BACKGROUND OF THE INVENTION Levulinic acid esters are known to be useful as plasticizers or solvents as well as in the prepara¬ tion of free levulinic acid by the hydrolysis of the ester. Levulinic acid is useful as a food-flavoring agent as well as an intermediate in the preparation of a variety of chemicals for industrial and pharmaceutical uses such as, for example, in the production of di- phenolic acid, a component of protective and decorative finishes and in the preparation of calcium levulinate, a particularly suitable form of calcium for intravenous injection. The degree of actual commercial use of 5 levulinic acid and its esters has nevertheless been rather limited because the available methods of prepara¬ tion of the acid and the esters are not very practical from a commercial point of view since the yields have been rather low and the'cost is relatively high. 0 The basic method for preparing levulinic acid esters is disclosed in U.S. Patent 2,763,665 and British Patent 735,693. This method comprises heating furfuryl alcohol at a temperature of 64°C to 220°C with a differ¬ ent alcohol under substantially anhydrous conditions 5 with agitation, in the presence of a catalyst such as hydrogen chloride or hydrogen bromide. It is important to maintain the concentration of furfuryl alcohol in the reaction mixture below 2% by volume of the other alco¬ hol. The other alcohol is used in the amount of at O least 4 molecular portions per molecular portion ^' of furfuryl alcohol. U.S. Patent 3,203,964 discloses an improvement of said process by incorporating in the reaction mixture a small amount of water, and also preferably premixing furfuryl alcohol with a portion of 5 the other alcohol prior to the addition of furfuryl

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alcohol to the reaction mixture. It is said that in this process the other alcohol can be used in amounts as low as 2 ols of the other alcohol per ol of furfuryl alcohol. Finally, British Patent 1,283,185 discloses a further modification of the basic process which com¬ prises heating furfuryl alcohol in water with either hydrochloric or oxalic acid catalysts and in the pres¬ ence of at least one mol of a water-soluble aliphatic ketone per mole of furfuryl alcohol.

SUMMARY OF THE DISCLOSURE

The instant invention is directed to an im¬ proved process for the manufacture of levulinic acid esters and the free acid. The process comprises the previously known steps of esterification of furfuryl alcohol with a different alcohol selected from the group consisting of an unsubstituted primary and secondary carbon-chain and carbon-ring alcohol in the presence of a strong acid catalyst such as hydrogen chloride, hydrogen bromide, or oxalic acid and the step of hydro- lysis of the resulting ester in the presence of water and an acid catalyst. The improvement of this invention resides in the isolation of the levulinate ester by vacuum distillation of the reaction mixture containing a high-boiling solvent. The instant process yields levulinate esters in high yields without the formation of hard resinous materials which are extremely difficult to remove from the reaction vessel. Levulinic acid can be prepared, according to the prior art, by the hydroly- sis of the ester. However, by using the pure distilled ester the hydrolysis time is very substantially decrease Furthermore, the resulting levulinic acid is obtained in a pure form without the need for recrystallization.

DETAILED DISCLOSURE The instant invention is directed to an im-

proved process for the manufacture of levulinic acid and its esters. More specifically, it is directed to a process for the manufacture of levulinic acid esters which comprises esterification of furfuryl alcohol with 5 a different alcohol selected from the group consisting of an unsubstituted primary and secondary carbon-chain and .carbon-ring alcohol in the presence of a strong acid catalyst and hydrolysis of the ester to levulinic acid, the improvement comprising isolating the levulinate

10 ester by vacuum distillation of the reaction mixture containing a high-boiling solvent.

As noted in the discussion of the prior art methods for the manufacture of levulinic acid, the esterification of furfuryl alcohol with a different

15 alcohol to yield a levulinate ester as well as the hydrolysis of the ester to levulinic acid is well known in the art. The disclosures of the patents mentioned in the discussion of the background of the invention de¬ scribe these steps in great detail and they are in- -

20 corporated herein by reference as to the disclosure dealing with those steps. In summary, the esterifica¬ tion step involves the reaction of furfuryl alcohol with a different alcohol in the presence of a strong acid catalyst such as hydrogen chloride, hydrogen bromide, or

25 oxalic acid. The next step in the prior art process is the hydrolysis of the crude ester. As a rule this step creates substantial practical problems because of the formation of a substantial amount of undesirable poly¬ mers in the reaction vessel. These polymers are very

30. difficultly soluble in a solvent such as acetone or toluene, which would be used for cleaning the still.

As disclosed in the prior art, any alcohol as defined above may be employed in preparing levulinate esters. As illustrative examples of such alcohols may

35 be mentioned aliphatic alcohols such as ethanol, ethanol, n-propanol, isopropanol, n-butanol, isobutanol,

n-pentanol, isopentanol, n-hexanol, n-octanol, n- decanol an alkoxy-alkanol, especially alkoxyethanol such as g-methoxy ethanol, β-ethoxyethanol and the like; or a cycloaliphatic alcohol such as cyclohexanol, tetra hydrofuryl alcohol and the like. Depending on the specific alcohol employed the reaction conditions may have _. to be varied. For example, if a lower alkyl alco¬ hol is used which has a relatively lower boiling point the reaction temperature may have to be lowered and consequently the reaction time lengthened. To compen¬ sate for the longer reaction time the pressure under which the reaction is carried out could be increased. Furthermore, with certain alcohols, such as lower alka- nols for example, conversion to the ester is not very satisfactory and the tar formation is excessive. For these practical reasons when levulinic acid is desired as the final product n-butanol is preferably employed i the esterification step.

The critical and inventive step of the instan process is to isolate the levulinic acid ester from the reaction mixture by adding to the reaction mixture a high-boiling solvent and then distilling the reaction mixture to give pure levulinic acid ester with the byproduct polymer tar and high-boiling solvents re- maining in the distillation vessel in liquid form that can easily be removed from the vessel and easily cleane with common solvents. A further advantage of the pro- ^' cess is that when pure levulinic acid ester is hydro- lyzed to levulinic acid, the time required to substan- tially complete the hydrolysis is greatly reduced as compared to that when crude ester reaction mixture is hydrolyzed. Furthermore, when the acid is prepared directly from the crude ester, the acid must be re- crystallized. The instant process yields pure aqueous solution of levulinic acid without recrystalization.

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The amount of a high-boiling solvent needed in the distillation step depends on the amount of resin formed and the properties of the solvent and the speci¬ fic ester formed. Usually the resin content is from about 2 to 10% by weight of the ester formed. Generally the ratio of the solvent to the resin should be in the range of from 1:1 to 6:1, and a more practical ratio being from 2:1 to 4:1. Expressed differently,' it may be said that the high-boiling solvent should be employed in the amount of from about 3% to 20% by volume or higher of the crude ester reaction mixture. For economic reasons it is preferable to employ as little as possible of the high-boiling solvent. Generally the high boiling solvent is required in the amount of from 4% to 10% by volume.

The high-boiling solvent employed should (1) boil higher than the levulinic acid ester being dis¬ tilled, (2) dissolve the tar, (3) be miscible with the levulinic acid ester being distilled, ^' (4) be inexpensive and (5) be readily available. Clearly, the latter two enumerated properties of the high boiling solvent are preferences for practical reasons only. However, the first three enumerated properties are of critical im¬ portance because otherwise the solvent would either co- distill with the ester or would not serve the purpose of maintaining tar by-products in a liquid form. It is preferable that the high boiling solvent have a boiling point of at least 20°C above the boiling point of the levulinate being distilled. It is clear that the greater the boiling point differential between the levulinate ester and the solvent, the lesser the chance that even a small amount of the solvent is codistilled with the levulinate ester. Illustrative examples of high-boiling solvents are triacetin, dimethyl phthalate, diethyl phthalate, diisobutyl phthalate, di-n-butyl phthalate and the like.

Levulinic acid is obtained. by the hydrolysis of the levulinate ester prepared as described above. If, according to the prior art, crude levulinate ester i hydrolyzed and then distilled to give levulinic acid, a substantial amount of tar remains in the reaction vessel which is very difficult to remove as already noted above. The hydrolysis of crude levulinate ester proceeds slowly and may require 30 to 60 hours- to reach completion. By employing the improved process of this invention pure levulinate ester is employed in the hydrolysis which permits the completion of the hydro¬ lysis step in about 3 to 6 hours, substantially improv¬ ing the efficiency of the equipment. The hydrolysis step is carried in the presence of dilute aqueous hydrochloric or hydrobromic acid, as is well known in the art. It can also be carried out both in the absence or prese ^' nce of an alcohol in the initial stages of hydrolysis and, in either a nitrogen or air atmosphere. The resulting product is aqueous levulinic acid which can be concentrated by the removal of water to yield pure levulinic acid.

The following examples further illustrate the process of the instant invention without introducing any limitation thereof.

EXAMPLE I

PREPARATION OF BUTYL LEVULINATE A 50 gal. (226.8 1.) glass lined reactor equipped with an agitator, a water-cooled condenser and a cooling/heating jacket was charged with 27.2 kg. of n- butyl alcohol. After turning on the agitator and water in the jacket for cooling, 4 lbs. (1.8 kg) of hydro¬ chloric acid (37%) was slowly added to the reaction vessel. The reactor was then purged with nitrogen, water was turned on in the condenser and steam turned on in the jacket to heat the reaction mixture to 96 -100°C.

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When the reaction mixture reached said temperature _steam was shut off and a mixture of 60 lbs. (27.2 kg.) of furfuryl alcohol and 60 lbs. (27.2 kg) of n-butanol was added to the reactor at a rate of 1 lb. (0.45 kg.) per minute, but varying the rate of addition so as to main¬ tain the temperature of the reaction mixture at 96 - 100°C. After completion of the addition of said mixture the jacket temperature was increased to 125°C at a rate of 2°C per minute and the reaction mi-Cture was allowed to reflux for one hour. Thereafter n-butanol was par¬ tially distilled off leaving behind crude butyl levulin¬ ate which had the following composition: 2.67% resin; 2.27% water; 30.54% butanόϊ; 64.0% butyl levulinate and 0.52% others. Following the above procedure another prepara¬ tion of butyl levulinate was carried out employing 50 lb. (22.7 kg.) of furfuryl alcohol ^" , 100 lb. (45.4 kg.) of n-butanol and 4.0 lb. (1.8 kg.) of 37% hydrochloric acid. After the removal of n-butanol, crude butyl ^• levulinate had the following composition: 4.31% of resin; 0.49% of water; 29.19% of butanol and 65.40% of butyl levulinate.

Based on many esterification runs it was found that the resin content of the crude ester may vary from 1.5% to 10% or even 20% depending on the temperature of the reaction, the concentration of the reagents and the rate of the addition of furfuryl alcohol to the reaction mixture.

EXAMPLES 2 - 5 In these examples crude butyl levulinate, pre¬ pared according to the above described procedure, was employed. Distillation of the crude ester yielded the following composition: 56.5% butyl levulinate; 31.7% n-butanol; and 11.8% resin. At the end of the distilla-

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tion the material remaining in the flask massively pre¬ cipitated. After cooling the material became hard and could not be removed except by mechanical means, and even that with great difficulty.

350 ml. (317.8 g.) of crude butyl levulinate and 35 ml. of a high boiling solvent (10% by volume of crude levulinate) as indicated in Table I below were distilled through a 30 cm. Vigerσux column yielding first n-butanol and thereafter pure butyl levulinate in the amount indicated in the Table.

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TABLE I

Levulinate*

Ex. o. Solvent b.p. ^Q C Head Temp. Pres. mm. Recovered

2 Triacetin 258-60° 82.5-88.5°C 1.45-1.9 57.7%

3 Dimethyl

Phthalate 282-85° 82 - 92°C 1.65-1.8 58.0%

4 Di-n-butyl

Phthalate 340° 85 - 90°C 1.7 -1.85 59.5%

10 5 Diethyl

Phthalate 296° 89 - 95°C 2.0 -2.2 57.8%

fBased on the total crude butyl levulinate

The resinous material remaining in the dis¬ tillation vessel was flowable and was readily dissolved in acetone.

EXAMPLE 6 The procedure of Example 2 was repeated, except that only 18 ml. of di-n-butyl phthalate (5% by volume of crude levulinate) was employed, 185.59 g. of pure butyl levulinate (84 - 89°C, 1.55-1.80 mm) was obtained. The resinous tar remaining in the distillation flask was easily removed and was -soluble in acetone.

EXAMPLE 7 The procedure of Example 2 was repeated, ex¬ cept that only 14.0 ml. of di-n-butyl phthalate (4% by volume of crude levulinate) was employed. 189.08 g. of pure butyl levulinate (83-88°C, 1.5-1.8 mm) was obtained

The resinous tar remining in the distillation vessel was flowable and was soluble in acetone.

EXAMPLE 8 T e procedure of Example 2 was repeated, ex- cepted that only 10.5 ml. of di-n-butyl phthalate (3% by volume of crude levulinate) was employed. 190.22 g. of pure butyl levulinate (82.5-88°C, 1.4-1.7 mm) was obtained. The resinous tar remaining in the distillation vessel was thick and not easily removed and was not readily soluble in acetone.

EXAMPLE 9 PREPARATION OF LEVULINIC ACID Pure butyl levulinate (100 g.) obtained after distillation as described in the above examples and 150 g. of distilled water were charged into a

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vessel equipped with a condenser. To this mixture was added concentrated hydrochloric acid (7.5 g.). The mixture was heated to the reflux temperature and main¬ tained at that temperature for 4 hours. During re- fluxing butanol was removed while water was returned t the reaction mixture. The remaining material is an aqueous solution of pure levulinic acid. Water can be distilled off to the extent desired. The aqueous acid solution had Gardner color index 2.5 which is pale straw yellow indicating high purity of the acid.