Process for the production of n-methylmorpholine having low hazen color number

Abstract

A process for preparing N-methylmorpholine, the process comprising continuously feeding morpholine, a formaldehyde solution and hydrogen into at least one reactor equipped with a heterogeneous hydrogenation catalyst comprising cobalt and copper, and subjecting the morpholine and formaldehyde to reductive amination to obtain N-methylmorpholine, wherein the fed formaldehyde solution is a methanolic formaldehyde solution.

Description

[0001] Process for the production of N-methylmorpholine having low Hazen color number

[0002] The invention relates to a process for preparing N-methylmorpholine and N-methylmorpholine oxide.

[0003] N-methylmorpholine (4-methylmorpholine; 4-methyltetrahydro-1 ,4-oxazine; NMM) is for instance used as a catalyst in polyurethane production. N-methylmorpholine is also the starting material for the production of N-methylmorpholine oxide, which is an important solvent for cellulose.

[0004] It can be prepared by the reductive amination of morpholine (MO) with formaldehyde in the presence of hydrogen.

[0005] N-methylmorpholine oxide (N-methylmorpholine N-oxide; NMMO) is for instance used as solvent for cellulose. Aqueous solutions of N-methylmorpholine N-oxide are technically used as a solubilizer in Lyocell fiber production, wherein pulp is dissolved and spun from a solution in N- methylmorpholine N-oxide and water.

[0006] N-methylmorpholine oxide can be obtained by oxidation of N-methylmorpholine.

[0007] The reductive amination of formaldehyde with morpholine can be carried out in batch mode or continuous mode.

[0008] DE 179 93 380 relates to the preparation of secondary and tertiary amines by the reaction of primary or secondary amines with formaldehyde followed by hydrogenation of the resultant N- methylolamine in the presence of a hydrogenation catalyst containing cobalt and manganese and / or chromium at elevated temperature and superatmospheric pressure. Example 3 of this document discloses the preparation of N-methylmorpholine from morpholine and formaldehyde. Accordingly, 855 parts of morpholine are reacted in a stirred vessel at 500 °C with 1140 parts of 30 % by weight aqueous formaldehyde solution while stirring, 50 parts per hour of the reaction solution obtained is metered to the top of a vertical high-pressure tube which is charged with a catalyst containing 14.4 % by weight of cobalt, 1 % by weight of chromium and 0.36 % by weight of phosphorus pentoxide on aluminium oxide. A temperature of 150 °C is maintained during the reaction, the pressure being kept at 250 atmospheres gauge by forcing in hydrogen. The discharge from the high-pressure tube is fractionally distilled. N-methylmorpholine having a boiling point of 114 °C is obtained in a yield of 90% of the theory.

[0009] DE 197 31 745 relates to a continuous process for preparing alkylamines by reacting amines and aliphatic aldehydes in the presence of free hydrogen on a fixed bed catalyst. Formaldehyde is preferably employed as the aldehyde. The document discloses as suitable amines diethylenetriamine, dipropylenetriamine, morpholine, piperidine, pyrrolidine, piperazine, 1 -methylpiperazine, and piperazine. The amines can be employed without added solvents or in the form of solutions. Alcohols are preferably not employed as solvents. It is preferable for no alcohols at all to be present in the reaction system. The amines are particularly preferably employed as aqueous solutions. For example, piperazine is employed as 30 to 80% by weight solution in water. The aldehydes are likewise preferably employed as aqueous solutions. In this case, the water content of the aldehyde solution is from 50 to 90%, specifically from 65 to 75% of the total weight of the solution.

[0010] When using an aqueous formaldehyde solution in the preparation of N-methylmorpholine from morpholine and formaldeyhde, significant amounts of N-formyl morpholine are formed as byproduct. Furthermore, the obtained product N-methylmorpholine is characterized by considerable yellowness.

[0011] It is an object of the present invention to provide for an efficient process for preparing N- methylmorpholine from morpholine and formaldehyde by reductive amination, wherein N- methylmorpholine is formed with very high selectivity, and an essentially pure and colorless raw product is obtained.

[0012] Summary of the invention

[0013] The problem is solved by a process for preparing N-methylmorpholine (NMM), the process comprising continuously feeding morpholine (MO), a formaldehyde solution and hydrogen into at least one reactor equipped with a heterogeneous hydrogenation catalyst and subjecting the morpholine and formaldehyde to reductive amination to obtain N-methylmorpholine, wherein the formaldehyde solution is a methanolic formaldehyde solution.

[0014] Detailed description of the invention

[0015] The reductive amination can be conducted in one or more reactor(s). Preferred reactors are tube reactors.

[0016] A key feature of the present invention is that the continuously fed formaldehyde solution is a methanolic formaldehyde solution. The term “methanolic formaldehyde solution” means that formaldehyde is employed in a solution that comprises methanol and preferably also water.

[0017] In such solution, the formaldehyde may exist in different forms. For instance, a certain amount, in general the major part, of formaldehyde exists as a hemiacetal (resulting from formaldehyde and methanol also referred to as hemiformal or 1 -methoxy-methanol) or polyoxymethylene having the formula HO-[CH2O]n-CH3 with n being an integer typically in the range from 2 to 10. The weight percentages (“wt.-%”) specified herein refer to the “theoretical” amount of formaldehyde, methanol, and water, thus, neglecting any potential reactions among formaldehyde, methanol, and water. Preferably, the methanolic formaldehyde solution comprises formaldehyde in the range from 30 to 70 wt.-%, methanol in the range from 20 to 50 wt.-% and water in the range from 5 to 25 wt.- %. The wt.-% is based on the total mass of the methanolic formaldehyde solution.

[0018] More preferably, the methanolic formaldehyde solution comprises formaldehyde in the range from 35 to 65 wt.-%, methanol in the range from 25 to 45 wt.-% and water in the range from 5 to 20 wt.-%.

[0019] Even more preferably, the methanolic formaldehyde solution comprises formaldehyde in the range from 40 to 60 wt.-%, methanol in the range from 30 to 45 wt.-% and water in the range from 5 to 15 wt.-%.

[0020] In a preferred embodiment, the amount of formaldehyde, methanol, and water in the methanolic formaldehyde solution is > 90 wt.-%, preferably > 95 wt.-%, more preferably > 98 wt.-%, even more preferably > 99 wt.-%, particularly preferably > 99.5 wt.-%.

[0021] Usually, the reaction temperature is in the range from 30 to 300°C, preferably from 30 to 250°C, more preferably from 30 to 200°C. The reaction temperature is even more preferably from 60 to 150°C, particularly preferably from 80 to 130°C, more particularly preferably from 90 to 130°C.

[0022] Unless not explicitly provided otherwise, all pressures in this application refer to the absolute pressure.

[0023] The reaction pressure is usually in the range from 50 to 300 bar, preferably from 50 to 250 bar, more preferably from 50 to 200 bar, even more preferably from 60 bar to 150 bar, particularly preferably from 80 to 150 bar, more particularly preferably from 90 to 140 bar.

[0024] The liquid hourly space velocity (LHSV) over the heterogeneous hydrogenation catalyst is usually in the range from 0.05 to 1 , preferably from 0.1 to 0.9, more preferably from 0.2 to 0.8, even more preferably from 0.3 to 0.7 kg of morpholine / (Lcat h). The LHSV refers to the amount of morpholine in kg which is fed over the volume (bed volume) of the heterogeneous hydrogenation catalyst in liter (abbreviated as Lcat.) per hour (h).

[0025] As the reaction is performed in presence of hydrogen, the reactor is preferably pressurized with hydrogen or mixtures of hydrogen with inert gases like nitrogen to obtain the pressure above. In a preferred embodiment, pure hydrogen is used. Any oxygen should be removed from the reactor before hydrogen is introduced into the reactor, for example by feeding an inert gas, first.

[0026] The amount of hydrogen is usually in the range from 100 to 3000 NL / (Lcat. ■ h), preferably from 200 to 2500, more preferably from 500 to 2000 NL / (Lcat. ■ h), even more preferably from 700 to 1800 NL / (Lcat. ■ h), particularly preferably from 1000 to 1700 NL / (Lcat. ■ h), more particularly preferably from 1200 to 1600 NL / (Lcat. ■ h). The amount of hydrogen is referred to the volume (bed volume) of the heterogeneous hydrogenation catalyst in liter (abbreviated as Lcat.) and hour (h).

[0027] NL means standard liters, i.e. liter (of molecular hydrogen) under standard conditions (S.T.P).

[0028] Standard conditions being understood as follows:

[0029] Standard pressure =1.01325 bar.

[0030] Standard temperature = 0 °C.

[0031] In a preferred embodiment the process according to the invention is operated in a recycle gas mode. This means that low volatile components (in particular hydrogen) are separated from the obtained NMM and recycled to the reaction. The separation is usually carried out in a high- pressure separator into which the reactor effluent is fed. A high-pressure separator is a device (for instance a vessel) that is operated at a pressure slightly below the reaction pressure. The recycle gas comprises preferably at least 10% by volume, particularly from 50 to 100% by volume, very particularly from 80 to 100% by volume of H2. The recycle gas flow rate is preferably in the range from 40 to 1500 m3(at operating pressure) / [m3of catalyst (bed volume)-h], in particular in the range from 100 to 700 m3(at operating pressure) / [m3of catalyst (bed volume)-h].

[0032] In a preferred embodiment the NMM is partially recycled to the reductive amination of MO and formaldehyde. A recycle of NMM to the reaction helps to control the temperature profile in the one or more reactor(s). The more NMM is recycled, the lower is the rise of the temperature in the one or more reactor(s). Such recycle is usually affected after low volatile components (in particular hydrogen) have been separated from the NMM. Such separation can be carried out in a high-pressure separator as described above.

[0033] The weight ratio of the recycled NMM to the combined amount of methanolic formaldehyde solution and MO being fed to the one or more reactors(s) can be in the range from 1 :1 to 10:1 , preferably from 2:1 to 8:1 . The weight ratio is calculated based on the mass flow rate of the respective streams comprising the NMM, MO and methanolic formaldehyde solution. It is to be noted that such streams may also comprise other components. For example, the methanolic formaldehyde solution stream may comprise water (as further specified below) which also contributes to the mass flow rate. In the same way the NMM stream may also comprise certain byproduct (as specified above), which contribute to the mass flow rate. The same is also true with respect to any impurities, which may be comprised in the MO.

[0034] It is to be noted, that the NMM obtained in accordance with the process according to the present invention is usually devoid of any starting material (formaldehyde and / or MO). In case small amounts thereof should be comprised in the recycled NMM any such amounts do not contribute to the molar ratio of formaldehyde to MO as defined in accordance with the present invention (i.e. the molar ratio is only based on fresh formaldehyde and MO). In general, the molar ratio of formaldehyde and morpholine in the feed to the reactor is in the range of from 0.8 : 1 to 1.5 : 1 , preferably from 0.9 : 1 to 1 .2 : 1 . The NMM obtained in accordance with the present invention is comprised in a reactor effluent which is withdrawn from the one or more reactor(s).

[0035] Besides NMM the reactor effluent comprises hydrogen, methanol, water, and by-products. The water is formed during the reaction and may further be comprised in the methanolic formaldehyde solution. Preferably, such reactor effluent is subjected to purification comprising the following steps:

[0036] (i) subjecting the reactor effluent to degassing, preferably in a high-pressure separator, to obtain a degassed stream;

[0037] (ii) subjecting the degassed stream as per step (i) to a distillation step to remove methanol and to obtain a stream comprising NMM and being depleted in methanol.

[0038] The degassing of step (i) particularly serves the purpose of removing hydrogen. Step (ii) is usually effected in one column.

[0039] With respect to the economy of the purification of the raw N M M , the use of methanolic formaldehyde is advantageous over the use of an aqueous formaldehyde solution. This is attributable to the fact that methanol has a lower boiling point than water, which therefore requires less energy for its removal via distillation.

[0040] The aqueous NMM mixture thus obtained can be employed in the production of NMMO as taught herein. The NMM obtained from step (ii) may also be subjected to a further distillation step to obtain a mixture of NMM and water. Such distillation is described in European patent application 23200776.5.

[0041] The process according to the invention is conducted in the presence of a heterogeneous hydrogenation catalyst. The term heterogeneous catalyst designates a solid catalyst, preferably in the form of particles, which is brought into contact with the liquid reaction mixture comprising the starting materials and any intermediates and NMM already obtained.

[0042] Any heterogeneous catalyst that has sufficient hydrogenation activity can be used for the manufacture of NMM in accordance with the present invention. It may be a supported or an unsupported catalyst. An unsupported catalyst is preferred.

[0043] Preferably, the heterogeneous hydrogenation catalyst is devoid or substantially devoid of any palladium. The amount of palladium is preferably less than 0.5 wt.-%, preferably less than 0.05 wt.-%, more preferably less than 0.01 wt.-%, based on the total weight of the heterogeneous hydrogenation catalyst.

[0044] The heterogeneous hydrogenation catalyst is installed in the reactor, preferably as fixed bed.

[0045] The heterogeneous hydrogenation catalyst comprises cobalt and copper. Preferably it comprises cobalt, copper and manganese, more preferably cobalt, copper, manganese and molybdenum, particularly preferably cobalt, copper, manganese, molybdenum and phosphorus.

[0046] The heterogeneous hydrogenation catalyst can for instance be prepared by applying precipitation or impregnation methods. Suitable heterogeneous hydrogenation catalysts and respective methods for their production are for instance taught in EP 2043996 B1 , WO 2011 / 067200 A1 , WO 2011 / 067199 A1 and EP 2780109 B1 (all BASF).

[0047] In general, the heterogeneous hydrogenation catalyst comprises

[0048] 5 to 90 wt.-%, preferably 10 to 60 wt.-%, cobalt,

[0049] 1 to 40 wt.-%, preferably 2 to 30 wt.-%, copper,

[0050] 0 to 30 wt.-% manganese,

[0051] 0 to 30 wt.-% molybdenum, and

[0052] 0 to 30 wt.-% phosphorus.

[0053] In a very preferred embodiment, the heterogeneous hydrogenation catalyst comprises

[0054] 5 to 90 wt.-%, particularly 10 to 60 wt.-% cobalt,

[0055] 1 to 40 wt.-%, particularly 2 to 30 wt.-% copper,

[0056] 0.1 to 30 wt.-%, particularly 1 to 10 wt.-% manganese,

[0057] 0.1 to 30 wt.-%, particularly 1 to 10 wt.-% molybdenum, and

[0058] 0.05 to 30 wt.-%, particularly 0.1 to 5 wt.-% phosphorus, based on the total weight of the heterogeneous hydrogenation catalyst.

[0059] The preparation of such catalysts is for instance taught in DE 2321101 (BASF). Accordingly, the catalyst is obtained by precipitation, followed by calcination. The catalyst thus obtained is activated by reduction in a hydrogen stream. It is to be noted, that any “wt.-%” as specified herein with respect to the composition of the heterogeneous hydrogenation catalyst refers the heterogeneous hydrogenation catalyst after the last of any heat treatments (for instance calcination) and prior to its reduction with hydrogen.

[0060] It follows from the nature of the preparation method, that the respective metals at least partially exist in an oxidized from. Nonetheless, the presence of a certain amounts of such metals in elementary form is not excluded. For instance, in the course of the preparation, one could, besides using respective metal nitrites, also apply certain amount(s) of respective metal(s) in elementary form. Usually, more than 90 wt.-%, preferably more than 95 wt.-%, more preferably more than 99 wt.-% (or even more than 99.5 wt.-%) of any respective metal exists in oxidized from. The phosphorus usually exists substantially in an oxidized form. Usually, one would not apply elementary phosphor in the catalyst preparation, but phosphorus in an oxidized form only (in particular phosphoric acid). Moreover, the formation of elementary phosphorus during pre- cipitation or calcination is unlikely to happen. Thus, preferably more than 99 wt.-%, more preferably more than 99.5 wt.-%, even more preferably more than 99.9 wt.-% of the respective phosphorus exist in oxidized form.

[0061] The heterogeneous hydrogenation catalyst preferably also contains oxygen. In a preferred embodiment the heterogeneous hydrogenation catalyst comprises oxygen and the cumulated amount of oxygen, cobalt, copper, manganese, molybdenum, phosphorus, and cobalt is

[0062] > 80 wt.-%, preferably > 90 wt.-%, more preferably > 95 wt.-% even more preferably > 97 wt.-%, particularly preferably > 98 wt.-%, based on the total weight of the heterogeneous hydrogenation catalyst.

[0063] The invention further relates to a process for preparing N-methylmorpholine oxide (NMMO), comprising the steps of:

[0064] (i) preparing N-methylmorpholine in accordance with the process of the invention as described above, and

[0065] (ii) subjecting the N-methylmorpholine obtained in step (i) to an oxidation reaction to obtain N-Methyl-morpholine oxide.

[0066] Oxidation step (ii) comprises reacting N-methylmorpholine with a less than stoichiometric amount of aqueous hydrogen peroxide in an aqueous medium in the presence of carbon dioxide as promotor.

[0067] In a preferred embodiment, oxidation step (ii) comprises reacting N-methylmorpholine with a less than stoichiometric amount of aqueous hydrogen peroxide in an aqueous medium while imposing on the aqueous medium a vapor space having a carbon dioxide partial pressure p (CO2) of less than 0.75 bar absolute, preferably less than 0.20 bar absolute. Although carbon dioxide is an effective promoter for the conversion of a tertiary amine into its amine oxide, applicants have found that amounts of free carbon dioxide in excess of a promoting amount can lead to undesired discoloration.

[0068] In an embodiment, the initial concentration of tertiary amine in the aqueous medium, that is the concentration of tertiary amine prior to the addition of hydrogen peroxide, is in the range from 40 to 95 vol.-%, preferably from 60 to 85 vol.-%. Any aqueous hydrogen peroxide can be used. In view of practical considerations, the concentration of hydrogen peroxide in the aqueous hydrogen peroxide that is added to the aqueous medium is in the range of from 10 to 70 wt.-%, preferably 29 to 51 wt.-%.

[0069] The following examples only serve for the purpose of the illustration of the present invention and shall therefore not limit it in whatsoever kind. EXAMPLES

[0070] Catalyst:

[0071] The catalyst was prepared in accordance with EP 0383132 or DE 2321101 (BASF SE). Its composition (based on the respective oxides) is as follows:

[0072] Cu (15.2 g), Mn (2.5 g), Mo (2.4 g), P (0.9 g), Co (16.5 g) in 100 g catalyst.

[0073] Examples 1 to 11

[0074] Example 1 (for comparison) and Example 2 (according to the invention)

[0075] A heated reactor with internal diameter 4 mm and a total volume of 60 mL was charged in the lower section with 2-3 wire mesh rings, followed by alternating 4 mm “Strang” of catalyst and 3 mm glass beads and finally 2-3 wire mesh rings. Prior to the reaction, the catalyst was activated at max. 270° C. Thereby, the catalyst was heated to 250 °C with a hydrogen flow of 90 NL / h. After 12 h at 250 °C, the reactor was cooled to 40 °C. A pressure of 150 bar was employed. 30 g / h of morpholine (MO) were metered through the reactor. The reactor was kept at a temperature and absolute pressure as set forth in Table 1 below. Additionally, formaldehyde was metered through a second pump. Aqueous formaldehyde with 35 wt.-% formaldehyde (FA) and 65 wt.-% water (entry 1 in Table 1 below) or methanolic formaldehyde solution with 55 wt.-% formaldehyde, 35 wt.-% methanol, and 10 wt.-% water (entry 2 in table 1 below), respectively. A recycle feed was in place which recycled raw product feed after the high-pressure separator to the reactor inlet, the mass ratio of MO feed to recycled feed being 2:1 (RLV). At different times, samples were taken from the reaction mixture and analyzed by means of gas chromatography. For this purpose, an "RTX-5 Amin" GC column was used with the following parameters: RTX-5 Amin((30 m x 0,32 mm x 1 ,5 pm) 60°C - 5°C / min - 280°C - 6 min. Flow: 1.5 NL / min N2.

[0076] The Hazen color number was determined on a Lico 620 Colorimeter (Company HACH).

[0077] The results are presented in Table 1 below.

[0078] Table 1

[0079] * Comparative Example (aqueous formaldehyde solution)

[0080] ** According to the invention (methanolic formaldehyde solution)

[0081] Examples 3 - 11

[0082] Examples 3-11 were performed analogue to Example 2. The process parameters, conversions and product selectivities are listed in Table 2 below.

[0083] Table 2

Claims

Claims1 . A process for preparing N-methylmorpholine, the process comprising continuously feeding morpholine, a formaldehyde solution and hydrogen into at least one reactor equipped with a heterogeneous hydrogenation catalyst comprising cobalt and copper, and subjecting the morpholine and formaldehyde to reductive amination to obtain N-methylmorpholine, wherein the fed formaldehyde solution is a methanolic formaldehyde solution.

2. The process according to claim 1 , wherein the heterogeneous hydrogenation catalyst comprises in the range of5 to 90 wt.-% cobalt,1 to 40 wt.-% copper,0 to 30 wt.-% manganese,0 to 30 wt.-% molybdenum, and0 to 30 wt.-% phosphorus, based on the total weight of the heterogeneous hydrogenation catalyst.

3. The process according to claim 1 or 2, wherein the heterogeneous catalyst comprises cobalt, copper, manganese, molybdenum and phosphor.

4. The process according to any one of claims 1 to 3, wherein the heterogeneous hydrogenation catalyst comprises in the range of5 to 90 wt.-% cobalt,1 to 40 wt.-% copper,0.1 to 30 wt.-% manganese,0.1 to 30 wt.-% molybdenum, and0.05 to 30 wt.-% phosphorus, based on the total weight of the heterogeneous hydrogenation catalyst.

5. The process according to any one of claims 1 to 4, wherein the fed methanolic formaldehyde solution comprises formaldehyde in the range from 30 to 70 wt.-%, methanol in the range from 20 to 50 wt.-% and water in the range from 5 to 25 wt.-%.

6. The process according any one of claims 1 or 5, wherein the molar ratio of formaldehyde and morpholine in the feed to the reactor is in the range from 0.8 : 1 to 1.5 : 1 , preferably from 0.9 : 1 to 1.2 : 1.

7. The process according to any one of claims 1 to 6, wherein the reductive amination is conducted in the liquid phase.

8. The process according to any one of claims 1 to 7, wherein N-methylmorpholine is partially recycled to the reductive amination of morpholine and formaldehyde.

9. The process according to claim 8, wherein the weight ratio of recycled N- methylmorpholine to the combined amount of morpholine and methanolic formaldehyde solution being fed to the one or more reactor(s) is in the range from 1 :1 to 10:1 , preferably from 2:1 to 8:1.

10. The process according to any one of claims 1 to 9, wherein the reaction temperature is in the range from 30 to 300 °C, preferably from 30 to 250 °C, more preferably from 30 to 200 °C, even more preferably from 60 to 150 °C, particularly preferably from 80 to 130 °C, more particularly preferably from 90 to 130 °C.11 . The process according to any one of claims 1 to 10, wherein the reaction pressure is in the range from 50 to 300 bar, preferably from 50 to 250 bar, more preferably from 50 to 200 bar, even more preferably from 60 bar to 150 bar, particularly preferably from 80 to 150 bar, more particularly preferably from 90 to 140 bar.

12. The process according to any one of claims 1 to 11 , wherein the liquid hourly space velocity over the heterogenous hydrogenation catalyst is in the range from 0.05 to 1 , preferably from 0.1 to 0.9, more preferably from 0.2 to 0.8, even more preferably from 0.3 to 0.7 kg of morpho- line / (Lkat h).

13. A process for preparing N-methylmorpholine oxide (NMMO), comprising the steps of:(i) preparing N-methylmorpholine in accordance with any one of claims 1 to 12, and(ii) subjecting the N-methylmorpholine obtained in step (i) to an oxidation reaction to obtain N-methyl-morpholine oxide.

14. The process of claim 13, wherein the oxidation reaction of step (ii) comprises reacting N- methylmorpholine with a less than stoichiometric amount of aqueous hydrogen peroxide in an aqueous medium in the presence of carbon dioxide as promotor.

15. The process of claim 14, wherein oxidation step (ii) comprises reacting N-methyl- morpholine with a less than stoichiometric amount of aqueous hydrogen peroxide in an aqueous medium while imposing on the aqueous medium a vapor space having a carbon dioxide partial pressure p (CO2) of less than 0.75 bar absolute, preferably less than 0.20 bar absolute.

Images