Process for preparing purified carboxylic acids

Details Process for preparing purified carboxylic acids Process for preparing purified carboxylic acids

1998-12-01

2001-07-03

Killos, Paul J. (Department: 1623)

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

Organic compounds

Carboxylic acids and salts thereof

C562S416000, C562S409000, C562S418000, C562S421000, C562S485000, C562S486000, C562S487000, C562S494000, C562S483000

Reexamination Certificate

active

06255525

ABSTRACT:

FIELD OF THE INVENTION
This invention relates generally to an improved process for preparing purified aromatic carboxylic acids. More particularly, this invention relates to an improved process for preparing purified aromatic carboxylic acids by contacting a crude aromatic carboxylic acid with hydrogen in the presence of a carbon catalyst.
BACKGROUND OF THE INVENTION
Aromatic carboxylic acids such as terephthalic acid, isophthalic acid and 2,6-naphthalenedicarboxylic acid are useful monomers for the preparation of polymeric materials, particularly polyester materials. For example, 2,6-naphthalenedicarboxylic acid can be reacted with ethylene glycol to prepare a high performance polyester, poly(ethylene-2,6-naphthalate) (PEN). Fibers and film manufactured from PEN have improved strength and superior thermal properties relative to other polyester materials. Films made from PEN demonstrate, for example, superior resistance to gas diffusion and particularly to the diffusion of carbon dioxide, oxygen and water vapor. Because of its exceptional properties, PEN is especially suitable for applications such as food and beverage containers, particularly for so-called “hot-fill” food and beverage containers, tire cord, magnetic recording tape and electronic components.
Methods for preparing 2,6-naphthalenedicarboxylic acid include the bromine-promoted, metal-catalyzed, liquid phase oxidation of 2,6-dialkylnaphthalenes. Such processes are disclosed in U.S. Pat. Nos. 3,870,754; 4,950,786; 4,933,491 and 5,183,933. The bromine-promoted, metal-catalyzed, liquid phase oxidation of 2,6-dialkylnaphthalenes, particularly 2,6-dimethylnaphthalene, produces a crude product containing a variety of impurities such as brominated 2,6-naphthalenedicarboxylic acid, 2-formyl-6-naphthoic acid, 2-naphthoic acid, trimellitic acid and other impurities. These impurities, particularly 2-formyl-6-naphthoic acid, are difficult to remove from crude 2,6-naphthalenedicarboxylic acid. Typically, customers desire that the crude 2,6-naphthalenedicarboxylic acid be purified before it is polymerized to form polymeric materials.
Methods for the purification of 2,6-naphthalenedicarboxylic acid are known. In the aforementioned U.S. Pat. No. 4,933,491, for example, 2,6-naphthalenedicarboxylic acid was purified after reacting the 2,6-naphthalenedicarboxylic acid with a lower alkanoic anhydride to produce a component that is soluble in excess alkanoic anhydride. The “solubilized” 2,6-naphthalenedicarboxylic acid was optionally treated with one or more purification procedures. U.S. Pat. No. 3,649,680 to McNarney discloses a process for purifying aromatic carboxylic acids wherein a mixture of water and an alkanol are added to an impure carboxylic acid paste, the carboxylic acid is separated from the alkanol/water mixture, and the purified carboxylic acid is subsequently washed with water. U.S. Pat. No. 3,671,578 to Ogata discloses a process for preparing 2,6-naphthalenedicarboxylic acid wherein the monoalkali salt of 2,6-naphthalenedicarboxylic acid is heated in water or a water-containing organic solvent, causing disproportionation thereof into 2,6-naphthalenedicarboxylic acid and the dialkali salt of 2,6-naphthalenedicarboxylic acid. U.S. Pat. No. 3,888,921 to Yamamoto et al., discloses a process for purifying 2,6-naphthalenedicarboxylic acid wherein an aqueous solution of a dialkali salt of crude 2,6-naphthalenedicarboxylic acid is prepared, then 40 to 97 mole percent of the dialkali salt is precipitated as a monoalkali salt while maintaining the pH of the aqueous solution at a value of not lower than 6.3, and converting the precipitate to 2,6-naphthalenedicarboxylic acid. It is disclosed in the Yamamoto et al. patent that the aqueous solution of the dialkali salt of 2,6-naphthalenedicarboxylic acid can be at a temperature of 60° C.-350° C. in the presence of potassium or sodium hydroxide, and it is disclosed that the solution can be treated with a reducing agent such as hydrogen gas, sodium dithionite, lithium aluminum hydride or sodium borohydride. U.S. Pat. No. 3,781,346 to Norton discloses a process for purifying naphthalene carboxylic acids comprising reacting a solid ammonium salt of the acid with steam at a temperature of from about 200° C. to about 300° C. U.S. Pat. No. 4,794,195 to Hayashi et al. discloses that it is impossible to purify crude naphthalenedicarboxylic acid to a high purity only by crystallization, and that it is necessary to combine the method of crystallization with other methods such as thermal treatment, oxidative treatment or reductive treatment. However, no specific means for conducting such treatments on 2,6-naphthalenedicarboxylic or other naphthalenedicarboxylic acid is disclosed. USSR Inventor's Certificate No. 486,008, to Kulakov et al. published on Jan. 15, 1976, discloses a method for purifying 2,6-naphthalenedicarboxylic acid by treating impure 2,6-naphthalenedicarboxylic acid having a particle size of 0.05-0.35 mm and containing up to 30% naphthalenemonocarboxylic acid with an aliphatic carboxylic acid at 180-250° C. U.S. Pat. No. 5,563,294 to Sikkenga et al., discloses a process for purifying 2,6-naphthalenedicarboxylic acid using hydrogen and a noble metal supported on carbon as a hydrogenation catalyst. U.S. Pat. No. 3,584,039 to Meyer discloses a process for preparing fiber-grade terephthalic acid by catalytic hydrogen treatment of dissolved impure terephthalic acid using a carbon catalyst containing noble metals. The processes described are either complicated or require the use of a potentially expensive noble metal catalyst. What is needed is a simpler process for purifying aromatic carboxylic acids, preferably one that does not require the use of potentially expensive metal catalysts.
SUMMARY OF THE INVENTION
Surprisingly, we have found that aromatic carboxylic acids may be purified by a relatively simple process that does not require the use of potentially expensive catalytic metals.
In our process, an impure aromatic carboxylic acid is purified by treating the impure acid at elevated temperature and pressure in the presence of a solvent, hydrogen gas, and a carbon catalyst which is essentially free or free of added metals such as noble metals. By “essentially free” we mean less than about 0.10 weight percent, preferably less than about 0.07 weight percent and more preferably less than about 0.03 weight percent metal based on the weight of the carbon catalyst. By “free” we mean that no hydrogenation or noble metal has been purposely or intentionally added to the carbon catalyst either before or after its manufacture.
Our invention is particularly suited to purifying naphthalenedicarboxylic acids, most preferably 2,6-naphthalenedicarboxylic acid. The preferred solvent typically is water.
Although it is known that carbon treatment can be used to remove color by adsorption of colored impurities, we have determined that a carbon catalyst essentially free or free of added noble metal hydrogenation components can be used in the presence of hydrogen gas to effectively hydrogenate aldehyde impurities in the crude aromatic carboxylic acid.
For example, 2-formyl-6-naphthoic acid (2-FNA) is an impurity that causes an increase in color of polyester made from 2,6-naphthalenedicarboxylic acid. 2-formyl-6-naphthoic acid in crude or impure 2,6-naphthalenedicarboxylic acid can be hydrogenated to 2-methyl-6-naphthoic acid in accordance with our invention by using a carbon catalyst essentially free of noble metal hydrogenation components. Thus, in addition to removing colored impurities, the process of this invention removes deleterious aldehyde components without the use of expensive noble metal hydrogenation components.
DETAILED DESCRIPTION OF THE INVENTION
In our invention, an aromatic carboxylic acid is contacted with a carbon catalyst in the presence of a solvent at an elevated temperature and pressure in the presence of hydrogen.
Although the aromatic carboxylic acids that can be purified by the process of this invention can be any aromatic carboxylic acid such as terephthalic acid, isop

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