Solvent-free oxidation of benzyl alcohol using Au–Pd catalysts prepared by sol immobilisation

We report the preparation of Au–Pd nanocrystalline catalysts supported on TiO2 and carbon prepared via a sol-immobilisation technique using three different preparation strategies; namely, simultaneous formation of the sols for both metals or initial formation of a seed sol of one of the metals followed by a separate step in which a coating sol of the second metal is added. The catalysts have been structurally characterised using a combination of transmission electron microscopy and X-ray photoelectron spectroscopy. The catalysts have been evaluated for the oxidation of benzyl alcohol under solvent-free conditions. The catalysts prepared using the sol immobilisation technique show higher activity when compared with catalysts prepared by impregnation, particularly as lower metal concentrations can be used. The Au–Pd catalysts were all more active than the corresponding monometallic supported Au or Pd catalysts. For 1 wt% Au–Pd/TiO2 the order of metal addition in the preparation was not observed to be significant with respect to selectivity or activity. However, the 1 wt% Au–Pd/carbon catalysts are more active but less selective to benzaldehyde than the TiO2-supported catalysts when compared at iso-conversion. Furthermore, for the carbon-supported catalyst the order of metal addition has a very marked affect on activity. The carbon-supported catalysts are also more significantly affected by heat treatment, e.g.calcination at 400 °C leads to the activity being decreased by an order of magnitude, whereas the TiO2-supported catalysts show a 50% decrease in activity. Toluene is observed as a by-product of the reaction and conditions have been identified that minimise its formation. It is proposed that toluene and benzaldehyde are formed by competing parallel reactions of the initial benzyl intermediate via an adsorbed benzylidene species that can either be hydrogenated or oxidised. Hence, conditions that maximise the availability of oxygen on the catalyst surface favour the synthesis of benzaldehyde.