Selective oxidation processes of organic substances in water by means of photocatalytic systems
Selective oxidation of benzyl alcohol into benzaldehyde in anoxic acidic aqueous solution, through a TiO2/Cu(II)/solar UV photocatalytic system, has been investigated both in a laboratory scaled reactor equipped with a high-pressure mercury lamp as well as in a solar pilot plant.
During the laboratory experiments, benzaldehyde gave best results, in terms of yield, equal to 35% with respect to the initial benzyl alcohol concentration. A partial conversion of benzaldehyde to benzoic acid has also been observed. Traces of hydroxylated by-products have also been detected. On the basis of the formation of these species, a production of HO radicals has been thus inferred.
The study has suggested that different operative parameters, such as the composition and amount of TiO2 photocatalyst, pH, ionic inorganic components in water, and the initial concentration of Cu(II) ions, play an important role in the photocatalytic selective oxidation of benzyl alcohol.
The mechanism of photocatalytic selective oxidation of benzyl alcohol into benzaldehyde and benzaldehyde into benzoic acid has been investigated in the presence of TiO2 catalyst and cupric ions, as electron acceptor, in water at a pH = 2.0 and under deaerated conditions.
A competitive adsorption has been proposed in which the aromatic substrates are adsorbed on the TiO2 surface and react with the positive holes. Whereas Cu(II) ions are reduced to Cu(0) by the photogenerated electrons.
A new kinetic model has been developed by writing a set of mass balance equations for the main species involved in the photocatalytic oxidation process. The resulting mathematical model has been used for the analysis of the data collected at different starting substrates’ concentrations. During each of the selective photoxidation runs, it satisfactorily predicts the concentrations of Cu(II) species, organic substrates, and intermediates. The effect of ionic components, which compete with benzyl alcohol and benzaldehyde for the reaction with positive holes on the catalyst surface and behave as scavengers towards HO radicals, has been taken into account in the model.
The values of some rate constants of the reactions of the holes with benzyl alcohol, benzaldehyde, Cu(II) species, and inorganic anions (sulfates and di-hydrogenophosphates), not available in the literature, have been estimated by a proper optimizing procedure.
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The conversion of hydroxybenzyl alcohols, methoxybenzyl alcohols and nitrobenzyl alcohol into the corresponding aldehydes has been attempted by using the same process.
The presence and position of substituent groups in the aromatic alcohols structure change the photocatalytic oxidation rates and product selectivities with respect to that previously observed for unsubstituted benzyl alcohol. In particular, the presence of both electron donating (hydroxy, methoxy groups) and electron withdrawing (nitro group) on the aromatic ring of the substrate causes a detrimental effect on the selectivity of the process with respect to that of benzyl alcohol.
The technical feasibility of selective photocatalytic oxidation of benzyl alcohol to benzaldehyde, in aqueous solutions and in presence of cupric ions, has been then investigated in a solar pilot plant with Compound Parabolic Collectors.
Under deaerated conditions, the presence of reduced copper species has been proved by XPS analysis. The results indicated that, at the end of the process, cupric species can be easily regenerated and reused, through a re-oxidation of reduced copper that is produced during the photolytic run, with air or oxygen in dark conditions.
A figure-of-merit (ACM), proposed by the International Union of Pure and Applied Chemistry (IUPAC) and based on the collector area, has been estimated, under the proposed conditions, with the aim to provide a direct link to the solar-energy efficiency independently of the nature of the system. Generally speaking, it can be considered that the lower ACM values are, the higher is the system efficiency.