Photocatalytic oxidation of aromatic alcohols to aldehydes in aqueous suspension of home-prepared titanium dioxide: 1. Selectivity enhancement by aliphatic alcohols

Abstract

Oxygenated aqueous suspensions of home-prepared (HP) and commercial TiO2 catalysts were used in a batch photoreactor for carrying out the oxidation of benzyl alcohol (BA) and 4-methoxybenzyl alcohol (MBA) under different operative conditions. HP catalysts were synthesized from TiCl4 and underwent a hydrolysis treatment of different times under mild conditions. The textural characterization of catalysts was carried out with XRD, SEM observations, BET surface area and porosity measurements. For both alcohols the main oxidation products were the corresponding aromatic aldehydes and CO2. The HP catalysts exhibited selectivity values towards the aldehyde production up to 28% (BA conversion: 50%) and 41% (MBA conversion: 65%), about four times higher than those of commercial TiO2. The addition of an aliphatic alcohol (methanol, ethanol, 2-propanol or tert-butanol) in small amounts with respect to water decreased the overall oxidation rate of aromatic alcohols but enhanced the selectivity for aldehyde formation up to 1.5 times. The reactivity results suggest that: (i) the aromatic alcohol molecules interact with the TiO2 surface in different ways that eventually determine two parallel reaction pathways (partial oxidation or mineralization); (ii) the aliphatic alcohols preferentially compete with aromatic alcohols for the mineralizing pathway.