Selectivity and mechanism of thermal decomposition of β-diketones on ZnO powder

Abstract

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 OH 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 CCF3 entity determines the preferred thermal decomposition pathways, as the CC bond in this group starts to react with a surface of ZnO around 400K, followed by immediate decomposition of the resulting CF3 group. Above 600K, 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 the vibrational signatures of the species produced and with the F-containing species recorded by XPS.