The oxidation of alcohols to aldehydes with O2 in place of stoichiometric oxygen donors is a crucial process for the synthesis of fine chemicals. However, the catalysts that have been identified so far are relatively inactive with primary alkyl alcohols. We showed that Au/Pd-TiO2 catalysts give very high turnover frequencies (up to 270,000 turnovers per hour) for the oxidation of alcohols, including primary alkyl alcohols. The addition of Au to Pd nanocrystals improved the overall selectivity and, using scanning transmission electron microscopy combined with x-ray photoelectron spectroscopy, we showed that the Au-Pd nanocrystals were made up of a Au-rich core with a Pd-rich shell, indicating that the Au electronically influences the catalytic properties of Pd.

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. The introduction of catalytic systems using oxygen from air is preferred for 'green' processing. Gold catalysis is now showing potential in selective redox processes, particularly for alcohol oxidation and the direct synthesis of hydrogen peroxide. However, a major challenge that persists is the synthesis of an epoxide by the direct electrophilic addition of oxygen to an alkene. Although ethene is epoxidized efficiently using molecular oxygen with silver catalysts in a large-scale industrial process, this is unique because higher alkenes can only be effectively epoxidized using hydrogen peroxide, hydroperoxides 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 ( approximately 98%) and significant conversions. Our finding significantly extends the discovery by Haruta 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.

Selective oxidation of primary carbon-hydrogen bonds with oxygen is of crucial importance for the sustainable exploitation of available feedstocks. To date, heterogeneous catalysts have either shown low activity and/or selectivity or have required activated oxygen donors. We report here that supported gold-palladium (Au-Pd) nanoparticles on carbon or TiO(2) are active for the oxidation of the primary carbon-hydrogen bonds in toluene and related molecules, giving high selectivities to benzyl benzoate under mild solvent-free conditions. Differences between the catalytic activity of the Au-Pd nanoparticles on carbon and TiO(2) supports are rationalized in terms of the particle/support wetting behavior and the availability of exposed corner/edge sites.

Supported Au catalysts are investigated for the oxidation of primary alcohols under solvent-free conditions in the absence of base. Three representative primary alcohols have been investigated: benzyl alcohol, octan-1-ol and geraniol using a range of supports for gold nanocrystals prepared using coprecipitation, deposition precipitation and impregnation. For benzyl alcohol and octan-1-ol selective oxidation to the corresponding aldehydes is observed, particularly with Au/CeO 2 , whereas for more acidic supports, e.g. Fe 2 O 3 , subsequent oxidation of the aldehydes to the corresponding acids, forming an ester (benzyl benzoate, octyl octanoate, respectively) by reaction with the alcohol, by a standard acid-catalysed mechanism. Alternatively, the mechanism of ester generation could involve hemiacetal formation between the aldehyde and residual alcohol, followed by direct oxidation to the observed ester. The reaction of geraniol is much more complex and the reaction is carried out in the presence and absence of acids to gain a full understanding of the interplay between oxidation and isomerisation reactions. Comparison with other active catalysts reveals that using Au catalysts in solvent free conditions gives very high turnover frequencies for the synthesis of the aldehydes with 100% selectivity (150 h )1 and 26 h )1 for benzyl alcohol and octan-1-ol, respectively), which are comparable to the best reported to date for these reactions.

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The oxidation of benzyl alcohol to benzaldehyde has been investigated in the absence of solvent using zeolite-supported Au and Au-Pd catalysts. Three zeolites were investigated, ZSM-5, zeolite b and zeolite Y, and these were contrasted with the titanoslicalite TS-1 and TiO 2 as supports. For the Au catalysts the best results are obtained with zeolite b as the support and the conversions were comparable or better than those observed with TiO 2 in terms of turn over frequencies. However, the selectivities observed with the acidic zeolites were lower than the non-acidic TS-1 and TiO 2 . This is due to the subsequent reaction of benzaldehyde via acid catalysed reactions to give benzyl benzoate and its dibenzyl acetal, and, in some cases dibenzylether. Initial catalysts were evaluated with a gold loading of 2 wt% and increasing this to 4 wt% showed the expected increase in activity, indicating that there is scope to improve the performance of these catalysts. The most active catalysts were prepared by impregnation and catalysts prepared by deposition precipitation were considerably less active. Introduction of Pd into the catalyst improved the activity without significantly affecting the selectivity.