NMR studies of the thermal degradation of a perfluoropolyether on the surfaces of γ-alumina and kaolinite

Solid-state nuclear magnetic resonance (NMR) methods are used to follow the thermal degradation of Krytox 1506, a common perfluoropolyether, following adsorption onto the surfaces of γ-Al₂O₃ and a model clay (kaolinite). The alumina studies are complemented with thermogravimetric analysis (TGA) to follow the degradation process macroscopically. Molecular-level details are revealed through ¹⁹F magic-angle spinning (MAS), ²⁷Al MAS, and ¹⁹F → ²⁷ Al cross-polarization MAS (CPMAS) NMR. The CPMAS results show the time-dependent formation of probable ^VI Al(O_6-nF_n) (n = 1, 2, 3) species in which the fluorine atoms are selectively associated with octahedrally coordinated aluminum atoms. For the alumina system, the changes in peak shapes of the CP spectra over time suggest the early formation of catalytically active degradation products, which in turn lead to the formation of additional perfluoropolyether degradation products. Similar to the alumina system, the kaolinite system also displays new resonances in both the ²⁷Al MAS and ¹⁹F → ²⁷Al CPMAS spectra after thermal treatment at 300°C for up to 20 h but reveals a more distinct species at -15.5 ppm that forms at the expense of an initial species (3 ppm), which is in greater abundance at shorter heating times.