Method for producing alkyl lactate

Abstract

Provided is a method of efficiently preparing alkyl lactate from by-products which are generated during a process of converting lactic acid into lactide, or from poly(lactic acid) (PLA).

Claims

1. A method of preparing alkyl lactate, the method comprising the steps of: reacting by-products generated during a process of converting lactic acid into lactide or poly(lactic acid) (PLA) directly with alcohol and an acidic catalyst to prepare alkyl lactate (a trans-esterification reaction step); neutralizing the prepared alkyl lactate to prepare a neutralized solution of pH 6 to pH 9 (a neutralization step); and recovering alkyl lactate from the neutralized solution (a recovery step), wherein the neutralization step is performed by ammonia gas.

2. The method of preparing alkyl lactate of claim 1, wherein the by-products include one or more selected from the group consisting of meso-lactide, L-lactide, D-lactide, lactic acid, and lactic acid oligomers.

3. The method of preparing alkyl lactate of claim 1, wherein the acidic catalyst of the trans-esterification reaction step is sulfuric acid, hydrochloric acid, or nitric acid.

4. The method of preparing alkyl lactate of claim 1, wherein the number of moles of the acidic catalyst of the trans-esterification reaction step is included at a molar ratio of 0.01 to 0.06, based on the number of moles of the lactic acid produced by hydrolysis of the by-products or the poly(lactic acid).

5. The method of preparing alkyl lactate of claim 1, wherein the alcohol of the trans-esterification reaction step is C1 to C4 alcohol.

6. The method of preparing alkyl lactate of claim 1, wherein the alcohol of the trans-esterification reaction step is methanol or ethanol.

7. The method of preparing alkyl lactate of claim 1, wherein the number of moles of the alcohol of the trans-esterification reaction step is included at a molar ratio of 2 to 5, based on the number of moles of the lactic acid produced by hydrolysis of the by-products or the poly(lactic acid).

8. The method of preparing alkyl lactate of claim 1, wherein the recovery step is performed by vacuum distillation of the neutralized solution.

9. The method of preparing alkyl lactate of claim 8, wherein the vacuum distillation is performed at a temperature of 30° C. to 90° C. and a pressure of 30 torr to 90 torr.

Description

MODE FOR INVENTION

(1) Hereinafter, the present disclosure will be described in more detail with reference Examples. However, these Examples are for illustrative purposes only, and the scope of the present disclosure is not intended to be limited by these Examples.

REFERENCE EXAMPLE

(2) A predetermined amount of by-products generated during a process of converting lactic acid into lactide, the by-products including meso-lactide, L/D-lactide, lactic acid oligomers, and lactic acid, or poly(lactic acid) (PLA) was taken, and dissolved in water, and then sodium hydroxide was added thereto to perform hydrolysis at 80° C. The hydrololysate was analyzed by high performance liquid chromatography (HPLC) to confirm that all components of the reactant were completely decomposed into lactic acid, and the number of mole of the produced lactic acid was calculated. The amounts of alcohol and an acidic catalyst used in the following Examples were determined by a molar ratio, based on the number of moles of the lactic acid produced when the by-products and the poly(lactic acid) were decomposed.

Example 1

(3) 50 g of by-products generated during a process of converting lactic acid into lactide, the by-products including 88% by weight of lactide (meso-lactide and L/D-lactide), 2% by weight of lactic acid oligomers, and 5% by weight of lactic acid, ethanol, and 98% sulfuric acid were introduced into a reactor, and then trans-esterification reaction was performed under stirring at 90° C. At this time, ethanol and 98% sulfuric acid were used in the number of moles of 2.2 times and 0.02 times with respect to the number of moles of lactic acid which was produced by degradation of all the components of the by-products, as calculated by the method of the Reference Example.

(4) HPLC analysis was performed to examine whether conversion of ethyl lactate from the reaction solution occurred. The HPLC analysis was performed using HPLC (Agilent, USA) equipped with a diode array detector (DAD) of a wavelength of 230 nm and an RP-18 C18 column (Merck, USA). As a mobile solvent, a concentration gradient of a 0.2% aqueous phosphoric acid solution and acetonitrile containing 0.2% phosphoric acid was used, and a flow rate was set to 1 mL/min. Through this analysis, it was confirmed that the conversion to ethyl lactate was 89%.

(5) The reaction mixture containing ethyl lactate which was produced by the reaction was cooled to room temperature, and then the pH was adjusted to 8 by bubbling ammonia gas into the reaction solution. Thereafter, vacuum distillation was performed at 30 torr to 50 torr and 50° C. to 80° C. to recover ethyl lactate. Karl-Fischer water analysis was performed to confirm that 1.23% of water was included, and liquid chromatography analysis was performed to confirm that the purity was 94%. The calculated final yield was 84%.

Example 2

(6) 50 g of by-products generated during a process of converting lactic acid into lactide, the by-products including 90% by weight of lactide (meso-lactide and L/D-lactide), 2% by weight of lactic acid oligomers, and 5% by weight of lactic acid, methanol, and 98% sulfuric acid were introduced into a reactor, and then trans-esterification reaction was performed under stirring at 90° C. At this time, methanol and 98% sulfuric acid were used in the number of moles of 2.2 times and 0.02 times with respect to the number of moles of lactic acid which was produced by degradation of all the components of the by-products, as calculated by the method of the Reference Example. After completion of the reaction, HPLC analysis was performed, and as a result, it was confirmed that conversion to methyl lactate was 89%.

(7) The reaction mixture containing methyl lactate which was produced by the reaction was cooled to room temperature, and then the pH was adjusted to 8 by bubbling ammonia gas into the reaction solution. Thereafter, vacuum distillation was performed in the same manner as in Example 1 to remove remaining methanol and trace water, thereby obtaining methyl lactate with water of 1.68% and the purity of 93%. The final yield was 84%.

Example 3

(8) 70 g of poly(lactic acid) having a molecular weight of 100,000 g/mol (NatureWorks LLC, USA), ethanol, and 98% sulfuric acid were introduced into a reactor, and then trans-esterification reaction was performed under stirring at 90° C. At this time, ethanol and 98% sulfuric acid were used in the number of moles of 2.5 times and 0.02 times with respect to the number of moles of lactic acid which was produced by degradation of all the components of the poly(lactic acid), as calculated by the method of the Reference Example. After completion of the reaction, HPLC analysis was performed, and as a result, it was confirmed that conversion to ethyl lactate was 95%.

(9) The reaction mixture containing ethyl lactate which was produced by the reaction was cooled to room temperature, and then the pH was adjusted to 7 by bubbling ammonia gas into the reaction solution. Thereafter, vacuum distillation was performed in the same manner as in Example 1 to remove remaining ethanol and trace water, thereby obtaining ethyl lactate with water of 2.16% and the purity of 95%. The final yield was 91%.

Example 4

(10) 700 g of by-products generated during a process of converting lactic acid into lactide, the by-products including 92% by weight of lactide (meso-lactide and L/D-lactide), 1% by weight of lactic acid oligomers, and 6% by weight of lactic acid, ethanol, and 98% sulfuric acid were introduced into a reactor, and then trans-esterification reaction was performed under stirring at 90° C. At this time, ethanol and 98% sulfuric acid were used in the number of moles of 2.5 times and 0.02 times with respect to the number of moles of lactic acid which was produced by degradation of all the components of the by-products, as calculated by the method of the Reference Example. After completion of the reaction, HPLC analysis was performed, and as a result, it was confirmed that conversion to ethyl lactate was 93%.

(11) The reaction mixture containing ethyl lactate which was produced by the reaction was cooled to room temperature, and then the pH was adjusted to 8 by bubbling ammonia gas into the reaction solution. Thereafter, vacuum distillation was performed in the same manner as in Example 1 to remove remaining ethanol and trace water, thereby obtaining 1,076 g of ethyl lactate. At this time, the obtained ethyl lactate was confirmed to contain water of 1.84%. The purity analyzed by liquid chromatography was 94%, and the final yield was 90%.

Example 5

(12) 750 g of by-products generated during a process of converting lactic acid into lactide, the by-products including 90% by weight of lactide (meso-lactide and L/D-lactide), 2% by weight of lactic acid oligomers, and 5% by weight of lactic acid, ethanol, and 98% sulfuric acid were introduced into a reactor, and then trans-esterification reaction was performed under stirring at 90° C. At this time, ethanol and 98% sulfuric acid were used in the number of moles of 2.5 times and 0.02 times with respect to the number of moles of lactic acid which was produced by degradation of all the components of the by-products, as calculated by the method of the Reference Example. After completion of the reaction, HPLC analysis was performed, and as a result, it was confirmed that conversion to ethyl lactate was 92%.

(13) The reaction mixture containing ethyl lactate which was produced by the reaction was cooled to room temperature, and then the pH was adjusted to 7 by bubbling ammonia gas into the reaction solution. Thereafter, vacuum distillation was performed in the same manner as in Example 1 to remove remaining ethanol and trace water, thereby obtaining 1,116 g of ethyl lactate. At this time, the obtained ethyl lactate was confirmed to contain water of 1.74%. The purity analyzed by liquid chromatography was 96%, and the final yield was 90%.

Comparative Example 1: Preparation of Alkyl Lactate by Neutralization Using a Basic Solution 1

(14) 750 g of by-products generated during a process of converting lactic acid into lactide, the by-products including 90% by weight of lactide (meso-lactide and L/D-lactide), 2% by weight of lactic acid oligomers, and 5% by weight of lactic acid, ethanol, and 98% sulfuric acid were introduced into a reactor, and then trans-esterification reaction was performed under stirring at 90° C. At this time, ethanol and 98% sulfuric acid were used in the number of moles of 2.2 times and 0.02 times with respect to the number of moles of lactic acid which was produced by degradation of all the components of the by-products, as calculated by the method of the Reference Example. After completion of the reaction, HPLC analysis was performed, and as a result, it was confirmed that conversion to ethyl lactate was 89%.

(15) A 10 N aqueous sodium hydroxide solution in an amount equivalent to sulfuric acid used in the above reaction was added to the reaction mixture containing ethyl lactate which was produced by the reaction. Thereafter, vacuum distillation was performed in the same manner as in Example to remove remaining ethanol and trace water, thereby obtaining 780 g of ethyl lactate, together with 490 g of gel-type lactic acid oligomers which were produced by a polymerization reaction of lactic acid by hydrolysis of ethyl lactate. The product was analyzed by liquid chromatography, and as a result, the purity of the obtained ethyl lactate was 75%, and the final yield was 73%.

Comparative Example 2: Preparation of Alkyl Lactate by Neutralization Using a Basic Solution 2

(16) 50 g of by-products generated during a process of converting lactic acid into lactide, the by-products including 90% by weight of lactide (meso-lactide and L/D-lactide), 2% by weight of lactic acid oligomers, and 5% by weight of lactic acid, ethanol, and 98% sulfuric acid were introduced into a reactor, and then trans-esterification reaction was performed under stirring at 90° C. At this time, ethanol and 98% sulfuric acid were used in the number of moles of 2.5 times and 0.02 times with respect to the number of moles of lactic acid which was produced by degradation of all the components of the by-products, as calculated by the method of the Reference Example. After completion of the reaction, HPLC analysis was performed, and as a result, it was confirmed that conversion to ethyl lactate was 96%.

(17) A 10 N aqueous sodium hydroxide solution in an amount equivalent to sulfuric acid used in the above reaction was added to the reaction mixture containing ethyl lactate which was produced by the reaction. Thereafter, vacuum distillation was performed in the same manner as in Example to remove remaining ethanol and trace water, thereby obtaining 62 g of ethyl lactate, together with gel-type lactic acid oligomers which were produced by a polymerization reaction of lactic acid by hydrolysis of ethyl lactate. The product was analyzed by liquid chromatography, and as a result, the purity of the obtained ethyl lactate was 76%, and the final yield was 76%.

RESULTS

(18) First, to examine the effect according to the kind of alcohol in the method of preparing alkyl lactate according to the present disclosure, ethanol and methanol were used as the alcohol in Examples 1 and 2, respectively, and the by-products having the compositions similar to each other were used as the raw materials to prepare ethyl lactate and methyl lactate, respectively. The conversion ratio, purity, and yield were calculated and compared. As a result, all showed the similar values, indicating that high conversion ratio, and high purity and yield of the products were achieved, irrespective of the number of carbon atoms of alcohol used.

(19) Further, to examine the effect according to the kind of the raw material in the method of preparing alkyl lactate according to the present disclosure, alkyl lactate was prepared by using poly(lactic acid) (Example 3), instead of the by-products generated during the process of converting lactic acid into lactide. As a result, the conversion ratio of 95%, the purity of 95%, and the final yield of 91% were obtained. All these values were equivalent to those of Examples 4 or 5 in which only the raw materials were different. In other words, even though non-pretreated poly(lactic acid) was used as the raw material, alkyl lactate could be prepared with high conversion ratio, and high purity and yield of the product.

(20) Furthermore, to examine possibility of mass-production, in Examples 4 and 5, the reaction was performed under conditions in which the amount of the raw material was increased by 10 times or more, as compared to that of Example 1. As a result, even though the reaction was performed by increasing, in the same ratio, the amount of the by-products generated during the process of converting lactic acid into lactide as the raw material and the amounts of alcohol and sulfuric acid as the catalyst to be reacted with the by-products, alkyl lactate could be prepared with the equivalent or higher conversion ratio, purity, and yield of the product.

(21) Meanwhile, in Examples 3 to 5, ethanol was used in the slightly increased amount, as compared with Example 1. As a result, when the number of moles of ethanol was increased from 2.2 times to 2.5 times with respect to the amount of lactic acid of the raw material, the conversion ratio into alkyl lactate by trans-esterification reaction showed about 4% increase.

(22) Further, even though the compositions of the components of the by-products used in Examples 1, 2, 4 and 5 were different from each other, all Examples showed high levels of the conversion ratio, purity, and yield of the product. Therefore, in the method of preparing alkyl lactate of the present disclosure, by-products having various composition ratios may be used.

(23) To examine the effect according to the difference in the neutralization step in the method of preparing alkyl lactate according to the present disclosure, Comparative Examples were prepared. In Comparative Examples, all procedures were performed in the same manner as in Examples, except for the neutralization step. In the neutralization step, the 10N aqueous sodium hydroxide solution corresponding to the equivalent weight of sulfuric acid was used, instead of ammonia gas. The conversion ratio, purity, and yield of alkyl lactate prepared by the methods of Comparative Examples were measured and compared with those of Examples, respectively. As a result, the conversion ratio was maintained at a high level of about 90% or more, whereas the purity of the purified alkyl lactate showed a reduction of about 20% or more, and the final yield also showed a reduction of 10% or more. Such a reduction of the purity and yield is attributed to formation of lactic acid oligomers as by-products. In other words, ethyl lactate is hydrolyzed into lactic acid by water generated during the neutralization step by sodium hydroxide, and the lactic acid is polymerized to produce gel-type lactic acid oligomers as by-products, thereby reducing the production yield and purity of ethyl lactate.

(24) However, in the method of preparing alkyl lactate of the present disclosure, water production by the neutralization reaction may be minimized by using ammonia gas in the neutralization step, thereby blocking the side-reaction and remarkably improving the purity and yield of alkyl lactate.