Oxidation Catalysts 1 Nitric Acid

Cyclohexane. The LPO of cyclohexane [110-82-7] suppUes much of the raw materials needed for nylon-6 and nylon-6,6 production. Cyclohexanol (A) and cyclohexanone (K) maybe produced selectively by using alow conversion process with multiple stages (228—232). The reasons for low conversion and multiple stages (an approach to plug-flow operation) are apparent from Eigure 2. Several catalysts have been reported. The selectivity to A as well as the overall process efficiency can be improved by using boric acid (2,232,233). K/A mixtures are usually oxidized by nitric acid in a second step to adipic acid (233) (see Cyclohexanol and cyclohexanone). 

Monosaccharides such as glucose and fmctose are the most suitable as starting materials. When starch is used, it is first hydrolyzed with oxahc acid or sulfuric acid into a monosaccharide, mainly glucose. It is then oxidized with nitric acid in an approximately 50% sulfuric acid solution at 63—85°C in the presence of a mixed catalyst of vanadium pentoxide and iron(III) sulfate. 

In the Dupont process, cyclohexane is reacted with air at 150 °C and 10 atm pressure in the presence of a soluble cobalt(II) salt (naphthenate or stearate). The conversion is limited to 8-10% in order to prevent consecutive oxidation of the ol-one mixture. Nonconverted cyclohexane is recycled to the oxidation reactor. Combined yields of ol-one mixture are 70-80%.83,84,555 The ol-one mixture is sent to another oxidation reactor where oxidation by nitric acid is performed at 70-80 °C by nitric acid (45-50%) in the presence of a mixture of Cu(N03)2 and NH4V03 catalysts, which increase the selectivity of the reaction. The reaction is complete in a few minutes and adipic acid precipitates from the reaction medium. The adipic acid yield is about 90%. Nitric acid oxidation produces gaseous products, mainly nitric oxides, which are recycled to a nitric acid synthesis unit. Some nitric acid is lost to products such as N2 and N20 which are not recovered. 

Nafion-silica nanocomposites (5%, 13%, 20%, 40%, and 80% loading) have been applied in the synthesis of a-tocopherol (160) in various solvents695 696 [Eq. (5.251)]. Under optimized conditions, Nafion SAC-40 proved to be the best catalyst (91% yield of a-tocopherol using 0.6 wt% of catalyst). Catalyst recycling was possible after reactivation with oxidizing agents (nitric acid or hydrogen peroxide). 

Peracetic acid can also be formed directly by liquid-phase oxidation at 5 to 50°C with a cobalt salt catalyst. Nitric acid oxidation of acetaldehyde yields glyoxal and the oxidation of p-xylene to terephthalic acid and of ethanol to acetic acid is activated by acetaldehyde. 

Sadykov VA, Isupova LA, Zolotarskii IA, Bobrova LN, Noskov AS, Parmon VN, Brushtein EA, Telyatnikova TV, Chernyshev VI, Lunin W. Oxide catalysts for ammonia oxidation in nitric acid production properties and perspectives. Applied Catalysis A General. 2000 204(l) 59-87. 

For many catalysts, the major component is the active material. Examples of such unsupported catalysts are the aluminosilicates and zeolites used for cracking petroleum fractions. One of the most widely used unsupported metal catalysts is the precious metal gauze as used, for example, in the oxidation of ammonia to nitric oxide in nitric acid plants. A very fast rate is needed to obtain the necessary selectivity to nitric oxide, so a low metal surface area and a short contact time are used. These gauze s are woven from fine wires (0.075 mm in diameter) of platinum alloy, usually platinum-rhodium. Several layers of these gauze s, which may be up to 3 m in diameter, are used. The methanol oxidation to formaldehyde is another process in which an unsupported metal catalyst is used, but here metallic silver is used in the form of a bed of granules. 

Homogeneous deposition precipitation (HDP) is explored for the preparation of carbon nanofiber supported ruthenium catalysts. First, carbon nanofibers (CNF, 177 m /g) are oxidized using nitric acid thus activating the graphitic carbon surfiice. Second, ruthenium (hydr)oxide is deposited homogeneously onto the CNF by hydrolysis of urea at 363K. 

By treating the activated carbon catalyst with nitric acid, both dehydration and dehydrogenation were enhanced however, dehydration was found to occur only on the outer surface, whereas dehydrogenation could also take place within the pores. It was found that there is an optimum concentration of acid groups above which the activity decreases [80]. In a more recent study with carbon catalysts oxidized with nitric acid and subsequently heat treated at different temperatures in the range 423 to 573 K, it has been shown that dehydration is controlled not only by the number and strength of the acid groups, but also by their accessibility [78]. 

The deactivation of the catalyst was slower the lower the reaction temperature, but at the cost of a much reduced catalytic conversion [175]. With PAN-ACF FE-300 the activity was stable at 573 K. Also with activated carbon produced from coal, the highest catalytic activities were found after treatment in hydrogen at 727 K followed by NH3 at 1173 K or after oxidation with nitric acid followed by NH3 treatment at 1073 K [176]. The activity increased on stepwise raising of the reaction temperature, but was much lower at a given temperature in the cooling cycle. 

The refined product from this treatment is oxidized by nitric acid in two stages at different temperatures, in the presence of catalysts and at pressures of 35-50 psi. First-stage oxidation is in the presence of an ammonium vanadate-copper oxide catalyst, and the exothermic reaction is maintained at 60-8d°C by cooling. Reaction time of 5 min is controlled by continuously bleeding off a stream to the second stage. The second oxidation stage is performed at about 105 C. Total nitric acid requirement is about 1 lb per lb of adipic acid made. 

The major part of these catalytic processes is carried out in fixed bed reactors. Some of the main fixed bed catalytic processes are listed in Table 11.1-1. Except for the catalytic cracking of gas oil, which is carried out in a fluidized bed to enable the continuous regeneration of the catalyst, the main solid catalyzed processes of today s chemical and petroleum refining industry appear in Table 11.1-1. However, there are also fluidized bed alternatives for phthalic anhydride— and ethylene dichloride synthesis. Furthermore, Table 11.1-1 is limited to fixed bed processes with only one fluid phase trickle bed process (e.g., encountered in the hydrodesulfurization of heavier petroleum fractions) are not included in the present discussion. Finally, important processes like ammonia oxidation for nitric acid production or hydrogen cyanide synthesis, in which the catalyst is used in the form of a few layers of gauze are also omitted from Table 11.1-1. 

The mixture of cyclohexanone and cyclohexanol can be converted to adipic acid in a second step by oxidation with nitric acid in the presence of metal compounds such as Cu or salts as homogeneous catalysts. 

Although air oxidation of the cyclohexanone-cyclohexanol mixtures on a Cu-Mn catalyst in acetic acid [140] is possible, the principal commercial operations entail oxidation with nitric acid. The reaction is usually carried out at 60 80°C and pressures of 0.1 to 0.4 MPa, employing 50-60% nitric acid and a copper-vanadium catalyst containing between 0.1 and 0.5% Cu and 0.1 and 0.2% V [141]. The yields of adipic acid are in the range of 90-96%. The main by-products are succinic acid and glutaric acid. Their concentration generally increases as the purity of the feed mixture decreases. The adipic acid is isolated by crystallization and purified by recrystallization from water. 

Catalytic conversion of off-gases, e.g., the reduction of nitrous oxide from nitric acid plants, with anunonia and the use of "three way" catalyst in automobile exhausts. 

Sadykov, V., Isupova, L., Zolotarskii, I., et al (2000). Oxide Catalysts for Ammonia Oxidation in Nitric Acid Production Properties and Perspectives, AppL Catal. A Gen., 204, pp. 59-87. 

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