FORM 2
THE PATENTS ACT, 1970
(39 of 1970)
&
THE PATENTS RULES, 2003
COMPLETE SPECIFICATION
(See section 10, rule 13)
/. Title of the invention'. POLYOL ESTER USEFUL AS LUBRICANT BASE, AND ITS PROCESS OF
PREPARATION.
2- Applicant(s)
NAME NATIONALITY ADDRESS
BHARAT PETROLEUM Indian Bharat Bhavan, 4 & 6
CORPORATION LIMITED Currimbhoy Road, Ballard
Estate, Mumbai-400 001, Maharashtra, India
3. Preamble to the description
COMPLETE SPECIFICATION
The following specification particularly describes the invention and the manner in which it
is to be performed.

FIELD OF THE INVENTION
The present invention relates to a process for preparation of a polyol ester by reacting a polyol with at least one monocarboxylic acid in the presence of an acid catalyst and an azeotropic solvent at a suitable temperature to obtain the polyol ester. The polyol is preferably a polyol free of beta hydrogen and the monocarboxylic acid used is a sterically hindered and/or an alpha substituted acid.
The polyol esters obtained by the process have excellent thermal, oxidative and hydrolytic stability as compared to conventional esters. They also have excellent low temperature fluidity and miscibility with hydrofluorocarbons. Thus, they are useful as a lubricant base for refrigeration application.
BACKGROUND OF THE INVENTION
Polyol esters are used in a wide range of fields such as cosmetics, pharmaceutical preparations, food, electronic equipment, printing, and lubrication, etc. Each field requires esters with certain qualities which make them suitable for use in that particular field. For example, for esters to be used for grease, durability and anti-evaporation properties at high temperatures are essential properties. For esters to be used for engine oil, properties such as longevity and good thermal and oxidative stability are required. For esters to be used for refrigerating machine oils, they should have properties such as high electric insulation, heat resistance, low acid and hydroxyl value, hydrolytic stability and heat stability at high temperatures. It is also required that they should have minimal or no contaminants and conductive impurities. In this context, esters having alkyl groups at alpha position from ester group are found to have excellent hydrolytic stability as well as low temperature properties.
Polyol esters are widely used along with hydroflorocarbon (HFC) for refrigerant application which has replaced chloroflorohydrocarbons (CFC) due to environmental concerns and implementation of stringent environmental norms regarding refrigerants. The compressor manufacturers have been forced to explore other possible environment friendly refrigerant compositions. Thus, it is important to judge the compatibility of polyol oil based refrigeration oil with new refrigerants.

Generally, polyol esters are prepared by reacting a polyol (polyhydric alcohol) with a fatty acid in presence of a catalyst under heat. The reaction proceeds at low rate even if the reaction is carried out at a high temperature of 200°C or higher.
US Patent no. 5820777 discloses process of preparation of polyol esters by reacting polyol such as neopentyl glycol, pentaerythritol, trimethylolpropane, dipentaerythritol, and polyalkylene glycol (PAG) with fatty acids in presence of acid or base catalyst under an inert gas atmosphere. The reaction requires prolonged reaction time at a temperature in the range of about 220°C to 230°C. The reaction also requires vacuum aid to achieve completion. Such reaction conditions often lead to formation of dark colored product which requires extensive purification steps to obtain esters with desired properties.
Use of base catalysts such as N,N'-dicyclohexylcarbodiimide-4-(N,N-dimethylamino) pyridine, triphenyl phosphine-2,2'-dipyridyl sulfide or similar for esterification also produces colored, blackish brown reaction product.
Attempts have been made to prepare esters by employing ion exchange resins and other compounds to improve the product characteristics. JP Publication no. 54-132502 discloses that a light-colored ester is obtained by esterification reaction of a carboxylic acid or its anhydride with an alcohol in presence of an acidic catalyst such as sulfuric acid or p-toluenesulfonic acid, by adding phosphorous acid, hypophosphorous acid or their salts, and boric acid to the reaction system. JP Publication no. 54-091589 discloses esterification of a polycarboxylic acid with a polyhyric alcohol and a terminator by using a catalyst consisting of one or more types of compounds, e.g. hydroxides of alkali (earth) metals, such as caustic soda or potassium hydroxide, metallic oxides, such as zinc oxide, metal carbonates, such as sodium carbonate, silica, protonic acid, such as phosphonic acid, supported on a carrier, such as silica, and also a hypophosphorous acid or its salt. Although these methods have some effects in improvement of product characteristics in terms of its color and oxidative properties, thermal stability of the product has been found to be insufficient.
JP Publication No. 07-309937 discloses that less colored polyester are produced by adding a stabilizer containing a phosphorus-containing compound, a phenol-

containing compound, a thioether-containing compound, an amine compound during the esterification reaction. However, it is difficult to remove these stabilizers from the reaction product. Another limitation is that these remaining stabilizers in the ester act as an accelerator to deterioration of the ester. As a result, sludge may be produced or prolong used at high temperature may cause discoloration.
Synthesis of polyol ester using sterically hindered and/or alpha branched C5-C12 monocarboxylic acids by employing aforementioned conditions not only demands extremely high reaction time but also leads to formation of partial esters with un-reacted hydroxyl groups due to low reactivity of such acids or dark colored esters. Preparation of pentaerythritol ester of 2-ethylhexanoic acid is reported to take several days for completion and resulted into colored impure product (A. Wahlstrom and L. Vamling, J. Chem. Eng. Data 2000, 45, 97). This is mainly due to extremely low esterification reaction rate constant at temperature below 200°C for 2-ethylhexanoic acid (Die Angewandte Makromolekulare Chemie 1989, 165, 9). The use of such partial esters as refrigeration oil is undesirable as they are often found to fail hydrolytic stability tests.
WO 2010/085545 discloses preparation of polyol ester by reacting monocarboxylic acid and neopentylpolyol in presence of at least one acid esterification catalyst at temperature between about 150°C and about 250°C in a two step process. The process leads to production of partial esters and vacuum aid is required to facilitate the reaction. Another limitation is that the reaction mixture has to be heated to a high temperature in the range of 200°C to 260°C for the completion of the reaction.
Therefore, the existing process of preparation of polyol esters have several limitations in terms of high reaction temperature range (230°C to 240°C), prolong reaction time and use of vacuum to force completion of the reaction . Furthermore, the prepared polyol esters have to be subjected to multiple purification steps to obtain product with desired properties.
Hence there is a need for a process for preparation of polyol esters under mild temperature conditions and short reaction completion time. Also, the product of the reaction do not require extensive purification steps and have desirable properties like excellent thermal, oxidative and hydrolytic stability compared to conventional esters.

SUMMARY
The present invention relates to a process for preparation of a polyol ester, said process comprising mixing a polyol with at least one monocarboxylic acid to obtain a reaction mixture, wherein the mole ratio of hydroxyl group of said polyol and said acid group is in the range of 1:2.0 to 1:2.5; and stirring the reaction mixture at a temperature in the range of 1 SOT to 200°C for about 5 to 10 hours to obtain the polyol ester.
The present invention also relates to a process for preparation of a polyol ester, said process comprising mixing a polyol with at least one monocarboxyiic acid to obtain a reaction mixture, wherein the mole ratio of hydroxyl group of said polyol and said acid group is in the range of 1:2.0 to 1:2.4 and wherein said reaction mixture comprises an acid catalyst and an azeotropic agent; and stirring the reaction mixture at a temperature in the range of 160°C to 175°C for about 5 to 10 hours to obtain the polyol ester.
The present invention further relates to the polyol ester produced by the process of the present invention.
These and other features, aspects, and advantages of the present subject matter will become better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.
STATEMENT OF INVENTION
Accordingly the invention provides a process for preparing a polyol ester, said process comprising: mixing a polyol with at least one monocarboxyiic acid to obtain a reaction mixture, wherein the mole ratio of hydroxyl group of said polyol and said acid group is in the range of 1:2.0 to 1:2.5; and stirring the reaction mixture at a temperature in the range of 150°C to 200°C for about 5 to 10 hours to obtain the polyol ester.
There is also provided a polyol ester prepared by the process of the present invention for use as a lubricant base for refrigeration application.

DETAIL DESCRIPTION OF THE INVENTION
The present invention provides a process for preparation of a polyol ester, said process comprising mixing a polyol with at least one monocarboxylic acid to obtain a reaction mixture, wherein the mole ratio of hydroxyl group of said polyol and said acid group is in the range of 1:2.0 to 1:2.5; and stirring the reaction mixture at a temperature in the range of 150°C to 200°C for about 5 to 10 hours to obtain the polyol ester.
The present invention also provides a process for preparation of a polyol ester, said process comprising: mixing a polyol with at least one monocarboxylic acid to obtain a reaction mixture, wherein the mole ratio of hydroxyl group of said polyol and said acid group is in the range of 1:2.0 to 1:2.4 and wherein said reaction mixture comprises an acid catalyst and an azeotropic agent; and stirring the reaction mixture at a temperature in the range of 160°C to 175°C for about 5 to 10 hours to obtain the polyol ester,
The process of the present invention further comprises neutralization and adsorption by the adsorbent of the polyol ester to obtain a purified polyol ester.
The range of the mole ratio of hydroxyl group of said polyol and said acid group in the process of the present invention of 1:2.0 to 1:2.4 is critical for the process of the present invention.
In an embodiment of the present invention, the process for preparation of a polyol ester comprises removing water continuously during stirring of the reaction mixture. The removal of water can be accomplished by using Dean Stark apparatus.
In another embodiment of the present invention, the polyol used for the preparation of a polyol ester comprises 2 to 6 hydroxyl groups.
In still another embodiment of the present invention, the polyol used for the preparation of a polyol ester comprises 2 to 4 hydroxyl groups.
In yet another embodiment of the present invention, the polyol used for the preparation of a polyol ester is selected from the group consisting of neopentyiglycol, trimethylolpropane, pentaerythritol and dipentaerythritol.
In an embodiment of the present invention, the polyol used for the preparation of a polyol ester is trimethylolpropane.

In another embodiment of the present invention, the polyol used for the preparation of a polyol ester is pentaerythritol.
In still another embodiment of the present invention, the polyol used for the preparation of a polyol ester is neopentylglycol.
In yet another embodiment of the present invention, the monocarboxylic acid used for the preparation of a polyol ester is a sterically hindered or an alpha substituted monocarboxylic acid.
In an embodiment of the present invention, the monocarboxylic acid used for the preparation of a polyol ester is a sterically hindered or an alpha substituted Cs to C12 monocarboxylic acid.
In another embodiment of the present invention, the monocarboxylic acid used for the preparation of a polyol ester is selected from the group consisting of valeric acid, isovaleric acid, 2-ethylbutanoic acid, 2-methylhexanoic acid, 3-methylhexanoic acid, 4-methylhexanoic acid, 5-methylhexanoic acid, 2,2-dimethylpentanoic acid, 2,2-dimethylbutanoic acid, 2-ethylpentanoic acid, 3-ethylpentanoic acid, isoheptanoic acid, 2-ethylhexanoic acid, 3,5,5-trimethylhexanoic acid, neononanoic acid and neodecanoic acid, or mixtures thereof.
In still another embodiment of the present invention, the monocarboxylic acid used for the preparation of a polyol ester is a mixture of recovered C4 to C6 acids from cyclohexane oxidation waste.
In yet another embodiment of the present invention, the monocarboxylic acid used for the preparation of a polyol ester is 2-ethylhexanoic acid.
In an embodiment of the present invention, the monocarboxylic acid used for the preparation of a polyol ester is 2,2-dimethylbutanoic acid.
In another embodiment of the present invention, the monocarboxylic acid used for the preparation of a polyol ester is a mixture of valeric acid, 2-methyl butyric acid, and 2-ethyl-hexanoic acid.
In still another embodiment of the present invention, any unreacted monocarboxylic acid left after the preparation of polyol ester is recovered and reused in the process.

In yet another embodiment of the present invention, the acid catalyst used for the preparation of a polyol ester is a Bronsted acid catalyst or a Lewis acid catalyst.
In an embodiment of the present invention, the acid catalyst used for the preparation of a polyol ester is a Lewis acid catalyst.
In another embodiment of the present invention, the Lewis acid catalyst used for the preparation of a polyol ester is selected from the group consisting of titanium-containing Lewis acid catalyst, tin-containing Lewis acid catalyst, antimony-containing Lewis acid catalyst, germanium-containing Lewis acid catalyst and zirconium-containing Lewis acid catalyst.
In still another embodiment of the present invention, the acid catalyst used for the preparation of a polyol ester is a Bronsted acid catalyst.
In yet another embodiment of the present invention, the Bronsted acid catalyst used for the preparation of a polyol ester is selected from the group consisting of mineral acids, organic acids and ion exchange resin or mixtures thereof.
In an embodiment of the present invention, the Bronsted acid catalyst used for the preparation of a polyol ester is selected from the group consisting of sulfuric acid, hydrochloric acid, p-toluenesulfonic acid, rnethanesulfonic acid, naphthalenesulfonic acid or mixtures thereof.
In another embodiment of the present invention, the Bronsted acid catalyst used for the preparation of a polyol ester is p-to!uenesulfonic acid.
In still another embodiment of the present invention, the acid catalyst used for the preparation of a polyol ester is taken in an amount in the range of 0.1 wt % to 2 wt % of the total weight of polyol and monocarboxylic acid.
In yet another embodiment of the present invention, the acid catalyst used for the preparation of a polyol ester is taken in an amount in the range of 1 wt % to 2 wt % of the total weight of polyol and monocarboxylic acid.
In an embodiment of the present invention, the acid catalyst used for the preparation of a polyol ester is taken in an amount of 1,5 wt % of the total weight of polyol and monocarboxylic acid.

In another embodiment of the present invention, the acid catalyst used for the preparation of a polyol ester is taken in an amount of 2 wt % of the total weight of polyol and monocarboxylic acid.
in still another embodiment of the present invention, the azeotropic solvent vsed for the preparation of a polyol ester is selected from the group consisting of aromatic hydrocarbons, aliphatic hydrocarbons and halogen containing hydrocarbons.
In yet another embodiment of the present invention, the azeotropic solvent used for the preparation of a polyol ester is selected from the group consisting of dodecane, benzene, toluene, xylene, chlorobenzene, carbon tetrachloride and chloroform.
In an embodiment of the present invention, the azeotropic solvent used for the preparation of a polyol ester is dodecane.
In another embodiment of the present invention, the adsorbent used for adsorption for the purification of polyol ester is selected from the group consisting of activated clay, acid white clay, activated carbon, zeolite, activated alumina, diatomaceous earth, silicon dioxide, aluminium oxide, manganese oxide, attapulgite clay and silica-alumina containing synthetic adsorbents; or mixtures thereof.
In still another embodiment of the present invention, the adsorbent used for adsorption for the purification of polyol ester is attapulgite clay.
In yet another embodiment of the present invention, the adsorbent used for adsorption for the purification of polyol ester is used in an amount in the range of 0.1 to 5.0 parts by weight with respect to one to ten parts by weight of the polyol ester.
In an embodiment of the present invention, the base used for neutralization for the purification of polyol ester is selected from the group consisting of alkali metal hydroxide, alkali metal salts and ammonium carbonate.
In another embodiment of the present invention, the base used for neutralization for the purification of polyol ester is an aqueous solution of alkali metal hydroxide, alkali metal salts and ammonium carbonate.
In still another embodiment of the present invention, the base used for neutralization for the purification of polyol ester is an aqueous solution of sodium

hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate, sodium hydrogen carbonate or ammonium carbonate.
In yet another embodiment of the present invention, the base used for neutralization for the purification of polyol ester is an aqueous solution of sodium hydrogen carbonate.
In an embodiment of the present invention, the concentration of base used for neutralization for the purification of polyol ester is in the range of 5 wt % to 10 wt%.
In another embodiment of the present invention, the concentration of base used for neutralization for the purification of polyol ester is 5 wt %.
An embodiment of the present invention provides a polyol ester prepared by the process of present invention.
In an embodiment of the present invention, the polyol ester is used as a lubricant base for refrigeration application.
In another embodiment of the present invention, the polyol ester is used as a lubricant base with hydro fluorocarbons for refrigeration application.
In still another embodiment of the present invention, the hydroffuorocarbons used with polyol ester for refrigeration application are selected from the group consisting of 1,1,1,2-tetrafluoroethane (HFC 134a), pentafluoroethane (HFC 125) and difluoromethane (HFC32); or mixtures thereof.
In yet another embodiment of the present invention, the polyol ester and hydro fluorocarbons are taken in a weight ratio in the range of )0:90 to 90:10 as a working composition for refrigeration application.
In the process of the present invention, the polyol and the monocarboxylic acid that have been adjusted to a suitable mole ratio, are subjected to esterification reaction in the presence of suitable amounts of acid catalyst such as p-toulene sulfonic acid and an azeotropic solvent. The reaction is performed without any vacuum aid usually at a temperature in the range of 150°C to 200°C, preferably in the range of J60°C to 175°C for a time period of about 5 to 10 hours, more preferably about 8 to 10 hours. Water is continuously removed during stirring of the reaction mixture. This can be accomplished

by using Dean Stark apparatus. After completion of reaction, the azeotropic solvent and the monocarboxyiic acid are recovered. The monocarboxylic acid is recycled.
The monocarboxylic acid used for the preparation of polyol ester in the present invention have 5 to 12 carbon atoms which includes sterically hindered or an alpha substituted monocarboxylic acid. Suitable monocarboxylic acids for use herein include isovaleric acid, 2-ethylbutanoic acid,, 2-methylhexanoic acid, 3-methylhexanoic acid, 4-methylhexanoic acid, 5-methylhexanoic acid, 2,2-dimethylpentanoic acid, 2,2-dimethylbutanoic acid, 2-ethylpentanoic acid, 3-ethylpentanoic acid, isoheptanoic acid, 2-ethylhexanoic acid, 3,5,5-trimethylhexanoic acid, neononanoic acid and neodecanoic acid. These monocarboxylic acids are used alone or in combination. Furthermore. mixture of recovered C4-C6 acids from cyclohexane oxidation waste can also be used.
Examples of polyol used in the present invention have 2 to 6 hydroxyl groups. Suitable polyol for use herein include neopentylglycol, trimethylolpropane. pentaerythritol and dipentaerythritol.
In the esterification reaction, the amounts of the polyol and the monocarboxylic acid are taken in an amount such that the mole ratio of hydroxyl groups of said polyol with respect to said acid group is in the range of1.0: 2.0 to 1.0:2.5, preferably in the range of 1.0: 2.0 to 1.0 to 2.4. In order to obtain an ester having a low hydroxyl value, it is necessary to perform an esterification reaction in presence of excessive monocarboxylic acid.
The purification of obtained polyol ester can be performed by conventional methods known in the art, such as neutralization with an aqueous base, adsorption with an adsorbent, steaming and distillation, which can be performed alone or in combination. Among these methods of purification, a combination of neutralization with a base and adsorption with an adsorbent is preferable in view of the thermal and oxidative stability of the produced ester.
The base used for neutralization in the present: invention includes alkali metal hydroxides such as sodium hydroxide and potassium hydroxide; alkali metal salts such as sodium carbonate, potassium carbonate, and sodium hydrogen carbonate; and ammonium salts such as ammonium carbonate, preferably sodium hydroxide and potassium hydroxide, more preferably sodium hydrogen carbonate. The amount of base used for

neutralization is 1 to 2 equivalents with respect to 1 equivalent of the acid value of the ester. The base used for neutralization is in the form of an aqueous solution having a concentration of 5 wt % to 10 wt %, more preferably 5 wt %.
The adsorbents used for adsorption in the present invention include activated clay, acid white clay, activated carbon, zeolite, activated alumina, diatomaceous earth, silicon dioxide, aluminum oxide, attapulgite clay, magnesium oxide and silica-alumina-containing synthetic adsorbents, preferably activated clay and silica-alumina-containing synthetic adsorbents, more preferably attapulgite clay. The preferred amount of an adsorbent used is 0.1 to 5.0 parts by weight with respect to one part by weight of the polyol ester.
In the process of present invention, the by-products are almost completely removed from the produced polyol ester, partly because of low reaction temperature, and also because of the presence of excess solvent which acts as a dispersion media during the reaction. As a result, the produced polyol esters have excellent thermal and oxidative stability. They are transparent and maintain their transparency for a longer time.
The use of polyol having 2 to 6 hydroxyl groups for the preparation of polyol ester of present invention result in esters with excellent heat resistance properties. Also, the use of sterically hindered or an alpha substituted C5 to C12 monocarboxylic acid for the preparation of polyol ester of present invention result in esters with excellent low temperature fluidity, hydroiytic stability, excellent miscibility and solubility with hydrofluorocarbons as compared to conventional esters.
It is preferable that the kinematic viscosity of the ester used as lubricating base stock for refrigeration compressor oil at 40°C is 8 mm2 /s to 100 mm2 /s, more preferably 10 mm /s to 70 mm /s. The acid value of the ester used as lubricating base stock for refrigerating compressor oil is 0.05 mg KOH/g or less in view of heat resistance and hydroiytic stability. Also, the preferable hydroxyl value of the ester used as lubricating base stock for refrigerating compressor oil is 5 mg KOH/g or less, more preferably 3 mg KOH/g or less in view of the compatibility with sealing material, the heat resistance and the hydroiytic stability. Furthermore, it is preferable that flock point for the targeted ester must be as low as possible with hydroflurocarbon to ensure miscibility between ester and refrigerant. The esters produced by the process of the invention have kinematic viscosity

at 40°C in the range of 8 mm2 /s to 47 mm2 /s. They also have excellent low temperature compatibility with various refrigerants.
ADVANTAGES
The previously described versions of the subject matter and its equivalent thereof have many advantages, including those which are described below.
The advantage of the present invention allows preparation of polyoi ester under mild conditions at a lower range of temperature i.e. in the range of 150°C to 200°C within few hours (5 to 10 hours). Also, the process of preparation does not require any vacuum aid which considerably reduces the cost of production. Moreover, the polyol esters produced by the process of invention are almost transparent, light coloured and of high quality. Hence, they do not require extensive purification steps and can be used for a long period of time even under severe use conditions. Another advantage of the present invention is that the polyoi esters produced have excellent thermal, oxidative and hydrolytic stability compared to conventional esters. Another additional advantage of polyoi esters of present invention is that they are heat resistant and have excellent low temperature fluidity. They are easily miscible and soluble with hydrofluorocarbons. Thus they are used as a lubricant base for refrigeration application in combination with hydrofluorocarbons.
EXAMPLES
The disclosure will now be illustrated with working examples, which is intended to illustrate the working of disclosure and not intended to take restrictively to imply any limitations on the scope of the present disclosure.
EXAMPLE 1
Preparation of trimethylolpropane-tri-2-ethyl-hexanoate
50 grams of trimethylolpropane was mixed with 427 ml of 2-ethyl-hexanoic acid in presence of 1.2 litre of dodecane with sufficient stirring. The mole ratio of hydroxyl group of polyoi to acid was 1:2.4. Then, 6.5 gms of p-toluene sulfonic acid (as a catalyst for esterification reaction) was further added to the mixture in an amount of 1.5 wt % of

the total weight of the polyol and the acid. The mixture was then stirred at 175°C for about 10 hours while removing water from the reaction system. After completion of reaction, the resultant reaction mixture was purified by neutralizing with 5% sodium hydrogen carbonate solution (aqueous) after which the azeotropic solvent was recovered by simple distillation. The unreacted acid was recovered in the form of its sodium salt which was acidified to recover the unreacted acid for its recycle. Finally, the polyol ester was treated over attpugulite clay to remove traces of acid and by-products. The thus washed polyol ester was dried over anhydrous sodium sulfate. Subsequently, almost transparent, light yellow colored and odorless trimethylolpropane-tri-2-ethyl-hexanoate was obtained with 98 % yield. Its purity was found to be 99.9 %, as determined by gas chromatography.
EXAMPLE 2
Preparation of pentaerythritol-tetra-2-ethyl-hexanoate:
50 gm of pentaerythritol was mixed with 561 ml of 2-ethyl-hexanoic acid in presence of 1.4 liters of dodecane with sufficient stirring. The mole ratio of hydroxyl group of polyol to acid was 1:2.4. Then, 8.4 gms of p-toluene sulfonic acid (as a catalyst for esterification reaction) was further added to the mixture in an amount of 1.5 wt % of the total weight of the polyol and the acid. The mixture was then stirred at 175°C for about 10 hours while removing water from the reaction system. After completion of reaction, the resultant reaction mixture was purified by neutralizing with 5% sodium hydrogen carbonate solution (aqueous) after which the azeotropic solvent was recovered by simple distillation. The unreacted acid was recovered in the form of its sodium salt which was acidified to recover the unreacted acid for its recycle. Finally, the polyol ester was treated over attpugulite clay to remove traces of acid and by-products. The thus washed polyol ester was dried over anhydrous sodium sulfate. Subsequently, almost transparent, light yellow colored and odorless pentaerythritol-tetra-2-ethyl-hexanoate was obtained with 98 % yield. Its purity was found to be 99.9 %, as determined by gas chromatography.
EXAMPLE 3
Preparation of neopentylglycol-di-2-ethyl-hexanoate

45 gm of neopentylglycol was mixed with 330 ml of 2-ethyl-hexanoic acid in presence of 916 ml of dodecane with sufficient stirring. The mole ratio of hydroxyl group of polyol to acid was 1:2.4. Then, 5.1 gms of p-toluene sulfonic acid (as a catalyst for esterification reaction) was further added to the mixture in an amount of 1.5 wt % of the total weight of the polyol and the acid. The mixture was then stirred at 175°C for about 8 hours while removing water from the reaction system. After completion of reaction, the resultant reaction mixture was purified by neutralizing with 5% sodium hydrogen carbonate solution (aqueous) after which the azeotropic solvent was recovered by simple distillation. The unreacted acid was recovered in the form of its sodium salt which was acidified to recover the unreacted acid for its recycle. Finally, the polyol ester was treated over attpugulite clay to remove traces of acid and by-products. The thus washed polyol ester was dried over anhydrous sodium sulfate. Subsequently, almost transparent, light yellow colored and odorless neopentylglycol -tetra-2-ethyl-hexanoate was obtained with 98 % yield. Its purity was found to be 99.9 %, as determined by gas chromatography.
EXAMPLE 4
Preparation of pentaerythritol -tetra-2,2-dimethylbutanoate
50 gm of pentaerythritol was mixed with 441 ml of 2,2-dimethylbutanoic acid in the presence of 1.2 litre of dodecane with sufficient stirring. The mole ratio of hydroxyl group of polyol to acid was 1:2.4, Then, 6.9 gms of p-toluene sulfonic acid (as a catalyst for esterification) was further added to the mixture in an amount of 1.5 wt % of the total amount of the polyol and the acid, The mixture was then stirred at 175°C for about 10 hours while removing water from the reaction system. After completion of reaction, the resultant reaction mixture was purified by neutralizing with 5% sodium hydrogen carbonate solution (aqueous) after which the azeotropic solvent was recovered by simple distillation. The unreacted acid was recovered in the form of its sodium salt which was acidified to recover the unreacted acid for its recycle. Finally, the polyol ester was treated over attpugulite clay to remove traces of acid and by-products. The thus washed polyol ester was dried in vaccum to obtain solid pentaerythritol-tetra-2,2-dimethylbutanoate with 98 % yield.

EXAMPLE 5
Preparation of trimethylolpropane-tri-2-ethyl-hexanoate
The recovered 2-ethylhexanoic acid of example I was recycled and reacted with trimethylolpropane as per the process described in example 1. Subsequently, almost transparent, light yellow colored and odorless trimethylolpropane-tri-2-ethyl-hexanoate was obtained with 98 % yield. Its purity was found to be 99.9 % as determined by gas chromatography.
EXAMPLE 6
Preparation of complex ester
24.1 gm of pentaerythritol was mixed with 25.0 ml of valeric acid, 14.2 ml of 2-methyl butyric acid, and 167 ml of 2-ethyl-hexanoic acid in presence of 564 ml of dodecane with sufficient stirring. The mole ratio of hydroxyl group of polyol to acid was 1:2. Then, 4.2 gms of p-toluene sulfonic acid (as a catalyst for esterification reaction) was further added to the mixture in an amount of 2 wt % of the total amount of the polyol and the acid. The mixture was stirred at 175°C for about 8 hours while removing water from the reaction system. After completion of reaction, the resultant reaction mixture was purified by neutralizing with 5% sodium hydrogen carbonate solution (aqueous) after which the azeotropic solvent was recovered by simple distillation. Finally, the polyol ester was treated over attpugulite clay to remove traces of acid and by-products. The thus washed polyol ester was dried over anhydrous sodium sulfate. Subsequently, almost transparent, light yellow colored and odorless pentaerythritol-tetra-complex ester was obtained with 98 % yield having acid and hydroxyl value of 0.01 and 1.5 mg KOH/g, respectively. Its purity was found to be 99.9 % as determined by gas chromatography.
EXAMPLE 7
Preparation of complex ester
4.0 gm of pentaerythritol was mixed with 6.2 ml (approximately 0.057 moles of acid groups) of mixture of recovered G4-C6 acids from cyclohexane oxidation waste and 28 ml of 2-ethyi hexanoic acid in the presence of 90 ml of dodecane with sufficient stirring. The mole ratio of hydroxyl group of polyol to acid was 1 ;2. The C4-C6 acid mixture was

recovered by neutralizing the alkaline waste of C4-C6 acids and separating the desired acid mixture by distillation. Then, 0.65 gms of p-toluene sulfonic acid (as a catalyst for esterjfication recation) was further added to the mixture in an amount of 2 wt% of the total amount of polyol and acid. The mixture was then stirred at 160°C for 4 hours and 175°C for about 5 hours while removing water from the reaction system. After completion of reaction, the resultant reaction mixture was purified by neutralizing with 5% Sodium hydrogen carbonate solution (aqueous) after which the azeotropic solvent was recovered by simple distillation. Finally, the polyol ester was treated over attapugulite clay to remove traces of acid and by-products. The thus washed polyol ester was dried over anhydrous sodium sulfate. Subsequently, almost transparent, light yellow colored and odorless pentaerythritol-tetra-complex ester was obtained with 98 % yield having acid and hydroxyl value of 0.01 and 1.5 mg KOH/g, respectively. Its purity was found to be 99.9 % as determined by gas chromatography.
EXAMPLE 8
Preparation of pentaerythritol-tetra-2-ethyl-hexanoate
50 gm of pentaerythritol was mixed with 444 ml of 2-ethyl-hexanoic acid in presence of 1.2 liters of dodecane with sufficient stirring. The mole ratio of hydroxyl group of polyol to acid was 1:1.9, Then, 6.8 gms of p-toluene sulfonic acid (as a catalyst for esterification reaction) was further added to the mixture in an amount of 1.5 wt % of the total weight of the polyol and the acid. The mixture was then stirred at 175°C for about 10 hours while removing water from the reaction system. After completion of reaction, the resultant reaction mixture was purified by neutralizing with 5% sodium hydrogen carbonate solution (aqueous) after which the azeotropic solvent was recovered by simple distillation. The unreacted acid was recovered in the form of its sodium salt which was acidified to recover the unreacted acid for its recycle. Filially, the polyol ester was treated over attapugulite clay to remove traces of acid and by products. The thus washed polyol ester was dried over anhydrous sodium sulfate. Subsequently, almost transparent, light yellow colored and odorless pentaerythritol-tetra-2-ethyl-hexanoate was obtained with 95 % yield,

EXAMPLE 9
Preparation of pentaerythritoI-tetra-2-ethyl-hexanoate
50 gm of pentaerythritol was mixed with 701 ml of 2-ethyl-hexanoic acid in presence of 1.8 liters of dodecane with sufficient stirring. The mole ratio of hydroxyl group of polyol to acid was 1:3.0. Then, 10.2 gms of p-toluene sulfonic acid (as a catalyst for esterification reaction) was further added to the mixture in an amount of 1.5 wt % of the total weight of the polyol and the acid. The mixture was then stirred at 175°C for about 8 hours while removing water from the reaction system. After completion of reaction, the resultant reaction mixture was purified by neutralizing with 5% sodium hydrogen carbonate solution (aqueous) after which the azeotropic solvent was recovered by simple distillation. The unreacted acid was recovered in the form of its sodium salt which was acidified to recover the unreacted acid for its recycle. Finally, the polyol ester was treated over attapugulite clay to remove traces of acid and by-products. The thus washed polyol ester was dried over anhydrous sodium sulfate. Subsequently, dark brown coloured pentaerythritol-tetra-2-ethyl-hexanoate was obtained with 96 % yield.
EXAMPLE 10
Preparation of complex ester
10.40 gm of pentaerythritol were mixed with 55.42 ml of valeric acid and 15.84 ml of 3,5,5-trimethyl hexanoic acid in the presence of 200 ml of dodecane with sufficient stirring. Then, 1.2 gms of p-toluene sulfonic acid (as a catalyst for esterification reaction) was further added to the mixture in an amount of 1.5 wt% of the total amount of polyol and acid. The mixture was stirred at 160°C for 4 hours and 175°C for about 5 hours while removing water from the reaction system. After completion of reaction, the resultant reaction mixture was purified by neutralizing with 5% sodium hydrogen carbonate solution (aqueous) after which the azeotropic solvent was recovered by simple distillation. Finally, the polyol ester was treated over attapugulite clay to remove traces of acid and by products. The thus washed polyol ester was dried over anhydrous sodium sulfate. Subsequently, almost transparent, light yellow colored and odorless

pentaerythritol-tetra-complex ester was obtained with 98 % yield. Its purity was found to be 99.9 % as determined by gas chromatography.
EXAMPLE 11
The esters prepared in examples 1-10 were evaluated for their properties as per ASTM /JOAC methods.
The phase compatibility (flock point) of the prepared polyol ester with hydroflurocarbons was evaluated as per the given method.
A requisite amount (1, 2, 3 or 4 ml) of lubricant to be tested was placed into a thermal shock resistant, volumetrically graduated 10 ml glass test tube (17 millimeters diameter and 145 mm long). The test tube was then stoppered and placed into a cooling bath regulated to -30°C. After the tube and its contents had equilibrated in the cooling bath for 5 minutes, sufficient refrigerant working fluid was added to give a total volume of 10 ml. At least 15 minutes after addition of refrigerant working fluid into the tube (during which time the tube and contents have been equilibrating in the cooling bath and the contents may have been agitated if desired), the tube contents are visually examined for evidence of phase separation. If there was any such phase separation, the tube was shaken to determine whether the combination can be rated as miscible or is totally unacceptable. If there was no evidence of phase separation at -30°C, the temperature of the cooling bath was lowered at a rate of 0.5 per minute until phase separation was observed. The temperature of first observation of phase separation, if within the range of the cooling equipment used., was then noted as the insolubility onset temperature. Table 1 shows the properties of various prepared esters.

Table 1: Properties of various prepared esters

Example No. 1 Vis. @
40°C,
mm2/s
ASTM-
D445 Pour point (°C)
ASTM-D97 Flock point (°C) with
R134a (R134a:oil
wt%) TAN mgKOH/g (oil) ASTM
D974 OH value AOCS method
Cd 13-60
1 -26 -48 -44 0.02 1.4
2 47 -42 -39 0.02 1.8
3 ~8 <-55 -60 0.02 1.5
4 ND ND ND ND 1.5
5 -27 -45 -44 0.02 1.9
6 -34 -33 -30 0.01 1.6
7 -19 -51 -42 0.01 1.6
8 ND ND ND ND 3.5
9 ND ND ND ND 1.5
10 -21 -45 -50(90:10) -30 (80:20) -27 (70:30) 0.02 1.8
(Blend of 1 & 3: 89:11
v/v) -22 -45 -48(90:10) -40 (80:20) -38 (70:30) -37 (60:40) 0.02 1.4
(Blend of 1 &3: 85:15
v/v) -15 -48 -50(90:10) -44 (80:20) -40 (70:30) -39 (60:40) 0.02 1.6
(Blend of 1 &3: 80:20
v/v) -10 -51 -50(90:10) -44 (80:20) -40 (70:30) -39 (60:40) 0.02 1.6
Commercial base oil -22 -42 -50(90:10) -30 (80:20) -27 (70:30) 0.02 1.8
ND: Not determined
Based on the evaluated properties, the prepared polyol esters in example nos. 1, 2, 3, 5, 6 and 7 were found to be ideal blend stocks for preparation of base oils for refrigeration application whereas polyol esters of example nos. 8 and 9 were found to be poor base stocks with respect to their hydroxyl value and color, respectively. Thus, polyol ester of example 1 and 3 were blended into various proportions (on volume basis) to produce base oil of ISO-22, ISO-15, and ISO-10 grade, respectively. For example, blend of polyol esters produced from example nos. 1 and 3 were found to have excellent low temperature

compatibility with R134a refrigerant from weight ratio of 90:10 to 60:40 (R134a:polyol ester) and was found to have better low temperature compatibility (lower than -35 °C) vis-a-vis commercial base oil as well as complex polyol ester (example 10) of ISO-22 grade. In view of this, the polyol esters produced using alpha branched monocarboxylic acids are ideal base stocks for producing base oils with superior low temperature compatibility with R134a refrigerant.
Although the subject matter has been described in considerable detail with reference to certain preferred embodiments thereof, other embodiments are possible. As such, the spirit and scope of the appended claims should not be limited to the description of the preferred embodiment contained therein.

We Claim:
1. A process for preparation of a polyol ester, said process comprising:
mixing a polyol with at least one monocarboxylic acid to obtain a reaction mixture, wherein the mole ratio of hydroxyl group of said polyol and said acid group is in the range of 1:2.0 to 1:2.5; and
stirring the reaction mixture at a temperature in the range of 150°C to 200°C for about 5 to 10 hours to obtain the polyol ester.
2. The process for preparation of a polyol ester as claimed in claim 1, said process
comprising:
mixing a polyol with at least one monocarboxylic acid to obtain a reaction mixture, wherein the mole ratio of hydroxyl group of said polyol and said acid group is in the range of 1:2.0 to 1:2.4 and wherein said reaction mixture comprises an acid catalyst and an azeotropic agent; and
stirring the reaction mixture at a temperature in the range of 160°C to 175°C for about 5 to 10 hours to obtain the polyol ester.
3. The process as claimed in claim 1 or 2, wherein said process comprises removing water continuously during stirring of the reaction mixture.
4. The process as claimed in claim \ or 2, wherein said polyol ester is further neutralized and adsorbed by an adsorbent to obtain a purified polyol ester.
5. The process as claimed in claim 1 or 2, wherein the polyol comprises 2 to 6 hydroxyl groups.
6. The process as claimed in claim 1 or 2, wherein the polyol is selected from the group consisting of neopentylglycol, trimethylolpropane, pentaerythritol and dipentaerythritol.
7. The process as claimed in claim 1 or 2, wherein the monocarboxylic acid is a sterically hindered or an alpha substituted C5 to C12 monocarboxylic acid.

8. The process as claimed in claim 7, wherein the monocarboxylic acid is selected from the group consisting of isovaleric acid, 2-ethylbutanoic acid, , 2-methylhexanoic acid, 3-methylhexanoic acid, 4-methylhexanoic acid, 5-methylhexanoic acid, 2,2-dimethylpentanoic acid, 2,2-dimethylbutanoic acid, 2-ethylpentanoic acid, 3-ethylpentanoic acid, isoheptanoic acid, 2-ethylhexanoic acid, 3,5,5-trimethylhexanoic acid, neononanoic acid and neodecanoic acid, or mixtures thereof.
9. The process as claimed in claim 2, wherein the acid catalyst is a Bronsted acid catalyst or a Lewis acid catalyst.
10. The process as claimed in claim 9, wherein the Lewis acid catalyst is selected from the group consisting of titanium-containing Lewis acid catalyst, tin-containing Lewis acid catalyst, antimony-containing Lewis acid catalyst, germanium-containing Lewis acid catalyst and zirconium-containing Lewis acid
catalyst.
11. The process as claimed in claim 9, wherein the Bronsted acid is selected from the group consisting of mineral acids, organic acids and ion exchange resin; or mixtures thereof.
12. The process as claimed in claim 9, wherein the Bronsted acid is selected from the group consisting of sulfuric acid, hydrochloric acid, p-toluenesulfonic acid, methanesulfonic acid and naphthalenesulfonic acid; or mixtures thereof.
13. The process as claimed in claim 2, wherein the acid catalyst is taken in an amount in the range of 1 wt % to 2 wt % of the total weight of polyol and monocarboxylic acid.
14. The process as claimed in claim 2, wherein the azeotropic solvent is selected from the group consisting of aromatic hydrocarbons, aliphatic hydrocarbons and halogen containing hydrocarbons.
15. The process as claimed in claim 2, wherein the azeotropic solvent is selected from the group consisting of dodecane, benzene, toluene, xylene, chlorobenzene, carbon tetrachloride and chloroform.

16. The process as claimed in claim 4, wherein the adsorbent is selected from the group consisting of activated clay, acid white clay, activated carbon, zeolite, activated alumina, diatomaceous earth, silicon dioxide, aluminium oxide, manganese oxide, attapulgite clay and silica-alumina containing synthetic adsorbents; or mixtures thereof.
17. The process as claimed in claim 4, wherein said polyol ester is neutralized with a base selected from the group consisting of alkali metal hydroxides, alkali metal salts and ammonium carbonate.
18. The process as claimed in any of the claims 1 to 17, wherein any unreacted monocarboxylic acid is recovered and reused in the process.
19. A polyol ester produced by the process as claimed in any of the claims 1 to 18.
20. The polyol ester as claimed in claim 19, wherein the polyol ester is used as a lubricant base for refrigeration application.
21. A process for preparation of a polyol ester for use as a lubricant base for refrigeration application substantially as herein described and illustrated in the examples.
22. A polyol ester for use as a lubricant base for refrigeration application substantially as herein described and illustrated in the examples.