Alcohols photocatalytic oxidation

Photocatalytic oxidation is a novel approach for the selective synthesis of aldehyde and acid from alcohol because the synthesis reaction can take place at mild conditions. These reactions are characterized by the transfer of light-induced charge carriers (i.e., photogenerated electron and hole pairs) to the electron donors and acceptors adsorbed on the semiconductor catalyst surface (1-4). Infrared (IR) spectroscopy is a useful technique for determining the dynamic behavior of adsorbed species and photogenerated electrons (5-7). 

The photocatalytic oxidation of alcohols constitutes a novel approach for the synthesis of aldehydes and acid from alcohols. Modification of Ti02 catalyst with Pt and Nafion could block the catalyst active sites for the oxidation of ethanol to CO2. Incorporation of Pt resulted in enhanced selectivity towards formate (HCOO ad)-Blocking of active sites by Nafion resulted in formation of significantly smaller amounts of intermediate species, CO2 and H2O, and accumulation of photogenerated electrons. The IR experimental teclmique has been extended to Attenuated Total Reflectance (ATR), enabling the study of liquid phase photocatalytic systems. 

Under the same reaction conditions, acetaldehyde and butyraldehyde displayed near-complete conversion (greater than 95%). The photocatalytic oxidation of the alcohol 1-butanol displayed similarly high conversion levels, although conversion of methanol was somewhat lower. The oxygenated compounds methyl-t-butyl ether (MTBE), methyl acrylate, 1,4 dioxane, and vinyl acetate displayed conversion levels ranging from 92% to 100%. The lowest conversion levels of the oxygenated compounds studied were seen with the ketones used [acetone and 2-butanone (methylethylketone)], which displayed conversions of approximately 80%. The initial conversion levels seen with -hexane were similar... 

Most photocatalytic studies conducted at low aromatic concentrations report no detectable concentrations of gas-phase intermediates [12,17,18]. Traces of intermediates may be present in the gas phase, but at levels below the detection limits of the analytical instruments employed in these studies. There is evidence, however, for either reaction intermediates or reaction by-products on the catalyst surface, even at these low concentrations. Catalyst discoloration, typically a yellowish or brownish color, is often reported following the photocatalytic oxidation of aromatic contaminants at low to moderate gas-phase concentrations [3,4,7,17,52]. These intermediates or reaction by-products may be largely trapped on the catalyst surface by the higher affinity of oxygenated species, like alcohols and aldehydes, for TiO, surfaces when compared to the aromatic parent compounds. 

A recent report on the regioselective oxidation of lactic acid also seemed consistent with this idea. Photocatalytic oxidation on platinized TiO2 led to decarboxylation, while that on platinized CdS led to pyruvic acid by oxidation of the alcohol group, Eq. (6) However, when potentiostatic oxidation of lactic acid was conducted in... 

It has recently been recognized that crystal structure and particle size can also influence photoelectrochemical activity. For example, titanium dioxide crystals exist in the anatase phase in samples which have been calcined at temperatures below 500 °C, as rutile at calcination temperatures above 600 °C, and as a mixture of the two phases at intermediate temperature ranges. When a range of such samples were examined for photocatalytic oxidation of 2-propanol and reduction of silver sulfate, anatase samples were found to be active for both systems, with increased efficiency observed with crystal growth. The activity for alcohol oxidation, but not silver ion reduction, was observed when the catalyst was partially covered with platinum black. On rutile, comparable activity was observed for Ag, but the activity towards alcohol oxidation was negligibly small . Photoinduced activity could also be correlated with particle size. 

Methvl-cvclohexane. The photocatalytic oxidation of neat-liquid methyl-cyclohexane gives selectivities equal respectively to 67 % in ketones, 4 % in alcohols, 2 % in cyclohexyl-formaldehyde and 27 % in C02- The following scheme indicates the percentages in the various... 

On the other hand, the heterogeneous photocatalytic oxidation of toluene, in aqueous suspensions of TiO, has been studied with some details, by Fujihira et al. (refs.4-5). These authors have reported that, in aqueous media, up to 2 h of UV-irradiation, benzaldehyde was one of the main products benzyl alcohol was de tected as traces and benzoic acid was not detected (refs.4-5). However, in our experimental conditions using neat-liquid toluene, not only benzaldehyde but both benzyl alcohol and benzoic acid were detected as main products. 

Richard found evidence that both holes and hydroxy radicals are involved in the photocatalytic oxidation of 4-hydroxybenzyl alcohol [32]. His results suggest holes and hydroxyl radicals have different regioselectivities in the photocatalytic transformation of this compound hydroquinone is thought to result from the direct oxidation by a valence-band hole, dihydroxybenzyl alcohol from the reaction with a hydroxyl radical, while 4-hydroxybenz-aldehyde is produced by both pathways. In the presence of a hydroxyl radical quencher, the formation of dihydroxybenzyl alcohol is completely inhibited while the formation of 4-hydroxybenzaldehyde is inhibited. 

Chemical differentiation between two oxidizable sites in the same molecule can also be achieved in organic photocatalytic reactions by choice of a different semiconductor and thus adjustment of the electrochemical band-edge positions. Consistent with this idea, the photocatalytic oxidation of lactic acid on UV-irradiated plati-nized-Ti02 leads to decarboxylation, presumably through the singly oxidized carboxyl radical. In contrast, the same reagent on irradiated platinized CdS leads to pyruvic acid by oxidation of the alcohol group (Eq. 10) [96]. 

In fact, primary alcohols can be selectively oxidized to the corresponding aldehydes, without appreciable overoxidation, whereas poorer yields were obtained with secondary aliphatic aromatic alcohols [106c. Similarly, phenols can be converted to quinones [118]. The efficiency of the photocatalytic oxidation depends on the pretreatment and size of the photocatalyst [119], and analogous conversions are also obtained on heteropolyacids, which can be considered soluble analogs of Ti02 suspensions [120]. 

Reports of the photosensitivity of POMs in the presence of organic compounds date to at least 1916 364 however, the first systematic study of POM photochemistry was the work of Yamase who studied the photochromism of alkyl- and dialkylammonium polymolybdates365 366 and subsequently the structural and spectroscopic (largely EPR) properties of irradiated alkylammonium molybdates.367-372 In key early work involving POMs as photocatalysts, Papaconstantinou reported the photocatalytic oxidation of alcohols by 02 in 1982 (Equation (34)),373 Darwent published the anaerobic photocatalytic production of H2 from alcohols later that same year (Equation (35)) 374 and Hill showed the breadth of the photocatalytic oxidation of organic compounds by POMs in 198 5.375 In the intervening years, many features of POM photochemistry have been elucidated. The key papers in this and other areas and their principal findings are summarized in Table 3 ... 

Tzhakis, M.D. et al., Decamngstate catalyst supported on silica and y-alumina Efficient photocatalytic oxidation of benzyl alcohols, 7. Catal., 252, 178, 2007. 

The photocatalytic oxidation of alcohols was performed in the presence of different loading amounts of HPW on MCM-41 (15, 30, and 50 wt.%) in three different ionic liquids (Scheme 14.45). 

Scheme 14.45 Photocatalytic oxidation of alcohols in ionic hquids...

Photocatalytic oxidation of lactic acid (600) on platinized Ti02 or CdS is strongly regioselective (Scheme 6.295).1553 Irradiation in the presence of Pt/TiCE at 360 420 nm leads to the cleavage oxidation products acetaldehyde and carbon dioxide, whereas pyruvic acid is exclusively obtained by irradiation of the Pt—CdS mixture at 440 520 nm. Since the standard potential of oxidation of acetic acid is known to be more positive than the valence band edge of CdS (CdS has a less positive valence band edge than does Ti02), both catalysts readily oxidize aliphatic alcohols and the CdS photocatalyst is apparently capable of specific oxidation. 

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