PREPARATION OF ISOCYANATES WHICH HAVE A CLEAR COLOR The present invention relates to a process for the preparation of isocyanates, particularly polyisocyanates of the diisocyanate series of
d pheni Imethane, which have a light color »Isocyanates are raw materials for the production of polyurethane. They are usually prepared by reacting the corresponding amines with phosgene. One of the most widely used isocyanates is 0 di phenyl ethane diisocyanate (MDI > and also its higher homologs (commonly known as crude MDI.) Due to the initial amine, which is a mixture of -diphenyl andia inas and pol faith. i lenpal ime i lenpol i ina (MDA), and the process of MD production which comprises the reaction of MDA with phosgene and the subsequent preparation of the reaction product after separation of the solvents and distillation removal of monomeric MDI, the obtained crude MDI has a dark color and leads to the production of polyurethane materials that D have a yellowish peeling.Since these discolorations are considered as quality defects, numerous attempts have been made to provide an MDS. Thus, document US-A ~ 5,3,4,495 describes a process for preparing MDI in which the phosgene is separated at low temperatures after phosgenation. and the remaining isocyanate is treated with an HCl gas to destroy the discoloration causing compounds, usually known as color precursors. EP-A-5BI 100 describes the addition of chemical reducing agents to the reaction product after the phosgenation, but before the separation of the solvent. However, such processes usually provide only an incomplete clearance of the MDI. A possible additional way to clear the color of MDS comprises the addition of additives to the crude product fosgenads. Accordingly, US Pat. No. 4,465,639 describes the addition of water, EP-A-538 500 describes the addition of two car-boi. 1, EP-A-445 602 describes the addition of alkanols and EP-A-467 125 describes the addition of polyethers to the product of the phosgenation reaction. These methods have the disadvantage that the reaction may be accompanied by unwanted side reactions. The treatment of the final MDI product after preparation is also known. Thus, EP-A-133 53S describes the purification of isocyanates by extraction. The documents EP-A-56Í 225 and £ _ß_¿.7¿. 391 disclose the subsequent hydrogenating treatment of MDI for the purpose of lightening the color, and EP-A-676 391 discloses the use of specific hydrogenation ion catalysts. These processes also result in an incomplete clearance of the isocyanates. A possible additional method to clarify an MDI is the 5 treatment of the MDA used as a raw material. EP-A-546 398 proposes the acidification of ΔDA with hydrochloric acid before phosgenation. However, the effect of this measure is unimportant. EP-446 781 describes a process for the preparation of clear MDI 10 by the hydrogenating treatment of MDA "However, the drastic conditions of such hydrogenation can lead to interfering side reactions. In addition, it is possible to alter the process conditions in the preparation of MDA. However, secondary reactions that can lead to V.D change in the other properties of the MDA can not be excluded in such a case. It is an object of the present invention to develop the process for preparing isacyanates, particularly crude MDI, which have a clear color, said process is simple, does not require intrusions in the process and provides effective color clearing. We have found that this object is achieved by treating the amines with solid inorganic substances that contain Lewis acid centers and / or acid centers
of Bronsted and by the subsequent phosgenation of the amines that have been treated in this manner. The present invention therefore provides a process for the preparation of isocyanates having a clear color, by reacting the amines with phosgene, wherein the amines are treated with solid inorganic substances containing Lewis acid centers and / or BrSnsted acid before reaction with phosgene. As solid inorganic substances containing Le? S acid centers and / or Bronsted acid centers, reference is made particularly to the use of compounds having a large internal surface area and can therefore come in contact with the liquid or the dissolved amines. It is preferred to use natural, artificially modified and synthetic oxide and / or silicates. Par icularly 15 alumino ilica os. Examples of these substances are activated aluminum oxide, activated silicas and silica gel. Activated silicates are preferred, particularly aluminum silicates such as zeolites, bentonite, kaolinites and mantonicides. Very particularly, activated materials of the type sntmori 1 lonitas are preferred. The activation can be carried out thermally or chemically. Chemical activation is described, for example, in P. Ku ar et. al., "Evolution of Poros i and and 25 Burface Acidity in Mantmo i 1 loni e Clay on Acid Activation,"
- U t-Uncle filtration, centrifugation-p > ~ H - =. R--. < -.-.-__ < - * (Evolution of porosity and surface acidity in ontmo clay 1 lanite during an acid activation), Ind. Eng. Chem. Res., Í 95, 1440-1448. The substances used in accordance with the present invention can be used not only in their natural form but, with particular preference, with acids and metal ions in accordance with that described, for example, in P. Laszlo, "Chemical Reactions on Clays (Reactions). Chemicals in Clay), Science 235, (1987), 1473. The size and the morphology of the particles of the solid inorganic substance is not an especially critical factor for the process of the present invention. in macroscopic forms that can be easily separated from the amine after treatment.In general terms, the finer the inorganic material, the better the color clearance action and the greater the separation difficulty, and vice versa. habitually by known methods for the removal of liquid system solids, for example by filtration medium, centrifugation Decanting The solid inorganic substances can be used either individually or as mixtures. When used as mixtures, it is helpful that at least one component comes from the group consisting of montmori 1 lonitas. In principle, it would also be possible to employ highly polymeric organic substances * having Lewis acid centers and / or Bronsted acid centers in place of the solid inorganic substances. However, the organic compounds do not play any role here because in the case of the known compounds of this type, it is not possible to rule out interference reactions of the polymer structure with the amines and / or the solvents. Co or amines subjected to a treatment in accordance with the present invention, with the solid inorganic substances,
All organic amines that can react with phosgene to provide isocyanates are suitable in principle. This treatment is particularly advantageous in the case of aromatic amines, particularly difeni I etandiamin - (MDA) and its higher homologs, since the polyisocyanate of
pol ifeni lenpol imetilens (rough MDI) tends particularly to strong discoloration. The treatment of amines must be carried out in the phase
* liquid, in order to ensure sufficient contact between the amine and the solid inorganic substances »
the constitution of amines, can be helpful to employ amines solutions. Solvents which can be used herein are all solvents which are inert in relation to the amines, for example esters such as for example ethyl acetate or butyl acetate, ethers as for example ether
However, it is preferred to use inert aromatic or aliphatic hydrocarbons or halogenated hydrocarbons, for example chlorobenzene, o-dicylarobenzene, toluene, xi logs, methyl tert-butyl or ketones such as propanone, butanone or cyclohexanone. , - 5-alkyl-1-benzenes and alkylanes, available for use in the subsequent phosgenation process and where both the amine and the isocyanate prepared from it are soluble, and which are not attacked in the process steps. of the treatment ie amines and phosgenation. The purification process of the present invention can be carried out by suspending the solid inorganic substances in the amine or the amine solution and by subsequently separating the solids. However, the solid is also fixed in a fixed bed and passing the amine or the amine solution over it. The first variant of the indicated process is carried out in one place in batches. Reaction vessels used aqμi are mixing vessels, for example stirred reactors. After the inorganic solid has acted on the amine, it is separated according to what has been described above. The treatment by means of a fixed bed was carried out mainly continuously, for example in columns or flow tubes. The treatment of the amine according to the present invention using inorganic solid substances is preferably carried out at a temperature between -10 ° C and 100 ° C, preferably from IO to 50 ° C, and under pressure. from Ol to 20 bars, preferably from 0.1 to 10 bars. The amount of inorganic solid substance should be from 0.1 to 30% by weight, preferably from 0.5 to 15% by weight, based on the amount of amine. The treatment time is from 1 second to 5 hours, preferably from 1 minute to 1 hour. The time depends, inter alia, on the type of amine used and on the effectiveness of the inorganic solid substance and can be determined by comparative experiments. The solvent can be separated after the treatment according to the present invention, but this is not absolutely necessary. Since the amine is used for the preparation of isocyanates, it is useful that the amine solution, after the
* separation of the solid inorganic substance, feed directly to the phosgenation. The amines treated by the process of the present invention can react without problems to provide isocyanates having a significantly lighter color than the color of the isocyanates prepared from untreated amines. The purpose of the present invention can be employed in. particularly useful way in the preparation of diphenylmethane diisocyanate and its known higher homologs with "crude MDI". The MDA mixture of two rings and multiple rings used as an amine, which is prepared according to known methods by condensation of aniline with formaldehyde in the presence of hydrochloric acid, comprises a large number of compounds that cause the dark color after the reaction of MDA to form MDI. He
The use of the process of the present invention allows a significant improvement in the color of the raw MDI. The obtained crude MDI can be processed to give paliurethanes that have a light color. The process of the present invention is simple to perform and can be easily integrated into existing plants for the production of isocyanates. Due to the very gentle treatment, it does not produce secondary reactions in the amine and therefore does not cause the acceptance of the quality of the isocyanate. After the treatment, the solid inorganic substances can be virtually separated 2G quantitatively from the amine. After separation, the solid inorganic substances can be regenerated by known and rejected methods, or they can be discarded. The invention is illustrated through the following examples. Examples 1 to 6 # In each case, 150 g of neutralized and dried MDA were dissolved from the acid condensation of aniline and formaldehyde in 30 ml of toluene. Then 20 g of the inorganic substance q? Table 1 and 5 the solution was stirred for 10 to 30 minutes. Subsequently, the organic substance was separated by means of a pressure filtration unit to one. anamometric pressure of 3 bar of nitrogen and the clear filtrate was released from the solvent toluene under reduced pressure. In each case, 100 g of the MDA treated in this way were dissolved in 1 liter of monolaobenzene and reacted at a temperature of 5O-80 ° C under atmospheric pressure in a stirred reactor of 6 liters with 200 g of phosgene, dissolved in 1.3 1 ds onoclorobenzene, to provide the carbamoyl chloride. The temperature of the reaction mixture was increased to 120 ° C over a period of 2 hours, during which the reaction to provide the isocyanate occurred. The remaining phosgene and most of the monochlorobepcene were subsequently removed and thereafter removed. remaining monochlorobenzene at 10 mbar and 120QC. The reaction mixture was then drained and subjected to thermal treatment after 10 mbar and ISO'C for 45 minutes in a rotary evaporator. The LC content of the MDI samples obtained in this way was determined in accordance with DIN 53 185, the * EHC (chlorohydrolyzate hydrolyzate) values were determined in accordance with ASTM D 4667-87 and the values
DHC (hard hydrolyzed chlorine) were determined in accordance with ASTM D 4663-87. 5 In addition, the iodine color index was determined in accordance with DIN 6162. For this purpose, samples diluted 1: 5 with monochlorobenzene were examined using a comparative Hellige comparator.
• - (with color discs corresponding to the iodo iO color indices.) A photometric determination of the color index was also carried out by means of a photometer
Do tor Lange in the iodo color index mode. The results are shown in Table 1. Table 1 15 Ex. Additive NCO EHC DHC I CN 1 Í 5 CN 1: 5 (.) (Pp 'f (ppm) * > * - *) 1 ont orí llonita 32.1 237 758 35 1 K 10 (20 g) 20 2 (C) - 32.2 316 915 80 41 3 Silicone seal 32.0 215 820 50 26 60 (20 g) 4 ontmori llonite 31.9 209 736 30 17 10 (20 g) 25 5 molecular sieve 31.8 190 730 35 21 10 A (20 g) 6 montmorilloni at 31.6 182 769 50 25 KSF (20 g)
Comparative SCN color index iodo CN color index photometically determined *) measured by the comparator * * measured by means of the photometer