Catalytic production of aryl alkyl hydroperoxides by manganese complexes

Abstract: This invention relates to a process for preparing organic hydroperoxides by selectively oxidizing aryl alkyl hydrocarbons to their corresponding organic hydroperoxides using as a catalyst certain tetranuclear manganese complexes. The process is carried out without appreciable decomposition by the catalyst of the resulting organic hydroperoxides, and without oxidation by the catalyst of other hydrocarbons present in the starting mixture. The process of the present invention is useful for producing organic hydroperoxides, which themselves are useful as starting materials for a number of commercial reactions, for example, phenol and acetone production.

Description: This invention relates to a process for preparing organic hydroperoxides by selectively converting aryl alkyl hydrocarbons in the presence of an oxygen-containing gas and certain tetranuclear manganese complexes as a catalyst. The production of organic hydroperoxides from aryl alkyl hydrocarbons in the presence of various transition metal salt complexes has been described in the literature (see, for example, US Patent No. 2,954,405 concerning the production of organic hydroperoxides by autooxidation of hydrocarbons in the presence of molecular oxygen and metal phthalocyanines as catalysts). In '405, certain phthalocyanines were found to catalyze autooxidation without significant decomposition of the organic hydroperoxides to undesirable side products, as was the case in many previous processes. However, when the '405 catalyst contains manganese, the reaction does, in fact, produce undesirable by-products. Similarly, US Patent No. 4,013,725 discloses a process for preparing hydroperoxides in a homogeneous system by autooxidizing secondary alkyl group-substituted methylbenzenes in the presence of water, a base, an oxygen containing gas, and a water- soluble chelate compound in which multidentate ligands are coordinated to at least one metal from the class of cobalt, nickel, manganese, copper, and iron. The multidentate ligand may include nitrogen containing polycarboxylic acids. The process may be distinguished from that of the present invention on several grounds. First, while both the organic ligand used in the process of the '725 patent and the present invention are amino acids and, hence, contain both amino and carboxyl groups, the process of the present invention uses a catalyst that contains an additional essential alkoxide group, which is not taught nor disclosed in the process of the '725 patent. Second, the process of the '725 patent requires the presence of significant amounts of water to dissolve the catalyst and to carry out the hydroperoxidation process. The present invention adds the catalyst in situ ; no water is required. Finally, the '725 patent requires continuous adjustment of the pH of the solution to a value between 5 and 7.5 using alkali salts. No additional reagents are required to adjust the pH in the process of the present invention. In US 5,025,101, applicants taught and claimed the tetranuclear manganese complexes and their method of preparation, which complexes are used as the catalyst in the process of the present invention. In '101, applicants found that the complexes were useful as hydrogen peroxide decomposers. Applicants have now discovered the unexpected result that these complexes are effective in the process of selectively producing organic hydroperoxides from aryl alkyl hydrocarbons, but that they do not catalyze the decomposition of those organic hydroperoxides. The result is unexpected since transition metal ions are generally known to decompose hydroperoxides catalytically; thus, the expectation is that a metal complex that vigorously decomposes them will also decompose organic hydroperoxides. This invention provides a process for preparing organic hydroperoxides comprising selectively oxidizing aryl alkyl hydrocarbons in situ to their corresponding organic hydroperoxides using as a catalyst certain tetranuclear manganese complexes. The process is carried out without decomposition of the resulting organic hydroperoxides to any appreciable extent. The process of the present invention is useful for producing organic hydroperoxides, which themselves are useful as starting materials for a number of commercial reactions; for example, the production of phenol and acetone from cumene hydroperoxide (also referred to as cumyl hydroperoxide). The process of the present invention is carried out in a heterogeneous system by contacting aryl alkyl hydrocarbons alone or a mixture of aryl alkyl hydrocarbons and other hydrocarbons that do not decompose or deactivate the catalyst, preferably aromatic hydrocarbons with an oxygen-containing gas and a tetranuclear manganese complex catalyst in catalytically-effective amounts. The oxygen containing gas is preferably air or oxygens, more preferably air. The catalyst is present in any form that renders the system heterogeneous, preferably in solid form. The tetranuclear manganese complexes used as catalysts in the process of this invention have the general formula: M₂[Mn₄(O)(OH)(O₂CR)₂L₂] wherein M is an alkali metal or an alkaline earth metal, preferably selected from magnesium, calcium, strontium, barium, potassium, sodium, lithium, and matures thereof, more preferably barium; R is selected from hydrogen and a hydrocarbyl group; and L is a ligand having the formula: In the catalyst, when R is a hydrocarbyl group, preferably it will have from 1 to 30 carbon atoms and, more preferably, is an alkyl group having from 1 to 10 carbon atoms. When R is an aralkyl group, it preferably will have from 7 to 10 carbon atoms. The catalyst's method of preparation is described, for example, in U.S. Patent No. 5,025,101. The catalyst in the process of the present invention is preferably used in the solid state form obtained from the crystallization process described in U.S. Patent No. 5,025,101. The aryl alkyl hydrocarbons employed as starting materials in this process may be obtained from commercial sources. The aryl alkyl hydrocarbons should have a melting point within the range of temperatures at which the process of the present invention is operated, or should be capable of being solubilized using an appropriate co-solvent that is inert to the process of the present invention. Further, the aryl alkyl hydrocarbons must contain a benzylic hydrogen, such that the hydrocarbon has the general formula: wherein R₁ and R₂ are organo groups, preferably lower alkyl groups or hydrogen groups and may be the same or different and Ar is an aromatic group, for example phenyl or naphthyl group, preferably phenyl: The solvent for the reaction is preferably an excess amount of the starting hydrocarbon mixture. The amount of catalyst used will vary depending upon the nature and amount of the organic starting material to be oxidized. In general from 0.001 to 0.5 parts by weight of catalyst per 100 parts of substrate and preferably from 0.1 to 0.2 parts per 100 parts of substrate are satisfactory. Contacting is preferably by any means that achieves intimate contact, such as rapid bubbling or mechanic agitation. The flow rate of gas will vary depending on the reaction temperature and pressure, but should be sufficient to convert the aryl alkyl hydrocarbons to the corresponding organic hydroperoxides. In general there appears not to be an upper limit on the flowrate. In the case of cumene hydroperoxide preferably the flow rate should be up to 10 liters/hr per 100g. of cumene, preferably at least 2 liters/hr and in all cases sufficient to accomplish the reaction. The contacting is preferably carried out with air rather than oxygen, at at least 0.5 atm. (0.51 bar) pressure. It is within the skill of one ordinarily skilled in the art to select the appropriate pressure to effect the optimum reaction rate. The reaction temperature may range from 0°C to 90°C, preferably from 60°C to 80°C. Temperatures at the lower end of the preferred range are more desirable. It is an important aspect of the process of the present invention that it results in the selective oxygenation of the aryl alkyl hydrocarbons in the starting material in the presence of the tetranuclear manganese complex and an oxygen containing gas, but does not oxygenate to any appreciable extent (i.e., within the limits of experimental error) aromatic or aliphatic hydrocarbons present in the starting material. The process also does not require the presence of an initiator in the form of a small amount of the resulting organic hydroperoxide, as has been the case in other processes. The catalyzed process of the present invention increases the reaction rate by a factor of approximately 10 over the process that uses cumene hydroperoxide as an initiator. It is also an important aspect of the present invention that while the tetranuclear manganese complex catalyzes the formation of organic hydroperoxides from the corresponding alkyl aromatic hydrocarbons it does not catalyze the decomposition of these organic hydroperoxides thus formed to any appreciable extent, i.e., within the limits of experimental error. For example, in the case of cumene hydroperoxide, no decomposition was found to occur at 65°C within the limits of experimental error (about 1%) even when the cumene hydroperoxide remained in contact with the catalyst for a period of 100 hours. This ability to catalyze the formation of organic hydroperoxides without also catalyzing their decomposition is unexpected given the fact that the catalyst was previously found to be useful in catalyzing the decomposition of hydrogen peroxide (H₂O₂), and given the fact that certain manganese salts of various nuclearity and topology are well known to those ordinarily skilled in the art to decompose organic hydroperoxides. The formation of the corresponding organic hydroperoxides can be monitored, for example, by analyzing aliquots by NMR, iodometric titration, chromatography or other means readily known to one skilled in the art. The organic hydroperoxides are readily recovered from the reaction mixture by conventional methods, for example distillation, and as a result, the process may be run in batch or continuously. In a continuous process, the aryl alkyl hydrocarbon starting material may be passed over a catalyst bed or otherwise contacted with the catalyst, the organic hydroperoxide withdrawn and the organic starting material recycled. When run in batch, the amount of starting material converted to organic hydroperoxide will depend on the time at which the reaction is stopped. Example 1 0.1g of solid barium tetranuclear manganese catalyst of the formula Ba₂[Mn₄(O)(OH) (O₂CCH₃)₂L₂] where L is a ligand as defined above, was added to 50g of neat cumene in a flask. Air was bubbled through the reaction mixture at a rate of 30ml/min. The reaction was run at 65°C, 1 atm. (1.01 bar) pressure. After 30 hours, the reaction was stopped. Analysis of the sample by titration showed approximately 40% of the cumene was converted to cumene hydroperoxide. The rate of conversion per hour was approximately 1.3% per hour. Table 1 shows the hydroperoxidation rates of certain aryl alkyl benzenes. The reactions were run under conditions specified in Example 1. Alkyl Benzene Hydroperoxidation Rate (Wt% per hour) isopropyl (cumene) 1.3 ethyl 0.2 sec-butyl 0.18 n-butyl 0.1 n-propyl 0.1 dimethyl (xylene) 0.01 Example 2 0.1g of solid barium tetranuclear manganese catalyst was added to 50g of 40 wt.% cumene hydroperoxide in cumene. The reaction was run at 65°C, 1 atm. (1.01 bar) and samples were taken at from 0 to 100 hours. Samples were analyzed by titration for the loss of cumene hydroperoxide. Within the limits of experimental error (approximately 1%), no decomposition of the cumene hydroperoxide was observed. In the accompanying drawings :- Figure 1 shows the weight percent of cumene hydroperoxide produced from cumene in the presence of air and the catalyst by the process of the present invention contrasted with the amount of cumene hydroperoxide produced in the presence of cumene, air and cumene hydroperoxide as an initiator. The figure shows that the catalyzed process of the present invention increases the reaction rate by a factor of approximately 10 over the process that uses cumene hydroperoxide as an initiator; and Figure 2 shows the lack of cumene hydroperoxide degradation by the catalyst.

Claims: A process for preparing organic hydroperoxides by selectively converting an aryl alkyl hydrocarbon to the corresponding organic hydroperoxide, comprising: contacting an aryl alkyl hydrocarbon wherein the aryl alkyl hydrocarbon contain a benzylic hydrogen of the general formula: wherein R₁ and R₂ are organo groups or hydrogen groups and may be the same or different from each other, and Ar is an aromatic group, with an oxygen-containing gas and a catalytically effective amount of a tetranuclear manganese complex catalyst having the formula: M₂[Mn₄(O)(OH)(O₂CR)₂L₂] wherein M is an alkali metal or an alkaline earth metal, R is hydrogen or a hydrocarbyl group, and L is a ligand having the formula: to produce the corresponding organic hydroperoxides. The process of claim 1 wherein M is selected from magnesium, calcium, strontium, potassium, sodium, lithium, barium and mixtures thereof. The process of claim 2 wherein M is barium. The process of any preceding claim wherein the R group of the tetranuclear manganese complex catalyst is a hydrocarbyl group selected from alkyl groups having from 1 to 30 carbon atoms and aryl alkyl groups. The process of any preceding claim wherein the aryl alkyl hydrocarbon is liquid at operating temperatures and pressures. The process of any preceding claim wherein the aryl alkyl hydrocarbon is soluble in an inert cosolvent. The process of any preceding claim wherein the aryl alkyl hydrocarbon is cumene.

Inventors: GORUN SERGIU MIRCEA, STIBRANY ROBERT TIMOTHY, GORUN, SERGIU MIRCEA, STIBRANY, ROBERT TIMOTHY
Applicants: EXXON RESEARCH ENGINEERING CO, EXXON RESEARCH AND ENGINEERING COMPANY
Application year: 1992
Main classification: C07C409
IPC codes: B01J31, C07B61, C07C409, C07C407