Increasing catalyst selectivity in continuous hydrogenation of nitro compounds by adding ammonia

Description

[0001] The present invention relates to a process for the continuous hydrogenation of a nitro compound to the corresponding amine in a liquid reaction mixture containing the nitro compound in a reaction space in the presence of a supported catalyst which contains as active component at least one element from groups 7 to 12 of the Periodic Table of the Elements, characterized in that ammonia is added to the reaction space during the hydrogenation.

[0002] Processes for the continuous hydrogenation of nitro compounds to the corresponding amines are known per se.

[0003] A frequently described difficulty in processes for the hydrogenation of nitro compounds to the corresponding amines is the release of large amounts of reaction heat and the associated potentially high reaction temperatures.

[0004] For example, DE 10 2008 063308 B4 describes a process for producing toluenediamine by hydrogenating dinitrotoluene, in which the reaction heat is used to generate steam, which can be fed into a steam network of an industrial plant and further utilized. Reaction temperatures greater than 100°C are essential for steam generation. The hydrogenation disclosed in DE 10 2008 063308 B4 is carried out at a temperature greater than or equal to 180°C.

[0005] However, a frequently described problem of high reaction temperatures during hydrogenation is undesirable side reactions, which also occur, for example, due to local overheating within the reactor.

[0006] WO 2014 / 108351 A1 describes a loop-Venturi reactor-type device for the continuous reaction of liquids with gases, in particular for hydrogenations, oxidations, or acetylations, e.g., for the production of toluenediamine by hydrogenation of dinitrotoluene. Local temperature peaks with large amounts of reaction heat released are avoided by modifying the arrangement of the reactor's heat exchanger tubes. Activated nickel catalysts according to WO 2008 / 145179 A1, for example, are used as catalysts. These catalysts consist of a doped Ni / Al alloy and are unsupported. One problem with such Ni / Al alloys is the formation of nickel aluminates, such as takovite or takovite-like compounds. Nickel aluminates are formed during the hydrogenation of nitroaromatics when the Ni / Al catalyst contains more than 5.5 wt.% Al. They form solids that settle on the walls of the reactor and peripheral equipment such as pumps.This can lead to a reduction in the system's heat transfer efficiency and even to system clogging. To dope the Ni / Al alloy of the activated nickel catalyst from WO 2008 / 145179 A1, one or more metals selected from the group consisting of Mg, Ce, Ti, V, Nb, Cr, W, Mn, Re, Fe, Ru, Co, Rh, Ir, Pt, Cu, Ag, Au, and Bi are used to reduce or completely prevent the formation of nickel aluminates, such as takovite or takovite-like compounds.

[0007] EP 1678118 B1 relates to a process for producing amines by catalytic hydrogenation of the corresponding nitro compounds. To reduce side reactions that lead to the formation of high-molecular-weight by-products or low-boiling components, the selectivity of the process is improved using a catalyst consisting of platinum and nickel.

[0008] DE 10 2005 041532 A1 also describes a process for the preparation of amines by catalytic hydrogenation of the corresponding nitro compounds with reduced side reactions and improved selectivity of the process using a catalyst consisting of platinum, nickel and an additional metal.

[0009] Another problem with the known processes is that after an extended reaction runtime, the product yield decreases due to catalyst aging. For example, impurities in the feedstock or the products of undesirable side reactions can contribute to catalyst aging. Catalyst aging can also be accelerated by an interruption in the feedstock supply, for example, during a shutdown.

[0010] When using a reactor with mass recirculation, a so-called loop reactor, there may be areas where complete conversion of the reactant occurs. In these areas, catalyst aging may be accelerated because the complete conversion of the reactant leads to increased formation of high-boiling components, such as various amino- and methyl-substituted diphenylamines, hydrazodiphenyls, and phenazines.

[0011] Furthermore, interrupting the supply of nitro compounds while maintaining the reaction conditions (such as temperature, pressure, hydrogen flow, and flow in the loop reactor (circulating flow)) and all other conditions leads to the formation of high-boiling components, such as various amino- and methyl-substituted diphenylamines, hydrazodiphenyls, and phenazines, which can deactivate the catalyst. If the process is continued by re-adding nitro compounds, the yield of amines is significantly lower and generally recovers only slowly and incompletely.

[0012] DE10105277A1 describes a process for the preparation of amines by hydrogenation of the corresponding nitro compounds, in which the hydrogen present in the reactor contains portions of at least one gas which is inert in the hydrogenation reaction.

[0013] The object of the present invention is to maintain catalyst activity over the long term in a process for the continuous hydrogenation of nitro compounds to the corresponding amines without having to reduce the feed rate of reactants or the temperature. Furthermore, the selectivity of the catalyst should be increased. Furthermore, it is an object of the present invention to reduce the formation of high-boiling components that lead to catalyst contamination and deactivation.

[0014] The object is achieved by a process for the continuous hydrogenation of a nitroaromatic to the corresponding amine in a liquid reaction mixture containing the nitro compound in a reaction space in the presence of a supported catalyst which contains as active component at least one element from groups 7 to 12 of the Periodic Table of the Elements, characterized in that ammonia is added to the reaction space during the hydrogenation, the amount of ammonia added being at least 200 mmol and at most 3000 mmol per kg of nitro compound added and the nitro compound to be hydrogenated being dinitrotoluene.

[0015] Surprisingly, it was found that the addition of ammonia during hydrogenation significantly increases the product yield of amines. At the same time, the formation of high-boiling components during the process is reduced and the catalyst lifetime is increased. At the same time, the ammonia can be easily removed from the reaction product, for example, by distillation methods.

[0016] Surprisingly, the formation of high-boiling components also decreases when ammonia is present in the reactor before the supply of nitro compounds is interrupted. The reaction yield of amines returns to its original level immediately after the reaction is continued by adding nitro compounds.

[0017] The process according to the invention for the continuous hydrogenation of a nitro compound to the corresponding amine in a liquid reaction mixture containing the nitro compound in a reaction space in the presence of a supported catalyst which contains as active component at least one element from groups 7 to 12 of the Periodic Table of the Elements is characterized in that ammonia is added to the reaction space during the hydrogenation.

[0018] Adding ammonia to the reaction chamber during hydrogenation means that the ammonia is added continuously with the other reactants, while the reaction itself, namely the hydrogenation, takes place. It does not involve introducing the desired amount of ammonia before the actual start of the reaction. The reaction chamber is understood to be the space within which the continuous hydrogenation of the nitro compound takes place. Generally, the active component of the supported catalyst contains at least one element from the group consisting of nickel, platinum, palladium, iron, and cobalt.

[0019] In a first preferred embodiment, the active component of the supported catalyst contains nickel in the form of nickel crystallites with a bimodal nickel crystallite size distribution and has a nickel content of 60 to 80 wt.% based on the total mass of the catalyst and a degree of reduction of at least 70%.

[0020] In a second preferred embodiment, the active component of the supported catalyst contains a mixture of nickel and platinum and optionally at least one additional metal. The hydrogenation catalyst of this second preferred embodiment preferably contains 1 to 5 wt.% platinum, 0.3 to 1.5 wt.% nickel, 0.05 to 1.5 wt.% of the at least one additional metal, and 94.65 to 97.45 wt.% support material, based on the total weight of the catalyst. Preferably, the at least one additional metal is chromium. Nitro compounds

[0021] Suitable nitro compounds are dinitrotoluene.

[0022] In a particularly preferred embodiment, 2,4-dinitrotoluene or 2,6-dinitrotoluene is used. Technical mixtures containing 2,4-dinitrotoluene and 2,6-dinitrotoluene are also suitable, with these mixtures preferably containing up to 35 wt.% of 2,6-dinitrotoluene with proportions of preferably 1 to 5 wt.%, preferably 1 to 4 wt.% of vicinal dinitrotoluene and preferably 0.5 to 1.5 wt.% of 2,5- and 3,5-dinitrotoluene, based on the total mixture.

[0023] The nitroaromatics mentioned are commercially available.

[0024] Furthermore, the nitroaromatics used can be obtained by chemical synthesis, as, for example, dinitrotoluenes can be obtained by the nitration of toluene. The resulting reaction product usually contains numerous impurities in addition to the desired nitro compound. For example, nitrating acid, including nitric acid, sulfuric acid, and nitrogen oxides, can be present in this reaction product. Degradation products, such as dinitrogen monoxide, hydrocyanic acid, carbon monoxide, or mixtures thereof, can also be present as impurities. Oxidation products, for example from undesired side reactions of the nitroaromatics, can also be present, such as aromatic carboxylic acids, such as nitrobenzoic acids, for example mononitrobenzoic acid, dinitrobenzoic acid, or their degradation products, as well as mixtures thereof.

[0025] Further impurities may be present in the form of high boilers, such as nitrocresol, such as mononitrocresol, dinitrocresol or trinitrocresol, and nitrophenol, such as dinitrophenol or trinitrophenol.

[0026] In general, purification of the resulting nitro compounds is therefore necessary to ensure their suitability as starting materials for subsequent processes, such as hydrogenation to the corresponding amines. Processes for the synthesis of nitro compounds and their further purification are generally known to those skilled in the art or are described for dinitrotoluene in US 2014 / 0039227 A1.

[0027] The purification is generally carried out in a multi-stage washing process comprising at least three washing steps: a washing step to remove the acids, a washing step in the presence of a base to remove weak acids and a neutral washing step to remove remaining alkaline substances.

[0028] The resulting washing solutions may, for example, contain nitrocresols or nitroaromatics, which can be separated from the nitroaromatics, for example, by precipitation using acidification, by treatment with activated carbon, by strongly basic exchange columns, by extraction using toluene or the aromatic to be nitrated, by oxidation using nitric acid and subsequent thermal decomposition, by decomposition using hydrogen peroxide or ozone, or by thermolysis.

[0029] After separating nitrating acid and potentially present nitrocresols and / or nitrobenzoic acids, the purification of the nitroaromatics can also be carried out in two washing steps, as described in US 2014 / 0039227 A1 for dinitrotoluene. In a first washing step comprising at least one extraction step, the crude mixture can be washed with a first washing acid containing nitric acid, nitrogen oxides, and sulfuric acid. Generally, the washing solution removed during the extraction has a total acid content of 20 to 40 wt.%, based on the total weight of this washing solution, and contains, for example, nitric acid, nitrogen oxides, such as nitrous acid, and sulfuric acid.

[0030] The resulting mixture, which contains the desired nitro compound, is treated, for example, in a second washing step with a second washing acid, in at least one further extraction step. In this extraction step, the discharged washing solution generally has a pH of ≤ 4. Typically, a mixture containing the desired nitro compound is obtained, which can be essentially free of nitric acid, sulfuric acid, and nitrogen oxides. The purified nitro compound obtained in this way can generally be used for hydrogenation to the corresponding amine.

[0031] The nitro compound obtained from the above-mentioned purification process or used for hydrogenation generally has a residual acid content of ≤ 300 ppm, such as sulfuric acid, and a pH of 2 to 4. In general, the nitro compound contains dinitrotoluene, ≤ 800 ppm nitrocresols or nitrophenols or mixtures thereof, ≤ 600 ppm nitrobenzoic acid, such as mononitrobenzoic acid or dinitrobenzoic acid or mixtures thereof, and a residual content of ≤ 300 ppm, preferably ≤ 200 ppm and particularly preferably ≤ 100 ppm nitric acid or nitrous acid or mixtures thereof, ≤ 50 ppm, preferably ≤ 10 ppm and particularly preferably ≤ 1 ppm hydrocyanic acid, ≤ 200 ppm, preferably ≤ 50 ppm and particularly preferably ≤ 25 ppm Nitrous oxide, ≤ 400 ppm, preferably ≤ 200 ppm and particularly preferably ≤ 50 ppm nitrogen monoxide, and ≤ 3 ppm sulfate.

[0032] In a preferred embodiment, the reaction mixture contains, in addition to the nitro compound to be hydrogenated, at least one high boiler from the group consisting of nitrocresols and nitrophenols. In a particularly preferred embodiment, the reaction mixture contains, in addition to the nitro compound to be hydrogenated, at least one high boiler from the group consisting of dinitrocresols, trinitrocresols, and nitrophenols.

[0033] In another preferred embodiment, the reaction mixture contains no high boilers from the group consisting of nitrocresols and nitrophenols in addition to the nitro compound to be hydrogenated. In another particularly preferred embodiment, the reaction mixture contains no high boilers from the group consisting of dinitrocresols, trinitrocresols, and nitrophenols in addition to the nitro compound to be hydrogenated. In a further preferred embodiment, the reaction mixture contains, in addition to the nitro compound to be hydrogenated, at least one compound from the group consisting of nitric acid, sulfuric acid, nitrogen oxides, dinitrogen monoxide, hydrocyanic acid, carbon monoxide, and nitrobenzoic acid or their degradation products.

[0034] For the hydrogenation process according to the invention, the nitro compound can be used in pure form, as a mixture with the corresponding mono-, di- or polyamine, as a mixture with the corresponding mono-, di- or polyamine and water, as a mixture with the corresponding mono-, di- or polyamine, water and an alcoholic solvent or as a mixture with the corresponding di- or polyamine, water, an alcoholic solvent and a catalyst-reactivating additive, wherein in each case mixtures of two or more of the above-mentioned nitro compounds, the corresponding amine compounds, the alcoholic solvent and the catalyst-reactivating additive can also be used.

[0035] Aprotic solvents, in particular dimethylformamide (DMF), dioxane or tetrahydrofuran (THF) or a mixture of two or more thereof, are preferably used as catalyst-reactivating additives.

[0036] Suitable alcoholic solvents are generally lower aliphatic alcohols containing 1 to 6 carbon atoms. Methanol, ethanol, or propanol are preferably used individually or in a mixture of two or more thereof. Ethanol is particularly preferred.

[0037] If a mixture described above is used, the weight ratio of amine compound to water is preferably in the range from 10:1 to 1:10, preferably in the range from 8:1 to 1:5 and particularly preferably in the range from 4:1 to 1:1 and the weight ratio of the amine / water mixture to at least one alcoholic solvent is preferably 1000:1 to 1:1, preferably 500:1 to 2.5:1 and particularly preferably 50:1 to 5:1.

[0038] The amount of alcoholic solvents and catalyst-reactivating additives used is not particularly limited in the process according to the invention and can be freely selected as required.

[0039] The process according to the invention for the hydrogenation of nitro compounds to the corresponding amines can also be carried out in the absence of solvents. This procedure simplifies the workup of the reaction mixture after hydrogenation, and side reactions with the solvent are completely eliminated. The purified nitro compound can generally be used for hydrogenation to the corresponding amines. However, the remaining impurities and the by-products formed during hydrogenation, such as high boilers, can contribute to catalyst aging. ammonia

[0040] The amount of ammonia added is at least 200 mmol per kg of nitro compound added, in particular at least 400 mmol, particularly preferably at least 1000 mmol, most preferably at least 1200 mmol.

[0041] The amount of ammonia added is at most 3000 mmol per kg of nitro compound added, in particular at most 2500 mmol, particularly preferably at most 2000 mmol, most preferably at most 1800 mmol.

[0042] Ammonia is added in an amount of 200 to 3000 mmol per kg of nitro compound added, in particular 400 to 2500 mmol, particularly preferably 1000 to 2000 mmol, most preferably 1200 to 1800 mmol.

[0043] In a first embodiment, the ammonia can be added as liquid ammonia in the preferred amounts mentioned above.

[0044] In a second embodiment, the ammonia can be added as an aqueous solution in the above-mentioned preferred amounts. The amount of ammonia, based on the total weight of the aqueous ammonia solution, is preferably at least 1 wt.%, in particular at least 5 wt.%, particularly preferably at least 10 wt.%, most preferably at least 20 wt.%.

[0045] The preferred method of adding ammonia is explained in more detail below as part of the process. catalyst

[0046] A supported catalyst is a catalyst in which an active component is located on an inactive component (the support material), in particular distributed over the support material. The active component is the catalytically active component.

[0047] Suitable supported catalysts for a process for the continuous hydrogenation of nitro compounds to the corresponding amines in the liquid phase are generally known to the person skilled in the art or are described, for example, in EP 1 678 118 B1, DE 10 2005 041 532 A1, WO 2008 / 138784 A1 and US 6,140,539.

[0048] Suitable active components for a catalyst for the hydrogenation of nitroaromatics to the corresponding amines are generally known to the person skilled in the art or are described, for example, in EP 1 678 118 B1, DE 10 2005 041 532 A1, WO 2008 / 138784 A1, EP 1161297 A1, EP 1165231 A1 and US 6,140,539.

[0049] Generally, the supported catalyst contains at least one element from groups 7 to 12 of the Periodic Table of Elements as an active component. Elements from groups 7 to 12 of the Periodic Table of Elements that are suitable as active components of the supported catalyst include iron, cobalt, nickel, ruthenium, rhodium, palladium, silver, osmium, iridium, platinum, gold, copper, rhenium, zinc, and / or manganese.

[0050] The active component of the supported catalyst preferably contains at least one element from groups 7 to 12 of the Periodic Table of the Elements, particularly preferably at least one element from the group consisting of nickel, platinum, palladium, iron and cobalt, very particularly preferably at least one element from the group consisting of nickel, platinum, palladium and cobalt and particularly preferably at least nickel.

[0051] In general, the catalyst contains 0 to 10 wt.%, preferably 1 to 5 wt.% of precious metal based on the total weight of the catalyst.

[0052] Optionally, the active component of the catalyst may contain chromium in addition to at least one element from groups 7 to 12 of the Periodic Table of Elements.

[0053] To suppress side reactions, it is preferable to conduct the process such that the catalyst is operated at its maximum loading. This can be controlled, for example, by the amount of nitro compound added, the amount of catalyst in the reaction mixture, the temperature, or the pressure. The maximum loading of the catalyst within the meaning of the invention is understood to mean the amount of hydrogenatable groups containing nitrogen atoms and oxygen atoms that can be hydrogenated by the catalyst under given pressure and temperature conditions. The groups containing nitrogen atoms and oxygen atoms can be nitro groups, nitroso groups, and nitrosamine groups, in addition to nitro groups.

[0054] In a first embodiment, as described in US 6,140,539, nickel can be used as the active component on a support, wherein the catalyst is stabilized, and the nickel crystallites have a bimodal nickel crystallite size distribution, a nickel content of 60 to 80 wt.%, based on the total mass of the catalyst, and a degree of reduction of at least 70%.

[0055] The degree of reduction is generally determined by post-reduction of the stabilized catalyst for one hour at 100°C.

[0056] In general, the two maxima of the bimodal nickel crystallite size distribution are between 30 and 80 angstroms and 81 and 150 angstroms. Preferably, the nickel content in the maximum range of 30 to 80 angstroms is ≥ 40 to < 100 wt.% based on the total mass of the catalyst.

[0057] Suitable support materials for this first embodiment include, for example, oxides and oxide mixtures of zirconium, hafnium, or silicon. The support preferably contains ZrO 2 , ZrO 2 HfO 2 , SiO 2 ·ZrO 2 or SiO 2 ·ZrO 2 HfO 2 or mixtures containing at least two of these substances. The support particularly preferably consists of these substances.

[0058] The SiO 2 content is preferably 0 to 20 wt.% based on the total mass of the catalyst.

[0059] Preferably, the ZrO 2 content is 0 to 40 wt.% based on the total mass of the catalyst.

[0060] The HfO 2 content is preferably 0 to 4 wt.% based on the total mass of the catalyst.

[0061] Processes for producing such a catalyst are generally known to the person skilled in the art or are described, for example, in US 6,140,539.

[0062] Preferably, the active component of the supported catalyst contains nickel in the form of nickel crystallites with a bimodal nickel crystallite size distribution and has a nickel content of 60 to 80 wt.% based on the total mass of the catalyst and a degree of reduction of at least 70%.

[0063] In a second embodiment, the process according to the invention for the continuous hydrogenation of nitro compounds to the corresponding amines uses a supported catalyst whose active component contains a mixture of nickel and platinum and optionally at least one additional metal, as described, for example, in EP 1678118 B1 or DE 10 2005 041 532 A1.

[0064] Generally, the active components of this catalyst are applied to the support material in the form of mixtures. Suitable mixtures contain nickel and platinum with an atomic ratio of nickel to platinum of preferably between 30:70 and 70:30, more preferably between 40:60 and 60:40, and most preferably between 45:55 and 55:45. Mixtures of nickel and platinum with a different atomic ratio are also useful, but often result in low product yields.

[0065] Preferably, at least one additional metal is added to the mixture containing nickel and platinum. Metals suitable as additional metals are generally known to those skilled in the art or are described, for example, in DE 10 2005 041 532 A1. The additional metal is preferably at least one metal from the group consisting of copper, cobalt, iron, zinc, manganese, and chromium, particularly preferably at least one metal from the group consisting of copper, cobalt, iron, and zinc.

[0066] The metal particles are generally polycrystalline. Their characterization is generally known to those skilled in the art or is described, for example, in DE 10 2005 041 532 A1.

[0067] The nature and methods for characterising the supported catalyst, the active material of which may be a mixture of nickel and platinum and optionally at least one additional metal, are generally known to the person skilled in the art or are described in EP 1678118 B1 or DE 10 2005 041 532 A1.

[0068] In general, the hydrogenation catalyst used in the process according to the invention based on nickel and platinum and at least one additional metal 1 to 5 wt% platinum, 0.3 to 1.5 wt% nickel, 0.05 to 1.5 wt% of at least one additional metal, and 94.65 to 97.45 wt% carrier material based on the total weight of the catalyst.

[0069] Particularly preferably, the hydrogenation catalyst used in the process according to the invention based on nickel and platinum and at least one additional metal consists of 1 to 5 wt% platinum, 0.3 to 1.5 wt% nickel, 0.05 to 1.5 wt% of at least one additional metal, and 94.65 to 97.45 wt% carrier material based on the total weight of the catalyst.

[0070] In general, the content of non-noble metals is 0 to 1.6 wt.%, preferably 0.1 to 0.9 wt.%, based on the total weight of the catalyst.

[0071] The materials suitable as supports for the catalysts of this second embodiment are generally known to the person skilled in the art or are described, for example, in EP 1678118 B1 or DE 10 2005 041 532 A1. In general, activated carbon, carbon black, graphite, or metal oxides, preferably hydrothermally stable metal oxides such as ZrO 2 and / or TiO 2 , or oxides of aluminum, such as Al 2 O 3 , or silicon, or other materials are used.

[0072] Preferably, graphite is used as a carrier, in which case HSAG (High Surface Area Graphite) with a surface area of ​​50 to 300 m 2 / g is particularly preferred.

[0073] The use of activated carbon as a carrier is particularly preferred. A particularly preferred embodiment is the use of physically or chemically activated carbon or carbon black, such as acetylene black, as a carrier.

[0074] Processes for producing the supported catalyst, the active material of which is a mixture of nickel and platinum and optionally at least one additional metal, are generally known to the person skilled in the art or are described in EP 1678118 B1 or DE 10 2005 041 532 A1.

[0075] The catalyst used in the process according to the invention based on a mixture of nickel and platinum and optionally at least one additional metal is preferably used in an amount of 0.01 to 10 wt.%, preferably 0.1 to 5 wt.% and particularly preferably 0.2 to 2 wt.%, based on the total weight of the reaction mixture.

[0076] The active component of the supported catalyst preferably contains a mixture of nickel and platinum and optionally at least one additional metal.

[0077] Preferably, the additional metal is at least one metal from the group consisting of copper, cobalt, iron, zinc, manganese and chromium, particularly preferably at least one metal from the group consisting of copper, cobalt, iron and zinc.

[0078] According to a third embodiment of the process according to the invention, catalysts can be used which contain, as the active component, a mixture of nickel, palladium, and an additional element selected from the group consisting of cobalt, iron, vanadium, manganese, chromium, platinum, iridium, gold, bismuth, molybdenum, selenium, tellurium, tin, and antimony on a support. This process can be used to produce toluenediamine by hydrogenating dinitrotoluene. The hydrogenation of nitro compounds to the corresponding amines using this catalyst is generally known to the person skilled in the art or is described, for example, in WO 2008 / 138784 A1.

[0079] The additional element is preferably selected from the group consisting of cobalt, iron, vanadium, bismuth and tin.

[0080] As a support for the catalyst of this third embodiment, the conventional and known materials can generally be used. Preference is given to activated carbon, carbon black, graphite, or metal oxides, preferably hydrothermally stable metal oxides such as ZrO 2 , TiO 2 , Al 2 O 3 . HSAG (high surface area graphite) with a surface area of ​​50 to 300 m 2 / g is preferred. Particular preference is given to activated carbons, in particular physically or chemically activated carbons, or carbon blacks, such as acetylene black.

[0081] In a further embodiment of the process according to the invention, the active component of the catalyst is not Raney nickel.

[0082] In general, the catalyst used in the process according to the invention is used in an amount of 0.01 to 10 wt.%, preferably 0.1 to 5 wt.% and particularly preferably 0.2 to 2 wt.%, based on the total weight of the reaction mixture.

[0083] The catalyst is typically introduced into the reactor in a reduced and passivated state. For the purposes of the invention, the reduced and passivated state means that the catalyst is activated after preparation, but then, for safety reasons, the active centers are passivated, for example, by passing oxygen or carbon dioxide over it. Also suitable is the removal and stabilization of the catalyst under an inert atmosphere or in a non-flammable solvent, for example, in water or a mixture of toluenediamine and water, or higher alcohols, such as butanol or ethylene glycol. Implementation

[0084] The reactors and reactor operating modes suitable for the process according to the invention are generally known to the person skilled in the art or are described, for example, in DE10 2005 041 532 A1, DE 10 2008 063 308 B4, WO 2000 / 035852 A1 or WO 2014 / 108351 A1.

[0085] The process parameters to be used for the process according to the invention, such as pressure and temperature, are also generally known to the person skilled in the art or are described, for example, in DE 10 2005 041 532 A1, DE 10 2008 063 308 B4, WO 2000 / 035852 A1 or WO 2014 / 108351 A1.

[0086] Suitable reactors include, for example, stirred tanks or tube bundle reactors or loop reactors, such as jet loop reactors, so-called loop Venturi reactors, or loop reactors with internal flow reversal as described in WO 2000 / 035852 A1, DE10 2005 041 532 A1, DE 10 2008 063 308 B4, or WO 2014 / 108351 A1. A loop reactor is preferably used for the process according to the invention.

[0087] Generally, the nitro compounds are added at a rate that, on the one hand, is adapted to the catalyst activity and, on the other hand, ensures sufficient mixing with the flow in the loop reactor, the so-called circulation flow. Typically, the catalyst activity is adjusted by adding sufficient amounts of catalyst so that the addition flow of the nitro compounds is aligned with the circulation flow, thus avoiding local excess concentrations of nitro compounds (e.g., greater than 10,000 ppm). In the case of a reactor with flow reversal, an internal circulation flow that is greater than the circulation flow can arise, for example, due to the impact of the fed reaction mixture on the reactor floor or due to internals.If the nitro compound is added to the internal circulation flow in this case, its addition flow depends on the size of the internal circulation flow in order to avoid local overconcentrations at a given conversion in the internal circulation loop.

[0088] In a preferred embodiment, after the first addition point for the nitro compound, there is a second addition point through which additional components, in particular the ammonia added according to the invention, are fed into the process.

[0089] The ammonia can generally be fed into the reactor and thus into the reaction chamber at any point. Preferably, ammonia is added separately from the nitro compounds fed into the reactor, rather than together with them. The addition preferably takes place in the direction of flow immediately after the addition of the nitro compound. This ensures good and rapid mixing throughout the entire reaction zone.

[0090] The weight-related space velocity used in the process according to the invention is preferably 5 to 100 kg (nitro compound) / kg (catalyst) / h, preferably 10 to 50 kg (nitro compound) / kg (catalyst) / h and particularly preferably 15 to 35 kg (nitro compound) / kg (catalyst) / h.

[0091] Suitable hydrogenation gas mixtures for the continuous hydrogenation of nitroaromatics to the corresponding amines are generally known to the person skilled in the art or are described, for example, in EP 1678 118 B1.

[0092] In general, gases that contain free hydrogen and do not contain harmful amounts of catalyst poisons, such as carbon monoxide, can be used as hydrogenation gas mixtures. Suitable hydrogenation gas mixtures are reformer offgases or mixtures of hydrogen with nitrogen and / or carbon dioxide. Hydrogen with a low inert gas content is preferred as the hydrogenation gas mixture.

[0093] The hydrogenation gas mixtures are generally fed in depending on the hydrogen consumption during the reaction, preferably by keeping the pressure prevailing in the reactor apparatus constant. Continuously removing a portion of the gas phase can prevent a buildup of inert components in the hydrogenation gas mixture or of inert gaseous reaction products.

[0094] In a first embodiment, the process according to the invention, as described in WO 2014 / 108351 A1 or DE 10 2008 063 308 B4, can be carried out at temperatures of 80 to 200°C, preferably of 110 to 190°C, particularly preferably of 150 to 190°C. In general, the process of this first embodiment is carried out at pressures of 10 to 50 bar, preferably of 15 to 35 bar. The resulting heat of reaction can be used with the aid of heat exchangers to obtain steam at an overpressure of at least 4 bar. The parameters required for carrying out this process are generally known to the person skilled in the art or are described, for example, in WO 2014 / 108351 A1 or DE 10 2008 063 308 B4.

[0095] Preferably, the process according to the invention for the hydrogenation of nitro compounds to amines, such as the hydrogenation of dinitrotoluene to toluenediamine derivatives, is carried out in a vertically extended reactor as described in WO 2014 / 108351 A1, which contains at least one mixing chamber, wherein the mixing chamber or the mixing chambers are each connected to a diffuser at their lower end.

[0096] Heat exchangers and cooling media suitable for this process are also generally known to the person skilled in the art or are described, for example, in WO 2014 / 108351 A1 or DE 10 2008 063 308 B4.

[0097] In a second embodiment, the process according to the invention can be carried out as described in WO 00 / 35852 A1 or DE 10 2005 041 532 A1 at temperatures of preferably 80 to 250° C, preferably 100 to 200° C and particularly preferably 120 to 150° C. In general, the process in this second embodiment is carried out at pressures of preferably 5 to 100 bar, preferably 10 to 40 bar and particularly preferably 20 to 25 bar.

[0098] Suitable reactors are generally known to the person skilled in the art or are described, for example, in WO 00 / 35852 A1 or DE 10 2005 041 532 A1. In general, stirred tank reactors or loop reactors, such as jet loop reactors, so-called loop Venturi reactors, or loop reactors with internal flow reversal, can be used.

[0099] In general, the amines formed during hydrogenation can be removed from the process continuously or discontinuously. The amines formed during hydrogenation are preferably removed from the process continuously.

[0100] The amines formed during hydrogenation can be removed at any point. Preferably, they are removed from the external circulation stream before the nitro compound is fed in. Since the hydrogenation of the nitro compound generally proceeds virtually quantitatively in the internal circulation stream under the conditions mentioned, the external circulation stream before the nitro compound is fed in essentially contains the corresponding pure amine, water, optionally solvent, and catalyst. The amine is generally separated from the withdrawn stream and fed to the purification stage. The catalyst and optionally water can be returned to the external circulation stream.

[0101] Preferably, the catalyst is suspended in the reaction medium. The reaction product is generally separated from the catalyst by membrane filtration. The membrane used for this purpose can preferably be installed in the outer circulation flow or, for example, in the continuously stirred tank. Alternatively, the catalyst can also be retained by sedimentation in a settler installed in the outer circulation flow or, for example, in the continuously stirred tank.

[0102] Membrane filtration is preferably carried out at a pressure on the suspension side of 5 to 50 bar, preferably 20 to 35 bar, a pressure difference between the suspension side and the permeate side of 0.3 bar to 5 bar, and a flow velocity on the suspension side of 1 to 6 m / s. The "suspension side" within the meaning of the invention is understood to mean the side of the membrane filter on which the catalyst-containing mixture is located. The "permeate side" within the meaning of the invention is understood to mean the side of the membrane filter on which the catalyst-free mixture is located.

[0103] Membrane filtration can be carried out continuously or discontinuously.

[0104] In continuous operation, at least a partial stream of the reaction mixture is generally continuously passed through a membrane filter. In a preferred embodiment of the process according to the invention, it is preferable to arrange the membrane filter in the external circuit of a circulation reactor.

[0105] In discontinuous filtration, the discharged reaction mixture is generally passed through a switchable purification stage consisting of at least one membrane filter and its own circulation pump. In another configuration of discontinuous filtration, the reaction mixture is passed through a membrane filter following the reaction.

[0106] The filter membranes used for the process can be made of ceramic (e.g. α-Al g2 O 3 ) or stainless steel (e.g. 1.4404) and, depending on the particle size of the catalyst used, preferably have number-weighted average pore diameters in the range from 10 nm to 20 micrometers, particularly preferably in the range from 20 nm to 10 micrometers and most preferably from 50 nm to 1 micrometer.

[0107] Suitable embodiments of membrane filtration, in particular cross-flow filtration, are known to the person skilled in the art and are described, for example, in WO2010 / 125025 or WO 2003 / 066571.

[0108] After separation of the catalyst, the amines formed are generally further purified. Processes for purifying the amines prepared according to the invention are generally known to those skilled in the art or are described, for example, in WO 2000 / 035852 A1. Suitable processes for purifying the amines formed include distillation or extraction. Examples

[0109] The following experiment was conducted in a miniplant pilot plant. This consisted of a loop reactor setup, one section (5.6 L) with an internal circulation flow driven by a motive jet (circulation flow consisting of product and catalyst), and another section designed as a tubular reactor (4.4 L). The entire setup was thermostatted with thermal oil to dissipate generated heat. DNT was mixed near the motive jet, and hydrogen was metered into the gas space above the internal circulation flow at a controlled pressure. The product formed was removed via a membrane retaining the catalyst, so that the liquid level in the reactor section with the internal circulation flow remained constant. A fixed amount of gas was removed above the gas space to prevent unlimited accumulation of gaseous products or impurities.

[0110] The reactor was loaded with 112 g (dry weight) of 3%Pt-1%Ni / C catalyst suspended in water and operated at 185°C, 25 bar gauge pressure, a circulation flow of 500 kg / h, and a DNT dosing rate of 2 kg / h. This resulted in a WHSV of 17.9 kg(DNT) / kg(cat) / h.

[0111] Ammonia was added by feeding a continuous volume flow of an aqueous solution into the reactor. The solution was introduced through a tube connected to the reactor head. The solution dripped into the reactor's internal circulation flow at the edge of the insertion tube, i.e., away from the DNT addition point. By varying the volume flow and the concentration of the ammonia aqueous solution, the ammonia concentrations described in Table 1 were adjusted. After steady-state conditions were established, several samples were taken and analyzed by gas chromatography.

[0112] After a run-in period of 250 h and significant catalyst aging, ammonia was added and the TDA yield gradually increased according to Table 1. The The first three entries in Table 1 show comparative examples. Table 1 mmol(NH3) / kg(DNT) Concentration of added NH 3 in water Selectivity TDA 0 97,8 13 2 wt.% 97,8 199 10 wt.% 98,2 397 10 wt.% 98,7 993 25% by weight 98,9 1322 25% by weight 99,1 1656 25% by weight 99,2 1918 25% by weight 99,0

Claims

1. A process for continuous hydrogenation of a nitroaromatic to the corresponding amine in a liquid reaction mixture comprising the nitro compound in a reaction space in the presence of a supported catalyst which comprises as the active component at least one element from groups 7 to 12 of the periodic table of the elements, wherein ammonia is added to the reaction space during the hydrogenation, wherein the added amount of ammonia is at least 200 mmol and at most 3000 mmol per kg of added nitro compound and the nitro compound for hydrogenation is dinitrotoluene.

2. The process according to claim 1, wherein the active component of the catalyst comprises at least one element from the group consisting of nickel, platinum, palladium, iron and cobalt.

3. The process according to claim 1 or 2, wherein the active component of the catalyst comprises chromium.

4. The process according to one or more of claims 1 to 3, wherein the active component of the supported catalyst comprises nickel in the form of nickel crystallites having a bimodal nickel crystallite size distribution and has a nickel content of 60 to 80 wt% based on the total mass of the catalyst and a degree of reduction of at least 70%.

5. The process according to one or more of claims 1 to 4, wherein the active component of the supported catalyst comprises a mixture of nickel and platinum in an atomic ratio of nickel to platinum of between 30:70 and 70:30 and optionally one or more additional metals.

6. The process according to claim 5, wherein the catalyst based on nickel and platinum and at least one additional metal and used in the process according to the invention comprises 1 to 5 wt% of platinum, 0.3 to 1.5 wt% of nickel, 0.05 to 1.5 wt% of the at least one additional metal and 94.65 to 97.45 wt% of support material based on the total weight of the catalyst.

7. The process according to claim 5 or 6, wherein the additional metal is at least one metal from the group consisting of copper, cobalt, iron, zinc, manganese and chromium.

8. The process according to claim 7, wherein the added amount of ammonia is at least 1000 mmol per kg of added nitro compound.

9. The process according to one or more of claims 1 to 8, wherein the added amount of ammonia is at most 2000 mmol per kg of added nitro compound.

10. The process according to one or more of claims 1 to 9, wherein the ammonia is added in liquid form.

11. The process according to one or more of claims 1 to 10, wherein the ammonia is added as an aqueous solution.

12. The process according to one or more of claims 1 or 11, wherein in addition to the nitro compound for hydrogenation the reaction mixture comprises at least one high boiler from the group consisting of dinitrocresols, trinitrocresols and nitrophenols.

13. The process according to one or more of claims 1 to 11, wherein in addition to the nitro compound for hydrogenation the reaction mixture comprises no high boilers from the group consisting of dinitrocresols, trinitrocresols and nitrophenols.

14. The process according to one or more of claims 1 to 13, wherein in addition to the nitro compound for hydrogenation the reaction mixture comprises at least one compound from the group consisting of nitric acid, sulfuric acid, nitrogen oxides, dinitrogen monoxide, hydrocyanic acid, carbon monoxide and nitrobenzoic acid or decomposition products thereof.

15. The process according to one or more of claims 1 to 14, wherein the hydrogenation is performed at a temperature of 80°C to 250°C.

16. The process according to one or more of claims 1 to 15, wherein the hydrogenation is performed in the absence of solvents.