AZOIC COMPOUNDS SYNTHESIS PROCESS

The present invention relates to a process for the synthesis of an azo compound, such as razo-bis-isobutyronitrile (AZDN or AIBN), by oxidation of a hydrazo compound.

AZDN is an azo compound commonly used in radical polymerization processes, in particular as an initiator or catalyst. It is also known as a blowing agent for the manufacture of PVC foams or silicone gaskets.

[0003] Azo compounds are conventionally produced by the chlorine oxidation of a hydrazo compound (WO 2006/067315). This process has also been applied to other azo compounds such as hydrazoamides (GB 976552). However, this synthesis process has the major drawback, in addition to the intrinsic dangerousness of chlorine, of generating hydrochloric acid as a by-product, so that the effluents cannot be easily recycled. It emerges that the chlorine process is therefore not suited to current environmental constraints.

To overcome this drawback, a process for the synthesis of azo compounds has been proposed using an oxidizing agent devoid of chlorine such as hydrogen peroxide (EP2821393). In order for the reaction to be sufficiently complete and rapid, this process requires the presence of an activating agent, generally a bromine compound such as a bromide, or else an iodine derivative, used in an acid medium. The reactivity of hydrogen peroxide, and thus the yield of the reaction, are generally improved by the use of metal catalysts in addition to bromine or iodine compounds, in particular catalysts based on molybdenum or tungsten (cf. CS232350 and A. PALOMO et al., “Afinidad”, (1985), 42 (397), 312-314) which have the advantage of being less toxic than tellurium, vanadium or selenium,

[0005] Despite its undoubted advantages over the chlorine process, the hydrogen peroxide process however also generates potentially harmful effluents for the environment, namely the residual water (also called mother liquors) resulting from filtration of the reaction medium to separate the AZDN therefrom. This residual water in fact contains a not insignificant quantity of the catalyst and of the activators used. It has therefore been suggested, not only for environmental issues, but also to improve the economics of the process, to recycle this residual water in the reaction, possibly after concentration (cf. for example CS 239407 and CS 237123).

[0006] In addition, the synthesis process also leads to the formation of ammonium ions. Now, in the presence of these ammonium ions, the catalyst tends to precipitate in the form of a yellow deposit which pollutes the azo compound formed and, consequently, decreases the purity of the latter.

[0007] Furthermore, the precipitation of the catalyst also causes instability of the residual water, thus limiting their recycling. Also, application EP2821393 proposes a process in which the addition, in the reaction medium, of a reducing agent such as hydrazine makes it possible to improve the stability of the residual water. However, this application does not completely solve the problems of catalyst stability, in particular when the residual water is concentrated before being recycled. Indeed, after several recycles, the precipitation of a species containing ammonium ions is observed while no ammonium ion has been introduced into the medium. These ammonium ions, accumulated during the process and resulting from the degradation of the nitrile functions of the azo derivative used, induce precipitation of the catalyst.

It is therefore necessary to find a process for the synthesis of azo compounds in which in particular the catalyst does not precipitate, thus leading to the synthesis of an azo compound in which the level of residual catalyst is greatly reduced and to better stability. of residual water.

[0009] Thus, the present invention proposes to resolve the problem of precipitation of the catalyst by adding a particular complexing agent which will be described in the description which follows. Other objects will also appear in said description of the present invention.

[0010] According to a first object, the invention relates to a process for synthesizing an azo compound, said process comprising the steps:

a) reacting an oxidizing agent with a hydrazo compound, at least one catalyst and at least one compound of formula (I):

(R 1 ) (R2) C (P03 (R3) 2 ) 2 (I)

in which

R 1 and R 2 , identical or different, are chosen independently of one another from the hydrogen atom, a saturated or unsaturated, linear, branched or cyclic hydrocarbon chain, optionally substituted, -OH and -O-alkyl , where “alkyl” represents a saturated hydrocarbon chain, optionally substituted, linear or branched, comprising from 1 to 6 carbon atoms; and

R 3 is chosen from the hydrogen atom and metal or ammonium ions; so as to form a solution containing an azo compound;

b) recovering all or part of the reaction mixture obtained in step a);

c) separating the reaction mixture recovered into a fraction containing the azo compound and a residual water fraction; and

d) recovering and optionally washing and drying the azo compound obtained.

In the above process, the compound of formula (I) acts as a complexing agent for the catalyst and will be designated, in the remainder of the application by the term "complexing agent".

By "optionally substituted" is meant that the hydrocarbon chain and the alkyl radical may be substituted by -OH, a halogen and N (R 4 ) (R5), R 4 and R 5 , identical or different, are chosen independently of one another from the hydrogen atom, an alkyl radical, linear or branched, comprising from 1 to 6 carbon atoms or else R 4 and R 5 , can form together, and with the atom d 'nitrogen to which they are attached, a 4, 5, 6 or 7 membered, preferably 5 or 6 membered, ring.

According to a preferred embodiment, within formula (I), Ri is -CH 3 and R 2 is -OH.

In formula (I), R 3 is chosen from -H and metal or ammonium ions. Said metal ions can be chosen, without limitation, from the metals represented by columns 1 to 13 of the periodic table of the elements, preferably from sodium, lithium, potassium, calcium, magnesium, aluminum, zinc, cadmium, manganese, nickel, cobalt, cerium, copper, tin, iron and chromium, more preferably from sodium, lithium, potassium, calcium, magnesium, especially among sodium and potassium. R 3 can also be chosen from ammonium ions and alkyl ammonium ions, in particular alkyl amines, alkylene amines and alkanolamines containing no more than two amine groups, such as ethylamine, diethylamine, propylamine, hexylamine, 2-ethylhexylamine, N-butylethanolamine and mono-, di- or tri-ethanolamines.

[0015] Within formula (I), the radicals R 3 can be identical or different. According to a preferred embodiment, they are all identical, and very particularly preferably, they all represent -H.

In a preferred embodiment, the complexing agent can be chosen from alkyldiphosphonic compounds, such as, for example, hydroxymethylene diphosphonic acid, 1-hydroxylethylidene-1, 1 -diphosphonic acid (HEDP) and 1-hydroxybutanediphosphonic acid, in their acid form or in the form of their salts. Of

particularly preferably, the organic complexing agent is 1-hydroxylethylidene-1, 1 -diphosphonic acid (HEDP) also known under the trade name Dequest ® 2010.

The complexing agent can also be zoledronic acid or one of its salts (zoledronate), that is to say the acid 1 -hydroxy-1 - (1 / - / - imidazol-1 -yl ) ethane-1, 1 -diyl] diphos-phonique or one of its salts respectively, these being marketed by the company NOVARTIS under the brands Zomate ® , Zomera ® , Aclasta ® and Reclast ® .

[0018] According to one embodiment of the invention, the molar mass of the complexing agent is less than 500 g. mol -1 , preferably less than 450 g. mol -1 , more particularly less than 400 g. mol 1 .

[0019] According to another embodiment, the complexing agent is soluble in an aqueous or hydro-organic medium, preferably in an aqueous medium, and more particularly in water.

The complexing agent can be used in a molar ratio of the complexing agent to the moles of molybdenum supplied by the catalyst greater than 0.05: 1, preferably between 0.1: 1 and 5: 1, more especially between 0.5: 1 and 2: 1.

The hydrazo compound oxidized during step a) of the process can be chosen from symmetrical hydrazo compounds carrying nitrogenous functions, in particular nitrile or amide functions, such as 2,2'-hydrazo-bis-isobutyronitrile, 2,2'-hydrazo-bis-methylbutyronitrile, 1, T-hydrazo-bis-cyclohexanecarbonitrile or 2,2'-hydrazo-dicarbonamide, preferably 2,2'-hydrazo-bis-isobutyronitrile.

The oxidizing agent present in step a) of the process can be of any type, and can be chosen from inorganic peroxide derivatives such as, for example, hydrogen peroxide, potassium or sodium persulphates, potassium or sodium monohydrogénopersulfates, dioxygen, organic peroxidic derivatives soluble in water such as for example peracetic acid. According to a preferred embodiment, the oxidizing agent is hydrogen peroxide.

The oxidizing agent is generally introduced into the aqueous solution at a temperature of 0 ° C to 40 ° C, preferably 0 ° C to 0 ° C, for a period ranging from 2 hours to 6 hours.

The oxidizing agent is generally used in a slight molar excess relative to the hydrazo compound. The molar ratio of the oxidizing agent to the hydrazo compound is thus advantageously between 1: 1 and 1.1: 1, preferably between 1.01: 1 and 1.05: 1, limits included.

The catalyst present during step a) of the process comprises a water-soluble compound chosen from salts and acids based on a catalytic metal from columns 5 and 6 of the periodic table of the elements and preferably chosen from molybdenum and tungsten, preferably molybdenum. Examples of such water-soluble compounds are in particular the alkali metal or ammonium salts of molybdenum, the alkali metal or ammonium salts of tungsten, phosphomolybdic acid and its alkali or ammonium salts, l phosphotungstic acid and its alkali metal or ammonium salts, molybdosulphates and mixtures thereof. Phosphomolybdic acid and its alkali salts are preferred, in particular sodium phosphomolybdate and potassium phosphomolybdate.

The catalyst can be used in a molar ratio of moles of metal provided by the catalyst to the hydrazo compound ranging for example from 0.005: 1 to 0.5: 1, preferably from 0.03: 1 to 0.1: 1.

In the process according to the invention, the complexing agent leads to a low precipitation of the catalyst, or even an absence of precipitation. In fact, it has been discovered, surprisingly, that the complexing agent of formula (I), by forming a complex with the catalyst, results in the absence of precipitation of said catalyst. Another advantage of the complexing agent is that it also allows the catalyst not to lose its reactivity, which allows said catalyst to be recycled and reused in the process without having lost its effectiveness. The complexing agent / catalyst complex is eliminated in the residual water thus allowing all, or almost all, of the catalyst to be recovered in the residual water without, however, altering the stability of said residual water.

According to a preferred embodiment, the solution in step a) is an acidic solution, the pH of which is preferably between 0 and 2. Thus, for the pH to be acidic, it is possible to add an organic or inorganic acid. Preferably, if the acid is an inorganic acid, it is chosen from hydrochloric acid, sulfuric acid, hydrobromic acid, phosphoric acid, and their mixtures, preferably hydrochloric acid. Preferably, if the acid is an organic acid, it is chosen from formic acid, acetic acid, and mixtures thereof. The acid can be used in a molar ratio of acid to hydrazo compound ranging, for example, from 1: 1 to 1: 5.

To further improve the kinetics of the reaction and / or the reaction yield, it is advantageous to carry out oxidation step a) in the presence of an activating agent. Said activating agent can be chosen from halogens and halides, such as for example, and without limitation, bromine, iodine, hydrobromic and hydroiodic acids and their salts, preferably chosen from hydrobromic acid and a alkali metal bromide such

as for example, potassium bromide and sodium bromide. It can be used in a molar ratio of activator to hydrazo compound ranging, for example, from 0.1: 1 to 1: 1, more particularly from 0.3: 1 to 0.7: 1.

[0030] A surfactant can optionally be added to the solution, in particular an anionic surfactant, in particular an alkyl sulfosuccinate such as di-2-ethylhexyl sulfosuccinate.

During step a) of oxidation, the order of introduction of the various compounds into this solution is not critical.

At the end of this step a), an aqueous solution is obtained containing an azo compound, at least one complexing agent / catalyst complex and a quantity of oxidizing agent. Said aqueous solution may optionally comprise an amount of an organic or inorganic acid and / or of an activating agent, and / or of a surfactant if these compounds have been added during step a).

It may be advantageous to add to the aqueous solution obtained at the end of step a) at least one reducing agent chosen from hydrazine, sodium sulphite, sodium bisulphite and their mixtures. According to a preferred embodiment, and in the case where the compound soluble in water is phosphomolybdic acid, the reducing agent is hydrazine.

Hydrazine has the advantage of forming only water and nitrogen gas in the effluents and in the recycling loop of the process, which prevents the accumulation of salts during recycling.

The reaction mixture then obtained at the end of step a) is then recovered in whole or in part, for example up to 30% to 60% by weight, in particular 45% to 55% by weight, at during step b) of the method according to the invention.

The reaction mixture optionally not recovered at the end of step b) can be recycled in step a), while the recovered reaction mixture is separated, in step c), into a fraction containing the azo compound and a residual water fraction. During step c), the azo compound can be separated by any technique known to those skilled in the art, preferably by filtration or centrifugation, and more particularly by centrifugation.

It may be advantageous to add to the fraction of residual water at least one reducing agent (even if it has already been added at the end of step a)). Said reducing agent is chosen from those already described above, and for example from hydrazine, sodium sulphite, sodium bisulphite and their mixtures.

[0038] According to a preferred embodiment, and in the case where the water-soluble compound is phosphomolybdic acid, the reducing agent is hydrazine. According to a preferred embodiment, the reducing agent is added at the end of step c). According to another preferred embodiment of the invention, the reducing agent is added at the end of step a) and at the end of step c).

The reducing agent is generally used in an amount necessary and sufficient to neutralize the excess oxidizing agent measured at the end of step a) or of step c) of the process.

The fraction containing the azo compound can be washed one or more times with water, in a step d), so as to recover an azo compound having a purity greater than 90% and washing water. This high degree of purity of the azo compound is obtained in particular by the absence of precipitation of the catalyst, this absence of precipitation being linked to the presence of the complexing agent which, as described above, forms a complex with the catalyst, said complex being eliminated in residual water.

The azo compound recovered after the separation step c) or the washing step d) can optionally undergo an additional purification step, for example, by recrystallization. This recrystallization can be carried out by any technique known to those skilled in the art such as, for example, that described in patent application WO 2006/072699 which specifies that the recrystallization can be carried out in a solvent such as, for example, chloride of methylene, methanol and methyl ethyl ketone. This step makes it possible to eliminate any possible trace of complexing agent.

In a subsequent step e) of the process, the fraction of residual water can be recycled in whole or in part in step a), it being understood that steps a) to e) are optionally repeated at least once, that is, the waste water fraction can be recycled at least twice. In a variant of the process according to the invention, the fraction of residual water can be concentrated, in particular by distillation, before being recycled. The distillate obtained can then be easily treated, for example incinerated, with a view to being discharged into the environment.

The method according to the invention allows, among other things, easy treatment of the effluents with a view to their recycling in the process and / or their treatment with a view to their discharge into the environment. To this end, it may comprise an additional step of treating all or part of the fraction of residual water produced in step c) and / or of the washing water produced in step d) using an adsorbent, such as activated carbon, to retain the catalytic metal.

The method according to the invention may further comprise a step of recovering the catalytic metal in the form of an aqueous solution, by treating the adsorbent with the aid of a basic aqueous solution, in particular of sodium hydroxide. It is industrially preferred to pass the fraction of residual water and / or of the washing water to be treated over a column containing the adsorbent, in a granulated form for example, then to recover the catalytic metal by passing a basic solution over this column. , according to well-known techniques for using these adsorbents.

The aqueous solution of catalytic metal can thus be recycled in step a) of the process, optionally after concentration, while the residual water resulting from the filtration of the adsorbent and / or the washing water of the adsorbent may be released into the environment. This makes it possible to recover and recycle an effective amount of catalyst, while discharging into the environment an effluent which is very poor in catalyst.

The method according to the invention makes it possible to obtain in a reasonable time a high particle size of the azo compounds produced, typically close to 150 μm (as measured by laser diffraction), which may prove to be advantageous in certain applications.

Thus, the process of the present invention is very particularly advantageous when the residual water and / or the catalyst and / or the activating agent, and in particular the residual water and / or all the components of the catalytic system, are recycled. . Among other things, these recycling operations have the great advantage of not discharging the catalytic system and in particular the expensive metal salts into the effluents, but also and above all harmful for the environment.

A subject of the invention is also the use as a complexing agent of a compound of formula (I) as defined above. According to one embodiment, said compound of formula (I) is used as a complexing agent for a catalyst, preferably in a process for the synthesis of an azo compound. According to one embodiment, said compound of formula (I) is used as a complexing agent in a process for the synthesis of an azo compound. Preferably, said compound of formula (I) is used as a complexing agent of a catalyst, for example of a catalyst as defined above, in a process for the synthesis of an azo compound comprising a step making react an oxidizing agent with a hydrazo compound, at least one catalyst and at least said compound of formula (I) so as to form a solution containing an azo compound. Preferably, said compound of formula (I) is used as a complexing agent for a catalyst, for example of a catalyst as defined above, in a

process for the synthesis of an azo compound, for example in the process as defined above.

A third subject of the invention is the residual waters comprising the complexing agent / catalyst complex described above, said residual waters being obtained during the process for the synthesis of an azo compound according to the invention.

The invention will be better understood in the light of the following examples, which are given for illustrative purposes only and in no way limit the scope of the invention.

EXAMPLES

In the following examples:

- DHC denotes hydrazo-bis-isobutyronitrile, a hydrazo compound obtained industrially by reaction of acetone cyanohydrin with hydrazine hydrate, filtration then washing with water, and stored in a refrigerator (T <10 ° C). Its moisture content is 12.7% and its purity is greater than 99% by analysis.

- The phosphomolybdic acid used is a product sold by the company Aldrich which corresponds to the formula H 3 [R (Mq3qio) 4] .cH 2 0 and is used as such (molar mass 1825.25 g / mol in anhydrous form) . The product used has a 50% molybdenum content.

- DOSS denotes di (2-ethylhexyl) sulfosuccinate.

- AZDN (or AIBN) denotes azo-bis-isobutyronitrile.

Method of sampling and determining the level of residual peroxide

In the following examples, the levels of residual peroxide are measured as follows. About 3 mL to 5 mL of the suspension reaction mixture is taken and filtered in order to remove the solid DHC and AZDN present. About exactly 1 gram of the filtered solution is weighed out and introduced into a 250 mL flask, 50 mL of distilled water, 15 mL of 30% by weight sulfuric acid and 15 mL of a solution of Kl at 30%. The bottle is stoppered and then left in the dark for 15 min. It is then titrated with a solution of sodium thiosulphate of normality 0.1 N until disappearance of the yellow color. The residual hydrogen peroxide (H 2 0 2 ) content is calculated as follows:

Volume of 0.1 N sodium thiosulfate (in mL)

% HO residual =

2 x 100 x mass of filtrate (in grams)

The determination of the residual molybdenum is carried out by the optical ICP technique (or Plasma Couplé Induction). The particle size measurement is carried out using a Masterziser® S device. The measurement is made on the wet crystals using water and a drop of Igepal® surfactant (ethoxylated nonylphenol) as dispersant, after 10 minutes of circulation in the measuring cell.

Example 1: Test for complexing agents

A mixture is prepared containing 100 ml_ of an aqueous solution of hydrogen bromide at 5% by mass, 1.5 g of phosphomolybdic acid (catalyst) and 1 g of complexing agent.

After 30 minutes, a first observation is made on the solubility of phosphomolybdic acid in the absence of ammonium bromide (NH 4 Br).

Then added 10 ml_ of a 37.5% aqueous solution of ammonium bromide. After 30 minutes, a second observation is made on the solubility of phosphomolybdic acid in the presence of ammonium bromide.

The results are collated in Table 1 below:

- Table 1 -

The tests demonstrate that the compounds other than those corresponding to formula (I) do not make it possible to avoid the appearance of a precipitate in the presence of ammonium ions. On the other hand, when the complexing agent meets the definition of formula (I), no precipitate is observed. This absence of precipitate reflects the stability of the phosphomolybdic acid in the presence of the complexing agent, in a medium containing ammonium ions. The complexing agent of formula (I), for example Dequest ® 2010, makes it possible to avoid the precipitation of phosphomolybdic acid in the presence of ammonium ions.

Another test, carried out under the same operating conditions as above, was carried out in order to determine the amount of compound of formula (I) sufficient to avoid the precipitation of phosphomolybdic acid in the presence of NH 4 Br: the amount of complexing agent of formula (I) was gradually reduced until the formation of a precipitate was observed.

This series of tests made it possible to demonstrate that, in a solution comprising containing 100 ml of an aqueous solution of hydrogen bromide at 5% by mass, 1.5 g of phosphomolybdic acid (catalyst) and 5 mL of an NH 4 Br solution at 37.5% by mass, a quantity of 0.35 g of complexing agent of formula (I), such as Dequest ® 2010, is sufficient to prevent the precipitation of phosphomolybdic acid , or a complexing agent / phosphomolybdic acid molar ratio of 0.2.

Example 2: Counterexample (process in the absence of complexing agent)

In a reactor with a capacity of 1.5 L and equipped with a stirring system allowing the mixing of a suspension, 126 g of AZDN (0.77 mol), 128 g of DHC are introduced (hydrazo-bis-isobutyronitrile) (0.77 mol), an aqueous solution composed of 635 g of water, 35 g of HBr, 7 g of phosphomolybdic acid (Mo content of 50%, or 0.036 mol of molybdenum ), and 0.1 g of DOSS (di- (2-ethylhexyl) sulfosuccinate). After starting the stirring and the double-jacket cooling system, the temperature is expected to stabilize at around 10 ° C.

Once the temperature of 10 ° C is reached, continuously introcLiit, 78.05 g of H 2 0 2 at 35% (0.80 mole of H 2 0 2 ) for 4 h. During the reaction, the medium is maintained at a temperature between 10 ° C and 12 ° C. The reaction is stopped approximately 20 to 30 minutes after having introduced the entire quantity of H 2 O 2 . The end of the reaction is made visible by the formation of bromine and by the increase in the redox potential (before the introduction of H 2 0 2 , the potential is about 400 mV and at the end of the reaction, that is - i.e. after the introduction of all of GH 2 0 2, the potential is about 800 mV). The redox potential is measured using a platinum redox probe.

The reactor suspension is then completely filtered. 290 g of crude AZDN are obtained containing 15% moisture, ie a yield of 95%. In addition, 20 mL of water is also used to rinse the filter and added to the residual water.

The residual filtration water obtained, ie 740 g, is neutralized with hydrazine hydrate in order to remove the excess peroxide and the bromine formed (by assaying 0.1% of H 2 equivalent is found 0 2 residual). The redox electrode is used to bring the potential back to its initial value, ie approximately 300 mV to 400 mV.

Half of the residual water, or 374 g, is set aside to be reused directly in the next test, while the other half undergoes a vacuum concentration step aimed at removing about 60% to 65% of water from this fraction of the residual water. This is done under a vacuum of 250 millibars (25 kPa) between 55 ° C and 60 ° C. For 325 g of residual water used, 235 g of an aqueous distillate fraction are thus obtained, and 90 g of the resulting concentrated solution. This resulting concentrated fraction, containing the catalytic system, can therefore be reused with the fraction of untreated residual water for the following cycle.

Half of the AZDN obtained is washed with water four times using 200 ml of water each time. The other half of the crude AZDN is kept as it is (with the fraction of residual mother liquors contained in the crystals) to be recycled during the cycle following the following example.

Thus, practically all the components of the catalytic system are recycled, apart from the fraction inherent in the losses of transfer and of sampling for assays as well as the residual water fraction retained in the crystals of the washed AZDN produced.

Example 3: Counterexample (process without complexing aqent and with recvclaqe)

145 g of the unwashed crude AZDN obtained previously (ie 0.78 mol of AZDN containing approximately 15 g of residual water in the crystals), 128 g of DHC are introduced into the reactor of the previous example. (hydrazo-bis-isobutyronitrile) (0.78 mol), 0.05 g of DOSS (di- (2-ethylhexyl sulfosuccinate)), 374 g of residual water resulting from the filtration of the reaction medium during the test previous test as well as the residual water concentrated during the previous test. The bromides contained in the recycled residual water fractions are assayed in order to determine the necessary make-up in the catalytic system. In order to maintain a reaction medium of the same composition as that of the preceding example, 4 g of HBr, 0.78 g of phosphomolybdic acid and 150 g of water are typically added,

This water, 150 g, will make it possible, in an industrial process, to more easily load the solid DHC in the form of an aqueous suspension from a buffer tank for example.

The reaction is then carried out as in the previous example, continuously flowing 78.05 g of H 2 0 2 at 35% over a period of 4 hours in the reaction medium maintained between 10 ° C and 12 ° C.

The operation is thus carried out by recycling in test n + 1 half of the AZDN and of the residual water obtained and concentrating it with the other half of the residual water in test n.

The first three recycling operations lead to obtaining white products with a purity greater than 95% by NMR analysis. During the fourth recycling there is a formation of a large deposit of yellow color. This deposit pollutes the AZDN obtained, washing with water does not allow this deposit to be eliminated.

It is also noted that in the residual water obtained after filtration, the deposit still forms over time.

The following concentration step was carried out on this residual water, after filtration to remove these crystals. At the end of the concentration, it is noted that these crystals have reformed in the bottom of the boiler and therefore pollute the concentrated solution of residual water.

Analysis of this deposit, by X-ray fluorescence and X-ray diffraction, shows that it is a crystalline compound of ammonium phosphomolybdate, these compounds are known to be very insoluble in acidic medium.

The formation of ammonium ions in the medium appears to be attributable to the hydrolysis of the cyanated by-products. The ammonium ions thus accumulate as they are recycled in the reaction medium, causing precipitation of the catalyst.

Example 4: Process in the presence of Dequest® 2010 (according to the invention)

[0077] The process is carried to the invention, according to the same procedure as described in Example 2-cons, but adding 20,7 g of a 60% aqueous solution of Dequest ® 2010 (or 1 , 65 moles of Dequest ® 2010 per mole of molybdenum) in the reaction medium before the introduction of the hydrogen peroxide solution.

A series of recycles is carried out as described in counter-example 3. In this example, 12 recycles are thus carried out, without noting the formation of a precipitate polluting the AZDN obtained or precipitating in the mother liquors or during the operations of concentration.

The supplements in the catalytic system are carried out throughout the process as indicated in counter-example 3, but in this example, for an additional 4 g of HBr, 1.4 g of Dequest are also added. ® 2010 (i.e. 2.35 g of 60% aqueous solution of Dequest ® 2010).

In the twelfth recycling, the yields of washed AZDN remain greater than 95%, the residual bromide content of the washed AZDN is 80 ppm to 90 ppm and the purity by NMR analysis is greater than 98%. The particle size remains constant and between 110 μm and 130 μm (number average diameter).

Example 5: Recrystallization of the AZDN obtained according to the process

The AZDN obtained according to Example 4 at the end of the twelfth recycling is washed 3 times with 200 ml of water and then dried at room temperature (residual humidity level 1% to 2% by weight). 20 g of this AZDN are then dissolved in 250 ml of methanol at 35 ° C. When the crystals are dissolved, the temperature is lowered to 20 C to recrystallize the AZDN. The mixture is then filtered cold, the fraction of methanol solvent recovered is set aside to be reused during the next recrystallization. The AZDN crystals recovered on the filter are washed with a 50 ml portion of water and then dried.

The operation is repeated by taking again 20 g of the AZDN obtained at the end of the twelfth recycling and using the fraction of methanol recovered previously, and possibly supplementing to compensate for the losses in solvent or completely dissolve the 20 g of AZDN engaged.

Five successive recrystallizations were thus carried out, each time re-engaging the fraction of methanol recovered. The search for traces of molybdenum by the ICP Optical method on an ICAP 6500 spectrometer device shows that all the AZDNs thus obtained are free of traces of residual molybdenum (detection limit of 1 ppm).

CLAIMS

1. A method of synthesizing an azo compound, said method comprising the steps:

a) reacting an oxidizing agent with a hydrazo compound, at least one catalyst and at least one compound of formula (I):

(R 1 ) (R2) C (P03 (R3) 2 ) 2 (I)

in which

R 1 and R 2 , identical or different, are chosen independently of one another from the hydrogen atom, a saturated or unsaturated, linear, branched or cyclic hydrocarbon chain, optionally substituted, -OH and -O-alkyl , where “alkyl” represents a saturated hydrocarbon chain, optionally substituted, linear or branched, comprising from 1 to 6 carbon atoms; and

R 3 is chosen from the hydrogen atom and metal or ammonium ions; so as to form a solution containing an azo compound;

b) recovering all or part of the reaction mixture obtained in step a);

c) separating the reaction mixture recovered into a fraction containing the azo compound and a residual water fraction; and

d) recovering and optionally washing and drying the azo compound obtained.

2. The method of claim 1, wherein the compound of formula (I) is selected from 1-hydroxy-1 - (1 / - / - imidazol-1 -yl) ethane-1, 1 -diyl] diphosphonic acid or one of its salts, hydroxymethylenediphosphonic acid, 1-hydroxylethylidene-1, 1 -diphosphonic acid (HEDP) and 1 -hydroxybutanediphosphonic acid, in their acid form or in the form of their salts, preferably the compound of formula (I) is 1-hydroxylethylidene-1, 1 -diphosphonic acid.

3. Method according to claim 1 or claim 2, in which the hydrazo compound is chosen from symmetrical hydrazo compounds bearing nitrogenous functions, in particular nitrile or amide functions, preferably from 2,2'-hydrazo-bis-. isobutyronitrile, 2,2'-hydrazo-bis-methylbutyronitrile, 1, 1 '-hydrazo-bis-cyclohexane-carbonitrile and 2,2'-hydrazo-dicarbonamide, preferably the hydrazo compound is 2,2'- hydrazo-bis-isobutyronitrile.

4. Method according to any one of the preceding claims, in which the oxidizing agent is chosen from inorganic peroxide derivatives, preferably from hydrogen peroxide, potassium or sodium persulphates, potassium mono-hydrogenopersulphates or sodium, dioxygen, per-acetic acid derivatives, more preferably the oxidizing agent is hydrogen peroxide.

5. Process according to any one of the preceding claims, in which the catalyst comprises a water-soluble compound chosen from salts and acids based on a catalytic metal from columns 5 and 6 of the periodic table of the elements and preferably chosen from molybdenum and tungsten, preferably molybdenum.

6. The method of claim 5, wherein the soluble compound is selected from alkali metal or ammonium salts of molybdenum, alkali metal or ammonium salts of tungsten, phosphomolybdic acid and its alkali salts or d. ammonium, phosphotungstic acid and its alkali metal or ammonium salts, molybdosulphates and their mixtures, preferably phosphomolybdic acid and its alkali metal or ammonium salts, in particular sodium phosphomolybdate and potassium phosphomolybdate.

7. Method according to any one of the preceding claims, in which at least one reducing agent is added to the aqueous solution obtained in step a).

8. Method according to any one of the preceding claims, in which at least one reducing agent is added to the waste water fraction.

9. The method of claim 7 or claim 8, wherein the reducing agent is selected from hydrazine, sodium sulfite, sodium bisulfite and mixtures thereof.

10. Use of a compound of formula (I) defined in claim 1 as a complexing agent for a catalyst, preferably in a process for the synthesis of an azo compound.

11. Use according to claim 10, wherein the compound of formula (I) is chosen from 1-hydroxy-1 - (1 / - / - imidazol-1 -yl) ethane-1, 1 -diyljdiphosphonic acid or a of its salts, hydroxymethylenediphosphonic acid, 1-hydroxylethylidene-1, 1 -diphosphonic acid (HEDP) and 1 -hydroxybutanediphosphonic acid, in their acid form

or in the form of their salts, preferably the compound of formula (I) is 1-hydroxylethylidene-1, 1 -diphosphonic acid.