Photocatalytic oxidations olefins

Chlorinated olefins, particularly trichloroethylene (TCE), appear uniquely suited to destruction by photocatalytic oxidation. The photocatalytic oxidation of TCE... 

Although TCE cofeeds are unable to increase the photocatalytic oxidation of benzene, the effects on the oxidation of branched aromatics are significant. However, practical considerations will hinder the use of TCE cofeeds. First, TCE and branched aromatic contaminants are not often present in the same airstream. Adding TCE or similar chlorinated olefins to an environment or airstream containing branched aromatic contaminants is not practical, because TCE is, itself, classi-... 

Photocatalytic oxidation over illuminated titanium dioxide has been demonstrated to be effective at removing low concentrations of a variety of hazardous aromatic contaminants from air at ambient temperatures. At low contaminant concentration levels and modest humidity levels, complete or nearly complete oxidation of aromatic contaminants can be obtained in photocatalytic systems. Although aromatic contaminants are less reactive than many other potential air pollutants, and apparent catalyst deactivation may occur in simations where recalcitrant reaction intermediates build up on the catalyst surface, several approaches have already been developed to counter these potential problems. The introduction of a chlorine source, either in the form of a reactive chloro-olefin cofeed or an HCl-pretreated catalyst, has been demonstrated to promote the photocatalytic oxidation of... 

In view of the above results, in photocatalytic oxidation of a series of 4-substituted diphenylethylenes, an increase in reactivity with decreasing Hammett s sigma constants (31) seems to arise not only from the lowering of the oxidation potentials of the olefins in this sequence but also from the general trend of the increase in the reactivity of olefins toward peroxyl radicals with increasing the electron donating ability of olefins (33). 

The coexistence of various pollutants does not have to be deleterious, but, in certain cases, can be quite beneficial. The first evidence for this claim came probably from the work of Lichtin et al. (1994) who found that the edition of 0.03% by volume to an air-stream containing 0.1% iso-octane caused an enhancement in the photocatalytic oxidation of the latter. Likewise, a significant rate enhancement was recorded in the photocatalytic degradation of chloroform and dichloromethane in the presence of TCE. Similar effects were recorded also with other chlorinated olefins, such as perchloroethylene (PCE) and trichloropropene (TCP), which enhanced the photooxidation of toluene in a manner similar to that of TCE (Sauer et al., 1995). 

RSCOCH, — RSOtsH. Swem and co-workers " developed an efficient synthesis of alkylsuBbnic acids involving photocatalytic addition of thiolacetic acid to a terminal olefin and oxidation of the adduct with 90% hydrogen peroxide in acetic... 

In summary, the addition of zeolite to Ti02 in the photocatalytic oxidation of alkylbenzenes and olefines studied shows two different behaviours. The use of both zeolite and semiconductor seems to be particularly interesting for increasing selectivity in the photooxidation. The various effects observed by comparing the results with and without zeolites are related to different mechanisms for the two types of substrates. 

In situ FTIR and diffuse reflectance infrared Fourier transform spectroscopic (DRIFTS) studies deal mainly with photocatalytic oxidation or reduction of nitric oxide (NO) as important pollutant and green house gas as well as the depollution or selective oxidation of organics such as olefinic, aliphatic, aromatic, and oxygenated hydrocarbons. 

Similar redox-combined processes have been reported. For example, it has been clarified by control experiments using a photoirradiated semiconductor electrode that the photocatalytic production of indazoles from substituted azobenzenes is based on the condensation of two intermediates formed through oxidation and reduction.38 40) In the case of oxidation of substituted olefins a similar redox combined mechanism is assumed cation and anion radicals are formed by the reaction of olefin with positive hole and of 02 with excited electron, respectively, and they react to produce a 4-membered ring intermediate, a dioxethane, to undergo bond cleavages into the desired products.4l) In the photocatalytic reactions, a positive hole and excited electron must react at the neighboring surface sites of a small semiconductor particle, enabling the combination of reduction and oxidation without the addition of an electrolyte, which is an indispensable component in electrolysis. However, in the particulate system the recombination of positive hole and electron is also likely, as well as... 

At the end of 1970 s we attempted an investigation of the action of semiconductor particles on medium sized organic compounds. In this article, we would like to describe some of the work done in our laboratory on the photocatalytic action of semiconductors on the oxidation of olefins and hydrocarbons, and on the isomerization of unsaturated systems. 

Although the absolute amount of the photocurrents is governed by various factors such as the oxidation potentials of olefins and the extent of adsorption of olefins on the electrode, the above findings show that the reactive olefins in the photocatalytic oxygenation exhibit photocurrents and the olefins which do not exhibit photocurrents are unreactive in the photocatalytic oxygenation. On the other hand, the olefins which exhibit photocurrents are not always reactive. For example, stilbene shows a higher photocurrent than DPE, but is not so reactive as DPE. The electron transfer to the excited semiconductor takes place more efficiently from stilbene than from DPE due to the lower oxidation potential of the former, but in the subsequent free radical reactions, stilbene is less reactive than DPE (33). 

Some other catalytic events prompted by rhodium or ruthenium porphyrins are the following 1. Activation and catalytic aldol condensation of ketones with Rh(OEP)C104 under neutral and mild conditions [372], 2. Anti-Markovnikov hydration of olefins with NaBH4 and 02 in THF, a catalytic modification of hydroboration-oxidation of olefins, as exemplified by the one-pot conversion of 1-methylcyclohexene to ( )-2-methylcycIohexanol with 100% regioselectivity and up to 90% stereoselectivity [373]. 3. Photocatalytic liquid-phase dehydrogenation of cyclohexanol in the presence of RhCl(TPP) [374]. 4. Catalysis of the water gas shift reaction in water at 100 °C and 1 atm CO by [RuCO(TPPS4)H20]4 [375]. 5. Oxygen reduction catalyzed by carbon supported iridium chelates [376]. - Certainly these notes can only be hints of what can be expected from new noble metal porphyrin catalysts in the near future. 

The most important photocatalytical reactions are oxidation reactions which include the oxygenation of unsaturated systems, but also oxidations of saturated carbons with or without incorporation of oxygen (C-H activation). The photooxygenation of olefins in the presence of Ti or Mo catalysts leads one-pot to epoxy alcohols (17) via singlet oxygen (eq. (9)) [82]. 

Zeolites involving transition metal ions within their framework and cavities are known to exhibit unique and fascinating properties for applications not only in catalysis but also for various photochemical processes [1,2]. Zeolites incorporated with transition metal ions in a hi ly dispersed state were found to exhibit photocatalytic reactivity for such reactions as oxidation using NO or O2 as an oxidant and the metathesis reaction of olefins. Also, the urgency of the reduction of global air pollution makes the need to address the elimination of CO as well as NO of great concern. Since CO poisons the catalysts and causes a decline in reactivity, it is vital that the reaction can proceed even in the presence of CO. 

---