USTC defluorinates PTFE and PFASs at low temperatures

A research team led by Prof. KANG Yanbiao from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences (CAS) has developed a supercapacitor (SC)-assisted electrophotocatalysis for the efficient defluorination of the polytetrafluoroethylene (PTFE) and poly- and perfluoroalkyl substances (PFASs) at low temperatures. They incorporated the strength of both electrochemical and photochemical energy, providing more possibilities for solving environmental problems caused by PTFE and PFASs. The study was published in Angewandte Chemie International Edition.

PTFE is widely used in various fields due to its excellent thermal and chemical stabilities. However, its high stability also makes it difficult to degrade and recycle. Commonly, high-energy consumption methods such as thermal cracking are used to treat PTFE, while defluorination degradation under low temperature conditions requires the use of strong reducing agents such as liquid alkali metals. Photocatalysis can compensate for the shortcomings of traditional methods under mild conditions, however, the defluorination rate of PTFE is less than 5%.

Prof. KANG's team has always been dedicated to the research on the activation and cleavage of inert carbon-heteroatom bonds. Based on this research, they have developed a highly efficient photocatalytic system based on twisted carbazole structures.

CBZ6, as the photoreductant, thanks to its twisted structure, has excellent single-electron transfer capabilities, thereby achieving the cleavage and transformation of inert carbon-heteroatom bonds in stable molecules such as PTFE.

However, in this process, due to the hydrophobic and oleophobic nature of PTFE, along with its insolubility in almost all organic solvents and high usage; the reaction system became a multi-phase reaction system mixed with various solids and liquids.

For photo-reactions, much insoluble solids reduced the permeability of light, which led to extremely high requirements for the light source and dispersion degree of the system. While this situation was unfavorable for carrying out large-scale reactions.

To address the above issues, the research team developed electrophotocatalysis system assisted by SCs. They used electrochemical methods to generate catalytically active species, replacing the poorly soluble reducing agents in photocatalytic reactions.

By effective electron injection to the carbon–fluorine bond of PTFE, the reduction defluorination of PTFE was established under mild conditions, with the help of synergistic effect of photochemistry and electrochemistry.

The electro-photoreduction catalytic system effectively avoided excessive using of co-reductants in the standalone photocatalytic reduction system. Moreover, the scale of the reaction was further expanded from milligram level to gram level.

At the same time, this catalytic system also shows good applicability to the defluorination of other small molecules containing PFASs.

In addition, supercapacitors have fast charging speed, high working efficiency, high energy ratio, ultra-high temperature resistance, and long cycle life. Therefore, they can be used outdoors with sunlight as the light energy source to achieve the defluorination reaction of PTFE.

In this study, various testing methods including Raman spectroscopy, X-ray photoelectron spectroscopy, and Fourier transform infrared spectroscopy were employed to characterize the solid products of PTFE defluorination.

Researchers thus confirmed that there were aliphatic structures, aromatic structures, and oxygen-containing functional groups in these products. In addition, they also possess regular carbon structures (Raman G peak) and irregular carbon structures (Raman D peak).

This study provides a new perspective for solving environmental problems caused by the hard degradation of PTFE and PFASs.