RU2847829C1 - Method for thermal decomposition of 4,4'-methylendiphenyldicarbamate to 4,4'-methylendiphenyldiisocyanate - Google Patents

Abstract

FIELD: chemical industry.

SUBSTANCE: invention relates to organic synthesis technology, specifically to a method for the thermal decomposition of 4,4'-methylenediphenyl dicarbonate (4,4'-MDK) to 4,4'-methylenediphenyl diisocyanate (4,4'-MDI), which is characterised in that a melt or suspension in an inert liquid carrier or powder of 4,4'-MDK and an inert gas are fed into a column for continuous thermal decomposition, the mixture is heated in the column to 200-400°C for the decomposition of 4,4'-MDK in the gas phase in a stream of inert gas at atmospheric pressure , and then the mixture is separated in a rectification column with the isolation of high-purity 4,4'-MDI.

EFFECT: development of an improved target method with high yield and purity of the obtained 4,4'-MDI.

5 cl, 5 ex

Description

The invention relates to a method for the thermal decomposition of 4,4'-methylene diphenyl dicarbamate and the production of 4,4'-methylene diphenyl diisocyanate, hereinafter 4,4'-MDI.

Methods are known for the decomposition of alkyl carbamates by thermolysis to obtain alkyl isocyanates.

A method for the thermal decomposition of diphenylmethanedicarbamate is known (patent CN 103772240, IPC C07C 263/04; C07C 265/14, published 07.05.2014). Diphenylmethanedicarbamate is subjected to thermal decomposition using zinc oxide as a catalyst, the amount of catalyst is from 0.01 to 5% by weight of the inert solvent, and the reaction temperature is from 150 to 300 °C, the amount of inert solvent is from 5 to 100 wt. % diphenylmethanedicarbamate. The catalyst based on zinc oxide is obtained as follows: first, activated carbon and a zinc salt solution are mixed and impregnated, and then dried and calcined to remove the activated carbon. A catalyst based on zinc oxide is obtained. The reaction temperature is 180-230°C, the reaction time is 30-60 min, and the amount of inert solvent is 5-15 times greater than the mass of diphenylmethanedicarbamate.

The disadvantages of the method include the periodicity of the method, the use of a catalyst and a low yield of the target product.

A method for the thermal decomposition of alkyl carbamates is known (patent RU 2655386, IPC C07C 263/04; C07C 265/04; C07C 265/10, published 05/28/2018), which consists in the fact that in a packed column with continuous supply of the corresponding 0-(2-hydroxyethyl)carbamate and an inert carrier gas to the top of the column, thermolysis is carried out at 200-450°C, followed by fractional condensation of the resulting target and by-products.

The disadvantage of the method is the low yield of the target product from 22% to 91%.

The closest technical solution to the claimed one is the method of thermal decomposition of 4,4'-methylene diphenyl dicarbamate (4,4'-MDC) (international application WO 2023285545, IPC C07C263/04; C07C265/12; C07C271/06, published on 19.01.2023). The method of decomposition of MDC is carried out at 250°C under pressure for 5, 15 and 30 minutes in an isopropanol solvent medium. Only at the 17th minute does 4,4'-methylene diphenyl diisocyanate appear in the reaction mixture, but at the same time, various oligomers with different chain lengths appear.

The disadvantage of the method is the use of liquid-phase decomposition of 4,4'-MDA, in which a suitable solvent is selected, but the yield of 4,4'-MDI remains low due to the formation of oligomers.

It is known that 4,4'-MDI includes isomers 2,2'-MDI and 2,4'-MDI, from which 4,4'-MDI is purified (patents RU 2796693, EP 3697756, EP 2640696).

From the analogs considered above, it follows that there is a need to develop new cost-effective technologies for the thermal decomposition of 4,4'-MDC to obtain 4,4'-MDI with a high yield and purity of the resulting product.

The objective of the invention is to develop a new improved method for the thermal decomposition of 4,4'-MDC to obtain 4,4'-MDI with high yield and purity.

The problem is solved using the method of thermal decomposition of 4,4'-methylene diphenyl dicarbamate of formula I to 4,4'-methylene diphenyl diisocyanate of formula II which consists in continuously feeding a melt or suspension in an inert liquid carrier or powder of 4,4'-methylene diphenyl dicarbamate and an inert gas into a column for carrying out thermal decomposition, in the column the mixture is heated to 200-400°C to decompose 4,4'-methylene diphenyl dicarbamate in the gas phase in a stream of inert gas at atmospheric pressure, then the mixture is separated in a distillation column to isolate 4,4'-methylene diphenyl diisocyanate with high purity.

Preferably, the flow of methanol and inert gas from the top of the distillation column is directed to condense methanol, and the bottom liquid with 4,4'-methylene diphenyl diisocyanate is separated from the bottom of the distillation column.

Preferably, the still liquid with 4,4'-methylene diphenyl diisocyanate is distilled to isolate and return unreacted 4,4'-methylene diphenyl dicarbamate and decomposition intermediate back to thermal decomposition.

Preferably, before feeding into the thermal decomposition column, the mixture is prepared in a mixer, into which a melt or suspension in an inert liquid carrier or powder of 4,4'-methylene diphenyl dicarbamate heated to 200-400°C and an inert gas are fed, and then mixed.

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

The main differences of the proposed method of thermal decomposition of 4,4'-MDC to 4,4'-MDI are as follows:

- decomposition is carried out in a continuous mode;

- a melt or suspension in an inert liquid carrier or 4,4'-MDK powder in a stream of inert gas is fed into the column;

- carrying out thermal decomposition in continuous mode;

- obtaining 4,4'-MDI with high yield and purity.

The technical result of the proposed invention is the development of a new improved method for the thermal decomposition of 4,4'-MDC to obtain 4,4'-MDI with high yield and purity.

Thermal decomposition of 4,4'-MDC to 4,4'-MDI is carried out 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 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 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%.

Thermal decomposition occurs with ideal mixing of 4,4'-MDK, with a continuous supply of inert gas in the reactor, each particle of 4,4'-MDK is in the same conditions for the same amount of time, which inhibits the formation of oligomers and impurities.

The invention is further disclosed in Examples 1-4.

Example 1.

Molten 4,4'-MDC and an inert gas (nitrogen is used as the inert gas) 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 in a distillation column, where the resulting 4,4'-MDI and methanol are separated. The methanol and inert gas flow exits from the top of the distillation column and is then cooled in a condenser. The condensed methanol is returned for the synthesis of, for example, MFC (O-methyl-N-phenylcarbamate). The bottom liquid, which contains 4,4'-MDI, unreacted 4,4'-MDC, 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, and the unreacted 4,4'-MDA and the decomposition intermediate are returned to the decomposition stage. The yield of 4,4'-MDI is 96.0%, and the purity is 99.9%.

Example 2.

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 in a distillation column, where the resulting 4,4'-MDI and methanol are separated. The methanol and inert gas stream exits the top of the distillation column and is then cooled in a condenser. The condensed methanol is recycled for synthesis, for example, of MFC (O-methyl-N-phenylcarbamate). The bottom liquid of the distillation column is collected, containing 4,4'-MDI, o-dichlorobenzene, unreacted 4,4'-MDC, and a decomposition intermediate containing one isocyanate and one carbamate group. The bottom liquid is distilled, and the o-dichlorobenzene, unreacted 4,4'-MDC, and the decomposition intermediate are returned to the decomposition stage. The yield of 4,4'-MDI is 96.8%, and the purity is 99.9%.

Example 3.

Similar to Example 2, except that a suspension of 4,4'-MDA in an inert liquid carrier—toluene (70%)—is fed into the mixer. The yield of 4,4'-MDI is 97%, and the purity is 99.9%.

Example 4.

Similar to example 1, the only difference is 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%.

Example 5. According to the prototype.

Thermal decomposition of 4,4'-MDK is carried out, the powder of which is mixed with the solvent diisopropanol in the amount necessary to form a liquid-phase suspension.

Thermal decomposition is carried out at 250°C for 30 minutes. The reaction mixture is cooled, and 4,4'-MDI is isolated. Analysis revealed that the reaction mixture contains impurities of various oligomers with varying chain lengths. The reaction mixture also contains 4,4'-MDA.

From the given examples it follows that the proposed invention claims a simple and technologically advanced method for the thermal decomposition of 4,4'-MDC with a high yield and purity of the resulting 4,4'-MDI without the use of expensive catalysts by selecting optimal conditions for the thermal decomposition.

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%.

Claims (5)

1. A method for the thermal decomposition of 4,4'-methylene diphenyl dicarbamate of formula I to 4,4'-methylene diphenyl diisocyanate of formula II , characterized in that a melt or suspension in an inert liquid carrier or powder of 4,4'-methylene diphenyl dicarbamate and an inert gas are continuously fed into a column for carrying out thermal decomposition, in the column the mixture is heated to 200-400°C to decompose 4,4'-methylene diphenyl dicarbamate in the gas phase 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 with high purity. 2. The method according to paragraph 1, characterized in that from the top of the distillation column the flow of methanol and inert gas is directed to condense the methanol, and from the bottom of the distillation column the bottom liquid with 4,4'-methylene diphenyl diisocyanate is separated. 3. The method according to paragraph 2, characterized in that the still liquid with 4,4'-methylene diphenyl diisocyanate is distilled to isolate and return unreacted 4,4'-methylene diphenyl dicarbamate and the decomposition semi-product back to thermal decomposition. 4. The method according to claim 1, characterized in that before feeding into the column for thermal decomposition, a mixture is prepared in a mixer, into which a melt or suspension in an inert liquid carrier or powder of 4,4'-methylene diphenyl dicarbamate, heated to 200-400°C, and an inert gas are fed, and then mixed. 5. The method according to any one of paragraphs 1-4, characterized in that the yield of 4,4'-methylene diphenyl diisocyanate is 95.0% with a purity of 98.0-99.9%.