Oxidation continuous selective

Active crystal face of vanadyl pyrophosphate for selective n-butane oxidation—Continued selectivity, 162,164r selectivity vs. face, 165,166/... 

Catalytic Oxidation for Straight-Chain Paraffinic Hydrocarbons. Synthetic fatty acids (SFA) are produced by Eastern European countries, Russia, and China using a manganese-catalyzed oxidation of selected paraffinic streams. The technology is based on German developments that were in use during World War II. The production volume in 1984 was estimated to be about 5.5 x ICf t/yr. The oxidation is highly exothermic and is carried out at about 105—125°C, mostly in continuous equipment. 

Various methods have been used for the reoxidation of vat leuco dyeings atmospheric skying, hypochlorite, chlorite and acidified dichromate are now rarely employed. Atmospheric oxidation can be difficult to control and thus uneven with some dyes it is also too slow, particularly for continuous methods. Sodium hypochlorite is used only for those few black dyes that tend to become dark green when oxidised with peroxide obviously hypochlorite should be avoided with the many chlorine-sensitive dyes. Similarly sodium chlorite, acidified to below pH 5 with acetic acid, can only be used with certain dyes, although with these it certainly gives rapid oxidation. Dye selectivity is also a drawback with... 

The investigation of the mechanism of olefin oxidation over oxide catalysts has paralleled catalyst development work, but with somewhat less success. Despite extensive efforts in this area which have been recently reviewed by several authors (9-13), there continues to be a good deal of uncertainty concerning the structure of the reactive intermediates, the nature of the active sites, and the relationship of catalyst structure with catalytic activity and selectivity. Some of this uncertainty is due to the fact that comparisons between various studies are frequently difficult to make because of the use of ill-defined catalysts or different catalytic systems, different reaction conditions, or different reactor designs. Thus, rather than reviewing the broader area of selective oxidation of hydrocarbons, this review will attempt to focus on a single aspect of selective hydrocarbon oxidation, the selective oxidation of propylene to acrolein, with the following questions in mind ... 

The discussion that follows first describes techniques for the airborne collection of a definable atmospheric phase for subsequent determinations, then continues with discussion of the analysis techniques themselves including those for nitrogen oxides and oxyacids, sulfur oxides, oxidants and selected organic species. Emphasis is on the modified instrumentation devised and used by Brookhaven National Laboratory staff to improve the selectivity and lower the limits of detection of these techniques (1). 

A continuous supply of electrons and protons to the catalyst makes the biological oxidation highly selective and efficient. The electrons are provided by nucleosides such as NADH and NADPH. The nucleosides donate a pair of electrons to co-factors such as flavins. The flavins are 2 electron/1 electron switches that transfer electrons one at a time to the Fe porphyrin catalyst which is enclosed in a protein matrix. 

Fig. 44. Etch rate of polysilicon, oxide, and photoresist as a function of hydrogen addition to a CF4 discharge. Etch rate stops on non-oxide materials due to polymer build up at high % H - Oxide continues to etch due to polymer removal by the available oxygen. This yields high selectivity of etching oxide over silicon. After [220].

Regarding their use as cracking and isomerization catalysts, bulk oxides such as clays and amorphous silica-aluminas have been widely displaced by molecular sieve compounds (e.g., zeolites, aluminophosphates), whose well-defined pore structures generally offer higher selectivity and flexibility. Nevertheless, bulk oxides continue to be used for various cracking and isomerization applications in the petroleum industry. 

The process described above in which a solute oxidizes preferentially to the parent element and forms a continuous layer on the surface is referred to as selective oxidation. The selective oxidation of elements which form a slowly growing, protective layer is the basis for the oxidation protection of all alloys and coatings used at high temperature. The only elements which consistently result in protective scales are Cr (chromia scale), Al (alumina scale), and Si (silica scale). Therefore, much research has been directed at finding alloy and coating compositions, which meet other property (e.g., mechanical) requirements and also form one of these scales. 

A more troublesome problem involves the operation of the Larox filter to produce the washed oxide. We selected the Larox filter because of its automation and the performance observed in a laboratory unit. The filter has proven reliable mechanically and does not require continuous operator attendance. However, the cake produced has a higher moisture content than originally expected. This high moisture is explained by the fine nature of the oxide. When we selected the filter, the secondary oxides expected as the feed to the plant were much coarser, filtered easily, and produced a cake that was dry enough to be readily handled. The Larox performance on the finer oxide needs to be improved. 

Fontijn, A., Sabadell, A.J., Ronco, R.J. Homogeneous chemiluminescent measurement of nitric oxide with ozone. Implications for continuous selective monitoring of gaseous air pollutants. Anal. Chem. 42, 575-579 (1970)... 

Organotin ethers continue to be employed in selective alkylations. In a study of the Bu2SnO-mediated alkylation of lactosides 1 and 2 aimed at obtaining the 3 -0-ethers, the penta-benzyl ether 2 afforded better yields than the corresponding benzoate 1, and siniilar conditions have been employed for the regioselective benzylation of octyl P-D-galactopyianoside at 0-3 Dibutyltin dimethoxide has been used in place of dibutyltin oxide for selective alkylation of carbohydrate diols. ... 

As with acid-base and complexation titrations, redox titrations are not frequently used in modern analytical laboratories. Nevertheless, several important applications continue to find favor in environmental, pharmaceutical, and industrial laboratories. In this section we review the general application of redox titrimetry. We begin, however, with a brief discussion of selecting and characterizing redox titrants, and methods for controlling the analyte s oxidation state. 

Single-reaction-step processes have been studied. However, higher selectivity is possible by optimizing catalyst composition and reaction conditions for each of these two steps (40,41). This more efficient utilization of raw material has led to two separate oxidation stages in all commercial faciUties. A two-step continuous process without isolation of the intermediate acrolein was first described by the Toyo Soda Company (42). A mixture of propylene, air, and steam is converted to acrolein in the first reactor. The effluent from the first reactor is then passed directiy to the second reactor where the acrolein is oxidized to acryUc acid. The products are absorbed in water to give about 30—60% aqueous acryUc acid in about 80—85% yield based on propylene. 

The first-stage catalysts for the oxidation to methacrolein are based on complex mixed metal oxides of molybdenum, bismuth, and iron, often with the addition of cobalt, nickel, antimony, tungsten, and an alkaU metal. Process optimization continues to be in the form of incremental improvements in catalyst yield and lifetime. Typically, a dilute stream, 5—10% of isobutylene tert-huty alcohol) in steam (10%) and air, is passed over the catalyst at 300—420°C. Conversion is often nearly quantitative, with selectivities to methacrolein ranging from 85% to better than 95% (114—118). Often there is accompanying selectivity to methacrylic acid of an additional 2—5%. A patent by Mitsui Toatsu Chemicals reports selectivity to methacrolein of better than 97% at conversions of 98.7% for a yield of methacrolein of nearly 96% (119). 

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. 

---