EP2043994A1

EP2043994A1 - Process for preparing alpha-hydroxycarboxylic acids

Info

Publication number: EP2043994A1
Application number: EP07729501A
Authority: EP
Inventors: Jochen Ackermann; Alexander May; Udo Gropp; Hermann Siegert; Bernd Vogel; Sönke BRÖCKER
Original assignee: Evonik Roehm GmbH
Current assignee: Evonik Roehm GmbH
Priority date: 2006-07-21
Filing date: 2007-05-25
Publication date: 2009-04-08
Also published as: CA2658590A1; DE102006034273A1; JP2009544640A; KR20090039730A; MX2009000597A; CN101489973A; TW200829548A; BRPI0715435A2; WO2008009503A1; US20090209781A1; RU2009105822A

Abstract

A continuous process for preparing alpha-hydroxycarboxylic esters, in which, as reactants, alpha-hydroxycarboxamide is reacted with an alcohol in the presence of a catalyst to obtain a product mixture which comprises alpha-hydroxycarboxylic ester, ammonia, unconverted alpha-hydroxycarboxamide, and also alcohol and catalysts; wherein a') reactant streams comprising, as reactants, an alpha-hydroxycarboxamide, an alcohol and a catalyst are fed into a pressure reactor; b') the reactant streams are reacted with one another in the pressure reactor at a pressure in the range from 1 bar to 100 bar; c') the product mixture which results from step b) and comprises alpha-hydroxycarboxylic ester, unconverted alpha-hydroxycarboxamide and catalysts, and also ammonia and alcohol, is discharged from the pressure reactor; and d') the product mixture is depleted in alcohol and ammonia, ammonia being distilled off at a pressure which is constantly kept greater than 1 bar, without the aid of additional stripping media. The continuous process can be employed on the industrial scale particularly advantageously.

Description

PROCESS FOR PREPARING ALPHA-HYDROXYCARBOXY ACID ESTERS

The present invention relates to processes for the production of alpha-hydroxycarboxylic acid esters on an industrial scale. In particular, the invention relates to a continuous process for the preparation of alpha-hydroxycarboxylic acid esters according to the preamble of claim 1.

Alpha-hydroxycarboxylic acid esters are valuable intermediates in the large-scale synthesis of acrylic acid esters and methacrylic acid esters, hereinafter referred to as alkyl (meth) acrylates. In turn, alkyl (meth) acrylates find their main application in the preparation of polymers and copolymers with other polymerizable compounds.

An overview of the common processes for the preparation of (meth) acrylic acid esters can be found in the literature such as Weissermel, Arpe "Industrial Organic Chemistry", VCH, Weinheim 1994, 4th Edition, p. 305 ff or Kirk Othmer "Encyclopedia of Chemical Technology" , 3rd edition, Vol. 15, page 357.

If the aim of the synthesis of methacrylic acid esters, such as, for example, methyl methacrylate, is the alpha-hydroxycarboxylic acid ester of 2-hydroxyisobutyrate (= HIBSM), a central intermediate for its preparation.

A generic method is known from EP 0 945 423. "Disclosed herein is a process for producing alpha-hydroxycarboxylic acid esters, which comprises the steps of reacting an alpha-hydroxycarboxylic acid amide and an alcohol in the presence of a catalyst in a liquid phase while maintaining the ammonia concentration in the reaction solution at 0, 1 wt .-% or less holds that emerging ammonia is removed as gas in a gas phase. To remove the ammonia from the reaction solution as a gas in the gas phase, it is distilled off from the reaction solution. For this purpose, the reaction solution is heated to boiling and / or a strip gas, ie an inert gas, is bubbled through the reaction solution.

The disadvantages of the process disclosed in EP 0 945 423 for the preparation of alpha-hydroxycarboxylic acid esters by alcoholysis of corresponding alpha-hydroxycarboxylic acid amides can be summarized as follows:

i. The simple distilling off of the ammonia according to a process variant indicated in EP 0 945 423 is not very effective. To realize this proposal, it requires a highly effective separation column and thus a special technical effort. ii. If additionally or exclusively an inert strip gas is used, the effectiveness of the removal of ammonia is improved, but at the expense of another process component, the handling of which means an additional expense. iii. If alpha-hydroxynobutyric acid amide and methanol are used as starting materials, it is very difficult to separate off ammonia and residual methanol formed under the conditions indicated in EP 0 945 423.

The almost always necessary use of an inert gas for removal of ammonia and the associated additional handling of another stream (separation stripping gas / ammonia) make the proposed procedure economically relatively uninteresting, which is also reflected in the fact that it hitherto to a technical implementation of the disclosed method lacking.

In view of the prior art, it was therefore an object of the present invention to provide processes for the preparation of alpha-hydroxycarboxylic acid esters, which can be carried out simply and inexpensively. Another object of the invention was to provide a process in which the alpha-hydroxycarboxylic acid esters can be obtained very selectively.

In addition, it was an object of the present invention to provide a process for the preparation of alpha-hydroxycarboxylic acid esters in which no by-products or only small amounts of by-products are produced. In this case, the product should preferably be obtained in high yields and, overall, with low energy consumption.

These and other non-explicitly stated objects, which, however, are readily derivable or deducible from the contexts discussed hereinbelow, are solved by methods having all features of claim 1. Advantageous modifications of the inventive methods are protected by the dependent claims appended to claim 1 posed.

Accordingly, the present invention relates to continuous processes for the preparation of alpha-hydroxycarboxylic acid esters in which alpha-hydroxycarboxamide is reacted with an alcohol in the presence of a catalyst to give a product mixture comprising alpha-hydroxycarboxylic acid esters, ammonia, unreacted alpha-hydroxycarboxamide and alcohol and catalyst; the process being characterized in that a ') starting material streams comprising, as starting materials, an alpha-hydroxycarboxylic acid amide, an alcohol and a catalyst are fed into a pressure reactor; b ') the educt streams in the pressure reactor at a pressure in the range of greater than 1 bar to 100 bar with each other; c ') discharging the product mixture resulting from step b') comprising alpha-hydroxycarboxylic acid ester, unreacted alpha-hydroxycarboxylic acid amide, ammonia, alcohol and catalyst from the pressure reactor; and d ') depletes the product mixture of alcohol and ammonia, wherein ammonia is distilled off at a pressure which is constantly greater than 1 bar, without the aid of additional striping agents.

The following advantages can be achieved, among others, by the measures according to the invention:

Surprisingly, the ammonia resulting from the reaction according to the invention can be separated off relatively easily from alcohol, for example methanol, which is used for the alcoholysis or methanolysis of the alpha-hydroxycarboxamide. This is possible, although alcohol or methanol and ammonia are very difficult to separate in dissolved form under normal conditions.

• During the separation, ammonia already accumulates in a very pure form and can thus be reused in various processes without further purification step. The alcohol also accumulates in such a way that it is present in process-suitable quality and can be recycled, for example, into a production process.

In this case, the process of the invention avoids the use of aids for the separation of the ammonia, above all the use of inert gases as a stripping agent for the ammonia becomes unnecessary. Accordingly, in the process according to the invention, no larger amount of additional inert gas flow, which in turn would have to be separated from the ammonia, is produced.

• The process according to the invention gives the alpha-hydroxycarboxylic acid esters in high yields and purities. This applies in particular in comparison with the processes described in EP-A-0945423, in which the α-hydroxycarboxylic acid amides subject to a very low current ammonia concentration in the liquid phase of an alcoholysis to the alpha-hydroxycarboxylic be subjected. Surprisingly, it could be stated that by using pressure in combination with a simple distillation / rectification, not only could the additional measure of stripping with inert gas be dispensed with, but also a higher ammonia concentration in the liquid phase is tolerable, without altogether higher To do without selectivities.

• The formation of by-products is unusually low. Furthermore, in particular taking into account the high selectivity, high conversions are achieved.

The process of the present invention also has an extremely low tendency to form by-products.

Furthermore, the method according to the invention can be carried out inexpensively, in particular with low energy consumption. Here, the catalysts used for the alcoholysis of the alpha-hydroxycarboxamide can be used over a long period of time, without the selectivity or the activity decreases. In this respect, the catalysts have a long service life.

Finally, the process of the present invention can be carried out particularly advantageously on an industrial scale.

In the process of the invention, alpha-hydroxycarboxylic acid esters are prepared by the reaction between the reactants alpha-hydroxycarboxamide and alcohol in the presence of a catalyst.

The alpha-hydroxycarboxylic acid amides useful in the reaction of the invention usually include all those Carboxylic acid amides which have at least one hydroxyl group in the alpha position to the carboxylic acid amide group.

Carboxylic acid amides, in turn, are well known in the art. Commonly included herein are compounds having groups of the formula - CONR'R "-, wherein R 'and R" independently represent hydrogen or a 1-30 carbon group comprising in particular 1-20, preferably 1-10 and especially 1-5 carbon atoms , The carboxylic acid amide may comprise 1, 2, 3, 4 or more groups of the formula - CONR'R "- These include, in particular, compounds of the formula R (-CONR'R"' _n , in which the radical R has 1-30 carbon atoms Group which comprises in particular 1- 20, preferably 1-10, in particular 1-5 and particularly preferably 2-3 carbon atoms, R 'and R "has the abovementioned meaning and n is an integer in the range of 1-10, preferably 1-4 and more preferably 1 or 2 represents.

The term "1 to 30 carbon atoms" denotes residues of organic compounds having 1 to 30 carbon atoms. In addition to aromatic and heteroaromatic groups, it also includes aliphatic and heteroaliphatic groups, such as, for example, alkyl, cycloalkyl, alkoxy, cycloalkoxy, cycloalkylthio and alkenyl groups. The groups mentioned can be branched or unbranched.

According to the invention, aromatic groups are radicals of mononuclear or polynuclear aromatic compounds having preferably 6 to 20, in particular 6 to 12, carbon atoms.

Heteroaromatic groups denote aryl radicals in which at least one CH group has been replaced by N and / or at least two adjacent CH groups have been replaced by S, NH or O. Preferred aromatic or heteroaromatic groups according to the invention are derived from benzene, naphthalene, biphenyl, diphenyl ether, diphenylmethane, diphenyldimethylmethane, bisphenone, diphenylsulfone, thiophene, furan, pyrrole, thiazole, oxazole, imidazole, isothiazole, isoxazole, pyrazole, 1,3,4-oxadiazole , 2,5-Diphenyl-l, 3,4-oxadiazole, 1,3,4-thiadiazole, 1,3,4-triazole, 2,5-diphenyl-l, 3,4-triazole, l, 2.5 -Triphenyl-l, 3,4-triazole, 1,2,4-oxadiazole, 1,2,4-thiadiazole, 1,2,4-triazole, 1,2,3-triazole, 1,2,3,4 Tetrazole, benzo [b] thiophene, benzo [b] furan, indole, benzo [c] thiophene, benzo [c] furan, isoindole, benzoxazole, benzothiazole, benzimidazole, benzisoxazole, benzisothiazole, benzopyrazole, benzothiadiazole, benzotriazole, dibenzofuran, dibenzothiophene , Carbazole, pyridine, bipyridine, pyrazine, pyrazole, pyrimidine, pyridazine, 1,3,5-triazine, 1,2,4-triazine, 1,2,4,5-triazine, tetrazine, quinoline, isoquinoline, quinoxaline, quinazoline , Cinnoline, 1,8-naphthyridine, 1,5-naphthyridine, 1,6-naphthyridine, 1,7-naphthyridine, Pht halazine, pyridopyrimidine, purine, pteridine or quinolizine, 4H-quinolizine, diphenyl ether, anthracene, benzopyrrole, benzooxathiadiazole, benzooxadiazole, benzopyridine, benzopyrazine, benzopyrazidine, benzopyrimidine, benzotriazine, indolizine, pyridopyridine, imidazopyrimidine, pyrazinopyrimidine, carbazole, aciridine, phenazine, benzoquinoline, Phenoxazine, phenothiazine, acridizine, benzopteridine, phenanthroline and phenanthrene, which may optionally be substituted.

Preferred alkyl groups include methyl, ethyl, propyl, isopropyl, 1-butyl, 2-butyl, 2-methylpropyl, tert-butyl, pentyl, 2-methylbutyl, 1,1 -Dimethylpropyl, hexyl, heptyl, octyl, 1,1,3,3-tetramethylbutyl, nonyl, 1-decyl, 2-decyl, undecyl, dodecyl, pentadecyl and the eicosyl group.

Preferred cycloalkyl groups include the cyclopropyl, cyclobutyl, cyclopentyl, cyclohexyl, cycloheptyl, and cyclooctyl groups, optionally substituted with branched or unbranched alkyl groups. Preferred alkenyl groups include the vinyl, allyl, 2-methyl-2-propylene, 2-butenyl, 2-pentenyl, 2-decenyl and 2-eicosenyl groups.

Preferred heteroaliphatic groups include the aforementioned preferred alkyl and cycloalkyl groups in which at least one carbon moiety is replaced by O, S or a group NR ⁸ or NR ⁸ R ⁹ , and R ⁸ and R ^{9 are} independently one to six carbon atoms having alkyl, a 1 to 6 carbon atoms having alkoxy or an aryl group.

Very particularly preferably according to the invention the carboxylic acid amides have branched or unbranched alkyl or alkoxy groups having 1 to 20 carbon atoms, preferably 1 to 12, suitably 1 to 6, in particular 1 to 4 carbon atoms and cycloalkyl or cycloalkyloxy groups having 3 to 20 carbon atoms, preferably 5 to 6 carbon atoms.

The radical R may have substituents. Among the preferred substituents are i.a. Halogens, in particular fluorine, chlorine, bromine, and alkoxy or hydroxy radicals.

The alpha-hydroxycarboxamides can be used in the process of the invention singly or as a mixture of two or three or more different alpha-hydroxycarboxamides. Particularly preferred alpha-hydroxycarboxylic acid amides include alpha-hydroxyisobutyric acid amide and / or alpha-hydroxyisopropionic acid amide.

Furthermore, in a modification of the process according to the invention, it is of particular interest to use those alpha-hydroxycarboxamides which are obtainable by cyanohydrin synthesis from ketones or aldehydes and hydrogen cyanide. In a first step, the carbonyl compound, for example a ketone, in particular acetone, or an aldehyde, for example acetaldehyde, propanal, butanal, with hydrocyanic acid to give the respective cyanohydrin implemented. It is particularly preferred to react acetone and / or acetaldehyde in a typical manner using a small amount of alkali or an amine as the catalyst. In a further step, the cyanohydrin thus obtained is reacted with water to the alpha-hydroxycarboxylic acid amide.

This reaction is typically carried out in the presence of a catalyst. Particularly suitable for this purpose are manganese oxide catalysts, as described, for example, in EP-A-0945429, EP-A-0561614 and EP-A-0545697. Here, the manganese oxide can be used in the form of manganese dioxide, which by treatment of manganese sulfate with potassium permanganate under acidic conditions (see Biochem.J., 50 p 43 (1951) and J.Chem.Soc, 1953, p 2189, 1953 ) or by electrolytic oxidation of manganese sulfate in aqueous solution. In general, the catalyst is often used in the form of powder or granules with a suitable grain size. Furthermore, the catalyst can be applied to a support. In particular, so-called slurry reactors or fixed-bed reactors can also be used here, which can also be operated as a trickle bed and are described inter alia in EP-A-956 898. Furthermore, the hydrolysis reaction can be catalyzed by enzymes. Suitable enzymes include nitrile hydratases. This reaction is exemplary in "Screening, Characterization and Application of Cyanide-resistant Nitrile Hydratases" Eng. Life. Be. 2004, 4, no. 6 described. In addition, the hydrolysis reaction can be catalyzed by acids, especially sulfuric acid. This is stated inter alia in JP Hei 4-193845.

Alcohols which can be used successfully in the process of the invention include all alcohols known to the person skilled in the art as well as precursor compounds of alcohols which under the stated conditions of pressure and temperature are capable of reacting with the alpha-hydroxycarboxamides in the sense of alcoholysis. Preferably, the reaction of the α-hydroxycarboxamide by alcoholysis with an alcohol, preferably 1-10 carbon atoms, particularly preferably 1 to 5 Carbon atoms. Preferred alcohols include methanol, ethanol, propanol, butanol, especially n-butanol and 2-methyl-1-propanol, pentanol, hexanol, heptanol, 2-ethylhexanol, octanol, nonanol and decanol. Methanol and / or ethanol is particularly preferred as the alcohol, with methanol being particularly useful. The use of precursors of an alcohol is possible in principle. For example, alkyl formates can be used. In particular, methyl formate or a mixture of methanol and carbon monoxide are suitable.

The reaction between alpha-hydroxycarboxamide and alcohol is carried out in the context of the invention in a pressure reactor. This is basically a reaction space to understand, which allows to maintain an overpressure during the implementation. Overpressure in this context means a pressure greater than atmospheric pressure, i. in particular greater than 1 bar. In the context of the invention, the pressure can be in a range from greater than 1 bar to less than or equal to 100 bar. It follows from the above that the pressure during the reaction / alcoholysis of the alpha-hydroxycarboxamide according to the invention as well as during the removal / removal of the ammonia from the product mixture is greater than atmospheric pressure or greater than 1 bar. In particular, this means that the ammonia formed during the reaction is distilled off from the mixture under a pressure of greater than 1 bar, wherein the use of auxiliaries such as strip gas for distillative removal of the ammonia is completely dispensed with.

For the purposes of the invention, the product mixture is depleted not only in ammonia but also in unreacted alcohol. Especially in the event that methanol is used for the alcoholysis, a product mixture results, inter alia, with the components, which are in principle very difficult to separate from one another, ammonia and methanol. In the simplest case, to deplete the product mixture of ammonia and alcohol, the said two components are removed directly as a mixture of substances from the product mixture. The two substances are then an after-connected separation operation, for example, subjected to a rectification. On the other hand, within the meaning of the invention it is also possible to separate off the two components alcohol (methanol) and ammonia in one process from the product mixture and at the same time to separate the two constituents ammonia and alcohol (methanol) from one another.

In a preferred process modification of the invention, it is of particular interest that the reaction step and the removal of the ammonia / alcohol from the product mixture are spatially separated from each other and carried out in different aggregates. For this purpose, one can for example provide one or more pressure reactors and connect them to a pressure distillation column. This is one or more reactors, which are arranged outside the distillation / reaction column in a separate area.

In the broadest sense, this includes continuous processes for the preparation of alpha-hydroxycarboxylic acid esters in which alpha-hydroxycarboxamide is reacted with an alcohol in the presence of a catalyst to give a product mixture comprising the alpha-hydroxycarboxylic acid esters, ammonia, unreacted alpha-hydroxycarboxamide and alcohol and Catalyst comprises; the process being characterized in that a ') starting material streams comprising, as starting materials, an alpha-hydroxycarboxylic acid amide, an alcohol and a catalyst are fed into a pressure reactor; b ') the educt streams in the pressure reactor at a pressure in the range of greater than 1 bar to 100 bar with each other; c ') discharging the product mixture resulting from step b') comprising alpha-hydroxycarboxylic acid ester, unreacted alpha-hydroxycarboxylic acid amide and catalyst from the pressure reactor; and d ') the product mixture depleted of alcohol and ammonia, wherein ammonia is distilled off at a pressure which is constantly greater than 1 bar, distilled off.

According to the above, a particularly expedient looks

Process modification that b '1) reacts the starting materials in the pressure reactor at a pressure in the range of 5 bar to 70 bar with each other; b'2) the product mixture resulting from step b'l) is depressurized to a pressure less than the pressure in the pressure reactor and greater than 1 bar; c 'l) the resulting from step b'2) relaxed product mixture in a

Feeds the distillation column; d) in the distillation column, ammonia and alcohol are distilled overhead, the pressure in the distillation column being maintained in the range from greater than 1 bar to less than or equal to 10 bar; and d'2) resulting from step d'1) depleted of ammonia and alcohol

Product mixture comprising alpha-hydroxycarboxylic acid ester, unreacted alpha-hydroxycarboxamide and catalyst discharged from the column.

According to this process variant, reaction of the educts and separation of ammonia / alcohol take place in two different spatially separated aggregates. In other words, reactor / reaction space and separation unit for the separation of ammonia / alcohol from the product mixture are separated from each other. This has the advantage that one can use different pressure ranges for the reaction / reaction of the educts and the subsequent separation of ammonia / alcohol. By separating the process into a reaction step in the pressure reactor under higher pressure than in a separation step in a pressure column, wherein both steps are conducted under overpressure, ie greater than 1 bar, it is not readily predictable manner in addition to the advantages discussed so far at the first variant of the method according to the invention, the separation effect again significantly improve and increase the efficiency of the separation of the ammonia / alcohol mixture.

The quality features mentioned can be further improved by repeating the reaction in the pressure reactor once or several times with the product mixture depleted in ammonia and alcohol in the bottom of the separation column (pressure distillation column), the reaction step being shifted to a plurality of pressure reactors connected in series are.

To this extent, a process variant is particularly preferred which is characterized in that e ') the product mixture discharged in step d 2) is compressed to a pressure in the range from 5 to 70 bar; f) the thus compressed according to step e ') compressed mixture for reaction in another pressure reactor and react again; and g ') repeats steps b'2), c' l), d'l) and d'2) according to the aforementioned list.

Accordingly, it is of particular interest that takes the depleted of ammonia and alcohol mixture from a bottom above the bottom of the first distillation column, compressed to a pressure greater than in the distillation column and then fed to a second pressure reactor, from where after reacting under Exposure to increased pressure and temperature and to obtain a product mixture which has been reacted twice, this in turn is depressurized to a pressure lower than in the second pressure reactor and greater than 1 bar and then into the first distillation column below the bottom from which the feed into the second pressure reactor but above the bottom of the first distillation column is recycled, where to obtain a depleted in ammonia and alcohol mixture ammonia and alcohol are distilled off again overhead. This process step can be repeated as desired, for example, three to four repetitions are particularly favorable. In this respect, preference is given to a process which is characterized in that the reaction in the pressure reactor, the expansion of the reacted mixture, the feed to the first distillation column, the depletion of ammonia and alcohol in the first distillation column, the removal of the depleted mixture, compression and Feed of the depleted mixture into a further pressure reactor is repeated several times, wherein a n-fold depleted in ammonia and alcohol product mixture is obtained at the bottom of the pressure distillation column depending on the number n of series-connected pressure reactors. Here, n can be a positive integer greater than zero. Preferably, n is in the range of 2 to 10.

An expedient method modification provides that the above-mentioned and defined steps e ') to g') are repeated several times.

Very specific process variants comprise carrying out the reaction and depletion four times using four series-connected pressure reactors to give a product mixture which has been depleted in ammonia and alcohol four times. Accordingly, this process variant is characterized in that steps e ') to g') are repeated at least twice so that the reaction is carried out in total in at least four pressure reactors connected in series.

For the specified process variant, different temperature ranges in column and reactor have proven to be particularly useful.

Thus, the pressure distillation column generally and preferably at a temperature in the range of about 50 ⁰ C to about 160 ⁰ C. The exact temperature is typically set by the boiling system as a function of the prevailing pressure conditions. The temperature in the reactor is preferably in the range of about 120 ⁰ C - 240 ⁰ C. It is particularly useful to lower the temperature from reactor to reactor, for example in steps in the range of 3 - 15 ° C, preferably 4 - 10 ^0th C and especially useful in 5 ° C increments. As a result, the selectivity of the reaction is positively influenced.

Another measure to increase the selectivity may also be to reduce the reactor volume from reactor to reactor. With decreasing reactor volume with increasing conversion one likewise obtains an improved selectivity.

As already mentioned above, it is advantageous to remove the product mixture to be taken from the pressure distillation column at certain points of the column. Here, the distance of the removal point to the bottom (column bottom) of the column is used for orientation as a relative location. Particularly expedient in the context of the invention is that the expanded product mixture according to step c 'l) after each re-conversion in a pressure reactor closer to the bottom of the distillation column is fed, based on the feed point of the feed of the previous step c' l) ,

For example, the ammonia released in the alcoholysis in the process of the invention can easily be recycled to an overall process for the preparation of alkyl (meth) acrylates. For example, ammonia can be converted to hydrocyanic acid with methanol. This is set forth, for example, in EP-A-0941984. Furthermore, the hydrocyanic acid can be obtained from ammonia and methane according to the BMA or Andrussow process, these processes being described in Ullmann's Encyclopedia of Industrial Chemistry 5th Edition on CD-ROM, keyword "Inorganic Cyano Compounds." The ammonia in an ammoxidation process, such as the large-scale synthesis of acrylonitrile from ammonia, oxygen and propene are attributed to the acrylonitrile synthesis is under the keyword Sohio Process in Indutrial Organic Chemistry by K. Weisermehl and H.-J. Arpe on page 307 ff. Described.

The reaction temperature can also vary over a wide range, with the rate of reaction generally increasing with increasing temperature. The upper temperature limit generally results from the boiling point of the alcohol used. Preferably, the reaction temperature is in the range of 40-300 ⁰ C, more preferably 120-240 ⁰ C.

For the present invention, according to one variant, any multi-stage pressure-resistant distillation column can be used, which preferably has two or more separation stages. The number of separation stages in the present invention refers to the number of plates in a tray column or the number of theoretical plates in the case of a packed column or a packed column.

Examples of a multistage distillation column with trays include those such as bubble trays, sieve trays, tunnel trays, valve trays, slotted trays, sieve slotted trays, sieve trays, nozzle trays, centrifugal trays, for a multistage distillation column with packing such as Raschig rings, Lessing rings, Pall Rings, Berl saddles, Intalox saddles and for a multistage distillation column with packings such as Mellapak (Sulzer), Rombopak (Kühni), Montz-Pak (Montz) and catalyst bag packs, for example Kata-Pak.

A distillation column having combinations of regions of soils, regions of packing or regions of packing may also be used.

The ammonia-depleted product mixture has inter alia the desired alpha-hydroxycarboxylic acid ester. For further isolation and purification of the ester can be in an appropriate process modification to the Remove ammonia-depleted product mixture through the bottom of the distillation column and feed to another second distillation column, where distilled off to obtain a depleted in both ammonia and alcohol mixture, the alcohol through the top of the column and preferably recirculated to a reactor.

For further isolation and recovery of the alpha-hydroxycarboxylic acid ester from the depleted in ammonia and alcohol mixture then a method is preferred in which the ammonia and alcohol depleted mixture is discharged through the bottom of the further distillation column and still another distillation column feeds, in which distilled off the alpha-hydroxycarboxylic acid over the top and the resulting mixture of ammonia, alcohol and alpha-hydroxycarboxylic ester depleted mixture, optionally after further purification steps, recycled to the reactor. The alpha-hydroxycarboxylic acid ester product obtained from the top of the column is of high purity and can, for example, be supplied in an extremely advantageous manner to further reaction steps for obtaining alkyl (meth) acrylates.

As described above, the distillation apparatus preferably has at least one region, called a reactor, in which at least one catalyst is provided. This reactor may, as described, preferably be within the distillation column.

In the context of the invention, it has been found that the described procedure can tolerate a broad spectrum of proportions of the educts. So you can the alcoholysis in a relatively large excess of alcohol or

under-run against the alpha-hydroxycarboxylic acid amide. Particular preference is given to processes in which the reaction of the educts takes place at a molar starting ratio of alcohol to alpha-hydroxycarboxylic acid amide in the range from 1: 3 to 20: 1. All particularly useful is the ratio 1: 2 to 15: 1 and more preferably 1: 1 to 10: 1.

Furthermore, preference is given to processes which are characterized in that hydroxyisobutyramide is used as the alpha-hydroxycarboxamide and methanol is used as the alcohol.

The reaction according to the invention takes place in the presence of a catalyst. The reaction can be accelerated, for example, by basic catalysts. These include homogeneous catalysts as well as heterogeneous catalysts.

Of very particular interest in carrying out the process according to the invention are water-resistant lanthanoid compounds as catalysts. The use of this type of homogeneous catalysts in a process of the invention is novel and leads to surprisingly advantageous results. The term "water resistant" means that the catalyst retains its catalytic ability in the presence of water, Accordingly, the reaction of the present invention can be carried out in the presence of up to 2% by weight of water without significantly affecting the catalytic ability of the catalyst. In this context, the term "essential" means that the reaction rate and / or the selectivity decreases by at most 50%, based on the reaction without the presence of water.

Lanthanoid compounds denote compounds of La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Td, Dy, Ho, Er, Tm, Yb, and / or Lu. Preferably, a lanthanoid compound is used which comprises lanthanum.

Preferably, the lanthanide compound has a solubility in water of at least 1 g / l, preferably at least 10 g / l at 25 ° C. Preferred lanthanoid compounds are salts which are preferably present in the oxidation state 3.

Particularly preferred water-resistant lanthanoid compounds are La (NCb) ₃ and / or LaCl ₃ . These compounds can be added as salts of the reaction mixture or formed in situ.

For the invention, it may be advantageous if at most 10 wt .-%, preferably at most 5 wt .-% and particularly preferably at most 1 wt .-% of the alcohol in the reaction phase are removed from the reaction system via the gas phase. By this measure, the reaction can be carried out particularly inexpensively.

Other homogeneous catalysts which can be successfully used in the present invention include alkali metal alcoholates and organometallic compounds of titanium, tin and aluminum. Preference is given to using a titanium alcoholate or tin alkoxide, such as, for example, titanium tetraisopropoxide or tin tetrabutyloxide.

A particular process variant involves using as catalyst a soluble metal complex containing titanium and / or tin and the alpha-hydroxycarboxylic acid amide.

Another specific modification of the process of the invention provides that the catalyst used is a metal trifluoromethanesulfonate. Preference is given to a metal trifluoromethanesulfonate, in which the metal is selected from the group consisting of the elements in the groups 1, 2, 3, 4, 11, 12, 13 and 14 of the Periodic Table. Of these, preference is given to using those metal trifluoromethanesulfonates in which the metal corresponds to one or more lanthanides. In addition to the preferred variants of homogeneous catalysis, processes using heterogeneous catalysts may also be appropriate. Among the successfully applicable heterogeneous catalysts include, inter alia, magnesium oxide, calcium oxide and basic ion exchangers and the like.

Thus, for example, preference may be given to processes in which the catalyst is an insoluble metal oxide which comprises at least one selected from among Sb, Sc, V, La, Ce, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Tc, Re, Fe, Co, Ni, Cu, Al, Si, Sn, Pb and Bi.

Alternatively, methods may be preferred wherein the catalyst used is an insoluble metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Co, Ni, Cu, Ga, In, Bi and Te.

General process sequence of a variant according to the invention with reference to FIG. 1

According to a particularly preferred embodiment, the alcoholysis, preferably methanolysis, can take place in the combination of a pressure rectification column and a plurality of pressure reactors shown in FIG. The Hydroxyisocarbonsäureamid, for example, Hydroxyisobutyramid, is fed via line (1) together with methanol via line (2) and a methanol / catalyst mixture via line (3) through line (4) a first pressure reactor (RI). Under the above-mentioned reaction conditions, a reaction mixture of the hydroxyisocarboxylic acid ester and ammonia, unreacted hydroxyisocaboxylic acid amide and methanol, catalyst and traces of a by-product are formed in the reactor (RI). This mixture is released after leaving the reactor (RI) to a lower pressure stage and passed via line (5) in a pressure column (KI). The column is preferably equipped with packings. There, the ammonia is separated from the reaction mixture with a portion of the methanol and recovered at the top as a distillate. The higher boiling components, the hydroxyisocarboxylic acid ester, the by-product and the unreacted hydroxyisobutyramide are withdrawn from the column with the remaining methanol, compressed to reactor pressure and fed to the second pressure reactor (R-2). The reaction is preferably carried out in 4 pressure reactors connected in series (RI to R-4). The product mixture leaving the column (KI) via the bottoms consists of the hydroxyisocarboxylic acid ester, traces of a by-product and the hydroxyisobutyramide. It is passed through line (9) into the still (K-2). There, the hydroxyisocarboxylic ester precipitates as distillate and is withdrawn via line (10). The Hydroxyisocarbonsäureamid / catalyst mixture leaves the column (K-2) via the bottom and is partially passed via the lines (12) and (4) back into the first pressure reactor (RI). A partial flow (11) is fed to a thin-film evaporator (DI). This allows the discharge of a mixture of amide, the high-boiling by-product and the catalyst via line (13).

The obtained in the column (K-I) as distillate ammonia / methanol mixture is compressed and fed via line (14) to another column (K-3). This separates the ammonia, which is obtained at the head, from the methanol, which is returned via the lines (15) and (4) in the first pressure reactor (Rl).

Hereinafter, the present invention will be explained in more detail by way of examples. Example 1:

In a laboratory test plant consisting of a starting material feed and a continuously operated stirred tank reactor, 157 g / h of a methanol / catalyst mixture with a catalyst fraction of 0.8% by weight and 35 g / h of alpha-hydroxyisobutyramide were fed over a test period of 48 h. The reaction was carried out using La (NCb) ₃ as a catalyst. The resulting product mixture was analyzed by gas chromatography. The relative to alpha-hydroxyisobutyramide molar selectivity to alpha-Hydroxyisobuttersäuremethylester was 98.7%, resulting in an ammonia concentration in the product mixture of 0.7 wt .-%.

Examples 2 - 7:

Table 1 shows further examples, which were carried out in the specified experimental apparatus at a molar Eduktverhältnis of MeOH: HIBA of 14: 1, but different reaction temperatures and residence times.

Table 1

Table 1 makes it clear that the selectivity to HIBSM (α-hydroxyisobutyric acid methyl ester) not only from the ammonia concentration in Reaction mixture in the reactor, but also on the reaction parameters residence time and temperature and thus depends on an exact reaction regime.

Example 8:

In the abovementioned laboratory test facility, a methanol / catalyst mixture with a catalyst fraction of 1.0% by weight and alpha-hydroxyisobutyramide in a molar ratio of 7: 1 were continuously metered in over an experimental period of 48 h. The reaction to HIBSM and ammonia was carried out at a pressure of 75 bar and a reaction temperature of 220 ⁰ C with a residence time of 5 min. The reaction was carried out using La (NCb) ₃ as a catalyst. The resulting product mixture was analyzed by gas chromatography. The molar selectivity to alpha-hydroxyisobutyrate based on alpha-hydroxyisobutyramide was 99%, resulting in an ammonia concentration in the product mixture of 0.63 wt .-%.

Examples 9-12:

In the abovementioned laboratory test plant, a methanol / catalyst mixture with a catalyst fraction of 0.9% by weight and alpha-hydroxyisobutyramide in a molar ratio of 10: 1 were continuously metered in over an experimental period of 48 h. The reaction to HIBSM and ammonia was carried out at a pressure of 75 bar and a reaction temperature of 200 and 220 ⁰ C with a residence time of 5 min or 10 min. The reaction was carried out using La (NCb) ₃ as a catalyst. The resulting product mixture was analyzed by gas chromatography. The molar selectivity to alpha-hydroxyisobutyric acid methyl ester based on alpha-hydroxyisobutyramide and the ammonia concentration in the product mixture are listed in Table 2. Table 2:

Claims

claims

A continuous process for the preparation of alpha-hydroxycarboxylic acid esters which comprises reacting alpha-hydroxycarboxylic acid amides with an alcohol in the presence of a catalyst to give a product mixture comprising alpha-hydroxycarboxylic acid esters, ammonia, unreacted alpha-hydroxycarboxamide, and alcohol and catalyst ; characterized in that one

a ') reactant streams comprising as starting materials an alpha-hydroxycarboxylic acid amide, an alcohol and a catalyst fed into a pressure reactor;

b ') the educt streams in the pressure reactor at a pressure in the range of 1 bar to 100 bar with each other;

c ') the product mixture resulting from step b) comprising alpha-hydroxycarboxylic acid ester, unconverted alpha-hydroxycarboxamide and catalyst and ammonia and alcohol are discharged from the pressure reactor; and

d ') depletes the product mixture of alcohol and ammonia, wherein ammonia is distilled off at a pressure which is constantly greater than 1 bar, without the aid of additional striping agents.

2. The method according to claim 1, characterized in that one

b '1) the educts in the pressure reactor at a pressure in the range of 5 bar to 70 bar with each other;

b'2) the product mixture resulting from step b'l) to a pressure less than the pressure in the pressure reactor and greater than 1 bar relaxed;

c 'l) feeds the relaxed product mixture resulting from step b'2) into a distillation column;

d) in the distillation column, ammonia and alcohol are distilled overhead, the pressure in the distillation column being maintained in the range from greater than 1 bar to less than or equal to 10 bar; and

d'2) the resulting from step dI) resulting in ammonia and alcohol depleted product mixture comprising alpha-hydroxycarboxylic acid ester, unreacted alpha-hydroxycarboxamide and catalyst discharged from the column.

3. The method according to claim 2, characterized in that one

e ') the product mixture discharged in step d'2) is compressed to a pressure in the range from 5 to 70 bar;

f) the thus compressed according to step e ') compressed mixture for reaction in another pressure reactor and react again; and

g ') the steps b'2), c' l), d 'l) and d' 2) according to claim 2 repeated.

4. The method according to claim 3, characterized in that the steps e ') to g') repeated several times.

5. The method according to claim 4, characterized in that the steps e ') to g') repeated at least twice, so that the Implementation is carried out in total in at least four series-connected pressure reactors.

6. The method according to any one of claims 3 to 5, characterized in that the relaxed product mixture according to step c'l) after each re-conversion in a pressure reactor closer to the bottom of the distillation column is fed, based on the feed point of the feed of the previous step c 'l).

7. The method according to any one of the preceding claims, characterized in that one carries out the reaction of the reactants at a molar starting ratio of alcohol to alpha-hydroxycarboxylic acid in the range of 1: 3 to 20: 1.

8. The method according to any one of the preceding claims, characterized in that at least one alpha-hydroxycarboxylic acid amide is used.

9. The method according to claim 11, characterized in that α-Hydroxyisobuttersäureamid and / or α-Hydroxyisopropionsäureamid were used.

10. The method according to any one of the preceding claims 1-9, characterized in that is used as the alpha-hydroxycarboxamide hydroxyisobutyramide and methanol as the alcohol.

11. The method according to at least one of the preceding claims, characterized in that the reaction is carried out at a temperature in the range of 120-240 ⁰ C.

12. The method according to any one of the preceding claims, characterized in that the residence time in the reaction of the starting materials in the range of 1 to 30 minutes.

13. The method according to any one of the preceding claims, characterized in that the reaction is catalyzed by at least one water-resistant lanthanoid compound.

14. The method according to claim 13, characterized in that the lanthanoid compound is a salt.

15. The method according to claim 13 or 14, characterized in that the lanthanoid compound is used in the oxidation state III.

16. The method according to any one of the preceding claims 13 -

15, characterized in that the lanthanoid compound shows a solubility in water of at least 10 g / l.

17. The method according to any one of the preceding claims 13 -

16, characterized in that the lanthanoid compound comprises lanthanum.

18. Process according to claim 17, characterized in that the lanthanoid compound comprises La (NCb) ₃ and / or LaCl ₃ .

19. The method according to any one of the preceding claims 1 to 12, characterized in that the catalyst used is a soluble metal complex containing titanium and / or tin and the alpha-hydroxycarboxylic acid amide.

20. The method according to any one of claims 1-12, characterized in that the catalyst used is a metal trifluoromethanesulfonate.

21. The method according to claim 20, characterized in that one uses a metal trifluoromethanesulfonate, wherein the metal is selected from the group consisting of the elements in the groups 1, 2, 3, 4, 11, 12, 13 and 14 of the Periodic Table.

22. The method according to claim 21, characterized in that one uses a metal trifluoromethanesulfonate, wherein the metal is one or more lanthanides.

23. The method according to any one of claims 1-12, characterized in that the catalyst is an insoluble metal oxide, which is at least one of the of Sb, Sc, V, La, Ce, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Tc, Re, Fe, Co, Ni, Cu, Al, Si, Sn, Pb and Bi group selected element.

24. The method according to any one of claims 1-12, characterized in that the catalyst used is an insoluble metal selected from the group consisting of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Fe, Co , Ni, Cu, Ga, In, Bi and Te.