Process for preparing methylenediphenyl diisocyanates

• US 8,318,971 B2
• Filed: 02/27/2009
• Issued: 11/27/2012
• Est. Priority Date: 03/01/2008
• Status: Active Grant

• Current Assignee: Covestro Deutschland AG
• Original Assignee: Bayer Materialscience AG (Covestro AG)
• Inventors: Pohl, Fritz, Bulan, Andreas, Weber, Rainer, Adamson, Richard, Böhm, Matthias, Six, Christian
• Primary Examiner(s): Puttlitz, Karl J
• Application Number:
  US12/394,149
• Publication Number:
  US 20090240076A1
• Time in Patent Office: 1,369 Days
• Field of Search: None
• US Class Current: 560/347
• CPC Class Codes: C07C 209/78 from carbonyl compounds, e.g. from formaldehyde, and amines having amino groups bound to carbon atoms of six-membered aromatic rings, with formation of methylene-diarylamines
• CPC Class Codes: C07C 211/50 with at least two amino groups bound to carbon atoms of six-membered aromatic rings of the carbon skeleton
• CPC Class Codes: C07C 263/10 by reaction of amines with carbonyl halides, e.g. with phosgene
• CPC Class Codes: C07C 265/14 containing at least two isocyanate groups bound to the same carbon skeleton
• CPC Class Codes: C25B 1/26 Chlorine; Compounds thereof by simultaneous production of alkali metal hydroxides and chlorine, oxyacids or salts of chlorine C25B1/34
• CPC Class Codes: C25B 15/08 Supplying or removing reactants or electrolytes; Regeneration of electrolytes

Abstract

The invention relates to a process for preparing methylenediphenyl diisocyanates which comprises at least the steps: A) reaction of aniline with formaldehyde in the presence of hydrochloric acid as catalyst to give a mixture of diamines and polyamines of the diphenylmethane series (MDA) and subsequent at least partial neutralization of the hydrochloric acid by means of alkali metal hydroxide, B) reaction of the mixture of diamines and polyamines of the diphenylmethane series obtained in step A) with phosgene to give a mixture of diisocyanates and polyisocyanates of the diphenylmethane series (MDI) and hydrogen chloride, wherein C) the hydrochloric acid which has been neutralized in step A) is separated off in the form of a solution containing alkali metal chloride and is subsequently at least partly fed to an electrochemical oxidation to form chlorine, alkali metal hydroxide and optionally hydrogen and D) at least part of the chlorine produced in step C) is used for preparing the phosgene used in step B).

Claims

1. A process for preparing methylenediphenyl diisocyanate, comprising

A) reacting aniline with formaldehyde in the presence of hydrochloric acid as catalyst to form a mixture of diamines and polyamines of the diphenylmethane series (MDA) and subsequently at least partially neutralizing said hydrochloric acid with alkali metal hydroxide;

wherein

A1) water introduced with the starting materials or formed during the reaction of aniline with formaldehyde in the presence of hydrochloric acid as catalyst to form a mixture of diamines and polyamines of the diphenylmethane series (MDA) is at least partly removed from the reaction mixture;

A2) after neutralization of the reaction mixture formed in A1), an aqueous solution comprising alkali metal chloride and an organic phase comprising said mixture of diamines and polyamines is obtained;

A3) the organic phase from A2) is separated off from the aqueous phase and worked up to give said mixture of diamines and polyamines; and

A4) the aqueous solution comprising alkali metal chloride remaining after A3) is optionally at least partly combined with any water removed in A1) and/or with aqueous phases obtained in the work-up of said organic phase in step A3);

purified; and

at least partly fed to the electrochemical oxidation C); and

wherein said aqueous solution comprising alkali metal chloride is adjusted to a pH of <

8 prior to purification in A4);

B) reacting said mixture of diamines and polyamines of the diphenylmethane series with phosgene to form a mixture of diisocyanates and polyisocyanates of the diphenylmethane series (MDI) and hydrogen chloride;

wherein

C) the hydrochloric acid neutralized in A) is separated off in the form of a solution comprising alkali metal chloride and at least a portion of which is subsequently fed to an electrochemical oxidation to form chlorine, alkali metal hydroxide, and optionally hydrogen; and

D) at least part of the chlorine formed in C) is used to prepare the phosgene used in B),

wherein said aqueous solution comprising alkali metal chloride is adjusted to a TOC value of <

200 ppm before the oxidation C).

2. The process of claim 1, wherein said alkali metal hydroxide in A) is sodium hydroxide.

3. The process of claim 1, wherein at least part of the alkali metal hydroxide formed in C) is recirculated to A) for neutralization of said hydrochloric acid.

4. The process of claim 1, wherein at least part of the hydrogen chloride formed in B) is converted into hydrochloric acid, at least part of which is used as said catalyst in A).

5. The process of claim 1, wherein the concentration of alkali metal chloride in the aqueous solution comprising alkali metal chloride formed in A) is from 2 to 24% by weight.

6. The process of claim 1, wherein the concentration of alkali metal chloride in the aqueous solution comprising alkali metal chloride formed in A) is from 5 to 15% by weight.

7. The process of claim 1, wherein said aqueous solution comprising alkali metal chloride is adjusted to an alkali metal chloride content of >

5% by weight by removal of water and/or by addition of alkali metal chlorides in solid form or in the form of a concentrated solution before being used in an electrochemical oxidation.

8. The process of claim 1, wherein the aniline/formaldehyde reaction in A) is carried out at a degree of protonation of <

30%.

9. The process of claim 1, wherein the electrochemical oxidation in C) is carried out as an alkali metal chloride electrolysis using ion exchange membranes and/or using gas diffusion electrodes as cathode.

10. The process of claim 9, wherein the alkali metal chloride electrolysis is carried out by the membrane electrolysis process using an ion exchange membrane, with the alkali metal chloride of the solution containing alkali metal chloride being sodium chloride and the ion exchange membrane having a water transport capacity of >

3 mol of water/mol of sodium.

11. The process of claim 9, wherein the alkali metal chloride solutions fed to the alkali metal chloride electrolysis are adjusted to a pH of <

7 prior to electrolysis.

12. The process of claim 9, wherein the concentration of alkali metal chloride in the solution comprising alkali metal chloride leaving the electrolysis cell is from 100 to 280 g/l.

13. The process of claim 9, wherein the concentration of the alkali metal hydroxide solution obtained from the electrolysis is from 13 to 33% by weight.