The present invention relates to processes for preparing methacrylic acid and/or alkyl methacrylates by heterogeneous-catalytic partial oxidation of organic compounds.

The preparation of methacrylic acid and/or alkyl methacrylates having 1 to 8 C atoms in the alkyl radical, more particularly methyl methacrylate (MMA), by heterogeneous-catalytic partial oxidation of isobutene and/or tert-butanol (TBA) in the gas phase has spread rapidly since 1982 especially in the Asiatic sphere. Also used in some cases is methyl tert-butyl ether (MTBE), which is available in large quantities, and which is cleaved beforehand into isobutene and methanol by a separate, upstream process. The methanol released in this procedure can be used again at least predominantly in the later esterification stage for the preparation of MMA. The process is characterized generally by continuous operation.

Processes of this kind are described in publications including DE 197 40 252, EP 0 376 1 17, EP 1 192 992, US 4 260 822, WO 02/081421 and WO 2004/089856.

The process has numerous advantages. As compared with the acetone cyanohydrin process, which is the otherwise most commonly operated conventional process, there is no need to use large and superstoichiometric amounts of concentrated sulphuric acid or to recycle it, a costly and inconvenient operation, for purification. Moreover, there is also no need for prussic acid, which has to be prepared separately and with special precautions on account of its toxicity.

A disadvantage, however, is found to be the relatively low operational yield of only around 62% to 68% on a molar basis, depending on process configuration and oxidation catalysts used, which typically are based on specific knowledge on the part of the manufacturers concerned. As a result of this relatively low operational yield, the process, despite its above- identified advantages, is fairly marginal in competition, depending on raw-materials prices and product prices; in other words, when all of the production costs are added up, there is no particular advantage over the acetone cyanohydrin process, with its apparent cost and inconvenience, which has a 91 % to 93% molar operational yield. By-products of the heterogeneously catalyzed partial oxidation in the gas phase are particularly oxidation products, beginning with CO/C0 ₂ and formaldehyde, and continuing through to higher-boiling compounds, such as phthalic acid, terephthalic acid and isophthalic acid. Unless they are gases, these by-products are found initially, in the course of operation, in the quench condensate of the off-stream from the heterogeneous-catalytic partial oxidation, this off-stream comprising predominantly water, and being recovered for the purpose of collecting the methacrylic acid produced. Some of this water is a product of the oxidation reactions. In addition, water is added to the reactant in order to stabilize the reaction mixture, or is added as fresh water as part of the quenching operation.

The crude methacrylic acid, which is the initial target product, is typically extracted as selectively as possible from this quench condensate by means of liquid-liquid extraction. The extractant used is typically heptane or a similarly boiling hydrocarbon, for example toluene, MMA or a mixture of these compounds. The extractant is distilled off continually from this extract, for repeated use, and is passed back to the extraction. In the water phase (extraction raffinate) running off from the liquid-liquid extraction, the oxidation by-products described above are present, as is a residual fraction, not recovered in the preliminary stage, of the target or intermediate-target product, methacrylic acid. This process wastewater, on account of its very high organic components content of up to 20% by weight, is typically burnt in special combustion units. In that case, all of the water present, which as is known has an extremely high vaporization enthalpy of 2257 kJ/kg, must necessarily be evaporated and heated to combustion temperature as well. This necessitates additional fuel in the form of heating gas, heating oil or other combustible materials. A result of this demand for fuel is that the process, which is otherwise very elegant and very well-situated in terms of raw materials, is not much superior in terms of competition, but instead must be classed more as marginal.

In light of the prior art, then, it is an object of the present invention to provide particularly economic processes for preparing methacrylic acid and/or alkyl methacrylates.

One particular object was to reduce the high energy requirements for work-up and for the elimination of the by-products. A further objective of the present invention may be seen as being that of specifying a process for preparing methacrylic acid and/or alkyl methacrylates which can be realized with particular simplicity and cost-effectiveness. These objects and others which, though not explicitly mentioned, can nevertheless be inferred or perceived from the introductory discussions herein, are achieved by a process for preparing methacrylic acid and/or alkyi methacrylates that has all of the features of Claim 1 . Useful modifications of the process of the invention are protected in dependent claims.

As a result of the fact that at least one carboxylic acid having 2 to 5 carbon atoms is isolated from the wastewater stream produced in a process for the continuously operated preparation of methacrylic acid and/or alkyi methacrylates having 1 to 8 C atoms in the alkyi radical, a particularly economic process is provided, surprisingly, for the continuously operated preparation of methacrylic acid and/or alkyi methacrylates having 1 to 8 C atoms in the alkyi radical, by heterogeneous-catalytic partial oxidation of isobutene and/or tert-butanol in the gas phase.

As a result of the measures taken according to the invention, moreover, it is possible to achieve the following advantages among others:

Through the process of the invention, the high energy demand associated with work-up and the elimination of by-products can be reduced significantly.

Furthermore, the process of the invention for preparing methacrylic acid and/or alkyi methacrylates can be realized in a particularly simple and cost-effective way.

The process of the present invention can be carried out in known units and on an industrial basis.

The process of the invention serves for the preparation of methacrylic acid and/or alkyi methacrylates having 1 to 8 C atoms in the alkyi radical by heterogeneous-catalytic partial oxidation of organic compounds in the gas phase. Organic compounds which can be used for preparing methacrylic acid and/or alkyi methacrylates having 1 to 8 C atoms in the alkyi radical are known in the art. Among others, for example, it is possible to use isobutene and/or tert-butanol. Furthermore, methyl tert-butyl ether (MTBE), which is available in large quantities, can be used as well, being cleaved beforehand into isobutene and methanol in a separate, upstream process. The methanol released in this operation can be used again at least predominantly in the later esterification stage for the preparation of MMA. The process is characterized generally by continuous operation.

These processes are well-established and are described in, for example, publications DE 197 40 252, EP 0 376 1 17, EP 1 192 992, US 4 260 822, WO 02/081421 and WO 2004/089856, some of these documents referring to additional literature.

In this process, typically, methacrylic acid is obtained first of all, and can be esterified in further process steps, in which case, preferably, alcohols having 1 to 8 carbon atoms, more preferably 1 to 4 carbon atoms, can be used, such as, for example, methanol, ethanol, n-propanol, isopropanol, butanol, pentanol, hexanol, cyclohexanol, heptanol and octanol.

In accordance with the process of the invention, at least one carboxylic acid having 2 to 5 carbon atoms is isolated from the wastewater stream produced in this process. As already set out, the methacrylic acid produced is recovered generally from what is called a quench condensate, comprising predominantly water. From this quench condensate, typically, the crude methacrylic acid which is the initial target product is extracted as selectively as possible by means, for example, of liquid-liquid extraction. The extractant used is typically heptane or a similarly boiling hydrocarbon, such as toluene, MMA or a mixture of these compounds. The wastewater stream for the purposes of the invention is the water phase from which a large part of the crude methacrylic acid has been removed (extraction raffinate). In accordance with the invention, at least one carboxylic acid having 2 to 5 carbon atoms is isolated from this wastewater stream. These carboxylic acids include acetic acid, propanoic acid, butanoic acid and/or pentanoic acid. The process of the invention permits in this case the isolation of the individual carboxylic acids or mixtures of these carboxylic acids.

In this case, in accordance with the invention, the carboxylic acids can be isolated directly, particularly as a mixture. Furthermore, the carboxylic acids can also be isolated as esters. For this purpose the carboxylic acids may first be separated from the water phase and concentrated, in order to shift the equilibrium of the reaction in the direction of the ester. Particularly suitable esters derive more particularly from alcohols having 1 to 8 carbon atoms, preferably 1 to 4 carbon atoms. Linear, branched or cyclic alcohols may be used in this reaction. These alcohols include more particularly methanol, ethanol, n-propanol, isopropanol, butanol, pentanol, hexanol, cyclohexanol, heptanol and octanol. The esterification is widely known per se and is set out in Ullmanns's Encyclopaedia of Industrial Chemistry, Fifth Edition, for example.

In a first step, for removing the carboxylic acids from the wastewater stream, an extraction can be carried out. For this purpose, for example, a liquid-liquid extraction can be carried out, the selected extractant being isolated from the carboxylic acids and higher-boiling accompanying compounds by means of a distillation procedure, and returned to the liquid- liquid extractor.

Examples of extractants which can be used include hydrophobic organic solvents, such as, for example, alcohols, ketones, aldehydes, esters, ethers, hydrocarbons and mixtures of these solvents with one another and also with further solvents. The expression "hydrophobic solvent" implies that the miscibility of this solvent with water ought to be extremely low. On the other hand, the extractant ought to be able to accommodate an extremely high fraction of by-products from the wastewater from the preparation of methacrylic acid. These by-products in some cases have a high fraction of polar groups.

The alcohols which can be employed as extractants include, among others, alcohols having 4 or more carbon atoms, preferably 4 to 12 and more preferably 4 to 9 carbon atoms. The alcohols may have a linear, branched or cyclic structure. Furthermore, the alcohols may comprise aromatic groups or substituents, such as halogen atoms. These alcohols comprise, for example, 1 -butanol, 2-butanol, isobutyl alcohol and other C ₄ alcohols; 1 -pentanol, isoamyl alcohol, tert-amyl alcohol, 2-pentanol and other C ₅ alcohols; 1 -hexanol, 2-methyl-1 -pentanol, 3-methyl-1 -pentanol, 2,2-dimethyl-1 -butanol, 2-ethyl-1 -butanol, 4-ethyl-1 -pentanol, 2-hexanol, 3-hexanol, 3-methyl-2-pentanol, 2,3-dimethyl-2-butanol, 2-methyl-3-pentanol, 3-methyl-3- pentanol, 4-methyl-2-pentanol, 2-methyl-2-pentanol and other C ₆ alcohols; n-heptyl alcohol, 2-methyl-1 -hexyl alcohol, 3-methyl-1 -hexyl alcohol, 4-methyl-1 -hexyl alcohol, 5-methyl-1 - hexyl alcohol, 2-ethyl-1 -pentanol, 3-ethyl-1 -pentanol, 2,2-dimethyl-1 -pentanol, 3,3-dimethyl-

1 - pentanol, 4,4-dimethyl-1 -pentanol, 2,3-dimethyl-1 -pentanol, 2,4-dimethyl-1 -pentanol, 3,4- dimethyl-1 -pentanol and other C ₇ alcohols; 1 -octanol, 2-methyl-1 -heptanol, 3-methyl-1 - heptanol, 4-methyl-1 -heptanol, 5-methyl-1 -heptanol, 2-octanol, 3-octanol, 4-octanol, 2- methyl-2-heptanol, 3-methyl-2-heptanol, 4-methyl-2-heptanol, 5-methyl-2-heptanol, 6-methyl-

2- heptanol, 2-methyl-3-heptanol, 3-methyl-3-heptanol and other C ₈ alcohols; and 1 -nonanol and other C ₉ alcohols; cyclopentanol, cyclohexanol and other cyclic alcohols; and benzyl alcohol and other aromatic alcohols.

The ketones which can be used as extractants include, for example, methyl ethyl ketone, diethyl ketone, methyl propyl ketone, methyl isopropyl ketone, methyl butyl ketone, methyl 1 - methylpropyl ketone, methyl-2-methylpropyl ketone, ethyl propyl ketone and other ketones having 4 or more carbon atoms, preferably 4 to 12 and more preferably 4 to 9 carbon atoms.

The aldehydes which can be used as extractants include, for example, butyraldehyde, valeraldehyde, benzaldehyde and other aldehydes having 4 or more carbon atoms, preferably 4 to 12 and more preferably 4 to 9 carbon atoms.

The ethers which can be used as extractants include, among others, diethyl ether, di-n- propyl ether, diisopropyl ether, di-n-butyl ether, diisobutyl ether and other ethers having 4 or more carbon atoms, preferably 4 to 12 and more preferably 4 to 9 carbon atoms.

Extractants which can be used include, for example, aliphatic, alicyclic and aromatic hydrocarbons. These hydrocarbons include, among others, pentane, hexane, more particularly n-hexane and 3-methylpentane, heptane, more particularly n-heptane and 3- methylhexane, octane, cyclopentane, cyclohexane, benzene, toluene, xylene and ethylbenzene.

With particular preference it is possible to use esters as extractants. Preferred esters have 2 to 5 carbon atoms in the acid residue and 1 to 8 carbon atoms in the alcohol residue. They include more particularly esters of ethanoic acid, propanoic acid, butanoic acid and pentanoic acid, such as, for example, methyl acetate, ethyl acetate, propyl acetate, butyl acetate, pentyl acetate, hexyl acetate, octyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, pentyl propionate, hexyl propionate, octyl propionate, methyl butyrate, ethyl butyrate, propyl butyrate, butyl butyrate, pentyl butyrate, hexyl butyrate and octyl butyrate, with ethyl acetate being particularly preferred.

The extractant preferably has a boiling point in the range from 10 to 200 °C, more preferably in the range from 20 to 120°C and very preferably in the range from 40 to 90 °C under atmospheric pressure (1024 mbar). The pressure at which the extraction for removing the carboxylic acids or carboxylic esters from the wastewater stream can be carried out is not critical. Hence the extraction may take place under subatmospheric, superatmospheric or atmospheric pressure. This extraction is preferably carried out under a pressure in the range from 0.02 to 10 bar, more preferably under a pressure in the range from 0.5 to 2 bar.

The extraction can be carried out in a wide temperature range. The extraction takes place preferably at a temperature in the range from 0 °C to Ι δΟ 'Ό, more preferably in the range from 10 to 90 °C and very preferably in the range from 20 °C to 50 °C.

To carry out the liquid-liquid extraction it is possible to use customary apparatus, preferably operated in countercurrent. This apparatus may feature, for example, bubble columns, packed columns, pulsed columns, columns with rotating internals and/or mixer-settler batteries.

For the purpose of removing the carboxylic acids and the higher-boiling organic compounds from the extractant it is possible to employ distillation procedures, which may take the form of a simple distillation or special variants. The special variants include, in particular, rectification, also called countercurrent distillation.

The carboxylic acids having 2 to 5 carbon atoms can be separated from higher-boiling accompanying components by means of at least one distillation procedure, more particularly by rectification. The higher-boiling accompanying components separated from the carboxylic acids and/or carboxylic acid derivatives may be passed on preferably for thermal utilization by combustion.

In accordance with one preferred embodiment of the process of the invention, carboxylic acids having 2 to 5 carbon atoms may be separated by means of rectification procedures. This may take place directly or after partial or complete esterification of these carboxylic acids with an alcohol having 1 to 8 carbon atoms, preferably 1 to 4 carbon atoms. It is possible here to use linear, branched or cyclic alcohols. These alcohols include more particularly methanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol, pentanol, hexanol, cyclohexanol, heptanol and octanol, with methanol, ethanol, n-butanol or isobutanol being preferred. The esters obtained in this way can be separated by rectification procedures. The distillation conditions are guided by factors including the carboxylic acids present in the wastewater stream, the intended separation efficiency, the apparatus used, and the alcohols used optionally for the esterification. Correspondingly, these conditions may lie within a wide range, and optimization is a very simple matter to the skilled person. Hence the distillation may be carried out at subatmospheric, superatmospheric or atmospheric pressure. The distillation procedures, as for example the rectification procedures, are carried out advantageously under a pressure in the range of 0.002-5 bar, more particularly 0.02 to 3 bar and more preferably 0.1 to 0.5 bar.

The distillation plants used preferably in the present process comprise, in particular, customary stills and special embodiments, examples being rectifiers. These distillation plants are general knowledge, and these plants may generally comprise columns having one or more separating stages. The number of separating stages for the purposes of the present invention means the number of trays in the case of a tray column, or the number of theoretical plates in the case of a column containing ordered packing or a column with random packing elements.

Examples of a multi-stage distillation column with trays include those such as bubblecap trays, sieve trays, tunnel-cap trays, valve trays, slotted trays, sieve-slotted trays, sieve- bubblecap trays, nozzle trays, centrifugal trays; examples of a multi-stage distillation column with random packing elements include those such as Raschig rings, Lessing rings, Pall rings, Berl saddles and Intalox saddles; and examples of a multi-stage distillation column with ordered packing include those of the Mellapak (Sulzer), Rombopak (Kuhni) and Montz-Pak (Montz) types and also packings with catalyst pockets, an example being Kata-Pak.

A distillation column with combinations of regions of trays, of regions of random packing elements or of regions with ordered packings may also be used.

The column may be fitted with internals. The column may preferably have a condenser for condensing the vapour, and a liquid-phase evaporator.

The organic compounds present in the wastewater produced after the extraction may preferably be isolated in an optional step by distillation procedures, including rectification procedures, for example, and/or by stripping, and returned to the process. The organic compounds include, in particular, extractants and also compounds having a low boiling point. In the case of stripping or expulsion, gases, such as air, nitrogen or water vapour, are passed through the wastewater, with organic compounds being transferred into the gas phase and separated off.

Surprisingly it has been found that the wastewater produced in the present process can be purified by means of biological treatment. In this context it must be stated that the aqueous phase obtained after the extraction step includes a substantially lower proportion of organic compounds than before the extraction step, with the compounds present in the wastewater generally having good biodegradability. The biological treatment preferably comprises at least one aerobic stage, more preferably at least one anaerobic stage and at least one aerobic stage.

With reference to Figure 1 , the process of the present invention is elucidated below in greater detail, without any intention that this should constitute a restriction.

Figure 1 shows a preferred plant for implementing the process of the invention. Line 1 guides a wastewater stream, from a process for preparing methacrylic acid and/or alkyl methacrylates having 1 to 8 C atoms in the alkyl radical by heterogeneous-catalytic partial oxidation of isobutene and/or tert-butanol in the gas phase, into an extractor 2.

The organic phase can be introduced via line 3 into a distillation plant 4. Line 5 allows the aqueous phase to be transferred into a unit for stripping, 6. The organic compounds recovered by stripping, including especially the extractant, can be passed back via line 7 into the extractor 2. The processed aqueous phase can then be transferred via line 8 into a biological treatment plant.

The organic phase transferred into the distillation plant 4 is separated, with the extractant in particular being separated from the other organic compounds. The extractant may preferably be selected such that its boiling point is lower than the boiling point of the majority of organic compounds present in the wastewater. The by-products obtained by the process for preparing methacrylic acid and/or alkyl methacrylates can be accumulated preferably in the liquid phase, while the extractant can be removed from the distillation unit 4 overhead. The extractant recovered can be passed back via line 9 into the extractor 2.

The by-products obtained in the liquid phase of the distillation plant 4 can be transferred via line 10 into a further distillation plant 1 1 . In the second distillation plant 1 1 , the by-products are separated, and more particularly the compounds having a high boiling point, which are often obtained in a low concentration as a by-product in the preparation of methacrylic acid, can be separated off.

The choice of distillation conditions and the number of removal points are dependent accordingly on factors including the by-product spectrum, and so economic considerations may be critical in the choice of distillation apparatus and in the compounds that are to be isolated.

The higher-boiling compounds may be taken from the liquid phase of the second still 1 1 , and methacrylic acid, for example, may be isolated by a distillation plant 13 from this liquid phase, depending on the choice of distillation parameters. The remainder of the liquid phase may be taken from the plant via line 14.

From the overhead stream in the distillation plant 1 1 it is possible to separate off organic compounds, more particularly carboxylic acids with a low boiling point. These carboxylic acids may be separated in a further distillation plant (not shown) or converted into the esters. In the plant depicted in Figure 1 , the concentrated carboxylic acids are transferred via line 12 into a reactor 15, in which the carboxylic acids are reacted with at least one alcohol, for example methanol, ethanol, n-butanol or isobutanol, to give the corresponding esters.

The products obtained are transferred from the reactor 15 via line 16 into a further distillation plant 17, for example a still or a rectifier, and separated. The higher-boiling carboxylic acids may be transferred via line 18 from the liquid phase of the distillation plant 17 into the reactor 15, while the low-boiling esters, the water obtained, and unreacted alcohol can be transferred via line 19 into a phase separator 20. In the phase separator 20, the alcohol used for the esterification may be separated from the product ester, by adding water via line 21 , for example, with a large proportion of the alcohol transferring to the aqueous phase.

The aqueous phase can be transferred via line 22 into a further distillation plant 23, the alcohol being separated from the water. The alcohol recovered may be introduced via line 24 into the reactor 15. The aqueous phase can be purified biologically or introduced into the phase separator 20. The alcohol, or azeotrope of alcohol and water, that is used in the present example has a lower boiling point than water, and so water is recovered from the liquid phase, and the alcohol is recovered overhead. With a different set-up, the water may also be removed overhead and the alcohol from the liquid phase.

The organic phase obtained in the phase separator 20 may be transferred via line 25 into a further distillation plant 26, in which the esters obtained are separated. The alcohol present in the organic phase can often be taken off overhead from the distillation plant and introduced into the phase separator 20 via line 27, for example. This separation may also take place, for example, in the form of an azeotrope.

In the liquid phase, the ester having the highest boiling point is isolated, and can be removed from the plant via line 28. Further desired esters of the carboxylic acid may be taken from the distillation plant 26 at various points, with resultant products being able to be transferred to additional plants 30, via lines, as for example line 29, and purified. These esters may be taken from the plant via line 31 , for example.

The distillation plants 4, 1 1 , 13, 17, 23 and 26 may be designed as conventional stills having one or more separating stages, or as rectifiers.

For example, ethyl acetate as extractant and ethanol as alcohol may be used for preparing the esters. In that case the plant may be operated such that carboxylic acids with a boiling point higher than that of acrylic acid can be taken from the liquid phase of the distillation plant 1 1 . Methacrylic acid may be isolated from the liquid phase via the distillation plant 13, for example. The further organic compounds from the liquid phase may be passed on for energy recycling. The carboxylic acids recovered overhead in the distillation plant 10, more particularly acetic acid and acrylic acid, can be converted with ethanol into the corresponding esters, for example ethyl acetate and ethyl acrylate. These esters are subsequently separated in distillation plant 26, with ethyl acrylate being recovered in the liquid phase and ethyl acetate at a middle point in the distillation plant; the ethyl acetate can be purified further in plant 30.