Pd on carbon nanotubes for liquid phase alcohol oxidation

Pd nanoparticles supported on carbon nanotubes (CNTs) showed a higher selectivity than Pd nanoparticles supported on activated carbon (AC) in the liquid phase oxidation of benzylic alcohol to benzaldehyde. Under solventless conditions a significant improvement in selectivity was observed for Pd/CNTs, whereas using Pd/AC a considerable over-oxidation of benzaldehyde was observed. Differently from other solvents cyclohexane improved significantly the selectivity to benzaldehyde for both catalysts. Characterisation by means of transmission electron microscopy revealed differences in metal dispersion between Pd/AC and Pd/CNTs that can be ascribed to textural, chemical and physical differences between active carbon and carbon nanotubes. The higher activity in the case of Pd on AC than on CNTs can be attributed to the improved Pd dispersion in the first case. On recycling Pd/CNTs resulted more stable (activity loss 50% in 7 runs) than Pd/AC (activity loss 70% in 7 runs) even a structural change of catalysts after reaction is observed. The Pd leaching and particle coalescence are the main reasons for the loss of activity. An extraordinary improving of catalyst life has been observed by alloying the Pd nanoparticles with Au, When CNTs are used as support the strong Pd leaching can be greatly limited and the activity/selectivity maintained at least for 8 runs.