Selective aerobic oxidation of hydrocarbons over supported gold catalysts

Abstract

The selective oxidation of hydrocarbons is of vital importance for the production of valuable chemicals from crude oil and natural gas resources. Unfortunately, when using molecular oxygen as an environmentally benign oxidant, these processes face tremendous difficulties, most importantly in controlling the selectivity. The aim of the research described in this PhD thesis is to contribute in the development and fundamental understanding of novel, more efficient, selective alkane oxidation routes with molecular oxygen. Part I of the thesis describes the potential gold-based catalysts for the selective production of hydrogen for fuel cell application by partial oxidation. Promotion of the gold-based catalysts with alkaline earth and lanthanum oxides resulted in a significant improvement of the catalyst performance. A detailed in-situ characterization of the gold catalysts revealed that the described promotion effect results from the altered redox properties of the gold nanoparticles. In Part II of this thesis the applicability of gold catalysts for the selective oxidation of cyclohexane and methane is studied. In contrast with literature results, the gold catalysts did not establish an improved performance as compared to the commercial cyclohexane autoxidation process. This strong deviation from the literature results was explained by the inadequate experimental procedures as were employed in the particular research articles. Also in the case of partial methane oxidation, no indication of selective C-H bond activation by the gold catalysts was found. Concluding, it is stated that there is no experimental evidence suggesting selective alkane oxidation with molecular oxygen to be possible over gold-based catalysts. In Part III of this thesis, a novel catalytic process is presented, according to which the yield of valuable cyclic oxygenates from cyclohexane oxidation can potentially be doubled. Cyclohexyl hydroperoxide, formed by the autoxidation of cyclohexane with molecular oxygen, was used to selectively epoxidize cyclohexene and cyclododecene over mesoporous titanium-silicates. Under optimized conditions, a selectivity of 170% based on peroxide conversion was obtained, which yields a large improvement when compared to the ~90% selectivity obtained in the commercially applied deperoxidation process. The catalysts showed a stable performance over four subsequent reaction cycles. Finally a mechanistic study into the epoxidation reaction is described. Different reactions, namely epoxidation, deperoxidation and allylic oxidation were identified and found to compete. In all three reactions the role of radical species was confirmed. Possible side reactions in cyclohexene epoxidation, like hydrolysis and isomerization of the epoxide, were found to be negligible. The participation of molecular oxygen was investigated, and found to play a major role in the observed solvent oxidation and the direct allylic oxidation. Eventually the formation of titanium-hydroperoxo groups as the active intermediate species was confirmed. A comprehensive mechanism of the epoxidation process is presented.