Photocatalytic oxidation of aromatic alcohols to aldehydes in aqueous suspension of home-prepared titanium dioxide

Abstract

Oxygenated aqueous suspensions of home-prepared (HP) and commercial TiO 2 catalysts were used in a batch photoreactor for carrying out the oxidation of benzyl alcohol (BA) and 4methoxybenzyl alcohol (MBA) under different operative conditions. HP catalysts were synthesized from TiCl 4 and underwent a hydrolysis treatment of different times under mild conditions. The textural characterisation 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 CO 2 . 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 TiO 2 . 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 TiO 2 surface in different ways that eventually determine two parallel reaction pathways (partial oxidation or mineralization); and (ii) the aliphatic alcohols preferentially compete with aromatic alcohols for the mineralizing pathway.

Photooxidation of ethanol on Cu-layer silicate/TiO2 composite thin films

“…The preparation of semiconductor oxides (TiO 2 , ZnO, SnO 2 ) and studies on their photocatalytic efficiencies represent an important possibility for the utilization of solar energy [1][2][3][4][5][6][7][8][9]. Since the excitation threshold of the TiO 2 utilized in our present study is 3.2 eV (λ g = 387 nm), nanoparticles require the application of a wavelength range relatively high energy, i.e.…”

Comparative analysis of ZnO-catalyzed photo-oxidation of p-chlorophenols

“…Many photoactive materials have been utilized to derive a high number of processes having varied applications [2][3][4]. Among these applications photo-oxidation stands at the forefront of contemporary research owing to its costeffectiveness and environmentally benign nature [5][6][7]. When water is used as medium for the photo-oxidation of organic compounds, their complete mineralization prevails owing to the presence of potent oxidizing species such as hydroxyl radical [8,9].…”

Enhancing selectivity in photocatalytic formation of p-anisaldehyde in aqueous suspension under solar light irradiation via TiO2 N-doping

“…The finding that the mineralization of adsorbed MBA molecules proceeds through steps in which the oxidation products are not released to the liquid phase suggests that this process should be favoured by using a catalyst with a hydrophobic surface. 18,19,21 It has been reported 18,21 that low crystalline catalysts have a hydrophilic surface while very crystalline catalysts have a hydrophobic surface. The hydrophilic/hydrophobic character is linked to the density of hydroxyl groups on the titania surface; this density decreases by increasing the treatment temperature of the catalyst.…”

“…This is indeed in agreement with previous literature. [18][19][20][21][22][23][24] The last series of experiments (Table 5) were performed under near-UV radiation. As the irradiance between 315 and 400 nm of this light source was 15-60 times higher than that of the other sources, the reactivities were the highest ones.…”

“…Hence, one of the main goals of current research is that of eliminating the environmentally harmful conditions of industrial processes. 15,16 Only recently, photocatalytic partial oxidation of aromatic alcohols to the corresponding aldehydes has been carried out under near-UV radiation in aqueous suspensions of home-prepared (HP) anatase, [17][18][19] brookite 20,21 and rutile TiO 2 22-24 in laboratory batch reactors and also in continuous reactors. 25,26 In those conditions HP photocatalysts showed selectivity values for aldehyde production far higher than those of commercial TiO 2 ones.…”