This invention relates to a process for catalytic hydrogenation of adiponitrile to hexamethylenediamine at elevated temperature and elevated pressure in the presence of catalysts based on elemental iron as catalytically active component and ammonia as solvent, which comprises
a) hydrogenating adiponitrile at from 70 to 220xc2x0 C. and from 100 to 400 bar in the presence of catalysts based on elemental iron as catalytically active component and ammonia as solvent to obtain a mixture comprising adiponitrile,
6-aminocapronitrile, hexamethylenediamine and high boilers until the sum total of the 6-aminocapronitrile concentration and the adiponitrile concentration is within the range from 1 to 50% by weight, based on the ammonia-free hydrogenation mixture,
b) removing ammonia from the hydrogenation effluent,
c) removing hexamethylenediamine from the remaining mixture,
d) separating 6-aminocapronitrile and adiponitrile from high boilers individually or together, and
e) returning 6-aminocapronitrile, adiponitrile or mixtures thereof into step a).
U.S. Pat. No. 3,696,153 discloses hydrogenating adiponitrile to hexamethylenediamine at temperatures of 100 to 200xc2x0 C. and pressures of about 340 atm in the presence of granulated catalysts comprising very predominantly iron and small amounts of aluminum oxide and in the presence of ammonia as solvent.
Hexamethylenediamine yields of 98.8%, 98.8%, 97.7% and 97.7% are reached in the examples of Table 1 (run 2) and Table 2 (runs 1 to 3) at pressures of 340 atm. Complete conversion is reported for the first three examples and 99.9% conversion for the fourth example. With regard to the life of the iron catalysts, Tables 1 and 2 merely reveal that catalyst activity is high at the end of the runs (after around 80 to 120 hours).
U.S. Pat. No. 4,064,172 discloses hydrogenating adiponitrile to hexamethylenediamine at pressures of 20 to 500 bar and temperatures of 80 to 200xc2x0 C. in the presence of iron catalysts synthesized from magnetite and in the presence of ammonia. A hexamethylenediamine yield of 98.2% is reported in Example 1.
U.S. Pat. No. 4,282,381 describes the hydrogenation of adiponitrile to hexamethylenediamine with hydrogen at temperatures of 110 to 220xc2x0 C. and a pressure of about 340 atm in the presence of ammonia and iron catalysts. The hydrogenation effluent contains 0.04 to 0.09% by weight of adiponitrile and 0.2 to 0.5% by weight of 6-aminocapronitrile.
McKetta, Encyclopedia of Chemical Processing and Design, Marcel Dekker Inc. 1987, volume 26, page 230, Table 3, confirms that a typical hydrogenation product contains 0.01 to 0.11% by weight of adiponitrile and 0.10 to 0.21% by weight of aminocapronitrile. Illustrations 2 and 4 reveal that these small aminocapronitrile quantities can be separated off and returned into the hydrogenation.
These processes suggest that the reaction conditions in the industrial production of hexamethylenediamine have to be directed to achieving complete conversion of the adiponitrile and of the 6-aminocapronitrile intermediate of the hydrogenation.
The disadvantage with this is that this requires a relatively high temperature and a very high reaction pressure. If the adiponitrile and 6-aminocapronitrile conversion decreases markedly in the course of the hydrogenation, it has to be pushed back up again by raising the temperature and optionally the reaction pressure and/or lowering the catalyst loading, or a not inconsiderable loss of product of value will be incurred.
If, to obtain complete conversion, the temperature cannot be further increased because of decreasing hexamethylenediamine selectivity and/or the pressure cannot be further increased for technical reasons, then the catalyst loading has to be reduced. However, this means that catalyst productivity, i.e., the amount of hexamethylenediamine produced per unit time, will decrease. If the productivity drops below a certain level, the hydrogenation plant has to be shut down and the iron catalyst moved and replaced with an unused or regenerated catalyst. The greater the frequency of such shutdowns required per year, the lower the hexamethylenediamine quantity which a given production plant can produce per year.
It is an object of the present invention to provide a process for the catalytic hydrogenation of adiponitrile to hexamethylenediamine in the presence of catalysts comprising very predominantly elemental iron and ammonia as solvent in an economical and technically simple manner while avoiding the disadvantages mentioned.
The process of the invention does not require complete adiponitrile and 6-aminocapronitrile conversion. This provides distinctly higher catalysts onstream times at lower pressures, fewer shutdowns for the hydrogenation plant and hence distinctly higher hexamethylenediamine productivities compared with the prior art.
It was unforeseeable and hence it is surprising that recycling 6-aminocapronitrile, adiponitrile or mixtures thereof into the hydrogenation stage does not cause any shortening of the catalyst onstream time. It is also surprising that the entire recycle does not cause any troublesome buildup of by-products in the system.
The adiponitrile used in the process of the invention can generally be prepared by conventional processes, preferably by reaction of butadiene with hydrocyanic acid in the presence of catalysts, especially nickel (0) complexes and phosphorus-containing cocatalysts, via pentenenitrile as intermediate.
The catalysts used can be conventional iron catalysts known for the production of hexamethylenediamine by hydrogenation of adiponitrile. Preferred catalyst precursors are those which comprise from 90 to 100% by weight, preferably from 92 to 99% by weight, based on the total mass of the catalyst precursor, of iron oxides, iron(II, III) oxide, iron(II) oxide, iron(II) hydroxide, iron(III) hydroxide or iron oxyhydroxide such as FeOOH. It is possible to use synthetic or naturally occurring iron oxides, iron hydroxides or iron oxyhydroxides, magnetite, which has the idealized formula of Fe3O4, brown ironstone, which has the idealized formula of Fe2O3 x H2O, or hematite, which has the idealized formula of Fe2O3.
Preferred catalysts are those which comprise
a) iron or a compound based on iron or mixtures thereof,
b) from 0.001 to 5% by weight based on a) of a promoter based on 2, 3, 4, 5 or 6 elements selected from the group consisting of aluminum, silicon, zirconium, titanium, vanadium and manganese, and
c) from 0 to 5% by weight based on a) of a compound based on an alkali metal or on an alkaline earth metal.
Further preferred catalyst precursors are those in which component b) comprises from 0.001 to 5% by weight, preferably from 0.01 to 4% by weight, especially from 0.1 to 3% by weight, of a promoter based on 2, 3, 4, 5 or 6 elements selected from the group consisting of aluminum, zirconium, silicon, titanium, manganese and vanadium.
Further preferred catalyst precursors are those in which component c) comprises from 0 to 5% by weight, preferably from 0.1 to 3% by weight, of a compound based on an alkali or alkaline earth metal preferably selected from the group consisting of lithium, sodium, potassium, rubidium, cesium, magnesium and calcium.
The catalysts can be supported or unsupported catalysts. Examples of suitable support materials are porous oxides such as aluminum oxide, silicon oxide, alumosilicates, lanthanum oxide, titanium dioxide, zirconium dioxide, magnesium oxide, zinc oxide and zeolites and also activated carbon or mixtures thereof.
Preparation is generally effected by precipitating precursors of component a) if desired together with precursors of the promoter components b) and if desired with precursors of the trace components c) in the presence or absence of support materials (depending on which type of catalyst is desired), if desired processing the resulting catalyst precursor into extrudates or tablets, drying and subsequently calcining. Supported catalysts are generally also obtainable by saturating the support with a solution of said components a), b) and if desired c), the individual components being added simultaneously or in succession, or by spraying said components a), if desired b) and c) onto the support in a conventional manner.
Suitable precursors for components a) are generally readily water-soluble salts of iron such as nitrates, chlorides, acetates, formates and sulfates, preferably nitrates.
Suitable precursors for components b) are generally readily water-soluble salts or complexes of the aforementioned metals and metalloids such as nitrates, chlorides, acetates, formates and sulfates, preferably nitrates.
Suitable precursors for components c) are generally readily water-soluble salts of the aforementioned alkali metals and alkaline earth metals such as hydroxides, carbonates, nitrates, chlorides, acetates, formates and sulfates, preferably hydroxides and carbonates.
Precipitation is generally effected from aqueous solutions, selectively by addition of precipitants, by changing the pH or by changing the temperature.
The catalyst prematerial thus obtained is usually dried, generally at from 80 to 150xc2x0 C., preferably at from 80 to 120xc2x0 C.
Calcination is customarily effected at temperatures within the range from 150 to 500xc2x0 C., preferably from 200 to 450xc2x0 C., in a gas stream comprising air or nitrogen.
After calcination, the catalyst material obtained is generally activated by exposing to a reducing atmosphere, for example by exposing it for from 2 to 100 hours to a hydrogen atmosphere or to a gas mixture comprising hydrogen and an inert gas such as nitrogen at from 200 to 500xc2x0 C., preferably at from 250 to 400xc2x0 C. The catalyst loading during this activating step is preferably 200 1 per liter of catalyst.
The activation of iron catalysts by reduction of iron oxides with hydrogen can be carried out in a conventional manner, for example as described in U.S. Pat. No. 3,758,584, with mixtures of hydrogen and ammonia at from 300 to 600xc2x0 C. or, as described in U.S. Pat. No. 4,480,051, in three steps, a first step of reducing the iron oxide with hydrogen or mixtures of hydrogen and ammonia, a second step of treating the resulting elemental iron with an oxygen-comprising gas, and then a third step of repeating the reduction of the first step.
U.S. Pat. No. 3,986,985 describes a deeper stabilization of reduced pyrophoric iron catalysts, for example in order that they may be transported. The original catalytic activity can be restored by a brief treatment of the stabilized catalyst with hydrogen.
The activation of the catalyst is advantageously carried out directly in the synthesis reactor, since this customarily dispenses with the otherwise necessary intermediary step, i.e., the passivation of the surface, customarily at from 20 to 80xc2x0 C., preferably at from 25 to 35xc2x0 C., by means of nitrogen-oxygen mixtures such as air. The activation of passivated catalysts is then preferably carried out in the synthesis reactor at from 180 to 500xc2x0 C., preferably at from 200 to 400xc2x0 C., in an atmosphere comprising hydrogen.
The catalysts may preferably be used as fixed bed catalysts in upflow or downflow mode or else as suspension catalysts.
The hydrogenation can be carried out batchwise, but is preferably carried out continuously using suspended, but preferably fixed bed, catalysts in the presence of ammonia.