WO1998000589A1

WO1998000589A1 - Process for preparation of an aqueous solution of n-methylmorpholin-n-oxide

Info

Publication number: WO1998000589A1
Application number: PCT/AT1997/000147
Authority: WO
Inventors: Wolfram Kalt; Johann Männer; Dieter Eichinger; Heinrich Firgo
Original assignee: Lenzing Aktiengesellschaft
Priority date: 1996-07-02
Filing date: 1997-07-01
Publication date: 1998-01-08
Also published as: NO980371D0; CA2230050A1; AT404033B; AU3327897A; ATA116596A; EP0874927A1; ID17544A; NO980371L; BR9706559A; CN1196762A; JPH11511764A

Abstract

The invention relates to a process for the preparation of a solution of N-methylmorpholin-N-oxide in water, which solution is used in the amine oxide process. The preparation process is characterised by the following steps: (a) an aqueous solution (A) is provided which contains N-methylmorpholine and is derived from the amine oxide process; (b) subsequently another aqueous solution (B) is added to said solution (A), and contains N-methylmorpholine, and a solution (C) is produced which (c) is treated with an oxidant to oxidise N-methylmorpholine to form N-methylmorpholin-N-oxide, a solution (D) of N-methylmorpholin-N-oxide being formed in water.

Description

Process for the preparation of an aqueous solution of N-methylmorpholine-N-oxide

The present invention relates to a process for producing a solution of N-methylmorpholine-N-oxide in water, which is used in the amine oxide process. The invention further relates to a method for producing cellulosic moldings, in particular fibers and films.

For several decades, processes for the production of cellulosic shaped articles have been sought which are to replace the viscose process which is used today on a large scale. An alternative, not least because of its better environmental compatibility, has emerged in the process of dissolving cellulose in an organic solvent without derivatization and molding, e.g. Fibers, foils and other moldings to extrude. Such extruded fibers were given the generic name Lyocell by BISFA (The International Bureau for the Standardization of man made fiberε). BISFA defines an organic solvent as a mixture of an organic chemical and water.

It has been found that a mixture of a tertiary amine oxide and water is particularly suitable as an organic solvent for the production of cellulosic moldings. N-Methylmorpholine-N-oxide (NMMO) is primarily used as the amine oxide. Other amine oxides are described, for example, in EP-A-0 553 070. A method for producing moldable cellulose solutions is known, for example, from EP-A-0 356 419. The production of moldable solutions of cellulose in tertiary amine oxides and their further processing into shaped bodies is generally referred to as amine oxide process for the purposes of the present description and the present patent claims. EP-A-0 356 419 describes an amine oxide process for producing spinnable cellulose solutions, which uses, among other things, a suspension of cellulose in liquid, aqueous N-methylmorpholine-N-oxide (NMMO) as the starting material. This method consists in the suspension being continuously and continuously converted into a moldable solution in a thin-film treatment apparatus. The moldable solution is then spun into filaments in a molding tool, for example a spinneret, which are passed through a precipitation bath.

The cellulose is precipitated in the precipitation bath. The tertiary amine oxide accumulates in the precipitation bath. The amine oxide content in the precipitation bath can be up to 30% by weight. For the economy of the amine oxide process, it is of crucial importance to recover the amine oxide as completely as possible and to use it again for the production of a moldable cellulose solution. It is therefore necessary to recover NMMO from the precipitation bath.

However, degradation products of the amine oxide process also accumulate in the precipitation bath with the amine oxide. These degradation products can be strongly colored and thus impair the quality of the cellulosic molded articles produced. Other substances in turn can pose an additional safety risk, since under certain conditions the amine oxide tends to undergo strongly exothermic decomposition reactions and these decomposition reactions can be induced or accelerated by certain substances. These substances must be removed from the precipitation bath to be processed before the concentration and separation of NMMO.

After these undesirable substances have been removed, water is drawn off from the cleaned precipitation bath, which is optionally combined with other process waters of the amine oxide process, such as vapor condensates which are produced in the production of the cellulose solution. This can for example by evaporation. In the bottom of this evaporation, highly concentrated, aqueous amine oxide is obtained, which is recycled back into the amine oxide process. The vapors of the evaporation mainly consist of water, in which considerable amounts of N-methylmorpholine, the main degradation product of the NMMO, are also dissolved. The vapors typically contain, for example, up to 1000 mg NMMO and 240 mg N-methylmorpholine per liter. These vapors are expediently concentrated, for example by reverse osmosis.

In order to close the cycles as far as possible and also to keep the losses of NMMO as low as possible, efforts are made to re-oxidize the N-methylmorpholine to NMMO. This can be done, for example, with a peroxidic oxidizing agent.

A process for the preparative preparation of tertiary amine oxides by oxidation of tertiary amines is known for example from EP-A - 0 092 862. According to this method, the amine oxide is oxidized in an aqueous solvent with molecular oxygen under pressure, which solvent has a pH value which is approximately equal to or higher than the pKa value of the tertiary amine.

DD-A-259 863 relates to the preparation of aqueous NMMO solutions by oxidation of N-methylmorpholine with H 0 and passing the reaction solution over one or more exchange columns which are filled with styrene / divinylbenzene copolymer containing sulfonate groups, and by adjusting one pH of the solution to values between 8 and 5 by adding phosphoric acid.

The oxidation of N-methylmorpholine with HO to NMMO is known for example from EP-A - 0 254 803. From DE-A -4 140 259 the production of NMMO is known, in which process the formation of nitrosamines is prevented by primary and secondary amines, for example, can be trapped with acid halides. EP-A-0 320 690 describes the production of essentially nitrosa-free amine oxides by oxidation using peroxides in the presence of a combination of C0 ₂ / ascorbic acid, which acts as a nitrosamine inhibitor. From EP-A-0 401 503 the oxidation with H_0 ₂ in water and a cosolvent, preferably a carboxylic acid ester, is known. According to FR-A-8 808 039, the oxidation is carried out with the addition of C0 ₂ , and according to US-A-5,216,154, the oxidation to NMMO is carried out in a pure C0 ₂ atmosphere.

In the amine oxide process, despite largely closed working methods, solvent losses occur, which are due on the one hand to dilution steps in fiber cleaning, the removal of the fiber itself, and in particular to the chemical degradation of NMMO described in the literature under thermal stress during the course of the process (Lang H. et al., Cell. Chem. Technol. 20, 1986, No. 3, page 289; and Taeger E. et al., Formulas, fibrous materials, finished goods 4, 1985, pages 14-22). Despite numerous attempts to optimize the degradation behavior (e.g. by using chemical stabilizers), it has so far not been possible to completely prevent such reactions. For these reasons, a regular supply of NMMO is necessary in the prior art. According to EP-A-0 448 924 by the applicant, some of the degradation products of the NMMO, in particular N-methylmorpholine, which are present in very dilute solutions, can be concentrated and then reoxidized in a known manner to NMMO, which is reintroduced into the amine oxide process.

This reoxidation of the decomposition product NMM contained in the vapors to NMMO reduces the losses of expensive NMMO, but does not prevent the amine oxide process from having to add up to a few percent NMMO per kilogram of pulp processed. A disadvantage of this procedure is the high price for technical NMMO, which has to be continuously added to the amine oxide process.

The present invention therefore aims to eliminate this disadvantage and to provide a method in which the addition of fresh NMMO can be largely reduced or even avoided.

This object is achieved with a process for the preparation of a solution of N-methylmorpholine-N-oxide in water which is used in the amine oxide process, which is characterized by the following steps:

(a) an aqueous solution (A) is provided which contains N-methylmorpholine and comes from the amine oxide process, after which

(b) this solution (A) is mixed with a further aqueous solution (B) which contains N-methylmorpholine, a solution (C) being formed which

(c) is treated with an oxidizing agent to oxidize N-methylmorpholine to N-methylmorpholine-N-oxide, whereby a solution (D) of

N-methylmorpholine-N-oxide is formed in water, which is used in the amine oxide process.

It has proven expedient to add at least as much N-methylmorpholine to solution (A) with solution (B) as is contained in solution (A).

A peroxide is best used as the oxidizing agent.

It is known that the aqueous solution (A) originating from the amine oxide process also contains morpholine in addition to N-methylmorpholine. This morpholine is a precursor of the toxic N-nitrosomorpholine in the course of oxidation. The inventors of the present invention have recognized that the formation of the Toxic N-nitrosomorpholine can be suppressed if the solution (C) to be treated with the oxidizing agent has a pH between 6.0 and 9.0. It has been shown that it is possible, simply by adjusting the pH of the oxidation mixture in the range mentioned, to suppress the formation of the toxic N-nitrosomorpholine and at the same time to achieve a maximum of oxidation of N-methylmorpholine to NMMO. The pH dependence of these two reaction routes can be seen in the attached figures and is described in the applicant's Austrian patent application A 1398/95.

It has proven to be extremely advantageous to bring the pH of the aqueous solution into the desired range by passing the solution to be worked up over a cation exchanger which can absorb morpholine. This measure has two important effects with regard to the reduction of nitrosamines. The cation exchanger selectively removes morpholine from the solution, which means that practically no more morpholine is available for the formation of new nitrosamines. By separating the morpholine, which has the highest basicity in comparison to the other components, the pH value of the solution is additionally lowered in the range in which the formation of NMMO reaches high values, but the formation of nitrosamines is still further inhibited.

The cation exchanger best has carboxyl groups or sulfonic acid groups for the separation of morpholine.

The small amounts of N-nitrosomorpholine, which are formed despite the above-mentioned pH adjustment, can be largely destroyed by the aqueous solution (C) being irradiated with ultraviolet light during or after treatment with the peroxidic oxidizing agent essentially has a wavelength of 254 nm. The presence of the peroxidic oxidizing agent is impaired not this destruction. The destruction of N-nitrosomorpholine with ultraviolet light is described in Austrian patent application A 1401/95.

There are known working instructions for the quantitative analysis of nitrosamines which use UV radiation and subsequent determination of the nitrites formed (DEG Shuker, SR Tannenbaum, Anal, formerly, 1983, 55_, 2152-2155; M. Rhighezza, MH Murello, AM Siouffi, J. Chro at., 1987, 410, 145-155; JJ Conboy, JH Hotchkiss, Analyst, 1989, 114, 155-159; B. Büchele, L. Hoffmann, J. Lang, Fresen. J.Anal Chem., 1990, 336, 328-333). However, these analytical procedures do not deal with the destruction of N-nitrosomorpholine.

H-O- is preferably used as the peroxidic oxidizing agent in the process according to the invention. This H 0 "is preferably used in the form of an aqueous solution with 20-50 wt .-% H_0. The H 0 is best used in an amount of 0.8 to 2 moles per mole of N-methylmorpholine.

The ultraviolet light with which the aqueous solution is irradiated comes best from a low-pressure mercury lamp. These low pressure lamps have an intensity maximum at 254 nm.

For the irradiation according to the invention with a low-pressure lamp, the lamp can be suspended in the container which contains the process water to be treated. The lamp can also be arranged in a different way. Furthermore, the irradiation can also be carried out, for example, while the solution to be irradiated is continuously pumped around in a thin-film UV reactor.

The radiation power can be, for example, 200 to 500 π-J / cm ² and is dependent on the design of the lamp and on the process conditions, in particular the temperature. Also this embodiment of the method according to the invention does not require any additional chemicals.

In order to remove unreacted N-methylmorpholine, which is still contained in the solution (D), it has proven to be expedient to subject the solution (D) to distillation. This produces vapors which, after condensation, result in a solution (E) which can at least partially be used as solution (B).

The invention further relates to a process for the production of cellulosic moldings by the amine oxide process, in which cellulose is dissolved in an aqueous NMMO solution to form a moldable solution, the solution obtained is shaped and, after molding, is passed into a precipitation bath, cellulosic moldings and a used precipitation bath are formed, which precipitation bath is worked up to recover NMMO, an aqueous solution is obtained which contains N-methylmorpholine and which, optionally together with vapor condensates formed in the amine oxide process, is subjected to oxidation to produce a new aqueous NMMO solution which is used again for the production of a moldable cellulose solution, which is characterized in that new N-methylmorpholine-containing solution obtained in the working up of the used precipitation bath is added, whereupon the solution is subjected to the oxidation.

In the process according to the invention, therefore, not NMMO, but N-methylmorpholine, is fed to the amine oxide process and together with that N-methylmorpholine, which is produced in the amine oxide process by degradation of NMMO and in the various process liquids, such as vapor condensates, which are used in the preparation of the moldable cellulose solution or accumulate during evaporation of the precipitation bath, is oxidized to NMMO. The person skilled in the art is free to determine the point in time at which N-methylmorpholine is added to those from the amine oxide process choose process water without affecting the result of the process according to the invention.

The invention is explained in more detail with the following examples. The abbreviations NMOR, NMMO, NMM and M used in the following stand for N-nitrosomorpholine, N-methylmorpholine-N-oxide, N-methylmorpholine and morpholine, respectively.

example 1

A mixture of vapor condensates, which are formed from the cellulose suspension in the preparation of the moldable cellulose solution and during the evaporation of the precipitation bath to be worked up and which contain NMMO and NMM, is filtered and concentrated in a reverse osmosis system. The retentate thus obtained contains, for example, about 7 kg of NMM and about 15 kg of NMMO per 1000 kg.

This solution is mixed with 44.3 kg NMM per 1000 kg and introduced into a reactor for oxidation. 79.5 kg of 22% H ₂ O _{2 are} added at 65 ° C. with stirring over 10 minutes. After a reaction time of 7 hours at 70 ° C., unreacted NMM is evaporated from the solution under reduced pressure of 100 mbar.

An aqueous solution which contains 51.8 kg of NMMO dissolved and remains in the amine oxide process, i.e. can be used to prepare the cellulose suspension.

The condensate obtained during evaporation contains unreacted NMM (19.5 kg) and is mixed with 24.8 kg NMM and added to the next batch of retentate.

Example 2 A mixture of vapor condensates, which are formed from the cellulose suspension in the preparation of the moldable cellulose solution and during the evaporation of the precipitation bath to be worked up and which contain NMMO and NMM, is filtered and concentrated on in a reverse osmosis system. The retentate thus obtained contains, for example, about 7 kg of NMM and about 15 kg of NMMO per 1000 kg.

This solution is mixed with 44.3 kg NMM per 1000 kg and then cleaned using a cation exchanger. The cleaned solution is placed in a reactor for oxidation. At 65 'C with stirring over 10 minutes, the addition of 79.5 kg 22% H ₂ 0 ₂ is carried out. After 7 hours reaction time at 70 ^* C, the solution is transferred to an additional container and irradiated for 10 hours by ultraviolet light having a wavelength of 254 nm. Unreacted NMM under reduced pressure of about 100 mbar and a temperature of 70 ^* C is then evaporated from the solution.

An aqueous solution containing 51.8 kg of NMMO remains in the distillation sump. This solution is fed to the amine oxide process, i.e. used to prepare the cellulose suspension.

Example 3

An aqueous solution containing 42 μ.q NMOR, 459 mg NMMO, 4300 mg NMM and 200 mg M per liter was in a UV reactor with a low-pressure mercury lamp (type Katadyn UV lamp EK-36, No. 79000 ; Manufacturer: Katadyn) irradiated (wavelength: 254 nm). The temperature of the aqueous solution was 60 ^* C. The concentration of NMOR was determined by HPLC (column: Hypersil ODS 250 x 4 mm; 50 ^* C; eluent: A = 0.6% acetonitrile; B = 49.7% H_0; gradient 1 ml / min; 10 min. 100% A; 7 min - 100% B; detector: UV 238 nm).

After an irradiation time of 150 minutes, the NMOR content in the process water decreased to 40 μg / l. After a further 150 minutes no NMOR was detectable.

After no more NMOR could be detected, the irradiation was stopped and checked again for NMOR at intervals of several hours. No NMOR could be detected, which proved that no NMOR can regress.

Example 4

An aqueous solution, which contained 25 μg NMOR, 2530 mg NMMO, 3923 mg NMM and 30 mg M per liter, was mixed with 30% H_0 ₂ to oxidize NMM to NMMO (mol NMM / mol H ₂ 0 ₂ = 1 / 1,2) and irradiated with UV light as described in Example 3. Within the first 90 minutes, the NMOR concentration rose to 45 μg / l, which is due to a quick reaction of the M in the solution. Then the concentration of NMOR decreased again sharply. No NMOR was detectable after 6 hours.

After a total oxidation time of 20 hours, the solution contained 5386 mg NMMO / liter. This corresponds to a yield of 62% of theory.

Example 5

7 aqueous solutions (50 ml) with 284 ppb NMOR, which contained 6097 mg NMM, 272 mg M and 1085 mg NMMO per liter, were adjusted to pH values 4, 6, 7, 8, 10, 12 and with HCl / NaOH 14 brought. Thereafter, aqueous hydrogen peroxide with 30 wt .-% H ₂ 0 ₂ in an amount was added to achieve an excess of 1.3 mol, based on NMM, and heated to 50 ^* C for 4 hours. The yield of newly formed NMMO and the concentration of NMOR were then determined by means of HPLC (see Example 3). The results are shown graphically in Figures 1 and 2.

Figure 1 shows the pH value as the abscissa and the yield of NMMO formed (% of theory) as the ordinate. It can clearly be seen that there is a maximum of approximately 50% in the range between 6.0 and 9.0. In Fig. 2 the pH value is also given as the abscissa and the concentration (in ppb) of NMOR in the solution after oxidation is given as the ordinate. It can be seen that the formation of the N-nitrosomorpholine only increases sharply from a pH of 8-9. In the range between 6.0 and 9.0, the formation of NMMO is thus maximized and, at the same time, the formation of the toxic N-nitrosomorpholine is minimized. This applies in particular to the pH range between 7.0 and 9.0.

Claims

Claims:

1. A process for the preparation of a solution of N-methylmorpholine-N-oxide in water, which is used in the amine oxide process, characterized by the following steps:

(c) treated with an oxidizing agent to oxidize N-methylmorpholine to N-methylmorpholine-N-oxide, thereby forming a solution (D) of N-methylmorpholine-N-oxide in water, which is used in the amine oxide process.

2. The method according to claim 1, characterized in that the pH of the solution (C) before treatment with the oxidizing agent is adjusted to a value between 6.0 and 9.0.

A method according to claim 1 or 2, characterized in that the solution (C) is passed through a cation exchanger which can absorb morpholine before treatment with the oxidizing agent.

A method according to claim 3, characterized in that the cation exchanger has carboxyl groups or sulfonic acid groups.

5. The method according to any one of claims l to 4, characterized in that a peroxide is used as the oxidizing agent.

Method according to one of claims 1 to 5, characterized in that the aqueous solution is irradiated with or after treatment with the oxidizing agent with ultraviolet light, which has a wavelength of essentially 254 nm.

7. The method according to any one of claims l to 6, characterized in that the solution (D) is subjected to distillation, vapors being formed which, after condensation, give a solution (E) which is used at least partially as solution (B).

8. A process for the production of cellulosic moldings by the amine oxide process, in which cellulose is dissolved in an aqueous NMMO solution to form a moldable solution, the solution obtained is shaped and, after molding, is passed into a precipitation bath, cellulose moldings and a used precipitation bath being formed, which precipitating bath is worked up to recover NMMO, whereby an aqueous solution is obtained which contains N-methylmorpholine and which, optionally together with vapor condensates formed in the amine oxide process, is subjected to an oxidation in order to produce a new aqueous NMMO solution which is again to be produced a formable cellulose solution is used,

characterized,

that the N-methylmorpholine-containing solution obtained when working up the used precipitation bath is new N-methylmorpholine is added, after which the solution is subjected to oxidation.