Protonated titanate nanotubes as solid acid catalyst for aldol condensation

- Protonated titanate nanotubes (HTiNTs) successfully catalyzed aldol condensation.
- This is due to the possession of strong acid sites (surface bridging OH groups).
- Polar solvents compete with substrates for active sites on the surface of HTiNTs.
- HTiNTs did not require any activation prior to the reaction.
- HTiNTs were reused several times and efficient even in a 1 g experiment.

One-dimensional protonated titanate nanostructures were tested as solid acid catalysts in the condensation reaction between benzaldehyde and cyclohexanone. Two morphologies were employed - nanotubes and nanoribbons. In situ diffuse reflectance infrared Fourier transform spectroscopy of adsorbed NH3 and NH3 temperature programmed desorption revealed that both protonated titanate nanotubes and nanoribbons possess Lewis and Brønsted acid sites with medium acid strength, whereas only nanotubes have also strong acid sites. Therefore only protonated titanate nanotubes revealed an efficient catalyst that was also successfully applied to other reaction systems with substituted benzaldehyde derivatives. Recycling studies showed no significant decrease in the catalytic activity of protonated titanate nanotubes in five cycles and even showed an excellent performance in the large scale experiment. In addition, protonated titanate nanotubes did not require any activation prior to the reaction. The mechanism is proposed to describe the condensation process over the catalyst.

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