Process for preparing substantially nitrosamine-free, peroxide-free, substantially colorless amine oxides

Abstract

Disclosed is a method for preparing a substantially nitrosamine-free, peroxide-free, substantially colorless amine oxide solution, the method comprising reacting a tertiary amine capable of forming an amine oxide with a less than stoichiometric amount of an aqueous hydrogen peroxide solution in an aqueous medium, a vapour space having a partial pressure p (CO2) of carbon dioxide of less than 0.75 bar absolute is applied on the aqueous medium at the same time. The process allows to obtain an amine oxide that is substantially free of nitrosamines, substantially free of peroxides, and substantially colorless. So far, in processes including only oxidation and distillation, a unique combination of such desirable characteristics has not been achieved.

Description

[0001] The present invention relates to a process for preparing a substantially nitrosamine-free, substantially peroxide-free, substantially colorless amine oxide.

[0002] Amine oxides are often used as oxidants, solvents or in consumer products such as shampoos, conditioners, detergents and hard surface cleaners. An industrially important representative of amine oxides is N-methylmorpholine N-oxide. Aqueous solutions of N-methylmorpholine N-oxide are technically used as solubilizers in the production of lyocell fibers, wherein the stock is dissolved in a solution of N-methylmorpholine N-oxide and water and spun from the solution.

[0003] Typically, amine oxides are produced by oxidation of tertiary amines, for example by using hydrogen peroxide as an oxidant. The methods known in the art typically use an excess of an oxidant (e.g. hydrogen peroxide) and are therefore often accompanied by disadvantages, such as contamination of the amine oxide product by unreacted oxidant (e.g. hydrogen peroxide).

[0004] Nitrosamines are formed as minor by-products when amine oxides are prepared using aqueous hydrogen peroxide. Although the amount of nitrosamines is very small, this small amount makes amine oxides unsuitable for many applications involving human contact. This is because nitrosamines are reported to be carcinogenic and / or mutagenic.

[0005] B. Balagam et al., Inorg. Chem. 2008, 47, 1173–1178, relates to the N-oxidation of amines with hydroperoxide and describes the use of CO in a round-bottom flask. 2 and H 2 O 2 N-Methylmorpholine oxidation.

[0006] T. Baumeister et al., React. Chem. Eng. 2019, 4, 1270–1276, relates to the continuous flow synthesis of amine oxides by oxidation of tertiary amines and describes the use of H in a microstructured reactor under flow conditions. 2 O 2 N-Methylmorpholine oxidation.

[0007] EP 0 307 184 describes a process for the oxidation of tertiary amines with hydrogen peroxide to form tertiary amine oxides. For this purpose, the tertiary amine is contacted with an aqueous hydrogen peroxide solution in the presence of a promoter formed by adding carbon dioxide to the reaction mixture. The reaction is carried out at a temperature below about 45° C., whereby the nitrosamine content of the tertiary amine oxide product is significantly reduced compared to the nitrosamine content of tertiary amine oxides produced by the same process at higher temperatures.

[0008] US 5,693,796 describes a method for reducing the nitrosamine content of N-methylmorpholine N-oxide. A catalytic amount of a carboxylic acid or a carboxylate salt thereof selected from formic acid, acetic acid, propionic acid, butyric acid and benzoic acid is added to an aqueous solution of N-methylmorpholine and oxidized with hydrogen peroxide.

[0009] EP 0 320 694 describes a process for preparing amine oxides which are substantially free of nitrosamines, the process comprising reacting a tertiary amine capable of forming an amine oxide with an aqueous hydrogen peroxide solution in the presence of a synergistic combination of ascorbic acid and a promoter formed by adding carbon dioxide to the reaction mixture. Excess hydrogen peroxide is preferred; such excess hydrogen peroxide remaining after the reaction is preferably 2 O 2 It must be destroyed by adding a reducing agent or a peroxide decomposition catalyst such as manganese dioxide.

[0010] US 5,216,154 describes the preparation of N-methylmorpholine N-oxide by contacting N-methylmorpholine with a molar excess of hydrogen peroxide in an atmosphere consisting essentially of carbon dioxide. The process uses carbon dioxide as a promoter to form N-methylmorpholine N-oxide containing very small amounts of nitrosamines.

[0011] US 6,166,255 describes the oxidation of tertiary amines with aqueous hydrogen peroxide in an aqueous reaction medium at about 15° C. to 25° C. using carbon dioxide as a catalyst. While stirring the reaction mixture, the temperature is allowed to rise adiabatically to about 50° C. to 100° C. In this manner, tertiary amine oxides having very low levels, if any, of nitrosamine impurities (e.g., less than 30 ppb) can be produced without the addition of metals and / or phosphorus-containing components.

[0012] As described in the above-mentioned references, by using carbon dioxide as a promoter, the problems regarding the undesirable nitrosamine content have been eliminated, or at least they have been reduced to acceptable levels. However, in addition to the nitrosamine contamination, there is also the problem of the formation of colored substances as reaction by-products, which can have a negative impact on the product quality even in low concentrations. The dark yellow-brown amine oxide solution thus obtained is not suitable for all applications unless additional purification steps are taken.

[0013] US 5,502,188 describes a process for producing an aqueous solution of N-methylmorpholine N-oxide having an APHA number below 200 based on an N-methylmorpholine N-oxide content of 50% by weight. For this purpose, an aqueous N-methylmorpholine solution having a water content of at least 35% by weight is oxidized with hydrogen peroxide. After the reaction, the concentration of the aqueous N-methylmorpholine N-oxide solution is adjusted by distilling off residual N-methylmorpholine and water. However, there is still a need to further reduce colored substances.

[0014] It was therefore an object of the present invention to provide an improved process for the preparation of amine oxides which results in substantially nitrosamine-free, substantially peroxide-free and substantially colourless amine oxides.

[0015] This object has been solved by a process for preparing an essentially nitrosamine-free, essentially peroxide-free, essentially colorless amine oxide solution. The process comprises reacting a tertiary amine capable of forming an amine oxide with a less than stoichiometric amount of an aqueous hydrogen peroxide solution in an aqueous medium, while applying to the aqueous medium a carbon dioxide partial pressure p(CO 2 ) of less than 0.75 bar absolute, preferably less than 0.20 bar absolute. 2 ) vapor space.

[0016] Although carbon dioxide is an effective promoter for the conversion of tertiary amines to their amine oxides, applicants have discovered that free carbon dioxide in amounts exceeding a promoting amount may result in undesirable discoloration. By the process of the present invention, it is possible that the amount of carbon dioxide (regardless of the form in which it is present) in the aqueous medium is not greater than the promoting amount of carbon dioxide. Without wishing to be bound by theory, it is believed that the promoting amount of carbon dioxide is present as peroxycarbonate or other adducts of hydrogen peroxide and carbonate. "Free" carbon dioxide species, i.e., excess carbon dioxide species that are not converted to peroxycarbonate by available hydrogen peroxide, is believed to be responsible for or at least contribute to the above-mentioned discoloration problem.

[0017] The process of the invention comprises reacting a tertiary amine capable of forming an amine oxide with an aqueous hydrogen peroxide solution in an aqueous medium. Typically, the aqueous solution of the tertiary amine is placed in a reaction vessel and the aqueous hydrogen peroxide solution is metered in over a variable time span.

[0018] The method of the present invention is applicable to any tertiary amine capable of forming an amine oxide. These are well known to the skilled person. They include amines having no hydrogen atoms bonded to the amine nitrogen atom. Such amines include trialkylamines; triarylamines; triarylalkylamines; mixed alkyl-aryl, alkyl-arylalkyl, aryl-arylalkyl or alkyl-aryl-arylalkylamines; tricycloalkylamines; alkyl-cycloalkylamines; aryl-cycloalkylamines; cyclic amines; for example: butyldimethylamine, hexyldimethylamine, isobutyldimethylamine, 2-ethylhexyldimethylamine, octyldimethylamine, decyldimethylamine, dodecyldimethylamine, tetradecyldimethylamine, hexadecyldimethylamine, eicosyldimethylamine, docosyldimethylamine, triacontyldimethylamine, tributylamine, butyldiethylamine, isobutyldiethylamine, decylbutylethylamine, hexadecylhexylmethylamine, eicosyldibutylamine, trioctylamine, trilaurylamine, di-dimethyl ... Dodecylethylamine, ditriacontylmethylamine, N,N-dimethylaniline, N-methyl-N-dodecylaniline, cyclopentyldimethylamine, cyclohexyldimethylamine, dicyclohexylmethylamine, cyclododecyldimethylamine, diphenylbutylamine, p-tolyldiethylamine, α-naphthylbutylmethylamine, benzylbutylmethylamine, α-methylbenzylbutylmethylamine, 4-butylbenzyloctylmethylamine, dibenzylbutylamine, 4-pentyl-benzyldibutylamine, N-butylmorpholine, N-methylmorpholine, N-methylpiperidine, N-dodecylpiperidine, N-octadecylpiperidine, N-triacontylpiperidine, N-methylpiperazine, N-butylpiperazine, N-octylpiperazine, N-phenylpiperidine, N-benzylpiperidine, N-cyclohexylpiperidine, pyridine, etc. In view of the high industrial demand for N-methylmorpholine N-oxide, an important aspect of the present invention is the embodiment in which the tertiary amine is N-methylmorpholine.

[0019] In an embodiment, the initial concentration of the tertiary amine in the aqueous medium (ie the concentration of the tertiary amine before the addition of hydrogen peroxide) is in the range of 40 to 95 vol.-%, preferably 60 to 85 vol.-%.

[0020] Any aqueous hydrogen peroxide solution may be used. In view of practical considerations, the concentration of hydrogen peroxide in the aqueous hydrogen peroxide solution added to the aqueous medium is in the range of 10 to 70 wt.-%, preferably 29 to 51 wt.-%.

[0021] According to the invention, a carbon dioxide partial pressure p(CO) of less than 0.75 bar absolute is applied to the aqueous medium. 2 ). “The partial pressure of carbon dioxide p(CO 2 )” is intended to mean maintaining a defined partial pressure of carbon dioxide in the vapor space above and in direct contact with the aqueous medium.

[0022] As far as we know, the prior art only discloses a method in which the carbon dioxide partial pressure p(CO2) is 1 bar absolute.2 ), the amount of carbon dioxide dissolved in the aqueous medium can be controlled: there is an equilibrium between the dissolved carbon dioxide and the carbon dioxide in the gas phase. A high carbon dioxide partial pressure results in a large amount of carbon dioxide dissolved in the aqueous medium, while a low carbon dioxide partial pressure results in a small amount of carbon dioxide dissolved in the aqueous medium.

[0023] According to the present invention, the carbon dioxide partial pressure p(CO 2 ) is less than 0.75 bar absolute and the dilution gas makes up the remainder of the total pressure.

[0024] In an embodiment, a purge of diluent gas is maintained in the vapor space above the aqueous medium and carbon dioxide is bubbled through the aqueous medium. The diluent gas is passed into the head space of the reaction vessel without passing it through the aqueous medium to maintain a selected carbon dioxide partial pressure p(CO) in the vapor space. 2 ). Carbon dioxide and a diluent gas may also be used intentionally to form an atmosphere under which the aqueous medium is maintained.

[0025] Carbon dioxide is bubbled through the aqueous medium, for example through a nozzle or a gas lance arranged at the bottom of the reaction vessel. Excess carbon dioxide comes out of the aqueous medium so that an equilibrium is established between the dissolved carbon dioxide and the carbon dioxide in the gas phase.

[0026] Bubbling carbon dioxide through the aqueous medium is preferred due to faster equilibrium and dissolution of carbon dioxide.It is also possible (although less preferred) to intentionally bubble a mixture of carbon dioxide and a diluent gas through the aqueous medium.

[0027] In an embodiment, the diluent gas is an inert gas, preferably nitrogen or argon.

[0028] The oxidation reaction of tertiary amines using hydrogen peroxide to produce amine oxides is exothermic. For example, the heat release of the oxidation reaction of N-methylmorpholine and hydrogen peroxide is about -160kJ / mol oxidized N-methylmorpholine. The exothermic characteristic leads to the formation of reaction heat. In order to control the reaction temperature within the desired range and ensure safe process conditions, it is mandatory to remove the reaction heat, such as by using a cooling jacket on the container being reacted. In order to alleviate the heat removal problem, aqueous hydrogen peroxide is usually added over a period of time, rather than all being added in a single time. Therefore, the addition time is determined by the effect of heat removal, and the heat removal effect is particularly affected by the height of batch size. Especially in the case of attempting a large batch size, reacting in an accurate and precise temperature range requires a considerable addition time of about 2 to 24 hours.

[0029] Dosing time and hydrogen peroxide dosing rate r(H 2 O 2 ) are related to each other. Hydrogen peroxide injection rate r(H2 O 2 ) can be expressed as the number of moles of hydrogen peroxide added per 1 mol of tertiary amine (based on the stoichiometric amount of the tertiary amine initially present) and per hour. In an embodiment, the hydrogen peroxide addition rate r (H 2 O 2 ) is in the range of 0.035 to less than 1 mol per 1 mol of the tertiary amine and per hour.

[0030] In another aspect of the present invention, it has been surprisingly found that for best results, the carbon dioxide partial pressure p(CO 2 ) and hydrogen peroxide addition rate r(H 2 O 2 ) is preferably coordinated. Therefore, in an embodiment, the carbon dioxide partial pressure p(CO 2 ) is given by

[0031] p(CO 2 )=r(H 2 O 2 ) / z

[0032] where z is between 1 and 4 hours -1 -bar -1 , preferably 1.5 to 3 hours -1 -bar -1 variables within the scope of .

[0033] It is believed that the hydrogen peroxide injection rate r (H 2 O 2 ) determines the concentration of unreacted hydrogen peroxide available in the aqueous medium, wherein a high dosing rate results in a higher concentration of hydrogen peroxide before it is consumed in the reaction. Within the limits defined by this equation, it is ensured that a high proportion of carbon dioxide, preferably substantially all of the carbon dioxide, is able to react with hydrogen peroxide to form peroxycarbonate. Thus, "free" carbon dioxide in the aqueous medium is avoided or at least significantly reduced.

[0034] Conveniently, the hydrogen peroxide is metered in at a substantially constant dosing rate. Alternatively, the hydrogen peroxide dosing rate may be varied within the limits of the above equation.

[0035] Preferably, the hydrogen peroxide addition rate is in the range of 0.035 to 0.5 mol per 1 mol of the tertiary amine per hour, more preferably 0.035 to 0.15 mol per 1 mol of the tertiary amine per hour.

[0036] For example,

[0037] (i) when the hydrogen peroxide addition rate is in the range of 0.5 to 0.25 mol per 1 mol of tertiary amine per hour, the carbon dioxide partial pressure is in the range of 0.06 to 0.5 bar absolute pressure,

[0038] (ii) when the hydrogen peroxide addition rate is in the range of less than 0.25 to 0.1 mol per 1 mol of tertiary amine per hour, the carbon dioxide partial pressure is in the range of 0.025 to 0.25 bar absolute pressure, and

[0039] (iii) when the hydrogen peroxide addition rate is in the range of less than 0.1 to 0.035 mol per 1 mol of the tertiary amine and per hour, the carbon dioxide partial pressure is in the range of 0.009 to 0.1 bar absolute pressure.

[0040] According to the invention, the total amount of hydrogen peroxide is less than the stoichiometric amount. Preferably, the total amount of hydrogen peroxide used is in the range of 50 to 99 mol-%, more preferably 80 to less than 90 mol-%, in particular 85 to less than 90 mol-%, per 1 mol of tertiary amine. This ensures that the hydrogen peroxide introduced into the reaction is substantially completely consumed. Thus, the amine oxide contains less than 50 ppm of hydrogen peroxide relative to the weight of the amine oxide. Preferably, the amine oxide contains less than 10 ppm of hydrogen peroxide. 2 O 2 The concentration of residual hydrogen peroxide can be determined by potentiometric titration with an acidic aqueous solution of cerium (IV) sulfate.

[0041] In an embodiment, the tertiary amine forms a minimum boiling azeotrope with water at a pressure in the range of 0.01 to 5 bar absolute. It is sufficient to form the azeotrope at a pressure within the mentioned range. In these cases, the unreacted amine can be removed as an azeotrope with water. For example, a suitable tertiary amine is N-methylmorpholine.

[0042] In an embodiment, the post-treatment of the reaction mixture comprises subjecting the tertiary amine oxide solution to distillation to remove water and / or water-azeotrope of unreacted tertiary amine. For this purpose, the azeotrope of unreacted amine and water is distilled directly from the crude reaction mixture together with compounds having a boiling point lower than the boiling point of the azeotrope, preferably in a distillation column.

[0043] Preferably, the distillation is carried out as a continuous distillation. Compounds having a boiling point lower than that of the azeotrope are withdrawn at the top of the distillation column, while the azeotrope is withdrawn via the first side-draw of the distillation column. In an embodiment, the distillation is carried out at a pressure in the range of 50 to 300 mbar.

[0044] The water-azeotrope of the unreacted tertiary amine removed is appropriately recycled to the reaction with hydrogen peroxide. If the water-azeotrope of the unreacted tertiary amine as obtained by distillation has a tertiary amine concentration outside the range of 40 to 95 vol.-%, the concentration of the tertiary amine can be adjusted. This means that if the concentration of the tertiary amine is too high, water is added, or if the concentration of the tertiary amine is too low, a tertiary amine is added.

[0045] Typically, the starting material of the method (i.e., tertiary amine) may contain unavoidable impurities. For example, N-methylmorpholine almost always contains traces of morpholine. Morpholine may be oxidized in a side reaction, thereby producing morpholine oxide as a colored byproduct. Such colored byproducts are crucial to the color of the amine oxide product and should therefore be avoided. In order to avoid the accumulation of these impurities, these impurities can be appropriately purged via an additional side draw of the continuous distillation column. The location of the additional side draw depends on the nature of the impurities, in particular the boiling point. In most cases, the additional side draw is located below the first side draw of the distillation column.

[0046] After distilling off substantially all of the unreacted amine, the remaining residue consists essentially of amine oxide and water. Amine oxides are usually stored and / or distributed as aqueous solutions. Advantageously and preferably, the resulting residue consisting essentially of amine oxide and water is a ready-to-use commercial product that does not require any additional purification or processing steps. However, if necessary, for example, in the case where it is necessary to distill off very large amounts of water in order to remove all of the unreacted amine, additional water may be added to the distillation column bottoms in order to adjust the concentration of the aqueous amine oxide solution in view of consumer demand and / or storage stability.

[0047] The reaction can be carried out over a wide temperature range. The temperature should be high enough to allow the reaction to proceed at a reasonable rate, but not so high as to cause decomposition of the reactants or products. In an embodiment, the method is carried out at a temperature of 20°C to 90°C, preferably 20°C to 65°C.

[0048] The process of the present invention produces an amine oxide that is substantially free of nitrosamines. In the context of the content of nitrosamines, the term "substantially free" means that one or more nitrosamines are present in the amine oxide as a contaminant in trace amounts of less than 50 ppb relative to the weight of the amine oxide.

[0049] The process of the present invention produces a substantially colorless amine oxide. In the context of the color change of the amine oxide, the term "substantially colorless" means that a 50 wt.-% aqueous solution of the amine oxide has an extinction of less than 2% / cm at 450 nm. The extinction is determined using a UV / VIS spectrometer apparatus (e.g., available from Perkin Elmer) at a temperature of 20°C.

[0050] Alternatively, the color change of the amine oxide can be characterized via APHA color number. Preferably, the amine oxide has an APHA color number of less than 100. The APHA color number is a standard method based on visual comparison of a sample with a solution of cobalt chloroplatinate having a known concentration. A color unit is a color unit produced by 1 mg platinum / L (in the form of chloroplatinate ions).

[0051] The process of the invention allows to obtain an amine oxide which is substantially free of nitrosamines, substantially free of peroxides and substantially colorless. This unique combination of desirable characteristics has not been achieved hitherto in processes involving only oxidation and distillation. The present invention therefore further relates to an N-methylmorpholine N-oxide having a nitrosamine content of less than 50 ppb and a peroxide content of less than 20 ppm relative to the weight of the N-methylmorpholine N-oxide, and an extinction at 450 nm of less than 2% / cm as a 50 wt.-% aqueous solution of N-methylmorpholine N-oxide.

[0052] Examples

[0053] The present invention can be further explained and illustrated based on the following examples.

[0054] Example 1

[0055] An aqueous solution of N-methylmorpholine (NMM, 63 wt.-%; 1.01 kg, 1.0 eq.) was placed in a 1.6 L double-walled glass reactor. The reactor was equipped with a cross-bar stirrer, a reflux condenser, a thermometer, a 2 (3.0) The glass tube equipped with a sintered filter head is connected to N 2 (Industrial grade) Gas inlet (w / o pipe) and H 2 O 2 Dosing line (Teflon tubing connected to a diaphragm metering pump). Feed H from a glass reservoir 2 O 2 The reaction was carried out at a pressure of 1.0 bar absolute.

[0056] The aqueous solution was heated to a temperature of 50° C. under a constant nitrogen flow (1.5 L / h). After reaching a temperature of 50° C., CO was added at a flow of 0.15 L / h. 2 From the ratio of flow to total pressure, CO can be calculated 2 The partial pressure is 91 mbar.

[0057] After 60 min, the mixture was stirred at 0.93 mL / min of H 2 O 2 Flow starts to add H 2 O 2 (50wt.-% H 2 O2 Aqueous solution, 395 g, 0.9 equiv.) (i.e., 0.15 mol / (mol·h); z = 1.65 hours -1 -bar -1 ). When adding H 2 O 2 During this period, the temperature was maintained at 50°C and CO was continuously added. 2 and N 2 .

[0058] When adding H 2 O 2 After completion, the H 2 O 2 The concentration of CO was maintained at 50°C with stirring. 2 and N 2 until the residual H 2 O 2 The concentration is less than 50ppm (about 1h).

[0059] Then, the addition of CO was stopped. 2 , and the reaction mixture was allowed to cool to room temperature. The reflux condenser was replaced with a distillation apparatus (condenser and vacuum pump) and water and NMM were distilled off at a sump temperature of 50° C. (2 fractions, first fraction, collected at about 150 mbar: 133 g, second fraction, collected at about 80 mbar: 188 g).

[0060] Water (308 g) was added to the distillation residue to obtain 1328 g of a 49.6 wt.-% aqueous solution of N-methylmorpholine N-oxide (NMMO) having an APHA color number of 22, an extinction at 450 nm of 1.2%, and an N-nitrosomorpholine content of less than 20 ppb.

[0061] Example 2

[0062] An aqueous solution of N-methylmorpholine (NMM, 63 wt.-%; 1.01 kg, 1.0 eq.) was placed in a 1.6 L double-walled glass reactor. The reactor was equipped with a cross-bar stirrer, a reflux condenser, a thermometer, a 2 (3.0) The glass tube equipped with a sintered filter head is connected to N 2 (Industrial grade) Gas inlet (w / o pipe) and H 2 O 2 Dosing line (Teflon tube connected to a diaphragm metering pump). Feed H from a glass reservoir 2 O 2 The reaction was carried out at a pressure of 1.0 bar absolute.

[0063] The aqueous solution was heated to a temperature of 50° C. under a constant nitrogen flow (1.5 L / h). After reaching a temperature of 50° C., CO was added at a flow of 0.50 L / h. 2 From the ratio of flow to total pressure, CO can be calculated 2 The partial pressure is 250 mbar.

[0064] After 60 min, the mixture was stirred at 0.93 mL / min of H 2 O 2 Flow starts to add H 2 O 2 (50wt.-% H 2 O 2 Aqueous solution, 395 g, 0.9 equivalents) (i.e., 0.15 mol / (mol·h); z = 0.6 hours -1 -bar -1 ). When adding H 2 O 2 During this period, the temperature was maintained at 50°C and CO was continuously added. 2 and N 2 .

[0065] When adding H 2 O 2 After completion, the H 2 O 2 The concentration of CO was maintained at 50°C with stirring. 2 and N 2 until the residual H 2 O 2 The concentration is less than 50ppm (about 30min).

[0066] Then, the addition of CO was stopped. 2 , and the reaction mixture was allowed to cool to room temperature. The reflux condenser was replaced with a distillation apparatus (condenser and vacuum pump) and water and NMM were distilled off at a tank temperature of 50° C. (2 fractions, first fraction, collected at about 150 mbar: 145 g, second fraction, collected at about 80 mbar: 162 g).

[0067] Water (312 g) was added to the distillation residue to obtain 1318 g of a 50.1 wt.-% aqueous solution of N-methylmorpholine N-oxide (NMMO) having an APHA color number of 296, an extinction of 4.3% at 450 nm, and an N-nitrosomorpholine content of less than 20 ppb.

[0068] Example 3

[0069] An aqueous solution of N-methylmorpholine (NMM, 63 wt.-%; 1.01 kg, 1.0 eq.) was placed in a 1.6 L double-walled glass reactor. The reactor was equipped with a cross-bar stirrer, a reflux condenser, a thermometer, a 2 (3.0) The glass tube equipped with a sintered filter head is connected to N 2 (Industrial grade) Gas inlet (w / o pipe) and H 2 O 2 Dosing line (Teflon tube connected to a diaphragm metering pump). Feed H from a glass reservoir 2 O 2 The reaction was carried out at a pressure of 1.0 bar absolute.

[0070] The aqueous solution was heated to a temperature of 50° C. under a constant nitrogen flow (1.5 L / h). After reaching a temperature of 50° C., CO was added at a flow of 0.05 L / h. 2 From the ratio of flow to total pressure, CO can be calculated 2 The partial pressure is 32 mbar.

[0071] After 60 min, the mixture was stirred at 0.93 mL / min of H 2 O 2 Flow starts to add H 2 O 2 (50wt.-% H 2 O 2 Aqueous solution, 395 g, 0.9 equivalents) (i.e., 0.15 mol / (mol·h), z = 4.69 hours -1 -bar -1 ). When adding H 2 O 2 During this period, the temperature was maintained at 50°C and CO was continuously added. 2 and N 2 .

[0072] When adding H 2 O 2 6 hours after the end, stop adding CO 2 , and the reaction mixture was allowed to cool to room temperature. The reflux condenser was replaced with a distillation apparatus (condenser and vacuum pump) and water and NMM were distilled off at a tank temperature of 50° C. (2 fractions, first fraction, collected at about 150 mbar: 143 g, second fraction, collected at about 80 mbar: 170 g).

[0073] Water (322 g) was added to the distillation residue, thereby obtaining 1329 g of a 49.6 wt.-% aqueous solution of N-methylmorpholine N-oxide (NMMO) having an APHA color number of 10, an extinction at 450 nm of 0.5%, an H2 O 2 content and 180ppb N-nitrosomorpholine content.

[0074] Comparative Example 1

[0075] N-Methylmorpholine (NMM, 101.1 g, 1.0 eq.) and water (37.5 mL; resulting in a mixture of 73 wt.-% NMM and 27 wt.-% water) were placed in a 250 mL double-walled glass reactor. The reactor was equipped with a cross-bar stirrer, a reflux condenser, a thermometer, a 2 (3.0) Pipeline assembly: glass tube with sintered filter and H 2 O 2 The dosing line (Teflon tube connected to a diaphragm metering pump) was used. The head space above the aqueous phase in the reactor (including the condenser) was about 500 mL, and H 2 O 2 The reaction was carried out at a pressure of 1.0 bar absolute.

[0076] The aqueous solution was saturated with constant CO without stirring. 2 The mixture was heated to a temperature of 50°C under a flow (1.0 L / h) for 2 hours. During this time, a total of 2 L of CO was fed. 2 , which corresponds to about 4 times the headspace volume of the reactor and is therefore sufficient to use pure CO 2 Basically replacing the previous atmosphere.

[0077] Then, the H 2 O 2 Flow starts to add H 2 O 2 (50wt.-% H 2 O 2 Aqueous solution, 61.2 g, 0.9 equivalents) (i.e., 0.225 mol / (mol·h); z = 0.23 hours -1 -bar -1 ). When adding H 2 O 2 During this time, the temperature was maintained at 50 °C and CO was continuously added at a flow rate of 0.1 L / h. 2 .

[0078] When adding H 2 O 2 After completion, the H 2 O 2 The concentration of CO was maintained at 50°C with stirring. 2 , until H 2 O 2The concentration is less than 50ppm (about 10min).

[0079] Then, the addition of CO was stopped. 2 , and the reaction mixture was allowed to cool to room temperature. The reflux condenser was replaced with a distillation apparatus (condenser and vacuum pump) and water and NMM were distilled off at a tank temperature of 50° C. (2 fractions, first fraction, collected at about 150 mbar: 21.9 g, second fraction, collected at about 80 mbar: 25.7 g).

[0080] Water (59.2 g) was added to the distillation residue to obtain 208.4 g of a 50.1 wt.-% aqueous solution of N-methylmorpholine N-oxide (NMMO) having an APHA color number of 297, an extinction of 4.9% at 450 nm, and an N-nitrosomorpholine content of less than 20 ppb.

[0081] Comparative Example 2

[0082] N-Methylmorpholine (NMM, 101.1 g, 1.0 eq.) and water (37.5 mL; resulting in a mixture of 73 wt.-% NMM and 27 wt.-% water) were placed in a 250 mL double-walled glass reactor. The reactor was equipped with a cross-bar stirrer, a reflux condenser, a thermometer, a 2 (3.0) Pipeline assembly: glass tube with sintered filter and H 2 O 2 Dosing line (Teflon tube connected to a diaphragm metering pump). Feed H from a glass reservoir 2 O 2 The reaction was carried out at a pressure of 1.0 bar absolute.

[0083] The aqueous solution was stirred under constant CO 2 The pressure was heated to 50°C under a flow of nitrogen (0.082 L / h) and nitrogen (0.353 L / h) for 2 hours. From the ratio of the flow to the total pressure, the CO 2 The partial pressure is 188 mbar.

[0084] Then, the H 2 O 2 Flow starts to add H 2 O 2 (50wt.-% H 2 O 2 Aqueous solution, 71.4 g, 1.05 equiv.) (i.e., 0.362 mol / (mol·h); z = 1.92 hours -1 -bar -1 ). When adding H 2 O 2During this period, the temperature was maintained at 50°C and CO was continuously added. 2 and N 2 .

[0085] When adding H 2 O 2 After the reaction, the mixture was stirred at 50 °C and CO was added. 2 and N 2 Continue for 16 hours. Then, stop adding CO 2 , and allowed the reaction mixture to cool to room temperature.

[0086] 24.0 g of water were added, resulting in 233.3 g of a 50.1 wt.-% aqueous solution of N-methylmorpholine N-oxide (NMMO) having an APHA color number of 33, an extinction at 450 nm of 0.6 % / cm and an N-nitrosomorpholine content of less than 20 ppb. The mixture contained 0.716 wt.-% hydrogen peroxide.

[0087] To an aliquot of the mixture (about 100 g) was added manganese dioxide (0.5 g), which resulted in vigorous gas formation. When gas formation ceased, the mixture was essentially free of hydrogen peroxide (as determined by cerometric titration), but had an APHA color number of 236. After standing at room temperature for 4 days, the APHA color number increased further, eventually exceeding the scale (>1000), and the extinction was 6.9% / cm. To another aliquot of the mixture (about 100 g) was added platinum on carbon (2.0 g, 57 wt.-% water, Pt content: 5.0 wt.-% of solids), which resulted in vigorous gas formation. When gas formation ceased, the mixture was essentially free of hydrogen peroxide (as determined by cerometric titration), and the APHA color number was 78. After standing at room temperature for 4 days, the APHA color number increased to 527, and the extinction was 3.9% / cm.

[0088] Example 4

[0089] Triethylamine (101.3 g, 1.0 eq.) and water (10.0 mL; resulting in a mixture of 91 wt.-% NMM and 9 wt.-% water) were placed in a 250 mL double-walled glass reactor. The reactor was equipped with a cross-bar stirrer, a reflux condenser, a thermometer, a thermometer connected to a CO 2 (3.0) Pipeline assembly: glass tube with sintered filter and H 2 O 2 Dosing line (Teflon tube connected to a diaphragm metering pump). Feed H from a glass reservoir 2 O 2 The reaction was carried out at a pressure of 1.0 bar absolute.

[0090] The aqueous mixture was heated to a temperature of 50° C. under a constant nitrogen flow (0.70 L / h). After reaching a temperature of 50° C., CO was added at a flow of 0.164 L / h. 2 From the ratio of flow to total pressure, CO can be calculated 2 The partial pressure is 190 mbar.

[0091] After 60 min, the mixture was stirred at 0.286 mL / min of H 2 O 2 Flow starts to add H 2 O 2 (50wt.-% H 2 O 2 Aqueous solution, 61.2 g, 0.9 equivalents) (i.e., 0.30 mol / (mol·h); z = 1.57 hours -1 -bar -1 ). When adding H 2 O 2 During this period, the temperature was maintained at 50°C and CO was continuously added. 2 and N 2 .

[0092] When adding H 2 O 2 After completion, the H 2 O 2 The concentration of CO was maintained at 50°C with stirring. 2 , until H 2 O 2 The concentration is less than 50ppm (about 180min).

[0093] Then, the addition of CO was stopped. 2 , and the reaction mixture was allowed to cool to room temperature. The reflux condenser was replaced with a distillation apparatus (condenser and vacuum pump), and water and triethylamine were distilled off at a bath temperature of 50° C. (1 fraction, collected at about 100 mbar: 35.3 g).

[0094] Water (47.6 g) was added to the distillation residue to obtain 210.1 g of a 49.7 wt.-% aqueous solution of triethylamine N-oxide having an APHA color number of 29, an extinction at 450 nm of 0.8%, and an N-nitrosodiethylamine content of less than 20 ppb.

Claims

1. A process for preparing a substantially nitrosamine-free, substantially peroxide-free, substantially colorless amine oxide solution, the process comprising reacting a tertiary amine capable of forming an amine oxide with a less than stoichiometric amount of aqueous hydrogen peroxide solution in an aqueous medium while applying to the aqueous medium a vapor space having a carbon dioxide partial pressure p(CO2) of less than 0.75 bar absolute.

2. The method of claim 1, wherein: A purge of diluent gas is maintained in the vapor space above the aqueous medium, and carbon dioxide is bubbled through the aqueous medium.

3. The method of claim 2, wherein: The diluent gas is an inert gas.

4. A method as claimed in any one of the preceding claims, wherein: The concentration of the tertiary amine in the aqueous medium is in the range of 40 to 95 vol.-%.

5. A method as claimed in any one of the preceding claims, wherein: The method comprises adding a 10 to 70 wt.-% aqueous hydrogen peroxide solution to the aqueous medium.

6. A method as claimed in any one of the preceding claims, wherein: The aqueous hydrogen peroxide solution is metered into the aqueous solution of the tertiary amine, and the hydrogen peroxide addition rate r(H2O2) is in the range of 0.035 to less than 1 mol per 1 mol of the tertiary amine and per hour.

7. The method of claim 6, wherein: The carbon dioxide partial pressure p(CO2) in bar absolute is given by p(CO2)=r(H2O2) / z where z is between 1 and 4 hours -1 -bar -1 variables within the scope of .

8. A method as claimed in any one of the preceding claims, wherein: The total amount of hydrogen peroxide used is from 50 to 99 mol-% per mol of tertiary amine.

9. A process as claimed in any preceding claim which is carried out at a temperature of from 20°C to 90°C.

10. A method as claimed in any one of the preceding claims, wherein: The tertiary amine forms a minimum boiling azeotrope with water at a pressure in the range of 0.01 to 5 bar absolute.

11. The method of claim 10, wherein: The tertiary amine is N-methylmorpholine.

12. The method according to claim 10 or 11, wherein: The tertiary amine oxide solution is subjected to distillation to remove water and / or water-azeotrope of unreacted tertiary amine.

13. The method of claim 12, wherein: The distillation is carried out at a pressure in the range from 50 to 300 mbar.

14. The method according to claim 12 or 13, wherein: The removed water-azeotrope of unreacted tertiary amine is recycled to the reaction with hydrogen peroxide.

15. A method as claimed in any one of the preceding claims, wherein: The amine oxide has a nitrosamine content of less than 50 ppb relative to the weight of the amine oxide.

16. A method as claimed in any one of the preceding claims, wherein: A 50 wt.-% aqueous solution of the amine oxide has an extinction at 450 nm of less than 2% / cm.

17. The method of claim 16, wherein: A 50 wt.-% aqueous solution of the amine oxide has an APHA color number of less than 100.