Photo-Kolbe reaction

A large variety of organic oxidations, reductions, and rearrangements show photocatalysis at interfaces, usually of a semiconductor. The subject has been reviewed [326,327] some specific examples are the photo-Kolbe reaction (decarboxylation of acetic acid) using Pt supported on anatase [328], the pho-... 

The photo-Kolbe reaction is the decarboxylation of carboxylic acids at tow voltage under irradiation at semiconductor anodes (TiO ), that are partially doped with metals, e.g. platinum [343, 344]. On semiconductor powders the dominant product is a hydrocarbon by substitution of the carboxylate group for hydrogen (Eq. 41), whereas on an n-TiOj single crystal in the oxidation of acetic acid the formation of ethane besides methane could be observed [345, 346]. Dependent on the kind of semiconductor, the adsorbed metal, and the pH of the solution the extent of alkyl coupling versus reduction to the hydrocarbon can be controlled to some extent [346]. The intermediacy of alkyl radicals has been demonstrated by ESR-spectroscopy [347], that of the alkyl anion by deuterium incorporation [344]. With vicinal diacids the mono- or bisdecarboxylation can be controlled by the light flux [348]. Adipic acid yielded butane [349] with levulinic acid the products of decarboxylation, methyl ethyl-... 

Spin trapping by PBN has also been employed to detect radical formation in a photo-Kolbe reaction in which acetic acid is irradiated (A > 360 nm) in the presence of platinized titanium dioxide powder (Kraeutler et al, 1978). The nitroxide observed was considered to be (PBN—Me ), but the published spectrum clearly shows the presence of a second species spectral overlap might therefore be an alternative to solvent polarity as an explanation of the discrepancy between the observed splitting parameters and those previously reported for this species. Where poor resolution obtains, it is important that... 

A variety of photocatalyzed decarboxylation reactions on Ti02 powder including the decomposition of acetate to methane and carbon dioxide and the breakdown of benzoic acid yielding predominantly CO2 have been reported by Bard and coworkers (23,24). Evidence for the occurrence of these "photo-Kolbe" reactions has stimulated the search for other organic reactions that might be photochemically initiated by excitation of semiconductors and extensive work in this area is in progress (25). 

Sakata, T Kawai.T. Hashimoto, K. (1984) Heterophotocatalysed reactions of organic acids and water. New reaction paths besides the Photo Kolbe Reaction J. Phys. Chem. 88 2344-2350... 

Carbon-carbon coupling of radicals observed in the photo-Kolbe reaction could also be observed with other surface generated radicals. Kisch and coworkers have shown, for example, that cyclic allylic ethers undergo alpha deprotonation under photoelectrochemical activation, producing radicals that can be oxygenated, Eq. (29). On colloidal zinc sulfide, hydrogen evolution accompanies the photocatalytic... 

Figure 12.14 shows the effect of ultiasound on the amount of the main products of the Photo-Kolbe reaction. The reaction appears to have been accelerated by ultrasonic irradiation. The product ratio of the sonophotocatalytic reaction, however, was not satisfactory. A reasonable value of methane (CH4) to carbon dioxide (C02) must be 1.0 for Photo-Kolbe reaction, as shown in Eq. (12.14). 

In this section, the sonophotocatalytic reaction of an organic compound is demonstrated. As a reactant, acetic acid is introduced. The reaction of acetic acid is a typical and a very famous photocatalytic reaction, callec the Photo-Kolbe reaction.345... 

Several authors (23,24) have reported photoassisted reactions between carbon and water yielding some hydrogen and C02. Photo-Kolbe reactions of carboxylates have also been demonstrated (25). However, neither addition of acetate to 0.08M nor an unintentional gross contamination of the 10M NaOH electrolyte with charred epoxy residue caused significant acceleration of hydrogen production in our experiments. The presence of carbon monolayers on SrTi03 shows the need for caution in evaluating photoreactions where the total product yield is on the order of one monolayer or less. 

While this reaction is substantially exothermic (6), it provides an intriguing approach to the production of fuels from renewable resources, as the required acids (including acetic acid, butyric acid, and a variety of other simple aliphatic carboxylic acids) can be produced in abundant yields by the enzymatic fermentation of simple sugars which are, in turn, available from the microbiological hydrolysis of cellulosic biomass materials ( ] ) These considerations have led us to suggest the concept of a "tandem" photoelectrolysis system, in which a solar photoelectrolysis device for the production of fuels via the photo-Kolbe reaction might derive its acid-rich aqueous feedstock from a biomass conversion plant for the hydrolysis and fermentation of crop wastes or other cellulosic materials (4). 

Despite these precautions, marked corrosion was still observed on some, but not all, of the n-SrTi03 photoanodes obtained from four different sources. The corrosion appeared to be most severe after several experiments (totalling typically 20 hours or more of use as an electrode) had been conducted under photo-Kolbe reaction conditions. A fine white film was also observed to form gradually on the irradiated areas of n-SrT103 when the acid electrolyte (typically 2N H2SO4) was used in the absence of added acetic acid. 

Figure 1. Comparison of n-SrTi03 photoelectrodes (A) before and (B) after corrosion under photo-Kolbe reaction conditions...

V 1 Range of Potentials Reported for Photo-Kolbe Reaction > (2CH3C02H + 2e - C2H6 + 2C02 + H2t at... 

Some additional insight as to possible mechanisms of the photocorrosion process can be gained from a more detailed consideration of the effects of pH on the band levels in SrTiC>3 and on the redox potentials of oxygen formation and the photo-Kolbe reaction. These data, along with the band levels for Ti02, are shown in Figure 5. It is important to remember that the photocorrosion process occurs in com-... 

The mechanistic principles which enable a semiconductor powder, suspended in a solution of substrates, to catalyze a photoreaction originate in the field of photoelectrochemistry. For a critical discussion of the basic assumptions made in the deduction of the resulting theories and their relevance to catalysis, the reader is referred to recent review articles [33-35]. The use of a metallized semiconductor particle as a kind of short-circuit micro-photoelectrochemical cell was introduced through the work of Kreutler and Bard on the photo-Kolbe reaction [36]. Differences from the macroelectrodes employed in more conventional photoelectrochemistry have been discussed [34, 37-40]. 

Stimulated by the early work of Bard et al. on the Ti02-catalyzed photo-Kolbe reaction [36], many papers appeared in subsequent years dealing with photocatalytic organic reactions [115] in the presence of colloidal or suspended semiconductor particles. They include cis-trans isomerizations [68, 93, 116-119], valence isomer-izations [120, 121], substitution and cycloaddition reactions [73, 80, 122-125], oxidations [126, 127], and reductions [128-130]. Characteristic of all these reactions is that in almost all cases well known compounds were formed, which were not isolated but only characterized by spectroscopic methods. The nature of the products can be rationalized within the mechanistic scheme of semiconductor photocatalysis type A, which means that at least one reduced and one oxidized compound are... 

Table 16. Major reaction products (pmol in 15 h) of photo-Kolbe reaction of 1 vol. /o Acetic acid (from H. Miyoshi and H. Yoneyama, J. Chem. Soc., Faraday Trans. I 1989, 85, 1873. Reproduced by permission of the Royal Society of Chemistry).

As already mentioned before, mainly irreversible reactions with organic compounds have been investigated at semiconductor particles. When organic molecules, for example alcohols, are oxidized by hole transfer, O2 usually acts as an electron acceptor or in the case of platinized particles, protons or H2O are reduced. A whole sequence of reaction steps can occur, which are frequently difficult to analyze because cross-reactions may also be possible at particles and a new product could be formed. Concerning the primary electron and hole transfer, certainly there should be no difference between particles and compact electrodes. Since sites at which reduction and oxidation occur are adjacent at a particle, the final product may be different. An interesting example is the photo-Kolbe reaction, studied for Ti02 electrodes and for Pt-loaded particles. Ethane at extended electrodes and methane at Pt/Ti02 particles have been found as reaction products upon photo-oxidation of acetic acid [56, 57]. The mechanism was explained by Kraeutler et al. as follows. 

The platination of semiconductor powders is a method for producing semiconductor-metal type photochemical diodes with an energy level scheme as shown in Fig. 11.14a. This was demonstrated for the first time with platinized Ti02 powders which showed excellent photocatalytic activity for the photodecarboxylation of acetate (the photo-Kolbe reaction), a process which has already been discussed in detail in Section 9.2.3 [80]. Various attempts have also been made to photocleave water by using semiconductor powders on which a catalyst such as Pt or RUO2 has been deposited. The relevant experiments usually failed, either because the semiconductor was... 

The possible involvement of R in the Kolbe reaction has been regularly investigated using ESR but, to date, positive evidence for radical intermediates has been lacking apart from two cases, firstly the photo-Kolbe reaction at irradiated semiconductor electrodes [76] and, secondly, the anodic oxidation of triphenylacetic acid at platinum electrodes in acetonitrile. The latter will be discussed in this section. 

Induced by the early work of Kraeutler and Bard [36] on the T1O2 catalyzed Photo-Kolbe reaction, many papers appeared thereafter dealing with photo-catalytic [115] organic reactions in the presence of colloidal or suspended... 

Another photocatalytic process that recently has been discussed, is the photo-Kolbe reaction (Eq. 11-12) ... 

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