CN1227223C - Method for preparing methionine - Google Patents

Abstract

A process for the preparation of methionine, comprising: hydrolyzing methionine amide in the presence of a titanium-containing catalyst to obtain ammonium methionine, wherein the catalyst has a porosity of 5-1000nm and a total pore volume of 0.2-0.55cm³Per g, surface area of 30-150m²(ii)/g; (b) in a second step, ammonia is removed and methionine is recovered from the ammonium methionine. The present invention also proposes an industrial process for the production of methionine which incorporates the aforementioned hydrolysis step.

Description

Method for preparing methionine

The present invention relates to a process for the production of methionine by hydrolysis of methionine amide using a titanium-containing catalyst, and the use of this process for the industrial production of methionine containing low salt by-products, and in some cases substantially free of salt by-products .

The hydrolysis of methionine amide to methionine is known. Specifically, European Patent Application No. 228938 discloses a method of hydrolyzing methionine amide with a strong base to obtain methionine. The problem with this method is that the acidification step with a strong acid also produces inorganic salts such as carbonates, hydrochlorides or sulfates. Additional purification steps are usually required to remove the salt.

French Patent Application No. 9814000 attempts to overcome the above-mentioned problems by using a titanium catalyst in the hydrolysis reaction. The use of titanium-based catalysts is also disclosed in Japanese Patent Application Nos. 03093753, 03093755 and 03093756.

We have found that methionine can be prepared in high yields using a specific titanium catalyst. Accordingly, the present invention provides a process for the preparation of methionine comprising: (a) hydrolyzing methionine amide in the presence of a titanium-containing catalyst to obtain ammonium methionine, said catalyst having a porosity of 5-1000nm, the total pore volume is 0.2-0.55cm ³ /g, and the surface area is 30-150m ² /g; (b) The second step is to remove ammonia and recover methionine from the ammonium methionine.

The method of the present invention for the preparation of methionine is superior to the known prior methods because in the method of the invention the methionine amide can be completely converted to methionine without additional work-up.

The method of the present invention involves the hydrolysis of methionine amide. Suitably, the amide is present in the form of an aqueous solution in an amount of 0.01-2 mol/kg, preferably 0.5-1 mol/kg.

The method of the present invention is a catalytic method using a titanium-containing catalyst. The porosity of the catalyst is 5-1000nm. In the present invention, porosity is defined as the distribution of pores within agglomerated crystallites. Preferably, the macropore distribution of the catalyst is 5-100 nm and 20-1000 nm. Preferably, the distribution is a bimodal distribution.

The pore volume of the catalyst is measured by mercury porosimetry, and the result is 0.2-0.55 cm ³ /g, preferably 0.25-0.45 cm ³ /g.

The catalyst must also have a surface area measured by the BET method of 30-150 m ² /g, preferably 40-120 m ² /g.

The catalyst can be used in any form of powder, granules or granules. When the catalyst is used in the process in the form of granules or granules, it may be in any suitable shape, such as extrudates, spherical particles and flakes. We have also found that the catalyst is most effective when used in the form of extrudates having a specific shape with three or four lobes. The particle size of the catalyst is suitably 0.05-4 mm, preferably 0.5-2 mm.

The catalyst may contain only one metal, namely titanium, or may contain one or more additional metals. When titanium alone is present, the catalyst may be titanium dioxide (TiO ₂ ). When the catalyst contains additional metals, suitable catalysts include Ti-W, Ti-Mo, Ti-Si-W, Ti-Nb-Mo, Ti-Zr, Ti-Al, Ti-Cr, Ti-Zn and Ti -V or a mixture thereof.

The catalyst may be prepared by any suitable method, for example, by mixing the dry ingredients and calcining them at a suitable temperature to form them into a suitable shape. Alternatively, after mixing and/or calcining the dry ingredients, water and/or acid are added to the titanium powder to form a paste. After kneading, the paste is extruded, and then calcined to extrude the resulting product.

The amount of catalyst used in the present invention will depend on the nature of the process and the physical properties of the catalyst. For use of the catalyst in powder form, a suitable amount may be 0.1-2 grams, preferably 0.5-1.5 grams of catalyst per gram of amide. For catalysts used in granular or particulate form and used on a continuous basis, the contact time may be from 0.5 to 60 minutes, preferably from 5 to 30 minutes.

In this method, the catalyst may lose activity after prolonged use. Regeneration can be performed in situ or ex situ. For in situ regeneration, the catalyst can be contacted with water or acidified water (ie water containing 0.01-5% mineral acid) at ambient temperature to the temperature at which the process is carried out, eg ambient temperature to 130°C. When regeneration is carried out off-site, this regeneration can be carried out by heating at a temperature of 200-500°C, preferably 300-400°C, in an oxygen-containing gas such as air or pure oxygen.

The process of the invention may suitably be carried out at a temperature of 50-150°C, preferably 80-130°C, and a pressure of 1-10 bar, preferably 1-5 bar.

In the second step of the reaction, methionine is liberated from methionine ammonium salt by removal of ammonia. This release can be achieved by any suitable method, such as steam stripping.

The method can be carried out in batch or continuous mode. Preferably, the process is carried out in a continuous plug flow mode using one or two or more reactors connected in series. Such a configuration is particularly preferred since it requires less catalyst, which is particularly welcome in industrial processes. The process may be carried out in any suitable reactor, for example in a fixed bed reactor or a fluidized bed reactor. Preferably, the process is carried out in a fixed bed reactor.

The amide can be obtained by existing methods. In the existing method, first, 2-hydroxy-4-methylthio-butyronitrile (HMTBN) is reacted with an aqueous solution of ammonia or ammonium to obtain 2-amino-4- Methylthiobutyronitrile (AMTBN). The 2-amino-4-methylthiobutyronitrile product is then reacted with a ketone in the presence of an alkali metal hydroxide to give the methionine amide. The method of the present invention can be combined with known methods to provide a novel method for the preparation of methionine.

Therefore, according to another aspect of the present invention, the present invention provides a method for preparing methionine, which can be used in industrial production, comprising:

(a) contacting 2-hydroxy-4-methylthiobutyronitrile with ammonia or a solution containing ammonia to produce a first product stream containing 2-amino-4-methylthiobutyronitrile;

(b) contacting said first product stream with a ketone and an alkali metal hydroxide to produce a second product stream comprising methionine amide, unreacted ketone, ammonia and water;

(c) removing unreacted ketones, ammonia and water from the second product stream;

(d) hydrolyzing the methionine amide in the presence of a titanium-containing catalyst having a porosity of 5-1000 nm and a total pore volume of 0.2-0.55cm ³ /g, the surface area is 30-150m ² /g;

(e) Liberation of methionine from methionine ammonium salt.

This process, which can be carried out on an industrial scale, involves the conversion of 2-hydroxy-4-methylthiobutyronitrile. This starting material may be obtained by any suitable method, for example, by the reaction of hydrocyanide (HCN) with methyl-4-methanal as disclosed in European Patent Application No. 739870, which is hereby incorporated as refer to.

In the first step of the industrial process according to the invention, 2-hydroxy-4-methylthiobutyronitrile is brought into contact with a solution of ammonia or ammonium and water to produce a mixture containing 2-amino-4-methylthiobutyronitrile. The molar ratio of ammonia to 2-hydroxy-4-methylthiobutyronitrile is suitably 3-10, preferably 4-7. When an aqueous solution of ammonia needs to be used, the concentration of the aqueous solution is suitably 25% by weight, preferably greater than 60% by weight. Preferably, the 2-hydroxy-4-methylthiobutyronitrile is contacted with pure ammonia.

Suitably, the first step of the process is carried out at a temperature of 40-80°C, preferably 70-75°C, and a pressure of 10-30 bar, preferably 15-25 bar. The reaction can be carried out in stirred or tubular reactors with plug flow, in particular with a heat exchange system, or using a combination of these two reactors.

At the end of the first step reaction, there may be an excess of unreacted ammonia present. The unreacted ammonia is preferably removed from the reactor. This can be done by rapid depressurization, or by using an inert gas such as nitrogen to carry the ammonia away. The temperature of this separation step is suitably lower than 60°C, preferably 10-40°C. The pressure can be atmospheric or slightly subatmospheric or slightly superatmospheric. A pressure of 0.1×10 ⁵ -0.5×10 ⁵ Pa is preferably used. The ammonia recovered from this reaction can then be concentrated or sent to a recovery section for further processing.

Then, in the presence of ketone and alkali metal hydroxide, the 2-amino-4-methylthiobutyronitrile obtained in the first step of the method is hydrated to obtain methionine amide. The concentration of the ketone is suitably 0.1-1 equivalent, preferably 0.2-0.5 equivalent. The concentration of the alkali metal hydroxide salt is suitably 0.05-0.5 equivalent, preferably 0.1-0.25 equivalent. The ketone is preferably acetone. Suitably, the alkali metal hydroxide is potassium hydroxide or sodium hydroxide, preferably sodium hydroxide.

This hydration step is suitably carried out at a temperature of 10-40°C, preferably 15-30°C. Suitably, the reaction is carried out at atmospheric pressure. The reaction is carried out in stirred or tubular reactors with a heat exchange system or in tower-packed reactors with suitable packing materials.

By-products from this particular reaction include the alkali metal salt of methionine, the residue 2-amino-4-methylthiobutyronitrile, the imidazolidinone (2,2′-dimethyl-5-(methylthio (ethyl)-4-imidazolidinone), water, ammonia, unreacted ketones and alkali metal hydroxides. The unreacted ketone, ammonia and at least some of the water in the product stream can then be separated from other components. To simplify this separation step, the product stream can be treated by distillation or stripping or by any other suitable separation technique. When the product stream is subjected to distillation or stripping, the strip containing the separated ketone, water and ammonia streams can be partially concentrated and the concentrated phase is then returned to the aminoamide synthesis reactor. This separation step can be carried out at atmospheric or elevated pressure. The remaining unconcentrated fraction also contains unreacted ketones, water and ammonia and can be sent to the recovery section for further processing.

The methionine amide without ketone and ammonia is then subjected to hydrolysis in the presence of a titanium-containing catalyst as previously described herein to obtain the ammonium salt of methionine. This salt is then treated as previously described herein to remove ammonia to yield methionine.

The process of the present invention may include a recovery portion capable of receiving unreacted and/or recovered ammonia, ketones and water from any step of the process. Suitably, the three components are separated by absorption and distillation in the recovery section. The absorption step can be performed using water or by an acid/base exchange reaction. The ammonia obtained after this treatment can then be recycled to the aminonitrile synthesis reactor, and the ketone and water can also be recycled to the aminonitrile reactor at the same time.

As mentioned above, the product stream obtained from the second step of the aforementioned process for industrial application produces by-products including alkali metal salts of methionine. These by-products are removed from the amide product stream by carrying out an additional step of contacting the product stream containing these salts with a resin to facilitate the process of exchange of the alkali metal ions with the resin. In a preferred embodiment of the process for industrial application of the present invention, the resin of the second product stream free of ketones and ammonia is contacted with the above-mentioned resin; that is, the step performed after step (c) is completed and before step (d) is carried out.

Alternatively, the resin is placed at the end of the overall process so that the final product, which contains methionine but no ammonia, contacts the resin.

When the product stream contacts the resin, the alkali metal of the alkali metal salt of methionine is retained on the ion exchange resin, resulting in a solution of methionine free of alkali metal ions. Suitable resins are acidic resins, especially sulfonic acid resins. Commercially available resins under the trade names Rohm & Haas IMAC C16P and Fluka Amberlist 15 are used. Carboxylic resins whose acids have a pKa of less than 6.2 are also suitable for use. Suitable resins are those available under the trade names Fluka Duolite C464 or Rohm & Haas IRC50. Carboxylic acid resins are preferably used.

Suitably, the product stream containing the alkali metal salt is continuously passed through the resin. When the resin is saturated with the alkali metal ions, the resin is suitably regenerated by displacing the metal ions. These metal ions can be displaced out by treatment with an acidic medium such as with a strong mineral acid such as sulfuric acid or hydrochloric acid. 2-14 moles of inorganic acid can be used per kilogram of resin, preferably 3-6 moles. Carboxylic resins can alternatively be regenerated by treatment with an aqueous medium of carbon dioxide at a pressure of typically 10-25 bar. In this regeneration process, 2-14 moles of acid are suitably used per kilogram of resin, preferably 3-6 moles of acid are used.

The finally obtained liquid free methionine can be used, or can be further processed to recover solid methionine if necessary. This can be achieved by isolating the methionine by any suitable separation method, such as simple crystallization after concentration or atomization after concentration or partial concentration, crystallization and grinding, or granulation after concentration.

Now, the present invention will be illustrated with reference to the following examples.

Embodiment 1: the preparation of titanium catalyst

Ten titanium-containing catalysts of the present invention and two other catalysts not of the present invention were prepared as follows.

(1) Catalyst 1: 55 g of wet powder of titanium dioxide was added to a Brabender ^™ mixer. A solution of nitric acid (6.26 g) and water (27.39 g) was slowly added to the powder, and the resulting mixture was stirred at 50 rpm for 30 minutes. Thereafter, the paste was extruded on a die having a diameter of 1.6 mm at a rate of 4 cm/min to obtain extrudates having a diameter of 1.6 mm.

The resulting extrudate was placed in an oven and the temperature was raised from 120°C to 480°C at a rate of 3°C/minute. The temperature was maintained at this level for 4 hours, after which the temperature was lowered to ambient at a rate of 5°C/min.

The weight loss of the paste was 38.5%.

(2) Catalyst 2: 59.2 g of wet powder of titanium dioxide was added to a Brabender ^™ mixer. A solution of nitric acid (5.65 g) and water (15.16 g) was slowly added to the powder, and the resulting mixture was stirred at 50 rpm for 30 minutes. Thereafter, the paste was extruded on a die having a diameter of 1.6 mm at a rate of 4 cm/min to obtain extrudates having a diameter of 1.6 mm.

The resulting extrudate was placed in an oven and the temperature was raised from 120°C to 480°C at a rate of 3°C/minute. The temperature was maintained at this level for 4 hours, after which the temperature was lowered to ambient at a rate of 5°C/min.

The weight loss of the paste was 40%.

(3) Catalyst 3: This catalyst was purchased from Procatalyse under the designation CRS31.

(4) Catalyst 4: This catalyst was purchased from Degussa under the designation 7708.

(5) Catalyst 5: 228g titanium dioxide wet powder, 9.12g methylcellulose and 4.56g polysaccharide were stirred in a Brabender ^™ mixer for 30 minutes. 119.39 g of water were then added to form a paste. The paste was kneaded for 120 minutes and then left for 1 hour. The paste was then extruded at a rate of 4 cm/min to obtain extrudates with a diameter of 1.00 mm. The resulting extrudate was then placed in an oven whose temperature was raised from 20°C to 140°C at a rate of 1°C per minute over a period of two hours. The temperature was then raised to 480°C at a rate of 3°C/min over a period of 4 hours.

The weight loss of the paste was 45%, and the percentage of methylcellulose and polysaccharide in the paste was 2%.

(6) Catalyst 6: Catalyst 6 was prepared using the procedure for preparing Catalyst 5, but the weight loss of the resulting paste was 45% and the diameter of the extrudate was 1.6 mm.

(7) Catalyst 7: This catalyst was prepared using the procedure for Catalyst 6, but the resulting paste had a weight loss of 45%, a percentage of methyl cellulose of 4% and an extrudate diameter of 1.6 mm.

(8) Catalyst 8: This catalyst was prepared using the procedure of Catalyst 1, but the resulting paste had a weight loss of 40%.

Comparative Catalyst 1 designated 7709, which is not part of the present invention, was purchased from Degussa.

Comparative Catalyst 2 designated Ti-0702, which is not part of the present invention, was purchased from Engelhard.

(9) Catalyst 9: 55 g of wet powder of titanium dioxide was added to a Brabender ^(TM) mixer. A solution of nitric acid (68% strength, 6.26 g) and water (20.57 g) was slowly added to the powder, and the resulting mixture was stirred at 50 rpm for 30 minutes. Afterwards, the paste was extruded on a cloverleaf die at a rate of 4 cm/min.

The resulting extrudate was placed in an oven and the temperature was raised from 120°C to 480°C at a rate of 3°C/minute. The temperature was maintained at this level for 4 hours, after which the temperature was lowered to ambient at a rate of 5°C/min. The calcined extrudate had an external diameter of 0.8 mm. The weight loss of the paste was 35%.

(10) Catalyst 10: This catalyst was prepared using the procedure for preparing Catalyst 9, but the outer diameter of the calcined extrudate was 1.6 mm.

Comparative Catalyst 1 designated 7709, which is not part of the present invention, was purchased from Degussa.

Comparative Catalyst 2 designated Ti-0702, which is not part of the present invention, was purchased from Engelhard.

The properties of the catalysts prepared above are summarized in Table 1.

Table 1: Characteristics of the catalysts catalyst shape External diameter (mm) Surface area (m ² /g) Pore volume (cm ³ /g) Mesopore porosity (mn) Mesopore porosity (nm) 1 extrudate 1.6 63 0.38 17 200 2 extrudate 1.4 79 0.30 12 100 3 extrudate 4.0 115 0.29 10 100 4 extrudate 3.2 45 0.39 30 - 5 extrudate 0.8 - 0.44 9 150-160 6 extrudate 1.4 - 0.43 8 150-160 7 extrudate 1.3 - 0.44 9 150-160 8 extrudate 1.5 63 0.45 17 200 9 clover shape 0.8 100 0.36 14 50 10 clover shape 1.6 100 0.34 14 50 Compare Catalyst 1 extrudate 3.2 12 - 35 - Compare Catalyst 2 flaky 3.3 177 0.36 6-20 -

Example 2: Hydrolysis of Methionine Amide Using Powdered Catalyst

Catalysts 1, 2, 3, 4 and 9 prepared as above and comparative catalysts 1 and 2 were pulverized and then used to hydrolyze methionine amide. The aqueous solution of methionine amide was added to a batch reactor with 10 g of catalyst suspended in water such that the initial ratio of catalyst to amide was 1 gram of catalyst per gram of methionine. The initial concentration of amide in the reactor was 0.5 mol/kg.

The reaction is carried out at 95-100°C and atmospheric pressure.

The resulting product was analyzed by HPLC and the yield and conversion were determined.

The results are given in Figure 1.

Example 3: Hydrolysis of Methionine Amide with Catalyst in Circulating Stationary Reactor

Catalysts 3, 5, 6 and 7 were used in extrudate form to hydrolyze methionine amide in the amounts indicated in Table 2. The catalyst is placed in a fixed bed reactor. 216 g of water were added. The temperature was raised to 96°C. Then 122.9 g (21.7% p/p) of methionine amide were added to give an initial concentration of amide of 0.5 mol/kg.

The obtained product was analyzed by HPLC to determine its yield and conversion.

The results are given in Figure 2.

Table 2 catalyst Catalyst weight (g) Feeding flow rate (kg/hour) diameter (mm) Calcination temperature (℃) 3 40 10-15 4 480 5 10 5 0.8 480 6 10 10-15 1.4 400 7 twenty two 10-15 1.3 480

Example 4: Hydrolysis of Methionine Amide Using a Catalyst in a Fixed Bed Reactor with Plug Flow

(a) At atmospheric pressure

Catalysts 4, 6 and 8 were used to hydrolyze methionine amides in extrudate form. A methionine solution with an initial concentration of 0.37-0.85 mol/kg was added to the reactor. The temperature of the reactor was set at 95°C. The catalyst in the amount shown in Table 3 was charged into the reactor, and then operated under the conditions shown in Table 3.

table 3 catalyst Catalyst weight (g) Concentration of amide (molar) Feeding flow rate (g/h) Outflow time (Time onStream)*(minutes) 4 30 0.37 168.8, 153.8 4.8, 5.3 4 30 0.37 210.8 3.8 6 5 0.37 223 0.6 6 5 0.37 162 0.8 6 30 0.37 145, 188.9, 212.8 5.5, 4.2, 3.7 6 30 0.83 178, 218, 250 3.2, 3.7, 4.5 8 40 0.84 120, 160, 220 4.8, 6.7, 8.9

^* Calculate outflow time (ts) as follows

ts=[60*Em*catalyst weight (grams)]/flow rate (grams/hour), wherein

Em=(weight of liquid)/(weight of dry catalyst+weight of liquid)

For the catalyst of the present invention, Em = 0.45.

The results are given in Figure 3.

(b) under elevated pressure

The above operation was repeated using catalysts of different diameters and weights. Amide conversions are given in Table 4 below.

Table 4 catalyst temperature(℃) Catalyst weight (g) Feeding flow rate (g/h) Conversion rate(%) 8 100 40 120 97.5 8 100 40 300 87 8 100 40 300 95.5 9 100 20 120 97.5 9 100 20 300 93.3 9 120 20 300 95.8 10 100 20 120 95.6 10 100 20 300 82.6 10 120 20 300 90.5 10 100 40 120 98.2 10 100 40 300 97 10 120 40 300 98

The calculation of outflow time is the same as the above example.

Embodiment 5: the method for industrial production methionine

Catalyst 9 prepared in Example 1 was used in an industrial process. Figure 5 represents the entire reaction scheme. In order to obtain methionine without alkali metal salts, resins are used in this process. As shown in Figures 6 and 7, the resin is placed in two positions, which are referred to as Embodiment 2 and Embodiment 3, respectively. Tables 5, 6 and 7 give the composition of the streams at each stage of the three methods, respectively.

Embodiment (1) - the synthesis of methionine: make 2-hydroxyl-4-methylthiobutyronitrile and ammonia react in reactor (A), obtain containing 2-amino-4-methylthiobutyronitrile mixture (composition 1). Unreacted ammonia is separated from this product stream in vessel (B) and sent to recovery section (C). The treated stream (composition 2) is sent to reactor (D). Ketone, water and sodium hydroxide are charged to reactor (D). The resulting product stream containing methionine amide (composition 3) is worked up in column (E) to remove unreacted ketone, ammonia and at least part of the water. The gas obtained by stripping is partially concentrated and sent to the recovery section (C). Water is added to the resulting amide solution (composition 4) and the resulting solution (composition 5) is then contacted with the titanium catalyst in reactor (F). The product stream containing ammonium methionine (composition 6) is treated to release ammonia which is stripped in the ammonia stripper (G) to obtain free methionine. The released ammonia is concentrated and sent to the recovery section (C). The resulting liquid free methionine (composition 7) can then be acidified and further processed to obtain solid methionine.

The recovery section (C) includes a first absorber, a heating column, a second absorber and a distillation column (not shown). The stripping gas obtained from the column (E) and the stripping gas obtained from the ammonium stripping column (G) are sent to the first absorber, which is added with ammonium dihydrogen phosphate solution (5.5w/w % ammonia, 24.5 w/w % H ₃ PO ₄ and 70 w/w % water). The gas stream entering the column was 40.6 w/w % ammonia, 8 w/w % acetone and 51.4 w/w % water. The absorber is equipped with a heat exchanger to transfer the heat of solution of ammonia. The liquid coming out of the bottom of _the absorber column at 111.8°C contained 7.8w/w% ammonia, 23.9w/w% _H3PO4 , 68.3w/w% water and traces of acetone. The gas exiting the top of the absorber column at 111.9°C contained 13.1% acetone, 0.9w/w% ammonia and 86w/w% water. This gas mixture is concentrated and recycled to the aminoamide synthesis reactor. The liquid mixture obtained from the bottom of the absorber column is fed to a heating column to liberate ammonia. The mixture was heated and stripped with steam at 130°C to recover ammonia. The gas mixture obtained from the top of the heating tower contained 18 w/w % ammonia and 82 w/w % water. This gas was concentrated and then mixed with unreacted ammonia stripped from vessel (B) in a water absorber (second absorber) to obtain a concentration of 25 w/w % ammonia, 74.9 w/w % water and 0.1 % acetone in water. The solution is distilled to recover pure ammonia, which is then recycled back to the aminonitrile synthesis reactor. The heat is removed from the liquid mixture exiting the bottom of the heating column before it is recycled to the absorption column.

Embodiment (2) - methionine synthesis using resin before hydrolysis: 2-hydroxy-4-methylthiobutyronitrile is reacted with ammonia in reactor (A) to obtain 2-amino-4 - A mixture of methylthiobutyronitrile (composition 1). Unreacted ammonia is separated off in vessel (B) and then sent to recovery section (C). The treated stream (composition 2) is sent to reactor (D). Acetone, water and sodium hydroxide are charged to reactor (D). The resulting methionine amide-containing product stream is worked up in column (E) to remove unreacted acetone, ammonia and at least part of the water. The stripped gas is partially concentrated and then sent to the recovery section (C). Water was added to the resulting amide solution (composition 4), and the resulting solution (composition 5) was then continuously contacted with the resin. The resulting sodium salt-free product stream (composition 6) is brought into contact with a titanium catalyst in reactor (F). The ammonium methionine-containing product stream (composition 7) is stripped in the ammonium stripper (G), releasing ammonia and separating free methionine. The free methionine liquid (composition 8) can be further processed to obtain solid methionine. The recovery part is as described in Embodiment 1.

Embodiment (3) - Synthesis of methionine using resin after hydrolysis: 2-hydroxy-4-methylthiobutyronitrile is reacted with ammonia in reactor (A) to obtain 2-aminomethylthio A mixture of butyronitriles (composition 1). Unreacted ammonia is separated from the resulting product stream in vessel (B) before it is sent to recovery section (C). The outgoing stream (composition 2) is sent to reactor (D). In acetone, water and sodium hydroxide are added to the reactor (D). The resulting methionine amide-containing product stream (composition 3) is worked up in column (E) to remove unreacted acetone, ammonia and at least part of the water. The gas obtained by stripping is partially concentrated and sent to the recovery section (C). Water is added to the resulting amide solution (composition 4) and the resulting stream (composition 5) is then contacted with a titanium catalyst in reactor (F). The product stream (composition 6) containing ammonium methionine and sodium methionine is treated in an ammonium stripper (G) to liberate ammonia and obtain free methionine. The resulting stream (composition 7) was then continuously contacted with resin. The resulting sodium salt-free stream (composition 8) can be further processed to obtain solid methionine. The recovery part is as described in Embodiment 1.

table 5 Component 1: Aminonitrile solution before ammonia separation Composition 2: Aminonitrile solution after separation of ammonia Composition 3: Remove acetone and _NH3 and amide solution before water dilution Composition 4: Amide solution after removal of acetone and _NH3 Composition 5: Amide solution after dilution with water Composition 6: The resulting solution after contact with _TiO2 Composition 7: Solution obtained after separation of ammonia %w/w Hydroxynitrile 0 0 0 0 0 0 0 Aminonitrile 50.70 67.10 0 0 0 0 0 Amide 0 0 26.10 30.90 10.20 0.03 0.03 MTN Na 0 0 5.97 7.36 2.43 2.43 2.43 MTN-NH ₄ 0 0 0 0 0 11.41 0 free MTN 0 0 0 0 0 0 10.25 NH ₃ 27.70 7.15 7.20 0.15 0.05 0.05 0 acetone 0 0 4.00 0.01 0.00 0.03 0 IDZ 0 0 1.20 1.30 0.43 0.33 0.27 water 21.60 25.75 55.53 60.28 86.89 85.72 87.02 T(°C) 70 20 35 102 57 100 100 pressure (bar) 20 1 1 1 1 3 1

table 6 Component 1: Aminonitrile solution before ammonia separation Composition 2: Aminonitrile solution after separation of ammonia Composition 3: Remove acetone and _NH3 and amide solution before water dilution Composition 4: Amide solution after removal of acetone and _NH3 Composition 5: Amide solution after dilution with water Composition 6: Amide solution after contact with resin Composition 7: The resulting solution after contact with _TiO2 Composition 8: Solution obtained after separation of ammonia %w/w Hydroxynitrile 0 0 0 0 0 0 0 0 Aminonitrile 50.70 67.10 0 0 0 0 0 0 Amide 0 0 26.10 30.90 9.90 9.90 0.04 0.04 MTN Na 0 0 5.97 7.36 2.36 0 0 0 MTN-NH ₄ 0 0 0 0 0 0 11.15 0 free MTN 0 0 0 0 0 2.06 2.06 12.07 NH ₃ 27.70 7.10 7.20 0.15 0.05 0 0 0 acetone 0 0 4.00 0.01 0.00 0 0.03 0 IDZ 0 0 1.20 1.30 0.42 0.42 0.32 0.26 water 21.60 25.80 55.53 60.28 87.27 87.62 86.40 87.63 T(°C) 70 20 30 100 50 50 100 100 pressure (bar) 20 1 1 1 1 1 3 1

table 7 Component 1: Aminonitrile solution before ammonia separation Composition 2: Aminonitrile solution after separation of ammonia Composition 3: Remove acetone and _NH3 and amide solution before water dilution Composition 4: Amide solution after removal of acetone and _NH3 Composition 5: Amide solution after dilution with water Composition 6: The resulting solution after contact with _TiO2 Composition 7: Solution obtained after separation of ammonia Composition 8: Solution obtained after contact with resin %w/w Hydroxynitrile 0 0 0 0 0 0 0 0 Aminonitrile 50.70 67.10 0 0 0 0 0 0 Amide 0 0 26.10 30.90 9.90 0.05 0.05 0.05 MTN Na 0 0 5.97 7.36 2.36 2.36 2.36 0 MTN-NH ₄ 0 0 0 0 0 11.14 0 0 free MTN 0 0 0 0 0 0 10.05 12.11 NH ₃ 27.70 7.10 7.20 0.15 0.05 0.05 0 0 acetone 0 0 4.00 0.01 0.00 0.03 0 0 IDZ 0 0 1.20 1.30 0.42 0.32 0.26 0.26 water 21.60 25.80 55.53 60.28 87.27 86.05 87.28 87.58 T(°C) 70 20 30 100 50 100 100 100 pressure (bar) 20 1 1 1 1 3 1 1

Claims (21)

1. A method for preparing methionine, characterized in that it comprises: (a) hydrolyzing methionine amide to obtain ammonium methionine in the presence of a titanium-containing catalyst having a bimodal pore distribution having a pore distribution of 5-100 nm and a pore distribution of 20-1000 nm, The total pore volume is 0.2-0.55cm ³ /g, and the surface area is 30-150m ² /g; (b) The second step is to remove ammonia and recover methionine from the ammonium methionine. 2. The method according to claim 1, characterized in that the particle diameter of the catalyst is 0.05-4mm. 3. The method according to claim 2, characterized in that the particle diameter of the catalyst is 0.5-2mm. 4. The method of claim 2, wherein the catalyst is in powder or granular form. 5. The method of claim 4, wherein the catalyst is in the form of fine particles selected from extrudates, spherical particles and flakes. 6. The method of claim 5, wherein the extrudate is a three-lobe or four-lobe type. 7. The method of claim 1, wherein the total pore volume is 0.25-0.45 ^cm3 /g. 8. The method of claim 1, wherein the surface area is 40-120 ^m2 /g. 9. The method according to claim 1, wherein the catalyst is selected from TiO ₂ , Ti-W, TiMo, Ti-Si-W, Ti-Nb-Mo, Ti-Zr, Ti-Al, Ti -Cr, Ti-Zn, Ti-V or mixtures thereof. 10. The method of claim 9, wherein the catalyst is _TiO2 . 11. The method of claim 1, wherein 0.1-2 grams of said catalyst is present per gram of amide. 12. The method of claim 1, wherein the hydrolysis is performed at 50-150°C. 13. The method of claim 1, wherein the hydrolysis is performed at a pressure of 10 ⁵ -10 ⁶ Pa. 14. The method of claim 1, wherein the ammonia is removed by steam stripping. 15. The method of claim 1, wherein the method comprises: (a) contacting the aqueous solution of methionine amide with a catalyst containing titanium to catalyze the methionine amide to form ammonium methionine, wherein the methionine amide in the aqueous solution is 0.01 per kilogram of the solution - the presence of a concentration of 2 molar methionine amide, said catalyst having a bimodal pore distribution with a pore distribution of 5-100 nm and a pore distribution of 20-1000 nm, with a total pore volume of 0.2-0.55 cm ³ /g, The surface area is 30-150m ² /g; (b) Ammonia is removed, and methionine is recovered from the ammonium methionine. 16. A method of producing methionine comprising: (a) contacting 2-hydroxy-4-methylthiobutyronitrile with ammonia or a solution containing ammonia to produce a first product containing 2-amino-4-methylthiobutyronitrile; (b) contacting the 2-amino-4-methylthiobutyronitrile with a ketone and an alkali metal hydroxide in a reactor to produce a second product comprising methionine amide; (c) removing all unreacted ketones, ammonia and water from the second product stream; (d) hydrolyzing the methionine amide to obtain a third product stream comprising ammonium methionine in the presence of a titanium-containing catalyst having a bimodal pore distribution and having pores of 5-100 nm Distribution and pore distribution of 20-1000nm, the total pore volume is 0.2-0.55cm ³ /g, and the surface area is 30-150m ² /g; (e) Liberation of methionine from methionine ammonium salt. 17. The method according to claim 16, characterized in that the methionine obtained in step (e) is brought into contact with an acidic resin. 18. The method according to claim 16, characterized in that the unreacted and/or recovered ammonia, ketone and water are sent to a recovery section before ammonia and ketone are separated from water. 19. The method of claim 18, wherein said separation is performed using absorption and distillation. 20. The method of claim 18, wherein the separated ammonia is recycled as part of the ammonia used in step (a). 21. The method of claim 18, wherein the separated ketone and water are recycled to the reactor of step (b).

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