Oxidation of Cumene

Cumene Route to Phenol and Acetone Chemistry Overview 

In the cumene process, cumene is produced from benzene and propene by Friedel-Crafts alkylation. In modern cumene plants, zeolite catalysts are used with high yields of more than 99.7% at temperatures and pressures of approximately 150 °C and 30 bar, respectively. The reaction heat is 98 kj/mol. 

In the cumene oxidation process, in a first reaction cumene is oxidized to CHP. In a second reaction, CHP is cleaved to phenol and acetone by using a strong mineral acid as catalyst. The reaction heat is 117kJ/mol for the oxidation and 252 kJ/mol for the cleavage. The oxidation is carried out at pressures ranging from atmospheric to approximately 7 bar and temperatures between 80 and 120 C. The cleavage is performed around atmospheric pressure and temperatures in the range between 40 and 80 °C. 

When storing and handling CHP at medium or even elevated temperatures, the heat release from the thermal decomposition must be efficiently removed in order to avoid any hazards from thermal explosion (runaway). Especially, in large reactors for cumene oxidation, the exothermic decomposition of CHP has to be taken into account during a shutdown process when there is no more mixing by aeration, so only limited heat removal to ambient takes place. The heat evolved from thermal decomposition is 270 kj/mol [8,9]. From process safety point of view, and also to understand the auto-catalyzed mechanism in the cumene oxidation, it is necessary to describe and quantify the thermal decomposition characteristics of CHP. 

Twigg [10] was the first to present a kinetic study for the thermal decomposition of CHP in cumene. Experiments were performed at temperatures between 110 and 160 °C. Pure CHP (98-99.5%) was diluted in purified cumene down to concentrations below 10 wt%. 

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Obtained synthetically by one of the following processes fusion of sodium ben-zenesulphonate with NaOH to give sodium phenate hydrolysis of chlorobenzene by dilute NaOH at 400 C and 300atm. to give sodium phenate (Dow process) catalytic vapour-phase reaction of steam and chlorobenzene at 500°C (Raschig process) direct oxidation of cumene (isopropylbenzene) to the hydroperoxide, followed by acid cleavage lo propanone and phenol catalytic liquid-phase oxidation of toluene to benzoic acid and then phenol. Where the phenate is formed, phenol is liberated by acidification. 

There are many variations of the basic process and the patent Hterature is extensive. Several key patents describe the technology (16). The process steps are oxidation of cumene to a concentrated hydroperoxide, cleavage of the hydroperoxide, neutralization of the cleaved products, and distillation to recover acetone. 

Oxidation of cumene to cumene hydroperoxide is usually achieved in three to four oxidizers in series, where the fractional conversion is about the same for each reactor. Fresh cumene and recycled cumene are fed to the first reactor. Air is bubbled in at the bottom of the reactor and leaves at the top of each reactor. The oxidizers are operated at low to moderate pressure. Due to the exothermic nature of the oxidation reaction, heat is generated and must be removed by external cooling. A portion of cumene reacts to form dimethylbenzyl alcohol and acetophenone. Methanol is formed in the acetophenone reaction and is further oxidized to formaldehyde and formic acid. A small amount of water is also formed by the various reactions. The selectivity of the oxidation reaction is a function of oxidation conditions temperature, conversion level, residence time, and oxygen partial pressure. Typical commercial yield of cumene hydroperoxide is about 95 mol % in the oxidizers. The reaction effluent is stripped off unreacted cumene which is then recycled as feedstock. Spent air from the oxidizers is treated to recover 99.99% of the cumene and other volatile organic compounds. 

Safety is a critical aspect in the design of phenol plants. Oxidation of cumene to CHP occurs at conditions close to the flammable limits. Furthermore, the CHP is a potentially unstable material which can violendy decompose under certain conditions. Thus, phenol plants must be carefully designed and provided with weU-designed control and safety systems. 

The oxidation step is similar to the oxidation of cumene to cumene hydroperoxide that was developed earlier and is widely used in the production of phenol and acetone. It is carried out with air bubbling through the Hquid reaction mixture in a series of reactors with decreasing temperatures from 150 to 130°C, approximately. The epoxidation of ethylbenzene hydroperoxide to a-phenylethanol and propylene oxide is the key development in the process. 

From cumene Almost all the phenol produced in the United States is prepared by this method. Oxidation of cumene takes place at the benzylic position to give a hydroperoxide. On treatment with dilute sulfuric acid, this hydroperoxide is converted to phenol and acetone. 

The most common precursor to phenolic resins is phenol. More than 95% of phenol is produced via the cumene process developed by Hock and Lang (Fig. 7.1). Cumene is obtained from the reaction of propylene and benzene through acid-catalyzed alkylation. Oxidation of cumene in air gives rise to cumene hydroperoxide, which decomposes rapidly at elevated temperatures under acidic conditions to form phenol and acetone. A small amount of phenol is also derived from coal. 

The Hock process includes the oxidation of cumene by air to hydroperoxides using large bubble columns and the cleavage of the hydroperoxide via acid catalysis, which is reaction [OS 82]. This process is used for the majority of world-wide phenol production and, as a secondary product, also produces large quantities of acetone [64]. Phenol is used, e.g., for large-scale polymer production when reacted in a polycondensation with formaldehyde. 

A very similar rearrangement takes place during the acid-catalysed decomposition of hydroperoxides, RO—OH, where R is a secondary or tertiary carbon atom carrying alkyl or aryl groups. A good example is the decomposition of the hydroperoxide (84) obtained by the air-oxidation of cumene [(l-methylethyl)benzene] this is used on the large scale for the preparation of phenol and acetone ... 

Hydratropaldehyde has been prepared by hydrolysis of phenylmethylglycidic ester,2 3 4 by chromyl chloride oxidation of cumene,5 by the elimination of halogen acid or water from halohydrins or glycols,5 8 and by the distillation at ordinary pressure of methylphenylethylene oxide.9,10... 

The last reaction occurs much rapidly than the disproportionation of two cumylperoxyl radicals and accelerates chain termination in oxidized cumene [15]. The addition of cumene hydroperoxide helps to avoid the influence of the cross termination reaction Me2PhCOO + CH302 on the oxidation of cumene and to measure the pure disproportionation of cumylperoxyl radicals [15]. 

Chain decomposition of ozone was also observed in the oxidation of cumene by an O3-O2 mixture [151]. The rate of ozone consumption was found to be... 

VV Shereshovets. Mechanism of Oxidation of Cumene by Mixture of Ozone-Oxygen. Ph.D. thesis Dissertation, Institute of Chemical Physics, Chernogolovka, 1978, pp 1-21 [in Russian]. 

IP Shevchuk. The Study of Solvent Influence on Liquid-Phase Oxidation of Cumene, 1,1-Diphe-nylethane and Nonene-1. PhD thesis, L vov Polytech. Inst. L vov, 1968. 

Figure 5.1 illustrates the effect of hexamethylbenzene that produces a secondary peroxyl radical on the oxidation of cumene [9]. 

The decomposition of carboxyl radical occurs very rapidly, and C02 is formed with a constant rate in the initiated co-oxidation of cumene and acid [104]. 

The mechanism of H02 formation from peroxyl radicals of primary and secondary amines is clear (see the kinetic scheme). The problem of H02 formation in oxidized tertiary amines is not yet solved. The analysis of peroxides formed during amine oxidation using catalase, Ti(TV) and by water extraction gave controversial results [17], The formed hydroperoxide appeared to be labile and is hydrolyzed with H202 formation. The analysis of hydroperoxides formed in co-oxidation of cumene and 2-propaneamine, 7V-bis(ethyl methyl) showed the formation of two peroxides, namely H202 and (Me2CH)2NC(OOH)Me2 [16]. There is no doubt that the two peroxyl radicals are acting H02 and a-aminoalkylperoxyl. The difficulty is to find experimentally the real proportion between them in oxidized amine and to clarify the way of hydroperoxyl radical formation. 

Hock and Kropf [253] studied cumene oxidation catalyzed by Pb02. They proposed that Pb02 decomposed cumyl hydroperoxide (ROOH) into free radicals (R0 , R02 ). The free radicals started the chain oxidation of cumene in the liquid phase. Lead dioxide introduced into cumene was found to be reduced to lead oxide. The reduction product lead oxide was found to possess catalytic activity. The following tentative mechanism was proposed. 

Emulsion oxidation of alkylaromatic compounds appeared to be more efficient for the production of hydroperoxides. The first paper devoted to emulsion oxidation of cumene appeared in 1950 [1], The kinetics of emulsion oxidation of cumene was intensely studied by Kucher et al. [2-16], Autoxidation of cumene in the bulk and emulsion occurs with an induction period and autoacceleration. The simple addition of water inhibits the reaction [6], However, the addition of an aqueous solution of Na2C03 or NaOH in combination with vigorous agitation of this system accelerates the oxidation process [1-17]. The addition of an aqueous phase accelerates the oxidation and withdrawal of water retards it [6]. The addition of surfactants such as salts of fatty acids accelerates the oxidation of cumene in emulsion [3], The higher the surfactant concentration the faster the cumene autoxidation in emulsion [17]. The rates of cumene emulsion oxidation after an induction period are given below (T = 353 K, [RH] [H20] = 2 3 (v/v), p02 = 98 kPa [17]). 

FIGURE 11.1 The kinetic curves of cumyl hydroperoxide formation in emulsion oxidation of cumene [8] at T — 358 K, H20 RH — 3 l (v/v) 1 N Na2C03 with input of 0.015mol L 1 H202 in the moments designated by arrows (curve 1), after 8 h (curve 2), and after 4 h (curve 3). 

Phenol formed in the system due to acid-catalyzed decomposition of hydroperoxide retards the cumene oxidation. The aqueous phase withdraws phenol from the hydrocarbon phase. This is the reason why the emulsion oxidation of cumene helps to increase the yield of hydroperoxide. The addition of hydrogen peroxide into the system helps to increase the yield of hydroperoxide. 

These reactions produce free radicals, as follows from the fact of consumption of free radical acceptor [42]. The oxidation of ethylbenzene in the presence of thiophenol is accompanied by CL induced by peroxyl radicals of ethylbenzene [43]. Dilauryl dithiopropionate induces the pro-oxidative effect in the oxidation of cumene in the presence of cumyl hydroperoxide [44] provided that the latter is added at a sufficiently high proportion ([sulfide]/[ROOH] > 2). By analogy with similar systems, it can be suggested that sulfide should react with ROOH both heterolytically (the major reaction) and homolytically producing free radicals. When dilauryl dithiopropionate reacts with cumyl hydroperoxide in chlorobenzene, the rate constants of these reactions (molecular m and homolytic i) in chlorobenzene are [42]... 

Hock Also known as the Hock Lang process, and the cumene peroxidation process. A process for converting isopropyl benzene (cumene) to a mixture of phenol and acetone m-di-isopropyl benzene likewise yields resorcinol, and p-di-isopropyl benzene yields hydro-quinone. The basis of the process is the liquid-phase air oxidation of cumene to cumene hydroperoxide ... 

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. 

From the recent advances the heteroatom-carbon bond formation should be mentioned. As for the other reactions in Chapter 13 the amount of literature produced in less than a decade is overwhelming. Widespread attention has been paid to the formation of carbon-to-nitrogen bonds, carbon-to-oxygen bonds, and carbon-to-sulfur bonds [29], The thermodynamic driving force is smaller in this instance, but excellent conversions have been achieved. Classically, the introduction of amines in aromatics involves nitration, reduction, and alkylation. Nitration can be dangerous and is not environmentally friendly. Phenols are produced via sulfonation and reaction of the sulfonates with alkali hydroxide, or via oxidation of cumene, with acetone as the byproduct. 

Phenol has been obtained by distillation from petroleum and synthesis by oxidation of cumene or toluene, and by vapor-phase hydrolysis of chlorobenzene (USITC 1987). In 1995, 95% of U.S. phenol production was based on oxidation of cumene except at one company that used toluene oxidation and a few companies that distilled phenol from petroleum (CMR 1996). In 1995 the total annual capacity of phenol production approached 4.5 billion pounds (CMR 1996). 

This two-step process involves oxidation of cumene to cumene hydroperoxide, which decomposes with the help of a little dilute acid into phenol and acetone. In the first step, cumene is fed to an oxidation vessel (as shown in Figure 7—5)> where it is mixed with a dilute aqueous sodium carbonate solution (soda ash with a lot of water). A small amount of sodium stearate is added, and the whole mixture becomes an emulsion. ... 

Catalytic activities of [Ru PW (H30)(0)3, ] -/03/CH3CN/80°C and of [Ru PMOjj(H30)(0)3, ] "/03/CH3CN were compared. The tungsten complex did not catalyse the aerobic oxidation of cumene and 1-octene to cumyl alcohol and 1-octene oxide while the Mo analogue did so the tungsten complex underwent structural change with to an inert form, while its molybdenum analogue did not... 

The co-oxidation of cumene (CH) and Tetralin (TH) can be represented by the following simplified reaction scheme ... 

Tetralin hydroperoxide (1,2,3,4-tetrahydro-l-naphthyl hydroperoxide) and 9,10-dihydroanthracyl-9-hydroperoxide were prepared by oxidizing the two hydrocarbons and purified by recrystallization. Commercial cumene hydroperoxide was purified by successive conversions to its sodium salt until it no longer increased the rate of oxidation of cumene at 56°C. All three hydroperoxides were 100% pure by iodometric titration. They all initiated oxidations both thermally (possibly by the bi-molecular reaction, R OOH + RH — R O + H20 + R (33)) and photochemically. The experimental conditions were chosen so that the rate of the thermally initiated reaction was less than 10% of the rate of the photoreaction. The rates of chain initiation were measured with the inhibitors 2,6-di-ter -butyl-4-methylphenol and 2,6-di-fer -butyl-4-meth-oxyphenol. None of the hydroperoxides introduced any kinetically first-order chain termination process into the over-all reaction. 

Co-oxidation of Cumene and Tetralin. The present method for determining cross-propagation constants is based on Thomas and Tolman s (34) observation that the oxidation of cumene is strongly inhibited by adding low concentrations of Tetralin hydroperoxide. These workers concluded that TOO radicals formed in the transfer reaction ... 

The cross-termination constant was obtained from the rates of oxidation of cumene-Tetralin mixtures in the region of the rate minimum (Table II). The rates, rate constants, and concentrations were substituted into Equation 9, giving fcC T = 1.0 X 106 Mole"1 sec."1 at 30°C. The rate constants for the oxidation of the cumene-Tetralin system are summarized in Table III. 

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