WO2012041918A2

WO2012041918A2 - Process for the manufacture of hydrogen peroxide

Info

Publication number: WO2012041918A2
Application number: PCT/EP2011/066893
Authority: WO
Inventors: Frédérique Jacqueline DESMEDT; Philip Hodge; Francine Janssens
Original assignee: Solvay Sa
Priority date: 2010-09-28
Filing date: 2011-09-28
Publication date: 2012-04-05
Also published as: WO2012041918A3

Abstract

The present invention relates to a process for the preparation of aqueous hydrogen peroxide from the anthraquinone (AO) loop process, comprising using at least one alkylanthraquinone of formula (I) wherein -R1, R2 and R3 are chosen from H, methyl radical, isopropyl radical, n-propyl radical, and tert-amyl radical, and -at least two of the three groups R1, R2 and R3 are different from H and are different from each other.

Description

Process for the manufacture of hydrogen peroxide

This application claims priority to European application No. 10181367.3 filed on September 28, 2010, the whole content of this application being incorporated herein by reference for all purposes.

The present invention relates to a process for the manufacture of hydrogen peroxide. In particular, it is related to the manufacture of hydrogen peroxide from the anthraquinone loop (AO) process, comprising using specific

alkylanthraquinones. The present invention also relates to a process for the synthesis of said specific alkylanthraquinones.

Hydrogen peroxide is one of the most important inorganic chemicals to be produced worldwide. The world production of H₂0₂ grew to 2.2 million tons

(100 % H₂0₂) in 2007. Its industrial applications includes textile, pulp and paper bleaching, organic synthesis (propylene oxide), the manufacture of inorganic chemicals and detergents, environmental and other applications.

Synthesis of hydrogen peroxide is predominantly achieved by using the Riedl-Pfleiderer process, also called anthraquinone loop process or AO process. A survey of this well-known anthraquinone process and its numerous

modifications is given in the "Ullmann's Encyclopedia of Industrial Chemistry", Chapter "Hydrogen Peroxide" from G. Goor et al, DOI :

10.1002/14356007.al3_443.pub2 (2007) and in the "Kirk-Othmer Encyclopedia of Chemical Technology", Chapter "Hydrogen Peroxide" from W. Eul et al, DOI : 10.1002/0471238961.0825041808051919.a01.pub2 (2001) which are incorporated herein by reference. For each of the distinct process steps, the Ullmann and Kirk-Othmer references disclose numerous different possibilities.

This well known cyclic process makes use typically of the auto-oxidation of at least one alkylanthrahydroquinone and/or of at least one

tetrahydroalkylanthrahydroquinone, most often 2-alkylanthraquinone, to the corresponding alkylanthraquinone and/or tetrahydroalkylanthraquinone, which results in the production of hydrogen peroxide. The term "alkylanthraquinone" is intended to denote 9, 10-anthraquinones substituted with at least one alkyl side chain of linear or branched aliphatic type comprising at least one carbon atom. These alkyl chains usually comprise less than 9 carbon atoms and preferably less than 6 carbon atoms. Typical examples of such alkylanthraquinones are 2-ethylanthraquinone, 2-isopropylanthraquinone, 2-sec- and

2-tert-butylanthraquinone, 1,3-, 2,3-, 1,4- and 2,7-dimethylanthraquinone, 2-iso- and 2-tert-amylanthraquinone, and mixtures thereof. Those alkylanthraquinones are disclosed e.g. in US 6,224,845. The term "tetrahydroalkylanthraquinone" is intended to denote alkyl-5,6,7,8-tetrahydro-9, 10-anthraquinones substituted with at least one alkyl side chain as defined for the alkylanthraquinones.

Alkylanthraquinones with one alkyl substituent are also disclosed e.g.

in GB 943 683.

The first step of the AO process is the reduction in an organic solvent of the chosen quinone (alkylanthraquinone or tetrahydroalkylanthraquinone) into the corresponding hydroquinone (alkylanthrahydroquinone or

tetrahydroalkylanthraquinone) using hydrogen gas and a catalyst. The mixture of organic solvents, hydroquinone and quinone species (working solution, WS) is then separated from the catalyst and the hydroquinone is oxidized using oxygen, air or oxygen-enriched air thus regenerating the quinone with simultaneous formation of hydrogen peroxide. The organic solvent of choice is typically a mixture of two types of solvents, one being a good solvent of the quinone derivative (for instance a mixture of aromatic compounds) and the other being a good solvent of the hydroquinone derivative (for instance a long chain alcohol). Hydrogen peroxide is then typically extracted with water and recovered in the form of a crude aqueous hydrogen peroxide solution, and the quinone is returned to the hydrogenator to complete the loop.

A large number of variations of the Riedl-Pfleiderer process have been described. They mainly relate to the optimization of the working solution using novel combinations of solvents and/or anthraquinone either in term of the anthraquinone species used, their respective proportions and/or in term of the nature or respective proportion of the solvent mixture.

For instance, US 4,428,923, US 3,041, 143 and US 5, 107,004 describe processes for the production of hydrogen peroxide by the AO process, using respectively 2-ethylanthraquinone, 2-amylanthraquinone and

2-neopentylanthraquinone.

Despite the numerous known alternatives, there is still a need for new anthraquinones which could exhibit advantages in the AO process, such as a higher stability, thus allowing a greater number of hydrogen peroxide production cycles to be carried out for a given amount of quinone. A higher solubility of the anthraquinone and of the corresponding hydroquinone in the solvents of the working solution could also lead to an improved productivity of the process, the concentration of the produced hydrogen peroxide being directly linked to the concentration of the anthraquinone and corresponding hydroquinone in the working solution. In addition, there is a need for anthraquinones exhibiting good properties that could be synthesized using an environmentally friendly process.

It is known to prepare substituted 9, 10-alkylanthraquinones from phthalic anhydride and alkylbenzenes, by Friedel-Crafts acylation in the presence of AICI3 catalyst, followed by acid-catalyzed cyclisation of the mixture of resulting benzophenone derivatives, but such reaction leads to the formation of aluminum salts which must be disposed of.

It is also known, for instance from US 6, 127,580, to prepare substituted 9, 10-anthraquinones starting from 1,4-naphtoquinone and substituted 1,3-dienes. In this process, in a first synthesis step, 1,4-naphthoquinone is reacted with a substituted 1,3-diene in a cycloaddition reaction (Diels- Alder reaction) to give a substituted tetrahydroanthraquinone (Diels- Alder or cycloaddition adduct). In a second synthesis step, the oxidation of the Diels- Alder adduct is carried out to give the substituted anthraquinone.

The purpose of the present invention is to provide an improved process for the preparation of hydrogen peroxide, in particular by the selection of alkylanthraquinones and corresponding alkylanthrahydroquinones leading to improved properties in the AO process. Particularly, said alkylanthraquinones and corresponding alkylanthrahydroquinones should have a good stability in the AO process as well as a good solubility in the solvents of the working solution. It would also be advantageous if said alkylanthraquinones could be synthesized by processes as environmentally friendly as possible, for instance starting from renewable raw materials, by avoiding the formation of by-products, and/or by limiting the amount of wastes.

The present invention therefore relates to a process for the preparation of aqueous hydrogen peroxide from the AO process, comprising using at least one alkylanthraquinone of formula I

Formula I wherein

- Rl, R2 and R3 are chosen from H, methyl radical, isopropyl radical, n-propyl radical, and tert-amyl radical, and

- at least two of the three groups Rl, R2 and R3 are different from H and are different from each other.

Indeed, it has been surprisingly found that the alkylanthraquinones as defined by formula I, as well as the corresponding hydrogenated quinones, exhibit a good stability in the AO process as well as a good solubility in the working solution solvents. They show a better productivity than e.g. the dialkylanthraquinones known from US 6,224,845.

The alkylanthraquinones of formula I can be used alone, combined together, or combined with other kinds of alkylanthraquinones, preferably alone or combined together.

In a first embodiment, one of the three groups Rl, R2 and R3 is H. In a second embodiment, one of the three groups Rl, R2 and R3 is an isopropyl or a tert-amyl radical. In a third embodiment, residue R2 is hydrogen. In an especially preferred embodiment,

- Rl is a methyl radical, R2 is H, and R3 is an isopropyl radical, or

- Rl is a methyl radical, R2 is an isopropyl radical, and R3 is H, or

- Rl is a n-propyl radical, R2 is an isopropyl radical, and R3 is H, or

- Rl is an isopropyl radical, R2 is a n-propyl radical, and R3 is H, or

- Rl is an isopropyl radical, R2 is H, and R3 is a n-propyl radical, or

- Rl is a methyl radical, R2 is H and R3 is a tert-amyl radical.

Said quinones correspond respectively to 1 -methyl -4- isopropylanthraquinone (MiPQ), l-methyl-3-diisopropylanthraquinone, l-n-propyl-3-isopropylanthraquinone, l-isopropyl-3-n-propylanthraquinone, 1 -isopropyl-4-n-propylanthraquinone, and 1 -methyl-4-tert-amylanthraquinone.

In the process of the present invention, the alkylanthraquinones of formula I can be dissolved in various types of solvents, especially in the solvents typically used in the working solution of the well known AO process. For instance, the alkylanthraquinones of formula I can be dissolved in a single solvent or in a mixed solvent comprising at least one aromatic solvent and at least one aliphatic or alicyclic alcohol, particularly in a mixed solvent. Aromatic solvents are for instance selected from benzene, toluene, xylene, tert- butylbenzene, trimethylbenzene, tetramethylbenzene, naphthalene,

methylnaphthalene mixtures of polyalkylated benzenes, and mixtures thereof. Aliphatic or alicyclic alcohols are for example selected from amyl alcohol, nonyl alcohol, isoheptyl alcohol, diisobutylcarbinol, methylcyclohexanol, and mixtures thereof. Useful single solvents are, among others, a ketone, an ester, an ether, or mixtures thereof.

It is thus provided a process for the preparation of aqueous hydrogen peroxide which comprises the following steps :

a) hydrogenation, in the presence of a catalyst, of a working solution comprising at least one organic solvent and at least one alkylanthraquinone of formula I to obtain the corresponding alkylanthrahydroquinone,

b) separation of the hydrogenated working solution comprising the

alkylanthrahydroquinone from the catalyst,

c) oxidation of the recovered hydrogenated working solution from step b) to form hydrogen peroxide,

d) separation, from the working solution, of said hydrogen peroxide during and/or subsequently to said oxidation step, preferably with an aqueous medium, and

e) recycling of the recovered working solution to step a).

The process of the present invention is typically conducted in the conditions usually used for the AO process. Thus, the hydrogenation reaction is most often conducted at a temperature from 0 to 80°C, preferably from 45 to 80°C, at a pressure from 0.2 barg to 5 barg. The hydrogenation reaction is further conducted in the presence of a hydrogenation catalyst that may be selected from the group consisting of palladium black, Raney nickel or supported palladium. The oxidation reaction is generally conducted at a temperature from 0 to 60°C, at atmospheric pressure or above. The anthraquinone is in general added into the working solution at a concentration as high as possible, commonly close to the solubility limit of the alkylanthrahydroquinone in the working solution.

In the process of the present invention, the alkylanthraquinones of formula I may be obtained by any method known in the art such as by Friedel- Crafts acylation followed by cyclisation. The alkylanthraquinones of formula I may also be prepared by a Diels- Alder (or cycloaddition) reaction followed by oxidation of the Diels- Alder (or cycloaddition) adduct in the presence of a base, namely by :

(a) reacting 1,4-naphthoquinone with a substituted 1,3-diene of formula II

Formula II

wherein Rl, R2 and R3 are as defined above for the alkylanthraquinones of formula I,

in the presence of a solvent, in a cycloaddition step to give the

corresponding Diels- Alder (or cycloaddition) adduct, and

(b) oxidizing the Diels- Alder adduct of step (a) with oxygen in the presence of a base to give the corresponding anthraquinone of Formula I.

Such processes for the preparation of alkylanthraquinones, starting from 1,4-naphthoquinone and the corresponding substituted 1,3-diene are known in the art and are described for instance in US 6, 127,580 or US 6,399,795 Bl, which are incorporated herein by reference.

In view of the above, the invention is also directed to hydrogen peroxide, purified or not, obtained or obtainable by using the process above described.

In another aspect, the present invention also relates to a process for the preparation of the anthraquinones of formula I, said process comprising :

(a) reacting 1,4-naphthoquinone with a substituted 1,3-cyclohexadiene of

formula III

Formula III

in the presence of a solvent, in a cycloaddition step to give the

corresponding Diels- Alder (or cycloaddition) adduct,

(b) oxidizing the Diels- Alder adduct of step (a) with oxygen in the presence of a base to give the corresponding oxidized Diels- Alder adduct, and (c) eliminating ethylene from the oxidized Diels- Alder adduct of step (b) by heating to give the anthraquinone of Formula I.

This new synthesis route has the advantage that no by-products are formed and there is no catalyst to be disposed of, which renders this route more environmentally friendly than classical Friedel-Crafts acylation, often used for the preparation of alkylanthraquinones. Compared to the known Diels- Alder (or cycloaddition) starting from substituted 1,3-diene of formula II, this process has the advantage that other starting materials are used, i.e. substituted

1,3-cyclohexadienes of formula III, which could be easier to obtain. This process is especially advantageous when starting from alpha-terpinene

(formula III wherein Rl is a methyl radical, R2 is H and R3 is an isopropyl radical) which is a natural product that can be isolated, amongst others, from cardamom and marjoram oils. This new synthesis route is thus especially environmentally friendly, leading to no by- products and being at least partially based on renewable and green raw materials.

In the cycloaddition step (a), the 1,4-naphthoquinone and the substituted 1,3-diene of formula II can be present in a stoichiometric ratio, or the substituted

1.3- diene can be present in an excess or in a deficient amount. Both products are preferably present in a substantially stoichiometric ratio or with a slight excess of the substituted 1,3-diene, such as with a molar ratio of substituted 1,3-diene /

1.4- naphthoquinone from 1,05: 1 to 1,5: 1, for instance about 1, 1 to 1,2: 1. The cycloaddition step (a) is commonly conducted at a temperature from 50 to 170°C, preferably from 70 to 130°C, typically from 80 to 110°C for instance about 90 or 100°C. The reaction time is usually adapted to attain almost completion of the cycloaddition reaction and can thus vary widely, for example from 0.5

to 100 hours. Step (a) can be carried out at atmospheric pressure, under reduced pressure or under elevated pressure, conveniently at atmospheric pressure. The cycloaddition step (a) can be conducted in the absence of any solvent, in particular if at least one of the 1,4-naphthoquinone or substituted 1,3-diene of formula II is in the form of a liquid under the reaction conditions, and preferably, if the other compound is in the form of a solid, provided that said other compound is at least partially soluble in the first liquid compound. The cycloaddition step (a) can also be conducted in the presence of a solvent. If the cycloaddition step (a) is conducted in the presence of a solvent, the starting components can be present as a solution or dispersion, i.e. as an emulsion, suspension or 3 -phase mixture (solid/liquid/liquid), it also being possible for the solvent to contain small amounts of one or both starting components in at least partially dissolved form. Preferably, the starting components and the solvent are selected such that, at least after heating to the reaction temperature, at least one component is present in solubilized form or in liquid form (thus forming an emulsion with the solvent), preferably both components are present in liquid form or in solubilized form. Step (a) can be carried out under air atmosphere or under a protective gas atmosphere, such as nitrogen or argon atmosphere.

Step (a) is most often carried out in the absence of a catalyst, but the use of a catalyst like a Lewis acid, for example boron trifluoride, is possible.

In step (b) of this process, it has now been found that it is recommended to add the base used in the oxidation step (b) progressively (or in portions) to the reaction medium of step (a), prior to the addition of oxygen. It has indeed been found that, if the base is added at once, as taught in prior art documents related to the process based on substituted 1,3-dienes such as US 6, 127,580

or US 6,399,795 Bl, when the oxygen is subsequently added a thermal runaway of the reaction is observed, which leads to unwanted spitting and explosion risks. On the contrary, if the base is added in portions to the reaction medium of step (a), prior to the addition of oxygen, the oxidation reaction is more controlled and the risk of thermal runaway can be avoided. In this aspect of the invention, it is therefore recommended to add the base in portions to the reaction medium, i.e. progressively. For instance, the base can be added by fractions which will depend on the amount of reaction medium and of base to be added, so that the total amount of base is added in a reasonable amount of time. For instance, 10 g of base can be added in 5 minutes (about 1 g each 30 seconds).

The base can be added into the reaction medium as a pure product or as a solution in part of the solvent, for instance with a concentration around 10 to 50 wt %, such as 25 wt %. Said base is typically added in an amount of from 0.01 to 10 mol per mol of the cycloaddition or Diels- Alder adduct, especially from 0.02 to 5 mol. Said base is conveniently selected such that it is soluble in the solvent. Suitable bases can for example be selected from the group consisting of ammonia, ammonium hydroxides and salts thereof, sodium acetate, mono-, di-, trialkylamines and cycloaliphatic amines, alkali metal and alkaline earth metal hydroxides, hydroxide forms of strong base anion exchange resins, and mixtures thereof, frequently at least from alkali and alkaline earth metal hydroxides. Amines are for example ethylamine, diethylamine, triethylamine, mono-, di- and tripropylamine, mono-, di- and tributylamine, ethylenediamine, diethylenetriamine, morpholine, N-methylmorpholine, isophoronediamine, l,4-diazabicyclo-2,2,2-octane, or l,8-diazabicyclo-5,4,0-undec-7-ene (DBU). Alkali and alkaline earth metal hydroxides are for instance sodium, potassium or lithium hydroxide, especially sodium hydroxide. Ammoniums can be selected, among others, from tetramethyl and tetrabutyl ammonium hydroxides and salts thereof with mineral acids. If a mixture of a polar and a nonpolar solvent is used, it can be advantageous to use a mixture of an inorganic and an organic base, as disclosed in US 6,399,795 Bl, which is incorporated herein by reference.

The oxidation step (b) can be conducted at a temperature from 0 to 170°C, especially from 20 to 130°C, more particularly from 20 to 50°C, for instance at room temperature. The reaction time is usually adapted to attain almost completion of the oxidation reaction and can thus vary widely, especially from 0.5 to 24 hours, frequently from 2 to 10 hours, for example from 4 to 6 hours. Step (b) can be carried out at atmospheric pressure, under reduced pressure or under elevated pressure, conveniently at atmospheric pressure. The oxygen can be selected from oxygen in pure form or in the presence of inert gases, for instance in the form of air. Said oxygen is advantageously brought into contact intensively with the liquid reaction medium. This can be achieved by passing the oxygen flow, preferably in finely divided form, through the liquid reaction medium. However, the oxygen can also be fed into the gas space above the reaction medium and brought into contact with the reaction medium by intensive stirring. In the oxidation step (b), the Diels- Alder adduct can be present as a solution or as a dispersion into the solvent, advantageously as a solution.

The solvents used in the oxidation step (b) and optionally in the

cycloaddition step (a) can for instance be selected from water ; alcohols such as methanol, ethanol, isopropanol, n-propanol, n-butanol, isobutanol, octanol, and diisobutylcarbinol ; ketones such as acetone and methyl ethyl ketone ; ethers such as tetrahydrofurane and dioxane ; esters such as ethyl acetate and methyl cyclohexyl acetate ; nonpolar hydrocarbons such as cyclohexane, benzene, toluene, xylene, trimethylbenzenes, and tetramethylbenzenes ; and mixtures thereof, including mixtures of a polar solvent and of a nonpolar hydrocarbon solvent. The solvent is frequently chosen from water, alcohols and mixtures thereof, for instance water, ethanol and mixtures thereof. If solvents are used in both the cycloaddition step (a) and in the oxidation step (b), they can be the same or different, conveniently the same. The amount of solvent is typically adapted to the solubility of the starting materials and to the solubility of the resulting oxidized Diels- Alder adduct. The amount of solvent can for instance represent from 50 to 95 wt % of the total reaction mixture.

In step (c) of the process of the invention, ethylene is eliminated from the oxidized Diels- Alder adduct of step (b) by heating. This is an exothermic reaction that must be carried out with care to avoid a dangerous evolution of gas. The heating can be performed at any temperature sufficient to allow elimination of ethylene from the oxidized Diels- Alder adduct of step (b), typically at a temperature from 100 to 200°C, particularly from 120 to 180°C, more particularly from 140 to 170°C. Said heating may be performed in the absence or in the presence of a solvent, preferably in the presence of a solvent. If a solvent is used, it is most often selected from organic solvents, especially from organic solvents having a high boiling point, in particular from organic solvents having a boiling point of at least 100°C, preferably more than 100°C, more preferably at least 120°C, most preferably at least 140°C. For instance, the solvent may be selected from diisobutylcarbinol, 1,2-dichlorobenzene, cyclohexyl acetate, diglyme, sulfolane, t-amyl benzene, t-butyl benzene, Exxsol 150, Shellsol 150, Solvesso 150, and mixtures thereof. If a solvent is used, its amount is typically adapted to the solubility of the oxidized Diels- Alder and to the solubility of the resulting anthraquinone. The amount of solvent can for instance represent from 50 to 95 wt % of the total reaction mixture.

Advantageously, a solvent is present and is selected from diisobutylcarbinol and 1,2-dichlorobenzene, particularly diisobutylcarbinol.

In a variant of the process of the present invention, the oxidized Diels- Alder adduct of step (b) may be dissolved in a first solvent and the resulting solution may be added progressively to a second solvent, heated at a temperature sufficient to allow elimination of ethylene from the oxidized Diels- Alder adduct. In this variant, the first solvent used to dissolve the oxidized Diels- Alder adduct of step (b) may be the same or different than the second heated solvent. If the first solvent is different from the second solvent, it is possible to use a solvent having a lower boiling point, for instance a solvent having a boiling point below 100°C. Said first solvent will then be eliminated from the reaction medium by evaporation when contacting the second heated solvent. This variant is especially advantageous as it allows the progressive heating of the oxidized Diels- Alder adduct of step (b), allowing a better control of the reaction and of the release of the ethylene. In a further variant of the process of the present invention, which may be combined with any of the preceding conditions, an inert gas may be passed through the reaction medium of step (c), to entrain the formed ethylene and the optionally present low boiling point solvent. Such inert gas may for instance be nitrogen or argon.

In a particular embodiment, additional steps can take place between the cycloaddition step (a) and the oxidation step (b), in which the Diels- Alder adduct formed in the cycloaddition step (a) is separated from the reaction medium, before being redissolved into a solvent and engaged into the oxidation step (b).

According to this particular embodiment, the present invention relates to a process for the preparation of the anthraquinones of formula I, said process comprising :

(a) reacting 1,4-naphthoquinone with a substituted 1,3-cyclohexadiene of

formula III

Formula III

in the presence of a solvent, in a cycloaddition step to give the

corresponding Diels- Alder (or cycloaddition) adduct,

(b) separating the Diels- Alder adduct from the reaction medium of step (a),

(c) dissolving the Diels- Alder adduct into a solvent,

(d) oxidizing the Diels- Alder adduct with oxygen in the presence of a base to give the corresponding oxidized Diels- Alder adduct, and

(e) eliminating ethylene from the oxidized Diels- Alder adduct by heating to give the anthraquinone of Formula I.

According to this particular embodiment, the separation step (b) may be conducted by any suitable separation method known in the art. For instance, the Diels- Alder adduct can be separated by crystallization at low temperature and subsequent solid/liquid separation, for example by centrifugation or filtration, optionally after removal of part of the solvent. According to this specific example, at least part of the solvent can be removed from the reaction medium, for instance by distillation or by evaporation under reduced pressure, in particular at least 30 wt % of the solvent amount, and most often at most 70 wt % of the solvent amount, for instance about half the solvent amount is removed. The resulting mixture can then be cooled down for example to -50 to 10°C, more often from -30 to 0°C, most often around -20 or -10°C, so that at least part of the Diels- Alder adduct crystallizes. The formed crystals can then be filtered, optionally washed, and dried. The drying can for example be conducted under vacuum at room temperature.

An additional advantage of removing at least part of the solvent by distillation or evaporation under vacuum in separation step (b) is the concomitant removal of at least part of volatile minor constituents that can be present in the reaction medium of step (a), at least part of said volatile minor constituents being removed with the distillate or evaporate. Said volatile minor constituents can be present in the starting components as minor constituents and/or may be formed during the cycloaddition step (a).

In this particular embodiment, the isolated Diels- Alder adduct is then dissolved into a solvent (step (c)) before being oxidized with oxygen in step (d), as described above.

In a further particular embodiment, at the end of the oxidation step leading to the oxidized Diels- Alder adduct and before the elimination of ethylene to lead to the anthraquinone of formula I, the reaction medium may optionally first be neutralized. Neutralization of the reaction medium, and in particular of the base added during the oxidation step, can be done by addition of an acid to the reaction medium, for instance by the addition of a diluted inorganic acid such as diluted H₂S0₄, diluted HC1 or diluted HN0₃, preferably diluted H₂S0₄. The acid is typically added in an amount such that a pH around 7 to 8 is attained.

The process for the preparation of the anthraquinones of formula I according to the present invention can be conducted in any kind of suitable reactor, in particular in a stirred vessel, in a loop reactor or in a bubble column, the reactor being advantageously equipped with reflux cooling.

In the process for the preparation of the anthraquinones of formula I, after elimination of ethylene from the oxidized Diels- Alder adduct, the formed anthraquinone of formula I can be recovered from the reaction medium by any suitable method known in the art. The anthraquinone can be removed in solid or in liquid form. By removal in solid form, it is intended that the anthraquinone can be removed from the reaction medium by crystallization at low temperature, for instance at room temperature or below, and subsequent solid/liquid separation, for example by centrifugation or filtration. Customary steps for the purification of the crystallizate can follow the removal, for example by washing with a suitable solvent and/or by recrystallization from an organic solvent. The crystallizate can also be purified by distillation after fusion. By removal in liquid form, it is intended that the anthraquinone can be removed from the reaction medium by extraction with an organic solvent or by separation of the organic phase, provided that the reaction medium is present respectively in the form of a mainly aqueous medium or in the form of an emulsion or of a two phase mixture. The anthraquinone can then be recovered from the organic phase by customary purification operations, such as extraction of the organic phase with water, distillation of the organic phase and/or crystallization of the anthraquinone from an organic solvent, for example by washing of the organic phase with water and subsequent distillation of the organic phase, for instance using a wiper blade evaporator, or by recrystallization from the organic phase.

The present invention also relates to the use of an anthraquinone of formula I for the manufacturing of hydrogen peroxide.

The present invention is further illustrated below without limiting the scope thereto.

Should the disclosure of any patents, patent applications, and publications which are incorporated herein by reference conflict with the description of the present application to the extent that it might render a term unclear, the present description shall take precedence.

Examples

Example 1 : preparation of l-methyl-4-isopropylanthraquinone (MiPQ)

14.93 g of 1, 4-naphthoquinone (0.0944 mol) were dissolved in 250 ml of ethanol and then 15.12 g of alpha-terpinene (0.111 mol) were added. The reaction mixture was heated under stirring and under reflux at about 90°C for approximately 80 h. The reaction mixture was left to cool down at room temperature. After the reaction mixture was cooled down to room temperature, about half of the solvent amount was removed by evaporation under reduced pressure at about 20°C. The resulting reaction mixture was put in the freezer overnight (about -20°C). As a result, white / grey crystals formed, which were then filtered and dried under vacuum. Said crystals corresponded to the

Diels- Alder adduct (mp : 101-102°C, yield : 25.29 g - 91 %). 2.32 g of the Diels- Alder adduct (7.51 mmol) were dissolved in 50 ml of absolute ethanol. 5 equivalents of sodium hydroxide (1.5 g -37.5 mmol) were added in portions (pellet by pellet) to the solution. Subsequently, an air stream was bubbled through the reaction mixture (low flow, with discrete bubbles). The reaction mixture was stirred at room temperature while bubbling air through the solution during 5 hours. As a result, the reaction mixture solidified and the color changed from dark green into purple. The resulting reaction mixture was worked-up by acidifying the mixture with diluted H₂SO₄ (concentration about 10 wt %) until a pH around 7 to 8 was attained, after which the precipitate was filtered off, washed with water and dried under vacuum at 60°C, and 1.78 g of the corresponding oxidized Diels- Alder adduct were recovered

(mp : 116-117°C, yield : 81 %).

The oxidized Diels- Alder adduct was dissolved in a minimum of dichloromethane (10 ml) and the resulting solution was added dropwise to diisobutylcarbinol (100 ml) at 160°C under magnetic stirring. As a result, the dichloromethane was immediately evaporated off and elimination of ethylene was observed by the presence of small bubbles forming into the reaction medium. The heating was continued until no more formation of bubbles was observed into the reaction medium (about 30 minutes), then the medium was left to cool down. The solvent was evaporated under vacuum and the final product, corresponding to l-methyl-4-isopropyl-anthraquinone, was recrystallized in ethanol at about -20°C (mp : 90°C, yield : 1.57 g - 79 %).

Testing of the alkylanthraquinones

Each alkylanthraquinone was dissolved in a mixture of solvents in an amount of 70 g of alkylanthraquinone per kg of solvent mixture. 200 g of the solution were hydrogenated at 75°C under hydrogen atmosphere in the presence of 2 g of catalyst (Pd reduced on alumina), up to a hydrogenation level of maximum 96 %. The temperature was then decreased to 55°C. The

hydrogenated solution was kept for about 48 hours at 55°C and presence of a precipitate was checked.

A sample of the hydrogenated solution (1-2 g) was filtered to eliminate the catalyst and was then oxidized with oxygen gas (bubbled through the solution) at room temperature during 30 minutes.

The amount of hydrogen peroxide produced was determined by

colorimetry using the following method. H₂0₂ calibration solutions containing 0 to 3 mg/100 ml of H₂0₂ were prepared by mixing 50 ml of a titanium salt reactant (titanium 1 g/1 in sulfuric acid, further diluted with water in a ratio 1 :3), a well-known quantity of H₂0₂ solution having a concentration of 1 g/kg and water up to a total volume of 100 ml. The wavelength of a spectrometer was fixed at 410 nm and the absorbance of the H₂0₂ calibration solutions was measured to establish a calibration curve. Samples (about 150 μΐ) of working solution were removed before the oxidation step, were oxidized with oxygen and the amount of hydrogen peroxide was determined by the colorimetric method after mixing with the titanium reactant.

The amount of hydrogenated anthraquinones (QH) was calculated as 1 mol of QH corresponds to 1 mol of H₂0₂.

The results are summarized in the table below. The alkylanthraquinone of example 1 were tested against comparative alkylanthraquinones, as detailed in said table.

Alkyl Solvent mixture Solubility Hydrogenated H202 radical (weight ratio) during the alkylanthraproduced of the hydrogenation quinone (g kg) anthraquin step (QH) (g kg)

one

Ex. 1 l-methyl-4- Solvesso^1M 150 / No precipitation 16 2.06 isopropyl diisobutylcarbinol at 96 % of

80:20 hydrogenation

Comp. 2-ethyl Solvesso^1M 150 / Precipitation at 17 2.40 Ex. 2 diisobutylcarbinol 61 % of

80:20 hydrogenation

Comp. 2-neopentyl Solvesso^1M 150 / Precipitation at 20 2.46 Ex. 3 diisobutylcarbinol 70 % of

80:20 hydrogenation

Comp. 2-neopentyl Solvesso / Sextate Precipitation 18 2.20 Ex. 4 80:20 even below

70 % of

hydrogenation

Comp Mix of Solvesso ^1M 150 / Precipitation at 35 4.50 Ex. 5 1.3- diEthyl diisobutylcarbinol 43 % of

(69 %), 80:20 hydrogenation

1.4- diEthyl

(27.8 %)

and

2,3-diEthyl

(3.2 %)

Comp Mix of Solvesso / Sextate Precipitation at 26 3.35 Ex. 6 1.3- diEthyl 70:30 34 % of

(69 %), hydrogenation

1.4- diEthyl

(27.8 %)

and

2,3-diEthyl

(3.2 %) Productivity is defined as quantity of H₂0₂ produced with given quantity of working solution (WS) and expressed in grams of H₂0₂ per kilogram of working solution. A typical productivity in the state-of-the-art AO processes is usually of maximum about 15 g H₂0₂ / kg of WS. The alkylanthraquinones according to the present invention thus allow a good H₂0₂ productivity while showing an improved solubility during the hydrogenation step, which is highly suitable in the AO process.

Claims

C L A I M S

1. Process for the preparation of aqueous hydrogen peroxide from the anthraquinone (AO) loop process, comprising using at least one

alkylanthraquinone of formula I

Formula I wherein

- Rl, R2 and R3 are chosen from H, methyl radical, isopropyl radical, n-propyl radical, and tert-amyl radical, and - at least two of the three groups Rl, R2 and R3 are different from H and are different from each other.

2. Process according to claim 1, wherein the alkylanthraquinone of formula I is dissolved in a mixed solvent comprising at least one aromatic solvent and at least one aliphatic or alicyclic alcohol. 3. Process according to claim 1 or 2, wherein the alkylanthraquinone of formula I has been prepared by :

(a) reacting 1,4-naphthoquinone with a substituted 1,

3-diene of formula II

Formula II wherein - Rl, R2 and R3 areas defined in claim 1, in the presence of a solvent, in a cycloaddition step to give the

corresponding Diels- Alder adduct, and

4. Process for the preparation of an anthraquinone of formula I

Formula I wherein

- Rl, R2 and R3 areas defined in claim 1, comprising :

(a) reacting 1,4-naphthoquinone with a substituted 1,3-cyclohexadiene of

formula III

Formula III wherein

- Rl, R2 and R3 are as defined in claim 1, in the presence of a solvent, in a cycloaddition step to give the

corresponding Diels- Alder adduct, (b) oxidizing the Diels- Alder adduct of step (a) with oxygen in the presence of a base to give the corresponding oxidized Diels- Alder adduct, and

(c) eliminating ethylene from the oxidized Diels- Alder adduct of step (b) by heating to give the anthraquinone of Formula I.

5. Process according to claim 4, wherein step (a) is conducted at a temperature from 50 to 170°C, preferably from 70 to 130°C, typically from 80 to 110°C, wherein step (b) is conducted at a temperature from 0 to 170°C, especially from 20 to 130°C, more particularly from 20 to 50°C, and wherein step (c) is conducted at a temperature from 100 to 200°C, particularly from 120 to 180°C, more particularly from 140 to 170°C.

6. Process according to claim 4 or 5, wherein the base added in step (b) is selected from the group consisting of ammonia, ammonium hydroxides and salts thereof, sodium acetate, mono-, di-, trialkylamines and cycloaliphatic amines, alkali metal and alkaline earth metal hydroxides, hydroxide forms of strong base anion exchange resins, and mixtures thereof, especially from alkali metal and alkaline earth metal hydroxides, and wherein said base is preferably added progressively to the reaction medium of step (a), prior to the addition of oxygen.

7. Process according to anyone of claims 4 to 6, further comprising, between steps (a) and (b), separating the Diels- Alder adduct from the reaction medium of step (a) and redissolving the Diels- Alder adduct into a solvent.

8. Process according to claim 7, wherein the Diels- Alder adduct is separated from the reaction medium of step (a) by crystallization and solid/liquid separation, optionally after removal of part of the solvent.

9. Process according to anyone of claims 4 to 8, wherein the solvents used in steps (a) and (b) are the same or different, preferably the same, and are selected from the group consisting of water, alcohols, ketones, ethers, esters, nonpolar hydrocarbons and mixtures thereof, in particular from water, alcohols and mixtures thereof.

10. Process according to anyone of claims 4 to 9, wherein, at the end of oxidation step (b), the reaction medium is neutralized by addition of an acid, preferably of a diluted inorganic acid.

11. Use of an anthraquinone of formula I

Formula I wherein - Rl, R2 and R3 are as defined in claim 1, for the preparation of hydrogen peroxide.

12. Process according to anyone of claims 1 to 10 or use according to claim 11, wherein one of the three groups Rl, R2 and R3 is H.

13. Process or use according to claim 12, wherein one of the three groups Rl, R2 and R3 is an isopropyl or a tert-amyl radical.

14. Process according to claim 12 or 13, wherein residue R2 is hydrogen.

15. Process according to anyone of claims 1 to 10 or use according to claim 11, wherein

- Rl is a methyl radical, R2 is H, and R3 is an isopropyl radical, or - Rl is a methyl radical, R2 is an isopropyl radical, and R3 is H, or

- Rl is a n-propyl radical, R2 is an isopropyl radical, and R3 is H, or

- Rl is an isopropyl radical, R2 is a n-propyl radical, and R3 is H, or

- Rl is an isopropyl radical, R2 is H, and R3 is a n-propyl radical, or

- Rl is a methyl radical, R2 is H and R3 is a tert-amyl radical.