Oxidation in the Gas Phase

This reaction is termolecular and is in the falloff region between second and third order at 1 atm pressure. The recommended high- and low-pressure limiting rate constants for reaction (4) at room temperature are k = 2 X 1012 cm3 molecule-1 s-1 and k ) = 4.0 X 10-31 [N2] cm6 molecule-2 s- 1 with Fv = 0.45 (see Chapter 5.A.2) (Atkinson et al., 1997b) or, alternatively, k., = 1.5 X 10-12 cm3 molecule-1 s-1 and k(l = 3.0 X 10-31 [M] cm6 molecule-2 s-1 with Fc = 0.6 (DeMore et al., 1997). The effective bimolecular rate constant at room temperature is thus k= (9.7 or 8.8) X 10-13 cm3 molecule -1 s, depending on which set of recommendations is used, and the corresponding lifetime of S02 with respect to OH at 1 X 106 radicals cm-3 is 13 days. 

The adduct free radical formed in reaction (4) [which has been detected directly using neutralization/reionization mass spectrometry (Egsgaard et al., 1988)] subsequently reacts with 02  

While the SO, formed reacts rapidly with water, forming sulfuric acid, 

While reaction of SO, with water vapor to form sulfuric acid is expected to be by far its major fate, there is also the possibility that some minor reaction paths could play a role under some circumstances. For example, S03 forms a complex with NH3 and ulti- 

FIGURE 8.4 Predicted energetics for various mechanisms for the S0j-H20 reaction transition states are marked with a i symbol (adapted from Morokuma and Muguruma, 1994). 

---

Flame Retardants. Because PVC contains nearly half its weight of chlorine, it is inherently flame-retardant. Not only is chlorine not a fuel, but it acts chemically to inhibit the fast oxidation in the gas phase in a flame. When PVC is diluted with combustible materials, the compound combustibiHty is also increased. Por example, plastici2ed PVC with > 30% plastici2er may require a flame retardant such as antimony oxide, a phosphate-type plastici2er, or chlorinated or brominated hydrocarbons (145,146). 

In the present chapter, we report about an inveshgahon of the catalyhc performance of rahle-type V/Sb and Sn/V/Sb/Nb mixed oxides in the gas-phase ammoxidation of n-hexane. These catalysts were chosen because they exhibit intrinsic mulhfunctional properties in fact, they possess sites able to perform both the oxidahve dehydrogenahon of the alkane to yield unsaturated hydrocarbons, and the allylic ammoxidahon of the intermediate olefins to the unsaturated lutriles. These steps are those leading to the formahon of acrylonitrile in propane ammoxidahon. The SnW/Sb/(Nb)/0 system is one of those giving the best performance in propane ammoxidahon under hydrocarbon-rich condihons (8,9). 

Geminox A direct process for converting butane to 1,4-butanediol. The butane is first oxidized in the gas phase to maleic anhydride, using BP s fluidized bed technology. The maleic anhydride is scrubbed with water and then catalytically dehydrogenated to butanediol. Developed in 1994 by BP Chemicals and Lurgi. Modifications of the process can be used to make tetrahydrofiuan and y-butyrolactone. The first plant will probably be built on BP s site at Lima, OH, for completion in 2000. 

The purpose of the catalytic converter is to oxidize some of the oxides in the gas phase. NO is reduced to elemental nitrogen. The principal reaction at the catalytic converter is oxidation of CO ... 

Iodine oxides and bromine oxides are solid compounds which are beyond the scope of this article and will therefore not be discussed in any detail. Clyne and Coxon410 have found that BrO decays in a similar manner to CIO, with the second order rate coefficient of the order of 2x 108 1.mole-1.sec-1. In view of the important role of CIO in the decomposition of chlorine oxides, it is conceivable that BrO may play a similar role in the decomposition of bromine oxides. However, no kinetic information on the decomposition of bromine oxides in the gas phase appears to be available at the present time. 

It is considerably more difficult to inhibit oxidation in the gas phase than in the liquid phase. At the high temperatures of gas-phase oxidations the rates of the chain-propagating and branching reactions are increased to a greater extent than the rates of the chain-terminating reactions. Initiation by surfaces can also constitute a serious problem. The majority of liquid-phase antioxidants which are effective at high temperatures are too involatile to be useful in the gas phase. However, inhibition can be achieved with aliphatic amines, which are generally rather ineffective inhibitors of low temperature liquid-phase oxidations. The mechanisms by which the different types of antioxidants inhibit oxidation are briefly described below. 

Figure 8.2 depicts the principle of such an experiment. In air mass A below the cloud, there are sulfate and trace metals such as selenium in suspended particles, and S02 and oxidants in the gas phase. As the air... 

One can estimate the relative contribution of the Criegee intermediate (Cl) to SOz oxidation in the gas phase in the troposphere. The absolute value of the rate constant for the reaction of the Cl with SOz is not known, with estimates ranging from 1.7 X 10 " to 3 X 10 15 cm2 molecule-1 s-1 (Hatakeyama and Akimoto, 1994). Using the highest value and a concentration of the Cl of 1 X 105 molecules cm"3, one obtains 10"6 s"1 for the first-order rate of removal of S02 by this reaction. This can be compared to the rate of removal of SOz by reaction with 1 X 106 OH radicals cm3, which is also 10 6 s-1 using the effective bimolecular rate constant cited earlier. Using the lower estimates for the CI-S02 rate constant, which is more reasonable, would lower its contribution proportionately. 

Figure 2 shows that with about 0.5M isobutane at 100 °C. the rates and rate laws for oxidation in the gas phase and in solution are similar. 

The second major theory of the mechanism of alkane oxidation in the gas phase is the aldehyde theory. The products of gaseous oxidation almost invariably contain aldehydes, and it has been proposed (41) that these are formed by decomposition of alkylperoxy radicals. 

Sulfur Dioxide. The formation of SO2 from CH3S and CH3SO was previously discussed. SO2 is currently understood to be oxidized in the gas phase by OH radicals in a series of steps leading to sulfuric add (H2SO4) ana is thus of major importance to add deposition processes. The chapter by Anderson et al. (this volume) describes new experimental studies of the mechanism of the oxidation of SO . 

The theoretical treatment of reaction (1), when catalyzed by paramagnetic centres, was developed by Wigner, 3>, and the reaction was shown to occur, using oxygen or nitric oxide in the gas phase, or oxygen, paramagnetic ions or free radicals in solution 14 1S>. 

Z. Su et al., Chiral self-recognition Direct spectroscopic detection of the homochiral and heterochiral dimers of propylene oxide in the gas phase. J. Am. Chem. Soc. 128, 17126-17131 (2006)... 

The highly exothermic reaction for the formation of MgF2 heats up the carbon soot formed to approx. 2200 K, which then emits the IR radiation. Moreover, in Mg rich formulations (m > 2), the evaporating Mg is oxidized in the gas phase (3100 K). In addition, the carbon which is formed from the reductive elimination of fluorine from Teflon can be oxidized further to CO or C02 by atmospheric oxygen ... 

Tab. 5.2 Current examples of research related to photo-initiated AOPs in aqueous phase. Ox intermediary oxidation products formed, detected during treatment and characterized. Research examples concerning VUV oxidation in aqueous phase and UV oxidation in the gas phase are presented in Chapter 7 (Table 7-2 and Table 7-4, respectively). For O3-UV based AOPs refer to Cottschalk et al. (2000)... 

Coan, C.R. and A.D. King. 1971. Solubility of water in compressed carbon dioxide, nitrous oxide, and ethane. Evidence for hydration of carbon dioxide and nitrous oxide in the gas phase. /. Am. Chem. Soc. 98 1857-1862. 

Cyanamide and melamine can be obtained by llCN oxidation in the gas phase on silica gel [41 j (Section 3.3) and the catalytic oxidation of HCN on platinum has been reported as the most convenient and least polluting method for HCN destruction (54). 

R. R. Baldwin and R. W. Walker, Discussion on Low Temperature Oxidation in The Gas Phase, Donnan Laboratories, Liverpool, 1969. 

Photodissociation of thietane 1-oxide in the gas phase yields cyclopropane and ethylene. It is suggested that singlet sulfur monoxide is produced except in the mercury-atom-sensitized decomposition that is supposed to yield triplet sulfur... 

---