Photocatalytic oxidations hydrocarbons

In this paper, we report a study of the photocatalytic oxidation in the neat-liquid phase and at room temperature of various model hydrocarbons containing one or two C6 rings (cyclohexane, methyl- and dimethyl-cyclohexane, cyclohexene, decalin and tetralin) over TiC>2 to further investigate the possibilities of heterogeneous photocatalysis in the field of fine chemicals. Bibliographic investigations gave no references on the photocatalytic oxidation of the various hydrocarbons cited above, except for cyclohexane (refs 6 - 8) with which wa have initiated the study of liquid cycloalkanes (ref. 9). 

Rosenthal J, Luckett TD, Hodgkiss JM, Nocera DG. Photocatalytic oxidation of hydrocarbons by a bis-iron(III)-/i-oxo Pacman porphyrin using 02 and visible light J Am Chem Soc 2006 128 6546-6547. 

Here the intervention of the hydrocarbon radical cation seems possible. Hydrocarbon photocatalyzed oxidations seem to depend significantly on the relative positions of the valence-band edge of the active photocatalyst and the oxidation potential of the substrate. For example, in contrast to the photocatalytic oxidation of toluene described above, lower activity was observed in neat benzene, despite the fact that its oxidation potential lies at or slightly below the valence-band edge. This observation implies the importance of radical cation formation (by photoinduced electron transfer across the irradiated interface) as a preliminary step to hydrocarbon radical formation. If the benzene is dispersed into a benzene-saturated aqueous solution into which the semiconductor is suspended, complete mineralization is attained [158]. Thus, to observe selective photoelectrochemistry, it is necessary to avoid primary formation of the highly reactive, nonselective hydroxyl radical (formed by water oxidation) by the use of an unreactive, but polar, organic solvent. 

A mechanism has been proposed to explain the heterogeneous photocatalytic oxidation of hydrocarbons in oxygen-containing solutions at platinized Ti02. This is based on the photogeneration of hydroxyl radicals at the Ti02 surface [equations (2)—(4), where h" = hole]. However, another paper suggests that the... 

Selective photocatalytic oxidation of hydrocarbon compounds over zeolites. 

We are advancing in the study of the photocatalytic oxidation of hydrocarbons using the TiOz/metal porphyrin hybrid catalyst and, herein, we report on the importance of surface tayloring of the TIO2 catalyst in order to improve the yields in cyclohexane oxidation. Results of the photo-oxidation of n-heptane and methylcyclohexane are also reported. In this case, the distribution of the photo-oxidation products provides some insight in how the regioselectivity is affected by the surface modification. 

We performed gas phase photocatalytic oxidation reactions of hydrocarbons by flowing a known mixture of heated humid air with organic vapor through an annular reactor (25). We used nanostructured TiOj coated using flame aerosol technique at optimized process conditions, which enabled us to achieve higher conversions and selectivities dian obtained by coating prepared by other methods. 

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. 

Izumi 1, Dunn WW, Wilboum KO, Fan F-RF, Bard AJ. (1980) Heterogeneous photocatalytic oxidation of hydrocarbons on platinized titanium dioxide powders. J. Phys. Chem., 84 3207-3210. 

Examples of reactions of industrial and environmental importance that run according MvK mechanism are alcohol selective oxidations, VOC total oxidations, selective catalytic reductions of NOx by ammonia or hydrocarbons, and more recently photocatalytic oxidation of hydrocarbons [5] etc. Revisitation of MvK mechanism for various reactions appeared in Ref. [6] For each one of these reactions, a metal oxide-based catalyst with well-developed redox properties has been optimized. 

Hashimoto, K., Kawai, T., and Sakata, T., Photocatalytic reactions of hydrocarbons and fossil fuels with water. Hydrogen production and oxidation, ]. Phys. Chem., 88, 4083,1984. 

Homogeneous, nanosized, copper-loaded anatase titania was synthesized by an improved sol-gel method [197], These titania composite photocatalysts were applied to the photoreduction of carbon dioxide to evaluate their photocatalytic performance. Methanol was found to be the primary hydrocarbon product [198], Under calcination conditions, small copper particles are well dispersed on the surface of anatase titania. According to XAS and XPS analysis, the oxidation state of Cu(I) was suggested to be the active species for C02 photoreduction [199], Higher copper dispersion and smaller copper particles on the titania surface are responsible for a great improvement in the performance of C02 photoreduction. 

That products of intermediate oxidation level can be detected in the photocatalytic reactions of hydrocarbons and fossil fuels is also consistent with a surface bound radical intermediate . Photocatalytic isotope exchange between cyclopentane and deuterium on bifunctional platinum/titanium dioxide catalysts indicates the importance of weakly adsorbed pentane at oxide sites. The platinum serves to attract free electrons, decreasing the efficiency of electron-hole recombination, and to regenerate the surface oxide after exchange. Much better control of the exchange is afforded with photoelectrochemical than thermal catalysis > ) As before, hydrocarbon oxidations can also be conducted at the gas-solid interface... 

The effects described in sections 2.1 to 2.4 are in agreement with the photocatalytic character of the cyclohexane oxidation chosen as model of the oxidation of cyclic hydrocarbons. 

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. 

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