阅读说明：本技术 制备甲基丙烯酸或甲基丙烯酸酯的方法 (Method for producing methacrylic acid or methacrylic acid esters ) 是由 A·迈 S·科里尔 M·特雷斯科 D·卡拉姆比亚 G·阿维拉 于 2019-07-11 设计创作，主要内容包括：本发明涉及制备甲基丙烯酸或甲基丙烯酸酯的方法。本发明针对的是从可由甘油或丙烷获得的丙烯醛开始制备甲基丙烯酸或甲基丙烯酸烷基酯的新方法。(The present invention relates to a process for the preparation of methacrylic acid or methacrylic acid esters. The present invention is directed to a novel process for the preparation of methacrylic acid or alkyl methacrylates starting from acrolein, obtainable from glycerol or propane.)

1. A process for preparing methacrylic acid or methacrylic acid esters, the process comprising the steps of:

a) the preparation of acrolein is carried out by reacting acrolein with a catalyst,

b) reacting acrolein of process step a) with hydrogen to produce propionaldehyde,

c) reacting the propionaldehyde from process step b) with formaldehyde to produce methacrolein, and

d) methacrolein is oxidized to methacrylic acid or methacrylic acid esters in the presence of an oxygen-containing gas and optionally an alcohol.

2. The process according to claim 1, wherein the acrolein of step a) is derived from a C3-based feedstock, and the C3-based feedstock is vaporized and then converted in the presence of a heterogeneous catalyst and a co-feed gas comprising at least one of the components selected from hydrogen, oxygen and water.

3. The process according to any one of claims 1 or 2, wherein in process step a) glycerol is evaporated as the C3 feedstock and converted to acrolein in the presence of water and optionally hydrogen, and the gaseous acrolein obtained in process step a) is not condensed but is converted directly in process step b) without intermediate separation of the acrolein.

4. The process according to any one of claims 1 to 3, wherein the product of process step b) is condensed and thereby separated from a gas stream comprising hydrogen and at least two of the components water, carbon monoxide, carbon dioxide, ethane, ethylene and propane, and wherein at least part of the gas stream is recycled in at least one of process steps a) or b) by using the gas stream as a co-feed.

5. The process according to any one of claims 1 to 2, wherein the acrolein of process step a) is prepared by oxidation of propylene in the gas phase in the presence of a catalyst comprising bismuth and molybdenum at a temperature of from 300 to 400 ℃.

6. The process according to claim 5, wherein the acrolein is hydrogenated in process step a) in the presence of acrylic acid formed as a by-product in process step a), and the acrylic acid is hydrogenated to propionic acid in process step b).

7. The process according to claim 6, wherein the acrylic acid and/or the propionic acid are reacted as cocatalyst in process step c).

8. The process according to any one of claims 1 to 7, wherein the acrolein of process step a) is separated from the acrylic acid by extraction and/or distillation, whereby the acrolein obtained is used in process step b).

9. A process according to claim 3, wherein acrolein of process step a) comprising glycerol as component C3 is hydrogenated in process step b) in the presence of hydroxyacetone, which is a by-product formed from glycerol, and which is reacted either in process step d) to form pyruvate methacrylate or in process step C) to form hydroxymethyl vinyl ketone.

10. The process according to claim 3, wherein the intermediate acrolein of process step a) before being hydrogenated in process step b) or the propanal of process step b) before being condensed in process step c) in the presence of formaldehyde is substantially purified free of hydroxyacetone.

11. The process according to any one of the preceding claims, wherein the intermediate of at least one of the process steps a), b) and/or c) is substantially separated from hydroxyacetone or hydroxymethyl vinyl ketone.

12. The process according to any one of the preceding claims, wherein process step c) is carried out in the presence of from 0.1 to 20 mol% of an organic base, preferably a secondary amine, and from 0.1 to 20 mol% of an acid, preferably an organic acid, the mol% values being in each case based on propionaldehyde, more preferably the molar ratio of the acid to the organic base is in the range from 20:1 to 1: 20.

13. The process according to any one of the preceding claims, wherein process step d) is an oxidative esterification of methacrolein which is carried out in the liquid phase at a pressure of from 1 to 100 bar and in the presence of a heterogeneous noble metal-containing catalyst comprising a metal and/or a metal oxide.

14. The process according to claim 13, wherein the heterogeneous oxidation catalyst comprises one or more ultra-finely divided metals having an average particle size of less than 20nm selected from the group consisting of gold, palladium, ruthenium, rhodium and silver.

15. The process according to any of the preceding claims, wherein process steps a) and b) are carried out simultaneously in one reactor.

16. The process according to any of the preceding claims, wherein process step b) is carried out in the presence of a noble metal catalyst and hydrogen.

Technical Field

The present invention relates to a process for the preparation of methacrylic acid or methacrylic acid esters. The present invention is directed to a novel process for the preparation of methacrylic acid or alkyl methacrylates starting from acrolein, obtainable from glycerol or propane.

Background

Methacrylic acid and methacrylates, such as Methyl Methacrylate (MMA), are used in a wide variety of applications. In addition, methyl methacrylate is an important building block for various specialty esters of methacrylic acid (MAA), which are prepared by transesterification with the corresponding alcohols.

The commercial production of methacrylic acid is carried out in particular by heterogeneously catalyzed gas phase oxidation of isobutene, tert-butanol, methacrolein or isobutyraldehyde. The gaseous reaction phase thus obtained is converted into an aqueous methacrylic acid solution by cooling and condensation, optionally separated from low boilers such as acetaldehyde, acetone, acetic acid, acrolein and methacrolein, and then introduced into a solvent extraction column to extract and separate methacrylic acid using a suitable extractant such as a short-chain hydrocarbon. The separated methacrylic acid is further purified, for example by distillation, to separate high-boiling impurities such as benzoic acid, maleic acid and terephthalic acid, in order to obtain pure methacrylic acid. Such known processes are described, for example, in EP 0710643, U.S. Pat. No. 4,618,709, U.S. Pat. No. 4,956,493, EP 386117 and U.S. Pat. No. 5,248,819.

Currently, Methyl Methacrylate (MMA) is mainly prepared from hydrocyanic acid and acetone via the resulting Acetone Cyanohydrin (ACH) as a main intermediate. The disadvantage of this process is the production of very large amounts of ammonium sulphate, which entails very high costs for its disposal. Other non-ACH-based processes are described in the relevant patent literature and are also carried out on a production scale. The starting materials used in this connection as starting materials include those based on C-4 compounds, for example isobutene or tert-butanol, which are converted in a plurality of stages into the desired methacrylic acid derivatives.

The general procedure here is to oxidize isobutene or tert-butanol in a first stage to give methacrolein, which is then reacted with oxygen to give methacrylic acid. The resulting methacrylic acid is then converted to MMA with methanol. Further details of the process are described in particular in the following documents: ullmann's Encyclopedia of Industrial Chemistry2012 (Ullmann's Encyclopedia of Industrial Chemistry 2012), Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Methacrylic Acid and Derivatives (Methacrylic Acid and Derivatives), DOI:10.1002/14356007.a 16-441. pub2, and Trends and Future of Monomer-MMA Technologies (Trends and Future of Monomer MMA technology), SUMITOMO KAGAKU 2004-II.

In one variant of the preparation process, ethylene may also be used as a starting material in place of the C4 building block (e.g. isobutene), and ethylene is first reacted with synthesis gas to give propionaldehyde; then reacting with formaldehyde to obtain the methacrolein. The resulting methacrolein is oxidized by air in the gas phase over a heterogeneous catalyst to give Methacrylic Acid, which is esterified with methanol to give MMA (Ullmann's Encyclopedia of Industrial Chemistry2012, Methacrylic Acid from Ethylene (Methacrylic Acid is prepared from Ethylene), and Trends and Future of Monomer-MMA Technologies, SUMITOMO KAGAKU 2004-II). This process has been operated by BASF since 1990 in a plant having a methacrylic acid production capacity of 40000 metric tons/year. According to the SUMITOMO article, this method was developed by BASF for specific needs and it is therefore difficult to apply the method in general for the preparation of larger amounts of MMA.

In a very modern variant of this process, which uses ethylene as starting material, methacrolein is converted directly to MMA by oxidative esterification in the presence of oxygen, methanol and a noble metal catalyst. Such a process can be found, for example, in WO 2014/170223.

Another method obtains MMA by: isobutene or tert-butanol is oxidized with atmospheric oxygen in the gas phase over a heterogeneous catalyst to give methacrolein, and methanol is subsequently used in the oxidative esterification of methacrolein. This process developed by ASAHI is described in particular in publications US 5,969,178 and US 7,012,039. This method is also described in the SUMITOMO article, which provides detailed information about the disadvantages of said method, which are particularly high energy consumption, which is caused in particular by unpressurized operating procedures.

Furthermore, other problems associated with all the above processes are, in particular, relatively unsatisfactory yields, high losses in the oxidation step and consequent CO₂Formation, and generally the consequent formation of by-products, requires complex steps to isolate the product: all processes using gas phase oxidation over heterogeneous catalyst systems starting from isobutene or from equivalent C-4-based feedstocks (e.g. TBA or MTBE) achieve yields of less than 90% and the relevant literature describes yields of less than 85% for the preparation of methacrolein starting from isobutene (e.g. table 5in Ullmann's Encyclopedia/Sumitomo, see above). The gas-phase process is naturally carried out at moderate pressures of from 1 to 2 bar absolute and produces a process gas which comprises only about 4-6% by volume of the product components. The separation of useful products from the inert gas ballast therefore entails high energy costs and consumes a large amount of cooling energy as well as steam for the multistage distillation work-up steps.

The preparation of MMA according to the process described so far produces relatively large amounts of waste, in particular waste gas or waste water, which require expensive disposal.

Furthermore, performing some of the processes described above requires very complex and therefore expensive equipment, which is accompanied by high capital expenditure and high maintenance costs.

The review article of SUMITOMO cited above describes the respective disadvantages in detail and can therefore be incorporated herein by reference.

Disclosure of Invention

Purpose(s) to

In view of the prior art mentioned, it was therefore an object of the present invention to provide an alternative process for preparing methacrylic acid or methacrylic esters which does not have the disadvantages of the conventional processes.

A particular object is to use raw materials obtained from natural sources in this process.

Another particular object is to enable the production of methacrylic acid or methacrylic esters with a relatively low energy use. In addition, the process should be carried out in a manner that provides a high level of environmental protection, so that the amount of waste obtained is very small.

It is another particular object of the present invention to develop a novel process for preparing methacrylates, in particular MMA or methacrylic acid, starting from the C3 building block. It is therefore important to improve the overall yield of methacrylic acid or methacrylic acid esters based on the C3 starting material used in the case of high product selectivity, for example by finding and combining individual reaction steps.

In addition, it should be possible to carry out the method with a very small number of steps, which should be simple and reproducible.

Furthermore, it should be possible to implement the method by using relatively simple and inexpensive equipment. The capital expenditure of the equipment should be correspondingly small. Maintenance of such equipment should be simple and inexpensive.

Other objects not specifically mentioned may be apparent from the overall context of the description and claims that follow.

Detailed Description

The above object is achieved by a method having all the features of claim 1, and also other objects not explicitly mentioned but easily derivable or deducible from the situation discussed in the introduction of the present description. The dependent claims 2 to 17 protect advantageous embodiments of the claimed method.

This novel process for the preparation of methacrylic acid or methacrylic acid esters comprises the following steps:

a) the preparation of acrolein is carried out by reacting acrolein with a catalyst,

b) reacting acrolein of process step a) with hydrogen to produce propionaldehyde,

c) reacting the propionaldehyde from process step b) with formaldehyde to produce methacrolein, and

d) methacrolein is oxidized to methacrylic acid or methacrylic acid esters in the presence of an oxygen-containing gas and optionally an alcohol.

Method step a)

It is particularly preferred that the acrolein of step a) is derived from a C3-based feedstock. In a preferred embodiment of the invention, the acrolein is derived from propylene. In a second even more preferred embodiment, the acrolein is derived from glycerol.

Independently of these embodiments, it is preferred to start with the evaporation of the C3-based feedstock, which is then converted to acrolein in the presence of a heterogeneous catalyst (contact) and a co-feed gas. This feed gas comprises at least one component selected from the group consisting of hydrogen, oxygen and water.

In the case where the C3-based feedstock is propylene, this reaction is preferably carried out in the gas phase at a temperature of from 300 to 400 ℃. It is particularly preferred to carry out the reaction in the presence of a catalyst comprising bismuth and molybdenum.

In the case where the C3-based feedstock is glycerol, it is preferred to evaporate this glycerol and convert it to acrolein in the presence of water and optionally hydrogen. It is particularly preferred not to condense the acrolein obtained but to convert it directly into propionaldehyde in process step b) without intermediate separation of the acrolein.

In the case of acrylic acid being formed as a by-product in process step a), the acrolein is hydrogenated in the presence of this acrylic acid in process step b). Whereby the acrylic acid is at least partially hydrogenated to propionic acid in parallel therewith. It is particularly preferred to transfer this propionic acid and/or acrylic acid together with the propionaldehyde to the reactor used in process step c). Here, the propionic acid and/or acrylic acid functions as a cocatalyst for reacting propionaldehyde with formaldehyde to form methacrolein. In this context, a particularly preferred embodiment of the present invention is that in which no additional acid is added to the reaction mixture of process step c) and the amount of propionic acid is sufficient to carry out process step c) effectively.

In one alternative thereof, the acrolein of process step a) is separated from the acrylic acid by extraction and/or distillation, whereby the acrolein obtained is used in process step b).

Method step b)

In process step b), acrolein from process step a) is reacted with hydrogen to produce propionaldehyde.

After the reaction, it is particularly preferred to condense the product of process step b). Thereby, the phase comprising propanal is separated from a gas stream comprising hydrogen and at least two of the following components, water, carbon monoxide, carbon dioxide, ethane, ethylene and propane. It is furthermore very advantageous to recycle at least part of the gas stream in at least one of the process steps a) or b) by using the gas stream as a co-feed.

It is particularly preferred to carry out process step b) in the presence of a noble metal catalyst and hydrogen.

For the embodiment of the invention in which the acrolein of process step a) comprises glycerol as component C3, the acrolein is hydrogenated in the presence of hydroxyacetone in process step b). Thus, the hydroxyacetone is formed as a by-product from acetol (acetole). For the following procedure, there are now two alternatives: the hydroxyacetone is reacted either in process step d) to form pyruvate methacrylate or in process step c) to form hydroxymethyl vinyl ketone. It is also possible to convert only a portion of the hydroxyacetone to acetol methacrylate, while the majority of the remaining hydroxyacetone eventually forms hydroxymethyl vinyl ketone.

Since both by-products have a similarly colored negative effect on the end product, it is preferred to carry out the intermediate acrolein in process step a) in process step b)It is substantially purified prior to hydrogenation. Alternatively or additionally, the propionaldehyde of process step b) is substantially purified from hydroxyacetone prior to condensing the propionaldehyde of process step b) in the presence of formaldehyde in process step c). Thus, other by-products, such as CO, can also be separated from the crude product₂Or H₂. At H₂In the case of (3), it is preferably recycled.

Alternatively or additionally, it is preferred that the intermediate of at least one of the process steps a), b) and/or c) is substantially separated from hydroxyacetone or hydroxymethyl vinyl ketone.

In a very particular embodiment of the invention, process steps a) and b) are carried out simultaneously in one reactor. It is particularly surprising that by doing so, the yield, effectiveness and selectivity of the overall process comprising steps a) to d) are still very good.

Method step c)

The claimed process comprises the preparation of methacrolein, especially by the reaction of propionaldehyde with formaldehyde. Methods suitable for this purpose are known to the person skilled in the art and are the subject of relevant review articles, for example in Ullmann's Encyclopedia of 5Industrial Chemistry2012, Wiley-VCH Verlag GmbH & Co.KGaA, Weinheim, Acrolein and Methacrrolein (Acrolein and Methacrolein), DOI:10.1002/14356007.a 01-149. pub 2. Further more detailed descriptions of this process can be found, for example, in WO 2014/170223 or in WO 2015/091173.

The reaction achieved by aldol condensation or Mannich condensation is not critical per se. However, preferred processes are those with high yields and low by-product formation. Preferably, this process step is carried out in the presence of water. It is suitable to use a reaction having a selectivity of at least 80%, preferably at least 90% and particularly preferably at least 92%, based on the amount of propionaldehyde used.

Provision may also be made for: the reaction according to step c) is carried out with a molar ratio of propionaldehyde to formaldehyde preferably in the range from 2:1 to 1:2, particularly preferably from 1.5:1 to 1: 1.5. Very particular preference is given to using an equimolar ratio of propionaldehyde to formaldehyde.

Preferred processes for preparing methacrolein starting from propionaldehyde and formaldehyde are described in particular in the following publications: US 7,141,702, DE 3213681 a1, US4,408,079, US2,848,499, JP 4173757a (JP 19900300135), JP 3069420B2 and EP 0317909 a2, and the teachings of said publications are hereby incorporated by reference into the present application for disclosure-related purposes.

The reaction of propionaldehyde with formaldehyde generally uses a catalyst, especially an inorganic acid or an organic mono-, di-or polycarboxylic acid, preferably an aliphatic monocarboxylic acid. Thus, the use of at least one organic acid for the reaction of propionaldehyde and formaldehyde is particularly preferred, and formic acid or acetic acid is even more preferred.

In principle, other organic acids can likewise be used, but they are generally less advantageous for reasons of price. The mineral acids used are generally sulfuric acid and phosphoric acid. Mixtures of acids may also be used.

The proportion of acid is from 0.1 to 20 mol%, advantageously from 0.5 to 10 mol%, preferably from 1 to 5 mol%, based on propionaldehyde.

The reaction of propionaldehyde with formaldehyde is in most cases additionally carried out in the presence of an organic base, preferably an amine, particularly preferably a secondary amine.

Examples of useful amines are: dimethylamine, diethylamine, methylethylamine, methylpropylamine, dipropylamine, dibutylamine, diisopropylamine, diisobutylamine, methylisopropylamine, methylisobutylamine, methyl-sec-butylamine, methyl (2-methylpentyl) amine, methyl (2-ethylhexyl) amine, pyrrolidine, piperidine, morpholine, N-methylpiperazine, N-hydroxyethylpiperazine, piperazine, hexamethyleneimine, diethanolamine, methylethanolamine, methylcyclohexylamine, methylcyclopentylamine, dicyclohexylamine or suitable mixtures.

The proportion of organic base, preferably secondary amine, is from 0.1 to 20 mol%, advantageously from 0.5 to 10 mol%, preferably from 1 to 5 mol%, based on propionaldehyde. The equivalent ratio of amine to acid is preferably chosen such that a pH of 2.5 to 9 is obtained in the reaction mixture before the reaction.

It can also be provided that the molar ratio of acid to organic base, preferably amine, is in the range from 20:1 to 1:20, preferably in the range from 10:1 to 1:10, particularly preferably in the range from 5:1 to 1:5, and particularly preferably in the range from 2:1 to 1: 2.

The reaction temperature of the reaction of propionaldehyde with formaldehyde at the outlet from the reaction zone is from 100 to 300 ℃, preferably from 130 to 250 ℃, preferably from 140 to 220 ℃, in particular from 150 to 210 ℃.

The reaction pressure is in the range from 2 to 300 bar, preferably from 5 to 250 bar, particularly preferably from 10 to 200 bar, advantageously from 15 to 150 bar, preferably from 20 to 100 bar and in particular from 40 to 80 bar. The pressure and temperature are adjusted so that the reaction always takes place below the boiling point of the reaction mixture, i.e. the reaction is carried out in the liquid phase. For the purposes of this application, all pressure data are in absolute pressure in bar.

The residence time or reaction time is preferably up to 25 minutes, advantageously from 0.01 to 25 minutes, preferably from 0.03 to 2 minutes, and particularly preferably in the range from 1 to 30 seconds. Advantageously, a tubular reactor is used as reactor with a residence time of less than 10 minutes. Here, the residence time means the time during which the reaction mixture reacts. Here, all components are provided at the reaction pressure and temperature, and the time can therefore be calculated from the distance between the mixing point and the depressurization point. The depressurization point is the point at which the mixture reaches a pressure below 5 bar from the reaction pressure.

The reaction mixture may contain, in addition to water, organic solvents such as propanol, dioxane, tetrahydrofuran and methoxyethanol.

It can also be provided that the reaction of propionaldehyde with formaldehyde according to step c) is carried out in the presence of preferably at least 0.1% by weight, preferably at least 0.2% by weight and particularly preferably at least 0.5% by weight, based on formaldehyde, of methanol to give methacrolein. Despite the relatively high methanol concentration, any complicated removal of methanol from the formaldehyde feed and/or during the methacrolein purification can be omitted by carrying out the subsequent step d) reaction as claimed.

According to a particular embodiment, formaldehyde and propionaldehyde may be mixed prior to bringing the starting materials to reaction pressure and/or temperature.

The reaction can be carried out as follows: maintaining a mixture of propionaldehyde, an amine, formaldehyde, and advantageously water and/or an acid and/or a base at the reaction temperature and the reaction pressure during the reaction time.

Step d)

According to the invention, methacrolein obtained in step c) is oxidized to methacrylic acid or methacrylic acid esters in the presence of an oxygen-containing gas and optionally an alcohol. Process step d) is thus preferably an oxidative esterification of methacrolein, which is carried out in the liquid phase at a pressure of from 1 to 100 bar and in the presence of a heterogeneous noble metal-containing catalyst. Furthermore, it is preferred that the heterogeneous catalyst comprises a metal and/or a metal oxide.

In a first embodiment of the invention, methacrolein is oxidized to methacrylic acid. In a further step, this methacrylic acid can be converted into alkyl methacrylate by reaction with an alcohol, in particular into MMA by reaction with methanol. Further details of the process are described in particular in the following documents: ullmann's Encyclopedia of Industrial Chemistry2012, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Metacrylic Acid and Derivatives, DOI:10.1002/14356007, a 16-441, pub2, and Trends and Future of Monomer-MMA Technologies, SUMITOMO KAGAKU 2004-II.

In a second alternative preferred embodiment according to the invention of step d) of the process, methacrolein is oxidized in the presence of an alcohol. According to the invention, and alternatively, methacrolein obtained in step c) is thus reacted in a direct oxidative esterification reaction to obtain a methacrylate, for example MMA. Details about this process step can be found, for example, in WO 2014/170223, WO 2015/076682, WO 2015/091173 or in WO 2015/091018.

The oxidation of methacrolein in the oxidative esterification reaction in step d) of the process according to the invention generally yields up to 30% by weight, preferably up to 15% by weight, particularly preferably up to 5% by weight, of methacrylic acid.

The oxidative esterification reaction is carried out with an oxidizing agent, and oxygen (O) is preferably used for this purpose₂). For cost reasons, air may be preferred and may contain varying proportions of oxygen; this is not critical to the invention.

In addition, at least one heterogeneous oxidation catalyst is used for carrying out the reaction according to step d), and these selectively accelerate the oxidation reaction as defined in more detail above. Suitable catalysts are well known to those skilled in the art and are described, for example, in the following publications: EP 0857512 a1, EP 1393800 a1, EP 2177267 a1 and EP 2210664 a1, and said publications are cited for the purpose of disclosure and the catalysts disclosed therein are incorporated into the present application. The publication also describes the reaction conditions, which are likewise incorporated into the present application.

The heterogeneous oxidation catalyst preferably comprises at least one noble metal and in most cases at least one metal oxide. Preference is given here to oxidation catalysts in which gold and/or palladium and/or ruthenium and/or rhodium and/or silver are present. Catalysts containing gold and/or palladium are particularly preferred.

In addition to the catalysts in the publications discussed above, it is preferred to use porous support materials. The noble metal is then dispersed on the surface of these primary support particles in the form of secondary nano-or microparticles. Particularly preferred specific surface areas (BET method) are generally at least 50m²A/g, preferably of at least 100m²/g。

It is particularly preferred that the heterogeneous oxidation catalyst comprises one or more ultra-finely divided metals having an average particle size of less than 20nm selected from the group consisting of gold, palladium, ruthenium, rhodium and silver. The catalysts mentioned above are particularly preferably based on gold and/or nickel oxides, whereas palladium catalysts are less preferred. Nickel-containing and gold-containing catalysts may preferably be lead-free.

The method of loading the carrier with the catalytically active component is not subject to any particular limitation. Suitable methods are inter alia coprecipitation, precipitation deposition, impregnation and vapour deposition.

It can be preferably provided that the water content of the reaction mixture used for the oxidative esterification in this embodiment of step d) is preferably at most 10% by weight, and preferably at most 5% by weight.

The amount of catalyst to be used varies depending on the composition of the feed mixture and the catalyst, depending on the reaction conditions, and depending on the type of reaction and the like. If the catalyst is used in the form of a slurry, it is preferable to use the catalyst in an amount of 0.01 to 0.5kg/l of the reaction system solution.

It is also preferred that the content of methacrolein in the reaction mixture used for the oxidative esterification in step d) is at least 5% by weight, preferably at least 15% by weight and particularly preferably at least 25% by weight. It can also be provided that the oxidative esterification reaction according to step d) is preferably carried out with a molar ratio of methanol to methacrolein in the range from 1:1 to 50:1, particularly preferably from 1.5:1 to 25:1, and particularly preferably from 2:1 to 10: 1.

The oxidative esterification reaction may be carried out in any conventional manner, for example in a liquid phase reaction or a trickle bed reaction. For example, any known reactor may be used, such as a bubble column reactor, a tubular reactor with an air stream, or a stirred reactor.

The pressure at which the reaction is carried out may vary widely. Surprising advantages can be achieved by reaction pressures in the range from 1 to 100 bar, preferably from 3 to 80 bar, more preferably from 4 to 50 bar and particularly preferably from 5 to 20 bar.

The pH of the reaction system is preferably maintained at 5 to 9, particularly 6.5 to 8, by adding at least one basic compound, preferably selected from alkali metal compounds and/or alkaline earth metal compounds, such as oxides, hydroxides, carbonates, carboxylates or the like.

The oxidative esterification reaction according to the second alternative step d) can be carried out at a temperature preferably in the range from 10 ℃ to 200 ℃, particularly preferably from 40 to 150 ℃ and particularly preferably from 60 to 120 ℃ and with a reaction time or residence time which varies depending on the other reaction conditions; however, it is preferably in the range of 10 minutes to 48 hours, preferably 30 minutes to 24 hours, and particularly preferably 45 minutes to 2 hours. More information on carrying out the oxidative esterification reaction according to step d) for carrying out MMA synthesis is found in particular in US4,249,019 or DE 3018071a 1.

The reaction product obtained in step d) can be worked up in a known manner to obtain pure MMA: the reacted reaction mixture obtained via oxidative esterification according to step d) can first be worked up by distillation, preferably with at least three distillation steps. Alternatively and/or additionally, the post-treatment comprises at least one phase separation and/or centrifugation process. In addition to the distillation step, other post-treatment techniques may be used.

It can also be provided that the reactor volume in step c) is smaller than the reactor volume in step d). The reactor volume here is based on the volume in step c) and step d), wherein the starting materials used in these steps are reacted in the liquid phase at the elevated pressure of the respective reaction to give the product. The ratio of the reactor volume in step c) to the reactor volume in step d) is advantageously in the range from 1:1000 to 1:100, preferably in the range from 1:800 to 1:200 and particularly preferably in the range from 1:500 to 1: 300.

A typical reactor volume of a continuously operated production plant may be, for example, for step c) a capacity of 0.1 to 0.5m³A tube/tube bundle reactor of, and for step d), a capacity of from 10 to 15m³Tubular/tube bundle reactors of, or having a capacity of from 50 to 100m³The continuously operating stirred tank of (a), but these data are not intended to represent any limitation.

Preference is given to carrying out steps a), b), c) and d) in a continuous process. The process of introducing the starting materials into the apparatus for carrying out the process according to the invention and removing the product from the apparatus is carried out continuously here over any desired period of time. However, the time period may be interrupted for maintenance and repair work.

Detailed Description

Examples

Synthesis of acrolein

Example 1: synthesis of acrolein from Glycerol as starting Material (method step a)

The following example is a reproduction of example 2 as disclosed in WO 2012/72697.

The glycerol is converted to acrolein over a catalyst bed, with hydrogen being used to reduce the partial pressure of the reactants. The catalyst layer used was composed of 56g of a catalyst supported on ZrO₂WO 10% by weight of above₃Composition of, the ZrO₂In the form of grains having a size of 20-30 mesh. The inlet liquid stream consisted of 20 wt% glycerol in water fed to the preheater at 0.3 g/min. A gaseous stream of 100ml/min hydrogen was also fed to the preheater. The liquid stream was preheated and vaporized to 280 ℃ prior to entering the reactor. The inlet to the reactor was maintained at 300 ℃ and a pressure of 5 bar gauge was applied to the reactor. The outlet stream is cooled in a condenser and the water is condensed. The liquid stream was collected in a sample container while the gas stream 11 was collected in a Tedlar (Tedlar) gas bag. The liquid samples were analyzed for hydrocarbons (propanol, propionaldehyde, propionic acid, etc.) using a GC equipped with FID and WAX columns. Analysis of gas samples for CO, CO with a two channel GC equipped with a TCD₂Ethylene, ethane, and the like. Glycerol is converted beyond the detection limit and acrolein is produced in amounts greater than 80%. The yield of hydroxyacetone was reduced to 5%, while CO and CO were₂The yield of (a) was substantially the same. Further, about 10% of n-propionaldehyde was formed as a by-product.

Example 2: synthesis of acrolein from Glycerol as starting Material (method step a)

The following example is a reproduction of example 1 as disclosed in US 2011/112330.

In a fixed catalyst bed, a cesium salt of tungstophosphoric acid (CsPW) was used for a 20 wt% aqueous solution of glycerol together with air. The fixed catalyst bed is heated at a temperature of 260 ℃ to 350 ℃ and the feed gas has a composition in mole percent: glycerol, oxygen, nitrogen, water 4.2:2.2:8.1: 85.5. GHSV of 2445h^-1. Acrolein was obtained in 93.1% yield.

Example 3: synthesis of acrolein from propene as starting Material (method step a)

The catalyst (Mo) was tested at 342 deg.C, a pressure of 15psi, and a total flow of 130 cc/min with a gas feed composition of nitrogen to oxygen to propylene to water ratio of 77:7.50:5.50:10₁Pd_{01.57e- 4}Bi_0.09Co_0.8Fe_0.2Al_0.123V_4.69e-3K_5.33e-3). The reaction product showed 99% conversion of propylene and 98% selectivity to acrolein.

Example 4: synthesis of acrolein from propene as starting Material (method step a)

The following synthesis is a reproduction of example 1 in EP 1460053.

A cyclic catalyst was used, having the following composition: mo: Bi: Co: Fe: Na: B: K: Si: O: 12:1:0.6:7:0.1:0.2:0.1:18: X (where X is a value determined by the oxidation state of the respective metal element). A mixed reaction raw material gas composed of 8 mol% of propylene, 67 mol% of air and 25 mol% of water vapor was fed from the top of the reaction tube of the fixed-bed multi-tube type reactor thereinto at 200 ℃ such that the reaction raw material gas was in contact with the catalyst for 3.5 seconds. In addition, the temperature of the niter (niter) was controlled so that the propylene conversion rate reached 98%. The yield of acrolein was 92.5%.

Example 5: synthesis of acrolein from propane as starting Material (method step a)

The following synthesis is a reproduction of example 1 in US 6,388,129.

Will consist of 90% by volume of O₂And 10% by volume of N₂The gas mixture of composition (modified air) and a recycle gas of 79.7mol/h with a composition of 87.7 vol% propane were converted to obtain propene via oxidative dehydrogenation of propane.

The propylene obtained by this process can be further converted to acrolein by the method as described in example 3 or example 4.

Example 6: synthesis of acrolein from allyl alcohol as starting Material (method step a)

The following synthesis is a reproduction of example 1 in US 2016/23995.

Synthesis of a catalyst with an Sb/Fe ratio of 0.6:

A0.05M solution was prepared by dissolving 2.21g of oxalic acid in 500ml of water at 80 ℃ with stirring. Once dissolution was complete, 140.97g of ferric nitrate nonahydrate were added to the oxalic acid solution while maintaining the temperature at 80 ℃. After the iron nitrate nonahydrate was completely dissolved, 30.51g of antimony (III) oxide was added. The resulting solution was evaporated while keeping the temperature at 80 ℃ under stirring until a viscous solution was obtained, which was subsequently dried in an air (ai1) oven at 120 ℃ for 72 hours. After drying, the resulting product is pressed into pellet form, which is subsequently ground to obtain a powdery product comprising particles having a size between 250 and 630 μm. These particles were then calcined under static air from ambient temperature up to 500 ℃ while a ramp gradient of 1 ℃/min was observed, and then a holding period at 500 ℃ for 8 hours. The catalyst was then left in the oven until the temperature returned to 50 ℃. A catalyst exhibiting an Sb/Fe ratio of 0.6 (i.e., x ═ 0.6) was obtained.

5g of the prepared catalyst was placed in a fixed bed reactor. The reaction was carried out with 7.2% by weight aqueous allyl alcohol. The reactor was heated to 400 ℃ and then the reactants (allyl alcohol/O) were added at atmospheric pressure₂/NH₃). The contact time of the reactants with the catalyst was about 0.1 seconds. The reaction time was 5 hours. The products obtained from the reaction were analyzed after trapping at the reactor outlet in a bubbler maintained at a low temperature (-4 ℃). The resulting liquid was then analyzed on a gas chromatograph equipped with a flame ionization detector. Allyl alcohol/O₂/NH₃The molar ratio is as follows: 1/1.6/0.4; conversion of allyl alcohol 87%, yield: 17% acrylonitrile, 52% acrolein, 5% acetaldehyde, 5% n-propionaldehyde, 1% acetonitrile.

Example 7: synthesis of acrolein from acetol as starting Material (method step a)

The following synthesis is a reproduction of example 7 in US 2016/23995.

Dehydration of acetol to acrolein via activated carbon and phosphoric acid at 290 ℃, with complete conversion of acetol and selectivity of acrolein > 55%.

Example 8: synthesis of n-propionaldehyde from acrolein as starting Material (method step b)

The following synthesis is a reproduction of example 1 in DE 755524.

In the molar ratio of aldehyde to hydrogen of 1: the experiment was carried out in the case of 2 and at an operating pressure of 5 bar. A 10 wt% aqueous solution of acrolein and hydrogen were fed to the preheater where the mixture was heated to about 150 ℃. The resulting mixed gaseous stream is then fed to a reactor containing a catalyst (at Al)₂O₃2% by weight of Pd-and a catalyst in which the Pd has been concentrated to the outermost surface of the catalyst (in Al)₂O₃Upper 0.18 wt% Pd)). Complete conversion of acrolein was observed. The selectivity to n-propanal was about 85%. The byproducts are hydroxyacetone, CO and CO₂。

Example 9: synthesis of Methacrolein (MAL) from n-propionaldehyde and formaldehyde as starting materials (method step c)

n-Propionaldehyde (PA) was continuously reacted with formaldehyde using Dimethylamine (DMA) and acetic acid (AcOH). 251g/h of PA and 349g/h of 37% formalin solution were premixed homogeneously (molar ratio 1: 1). 18.7g/h of a catalyst solution having 24.8% dimethylamine and 37.9% acetic acid was passed to preheater 12. The two streams are heated to a temperature of 170 ℃ before they are combined. The preheated streams were combined in a T-mixer directly connected to a tubular reactor (1/16 inch tube, length 4.2 m). The temperature of the reactor was controlled by an oil bath operating at 180 ℃, the residence time was 10 seconds, and the pressure in the tubular reactor was 70 bar. Downstream of the tubular reactor, the mixture is depressurized in a valve 14 and introduced into a column 15. 335g/h of the material discharged at the bottom of the column are returned to the reactor 13 and 370g/h of the material discharged at the bottom of the column are disposed of in the form of waste water. After condensation of the overhead stream in condenser 16 and phase separation in 17, a methacrolein-rich phase having a methacrolein content of 96.5% is discharged as product 111, and the aqueous material discharged from the phase separator is returned to column 15. The conversion was 99.9%, and the yield was 98.1%, based on n-propionaldehyde.

Example 10: synthesis of MMA or methacrylic acid from methacrolein as starting Material (method step d)

Synthesizing a catalyst carrier: in a beaker, 21.36g Mg (NO)₃)₂·6H₂O、31.21g Al(NO₃)₃·9H₂O was dissolved in 41.85g of water by stirring. 1.57g of 60% strength HNO are added with stirring₃. 166.67g of silica sol (c)1530AS, 30% by weight SiO₂The particle size: 15nm) was placed in a 500ml three-necked flask and cooled to 15 ℃ while stirring. 2.57g of 60% strength HNO were slowly added thereto with vigorous stirring₃. The previously prepared nitrate solution was added to the sol at 15 ℃ over 45 minutes. When the addition was complete, the mixture was heated to 50 ℃ over 30 minutes and aged (while stirring) at this temperature for 24 hours. Thereafter, the mixture was spray dried at 130 ℃. The dried powder (spherical, average particle size 60 μm) was heated to 300 ℃ in the form of a thin layer within 2 hours. The temperature was maintained at 300 ℃ for 3 hours, and the temperature was raised to 600 ℃ over 2 hours and maintained there for 3 hours. Metal impregnation of catalyst support: a suspension of 10g of the carrier prepared previously was mixed with 33.3g of water and heated to 90 ℃. It was kept at this temperature for 15 minutes, then Co (NO) was added₃)₂*6H₂A solution of O (569mg,1.95mmol) in 8.3g of water, preheated at 90 ℃ is then stirred for 30 minutes at 90 ℃. After cooling to room temperature, the mixture was filtered and washed six times with 50mL of water each time. The material was dried at 105 ℃ for 10 hours and then carefully ground. Finally, the material was heated from 18 ℃ to 450 ℃ over 1 hour, and held at this temperature for 5 hours.

Noble metal impregnation of metal catalyst: 10g of the previously prepared cobalt catalyst was heated to 90 ℃ in 33.3g of water and kept at this temperature for 15 minutes while stirring. Slow addition of HAuCl₄*3H₂O (205mg) in 8.3g water at 90 ℃ preheat the solution, then when the addition is complete, the mixture is stirred at 90 ℃ for 30 minutes, andand finally cooling to room temperature. The material was isolated by filtration and washed six times with 50mL of water each time. The material was dried at 105 ℃ for 10 hours, carefully ground, and finally calcined at 450 ℃ for 5 hours.

Continuous conversion of methacrolein to MMA/MAS: the pH of the feed containing 42.5 wt.% methacrolein in MMA solution was adjusted to 7 by addition of NaOH in MeOH solution. The feed was continuously added to a stirred and gasified (with air) tank reactor at 10 bar pressure and 80 ℃. The reactor was preloaded with 20g of the previously prepared gold-cobalt catalyst. In addition to the feeds of MeOH and methacrolein, a second feed with 1 wt% NaOH in MeOH was continuously added to the reactor to maintain the pH at 7.0. The reactor was operated at a constant volume level and excess volume was continuously removed via a filter to keep the catalyst inside the reactor. After 2000 hours TOS, the catalyst still had a conversion of 73.8% methacrolein with a selectivity to MMA of 95.5%. Methacrylic acid was additionally prepared with a selectivity of 1%.

Example 11: synthesis of MMA or methacrylic acid from methacrolein as starting Material (method step d)

In this example, a catalyst containing Ni and Au was used.

Synthesizing a catalyst carrier: in a beaker, 21.36g Mg (NO)₃)₂*6H₂O、31.21g Al(NO₃)₃·9H₂O was dissolved in 41.85g of water by stirring. 1.57g of 60% HNO was added while stirring₃. 166.67g of silica sol (c)1530AS, 30% by weight SiO₂The particle size: 15nm) was placed in a 500mL three-necked flask and cooled to 15 ℃ while stirring. 2.57g of 60% HNO were slowly added thereto under vigorous stirring₃. The previously prepared nitrate solution was added to the solution over 45 minutes at 15 ℃. When the addition was complete, the mixture was heated over 30 minutesTo 50 ℃ and aged at this temperature for 24 hours (while stirring). Thereafter, the mixture was spray dried at 130 ℃. The dried powder (spherical particles, average particle size 60 μm) was heated in the form of a thin layer to 300 ℃ over 2 hours, held at 300 ℃ for 3 hours, heated to 600 ℃ over 2 hours and finally held at this temperature for a further 3 hours.

Metal and noble metal impregnation of the catalyst support: a suspension of 10g of the carrier prepared previously was mixed with 33.3g of water and heated to 90 ℃. Held at this temperature for 15 minutes and then HAuCl was added₄*3H₂O (205mg) and Ni (NO)₃)₂*6H₂A solution of O (567mg,1.95mmol) in 8.3g of water preheated at 90 ℃ is then stirred for 30 minutes at 90 ℃. After cooling to room temperature #, the mixture was filtered and washed six times with 50mL of water each time. The resulting material was dried at 105 ℃ for 10 hours and then carefully ground. Finally, the material was heated from 18 ℃ to 450 ℃ over 1 hour, and held at this temperature for 5 hours.

Continuous conversion of methacrolein to MMA/MAS: the pH of the feed containing 42.5 wt.% methacrolein in MMA solution was adjusted to 7 by addition of NaOH in MeOH solution. The feed was continuously added to a stirred and gasified (with air) tank reactor at 10 bar pressure and 80 ℃. The reactor was preloaded with 20g of the previously prepared gold-nickel catalyst. In addition to the feeds of MeOH and methacrolein, a second feed with 1 wt% NaOH in MeOH was continuously added to the reactor to maintain the pH at 7.0. The reactor was operated at a constant volume level and excess volume was continuously removed via a filter to keep the catalyst inside the reactor. After 2000 hours TOS, the catalyst still had a conversion of 73.8 wt% methacrolein with a selectivity to MMA of 95.5%. Methacrylic acid was additionally prepared with a selectivity of 1%.

Example 12: synthesis of MMA or methacrylic acid from methacrolein as starting Material (method step d)

In this example, a catalyst containing Pd and Pb was used. The synthesis is based on example 1 as disclosed in US 6,680,405.

In a 4 liter reactor equipped with a condenser and a stirrer, 350g of a catalyst (calcium carbonate catalyst containing 5% by weight of palladium, 1% by weight of lead and 1% by weight of iron) and a reaction liquid composed of 700g of methacrolein and 1280g of methanol were charged. Methyl methacrylate was synthesized by continuing the reaction at a bath temperature of 80 ℃ and a pressure of 400kPa abs while blowing air and nitrogen gas at rates of 4.77Nl/min and 5.0Nl/min, respectively, for 4 hours. The reaction product was collected and analyzed, and as a result, the conversion of methacrolein and the selectivity of methyl methacrylate were found to be 75.1% and 85.2%, respectively.

Example 13: synthesis of MMA or methacrylic acid from methacrolein as starting Material (method step d)

In this example, a catalyst containing Pd and Pb was used. The synthesis is based on example 1 as disclosed in US 2014/206897.

50.1g of methacrolein was added to the reactor together with 25.2g of methanol (molar ratio of methanol to methacrolein was about 1.1). Approximately 1g of catalyst (e.g., comprising 3 wt% palladium and 2 wt% lead on silica) was added to the solution. The stirrer was turned on and the solution was heated to about 50 ℃. The oxygen flow was started at about 6 milliliters per minute (mL/min). The reactor was opened to atmospheric pressure. The reaction was allowed to proceed for about 4 hours. This resulted in a methacrolein conversion of about 50% with a selectivity to methyl methacrylate of about 90%.

Example 14: oxidation of methacrolein to methacrylic acid in the gas phase (process step d)

Preparation of aqueous slurry a 1: in 105g of ion-exchanged water heated to 40 ℃ were dissolved 38.2g of cesium nitrate [ CsNO ]₃]12.8g of 75% by weight orthophosphoric acid and 12.2g of 67.5% by weight nitric acid to form a liquid alpha. Separately, 138g of ammonium molybdate tetrahydrate [ (NH)₄)₆Mo₇O₂₄·4H₂O]Dissolved in 154g of ion-exchanged water heated to 40 ℃ and then 3.82g of ammonium metavanadate [ NH ]₄VO₃]Suspended therein to form a liquid beta. The liquid α was added dropwise to the liquid β while stirring, and the temperatures of the liquids α and β were maintained at 40 ℃ to obtain an aqueous slurry a 1. The atomic ratios of the metal elements (i.e., phosphorus, molybdenum, vanadium, and cesium) contained in the aqueous slurry a1 were 1.5, 12, 0.5, and 3.0, respectively, and thus the atomic ratio of cesium to molybdenum was 3.0: 12.

Preparation of aqueous slurry B1: 14.6g of 75 wt% orthophosphoric acid and 13.9g of 67.5 wt% nitric acid were dissolved in 120g of ion-exchanged water heated to 40 ℃ to form a liquid a. Separately, 158.2g of ammonium molybdate tetrahydrate was dissolved in 176g of ion-exchanged water heated to 40 ℃ and then 4.37g of ammonium metavanadate was suspended therein to form liquid b. Liquid a was added dropwise to liquid B while stirring, and the temperatures of liquids a and B were maintained at 40 ℃ to obtain aqueous slurry B1. The atomic ratios of the metal elements (i.e., phosphorus, molybdenum, and vanadium) contained in the aqueous slurry B1 were 1.5, 12, and 0.5, respectively, and thus the atomic ratio of cesium to molybdenum was 0: 12.

Preparation of aqueous slurry M1: the entire amount of aqueous slurry B1 was mixed with the entire amount of aqueous slurry a1, and then the mixture was stirred in a closed vessel at 120 ℃ for 5 hours. Then, 10.2g of antimony trioxide [ Sb ] was added to the mixture₂O₃]And 10.1g of copper nitrate trihydrate [ Cu (NO)₃)₂，3H₂O]Suspension in 23.4g of ion-exchanged water, and the mixture was further stirred in a closed vessel at 120 ℃ for 5 hours to obtain an aqueous slurry M1. The aqueous slurry M1 was dried by heating it in air at 135 ℃ to evaporate water therefrom. To 100 parts by weight of the dried product were added 4 parts by weight of ceramic fiber, 17 parts by weight of ammonium nitrate and 7.5 parts by weight of ion-exchanged water, and the mixture was kneaded and extrusion-molded into cylinders each having a diameter of 5mm and a height of 6mm, respectively. The molded cylinders were dried at 90 ℃ and 30% relative humidity for 3 hours and then calcined by holding them in a flow of air at 390 ℃ for 4 hours and then in a flow of nitrogen at 435 ℃ for 4 hours to obtain the catalyst. The catalyst comprises a heteropoly acid compound and is used for the heteropoly acid reactionThe atomic ratio of the metal elements other than oxygen (i.e., phosphorus, molybdenum, vanadium, antimony, copper and cesium) contained in the compound was 1.5:12:0.5:0.5:0.3:1.4, respectively, and thus the atomic ratio of cesium to molybdenum was 1.4: 12.

9g of the catalyst synthesized as described above was charged into a glass microreactor having an inner diameter of 16mm, and a starting gas composed of 4% by volume of methacrolein, 12% by volume of molecular oxygen, 17% by volume of water vapor and 67% by volume of nitrogen, which was prepared by mixing methacrolein, air, water vapor and nitrogen, was allowed to stand for 670h^-1Is fed into the reactor and the reaction is carried out for one hour at an oven temperature of 355 c (the temperature of the oven used to heat the microreactor). Then, a starting gas having the same composition as described above was fed into the microreactor at the same space velocity as described above, and the reaction was restarted at a furnace temperature of 280 ℃. After 1 hour from the restart of the reaction, the outlet gas (gas after the reaction) was sampled and analyzed by gas chromatography, and the conversion (percentage) of methacrolein, the selectivity (percentage) for methacrylic acid of 80%, and the yield of methacrylic acid of 77% were obtained at a conversion of 96%.

Example 16: direct conversion of glycerol to n-propanal without condensation

Crude glycerol (containing water, containing salts (mainly NaCl and KCl)) is converted to acrolein over a catalyst bed, with hydrogen being used to reduce the partial pressure of the reactants. A catalyst layer composed of 56g of a catalyst supported on ZrO was used₂Above 10% by weight of WO₃Composition of, the ZrO₂Is a grain with a size of 20 to 30 mesh. The inlet liquid stream consisted of 20 wt% crude glycerol in water fed to the preheater at 0.3 g/min. A gas stream containing at least 100ml/min of hydrogen is also fed to the preheater. The liquid stream is preheated and vaporized to a temperature of 250 to 300 ℃ (ideally 280 ℃) prior to entering the reactor. The inlet to the reactor is maintained at 275 to 325 c (ideally 300 c) and a pressure in the range of 1 to 5 barg is applied to the reactor. The outlet stream is formed from a large amount of acrolein and by-productsThe materials n-propionaldehyde (propionaldehyde) and acetol (except for most of the water) are made up and further cooled (but not condensed) to a temperature in the range of 130 to 200 ℃ (ideally 170 to 180 ℃). Feeding the stream directly to a reactor containing an eggshell catalyst (at Al)₂O₃Upper 2 wt% Pd). The composition of the stream leaving the reactor does not contain any glycerol or acrolein. The stream is cooled to condense water and n-propanal, and then the pressure is reduced to 0 to 3 barg. Thereby obtaining liquid and gaseous streams. The gaseous stream is split into a purge stream (purge) (1 to 20 vol% of the total volume) -discarded-and a recycle stream, which is used as hydrogen feed to the first reactor. The gaseous feed is typically composed of 26 wt% n-propanal, 1 wt% H₂O, 21% by weight of H₂And 52 wt% CO, and is mixed with a second gaseous feed from the distillation of n-propanal, which is typically composed of 55 wt% n-propanal, 1 wt% H₂O, 4% by weight of H₂1.5% by weight of CO, 23% by weight of ethane and 2% by weight of ethylene. The gas stream obtained is enriched with pure hydrogen to meet the requirements as hydrogen feed to the first reactor. Surprisingly, none of the components in this feed had a measurable negative impact on the performance of the first two steps. The liquid stream obtained before (together with the gaseous stream) generally consists of 12.1% by weight of n-propanal, 87.5% by weight of water and 0.2% by weight of acetol. In addition to this, some dissolved gases are present, such as 0.2 wt% ethane and ethylene. The liquid was fed to the lower third of the distillation column to separate n-propionaldehyde as a first-run product, 96.5% pure, the remainder being 2.7% water and dissolved gases, which were separated by cooling the mixture to condense n-propionaldehyde and water as final products. The gaseous streams are combined as previously described. The water leaving the column as a bottoms product contains mainly water and high boiling by-products, such as acetol. Surprisingly, this stream cannot be used as a water feed to the first reactor and must be discarded. The n-propionaldehyde thus obtained is converted into methacrolein in a next step。