Microwave-assisted acetylation of amines

Abstract

The present invention relates to a continuous process for the acetylation of diamines or polyamines, characterized in that a mixture containing the components a) one or more diamines or one or more polyamines, b) Acetic anhydride and c) Acetic acid in a monomodal microwave system under microwave irradiation of a standing wave.

Description

The present invention relates to a microwave-assisted process for the complete acetylation of amines, in which the reaction time is seconds to minutes, undesired side reactions and impurities are virtually completely omitted and high conversions are achieved with reduced energy consumption.Tetraacetylethylenediamine (TAED) is an important ingredient for laundry detergents and cleaners and is used as active ingredient in human hygiene biocidal products.TAED is prepared from ethylenediamine and acetic anhydride in a two-stage process by known processes DE 28 16 174 A1, DE 21 33 458 B2. The first reaction yields the symmetrical diacetylated N,N'-diacetylethylenediamine (DAED). The second reaction, on heating DAED for several hours with an excess of acetic anhydride at 120°C to 190°C, results in an equilibrium distribution between TAED, DAED and triacetylethylenediamine as well as dark brown polymers produced by ketene which is formed as a byproduct. A disadvantage is a high outlay for obtaining pure TAED.WO 2011 / 000462 A1 discloses the preparation of amides of aliphatic carboxylic acids in which at least one carboxylic acid ester is reacted with an amine, for example N,N-dimethylaminopropylamine, to form the carboxamide under microwave irradiation in a reaction tube whose longitudinal axis is located in the direction of propagation of the microwaves of a monomode microwave applicator.WO 2009 / 121485 A1 claims a continuous process for preparing fatty acid alkanolamides by reacting a fatty acid with alkanolamine under microwave irradiation.The known processes have disadvantages in that a high energy input is necessary for the preparation of tetraacetylethylenediamine (TAED) and in that a series of undesirable by-products are formed, which adversely affect the efficiency of these processes. The isolation of pure TAED requires a high outlay.It is therefore an object of the present invention to provide an improved process for acetylation of amines which does not exhibit the above-mentioned disadvantages.This object is achieved according to the invention by a continuous process for acetylation of diamines or polyamines, characterized in that a mixture comprising components a) one or more diamines or one or more polyamines, b) acetic anhydride and c) acetic acid is reacted in a monomodal microwave installation with microwave irradiation of a standing wave.A continuous process is understood to mean that a reaction medium is passed through a reactor with constant flow. The reactor can be configured as desired, preferably having a tubular geometryFor the purposes of the present invention, "monomodal microwave installation" (also "single-mode microwave installation") is understood to mean a microwave installation which is characterized in that it contains a microwave reactor in which a standing electromagnetic wave is built up which runs in one direction.Surprisingly, it has been found that acetylation of diamines and / or polyamines with acetic anhydride leads to substantially complete acetylation of the nitrogen group if a mixture of components a) diamines or polyamines, b) acetic anhydride and c) acetic acid is reacted in a continuous process in a monomodal microwave installation with electromagnetic microwave irradiation of a standing wave. This realizes a significantly higher yield with largely reduced by-products.The process according to the invention advantageously achieves virtually complete acetylation and the formation of dark brown polymers is significantly reduced. Upon cooling the reaction product, a crystalline product is obtained which is purified by washing with cold acetic acid or a mixture of acetic acid / acetic anhydride. A light beige product is thus obtained. Energy-intensive distillation steps can be saved. Incompletely reacted reaction components can be used without further workup in the production process; the continuous formation of dark-colored polymers in the process according to the prior art is omitted in the process according to the invention. This results in virtually 100 percent conversion of component a).In a preferred embodiment of the process according to the invention, component a) is N,N'-diacetylethylenediamine (DAED) which is reacted to give N,N,N',N'-tetraacetylethylenediamine (TAED) of the formula (I) The reaction of DAED with acetic anhydride in the presence of acetic acid with the process according to the invention advantageously leads to complete acetylation of the amine groups with optimum process control. An increasing accumulation of dark colored polymers in the reaction mixture is avoided.Upon cooling the reaction product crystalline TAED is obtained which is purified by washing with cold acetic acid or a mixture of acetic acid / acetic anhydride to give a light beige product. Energy-intensive distillation steps can be saved.Unreacted reaction components can be recycled into the reaction process without purification steps.Particular preference is given to a continuous process according to the invention in which the yield of N,N,N',N'-tetraacetylethylenediamine of the formula (I) is ≥90%, based on the amount of N,N'-diacetylethylenediamine used.A further preferred embodiment provides that the molar ratio of component a) to component b) is in the range from 1:2 to 1:4, preferably in the range from 1:2.5 to 1:4, particularly preferably 1:2.8 to 1:3.Yields of TAED ≥ 95% can thus be achieved.The molar ratios of components a) and b) preferably used in the process of the invention also suppress undesirable by-products. A marked reduction of waste materials is achieved.It is further preferred according to the invention if the molar ratio of component c) to the molar ratio of component a) is in the range from 0.01:1 to 1:1.To optimize the yield and the purity of TAED, the reaction mixture obtained by the process according to the invention is cooled to below 80° C. to 20° C., preferably with stirring, and TAED crystallizes out. Needle formation can be suppressed by stirring during crystallization.The crystallized TAED is separated from the liquid residue on a band filter and washed with acetic acid cooled to 15°C to 0°C or a mixture of acetic acid and acetic anhydride cooled to 15°C to 0°C. The liquid residue is combined with the acetic acid used for washing or a mixture of acetic acid and acetic anhydride used for washing.Essentially, the combined mixture contains acetic acid, acetic anhydride, optionally DAED, and the intermediate triacetylethylenediamine. The combined mixture can be used for a renewed production process of the process according to the invention. The preferred molar ratios of components a) b) and c) are advantageously to be adjusted. In this way, a virtually 100% yield of TAED from the components DAED and acetic anhydride and acetic acid can be realized.With the molar ratios of components a) and c) preferably used in the process according to the invention, virtually complete acetylation of amines is achieved. In particular, the formation of TAED is achieved in amounts >98% by weight, based on DAED used.A further development of the method according to the invention provides that the reaction time in the monomodal microwave installation is in the range from 1 second to 60 minutes, preferably 30 seconds to 40 minutes, particularly preferably 1 minute to 30 minutes.The short reaction times represent a considerably lower energy consumption compared to conventional processes.It has been found to be advantageous if the mixture comprising components a), b) and c) reaches a temperature of 120° C. to 200° C., preferably 150° C. to 195° C., particularly preferably 180° C. to 190° C. In the electromagnetic field, the microwave energy is coupled directly into the molecules of the reaction mixture and the required reaction temperature is achieved within seconds.In one embodiment of the process according to the invention, the mixture comprising components a), b) and c) reaches a pressure of 2 bar to 20 bar, preferably 3 bar to 15 bar, particularly preferably 4 bar to 8 bar, during the microwave irradiation. The pressure setting is achieved by means of a high-pressure pump and supply of inert gas, preferably nitrogen.In a further preferred embodiment, the continuous process according to the invention is carried out in a manner such that the irradiation of the mixture of components a), b) and c) with microwaves is carried out in a reaction tube whose longitudinal axis is located in the direction of propagation of the microwaves in a monomodal microwave applicator. This construction enables the standing electromagnetic wave to have more than one wavelength of the microwave radiation used. The coupling of energy to the reaction material can thus be increased.A modification of the continuous process according to the invention provides that the reaction medium is transported through a rectangular waveguide in a cavity parallel to the propagation direction of a standing electromagnetic wave in the TE10 mode (transverse electric mode) or perpendicular to the propagation direction of a standing electromagnetic wave in the TE10 mode, wherein standing waves have formed in the rectangular waveguide and the length of the waveguide is between half a wavelength and up to 20 wavelengths, preferably 2 to 10 wavelengths, particularly preferably 3 to 6 wavelengths of the standing wave. Typically, the cross section of the waveguide has a width of half a wavelength and a height of a quarter of the wavelength of the standing electromagnetic wave, so that the latter forms a TE10mode. The number of wavelengths allows the optimum coupling of energy into the reaction medium to be controlled.A particularly preferred continuous process according to the present invention is characterized in that the number of field maxima of the standing wave is 2 to 14, preferably 3 to 8, particularly preferably 4 to 6. Thus, for the reaction mixture containing DAED, acetic anhydride and acetic acid, optimum coupling of energy to the reaction mixture is achieved.Further objects, features, advantages and possible applications will become apparent from the following description of exemplary embodiments which do not restrict the invention. All features described form the subject matter of the invention, either alone or in any combination, even independently of their summary in the claims or their reference back.Preferably, the mixture of components a), b) and c) is irradiated with microwaves in a substantially microwave-transparent reaction tube which is located within a waveguide connected to a microwave generator. The reaction tube is preferably aligned axially with the central axis of symmetry of the hollow conductor.The hollow conductor functioning as a microwave applicator is preferably shaped as a cavity resonator. Furthermore, the microwaves not absorbed in the waveguide are preferably reflected at its end. By shaping the microwave applicator as a resonator of the reflection type, a local increase in the electric field strength with the same power supplied by the generator and an increased energy utilization are achieved.The cavity resonator is preferably operated in the E01nmode, wherein n stands for an integer and specifies the number of field maxima of the standing microwave along the central axis of symmetry of the resonator. In this operation, the electric field is directed towards the central axis of symmetry of the cavity resonator. It has a maximum in the region of the central axis of symmetry and decreases towards the lateral surface to the value zero. This field configuration is rotationally symmetrical about the central axis of symmetry. Depending on the desired flow rate of the reaction material through the reaction tube, the required temperature and the required residence time in the resonator, the length of the resonator relative to the wavelength of the microwave radiation used is selected. Preferably, n is an integer from 1 to 20, particularly preferably from 2 to 14, in particular from 4 to 6.The microwave energy can be radiated into the waveguide functioning as a microwave applicator via suitably dimensioned holes or slots. In an embodiment which is particularly preferred according to the invention, the ammonium salt is irradiated with microwaves in a reaction tube which is located in a hollow conductor with coaxial transition of the microwaves.Microwave devices which are particularly preferred for this method are constructed from a cavity resonator, a coupling device for coupling a microwave field into the cavity resonator and each with an opening on two opposite end walls for passing the reaction tube through the resonator. The microwaves are preferably coupled into the cavity resonator via a coupling pin which projects into the cavity resonator. The coupling pin is preferably formed as a preferably metallic inner conductor tube functioning as a coupling antenna.In a particularly preferred embodiment, this coupling pin protrudes through one of the front-side openings into the cavity resonator. Particularly preferably, the reaction tube adjoins the inner conductor tube of the coaxial transition and, in particular, it is guided through its cavity into the cavity resonator. The reaction tube is preferably aligned axially with a central axis of symmetry of the cavity resonator, for which purpose the cavity resonator preferably has a central opening each on two opposite end walls for the passage of the reaction tube.The microwaves can be fed into the coupling pin or into the inner conductor tube functioning as a coupling antenna, for example, by means of a coaxial connecting line. In a preferred embodiment, the microwave field is supplied to the resonator via a hollow conductor, wherein the end of the coupling pin protruding from the cavity resonator is guided into the hollow conductor in an opening located in the wall of the hollow conductor and draws microwave energy from the hollow conductor and couples it into the resonator.The following example is intended to illustrate the invention without limiting it thereto.Example: Preparation of N,N,N',N'-tetraacetylethylenediamineThe microwave-assisted reaction was carried out in an apparatus which contained, as reaction tube, a borosilicate glass tube (length: 160 cm, diameter inside 1.0 cm) which was located axially symmetrically in a cylindrical cavity resonator (90 cm x 10 cm) (irradiation zone). At one of the end faces of the cavity resonator, this borosilicate glass tube runs through the cavity of an inner conductor tube functioning as a coupling antenna. The microwave field generated by a magnetron and having a frequency of 2.45 GHz was coupled into the cavity resonator by means of the coupling antenna (E01 cavity applicator; mono-mode), in which a standing wave formed. The microwave system was under N 2- atmosphere.The microwave power was adjusted over the duration of the test in each case in such a way that the desired temperature of the reaction mixture at the end of the reaction tube was kept constant. The mentioned microwave power represents the temporal mean value of the radiated microwave power. The temperature measurement of the reaction mixture was carried out directly at the end of the reaction tube.Microwave energy not directly absorbed by the reaction mixture was reflected at the end face of the cavity resonator opposite the coupling antenna; the microwave energy not absorbed by the reaction mixture even during the return and reflected back in the direction of the magnetron was conducted with the aid of a prism system (circulator) into a vessel containing water. From the difference between the energy introduced and the power loss (determined by heating the water load), the microwave energy introduced into the reaction mixture was calculated.Example: Microwave Assisted Production of TAED on Industrial ScaleIn a 150 l receiver with gas inlet tube, stirrer, internal thermometer and pressure compensation, 20 kg (138.88 mol) of N,N'-diacetylethylenediamine (DAED), 42.536 kg (416.64 mol) of acetic anhydride and 840 g of concentrated acetic acid were homogenized with stirring and heated to 80° C.The reaction mixture thus obtained was pumped continuously through the apparatus at 15 l / h at a working pressure of 4 bar and exposed to a microwave power of 2.5 kW, 90% of which were absorbed by the reaction material. The residence time of the reaction mixture in the reaction tube was about 55 seconds. At the end of the reaction tube, the reaction mixture had a temperature of 195° C.The reaction mixture was then cooled to 80° C. in the heat exchanger, expanded to atmospheric pressure in the separator and cooled further to room temperature with slow stirring.Stirring during the cooling phase suppresses the formation of larger crystals and precipitates a small-grain product consisting essentially of TAED. The product is filtered off and washed with 15 L acetic acid / acetic anhydride mixture at 10 °C. The yield of TAED was 92 wt.-% based on the N,N'-diacetylethylenediamine used.The filtrate and acetic acid / acetic anhydride wash are combined and the molar composition of the reddish mixture determined analytically. The composition is adjusted to the molar ratio required for the synthesis by addition of DAED and acetic anhydride and acetic acid and is transferred back to the microwave apparatus without further processing steps. The reaction solution and the washing solution can be repeatedly fed into the microwave process without discoloration.The recycling of filtrate and wash water of acetic acid / acetic anhydride to the production process was repeated 8 times without forming dark colored polymers. This allowed the conversion of DAED to TAED to be achieved by nearly 100 percent.Analysis was carried out by means of 1H-NMR spectroscopy at 500 MHz in CDCl 3.References included in the specificationThis list of documents cited by the applicant has been produced in an automated manner and is only included for the better information of the reader. The list is not part of the German patent application or utility model application. The DPMA does not take any adhesion for any faults or omissions.Patent Literature citedDE 28 16 174 A1

[0003] DE 21 33 458 B2

[0003] WO 2011 / 000462 A1

[0004] WO 2009 / 121485 A1

[0005]

Claims

Continuous process for acetylation of diamines or polyamines, characterised in that a mixture comprising components a) one or more diamines or one or more polyamines, b) acetic anhydride and c) acetic acid is reacted in a monomodal microwave installation with microwave irradiation of a standing wave.The continuous process according to claim 1, wherein component a) is N,N'-diacetylethylenediamine which is reacted to give N,N,N',N'-tetraacetylethylenediamine according to formula (I) Continuous process according to Claim 1 or 2, wherein the yield of N,N,N',N'-tetraacetylethylenediamine of the formula (I) is ≥ 90%, based on the amount of N,N'-diacetylethylenediamine used.The continuous process according to any one of claims 1 to 3, wherein the molar ratio of component a) to component b) is in the range of from 1:2 to 1:4.The continuous process according to any one of claims 1 to 4, wherein the molar ratio of the component c) to the molar ratio of the component a) is in the range of 0.01:1 to 1:1.The continuous process of any one of claims 1 to 5, wherein the reaction time in the monomodal microwave system is in the range of from 1 second to 30 minutes.The continuous process according to any one of claims 1 to 6, wherein the mixture containing components a), b) and c) reaches a temperature of 120°C to 200°C during microwave irradiation.The continuous process according to any one of claims 1 to 7, wherein the mixture containing components a), b) and c) reaches a pressure of 2 bar to 20 bar during microwave irradiation.The continuous process according to any one of claims 1 to 8, wherein the irradiation of the mixture of components a), b) and c) with microwaves is carried out in a reaction tube whose longitudinal axis is located in the microwave propagation direction in a monomodal microwave applicator.Continuous process according to any of claims 1 to 9, wherein the number of field maxima of the standing wave is 1 to 20, preferably from 2 to 14, particularly preferably 4 to 6.