ALKYLENE CARBONATE PURIFICATION METHOD BACKGROUND OF THE INVENTION FIELD OF THE INVENTION The present invention relates to the purification of an alkylen carbonate such as propylene carbonate by contact with a solid adsorbent such as basic alumina having a surface area of 50-400 m2/gram.

DESCRIPTION OF THE PRIOR ART It is known to form an alkylen carbonate by the reaction of an alkylen oxide with carbon dioxide. See, for example, U. S. Patent Nos. 2,773, 070, 2,873, 282,4, 786,741, 5,179, 214,5, 283,356 and the like. Appropriate catalyst and reaction conditions are known and taught, for example, in the above references.

A problem which has been associated with prior practices has been the color instability of the product alkylen carbonate as well as contamination of the product alkylen carbonate with various impurities.

It has been proposed, for example in U. S. Patent Nos. 5,179, 214 and 5,282, 356 that in a continuous reaction system the alkylen oxide and carbon dioxide be introduced into a continuous reactor containing catalyst with continuous recirculation of a portion of the reaction mixture and continuous withdrawal and flashing of another portion of the reaction mixture to remove unreacted alkylen oxide and carbon dioxide; the removed materials can be compressed and returned to the reaction mixture. The residue from flashing is distilled to separate product carbonate from the catalyst solution. A problem in continuous systems has been that higher temperatures are employed than in batch systems in order to reduce unreacted alkylen oxide levels but this has the disadvantage of forming light and heavy impurities. Elaborate alkylen oxide distillation procedures are required to produce satisfactory product.

In U. S. Patent No. 5,631, 386, it is proposed to remove residual alkylen oxide from the alkylen carbonate by stripping with an inert gas such as carbon dioxide.

WO 01/66510 A2 describes a process to provide high purity alkylen carbonate through use of multiple distillations. It is disclosed that the high purity alkylen carbonate may be further purified by use of high surface area carbon to

improve the UV absorbance of the alkylen carbonate. However, unlike the process of this invention, the process of WO 01/66510 A2 is not effective for the removal of catalyst residues.

BRIEF DESCRIPTION OF THE INVENTION It has now been found that color forming impurities including nitrogen and bromine compounds derived from the catalyst used to produce the alkylen carbonate can be removed from the alkylen carbonate by contact with a high surface area alumina or silica.

DETAILED DESCRIPTION Alkylen carbonates treated by the invention, are prepared using known reagents, catalysts and reaction conditions. See, for example, U. S. Patent Nos.

2,773, 070,2, 873,282, 4,786, 741,5, 179,214, 5,283, 356 and the like.

The invention is especially applicable to the treatment of propylene carbonate and ethylene carbonate prepared by the tetraethyl ammonium bromide catalyzed reaction of propylene oxide with carbon dioxide and ethylene oxide with carbon dioxide.

As described in U. S. Patent No. 5,283, 356, the reaction of an alkylen oxide and carbon dioxide to form alkylen carbonate may be carried out at a temperature of from about 100° to about 225° C or higher, preferably from about 175° to about 215° C. The reaction may be carried out at atmospheric pressure or, advantageously, under a pressure of about 300 psig or greater. More preferably, the reaction is carried out under a pressure of about 300 to about 3000 psig. The reaction may be conducted either batch-wise or continuously.

In a continuous reaction, the alkylen oxide and carbon dioxide are introduced to a continuous reactor containing the catalyst, from which a portion of the reaction mixture may be continuously recirculated through the reactor.

Another portion of this reaction mixture is continuously withdrawn and treated to remove unreacted alkylen oxide from product alkylen carbonate. Alternatively, the continuous reaction can be carried out on a once through basis with suitable heat removal.

Alternatively, batches of the alkylen oxide and catalyst may be introduced into an autoclave or kettle type reactor. The desired pressure may be built up by introducing carbon dioxide. Typically, the reaction mixture is heated to reaction temperature, agitated, and held under a super atmospheric pressure

of carbon dioxide. After the bulk of the alkylen oxide has reacted, the reaction mixture can be treated to remove unreacted alkylen oxide.

The alkylen oxide and carbon dioxide should be mixed in proportion to provide an excess of carbon dioxide over and above the stoichiometric amount required for reaction. This excess may be on the order of from about 1.1 moles of carbon dioxide per mole of alkylen oxide to about 10 moles of carbon dioxide per mole of alkylen oxide. An excess of alkylen oxide should be avoided, because it results in undesired by-products, chiefly alkylen oxide polymer, and because explosive conditions may result.

After completion of the desired reaction between the alkylen oxide and carbon dioxide to form alkylen carbonate, the reaction mixture is treated to remove residual unreacted alkylen oxide. Advantageously, the system pressure is reduced and carbon dioxide and alkylen oxide are vented from the system. Even after such venting the reaction mixture contains unacceptable levels of alkylen oxide, usually from 0. 1% to 1% or more as against acceptable levels of 0.06% or less. Simple flashing and removal of flashed alkylen oxide and carbon dioxide is generally ineffective in producing acceptable product. The stripping procedure of U. S. Patent No, 5,631, 386 can advantageously be used.

Alkylen carbonates prepared by the above known procedures tend to develop discoloration and this has an important and adverse effect on the sale and use of such products. Although the exact cause of the color instability is not known with certainty, it is believed that at least a factor contributing to the color instability is the presence in the alkylen carbonate of small amounts of nitrogen and/or halide compounds derived from the catalyst used in production of the alkylen carbonate and an important feature of the invention is the removal of such impurities.

Now, in accordance with the present invention, in order to reduce the level of impurities such as nitrogen compounds and to improve the color and color stability of an alkylen carbonate, the carbonate is contacted in the liquid phase with a high surface area silica or alumina adsorbent effective to improve color and reduce the impurities level of the carbonate. Adsorbents used in practice of the invention have a surface area of 50 to 400 m2/g. Basic alumina is useful as are conventional silica and alumina having the designated surface area.

The adsorptive contact is conveniently carried out at temperatures in the range of about 15° C to 90° C, preferably 20° C to 40° C. Flow rates of about 1 to 10 volumes of alkylen carbonate per volume of adsorbent per hour, preferably 2 to 5 are preferred.

It is generally preferred to employ plural adsorbent contact beds so that a depleted bed can be regenerated while a fresh bed is used. Regeneration can be by washing as with water followed by drying or by stripping with a heated inert gas such as steam, nitrogen or the like.

Basic alumina as used herein refers to alumina having a surface area of 50-400 m2/g which has been impregnated with a basic solution having a pH of at least 9 and dried. The basic solution may suitably be a solution of an alkali metal or ammonium compound such as one selected from hydroxides, carbonates, bicarbonates, phosphates, and organic acid salts. Suitable basic compounds that may be employed include sodium, potassium or ammonium carbonate, hydroxide, bicarbonate, nitrate, formate, acetate, benzoate or citrate.

The most preferred basic compound for use is potassium carbonate.

A modified alumina for use in the invention may be prepared by making a solution of the chosen basic compound having an appropriate pH as described above and adding the solution to an alumina in a volume just sufficient to fill the pores of the alumina without producing surface wetness. The concentration and the amount of the solution may be chosen to produce a loading of the compound on the alumina of from 1 to 10% on a dry weight basis.

In addition to basic alumina as described above, conventional alumina and silica having the designated surface area can be employed.

The following examples illustrates the invention: EXAMPLE 1 A 1 cm ID column was packed with Aldrich basic alumina (150 mesh) (38.4 g, 50 cc, SA=155 m2/g). A feed of propylene carbonate (APHA=431,1400 ppm Br and 250 ppm N) was passed through the bed at a LHSV of 2/hr and product was collected in 50 cc cuts. The following results were obtained: Table 1 Cut # APHA Color %'N Removed % Br Removed 1 45 99 98 2 110 93 3 348 40 36 4 272 4 0

The above results demonstrate the effectiveness of the adsorption treatment of the present invention in improving color and reducing impurities content of propylene carbonate.

In addition, accelerated aging tests showed that color did not develop even after 2 months at 50° C.

EXAMPLE 2 A 1 cm ID column was packed with Aldrich acidic alumina (150 mesh) (46.6 g, 49 cc, SA=155m2/g). A feed of propylene carbonate (APHA=431,1020 ppm Br and 180 ppm N) was passed through the bed at a LHSV of 1/hr and product was collected in 50 cc cuts. The following results were obtained: Table 2 Cut # APHA Color % N Removed % BR Removed 1 32 99 93 2 76 >99 94

EXAMPLE 3 A 1 cm ID column was packed with Aldrich neutral alumina (150 mesh) (44.3 g, 47cc, SA=155 m2/g). A feed of propylene carbonate (APHA=431,1020 ppm Br and 180 ppm N) was passed through the bed at a LHSV of 2/hr and product was collected in 50 cc cuts. The following results were obtained:

Table 3 Cut # APHA Color % N Removed % Br Removed 1 35 >99 94 2 85 >99 95 3 221 79 71 4 230 22 12 EXAMPLE 4 A 1 cm ID column was packed with Engelhard A13996R Alumina (14x30 mesh) (23.4 g, 46 cc, SA=196 m2/g). A feed of propylene carbonate (APHA=431,1020 ppm Br and 180 ppm N) was passed through the bed at a LHSV of 2/hr and product was collected in 50 cc cuts. The following results were obtained: Table 4 Cut # APHA Color % N Removed % Br Removed 1 127 96 91 2 219 82 71 3 265 56 41 4 296 39 22 EXAMPLE 5 A 1 cm ID column was packed with Grace silica (14x30 mesh) (16.3 g, 44 cc, SA=320 m2/g). A feed of propylene carbonate (APHA) =431,1020 ppm Br and 180 ppm N) was passed through the bed at a LHSV of 2/hr and product was collected in 50 cc cuts. The following results were obtained: Table 5 Cut # APHA Color % N Removed % Br Removed 1 99 98 90 2 154 >99 80 3 231 92 80 4 308 17 71

EXAMPLE 6 A 1 cm ID column was packed with Engelhard E149T alumina (14x30 mesh) (29.3 g, 44 cc, SA=50 m2/g). A feed of propylene carbonate (APHA=431, 1020 ppm Br and 180 ppm N) was passed through the bed at a LHSV of 2/hr and product was collected in 50 cc cuts. The following results were obtained: Table 6 Cut # APHA Color % N Removed % Br Removed 1 187 83 71 2 317 22 12 3 348 6 2 4 363 6 2 COMPARATIVE EXAMPLE 1 (low surface area alumina) A 1 cm ID column was packed with Alcoa T64 alumina (90.8 g, 47 cc, SA=0.04 m2/g). A feed of propylene carbonate (APHA=430,1020 ppm Br) was passed through the bed at a LHSV of 2/hr and product was collected in 50 cc cuts. The following results were obtained: Comparative Table 1 Cut # APHA Color % N Removed % Br Removed 140902 2 413 6 2 3 413 0 2 4 411'0 2 COMPARATIVE EXAMPLE 2 (activated carbon) A 1 cm ID column was packed with Calgon CAL 12x40 Carbon (14.0 g, 41 cc, SA=1100 m2/g). A feed of propylene carbonate (APHA=430,1020 ppm Br) was passed through the bed at a LHSV of 2/hr and product was collected in 50cc cuts. The following results were obtained: Comparative Table 2 Cut # APHA CoWor °/O N Removed % Br Removed 1 43 39 31 2 71 28 12 3 104 22 12 4 119 17 12

COMPARATIVE EXAMPLE 3 (activated carbon) A 1 cm ID column was packed with Calgon APC 12x40 Carbon (14.0 g, 41 cc, SA=1350 m2/g). A feed of propylene carbonate (APHA=430,1020 ppm Br) was passed through the bed at a LHSV of 2/hr and product was collected in 50 cc cuts. The following results were obtained: Comparative Table 3 Cut # APHA Color % N Removed % Br Removed 1 53 39 31 2 82 28 12 3 114 17 12 4 126 22 12

The above data show the effective results according to the invention as well as the comparative ineffectiveness of low surface area adsorbents and high surface area carbon by comparison.