Photooxidation, liquid phase

Larson, S.A., and Falconer, J.L. Characterization of Ti02 Used in Liquid Phase and Gas Phase Photooxidation of Trichloroethylene, in D.F. Ollis, and H. Al-Ekabi (Eds.), Photocatalytic Purification of and Treatment of Water and Air . Elsevier Science Publishers, Amsterdam (1993), p. 473. 

The authors concur in saying that the primary molecular product of photooxidation of an aldehyde in the liquid phase is the corresponding peracid ... 

If the temperature is not too high, the peracid formed is itself the principal product of the reaction, at least in the early stages of oxidation. It may therefore be said that in concentrating their attention on the initial rate of photooxidation of an aldehyde in the liquid phase, the authors have studied the kinetics of the primary reaction—the oxidation of this aldehyde to the corresponding peracid. 

Fillet, Niclause, and Letort were thus led to suggest the chain mechanism by free radicals set out below, as a means of representing the photooxidation, in the liquid phase, of ethanal into peracetic acid. 

Fig. 1. Influence of the oxygen pressure on the initial rate on photooxidation of n-heptanal. Temperature of the liquid phase 3°C. Volume of the liquid phase 5 cm. / = 4 X 10 7einstein(5em. )" min. i(I)Pureheptanal(two series of experiments) (2) 20% solution by volume of heptanal in n-decane. The curves plotted are theoretical curves, calculated on the basis of the following values (at 3°C.) ...

With the aid of a dynamic apparatus, Bowen and Tietz studied the photochemical oxidation of gaseous ethanal at around 25°C. and for near-unity values of the ratio of concentration [OjJ/IRCHO]. The primary molecular product of the reaction is, as in the liquid phase, peracetic acid. The rate of this photooxidation is independent of the oxygen concentration and is proportional to the concentration of aldehyde and to the square root of the absorbed light intensity la- The total quantum yield of the reaction is of the order of magnitude of 100. ... 

These experimental data are already sufficient to indicate that the photooxidation of gaseous ethanal takes place via a chain reaction. Accepting the processes of propagation suggested by Backstrom, von Elbe and Lewis wrote out a kinetic scheme which seems to be a special case of the mechanism we have already referred to in dealing with photooxidation of ethanal in the liquid phase, i.e., the case where termination processes (t4 and ts) are negligible in comparison with termination process (ta). 

The mechanism suggested by McDowell and Sharpies is identical with that already proposed by Fillet, Niclause, and Letort to represent the photooxidation of this aldehyde in the liquid phase, and McDowell and Sharpies have been led to conclude that in their experiments (as in those... 

In the case of photooxidation of heptanal (R = w-CeHis) in the liquid phase, a recent study using infrared spectrophotometry - also establishes the secondary formation of a peroxide X and assigns to it formula (B). 

Moreover, it has been shown in certain cases that the oxidation of an aldehyde in liquid phase bears the marks of self-inhibition, -this is particularly true in the case of the photooxidation of butanaP and of heptanal. - But the identity of the reaction product responsible for this auto-inhibition is still much contested. 

According to Melville and his co-workers, the thermal oxidation of n-decanal and benzaldehyde in liquid phase at around 10°C. must also involve a chain mechanism which would differ from that of photooxidation only in respect to the process of initiation in the case of the thermal reaction, the initiation process would be ... 

Melville and Richards to interpret the photooxidation of isopropylbenzene in liquid phase. 

Anatase is much more active than rutile in the liquid-phase photooxidation of 2-propanol, and the observed reactivity compared well with that of gaseous alcohol on platinized titania [106a]. Because most early studies employed the alcohol as a sacrificial reagent for the photochemical production of hydrogen, the organic product was often not analyzed and little effort was devoted to the selective activation of alcohols in the presence of other functional groups. In the oxidation of car-... 

Intermediates in colloidal titania-catalysed photoreactions have been detected, and a study of the heterogeneous and homogeneous photooxidation of various organic compounds in both gas and liquid phases has associated the photocatalytic centre of the catalyst with the Ti-O bond. During light absorption the Ti ion changes its oxidation state and coordination number. The photocatalytic oxidation of ethane has been studied at room temperature, au d in particular the effect on the rate of ethame oxidation of the partial pressure of react ults, the UV light... 

A further point of discussion, however, is whether the often cited reactions (6, 7, 8) of TMP derivatives (and their conversion products) with peroxy radicals may sufficiently compete with the propagation steps (5) in Scheme I. Indeed, TMP-derivatives including NOR are known to be rather weak scavengers of peroxy radicals (7) in the liquid phase. Based on considerations which take into account that rapid randomization of macroradicals is largely restricted in the solid polymer, Carlson and Wiles, however, concluded that fast radical scavenging would in fact not be needed for efficient inhibition of long chain polymer photooxidation processes (2, 8, 9). 

At the present time, a discussion of the results of our model investigations in terms of possible consequences for polypropylene must be completely speculative. Apart from the differences expected between liquid phase and solid polymer photooxidation kinetics, differences in the chemical structure between our model substance, isooctane, and the structural unit of polypropylene have to be also considered. With respect to the number of CH,-groups per structural unit, isooctane and polypropylene differ by a ratio of 5 1. 

The basic mechanism of the removal of SO2 from the atmosphere is its oxidation to SO3. In the day-time, at a low relative humidity, the SO2 oxidation is prevalent in the presence of nitrogen oxides or possibly of intermediate products of the photooxidation of hydrocarbons. In the night and under conditions of high humidity, SOj is absorbed in water drops and the oxidation occurs in the liquid phase. SO3, produced during the oxidation in the gas phase reacts with the atmospheric water vapour, producing an aerosol of HjSO. The aerosol particles in the atmosphere gradually increase in size and after several days they are as large as 0.1 to... 

K. Teramura, T. Tanaka, M. Kani, T. Hosokawa, T. Funabiki, Selective photooxidation of neat cyclohexane in the liquid phase over V2O5/AI2O3 , Journal of Molecular Catalysis A Chemical, 208, 299-305, (2004). 

Their use has, however, an important drawback that is related to the rather low surface areas and not easy preparation methods. But the recent achievements in the synthesis of metal-modified mesoporous silicas and development of new methods for the synthesis of oxides with higher surface areas diversified the number of catalysts and applications of mixed oxides as catalysts in liquid-phase oxidations. The performances recently reported opened new perspectives for the green and sustainable oxidation using such materials and extended the interest for the application of these reactions in industrial organic synthesis and water decontamination. Furthermore, coupling the photooxidation with Fenton and ozonation processes provides extremely attractive techniques in advanced oxidation processes for eliminating organic contaminants in wastewaters. 

Once initiated, photooxidation may proceed through a free-radical chain mechanism analogous to that commonly used to explain the thermal oxidation of alkanes in the liquid phase. 

FIGURE 9.63 Plots of om-phase-normalized gas-particle partitioning constant log Kp iun vs logarithm of the subcooled liquid vapor pressure, log pL, for a series of semivolatile PAHs partitioning on ( ) dioctyl phthalate (DOP) or (a) secondary organic aerosol (SOA) from photooxidized gasoline vapor. PAHs are as follows naphthalene, A acenaphthalene, B fluorene, C and C phenanthrene, D and D anthracene, E and E fluoranthene, F and F pyrene, G and G chrysene, H (adapted from Liang el al., 1997). 

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