Claims:1. A process for preparing an organic carboxylic ester from an organic carboxylic acid selected from the group consisting of acetic acid, propanoic acid, and butanoic acid; said process comprising:
a. dissolving equimolar quantities of a hydrogen donor compound selected from the group consisting of para-toluenesulfonic acid, oxalic acid, maleic acid, methanesulfonic acid and tartaric acid, and choline chloride in a medium comprising an alcohol to obtain an ionic fluid containing an in-situ generated ionic compound; and
b. reacting the ionic fluid with an organic carboxylic acid in the presence of the ionic compound under constant stirring at a temperature in the range of 20 oC to 50 oC to obtain an organic carboxylic ester.

2. The process as claimed in claim 1, wherein the alcohol is at least one selected from the group consisting of methyl alcohol, ethyl alcohol, iso-propyl alcohol, and n-butyl alcohol.

3. The process as claimed in claim 1, wherein the proportion of the organic carboxylic acid and the alcohol expressed in terms of molar ratio is in the range of 1:20 to 20:1.
4. The process as claimed in claim 1, wherein the esterification reaction in the method step (b) is carried out at a temperature in the range of 25 oC to 27 oC. , Description:This is a divisional application in furtherance to the first application number 1585/MUM/2011 dated 27/05/2011.

FIELD OF THE INVENTION:
The present invention relates to a process for preparing an organic carboxylic ester from an organic carboxylic acid in the presence of ionic fluids.

BACKGROUND:
Ionic fluids are examples of viscous molten salts which are liquid at or below ambient temperatures. In the last two decades, ionic fluids have found profound use in a number of applications such as solvents, co-solvents and catalysts, because of a number of interesting and useful properties and utilities associated with them. Diversified areas including electrochemistry, synthesis of chemical compounds, dyes, batteries, fuel cells, photovoltaic devices, electro-deposition processes, semi-conductor cleaning, pyrolysis, gasification, and applications involving cellulose dissolution have observed successful implementation of ionic fluids in place of conventional molecular solvents.
The need to replace conventional molecular acid catalysts with ionic fluid arises from the urge to develop environmentally safe processes. Ionic fluids exhibit very low or zero vapor pressure and thus, in contrast to many conventional molecular solvents, do not produce any vapors.

EXISTING KNOWLEDGE:
United States Patent No. 5892124 discloses quaternary ammonium or phosphonium based ionic fluid having general formula of Q+A- wherein A- represents various anions including tetrachloroaluminate and trichlorozincate. The feasibility of ionic fluid is disclosed for Diel-Alder reactions.
United States Patent No. 6573405 discloses quaternary ammonium based ionic compound formed by the reaction of a quaternary ammonium compound of formula R1R2R3R4+N+X- or a mixture of two or more thereof, with a halide of zinc, tin or iron, or a mixture of two or more thereof. The preferred ionic compound derived from choline chloride and zinc chloride for electrochemical, electrodeposition, electrochromics and dissolution of metal oxides, battery and Diel-Alder reactions is disclosed.
Another United States Patent No. 7183433 discloses quaternary ammonium based ionic compound derived from the reaction of a quaternary ammonium compound of formula R1R2R3R4N+X- or mixture of two or more thereof and urea; for example choline chloride and urea. Similarly, attempts were also made to synthesize ion liquid of quaternary ammonium compound of the formula R1R2R3R4N+X- with specific hydrogen donor compounds chosen from carboxylic acids, amides, ethers, esters, aldehydes, ketones, alcohols, and carbohydrates. Anions of quaternary ammonium compound of the formula R1R2R3R4N+X- were chloride, nitrate and tetraborate. The aforementioned US Patent further discloses the effect of anion X- by varying anions with symmetric amine salts.
The esterification of alcohols and carboxylic acids to produce respective alkyl esters is known to be carried out in the presence of acid catalysts. Employment of cation exchange resin is largely reported to facilitate esterification reactions. However there are few processes which use mineral acids, mostly sulfuric acid and organic catalyst such as phosphoric acid, para-toluenesulfonic acid (PTSA). Acidic inorganic catalysts are also employed in some instances. The prior-art methods for the esterification of alcohols are carried out at temperatures in the range of 50 to 150 oC. The esterification reactions are equilibrating reactions; therefore removal of one of the product is highly desirable to move the reaction forward thereby increasing the conversions.
Esterification reactions as reported in European Patent No.0087870, US2719166, US3014066, US4435595, US4481146, US4939294, US5430178, US5518699, US5998658, US4939294, US5430178, US5518699, US5998658, US6028215, US6458992, US6693123 and United States Patent Application No.2006/0041162 include high temperature reactions to achieve acceptable per mass conversions, leading to large reflux ratio.
The hitherto reported reactions however suffer from numerous drawbacks which include requirement of high temperature conditions and being non-ecofriendly.
Therefore, there is felt the need for the development of a new catalyst system for the preparation of organic carboxylic esters by reaction of alcohols and organic carboxylic acids which is cost-effective and environmentally friendly.

OBJECTS OF THE INVENTION:
It is an object of the present invention to provide a process for preparing an organic carboxylic ester from an organic carboxylic acid in the presence of an ionic fluid.
Another object of the present invention is to provide a process for preparing an organic carboxylic ester from an organic carboxylic acid and an alcohol with a high conversion rate.
Still another object of the present invention is to provide a process for preparing an organic carboxylic ester from an organic carboxylic acid and an alcohol that is carried at a low temperature.
Yet another object of the present invention is to provide a cost efficient process for preparing an organic carboxylic ester from an organic carboxylic acid and an alcohol.
A further object of the present invention is to provide an environmentally safe process for preparing an organic carboxylic ester from an organic carboxylic acid and an alcohol.

DEFINITIONS:
As used in the present specification, the following words and phrases are generally intended to have the meanings as set forth below, except to the extent that the context in which they are used indicates otherwise.
The phrase “ionic fluid” is used herein to refer to a prepared solvate in a medium, whereby an ionic compound is formed in-situ by dissolving the mixture of a hydrogen donor compound and a quaternary ammonium salt in the medium.

SUMMARY OF THE INVENTION:
In accordance with the present disclosure, there is provided a process for preparing an organic carboxylic ester from an organic carboxylic acid selected from the group consisting of acetic acid, propanoic acid, and butanoic acid; said process comprising the following steps:
a. dissolving equimolar quantities of a hydrogen donor compound selected from the group consisting of para-toluene sulfonic acid, oxalic acid, maleic acid, methane sulfonic acid and tartaric acid, and choline chloride in a medium comprising an alcohol to obtain an ionic fluid containing an in-situ generated ionic compound; and
b. reacting the ionic fluid with an organic carboxylic acid in the presence of the ionic compound under constant stirring at a temperature in the range of 20 oC to 50 oC to obtain an organic carboxylic ester.
In accordance with the embodiments of the present disclosure, the alcohol is at least one selected from the group consisting of methyl alcohol, ethyl alcohol, iso-propyl alcohol, and n-butyl alcohol.
In accordance with the embodiments of the present disclosure, the organic carboxylic ester is at least one selected from the group consisting of methyl acetate, ethyl acetate, iso-propyl acetate, n-butyl acetate, amyl acetate and hexyl acetate.
The proportion of organic carboxylic acid and alcohol expressed in terms of molar ratio is in the range of 1:20 to 20:1.
The esterification reaction in the method step (b) is carried out at a temperature in the range of 25 oC to 27 oC.

DETAIL DESCRIPTION OF THE INVENTION:
The present disclosure envisages a process for preparing an organic carboxylic ester from an organic carboxylic acid selected from the group consisting of acetic acid, propanoic acid, and butanoic acid, and an alcohol in the presence of an ionic compound.
The ionic compounds with cations and anions connected by hydrogen bond are reported to have supramolecular structural organization (ref.: Oliver-Bourbigou, H., et al., Applied Catalysis A: General, 373, 1-56, 2010; Deetlefs, m., et al., J. Physical Chemistry B.110, 12055-12061; Canongia Lopez, J.N. and Padua, A.A.H., J. Physical Chemistry B.110, 3330-3335, 2006). The continuous micro domain structures formed due to the network of hydrogen bond seems to be favorable for acid catalyzed reactions.
In the prior-art, the preparation of a deep eutectic ionic compound in larger quantities is found to be tedious. The resultant ionic compound being very viscous in nature, the process for making the ionic compound is time consuming and it also requires high temperature.
The ionic compounds used for the preparation of ionic fluids in the present disclosure are derived from a quaternary ammonium salt and a hydrogen donor compound.
The ionic fluid which is used as an acid catalyst in accordance with the present disclosure is prepared by a simple process at a low temperature that overcomes the drawbacks associated with the preparation of ionic compounds as reported in the prior art.
In accordance with the present disclosure, the process for preparing organic carboxylic ester from organic carboxylic acid in the presence of prepared ionic fluid is carried out as follows:
Equimolar quantities of a hydrogen donor compound and a quaternary ammonium salt are dissolved in a medium under constant stirring to obtain an ionic fluid containing in-situ generated ionic compound.
The medium in accordance with the present disclosure comprises an alcohol.
To the obtained ionic fluid, the organic carboxylic acid is added under constant stirring at a temperature in the range of 20 oC to 50 oC and at atmospheric pressure.
In accordance with the present disclosure, the alcohol is at least one selected from the group consisting of methyl alcohol, ethyl alcohol, iso-propyl alcohol and n-butyl alcohol.
In accordance with the present disclosure, the organic carboxylic acid is at least one selected from the group consisting of acetic acid, propanoic acid and butanoic acid.
After adding the organic carboxylic acid, the obtained reaction mixture is stirred at the pre-determined temperature until an equilibrium condition is achieved.
In accordance with one of the embodiment of the present disclosure, the method step of esterification is preferably carried out at a temperature in the range of 25 oC to 27 oC.
In accordance with the present disclosure, the proportion of the organic carboxylic acid and the alcohol expressed in terms of molar ratio is in the range of 1:20 to 20:1.
For comparison, the esterification of the organic carboxylic acid and the alcohol to respective organic carboxylic ester was carried out in the presence of the hydrogen donor compound alone, under the same reaction condition as maintained during esterification of the organic carboxylic acid and the alcohol in the presence of the ionic-fluid of the present disclosure.
The analytical data for the esterification of organic carboxylic acid and alcohol in the presence of in-situ generated ionic fluid and in the presence of the hydrogen donor compound alone is tabulated in Table 1 to Table 8 of the present disclosure.
From the data as provided in Table 1 to Table 8, it is evident that equilibrium conversion is higher in both the cases i.e., for the esterification carried out in the presence of in-situ generated ionic fluid and in the presence of the hydrogen donor compound alone.
However, in some instances rate of the esterification carried out in the presence of ionic fluid is faster; for example in the case of esterification of ethanol carried out in the presence of ionic fluid derived from oxalic acid, maleic acid and tartaric acid (ref: Table 3 and Table 4 of the present disclosure).
In case of an iso-propyl alcohol, considerably high rate of conversion is achieved in the presence of in-situ generated ionic fluids derived from para-toluene sulfonic acid, oxalic acid, maleic acid and tartaric acid, in comparison to the esterification carried out in the presence of only the hydrogen donor compound (ref: Table 5 and Table 6 of the present disclosure).
The esterification of organic carboxylic acid and alcohol does not require high temperature conditions to achieve high conversion rates. The miscibility of the two reactants along with catalyst is one of the reasons to achieve the high equilibrium conversions at 26 oC.
In some instances, there is faster equilibrium in both the cases, nevertheless the presence of ionic compound as catalyst system obviates the drawbacks associated with use of conventional acid catalyst systems.
The equilibrium conversion achieved in the case of esterification of organic carboxylic acid and alcohol carried out in the presence of in-situ generated ionic fluid eliminates the need of complex reactive distillation columns thereby reducing capital and operational investments.
In accordance with the present disclosure, the hydrogen donor compounds are selected from the group consisting of methane sulfonic acid (MSA), para-toluenesulfonic acid (PTSA), oxalic acid, maleic acid and tartaric acid.
The quaternary ammonium salt as used herein the present disclosure is choline chloride.
The in-situ formation of ionic compound in suitable medium in accordance with the present disclosure is confirmed by the enhanced solubility of hydrogen donor compound in a suitable medium. For example, solubility of oxalic acid in water at 25 oC is 14%, which is enhanced to 30 % in the presence of choline chloride. This phenomenon clearly indicates the formation of ionic compound through hydrogen bond enhancing the solubility of oxalic acid at 25 oC. Otherwise, a water temperature of about 40-45 oC is required to solubilize 30 % of oxalic acid in water.
The inventors of the present disclosure have surprisingly found out that the product obtained by the process of the present disclosure remains free of residual hydrogen donor compound. This obviates the need for neutralization of the reaction mixture before isolation of the product as is required in the known processes.
The present disclosure will be further described with reference to the following non-limiting examples.
The laboratory scale experiments provided herein can be scaled up to industrial or commercial scale.
Example 1:
Toluene-4-sulfonic acid monohydrate (PTSA), a hydrogen donor compound (5.7 gm) and choline chloride (4.2 gm) in equal molar ratio was added to 15 ml of methyl alcohol under constant stirring to obtain an ionic fluid. To the prepared ionic fluid, 21.5 ml of acetic acid was added. The reaction was carried out at 26 oC. Methyl alcohol was esterified to methyl acetate and water, and equilibrium conversion was found to be 78 %.

Example 2-5:
The procedure of example 1 was followed with different hydrogen donors in lieu of PTSA and the esterification was carried out. The details are provided in Table-1.
Table 1: Esterification of methanol at 26 oC with ionic compounds
Example No. Acetic acid, ml Methanol, ml Hydrogen donor, gm Choline Chloride, gm Reaction Time, min Conversion of methanol %
1 21.5 15 PTSA(5.7) 4.2 30 78
2 21.5 15 Oxalic acid (3.78) 4.2 30 74
3 21.5 15 Maleic acid (3.48) 4.2 30 77
4 21.5 15 Tartaric acid (4.5) 4.2 30 65
5 21.5 15 Methane sulfonic acid (2.9) 4.2 15 75

Example 6-10:
The procedure of example 1 was followed except for the acids (hydrogen donor compounds) only were used as catalyst for the esterification (no ionic compounds). The details are provided in Table-2.
Table 2: Esterification of methanol at 26 oC with hydrogen donor alone
Example No. Acetic acid, ml Methanol, ml hydrogen donor, gm Reaction Time, min Conversion of methanol %
6 21.5 15 PTSA(5.7) 30 70
7 21.5 15 Oxalic acid (3.78) 30 65
8 21.5 15 Maleic acid (3.48) 30 61
9 21.5 15 Tartaric acid (4.5) 30 28
10 21.5 15 Methane sulfonic acid (2.9) 30 79

Example 11-15:
The procedure of example 1 was followed except, methyl alcohol was replaced with ethyl alcohol. Ethyl alcohol was esterified to ethyl acetate and water. The conversion details are provided in Table-3.

Table 3: Esterification of ethanol at 26 oC with ionic compounds
Example No. Acetic acid, ml Ethanol, ml Hydrogen donor, gm Choline Chloride, gm Reaction Time, min Conversion of ethanol %
11 14.8 15 PTSA(5.7) 4.2 25 70
12 14.8 15 Oxalic acid (3.78) 4.2 25 69
13 14.8 15 Maleic acid (3.48) 4.2 30 66
14 14.8 15 Tartaric acid (4.5) 4.2 30 38
15 14.8 15 Methane sulfonic acid (2.9) 4.2 18 75

Example 16-20:
The procedure of example 1 was followed except for the acids (hydrogen donor compounds) only were used as catalyst for the esterification (no ionic compounds). The details are provided in Table-4.
Table 4: Esterification of ethanol at 26 oC with hydrogen donor
Example No. Acetic acid, ml Ethanol, ml hydrogen donor, gm Reaction Time, min Conversion of ethanol %
16 14.8 15 PTSA(5.7) 30 69
17 14.8 15 Oxalic acid (3.78) 30 53
18 14.8 15 Maleic acid (3.48) 30 44
19 14.8 15 Tartaric acid (4.5) 30 10
20 14.8 15 Methane sulfonic acid (2.9) 30 73

Example 21-25:
The procedure of example 1 was followed except, methyl alcohol was replaced with isopropyl alcohol. Iso-propyl alcohol was esterified to propyl acetate and water. The conversion details are provided in Table-5.
Table 5: Esterification of iso-propyl alcohol at 26 oC with ionic compounds
Example No. Acetic acid, ml Iso-propanol, ml Hydrogen donor, gm Choline Chloride, gm Reaction Time, min Conversion of IPA, %
21 11.2 15 PTSA(5.7) 4.2 30 82
22 11.2 15 Oxalic acid (3.78) 4.2 30 64
23 11.2 15 Maleic acid (3.48) 4.2 30 62
24 11.2 15 Tartaric acid (4.5) 4.2 30 32
25 11.2 15 Methane sulfonic acid (2.9) 4.2 20 68

Example 26-30:
The procedure of example 1 was followed except for the acids (hydrogen donor compounds) only were used as catalyst for the esterification (no ionic compounds). The details are provided in Table-6.

Table 6: Esterification of iso-propyl alcohol at 26 oC with hydrogen donor
Example No. Acetic acid, ml Iso-propanol, ml hydrogen donor, gm Reaction Time, min Conversion of IPA, %
26 11.2 15 PTSA(5.7) 30 66
27 11.2 15 Oxalic acid (3.78) 30 23
28 11.2 15 Maleic acid (3.48) 30 12
29 11.2 15 Tartaric acid (4.5) 30 8
30 11.2 15 Methane sulfonic acid (2.9) 30 69

Example 31-35:
The procedure of example 1 was followed except, methyl alcohol was replaced with n-butyl alcohol. n-butyl alcohol was esterified to butyl acetate and water. The conversion details are provided in Table-7.
Table 7: Esterification of n-butanol at 26 oC with ionic compounds
Example No. Acetic acid, ml Butanol, ml Hydrogen donor, gm Choline Chloride, gm Reaction Time, min Conversion of butanol %
31 9.4 15 PTSA(5.7) 4.2 20 76
32 9.4 15 Oxalic acid (3.78) 4.2 30 67
33 9.4 15 Maleic acid (3.48) 4.2 20 62
34 9.4 15 Tartaric acid (4.5) 4.2 30 23
35 9.4 15 Methane sulfonic acid (2.9) 4.2 20 78

Example 36-40:
The procedure of example 1 was followed except for the acids (hydrogen donor compounds) only were used as catalyst for the esterification (no ionic compounds). The details are provided in Table-8.
Table 8: Esterification of n-butanol at 26 oC with hydrogen donor
Example No. Acetic acid, ml Butanol, ml hydrogen donor, gm Reaction Time, min Conversion of butanol, %
36 9.4 15 PTSA(5.7) 30 76
37 9.4 15 Oxalic acid (3.78) 30 57
38 9.4 15 Maleic acid (3.48) 30 63
39 9.4 15 Tartaric acid (4.5) 30 3
40 9.4 15 Methane sulfonic acid (2.9) 30 78

TECHNICAL ADVANTAGES:
Technical advantages of the present disclosure lie in providing a process for preparing an organic carboxylic ester from an organic carboxylic acid and an alcohol in the presence of an ionic fluid comprising:
1. in-situ generation of an ionic fluid, thereby eliminating the process step of
synthesizing deep eutectic ionic compound separately;
2. time and energy saving process over conventional processes;
3. cost efficient, eco-friendly and safe processes over conventional processes.

“Whenever a range of values is specified, a value up to 10 % below and above the lowest and highest numerical value respectively, of the specified range, is included in the scope of the invention”.
Throughout this specification the word “comprise”, or variations such as “comprises” or “comprising”, will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but not the exclusion of any other element, integer or step, or group of elements, integers or steps.
The use of the expression “at least” or “at least one” suggests the use of one or more elements or ingredients or quantities, as the use may be in the embodiment of the disclosure to achieve one or more of the desired objects or results.
Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form a part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.
The numerical values mentioned for the various physical parameters, dimensions or quantities are only approximations and it is envisaged that the values higher/lower than the numerical values assigned to the parameters, dimensions or quantities fall within the scope of the disclosure, unless there is a statement in the specification specific to the contrary.
While considerable emphasis has been placed herein on the preferred embodiments, it will be appreciated that many embodiments can be made and that many changes can be made in the preferred embodiments without departing from the principles of the invention. These and other changes in the preferred embodiments as well as other embodiments of the invention will be apparent to those skilled in the art from the disclosure herein, whereby it is to be distinctly understood that the forgoing descriptive matter to be implemented merely as illustrative of the invention and not as limitation.