Selectivity and Mechanism of Thermal Decomposition of β-diketones on ZnO Powder

The thermal chemistry of β-diketones underlies a number of catalytic processes related both to the catalytic reactions yielding commodity chemicals and to the production of supported transition metal catalysts themselves. The mechanisms of decomposition during thermal transformation of three β-diketones, acetylacetone (acacH), 1,1,1-trifluoroacetylacetone (tfacH), and 1,1,1,5,5,5-hexafluoroacetylacetone (hfacH), were studied on ZnO powder surface using Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and Density functional theory (DFT) computational investigation. The initial O-H dissociation leads to the formation of corresponding β-diketonates in all the cases investigated. These diketonates are important surface intermediates that can be generated in a controlled manner in these experiments. The presence on the C-CF₃ entity determines the preferred thermal decomposition pathways, as the C-C bond in this group starts to react with a surface of ZnO around 400 K, followed by immediate decomposition of the resulting CF₃ group. Above 600 K, the presence of the CF₃-substituent leads to the formation of ketene-like structures observed by vibrational spectroscopy. The reaction mechanisms examined with the help of DFT calculations are correlated with vibrational signatures of the species produced and with the F-containing species recorded by XPS.