PROCESS FOR REMOVING HALIDE IMPURITIES BY RESIN TREATMENT FIELD OF THE INVENTION The present invention relates to the substantially complete removal of halide impurities from halide contaminated carboxylic acids and anhydrides. The invention is particularly effective in removing halide impurities from halide-containing carboxylic acids and anhydrides such as the reaction products of the carbonylation of lower-alkyl alcohols, esters, and ethers.

BACKGROUND OF THE INVENTION Carbonylation is a process used to manufacture a variety of commercially important chemicals. Such chemicals include acetic acid, acetic anhydride, and homologs thereof. A particularly important carboxylic acid produced by carbonylation is acetic acid. The chemical industry manufactures millions of tons of acetic acid each year for use in the manufacture of a variety of other useful chemicals such as vinyl acetate and cellulose acetate. For example, vinyl acetate is the raw material for polyvinyl acetate, a thermoplastic used in the manufacture of adhesives, water-based paints, and latex paints. Cellulose acetate is used in the manufacture of a variety of products such as plastic fibers, films, and sheets. The following illustrates the general reaction for the carbonylation of methanol with carbon monoxide to form acetic acid: Carbonylation processes typically employ metal catalysts such as cobalt, nickel, iridium or rhodium in a liquid reaction medium. For example, in the carbonylation of methanol to make acetic acid, rhodium is generally employed as the catalyst by contacting the reactants with rhodium in a reaction medium. Typically contact is achieved by dissolving the rhodium in the reaction medium.

The reaction medium is generally an organic solution, i. e., the product, containing the starting materials and the catalyst. In addition, halogen-containing promoters are typically employed in these reaction systems. Alkyl halides are common promoters. Alkyl iodides, most notably methyl iodide, are typically employed as promoters in acetic acid production. For the rhodium catalyzed acetic acid synthesis, the active species is believed to be [Rh (CO) 2I2-. In

addition, iodide ions, as provided by iodide salts, such as inorganic salts, sodium iodide, potassium iodide, lithium iodide, ferric iodide, manganese iodide, cobalt iodide, and the like, or organic salts, N-methylpyridinium iodide, and the like, and mixtures thereof, can be added to the reaction medium as co-promoters or catalyst stabilizers. Iodide salts can also become a part of the reaction mixture as a result of corrosion of the metallurgy of the reaction train.

These iodide salts are particularly useful to allow the water concentration in the reaction solution to be reduced and the concentration of reaction intermediates to be increased, thereby allowing for a greatly enhanced reactor throughput to be achieved.

Although halide promoters and stabilizers such as methyl iodide and I-salts improve the efficiency and productivity of carbonylation processes, the presence of halide compounds in the carbonylation products is undesirable. It is typical that, even after extensive purification, carbonylation reaction products typically contain contaminants from the original halide compounds present in the reaction medium during the carbonylation reaction and new halide compounds generated during the carbonylation reaction. For example, in the carbonylation of methanol with carbon monoxide using methyl iodide and lithium iodide, the carbonylation reaction products contain a variety of iodide compounds, generally C2 l2 alkyl iodides for example methyl iodide, ethyl iodide, butyl iodide, and hexyl iodide.

The carbonylation product, such as acetic acid, is most often a starting material in a subsequent process employing a halide-sensitive catalyst. For example, a significant amount of the world supply of acetic acid is used in the production of vinyl acetate by processes using sensitive, expensive catalysts containing metals especially precious metals such as gold and palladium. Since halide compounds, especially iodide compounds, deactivate or"poison"gold and palladium catalysts, and since this deactivation is cumulative and irreversible, starting materials that are essentially halide-free are required. The industry standard for halide contaminants in acetic acid is 10 parts per billion (ppb) or less. However, as indicated above, poisoning effects on precious metal catalysts are generally cumulative and irreversible as in the case of iodide contamination of catalysts for vinyl acetate production. Consequently, less than 1 ppb halide content is desired.

The problems associated with halide impurities in organic liquids have prompted the search for several solutions for removing the impurities. One method for removing halide impurities is a distillation process using peracids such as potassium permanganate to fix the iodide prior to distillation. Other methods involve the use of liquid-phase extraction processes

with non-aromatic hydrocarbons such as alkanes and cycloalkanes.

A number of methods are directed to treating carbonylation reaction products with reagents that form iodide salts. For example, many organo-nitrogen, organo-phosphorous and organo-sulfur species are known to quaternize, or form ionic salts, with alkyl halides such as methyl iodide and hexyl iodide. However, these compounds have not demonstrated the level of halide removal efficiency demanded by the industry.

One approach to enhancing the halide removal capability of quaternizing compounds such as alkyl or aryl phosphines or heterocyclic aromatic nitrogen compounds is described in U. S. Patent No. 4,664,753, incorporated herein by reference. The iodide removal approach described involves a homogeneous system wherein iodide contaminates contained in the carbonylation reaction products are treated with a combination of alkyl or aryl phosphines or a heterocyclic aromatic nitrogen compound and at least one of the metals copper, silver, zinc or cadmium, or metal salts thereof, such as copper (II) acetate or silver acetate, to fix the iodide in a non-volatile form. A second step is required to remove the iodide from the carbonylation products. In the second step, the carbonylation products are separated from the fixed iodide by distillation of the carbonylation products. Since a stoichiometric excess of metal and phosphine or aromatic nitrogen compound is required to achieve substantial iodide removal, this homogeneous system requires recovery and disposal of toxic metal residues. Furthermore, the distillation required to separate the carbonylation product from the halide precipitate is an additional step in the production process, increasing its duration and expense.

The requirement for a second distillation has been obviated by passing the iodide- containing product through a bed of the above metal-containing phosphine or nitrogenous compounds wherein the compounds are immobilized as a part of a polymer. Examples of such compounds are disclosed in U. S. Patent 5,300,685, and include poly 4-vinyl pyridine, polyphenylene sulfide, and polybenzimidazole each of which have been coordinatively bonded to a silver last.

Another approach suggests avoiding the combination of alkyl and aryl phosphines or aromatic nitrogen compounds and metals or metal salts. This approach suggests treating carboxylic acid or carboxylic acid anhydrides with an unsupported silver salt"scavenger,"e. g., silver acetate, in the absence of alkyl or aryl phosphines and heterocyclic aromatic nitrogen compounds. A high stoichiometric excess of metal salt to the halide impurities is required to achieve significant halide removal. Also, as in the treatment/precipitation procedures described,

an additional distillation step is required to purify the carbonylation product. Moreover, the scavenger residue must be recovered. In addition, the process does not achieve sufficient halide removal for demanding end uses such as vinyl acetate production.

A particularly effective approach, which avoids many of the problems associated with distillation and extraction processes, employs a strong acid cation exchange resin. One such resin is a sulfonated copolymer of styrene and divinylbenzene in which at least a portion of the active sites are exchanged to the silver or mercury form through the exchange of a silver or mercury salt, such as silver acetate or mercuric acetate. In this process, the silver or mercury exchanged resin can be placed in a column and the halide contaminated liquid passed through the column. Greater than about 99.99% removal of iodide compounds from iodide contaminated acetic acid can be achieved through this process as reported in U. S. Patent No. 4,615,806, incorporated herein by reference. Although each of these approaches offer various degrees of halide removal effectiveness and commercial feasibility, still other effective, efficient, and commercially desirable halide impurity removal processes are desired.

SUMMARY OF THE INVENTION The present invention relates to a method for removing halide, preferably iodide compounds from a non-aqueous organic carbonylation product. The product may be a vapor or a liquid product such as acetic acid and acetic anhydride. The product is contacted with a metal salt which complexes with an alkyl iodide to remove the alkyl iodide from solution. The metal salt is, in particular, a salt of a weak acid contained on the pendant portion of an appropriately substituted macroporous resin such as a styrene-divinylbenzene copolymer structure. More particularly, the metal salt comprises a styrene-divinylbenzene resin containing at least one recurring pendant weak acid salt of such metal ion within the pendant group or between adjacent pendant groups. This metal salt is thereafter capable of forming an insoluble compound with the iodide-containing impurities in the carbonylation product when placed in contact with the product. The present invention is an alternative to that taught by US 4,615,806 or US 5,300,685 in that a new functionality has been discovered to be useful in the purification process.

DETAILED DESCRIPTION The present invention is broadly applicable for the removal of halide compounds and particularly iodide compounds, and is an alternative to the use of sulfonic acid cation exchange

resins. The present invention relates to a method for removing halide compounds from a non- aqueous organic medium comprising contacting the medium in liquid or vapor form containing said halide compounds with a metal salt characterized in that the salt function comprises the pendant weak acid salt of a styrene-divinylbenzene macroporous resin, and the metal being one that will react with the iodide impurity to precipitate the halide-containing compound to remove it from solution, thereby purifying the carbonylation product, and maintaining the contact between the vapor or liquid for a time sufficient for metal halide precipitates to form, and separating the carbonylation product from the polymeric resin.

The method is generally applicable to products produced by the carbonylation of methanol, dimethyl ether, or methyl acetate, or mixtures thereof with carbon monoxide in the presence of a rhodium catalyst, an alkyl iodide, and an ionic iodide salt promoter to form acetic acid or acetic anhydride or mixtures thereof. The term"non-aqueous"referred to herein means that water is not present to any significant extent in the organic medium which is being processed. In a preferred embodiment, the medium is acetic acid, acetic anhydride, mixtures of acetic acid and acetic anhydride, or other carbonylation products. When acetic acid is processed in accordance with the present invention, it usually has no more than about 0.15% by wt. of water present.

Metal salts useful for the method of this invention include salts of macroporous polymeric resins containing the following pendant functionalities where X is a polymeric backbone structure including but not limited to a styrene divinylbenzene copolymer: X- (CR2),-NH- (CR-COOM X-(CR2)n-S-(CR2),-COOM X-(CR2) n-O-(CR2) n-COOM wherein R is H or lower alkyl, M is the iodide-precipitating metal described and discussed hereinbelow, and n equals 1-3, p equals 1-6, preferably p equals 1-3."Lower"herein refers to C, 6. It is obvious to one skilled in the art that the examples shown depict monovalent metals.

Where the metal is divalent or polyvalent, the salt will contain more than one weak acid entity per metal ion.

The polymeric resin containing the metal salt (hereinafter called"metal-containing resin to distinguish it from the hydrogen or alkali metal form of the resin available commercially) is contacted with the carbonylation product containing halide impurities. The halide impurities are fixed in the form of precipitated metal halides trapped in the polymeric resin matrix. The metal- containing resin can be employed in a fixed-bed process wherein liquid or vaporous feedstock is continuously passed through the resin bed for removal of halide contaminants. Alternatively, the metal-containing resin can be slurried with the feedstock containing the iodide impurities to remove the impurities and the liquid, depleted in impurities, can be decanted from the resin.

Thus, one particular advantage of the present invention is the ability to trap the metal halide within the resin matrix in one step. This ability obviates a subsequent distillation step required to separate the metal halide and excess reagents from the carbonylation product. Moreover, the recovery and disposal of toxic residues is simplified. Of particular importance and value is the use of the present invention to remove iodine values from acetic acid, acetic anhydride, and mixtures thereof manufactured by the carbonylation of methanol in the presence of alkyl iodides and iodide salt promoters, i. e., lithium iodide.

The metal-containing resin is prepared by replacing the hydrogen or alkali metal form of the polymer as commercially available with a metal ion suitable for the precipitation of iodide contaminants in a suitable solvent. This technique is well known in the ion-exchange industry.

"Solvent"as used herein refers to a solvent for the metal salt.

The metal-containing resin salts useful in practicing the invention are generally salts of cations which can cause the precipitation of the iodide contaminant allowing the precipitated contaminant to be adsorbed in the interstices of the macroporous resin. In addition, the desirability of the metal salt is governed by the insolubility of the metal halide formed in the particular medium. General, but non-limiting examples of metals salts include silver, mercury, copper, and lead salts sufficiently soluble in the solvent to allow exchange onto the resin.

Examples of particularly preferred metal salts include silver or mercury salts such as silver acetate, silver nitrate, or mercuric acetate.

Thus, for example, the resins useful for the practice of this invention can include:

X-CH2-CH2-COO-Ag+ X-(CH2-COO-) 2 Hg++ X-(CH2-CH2-CH2-CO0) 2-Cu++ X-CH2-NH-CH2-COO-Ag+ X-CH2-N- (CH2-CH2-COO-Ag+) 2 X-(CH2-CH2-S-CH2-CO0) 2-Hg++ X-CH2-O-CH2-COO-Pb+ and the like The solvent can include the liquid from which the iodide will be removed according to the method of this invention, such as acetic acid, acetic anhydride, or acetic acid/acetic anhydride mixtures. The solvent may also include water, methanol, methyl acetate, or mixtures thereof, other solvents for the metal salt.

The polymer is mixed with the metal salt solution and allowed to exchange in a manner well-known to the art. Preferably the polymeric resin is first conditioned in the solvent by immersing the polymer in the solvent to form a mixture or slurry and applying a vacuum on the head space above the polymer-solvent mixture to facilitate removal of trapped gases such as air in the polymer matrix and to allow the solvent to flow more easily into the resin matrix. After the resin is conditioned in the solvent, the metal salt or a solution of the metal salt is added to the conditioned polymer mixture or slurry. The metal salt solution can be a solution of the metal salt in the solvent used to prepare the resin or another compatible solvent. The mixture or slurry is then stirred and heated as necessary to allow the metal salt to exchange with the hydrogen or alkali metal contained on the resin. The mixture of resin and solvent is then cooled if needed, filtered, and washed with additional solvent.

As discussed hereinabove, the metal-containing resin is then contacted with the halide containing liquids or vapor in any suitable manner. The metal-containing resin can be employed in a fixed-bed system and halide-containing liquid passed through the resin bed to remove halide impurities. The metal-containing resin can be slurried with the halide-containing liquid and the purified liquid decanted therefrom. Alternatively, halide-containing vapors could be passed

through a column containing the resin. The metal-containing resin is then held in contact for sufficient time to allow the metal halide precipitates to form. No special processing parameters are contemplated. For example, ambient room temperature would be an adequate processing temperature as would temperatures found in a typical carbonylation-process product stream.

Since operation at elevated temperatures is desirable in many carbonylation processes, the thermal stability of polymers useful in the present invention is advantageous.

The following examples further illustrate the practice of the present invention.

Example 1 Resin Preparation The dry resin in sodium form, 100 g. was washed with deionized water five times until the effluent was clear. The water wash was decanted therefrom and the resin was transferred to a flask equipped with a stirrer. 30.6 g of silver acetate and 200 ml water was added. The slurry was stirred for about 24 hours, the aqueous solvent was removed, and the resin washed until the effluent was clear.

Removal of Hexyl Iodide from Acetic Acid A stock solution of 500 ppm hexyl iodide in acetic acid was prepared. Ten ml of each of the silver exchanged resins was added to a container of 100 g of the stock solution. The slurry was mechanically shaken for one hour and then allowed to sit for 24 hours. The supernatant was removed and tested for hexyl iodide. Silver-loaded resin % Hexyl iodide removed Amberlite* IRC-718 99.8 Diaion* * CR-1 1 92 0 1 Both resins in their Ag+ form are defined by the structure: X-CH2-N- (CH2-COO-Ag+) 2 depending on the actual level of silver exchanged and

wherein X is a styrene-divinylbenzene copolymer backbone structure.

* Amberlite is the registered trademark of Rohm and Haas Company * * Diaion is the registered trademark of Mitsubishi Chemical America, Inc Analysis of the treated samples was conducted by gas chromatography using an electron capture detector. The detection limit for the hexyl iodide was approximately 1 ppb. Samples that had high hexyl iodide concentrations after treatment (>50 ppb) were diluted with iodide-free acetic acid before analysis to make the resulting hexyl iodide <50 ppb to maintain linearity of the analysis. The results were then multiplied by the appropriate dilution factor.