Tunable gold catalysts for selective hydrocarbon oxidation under mild conditions

Good as gold Under normal circumstances gold is far too noble to act as an effective catalyst, but that changes when it is present as finely divided nanoparticles. Gold in that form is finding particular favour as a ‘green’ catalyst that might help replace peroxide-based oxidation processes that produce noxious waste products. A new catalyst, made of gold nanocrystals supported on carbon, has been found to be highly effective for the selective oxidation of alkenes to more valuable chemical compounds, using only oxygen present in air and minute amounts of an initiator. Oxidation is an important method for the synthesis of chemical intermediates in the manufacture of high-tonnage commodities, high-value fine chemicals, agrochemicals and pharmaceuticals: but oxidations are often inefficient 1 . The introduction of catalytic systems using oxygen from air is preferred for ‘green’ processing 2 . Gold catalysis is now showing potential in selective redox processes 3 , 4 , 5 , 6 , particularly for alcohol oxidation 7 , 8 , 9 , 10 and the direct synthesis of hydrogen peroxide 11 , 12 . However, a major challenge that persists is the synthesis of an epoxide by the direct electrophilic addition of oxygen to an alkene 13 . Although ethene is epoxidized efficiently using molecular oxygen with silver catalysts in a large-scale industrial process 14 , this is unique because higher alkenes can only be effectively epoxidized using hydrogen peroxide 15 , 16 , 17 , hydroperoxides 16 or stoichiometric oxygen donors. Here we show that nanocrystalline gold catalysts can provide tunable active catalysts for the oxidation of alkenes using air, with exceptionally high selectivity to partial oxidation products (∼98%) and significant conversions. Our finding significantly extends the discovery by Haruta 18 , 19 that nanocrystalline gold can epoxidize alkenes when hydrogen is used to activate the molecular oxygen; in our case, no sacrificial reductant is needed. We anticipate that our finding will initiate attempts to understand more fully the mechanism of oxygen activation at gold surfaces, which might lead to commercial exploitation of the high redox activity of gold nanocrystals.