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The invention relates to a method for synthesis of aliphatic isocyanates from aromatic isocyanates in substantially 3 stages. In particular, the invention relates to a method for synthesis of bis{4-isocyanatocyclohexyl}methane (H12MDI) from bis{4-isocyanatophenyl}methane (MDI). More especially, the

The invention relates to a method for synthesis of aliphatic isocyanates from aromatic isocyanates in substantially 3 stages. In particular, the invention relates to a method for synthesis of bis{4-isocyanatocyclohexyl}methane (H12MDI) from bis{4-isocyanatophenyl}methane (MDI). More especially, the invention relates to a method for synthesis of H12MDI with a trans-trans isomer content of <30%, preferably of <20%, particularly preferably of 5 to 15%.

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1. A method for synthesizing an aliphatic isocyanate from an aromatic isocyanate which has one or more aromatic rings and one or more isocyanate groups bonded directly, indirectly or both directly and indirectly to one or more of the aromatic rings, the method comprising:i. urethanizing the aromatic

1. A method for synthesizing an aliphatic isocyanate from an aromatic isocyanate which has one or more aromatic rings and one or more isocyanate groups bonded directly, indirectly or both directly and indirectly to one or more of the aromatic rings, the method comprising:i. urethanizing the aromatic isocyanate to form an aromatic urethane, wherein the urethanizing is carried out continuously or batchwise, in the presence or absence of a solvent or solvent mixture, and in the presence or absence of a catalyst, at temperatures of 20° C. to 160° C., and under normal pressure; ii. hydrogenating the aromatic urethane with hydrogen in the presence of a supported catalyst to form a hydrogenated urethane, where the supported catalyst comprises as an active metal, applied on a catalyst support, ruthenium, alone or together, with at least one metal of Group I, VII or VIII of the Periodic Table, wherein the proportion of the active metal is 0.01 to 20 wt % relative to the weight of the supported catalyst, and wherein the catalyst support has a BET surface area from greater than 30 m2/g to less than 70 m2/g, and more than 50% of the pore volume of the catalyst support is comprised of macropores with a pore diameter of larger than 50 nm, and less than 50% of the pore volume of the catalyst support is comprised of mesopores with a pore diameter of 2 to 50 nm; and iii. dissociating the hydrogenated urethane to the aliphatic isocyanate. 2. The method according to claim 1, wherein the aromatic isocyanate is at least one selected from the group consisting of bis{4-isocyanatophenyl}methane, 2-isocyanatophenyl-4′-isocyanatophenylmethane, 2-isocyanatophenyl-2′-isocyanatophenylmethane (MDI), a polynuclear methylene-bridged isocyanatophenyl (PMDI), 4,4′-dimethyl-3,3′-diisocyanatodiphenylmethane, 2,4′-dimethyl-3,3′-diisocyanatodiphenylmethane, 2,2′-dimethyl-3,3′-diisocyanatodiphenylmethane, 1,2-diisocyanatobenzene, 1,3-diisocyanatobenzene, 1,4-disocyanatobenzene, 2,4-diisocyanatotoluene, 2,6-diisocyanatotoluene, 1,6-diisocyanatonaphthalene, xylylene diisocyanate (MXDI) and tetramethylxylylene diisocyanate (TMXDI).3. The method according to claim 1, wherein the urethanizing step is carried out in an alcohol.4. The method according to claim 1, wherein the urethanizing step is carried out in n-butanol.5. The method according to claim 1, wherein the active metal applied on the catalyst support has a depth of penetration into the catalyst support of from 20 to 500 μm.6. The method according to claim 1, wherein the active metal applied on the catalyst support has a depth of penetration into the catalyst support of from 25 to 250 μm.7. The method according to claim 1, wherein a ratio of the surface area of the active metal, determined by CO pulse chemisorption, to the surface area of the catalyst support, determined by the BET method, is greater than 0.01.8. The method according to claim 1, wherein a ratio of the surface area of the active metal, determined by CO pulse chemisorption, to the surface area of the catalyst support, determined by the BET method, is from 0.03 to 0.3.9. The method according to claim 1, wherein the catalyst support is at least one selected from the group consisting of a crystalline oxide, an amorphous oxide, a crystalline silicate and an amorphous silicate.10. The method according to claim 1, wherein the catalyst support is at least one selected from the group consisting of Al2O3, SiO2, TiO2, ZrO2, MgO, ZnO and an aluminosilicate.11. The method according to claim 1, wherein the catalyst support has a BET surface area of from 32 to 67 m2/g, the depth of penetration of the active metal ranges from 50 to 200 μm and the Ru content is from 0.2 to 3 wt % relative to the catalyst, and at least 55% of the pore volume of the catalyst support is comprised of macropores and less than 45% is comprised of mesopores.12. The method according to claim 1, wherein the hydrogenating step is carried out in a suspension or fixed-bed hydrogenation reactor, continuously or batchwise, at a temperature of from 20 to 250° C., and a hydrogen partial pressure of from 1 to 30 MPa.13. The method according to claim 1, wherein the hydrogenating step is carried out in a fixed-bed reactor.14. The method according to claim 1, wherein the hydrogenating step is carried out in a tube reactor by a trickling-bed procedure.15. The method according to claim 1, wherein the supported catalyst was produced by applying the active metal ruthenium onto the catalyst support by spraying the catalyst support with a ruthenium solution at a temperature of at least 80° C., with subsequent heat treatment and activation of the catalyst by reduction in a hydrogen-containing gas.16. The method according to claim 1, wherein the supported catalyst was produced by applying the active metal ruthenium onto the catalyst support by spraying the catalyst support with a ruthenium nitrosylnitrate solution at a temperature of at least 80° C., with subsequent heat treatment and activation of the catalyst by reduction in a hydrogen-containing gas.17. The method according to claim 1, wherein the aromatic urethane is at least one selected from the group consisting of a C1-C6 dialkyl 4,4′-methylenedicarbanilate, a C1-C6 dialkyl 2,4′-methylenedicarbanilate, a C1-C6 dialkyl 2,2′-methylenedicarbanilate, a C1-C6 polynuclear methylene-bridged alkyl carbanilate (PMDU), a C1-C6 dialkyl 4,4′-methylene-3,3′-dicarbanilate, a C1-C6 dialkyl 2,4′-methylene-3,3′-dicarbanilate, a C1-C6 dialkyl 2,2′-methylene-3,3′-dicarbanilate, a C1-C6 dialkyl 1,2-phenyldicarbamate, a C1-C6 dialkyl 1,3-phenyldicarbamate, a C1-C6 dialkyl 1,4-phenyldicarbamate, a C1-C6 dialkyl 2,4-toluenedicarbamate, a C1-C6 dialkyl 2,6-toluenedicarbamate, a C1-C6 dialkyl 1,6-naphthalenedicarbamate, a urethane corresponding to MXDI, and a urethane corresponding to TMXDI.18. The method according to claim 1, wherein the aromatic urethane is at least one selected from the group consisting of a dialkyl 4,4′-(C1 to C4)alkanedicarbanilate, a dialkyl 2,4′-(C1 to C4)alkanedicarbanilate, and a dialkyl 2,2′-(C1 to C4)alkanedicarbanilate.19. The method according to claim 12, wherein the hydrogenated aromatic urethane is a dibutyl 4,4′-methylenedicarbanilate.20. The method according to claim 1, wherein the hydrogenated aromatic urethane has a trans-trans isomer content of <30% and is synthesized from a bridged binuclear starting product.21. The method according to claim 1, wherein the hydrogenated aromatic urethane has a trans-trans isomer content of 5 to 15% and is synthesized from a bridged binuclear starting product.22. The method according to claim 1, wherein dibutyl 4,4′-methylenedicarbanilate is hydrogenated to form dibutyl 4,4′-methylenedicyclohexylcarbamate having a trans-trans isomer content of <30%.23. The method according to claim 1, wherein dibutyl 4,4′-methylenedicarbanilate is hydrogenated to form dibutyl 4,4′-methylenedicyclohexylcarbamate having a trans-trans isomer content of 5 to 15%.24. The method according to claim 1, wherein the hydrogenating is carried out in a solvent or solvent mixture.25. The method according to claim 1, wherein the hydrogenating is carried out in a solvent comprising at least one of an alcohol or an ether.26. The method according to claim 25, wherein the solvent comprises an alcohol, and the alcohol corresponds to the alcohol group of the urethane.27. The method according to claim 25, wherein the solvent comprises n-butanol.28. The method according to claim 25, wherein the solvent comprises tetrahydrofuran.29. The method according to claim 1, wherein the dissociating step takes place in the gas or liquid phase, with or without catalyst, in the presence or absence of solvents, continuously or batchwise.30. The method according to claim 1, wherein the dissociating step takes place in, combined cracking and rectification column.31. The method according to claim 1, wherein the dissociating step takes place in liquid phase without additional solvent.32. The method according to claim 1, wherein the dissociating step taken place in the presence of at least one catalyst.33. The method according to claim 32, wherein the dissociating step takes place in the presence of 1 to 2,000 ppm of the catalyst, relative to the volume of the mixture in a cracking reactor.34. The method according to claim 33, wherein the cracking reactor comprises a cracking column including a bottom, and dissociating further comprises drawing off one or more secondary products from the bottom of the cracking column.35. The method according to claim 33, wherein the cracking reactor comprises a combined cracking and rectification column, and dissociating further comprisespurifying a raw isocyanate drawn off from the combined cracking and rectification column by vacuum distillation, and wherein one or more first runnings and distillation residues may be recycled to the combined cracking and rectification column. 36. The method according to claim 1, wherein the dissociating step takes place at a temperature of 200 to 300° C.37. The method according to claim 1, further comprisingpre-purifying before dissociating, wherein the solvents are first separated by nitrogen stripping and then the concentrations of further secondary components are reduced by means of a two-stage combination of short-path evaporation and thin-film evaporation. 38. The method according to claim 1, wherein the method is carried out completely continuously, semi-continuously or in batches.39. The method according to claim 1, wherein the aromatic isocyanate is bis{4-isocyanatophenyl}methane (MDI), the aliphatic isocyanate is bis{4-isocyanatocyclohexyl}methane (H12MDI) with a trans-trans isomer content of <30%, and the method comprises:i. urethanizing the MDI to form dialkyl 4,4′-methylenedicarbanilate (MDU), ii. hydrogenating the MDU to form a hydrogenated MDU, and iii. dissociating the hydrogenated MDU to form H12MDI. 40. The method according to claim 1, wherein dissociating the hydrogenated urethane forms a cycloaliphatic isocyanate.41. The method according to claim 1, wherein the aromatic isocyanate is at least one selected from the group consisting of a compound of formula I, formula II, formula IIII, formula IV, formula V, formula VI, formula VII, and formula VIII: 42. The method according to claim 17, wherein the aromatic urethane is at least one selected from the group consisting of a compound of formula I, formula II, formula III, formula IV, formula V, formula VI, formula VII and formula VIII: wherein R is C1-C6 alkyl group.43. The method according to claim 1, wherein the urethanizing step is carried out at temperatures of 20° C. to 120° C.44. The method according to claim 1, wherein the catalyst support has a BET surface area from 32 m2/g to less than 70 m2/g.