DE1112508B - Process for maintaining the catalytic effectiveness of cobalt salts during the oxygen oxidation of aromatic hydrocarbons - Google Patents

Description

The invention relates to a method for maintaining the catalytic effectiveness of cobalt salts during the oxygen oxidation of aromatic hydrocarbons containing oxidizable side chains to mixtures of aromatic carboxylic acids, unreacted starting material and non-acidic oxidation intermediates.

It is known to use a wide variety of aromatic compounds, such as toluene, xylene, cumene, cymene, t-butyltoluene; d. H. general aromatic hydrocarbons, which has an aliphatic carbon atom bearing at least one hydrogen atom in the aromatic Contain core, with oxygen in the presence of an oxidation catalyst, usually a cobalt compound, to oxidize, with one or more of the aliphatic substituents in a carboxyl group be transferred. In these processes only partial oxidation takes place, i. H., a large and usually predominant part of the aromatic starting material undergoes oxidation unchanged. The unconverted starting material must work economically separated and fed back into the process. Furthermore, it is proportionate small part, which is oxidized per pass, in turn only a part into the desired carboxylic acid convicted. The other reaction products can be divided into two groups: first, intermediates, such as alcohols, aldehydes and esters, which when returned to the oxidation stage to the acids Can be oxidized, and secondly, by-products that are not converted into acids after recycling can be transferred and also poison the catalyst.

Furthermore, it was from British Patents 757,455 and 757,658 known to withdraw the raw Alkylbenzolmonocarbonsäuren contained in the obtained in the oxidation of alkylbenzenes oxidizing mixture by extraction with aqueous alkaline solutions having a _pH value of 7 and 8, and this from this in a known manner, for. B. by acidification with mineral acids, to be deposited.

The organic phase remaining after the extraction with weak alkali contains a larger proportion of the unreacted hydrocarbon, intermediate products which, upon further oxidation, are converted into the Acid can be converted, and by-products not oxidizable to the acid, of which at least some give rise to poisoning of the oxidation catalyst.

If one wants to maintain the oxygen oxidation of the alkylbenzenes

the catalytic effectiveness
of cobalt salts during the oxygen oxidation of aromatic hydrocarbons

Applicant:

The Dow Chemical Company,
Midland, me. (V. St. A.)

Representative: Dr.-Ing. H. Ruschke, Berlin-Friedenau,

and Dipl.-Ing. K. Grentzenberg,
Munich 27, Pienzenauer Str. 2, patent attorneys

Claimed priority:
V. St. v. America 6 May 1957 (No. 657 128)

William Hunt Taplin and Guy Hendrickson Harris,

Concord, Calif. (V. St. Α.),
have been named as inventors

by recycling the unreacted starting materials and oxidation intermediates into the starting mixture continuous, so it is necessary to remove the by-products that poison the catalyst to separate.

It has now been found that the effectiveness of the cobalt catalyst used in the oxidation with oxygen can be maintained by using that obtained after the weak alkali extraction Subject mixture to further extraction with strong alkali.

The invention therefore relates to a continuously operating process in which the oxidation mixture obtained from the partial oxidation of an aromatic hydrocarbon with molecular oxygen, preferably with air, in the presence of a cobalt salt is first obtained with an aqueous solvent of Pj ₁ 5 to 9, preferably 7 to 8, is brought into contact. The acid contained in the mixture is converted into its salt and dissolved in the aqueous phase. The aqueous phase is then separated from the organic phase and from this the. Acid in a known manner by acidification with a mineral

109 677/222

Ralsäure deposited and isolated by filtering or centrifuging.

The remaining organic phase contains the unreacted aromatic starting material, oxidation intermediates, by-products and catalyst poisons. By treating this organic phase or part thereof with an aqueous alkaline solution (p _H> 10) impurities and other catalyst poisons are dissolved. After the aqueous phase has been separated off, the organic phase is purified to such an extent that it can be returned to the oxidation vessel.

Since the formation of impurities and catalyst poisons is generally slower than that Formation of the carboxylic acid, it is preferable to use only a part, e.g. B. a third or half of the to subject the mixture flowing out of the reaction vessel to the second extraction, while the remainder is returned to the reaction vessel immediately after the first extraction.

The method according to the invention is shown schematically on the basis of a drawing (flow diagram).

It can be seen from the drawing that the aromatic hydrocarbon, the catalyst and oxygen are fed into a reaction vessel 1. From there, the flowing reaction mixture passes into the Extraktor2 in which it is brought at _pH 5 to 9 with a weak aqueous alkali into contact. The aqueous phase of the extract contains the salt of the alkylbenzene monocarboxylic acid, from which it can be separated out in a known manner by acidification and filtration.

The effluent from the Extraktorl organic phase is passed in whole or in part in the Extraktor3, where it is brought into contact with a strong aqueous alkali (p _H> 10), said by-products and catalyst poisons are removed. The organic phase is then returned to the oxidation vessel. The part of the organic phase obtained from the extractor 2 which may not have been sent through the extractor 3 is returned directly to the reaction vessel.

As the solvent in the extractor 2, an aqueous buffer solution having a _pH value is preferably applied 7 to 8 Alkali metal carbonates and bicarbonates dissolved in water are suitable for this purpose, sodium bicarbonate being preferred. It can also be used solutions of alkali metal hydroxides, wherein the amount to be added by the _pH value of the aqueous phase is prescribed in the extractor. Usual p _H -Meß- or -Anzeigeverfahren can be used to control the solvent addition in the extractor 2, so that the _pH value of the aqueous phase is kept 5 to 9 The use of alkali metal hydroxide solutions in such great dilution that the _pH value of the added solution amounts to approximately to of the aqueous phase, has drawbacks in practice because of the large to handle volumes of solvent.

The amount of solvent used in the extractor 2 is not critical. However, it is note that the amount is sufficient to cover all of the carboxylic acid contained in the oxidation mixture to neutralize or to dissolve. A large surplus is for economic reasons unfavorable because the excess part is lost. The best way to work is to use the Selects the alkalinity of the aqueous solvent so that the entire proportion of the carboxylic acid formed is extracted will.

The aqueous solvent used for the extractor 3 is intended to use a _pH value of at least 10 and preferably have at least 11, that is, it is necessary, a alkali metal hydroxide, preferably sodium hydroxide. Concentration and amount of aqueous alkaline solvent are as long as the phase to be treated is brought organic with an aqueous phase into intimate contact without significance, whose _pH value is maintained at at least 10 degrees.

The invention is illustrated by the following examples.

example 1

An oxidation mixture, such as that obtained by oxidizing 11.33 kg of commercially available o-xylene for 25 hours, which contained about 9% m-xylene, at 140 ° C with the help of 64.5 g of cobalt ethyl hexanate had been obtained as a catalyst, was used to extract the Toluic acid brought into contact with a sufficient amount of 8% aqueous sodium bicarbonate solution. the organic phase was separated from the bicarbonate solution and then exhausted in the same way extracted with 80% sodium hydroxide solution. The organic phase was then fresh with the lot Xylene and catalyst to restore the original amounts, supplements and the oxidation in Continued continuously, maintaining a temperature of 140 ° C and 19 moles of air were initiated per hour. One third of the reaction mixture was removed every 2.5 hours and with the same volume of 8% aqueous sodium bicarbonate solution and every third time the organic Phase extracted with the same volume of 8% aqueous sodium hydroxide solution. The organic phases were then along with enough fresh xylene and catalyst to supplement losses or the amount used is returned to the reaction vessel.

This procedure was discontinued after 109 hours. During the whole experiment the The effectiveness of the catalyst is monitored by determining the oxygen content in the escaping gas. This never rose above 4% and was only 2.5% at the end of the experiment, from which it can be seen that the catalyst had retained its effectiveness, d. H. by the procedure according to the invention none Catalyst poisons arise.

A total of 14.4 kg of xylene were consumed. After acidifying the bicarbonate extracts fell 15.2 kg of practically pure toluic acid, of which 95% consisted of the o-isomer.

Example 2

That which remained in the reaction vessel after completion of the experiment described in Example 1 The reaction mixture was heated to 140 ° C. and the experiment was continued in the same way, with the difference that after each bicarbonate extraction was extracted with sodium hydroxide. In the course the catalyst remained highly effective after 100 hours; the toluic acid was in the same yield and purity as in Example 1 obtained.

Example 3

In otherwise the same manner as in Example 2, the bicarbonate solution was replaced by an aqueous solution of sodium carbonate (p _H 7.5). The results were practically identical to those of Example 2.

Example 4

In an otherwise identical experiment as in Example 2, the bicarbonate solution fed to the first extractor was replaced by an approximately 8% strength sodium hydroxide solution. The addition of the sodium hydroxide was such that the _pH value always remained the aqueous phase at about 7.5. The results were practically identical to those of Example 2.

Example 5

Under otherwise identical conditions as described in Example 1, the extraction with sodium hydroxide solution was carried out omit in the second stage, d. H. those after bicarbonate extraction in the first Stage obtained organic phase returned immediately.

The oxidation proceeded normally during the first 38 hours, that is, less than 5% oxygen was contained in the escaping gas. With the increase in the concentration of the catalyst poisons, however, the effectiveness of the catalyst gradually fell, so that the oxygen content in the escaping gas rose to 10% over the next 20 hours. However, repeated addition of larger amounts of fresh catalyst could not restore the effectiveness, so that after a further 20 hours there was hardly any oxidation. Was replaced as the bicarbonate solution by a sodium carbonate solution (p _H 9.4), the efficiency of the method could not be increased again, although the use of such a solution in an experiment in which the extraction in the second stage with a sodium carbonate solution (p _H Ξ> 10) was made completely satisfactory.

Example 6

45

Under otherwise the same condition as described in Example 1, an excess of a 5 ° / o aqueous sodium hydroxide solution was used as the sole extracting agent so that the _pH value of the aqueous phase was greater than 10 degrees.

Even if the catalyst poisons were effectively separated from the reaction mixture and the activity of the catalyst remained unchanged in the process, the toluic acid obtained by acidifying the aqueous alkaline solution was strongly colors and less than 90% pure.

Example 7

From an oxidation mixture, as it had been obtained by air oxidation of 340 g of p-tert-butyltoIuene at 140 ° C in the presence of 2.23 g of cobalt ethylhexanate and 50 ecm of benzene - for better separation of the water with the aid of a water separator - were in At approximately 8-hour intervals, parts of the contents were removed for analysis and for alkaline extraction of the p-tert-butylbenzoic acid and by-products formed. If the product removed from the reaction vessel was extracted with saturated aqueous sodium carbonate solution and the aqueous extract was then acidified, pure, white p-tert-butylbenzoic acid was obtained in good yield. If, instead of a sodium carbonate solution, a saturated aqueous solution of sodium carbonate was used for the extraction, practically the same results were obtained, although the p-tert-butylbenzoic acid obtained was somewhat less pure. However, when, instead of a bicarbonate or carbonate using a 5 ° / o aqueous sodium hydroxide solution without adjusting the p _H -value, so obtaining a strongly discolored and contaminated p-tert-butylbenzoic acid.

So only a single extraction was carried out and aqueous sodium bicarbonate was used as the extractant or carbonate was used, the end product obtained was very pure, but accumulated gradually catalyst poisons, while with strong alkali as extraction agent the catalyst poisons were separated, but the end product was quite impure. By using the two-stage extraction, as described in Example 1, in the case of sodium carbonate or bicarbonate in the first and Sodium hydroxide used in the second stage will consistently be an end product of high purity generated and at the same time an accumulation of catalyst poisons avoided.

Similar (as described above) results are obtained when using the method according to the invention in the catalytic air oxidation of aromatic hydrocarbons, such as m- and p-xylene, tri- and tetramethylbenzenes, cumene, cymene, tert-butyltoluene, tert-amyltoluene, d. H. Connections of general formula

CH ₃

-L [(C "H _2K ) -

in which η is an integer from 1 to 5 and χ is an integer from 0 to 4 and not greater than 1 when n = 2 or greater than 2, applies.

In order to avoid complex carboxylic acid mixtures being formed as end products, only those hydrocarbons which have no more than one aliphatic side chain with more than one carbon atom are preferably used, i.e. if η is greater than 1, χ should not be greater than 1 .

Claims (3)

PATENT CLAIMS: 1. A process for maintaining the catalytic effectiveness of cobalt salts during the oxygen oxidation of aromatic, oxidizable alkyl side chains containing hydrocarbons as the only substituents on the aromatic nucleus to form mixtures of aromatic carboxylic acids, unreacted starting material and non-acidic oxidation intermediates, characterized in that the one obtained in the oxidation mixture was extracted first with the aqueous solution of a weak alkali while maintaining a p _H -value 5-9, separating the aqueous phase and then the organic phase with the aqueous solution of a strong alkali under Aufrechterhai- tung a p _H -value of at least extracted 10th 2. The method according to claim 1, characterized in that that the weak alkali is sodium carbonate and the strong alkali is sodium hydroxide used. 3. The method according to claim 1 or 2, characterized in that one in continuous Way works. References considered: British Patent Nos. 757 455, 757 658. 1 sheet of drawings