HK1119664A - Methods for producing alkyl(meth)acrylates - Google Patents

Description

Process for preparing alkyl (meth) acrylates

The present invention relates to a process for preparing alkyl (meth) acrylates.

The main field of application of acrylates and methacrylates, hereinafter referred to as alkyl (meth) acrylates, is the preparation of polymers and copolymers with other polymerizable compounds.

In addition, methacrylic esters, such as methyl methacrylate, are important building blocks for a number of special esters based on Methacrylic Acid (MAS), which are prepared by transesterification with the corresponding alcohols.

Methyl Methacrylate (MMA) and methacrylic acid are now produced predominantly from hydrocyanic acid and acetone via the Acetone Cyanohydrin (ACH) formed as the central intermediate.

Other processes using other raw material bases than ACH are described in the relevant patent literature and are realized during this on a production scale. In this connection, C-4-based starting materials such as isobutene or tert-butanol, which are converted into the desired methacrylic acid derivatives in a plurality of process steps, are currently used as reactants.

The use of propylene as a base feedstock to obtain methacrylic acid in suitable yields via step hydroformylation (to isobutyric acid) and dehydrooxidation has also been extensively investigated.

Propionaldehyde or propionic acid, which are obtained in industrial processes starting from ethylene and C-1 structural units such as carbon monoxide, are known as base feedstocks. In these processes, in the aldol condensation reaction with formaldehyde, the β -hydroxy-carbonyl compound generated in situ reacts to form the corresponding α, β -unsaturated compound upon dehydration. A general outline of the customary processes for preparing methacrylic acid and esters thereof can be found in the literature, for example in Weisselmel, Arpe "Industrial IENTERGANISCHE Chemie", VCH, Weinheim 1994, 4 th edition, page 305 onward or in Kirk Othmer "Encyclopedia of Chemical Technology", 3 rd edition, volume 15, page 357.

It is generally known that an industrial process based on ACH uses a high concentration of sulfuric acid (about 100% by weight of H) in the first step of the reaction, the so-called amidation₂SO₄) At a temperature of between 80 ℃ and about 110 ℃.

Representative of such a process is, for example, U.S. Pat. No. 4,529,816, in which ACH amidation is carried out at a temperature of 100 ℃ in ACH: H₂SO₄Is carried out in a molar ratio of about 1: 1.5 to 1: 1.8. The process steps of the associated method for this process are: a) amidation; b) transformation; and c) esterification.

In the amidation, as the main product of the reaction, theSIBAPivaloyl-alpha-hydroxyisobutyramide-bisulfate and MASA × H₂SO₄As a solution in excess sulfuric acid. Furthermore, HIBA XH is also obtained in a typical amidation solution₂SO₄α -hydroxyisobutyramide-bisulfate in a yield of < 5% relative to ACH. In the substantially complete conversion of ACH, the amide is selectively rightwardThe reaction was carried out in about 96-97% yield (total sum of the intermediates).

However, in this step, as by-products, non-negligible amounts of carbon monoxide, acetone, sulfonation products of acetone and cyclocondensation products of acetone with various intermediates are thus formed.

The proportion of HIBA in the amidation mixture, in addition to SIBA, is also adjusted depending on the water content of the sulfuric acid used. For example, if 97 wt.% sulfuric acid (1.5 equivalents H relative to ACH) is used₂SO₄) Then 25 wt.% HIBA is produced which is no longer selective and can be fully reacted to MASA in the conversion. The higher water content in the amidation at temperatures of 90 ℃ to 110 ℃ also results in a higher HIBA content which can be converted only relatively nonselectively by conventional conversion into the target intermediate MASA XH₂SO₄。

The aim of the conversion is to react the SIBA and HIBA as completely as possible to MASA, which is carried out under the beta-elimination method of sulfuric acid (excess sulfuric acid as solvent).

In the process step conversion, sulfuric acid (anhydrous) solutions of HIBA, SIBA and MASA (each present as bisulfate salts) are reacted at elevated temperatures of 140 ℃ to 160 ℃ and short residence times of about 10 minutes or less.

The conversion mixture of this process is characterized by a high excess of sulfuric acid and the presence of the main product MASA XH in solution at a concentration of about 30 to 35 wt.% (depending on the excess of sulfuric acid used, respectively)₂SO₄。

For substantially complete SIBA H₂SO₄Conversion As MASA XH₂SO₄The yield was about 94-95%. Including losses in amidation due to the above-mentioned side reactions, which provide only 90-92% MASA (relative to ACH) for the subsequent esterification to Methyl Methacrylate (MMA) which is desired as product.

As a by-product in this process step, the severe reaction conditions determine that the intermediates form a very large number of condensation and addition products with each other.

The goal of the esterification is to proceed from MASA XH₂SO₄The conversion is reacted as completely as possible to MMA. Esterification is carried out by adding a mixture consisting of water and methanol to the MASA-sulfuric acid solution and at least partly via Methacrylic Acid (MAS) as an intermediate. The reaction can be operated under pressure or without pressure.

Here, MMA, MAS and the ammonium bisulfate solution formed are obtained in sulfuric acid solution by subjecting the conversion solution to saponification/esterification at temperatures of generally between 90 ℃ and 140 ℃ over a reaction time of one or more hours.

Methanol selectivity in this step is only about 90% or less due to the reaction conditions in the presence of free sulfuric acid, at which point dimethyl ether forms a by-product by condensation of methanol.

At substantially complete MASA × H₂SO₄In the case of conversion, the esterification is carried out with an MMA yield of about 98 to 99% (relative to the MASA used) (total selectivity for MAS + MMA). Including losses in amidation and conversion due to the abovementioned side reactions, it is thus possible, in the case of optimum reaction operation, to obtain a MMA yield of up to 90% relative to ACH in the overall process over all steps.

In addition to the poor overall yield of the above-described process, which is associated with the production of significant amounts of waste and off-gases, especially on a production scale, the process has the disadvantage that sulfuric acid, which far exceeds the stoichiometric amount, must be used. But also from the process acid containing ammonium bisulfate and sulfuric acid, which is regenerated in the sulfuric acid contact apparatus, tar-like, solid condensation products are separated, which prevents trouble-free transport of the process acid and must be excluded at considerable cost.

Due to the significant yield loss of the above-mentioned process disclosed in U.S. Pat. No. 4,529,816, some proposals have been made for amidation and hydrolysis of ACH in the presence of water, when the hydroxyl functions in the molecular structure remain present at least in the first step of the reaction.

The proposal is that the optional amidation is carried out in the presence of water, depending on whether methanol is present or not, i.e. either leads to the formation of methyl 2-hydroxyisobutyrate (═ HIBSM) or to the formation of 2-hydroxyisobutyric acid (═ HIBS).

2-Hydroxyisobutyric acid is an important intermediate for the preparation of methacrylic acid and the methacrylates derived therefrom, especially methyl methacrylate.

Another alternative for preparing esters of 2-hydroxyisobutyric acid, in particular methyl 2-hydroxyisobutyrate, starting from ACH is described in JP Hei 4-193845. In JP-A-4-193845 ACH is first amidated with 0.8 to 1.25 equivalents of sulfuric acid in the presence of less than 0.8 equivalents of water at temperatures below 60 ℃ and subsequently reacted with more than 1.2 equivalents of an alcohol, especially methanol, at temperatures above 55 ℃ to form HIBSM or the corresponding ester. The presence of viscosity-reducing media which are stable to the reaction matrix is not discussed.

A disadvantage and problem of this process is that the industrial conversion develops a particular viscosity at the end of the reaction.

Some studies on the evaluation and conversion of methyl methacrylate by dehydration from HIBSM are described in the patent literature.

For example, in EP 0429800, HIBSM or a mixture of HIBSM and the corresponding α or β -alkoxy ester in the gas phase is reacted over a heterogeneous catalyst composed of a crystalline aluminosilicate and a mixed feed composed of an alkali metal element on the one hand and a noble metal on the other hand, in the presence of methanol as a co-feed. Although the conversion and selectivity of the catalyst are good at least at the start of the reaction, very severe catalyst deactivation with a concomitant decrease in yield occurs with increasing reaction time.

A similar study was carried out in EP 0941984, which describes a chemical reaction of SiO₂Of heterogeneous catalysts consisting of alkali metal phosphatesIn the presence of HIBSM, the gas phase is dehydrated as a substep of MMA synthesis. However, overall, this multistep process is complicated, requires high pressures in the substeps, thus requires expensive equipment, and gives very unsatisfactory yields.

In addition to the above work for dehydrating HIBSM and related esters to the corresponding α - β unsaturated methacrylic compounds in the vapor phase, it has also been suggested to conduct the reaction in the liquid phase.

The preparation of MAS starting from 2-hydroxyisobutyric acid is described, for example, in US 3,487,101, in which various methacrylic acid derivatives, in particular methacrylic acid and methacrylic esters, are prepared starting from 2-hydroxyisobutyric acid in the liquid phase, characterized in that the reaction of HIBS to methacrylic acid is carried out in the presence of dissolved basic catalysts at high temperatures of 180 ℃ to 320 ℃ in the presence of high-boiling esters (e.g. dimethyl phthalate) and internal anhydrides (e.g. phthalic anhydride). According to the patent, MAS-selectivity reached 98% when the HIBS conversion was > 90%. With regard to the long-term stability of the liquid catalyst solutions, in particular, no exhaustion of the anhydrides used is specified.

Dehydration of HIBSM in the presence of high concentrations of sulfuric acid (90-100 wt.%) is also described in JP 184047/1985. Among them is the high consumption of sulfuric acid and the forced production of large amounts of aqueous sulfuric acid solution, which is formed by the release of water from the HIBSM during the reaction. This process is not economically significant due to the amount of waste acid.

DE-OS 1191367 relates to the preparation of methacrylic acid in the liquid phase starting from 2-hydroxyisobutyric acid, characterized in that the conversion of HIBS into methacrylic acid is carried out in the presence of polymerization inhibitors (e.g.copper powder) and in the presence of a catalyst mixture consisting of metal halides and alkali metal halides at elevated temperatures of 180-220 ℃. According to the patent, MAS selectivity reaches > 99% when HIBS conversion is > 90%. The best results are achieved with a catalyst mixture consisting of zinc bromide and lithium bromide. It is known that the use of halide-containing catalysts at high temperatures places high demands on the process materials used, and that this problem also arises in subsequent plant components with regard to by-products of prolonged halogenation in the distillate.

EP 0487853 describes the preparation of methacrylic acid starting from acetone cyanohydrin, characterized in that, in a first step, ACH is reacted with water at mild temperatures in the presence of a heterogeneous hydrolysis catalyst, in a second step, 2-hydroxyisobutyramide is reacted with methyl formate or methanol/carbon monoxide to give formamide and methyl hydroxyisobutyrate, in a third step, HIBSM is saponified with water in the presence of a heterogeneous ion exchanger to give hydroxyisobutyric acid, and in a fourth step, HIBS is dehydrated by reaction in the liquid phase at elevated temperature in the presence of a soluble alkali metal salt. Methacrylic acid-to-make ex HIBS is described as having an approximately quantitative selectivity of 99% at high conversion. The multiple reaction steps necessary and the necessity of intermediate isolation of the intermediates, in particular also the process steps at elevated pressure, make the process complicated and therefore ultimately uneconomical. Furthermore, formamide must be prepared, which is often regarded as an undesirable by-product and must be eliminated in an expensive manner.

DE-OS 1768253 describes a process for preparing methacrylic acid by dehydration of alpha-hydroxyisobutyric acid, characterized in that HIBS is reacted in the liquid phase at a temperature of at least 160 ℃ in the presence of a dehydration catalyst consisting of a metal salt of alpha-hydroxyisobutyric acid. Particularly suitable in this case are the alkali metal and alkaline earth metal salts of HIBS, which are prepared in situ in the HIBS melt by conversion into the appropriate metal salts. According to the patent, MAS yields up to 95% ex HIBS are described, in which case the feed for the continuous process consists of HIBS and about 1.5% by weight of HIBS-alkali metal salt.

RU 89631 relates to a process for the preparation of methacrylic acid starting from 2-hydroxyisobutyric acid by separation of water in the liquid phase, characterized in that the reaction is carried out with an aqueous solution of HIBS (up to 62% by weight of HIBS in water) in the absence of a catalyst, under pressure and at elevated temperature from 200 ℃ to 240 ℃.

It is also known that, for the preparation of 2-hydroxyisobutyric acid starting from Acetone Cyanohydrin (ACH), the saponification of the nitrile function can be carried out in the presence of mineral acids (see J.Brit.chem.Soc. (1930); chem.Ber.72(1939), 800).

Representative of such a method is, for example, Japanese patent laid-open publication No. Sho 63-61932, in which ACH is saponified to 2-hydroxyisobutyric acid in a two-step process. In this case, ACH is first reacted in the presence of 0.2 to 1.0mol of water and 0.5 to 2 equivalents of sulfuric acid to form the corresponding amide salt. In this step, when smaller water and sulfuric acid concentrations are used, which is necessary for better yields, shorter reaction times and smaller amounts of process acid discharged, i.e.there arises a serious problem of stirrability of the amidation mixture, especially at the end of the reaction time, due to the high viscosity of the reactants.

If the molar water quantity is increased to ensure a low viscosity, the reaction is slowed down considerably and side reactions occur, in particular the cleavage of ACH into the reactants acetone and hydrocyanic acid, which react further under the reaction conditions to subsequent products. According to the provisions of Japanese patent laid-open No. Sho 63-61932, and in the case of elevated temperatures, although it is possible to control the viscosity of the reaction mixture and the corresponding reactants, although stirrable due to the decrease in viscosity, increase significantly the side reactions even at mild temperatures, which finally manifests itself in very usual yields (see comparative examples).

If the operation is carried out at lower temperatures of < 50 ℃, an alternative reaction embodiment is ensured, with the result that at the end of the reaction time, owing to the increased concentration of the sparingly soluble amide salts under the reaction conditions, a sparingly stirrable suspension is first formed, following which the reaction mass is completely solidified.

In the second step of Japanese patent laid-open No. Sho 63-61932, water is added to the amidation solution and hydrolyzed at a temperature higher than the amidation temperature, whereby the amide salt formed after amidation forms 2-hydroxyisobutyric acid with release of ammonium bisulfate.

The key to the economics of the industrial process is that, in addition to the selective production of the desired product, HIBS, is separated from the reaction matrix during the reaction or from the remaining process acid.

In JP 57-131736, a method of separating α -oxyisobutyric acid (═ HIBS), this problem is addressed by treating a reaction solution containing α -hydroxyisobutyric acid and acidic ammonium bisulfate, which is separated by hydrolysis after the reaction between acetone cyanohydrin, sulfuric acid and water, with an extractant into which 2-hydroxyisobutyric acid is transferred and which remains in the aqueous phase.

According to this process, the sulfuric acid remaining free in the reaction medium before extraction is neutralized by treatment with an alkaline medium, thereby increasing the degree of extraction of HIBS in the organic extraction phase. The necessary neutralization requires a considerable consumption of amine base or inorganic base and thus produces a large discharge of the corresponding salts, which cannot be eliminated ecologically and economically.

The disadvantages of the process of JP-A-57-131736 (reaction sequence: amidation-transformation-hydrolytic esterification) for preparing MMA by means of methacrylamide-hydrogen sulfate salt include the following:

a.) use of a high molar excess of sulfuric acid relative to ACH (about 1.5-2 equivalents sulfuric acid per equivalent ACH in an industrial process).

b.) high yield loss in the amidation step (about 3-4%) and the conversion step (about 5-6%), which finally represents a maximum methacrylamide sulfate yield, about 91%.

c.) a large bleed stream in the form of an aqueous sulfuric acid solution in which ammonium bisulfate and organic by-products are dissolved. From the process, there is an undefined tar residue which is discharged from the acid discharge and which has to be worked up or eliminated in a complicated manner.

The disadvantages of the process of JP Sho 57-131736 (reaction sequence: amidation-hydrolysis HIBS synthesis-MAS synthesis-hydrolysis esterification) for the preparation of MMA by hydroxyisobutyric acid as central intermediate include the following:

a.) although a small molar excess of sulfuric acid relative to ACH is used (only about 1.0 equivalents sulfuric acid per equivalent ACH), the viscosity and stirrability of the amidation medium are of serious concern until the reactants are fully cured; the proposed dilution of the amidation with an alcohol (methanol) or a different ester leads to incomplete ACH reactions under the reaction conditions, to a significant increase in side reactions or to chemical decomposition of the diluent.

b.) high yield losses in the amidation step (about 5-6%) and the formation of an extractant phase containing water and HIBS using a cumbersome organic solvent extraction, which has to be worked up distillatively with high energy consumption to isolate HIBS. About 2kg of process acid-effluent were produced per kg of HIBS, containing about 34% by weight of water in addition to 66% by weight of ammonium bisulfate (see Japanese publication Sho 57-131736, example 4). The capacity of a sulfuric acid contact plant (SK plant) in which the discharged salt solution is regenerated with a high content of water requires a large energy expenditure is significantly limited.

Common to all these methods is that the separation of HIBS from an aqueous reaction matrix containing ammonium bisulfate is cumbersome. Too high a water content in the HIBS-containing extract phase also leads to a lingering of ammonium bisulfate to the subsequent MAS step, which on an industrial scale is no longer possible to operate continuously for a reasonable time. The high energy consumption in regenerating the highly concentrated aqueous process acid and extract streams renders the proposed process uneconomical, and does not provide a real alternative to established process regimes which, although non-selective, are targeted by simple, less technically demanding operation.

In view of the prior art, it is an object of the present invention to provide a process for preparing alkyl (meth) acrylates which can be carried out simply and at low cost.

Another object of the present invention is to provide a process which makes it possible to obtain alkyl (meth) acrylates very selectively.

Further, it is an object of the present invention to provide a method for preparing alkyl (meth) acrylates, which produces only a small amount of by-products. The products should be obtained in as high a yield as possible and with a low overall energy consumption.

A further object of the present invention is to provide a process for preparing alkyl (meth) acrylates which can be carried out particularly simply and at low cost.

These objects and others not mentioned explicitly but which can be derived or deduced directly from the discussion herein are solved by a method having all the features of patent claim 1. The improvements which are aimed at for the method of the invention are protected in the subclaims which depend on claim 1.

The subject of the present invention is therefore a process for the preparation of alkyl (meth) acrylates comprising a step of transesterification of an alkyl a-hydroxycarboxylic acid with (meth) acrylic acid, thereby obtaining the alkyl (meth) acrylate as well as the a-hydroxycarboxylic acid, and dehydrating the a-hydroxycarboxylic acid, thereby obtaining (meth) acrylic acid.

The measures according to the invention also make it possible to achieve the following advantages:

the process avoids the use of large amounts of sulfuric acid as a reactant. Thus, no significant amount of ammonium bisulfate is produced in the process of the present invention.

By the process of the present invention, alkyl (meth) acrylates are obtained in high yield. This is in particular met in comparison with the process described in EP-A-0941984, in which the alkyl α -hydroxycarboxylic acid is directly dehydrated to alkyl (meth) acrylate. It was surprisingly found that by the additional reaction step of transesterification of the alkyl ester of an alpha-hydroxycarboxylic acid with (meth) acrylic acid, an overall higher selectivity is achieved.

By-products are very rarely formed here. In addition, high conversions are achieved, in particular, with high selectivities being taken into account.

The process of the invention produces small amounts of by-products.

The process according to the invention can be carried out at low cost, in particular with low energy consumption. In this case, the catalyst used for dehydration and transesterification can be used for a long time without lowering the selectivity or activity.

The process of the invention can be carried out industrially.

According to the invention, an alkyl ester of an alpha-hydroxycarboxylic acid is reacted with (meth) acrylic acid. The (meth) acrylic acids which can be used for this purpose are known and commercially available. In addition to acrylic acid (Propenhalene ure) and methacrylic acid (2-Methylpropen ure), particular substituents-containing derivatives belong thereto. Suitable substituents are, in particular, halogens, such as chlorine, fluorine and bromine, and also alkyl groups, preferably having 1 to 10, particularly preferably 1 to 4, carbon atoms. Among these are beta-methacrylic acid (Butenbash ure), alpha, beta-dimethylacrylic acid, beta-ethylacrylic acid and beta, beta-dimethylacrylic acid. Acrylic acid (Propenhale ure) and methacrylic acid (2-Methylpropen ure) are preferred, with methacrylic acid being particularly preferred.

Alkyl esters of alpha-hydroxycarboxylic acids are known for use herein, the alcohol group of the esters preferably containing from 1 to 20 carbon atoms, in particular from 1 to 10 carbon atoms, particularly preferably from 1 to 5 carbon atoms. Preferred alcohol radicals are in particular derived from methanol, ethanol, propanol, butanol, in particular n-butanol and 2-methyl-1-propanol, pentanol, hexanol and 2-ethylhexanol, with methanol and ethanol being particularly preferred.

The acid groups of the alkyl esters of alpha-hydroxycarboxylic acids used for the transesterification are preferably derived from (meth) acrylic acid, which can be obtained by dehydration of the alpha-hydroxycarboxylic acids. For example, if methacrylic acid is used, then α -hydroxyisobutyrate is used. For example, if acrylic acid is used, it is preferred to use α -hydroxyisopropyl acid.

The alkyl esters of alpha-hydroxycarboxylic acids preferably used are methyl alpha-hydroxypropionate, ethyl alpha-hydroxypropionate, methyl alpha-hydroxyisobutyrate and ethyl alpha-hydroxyisobutyrate.

Such alkyl esters of alpha-hydroxycarboxylic acids are frequently and inexpensively obtained from the corresponding cyanohydrins. In this case, the purity of the cyanohydrin is not critical. Thus, the hydrolysis can be carried out using purified or unpurified cyanohydrins. Thus, the alkyl esters of α -hydroxycarboxylic acids used according to the invention can be prepared from ketones and aldehydes as well as hydrocyanic acid and the corresponding alcohols.

In the first step, a carbonyl compound, for example a ketone, in particular acetone, or an aldehyde, for example acetaldehyde, propionaldehyde, butyraldehyde, is reacted with hydrocyanic acid to form the corresponding cyanohydrin. It is particularly preferred here for acetone and/or acetaldehyde to be reacted in a typical manner using small amounts of alkali metals or amines as catalysts.

In a further step, the cyanohydrin thus obtained is reacted with water to give the hydroxycarboxylic acid amide.

Typically, the reaction is carried out in the presence of a catalyst. Suitable for this purpose are, in particular, manganese oxide catalysts, such as those described in EP-A-0945429, EP-A-0561614 and EP-A-0545697. Manganese oxides in the form of manganese dioxide, which are obtained by treating manganese sulfate with potassium permanganate under acidic conditions (see biochem.j., 50s.43(1951) and j.chem.soc, 1953, s.2189, 1953) or by electrolytically oxidizing manganese sulfate in aqueous solution, can be used here. Catalysts are generally used, often in the form of powders or granules having a suitable particle size. The catalyst may furthermore be coated on a support. In particular, so-called Slurry reactors or fixed-bed reactors, as described in particular in EP-A-956898, can also be used here.

The hydrolysis reaction may furthermore be catalyzed by enzymes. Suitable enzymes include, in particular, nitrile hydratase. This reaction is described, for example, in "Screening, charaterization and Application of cyanide-resistant Nitrile Hydratases" Eng.Life.Sci.2004, 4, No. 6.

Furthermore, the hydrolysis reaction can also be catalyzed by acids, in particular sulfuric acid. This is particularly true in JP Hei 4-193845.

The water necessary for the hydrolysis of cyanohydrins can often be used as solvent. The molar ratio of water to cyanohydrin is preferably at least 1, particularly preferably from 0.5: 1 to 25: 1, very particularly preferably from 1: 1 to 10: 1.

The water used for hydrolysis may be of higher purity. Although this property is not mandatory. It is therefore possible to use used or industrial water which contains impurities to a large extent in addition to fresh water. Thus, it is also possible to use recycled water for the hydrolysis.

In addition, other components may also be present in the reaction mixture for the hydrolysis of cyanohydrin. Of this are in particular aldehydes and ketones, in particular any aldehydes and ketones used for preparing cyanohydrins. For example acetone and/or acetaldehyde may be included in the reaction mixture. Such as those described in US 4018829-a. The purity of the aldehyde and/or ketone to be added is generally not particularly critical. These substances may therefore contain impurities, in particular alcohols such as methanol, water and/or methyl a-Hydroxyisobutyrate (HIBSM). The amount of carbonyl compounds, in particular acetone and/or acetaldehyde, in the reaction mixture can be used in a wide range. The carbonyl compound is preferably used in an amount of 0.1 to 6Mol, preferably 0.1 to 2Mol per Mol of cyanohydrin.

The hydrolysis reaction may be carried out at a temperature of usually 10 to 150 ℃, preferably 20 to 100 ℃, particularly preferably 30 to 80 ℃.

The reaction can be carried out, for example, in a fixed-bed reactor or in a suspension reactor.

The resulting reaction mixture generally comprises, in addition to the desired hydroxy acid amide, further constituents, in particular unreacted cyanohydrin and optionally acetone and/or acetaldehyde. Thus, the reaction mixture can be purified, so that unreacted cyanohydrin can be split into acetone and hydrocyanic acid so as to be reused for the preparation of cyanohydrin. The same applies to the acetone and/or acetaldehyde separated off.

In addition, the purified reaction mixture containing the hydroxy acid amide is washed off from the other components by passing through an ion exchange column.

Cation exchangers and anion exchangers can be used in particular here. Ion exchangers suitable for this are known. For example, suitable cation exchangers can be obtained by sulfonating styrene-divinylbenzene copolymers. The basic anion exchangers comprise quaternary ammonium groups covalently bonded to a styrene-divinylbenzene copolymer.

The procedure for preparing alphｃA-hydroxycarboxylic acid amides is described in detail in particular in EP-A-0686623.

In a next step, the thus obtained α -hydroxycarboxylic acid amide may be reacted to an α -hydroxycarboxylic acid alkyl ester. This can be achieved, for example, by using alkyl formates. Particularly suitable is methyl formate, or ｃA mixture of methanol and carbon monoxide, the reaction being described, for example, in EP-A-0407811.

The reaction of the α -hydroxycarboxylic acid amides is preferably carried out by alcoholysis with alcohols preferably having from 1 to 10 carbon atoms, particularly preferably from 1 to 5 carbon atoms. Preferred alcohols are, in particular, methanol, ethanol, propanol, butanol, in particular n-butanol and 2-methyl-1-propanol, pentanol, hexanol, heptanol, 2-ethylhexanol, octanol, nonanol and decanol. Particular preference is given to using methanol and/or ethanol as alcohol, methanol being more particularly preferred. The reaction of carboxylic acid amides with alcohols in order to obtain carboxylic acid esters is generally known.

The reaction can be accelerated, for example, by basic catalysts. This includes homogeneous catalysts as well as heterogeneous catalysts.

Among the homogeneous catalysts are alkali metal alkoxides and organometallic compounds of titanium, tin and aluminum. Preference is given to titanium alkoxides or tin alkoxides, for example titanium tetraisopropoxide or tin tetrabutoxide. Belonging to heterogeneous catalysts, in particular magnesium oxide, calcium oxide and basic ion exchangers, as described above.

The molar ratio of the alpha-hydroxycarboxylic acid amide to the alcohol, for example alpha-hydroxyisobutyramide, to methanol is not critical, and is preferably from 2: 1 to 1: 20.

The reaction temperature can likewise be within a wide range, the reaction rate generally increasing with increasing temperature. The above temperature limits are generally produced by the boiling point of the alcohol used. The reaction temperature is preferably from 40 to 300 ℃ and particularly preferably from 160 ℃ to 240 ℃. Depending on the reaction temperature, the reaction can be carried out at low pressure or overpressure. Preferably, the reaction is carried out at a pressure in the range from 0.5 to 35 bar, particularly preferably from 5 to 30 bar.

The ammonia formed is generally removed from the reaction system, in which case the reaction is usually carried out at the boiling point.

The ammonia released in the alcoholysis can be returned to the entire process in a simple manner. For example, ammonia may be reacted with methanol to hydrocyanic acid. This is described, for example, in EP-A-0941984. Furthermore, hydrocyanic acid may be obtained from ammonia and methane according to the BMA or Andrussow process, which are described in Ullmann's Encyclopedia of Industrial Chemistry 5. aflage aufCD-ROM, Stichworld "organic Cyano Compounds".

In the next step, the alkyl α -hydroxycarboxylic acid is reacted with (meth) acrylic acid to give the alkyl (meth) acrylate and the α -hydroxycarboxylic acid.

The reaction mixture may contain, in addition to the reactants, other ingredients such as solvents, catalysts, polymerization inhibitors and water.

The reaction of the alkyl hydroxycarboxylic acid with (meth) acrylic acid can be catalyzed by at least one acid or at least one base. Both homogeneous and heterogeneous catalysts can be used here. Particularly suitable acidic catalysts are, in particular, mineral acids, such as sulfuric acid or hydrochloric acid, and organic acids, such as sulfonic acids, in particular p-toluenesulfonic acid, and acidic cation exchangers.

Particularly suitable cation exchanger resins include, in particular, styrene-divinylbenzene polymers containing sulfonic acid groups. Particularly suitable cation exchanger resins are commercially available from Rohm & Haas under the trade name Amberlyst ® and from Bayer under the trade name Lewatit ®.

The concentration of the catalyst is preferably from 1 to 30% by weight, particularly preferably from 5 to 15% by weight, based on the sum of the alpha-alkylhydroxycarboxylic acid ester used and the (meth) acrylic acid used.

Polymerization inhibitors which can preferably be used are, in particular, phenothiazine, tert-butylcatechol, hydroquinone monomethyl ether, hydroquinone, 4-hydroxy-2, 2, 6, 6-Tetramethylpiperidinyloxy (TEMPOL) or mixtures thereof; the action of these inhibitors can be partially improved by the use of oxygen. The polymerization inhibitor can be used in a concentration of 0.001 to 2.0% by weight, particularly preferably 0.01 to 0.2% by weight, based on the sum of the alpha-alkylhydroxycarboxylic acid ester used and the (meth) acrylic acid used.

The reaction is preferably carried out at a temperature of from 50 ℃ to 200 ℃, particularly preferably from 70 ℃ to 130 ℃, in particular from 80 ℃ to 120 ℃, very particularly preferably from 90 ℃ to 110 ℃.

Depending on the reaction temperature, the reaction can be carried out at low pressure or overpressure. Preferably, the reaction is carried out at a pressure in the range from 0.02 to 5 bar, in particular from 0.2 to 3 bar, particularly preferably from 0.3 to 0.5 bar.

The molar ratio of (meth) acrylic acid to alkyl α -hydroxycarboxylic acid is preferably from 4: 1 to 1: 4, in particular from 3: 1 to 1: 3, particularly preferably from 2: 1 to 1: 2.

The selectivity is preferably at least 90%, particularly preferably 98%. Selectivity is defined as the ratio of the sum of the amounts of alkyl (meth) acrylate and α -hydroxycarboxylic acid formed to the sum of the amounts of alkyl (meth) hydroxycarboxylic acid and (meth) acrylic acid reacted.

According to a particular aspect of the invention, the transesterification can be carried out in the presence of water. The water content is preferably from 0.1 to 50% by weight, particularly preferably from 0.5 to 20% by weight, very particularly preferably from 1 to 10% by weight, based on the weight of the alkyl α -hydroxycarboxylic acid used.

By adding small amounts of water, the selectivity of the reaction can surprisingly be increased. Despite the addition of water, the formation of methanol remained surprisingly small.

When the water concentration is from 10 to 15% by weight, based on the weight of the alkyl alpha-hydroxycarboxylic acid used, methanol is preferably formed in an amount of less than 5% by weight at a reaction temperature of 120 ℃ and a reaction time or residence time of 5 to 180 minutes.

The transesterification can be carried out batchwise or continuously, with a continuous process being preferred.

The reaction time for the transesterification depends on the molar amounts used and on the reaction temperature, and these parameters can be within wide limits. The reaction time for the transesterification of the alkyl α -hydroxycarboxylic acid with (meth) acrylic acid is preferably from 30 seconds to 15 hours, particularly preferably from 5 minutes to 5 hours, and very particularly preferably from 15 minutes to 3 hours.

In the continuous process, the residence time is preferably from 30 seconds to 15 hours, particularly preferably from 5 minutes to 5 hours, very particularly preferably from 15 minutes to 3 hours.

When methyl methacrylate is prepared from methyl a-hydroxyisobutyrate, the temperature is preferably from 60 to 130 ℃, particularly preferably from 80 to 120 ℃, very particularly preferably from 90 to 110 ℃. The pressure is preferably from 50 to 1000 mbar, particularly preferably from 300 to 800 mbar. The molar ratio of methacrylic acid to methyl a-hydroxyisobutyrate is preferably from 2: 1 to 1: 2, in particular from 1.5: 1 to 1: 1.5.

For example, the transesterification is carried out in the apparatus shown in FIG. 1. A hydroxycarboxylic acid ester, such as methyl hydroxyisobutyrate, is fed via line (1) to a fixed bed reactor (3) containing a cation exchange resin. (meth) acrylic acid, for example 2-methacrylic acid, is fed to the fixed-bed reactor (3) via line (2) or line (17). Line (2) may be connected to other lines, such as line (9) and line (13), in order to reduce the number of reactor inlet lines. However, lines (9), (13) and/or (17) can also be passed directly into the fixed-bed reactor. Under the reaction conditions described above, a reaction mixture is formed which, in addition to methanol and unreacted methyl hydroxyisobutyrate and methacrylic acid, also contains the reaction products hydroxyisobutyric acid and methyl methacrylate. The reaction mixture is introduced via line (4) into a still (5). In the still (5), water, methyl methacrylate and methanol are obtained as distillate, which is fed via line (7) as overhead product to the phase separator (8). Methyl methacrylate and methanol are collected in the upper phase, and they are taken out of the system through a line (10). In the lower phase of the phase separator (8) in particular water is collected which is removed from the system via line (11) or can be fed into the fixed-bed reactor (3) via line (9).

Methyl hydroxyisobutyrate, hydroxyisobutyric acid and methacrylic acid are obtained from the bottom of the column and can be introduced into the second still (12) through line (6). Methyl hydroxyisobutyrate and methacrylic acid are distilled off here and returned to the transesterification via line (13). Hydroxyisobutyric acid contained in the bottom of the distillation column is introduced into a dehydration reactor (15) through a line (14). The methacrylic acid thus obtained may be fed to the above-mentioned transesterification via the line (17) or taken out of the system via the line (16).

According to a particularly preferred embodiment, the transesterification can be carried out in a still. The catalyst can be added at any zone of the still at this point. For example, the catalyst may be prepared to be provided in the column bottom region or in the column region. However, the reactants should be contacted with the catalyst. Furthermore, the catalyst may be provided in a separate zone of the distiller, in which case this zone is connected to other zones of the distiller, such as the bottom of the tower and/or the column. This separate arrangement of catalyst zones is preferred.

By this preferred approach, the selectivity of the reaction can surprisingly be increased. In this connection, it was established that the pressure of the reaction can be adjusted independently of the pressure inside the distillation column. So that the boiling temperature can be kept low without a corresponding increase in reaction time or residence time. In addition, the reaction temperature was changed by other regions. Thereby shortening the reaction time. In addition, the volume of the catalyst can be chosen arbitrarily, without having to take into account the geometry of the column. In addition, for example, other reactants may also be added. All these measures contribute to an increase in selectivity and productivity, thereby achieving a surprising synergistic effect.

Here, an alkyl ester of an alpha-hydroxycarboxylic acid, for example methyl alpha-hydroxyisobutyrate, is fed to a still. (meth) acrylic acid, such as methacrylic acid, is also introduced into the still. The distillation conditions are preferably designed such that exactly one product is removed from the still by distillation, while the second product remains at the bottom and is continuously removed from the bottom. When an alcohol having a smaller carbon number is used, particularly ethanol or methanol, it is preferable to take the alkyl (meth) acrylate out of the reaction mixture by distillation. The reactants are recycled to the catalyst zone. Thereby continuously forming alkyl (meth) acrylate and alpha-hydroxycarboxylic acid.

A preferred embodiment of reactive distillation is schematically illustrated in figure 2. The reactants can be introduced into the distillation column (3) via a common line (1) or separately via the two lines (1) and (2). Preferably, the addition of the reactants is carried out via separate lines. In this case, the reactants may be added at the same stage or at any position of the column.

The temperature of the reactants can be adjusted via heat exchangers in the transfer lines, the equipment required for this being not shown in FIG. 1. In a preferred variant, the reactants are metered separately into the column and the metering of the low-boiling components is carried out below the point at which the high-boiling compounds are conveyed. In this case, it is preferred that the low-boiling components be added in the vapor state.

For the present invention, any multi-stage distillation column (3) having two or more separation stages may be used. The number of separation stages used in the present invention is the number of trays in a tray column or, in the case of a conventional packed column (Packungskolone) or a random packed column (KolonneFa llk ö rpern), the number of theoretical separation stages.

Examples of the multistage distillation column having trays include those having bubble cap trays, sieve trays, channel bubble cap trays, valve trays, channel trays, slotted sieve trays, bubble cap sieve trays, jet trays, centrifugal trays; for multi-stage distillation columns with random packing, those such as raschig rings, luxing rings, pall rings, bell saddles, intel rock saddles; and for multistage distillation columns with structured packing such as those of Mellapak (Sulzer), Rombopak (Ku hni), Montz-Pak (Montz) and structured packing with catalyst bags such as Kata-Pak.

Distillation columns with combinations of tray zones, random packing zones, or random packing zones may also be used.

The column (3) may be equipped with internals. The column preferably has a condenser (12) for condensing the vapor and a bottom evaporator (18).

The distillation apparatus preferably has at least one zone, hereinafter referred to as reactor, in which at least one catalyst is contained. The reactor may be within a distillation column. However, the reactor is preferably arranged outside the column (3) in a separate zone, for which one of the preferred embodiments is explained in detail in fig. 2.

In order to carry out the transesterification in a separate reactor (8), a portion of the liquid phase flowing downwards can be collected in the column by means of a collector and conducted away from the column as a substream (4). The location of the trap is determined by the concentration profile of the individual components in the column. The concentration profile can be adjusted by means of temperature and/or reflux. The collector is preferably arranged such that: so that the stream carried out of the column contains both reactants, more preferably a sufficiently high concentration of reactants, and most preferably an acid to ester molar ratio of 1.5: 1 to 1: 1.5. In addition, it is possible to equip the distillation column with a plurality of collectors at different locations, in which case the molar ratio can be adjusted by the amount of reactants withdrawn.

In addition, other reactants, such as water, can be metered into the stream withdrawn from the column to adjust the acid/ester product ratio in the cross-transesterification reaction or to increase the selectivity. The water can be fed in from the outside through a conduit (not shown in fig. 1) or from the phase separator (13). The pressure of the water-enriched stream (5) can then be increased by means of a pressure increasing device (6), such as a pump.

The increase in pressure may reduce or prevent the formation of water vapor in the reactor, e.g., a fixed bed reactor. This enables a uniform flow through the reactor and wetting of the catalyst particles. The stream can be guided through a heat exchanger (7) and the reaction temperature adjusted. The stream may be heated or cooled if desired. The product ratio of ester to acid can additionally be adjusted by means of the reaction temperature.

The transesterification is carried out in a fixed-bed reactor (8) over a catalyst. The flow through the reactor may be downward or upward. A portion of the reactor output stream (9) comprising product and unconverted reactants is first passed through a heat exchanger (10) and adjusted to a temperature favorable for introduction into the distillation column, the portion of the components in the reactor output stream depending on the residence time, the amount of catalyst, the reaction temperature and the reactant ratio and the amount of water added. The temperature preferably set corresponds to the temperature of the point of introduction of the stream in the distillation column.

The point at which the stream leaving the reactor is returned to the column may be above or below the point at which the reactor feed is withdrawn, but will preferably be above. Before being led back into the column, the stream can be depressurized through a valve (11) which preferably establishes the same degree of pressure as in the column. In this context, the distillation column preferably has a lower pressure. This configuration provides the advantage of reducing the boiling point of the components to be separated, since the distillation can be carried out at a lower temperature level, thereby saving energy and being carried out more gently thermally.

The product mixture is then separated in a distillation column (3). The low boilers, preferably the esters formed in the transesterification, are separated off via the top. The distillation column is preferably operated as follows: so that the water fed upstream of the fixed-bed reactor is likewise separated off as overhead product. The vapor stream discharged at the top is condensed in a condenser (12) and then separated in a decanter (13) into an aqueous phase and a phase containing the product esters. The aqueous phase can be discharged for processing via conduit (15) or returned to the reaction in whole or in part via conduit (17). The stream of the ester-containing phase can be partly introduced into the column as reflux (16) or partly discharged from the still via conduit (14). High boilers, preferably the acid formed in the cross-transesterification, are discharged from column (19) as a bottom stream.

The selectivity of the reaction can be surprisingly increased by this preferred embodiment. In this connection, it was confirmed that the pressure of the reaction can be adjusted independently of the pressure inside the distillation column. So that the boiling temperature can be kept low without a corresponding increase in reaction time or residence time. Furthermore, the reaction temperature can be varied within wide limits. Thereby shortening the reaction time. Furthermore, the volume of the catalyst can be chosen arbitrarily, without having to take into account the geometry of the column. In addition, for example, other reactants may also be added.

The alpha-hydroxycarboxylic acids obtained from the reaction, for example hydroxyisobutyric acid, can be dehydrated in a known manner. In general, an α -hydroxycarboxylic acid, for example α -hydroxyisobutyric acid, is heated to a temperature of 160-300 ℃ and particularly preferably 200 to 240 ℃ in the presence of at least one metal salt, for example an alkali metal salt and/or an alkaline earth metal salt, whereby (meth) acrylic acid and water are generally obtained. Suitable metal salts include, inter alia, sodium hydroxide, potassium hydroxide, calcium hydroxide, barium hydroxide, magnesium hydroxide, sodium sulfite, sodium carbonate, potassium carbonate, strontium carbonate, magnesium carbonate, sodium bicarbonate, sodium acetate, potassium acetate, and sodium dihydrogen phosphate.

The dehydration of the alpha-hydroxycarboxylic acid can preferably be carried out at a pressure in the range from 0.05 bar to 2.5 bar, particularly preferably in the range from 0.1 bar to 1 bar.

According to a particular aspect of the present invention, the pressure at the time of dehydration is substantially the same as the pressure at the time of transesterification of the above-mentioned alkyl α -hydroxycarboxylic acid ester with (meth) acrylic acid, but is not limited thereto. The pressure difference between transesterification and dehydration is preferably less than 0.1 bar, particularly preferably less than 0.05 bar. According to a particular embodiment of the invention, the (meth) acrylic acid obtained in the gaseous state is introduced into the transesterification without condensation and renewed vaporization.

The dehydration of alpha-hydroxycarboxylic acids is described, for example, in DE-A-1768253.

The (meth) acrylic acid thus obtained can be reused for the preparation of alkyl (meth) acrylates. Furthermore, (meth) acrylic acid is a commercial product. Surprisingly, therefore, the apparatus for preparing the alkyl (meth) acrylate can also be used for preparing (meth) acrylic acid, in which case the product ratio of alkyl (meth) acrylate to (meth) acrylic acid can be easily controlled by the concentration of water during the transesterification of the alkyl (meth) acrylate with the alkyl (meth) acrylate and/or by the reaction temperature.

Thus, in general, an alkyl (meth) acrylate can be obtained simply and at low cost from a carbonyl compound, hydrocyanic acid, and alcohol by a method comprising the steps of:

A) forming at least one cyanohydrin by reaction of at least one carbonyl compound with hydrocyanic acid;

B) hydrolyzing said cyanohydrin(s) to form at least one α -hydroxycarboxylic acid amide;

C) alcoholysis of the α -hydroxycarboxylic acid amide or amides to yield at least one alkyl α -hydroxycarboxylic acid ester;

D) (ii) transesterifying the alpha-hydroxycarboxylic acid alkyl ester or esters with (meth) acrylic acid to form at least one alkyl (meth) acrylate and at least one alpha-hydroxycarboxylic acid;

E) dehydrating the alpha-hydroxycarboxylic acid or acids to form (meth) acrylic acid.

The present invention is explained in more detail below by way of examples and comparative examples.

Example 1

4619g of methyl alpha-Hydroxyisobutyrate (HIBSM) and 3516g of Methacrylic Acid (MAS) were charged over a period of 48 hours in the reactive still shown in FIG. 2. The reaction is carried out at a temperature of 120 ℃ and a pressure of 250 mbar. The alpha-hydroxyisobutyric acid formed is removed from the bottom of the column. Methyl Methacrylate (MMA) was distilled off. The reaction was carried out in the presence of 16% by weight of water based on the weight of methyl alpha-hydroxyisobutyrate. The reaction was carried out using an acidic catalyst (cation exchanger; Lewatit ® -Typ K2431 from Bayer).

The selectivity, defined as the ratio of the amounts of Methyl Methacrylate (MMA) and alpha-hydroxyisobutyric acid (HIBS) formed to the amounts of HIBSM and MAS reacted, was 99%.

The alpha-hydroxyisobutyric acid obtained from this process is dehydrated according to DE-OS 1768253.

Overall, the selectivity is 98.5%, which is defined as the ratio of the amount of substance of MMA formed to the amount of substance of HIBSM reacted.

Comparative example 1

Methyl methacrylate is prepared by dehydration of methyl alpha-hydroxyisobutyrate. The reaction is carried out according to EP-A-0941984. To a mixture of 20g of sodium dihydrogen phosphate and 80g of water, 60g of silica gel was added. Water is removed from the mixture under reduced pressure. The residue was dried at 150 ℃ overnight to give a catalyst. 10g of the resulting catalyst was charged into a quartz tube equipped with a vaporizer. The quartz tube was heated with a furnace and the temperature of the catalyst layer was about 400 ℃. A mixture of methanol and methyl a-hydroxyisobutyrate (2: 1) was continuously vaporized at a rate of 10 g/hr and conducted through the catalyst layer. The reaction selectivity, defined as the ratio of the amount of MMA formed to the amount of HIBSM reacted, was 88%.

Examples 2 to 18

Example 1 was substantially repeated, except that no water was added to the reaction mixture. The reaction is carried out under the conditions described in table 1, in particular with respect to temperature, residence time and molar ratio of the reactants. The reaction selectivity, defined as the ratio of the amounts of MMA and HIBS formed to the amounts of HIBSM and MAS reacted, is likewise shown in Table 1.

TABLE 1

Examples	The reaction temperature is [ deg.C]	Molar ratio HIBSM/MAS	Residence time [ min ]]	Selectivity [% ]]
					2	120	1.00	28.33	93.21
3	90	1.00	42.50	95.06
					4	100	1.00	42.50	94.81
5	110	1.00	42.50	94.64
					6	120	1.00	42.50	90.67
7	90	1.00	85.00	95.53
					8	100	1.00	85.00	94.95
9	110	1.00	85.00	93.55
					10	120	1.00	85.00	91.78
11	90	1.00	170.00	94.83
					12	100	1.00	170.00	94.06
13	90	2.0	42.50	91.61
					14	100	2.0	42.50	91.73
15	90	2.0	85.00	90.63

16	100	2.0	85.00	90.30
					17	120	0.50	28.33	92.05
18	120	0.50	42.50	92.62

Examples 19 to 38

Example 1 was substantially repeated, but the reaction was carried out under the conditions described in table 2, in particular with respect to temperature and residence time. The molar ratio of HIBSM/MAS was 1: 1. In addition, different portions of water were added, which are also shown in table 2. The reaction selectivity, defined as the ratio of the amounts of MMA and HIBS formed to the amounts of HIBSM and MAS reacted, and the molar ratio of HIBS to MMA are likewise stated in Table 2.

TABLE 2

Examples	The reaction temperature is [ deg.C]	Molar ratio H2O/HIBSM	Residence time [ min ]]	Selectivity [% ]]	Molar ratio HIBS/MMA
						19	90	0.20	42.5	98.61	1.33
20	100	0.20	42.5	98.18	1.21
						21	110	0.20	42.5	97.44	1.11
22	120	0.20	42.5	96.27	0.99
						23	90	0.20	85	98.34	1.18
24	100	0.20	85	97.66	1.09
						25	110	0.20	85	96.56	1.02
26	100	0.20	170	96.95	1.00
						27	90	0.50	42.5	98.80	1.61
28	100	0.50	42.5	98.64	1.36
						29	110	0.50	42.5	98.21	1.22
30	120	0.50	42.5	97.58	1.08
						31	90	0.50	85	98.76	1.39
32	100	0.50	85	98.35	1.20
						33	110	0.50	85	97.78	1.10
34	100	0.50	170	98.08	1.10
						35	90	1.00	50.0	99.41	2.090
36	100	1.00	50.0	99.65	1.618
						37	110	1.00	50.0	99.82	1.360
38	120	1.00	50.0	99.54	1.319

The above examples show that alkyl (meth) acrylates can be formed with very high selectivity by means of the invention, the ratio of alkyl (meth) acrylate to alpha-hydroxycarboxylic acid being close to 1 even at higher water concentrations. Thus, less methanol is formed. The molar ratio of alkyl (meth) acrylate to alpha-hydroxycarboxylic acid can also be controlled by temperature.

Claims (22)

1. A process for producing an alkyl (meth) acrylate, which comprises the steps of transesterifying an alkyl (meth) acrylate with (meth) acrylic acid to obtain an alkyl (meth) acrylate and an alpha-hydroxycarboxylic acid, and dehydrating the alpha-hydroxycarboxylic acid to obtain (meth) acrylic acid.

2. The process according to claim 1, characterized in that the alkyl α -hydroxycarboxylic acid is obtained by alcoholysis of a hydroxycarboxylic acid amide.

3. A process according to claim 2, characterized in that the hydroxycarboxylic acid amide is obtained by hydrolysis with cyanohydrin.

4. A process according to claim 3, characterized in that the cyanohydrin is acetone cyanohydrin.

5.A process according to claim 3 or 4, characterized in that the hydrolysis is carried out using a catalyst.

6. The method of claim 5, wherein the catalyst comprises manganese oxide, sulfuric acid, or an enzyme.

7. The process according to at least one of the preceding claims 2 to 6, characterized in that the alcohol used for the alcoholysis of hydroxycarboxylic acid amides contains from 1 to 10 carbon atoms.

8. Process according to claim 7, characterized in that the alcohol is methanol and/or ethanol.

9. Process according to at least one of the preceding claims 2 to 8, characterized in that the alcoholysis is carried out at a temperature of 160-240 ℃.

10. The process according to at least one of the preceding claims 2 to 9, characterized in that alcoholysis is carried out at a pressure of 5 to 30 bar.

11. The process according to at least one of the preceding claims 2 to 10, characterized in that alcoholysis is carried out using at least one basic catalyst.

12. The process according to at least one of the preceding claims, characterized in that the transesterification of the alkyl α -hydroxycarboxylic acid with (meth) acrylic acid is catalyzed by an acid.

13. The process according to claim 12, characterized in that the acid is an ion exchanger.

14. The process according to claim 12 or 13, characterized in that the transesterification is carried out in a still.

15. The process according to at least one of the preceding claims, characterized in that the transesterification of the alkyl α -hydroxycarboxylic acid with (meth) acrylic acid is carried out at a pressure of 100 mbar to 3 bar.

16. The process according to at least one of the preceding claims, characterized in that the transesterification of the alkyl α -hydroxycarboxylic acid with (meth) acrylic acid is carried out at a temperature of 70 to 130 ℃.

17. The process according to at least one of the preceding claims, characterized in that the transesterification of the alkyl α -hydroxycarboxylic acid with (meth) acrylic acid is carried out in the presence of water.

18. The process according to claim 17, wherein the water concentration is from 0.1 to 50% by weight, based on the weight of the alkyl α -hydroxycarboxylic acid.

19. The process according to at least one of the preceding claims, characterized in that the molar ratio of alkyl α -hydroxycarboxylic acid ester to (meth) acrylic acid is 3: 1 to 1: 3 when the alkyl α -hydroxycarboxylic acid ester is transesterified with (meth) acrylic acid.

20. The process according to at least one of the preceding claims, characterized in that the reaction time is 5 minutes to 5 hours when the alkyl α -hydroxycarboxylic acid ester is transesterified with (meth) acrylic acid.

21. The process according to at least one of the preceding claims, characterized in that the dehydration of the α -hydroxycarboxylic acid and the transesterification of the α -hydroxycarboxylic acid alkyl ester with (meth) acrylic acid are carried out under the same pressure.

22. The process according to at least one of the preceding claims, characterized in that the (meth) acrylic acid obtained in gaseous form by dehydration of the α -hydroxycarboxylic acid is introduced into the transesterification without condensation and revaporization.