Effect of Electron Density of Pt Catalysts Supported on Alkali Titanate Nanotubes in Cinnamaldehyde Hydrogenation

Pt nanoparticles of diameter 0.5-1.5 nm were dispersed homogeneously on the inner and outer surfaces of alkali metal titanate nanotubes (M2Ti3O7, M = Li+, Na+, and K+) by ion exchange with Pt(NH3)(2)Cl-2 followed by calcination in ambient air at 673 K and reduction in H-2 at 448 K. Both XPS and DRIFTS revealed that these titanate nanotubes donated their electrons to the Pt particles, yielding a negative Pt oxidation state, and that the degree of electron donation increased with the size of the alkali metal cation. The high electron density of the Pt particles promoted the activation of both C=C and C=O functional groups in cinnamaldehyde. Therefore, the order of the catalytic activity in the hydrogenation of cinnamaldehyde was Pt/KTNTs > Pt/NaTNTs > Pt/LiTNTs. Since the Pt particles were very small, Pt/MTNTs selectively hydrogenated the C=C bond, yielding a hydrocinnamaldehyde with selectivities of 80-87% (at cinnamaldehyde conversions of 27-50%). The alkali metal ions of the titanate nanotubes were able to modify the mode of adsorption of cinnamaldehyde. Therefore, when a KOH promoter was added to the reaction mixture, cinnamyl alcohol became the main product and the cinnamyl alcohol selectivities were 77-83% (at cinnamaldehyde conversions of 49-74%).