Cyclohexyl hydroperoxide, decomposition

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. 

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 goal here was to find new solid catalysts for cyclohexyl hydroperoxide (chhp) decomposition in cyclohexanol and cyclohexanone. The requirement list had foreseen a study on silica-supported metals of groups 4 and 5 and the need for a heterogeneous catalyst without metal Bxiviation. 

The cobalt-catalyzed oxidation of cyclohexane takes place through cyclohexyl hydroperoxide with the cobalt catalyst acting primarily in the decomposition of the hydroperoxide to yield the products 870 877... 

Decomposition of the hydroperoxides would be considered in terms of the following general scheme for example, for cyclohexyl hydroperoxide ... 

Generally, the issue of whether a truly solid Cr catalyst has been created for the aforementioned reactions is unresolved. This point is illustrated most clearly by all the work that has been devoted, in vain, to Cr molecular sieves (55-57). Particularly the silicates Cr-silicalite-1 and Cr-sihcahte-2 and the aluminophosphate Cr-AlPO-5 have been investigated. These materials have been employed, among others, for alcohol oxidation with t-BuOOH, for allylic (aut)oxidation of olefins, for the autoxidation of ethylbenzene and cyclohexane, and even for the catalytic decomposition of cyclohexyl hydroperoxide to give mainly cyclohexanone ... 

Decomposition of cyclic hydroperoxides. Cyclohexyl hydroperoxide (1) is decomposed mainly to (E)-2-hexenal by this salt and a trace of FeSO -VHaO in a two-phase system of water and an organic solvent. Highest yields (58%) are obtained with nitrobenzene. Na2PdCl4 is the most satisfactory water-soluble eomplexof PdX, for this purpose. The reaction is general, but yields are low from Cg-Ci2 cyclic hydroperoxides. Cyclopentyl hydroperoxide is converted into the corresponding enal in 73.6% yield. 

The synthesis of cyclohexanone, which is an intermediate in the manufacture of nylon 6 and nylon 6,6 is an important industrial process [1], One of the major current routes for the synthesis of cyclohexanone is the liquid-phase autoxidation of cyclohexane at 125-160 °C and 10 bar followed by the selective decomposition of the intermediate cyclohexyl hydroperoxide, using a soluble cobalt catalyst, to a mixture of cyclohexanol and cyclohexanone [2]. These severe conditions are necessary due to the low reactivity of cyclohexane towards autoxidation. Due to the high reactivity of the products in the autoxidation step conversions must be kept low (<10%) [3,4]. Heterogeneous catalysts potentially offer several advantages over their homogeneous counterparts, for example, ease of recovery and recycling and enhanced stability. Recently we found that chromium substituted aluminophosphate-5 and chromium substituted silicalite-1 (CrS-1) are active, selective and recyclable catalysts for the decomposition of cyclohexyl hydroperoxide to cyclohexanone [5j. 

However, if there is only one channel of the ol and one formation in the course of the oxidation, i.e., gradual decomposition of the alkyl hydroperoxide to produce both products again in approximately equal amounts, the concentrations of ol and one in the chromatogram of the untreated sample should be equal. This situation has been noticed in many cases of alkane oxidation. If the concentrations of ol and one, determined by GC, in the untreated sample are different, it testifies that the real amounts of products are not equal and either the alkyl hydroperoxide decomposes to produce predominantly the alcohol (or, on the contrary, the ketone) or there is an additional channel leading to cyclohexanol or cyclohexanone. This third simplified (without determination of ) method can give relatively precise values of the real concentrations of the alkyl hydroperoxide (as well as cyclohexanol and cyclohexanone) only if all conditions mentioned above are valid. For example, if the chromatogram of a solution before the reduction exhibits two peaks of approximately equal area for cyclohexanol and cyclohexanone, and after the reduction only cyclohexanol is determined by the GC, these data testify that only cyclohexyl hydroperoxide is present in the solution and its concentration can be determined precisely. 

The free radical yield/for AIBN in styrene and various solvents at 50°C is /= 0.5. Because of induced decomposition,/varies strongly with solvent in the case of BPO. If, for steric reasons, the primary free radicals cannot recombine, then the free radical yield can, according to conditions, increase up to/= 1. Thus, the start reaction is rarely a simple function of added initiator concentration, since it depends on free radical yield and may also depend on induced decomposition. Because of this, faster initiator decomposition need not necessarily produce faster polymerization. For example, dibenzoyl peroxide decomposes a 1000 times faster in benzene than cyclohexyl hydroperoxide, but only polymerizes styrene five times as fast. 

An investigation of the kinetics of the decomposition of cyclohexyl hydroperoxide at 60-70 in the presence of vanadyl acetylacetonate was recently carried out [355]. Cyclohexanol and cyclohexanone were formed in roughly a 1 1 ratio. The initial rate of decomposition was first order in initial concentrations of hydroperoxide and vanadium complex at [ROOM] <5 x 10 M and [VO(acac)2] < 1 x 10" M. The initial rate of decomposition changed from first order in [ROOH] to zero order giving evidence of complex formation prior to hydroperoxide decomposition. Using a chemiluminescence method the authors [355] concluded that only about 20% of the cyclohexyl hydroperoxide which decomposed gave free radicals. 

The decomposition of cyclohexylhydroperoxide was also studied in the presence of molybdenum and chromium complexes [356]. The decomposition of cyclohexylhydroperoxide in benzene catalyzed by [Mo02(acac)2], has many characteristics of the [VO(acac)2]-catalyzed reaction [355]. The ketone/alcohol ratio in the product was 1 and the kinetic pattern of reaction is similar. When chromium(III) acetylacetonate is used, however, there is a substantial difference. The chromium complex selectively converts cyclohexyl hydroperoxide to cyclohexanone. It is suggested that in this case the extent of release of free radicals to the solution is small [356]. The ketone/alcohol ratio in this case is " 13.7. The predominant formation of cyclohexanone on decomposition of cyclohexyl hydroperoxide in the presence of [Cr(acac)3] is no doubt related to the much higher yield of ketone obtained in cyclohexane oxidation in the presence of chromium complexes than observed when Mo or V compounds are used as catalysts [356]. 

Reports have appeared on the rates of decomposition of cyclohexyl hydroperoxide (an intermediate in the industrial oxidation of cyclohexaneto cyclohexanol and cyclohexanone catalyzed by Ru(porp)CO and Ru(porp)(0)i systems (porp = rCPP, mCrPP, TDCPP, TMCPP, TMP, TPP) either in solution or anchored to polystyrene or silica . The systems were studied in 20 1 cyclohexane/CH2Cl2 at 25°C, when decompositions in the 28-66% range were observed after 2 h, and close to 100% after 48 Several, plausible reaction pathways were... 

Cobalt not only catalyses the oxidation reaction, but it also catalyses the decomposition of the first reaction product, i.e. cyclohexyl hydroperoxide (CHHP). However, the exact role, if any, of cobalt in the activation of cyclohexane has always been a subject of dispute. 

Figure 16.18. Cobalt catalysed decomposition of cyclohexyl hydroperoxide under oxidation conditions.

The new process variant investigated at DSM is also based on suppression of the anol and anone concentrations, which is achieved by leaving out the catalyst. Application of appropriately chosen conditions counteracts decomposition of the cyclohexyl-hydroperoxide intermediate (PER), thus lowering the anol and anone concentrations and as a consequence improving the efficiency. Depending on the degree of cyclohexane conversion, efficiencies of 85-95 % can now be obtained. Of course, the degree to which the overall efficiency is increased after the decomposition of the peroxide depends on the selectivity of the peroxide conversion step, but this will not be considered here. 

Titanium-catalysed reaction of cyclohexyl hydroperoxide with alkenes involves two pathways, epoxidation of alkene and thermal decomposition of the hydroperoxide. The formation of radicals seems to play a role in both reactions. Epoxidation was found to take place solely at the catalyst. Absorption spectroscopy provided proof for the formation of titanium-hydroperoxide species as the active catalytic site for the direct epoxidation reaction. ... 

For the formal deoxygenation (decomposition) reaction 5, there is an enthalpy of formation value for every alcohol that matches a hydroperoxide . Using our exemplary groups, R = 1-hexyl, cyclohexyl and ferf-butyl, the liquid enthalpies of reaction are —77.9, —75.0 and —65.6 kJmoR, respectively (there is no liquid phase enthalpy of formation reported for f-butyl peroxide from Reference 4). The secondary hydroperoxides enthalpies of reaction average —77 7 kJmoR. For the three instances where there are also gas phase enthalpies of formation, the enthalpies of reaction are almost identical in the gas and liquid phases. The 1-heptyl (—60.3 kJmoR ) and 1-methylcyclohexyl (—50.6 kJmoR ) enthalpies of reaction are again disparate from the 1-hexyl and tert-butyl. If the enthalpy of reaction 5 for 1-hexyl hydroperoxide is used to calculate an enthalpy of formation of 1-heptyl hydroperoxide, it is —325 kJmoR, almost identical to values derived for it above. The enthalpies of reaction for the liquid and gaseous phases for the tertiary 2-hydroperoxy-2-methylhex-5-en-3-yne are —78.2 and —80.9 kJmoR, respectively. For gaseous cumyl hydroperoxide, the enthalpy of reaction is —84.5 kJmoR. ... 

The decarboxylation of decanedioic acid was studied in co-oxidation with cyclohexanol [106]. This reaction was seen to proceed through the attack of radicals (H02 and cyclohexyl-hydroxyperoxyl radical), see Chapter 7) on the C—H bonds of the acid and decomposition of the formed hydroperoxide. It was found that kpl2 = 3.8 x 104 exp(-50.2/7 7 ) L mol-1 s-1 and kd = 1.4 x 1015 exp(—108.8/ ) s The quasistationary concentration of the intermediate hydroperoxide decomposing with the formation of C02 was estimated to be as small as... 

The metaboric acid is fed to the oxidation train continuously and the mole ratio of boron added to O2 utilized is kept in the 0.65 to 1 range. The primary role of the metaboric acid is to esterify the cyclohexanol, thereby preventing selectivity robbing overoxidation. The boric acid also serves to catalyze the de-peroxidation of the cyclohexylhydroperoxide to cyclohexanol in high yield (-95%) at the expense of other uncatalyzed decomposition products such as cyclohexanone. This effect arises from the ability of the boron compound to reduce the intermediate hydroperoxide to the corresponding cyclohexyl borate ester, dioxygen, and water (Scheme l)M... 

Fig. 10. Temperature dependence of the rate of decomposition of the hydroperoxide of pure polypropylene (1) and in the presence of N-cyclohexyl-N -phenyl-p-phenylenediamine (2). The squares on curve 1 represent the results obtained in iodometric titration the points correspond to the data obtained from the rate of formation of the products.

It was also shown recently that secondary amines (such as diphenylamine and N-phenyl-N -cyclohexyl-p-phenylenediamine [21]) in the solid phase are capable of reacting not only with radicals ROO, but also with pol5rmer hydroperoxide, forming water as the basic reaction product, thereby greatly accelerating decomposition of the hydroperoxide ... 

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