C F Bond Research Articles

Forever no more: Complete mineralization of per- and polyfluoroalkyl substances (PFAS) using an optimized UV/sulfite/iodide system

As the global issue of PFAS contamination in water continues to grow there exists a need for technologies capable of fully mineralizing PFAS in water, with destruction being measured as both a loss of the initial PFAS and a quantitative recovery of the resultant fluoride ions. This study investigates the use of sulfite and iodide in a bicarbonate-buffered alkaline system activated with ultraviolet (UV) light to destroy PFAS. The UV/sulfite/iodide system creates a reductive environment through the generation of aqueous electrons, which can degrade PFAS. The extent of degradation and defluorination was explored for perfluorooctane sulfonic acid (PFOS), perfluorooctanoic acid (PFOA), 6:2 fluorotelomer sulfonic acid (6:2 FTS), and perfluorobutane sulfonic acid (PFBS). An initial UV/sulfite/iodide system achieved 100 % degradation and > 90 % defluorination for PFOS, PFOA, and 6:2 FTS, but was not capable of completely degrading PFBS. Transformation product elucidation experiments were performed for PFOS under different UV systems, and 6:2 FtSaB using the initial UV/sulfite/iodide system. Several transformation products were identified including -nF/+nH PFOS (n = 1–13), -F/+H shorter-chain PFSAs, 6:2 fluorotelomer sulfonamidoamine (6:2 FtSaAm), 6:2 fluorotelomer sulfonamide, and 6:2 fluorotelomer unsaturated sulfonamide. Novel identification of -F/+H perfluoropropane sulfonic acid (PFPS) and -F/+H perfluoroethane sulfonic acid (PFES) following degradation of PFOS confirms CC bond cleavage, and different isomers of -F/+H PFOS confirms the potential for CF bond cleavage to occur throughout the perfluoroalkyl chain. Additional optimization experiments were performed aiming to fully degrade PFBS. The optimal protocol found in this study involved an elevated initial sulfite concentration and adding additional sulfite at regular intervals during UV-activation, achieving >99.9 % destruction and complete quantitative defluorination of PFBS.

A Review: Per- and Polyfluoroalkyl Substances-Biological Degradation.

Per- and polyfluoroalkyl substances (PFASs), highly stable synthetic organic compounds with multiple carbon-fluorine bonds, are emerging as environmental contaminants, toxic, bioaccumulative, and environmentally persistent. PFASs are strongly resistant to biological and chemical degradation, and therefore PFASs present a challenge to researchers and scientists for a better understanding and application of remediation methods and biodegradation of PFASs and have become subject to strict government regulations. The review summarizes the recent knowledge of bacterial and fungal degradation of PFASs, as well as the enzymes involved in the processes of transformation/degradation of PFASs.

Facile synthesis of fluorinated graphene for surface self-assembly of aluminum hydride

A facile method, refluxing with hydrothermal process, was used to synthesize the fluorinated graphene sheets (FGS) with high F/C ratio. Then, the surface of aluminum hydride (AlH3) was coated with FGS by means of liquid self-assembly process. The structure and performance of FGS were characterized through fourier transform infrared spectra (FTIR), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscope (TEM) and elemental analyzer. FTIR spectra show that the FGS have obvious absorption peak of CF bond. Raman spectra display many defects on FGS. XPS spectra and elemental analysis also prove the existence of CF bond, and the content of fluorine is more than 25%. From SEM photos we can see that FGS have thin layers with some curled wrinkles. TEM photos demonstrate intuitively the FGS has a few layers. At last, the aluminum hydride (AlH3) was coated with FGS by means of liquid self-assembly process, and AlH3 samples before and after treated were characterized through hydrogen content (H%) analysis, X-ray diffraction (XRD), FTIR, SEM and differential thermal analysis (DTA), ect. The results demonstrate that there is only a little FGS on the surface of AlH3, which won't affect the effective H% and release efficiency of hydrogen. AlH3@FGS also reduces the mechanical sensitivity of solid propellant with AlH3, which will remarkably promote the application of AlH3 in the novel high-energy solid propellant.

11 C-Cyanation of Aryl Fluorides via Nickel and Lithium Chloride-Mediated C-F Bond Activation.

Aryl fluorides are expected to be useful as radiolabeling precursors due to their chemical stability and ready availability. However, direct radiolabeling via carbon-fluorine (C-F) bond cleavage is a challenging issue due to its significant inertness. Herein, we report a two-phase radiosynthetic method for the ipso-11 C-cyanation of aryl fluorides to obtain [11 C]aryl nitriles via nickel-mediated C-F bond activation. We also established a practical protocol that avoids the use of a glovebox, except for the initial preparation of a nickel/phosphine mixture, rendering the method applicable for general PET centers. This method enabled the efficient synthesis of diverse [11 C]aryl nitriles from the corresponding aryl fluorides, including pharmaceutical drugs. Stoichiometric reactions and theoretical studies indicated a significant promotion effect of lithium chloride on the oxidative addition, affording an aryl(chloro)nickel(II) complex, which serves as a precursor for rapid 11 C-cyanation.

Cross- and Multi-Coupling Reactions Using Monofluoroalkanes.

Carbon-fluorine bonds are stable and have demonstrated sluggishness against various chemical manipulations. However, selective transformations of C-F bonds can be achieved by developing appropriate conditions as useful synthetic methods in organic chemistry. This review focuses on C-C bond formation at monofluorinated sp3 -hybridized carbons via C-F bond cleavage, including cross-coupling and multi-component coupling reactions. The C-F bond cleavage mechanisms on the sp3 -hybridized carbon centers can be primarily categorized into three types: Lewis acids promoted F atom elimination to generate carbocation intermediates; nucleophilic substitution with metal or carbon nucleophiles supported by the activation of C-F bonds by coordination of Lewis acids; and the cleavage of C-F bonds via a single electron transfer. The characteristic features of alkyl fluorides, in comparison with other (pseudo)halides as promising electrophilic coupling counterparts, are also discussed.

PFAS–Container Interaction: A Case Study of PFOS Defluorination with Acoustic Cavitation

Perfluoroalkyl and polyfluoroalkyl substances (PFAS) are a large class of ubiquitous and persistent environmental contaminants. Due to the extremely stable carbon-fluorine bonds, PFAS are difficult to destruct. Increasing research efforts are focusing on PFAS defluorination, and one of the key challenges is tracking down the breakdown products and achieving a reasonable mass balance. In this study, we demonstrate one vital but often overlooked aspect of PFAS destruction studies, using perfluorooctanesulfonic acid (PFOS) destruction with acoustic cavitation as a case. This study showed that PFOS–container interactions can significantly impact the destruction efficiency and provided an example whereby these interactions may be prevented using surface coating. A showcase of mass balance for PFAS defluorination using acoustic cavitation was also investigated by employing targeted analysis [liquid chromatography-mass spectrometry (LC-MS)] and complementary untargeted analysis (total organofluorine measurement). This study demonstrated the necessity for a thorough consideration of all aspects in PFAS destruction experiments and the necessity of early adoption of total organofluorine measurements.

Chemo- and regioselectivities of the TBAF-catalyzed C[sbnd]F bond allylation of trifluoromethylalkenes: A theoretical view

Selective CF bond activation and functionalization of readily available polyfluorinated groups is a classical and effective way to access high value-added organofluorine compounds. Insights into origin of the selectivities could provide valuable guidance to precise manipulation and so as to get desired products. In this study, the mechanism and causation of the selective mono-CF bond allylation of trifluoromethylalkenes with tetrabutylammonium fluoride (TBAF) as catalyst has been investigated by using the density functional theory (DFT) methods. It is unveiled that the SN2′-type nucleophilic substitution going through the addition-elimination mechanism is more favorable than the concerted SN2 pathway. According to the calculation results, allyl anions would kinetically identify the trifluoromethylalkene, ammonium cation (TBA+), and difluoromethylalkene in turn, which guarantees the mono-CF bond activation under the conditions of excessive nucleophiles. Furthermore, as a specific point, the regioselectivity of the reaction using asymmetric allyl anion as the nucleophile has also been studied. The Parr function calculations and non-covalent interaction analysis reveals that the more significant nucleophilicility of the benzyl carbon atom in allyl anion and the π…π interaction between the aryl substituents on the two reacting fragments leads to favor of the reaction at the benzyl site.

Extreme UV Resist Exhibiting Synergism between Chemical and Physical Crosslinking Mechanisms.

Carbon-fluorine bonds in fluorinated molecules can undergo homolytic cleavage reactions when electrons are injected, and the resulting radicals combine to form network structures characterized by reduced solubility. This crosslinking chemistry suggests a new category of patterning materials that function under electron beam (e-beam) and extreme ultraviolet (EUV) lithographic conditions. Although this chemistry enables the production of 50 nm or smaller-sized features of simple fluoroalkylated polymers, it is limited by the need for relatively large amounts of irradiation energy to achieve required solubility changes. Therefore, this study was undertaken to devise a sensitivity-enhancing strategy based on a synergistic combination of radical crosslinking and hydrogen-bonding interactions between highly fluoroalkylated copolymers. An alternating copolymer was synthesized using tert-butoxystyrene and a fluoroalkylated maleimide, the former of which produces active hydrogens through catalytic acidolysis reactions. When the polymer was blended with a catalytic amount of a photoacid generator and subjected to lithographic patterning tests under e-beam and EUV irradiation, the deprotection reactions of tert-butoxy moieties proceeded at room temperature and led to a solubility decrease. We presume the small number of hydroxyl moieties produced formed an intermolecular hydrogen-bonding network, which acted synergistically with the covalent crosslinks generated by C-F bonds. When 30 nm features of copolymer thin films were fabricated by EUV lithography, sensitivity was improved by 25-34% without significant deterioration of pattern quality, especially line-edge roughness. These results demonstrate that EUV resists with improved patterning capabilities can be achieved by combining catalytic acidolysis reactions and noncatalytic crosslinking chemistry.

Synthesis of trifluoromethylated allenes via visible light-promoted bis(trifluoromethylation) of 1,3-enynes

A visible light-promoted/copper-mediated 1,4-bis(trifluoromethylation) method of easily available 1,3-enynes to construct allenic Csp2−CF3 bonds has been described. The reaction proceeded under mild conditions, has broad substrate scope and good functional group tolerance. Photocatalyst is not required in this protocol, and O2 plays an important role in facilitating the trifluoromethyl group transfer and improving the yield. Mechanistic studies revealed that a radical process is involved in the reaction.

Posttranslational, site-directed photochemical fluorine editing of protein sidechains to probe residue oxidation state via 19F-nuclear magnetic resonance.

The fluorination of amino acid residues represents a near-isosteric alteration with the potential to report on biological pathways, yet the site-directed editing of carbon-hydrogen (C-H) bonds in complex biomolecules to carbon-fluorine (C-F) bonds is challenging, resulting in its limited exploitation. Here, we describe a protocol for the posttranslational and site-directed alteration of native γCH2 to γCF2 in protein sidechains. This alteration allows the installation of difluorinated sidechain analogs of proteinogenic amino acids, in both native and modified states. This chemical editing is robust, mild, fast and highly efficient, exploiting photochemical- and radical-mediated C-C bonds grafted onto easy-to-access cysteine-derived dehydroalanine-containing proteins as starting materials. The heteroaryl-sulfonyl reagent required for generating the key carbon-centered C• radicals that install the sidechain can be synthesized in two to six steps from commercially available precursors. This workflow allows the nonexpert to create fluorinated proteins within 24 h, starting from a corresponding purified cysteine-containing protein precursor, without the need for bespoke biological systems. As an example, we readily introduce three γCF2-containing methionines in all three progressive oxidation states (sulfide, sulfoxide and sulfone) as D-/L- forms into histone eH3.1 at site 4 (a relevant lysine to methionine oncomutation site), and each can be detected by 19F-nuclear magnetic resonance of the γCF2 group, as well as the two diastereomers of the sulfoxide, even when found in a complex protein mixture of all three. The site-directed editing of C-H→C-F enables the use of γCF2 as a highly sensitive, 'zero-size-zero-background' label in protein sidechains, which may be used to probe biological phenomena, protein structures and/or protein-ligand interactions by 19F-based detection methods.

Thermal kinetics of PFAS and precursors in soil: Experiment and surface simulation in temperature-time plane

Per- and polyfluoroalkyl substances (PFAS) are chemically and thermally stable due to the presence of carbon-fluorine (C–F) bond in their molecular structures, hence have been previously formulated as firefighting ingredients. During the firefighting process, however, owing to the high temperature, PFAS can be potentially degraded, particularly for PFAS precursors that contain non-C-F bonds, which is studied herein by exposing PFAS-contaminated soil in a muffle furnace oven. Different temperatures and time intervals are applied to the real soil sample to mimic the firing process and to evaluate the degradation and conversion of PFAS. This thermal treatment can not only degrade precursors (e.g. 6:2 fluorotelomer sulphonate), but also degrade perfluoroalkyl carboxylates (PFCA, e.g. perfluorooctanoic acid PFOA) and perfluoroalkyl sulfonates (PFSA, e.g. perfluorooctane sulfonate PFOS). The concentration dependence of the PFAS on temperature and time is fitted using a 2D Gaussian surface to simulate the complex thermal kinetic, and to compare with the traditional approach such as thermogravimetric analysis (TGA) (1D dependence on temperature only). The 2D simulation can directly visualise the thermal kinetic of individual or sum PFAS in the complex temperature-time plane, which depends on the sample background and particularly on the coexist PFAS precursors. Overall, this study provides a simple approach to monitor and optimise the thermal treatment of the PFAS-contaminated soil.

Nonlethal detection of PFAS bioaccumulation and biomagnification within fishes in an urban- and wastewater-dominant Great Lakes watershed

Per- and polyfluoroalkyl substances (PFAS) are synthetic endocrine disruptors that are particularly stable and pervasive due to strong carbon-fluorine bonds. They are known to bioaccumulate in protein-rich tissues of fish, and most cannot be eliminated with cooking. Despite studies linking PFAS to adverse health outcomes, there is a lack of international regulations of PFAS as a hazardous material. To investigate PFAS in an aquatic food web and the potential human health implications, we analyzed the concentrations of 40 PFAS from muscle biopsy and serum samples of fish representing different trophic levels along the Lake Huron – Lake Erie Corridor. In Summer (2021), walleye (Sander vitreus; WAE), yellow perch (Perca flavescens; YEP) and round gobies (Neogobius melanostomus; ROG) were collected for analysis from the Detroit River (contaminated site) and St. Clair River (reference site). Eight PFAS congeners were detected in muscle and 15 congeners in serum, leading to the novel detection in Great Lakes fish of 7:3 FTCA in muscle and PFHpS, PFNS, MeFOSAA, and EtFOSAA in serum. PFOS was detected in 100% of muscle and serum pools across all species at concentrations lower than those associated with fish toxicity. Muscle PFOS concentration in DR WAE fell under the 8 meals per month (>13 ng–19 ng) fish consumption advisory according to the State of Michigan. Log bioaccumulation factor was significantly different (p = 0.01) among species in DR, driven by higher log BAF for WAE (3.8 ± 0.1) compared to ROG (3.2± 0.02). Biomagnification factor greater than 1 for all species in both rivers indicates that PFOS is biomagnifying in SCR and DR food webs. Successful detection and quantification of PFAS in the muscle and serum of three fish species demonstrates the potential for using this nonlethal sampling method to monitor PFAS and better understand ecological and human health impacts of PFAS exposure.

Cyclic Dinucleotide-Based Enantioselective Fluorination in Water.

The diverse structures of DNA serve as potent chiral scaffolds for DNA-based asymmetric catalysis, yet in most cases tens to hundreds of nucleotides in DNA hybrid catalysts hinder the deep insight into their structure-activity relationship. Owing to the structural simplicity and design flexibility of nucleotides, nucleotide-based catalysts have been emerging as a promising way to obtain fine structural information and understand the catalytic mechanisms. Herein, we found that a cyclic dinucleotide of cyclic di-AMP (c-di-AMP) and 1,10-phenanthroline copper(II) nitrate (Cu(phen)(NO3)2) are assembled to a c-di-AMP-based catalyst (c-di-AMP/Cu(phen)(NO3)2), which could fast achieve enantioselective fluorination in water with 90-99% yields and up to 90% enantiomeric excess (ee). The host-guest interaction between c-di-AMP and Cu(phen)(NO3)2 has been proposed mainly in a supramolecular interaction mode as evidenced by spectroscopic techniques of ultraviolet-visible, fluorescence, circular dichroism, and nuclear magnetic resonance. Cu(phen)(NO3)2 tightly binds to c-di-AMP with a binding constant of 1.7 ± 0.3 × 105 M-1, and the assembly of c-di-AMP/Cu(phen)(NO3)2 shows a modest rate enhancement to carbon-fluorine bond formations as supported by kinetic studies.

Experimental study on the effect of flow channel parameters on the durability of PEMFC stack and analysis of hydrogen crossover mechanism

Polymer electrolyte membrane fuel cell (PEMFC) is a leading technology with a bright future in the field of transportation. The bipolar plate structure has a profound impact on the mass and heat transfer of PEMFC. In this study, the durability tests of two stacks with different channel depths are carried out for 1000 h respectively on the 1 kW fuel cell stack test platform. We detect the overall performance change of the stack and characterize the membrane electrode assembly at the end of the test. Based on the first principle calculation, the mechanism of membrane degradation and hydrogen crossover is analyzed, and some durability experimental results are explained. The results show that the durability of the PEMFC with a shallow flow channel is worse, the voltage drops significantly, and the hydrogen crossover increases significantly after 500 h. The first principle calculation results reveal that the chemical structure change of the membrane is caused by the free radical attack. Hydrogen crossover promotes ∙H to attack carbon-fluorine bonds. This paper reveals the mechanism of hydrogen crossover and its influence on the catalyst layer, which is helpful to improve the durability of PEMFC.

Transparent wafer-scale self-standing fluorinated graphene films

The fluorination of self-standing graphene oxide (GO) films under mild conditions at 70 °C for 1h using a mixture of elemental fluorine and nitrogen gases (0.35 F2/0.65 N2 v/v) results in transparent, highly fluorinated (F/C = 1 as seen by XPS) graphene oxide (FGO) films which maintain initial densely packed structure and mechanical strength. Most of the oxygen atoms in the GO film were replaced by fluorine after fluorination, and two types of carbon-fluorine bonds coexist, i.e. primarily weakened C⋯F bonds and also pure covalent C–F bonds as evidenced by FTIR, XPS, NMR, and cyclic voltammetry when FGO is used as electrode in Li/electrolyte (PC:EC:3DMC/LiTFSI 1 M)/FGO cell similar to a lithium battery. To further understand the fluorination mechanism and to distinguish the reaction steps, xenon difluoride XeF2 was used as fluorinating agent for GO and in situ EPR and FTIR analyses suggest hydroxyl groups as preferential reactive sites. These self-standing transparent FGO films exhibit unusual electrochemical properties in lithium battery and can be used as hydrophobic lubricant membranes because of the low and stable coefficient of friction evidenced in the present work.

Perfluorooctanoic acid (PFOA) removal from real landfill leachate wastewater and simulated soil leachate by electrochemical oxidation process

Poly and perfluoroalkyl compounds (PFASs) are a group of chemicals that are widely used and are difficult to purify. Within this group, perfluorooctanoic acid (PFOA), known for its highly polar and strong carbon–fluorine bonds, is a frequently encountered species in surface water, drinking water, and groundwater. In this study, we investigated the efficiency of electrooxidation (EO) in PFOA removal, optimization of EO parameters, and groundwater simulation in a realistic scenario. The EO optimization experiments were performed with a boron-doped diamond (BDD) anode for different values of pH, current density, and inlet concentration, and the effects of different anode materials were investigated for comparison. Under optimum conditions, total organic carbon (TOC) removal of up to 90% was achieved. In the groundwater simulation, we applied optimized EO parameters after obtaining leachates from the soil. A TOC removal of up to 86% was obtained in the EO of simulated groundwater contaminated with PFOA. In the case of realistic leachate simulation, four different leachate treatments were applied to PFOA-contaminated soil, and high TOC removal was achieved in all matrices. Additionally, the EO with BDD was applied to landfill leachate wastewater supplemented with PFOA to monitor the effectiveness of the process. A TOC removal of 85% was achieved, and it was observed that the number of free F-ions increased. The results showed that the BDD EO has a high potential for the treatment of PFOA-contaminated groundwater. • A realistic simulation was designed to obtain PFOA-contaminated groundwater. • Parameter optimization for BDD EO was performed. • The effects of different anodes (DSA) in the EO was investigated. • TOC removal efficiencies for water and wastewater were 90% and 78%, respectively. • TOC removal of 85% was achieved for PFOA contaminated groundwater.

When Chemicals Go to the Dark Side

When Chemicals Go to the Dark Side

Weatherable, solvent-soluble, paintable and transparent fluoropolymers for daytime radiative cooling

A fluoropolymer, LUMIFLONTM (LF), with high transparency at solar wavelengths is reported for use in efficient daytime radiative cooling (RC). The RC property is even present in thin films (∼50μm-thick). The fluoropolymer is weatherable, solvent-soluble, and paintable. The RC property stems from vibrations of the carbon–fluorine bond at 7.8 – 10μm (1000 – 1300 cm−1). As LF is transparent, it can be used for the painting of objects and buildings without changing their appearance. It is also promising for the refrigeration of solar cells without using electricity. An analysis evaluating RC materials was established by in-house measurements, in which liquid nitrogen was used as a refrigerant. RC materials such as LF and poly(dimethylpolysiloxane) were explored. Further, field RC measurements were performed and the results were analyzed. It was found that LF shows high RC efficiency in practical use.

Ir-Catalyzed Chemo-, Regio-, and Enantioselective Allylic Enolization of 6,6-Dimethyl-3-((trimethylsilyl)oxy)cyclohex-2-en-1-one Involving Keto-Enol Isomerization.

The utilization of 6,6-dimethyl-3-((trimethylsilyl)oxy)cyclohex-2-en-1-one made from an unsymmetrical 4,4-dimethylcyclohexane-1,3-dione in iridium-catalyzed allylic enolization involving keto-enol isomerization is accomplished under mild conditions. The chemoselectivity, regioselectivity, and enantioselectivity are facilitated by the quaternary carbon and adjusting the reaction conditions. This method provides the substituted 2-(but-3-en-2-yl)-3-hydroxy-6,6-dimethylcyclohex-2-en-1-ones in good to high yields with high level of chemo-, regio-, and enantioselectivities. The chiral carbon-fluorine bond formation is induced by an adjacent chiral carbon center of the allylated 3-hydroxy-6,6-dimethylcyclohex-2-en-1-one, as well.

KOH modification of fluorinated graphite and its reaction mechanism

KOH electrochemical method and heating method were employed to modify fluorinated graphite and explore the modification mechanism. The chemical composition and microstructure of the products were characterized and analyzed before and after the reaction. As the electrochemical reaction time or heating temperature increased, the carbon fluorine bond gradually underwent a nucleophilic reaction with KOH according to its reactivity, promoting the formation of fluorine ions in the residual product and carbon oxygen bonds in the corresponding oxidized fluorinated graphite (OFG). The electrochemical method with the anode on the bottom and the heating method were insufficient to allow the isolated carbon fluorine bond to react, retaining some carbon fluorine bonds. By positioning the anode on top, electron transfer significantly accelerates the activation of the carbon fluorine bond, which then reacts completely. According to theoretical simulation calculations, electronegative groups around the carbon fluorine bond can effectively enhance its reactivity.