Application of hydrothermally produced TiO2 nanotubes in photocatalytic esterification of oleic acid

• A hydrothermal method was employed to synthesize TiO2 nanotubes.
• TiO2 nanotubes were studied for photocatalytic esterification of oleic acid.
• Optimum conditions were obtained at a concentration of 15% (w/w) and a molar ratio 3:1 (methanol:oleic acid).
• The greater number of hydroxyl groups may have contributed to a low yield of ester versus P25.

This study investigated the use of TiO2 nanotubes (TNTs) as photocatalysts in the esterification of fatty acids for biodiesel production. The TNTs were synthesized via a hydrothermal route and evaluated for their crystallinity, morphology, surface area and photocatalytic activity compared with a TiO2 P25 standard. Optimum photocatalytic conditions were obtained using a 15% concentration of catalyst (w/w) and a 3:1 molar ratio of methanol to oleic acid. The highest yield of methyl oleate obtained was 86.0% when P25 was used as a photocatalyst. The lowest band gap energy was obtained with the TNT sample synthesized at 110 °C for 48 h (Eg = 3.08 eV), which also exhibited the highest rate of oleic acid esterification (59.3%) among all the investigated TNTs. We also observed that, in addition to the band gap, other factors such as the crystalline phase of the TNTs and their surface area were important in photocatalytic performance.