Metal-organic Framework Catalyst Research Articles

MIL-101(Cr)/aminoclay nanocomposites for conversion of CO2 into cyclic carbonates.

We present the use of an amine functionalized two-dimensional clay i.e., aminoclay (AC), in the chemistry of a three-dimensional metal-organic framework (MOF) i.e., MIL-101(Cr), to prepare MIL-101(Cr)/AC composites, which are exploited as catalysts for efficient conversion of CO2 gas into cyclic carbonates under ambient reaction conditions. Three different MOF nanocomposites, denoted as MIL-101(Cr)/AC-1, MIL-101(Cr)/AC-2, and MIL-101(Cr)/AC-3, were synthesized by an in situ process by adding different amounts of AC to the precursor solutions of the MIL-101(Cr). The composites were characterized by various techniques such as FT-IR, PXRD, FESEM, EDX, TGA, N2 adsorption, as well as CO2 and NH3-TPD measurements. The composites were exploited as heterogeneous catalysts for CO2 cycloaddition reactions with different epoxides and the catalytic activity was investigated at atmospheric pressure under solvent-free conditions. Among all the materials, MIL-101(Cr)/AC-2 shows the best catalytic efficiency under the optimized conditions and exhibits enhanced efficacy compared to various MIL-101(Cr)-based MOF catalysts, which typically need either high temperature and pressure or a longer reaction time or a combination of all the parameters. The present protocol using MIL-101(Cr)/AC-2 as the heterogeneous catalyst gives 99.9% conversion for all the substrates into the products at atmospheric pressure.

Sustainable process intensification: Ultrasonic preparation of MIL-88A for benzoxazole synthesis

This study provides a biodegradable fumaric acid-based MIL-88A metal-organic framework, a cost-effective and non-toxic catalyst, for biologically active benzoxazoles synthesis. Initially, the ultrasonic approach is used for MIL-88A preparation at room temperature, significantly reducing the synthesis time compared to conventional heating protocols. The ultrasonic approach is once again used for the three-component production of benzoxazoles in the presence of MIL-88A catalyst. Subsequently, the benzoxazole reaction is accelerated and yields are also improved. Impressive yields ranging between 84 % and 96 % were obtained within one hour at room temperature for the three-component synthesis of benzoxazoles. Characterization techniques, including FT-IR, TEM, XRD, and SEM-EDS mapping, confirm the successful synthesis of MIL-88A. Moreover, a large-scale reaction for benzoxazole synthesis can be performed with minimal yield loss, indicating its practical application. Notably, fumaric acid, found in many fruits and vegetables, serves as the precursor for catalyst synthesis. Catechol, sourced from various plant sources, is employed for three-component production of benzoxazoles. By integrating sustainable techniques, including ultrasound-assisted synthesis and naturally available substrates, an innovative strategy was developed utilizing a renewable catalyst for an eco-friendly reaction.

Engineering the Activity and Stability of the Zn-MOF Catalyst via the Interaction of Doped Zr with Zn.

Metallic atoms within metal-organic framework (MOF) materials exhibit a distinctive and adaptable coordination structure. The three-dimensional (3D) pore configuration of MOFs enables the complete exposure of metal active sites, rendering them prevalent in various catalytic reactions. In this study, zinc (Zn) atoms within Zn-based MOF materials, characterized by an abundance of valence electrons, are utilized for the transesterification of dimethyl carbonate (DMC). Additionally, the introduction of zirconium (Zr) effectively addresses the susceptibility of the MOFs' crystal structure to dissolution in organic solvents. The formulated catalyst, Zn-10%Zr-MOF(300), demonstrates remarkable catalytic performance with 91.5% DMC selectivity, 61.9% propylene carbonate (PC) conversion, and 56.6% DMC yield. Impressively, the catalyst maintains its high performance over five cycles. Results indicate that Zr interacts with Zn, forming new coordination bonds and enhancing the catalyst crystal structure stability. Moreover, electron transfer intensifies the alkalinity of the active Zn atoms, enhancing the overall catalyst performance. This research informs the development of transesterification heterogeneous catalysts and broadens the application scope of MOF catalysts.

Tuning Two-Dimensional Phthalocyanine Dual Site Metal-Organic Framework Catalysts for the Oxygen Reduction Reaction.

Metal-organic frameworks (MOFs) offer an interesting opportunity for catalysis, particularly for metal-nitrogen-carbon (M-N-C) motifs by providing an organized porous structural pattern and well-defined active sites for the oxygen reduction reaction (ORR), a key need for hydrogen fuel cells and related sustainable energy technologies. In this work, we leverage electrochemical testing with computational models to study the electronic and structural properties in the MOF systems and their relationship to ORR activity and stability based on dual transitional metal centers. The MOFs consist of two M1 metals with amine nodes coordinated to a single M2 metal with a phthalocyanine linker, where M1/M2 = Co, Ni, or Cu. Co-based metal centers, in particular Ni-Co, demonstrate the highest overall activity of all nine tested MOFs. Computationally, we identify the dominance of Co sites, relative higher importance of the M2 site, and the role of layer M1 interactions on the ORR activity. Selectivity measurements indicate that M1 sites of MOFs, particularly Co, exhibit the lowest (<4%), and Ni demonstrates the highest (>46%) two-electron selectivity, in good agreement with computational studies. Direct in situ stability characterization, measuring dissolved metal ions, and calculations, using an alkaline stability metric, confirm that Co is the most stable metal in the MOF, while Cu exhibits notable instability at the M1. Overall, this study reveals how atomistic coupling of electronic and structural properties affects the ORR performance of dual site MOF catalysts and opens new avenues for the tunable design and future development of these systems for practical electrochemical applications.

Tailored Metal-Organic Frameworks for Water Purification: Perfluorinated Fe-MOFs for Enhanced Heterogeneous Catalytic Ozonation.

An industrially viable catalyst for heterogeneous catalytic ozonation (HCO) in water purification requires the characteristics of good dispersion of active species on its surface, efficient electron transfer for ozone decay, and maximum active species utilization. While metal-organic frameworks (MOFs) represent an attractive platform for HCO, the metal nodes in the unmodified MOFs exhibit low catalytic activity. Herein, we present a perfluorinated Fe-MOF catalyst by substituting H atoms on the metalated ligands with F atoms (termed 4F-MIL-88B) to induce structure evolution. The Lewis acidity of 4F-MIL-88B was enhanced via the formation of Fe nodes, tailoring the electron distribution on the catalyst surface. As a result of catalyst modification, the rate constant for degradation of the target compounds examined increased by ∼700% compared with that observed for the unmodified catalyst. Experimental evidence and theoretical calculations showed that the modulated polarity and the enhanced electron transfer between the catalyst and ozone molecules contributed to the adsorption and transformation of O3 to •OH on the catalyst surface. Overall, the results of this study highlight the significance of tailoring the metalated ligands to develop highly efficient and stable MOF catalysts for HCO and provide an in-depth mechanistic understanding of their structure-function evolution, which is expected to facilitate the applications of nanomaterial-based processes in water purification.

In-situ electrochemical activation of a nickel metal-organic framework modified TiO2 photoanode for urea photoelectrocatalysis

Metal-organic frameworks (MOFs) possess a large specific surface area, porosity, and structural diversity, making them ideal candidates for catalysing the urea oxidation reaction (UOR). Even though MOF-based catalysts have been broadly investigated in urea electrocatalysis, few MOF catalysts are used for urea photoelectrocatalysis. The latter requires the MOF-based catalysts to efficiently separate and transport the photogenerated holes at the interface of photoanode-electrolyte junctions. In this study, a sulphur-containing organic ligand of 1,4-benzenedithiol (BDT) is innovatively employed to synthesize two-dimensional Ni-BDT MOFs from Ni(OH)2 nanosheets precursor. After the in-situ electrochemical activation of TiO2@Ni-BDT, the resultant MOF derivative modified TiO2 photoanode named as TiO2@Ni-BDT-A exhibits 3.83 times photocurrent in comparison with the pristine TiO2. A high photovoltage of 0.73 V and an impressive 82 % photocurrent retention after a long-term stability test can also be observed. The improvements are attributed to the enhanced surface active sites and the active species of Ni(2−σ)+−SOx, which can efficiently promote charge separation, increase conductivity, improve donor concentration, and lower the charge transfer resistance. This work develops a sulphur-containing Ni-MOF as a co-catalyst for efficient urea photoelectrocatalysis, and an effective strategy of in-situ electrochemical activation to boost photoanode activity.

Plasma-Driven Efficient Conversion of CO2 and H2O into Pure Syngas with Controllable Wide H2/CO Ratios over Metal-Organic Frameworks Featuring In SituEvolved Ligand Defects.

While the mild production of syngas (a mixture of H2 and CO) from CO2 and H2O is a promising alternative to the coal-based chemical engineering technologies, the inert nature of CO2 molecules, unfavorable splitting pathways of H2O and unsatisfactory catalysts lead to the challenge in the difficult integration of high CO2 conversion efficiency with produced syngas with controllable H2/CO ratios in a wide range. Herein, we report an efficient plasma-driven catalytic system for mild production of pure syngas over porous metal-organic framework (MOF) catalysts with rich confined H2O molecules, where their syngas production capacity is regulated by the in situ evolved ligand defects and the plasma-activated intermediate species of CO2 molecules. Specially, the Cu-based catalyst system achieves 61.9 % of CO2 conversion and the production of pure syngas with wide H2/CO ratios of 0.05:1-4.3:1. As revealed by the experimental and theoretical calculation results, the in situ dynamic structure evolution of Cu-containing MOF catalysts favors the generation of coordinatively unsaturated metal active sites with optimized geometric and electronic characteristics, the adsorption of reactants, and the reduced energy barriers of syngas-production potential-determining steps of the hydrogenation of CO2 to *COOH and the protonation of H2O to *H.

Bimetallic Center Metal–Organic Framework Catalysts for the Two-Electron Oxygen Reduction Reaction

Bimetallic Center Metal–Organic Framework Catalysts for the Two-Electron Oxygen Reduction Reaction

Plasma‐Driven Efficient Conversion of CO2 and H2O into Pure Syngas with Controllable Wide H2/CO Ratios over Metal‐Organic Frameworks Featuring In Situ Evolved Ligand Defects

While the mild production of syngas (a mixture of H2 and CO) from CO2 and H2O is a promising alternative to the coal‐based chemical engineering technologies, the inert nature of CO2 molecules, unfavorable splitting pathways of H2O and unsatisfactory catalysts lead to the challenge in the difficult integration of high CO2 conversion efficiency with produced syngas with controllable H2/CO ratios in a wide range. Herein, we report an efficient plasma‐driven catalytic system for mild production of pure syngas over porous metal‐organic framework (MOF) catalysts with rich confined H2O molecules, where their syngas production capacity is regulated by the in situ evolved ligand defects and the plasma‐activated intermediate species of CO2 molecules. Specially, the Cu‐based catalyst system achieves 61.9 % of CO2 conversion and the production of pure syngas with wide H2/CO ratios of 0.05:1‐4.3:1. As revealed by the experimental and theoretical calculation results, the in situ dynamic structure evolution of Cu‐containing MOF catalysts favors the generation of coordinatively unsaturated metal active sites with optimized geometric and electronic characteristics, the adsorption of reactants, and the reduced energy barriers of syngas‐production potential‐determining steps of the hydrogenation of CO2 to *COOH and the protonation of H2O to *H.

A Bioinspired Copper-Pair Catalyst in Metal-Organic Frameworks for Molecular Dioxygen Activation and Aerobic Oxidative C-N Coupling.

Open Cu sites were loaded to the UiO-67 metal-organic framework (MOF) skeleton by introduction of flexible Cu-binding pyridylmethylamine (pyma) side chains to the biphenyldicarboxylate linkers. Distance between Cu centers in the MOF pores was tuned by controlling the density of metal-binding side chains. "Interacted" Cu-pair or "isolated" monomeric Cu sites were achieved with high and low (pyma)Cu side chain loading, respectively. Spectroscopic and theoretical studies indicate that "interacted" Cu pairs can effectively bind and activate molecular dioxygen to form Cu2O2 clusters, which showed high catalytic activity for aerobic oxidative C-N coupling. On the contrary, MOF catalyst bearing isolated monomeric Cu sites only showed modest catalytic activity. Enhancement in catalytic performance for the Cu-pair catalyst is attributed to the remote synergistic effect of the paired Cu site, which binds molecular dioxygen and cleaves the O═O bond in a collaborative manner. This work demonstrates that noncovalently interacted metal-pair sites can effectively activate inert small molecules and promote heterogeneous catalytic processes.

Efficient green synthesis of n-amyl acetate in liquid phase over metal-organic framework catalysts

This investigation describes, for the first time, the efficient and selective liquid-phase green synthesis of n-amyl acetate over UiO-66 and NH2-UiO-66 metal-organic framework catalysts. By employing UiO-66 and NH2-UiO-66 solid acid catalysts, n-amyl acetate can be produced without the corrosive impact and separation problems associated with homogeneous catalysis by mineral acids. Thermal, structural, textural, and morphological properties of the catalysts were evaluated by TG-DTA, XRD, FTIR, Raman spectroscopy, BET-surface area, SEM and HRTEM. Their surface acidities were characterized by the dehydration of isopropanol and the chemical adsorption of suitable probs. The effects of reflux time, acid: alcohol molar ratio, and catalyst load were extensively studied. Under the optimized conditions, UiO-66, and NH2-UiO-66 catalysts, respectively offered 85 and 67 % conversions, and 100 % selectivity into n-amyl acetate. Activity of catalysts was well correlated with the total number of acidic sites, SBET and the kinetic rate constants of the esterification reaction. Analysis of the pre-adsorption studies of the reactants on the catalyst surface demonstrated that the Langmuir-Hinshelwood mechanism was followed in this reaction. The UiO-66 catalyst was regenerated numerous times with nearly the same activity and 100 % selectivity.

Tracking the role of compressive strain in bowl-Like Co-MOFs structural evolution in water oxidation reaction

Dynamic structural evolution of catalyst active sites is crucial to for transition metal-catalyzed oxygen evolution reaction (OER). Yet, the elucidation of their mechanism behind the compressive-strain-induced reconstruction during OER process remains enigmatic. Here, we construct a bowl-like cobalt-based metal-organic frameworks (BL-Co-MOFs) catalyst with compressive strain to investigate the effect of strain on the structural evolution process and the OER mechanism. In situ techniques directly observe that compressive strain exists in the reconstruction of the potential-driven CoOOH-like active phase with contractile interatomic CoCo distance, promoting a direct OO radical coupling over the Co sites (Co−O−O−Co) during the OER process. This disrupts the constrains of the inherent linear scaling relationship and achieves the kinetic-faster oxide path mechanism (OPM) during OER process. Consequently, the BL-Co-MOFs exhibit a huge mass activity of 12516 A gmetal−1 and a large turnover frequency of 6888 h−1 at a low overpotential of 275 mV (300 mA cm−2).

Highly defective ultra-small tetravalent MOF nanocrystals

The size and defects in crystalline inorganic materials are of importance in many applications, particularly catalysis, as it often results in enhanced/emerging properties. So far, applying the strategy of modulation chemistry has been unable to afford high-quality functional Metal–Organic Frameworks (MOFs) nanocrystals with minimized size while exhibiting maximized defects. We report here a general sustainable strategy for the design of highly defective and ultra-small tetravalent MOFs (Zr, Hf) crystals (ca. 35% missing linker, 4–6 nm). Advanced characterizations have been performed to shed light on the main factors governing the crystallization mechanism and to identify the nature of the defects. The ultra-small nanoMOFs showed exceptional performance in peptide hydrolysis reaction, including high reactivity, selectivity, diffusion, stability, and show emerging tailorable reactivity and selectivity towards peptide bond formation simply by changing the reaction solvent. Therefore, these highly defective ultra-small M(IV)-MOFs particles open new perspectives for the development of heterogeneous MOF catalysts with dual functions.

Modulating 3d Charge State via Halogen Ions in Neighboring Molecules of Metal-Organic Frameworks for Improving Water Oxidation.

Modulating the coordination environment of the metal active center is an effective method to boost the catalytic performances of metal-organic frameworks (MOFs) for oxygen evolution reaction (OER). However, little attention has been paid to the halogen effects on the ligands engineering. Herein, a series of MOFs X─FeNi-MOFs (X = Br, Cl, and F) is constructed with different coordination microenvironments to optimize OER activity. Theoretical calculations reveal that with the increase in electronegativity of halogen ions in terephthalic acid molecular (TPA), the Bader charge of Ni atoms gets larger and the Ni-3d band center and O-2p bands move closer to the Fermi level. This indicates that an increase in ligand negativity of halogen ions in TPA can promote the adsorption ability of catalytic sites to oxygen-containing intermediates and reduce the activation barrier for OER. Experimental also demonstrates that F─FeNi-MOFs exhibit the highest catalytic activity with an ultralow overpotential of 218mV at 10mAcm-2, outperforming most otate-of-the-art Fe/Co/Ni-based MOFs catalysts, and the enhanced mass activity by seven times compared with that for the sample before ligands engineering. This work opens a new avenue for the realization of the modulation of NiFe─O bonding by halogen ion in TPA and improves the OER performance of MOFs.

Phosphine-incorporated Metal-Organic Framework for Palladium Catalyzed Heck Coupling Reaction.

As an emerging material with the potential to combine the high efficiency of homogeneous catalysts and high stability and recyclability of heterogeneous catalysts, metal-organic frameworks (MOFs) have been viewed as one of the candidates to produce catalysts of the next generation. Herein, we heterogenized the highly active mono(phosphine)-Pd complex on surface of UiO-66 MOF, as a catalyst for Suzuki and Heck cross coupling reactions. The successful immobilization of these Pd-monophosphine complexes on MOF surface to form UiO-66-PPh2-Pd was characterized and confirmed via comprehensive set of analytical methods. UiO-66-PPh2-Pd showed high activity and selectivity for both Suzuki and Heck Cross Coupling Reactions. This strategy enabled facile access to mono(phosphine) complexes which are challenging to design and require multistep synthesis in homogeneous systems, paving the way for future MOF catalysts applications by similar systems.

Recent advances on metal-organic frameworks (MOFs) and their applications in energy conversion devices: Comprehensive review

Metal-organic frameworks (MOFs) have enticed huge interest over the years in a wide range of applications, including electrochemical energy storage/conversion devices, due to their controllable porous structure, tuneable composition, excellent thermal/chemical stabilities, and facile synthesis. However, conductivity enhancement and synthesis of redox-active MOFs are two key challenges hindering their large-scale applications in electrochemistry. Redox-active MOFs can be prepared using redox-active ligands and metal ions, which in turn leads to an additional benefit of π-stacking interactions. Conductivity improvements through favourable overlap of energy and orientation of both metal and ligand, π-π stacking, and the incorporation of a guest molecule to induce free charge carriers and reduce band gaps are key strategies. This review provides a detailed assessment of various synthesis techniques followed by post-production treatments to improve MOF's conductivity. Advancements in computational methods & machine learning for MOFs as well as the econmics of MOF's production are introduced. The use of MOFs and MOF-based nanomaterials in batteries, supercapacitors, and fuel cells, as well as the progress in using MOF and MOF-based catalysts for CO2 reduction and as a photocatalyst for hydrogen production, have been scrutinized by highlighting their benefits and shortcomings. Finally, the challenges MOFs and MOF-based materials face and their prospects when adopted as active materials in energy storage/conversion devices, as well as CO2 reduction and green hydrogen production, have also been elaborated.

Copper-doped ZIF-67/wood aerogel for effective peroxymonosulfate activation

Metal-organic frameworks (MOFs), composed of metal ions and organic linkers, have garnered enormous attention in catalytic application due to their exceptional characteristics such as high specific surface area, low density, large porosity and structural diversity. However, the recycling and performance improvement of MOF should be taken into account in material design. In this work, we fabricated copper-doped metal-organic framework/wood aerogel composites to enhance the activation of peroxymonosulfate (PMS) for the degradation of organic contaminants. Wood Aerogel (WA) was used as suitable carrier to solve the recycling problem, and copper-doped ZIF-67 was developed to enhance the catalytic performance. Copper-doped ZIF-67 (ZIF-67B) was loaded into the cavities of a wood aerogel by in situ wet chemical precipitation, followed by freeze drying to obtain the composite. The effects of copper doping ratio, temperature, catalyst dosage, initial pH and other operational parameters on the degradation of organic pollutants were comprehensively investigated. The catalytic capacity of ZIF-67B@WA (CoxCu(1-x)) was investigated by activating PMS for the removal of Rhodamine B (RhB). The results demonstrate that the degradation efficiency of ZIF-67B@WA (CoxCu(1-x)) towards RhB can achieve 100 % within 15 min, showing exceptional catalytic performance compared with ZIF-67@WA. Even after four consecutive cycles, the degradation efficiency remains at 87.5 %, indicating that the catalyst has good cycle stability. Additionally, possible degradation mechanisms are proposed in this study, in which WA provides high loading active site/area for MOF catalyst and Cu doping can accelerate electron transfer and facilitate the redox cycling of Co2+ and Co3+, thereby enhancing catalytic efficiency. This work expands the application scope of MOF materials in environmental remediation and holds promising prospects.

Confinement Effect in Metal-Organic Framework Cu3(BTC)2 for Enhancing Shape Selectivity of Radical Difunctionalization of Alkenes.

The radical difunctionalization of alkenes plays a vital role in pharmacy, but the conventional homogeneous catalytic systems are challenging in selectivity and sustainability to afford the target molecules. Herein, the famous readily available metal-organic framework (MOF), Cu3(BTC)2, has been applied to cyano-trifluoromethylation of alkenes as a high-performance and recyclable heterogeneous catalyst, which possesses copper(II) active sites residing in funnel-like cavities. Under mild conditions, styrene derivatives and various unactivated olefins could be smoothly transformed into the corresponding cyano-trifluoromethylation products. Moreover, the transformation brought about by the active copper center in confined environments achieved regio- and shape selectivity. To understand the enhanced selectivity, the activation manner of the MOF catalyst was studied with control catalytic experiments such as FT-IR and UV-vis absorption spectroscopy of substrate-incorporated Cu3(BTC)2, which elucidated that the catalyst underwent a radical transformation with the intermediates confined in the MOF cavity, and the confinement effect endowed the method with pronounced selectivities.

Multifunctional Metal-Organic Frameworks as Catalysts for Tandem Reactions.

Owing to well-defined structure as well as easy synthesis and modification, metal-organic frameworks (MOFs) have emerged as promising catalysts for tandem reactions. In this article, we aim to summarize the development of multifunctional MOFs, including mixed metal MOFs, MOFs that are synergistically catalyzed by metal nodes and organic linkers, MOFs loaded with metal nanoparticles, etc, as heterogenous catalysts for tandem reactions over the past five years. This concept briefly discusses on present challenges, future trends, and prospects of multifunctional MOFs catalysts in tandem reactions.

Hierarchically Structured Metal-Organic Framework Polymer Composites for Chemical Warfare Agent Degradation.

Metal-organic frameworks (MOFs) have captured the imagination of researchers for their highly tunable properties and many potential applications, including as catalysts for a variety of transformations. Even though MOFs possess significant potential, the challenges associated with processing of these crystalline powders into usable form factors while retaining their functional properties limit their end use applications. Herein, we introduce a new approach to construct MOF-polymer composites via 3D photoprinting to overcome these limitations. We designed photoresin composite formulations that use polymerization-induced phase separation to cause the MOF catalysts to migrate to the surface of the printed material, where they are accessible to substrates such as chemical warfare agents. Using our approach, MOF-polymer composites can be fabricated into nearly any shape or architecture while retaining both the excellent catalytic activity at 10 wt % loading of the MOF components and the flexible, elastomeric mechanical properties of a polymer.