Organic compounds -- part of the class 532-570 series – Organic compounds – Oxygen containing
Reexamination Certificate
1999-08-16
2001-05-22
O'Sullivan, Peter (Department: 1621)
Organic compounds -- part of the class 532-570 series
Organic compounds
Oxygen containing
C568S872000
Reexamination Certificate
active
06235948
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns a process for the removal of impurities, especially color forming impurities, from 1,3-propanediol.
TECHNICAL BACKGROUND OF THE INVENTION
1,3-Propanediol is a precursor for polyether glycols, polyester bomopolymers and copolymers, and thermoplastic elastomers. The quality of these products is in general dependent on the quality of the raw materials. For some applications, such as fibers, color quality is a major concern. It is known that products derived from 1,3-propanediol have suffered from discoloration. Commercially available samples of 1,3-propanediol starting materials have resulted in brown colored polyether glycols. Disclosed attempts to remove color from the products made from available 1,3-propanediol have been laborious and expensive. Even after extensive purification processes, many products retain a yellow color.
U.S. Pat. No. 2,520,733 discloses a process for the purification of polyols prepared from 1,3-propanediol in the presence of acid catalyst (2.5 to 6% by weight) and at a temperature from about 175° C. to 200° C. This purification process involves percolation of the polymer through Fuller's earth followed by hydrogenation. Even after this extensive purification process, the final product remains light yellow in color.
U.S. Pat. No. 3,326,985 discloses a procedure for the preparation of poly(1,3-propanediol) of molecular weights in the range of 1200-1400 possessing improved color by vacuum stripping, under nitrogen, poly(1,3-propanediol) of lower molecular weight.
U.S. Pat. No. 5,659,089 discloses a process for the preparation of poly(2-methyl-1,3-propanediol) by the etherification of 2-methyl-1,3-propanediol. No diol purification process is given. Any available grade of diol can be used in the etherification process.
U.S. Pat. No. 5,527,973 discloses a process for providing a purified 1,3 propanediol which can be used as a starting material for low color polyester. That process has several disadvantages including the use of large equipment and the need for dilution with large quantities of water which is difficult to remove from the product.
The process of the present invention alleviates the problem of discolored polymer production in a relatively simple and economical process. The process of the present invention involves purification of, and removal of color precursors from the starting reactant 1,3-propanediol rather than treating the resulting products. The purified diol from this process is useful for preparing a variety of polymers that include polyether glycols, polyesters and thermoplastic elastomers having excellent color characteristics.
SUMMARY OF THE INVENTION
Disclosed is a process for the purification of 1,3-propanediol comprising the steps of
a) contacting 1,3-propanediol with an acid catalyst at a temperature above that required for impurities to react and below that required for extensive ether formation; and
b) separating purified 1,3-propanediol from impurities, reacted impurities and the acid catalyst.
DETAILED DESCRIPTION OF THE INVENTION
In the preparation of low molecular weight polyether glycols via the dehydration of 1,3-propanediol, polyol quality is an important item. For some applications, such as the production of fibers, color is a major concern. In the past, attempts to prepare polytrimethylene glycols having good color properties have been unsuccessful. This invention discloses a process for the preparation of purified 1,3-propanediol so that products made from it, polyols, polyester homopolymers and copolymers, and thermoplastic elastomers, have excellent color characteristics.
Color-free, i.e., chromophore-free, polyether glycols have been prepared from 1,3-propanediol that has been purified of chromophore precursors by a two step process. The first step in the treatment of the diol to free it of color formers (referred to herein as “color precursors”) involves the contacting of the 1,3-propanediol with an acid catalyst at elevated temperatures. While not wishing to be bound by any mechanism, applicant believes this step converts color precursors and other impurities to colored and uncolored derivatives that, due to their changed chemical nature, are easily separable from 1,3-propanediol. The second step of the purification process involves the separation of the desirable, purified 1,3-propanediol from the formed colored and uncolored derivatives, from the acid catalyst employed to effect the conversion and, potentially, from residual impurities.
An acid catalyst is employed in the treatment process. This acid catalyst promotes the conversion of chromophore precursors to chromophores. The type of acid, (homogeneous or heterogeneous), nature (strong or weak) and the amount of acid catalyst can be varied widely. Although soluble acid catalysts, including inorganic acids and organic sulfonic acids, can be used, heterogeneous acid catalysts are usually preferred because they can be removed more easily and can easily be recycled. Soluble catalysts, if used, can be removed by extraction or neutralization. Solid heterogeneous acid catalysts can be removed by filtration. Fixed heterogeneous catalyst may also be used, i.e., in a continuous process, provided that contact time is sufficient. Suitable heterogeneous catalysts are perfluorinated ion-exchange polymers (PFIEP) containing pendant sulfonic acid groups or pendant carboxylic acid groups, including Nafion® catalyst, obtainable from the DuPont Company, which is an example of perfluorinated ion-exchange polymers containing pendant sulfonic acid groups, silica or alumina supported Nafion® catalyst, Nafion® Superacid catalyst, a porous nanocomposite of Nafion® resin and silica, acid zeolites, or acid clays.
The amount of acid catalyst used herein is not critical, as long as sufficient catalyst is present. Small amounts of catalyst may necessitate extended treatment times. In the case of commercial Nafion® NR50 catalyst, 1% by weight of catalyst versus the weight of 1,3-propanediol was convenient in a batch treatment process. The amount of catalyst employed, in batch treatment processes, will generally be within the range of about 0.1 to 5 wt % based on the amount of 1,3-propanediol used.
The temperature for the acid treatment step is selected such that the reaction that converts chromophore precursors readily occurs, but the dehydration reaction of 1,3-propanediol is essentially avoided. The process of the invention is carried out by heating the 1,3-propanediol and the acid catalyst at a temperature within the range of about 100-160° C. for 0.1-3 hours under nitrogen atmosphere. At temperature greater than 170° C., the acid can function as a dehydrating catalyst that could lead to the formation of polyether glycols from the 1,3-propanediol. Preferably, the reaction temperature is maintained within the range of 130° C.-150° C. so that chromophore precursor reaction occurs but essentially no dimers or trimers of 1,3-propanediol are formed.
The acid treatment is most conveniently carried out at atmospheric or above atmospheric pressures.
In a preferred mode of operation, after the color precursors are transformed, the mixture is cooled to room temperature, and the solid catalyst is removed either by filtration or by decantation. The filtered mixture is then distilled under reduced pressure, and the 1,3-propanediol is collected. The colored impurities and other impurities are left in the distillation flask. The absence of acid catalyst during the vacuum distillation process limits the occurrence of 1,3-propanediol dehydration reactions.
The purification of 1,3-propanediol, as described above, can be carried out in either a batch process or a continuous process. The treatment maybe carried out in an agitated system or, if the acid catalyst is in solid form or adhered/attached to a solid support, the treatment may be carried out by passing the diol over or through a bed of the solid acid catalyst.
The process can be conducted as a stand alone process, carried out on available 1,3-propanediol. Alternatively, it may be integrated with
Sunkara Hari Babu
Umile, II Robert John
E. I. Du Pont de Nemours and Company
O'Sullivan Peter
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