RU2848366C1 - A phosphorus-free method for obtaining 4,4'-methylendiphenyldiisoccyanate - Google Patents

Abstract

FIELD: chemical industry.

SUBSTANCE: invention relates to a new method for producing 4,4'-methylenediphenyl diisocyanate (MDI). The phosgene-free method for producing 4,4'-methylenediphenyl diisocyanate is characterised in that diphenylurea is obtained from urea and aniline in an aromatic hydrocarbon medium, then the resulting diphenylurea is reacted with methanol in excess to obtain O-methyl-N-phenylcarbamate; to obtain 4,4'-methylenediphenyl dicarbonate, O-methyl-N-phenylcarbamate is reacted in a melt with anhydrous formaldehyde derivatives using acid catalysts, and the thermal decomposition of the melt or suspension in an inert liquid carrier or powder of 4,4'-methylenediphenyl dicarbonate is carried out in a continuous mode in a stream of inert gas, and 4,4'-methylenediphenyl diisocyanate is isolated by rectification.

EFFECT: new phosgene-free method for the synthesis of 4,4'-MDI, suitable for industrial-scale implementation.

9 cl, 4 ex

Description

The invention relates to a new method for producing 4,4'-methylene diphenyl diisocyanate, in which the main raw materials for its production are ammonia, carbon dioxide (or carbamide (urea)), aniline, and can be used in the manufacture of varnishes, polyurethane foam adhesives and sealants, synthetic leather, synthetic rubber, etc.

In known methods, including industrial ones, the synthesis of isocyanates is carried out by phosgenation of the corresponding primary amines; for example, methylene diphenyl diisocyanate (4,4'-MDI) is obtained from methylene diphenyl diamine by interaction with phosgene.

A method for producing methylene diphenyl diisocyanate is known (patent CN 106748887, IPC C07C 263/10; C07C 265/14, published 05/31/2017). The method includes the following steps: carrying out a reaction of salt formation from m-xylylenediamine and gaseous hydrogen chloride to obtain m-xylylenediamine hydrochloride; mixing m-xylylenediamine hydrochloride, an organic phosphorus compound and solid phosgene to obtain methylene diphenyl diisocyanate. In the process of obtaining

An organic phosphorus compound additive is introduced into methylene diphenyl diisocyanate.

A method is known for producing 4,4'-methylene diphenyl diisocyanate and the corresponding oligomer with improved selectivity and yield (US Patent 4,297,294, IPC C07C 265/12; C07D 251/04, published 10/27/1981), which includes the condensation of formaldehyde and aniline under neutral or basic conditions to produce an aniline condensate, followed by a reaction of the aniline condensate with additional aniline and a protic salt of aniline to produce 4,4'-methylenedianiline and the corresponding oligomer, followed by conventional phosgenation.

A method for producing methylene diphenyl diisocyanate (MDI) by decomposing methylene diphenyl dicarbamate (MDC) is known (patent CN 103772240, IPC C07C 263/04; C07C 265/14, published on May 31, 2017). Zinc oxide is used as a catalyst, the amount of the catalyst used is 0.01-5% of the weight of the inert solvent, the reaction temperature is 150-300 °C, the amount of the inert solvent used is 5-100 times the weight of methylene diphenyl dicarbamate (MDC), the method for preparing a catalyst from zinc oxide is as follows: first, activated carbon is mixed and dipped in a zinc salt solution, then dried, calcined to remove activated carbon to prepare a catalyst from zinc oxide.

The resulting 4,4'-MDI includes a significant amount, up to 30%, of various isomeric methylene diphenyl diisocyanate monomers, like the analogs disclosed above.

A method for producing di- and polyamines of the diphenylmethane series is known (patent RU 2501784, IPC C07C 209/78; C07C 263/10; C07C 211/50; C07C 265/14, published on 20.12.2013). The method involves the reaction of aniline and formaldehyde in the presence of an acid catalyst, wherein aniline is used, containing in total from 0.0001 to 0.25 wt. %, based on the weight of the aniline used, compounds having at least one carbonyl group or representing reaction products of such carbonyl compounds with aniline. The proposed method makes it possible to significantly improve the color of the resulting product. The invention also relates to a method for producing di- and polyisocyanates of the diphenylmethane series, in which the di- and polyamines of the diphenylmethane series, obtained by the method proposed above, react with phosgene.

A disadvantage of the disclosed analogs is the use of a phosgenation stage; in addition, the known methods produce 4,4'-MDI with impurities of dimers, oligomers, polymers, and ortho-isomers, which require purification.

For the purification of 4,4'-methylene diphenyl diisocyanate from a crude mixture (patent RU 2796693, IPC C07C 263/20; C07C 265/14; B01D 1/22; B01D 3/26; B01D 5/00; B01D 9/02, published 05/29/2023), a unit is known for producing purified 4,4'-methylene diphenyl diisocyanate from a mixture of various isomeric methylene diphenyl diisocyanate monomers. The installation comprises a distillation device that comprises a) a distillation column including structured packing, b) a source for a mixture of different isomeric methylene diphenyl diisocyanate monomers, c) an evaporator, d) a head vapor condenser, e) a head vacuum system, and f) a flow-controlled reflux system. The head vapor condenser has a shell-and-tube configuration and is designed to directly subcool the condensate to a temperature below 47°C. The flow-controlled reflux system contains a heater that is designed to reheat a partial flow of condensate, which is formed in the head vapor condenser, to 190°C. The invention also relates to a method for producing purified 4,4'-methylene diphenyl diisocyanate from an unrefined mixture containing different isomeric methylene diphenyl diisocyanate monomers, as well as the corresponding dimers, oligomers, and polymers. The method is carried out in the above-mentioned unit by feeding a mixture of various isomeric methylene diphenyl diisocyanate monomers into the evaporator. The invention also relates to a purified 4,4'-methylene diphenyl diisocyanate composition obtained as the condensed overhead product of the unit's distillation column.

The disadvantage of this method is its complexity and high energy costs.

The closest technical solution to the claimed one is a method for continuously producing a product with a target 4,4'-methylene diphenyl diisocyanate content of 95-98% in a displacement reactor (patent RU 2750198, IPC C07C 263/10; C07C 265/14, published on June 23, 2021). The method is characterized by phosgenation of a 10-30 wt. % methylene diphenyl diamine solution with a triphosgene solution containing from 10 to 30 wt. % at a temperature of 0-10 °C and further heating the reaction mixture to a temperature of 120-130 °C. Methylene diphenyl diamine with a content of 95.1-98.7% of all methylene diphenyl diamine isomers is used as the feedstock, which contains non-target isomers of 2,4- and 2,2-methylene diphenyl diamine in quantities of less than 10 and 3 wt.%, respectively, and chlorobenzene is used as the solvent.

In the proposed method, toxic phosgene is replaced by triphosgene, but there are problems with purifying 4,4'-methylene diphenyl diisocyanate from non-target isomers.

The objective of the invention is to develop a new phosgene-free method for synthesizing 4,4'-MDI, suitable for implementation on an industrial scale using commercially available raw materials, reagents, standard process equipment, simple chemical and technological methods of synthesis, with the production of a new product 4,4'-MDI with a high yield and purity, the qualitative composition of which does not require purification from impurities.

The stated problem is solved using a phosgene-free method for producing 4,4'-methylene diphenyl diisocyanate, which consists in the fact that diphenylurea is obtained from urea and aniline in an aromatic hydrocarbon medium, then the resulting diphenylurea is reacted with methanol in an excess of methanol to obtain O-methyl-N-phenylcarbamate; to obtain 4,4'-methylene diphenyl dicarbamate, O-methyl-N-phenylcarbamate is reacted in a melt with anhydrous derivatives of formaldehyde using acid catalysts, and thermal decomposition of the melt or suspension in an inert liquid carrier or powder of 4,4'-methylene diphenyl dicarbamate is carried out in a continuous mode in a stream of inert gas and 4,4'-methylene diphenyl diisocyanate is isolated by rectification.

Preferably, urea is obtained from ammonia and carbon dioxide.

Preferably, the interaction of urea and aniline is carried out in a molar ratio of 1:2 to 1:3 at a temperature of 110-180°C with constant distillation of the released ammonia.

Preferably, the reaction of diphenylurea with methanol is carried out at a molar ratio of the diphenylurea:methanol reagents equal to 1:(6-15) and a temperature of 120-190°C.

Preferably, the reaction is carried out in an autoclave; after the process is complete, the autoclave is cooled and the products of the process are subjected to a two-stage rectification. In the first column, methanol is separated from the high-boiling products: O-methyl-N-phenylcarbamate and aniline; in the second column, the high-boiling products are separated; the isolated aniline is returned to the stage of obtaining diphenylurea.

Preferably, the reaction of O-methyl-N-phenylcarbamate with anhydrous derivatives of formaldehyde is carried out using acid catalysts at a molar ratio of the reagents equal to (2-10):1:(0.25-2), and the reaction is carried out at a temperature of 50-150°C in molten O-methyl-N-phenylcarbamate.

Preferably, sulfonic acids and sulfuric acid are used as acid catalysts.

Preferably, the mixture is heated to 200-400°C to thermally decompose 4,4'-methylene diphenyl dicarbamate in an inert gas stream at atmospheric pressure, then the mixture is separated in a distillation column to isolate 4,4'-methylene diphenyl diisocyanate.

The yield of 4,4'-methylene diphenyl diisocyanate is 95.0% with a purity of 98.0-99.9%.

The technical result of the proposed invention is the development of a new phosgene-free method for synthesizing 4,4'-MDI, suitable for implementation on an industrial scale using commercially available raw materials, reagents, standard process equipment, simple chemical and technological methods of synthesis with the production of a new product 4,4'-MDI with a high yield and purity, the qualitative composition of which does not require purification from impurities.

A method is proposed for obtaining 4,4'-methylene diphenyl diisocyanate (hereinafter 4,4'-MDI) with a purity of more than 98%, which consists in the fact that 4,4'-MDI is obtained in five stages from available raw materials (carbon dioxide, ammonia, aniline, methanol, anhydrous derivatives of formaldehyde) according to the scheme:

where Me is the alkyl group _CH3 , MeOH is methanol ( _CH3OH ).

The structure of the compounds was confirmed by ¹ H NMR spectroscopy.

The proposed method features high yields at each stage, process selectivity, and the absence of significant waste. It also eliminates the phosgenation step. Furthermore, non-target byproducts released at any stage are recycled back into the process (described above). The proposed method is carried out as follows.

In the first stage, urea is synthesized from ammonia and carbon dioxide:

The specified method of production is industrial and is widely used in the production of urea.

The production of urea from ammonia and carbon dioxide is widely disclosed in the technical literature (patent RU 2172732).

Thus, at the first stage, the presence of urea is ensured.

In the second stage, N,N'-diphenylurea (hereinafter DFM) is obtained from urea and aniline:

The reaction is carried out at a urea to aniline molar ratio of 1:2 (2-3) by heating in aromatic hydrocarbons (toluene, xylene, chlorobenzene, o-dichlorobenzene) at 110-180°C for 10-72 hours with distillation of the released ammonia, which is then used in the first stage of urea synthesis. Upon completion of the reaction, diphenylurea is filtered and dried. For improved purity, it is recrystallized from ethanol. The yield of diphenylurea is 89-96%. The reaction filtrate, which contains excess aniline in an inert solvent, can be reused. Thus, excess aniline and solvent are continually returned to the process.

In the third stage of the synthesis of 4,4'-MDI, O-methyl-N-phenylcarbamate (MPC) is obtained from DFM according to the scheme:

The reaction is carried out in a metal autoclave at a methanol to DFM molar ratio of (6-15):1. The best yields are achieved at temperatures of 120-190°C and reaction times of 1-8 hours. The target MFC is isolated by a two-stage distillation method. In the first distillation column, methanol is distilled off, and in the second, MFC and aniline are distilled under reduced pressure. The MFC yield is 78-89%. The recovered aniline is returned to the second stage.

The fourth stage - the synthesis of 4,4'-methylene diphenyl dicarbamate (hereinafter 4,4'-MDC) from MFC is carried out according to the scheme:

The synthesis is carried out in molten MFC, which is used as a reactant and a solvent. Anhydrous formaldehyde derivatives are used as the second reactant. The reaction is carried out under catalysis by sulfonic acids: trifluoromethanesulfonic acid, p-sulfobenzoic acid, cysteine acid, benzenesulfonic acid, methanesulfonic acid, toluenesulfonic acid, and sulfuric acid. The molar ratio of the MFC reagents, formaldehyde derivative and catalyst varies within the range: (2-10):1:(0.25-2). The reaction temperature and time are 50-150°C and 1-10 hours, respectively. After the reaction, the reaction mass is diluted with an inert solvent (benzene, toluene, chloroform, dichloromethane, ethyl acetate) and the product is filtered and dried. The yield of 4,4'-MDC is 71-95%. According to NMR spectra, the product does not contain any ortho-isomers. The filtrate is washed with an aqueous sodium bicarbonate solution to remove the acid, and the solvent is distilled off. The aqueous sodium salt solution (the main production waste) is disposed of. The MFC remaining after solvent distillation is reused in the process. The solvents are recovered by distillation.

The fifth stage of the process is the thermal decomposition of 4,4'-MDC to 4,4'-MDI, which is carried out according to the scheme:

The reaction is carried out continuously at atmospheric pressure in an inert gas stream. A melt or suspension in an inert liquid carrier, or 4,4'-MDC powder, is mixed with an inert gas stream and sent to a decomposition column, where the mixture is heated to 200-400°C. This leads to the decomposition of 4,4'-MDC. According to the invention, an inert liquid carrier is understood to be any liquid carrier chemically inert with respect to 4,4'-MDC. Benzene, toluene, xylene, chlorobenzene, and other known similar inert carriers can be used as inert liquid carriers. Carrying out the reaction in an inert gas stream eliminates the possibility of interaction between the molecules of the resulting 4,4'-MDI and prevents their di- and trimerization. Dilution with an inert gas shifts the equilibrium reaction toward the product and ensures uniform heating of the reaction space of the column. After decomposition, 4,4'-MDI and methanol are distilled, and the 4,4'-MDI is rectified. Methanol is returned to the third stage of the process. The yield of 4,4'-MDI reaches over 95%, and the purity, determined by derivatives (the addition product of dipropylamine to 4,4'-MDI, 4,4'-Bis(N,N-dipropylaminocarbonylamino)-diphenylmethane), exceeds 98%.

The proposed method is distinguished by the absence of the need for separation of 4,4'-MDI isomers and, accordingly, separation columns, which significantly complicate and increase the cost of traditional phosgene technology.

Thus, the proposed technology for the synthesis of 4,4'-MDI is distinguished by the fact that:

1. It is carried out in five stages from large-tonnage industrially available products of basic organic synthesis: ammonia and carbon dioxide (or urea, as a result of the reaction), aniline, formaldehyde derivatives, methanol;

2. Low waste. All by-products are returned to the process at each stage. The main, safe waste is aqueous solutions of catalyst salts—sulfonic acids;

3. 4,4'-MDC is synthesized in a melt of MFC, using it as both a solvent and a reagent. Anhydrous formaldehyde derivatives are used as the second reagent. Catalysis is accomplished with inexpensive and industrially available sulfonic acids. The 4,4'-MDC obtained in this manner is free of ortho-isomer and oligomer impurities and is synthesized with a high degree of purity, without loss in the form of byproducts.

4. The decomposition of 4,4'-MDC to 4,4'-MDI is carried out continuously at atmospheric pressure and dilution of the reaction mass with an inert gas, which eliminates the di- and trimerization of the resulting isocyanate groups and leads to the formation of 4,4'-MDI with a purity of more than 98%.

5. The technology eliminates the phosgenation step, which is typically required and used in common 4,4'-MDI production processes. This dramatically reduces the hazard of producing 4,4'-MDI, a component of various polyurethane foams—one of the most widely produced organic chemistry products in recent years. Eliminating the need for phosgene, which is known, among other things, as a chemical warfare agent, and which is an odorless, transparent gas, significantly reduces both the hazard and enhances the environmental benefits of the patented technology. Furthermore, the elimination of the phosgenation step reduces the complexity of the process's equipment and technology design.

The invention is further disclosed in Examples 1-4.

Example 1. First stage.

Urea is produced from ammonia and carbon dioxide using a well-known industrial process. This ensures the availability of feedstock for producing urea.

Second stage.

Obtaining diphenylurea.

A flask equipped with a reflux condenser is charged with 15.0 g (0.25 mol) of urea, 46.5 g (0.5 mol) of aniline, and 200 ml of o-dichlorobenzene. The mixture is boiled for 10 hours. After this time, the reaction mixture is cooled, and the precipitate is filtered off. The filtrate is reused, and a fresh portion of urea and aniline is added to it. The precipitate is washed twice with 30 ml of toluene, air-dried, and recrystallized from ethanol. 50.9 g is obtained (yield - 96.0%, purity 99.9%).

The third stage.

Obtaining O-methyl-N-phenylcarbamate.

A steel autoclave is charged with 45.00 g (0.21 mol) of DFM and 40.3 g (1.26 mol) of methanol. The autoclave is hermetically sealed and heated at 180°C for 2 hours. After the specified time, the autoclave is cooled and opened. 10 ml of methanol is added, and unreacted DFM (5.2 g) is filtered off. The residue is subjected to two-stage distillation. In the first column, methanol and high-boiling products—MFK and aniline—are distilled. Methanol is recycled. In the second column, MFK and aniline are distilled under vacuum (30 mmHg). 22.6 g (78.0% of the reacted DFM) of MFC with a purity of 99.9% and 18.7 g of aniline are obtained, which are sent to the DFM synthesis stage.

The fourth stage.

Obtaining 4,4'-methylene diphenyl dicarbamate.

In a flask equipped with a stirrer, reflux condenser and thermometer, 35.0 g (0.232 mol) of O-methyl-N-phenylcarbamate, 3.48 g (0.116 mol) of anhydrous formaldehyde derivative and 11.0 g (0.058 mol) of toluenesulfonic acid hydrate are charged. The mixture is heated with stirring to a temperature of 100 °C and stirred at this temperature for 4 hours. After this, the reaction mass is cooled to room temperature, 100 ml of ethyl acetate are added. The precipitate is filtered and dried. After drying, 29.3 g (80.0%) of 4,4'-MDC are obtained (ortho-isomers and by-products are absent). The organic phase is separated and distilled under vacuum. The residue containing MFC and toluenesulfonic acid is reused. Fifth stage.

Obtaining 4,4'-methylene diphenyl diisocyanate.

Molten 4,4'-MDA and an inert gas (nitrogen) are fed into a heated steel packed mixer heated to 200-400°C. The resulting mixture is then fed into a steel column heated to an internal temperature of 200-400°C. The mixture is then sent for separation to a distillation column, where the resulting 4,4'-MDI and methanol are separated. The methanol and inert gas flow exits the top of the distillation column and is then cooled in a condenser. The condensed methanol is returned to the third stage. The bottom liquid, containing 4,4'-MDI, unreacted 4,4'-MDA, and a decomposition intermediate containing one isocyanate and one carbamate group, is collected from the bottom of the distillation column. The still liquid is distilled, unreacted 4,4'-MDC and the decomposition semi-product are returned to the decomposition stage.

A pure transparent white product is obtained - the yield of 4,4'-MDI is 96%, the purity is 99.9%.

Example 2.

The first stage is similar to example 1.

Second stage.

Obtaining diphenylurea.

A flask equipped with a reflux condenser is charged with 15.0 g (0.25 mol) of urea, 59.8 g (0.75 mol) of aniline, and 300 ml of chlorobenzene. The mixture is boiled for 15 hours. After this time, the reaction mixture is cooled, and the precipitate is filtered off. The filtrate is rectified, returning 290 ml of chlorobenzene and 37.9 g of aniline back to the process. The precipitate is air-dried and recrystallized from ethanol. 22.2 g are obtained (yield of reacted aniline - 89.0%, purity 99.9%).

The third stage.

Obtaining O-methyl-N-phenylcarbamate.

A steel autoclave is charged with 45.00 g (0.21 mol) of DFM and 100.8 g (3.15 mol) of methanol. The autoclave is hermetically sealed and heated at 120°C for 8 hours. After the specified time, the autoclave is cooled and opened. The residue is subjected to two-stage distillation. In the first column, methanol and high-boiling products - MFC and aniline - are distilled. Methanol is recycled. In the second column, MFC and aniline are distilled under vacuum (30 mm Hg). 26.7 g (84.3%) of MFC with a purity of 99.9% and 18.8 g of aniline are obtained, which are sent to the DFM synthesis stage.

The fourth stage.

Obtaining 4,4'-methylenediphenyldicarbamate.

In an autoclave equipped with a stirrer, 50.0 g (0.331 mol) of O-methyl-N-phenylcarbamate, 0.993 g (33.1 mmol) of anhydrous formaldehyde derivative and 1.58 g (8.3 mmol) of toluenesulfonic acid hydrate are charged. The mixture is heated with stirring to a temperature of 130 °C and stirred at this temperature for 1 hour. After this, the reaction mass is cooled to room temperature and diluted with 100 ml of chloroform. The precipitate is filtered and dried. After drying, 7.4 g (71.0%) of 4,4'-MDC are obtained (ortho-isomers and by-products are absent). The filtrate is washed with a saturated solution of sodium bicarbonate until the medium is neutral. The organic phase is separated and distilled under vacuum. The residue - 41.1 g of MFC - is reused.

Fifth stage.

Obtaining 4,4'-methylene diphenyl diisocyanate.

A heated steel packed mixer heated to 200°C is fed with a suspension of o-dichlorobenzene in an inert liquid carrier and an inert gas (argon). The resulting mixture is then fed into a steel column heated to an internal temperature of 200-400°C. The mixture is then sent for separation to a distillation column, where the resulting 4,4'-MDI and methanol are separated. The methanol and inert gas flow exits the top of the distillation column and is then cooled in a condenser. The condensed methanol is returned to the third stage. The bottom liquid, which contains 4,4'-MDI, o-dichlorobenzene, unreacted 4,4'-MDA, and a decomposition intermediate containing one isocyanate and one carbamate group, is collected from the bottom of the distillation column. The still liquid is distilled, o-dichlorobenzene, unreacted 4,4'-MDC and the decomposition intermediate are returned to the decomposition stage.

A pure transparent white product is obtained - the yield of 4,4'-MDI is 96.8%, the purity is 99.9%.

Example 3.

Similar to example 2, only at the fifth stage a suspension is fed into an inert liquid carrier - toluene.

A pure transparent white product is obtained - the yield of 4,4'-MDI is 97%, the purity is 99.9%.

Example 4.

Similar to example 1, only the fifth stage differs in that 4,4'-MDK powder and inert gas are fed into the mixer.

A pure transparent white product is obtained - the yield of 4,4'-MDI is 97%, the purity is 99.9%.

Thus, in the proposed phosgene-free method for producing 4,4'-methylene diphenyl diisocyanate, a new product was obtained that is practically free of isomer impurities, has a transparent white color and does not require special methods to improve its color.

Claims (9)

1. A phosgene-free method for producing 4,4'-methylene diphenyl diisocyanate, characterized in that diphenylurea is obtained from urea and aniline in an aromatic hydrocarbon medium, followed by a reaction of the resulting diphenylurea with methanol in an excess of methanol to obtain O-methyl-N-phenylcarbamate; to obtain 4,4'-methylene diphenyl dicarbamate, O-methyl-N-phenylcarbamate is reacted in a melt with anhydrous derivatives of formaldehyde using acid catalysts, and thermal decomposition of the melt or suspension in an inert liquid carrier or powder of 4,4'-methylene diphenyl dicarbamate is carried out in a continuous mode in a stream of inert gas and 4,4'-methylene diphenyl diisocyanate is isolated by rectification. 2. The method according to paragraph 1, characterized in that urea is obtained from ammonia and carbon dioxide. 3. The method according to paragraph 1, characterized in that the interaction of urea and aniline is carried out in a molar ratio of 1:2 to 1:3 at a temperature of 110-180°C with constant distillation of the released ammonia. 4. The method according to claim 1, characterized in that the reaction of diphenylurea with methanol is carried out at a molar ratio of the reagents diphenylurea:methanol equal to 1:(6-15), and a temperature of 120-190°C. 5. The method according to paragraph 4, characterized in that the reaction is carried out in an autoclave, after completion of the process the autoclave is cooled and the process products are subjected to two-stage rectification, in the first column methanol is separated from high-boiling products: O-methyl-N-phenylcarbamate and aniline, in the second column the high-boiling products are separated, the separated aniline is returned to the stage of obtaining diphenylurea. 6. The method according to claim 1, characterized in that the reaction of O-methyl-N-phenylcarbamate with anhydrous derivatives of formaldehyde is carried out using acid catalysts at a molar ratio of reagents equal to (2-10):1:(0.25-2), and the reaction is carried out at a temperature of 50-150°C in molten O-methyl-N-phenylcarbamate. 7. The method according to paragraph 6, characterized in that sulfonic acids and sulfuric acid are used as acid catalysts. 8. The method according to claim 1, characterized in that the mixture is heated to 200-400°C for thermal decomposition of 4,4'-methylene diphenyl dicarbamate in a stream of inert gas at atmospheric pressure, then the mixture is separated in a distillation column with the isolation of 4,4'-methylene diphenyl diisocyanate. 9. The method according to any one of paragraphs 1-8, characterized in that the yield of 4,4'-methylene diphenyl diisocyanate is 95.0% with a purity of 98.0-99.9%.