This research work is centred on the selective oxidation of alkanes or cyclic alkanes by using molecular oxygen, and developing supported metal nanoparticles catalysts in a solvent-free system under mild conditions. It aims to design of novel supported metal nanoparticles for the direct oxidation of hydrocarbons to synthesize the corresponding oxygen-containing products like alcohols and ketones, which are valuable precursors for the manufacture of fibres, nylon and their derivatives. To this scope, Ag supported metal particles were developed, and these catalysts were capable of activating O2 and the organic substrate.
In particular, a novel supported metal catalyst Ag/Nb2O5 was developed for the oxidation of cyclooctane and cyclohexane, which to the best of our knowledge it is firstly reported, developed and applied for catalytic purposes. A systematic study was conducted to investigate both the catalytic activity of Ag and Nb2O5 in cyclooctane oxidation, as well as the possibility of existence of other active species in the parent Nb2O5 or cyclooctane. This included a systematic evaluation on the effect of traces of water or alkyl hydroperoxides in cyclooctane and characterization of metal and support by means of XPS, XRD and TEM.
By using this new catalyst, it was possible to achieve conversion values of 81%, 13% for the substrates, cyclooctane, cyclohexane, respectively. With a total selectivity of ~70%, ~75% to the combination of ketone and alcohol (mainly ketone) generating from these substrates, posing a potential application of this catalyst in the production of oxygen containing products, especially for ketone. Given these promising results, the roles of supported Ag particles and Nb2O5 in the oxidation process were systematically investigated. It was found that Ag species (Ag0, Ag+) could activate molecular oxygen to give reactive superoxide species, or Ag+ was responsible for the direct dissociation of C-H bond, whereas metallic Ag0 was proved to be effective for the abstraction of α-H in the crucial intermediates alkyl hydroperoxide to generate the corresponding ketones as a higher selectivity to ketones was found in comparison with Nb2O5 in the oxidation of cyclooctane and cyclohexane. Nb2O5 was also having an active role in the decomposition of alkyl hydroperoxides. In addition, based on the studies about the different reaction performance of Ag/Nb2O5 prepared by different methods: wet impregnation(WI), deposition precipitation(DP) and sol immobilization(SI), the impregnation protocol was the one that exhibited the highest catalytic activity towards C-H activation, which was probably because the method led to a relatively larger amount of Ag+ species versus Ag0 and correlated to a higher Ag loading. Moreover, in view of these results, a bimetallic supported catalyst with the incorporation of Fe was developed, with the aim to have a metallic partner promoting the initiation of the reaction and a metallic partner promoting the selectivity of the reaction, and therefore in practical terms to have a bifunctional catalyst. In this case supported Ag-Fe/Nb2O5 prepared by wet impregnation method exhibited a superior reactivity with an enhanced selectivity to ketone (~60%) and a higher K/A (ketone to alcohol) molar ratio (3.0) in comparison with supported monometallic Ag/Nb2O5.
According to the collected results, a simplified reaction scheme was proposed for the understanding of the catalytic performances of Ag/Nb2O5 to provide insight of the effects of active species in the oxidation process, laying foundation for the further exploitation of this catalyst into the oxidation of other hydrocarbons or alcohols.
