WO2014033413A1

WO2014033413A1 - Method for producing methacrolein and/or methacrylic acid

Info

Publication number: WO2014033413A1
Application number: PCT/FR2013/052014
Authority: WO
Inventors: Jean-Luc Dubois
Original assignee: Arkema France
Priority date: 2012-09-03
Filing date: 2013-09-02
Publication date: 2014-03-06
Also published as: FR2994973B1; FR2994973A1

Abstract

The invention relates to a method for the synthesis of methacrolein and/or methacrylic acid, consisting of: a first step in which a charge comprising a compound having formula R-CH(CH3)-CH2OH, wherein R is either CH₂OH (2- methyl-1,3-propanediol) or CHO (3-hydroxy-2-methyl-propanaldehyde), is subjected to a dehydration reaction, optionally in the presence of a dehydration catalyst; and a second step in which the effluent from the first step is subjected to an oxidation reaction in the presence of an oxidation catalyst, producing methacrolein and/or methacrylic acid. The method of the invention uses, as a raw material, a by-product or an intermediate product from the synthesis of butanediol-1,4 by means of hydroformylation of allyl alcohol.

Description

PROCESS FOR THE PRODUCTION OF METHACROLEIN AND / OR ACID

METHACRYLIC

The present invention relates to a process for the synthesis of methacrolein and / or methacrylic acid from 2-methyl-1,3-propanediol.

Methacrylic acid synthesis processes, known for more than a century, originally used cyanohydrin compounds as raw materials. Firstly, ethylene cyanohydrin (2-cyanoethanol of formula CN-CH ₂ -CH ₂ OI-1) was used, followed by cyanohydrin of acetone (2-cyano-2-propanol of formula CH). ₃ -C (CN) OH-CH ₃ also known as acetone cyanohydrin) which constitutes the conventional method in the field.

For the description of this process reference may be made to the techniques of the Engineer, Process Engineering, Processes, J 6400 1 -6. After the synthesis of acetone cyanohydrin by reacting acetone with hydrogen cyanide, the latter is reacted with sulfuric acid, which gives rise, by a strongly exothermic reaction, to the formation of oxyisobutyramide. monosulfate, which is converted into sulfuric methacrylamide. The latter is then hydrolysed to form methacrylic acid.

In another form of the process via acetone cyanohydrin, illustrated by patent EP 407 811, the latter is hydrated to α-hydroxyisobutyramide [(H ₃ C) (OH) (CH ₃ ) C-CO-NH ₂ ] which, reacting with methyl formate [HCOOCH ₃ ], produces the methyl α-hydroxyisobutyrate [(H ₃ C) (OH) (CH ₃ ) -COOCH ₃ ] which is dehydrated to methyl methacrylate by hydrolysis to methacrylic acid.

Methacrylic acid can also be obtained by transiting methacrolein as an intermediate product which is oxidized to methacrylic acid.

Methacrolein can come from the oxidation of isobutylene (patent applications EP 1 994 978 and EP 1 995 231). It can also be obtained from propylene hydroformylated in the presence of hydrogen and carbon monoxide to form methacrolein precursor isobutyraldehyde (GB Patent 2,094,782). It can also be obtained by decomposition of the Mannich base of propionaldehyde, formaldehyde or dimethylamine. Finally, it is proposed in FR 2137827 to synthesize methacrolein from 2-methylene-1,3-propanediol. It has also been proposed in US Pat. No. 2,811,545 to use isobutane of natural gas as raw material. Isobutane is in a first step oxidized to isobutylene glycol of formula CH ₃ -C (CH ₃ ) OH-CH ₂ OH. In a second step isobutylene glycol is oxidized to yield 2-hydroxyisobutyric acid CH ₃ - C (CH ₃ ) OH-COOH (or 2-hydroxy-2-methyl-propionic acid) which, in a third step is dehydrated to methacrylic acid.

Very recently, it has been proposed a novel route of synthesis of methacrylic acid using as raw material 2-methyl-1,3-propanediol. An article by Sang-Hyun Pyo et al. was published on April 20, 2012 on the web by the Royal Society of Chemistry entitled "A new route for the synthesis of methacrylic acid from 2-methyl-1,3-propanediol by integrating biotransformation and catalytic dehydration". In this paper, the authors describe a process in which in a first step 2-methyl-1,3-propanediol is oxidized by bio-conversion to 3-hydroxy-2-methyl-propionic acid via 3-hydroxy aldehyde. -2-methyl-propionic in the presence of a microorganism consisting of "Gluconobacter oxydans" cells carrying alcohol dehydrogenase (ADH) and aldehyde dehydrogenase (ALDH) enzymes. In the second step 3-hydroxy-2-methyl-propionic acid is dehydrated to methacrylic acid in the presence of a titanium dioxide catalyst.

The object of the present invention is to propose a new process for the synthesis of methacrylic acid using 2-methyl-1,3-propanediol or 3-hydroxy-2-methyl-propanal (also called 3-hydroxy) as raw material. 2-methylpropanaldehyde) which are, inter alia, a co-product or an intermediate product in the process of synthesizing 1,4-butanediol by hydroformylation of the allyl alcohol. 2-methyl-1,3-propanediol (CAS 2163-42-0, MW 90.12) is a branched saturated diol and does not correspond to 2-methylene-1,3-propane diol, unsaturated (CAS 3513 -81-3, MW 88) used in the prior art to synthesize methacrolein.

This type of process must, to go to the industrial stage, be able to be implemented under conventional operating conditions. It differs from the usual processes by the use of a different raw material and naturally by a different reaction scheme. Moreover, this new process makes it possible to overcome the bioconversion step described above which poses delicate problems when looking for industrial productions. In fact, the bioconversion stage requires specific investments and furthermore leads to a salt because of the pH of the fermentation close to 7, which must then be reconverted into acid, generally using sulfuric or hydrochloric acid - thus stoichiometrically generating as much salt as hydroxyisobutyric acid - to obtain the acid that must still be dehydrated in a specific plant using a heterogeneous catalyst.

The proposed new process combines a dehydration reaction and an oxidation reaction into a single industrial plant without the generation of inorganic salts. These reactions can be carried out in a single two-stage reactor or two consecutive reactors. The process of the invention differs from that of US Pat. No. 2,811,545 because of a different raw material, the choice of the oxidant used as well as the order of the steps, oxidation followed by dehydration in the case of US Pat. 2,811, 545.

The dehydration reaction can be carried out in the absence of a catalyst, thus the process of the invention can also consist of a one-step process, by oxidation-hydration reaction leading directly to methacrylic acid and which can be implemented. in a single reactor.

The oxidation reaction of the dehydrated product leads to the formation of methacrylic acid, but alternatively, the oxidation reaction can lead to another oxidized form, namely to methacrolein. The process according to the invention can then consist of a methacrolein synthesis process by dehydration reaction followed by an oxidation step, or in the preceding case where the dehydration is carried out in the absence of a catalyst, by reaction of oxydehydration.

The subject of the invention is thus a process for the synthesis of methacrolein and / or methacrylic acid consisting in a first charge-loading step comprising a compound of formula R-CH (CH ₃ ) -CH ₂ OH where R is either CH ₂ OH (2-methyl-1,3-propanediol), or CHO (3-hydroxy-2-methyl-propanaldehyde) to a dehydration reaction, optionally in the presence of a dehydration catalyst, then in a second step to subjecting the first stage effluent to an oxidation reaction in the presence of an oxidation catalyst leading to methacrolein and / or methacrylic acid.

The various reaction schemes can be summarized as follows, depending on whether the reactive product is 2-methyl-1,3-propanediol, or 3-hydroxy-2-methylpropanaldehyde: (1) CH ₂ OH-C (CH ₃ ) H -CH ₂ OH CH ₂ = C (CH ₃ ) -CH ₂ OH + H ₂ O

(2) CH ₂ = C (CH ₃ ) -CH ₂ OH + O ₂ -> CH ₂ = C (CH ₃ ) -COOH + H ₂ O

or (2 ') CH ₂ = C (CH ₃ ) -CH ₂ OH + O ₂ -> CH ₂ = C (CH ₃ ) -CHO + H ₂ O

Or (3) CH ₂ OH-C (CH ₃ ) H-CH ₂ OH + O ₂ -> CH ₂ = C (CH ₃ ) -COOH + H ₂ O

(3 ') CH ₂ OH-C (CH ₃ ) H-CH ₂ OH + O ₂ -> CH ₂ = C (CH ₃ ) -CHO + H ₂ O

Or

(1) CHO-C (CH ₃ ) H-CH ₂ OH CH ₂ = C (CH ₃ ) -CHO + H ₂ O

(2) CH ₂ = C (CH ₃ ) -CHO +1 ₂ 0 ₂ -> CH ₂ = C (CH ₃ ) -COOH

The operating conditions of the process are as follows:

According to a first embodiment of the invention, the dehydration step 1 is carried out at a temperature T such that 200 <T <350 ° C, preferably between 250 and 320 ° C, and under a pressure of between 0 , 5 and 5 absolute bar, preferably between 1.5 and 3.5 bar absolute in the presence of a solid catalyst of acid type. Preferably, the reactive stream contains oxygen or an oxygen-containing gas, and an inert gas such as CO ₂ or N ₂ . The oxygen content in the reactive flow may range from 0 to 20 mol%, preferably from 0 to 10 mol%. 2-methyl-1,3-propane diol (MPDO) is preferably used in the form of an aqueous solution of concentration ranging from 5% to 100% by weight, for example an aqueous solution of 10% to 50% by weight of MPDO.

The term solid acid catalyst means a solid consisting of a porous material insoluble in the reaction medium having a specific surface area greater than 5 m ² / g and preferably greater than 30 m ² / g and having few basic sites of surface such that the acid-type solid catalyst has an adsorption of less than 20 micromole SO ₂ / g.

The catalyst will generally be made from a substantially acidic solid material, which may optionally include basic sites due for example to surface impurities.

It may also be manufactured from an amphoteric solid material comprising both acidic and basic sites, and even from a basic solid material. In the latter two cases, the materials will be treated so as to minimize (limit) the amount of basic surface sites. It is clear that those skilled in the art would be deterred from choosing in the dehydration process used in the process of the invention the latter two types of materials as catalyst without prior modifications.

In order to obtain this partial or total elimination of the basic sites, the amphoteric or basic solid material will be subjected to cycles (contact with acid precursor solutions - acid neutralization base) / drying / calcination prior to the implementation of the catalyst in the process and also, if necessary, during the regeneration of the catalyst.

The basicity of a material may be related to its composition, for example its crystalline structure or to surface defects such as the presence of impurities such as alkali or alkaline earth metals.

The presence of basic sites and their amounts can be determined by any known method, for example by adsorption of an acid compound such as CO ₂ or SO ₂ and by micro-calorimetric measurements of the adsorption energy. The presence of acid sites and their amounts can in turn be measured by adsorption of a basic compound such as ammonia for example.

As examples of basic or amphoteric materials that can be used in the process of the invention, mention may be made of mixed oxides such as zirconium / lanthanum oxide, metal hydroxides such as hydroxyapatite or lamellar hydroxides such as hydrotalcites. Using the adsorption method of S0 ₂ , the materials considered basic or amphoteric are those reaching an adsorption greater than 20 micromoles SO ₂ / g.

When the material used is an acid but having basic sites (amphoteric or acidic solids with basic impurities), the acidity can be measured by NH ₃ adsorption or the Hammett acidity measurement method.

The amphoteric or basic materials, modified by impregnation of acidic compounds, neutralizing all or part of the basic contribution of the solid material, which are suitable are homogeneous or multiphase materials, insoluble in the reaction medium, which have an adsorption of less than 20 micromole S0 ₂ / g, an adsorption of ammonia greater than 20 micromoles of NH ₃ / g and preferably greater than 100 micromoles / g of NH ₃ / g, or have a Hammett acidity, denoted H ₀ , As reported in US Pat. No. 5,387,720 which refers to the article by K. Tanabe et al in "Studies in Surface Science and Catalysis", Vol 51, 1989, Chapters 1 and 2, the acidity of Hammett is determined by amine titration. using indicators or adsorbing a base in the gas phase.

The materials at the base of these catalysts can be chosen from natural or acidic siliceous materials, acidic zeolites, inorganic supports, such as oxides, coated with inorganic acids, mono, di, tri or polyacids, oxides. or mixed oxides, often basic or amphoteric, which can also be covered by inorganic acids, mono, di, tri or polyacids or (preferably) hetero-polyacids or hetero-polyacids salts solid catalyst hetero-polyacids which can themselves be deposited on supports.

These catalysts may be constituted by a heteropoly acid salt in which protons of said heteropoly acid are exchanged with at least one cation selected from the elements belonging to Groups 1 to 16 of the Periodic Table of Elements, these heteropoly acid salts containing at least one element selected from the group consisting of W, Mo and V.

Among the mixed oxides, mention may be made especially of those based on iron and phosphorus, those based on vanadium and phosphorus, those based on aluminum and phosphorus, boron and phosphorus, phosphorus or silicon and tungsten and those containing cesium, phosphorus and tungsten.

The catalysts may in particular be chosen from zeolites, Nafion® composites (based on fluorinated acid sulphonic acid), chlorinated aluminas, acids and salts of phosphotungstic and / or silicotungstic acids, and various metal oxide solids. such as tantalum oxide Ta ₂ 0 _5, niobium oxide Nb ₂ 0 _5, alumina Al ₂ 0 _3, titanium oxide Ti0 _2, Zr0 ₂ zirconia, tin oxide Sn0 _2, silica Si0 ₂ or Si0 ₂ silicoaluminate -Al ₂ 0 ₃ , impregnated with acidic functions such as borate B0 ₃ , sulfate SO ₄ , tungstate WO ₃ , phosphate PO ₄ , silicate SiO ₂ or molybdate MoO ₃ or a mixture of these compounds.

The foregoing catalysts may further comprise a metal promoter such as Au, Ag, Cu, Pt, Rh, Pd, Ru, Sm, Ce, Yt, Se, La, Zn, Mg, Fe, Co, Ni. The preferred catalysts are zirconium phosphates, tungsten zirconias, zirconia silicones, titanium, zirconium, aluminum or tin oxides impregnated with tungstate, silicotungstate or phosphotungstate, phosphated aluminas or silicas, heteropolyacids or sodium salts. heteropolyacids, iron phosphates and iron phosphates comprising a promoter, mixed vanadium-phosphorus oxides.

The oxidation step 2 is conducted in the gas phase using oxygen as oxidizing agent at a temperature of between 200 ° C. and 500 ° C., preferably between 200 ° C. and 350 ° C., under a pressure of between 0.5 ° C. and 10 bar absolute, preferably between 0.5 and 5 bar absolute, more preferably between 1 and 3 bar, and in the presence of solid oxidation catalysts, especially in the presence of solid catalysts containing Mo as the main active element.

The oxidation catalysts that can be used in the process of the invention will comprise molybdenum and at least one element chosen from P, Si, W, Ti, V, Nb, Ta, Cr, Mn, Fe, Co, Ni, Cu, Zn. , Ga, In, Ti, Sb, Ag, As, Ge, B, Bi, La, Ba, Sb, Ce Te, Pb, selected from the group consisting of mixed oxides containing molybdenum and heteropolyacids containing molybdenum.

These catalysts may be represented by the following general formula.

in which

A is at least one cation selected from the elements of Groups 1 to 16 of the

Periodic Classification of Elements and Lanthanides, preferably a cation of an alkali metal such as Cs, Rb or K,

X is P or Si and preferably P

Z is at least one element selected from the group consisting of W, Ti, Zr, V, Nb, Ta, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, In, Ti, Sn, Ag, As, Ge, B, Bi, La, Ba, Sb, Te, Ce and Pb and preferably Cu, Fe, Bi, Co, Ni, W, V, Cr, Sb, Mn, Ce,

O is oxygen a, b, c and d are indices consisting of satisfying whole or decimal numbers to the following ranges

0 <a <9 and preferably 0 <a <9

0 <b <2 and preferably 0.1 <b <1, 5

0 <c <12 and preferably 5 <c <12

0 <d <12 and preferably 0 <d <4

such that a + b + d> 0, and e is a number determined by the total degree of oxidation of the elements.

The catalyst used during this oxidation step may be a type of catalyst Fe-Mo-0 doped or not generally used during the oxidation of methanol to formaldehyde. Examples of iron molybdate catalysts (Fe-Mo-O) are described in Catalysis Review, Vol 47 (2004), pp 125-174. These catalysts are commercially available. For example, Sud Chemie supplies several grades of these catalysts under the trade name FAMAX ^® : FAMAX J5, FAMAX MS, FAMAX HS, FAMAX TH.

The propylene oxidation catalysts could be used as described in patents EP 900774, EP 1 125911, EP 1987877, EP1074938, US Pat. No. 6,268,529 and US Pat. No. 4,837,940. The catalyst then comprises molybdenum and at least one element selected from P, Si, W, Cr, Mn, Fe, Co, Ni, Bi, Sb, Ce.

Examples of particularly effective catalysts are mixed oxides and heteropoly acid salts each based on FeMo, CsPMo, CsPWMo.

The solid catalysts that can be used in this stage of the process can be prepared using techniques well known to those skilled in the art, for example those described in the application WO 09/128555 (heteropolyacids) or in "Catalysis Review, Vol 47 ( 2004), pp. 125-174 "and references thereto.

The solid component of the catalyst thus obtained is then generally calcined before being used in the process.

The calcination may be carried out in air or under an inert gas such as nitrogen, helium and argon or in a mixed atmosphere of oxygen and an inert gas. This calcination is usually conducted in an oven such as a muffle furnace, a rotary kiln or a fluidized bed furnace. The combustion temperature is generally between 150 and 900 ° C, preferably between 200 and 800 ° C and more preferably between 200 and 600 ° C. The duration of the calcination is generally between 0.5 and 10 h.

The oxidation catalysts may be in bulk form and used in this case without support.

The catalysts can also be deposited on an inactive support whose amount represents from 30 to 90% and preferably at least 50% of the total weight of the catalyst.

It is possible to use as support any material such as silica, alumina, magnesia, titanium oxide, zirconia, silicon carbide, silicates, diatomaceous earths, borates or carbonates provided that the materials are stable to the operating conditions to which the catalysts are subjected.

The bulk catalyst or the supported catalyst may be in the form of a granule or powder and may be in any form such as sphere, grain, trilobal and quadrilobe hollow cylinders, and in the form of cylinders, extruded or compressed optionally using a pelletizing agent.

The particle size will be, for example, 0.1 to 10 mm for use in a fixed bed and less than 1 mm for a fluidized bed.

Preferably, catalysts supported for the second step of the process according to the invention are used.

According to a second embodiment of the invention, the dehydration reaction of 2-methyl-1,3-propanediol is carried out in the absence of a dehydration catalyst, the oxidation step 2 then directly constitutes a step of dehydration. 'oxydehydration. It is then possible to obtain methacrylic acid in a single reactor from a reactive stream comprising 2-methyl-1,3-propanediol, oxygen, and optionally an inert gas, in the presence of an oxidation catalyst as described above.

Alternatively, the process according to the invention is carried out with an oxidation catalyst leading to the oxidized form methacrolein. Such catalysts correspond to the general formula shown above and can be selected for example from propylene oxide active catalysts for acrolein. The conditions described above, relating to 2-methyl, 1 -3 propane diol, apply to the use of 3-hydroxy-2-methyl-propanaldehyde as raw material to lead to methacrolein and / or acid methacrylic.

The following examples illustrate the present invention without, however, limiting its scope.

Example 1 Manufacture of Methacrylic Acid from 2-methyl-1,3-propanediol The catalysts used in this example are as follows:

- For the first step, using a tungsten zirconia (90.7% Zr0 ₂ - 9.3% W0 ₃ ) Daiichi Kigenso (supplier H 1417 reference). The catalyst has a loss on ignition at 1000 ° C. of 1.75% and a specific surface area of 47.4 m ² / g (BET, 1 point).

The catalyst of step 2 is prepared according to the method described below:

In 1000 ml of deionized water, 167 g of molybdenum oxide, 7.37 g of vanadium pentoxide and 14.675 g of 85% phosphoric acid are added, and the solution is refluxed (90 ° -100 ° C. ) for 5 hours to obtain a clear dark red solution.

Then 5.06 g of antimony trioxide was added and refluxing was continued at 90-100 ° C for 2 hours to dissolve the antimony trioxide. During this step, a succession of redox reactions takes place during which the oxidation states of the different elements adjust each other. It operates in an inert atmosphere of nitrogen. The resulting solution is a very dark blue.

Then the solution is cooled to room temperature. To the resulting solution is added a solution of cesium acetate containing 11.1 g of Cs acetate in 120 ml of pure water and a solution of 12.85 g of ammonium acetate in 120 ml of water. pure water by maintaining vigorous agitation. A suspension is formed, to which a solution containing 9.25 g of cupric acetate in 140 ml of pure water is added. The resulting suspension is left to mature for 1 hour at room temperature with stirring. She takes a blue-green color. Finally, the water is evaporated in a water bath to obtain a solid, which will be calcined under a stream of air at 310 ° C. for 5 hours. The solid obtained has the following molar composition Mo / V / P / Cu / Sb / Cs: 10 / 0.7 / 1, 1 / 0.4 / 0.3 / 0.5.

The measurements of the acidity and the basicity of the catalyst were carried out by adsorption of S0 ₂ (basicity) and NH ₃ (acidity) and by micro-calorimetric measurements of the adsorption energy. The operating conditions were as follows.

The tests were conducted at 150 ° C in a calorimeter (Setaram C80) connected to a conventional volumetric apparatus equipped with a Barocel capacitive pressure gauge for pressure measurements. The samples were pretreated in a quartz cell by heating overnight under vacuum at 300 ° C. This temperature was reached by increasing the temperature at a rate of 1 ° C / min. The differential adsorption heats were measured as a function of the repetitive rate of recovery of small doses of the respective gases on the sample until an equilibrium pressure of about 67 Pa was reached. The sample was degassed for 30 minutes. at the same temperature and a second similar adsorption series was carried out until an equilibrium pressure of about 27 Pa was reached. The difference between the amounts adsorbed between the first and second adsorption (at 27 Pa) represents the irreversibly adsorbed amount of the respective gases which provides an estimate of the number of strong sites respectively acid or base.

The process according to the invention is carried out according to the following procedure:

Two reactors in series of 2.54 cm diameter and 1 m long immersed in baths of molten salt at temperatures of 300 and 315 ° C respectively are fed with a VHV of 1000 h ^"1 with a mixture 02/2 -methyl-1,3-propanediol / H ₂ O / N 2 in the molar proportions 21 1/10/12 (the reactor is supplied with an aqueous solution containing 30% by weight of 2-methyl-1,3-propanediol). first reactor is charged with Daiichi Kigenso tungsten zirconia, having an acidity of 150 micromoles of ammonia per gram, and a basicity of 17 micromoles of S0 ₂ per gram, and the second of the catalyst Mo / V / P / Cu / Sb The heat point in the second catalyst bed is 330.degree.

After 3 hours of operation, the conversion of 2-methyl-1,3-propanediol is 99%, and the yield of methacrylic acid is 37.5%. Example 2 Manufacture of methacrylic acid from 2-methyl-1,3-propanediol without a first-stage catalyst.

The procedure is as above, but in the absence of catalyst for the first step. This catalyst is replaced by chemically inert steatite beads.

The contact time in the steatite bead zone maintained at 275 ° C. is 20 seconds.

The yield of methacrylic acid is 24%.

Example 3 Manufacture of methacrolein from 2-methyl-1,3-propane diol without a first-stage catalyst

The procedure is as in Example 2, but using as catalyst oxidation, the catalyst ACF-4 Nippon Shokubai, which is a catalyst used in particular for the oxidation of propylene to acrolein. The reaction is carried out at 250 ° C with a contact time of 2 seconds (in the zone containing the catalyst).

The yield of methacrolein is 42%

Claims

1) Process for the synthesis of methacrolein and / or methacrylic acid consisting in a first charge-loading step comprising a compound of formula R-CH (CH ₃ ) -CH ₂ OH where R is either CH ₂ OH (2- methyl-1,3-propanediol), or CHO (3-hydroxy-2-methyl-propanaldehyde) to a dehydration reaction, optionally in the presence of a dehydration catalyst, then in a second stage to submit the effluent of first step to an oxidation reaction in the presence of an oxidation catalyst leading to methacrolein and / or methacrylic acid.

2) Process according to claim 1 characterized in that the dehydration step is conducted at a temperature T such that 200 <T <350 ° C, preferably between 250 and 320 ° C and at a pressure between 0.5 and 5 absolute bar, preferably between 1, 5 and 3.5 bar absolute.

3) Process according to claim 1 or 2 characterized in that the dehydration reaction is carried out in the absence of catalyst.

4) Process according to claim 1 or 2 characterized in that the dehydration reaction is carried out in the presence of a solid catalyst of acid type.

5) Process according to claim 4 characterized in that the dehydration catalyst consists of a porous material insoluble in the reaction medium having a specific surface greater than 5 m ² / g and preferably greater than 30 m ² / g and comprising few basic surface sites such that the acid-type solid catalyst has an adsorption of less than 20 micromoles SO ₂ / g and an adsorption of ammonia greater than 20 micromoles NH ₃ / g and preferably greater than 100 micromoles NH ₃ /boy Wut.

6) Process according to claim 4 or 5 characterized in that the catalyst is selected from zeolites, Nafion® composites (based on sulfonic acid fluoropolymers), chlorinated aluminas, acids and phosphotungstic acid salts and and / or silicotungstics, and various solids of the metal oxide type, such as tantalum oxide Ta ₂ O ₅ , niobium oxide Nb ₂ O ₅ , alumina Al ₂ O ₃ , titanium oxide TiO ₂ , zirconia ZrO ₂ , tin oxide SnO ₂ , silica SiO ₂ or silicoaluminate SiO ₂ -Al ₂ O ₃ , impregnated with acidic functions such as borate B0 ₃ , sulfate S0 ₄ , tungstate WO ₃ , phosphate PO ₄ , silicate SiO ₂ or molybdate MoO 3 or a mixture of these compounds.

7) Process according to any one of claims 4 to 6 characterized in that the catalyst is selected from phosphated zirconia, tungsten zirconia, silicon zirconia, titanium oxide, zirconium, aluminum or tin impregnated tungstate, silicotungstate or phosphotungstate, phosphated aluminas or silicas, heteropolyacids or heteropolyacid salts, iron phosphates and iron phosphates comprising a promoter, mixed vanadium-phosphorus oxides. 8) Process according to any one of the preceding claims characterized in that the oxidation step is conducted in the gas phase using oxygen as an oxidizing agent at a temperature between 200 ° C and 500 ° C, preferably between 200 and 350 ° C, at a pressure between 0.5 and 10 bar absolute, preferably between 0.5 and 5 bar absolute .. 9) A method according to any one of the preceding claims characterized in that the step d The oxidation is carried out in the presence of solid oxidation catalysts containing Mo as the main active element.

10) Process according to any one of the preceding claims, characterized in that the oxidation catalysts comprise molybdenum and at least one element chosen from P, Si, W, Ti, V, Nb, Ta, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, In, Tl, Sb, Ag,

As, Ge, B, Bi, La, Ba, Sb, Ce Te, Pb, in the form of mixed oxides containing molybdenum or heteropolyacids containing molybdenum.

1 1) Process according to any one of the preceding claims characterized in that the oxidation catalysts may be represented by the following general formula.

A is at least one cation chosen from the elements of Groups 1 to 16 of the Periodic Table of Elements and the lanthanides, preferably an alkali metal cation such as Cs, Rb or K,

X is P or Si and preferably P Z is at least one element selected from the group consisting of W, Ti, Zr, V, Nb, Ta, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, In, Ti, Sn, Ag, As, Ge, B, Bi, La, Ba, Sb, Te, Ce and Pb and preferably Cu, Fe, Bi, Co, Ni, W, V, Cr, Sb, Mn, Ce,

O is the oxygen a, b, c and d are indices consisting of integers or decimals satisfying the following ranges

0 <a <9 and preferably 0 <a <9

0 <b <2 and preferably 0.1 <b <1, 5

0 <c <12 and preferably 5 <c <12

0 <d <12 and preferably 0 <d <4 such that a + b + d> 0 and e is a number determined by the total degree of oxidation of the elements.

12) Process according to any one of the preceding claims, characterized in that the oxidation catalysts are used in bulk form.

13) Process according to any one of the preceding claims characterized in that the oxidation catalysts are deposited on a support whose amount represents from 30 to 90% and preferably at least 50% of the total weight of the catalyst.