Selective hydrogenation. II. m-Dinitrobenzene to m-nitroaniliine using palladium on carbon as catalyst

Article abstract of DOI:10.1021/op990039s

In the present work m-dinitrobenzene (m-DNB) was hydrogenated to m-nitroaniline (m-NA) using palladium on carbon as catalyst. The reaction conditions were standardized to obtain m-NA with high selectivity. It was possible to obtain m-NA with 95% selectivity at 90% conversion and formation of m-phenylenediamine (m-PD) was restricted using suitable conditions. Kinetic studies of the reaction at different initial concentrations of substrate, catalyst loading, temperature, as well as pressure, were studied.

Full text of DOI:10.1021/op990039s

Organic Process Research & Development 2001, 5, 263−266
Selective Hydrogenation. II. m-Dinitrobenzene To m-Nitroaniliine Using
Palladium on Carbon As Catalyst
Veena L. Khilnani and S. B. Chandalia*
UniVersity Department of Chemical Technology, UniVersity of Mumbai, Matunga (CR), Mumbai-400 019, India
Abstract:
to m-NA using noble metal catalysts. The use of noble metal
catalysts may be more desirable for obtaining high selectivity.
However, the crucial aspect appears to be not only the choice
of catalyst system but also the manipulation of the reaction
conditions and the selection of an appropriate solvent9 for
obtaining m-NA selectively.
In the current work m-DNB was hydrogenated to m-NA
under suitable conditions using Pd/C as catalyst so that high
selectivity at comparatively higher conversions could be
obtained from viewpoint of process, research, and develop-
ment.
In the present work m-dinitrobenzene (m-DNB) was hydroge-
nated to m-nitroaniline (m-NA) using palladium on carbon as
catalyst. The reaction conditions were standardized to obtain
m-NA with high selectivity. It was possible to obtain m-NA with
95% selectivity at 90% conversion and formation of m-
phenylenediamine (m-PD) was restricted using suitable condi-
tions. Kinetic studies of the reaction at different initial concen-
trations of substrate, catalyst loading, temperature, as well as
pressure, were studied.
Introduction
Experimental Section
m-Nitroaniline (m-NA) is used extensively as a dye-
intermediate in azo and leuco dyes, as a corrosion inhibitor
in the mechanical and electronic industry, in polymers for
urethane synthesis, and also to a limited extent in pharma-
ceuticals.
Materials. m-Dinitrobenzene, acetone, ethanol (procured
from S.D. Fine Chemicals, Bombay, India), and 5% Pd/C
(from Arora Mathews, Calcutta, India) were used for the
above study.
Experimental Set-Up. The hydrogenation of m-DNB was
carried out in the reaction set-up as described in the previous
paper.3
Experimental Procedure. Predetermined quantity of
m-DNB in the suitable solvent and the catalyst were placed
in the autoclave which was repeatedly purged with hydrogen.
Subsequently, the reaction was carried out as reported in the
previous paper.3
Analysis. The reaction was monitored by analyzing the
samples withdrawn at regular intervals by gas chromatog-
raphy. The analysis conditions used were as follows:
column: S.S., 3.2 mm i.d., 4 m length; stationary phase, 10%
OV-17 on Chromosorb-W; carrier gas, nitrogen; flow rate,
30 mL/min.; detector, F.I.D.; oven temperature, 250 °C
isothermal; injector temperature, 300 °C; detector tempera-
ture, 300 °C.
Several chemical processes for the preparation of m-NA
are known. The most common methods are based on the
use of sulphides1 and metal acid.2
As mentioned in the previous paper,3 these methods of
reduction are hazardous to the environment and require high
cost for waste disposal, and work-up of the reaction mixture
is cumbersome. Therefore, catalytic hydrogenation should
be the method of choice for the preparation of m-NA from
m-dinitrobenzene (m-DNB), provided high selectivities with
respect to m-NA could be realized.
The catalytic hydrogenation of m-DNB leads to the
formation of m-phenylenediamine (m-PD). However, for the
selective hydrogenation of m-DNB to m-NA, modification
of the catalyst system or addition of regulator or proper
selection of reaction conditions is required.
Raney copper,4 Raney nickel,5 and Ni-Al6 catalyst are
used for the selective hydrogenation of substituted m-DNBs.
Various supports for the noble metal catalyst systems have
been employed for the selective hydrogenation of m-DNB
to m-NA.
Another approach is to introduce additives7,8 in the
reaction mixture for the selective hydrogenation of m-DNB
Results and Discussions
Effect of Speed of Agitation. The speed of agitation was
studied in the range of 500-1500 rpm. It had negligible
effect on overall conversion of m-DNB in the range of 1000-
1500 rpm. With the mass-transfer effects being unimportant
at speeds above 1000 rpm, all further experiments were
carried out at 1080 rpm (Table 1). However, it may be
emphasized that, depending on the size of the commercial
reactor, the speed of agitator, and the number of impellers
(1) Groggins, P. H. Unit Process in Organic Synthesis, 5th ed.; McGraw-Hill:
New York, 1958.
(2) Wulfmann D. S.; Copper, C. F. Synthesis 1978, 924.
(3) Khilnani, V.; Chandalia, S. B. Org. Process Res. DeV. 2001, 5, 257-262.
(4) Jones, W. H.; Benning, W. F.; Davis, P. Ann. N.Y. Acad. Sci. 1969, 158(2),
471.
(5) Ono, A.; Terasakai, S.; Tsuruoka,Y. Chem. Ind. 1983, 12, 477.
(6) Masenova, A.T.; Bizhanov, F. B. IzV. Akad. Nauk Kaz. SSR, Ser. Khim.
1993, 2, 32; Chem. Abstr. 1993, 118, 149777w.
(8) Theoridis, G. U.S. Patent 5,105,012, 1992; Chem. Abstr. 1992, 117,
899451s.
(7) Shmomina, V. P.; Moldabaev, U. Khim. Khim. Tekhnol. 1968, 78, 85. Chem.
Abstr. 1971, 74, 22347y.
(9) Belous, L. P.; Rogovik, V. M. Katal. Katal. 1977, 15, 51; Chem. Abstr.
1978, 89, 6033x.
10.1021/op990039s CCC: $20.00 © 2001 American Chemical Society and The Royal Society of Chemistry
Published on Web 03/22/2001
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Table 1. Effect of speed of agitationa
no
rpm
% overall conversion
1
2
3
500
1080
1500
9.5
31.2
32.2
a m-DNB, 0.8929 gmol/L; solvent, acetone; catalyst, 5% Pd/C; catalyst
loading, 1.0% g/L; temperature, 31 °C; pressure, 34 atm; reaction volume, 500
mL; reaction time, 90 min.; catalyst, 5% Pd/C; pressure, 34 atm; speed of
agitation, 1080 rpm; reaction volume, 500 mL.
Figure 2. Effect of catalyst concentration. m-DNB, 0.8929
gmol/L; solvent, acetone; catalyst, 5% Pd/C; pressure, 34 atm;
speed of agitation, 1080 rpm; reaction volume, 500 mL.
Preliminary Studies. Preliminary studies were carried
out on the hydrogenation of m-DNB to m-NA using 5% Pd/C
with ethanol as the solvent. The catalyst concentration was
kept low (0.01% w/v), and temperature was varied from 30
to 90 °C, but no reaction took place. However, at 90 °C
with a much higher catalyst loading (0.1% w/v), the reaction
proceeded, but more of m-PD was obtained rather than
desired m-NA. Lowering the temperature lowered the m-PD
formation and enhanced the selectivity with respect to m-NA
although the conversion obtained was much lower for the
given period of reaction. At a catalyst loading of 0.1% w/v
and the temperature of 31 °C the reaction was carried out
using acetone as solvent. The selectivity was comparable,
but the reaction rate was very high, since for the same period
of reaction 90% overall conversion was obtained with
acetone as compared to 25% overall conversion obtained with
ethanol as a solvent (see Table 2). A detailed and systematic
investigation is required before a proper explanation can be
given about the effect of type of solvent on the reaction rate.
Effect of Temperature. As expected, with the rise in
temperature, the reaction rate increased markedly. When the
temperature was increased from 20° to 40 °C, the conversion
of m-DNB increased from 7 to 93% in 1 h, and selectivity
decreased marginally (Figure 4). Therefore, further reactions
were carried out at 31 °C.
Figure 1. Effect of catalyst loading on initial rates of reaction.
m-DNB, 0.8929 gmol/L; solvent, acetone; catalyst, 5% Pd/C;
pressure, 34 atm; speed of agitation, 1080 rpm; reaction volume,
500 mL.
used, and the catalyst loading, the mass-transfer effects may
play a significant role, and this needs to be assessed during
scale-up.
Effect of Catalyst Loading. The catalyst loading ex-
pressed as weight percent of catalyst based on the total
reaction volume was varied from 0.5 to 2.0% g/L (Figure
1). As the catalyst loading was increased, the reaction rates
increased significantly. In view of the observations that the
rate of reaction is independent of the speed of agitation
(above 1000 rpm) and that it varies linearly with the catalyst
loading (Figure 2), it appears that the overall reaction is
controlled by reaction on the surface of the catalyst. For the
conditions employed, the role of diffusion of hydrogen from
the bulk liquid to the solid is estimated to be unimportant,
and the data appears to represent the true kinetics of the
process.
Effect of Partial Pressure of Hydrogen. The effect of
hydrogen pressure was studied in the range of 10-34 atm.
As seen in the Figure 3, there was marginal increase in the
conversion to the product, but with a further increase in the
pressure to 34 atm, the overall conversion of m-DNB was
There was a significant decrease in selectivity at high
catalyst loading mainly because of relatively high levels of
overall conversion due to the high reaction rate.
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Table 2. Effect of solventa
catalyst
%overall
% selectivity wrt.
concentration
conversion
(%w/v)
solvent temp. (°C) of m-DNB m- NA m-PD
0.01
ethanol
30
50
70
90
90
31
31
-
-
-
-
-
-
-
-
97.5
25.0
90.0
-
-
58.5
84.1
86.0
-
-
41.5
15.9
14.0
0.1
0.1
ethanol
acetone
a m-DNB ) m-dinitrobenzene. m-NA ) m-nitroaniline. m-PD ) m-phen-
ylenediamine. m-DNB, 0.2976 gmol/L; pressure, 34 atm; reaction volume, 500
mL; speed of agitation, 1080 rpm.
Figure 4. Effect of temperature. m-DNB, 0.8929 gmol/L;
catalyst, 5% Pd/C; catalyst loading, 0.1% w/v; pressure, 34 atm;
speed of agitation, 1080 rpm; solvent, acetone; reaction volume,
500 mL.
Figure 3. Effect of pressure. m-DNB, 0.8929 gmol/L; catalyst,
5% Pd/C; catalyst loading, 0.1% w/v; speed of agitation, 1080
rpm; solvent, acetone; reaction volume, 500 mL.
almost unaffected. Thus, in the pressure range of 10-34 atm,
the reaction is almost independent of partial pressure of
hydrogen.
Effect of Substrate Concentration and Period of
Reaction. The effect of period of reaction for the hydrogena-
tion of m-DNB in acetone as the medium using different
catalyst loading, different pressures and different tempera-
tures as well as different initial concentrations of m-DNB
was studied. These are presented in Figures 1 and 3-5. In
most of these cases, the selectivity obtained with respect to
m-NA was above 90% indicating that all of these variables
do not affect the selectivity in a marked way.
The kinetic data in most of the cases also indicated that
with increase in the period of reaction, the rate of reaction
increased substantially, presumably because the decrease in
the concentration of m-DNB enhances the reaction rates. The
Figure 5. Effect of substrate concentration. Catalyst, 5% Pd/
C; catalyst, loading, 0.1% w/v; temperature, 31 °C; pressure,
34 atm; speed of agitation, 1080 rpm; solvent, acetone; reaction
volume, 500 mL.
same observation was made when the results with different
initial concentrations of m-DNB were compared at various
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Table 3. Effect of catalyst reusuabilitya
The salts maybe neutralized to obtain free amines, and
these may be isolated and separated by fractional distillation
under high vacuum The difference in the boiling points of
m-nitroaniline and m-phenylenediamine is 20 °C at 10
mmHg.
Scale-Up Aspects. Under the conditions recommended
for the process, the kinetic data represent the intrinsic kinetics
of the process. Hence, the scale-up should pose no problem.
While designing the industrial equipment, it should be
ascertained that the mass-transfer effects do not play any
significant role so that the selectivity would not change. This
could be done by proper scale-up and design of the agitator
system.
It may be mentioned that on industrial scale, when the
diameter of the impeller is large, the speed of the agitation
would be correspondingly low. The scale-up is based on
equal power consumption per unit volume of the reactor.
The choice of the batch versus continuous process would
be governed by the capacity of the proposed plant.
% selectivity wrt
% overall conversion
no catalyst time (h)
of m-DNB
m-NA
m-PD
1
2
fresh
used
2.5
5.0
93.7
13.5
96.4
100
3.6
-
a m-DNB, 0.8929 gmol/L; catalyst 5% Pd/C; catalyst loading 1.0% g/L;
temperature, 31 °C; pressure, 34 atm; speed of agitation, 1080 rpm; reaction
time, 4h.
levels of conversion (at different time intervals). This is
contrary to usual expectations and could be explained if
kinetic data are correlated by a suitable rate equation based
on Langmuir-Hinshelwood mechanism because the present
case is that of solid-catalyzed liquid-phase reaction. A
detailed work in this direction was not carried out, and hence,
a suitable rate expression is not proposed.
Reusability of the Catalyst. The catalyst was reused after
decanting and washing with fresh solvent. Results in Table
3 show that the activity of catalyst decreased considerably,
while there was a marginal enhancement in selectivity. The
finding that the selectivity could be marginally better for
reused catalyst is commercially not attractive. Since the
conversion obtained is drastically reduced even when the
reaction period is increased by a factor of 2 in the case of
reused catalyst.
Conclusions
(1) m-DNB gave 90% overall conversion at 95% selectiv-
ity with respect to m-NA in 2.5 h.
(2) It higher catalyst loading, selectivity with respect to
m-NA decreases, and therefore lower catalyst loading (0.1%
w/v) was used.
(3) Pressure had no effect on the reaction rate, in the range
of 10-34 atm.
Isolation. No work was done regarding isolation of
product. It should be possible to convert m-nitroaniline and
m-phenylenediamine to the corresponding hydrochloride salts
and separate these salts as aqueous solution so that uncon-
verted m-dinitrobenzene is separated.
Received for review May 4, 1999.
OP990039S
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