Storage-stable 2-hydroxy-4-(methylthio)butyronitrile

1. A 2-hydroxy-4-methylthiobutyronitrile mixture, comprising:

86 to 97% by weight of 2-hydroxy-4-methylthiobutyronitrile;

2 to 14% by weight of water; and

0.05 to 0.5% by weight of HCN;

wherein

a pH of the 2-hydroxy-4-methylthiobutyronitrile mixture is from 1 to 4, measured with a pH electrode at 23° C.

2. The 2-hydroxy-4-methylthiobutyronitrile mixture according to claim 1 wherein

a loss of 2-hydroxy-4-methylthiobutyronitrile in the mixture on storage for 128 days is less than 1% by weight.

3. The 2-hydroxy-4-methylthiobutyronitrile mixture according to claim 2 wherein

the loss of 2-hydroxy-4-methylthiobutyronitrile in the mixture on storage for 128 days is from 0.01 to 0.73% by weight.

4. The 2-hydroxy-4-methylthiobutyronitrile mixture according to claim 1 wherein

the content of 2-hydroxy-4-methylthiobutyronitrile is 88 to 92% by weight, and

the content of water is from 3 to 12% by weight.

5. The 2-hydroxy-4-methylthiobutyronitrile mixture according to claim 1 wherein

the content of 2-hydroxy-4-methylthiobutyronitrile is 88 to 92% by weight, and

the content of HCN is from 0.1 to 0.3% by weight.

6. The 2-hydroxy-4-methylthiobutyronitrile mixture according to claim 1 wherein

the content of 2-hydroxy-4-methylthiobutyronitrile is 88 to 92% by weight, and

the pH is from 2 to 3.

7. A method for producing the 2-hydroxy-4-(methylthio)butyronitrile mixture according to claim 1 , comprising:

a) reacting 3-methylmercaptopropionaldehyde with hydrogen cyanide in the presence of a base to form a reaction mixture comprising the 2-hydroxy-4-methylthiobutyronitrile;

b) adjusting the water content of the reaction mixture to 2 to 14% by weight by water addition, optionally during the reaction, after the reaction or during and after the reaction;

c) adjusting the HCN content of the reaction mixture to 0.05 to 0.5% by weight by HCN addition, optionally during the reaction, after the reaction or during and after the reaction; and thereafter

d) adding acid to set the pH to 1-4.

8. The method according to claim 7 , wherein the molar ratio of hydrogen cyanide to 3-methylmercaptopropionaldehyde is from 0.98 to 1.03.

9. The method according to claim 7 , wherein in d) the pH is set to from 2 to 3.

10. The method according to claim 7 , wherein in a),

the reaction of 3-methylmercaptopropionaldehyde with hydrogen cyanide in the presence of a base is conducted in a main reaction zone of a reactor,

residual amounts of gaseous hydrogen cyanide which leave the main reaction zone are absorbed in an absorption and post-reaction zone of the reactor, wherein the absorption and post-reaction zone comprises a mixture of 3-methylmercaptopropionaldehyde, catalyst, and optionally, 2-hydroxy-4-methylthiobutyronitrile and

the residual hydrogen cyanide reacts with the 3-methylmercaptopropionaldehyde to form 2-hydroxy-4-methylthiobutyronitrile.

11. The method according to claim 10 , wherein the mixture of 3-methylmercaptopropionaldehyde, catalyst, and optionally, 2-hydroxy-4-methylthiobutyronitrile in the absorption and post-reaction zone originates at least partially from the main reaction zone.

12. The method according to claim 10 , wherein the main reaction zone comprises at least one of a stirred reactor, a loop reactor and a jet pump.

13. The method according to claim 10 , wherein the absorption and post-reaction zone comprises at least one device for contacting a gas with a liquid, the device being selected from the group of devices consisting of a column, a tray column, a packed-bed column, a bubble-column reactor, a droplet column, a reactor having a mechanically agitated container, a submerged jet reactor and a jet pump.

14. The method according to claim 10 , wherein the hydrogen cyanide is introduced into the main reaction zone as a product gas from a plant for producing hydrogen cyanide, the plant being connected to the reactor.

15. The method according to claim 14 , wherein a hydrogen cyanide content of the product gas is 1 to 99% by weight.

16. The method according to claim 7 , wherein the catalyst is selected from the group consisting of a low-molecular-weight amine, a heterogeneous amine, a solution of an inorganic base, and a mixture of an acid and low-molecular-weight amine.

17. The method according to claim 16 , wherein the catalyst is a low-molecular-weight amine, which is selected from the group consisting of a tri-(C 1 -C 12 -alkyl)amine, a dialkylaralkylamine, a dialkylarylamine and a heterocyclic amine.

18. The method according to claim 16 , wherein the catalyst is a mixture of an acid and low-molecular-weight amine, the acid being at least one selected from the group consisting of an organic acid, an organic sulfonic acid, and a mineral acid.

19. A method for producing the 2-hydroxy-4-(methylthio)butyronitrile mixture according to claim 1 , comprising:

a) preparing a mixture of 3-methylmercaptopropionaldehyde and a base having a water content of 2 to 14% by weight;

b) adjusting the pH of the mixture to 4.5 to 6.0;

c) adding HCN to the mixture having a pH of 4.5 to 6.0;

d) reacting the 3-methylmercaptopropionaldehyde with the HCN in the presence of the base at a temperature of from 20 to 80° C. to form a reaction mixture comprising the 2-hydroxy-4-methylthiobutyronitrile;

e) adjusting the HCN content of the reaction mixture to 0.05 to 0.5% by weight by HCN addition; and thereafter,

f) adding acid to set the pH to 1-4,

wherein pH adjustment is with a pH electrode at 23° C.,

wherein a molar ratio of hydrogen cyanide to 3-methylmercaptopropionaldehyde in d) is from 0.98 to 1.03.

20. The method according to claim 19 , wherein the molar ratio of hydrogen cyanide to 3-methylmercaptopropionaldehyde is from 0.99 to 1.01.

21. The method according to claim 19 , wherein in d),

the reaction of 3-methylmercaptopropionaldehyde with hydrogen cyanide in the presence of a base is conducted in a main reaction zone of a reactor,

residual amounts of gaseous hydrogen cyanide which leave the main reaction zone are absorbed in an absorption and post-reaction zone of the reactor, wherein a temperature of the absorption and post-reaction zone is from 0 to 30° C., the absorption and post-reaction zone comprises a mixture of 3-methylmercaptopropionaldehyde, catalyst, and optionally, 2-hydroxy-4-methylthiobutyronitrile and

at least a part of the residual hydrogen cyanide reacts with the 3-methylmercaptopropionaldehyde to form 2-hydroxy-4-methylthiobutyronitrile.

22. A method for producing D,L-methionine or 2-hydroxy-4-methylthiobutyric acid, comprising: employing the 2-hydroxy-4-(methylthio) butyronitrile mixture according to claim 1 as a starting material source or as an intermediate to prepare D,L-methionine or 2-hydroxy-4-methylthiobutyric acid.