Electrocatalytic Dechlorination of Dichloromethane in Water Using a Heterogenized Molecular Copper Complex

Plant-substrate biochar properties critical for mediating reductive debromination of 1,2-dibromoethane

Dibromoethane is a widespread, persistent organic pollutant. Biochars are known mediators of reductive dehalogenation by layered FeII-FeIII hydroxides (green rust), which can reduce 1,2-dibromoethane to innocuous bromide and ethylene. However, the critical characteristics that determine mediator functionality are lesser known. Fifteen biochar substrates were pyrolyzed at 600 °C and 800 °C, characterized by elemental analysis, X-ray photo spectrometry C and N surface speciation, X-ray powder diffraction, specific surface area analysis, and tested for mediation of reductive debromination of 1,2-dibromoethane by a green rust reductant under anoxic conditions. A statistical analysis was performed to determine the biochar properties, critical for debromination kinetics and total debromination extent. It was shown that selected plant based biochars can mediate debromination of 1,2-dibromoethane, that the highest first order rate constant was 0.082/hr, and the highest debromination extent was 27% in reactivity experiments with 0.1 µmol (20 µmol/L) 1,2-dibromoethane, ≈ 22 mmol/L FeIIGR, and 0.12 g/L soybean meal biochar (7 days). Contents of Ni, Zn, N, and P, and the relative contribution of quinone surface functional groups were significantly (p < 0.05) positively correlated with 1,2-dibromoethane debromination, while adsorption, specific surface area, and the relative contribution of pyridinic N oxide surface groups were significantly negatively correlated with debromination.

Ambient Electroreductive Carboxylation of Unactivated Alkyl Chlorides and Polyvinyl Chloride (PVC) Upgrading

Electrosynthesis of alkyl carboxylic acids upon activating stronger alkyl chlorides at low-energy cost is desired in producing carbon-rich feedstock. Carbon dioxide (CO2), a greenhouse gas, has been recognized as an ideal primary carbon source for those syntheses, and such events also mitigate the atmospheric CO2 level, which is already alarming. On the other hand, the promising upcycling of polyvinyl chloride to polyacrylate is a high energy-demanding carbon-chloride (C-Cl) bond activation process. Molecular catalysts that can efficiently perform such transformation under ambient reaction conditions are rarely known. Herein, we reveal a nickel (Ni)-pincer complex that catalyzes the electrochemical upgrading of polyvinyl chloride to polyacrylate in 95 % yield. The activities of such a Ni electrocatalyst bearing a redox-active ligand were also tested to convert diverse examples of unactivated alkyl chlorides to their corresponding carboxylic acid derivatives. Furthermore, electronic structure calculations revealed that CO2 binding occurs in a resting state to yield an η2-CO2 adduct and that the C-Cl bond activation step is the rate-determining transition state, which has an activation energy of 19.3 kcal/mol. A combination of electroanalytical methods, control experiments, and computational studies were also carried out to propose the mechanism of the electrochemical C-Cl activation process with the subsequent carboxylation step.

Dichloromethane as C1 Synthon for the Photoredox Catalytic Cyclopropanation of Aromatic Olefins

Dichloromethane, as a readily available and inexpensive C1 synthon is proposed as a powerful building block for cyclopropanation of alkenes under mild conditions. Herein, we report a highly efficient and versatile dual photoredox system, involving a nickel aminopyridine coordination complex and a photocatalyst, for the cyclopropanation of aromatic olefins using dichloromethane, under visible-light irradiation. The cyclopropanation protocol has been successfully applied at gram scale. Mechanistic studies suggest a Ni(II) pyridyl radical complex as the key intermediate for the homolytic cleavage of the Csp3-Cl bond, generating a chloromethyl radical that is captured by the olefin coupling partner. Our findings also highlight the versatility of this methodology. By directing the radical/polar crossover process, we were able to selectively drive the reaction towards either the formation of cyclopropyl derivatives or the corresponding non-cyclic alkyl chloride products. The methodology also successfully apply to geminal dichloroalkanes, including the formation of spiro[2,2] compounds. Moreover, our methodology extends to the synthesis of deuterium-labelled cyclopropanes, demonstrating its utility in isotopic labelling and broadening its applicability in chemical synthesis and drug development.

CO2-Promoted Electrocatalytic Reduction of Chlorinated Hydrocarbons

Electrochemical reactions and their catalysis are important for energy and environmental applications, such as carbon neutralization and water purification. However, the synergy in electrocatalysis between CO2 utilization and wastewater treatment has not been explored. In this study, we find that the electrochemical reduction of chlorinated organic compounds such as 1,2-dichloroethane, trichloroethylene, and tetrachloroethylene into ethylene in aqueous media, which is a category of challenging reactions due to the competition of H2 evolution, can be substantially enhanced by simultaneously carrying out the reduction of CO2 on an easily prepared and cost-effective Cu metal catalyst. In the case of 1,2-dichloroethane dechlorination, a 6-fold improvement in Faradaic efficiency and a 19-fold increase in partial current density are demonstrated. Through electrochemical kinetic studies, in situ Raman spectroscopy, and computational simulations, we further find that CO2 reduction reduces hydrogen coverage on the Cu catalyst, which not only exposes more active sites for the dechlorination reaction but also enhances the effective reductive potential on the catalyst surface and reduces the kinetic barrier of the rate-determining step.

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.

Mimicking reductive dehalogenases for efficient electrocatalytic water dechlorination

Electrochemical technology is a robust approach to removing toxic and persistent chlorinated organic pollutants from water; however, it remains a challenge to design electrocatalysts with high activity and selectivity as elaborately as natural reductive dehalogenases. Here we report the design of high-performance electrocatalysts toward water dechlorination by mimicking the binding pocket configuration and catalytic center of reductive dehalogenases. Specifically, our designed electrocatalyst is an assembled heterostructure by sandwiching a molecular catalyst into the interlayers of two-dimensional graphene oxide. The electrocatalyst exhibits excellent dechlorination performance, which enhances reduction of intermediate dichloroacetic acid by 7.8 folds against that without sandwich configuration and can selectively generate monochloro-groups from trichloro-groups. Molecular simulations suggest that the sandwiched inner space plays an essential role in tuning solvation shell, altering protonation state and facilitating carbon-chlorine bond cleavage. This work demonstrates the concept of mimicking natural reductive dehalogenases toward the sustainable treatment of organohalogen-contaminated water and wastewater.

Rod-Like Nanostructured Cu-Co Spinel with Rich Oxygen Vacancies for Efficient Electrocatalytic Dechlorination

Dichloromethane (CH₂Cl₂) hydrodechlorination to methane (CH₄) is a promising approach to remove the halogenated contaminants and generate clean energy. In this work, rod-like nanostructured CuCo₂O₄ spinels with rich oxygen vacancies are designed for highly efficient electrochemical reduction dechlorination of dichloromethane. Microscopy characterizations revealed that the special rod-like nanostructure and rich oxygen vacancies can efficiently enhance surface area, electronic/ionic transport, and expose more active sites. The experimental tests demonstrated that CuCo₂O₄-3 with rod-like nanostructures outperformed other morphology of CuCo₂O₄ spinel nanostructures in catalytic activity and product selectivity. The highest methane production of 148.84 μmol in 4 h with a Faradaic efficiency of 21.61% at -2.94 V (vs SCE) is shown. Furthermore, the density function theory proved oxygen vacancies significantly decreased the energy barrier to promote the catalyst in the reaction and O_v-Cu was the main active site in dichloromethane hydrodechlorination. This work explores a promising way to synthesize the highly efficient electrocatalysts, which may be an effective catalyst for dichloromethane hydrodechlorination to methane.

Efficient electrocatalytic valorization of chlorinated organic water pollutant to ethylene

Electrochemistry can provide an efficient and sustainable way to treat environmental waters polluted by chlorinated organic compounds. However, the electrochemical valorization of 1,2-dichloroethane (DCA) is currently challenged by the lack of a catalyst that can selectively convert DCA in aqueous solutions into ethylene. Here we report a catalyst comprising cobalt phthalocyanine molecules assembled on multiwalled carbon nanotubes that can electrochemically decompose aqueous DCA with high current and energy efficiencies. Ethylene is produced at high rates with unprecedented ~100% Faradaic efficiency across wide electrode potential and reactant concentration ranges. Kinetic studies and density functional theory calculations reveal that the rate-determining step is the first C-Cl bond breaking, which does not involve protons-a key mechanistic feature that enables cobalt phthalocyanine/carbon nanotube to efficiently catalyse DCA dechlorination and suppress the hydrogen evolution reaction. The nanotubular structure of the catalyst enables us to shape it into a flow-through electrified membrane, which we have used to demonstrate >95% DCA removal from simulated water samples with environmentally relevant DCA and electrolyte concentrations.

A comparison study of Fenton-like and Fenton reactions in dichloromethane removal

Dichloromethane (DCM), as a low-chlorinated organic compound, is hardly to be degraded through the reductive dechlorination pathway. In this study, the removal of DCM in Fenton-like system, using activated carbon fibres-supported zero-valence Fe/Ni nanoparticles (ACF-Fe/Ni) as catalysts, was investigated and compared with that of a traditional Fenton system (Fe2+/H2O2). The influence of vital parameters, including initial solution pH, DCM concentration, catalyst and H2O2 dosages, temperature and cosolute on the removal of DCM, was systematically studied. The results showed that 94.2% of DCM with an initial concentration of 5 mg/L could be removed in the Fenton-like reaction under the optimum condition: initial pH of 2.0, 0.4 g/L of ACF-Fe/Ni, 10 mM of H2O2 and a temperature of 30°C. In comparison, the removal of DCM in the Fenton-like system was faster than that of the Fenton system and the corresponding activation energies were 39.69 and 33.82 kJ/mol, respectively. The coexistence of solute was adverse to the removal of DCM in both Fenton-like and Fenton systems. Moreover, the active species for DCM removal in the Fenton-like system was confirmed as hydroxyl radical (·OH) via the quenching experiment and electron paramagnetic resonance measurement. The incomplete mineralisation (41.7%) of DCM after reaction indicated that the Fenton-like technology had the potential to realise DCM's non-toxic and harmless conversion and organic intermediates formed needed to take longer time to be decomposed.

Water Purification of Classical and Emerging Organic Pollutants: An Extensive Review

The main techniques used for organic pollutant removal from water are adsorption, reductive and oxidative processes, phytoremediation, bioremediation, separation by membranes and liquid–liquid extraction. In this review, strengths and weaknesses of the different purification techniques are discussed, with particular attention to the newest results published in the scientific literature. This study highlighted that adsorption is the most frequently used method for water purification, since it can balance high organic pollutants removal efficiency, it has the possibility to treat a large quantity of water in semi-continuous way and has acceptable costs.