Process to separate rhodium from an organic mixture

Details Process to separate rhodium from an organic mixture Process to separate rhodium from an organic mixture

2000-06-01

2001-05-15

Bos, Steven (Department: 1754)

Chemistry of inorganic compounds

Treating mixture to obtain metal containing compound

Platinum group metal

Reexamination Certificate

active

06231821

ABSTRACT:

The invention relates to a process to separate rhodium from an organic starting mixture comprising an organic solvent, C
6
-dicarboxylic acids and iodide compounds.
Mixtures comprising rhodium, C
6
-dicarboxylic acids and iodide compounds are for example obtained as purge streams in the preparation of pentenoic acid as described in EP-A-405433. This patent publication describes the carbonylation of butadiene with water in the presence of carbon monoxide and a catalyst system comprising of rhodium and an iodide promotor compound.
It has been found to be a problem to remove the valuable rhodium from the purge stream. No methods which can be used on a commercial scale have been described for this particular problem. A practical method for removing the rhodium from the purge stream is highly desirable when one wants to operate a commercial plant based on the process according to EP-A-405433. The rhodium obtained from the purge stream is advantageously reused in the carbonylation reactor.
The object of this invention is to obtain a practicable method for removing rhodium from a starting mixture as described above.
This object is achieved in that rhodium is separated by extracting the organic starting mixture with an aqueous solvent containing C
6
-dicarboxylic acids and an iodide compound resulting in an organic raffinate poor in rhodium and an aqueous extract rich in rhodium.
It has been found that rhodium can be successfully removed from the organic mixture indicated above by extraction. The extraction can be simply operated in a continuous mode on a large scale. When performing a process according to EP-A-405433 it has been found that the resulting aqueous extract containing the rhodium can advantageously be directly returned to the carbonylation reactor.
Without wanting to limit the invention by the following theory it has been found that the extraction efficiency of rhodium from the organic phase to the aqueous phase is highly influenced by the content of the C
6
-dicarboxylic acids in the two immiscible phases. If the content of C
6
-dicarboxylic acids in the aqueous phase is near zero the extraction coefficient of rhodium (expressed in K
Rh
=Rh(ppm)
water
/Rh(ppm)
organic
) over the two phases is also low. In contrast a high and preferred K
Rh
is found to exist when the content of C
6
-dicarboxylic acids in the aqueous phase is above a certain level. When performing the extraction it is therefore important to maintain a certain level of dicarboxylic acids in the aqueous phase at all times in order to keep rhodium in the aqueous phase. The content of C
6
-dicarboxylic acids in the aqueous phase can be increased provided that a phase separation will occur. Preferably the C
6
-dicarboxylic acids in the aqueous phase are in equilibrium with the C
6
-dicarboxylic acids in the organic phase. This results in that during the extraction (almost) no net exchange of dicarboxylic acids between the two phases will take place.
The ratio of C
6
-dicarboxylic acids concentration in the aqueous solvent to the C
6
-dicarboxylic acids concentration in the organic starting mixture is preferably between 1:1 and 5:1. Preferably this ratio is lower than 3:1. Most preferably this ratio is equal to the two-phase equilibrium ratio of the dicarboxylic acids. The equilibrium ratio will depend for example on the composition of the organic starting mixture, temperature and pressure. The C
6
-dicarboxylic acid in the organic starting mixture is preferably between 10-60 wt. % and in the aqueous solvent between 30-70 wt. %. The C
6
-dicarboxylic acids are adipic acid, 2-methyl glutaric acid, ethyl succinic acid, dimethyl succinic acid or mixtures thereof.
The organic solvent should be substantially immiscible with water, C
6
-dicarboxylic acids should dissolve in the solvent and it should be inert during a carbonylation reaction. Preferably the organic solvent is a C
5
-C
13
mono-carboxylic acid or mixtures thereof. More preferably C
8
-C
13
mono-carboxylic acids or mixtures of these acids are used. C
9
-carboxylic acids will generally be present when the organic starting mixture is derived from a carbonylation process to prepare pentenoic acid, because C
9
-carboxylic acids are by-products of this reaction. The C
9
-carboxylic acids are for example nonanoic acid, nonenoic acid, and branched or cyclic C
9
-carboxylic acids, or mixtures thereof.
Preferably the aqueous solvent also contains the corresponding C
5
-C
13
mono-carboxylic acids. The content of these mono-carboxylic acids in the aqueous phase can be increased provided that a phase separation will occur. The ratio of mono-carboxylic acids concentration in the aqueous solvent to the mono-carboxylic acids concentration in the organic starting mixture is preferably between 1:5 and 1:20. Most preferably this ratio is equal to the two-phase equilibrium ratio of the mon-carboxylic acids. The equilibrium ratio will depend for example on the composition of the organic starting mixture, temperature and pressure. The mono-carboxylic acid concentration in the organic starting mixture is preferably between 40-90 wt. % and in the aqueous solvent between 5-20 wt. %.
The C
6
-dicarboxylic acids and the C
5
-C
13
mono-carboxylic acids (if present) may be added prior to the extraction to pure water to obtain the aqueous solvent to be used in the extraction. The desired composition of the aqueous solvent can also be obtained by performing the exctraction with only water (d): By adding a number of exchange stages in a counter current extraction column (A′) relative to the situation in which a aqueous/C
6
-dicarboxylic acid mixture is directly used (A), it has been found that the desired aqueous composition according to the invention will be obtained at an intermediate position (c) in the extraction column. If the rhodium content is sufficiently low at that position (b) in the column the content of rhodium in the organic raffinate (e) will also be low. The letters A, A′ and (a)-(f) refer to FIG.
1
. In this Figure stream (f) is the aqueous extract.
If rhodium is to be removed from a purge stream of a carbonylation process as for example described in EP-A-405433 it can be necessary to adjust the composition of the organic mixture before performing the extraction according to the invention. The adjustment of the composition is needed in order to achieve the preferred concentration and ratio's of the mono- and/or di-acids concentration over the phases as described above. A purge stream will generally contain between 10-10000 ppm rhodium, between 10-90 wt % C
6
-dicarboxylic acids, between 10-90 wt. % C
5
-C
13
mono-carboxylic acids and between 10 and 10000 ppm iodide promotor compounds. The purge stream may also contain other high boiling by-products, for example tars, of the carbonylation reaction. In a continuous process also some water may be present in the organic starting mixture because some water will be present in the C
5
-C
13
mono-carboxylic acids stream used to mix with the purge stream before extraction.
The aqueous extract will contain some C
6
-dicarboxylic acids and mono-carboxylic acids (if present), due to their solubility in the water phase. The organic rafinate will, apart from the C
6
-dicarboxylic acids and mono-carboxylic acids, also contain water due to its solubility in the organic phase. This water is preferably removed from the organic raffinate together with part of the C
6
-dicarboxylic acids and mono-carboxylic acids. This separation is preferably performed by distillation. Preferably three product streams are obtained in the distillation, namely a water stream, a mono-carboxylic acid stream and a C
6
-dicarboxylic acid stream. The purity of these product streams is not critical, therefore making a distillation in one column possible. The water stream and part of the C
6
and optionally the mono-carboxylic-products are preferably reused in the extraction. The remaining C
6
-dicarboxylic acids and mono-carboxylic acids can be disposed of.
The amount of C
6
-dicarboxylic acids and mono-carboxylic acids in the aqu

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