Photo-Initiated Oxidations

A recent contribution to the chemoselectivity problem is the composite photocatalysis developed by Maldotti et al. [117]. Cyclohexene la was subjected to photo-initiated oxidation in Nation membranes containing Pd(II) porphyrin Pd(4-TMPyP) [4-TMPyP = weso-tetrakis(N-methyl-4-pyridyl)porphyrin] and Fe(III) porphyrin 11 to... 

On the other hand, it is interesting to use the supercritical C02 as an alternative solvent for various photochemical reactions, particularly for photo-initiated oxidation reactions, due to the non-ignitability of C02. Thus we are studying photo-induced oxidation reactions of ternary mixtures of hydrocarbon/02/C02. Partially oxidized products have been found to be produced in the case of C Hg, ethylene (C2H4) and cyclohexane (C6H,2) which have been studied so far. The present paper mainly reports the case of C Ii, in order to understand the behavior of typical olefinic hydrocarbons which are expected to trap some active species during the reaction. 

The special features of UV treatment technologies of water and air that are favored by manufacturers, researchers and developers are collected in Tab. 2-1 (cf FIGAWA, 1997, Prengle Jr et al., 1996, Freeman and Harris, 1995, von Sonntag, 1995, Meyer and Fischer, 1994). From this table it is evident that a remarkable difference in the numerical handhng of UV disinfection and photo-initiated oxidation techniques ex-... 

As already menhoned above, AOPs mainly rely on photo-inihated oxidahons (Fig. 5-10, reactions 4). However, depending on the particular situahon of water or air contamination, photo-induced oxidation and photooxygenahon reachons may compete effechvely with photo-initiated oxidations generating a complex mechanishc scheme (cf. Zepp, 1988). 

Fig. 5.12 Idealized scheme of photo-initiated oxidation reactions based on the presence of auxiliary oxidants and corresponding nomenclature.

Fig. 7.21 Simplified scheme of interrelated photo-initiated oxidation and secondary radical reactions in the gas phase (atmospheric...

The photo-initiated oxidation of persistent organic compounds in nickel containing waters of low optical transmittance (I <1 mm) by the H2O2-UV AOP could be satisfactorily achieved by using the photoreactor presented in Fig. 8-10 (Sorensen and Weckenmann, 1998). 

When the peroxyl population was quickly destroyed by 2-methyl -2-nitrosopropane treatment, post-y oxidation still occurred (Figure 7). In addition, film which had been pre-oxidized by xenon-arc photo-initiated oxidation showed an identical, slow, thermal oxidation during storage, as well as a rapid embrittlement in the accelerated aging at 60°C (Figure 6). The photo-oxidation of PP is well... 

As well as influencing chromophore types and levels, variations in production conditions result in fibers of varying morphology (orientation, crystallinity, and supramolecular order) (3). Because morphology itself may markedly effect photooxidative sensitivity, the extent of this effect was explored by first studying the susceptibility of the various filaments to y-initiated oxidation. The y-initiation is believed to be completely random and it is not dependent on trace quantities of chromo-phores or the build-up of oxidation products, in contrast to photooxidation, yet it is followed by the same propagation and termination steps as photo-initiated oxidations (2,4). 

Costa L., M.P. Luda, and L. TrossareUi. 1997a. UHMWPE. 2. Thermally and photo initiated oxidation. Polym Degr Stab 58 41-54. 

In yet other polymers, such as with bisphenol polycarbonates both photodegradation and photo-initiated oxidative reactions take place simultaneously. The direct photoreaction is a Erie s rearrangement reaction, while the oxidative reactions result in chain scission and discoloration of the polymer. The chemical pathways involved in the latter process are not well understood. 

Photo-initiated oxidation accompanied by scission With PE and PP, light-initiated oxidation is the predominant reaction, and extensive main-chain scission, sometimes accompanied by cross-linking, takes place with rapid loss of mechanical integrity. The autoxidation cycle associated with photo-initiated scission is discussed in detail in Eigure 6.3. 

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