Batch process for producing methanol formaldehyde solutions by depolymerization of paraformaldehyde

Abstract

A batch process for preparing a methanol formaldehyde solution by depolymerizing paraformaldehyde in the presence of methanol and water, the process comprising the steps of (a) providing a mixture comprising paraformaldehyde, water and methanol, (b) subjecting the mixture obtained in step (a) to a first heating step, wherein the mixture is held in a temperature range of 50 DEG C to 90 DEG C for a period of 5 to 300 minutes, (c) subjecting the mixture obtained in step (b) to a second heating step wherein the mixture is held in a temperature range of 100 DEG C to 130 DEG C for a period of 0.1 to 48 hours.

Description

[0001] This invention relates to a batch method for preparing a methanol-formaldehyde solution by depolymerizing paraformaldehyde in the presence of methanol and water. The invention further relates to methods for preparing N-methylmorpholine, methods for preparing N-methylmorpholine oxide, and methods for preparing pentamethyldiethylenetriamine.

[0002] Formaldehyde is produced by selectively oxidizing methanol on a large scale. Formaldehyde can be obtained as an aqueous solution, a methanol solution, or paraformaldehyde. Because paraformaldehyde is a solid, it cannot be readily used in any continuous process. Aqueous formaldehyde and methanol-formaldehyde can be used in both batch and continuous processes. The use of methanol-formaldehyde can have certain advantages over aqueous formaldehyde. For example, it has recently been found that yellowing can be avoided when using methanol-formaldehyde in NMM production. Another advantage is the fact that separating methanol from the reaction products requires less energy than separating water. This is due to the lower boiling point of methanol. However, the availability of methanol-formaldehyde solutions is limited, primarily because producing methanol-formaldehyde solutions from methanol is more complex than producing aqueous formaldehyde solutions (see US3629997). Therefore, the availability of methanol-formaldehyde solutions is low.

[0003] Paraformaldehyde can be synthesized from an aqueous formaldehyde solution via the polymerization of formaldehyde. Paraformaldehyde typically forms a white precipitate. Its chemical formula is OH(CH₂O). n H, where n is typically in the range of 8 to 100.

[0004] Formaldehyde is used, for example, to prepare N-methylmorpholine (NMM) and pentamethyldiethylenetriamine (PMDETA) by reducing and amination of formaldehyde with morpholine (MO) and diethylenetriamine (DETA) respectively in the presence of hydrogen and a hydrogenation catalyst. NMM and PMDETA are used as catalysts in polyurethane production. The use of formaldehyde as a methanol-formaldehyde solution has been found to be advantageous.

[0005] CN 110627654 A discloses a batch method for methylating amines selected from ethylenediamine, cyclohexylamine, aniline or benzylamine using paraformaldehyde.

[0006] CN 111675677 B discloses a batch method for producing N-methylmorpholine using paraformaldehyde but without the use of metal catalysts or reducing agents.

[0007] CN 106957232 A discloses a batch method for manufacturing N-monomethylamine compounds using paraformaldehyde.

[0008] According to GB 737,023, an aqueous solution of formaldehyde is distilled to form formaldehyde vapor containing water. The formaldehyde vapor is then mixed with alcohol vapor (e.g., methanol vapor), which absorbs the water and condenses with it in a first stage, while a portion of the formaldehyde vapor condenses with additional methanol only later in a second stage to form a product with a high formaldehyde concentration (i.e., 70% or greater). This known method is very complex. Separating the water vapor from the formaldehyde by adding methanol vapor and condensation requires cumbersome equipment.

[0009] One object of the present invention is to provide a simple and economical method for preparing methanol-formaldehyde solution.

[0010] This objective is achieved by a batch method for preparing a methanol-formaldehyde solution by depolymerizing paraformaldehyde in the presence of methanol and water, the method comprising the following steps:

[0011] (a) Provide a mixture comprising paraformaldehyde, water and methanol,

[0012] (b) subjecting the mixture obtained in step (a) to a first heating step, wherein the mixture is maintained at a temperature range of 50°C to 90°C for a period of 5 to 300 minutes.

[0013] (c) subjecting the mixture obtained in step (b) to a second heating step, wherein the mixture is held at a temperature range of 100°C to 130°C for a period of 0.1 to 48 hours.

[0014] The inventors have discovered an improved method for producing a methanol-formaldehyde solution by applying two heating steps (b) and (c).

[0015] In the context of this invention, the term "heating" means supplying thermal energy in the form of heat. For example, so much heat can be supplied that the temperature of the mixture rises. Heat can also be supplied in an amount necessary to compensate for heat dissipated into the environment and / or a corresponding condenser, typically equipped in a suitable batch reactor.

[0016] Without being bound by any theory, it is assumed that maintaining the mixture in the temperature range of 50°C to 90°C for 5 to 300 minutes as described in step (b) results in the formation of a solution containing a hemiacetal of formaldehyde and methanol and / or its higher polyoxymethylene homologues (which are compounds with higher boiling points than methanol itself). However, the depolymerization of paraoxymethylene is incomplete in this temperature range. According to step (c), complete depolymerization is achieved in the higher temperature range of 100°C to 130°C. Due to the previous formation of a higher-boiling-point mixture containing a hemiacetal of formaldehyde and methanol (also referred to as “hemiacetal” hereinafter) and / or its higher polyoxymethylene homologues, methanol evaporation is greatly reduced in the higher temperature range. However, when the mixture of paraoxymethylene, water, and methanol is directly heated to the temperature range of 100°C to 130°C, methanol continues to evaporate. This requires a very high condenser load, which is necessary to ensure a significant amount of methanol (solvent) in the reactor. A high condenser load means the removal of a large amount of heat energy per unit time, and is therefore energy inefficient. Furthermore, this can be critical in the context of operational safety. For example, in the event of a power outage (resulting in the condenser malfunctioning), methanol in the reaction mixture evaporates very rapidly (due to the high temperatures of 100°C to 130°C in the mixture). Without sufficient solvent in the mixture, the likelihood of overheating and other undesirable events increases significantly. Therefore, intensive monitoring of the reaction would be necessary in such situations.

[0017] As outlined above, a key aspect of this invention is to first maintain the mixture within a temperature range of 50°C to 90°C. It is believed that within such a temperature range, the formation of hemiacetals and / or their higher polyoxymethylene homologues occurs at a significant rate. To ensure sufficient formation of hemiacetals and / or their higher polyoxymethylene homologues, the mixture needs to be maintained within this temperature range for at least 5 minutes. One way to achieve step (b) is to directly heat the mixture to a specific temperature (e.g., 80°C) within the 50°C to 90°C range according to step (b) and maintain it at that temperature for a certain period of time (e.g., 150 minutes). Another way to achieve step (b) is to gently heat the mixture such that the temperature increases slowly. For example, the mixture can be heated such that the temperature increases from 50°C to 90°C over a period of 90 minutes. In principle, it is also possible to directly heat to a temperature of, for example, 70°C, and then slowly heat to a temperature of, for example, 85°C for a period of 100 minutes. It should be further noted that heating can also be interrupted, although this is not preferred. For example, if the reaction is heated directly to a temperature of 85°C, held at this temperature for 60 minutes, and then cooled (by not heating) to 60°C for a period of 60 minutes, then it is not critical and therefore conforms to the present invention.

[0018] The same applies to step (c).

[0019] Preferably, the mixture is stirred in step (b) and step (c).

[0020] Steps (b) and (c) can be performed, for example, at an average heating rate of 2 to 200°C / h, preferably at an average heating rate of 5 to 100°C / h, and more preferably at an average heating rate of 10 to 50°C / h.

[0021] The absolute pressure is typically in the range of 0.5 to 20 bar, preferably 0.5 to 10 bar, more preferably 0.5 to 5 bar, particularly preferably 0.8 to 3 bar, especially 0.9 to 2 bar, for example 1.0 to 1.5 bar.

[0022] The mixture provided in step (a) typically comprises 20 to 69 wt.% paraformaldehyde, 30 to 69 wt.% methanol, and 1 to 20 wt.% water. Preferably, the mixture provided in step (a) comprises 35 to 65 wt.% paraformaldehyde, 30 to 45 wt.% methanol, and 5 to 20 wt.% water. Even more preferably, the mixture provided in step (a) comprises 40 to 60 wt.% paraformaldehyde, 30 to 45 wt.% methanol, and 5 to 15 wt.% water.

[0023] The mixture provided in step (a) is typically present at ambient temperature. This depends on the location of the corresponding production facility. Typically, it has a temperature range of 1°C to 49°C, preferably 5°C to 45°C.

[0024] The temperature range according to step (b) is typically 60°C to 90°C, preferably 70°C to 90°C, and more preferably 75°C to 85°C. Generally, a higher temperature range is preferred because it provides a higher formation rate of hemiacetals and their higher polyoxymethylene homologues. Therefore, the time period required to achieve such a temperature range can be shorter.

[0025] The time period according to step (b) is typically 30 to 250 minutes, preferably 60 to 200 minutes, more preferably 90 to 200 minutes, and even more preferably 120 to 200 minutes.

[0026] The temperature range for step (c) is typically 110°C to 130°C, preferably 110°C to 125°C, and more preferably 110°C to 120°C.

[0027] The time period according to step (c) is typically 0.5 to 36 hours, preferably 1 to 24 hours, more preferably 1 to 12 hours, and even more preferably 2 to 12 hours.

[0028] The reaction is preferably carried out in a reactor, typically an STR (stirred tank reactor, also known as an agitated vessel). It is typically carried out in air or under a nitrogen atmosphere, with a nitrogen atmosphere being preferred. According to the invention, the reactor is operated in batch mode, meaning that all components of the mixture are added to the vessel and agitation is initiated. Those skilled in the art can readily select a suitable agitator with appropriate geometry to achieve thorough mixing. Such a reactor is preferably equipped with a condenser. A condenser is a device for condensing vapor into a liquid using a coolant, which can be selected from water, air, or brine. Those skilled in the art can readily select a suitable condenser.

[0029] In the methanol-formaldehyde solution obtained according to the present invention, formaldehyde can exist in different forms. For example, a certain amount (usually the major component) of formaldehyde is present as a hemiacetal (produced from formaldehyde and methanol, also called hemiacetal or 1-methoxy-methanol) or has the formula HO-[CH2O]. n The presence of polyoxymethylene (POM) in the formaldehyde group is -CH3 (where n is typically an integer in the range of 2 to 10). The weight percentages (“wt.-%”) specified herein refer to the “theoretical” amounts of formaldehyde, methanol, and water, and therefore any potential reactions between formaldehyde, methanol, and water are ignored.

[0030] Preferably, the methanol-formaldehyde solution obtained according to the present invention comprises 20 to 69 wt.% formaldehyde, 30 to 69 wt.% methanol, and 1 to 20 wt.% water. wt.% is based on the total mass of the methanol-formaldehyde solution.

[0031] More preferably, the methanol-formaldehyde solution comprises 35 to 65 wt.% formaldehyde, 30 to 45 wt.% methanol, and 5 to 20 wt.% water.

[0032] Even more preferably, the methanol-formaldehyde solution contains 40 to 60 wt.% formaldehyde, 30 to 45 wt.% methanol, and 5 to 15 wt.% water.

[0033] In a preferred embodiment, the amounts of formaldehyde, methanol, and water in the methanol-formaldehyde solution are ≥ 90 wt.-%, preferably ≥ 95 wt.-%, more preferably ≥ 98 wt.-%, even more preferably ≥ 99 wt.-%, and particularly preferably ≥ 99.5 wt.-%.

[0034] Another object of the present invention is to provide a simple and economical method for preparing N-methylmorpholine (NMM). A further object of the present invention is to provide a simple and economical method for preparing N-methylmorpholine oxide (NMMO). A further object of the present invention is to provide a simple and economical method for preparing pentamethyldiethylenetriamine.

[0035] The present invention also relates to a method for preparing N-methylmorpholine (NMM), the method comprising the following steps:

[0036] (i) Provide formaldehyde as a methanol-formaldehyde solution obtained by the method of the present invention as described above.

[0037] (ii) Provide morpholine.

[0038] (iii) subjecting the formaldehyde provided in step (i) and the morpholine provided in step (ii) to continuous reductive amination in the presence of hydrogen and a heterogeneous hydrogenation catalyst fixed in a reactor to obtain N-methylmorpholine.

[0039] The methanol-formaldehyde solution fed into step (i) is prepared by the method of the present invention as described above, and contains formaldehyde, methanol and water within the range indicated above.

[0040] This method is preferably performed continuously.

[0041] The key to this method is to provide a paraformaldehyde-based methanol-formaldehyde solution using the simple and economical method of the present invention as described above.

[0042] In a preferred embodiment, the heterogeneous hydrogenation catalyst is used as a fixed-bed catalyst.

[0043] This method for producing NMM is more efficient than that using paraformaldehyde because amination can be carried out continuously. The corresponding method using paraformaldehyde in amination cannot be operated continuously (because paraformaldehyde is a solid). Since amination requires a heterogeneous catalyst, the latter needs to be separated, which is technically and economically less efficient than using an immobilized catalyst as in step (iii). Furthermore, the continuous method offers better space-time yields (i.e., higher) compared to batch methods.

[0044] In a preferred embodiment, methanol is separated from the N-methylmorpholine obtained in step (iii) and recycled to step (a) of the methanol-formaldehyde production method according to the invention. In this way, the combination of the method for producing methanol-formaldehyde solution and NMM production becomes even more efficient because methanol can be continuously reused.

[0045] Typically, the morpholine provided in step (ii) is prepared by reacting diethylene glycol and ammonia in the presence of hydrogen and a heterogeneous hydrogenation catalyst. The preparation of morpholine from diethylene glycol and ammonia is described, for example, in WO 2011 / 067199 A1, WO2008 / 037587 A1, WO 2008 / 037589 A1 and WO 2008 / 037590 A1 (all by BASF).

[0046] The continuously fed formaldehyde solution is a methanol-formaldehyde solution prepared according to the present invention.

[0047] Typically, the reaction temperature is in the range of 30°C to 300°C, preferably 30°C to 250°C, and more preferably 30°C to 200°C. The reaction temperature is even more preferably 60°C to 150°C, particularly preferably 80°C to 130°C, and especially 90°C to 130°C.

[0048] Unless otherwise expressly stated, all pressures in this application refer to absolute pressures.

[0049] The reaction pressure is typically in the range of 50 to 300 bar, preferably 50 to 250 bar, more preferably 50 to 200 bar, even more preferably 60 to 150 bar, particularly preferably 80 to 150 bar, and especially 90 to 140 bar.

[0050] The method for preparing N-methylmorpholine is carried out in the presence of a heterogeneous hydrogenation catalyst. The term heterogeneous catalyst refers to a solid catalyst, preferably in particulate form, which is brought into contact with a liquid reaction mixture comprising the starting material and any intermediates, as well as the obtained NMM.

[0051] Any heterogeneous catalyst with sufficient hydrogenation activity can be used to manufacture NMM according to the present invention. It can be a supported or unsupported catalyst. Unsupported catalysts are preferred.

[0052] Preferably, the heterogeneous hydrogenation catalyst contains no or substantially no palladium. The amount of palladium is preferably less than 0.5 wt.-%, more preferably less than 0.05 wt.-%, and even more preferably less than 0.01 wt.-%, based on the total weight of the heterogeneous hydrogenation catalyst.

[0053] The heterogeneous hydrogenation catalyst is placed in the reactor, preferably as a fixed bed.

[0054] Heterogeneous hydrogenation catalysts may contain cobalt.

[0055] Preferably, the heterogeneous hydrogenation catalyst comprises cobalt and / or copper, more preferably cobalt, copper and / or manganese, even more preferably cobalt, copper, manganese and / or molybdenum, and particularly preferably cobalt, copper, manganese, molybdenum and / or phosphorus. In any of the foregoing combinations, the conjunction “and” used in any of the foregoing embodiments is preferred.

[0056] Heterogeneous hydrogenation catalysts can be prepared, for example, by precipitation or impregnation methods. Suitable heterogeneous hydrogenation catalysts and corresponding production methods are taught, for example, in EP 2043996 B1, WO 2011 / 067200 A1, WO 2011 / 067199A1 and EP 2780109 B1 (all by BASF).

[0057] In a highly preferred embodiment, the heterogeneous hydrogenation catalyst comprises, based on the total weight of the heterogeneous hydrogenation catalyst,

[0058] 5 to 90 wt.-%, preferably 10 to 60 wt.-%, of cobalt.

[0059] 1 to 40 wt.-%, preferably 2 to 30 wt.-%, of copper.

[0060] 0.1 to 30 wt.%, preferably 1 to 10 wt.%, of manganese.

[0061] 0.1 to 30 wt.-%, preferably 1 to 10 wt.-%, of molybdenum, and

[0062] 0.05 to 30 wt.-%, preferably 0.1 to 5 wt.-%, of phosphorus.

[0063] The preparation of such catalysts is taught, for example, in DE 2321101 (BASF). The catalyst is thus obtained by precipitation followed by calcination. The resulting catalyst is activated by reduction in a hydrogen stream. It should be noted that any “wt.-%” specified herein regarding the composition of heterogeneous hydrogenation catalysts refers to heterogeneous hydrogenation catalysts after the final heat treatment (e.g., calcination) and before their reduction with hydrogen.

[0064] The nature of the preparation method indicates that the corresponding metal is at least partially present in oxidized form. Nevertheless, the presence of a certain amount of such metal in elemental form is not excluded. For example, in the preparation process, in addition to using the corresponding metal nitrite, a certain amount of the corresponding metal in elemental form may be applied. Typically, more than 90 wt.-%, preferably more than 95 wt.-%, more preferably more than 99 wt.-% (or even more than 99.5 wt.-%) of any corresponding metal is present in oxidized form. Phosphorus is generally present essentially in oxidized form. Typically, elemental phosphorus is not applied in catalyst preparation; only phosphorus in oxidized form (especially phosphoric acid) is applied. Furthermore, the formation of elemental phosphorus cannot occur during precipitation or calcination. Therefore, it is preferable that more than 99 wt.-%, more preferably more than 99.5 wt.-%, and even more preferably more than 99.9 wt.-% of the corresponding phosphorus is present in oxidized form.

[0065] The heterogeneous hydrogenation catalyst preferably also contains oxygen. In a preferred embodiment, the heterogeneous hydrogenation catalyst comprises oxygen and cumulative amounts of oxygen, cobalt, copper, manganese, molybdenum, and phosphorus, and based on the total weight of the heterogeneous hydrogenation catalyst, cobalt is ≥ 80 wt.-%, preferably ≥ 90 wt.-%, more preferably ≥ 95 wt.-%, even more preferably ≥ 97 wt.-%, particularly preferably ≥ 98 wt.-%.

[0066] The present invention further relates to a method for preparing N-methylmorpholine oxide (NMMO), the method comprising the following steps:

[0067] (i) The preparation of N-methylmorpholine according to the method of the present invention as described above, and

[0068] (ii) subject the N-methylmorpholine obtained in step (i) to an oxidation reaction to obtain N-methylmorpholine oxide.

[0069] Oxidation step (ii) involves reacting N-methylmorpholine with less than a stoichiometric amount of aqueous hydrogen peroxide in an aqueous medium in the presence of carbon dioxide as a promoter.

[0070] In a preferred embodiment, oxidation step (ii) comprises reacting N-methylmorpholine with less than a stoichiometric amount of aqueous hydrogen peroxide in an aqueous medium while applying a vapor space to the aqueous medium 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 tertiary amines to their amine oxides, the applicant has found that amounts of free carbon dioxide exceeding the promoting amount can cause undesirable discoloration.

[0071] In the embodiments, the initial concentration of N-methylmorpholine in the aqueous medium (i.e., the concentration of N-methylmorpholine before the addition of hydrogen peroxide) is in the range of 40 to 95 vol.-%, preferably 60 to 85 vol.-%. Any aqueous hydrogen peroxide can be used. For practical considerations, the concentration of hydrogen peroxide in the aqueous hydrogen peroxide added to the aqueous medium is in the range of 10 to 70 wt.-%, preferably 29 to 51 wt.-%.

[0072] This method is preferably performed continuously.

[0073] The present invention further relates to a method for preparing pentamethyldiethylenetriamine (PMDETA), the method comprising the following steps:

[0074] (i) Provide formaldehyde as a methanol-formaldehyde solution obtained by the method of the present invention as described above.

[0075] (ii) Provide diethylenetriamine,

[0076] (iii) The formaldehyde provided in step (i) and the diethylenetriamine (DETA) provided in step (ii) are subjected to reductive amination in the presence of hydrogen and a hydrogenation catalyst to obtain pentamethyldiethylenetriamine.

[0077] The methanol-formaldehyde solution fed into step (i) is prepared by the method of the present invention as described above, and contains formaldehyde, methanol and water within the range indicated above.

[0078] This method is preferably performed continuously.

[0079] The key to this method is to provide a paraformaldehyde-based methanol-formaldehyde solution using the simple and economical method of the present invention as described above.

[0080] In a preferred embodiment, the heterogeneous hydrogenation catalyst is used as a fixed-bed catalyst.

[0081] This method for producing PMDETA is more efficient than that using paraformaldehyde because amination can be carried out continuously. The corresponding method using paraformaldehyde in amination cannot be operated continuously (because paraformaldehyde is a solid). Since amination requires a heterogeneous catalyst, the latter needs to be separated, which is technically and economically less efficient than using an immobilized catalyst as in step (iii). Furthermore, the continuous method offers better space-time yields (i.e., higher) compared to batch methods.

[0082] In a particularly preferred embodiment, the molar ratio of formaldehyde to DETA is in the range of 4.5:1 to < 5.9:1.

[0083] In a preferred embodiment, PMDETA is partially recycled to the reductive amination of DETA and formaldehyde. Preferably, the weight ratio of the recycled PMDETA to the combined amount of DETA and methanol-formaldehyde solution fed into one or more reactors is in the range of 1:1 to 10:1, preferably 2:1 to 8:1.

[0084] The reaction temperature is typically in the range of 30°C to 300°C, preferably 30°C to 250°C, more preferably 30°C to 200°C, even more preferably 60°C to 150°C, particularly preferably 80°C to 130°C, and especially 90°C to 130°C.

[0085] The reaction pressure is in the range of 50 to 300 bar, preferably 50 to 250 bar, more preferably 50 to 200 bar, even more preferably 60 to 150 bar, particularly preferably 80 to 150 bar, and especially 90 to 140 bar.

[0086] The heterogeneous hydrogenation catalyst preferably comprises cobalt and / or copper. Preferred heterogeneous hydrogenation catalysts are those described above in conjunction with the preparation of NMM. Based on the total weight of the heterogeneous hydrated catalyst, particularly preferred catalysts contain...

[0087] 5 to 90 wt.-%, preferably 10 to 60 wt.-%, of cobalt.

[0088] 1 to 40 wt.-%, preferably 2 to 30 wt.-%, of copper.

[0089] 0.1 to 30 wt.%, preferably 1 to 10 wt.%, of manganese.

[0090] 0.1 to 30 wt.-%, preferably 1 to 10 wt.-%, of molybdenum, and

[0091] 0.05 to 30 wt.-%, preferably 0.1 to 5 wt.-%, of phosphorus.

[0092] The following examples are for illustrative purposes only and should not be construed as limiting the invention in any way. Example

[0093] Example 1:

[0094] Water containing approximately 96 wt.% paraformaldehyde (57 wt.%) was added to a stirred vessel equipped with a condenser. Water (8 wt.%) was added and the reactor was placed under a nitrogen atmosphere. MeOH (35 wt.%) was added and stirring was started (100 rpm). The mixture was heated to 80°C and stirred for 150 min, then stirred at 115°C for 240 min. After cooling, a clear solution containing 55 wt.% formaldehyde was obtained (measured as free formaldehyde by UV-Vis spectroscopy).

[0095] Example 2:

[0096] Water containing approximately 77 wt.% paraformaldehyde (55 wt.%) was added to a stirred vessel. Water (10 wt.%) was added and the reactor was set under a nitrogen atmosphere. MeOH (35 wt.%) was added and stirring was started (100 rpm). The mixture was heated to 80°C and stirred for 150 min, then stirred at 115°C for 240 min. After cooling, a clear solution containing 42.4 wt.% formaldehyde was obtained (measured as free formaldehyde by UV-Vis spectroscopy).

[0097] Example 3:

[0098] Water containing paraformaldehyde (65 wt.%) with a paraformaldehyde content of approximately 92 wt.% was added to a stirred vessel equipped with a condenser, and the reactor was set under a nitrogen atmosphere. MeOH (35 wt.%) was added, and stirring was started (100 rpm). The mixture was heated to 80°C and stirred for 150 min, then stirred at 115°C for 240 min. After cooling, a clear solution containing 60 wt.% formaldehyde was obtained (measured as free formaldehyde by UV-Vis spectroscopy).

[0099] Example 4 (Evidence for the formation of formaldehyde hemiacetal and its higher polyoxymethylene homologues):

[0100] Water containing approximately 96 wt.-% paraoxymethylene (57 wt.-%) was added to a stirred vessel equipped with a condenser. Water (8 wt.-%) was added and the reactor was set under a nitrogen atmosphere. MeOH (35 wt.-%) was added and stirring was started (100 rpm). The mixture was heated to 60°C and stirred for 120 min, and 13C NMR spectra were measured. The spectra indicated the formation of methoxymethanol (CAS = 4461-52-3; formaldehyde hemiacetal) and its higher paraoxymethylene homologues. No indication of higher molecular structures or free formaldehyde was found in the solution.

[0101] Example 5 (Comparison):

[0102] Water containing paraformaldehyde (55 wt.%) with a paraformaldehyde content of approximately 96 wt.% was added to a stirred vessel equipped with a condenser. Water (10 wt.%) was added and the reactor was set under a nitrogen atmosphere. MeOH (35 wt.%) was added and stirring was started (100 rpm). The mixture was heated to 80°C and stirred for 150 min + 240 min (i.e., a total of 390 min). After cooling, a white suspension was obtained. This indicates that the paraformaldehyde has not completely depolymerized. Using this suspension in a continuous setup is not feasible due to pump and other equipment blockages.

[0103] Discussion of the results:

[0104] When heated to temperatures in the range of 50°C to 90°C, depolymerization is incomplete (see Comparative Example 5). When directly heated to temperatures in the range of 100°C to 130°C, methanol continues to evaporate (requiring very high condenser loads). There is too much methanol in the gas phase. Therefore, depolymerization proceeds unevenly. Initial heating to 60°C to 90°C leads to the formation of a hemiacetal (i.e., methanol is converted into a compound with a higher boiling point). If this conversion has already been completed, complete depolymerization is achieved at higher temperatures of 100°C to 130°C.

Claims

1. A batch process for preparing a methanol formaldehyde solution by depolymerization of paraformaldehyde in the presence of methanol and water, the process comprising the following steps: (a) providing a mixture comprising paraformaldehyde, water and methanol, (b) subjecting the mixture obtained in step (a) to a first heating step, wherein the mixture is held in a temperature range of 50 °C to 90 °C for a time period of 5 to 300 minutes, (c) subjecting the mixture obtained in step (b) to a second heating step, wherein the mixture is held in a temperature range of 100 °C to 130 °C for a time period of 0.1 to 48 hours.

2. The method of claim 1, wherein, The temperature range according to step (b) is 60 °C to 90 °C, preferably 70 °C to 90 °C, more preferably 75 °C to 85 °C.

3. The method of claim 1 or 2, wherein, The time period according to step (b) is 30 to 250 minutes, preferably 60 to 200 minutes, more preferably 90 to 200 minutes, even more preferably 120 to 200 minutes.

4. The method of any one of claims 1 to 3, wherein, The temperature range according to step (c) is 110 °C to 130 °C, preferably 110 °C to 125 °C, more preferably 110 °C to 120 °C.

5. The method of claim 4, wherein, The time period according to step (c) is 0.5 to 36 hours, preferably 1 to 24 hours, more preferably 1 to 12 hours, even more preferably 2 to 12 hours.

6. The method of any one of claims 1 to 5, wherein, The absolute pressure is in the range of 0.5 to 20 bar, preferably 0.5 to 10 bar, more preferably 0.5 to 5 bar, particularly preferably 0.8 to 3 bar, in particular 0.9 to 2 bar, for example 1.0 to 1.5 bar.

7. The method of any one of claims 1 to 6, wherein, The mixture provided in step (a) comprises 20 to 69 wt.-% of paraformaldehyde, 30 to 69 wt.-% of methanol and 1 to 20 wt.-% of water.

8. A process for preparing N-methylmorpholine, the process comprising the following steps: (i) providing formaldehyde as a methanol formaldehyde solution obtained by a process according to any one of claims 1 to 7, (ii) providing morpholine, (iii) subjecting the formaldehyde provided in step (i) and the morpholine provided in step (ii) to a reductive amination in the presence of hydrogen and a hydrogenation catalyst to obtain N-methylmorpholine.

9. The method of claim 8, wherein, The morpholine provided in step (ii) is prepared by reacting diethylene glycol and ammonia in the presence of hydrogen and a heterogeneous hydrogenation catalyst.

10. A process for preparing N-methylmorpholine oxide, the process comprising the following steps: (i) preparing N-methylmorpholine according to the process of claim 8 or 9, and (ii) subjecting the N-methylmorpholine obtained in step (i) to an oxidation reaction to obtain N-methylmorpholine oxide.

11. The method of claim 10, wherein, The oxidation reaction of step (ii) comprises reacting N-methylmorpholine with less than the stoichiometric amount of aqueous hydrogen peroxide in an aqueous medium in the presence of carbon dioxide as a promoter.

12. The method of claim 11, wherein, The oxidation step (ii) comprises reacting N-methylmorpholine with less than stoichiometric amounts of aqueous hydrogen peroxide in an aqueous medium, while applying a vapor space of carbon dioxide partial pressure p(C02) of less than 0.75 bar absolute, preferably less than 0.20 bar absolute, over the aqueous medium.

13. A process for the preparation of pentamethyldiethylenetriamine, comprising the steps of: (i) providing formaldehyde as a methanolic formaldehyde solution obtained by the process according to any one of claims 1 to 7, (ii) providing diethylenetriamine, (iii) subjecting the formaldehyde provided in step (i) and the diethylenetriamine provided in step (ii) to reductive amination in the presence of hydrogen and a hydrogenation catalyst to obtain pentamethyldiethylenetriamine.