Evidences in favour of a single electron transfer (SET) mechanism in the TiO2 sensitized photo-oxidation of α-hydroxy- and α,β-dihydroxybenzyl derivatives in water (original) (raw)
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Physical Chemistry Chemical Physics, 2010
The TiO 2 photosensitized oxidation in water of a series of X-ring substituted benzyl alcohols gives the corresponding benzaldehyde. Kinetic evidence (from competitive experiments) suggests a single electron transfer (SET) mechanism with a changeover of the electron abstraction site from the aromatic moiety (X = 4-OCH 3 , 4-CH 3 , H and 3-Cl) to the hydroxylic group (X = 3-CF 3 and 4-CF 3 ), probably due to the preferential adsorption of the above OH group on the TiO 2 surface. The same photo-oxidation of a series of 1-(X-phenyl)-1,2-ethanediols and of 2-(X-phenyl)-1,2-propanediols gives the corresponding benzaldehyde and acetophenone, respectively, accompanied by formaldehyde, whereas a series of symmetrically X-ring-substituted 1,2-diphenyl-1,2-ethanediols yields the corresponding benzaldehyde (substrate/product molar ratio = 0.5). The relative rate values suggest a SET mechanism in all of the series, with electron abstraction from one of the two OH groups of all the considered diols, probably due to the much higher adsorption of the above groups (due to the chelation effect) on the semiconductor. Further confirmation of this mechanistic behaviour has been obtained from laser flash photolysis experiments.
Journal of Chemical Research, 2002
The TiO 2 -sensitised photomineralisation of organic compounds as pollutants in waste water is an environmentally friendly system. 1 In this context, it is useful to acquire further information about the mechanistic behaviour of differently functionalised substrates. In particular, knowledge about the kinetically significative steps of the primary oxidation products and of the reaction stoichiometry (the number of electrons exchanged per molecule of reacted substrate) is useful. The use of the TiO 2 photoanode (as a film supported on a conducting material) in an electrolytic cell could be a way to evaluate both of these aspects. In effect, this photoelectrochemical technology allows the experimentally-observed electrical parameters (current efficiency and density) to be used to acquire further mechanistic information relative to the process. To date, this technology has been principally used in aqueous medium. 2 In some works, acetonitrile was considered as an alternative solvent; 3 this organic solvent, in contrast to water, does not signficantly compete with the substrate oxidation and therefore allows current efficiencies nearer to unity to be obtained, 3a,b making the process more useful for both synthetic and mechanistic purposes. It has also been shown that the TiO 2 -photosensitised oxidation mechanism should be the same as in aqueous medium, at least with respect to the primary oxidation steps of low reduction potential substrates. 4 In this work we have undertaken a study in CH 3 CN of the photoelectrochemical oxidation of 4-(1) and 3-methoxybenzyl alcohol (2) together with 4-(3) and 3-methoxybenzyltrimethylsilane (4) sensitised by TiO 2 (with 'anatase' or 'rutile' crystal structure), as a film supported on titanium anodes prepared as reported. 5 The analysis of the reaction products together with current efficiency (yield) and density measurements have provided further information about the mechanistic behaviour of these photoelectrochemical processes.
Journal of Photochemistry and Photobiology A-chemistry, 2004
The quantum yields (Φ) of the colloidal TiO 2 -sensitized photooxidation of 4-(1) and 3-methoxybenzyl alcohol (2) together with 4-methoxybenzyltrimethyl-(3) and 4-methoxybenzyltriisopropylsilane (4) were determined in CH 3 CN, in the presence of HClO 4 for 3 and 4. The true quantum yields (Φ 0 ) of 1, 2 and 3, obtained from a Langmuir-Hinshelwood isotherm treatment of Φ at different substrate concentrations, are linearly correlated with I −1/2 A , where I A is the light intensity. According to the previously suggested mechanisms for alcohols and silanes, a kinetic scheme that justifies this correlation is suggested. It is shown that the ratio of the slopes (from the Φ 0 versus I −1/2 A plots) for 1 and 2 is equal to the Φ 1 0 /Φ 2 0 ratio at any I A ; this ratio depends on the rate constants in the kinetic scheme, in this case principally on the electron transfer constant, k et . On the contrary, the Φ 3 0 /Φ 4 0 ratio depends on k p , the cation radical desilylation rate constant, confirming a steric hindrance to nucleophylic assistance in the C-Si fragmentation by the bulky isopropyl group in the SiR 3 moiety. Differently from Φ 0 , the adsorption constants on the semiconductor under irradiation (K D , obtained from the above isotherm treatment) are independent of I A . Moreover, as K 1 D = K 2 D and K 3 D = K 4 D , the structural modifications within the two alcohols and within the two silanes should be far enough away from the adsorption site. For all the substrates, K (the dark adsorption constant) is five times greater than K D , showing that this change does not depend on the substrate structure but is the result of different experimental conditions.
Journal of Physical Organic Chemistry, 2000
The quantum yields (f obs ) relating to the colloidal TiO 2 -sensitized photo-oxidation of ring methoxysubstituted benzylic alcohols (D) were determined. The true quantum yields (f 0 ) were obtained from a Langmuir-Hinshelwood isotherm treatment of f obs at different [D]. In the light of the suggested photo-oxidation mechanism, an equation was deduced where f 0 is expressed as a function of the electron transfer (k et ), back electron transfer (k Àet ) and benzylic deprotonation (k p ) rate constants. In particular, the lower f 0 value of 3-CH 3 O-vs 4-CH 3 O-benzyl alcohol (1, with lower E p ) should principally depend on the difference in k et , while the decrease in f 0 on going from alcohol 1 to the a-methyl derivative (4) should be due to the difference in k p (stereoelectronic effect). The adsorption equilibrium constants under irradiation (K D ) were also obtained from the above equation and the values are similar for the considered substrates, except for 4. In fact, this substrate shows a lower K D value, probably because of the steric hindrance of the methyl group to the OH adsorption (preferential site) on TiO 2 . Finally, both the inter-and intramolecular deuterium isotope effect (k H /k D = 1.8 and 1.6, respectively) are consistent with a kinetically significant C a -H bond breaking following the electron-transfer step.
Journal of Photochemistry and Photobiology A: Chemistry, 1995
The photocatalysed oxidation of phenol, 2-chlorophenol and pentachlorophenol was re-examined under conditions in which TiO2 anatase was sensitized by CdS in air-equilibrated aqueous media; this was to assess whether or not the interparticle electron transfer pathway, first discovered a decade ago (N. Serpone, E. Borgarello and M. Gratzel, J. Chem. Soc., Chem. Commun., (1984) 342) and subsequently applied to enhance reductive processes on titania, could also be applied to photooxidative processes. The results indicate that combinations such as CdS/TiO2 lead to an enhanced rate of disappearance of the initial substrate by a factor greater than two, consistent with the notion that (irradiated) CdS electrons are vectorially displaced onto the non-illuminated TiO2 particulates. Cadmium sulphide is a poorer photo-oxidation catalyst than titania. Other semiconductor materials have also been examined under a variety of conditions of pH and irradiation wavelength. The data also show that when both semiconductors in a coupled system are illuminated simultaneously and their valence and conduction bands are suitably disposed, both electron and hole transfer occur (as in the CdS/TiO2, ZnOFFiO2, TiOJFe203 and ZnO/Fe203 couples), which will influence the efficiency of photo-oxidation. NzO-saturated aqueous dispersions of TiO2 have no effect on the photo-oxidation of phenol, although it was expected that nitrous oxide would scavenge the photogenerated electrons in a manner similar to chemisorbed molecular oxygen, and enhance the efficiency. It is suggested that the role of oxygen in photo-oxidations may be more than just a simple electron scavenger.
Journal of Photochemistry and Photobiology A: Chemistry, 2001
The early degradation product distributions from TiO 2 -mediated photocatalytic degradations of a series of multiply hydroxylated benzenes and their methoxylated analogs is reported. The methoxylated compounds show a distinct trend away from ring-opening reactions that are attributed to electron transfer chemistry towards hydroxylations and demethylations that are attributed to hydroxyl-type chemistry. The initial rates of the reactions suggest that this is not simply due to the availability of the extra carbon sites. It is instead hypothesized that the compounds with hydroxyl groups are bound to the TiO 2 in such a way as to facilitate electron transfer and its subsequent chemistry. Addition of isopropyl alcohol to the degradations slows the degradations of the methoxylated compounds more than it does the hydroxylated compounds and particularly inhibits the demethylation and hydroxylation reactions. It is suggested that the question of whether an organic substrate must be bound to the photocatalyst at the time of excitation may be dependent on the type of chemistry being observed.
Journal of Catalysis, 2010
The highly efficient and selective photocatalytic oxidation of benzyl alcohol and its derivatives substituted with-OCH 3 ,-CH 3 ,-C(CH 3) 3 ,-Cl,-CF 3 and-NO 2 into corresponding aldehydes has been successfully carried out on TiO 2 in the presence of O 2 under visible light irradiation. The photocatalytic activity for the formation of the aldehyde was evaluated by a pseudo-first-order reaction, and it was found that the activity is enhanced by phenyl-ring substitution with the electron-releasing groups (-OCH 3 ,-CH 3 ,-C(CH 3) 3) and the electron-withdrawing groups (-Cl,-CF 3 and-NO 2). The effects of the substituents and their orientation on the photocatalytic performance of selective oxidation reaction are discussed here. It was shown that the photocatalytic activities are influenced not only by the oxidative potentials of the reactants but also by the stability of the resonant structures of the benzylic alcohol radicals formed by oxidation with a hole, leading to further reactions to form corresponding aldehydes.
Journal of Photochemistry and Photobiology A-chemistry, 2011
The photosensitized electrochemical oxidation at a TiO 2 /Ti anode is, in itself, a "green" chemical strategy as the semiconductor remains unchanged during the experiments, the metal is safe, low energies are involved and experimental conditions are mild. In this context, the oxidation of 4-methoxybenzyl alcohol to 4-methoxybenzaldehyde has been carried out on the light of further "green chemistry" requirements: water as medium, NaCl as supporting electrolyte, exclusion of the reference electrode, Ti cathode (less expensive than the traditional Pt one) and Ti mesh (to optimize the cell geometry). In this study some chemical (alcohol conversion to aldehyde and material recovery) and electrochemical (current efficiency and density) parameters have been used to select the reaction conditions that are more suitable for this purpose. As concern the cell, the best geometry was realized with an immersion lamp, coaxial with a mesh cylindrical TiO 2 /Ti anode and a mesh Ti cathode.
Journal of Photochemistry and Photobiology A: Chemistry, 2016
The TiO 2 photosensitized oxidation of benzyl methyl sulfides (X-C 6 H 4 CH 2 SCH 3) and benzyl thiols (X-C 6 H 4 CH 2 SH) has been investigated in Ar-saturated CH 3 CN. Steady-state irradiation produced benzaldehydes or dibenzylsulfides as oxidation products with sulfides and thiols, respectively. The results obtained through kinetic competitive experiments, aimed to evaluate the ring substituent effect on the reactivity, suggested the involvement of radical cation intermediates, formed by the favorable electron transfer from the substrate to the TiO 2 photogenerated hole, which reasonably deprotonate to give the final products. This process was poorly affected by the adsorption of the substrate at the TiO 2 surface, as demonstrated by similar results, both in terms of products and reactivity, collected for the homogeneous photooxidation of the same substrates sensitized by N-methoxyphenanthridinium hexafluorophosphate (MeOP + PF 6 À). This behavior is likely due to the low hydrogen-bond acceptor ability of divalent sulfur systems. Density functional theory calculations pointed out that the most stable conformations of X-C 6 H 4 CH 2 SH + are characterized by having the CÀ ÀS bond almost collinear with the p system of the aromatic ring and by a significant charge and spin delocalization involving both the phenyl ring and the sulfur atom. 2016 Elsevier B.V. All rights reserved.