Oxidation, selective

Selective oxidation of hydrocarbons is of key importance in functionalization of hydrocarbon molecules. It is always a multi-step process with consecutive abstraction of hydrogen and addition of oxygen atoms. The difficulty of this reaction is, undoubtedly, that the process should go through these many steps, but also should stop at the desired product. Such requirements can be met by complex mixed-metal oxides, and the XPS characterization of two selected examples is briefly reviewed here. 

Selective oxidation is used in the activation of raw materials to provide useful products and chemical intermediates. Catalytic methods for the oxidation of organic molecules of this type are of growing interest with regard to eco-sustainable chemical processes [18,356]. Heterogeneous catalysts may be used under mild conditions with gaseous oxygen or air in aqueous dispersion to promote these transformations effectively. 

Rossi and co-workers have studied the oxidation of organic substrates in the liquid phase, and shown that gold on carbon is the preferred catalyst [27,357-364]. One of the best preparation methods is the immobilisation of colloidal particles [27,140, 357-360, 362-364], which can readily be prepared 

The catalytic oxidation of vicinal diols in water is a valuable target, for example, in the synthesis of glycolic acid and lactic acid via the 

Current chemical methods involve the use of toxic or corrosive reagents as well as high-pressure equipment. An alternative fermentation process, available for lactic acid production, is affected by low productivity and severe purification problems. 

Rossi and co-workers have also found that phenylethane-1,2-dial, a cheap starting material, can be oxidised under mild conditions using 1% An on carbon powder, prepared from a colloidal sol, where the average gold particle diameter was 7 nm. Gold on carbon thus represents a promising alternative to present methods for the synthesis of mandelic acid, an important pharmaceutical intermediate [356,363]. 

Recently, a silica-modified magnetite particle-supported bifunctional hybrid material (IL/SMNP) combining TEMPO (2,2,6,6-tetramethylpiperidine-l-oxyl) based IL and polyoxometalate moieties was developed for aerobic oxidation of alcohol [82]. The involvement of the TEMPO moiety improves the catalytic activity dramatically. Greater than 70% yields compared to the corresponding aldehydes were obtained even using primary ahphatic alcohols such as 1-octanol as starting material. 

some trivalent rare earths (Ce, Pr, Tb) may also become tetravalent, and some others (Sm, Eu, Yb) may become divalent. This creates the possibility to use selective oxidation or reduction of these elements to separate them, because the divalent and tetravalent state the elements show marked differences in behavior from the trivalent state (Gupta and Krishnamurthy 2005). 

Cerium can be removed from the rare earths mixture after oxidation of naturally occurring Ce(lll) to Ce(lV). This valence change occurs, for instance, when bast-naesite is heated in air at 650 °C, or when the rare earth hydroxides are dried in air at temperatures of 120-130°. In aqueous hydroxide suspensions, oxidation can be achieved by chlorination or electrolysis. Also application of ozone is possible for cerium removal. The ozone causes an oxidation precipitation process. 

Once oxidized, the Ce(lV) can be removed from the trivalent rare earths in the hydroxide-oxide mixture in two ways  

4 Mineral Processing and Extractive Metallnrgy of the Rare Earths 

Subsequently, selective precipitation is carried out (Gupta and Krishnamurthy 2005). 

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Pu (86 years) is formed from Np. Pu is separated by selective oxidation and solvent extraction. The metal is formed by reduction of PuF with calcium there are six crystal forms. Pu is used in nuclear weapons and reactors Pu is used as a nuclear power source (e.g. in space exploration). The ionizing radiation of plutonium can be a health hazard if the material is inhaled. 

A topic of current interest is that of methane activation to give ethane or selected oxidation products such as methanol or formaldehyde. Oxide catalysts are used, and there may be mechanistic connections with the Fischer-Tropsch system (see Ref. 285). 

Selective oxidation of a,p-unsatutrated (allylic, benzylic, acetylenic) alcohols. 

Selective oxidation of secondary alcohols to ketones is usually performed with CrOj/HjSO, I I in acetone (Jones reagent) or with CrOjPyj (Collin s reagent) in the presence of acid-sensitive groups (H.G. Bosche, 1975 C. Djerassi, 1956 W.S. Allen, 1954). As mentioned above, a,)S-unsaturated secondary alcohols are selectively oxidized by MnOj (D.G. Lee, 1969 D. Arndt, 1975) or by DDQ (D. Walker, 1967 H.H. Stechl, 1975). 

Alcohols are oxidized slowly with PdCh. Oxidation of secondary alcohols to ketones is carried out with a catalytic amount of PdCh under an oxygen atmo-sphere[73.74]. Also, selective oxidation of the allylic alcohol 571 without attacking saturated alcohols is possible with a stoichiometric amount of PdfOAc) in aqueous DMF (1% H OifSll],... 

The alkyl derivatives of thiazoles can be catalytically oxidized in the vapor phase at 250 to 400°C to afford the corresponding formyl derivatives (21). Molybdenum oxide, V2O5, and tin vanadate are used as catalysts either alone or with a support. The resulting carbonyl compounds can be selectively oxidized to the acids. 

D. J. HucknaU, Selective Oxidation of Hydrocarbons, Academic Press, Inc., London, 1974. 

Sta.rting from Phenol. Phenol can be selectively oxidized into -benzoquinone with oxygen. The reaction is catalyzed by cuprous chloride. At low catalyst concentration, the principal drawback of this method is the high pressure of oxygen that is required, leading to difficult safety procedures. It appears that a high concentration of the catalyst (50% of Cu(I)—phenol) allows the reaction to proceed at atmospheric pressure (58). 

A second type of siUcon nitride, called stoichiometric siUcon nitride, is deposited at much higher temperatures using CVD or LPCVD in the form of Si N. Stoichiometric siUcon nitride can be used as a mask for the selective oxidation of siUcon. Here the siUcon nitride is patterned over a siUcon substrate, and the exposed siUcon is oxidized. The siUcon nitride oxidizes very slowly compared to the siUcon. 

Selective Oxidation. Cerium, the most abundant lanthanide, can be separated easily after oxidation of Ce(III) to Ce(IV), simplifying the subsequent separation of the less abundant lanthanides. Oxidation occurs when bastnaesite is heated in air at 650°C or when the hydroxides are dried in air... 

Sla.g ReHning. Unwanted constituents can be removed by transfer into a slag phase. Slag refining is also used for operations in which the Hquid metal is maintained in contact with a slag or a molten salt. This second immiscible Hquid is usually more oxidizing than the metallic phase and selective oxidation of the impurities renders them soluble in the slag or molten salt. Impurities that are less easily oxidized remain in the Hquid metal. 

Cyanide Wastes. Ozone is employed as a selective oxidant in laboratory-scale synthesis (7) and in commercial-scale production of specialty organic chemicals and intermediates such as fragrances, perfumes (qv), flavors, antibiotics (qv), hormones (qv), and vitamins (qv). In Japan, several metric tons per day (t/d) of piperonal [120-57-0] (3,4-methylenedioxybenzaldehyde) is manufactured in 87% yield via ozonolysis and reduction of isosafrole [93-16-3], Piperonal (or heHotropine [120-57-0]) has a pleasant odor and is used in perfumery. Oleic acid [112-80-1/, CH3(CH2 )7CH—CH(CH2 ). C02H, from tall oil (qv) is ozonated on a t/d scale to produce pelargonic, GgH2yG02H, and azelaic, H02G(GH2)yG02H, acids. Oleic acid also is ozonated in Japan... 

To date (ca 1996) many potentially usefiil sucrose derivatives have been synthesized. However, the economics and complexities of sucrochemical syntheses and the avadabiLity of cheaper substitutes have limited their acceptance hence, only a few of them are in commercial use. A change in the price and availability of petroleum feedstocks could reverse this trend. Additional impetus may come from regioselective, site-specific modifications of sucrose to produce derivatives to facilitate and improve the economics of sucrochemical syntheses. For example, the microbe yigwbacterium tumifaciens selectively oxidizes sucrose to a three-keto derivative, a precursor of alkylated sucroses for detergent use (50). Similarly, enzymes have been used for selective synthesis of specific sucrose derivatives (21). 

A derivative of the Claus process is the Recycle Selectox process, developed by Parsons and Unocal and Hcensed through UOP. Once-Thm Selectox is suitable for very lean acid gas streams (1—5 mol % hydrogen sulfide), which cannot be effectively processed in a Claus unit. As shown in Figure 9, the process is similar to a standard Claus plant, except that the thermal combustor and waste heat boiler have been replaced with a catalytic reactor. The Selectox catalyst promotes the selective oxidation of hydrogen sulfide to sulfur dioxide, ie, hydrocarbons in the feed are not oxidized. These plants typically employ two Claus catalytic stages downstream of the Selectox reactor, to achieve an overall sulfur recovery of 90—95%. 

Titanium Silicates. A number of titanium siUcate minerals are known (160) examples are Hsted in Table 19. In most cases, it is convenient to classify these on the basis of the connectivity of the SiO building blocks, eg, isolated tetrahedra, chains, and rings, that are typical of siUcates in general. In some cases, the SiO units may be replaced, even if only to a limited extent by TiO. For example, up to 6% of the SiO in the garnet schorlomite can be replaced by TiO. In general, replacement of SiO by TiO bull ding blocks increases the refractive indices of these minerals. Ti has also replaced Si in the framework of various zeofltes. In addition, the catalytic activity of both titanium-substituted ZSM-5 (TS-1) and ZSM-11 (TS-2) has received attention (161), eg, the selective oxidation of phenol, with hydrogen peroxide, to hydroquinone and catechol over TS-1 has been operated at the 10,000 t/yr scale in Italy (162). 

In a first step, JS ocardia asteroides selectively oxidizes only (3)-pantolactone to ketopantolactone (19), whereas the (R)-pantolactone remains unaffected (47). The accumulated ketopantolactone is stereospecificaHy reduced to (R)-pantolactone in a second step with Candidaparapsilosis (product concentration 72 g/L, 90% molar yield and 100% ee) (48). Racemic pantolactone can also be converted to (R)-pantolactone by one single microbe, ie, Jiodococcus erythropolis by enantioselective oxidation to (3)-pantolactone and subsequent stereospecific reduction in 90% yield and 94% ee (product concentration 18 g/L) (40). 

In order to circumvent this problem, there has been significant activity directed toward the search for a less environmentally toxic and more selective oxidizing agent than chromium. For example, Hoechst has patented a process which uses organorhenium compounds. At a 75% conversion, a mixture of 86% of 2-methyl-l,4-naphthoquinone and 14% 6-methyl-l,4-naphthoquinone was obtained (60). Ceric sulfate (61) and electrochemistry (62,63) have also been used. 

Bromide ndIodide. The spectrophotometric determination of trace bromide concentration is based on the bromide catalysis of iodine oxidation to iodate by permanganate in acidic solution. Iodide can also be measured spectrophotometricaHy by selective oxidation to iodine by potassium peroxymonosulfate (KHSO ). The iodine reacts with colorless leucocrystal violet to produce the highly colored leucocrystal violet dye. Greater than 200 mg/L of chloride interferes with the color development. Trace concentrations of iodide are determined by its abiUty to cataly2e ceric ion reduction by arsenous acid. The reduction reaction is stopped at a specific time by the addition of ferrous ammonium sulfate. The ferrous ion is oxidi2ed to ferric ion, which then reacts with thiocyanate to produce a deep red complex. 

Displacement of the sulfinyl group of penems (85), obtained by regio selective oxidation of (74, R = R" = CH2CHCH2) (Fig. 5)... 

Oxidation. Benzene can be oxidized to a number of different products. Strong oxidizing agents such as permanganate or dichromate oxidize benzene to carbon dioxide and water under rigorous conditions. Benzene can be selectively oxidized in the vapor phase to maleic anhydride. The reaction occurs in the presence of air with a promoted vanadium pentoxide catalyst (11). Prior to 1986, this process provided most of the world s maleic anhydride [108-31 -6] C4H2O2. Currendy maleic anhydride is manufactured from the air oxidation of / -butane also employing a vanadium pentoxide catalyst. 

The washed slime is dried and melted to produce slag and metal. The slag is usually purified by selective reduction and smelted to produce antimonial lead. The metal is treated ia the molten state by selective oxidation for the removal of arsenic, antimony, and some of the lead. It is then transferred to a cupel furnace, where the oxidation is continued until only the silver—gold alloy (dorn) remains. The bismuth-rich cupel slags are cmshed, mixed with a small amount of sulfur, and reduced with carbon to a copper matte and impure bismuth metal the latter is transferred to the bismuth refining plant. 

Intraparticle mass transport resistance can lead to disguises in selectivity. If a series reaction A — B — C takes place in a porous catalyst particle with a small effectiveness factor, the observed conversion to the intermediate B is less than what would be observed in the absence of a significant mass transport influence. This happens because as the resistance to transport of B in the pores increases, B is more likely to be converted to C rather than to be transported from the catalyst interior to the external surface. This result has important consequences in processes such as selective oxidations, in which the desired product is an intermediate and not the total oxidation product CO2. 

The selective oxidation is catalyzed by silver, which is the only good catalyst. Other olefins are not converted selectively to the epoxides in the presence of silver. However, propylene epoxidation is appHed commercially the catalysts are either molybdenum complexes in solution or soHd Ti02—Si02 (see... 

The primary determinant of catalyst surface area is the support surface area, except in the case of certain catalysts where extremely fine dispersions of active material are obtained. As a rule, catalysts intended for catalytic conversions utilizing hydrogen, eg, hydrogenation, hydrodesulfurization, and hydrodenitrogenation, can utilize high surface area supports, whereas those intended for selective oxidation, eg, olefin epoxidation, require low surface area supports to avoid troublesome side reactions. 

Various substituted N-hiomo- and A/-chloroureas have also been prepared (97). These compounds are useful for synthesis of oxazoUdinones, and also hydrazine, hydrazo, and azo compounds. A/-Bromourea [51918-81 -1] is useful for selective oxidation of sugar derivatives (98). 

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