GB2063261A - Esterification of Carboxylic Acids with Alcohols - Google Patents

Abstract

An improved method for the preparation of esters comprises contacting an organic carboxylic acid having 1 to about 18 carbon atoms with an alcohol having about 1 to about 20 carbon atoms in the presence of a catalytic amount of a perfluorosulfonic acid resin which is a copolymer of tetrafluoroethylene and sulfonyl fluoride vinyl ether.

Description

SPECIFICATION Esterification of Carboxylic Acids with Alcohols This invention relates to the esterification of carboxylic acids with alcohols and in particular to the use of perfluorosulfonic acid resins as catalysts.

Organic esters represent a class of compounds which serve many useful purposes. Acetate esters are solvents for a multitude of products as well as useful extractants in many processes. Acrylic esters are monomers which can be formulated into a variety of polymeric end products. Other higher hydrocarbon acids can be esterified to form essential fragrances and high performance products.

Traditional esterification processes involving the catalyzed reaction of a carboxylic acid with an alcohol routinely employ a soluble catalyst such as sulfuric acid, toluenesulfonic acid, phosphoric acid and the like. These processes require either neutralization of the catalyst prior to recovery of the esterified product or vaporization of the esterified product away from the catalyst prior to purification and recovery. Each of these options is wasteful from an economic point of view.

Neutralization of the catalyst causes neutralization of unreacted carboxylic acid starting product, lowers the reaction efficiency, and requires constant addition of catalyst to the reactor. On the other hand, vaporization of the product away from the catalyst also causes removal of starting material with the product. Such a technique can involve a large recycle volume with concurrent energy waste.

Prior art solutions of these problems include among others, the use of a catalyst that can be filtered or isolated from the reaction mixture by mechanical methods. Thus a catalyst which is soluble in the reaction solution at high temperatures but insoluble at low temperatures is beneficial.

Another solution has been the use of ion exchange resins which are insoluble and therefore separable from the reaction components.

However each of these solutions introduces still another problem. Thus for example, the stannous catalysts require temperatures in the neighborhood of 2000C. in order to be effective. The ion exchange resins are relatively brittle and subject to mechanical degradation.

By practice of the present invention there may be provided a catalyst system for the preparation of esters which obviates the catalyst difficulties of the prior art.

According to the present invention there is provided a facile method of preparing esters, which comprises contacting an organic carboxylic acid having 1 to about 1 8 carbon atoms with a saturated aliphatic alcohol containing about 1 to about 20 carbon atoms, in a mole ratio of carboxylic acid to alcohol of from 5:1 to about 1:5, at a temperature of about 300C to about 2000C., at a pressure of about 1 mm Hg up to about 50 atmospheres, and in the presence of a catalytic amount of a perfluorosulfonic acid resin which is a copolymer of tetrafluoroethylene and a sulfonyl fluoride vinyl ether.

Although the reaction temperature may vary from about 300C. to about 2000C., it is preferred to carry out the esterification at a temperature of from about 70 C. to about 1000C.

Although the pressure can vary from about 1 mm Hg to about 50 atmospheres, it is preferred to carry out the esterification at a pressure of about 1 to about 5 atmospheres.

The carboxylic acids which are most suitable for use in this invention include formic, acetic, propionic, and higher saturated aliphatic acids; diprotic acids, such as, oxalic, malonic, succinic, glutaric, adipic, and the like; unsaturated acids, such as, acrylic acid, methacrylic acid, crotonic acid, and like higher unsaturated acids; as well as aromatic acids, such as, benzoic acid, phthalic acid, and the like.

The perfluorosulfonic acid resins used as catalysts in this invention are commercially available under the trademark Nafion from the Du Pont de Nemours Company at Wilmington, Delaware. Suitable variations of these resins are described in U.S. Patent Specification No. 4,065,512 and in Du Pont "Innovation", Volume 4, No. 3, Spring 1973.

The term "catalytic amount of perfluorosulfonic acid resin" is meant to mean concentrations based on the total reaction mixture of about 0.001% to about 50% by weight.

It is surprising to find that the perfluorosulfonic acid resins are as active as conventional acid esterification catalysts. In addition, the Nafion catalyst resins are extremely resilient and do not deteriorate in physical form to a powder during use as do the ion exchange resins. No signs of wear or tear have been observed in these catalysts after many hours of use making esters. Furthermore, because of this great durability, the Nafion catalysts are ideally suited for use as esterification catalysts in a fluidized bed-type of reactor.

As polymer supported catalysts, Nafion is also well suited for use in packed bed reactors.

No special equipment is required for carrying out the ester preparation of this invention and therefore conventional reactors, stirrers, heating sources, distillation apparatus, and the like can be employed. This invention may be practiced as either a batch or continuous process. However, perhaps the greatest advantage of the Nafion resin over other acid catalysts in these esterification reactions is that lower reaction temperatures are possible. This is extremely important in the case of heat-sensitive products, such as, the acrylic esters. It has been found that the production rate can be maintained by increasing the catalyst concentration and decreasing the reaction temperature. This offers a significant economic advantage in that less fouling occurs and lower levels of in-process inhibitors are required.

The saturated aliphatic alcohols of this invention include not only methanol and its homologues, including for example, ethanol, n-propanol, n-butanol, n-octanol, n-decanol, n-octadecanol and the like; but also structural isomers, such as, isopropanol, t-butanol, isooctanol, isodecanol, isooctadecanol, and the like; glycol ethers, such as, Cellosolve (trademark of Union Carbide Corporation for the monomethylether of ethylene-glycol), Carbitol (trademark of Union Carbide Corporation for the monomethyl ether of ethylene diglycol) and the like.

The present invention will now be further described by way of the following examples. All parts and percentages are by weight unless otherwise specified.

Example 1 Formation of Ethyl Acrylate A 250 ml flask containing 72.1 g acrylic acid and 46.1 g ethyl alcohol was placed in a 680C.

constant temperature bath. When the contents had reached a constant temperature, 5.9 g (5.3 xl 0-3 equivalents) Nafion 811 tubing in sections 1 cm long were added to the flask. The contents of the flask were stirred and samples were removed periodically for determination of the kinetics of the reaction.

The data thus obtained are presented below under the headings, Reaction Time, Acid Concentration and Ester Concentration.

Reaction Time Acid Concentration Ester Concentration (Minutes) {Moles/Liter) (Moles/Liter) 0 7.58 0.0 5 7.48 0.1 10 7.37 0.21 20 7.16 0.42 30 6.95 0.63 40 6.72 0.86 60 6.33 1.25 90 6.13 1.45 120 5.92 1.76 150 5.71 1.87 The integrated rate expression for a second order reversible of the type

with the condition that tcAo] - [cBo] and [CcO] [CDo] is:

where X=CAOCA and CAO is the initial concentration of A.

A plot of the left hand side of this equation vs. t gives a slope equal to 2k1CAO/#K. This This allows evaluation of k1,the forward rate constant.

The slope was determined to be 4.5x 10-3 min Therefore, 4.5x 1 0-30 4.5 X 1 0-1 .40) 6.3 x 1 0-31 4.2 x 1 0-41 k1= min 2CAo min 2 (7.58 mole) 15.2 min mole min mole Published data by V.R. Dnanuka, V.C. Malshe, and S.E. Chandalia; Chem. Eng. Sci., 32,551-6 (1977), for the esterification of acrylic acid with ethanol using H2S04, (5.02x 10-2 mole/liter), as the catalyst at 820C. gave a 6.15x10-41 k1= min mole These results indicate that the polymer-bound catalyst is as good as the conventional H2SO4 catalyst.

Example 2 Formation of Cellosolve Acetate Using H2SO4 A two liter flask was charged with 480 9 acetic acid, 675 g Cellosolve solvent and placed in a 70"0. constant temperature bath. When the flask contents reached constant temperature, 1.80 9 of H2SO4 dissolved in 45.5 g Cellosolve solvent was added to the flask. Samples were withdrawn periodically for determination of kinetic parameters. The data obtained are shown below.

Reaction Time Acid Concentration Ester Concentration (Minutes) {Moles/Liter) (Moles/Liter) 0 6.30 0 2 5.97 .33 9 5.61 .69 17 5.16 1.14 26 4.87 1.42 34 4.63 1.68 39 4.50 1.80 62 4.08 2.22 82 3.86 2.44 2845 2.86 3.44 Treatment of this data using equation 1 gave: 1.6x10-2 2klCAo slope= =- min 1.6x10-2iT 10-2(1.24) liter 1.57x10-31 k,= = = min 2 CAO 2 (6.3) Moles Min Moles Min Example 3 Preparation of Cellosolve Acetate Using Nafion 811 A 250 ml flask was charged with 67.6 9 Cellosolve solvent and 48.2 9 acetic acid and heated to 700C. When a constant temperature was reached, 1.8 g Nafion 811(1 .45x 10-2 Equivalent/L) tubing which had been cut into 1 cm lengths was added, and the resulting mixture was agitated with an overhead stirrer.Samples were withdrawn periodically for determination of kinetic parameters. The data obtained are presented below.

Reaction Time Acid Concentration Ester Concentration (Minutes) {Moles/Liter) (Moles/Liter) 0 6.50 0.0 2 6.37 0.13 6 6.20 0.30 10 6.07 0.43 15 5.96 0.54 20 5.83 0.67 30 5.51 0.99 40 5.36 1.14 50 5.23 1.27 60 5.01 1.49 97 4.51 1.99 120 4.28 2.22 150 3.95 2.55 The slope of the line is equal to 2 CAok as shown in equation 1. Thus: 7.67x103 2klCAo Min 7.67x103(1.24) 7.32x1041 k,= Min 2(6.5 Mole/Liter) Mole Min Although the k, value derived for the Nafion-catalyzed Example 3 is lower than that for H2SO4 - catalyzed Example 2, nevertheless, it is indicative of a very strong acid catalyst which is well suited for this application.

Example 4 Preparation of Cellosolve Acetate Using Amberlyst XN-1010 A 500 ml flask was charged with 135.2 g Cellosolve solvent and 96.4 g acetic acid and heated to 700C. When a constant temperature was attained, 3.8 9 (5.6x10-2 Equivalent/L) Amberlyst XN-1010 resin was added and the resulting mixture was agitated with an overhead stirrer. Samples were withdrawn periodically for determination of kinetic parameters. The data are presented below.

Reaction Time Acid Concentration Ester Concentration (Minutes) {Moles/Liter) {Moles/Liter) 0 6.58 0 2 6.43 0.15 5 6.33 0.25 12 6.12 0.46 20 5.88 0.70 25 5.78 0.80 30 5.68 0.90 40 5.48 1.10 50 5.25 1.33 60 5.00 1.58 90 4.58 2.00 120 4.25 2.33 180 3.93 2.650 From a plot of the above data, the slope of the line is equal to 2C k v'w as shown in Equation 1.

Thus: 8.5x 10-3 2klCAo min 8.5x10-3(1.24) 8.0x10-4 liter k,- = min 2(6.58 mole/liter) mole min Thus the k, for the Amberlyst XN-1010 resin is only slightly better than Nafion when used on an equal weight basis. But when compared on the equivalents of acid catalyst used, Nafion is much superior, as shown in the calculation below.

Nafion 7.32x104 liter k,= mole min 1.45 X 10-2 Eq Acid equivalents= liter k, =5.04x 10-2 Acid Equivalent Amberlyst XN-1010 8.0x 10-4 liter k,= mole min 5.6x 10-2 moles Acid equivalents= liter k, 8.0x 10-4 = = 1.43x10-2 Acid Equivalent 5.6x 10-2 Effectiveness of Nafion over Amberlyst Nafion 5.04x 10-2 =3.52 Amberlyst 1 .43x 10-2

Claims (13)

Claims

1. A method of preparing esters, which comprises contacting an organic carboxylic acid having one to about 1 8 carbon atoms with a saturated aliphatic alcohol having about one to about 20 carbon atoms, in a mole ratio of carboxylic acid to alcohol of from about 5:1 to about 1 :5, at a temperature of about 300C. to about 2000 C., at a pressure of 1 mm Hg up to about 50 atmospheres, and in the presence of a catalytic amount of a perfluorosulfonic acid resin which is a copolymer of tetrafluoroethylene and a sulfonyl fluoride vinyl ether.

2. A method as claimed in claim 1, wherein the mole ratio of carboxylic acid to alcohol is about 1:1.

3. A method as claimed in claim 1 or 2, wherein the carboxylic acid is acrylic acid.

4. A method as claimed in claim 1 or 2, wherein the carboxylic acid is acetic acid.

5. A method as claimed in any of claims 1 to 4, wherein the alcohol is ethanol.

6. A method as claimed in any of claims 1 to 4, wherein the alcohol is a monoalkyl ether of a glycol.

7. A method as claimed in claim 6, wherein the monoalkyl ether of a glycol is ethylene glycol monomethyl ether.

8. A method as claimed in any of claims 1 to 4, wherein the alcohol is a monoalkyl ether of a glycol ether.

9. A method as claimed in claim 8 wherein the monoalkyl ether of a glycol ether is the monomethyl ether of ethylene diglycol.

10. A method as claimed in any of claims 1 to 9, wherein the temperature is in the range of about 700C. to about 1000C.

11. A method as claimed in any of claims 1 to 10, wherein the pressure is in the range of about 1 to about 5 atmospheres.

1 2. A method as claimed in claim 1 and substantially as hereinbefore described with reference to any of the Examples.

13. Esters whenever prepared by a method as claimed in any of claims 1 to 12.