Oxidation, gas-phase

McKee and Mimeault [8] to be produced only in the initial stages of oxidation, with the surface becoming smoother and much thinner as concentric layers of carbon were removed throughout the oxidation process. The presence of Pb or Cu salts deposited on the fiber surface prior to oxidation initiated a catalytic etching, which needed careful control in order to avoid rapid degradation. 

An increase in specific surface area and ILSS was obtained by Druin et al [9] by using an inert atmosphere containing a small amount of O2. 

Source Reprinted with permission from Vukov AJ, J Serb Chem Soc, 55, 333, 1990. Copyright 1990, Serbian Chemicai Society. 

Steam gave an initial loss of up to about 15% and then remained unchanged. The authors believe that steam results in fiber bum-off producing microporosity and a reduction in fiber diameter, whilst CO2 produces microporosity without a reduction in fiber diameter. However, with CO2, internal pitting takes place with a significant reducing effect on fiber strength. 

On the other hand, UHV measurements showed that the Ru surface can be used as a kind of storage, able to accommodate large amounts of atomic oxygen. Other transition metals also exhibit this ability, but the exceptional property of Ru surfaces is due to the fact that oxygen can be completely removed by simply heating the sample up to about 1700 K without irreversibly incorporating oxide in the bulk. The oxidation of the Ru(OOOl) surface in UHV at low O2 pressure facilitates the formation of (2 X 2)-0 and (2 x l)-0 superstructures at coverages of 0.25 and 

respectively. Both superstructures have minimal catalytic 

RuO2(110) single-crystal surface is depicted in Fig. 8. LEED and STM data for UHV oxidation of Ru(OOOl) suggest that RUO2 overlayer is not pseudomorphic with the Ru substrate. Between the RUO2 domains, the Ru surface is covered with a monolayer of RuOH, which is the precursor to Ru oxidation. 

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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]. 

Isobutjiene [115-11-7] or tert-huty alcohol can be converted to methacrylic acid in a two-stage, gas-phase oxidation process via methacrolein as an intermediate. The alcohol and isobutjiene may be used interchangeably in the processes since tert-huty alcohol [75-65-0] readily dehydrates to yield isobutjiene under the reaction conditions in the initial oxidation. Variations of this process have been commercialized by Mitsubishi Rayon and by a joint venture of Sumitomo and Nippon Shokubai. Nippon Kayaku, Mitsui Toatsu, and others have also been active in isobutjiene oxidation research. 

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). 

Direct Oxidation of Propylene to Propylene Oxide. Comparison of ethylene (qv) and propylene gas-phase oxidation on supported silver and silver—gold catalysts shows propylene oxide formation to be 17 times slower than ethylene oxide (qv) formation and the CO2 formation in the propylene system to be six times faster, accounting for the lower selectivity to propylene oxide than for ethylene oxide. Increasing gold content in the catalyst results in increasing acrolein selectivity (198). In propylene oxidation a polymer forms on the catalyst surface that is oxidized to CO2 (199—201). Studies of propylene oxide oxidation to CO2 on a silver catalyst showed a rate oscillation, presumably owing to polymerization on the catalyst surface upon subsequent oxidation (202). 

Gas-phase oxidation of propylene using oxygen in the presence of a molten nitrate salt such as sodium nitrate, potassium nitrate, or lithium nitrate and a co-catalyst such as sodium hydroxide results in propylene oxide selectivities greater than 50%. The principal by-products are acetaldehyde, carbon monoxide, carbon dioxide, and acrolein (206—207). This same catalyst system oxidizes propane to propylene oxide and a host of other by-products (208). 

Propylene oxide is also produced in Hquid-phase homogeneous oxidation reactions using various molybdenum-containing catalysts (209,210), cuprous oxide (211), rhenium compounds (212), or an organomonovalent gold(I) complex (213). Whereas gas-phase oxidation of propylene on silver catalysts results primarily in propylene oxide, water, and carbon dioxide as products, the Hquid-phase oxidation of propylene results in an array of oxidation products, such as propylene oxide, acrolein, propylene glycol, acetone, acetaldehyde, and others. 

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). 

Gas-phase oxidation of thiols has been discussed in some depth (33). This review mainly emphasi2es atmospheric processes, but a section on nitrogen oxides and thiols appears to be broadly appHcable. The atmospheric oxidation chemistry of thiols is quite different from that of alcohols. 

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). 

After several years and millions of dollars were spent to develop a homogenous gas-phase oxidation process to produce propylene oxide, the development was terminated. This was a reaction classified as a... 

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). 

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] ... 

O.A. Mar ina, V.A. Sobyanin, V.D. Belyaev, and V.N. Parmon, The effect of electrochemical oxygen pumping on catalytic properties of Ag and Au electrodes at gas-phase oxidation ofCH4, Catalysis Today 13, 567-570 (1992). 

Let s write the rate laws for the steps in a mechanism proposed for the gas-phase oxidation of NO to N02. Its overall rate law has been determined experimentally ... 

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. 

Tsukahara, H., Ishida, T., and Mitsufumi, M., Gas-phase oxidation of nitric oxide chemical kinetics and rate constant, Nitric Oxide, 3, 191-198 (1999). 

Example 4.5 Suppose the recycle reactor in Figure 4.2 is used to evaluate a catalyst for the manufacture of sulfuric acid. The catal5Tic step is the gas-phase oxidation of sulfur dioxide ... 

In this chapter, we focus on the methods to deposit gold NPs on a number of materials and on gas-phase oxidation of methanol, its decomposed derivatives, and pollutants in ambient air at room temperature. 

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. 

Brubaker WW, RA Hites (1998) Gas-phase oxidation products of biphenyl and polychlorinated biphenyls. Environ Sci Technol 32 3913-3918. 

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]. 

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],... 

It is well known also that higher alkanes suffer radical gas phase oxidation above 723 K. Therefore, their use requires catalysts active and selective for deNOx at lower temperatures. The mechanism of NOx elimination is still debated a redox mechanism involving Cu ions is probable, and isolated Cu cations exchanged into MFI [4,5] or mordenite [6] have been found to be more active than CuO clusters. It must be emphasized, however, that acid zeolites exhibit good activity at high temperature, and acid mechanisms have been proposed [7-10]. In presence of Cu this acid mechanism disappears probably due to the decrease of the acidity of mordenite upon Cu exchange [6]. According to... 

The catalytic conversion of NO was investigated Grst in absence of catalyst (blank). The results reported in Fig. 1 show that the homogeneous gas phase oxidation of the alkane starts at 650 K. No reduction of NO is observed in the homogeneous process, then the production of N2 can be ascribed to the catalytic reduction. The catalytic properties were determined by temperature programmed reaction (ramp 2 K min-l). The temperature was increased from 523 to 673 K and back. 

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