WO2017012952A1 - Method for the hydroxylation of a phenolic substrate - Google Patents

Abstract

The invention relates to a method for the catalytic hydroxylation of a phenolic substrate, by hydrogen peroxide, into at least one dihydroxylated product, especially phenol into hydroquinone (HQ) and pyrocatechol (PC), characterised in that water is removed throughout all or part of said hydroxylation reaction.

Description

 METHOD OF H Y D RO X Y L A T I O OF A PHENOLIC SUBSTRATE

TECHNICAL FIELD OF THE INVENTION

 The present invention relates to a novel process for the hydroxylation of phenols and phenol ethers with hydrogen peroxide.

STATE OF THE ART

The ^{hydroxylation} reaction of phenol leads to ^the obtaining of two isomers ie 1, 4-dihydroxybenzene or hydroquinone (HQ) and 1, 2-dihydroxybenzene or pyrocatechol (PC) which are compounds with strong industrial potential.

 Hydroquinone is a product used in many fields of application as a polymerization inhibitor, an antioxidant in elastomers or as a synthetic intermediate. Another area of application is photography.

 Pyrocatechol is a product widely used as a polymerization inhibitor or antioxidant in elastomers, olefins, polyolefins or polyurethanes or as a tanning agent.

Given this wide field ^of operation, it is necessary to produce these products on an industrial scale and have optimized manufacturing processes.

 Conventionally, hydroquinone and pyrocatechol are produced by hydroxylation of phenol with hydrogen peroxide, in the presence of an acid catalyst, strong protonic acid or a solid catalyst with acidic properties such as zeolite TS-1.

Thus, FR 2,071,464, proposes ^to carry out this hydroxylation by the ^hydrogen peroxide in the presence ^of a strong pro tick acid such as for example, ^sulfuric acid, chlorosulfonic acid, perchloric acid sulphonic acids, for example methanesulphonic acid, trifluoromethanesulphonic acid, toluenesulphonic acid or phenolsulphonic acid.

This ^{hydroxylation} reaction of phenol is generally carried out in a mixing apparatus, such as stirred reactor or cascaded stirred tank reactors.

 However, these conventional methods of preparation need to be optimized in terms of productivity, energy efficiency and performance.

On the other hand, the hydroxylation reaction of a phenolic substrate simultaneously produces, in general, a hydroxylated product in para of the hydroxy or alkoxy functional group and a hydroxylated product in ortho of the hydroxy or alkoxy functional group. For example, the hydroxylation of phenol usually co-produces hydroquinone and catechol. The combined production of these products should preferably be optimized to improve the yield of each product but also to modify and adjust their proportions, since the market demand for these products can vary independently.

JP 2002-322108 provides a method for ^preparing alkyl catechol by hydroxylation of phenols by hydrogen peroxide, which have the advantage of producing little by-products. This method is, however, implemented only on para-substituted alkyl-substituted phenol substrates, such as p-tert-butylphenol. Similarly, the document EP 1 481 730 describes a hydroxylation of 2,3,6-trimethylphenol, which is totally substituted in ortho. As a result, the orientation problem in ortho or para hydroxylation does not arise.

 The present invention specifically aims to meet this need.

BRIEF DESCRIPTION OF THE INVENTION

So against all expectations, the inventors have found that it is possible to increase signiflcativement the kinetics of this ^{hydroxylation} reaction and change the ratio of ortho-hydroxylated products / para-hydroxylated products subject to carry it in special conditions. The term "orthohydroxylated product" refers to a hydroxylated product ortho to the hydroxy or alkoxy functional group, while the term "para-hydroxylated product" refers to a hydroxylated product para to the hydroxy or alkoxy functional group.

More specifically, the present invention is primarily a method ^of catalytic hydroxylation ^of phenol substrate ic by the ^hydrogen peroxide e product (s) di-hydroxyl (s), in particular the phenol hydroquinonc (HQ) and catechol . (PC), characterized in that it is carried out a removal of water during all or part of said hydroxylation reaction.

According to an advantageous embodiment, the removal of water is performed simultaneously with the ^{hydroxylation} reaction. and preferably all u along the ^{hydroxylation} reaction.

Advantageously, ^the removal of water is carried out continuously. The method of the invention particularly derives from the unexpected finding by the inventors of an inhibitory effect of water on the kinetics of the ^{hydroxylation} reaction.

However, this water is present in the hydroxylation medium for several reasons. Firstly, the hydrogen peroxide is used in the form of an aqueous solution, generally at 30% by weight, or even up to more than 70% by weight.

 In addition, the hydroxylation reaction results in the generation of one mole of water per mole of diphenol produced.

 Finally, undesirable side reactions, such as the oxidation of the products pyrocatechol and hydroquinone, also lead to the generation of water molecules.

 The inventors have found that the elimination of the water, preferably in continuous mode during the hydroxylation reaction, makes it possible to accelerate the hydroxylation and thus to increase the productivity of the corresponding process.

 In addition, the inventors have found that the removal of water during the hydroxylation reaction had an effect on the ratio of ortho-hydroxylated products / para- hydroxylated products.

 This removal of water can be carried out according to different embodiments.

According to a first variant, the water is removed by adsorption via a hydrophilic material. As detailed below, this adsorption can be carried out either by direct introduction of the adsorbent material into the reactor in which the hydroxylation reaction is conducted, or by loop circulation of the reaction medium in a device separate from the reactor and loaded with such an adsorbent material.

 According to a second variant, this elimination can be carried out by vaporization of the liquid water present in the reaction medium. This vaporization can be carried out by heating all or part of the liquid medium constituting the reaction medium and / or under reduced pressure thereof.

 According to a preferred embodiment, the catalytic hydroxylation process according to the invention is carried out by catalytic distillation (also called reactive distillation).

 Reactive distillation is a known technique to allow the simultaneous realization of the two key steps of a synthesis process, namely the actual chemical reaction and the physical separation of the products in a single device, namely a distillation column.

In the context of the invention, the reactive distillation is carried out with at least one, and preferably a single distillation column, advantageously with trays or packed, and preferably packed, this column being equipped at the top of a condenser and at the base of a boiler. These condensers and boilers can be an integral part of the column. When they are external to the column, as shown in Figure 1, the set they constitute with the column will be further referred to in the sense of the invention catalytic distillation unit.

 Figure 1 shows an illustration of a distillation unit suitable for the invention.

 The distillation column is implemented in a vertical form. It can be defined as consisting of at least:

 a reaction zone (R) provided in particular with at least one feed means (3) as starting materials for the hydroxylation reaction;

 - An upper zone (S) at the top of the reaction zone and connected, directly or indirectly, to a condenser (6); and

 - A lower zone (I) at the base of the reaction zone and connected directly or indirectly to a boiler (10).

 The distillation zone (R) is more particularly dedicated to the reaction and comprises at least one distillation tray or a lining.

 The distillation zone (S) is more particularly dedicated to the recovery of a vapor mixture essentially composed of water and of the phenolic substrate, and to the evacuation thereof to the condenser (6).

 The condenser allows the formation of a water distillate / phenolic substrate by liquefaction of the water vapor / phenolic substrate mixture and makes it possible, by subsequent treatment of this distillate, to remove the water prior to reinjection of the distillate thus purified, by reflux at the top of the column.

 The lower distillation zone (I) is more particularly dedicated to feeding the boiler in a reaction medium and feeding the downstream of the process with the hydroxylation products of the phenolic substrate, for example hydroquinone and pyrocatechol. in the case of phenol.

 The boiler is intended to vaporize all or part of the reaction medium and to replenish the bottom of the column in a reaction medium in totally or partially vaporized form.

 This particular embodiment of the method according to the invention is advantageous in several ways.

 First, it makes it possible to proceed throughout the reaction, and thus in continuous mode, to a depletion of the reaction medium in water.

The simultaneous realization of the chemical reaction and the separation of the products represents from a economic point of view a significant gain in terms of investment, operating costs and flexibility of the ratio of ortho-hydroxylated products / para- hydroxylated products.

 In view of the increase in reaction kinetics, the reaction time is reduced with immediate benefit on the formation of undesirable tar by-products. Indeed, during the hydroxylation of phenol with hydrogen peroxide in the presence of a strong acid type catalyst, hydroquinone (HQ), pyrocatechol (PC), water but also different high molecular weight compounds, called "tars" or "heavy". More precisely, these tars derive essentially from the secondary oxidation of part of the pyrocatechol and hydroquinone by hydrogen peroxide. However, in the case of a process according to the invention, the formation of these tars is advantageously more limited because the contact time of the hydroxylation products with the other reagents is reduced.

 Reactive distillation technology is also of interest inasmuch as it is compatible with a low pressure process which advantageously makes it possible to lower the reaction temperature.

 Finally, it is compatible with a homogeneous but also heterogeneous catalysis of hydroxylation.

More specifically, a process according to the invention implementing a reactive distillation may comprise at least the steps consisting in:

 contacting at least one phenolic substrate, hydrogen hydroxide, preferably in the form of an aqueous solution, and at least one hydroxylation catalyst at a reactive distillation column which comprises at least :

 a) a reaction zone, advantageously provided with distillation trays or preferably a packing,

 b) an upper zone disposed at the top of said reaction zone, equipped at the outlet of a condenser and a reflux feed pipe, and

 c) a lower zone, disposed at the base of said reaction zone, connected to a boiler;

 - imposing on the distillation column thermal conditions and pressure conducive to carrying out said hydroxylation and the accumulation in the upper zone, a vapor phase containing at least water and said phenolic substrate;

recovering at the outlet of the condenser a distillate formed by liquefaction of the water vapor / phenolic substrate phase; said method being characterized in that water is removed from said distillate and that the phenolic substrate-concentrated distillate thus obtained is refluxed in said column, and in particular in the upper zone of said column. DETAILED DEFINITION OF THE INVENTION

 In the following description, the expression "between ... and ..." must be understood as including the boundaries quoted.

 Phenolic substrate

 The process of the invention is well suited to the hydroxylation of phenol or a phenol ether, but also to phenols or substituted phenol ethers.

 As previously meant, the term "phenolic substrate" or "phenolic compound" may be used herein to mean either phenol, and phenols or substituted phenol ethers.

 By "phenol or substituted phenol ether" is meant a phenol or a phenol ether in which one of the hydrogen atoms of the aromatic ring is replaced by one or more substituents.

 Generally, by more than one substituent, less than four substituents are defined per aromatic ring.

 Any substituent may be present to the extent that it does not interfere with the reaction of the invention.

 Thus, the process of the invention is well adapted to apply to phenolic substrates of general formula (I):

in which :

 - symbolized a benzene or naphthalenic ring;

 R 1 represents a hydrogen atom, an alkyl, cycloalkyl, aryl or aralkyl group;

- R-2 represents a hydrogen atom or one or more substituents, identical or different;

 - n, number of substituents per aromatic ring, is a number less than or equal to 4.

In the formula (I), the ORi group is an ether group when Ri is different from a hydrogen atom. The number of substituents per aromatic ring is variable and generally less than or equal to 4, preferably equal to 0, 1, 2 or 3.

 Preferred examples of substituents are given for formula (Ia).

Thus, the process of the invention is well suited to phenolic substrates corresponding to formula (I) in which A represents a benzene ring and which is represented more particularly by general formula (Ia):

in which :

 n is a number from 0 to 4, preferably equal to 0, 1 or 2;

 R 1 represents a hydrogen atom, an alkyl, cycloalkyl, aryl or aralkyl group;

R ₂ , which may be identical or different, represent an alkyl group, an alkoxy group, a hydroxyl group, a halogen atom or a halogeno- or perhalo-alkyl group.

 The process of the invention can be applied to substrates corresponding to formula (la) in which n is equal to 0. According to one embodiment, the phenolic substrate is represented by general formula (la):

in which :

 n is 0;

 R 1 represents a hydrogen atom, an alkyl, cycloalkyl, aryl or aralkyl group.

The phenolic substrate can thus be represented by the general formula (Ib)

wherein R 1 is hydrogen, alkyl, cycloalkyl, aryl or aralkyl.

According to one embodiment, the phenolic substrate according to the invention is a phenolic substrate whose para position is free and at least one of the positions in ortho is free. It may be a phenolic compound within the meaning of the invention having the para position and the two ortho-free positions, or a phenolic compound within the meaning of the invention having the para position and a single ortho-free position.

 Thus, the phenolic substrate can be represented by the general formula (Ic) or the general formula (Id)

in which :

R 1 represents a hydrogen atom, an alkyl, cycloalkyl, aryl or aralkyl group; - R ₃ , R 4 and R ₅ represent, independently of each other, a hydrogen atom, an alkyl group, an alkoxy group, a hydroxyl group, a halogen atom, a halogeno- or perhalo-alkyl group.

The method of the invention applies preferentially to substrates corresponding to formula (la) in which n is equal to 0 or 1; ¾ represents a hydrogen atom or an alkyl group having 1 to 4 carbon atoms; R ₂ represents a hydrogen atom, an alkyl or alkoxy group having 1 to 4 carbon atoms.

In formulas (I) and (Ia), the term "alkyl" means a saturated linear or branched hydrocarbon chain C1-C15, preferably C1-C1 _0. Examples of preferred alkyl groups include methyl, ethyl, propyl, isopropyl, butyl, isobutyl and t-butyl.

 By "alkoxy" is meant an alkyl-O- group in which the term alkyl has the meaning given above. Preferred examples of alkoxy groups are methoxy or ethoxy.

 By "cycloalkyl" is meant a cyclic, monocyclic C3-C8 hydrocarbon group, preferably a cyclopentyl or cyclohexyl group.

"Aryl" means an aromatic mono- or polycyclic group, preferably mono- or bicyclic C6-C2 _0, preferably phenyl or naphthyl. When the group is polycyclic, that is to say that it comprises more than one ring nucleus, the ring nuclei can be condensed two by two or attached in pairs by bonds σ. Examples of (C6-C18) aryl groups include phenyl and naphthyl. By "aralkyl" is meant a linear or branched hydrocarbon-based group carrying a C ₇ -C ₁₂ monocyclic aromatic ring, preferably benzyl, the aliphatic chain comprising 1 or 2 carbon atoms.

 The term "haloalkyl group" means an alkyl group as defined above, one or more hydrogen atoms of which are replaced by a halogen atom, preferably a fluorine atom.

 The term "perhaloalkyl group" means an alkyl group comprising from 1 to 10 carbon atoms and from 3 to 21 halogen atoms, preferably fluorine, and more particularly the trifluoromethyl group.

 By "halogen atom" is meant fluorine, chlorine and bromine.

By way of illustration of phenolic substrates of formula (I) that can be used in the process of the invention, mention may be made more particularly of:

 those corresponding to formula (I) in which n is equal to 0, such as phenol, anisole or phenetole;

 those corresponding to formula (I) in which n is equal to 1, such as o-cresol, m-cresol, p-cresol, 2-ethylphenol, 3-ethylphenol, 2-propylphenol, 2 -sec-butylphenol, 2-tert-butylphenol, 3-tert-butylphenol, 4-tert-butylphenol, 2-methoxyphenol, 3-methoxyphenol, 4-methoxyphenol, 2-ethoxyphenol, methyl salicylate 2-chlorophenol, 3-chlorophenol or 4-chlorophenol;

 those corresponding to formula (I) in which n is equal to 2, such as 2,3-dimethylphenol, 2,5-dimethylphenol, 2,6-dimethylphenol, 3,5-dimethylphenol, 2, 3-dichlorophenol, 2,5-dichlorophenol, 2,6-dichlorophenol, 3,5-dichlorophenol, 2,6-ditert-butylphenol, 3,5-ditert-butylphenol;

 those corresponding to formula (I) in which n is equal to 3, such as 2,3,5-trimethylphenol, 2,3,6-trimethylphenol, 2,3,5-trichlorophenol or 2,3 6-trichlorophenol;

 those corresponding to formula (I) in which A represents a naphthalenic ring, such as 1-hydroxynaphthalene.

The phenolic substrate may in particular be chosen from the group consisting of phenol, anisole, phenetole, o-cresol, m-cresol, 2-ethylphenol, 3-ethylphenol, 2-propylphenol, 2-sec. -butylphenol, 2-tert-butylphenol, 3-tert-butylphenol, 2-methoxyphenol, 3-methoxyphenol, 2-ethoxyphenol, methyl salicylate, 2-chlorophenol, 3-chlorophenol, 2, 3-dimethylphenol, 2,5-dimethylphenol, 3,5-dimethylphenol, 2,3-dichlorophenol, 2,5-dichlorophenol, 3,5-dichlorophenol, 3,5-ditert-butylphenol, 2,3,5-trimethylphenol, 2,3-dichlorophenol 5-trichlorophenol, and 1-hydroxynaphthalene; more preferably in the group consisting of phenol, anisole, phenetol, Γο-cresol, m-cresol, 2-tert-butylphenol, 3-tert-butylphenol, 2-methoxyphenol and 2-ethoxyphenol.

 Among the abovementioned phenolic substrates, phenol, ο-cresol, m-cresol, p-cresol, anisole, phenetole, 2-methoxyphenol (guaiacol) or 2-ethoxyphenol (the guétol), and more preferably phenol, anisole, phenétole, 2-methoxyphenol (guaiacol) or 2-ethoxyphenol (guétol).

 The present process is particularly suitable for the preparation of hydroquinone and pyrocatechol from phenol.

According to the process of the invention, the phenolic compound is reacted with hydrogen peroxide in the presence of a catalyst and optionally a co-catalyst.

 Advantageously, the phenolic substrate and the hydrogen peroxide solution are introduced in the form of a mixture and separately to the catalyst.

Catalyst

 Two types of catalyst can be used, either a homogeneous catalyst or a heterogeneous catalyst. The homogeneous catalyst is in the same physical state as the reaction mixture and the heterogeneous catalyst is in a different physical state from that of the mixture.

 In the case of a homogeneous catalyst which thus mixes with the reaction mixture, an additional step is then necessary to recover the catalyst or to purify the catalyst products. This additional step increases the investment and operation cost.

On the other hand, the heterogeneous catalyst has the advantage of not mixing with the reaction medium, and therefore of not requiring such a step. On the contrary, it is necessary to consider a mode of immobilization of this catalyst in the reaction zone or downstream of said zone. a) Homogeneous catalyst For its part, a homogeneous catalyst may advantageously be introduced, jointly or otherwise, with the other reactants, and more particularly at the level of the reaction zone.

 Advantageously, it is a strong acid.

 By strong acid is meant in the present invention, an acid having a pKa in water of less than -0.1 and, preferably, less than -1.0.

 PKa is defined as the ionic dissociation constant of the acid / base pair, when water is used as the solvent. Among the acids corresponding to this definition, it is preferable to use those which are stable with respect to oxidation by hydrogen peroxide.

 Mention may more particularly be made of halogenated or non-halogenated oxyacids such as sulfuric acid, phosphoric acid, pyrosulphuric acid, perchloric acid, aliphatic or aromatic sulphonic acids, such as, for example, methanesulphonic acid, ethanesulphonic acid, benzenesulphonic acid, bis-trifluoromethanesulphonimide acid, toluenesulphonic acids, naphthalenesulphonic acids, benzenedisulphonic acids, naphthalenedisulphonic acids, halosulphonic acids such as fluorosulphonic acid, chlorosulphonic acid or trifluoromethanesulphonic acid .

 Among the above-mentioned acids, sulfuric acid, perchloric acid, methanesulfonic acid, trifluoromethanesulfonic acid, toluenesulphonic acid, phenolsulphonic acid or bis-trifluoromethanesulphonimide acid are preferably used.

 According to a variant of the process of the invention, it is possible to use as a strong protonic acid a hydroxyaromatic sulphonic acid as described in WO 2009/150125.

 As preferred examples of hydroxyaromatic sulphonic acids preferably used in the process of the invention, there may be mentioned the acids corresponding to the following formula:

in which :

 x is 1, 2 or 3, preferably 1 or 2;

 - y is 1 or 2;

z is a number from 0 to 4, preferably equal to 0, 1 or 2; M represents a hydrogen atom, sodium or potassium;

 R represents an alkyl or alkoxy group having from 1 to 4 carbon atoms or a carboxylic group.

 Among the acids that are suitable for the process of the invention, mention may be made more particularly of hydroxybenzenesulphonic acids, sulphonated hydroxybenzoic acids, hydroxybenzenedisulphonic acids, dihydroxybenzenedisulphonic acids, hydroxytoluenesulphonic acids, hydroxynaphthalenesulphonic acids, hydroxynaphthalenedisulphonic acids and their mixtures.

 Among the hydroxybenzenesulphonic acids, it is preferable to use 4-hydroxybenzenesulphonic acid, 2-hydroxybenzenesulphonic acid, 5-sulphosalicylic acid or mixtures thereof.

 Preferred examples of dihydroxybenzenesulfonic acids used include sulfonic acids resulting from the sulfonation of hydroquinone (1,4-dihydroxybenzene), pyrocatechol (1,2-dihydroxybenzene), and resorcinol (1, 3-dihydroxybenzene).

 The preferred dihydroxybenzenedisulphonic acids are 5,6-dihydroxy-1,3-benzenedisulphonic acid, 4,6-dihydroxy-1,3-benzenesulphonic acid and 2,5-dihydroxy-1,4-dihydroxy acid. benzenedisulfonic.

 The hydroxyaromatic sulfonic acids are available in solid, liquid or aqueous solution, the concentration of which may vary between 5% and 95% by weight, preferably between 50% and 70% by weight.

 According to another variant of the process of the invention, it is possible to use a mixture of at least two strong protonic acids, as described in WO 2010/115784.

 The mixture comprises two acids (A) and (B) respectively having specific pKa: the acid (B) being much stronger than the acid (A).

 The mixture can include:

 a strong acid (A) having a pKa (S) greater than or equal to that of sulfuric acid and an ApKa (S) with respect to sulfuric acid less than or equal to 4 and greater than or equal to 0; and

another acid (B) chosen from superacids.

 Acid (A) has a pKa (S) greater than or equal to that of sulfuric acid:

(S) representing the organic solvent which is nitrobenzene.

Acid (B) is a super acid defined as having a pKa (S) lower than that of sulfuric acid. PKa (S) is defined as the ionic dissociation constant of the acid / base pair in a solvent (S).

 The pKa of the acids is defined by reference to a potentiometric measurement carried out in a solvent which is nitrobenzene (S) and whose measurement protocol is explained before the examples of WO 2010/115784.

 The acids involved in said mixture are defined by a difference of pKa, ApKa, which corresponds for the same solvent, the difference between the pKa of the chosen acid and the pKa of sulfuric acid.

 The acid (A) used has an ApKa (S) with respect to the sulfuric acid of less than or equal to 4 and greater than or equal to 0. Even more preferentially, the acid (A) has an ApKa (S) with respect to sulfuric acid less than or equal to 3 and greater than or equal to 0.

 Examples of acids (A) that may be mentioned include sulfuric acid, aliphatic or aromatic sulphonic acids such as, for example, methanesulphonic acid, ethanesulphonic acid, benzenesulphonic acid, toluenesulphonic acids and naphthalenesulphonic acids. .

 Another class of acids (A) are hydroxybenzenesulfonic acids, sulphonated hydroxybenzoic acids, hydroxybenzenedisulphonic acids, dihydroxybenzenesulphonic acids, hydroxytoluenesulphonic acids, hydroxynaphthalenesulphonic acids, hydroxynaphthalenedisulphonic acids and mixtures thereof.

 Among the aforementioned acids, the preferred acids are the acid

4-hydroxybenzenesulfonic acid, 2-hydroxybenzenesulfonic acid, 5-sulfosalicylic acid, sulfonic acids resulting from the sulfonation of hydroquinone (1,4-dihydroxybenzene), pyrocatechol (1,2-dihydroxybenzene), and resorcinol (1,3-dihydroxybenzene), 5,6-dihydroxy-1,3-benzenedisulphonic acid, 4,6-dihydroxy-1,3-benzenedisulphonic acid, 2,5-dihydroxy-1 acid, 4-benzenedisulfonic.

 Other examples of acids that may be mentioned include perhaloacetic acids such as trichloroacetic acid and trifluoroacetic acid.

 With regard to the second component (B) of the acid mixture, it is a superacid, that is to say an acid having a pKa (S) lower than that of sulfuric acid and which has therefore a negative ApKa.

 The lower bound is not critical but generally the ApKa in nitrobenzene is greater than or equal to - 12.

The superacids chosen preferentially have an ApKa less than or equal to -0.1 and preferably greater than or equal to -8. Exemplary superacids (B) include perchloric acid, halosulfonic acids such as fluorosulfonic acid, chlorosulfonic acid, perhaloalkanesulfonic acids, preferably trifluoromethanesulfonic acid.

 Also as superacids (B), there may be mentioned inter alia, trifluoromethanesulfinic acid and bis-trifluoromethanesulfonimide acid.

 Preferred acid pairs (A) and (B) are perchloric acid and sulfuric acid, perchloric acid and 4-hydroxybenzenesulfonic acid, trifluoromethanesulphonic acid and 4- hydroxybenzenesulfonic acid, bis-trifluoromethanesulfonimide acid and 4-hydroxybenzenesulfonic acid. The proportion in the mixture of different acids can vary widely. Thus, it is possible to use mixtures comprising:

 from 60% to 95%, preferably from 80% to 95% by moles of an acid (A);

 from 5% to 40%, preferably from 5% to 20% by mole of an acid (B). Each percentage of acid expresses the ratio (expressed in%) between the number of moles of the acid considered and the number of moles of the sum of the two acids (A) and (B).

 The acids that may be used in the mixture are commercially available in solid, liquid or aqueous solution form whose concentration may vary between 5% and 95% by weight, preferably between 50% and 70% by weight. .

 Thus, according to a preferred embodiment of the invention, the process implements, as a catalyst, a strong protonic acid having a pKa in water of less than -0.1 and preferably less than -1.0. , or a mixture of protonic acids.

 Advantageously, the acidic catalyst is sulfuric acid, perchloric acid, methanesulphonic acid, trifluoromethanesulphonic acid, toluenesulphonic acid, phenolsulphonic acid, bis-trifluoromethanesulphonimide, or a mixture of perchloric acid and sulfuric acid, of perchloric acid and of 4-hydroxybenzenesulfonic acid, trifluoromethanesulfonic acid and 4-hydroxybenzenesulfonic acid, or bis-trifluoromethanesulfonimide acid and 4-hydroxybenzenesulfonic acid.

The strong protonic acid or the mixture of acids can be used in the process of the invention, in a quantity expressed by the ratio of the number of proton equivalents to the number of moles of phenolic substrate which advantageously varies between 0.002. % and 0.15%. Thus, said molar ratio is preferably selected from 0.01% to 0.07%. b) Heterogeneous catalyst As a representative of the heterogeneous catalysts that are suitable for the invention, mention may in particular be made of zeolites of titano-silicalite type, such as commercial zeolites TS-1 and TS-2.

 In the process variant according to the invention, conducting the hydroxylation reaction by reactive distillation, this catalyst can in particular be used in an immobilized form at the level of a material integrated in the column, such as, for example, a padding or a tray.

 Thus, ion exchange resins, directly charged with acidic ions, can also be used as catalysts of the reaction of the present invention, and more particularly strong cationic resins. Among these strong cationic resins, the sulfonic resins are suitable for this use, with for example as commercial products the Amberlyst 15, Dowex 650C or Nafion resins. c) Co-catalyst

According to a particular variant of the process of the invention, the hydroxylation of the phenolic substrate is carried out in the presence of a co-catalyst which is a ketone compound and more particularly of formula (III):

in which :

R _a and R _b , identical or different, represent hydrocarbon groups having from 1 to 30 carbon atoms or together form a divalent group, optionally substituted with one or more halogen atoms or stable functional groups under the conditions of the reaction ;

X represents a covalent bond, a group -CO-, a group -CHOH or a group - (R) _n -, R representing an alkylene group preferably having from 1 to 4 carbon atoms and n is an integer chosen from 1 and 16.

In formula (III), R _a and R _b are more particularly:

 linear or branched alkyl groups;

 linear or branched alkenyl groups;

 cycloalkyl or cycloalkenyl groups having from 4 to 6 carbon atoms;

 mono- or polycyclic aryl groups; in the latter case, the cycles forming between them an ortho- or ortho- and peri-condensed system or being interconnected by a valency bond;

arylalkyl or arylacenyl groups; - R _a and R _b may together form an alkylene or alkenylene group having from 3 to 5 carbon atoms, optionally substituted by a lower carbon alkyl group or a cycloalkyl or cycloalkenyl group having 4 to 6 carbon atoms; 2 to 4 carbon atoms of the alkylene or alkenylene groups may be part of one or two benzene rings optionally substituted with 1 to 4 hydroxyl groups and / or alkyl and / or alkoxy low carbon condensation.

 In the following description of the invention, the term "alkyl group of low carbon condensation", a linear or branched alkyl group generally having 1 to 4 carbon atoms.

 The abovementioned hydrocarbon groups may be substituted with 1 or more, preferably 1 to 4, low carbon condensation alkyl groups or functional groups such as hydroxyl groups, low carbon alkoxy, hydroxycarbonyl, alkyloxycarbonyl groups having from 1 to 4 atoms. carbon in the alkyl group, a nitrile, sulfonic, nitro group or with one or more halogen atoms, and especially chlorine and bromine.

Preferably, R _a and R _b are more particularly:

 linear or branched alkyl groups having from 1 to 10 carbon atoms;

 linear or branched alkenyl groups having from 2 to 10 carbon atoms;

 cycloalkyl or cycloalkenyl groups having from 4 to 6 carbon atoms;

 phenyl groups optionally substituted with 1 to 4 alkyl and / or hydroxyl and / or alkoxy groups;

 phenylalkyl or phenylacenyl groups comprising 1 (or 2) to 10 carbon atoms in the aliphatic part, and even more particularly from 1 (or 2) to 5 carbon atoms in the aliphatic part;

- R _a and R _b may together form an alkylene or alkenylene group having 3 to 5 carbon atoms, optionally substituted with 1 to 4 alkyl groups low carbon condensation. Thus, the dialkylketone-type ketone compounds of formula (III) in which R _a and R _b represent a linear or branched alkyl group having from 1 to 8 carbon atoms are particularly used.

Among all the ketonic compounds corresponding to formula (III), those which correspond to formula (III) in which R _a and R _b represent an optionally substituted phenyl group are preferably chosen.

Said ketone compounds may be represented by the following formula (IIIa):

in which :

- R _c and R _d , identical or different represent a hydrogen atom or a substituent, preferably an electron-donor group;

- nor, n ₂ , identical or different is a number equal to 0, 1, 2 or 3;

optionally, the two carbon atoms located at the α-position with respect to the two carbon atoms bearing the -CO group can be bonded together by a covalent bond or by a -CH ₂ - group, thus forming a ketone ring which can be saturated but also unsaturated.

 The substituent is chosen such that it does not react under the acid conditions of the invention. It is preferably an electro-donor group.

 By "electron donor group" is meant a group as defined in Jerry MARCH - Advanced Organic Chemistry, Chapter 9, pages 273 and 275 (1992).

 Examples of substituents that are well suited to the invention are the following:

 linear or branched alkyl groups having from 1 to 4 carbon atoms;

 the phenyl group;

 alkoxy groups comprising a linear or branched alkyl chain having from 1 to 4 carbon atoms or a phenoxy group;

 the hydroxyl group;

 - the fluorine atom.

Examples of ketone compounds which are particularly suitable for the invention include, in particular, ketone compounds corresponding to the general formula (IIIa) in which R _c and R _d , which are identical or different, represent a hydrogen atom or a substituent such that above, preferably in position 4,4 'and ¾, n ₂ , identical or different are equal to 0 or 1.

 Preferred is ketone compounds of the formula

(IIIa) in which R _c and R _d , identical or different, represent a hydrogen atom, a methyl, ethyl, tert-butyl, phenyl, a methoxy or ethoxy group, a hydroxyl group, preferably in the 3,3-position; 'or 4,4'.

 As specific examples of ketones that can be used in the process of the invention, there may be mentioned more particularly:

benzophenone; 2-methylbenzophenone;

 2,4-dimethylbenzophenone;

 4,4'-dimethylbenzophenone;

 2,2'-dimethyl benzophenone;

 4,4'-dimethoxybenzophenone;

 4-hydroxybenzophenone;

 4,4'-dihydroxybenzophenone;

 4-benzoylbiphenyl.

 The amount of ketone compound involved expressed by the ratio between the number of moles of ketone compound and the number of moles of phenolic compound can vary between 0.01% and 20%, and preferably between 0.1% and 2%. .

Thus, according to a variant of the invention, a phenol or a phenol ether is reacted with hydrogen peroxide in the presence of a strong protonic acid and optionally a ketone.

Hydrogen peroxide

 The hydrogen peroxide used according to the invention may be in the form of an aqueous solution or an organic solution.

 Aqueous solutions being commercially more readily available, they are preferably used.

 The concentration of the aqueous solution of hydrogen peroxide, although not critical in itself, is chosen so as to introduce the least possible water into the reaction medium.

An aqueous solution of hydrogen peroxide having an H ₂ O ₂ concentration of at least 20% by weight and preferably between 20% and 90% by weight, advantageously ranging from 30% to 90%, is generally used. by weight, preferably from 30% to 70% by weight, more preferably from 40% to 70% by weight, and even more preferably from 45% to 70% by weight.

In conventional processes, the amount of hydrogen peroxide must generally be adjusted to a minimum so as not to favor the formation of the products of over-oxidation in the image of tars formed by reaction of this hydrogen peroxide with the products of dihydrogen peroxide. -hydroxylation expected. When the elimination of water is carried out continuously from the hydroxylation reaction, the increase of the kinetics of reaction makes it possible to reduce the contact time of the dihydroxylation products with hydrogen peroxide, and therefore this The limiting factor is advantageously at least partly overcome according to the present invention.

 The method according to the invention therefore makes it possible to envisage the use of a molar ratio of hydrogen peroxide / phenolic substrate of 0.05 to 1, and therefore different from the conventional ratios ranging from 0.01 to 0.3 and, preferably from 0.03 to 0.10.

 In the context of an implementation of the process according to the invention by reactive distillation, this ratio is advantageously adjustable via the reflux ratio imposed on the distillation column.

 Using a hydrogen peroxide / phenolic substrate molar ratio higher than conventional ratios is advantageous for several reasons.

 It makes it possible to reduce the amount of phenolic substrate present in the reaction medium for the same quantity of reaction products. This therefore results in a lower overall reaction medium flow rate for a given quantity of products, which intensifies the process and therefore makes it more capitalistic.

 Moreover, the phenolic substrate introduced in excess of the reaction is separated from the reaction medium downstream of the reaction and then recycled to this same reaction. The fact of using a hydrogen peroxide / phenolic substrate molar ratio higher than the conventional ratios, and thus a smaller amount of phenolic substrate to the reaction, also makes it possible to significantly reduce the flow of phenolic substrate to be separated and recycled. This results in a reduction in the size of the equipment used to achieve this separation, which makes the process more capitalistic.

 Finally, the decrease in the flow rate of phenolic substrate to be separated also makes it possible to reduce the energy expenditure related to this separation, the invention thus making it possible to reduce the operating costs.

Water

 As mentioned above, the inventors have characterized an inhibitory effect of water on the kinetics of reaction.

Thus, it is preferable to preferentially choose an initial content of the water medium of less than 20% by weight and preferably less than 10% by weight. The weight contents of water indicated are expressed relative to the mixture phenolic substrate - hydrogen peroxide - water. This initial water corresponds to the water introduced with the reagents and in particular with hydrogen peroxide.

 As previously stated, this continuous elimination of water makes it possible: to accelerate the reaction kinetics by a factor of 5 to 100 depending on the temperature, and if necessary the pressure retained for the hydroxylation reaction, in particular within a reactive distillation column;

 to significantly reduce the residence time in the reactor or the column;

 to make it possible to lower the reaction temperature from about 15 ° C. to 35 ° C. with a positive impact on the rate of formation of undesirable side products such as tars; and

 to modify the ratio of ortho-hydroxylated products / para-hydroxy products.

 The lowering of the temperature is of course interesting in terms of productivity gain.

The inventors have found that the process according to the invention makes it possible to favor the formation of the para-hydroxylated isomer with the orthohydroxylated isomer, in particular the formation of the hydroquinone (HQ) isomer with the pyrocatechol isomer (PC ) in the case of phenol. Thus such a method allows access to ratios (PC) / (HQ) of the order of 1.3, against about 1.5 with conventional methods. Preferably, the method according to the invention makes it possible to reach a ratio (PC) / (HQ) less than or equal to 1.3, more preferably less than or equal to 1.0. More generally, the inventors have found that the process according to the invention makes it possible to reduce the ratio (ortho-hydroxylated product) / (para-hydroxylated product) by at least 0.1 point, more preferentially by at least 0 , 2 point, and more preferably at least 0.3 point. The ratio (ortho-hydroxylated product) / (para-hydroxylated product) is defined by the ratio between the number of moles of ortho-hydroxylated product formed and the number of moles of para-hydroxylated product formed.

Complexing agent of metal ions

 An advantageous variant of the process of the invention also considers the presence of a complexing agent for the metal ions present in the reaction medium, since these are detrimental to the good progress of the process of the invention, in particular in the case of phenols where the yields of hydroxylation products are low. Therefore, it is preferable to inhibit the action of metal ions.

 The metal ions that are detrimental to the progress of the hydroxylation are transition metal ions and more particularly iron, nickel, copper, chromium, cobalt, manganese and vanadium ions.

 The metal ions are provided by the reagents and in particular the starting substrates and the equipment used. To inhibit the action of these metal ions, it suffices to conduct the reaction in the presence of one or more stable complexing agents with respect to hydrogen peroxide and giving complexes that can not be decomposed by the strong acids present and in which the metal can no longer exert chemical activity.

 By way of nonlimiting examples of complexing agents, it is possible to use, in particular, the various phosphoric acids such as, for example, orthophosphoric acid, metaphosphoric acid, pyrophosphoric acid, polyphosphoric acids, phosphonic acid, such as (1-hydroxyethylidene) diphosphonic acid, phosphonic acid, ethylphosphonic acid and phenylphosphonic acid.

 It is also possible to use the esters of the abovementioned acids and, more particularly, ortho-phosphates of mono- or dialkyl, of mono- or dicycloalkyl, of mono- or dialkylaryl, for example ethyl phosphate or of diethyl, hexyl phosphate, cyclohexyl phosphate and benzyl phosphate.

 Preferably, one operates in the presence of a complexing agent of transition metal ions, stable under the reaction conditions, such as phosphoric acids, pyrophosphoric acids, phosphonic acids and their acid esters.

 The amount of complexing agent depends on the content of the reaction medium in metal ions.

There is obviously no upper limit, the amount of complexing agents present may be largely in excess of that required to complex the metal ions. Generally, an amount representing 0.01% and 2%, preferably 0.01 to 0.3% by weight of the reaction medium, is suitable. DETAILED DEFINITION OF THE PROCESS

 As is apparent from the foregoing, the method according to the invention can be implemented according to different embodiments which are clearly within the competence of those skilled in the art.

 Thus, it is possible to put the reaction medium in contact with a hydrophilic adsorbent material.

 As an illustration of this type of material, molecular sieves such as microporous alumino-silicate with pore sizes of 3 Å or 4 Å can notably be mentioned.

 This material can be directly disposed within the reactor in which the hydroxylation is carried out.

 In another variant, this material may be external to the reactor. The contact of the reaction medium with this material is then achieved by imposing a circulation in continuous mode or not, through the material.

 For example, the material can be loaded within a column and the medium is circulated from the reactor through the height of the column and then at the column outlet is redirected to the reactor.

 The material may also be implemented in the form of a tangential membrane installed outside the reactor and in which the reaction medium circulates and then recycled to the reactor. The water separated by tangential membrane filtration is then recovered outside the tube constituted by the membrane.

 According to a preferred embodiment, the process according to the invention uses a reactive distillation column comprising at least the following 3 distillation zones:

 a reaction zone (R) at which, in particular, the supply of starting materials is carried out;

 an upper zone (S) located at the top of the reaction zone dedicated to recovering a water vapor / phenolic substrate mixture and conveying it to the condenser; and

 - A lower zone (I), located at the base of the reaction zone dedicated to allow the recovery of the hydroxylation products via a boiler.

FIG. 1 shows an illustration of a reactive distillation column that is suitable for the process of the invention and specifies in particular these three zones, namely: a reaction zone (R), an upper zone (S), located at the top of the central zone (R) and a lower base zone (I). The chemical reaction and the distillation, one favoring the other, progress in all three zones, the chemical reaction being more favored in the zone located below the feeding zone.

 The reactive distillation column may be any type of distillation column, including a vacuum distillation column, a steam distillation column, or preferably a fractionation column.

 The internal configuration of the column is not particularly limited and a wide variety of configurations can be used, as is apparent from the state of the art. The inside of a column may include, but not be limited to, trays, random packing elements, structured packing elements, baffles, varying diameters throughout the column, each with or without a catalyst .

 The internal configuration can also be distinct depending on the function of the section of the column. For example, trays may be used in one section and structured packing in another section of the column. Techniques known in the state of the art for extending the liquid holding time can also be used. The appropriate liquid medium holding times (excluding reboiler) range from 1 minute to 60 minutes, or from 3 minutes to 20 minutes.

The reaction zone (R) is equipped with one or more feed lines (3) for introducing all or part of the reagents. This or these supply ducts may advantageously be connected to a distributor for homogeneously distributing the starting reagents over the entire surface of the distillation tray or the lining.

 The top or zone (S) of the column is equipped with a pipe (4) for evacuating the vapors of the most volatile constituents and a pipe (5) for the introduction of a liquid reflux consisting of a fraction of condensed vapors in a condenser (6). The condenser is connected to a device (7) for extracting the water present in the water distillate / phenolic substrate recovered at the outlet of the condenser, for example a device of the azeotropic distillation type with a third body. The phenolic substrate thus purified is re-fed to the top of the column, preferably via a distributor, while the water is extracted via a pipe (11).

The base or zone (I) of the column is equipped with a pipe (8) for the evacuation of the less volatile constituents in liquid form in a boiler and advantageously a pipe (9) for the introduction of the vapor generated by vaporization partial bottom liquid in the boiler. The water-free mixture containing the reaction products is extracted through a line (12).

Thus, the reactive distillation process of the invention comprises at least the steps of:

 contacting at least one phenolic substrate, hydrogen hydroxide, preferably in the form of an aqueous solution, and at least one hydroxylation catalyst at a reactive distillation column which comprises at least a) a reaction zone, advantageously provided with distillation trays or preferably with a packing,

 b) an upper zone disposed at the top of said reaction zone, equipped at the outlet of a condenser and a reflux feed pipe, and

 c) a lower zone, disposed at the base of said reaction zone, connected to a boiler;

 - imposing on the distillation column thermal conditions and pressure conducive to the realization of said hydroxylation and the accumulation in the upper zone, a vapor phase containing at least water and the phenolic substrate;

 recovering at the outlet of the condenser a distillate formed by liquefaction of the water vapor / phenolic substrate phase;

 recovering at the level of the boiler a liquid reaction medium concentrated in dihydroxylation products;

said process being characterized in that water is removed from said distillate and that the phenolic substrate-concentrated distillate thus obtained is re-fed at reflux into said column. The supply of reagents in the zone (R) is carried out by conventional means such as, for example, a pump, and more particularly a centrifugal pump or a volumetric pump.

 These reagents can be introduced into the reaction zone of the column separately or as a mixture, pure or diluted.

Formation of said vapor phase is ensured by loop circulation of reaction medium between the lower zone of the column and the boiler.

The hydroxylation products are recovered from the reaction medium which is recovered in this same lower zone of the column. From a practical point of view, a preferred embodiment of the reagents is to prepare beforehand a mixture in a static mixer by introducing therein separately the phenolic compound optionally supplemented with a complexing agent, optionally all or part of the acid catalyst, optionally all or part of the cocatalyst, and the solution of hydrogen peroxide.

 The mixing operation is conducted at a temperature sufficient for the phenol or starting phenol ether to remain liquid. Said temperature is chosen according to the melting temperature of the phenolic substrate.

 In the case of phenol, for example, the mixing operation is carried out at a temperature generally above 41 ° C, or even 42 ° C.

 The temperature of this mixing operation is advantageously chosen at most equal to 85 ° C., preferably between 45 ° C. and 60 ° C.

 The temperature during the mixing operation is chosen differently depending on whether the catalyst is introduced during this mixing step or not.

 The mixture can be carried out under atmospheric pressure or under reduced pressure, but higher pressures can also be envisaged. For example, pressures between 1 bar and 200 bar absolute may be suitable.

 This step can be carried out under an inert atmosphere, for example under nitrogen or even under argon, nitrogen being preferred especially in view of its reduced cost.

 After this operation of bringing the reagents into contact, the mixture thus formed is introduced into the reaction zone of the reactive distillation column.

 This power supply is preferably carried out via a distributor. The hydroxylation reaction may be conducted in the reactive distillation column at atmospheric pressure or at a higher pressure but preferably under a reduced pressure. For example, pressures between 1 mbar and 1 bar absolute may be suitable, and preferably between 5 mbar and 500 mbar absolute.

 The adjustment of the distillation column to a pressure different from atmospheric pressure clearly falls within the skill of those skilled in the art.

 The hydroxylation reaction can also be carried out under an inert atmosphere, preferably under a nitrogen atmosphere.

The reactive distillation process according to the invention is advantageously carried out continuously. The adjustment of flow rates at the feed zones, the condenser and the boiler is clearly within the skill of those skilled in the art.

 In the reaction zone, hydroxylation forms the expected di-hydroxylated products and water as a co-product. The reaction mixture therefore contains these products, and also contains the phenolic substrate and the unreacted hydrogen peroxide. At least a portion of the reaction mixture is in liquid form in the column. Thus, at least a portion of the expected di-hydroxylated products is liquid. In contrast, at least a portion of the water and at least a portion of the unreacted phenolic substrate is in the form of vapor. For this purpose, the temperature inside the column may be between a high temperature of 225 ° C and a low temperature of 20 ° C depending on the composition in the column.

 The upper zone is for its part heated to a temperature above the boiling point of at least the phenolic substrate so that at least a portion of it vaporizes in the column and progresses towards the top of the column and then the condenser where it is liquefied.

 The condensate thus formed is collected in a reflux vessel or tank and subjected to a separation step to remove the water. This separation can be carried out by any conventional technique such as decantation, secondary distillation, evaporation, or centrifugation, with a third body if necessary. The non-aqueous phase of the condensate is then returned to the column as reflux.

 It is therefore carried out continuously, the hydroxylation reaction according to the invention, a removal of water via the treatment of distillate water / phenolic substrate collected at the outlet of the condenser.

 It is also carried out continuously withdrawal of the liquid reaction medium in zone (I) of the distillation column.

 The temperature of the reaction mixture in the liquid reservoir in zone (I) at the bottom of the column is lower than the boiling point of the di-hydroxylated products and higher than the boiling point of the phenolic substrate.

 The boiler can advantageously be heated to a temperature ranging from 100 ° C. to 250 ° C.

The invention will now be described by means of the following example given by way of illustration and not limitation of the invention. EXAMPLE

 A process according to the invention carried out with the device of FIG. 1 with a distillation column of 7 theoretical plates:

 operating at a pressure of 10 mbar;

 - fed with hydrogen peroxide and phenol in a ratio of 6 mol% of hydrogen peroxide with respect to phenol and in the presence of an acid catalyst at a rate of 18ppm molar relative to phenol;

 with a temperature at the bottom of the column of 65 ° C .; and

 a water content at the bottom of the column of 400 ppm.

 This method makes it possible to obtain a kinetic gain of a factor of 10 compared with a conventional reactor process for the production of hydroquinone and pyrocatechol.

 With such a process, we can see:

 an increase in selectivity for diphenols relative to phenol RT (PC + HQ) / PhOH of 4%. RT (PC + HQ) / PhOH corresponds to the ratio between the number of moles of diphenols formed (pyrocatechol + hydroquinone) and the number of moles of phenol transformed;

an increase in selectivity for diphenols relative to hydrogen peroxide RT (PC + HQ) / H ₂ O ₂ of 2.5%. RT (PC + HQ) / H ₂ 0 ₂ corresponds to the ratio between the number of moles of diphenols formed (pyrocatechol + hydroquinone) and the number of moles H ₂ 0 ₂ consumed; and

 a drop in the PC / HQ ratio of 0.2 points, the PC / HQ ratio being defined by the ratio between the number of moles of pyrocatechol formed and the number of moles of hydroquinone formed.

Claims

A process for the catalytic hydroxylation of a phenolic substrate by hydrogen peroxide to a di-hydroxylated product (s), said phenolic substrate being represented by the general formula (Ia):

in which :  n is 0;  R 1 represents a hydrogen atom, an alkyl, cycloalkyl, aryl or aralkyl group; characterized in that a removal of water is carried out during all or part of said hydroxylation reaction.  2. Method according to claim 1, characterized in that the removal of water is carried out simultaneously with the hydroxylation reaction, and preferably throughout the hydroxylation reaction. 3. The method of claim 1 or claim 2, characterized in that ^the elimination of ^water is carried out continuously.  4. Method according to any one of claims 1 to 3, characterized in that the phenolic substrate is phenol, anisole, or phenetole.  5. Method according to any one of claims 1 to 4, characterized in that it implements as a catalyst, a strong protonic acid having a pKa in water less than - 0.1 and preferably lower at -1.0, or a mixture of protonic acids.  6. Process according to claim 5, characterized in that the acidic catalyst is sulfuric acid, perchloric acid, methanesulphonic acid, trifluoromethanesulphonic acid, toluenesulphonic acid, phenolsulphonic acid, bis-trifluoromethanesulphonimide, or a mixture of perchloric acid and sulfuric acid, perchloric acid and 4-hydroxybenzenesulfonic acid, trifluoromethanesulfonic acid and 4-hydroxybenzenesulfonic acid, or bis-trifluoromethanesulfonimide acid and 4-hydroxybenzenesulfonic acid.  7. Process according to any one of Claims 1 to 6, characterized in that a cocatalyst which is a ketonic compound which corresponds to the formula is additionally added. (III):

in which : - R _a and ¾, identical or different, represent hydrocarbon groups having from 1 to 30 carbon atoms or together form a divalent group, optionally substituted with one or more halogen atoms or stable functional groups under the conditions of the reaction; X represents a covalent bond, a group -CO-, a group -CHOH or a group - (R) _n -, R representing an alkylene group preferably having from 1 to 4 carbon atoms and n is an integer chosen from 1 and 16.  8. Process according to any one of Claims 1 to 7, characterized in that the reaction is carried out in the presence of a complexing agent of transition metal ions which are stable under the reaction conditions, such as phosphoric acids, pyrophosphoric acids, phosphonic acids and their acid esters.  9. Process according to any one of claims 1 to 8, characterized in that the water is removed by adsorption via a hydrophilic material.  10. Method according to any one of claims 1 to 8, characterized in that the water is removed via the vaporization of the liquid water present in the reaction medium.  11. The method of claim 10, said hydroxylation being carried out by reactive distillation.  The method of claim 11, comprising at least the steps of:  contacting at least one phenolic substrate, hydrogen hydroxide, preferably in the form of an aqueous solution, and at least one hydroxylation catalyst at a reactive distillation column which comprises at least :  a) a reaction zone, advantageously provided with distillation trays or preferably a packing,  b) an upper zone disposed at the top of said reaction zone, equipped at the outlet of a condenser and a reflux feed pipe, and  c) a lower zone, disposed at the base of said reaction zone, connected to a boiler; - imposing on the distillation column thermal conditions and pressure conducive to the realization of said hydroxylation and the accumulation in the upper zone, a vapor phase containing at least water and the phenolic substrate; and recovering at the outlet of the condenser a distillate formed by liquefaction of the water vapor / phenolic substrate phase; characterized in that water is removed from said distillate and that the phenolic substrate-concentrated distillate thus obtained is refluxed in said column.  13. Process according to any one of claims 1 to 12, characterized in that the phenolic substrate and the hydrogen peroxide solution are introduced in the form of a mixture and separately to the catalyst.  14. Method according to any one of claims 1 to 13, characterized in that it is a catalytic hydroxylation process of phenol hydroquinone and pyrocatechol.  15. The method of claim 14, characterized in that the ratio (pyrocatechol) / (hydroquinone) obtained is less than or equal to 1.3, preferably less than or equal to 1.0.