DESC:FIELD OF INVENTION

The present invention relates to a process for preparation of paraldehyde. More particularly, the present invention relates to a process for preparation of paraldehyde from acetaldehyde. The developed process produces paraldehyde that is almost free of metal impurities and therefore the synthesized paraldehyde is suitable for electronic applications.

BACKGROUND OF THE INVENTION.
Paraldehyde is the cyclic trimer of acetaldehyde molecules. Formally, it is a derivative of 1,3,5-trioxane having following structure of Formula I:

Formula I
Applications of Paraldehyde are very wide; such as in resin manufacture and in various other electronic based processes as a solvent.
The preparation of Paraldehyde is known per se. Paraldehyde is produced by polymerizing acetaldehyde (trimerization) with a trace of sulfuric acid; the resulting liquid is then neutralized with calcium carbonate and purified by fractional distillation.
The known synthesis process is highly exothermic, thus slow addition of acetaldehyde and requirement of careful temperature control become crucial steps making the overall synthesis process very time consuming. Also, the homogeneous acid catalyst must be neutralised to avoid depolymerisation of paraldehyde before separation by distillation process. Therefore, the conventional process is particularly disadvantageous as it generates chemical effluent due to neutralisation and washing of products to remove salts. Additionally, the product yield of said process is very low because of the inability to recover and recycle unreacted starting material.
US 2,479,559 discloses a process of polymerisation of acetaldehyde to paraldehyde using a fixed bed of acidic cation exchanger (sulfonated phenol formaldehyde resins) and then directly to a column to distil the reaction mixture. Wherein, the rate at which acetaldehyde is introduced into the reaction column is largely controlled by the size of the column and the efficiency of the cooling system employed because the polymerisation process is exothermic. This process provides yield of about 92%.
US 3,627,786 discloses a method for continuous production of paraldehyde polymerization (trimerization) of acetaldehyde wherein a cation exchange sulfonic resin, is maintained in a highly dispersed and fluidized state in and by the stream of acetaldehyde passing upwards through the reactor, where yield is about 92%. Further, this process is costly as it involves use of high energy consuming fluidisation step.
In view of disadvantages associated with the above prior arts, the present invention proposes an improved process for synthesis of paraldehyde in a continuous manner, with zero effluent generation and devoid of neutralisation step. High yield of Paraldehyde containing low metal impurities is obtained by the proposed process.

OBJECTIVES OF THE INVENTION
• The primary objective of the present invention aims to provide an effective process for synthesis of paraldehyde with increased product yield.
• One more objective of present invention is to provide the process for preparation of electronic grade paraldehyde wherein, the metal impurities are avoided in the final product.
• Another objective of the present invention is to optimise the design and arrangement of the apparatus for making it suitable for industrially feasible operating process.

SUMMARY OF THE INVENTION
In accordance to first embodiment, the present invention provides the process for preparation of paraldehyde from acetaldehyde. The developed process produces paraldehyde that is almost free of metal impurities and hence suitable for electronic applications.

In an embodiment, the process for synthesis of paraldehyde suitable for electronic grade application comprising of:
(a) providing a liquid-phase reactor containing heterogeneous catalyst bed of solid cation exchange resin having macroporous spherical beads,
(b) contacting the catalyst bed with liquid phase of acetaldehyde feed under reactor conditions of temperature between 15 to 30 deg C and pressure of 0.5 to 2 kg/cm2;
Characterized in that the weight ratio of substrate acetaldehyde to catalyst is between 5 to 6.
In present invention, the catalyst is solid cation exchange resin selected from crosslinked Polystyrene Sulphonic acid having sulphonic acid functional groups throughout the entire structure of polystyrene polymer.
According to one of embodiment of present invention, the substrate acetaldehyde is contacting with heterogenous catalyst bed at circulation rate ranging between 4 m3/hr to 5 m3/hr.
According to second embodiment of present invention, the temperature for step b) is selected at the range from 15 deg C to 20 deg C and pressure selected from 0.5 to 1.5 kg/cm2
DETAILED DESCRIPTION OF THE INVENTION
Before the present invention is described, it is to be understood that the terminology used in the description is for the purpose of describing the particular embodiments only and is not intended to limit the scope of the present invention. The described embodiments of the invention are given for the purpose of illustration rather than limitation of the invention.
The present invention relates to an improved process for preparation of paraldehyde. More specifically, the present invention provides an efficient process for synthesis of electronic grade Paraldehyde that is almost free of metal impurities.
The inventors of the present invention have improved the process for making Paraldehyde suitable for electronic grade applications, designed to operate continuously, devoid of neutralisation step with zero effluent generation.
The present invention as embodied and broadly described herein discloses use of heterogeneous catalyst to control metal impurities in final product while maximizing product yield and purity including increased selectivity and conversion rate from Acetaldehyde to Paraldehyde.
The present invention provides an improved process for preparation of Paraldehyde, wherein modified heterogeneous cation exchange resin is used as fixed bed instead of homogeneous mineral acid. The cation exchange resin is selected from a beads of crosslinked Polystyrene with Sulphonic acid. The said resin is easy to recycle and no need to neutralise and washing of final product. Hence, the present invention provides advantage of recovery of catalyst that can be reused for several cycles.
In an embodiment, the process for synthesis of paraldehyde suitable for electronic grade application comprising of:
(a) providing a liquid-phase reactor containing heterogeneous catalyst bed of solid cation exchange resin having macroporous spherical beads,
(b) contacting the catalyst bed with liquid phase of acetaldehyde feed under reactor conditions of temperature between 15 to 30 deg C and pressure of 0.5 to 2 kg/cm2;
Characterized in that the weight ratio of substrate acetaldehyde to catalyst is between 5 to 6.
In present invention, the catalyst is solid cation exchange resin selected from crosslinked Polystyrene Sulphonic acid having sulphonic acid functional groups throughout the entire structure of polystyrene polymer.
According to one of embodiment of present invention, the substrate acetaldehyde is contacting with heterogenous catalyst bed at circulation rate ranging between 4 m3/hr to 5 m3/hr.
According to second embodiment of present invention, the temperature for step b) is selected at the range from 15 deg C to 20 deg C and pressure selected from 0.5 to 1.5 kg/cm2
Most importantly, the selection of heterogeneous catalyst to be used in present process along with desired temperature and pressure parameter is very important, that leads to increasing the yield of synthesized Paraldehyde to more than 96 %, more preferably more than 98% and metal impurity is also reduced to minimal making the final product useful for electronic applications.
The process involves, continuously contacting substrate acetaldehyde with fixed bed of solid cation exchange resin packed in a column type reactor at temperature range between 15 to 40 ºC at specified pressure.
After reaction, unreacted acetaldehyde is recovered by distillation and recirculated again through same catalyst bed to increase the conversion rate of said process.
Furthermore, the modification in weight of substrate to catalyst along with temperature parameter and effective pressure parameter have been incorporated in the present invention in order to build a commercial production of Paraldehyde in cost effective manner. The said process is devoid of industrial effluent and hence more environment friendly.
The advantages of present invention are, it is continuous process and recovery of unreacted acetaldehyde is possible. Recycle of catalyst is possible. It requires small size equipment, zero effluent process and more productive compared to conventional process. Most importantly, reduction in metal impurity in final product is obtained making the product usable for electronic grade applications.
It is to be appreciated that conventional sulfuric acid homogeneous catalyst base process and heterogeneous catalyst pack bed or fluidized bed operate using either non-recyclable catalyst or requirement of specific fluidized bed equipment with very precise control parameters. The present invention thus offers a surprising and non-obvious contribution to the art since the invention pertains to novel catalyst that possesses greater economic production at commercial plant as compared to the conversion of Acetaldehyde to Paraldehyde used in conventional fluidized bed reactor heterogeneous catalysts or homogeneous acidic catalysts.
The foregoing description of the invention has been set merely to illustrate the invention and is not intended to be limiting. Since modifications of the disclosed embodiments incorporating the spirit and substance of the invention may occur to person skilled in the art, the invention should be construed to include everything within the scope of the disclosure.
The present invention is further described with the help of the following examples, which are given by way of illustration all the parts, percent’s and ratios are by weight unless otherwise indicated and therefore should not be construed to limit the scope of the invention in any manner.
EXAMPLES:
Example 1: Synthesis of Paraldehyde as per present invention:
The reactor column was packed with heterogeneous catalyst and the acetaldehyde liquid feed stream was passed through the reactor system packed with catalyst bed under controlled circulation rate. The reactor column was maintained at controlled temperature and pressure as mentioned in following table. Accordingly the final product Paraldehyde was recovered and results are also recorded and reported in following table.
Type of reaction Solid catalyst bed
Catalyst Type Heterogeneous catalyst
Catalyst Crosslinked Polystyrene with Sulphonic acid

Modified process parameter with catalyst parameter
Temperature of catalyst : 15 - 20 deg C, Pressure : 0.5 - 1.5 kg/cm2,
Circulation rate:3 m3/hr
Weight Ratio of substrate to catalyst 5.74
Reaction Time 1 Hr
Metal Impurities:
Arsenic
Cadmium
Lead
Mercury
Zinc
Copper
Tin Less Than 1ppb
Yield 96%
Purity 99.5%
Conversion of acetaldehyde 90%
Catalyst Throughput 4.9 gm/hr/ml of catalyst
TABLE NO. 1

Example 2: Effect of Temperature and Pressure change on yield:
This is a comparative example that is made in the same manner as example 1, except the temperature and pressure of catalyst packed column was varied as mentioned in following table.
Parameter Changed Parameter Effect on Yield
Temperature 30 - 40 deg c decreased selectivity from 96 to 80%
Pressure 0.1 – 0.4 kg/cm2 decreased selectivity from 96 to 89%
TABLE NO. 2
Example 3: Synthesis of Paraldehyde as per conventional method using sulphuric acid:
The reactor was filled with acetaldehyde and reaction was carried out by controlled addition of sulphuric acid in the said reactor. After neutralisation with base the final paraldehyde product was separate and results are also recorded and reported in following table.

Type of reaction Batch reactor
Catalyst Type Homogenous catalyst
Catalyst Sulphuric acid

Modified process parameter with catalyst parameter
Temperature of reaction mass : 25 - 35 deg C, Pressure : 0.5 - 1 kg/cm2,
Weight Ratio of substrate to catalyst 1% on total mass of reaction
Reaction Time 10 Hr
Metal Impurities:
Arsenic
Cadmium
Lead
Mercury
Zinc
Copper
Tin Less Than 0.1 ppm
Yield 80%
Purity 99%
Conversion of acetaldehyde 88%
TABLE NO. 3
Example 4: Synthesis of Paraldehyde as per prior art US 2479559:
This is a comparative example that is reported and disclosure is made in the following table.
Type of reaction Solid catalyst bed
Catalyst Type Heterogeneous catalyst
Catalyst Sulfonated phenol formaldehyde resin
Modified process parameter with catalyst parameter
Temperature of catalyst : 16 - 20 deg C, Pressure : not disclosed
Weight Ratio of substrate to catalyst 5.00
Reaction Time 4 Hr
Metal Impurities: Not detected and not disclosed
Yield not disclosed
Purity not disclosed
Conversion of acetaldehyde 75%
Catalyst Throughput 2.4 gm/hr/ml of catalyst
TABLE NO. 4
Example 5: Synthesis of Paraldehyde as per prior art US 3627786:
This is a comparative example that is reported and disclosure is made in the following table.
Type of reaction Fluidized bed
Catalyst Type Heterogeneous catalyst
Catalyst Cation exchange sulfonic resin
Modified process parameter with catalyst parameter
Temperature of catalyst : 25 - 45 deg C, Pressure : 1 - 3 kg/cm2
Weight Ratio of substrate to catalyst 0.34
Reaction Time 1 Hr
Metal Impurities: Not detected and not disclosed
Yield 92%
Purity not disclosed
Conversion of acetaldehyde 78%
Catalyst Throughput 0.98 gm/hr/ml of catalyst
TABLE NO. 5
,CLAIMS:1. A process for synthesis of paraldehyde suitable for electronic grade application comprising of:
(a) providing a liquid-phase reactor containing heterogeneous catalyst bed of solid cation exchange resin having macroporous spherical beads,
(b) contacting the catalyst bed with liquid phase of acetaldehyde feed under reactor conditions of temperature between 15 to 30 deg C and pressure of 0.5 to 2 kg/cm2.

2. The process for synthesis of paraldehyde as claimed in claim 1, wherein said solid cation exchange resin is crosslinked Polystyrene Sulphonic acid having sulphonic acid functional groups throughout the entire structure of polystyrene polymer.
3. The process for synthesis of paraldehyde as claimed in claim 1, wherein acetaldehyde is contacting with heterogenous catalyst bed at circulation rate ranging between 4 m3/hr to 5 m3/hr.
4. The process for synthesis of paraldehyde as claimed in claim 1, wherein the temperature is selected at the range from 15 deg C to 20 deg C.
5. The process for synthesis of paraldehyde as claimed in claim 1, wherein the reaction is carried out at pressure from 0.5 to 1.5 kg/cm2
6. The process for synthesis of paraldehyde as claimed in claim 1, wherein the weight ratio of substrate acetaldehyde to catalyst is between 5 to 6.