Rhodium-catalyzed electrochemical hydrodefluorination: A mild approach for the degradation of fluoroaromatic pollutants

Influence of chemical structures on reduction rates and defluorination of fluoroarenes during catalytic reduction using a rhodium-based catalyst

Continuous growth in fluoroarene production has led to environmental pollution and health concerns owing to their persistence, which is attributed to the stable C-F bond in their structures. Herein, we investigated fluoroarene decomposition via hydrodefluorination using a rhodium-based catalyst, focusing on the effects of the chemical structure and functional group on the defluorination yield. Most compounds, except (pentafluoroethyl)benzene, exhibited full or partial reduction with pseudo-first-order rate constants in the range of 0.002-0.396 min-1 and defluorination yields of 0%-100%. Fluoroarenes with hydroxyl, methyl, and carboxylate groups were selected to elucidate how hydrocarbon and oxygen-containing functional groups influence the reaction rate and defluorination. Inhibition of the reaction rate and defluorination yield based on functional groups increased in the order of hydroxyl < methyl < carboxylate, which was identical to the order of the electron-withdrawing effect. Fluoroarenes with polyfluoro groups were also assessed; polyfluoro groups demonstrated a different influence on catalyst activity than non-fluorine functional groups because of fluorine atoms in the substituents undergoing defluorination. The reaction kinetics of (difluoromethyl)fluorobenzenes and their intermediates suggested that hydrogenation and defluorination occurred during degradation. Finally, the effects of the type and position of functional groups on the reaction rate and defluorination yield were investigated via multivariable linear regression analysis. Notably, the electron-withdrawing nature of functional groups appeared to have a greater impact on the defluorination yield of fluoroarenes than the calculated C-F bond dissociation energy.

Molecular Cu Electrocatalyst Escalates Ambient Perfluorooctanoic Acid Degradation

Groundwater reservoirs contaminated with perfluoroalkyl and polyfluoroalkyl substances (PFASs) need purifying remedies. Perfluorooctanoic acid (PFOA) is the most abundant PFAS in drinking water. Although different degradation strategies for PFOA have been explored, none of them disintegrates the PFOA backbone rapidly under mild conditions. Herein, we report a molecular copper electrocatalyst that assists in the degradation of PFOA up to 93% with a 99% defluorination rate within 4 h of cathodic controlled-current electrolysis. The current-normalized pseudo-first-order rate constant has been estimated to be quite high for PFOA decomposition (3.32 L h-1 A-1), indicating its fast degradation at room temperature. Furthermore, comparatively, rapid decarboxylation over the first 2 h of electrolysis has been suggested to be the rate-determining step in PFOA degradation. The related Gibbs free energy of activation has been calculated as 22.6 kcal/mol based on the experimental data. In addition, we did not observe the formation of short-alkyl-chain PFASs as byproducts that are typically found in chain-shortening PFAS degradation routes. Instead, free fluoride (F-), trifluoroacetate (CF3COO-), trifluoromethane (CF3H), and tetrafluoromethane (CF4) were detected as fragmented PFOA products along with the evolution of CO2 using gas chromatography (GC), ion chromatography (IC), and gas chromatography-mass spectrometry (GC-MS) techniques, suggesting comprehensive cleavage of C-C bonds in PFOA. Hence, this study presents an effective method for the rapid degradation of PFOA into small ions/molecules.

Electrosynthetic C-F bond cleavage

Fluorinated organic compounds are common among pharmaceuticals, agrochemicals and materials. The significant strength of the C-F bond results in chemical inertness that, depending on the context, is beneficial, problematic or simply a formidable synthetic challenge. Electrosynthesis is a rapidly expanding methodology that can enable new reactivity and selectivity for cleavage and formation of chemical bonds. Here, a comprehensive overview of synthetically relevant electrochemically driven protocols for C-F bond activation and functionalization is presented, including photoelectrochemical strategies.

Photochemical and Electrochemical Strategies for Hydrodefluorination of Fluorinated Organic Compounds

Hydrodefluorination (HDF) is a very important fundamental transformation for conversion of the C-F bond into the C-H bond in organic synthesis. In the past decade, much progress has been achieved with HDF through the utility of low-valent metals, transition-metal complexes and main-group Lewis acids. Recently, novel methods have been introduced for this purpose through photo- and electrochemical pathways, which are of great significance, due to their considerable environmental and economical advantages. This Review highlights the HDF of fluorinated organic compounds (FOCs) through photo- and electrochemical strategies, along with mechanistic insights.

Feasibility study on the electrochemical reductive decomposition of PFOA by a Rh/Ni cathode

The discharge of widely used per- and poly-fluorinated compounds (PFCs) leads to their environmental prevalence, bioaccumulation and biotoxicity; and attracts researches focusing on their treatment in wastewater. Electrochemical reductive treatment is a promising alternative due to its milder reaction conditions and easy operation. The feasibility of electrochemical reductive decomposition of PFOA using a Rh/Ni cathode was explored. The Rh/Ni cathode was fabricated by coating Rh³⁺ on Ni foil through electrodeposition. The Rh coating was primarily elemental and in a Rh(111) crystalline form. PFOA decomposition and defluorination were observed when using the Rh/Ni cathode where DMF was the solvent and the cathode potential was -1.25 V. A hydrodefluorination reaction was considered having occurred. Because possessing d electrons and empty d orbitals, the Rh coating enhanced PFOA adsorption onto the cathode surface and facilitated CF bond activation through Rh···F interactions. Moreover, the Rh(111) crystal helped chemisorb the generated H* and supply it participating in PFOA decomposition. With the continuous interaction of cathode-supplied electrons, CF bond would ultimately dissociate and transform to CH bond by H* substitution. Adding FeCp₂* as a supporting electrolyte enhanced PFOA decomposition by working as the shuttle facilitating PFOA migration to the cathode surface.

Equivalence of difluorodichloromethane (CFC-12) hydrolysis catalyzed by solid acid(base) MoO3(MgO)/ZrO2

In this paper, a solid acid(base) MoO3(MgO)/ZrO2 was prepared for the catalytic hydrolysis of difluorodichloromethane (CFC-12). The effects of the catalyst preparation method, calcination temperature, and hydrolysis temperature on the conversion rate of CFC-12 were studied. The catalysts were characterized by XRD, N2 isotherm adsorption desorption, NH3-TPD, and CO2-TPD. Meanwhile, the equivalence of the catalytic activity of MoO3(MgO)/ZrO2 for CFC-12 was studied. Research shows that the solid acid MoO3/ZrO2 and solid base MgO/ZrO2 catalyzed hydrolysis of CFC-12 is equivalent; the solid acid MoO3/ZrO2 is calcined at 600 °C for 3 h and the solid base MgO/ZrO2 is calcined at 600 °C for 6 h (co-precipitation) and 700 °C for 6 h (impregnated) at a catalytic hydrolysis temperature of 300-400 °C and CFC-12 concentration of 4%. The catalytic hydrolysis products obtained were CO, HCl, and HF, and the CFC-12 conversion rate almost reached 100%.

Potential-Driven Electron Transfer Lowers the Dissociation Energy of the C-F Bond and Facilitates Reductive Defluorination of Perfluorooctane Sulfonate (PFOS)

The widespread environmental occurrence of per- and polyfluoroalkyl substances (PFAS) has attracted significant regulatory, research, and media attention because of their toxicity, recalcitrance, and ability to bioaccumulate. Perfluorooctane sulfonate (PFOS) is a particularly troublesome member of the PFAS family due to its immunity to biological remediation and radical-based oxidation. In the present study, we present a heterogeneous reductive degradation process that couples direct electron transfer (ET) from surface-modified carbon nanotube electrodes (under low potential conditions) to sorbed PFOS molecules using UV-generated hydrated electrons without any further chemical addition. We demonstrate that the ET process dramatically increases the PFOS defluorination rate while yielding shorter chain (C₃-C₇) perfluorinated acids and present both experimental and ab initio evidence of the synergistic relationship between electron addition to sorbed molecules and their ability to react with reductive hydrated electrons. Because of the low energy consumption associated with the ET process, the use of standard medium-pressure UV lamps and no further chemical addition, this reductive degradation process is a promising method for the destruction of persistent organic pollutants, including PFAS and other recalcitrant halogenated organic compounds.