Highly Active Ultrasmall Ni Nanoparticle Embedded Inside a Robust Metal–Organic Framework: Remarkably Improved Adsorption, Selectivity, and Solvent-Free Efficient Fixation of CO2

Silver Nanoparticle-Functionalized Postsynthetically Modified Thiol MOF UiO-66-NH-SH for Efficient CO2 Fixation

Thiols are essential functional groups imparting unique properties, such as reactivity and selectivity, to many vital enzymes and biomolecules. The integration of electronically soft thiol groups within metal-organic frameworks (MOFs) yields elevated reactivity and a pronounced affinity for soft metal ions. However, the scarcity of thiol-based ligands and synthetic challenges hinder the advancement of thiol-based MOFs. To bypass the difficulties of synthesizing thiol MOFs by a direct reaction between thiol-based ligands and corresponding metal salts, postsynthetic modification (PSM) of MOFs is an efficient strategy to introduce thiol functionality. Herein, we have introduced Ag nanoparticles in postsynthetically modified thiol MOFs UiO-66-NH-SH (1) (synthesized by reaction between UiO-66-NH2 and thioglycolic acid) and UiO-66-NH-SH (2) (synthesized by reaction between UiO-66-NH2 and 3-mercaptopropionic acid) to synthesize a series of heterogeneous catalysts for CO2 fixation. Catalysts Cat 1-2 and Cat 3 - 4 were synthesized from UiO-66-NH-SH (1) and UiO-66-NH-SH (2), respectively, by using varying concentrations of silver (AgNO3). Catalyst Ag@UiO-66-NH-SH (1) (Ag = 3.45%; namely Cat 2) shows the highest efficiency for the catalytic conversion of propargylic alcohol and terminal epoxide to the corresponding cyclic carbonates. Finally, a rationalized reaction mechanism is proposed by correlating our results with the current literature. This work presents a viable strategy to utilize the thiol functionality of MOFs (avoiding the complexities associated with synthesizing thiol MOFs directly from thiol ligands) as a platform for introducing catalytically active metal centers and applying them as a heterogeneous catalyst for CO2 fixation reactions.

Coordination Unsaturation and Basic Site-Immobilized Nanochannel in a Chemorobust MOF for 3-Fold-Increased High-Temperature Selectivity and Fixation of CO2 under Mild Conditions with Nanomolar Recognition of Roxarsone

A multifaceted metal-organic framework (MOF) with task-specific site-engineered pores can promise high-temperature and moisture-tolerant capture and non-redox fixation of CO2 under mild conditions as well as ultrasensitive detection of carcinogenic contaminants in water. Herein, we report a pillar-bilayered MOF that holds a nanochannel with contrasting functionalities for both these sustainable applications with improved performance characteristics. The twofold entangled robust framework exhibits CO2 adsorption at elevated temperatures with considerable MOF-gas interaction. Interestingly, CO2 selectivity unveils nearly a 3-fold improvement upon the rise of temperature, affording a CO2/N2 value of 820 at 313 K, which outperforms many porous adsorbents. Additionally, breakthrough simulation establishes complete separation and attests the potential of this MOF in the separation of flue gas mixture. Importantly, minor CO2 loss during multiple capture-release cycles and under a relative humidity of 75% promise practical usability of the material. Density functional theory (DFT) not only portrays the atomistic level snapshots of temperature-triggered CO2 inclusion inside this microporous vessel alongside the role of diverse CO2-philic sites but also validates the basis of N2-phobicity of an azo-functionalized linker on such increased selectivity. The guest-free MOF further demonstrates non-redox and recyclable CO2 fixation with wide epoxide tolerance under solvent-free mild conditions and even works at atmospheric pressure and room temperature. The crucial roles of high-density acid-base sites in both adsorption and catalysis are supported by control experiments and by comparing the activity of an unfunctionalized MOF. The hydrolytic stability and strong luminescence signature benefit the framework in aqueous-phase selective and fast responsive detection of detrimental roxarsone (ROX) with high quenching (7.56 × 104 M-1) and very low sensitivity (68 nM). Apart from varying degrees of an energy-transfer mechanism, the fluorosensing of ROX is comprehensively supported by in-depth DFT studies that manifest alteration of MOF energy levels in the presence of organoarsenic compounds and depict MOF-analyte supramolecular interactions.

Redox-Active and Urea-Engineered-Entangled MOFs for High-Efficiency Water Oxidation and Elevated Temperature Advanced CO2 Separation Cum Organic-Site-Driven Mild-Condition Cycloaddition

Development of the multifaceted metal-organic framework (MOF) with in situ engineered task-specific sites can promise proficient oxygen evolution reaction (OER) and high-temperature adsorption cum mild-condition fixation of CO₂. In fact, effective assimilation of these attributes onto a single material with advance performance characteristics is practically imperative in view of renewable energy application and carbon-footprint reduction. Herein, we developed a three-fold interpenetrated robust Co(II) framework that embraces both redox-active and hydrogen-bond donor moieties inside the microporous channel. The activated MOF demonstrates notable OER catalysis in alkaline medium via quasi-reversible Co²⁺/Co³⁺ couple and unveils low overpotential with impressive 53.5 mV/dec Tafel slope that overpowers some benchmark, commercial, as well as contemporary materials. In particular, significantly increased turnover frequency (3.313 s^-1 at 400 mV) and fairly low charge-transfer resistance (3.02 Ω) compared to Co₃O₄, NiO, and majority of redox-active MOFs together with 91% Faradaic efficiency and notable framework durability after multiple OER cycles endorse high-performance water oxidation. Pore-wall decked urea groups benefit appreciable CO₂ adsorption even at elevated temperatures with considerable MOF-CO₂ interactions and exhibit recurrent capture-release cycles at diverse temperatures. Interestingly, CO₂ selectivity displays radical upsurge with temperature rise, affording 40% improved CO₂/N₂ value of 200 at 313 K, which outperforms many porous adsorbents and delineates real-time CO₂ scavenging potential. The guest-free MOF effectively catalyzes solvent-free CO₂ cycloaddition with broad substrate tolerance and satisfactory reusability under relatively mild condition. Opposed to the common Lewis acid-mediated reaction, two-point hydrogen-bonding activates the substrate, as supported from controlled experiments, juxtaposing the performance of an un-functionalized MOF and fluorescence modification-derived framework-epoxide interaction, providing valuable insights on unconventional cycloaddition route in the MOF.

Largely Entangled Diamondoid Framework with High-Density Urea and Divergent Metal Nodes for Selective Scavenging of CO2 and Molecular Dimension-Mediated Size-Exclusive H-Bond Donor Catalysis

Pore environment modulation with high-density polarizing groups in metal-organic frameworks (MOFs) can effectively accomplish selective and multicyclic carbon dioxide (CO₂) adsorption, whereas the incorporation of task-specific organic sites inside these porous vessels promise to evade self-quenching, solubility, and recyclability issues in hydrogen-bond donating (HBD) catalysis. However, concurrent amalgamation of both these attributes over a single platform is rare but extremely demanding in view of sustainable applications. We designed a robust diamondoid framework CSMCRI-17 (CSMCRI = Central Salt and Marine Chemicals Research Institute) from the mixed-ligand assembly of azo group-containing dicarboxylate ligand, urea-functionalized pyridyl linker, and Zn(II) nodes with specific divergent coordination. Seven-fold interpenetration to the microporous structure largely augments N-rich functionality that facilitates high CO₂ uptake in the activated form (17a) with good CO₂ selectivity over N₂ and CH₄ that outperform many reported materials. The framework displays very strong CO₂ affinity and no reduction in adsorption capacity over multiple uptake-release cycles. Benefitting from the pore-wall decoration with urea functionality from the pillaring strut, 17a further demonstrates hydrogen-bond-mediated Friedel-Crafts alkylation of indole with β-nitrostyrene under mild conditions, with multicyclic usability and excellent reactivity toward wide ranges of substituted nucleophiles and electrophiles. Interestingly, interpenetration-generated optimum-sized pores induce poor conversion to sterically encumbered substrate via molecular dimension-mediated size selectivity that is alternatively ascribed from additional control experiments and support the occurrence of HBD reaction within the MOF cavity. The catalytic path is detailed in light of the change of emission intensity of the framework by the electrophile as well as the judicious choice of the substrate, which authenticates the prime role of urea moiety-governed two-point hydrogen bonding.

Selective and Multicyclic CO2 Adsorption with Visible Light-Driven Photodegradation of Organic Dyes in a Robust Metal-Organic Framework Embracing Heteroatom-Affixed Pores

Pore environment modulation with polarizing groups is one of the essential prerequisites for selective carbon dioxide (CO₂) adsorption in metal-organic frameworks (MOFs), wherein judicious installation of the photocatalytic feature can promise visible light-triggered degradation of toxic organic dye molecules. However, astute amalgamation of both these attributes over a single MOF is rather rare, yet much anticipated in view of sustainable applications. Pore engineering is effectively harnessed in a Zn(II)-based three-dimensional (3D) MOF, CSMCRI-16 (CSMCRI = Central Salt and Marine Chemicals Research Institute), through mixed-ligand assembly of a N-rich linker (L), 4,4'-oxybis(benzoic acid) (H₂oba) ligand, and [Zn₂(CO₂)₄N₂] paddle-wheel secondary building units (SBUs). The noninterpenetrated structure contains unbound nitrogen and accessible oxygen atom-decorated porous channels and exhibits admirable stability in diverse organic solvents, open air, and at elevated temperatures. The heteroatom-decorated porous channels facilitated excellent CO₂ uptake in the activated MOF (16a) with high selectivity over N₂ (CO₂/N₂: 155.3) at 273 K. The framework further exhibits reasonable CO₂ affinity and multicyclic CO₂ sorption recurrence without a significant loss in the uptake capacity. Benefitting from the presence of the [Zn₂(CO₂)₄N₂] cluster in conjugation with π-conjugated organic ligands, the extended 3D network revealed an optical band gap energy of 2.55 eV, which makes the MOF an efficient photocatalyst toward the degradation of the cationic dyes crystal violet (CV) and methylene blue (MB) in the presence of a simple 40 W visible light lamp without any assistance of external oxidants. The catalyst exhibits multicyclic performance and short reaction time in addition to the fact that catalytic efficiencies (CV: 97.2%, MB: 97.8%) are comparable to those of contemporary materials.

Brønsted Acid-Functionalized Ionic Co(II) Framework: A Tailored Vessel for Electrocatalytic Oxygen Evolution and Size-Exclusive Optical Speciation of Biothiols

Metal-organic frameworks (MOFs) not only combine globally demanded renewable energy generation and environmental remediation onto a single platform but also rationalize structure-performance synergies to devise smarter materials with remarkable performance. The robust and non-interpenetrated cationic MOF exemplifies a unique bifunctional scaffold for the efficient electrochemical oxygen evolution reaction (OER) and ultrasensitive monitoring of biohazards. The microporous framework containing Brønsted acid-functionalized [Co₂(μ₂-OH)(CO₂)₂] secondary building units (SBUs) exhibits remarkable OER performance in 1 M KOH, requiring 410 mV overpotential to obtain 10 mA cm^-2 anodic current density, and a low Tafel slope of 55 mV/dec with 93.1% Faradaic efficiency. Apart from the high turnover frequency and electrochemically assessable surface area, steady OER performance over 500 cycles under potentiodynamic and potentiostatic conditions result in long-term catalyst durability. The highly emissive attribute from nitrogen-rich fluorescent struts benefits the MOF in recyclable and selective fluoro-detection of three biothiols (l-cysteine, homocysteine, and glutathione) in water with a fast response time. In addition to colorimetric monitoring in the solid and solution phases, control experiments validate size-exclusive biothiol speciation through molecular-dimension-mediated pore diffusion. The role of SBUs in the OER mechanism is detailed from density functional theory-derived free energy analysis, which also validates the importance of accessible N-sites in sensing via portraying framework-analyte supramolecular interactions.

Urea-engineering mediated hydrogen-bond donating Friedel−Crafts alkylation of indoles and nitroalkenes in dual-functionalized and microporous metal-organic framework with high recyclability and pore-fitting-induced size-selectivity

As an effective alternative to Lewis acid activation, hydrogen-bond donating (HBD) organo-catalysis denotes a powerful construction tool to important classes of carbon–carbon bonds, wherein metal-organic frameworks (MOFs) alleviate issues like...

Design syntheses of metal–organic layers with rich N-sites for CO2 chemical fixation

Presented here is a series of metal–organic layers in which azolate derivatives as terminal ligands supply numerous uncoordinated N active sites and exhibit high catalytic activity.

Construction of two novel non-penetrating Co-MOFs derived from designed 2,4,6-tri(2,4-dicarboxyphenyl) pyridine: synthesis, structure and gas adsorption properties

The strategy of extending ligands and reducing symmetry provide a facile access to obtain a wide variety of linkers for the construction of MOFs bearing diverse structures and intriguing properties....

Organic transformations in the confined space of porous organic cage CC2; catalysis or inhibition

Porous organic cages have shape persistent cavities which provide a suitable platform for encapsulation of guest molecules with size suitably fitting to the cavity. The interactions of the guest molecule with the porous organic cage significantly alter the properties of the guest molecule. Herein, we report the effect of encapsulation on the kinetics of various organic transformations including 2 + 4 cycloaddition, 1,5-sigmatropic, 6π-electrocyclization, ring expansion, cheletropic, dyotropic, trimerization and tautomerization reactions. Non-bonding interactions are generated between the CC2 cage and encapsulated species. However, the number and nature/strength of interactions are different for reactant and TS with the CC2 cage and this difference detects the reaction to be accelerated or slowed down. A significant drop in the barrier of reactions is observed for reactions involving strong interactions of the transition state within the cage. However, for some reactions such as the Claisen rearrangement, reactants are stabilized more than the transition state and therefore an increase in activation barrier is observed. Furthermore, non-covalent analyses of all transition states (inside the cage) confirm the interaction between the CC2 cage and substrate. The current study will promote further exploration of the potential of other porous structures for similar applications.

Porous organic cages have shape persistent cavities which provide a suitable platform for encapsulation of guest molecules with size suitably fitting to the cavity.

Intrinsic-Unsaturation-Enriched Biporous and Chemorobust Cu(II) Framework for Efficient Catalytic CO2 Fixation and Pore-Fitting Actuated Size-Exclusive Hantzsch Condensation with Mechanistic Validation

Carbon dioxide (CO₂) utilization and one-pot Hantzsch condensation denote two important protocols pertinent to sustainable agenda because of the obvious advantages like reduction in chemical usage, short reaction time, and minimum waste generation. To this end, the astute combination of optimum-sized pore structure with built-in Lewis acid center in metal-organic frameworks (MOFs) can bring about such reactions under energetically favorable conditions and offer a step forward to size-exclusive catalysis. The chemoresistant and twofold interpenetrated Cu(II) framework CSMCRI-13 (CSMCRI = Central Salt & Marine Chemicals Research Institute) is built from a C₃-symmetric tricarboxylate ligand and an N,N'-donor linker that undergo incisive amalgamation of the paddle-wheel [Cu₂(COO)₄] secondary building unit (SBU) and the intrinsically unsaturated Cu(II) node with four coordination. The microporous structure features a dual-pore containing cage-like network with free oxygen-atom-enriched cavities and exhibits appreciable CO₂ adsorption with moderate MOF-CO₂ interaction in activated form (13a). Benefitting from both, the coordinatively frustrated metal center containing MOF acts as a highly synergistic and solvent-free catalyst in CO₂ cycloaddition reaction under an 8 bar CO₂ pressure at 70 °C in 6 h. The catalyst furnished admirable reactivity and fair recyclability with a wide range of substrates, wherein sterically encumbered and long-chain epoxides produced poor conversion. This MOF further executes highly regenerable Hantzsch condensation reaction under mild condition with superior activity to contemporary materials, where most of the 1,4-dihydropyridine derivatives are additionally characterized through the single-crystal X-ray diffraction analysis. Importantly, mechanistic proof of the tricomponent condensation involving built-in Lewis acid sites is validated from several control experiments and in-depth analytical studies. To the best of the single-step multicomponent reaction, substrate molecules having incompatible molecular dimension to that of pore size of the framework resulted insignificant conversion and demonstrated the first-ever pore-fitting-induced size selectivity in Hantzsch condensation.

Chemically Robust and Bifunctional Co(II)-Framework for Trace Detection of Assorted Organo-toxins and Highly Cooperative Deacetalization-Knoevenagel Condensation with Pore-Fitting-Induced Size-Selectivity

Acute detection of assorted classes of organo-toxins in a practical environment is an important sustainable agenda, whereas cooperative and recyclable catalysis can mitigate hazards by minimizing energy requirements and reducing waste generation. We constructed an acid-/base-stable Co(II)-framework with a unique network topology, wherein unidirectional porous channels are decorated by anionic [Co₂(μ₂-OH)(COO)₄(H₂O)₃] secondary building units and neutral [CoN₂(COO)₂] nodes. An intense luminescent signature of the hydrolytically robust framework is harnessed for the selective, fast-responsive, and regenerable detection of two detrimental organo-aromatics, 4-aminophenol (4-AP) and 2,4,6-trinitrophenol (TNP). Alongside remarkable quenching, their nanomolar detection limits (4-AP: 99.5 nM; TNP: 67.2 nM) rank among the lowest reported values in water and corroborate their ultra-sensitivity. Density functional theory (DFT) calculations verify the electron-transfer route of sensing through portraying redistribution of energy levels of molecular orbitals in a three-dimensional network by each analyte and further envisages non-covalent host-guest interactions. Benefiting from the concurrent existence of an open-metal site and a triphenylamine-moiety-functionalized ligand, the activated framework acts as an outstandingly cooperative heterogeneous catalyst in deacetalization-Knoevenagel condensation under mild conditions. The acid-base dual catalysis is detailed for the first time from combined inputs of control experiments and DFT validations. To the best of tandem reaction, larger-sized substrate exhibits insignificant conversion, and certifies rarest pore-fitting induced size-selectivity.

Structural Dynamism-Actuated Reversible CO2 Adsorption Switch and Postmetalation-Induced Visible Light Cα-H Photocyanation with Rare Size Selectivity in N-Functionalized 3D Covalent Organic Framework

The impact of dimensionality and flexibility on anticipated properties has prompted major research focus to three-dimensional covalent organic frameworks (3D COFs), where astute functionalization of porous channels for dynamic CO₂ adsorption as well as size-exclusive C-H activation under eco-friendly condition are the most intriguing advanced applications. Herein, we report an imine-based, diamondoid COF that embraces one-dimensional porous channels in spite of ninefold interpenetration. A combination of intrinsic microporosity and pore wall decoration with accessible N atoms from linear strut renders this 3D COF display reasonable CO₂ affinity with decent selectivity (CO₂/N₂: 64.2; CO₂/CH₄: 10.5) alongside worthy multicyclic CO₂ uptake-release recurrence. Interestingly, the COF undergoes solvent-assisted alteration to a pore-stretched structure via -C═N- "pedal" motion with a concomitant enhancement in CO₂ uptake, where steady reversibility of such structural dynamism instigates unprecedented CO₂ adsorption switch up to seven consecutive cycles. Integration of 2,2'-bipyridyl units benefits anchoring of homogeneous catalyst to device first-ever Ru(Bpy)₂²⁺ hooked diamondoid COF (Ru-COF), which performs visible-light-triggered oxidative cyanation of tertiary amines at room temperature, using molecular oxygen as a selective oxidant in green solvent H₂O. The photocatalyst-engineered COF manifests excellent recyclability and comparable activity to that of homogeneous catalyst. To the best of Ru-COF, atom-economic photocyanation is realized via in situ generated iminium ion, wherein larger-sized substrates exhibit insignificant conversion of α-aminonitriles and validate rarest size selectivity in oxidative Strecker reaction. This study not only demonstrates potential of 3D COF as next-generation dynamic CO₂ adsorbent but also sheds light on tailor-made fabrication of smart functional material for promising catalytic applications through an environmentally benign route.

Pore-Functionalized and Hydrolytically Robust Cd(II)-Metal-Organic Framework for Highly Selective, Multicyclic CO2 Adsorption and Fast-Responsive Luminescent Monitoring of Fe(III) and Cr(VI) Ions with Notable Sensitivity and Reusability

Metal-organic frameworks (MOFs) show a distinctive pre-eminence over other heterogeneous systems for adsorption of carbon dioxide (CO₂) gas and fluorescence detection of water contaminating ions, where integration of both these attributes along with enhancement of pore functionality and water stability is crucial for potential applications related to environmental remediation. Pore functionalization has been achieved in a 2-fold interpenetrated, mixed-ligand Cd(II)-framework [Cd_1.5(L)₂(bpy)(NO₃)]·2DMF·2H₂O (CSMCRI-5) (HL = 4-(4-carboxyphenyl)-1,2,4-triazole, bpy = 4,4'-bipyridine, DMF = dimethylformamide, CSMCRI = Central Salt & Marine Chemicals Research Institute) by utilizing a bifunctional ligand HL. The bpy-pillared framework, containing diverse Cd(II) nodes, optimum sized voids, and free N-atom affixed one-dimensional porous channels, shows notable structural robustness in diverse organic solvents and water. In spite of a negligible surface area, the activated MOF (5a) exhibits good CO₂ uptake and highly selective CO₂ adsorption over N₂ (259.94) and CH₄ (14.34) alongside minor loss during multiple CO₂ adsorption-desorption cycles. Luminescence studies demonstrate extremely selective and ultrafast sensing of Fe³⁺ ions in the aqueous phase with notable quenching (1.13 × 10⁴ M^-1) as well as an impressive 98 ppb limit of detection (LOD). Importantly, Fe³⁺ detection is exclusively retained under simulated physiological conditions. The framework further serves as a quick-responsive scaffold for toxic CrO₄^2- and Cr₂O₇^2- anions, where individual quenching constants (CrO₄^2-: 1.73 × 10⁴ M^-1; Cr₂O₇^2-: 5.42 × 10⁴ M^-1) and LOD values (CrO₄^2-: 280 ppb; Cr₂O₇^2-: 320 ppb) rank among the best sensory MOFs for aqueous phase detection of Cr(VI) species. It is imperative to stress vivid monitoring of all these aqueous pollutants by a handy paper-strip method, besides outstanding applicability of 5a toward their recyclable detection. Mechanism of selective quenching is comprehensively investigated in light of the absorption of the excitation/emission energy of the host framework by an individual studied analyte.

Stepwise and hysteretic sorption of CO2 in polycatenated metal–organic frameworks

Two isostructural 3D dynamic MOFs with different metal sites have been constructed, which exhibit temperature-dependent stepwise and hysteretic sorption behaviors toward CO₂ around room temperature.