Note: Descriptions are shown in the official language in which they were submitted.
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Process for the catalytic hydrogenation of aromatic nitro compounds
The present invention relates to a process for the hydrogenation of
unsubstituted or
substituted aromatic nitro compounds with hydrogen in the presence of known
hydrogenation catalysts, typically Rh, Ru, Pt, Pd, Ir, Ni or Co, at which
hydrogenation
catalytic amounts of at least one vanadium compound must be present. The
invention also
relates to the use of vanadium compounds in the catalytic hydrogenation of
aromatic nitro
compounds with hydrogen in the presence of known hydrogenation catalysts.
The catalytic hydrogenation of aromatic nitro compounds is a reaction which is
industrially
important, for example for the preparation of intermediates for agrochemicals,
dyes and
fluorescent whitening agents. For the preparation of stilbene fluorescent
whitening agents,
for example, 4,4'-dinitrostilbene-2,2'-disulfonic acid has to be reduced to
4,4'-diamino-
stilbene-2,2'-disulfonic acid, which may be achieved by classical reduction
processes or by
catalytic hydrogenation. The preparation of azo dyes requires large amounts of
diazonium
salts which in turn are prepared from the corresponding amines.
Catalytic hydrogenations of aromatic nitro compounds to the corresponding
aromatic
amines proceed via several intermediary stages. Important among these are the
corresponding nitroso compounds and, in particular, the hydroxylamine
intermediate, as is
described, inter alia, by M. Freifelder in Handbook of Practical Catalytic
Hydrogenation,
Verlag Wiley-Interscience, New York, 1971.
This hydroxylamine intermediate poses a special problem in practice, because
under
specific conditions it can accumulate in large amounts in reaction solutions.
This applies in
particular to aromatic nitro compounds, the hydrogenation of which results in
relatively
stable arylhydroxylamines. This is particularly critical when the
hydrogenation is carried out
in a slurry batch reactor. In the extreme case, several tons of
arylhydroxylamine can thus be
formed.
Arylhydroxylamines are in many respects problematical. For one thing it is
known that such
compounds are often thermally instable and can disproportionate during heating
with or
without H2 with strong emission of heat. The liberated heat can trigger
further decompo-
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sition reactions which can then result in incidents with heavy explosions. W.
R. Tong et al,
AICHE Loss Prev. 1977, (11), 71-75 describe such an incident during the
reduction of 3,4-
dichloronitrobenzene to 3,4-dichloroaniline.
This instability makes a thorough and elaborate thermal examination of
hydrogenation
mixtures imperative. In particular, the thermal behaviour of the possible
hydroxylamine
intermediates must be thoroughly examined. F. Stoessel, J. Loss Prev. Process
Ind., 1993,
Vol 6, No 2, 79-85 describes this procedure, using the hydrogenation of
nitrobenzene to
aniline as an example.
Arylhydroxylamines are also known as strong carcinogens and therefore
constitute a high
hazard potential in the case of interrupted or incomplete hydrogenation (J. A.
Miller, Cancer
Res. 3 (1970),559).
The preparation of a pure amine constitutes a third complex of problems. If,
during the
hydrogenation or at the end of the reaction, significant amounts of
arylhydroxylamine are
present, then this may lead to condensations with formation of unwanted and
dyed azo or
azoxy products. Since the amount of arylhydroxylamine can change from batch to
batch,
the resulting product quality differs in purity and aspect.
The problems indicated above are further aggravated by the fact that the
resulting
concentrations or even the maximum possible concentrations of this
hydroxylamine
intermediate cannot be predicted even in processes which are known and well-
studied. The
presence of impurities in the trace range can trigger the spontaneous
accumulation of
hydroxylamine intermediates in unpredictable manner. In, for example,
Catalysis of Organic
Reactions, Vol 18, (1988), 135, J.R.Kosak relates that the simple addition of
1% of NaNO3
increases the accumulation during the hydrogenation of 3,4-
dichloronitrobenzene from the
initial < 5% to about 30%.
To solve these problems, different processes have been proposed in the prior
art.
DE-OS-25 19 838, for example, discloses a continuous process for the catalytic
hydro-
genation =
of nitro compounds to the corresponding amino compounds in which the catalyst
particles of 0.5 to 3 mm are arranged in a fixed bed and the nitro compounds
are carried in
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the trickling phase. The catalyst is preferably applied to a carrier,
typically aluminium oxide
or silicic acid.
A similar continuous process is disclosed in DE-OS-22 14 056. In this process
the nitro
compound is also lead over the fixed catalyst. Said catalyst consists of
aluminium spinel as
carrier on which palladium and vanadium or vanadium compounds are fixed.
DE-OS 28 49 002 discloses a process for the hydrogenation of nitrobenzene in
the vapour
phase in a continuous process in the presence of a multicomponent carrier
catalyst
comprising 1-20g of a noble metal and 1-20g of, for example, vanadium or of a
vanadium
compound per litre of carrier material.
For the reduction of aromatic nitro compounds in a batch reaction, US-A-4 212
824
proposes the use of an iron-modified platinum catalyst for the hydrogenation.
In practice,
however, this iron-modified platinum catalyst cannot entirely satisfy. In many
cases the
formation of hydroxylamine is, on the one hand, not completely prevented but,
on the other
hand, the rate of hydrogenation can be markedly slowed down.
These proposals of the prior art all have in common that the actual catalyst
is modified
such, and its activity is thereby adjusted such, that no great amount of
accumulation of
hydroxylamine can occur, in particular in the continuous process. In the
continuous
processes this is in any case substantially less critical than in batch
processes because
continuous processes have a substantially lower amount of educt and product in
the actual
reaction volume. On the other hand, continuous processes are only economical
in the case
of products with large tonnages so that there is still a desire for an easily
controllable
reaction, essentially without hydroxylamine accumulation. This is particularly
important with
respect to batch reactions.
Furthermore, the preparation of the above-described fixed bed catalysts
involves a great
= amount of expenditure and is complicated, which also reduces the economy of
such
operational processes.
Surprisingly, it has now been found that in the catalytic hydrogenation of
aromatic nitro
compounds the accumulation of hydroxylamines can be almost completely
prevented by the
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addition of catalytic amounts of vanadium compounds, which usually results in
concen-
trations of < 1% of hydroxylamine. This result can be achieved with any
commercially obtainable hydrogenation catalyst. A
special pretreatment or modification of the catalyst, as known from the prior
art, is not
necessary.
The resulting hydrogenated products are whiter (purer) than those obtained
without the
addition of the vanadium compound because almost no azo or azoxy compounds are
obtained. The hydrogenation, in particular the final phase, proceeds faster
than without said
addition. Accordingly, substantial advantages result with respect to quality
constancy and
economy.
Compared to the prior art, this invention has the substantial advantage that
catalytic
amounts of a vanadium compound can be easily dissolved or dispersed in the
reaction
medium, affording excellent hydrogenation results.
One object of the invention is a process for the catalytic hydrogenation of
aromatic nitro
compounds in solution or in melt in the presence of hydrogen and at least one
noble metal
catalyst, nickel catalyst or cobalt catalyst, in which process a catalytic
amount of at least
one vanadium compound is present, wherein the vanadium has the oxidation state
0, II, III,
IV or V.
A preferred process is that wherein the vanadium compound is dissolved or
dispersed in
catalytic amounts in the reaction medium; preferably it is dissolved.
Another likewise preferred process is obtained when the vanadium compound is
mixed with
the catalyst or is applied thereto.
It is also preferred to apply the vanadium compound first to a suitable
carrier and then to
disperse it in this form in the reaction medium.
Suitable carrier materials are, for example, all those used for the
preparation of commercial
hydrogenation catalysts in powdered form, such as those indicated below.
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Application to the catalyst or to the carrier material is carried out in
simple manner, typically
by dissolving the vanadium compounds, suspending the catalyst or the carrier
material in
the solution and subsequent filtration.
If the vanadium compounds are not soluble in the reaction medium, then they
can also be
mixed in disperse slurried form with the slurried catalyst and filtered
together.
Suitable vanadium compounds of the oxidation state 0, 11, III, IV or V are
elemental
vanadium as well as purely inorganic compounds, but organic complexes with,
for example,
oxalate or acetylacetonate are also possible.
Preferred vanadium compounds are V205 or those which constitute a purely
inorganic salt,
oxo salt or the hydrate of a purely inorganic salt or oxo salt. Typical
examples are VOCI3,
VCIs ,[VO(SCN)4 ]2-, VOSOa, NH4VO3 , VCI3 , VCI2 or the corresponding halides
with F or
Br. The compounds are obtained in aqueous solution in different hydrate forms,
depending
on the pH (F. A. Cotton, G. Wilkinson, Anorganische Chemie, Veriag Chemie
Weinheim
1968, 2nd edition, pages 757-766).
Particularly preferred vanadates or hydrates of vanadates are those of
oxidation state V.
The ammonium, lithium, sodium or potassium vanadates, or a hydrate of these
vanadates,
are very particularly preferred.
It is preferred to use the vanadium compound in an amount of 1 - 2000ppm,
particularly
preferably in an amount of 5 - 500ppm, based on the aromatic nitro compound to
be
hydrogenated.
The weight ratio of vanadium compound to catalyst is preferably from 1:1 to
1:10 000,
particularly preferably from 1:10 to 1:1000 and, very particularly preferably,
from 1:50 to
1:750.
The aromatic nitro compounds can be substituted by any groups that are inert
during the
= hydrogenation or also by further groups which can be hydrogenated, e.g.
olefinic groups. A
concomitant hydrogenation of all groups may sometimes be desired.
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The aromatic nitro compounds can comprise one or more than one nitro group.
Some examples of aromatic nitro compounds are aromatic hydrocarbons, typically
benzenes, polycyclic hydrocarbons (also partially hydrogenated ones such as
tetralin),
biphenyls, cyclopentadienyl anion and cycloheptatrienyl anion,
heteroaromatics, typically
pyridines, pyrroles, azoles, diazines, triazines, triazoles, furans,
thiophenes and oxazoles,
condensed aromates, typically naphthalene, anthracene, indoles, quinolines,
isoquinolines,
carbazoles, purines, phtalazines, benzotriazoles, benzofurans, cinnolines,
quinazoles,
acridines and benzothiophenes. Said compounds will also be understood to
include
conjugated aromatic systems such as stilbenes or cyanines under the condition
that the
nitro group is bonded to the aromatic part of the conjugated aromatic system.
A preferred subgroup is formed by aromatic nitro compounds, wherein the
aromatic radical
is substituted by electrophilic groups.
Electrophilic groups are typically halogen, sulfonic acid radicals and their
derivatives,
carboxylic acid radicals or their derivatives, such as ester, acid chloride or
nitriles.
Halogen is fluoro, chloro, bromo or iodo. Fluoro, chloro or bromo are
preferred.
Preferred electrophilic groups are halogen, -SO3M, -COX, wherein M is hydrogen
or an
alkali metal and X is halogen or O-C,-C,Zalkyl.
C1-C12AIkyI can be methyl, ethyl, isopropyl, n-propyl, n-butyl, isobutyl, sec-
butyl, tert-butyl as
well as the different isomeric pentyl, hexyl, heptyl, octyl, nonyl, decyl,
undecyl and dodecyl
radicals.
Very particularly preferred is the aromatic nitro compound 4,4'-
dinitrostilbene-2,2'-disulfonic
acid or a compound of formula II, 111 or IV
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NOz NOz
S ~CH NOz CI ~ ~ O CI
N 3
J \ I NO
z
(II), C~ (III) CI (IV).
In principle, the process is suitable for all reductions of aromatic nitro
groups to aromatic
amines carried out on a large industrial scale. Typical examples are
intermediates for
agrochemicals, fluorescent whitening agents and dyes.
The process of this invention is particularly suitable for the preparation of
aromatic amino
compounds, such as those disclosed, inter alia, in EP-A-42357, which are used
for the
preparation of diazonium salts in the synthesis of azo dyes.
The reaction can be carried out in solution in a suitable solvent which is
inert during the
reaction, but can also be carried out in the melt of the educt.
Suitable solvents are typically water, alcohols, such as methanol, ethanol, n-
propanol,
isopropanol, n-butanol, the isomeric butanols and cyclohexanol, ethers, esters
and ketones,
typically diethyl ether, methyl-tert-butyl ether, tetrahydrofuran, dioxane,
dimethoxyethane,
ethyl acetate, butyl acetate, butyrolactone, acetone, methyl ethyl ketone,
methyl-isobutyl
ketone or cyclohexanone, carboxylic acids, typically acetic acid and propionic
acid,
dipolar/aprotic solvents, such as dimethyl formamide, N-methylpyrrolidone,
dimethyl-
acetamide, sulfolane, dimethyl sulfoxide or acetonitrile, apolar solvents,
typically toluene or
xylene, chlorinated aromatic hydrocarbons, typically methylene chloride, C3-
CAlkane or
cyclohexane.
These solvents can be used in pure form or in the form of mixtures.
= The noble metal catalyst can contain rhodium, ruthenium, iridium, palladium
or platinum as
noble metals. Nickel catalysts or cobalt catalysts are also suitable.
The nickel catalyst can be, for example, Raney nickel.
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In a preferred embodiment of this invention, the noble metal catalyst is
platinum, palladium,
iridium, rhodium or ruthenium in metallic or oxidised form which is applied to
a carrier. The
metallic form is particularly preferred.
Platinum or palladium are very particularly preferred.
Particularly suitable carriers are activated carbon, silicic acid, silica gel,
aluminium oxide,
calcium carbonate, calcium phosphate, calcium sulfate, barium sulfate,
titanium oxide,
magnesium oxide, iron oxide, lead oxide, lead sulfate or lead carbonate.
Activated carbon,
silica gel, aluminium oxide or calcium carbonate are very particularly
suitable.
It is preferred to use the noble metal catalyst in an amount of 0.1 to 5 % by
weight, based
on the aromatic nitro compound.
The process is preferably carried out at a pressure of 1=105-2=10' pascal.
The process is preferably carried out in the temperature range of 0 - 300 C,
particularly
preferably of 20 - 200 C.
The process can be carried out as a batch or continuous process. The batch
process is
preferred.
The invention also relates to the use of vanadium compounds according to claim
1 for the
catalytic hydrogenation of aromatic nitro compounds in solution or in melt in
the presence of
hydrogen and at least one noble metal catalyst, nickel catalyst or cobalt
catalyst.
The following Examples illustrate the invention in more detail. The reaction
rates were
determined by NMR spectroscopy and the percentages are by weight.
Example 1
A 300m1 autoclave equipped with a sparger is charged, under pressure, with 77g
of the
compound of formula II
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NOZ
S\
N~CH3
(II).
110.5 ml of tetrahydrofuran absolute (Merck p.a.), 530mg of 5% Pd/C (Johnson
Matthey
87L) and 19.4mg of NH4VO3 are then added. The air in the autoclave is replaced
with N2
and the reaction mixture is heated to 120 C. At 120 C, N2 is replaced with
H2 (20bar) and
the sparger is started.
After a reaction time of 120 minutes, 100% of the amino compound are obtained
and 0% of
hydroxylamine. Throughout the reaction no formation of hydroxylamine can be
detected.
Comparison Example 1a
Example 1 is repeated, but without the addition of NH4VO3.
After a reaction time of 150 minutes, 84% of the amino compound and 16% of
hydroxyl-
amine are obtained. The maximum concentration of hydroxylamine during the
reaction is
41%.
Example 2. Preparation of aniline-2-sulfonic acid-(N-cyclohexvl-N-methyl amide
NOZ
SOZN + 3 H2 (1.1 bar), MeOH NHZ
/
I CH3
modifier, 40 -50 C, 5% Pd/C SOZN ~'\ CH3
A 500 mi shaker flask is charged with 13.0 g of nitrobenzene-2-sulfonic acid-
(N-cyclohexyl-
N-methyl)amide, 130 g of methanol, 0.895 g of 5% Pd/C and vanadium modifier
(Table 1).
The shaker flask is evacuated 3 times and flushed with hydrogen. The
temperature is
elevated to 40 - 50 C and the reaction is started (1.1 bar of hydrogen).
During the reaction,
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4- 5 samples are taken to check the reaction. These samples as well as the
reaction
product are analysed with 1 H-NMR. The results listed in Table 1 are obtained.
Table 1
Exp. Modifier Amount tR * [Amine] [Hydroxyl- Hydroxyl-
acc. to tR amine] acc. amine
to tR max
2a - - 275min 90% 10% 33%
2b NH4VO3 1.9mg 110min 100% 0% 10%
2c VOSO4=5H2O 4.1 mg 77 min 100 % 0% 14 %
2d 3% V205 / Si02 30 mg 89 min 95 % 5% 22 %
2e NH4VO3/C 2.5mg 110min 100% 0% 8%
* tR = reaction time
** NH4VO3 deposited on activated carbon
Example 3. Preparation of 3-amino-4-chloro-acetanilide
ci
ci ci
NOZ
6 H2 (18 bar), MeOH NH2 AC20 NHZ
modifier, 60 C, 1% PUC
NO2
NHZ NHCOCH3
Example 3a. A hydrogenation reactor is charged with 15 parts of sodium
acetate, 60 parts
of NaHCO3, 1320 parts of MeOH and 1015 parts of 1-chloro-2,4-
dinitrochlorobenzene under
nitrogen at 50 C and then 11 parts of 1% Pt/C, 0.15 parts of NH4VO3 and 66
parts of water
are added.The hydrogenation is carried out at 60 C and 18 bar. The product is
isolated as
3-amino-4-chloroacetanilide (785 parts, 85% of theory).
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Example 3b. A 0.3 I Hastalloy B autoclave is charged with 40.8g of 1-chloro-
2,4-dinitro-
chlorobenzene, 120 ml of methanol and 0.21 g of 5% Pt/C catalyst. The mixture
is flushed
with nitrogen and then hydrogenated with hydrogen at 60 C and 10 bar. The
selectivity with
respect to dehalogenation is 66%.
~
Example 4. Preparation of 2.4.4'-trichloro-2'-aminodiphenyi ether (TADE)
Ci NOZ
CI NNZ
O
:::: r),M60 C, \
TNDE 1% Pt and 0.1 % Cu/C CI ci
TADE
A 2 1 steel autoclave is charged with 330 g of 2.4,4'-trichloro-2'-
nitrodiphenyl ether, 330 g of
MeOH, 2.8 g of 1% Pt + 0.1 % Cu/C. The autoclave is closed and flushed with
nitrogen. The
hydrogenation is carried out at hydrogen pressure of 12 bar and at 60 C. After
consumption
of specific percentages of the calculated amount of hydrogen, the
hydrogenation is
interrupted and a sample is taken from the reaction mixture. The sample is
heated in the
DSC temperature-programmed at 4 C/min and the liberated energy of
decomposition is
measured. The disproportionation of the arylhydroxylamine is given a thermal
signal which
is already visible at <100 C. The decomposition of the nitro compound still
present in the
reaction mixture (rm) starts at > 200 C. The results are listed in Table 2.
Table 2
Exp. Modifier H2 Energy of Energy of
Consump- decomposition decomposition
tion < 100 C > 200 C
[% of [kJ/kg rm] [kJ/kg rm]
theory]
4a none 75% -127 -1423
4b 110 mg NH4VO3 60% 0 -1273
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The risk that a spontaneous decomposition of the accumulated arylhydroxylamine
will
trigger the decomposition of the nitro compound can be remarkably reduced.
Example 5. Preparation of sodium 4,4'-diaminostilbene-2,2'-disulfonate (DASI
NHZ
NOZ
SO3Na
S03Na 6 H2 (1-5 bar)
PUC, H20
(pH 6.5 - 7.0) SONa
SONa 1.5 bar H2 DAS
NHZ
NOZ DNS
SO3Na
+ CHO
j::
NHZ
ABAS
A 300 mi steel autoclave is charged with 48 g of sodium 4,4'-dinitrostilbene-
2,2'-disulfonate,
174 g of water, 0.15 ml of 0.5 M H2SO4, 1.4 g of activated carbon, 64 mg of 5%
Pt/C and
12 mg of NH4VO3. The autoclave is closed and flushed with nitrogen. The
hydrogenation is
carried out at 70 C with the controlled addition of hydrogen of 2.5 NI/h (max.
4-5 bar
hydrogen). After the hydrogenation is terminated, the autoclave is rendered
inert, the
catalyst is filtered off and the reaction mixture is analysed with HPLC. The
results are listed
in Table 3.
Table 3
Exp. NH4VO3 Carbon Content ABAS Azo and Unknown
DAS azoxy
compounds
5a - - 94.9% 3.5% 0.3% 1.3 %
5b - 1.44 g 93.8% 4.1 % 0.4% 1.7 %
5c 12 mg - 94.7% 3.9% 0.3% 1.1 %
5d 12 mg 1.44 g 98.3% 1.0% 0.4% 0.3%
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The selectivity of the hydrogenation of DNS is highly dependent on the
availability of the
hydrogen on the surface of the catalyst. The hydrogenation is therefore
preferably carried
out under high pressure in well-gassifying reactors. In accordance with the
described
process it is possible to carry out the hydrogenation at a low H2 partial
pressure and still
obtain good product quality. The hydrogenated product is an intermediate for
the
preparation of fluorescent whitening agents. The rate of reaction and
therefore also the
heat flow resulting from the hydrogenation can thus be controlled via the H2
dosage.
a
