EP4143154A1

EP4143154A1 - Method for producing hydroxyalkyl(meth)acrylic acid esters by oxidative splitting of methacrolein acetals

Info

Publication number: EP4143154A1
Application number: EP21719143.6A
Authority: EP
Inventors: Andreas RÜHLING; Steffen Krill; Florian Zschunke; Belaid AIT AISSA
Original assignee: Roehm GmbH Darmstadt
Current assignee: Roehm GmbH Darmstadt
Priority date: 2020-04-30
Filing date: 2021-04-19
Publication date: 2023-03-08
Also published as: EP3904327A1; JP2023523465A; US20230174456A1; KR20230002780A; TW202146370A; MX2022013202A; WO2021219409A1; CN115461319A

Abstract

The present invention relates to a method for producing hydroxyalkyl(meth)acrylic esters, in particular hydroxyethylmethacrylate (HEMA), comprising, in a first reaction stage, the conversion of (Meth)acrolein with at least one multivalent alcohol, in particular ethylene glycol, in the presence of a first catalyst K1, wherein a first reaction product containing a cyclical acetal is obtained and, in a second reaction stage, the conversion of the first reaction product with oxygen in the presence of a second catalyst K2, wherein a second reaction product, containing at least one hydroxyalkyl(meth)acrylic ester, is obtained, wherein, after the first reaction stage, water and optionally additional components, in particular (meth)acrolein and/or the multivalent alcohol, for example ethylene glycol, are removed at least in part from the first reaction product.

Description

Process for the preparation of hydroxyalkyl (meth) acrylic acid esters by oxidative

Cleavage of methacrolein acetals

description

The present invention relates to a process for the preparation of hydroxyalkyl (meth) acrylic esters, in particular hydroxyethyl methacrylate (HEMA), comprising in a first reaction stage the reaction of (meth) acrolein with at least one polyhydric alcohol, in particular ethylene glycol, in the presence of a first catalyst K1, wherein a first reaction product containing a cyclic acetal is obtained, and in a second reaction stage the reaction of the first reaction product with oxygen in the presence of a second catalyst K2, a second reaction product containing at least one hydroxyalkyl (meth) acrylic ester being obtained, after the first Reaction stage water and optionally further components, in particular (meth) acrolein and / or the polyhydric alcohol, for example ethylene glycol, are at least partially removed from the first reaction product.

Hydroxyalkyl esters based on methacrylic acid and / or acrylic acid are technically important monomers or comonomers for the production of polymethyl (meth) acrylates. Hydroxyalkyl (meth) acrylic esters and polymers made from them are used for various applications, for example for paints, adhesives, contact lenses, polymer crosslinkers and materials for 3D printing. Hydroxyethyl methacrylate (HEMA) in particular is of technical importance.

State of the art

The prior art often describes the production of HEMA starting from methacrylic acid and ethylene oxide using chromium catalysts, for example WO 2012/116870 A1, JP 5 089 964 B2 and US 2015/01267670. A stabilizer is often added to the very reactive hydroxyalkyl (meth) acrylic ester monomer (for example EP-B 1 125 919). In addition to the production of HEMA starting from methacrylic acid, the prior art also describes the production of other hydroxyalkyl (meth) acrylic esters, using, for example, propylene oxide or other substituted oxiranes, e.g. JP 2008143814, JP 2008127302. acrylic acid carried out with the corresponding oxiranes using homogeneous catalysts, the (meth) acrylic acid, partially or completely in the presence of the dissolved catalyst and a stabilizer, being initially charged and the oxirane being metered in in gaseous or liquid form. In addition to the dosage, the mixing of the reactants and the catalyst is also important so that the desired hydroxyalkyl (meth) acrylic ester is obtained in these reactions in high, constant product quality, and so that the system can be used continuously and easily cleaned. An embodiment suitable for this is described, for example, in WO 2012/1168770 A1, the reactor being equipped with an injector mixing nozzle and a circulation line. Towards the end of the reaction, the desired hydroxyalkyl (meth) acrylic ester is typically present in high concentration, while the starting material methacrylic acid is greatly reduced or no longer present.

The subsequent work-up and isolation of the hydroxyalkyl (meth) acrylic esters is often carried out by distillation and can be operated continuously (e.g. described in DE 10 2007 056926 A1 and EP 1090904 A2).

For commercial products, purification by distillation is usually carried out up to a purity of more than 97%, with the acid-based secondary components and crosslinking agents being further reduced for special applications. The hydroxyalkyl (meth) acrylic ester content is usually more than 99%. Such a purification takes place in several distillation steps and is described, for example, in EP 2427421 B1. Regardless of the desired product quality, at least one stabilizer often has to be added during the purification. The product conforming to the specification can contain, for example, hydroquinone monomethyl ether and 4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxyl (TEMPOL) or combinations of several stabilizers (for example described in WO 2010/105894 A2). There are also product compositions described which, in addition to stabilizers, also by-products of the reaction, such as Hydroxyethyl acetate, and which should have a particular storage stability (for example, described in EP 1125919 A2).

The catalysts used in the reaction are often based on chromium salts, as shown, for example, in the documents WO 2012/116870 A1, JP 5 089 964 B2 and US 2015/01267670. In addition to chromium catalysts, catalysts based on iron salts, for example in US 4,365,081, are also described. A large number of other metal-containing, soluble catalysts from the group of transition metals with a wide variety of counterions or ligands are described in the literature.

The problem with these known and common manufacturing processes is the use of ethylene oxide, which is a very toxic, carcinogenic, extremely flammable and explosive compound. Consequently, these processes place very high demands on the handling, transport, storage, etc. of ethylene oxide. The transport and use of ethylene oxide are heavily regulated in the EU and other countries, e.g. the USA, and are therefore time-consuming. For example, information pertaining to the United States is described in the EPA Clean Air Act. A tightening of the requirements for handling ethylene oxide in the USA is likely on the basis of the upcoming update of the "National Emission Standards for Hazardous Air Pollutants (NESHAP)" in 2020.

The expansion of capacities for the production of HEMA is therefore often limited due to the local availability and regulation of ethylene oxide (e.g. EPA Clean Air Act for USA).

In the known processes, the use of ethylene oxide is to be assessed critically in particular when ethylene oxide is present in excess or stationary in comparatively high concentrations. This is the case in most of the methods described in the prior art. Thus, increased safety precautions must be taken, in particular the mixing and steady-state concentration of ethylene oxide must be strictly regulated by setting the metering times and metering rates as well as the temperature control and removal of the reaction exotherm. Another disadvantage of the production processes of the prior art is the use of toxic and carcinogenic chromium salts, often in the + VI oxidation state. The first chromium salts of other oxidation states are already classified as critical with regard to use and disposal (see, for example, file number WD5-3000-044 / 19 of the German Bundestag and REACH Annex XVII on chromium + VI compounds). It is to be expected that further bans, requirements and regulations for stricter occupational safety will follow in the future, which will at least result in additional work. Residues of the chromium salts occur in all established processes, in particular in the processing as bottom residue from the distillation. This sump residue has to be disposed of in a complex and careful manner to ensure that no chromium gets into the environment. In summary, this results in disadvantages of the established processes with regard to the high costs for the use of the catalysts and the resulting disposal of the catalyst residues, as well as high investment costs for the production plant and high, ongoing operating costs due to long reaction times.

EP 0 704 441 A2 describes a process for the production of 2-vinyl-1,3-dioxolane by reacting acrolein with ethylene glycol in the presence of a solid acidic catalyst. It is also pointed out here that the reaction has a higher selectivity at low temperatures of below 20 ° C. It is described that by-products are increasingly formed at elevated temperatures due to the addition of the alcohol to the 4-position of acrolein in the sense of a Michael-like reaction. When polyhydric alcohols are used, it is also to be expected that oligomeric and polymeric compounds will be formed, which would make both the conduct of the reaction and the purification more difficult. This publication does not make any clear statements on this.

EP 0 485 785 A1 describes a process for the production of alpha, beta unsaturated acetals, in particular starting from methacrolein, methanol being used as the alcohol. In this case, methanol and methacrolein are separated off from the acetal by distillation and reacted at room temperature. Unreacted starting material runs together with the acetal into the bottom receiver of the column. That However, the process described here cannot be used for dihydric alcohols such as ethylene glycol, since the dihydric alcohol has a higher boiling point than the corresponding dioxolane. Correspondingly, large amounts of by-products, analogous to the description of EP 0 704 441 A2, are to be expected for such a reaction.

The document JPH11315075A describes the reaction of methacrolein with ethylene glycol using a heterogeneous catalyst, e.g. a zeolite or mixtures of silicon dioxide and aluminum oxide and an azeotropic entrainer such as cyclohexane or toluene. Only batch processes are described here which, due to the nature of the entrainers chosen, have to be operated at high temperatures.

Japanese patent application JP 43-11205 describes the oxidative cleavage of a cyclic acetal in order to obtain an unsaturated hydroxy ester based on acrylate. The oxidative cleavage of a cyclic acetal based on methacrolein is also described, as a result of which 2-hydroxyethyl methacrylate is obtained. However, a conversion of only 25% is obtained here within 6 hours, which, based on the space-time yield, is to be regarded as inadequate and not very economical. The production of the cyclic acetals is not described in the document.

The Japanese patent application JP 2009274987 describes a process for the production of hydroxyalkyl (meth) acrylates, e.g. hydroxyethyl (meth) acrylate or hydroxypropyl (meth) acrylate, by oxidizing a cyclic acetal in the presence of a special heterogeneous noble metal-containing catalyst. However, the processes described here are unsuitable for large-scale industrial use, in particular in a continuous process, in particular because of the long reaction times.

The chemoselective oxidation of aromatic acetals by oxygen with the formation of hydroxyalkyl esters using a palladium catalyst is also described in Sawama et al. (Org. Lett. 2016, 18, 5604). The conversion of non-aromatic substrates and especially of alpha, beta-unsaturated Connections, is not described here. According to Sawama et al. methanol or ethylene glycol is described as the best solvent.

The document US Pat. No. 9,593,064 B2 (S.S. Stahl et al.) Describes palladium catalysts on activated carbon, which are supplemented by two dopants, for example bismuth and tellurium. The catalysts are used to produce esters by direct oxidative esterification of organic alcohols in the presence of methanol or ethanol. Cyclic acetals are not described as substrates.

In view of the prior art, it is therefore the object of the present invention to provide a process for the production of hydroxyalkyl (meth) acrylic esters, in particular hydroxyethyl methacrylate (HEMA), with ethylene oxide and optionally also methacrylic acid being replaced by other starting materials which are safe, inexpensive and are readily available globally. In addition, the production process should in particular be competitive with known processes of the prior art and not have the disadvantages of conventional processes described above.

A further object of the present invention is to provide a hydroxyalkyl (meth) acrylic ester product which meets the usual requirements for purity and the content of secondary components or which can be further purified with as little effort as possible so that it meets the corresponding requirements. In particular, a hydroxyalkyl (meth) acrylic ester product is to be provided which has the lowest possible content of crosslinking by-products (compounds with two or more C -C double bonds), e.g. ethylene dimethacrylate.

It has surprisingly been found that the above-described objects are achieved by the method according to the invention. In particular, it has been found that methacrolein and ethylene glycol can be used as starting materials in the production of hydroxyethyl methacrylate (HEMA), a cyclic acetal initially being formed with elimination of water, which is then also formed Oxygen is catalytically oxidized, thereby selectively obtaining HEMA. Ethylene glycol and other polyhydric alcohols are typically inexpensive and available worldwide. The production of methacrolein from propionaldehyde and formalin is known to the person skilled in the art and is implemented on an industrial scale. In addition, methacrolein can be obtained by oxidation of isobutene or starting from t-butanol. Processes for the production of methacrolein are described, for example, in Ullmanns Encyclopedia of Industrial Chemistry, 2012, Wiley-VCH Verlag GmbH, Weinheim (DOI: 10.1002 / 14356007.a01_149.pub2).

Description of the invention

The present invention relates to a process for the preparation of hydroxyalkyl (meth) acrylic esters comprising the steps: a. Reaction of (meth) acrolein with at least one polyhydric alcohol in the presence of a first catalyst K1, a first reaction product containing at least one cyclic acetal being obtained; b. at least partially removing water from the first reaction product; c. Reaction of the first reaction product with oxygen in the presence of a second catalyst K2, a second reaction product containing at least one hydroxyalkyl (meth) acrylic ester being obtained.

In a preferred embodiment, it is a continuous or semicontinuous, preferably a continuous, process for the preparation of hydroxyalkyl (meth) acrylic esters.

The expression “(meth) acrylate” or “(meth) acrylic ester” encompasses acrylate and / or methacrylate in the context of the invention. Accordingly, the expression “(meth) acrolein” in the context of the invention includes acrolein and / or methacrolein.

In a preferred embodiment, ethylene glycol is used as the polyhydric alcohol. The hydroxyalkyl (meth) acrylic ester is preferably Hydroxyethyl methacrylate (HEMA), in particular the cyclic acetal is methacrolein-ethylene glycol acetal (2-isopropenyl-1,3-dioxolane).

The invention further comprises the use of alkyl-substituted glycols as polyhydric alcohol instead of ethyl glycol. In particular, the invention also relates to the production of hydroxypropyl (meth) acrylic ester, in particular 2-hydroxypropyl (meth) acrylic ester (HPMA), in particular propanediol, e.g. 1,2-propanediol, being used as the polyhydric alcohol.

Step a - conversion to the cyclic acetal

The process according to the invention comprises in step a the reaction of (meth) acrolein with at least one polyhydric alcohol in the presence of a first catalyst K1, a first reaction product containing at least one cyclic acetal being obtained. Typically, water is obtained as a further product of acetal formation.

For the purposes of the present invention, a polyhydric alcohol is a compound, in particular an organic compound, which comprises two or more hydroxyl groups (—OH). The polyhydric alcohol is preferably at least one alcohol comprising 2 to 10 carbon atoms, preferably 2 to 5 carbon atoms, particularly preferably 2 to 3 carbon atoms, and comprising two or more hydroxyl groups, preferably two or three hydroxyl groups, particularly preferably two hydroxyl groups. The hydroxyl groups can preferably be constituted in 1, 2-; 1, 3- or 1, 4-position in the polyhydric alcohol, e.g. in the diol. The hydroxy groups are particularly preferably in the 1,2-position in the polyhydric alcohol. In addition, the polyhydric alcohol can have further functional groups, for example alkoxy groups, aryl groups, phosphonate groups, phosphate groups, alkenyl groups, alkynyl groups, masked carbonyl groups or ester units.

The at least one polyhydric alcohol is particularly preferably selected from ethylene glycol, propylene glycol, butanediol and / or glycerol. In a preferred Embodiment the at least one polyhydric alcohol is ethylene glycol. In a preferred embodiment, only a polyhydric alcohol as described above is used. The cyclic acetal preferably has a structure according to formula (I): where Y is a C ₂ -Cio-alkylene group, preferably a C ₂ -C ₄ -alkylene group, particularly preferably a C ₂ -C ₃ -alkylene group; R ¹ and R ^{2 are} independently selected from H, Ci-C ₂ o-alkyl, Ci-C ₂ o-hydroxyalkyl, Ci-C ₂ o-alkoxy, and C ₆ -C ₂₀ aryl; R ^{3 is} H or Ci-C ₂ o-alkyl, preferably H or methyl; R ⁴ and R ^{5 are} independently selected from H, Ci-C ₂ o-alkyl, Ci-C ₂ o-hydroxyalkyl and C ₆ -C ₂₀ aryl. R ¹ and R ² are preferably independently selected from H, C ₁ -C ₆ - alkyl, Ci-C6-hydroxyalkyl, and C ₆ -C ₂ aryl. Preferably, R ⁴ and R ^{5 are} independently selected from H, Ci-C6-alkyl, Ci-C6-hydroxyalkyl and C ₆ -Ci ₂ -aryl, particularly preferably from H and Ci-C6-alkyl. R ⁴ and R ^{5 are} particularly preferred. R ³ is particularly preferably methyl.

The cyclic acetal particularly preferably has a structure according to formula (II): with R ¹ , R ² , R ³ , R ⁴ and R ⁵ as defined above. The reaction of (meth) acrolein with at least one polyhydric alcohol to form the cyclic acetal in step a is typically an equilibrium reaction, the conversion in particular being 10 to 75%, preferably 15 to 50%. Typically, the conversion in a continuous process relates to the conversion per pass in step a.

The reaction in step a is preferably carried out in the presence of at least one acidic compound as catalyst K1, in particular selected from Brönsted acids and Lewis acids. The at least one catalyst K1 is preferably selected from the group consisting of phosphoric acid, sulfuric acid, sulfonic acids, carboxylic acids (e.g. formic acid, acetic acid), lanthanoid salts, metal salts of the elements of groups 3 to 15 of the periodic table, and ion exchange resins containing at least one acidic group from phosphonic acid (-P (= 0) (OH) ₂ ), sulfonic acids (-S (= 0) ₂ 0H) and carboxylic acids (-COOH) (e.g. acetic acid (-CH2-CH2- C (= 0) OH)). The at least one catalyst K1 is particularly preferably selected from the group consisting of phosphoric acid, sulfuric acid, sulfonic acids, carboxylic acids (e.g. formic acid, acetic acid) and ion exchange resins containing at least one acidic group selected from sulfonic acids (-S (= 0) ₂ OH) and carboxylic acids ( -COOH).

It is also possible to use at least one Lewis acid as catalyst K1, for example selected from lanthanoid salts and metal salts of the elements of groups 3 to 15 of the periodic table, in particular the compounds can be halides, hydroxides, mesilates, triflates, carboxylates and / or act chalcogenides. For example, at least one Lewis acid can be used as catalyst K1 selected from halides, hydroxides, mesilates, triflates, carboxylates and chalcogenides (preferably selected from halides) of alkali metals (in particular Li ⁺ , Na ⁺ , K ⁺ ), alkaline earth metals (in particular Be ^{2 +} , Mg ²⁺ , Ca ²⁺ ), B ³⁺ , Al ³⁺ , ln ³⁺ , Sn ²⁺ , Sn ⁴⁺ , Si ⁴⁺ , Sc ³⁺ , Ti ⁴⁺ , Pd ²⁺ , Ag ⁺ , Cd ²⁺ , Pt ²⁺ , Au ⁺ , Hg ²⁺ , In ³⁺ , Tl ³⁺ , Pb ²⁺ ; and from organic salts, for example alcoholates, of Ti ⁴⁺ , Sn ⁴⁺ and B ³⁺ , for example Ti (OH) ₄ , B (OR) ₃ , Sn (OR) ₄ , with R being Ci-Cio-alkyl. For example, a known Lewis acid, selected from BCI ₃ , BF ₃ , B (OH) ₃ , B (CH ₃ ) ₃ , AICI ₃ , AIF ₃ , TiCU, Ti (OH) ₄ , ZnCb, SiBr ₄ , SiF ₄ , PF5, Ti (OR) ₄ , B (OR) ₃ , and Sn (OR) ₄ , where R is Ci-Cio-alkyl, can be used.

The catalyst K1 is preferably one or more Brönsted acids, which preferably have a pKa value of less than or equal to 5, particularly preferably of less than or equal to 2.

The catalyst K1 preferably comprises at least one acidic group which has a pKa value of less than or equal to 5, particularly preferably of less than or equal to 2.

In a preferred embodiment, the reaction in step a takes place in the presence of at least one acidic compound as catalyst K1, selected from Brönsted acids and Lewis acids, the catalyst being present in heterogeneous or homogeneous form.

Furthermore, the catalyst K1 is preferably a heterogeneous catalyst. For example, the catalyst can be applied to a polymer matrix, composite matrix and / or an oxidic carrier material. The use of heterogeneous polyacids, for example based on molybdates, is also preferred. The advantage of using a heterogeneous catalyst K1 is, in particular, that the catalyst K1 and the reaction mixture can be separated from one another more easily and / or that the catalyst K1 can be reused (or generally operated) over a longer period of time.

In a preferred embodiment, (meth) acrolein is reacted with the at least one polyhydric alcohol in step a at a molar ratio of (meth) acrolein to polyhydric alcohol (s) in the range from 1:50 to 50: 1, preferably 1 : 10 to 10: 1, particularly preferably from 1: 3 to 3: 1.

The reaction of (meth) acrolein with the polyhydric alcohol in step a is preferably carried out in a temperature range from -50.degree. C. to 100.degree. C., particularly preferably from -10.degree. C. to 30.degree. C., particularly preferably from 0 to 20.degree. The implementation is preferably carried out of (meth) acrolein with the polyhydric alcohol in step a in a pressure range from 0.5 to 10 bar (absolute), particularly preferably from 1 to 5 bar (absolute). In particular, the reaction in step a takes place at a temperature and / or at a pressure within the ranges given above.

In a preferred embodiment, the reaction of (meth) acrolein with the polyhydric alcohol (for example ethylene glycol) in step a can be carried out in the presence of a solvent. Typically, a solvent can be selected from linear or cyclic alkanes (e.g. hexane, octane, cyclohexane), aromatic hydrocarbons (e.g. toluene, benzene), halogenated hydrocarbons (e.g. chloroform, carbon tetrachloride, hexachloroethane, hexachlorocyclohexane, chlorobenzene), alcohols (e.g. methanol, ethanol, n-butanol, tert-butanol), ethers (for example diisopropyl ether, tetrahydrofuran, 1,3-dioxane) or mixtures thereof can be used. The person skilled in the art is also familiar with other polar solvents which are chemically inert in the reaction after step a. The reaction in step a typically takes place in a reaction mixture containing 1 to 90% by weight, preferably 5 to 50% by weight, based on the entire reaction mixture, of at least one solvent.

In a particularly preferred embodiment, the reaction in step a is carried out without a solvent.

Step b - removing water from the first reaction product

The method according to the invention comprises in step b the at least partial removal of the reaction product water from the first reaction product. Preferably, step b of the method according to the invention also comprises the at least partial removal of the unreacted, polyhydric alcohol, e.g. ethylene glycol, and / or the unreacted (meth) acrolein from the first reaction product obtained in step a.

The removal of water and optionally polyhydric alcohol and optionally (meth) acrolein is preferably carried out by at least one distillation step (in particular in the form of a distillation column, for example columns 7 and 10). In a preferred embodiment, step b comprises that unconverted (meth) acrolein and / or unconverted polyhydric alcohol, typically together with the water, are separated off from the first reaction product, optionally separated from the water, and returned to the reaction in step a .

Step b particularly preferably comprises that in a first separation step, preferably in a distillation step, (meth) acrolein and at least partially water are separated from the first reaction product. In particular, in this first separation step there is a distillative separation of (meth) acrolein and its azeotrope with water from the first reaction product. In particular, the first separation step takes place in a distillation column (e.g. column 7).

Step b furthermore preferably comprises that in at least two separation steps, preferably two distillation steps, water, (meth) acrolein and polyhydric alcohol are separated off from the first reaction product containing at least one cyclic acetal.

Step b particularly preferably comprises that (e.g. in a second separation step) the polyhydric alcohol and optionally water and optionally high boilers are at least partially separated off from the reaction product, preferably in a distillation step (e.g. column 10).

In a preferred embodiment, step b comprises that in at least two separation steps water, unconverted (meth) acrolein and unconverted polyhydric alcohol are separated off from the first reaction product containing at least one cyclic acetal, wherein in a first separation step, preferably in a distillation step ( e.g. column 7), (meth) acrolein and at least partially water (e.g. (meth) acrolein and its azeotrope with water), are separated from the first reaction product, and in a second separation step, preferably in a distillation step (e.g. column 10), the polyhydric alcohol and optionally water and optionally high boilers are at least partially separated from the first reaction product.

The (meth) acrolein is preferably separated off in step b in such a way that in the reaction mixture in step c less than 10% by weight, preferably less than 8% by weight, particularly preferably less than 5% by weight, (Meth) acrolein, based on the total reaction mixture in step b, are included.

The removal of water in step b is preferably carried out in such a way that less than 5% by weight, preferably less than 2% by weight, particularly preferably less than 1% by weight, of water is obtained in the reaction mixture in step c on the entire reaction mixture in step b.

The polyhydric alcohol is preferably separated off in step b in such a way that less than 10% by weight, preferably less than 8% by weight, particularly preferably less than 5% by weight, of polyhydric alcohol in the reaction mixture in step c , based on the total reaction mixture in step c. In step b, the polyhydric alcohol is particularly preferably separated off almost completely from the first reaction product.

Step c - Oxidation

The process according to the invention comprises in step c the reaction of the first reaction product, containing at least one cyclic acetal (in particular 2-isopropenyl-1,3-dioxolane), with oxygen in the presence of a second catalyst K2, a second reaction product containing at least one hydroxyalkyl ( meth) acrylic ester.

Step c typically comprises an oxidative esterification of the cyclic acetal obtained in reaction step a. Typically, the hydroxyalkyl (meth) acrylic ester is obtained by oxidative ring opening of the cyclic acetal. The hydroxyalkyl (meth) acrylic ester preferably has a structure according to formula (III): where Y is a C ₂ -Cio-alkylene group, preferably a C ₂ -C ₄ -alkylene group, particularly preferably a C ₂ -C ₃ -alkylene group; R ¹ and R ^{2 are} independently selected from H, Ci-C ₂ o-alkyl, Ci-C ₂ o-hydroxyalkyl, Ci-C ₂ o-alkoxy and C ₆ -C ₂₀ aryl; R ^{3 is} H or Ci-C ₂ o-alkyl, preferably H or methyl; R ⁴ and R ^{5 are} independently selected from H, Ci-C ₂ o-alkyl, Ci-C ₂ o-hydroxyalkyl and C ₆ -C ₂₀ aryl. R ¹ and R ² are preferably independently selected from H, C ₁ -C ₆ - alkyl, Ci-C6-hydroxyalkyl, and C ₆ -C ₂ aryl. Preferably, R ⁴ and R ^{5 are} independently selected from H, Ci-C6-alkyl, Ci-C6-hydroxyalkyl and C ₆ -Ci ₂ -aryl, particularly preferably from H and Ci-C6-alkyl. R ⁴ and R ^{5 are} particularly preferred. R ³ is particularly preferably methyl.

The hydroxyalkyl (meth) acrylic ester particularly preferably has a structure according to formula (IV): with R ¹ , R ² , R ³ , R ⁴ , R ⁵ as defined above.

The reaction in step c is preferably carried out in the presence of a metal- and / or semimetal-containing, heterogeneous catalyst system as catalyst K2, particularly preferably a noble metal-containing, heterogeneous catalyst system as catalyst K2.

In the context of the present invention, the term noble metal includes the elements ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt), Silver (Ag), gold (Au), and rhenium (Re), especially gold (Au), silver (Ag) and the platinum metals (Ru, Rh, Pd, Os, Ir, Pt).

The at least one catalyst K2 is preferably a heterogeneous catalyst containing one or more support materials and one or more active components, the support material being selected from activated carbon, silicon dioxide, aluminum oxide, titanium dioxide, alkali metal oxides, alkaline earth metal oxides and mixtures thereof, and the active component at least Contains an element selected from palladium (Pd), platinum (Pt), iridium (Ir), rhodium (Rh), ruthenium (Ru), gold (Au), cobalt (Co), nickel (Ni), zinc (Zn) , Copper (Cu), iron (Fe), selenium (Se), tellurium (Te), arsenic (As), antimony (Sb), bismuth (Bi), germanium (Ge), tin (Sn) and lead (Pb) , whereby the elements can be elemental, as an alloy or in the form of their compounds in any oxidation state (preferably in the form of their oxides).

In particular, the carrier material is silicon dioxide and / or aluminum oxide, preferably aluminum oxide.

The active component particularly preferably contains at least one element selected from palladium (Pd), platinum (Pt), iridium (Ir), rhodium (Rh), ruthenium (Ru), gold (Au), bismuth (Bi) and / or tellurium (Te), where the elements can be in elemental form, as an alloy or in the form of their compounds in any oxidation state (preferably in the form of their oxides). The active component of the catalyst K2 particularly preferably comprises palladium.

The at least one catalyst K2 is particularly preferably a heterogeneous catalyst containing silicon dioxide and / or aluminum oxide as support material and containing at least one element selected from palladium, bismuth and tellurium as active component, the elements being elementary, as an alloy or in the form of their compounds in any oxidation state (preferably in the form of their oxides) can be present.

In a preferred embodiment, the active component of the catalyst K2 is the combination of at least one noble metal, in particular selected from gold (Au), silver (Ag), palladium (Pd), platinum (Pt), iridium (Ir), rhodium (Rh) and ruthenium (Ru); and at least one further element (dopant), in particular selected from selenium (Se), tellurium (Te), arsenic (As), antimony (Sb), bismuth (Bi), germanium (Ge), tin (Sn) and lead (Pb ), whereby the elements can be elemental, as an alloy or in the form of their compounds in any oxidation state (preferably in the form of their oxides).

In a particularly preferred embodiment, the active component of the catalyst K2 is the combination of palladium (Pd) and at least one further element (dopant) selected from selenium (Se), tellurium (Te), antimony (Sb), and Bismuth (Bi), where the elements can be in elemental form, as an alloy or in the form of their compounds in any oxidation state (preferably in the form of their oxides).

Furthermore, the at least one catalyst K2 is preferably a heterogeneous catalyst containing silicon dioxide and / or aluminum oxide as support material and an active component, the active component being palladium and at least one further element (dopant) selected from selenium (Se), tellurium (Te) and bismuth (Bi), where the elements can be elemental, as an alloy or in the form of their oxides in any oxidation state.

The catalyst K2 preferably contains at least 70% by weight, preferably 70 to 99.9% by weight, based on the total catalyst mass, one or more support materials and at most 30% by weight, preferably 0.1 to 30% by weight , based on the total catalyst mass, one or more active components.

In a preferred embodiment, the heterogeneous second catalyst K2 is used in step c in the form of a powder. In this case, the reaction in step c is preferably carried out in the presence of a dispersed, powdery catalyst K2. Typically, the amount of catalyst K2 in step c, based on the total mass of reaction mixture, is 0.1 to 30% by weight, preferably 1.0 to 20% by weight and particularly preferably 2.0 to 15% by weight .

In an alternative embodiment, step c is carried out in a fixed-bed or trickle-bed reactor, the ratio of catalyst to reaction mixture with the parameter LFISV (Liquid-Flourly-Space-Velocity) known to the person skilled in the art, for example in L liquid / (kg catalyst x hr ) or in kg liquid / (kg catalyst x hr). The LFISV is typically 0.05 to 15, preferably between 0.1 to 10 and particularly preferably between 1 and 5.

In the reaction in step c, preference is given to bringing a reaction mixture comprising the first reaction product and the second catalyst K2 into contact with an oxygen-containing gas. This can be achieved in reactors or units in the reactor periphery known to the person skilled in the art, such as bubble columns, gassed stirred tank reactors and trickle bed reactors.

Typically, during the reaction in step c, the reaction mixture is brought into contact with an oxygen-containing gas, an oxygen-containing exhaust gas being obtained. The oxygen-containing exhaust gas typically has an oxygen content in the range from 1 to 10% by volume, preferably 1 to 5% by volume, based on the total oxygen-containing exhaust gas.

Typically, during the reaction in step c, the reaction mixture is brought into contact with an oxygen-containing gas, the oxygen-containing gas having an oxygen content in the range from 1 to 40% by volume, preferably 5 to 22% by volume. , based on the total oxygen-containing gas. Air can preferably be used as the oxygen-containing gas in step c.

Particularly preferably, an oxygen-containing exhaust gas is obtained in the reaction in step c, the oxygen-containing exhaust gas being cooled in at least one stage, and the oxygen-containing exhaust gas having an oxygen content in the range from 1 to 10% by volume. , preferably 1 to 5% by volume, based on the total Oxygen-containing exhaust gas. In particular, by cooling in at least one stage, condensation and separation of organic components in the exhaust gas can be achieved, which can then preferably be returned to the process.

In a preferred embodiment, a reaction mixture containing the first reaction product and the second is used in the reaction in step c

Catalyst K2 is used which has a polyhydric alcohol (s) content of less than 10% by weight, preferably less than 8% by weight, particularly preferably less than 5% by weight, based on the reaction mixture.

Catalyst K2 is used which has a (meth) acrolein content of less than 10% by weight, preferably less than 8% by weight, particularly preferably less than 5% by weight, based on the reaction mixture.

The reaction of the first reaction product with oxygen in step c is preferably carried out in a temperature range from 0.degree. C. to 120.degree. C., particularly preferably from 50.degree. C. to 120.degree. C., particularly preferably from 60.degree. C. to 100.degree. The reaction of the first reaction product with oxygen in step c is preferably carried out in a pressure range from 0.5 to 50 bar (absolute), particularly preferably from 1 to 50 bar (absolute), particularly preferably from 2 to 30 bar (absolute). In particular, the reaction in step c takes place at a temperature and / or a pressure within the ranges given above.

In a preferred embodiment, the reaction of the first reaction product, comprising at least one cyclic acetal, with oxygen in step c can be carried out in the presence of a solvent. Typically, a solvent can be selected from linear or cyclic alkanes (e.g. hexane, octane, cyclohexane), aromatic hydrocarbons (e.g. toluene, benzene), halogenated hydrocarbons (e.g. chloroform, carbon tetrachloride, hexachloroethane, hexachlorocyclohexane, chlorobenzene), alcohols (e.g. methanol, ethanol, n-butanol, tert-butanol), ethers (e.g. diisopropyl ether, tetrahydrofuran, 1,3-dioxane), esters (e.g. methyl acetate, ethyl acetate), nitriles (e.g. acetonitrile) or mixtures thereof. Preferred esters are Ci-C2o-alkyl esters, preferably Ci-Cio-alkyl esters, of aliphatic and aromatic Ci-C2o-carboxylic acids, for example formic acid, acetic acid, propanoic acid, butanoic acid, pentanoic acid, hexanoic acid and benzoic acid. The reaction in step c typically takes place in a reaction mixture containing 1 to 90% by weight, preferably 5 to 50% by weight, based on the entire reaction mixture, of at least one solvent.

Typically, the oxidative conversion of the cyclic acetal in step c does not take place completely. The process according to the invention preferably comprises a work-up step d, the cyclic acetal being at least partially separated off from the second reaction product containing hydroxyalkyl (meth) acrylic ester and optionally being returned to the reaction in step c. For example, the cyclic acetal can be separated off from the second reaction product in one or more distillation steps (distillation columns).

The process according to the invention can optionally comprise further steps for processing and / or purifying the second reaction product containing at least one hydroxyalkyl (meth) acrylic ester. The further steps for preparation and / or purification typically include distillation steps and / or extraction steps and / or crystallization steps.

In a preferred embodiment, a crude product containing hydroxyalkyl (meth) acrylic ester is obtained from the second reaction product in at least one distillation step (e.g. column 16). Typically, in this distillation step, unconverted acetal and optionally solvent can be separated off from the second reaction product and optionally returned to the oxidation after step c).

In a preferred embodiment, the reaction in step a takes place in a temperature range from -50 ° C. to 100 ° C., preferably -10 ° C. to 30 ° C., in particular preferably 0 ° C to 20 ° C, and the reaction in step c in a temperature range from 0 ° C to 120 ° C, preferably 50 ° C to 120 ° C, particularly preferably 60 ° C to 100 ° C, the reaction in Step a takes place at a temperature which is at least 15 ° C. lower than the temperature of the reaction in step c.

In a preferred embodiment, the reaction in step a takes place in a pressure range from 0.5 to 10 bar (absolute), preferably 1 to 5 bar (absolute), and the reaction in step c in a pressure range from 1 to 50 bar (absolute) , preferably 2 to 30 bar (absolute), the reaction in step c taking place at a pressure which is at least 0.1 bar absolute, preferably at least 0.5 bar absolute, higher than the pressure of the reaction in step a.

The process according to the invention can typically comprise the addition of one or more stabilizers, for example selected from polymerization inhibitors, free radical scavengers and antioxidants, in particular stabilizers as described in EP-B 1 125919. For example, the stabilizer can be selected from phenol, substituted phenols (eg 4-methoxyphenol), hydroquinone, alkyl-substituted hydroquinones (eg methyl hydroquinone, tert-butyl hydroquinone, 2,6-di-tert-butyl parahydroquinone, 2.5 -di-tert-butyl-hydroquinone); saturated hydroxyalkyl carboxylates (e.g. hydroxyethyl acetate, hydroxyethyl propionate, hydroxyethyl isobutyrate, hydroxypropyl acetate) and N-oxyl compounds (e.g. piperidino-oxyl compounds such as 4-hydroxy-2, 2,6,6-tetramethylpiperidines -1-oxyl).

Typically, a stabilizer can be added as described above in one or more of steps a, b and / or c. A stabilizer can also be added, as described above, before or after one of steps a, b and / or c. Typically, one or more stabilizers can be added to the first reaction product. The stabilizer can be added, for example, after the reaction in step a, for example in step b. Description of the figures

Figure 1 shows an example of a possible schematic flow diagram of the method according to the invention. The terms have the following meaning:

1 (meth) acrolein stream on 3 or 7

2 dialcohol stream

3 Reactor, first stage, formation of the cyclic acetal

4 optional recycling stream ((meth) acrolein)

5 decanters

6 Separation of water

7 distillation column, separation of (meth) acrolein and water

8 raw stream first reaction product (cyclic acetal)

9 optional recycling stream (dialcohol)

10 distillation column, separation of acetal from dialcohol and high boilers

11 Acetal stream to the second reaction stage, here optional addition of stabilizer and solvent to the second reaction stage possible

12 Air dosing for the oxidation of the acetal

13 Reactor, second stage, formation of the hydroxyalkyl (meth) acrylic ester

14 optional recycling stream (acetal and / or solvent)

15 exhaust pipe

16 distillation column, separation of acetal and / or solvent

17 Removal of high boilers from the first reaction stage

18 crude stream second reaction product (hydroxyalkyl (meth) acrylic ester)

Experimental part

Example 1a - Continuous production of the cyclic acetal (2-isopropenyl-1,3-dioxolane) based on methacrolein and ethylene glycol / methacrolein in a distillation column A jacketed loop reactor with an active amount of catalyst of 5 kg and a total volume of 25 L was used. A sulfonic acid resin from Lanxess (K2431) was used as the catalyst (K1). The reactor was controlled via the jacket (operation with ARAL Antifreeze coolant) so that the internal temperature was 2-3 ° C. The reactor was connected to a distillation column (DN 150 mm, height 6 m) which was equipped with a Sulzer DX type packing (HETP 60 mm, ~ 16.6 theoretical plates per meter of packing height).

Ethylene glycol (EG) (15 kg / h, 242 mol / h) was mixed with 100 ppm TEMPOL (4-hydroxy-2,2,6,6-tetramethylpiperidine-1-oxyl) and metered directly into the reactor (3), whereas the methacrolein (MAL) was fed to a distillation column (7) together with the reactor discharge. This enabled both the methacrolein (MAL) and the reaction product to be dehydrated, which was advantageous for the conduct of the reaction. The hetero-azeotrope of methacrolein and water collected in the top of the column, which breaks down into two phases after cooling. The methacrolein was returned to the reactor (3) via a decanter (5). The amount of fresh methacrolein was adjusted so that the amount of methacrolein that was fed into the reactor per hour was 17.25 kg (246 mol). The molar ratio of methacrolein to ethylene glycol was thus 1.02 and the LHSV (liquid hourly space velocity) was 6.4 ((kg MAL + kg EG) / (kg cat. ^* Hr)).

The contents of the loop reactor were circulated via a pump so that the internal circulation ensured that the reactants were thoroughly mixed; at low circulating currents, the formation of two phases could be observed. To improve the mixing, a static mixer was installed upstream of the catalyst bed. The residence time in the reactor was just under 45 minutes and the reaction mixture at the reactor outlet had a composition of 40% by weight methacrolein, 34% by weight ethylene glycol, 21% by weight acetal, 3% by weight water and 2% by weight. -% minor components. The secondary components are, in particular, high-boiling addition products of glycol or water with the acetal. The reactor was started up over a period of 3 hours and then operated continuously and stably for 12 hours with these parameters. The conversion of methacrolein was 26% and the selectivity for acetal was 92%.

Example 1b - Continuous separation of methacrolein and water from the product mixture from Example 1a

The reaction discharge from Example 1a (32.25 kg / h) was mixed with the fresh methacrolein and driven into a distillation column (7) (DN 150mm, height 6m), which was supplied with a packing of the Sulzer DX (HETP 60 mm, ~ 16, 6 theoretical trays per meter of packing height). The column was operated at a pressure of 90 mbar, a bottom temperature of 90 ° C., a distillate temperature of 5 ° C. and a reflux ratio of 1. A decanter (5) was connected to the top of the column, by means of which the hetero-azeotrope obtained was separated from methacrolein and water (98.8% by weight of MAL and 1.2% by weight of water).

The aqueous phase of the decanter consisted of 93.9% by weight of water and 6.1% by weight of methacrolein. The aqueous phase was stripped discontinuously, a methacrolein-free, aqueous sump being obtained. This swamp can be treated biologically or incinerated. The organic phase of the decanter (5) was recycled into the reaction (reactor 3).

The bottom of the distillation column (7) (18.4 kg / h) consisted of the cyclic acetal (37% by weight), ethylene glycol (60% by weight) and the high-boiling by-products mentioned under 1a (3% by weight) .

The procedure in which fresh methacrolein was added to the distillation column (7) ensured that the azeotrope of methacrolein and water was almost completely removed from the bottom of the column 7. The methacrolein in the bottom discharge (from 7) was thus reduced to a level below 1000 ppm. Example 1c - Continuous production of the cyclic acetal (2-isopropenyl-1,3-dioxolane) based on methacrolein and ethylene glycol / methacrolein in the reactor

A jacketed loop reactor with an active amount of catalyst of 5 kg and a total volume of 25L was used. A sulfonic acid resin from Lanxess (K2431) was used as the catalyst (K1). The reactor was controlled via the jacket (operation with ARAL Antifreeze coolant) so that the internal temperature was 2-3 ° C.

The ethylene glycol (15 kg / h, 242 mol / h) and the methacrolein (17.25 kg / h, 246 mol / h) were metered into the reactor (3) with 100 ppm TEMPOL. The molar ratio of methacrolein to ethylene glycol was thus 1.02 and the LHSV was 6.4 ((kg MAL + kg EG) / (kg cat. ^* Hr)). The contents of the loop reactor were circulated via a pump so that the internal circulation ensured that the reactants were thoroughly mixed; at low circulating currents, the formation of two phases could be observed. To improve the mixing, a static mixer was installed upstream of the catalyst bed.

The residence time in the reactor (3) was just under 45 minutes and the reaction mixture at the outlet had a composition of 40% by weight methacrolein, 34% by weight ethylene glycol, 21% by weight acetal, 3% by weight water and 2% by weight .-% secondary components. The secondary components are, in particular, high-boiling addition products of glycol or water with the acetal.

The reactor was started up over a period of 3 hours and then operated continuously and stably for 12 hours with these parameters. The conversion of methacrolein was thus 26% and the selectivity for acetal was 92%.

Example 1d - Continuous separation of methacrolein and water from the product mixture from Example 1c The reaction discharge from Example 1c (32.25kg / h) was driven into a distillation column (7) (DN 150mm, height 6m), which was equipped with a packing of the Sulzer DX (HETP 60 mm, ~ 16.6 theoretical plates per meter of packing height) was equipped. The column was operated at a pressure of 85 mbar, a bottom temperature of 88 ° C., a distillate temperature of 5 ° C. and a reflux ratio of 2.5. A decanter (5) was connected to the top of the column, by means of which the hetero-azeotrope obtained was separated from methacrolein and water (98.8% by weight of MAL and 1.2% by weight of water).

The aqueous phase of the decanter consists of 93.5% by weight of water; 6.1 wt% methacrolein and 0.4 wt% acetal. The aqueous phase was stripped discontinuously, a methacrolein-free, aqueous sump being obtained. This swamp can be treated biologically or incinerated. The organic phase of the decanter was recycled into reaction (3) and contained 2.3% by weight of acetal.

The bottom of the distillation column (7) (18.4 kg / h) consisted of the cyclic acetal (36% by weight), ethylene glycol (61% by weight) and the high-boiling by-products mentioned under 1a (3% by weight) . A depletion of methacrolein and water in the sump to below 1000 ppm was achieved, but part of the acetal was lost in the decanter (5).

Example 1e - discontinuous production of the cyclic acetal (2-isopropenyl-1,3-dioxolane) based on methacrolein and ethylene glycol / with the aid of an inert, azeotropic entrainer

315 g of methacrolein (4.5 mol), 279 g of ethylene glycol (4.5 mol), 500 g of hexane and 5.2 g of phosphoric acid (1 mol%) were placed in a glass apparatus with a Dean-Stark apparatus attached. The reaction mixture was stabilized with 0.4 g each of TEMPOL and 0.4 g of 4-methoxyphenol. The mixture was heated to 80 ° C. for 6 hours, so that water released in the reaction was removed by means of hexane as an entrainer. In addition, a water-rich fraction and the condensed organic fraction collected in the separation part of the Dean-Stark apparatus Fraction consisting of hexane and methacrolein was permanently returned to the reaction section. The reaction was terminated after 6 hours.

The conversion of methacrolein was approx. 70% and the selectivity was approx. 48%. Shortly after the start of the reaction, a dark colored cloudiness formed in the reaction vessel at the phase boundary between methacrolein / hexane and ethylene glycol, which increased as the reaction progressed. The turbidity was caused in particular by high-boiling polymers of methacrolein and glycol or addition products of glycol to the 4-position of methacrolein or its acetal. When the amount of catalyst was increased or the reaction was scaled up on a larger scale, the formation of these high boilers accelerated.

In principle, unsatisfactory acetal yields were obtained in this procedure, especially with regard to the space-time yield. The principle of water separation by means of entrainer and separation by means of methacrolein-water azeotrope is functional, however.

Example 2 - Synthesis of the catalyst (K2) for the oxidation of 2-isopropenyl-1,3-dioxolane to hydroxyethyl methacrylate (HEMA)

Example 2a

0.90 g of bismuth nitrate pentahydrate and 0.36 g of telluric acid were suspended in a glass apparatus with stirring. HNO3 (60%) was added dropwise until everything was dissolved and the formation of unstable suboxides was prevented. 20.0 g of palladium on alumina (5% by weight of Pd) were added and the suspension was heated to 60.degree. After the temperature had been reached, the mixture was stirred for one hour. 10.0 g of a hydrazine monohydrate solution were added dropwise. The suspension was heated to 90 ° C. and stirred for a further hour. After cooling to room temperature, the black solid was filtered off and washed with 4 portions of 100 mL each of distilled water. The conductivity of the last Washing solution was below 100 pS / cm, which showed that the dopants were absorbed almost quantitatively.

The solid was dried for 10 h at 105 ° C. and the final catalyst was obtained. The stoichiometry was Pd1.00Bi0.20Te0.17 @ AI2O3.

Example 2b

The synthesis was carried out analogously to Example 2a without the addition of telluric acid.

Example 2c

The synthesis was carried out analogously to Example 2a without the addition of bismuth nitrate pentahydrate.

Example 2d

The synthesis was carried out analogously to Example 2a with the addition of twice the amount of telluric acid.

Example 2e

The synthesis was carried out analogously to Example 2a with the addition of twice the amount of bismuth nitrate pentahydrate.

Example 2f

The synthesis was carried out analogously to Example 2a with the addition of twice the amounts of bismuth nitrate pentahydrate and telluric acid.

Example 3 - Oxidation of 2-isopropenyl-1,3-dioxolane to hydroxyethyl methacrylate (HEMA) Example 3a

400 mg of catalyst (K2) from Example 2a and a 25% by weight solution of 2-isopropenyl-1,3-dioxolane in toluene, stabilized with 200 ppm of TEMPOL, were weighed into a 130 mL steel autoclave with a stirrer. The autoclave was closed, pressurized to 37 bar with 7% oxygen in nitrogen (0.6 equivalents of oxygen per equivalent of acetal) and placed in a preheated oil bath at 70.degree. The reaction was stirred for 4 h and then stopped by cooling with dry ice. The autoclave was carefully let down and the reaction mixture was analyzed by gas chromatography (GC).

The conversion was 54% and the selectivity to 2-hydroxyethyl methacrylate was 84%. This corresponds to a space-time yield of 3.8 mol HEMA / kg catalyst per hour. Ethylene dimethacrylate could not be detected by gas chromatography (GC).

Example 3b

The oxidation was carried out as in Example 3a, but ethyl acetate was used as the solvent. After a reaction time of 2 hours, the conversion was 91%, the selectivity was 88% and the space-time yield was 19.6 mol HEMA / kg catalyst per hour. Ethylene dimethacrylate could not be detected by gas chromatography.

Example 3c

The oxidation was carried out as in Example 3a, but with the catalyst from Example 2b. The conversion was 12%, the selectivity 79%.

Example 3d

The oxidation was carried out as in Example 3a, but with the catalyst from Example 2c. The conversion was 73%, the selectivity was 78% and the space-time yield was 7.1 mol HEMA / kg catalyst per hour. Example 3e

The oxidation was carried out as in Example 3a, but with the catalyst from Example 2d. The conversion was 20%, the selectivity was 84% and the space-time yield was 2.1 mol HEMA / kg catalyst per hour.

Example 3f

The oxidation was carried out as in Example 3a, but with the catalyst from Example 2e. The conversion was 83%, the selectivity 59% and the space-time yield 6.1 mol HEMA / kg catalyst per hour.

Example 3g

The oxidation was carried out as in Example 3a, but with the catalyst from Example 2f. The conversion was 59%, the selectivity 71% and the space-time yield 5.3 mol HEMA / kg catalyst per hour.

Example 3h

The oxidation was carried out as in Example 3a, but ethylene glycol was used as the solvent. After a reaction time of 2 hours, the conversion was 99%, the selectivity was 55% and the space-time yield was 7.06 mol HEMA / kg catalyst per hour.

The hydrogenation of 2-hydroxyethyl methacrylate was observed as the main side reaction. The selectivity of this reaction was 30%. The example demonstrates that for the conduct of the reaction and to achieve high selectivities and yields it is advantageous to control and reduce the concentration of alcohol (starting material first stage), since otherwise the side reaction to the undesired, hydrogenated by-product can increase.

Example 3i

The oxidation was carried out as in Example 3a, but the reaction was carried out at atmospheric pressure and the amount of gas selected so that an excess of oxygen was still present. After 4 hours of reaction time was almost no turnover available. The reaction time was extended to 48 hours, a conversion of about 50% being found and the selectivity being 83%.

This shows that the reaction is possible at lower pressures, but is economically unprofitable.

Example 3j

The oxidation was carried out as in Example 3a, but the reaction was carried out at 50.degree. After a reaction time of 4 hours, the conversion was 29% and the selectivity was 83%. When the temperature was lowered, an almost linear decrease in the rate of the reaction was found.

This shows that the reaction is possible at low temperatures, but economically unprofitable.

Claims

Expectations

1. A process for the preparation of hydroxyalkyl (meth) acrylic esters comprising the steps: a. Reaction of (meth) acrolein with at least one polyhydric alcohol in the presence of a first catalyst K1, a first reaction product containing at least one cyclic acetal being obtained; b. at least partially removing water from the first reaction product; c. Reaction of the first reaction product with oxygen in the presence of a second catalyst K2, a second reaction product containing at least one hydroxyalkyl (meth) acrylic ester being obtained.

2. The method according to claim 1, characterized in that it is a continuous process for the production of hydroxyalkyl (meth) acrylic esters.

3. The method according to claim 1 or 2, characterized in that the reaction in step a in the presence of at least one acidic compound as catalyst K1, selected from Brönsted acids and Lewis acids, takes place, the catalyst K1 being in heterogeneous or homogeneous form and comprises at least one acidic group which has a pKa value of less than or equal to 5.

4. The method according to any one of claims 1 to 3, characterized in that the at least one catalyst K1 is selected from the group consisting of phosphoric acid, sulfuric acid, sulfonic acids, carboxylic acids and ion exchange resins containing at least one acidic group selected from sulfonic acids and carboxylic acids.

5. The method according to any one of claims 1 to 4, characterized in that the reaction in step a at a molar ratio of (meth) acrolein to polyhydric alcohol (s) in the range from 1:50 to 50: 1, preferably from 1: 10 to 10: 1, particularly preferably from 1: 3 to 3: 1, takes place.

6. The method according to any one of claims 1 to 5, characterized in that in step b unconverted (meth) acrolein and / or unconverted polyhydric alcohol are separated from the first reaction product and returned to the reaction after step a.

7. The method according to any one of claims 1 to 6, characterized in that in step b in a first separation step (meth) acrolein and at least partially water are separated from the first reaction product.

8. The method according to any one of claims 1 to 7, characterized in that in step b in at least two separation steps water, unreacted (meth) acrolein and unreacted polyhydric alcohol are separated from the first reaction product containing at least one cyclic acetal.

9. The method according to any one of claims 1 to 8, characterized in that the reaction in step c is carried out in the presence of a metal- and / or semimetal-containing, heterogeneous catalyst system as catalyst K2.

10. The method according to any one of claims 1 to 9, characterized in that the at least one catalyst K2 is a heterogeneous catalyst containing one or more support materials and one or more active components, the support material being selected from activated carbon, silicon dioxide, aluminum oxide, titanium dioxide , Alkali metal oxides, alkaline earth metal oxides and mixtures thereof, and wherein the active component contains at least one element selected from palladium, platinum, iridium, rhodium, ruthenium, gold, cobalt, nickel, zinc, copper, iron, selenium, tellurium, arsenic, antimony, Bismuth, germanium, tin and lead, being the elements elemental, as an alloy or in the form of their compounds in any oxidation state.

11. The method according to any one of claims 1 to 10, characterized in that the at least one catalyst K2 is a heterogeneous catalyst containing silicon dioxide and / or aluminum oxide as support material and containing at least one element selected from palladium, bismuth and tellurium as active component , whereby the elements can be elemental, as an alloy or in the form of their compounds in any oxidation state.

12. The method according to any one of claims 1 to 11, characterized in that the at least one catalyst K2 is a heterogeneous catalyst containing silicon dioxide and / or aluminum oxide as support material and an active component, the active component palladium and at least one further element being selected from selenium, tellurium and bismuth, whereby the elements can be elemental, as an alloy or in the form of their oxides in any oxidation state.

13. The method according to any one of claims 1 to 12, characterized in that during the reaction in step c a reaction mixture containing the first reaction product and the second catalyst K2 is brought into contact with an oxygen-containing gas, wherein in step c an oxygen -containing exhaust gas is obtained, wherein the oxygen-containing exhaust gas is cooled in at least one stage, and wherein the oxygen-containing exhaust gas has an oxygen content in the range from 1 to 10 vol .-%, based on the total oxygen-containing exhaust gas .

14. The method according to any one of claims 1 to 13, characterized in that in the reaction in step c, a reaction mixture containing the first reaction product and the second catalyst K2 is used, which has a polyhydric alcohol (s) content of less than 10 wt .-%, based on the reaction mixture in step c.

15. The method according to any one of claims 1 to 14, characterized in that the reaction in step a takes place in a temperature range from -50 ° C to 100 ° C, the reaction in step c in a temperature range from 0 ° C to 120 ° C takes place, and the reaction in step a takes place at a temperature which is at least 15 ° C lower than the temperature of the

Implementation in step c.

16. The method according to any one of claims 1 to 15, characterized in that the reaction in step a takes place in a pressure range of 0.5 to 10 bar, the reaction in step c takes place in a pressure range of 1 to 50 bar, and wherein the Reaction in step c takes place at a pressure which is at least 0.1 bar higher than the pressure of the reaction in step a.