DE102010002603A1 - Producing mono- and dicarboxylic acids, useful in pharmaceutical and plastic industries, comprises oxidatively splitting oxidized derivatives of vegetable oil or fat with molecular oxygen or air using gold-containing catalyst and solvent - Google Patents

Abstract

Producing mono- and dicarboxylic acids from oxidized derivatives of plant oils or fats, comprises oxidative splitting of oxidized derivatives of vegetable oils or fats with molecular oxygen or air in the presence of a gold-containing catalyst and a solvent at pH values of 7-14.

Description

Field of the invention

The invention relates to a catalytic process for the preparation of mono- and dicarboxylic acids from oxidized derivatives of vegetable oils and fats by oxidative cleavage with molecular oxygen or air.

Technological background and state of the art

Renewable raw materials, especially vegetable oils and fats, are again gaining importance as a basis for the production of fine chemicals and chemical intermediates. In addition, energy use is also being driven forward, for example in the form of biodiesel. Renewable resources thus compete with petrochemical resources, and in some areas their substitution can already be observed. Vegetable oils and fats contain a high proportion of unsaturated fatty acid residues. Thus, not only a chemical change in the ester group is possible, but in many ways reactions can take place at the double bonds. These include hydrogenations, C-C bond and ring closure reactions, isomerizations, carbonylations and, in particular, oxidations. Noncatalytic and catalytic oxidations give access to epoxides, di- and polyols, keto compounds, and mono- and dicarboxylic acids with medium chain lengths (see eg A. Köckritz, A. Martin, Eur. J. Lipid Sci. Tech. 110 (2008) 812-824 ). The former find extensive applications in the production of polyesters, polyethers, epoxies, surfactants or oligomeric ethers.

In particular, dicarboxylic acids such as adipic acid, suberic acid, azelaic acid, sebacic acid and 1,12-dodecanedioic acid are mainly used for the production of polyamides, polyurethanes and polyesters, of lubricants, plasticizers, adhesives, pharmaceutical products and chemical intermediates.

Also monocarboxylic acids with chain lengths of 6 or more carbon atoms have great technical importance. For example, pelargonic acid is used in the production of lubricating oils, peroxides, perfumes and alkyd resins. Fatty acids in the true sense with 12 or more carbon atoms are used in the production of fatty alcohols, amines, fatty acid esters, metal soaps, plastics, detergents, soaps, cosmetics, alkyd resins, paints, for textile, leather and paper finishing and for the production of lubricants, candles and rubber materials used.

Previous technical processes for the preparation of aliphatic dicarboxylic acids include, in particular, the ring-opening oxidation of cyclic compounds, the oxidative cleavage of unsaturated monocarboxylic acids, the base-catalyzed cleavage of substituted monocarboxylic acids, the hydrogenation of unsaturated dicarboxylic acids and the oxidation and carbonylation of linear diols.

Considering fatty acids and their derivatives as a well-available raw material source in large quantities, the industrially introduced ozonolysis of unsaturated fatty acids (but also of cyclic olefins) is an important method for the production of dicarboxylic acids. However, it carries considerable risks due to the toxicity of ozone and the risk of explosion.

Sebacic acid is z. B. according to WO98 / 35925 obtained by basic cleavage of ricinoleic acid under pressure and at high temperatures.

The oxidative cleavage of ricinoleic acid with nitric acid leads to suberic acid.

The oxidative degradation of saturated fatty acids with N ₂ O ₄ or mixtures of HNO ₃ / N ₂ O ₄ leads to mixtures of dicarboxylic acids. For example, from palmitic acid pimelic acid and stearic acid sebacic obtained in addition to other dicarboxylic acids. By ozonolysis of palmitic acid, cork acid, sebacic acid and azelaic acid are available alongside other dicarboxylic acids.

For the synthesis of azelaic acid and brassylic acid, a number of methods have been described. The most important is the ozonolysis of oleic acid or erucic acid. In addition, oleic acid can also be used in the presence of RuO ' ₄ or ruthenates ( Y. Nakano et al. J. Am. Oil Chem. Soc. 59 (1982) 163 , of Ru catalysts and Cl ₂ as the oxidant, KMnO ₄ , by means of NaOCl and RuO ₄ or OsO ₄ as catalyst or HNO ₃ Azelaic be cleaved. The fermentation of pelargonic acid also leads to azelaic acid. Brassylic acid can also be obtained from erucic acid by oxidative cleavage with HNO ₃ .

Various variants of carbonylation reactions also make it possible to obtain dicarboxylic acids from fatty acids. Thus, C19-dicarboxylic acids can be obtained from oleic acid or oleates. Industrially used are the Reppe carbonylation, the Koch reaction and the hydroformylation with subsequent oxidation. However, many of these methods synthesize predominantly branched dicarboxylic acids.

In summary, these known methods often use environmentally harmful oxidants, they are sometimes not very selective or multi-stage processes must be used for the production of dicarboxylic acids.

In the interests of a sustainable method for the synthesis of the desired dicarboxylic acid and for reasons of atom economy, it would be desirable to use oxidants such as molecular oxygen or air. In the scientific literature, a few examples of the oxidative conversion of fatty acids and their derivatives with these oxidizing agents are known. Thus, the cleavage of vicinal dihydroxy fatty acids with lacunary polyoxometalates from H ₂ WO ₄ and Co (OAc) ₂ in the WO2007 / 039481 described. Co or else Mn, Fe, Ni, V, Mo, Cr, Nb and Ta were introduced into zeolite lattice and used for the aerobic cleavage of unsaturated carboxylic acids or olefins in the JP 2005-255652 used. Cleavage of methyl oleate with RuCl ₃ / PV ₆ Mo ₆ O ₄₀ ^9- and isobutyraldehyde in BuCN resulted in 55% azelaic and pelargonic acid according to JP 2005-325035 ,

An inexpensive, effective and selective production of mono- and dicarboxylic acids of medium chain length would open up new or extended fields of application, especially in the field of plastics and synthetic fibers. However, it is apparent from the prior art that there is no satisfactory solution for the industrial use in terms of yields, selectivities or suitable for the aerobic oxidative conversion of oxidized derivatives of vegetable oils and fats or the corresponding fatty acid derivatives to shorter-chain mono- and dicarboxylic acids Reaction conditions are. Therefore, with the present invention, such a method is offered.

Summary of the invention

The object of the present invention is therefore to provide a new effective process for the production of mono- and dicarboxylic acids from oxidized derivatives of vegetable oils and fats and their derivatives by oxidative cleavage with molecular oxygen or air.

According to the invention, the process for the preparation of mono- and dicarboxylic acids from oxidized derivatives of vegetable oils or fats consists in reacting the oxidized derivatives of vegetable oils or fats in the presence of a gold-containing catalyst in a solvent at pH values in the range from pH 7 to pH 14 with molecular oxygen in contact.

Vegetable oils which are suitable for the invention are, above all, those which contain monounsaturated higher fatty acids from C ₁₂ , in particular from C ₁₈ . These vegetable oils include sunflower oil, rapeseed oil, peanut oil, olive oil, castor oil, sesame oil, soybean oil, corn oil, palm oil, linseed oil, walnut oil, wheat germ oil, grape seed oil, evening primrose oil, thistle oil, hemp oil, jojoba oil, tung oil, cottonseed oil and mixtures thereof. Particularly preferred are sunflower oil, rapeseed oil, peanut oil, soybean oil, palm oil and linseed oil, especially sunflower oil.

Particularly suitable fatty acids as starting material for oxidized derivatives of the unsaturated fatty acids are oleic acid, elaidic acid, icosenic acid, cetoleic acid, erucic acid, linoleic acid and α- and γ-linolenic acid, in particular oleic acid, icosenoic acid and erucic acid.

Suitable compounds of fats are triglycerides such. B. Trioleate from high-oleic sunflower oil.

"Oxidized derivatives of vegetable oils or fats" in the context of the invention are epoxidized fatty acids or epoxidized fatty acid derivatives or higher saturated and unsaturated monocarboxylic acids (C ₁₂ to C ₂₂ , in particular from C ₁₈ ) or esters thereof which have one or more hydroxide functions in the chain , In the case of the epoxidized fatty acids or fatty acid derivatives, the degree of epoxidation is at least included 80%, in particular higher than 90%, in order to achieve good conditions for the subsequent steps in terms of yield and selectivity.

The epoxidation is usually carried out with an in situ generated peracid from H ₂ O ₂ and formic acid or acetic acid, the acid being circulated.

Such epoxidized fatty acids or fatty acid derivatives are then converted by hydrolytic ring opening into the corresponding acids having one or more OH groups on the chain.

The introduction of OH groups is also possible via the direct hydroxylation of double bonds, z. B. with Os, Ru and Mn compounds.

Particularly preferred for the purposes of the invention are diols which are selectively cleaved in the subsequent catalytic reaction with O ₂ in two shorter chain carboxylic acids, of which at least one acid is a dicarboxylic acid.

So z. B. 9,10-dihydroxystearic acid or its methyl ester cleaved in pelargonic acid and azelaic acid and in the same way a highly-oleic acid-containing epoxidized sunflower oil. 13,14-Dihydroxybehensäure is cleaved in pelargonic acid and brassylic acid. A trihydroxy acid such as 9,10,12-trihydroxystearic acid is cleaved into Oenanthic acid and Azelaic acid.

The catalytic reaction itself proceeds with molecular oxygen or with air and can be carried out at atmospheric pressure or overpressure in the range of 1-50 bar, more preferably 1-10 bar, in particular 1-6 bar.

The reaction temperature is in the range of 0-180 ° C, preferably 60-150 ° C, more preferably 70-120 ° C, especially 70-95 ° C.

Suitable solvents for the catalytic reaction are water, lower aliphatic alcohols having 1 to 5 carbon atoms or polyhydric alcohols. The preferred C ₁ -C ₅ alcohols include methanol, ethanol, n-propanol, propanol, n-butanol, tert-butanol, n-pentanol, i-pentanol. Particularly preferred are ethanol and methanol.

Preferred polyhydric alcohols are glycerin and ethylene glycol.

A particularly preferred solvent is water.

The gold catalyst according to the invention can be present as a gold colloid in the solvent or as a gold particle on a support material.

Suitable support materials are carbon, alumina, titania, zirconia, silica, magnesia, ceria, niobium, tantalum, vanadium, other rare earth oxides, or mixtures thereof.

Other oxides of the rare earth metals include in particular the oxides of scandium, yttrium, lanthanum, samarium and gadolinium.

It is also possible to use catalysts which are mixtures of gold with other noble metals, eg. With platinum, palladium, iridium, rhodium or ruthenium. One or two other precious metals may be components of the gold catalyst. Particularly preferred are Au / Pt or Au / Pd catalysts.

The supported catalysts are prepared by conventional methods by impregnation, precipitation or impregnation. Suitable methods are deposition precipitation, incipient wetness impregnation (IMP) and direct anionic exchange. In the case of "deposition Precipitation" example, the particulate carrier material is brought into contact with a solution of a gold-containing precursor in the presence of urea or NaOH and precipitated after prolonged stirring with a citrate and then calcined the dried precursor material. In this example, concentrations of 0.3-2.5 wt .-% Au are achieved on the support.

The catalytic reaction is carried out at a pH of pH 7 to pH 14. Particular preference is given to pH 8.5 to pH 13, in particular pH 9 to pH 12. The pH values are to be selected in a suitable manner so that the stability of the carrier at the corresponding pH value is ensured in the case of a supported catalyst.

If alcohols are chosen as solvents, 2-6 equivalents, based on the molar amount of an oxidized fatty acid derivative, of a base, preferably NaOH or KOH or mixtures thereof are added.

The reaction is carried out under atmospheric pressure up to a pressure of 50 bar at temperatures up to 180 ° C, preferably 140 ° C discontinuously in a stirred tank. In addition to discontinuous operation, continuous process control is also possible. The oxidized fatty acid derivative or the oxidized fatty acid is added in discontinuous operation to an aqueous solution of a specific pH between pH 7 and pH 14, which contains the catalyst, and after addition of the substrate, the oxygen-containing gas in the required pressure to the reactor pressed or at normal pressure through the reaction solution. During the reaction, the pH is kept constant by means of a suitable titration apparatus, or the required amount of base is added before the beginning of the reaction. The reaction conditions are maintained for the required time. The progress of the reaction can be monitored by sampling and analysis of the sample. After completion of the reaction, the reaction products are extracted from the aqueous solution after adjusting an acidic pH. The analysis of the reaction products is carried out after appropriate derivatization by GC / MS.

The inventive method offers significant advantages over known methods. Renewable raw materials in the form of fatty acid derivatives are used as starting materials, which is advantageous from an ecological point of view. It can be worked in water as a solvent even at atmospheric pressure. The oxidant oxygen is used atom-economically and does not provide waste products. Non-toxic gold-containing catalysts can be supported and thus easily separated.

The invention will be explained in the following exemplary embodiments, which, however, do not restrict the invention.

The reaction of oxidized derivatives of vegetable oils and fats is achieved at mild reaction temperatures with the addition of molecular oxygen in the presence of a suitable catalyst. The process of the invention will be explained using the example of 9,10-dihydroxymethyl stearate 1, but this does not restrict the invention. It has surprisingly been found that an oxidative cleavage of the vicinal diol 1 to form saturated mono- and dicarboxylic acids 2 and 3 succeeds. Scheme 1 shows this reaction.

The use of epoxidized fatty acid derivatives leads under the reaction conditions to the desired dicarboxylic acids and monocarboxylic acids.

example 1

Example 1a Preparation of the catalyst

10 g of Al ₂ O ₃ (Puralox HP 14/150, Sasol) are introduced into 200 ml of distilled water in a 1 l three-necked flask with pH control, and a solution of 200 mg H AuCl ₄ .3H ₂ O in 200 ml H ₂ O is added with stirring at room temperature. A solution of 750 mg urea in 2 ml dist. H ₂ O is added. The suspension is heated to 70 ° C and stirred overnight at this temperature.

After 24 h, 330 mg of urea in 2 ml of dist. Dissolved H ₂ O and the solution was added to the suspension, which is again stirred at 70 ° C overnight. The same procedure is repeated again after about 24 h. The suspension is cooled to room temperature with stirring.

Thereafter, 927 mg of magnesium hydrogen citrate (MgHC ₆ H ₅ O ₇ .5H ₂ O) in 50 ml of dist. Dissolved H ₂ O and adjusted the pH to 7 using 0.1 N NaOH. The solution is added to the suspension at once with stirring.

It is then centrifuged, the precipitate washed with water (3 × 200 ml) and dried first at room temperature for 17 hours, then at 50 ° C. for 4 hours. The precursor material is dried under air for 3 h at 250 ° C (heating rate 1 K / min). This gives 1% Au / Al ₂ O ₃ (urea) as the supported catalyst.

Example 1b Preparation of 9,10-dihydroxymethyl stearate

1. Preparation of the oxidation solution:

50 ml H ₂ O ₂ are added to 116 ml tBuOH. 33 mg MgSO ₄ (anhydrous) are added. With a water bath and ice bath care must be taken that the temperature of the solution does not fall below 30 ° C and rises above 40 ° C. The solution is stirred for 3 hours and then filtered.

2. Dihydroxylation

1.5 mmol (381 mg) of methyltrioxorhenium (MTO) are added to 80 ml of the oxidation solution (s.o.). 0.15 mol of oleic acid are added. The solution is stirred for 24 hours at 50.degree. After completion of the reaction, excess peroxide is decomposed with sodium sulfite. The reaction products are recrystallized first with hexane and then with methyl tert-butyl ether (MTBE).

Example 1c Catalytic Reaction to the Carboxylic Acids

In a 500 ml autoclave, 3.30 g of methyl 9,10-dihydroxystearate (90%) and 200 ml of distilled water are heated to 80 ° C. The pH is adjusted to 10 with 0.5 M NaOH. After addition of 620 mg Au / Al ₂ O ₃ (0.64% Au, 0.02 mmol Au), the autoclave is charged with 5 bar of pure oxygen gas and the solution is stirred for 4 hours. The pH is kept constant during the reaction via a pH-stat by metering in with 0.5 M NaOH. After completion of the reaction, the autoclave is decompressed and cooled to room temperature. The reaction suspension is filtered, the filtrate is acidified with dilute hydrochloric acid and extracted three times with tert-butylmethyleter. The organic phase is washed with water, dried over MgSO ₄ and the yields of products are determined by gas chromatography.
Yield of pelargonic acid: 72% d. Th; Yield of azelaic acid: 64% d. Th.

Example 2

In a 100 ml autoclave, add 316 mg of dihydroxystearic acid (90%) along with 20 ml of 0.25 M NaOH and 61 mg Au / Al ₂ O ₃ (0.64% Au, 0.02 mmol Au). The mixture is heated to 70 ° C under argon and with stirring. The autoclave is then rinsed with synthetic air and the reaction is started by pressing in 25 bar of synthetic air while stirring. After a reaction time of 9 hours, the reaction is carried out analogously to Example 1.

The catalyst is prepared similarly as in Example 1a. The dihydroxystearic acid is prepared as described in Example 1b.
Yield of pelargonic acid: 62% of theory. Th; Yield of azelaic acid: 59% d. Th.

Example 3

3.11 g of epoxidized high oleic acid HO sunflower oil (90% oleic acid) is reacted with Au / Al ₂ O ₃ (2.13% Au, 0.02 mmol Au) analogously to Example 1.

The catalyst is prepared similarly as in Example 1a. The epoxidized HO sunflower oil is converted by ring cleavage and ester cleavage by means of the base used at 80 ° C, 8 bar O ₂ and 4 h reaction time in azelaic and pelargonic acid
Yield of pelargonic acid: 5% of theory Th; Yield of azelaic acid: 10% of theory Th.

Example 4

3.73 g of 13,14-dihydroxybehenic acid (90% of DHB) is reacted analogously to Example 1 with Au / Al ₂ O ₃ (2.13% Au, 0.02 mmol Au) at 70 ° C. and pH = 9 , processed and analyzed.

The catalyst is prepared similarly as in Example 1a. The 13,14-Dihydroxybehensäure is prepared analogously to Example 1b using 0.15 mol of erucic acid. The solution is stirred for 24 h at 60.degree.
Yield of pelargonic acid: 12% d. Th; Yield of brassylic acid: 27% of theory. Th.

Example 5

3.17 g of 9,10-trihydroxystearic acid (80% strength THS) is reacted analogously to Example 1 with Au / Al ₂ O ₃ (2.13% Au, 0.02 mmol Au) at 70 ° C. and pH = 9 , processed and analyzed.

The catalyst is prepared similarly as in Example 1a. The trihydroxystearic acid is prepared analogously to Example 1b using 0.15 mol of ricinoleic acid. The solution is stirred for 24 h at 40.degree.
Yield of Oenanthic Acid: 15% d. Th; Yield of azelaic acid: 16% d. Th.

The following table lists examples 6-26. The percentages according to the name of the reactant relate to the proportion of the starting material in the total mass used [% by mass]. The corresponding basic pH values in Examples 6-23 are achieved by the corresponding 4-12-fold molar excess of NaOH relative to DSME or DS before the start of the reaction.
Legend: DHB = dihydroxybehenic acid; THS = trihydroxystearic acid

The catalyst in Example 21 is the same catalyst as in Example 20, which was separated after the reaction, washed and then used a second time in Example 21 without further treatment. Examples 6-26 Ex. analogous to reactant m _{starting material} [mg] pH * n _NaOH / n _{starting material} catalyst 6 Ex. 2 DSME (90%) 661 4 Au / CeO ₂ 7 Ex. 2 DSME (90%) 661 4 Au / TiO2 8th Ex. 2 DSME (90%) 661 4 Au / Al ₂ O ₃ 9 Ex. 2 DSME (90%) 661 4 Au / MgO 10 Ex. 2 DSME (90%) 661 4 Au / Fe ₂ O ₃ 11 Ex. 2 DSME (90%) 661 4 Au / ZrO ₂ 12 Ex. 2 DSME (90%) 330 2 Au / Al ₂ O ₃ 13 Ex. 2 DSME (90%) 330 6 Au / Al ₂ O ₃ 14 Ex. 2 DSME (90%) 330 8th Au / Al ₂ O ₃ 15 Ex. 2 DSME (90%) 330 10 Au / Al ₂ O ₃ 16 Ex. 2 DSME (90%) 330 12 Au / Al ₂ O ₃ 17 Ex. 2 DSME (90%) 330 5 Au / Al ₂ O ₃ 18 Ex. 2 DSME (90%) 330 5 Au / Al ₂ O ₃ 19 Ex. 2 DSME (90%) 330 5 Au / Al ₂ O ₃ 20 Ex. 2 DS (90%) 317 5 Au / Al ₂ O ₃ 21 Ex. 2 DS (90%) 317 5 Recycling of Cat. Ex. 20 22 Ex. 2 DS (90%) 317 5 Au / Al ₂ O ₃ 23 Ex. 2 DS (90%) 317 5 Au / Al ₂ O ₃ 24 Example 1 DS (90%) 317 9 Au / Al ₂ O ₃ 25 Example 1 DS (90%) 317 10 Au / Al ₂ O ₃ 26 Example 1 DS (90%) 317 11 Au / Al ₂ O ₃ Ex. Au [%] n _Au [mmol] T [° C] t [min] P _O₂ [bar] Yield _PS [%] Yield _AS [%] 6 0.36 0,004 80 640 5 9 6 7 0.85 0,004 80 540 5 22 19 8th 0.64 0,004 80 540 5 37 29 9 0.18 0,004 80 540 5 24 20 10 1.06 0,004 80 540 5 32 25 11 0.29 0,004 80 540 5 26 20 12 0.64 0,002 80 260 5 18 15 13 0.64 0,002 80 260 5 39 34 14 0.64 0,002 80 260 5 42 38 15 0.64 0,002 80 260 5 51 43 16 0.64 0,002 80 260 5 38 32 17 0.64 0,002 60 260 5 16 20 18 0.64 0,002 70 260 5 58 65 19 0.64 0,002 100 260 5 60 72 20 0.64 0,002 70 540 5 50 49 21 70 540 71 66 22 2.13 0,004 70 540 5 46 50 23 3.2 0.01 70 540 5 48 55 24 2.13 0,002 70 240 6 50 50 25 2.13 0,002 70 240 5 56 54 26 2.13 0,002 70 240 5 55 53 Example: example; DSME: 9,10-Dihydroxystearic acid methyl ester
DS: dihydroxystearic acid; PS: pelargonic acid;
AS: azelaic acid;
* pH is kept constant during the reaction by dropwise addition of 0.1 M NaOH

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• WO 98/35925 [0007]
• WO 2007/039481 [0013]
• JP 2005-255652 [0013]
• JP 2005-325035 [0013]

Cited non-patent literature

• A. Köckritz, A. Martin, Eur. J. Lipid Sci. Tech. 110 (2008) 812-824 [0002]
• Y. Nakano et al. J. Am. Oil Chem. Soc. 59 (1982) 163 [0010]

Claims (10)

Process for the preparation of mono- and dicarboxylic acids from oxidized derivatives of vegetable oils or fats, characterized by an oxidative cleavage of oxidized derivatives of vegetable oils or fats with molecular oxygen or air in the presence of a gold-containing catalyst and a solvent at pH values in the range of pH 7 to pH 14. A method according to claim 1, characterized in that as vegetable oils are used those which are selected from sunflower oil, rapeseed oil, peanut oil, olive oil, sesame oil, castor oil, soybean oil, corn oil, palm oil and mixtures thereof. A method according to claim 1, characterized in that are used as oxidized derivatives of saturated or unsaturated C ₁₂ -C ₂₂ fatty acids having one or more epoxide or hydroxide functions in the chain. A method according to claim 1, characterized in that the gold-containing catalyst is introduced in the form of a gold colloid or as gold particles on a support material. A method according to claim 1, characterized in that a second or third noble metal is introduced in addition to gold in the catalyst, wherein the second or third noble metal from the group of platinum, palladium, iridium, rhodium and ruthenium is selected. A method according to claim 4, characterized in that a support material from the group carbon, alumina, titania, zirconia, ceria, niobium, vanadium, tantalum, silica, magnesia, rare earth oxides or their mixtures is used. A method according to claim 1, characterized in that the molar ratio of gold to reactive groups of the oxidized derivative 0.01-30 wt .-%, preferably 0.05-5 wt .-% is. A method according to claim 1, characterized in that the solvent used is water, glycerol, ethylene glycol or a lower aliphatic alcohol having 1-5 carbon atoms, preferably water. A method according to claim 1, characterized in that it is carried out at a pressure in the range of 1 to 50 bar, preferably 1-10 bar. A method according to claim 1, characterized in that one works at a temperature in the range of 0-180 ° C, preferably 60-120 ° C.