Organic compounds -- part of the class 532-570 series – Organic compounds – Carboxylic acids and salts thereof
Reexamination Certificate
2000-08-22
2002-07-30
Killos, Paul J. (Department: 1623)
Organic compounds -- part of the class 532-570 series
Organic compounds
Carboxylic acids and salts thereof
C562S481000
Reexamination Certificate
active
06426431
ABSTRACT:
FIELD OF THE INVENTION
This invention is related to the production of 2,6-naphthalene dicarboxylic acid (2,6-NDA). More particularly, this invention is related to a reaction step known in the art for the purification of 2,6-NDA in which the monosalt of 2,6-NDA, KHNDA, is heated in water or aqueous solvent to cause the disproportionation thereof, and to the stoichiometric limitation of the reaction to produce 50% 2,6-NDA on a molar basis. The present invention is a method for increasing the yield at least about 20% or more beyond the stoichiometric limitations.
BACKGROUND OF THE INVENTION
Aromatic dicarboxylic acids are highly useful organic compounds. They are often used as monomers for the preparation of polymeric materials. 2,6-naphthalene dicarboxylic acid is a particularly useful aromatic carboxylic acid, because it can be reacted with ethylene glycol to prepare poly(ethylene-2,6-naphthalate), PEN. Fibers and films manufactured from PEN display improved strength and superior thermal properties compared with other polyester materials such as polyethylene terephthalate. High strength fibers made from PEN can be used to make tire cords, and films made from PEN are advantageously used to manufacture magnetic recording tape and components for electronic applications.
It is known in the art to prepare aromatic dicarboxylic acids such as 2,6-NDA by primarily two methods. One is the liquid phase, metal catalyzed oxidation of an alkyl or acyl substituted aromatic compound. This method is described, for example, in U.S. Pat. Nos. 2,833,816; 3,856,855; 3,870,754; 4,933,491; and 4,950,786.
Alternatively, naphthalene monocarboxylic acid and naphthalene dicarboxylic acids other than 2,6-naphthalene dicarboxylic acid can be converted to 2,6-NDA, using a disproportionation reaction in the case of the monocarboxylic acids, or a rearrangement reaction in the case of other naphthalene dicarboxylic acids. Henkel and Cie first patented a reaction of naphthoic acid salts to 2,6 NDA. (See U.S. Pat. Nos. 2,823,231 and 2,849,482).
Currently most commercial processes use the oxidation method, though it requires expensive feedstock, forms oxidation products with impurities trapped within, and the product usually has to be submitted to esterification, so that the product is 2,6-naphthalene dicarboxylate, rather than 2,6-NDA. In view of the problems with the oxidation process and product, there has been much research on alternative routes to 2,6-NDA based on disproportionation. After a disproportionation or rearrangement reaction the dialkali metal salts have to be separated and directed through several steps in order to obtain the desired pure 2,6 -NDA. It is known to add mineral acid to free the 2,6-NDA. Neutralization of the disalt produced in the Henkel reaction is difficult without compromising the purity of the product 2,6-NDA. Other methods involve benzene extraction, evaporation, and sublimation. See for example U.S. Pat. Nos. 2,828,231, 2,849,482, 3,631,096.
Several references in the art have taught it is advantageous in the recovery of 2,6-NDA to precipitate the monoalkali salt of 2,6-NDA and disproportionate it to produce free 2,6-NDA. Precipitation of the monosalt with an acid such as CO
2
, followed by disproportionation of the monosalt to make 2,6-NDA is a route that has the potential to result in high purity 2,6-NDA. Unfortunately this route only produces half a mole of 2,6-NDA from each mole of the 2,6 disalt present in the disproportionation product.
U.S. Pat. No. 3,671,578, to Teijin, discloses the monoalkali salt of 2,6-naphthalene dicarboxylic acid is disproportionated when heated in water or water-containing organic solvent, to form free dicarboxylic acid and by-product dialkali salt, and the former acid is precipitated.
This reference teaches the use of atmospheric pressure, or “slightly elevated” (Col. 3, line 68-70), and does not address the idea of increasing yield beyond stoichiometric limitations.
U.S. Pat. No. 3,888,921, to Teijin Ltd., discloses a method for purifying a dialkali salt of crude 2,6-naphthalene dicarboxylic acid comprising precipitating 40 to 97 mol percent of the dialkali 2,6-naphthalene dicarboxylate dissolved in an aqueous solution substantially as a monoalkali salt of the 2,6 -naphthalenedicarboxylic acid while maintaining the pH of said aqueous solution at a value not lower than 6.3, and separating the precipitate, and converting the separated precipitate to a 2,6-naphthalene dicarboxylic acid by disproportionation.
Canadian Patent 864587 discloses a process for the preparation of 2,6-NDA which comprises heating a monoalkali salt of 2,6-NDA in water or water-containing organic solvent causing disproportionation thereof into 2,6-NDA and a dialkali salt and separating the 2,6-NDA by a method that includes dissolving a rearrangement reaction product containing dialkali salt of 2,6-naphthalene dicarboxylic acid in warm water, filtering off the insoluble matter therefrom, concentrating the remaining solution, whereby the filtrate is concentrated to such a degree that the precipitation yield of the dialkali salt precipitated when the concentrated liquid is cooled to room temperature reaches at least 70% and the purity of said precipitate exceeds 99% passing gaseous carbon dioxide through the aqueous solution of the precipitate recovered from the concentrated liquid, and recovering the resulting precipitate,and the mother liquour containing the side product dialkali salt of 2,6-naphthalene dicarboxylic acid is recycled into the carbon dioxide reaction step. This reference teaches at page 7, line 12, “Atmospheric pressure is employable, but the reaction can be . . . at elevated pressures and temperatures above 100° C.”. This reference does not address the possibility of increased yields.
U.S. Pat. No. 5,175,354 discloses a process for preparing 2,6-NDA by reacting at least one of 2,6-NDA dipotassium salt and 2,6-NDA monopotassium salt with a benzenecarboxylic acid to yield solid 2,6-NDA and an aqueous solution of a benzenecarboxylic acid potassium salt, separating the solid 2,6-NDA from the aqueous solution, recovering the benzenecarboxylic acid potassium salt from the aqueous solution, reacting the benzenecarboxylic acid potassium salt with naphthalene to yield 2,6-NDA dipotassium salt, and recycling the 2,6-NDA dipotassium salt.
Although the step of heating the monopotassium salt of 2,6-NDA in water or aqueous solvent to disproportionate it to produce 2,6-NDA has been disclosed in the art, a disadvantage is that the maximum yield of 2,6-NDA has appeared to be limited by stoichiometry to 50% on a molar basis. None of the references found address the concept of increasing yields well beyond the stoichiometric limitations. It would be a distinct advance in the art if it were possible to substantially improve the yield of 2,6-NDA. This would be extremely valuable economically in any integrated process for producing 2,6-NDA.
SUMMARY
In accordance with the foregoing the present invention is a method of substantially increasing the yield of 2,6-NDA which comprises:
a) Dissolving the disproportionation reaction product of potassium naphthoate, containing the dialkali salt of 2,6-NDA(K2NDA) in water to form an aqueous solution and filtering off insoluble matter from the resulting solution;
b) Reacting said aqueous K2NDA solution at a CO
2
pressure of about 0-200 psig and a temperature of about 0-50° C. to selectively precipitate the monopotassium salt of 2,6-NDA (KHNDA);
c) Suspending said monopotassium salt in water in a weight ratio higher than 8:1, water: monosalt; and
d) Heating said monopotassium salt in water at a temperature above 100° C. and at a CO
2
pressure above 100 psig causing disproportionation to form 2,6-NDA and K2NDA.
The present invention provides a method of increasing the yields from the stoichiometric limitation of 50% to as high as 72%.
DETAILED DESCRIPTION OF THE INVENTION
In the present invention we have discovered that there are critical conditions for the disproportionation of KHNDA to 2,6-NDA which substantially increase the molar yield.
Brownscombe Thomas F.
Diaz Zaida
Killos Paul J.
Mossi & Ghisolfi Overseas S.A.
Renner Kenner Greive Bobak Taylor & Weber
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