Metal-Organic Framework-Derived Electrocatalysts Competent for the Conversion of Acrylonitrile to Adiponitrile.

Electrochemical conversion of acrylonitrile (AN) to produce adiponitrile (ADN), the raw material for the production of Nylon 66, has become a crucial process owing to the increasing market demand of Nylon 66. Although the metallic Pb or Cd electrodes are commonly used for this reaction, the use of electrocatalysts or electrodes modified with catalysts has been barely investigated. In this study, nanoporous and electrically conductive metal-organic framework (MOF)-derived materials composed of Pb, PbO, and carbon are synthesized by carbonizing a Pb-based MOF through thermal treatments, and these MOF-derived materials are served as electrocatalysts for the electrosynthesis of ADN. The crystallinity, morphology, elemental composition, porosity, electrical conductivity, and electrochemically active surface area of each MOF-derived material are investigated. Mass-transport-corrected Tafel analysis is used to probe the enhanced kinetics for the electrochemical reduction of AN occurring at the electrode modified with the MOF-derived material. Electrolytic experiments at various applied potentials are conducted to quantify the production rate and Faradaic efficiency toward ADN, and the result shows that the MOF-derived materials can act as electrocatalysts to initiate the electrochemical reduction of AN to produce ADN at a reduced overpotential. The optimal MOF-derived electrocatalyst can achieve a Faradaic efficiency of 67% toward ADN at an applied potential of -0.85 V versus reversible hydrogen electrode─a much lower overpotential compared to that typically required for this reaction without the use of catalysts. Findings here shed light on the design and development of advanced electrocatalysts to boost the performances for the electrosynthesis of ADN.

Probing the electronic and ionic transport in topologically distinct redox-active metal-organic frameworks in aqueous electrolytes.

Three topologically distinct zirconium-based metal-organic frameworks (Zr-MOFs) constructed from redox-innocent linkers, MOF-808, defective UiO-66, and CAU-24, are synthesized, and the spatially dispersed redox-active manganese sites are post-synthetically immobilized on the hexa-zirconium nodes of these Zr-MOFs. The crystallinity, morphology, porosity, manganese loading, and bulk electrical conductivity of each material are studied. The redox-hopping-based electrochemical reaction between the installed Mn(III) and Mn(IV) occurring within the thin films of these MOFs in aqueous electrolytes is investigated, in the presence of various concentrations of Na2SO4 in the electrolytes. Cyclic voltammetry is used to qualitatively study the redox-hopping process, and chronoamperometry is used to quantify the electrochemically active fractions of manganese sites within the MOF thin film as well as the values of apparent diffusivity for the redox-hopping process. By adjusting the concentration of Na2SO4 in the electrolyte, the rate-determining step for the redox-hopping process can be tuned from ionic transport to electronic transport, and the Mn-decorated MOF-808, which possesses the largest pore size, can achieve the highest value of apparent diffusivity. Findings here shed light on the selection of Zr-MOF as well as the choice of electrolyte concentration for the applications of MOFs in supercapacitors and electrocatalysis relying on such redox-hopping processes in aqueous electrolytes.

Proton-Conductive Cerium-Based Metal-Organic Frameworks.

In this study, proton-conducting behaviors of a cerium-based metal-organic framework (MOF), Ce-MOF-808, its zirconium-based isostructural MOF, and bimetallic MOFs with various Zr-to-Ce ratios are investigated. The significantly increased proton conductivity (σ) and decreased activation energy (Ea) are obtained by substituting Zr with Ce in the nodes of MOF-808. Ce-MOF-808 achieves a σ of 4.4 × 10-3 S/cm at 25 °C under 99% relative humidity and an Ea of 0.14 eV; this value is among the lowest-reported Ea of proton-conductive MOFs. Density functional theory calculations are utilized to probe the proton affinities of these MOFs. As the first study reporting the proton conduction in cerium-based MOFs, the finding here suggests that cerium-based MOFs should be a better platform for the design of proton conductors compared to the commonly reported zirconium-based MOFs in future studies on energy-related applications.