WO2016008904A1

WO2016008904A1 - METHOD FOR PURIFYING RAW γ-BUTYROLACTONE

Info

Publication number: WO2016008904A1
Application number: PCT/EP2015/066114
Authority: WO
Inventors: Rolf Pinkos; Olga OSETSKA; Irene DE WISPELAERE; Wolf-Steffen Weissker; Jan Eberhardt
Original assignee: Basf Se
Priority date: 2014-07-16
Filing date: 2015-07-15
Publication date: 2016-01-21

Abstract

The present invention relates to a method for purifying raw γ-butyrolactone that comprises at least one of the compounds 3-tetrahydrofuran-2-yl-propan-1-ol or 1,3-dioxepan-2-yl-methanol.

Description

Method for purifying raw γ-butyrolactone

BACKGROUND OF THE INVENTION The present invention relates to a method for purifying raw γ-butyrolactone that comprises at least one of the compounds 3-tetrahydrofuran-2-yl-propan-1 -ol or 1 ,3-dioxepan-2-yl-methanol.

STATE OF THE ART γ-Butyrolactone (GBL) is used inter alia as a solvent for various reactions, as a versatile solvent for polymers, as a synthetic intermediate for the production of polyamides and herbicides and as a precursor for the synthesis of various pyrrolidones. It is known to synthesize γ-butyrolactone from 1 ,4-butanediol via dehydrocyclization

(see e.g. K. Weissermel, H.-J. Arpe, Industrielle Organische Chemie, 5th edition, 1998, page 1 14). US 6,521 ,763 describes a corresponding process for preparing

γ-butyrolactone by reaction of 1 ,4-butanediol over a copper catalyst. Generally, very pure 1 ,4-butanediole is used in said synthesis. Therefore, it usually results in γ-butyrolactone with a high purity above 99.9 wt.%.

In the industrial preparation of γ-butyrolactone increasing use is made of hydrocarbons having at least four carbon atoms as economical raw materials. Important starting materials are n-butane, n-butenes, benzene, etc. In a first stage, the hydrocarbons are oxidized to give maleic anhydride. In particular this oxidation is performed in the gas phase over heterogeneous catalysts. An overview of current processes is given in Ullmann's Encyclopedia of Industrial Chemistry, 6th Edition, 1999 Electronic Release, Chapter "Maleic and Fumaric Acids, Maleic Anhydride". Depending on the work-up chosen, maleic anhydride or a successor product like maleic acid, its diesters, its monoesters or mixtures thereof are obtained. These compounds are subsequently hydrogenated, usually in the liquid or gas phase over heterogeneous catalysts.

K. Weissermel, H.-J. Arpe, Industrielle Organische Chemie, 4th edition, 1994, pages 399 - 401 describes the synthesis of maleic anhydride by oxidation of butane and the stepwise hydrogenation of maleic anhydride to succinic anhydride, γ-butyrolactone, tetrahydrofurane and 1 ,4-butanediol.

WO 86/07358 describes a process for the production of γ-butyrolactone by

hydrogenolysis of a dialkyl ester of a C₄ dicarboxylic acid in two subsequent hydrogenolysis zones. In a concrete embodiment diethyl maleate is employed. The first reaction mixture still contained significant amounts of diethyl succinate which is the converted to a final product containing ethanol (i.e. the alcohol of the employed diester), tetrahydrofuran, n-butanol, γ-butyrolactone, 1 ,4-butanediol and minor by-products.

The preparation of γ-butyrolactone via oxidation of a C₄ hydrocarbon feedstock and hydrogenation of the resulting C₄ dicarboxylic acid or a derivative thereof leads to product mixtures that afford a complex purification to isolate the contained

γ-butyrolactone. Generally, γ-butyrolactone prepared by this route cannot be isolated with the same purity as γ-butyrolactone that has been synthesized from

1 ,4-butanediole.

It has been found that especially γ-butyrolactone synthesized from C₄ hydrocarbons and in particular butane as starting material, wherein a dialkyl ester of a C₄ dicarboxylic acid is subjected to a hydrogenolysis comprises 3-tetrahydrofuran-2-yl-propan-1 -ol (compound I) and/or 1 ,3-dioxepan-2-yl-methanol (compound II) as impurities:

Other typical by-products of this synthesis route are tetrahydrofuran, 1 ,4-butandiol, the alcohol component of the dialkyl ester, dialkyl succinates and 4-alkoxybutanols.

However, the last-mentioned by-products are typically contained in the reaction product in amounts below 100 ppm and may be easily removed via distillation. On the contrary, it has been found that no significant amounts of compounds I and II may be removed via distillation.

An essential requirement placed on the synthesis of fine chemicals is the consistently high quality of the product. In this connection it is not acceptable that a product contains by-products that are detrimental to the desired use of the product and/or that deteriorate during storage or use. Both compounds I and II hinder the further use of the γ-butyrolactone. Compound I easily reacts with oxygen to form peroxides. Said peroxides may cause several undesirable reactions, especially producing coloured by-products. Compound II reacts with water under formation of a reactive aldehyde. Said aldehyde may also form coloured by-products. Already small amounts of compounds I and II in the ppm-range may result in said undesired colouration. Any product based on γ-butyrolactone that comprises impurities of compound I and/or II is practically unsaleable.

Therefore, it is an object of the invention to provide a method for separating compounds I and II from γ-butyrolactone, especially without producing any further problematic by-products and without lowering the yield of γ-butyrolactone.

Surprisingly, it has now been found that this object is achieved if a raw γ-butyrolactone that comprises at least one of the compounds I or II is homogeneously or

heterogeneously brought into contact with an acidic compound.

SUMMARY OF THE INVENTION

The invention provides a method for purifying γ-butyrolactone, wherein a) a raw γ-butyrolactone is provided that comprises at least one of the compounds I or II

b) the raw γ-butyrolactone is subjected to a treatment with at least one acidic

compound, wherein, compared to the starting material, a product mixture depleted in at least one of the compounds I or II is obtained, c) a purified γ-butyrolactone is separated from the product mixture obtained in step b).

The invention further relates to a purified γ-butyrolactone, obtainable by a process as defined above and in the following, comprising the compounds I and II

I II in a total amount of not more than 150 ppm, preferably not more than 100 ppm, in particular not more than 50 ppm, based on the total weight of the purified

γ-butyrolactone. DESCRIPTION OF THE INVENTION

For the purposes of the invention, a "raw γ-butyrolactone" denotes a composition of γ-butyrolactone before purification to remove or lower the content of compounds I and/or II by the method of the invention. The raw γ-butyrolactone may be a composition as is generally formed in the synthesis of γ-butyrolactone and may comprise further products of value (e.g. tetrahydrofuran or 1 ,4-butanediol), starting materials or intermediates (e.g. maleic anhydride, maleic monoesters or maleic diesters), one or more impurities different from I and II (e.g. dialkyl succinates), catalysts and other auxiliaries employed for the synthesis. The term "raw γ-butyrolactone" also

encompasses compositions formed in the synthesis of γ-butyrolactone that have been subjected to a pre-purification to separate off at least one undesired component different from compounds I and II.

For the purposes of the invention, a "purified γ-butyrolactone" denotes a composition of γ-butyrolactone after purification to remove or lower the content of compounds I and/or II by the method of the invention. The "purified γ-butyrolactone" obtained in step c) of the process of the invention may still comprise further components different from compounds I and/or II, selected from the afore-mentioned products of value different from γ-butyrolactone, starting materials, intermediates, impurities, catalysts and auxiliaries. Thus, in a special embodiment the obtained purified γ-butyrolactone is subjected to a post-purification to separate off further undesired component different from compounds I and II.

Without being bound by a certain theory it is believed that the acidic compound acts as a catalyst that catalyses the reaction of compounds I and II to other compounds that may be easily separated from γ-butyrolactone, especially via distillation.

Preferably, the raw γ-butyrolactone provided in step a) has a γ-butyrolactone content of 50 to 99.9 wt.%, preferably 75 to 99.9 wt.%, based on the total weight of the raw γ-butyrolactone.

Preferably, the raw γ-butyrolactone provided in step a) has a total content of the compounds I and II of 100 to 5000 ppm, preferably 150 to 2000 ppm, based on the total weight of the raw γ-butyrolactone. In a special embodiment, the raw γ-butyrolactone provided in step a) has a

γ-butyrolactone content of at least 95 wt.%, more preferably at least 97 wt.%, in particular at least 99.0 wt.%, especially at least 99.3 wt.%, based on the total weight of the raw γ-butyrolactone. According to this embodiment, the raw γ-butyrolactone provided in step a) is preferably essentially free from compounds different from compounds I and II having a lower or higher boiling point than that of γ-butyrolactone. In the context of the invention, essentially free from compounds having a lower or higher boiling point than that of γ-butyrolactone means that the raw γ-butyrolactone comprises at most 5% by weight, preferably at most 3% by weight, particularly at most 1 % by weight, especially at most 0.7% by weight of compounds (different from I and II) having a lower or higher boiling point than that of γ-butyrolactone. According to this embodiment in step a) of the process of the invention the reaction product from the synthesis of γ-butyrolactone can be subjected to a distillative separation of a fraction enriched in γ-butyrolactone and depleted in lower and/or higher boiling components to provide the raw γ-butyrolactone. This variant allows the separation of a purified γ-butyrolactone in step c) with a γ-butyrolactone content of preferably more than 99.7 wt.%, more preferably of more than 99.8 wt.%, most preferably of more than 99.9 wt.%.

The composition of the raw γ-butyrolactone provided in step a) can vary widely and depends on the way in which it is produced. The process by means of which the raw γ-butyrolactone has been obtained is not critical for the success of the process of the present invention.

In a preferred embodiment, the raw γ-butyrolactone provided in step a) comprises γ-butyrolactone, at least one of the compounds I or II and at least one further compound, preferably selected from tetrahydrofuran, 1 ,4-butanediol, succinic acid esters, 4-hydroxybutyric acid esters and others. In general, the concentration of these further compounds is from 0 to 30 wt.%, more preferably 0 to 15 wt.%, in particular 0 to 5 wt.%, based on the total weight of the raw γ-butyrolactone.

The raw γ-butyrolactone provided in step a) is preferably obtained from the

hydrogenation of a dialkylester of a C₄ dicarboxylic acid. Preferred C₄ dicarboxylic acids are maleic acid, fumaric acid, succinic acid and mixtures thereof. Maleic acid is particularly preferred. The raw γ-butyrolactone provided in step a) is preferably obtained from the hydrogenation of a di-(Ci-C₄-alkyl)ester of maleic acid. Examples of suitable esters are the dimethyl, diethyl, di-n-propyl, di-i-propyl, di-n-butyl, di-i-butyl and di-sec-butyl esters of maleic, fumaric and succinic acid and mixtures thereof. Preferred esters are the dimethyl, diethyl, di-n-propyl, di-i-propyl, di-n-butyl, di-i-butyl and di-sec- butyl esters of maleic acid and mixtures thereof. Especially preferred esters are dimethyl maleate and diethyl maleate. The raw γ-butyrolactone provided in step a) is preferably obtained by a

heterogeneously catalyzed gas-phase hydrogenation of the above-mentioned components, as is known, for example, from WO 97/24346, WO 97/43242 and WO 97/43234. The product stream obtained in the hydrogenation is conveyed from the hydrogenation reactor and is generally cooled. In the preferred gas-phase process, the desired γ-butyrolactone, tetrahydrofuran, 1 ,4-butandiol, compounds I and II, water and the major part of the further by-products are condensed out in this way. Unreacted hydrogen, inert gases (e.g. nitrogen, methane and noble gases) and very low-boiling by-products remain in the gas phase and are recycled or separated off. The liquid mixture obtained in this way can then be passed to a continuous fractional distillation to provide the raw γ-butyrolactone. A suitable distillation process is described in

US 6,846,398.

In a typical composition, the raw γ-butyrolactone provided in step a) comprises the following compounds, based in each case on the total weight of the raw compound: γ-butyrolactone: 95 to 99.9 wt.%,

tetrahydrofuran: 0 to 0.1 wt.%,

1 ,4-butandiol: 0 to 5 wt.%,

compounds I and II: 150 to 2000 ppm,

alcohols from the diesters used: 0 to 0.1 wt.%,

succinic anhydride: 0 to 0.1 wt.%,

succinic esters: 0 to 5 wt.%,

4-hydroxy butyric acid esters: 0 to 2 wt.%,

water: 0 to 0.5% by weight.

The afore-mentioned values are typical of a raw γ-butyrolactone that has already been depleted in tetrahydrofuran and 1 ,4-butandiol.

In principle, any acidic catalyst can be used for the acidic treatment in step b), i.e. any substance having Bronstedt or Lewis acidity. Examples of suitable catalysts are protic acids, such as hydrochloric acid, sulfuric acid, phosphoric acid, methanesulfonic acid and p-toluenesulfonic acid, acidic molecular elemental compounds, such as aluminum chloride, boron trifluoride, zinc chloride, phosphorus pentafluoride, arsenic trifluoride, tin tetrachloride, titanium tetrachloride and antimony pentafluoride, heteropolyacids, oxidic acidic solids, such as zeolites, silicates, aluminates, aluminosilicates, clays and acidic ion exchangers.

In a first embodiment in step b) at least one acidic compound is homogeneously mixed with the raw γ-butyrolactone.

In this embodiment the employed acidic compounds are at least partly soluble in the raw γ-butyrolactone under the treatment conditions. The acidic compound is preferably selected from sulphuric acid, phosphoric acid, sulphonic acids, heteropoly acids and mixtures thereof. More preferably, the acidic compound is selected from sulphuric acid, phosphoric acid, methane sulphonic acid, p-toluene sulphonic acid, wolframo phosphoric acid and mixtures thereof. Preferably, the acidic compound is added to the raw γ-butyrolactone in an amount of from 0.001 to 2 parts by weight %, more preferably 0.05 to 1 parts by weight, based on the γ-butyrolactone content of the raw γ-butyrolactone.

Preferably, in step b) the raw γ-butyrolactone is homogeneously treated with the acidic compound at a temperature in the range from 30°C to 300°C, more preferably in the range from 50°C to 250°C, most preferably in the range from 80°C to 200°C.

If the at least one acidic compound is homogeneously mixed with the raw

γ-butyrolactone in step b), the obtained purified γ-butyrolactone is preferably separated from the product mixture by distillation.

In a second embodiment (preferred) in step b) at least one acidic compound is heterogeneously mixed with the raw γ-butyrolactone. In this embodiment the employed acidic compounds are essentially unsoluble in the raw γ-butyrolactone under the treatment conditions. In terms of the invention the term essentially unsoluble means that the employed acidic compounds under the treatment conditions of step b) have a solubility in the reaction mixture of not more than

0.010 g/L, more preferably not more than 0.0001 g/L.

Preferably, if step b) is carried out in a batch mode, the at least one acidic compound is heterogeneously added to the raw γ-butyrolactone in an amount of from 0.1 to 30 parts by weight %, more preferably 0.5 to 20 parts by weight %, based on the

γ-butyrolactone content of the raw γ-butyrolactone. Preferably, if step b) is carried out continuously, the at least one acidic compound is heterogeneously added to the reaction system, and the raw γ-butyrolactone is fed to the reaction system to achieve a catalyst loads in the range of 0.01 to 10 kg, preferably 0.1 to 2 kg, more preferably 0.2 to 1 kg feed/litre catalyst per hour.

The acidic compound preferably comprises at least one element selected from the group consisting of B, Al, C, Si, P, S, La, Ce, Ti, Zr, V, Cr, Mo, W and Fe. The at least one element preferably at least partly forms a compound with oxygen and/or sulphur.

Preferably, the acidic compound for the heterogeneous treatment of the raw γ-butyrolactone is selected from zeolites, S1O2, silicates, AI2O3, aluminates, La203, Ce203, T1O2, Zr02, Ο2Ο3, Fe203, clays, aluminosilicates, wherein the aforementioned compounds optionally comprise phosphoric acid groups and/or sulphuric acid groups and mixtures of said compounds.

The acidic compound used in step b) preferably comprises at least one silicate with acidic groups, selected from among sheet silicates, framework silicates and combinations thereof. Preferred sheet silicates are clay minerals. These include, for example, two-layer, three-layer and four-layer clay minerals which differ in terms of the sequence of their tetrahedral and octahedral layers. Suitable clay minerals are, for example, aluminum silicates which are made up of layers of S1O2 tetrahedra and layers of AI2O3 octahedra, with part of the silicon in the layer of tetrahedra being able to be replaced by trivalent cations, preferably aluminum, and/or part of the aluminum in the layer of octahedra being able to be replaced by divalent cations, e.g. magnesium. The acidic compound is then preferably selected from among bentonite, kaolinite, montmorrillonite, attapulgite, hectorite, sepiolite, pillared clays and combinations thereof.

The preparation of "pillared clays" is comprehensively described in, for example, Figuras, Catal. Rev. Sci. Eng., 30(3) (1988), pages 457 to 499, or Jones, Catal. Today (2) (1988) 357. These documents are hereby incorporated by reference. Pillared clays are especially suitable for producing acidic compounds, since in the pillared clays the individual layers are supported by one another. Pillared clays (PILCs) are e.g. made up of layers, such as montmorillonite, beidellite, hectorite or saponite, between which oxides are intercalated in the form of pillars. Suitable acidic compounds are also acidic zeolites. They are preferably selected from zeolites Beta, ZSM-5, ZSM-22, ZSM-23, MCM-22, and MCM-49.

In one embodiment, the acidic compound is suspended in the raw γ-butyrolactone. According to this embodiment, the acidic compound preferably consists of particles and at least 80 wt.% of the acidic compound, based on the total weight of the acidic compound, has a particle size in the range from 0.01 mm to 1 mm, more preferably in the range from 0.02 mm to 0.1 mm. In a further embodiment, the raw γ-butyrolactone is brought into contact with a bed of the acidic compound. According to this embodiment the acidic compound preferably consists of particles and at least 80 wt.% of the acidic compound, based on the total weight of the acidic compound, has a particle size in the range from 0.5 mm to 20 mm, more preferably in the range from 1 mm to 10 mm, most preferably in the range from 1 .5 mm to 7 mm.

In a further embodiment, in step b) the raw γ-butyrolactone is brought into contact with a strongly acidic cation exchanger. In one preferred embodiment of the invention said compounds comprise -COOH groups. In another preferred embodiment of the invention said compounds comprise -SO3H groups.

The term strongly acidic cation exchanger is understood to mean a cation exchanger in the H⁺ form having strongly acidic groups. Strongly acidic groups are generally sulfonic acid groups. The acidic groups are generally attached to a polymer matrix which may be, for example, in gel form or macroporous. A preferred embodiment of the method according to the invention is accordingly characterized in that a strongly acidic cation exchanger having sulfonic acid groups is used. Suitable strongly acidic cation exchangers are described in WO 2010/133473 and WO 201 1/154330 which are hereby fully incorporated by reference.

Suitable for use in step b) are strongly acidic ion exchangers (e.g. Amberlyst,

Amberlite, Dowex, Lewatit, Purolite, Serdolit) which are based on polystyrene and the copolymers of styrene and divinylbenzene as support matrix, comprising sulfonic acid groups in H⁺ form and ion exchange groups functionalized with sulfonic acid groups (-SO3H). The ion exchangers differ in the structure of their polymer skeleton and a distinction is made between gel-like and macroporous resins. In a specific embodiment, a perfluorinated polymeric ion exchange resin is used in step a). Such resins are marketed, for example under the name Nafion ® by DuPont. An example of such a perfluorinated polymeric ion exchange resin which may be mentioned is Nation ® NR- 50.

Suitable commercially available strongly acidic cation exchangers for the reaction in step a1 ) are known, for example, under the trade names Lewatit ® (Lanxess), Purolite ® (The Purolite Company), Dowex ® (Dow Chemical Company), Amberlite ® (Rohm and Haas Company), Amberlyst (TM) (Rohm and Haas Company). Preferred strongly acidic cation exchangers are: Lewatit ® K 1221 , Lewatit ® K 1461 , Lewatit ® K 2431 , Lewatit ® K 2620, Lewatit ® K 2621 , Lewatit ® K 2629, Lewatit ® K 2649, Amberlite ® FPC 22, Amberlite ® FPC 23, Amberlite ® IR 120, Amberlyst (TM) 131 , Amberlyst (TM) 15, Amberlyst (TM) 31 , Amberlyst (TM) 35, Amberlyst (TM) 36, Amberlyst (TM) 39, Amberlyst (TM) 46, Amberlyst (TM) 70, Purolite ® SGC650, Purolite ® C100H, Purolite © C150H, Dowex © 50X8, Serdolit ® red and Nation ® NR-50. The strongly acidic ion exchange resins are generally regenerated using hydrochloric acid and/or sulfuric acid.

Preferably in step b) the raw γ-butyrolactone is heterogeneously treated with the acidic compound at a temperature in the range from 50°C to 300°C more preferably in the range from 80°C to 250°C, most preferably in the range from 100°C to 200°C.

If in step b) the raw γ-butyrolactone is heterogeneously treated with the acidic compound this can be performed batch-wise or continuously. Preferably, in this embodiment the at least one acidic compound is continuously brought into contact with the raw γ-butyrolactone. Preferably, 0.01 kg to 10 kg, more preferably, 0.1 kg to 2 kg, in particular 0.2 kg to 1 kg, raw γ-butyrolactone is contacted with 1 litre of acidic compound per hour.

Contacting the acidic compound as a solid phase catalyst with the raw γ-butyrolactone is preferably performed by leading a stream of raw γ-butyrolactone over the solid acidic compound. Shaft reactors or tube reactors which are operated with a sump or a trickle- bed may especially be used for the solid phase catalysis. The reaction temperature is preferably already set in the feed of the reactor. This allows the reactor to be constructed with a thermal isolation that is optionally augmented by trace heating. No mayor heating element is needed to raise the temperature of the reaction mixture inside the reactor.

Gas-phase stripping of compounds is preferably suppressed during treatment of the raw γ-butyrolactone with the at least one acidic compound as a solid phase catalyst. This is especially realized by putting the reaction mixture under pressure. Whether the reaction mixture shall be put under pressure may be selected depending on the temperature of the reaction mixture and depending on the boiling point of the compounds present in the reaction mixture. In case the reaction mixture comprises water it shall be put under pressure at a temperature above 100°C. E.g. a pressure of 1 MPa can be selected at a temperature of 150°C in the presence of water.

If the at least one acidic compound is heterogeneously mixed with the raw

γ-butyrolactone in step b), the obtained purified γ-butyrolactone is preferably separated from the product mixture by filtration, distillation, or a combination thereof.

In all aspects of the invention the raw γ-butyrolactone is treated with the at least one acidic compound for preferably 0.1 hours to 10 hours, more preferably 0.2 hours to 5 hours, most preferably 0.5 hours to 4 hours.

Preferably, the raw γ-butyrolactone is continuously treated with the at least one acidic compound in all aspects of the invention.

After treating the raw γ-butyrolactone with the at least one acidic compound it is preferably purified via column distillation, especially using a dividing wall column. This allows separation of low boilers at the column head and separation of high boilers at the bottom of the column. Even if the raw γ-butyrolactone is already free of high boilers and low boilers new high boilers and low boilers may be produced during treatment of the raw γ-butyrolactone with the acidic compound. Said newly produced high boilers and low boilers can be removed via the column distillation.

Preferably, the purified γ-butyrolactone obtained in step c) comprises the compounds I and II in total in an amount of not more than 150 ppm, preferably not more than 100 ppm, most preferable not more than 50 ppm based on the total weight of the purified γ-butyrolactone.

The examples below serve to illustrate the invention without limiting it in any way.

EXAMPLES

Raw γ-butyrolactone was synthesized according to WO 86/07358 A1 to obtain γ-butyrolactone with a purity of 99.86 wt.%. It contains 250 ppm of compound I, 250 ppm of compound II and 900 ppm of further impurities comprising at least three different compounds. Examples E1 - E7

5 g of the raw γ-butyrolactone was treated with a mass (m) of acidic compounds to form a homogeneous (E1 and E2) or heterogeneous (E3 - E7) reaction mixture. The reaction mixture was stirred for 3 hours under inert nitrogen atmosphere in a glass flask at constant temperature (T). Then, the reaction mixture was analyzed via gas chromatography to detect the decrease (d) of compounds I and II in the raw

γ-butyrolactone. The results are listed in table 1.

Example E8

100 ml of the acidic compound (2.5 mm extrudate) where placed in a 270 ml reactor. The remaining volume of the reactor was filled with inert glass rings. Raw

γ-butyrolactone was fed from a storage flask to the reactor via a sump feed at the reaction temperature. 0.3 g raw γ-butyrolactone was fed per ml of acidic compound per hour for 300 hours. The discharge of the reactor was analyzed via gas chromatography to detect the decrease (d) of compounds I and II in the raw γ-butyrolactone. The result is listed in table 1 .

900 g of the discharge were fractionally distilled over an 80 cm packed column with a reflux ratio of 2. The resulting γ -butyrolactone was analyzed via gas chromatography to have a purity of 99.96%. 10 ppm of compound I was found. Compound II was not detectable. Based on the raw γ-butyrolactone the yield of γ-butyrolactone was 97%.

Comparative example

900 g of raw γ-butyrolactone were fractionally distilled over an 80 cm packed column with a reflux ratio of 2. The resulting γ-butyrolactone was analyzed via gas

chromatography to have a purity of 99.90%. 90 ppm of compound I was found.

240 ppm of compound II was found. Based on the raw γ-butyrolactone the yield of γ-butyrolactone was 97%.

Table 1

Example acidic compound m [g] T [°C] d(l) [%] d(ll) [%]

E1 sulphuric acid 0.04 60 100 85

E2 wolframoposphoric acid 0.05 120 100 89 E3 titan silicalite 0.05 150 100 17

E4 amberlite 252H 0.5 120 100 87

E5 zeolithe ZSM-5 (H form) 0.5 150 66 91

E6 β-zeolithe CP-814C 0.05 150 100 86

E7 bentonite K10, activated with HCI 0.5 150 100 85

E8 bentonite K10, activated with HCI * 170 100 100

^* the acidic compound is added by volume and not by mass (see description β-Zeolithe CP-814C was purchased from Zeolyst International, USA

Bentonite K10 was purchased from Sud-Chemie AG, Germany

Claims

Claims:

1 . A method for purifying γ-butyrolactone, wherein a) a raw γ-butyrolactone is provided that comprises at least one of the

compounds I or II

b) the raw γ-butyrolactone is subjected to a treatment with at least one acidic compound, wherein, compared to the starting material, a product mixture depleted in at least one of the compounds I or II is obtained, c) a purified γ-butyrolactone is separated from the product mixture obtained in

Method according to claim 1 , wherein the raw γ-butyrolactone provided in step a) has a γ-butyrolactone content of 50 to 99.9 wt.%, preferably 75 to 99.9 wt.%, based on the total weight of the raw γ-butyrolactone.

Method according to claim 1 or 2, wherein the raw γ-butyrolactone provided in step a) has a total content of the compounds I and II of 100 to 5000 ppm, preferably 150 to 2000 ppm, based on the total weight of the raw γ-butyrolactone.

Method according to any of the preceding claims, wherein the raw γ-butyrolactone provided in step a) additionally comprises at least one compound, selected from tetrahydrofuran, 1 ,4-butanediol and mixtures thereof.

Method according to any of the preceding claims, wherein the raw γ-butyrolactone provided in step a) is obtained from the hydrogenation of a dialkylester of a C₄ dicarboxylic acid, preferably from the hydrogenation of a di-(Ci-C₄-alkyl)ester of maleic acid.

6. Method according to any of claims 1 to 5, wherein in step b) at least one acidic compound is homogeneously mixed with the raw γ-butyrolactone.

Method according to claim 6, wherein the acidic compound is selected from the group consisting of sulphuric acid, phosphoric acid, sulphonic acids, heteropoly acids and mixtures thereof.

8. Method according to claim 6 or 7, wherein the acidic compound is added to the raw γ-butyrolactone in an amount of from 0.001 to 2 parts by weight %, more preferably 0.05 to 1 parts by weight, based on the γ-butyrolactone content of the raw γ-butyrolactone.

9. Method according to any of claims 1 to 5, wherein in step b) at least one acidic compound is heterogeneously mixed with the raw γ-butyrolactone.

10. Method according to claim 9, wherein the acidic compound comprises at least one element selected from the group consisting of B, Al, C, Si, P, S, La, Ce, Ti, Zr, V, Cr, Mo, W and Fe.

1 1 . Method according to claim 9 or 10, wherein the acidic compound is selected from zeolites, S1O2, silicates, AI2O3, aluminates, La203, Ce203, ΤΊΟ2, Zr02, Ο2Ο3, Fe203, clays, aluminosilicates, wherein the aforementioned compounds optionally comprise phosphoric acid groups and/or sulphuric acid groups, and mixtures of said compounds.

12. Method according to any of claims 9 to 1 1 , wherein the acidic compound is

suspended in the raw γ-butyrolactone.

13. Method according to claim 12, wherein the acidic compound consists of particles and at least 80 wt.% of the acidic compound, based on the total weight of the acidic compound, has a particle size in the range from 0.01 mm to 1 mm.

14. Method according to any of claims 9 to 1 1 , wherein the raw γ-butyrolactone is brought into contact with a bed of the acidic compound.

15. Method according to claim 14, wherein the acidic compound consists of particles and at least 80 wt.% of the acidic compound, based on the total weight of the acidic compound, has a particle size in the range from 0.5 mm to 20 mm.

Method according to claim 9, wherein in step b) the raw γ-butyrolactone is brought into contact with a strongly acidic cation exchanger.

17. Method according to any of claims 9 to 16, wherein the acidic compound is added to the raw γ-butyrolactone in an amount of from 0.1 to 30 parts by weight %, more preferably 0.5 to 20 parts by weight %, in particular 1 to 210 parts by weight %, based on the γ-butyrolactone content of the raw γ-butyrolactone.

18. Method according to any of claims 9 to 17, wherein in step b) the at least one acidic compound is continuously brought into contact with the raw γ-butyrolactone.

19. Method according to claim 18, wherein 0.01 kg to 10 kg raw γ-butyrolactone is contacted with 1 litre of acidic compound per hour.

20. Method according to claim 18 or 19, wherein gas-phase stripping of compounds is suppressed during treatment of the raw γ-butyrolactone with the at least one acidic compound.

21 . Method according to any of claims 9 to 20, wherein in step c) the purified

γ-butyrolactone is separated from the acidic compound by filtration, distillation or a combination thereof.

22. Method according to any of the preceding claims, wherein the raw γ-butyrolactone is treated with the at least one acidic compound for 0.1 hours to 10 hours.

23. Method according to any of the preceding claims, wherein the purified

γ-butyrolactone obtained in step c) comprises the compounds I and II in total in an amount of not more than 150 ppm, preferably not more than 150 ppm, most preferable not more than 50 ppm based on the total weight of the purified γ-butyrolactone.

24. A purified γ-butyrolactone, obtainable by a process according to any of claims 1 to 23, comprising the compounds I and II

II in a total amount in total in an amount of not more than 150 ppm, preferably not more than 100 ppm, most preferable not more than 50 ppm, based on the total weight of the purified γ-butyrolactone.