Enhanced sorption in an indium-acetylenedicarboxylate metal-organic framework with unexpected chains of cis-μ-OH-connected {InO6} octahedra

Single crystals of the new metal-organic framework (MOF) In-adc (HHUD-4) were obtained through the reaction of linear acetylenedicarboxylic acid (H₂adc) with In(NO₃)₃·xH₂O as a racemic conglomerate in the chiral tetragonal space groups P4₃22 and P4₁22. Fundamentally different from other MOFs with linear linkers and trans-μ-OH-connected infinite {MO₆} secondary building units as in the MIL-53-type, the linear adc^2- linker leads to the formation of cis-μ-OH connected {InO₆} polyhedra, which have otherwise only been found before for V-shaped ligands, as in CAU-10-H. A far-reaching implication of this finding is the possibility that trans-μ-OH/straight MIL-53-type MOFs will have polymorphs of CAU-10-H cis-μ-OH/helical topology and vice versa. HHUD-4 is a microporous MOF with a BET surface area of up to 940 m² g^-1 and a micropore volume of up to 0.39 cm³ g^-1. Additionally, HHUD-4 features good adsorption uptakes of 3.77 mmol g^-1 for CO₂ and 1.25 mmol g^-1 for CH₄ at 273 K and 1 bar, respectively, and a high isosteric heat of adsorption of 11.4 kJ mol^-1 for H₂ with a maximum uptake of 6.36 mmol g^-1 at 77 K and 1 bar. Vapor sorption experiments for water and volatile organic compounds (VOCs) such as benzene, cyclohexane and n-hexane yielded uptake values of 135, 269, 116 and 205 mg g^-1, respectively, at 293 K. While HHUD-4 showed unremarkable results for water uptake and low stability for water, it exhibited good stability with steep VOC uptake steps at low relative pressures and a high selectivity of 17 for benzene/cyclohexane mixtures.

Modulation of the Sorption Characteristics for an H-bonded porous Architecture by Varying the Chemical Functionalization of the Channel Walls

Five isostructural microporous supramolecular architectures prepared by H-bonded assembly between the hexa-anionic complex [Zr₂ (Ox)₇ ]^6- (Ox=oxalate, (C₂ O₄ )^2- ) and tripodal cations (H₃ -TripCH₂ -R)³⁺ with R=H, CH₃ , OH and OBn (Bn=CH₂ Ph) are reported. The possibility to obtain the same structure using a mixture of tripodal cations with different R group (R=OH and R=CH₃ ) has also been successfully explored, providing a unique example of three-component H-bonded porous framework. The resulting SPA-1(R) materials feature 1D pores decorated by R groups, with apparent pore diameters ranging from 3.0 to 8.5 Å. Influence of R groups on the sorption properties of these materials is evidenced through CO₂ and H₂ O vapor sorption/desorption experiments, as well as with I₂ capture/release experiments in liquid media. This study is one of the first to demonstrate the possibility of tuning the porosity and exerting precise control over the chemical functionalization of the pores in a given H-bonded structure, without modifying the topology of the reference structure, and thus finely adjusting the sorption characteristics of the material.

The Complexity of Comparative Adsorption of C6 Hydrocarbons (Benzene, Cyclohexane, n-Hexane) at Metal-Organic Frameworks

The relatively stable MOFs Alfum, MIL-160, DUT-4, DUT-5, MIL-53-TDC, MIL-53, UiO-66, UiO-66-NH₂, UiO-66(F)₄, UiO-67, DUT-67, NH₂-MIL-125, MIL-125, MIL-101(Cr), ZIF-8, ZIF-11 and ZIF-7 were studied for their C₆ sorption properties. An understanding of the uptake of the larger C₆ molecules cannot simply be achieved with surface area and pore volume (from N₂ sorption) but involves the complex micropore structure of the MOF. The maximum adsorption capacity at p p₀^-1 = 0.9 was shown by DUT-4 for benzene, MIL-101(Cr) for cyclohexane and DUT-5 for n-hexane. In the low-pressure range from p p₀^-1 = 0.1 down to 0.05 the highest benzene uptake is given by DUT-5, DUT-67/UiO-67 and MIL-101(Cr), for cyclohexane and n-hexane by DUT-5, UiO-67 and MIL-101(Cr). The highest uptake capacity at p p₀^-1 = 0.02 was seen with MIL-53 for benzene, MIL-125 for cyclohexane and DUT-5 for n-hexane. DUT-5 and MIL-101(Cr) are the MOFs with the widest pore window openings/cross sections but the low-pressure uptake seems to be controlled by a complex combination of ligand and pore-size effect. IAST selectivities between the three binary mixtures show a finely tuned and difficult to predict interplay of pore window size with (critical) adsorptive size and possibly a role of electrostatics through functional groups such as NH₂.

Synthesis and Characterization of a Crystalline Imine-Based Covalent Organic Framework with Triazine Node and Biphenyl Linker and Its Fluorinated Derivate for CO2/CH4 Separation

A catalyst-free Schiff base reaction was applied to synthesize two imine-linked covalent organic frameworks (COFs). The condensation reaction of 1,3,5-tris-(4-aminophenyl)triazine (TAPT) with 4,4'-biphenyldicarboxaldehyde led to the structure of HHU-COF-1 (HHU = Heinrich-Heine University). The fluorinated analog HHU-COF-2 was obtained with 2,2',3,3',5,5',6,6'-octafluoro-4,4'-biphenyldicarboxaldehyde. Solid-state NMR, infrared spectroscopy, X-ray photoelectron spectroscopy, and elemental analysis confirmed the successful formation of the two network structures. The crystalline materials are characterized by high Brunauer-Emmett-Teller surface areas of 2352 m²/g for HHU-COF-1 and 1356 m²/g for HHU-COF-2. The products of a larger-scale synthesis were applied to prepare mixed-matrix membranes (MMMs) with the polymer Matrimid. CO₂/CH₄ permeation tests revealed a moderate increase in CO₂ permeability at constant selectivity for HHU-COF-1 as a dispersed phase, whereas application of the fluorinated COF led to a CO₂/CH₄ selectivity increase from 42 for the pure Matrimid membrane to 51 for 8 wt% of HHU-COF-2 and a permeability increase from 6.8 to 13.0 Barrer for the 24 wt% MMM.

Metal-organic framework structures of fused hexagonal motifs with cuprophilic interactions of a triangular Cu(I)3(pyrazolate-benzoate) metallo-linker

The reaction of the N,O-heteroditopic bifunctional ligand 4-(3,5-dimethyl-1H-pyrazol-4-yl)benzoic acid (H2mpba) with Cu(NO3)2·2.5H2O and Zn(NO3)2·4H2O or Zn(CH3COO)2·2H2O in N,N-dimethylformamide (DMF) results in concomitant formation of three bimetallic metal-organic frameworks (MOFs) with...

Biphenyl-Based Covalent Triazine Framework/Matrimid® Mixed-Matrix Membranes for CO2/CH4 Separation

Processes, such as biogas upgrading and natural gas sweetening, make CO₂/CH₄ separation an environmentally relevant and current topic. One way to overcome this separation issue is the application of membranes. An increase in separation efficiency can be achieved by applying mixed-matrix membranes, in which filler materials are introduced into polymer matrices. In this work, we report the covalent triazine framework CTF-biphenyl as filler material in a matrix of the glassy polyimide Matrimid^®. MMMs with 8, 16, and 24 wt% of the filler material are applied for CO₂/CH₄ mixed-gas separation measurements. With a CTF-biphenyl loading of only 16 wt%, the CO₂ permeability is more than doubled compared to the pure polymer membrane, while maintaining the high CO₂/CH₄ selectivity of Matrimid^®.

Cucurbit[6]uril@MIL-101-Cl: loading polar porous cages in mesoporous stable host for enhanced SO2 adsorption at low pressures

The robust cucurbituril-MOF composite CB6@MIL-101-Cl was synthesized by a wet impregnation method and a concomitant OH-to-Cl ligand exchange {CB6 = cucurbit[6]uril, 31 wt% content in the composite, MIL-101-Cl = [Cr₃(O)Cl(H₂O)₂(BDC)₃], BDC = benzene-1,4-dicarboxylate}. MIL-101-Cl was formed postsynthetically from standard fluorine-free MIL-101 where Cr-OH ligands were substituted by Cl during treatment with HCl. CB6@MIL-101-Cl combines the strong SO₂ affinity of the rigid CB6 macrocycles and the high SO₂ uptake capacity of MIL-101, and shows a high SO₂ uptake of 438 cm³ g^-1 (19.5 mmol g^-1) at 1 bar and 293 K (380 cm³ g^-1, 17.0 mmol g^-1 at 1 bar and 298 K). The captured SO₂ amount is 2.2 mmol g^-1 for CB6@MIL-101-Cl at 0.01 bar and 293 K (2.0 mmol g^-1 at 298 K), which is three times higher than that of the parent MIL-101 (0.7 mmol g^-1) under the same conditions. The near zero-coverage SO₂ adsorption enthalpies of MIL-101 and CB6@MIL-101-Cl are -35 kJ mol^-1 and -50 kJ mol^-1, respectively, reflecting the impact of the incorporated CB6 macrocycles, having higher affinity towards SO₂. FT-IR spectroscopy confirms the interactions of the SO₂ with the cucurbit[6]uril moieties of the CB6@MIL-101-Cl composite and SO₂ retention for a few minutes under ambient air. Comparative experiments demonstrated loss of crystallinity and porosity after dry SO₂ adsorption for MIL-101, while CB6@MIL-101-Cl exhibits nearly complete retention of crystallinity and porosity under the exposure to both dry and wet SO₂. Thus, CB6@MIL-101-Cl can be an attractive adsorbent for SO₂ capture because of its excellent recycling stability, high capacity and strong affinity toward SO₂ at low pressure.

Cucurbituril-Encapsulating Metal-Organic Framework via Mechanochemistry: Adsorbents with Enhanced Performance

The first examples of monolithic crystalline host-guest hybrid materials are described. The reaction of 1,3,5-benzenetricarboxylic acid (H3 BTC) and Fe(NO3 )3 ⋅9 H2 O in the presence of decamethylcucurbit[5]uril ammonium chloride (MC5⋅2 NH4 Cl⋅4 H2 O) directly affords MC5@MIL-100(Fe) hybrid monoliths featuring hierarchical micro-, meso- and macropores. Particularly, this "bottle-around-ship" synthesis and one-pot shaping are facilitated by a newly discovered Fe-MC5 flowing gel formed by mechanochemistry. The designed MC5@MIL-100(Fe) hybrid material with MC5 as active domains shows enhanced CH4 and lead(II) uptake performance, and selective capture of lead(II) cations at low concentrations. This shows that host-guest hybrid materials can exhibit synergic properties that out-perform materials based on individual components.

A Highly-Efficient Oxygen Evolution Electrocatalyst Derived from a Metal-Organic Framework and Ketjenblack Carbon Material

The composite of the metal-organic framework (MOF) Ni(Fe)-MOF-74 and the highly conductive carbon material ketjenblack (KB) could be easily obtained from the in-situ MOF synthesis in a one-step solvothermal reaction. The composite material features a remarkable electrochemical oxygen evolution reaction (OER) performance where the overpotential at 10 mA/cm2 and the current density at 1.7 VRHE are recorded as 0.274 VRHE and 650 mA/cm2 , respectively, in 1 mol/L KOH. In particular, the activation of nickel-iron clusters from the MOF under an applied anodic bias steadily boosts the OER performance. Although Ni(Fe)-MOF-74 goes through some structural modification during the electrochemical measurements, the stabilized and optimized composite material shows excellent OER performance. This simple strategy to design highly-efficient electrocatalysts, utilizing readily available precursors and carbon materials, will leverage the use of diverse metal-organic complexes into electrode fabrication with a high energy conversion efficiency.

Photocatalytic asymmetric epoxidation of trans-stilbene with manganese–porphyrin/graphene-oxide nanocomposite and molecular oxygen: axial ligand effect

An efficient, visible light-driven manganese–porphyrin photocatalyst was developed for the asymmetric epoxidation of trans-stilbene by molecular oxygen under mild conditions.

Discovery of a novel thermostable Zn2+ -dependent alcohol dehydrogenase from Chloroflexus aurantiacus through conserved domains mining

AIMS

The purpose of the study was to demonstrate feasibility of the Conserved Domains Database (CDD) for identification of novel biocatalysts with desirable properties from a class of well-characterized biocatalysts.

METHODS AND RESULTS

The thermostable ADH from Sulfolobus solfataricus with a broad substrate range was applied as a template for the search for novel thermostable ADHs via CDD. From the resulting hits, a putative ADH gene from the thermophilic organism Chloroflexus aurantiacus was cloned and expressed in Escherichia coli. The resulting enzyme was purified and characterized. With a temperature activity optimum of 70°C and a broad substrate spectrum especially for diketones, a versatile new biocatalyst was obtained.

CONCLUSIONS

Database-based mining in CDD is a suitable approach to obtain novel biocatalysts with desirable properties. Thereby, the available diversity of similar but not equal enzymes within this class can be increased.

SIGNIFICANCE AND IMPACT OF THE STUDY

For industrial applications, there is a demand for larger diversity of similar well-characterized enzymes in order to test them for a given process (biodiversity screening). For fundamental science, the comparison of enzymes with similar function but different sequence can provide insight into structure function relationships or the evolution of enzymes. This study gives a good example on how this demand can be efficiently met.