Photoxidation of Benzyl Alcohol with Heterogeneous Photocatalysts in the UV Range: The Complex Interplay with the Autoxidative Reaction

Abstract

In conventional studies of heterogeneous photocatalytic oxidation, it is crucial to evaluate by means of control experiments the relevance of the “non‐catalyzed” autoxidation reaction under irradiation. If the autoxidation is found to be negligible, it is usually considered that it can be safely disregarded. However, in the case of aromatic aldehydes’ synthesis such as benzaldehyde, irradiating with UV light may lead to a more complex mechanism than previously thought. Herein, we prove that neglecting the autoxidative reaction can lead to a misinterpretation of the mechanism as well as an overestimation of the catalyst's efficiency, even if the control experiment exhibits zero yield. As an example, we studied the synthesis of benzaldehyde from benzyl alcohol by UV‐A irradiation with and without TiO2 and CdS nanoparticles; without catalyst, after a long induction time, the aldehyde enhances its own production through an autocatalytic reaction activated by the same light wavelengths utilized for the nanocatalysts. The long induction time leads to considering the autocatalysis to be deceptively negligible. However, the nanoparticles act as a fast initiator of the benzaldehyde autocatalysis, in such a way that the oxidation rate reached a similar performance with nanoparticles and after removing them. This suggests that the commonly ignored autocatalysis of benzaldehyde indeed is a relevant parallel pathway to the heterogeneous catalysis mechanism.

Revealing the contribution of singlet oxygen in the photoelectrochemical oxidation of benzyl alcohol

“…This formation of 1 O 2 explains the nonelectrochemical oxidation of 1, depicted in Fig. 3c; therefore we can exclude the hypothesis that other pathways, such as the autocatalytic oxidation of 1 reported previously, 30 is taking place in our case. The reaction depicted in Fig.…”

“…We thought that this non-electrochemical oxidation could be due to the following reasons: (i) thermal effect, (ii) photocatalytic activity of BiVO 4 , (iii) activation by cOH radicals, 8 (iv) light-driven photooxidation, or (v) autocatalytic oxidation of benzyl alcohol, as suggested in previous studies. 30 Considering that in the chronoamperometry, aer illuminating for 13 hours, the solutions reach a temperature of 313 K, we performed the reaction at this temperature, without applying bias or illumination. Under these conditions, no reaction was detected (Table Scheme 1 1, entry 2); therefore, the overoxidation due to a thermal process can be discarded.…”

“…This autooxidation was suggested to be a consequence of the photoactivation of small amounts of benzaldehyde present in the reaction media. 30 Aiming to shed some light on the role of UV irradiation in the photocatalytic oxidation of alcohols, herein we carried out a study considering all plausible mechanistic pathways. In the rst set of experiments, we detected a photoelectrode-free oxidation of benzyl alcohol, with concomitant formation of H 2 O 2 as a persistent side-product.…”

Metal/semiconductor interfaces in nanoscale objects: synthesis, emerging properties and applications of hybrid nanostructures

“…Colloidal-phase photocatalytic reactions are mainly oxidation or reduction reactions (possibly with radical intermediates) taking place in aqueous or organic media. Though organic-phase photocatalytic oxidation reactions exist (e.g., 2mercaptobenzothiazole (MBT) oxidation demonstrated using cesium-lead-halide perovskite QDs in hexane, 334 benzyl alcohol oxidation using titania and titania-iron oxide 335 or using CdS and/or TiO 2 in MeCN (acetonitrile) and or MeCN-tri-uorotoluene mixtures 336 and alcohol dehydrogenation in MeCN 67 ), they are less common for M/SC hybrids as the metal serves as the electron sink. Therefore, when a SC/M hybrid is used, the electrons from the sink are consumed for the formation of radical species such as cO 2 from dioxygen, which are consumed during the oxidation reaction.…”

Unraveling the impact of the Pd nanoparticle@BiVO4/S-CN heterostructure on the photo-physical & opto-electronic properties for enhanced catalytic activity in water splitting and one-pot three-step tandem reaction

“…Noble metal-and non-noble metalmediated and C 3 N 4 -based photocatalysts have been developed for the selective oxidation of benzyl alcohol to benzaldehyde. [25][26][27] Further, catalytic hydrogenation of nitroaromatics using Pt-, Au-, Pd-, and Ir-based systems has been developed using H 2 as a reducing agent. [28][29][30] It is also possible to use the catalytic transfer hydrogenation strategy to reduce nitroaromatics in the presence of hydrogen donors, such as hydrazine, formic acid, and NaBH 4 .…”