MXPA00000015A - Method for esterification of (meth)acrylic acids - Google Patents

Abstract

The invention relates to a method for producing alkyl(meth)acrylate by continuous reaction of an aqueous solution containing (meth)acrylic acid obtained directly from gas-phase oxidation by reacting with a straight-chain or branched alkanol with 1 to 6 C atoms or a mixture of two or more thereof in the presence of an esterification catalyst. The inventive method is characterized in that alkanol is introduced into aqueous solution containing (meth)acrylic acid in the form of a gas.

Description

ESTERIFICATION OF ACID (MET) ACRYLIC The present invention relates to a process for preparing an alkyl (meth) acrylate by continuous reaction of an aqueous solution containing (meth) acrylic acid obtained directly from the oxidation in gas phase with an alcohol. The term "(meth) acrylic acid" used in this application refers to both acrylic acid and ethacrylic acid. Methacrylic acid can be obtained, inter alia, by gas phase oxidation of 1-butene, isobutene, isobutyraldehyde, isobutyric acid, MTBE and / or ethochleline with oxygen or oxygen-containing gases in the presence of catalysts such as multimetal oxides containing the molybdenum elements and vanadium in the form of oxide. Preference is given at the start from methacrolein. The oxidation is carried out at elevated temperatures and also, due to the high heat of the reaction, preferably with dilution of the reactants with inert gases such as N2, C02 and / or hydrocarbons and / or water vapor. However, this process produces non-pure methacrylic acid, but the reaction mixture containing, in addition to methacrylic acid, the unreacted starting material such as methacrolein, water vapor, inert diluent gas (for example nitrogen) and by-products (by examples are carbon oxides), lower aldehydes such as formaldehyde, high-boiling constituents such as citraconic acid and also acids such as acetic acid, acrylic acid, formic acid and / or propionic acid, in particular acetic acid (see, for example EP-A 0 253 409 and DE-A 19 62 431). However, other possible initial compounds are those from which the initial material is first formed, for example etacrolein, as an intermediate in gas phase oxidation. Examples that may be mentioned are tert-butanol methyl ether (MTBE). To prepare methacrolein, it is also possible to condense formaldehyde and propionaldehyde and obtain methacrolein by subsequent distillation. Such a process is described in EP-B 0 058 927. The methacrolein obtained in this way can then be reacted in a conventional manner by catalytic, gas phase oxidation to form methacrylic acid. This reaction is described, inter alia, in EP-B 0 297 445. The acrylic acid can, inter alia, be obtained analogously starting from the corresponding C3 compounds, in particular propene and / or acrolein. In general, the reaction mixture obtained in a process as described above for the preparation of (meth) acrylic acid is first subjected to a condensation step, giving an aqueous solution containing (meth) acrylic acid.

The esterification of (meth) acrylic acid with an alkanol such as methanol to give an alkyl (meth) acrylate such as methyl (meth) acrylate and water is an equilibrium reaction that moves increasingly towards the side of the initial materials with increasing water content. It is known that these equilibrium reactions can be displaced to the side of the product by the use of an excess of one of the initial components (usually the alkanol) or by eliminating the products of the reaction (water and alkyl (meth) acrylate) In addition, it is known that the esterification can be considerably accelerated by the addition of suitable acids Thus, in JP-A 7 301 369 a mixture of methacrylic acid with an excess of alkanol is esterified ( molar excess: 1.2-5.0) in the presence of an acid catalyst Furthermore, numerous esterification processes are known in which the aqueous solution of methacrylic acid is used.In these processes, methacrylic acid is generally separated from water by extraction and subsequently esterified with an alkanol Methacrylic acid and water can not be separated by distillation due to the formation of an azeotrope of these two compounds. Thus, DE-A 29 07 602 describes a process for the preparation of alkyl (meth) acrylates from dilute aqueous solutions of methacrylic acid, in which the methacrylic acid is separated from the water by extraction with an inert solvent with high boiling point and subsequent distillation and then esterified with an alkanol in the presence of an acid catalyst. These processes all have the disadvantage of an additional separation problem that is caused by the introduction of the extractant and makes a series of separation steps necessary to obtain the (meth) acrylic acid or the (meth) acrylic ester without residues of the extractant. In addition, US-A 43 31 813 describes the catalytic preparation of methyl methacrylate from an aqueous solution of isobutyric acid and methanol in the gas phase at 410 ° C and a pressure of 1 × 10 5 Pa. The direct esterification of a solution aqueous methacrylic acid is described in DE-A 31 46 191. According to this document, an esterification mixture containing an aqueous solution of methacrylic acid (content of methacrylic acid: 5-60% by weight) is continuously reacted with an alkanol in the presence of sulfuric acid or an organic sulfonic acid to obtain the corresponding ester and the latter subsequently distilled. An object of the present invention is to provide another improved process for preparing an alkyl (meth) acrylate by continuous reaction of an aqueous solution containing (meth) acrylic acid obtained directly from the gas phase oxidation of (meth) acrolein, whose process It makes it possible to convert aqueous solutions of (meth) acrylic acid with a water content of up to 90% by weight, in the presence of different secondary components such as the unreacted starting material, such as (meth) acrolein (up to about 1% by weight), acetic acid (up to about 10% by weight), formic acid (up to about 1.5% by weight) and high boiling constituents in the corresponding alkyl (meth) acrylate by reaction with a suitable alkanol. In addition, the process of the present invention must be carried out in a more favorable way from the energy point of view compared to the processes known to date. We have found that this objective is achieved by the process of the present invention. Accordingly, the present invention provides a process for preparing an alkyl (meth) acrylate by continuous reaction of an aqueous solution containing (meth) acrylic acid obtained directly from the gas phase oxidation with a linear or branched alkanol having 1 to 6 carbon atoms or a mixture of two or more thereof, in the presence of a catalyst for esterification, wherein the alkanol is introduced in gaseous form into the aqueous solution containing (meth) acrylic acid. As already mentioned above, according to the process of the present invention, an aqueous solution containing (meth) acrylic acid obtained directly from gaseous phase oxidation of, for example, (meth) acrolein is reacted. These aqueous solutions generally have a (meth) acrylic acid content of from about 10 to about 90% by weight, preferably from about 40 to about 90% by weight and, most preferably, from about 50 to about 90% by weight. The water content of these solutions is, therefore, generally from about 10 to about 90% by weight, preferably from about 10 to about 60% by weight and, most preferably, from about 10 to about 50% by weight. The most important secondary constituents are the initial material that did not react, such as (meth) acrolein up to a content of about 1% by weight, acetic acid to a content of about 10% by weight of formic acid to a content of 1.5% by weight and also the constituents of high boiling point, where the percentages above mentioned add up to 100% by weight. In the process of the present invention, it is particularly surprising that the presence of the unreacted starting material, such as (meth) acrolein, ie, an aldehyde having a CC double bond as another reactive group, does not give rise to the formation of residues in the reaction mixture under the selected reaction conditions, although it is known that, for example, the aldehydes tend to form resin in the presence of acids. In addition, the aqueous solution containing (meth) acrylic acid used can also contain a small amount, ie, up to about 1% by weight, of relatively non-volatile constituents such as polymerization inhibitors. During the course of the reaction, these relatively non-volatile constituents accumulate in the reaction mixture and can be removed, for example, by continuously taking part of the waste. The waste can, if desired, in turn be treated by conventional methods, particularly in distillation processes. The volatile constituents, for example, water, (meth) acrylic acid and alkyl (meth) acrylate, present in the residues that have been taken may return to the reaction after separation of the constituents with low boiling point. To prepare the aqueous solution containing (meth) acrylic acid which reacts in the present process, the gaseous products obtained from the gas phase oxidation can, in one embodiment, be treated in water, as a general rule giving solutions with a water content relatively high of generally more than 50% by weight. In another embodiment, gaseous products obtained from gas phase oxidation can be condensed by cooling, directly giving aqueous solutions containing (meth) acrylic acid with a relatively low water content of generally up to 50% by weight. The catalyst used is sulfuric acid or sulfonic acid, for example benzenesulfonic acid, toluenesulfonic acid or methanesulfonic acid, giving preference to toluenesulfonic acid. The proportion of the catalyst in the reaction mixture is calculated to be present in a concentration of from about 0.5 to 2 mol / 1, preferably from about 1 to 1.5 mol / 1, of the reaction mixture. Although most, or even all, of the added catalyst is constantly regenerated within the reaction, other relatively small amounts of catalyst can be metered in continuously or discontinuously during the reaction. In the process of the present invention, the aqueous solution containing (meth) acrylic acid, if desired together with small amounts of catalyst, is continuously fed to the reaction apparatus, the amount of the reaction mixture being present in the steady state from 5 to 200% by weight, preferably from 50 to 150% by weight, based on the aqueous solution containing (meth) acrylic acid fed per hour. Simultaneously with the aforementioned aqueous solution, a linear or branched alkanol having from 1 to 6 carbon atoms is continuously added as a gas to the reaction mixture in an amount at least equimolar, if desired in an excess of up to 4 times the stoichiometric amount, based on the amount of (meth) acrylic acid fed. The preferred molar ratio of (meth) acrylic acid to alkanol is about 1: 1.2 to 2.0. In the process of the present invention, the alkanol used is preferably methanol, ethanol, isopropanol, n-propanol, isobutanol, tert-butanol or n-butanol, more preferably n-butanol, tert-butanol or methanol. The alkanol- used can also contain a small amount of water. Of course, it is also possible to use mixtures of two or more albandes. When the alkanol is fed, the feeding conditions, i.e., in particular the temperature and the pressure, are established so that the alkanol is fed in gaseous form to the reaction mixture. This makes it possible to significantly reduce the total energy consumption of the esterification and the subsequent vaporization of the product, possibly together with the water, in comparison with the addition of alkanol in liquid form.

The reaction mixture is maintained at the boiling point under atmospheric or superatmospheric pressure and from about 70 to about 170 ° C. In general, pressures from approximately 6 x 103 to 2 x 105 Pa are necessary for this. The reaction mixture forms the residues of a distillation unit, where part of the residues, when the non-volatile residues accumulate, as defined above, are removed, purified and, if desired, recirculated. The residues obtained in the process of the present invention are heated by means of a suitable heating apparatus and kept boiling -to an extent that the amount of the reaction mixture remains approximately constant. The lower part of the distillation unit is preferably configured with a forced circulation vaporizer-depressurization or convection vaporization to ensure good mixing of the reaction mixture without. mechanical interference. The distillate containing an alkyl (meth) acrylate leaving the bottom of the unit preferably is taken through a distillation column. This distillate contains a mixture of the alkyl (meth) acrylate-water azeotrope and the alkanol used. For methyl methacrylate, the azeotrope methacrylate methyl-water at atmospheric pressure has the composition 86% by weight of methyl methacrylate and 14% by weight of water. The distillation preferably takes place in such a way that the acid (met) acrylic which is generally in the same way present in the feed for the distillation is separated from the distillate. That is, in particular, it is obtained when a part of the alkyl (meth) acrylate is returned to the distillation. Thus, the azeotrope is released from acid (met) acrylic when it is separated. In accordance with the present invention, it has further proved advantageous that the azeotrope be present as a phase when it is separated. The azeotrope is no longer present as "a phase" if the azeotrope is separated into a phase of methyl (meth) acrylate and a water phase. Furthermore, it has proved especially advantageous according to the invention that the azeotrope is present as a phase as well as that it is free of (meth) acrylic acid when it is separated. It is particularly preferred that the azeotrope be present as a phase within the column, thus achieving an especially good separation. If the separation is carried out by means of a distillation column, it is particularly preferred that the azeotrope be present in the upper part of the column as a phase or free of (meth) acrylic acid or preferably be present as a phase and free of (meth) acrylic acid. In this sense, it has been found that temperatures from 50 to 100, preferably from 60 to 80 and, more preferably from 65 to 75 ° C in the head part are particularly suitable. Furthermore, it is advantageous if the separation is carried out at normal pressure, preferably in the range of ± 300 mbar, preferably ± 200 mbar, around the normal pressure. According to the present invention, the azeotrope is considered free from (meth) acrylic acid if the azeotrope contains less than 10, preferably less than 1 and more preferably less than 0.1% by weight of (meth) acrylic acid. After condensation, the distillate is separated into a higher organic phase containing alkyl (meth) acrylate and a lower layer of water containing the alkanol by means of a suitable apparatus. This separation can be carried out in one or more stages. A part, usually the main part, of the organic phase is preferably returned to a column, part of it preferably in the upper section of the column, which improves the functioning of the separation to obtain the (met) alkyl acrylate in higher purity. The organic phase can, in addition to the alkyl (meth) acrylate as the main product, further contain high and low boiling constituents which can be separated from each other in a known manner by a plurality of distillation steps. The aqueous solution containing frequently used methacrylic acid contains a small amount of isobutyric acid and especially acetic acid. This is esterified together with methacrylic acid and can be separated from the corresponding ester mixture by distillation, for which a distillation column having a suitably large number of theoretical plates is required. The aqueous phase of the distillate contains, in addition to water, a significant amount of the unreacted alkanol and, depending on the solubility, secondary constituents such as acetone or other aldehydes or ketones, as well as acetic and formic acids. Of course, the alkanol can be separated from the aqueous phase by conventional methods, for example by fractional distillation, and returned to the reaction in gaseous form. In accordance with the present invention, however, it is preferred that a portion of the alkanol contained in the aqueous phase be removed from the aqueous phase. This preferential separation is carried out by distillative separation of the alkanol from the aqueous phase. For this purpose the use of a distillation column is preferred. In accordance with the present invention, it is preferred to remove from the aqueous phase when less than one-third, preferably at least half, and most preferably at least two-thirds of the alkanol contained in the aqueous phase. Normally, the aqueous phase contains from 1 to 30, preferably from 5 to 20 and even more preferably from 10 to 15% by weight of alkanol, preferably methanol, 0.1 to 10, preferably from 1 to 7 and even higher preferably from 2 to 5% by weight of the alkyl (meth) acrylic ester, preferably alkyl (meth) acrylate and up to 100% by weight of the aqueous phase of the water. For the distillative separation of alkanol and, in particular, for the distillative separation of methanol, it has proven particularly advantageous according to the invention that the aqueous phase is maintained at a temperature of from 90 to 130, preferably from 95 to 120 and, greater preference, from 100 to 115 ° C. In the upper part of the distillation column, a temperature in the range from 60 to 90, preferably from 65 to 85 and, more preferably, from 70 to 80 ° C, has been found suitable. Furthermore, it has proven particularly advantageous that the distillative separation of the alkanol is carried out at a pressure of from 1 to 3, preferably from 1.1 to 3 and, more preferably, from 1.2 to 1.4 bar. The alkanol withdrawn from the aqueous phase can be introduced into the aqueous solution containing (meth) acrylic acid. This can be introduced in the form of the liquid alkanol, as well as in the gaseous alkanol form, it being preferred to introduce the alkanol in its gaseous form in the aqueous solution containing (meth) acrylic acid. Both the separation of the alkanol from the aqueous phase and the introduction of the alkanol into the aqueous solution containing (meth) acrylic acid can be carried out continuously or batchwise, preferably continuously. The removal of the alkanol from the aqueous phase and the introduction of the alkanol into the aqueous solution containing (meth) acrylic acid have the advantage that the unreacted alkanol contained in the aqueous phase can be introduced into the reaction in the aqueous solution which contains (meth) acrylic acid. Thus, alkanol can be used in an economical way thus saving resources. To prevent polymerization, it is possible to add a polymerization inhibitor, such as hydroquinone, phenothiazine or hydroquinone monomethyl ether, preferably in the upper part of the distillation apparatus. This gradually reaches the residues of the distillation and is eliminated during the treatment of the latter. The present invention is illustrated by the following examples. Examples 1 to 5 which are in accordance with the present invention were made in a unit for the continuous esterification of (meth) acrylic acid with methanol. It consists of a dosing section, that is, a tank for the aqueous solution containing (meth) acrylic acid, plus a feeding installation for gaseous methanol, a reaction section with a reaction volume from 500 to 1000 ml. , a column packed with a length of 100 cm and a normal width of 5 cm as a distillation unit and a phase separation stage. At first, a reaction mixture containing the aqueous solution containing (meth) acrylic acid obtained directly from the gas phase oxidation of methacrolein, methanol and p-toluenesulfonic acid as catalyst were placed in the reaction section and were brought to the conditions of the operation. Subsequently, the gaseous methanol and the aqueous solution containing methacrylic acid were dosed continuously at a rate such that the volume of the residues remained constant. The methyl methacrylate formed during the reaction was taken at the top of the distillation unit as an azeotrope with water and was separated into an organic phase and an aqueous phase separator. Part of the organic phase was returned with separation to the top of the distillation column to prevent the methacrylic acid from being distilled. The phenothiazine as stabilizer was dosed to the separation or backflow stream to avoid polymerization in the distillation column.

The operating conditions and mass flows that were used in Examples 1 to 5, according to the present invention, and the methacrylic acid concentrations used in the examples are shown in Table 1 below. The composition of the aqueous solutions containing methacrylic acid used as feed are as shown in Table 2.

Table 1 - Operating conditions and mass flows The following table shows the operating conditions and mass flows for different concentrations of methacrylic acid: Table 2 - Comparison of aqueous solutions containing acid (et) acrylic Composition of MAA feed streams: Examples 6 to 12 related to the continuous esterification of aqueous methacrylic acid were carried out in a mini-plant consisting of a dosing section (a deposit for the initial materials and the stabilizing solution), a reaction section (reactor with vaporizer by convection) , a distillation section and a sedimentation stage. At first, one liter of the reaction solution was introduced into the reaction vessel and heated to the operating temperature (95 to 110 ° C). In the reactor, the aqueous methacrylic solution (liquid) and methanol containing water (gaseous) were dosed. In addition, a stabilizer (phenothiazine) and a catalyst (p-toluenesulfonic acid) were added. The methyl methacrylate formed during the reaction was removed at the top of the distillation column as an azeotrope with water, as well as excess methanol. The condensate from the distillation was separated in an aqueous phase and an organic in a phase separator by addition of water. Part of the organic phase was returned as recoil to the top of the distillation column. Part of the residue from the reaction section was sometimes separated due to secondary reactions. The aqueous phase was fed to a distillation column in which at a temperature in the lower part of 100 to 115 ° C and a temperature in the upper part of 60 to 80 ° C, at 1.3 bar the distillation was carried out. The top product also contained methanol of 2 to 12% water. The operating conditions and the results of the analysis obtained in the examples according to the present invention can be taken from Tables 3a to 3c. In the following tables, the operation and the results of the analysis are composed of different temperatures in the lower part, proportions MAA: MeOH and return flows.

Table 3a Table 3b Table 3c 2 ^ The abbreviations in Tables 3a to 3c mean: MAA feed: methacrylic acid solution in 15% by weight of water MeOH feed: gaseous dosing of methanol with 10% by weight of water H20: feed of water in the settler stage to improve the phase separation of the condensate.

Claims (1)

1. CLAIMS A process for preparing an alkyl (meth) acrylate by continuous reaction of an aqueous solution containing (meth) acrylic acid obtained directly from the gas phase oxidation with a linear or branched alkanol having from 1 to 6 carbon atoms or a mixing two or more thereof in the presence of an esterification catalyst, wherein the alkanol is introduced in gaseous form into the aqueous solution containing (meth) acrylic acid. The process as recited in claim 1, wherein the alkanol is selected from the group consisting of methanol, butanol, tert-butanol and mixtures of two or more thereof. The process as mentioned in claim 1 or 2, wherein the catalyst for the esterification is p-toluenesulfonic acid. The process as recited in claim 1 to 3, wherein the alkyl (meth) acrylate is, in a separation step, separated as a distillate containing the azeotrope of alkyl (meth) acrylate with water. The process as recited in claim 4, wherein the azeotrope is present as a phase or free of (meth) acrylic acid or is present as a phase and free of (meth) acrylic acid. The process as mentioned in claims 4 or 5, wherein the distillate is separated into an organic phase and an aqueous phase. The process as mentioned in claim 6, wherein at least a part of the organic phase is returned to the separation stage. The process as mentioned in claim 6, wherein at least a part of the alkanol contained therein is removed from the aqueous phase. The process as recited in claim 8, wherein the alkanol is introduced into the aqueous solution containing (meth) acrylic acid. The process as mentioned in any of claims 1 to 9, wherein the reaction is carried out in a vaporizer by convection.