Reaction mechanism of polyvinylidene fluoride in supercritical water: product distribution, role of supercritical water, F-element migration pathway and hydrogen formation mechanism

Supercritical water gasification (SCWG) technology is expected to cleanly and efficiently treat waste polyvinylidene difluoride (PVDF), however, the conversion mechanism of PVDF in supercritical water (SCW) is still unclear. This paper employed experiments and molecular dynamics simulation to study the reaction properties of PVDF in SCW including the hydrogen supplying role of SCW, the F-element migration pathway and the mechanism of H₂ formation. Detailed analysis of the product distribution identified the mechanisms affecting temperature, reaction time and feedstock concentration on the supercritical water gasification of PVDF. The H₂ yield of 34.04 mol/kg and carbon gasification efficiency (CE) of 92.87 % were achieved in the experiment confirming the potential of PVDF gasification for H₂ production. The comparison of experimental and simulation results of PVDF pyrolysis and SCWG proved that SCW acted as a hydrogen donor to improve the conversion efficiency of PVDF. The attack of SCW can accelerate the C–C breakage in PVDF to form intermediates (INT), which decreased the energy barrier of decomposition by 26.2 %–326 kJ/mol compared to pyrolysis without SCW. The F-element in PVDF was assigned to HF and organic fluorides after a series of free radical reactions. H₂ was formed mainly through H radical coupling and water participation in the reaction. This study may reveal the reaction mechanism of PVDF in SCW and provide a theoretical framework for the development of F pollution control strategies.