RU2841154C1 - Method for catalytic production of methionine analogue - Google Patents

Abstract

FIELD: chemistry.

SUBSTANCE: invention relates to a method of producing a hydroxy acid selected from 2-hydroxy-4-methylthiobutyric acid (HMTBA) or 2-hydroxy-4-methylselenobutyric acid (HMSeBA), which are of great interest for animal nutrition. Method involves catalytic conversion of a hydroxynitrile compound selected from 2-hydroxy-4-methylthiobutyronitrile or 2-hydroxy-4-methylselenobutyronitrile, respectively, in the presence of water and at least one weak acid. Weak acid is selected from formic acid, acetic acid, propionic acid, linear or branched butanoic acid, pentanoic acid, carbonic acid, glycolic acid, thioacetic acid, cyanoacetic acid, lactic acid, pyruvic acid and oxalic acid and one catalyst containing at least one of: aluminium oxide, titanium dioxide and zirconium dioxide. Catalyst contains at least one of: aluminium oxide, titanium dioxide and zirconium dioxide.

EFFECT: combining the hydration and hydrolysis steps into one catalytic step to obtain HMTBA or its selenium analogue with high output.

9 cl, 2 dwg, 15 ex

Description

The invention relates to an improvement in the method for producing 2-hydroxy-4-methylthiobutyric acid, an analogue of methionine, or its selenium analogue, 2-hydroxy-4-methylselenobutyric acid, from 2-hydroxy-4-methylthiobutyronitrile or 2-hydroxy-4-methylselenobutyronitrile, respectively.

2-hydroxy-4-methylthiobutyric acid (HMTBA), which is the hydroxy analogue of methionine, its salts, its chelates, in particular metal chelates (Zn, Ca, Mn, Mg, Cu, Na, etc.) and its esters such as isopropyl and tert-butyl esters of HMTBA are widely used in animal nutrition. Selenium derivatives of the said acid, its salts, its chelates and its esters are also of great interest in animal nutrition.

The preparation of 2-hydroxy-4-methylthiobutyronic acid can be accomplished by a variety of methods, including various synthetic intermediates, and in particular 2-hydroxy-4-methylthiobutyronitrile (HMTBN) and 2-hydroxy-4-methylthiobutyramide (HMTBM).

Document US2001/0001105A1 describes a continuous process for the synthesis of 2-hydroxy-4-methylthiobutyric acid (HMTBA) from 2-hydroxy-4-methylthiobutyronitrile (HMTBN), according to which in a first step HMTBN is hydrolyzed to 2-hydroxy-4-methylthiobutyramide (HMTBM) in the presence of an aqueous solution of a mineral acid such as sulfuric acid, then in a second step HMTBM is hydrolyzed to HMTBA. This process has the disadvantage of using large quantities of sulfuric acid, the latter being usually used in excess with respect to HMTBN, which leads to the formation of large quantities of by-products such as ammonium bisulfate, which must be separated and which are also difficult to recycle. This method also requires a long residence time of several hours.

According to document WO2004/089863A1, a process is known for the preparation of the ammonium salt of HMTBA from the nitrile precursor of HMTBA, HMTBN, according to which HMTBN in aqueous solution in the presence of a titanium-based catalyst was converted into the ammonium salt of HMTBA in a single step. This synthesis also leads to the formation of methionine and HMTBM, and the reported yields of ammonium salts of HMTBA are in the range of 1%. They are too low to envisage the possibility of using this process on an industrial scale.

The present invention represents an alternative to existing processes that eliminates the use of sulfuric acid and combines the hydration and hydrolysis steps into a single catalytic step, resulting in an exceptional and unexpectedly high yield of HMTBA or its selenium analogue.

According to the invention, it has been found that the intermediate hydroxynitrile (HMTBN or its selenium equivalent) can be converted into 2-hydroxy-4-methylthiobutyric acid (or into seleno-hydroxymethionine) in one step in the presence of at least one catalyst and a weak acid. The availability and efficiency of its conversion are such that its conversion is possible in the industrial production of the hydroxy analogue of methionine. Compared with the known synthesis methods and the developed improvements that can be made to them, but which are not sufficiently useful for modifying traditional industrial processes, the present invention represents a real advance. Significant yields were obtained in a very short time, and this method avoids the use of sulfuric acid and the formation of by-products or intermediates in the synthesis.

The invention relates to a method for producing a hydroxy analogue of methionine or a hydroxy analogue of selenomethionine by catalytic conversion of 2-hydroxy-4-methylthiobutyronitrile or 2-hydroxy-4-methylselenobutyronitrile, respectively, wherein said conversion is carried out in the presence of water and at least one weak acid and one catalyst containing at least one of: aluminum oxide, titanium dioxide and zirconium dioxide.

According to the invention, the hydroxynitrile compound is directly converted into a hydroxy acid after converting the ammonium salt of the hydroxy acid according to conventional methods without having to resort to a cumbersome separation step, which results in significant economic benefits.

Before describing the invention in more detail, definitions of some terms used in this text are given.

The term hydroxy acid is used interchangeably with 2-hydroxy-4-methylthiobutyric acid or 2-hydroxy-4-methylselenobutyric acid, considered together or separately. Similarly, the term hydroxy nitrile refers to both 2-hydroxy-4-methylthiobutyronitrile and 2-hydroxy-4-methylselenobutyronitrile, considered together or separately, and the term hydroxy amide refers to 2-hydroxy-4-methylthiobutyramide or 2-hydroxy-4-methylselenobutyramide, considered together or separately.

According to the invention, a weak acid is any organic or mineral acid whose pK _a constant is at least 1 and not more than 10 at 25 °C, or any compound or mixture that behaves in this manner. As an illustration of a weak organic acid, mention may be made of carboxylic acids and polyacids that may or may not perform one or more functions, for example, selected from OH, C=O, such as acetic acid, formic acid, and as an illustration of a weak mineral acid, mention may be made of phosphoric acid, dihydrogen phosphate, hydrofluoric acid (HF), hypochlorous acid (HOCl), boric acid (H ₃ BO ₃ ), sulfurous acid (H ₂ SO ₃ ), hydrocyanic acid (HCN).

The term "catalyst" as used herein generally refers to the active phase of the catalyst, without excluding the fact that the catalyst may be doped and/or supported.

By aluminum oxide, titanium dioxide and zirconium dioxide are meant all polymorphs, where applicable, of aluminum oxide Al ₂ O ₃ , titanium dioxide TiO ₂ and zirconium dioxide ZrO ₂ , respectively, these forms being well known to those skilled in the art. The catalyst may also be a two-fold or three-fold combination of aluminum oxide, titanium dioxide and zirconium dioxide. It may also contain any other substance that contributes to its catalytic function.

The characteristics, applications and advantages of the invention are set out below in more detail, taking into account that these characteristics can be considered independently of each other or in combination, regardless of the combination.

The weak acid is preferably an organic acid having one or more carboxyl groups or a mineral acid having a pK _a of at least 1 at 25 °C and preferably not more than 10, more preferably not more than 7 at 25 °C. In practical interest, it has a boiling point of 170 °C or less and preferably 150 °C or less or even 120 °C or less, so that it can be more easily separated from the reaction medium, usually by distillation. Such acids which can be used according to the invention are in particular selected from formic acid, acetic acid, propionic acid, linear or branched butanoic acid, pentanoic acid, carbonic acid, glycolic acid, thioacetic acid, cyanoacetic acid, lactic acid, pyruvic acid, oxalic acid, methionine or its selenium equivalent, or a hydroxy analogue of methionine or its selenium equivalent. These acids can be used alone or in any mixture with each other. According to one variant, they are selected from formic acid, acetic acid, propionic acid, linear or branched butanoic acid, pentanoic acid, carbonic acid, glycolic acid, thioacetic acid, cyanoacetic acid, lactic acid, pyruvic acid and oxalic acid. Preferably, the acids used are formic acid, acetic acid and/or propionic acid. According to another embodiment of the invention, the weak acid is a mineral acid, such as phosphoric acid, dihydrogen phosphate, used alone or as a mixture.

Said weak acid is present in at least one molar equivalent with respect to the hydroxynitrile or even in excess, preferably the molar ratio of the weak acid to the hydroxynitrile is from 0.001 to 50 or even from 0.001 to 30 and even from 0.001 to 10 or even from 0.001 to 1. In practice, the molar concentration of the weak acid in the reaction medium varies from 0.05 to 10 M, preferably from 0.1 to 2 M and even more preferably from 0.2 to 1 M.

According to the invention, the catalyst is selected from aluminum oxide, titanium dioxide and zirconium dioxide; this compound is at least the active phase of the catalyst, optionally the support. Thus, if the catalyst does not consist entirely of one or more of the said oxides, it can contain any other compound that does not affect the characteristics of the catalyst or even enhances them. In one embodiment, the catalyst consists of one of the said oxides.

The catalyst can be doped and/or supported. It can be doped with any element or compound commonly used and well known to those skilled in the art. As an illustration, the catalyst can be doped with one or more compounds selected from sulfates (SO ₄ ), phosphates (PO ₄ ), tungstates (WO ₃ ), borates (B ₂ O ₃ ), heteropoly acids corresponding to one of the formulas H _n XM ₁₂ O ₄₀ and H _n X ₂ M ₁₈ O ₆₂ , in which n is an integer, preferably not exceeding 10, X is Si, Ge, P or As, and M is Mo or W, such as phosphomolybdic acid of the formula H ₆ P ₂ Mo ₁₈ O ₆₂ , as well as any other doping compound that provides acidity to the catalyst. The following compounds PO ₄ , SO ₄ and H ₆ P ₂ Mo ₁₈ O ₆₂ are preferred. If the catalyst does not consist of aluminum oxide, titanium dioxide and/or zirconium dioxide, it can also be supported on any other compounds commonly used and well known to those skilled in the art, and in particular silicon dioxide and silicoaluminates.

According to the invention, all the solid catalysts mentioned above may be in powder form or, preferably, in the form of granules, extrudates, tablets, trefoils or any other form that allows them to be used in a continuous fixed bed reactor or other or in batch mode in an open reactor or a reactor under pressure.

Said catalyst preferably has a specific surface area of at least 10 ^m2 /g. Below this limit, the productivity of the catalyst drops off rapidly, especially with a decrease in the selectivity for the hydroxy acid in favor of the selectivity for the hydroxy amide and a decrease in the conversion of the hydroxy nitrile. This observation applies to the selenium equivalent. Preferably, the specific surface is at least 50 ^m2 /g. The upper limit of the specific surface is not critical in the context of the invention, the latter being determined by commercially available active phases. The values of the specific surface area indicated in the present text are determined by the most common method, namely the physical adsorption of nitrogen, and are calculated by the BET (Bruner-Emmett-Teller) method.

In a preferred embodiment of the method according to the invention, the catalyst is present in a weight concentration of from 0.1 to 200% based on the weight of HMTBN, preferably from 0.5 to 100% and even better from 1 to 50%.

According to the invention, various equipment for carrying out the reaction in a batch or continuous mode can be provided: the solid catalyst, doped or not, can be immobilized in the reactor in the form of grains or extrudates or in any other form or be applied to a metal foam. The reactor associated with this type of catalyst is preferably a tubular or multi-tubular fixed bed operating in a jet or flooded isothermal or adiabatic mode, or an exchange reactor coated with a catalyst.

The conversion of HMTBN in the context of the present invention is preferably carried out at a temperature in the range of 20 to 200 °C, preferably from 50 to 180 °C, and even better from 80 to 170 °C. It was noted that during the reaction period of from about 10 minutes to 3 hours, at a temperature below 20 °C the reaction slows down significantly, and that from 180 °C, the more the temperature increases, the more the selectivity for methionine and the dinitrile polypeptide and methionine increases to the detriment of the selectivity for 2-hydroxy-4-methylthiobutyric acid. The selectivity for the hydroxy acid is highest in the range of 100 to 180 °C.

In the context of the invention, the contact time of the reaction mixture containing water, hydroxynitrile and acid with the catalyst is from 30 seconds to 1 hour, preferably from 1 minute to 30 minutes and even more preferably from 2 minutes to 20 minutes.

Hydroxynitrile is usually in aqueous solution. It can be prepared for the process. The weak acid used in the reaction can be added to the aqueous solution of hydroxynitrile or added through a mixer before feeding to the catalytic reactor.

The concentration of hydroxynitrile can influence the efficiency of the process, especially when it is too high. Thus, according to an embodiment of the invention, hydroxynitrile is in an aqueous solution at a concentration of 0.01 to 10 M, preferably 0.05 to 1 M. It has been noted that beyond 1 M, if the conversion to hydroxynitrile remains strong, the selectivity for hydroxy acid decreases, and for hydroxyamide, dinitrile and even polypeptide, respectively, increases.

The invention also relates to the continuous implementation of the process according to the invention. According to this variant, the process was carried out under a pressure which may be from 1 to 20 bar, preferably from 2 to 10 bar. The invention thus relates to a device comprising a reservoir for a hydroxynitrile solution, to which a weak acid was added. The resulting hydroxy acid solution was pumped into a reactor which contained a catalyst and which was heated by means of a sleeve or a furnace at a temperature of from 80 to 180 °C. The reaction medium was discharged into a gas-liquid separator from which gases would be removed and from which the liquid was treated to recover the hydroxy acid. The solution was then evaporated to separate the excess used water and the weak acid, which may or may not form an azeotrope, wherein these compounds were then preferably recycled in the process. The phase containing the hydroxy acid may then be stripped to remove all or part of the ammonia which constitutes the ammonium salt of the hydroxy acid. Additional electrodialysis steps or other techniques known to those skilled in the art may be provided to completely recover the hydroxy acid in its acid form (2-hydroxy-4-methylthiobutyric acid or the corresponding selenic acid) and to completely recycle the ammonia formed during the hydrolysis of the intermediate HMBTM in the catalysis step. Preferably, the recovered ammonia will be recycled before the process for producing the hydroxy acid, as in the synthesis of HCN.

The invention and its advantages over the prior art are illustrated in the following examples in support of the following figures:

[Fig. 1] illustrates the conversion of HMTBN, selectivity to HMTBA, selectivity to methionine and selectivity to HMTBM as a function of reaction time according to the method of the invention under the conditions described in Example 1.

[Fig. 2] illustrates the conversion of HMTBN, selectivity to HMTBA, selectivity to methionine and selectivity to HMTBM as a function of reaction time according to the method of the invention under the conditions described in Example 2.

In the experimental part that follows:

Examples 1-6 illustrate various embodiments of the method according to the invention;

Examples 7 and 8 illustrate a procedure for doping a catalyst to obtain a doped catalyst that can be used in the process of the invention, according to a variant such as that which is the subject of Example 6;

Example 9 illustrates the step of separating a hydroxy acid salt into a hydroxy acid; and

Examples 10-15 illustrate methods beyond the scope of the present invention by comparison.

Example 1: Preparation of HMTBA in the presence of titanium dioxide and acetic acid according to the invention

The hydrolysis reaction of HMTBN and the conditions under which it is carried out are described in the diagram below.

In a 1-liter screw-cap vial, 14.0 g HMTBN was added with 2000 ml _H2O and 60 mg acetic acid. The solution was stirred at room temperature with a nitrogen stream and introduced into a tubular reactor heated to 120 °C at a flow rate of 0.05 ml/min (contact time 24 minutes) and containing 60 g _TiO2 (anatase, 150 ^m2 /g, Norpro, ST 61120).

After the reaction, HPLC (high performance liquid chromatography) is carried out. The yield of HMTBA is 88%.

Example 2: Preparation of HMTBA in the presence of titanium dioxide and acetic acid according to the invention

The hydrolysis reaction of HMTBN and the conditions under which it is carried out are described in the diagram below.

In a 1 liter screw cap vial, 13.1 g of HMTBN were added with 990 ml of _H2O and 10 ml of acetic acid. The solution was stirred at room temperature with a nitrogen stream and the solution was introduced into a tubular reactor heated to 160 °C at a flow rate of 0.1 ml/min (contact time 10 minutes) and containing 4 g of _TiO2 (anatase, 150 ^m2 /g, Norpro, ST 61120). The resulting HMTBA salt was converted to HMTBA by distilling off ammonia according to the procedure illustrated in Example 9.

After the reaction, HPLC was performed: the conversion of HMTBN, selectivity to HMTBA, selectivity to methionine and selectivity to HMTBM as a function of time are shown in [Fig. 1].

The yield of HMTBA is 95% and 5% methionine.

Example 3: Preparation of HMTBA in the presence of titanium dioxide and acetic acid according to the invention

The hydrolysis reaction of HMTBN and the conditions under which it is carried out are described in the diagram below.

In a 1-liter screw-cap vial, 13.1 g of HMTBN were added with 800 ml of _H2O and 200 ml of acetic acid. The solution was stirred at room temperature with a nitrogen stream, and the solution was introduced into a tubular reactor heated to 160 °C at a flow rate of 0.1 ml/min (contact time 10 min). The reactor contained 4 g of _TiO2 (anatase, 150 ^m2 /g, Norpro, ST 61120). The resulting HMTBA salt was converted to HMTBA by distilling off ammonia according to the procedure illustrated in Example 9.

After the reaction, HPLC was performed: the conversion of HMTBN, selectivity to HMTBA, selectivity to methionine and selectivity to HMTBM as a function of time are shown in [Fig. 2].

The yield of HMTBA is 89% and 11% methionine.

Example 4: Preparation of HMTBA in the presence of titanium dioxide and acetic acid according to the invention

The hydrolysis reaction of HMTBN and the conditions under which it is carried out are described in the diagram below.

In a 1-liter screw-cap vial, 13.1 g HMTBN was added with 990 ml _H2O and 10 ml formic acid. The solution was stirred at room temperature with a nitrogen stream, and the solution was introduced into a tubular reactor heated to 160 °C at a flow rate of 0.1 ml/min (contact time 10 min). The reactor contained 4 g _TiO2 (anatase, 150 ^m2 /g, Norpro, ST 61120).

After the reaction, HPLC was performed after 2 hours.

The resulting HMTBA salt was converted to HMTBA by distillation of ammonia according to the procedure illustrated in Example 9.

The yield of HMTBA is 90% and 10% methionine.

Example 5: Preparation of HMTBA in the presence of sulfate-doped aluminum oxide and acetic acid according to the invention

The hydrolysis reaction of HMTBN and the conditions under which it is carried out are described in the diagram below.

In a 1 liter screw cap vial, 13.1 g of HMTBN were added with 800 ml of H ₂ O and 200 ml of acetic acid. The solution was stirred at room temperature with a nitrogen stream, and the solution was introduced into a tubular reactor heated to 160 °C at a flow rate of 0.1 ml/min (contact time 10 min). The reactor contained 4 grams of Al ₂ O ₃ (gamma, 300 m ² /g, IFPEN, 33006 GFSA 401, alumina doped with 10% by weight of sulfate function).

After the reaction, HPLC was performed after 2 hours.

The resulting HMTBA salt was converted to HMTBA by distillation of ammonia according to the procedure illustrated in Example 9.

The yield of HMTBA is 96% and 4% methionine.

Example 6: Preparation of HMTBA in the presence of zirconium dioxide and acetic acid according to the invention

The hydrolysis reaction of HMTBN and the conditions under which it is carried out are described in the diagram below.

In a 1-liter screw-cap vial, 13.1 g of HMTBN were added with 800 ml of H ₂ O and 200 ml of acetic acid. The solution was stirred at room temperature with a nitrogen stream, and the solution was introduced into a tubular reactor heated to 160 °C at a flow rate of 0.1 ml/min (contact time 10 min). The reactor contained 6 grams of ZrO ₂ (monoclinic, 100 m ² /g, Norpro, XZ 16075).

After the reaction, HPLC was performed after 2 hours.

The resulting HMTBA salt was converted to HMTBA by distillation of ammonia according to the procedure illustrated in Example 9.

The yield of HMTBA is 74% and 16% methionine.

Example 7: Preparation of TiO ₂ doped with 10 wt.% sulfate (SO ₄ ) using sulfuric acid.

20 g of TiO ₂ powder (anatase, 150 m ² /g, Norpro, ST 61120) were added to a 1-liter flask with 500 ml of water and 2.04 g of sulfuric acid. The solution was stirred for 2 hours at room temperature, then the water was evaporated. Then the obtained powder was dried at 200 °C for 3 hours and then calcined at 700 °C in air for 2 hours. Elemental analysis was carried out to measure sulfur, the amount of sulfur of 3.4 wt.% of the catalyst was observed, which corresponds to the amount of sulfate of 10 wt.%.

Example 8: Preparation of TiO ₂ doped with 10 wt.% sulfate (SO ₄ ) with ammonium sulfate

20 g of TiO ₂ powder (anatase, 150 m ² /g, Norpro, ST 61120) were added to a 1-liter flask with 500 ml of water and 2.78 g of ammonium sulfate. The solution was stirred for 2 hours at room temperature, then the water was evaporated. Then the obtained powder was dried at 200 °C for 3 hours and then calcined at 700 °C in air for 2 hours. Elemental analysis was carried out to measure sulfur, the amount of sulfur of 3.2 wt.% of the catalyst was observed, which corresponds to the amount of sulfate of 9.8 wt.%.

Example 9: Conversion of ammonium salt HMTBA to HMTBA

The solution of ammonium salt HMTBA obtained according to the invention was concentrated in organics to an organic content of 87 wt.%. The temperature of the medium varies from 100 to 130 °C (atmospheric pressure). After this concentration step, the conversion in the amount of HMTBA is 21% (mol). Then, a steam distillation phase was carried out. Water for distillation was introduced in liquid form. The organic content was maintained constant at 87 wt.%. The temperature is in the range from 115 to 121 °C. The distillation rate is from 3.8 to 4.3 ml/min. After 200 minutes under these conditions, a conversion to HMTBA of 47% was obtained. An additional distillation step was carried out to obtain HMTBA with a yield of about 100%.

Example 10: Preparation of HMTBA in the presence of acetic acid, not part of the invention

The hydrolysis reaction of HMTBN and the conditions under which it is carried out are described in the diagram below.

0.131 g of HMTBN was added to a 20 ml screw cap vial with 8 ml of H ₂ O and 2 ml of acetic acid. The solution was stirred at 160 °C for ten minutes.

The solution was analyzed by HPLC, no reaction was observed.

Example 11: Preparation of HMTBA in the presence of formic acid, not part of the invention

The hydrolysis reaction of HMTBN and the conditions under which it is carried out are described in the diagram below.

0.131 g of HMTBN was added to a 20 ml screw cap vial with 9 ml of H ₂ O and 1 ml of acetic acid. The solution was stirred at 160 °C for ten minutes.

The solution was analyzed by HPLC, no reaction was observed.

Example 12: Preparation of HMTBA in the presence of titanium dioxide according to the prior art

The hydrolysis reaction of HMTBN and the conditions under which it is carried out are described in the diagram below.

In a 1-liter screw-cap vial, 13.1 g of HMTBN was added with 1000 ml of _H2O . The solution was stirred at room temperature with a nitrogen stream and the solution was introduced into a tubular reactor heated to 160 °C at a flow rate of 0.1 ml/min (contact time 10 minutes) and containing 4 g _{of TiO2} (anatase, 150 ^m2 /g, Norpro, ST 61120).

After the reaction, HPLC was performed, the yield of HMTBA was 1% and 15% methionine.

Example 13: Preparation of HMTBA in the presence of gamma-phase alumina according to the prior art

The hydrolysis reaction of HMTBN and the conditions under which it is carried out are described in the diagram below.

In a 10 ml screw cap vial, 0.4 g of γ-Al ₂ O ₃ (300 m ² /g) was added, followed by 0.1 g of HMTBN (97%) with 1 ml of water. The solution was heated at 90 °C for 60 min, after which the solution was filtered and analyzed by proton NMR (nuclear magnetic resonance).

No HMTBN conversion observed

Example 14: Preparation of HMTBA in the presence of gamma-phase alumina according to the prior art

The hydrolysis reaction of HMTBN and the conditions under which it is carried out are described in the diagram below.

In a 20 ml screw cap vial, 0.4 g _of γ- _Al2O3 (300 ^m2 /g) was added followed by 1.1 g of HMTBN (97%) with 10 ml of water. The solution was heated at 90 °C for 18 h, after which the solution was filtered and analyzed by proton NMR.

The yield of HMTBM is 30% and HMTBA is 6%.

Example 15: Preparation of HMTBA in the presence of anatase _TiO2 according to the prior art

The hydrolysis reaction of HMTBN and the conditions under which it is carried out are described in the diagram below.

In a 10 mL screw cap vial, 1 g TiO ₂ (as anatase) (90 m ² /g) was added followed by 1.1 g HMTBN (97%) with 1 mL water. The solution was heated at 90 °C for 96 h, after which the solution was filtered and analyzed by proton NMR.

No traces of HMTBM or HMTBA were observed.

A comparison of the results of Examples 1-6 according to the invention with the results obtained in the process carried out without a catalyst (Examples 10 and 11) or without a weak acid (Examples 12-15) demonstrates a significant improvement in the characteristics of the production of hydroxy acid in the process according to the invention, which is also unexpected.

Claims (9)

1. A method for producing a hydroxy acid selected from 2-hydroxy-4-methylthiobutyric acid (HMTBA) or 2-hydroxy-4-methylselenobutyric acid (HMSeBA) by catalytically converting a hydroxynitrile compound selected from 2-hydroxy-4-methylthiobutyronitrile or 2-hydroxy-4-methylselenobutyronitrile, respectively, wherein said conversion is carried out in the presence of water and at least one weak acid selected from formic acid, acetic acid, propionic acid, linear or branched butanoic acid, pentanoic acid, carbonic acid, glycolic acid, thioacetic acid, cyanoacetic acid, lactic acid, pyruvic acid and oxalic acid, and one catalyst containing at least one of: aluminum oxide, titanium dioxide and zirconium dioxide. 2. The method according to claim 1, wherein said weak acid is selected from acetic acid, formic acid and propionic acid. 3. The method according to claim 1 or 2, wherein the weak acid is in at least one molar equivalent relative to the hydroxynitrile or even in excess, preferably the molar ratio of the weak acid to 2-hydroxy-4-methylthiobutyronitrile or 2-hydroxy-4-methylselenobutyronitrile is from 0.001 to 50. 4. The method according to any one of claims 1 to 3, wherein the catalyst has a mass concentration of from 0.1 to 200% relative to the mass of HMTBN (2-hydroxy-4-methylthiobutyronitrile), preferably from 0.5 to 100% and even better from 1 to 50%. 5. The method according to any one of claims 1 to 4, wherein 2-hydroxy-4-methylthiobutyronitrile or 2-hydroxy-4-methylselenobutyronitrile is in an aqueous solution at a concentration of 0.01 to 10 M, preferably 0.05 to 1 M. 6. The method according to any one of claims 1 to 5, wherein said catalyst has a specific surface area according to the BET (Bruner-Emmett-Teller) method of at least 10 ^m2 /g, preferably at least 50 ^m2 /g. 7. The method according to any one of claims 1 to 6, wherein said catalyst is doped, preferably it is doped with one or more compounds selected from sulfates, phosphates, borates, tungstates, heteropoly acids having one of the formulas H _n XM ₁₂ O ₄₀ and H _n X ₂ M ₁₈ O ₆₂ , in which n is an integer, preferably not exceeding 10, X is Si, Ge, P or As, and M is Mo or W, such as phosphomolybdic acid of the formula H ₆ P ₂ Mo ₁₈ O ₆₂ , as well as any other doping compound that provides acidity to the catalyst. 8. The method according to any one of claims 1 to 7, wherein the conversion is carried out at a temperature in the range from 20 to 200 °C, preferably from 50 to 180 °C, and more preferably from 80 to 170 °C. 9. The method according to any of paragraphs 1-8, which is carried out continuously.