Note: Descriptions are shown in the official language in which they were submitted.
CA 0223l~36 l998-04-02
0050/4625B
Preparation of a hydrogenation catalyst
The present invention relates to an improved process for the
5 preparation of a hydrogenation catalyst by reducing platinum in
the oxidation state of +4 with a selective reducing agent in an
acidic aqueous medium in the presence of a carbon-containing
carrier to platinum in the oxidation state +2, subsequently
poisoning the resulting platinum with a sulfur-containing
10 selective reducing agent and then reducing the platinum partially
poisoned in this manner to metallic platinum and then working up
in a manner known per se.
The present invention furthermore relates to the use of alkali
15 metal formates for the preparation of hydrogenation catalysts,
hydros~enation catalysts prepared according to the invention, a
process for the preparation of hydroxylammonium salts and a
process for regenerating hydrogenation catalysts based on
platinum .
Noble metals, such as palladium, platinum or ruthenium, which are
applied to various carriers, such as silica, alumina, graphite or
active carbon, as disclosed in "Katalytische Hydrierungen im or-
ganisch chemischen Laboratorium~', E'.Zimalkowski, Ferdinand Enke
25 Verlag, Stuttgart (1965), are suitable for hydrogenating organic
and inorganic compounds.
The high dispersion of the noble metal on the catalyst carrier is
essent:ial for the activity of these catalysts. The fact (see
30 Struct:ure of Metallic Catalyst, J.R. Anderson, Academic Press
(1975), page 164 et seq.), that, under the reaction conditions,
the particle size of the applied noble metal increases as a
result of agglomeration, the dispersion decreases and the
elemental noble metal becomes detached from the carrier material
35 is disadvantageous with regard to process engineering.
German Patent 1,088,037 descri~es a process for the preparation
and regeneration of a special catalyst for obtaining
hydroxylamine by first reducing platinum in the oxidation state
40 of +4 with a selective reducing agent in an acidic aqueous medium
in the presence of a carbon-containing carrier to platinum in the
o~cida~ion state +2, then partially poisoning the resulting
platinum with a sulfur-c:ontaining selective reducing agent and
then carrying out the reduction of the resulting poisoned
45 platinum to metallic platinum with a formate and then working up
in a rnanner known per se.
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The diisadvantaye of this process is the insufficient selectivity
with respect to hydroxylamine, the excessively large amount of
ammonia and nitrous oxide obtained and the insufficient
conversion of nitric oxide and the unsatisfactory space-time
5 yield.
DE-C 40 22 853 states that the selectivity with respect to
hydroxylamine in the hydrogenation of nitric oxide can be
increased by using graph:ite-supported platinum catalysts in which
10 the graphite has a particle size of 1-600 ~m.
German Patent 956,038 discloses graphite-supported platinum
catalysts which are obtained by precipitating platinum onto
suspended graphite carriers, with or without the addition of
15 poisons, such as sulfur compounds, selenium compounds, arsenic
compounds or tellurium compounds. Such catalysts are suitable for
the catalytic hydrogenation of nitric oxide. These catalysts have
the disadvantage that the reactivity and selectivity rapidly
decreases during prolonged use.
DE-C 40 22 851 states that selectivity is related to the apparent
density, the compressive strength and the porosity of the
graphite carrier in the preparation of hydroxylamine by
hydrogenation of nitric oxide in the presence of
25 graphite-supported platinum catalysts.
The catalysts used in the processes of the abovementioned German
patents have the disadvantage that, owing to agglomeration of the
active components, only relatively short catalyst lives can be
30 achieved.
DE-A 93 11 420 describes the preparation of a hydrogenation
catalyst which is obtainable by treating a platinum metal salt
with finely divided sulfur in the presence of a dispersant and
35 subseSIuently reducing the platinum metal salt to metallic
platinum. Although sodium formate is also mentioned as a reducing
agent, according to DE-A 43 11 420 formic acid is particularly
prefer-red. Tests with sodium formate were not carried out in the
corresponding German patent. There is also no indication that the
40 particle size of the platinum has a decisive effect on the
mechanical stability of the catalyst, the selectivity and the
byprocluct spectrum.
It is an object of the p,resent invention to provide an improved
45 process for the preparation of hydrogenation catalysts, which
ensures longer lives for the catalysts used in conjunction with
at least the same selectivity and a high space-time yield.
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Furthermore, the byproducts should be minimized, in particular
nitrous oxide and ammonia.
We have found that this object is achieved by an improved process
5 for the preparation of a hydrogenation catalyst by reducing
platinum in the oxidation state of -~4 (Pt (IV~) with a selective
reducing agent in an acidic aqueous medium in the presence of a
carbon-containing carrier to platinum in the oxidation state +2
(Pt (II)), then poisoning the resulting platinum with a
10 sulfur-containing select:ive reducing agent and then reducing the
platinum partially poisoned in this manner to metallic platinum
(Pt (0)) and then workin~ up in a manner known per se, wherein
(a) Pt (II) is partially poisoned with a sulfur-containing
selective reducing agent, said reducing agent being used in
an amount which corresponds to 15 - 70 mol% of the amount of
a selective reducing agent which would be needed to reduce Pt
(]:V) to Pt (II), provided that the amount of Pt (IV)
corresponds to the amount of Pt (II) used and to be poisoned,
and the partially poisoned Pt (II) is then reduced with an
a]kali metal formate to Pt (0), or
(b) p]atinum in an oxidation state higher than +2 is partially
poisoned and is reduced subsequently or simult.aneously by
means of an alkali metal formate to Pt (0).
We have also found the use of alkali metal formates for the
preparation of hydrogenation catalysts, hydrogenation catalysts
prepared according to the invention, a process for the
30 preparation of hydroxylammonium salts and a process for the
regeneration of hydrogenation catalysts based on platinum.
Accord,ing to the invention, in variant (a) platinum in an
oxidat.ion state of +2 is poisoned with a sulfur-containing
35 select.ive reducing agent in an acidic aqueous medium in the
presence of a carbon-containing carrier. The platinum partially
poisor,ed in this manner is then reduced to metallic platinum with
an alk:ali metal formate and then worked up in a manner known per
se.
The amount of sulfur-containing selective reducing agent is
choser, so that it is used in an amount which corresponds to 15 -
70, preferably 20 - 65, mol~ of the amount of a selective
reduci.ng agent which would be needed to reduce Pt (IV) to Pt
45 (II), provided that the amount of Pt (IV) corresponds to the
amount of Pt (II) used and to be poisoned.
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In variant ~b~, platinum in an oxidation state higher than +2 is
poison.ed, according to the invention, and is subsequently or
simult.aneously reduced by means of an alkali metal formate to Pt
(O) .
Accorcling to the invention, alkali metal formates, preferably
lithium formate, sodium formate and potassium formate,
particularly preferably sodium formate, are used.
10 In a ~)articular embodiment, platinum in the oxidation state +2,
which can be obtained by reducing platinum in an oxidation state
higher than +2, particularly preferably Pt (IV), with a selective
reducing agent, is used in variant (a).
15 As a rule, dithionites, in particular sodium dithionite
(Blankit~), derivatives of sulfoxylic acid, in particular the
product obtained by the action of formaldehyde on sodium
hyposulfite known under the name Rongalit~, sulfurous acid and
sulfil:es can be used as the selective reducing agent.
In general, the selective reducing agent is used in an amount
equivalent to the dissol.ved platinum having an oxidation state
higher than 2, preferab].y +4.
25 When a sulfur-containincl selective reducing agent is used, in
variant (a), as a poison which is capable of reducing platinum in
the oxidation state +4 t:o platinum in the oxidation state +2
(e.g. Blankit~), the reduction is carried out, according to the
invention, initially on:Ly to platinum in the oxidation state +2.
30 Observations to date have shown that the end point of this reac-
tion is detectable from a large change in the potential which is
measured during the reduction. The partial poisoning is then car-
ried out, according to l:he invention, by adding a certain amount
over and above that required for reducing Pt (>II), preferably Pt
35 (IV), to Pt (II) and corresponding to 15 - 70, preferably 20 -
65, m~l% of the amount used for reducing Pt (IV) to Pt (II). The
reduction with the alka:Li metal formate is then carried out ac-
cording to the invention.
40 If the poison used is a compound which is not capable of reducing
platinum in an oxidation state greater than +2, in particular Pt
(IV), to Pt (II) (variant (b)), then, according to the invention,
the platinum is first poisoned and is reduced subsequently or
simultaneously to platinum (0) (metallic platinum) with an alkali
45 metal formate as reducing agent.
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Such poisons are conventional poisons based on sulfur, selenium,
arsenic or tellurium, such as sodium dithionite, alkali metal
thiosulfates, hydrogen sulfide, alkali metal sulfide, alkali
metal polysulfides, thiourea, telluric acid or arsenic acid or
5 combinations thereof. Elemental sulfur, in particular wettable
sulfur, which has a particle size of less than 500 ~m, preferably
less than 50 ~m, is particularly preferably used.
The molar ratio of platinum to sulfur, selenium, arsenic or
10 tellurium to be poisoned is usually chosen in the range from 20:1
to 3:1, preferably from 10:1 to 5:1.
The partial poisoning is usually carried out by methods known per
se, as described, for example, in DE-C 40 22 853.
The reduction with alkali metal formate to give metallic platinum
is effected preferably after the partial poisoning has been
carried out.
20 Partic:ularly suitable platinum ~IV) compounds are the
water-soluble compounds, such as hexachloroplatinic acid and its
alkali. metal and ammonium salts, such as disodium, dipotassium
and diammonium hexachloroplatinate.
25 The molar ratio of platinum used to the alkali metal formate is
usually chosen in the range from 1000:1 to 10:1, preferably from
100:1 to 20:1.
The carbon-containing carrier used is as a rule suspended
30 graphi.te or an active carbon, in particular electrographite
grades, particularly preferably those electrographite grades
which have a particle size of from 0.5 to 600 ~m, preferably from
1 to 70, particularly preferably from 2 to 50 ~m. The amount of
platinum is in general from 0.2 to 2, preferably from 0.5 to 1%
35 by wei.ght, based on the total weight of graphite-supported
platinum catalyst.
Accorcling to the invention, the reduction of the platinum is
carried out in an aqueous solution, the weight ratio of water to
40 platinum being chosen to be, as a rule, from 1000:1 to 100:1,
preferably from 500:1 to 100:1.
Furthermore, the reduction is carried out in a slightly acidic
range, the pH usually being from 4.5 to less than 7, preferably
45 from 5 to 6. The pH is generally established by adding buffer
CA 02231~36 1998-04-02
0050/~e6258
salts, such as alkali metal acetate, in particular sodium
acetate.
In a preferred embodiment of variant (a), Blankit~ (sodium di-
5 thionite) is used as selective reducing agent. As a rule, the
amount of Blankit~ added is just sufficient for the potential of
the solution, measured by means of a glass electrode, to be from
420 to 500 mV, preferably from 440 mV to 480 mV. At the end of
the reduction of the platinum (IV) to platinum (II), which
10 observations to date have shown to be detectable from a large
change in the potential, an amount of Blankit~ over and above
that recIuired for reducing the platinum (IV) to platinum (II) is
added until a certain desired potential is reached. This poten-
tial characterizes the poisoning state of the catalyst and is
15 usually from 440 mV to 200 mV, preferably from 270 mV to 340 mV.
The mo:Lar ratio of alkali metal formate to platinum is chosen to
be in general from 1000:l to 10:1, preferably from 100:1 to 20:1.
20 The temperature during the reduction is chosen to be in general
from 50 to 95~C, preferab~ly from 60 to 90OC.
Furthe:rmore, atmospheric pressure is advantageously employed.
25 The pH after the reduction to metallic platinum depends
essentially on the type of reducing agent chosen and is usually
from S to 8, particularly preferably from S to 6.5.
After the end of the reduction, the catalyst is worked up as a
30 rule in the usual manner, for example by filtering it off from
the reaction mixture and advantageously washing it with water,
preferably until the wash water is neutral.
Observ,~tions to date have shown that the size of the platinum
35 particles prepared according to the invention is in general not
greater than 3.5 nm, obtained by determining the line width at
half height by x-ray diff-raction.
Observations to date have shown that the catalysts obtained by
40 the novel process are suitable for the hydrogenation of both
organi,- and inorganic cornpounds.
The novel catalysts are preferably used for hydrogenating
olefinically or acetylen:Lcally unsaturated compounds and for
45 hydrogenating carboxylic acids, aldehydes or ketones to the
corresjponding alcohols or nitriles to the corresponding amines.
Furthermore, the novel catalysts are suitable for hydrogenating
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inorganic substances, such as oxygen, but in particular for the
preparation of hydroxylar~onium salts by hydrogenating nitric
oxide in aqueous mineral acids.
5 In the preparation of hydroxylammonium salts, a molar ratio of
hydrogen to nitric oxide of from 1.5:1 to 6:1, preferably from
3.5 to 5:1, is as a rule maintained. Observations to date have
shown that particularly c~ood results are obtained if it is
ensured that a molar rat:io of hydrogen to nitric oxide of from
10 3.5:1 to 5:1 is maintained in the reaction zone.
Advantageously used acids are strong mineral acids, such as
nitric acid, sulfuric acid or phosphoric acid, or aliphatic
Cl-Cs-rnonocarboxylic acicls, such as formic, acetic, propionic,
15 butyric and valeric acid, preferably formic acid and acetic acid.
Acidic salts, such as ammonium bisulfate, are also suitable. As a
rule, from 4 to 6 normal aqueous acids are used, and the acid
concentration is usually not permitted to fall below 0.2 normal
in the course of the reaction.
The hydrogenation of nitric oxide is carried out in general at
from 30 to 80~C, prefera'bly from 35 to 60~C. Furthermore, the
pressure during the hydrogenation is usually chosen to be from 1
to 30, preferably from 1 to 20, bar (absolute).
The ratio of mineral acid to catalyst depends essentially on the
platinum metal and on the reactor pressure and, in the case of
platinum is in general from 1 to 100, preferably from 30 to 80, g
of platinum-graphite catalyst per liter of mineral acid.
In a further preferred embodiment, in particular in the
preparation of hydroxylammonium salts, the catalyst is treated
with h,ydrogen (activation) before the hydrogenation in acidic
solution, advantageously in the mineral acid in which the
35 hydrogenation is to be carried out.
Spent platinum metal catalysts can be regenerated with the aid of
the novel process by bringing the platinum metal of the catalyst
into solution, usually by means of an acid or an acid mixture,
40 and, if required, separating off insoluble components. The
platinum metal salt solution obtained is then neutralized, and
the p]atinum metal salt is then treated by the novel process
described above.
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Observations to date have shown that the novel catalysts are
superior to known catalysts for the same purpose, with regard to
activit:y, selectivity and catalyst life.
5 Examples
The size of the graphite particles was determined using a MALVERN
Mastersizer ~cf. also Verfahrenstechnik 24 (1990), 36 et seq.).
The Fraunhofer diffraction at a wavelength of 633 nm was
10 measured. Particle size clistribution was determined in a range
from 1 to 600 ~m by choosing an auxiliary lens having a focal
length f= 300 nm.
For the measurement, a pinch of the powders being investigated
15 was added to a liter of 0.1% strength by weight of aqueous Neka-
nil 910 solution (BASF AG; Nekanil 910 is a nonylphenol reacted
with f:rom 9 to 10 mol of ethylene oxide; properties: transparent,
viscous liquid; nonionic, density at 20~C: 1.04 g/cm3; pour point:
below -10~C; pH of a 1% strength by weight solution from 6.5 to
20 8.5). ]3efore the measurement, the resulting mixture to be
investigated was subjected to an ultrasonic treatment for 1 min.
The size of the platinum particles was determined by means of
x-ray diffraction by determining the line width at half height.
Example 1
a) 40 g of a graphite from Becker-Pennrich, having a particle
size of from 28 to 50 ~m, and 0.5310 g of hexachloroplatinic (IV)
30 acid 6-hydrate was stirr~3d overnight at 80~C with 40 ml of an
aqueous solution which contained 3.87 ml of concentrated
hydrochloric acid and 0.137 ml of concentrated nitric acid. Sodium
carbonate was added to the resulting suspension until a pH of
2.75 was reached. 2.5 g of sodium acetate was then added for
35 buffering. 4.58% strength by weight aqueous sodium dithionate
solution was then added in an amount sufficient to reduce
platinum4+ to platinum2+ (detectable from a large change in the
potential at 460 mV).
40 In order to poison the catalyst with sulfur, the same sodium
dithionite solution whicn was used for reducing the platinum4+ to
platinum2+ was added in an amount which corresponded to 60 mol% of
the amount which was used for reducing Pt4+ to Pt2+- The potential
of the solution then obt,~ined, determined by means of a glass
45 electrode, was 355 mV.
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0050/4:625~
14.1 g of a 40% strength by weight aqueous sodium formate
solution (83 mmol) were then added to the suspension thus
obtained, and stirring was carried out for 4 hours at 80~C. After
this time, the platinum was no longer detectable with hydrazine
5 hydrate (gives a black precipitate in alkaline solution in the
presence of platinum).
The calalyst thus prepared was isolated from the reaction mixture
by fill:ration through a glass frit and was washed with distilled
10 water until the pH of the wash water was no longer in the acidic
range. The dried catalyst contained 0.5% by weight of platinum.
b) 4.8 g of the catalyst prepared under a) were suspended in
120 ml of 4.3 N sulfuric acid, and 7.75 l/h of a mixture of 35%
15 by volume of nitric oxide and 65% by volume of hydrogen were
passed in at 40~C with vigorous stirring (3500 rpm). After 4
hours, the catalyst was separated off and the liquid phase
analyzed. Thereafter, 120 ml of 4.3 N sulfuric acid were added to
the catalyst separated off, and the reaction was continued. This
20 proces,3 was repeated every four hours. The reaction was
terminated after the selectivity with respect to nitrous oxide
formed exceeded the set upper limit of 10%. The experimental
result,s are shown in the table below.
25 Comparative Example 1
The procedure was as in E~xample 1, except that poisoning was
carried out using 56 mol9D, based on the amount which was used for
the reduction to platinunn (II), of sodium dithionite, and 6.25 ml
30 of con,-entrated formic acid were used for the precipitation to
give the zero-valent plal:inum. The results obtained are shown in
the table below.
Example 2
a) 40 g of a graphite from Becker-Pennrich, having a particle
size of from 28 to 50 ~m, and 0.5310 g of hexachloroplatinic
(IV) acid 6-hydrate was stirred overnight at 80~C with 40 ml
of an aqueous solution which contained 3.87 ml of
concentrated hydrochloric acid and 0.87 ml of concentrated
nitric acid. Sodium carbonate was added to the resulting
suspension until a pH of 2.75 was reached. 2.5 g of sodium
acetate were then added for buffering. Thereafter, 6.25 mg of
elemental sulfur were added and, after a waiting time of 2
minutes, 14.1 g of a 40% strength by weight aqueous sodium
formate solution (83 mmol~ were added to the resulting
suspension and stirring was carried out for ~ hours at 80~C.
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0050/~6258
After this time, the platinum was no longer detectable with
hydrazine hydrate (gives a black precipitate in alkaline
solution in the presence of platinum).
The catalyst thus prepared was isolated from the reaction
mixture by filtration through a glass frit and was washed
with distilled water until the pH of the wash water was no
longer in the acidic range. The dried catalyst contained 0.5%
by weight of platinum.
b) The catalyst was tested as described under lb).
Comparative Example 2 - similar to DE-A 43 11 420
15 The procedure was as in Example 2, except that 6.25 mg of
elemental sulfur were used for poisoning, and 6.25 ml of
concentrated formic acid were used for the precipitation to give
the zero-valent platinum. The results obtained are shown in the
table below.
Example 1Comparison 1.
Number of cycles 11 4
Seleclivity NH2OH [%] 90.68 85.53
25 SeleclivitY NH3 [%] 4.96 6.36
Seleclivity N2O [%] 4.38 8.11
NO-conversion [%] 91.91 89.88
Space - time yleld 0.842 0.774
30 Pt-particle size [nm] 2.9 4.0
Example 2 Comparison 2.
Number of cycles 11 3
Selec~ivity NH2OH [%] 86.53 81.76
35 Selec!ivity NH3 [%] 8.24 12.04
SeleclivitY N2O [%] 5.23 6.20
NO-conversion [%] 90.68 90.75
Space - time yield 0.793 0.749
40 Pt-pa:cticle size [nm] 3.5 4.0
