The electrocatalytic reduction of CO2 has been investigated using four Cu‐based metal–organic porous materials supported on gas diffusion electrodes, namely, (1) HKUST‐1 metal–organic framework ...(MOF), Cu3(μ6‐C9H3O6)2n; (2) CuAdeAce MOF, Cu3(μ3‐C5H4N5)2n; (3) CuDTA mesoporous metal–organic aerogel (MOA), Cu(μ‐C2H2N2S2)n; and (4) CuZnDTA MOA, Cu0.6Zn0.4(μ‐C2H2N2S2)n. The electrodes show relatively high surface areas, accessibilities, and exposure of the Cu catalytic centers as well as favorable electrocatalytic CO2 reduction performance, that is, they have a high efficiency for the production of methanol and ethanol in the liquid phase. The maximum cumulative Faradaic efficiencies for CO2 conversion at HKUST‐1‐, CuAdeAce‐, CuDTA‐, and CuZnDTA‐based electrodes are 15.9, 1.2, 6, and 9.9 %, respectively, at a current density of 10 mA cm−2, an electrolyte‐flow/area ratio of 3 mL min cm−2, and a gas‐flow/area ratio of 20 mL min cm−2. We can correlate these observations with the structural features of the electrodes. Furthermore, HKUST‐1‐ and CuZnDTA‐based electrodes show stable electrocatalytic performance for 17 and 12 h, respectively.
Closing the loop: Metal–organic porous materials are effective electrocatalysts for the continuous electrochemical conversion of CO2 to alcohols, a process that could promote the transition to a low‐carbon economy. The modularity of these systems yields many opportunities for further performance improvements and opens new directions in electrocatalysis.
Abstract
The doping of zirconium based EHU-30 and EHU-30-NH
2
metal–organic frameworks with copper(II) yielded a homogeneous distribution of the dopant with a copper/zirconium ratio of 0.04–0.05. The ...doping mechanism is analysed by chemical analysis, microstructural analysis and pair distribution function (PDF) analysis of synchrotron total scattering data in order to get deeper insight into the local structure. According to these data, the Cu(II) atoms are assembled within the secondary building unit by a transmetalation reaction, contrarily to UiO-66 series in which the post-synthetic metalation of the MOF takes place through chemical anchorage. The resulting materials doubled the overall performance of the parent compounds for the CO
2
electroreduction, while retained stable the performance during continuous transformation reaction.
The use of covalent organic frameworks (COFs) in practical applications demands shaping them into macroscopic objects, which remains challenging. Herein, we report a simple three‐step method to ...produce COF aerogels, based on sol‐gel transition, solvent‐exchange, and supercritical CO2 drying, in which 2D imine‐based COF sheets link together to form hierarchical porous structures. The resultant COF aerogel monoliths have extremely low densities (ca. 0.02 g cm−3), high porosity (total porosity values of ca. 99 %), and mechanically behave as elastic materials under a moderate strain (<25–35 %) but become plastic under greater strain. Moreover, these COF aerogels maintain the micro‐ and meso‐porosity of their constituent COFs, and show excellent absorption capacity (e.g. toluene uptake: 32 g g−1), with high removal efficiency (ca. 99 %). The same three‐step method can be used to create functional composites of these COF aerogels with nanomaterials.
A three‐step method produces COF aerogel monoliths, based on sol–gel transition, solvent‐exchange, and supercritical CO2 drying. 2D imine‐based COF sheets link together to form hierarchical porous structures. The aerogels have extremely low densities, high porosity, and mechanically behave as elastic or plastic materials under different strain. They show excellent absorption capacity with high removal efficiency.
Zirconium based metal-organic gels are obtained through a rapid method at room temperature, employing green solvents, in which the role of water is important. These porous materials, decorated with ...Brønsted acid sites, show outstanding thermal and chemical stability prompting them as stable catalyst in the continuous electroreduction of CO2.
Zirconium based metal-organic gels are obtained through a green and rapid method at room temperature in which the role of water seems to be crucial. Display omitted
•Green and rapid method for achieving zirconium based metal-organic gels (MOGs) at room temperature.•Porous structure with thermal and chemical stability together with Brønsted acid sites.•Zr-MOGs as electrodes in the electrocatalytic reduction of CO2 to formic acid.
Several synthetic approaches have been employed to obtain novel {ScM(μ4-pmdc)2(H2O)2·solv} n EHU1(Sc,M) (where M = Li, Na; pmdc = pyrimidine-4,6-dicarboxylate; solv = corresponding solvent) ...compounds. The synthesis method is crucial to determine the type of alkaline that could be hosted in the structure as well as the crystallinity, adsorption performance, and ionic conductivity of the resulting materials. Compared with other synthetic methods, a heat-assisted solvent-free procedure has proven to be the most effective route, giving materials with adsorption capacities close to those expected from GCMC (Grand Canonical Monte Carlo) calculations. Despite the presence of alkaline ions in the framework, the pristine materials exhibit rather low conductivity values of ca. 10–7 S cm–1. The concentration of charge carriers has been increased by means of a doping approach that incorporates divalent transition metal ions to the structure and forces an increase of the alkaline ions, thus raising the ionic conductivity by 1 order of magnitude. Additionally, soaking the samples in solutions containing alkaline salts led to materials possessing an even higher number of carriers achieving conductivity values among the best results reported for MOFs at room temperature, i.e., 4.2 × 10–4 and 9.2 × 10–5 S cm–1 for EHU1(Sc,Li) and EHU1(Sc,Na) obtained by the solvent-free procedure, respectively.
Metal‐organic gels (MOGs) appear as a blooming alternative to well‐known metal‐organic frameworks (MOFs). Porosity of MOGs has a microstructural origin and not strictly crystalline like in MOFs; ...therefore, gelation may provide porosity to any metal‐organic system, including those with interesting properties but without a porous crystalline structure. The easy and straightforward shaping of MOGs contrasts with the need of binders for MOFs. In this contribution, a series of MOGs based on the assembly of 1D‐coordination polymer nanofibers of formula M(DTA)n (MII: Ni, Cu, Pd; DTA: dithiooxamidato) are reported, in which properties such as porosity, chemical inertness, mechanical robustness, and stimuli‐responsive electrical conductivity are brought together. The strength of the MS bond confers an unusual chemical resistance, withstanding exposure to acids, alkalis, and mild oxidizing/reducing chemicals. Supercritical drying of MOGs provides ultralight metal‐organic aerogels (MOAs) with densities as low as 0.03 g cm−3 and plastic/brittle behavior depending on the nanofiber aspect ratio. Conductivity measurements reveal a semiconducting behavior (10−12 to 10−7 S cm−1 at 298 K) that can be improved by doping (10−5 S cm−1). Moreover, it must be stressed that conductivity of MOAs reversibly increases (up to 10−5 S cm−1) under the presence of acetic acid.
A series of metal‐organic gels and aerogels based on the assembly of 1D‐coordination polymer nanofibers of formula M(DTA)n (MII: Ni, Cu, Pd; DTA: dithiooxamidato) are reported, in which properties such as porosity, chemical inertness, mechanical robustness, and stimuli‐responsive electrical conductivity are brought together into an unprecedented material. Gelation approach provides a valuable tool to achieve metal‐organic mesoporous materials.
The hybrid compound Cu(cyclam)(H2O)20.5{Cu(cyclam)}1.5{B-H2As2Mo6O26(H2O)}·9H2O (1) (cyclam = 1,4,8,11-tetraazacyclotetradecane) was synthesized in aqueous solution by reacting the {Cu(cyclam)}2+ ...complex with a mixture of heptamolybdate and an arsenate(V) source. Crystal packing of 1 exhibits a supramolecular open-framework built of discrete covalent molybdoarsenate/metalorganic units and additional Cu(cyclam)(H2O)22+ cations, the stacking of which generates squarelike channels parallel to the z axis with an approximate cross section of 10 × 11 Å2 where all the hydration water molecules are hosted. Thermal evacuation of solvent molecules yields a new anhydrous crystalline phase, but compound 1 does not preserve its single-crystalline nature upon heating. However, when crystals are dehydrated under vacuum, they undergo a structural transformation that proceeds via a single-crystal-to-single-crystal pathway, leading to the anhydrous phase {Cu(cyclam)}2(A-H2As2Mo6O26) (2). Total dehydration results in important modifications within the inorganic cluster skeleton which reveals an unprecedented solid-state B to A isomerization of the polyoxoanion. This transition also involves changes in the CuII bonding scheme that lead to covalent cluster/metalorganic layers by retaining the open-framework nature of 1. Compound 2 adsorbs ambient moisture upon air exposure, but it does not revert back to 1, and the hydrated phase {Cu(cyclam)}2(A-H2As2Mo6O26)·6H2O (2h) is obtained instead. Structural variations between 1 and 2 are reflected in electron paramagnetic resonance spectroscopy measurements, and the permanent microporosity of 2 provides interesting functionalities to the system such as the selective adsorption of gaseous CO2 over N2.
The development of intracrystalline mesoporosity within zeolites has been a long-standing goal in catalysis as it greatly contributes to alleviating the diffusion limitations of these widely used ...microporous materials. The combination of in situ synchrotron X-ray diffraction and liquid-cell transmission electron microscopy enabled the first in situ observation of the development of intracrystalline mesoporosity in zeolites and provided structural and kinetic information on the changes produced in zeolites to accommodate the mesoporosity. The interpretation of the time-resolved diffractograms together with computational simulations evidenced the formation of short-range hexagonally ordered mesoporosity within the zeolite framework, and the in situ electron microscopy studies allowed the direct observation of structural changes in the zeolite during the process. The evidence for the templating and protective role of the surfactant and the rearrangement of the zeolite crystal to accommodate intracrystalline mesoporosity opens new and exciting opportunities for the production of tailored hierarchical zeolites.
A novel imidazolium halometallate molten salt with formula (trimim)FeCl
4
(trimim: 1,2,3-trimethylimidazolium) was synthetized and studied with structural and physico-chemical characterization. ...Variable-temperature synchrotron X-ray powder diffraction (SXPD) from 100 to 400 K revealed two structural transitions at 200 and 300 K. Three different crystal structures were determined combining single crystal X-ray diffraction (SCXD), neutron powder diffraction (NPD), and SXPD. From 100 to 200 K, the compound exhibits a monoclinic crystal structure with space group
P
2
1
/
c
. At 200 K, the former crystal system and space group are retained, but a disorder in the organic cations is introduced. Above 300 K, the structure transits to the orthorhombic space group
Pbcn
, retaining the crystallinity up to 400 K. The study of the thermal expansion process in this temperature range showed anisotropically evolving cell parameters with an axial negative thermal expansion. Such an induction occurs immediately after the crystal phase transition due to the translational and reorientational dynamic displacements of the imidazolium cation within the crystal building. Electrochemical impedance spectroscopy (EIS) demonstrated that this motion implies a high and stable solid-state ionic conduction (range from 4 × 10
−6
S cm
−1
at room temperature to 5.5 × 10
−5
S cm
−1
at 400 K). In addition, magnetization and heat capacity measurements proved the presence of a three-dimensional antiferromagnetic ordering below 3 K. The magnetic structure, determined by neutron powder diffraction, corresponds to ferromagnetic chains along the
a
-axis, which are antiferromagnetically coupled to the nearest neighboring chains through an intricate network of superexchange pathways, in agreement with the magnetometry measurements.
We present a novel halometallate molten salt based on imidazolium cation with two structural transitions from 100 to 400 K which has been studied by X-ray and neutron diffraction techniques. Furthermore, the magnetic structure at low temperature and the ionic conductivity is also described.
This contribution addresses standing questions about the nature and consequences of the ion self-assembly and magnetic structures, as well as the molecular motion of the crystalline structure as a ...function of the temperature, in halometalate materials based on imidazolium cation. We present the magnetic structure and magnetostructural correlations of 1-ethyl-2,3-dimethylimidazolium tetrachloridoferrate, (Edimim)FeCl4, resolved by neutron diffraction studies. Single-crystal, synchrotron powder X-ray diffraction and powder neutron diffraction techniques have been combined to follow the temperature evolution on its crystallographic structure from 2 K close to its melting point (340 K). In this sense, slightly above room temperature (307 K) (Edimim)FeCl4 presents a single-crystal to single-crystal transition (SCSC), from phase I (space group P21/n) to phase II (P21/m), accompanied by a notable increase in the disorder of the imidazolium cation, as well as in the metal complex anion. The temperature evolution and solid-phase transitions of the presented compound were followed in detail by synchrotron X-ray powder diffraction (SXPD), which confirms the occurrence of another phase transition at 330 K, phase III (P21/m), the crystal structure of which was elucidated from the SXPD pattern. Moreover, this material presents an anisotropic thermal expansion with a switch from axial positive to negative thermal expansion coefficients as the temperature is raised above the first phase transition, which has been correlated with the molecular motion of the imidazolium-based molecules, producing not only a shortening of the counterion···counterion distances but also the occurrence of different quasi-isoenergetic crystal structures as a function of the temperature.
PDF
