Oxidation catalyzed by cobalt

It was shown in the previous section that hydrocarbon oxidation catalyzed by cobalt salts occurs under the quasistationary conditions with the rate proportional to the square of the hydrocarbon concentration and independent of the catalyst (Equation [10.9]). This limit with respect to the rate is caused by the fact that at the fast catalytic decomposition of the formed hydroperoxide, the process is limited by the reaction of R02 with RH. The introduction of the bromide ions into the system makes it possible to surmount this limit because these ions create a new additional route of hydrocarbon oxidation. In the reactions with ROOH and R02 the Co2+ ions are oxidized into Co3+, which in the reaction with ROOH are reduced to Co2+ and do not participate in initiation. 

The effect of bromide ion was more pronounced in polystyrene oxidation. Although polystyrene in a 1/1 mixture by volume of chlorobenzene and acetic acid is barely autoxidized at 100°C. in the presence of cobalt salt or initiators, the oxidation catalyzed by cobalt is so strongly accelerated by bromide ion that it proceeds rapidly even at temperatures as low as 45°C. (Figure 10). 

The role of Coball-dioxygen complexes in autooxidations other than phenol oxidation is less certain, and ostensibly similar reactions appear to follow radically different pathways. Thus, in the oxidation of thiols to disulfide catalyzed by Co11 species catalysis by the phthalocyanine complex [Con(TSPc)]4 apparently proceeds via a Co1 intermediate and without participation of Co—02 species,680 whereas catalysis by [CoH(TPP)] appears to involve initial formation of an >/ cobalt-dioxygen complex from which Of is displaced by thiolate.681 Several reviews giving extensive coverage to oxidations catalyzed by cobalt(II) complexes are available.649,650,682 683... 

The oxidation of methacrolein catalyzed by cobalt, added in the form of cobaltic acetate, usually produces methacrylic acid, but the reaction may be catalyzed by various transition metals [35] (Table 6). The kinetic study of the oxidation catalyzed by cobalt at concentrations of between 5 X 10-3 and 40 X 10"3 mole l 1 with aldehyde concentrations of 0.5 < [RCHO] < 4 mole l-1 shows that the rate of oxidation is independent of... 

Figure 5.4. Induction period vs. concentration of A -phenyl-y6-naphtylamine in the reaction of ethylbenzene oxidation catalyzed by cobalt (II) acetate (1.1310 M) at 60 °C. Data by [52],...

Figure 64. Proposed reaction mechanism for DTBP oxidation catalyzed by cobalt cyclidene...

An aerobic oxidation of tertiary amines to N-oxides catalyzed by Cobalt Schiff-base complexes was recently reported [98]. The reaction was run at room temperature with 0.5 mol% of the cobalt catalyst [Eq. (26)]. The presence of molecular sieves (5 A) enhanced the rate of the reaction. With this procedure various pyridines were oxidized to their corresponding N-oxides in yields ranging from 50 to 85 %. Electron-deficient pyridines such as 4-cyanopyridine gave a slow reaction with only 50% yield. 

The breadth of reactions catalyzed by cobalt compounds is large. Some types of reactions are hydrotreating petroleum (qv), hydrogenation, dehydrogenation, hydrodenitrification, hydrodesulfurization, selective oxidations, ammonoxidations, complete oxidations, hydroformylations, polymerizations, selective decompositions, ammonia (qv) synthesis, and fluorocarbon synthesis (see Fluorine compounds, organic). 

The present study concerns the interaction of propene molecules with cobalt sites in CoZSM-5. The experiments of CO and NO sorption evidenced that this zeolite contained practically only Co2+ in exchange position and Co3+ in oxide form. Propene is a reactant in several reactions catalyzed by cobalt containing zeolites (like reduction of NO, amonoxidation of propene and others). 

The question about the competition between the homolytic and heterolytic catalytic decompositions of ROOH is strongly associated with the products of this decomposition. This can be exemplified by cyclohexyl hydroperoxide, whose decomposition affords cyclo-hexanol and cyclohexanone [5,6]. When decomposition is catalyzed by cobalt salts, cyclohex-anol prevails among the products ([alcohol] [ketone] > 1) because only homolysis of ROOH occurs under the action of the cobalt ions to form RO and R02 the first of them are mainly transformed into alcohol (in the reactions with RH and Co2+), and the second radicals are transformed into alcohol and ketone (ratio 1 1) due to the disproportionation (see Chapter 2). Heterolytic decomposition predominates in catalysis by chromium stearate (see above), and ketone prevails among the decomposition products (ratio [ketone] [alcohol] = 6 in the catalytic oxidation of cyclohexane at 393 K [81]). These ions, which can exist in more than two different oxidation states (chromium, vanadium, molybdenum), are prone to the heterolytic decomposition of ROOH, and this seems to be mutually related. 

Celanese LPO [Liquid phase oxidation] A process for making acetic acid by oxidizing n-butane in the liquid phase, catalyzed by cobalt acetate. Developed by Hoechst Celanese and operated in the United States and The Netherlands. See also DF. 

The autoxidation of hydrocarbons catalyzed by cobalt salts of carboxylic acid and bromide ions was kinetically studied. The rate of hydrocarbon oxidation with secondary hydrogen is exactly first order with respect to both hydrocarbon and cobalt concentration. For toluene the rate is second order with respect to cobalt and first order with respect to hydrocarbon concentration, but it is independent of hydrocarbon concentration for a long time during the oxidation. The oxidation rate increases as the carbon number of fatty acid solvent as well as of cobalt anion salt are decreased. It was suggested that the cobalt salt not only initiates the oxidation by decomposing hydroperoxide but also is responsible for the propagation step in the presence of bromide ion. 

The activation energy of over-all oxidation catalyzed by 0.02M cobalt acetate and 0.04M NaBr is very small—8.3 kcal./mole for ethylbenzene, 8.7 for p-xylene, and 14.9 for n-dodecane. 

The broken line in Figure 2 shows that the steady rate of oxidation of 4.07M ethylbenzene in acetic acid, catalyzed by cobalt acetate, reaches a limiting rate of 2.2 X 10 r> mole liter"1 sec. 1 (after correcting for the dielectric effect (5,9), 1.8 X 10 5), which is in excellent agreement with the theoretical limiting rate of 1.85 X 10"r> mole liter 1 sec. 1 as calculated by 32(RH)2/2 6. 

In reactions catalyzed by cobalt and bromide the oxidation rates of ethylbenzene, cumene, and Tetralin start to decrease after several percent conversion and are roughly proportional to the hydrocarbon concentration during the oxidation. 

Oxidation Products. Although the ratio of hydroxyl to carbonyl products is 1/1 or nearly so in the ordinary metal salt-catalyzed autoxi-dation of hydrocarbons, higher proportions of carbonyl compounds are obtained in autoxidations catalyzed by cobalt and bromide ion—e.g.,... 

Partial Pressure of Oxygen. It was reported that the oxidation rate of p-toluic acid catalyzed by cobalt and bromide ion at 130°C. is half order with respect to the partial pressure of oxygen, and the initiating reaction is suggested to be the following. 

The induction period in the oxidation of ethylbenzene catalyzed by cobalt and sodium bromide in the presence of 2,6-di-fert-butyl-p-cresol indicates that the direct initiation is negligible compared with the rate of initiation by the cobalt-catalyzed decomposition of hydroperoxide. 

He J, Wu T, Jiang T et al (2008) Aerobic oxidation of secondary alcohols to ketones catalyzed by cobalt(II)/ZnO in poly(ethylene glycol)/C02 system. Catal Commun 9(13) 2239-2243... 

Ito MM, Akita K, Inoue H. Wet oxidation of oxygen and nitrogen containing organic compounds catalyzed by cobalt(III) oxide. Ind Eng Chem Rev 1989 28(suppl 7) 894-899. 

In the presence of bromide ion there is apparently no direct reaction of Co(III) with the hydrocarbon substrate, in contrast to cobalt-catalyzed autoxidations carried out in the absence of bromide. That different mechanisms are operating is illustrated by the relative rates of oxidation of alkylbenzenes catalyzed by cobalt acetate alone compared to those obtained in the presence of added bromide ion (Table VIII). In the presence of bromide ion, the relative reactivities are consistent with a mechanism involving attack by bromine atoms but not one involving electron transfer. Individual discrepancies in selectivities between bromine atom and the species active in the Co(0 Ac)2-NaBr system (Table VIII) were attributed to a bromine complex,... 

Despite the enormous importance of zeolites (molecular sieves) as catalysts in the petrochemical industry, few studies have been made of the use of zeolites exchanged with transition metal ions in oxidation reactions.6338- 634a-f van Sickle and Prest635 observed large increases in the rates of oxidation of butenes and cyclopentene in the liquid phase at 70°C catalyzed by cobalt-exchanged zeolites. However, the reactions were rather nonselective and led to substantial amounts of nonvolatile and sieve-bound products. Nevertheless, the use of transition metal-exchanged zeolites in oxidation reactions warrants further investigation. 

In some applications, such as homogeneous oxidations catalyzed by Co(salen) complexes, Raman, UV-vis and ATR spectroscopies could not be applied simultaneously because the cobalt concentrations needed to be optimized individually for each technique (Kervinen et al., 2005 Tinnemans et al., 2006). 

In the past, acetic acid was produced by aerobic oxidation of n-butane (and also of light naphtha, which is mainly a mixture of liquid -alkanes up to C9H2o). The process catalyzed by cobalt(II) acetate, closely resembled the one-step... 

Cobalt(II) salts are effective catalysts for the oxidation of 1,2-glycols with molecular oxygen in aprotic polar solvents such as pyridine, 4-cyanopyridine, benzonitrile, DMF, anisole, chlorobenzene and sulfolane. Water, primary alcohols, fatty acids and nitrobenzene are not suitable as solvents. Aldehydic products are further oxidized under the reaction conditions. Thus, the oxidative fission of rram-cyclo-hexane-l,2-diol gives a mixture of aldehydes and acids. However, the method is of value in the preparation of carboxylic acids from vicinal diols on an industrial scale for example, decane-1,2-diol is cleaved by oxygen, catalyzed by cobalt(II) laurate, to produce nonanoic acid in 70% yield. ... 

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