Autoxidation of ethylbenzene

The reverse micelles stabilized by SDS retard the autoxidation of ethylbenzene [27]. It was proved that the SDS micelles catalyze hydroperoxide decomposition without the formation of free radicals. The introduction of cyclohexanol and cyclohexanone in the system decreases the rate of hydroperoxide decay (ethylbenzene, 363 K, [SDS] = 10 3mol L [cyclohexanol] =0.03 mol L-1, and [cyclohexanone] = 0.01 mol L 1 [27]). Such an effect proves that the decay of MePhCHOOH proceeds in the layer of polar molecules surrounding the micelle. The addition of alcohol or ketone lowers the hydroperoxide concentration in such a layer and, therefore, retards hydroperoxide decomposition. The surfactant AOT apparently creates such a layer around water moleculesthat is very thick and creates difficulties for the penetration of hydroperoxide molecules close to polar water. The phenomenology of micellar catalysis is close to that of heterogeneous catalysis and inhibition (see Chapters 10 and 20). 

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 ... 

We turned our attention next to the autoxidation of ethylbenzene (EB) to the corresponding hydroperoxide (EBHP) which constitutes the first step in the SMPO (styrene monomer propene oxide) process for the co-production of styrene and propene oxide from ethylbenzene and propene (Scheme 7). The overall selectivity to propene oxide obviously depends on the selectivity to EBHP in the first step, which is believed to be 80-85% in the commercial process. This is lower than for cumene as a result of secondary (in the case of EB) versus tertiary (in the case of cumene) C-H bond oxidation. The main byproduct in the autoxidation of ethylbenzene is acetophenone (16). From an economic viewpoint die production of acetophenone should be kept as low as possible. 

The mechanism of the autoxidation of ethylbenzene has been thoroughly studied, see NM Emanuel, D Gal, Modelling of Oxidation Processes, prototype the oxidation of ethylbenzene, Budapest Akademiai Kiado, 1986. 

Russell (10) suggested that the bimolecular self-reaction of S-RO2 involves the concerted decomposition of a cyclic tetroxide formed by combination of the radicals. This mechanism was deduced from a consideration of the results of a kinetic and product study of the autoxidation of ethylbenzene. Thus Russell found that almost one molecule of acetophenone is produced per two kinetic chains and that CeHsCHCCHa)O2 interact to form non-radical products nearly twice as fast as CsHsCDCcHs) O2. The former result is only compatible with (29) if all the alkoxy radicals disproportionate in the solvent cage (30) while the deuterium isotope effect requires a H-atom transfer reaction to be rate controlling, which is unlikely for the radical pathway. 

A review of recent advances in transition-metal-catalysed oxidations by molecular oxygen has highlighted the scope and limitations, as well as the meehanisms of these reactions. " " An overview of the fundamental studies on a new method of synthesis of nicotinic acid by the gas-phase catalytic oxidation of -picoline by oxygen has been presented. The reactivity of vanadium species has been considered in order to discover the nature of the active catalyst. Kinetic equations for -picoline oxidation on vanadia-titania catalysts have been discussed. " The effect of quaternary ammonium salts or macrocyclic ethers on the autoxidation of ethylbenzene or the decomposition of the a-phenylethyl hydroperoxide intermediate catalysed by Ni(II) or Fe(III) acetylacetonates has been reviewed. ... 

The reverse emulsion stabilized by sodium dodecylsulfate (SDS, R0S03 Na+) retards the autoxidation of dodecane [24] and ethylbenzene [21,26,27]. The basis for this influence lies in the catalytic decomposition of hydroperoxides via the heterolytic mechanism. The decay of hydroperoxides under the action of SDS reverse micelles produces olefins with a yield of 24% (T=413 K, 0.02mol L 1 SDS, dodecane, [ROOH]0 = 0.08 mol L 1) [27], The thermal decay gives olefins in negligible amounts. The decay of hydroperoxides apparently occurs in the ionic layer of a micelle. Probably, it proceeds via the reaction of nucleophilic substitution in the polar layer of a micelle. 

Hydrocarbon Concentration. The steady rate of hydrocarbon oxidation is exactly first order with respect to hydrocarbon concentration, but it tends to be independent of this concentration below l.OAf (Figure 4). The cobalt-catalyzed autoxidation of Tetralin (6) and ethylbenzene at 0.05M cobalt in the absence of bromide is exactly second order with respect to hydrocarbon concentration. 

The materials showed some activity in the autoxidation of alkylaromatics such as ethylbenzene at 403 118 K, even though at these temperatures there is a considerable blank background reaction. The stability of a salicylidene imine under the conditions of high-temperature autoxidation is questionable in any event. 

More recent investigations have shown that these reactions involve metal-catalyzed decomposition of hydroperoxides via the usual redox cycles. Thus, inhibition, polymerization and product studies in the RhCl(Ph3 P)3-catalyzed autoxidation of cyclohexene,136 ethylbenzene,136 and diphenylmethane137 were compatible only with metal-catalyzed decomposition of the alkyl hydroperoxide and not a direct reaction of the metal-dioxygen complex with substrate. Complexes Rh(III) (acac)3, Rh(III) (2-ethylhexanoate)3, and Co(II) (2-ethylhexanoate)2, gave results that were almost the same as those obtained with RhCl(Ph3P)3. The redox cycle may involve Rh(II) and Rh(III) ... 

Kropf 161, 197-199) has studied the autoxidation of cumene, p-nitro-cumene, toluene, ethylbenzene, diphenylmethane, p-xylene, p-cymene, m-diisopropylbenzene, phenylcyclopentane, and phenylcyclohexane. The autoxidation proceeds in most cases via the formation of a hydroperoxide, for example,... 

We have developed an effective method for the selective autoxidation of alky-laromatic hydrocarbons to the corresponding benzylic hydroperoxides using 0.5 mol% NHPI as a catalyst and the hydroperoxide product as an initiator. Using this method we obtained high selectivities to the corresponding hydroperoxides, at commercially viable conversions, in the autoxidation of cyclohexylbenzene, cumene and ethylbenzene. The highly selective autoxidation of cyclohexylbenzene to the 1-hydroperoxide product provides the basis for a coproduct-free route to phenol and the observed inq)rovements in ethylbenzene hydroperoxide production provide a basis for in roving the selectivity of the SMPO process for styrene and propene oxide manufacture. 

If there is a CH2 or a CH group in a-position to an aromatic system, it is attacked preferentially by oxygen with formation of a substituted benzyl hydroperoxide, 317 examples being tetralin, ind ne, fluorene, cumene, p-xylene, and ethylbenzene. Temperatures required for autoxidation of such compounds are lower than for alkanes. 

Fig. 4, Chemiluminescence intensity vs. time plot of ethylbenzene autoxidation in benzene at 40°. Initiator dicyclohexylperoxycarbonate (5,2 x 10 M). The straight line was drawn assuming an initial oxygen concentration calculated from solubility data in the literature and assuming that at the drop , oxygen concentration equals zero [12]).

Intensity vs. time plot of the chemiluminescence of ethylbenzene autoxidation in benzene, at 40°C. Starter dicyclohexyl peroxycarbonate (5.2x10 M). After Vasil ev and Rusina [69]. 

Commercially, autoxidation is used in the production of a-cumyl hydroperoxide, tert-huty hydroperoxide, -diisopropylbenzene monohydroperoxide, -diisopropylbenzene dihydroperoxide, -menthane hydroperoxide, pinane hydroperoxide, and ethylbenzene hydroperoxide. 

It is seen from Table 11.1 that surfactant cetyltrimethylammonium bromide (CTAB, RN MexBr ) exerts a positive catalytic effect on ethylbenzene autoxidation. The kinetic study of this phenomenon [21,27] showed that the acceleration was caused by the additional reaction of hydroperoxide with the bromide ion of CTAB to form free radicals [30],... 

By-products formed during their preparation (e.g., ethylbenzene and divinyl-benzenes in styrene acetaldehyde in vinyl acetate) added stabilizers (inhibitors) autoxidation and decomposition products of the monomers (e.g., perox-... 

The hypothesis of a bimolecular initiation reaction for liquid phase autoxida-tions was extended beyond cyclohexanone as a reaction partner. Also other substances featuring abstractable H-atoms are able to assist in this radical formation process. The initiation barrier was found to be linearly dependent on the C-H bond strength, ranging from 30 kcal/mol for cyclohexane to 5 kcal/mol for methyl linoleate [14, 15]. Substrates that yield autoxidation products that lack weaker C-H bonds than the substrate (e.g., ethylbenzene) do not show an exponential rate increase as the chain initiation rate is not product enhanced [16]. 

By-products formed during their preparation (e.g., ethylbenzene and divin-ylbenzenes in styrene acetaldehyde in vinyl acetate) added stabilizers (inhibitors) autoxidation and decomposition products of the monomers (e.g., peroxides in dienes, benzaldehyde in styrene, hydrogen cyanide in acrylonitrile) impurities that derive from the method of storage of the monomer (e.g., traces of metal or alkali from the vessels, tap grease etc.) dimers, trimers, and polymers that are generally soluble in the monomer, but sometimes precipitate, for example, polyac-rylOTiitrile from acrylonitrile. Likewise, in polycondensation reactions it is important to remove reactive impurities because they can cause considerable interference during the polyreaction. 

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