Process for improving yields in a disproportionation...

Details Process for improving yields in a disproportionation... Process for improving yields in a disproportionation...

2000-08-22

2002-09-17

Padmanabhan, Sreeni (Department: 1623)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carboxylic acids and salts thereof

C562S481000, C562S488000, C562S490000, C562S492000

Reexamination Certificate

active

06452045

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to disproportionation/isomerization reactions, such as the Henkel reaction. More particularly, this invention relates to the disproportionation of the salt of a monocarboxylic acid, such as potassium naphthoate, to form the salt of a dicarboxylic acid, such as the salt of isomers of naphthalene dicarboxylic acid. Still more particularly, this invention relates to a combination of modifications of the elements of a disproportionation reaction that provides improvements in product yield of as much as 40%.
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 (2,6-NDA) 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 in the late 1950s. (See U.S. Pat. No. 2,823,231 and U.S. Pat. No. 2,849,482). In these references, the product yield of the disproportionation reaction is about 65% at best. Of course it would be desirable to improve on these yields.
Currently in the art 2,6-NDA for commercial use is prepared by oxidation, even though that route is plagued with difficulties. The most common process for making 2,6 NDA starts with relatively expensive o-xylene and butadiene feedstocks, as discussed, for example, in U.S. Pat. No. 5,510,563 and U.S. Pat. No. 5,329,058 and incurs substantial yield losses of these starting materials. Following the synthesis and purification of 2,6 dimethylnaphthalene (2,6 DMN), 2,6 DMN is oxidized to produce crude NDA product which forms as a solid with impurities trapped within. In order to remove these impurities to a sufficiently low level acceptable for polymerization, the 2,6 NDA product must be purified via multiple steps. These steps typically involve esterification, so that the end product is 2,6-naphthalene dicarboxylate, an ester, rather than the preferred 2,6 napthalene dicarboxylic acid. Esterification to naphthalene dicarboxylate (NDC) is necessary to eliminate the impurities, as discussed in U.S. Pat. No. 5,254,719 and 4,886,901.
Various improvements in a route to NDA based on disproportionation/rearrangement have been claimed in the art. In this work Henkel preparations which claimed to have improved yields depended on catalysts containing halogen-containing corrosive salts or other toxic and irritating materials. Other research that used zinc as a catalyst reported the making of zinc salts of the aromatic acids, a process involving added capital and difficulty in recycling the zinc. Yields of these processes were, even so, low due to failure to observe the effect of the critical components of the reaction and the effect of their ranges on the result of the reaction.
U.S. Pat. No. 2,919,273 claims improvements in yield in the thermal rearrangement of salts of cyclic carboxylic acids using as the catalyst salts of catalytically-active bivalent metals, wherein said metals are present in the form of compounds of the general formula selected from the group consisting of Alk
2
(MeX
2
Y
2
) and Alk(MeX
2
), where Alk is an alkali metal cation, Me is a bivalent catalytically active metal selected from the group consisting of cadmium, zinc and lead, and X is a halogen ion and Y is an ion selected from the group consisting of halogen and carbonate ions. This reference does not specifically address or have examples of disproportionation of potassium naphthoate.
The object of U.S. Pat. No. 3,641,130 is to obtain greater efficiency in a disproportionation reaction and claims an advantage carrying out said reaction in the presence of at least one adjuvant compound of the formula R—X—M, wherein R is an alkyl, cycloalkyl, aryl radical, or combination thereof having from 1 to 15 carbon atoms therein, X is oxygen or sulfur, and M is hydrogen or an alkali metal.
U.S. Pat. No. 3,751,456 claims an improvement in a disproportionation reaction using as a catalyst a mixture of cadmium iodide and sodium iodide.
U.S. Pat. No. 3,875,218 claims advantages using as a catalyst a metal salt of an aromatic carboxylic acid, said metal being selected from the group consisting of zinc, cadmium, mercury, lead, and iron. The examples demonstrate the use of zinc benzoate.
U.S. Pat. No. 4,820,868 claims an improvement in a process for the preparation of a naphthalene 2,6 -dicarboxylic acid dialkali metal salt comprising using naphthalene as a reaction medium, wherein the amount of naphthalene is 0.5 to 10 times the amount of the starting material by weight.
There is still a need in the art for greater conversion and yield in the production of 2,6-NDA, the preferred monomer for the production of polyethylenenaphthalate (PEN).
An object of the present invention is to improve rates of conversion to 2,6- naphthalene dicarboxylic acid (2,6-NDA). Another object is to increase the total yield of 2,6-NDA.
SUMMARY
The present invention provides a method for increasing the disproportionation yields (Henkel II) of the dipotassium salts of 2,6-NDA from ca. 70% to as much as 95-100%, or more, resulting in a far more economical process. (Yields in excess of 100% basis the formal disproportionation reaction are possible provided some of the naphthoate salt starting material is carboxylated instead of disproportionated).
In accordance with the foregoing the present invention provides a process for disproportionation of potassium naphthoate to the dipotassium salt isomers of 2,6-NDA with reproducible improved yields which comprises reacting naphthoic acid in the presence of excess base to produce a feed comprising a finely dispersed disordered salt mixture of excess base salts and naphthoic acid salts, wherein the feed is prepared by the steps of:
a) Reacting naphthoic acid in the presence of at least 0.001 to 0.30 moles of excess base (excess beyond the amount necessary to form naphthoate salt of 1:1 acid to cation ratio) selected from the group consisting of carbonates and bicarbonate to form a solid salt;
b) Drying said salt mixture by a method which forms a highly mixed disordered salt mixture characterized by:
(1) A differential scanning calorimeter(DSC) signature characterized by low melting peaks not previously observed in potassium naphthoate salt; and/or
(2) A characteristic Xray diffraction pattern.
c) Disproportionating said solid salts in the presence of a disproportionation catalyst to form the dipotassium salts of 2,6-NDA.
DETAILED DESCRIPTION OF THE INVENTION
When preparing feed salts for the Henkel disproportionation reaction in literature of the prior art, care is usually taken to prepare organic salts of precise stoichiometry. Raecke, (See U.S. Pat. No. 2,823,231) for example, back-titrated potassium acid salts with aqueous HCl to produce a solution of pH 6-7, resulting in a ratio of about 0.96(or less) to 1.00 potassium: aromatic acid. Using this sort of procedure yields are typically 60-70% of theoretical. In the present inven

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