Process for the synthesis of hydrogen peroxide

Abstract: A process is described for the synthesis of hydrogen peroxide by the reaction of oxygen and a primary or secondary alcohol in an alcohol/water biphasic system, in the presence of a catalytic complex consisting of: (a) an organic or inorganic salt of palladium; (b) an aromatic nitrogenated mono or polydentate ligand (Chel) capable of binding itself to the palladium atom; and optionally (c) an organic or inorganic acid.

Description: The present invention relates to a process for the synthesis of hydrogen peroxide by the reaction of oxygen with a primary or secondary alcohol in an alcohol/water biphasic system, which uses a particular catalytic complex. Hydrogen peroxide is a commercially important compound which is widely used as a bleach in the textile and paper industries, in the chemical industry in oxidation processes and in the environmental field as a biocide. Processes for the preparation of hydrogen peroxide by the oxidation of secondary alcohols such as isopropanol and methylbenzylalcohol (US 2.871.102, EP-378.388) or high-boiling secondary alcohols such as diaryl methanol (US 4.303.632) with oxygen, are known and used industrially. These known processes however substantially suffer from disadvantages deriving from the necessity of operating at high reaction temperatures (generally between 100 and 250°C), the partial oxidation of the ketone which is formed as main co-product, the requirement of the presence of a radical generator (for example peroxide) as initiator, the presence of a hydrogen peroxide stabilizer (orthophosphoric or sodium pyrophosphate) and in some cases a salt of a metal such as Co, Fe and Cr. In addition, these processes which are carried out in homogeneous phase are complicated by the necessity of separating and recovering the hydrogen peroxide from the reaction mixture, containing the by-products which are formed in the oxidation reaction. It is one object of the present invention to obtain hydrogen peroxide by a simple, economical process with an easy industrial embodiment, which does not have the disadvantages of the processes of the known art. It has now been found, according to the present invention, that the above object can be reached if the reaction between oxygen and alcohol is carried out in the presence of small quantities of a very active and selective catalytic system, consisting of a palladium salt, an aromatic nitrogenated ligand and optionally an organic or inorganic acid. The use of this catalyst has substantial advantages and in particular the possibility of operating at low temperatures and of also using primary alcohols. In addition, the use of a biphasic system, in which the aqueous solution contains only hydrogen peroxide, allows the organic phase to be reused as an operating solution for subsequent reactions and the possible direct use of hydrogen peroxide in aqueous phase. This has evident benefits from an economical point of view. In accordance with this, the present invention relates to a process for the synthesis of hydrogen peroxide by the reaction of oxygen with a primary or secondary alcohol in an alcohol/water biphasic system, in the presence of a catalytic complex consisting of: (a) an organic or inorganic salt of palladium; (b) an aromatic nitrogenated mono or polydentate ligand (Chel) capable of binding itself to the palladium atom; and optionally (c) an organic or inorganic acid. The catalytic complex, soluble in the organic phase, can be prepared in situ , or it can be a pre-formed complex. In addition the catalytic complex can be supported on inert porous carriers selected from cation exchange resins of the macroporous polymeric type. In the latter case the synthesis of hydrogen peroxide can be carried out without free acid. Monodentate nitrogenated ligands suitable for the purposes of the present invention can be nitrogenated heterocyclic compounds such as pyridine and its derivatives such as, for example, alkylpyridine. Bidentate nitrogenated ligands can be selected from those having general formula (I) wherein: R represents a C 2 -C 4 alkyl radical possibly substituted with an alkyl or aryl group; R 1 -R 4 , the same or different, each represent a C 1 -C 10 alkyl radical, a C 3 -C 10 cycloalkyl radical or a C 6 -C 12 aromatic radical possibly substituted with a C 1 -C 4 alkyl or alkoxyl radical; or from those having general formula (II): wherein: X and Y, the same or different, represent a bridged organic group having at least three atoms in the bridge of which at least two are carbon atoms. When, as well as the carbon atoms, groups X and Y contain other atoms, these are preferably selected from oxygen or nitrogen. Examples of polydentate nitrogenated ligands are: 2,2'-bipyridyl, 4,4'-dimethyl-2,2'bipyridyl, 4,4'-diphenyl-2,2'bipyridyl, 5-methyl-2,2'bipyridyl, 5,5'-dimethyl-2,2'-bypyridyl, 1,10-phenanthroline, 4-methyl-1,10-phenanthroline, 5-methyl-1,10-phenanthroline, 4,7-dimethyl-1,10-phenanthroline, 3,8-dimethyl-1,10-phenanthroline, 4,7-diphenyl-1,10-phenanthroline, 4,7-dichloro-1,10-phenanthroline, 3,4,7,8-tetramethyl-1,10-phenanthroline, 1,2-N,N,N',N'-tetramethylpropanediamine, 1,3,N,N,N',N'-tetramethylethylenediamine, 4,4'-dimethyl-5,5'-bioxazole, 2,2'-bipyrimidine, 2,2'-tetrahydro-bioxazole, 2,9-dichloro-1,10-phenanthroline, 2,9-dibutyl-phenanthroline, 2,9-di-methylphenanthroline, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, 2,4,6-tri-(2-pyridyl)-1,3,5-triazine and 6,7-dimethyl-2,3-di(2-pyridyl)-quinoxaline. Preferred ligands for the purposes of the present invention are 2,4,6-tri-(2-pyridyl)-1,3,5-triazine, 6,7-dimethyl-2,3-di(2-pyridyl)-quinoxaline, and phenanthrolines substituted in position 2,9 such as for example: 2,9-dichloro-1,10-phenanthroline, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline and 2,9-dibutyl- phenanthroline. Advantageous results are obtained using molar ratios Chel/Pd ranging between 1:1 and 20:1, preferably between 1:1 and 3:1. Examples of palladium salts which can be used in the process of the present invention can be selected from palladium acetate, palladium nitrate, palladium sulfate. Palladium acetate is preferred. The quantity of palladium salt used in the process of the present invention is generally between 0.0001 and 0.05 moles per litre of organic phase, preferably between 0.0005 and 0.001 moles per litre of organic phase. Examples of acids which can be used in the process of the present invention are selected from phosphoric acid, acetic acid, sulfuric acid, hydrochloric acid, benzoic acid, p-toluic acid, trimethylbenzoic acid, methanesulfonic acid, benzenesulfonic acid and diphenylphosphinic acid or from perfluorinated acids having the formula C n F 2n+1 COOH with n between 1 and 15. Perfluoro-octanic acid and trifluoroacetic acid are preferred. The quantity of acid used in the process of the present invention is selected so as to obtain a molar ratio acid/palladium ranging from 1/1 to 60/1, preferably between 2/1 and 20/1. The alcohols are selected from linear or branched, primary or secondary aliphatic alcohols substituted by alkyl or aryl residues, immiscible with water. Examples of these alcohols are n-amyl alcohol, n-butanol, cyclohexanol, phenylethyl alcohol, phenylpropyl alcohol, n-octanol, benzyl alcohol, methoxy-benzyl alcohol. Cyclohexanol, phenylethyl alcohol and n-octanol are preferred. The quantity of oxygen used in the reaction is selected so as to maintain a total pressure of more than 1 atm and preferably between 5 and 100 atm. A quantity of O 2 ranging from 5 to 50 atm is conveniently used. According to an embodiment of the process of the present invention, the reaction can be carried out using air instead of pure oxygen. The quantity of water is selected so as to generate a double phase. The volumetric ratio water:alcohol is conveniently between 30:70 and 70:30 and is preferably 60:40. In the process of the present invention the alcohol is oxidized by the palladium which in turn is oxidized to form hydrogen peroxide according to the following scheme: 2R 2 CHOH + Pd 2+ → 2R 2 C=O + Pd 0 + 2H + Pd 0 +O 2 + 2H + → Pd 2+ + H 2 O 2 The ketone generated during the reaction, soluble in the organic phase, does not interfere in the subsequent reaction cycles. The ketone can be reduced, however, to alcohol using the known techniques, or it can be used as reagent in organic synthesis processes. For example, when the alcohol is cyclohexanol, the corresponding ketone, cyclohexanone, can be used for the production of cyclohexanoxine (DE-1245371, EP-208311) and therefore caprolactam. The process of the present invention can be carried out in batch or in continuous using a preformed catalytic complex prepared in situ or supported. For example, in a batch process which uses a catalytic complex prepared in situ , the palladium salt and nitrogenated ligand are dissolved in the organic solvent and the resulting mixture is reacted at room temperature, under stirring, for a time which is necessary to guarantee the complete formation of the ligand/palladium catalytic complex. A primary or secondary alcohol insoluble in water and an aqueous solution of an acid are added to this solution so as to obtain a biphasic system and the mixture is brought to a temperature ranging from 20 to 70°C. Oxygen is then fed at the desired pressure, defined above and the mixture is reacted, under vigorous stirring, at a constant temperature. Solvents which can be used for the preparation of the catalyst used in the process of the present invention can be aliphatic, aromatic hydrocarbons, aliphatic or aromatic chlorinated hydrocarbons, linear or cyclic ethers immiscible with water. Examples of solvents suitable for the purposes of the present invention can be selected from benzene, toluene, chlorobenzene, dichloromethane, acetophenone, chloronaphthalene, 1,2,4-trichlorobenzene, methyl-terbutyl-ketone, methylisobutyl-ketone, p-fluoroacetophenone, p-methylacetophenone, anisol, benzonitrile. The reaction is typically carried out at a temperature of between 20° and 100°C and for a period of time ranging from 15 minutes to 10 hours. It is preferable to operate at a temperature ranging from 20° to 70°C and for a period of time ranging from 30 minutes to 3 hours. At the end of the reaction the aqueous phase, containing the hydrogen peroxide, is separated from the organic phase, containing the catalytic complex and the ketone produced during the reaction, by means of the usual separation techniques. The aqueous phase can be used directly or it can be further concentrated using the known techniques as described for example in patents DE-2.233.159, DE-2.125.199 and DE-1.945.754. The organic phase can be reused in a subsequent synthesis cycle of hydrogen peroxide operating as described above. The process in continuous can be carried out according to the conventional techniques removing the hydrogen peroxide and reintegrating the reagents. The term "hourly turnover of palladium", as used in the present description, relates to the efficiency of the catalyst and is calculated as a molar ratio between the hydrogen peroxide produced in 1 hour and the palladium charged. The following examples, whose sole purpose is to describe the present invention in greater detail, should not be considered as restricting its scope in any way. Example 1 11.2 mg (0.05 mmoles) of palladium acetate, 36 mg (0.1 mmoles) of 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (batocuproine) (molar ratio ligand:palladium = 2:1) and 5 ml of chlorobenzene are charged into a 50 ml glass flask. The mixture is maintained under stirring, at room temperature for 2 hours. The operating solution thus obtained is kept unaltered for several days. 0.5 ml of operating solution, corresponding to 0.005 mmoles of catalyst, are then diluted in 4.5 ml of cyclohexanol. 25 mg (0.06 mmoles) of perfluoro-octanic acid (molar ratio acid:palladium = 12:1) are added to this solution and the mixture is left under stirring for 1 hour at 60°C. 7 ml of water are then added to the mixture and the whole mixture is transferred to an autoclave equipped with a glass ampule and magnetic stirrer. The reactor is pressurized with 5 atms of oxygen and the reaction is carried out under vigorous stirring, at a temperature of 70°C. After 1 hour the reactor is depressurized and the aqueous phase is separated from the organic phase. An aliquot (0.5 ml) of the organic phase is titrated with a solution of potassium permanganate to determine the concentration of hydrogen peroxide, which proves to be equal to 0.54% by weight. This corresponds to an hourly turnover of palladium of 221. Examples 2-10 Effect of the alcohols The reaction is carried out as described in example 1, but using different types of alcohols. The results are shown in table 1. Ex. Alcohol H 2 O 2 TN hr -1 2 1-phenylethanol 0.37 152 3 1-phenylpropanol 0.10 44 4 1-phenylpropan-2-ol 0.30 123 5 2-octanol 0.08 34 6 n-pentanol 0.26 105 7 2-ethyl-hexanol 0.34 138 8 n-octanol 0.33 134 9 benzyl alcohol 0.31 127 10 4-methoxybenzyl alc. 0.08 35 Examples 11-15 Effect of the acid The reaction is carried out under the same operating conditions as example 1, but using various types of acid. The results obtained are indicated in table 2. Ex. Acid H 2 O 2 TN hr -1 11 CF 3 COOH 0.36 148 12 HCl 0.12 49 13 CH 3 COOH 0.18 76 14 H 2 SO 4 0.19 78 15 H 3 PO 4 0.22 91 Examples 16-19 Effect of the oxygen pressure The reaction is carried out under the same operating conditions as example 1, but varying the oxygen pressure. The results are indicated in table 3. Ex. pO 2 atm H 2 O 2 TN hr -1 16 5 0.54 221 17 10 0.545 223 18 20 0.55 226 19 50 0.56 230 The values shown in table 3 show that the reaction is not particularly influenced by the oxygen pressure. Examples 20-21 The reaction is carried out as described in example 1, but using different nitrogenated ligands. The results are indicated in table 4. Ex. ligand H 2 O 2 TN hr -1 20 2,9-dimethylphenanthroline 0.28 115 21 2,4,6-tri-(2-pyridyl)-1,3,5-triazine 0.16 66 Examples 22-23 The reaction is carried out under the same operating conditions as example 1, but at different temperatures. The results are shown in table 5. Ex. T°C H 2 O 2 TN hr -1 22 30 0.24 101 23 50 0.42 173 Example 24 Preparation of the supported catalyst 1 g of strong cation exchange resins of the type Amberlite R IR200 in acid form and Amberlist R 15 (Rhom & Haas) is treated with methanol and chlorobenzene and then dried under vacuum. The resins are then each suspended in 3 ml of operating solution prepared as described in example 1, corresponding to 0.03 mmoles of catalyst. The suspensions are left under stirring for 3 hours at room temperature, until the complete decolouring of the solution. The resins supported by the reaction mixtures are then separated by filtration and dried under vacuum. The resins thus prepared contain 0.03 mmoles of catalyst per g of resin, corresponding to 3 ‰ of Pd. Example 25 Reaction with the supported catayst 0.166 g of heterogeneous catalyst, prepared as described in example 26, corresponding to 0.005 mmoles of palladium, 5 ml of cyclohexanol and 7 ml of distilled water are charged into an autoclave equipped with a glass ampule and magnetic stirrer. The reactor is pressurized with 50 atms of oxygen and the reaction is carried out under vigorous stirring, at a temperature of 70°C. After 1 hour the reactor is depressurized and the aqueous phase is separated from the organic phase. An aliquot (0.5 ml) of the aqueous phase is titrated with a solution of potassium permanganate to determine the concentration of hydrogen peroxide. The results are shown in table 6. Type of resin H 2 O 2 TN hr -1 Amberlite R IR200 0.13 55 Amberlist R 15 0.16 66 Example 26 100 mg (0.44 mmoles) of palladium acetate and, subsequently, 193 mg (0.53 mmoles) of batocuproine are charged into a 250 ml glass flask containing 10 ml of toluene. The solution is left under stirring, at room temperature for 30 minutes, 1.5 ml of trifluoroacetic acid are then added and the mixture is reacted for 2 hours at room temperature. The solution is cooled on ice and hexane is added until the precipitation of yellow crystals. These are left for a night at 4°C, and the crystals of palladium-batocuproine-trifluoroacetate are then filtered. After drying under vacuum, 183 mg of product are obtained, with a yield of 60%. Example 27 3 mg (0.004 mmoles) of the catalytic complex prepared as in example 28, are dissolved in 5 ml of cyclohexanol. 7 ml of a solution of trifluoroacetic acid at 0.1% are then added and the reactor is pressurized with 5 atms of oxygen. The reaction is carried out for 1 hour under vigorous stirring maintaining the temperature at 70°C. At the end of the reaction hydrogen peroxide is obtained at a concentration of 0.45% corresponding to a turnover of palladium of 232 cycles. Example 28 The reaction is carried out under the same experimental conditions as example 1, but using 50 atms of air. After 1 hour hydrogen peroxide is obtained at 0.3%, corresponding to a turnover of palladium of 124 cycles. Example 29 (comparative) 5 ml of cyclohexanol, 7 ml of distilled water and 25 mg of perfluoro-octanic acid are charged into an autoclave equipped with a glass ampule and magnetic stirrer. The reactor is pressurized with 5 atms of oxygen and the reaction is carried out under vigorous stirring, at a temperature of 70°C for 1 hour. After depressurization, an aliquot (0.5 ml) of the aqueous phase is titrated with a solution of potassium permanganate, but no traces of the formation of hydrogen peroxide are revealed. The reaction is repeated under the same conditions charging 50 atms of oxygen, but also in this case there is no formation of hydrogen peroxide.

Claims: A process for the synthesis of hydrogen peroxide by the reaction between oxygen and a primary or secondary alcohol in an alcohol/water biphasic system, characterized in that it is carried out in the presence of a catalytic complex consisting of: (a) an organic or inorganic salt of palladium; (b) an aromatic nitrogenated mono or polydentate ligand (chel) capable of forming stable complexes with the palladium atom. The process according to claim 1, characterized in that the catalytic complex additionally comprises an organic or inorganic acid. The process according to claim 1, characterized in that the palladium salt is selected from palladium acetate, palladium nitrate and palladium sulfate. The process according to claim 3, characterized in that the palladium salt is palladium acetate. The process according to claim 1, characterized in that the monodentate nitrogenated ligand is a nitrogenated heterocyclic compound selected from pyridine and alkylpyridine. The process according to claim 1, characterized in that the polydentate nitrogenated ligand is selected from compounds having formula (I) wherein: R represents a C 2 -C 4 alkyl radical possibly substituted with an alkyl or aryl group; R 1 -R 4 , the same or different, each represent a C 1 -C 10 alkyl radical, a C 3 -C 10 cycloalkyl radical or a C 6 -C 12 aromatic radical possibly substituted with a C 1 -C 4 alkyl or alkoxyl radical. The process according to claim 1, characterized in that the nitrogenated ligand is selected from compounds having formula (II) wherein: X and Y, the same or different, represent a bridged organic group having at least three atoms in the bridge of which at least two are carbon atoms. The process according to claim 7, wherein when, as well as the carbon atoms, groups X and Y contain other atoms, these are preferably selected from oxygen or nitrogen. The process according to claim 1, characterized in that the polydentate nitrogenated ligand is selected from 2,2'-bipyridyl, 4,4'-dimethyl-2,2'bipyridyl, 4,4'-diphenyl-2,2'bipyridyl, 5-methyl-2,2'bipyridyl, 5,5'-dimethyl-2,2'-bypyridy-1,1,10-phenanthroline, 4-methyl-1,10-phenanthroline, 5-methyl-1,10-phenanthroline, 4,7-dimethyl-1,10-phenanthroline, 3,8-dimethyl-1,10-phenanthroline, 4,7-diphenyl-1,10-phenanthroline, 4,7-dichloro-1,10-phenanthroline, 3,4,7,8-tetramethyl-1,10-phenanthroline, 1,2-N,N,N',N'-tetramethylpropanediamine, 1,3,N,N,N',N'-tetramethylethylenediamine,4,4'-dimethyl-5,5'-bioxazole, 2,2'-bipyrimidine, 2,2'-tetrahydro-bioxazole, 2,9-dichloro-1,10-phenanthroline, 2,9-dibutyl-phenanthroline, 2,9-di-methylphenanthroline, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, 2,4,6-tri-(2-pyridyl)-1,3,5-triazine and 6,7-dimethyl-2,3-di(2-pyridyl)-quinoxaline. The process according to claim 9, characterized in that the ligand is selected from 2,4,6-tri-(2-pyridyl)-1,3,5-triazine, 6,7-dimethyl-2,3-di(2-pyridyl)-quinoxaline, and phenanthrolines substituted in position 2,9 such as 2,9-dichloro-1,10-phenanthroline, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline and 2,9-dibutyl-phenanthroline. The process according to the claims from 1 onwards, characterized in that the molar ratio ligand:palladium is between 1:1 and 20:1. The process according to claim 11, characterized in that the molar ratio ligand:palladium is between 1:1 and 3:1. The process according to claim 1, characterized in that the concentration of palladium salt is between 0.0001 and 0.05 moles of palladium salt per litre of organic phase. The process according to claim 13, characterized in that the concentration of palladium salt is between 0.0005 and 0.001 moles of palladium salt per litre of organic phase. The process according to claim 2, characterized in that the acid is selected from phosphoric acid, acetic acid, sulfuric acid, hydrochloric acid, benzoic acid, p-toluic acid, trimethylbenzoic acid, methanesulfonic acid, benzenesulfonic acid and diphenylphosphinic acid or from perfluorinated acids having the formula C n F 2n+1 COOH with n between 1 and 15. The process according to claim 15, wherein the acid is selected from perfluoro-octanoic acid and trifluoroacetic acid. The process according to claim 2, characterized in that the molar ratio acid:palladium is between 1:1 and 60:1. The process according to claim 17, characterized in that the molar ratio acid:palladium is between 2:1 and 20:1. The process according to claim 1, characterized in that the alcohols are selected from linear or branched, primary or secondary aliphatic alcohols substituted by alkyl or aryl residues, immiscible with water. The process according to claim 19, characterized in that the alcohols are selected from n-amyl alcohol, n-butanol, cyclohexanol, phenylethyl alcohol, phenylpropyl alcohol, n-octanol, benzyl alcohol, methoxy-benzyl alcohol. The process according to claim 20, wherein the alcohol is selected from cyclohexanol, phenylethyl alcohol and n-octanol. The process according to claim 1, characterized in that the reaction is carried out at an oxygen pressure ranging from 1 to 100 atm. The process according to claim 22, characterized in that the pressure is between 5 atm and 70 atm. The process according to claim 1, wherein the reaction is carried out using air as oxygen source. The process according to claim 1, characterized in that the reaction is carried out at a temperature ranging from 20 to 100°C. The process according to claim 25, wherein the temperature is between 30°C and 70°C. The process according to claim 1, characterized in that the catalytic complex is used in the form of preformed complex. The process according to claim 1, characterized in that the catalytic complex is prepared in situ in an organic solvent immiscible with water selected from aliphatic, aromatic hydrocarbons, aliphatic or aromatic chlorinated hydrocarbons, linear or cyclic ethers, ketones, or their mixtures. The process according to claim 28, characterized in that the organic solvent is selected from benzene, toluene, chlorobenzene, anisol, benzonitrile, chloronaphthalene, methyl-terbutyl-ketone, methyl-isobutyl-ketone, 1,2,4-trichlorobenzene, acetophenone, p-fluoroacetophenone, p-methylacetophenone. The process according to claim 1, characterized in that the catalytic complex is immobilized on an inert porous carrier selected from cation exchange resins of the macroporous polymeric type.

Inventors: BIANCHI DANIELE, BORTOLO ROSSELLA, D ALOISIO RINO, QUERCI CECILIA, SOATTINI SERGIO, BIANCHI, DANIELE, BORTOLO, ROSSELLA, D'ALOISIO, RINO, QUERCI, CECILIA, SOATTINI, SERGIO
Applicants: ENICHEM SPA, ENICHEM S.P.A.
Application year: 1997
Main classification: C01B15
IPC codes: C01B15