Photocatalytic oxidation of cyclohexane by titanium dioxide: Catalyst deactivation and regeneration

Two commercially available TiO2 catalysts were compared in the selective photocatalytic oxidation of cyclohexane: Hombikat UV100 (as received (H), and after calcination at 600 °C (H600)), and Solaronix, S450. Hombikat UV100 shows the highest initial activity on a g−1 catalyst basis, followed by H600 and S450 with very similar activity profiles. All catalysts suffer from deactivation. By in situ ATR and DRIFT spectroscopy, it is demonstrated that the extent, nature, and thermal stability of carboxylates and carbonates formed on the surface of the three catalysts are quite different. The extent of carboxylate formation was significantly smaller on the surfaces of H600 and S450, when compared to H. The thermal stability of the surface species at 400 °C decreased in the order H > H600 > S450. Complete removal of carboxylate and carbonate species from the surface was only achieved in the case of S450, which resulted in complete regeneration of activity, as demonstrated in both a slurry reactor (top illumination reactor) and internally illuminated monolith reactor (IIMR). The differences in surface chemistry and regenerability are discussed on the basis of lattice defects, affecting the opto-electronic properties, and defects/irregularities on the surface, affecting (thermal) stability of surface-adsorbed species. Solaronix S450 is the preferred catalyst for the desired conversion, the favorable surface properties being an important first step toward the practical application of photocatalysis in selective liquid-phase photo-oxidation processes and in particular toward employment of reactors with immobilized TiO2 in the photocatalytic oxidation of cyclohexane.

The catalyst stability in photocatalytic oxidation of cyclohexane and surface chemistry of TiO2 are a strong function of the hydrophilicity of the surface and the crystal ‘quality’, affecting the concentration of reactive holes. Well-defined crystals with hydrophobic surfaces induce favorable catalyst properties.Figure optionsDownload high-quality image (99 K)Download as PowerPoint slideResearch highlights
► In the selective photocatalytic oxidation of cyclohexane, severe catalyst deactivation is causing the current levels of conversion to be below 2%.
► Hombikat-based TiO2 as received (H) shows the highest rate of cyclohexanone formation in photocatalytic oxidation of cyclohexane in a slurry-based reactor illuminated from the top, compared to H600 (Hombikat calcined at 600 °C) and Solaronix S450.
► However, deactivation of H and H600 was found to be irreversible, while for S450, the surface population after photocatalytic reaction (ratio of carboxylates over carbonates) was different, and the thermal stability of these species was found to be significantly lower.
► This allowed regeneration of this material by heat treatment in air at 400 °C. The regenerated S450 catalyst was successfully used both in slurry (Top Illumination Reactor) and in immobilized applications (Internally Illuminated Monolith Reactor).