Heterogeneous gold, palladium and copper based catalysts for liquid phase oxidation of methane

Ab Rahim, Mohd Hasbi 2011. Heterogeneous gold, palladium and copper based catalysts for liquid phase oxidation of methane. PhD Thesis, Cardiff University.

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Abstract

The oxidation of lower alkanes especially methane to methanol under mild reaction conditions is one of the most challenging task for industry and academia. At present, indirect utilisation via synthesis gas is the only commercially viable process for methanol production. Therefore, this study intends to investigate the direct oxidation of methane to methanol using a novel low temperature approach. Recently, gold based supported catalysts have been found to be highly effective oxidation catalysts where a number of important discoveries have been made such as in hydrogen peroxide synthesis and selective oxidation of alcohols to aldehydes. Due to these recent advances, further work into the oxidation of carbon-hydrogen bonds especially methane by gold and gold-palladium alloyed nanoparticles was the central topic of this study. As a proof of concept for the following studies, oxidation of primary C-H bonds in toluene and toluene derivatives were carried out in a high pressure stirred autoclave with molecular oxygen as oxidant. It was evident that Au-Pd supported catalyst is capable in oxidising primary C-H bonds on toluene and toluene derivatives at lower temperature with high catalytic activity based on turnover number (TON) compared to available heterogeneous catalysts reported in literature. However, these catalysts are ineffective in the oxidation of methane with oxygen under mild conditions with water as solvent and temperature below 90ᵒC. In view of this, hydrogen peroxide has been used as oxidant and it was shown that Au-Pd supported nanoparticles are active for the oxidation of methane giving high selectivity to methanol especially in the reactions carried out with hydrogen peroxide generated using an in-situ approach. Methane oxidation reactions were carried out in aqueous medium. The main products were methanol, methyl hydroperoxide and only carbon dioxide as overoxidation product. Investigations of reaction conditions such as concentration of oxidant, reaction time, reaction temperature and pressure of methane were investigated. It was found that the activity and selectivity of the catalyst was highly dependant on these variables. Oxygenate productivity was found to increase by increasing the H2O2 or H2O2 concentration and methane pressure. Longer reaction times were detrimental to the methanol selectivity where overoxidation reaction occurred. Interestingly, the Au-Pd catalytic system was able to oxidise methane to methanol at temperatures as low as 2ᵒC. The applicability of the developed catalytic system was tested on ethane oxidation reaction and it successfully produced ethanol as the major product. The oxygenate productivity was higher as compared to methane due to the solubility factor and the difference in the strength of carbon-hydrogen bonds. The catalyst preparation method and pretreatment were shown to be very important in the formation of active catalysts. The Au-Pd alloy having Au core-palladium shell structure with PdO dominance on the surface and bigger particle size was preferred than analogue catalyst consists of Au and Pd in metallic state with smaller particle size. In addition to that, the choice of support is crucial and this study discovered TiO: as a preferred support where it could assist in stabilising the active hydroperoxy species. The Au:Pd ratio was also found to be an important variable, and equal weight ratio between Au and Pd was shown to be the optimised ratio for methane oxidation either using addition of H2O2 or in-situ H2O2 approach. The synergistic effect of Au and Pd was confirmed by superior catalytic activity compared to monometallic catalysts. Reaction mechanism was proposed and it was based on catalytic evaluation data, stability of the products and oxidation with radical scavengers. The proposed mechanism was in line with the theoretical modelling studies on similar catalytic systems. Optimisation of Au based supported catalyst with copper as co-metal supported on TiO2 was shown to improve the oxygenate productivity and methanol selectivity as well as enhanced the H2O2 utilisation. In particular, trimetallic 5wt%AuPdl.0wt%Cu/TiO2 synthesised via impregnation method and calcined in static air gave more than double turn over frequency (TOF = 1.404) with methanol selectivity around 83% as compared to bimetallic 5wt%Au-Pd/TiO2 catalyst (TOF = 0.692, methanol selectivity = 49%). It was suggested in this study that copper is responsible in enhancing the formation of intermediate methyl hydroperoxide species and in some extent to block the non-selective sites for hydrogen peroxide decomposition and hydrogenation by disrupting the surface structure of Au-Pd alloy whilst at the same time maintaining the active sites (Au-Pd alloy) responsible for selective formation of methanol. The oxidation state of copper was shown to be the main factor in controlling the catalytic activity and selectivity. Copper in a combination of multiple oxidation states was preferred than single oxidation state.

Item Type:	Thesis (PhD)
Status:	Unpublished
Schools:	Chemistry
Subjects:	Q Science > QD Chemistry
ISBN:	9781303222887
Date of First Compliant Deposit:	30 March 2016
Last Modified:	25 Jul 2022 10:07
URI:	https://orca.cardiff.ac.uk/id/eprint/55125

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