Strategies in Metal– Organic Framework‐based Catalysts for the Aerobic Oxidation of Alcohols and Recent Progress

Fluorescent and Catalytic Properties of a 2D Lamellar Zn Metal-Organic Framework with sql Network Structure

A two-dimensional (2D) lamellar Zn metal-organic framework (Zn-MOF, 1) with a fluorescent 1,6-di(pyridin-3-yl)pyrene (3-DPPy) and 1,4-benzenedicarboxylate (BDC2-) bridging linkers was prepared and structurally characterized. The chemical formula of 1 is [Zn(μ-3-DPPy)(μ-BDC)]n. The mononuclear Zn(II) ion, acting as a node, is tetrahedrally coordinated with two 3-DPPy and two BDC linkers. The coordination environment of Zn(II) is a distorted tetrahedral geometry. The Zn-MOF is the sql network structure based on topology analysis. The undulated 2D sheets of 1 tightly pack together to form a lamellar structure. The pyrene moieties are parallelly oriented to each other. The Zn-MOF is not porous, possibly because the mononuclear Zn(II) node did not form cluster-based secondary building units due to the less symmetric 3-DPPy. The steady-state fluorescence measurements indicate that the fluorescence signal of the 1 is slightly blue-shifted compared to the free 3-DPPy in the solid state. The excimer emission band at 463 nm for crystalline 3-DPPy is shifted to 447 nm for 1. The value of 447 nm is also a blue-shift value compared to nonsubstituted pyrene crystals (470 nm). Despite its nonporosity, the surface Lewis acidic sites of 1 could catalyze the transesterification of esters. Surface defect sites are responsible for this catalytic activity.

Generation and Stabilization of a Dinickel Catalyst in a Metal-Organic Framework for Selective Hydrogenation Reactions

Although many monometallic active sites have been installed in metal-organic frameworks (MOFs) for catalytic reactions, there are no effective strategies to generate bimetallic catalysts in MOFs. Here we report the synthesis of a robust, efficient, and reusable MOF catalyst, MOF-NiH, by adaptively generating and stabilizing dinickel active sites using the bipyridine groups in MOF-253 with the formula of Al(OH)(2,2'-bipyridine-5,5'-dicarboxylate) for Z-selective semihydrogenation of alkynes and selective hydrogenation of C=C bonds in α,β-unsaturated aldehydes and ketones. Spectroscopic studies established the dinickel complex (bpy⋅- )NiII (μ2 -H)2 NiII (bpy⋅- ) as the active catalyst. MOF-NiH efficiently catalyzed selective hydrogenation reactions with turnover numbers of up to 192 and could be used in five cycles of hydrogenation reactions without catalyst leaching or significant decrease of catalytic activities. The present work uncovers a synthetic strategy toward solution-inaccessible Earth-abundant bimetallic MOF catalysts for sustainable catalysis.

Green and Red Photoluminescent Manganese Bromides with Aminomethylpyridine Isomers

Two positional isomers, 4-amino-3-methylpyridine and 3-amino-5-methylpyridine, produce 4-amino-3-methylpyridinium and 5-methylpyridin-3-aminium, respectively, under acidic conditions. The two protonated isomers create different hydrogen bonding networks, resulting in different coordination environments of the [MnX₄]^2- unit embedded in molecular compounds such as 4-amino-3-methylpyridinium manganese bromide, [(C₆H₉N₂)₂MnBr₄] and 5-methylpyridin-3-aminium manganese bromide, [(C₆H₉N₂)₄MnBr₄(H₂O)·(MnBr₄)]. Both compounds can be prepared using the slow evaporation method or mechanochemical synthetic procedures. Single-crystal structure analysis of [(C₆H₉N₂)₂MnBr₄] and [(C₆H₉N₂)₄MnBr₄(H₂O)·(MnBr₄)] revealed different manganese halide units, including tetrahedral and tetrahedral with distorted trigonal bipyramidal structures, which emit photoluminescence in the green (527 nm) and red (607 nm) regions, respectively. Electronic structure calculations were conducted to support the validity and interpretation of the UV-vis and photoluminescence (PL) spectral data. Thin films deposited using the [(C₆H₉N₂)₂MnBr₄] precursor also exhibit PL properties. The diverse pseudo-three-dimensional networks can be constructed by using positional isomers with different hydrogen bonding pathways and π-π stacking of organic units, in which the design strategy successfully enables the tuning of various optical properties.

4,6-Diamino-2-thiopyrimidine-based Cobalt Metal Organic Framework (Co-DAT-MOF): green, efficient, novel and reusable nanocatalyst for synthesis of multicomponent reactions

In this study, Co-DAT-MOF powder was prepared via the solvothermal method using 4, 6-diamino-2-thiopyrimidine as the organic linker and Co(NO₃)₂·6H₂O. The synthesized catalysts are characterized using XRD, FT-IR, TGA, SEM, BET, NH₃-TPD, and ICP-OES techniques. SEM analysis clearly indicated the formation of nanosheet microspheres. NH₃-TPD-MS was employed as a means of identifying the various strengths of acid sites and their relative abundance in an attempt to explain the effect of the catalyst surface acid sites. We identified a new acidic feature in Co-DAT-MOF catalyst, related to the presence of desorption peaks in the NH₃-TPD profiles. The activity of Co-DAT-MOF catalyst for the synthesis of multicomponent reactions correlates with lewis acidity. In addition, Co-DAT-MOF exhibited excellent performance for the synthesis of pyrroloacridine-1(2H)-one and chromeno [2, 3- d] pyrimidin-8-amines, as well as good reusability and recyclability.

Crystalline Metal-Peptide Networks: Structures, Applications, and Future Outlook

Metal-peptide networks (MPNs), which are assembled from short peptides and metal ions, are considered one of the most fascinating metal-organic coordinated architectures because of their unique and complicated structures. Although MPNs have considerable potential for development into versatile materials, they have not been developed for practical applications because of several underlying limitations, such as designability, stability, and modifiability. In this review, we summarise several important milestones in the development of crystalline MPNs and thoroughly analyse their structural features, such as peptide sequence designs, coordination geometries, cross-linking types, and network topologies. In addition, potential applications such as gas adsorption, guest encapsulation, and chiral recognition are introduced. We believe that this review is a useful survey that can provide insights into the development of new MPNs with more sophisticated structures and novel functions.

Hierarchical Metal-Organic Aerogel as a Highly Selective and Sustainable CO2 Adsorbent

Typical amorphous aerogels pose great potential for CO₂ adsorbents with high surface areas and facile diffusion, but they lack well-defined porosity and specific selectivity, inhibiting utilization of their full functionality. To assign well-defined porous structures to aerogels, a hierarchical metal-organic aerogel (HMOA) is designed, which consists of well-defined micropores (d ∼ 1 nm) by coordinative integration with chromium(III) and organic ligands. Due to its hierarchical structure with intrinsically flexible coordination, the HMOA has excellent porous features of a high surface area and a reusable surface with appropriate binding energy for CO₂ adsorption. The HMOA features high CO₂ adsorption capacity, high CO₂/N₂ IAST selectivity, and vacuum-induced surface regenerability (100% through 20 cycles). Further, the HMOA could be prepared via simple ambient drying methods while retaining the microporous network. This unique surface-tension-resistant micropore formation and flexible coordination systems of HMOA make it a potential candidate for a CO₂ adsorbent with industrial scalability and reproducibility.

Patchwork Metal-Organic Frameworks by Radical-Mediated Heterografting of Star Polymers for Surface Modification

We report a synthetic methodology for decorating a surface of metal-organic frameworks (MOFs) with polymers through postsynthetic modification. Well-defined polymers with reversibly deactivated radical species at their chain end were reacted with vinyl-functionalized MOFs in the presence of a radical initiator. The radical addition forms a C-C bond between the polymer end with the functional group at the MOF ligand. We used sterically bulky star polymers containing electron-deficient maleimide chain ends, which facilitated modification of the external surface, yielding polymer-grafted MOF composite particles. A patchy MOF particle can also be obtained by simultaneously grafting two polymers and jammed at the immiscible liquid-liquid interface. We further show that the selective removal of a sacrificial polymer would partially expose the surface of MOFs to external environment, which hinders the uptake of macromolecular guests above the critical hydrodynamic size. Overall, four polymer@MOF composites have successfully been achieved through the present postsynthetic patchworks on MOFs with star polymers and selective etching process.

Silver-Nanoparticle-Assisted Modulation of NH3 Desorption on MIL-101

Ammonia has recently emerged as a promising hydrogen carrier for renewable energy conversion. Establishing a better understanding and control of ammonia adsorption and desorption is necessary to improve future energy generation. Metal-organic frameworks (MOFs) have shown improved ammonia capacity and stability over conventional adsorbents such as silica and zeolite. However, ammonia desorption requires high temperature over 150 °C, which is not desirable for energy-efficient ammonia reuse and recycling. Here, we loaded silver nanoparticles from 6.6 to 51.4 wt% in MIL-101 (Ag@MIL-101) using an impregnation method to develop an efficient MOF-based hybrid adsorbent for ammonia uptake. The incorporation of metal nanoparticles into MIL-101 has not been widely explored for ammonia uptake, even though such hybrid nanostructures have significantly enhanced catalytic activities and gas sensing capacities. Structural features of Ag@MIL-101 with different Ag wt% were examined using transmission electron microscopy, X-ray powder diffraction, and infrared spectroscopy, demonstrating successful formation of silver nanoparticles in MIL-101. Ag@MIL-101 (6.6 wt%) showed hysteresis in the N₂ isotherm and an increase in the fraction of larger pores, indicating that mesopores were generated during the impregnation. Temperature-programmed desorption with ammonia was performed to understand the binding affinity of ammonia molecules on Ag@MIL-101. The binding affinity was the lowest with Ag@MIL-101 (6.6 wt%), including the largest relative fraction in the amount of desorbed ammonia molecules. It was presumed that cooperative interaction between the silver nanoparticle and the MIL-101 framework for ammonia molecules could allow such a decrease in the desorption temperature. Our design strategy with metal nanoparticles incorporated into MOFs would contribute to develop hybrid MOFs that reduce energy consumption when reusing ammonia from storage.

Structural properties of [Cu(II)3L6] cages: bridged polyatomic anion effects on unprecedented efficiency of heterogeneous catechol oxidation

Self-assembly of CuX₂ (X^- = BF₄^-, ClO₄^-, PF₆^-, and SbF₆^-) with a bidentate ethylmethylbis(3-pyridine)silane ligand (L) in the presence of additional polyatomic anions (X' = SiF₆^2- and PF₆^-) gives rise to single crystals consisting of the X'@[Cu(II)₃L₆] cage motif. These cages exist as discrete or anion-bridged 3D networks depending on outside anions. The anion-bridged 3D networks interpenetrate in a four-fold fashion, and show, to our best knowledge, the most effective heterogeneous catalysis in 3,5-di-tert-butylcatechol oxidation reaction within 20 min at room temperature. Surprisingly, the heterogeneous catalysis is more effective than its corresponding homogeneous catalysis. Such notable catalytic effects can be explained by the maintenance of 3D inter-cage Cu⋯Cu distance as a catalytic center.

Binding Materials for MOF Monolith Shaping Processes: A Review towards Real Life Application

Metal–organic frameworks (MOFs) could be utilized for a wide range of applications such as sorption, catalysis, chromatography, energy storage, sensors, drug delivery, and nonlinear optics. However, to date, there are very few examples of MOFs exploited on a commercial scale. Nevertheless, progress in MOF-related research is currently paving the way to new industrial opportunities, fostering applications and processes interconnecting fundamental chemistry with engineering and relevant sectors. Yet, the fabrication of porous MOF materials within resistant structures is a key challenge impeding their wide commercial use for processes such as adsorptive separation. In fact, the integration of nano-scale MOF crystallic structures into bulk components that can maintain the desired characteristics, i.e., size, shape, and mechanical stability, is a prerequisite for their wide practical use in many applications. At the same time, it requires sophisticated shaping techniques that can structure nano/micro-crystalline fine powders of MOFs into diverse types of macroscopic bodies such as monoliths. Under this framework, this review aims to bridge the gap between research advances and industrial necessities for fostering MOF applications into real life. Therefore, it critically explores recent advances in the shaping and production of MOF macro structures with regard to the binding materials that have received little attention to date, but have the potential to give new perspectives in the industrial applicability of MOFs. Moreover, it proposes future paths that can be adopted from both academy and industry and can further boost MOF exploitation.

Construction of Stable Metal-Organic Framework Platforms Embedding N-Heterocyclic Carbene Metal Complexes for Selective Catalysis

We report a bottom-up approach to immobilize catalysts into MOFs, including copper halides and gold chloride in a predictable manner. Interestingly, the structures of MOFs bearing NHC metal complexes maintained a similar 4-fold interpenetrated cube. They exhibited exceptionally high porosity despite the interpenetrated structure and showed good stability in various solvents. Moreover, these MOFs possess high size activity depending on the size of the substrates in various reactions, compared to homogeneous catalysis. Also, the high catalytic activity of MOFs can be preserved 4 times without significant loss of crystallinity. Incorporation of the various metal complexes into MOFs allows for the preparation of functional MOFs for practical applications.

TEMPO-radical-bearing metal-organic frameworks and covalent organic frameworks for catalytic applications

It is known that 2,2,6,6-tetramethylpiperidinyl-1-oxy (or TEMPO) is a stable, radical-containing molecule, which has been utilized in various areas of organic synthesis, catalysis, polymer chemistry, electrochemical reactions, and materials chemistry. Its unique stability, attributable to its structural features, and molecular tunability allows for the modification of various materials, including the heterogenization of solid materials. Metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are porous and tunable because of their ligand or linker portion, and both have been extensively studied for use in catalytic applications. Therefore, synergistically combining the chemistry of TEMPO with the properties of MOFs and COFs is a natural choice and should allow for significant advancements, including improved recyclability and selectivity. This article focuses on TEMPO-bearing MOFs and COFs for use in catalytic applications. In addition, recent strategies related to the use of these functional porous materials in catalytic reactions are also discussed.