Gas-Phase Oxidation with

Although N2O is still an expensive oxidant, the chemistry concerning its application in selective oxidation catalysis has attracted much attention. Actually, many studies have shown that N2O can be used as an efficient oxidant for the selective oxidation of hydrocarbons such as benzene to phenol, propane to propene, and methane to methanol. 

Duma and Honicke were the first to report the successful use of N2O in propene epoxidation. A PO yield of 5% was obtained over silica-supported iron oxide catalysts promoted with Na ions [43bj. The pore shape and diameter of the support as well as iron oxide dispersion are crucial parameters in the reaction [43b,cj. Doping vdth alkali metal may also considerably affect the Fe dispersion, and favor epoxidation over allylic oxidation [43fj. Further modification by boron can also significantly enhance the catalytic performance of the K-doped FeO /SBA-lS catalyst [43gj. 

The reaction pattern includes the formation of PO, its consecutive isomerization to propanal, acetone and ally alcohol on acidic sites and combustion [43aj. Propanal and acrolein are also primary products. The formation of lower alkanes, alkenes, acetaldehyde and methanol results from cracking and oxidative C—C bond cleavage of propene and products. Additional side-reactions may occur in the gas phase, including radical-type oxidation of propene to acrolein, hexadiene and other byproducts. Alkyl dioxanes and alkyl dioxolanes may form via dimerization reactions of PO on acidic catalysts. Indeed, major by-products are heavy compounds that 

The best yield reported in the literature is 13.3%, with selectivity of about 60% obtained with silica-supported K-promoted iron oxide catalysts modified by amines [43c]. The same catalyst is inactive in propene oxidation with air. However, the use of ammonia/air mixtures leads to a considerably enhanced conversion with respect to air only, with 60% selectivity for the epoxide. This observation suggests a mechanism whereby ammonia is first oxidized to nitrous oxide, which subsequently produces the active oxygen species for epoxidation. 

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In the 1980 s zeolites attracted a renewed attention. They were shown to be rather promising catalysts if, instead of O2, a chemically pre-modified oxygen entering the oxygen-containing molecules is used. The most known example is an excellent catalytic performance of titanium silicalites in the liquid phase oxidations with H2O2 [5]. A gas phase oxidation with nitrous oxide is another approach in this field being intensively developed in the last years [2],... 

The second factor is the unique feature of N20 as an oxygen donor - first demonstrated with the gas-phase oxidation of lower alkanes over metal oxides. Later, the high catalytic efficiency of the FeZSM-5 zeolites was discovered, suggesting an opportunity for developing new processes of gas-phase oxidation with N20, especially the hydroxylation of benzene and other aromatics to the corresponding phenols. 

Before passing on to the analysis of gas-phase oxidation with hydrogen peroxide, we must first obtain information about its dissociation. Hydrogen peroxide easily dissociates to water and molecular oxygen, which is the typical feature, very useful in some cases and unwanted in another. H202 can dissociate in different ways, but all of them are described by the general material balance equation as follows ... 

Durene is predominantly oxidized to pyromellitic dianhydride this anhydride can also be produced by oxidation of the corresponding triisopropyltoluenes and diisopropylxylenes. The favored process is gas-phase oxidation with V2O5 as a catalyst, at temperatures from 400 to 600 ""C. 

World capacity for anthraquinone is currently around 30,000 tpa. Chromic acid oxidation processes, which account for around 40%, have the largest share. They are followed by gas-phase oxidation, with around 30%, and Friedel-Crafts acyla-... 

With higher alkenes, three kinds of products, namely alkenyl acetates, allylic acetates and dioxygenated products are obtained[142]. The reaction of propylene gives two propenyl acetates (119 and 120) and allyl acetate (121) by the nucleophilic substitution and allylic oxidation. The chemoselective formation of allyl acetate takes place by the gas-phase reaction with the supported Pd(II) and Cu(II) catalyst. Allyl acetate (121) is produced commercially by this method[143]. Methallyl acetate (122) and 2-methylene-1,3-diacetoxypropane (123) are obtained in good yields by the gas-phase oxidation of isobutylene with the supported Pd catalyst[144]. 

The reaction of methyl propionate and formaldehyde in the gas phase proceeds with reasonable selectivity to MMA and MAA (ca 90%), but with conversions of only 30%. A variety of catalysts such as V—Sb on siUca-alumina (109), P—Zr, Al, boron oxide (110), and supported Fe—P (111) have been used. Methjial (dimethoxymethane) or methanol itself may be used in place of formaldehyde and often result in improved yields. Methyl propionate may be prepared in excellent yield by the reaction of ethylene and carbon monoxide in methanol over a mthenium acetylacetonate catalyst or by utilizing a palladium—phosphine ligand catalyst (112,113). 

AHyl alcohol can be easily oxidized to yield acrolein [107-02-8] and acryhc acid [79-10-7]. In an aqueous potassium hydroxide solution of RuQ., aHyl alcohol is oxidized by a persulfate such as K2S20g at room temperature, yielding acryhc acid in 45% yield (29). There are also examples of gas-phase oxidation reactions of ahyl alcohol, such as that with Pd—Cu or Pd—Ag as the catalyst at 150—200°C, in which ahyl alcohol is converted by 80% and acrolein and acryhc acid are selectively produced in 83% yield (30). 

Noncatalytic oxidation of propylene to propylene oxide is also possible. Use of a small amount of aldehyde in the gas-phase oxidation of propylene at 200—350°C and up to 6900 kPa (1000 psi) results in about 44% selectivity to propylene oxide. About 10% conversion of propylene results (214—215). Photochemical oxidation of propylene with oxygen to propylene oxide has been demonstrated in the presence of a-diketone sensitizers and an aprotic solvent (216). 

A high purity titanium dioxide of poorly defined crystal form (ca 80% anatase, 20% mtile) is made commercially by flame hydrolysis of titanium tetrachloride. This product is used extensively for academic photocatalytic studies (70). The gas-phase oxidation of titanium tetrachloride, the basis of the chloride process for the production of titanium dioxide pigments, can be used for the production of high purity titanium dioxide, but, as with flame hydrolysis, the product is of poorly defined crystalline form unless special dopants are added to the principal reactants (71). 

Oxidation. The chlorine atom [22537-15-17-initiated, gas-phase oxidation of vinyl chloride yields 74% formyl chloride [2565-30-2] and 25% CO at high oxygen [7782-44-7], O2, to CI2 ratios it is unique among the chloro olefin oxidations because CO is a major initial product and because the reaction proceeds by a nonchain path at high O2/CI2 ratios. The rate of the gas-phase reaction of chlorine atoms with vinyl chloride has been measured (39). 

Industrially it is now made by direct gas-phase oxidation of HCN with O2 (over a silver catalyst), or with CI2 (over activated charcoal), or NO2 (over CaO glass). (CN)2 is fairly stable in H2O, EtOH and Et20 but slowly decomposes in solution to give HCN, HNCO, (H2N)2C0 and H2NC(0)C(0)NH2 (oxamide). Alkaline solutions yield CN and (OCN) (cf. halogens). 

Repeated attempts to obtain the band at 1030 cm 1 in spectra of the respective solids of various compositions did not furnish the desired result. Nevertheless, the band was observed in IR transmission spectra of gaseous components that separated from molten K2NbF7 and were collected in a standard gas phase cell with Csl windows appropriate for IR measurements. Fig. 85 presents the structure of the band and exact wave numbers of its components. Storage of the gas in the cell for several days resulted in a yellow deposit on the windows due to oxidation and subsequent separation of iodine. Analysis of available reported data [364 - 367] enables to assign the band observed at -1030 cm 1 to vibrations of OF radicals. It should be emphasized that a single mode was observed for OF in the argon matrix while in the case of nitrogen, two modes were indicated [367]. 

Ethylene oxide is produced by a heterogeneous catalyzed gas phase oxidation of ethylene with pure oxygen at temperatures of 240-290°C and 5-25 bar [63] ... 

R23 is the only significant removal process for N02 and serves as well as a radical sink reaction for HO. Sulfur dioxide (with higher water solubility than NO2.) is also oxidized to sulfuric acid in aerosols and fog droplets (71,72,73,74) its gas-phase oxidation via R24 does not constitute a radical sink, since H02 is regenerated. 

Figure 4-13 shows an example from a three-dimensional model simulation of the global atmospheric sulfur balance (Feichter et al, 1996). The model had a grid resolution of about 500 km in the horizontal and on average 1 km in the vertical. The chemical scheme of the model included emissions of dimethyl sulfide (DMS) from the oceans and SO2 from industrial processes and volcanoes. Atmospheric DMS is oxidized by the hydroxyl radical to form SO2, which, in turn, is further oxidized to sulfuric acid and sulfates by reaction with either hydroxyl radical in the gas phase or with hydrogen peroxide or ozone in cloud droplets. Both SO2 and aerosol sulfate are removed from the atmosphere by dry and wet deposition processes. The reasonable agreement between the simulated and observed wet deposition of sulfate indicates that the most important processes affecting the atmospheric sulfur balance have been adequately treated in the model. 

After gas-phase oxidation reaction finished, the reactor wall surfece was coated with a thick rough scale layer. The thickness of scale layer along axial direction was varied. The scale layer at front reactor was much thicker than that at rear. The SEM pictures were shown in Fig. 1 were scale layers stripped from the reactor wall surface. Fig. 1(a) was a cross sectional profile of scale layer collected from major scaling zone. Seen from right side of scale layer, particles-packed was loose and this side was attached to the wall surface. Its positive face was shown in Fig. 1(b). Seen from left side of scale layer, compact particles-sintered was tight and this side was faced to the reacting gases. Its local amplified top face was shown in Fig. 1(c). The XRD patterns were shown in Fig. 2(a) were the two sides of scale layer. Almost entire particles on sintered layer were characterized to be rutile phase. While, the particle packed layer was anatase phase. 

The chemistry of vinyl acetate synthesis from the gas-phase oxidative coupling of acetic acid with ethylene has been shown to be facilitated by many co-catalysts. Since the inception of the ethylene-based homogeneous liquid-phase process by Moiseev et al. (1960), the active c ytic species in both the liquid and gas-phase process has always been seen to be some form of palladium acetate [Nakamura et al, 1971 Augustine and Blitz, 1993]. Many co-catalysts which help to enhance the productivity or selectivity of the catalyst have appeared in the literature over the years. The most notable promoters being gold (Au) [Sennewald et al., 1971 Bissot, 1977], cadmium acetate (Cd(OAc)j) [Hoechst, 1967], and potassium acetate (KOAc) [Sennewald et al., 1971 Bissot, 1977]. 

SCHEME 2.16 Additional reaction pathway for the generation of the quinone methide in the gas phase oxidation of 2-methylphenyl radical, investigated by the hybrid functional MPW1K (reproduced from Ref. [23] with permission from American Chemical Society). 

Immediate ignition in the gas phase occurs with ammonia, dinitrogen oxide or hydrogen sulfide. 

Oxidation rate constant k = 4.58 x 10-12 cm3 molecule-1 s-1 for the gas phase reaction with OH radical at room temp. (Ohta Ohyama 1985 Atkinson 1989) ... 

The measurement of oxygen diffusion is usually made by the use of O18 as the labelling isotope. If a gas containing an initial concentration C, of O18 in O16, and Co is the initial concentration of O18 in a right cylinder oxide sample of thickness 21, and a is the ratio of oxygen atoms in the original gas phase compared with that in the solid, then after a time t, when the O18 concentration in the gas phase is C /... 

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