PREPARATION OF CARBOXYLIC ACID ESTERS OF ALKYLENE GLYCOL MONOALKYL ETHERS

Abstract: An improved method for the preparation of monocarbox­ ylic acid esters of alkylene glycol monoalkyl ethers by the acid catalyzed esterification of the alkylene glykol monoalkyl ether with a carboxylic acid employing azeotropic distillation of water produced by or added to the reaction with an inert water-azeotroping solvent with reflux of the solvent phase and reflux of the water phase to control column overhead tem­ peratures and greatly reduce loss of reactants giving high con­ version and selectivity of product. The process is especially suitable for the preparation of propylene glycol monomethyl ether acetate.

Description: PREPARATION OF CARBOXYLIC ACID ESTERS OF ALKYLENE GLYCOL MONOALKYL ETHERS BACKGROUND OF THE INVENTION The present invention is directed to a novel improved method for the preparation of monocarboxylic acid esters of alkylene glycol monoalkyl ethers such as, for example, propylene glycol monomethyl ether acetate,by the acid catalyzed esterification of acetic acid with propylene glycol monomethyl ether. The esters produced by the process of this invention find many important commercial applications and particularly as solvents for inks and coating polymers and in cleansers. Prior art processes are known for the preparation of glycol ether esters by the acid catalyzed esterification of mono-, di- and triglycol alkyl ethers with a carboxylic acid such as acetic acid. The reaction liberates water which in addition to unreacted reactants (acid and glycol ether) causes operational as well as purification and separation problems. Commonly, the transesterification of ethyl acetate with the glycol ether is employed to produce the glycol ether acetate. French Demande 2,187,757 describes the continuous preparation of esters of glycol monoethers with lower fatty acids. U.S. Patent No. 3,404,175 describes a process of forming and separating ethers or esters by plural stage distillation with removal of water as an azeotrope. German Offenlegungsschrift 1,919,527 shows the continuous production of esters IAcOCH2CH2OMe and AcOtCH2)2OEt) by maintaining the lower portion of the distillation column at )atmospheric pressure. U.S. 3,637,794 discloses esters prepared by successive reactionsof boric acid with glycol monoethers and polyols. SUMMARY OF THE INVENTION This invention relates to a method for the preparation of monocarboxylic acid esters of alkylene glycol monoalkyl ethers, and especially the preparation of propylene glycol monomethyl ether acetate, and involves reacting at least stoichiornetric quantities of a monocarboxylic acid and a glycol ether in the presence of an acid catalyst with a water azeotroping agent such as toluene. Water added with the reactants and water produced by the esterification reaction is continuousiy stripped out of the reaction via a distillation column using an inert water-azeotroping solvent. At the same time, the azeotroping solvent is refluxed to the distillation column along with a reflux of the water phase by means of a phase separator, allowing for discharge of some of the water. enough of the water phase is refluxed along with the azeotropic solvent to lower and maintain the column overhead temperature of the distillation column and thus greatly reduce the loss of reactant glycol ether and carboxylic acid as well as product ester thus obtaining high conversions and high selectivity. It is therefore an object of this invention to provide an improved method for the preparation of monocarboxylic acid esters of glycol ethers in high yield and selectivity. It is another object of this invention to lower the overhead temperature of a distillation unit employed in an acid catalyzed esterification reaction of glycol ethers with monocarboxylic acids by azeotroping water with an azeotroping solvent with reflux of the solvent and at least partial reflux of the water to the distillation unit to provide minimum losses of reactants and product. These and other objects and advantages of this invention will become apparent from the description and from the claims. BRIEF DESCRIPTION OF THE DRAWING The figure is a schematic diagram of a specific flow system which can be used to carry out the process of this invention. DETAILED DESCRIPTION OF THE INVENTION AND DRAWING In accordance with this invention an alkylene glycol monoalkyl ether ester is produced by reacting a monocarboxylic acid with an alkylene glycol monoalkyl ether of the general formulae R(OCH2CHR')nOH and EMI3.1 wherein R is a straight or branched chain alkyl group containing from l to 10 carbon atoms, R' is hydrogen or a methyl group and n is an integer of from 1 to 3 at elevated temperatures and preferably atmospheric pressure in the presence of an acid catalyst and a minimum amount of an azeotroping solvent for the removal of water added with the reactants and water produced by the reaction, with essentiallY total reflux of the azeotroping solvent phase and preferably a partial reflux of the water phase to the distillation column. By removing end refluxing at least part of the water as it is formed by the reaction, the overhead temperature in the distillation column is reduced and maint:ained within a desirable range of fron about 6:) C. to 950C. in order to minimize the loss of glycol ether and carboxylic acid reactants as well as ester product in the water dLstillate. Without at least a partial reflux of the voter phase to the distillation column, overhead temperatures generally will increase up to as high as 125 C. depending on the ester being produced. Lowering or minimizing the amount of azeotroping solvent charged to the reaction system allows the reaction pot temperatures to increase to within a range of from about 80 C. to 165 C., preferably 115 C. to 145 C., thus increasing the reaction rate to permit a reduction in the amount of acid catalyst employed. Generally the amount of azeotroping solvent used will be between about 1 to 25 weight per cent and preferably between 3 to 10 weight per cent of the total reaction mixture. The acid catalyzed esterification reaction may be carried out in any suitable reactor which may be equipped with means for agitation, a means for regulating temperature and a means, such as distillation column, a condenser and a phase separator or decanter, for removing the solvent-water azeotrope with a leans for returning the azeotroping solvent and the water to the distillation column. A general procedure for carrying out the reaction is to charge the glycol ether, carboxylic acid, azeotroping solvent and acid catalyst into the reaction vessel, and then heat or maintain the mixture at the desired reaction temperatures for an appropriate period while azeotroping of the water with a reflux of the azeotroping solvent and at least a partial reflux of t!,e water removed. The reaction may be carricd out as o thatch, semi-continuous or continuous process. The reaction products are recovered and treated by any conventional method uch as stripping of unreacted glycol ether and carboxylic acid and neutralization of the acid catalyst followed by distillation to recover the product ester. The alkylene glycol ronoalkyl ethers which may be employed as reactants in the esterification reaction conform to the general formula R(OCH2-CHR')nOH and the isomer EMI5.1 as herein above described and which may be prepared for example by the process disclosed in ).S. Patent Nos. 2,816,932 and 4,299,997. Representative alkylene glycol monoalkyl ethers within the above noted formula and suitable for use in this invention include, for example, ethylene glycol and diethylene glycol mono-methyl, -ethyl, -propyl, -isopropyl, -butyl, -tertiary-butyl, -isobutyl, -amyl, -hexyl, -octyl, -decyl, etc., ethers, propylene glycol, dipropylene glycol and tripropylene glycol mono-metilyl, -ethyl, -propyl, -isopropyl, -butyl, -amyl, -octyl, -decyl, etc., ethers. The monocarboxylic acid employed as reactants in the esterification reaction according to the invention include monocarboxylic and halogenated monocarboxylic acids having from 1 to 10 carbon atoms. Representative monocarboxylic acids include for example, formic, acetic, propionic, n-butyric, isobutyric, n-valoric, caproic, pelargonic, trimethylacetic, chloroacetic, bromoacetic, chloropropionic, etc., acids. The inert azeotroping solvent for use in the process of this invention include those solvents forming 'J 1 1 a binary minimum-boiling azeotrope with water within the esferificatiorl reaction temperature rane of from about 800C. to 1650C. at atmospheric, subatmospheric or superatmospheric pressures. Representative azeotroping solvents include, for example, aromatic hydrocarbons such as benzene, toluene, xylene, chlorobenzene, bromobenzene, alkanes and cycloalkanes such as n-pentane, trimethylpentane, isopentane, n-hexane, n-heptane, nonane, 3,4-dimethylhexane, 3-ethylhexane, cyclohexane, methyl cyclohexane, cyclopentane, cyclooctane, halogenated aliphatic compounds such as chloroform, carbon tetrachloride, 1,2-dichloroethane, trichloroethane, etc., as well as diethylketone. The acid catalysts employed in the present in invention are known and include, for example, the mineral acids such as concentrated sulfuric acid, hydrochloric acid, nitric acid, etc., Lewis acids such as boron trifluoride, antimony pentafluoride, etc., organic sulfonic and halogenated sulfonic acids such as methane, ethane, and butane sulfonic acids, trifluoromethane sulfonic acid, trichloromethane sulfonic acid, o- or p-toluene sulfonic acid, benzene sulfonic acid, etc., as well as strongly acidic sulfonated aromatic ion exchange resins and perfluoroalkane sulfonic acid resins. The acid catalysts are generally employed in concentrations of from about 0.01 to 5 weight per cent, preferably 0.1 to 2.0 weight per cent, based on the total reaction mixture, which concentrations within this range vary with te particular acid employed. The reaction pot temperature of the present invention will range between about 800C. and 165 C. It is prefferred however to operate at temperatures of from 115 C. to 145 C. to obtain a convenient rate of esterification and maintain the water azeotrope. Pot reaction temperatures will of course be based on the azeotroping solvent and pressure, if any, employed in the reaction. The overhead temperatures of the distillation column are, as hereinabove described, maintained between a range of about 650C. and 95 C. with a reflux of water formed by the reaction. Between about 10 and 95 and preferably between 25 and 75 weight per cent of the water removed is refluxed to the upper portion and trays of the distillation column in order to reduce and maintain the overhead temperature in the distillation column at between about 65 C. and 950C. until at least near completion, e.g., approximately 80 per cent reaction completion and thus depletion of water in the system, at which time the overhead temperature of the distillation column will gradually increase to between about 100 C. to 1250C. which is equivalent to operating in the absence of a water reflux. The process is preferably carried out at atmospheric pressure. However, subatmospheric pressure of about 50 mm Hg up to atmospheric as well as superatmospheric pressures of up to 50 psig may be used if desirable. In order to describe the invention in greater detail reference is made to the drawing wherein an alkylene glycol monoalkyl ether, a carboxylic acid, acid catalyst and azeotroping solvent mixture is fed via line 1 to the reactor pot 2 which is then heated either by heating coils 3 within the reactor or by recirculation of the reaction mixture through line 4 to reboiler 5 and trump 7 via line 6 back into the reactor to maintain the pOr reaction te,nperature. Azeotroping solvent, water and small amounts of unreacted glycol ether and monocarboxylic acid as well as some ester product pass into distillation column 8 which is maintained preferably at atmospheric pressure. A predominantly water solvent azeotrope is taken off the top of the distillation column 8 via line 9 to condenser 10. The condensed watersolvent mixture is then fed to phase separator 12 via line 11. The azeotroping solvent-water mixture is separated in phase separator 12. The solvent phase is then removed and refluxed to the distillation column 8 via pump 17 and line 18 while the water phase is taken off and controlled by pump 13 and partially removed via valve 16 through line 14 for discharge and partially refluxed back to the distillation column via valve 15 and line 19. After completion of the esterification reaction, any unreacted glycol ether and monocarboxylic acid is essentially stripped from the remaining ester product in reactor pot 12 which product is then generally treated with, for example, excess sodium carbonate under agitation to neutralize acid catalyst such as para-toluene sulfonic acid and any free monocaThoxylic acid remaining. The organic product phase may, for example, be distilled under vacuum and recovered. Although the method of the present invention will be directed principally to the preparation of the acetic acid ester of propylene glycol monomethyl ether and dipropylene glycol monomethyl ether it is not intended that the method L)e limited to such ester preparation and Lhose skilled in the art will recognize that the present method is broadly applicable to the preparation of other esters such as propylene glycol monobutyl acetate, dipropylene glycol monooctyl butyrate, ethylene glcyol :nonoethyl formate, etc., using the appropriate glycol ether and monocarboxylic acid. The following examples are provided to illustrate the invention in accordance with the principles of this invention but are not to be construed as limiting the invention in anyway except as indicated by the appended claims. EXAMPLE 1 A liquid mixture of 400 g. (4.44 mols) of propylene glycol monomethyl ether, 255 g. (4.25 moles) glacial acetic acid, 60 g. toluene and 2.00 g. concentrated sulfuric acid (95.7%) was charged to a 1000 ml 3 neck round bottom reaction flask with a heating mantle and equipped with a thermometer, magnetic stirrer and a 1" diameter 25 tray Oldershaw distillation column with a distilling head and thermometer. The reaction was run for a period of 2 hours at a pot temperature, which was gradually increased, from between 1150C. to 1350C. at the end of the run. Water was removed by water-toluene azeotrope. 30 per cent by weight of water removed during the reaction was refluxed to the top of the distillation column along with total reflux of the toluene to maintain an overhead distillation column temperature of 82 -88 C. which was maintained to within 10 minutes of reaction completion when the temperature -9- increased to 120 C.-125 C. Analysis of the product remaining in tlie reactor l)y gas ciromatograpiiy and titration showed a 90 per cent conversion of the propylene glycol monomethyl ether and a 93 per cent conversion based on the acetic acid. Selectivity to the propylene glycol monomethyl ether acetate was 99+ per cent. Only 2 weight per cent of the propylene glycol monomethyl ether and 0.3 weight per cent of the acetic acid charged to the reactor was lost in the aqueous phase taken overhead. EXAMPLE 2 (Comparative) A mixture of 40 g. of propylene glycol monomethyl ether, 25.5 g. of glacial acetic acid, 20 g. toluene and 0.40 g. concentrated sulfuric acid was charged to similar reaction equipment as used in Example 1 but with no provision for the reflux of water to the distillation unit. The reaction was run for 2 hours at a pot temperature of between 118 C. and 122 C. with water removed overhead with Dean Stark apparatus and the toluene refluxed. The overhead temperature of the distillation column was between 1030 and 104 C. during the reaction. Analysis of the product ester showed only 84 per cent conversion of the propylene glycol monomethyl ether and acetic acid. Selectivity based on the propylene glycol monomethyl ether was 95 weight per cent. 8 weight per cent propylene glycol monomethyl ether and 10 weight per cent acetic acid including some product ester was lost in the aqueous phase removed. EXAMPLE 3 The procedure of Example 1 was repeated employing 670 g. of dipropylene glycol moomethyl ether, 255 g. glacial acetic acid, 40 q toluene and 4.6 g. àra-toluene ulfonic acid monohydrate as catalyst. The reaction was run for two hours at a pot temperature of between 115 C. and 135 C. with 35 per cent of the total water removed during the reaction beiny refluxed to the top of the distillation column along with a total reflux of the toluene. The overhead temperature of the distillation column was reduced and maintained at 820-880C. to within 10 minutes of completion of the reaction when the temperature of the overhead increased to "-'1050C. Analysis of the product by gas chromatography and titration showed 99 per cent selectivity with an 89 per cent conversion of reactants. Loss of glycol ether and acid reactants to the aqueous phase was only 2.7 weight per cent and 0.5 weight per cent respectively. EXAMPLE 4 The procedure of Example 1 was repeated using a reactor charge of 400 g. propylene glycol monomethyl ether, 255 g. glacial acetic acid, 40 g. toluene and 3.30 g. p-toluene sulfonic acid monohydrate as catalyst. The reaction was run for 2 hours at a pot temperature, which was gradually increased, from 115 C. to 1350C. toward the end of the run. 30 per cent of the total water removed during the reaction was ref fluxed to the overhead portion of the distillation column reducing and maintaining the temperature overhead at about 840C. to within 10 minutes of the completion of the reaction when the temperature increased to # 1030C. Analysis of product showed a 90 per cent conversion of reactants with 99+ per cent selectivity. Loss of ether and acid reactants to the aqueous rhi ie was cnly 2 and 0.4 weight ier cent respectively. EXAMPLE 5 (Comparative) Example 4 was repeated using the same reactants, as well as the same amount of reactants. The reaction was run for 3 hours at a pot erature of 1160C.-1350C. Water removed during distillation was discharged from the system via Dean Stark apparatus with a reflux of toluene. Without the water reflux the overhead temperature in the distillation column remained at N 1040C. Analysis of the product by gas chromatograph showed 82 per cent conversion of reactants with 99 per cent selectivity. Loss of the glycol ether and acid reactants was 3.9 and 5.0 weight per cent respectively. EXAMPLE 6 The procedure of Example 1 was repeated using a reactor charge of 4000 g. of propylene glycol monomethyl ether, 2400 g. of glacial acetic acid, 1800 g. of toluene and 250 g. of sulfonated polyaromatic ion exchange resin (sold, for example, commercially as "Amberlyst 15" by Rohm and Haas Co.) which resin has a bulk density of approximately 595 g/l., a hydrogen ion concentration of approximately 4.9 milliequivalents/g. dry, a surface area of from about 40 to 50 m2/g. and an average pore diameter of from about 200 to 600 Angstrom units. The reaction was run for 4 hours at a reaction pot temperature of 1100C. Approximately 25 per cent of the water removed during azeotropic distillation was refluxed into the overhead portion of the distillation column reducing and maintaining the overhead temperature of the column to 83 -84tC. until toward the end of the reaction and depletion of water in the system when the temperature in the column overhead rose to(J1030C. Analysis of the product showed 83 per cent conversion with 84 per cent selectivity. Loss of glycol ether and acid reactants were 4.0 and 0.4 respectively. EXAMPLE 7 (Comparative) Example 6 was repeated using the same as well as amounts of reactants except that total water removed during distillation was discharged from the system via Dean Stark apparatus with a total reflux of toluene. Without the water reflux overhead, temperature in the distillation column remained at 1040C. during the reaction. Analysis of the product showed an 82 per cent conversion of reactants with 77 per cent selectivity. Loss of glycol ether and acid reactants were 5.0 and 5.0 respectively.

Claims: CLAIMS 1. A method for the preparation of a monocarboxylic acid ester of an alkylene glycol monoalkyl ether which comprises the steps of: reacting at a reaction pot temperature of from about 8O0C. to 165 C., a monocarboxylic or halogenated monocarboxylic acid having from I to 10 carbon atoms with an alkylene glycol monoalkyl ether having the formulae R(OCH2CHR' )nOH and EMI14.1 wherein R is a straight or branched chain alkyl group containing from 1 to 10 carbon atoms, Rl is hydrogen or a methyl group and n is an integer of from 1 to 3, in the presence of an acid catalyst and a water-azeotroping solvent which forms a binary minimum-boiling azeotrope with water within the reaction temperature range; ; removing water via a distillation column during said reaction by azeotropic distillation with said water-azeotroping solvent; separating the water-solvent azeotrope forming a solvent phase and a water phase; refluxing the solvent phase and the water phase to the overhead section of the distillation column to reduce and maintain an overhead temperature in said column at between about 650C. and 950C.; and recovering the desired monocarboxylic acid ester. 2. A method according to Claim 1 wherein the monocarboxylic acid is acetic acid. 3. A method according to Claim 1 or Claim 2 characterised in that the alkylene glycol monoalkyl ether is propylene glycol monomethyl ether or dipropylene glycol monomethyl ether. 4. A method according to any one of Claims 1 to 3 characterised in that the acid catalyst is selected from sulfuric acid, p-toluene sulfonic acid and sulfonated polyaromatic ion exchange resins and is preferably p-toluene sulfonic acid. 5. A method as claimed in any one of Claims 1 to 4 characterised in that the reaction temperature is between 1150C and 1450C. 6. A method as claimed in any one of Claims 1 to 5 characterised in that between about 10 and 95 weight percent, and preferably between about 25 and 75 weight percent, of the water phase is refluxed to the overhead section of the distillation column. 7. A method as claimed in any one of Claims I to 6 characterised in that the water-azeotroping solvent is toluene. 8. A'method as claimed in any one of Claims 1 to 7 characterised in that between about 1 to 25 weight percent, and preferably between about 3 and 10 weight percent, water-azeotroping solvent is employed in the reaction based on the total reaction mixture. 9. A method as claimed in any one of Claims 1 to 8 characterised in that the acid catalyst is employed in a concentration of from about 0.01 to 5 weight percent, and preferably about 0.1 to 2.0 weight percent, based on the total reaction mixture. .10. A method as claimed in any one of Claims I to 9 characterised in that the reaction is carried out under a superatmospheric pressure of up to 50 psig. 11. A method for the preparation of propylene glycol monomethyl ether acetate which comprises the steps of reacting at a reaction pot temperature of from about 115 C to 145 C acetic acid with propylene glycol monomethyl ether, in the presence of p-toluene sulfonic acid catalyst and toluene waterazeotroping solvent; removing water via a distillation column during said reaction by azeotropic distillation with said toluene; separating the water-toluene azeotrope forming a water phase and a toluene phase; ref fluxing the toluene phase and between about 25 and 75 weight percent of the water phase to the overhead section of the distillation column to reduce and maintain an overhead temperature in said column at between about 820C and 880C and recovering the desired propylene glycol monomethyl ether acetate.

Inventors: GUPTA, VIJAI P.
Applicants: ATLANTIC RICHFIELD COMPANY
Application year: 1984
Main classification: C07C67
IPC codes: C07C69, B01D3, C07C67