GB1220542A - Purification of organic compounds - Google Patents

Abstract

1,220,542. Purifying terephthalic acid. CELANESE CORP. 12 Feb., 1969 [15 Feb., 1968 (3); 29 Nov., 1968 (4)], No. 7495/69. Heading C2C. The invention relates to a method for improving the purity of a crude terephthalic acid comprising (a) heating a combination of crude terephthalic acid and an inert liquid medium to a temperature such that no more than about 50% of the terephthalic acid remains as a solid, the temperature being at least 275‹ C. when in the presence of an iodine catalyst, a bromine catalyst, or a Group VIII noble metal catalyst and being at least 325‹ C. in other cases, the pressure being sufficient to maintain a liquid phase at temperatures below the critical temperature; (b) cooling the terephthalic acid and liquid medium so as to crystallize terephthalic acid; and (c) recovering the thus crystallized, purified terephthalic acid. The process may be carried out with or without a catalyst and suitable catalysts include those which will provide an alkali metal or a metal from Groups IB, IIB e.g. metals copper, silver, gold, zinc, cadmium, and mercury, or Group VIII of Mendeleev's Periodic Table, e.g. those metals having atomic numbers below 29 such as iron, cobalt and nickel and those having atomic numbers above 43 such as ruthenium, rhodium, palladium, osmium, iridium, or platinum. The catalyst may provide the desired metal as the free metal itself, such as a palladium on charcoal catalyst, or may provide the desired metal in the form of a chemical compound such as cobalt acetylacetone or sodium acetate. When using a non-supported catalyst, that is when using a chemical compound as the source of the catalyst, the catalyst is preferably soluble under the reaction conditions. Of the various types of metal containing compounds that may be used as catalysts in the present invention it is preferred to use metal chelates, metal salts of organic carboxylic acids, for example C 1 to C 12 acids, or inorganic salts such as phosphates, bromides, sulphates and chromates. When using a supported catalyst it is preferred to use a supported noble metal catalyst due to catalyst recovery problems presented when using the non-noble metals. Other catalysts which may be utilized include iodine and bromine catalysts which may be obtained by adding elemental iodine (I 2 ) or elemental bromine (Br 2 ) to the terephthalic acid-liquid mixture or it may be obtained by the addition of an iodine- or bromine-containing compound which will decompose or ionize to give iodine or bromine (probably as HI or HBr). The inert, liquid medium should be one which has a boiling point above 20‹ C. and a melting point below 200‹ C. and can be water, organic compounds or mixtures of organic compounds and water. Usually the organic medium will be selected from C 5 + hydrocarbons, carboxylic acids, hydrocarbon ethers, halohydrocarbon ethers, and mixtures thereof, and of the hydrocarbons and halohydrocarbons that may be used, aromatics are preferred. In an example an autoclave constructed of titanium was charged with crude terephthalic acid containing 4500 p.p.m. of 4-carboxybenzaldehyde and 2000 p.p.m. of toluic acid, glacial acetic acid, demineralized water and a catalyst consisting of 0.5 weight per cent ruthenium on carbon, 4 x 8 mesh, and was then sealed and quickly heated to 350‹ C. and maintained at that temperature for 10 minutes at which time the autoclave was rapidly cooled to about 100‹ C. so as to crystallize the terephthalic acid, both the cooling and heating being accompanied by rocking agitation. The contents of the autoclave were removed, filtered to isolate the solid terephthalic acid and catalyst, and then the solids were washed with water to remove the acetic acid present. In order to separate the terephthalic acid from the catalyst, a dilute solution of ammonium hydroxide was added so as to dissolve the terephthalic acid present and then the catalyst separated by filtration. Hydrochloric acid was added to the mother liquor so as to spring the terephthalic acid from solution in the dilute ammonium hydroxide, and then the terephthalic acid recovered by filtration and washed with water. Analysis of the thus purified terephthalic acid showed it to contain about 60 p.p.m. of 4- carboxybenzaldehyde and 31 p.p.m. toluic acid. Other experiments have shown that the dissolution of crude terephthalic acid in ammonia and springing with hydrochloric acid does not remove 4-carboxybenzaldehyde impurities at all.