WO2008031755A1 - Process for preparing isocyanates - Google Patents

Abstract

The invention relates to processes for preparing isocyanates by splitting urethane, said process comprising the following process steps: (i) provision of synthesis gas from natural gas, coal, heavy oil or biomass gasification and production of methanol from the synthesis gas, (ii) reaction of the methanol by oxidative carbonylation or with urea to give dimethyl carbonate (DMC), (iv) preparation of urethane by reacting amine with the dimethyl carbonate, (v) splitting of the urethane to give the isocyanate and methanol, (vi) recycling of the methanol to the preparation of the dimethyl carbonate.

Description

 Process for the preparation of isocyanates

description

The invention preferably relates to integrated processes for the preparation of isocyanates by cleavage of urethane, the process comprising the following process steps:

(i) providing synthesis gas from natural gas, coal, heavy oil or biomass, preferably natural gas or coal, by gasification and production of methanol and optionally electrical energy from the synthesis gas,

(ii) reacting the methanol by oxidative carbonylation or with urea to form dimethyl carbonate (DMC), (iv) preparing urethane by reacting amine with the dimethyl carbonate,

(v) cleavage of urethane to isocyanate and methanol,

(vi) recycling the methanol to produce the dimethyl carbonate.

The synthesis of isocyanates, which are important basic products for the preparation of polyurethanes, is carried out industrially by the reaction of the corresponding amine with carbonyl dichloride (CDC). The yields are more than 90% based on the amine used. With regard to the chlorine cycle associated with this process as well as the desire for absolutely chlorine-free isocyanate, there have been efforts again and again to produce isocyanates without chlorine.

In addition to the direct carbonylation of the nitro compounds underlying the amines with carbon monoxide to the isocyanate, the reductive alkoxycarbonylation of nitro compounds in the presence of an alcohol to the corresponding urethane, which is then cleaved to the isocyanate, and the oxidative alkoxycarbonylation of the amine to urethane, which then to Isocyanate is cleaved, the reaction of the amine with dialkyl carbonates to the corresponding urethane, which is then cleaved to isocyanate, generally known.

WO 02/24634 describes the reaction starting from the formamide of the amine, which is reacted with diphenyl carbonate directly over the intermediately formed urethane to the isocyanate. Disadvantages are the moderate yield and the handling of toxic phenol melts. In JP 2004262835, the formamide is likewise reacted with diaryl carbonate in a one-pot reaction with the isocyanate. According to JP 10007641, the formamide is reacted with dimethyl carbonate (DMC) to urethane, which is then cleaved. Another dialkyl carbonate based process is described in WO 01/56977. This z. B.Toluylendiamin with DMC with the addition of salts such. B. zinc acetate is reacted as a catalyst for Toluylendiurethan (TDU). TDU is then split in a system of two pyrolysis reactors, the first reactor being a film evaporator.

All of the abovementioned processes have lower yields than the reaction of the amine with CDC, which means that these alternative processes can not be commercially operated on an industrial scale.

The object of the present invention was thus to develop a chlorine-free process for the preparation of isocyanates which is economically competitive and, moreover, is operated simply and robustly.

This problem could be solved by the method presented at the beginning. Since it is also possible to use another dialkyl carbonate instead of the dimethyl carbonate, the present invention also provides a process for the preparation of isocyanates by cleavage of urethane, the process comprising the following process steps:

(i) providing synthesis gas from natural gas, coal, heavy oil or biomass, preferably natural gas or coal, by gasification and production of methanol and optionally electrical energy from the synthesis gas, (ii) conversion of the methanol by oxidative carbonylation or with urea to dimethyl carbonate (DMC )

(iii) reacting the dimethyl carbonate with an alcohol having 2 to 6 carbon atoms to the corresponding dialkyl carbonate,

(iv) preparation of urethane by reaction of amine with the dialkyl carbonate,

(v) cleavage of the urethane to the isocyanate and alcohol,

(vi) recycling the methanol to produce the dimethyl carbonate,

(vii) recycling the alcohol having 2 to 6 carbon atoms to produce the dialkyl carbonate.

A higher dialkyl carbonate, for. As dibutyl carbonate, has the advantage that the corresponding formed in (iv) urethane has a lower melting point than the DMC derived methylurethane. The lower melting point simplifies the technical handling. Furthermore, in step (v), the recombination of the alcohol formed in the cleavage of the urethane ethane is inhibited when a higher alcohol is used. On the other hand, with higher alcohols there is the risk of fragmentation of the urethane under the high cleavage temperatures of the cleavage reactor in which the urethane is determined. intended to be cleaved in isocyanate and alcohol. Fragmentation produces amine, CO2 and olefin. In particular, the amine interferes, however, since it reacts with the isocyanate in rapid reaction to ureas and biurets. These reduce the yield and reduce the availability of the system due to the formation of solids. Therefore, the use of DMC is the preferred variant.

The inventive method is characterized not only by an optimization of the individual stages, but also represents an integrated method that only in its entirety allows economic operation.

The individual stages of the process and the starting materials are presented by way of example below.

(I) providing synthesis gas from natural gas, coal, heavy oil or biomass by coal gasification and production of methanol and optionally electrical energy from the synthesis gas

According to the invention, the synthesis gas used for the preparation of the methanol is produced on the basis of natural gas, preferably natural gas, which is referred to as "flared gas" or "stranded gas" z. B. is economically available in oil production areas, or on the basis of coal, the synthesis gas is preferably produced in world-scale coal gasification facilities. The use of heavy oils and biomass is possible. Preference is given to the provision of synthesis gas from natural gas. However, the provision of synthesis gas from coal is also preferred. Preference is also the provision of synthesis gas from heavy oils or biomass.

In the present specification, synthesis gas is understood as meaning a gas mixture containing carbon monoxide and hydrogen. The production of synthesis gas from natural gas or coal is well known. The technology used depends in particular on the production of coal on the type, composition and dosage form of the raw material. Summaries put next to ^Ullmann's Encyclopedia of Industrial Chemistry (1986 ^Ullmann's Encyclopedia of Industrial Chemistry 1986 volume A7 "Coal liquefaction" and ^Ullmann's Encyclopedia of Industrial Chemistry 1976 Volume 14 "coal, gas generation") z. Examples include the following: JP Lange "Methanol synthesis: a short review of technology improvements" Catalysis today 64 (2001), 3-8, JR Rostrup-Nielsen "New aspects of syngas production and use" Catalysis today 63 (2000), Catalysis today 18 (1993), T. Sundset et al., "Evaluation of natural gas-based synthesis gas production technologies" Catalysis today 21 (1994).

EP 0 533 231 B1 directly describes such a process for the production of synthesis gas for the synthesis of methanol. The aim is to produce a synthesis gas, which Has the appropriate stoichiometry and therefore can be fed to the methanol synthesis without further treatment.

Among the methods of synthesis gas production from natural gas stands out in particular a two-stage process for the production of synthesis gas from natural gas, which is often referred to as "combined" or "primary & secondary reforming". In the first stage, a steam reforming of methane is performed (so-called. Steam methane reforming SMR) and in the serially connected second stage, a catalytic partial oxidation (so-called Catalytic partial oxidation CPOX or Autothermal reforming ATR).

The SMR step is characterized by the fundamental endothermic reaction of syngas generation:

CH ₄ + H ₂ O -> CO + 3H ₂

It is a reaction conducted in a fixed catalyst bed at about 600 to 1000 ^° C. and about 15 to 50 bar. Preference is given to using Ni, Rh or Ru based catalysts.

In the ATR stage, the residual conversion of methane is preferably with pure or enriched oxygen in a similar pressure range as above. The main exothermic reactions are:

CH ₄ + 0.5 O ₂ -> CO + 2 H ₂ CH ₄ + 2 O ₂ -> CO ₂ + 2 H ₂ O

The temperatures are preferably from 2000 to 800 ^° C. The reaction is generally conducted adiabatically in a fixed catalyst bed reactor. Preference is given to using Ni, Rh and / or Ru-based catalysts.

The production of synthesis gas from coal or carbonaceous solids, oils or liquids can be carried out in fluidized beds, moving beds or dust gasifiers and exists worldwide in a variety of configurations. Since the hydrogen content of the feedstock is smaller than that of natural gas, the higher the CO content of the synthesis gas produced in this way, either hydrogen must be supplied externally or by the shift reaction

CO + H ₂ O -> CO ₂ + H ₂

be generated. The reaction (in the production of synthesis gas from coal) is preferably operated at pressures in the range of 1 to 80 bar. The outlet temperatures of the reaction gas are preferably 700 to 1500 ⁰ C. The gasification processes for coal can be transferred in principle to the gasification of biomass.

Dealing with solids and viscous liquids requires extensive logistics and is expensive. The gasification of coal is preferably part of a larger network that includes the production of electrical energy, wherein the synthesis gas is used as fuel in turbines for power generation (eg., So-called "Integrated Gasification Combined-cycle IGCC" method) at

Locations where there is no flared or stranded gas or where natural gas is expensive, the gasification of coal, heavy oil or biomass, the preferred option.

Therefore, the preparation of methanol from synthesis gas based on natural gas as described above and the further reaction of a part of the methanol in the direction of isocyanate, furthermore the production of synthesis gas from coal, heavy oil or biomass and the conversion of this synthesis gas as described above are part of this invention Methanol and further reaction of a portion of the methanol toward isocyanate.

Part of the present preferred integrated process, d. H. Method in which the individual stages are directly coupled to each other, d. H. be carried out in close proximity, the production, preferably large-scale production of methanol from the synthesis gas, which was preferably generated according to the above step. The production of methanol from synthesis gas is well known and widely described. However, the methanol is preferably prepared using the process described in EP 067 491 B1, that in Energia 166 (2002), 63-68 or one of the processes described in JP Lange, Chimie Nouvelle 13 (1995), 1433-6 comes. The capacities of these methanol plants amount to 2500 to 10000 tons per day and can thus make CO and methanol particularly advantageous due to the use of the so-called economies of scale. EP-A 1 348 685 discloses a particular embodiment of this process, which has low carbon dioxide emissions. Recently, there is also the possibility of converting natural gas directly into methanol in a single-step synthesis. Such a method is described in M. Golombok, W. Teunissen; Industrial & Engineering Chemistry Research 42 (2003), 5003-5006 and in M. Golombok, T. Nijbaker, M. Raimondi; Industrial & Engineering Chemistry Research 43 (2004), 6001-6005. The Liquid Phase Methanol LPMEOH ™ process links an IGCC process to methanol production.

The synthesis of methanol from synthesis gas is carried out in one or more stages, preferably in two stages on a fixed bed contact, wherein in several stages at each stage the same or specially adapted catalysts are used. The underlying exothermic equilibrium reactions are:

CO + 2H ₂ -> CH ₃ OH CO ₂ + 3 H ₂ -> CH ₃ OH + H ₂ O

As reactors, tube bundle reactors or fixed beds with internal heat exchangers are preferably used. Also several adiabatic fixed beds in series with intermediate heat exchangers or feeds of cold quenching gas between see the fixed beds are used. In multi-stage operation in tube bundle reactors, the main sales take place in the first stage. In the second and higher stages, the approximation to the thermodynamic equilibrium conditions takes place. For a copper-based catalyst in a two-stage process the first stage is preferably from 180 to 350 ⁰ C, and has operated in the reaction part via a tube bundle reactor, a first rising then falling profile. The second stage advantageously has a falling temperature profile with the lowest temperature at the reactor outlet. As an alternative to Cu-based catalysts, it is also possible to use Pd, rare earths, Zn, Al and / or Cr-based catalysts.

The synthesis gas and the methanol stage can be in a heat combination.

(ii) Reaction of the Methanol by Oxidative Carbonylation or with Urea to Dimethyl Carbonate (DMC)

For the production of dimethyl carbonate, a number of methods have been known and described in various ways. According to the present invention, for the present robust, economical, integrated and also simple process, dimethyl carbonate is preferably prepared by oxidative carbonylation of methanol over copper chloride catalysts, by oxidative carbonylation of methanol with nitrogen oxide as redoxagent to copper chloride catalysts or by reacting urea with methanol. The oxidative carbonylation of methanol over copper chloride catalysts with nitrogen oxide as redox agent (US Pat. No. 5,885,917) and the reaction of urea with methanol are preferred (US Pat. No. 5,902,894, WO 95/17369). However, it is also possible to prepare the dimethyl carbonate in step (ii) by reacting methanol with carbon monoxide and oxygen in the presence of copper-containing catalysts, as described, for example, in US Pat. B. in US 5,210,269 is described. The dimethyl carbonate in step (ii) is preferably prepared by reacting methanol with carbon monoxide and oxygen in the presence of catalysts containing NO and copper.

If the dimethyl carbonate is prepared by reacting urea with methanol, the process product in addition to dimethyl carbonate and methyl carbamate and Contain alcohol. This process product containing methyl carbamate can also be used according to the invention in step (iv) and, if appropriate, (iii).

WO 2003/106393 claims the simultaneous production of raw materials for methanol synthesis, ammonia synthesis, urea synthesis and CO. On this basis, methanol can be reacted very advantageously with urea either as described above to DMC or DMC / Methylcarbamatmischungen. Methods of this type are z. As described in WO 2005/028414, WO 2005/06611 1 and part of EP-A 0 566 925 and EP-A 0 643 042.

WO 2005/051939 describes the preparation of dimethyl carbonate as coproduct of 1,2-propanediol production. This way of producing DMC is, in principle conceivable, but not directly preferred, since a co-product is formed, so that the advantage depends on the use and marketing of the same

(iii) transesterification of the dimethyl carbonate with an alcohol having 2 to 6 carbon atoms to the corresponding dialkyl carbonate

Before the implementation of the amine to urethane, the dimethyl carbonate can be reacted with alcohol, preferably monoalcohol to the corresponding dialkyl carbonate. In this case, methanol is replaced by the corresponding alkanol in the dialkyl carbonate. Suitable alcohols are preferably alkanols having between 2 and 6 carbon atoms, preferably ethanol, propanol, isopropanol, n-butanol, more preferably n-butanol and / or ethanol. More specifically, in the step (iv), the dimethyl carbonate is reacted with n-butanol or ethanol to dibutyl carbonate or diethyl carbonate. This reaction can be carried out in conventional processes, as described for the transesterification of DMC production. Such are exemplified in US 4,181,676, US 4,307,032, US 4,661,609. Preferably, however, the transesterification takes place in a catalytic reactive distillation, wherein the low-boiling methanol goes off overhead and the high boilers dialkyl carbonate is withdrawn from the bottom. Such a reactive distillation can be carried out analogously to that, as it is exemplified in EP 126,288. Preferably, the methanol in the step (ii), d. H. the preparation of the dimethyl carbonate recycled.

(iv) Preparation of urethane by reaction of amine with the dialkyl carbonate

For the preparation of the urethane can be in the reaction with the dialkyl, z. As the dimethyl carbonate, generally for the production of isocyanates known amines, preferably use diamines. Preference is given to the amines which are or have already been used for the preparation of isocyanates. Preference is given in step (iv) as the amine 2,2'-, 2,4'- and / or 4,4'-diamino-diphenylmethane (MDA), 2,4- and / or 2,6-toluenediamine (TDA), 1-amino-3,3,5-trimethyl-5-amino-methylcyclohexane (isophorone diamine, IPDA) and / or hexamethylenediamine (HDA) ,

The reaction of amine with dimethyl carbonate is generally known and described in detail, for example, in WO 01/056977, page 4, line 11 to page 11, line 15. Advantageously, the reaction takes place in a continuous process. In a multi-stage stirred tank cascade with n boilers and n = 1 to 20, the amine is preferably reacted with the DMC in the presence of a catalyst at from 120 to 300 ^° C. The catalyst used may be those mentioned in EP 0 566 925 and EP 0 643 042 (page 7, column 40 and page 8, column 38), WO 01/056977, preferably in amounts of from 100 ppm to 5% by weight of the used amine.

The molar ratio of the DMC to the amine (DMC: amine) is preferably 5: 1 to 50: 1.

The cascade is preferably characterized in that each or every nte with n = 2 to 10 reactors has an attached column in which the alcohol can be discharged.

The design of the stirred tank is carried out according to the expert's known criteria, but preferably as

Boiler with external Umpumpkreis and heat exchanger, wherein the mixing energy is preferably introduced into the boiler via a jet or

Flow tube, which is preferably segmented by the installation of diaphragms, wherein the space between two diaphragms is considered technically as a stirred tank analog, wherein the mixing energy is introduced by one or more upstream pumps and wherein the flow tube is followed by a column in which Reaction product methanol can be separated, and to which the next section (flow tube + column) can connect in series.

The discharge from the stirred tank cascade is preferably flashed and the excess DMC and the alcohol are removed by means of suitable measures known to the person skilled in the art. The urethane is fed as a melt or in a mixture with a high-boiling solvent of the cleavage.

MDA urethanes can still be subjected to condensation with formaldehyde after this stage, as described for aniline in EP-A 1 270 544. The oligomers thus obtained are then fed to the cleavage. (v) Cleavage of urethane to isocyanate and alcohol

The cleavage of urethane as a product of step (v) is well known and diverse z. In EP 0 524 554, EP 0 566 925, EP 0 643 042, EP 1 497 259, WO 01/56977, WO 03/84921, DE 199 07 648 and DE 198 55 959. The cleavage is usually carried out in the liquid phase and / or in the gas phase optionally in the presence of solvent at temperatures of generally 200 to 400 ⁰ C and a pressure between 2 mbar and 2000 mbar.

Preferably, the cleavage of the urethane in step (v) is performed such that a

Fluidized bed process (DE 199 07 648) or a helical tube reactor (DE 198 55 959) are used. In this case, in the case of the oligomers of MDA preferred the helical tube in question. Compared with the thin film evaporator, these methods have the advantage of cheaper investment and simpler construction. Due to the high flow rates and short residence times in the helical tube, caking and fouling are prevented. In the case of the fluidized bed deposits on the fluidized material can be eliminated by discharging and burning. Falling film evaporators require the shutdown of the system and cleaning or parallel construction of the aggregates. However, it is also possible to carry out the cleavage in this or in the liquid phase with or without a solvent.

(vi) recycling the methanol to produce the dimethyl carbonate

That in stage (iii), d. H. the reaction of the dimethyl carbonate with an alcohol having 2 to 6 carbon atoms to the corresponding dialkyl carbonate, in the step (iv), d. H. in the preparation of the urethane by means of dimethyl carbonate, and optionally in the cleavage of the urethane in step (v) resulting methanol according to the invention in the stage (ii), d. H. the preparation of the dimethyl carbonate, recycled.

(vii) recycling the alcohol having 2 to 6 carbon atoms to produce the dialkyl carbonate

If the preparation involves step (vii), the alcohol having more than 2 carbon atoms, which is formed during the preparation and decomposition of the urethane, is fed into step (iii), i. H. used for the reaction with the dimethyl carbonate.

The invention will be explained in more detail by the following example.

example

1000 g / h of TDA 80/20 as a mixture of 2,4 and 2,6 isomers in the ratio 80:20 and 14,75 kg / h of DMC are pumped into the first boiler of one of six boilers existing stirred tank cascade promoted. Furthermore, 10 g / h of the catalyst Zn acetate are mixed in a portion of the DMC continuously added. Each of the six boilers has a volume of 50 l and has a pumped circulation with heat exchanger and an attached pressure column, in which methanol and DMC are separated. The methanol is removed from the system and a mixture of DMC, possibly with small amounts of methanol, runs back into the kettle. The first three boilers are operated in series at approx. 160 ^° C and approx. 7 bar each. The boilers are arranged so that the liquid phase flows gravimetrically from the first to the second into the third. The hold up of the liquid phase in each vessel is 30 to 34 liters. The drain from the third boiler is radiometrically controlled with a pump in the fourth boiler promoted. The fourth, fifth and sixth boilers are operated in series at approx. 180 ^° C and approx. 10 bar each. The boilers are arranged so that the liquid phase flows gravimetrically from the fourth to the fifth to the sixth. The hold up of the liquid phase in each vessel is 30 to 34 liters. The discharge from the sixth kettle is flashed into a column. The bottom of the column is maintained at about 175 ⁰ C and contains the TDU (91%) high-boiling components (> 8%) and the catalyst. The methanol / DMC mixture goes off overhead and can be worked up by distillation, but this did not take place in the present experimental case. It can also be traced back to the DMC system. The melt including the high-boiling components and the catalyst is conveyed with a small residence time of 20 min via a gear pump directly into a fluidized bed and injected. The fluidized bed contains fine steatite, which is held by a heat exchanger at a temperature of 350 ⁰ C. The steatite is held in suspension by nitrogen at 1 bar. The molar ratio of nitrogen to TDU flow through the reactor is about 4: 1. At a downstream condenser which is operated at 80 ⁰ C, 73% of the TDU used are ^'s taken off as TDI. On a second condenser, the total condensation takes place, wherein a mixture of alcohol, TDU and TDI is withdrawn semi-urethane. The latter arise from partial recombination of the TDI with the alcohol. By separation of the methanol and recycling of TDU and TDI semi-urethane, the yield of cleavage can be increased to 95%. The fluidized material is removed at regular intervals in small batches from the fluidized bed and burned in a second fluidized bed reactor with air supply.

The overall yield of the process is about 86% based on TDA and thus below that of phosgenation. However, this disadvantage can be compensated for by providing the CO building block via inexpensive DMC via the low-cost synthesis gas building block carbon monoxide by coupling with a world-scale synthesis gas plant.

Claims

claims A process for the preparation of isocyanates by cleavage of urethane, characterized in that the process comprises the following process steps: (i) providing synthesis gas from natural gas, coal, heavy oil or biomass by gasification and production of methanol from the synthesis gas, (ii) conversion of the methanol by oxidative carbonylation or with urea to dimethyl carbonate (DMC), (iv) preparation of urethane by reaction of amine with the dimethyl carbonate, (v) cleavage of the urethane to the isocyanate and methanol, (vi) recycling of the methanol to produce the dimethyl carbonate. 2. A process for the preparation of isocyanates by cleavage of urethane, characterized in that the process comprises the following process steps: (i) providing synthesis gas from natural gas, coal, heavy oil or biomass by gasification production of methanol from the synthesis gas, (ii) conversion of the methanol by oxidative carbonylation or with urea to dimethyl carbonate (DMC), (iii) reacting the dimethyl carbonate with an alcohol having 2 to 6 carbon atoms to give the corresponding dialkyl carbonate, (iv) preparing urethane by reacting amine with the dialkyl carbonate, (v) cleavage of the urethane to the isocyanate and alcohol, (vi) recycling of the methanol to produce the dimethyl carbonate, (vii) recycling of the alcohol having 2 to 6 carbon atoms to produce the dialkyl carbonate. 3. The method according to claim 1 or 2, characterized in that in step (i) produces the synthesis gas in a two-stage process, wherein in the first stage, a steam reforming of the methane is carried out and in the second stage, a catalytic catalytic partial oxidation. 4. The method according to claim 3, characterized in that one carries out the steam reforming of methane with a fixed catalyst bed with Ni, Rh and / or Ru based catalysts at temperatures between 600 and 1000 ⁰ C and a pressure between 15 and 50 bar. 5. The method according to claim 3, characterized in that one carries out the catalytic partial oxidation at temperatures between 2000 and 800 ⁰ C under adiabatic conditions in a fixed catalyst bed reactor, wherein one uses Ni, Rh and / or Ru based catalysts. 6. The method according to claim 1 or 2, characterized in that the production of methanol with synthesis gas in step (i) methanol synthesis of synthesis gas takes place in two stages on a fixed bed contact. 7. The method according to claim 1 or 2, characterized in that the production of methanol with synthesis gas in step (i) methanol synthesis from synthesis gas in a two-stage process with copper-based catalysts, wherein the first stage operated at a temperature between 180 to 350 ⁰ C. is and the first stage via a tube bundle reactor has a first rising then falling temperature profile and in the second stage a falling Temperature profile is present with the lowest temperature at the reactor outlet. 8. The method according to claim 1 or 2, characterized in that the preparation of the synthesis gas and the production of the methanol are carried out in a heat Verbund. 9. The method according to claim 1 or 2, characterized in that one prepares the dimethyl carbonate in step (ii) by reacting methanol with carbon monoxide and oxygen in the presence of NO and copper-containing catalysts. 10. The method according to claim 1 or 2, characterized in that reacting the dimethyl carbonate with n-butanol or ethanol to dibutyl carbonate or diethyl carbonate in step (iii). 1 1. The method of claim 1 or 2, characterized in that one carries out in step (iii) the transesterification of the dimethyl carbonate in a reactive catalytic distillation, wherein the low boilers go off methanol overhead and the high boilers dialkyl carbonate is withdrawn from the bottom. 12. The method according to claim 1 or 2, characterized in that in step (iv) as the amine 2,2'-, 2,4'- and / or 4, 4'-diamino-diphenylmethane (MDA), 2.4 - and / or 2,6-toluenediamine (TDA), 1-amino-3,3,5-trimethyl-5-amino-methyl-cyclohexane (isophorone diamine, IPDA) and / or hexamethylenediamine (HDA) is used. 13. The method according to claim 1 or 2, characterized in that one carries out the reaction of the amine with the dimethyl carbonate or dialkyl carbonate in step (iv) such that one carries out the reaction in a continuous process with a stirred tank cascade with 2 to 20 boilers in Presence of a catalyst at a temperature between 120 and 300 ⁰ C, wherein the stirred tank each having an external Umpumpkreis and heat exchanger. 14. The method according to claim 1 or 2, characterized in that the reaction of the amine with the dimethyl carbonate or the dialkyl carbonate in Step (iv) is carried out by carrying out the reaction in a continuous process with a flow tube segmented by the installation of orifices, wherein the mixing energy is introduced by one or more upstream pumps and wherein a column adjoins the flow tube, in which the reaction product methanol is separated. 15. The method according to claim 1 or 2, characterized in that one carries out the cleavage of the urethane in step (v) in the liquid phase and / or in the gas phase at temperatures between 200 and 400 ⁰ C and a pressure between 2 mbar and 2000 mbar. 16. The method according to claim 15, characterized in that one carries out the cleavage of the urethane in step (v) in a fluidized bed 17. The method according to claim 15, characterized in that the urethane based on the reaction of 2,2'-, 2,4'- and / or 4,4'-diamino-diphenylmethane (MDA) with dimethyl carbonate and the cleavage of Urethane in step (v) in a helical tube performs.