WO2012143331A1

WO2012143331A1 - Catalyzed hydroxylamine preparation

Info

Publication number: WO2012143331A1
Application number: PCT/EP2012/056924
Authority: WO
Inventors: Marijke Hilde Leen GROOTHAERT; Johan Thomas Tinge
Original assignee: Dsm Ip Assets B.V.; Guit, Rudolf Philippus Maria
Priority date: 2011-04-22
Filing date: 2012-04-16
Publication date: 2012-10-26
Also published as: TW201247528A; CN103492314B; CN103492314A; KR20140024872A

Abstract

Method for preparing hydroxylamine in a continuous process, comprising hydrogenating nitrate in a reaction zone comprising a liquid phase, a gas phase and a heterogeneous hydrogenation catalyst, in which reaction zone hydroxylamine and nitrous oxide (N20) are formed, characterized in that the nitrous oxide concentration in the gas phase, is monitored and is maintained within a predetermined range by means of controlled addition of a promoter for the heterogeneous hydrogenation catalyst and/or controlled addition of further amounts of heterogeneous hydrogenation catalyst to the reaction zone; a method for preparing an oxime and a method of preparing a lactam comprising said method for preparing hydroxylamine.

Description

CATALYZED HYDROXY LAMINE PREPARATION

The invention relates to a method for preparing hydroxylamine in a continuous process, to a method for preparing an oxime, and to a method for preparing a lactam.

Hydroxylamine (hereinafter also referred to as ΉΥΑΜ") is a commonly used reagent in a myriad of organic and inorganic reactions. It is in particular suitable for use in the preparation of oximes, in particular cyclohexanone oxime, which may thereafter be converted into caprolactam via Beckmann

rearrangement. Beckmann rearrangement processes for the preparation of

caprolactam are generally known in the art, e.g. from Ullmann's Encyclopedia of Industrial Chemistry, for instance the 7^th edition (2005) (DOI:

10.1002/14356007.a05.031). Other oximes of which the preparation using

hydroxylamine has been described include cyclodedocecanone oxime (e.g. EP-A 1 ,329,448) and butanone oxime.

Methods of preparing hydroxylamine are also commonly known in the art. Further, various patent rights have been published on this subject. GB-A 1 ,287,303 and US 5,364,609, for example, relate to processes wherein nitrate is reduced in a phosphate buffer solution using molecular hydrogen.

The HPO^® cyclohexanone oxime process of DSM (see e.g. H.J.

Damme, J.T. van Goolen and A.H. de Rooij, Cyclohexanone oxime made without byproduct (NH₄)₂S0₄, July 10, 1972, Chemical Engineering; pp 54/55 or Ullmann's Encyclopedia of Industrial Chemistry (2005) at page 6/7 under chapter Caprolactam, makes use of two recycling liquors - an inorganic liquid and an organic liquid - in which several reactions and operations take place. The inorganic liquid is an aqueous phosphoric acid and ammonium nitrate solution which is fed to the hydrogenation reactor, where hydroxylamine is produced. Hydroxylamine is formed via reduction of nitrate ions with hydrogen, which is catalyzed by a heterogeneous hydrogenation catalyst (palladium-containing catalyst with carbon as carrier). In general, a promoter for the heterogeneous hydrogenation catalyst is added in order to improve the performance of the catalyst.

Gaseous hydrogen is contacted, in a gas-liquid reactor, with a circulating inorganic liquid containing nitrate ions, together with a buffering acid and the catalyst. The hydrogen containing gas phase is circulated over a bubble column type reactor by a circulation compressor. Fresh hydrogen is fed to the circulation gas, a small amount being withdrawn from the system to maintain a constant partial hydrogen pressure. Inert gaseous components in the fresh hydrogen and the produced gaseous by-products nitrogen (N₂) and nitrous oxide (N₂0) disappear via the gas purge.

The gas-liquid suspension is circulated by the Mammoth pump principle from the gassed reactor section, over gas-liquid separators, to the filter candles in the filtration section and via a heat exchanger for the removal of heat of reaction, back to the gassed reactor section.

The inorganic liquid obtained after filtration is then contacted in the oximation section with an organic liquid, being a mixture of toluene and cyclohexanone. Here the cyclohexanone is practically quantitatively converted into cyclohexanone oxime. The obtained cyclohexanone oxime-containing organic phase is distilled to recover the toluene.

The inorganic liquid leaving the oximation section has to be purified thoroughly to protect the catalyst in the hydroxylamine reactor. This is done by extraction with toluene, followed by stripping with steam. In the stripping column the water that is co-produced during the preparation of hydroxylamine and cyclohexanone oxime is also removed. A small amount of ammonia by-product remains in solution, but is prevented from building up by conversion to nitrogen in a nitrous gas (mixture of N0₂ and NO) absorber.

Finally, the consumed amount of nitrate should be compensated.

Nitrous gas required in the process is produced in an ammonia combustion unit.

W098/18717A1 describes a process for producing hydroxylamine by catalytic reduction of nitrate ions, and mentions the subsequent production of cyclohexanone oxime by reaction with cyclohexanone. It teaches that the selectivity of the palladium or platinum reduction catalyst can be enhanced by incorporation of a small quantity of halogen.

EP0773189A1 also describes such a process for producing, hydroxylamine, cyclohexanone oxime. Also, rearrangement of cyclohexanone to caprolactam is mentioned. It teaches that a nitrate reduction catalyst wherein the platinum and palladium concentrations are substantially the same, leads to improved selectivity.

EP1275616A1 also describes a process for producing hydroxylamine by catalytic reduction of nitrate ions. Efficiency of the system is improved by withdrawing gas mixture from the reaction, removing non-hydrogen compounds from this stream and recycling the hydrogen enriched phase to the reaction. US5155081 describes a platinum on graphite catalyst and its use as in a process for producing hydroxylamine, by reduction of nitrogen oxide gas, rather than from a nitrate solution. It recognizes that after time, selectivity of the catalyst deteriorates, as indicated by an increase in N₂0 in the off-gas.

Although the preparation of hydroxylamine from nitrate has been known for many decades and ways to improve known preparation methods have been investigated thoroughly over the years, presently known industrial processes, which are generally of a continuous nature, still suffer from drawbacks.

In particular, in known continuous processes a decrease in selectivity towards the conversion into hydroxylamine (hydroxylamine selectivity) in time is a problem. In addition, the heterogeneous hydrogenation catalyst slowly loses activity. This results in the need for increasing the feed of starting compounds (nitrate, hydrogen) into the reactor wherein hydroxylamine is formed in order to maintain the same hydroxylamine production rate. This is not only disadvantageous because valuable starting materials are lost without being converted into hydroxylamine, but also results in one or more unwanted by-products. Therefore there exists a need to compensate the decrease in selectivity and activity of the heterogeneous

hydrogenation catalyst in a hydroxylamine-producing reaction. Typically this could be done by adding heterogeneous hydrogenation catalyst and/or adding a promoter for that catalyst.

It is a further object of the present invention to provide a method for preparing hydroxylamine in a continuous process that may serve as an alternative to known methods, in particular a method that overcomes one or more drawbacks of a known method, such as that referred to above.

It is a further object to provide a method for preparing hydroxylamine that produces less by-product(s), especially less ammonium, that has to be converted to nitrogen with expensive nitrous gas, and water that has to be removed via evaporation, compared to a conventional process operated in the same production facility.

More in particular, it is an object of the invention to prepare hydroxylamine with improved hydroxylamine selectivity while maintaining a target hydroxylamine production rate compared to a conventional process operated in the same production facility. This will benefit in a more environmentally friendly process, and a cheaper process compared to a conventional process operated in the same production facility. One or more further objects may become apparent from the remainder of the description.

It has now surprisingly been found possible to address one or more of said objects by adding hydrogenation catalyst or promoter in a specifically controlled manner, in a continuous process wherein hydroxylamine is prepared.

Accordingly, the present invention provides a method for preparing hydroxylamine in a continuous process, comprising hydrogenating nitrate in a reaction zone comprising a liquid phase, a gas phase and a heterogeneous hydrogenation catalyst, in which reaction zone hydroxylamine and nitrous oxide (N₂0) are formed, characterized in that the nitrous oxide concentration in the gas phase, is monitored and is maintained within a predetermined range by means of controlled addition of a promoter for the heterogeneous hydrogenation catalyst and/or controlled addition of further amounts of heterogeneous hydrogenation catalyst to the reaction zone.

It has been found that by controlling the addition of promoter and/or hydrogenation catalyst in the course of the continuous process based on the nitrous oxide concentration in the gas phase in the reaction zone, it is possible to produce hydroxylamine with good selectivity while maintaining hydroxylamine production rate for a prolonged time compared to a method according to the prior art, as mentioned above.

Further, advantageously a method according to the invention requires a lower amount of starting compounds (nitrate, hydrogen).

Further, it has been found possible to reduce the amount of unwanted by-product(s), for example ammonia, in accordance with the invention.

It has been found that compared to a method wherein the

concentration of N₂0 in the gas phase is not measured, the drop of selectivity over time of the hydrogenation of nitrate towards hydroxylamine producing reaction can be reduced.

It has further been found that a method according to the invention is advantageous with respect to the carbon footprint of the preparation of hydroxylamine, making e.g. the HPO^® process even better than before.

The term "or" as used herein means "and/or" unless specified other wise.

The term "a" or "an" as used herein means "at least one" unless specified otherwise.

When referring to a 'noun' (e.g. a compound, an additive etc.) in singular, the plural is meant to be included, unless specified otherwise.

As used herein, 'hydroxylamine selectivity' (the selectivity towards the production of hydroxylamine) is defined as follows: molar ratio of the amount of hydroxylamine produced in the reaction zone divided by half the amount of H⁺ consumed in the reaction zone.

The activity of the catalyst is defined herein as grams hydroxylamine produced per gram catalyst per hour. In other words activity is related to the rate of production of hydroxylamine.

As used herein weight of catalyst refers to the dry weight of the catalyst including the weight of any support that the active catalytic material is on.

The preparation of hydroxylamine can suitably be carried out in a known continuous reactor for preparing hydroxylamine. In an embodiment, the reaction is carried out in a reactor providing well mixed gas/liquid systems. Such systems are well-known in the art and include stirred tank reactors, internal loop reactors, external loop reactors, and bubble column reactors. In a preferred embodiment, a bubble column reactor is used. Good results have in particular been achieved using a bubble column reactor with external gas-lift.

The concentration of the substance or a concentration derived parameter (for example a ratio of concentrations of substances) in a liquid phase in the reaction zone can be determined by determining that parameter in a sample taken from the process liquid leaving the reaction zone.

The hydrogenation catalyst used in accordance with the invention can in principle be any catalyst suitable for catalyzing the preparation of hydroxylamine from nitrate. Various catalysts are known in the art. In particular the hydrogenation catalyst may be a metal catalyst. Preferably, the heterogeneous hydrogenation catalyst is a catalyst comprising palladium or a catalyst comprising platinum, preferably a catalyst comprising palladium. In particular, good results have been achieved with a catalyst comprising palladium. The catalyst comprising palladium may comprise a minor amount of one or more other catalytic substances capable of catalyzing the hydrogenation of nitrate. If a catalyst comprising palladium is used, it usually comprises less than 2 wt. %, in particular less than 0.2 wt. %, more in particular 0.02 wt. % or less of other metals based on the weight of the entire catalyst including any carrier. An example of such a metal that in particular may be present is platinum. The catalyst is usually provided on a carrier. In particular, the heterogeneous hydrogenation catalyst comprises a carbon carrier. Good results have been achieved with a catalyst comprising palladium on a carbon carrier.

Typically, the catalyst is provided with a promoter for the catalyst. Typically a promoter is selected from the group of germanium(IV)oxide, cadmium oxide, indium oxide and tin(IV)oxide. Preferably it is germanium(IV)oxide. The promoter can be added at any time during the method of preparing hydroxylamine, as desired.

Promoters for hydrogenation catalysts (which may also be referred to as activators) increase the activity of a hydrogenation catalyst, although they are generally not catalytically active themselves. In principle any promoter suitable to increase the activity of the hydrogenation catalysts may be used.

In accordance with the invention, promoter or heterogeneous hydrogenation catalyst is continuously or intermittently added to the reaction zone, as mentioned above. Adding promoter is in particular advantageous for increasing the hydroxylamine production rate in terms of product rate per kg catalyst in the reaction zone, as it increases the activity of the catalyst already present in the reaction zone. Adding hydrogenation catalyst is in particular useful for increasing the hydroxylamine production rate without impairing the selectivity. If desired, promoter and

heterogeneous hydrogenation catalyst can be added simultaneously. Preferably, catalyst is added together with promoter, whereas promoter may also advantageously be added without catalyst.

For a carbon-supported palladium catalyst a ratio by weight of from 0.001 to 0.01 Ge0₂: Pd/C is typical. Preferably the ratio is from 0.002 to 0.008; more preferably from 0.004 to 0.006.

In a preferred embodiment a method is provided, wherein (a) the temperature at which the hydrogenation is carried out is increased from a first temperature to a second temperature, thereby maintaining a target hydroxylamine production rate, (b) adding promoter and/or heterogeneous hydrogenation catalyst, then (c) reducing the temperature to a third temperature below the second

temperature, thereby maintaining the hydroxylamine production rate, and (d) then repeating (a), (b) and (c) a plurality of times.

The adjustment of temperature can be done gradually or stepwise (e.g. in increments of about from 0.5 to 2 °C), in order to keep the production rate of hydroxylamine at a satisfactory level over time. The plurality of times is not critical; it depends on the period of the production run. It may be 2 or more, 10 or more, or 25 or more. It may be 10000 or less, 1000 or less, 100 or less or 50 or less.

An embodiment wherein temperature is adjusted is in particular advantageous for producing hydroxylamine at a high selectivity, whilst maintaining hydroxylamine production rate.

In a particularly preferred method, if the hydroxylamine production rate becomes too low to maintain the target hydroxylamine production rate, the following adjustments are made in chronological order:

1) if the temperature is below a predetermined maximum temperature, then the temperature in the reaction zone is increased to a higher temperature, thereby increasing the production rate;

2) if the nitrous oxide concentration is within the predetermined range, then

promoter is added, thereby increasing the production rate;

3) if the temperature has reached the predetermined maximum temperature and nitrous oxide concentration exceeds the predetermined range, then add catalyst (usually in combination with promoter);

4) if the production rate becomes higher than the target production rate, reduce the temperature.

Such embodiment is in particular advantageous for producing hydroxylamine at a high selectivity, whilst maintaining hydroxylamine production rate. Also, it has been found that in such a method generally less promoter is used compared to a comparable method according to the invention wherein the nitrous oxide concentration is kept constant.

The hydroxylamine is usually prepared at a temperature in the range of from 20 to 70 °C, preferably in the range of from 30 to 60 °C, in particular in the range of from 35 to 55 °C. Thus, a predetermined minimum or maximum temperature as referred to herein may in particular be chosen within any of these ranges, with the proviso that a predetermined maximum temperature usually is at least 35 °C, preferably in the range of from 40 to 60 °C, in particular in the range of from 45 to 55 °C.

In further preferred embodiments, methods are provided, wherein, in addition to or instead of steps (a) to (d) above, (e) the hydrogen partial pressure at which the hydrogenation is carried out is increased from a first pressure to a second pressure, thereby maintaining the hydroxylamine production rate, until the second hydrogen partial pressure is reached, (f) adding promoter and/or heterogeneous hydrogenation catalyst, then (g) decreasing the hydrogen partial pressure to a third pressure below the second to maintain the target hydroxylamine production rate after adding the promoter and/or catalyst, and then (h) repeating (e), (f) and (g) a plurality of times.

The adjustment of hydrogen partial pressure can be done gradually or stepwise (e.g. in increments of from about 0.5 to 2 MPa per step), in order to keep the production rate of hydroxylamine at a satisfactory level over time. The plurality of times is not critical; it depends on the period of the production run. It may be 2 or more, 10 or more, or 25 or more. It may be 10000 or less, 1000 or less, 100 or less or 50 or less.

Hydrogen can be fed into the reaction zone in a manner and at a concentration (hydrogen partial pressure) known per se. Preferably, the pressure is at least approximately 0.5 MPa, more preferably at least approximately 1.0 MPa. Usually, the hydrogen pressure is approximately 1000 MPa or less. Accordingly, in an embodiment wherein the hydrogen partial pressure is adjusted during the preparation of hydroxylamine, a hydrogen partial pressure is generally chosen within the range of from 0.5 to 10 MPa, in particular in the range of from 0.8 to 2.5 MPa.

Preferably, in any repetition of step (b) or (f) either heterogeneous hydrogenation catalyst or promoter or both heterogeneous hydrogenation catalyst and promoter are added. However, more preferably the method comprises a method step wherein promoter and heterogeneous hydrogenation catalyst are added

simultaneously.

In one preferred embodiment the selectivity of the hydrogenation of nitrate towards hydroxylamine is maintained within a predetermined range, by increasing the ratio of promoter to heterogeneous hydrogenation catalyst in the reaction zone if a predetermined minimum production rate is reached and by decreasing the ratio of promoter to heterogeneous hydrogenation catalyst in the reaction zone if a predetermined minimum selectivity is reached.

Preferably, the weight to weight ratio of promoter to catalyst is maintained in the range of from 2 to 7 mg/g, in particular in the range of from 2.5 to 6 mg/g more in particular in the range of from 3 to 5 mg/g.

The nitrous oxide (N₂0) concentration can generally be monitored and kept within a predetermined range adequately by determining the N₂0

concentration in the gas phase purged from the reaction zone.

The monitoring of the nitrous oxide concentration in the gas phase can be done using a detector for nitrous oxide in gases, as are known in the art perse. When the concentration of nitrous oxide is outside the predetermined range, promoter and/or heterogeneous hydrogenation catalyst are added into the reaction zone. This may be done manually, or in an automated manner. In case of an automated manner, if the predetermined range is exceeded, a controller unit will signal a dosing unit to dose promoter or catalyst.

The promoter or catalyst may be added continuously. It is preferred to add promoter and/or catalyst intermittently. In case both promoter and catalyst are added in the course of the method, they may be added simultaneously or in an essentially alternating manner. Alternating additions includes one or more subsequent additions of promoter (each generally triggered by reaching a predetermined maximum N₂0 concentration), followed by one or more additions of catalyst (each generally triggered by reaching a predetermined maximum of the N₂0 concentration), followed by one or more subsequent additions of promoter (each generally triggered by reaching a predetermined maximum of the N₂0 concentration).

Usually, the nitrous oxide concentration in the gas phase is maintained at from 0 to 1 volume %, in particular at from 0.01 to 0.5 volume %, more in particular at from 0.05 to 0.3 volume %, most particularly at from 0.08 to 0.2 volume %. Volume % of nitrous oxide is based on the total volume of the gas phase.

Nitrous oxide concentration is determined from the gas stream leaving the reactor, and is well represented by the figure so measured. This is not necessarily identical to the concentration of nitrous oxide in the reactor. In practice some of the exit gas may be recycled such that nitrous oxide is recirculated back into the reactor.

Advantageously, if catalyst is added, in particular a catalyst comprising palladium on a carbon carrier, preferably an activated carbon carrier, the amount of catalyst added in any step is any suitable range, for example, the range of from 1 to 100 kg catalyst per dosage, for example in the range of from 10 to 90 kg catalyst per dosage, more in particular from 20 to 80 kg catalyst, e.g. about 50 kg catalyst per dosage. Advantageously, if promoter is added, in particular Ge0₂, the amount of promoter added at an occasion at which the predetermined maximum nitrous oxide concentration is exceeded is in the range of from 10 to 1000 g promoter per dosage, in particular in the range of from 50 to 500 g promoter per dosage, more in particular from 75 to 300 g promoter, e.g. about 140 g promoter per dosage.

Advantageously, on average, the weight to weight ratio of added promoter to added catalyst added overall in a method according to the invention during a single run time is in the range of from 0.5 to 50 mg promoter per g catalyst, in particular from 1 to 20 mg promoter per g catalyst, more in particular from 2 to 10 mg promoter per g catalyst. In particular, for a catalyst comprising Pd and Ge0₂ as promoter, said ratio preferably is in the range of from 2 to 8 mg/g, in particular in the range of from 2.5 to 7 mg/g more in particular in the range of from 3 to 6 mg/g.

In accordance with the invention the nitrate concentration can be chosen within a wide range. Usually, the nitrate concentration in the reaction zone is 2 mol/kg or less, in particular 1.0 mol/kg or less, as determined in the liquid leaving the reaction zone. In a preferred embodiment, the nitrate concentration is 0.9 mol/kg or less, in particular 0.8 mol/kg or less. Particularly good results have been achieved with a nitrate concentration of about 0.70 or less. Usually, the nitrate concentration is at least 0.3 mol/kg, in particular at least 0.4 mol/kg. Preferably, the nitrate concentration is at least 0.45 mol/kg, more preferably at least 0.50 mol/kg.

Typically, the hydrogenation is carried out in a phosphate buffer solution. The phosphate is usually provided as phosphoric acid or a hydrogen phosphate salt (which may be formed by adjusting the pH of a phosphoric acid solution with an appropriate base, such as a hydroxide or ammonia). The molar ratio of nitrate to phosphate usually is at least 0.05, preferably at least 0.10. Excellent results are achieved at values of at least 0.15, more in particular of at least 0.20. The molar ratio of nitrate to phosphate preferably is 0.40 or less, in particular 0.35 or less, more in particular 0.30 or less.

Hydrogen can be fed into the reaction zone in a manner and at a concentration (hydrogen pressure) known per se. Preferably, the pressure is at least approximately 0.5 MPa, more preferably at least approximately 1.0 MPa. Usually, the hydrogen pressure is 10 MPa or less.

In an advantageous embodiment, the buffer ratio is chosen within a specific range. Herein the buffer ratio is defined as:

([H⁺] + [HYAM])/[phosphate] wherein:

[H⁺] = molar concentration of H⁺ in mol/kg in the aqueous liquid leaving the reaction zone;

[HYAM] = hydroxylamine concentration in mol/kg in the aqueous liquid leaving the reaction zone; and [phosphate] = total concentration of phosphate (including phosphate in H₃P0₄, monohydrogen phosphate and dihydrogen phosphate) in mol/kg, in the aqueous liquid leaving the reaction zone.

[H⁺], [HYAM] and [phosphate] concentrations are all determined by equilibrium titration of one sample, by subsequently titrating a sample of the liquid from the reaction zone at 25 °C with 0.25 N aqueous NaOH solution) to get the [H⁺] concentration ("free acid") at the first equilibrium point (at a pH of about 4.2); next molar excess of acetone is added to the sample, to convert hydroxylamine into an oxime and H⁺, and equilibrium titration is continued so as to subsequently reach three further equivalence points, the first of which is corresponding to the free acid coming from hydroxylamine (and thus provides the value for [HYAM] in the sample); the second of which provides the value for [phosphate], and the last of which provides a value for ammonium. The latter value, however, is not needed here.

In particular for a low risk of crystallization taking place, also at a relatively high production capacity, the buffer ratio preferably is in the range of from 0.4 to 0.8, in particular in the range of from 0.45 to 0.70, more in particular in the range of from 0.50 to 0.65 mol/mol.

The molar H⁺ concentration during the reaction is usually in the range of from 0.1 to 1 mol/kg, in particular in the range of from 0.4 to 0.8 mol/kg, more in particular in the range of from 0.50 to 0.65 mol/kg.

The present invention further provides a method for preparing an oxime, comprising reacting hydroxylamine obtained in a method according to the invention with an alkanone, in particular an alkanone selected from the group of cyclohexanone, cyclododecahexanone and butanone.

A (cyclic) oxime obtained in accordance with the invention, may in particular be used for preparing a lactam. This can be accomplished by Beckmann rearrangement, in a manner known per se.

Thus cyclohexanone oxime obtained in accordance with the invention may be used in the preparation of caprolactam. Accordingly, the invention is further directed to a method for preparing caprolactam, comprising subjecting cyclohexanone oxime, obtained in a method according to the invention to Beckmann rearrangement, thereby forming caprolactam. The preparation of caprolactam may be carried out in a manner known per se, e.g. as described in the above identified prior art, of which the contents are incorporated herein by reference with respect to suitable conditions.

Likewise, laurolactam is obtained in an embodiment of the invention by a method comprising subjecting cyclododecanone oxime, obtained in accordance with the invention, to Beckmann rearrangement, thereby forming laurolactam. This preparation step may also be carried out in a manner known per se.

The method for preparing a (cyclic) oxime, for example

cyclohexanone oxime and, if desired, the method for making a lactam therefrom, for example caprolactam, are usually integrated in a single plant wherein hydroxylamine, oxime and, if desired, lactam are prepared in a continuous process.

The method is illustrated by, but not limited to, the following examples.

Comparative Example

Hydroxylamine production was carried out in an existing DSM HPO^® plant comprising a gas lift loop reactor with a gassed riser section, a liquid-gas separation section, a gas recycle section, a filtration section to separate part of the circulating liquid as an aqueous product solution from the heterogeneous

hydrogenation catalyst-containing reactor liquid. Concentrations of the aqueous process liquid (aqueous process liquid leaving the reaction zone of the nitrate hydrogenation reactor) were determined at the outlet for said aqueous product solution immediately after the filtration section, where they closely correspond to the concentrations in the reaction zone. Fresh hydrogen was fed to the reactor and a gas purge was applied such that the hydrogen partial pressure was maintained at approximately 1.5 MPa in the top section of the reactor. The average reactor temperature was maintained at approximately 53 °C. As heterogeneous hydrogenation catalyst 10 wt% Pd/C (activated carbon) was used. As promoter Ge0₂ was used. The target hydroxylamine production rate is maintained at a level of approximately 46,500 ton hydroxylamine per year.

The composition of the aqueous process liquid leaving the reaction zone of the nitrate hydrogenation reactor was maintained at:

Nitrate concentration: approximately 1.00 mol/kg solution Total phosphate concentration: approximately 3.10 mol/kg solution

Hyam concentration: approximately 1.30 mol/kg solution

Ammonia concentration: approximately 2.20 mol/kg solution

H⁺ concentration: approximately 0.60 mol/kg solution After start-up of the nitrate hydrogenation reactor, the catalyst hold up was 800 kg Pd/C (calculated as dry weight of catalyst) and at the start the amount of promoter supplied was 2.24 kg Ge0₂.

After start-up of the nitrate hydrogenation reactor, the selectivity of the hydrogenation of nitrate towards hydroxylamine after start-up of the nitrate hydrogenation reactor was calculated to be approximately 85.9 %.

During this production run batches of 140 gram Ge0₂ each were added to the nitrate hydrogenation reactor when the targeted hydroxylamine production rate could not be reached anymore and the ratio Ge0₂ to catalyst remained below 9 grams Ge0₂ per kg Pd/C catalyst (calculated as dry weight of catalyst) after each such addition. However, as soon as the ratio Ge0₂ to catalyst became higher than 9 grams Ge0₂ per kg Pd/C catalyst (calculated as dry weight of catalyst) and the target hydroxylamine production rate could not be reached anymore, then a batch of 50 kg Pd/C catalyst (calculated as dry weight of catalyst) together with 140 gram Ge0₂ was added to the nitrate hydrogenation reactor.

After a period of 12 months the production run was stopped. At that moment, the selectivity of the hydrogenation of nitrate towards hydroxylamine, had dropped to approximately 79.8 %. The total amount of catalyst added to the nitrate hydrogenation reactor, including the amount of catalyst added during start-up, was 2550 Pd/C (calculated as dry weight of catalyst) and the total amount of promoter supplied, including the amount of promoter added during start-up, was 22.96 kg Ge0₂.

The average decay of the selectivity of the hydrogenation of nitrate towards hydroxylamine was during this production run approximately 0.5 % per month.

This comparative example when compared with the Examples shows that hydroxylamine preparation without monitoring N₂0 concentration in the gas phase leads to a substantial decay of the selectivity of the hydrogenation of nitrate towards hydroxylamine.

Example 1

Hydroxylamine production was carried out in the same DSM HPO^® plant as described in the Comparative Example. All process conditions were the same as in the Comparative Example unless mentioned explicitly. Again, as heterogeneous hydrogenation catalyst 10 wt% Pd/C was used and as promoter Ge0₂ was used. The target hydroxylamine production rate was again approximately 46,500 ton

hydroxylamine per year. After start-up of the nitrate hydrogenation reactor, the hold up of catalyst was 800 kg (calculated as dry weight) and at the start the amount of promoter supplied was 2.24 kg Ge0₂.

After start-up of the nitrate hydrogenation reactor, the selectivity of the hydrogenation of nitrate towards hydroxylamine was calculated to be approximately 86.0 %.

During this production run the nitrous oxide concentration in the gas phase (the gas leaving the top of the hydrogenation reactor) was used as tool to determine the amount of promoter and/or promoter and catalyst that were to be added batch-wise.

During this production run batches of 140 gram Ge0₂ each were added to the nitrate hydrogenation reactor when the target hydroxylamine production rate could not be reached anymore and the nitrous oxide concentration in the gas phase remained below 0.3 vol.% after each such addition. However, as soon as the nitrous oxide concentration in the gas phase became higher than 0.3 vol.% and the target hydroxylamine production rate could not be reached anymore, then a batch of 50 kg Pd/C catalyst (calculated as dry weight of catalyst) together with 140 gram Ge0₂ was added to the nitrate hydrogenation reactor.

After a period of 18 months the production run was stopped. At that moment, the selectivity of the hydrogenation of nitrate towards hydroxylamine, had dropped to approximately 80.5 %. The total amount of catalyst added to the nitrate hydrogenation reactor, including the amount of catalyst added during start-up, was 2800 Pd/C (calculated as dry weight of catalyst) and the total amount of promoter supplied, including the amount of promoter added during start-up, was 19.74 kg Ge0₂.

The average decay of the selectivity of the hydrogenation of nitrate towards hydroxylamine was during this production run approximately 0.3 % per month.

Example 1 shows that hydroxylamine preparation with monitoring N₂0 concentration in the gas phase and keeping the N₂0 concentration in a certain range leads to less decay of the selectivity of the hydrogenation of nitrate towards hydroxylamine.

Example 2

Example 1 was repeated but with the following differences. After start-up of the nitrate hydrogenation reactor, the selectivity of the hydrogenation of nitrate towards hydroxylamine was calculated to be approximately 85.7 %.

During this production run batches of 140 gram Ge0₂ each were added to the nitrate hydrogenation reactor when the target hydroxylamine production rate could not be reached anymore and the nitrous oxide concentration in the gas phase remained below 0.2 vol.% after each such addition. However, as soon as the nitrous oxide concentration in the gas phase became higher than 0.2 vol.% and the target hydroxylamine production rate could not be reached anymore, then a batch of 50 kg Pd/C catalyst (calculated as dry weight) together with 140 gram Ge0₂ was added to the nitrate hydrogenation reactor.

After a period of production of 10 months the production run was stopped. At that moment, the selectivity of the hydrogenation of nitrate towards hydroxylamine had dropped to approximately 83.6 %. The total amount of catalyst added to the nitrate hydrogenation reactor, including the amount of catalyst added during start-up, was 2900 Pd/C (calculated as dry weight) and the total amount of promoter supplied, including the amount of promoter added during start-up, was 14.56 kg Ge0₂.

The average decay of the selectivity of the hydrogenation of nitrate towards hydroxylamine was during this production run approximately 0.2 % per month.

Example 2 shows that hydroxylamine preparation with monitoring N₂0 concentration in the gas phase and keeping the N₂0 concentration in a certain range leads to less decay of the selectivity of the hydrogenation of nitrate towards hydroxylamine.

Claims

Method for preparing hydroxylamine in a continuous process, comprising hydrogenating nitrate in a reaction zone comprising a liquid phase, a gas phase and a heterogeneous hydrogenation catalyst, in which reaction zone hydroxylamine and nitrous oxide (N₂0) are formed, characterized in that the nitrous oxide concentration in the gas phase, is monitored and is maintained within a predetermined range by means of controlled addition of a promoter for the heterogeneous hydrogenation catalyst and/or controlled addition of further amounts of heterogeneous hydrogenation catalyst to the reaction zone.

Method according to claim 1 , wherein the nitrous oxide concentration in the gas phase is maintained at from 0 to 1 volume %, in particular at from

0.01 to 0.5 volume %, more in particular at from 0.05 to 0.3 volume %, most particularly at from 0.08 to 0.2 volume %.

Method according to claim 1 or 2, wherein the heterogeneous hydrogenation catalyst is a catalyst comprising palladium or a catalyst comprising platinum, preferably a catalyst comprising palladium.

Method according to any of the preceding claims, wherein the heterogeneous hydrogenation catalyst comprises a carbon carrier.

Method according to any of the preceding claims, wherein the promoter is selected from the group of germanium(IV)oxide, cadmium oxide, indium oxide and tin(IV)oxide, preferably germanium(IV)oxide.

Method according to any of the preceding claims, wherein the promoter and/or heterogeneous hydrogenation catalyst is added intermittently.

Method according to any of the preceding claims, wherein (a) the temperature at which the hydrogenation is carried out is increased from a first temperature to a second temperature, thereby maintaining a target hydroxylamine production rate, (b) adding promoter and/or heterogeneous hydrogenation catalyst, then (c) reducing the temperature to a third temperature below the second temperature, thereby maintaining the hydroxylamine production rate, and (d) then repeating (a), (b) and (c) a plurality of times.

Method according to any preceding claim, wherein (e) the hydrogen partial pressure at which the hydrogenation is carried out is increased from a first pressure to a second pressure, thereby maintaining the hydroxylamine production rate, until the second hydrogen partial pressure is reached, (f) adding promoter and/or heterogeneous hydrogenation catalyst, then (g) decreasing the hydrogen partial pressure to a third pressure below the second to maintain the target hydroxylamine production rate after adding the promoter and/or catalyst, and then (h) repeating (e), (f) and (g) a plurality of times. Method according to claim 7 or claim 8, wherein in any repetition of step (b) or (f) either heterogeneous hydrogenation catalyst or promoter or both heterogeneous hydrogenation catalyst and promoter are added.

Method according to any of the preceding claims, comprising a method step wherein promoter and heterogeneous hydrogenation catalyst are added simultaneously.

Method according to any of the preceding claims, wherein the selectivity of the hydrogenation of nitrate towards hydroxylamine is maintained within a predetermined range, by increasing the ratio of promoter to heterogeneous hydrogenation catalyst in the reaction zone if a predetermined minimum production rate is reached and by decreasing the ratio of promoter to heterogeneous hydrogenation catalyst in the reaction zone if a predetermined minimum selectivity is reached.

Method according to any of the preceding claims, wherein the nitrous oxide concentration in the gas phase is monitored in the gas phase purged from the reaction zone.

Method for preparing an oxime, comprising a method defined in any one of claims 1 to 12, followed by reacting the hydroxylamine with an alkanone, in particular an alkanone selected from cyclohexanone, cyclododecahexanone and butanone.

Method for preparing a lactam, comprising a method defined in claim 13, wherein the alkanone is a cycloalkanone and the oxime is a cycloalkanone oxime, followed by subjecting the cycloalkanone oxime to Beckmann rearrangement.

Method according to claim 14, wherein the cycloalkanone oxime is

cyclohexanone oxime and the lactam is caprolactam; or wherein the cycloalkanone oxime is cyclododecanone oxime and the lactam is

laurolactam.