Catalysis with Metal Nanoparticles Immobilized within the Pores of Metal–Organic Frameworks

Metal–organic frameworks (MOFs) are highly ordered crystalline porous materials prepared by the self-assembly of metal ions and organic linkers having low-density framework structures of diversified topologies with tunable pore sizes and exceptionally large surface areas. Other than outstanding gas/molecule storage properties, loading of metal nanoparticles (MNPs) into the pores of MOFs could afford heterogeneous catalysts having advantages of controlling the particle growth to a nanosize region, resulting in highly active sites and enhanced catalytic performances, and these entrapped MNPs within MOF pores could be accessed by reactants for chemical transformations. This is a rapidly developing research area, and this Perspective addresses current achievements and future challenges for diverse MOF-immobilized MNPs within their pores, focusing especially on their preparation, characterization, and application as heterogeneous catalysts

From Metal–Organic Framework to Nitrogen-Decorated Nanoporous Carbons: High CO₂ Uptake and Efficient Catalytic Oxygen Reduction

High-surface-area N-decorated nanoporous carbons have been successfully synthesized using the N-rich metal–organic framework ZIF-8 as a template and precursor along with furfuryl alcohol and NH₄OH as the secondary carbon and nitrogen sources, respectively. These carbons exhibited remarkable CO₂ adsorption capacities and CO₂/N₂ and CO₂/CH₄ selectivities. The N-decoration in these carbons resulted in excellent activity for the oxygen reduction reaction. Samples NC900 and NC1000 having moderate N contents, high surface areas, and large numbers of mesopores favored the four-electron reduction pathway, while sample NC800 having a high N content, a moderate surface area, and a large number of micropores favored the two-electron reduction process

Metal–Organic Framework-Immobilized Polyhedral Metal Nanocrystals: Reduction at Solid–Gas Interface, Metal Segregation, Core–Shell Structure, and High Catalytic Activity

For the first time, this work presents surfactant-free monometallic and bimetallic polyhedral metal nanocrystals (MNCs) immobilized to a metal–organic framework (MIL-101) by CO-directed reduction of metal precursors at the solid–gas interface. With this novel method, Pt cubes and Pd tetrahedra were formed by CO preferential bindings on their (100) and (111) facets, respectively. PtPd bimetallic nanocrystals showed metal segregation, leading to Pd-rich core and Pt-rich shell. Core–shell Pt@Pd nanocrystals were immobilized to MIL-101 by seed-mediated two-step reduction, representing the first example of core–shell MNCs formed using only gas-phase reducing agents. These MOF-supported MNCs exhibited high catalytic activities for CO oxidation

Construction of Non-Interpenetrated Charged Metal–Organic Frameworks with Doubly Pillared Layers: Pore Modification and Selective Gas Adsorption

The rigid and angular tetracarboxylic acid 1,3-bis­(3,5-dicarboxyphenyl)­imidazolium (<b>H</b>_<b>4</b><b>L</b>^<b>+</b>), incorporating an imidazolium group, has been used with different pyridine-based linkers to construct a series of non-interpenetrated cationic frameworks, {[Zn₂(<b><i>L</i></b>)­(bpy)₂]·(NO₃)·(DMF)₆·(H₂O)₉}_<i>n</i> (<b>1</b>), {[Zn₂(<i><b>L</b></i>)­(dpe)₂]·(NO₃)·(DMF)₃·(H₂O)₂}_<i>n</i> (<b>2</b>), and {[Zn₂(<i><b>L</b></i>)­(bpb)₂]·(NO₃)·(DMF)₃·(H₂O)₄}_<i>n</i> (<b>3</b>) [<b><i>L</i></b> = <b>L</b>^<b>3–</b>, DMF = <i>N</i>,<i>N</i>′-dimethylformamide, bpy = 4,4′-bipyridine, dpe = 1,2-di­(4-pyridyl) ethylene, bpb = 1,4-bis­(4-pyridyl)­benzene]. The frameworks consist of {[Zn₂(<i><b>L</b></i>)]⁺}_<i>n</i> two-dimensional layers that are further pillared by the linker ligands to form three-dimensional bipillared-layer porous structures. While the choice of the bent carboxylic acid ligand and formation of double pillars are major factors in achieving charged non-interpenetrated frameworks, lengths of the pillar linkers direct the pore modulation. Accordingly, the N₂ gas adsorption capacity of the activated frameworks (<b>1a–3a</b>) increases with increasing pillar length. Moreover, variation in the electronic environment and marked difference in the pore sizes of frameworks permit selective CO₂ adsorption over N₂, where <b>3a</b> exhibits the highest selectivity. In contrast, the selectivity of CO₂ over CH₄ is reversed and follows the order <b>1a</b> > <b>2a</b> > <b>3a</b>. These results demonstrate that even though the pore sizes of the frameworks are large enough compared to the kinetic diameters of the excluded gas molecules, the electronic environment is crucial for the selective sorption of CO₂

Gas Adsorption and Magnetic Properties in Isostructural Ni(II), Mn(II), and Co(II) Coordination Polymers

Two new isostructural 3D porous coordination polymers {[Ni₂(DBIBA)₃]·Cl·18H₂O}_<i>n</i> (<b>1</b>), {[Mn₂(DBIBA)₃]·Cl·3H₂O}_<i>n</i> (<b>2</b>) (DBIBA = 5-di­(1<i>H</i>-benzo­[d]­imidazol-1-yl)­benzoate) have been synthesized by acidic hydrolysis of cyanide functional group of 5-di­(1<i>H</i>-benzo­[d]­imidazol-1-yl) benzonitrile under solvothermal conditions. Both the compounds have been characterized by X-ray crystallography, IR spectroscopy, thermogravimetry, powder X-ray diffraction, and elemental analysis. They form isoreticular structures with binodal 4,6-connected net having 1D hexagonal channels. These channels are occupied by water molecules. Frameworks are stabilized by hydrogen bonding interactions with anion and π–π interactions in the hexagonal cavity. Desolvated <b>1</b> exhibits selective CO₂, while <b>2</b> shows N₂, H₂, and CO₂ gas adsorption properties without any selectivity. This difference in gas adsorption properties is attributed to different degrees of flexibility of the framework. Magnetic susceptibility measurements at low temperature show weak ferromagnetic behavior in both coordination polymers <b>1</b> and <b>2</b> due to spin canting phenomenon. Complex {[Co₂(DBIBA)₃]·Cl·9H₂O}_<i>n</i> (<b>3</b>), which was previously reported by us, was also subjected to variable temperature magnetic susceptibility measurements

Gas Adsorption and Magnetic Properties in Isostructural Ni(II), Mn(II), and Co(II) Coordination Polymers

Two new isostructural 3D porous coordination polymers {[Ni₂(DBIBA)₃]·Cl·18H₂O}_<i>n</i> (<b>1</b>), {[Mn₂(DBIBA)₃]·Cl·3H₂O}_<i>n</i> (<b>2</b>) (DBIBA = 5-di­(1<i>H</i>-benzo­[d]­imidazol-1-yl)­benzoate) have been synthesized by acidic hydrolysis of cyanide functional group of 5-di­(1<i>H</i>-benzo­[d]­imidazol-1-yl) benzonitrile under solvothermal conditions. Both the compounds have been characterized by X-ray crystallography, IR spectroscopy, thermogravimetry, powder X-ray diffraction, and elemental analysis. They form isoreticular structures with binodal 4,6-connected net having 1D hexagonal channels. These channels are occupied by water molecules. Frameworks are stabilized by hydrogen bonding interactions with anion and π–π interactions in the hexagonal cavity. Desolvated <b>1</b> exhibits selective CO₂, while <b>2</b> shows N₂, H₂, and CO₂ gas adsorption properties without any selectivity. This difference in gas adsorption properties is attributed to different degrees of flexibility of the framework. Magnetic susceptibility measurements at low temperature show weak ferromagnetic behavior in both coordination polymers <b>1</b> and <b>2</b> due to spin canting phenomenon. Complex {[Co₂(DBIBA)₃]·Cl·9H₂O}_<i>n</i> (<b>3</b>), which was previously reported by us, was also subjected to variable temperature magnetic susceptibility measurements

Two New Coordination Polymers with Co(II) and Mn(II): Selective Gas Adsorption and Magnetic Studies

A novel bifurcated ligand 3,5-di­(1<i>H</i>-benzo­[<i>d</i>]­imidazol-1-yl)­benzonitrile (DBIBN) has been synthesized, which reacts with Co­(II) and Mn­(II) salts to form new coordination polymers {[Co₂(DBIBA)₃]·Cl·9H₂O}_<i>n</i> (<b>1</b>) and {[Mn₃(DBIBA)₆]}_<i>n</i> (<b>2</b>), (DBIBA = 3,5-di­(1<i>H</i>-benzo­[d]­imidazol-1-yl)­benzoate), in which DBIBN get hydrolyzed into DBIBA under solvothermal conditions. Both the complexes have been characterized by single-crystal X-ray crystallography (XRD), IR spectroscopy, elemental analysis, thermogravimetry, and X-ray powder diffraction (PXRD). Complex <b>1</b> is a 3D coordination polymer composed of binuclear Co­(II) units, having (4,6) connected net. On the other hand, complex <b>2</b> is a layered structure with trinuclear Mn­(II) units, which has 4-connected 4⁴ <i><b>sql</b></i> topology. TGA and PXRD measurements show that the framework <b>1</b> is stable after desolvation. Desolvated framework <b>1</b> showed selective adsorption of CO₂ over N₂, H_2, and Ar. Variable temperature magnetic susceptibility measurements show that complex <b>1</b> exhibits weak antiferromagnetic behavior

Two New Coordination Polymers with Co(II) and Mn(II): Selective Gas Adsorption and Magnetic Studies

A novel bifurcated ligand 3,5-di­(1<i>H</i>-benzo­[<i>d</i>]­imidazol-1-yl)­benzonitrile (DBIBN) has been synthesized, which reacts with Co­(II) and Mn­(II) salts to form new coordination polymers {[Co₂(DBIBA)₃]·Cl·9H₂O}_<i>n</i> (<b>1</b>) and {[Mn₃(DBIBA)₆]}_<i>n</i> (<b>2</b>), (DBIBA = 3,5-di­(1<i>H</i>-benzo­[d]­imidazol-1-yl)­benzoate), in which DBIBN get hydrolyzed into DBIBA under solvothermal conditions. Both the complexes have been characterized by single-crystal X-ray crystallography (XRD), IR spectroscopy, elemental analysis, thermogravimetry, and X-ray powder diffraction (PXRD). Complex <b>1</b> is a 3D coordination polymer composed of binuclear Co­(II) units, having (4,6) connected net. On the other hand, complex <b>2</b> is a layered structure with trinuclear Mn­(II) units, which has 4-connected 4⁴ <i><b>sql</b></i> topology. TGA and PXRD measurements show that the framework <b>1</b> is stable after desolvation. Desolvated framework <b>1</b> showed selective adsorption of CO₂ over N₂, H_2, and Ar. Variable temperature magnetic susceptibility measurements show that complex <b>1</b> exhibits weak antiferromagnetic behavior

Single-Atomic Co–N₄ Sites with CrCo Nanoparticles for Metal–Air Battery-Driven Hydrogen Evolution

Designing highly active and robust earth abundant trifunctional electrocatalysts for energy storage and conversion applications remain an enormous challenge. Herein, we report a trifunctional electrocatalyst (CrCo/CoN4@CNT-5), synthesized at low calcination temperature (550 °C), which consists of Co–N4 single atom and CrCo alloy nanoparticles and exhibits outstanding electrocatalytic performance for the hydrogen evolution reaction, oxygen evolution reaction, and oxygen reduction reaction. The catalyst is able to deliver a current density of 10 mA cm–2 in an alkaline electrolytic cell at a very low cell voltage of ∼1.60 V. When the catalyst is equipped in a liquid rechargeable Zn–air battery, it endowed a high open-circuit voltage with excellent cycling durability and outperformed the commercial Pt/C+IrO2 catalytic system. Furthermore, the Zn–air battery powered self-driven water splitting system is displayed using CrCo/CoN4@CNT-5 as sole trifunctional catalyst, delivering a high H2 evolution rate of 168 μmol h–1. Theoretical calculations reveal synergistic interaction between Co–N4 active sites and CrCo nanoparticles, favoring the Gibbs free energy for H2 evolution. The presence of Cr not only enhances the H2O adsorption and dissociation but also tunes the electronic property of CrCo nanoparticles to provide optimized hydrogen binding capacity to Co–N4 sites, thus giving rise to accelerated H2 evolution kinetics. This work highlights the importance of the presence of small quantity of Cr in enhancing the electrocatalytic activity as well as robustness of single-atom catalyst and suggests the design of the multifunctional robust electrocatalysts for long-term H2 evolution application

Immobilizing Highly Catalytically Active Pt Nanoparticles inside the Pores of Metal–Organic Framework: A Double Solvents Approach

Ultrafine Pt nanoparticles were successfully immobilized inside the pores of a metal–organic framework, MIL-101, without aggregation of Pt nanoparticles on the external surfaces of framework by using a “double solvents” method. TEM and electron tomographic measurements clearly demonstrated the uniform three-dimensional distribution of the ultrafine Pt NPs throughout the interior cavities of MIL-101. The resulting Pt@MIL-101 composites represent the first highly active MOF-immobilized metal nanocatalysts for catalytic reactions in all three phases: liquid-phase ammonia borane hydrolysis, solid-phase ammonia borane thermal dehydrogenation, and gas-phase CO oxidation