Indirect oxidation processes

The equivalent to allylic oxidation of alkenes, but with allylic transposition of the carbon-carbon double bond, can be carried out by an indirect oxidative process involving addition of an electrophilic arylselenenyl reagent, followed by oxidative elimination of selenium. In one procedure, addition of an arylselenenyl halide is followed by solvolysis and oxidative elimination. 

In contrast to the direct process, an indirect process is one in which a foreign molecule or ion serves to shuttle electrons between the electrode and the substrate molecules. An indirect oxidation process may also involve the transfer of a hydrogen atom from a suitable substrate to a radical generated electrochemically. Indirect processes are typically observed for saturated aliphatic hydrocarbons and substrates that are more difficult to oxidize than the solvent-supporting electrolyte system. 

Aliphatic gem-dinitro compounds can be prepared by anodic oxidation of a nitro-paraffin in aqueous alkaline solution in the presence of an excess of N02 [201]. Yields are considerably better when Ag" is the oxidizing agent [202], so that an indirect oxidation process involving the generation of Ag" " at an Ag anode was found to give excellent results [203]. 

To overcome the drawbacks of the direct conversion of n-butenes. Bayer has developed an indirect oxidation process that helps to improve the overall selectivity of the operation, and whichconsists in passing through theantermediate formation of secondary butyl acetate. Acetic arid is manufactured in two steps according to the following reaction mechanism ... 

Figure 2.5 shows two pathways of ozonation of water competing for oxidation, direct oxidation is slower than hydroxy radical mechanism but concentration of aqueous ozone is higher as compared to hydroxy radical where ozone concentration is less in indirect oxidation process (EPA 1999). 

Synthesis. The total aimual production of PO in the United States in 1993 was 1.77 biUion kg (57) and is expected to climb to 1.95 biUion kg with the addition of the Texaco plant (Table 1). There are two principal processes for producing PO, the chlorohydrin process favored by The Dow Chemical Company and indirect oxidation used by Arco and soon Texaco. Molybdenum catalysts are used commercially in indirect oxidation (58—61). Capacity data for PO production are shown in Table 1 (see Propylene oxide). 

Indirect (French) Process. Ziac metal vapor for burning is produced ia several ways, one of which iavolves horizontal retorts. Siace all the vapor is burned ia a combustion chamber, the purity of the oxide depends on that of the ziac feed. Oxide of the highest purity requires special high grade ziac and less-pure products are made by blending ia Prime Western and even scrap ziac. 

Where small quantities of high-purity steam is required for electronic chip, pharmaceutical, sterilization, food preparation, and similar process applications, a small risk of steam contamination may exist. This may be caused directly by the use of amine treatments or indirectly through process contaminants or the transport of iron oxides. Consequently, alternative arrangements for steam generation are made. 

An alternative to the direct anodic oxidation of organic contaminants are the methods of indirect oxidation with the aid of oxidizers formed electrochemically in situ. These oxidizers (or mediators) can be obtained in both anodic and cathodic processes. Anodic agents are the salts of hypochloric acid (hypochlorites), the permanganates, the persulfates, and even ozone. 

The results have been compared with the earlier proposal of a dual-pathway mechanism for Cl oxidation, and, together with previous experimental and theoretical results, summarized in a comprehensive reaction scheme that explicitly includes also the (reversible) exchange between adsorbed species, dissolved product species in the catalyst layer, and similar species in the bulk electrolyte. The traditional dualpathway mechanism, where both the direct and indirect pathways lead to CO2 formation, has beenextended by adding a third pathway that accounts for formation and desorption of incomplete oxidation products. In the mechanistic discussion, we have focused on the role in and contribution to the Ci oxidation process of the formation/desorption and re-adsorption plus further oxidation of incomplete oxidation products. This not only leads to faradaic currents exceeding that for CO2 formation, but may result in additional COad and CO2 formation, via adsorption and oxidation of the incomplete oxidation products. 

The Ce(III) thus obtained will next be oxidized at its decomposition potential (nearly 1.4 V) this overall process of direct and indirect oxidation continues until Fe(II) has been completely converted into Fe(III) at this stage no Ce(III) remains, but only the amount of Ce(IV) previously added in order to stop the anodic oxidation in time the potential of the Pt-WE (vs. SCE-RE) should automatically interrupt the electrolysis beyond say 1.5 V, i.e., well below the decomposition potential of H20. 

Fig. 9. Indirect electrosynthetic process for the oxidation of anthracene to anthraquinone [10]. The charcoal column serves to remove organics which lead to corrosion and poisoning of the lead dioxide (Pb02) anode...

Fig. 13. Continuous indirect oxidation of toluene and toluene derivatives using a cascade process. Pilotstate [140]...

Indirect sulfonating reagents, 23 522-523t Indirect vacuum gauges, 20 657 Indirect (French) zinc oxide process, 26 612-613 Indisan, 24 498... 

Some of these processes have been developed for technical conversions and have been summarized in Ref. [228, 229]. The anodic technical production of t-butylbenzaldehyde has been coupled with the cathodic reduction of phthahc anhydride to phthalide in a paired electrosynthesis in a capillary gap cell [230]. Indirect oxidations with Mn +/Mn + or as mediators... 

The conversion of aromatic compounds comprises coupling, nuclear and ben-zylic substitution, and in some cases, addition. Homo- and in a more limited scope, heterocoupling is achieved for unsubstituted and substituted aromatic compounds in direct or indirect anodic processes. Chemically, there is a limited variety of expensive oxidation reagents available, but a large scope of transition... 

This indirect oxidation route takes two steps. In the first, a hydrocarbon, such as iso butane or ethylbenzene, is oxidized. The source of the oxygen is air. The reaction takes place just by mixing the ingredients and heating them to 250-300°F at 50 psi, producing a hydroperoxide. In the second step, the oxidized hydrocarbon reacts with propylene in a liquid phase and in the presence of a metal catalyst at 175-225°F and 550 psi to produce PO yields of better than 90%. The process flow is shown in Figure 11—3. 

There is another route to propylene oxide and to styrene not shown on the figure. Indirect oxidation of isobucane or of ethylbenzene just doesn t fit neatly on the chart, but both are commercial processes, the former yielding PO and TBA, the latter giving PO and styrene. 

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