Cumene oxidation rate

The increase in the amount of catalyst introduced in oxidized cumene (353 K) increases the oxidation rate, decreases the amount of the formed hydroperoxide, and increases the yield of the products of hydroperoxide decomposition methylphenyl ethanol and acetophenone. Similar mechanism was proposed for catalysis by copper phthalocyanine in cumene oxidation [254],... 

The effect of jumping of the maximal hydroperoxide concentration after the introduction of hydrogen peroxide is caused by the following processes. The cumyl hydroperoxide formed during the cumene oxidation is hydrolyzed slowly to produce phenol. The concentration of phenol increases in time and phenol retards the oxidation. The concentration of hydroperoxide achieves its maximum when the rate of cumene oxidation inhibited by phenol becomes equal to the rate of hydroperoxide decomposition. The lower the rate of oxidation the higher the phenol concentration. Hydrogen peroxide efficiently oxidizes phenol, which was shown in special experiments [8]. Therefore, the introduction of hydrogen peroxide accelerates cumene oxidation and increases the yield of hydroperoxide. 

The experiments on emulsion cumene oxidation with AIBN as initiator proved that oxidation proceeds via the chain mechanism inside hydrocarbon drops [17]. The presence of an aqueous phase and surfactants compounds does not change the rate constants of chain propagation and termination the ratio (fcp(2fct)-1/2 = const in homogeneous and emulsion oxidation (see Chapter 2). Experiments on emulsion cumene oxidation with cumyl hydroperoxide as the single initiator evidenced that the main reason for acceleration of emulsion oxidation versus homogeneous oxidation is the rapid decomposition of hydroperoxide on the surface of the hydrocarbon and water drops. Therefore, the increase in the aqueous phase and introduction of surfactants accelerate cumene oxidation. 

With cumene hydroperoxide the situation is more complex. Traylor and Russell (35) found that adding this hydroperoxide to cumene increased the oxidation rate of the cumene (cf. Table I). This phenomenon... 

The experimentally measured direct chain termination constant was found to be 5.5 X 103 Mole"1 sec."1. However, this value was not considered very accurate because there is a large correction to the measured oxidation rates for oxygen evolved in the self-reactions of COO radicals at the relatively high photo-initiation rates required to reduce the importance of thermal initiation from the added COOH. A more accurate value of 2.9 X 103 mole"1 sec."1 was calculated from the limiting value of fcpC/[2fc fdirect)]"1/2 at high [COOH] for the AIBN thermally initiated reaction at 30 °C. combined with the measured value of Jcp for neat cumene (0.18 Mole"1 sec."1). 

Hydrogen Atom Transfer from Hydroperoxides to Peroxy Radicals. The reaction of cumylperoxy radicals with Tetralin hydroperoxide (Reaction 10) can be studied at hydroperoxide concentrations below those required to reduce the oxidation rate to its limiting value. The rate of oxidation of cumene alone can be represented by ... 

Chain Termination in the Oxidation of Cumene. Traylor and Russell (35) assume that the acceleration in the rate of oxidation of CH which is produced by added COOH is solely caused by a chain transfer reaction between CO radicals and COOH. This assumption implies that all CH3OO radicals enter into termination via Reaction 13. However, Thomas (32) has found that acetophenone is formed even in the presence of sufficient COOH to raise the oxidation rate of CH to its limiting value. (The receipt of Thomas manuscript prior to publication stimulated the present calculations.) From this fact, and from a study of the acetophenone formed during the AIBN-induced decomposition of COOH, Thomas concludes that the accelerating effect of added COOH is primarily caused... 

Reactivity ratios for all the combinations of butadiene, styrene, Tetralin, and cumene give consistent sets of reactivities for these hydrocarbons in the approximate ratios 30 14 5.5 1 at 50°C. These ratios are nearly independent of the alkyl-peroxy radical involved. Co-oxidations of Tetralin-Decalin mixtures show that steric effects can affect relative reactivities of hydrocarbons by a factor up to 2. Polar effects of similar magnitude may arise when hydrocarbons are cooxidized with other organic compounds. Many of the previously published reactivity ratios appear to be subject to considerable experimental errors. Large abnormalities in oxidation rates of hydrocarbon mixtures are expected with only a few hydrocarbons in which reaction is confined to tertiary carbon-hydrogen bonds. Several measures of relative reactivities of hydrocarbons in oxidations are compared. 

Several workers have pointed out the easy exchange of alkylperoxy radicals with hydroperoxides (16, 24, 34, 35) and how it may affect rates of co-oxidation, at least of cumene. Howard et al. (24) show how even 0.1M hydroperoxide affects both oxidation rate and reactivity ratios. When rarh = 1, this exchange will make no difference in the apparent radical reactivities. Russell and Williamson have necessarily used high conversions in most of their co-oxidations. Alagy et al. have not specified their conversions. 

The results of a study of the zinc diisopropyl dithiophosphate-inhib-ited oxidation of cumene at 60°C. are shown in Figures 1 to 3. The initial oxidation rate is directly proportional to the AIBN concentration, but the dependence of initial rate on the cumene concentration or the reciprocal of the zinc salt concentration, although reasonably linear, is not in direct proportion. 

Figure 2. Initial oxidation rate of 7.2M cumene at 60°C. containing 0.06M A1BN as a function of ZnP concentration...

Figure 3. Initial oxidation rate as a function of concentration of cumene in tert-butylbenzene at 60°G. containing 0.06M AIBN and 0.02M ZnP...

Traylor and Russell (30) have shown recently that similar reactions for the cumyloxy radical are important in cumene oxidation at 60 °C., and Hendry (12) has provided some quantitative data. At low concentrations of hydrocarbon, Reaction 9 is favored over Reaction 7 (propagation by tert-BuO ), and significant numbers of methyl radicals are formed and converted to Me02 radicals. Chain termination thus shifts from the slow termination by 2 tert-Bu02 (Reaction 6) to Reaction 10, which has a rate constant several hundredfold larger (21). The apparent order of the oxidation in isobutane is then 3/2 a similar relation applies to gas-phase oxidations and is discussed there. 

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. 

Cumene oxidized relatively slowly, at about 1/13 the rate of p-xylene. This was not caused by the formation of phenol, as might be expected by an acid-catalyzed rearrangement of cumene hydroperoxide. No phenol or product clearly derived from phenol, as by radical attack or by oxidation to a quinone, was detected at any time in the reaction mixture. The two major products were a-methylstyrene and 2-phenylpropylene oxide their concentrations increased with time. The group at Shell also observed the formation of a-methylstyrene and 2-phenylpropylene oxide among the products of cumene oxidation in butyric acid at 140°C. with cobalt and manganese catalysts (30). 

This process competes favorably with benzylic hydrogen abstraction in toluene, less in ethylbenzene, and least in cumene (31). Such reactions do not seem significant in the oxidation of benzene derivatives. However, naphthalene reacts about 20 times as rapidly with phenyl radical as does benzene (16), and radical addition to the naphthalene nucleus may at least partly account for the slow oxidation rate in the methylnapthalenes. Among the minor products from both methylnaphthalene oxidations were compounds of molecular weight 296 ... 

The propagation step, Eq. (4), is much slower than Eq. (3) as an example, its rate constant kp is 0.18 M 1 sec-1 for cumene at 303K. Values of kp can vary considerably for different substrates, as shown by the oxidation rates of substituted toluenes (8). With respect to toluene, taken as 1.0, the reactivity of 4-nitrotoluene toward ROO is 0.33 and that of / -xylene is 1.6. A homolytic process like the fission of the C-H bond should be essentially apolar, but data for substituted toluenes correctly suggest that the hydrogen radical abstraction is favored by electron-donor substituents and that in the transition state the carbon atom involved has a partial positive charge. The difference in kp between different molecules or different groups of the same molecule is the reason of the selectivity of autoxidation. 

In recent years much emphasis has been placed on studies of co-oxidations, since they can provide quantitative data about fundamental processes (such as the relative reactivities of peroxy radicals toward various hydrocarbons48-50), which are difficult to obtain by other methods. Co-oxidations are also quite important from a practical viewpoint since it is possible to utilize the alkylperoxy intermediates for additional oxidation processes instead of wasting this active oxygen. That the addition of a second substrate to an autoxidation reaction can produce dramatic effects is illustrated by Russell s observation51 that the presence of 3 mole % of tetralin reduced the rate of cumene oxidation by two-thirds, despite the fact that tetralin itself is oxidized 10 times faster than cumene. The retardation is due to the higher rate of termination of the secondary tetralyl-peroxy radicals compared to the tertiary cumylperoxy radicals (see above). 

In contrast to silver-catalysed cumene oxidation, the evidence concerning the mechanism of copper-catalysed reactions favours radical initiation via surface hydroperoxide decomposition. Gorokhovatsky has shown that the rate of ethyl benzene oxidation responds to changes in the amount of copper(ii) oxide catalyst used, in a manner which is characteristic of this mechanism. Allara and Roberts have studied the oxidation of hexadecane over copper catalysts treated in various ways to produce different surface oxide species, Depending on the catalyst surface area and surface oxide species present, a certain critical hydroperoxide concentration was necessary in order to produce a catalytic reaction. At lower hydroperoxide levels, the reaction was inhibited by the oxidized copper surface. XPS surface analysis of the copper catalysts showed a... 

Ag 10%Au cumene oxidation increased rate of formation of cumene hydroperoxide... 

It is known that manganese salts cause oxidation of hydrocarbons, like cumene, by initiating free radical chain reactions. However, this is normally done by catalytic decomposition of trace amounts of hydroperoxides found in the hydrocarbons. In our case, the catalyst does not seem to decompose CHP, as demonstrated in an independent experiment (see above). If it did, the rate of decomposition should increase in time as the reaction progresses leading to an increase in the autoxidation rate. While we do observe for cumene an initiation period up to the accumulation of 3-5% hydroperoxide, from that point on up to greater than 50% CHP accumulation, the oxidation rate is constant. This initiation period may be due to surface activation of the catalyst. 

Thus, in the oxidation of cumene, we are faced with the problem of increasing the conversion and rate of cumene oxidation while retaining the high selectivity 97-99%. 

These works appeared actually simultaneously with ours. In presence of heterogenised variable-valence metal compounds the values and of cumene oxidations to hydroperoxide are lower. For example, at catalysis by silica - and polymer (poly-4-vinylpyridine) - supported Cu(OAc)2, the increase in the rate of cumene oxidation by 68% was observed, and was equal 92% at C = 22% [36]. 

Since the rate of cumene oxidation at 100° is rather high, on this system, on-visible, it is necessary to work with small concentrations of nickel catalyst, which can facilitate a high yield of the hydroperoxide [8, 39]. It is evident from analysis of scheme of catalyzed hydrocarbons oxidation, including participation of catalyst in chain initiation reaction under catalyst interaction with ROOH and also in chain propagation (Ct + RO ), that with decrease in [Ct the rate of reaction should be decreased, and [ROOH] should be increased [8, 15]. It has appeared that together with... 

The catalytic properties of Co in the hydrocarbon oxidation have been the subject of intensive investigations [107], It has been established that during the cumene-AcOH ozonolysis in 1 1 (v v) in the presence of Co(AcO)2 the oxidation reaction is accelerated (Fig. 17).In contrast to the noncatalysed process in the catalyzed by transition metal salts the ozonolysis is characterized by 1) absence of ozonides formation that is indicative of the absence of ozone interaction with the phenyl ring and 2) the main product is DMPC, the accumulation rate of which proportional to the concentration of Co after the 10 min. The initial rates of CHP formation do not vary with the changes in Co + but after the 15 min the rates increase with [Co ]. It can be seen from Table 9 that if we assume the ozonolysis of pure cumene as a reference then the addition of AcOH results in autoretardation of the oxidation rate and to reduction of the products yield. The ratio [IP]/[03 reaches value of 6.9. 

Initiation rate (Wi) was calculated from equation (1) where Wox -cumene oxidation initial rate, kp, kt - rate constants of chain propagation and termination correspondingly. 

Initiation rate values for liquid phase radical chain cumene oxidation initiated by peroxide compounds I and II in the presence of Et4N Br are presented below. 

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