Note: Descriptions are shown in the official language in which they were submitted.
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Mo-5231
LeA 33,225
PROCESS FOR PREPARING CATALYSTS COMPRISING NOBLE
METALS ON CARBON-CONTAINING SUPPORT MATERIALS
FIELD OF THE INVENTION
The present invention relates to a process for preparing catalysts
comprising noble metals on a carbon-containing support material, to the
catalysts obtained in this way and to their use for preparing substituted
cyclohexanones.
BACKGROUND OF THE INVENTION
The preparation of organic compounds is carried out on an
. industrial scale in the presence of catalysts containing noble metals. An
important field of application is, for example, hydrogenation reactions.
Activity and selectivity of such catalysts can be influenced by additional
promoters and modifiers. Promoters increase the catalytic activity,
whereas modifiers change the selectivity by partial poisoning of the
catalyst. Depending on the desired reaction, particular components can
act as promoters in one case and as modifiers in another case. In general,
the catalytically active noble metals are applied to an inert support
material. Compared with unsupported noble metal catalysts, supported
catalysts have an increased thermal and mechanical stability and lead to a
significantly increased active surface area of the corresponding noble
metals.
Carbon-containing support materials are becoming increasingly
important in catalysis. This is due not only to their virtually unlimited
availability and the favourable price but also to the catalytically
advantageous properties. Thus, graphites, activated carbons and carbon
blacks suitable for catalytic use have a high resistance to acids and bases,
high melting points and sometimes very good electrical conductivity.
Another advantage is that the physical and chemical properties such as
the porosity or the functional groups present on the surface can be
modified within wide ranges.
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Activated carbons in particular have proven useful as support
materials for catalysts containing noble metals. The activated carbons are
highly porous, finely crystalline substances which contain microcrystalline
regions derived from the graphite lattice which are predominantly present
in a disordered arrangement. Activated carbons are obtained from carbon-
containing materials such as wood, nut shells, fruit stones, peat, charcoal,
hard coal or brown coal or petroleum products by carbonization with
subsequent chemical activation or gas activation. The removal of
amorphous carbon occurring in the activation step leads to formation of a
pore system and thus to an increase in this specific surface area.
Activated carbons having a pore volume of up to 1.5 ml/g and a specific
surface area of up to 2,500 m2/g are formed. The specific surface area can
be determined from the nitrogen adsorption isotherms via evaluation by
the method of Brunauer, Emmett and Teller (BET) (DIN 66132). Activated
carbons contain, apart from carbon, small amounts of oxygen and
hydrogen which are present in the form of various functional groups such
as phenol, carbonyl, carboxyl, lactone, quinone and ether groups. The
type and number of functional groups present on the activated carbon
surface is determined by the raw material and the activation process.
Activated carbons additionally contain a series of mineral constituents
such as silicas and alkali metal and alkaline earth metal salts. Their
content is indicated by the manufacturers as "ash content".
The preparation of palladium-containing coated catalysts on
activated carbon as support is described in U.S. Pat. No. 3,271,327. Here,
an aqueous solution of palladium chloride and hydrochloric acid is first
adjusted to a pH of from 4 to 6.5 by addition of a base. This solution is
then brought into contact with the support material, held at a temperature
of about 21-30°C for a certain impregnation time and is subsequently
dried
at temperatures below 60°C. Reduction of the catalysts prior to use in
hydrogenation reactions is not necessary; rather, they are activated in situ
during the catalysed hydrogenations by the hydrogen present. However,
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the catalysts prepared in this way still capable of improvement in terms of
their activity and the selectivity profile of the catalysed reactions.
The use of palladium-containing coated catalysts in the
hydrogenation of substituted or unsubstituted phenols to prepare the
corresponding cyclohexanones is known. Thus, for example, the
preparation of 2,6-dimethylaniline from 2,6-dimethylphenyl using a
palladium-containing catalyst in the presence of ammonia is described in
J. Mol. Cat. 55 (1989) 415-428. This gives, firstly, 2,6-dimethylcyclo-
hexanone which then reacts further with ammonia. The palladium-
containing coated catalyst used is colloidal palladium hydroxide on
sibunite carbon as support material. To prepare this catalyst, an aqueous
sodium carbonate solution is combined with an aqueous solution of
H2PdCl2, forming colloidal palladium hydroxide. This solution is
subsequently added to the porous sibunite support material.
Owing to the increasing importance of catalysts containing noble
metals for hydrogenation reactions, it is an object of the present invention
to provide supported catalysts containing noble metals which lead to
improved results in hydrogenation reactions, particularly, the
hydrogenation of substituted or unsubstituted phenols.
SUMMARY OF THE INVENTION
The present invention provides a process for preparing noble metal
catalysts which comprise at least one noble metal of the platinum metal
group and/or compounds thereof as catalytically active component and, if
desired, promoters and/or modifiers on a carbon-containing support
material, in which the process is wherein the carbon-containing support
material is first treated with alkali metal salts or alkaline earth metal
salts of
organic or inorganic acids and the catalytically active noble metal
components and, if desired, promoters and/or modifiers are subsequently
applied to the treated carbon-containing support material.
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DESCRIPTION OF THE INVENTION
The process of the invention gives catalysts which, when used in
the hydrogenation of substituted or unsubstituted phenols to form the
corresponding cyclohexanones, display excellent activities and
selectivities. An essential aspect of the process of the invention is that the
carbon-containing support material is treated with alkali metal or alkaline
earth metal salts of organic or inorganic acids prior to application of the
catalytically active noble metal components and, if desired, promoters
and/or modifiers. Here, the sodium and potassium salts of organic or
inorganic acids have been found to be particularly useful. As organic acid,
preference is given to using formic acid; as inorganic acids, preference is
given to using hydrohalic acids, in particular, hydrochloric acid, or nitric
acid. Particular preference is given to sodium formate, sodium chloride
and sodium nitrate. The salt solutions employed usually have a
concentration of 1-100 mmol /I of solution.
Carbon-containing support materials which can be used in the
process of the invention are, for example, graphite, activated carbons or
carbon blacks. Preference is given to using activated carbon, usually in
powder form, i.e., finely divided form having a mean particle diameter of
from 15 to 30 Nm, or as shaped bodies having dimensions of from 1 to 5
mm. They generally have a specific surface area of up to 2500 m2/g,
preferably 300 - 1200 mZ/g, a total pore volume of more than 0.5 ml/g and
a residual ash content of <5% by weight. The activated carbon can be
used in the untreated state, but in many cases it has been found to be
useful to carry out a pre-treatment. Such a pre-treatment usually
comprises washing the activated carbon with strong mineral acids, such
as, hydrochloric acid or nitric acid. This significantly reduces the
sometimes high ash content of commercial activated carbons (up to 20%)
which originates from the mineral components of the starting materials.
Compounds containing alkaline earth metals and heavy metals are
removed. Furthermore, the washing procedure also refunctionalises the
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support surface. Both have an advantageous effect on the activity of the
finished catalyst. After such washing, the residual ash contents of the
activated carbon used are typically less than 2% by weight. According to
German Offenlegungsschrift 43 08 101, it can also be advantageous to
subject the activated carbon support which has been washed with nitric
acid to an oxidative pre-treatment with hydrogen peroxide or sodium
hypochlorite prior to addition of the salts and the application of the noble
metals. This oxidative pre-treatment is preferably carried out in the form of
a wash with an aqueous solution containing from 0.1 to 30% by weight of
hydrogen peroxide or sodium hypochlorite at room temperature for from
100 to 300 minutes.
The process of the invention can be carried out in various
embodiments. In an embodiment A,
1 ) a 10-70% strength, preferably 50% strength, suspension of the
carbon-containing support material in water is prepared;
2) the aqueous solution of an alkali metal or alkaline earth metal salt of
an organic or inorganic acid is added thereto;
3) a pH in the range from 1 to 6.5, preferably from 2 to 6, is set;
4) an aqueous solution of the water-soluble noble metal compounds,
which has previously preferably been set to the same pH as the
suspension of the carbon-containing support material, is added to
the suspension; and
5) the catalyst is filtered off and dried.
In this case, the catalyst contains the catalytically active noble metal
component in the form of the corresponding cations on the carbon-
containing support material, in the case of palladium as Pd2+ ions.
A second embodiment B is characterised in that, subsequent to the
steps 1 ) to 4) of embodiment A,
6) the water-soluble noble metal compounds are converted into
compounds which are sparingly soluble or insoluble in water by
addition of a base; and
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7) the catalyst is filtered off, washed and dried.
In both embodiments A and B, the catalyst obtained can be used
without further reduction as catalyst in hydrogenation reactions where
activation by the hydrogen present then takes place in situ.
A third embodiment C is characterised in that, subsequent to the
steps 1 ) to 4) of embodiment A,
8) a reduction of the noble metal compounds is carried out; and
9) the catalyst is filtered off, washed and dried.
The pH of the suspension of the carbon-containing support material
and of the aqueous solution of the noble metal compounds is set by
addition of aqueous inorganic acids, for example, HCI, or aqueous bases,
for example, alkali metal hydroxides, in particular, NaOH. The aqueous
solution of the noble metal compounds is usually added to the suspension
of the carbon-containing support material while stirring and while
controlling the pH. Preferably, the pH is kept approximately constant at the
previous pH of the two starting solutions, i.e., the suspension of the
support material and the solution containing the noble metals. The reaction
temperature is usually in the range from 20 to 90°C, preferably, from
20 to
50°C. The reaction time here is from 1 minute to 24 hours, preferably,
from
60 to 90 minutes.
In step 8 of embodiment C, the noble metal cations are converted
into the corresponding noble metal. Reducing agents which can be used
include hydrazine, sodium formate, sodium boranate, formaldehyde or
hydrogen. The reduction temperature is generally at from 0 to 100°C,
preferably, from 0 to 50°C. If a gas is used as reducing agent, it has
been
found to be useful to dilute this beforehand using an inert gas, such as,
nitrogen, carbon dioxide or a noble gas. The reduction can also be carried
out in the liquid phase at a temperature of from 0 to 50°C, preferably,
from
0 to 25°C. As reducing agents, it is possible to use aqueous solutions
of
hydrazine, formic acid or alkali metal borohydrides. The reduction is,
preferably, carried out by means of formaldehyde in an aqueous alkaline
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medium, in particular, at a pH of from 8 to 9, and at temperatures of from 0
to 10°C.
As catalytically active noble metals of the platinum metal group, it
has been found to be useful to employ platinum, palladium, rhodium,
ruthenium, osmium or iridium or combinations thereof. Particular
preference is given to using platinum, palladium, rhodium and ruthenium,
in particular, palladium. In the process of the invention, the platinum
metals are used in the form of water-soluble compounds. Here, the readily
available tetrachloropalladic acid, hexachloroplatinic acid, palladium
nitrate, hydrated palladium oxide and palladium acetylacetonate and also
amine complexes of palladium have been found to be useful. Tetrachloro-
palladic acid and hexachloroplatinic acid can be prepared by addition of
concentrated hydrochloric acid to palladium(II) chloride and platinum(IV)
chloride, respectively, to form a 0.1 - 5 M, preferably 1-1.5 M, aqueous
solution. The noble metal content of the catalyst is from 0.1 to 20% by
weight, preferably, from 0.1 to 10% by weight, based on the carbon-
containing support material.
If the catalyst is also to contain promoters and/or modifiers, water
soluble compounds of these promoters and/or modifiers are added to the
aqueous solution of the noble metal compounds. Known promoters and
modifiers are, for example, the noble metals Au, Ag, Hg or Ru and also S,
Pb, Bi, Cu, Fe or Ni.
The invention further provides the catalysts obtainable via the
process of the invention and also provides for the use of these catalysts in
hydrogenation reactions, preferably, in the hydrogenation of substituted or
unsubstituted phenols to form the corresponding cyclohexanones.
The hydrogenation of substituted or unsubstituted phenols using
hydrogen to give the corresponding cyclohexanones is usually carried out
in the liquid phase in a manner known to those skilled in the art at a
reaction temperature of from 100 to 180°C, preferably, from 140 to
160°C,
and a pressure of from 0.1 to 5 MPa, preferably from 0.5 to 2 MPa. The
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catalyst of the invention is used here in an amount of from 0.1 to 5% by
weight, preferably, from 0.5 to 2% by weight and in particular from 0.75 to
1 % by weight, based on the phenol used. The reaction is carried out in
conventional autoclaves. The hydrogenation can also be carried out in the
gas phase, e.g., in a flow tube at a reaction temperature of from 100 to
180°C, preferably, from 140 to 160°C, and a pressure of from 0.1
to 0.5
MPa. In this case, hydrogen and phenol are used in a molar ratio of (2-
5):1, preferably, (3-5):1. An even greater excess of hydrogen is possible,
but leads to a poorer selectivity owing to increased formation of the
corresponding cyclohexanol. The phenols to be hydrogenated may be
substituted by one or more radials which are inert under the hydrogenation
conditions. Examples are halogen radicals and also straight-chain or
branched C~-Ca alkyl radicals, preferably methyl, ethyl, propyl, i-propyl, n-
butyl, i-butyl or t-butyl radicals.
The invention is further illustrated but is not intended to be limited by
the following examples in which all parts and percentages are by weight
unless otherwise specified.
EXAMPLES
Example 1 Preparation of catalyst 1 )
10 g of activated carbon were suspended in 80 ml of water at room
temperature and, after addition of 4 mmol of sodium formate, the pH was
adjusted to 2 by addition of HCI (solution 1 ). In a second vessel, solution 2
comprising 3.6 ml of a 1 M tetrachloropalladic acid and 40 ml of water was
likewise adjusted to a pH of 2 by addition of NaOH. Subsequently, solution
2 was quickly added to solution 1 while stirring vigorously and deviations
from the pH of 2 were corrected. After 1 hour, the reaction solution was,
while continuing to stir, cooled to 0°C by means of a cooling bath,
0.51 ml
of a 36.5% strength formaldehyde solution was added and a pH of > 8 was
quickly set using NaOH. After removing the cooling bath, the solution was
stirred further until it had again reached room temperature. Subsequently,
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the catalyst obtained was filtered off and dried under reduced pressure
over a desiccant.
Comparative Example 1 (Preparation of catalyst C1 )
Comparative Example 1 was carried out using a procedure
analogous to that of Example 1, but no sodium formate was added to the
activated carbon after suspension.
Example 2 ,Preparation of catalvst 2)
Catalyst 2 was prepared by a procedure analogous to that for
preparing catalyst 1, but the pH of solutions 1 and 2 was in each case
adjusted to 6 and this value was also kept largely constant during the
addition of solution 2 to solution 1.
Comparative Example 2 (Preparation of catalyst C2~
Catalyst C2 was prepared using a procedure analogous to that for
preparing catalyst C1, but the pH of solutions 1 and 2 was in each case
adjusted to 6.
Examples 3 and 4 and Comparative Examples C3 and C4 (Hydrogenation
of henol
The performance of the catalysts 1, 2, C1 and C2 was studied in a
liquid-phase intermediate-pressure hydrogenation of phenol to
cyclohexanone.
In each case, 2.25 g of the catalyst together with 300 g of liquefied
phenol were introduced into a 1 I autoclave preheated to 80°C and were
subsequently heated to 140°C under a nitrogen atmosphere with the
stirrer
not running. After the reaction temperature had been reached, nitrogen
was replaced by hydrogen and a reactor pressure of 5 bar was set. The
beginning of the reaction was characterised by the switching-on of the
stirrer (stirrer speed: 1000 rpm). The hydrogen uptake during the reaction
was monitored by means of the decreasing gas pressure in two hydrogen
reservoirs which ensure a constant gas supply. On reaching a total
hydrogen uptake of 98.5%, the reaction was terminated by stopping the
stirrer and cooling the autoclave.
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The reaction period used for calculating the activity was the reaction
time between the 15th and 20th minute. The activity was reported in [I of
[H2]/ g Wit, * min ~t~,2 t2.s min]]~ The selectivity was given by the ratio of
the
rate constants k~/k2, where k~ characterised the reaction of phenol to
cyclohexanone and k2 characterised the subsequent reaction of
cyclohexanone to cyclohexanol.
The results of the hydrogenations are summarised in Table 1
below.
Exam le 3 C3 4 C4
Catalyst 1 C1 2 C2
(Synthesis(Synthesis(Synthesis (Synthesis
at at
at pH = at pH=2) pH=6 using pH=6)
2
using formate)
formate
Reaction
roduct
Phenol % 0 0 0 0
Cyclohexanone88.2 79 94.5 89.6
Cyclohexanol 11.6 20 5.4 10.2
Selectivity 36.58 10.63 62.0 30.38
k~/k2
Activit 0.66 0.11 1.0 0.97
t tc min 328 649 187.4 257
TABLE 1
The experiments carried out on the hydrogenation of phenol to
cyclohexanone demonstrate that the catalysts prepared by the process of
the invention with addition of sodium formate had a significantly higher
activity and selectivity than the catalysts prepared without salt addition and
at the same time make possible a greatly shortened reaction time.
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Although the invention has been described in detail in the foregoing
for the purpose of illustration, it is to be understood that such detail is
solely
for that purpose and that variations can be made therein by those skilled in
the art without departing from the spirit and scope of the invention except as
it may be limited by the claims.
