WO2000023377A1

WO2000023377A1 - Method for preparing a mel-type titanium silicalite, resulting product and uses in catalysis

Info

Publication number: WO2000023377A1
Application number: PCT/FR1999/002478
Authority: WO
Inventors: Jean-Louis Grieneisen; Henri Kessler; Eric Fache; Anne-Marie Le Govic; Michel Costantini
Original assignee: Rhodia Chimie
Priority date: 1998-10-19
Filing date: 1999-10-13
Publication date: 2000-04-27
Also published as: FR2784672A1; CN1324325A; NO20011920D0; AU6206799A; FR2784672B1; NO20011920L; JP2002527347A; EP1131264A1

Abstract

The invention concerns a novel method for preparing MEL-type titanium silicalite, the resulting product and its different uses in the field of catalysis. The inventive method for preparing MEL-type titanium silicalite is characterised in that it consists in: impregnating an amorphous xerogel (TiO2-SiO2) comprising at least silicon and titanium elements with a solution comprising at least a structural agent and at least a mobilizer OH-; the impregnating solution comprising at least an efficient amount of a compound of the type 3,5 dimethylpiperidinium used as structural agent; heating said reaction mixture up to the zeolite crystallisation temperature; then recuperating and calcining the resulting zeolite. The zeolite crystals obtained present a morphology resulting from their preparation method.

Description

PROCESS FOR THE PREPARATION OF A SILICALITY OF TITANIUM OF THE MEL TYPE PRODUCT OBTAINED AND ITS APPLICATIONS IN CATALYSIS.

The present invention relates to a new process for preparing a titanium silicalite of MEL type, the product obtained and its various applications in the field of catalysis.

Titanium silicalites (TS-2) are known products and widely described in the literature, in particular in "Atlas of zeolites structure types by WM Meier and DH Oison published by the Structure Commission of the International Zeolite Association (1992), p 132. "

These are zeolites listed as having an MEL structure. One of the first processes for preparing TS-2, according to BE-A- 1 001 038, consists in carrying out the crystallization of a product resulting from the reaction of a source of silicon, in particular of o-silicate tetraethyl, a source of titanium oxide, in particular a titanium tetraalkoxide, a mobilizing agent consisting of OH- ions and a structuring agent, preferably tetrabutylammonium hydroxide. TS-2 prepared according to this synthetic route does not really have an MEL type structure, but is formed of MEL / MFI intergrowths, resulting from poor stacking of "pentasil" sheets. These numerous defects can be harmful in catalysis.

To overcome this drawback, it has been proposed according to WO96 / 34827 to prepare a TS-2 zeolite with a phase of pure MEL type, by using a particular structuring agent, namely 3,5-dimethyl-N, N-diethylpiperidinium.

The process described consists in preparing a reaction mixture comprising both a source of titanium oxide, a source of silicon oxide and a structuring agent comprising at least one compound of the 3,5-diιmethylpiperidinium type; maintaining the aqueous solution under conditions sufficient to form zeolite crystals, then recovering the latter.

The disadvantage from which this process suffers is that the crystallization times are relatively long and durations of 15 days are mentioned in the examples. In addition, the amount of structuring agent used is substantial.

The objective of the present invention is to provide a more industrial process for the preparation of pure TS-2 which involves a lesser amount of structuring agent and a much shorter crystallization step. A process has now been found, and this is the subject of the present invention, for the preparation of a titanium silicalite which consists:

- impregnating an amorphous xerogel (Tiθ2-Siθ2) comprising at least the silicon and titanium elements with a solution comprising at least one structuring agent and at least one mobilizing agent OH "> the impregnation solution comprising at least one effective amount of a compound of the 3,5-dimethylpiperidinium type used as a structuring agent,

- subjecting the reaction mixture to heating up to the crystallization temperature of the zeolite,

- then recover and calcine the zeolite obtained.

The titanium silicalite obtained according to this impregnation technique has a MEL type structure. It is based on silicon oxide and titanium oxide and its chemical composition, after calcination corresponds to the following formula:

^S '(96-x) ^Ti x ° 192 (D in which x is between 0.1 and 6, preferably between 0.1 and 4.

The titanium silicalite obtained according to the process of the invention has a quadratic crystal system and an X-ray diffraction diagram defined in table (I).

In this table, the extreme values of the different reticular equidistances d _n kl are given and correspond to the limit concentrations of titanium incorporated into the framework of the zeolite or more precisely to the Ti / Si ratio.

Indeed, the identification of titanium silicalite can in particular and advantageously be carried out by establishing their X-ray diffraction diagram.

This diffraction diagram can be obtained using a diffractometer using the conventional method of powders with K radiation from copper.

From the position of the diffraction peaks represented by the angle 2, we calculate by the Bragg relation, the reticular equidistances d ^ kl characteristic of the sample.

The estimation of the measurement error (d _n k |) is calculated, as a function of the absolute error (2), by the Bragg relation.

An absolute error (2) equal to 0.2 ° is commonly accepted.

The relative intensity l / l ₀ assigned to each value of dhki is estimated from the height of the corresponding diffraction peak. A symbol scale is often used to characterize this intensity, namely FF = very strong, F = strong, mF = medium to strong, mf ≈ medium to weak, f = weak, ff = very weak.

Table (I)

The morphology of the crystals is demonstrated by scanning electron microscopy

The crystals are in the form of the following medium-sized parallelepipeds:

- thickness between 50 and 500 nm, preferably between 100 and 250 nm,

- length between 100 and 1000 nm, preferably between 300 and 500 nm,

- width between 50 and 500 nm, preferably between 100 and 250 nm. This type of morphology is characteristic of the process of the invention, namely a synthesis in OH medium ^" and the technique of impregnating xerogel.

In accordance with the process of the invention, the zeolite is prepared according to a process which consists of:

- impregnating an amorphous xerogel (Tiθ2-Siθ) comprising at least ^' the silicon and titanium elements with a solution comprising at least one structuring agent and at least one mobilizing agent OH "^'> the impregnation solution comprising at least one effective amount a compound of the 3,5-dimethylpiperidinium type used as a structuring agent,

- then recover and calcine the zeolite obtained.

By xerogel is meant a dry, amorphous gel composed of a mixed oxide Tiθ2

- SiO ₂ .

First, we prepare the xerogel. A preferred method of preparation consists in hydrolyzing in an acid medium a source of silicon and then adding a source of titanium. The soil thus prepared is gelled by adding a base or by heating. The gel is dried at an appropriate temperature.

Many sources of the silicon element with oxidation state +4 can be used. By way of example, mention may be made of silicas in the form of hydrogels, aerogels, xerogels, colloidal suspensions, silicas resulting from precipitation from solutions of soluble silicates or from hydrolysis of silicic esters as Si (OCH ₃ ) 4, Si (OC2H ₅ ) ₄ . It is also possible to use hydrolyzable tetravalent silicon compounds such as silicon halides or the like.

The source of silicon is preferably chosen from alkylsilicates, tetraethylsilicate being that which is most preferred.

Among the sources of titanium oxide, there may be mentioned, by way of example, titanium oxides and hydroxides, crystallized or amorphous, tetravalent titanium compounds which can be hydrolysed such as halides (TiCL,), organic derivatives titanium, such as, for example, alkyl o-titanates, preferably tetraethyl o-titanate or tetrabutyl o-titanate, soluble titanium salts such as TiOS0 ₄ , TiOCI ₂ , (NH ^ TiO ^ O ^. It is also possible to use as sources of silica or titanium oxide, compounds comprising the elements Si and Ti such as, for example, glasses or gels based on the oxides of these two elements.

The sources of silica and titanium oxide can be used in the soluble form or in powdery solids but also in the form of agglomerates such as, for example, extruded pellets which can be transformed into titanozeosilite of desired structure without modification of shape.

The molar ratio Tiθ2 Siθ2 in this xerogel is preferably between 25 and 200, more preferably between 35 and 100. Under the preferential conditions, a source of silicon is hydrolyzed in an acid solution (for example, dilute HCl), then a source of titanium in the same solution, in order to obtain a clear solution.

The hydrolysis of the silicon source is preferably started with a dilute acid having a concentration advantageously between 0.01 and 2 N.

The amount of acid involved is such that the H2θ / Si ratio is between 2 and 10 and preferably between 3 and 5.

The source of titanium is added.

A preferred variant consists in diluting the source of titanium by adding an organic solvent, for example an alcohol with low carbon condensation, for example from 1 to 5 carbon atoms, and preferably isopropanol is used.

The amount of solvent represents from 50% to 85% of the volume represented by the source of titanium and of the organic solvent. The hydrolysis operation can be carried out from 0 ° C to room temperature (generally 15 ° C - 25 ° C). It is also possible to exceed this temperature.

After adding the titanium source, it is advantageous to remove the organic solvent by heating generally 50 to 80 ° C. for the alcohols. A clear solution is obtained.

The pH of said solution is then brought to a value which allows the coprecipitation of the soil Tiθ2-Siθ2-

A base is therefore used which can be of mineral origin, for example NH OH or of organic origin. In the latter case, it is preferred to use an organic compound which also has the role of structuring agent and preferably, a compound of the 3,5-dimethylpiperidinium type.

The basic solution used generally has a high concentration preferably between 10 and 40% by weight. The amount of base added is such that the pH of the solution obtained is between approximately 5 and approximately 7.

The operation is carried out at a temperature advantageously between 0 and 25 ° C. The gel obtained is subjected to drying at a temperature advantageously chosen between 50 and 120 ° C., under atmospheric pressure or under reduced pressure chosen between 1 mm of mercury and atmospheric pressure.

A xerogel is obtained which has a large specific surface area varying most often between 400 m ² / g and 700 m ² / g. Its pore volume is most often between 0.25 and 0.5 cm ³ / g.

The pore size varies widely between 5 and 100 Å but the xerogel has a large microporous volume since almost 80% of the volume is constituted by micropores having from 3 to 20 Å.

In the following step, the xerogel obtained is impregnated using a solution comprising an organic compound used as a structuring agent and a mobilizing agent, of the OH ^" type.

The mobilizing agent OH ^" is introduced in the form of weak and / or strong base (s) preferably containing no alkaline cations. Mention may be made of amines and quaternary ammonium hydroxides. Examples are given below.

As for the structuring agent, it directs and stabilizes the formation of the zeolite. As examples of structuring agents suitable for the invention, mention may be made of those which correspond to the following formula:

in which :

- R _a and Rb, identical or different, represent an alkyl group, linear or branched, having from 1 to 7 carbon atoms; the groups R _a and Rb, can together form a ring system which includes the nitrogen atom, comprising from 4 to 7 carbon atoms, said system being able to be substituted or not by alkyl, linear or branched groups having from 1 to 3 carbon atoms,

- X ^" is an anion.

As 3,5-DMP compounds, there may be mentioned 3,5-dimethyl-N, N-diethylpiperidinium, 3,5-dimethyl-N-methyl-N-ethylpiperidinium. As spiro-3,5-dimethyipiperidinium compounds, mention may be made of the compounds 1-azonia-3,5,7-trimethyl-spiro [5.4] decane.

As for the nature of the anion X ^" , it must not disturb the formation of the zeolite. The anion can be essentially a halide or hydroxide anion. As examples of halide, we consider fluoride, chloride, bromide , iodide or combinations thereof. Thus, the illustrative anions are: hydroxide, acetate, sulfate, carboxylate, tetrafluoroborate, halides such as fluoride, chloride, bromide, iodide. Iodides and hydroxides are the preferred anions. of the process of the invention consists in associating 3,5-

DMP with tetraikylammonium hydroxide designated "HTA". Indeed, it has been found that the addition of a small amount of tetraikylammonium hydroxide makes it possible to considerably reduce the rate of crystallization of the zeolite. As more specific examples of tetraikylammonium hydroxides, mention may in particular be made of tetraethylammonium, tetrapropylammonium and tetrabutylammonium hydroxides.

When a 3,5-DMP and HTA mixture is used, the amount of HTA used is such that the HTA / 3.5-DMP molar ratio varies between 0.5 and 2.10 ^-3 and, preferably, between 0.1 and 0.01 The second HTA structuring agent is added in any way to the

3,5-DMP.

After the dried solid has been impregnated with a solution comprising the structuring agent and the mobilizing agent, the reaction mixture is crystallized at an appropriate temperature, with stirring. The composition of the reaction mixture is characterized by a low water content.

The amount of water is determined so that it allows wetting of the xerogel.

The H2θ / Siθ2 ratio is less than 10, preferably between 3 and 10, and more preferably between 4 and 6.

The OH7Siθ2 molar ratio advantageously varies between 0.05 and 1.0, preferably between 0.05 and 0.5.

It has been found that the amount of structuring agent to be used must be different depending on whether the 3,5-DMP is used alone or else in admixture with another "HTA" structuring agent.

Surprisingly, it has been found that the quantity of structuring agent expressed by the molar ratio Q / Si0 ₂ (in which Q represents the 3,5-DMP) must be at most equal to 0.13, preferably between 0.05 and 0.125, and even more preferably between 0.075 and 0.125.

When Q is a mixture of 3,5-DMP and HTA, the molar ratio Q / Siθ2 is between 0.125 and 1.0, preferably between 0.15 and 0.20. The crystallization of the zeolite can be obtained by heating the xerogel for the time necessary for crystallization, according to the standard procedure for the synthesis of zeolite well known to those skilled in the art.

Appropriate temperatures are between 90 and 210 ° C, preferably between 120 and 190 ° C and more preferably between 150 and 170 ° C. As an indication, the duration of heating can be between 6 hours and

About 250 hours.

This heating and this crystallization are preferably carried out in a container or autoclave coated with a layer such as for example polytetrafluoroethane. A variant of the process of the invention consists in adding crystallized seeds of determined structure, MEL, in a proportion which does not exceed a few weight percentages (generally <5%) relative to the weight of SiO + TiO engaged, facilitates crystallization zeolite. As seeds of crystallization, one can use any zeolite having the MEL structure, whatever its chemical composition.

It is preferable to use a silicalite (silicalite-2) which is a zeolite equivalent to titanium silicalite, but containing only silicon in its framework. It is also possible to use seeds of titanium silicalite MEL originating from a previous manufacture. It should be noted that when only 3,5-DMP is used as a structuring agent, it is particularly advantageous to add seeds of crystallization.

At the end of the hydrothermal treatment, the material obtained is separated according to conventional solid / liquid separation techniques, preferably by filtration. It may be advantageous to carry out a washing operation, preferably with deionized water.

The material obtained is subjected to drying at a temperature preferably chosen between 50 and 120 ° C., under atmospheric pressure or under reduced pressure chosen between 1 mm of mercury and atmospheric pressure. Finally, it is calcined preferably in air, at a temperature of at least

350 ° C, between 400 and 600 ° C, preferably between 450 and 600 ° C.

The duration of the calcination, given below for information, is most often between 2 and 12 hours. The zeolites obtained according to the process of the invention have catalytic properties which allow their use as catalysts or catalyst supports for reactions of transformation of various organic compounds.

They can find numerous applications in different oxidation reactions, in particular in oxidation reactions, of disproportionation of aromatic compounds such as toluene; conversion of aliphatic carbonyl or olefin compounds; the hydroxylation of aromatic compounds; formylation of aromatic compounds; epoxidation of olefins; cyclization of organic compounds.

A preferred application of the zeolites of the invention is their use for the hydroxylation reactions of phenolic compounds.

The process of the invention advantageously applies to phenolic compounds which are soluble in the medium, under the reaction conditions.

It is obviously possible to tolerate a certain insolubility but it is preferred that the phenolic compound is soluble in an amount of at least 90% by weight.

The present invention applies to phenolic compounds of general formula (III):

in said formula (III):

- R- _| , R2, R3 and R4, identical or different, represent a hydrogen atom or any substituent,

- two groups Ri and R2 and / or R3 and R placed on two vicinal carbon atoms can form together and with the carbon atoms which carry them a ring,

- R 'represents a hydrogen atom or a hydrocarbon radical having from 1 to 24 carbon atoms, which can be a saturated or unsaturated, linear or branched acyclic aliphatic radical; a saturated or unsaturated, monocyclic or polycyclic cycloaliphatic radical; a saturated or unsaturated, linear or branched aliphatic radical, carrying a cyclic substituent.

By cyclic substituent is meant a saturated, unsaturated or aromatic carbocycle having, generally, from 4 to 7 carbon atoms, and preferably 6 carbon atoms. The process of the invention applies to any phenolic compound corresponding to the general formula (III) and, more particularly, to the phenolic compounds of formula (III) in which R 'represents: a hydrogen atom. a linear or branched alkyl radical having from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms, and more particularly a methyl or ethyl radical,. a cyclohexyl radical,. a benzyl radical. The phenolic compound of formula (III) may carry one or more substituents R- |, R2, R3 and R ₄ . Examples of substituents are given below, but this list is in no way limiting. Any substituent can be present on the cycle as long as it does not interfere with the desired product. The process of the invention applies more preferably to phenolic compounds of formula (III) in which:

- R- |, R2, R3 and R ₄ , identical or different, represent R ₀ , one of the following groups:

. a hydrogen atom,. an alkyl radical, linear or branched, having from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms, such as methyl, ethyl, propyl, isopropyl, butyl, sec-butyl, tert-butyl,

. a linear or branched alkenyl radical having from 2 to 6 carbon atoms, preferably from 2 to 4 carbon atoms, such as vinyl, allyl,. a linear or branched alkoxy radical having from 1 to 6 carbon atoms, preferably from 1 to 4 carbon atoms such as the methoxy, ethoxy, propoxy, isopropoxy, butoxy, radicals. an acyl group having from 2 to 6 carbon atoms,. a radical of formula: -R5-OH

-R ₅ -COOR ₆ -R ₅ -X -R5-CF3 in said formulas, R5 represents a valence bond or a divalent, linear or branched, saturated or unsaturated hydrocarbon radical, having from 1 to

6 carbon atoms such as, for example, methylene, ethylene, propylene, isopropylene, isopropylidene; Rρ represents a hydrogen atom or a linear or branched alkyl radical having from 1 to 6 carbon atoms; X symbolizes a halogen atom, preferably a chlorine, bromine or fluorine atom. - R- |, R ₂ , R3 and R ₄ , identical or different, represent R ₇ , one of the following more complex radicals:. a saturated or unsaturated carbocyclic radical having from 4 to 7 carbon atoms, preferably a cyclohexyl radical,. a radical of formula

in which R5 represents a valential bond or a divalent, linear or branched, saturated or unsaturated hydrocarbon radical having from 1 to 6 carbon atoms such as, for example, methylene, ethylene, propylene, isopropylene, isopropylidene and R ₀ having the meaning given above and m is an integer from 0 to 4,. a radical - R ₅ - A - R ₈ in which R ₅ has the meaning given above, R ₈ represents a linear or branched alkyl radical having from 1 to 6 carbon atoms or a radical of formula

and A symbolizes one of the following groups: - O -, - COO -, - OCOO -, - SO ₂ -, - CO - N -,

IR ₉ in these formulas, Rg represents a hydrogen atom or an alkyl radical having from 1 to 4 carbon atoms, a cyclohexyl or phenyl radical.

- two groups R- _| and R2 and / or R3 and R placed on two vicinal carbon atoms can form together and with the carbon atoms which carry them an unsaturated or aromatic carbocycle having from 4 to 7 carbon atoms and, preferably, 6 carbon atoms,

Among the compounds of formula (III), use is more particularly made of those corresponding to formula (III) in which:

- R 'represents a hydrogen atom - R- _j , R2, R3 and R ₄ , identical or different, represent one of the following groups:

. a hydrogen atom,

. an alkyl radical, linear or branched having from 1 to 4 carbon atoms ,.

. a linear or branched alkoxy radical having from 1 to 4 carbon atoms,. a hydroxyl group, . a halogen atom,

. a group -CF ₃

. a cyclohexyl radical,

. a phenyl radical, - two groups R 1 and R 2 and / or R ₃ and R ₄ placed on two vicinal carbon atoms can form together and with the carbon atoms which carry them a benzene ring.

Even more preferably, the compounds of formula (III) are chosen in which R ′ represents a hydrogen atom and one of the radicals R ^ R2, R3 and R ₄ represents a hydroxyl group, a methyl radical or a methoxy radical and the other 3 represent a hydrogen atom.

By way of illustration of phenolic compounds of formula (III) capable of being used in the process of the invention, there may be mentioned more particularly: - those corresponding to formula (III) in which R- |, R2, R3 and R represent a hydrogen atom, such as phenol or anisole,

- those corresponding to formula (III) with a substituent on the benzene ring, such as o-cresol, m-cresol, p-cresol, 2-methoxyphenol, 2-ethylphenol, 3-ethylphenol, 2-propylphenol, 2-sec-butylphenol, 2-tert-butylphenol, 3- tert-butylphenol, 4-tert-butylphenol, 2-methoxyphenol, 3-methoxyphenol, 4-methoxyphenol, salicylate methyl, 2-chlorophenol, 3-chlorophenol, 4-chlorophenol,

- those corresponding to formula (III) with two substituents on the benzene ring, 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 (III) with three substituents on the benzene ring, such as 2,3,5-trimethylphenol, 2,3,6-trimethylphenol, 2,3,5-trichlorophenol, 2,3 , 6-trichlorophenol,

- those corresponding to formula (III) in which R- | and R2 form a benzene ring, such as 1-hydroxynaphthalene,

- Those corresponding to formula (III) in which R ^ represents a radical of type R ₇ , such as 2-phenoxyphenol, 3-phenoxyphenol. Among the phenolic compounds of formula (III) which may be used in the process of the invention, there may be mentioned, without limitation, phenol, o-cresol, m-cresol, p-cresol. In accordance with the process of the invention, the phenolic compound of formula (III), the hydrogen peroxide and the zeolitic catalyst are used during the hydroxylation process.

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

Preferably, aqueous solutions being commercially more readily available are used.

The concentration of the aqueous hydrogen peroxide solution is not critical. One can use an aqueous solution of hydrogen peroxide having a concentration ranging for example from 20% to 70% by weight of H ₂ θ2-

For reasons of convenience of implementation, it is preferable to use a dilute solution of hydrogen peroxide, its concentration can advantageously be between 20% and 40% by weight.

The amount of hydrogen peroxide can range up to 1 mole of H2O2 for 1 mole of phenolic compound of formula (III).

It is however preferable to obtain an industrially acceptable yield to use a molar ratio hydrogen peroxide / phenolic compound of formula (III) of 0.01 to 0.4 and, preferably, of 0.10 to 0.25.

In accordance with the process of the invention, the reaction is carried out in an aqueous medium.

Most of the water is introduced at the start of the reaction and the other part is provided by the hydrogen peroxide solution.

The total weight content of water expressed relative to the phenolic compound of formula (III) can vary between 30% and 100%, preferably between 40% and 70%.

The amount of zeolitic catalyst which is used in the process of the invention can vary within wide limits.

When the process is carried out batchwise, the catalyst can represent, by weight relative to the phenolic compound of formula (III) used, from 0.1 to 25%, preferably from 3 to 10%. However, if the process is carried out continuously, for example by reacting a mixture of phenolic compound (III), of hydrogen peroxide solution on a fixed bed of catalyst, these catalyst / phenolic compound ratios of formula (III) do not make sense and at a given time, there may be an excess weight of catalyst compared to the phenolic compound of formula (III).

It is also possible to carry out the hydroxylation reaction of the phenolic compound of formula (III) in a solvent for said compound which is preferably miscible or partially miscible with water. By way of examples of such solvents, mention may be made of water, alcohols such as methanol, ethanol, isopropanol, tert-butanol; ketones such as acetone or methyl isobutyl ketone; nitriles such as acetonitrile; carboxylic acids such as acetic acid; esters of carboxylic acids such as propyl acetate; ethers such as methyltertiobutylether; polar aprotic solvents such as tetrahydrothiophene dioxide (sulfolane), ethylene glycol carbonate, propylene glycol carbonate, N-methylpyrrolidone.

In accordance with the process of the invention, the hydroxylation of the phenolic compound of formula (III) is carried out at a temperature which can be between 45 ° C. and 150 ° C.

A preferred variant of the process of the invention consists in choosing the temperature between 50 ° C and 120 ° C, and even more preferably around 80 ° C. The reaction is advantageously carried out at atmospheric pressure.

It is also possible to operate at a higher temperature and under pressure greater than atmospheric pressure.

From a practical point of view, the method according to the invention is simple to implement continuously or discontinuously. Preferably, the following different reagents are introduced in any order, phenolic compound of formula (III), and catalyst.

The reaction medium is brought to the desired temperature and then the hydrogen peroxide solution is added gradually.

At the end of the reaction, the zeolitic catalyst is separated according to conventional solid / liquid separation techniques, preferably by filtration and then the untransformed phenolic compound, are separated from the hydroxylation products by the usual means, in particular by distillation and are returned to the reaction zone.

Examples of the invention are given below.

Examples 1 to 7 which follow illustrate the invention without however limiting it.

Examples

In the examples, the following abbreviations mean: - TEOS = tetraethylsilicate,

- TBOT ≈ tetrabutyl o-titanate,

- TPAOH = tetrapropyiammonium hydroxide,

- TPABr = tetrapropyiammonium bromide. - 3,5-DMDEPI = 3,5-dimethyl-NN-diethylpiperidinium iodide

- 3,5-DMDEPOH = 3,5-dimethyl-N, N-diethylpiperidinium hydroxide

number of moles of hydrogen peroxide transformed TT =% number of moles of hydrogen peroxide introduced

number of moles of hydroquinone formed

RT _HQ =% number of moles of hydrogen peroxide transformed

number of moles of pyrocatechol formed

RT _pc =% number of moles of hydrogen peroxide transformed

Example 1,

Synthesis of 3.5-dimethvl-N.N-diethvl Diperidinium hvdroxvde.

37.8 g of 3,5-dimethylpiperidine (cis / trans ratio = 3) are placed in a 1 liter three-necked flask equipped with a stirring system, a dropping funnel and a reflux condenser. , 48.2 g of potassium bicarbonate and

330 cm ³ of dry methanol.

150 g of ethyl iodide are added dropwise.

The mixture is then placed in an oil bath and heated to reflux (50 -

55 ° C) for 3 days. After treatment of the reaction medium and recrystallization of the solid from an acetone / methanol mixture, 53 g of 3,5-dimethyl-N, N-diethylpiperidinium iodide (3,5-DMDEPI) are recovered.

3,5-DMDEPI is characterized by proton nuclear magnetic resonance

(cis / trans ratio = 4.2) and contains less than 50 ppm of potassium and sodium. I70H ion exchange ^" for obtaining 3,5-dimethyl- hydroxide

N.N-diethylpiperidinium (3,5-DMDEPOH) is produced from silver oxide according to the following procedure: a solution containing 50 g of

3,5-DMDEPI and 96 g of water. 20 g of Ag2θ are added and the suspension is stirred

1 hour at 50 ° C, protected from light. The solution of 3,5-DMDEPOH at about 25% (by weight) is recovered by centrifugation.

Example 2.

Preparation of an SiO -TiO Si / Ti xerogel = 45) 42 g of TEOS are hydrolyzed with 14.5 g of an aqueous hydrochloric acid solution at 0.05 mol / l while stirring the mixture for 1 hour at room temperature.

The solution is then cooled to 0 ° C. and then a solution containing 1.53 g of TBOT and 9.15 g of isopropanol is added dropwise.

At the end of the addition, the solution is stirred for 1 hour at room temperature and then 2 hours at 70 ° C. in order to remove the alcohols (alcohols released by hydrolysis + isopropanol).

The clear soil thus obtained is gelled by adding 2.25 g of a 25% solution of 3,5-DMDEPOH.

The gel is then dried overnight in an oven at 110 ° C.

The Siθ2-TiO ₂ xerogel thus prepared is finely ground in a mortar before use.

Preparation of titanium silicalite (TS-2) 0.08 g of titanium silicalite seeds (TS-2) are added to 5.0 g of an aqueous solution of 3,5-DMDEPOH at 25% by weight then 4 g of Siθ xerogel -

TiO ₂ .

The mixture is stirred for approximately 10 minutes.

The molar composition reduced to one mole of silica of the reaction mixture is as follows:

1 SiO ₂ ; 0.0222 TiO ₂ ; 0.1 3.5-DMDEPOH; 2% of germs.

The reaction mixture is then crystallized in an autoclave coated internally with polytetrafluoroethane, by heating at 170 ° C for 9 days with stirring (rotary oven). After crystallization, the solid phase is separated by filtration, washed with water and dried at 80 ° C.

After calcination at 560 ° C. for 8 hours, the solid phase is identified by its X-ray diffraction spectrum given in table (II).

Titanium silicalite (TS-2) is in the form of parallelepipedic crystals of size 150 x 150 x 400 nm approximately.

Chemical analysis carried out on the solid obtained gives 1.78% of titanium.

The infrared spectrum of the solid comprises a band at 960 cm ^" characteristic of titanium silicalites.

Its UV-Visible spectrum is devoid of the bands characteristic of the presence of extra-reticular Tiθ2.

Example 3

Preparation of titanium silicalite (TS-2)

9 g of a solution of 3.5-DMDEPOH at 25% by weight and 0.4 g of a solution of TBAOH at 40% are mixed. Then added 0.08 g of titanium silicalite seeds (TS-2) and then 4.0 g of the Siθ2-Ti0 ₂ xerogel prepared in Example 2.

The impregnated xerogel is stirred for approximately 10 minutes.

1 Si0 ₂ ; 0.0222 Ti0 _2; 0.183.5-DMDEPOH; 0.01 TBAOH; 2% of germs.

The reaction mixture is then crystallized in an autoclave coated internally with polytetrafluoroethane, by heating at 170 ° C for 7 days with stirring (rotary oven). After crystallization, the solid phase is separated by filtration, washed with water and dried at 80 ° C.

After calcination at 560 ° C for 8 hours, the solid is characterized by X-ray diffraction.

Titanium silicalite (TS-2) is in the form of spherical particles of 0.5 μm polycrystalline.

Chemical analysis carried out on TS-2 gives 1.20% titanium.

Example 4

Preparation of titanium silicalite (TS-2) Example 2 is repeated, but with 6.25 g of 3.5-DMDEPOH at 25% instead of 5.0 g.

1 Siθ2; 0.0222 TiO ₂ ; 0.125 3.5-DMDEPOH; 2% of germs. The reaction mixture is then crystallized in an autoclave coated internally with polytetrafluoroethane, by heating at 170 ° C for 8 days with stirring (rotary oven).

After crystallization, the solid phase is separated by filtration, washed with water and dried at 80 ° C. After calcination at 560 ° C for 8 hours, the solid is characterized by X-ray diffraction and by scanning electron microscopy.

The physico-chemical characteristics are close to those of the catalyst prepared in Example 2.

Example 5.

Preparation of titanium silicalite (TS-2)

Example 3 is repeated but the hydrothermal synthesis is carried out statically and for 3 days (instead of 8 days in Example 3). After crystallization, the solid phase is separated by filtration, washed with water and dried at 80 ° C.

After calcination at 560 ° C for 8 hours, the solid is characterized by X-ray diffraction. Titanium silicalite (TS-2) is in the form of spherical particles of 0.5 μm polycrystalline.

Chemical analysis carried out on titanium silicalite (TS-2) gives 1.35% titanium.

Example 6

3 In a Pyrex glass reactor of 50 cm, equipped with a central stirring, a refrigerant connected to a gasometer, a regulated heating system and an injection system, we charge:

- 13.2 g of phenol - 8.0 g of water

- 0.8 g of titanium silcalite (TS-2) prepared in Example 3

Heated to 80 ° C with stirring, then injected in 2 hours, 3.8 g of an aqueous solution of hydrogen peroxide at 30% by weight.

Thirty minutes after the end of the pouring, the hydrogen peroxide was completely consumed (according to iodometric dosage).

The determination of diphenols by high performance liquid chromatography (HPLC) gives the following results:

- yield of pyrocatechol relative to transformed H2O2 (RT): 23.6%

- yield of hydroquinone compared to transformed H2O2 (RT): 52.4% - total yield of diphenols: 76%

Example 7

Example 6 is repeated but the titanium silicalite (TS-2) of Example 3 is replaced by that of Example 4. About 30 minutes after the end of the pouring, the hydrogen peroxide has been completely consumed .

- yield of pyrocatechol relative to transformed H2O2 (RT): 22.3% - yield of hydroquinone relative to transformed H2O2 (RT): 55.7%

- total yield of diphenols: 78%

Claims

1 - Process for the preparation of a titanium silicalite of the MEL type, characterized in that it consists in: - impregnating an amorphous xerogel (Tiθ2-Siθ2) comprising at least the silicon and titanium elements with a solution comprising at least one agent structuring agent and at least one mobilizing agent OH ^"' > the impregnating solution comprising at least one effective amount of a compound of the 3,5-dimethylpiperidinium type used as structuring agent, - subjecting the reaction mixture to heating until the crystallization temperature of the zeolite, - then recovering and calcining the zeolite obtained.

2 - Process according to claim 1 characterized in that one carries out the preparation of xerogel by hydrolysis in acidic medium, a source of silicon and addition of a source of titanium leading to a sol which is gelled by addition of 'a base or by heating and finally drying the gel.

3 - Method according to one of claims 1 and 2 characterized in that the source of silicon is an alkylsilicates, preferably tetraethylsilicate and the source of titanium is an alkyl o-titanate, preferably o-titanate tetraethyl or tetrabutyl o-titanate.

4 - Method according to one of claims 1 and 2 characterized in that the molar ratio Tiθ2 Siθ2 in the xerogel is preferably between 25 and

200, more preferably between 35 and 100.

5 - Process according to claim 2 characterized in that one dilutes the source of titanium by adding an organic solvent, preferably an alcohol with low carbon condensation, and more preferably isopropanol.

6 - Process according to claim 2 characterized in that a base is added in an amount is such that the pH of the solution obtained is between about 5 and about 7.

7 - Process according to claim 6 characterized in that the base used is a base which may be of mineral origin, preferably NH ₄ OH or a base of organic origin, preferably a compound of 3,5-dimethylpiperidinium type.

8 - Method according to one of claims 1 to 7 characterized in that the xerogel has a specific surface varying between 400 m ² / g and 700 m ² / g.

9 - Process according to claim 8 characterized in that the xerogel has a pore volume between 0.25 and 0.5 cm ³ / g: 80% of the pore volume being constituted by micropores having from 3 to 20 A.

10 - Process according to claim 1 characterized in that the mobilizing agent OH "introduced into the impregnation solution is in the form of base (s) weak (s) and / or strong (s) preferably containing no alkaline cations, preferably an amine or a quaternary ammonium hydroxide.

11 - Method according to claim 1 characterized in that the structuring agent introduced into the impregnation solution corresponds to the following formula:

in which :

- R _a and R _D , identical or different, represent an alkyl group, linear or branched, having from 1 to 7 carbon atoms; the groups R _a and R _D , can together form a ring system which includes the nitrogen atom, comprising from 4 to 7 carbon atoms, said system being able to be substituted or not by alkyl, linear or branched groups having from 1 with 3 carbon atoms,

- X ^" is an anion.

12 - Process according to claim 11 characterized in that the structuring agent is 3,5-dimethyl-N, N-diethylpiperidinium, 3,5-dimethyl-N-methyl-N-ethylpiperidinium.

13 - Method according to one of claims 11 and 12 characterized in that one associates 3,5-DMP with a tetraaikylammonium hydroxide, preferably a tetraethylammonium hydroxide, tetrapropyiammonium, tetrabutylammonium.

14 - Method according to one of claims 11 to 13 characterized in that one implements a mixture of 3,5-DMP and HTA; the amount of HTA being such that the HTA / 3.5-DMP molar ratio varies between 0.5 and 2.10 ^-3 and, preferably, between 0.1 and 0.01.

15 - Method according to claim 1 characterized in that the H2θ / Siθ2 ratio is less than 10, preferably between 3 and 10, and more preferably between 4 and 6.

16 - Process according to claim 1 characterized in that the OH7Siθ2 molar ratio varies between 0.05 and 1.0, preferably between 0.05 and 0.5.

17 - Process according to claim 1 characterized in that the molar ratio Q / SiO ₂ (in which Q represents 3,5-DMP) is at most equal to 0.13, preferably between 0.05 and 0.125, and even more preferably between 0.075 and 0.125; the molar ratio Q / Siθ (in which Q represents a mixture of 3,5-DMP and HTA) is between 0.125 and 1.0, preferably between 0.15 and 0.20.

18 - Method according to one of claims 1 to 17 characterized in that the crystallization of the zeolite is obtained by heating the xerogel, with stirring, at a temperature between 90 and 210 ° C, preferably between 120 and 190 ° C and, more preferably, between 150 and 170 ° C.

19 - A method according to claim 18 characterized in that one adds seeds of crystallization.

20 - Method according to one of claims 1 to 19 characterized in that the material obtained is subjected to drying at a temperature preferably chosen between 50 and 120 ° C, under atmospheric pressure or under reduced pressure chosen between 1 mm of mercury and atmospheric pressure.

21 - Method according to one of claims 1 to 20 characterized in that the dried material is preferably calcined in air, at a temperature of at least 350 ° C, between 400 and 600 ° C, preferably between 450 and 600 ° C. 22 - Titanium silicalite based on silicon oxide and titanium oxide having a MEL type structure and responding after calcination to the following formula:

If (96-x) ^Ti x ° 192 0 in which x is between 0.1 and 6, preferably between 0.1 and 4 and having a quadratic crystal system and an X-ray diffraction diagram defined in the table ( I):

23 - Zeolite according to Claim 22, characterized in that its crystals are in the form of parallelepipeds of the following average size:

- thickness between 50 and 500 nm, preferably between 100 and 250 nm,

- length between 100 and 1000 nm, preferably between 300 and 500 nm,

- width between 50 and 500 nm, preferably between 100 and 250 nm. 24 - Use of a zeolite described in one of claims 22 and 23 as catalyst or catalyst support for transformation reactions of various organic compounds.

25 - Use of a zeolite obtained according to the process described in one of claims 1 to 21 as catalyst or catalyst support for transformation reactions of various organic compounds.

26 - Use according to one of claims 24 and 25 in a process for the hydroxylation of a phenolic compound, with hydrogen peroxide in the presence of an effective amount of a zeolite.

27 - Use according to claim 26 characterized in that the phenolic compound corresponds to the formula

in said formula (III):

- R- ₍ , R2, R3 and R ₄ , identical or different, represent a hydrogen atom or any substituent,

28 - Use according to claim 27 characterized in that the phenolic compound is phenol.

29 - Use according to one of claims 26 to 28 characterized in that the catalyst represents by weight relative to the phenolic compound of formula (III) engaged, from 0.1 to 25%, preferably from 3 to 10% .