involves one or more reactions on a pre‐formed MOF, and it can be used to prepare MOFs that are not accessible by direct combination of metal and linker. This review explores the methods and strategies that have been developed for post‐synthetically modifying MOFs, concentrating on four classes of reaction: covalent transformations of the linker, coordination of a metal centre to a linker, modification of the inorganic part of the MOF and exchange of counter‐ions. Examples of the use of the modified MOFs are given, with a focus on their utility in catalysis.