Mixed-Valence Cobalt(II/III) Metal–Organic Framework for Ammonia Sensing with Naked-Eye Color Switching

The construction of colorimetric sensing materials with high selectivity, low detection limits, and great stability provides a significant way for facile device implementation of an ammonia (NH₃) sensor. Herein, with excellent alkaline stability and exposed N sites in molecule as well as with naked-eye color switching nature generated from changeable cobalt (Co) valence, a three-dimensional mixed-valence cobalt­(II/III) metal–organic framework (<b>FJU-56</b>) with tris-(4-tetrazolyl-phenyl)­amine (H₃L) ligand was synthesized for colorimetric sensing toward ammonia. The activated <b>FJU-56</b> demonstrates a limit of detection of 1.38 ppm for ammonia sensing, with high selectivity in ammonia and water competitive adsorption, and shows outstanding stability and reversibility in the cyclic test. The NH₃ or water molecules binding to the exposed N sites with the hydrogen-bond are observed by single-crystal X-ray diffraction, determining that the attachment of guest molecules to the <b>FJU-56</b> framework changes the valence of Co ions with a naked-eye color switching response, which provides an ocular demonstration for ammonia capture and a valuable insight into ammonia sensing

Mixed-Valence Cobalt(II/III) Metal–Organic Framework for Ammonia Sensing with Naked-Eye Color Switching

The construction of colorimetric sensing materials with high selectivity, low detection limits, and great stability provides a significant way for facile device implementation of an ammonia (NH₃) sensor. Herein, with excellent alkaline stability and exposed N sites in molecule as well as with naked-eye color switching nature generated from changeable cobalt (Co) valence, a three-dimensional mixed-valence cobalt­(II/III) metal–organic framework (<b>FJU-56</b>) with tris-(4-tetrazolyl-phenyl)­amine (H₃L) ligand was synthesized for colorimetric sensing toward ammonia. The activated <b>FJU-56</b> demonstrates a limit of detection of 1.38 ppm for ammonia sensing, with high selectivity in ammonia and water competitive adsorption, and shows outstanding stability and reversibility in the cyclic test. The NH₃ or water molecules binding to the exposed N sites with the hydrogen-bond are observed by single-crystal X-ray diffraction, determining that the attachment of guest molecules to the <b>FJU-56</b> framework changes the valence of Co ions with a naked-eye color switching response, which provides an ocular demonstration for ammonia capture and a valuable insight into ammonia sensing

Highly Selective Adsorption of C₂/C₁ Mixtures and Solvent-Dependent Thermochromic Properties in Metal–Organic Frameworks Containing Infinite Copper-Halogen Chains

Separation of light hydrocarbon mixtures is a very important but challenging industrial separation task. Here, we have synthesized two isostructural cationic metal–organic frameworks {[(Cu­(Btz)­X]·X·6H₂O·0.25DMSO} (<b>FJU-53</b>, Btz = 1,4′-Bis­(4<i>H</i>-1,2,4-triazol-4-yl)­benzene, X = Cl or Br, DMSO = dimethyl sulfoxide) containing infinite copper-halogen chains and first demonstrated that the adsorption selectivity toward C₂/C₁ mixtures in the charged MOFs can be improved by tuning counter-anions. <b>FJU-53</b> exhibits the highest selectivity for C₂H₂/CH₄ separation at 296 K and 1 atm, and exceptional chemical stability in aqueous solutions with pH ranging from 1 to 13. In addition, <b>FJU-53</b> also shows the attractive solvent- and halogen-dependent thermochromic behaviors. Its thermochromic mechanism is attributed to the thermally induced vibration of the infinite [(CuX)_<i>n</i>]^<i>n</i>+ chains, remarkably different from that for the traditional copper­(II) halide materials, the thermochromism for which comes from the coordination geometry transformation or Jahn–Teller distortion

Straightforward Loading of Imidazole Molecules into Metal–Organic Framework for High Proton Conduction

A one-step straightforward strategy has been developed to incorporate free imidazole molecules into a highly stable metal–organic framework (<b>NENU-3</b>, ([Cu₁₂(BTC)₈(H₂O)₁₂]­[HPW₁₂O₄₀])·Guest). The resulting material <b>Im@(NENU-3)</b> exhibits a very high proton conductivity of 1.82 × 10^–2 S cm^–1 at 90% RH and 70 °C, which is significantly higher than 3.16 × 10^–4 S cm^–1 for <b>Im-Cu@(NENU-3a)</b> synthesized through a two-step approach with mainly terminal bound imidazole molecules inside pores. Single crystal structure reveals that imidazole molecules in <b>Im-Cu@(NENU-3a)</b> isolate lattice water molecules and then block proton transport pathway, whereas high concentration of free imidazole molecules within <b>Im@(NENU-3)</b> significantly facilitate successive proton-hopping pathways through formation of hydrogen bonded networks

Microporous Metal–Organic Framework Stabilized by Balanced Multiple Host–Couteranion Hydrogen-Bonding Interactions for High-Density CO₂ Capture at Ambient Conditions

Microporous metal organic frameworks (MOFs) show promising application in several fields, but they often suffer from the weak robustness and stability after the removal of guest molecules. Here, three isostructural cationic metal–organic frameworks {[(Cu₄Cl)­(cpt)₄(H₂O)₄]·3X·4DMAc·CH₃OH·5H₂O} (<b>FJU-14</b>, X = NO₃, ClO_4, BF₄; DMAc = <i>N</i>,<i>N</i>′-dimethylacetamide) containing two types of polyhedral nanocages, one octahedron, and another tetrahedron have been synthesized from bifunctional organic ligands 4-(4<i>H</i>-1,2,4-triazol-4-yl) benzoic acid (Hcpt) and various copper salts. The series of MOFs <b>FJU-14</b> are demonstrated as the first examples of the isostructural MOFs whose robustness, thermal stability, and CO₂ capacity can be greatly improved via rational modulation of counteranions in the tetrahedral cages. The activated <b>FJU-14-BF</b>_<b>4</b><b>-a</b> containing BF₄^– anion can take CO₂ of 95.8 cm³ cm^–3 at ambient conditions with an adsorption enthalpy only of 18.8 kJ mol^–1. The trapped CO₂ density of 0.955 g cm^–3 is the highest value among the reported MOFs. Dynamic fixed bed breakthrough experiments indicate that the separation of CO₂/N₂ mixture gases through a column packed with <b>FJU-14-BF</b>_<b>4</b><b>-a</b> solid can be efficiently achieved. The improved robustness and thermal stability for <b>FJU-14-BF</b>_<b>4</b><b>-a</b> can be attributed to the balanced multiple hydrogen-bonding interactions (MHBIs) between the BF₄^– counteranion and the cationic skeleton, while the high-density and low-enthalpy CO₂ capture on <b>FJU-14-BF</b>_<b>4</b><b>-a</b> can be assigned to the multiple-point interactions between the adsorbate molecules and the framework as well as with its counteranions, as proved by single-crystal structures of the guest-free and CO₂-loaded <b>FJU-14-BF</b>_<b>4</b><b>-a</b> samples

Microporous Metal–Organic Framework Stabilized by Balanced Multiple Host–Couteranion Hydrogen-Bonding Interactions for High-Density CO₂ Capture at Ambient Conditions

Microporous metal organic frameworks (MOFs) show promising application in several fields, but they often suffer from the weak robustness and stability after the removal of guest molecules. Here, three isostructural cationic metal–organic frameworks {[(Cu₄Cl)­(cpt)₄(H₂O)₄]·3X·4DMAc·CH₃OH·5H₂O} (<b>FJU-14</b>, X = NO₃, ClO_4, BF₄; DMAc = <i>N</i>,<i>N</i>′-dimethylacetamide) containing two types of polyhedral nanocages, one octahedron, and another tetrahedron have been synthesized from bifunctional organic ligands 4-(4<i>H</i>-1,2,4-triazol-4-yl) benzoic acid (Hcpt) and various copper salts. The series of MOFs <b>FJU-14</b> are demonstrated as the first examples of the isostructural MOFs whose robustness, thermal stability, and CO₂ capacity can be greatly improved via rational modulation of counteranions in the tetrahedral cages. The activated <b>FJU-14-BF</b>_<b>4</b><b>-a</b> containing BF₄^– anion can take CO₂ of 95.8 cm³ cm^–3 at ambient conditions with an adsorption enthalpy only of 18.8 kJ mol^–1. The trapped CO₂ density of 0.955 g cm^–3 is the highest value among the reported MOFs. Dynamic fixed bed breakthrough experiments indicate that the separation of CO₂/N₂ mixture gases through a column packed with <b>FJU-14-BF</b>_<b>4</b><b>-a</b> solid can be efficiently achieved. The improved robustness and thermal stability for <b>FJU-14-BF</b>_<b>4</b><b>-a</b> can be attributed to the balanced multiple hydrogen-bonding interactions (MHBIs) between the BF₄^– counteranion and the cationic skeleton, while the high-density and low-enthalpy CO₂ capture on <b>FJU-14-BF</b>_<b>4</b><b>-a</b> can be assigned to the multiple-point interactions between the adsorbate molecules and the framework as well as with its counteranions, as proved by single-crystal structures of the guest-free and CO₂-loaded <b>FJU-14-BF</b>_<b>4</b><b>-a</b> samples

Metastable Interwoven Mesoporous Metal–Organic Frameworks

Three isostructural interwoven 3,4-connected mesoporous metal–organic frameworks of pto-a topology (<b>UTSA-28-Cu</b>, <b>UTSA-28-Zn</b>, and <b>UTSA-28-Mn</b>) were synthesized and structurally characterized. Because of their metastable nature, their gas sorption properties are highly dependent on the metal ions and activation profiles. The most stable, <b>UTSA-28a-Cu</b>, exhibits promising gas storage and separation capacities

Triple Framework Interpenetration and Immobilization of Open Metal Sites within a Microporous Mixed Metal–Organic Framework for Highly Selective Gas Adsorption

A three-dimensional triply interpenetrated mixed metal–organic framework, Zn₂(BBA)₂(CuPyen)·G_<i>x</i> (<b>M’MOF-20</b>; BBA = biphenyl-4,4′-dicarboxylate; G = guest solvent molecules), of primitive cubic net was obtained through the solvothermal reaction of Zn­(NO₃)₂, biphenyl-4,4′-dicarboxylic acid, and the salen precursor Cu­(PyenH₂)­(NO₃)₂ by a metallo-ligand approach. The triple framework interpenetration has stabilized the framework in which the activated <b>M’MOF-20a</b> displays type-I N₂ gas sorption behavior with a Langmuir surface area of 62 m² g^–1. The narrow pores of about 3.9 Å and the open metal sites on the pore surfaces within <b>M’MOF-20a</b> collaboratively induce its highly selective C₂H₂/CH₄ and CO₂/CH₄ gas separation at ambient temperature

High Anhydrous Proton Conductivity of Imidazole-Loaded Mesoporous Polyimides over a Wide Range from Subzero to Moderate Temperature

On-board fuel cell technology requires proton conducting materials with high conductivity not only at intermediate temperatures for work but also at room temperature and even at subzero temperature for startup when exposed to the colder climate. To develop such materials is still challenging because many promising candidates for the proton transport on the basis of extended microstructures of water molecules suffer from significant damage by heat at temperatures above 80 °C or by freeze below −5 °C. Here we show imidazole loaded tetrahedral polyimides with mesopores and good stability (Im@Td-PNDI <b>1</b> and Im@Td-PPI <b>2</b>) exhibiting a high anhydrous proton conductivity over a wide temperature range from −40 to 90 °C. Among all anhydrous proton conductors, the conductivity of <b>2</b> is the highest at temperatures below 40 °C and comparable with the best materials, His@[Al­(OH)­(1,4-ndc)]_<i>n</i> and [Zn₃(H₂PO₄)₆(H₂O)₃]­(Hbim), above 40 °C

Triple Framework Interpenetration and Immobilization of Open Metal Sites within a Microporous Mixed Metal–Organic Framework for Highly Selective Gas Adsorption

A three-dimensional triply interpenetrated mixed metal–organic framework, Zn₂(BBA)₂(CuPyen)·G_<i>x</i> (<b>M’MOF-20</b>; BBA = biphenyl-4,4′-dicarboxylate; G = guest solvent molecules), of primitive cubic net was obtained through the solvothermal reaction of Zn­(NO₃)₂, biphenyl-4,4′-dicarboxylic acid, and the salen precursor Cu­(PyenH₂)­(NO₃)₂ by a metallo-ligand approach. The triple framework interpenetration has stabilized the framework in which the activated <b>M’MOF-20a</b> displays type-I N₂ gas sorption behavior with a Langmuir surface area of 62 m² g^–1. The narrow pores of about 3.9 Å and the open metal sites on the pore surfaces within <b>M’MOF-20a</b> collaboratively induce its highly selective C₂H₂/CH₄ and CO₂/CH₄ gas separation at ambient temperature