Process for recovering water-soluble rhodium-containing hydroformylation catalysts.

The present invention relates to the recovery of hydroformylation catalysts based on complex rhodium compounds from their solutions in water.

The use of water as the reaction medium in the hydroformylation of olefins with synthesis gas offers the advantage of being able to remove the water-insoluble reaction products from the reaction mixture by simple phase separation. In this way, the thermal stress on the aqueous catalyst solution is reduced, since stripping of the aldehydes formed is not necessary. Therefore, higher olefins can also be successfully converted in this way, the high-boiling aldehydic reaction products of which would lead to thermal decomposition of the active hydroformylation catalyst when stripped.

A hydroformylation process that works with water as the reaction medium is described in DE-PS 26 27 354. Rhodium in metallic form or as a compound together with water-soluble phosphines is used as the catalyst system. The water solubility of the phosphines is due to the presence of sulfonic acid groups in the molecule. The phosphines are preferably used in the form of the alkali metal, ammonium or alkaline earth metal sulfonates.

If the process is carried out continuously, the catalyst solution is subject to a number of influences which ultimately result in a reduction in the activity of the catalyst system due to poisoning. In this context, the catalyst poisons include, for example, iron carbonyl, which is formed by the action of synthesis gas on the synthesis gas transport lines or the construction material of the reactor, and higher-boiling condensation products which arise from the aldehydes. Furthermore, the sulfonated phosphines are oxidized to the corresponding sulfonated phosphine oxides or broken down to aromatic sulfonic acids. To a certain extent, phosphine sulfides are also formed from the sulfur-containing compounds contained in the synthesis gas and through the reduction of sulfonate groups.

Neither phosphine oxides or phosphine sulfides nor aromatic sulfonic acids are suitable as part of the hydroformylation catalyst.

It is therefore necessary to replace the deactivated aqueous catalyst solution with fresh solution from time to time. The used catalyst solution contains the still active sulfonated phosphine in the form of the alkali salts and rhodium, which have to be recovered to ensure the economy of the process.

A process for the separation of aromatic sulfonic acids from sulfuric acid and sulfates, which are present together in aqueous solution, is described in US Pat. No. 3,919,703. It consists in treating the aqueous solution with a water-insoluble aliphatic amine in which the sulfonic acids dissolve, separating the water and amine phases and then extracting the sulfonic acids from the amine. The additional recovery of a metal, namely rhodium, which is present in a very low concentration, and its separation from a second metal, iron, which also belongs to the same group of the periodic table, is not described in this publication.

A process for the separation of water-soluble salts of aromatic sulfonic acids is the subject of EP-A 00 41 134. The sulfonation mixture diluted with water is treated with an amount of water-insoluble amine equivalent to the sulfonic acid, which forms a lipophilic salt with the sulfonic acid. Then the resulting two phases are separated and the phase containing the ammonium salt is treated with the stoichiometric amount of a water-soluble base, the sulfonic acid salt of which is to be prepared. The sulfonic acid salt is obtained in aqueous solution from which it can be isolated. The process assumes that the ammonium salt is liquid below the boiling point of the aqueous sulfuric acid.

DE-PS 29 11 193 describes a process for the recovery of rhodium from residues of the oxo synthesis. Here, the rhodium is precipitated by adding elemental sulfur or sulfur-releasing compounds, and the precipitate is worked up in a manner known per se by means of a pyrosulfate melt over rhodium sulfate to give rhodium or rhodium compounds. This process is particularly suitable for working up residues that contain free triphenylphosphine in addition to rhodium.

It proves to be a disadvantage that the rhodium is not obtained in the form of a product which can be reused directly as a hydroformylation catalyst, but must first be reworked accordingly. It is not possible to recover the phosphine ligand using this method.

The task was therefore to develop a process which allows the recovery of the water-soluble rhodium and phosphine-containing catalyst system.

The above object is achieved by a process for the recovery of water-soluble rhodium and phosphines and, in addition, catalyst systems containing alkali metal and / or alkaline earth metal and / or ammonium ions, first adding at least an equivalent amount of acid to the aqueous solution of the catalyst system, then extracted with an amine dissolved in an organic solvent, brings the separated organic phase into intimate contact with the aqueous solution of an inorganic base and finally separates the aqueous phase.

The process according to the invention not only ensures that rhodium and also the phosphines are recovered almost quantitatively, but also that impurities such as iron and other metal compounds, halides, phosphine oxides, sulfides, aromatic sulfonic acids are removed.

In the process according to the invention, the used catalysts containing impurities and inactive substances can be used in the cleaning process without prior work-up. The process can be described as follows:

<ul id="ul0001" list-style="none"><li>The aqueous catalyst solution to be cleaned is first acidified. For this purpose, they are mixed with at least as many acid equivalents as are necessary to convert the acid residues present as salts into the acid form. An excess of acid is not harmful, but is unnecessary. The amount of acid required should be determined analytically beforehand. Suitable for converting the salts into the acid are all acids which, because of their acid strength, convert the sulfonic acid salts into the free sulfonic acids to such an extent that they can be extracted under the chosen reaction conditions, and which are less well extracted under the chosen reaction conditions due to their hydrophilic character are called the sulfonic acids.</li></ul>

Both inorganic and organic acids can be considered. Inorganic acids such as hydrochloric acid, sulfuric acid, nitric acid, phosphoric acid and organic acids such as acetic acid, formic acid and oxalic acid are suitable. Sulfuric acid and acetic acid are used with particular success.

When using polyvalent acids, the amount of acid required to convert the catalyst solution is expediently only related to the conversion of the first acid stage.

<ul id="ul0002" list-style="none"><li>According to a particular embodiment of the procedure according to the invention, the required amount of acid in the form of an amine salt is added to the catalyst solution. The salts of those amines which are also used as extractants are suitable. In this case, the amine salts of the sulfonic acids are formed immediately.</li><li>After acidification of the catalyst solution, rhodium and the sulfonated phosphines are extracted with the aid of an amine in the second step. The amines used as extractants should be liquid and as little soluble in water as possible.</li></ul>

The salt formed from the amine and the sulfonic acid generally has a low solubility in water. If the resulting salt is more soluble in water than in the extractant, the amine is not very suitable for extraction.

Possible largely water-insoluble amines which form a sufficiently lipophilic salt with the sulfonic acid are largely water-insoluble homo- and heterocyclic aliphatic, aromatic, araliphatic and preferably open-chain branched or unbranched aliphatic amines with a total of 10 to 60, preferably 13 to 36 carbon atoms , e.g. Isotridecylamine (mixture of isomers), di-2-ethylhexylamine, tri-n-hexylamine, tri-n-octylamine, tri-isoctylamine, tri-iso-nonylamine (mixture of isomers), di-isononyl-2-phenylpropylamine, isononyl- di-2-phenylpropylamine, tri-iso-tridecylamine (mixture of isomers), NN-dimethyl-hexadecylamine, NN-dimethyl-octadecylamine. In particular, the extractant used isotridecylamine, tri-n-octylamine or tri-iso-octylamine.

As such, the amines can be used as extractants. However, they are advantageously dissolved in an organic solvent which is immiscible or only slightly miscible with water. It is used in an amount such that the amine is in a 0.5 to 50 vol Vigen solution. The concentration is essentially limited by the solubility of the amine sulfonic acid salts in the solvent to be taken into account during the extraction and by the viscosity of the salt solution obtained. A suitable diluent is therefore chosen based on physical rather than chemical aspects. If possible, the diluent should meet the following requirements: it should have low water solubility, its flash point should be above 25 ° C, the evaporation losses should be low, it should also have no tendency to form emulsions and should only be carried along a little into the aqueous phase. In addition, it should be inert, non-toxic and inexpensive, show good hydrodynamic behavior and have only a low extraction capacity for the impurities contained in the catalyst solution. Suitable solvents are kerosene-like fractions, aromatic fractions, C₄-C₂₀-Alcohols, C₈-C₂₀Ether, especially toluene or kerosene-like fractions. With their help it is possible not only to separate inorganic substances such as iron compounds and halides, but also the catalytically inactive substances such as phosphine oxides, phosphine sulfides and aromatic sulfonic acids.

The amine is used in an amount of 0.1 to 1.0 mol per equivalent of sulfonic acid and in particular from 0.25 to 0.75 mol per equivalent of sulfonic acid.

According to a particularly proven embodiment of the procedure according to the invention, the extraction of rhodium and the phosphines is carried out not in one but in several steps, by adding not all of the amine at once, but in portions of the acidified catalyst solution. In the first stage, 0.25 to 0.5 mol of amine per equivalent of sulfonic acid is then added to the solution, and in the second stage a further 0.25 to 0.5 mol, in a further third stage, if necessary, that remaining amine are added to the acidified catalyst solution. In this variant of the method according to the invention, more than 90% of the rhodium present in the solution is removed in the first stage. In this context it is also possible to have different levels. Use amines to selectively extract the components of the solution.

During the extraction, all of the rhodium and the water-soluble phosphine are converted into the amine phase. The acidified aqueous phase only contains the water-soluble inorganic and organic impurities and can be discarded.

To recover the rhodium and phosphines now contained in the organic amine phase, the amine phase is brought into intimate contact with the aqueous solution of an inorganic base in a third step. Sodium hydroxide is used as the inorganic base, Potassium hydroxide, sodium carbonate, potassium carbonate and alkaline earth metal hydroxides, especially sodium hydroxide and potassium hydroxide. The base is used as a 0.1 to 10% by weight solution and, based on the amine present, is used in a stoichiometric amount or in a slight excess. In this treatment, rhodium and the phosphines migrate back into the aqueous phase, and an aqueous solution is formed which can be used again as a catalyst solution immediately or after appropriate dilution or supplementation.

The extraction with an amine solution and the re-extraction with an aqueous alkali can be carried out in a known manner, for example in a continuously operating countercurrent extraction system.

In practice, the process according to the invention is carried out by acidifying the used, aqueous catalyst solution with the required amount of acid. Then the amine, optionally dissolved in a solvent, is added. Intensive mixing of the two phases, which are practically insoluble in one another, ensures that both the rhodium and the phosphines largely pass into the amine phase. The two phases are allowed to settle and the upper organic phase is separated from the lower aqueous phase. The organic phase is then treated with the aqueous solution of an inorganic base. It is also important here that the immiscible phases come into intimate contact so that rhodium and the phosphines largely pass from the organic phase into the aqueous phase. The aqueous phase is then separated off and can be used directly as a catalyst solution in the synthesis.

Example I

100 g of a used catalyst solution per kg of solution<ul id="ul0003" list-style="none"><li>0.155 g rhodium</li><li>0.160 g iron</li><li>9.0 g Na triphenylphosphine trisulfonate (TPPTS)</li><li>22.0 g Na triphenylphosphine oxide trisulfonate (TPPOTS)</li><li>2.0 g Na triphenylphosphine sulfide trisulfonate (TPPSTS)</li><li>4.0 g Na triphenylphosphine oxide disulfonate (TPPODS)</li></ul>contains, are treated in a shaking funnel for 30 minutes with 17.04 g of extractant in 82.96 g of toluene. The extractant is obtained by dissolving<ul id="ul0004" list-style="none"><li>100 g of triisooctylamine and 30 g of sulfuric acid in 900 g of toluene</li></ul>receive. The amount of amine used as the extractant corresponds to 0.25 mol of amine per val. Sulfonate. After the toluene phase has been separated off, the extraction is repeated a further five times with the same amount of extractant and toluene.

In a three-necked flask equipped with a bottom drain valve, 0.1 N aqueous NaOH solution is added to the organic phases separately, with intensive stirring, until a pH of 8 is reached. The aqueous phases containing the entire catalyst system are then separated from the organic phases and, if necessary after dilution with water, can be used again in the synthesis.

Example II

100 g of the used catalyst solution of Example I are acidified with 39.3 g of 10 V sulfuric acid and then extracted six times with 16.54 g of extractant (obtained from 100 g of triisooctylamine and 900 g of toluene) dissolved in 60 g of toluene. The further treatment of the separated organic phases is carried out as in Example I.

Example III

100 49.2 g of 40 Viger sulfuric acid are added to g of the used catalyst solution of Example I. The acidified solution is treated as in Example II.

Example IV

100 20.2 g of 10% sulfuric acid are added to g of the used catalyst solution of Example I. The acidified solution is extracted six times with 16.54 g of extractant (obtained from 100 g of tri-n-octylamine and 900 g of toluene) dissolved in 60 g of toluene. The organic phases are further processed as in Example I.

Example V

100 g of the used catalyst solution of Example I is acidified with 20.2 g of 10 Viger sulfuric acid and then dissolved six times with 60.5 each of 16.54 g of extractant (obtained from 38.1 g of tri-n-hexylamine and 461.9 g of toluene) g of toluene extracted. The amount of amine used as the extractant corresponds to 0.25 mol of amine per Val sulfonate.

Otherwise the procedure is as in Example I.

Example VI

100 g of the used catalyst solution of Example I are acidified with 20.2 g of 10 Viger sulfuric acid and then dissolved six times with 60.5 each of 16.54 g of extractant (obtained from 52.4 g of tri-n-butylamine and 947.6 g of toluene) g of toluene extracted. The amount of amine used as the extractant corresponds to 0.25 mol of amine per val of sulfonate. Otherwise, the procedure is as in Example I.

Surprisingly, no rhodium is extracted from the aqueous solution of the catalyst system with the extractant used.

Example VII

100 g of the used catalyst solution of Example I acidified with 20.2 g of 10 l sulfuric acid and then six times with 16.54 g of extracting agent (obtained from 34.14 g of di-2-ethylhexylamine and 465.86 g of toluene) dissolved in 60 g each Toluene extracted. The amount of amine used as the extractant corresponds to 0.25 mol per val of sulfonate. Otherwise the procedure is as in Example I.

The results of Examples I to VII are summarized in Table 1. They show that the ratio of acid added to sulfonate groups present has little effect on the extraction effect (I to III), but that there are differences between the amines (IV, V, VII compared to VI). Rhodium is extracted quantitatively and with high selectivity from used aqueous catalyst solutions by sufficiently hydrophobic amines, ie amines which are more hydrophobic than tri-n-butylamine.<tables id="tabl0001" num="0001"><img file="EP0103845A2_D0001.tif" /></tables>

Example VIII

100 20.2 g of 10 Viger sulfuric acid are added to g of the used catalyst solution of Example 1.

A solution of 61.88 g of isotridecylamine in 438.12 g of toluene is used as the extractant.

In 3 successive extraction steps, the catalyst solution acidified with sulfuric acid is initially treated with 7.54 g of extractant and 70 g of toluene (corresponding to 0.25 mol of amine / val sulfonate) = 1st re-extract

7.54 g extractant and 70 g toluene (corresponding to 0.25 mol amine / val sulfonate) = 2nd re-extract and finally with

15.08 g extractant and 70 g toluene (corresponding to 0.50 mol amine / val sulfonate) = 3rd re-extract extracted.

The catalyst system consisting of rhodium and phosphine is re-extracted from the toluene solutions by adding enough 1N sodium hydroxide solution to adjust the pH to 12.0. The results are shown in Table 2.

As can be seen, rhodium can largely be separated from TPPOTS by partial amine extraction. At the same time, TTPPTS is almost completely recovered.<tables id="tabl0002" num="0002"><img file="EP0103845A2_D0002.tif" /></tables>

Examples IX-XXVI

A used catalyst solution contains 345 mg Rh, 270 mg Fe, 19.2 g Na per kg₃-Triphenylphosphine trisulfonate, 38.0 g Na₃Triphenylphosphine oxide trisulfonate; 0.7 g Na₃-Triphenyl-phosphine sulfide trisulfonate, 2.0 g Na₂-Triphenylphosphine disulfonate, and 7.7 g triphenylphosphine oxide disulfonate.

Three 100 g samples of catalyst solution are each concentrated with 6 g. Sulfuric acid added. One sample is extracted with 13.83 g of triisooctylamine, the other with 9.44 g of di-2-ethylhexylamine and the third with 7.80 g of isotridecylamine and 100 ml of the solvent listed in Table 3, corresponding to 1.0 mol of amine 1 val of sulfonate each.

The catalyst system is re-extracted from the organic phases by adding 1N sodium hydroxide solution until the pH is 8.0 or 12.0 (isotridecylamine).

All experiments are carried out in a nitrogen atmosphere, the solvents used were previously under N₂- Protection has been heated to the boiling point.

When using a mixture of aliphatic hydrocarbons (boiling range 140 - 170 ° C) or cyclohexane. or diethyl ether as solvent, two separate organic phases are obtained, the lighter, thinner phase is largely free of rhodium, the heavier, more viscous phase contains the majority of the rhodium. Both phases are used together for re-extraction.

The results of the tests are summarized in Table 3. They show that a wide variety of inert solvents can be used as diluents for the amine. The information applies to 100 g re-extract and 100 g waste water.<tables id="tabl0003" num="0003"><img file="EP0103845A2_D0003.tif" /></tables><tables id="tabl0004" num="0004"><img file="EP0103845A2_D0004.tif" /></tables>

Examples XXVII - XXVIII

In each case 100 g of the used catalyst solution of Examples IX - XXVI are concentrated with 6 g. Acidified sulfuric acid or with 3 g of acetic acid and then extracted with 100 g of isotridecylamine / toluene solution. The re-extraction of the rhodium catalyst is carried out with sodium hydroxide solution - addition of sodium hydroxide solution until the pH is adjusted to 12.

The two experiments, the results of which are summarized in Table 4, demonstrate that both inorganic and organic acids can be used to acidify the used catalyst solution. At the same time it can be seen that partial extraction of TPPOTS is much better.

<tables id="tabl0005" num="0005"><img file="EP0103845A2_D0005.tif" /></tables>

Example XXIX

100 g of the catalyst solution of experiments IX-XXVIII are acidified with 3 g of acetic acid. The catalyst solution is then extracted with 100 g of triisooctylamine / toluene solution. The re-extraction of the rhodium catalyst is carried out in a known manner with sodium hydroxide solution.

The results of the experiment are shown in Table 5. They illustrate the high selectivity of the separation of rhodium and TPPTS from TPPOTS.<tables id="tabl0006" num="0006"><img file="EP0103845A2_D0006.tif" /></tables>