Switchable Conductive MOF–Nanocarbon Composite Coatings as Threshold Sensing Architectures

Switchable metal–organic frameworks (MOFs) showing pronounced and stepwise volume changes as a response toward external stimuli such as partial pressure changes were integrated into electron conductive composites to generate novel threshold sensors with pronounced resistivity changes when approaching a critical partial pressure. Two “gate pressure” MOFs (DUT-8­(Ni), DUT = Dresden University of Technology, and ELM-11, ELM = Elastic Layer-structured MOF) and one “breathing” MOF (MIL-53­(Al), MIL = Material Institute Lavoisier) are shown to cover a wide range of detectable gas concentrations (∼20–80%) using this concept. The highest resistance change is observed for composites containing a percolating carbon nanoparticle network (slightly above the percolation threshold concentration). The volume change of the MOF particles disrupts the percolating network, resulting in a colossal resistance change up to 7500%. Repeated threshold detection is particularly feasible using MIL-53­(Al) due to its high mechanical and chemical stability, even enabling application of the composite sensor concept in ambient environment for the detection of volatile organic compounds at high concentration levels

Advanced Structural Analysis of Nanoporous Materials by Thermal Response Measurements

Thermal response measurements based on optical adsorption calorimetry are presented as a versatile tool for the time-saving and profound characterization of the pore structure of porous carbon-based materials. This technique measures the time-resolved temperature change of an adsorbent during adsorption of a test gas. Six carbide and carbon materials with well-defined nanopore architecture including micro- and/or mesopores are characterized by thermal response measurements based on <i>n</i>-butane and carbon dioxide as the test gases. With this tool, the pore systems of the model materials can be clearly distinguished and accurately analyzed. The obtained calorimetric data are correlated with the adsorption/desorption isotherms of the materials. The pore structures can be estimated from a single experiment due to different adsorption enthalpies/temperature increases in micro- and mesopores. Adsorption/desorption cycling of <i>n</i>-butane at 298 K/1 bar with increasing desorption time allows to determine the pore structure of the materials in more detail due to different equilibration times. Adsorption of the organic test gas at selected relative pressures reveals specific contributions of particular pore systems to the increase of the temperature of the samples and different adsorption mechanisms. The use of carbon dioxide as the test gas at 298 K/1 bar provides detailed insights into the ultramicropore structure of the materials because under these conditions the adsorption of this test gas is very sensitive to the presence of pores smaller than 0.7 nm

Aqueous Solution Process for the Synthesis and Assembly of Nanostructured One-Dimensional α‑MoO₃ Electrode Materials

A low-temperature aqueous solution synthesis of nanostructured one-dimensional (1D) molybdenum trioxide (MoO₃) was developed. The subsequent self-assembly of the fibers to form large-scale freestanding films was achieved without any assistance of organic compounds. Indeed, the whole process, from synthesis to assembly, does not require toxic organic solvents. As an example of the application of our synthesized materials, we built two types of half-cell lithium-ion batteries: (i) the cathode made out of 1D MoO₃, having the width in 50–100 nm, with the length in micro scale, and with thickness in ∼10 nm, and (ii) the anode made out of the macroscopic oxide papers consisting of 1D MoO₃ and carbon materials. As a cathode material, 1D MoO₃ showed a high rate capability with a stable cycle performance up to 20 A g^–1 as a result of a short Li⁺ diffusion path along the [101] direction and less grain boundaries. As an anode material, the composite paper compound showed a first specific discharge capacity of 800 mAh g^–1. These findings indicate not only an affordable, eco-efficient synthesis and assembly of nanomaterials but also show a new attractive strategy toward a possible full aqueous process for a large-scale fabrication of freestanding oxide paper compounds without any toxic organic solvent

Zr- and Hf-Based Metal–Organic Frameworks: Tracking Down the Polymorphism

Six novel Zr­(IV)- and Hf­(IV)-based MOFs, namely DUT-67, DUT-68, and DUT-69 (DUT, Dresden University of Technology) were obtained using a modulated synthesis approach with the acetic acid as a modulator and the bent 2,5-thiophenedicarboxylate (tdc^2–) as a ligand. The modulator not only increases the size of the MOF crystallites but also plays a role of a structure directing agent, affecting both the secondary building unit (SBU) connectivity and topology of the resulting frameworks. The structure of DUT-67 is based on the <b>reo</b> underlying net, characteristic for its cuboctahedral and octahedral pores and is therefore isoreticular to DUT-51. The DUT-68 material has a more complicated hierarchical pore system including rhombicuboctahedral mesopore, surrounded by cuboctahedral, square-antiprismatic and octahedral microcages. DUT-69 is the first example of Zr-based MOF containing 10-connected SBU. DUT-69 has <b>bct</b> topology, possessing octahedral cages and channels running along one crystallographic direction. In accordance with X-ray single crystal analysis, the pores of DUT-67 and DUT-68, which were obtained at high modulator concentrations, are partially occupied by additional clusters. All novel materials are found to be robust, hydrophilic, chemically, and thermally stable. The BET specific surface area amounts to 1064 and 810 m²·g^–1 for DUT-67­(Zr) and DUT-67­(Hf), 891 and 749 m²·g^–1 for DUT-68­(Zr) and DUT-68­(Hf), and 560 and 450 m²·g^–1 for DUT-69­(Zr) and DUT-69­(Hf), respectively

Gas Storage in a Partially Fluorinated Highly Stable Three-Dimensional Porous Metal–Organic Framework

A partially fluorinated linear rigid linker, 2,2′-bis-trifluoromethyl-biphenyl-4,4′-dicarboxylic acid (H₂bfbpdc), has been synthesized. This linker forms a porous three-dimensional (3D) metal–organic framework, {[Zn₄O­(bfbpdc)₃(bpy)_0.5(H₂O)]·(3DMF)­(H₂O)}_<i>n</i> (<b>1</b>), in the presence of the colinker 4,4′-bipyridine (bpy) and Zn­(NO₃)₂·6H₂O under solvothermal condition. Single crystal X-ray analysis shows that <b>1</b> contains a 3D channel structure with highly polar pore surfaces decorated with pendant trifluoromethyl groups of bfbpdc^2– linker. Thermogravimetric analysis (TGA) and variable temperature powder X-ray diffraction (VTPXRD) exhibit high thermal stability of the framework. The solvent molecules present in the voids can be removed by heating, maintaining the integrity of the structure to afford a porous framework. Gas (N₂, H₂, CH₄, and CO₂) and water adsorption studies were performed on this framework

Zr- and Hf-Based Metal–Organic Frameworks: Tracking Down the Polymorphism

Six novel Zr­(IV)- and Hf­(IV)-based MOFs, namely DUT-67, DUT-68, and DUT-69 (DUT, Dresden University of Technology) were obtained using a modulated synthesis approach with the acetic acid as a modulator and the bent 2,5-thiophenedicarboxylate (tdc^2–) as a ligand. The modulator not only increases the size of the MOF crystallites but also plays a role of a structure directing agent, affecting both the secondary building unit (SBU) connectivity and topology of the resulting frameworks. The structure of DUT-67 is based on the <b>reo</b> underlying net, characteristic for its cuboctahedral and octahedral pores and is therefore isoreticular to DUT-51. The DUT-68 material has a more complicated hierarchical pore system including rhombicuboctahedral mesopore, surrounded by cuboctahedral, square-antiprismatic and octahedral microcages. DUT-69 is the first example of Zr-based MOF containing 10-connected SBU. DUT-69 has <b>bct</b> topology, possessing octahedral cages and channels running along one crystallographic direction. In accordance with X-ray single crystal analysis, the pores of DUT-67 and DUT-68, which were obtained at high modulator concentrations, are partially occupied by additional clusters. All novel materials are found to be robust, hydrophilic, chemically, and thermally stable. The BET specific surface area amounts to 1064 and 810 m²·g^–1 for DUT-67­(Zr) and DUT-67­(Hf), 891 and 749 m²·g^–1 for DUT-68­(Zr) and DUT-68­(Hf), and 560 and 450 m²·g^–1 for DUT-69­(Zr) and DUT-69­(Hf), respectively

Modular Construction of a Porous Organometallic Network Based on Rhodium Olefin Complexation

We describe the rational design and synthesis of the first member of a new class of microporous materials. It is built from rhodium and a polyolefinic ligand featuring a rigid tetraphenylsilane backbone via metal olefin complexation, creating a truly organometallic network. The resulting framework, denoted as DUT-37 (Dresden University of Technology no. 37) exhibits considerable porosity and unprecedented stability under ambient conditions. Furthermore, it is catalytically active in transfer hydrogenation

Proline Functionalized UiO-67 and UiO-68 Type Metal–Organic Frameworks Showing Reversed Diastereoselectivity in Aldol Addition Reactions

Functionalization of dicarboxylate linkers with proline was used to generate catalytically active metal–organic frameworks (MOFs) for diastereoselective aldol addition. Due to high robustness and chemical stability, zirconium based MOFs, namely UiO-67 and UiO-68, were chosen as catalyst hosts. During the MOF synthesis, utilizing Boc protected proline functionalized linkers H₂bpdc-NHProBoc and H₂tpdc-NHProBoc, <i>in situ</i> deprotection of the Boc groups without racemization is achieved, enabling direct application of the enantiopure, homochiral MOFs in catalytic reaction, without further postsynthetic treatment. Solvent screening and kinetic studies as well as cycling tests were used to evaluate the conditions for diastereoselective aldol addition using a model reaction of 4-nitrobenzaldehyde and cyclohexanone. High yields (up to 97%) were achieved in reasonable reaction time using ethanol as solvent. In comparison to homocatalytic reactions catalyzed by l-proline and its derivatives, MOFs showed opposite diastereoselectivity attributed to the catalytic sites in confined pore space rendering this class of materials as promising catalysts for fine chemicals production

Crystallographic Information File from The modulator driven polymorphism of Zr(IV) based metal-organic frameworks

Crystallographic data for DUT-12

Modular Construction of a Porous Organometallic Network Based on Rhodium Olefin Complexation

We describe the rational design and synthesis of the first member of a new class of microporous materials. It is built from rhodium and a polyolefinic ligand featuring a rigid tetraphenylsilane backbone via metal olefin complexation, creating a truly organometallic network. The resulting framework, denoted as DUT-37 (Dresden University of Technology no. 37) exhibits considerable porosity and unprecedented stability under ambient conditions. Furthermore, it is catalytically active in transfer hydrogenation