Liquid purification or separation – Processes – Ion exchange or selective sorption
Reexamination Certificate
2001-11-15
2003-05-20
Barry, Chester T. (Department: 1724)
Liquid purification or separation
Processes
Ion exchange or selective sorption
C210S688000, C210S912000
Reexamination Certificate
active
06565754
ABSTRACT:
The present invention relates to a novel process for the production and subsequent purification of monocarboxylic and polycarboxylic aromatic acids by the catalytic oxidisable substituent group attached directly to the carbon atom of the corresponding aromatic nucleus.
FIELD OF THE INVENTION
Processes for the production of monocarboxylic and polycarboxylic aromatic acids are well known in the literature. They are normally carried out in liquid phase, operating either continuously or discontinuously and using as substrates aromatic compounds carrying at least one oxidisable substituent group attached directly to the carbon atom of the corresponding aromatic nucleus, where the expression “oxidisable substituent group” is intended to indicate any substituent in which a carbon atom is bonded directly to the aromatic nucleus and which, as a result of oxidation, is converted into a carboxyl group.
The oxidising agent is generally gaseous molecular oxygen, preferably diluted with an inert gas; for obvious reasons of practicality, air (optionally enriched with molecular oxygen) is the gaseous mixture most commonly used for this purpose. The oxidation reaction is normally carried out using as solvent an aqueous organic acid, preferably acetic acid, normally with a water content of from 2 to 15%.
Those reactions are carried out in the presence of a catalytic complex generally composed of one or more metals, normally in the form of salts soluble in the reaction solvent, and a suitable activator. The metal performs the function of catalysing the actual oxidation reaction while the activator is used to return the metal (which undergoes a reduction in its oxidation number during catalysis) to its original valency, thus enabling it to regain and exert its catalytic activity. The activator may itself be a metal, in which case it also will preferably be present in the form of a salt soluble in the reaction medium; alternatively, it is possible to use organic compounds having carbonyl functions, such as ketones or aliphatic aldehydes, preferably acetaldehyde, or molecular bromine.
British patent GB-1063964 describes a process for the production of monocarboxylic and polycarboxylic aromatic acids at temperatures of preferably from 80 to 130° C. and pressures of from 1 to 60 kg/cm
2
by means of a catalytic complex substantially based on zirconium and cobalt.
U.S. Pat. No. 5,112,992 describes the production of aromatic acids at temperatures of from 100 to 275° C., using metals from groups IIIA and IVA of the periodic table of elements (groups IIIB and IVB according to the new notation adopted, for example, by Perry,
Chemical Engineers' Handbook,
VI edition, 1984), in particular zirconium and hafnium, in order to increase the kinetics of oxidation reactions that use catalytic complexes based on cobalt and manganese in the presence of bromine as the activator.
International patent application WO 98/29378 describes a catalytic complex comprising:
1. at least one metal having a valency higher than 2 which belongs to group VIIIA of the periodic table of elements, preferably ruthenium, iridium, palladium, platinum; and/or at least one metal from group VIIA, preferably rhenium; and/or cerium; and
2. at least one metal from group IVA of the periodic table of elements, preferably zirconium and/or hafnium;
where the catalytic pair cerium-zirconium constitutes the preferred complex for implementing the invention.
N.B. groups VIIIA, VIIA and IVA indicated above correspond, respectively, to groups VIII, VIIB and IVB according to the new notation adopted, for example, by Perry,
Chemical Engineers' Handbook,
VI edition.
The catalytic complex described in WO 98/29378 is used for the production of monocarboxylic and polycarboxylic aromatic acids, preferably from meta- and para-xylene, operating at temperatures of from 90 to 150° C., preferably from 105 to 115° C., and at pressures of from 1 to 20 barg, preferably from 2 to 5 barg.
Monocarboxylic and polycarboxylic aromatic acids have a very low solubility in the solvents conventionally used in oxidation reactions of the type described above; therefore, they can be readily isolated from the reaction medium by simple filtration.
Once separated by filtration, the crude acid so obtained normally has to be purified of any secondary products of the oxidation reaction, generally aromatic aldehydes, formed by partial oxidation of the aromatic starting derivatives and, as such, difficult to separate from the desired end products; the crude terephthalic acid, for example, generally obtained by oxidation of para-methylacetophenone or para-xylene, normally contains a not inconsiderable amount of para-carboxybenzaldehyde which, because it is relatively similar to terephthalic acid from a physico-chemical point of view, cannot be completely separated from the latter by simple crystallisation.
The purification of the monocarboxylic and polycarboxylic aromatic acids is thus normally carried out by subjecting the crude product in aqueous solution (generally 20%) to a hydrogenation reaction with gaseous hydrogen in the presence of suitable catalysts, in order to convert the impurities into derivatives that are more easily separable from the desired end product, normally by crystallisation; this reaction is normally carried out at rather high temperatures (270° C. as regards the purification of terephthalic acid, approximately 220° C. for the purification of isophthalic acid).
Hydrogenation catalysts, which have to be capable of selectively hydrogenating the impurities while leaving the aromatic acid unaltered, are well known in the art; generally, they are metals belonging to group VIII (Perry,
Chemical Engineers' Handbook,
VI edition), preferably platinum, rhodium or palladium, normally supported on inert materials, such as on carbon or alumina; catalysts of this type are described in U.S. Pat. Nos. 3,522,298; 3,542,863; 3,584,039; 3,591,629; 3,607,921; 3,726,915; 3,799,976; 4,260,817.
U.S. Pat. No. 4,126,638 describes in particular a process for the purification of dicarboxylic aromatic acids by heterogeneous catalysts based on platinum, palladium, ruthenium, rhodium, iridium and/or osmium, operating in aqueous solution containing from 1 to 7% by weight of alcohol at a temperature of from 110 to 350° C.
U.S. Pat. No. 4,629,715 describes a process for the purification of crude terephthalic acid in aqueous solution by means of a heterogeneous catalyst based on palladium and rhodium, operating at temperatures of from 100 to 350° C. and at pressures of from 63 to 84 barge.
DESCRIPTION OF THE INVENTION
When only organic activators are used in the oxidation reaction, the crude aromatic acids so produced do not contain residual products which could damage the hydrogenation catalyst. However, the use of hafnium salts and in particular zirconium salts is at the root of a highly undesirable phenomenon which substantially limits the efficiency of the entire production process.
As described in the already mentioned U.S. Pat. No. 5,112,592 and WO 98/29378, the crude aromatic acids in fact contain a small portion of the above-mentioned zirconium and hafnium salts, generally acetates, as further impurities; those salts are unfortunately readily hydrolysable in aqueous solution, above all if subjected to elevated temperatures, of the order of from 200 to 300° C. Therefore, when the aqueous solution of the crude acid is introduced into the purification reactor, the hafnium and/or zirconium salts hydrolyse and precipitate on the hydrogenation catalyst in the form of insoluble colloidal polyoxides with the consequent poisoning of the catalyst (the service life of which is normally reduced by from 60 to 80%).
That disadvantage is normally limited by maintaining the concentration of the salts in the oxidation mixture at a value lower than 250 ppm, with obvious disadvantages from the point of view of production.
The object of the present invention is therefore to provide a process for the production of monocarboxylic and/or polycarboxylic aromatic acids which does not have the d
Codignola Franco
Moro Antonio
Barry Chester T.
Nixon & Vanderhye
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