Cyclohexane hydroperoxide

The mechanism of cyclohexane oxidation involves cyclohexane hydroperoxide as a key intermediate. 

The cyclohexane hydroperoxide then undergoes a one-electron transfer with cobalt or manganese (II). Chain transfer of the cyclohexyloxyl radical gives cyclohexanol or P-scission gives cyclohexanone. 

Photochemical reactions of materials enclosed in zeolites can lead to different proportions of products, or in some cases, to different products than those run in solutions.207 The distribution can vary with the zeolite. The enhanced selectivity in the oxidation of hydrocarbons with oxygen208 was mentioned in Chap. 4. The oxidation of cyclohexane in NaY zeolite with oxygen and visible light to yield cyclohexane hydroperoxide with complete selectivity at more than 40% conversion may have considerable industrial potential. Heating the hydroperoxide yields only cyclohexanone which can be oxidized to adipic acid for use in making nylon 6,6.209... 

The radical process begins with the radical-transfer agents R and ROO" (R = CgHii). Cobalt acts as an electron-transfer catalyst and redox initiator in the process. In a one-electron step, the oxidation state of the metal varies between +2 and +3, and radicals are released from the cyclohexane hydroperoxide. Since the cobalt is also involved in a cyclic process, its function is purely catalytic, and thus only small amounts of catalyst are required. Other metals such as V, Cr, Mo, Mn can also be used. Industrial variants of the process have been developed by companies such as BASF, Bayer, DuPont, ICI, Inventa, Scientific Design, and Vickers-Zimmer [T9]. 

Kaufman [in "Ojqrgenases" (O. Hayaishi, ed.), p. 170. Academic Press, New York, 1962] has summarized four proposals for a general mechanism of oxygenation, of which one is the formation and reduction of an intermediate hydroperoxide depicted here. B. Imelik [Compt. Rend, 226, 2082 (1948)] reported that Pseudomonas aeruginosa caused the transient formation of cyclohexane hydroperoxide when cyclohexane was the substrate. 

Hydroperoxide Process. The hydroperoxide process to propylene oxide involves the basic steps of oxidation of an organic to its hydroperoxide, epoxidation of propylene with the hydroperoxide, purification of the propylene oxide, and conversion of the coproduct alcohol to a useful product for sale. Incorporated into the process are various purification, concentration, and recycle methods to maximize product yields and minimize operating expenses. Commercially, two processes are used. The coproducts are / fZ-butanol, which is converted to methyl tert-huty ether [1634-04-4] (MTBE), and 1-phenyl ethanol, converted to styrene [100-42-5]. The coproducts are produced in a weight ratio of 3—4 1 / fZ-butanol/propylene oxide and 2.4 1 styrene/propylene oxide, respectively. These processes use isobutane (see Hydrocarbons) and ethylbenzene (qv), respectively, to produce the hydroperoxide. Other processes have been proposed based on cyclohexane where aniline is the final coproduct, or on cumene (qv) where a-methyl styrene is the final coproduct. 

Homogeneous Oxidation Catalysts. Cobalt(II) carboxylates, such as the oleate, acetate, and naphthenate, are used in the Hquid-phase oxidations of -xylene to terephthaUc acid, cyclohexane to adipic acid, acetaldehyde (qv) to acetic acid, and cumene (qv) to cumene hydroperoxide. These reactions each involve a free-radical mechanism that for the cyclohexane oxidation can be written as... 

Oxidation catalysts are either metals that chemisorb oxygen readily, such as platinum or silver, or transition metal oxides that are able to give and take oxygen by reason of their having several possible oxidation states. Ethylene oxide is formed with silver, ammonia is oxidized with platinum, and silver or copper in the form of metal screens catalyze the oxidation of methanol to formaldehyde. Cobalt catalysis is used in the following oxidations butane to acetic acid and to butyl-hydroperoxide, cyclohexane to cyclohexylperoxide, acetaldehyde to acetic acid and toluene to benzoic acid. PdCh-CuCb is used for many liquid-phase oxidations and V9O5 combinations for many vapor-phase oxidations. 

Inspired by Gif or GoAgg type chemistry [77], iron carboxylates were investigated for the oxidation of cyclohexane, recently. For example, Schmid and coworkers showed that a hexanuclear iron /t-nitrobenzoate [Fe603(0H) (p-N02C6H4C00)n(dmf)4] with an unprecedented [Fe6 03(p3-0)(p2-0H)] " core is the most active catalyst [86]. In the oxidation of cyclohexane with only 0.3 mol% of the hexanuclear iron complex, total yields up to 30% of the corresponding alcohol and ketone were achieved with 50% H2O2 (5.5-8 equiv.) as terminal oxidant. The ratio of the obtained products was between 1 1 and 1 1.5 and suggests a Haber-Weiss radical chain mechanism [87, 88] or a cyclohexyl hydroperoxide as primary oxidation product. 

A recent stndy (13,27) describes the use of Co-Si-TUD-1 for the liquid-phase oxidation of cyclohexane. Several other metals were tested as well. TBHP (tert-butyl hydroperoxide) was used as an oxidant and the reactions were carried out at 70°C. Oxidation of cyclohexane was carried out using 20 ml of a mixture of cyclohexane, 35mol% TBHP and 1 g of chlorobenzene as internal standard, in combination with the catalyst (0.1 mmol of active metal pretreated overnight at 180°C). Identification of the products was carried out using GC-MS. The concentration of carboxylic side products was determined by GC analysis from separate samples after conversion into the respective methyl esters. Evolution and consumption of molecular oxygen was monitored volumetrically with an attached gas burette. All mass balances were 92% or better. 

Results of the cyclohexane oxidation tests are shown in Table 41.4. Mono-oxygenated products are cyclohexanone, cyclohexanol and cyclohexyl hydroperoxide. Cu and Cr were very active, but subsequent tests showed considerable leaching for both metals, whereas Co-Si-TUD-1 did not show ai r leaching. Tests with different Co loadings indicate that the lowest Co concentration has the best conversion and ketone selectivity. Isolated cobalt species are most efficient for the conversion of cyclohexane, as agglomeration of Co reduces... 

See Oxygen Cyclohexane-1,2-dione bis(phenylhydrazone) See Other a-PHENYLAZO HYDROPEROXIDES... 

Cyclohexyl radicals react with cyclohexyl hydroperoxide to yield Cyclohexane and the cyclohexyl peroxy radical ... 

Oxidation of organic compounds by dioxygen is a phenomenon of exceptional importance in nature, technology, and life. The liquid-phase oxidation of hydrocarbons forms the basis of several efficient technological synthetic processes such as the production of phenol via cumene oxidation, cyclohexanone from cyclohexane, styrene oxide from ethylbenzene, etc. The intensive development of oxidative petrochemical processes was observed in 1950-1970. Free radicals participate in the oxidation of organic compounds. Oxidation occurs very often as a chain reaction. Hydroperoxides are formed as intermediates and accelerate oxidation. The chemistry of the liquid-phase oxidation of organic compounds is closely interwoven with free radical chemistry, chemistry of peroxides, kinetics of chain reactions, and polymer chemistry. 

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. 

The reaction of ions with peroxyl radicals appears also in the composition of the oxidation products, especially at the early stages of oxidation. For example, the only primary oxidation product of cyclohexane autoxidation is hydroperoxide the other products, in particular, alcohol and ketone, appear later as the decomposition products of hydroperoxide. In the presence of stearates of metals such as cobalt, iron, and manganese, all three products (ROOH, ROH, and ketone) appear immediately with the beginning of oxidation, and in the initial period (when ROOH decomposition is insignificant) they are formed in parallel with a constant rate [5,6]. The ratio of the rates of their formation is determined by the catalyst. The reason for this behavior is evidently related to the fast reaction of R02 with the... 

Fire and explosion hazards of processes involving the oxidation of hydrocarbons are reviewed, including oxidation of cyclohexane to cyclohexanone/cyclohexanol, ethylene to ethylene oxide, of cumene to its hydroperoxide, and of p-xylene to terephthalic acid. 

Bromophenylazo)-2-propyl hydroperoxide, 3156 l,2-Dihydroperoxy-l,2-bis(benzeneazo)cyclohexane, 3762 (Y-Phcnylazobcnzyl hydroperoxide, 3609 a-Phenylazo-4-bromobenzyl hydroperoxide, 3607 1-Phenylazocyclohexyl hydroperoxide, 3539 a-Phenylazo-4-fluorobenzyl hydroperoxide, 3608 3,3,5-Triphenyl-4,4-dimethyl-5-hydroperoxy-4,5-dihydro(3H)pyrazole, 3841... 

The detailed mechanism for these Co AlPO-18- and Mn ALPO-18-cata-lyzed oxidations are unknown, but as previously pointed out vide supra) and by analogy to other metal-mediated oxidations a free-radical chain auto-oxidation (a type IIaRH reaction) is anticipated [63], This speculation is supported by several experimental observations that include (1) an induction period for product formation in the oxidation of n-hexane in CoAlPO-36, (2) the reduction of the induction period by the addition of free-radical initiators, (3) the ability to inhibit the reaction with addition of free-radical scavengers, and (4) the direct observation of cyclohexyl hydroperoxide in the oxidation of cyclohexane [62],... 

Vanoppen et al. [88] have reported the gas-phase oxidation of zeolite-ad-sorbed cyclohexane to form cyclohexanone. The reaction rate was observed to increase in the order NaY < BaY < SrY < CaY. This was attributed to a Frei-type thermal oxidation process. The possibility that a free-radical chain process initiated by the intrazeolite formation of a peroxy radical, however, could not be completely excluded. On the other hand, liquid-phase auto-oxidation of cyclohexane, although still exhibiting the same rate effect (i.e., NaY < BaY < SrY < CaY), has been attributed to a homolytic peroxide decomposition mechanism [89]. Evidence for the homolytic peroxide decomposition mechanism was provided in part by the observation that the addition of cyclohexyl hydroperoxide dramatically enhanced the intrazeolite oxidation. In addition, decomposition of cyclohexyl hydroperoxide followed the same reactivity pattern (i.e., NaY < BaY... 

The mechanism of this reaction involves free radical oxidation of butane to butane hydroperoxide, which decomposes to acetaldehyde via P scissions. It is similar to the oxidation of cyclohexane to cyclohexanol and cyclohexanone, which will be discussed in Chapter 11, Section 4. 

Figure 11.2 The large tower on the right is the cyclohexane oxidation chamber and purification unit to convert cyclohexane to the hydroperoxide and then to cyclohexanone/cyclohexanol. An elevator leads to the top platform of this narrow tower, where an impressive view of this and other surrounding plants can be obtained. (Courtesy of Du Pont)...

Ethyl benzene is to be converted into the hydroperoxide (used to make styrene and propylene oxide) by bubbling air through a 1 molar solution in cyclohexane at 25°C in the reaction... 

PhI02 is rather bulky and plugs the pores, thus preventing further access of reactants to the active sites [49-50,63-64]. Therefore turn-overs are quite low when PhIO is used as oxidant. For the oxidation of methyl cyclohexane on TMPcY [49-50,63-64] and of cyclohexane on Fet.BuPcY [67] turn-overs are 5.6 and 7.6 respectively. It should be noted that the reported turn-overs for oxidations with PhIO correspond to conversions of less than 1 substrate molecule per two supercages, or to total conversions of less than 0.1 %. Therefore the observed activities and selectivities may be influenced by sorption effects. Furthermore iodosobenzene is a rather expensive oxidant and not practical to use because of its low solubility in solvents. Therefore some researchers tend to use other oxidantia such as air [65,66] and tertiary butyl hydroperoxide (t-ButOOH) [57]. In the oxidation of n-octane with t-ButOOH turn-overs as high as 6000 have been reported [57]. 

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