Note: Descriptions are shown in the official language in which they were submitted.
5 0 ~
_roces~ for the recovery of rhodium from distillation
residues of products of the oxo synthesis
The pr~sent invention relates to an improved process for
the recovery of rhodium from the distillation residues o~
products of the oxo synthesis.
The preparation of aldehydes and alcohols by catalytic
addition of carbon monoxide and hydrogen onto olefinic
double bonds (hydroformylation) is known. Modern pro-
ce~ses operate with metallic rhodium or with rhodium
compounds as catalysts, which are employed by themselves
or in combination with complexing ligands, for example
organic phosphines or esters of phosphorous acid. Accord-
ing to the unanimous opinion of experts, hydridocarbonyl
compounds of rhodium which can be represented by the
lS general formula ~[Rh(CO)4~Lx], in which L is a ligand and
x is 0 or an integer from 1 to 3, are active as the
catalyst under the reaction condi~ions.
Compared with the conventional oxo synthesis using cobalt
catalysts, the use of rhodium catalysts has a number of
advantageq. The activity of rhodium catalysts i9 higher
than that o~ cobalt catalysts, and terminal olefins are
converted into unbranched aldehycles in the presence of
rhodium (in the form of rhodium complex compounds) to a
greater extent than in the presence of cobalt. Moreover,
production plants can be operated largely without
problems when rhodium catalysts are used, which applies
in particular to the synthesis procedure and the dis-
charge of the products.
A determining factor for the profitability of the rhodium
process is~the removal and recovery of the noble metal as
far as possible without losses, regardless of whether it
has been employed as the catalyst with or without an
additional complexing agent~ After the end of the
reaction, the rhodium is present as a carbonyl compound,
which may also contain other ligands, dissolved in the
hydroformylation product.
.
- ~ .
. ` ~ .
2 ~
- 2 --
For working up, the crude product from the synthe~is is
first let down to normal pressure in one or more stages,
dissolved synthesis gases being relea ed. The rhodium i8
removed either directly from the crude product which has
been let clown or from the residue of the crude product
distillation. The first route is taken if the rhodium has
been employed as the catalyst in the preceding hydro-
formylation stage without an additional complexing agent.
The second variant is used if the rhodium catalyst a1BO
contains other ligands in addition to carbon monoxide,
for example phosphines or phosphites, bonded as com-
plexes. It can also be used if the hydroformylation had
indeed been carried out with rhodium by itself, but a
complexing agent had been added to the crude product
after letting down in order to stabilize the rhodium, or
it is ensured otherwise, for example by distillation
under prescure, that the rhodium does not escape from the
material for distillation or the distillation residue in
the form of volatile compounds.
Regardless of the form of working up the reaction mixture
chosen, it should be taken into laccount that the noble
metal iR present in the crude procluct in a concentration
of only a few ppm, and its removal therefore requires
very circumspect operation. Additional difficulties may
also arise from the fact that the rhodium is partly
converted into the metal or forms polynuclear carbonyls
duxing the letting-down operation, especially if it has
be~n employed without a ligand. A hetero~eneous system
then forms, which comprises the liguid organic phase and
the solid phase containing rhodium or rhodium compounds.
Under these circumstances, it is not surprising that the
recovery o~ rhodium from the products of the oxo syn-
thes~s, including the residues of crude oxo products, has
been investigated on many occasions. The studies have led
to the development of numerous processes, a few of which
have also found use on an industrial scale.
:
,.~: . '`:. . .'
2 ~ 9
- 3 -
According to a proceqs which is proven in practiae for
the recovery of rhodium contained in the residues of the
distillatio~ of products of the oxo synthesi3 bonded a~
a complex with an organic phosphorus(III) compound, the
residues are treated with oxygen or an oxygen-containing
gas at 60 to 120C under normal pressure or under pres-
sure in the presence o~ a C2- to C5-monocarboxylic acid
and the alkali metal salt of a C2- to Cs-monocarboxylic
acid, and the rhodium compound is then extracted with
water (cf. EP 424 736 A1).
US Patent 42 92 196 relates to the extraction of metals
of group 8, which are present as catalysts in the form of
metal carbo~yls or organometallic compounds dissolved
homogeneously in hydroformylation products, using water~
soluble, nitrogen-containing compounds. The removal is
carried out at temperatures which lie between room
temperature and 100C and in a range from normal pressure
up to an inareased pressure of 7 MPa. Ammonia, ammonium
hydroxide and amines are employed as the nitrogen-
containing compounds for the extraction.
Another process for the removal of rhodium from theproducts of the oxo synthesis which i6 based on extrac-
tion with a complexing reagent is described in
EP 147 824 B1. Water-~oluble sulfonates or carboxylates
of organic phosphines in the form of an agueous solution
which is immiscible with the crude oxo product ar~
employed a~ complexing reagents.
In the last two casPs described, the metal carbonyl
compounds or organometallic compounds present in the
hydroformylation products are treated with the extraction
agent without prior cleavage of the coordinative or
metal-carbon bonds.
The known processes allow up to 90% of the rhodium
originally employed to be recovered in an industrial
3~ procedure, the remainder of the noble metal being lost.
.
,
.
.
. .
,' ' ' . ~ :
21~0~
-- 4 --
Problems during working up of the rhodium extracts some-
times occur because the rhodium concentration in the
aqueous solutions is low and either large volumes of
liquid have to be treated or the ~olution~ must first be
concentr~ted. There is therefore interest in perfecting
the recovery of rhodium from the products of the oxo
synthesis, reducing the rhodium loss~s further and
simplifying the handling of the extracts.
The invention achieves the object described above by a
process or the recovery of rhodium which is contained in
the distillation residue of products of the oxo syn-
thesis, if appropriate bonded as a complex, by treatment
of the residue with oxygen or an oxygen-containing gas at
60 to 120C, under normal pressure or under pressure in
the presence of a C2- to Cs-monocaxboxylic acid and of the
alkali metal salt of a C2- to Cs-monocarboxylic acid and
subsequent extraction of the rhodium present as a water-
soluble compound. This proces~ comprises extracting the
residue with the aqueous solution of a reagent which
complexes with rhodium, if appropriate after it has
already been extracted beforehand with water by itself.
The novel process ensures that a very high proportion of
the `rhodium contained in the crude oxo product is
removed. With the aid of this process it is possible for
rhodium also to be recovered if, for example, water by
itself is ineffective or has only little effect when very
low metal concentrations are present. In this process,
the rhodium is obtained either mainly as a binary com-
pound which i easy to process further or exclusively as
a catalytically active, water-soluble complex compound.
In this connection, it should be emphasized in particular
that the extraction solution contains rhodium in a
concentration which allows its working up or its reuse
without problems.
The novel process starts from the residues of the hydro-
formylation of olefinically unsaturated compounds, which
2 ~
-- 5 --
resi~ues ar~ obtained as the distillation bottom product
after the removal of the aldehydes and alcohols by
di~tillation. They essentially comprise higher molecular
weight compounds which have been formed from the
aldehydes by aldol condensation and can also split off
watex in a secondaxy reaction to form unsaturated com-
pounds. The nature of the compounds which have been
hydroformylated is of no significance for the procedure
claimed. Accordingly, both residues which result from the
reaction of olefins with carbon monoxide and hydrogen and
residues which are formed from the reaction of
olefinically unsaturated compounds which also contain
functional groupæ in the molecule can be employed. The
novel process is of primary importance for the recovery
of rhodium from the residues of hydroformylation of
olefins having 2 to 12 carbon atoms, corresponding to the
economic importance of the aldehydes prepared from them.
In addition to aldehydes and alcohols and the saturated
and unsaturated condensation products, the mixtures to be
processed can also contain, as essential constituents,
solvents and furthermore compounds which react with
rhodium compounds or rhodium carbonyl compounds to form
complexes and are usually present in excess compared with
the rhodium. These compounds include organic
phosphorus(III) compounds, in par.ticular phosphines and
phosphites, preferably the aryl compounds, such as
triphenylphosphine and triphenyl phosphiteO
According to the invention, the distillation residue is
treated with oxygen in order to convert the rhodium
present as a carbonyl compound or bonded as -another
complex into a form which is easily extractable. The
oxidizing agent is employ~d in the pure form or as an
oxygen-containing gas mixture, in particular air. The
amount of oxygen can be varied within wide limits. It
depends on the rhodium concentration and on the concen-
tration of the ligands, that is to say preferably the
phosphorus(III) compounds in the residue. It is advisable
to use 100 to 2000, in particular 300 to 1200 mol of
... . . . .
. . .
.
. . .
.. . . . , ~ , .
: . : ~ : ~' ,:
....
- : .
2 ~ 9
-- 6 --
oxygen per mol of rhodium and per mol of ligand.
According to the invention, the treatment of the distil-
lation residua with oxygen is carried out in the pre ence
of a saturated straight-chain or branched monocarboxylic
acid having 2 to 5 carbon atoms. Examples of suitable
acids are acetic acid, propionic acid, n butyric acid,
i-butyric acid and n-valeric acid. Acetic acid and
propionic acid have proven to be particularly suitable.
They are employed in the commercially available form and
in an amount such that about 2 to 150, preferably 3 to
50 mol of acid are pre~ent per mol of rhodium. The acid
is added to the mixture before the reaction with oxygen,
regardless of whether acid can likewise be formed in the
course of the reaction because of the aldehyde present.
Another important feature of the process according to the
invention is the presence of an alkali metal carboxylate
in the starting material during the oxygen treatment.
Alkali metal carboxylates which are used in the context
of the novel process are salts of saturated straight-
chain or branched monocarboxylic acids having 2 to 5
carbon atoms. The sodium and potassium salts of acetic
acid, of propionic acid, of n- and iso-butyric acid and
of n-valeric acid have proved to be particularly ~uit-
able. They are used in an amount of 10 to 250, preferably
20 to 180 mol per mol of rhodium. The commercially
available salts are suitable, but these gradually dis-
solve only in the course of the oxidation. It is there-
fore more advantageous to add to the residue free acid
and the equivalent amount of alkali metal hydroxide,
which are immediately dissolved homogeneously and there-
fore become fully effective.
The reaction with oxygen is carried out at 60 to 120,
preferably 80 to 100C. It can be carried out under
normal pressure or under pressure, pressures of between
0.2 and 1.0 MPa having proved to be particularly
suitable~
:~
- ' ;
: -
.
2 1 ~ 9
The reaction time depe~ds on the rhodium concentration
and the ligand concentration in the starting material. It
is furthermore determined by the amount of oxygen
employed and by the reaction temperature and pressure.
High concentrations of the dissolved substances require
longer treatment times than low concentrations. A large
supply of oxygen and increased pressure reduce the
reaction time, as does intensive mixing of the residue
with the oxygen. Temperatures in the lower and upper
region of the range claimed are somewhat less effective
than those in the middle temperature region.
The reaction of the distillation residues can be carried
out continuously or discontinuously in conventional
apparatuses. The oxygen or the oxygen-containing gas is
passed into the reactor via distribution devices, and
uniform mixing of the liquid and gaseous phase is
assisted by stirring, if appropriate.
After conclusion of the treatment with oxygen, the
organic phase is extracted with the aqueous solution of
a reagent which bonds rhodium as a complex. According to
the invention, compounds which enter into stable water-
soluble complex compounds with 1:he rhodium under the
prevailing temperature and pxessure conditions are called
complexing reagents. The greater the tendency of the
complexing reagents to re~ct with the rhodium, the more
complete the removal of the noble metal from the crude
product or distillation residue pretreated with oxygen.
The complexing aqents can be mono- or polydentate, that
is to say can occupy one or more coordination sites on
the central atom.
Preferred complexing agents for the rhodium are compounds
of nitrogen and of phosphorus which contain nitrogen or
phosphorus atoms capable of forming coordination bonds
with the rhodium, i~e. atoms which have free electron
pairs. Examples of nitrogen compounds are ammonia, water-
soluble primary, secondary or tertiary amines and
-: ~ . . . . . .. .
' .', ' ' ' ', . ~: .
.. .
.
~ :
~ r~ ~9
-- 8 --
diamines, alcoholamines and aminocarboxyic acids. Poss-
ible phosphorus compounds are, in particular, sulfonated
or carboxylated arylphosphines or aryldiphosphines.
Iminodiacetic acid, nitrilotriacetic acid, ethylene-
diaminetetraaceti~ acid and triphenylphosphinetri-
sulfonates and triphenylphosphinedisulfonates of the
alkali metals and of ammonium are preferred.
The complexing reagents are used in excess with respect
to the rhodium. Since they can be circulated, the level
of the excess can be as desired, but at least 5 mol of a
monodentate complexing agent or at least ~ mol of an
n-dentate complexing agent must be present per mol of
rhodium. It has proven suitable to use 10 to 50 mol of a
monodentate complexing agent or ' to 50 mol of an
n-dentate complexing agent per mol of rhodium.
The concentration of the complexing reagent in the
solvent can be varied within wide limits. It depends in
particular on the extent to which the rhodium is to be
concentrated. Accordingly, not on:Ly very dilute but, if
appropriate, even saturated solut:ions can be used. As a
rule, solutions which contain 0.5 to 25% by weight of the
complexing agent, based on the solution, are employed.
The extraction of the rhodium with the dissolved complex-
ing agent is carried out at temperatures of 20 to 120C.
The removal can be carried out under normal pressure, but
also under increased pressures of up to 1.0 MPa, the
range from 0.2 to l.0 MPa being preferred. ~;
In many cases, a single treatment of the distillation
residue with the solution of the complexing reagent is
sufficient. The solution can of course be recirculated in
order to bring the removal of rhodium to completion and
to increase the rhodium concentration in the extraction
agent by several treatments of the organic phase. A
multi-stage extraction is also possible. An extraction
with water can precede the extraction with the aqueous
.
.: -
?
:. :
c~
- 9 -
solution of a complexing reagent. It is u~ually carried
out with deionized water at 20 to 120C under normal
pressure or under pressures of up to 1.0 MPa, preferably
O.2 to ~.O MPa, in particular in several stages. The
amount of water employed depends on the distribution
equilibrium of the substance to be extracted between the
organic and aqueous phase and the xhodium concentration
required in the ~queous phase. The aqueous extraction
solution can be circulated until the distribution equili-
brium is established and thus used repeatedly for removalof rhodium in order to achieve concentration of the metal
in the solution. In general, two to three extraction
stages are envisaged. The transition from one extraction
agent to the other is to be matched to the individual
circumstances. Deciding factors are, in particular, the
decrease in the effectiveness of the pure water for the
removal of rhodium, which is detectable from the amount
of rhodium absorbed per unit volume of water, and the
rhodium concentration required in the aqueous extraction
solution.
The novel process can be carried out batchwise or
continuously in the reactors c:ustomary for solvent
extraction. The extraction can be carried out either in
cocurrent or in countercurrent.
Further treatment or further use of the phases containing
the rhodium removed depends on the particular circum-
stances. The rhodium extracted as a binary compound can
thus be precipitated from an aqueous solution as a salt
of a higher carboxylic acid. It is likewise possible for
the aqueous solution of the binary rhodium compound to be
reused as the catalyst pha~e in the oxo synthesis, for
example after addition of a water-soluble phosphine. The
aqueous phase containing rhodium bonded as a complex is
preferably used as a catalyst, if appropriate after
addition of further rhodium and/or further complexing
agent.
- : . : .,
. :' , ~ ~: '
~,
.. ....
~ .. '~ "''
2 1 ~
-- 10 --
The invention is illu~trated in the following examples.
However, thP intention is not to limit it to the~e
specific embodiments.
Example 1
90.3 kg of a distillation residue of ethylene hydro-
formylation are diluted in a stirred reactor with toluene
in an amount such that the rhodium concentration in the
solution is 70 ppm. 534 g of Na propionate and 55 g of
propionic acid are added, and 120 m3 of air are passed
through the stirred contents of the reactor at 80C under
a pressure of 0.25 MPa over a period of 6 hours. The
mixture is then cooled to below 60C and stirred with
25 l of water under normal pressure for 15 minutes, and ^~
the aqueous and organic phase are separatecl from one
another. This treatment is repeated twice with 10 l of
water each time. It leads to the removal of a total of
83.5% of the rhodium originally contained in the organic
phase. Thereafter, the organic phase, which still con-
tains rhodium in a concentration of 13.5 ppm, is stirred
under normal pressure at room temperature for 15 minutes
with an amount of a trisodium triphenylphosphine-
tri~ulfonate solution (1% strengt:h by weight in water)
such that 32.2 mol of the phosphine are present per mol
of rhodium. The amount of rhodium removed is increased by
this extraction step to 88.8% in total, and the organic
phase still contains the metal in a concentration of
10.2 ppm. ;
Example 2
61 kg of a distillation residue of ethylene hydroformyl-
ation is diluted in a stirred tank with toluene in an
amount such that the rhodium concentration in the
solution is 70 ppm. 655 g of Na propionate and 67 g of
propionic acid are added, and 120 m3 of air are passed
through the stirred contents of the xeactor at 80C under
a pressure of 0.25 MPa over a period of 6 hours. The
. ::: ~ '
. :.
,, . .. 1'
; . .. . , .
2 ~ 9
mixture is then cooled to below 60C and stirred with
25 l of water under normal pressure for 15 minutes, and
the aqueous and organic phase are separated from one
another. ~his treatment is repeated three times with 10 l
of water each time. It leads to the removal of a total of
8~.9% of the rhodium ori~inally contained in the organic
phase~ Thereafter, the organic phase, which still con-
tains rhodium in a concentration of 12.8 ppm, is stirred
under normal pressure ~t room temperature for 15 minutes
with an amount of a trisodium triphenylphosphinetri-
sulfonate solution (1% strength by weight in water) such
that 28.3 mol of the phosphine are present per mol of
rhodium. The amount of rhodium removed is increased by
this extraction step to 91.4% in total, the organic phase
still containing the metal in a concentration of 7 4 ppm.
Example 3
150 kg of a distillation residue of ethylene
hydroformylation are diluted in a stirred reaction with
toluene in an amount such that the rhodium concentration
in the solution is 70 ppm. 557 g of Na propionate and
58 g of propionic acid are added and 120 m3 of air are
passed through the stirred reactor at 80C under a
pressure of 0.25 MPa over a p~riod of 6 hours. The
mixture is then cooled to below 60C and stirred wikh
25 l of water under normal pressure for 15 minutes, and
the aqueous and organic phase are separated from one
~nother. This treatment is repeated three times with 10 1
of water each time. It leads to the r~moval of a total of
72.7% of the rhodium originally contained in the organic
3C phase. Thereafter, the organic phase, which still con-
tains rhodium in a concentration of 19.7 ppm, is stirred
under normal pressure at room temperature for 15 minutes
with an amount of an ethylenediamine-tetraacetic acid
solution (1% strength by weight in water) such that
10.7 mol of the diamine (corresponding to 21~4 mol of the
amine nitrogen capable of complexing) are present per mol
of rhodium. The amount of rhodium removed is increased by
this extraction step to 31.7% in total, the organic phase
still containing the metal in a concentration of 14.9 ppm.
.
:, - ~
: , ' ' , . :
- 12 - 21~0~
Exam~e 4
150 kg of a distillation residue of ethylene
hydroformylation are diluted in a stirred reactor with
toluene in an amount such that the rhodium concentration
in the solution is 70 ppm. 557 g of Na propionate and
58 g of propionic acid are added and 120 m3 of air are
passed through the stirred contents of the reactor at
80C under a pressure of 0.25 MPa over a period of
6 hours. The mixture is then coolad to below 60C and
stirred under normal pressure at room temperature for
15 minutes with 25 1 of water under normal pressure for
15 minutes, and the aqueous and organic phase are separ-
ated from one another. This treatment is repeated three
times with 10 1 of water each time. It leads to th~
removal of a ~otal of 76.7% of the rhodium originally
contained in the organic phase. Thereafter, the organic
phase, which still contains rhodium in a concentration of
19.7 ppm, is stirred under normal pressure at room
temperature for 15 minutes with an amount of nitrilo-
triacetic acid solution (1% strength by weight in water)
such that 10.9 mol of the phosphine are present per mol
of rhodium. The amount of rhodium removed is increased by
this extraction step to 83.3% in total, the organic phase
still containing the metal in a concentration of
12.8 ppm.
;, ~
21~4~09
Example 5
70 kg of a distillation residue of methyl acry-
late hydroformylation are diluted in a stirred reactor
with toluene in an amount such that the rhodium concen-
tration in the solution is 70 ppm. 737 g of Na propionateand 76 g of propionic acid are added and 100 m3 of air
are passed through the stirred contents of the reactor at
100C under a pressure of 0.25 MPa over a period of
6 hours. The mixture is then cooled to below 60C and
stirred with 32 l of water under normal pressure for
15 minutes, and the aqueous and organic phase are
separated from one another. This treatment is repeated
once with 10 l o~ water. It leads to the removal of a
total of 81.4% of the rhodium originally contained in the
organic phase. Thereafter, the organic phase, which still
contains rhodium in a concentration of 15.5 ppm, is
stirred under normal pressure at room temperature for
15 minutes with an amount of a trisodium triphenyl-
phosphinetrisulfonate solution (1~ strength by weight in
water) such that 14.5 mol of the phosphine are present
per mol of rhodium. The amount of rhodium removed is
increased by this extraction step to 84.5% in total, the
organic phase still containing the metal in a
concentration of 14.8 ppm.
Example 6
140 kg of a distillation residue of methyl
acrylate hydroformylation are diluted in a stirred tank
with toluene in an amount such that the rhodium concen-
tration in the solution is 5.5 ppmO 701 g of Na
propionate and 77 g o~ propionic acid are added and
100 m3 of air are passed through the stirred contents of
the reactor at 100C under a pressure of 0.25 MPa over a
period of 6 hours. The mixture is then cooled to below
60C and stirred with 10 l of water under normal pressure
for 15 minutes, and the aqueous and organic phase are
separated from one another. This treatment leads to the
removal of 67.4~ of the rhodium originally contained in
the organic phase. Thereafter, the organic phase, which
still contains rhodium in a concentration of 4.3 ppm, is
.. . .
- 14 - ~ ~ ~ 09
stirred under normal pressure at room temperature for
lS minutes with an amount of a trisodium triphenyl-
phosphinetri~ulfonate solution such that 45.2 mol of the
phosphine are present per mol of rhodium~ The amount of
rhodium removed is increased by this extraction step to
79~ in total, the organic phase still containing the
metal in a concentration of 3.6 ppm.
. .
