Methane is the main component of natural gas and a by-product from oil refining and chemical processing. It is therefore desirable that novel technologies and infrastructure are developed to convert methane into value-added chemicals. Direct conversion to oxygenated products is highly desirable but challenging owing to its chemistry. Currently, primary utilization of methane takes place via indirect routes involving production of synthesis gas which is highly energy intensive. Direct conversion to methanol at low temperatures is an atom-efficient method for utilization. One strategy for catalyst design involves spatial separation of the oxidation reaction and the active-site regeneration, thus closing the catalytic cycle. However, this requires use of preformed reactive oxygen species, with many systems requiring highly acidic media like Periana catalyst which uses sulfuric acid for oxidation. Conceptually similar is the use of zeolite catalyst with hydrogen peroxide as oxidant. Hydrogen Peroxide can also be synthesised in situ, hence creating a catalytic system for methane oxidation. Gold-Palladium catalysts have been shown to be highly effective for the direct synthesis of hydrogen peroxide. It has been shown that supported AuPd and AuPdCu catalyse the low temperature oxidation of methane in liquid phase. In this work, gold-palladium colloidal catalysts are shown to be active for the oxidation of methane in water at 50oC using hydrogen peroxide as the oxidant. Activity and selectivity of these nanoparticles are compared with those of supported catalysts prepared by sol-immobilization. The effect of supporting the nanoparticles is investigated, specifically its effect on peroxide decomposition. EPR studies show that these reactions proceed via a radical mechanism. These colloidal catalysts involve use of a stabilizing ligand, which afford a greater particle size control with narrow particle size distribution (2-5 nm). The reaction is shown to be highly selective towards the primary oxygenated products namely methyl hydroperoxide, methanol and formic acid, with highest selectivity towards methyl hydroperoxide. Over-oxidation leads to formation of carbon dioxide. With time on line studies and optimization of the reaction conditions, primary oxygenate selectivity of ca. 90% can be be achieved. Catalyst preparation, characterization and effect of reaction conditions will be discussed.

Low temperature selective oxidation of methane using unsupported gold-palladium colloidal catalysts

2019 Fall National Meeting

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