METHOD FOR HYDRATION OF A MIXTURE IN THE PRESENCE OF A COLORLESS AMIN

Description

[0001] The present invention relates to a process for the hydrogenation of a mixture (G1) in the presence of a catalyst, wherein the mixture (G1) contains at least one colorless amine and at least one color-providing component. The hydrogenation step initially partially, preferably completely, hydrogenates the at least one color-providing component, while the colorless amine, in particular aniline, is not hydrogenated or is hydrogenated only to a very small extent. The color-providing components causing the discoloration of the amine, which is essentially colorless per se, are thus removed by hydrogenation, thereby purifying the colorless amine or the mixture (G1). Subsequently, the colorless amine is converted to a diphenylmethane derivative in at least one further step.

[0002] Processes for the purification of amines and for the removal of color-providing components, e.g., in aniline, are described in the prior art.

[0003] US Pat. No. 3,222,310 discloses a process for inhibiting color formation in amines. In this process, colorless aniline is treated with alkali metal borohydrides, resulting in a color-stable reaction even after contact with atmospheric oxygen. In this disclosure, the aniline used is already colorless; the removal of color-imparting components is not described, nor is the use of catalysts.

[0004] US 2,019,032 discloses a process for aromatic amines, exemplified by aniline, which exhibit only a barely perceptible color change upon contact with atmospheric oxygen by adding small amounts of maleic anhydride before a final distillation. Hydrogen is not mentioned.

[0005] US 2009 / 0240077 A1 describes a process for preparing di- and polyamines of the diphenylmethane series by reacting aniline and formaldehyde in the presence of an acidic catalyst, in which the aniline used contains in total less than 0.5 wt.%, based on the weight of the aniline used, of compounds which contain at least one carbonyl group or which are formed by reacting these compounds containing at least one carbonyl group with aniline.

[0006] WO 2010 / 057909 discloses a process for preparing an isocyanate, comprising hydrogenating a mixture (Gi) containing an amine in the presence of a copper-containing hydrogenation catalyst to obtain a mixture (Gii) containing the amine, and reacting the mixture (Gii) with phosgene to obtain a mixture (Gii) containing the isocyanate. The amine present in the mixture (Gi) is preferably a di- or polyamine of the diphenylmethane series, such as MDI, which is obtained, for example, by reacting aniline with formaldehyde. WO 2010 / 057909 further relates to the isocyanate preparable by this process. It is not disclosed that the mixture (Gi) itself may contain a coloring substance. The mixture (Gi) typically contains at least one precursor to a coloring substance, meaning that a coloring substance only results from it in the phosgenation stage.Examples include N-formylated diamines and / or N-formylated polyamines of the diphenylmethane series or compounds of the 3,4-dihydroquinazoline class. Also, no impurities from the production of the precursor of the mixture (Gi) are mentioned, i.e., impurities that may arise during the production of, for example, colorless monoamines such as aniline.

[0007] US Pat. No. 5,889,070 A discloses a process for producing a polyamine mixture obtained by reacting aniline with formaldehyde in the presence of an acid catalyst. The process comprises a step in which the polyamine mixture is reduced by catalytic or chemical hydrogenation in the presence of a metal catalyst. Prior to this step, the polyamine mixture is purified. It is not disclosed that hydrogenation is carried out prior to the conversion to a polyamine or that a conversion to a diphenylmethane derivative is carried out after hydrogenation.

[0008] The processes described in the prior art demonstrate that there is a need for colorless or nearly colorless amines, particularly to provide them as the purest possible starting material for further industrial use. The processes described in the prior art have the disadvantage, for example, that color formation in an amine such as aniline is merely inhibited, or that it is not a colorless amine such as aniline that is purified, but rather its derivatives in the subsequent course of syntheses in which an amine such as aniline is used. Accordingly, there is a need for a process for purifying amines that provides colorless or nearly colorless amines.

[0009] The object of the present invention is therefore to provide a process for the purification of amines, whereby an amine-containing mixture which is colorless or almost colorless can be obtained.

[0010] The object is achieved by a process for the hydrogenation of a mixture (G1) which contains at least one colorless amine and at least one color-providing component, wherein the at least one color-providing component is selected from cycloaliphatic carbonyl compounds, cycloaliphatic amines, cycloaliphatic imines and / or C6-ring aromatics, characterized in that the mixture (G1) is hydrogenated in the presence of a catalyst to obtain a mixture (G2) in which the at least one color-providing component is at least partially or completely hydrogenated, that the colorless amine contained in the mixture (G2) is converted in at least one further step to a diphenylmethane derivative and the diphenylmethane derivative is optionally converted in at least one further step to an aromatic isocyanate,Preferably, the colorless amine is first converted into a diphenylmethane derivative and this is then converted into an aromatic isocyanate by phosgenation.

[0011] A major advantage of the process according to the invention is that it enables purification of the mixture (G1). This is particularly important for further processing of the mixture (G2) obtained by hydrogenation, since the purer the components of the mixture (G2), the purer the subsequent products obtained by reacting the mixture (G2). This also contributes significantly to reducing the costs of longer syntheses. Therefore, it is particularly valuable to purify reactants that are used at the very beginning of longer syntheses (such as aniline).

[0012] A further advantage of the process according to the invention is that time-consuming and costly processing after hydrogenation can be avoided.

[0013] An additional advantage of the process according to the invention is that the proportion of amine in the mixture does not decrease or decreases only slightly during purification.

[0014] Thus, according to the invention, it is particularly advantageous if catalysts containing copper and / or palladium are used in the hydrogenation of the mixture (G1), which improves the color values ​​of the amine and thus provides a colorless or virtually colorless mixture (G2) containing the corresponding amine. Furthermore, it is advantageous that the proportion of the colorless amine in the mixture does not decrease, or decreases only slightly, during the hydrogenation.

[0015] A further advantage of the process according to the invention is that, if the purified amine is, for example, aniline, it can be used, for example, for the synthesis of the compounds methylenedianiline (MDA) and methylenediphenyl diisocyanate (MDI), which then also have an advantageous color, which is particularly important for further industrial use.

[0016] A further advantage of the process according to the invention is that by purifying a component such as an amine, which is so fundamental for many processes, further purification steps in subsequent processes can be eliminated and these processes thus become more economical.

[0017] In addition, it is also advantageous that the mixture (G2) obtained by hydrogenation has a high long-term color stability.

[0018] In the context of the present invention, the term "purification" means a process in which the degree of purity of the amine present in mixture (G1) is improved, in particular by hydrogenation of color-providing components present in the mixture.

[0019] The terms "colorless" and "colorless amine" as used in this application refer to the color number L*, measured according to DIN 5033 (2009), of at least one compound, which is at least 90, preferably at least 95, particularly preferably at least 98.

[0020] The terms "color-providing" and "color-providing component" as used in this application refer to the color number L*, measured according to DIN 5033 (2009), of at least one compound which is less than 90, preferably at most 80, more preferably at most 75.

[0021] The process according to the invention for the hydrogenation of amines is defined in more detail below.

[0022] A mixture (G1) containing at least one colorless amine and at least one color-providing component is hydrogenated in the presence of a catalyst.

[0023] The mixture (G1) contains at least one colorless amine. Colorless amines as such are known to the person skilled in the art.

[0024] The at least one colorless amine can be a primary amine, an aromatic amine, and / or a monoamine, preferably a primary aromatic amine, a primary monoamine, and / or an aromatic monoamine, particularly preferably a primary aromatic monoamine, most preferably aniline. In particular, the colorless amine is aniline.

[0025] The at least one colorless amine contained in the mixture (G1) is preferably present in an amount of at least 80% by weight, particularly preferably at least 90% by weight, very particularly preferably at least 95% by weight, based on the total weight of the mixture (G1).

[0026] The term "total weight of the mixture (G1)" as used in the present application refers to the sum of the weights of the at least one colorless amine and the at least one color-imparting component in the mixture (G1). Any additional constituents (components) such as solvents are not included here, but are additionally stated in relation to the total weight of the mixture (G1).

[0027] Color-providing components in connection with the present invention are known to the person skilled in the art from the prior art. These occur, for example, in the mixture (G1) due to the process or are formed, for example, by contact of the mixture (G1) or the components contained therein with oxidizing media, for example, atmospheric oxygen.

[0028] The at least one color-providing component is selected from cycloaliphatic carbonyl compounds, cycloaliphatic amines, cycloaliphatic imines and / or C6-ring aromatics.

[0029] For the purposes of the present invention, cycloaliphatic carbonyl compounds, cycloaliphatic amines, and cycloaliphatic imines are understood to mean both the corresponding saturated and unsaturated compounds. These compounds may optionally be substituted, for example, with one or more alkyl or hydroxy radicals.

[0030] In the context of this application, the term "C6-ring aromatic" refers to both unsubstituted and substituted benzene.

[0031] If it is a substituted benzene, it can be substituted at one or more positions. The substituents are, independently of one another, preferably alkyl, hydroxy, halogen, amino, and / or nitro residues.

[0032] The at least one color-providing component in the mixture (G1) is preferably selected from cyclohexylphenylamine derivatives, cyclohexanone derivatives, cyclohexenone derivatives, cyclohexylamine derivatives, N-methylcyclohexylamine derivatives, toluidine derivatives, nitrobenzene derivatives, 2-aminophenol derivatives, 1,2-phenylenediamine derivatives, diphenylamine derivatives, oxidized N-methylaniline derivatives or non-oxidized N-methylaniline derivatives, particularly preferably from cyclohexylphenylamine derivatives, cyclohexylamine derivatives, N-methylcyclohexylamine derivatives, oxidized N-methylaniline derivatives or non-oxidized N-methylaniline derivatives.

[0033] The term "derivative" encompasses both the respective parent compounds and the compounds derived from them, in which, for example, one or more hydrogen atoms are replaced by other functional groups. For example, if the compound is a cyclohexanone derivative, the term "derivative" encompasses both the pure cyclohexanone (parent compound) and the compounds derived from it, such as 3-methylcyclohexanone.

[0034] The at least one coloring component contained in the mixture (G1) is preferably present at a maximum of 5% by weight, particularly preferably at a maximum of 2% by weight, very particularly preferably at a maximum of 1% by weight, based on the total weight of the mixture (G1).

[0035] According to one embodiment of the present invention, the mixture (G1) is recycled aniline, wherein the recycled aniline contains at least one color-providing component selected from cycloaliphatic carbonyl compounds, cycloaliphatic amines, cycloaliphatic imines and / or C6-ring aromatics.

[0036] Furthermore, the mixture (G1) may contain, in addition to the at least one colorless amine and the at least one color-providing component, further constituents such as solvents. If such constituents (components) are present in the mixture (G1), the corresponding amounts of these constituents are additionally stated in relation to the total weight of the mixture (G1). Preferably, the mixture (G1) contains no further constituents or contains them in an amount of not more than 10%, based on the total weight of the mixture (G1), i.e., the sum of colorless amines and color-providing components.

[0037] The mixture (G1) is hydrogenated in the presence of a catalyst to obtain a mixture (G2) in which the at least one color-providing component is at least partially or completely hydrogenated.

[0038] The expression "at least partially hydrogenated" as used in this application means that the at least one color-providing component has been hydrogenated to a percentage of at least 50 wt.%, preferably at least 60 wt.%, more preferably at least 70 wt.%, based on the total weight of the at least one color-providing component in mixture (G1).

[0039] After hydrogenation, the mixture (G2) contains a large part of the colorless amine (from the mixture (G1)), for example at least 90% by weight, preferably at least 94% by weight, more preferably at least 96% by weight, particularly preferably at least 99% by weight of the at least one colorless amine, based on the total amount of the colorless amine contained in the mixture (G1).

[0040] The color of a mixture can be determined using various methods, for example, using the CIE color scale. This is defined by the values ​​L*, a*, and b*. L* is a measure of the brightness of the sample (0–100, 100 being the brightest, i.e., colorless), a* denotes the green (negative values) or red (positive values) component, and b* denotes the blue (negative values) or yellow (positive values) component. For a* and b*, a value of 0 denotes a colorless sample.

[0041] The term "brightness" is equivalent to the term "colorlessness" in the context of the present invention.

[0042] In the process according to the invention, the values ​​of the color numbers L* and b* are particularly important.

[0043] The mixtures (G1) according to the invention preferably have a color number L*, measured according to DIN 5033 (2009), of at least 50.

[0044] The at least one colorless amine contained in mixture (G1) preferably has a color number L*, measured according to DIN 5033 (2009), of at least 90, preferably at least 95, particularly preferably at least 98.

[0045] The mixtures (G2) according to the invention preferably have a color number L*, measured according to DIN 5033 (2009), of at least 90, particularly preferably of at least 95, very particularly preferably of at least 98.

[0046] The mixtures (G2) according to the invention preferably have a color number b *< , measured according to DIN 5033 (2009), of at most 20, particularly preferably of at most 15, very particularly preferably of at most 10.

[0047] In the process of the present invention, the mixture (G1) is hydrogenated in the presence of a catalyst.

[0048] In principle, all suitable hydrogenation catalysts known to those skilled in the art are suitable catalysts. In the context of the present invention, preference is given to using catalysts that do not hydrogenate primary amines or that hydrogenate them only slightly.

[0049] In the process according to the invention, catalysts containing copper, palladium, cobalt, rhenium, and / or manganese, particularly preferably copper and / or palladium, are preferably used. The aforementioned elements, such as copper, thus form the catalytically active component of the corresponding catalyst, which may also contain other components such as support materials.

[0050] The catalyst used in the process according to the invention may contain copper in an amount in the range of 0.1 to 100 wt.%, preferably in the range of 20 to 80 wt.%, based on the total weight of the catalyst and calculated as metal, and / or palladium in an amount in the range of 0.1 to 100 wt.%, preferably in the range of 0.1 to 30 wt.%, based on the total weight of the catalyst and calculated as metal.

[0051] The term "calculated as metal" means that the information refers to the elemental metal.

[0052] If the catalyst contains copper, the copper in the catalyst may be present as a metal or as a copper compound or as a mixture of at least two copper compounds, or as a mixture of metal and a copper compound or as a mixture of metal and at least two copper compounds.

[0053] Preferred copper compounds are copper chromate, copper chromite, copper oxide, copper nitrate, copper sulfate, copper chloride, copper bromide, copper iodide, copper carbonate, copper acetyl acetate, copper alkoxide, copper aryloxide or copper carboxylate, particularly preferably copper chromite or copper oxide.

[0054] If the catalyst contains palladium, the palladium may be present in the catalyst as a metal or as a palladium compound or as a mixture of at least two palladium compounds, or as a mixture of metal and a palladium compound or as a mixture of metal and at least two palladium compounds.

[0055] The catalysts used in the process according to the invention can be either supported or unsupported. The support material is a support material that is inert or substantially inert to the reaction conditions and is preferably selected from the group consisting of carbon, silicon oxide, aluminum oxide, manganese oxide, cerium oxide, zirconium oxide, lanthanum oxide, titanium oxide, and mixtures of two or more of these materials. The catalyst is preferably supported or in the form of a solid metal catalyst. The catalyst can be used either as a powder or as a shaped body, preferably as a shaped body.

[0056] Hydrogen gas (H 2 ), particularly preferably technically pure hydrogen, is preferably used as the hydrogenating agent in the process according to the invention.

[0057] "Technically pure" means a hydrogen content of preferably at least 99% by weight, particularly preferably at least 99.5% by weight, based on the total weight of the hydrogen.

[0058] Furthermore, the hydrogenating agent can also be a mixture of hydrogen and an inert gas, where the inert gas can be helium, argon, carbon dioxide, nitrogen, or neon. Formic acid or other comparable hydrogen-releasing compounds can also be used.

[0059] The hydrogenation process can be carried out continuously or discontinuously (batch process); preferably, the process according to the invention is carried out continuously.

[0060] When carried out batchwise, the hydrogenation can be carried out, for example, in a stirred tank or stirred autoclave, a loop reactor, a jet loop reactor, a bubble column, or a fixed-bed reactor with a pumped circulation system. Preferably, the hydrogenation is carried out in a stirred tank or stirred autoclave. When carried out batchwise, the mixture (G1) and the catalyst are preferably introduced completely into the reactor. The catalyst can be introduced as a fixed bed or in another form. In the process according to the invention, the preferred embodiment is a suspension of catalyst in the mixture (G1).

[0061] In the continuous process, the hydrogenation is carried out, for example, in a continuously operated stirred tank reactor, a continuously operated loop reactor, a continuously operated jet loop reactor, a continuously operated bubble column, a continuously operated fixed-bed reactor with pumped circulation, or a stirred tank cascade. In the continuous process, the mixture (G1) is continuously fed in, and the resulting product mixture (G2), which contains the purified amine, is removed. In the continuous process, the catalyst is preferably present as a fixed bed in the reactor and is only renewed and / or regenerated as needed.

[0062] The hydrogenation of the mixture (G1) can be carried out at temperatures known from the prior art. Preferably, the hydrogenation of the mixture (G1) is carried out at a temperature between 90°C and 300°C, more preferably between 90°C and 200°C, most preferably between 90°C and 150°C.

[0063] The hydrogenation time is preferably between 1 and 100 hours, particularly preferably between 5 and 80 hours, most particularly preferably between 10 and 50 hours.

[0064] The hydrogenation is preferably carried out until the mixture (G2) with the desired quality is achieved.

[0065] The term "desired quality" in this context means that the mixture (G2) after the hydrogenation step has a better color or a higher L* value and / or a b* value as low as possible (0 is the optimum) than the mixture (G1).

[0066] The resulting product mixture (G2) can be processed in a conventional manner known to the person skilled in the art.

[0067] For example, the resulting product mixture (G2) can be further worked up by a solid-liquid separation or by distillation, preferably by distillation, in order to isolate the at least one colorless amine, to separate the catalyst and / or to remove the hydrogenation products from the mixture (G2).

[0068] In the case of a discontinuous process, the separation of the catalyst from the product mixture (G2) can be carried out, for example, by a solid-liquid separation such as filtration, sedimentation or centrifugation.

[0069] The distillation of the mixture (G2) is preferably carried out using evaporators and / or columns. Columns known to the person skilled in the art are suitable for this purpose. Preference is given to packed columns, tray columns with sieve trays, columns with dual-flow trays, columns with bubble-cap trays, or rectification columns equipped with valve trays, dividing-wall columns, or thin-film and falling-film evaporators. The column used preferably has a theoretical number of trays (N th ) of 1 to 100, particularly preferably 1 to 50. The distillation is preferably carried out in the absence of oxygen.

[0070] The term "absence of oxygen" means that the oxygen volume fraction is less than 1%, preferably less than 0.01%, particularly preferably less than 0.01% and most preferably less than 0.001%, based on the total volume of the distillation column.

[0071] The distillation is preferably carried out at a head temperature and a head pressure of 1 to 100 mbar. The process can be carried out batchwise or continuously.

[0072] In the discontinuous process, the mixture (G2) is placed in a reactor, preferably under exclusion of air, and heated. Once steady-state is reached, fractions can be removed.

[0073] In continuous distillation, the column is adjusted to achieve a reflux ratio of 1 to infinity, preferably 1 to 10.

[0074] The reflux ratio is defined as the mass ratio of condensate returned to the column and withdrawn as distillate at the top of a distillation column.

[0075] The continuous process is carried out using a column, which is preferably equipped with an evaporator and a top condenser.

[0076] In at least one further process step, the colorless amine present in mixture (G2) is converted to a diphenylmethane derivative, and the diphenylmethane derivative can optionally be converted to an aromatic isocyanate in at least one further step. Preferably, the colorless amine is first converted to a diphenylmethane derivative, which is then converted to an aromatic isocyanate by phosgenation.

[0077] For example, if the colorless amine is aniline, the aniline is converted to methylenedianiline (MDA) in at least one further step.

[0078] The methylenedianiline (MDA) can then be further converted to methylene diphenyl diisocyanate (MDI).

[0079] The synthesis of MDI typically occurs in a two-step process, in which aniline is first reacted with formaldehyde to form MDA, followed by a second reaction with phosgene. The phosgenation of MDA is known to those skilled in the art and is described, for example, in H. Ullrich, "Chemistry and Technology of Isocyanates," John Wiley, 1996, or WO 2010 / 057909.

[0080] According to one embodiment of the invention, the mixture (G2) contains at least 90% by weight, preferably at least 94% by weight, more preferably at least 96% by weight, particularly preferably at least 99% by weight, of aniline, wherein the aniline is converted in at least one further step to methylenedianiline (MDA), which is preferably subsequently converted to methylenediphenyl diisocyanate (MDI).

[0081] In a further preferred embodiment of the present invention, the mixture (G1) is hydrogenated in the presence of a copper-containing and / or palladium-containing catalyst. The mixture (G1) preferably contains at least 90 wt. %, more preferably at least 94 wt. %, even more preferably at least 96 wt. %, particularly preferably at least 99 wt. %, of aniline (based on the total weight of the mixture (G1)).

[0082] After hydrogenation, the mixture (G2) preferably contains at least 90 wt.% aniline. It is preferred that the aniline present in the mixture (G2) be <2 wt.%, even more preferably <1 wt.%, and particularly preferably <0.5 wt.% less than the mixture (G1) before hydrogenation. Furthermore, in this embodiment, it is preferred that the aniline present in the mixture (G2) is optionally purified and first converted to MDA using formaldehyde, after which MDA is subsequently converted to MDI. Examples

[0083] The following examples illustrate how the process according to the invention can be carried out. However, the process according to the invention is not limited to the reactions given in the examples. Examples 1-7: Variation of the catalyst General experimental procedure

[0084] Aniline (150 g) is charged into a 300 mL autoclave. A catalyst (1.5 g) is added. The autoclave is purged several times with nitrogen, which is then replaced with pure hydrogen. After heating to 100 °C, the hydrogen pressure is adjusted to 40 bar, and the mixture is stirred at this pressure and temperature for 12 hours. The autoclave is then cooled and depressurized, and the aniline is removed, from which the color number is determined. Table 1 Example catalyst Color number of aniline before hydrogenation Aniline content before hydrogenation Color number after hydrogenation Aniline content after hydrogenation L* : a* : b* [GC area%] L* : a* : b* [GC area%] 1 60% CuO, 40% Cr2O3 67,3 : 23,3: 66,2 96,6 90,3 : 1,9 : 11,5 96,1 2 65% CuO, 5% La, 30% Al 2 O 3 67,3 : 23,3: 66,2 96,6 91,9 : -1,5 : 8,2 96,5 3 50% CuO 50% Al 2 O 3 67,3 : 23,3: 66,2 96,6 96,6 : -0,5 : 7,8 96,6 4 5% Pd 95% C 67,3 : 23,3: 66,2 96,6 98,5 : -0,8 : 5,2 94,2 5 65% Co, 25% Cu, 10% Mn 67,3 : 23,3: 66,2 96,6 98,2: -3,1 : 12,4 96,0 6 Raney-Ni 67,3 : 23,3: 66,2 97 80:10:30 51 7 5% Ru 95% ZrO 2 67,3 : 23,3: 66,2 97 82:-0.5:15 84

[0085] Table 1 shows that the use of Cu-containing catalysts is particularly advantageous, as the color number can be significantly improved while the aniline content remains virtually unchanged. The use of Ni- or Ru-containing catalysts, on the other hand, leads to a loss of aniline but improves the color number. In both Table 1 above and the following text, the total catalyst weight always equals 100% (calculated as wt.%). If the respective values ​​are less than 100%, the missing percentages refer to the support material. Example 8: Variation of reaction time General experimental procedure

[0086] Aniline (150 g) is charged into a 300 mL autoclave. A 50% CuO 50% Al 2 O 3 catalyst (1.5 g) is added. The autoclave is purged several times with nitrogen, and the nitrogen is then replaced with pure hydrogen. After heating to 100 °C, the hydrogen pressure is adjusted to 40 bar, and the mixture is stirred for the specified time at the specified pressure and temperature. The autoclave is then cooled and depressurized, and the aniline is removed, from which the color number is determined. Table 2 Example Reaction time Color number of recycled aniline before hydrogenation Aniline content before hydrogenation Color number after hydrogenation Aniline content after hydrogenation [h] L* : a* : b* [GC area%] L* : a* : b* [GC area%] 2 12 h 67,3 : 23,3: 66,2 96,6 91,9 : -1,5 : 8,2 96,5 8 48 h 67,3 : 23,3: 66,2 96,6 96,8 : 0,0 : 8,1 96,4

[0087] The experiments in Table 2 show that the reaction time has a significant influence on the resulting color number L*, increasing with increasing reaction time while the aniline content remains constant. However, the aniline content decreases with increasing reaction time. Examples 9-12: Variation of the reaction temperature General experimental procedure

[0088] Aniline (150 g) is charged into a 300 mL autoclave. A 50% CuO 50% Al 2 O 3 catalyst (1.5 g) is added. The autoclave is purged several times with nitrogen, and the nitrogen is then replaced with pure hydrogen. After heating to the specified temperature, the hydrogen pressure is adjusted to 40 bar, and the mixture is stirred for 12 h at the specified pressure and temperature. The autoclave is then cooled and depressurized, and the aniline is removed, from which the color number is determined. Table 3 Example Reaction temperature Color number of aniline before hydrogenation Aniline content before hydrogenation Color number after hydrogenation Aniline content after hydrogenation [°C] L* : a* : b* [GC area%] L* : a* : b* [GC area%] 2 100 67,3 : 23,3: 66,2 96,6 91,9 : -1,5 : 8,2 96,5 9 130 67,3 : 23,3: 66,2 96,6 98,2 : 0,1 : 4,9 96,6 10 150 67,3 : 23,3: 66,2 96,6 98,4 : -0,3 : 4,9 97,1 11 180 67,3 : 23,3: 66,2 96,6 99,5 : -0,5 : 3,0 96,7 12 200 67,3 : 23,3: 66,2 96,6 99,2 : -0,4 : 2,5 96,7

[0089] The results in Table 3 show that the color number can be improved by increasing the reaction temperature. At 150 °C (Example 10), the aniline content is also highest. Example 13: Distillation to improve the color number

[0090] The color number of the hydrogenated samples can be further increased by distillation. For this purpose, aniline from Example 10 is distilled using a short-path distillation at 35 mbar and a bottom temperature of 72 °C. The main run has a color number of L* = 100.0, a* = -0.1, b* = 0.3. Example 14: Continuous hydrogenation

[0091] 34 g of catalyst (65% CuO, 5% La, 30% Al 2 O 3 ) are charged into a tubular reactor and inertized with nitrogen. The catalyst is then activated by passing hydrogen over it. The aniline is passed through the reactor at varying catalyst loading, varying temperatures, and 20 bar hydrogen pressure. The color number and composition of the effluent are determined at regular intervals. Table 4 summarizes some of the results of this experiment. Table 4 Duration Temperature [°C] Catalyst load [kg feed / L cat*h] Proportion of aniline in the mixture [GC area%] L* a* b* Inflow --- --- 95.4 92.9 -5,5 40.7 238 100 0.30 95.5 94.1 -0.1 15.5 309 100 1.00 95.3 95.3 0.1 10.8 380 150 0.30 95.4 98.1 -0.7 5.6 479 125 0.30 95.4 97.1 -0.4 7.9 551 125 1.00 95.4 95.2 -0.2 11.5 575 150 1.00 95.4 96.9 -0.7 9.0 671 170 0.65 95.1 95.3 -1.1 10.8 695 185 0.65 94.5 97.3 -1.1 8.7 719 200 0.65 94.1 97.9 -0.9 6.8 815 200 1.00 94.9 97.1 -0.5 8.5 860 150 0.65 95.3 97.0 0.2 6.8 884 150 0.65 95.3 96.8 0.1 7.7 912 150 0.65 95.4 97.4 0.2 6.0 1028 150 0.65 95.3 96.9 0.1 7.6 1148 150 0.65 95.4 95.9 0.0 9.7 1202 150 0.65 95.4 95.7 -0.1 11.5

[0092] The tests according to Table 4 show that the catalyst used is stable in the long term. Example 15: Continuous hydrogenation

[0093] 21 g of catalyst (50% CuO 50% Al 2 O 3 ) are charged into a tubular reactor and inertized with nitrogen. The catalyst is then activated by passing hydrogen over it. The recycled aniline is passed through the reactor at varying catalyst loadings, varying temperatures, and a hydrogen pressure of 20 bar. The color number and composition of the effluent are determined at regular intervals. Table 5 summarizes some of the results of this experiment. Table 5 Duration Temperature [°C] Catalyst load [kg feed / L cat*h] Proportion of aniline in the mixture [GC area%] L* a* b* Inflow --- --- 95.5 86.6 -1.2 59.6 113 150 0.65 95.2 88.6 1.6 29.0 139 170 0.65 95.1 89.9 0.6 25.8 161 190 0.65 94.7 91.6 -0.1 22.8 281 190 0.65 95.0 84.4 2.5 25.0 305 200 0.65 94.8 93.5 -1.3 19.7 401 210 0.65 94.8 94.9 -0.3 14.3 427 220 0.65 94.9 94.30 -0.4 16.6

[0094] At the same catalyst load, the L* value increases with increasing temperature, but the proportion of aniline decreases. Example 16: Synthesis of MDI with color-improved aniline

[0095] Aniline is converted into MDI according to the following procedure.

[0096] The synthesis of MDA is carried out in a 2 L jacketed reactor equipped with a propeller stirrer. The aniline from Example 13 is treated with aqueous hydrochloric acid (32 wt%) and heated to 90 °C. A 36.5 wt% aqueous formaldehyde solution is then added, maintaining the reaction temperature at 90 °C. After complete addition of the formaldehyde, the reaction mixture is heated to 120 °C for 2 h, then neutralized with 50% aqueous NaOH solution and then washed twice with water.

[0097] The resulting MDA is added to 1300 mL of monochlorobenzene and added dropwise to a solution of monochlorobenzene and phosgene (100 g) at 50 °C over 60 minutes. The reaction mixture is then heated to 100 °C for 30 minutes until it becomes clear. Excess phosgene is removed by applying a 20 mbar vacuum at 75 °C. The resulting MDI is dechlorinated first for 60 minutes at 100 °C and 50 mbar, then for 60 minutes at 180 °C and 20 mbar. After cooling to room temperature, the color, NCO number, and chlorine content are determined. Table 6 Example aniline Color number of MDI L* : a* : b* NCO number of the MDI 11 aniline 84,3 : -3,2 : 54,9 32,4 13 Aniline from Example 11 95,0 : -7,2 : 33,2 32,7

[0098] The examples in Table 6 show that the color of MDI can be significantly improved by using hydrogenated aniline. The NCO number, an important quality criterion for MDI, remains virtually unchanged.

Claims

1. A process for hydrogenating a mixture (G1) comprising at least one colorless amine and at least one color-imparting component, where the at least one color-imparting component is selected from among cycloaliphatic carbonyl compounds, cycloaliphatic amines, cycloaliphatic imines and / or C6 ring aromatics, wherein the mixture (G1) is hydrogenated in the presence of a catalyst to give a mixture (G2) in which the at least one color-imparting component is at least partially or completely hydrogenated, the colorless amine present in the mixture (G2) is converted in at least one further step into a diphenylmethane derivative, and optionally, in at least one further step, the diphenylmethane derivative is converted into an aromatic isocyanate, with the colorless amine preferably being firstly converted into a diphenylmethane derivative and this subsequently being converted by phosgenation into an aromatic isocyanate.

2. The process according to claim 1, wherein the at least one colorless amine is a primary amine, an aromatic amine and / or a monoamine, preferably a primary aromatic amine, a primary monoamine and / or an aromatic monoamine, particularly preferably a primary aromatic monoamine, very particularly preferably aniline.

3. The process according to claim 1 or 2, wherein the mixture (G2) has a color number L* of at least 90, preferably at least 95, particularly preferably at least 98, and / or the mixture (G2) has a color number b* of not more than 20, preferably not more than 15, particularly preferably not more than 10.

4. The process according to any of claims 1 to 3, wherein at least 90% by weight, preferably at least 94% by weight, more preferably at least 96% by weight, particularly preferably at least 99% by weight, of the at least one colorless amine, based on the total amount of the colorless amine present in the mixture (G1), is present in the mixture (G2) after the hydrogenation.

5. The process according to any of claims 1 to 4, wherein at least one colorless amine is present in the mixture (G1) in an amount of at least 80% by weight, preferably at least 90% by weight, particularly preferably at least 95% by weight, based on the total weight of the mixture (G1).

6. The process according to any of claims 1 to 5, wherein the at least one color-imparting component is present in the mixture (G1) in an amount of not more than 5% by weight, preferably not more than 2% by weight, particularly preferably not more than 1% by weight, based on the total weight of the mixture (G1).

7. The process according to any of claims 1 to 6, wherein the catalyst comprises copper, palladium, cobalt, rhenium and / or manganese, preferably copper and / or palladium.

8. The process according to claim 7, wherein the catalyst comprises i) copper in an amount in the range from 0.1 to 100% by weight, preferably in the range from 20 to 80% by weight, based on the total weight of the catalyst and calculated as metal, and / or ii) palladium in an amount in the range from 0.1 to 100% by weight, preferably in the range from 0.1 to 30% by weight, based on the total weight of the catalyst and calculated as metal.

9. The process according to claim 7 or 8, wherein i) if the catalyst comprises copper, the copper is present as metal, as copper compound, as a mixture of at least two copper compounds, as a mixture of metal and a copper compound or as a mixture of metal and at least two copper compounds, where the copper compounds are preferably selected from among copper chromate, copper chromite, copper oxide, copper nitrate, copper sulfate, copper chloride, copper bromide, copper iodide, copper carbonate, copper acetylacetate, copper alkoxide, copper aryloxide and copper carboxylate, particularly preferably selected from among copper chromite and copper oxide, and / or ii) if the catalyst comprises palladium, the palladium is present as metal, as palladium compound, as a mixture of at least two palladium compounds, as a mixture of metal and a palladium compound or as a mixture of metal and at least two palladium compounds.

10. The process according to any of claims 1 to 9, wherein the at least one color-imparting component is selected from among cyclohexylphenylamine derivatives, cyclohexanone derivatives, cyclohexenone derivatives, cyclohexylamine derivatives, N-methylcyclohexylamine derivatives, toluidine derivatives, nitrobenzene derivatives, 2-aminophenol derivatives, 1,2-phenylenediamine derivatives, diphenylamine derivatives, oxidized N-methylaniline derivatives or unoxidized N-methylaniline derivatives, with the at least one color-imparting component particularly preferably being selected from among cyclohexylphenylamine derivatives, cyclohexylamine derivatives, N-methylcyclohexylamine derivatives, oxidized N-methylaniline derivatives or unoxidized N-methylaniline derivatives.

11. The process according to any of claims 1 to 10, wherein i) the temperature during the hydrogenation is in the range from 90°C to 300°C, preferably from 90°C to 200°C, particularly preferably from 90°C to 150°C, and / or ii) the duration of the hydrogenation is in the range from 1 to 100 hours, preferably from 5 to 80 hours, particularly preferably from 10 to 50 hours, and / or iii) the hydrogenation is carried out using hydrogen gas (H2), in particular technical-grade hydrogen, where the technical-grade hydrogen preferably has a hydrogen content of at least 99% by weight, more preferably 99.5% by weight, based on the total weight of the hydrogen, and / or iv) the process is carried out continuously or batchwise, preferably continuously.

12. The process according to any of claims 1 to 11, wherein i) the catalyst is present in supported or unsupported, preferably supported, form, where the support material is preferably selected from the group consisting of carbon, silicon oxide, aluminum oxide, manganese oxide, cerium oxide, zirconium oxide, lanthanum oxide, titanium oxide and mixtures of two or more of these materials, and / or ii) the catalyst is used as powder or as shaped bodies, preferably as shaped bodies.

13. The process according to any of claims 1 to 12, wherein the mixture (G2) is worked up further, preferably worked up further by distillation, in order to isolate the at least one colorless amine, to separate off the catalyst and / or to remove the hydrogenation products from the mixture (G2).

14. The process according to claim 1, wherein the mixture (G2) comprises at least 90% by weight, preferably at least 94% by weight, more preferably at least 96% by weight, particularly preferably at least 99% by weight, of aniline and the aniline is converted in at least one further step into methylenedianiline (MDA).

15. The process according to claim 14, wherein methylenedianiline (MDA) is converted into methylene diphenyl diisocyanate (MDI).

16. The process according to any of claims 1 to 15, wherein the mixture (G1) is recycled aniline, where the recycled aniline comprises at least one color-imparting component selected from among cycloaliphatic carbonyl compounds, cycloaliphatic amines, cycloaliphatic imines and / or C6-ring aromatics.