The present invention relates to a process for the preparation of an ester using a polyaniline salt as the catalyst. The present invention more particularly relates to a process for producing aliphatic esters by the direct esterification of aliphatic mono carboxylic acids with aliphatic monohydric alcohols over polyaniline salts as catalysts.
Esters are useful in a wide variety of industrial applications, such as for use in making coatings, adhesives, resins, fragrances, perfumes, plasticizers etc. Esterification is a well known equilibrium limited reaction involving reaction of a mono-, di- or polycarboxylic acid (or, in suitable cases, an acid anhydride) with an alcohol or phenol component. Such an alcohol or phenol component can be mono, di- or polyhydric.
Several synthetic routes exist to make esters, but most of them are not suitable to meet the stringent specifications which are being applied in the chemical industry. The most acceptable method of making an ester is to react an acid with an alcohol in the presence of catalyst.
Esterification is one of the most fundamental and important reactions in organic synthesis. Conventionally, the processes of making esters from acids and alcohols can be classified into the following three main categories: (R. C. Larock, Comprehensive Organic Transformations, VCH, New York, 1989, page 966, references therein, Encyclopedia of Chemical Technology, Vol 9, 4th Edition, Wiley Interscience Publications, Page 755, references therein: Ullmann""s Encyclopedia of Industrial Chemistry, Vol. 10, Vth Edition, 1987, Page 281, references therein; Vogel""s, Text book of Practical Organic Chemistry, Longman Group Ltd., England, Vth Edition, 1996).
(a) Liquid-phase esterification reaction utilizing a liquid catalyst: This type of processes utilize liquid phase acid, such as sulfuric acid, phosphoric acid, sulfonic acid, or p-toluene sulfonic acid, as catalysts.
(b) Liquid phase esterification reaction utilizing a solid catalyst: This type of processes typically utilize inorganic salts, cationic ionic exchange resin and solid acid catalyst etc.
(c) Gas phase esterification reaction: This type of processes utilize a variety of catalysts such as heteropolyacids, liquid phase acids carried by a solid carrier, and zeolite in a gas phase reaction.
One of the problems associated with the liquid-catalyst liquid-phase esterification reaction is that the acidic liquid catalysts of sulfuric acid or p-toluene sulfonic acid cause corrosion in the reactor. These liquid acid catalysts are also discharged along with the reaction products, thus causing serious waste disposal and pollution problems. The drawbacks of using mineral acid as catalyst are: (i) Catalyst can not be reused, (ii) Disposal of acid is not environmentally safe and it is not economical, (iii) Low selectivity is frequently observed, (iv) Corrosion of the reaction vessel and reactors, (v) Not easy to handle and (iv) High inventory of the catalyst.
The solid-catalyst, liquid-phase esterification reaction, which typically utilizes a cationic ionic exchange resin as catalyst, ameliorates the corrosion and waste disposal problems experienced with the liquid-catalyst liquid-phase processes, and results in simplified separation procedure required between the reaction product and catalysts. However, cationic ion-exchange resins typically exhibit relatively poor heat-resistance, and they often lose substantial activity after being subject to heat. Once the catalytic activity of the cationic ion-exchange resins is reduced, it is difficult to be regenerated.
In the gas phase esterification reaction, the reaction conditions are maintained so that all the reactants and products are in the gas phase. Typically, inorganic materials are utilized as catalysts which typically exhibit excellent heat resistance and can be easily separated from the reaction products. However, the gas phase reaction necessitates a relatively large reaction vessel, resulting in large capital investment cost. Furthermore, if the gas phase esterification reaction is utilized to produce unsaturated carboxylic esters, the high reaction temperature often causes undesired by-products of polymers or oligomers to be produced. In certain instances, the high reaction temperature has caused the alcohol molecules to be dehydrated to become ethers. These side-reactions will tend to cause the reaction catalysts to lose their activity and result in operational difficulties.
The main object of the present is to provide a process for preparation of esters using polyaniline-salts as catalysts which obviates the drawbacks as detailed above.
It is another object of the invention to provide a process for the preparation of an ester which is environmentally safe.
It is a further object of the invention to provide a process for the preparation of an ester that allows catalyst recycling.
It is still another object of the invention to provide a process that is economical and efficient.
Another object of the present invention is to produce aliphatic esters by the direct esterification of aliphatic mono carboxylic acids with aliphatic monohydric alcohols over polyaniline salts as catalysts.
Accordingly, the present invention relates to a process for preparation of an ester said process comprising direct esterification of an aliphatic mono carboxylic acid with an aliphatic mono hydric alcohol in presence of a polyaniline salt catalyst at a temperature in the range of 40 to 80xc2x0 C. for a time period in the range of 8 to 24 hrs. and separating the ester so obtained from the reaction mixture.
In one embodiment of the present invention, the aliphatic mono carboxylic acid used is selected from aliphatic mono carboxylic acids having C2-C20 carbon atoms.
In an another embodiment of the present invention, the aliphatic mono hydric alcohol used is selected from mono hydric alcohols having C1-C20 carbon atoms.
In a further embodiment of the invention, the aliphatic monohydric alcohol is selected from iso propyl alcohol and tertiary butyl alcohol.
In yet another embodiment of the present invention, the polyaniline salt catalyst used is selected from polyaniline-sulfate, polyaniline-hydrochloride, polyaniline-nitrate, polyaniline-p-toluene sulfonate and polyaniline-phosphoric acid salt.
In another embodiment of the invention the reaction is carried out in the temperature range of 70 to 80xc2x0 C.
In yet another embodiment of the invention, the reaction is carried out for a period of 20 to 24 hrs.
In still another embodiment of the present invention, the catalyst amount used is in the range of 100 to 400 mg.
In yet another embodiment of the present invention, the amount of alcohol used is in the range of 1 ml to 5 ml.
These embodiments will be apparent from the ensuing detailed description of the present invention.
The process of esterification may be carried out by reacting aliphatic mono carboxylic acid with aliphatic mono hydric alcohol in presence of catalyst, removing the catalyst by conventional methods. The ester can be isolated such as filtration followed by isolation of esters by conventional column chromatography using adsorbent such as silica gel and solvent such as chloroform, ethylacetate, hexane and mixture there off.