The hydrodynamics of a three-phase micro-packed bed reactor and its effect on catalysed benzyl alcohol oxidation with pure oxygen were studied in a silicon–glass microstructured reactor. The microreactor was operated at 120°C and 1barg and contained a channel with a 300μm×600μm cross-section, packed with 1wt% Au–Pd/TiO2 catalyst, 65μm in average diameter. Improvements in the conversion of benzyl alcohol and selectivity to benzaldehyde were observed with increasing gas-to-liquid ratio, which coincided with a change in the flow pattern from a liquid-dominated slug to a gas-continuous flow regime. The observed enhancement is attributed to improved external mass transfer, associated with an increase in the gas–liquid interfacial area and reduction in the liquid film thickness that occur with gradual changes in the flow pattern. A maximum selectivity of 93% to benzaldehyde was obtained under partial wetting – which introduced the added benefit of direct gas–solid mass transfer – outperforming the selectivity in a conventional glass stirred reactor. However, this was at the expense of a reduction in the conversion. A response surface model was developed and then used to predict optimal operating conditions for maximum benzaldehyde yield, which were in the gas-continuous flow regime. This corresponded to relatively high gas flow rate in conjunction with moderate liquid flow rate, ensuring sufficient catalyst wetting with a thin film to reduce transport resistances

We report the catalytic performance of Au–Pd nanoparticles prepared via a sol immobilisation technique for the catalytic hydrogenation of cinnamaldehyde under mild reaction conditions. We synthesised a series of bimetallic Au–Pd colloidal supported nanoparticles with different Au : Pd molar ratios and optimized the experimental parameters to achieve the best catalyst performance. The optimum catalytic activity for the hydrogenation of cinnamaldehyde was observed for Au₅₀Pd₅₀/TiO₂ (with a Au : Pd molar ratio of 1 : 1), while the monometallic Pd/TiO₂ was the most selective towards hydrocinnamaldehyde. The catalysts have been structurally characterised and FTIR analysis showed that the presence of adsorbed carbonyl surface species in used catalyst materials is coupled with Pd leaching, which is the main reason for catalyst deactivation. The effect of calcination on the most active Au–Pd/TiO₂ was studied in the range 110–400 °C and a direct correlation between the rise in calcination temperature and catalyst stability and selectivity was observed. These results emphasise the importance of tuning the Au–Pd molar ratio and understanding the metal–support interaction of catalysts synthesised for hydrogenation reactions, such as cinnamaldehyde hydrogenation.

The synthesis of supported bimetallic nanoparticles with well-defined size and compositional parameters has long been a challenge.