Kinetics of liquid-phase catalytic hydrogenation of 4-chioro-2-nitrophenol

Article abstract of DOI:10.1021/op980052x

Process parameters were studied to increase the selectivity of the haloamino compound by preventing the formation of dehalogenated product for the selective hydrogenation of 4-chloro-2-nitrophenol on 5% Pd/C catalyst. At a conversion ratio of 87%, 96% selectivity was achieved in 5 h. Kinetic interpretations have been made for this liquid-phase hydrogenation reaction.

Full text of DOI:10.1021/op980052x

Organic Process Research & Development 1999, 3, 201−205
Kinetics of Liquid-Phase Catalytic Hydrogenation of 4-Chloro-2-nitrophenol
Sudip Mukhopadhyay,* Ganesh K. Gandi, and Sampatraj B. Chandalia
Chemical Engineering DiVision, UniVersity Department of Chemical Technology, UniVersity of Mumbai,
Matunga, Mumbai-400 019, India
Abstract:
tendency for its formation needs to be reduced, by choice
of the proper reaction conditions, the type of catalyst, and
the solvent.
The information available regarding this processes is
inadequate and is mostly confined to the patent literature.
Hence, the present work was undertaken to ascertain the
suitable process conditions and kinetics for the manufacture
of the desired product, 2-amino-4-chlorophenol.
Process parameters were studied to increase the selectivity of
the haloamino compound by preventing the formation of
dehalogenated product for the selective hydrogenation of
4
-chloro-2-nitrophenol on 5% Pd/C catalyst. At a conversion
ratio of 87%, 96% selectivity was achieved in 5 h. Kinetic
interpretations have been made for this liquid-phase hydroge-
nation reaction.
Experimental Section
Experimental Setup. Experiments were carried out in
an autoclave of 100 mL capacity, made of Hastelloy. The
autoclave was equipped with a four-bladed magnetically
driven impeller, and a cooling system was arranged in it.
The autoclave was heated externally by a heating element,
and the temperature of the reaction was regulated by a
temperature indicator controller. The pressure gauge, pressure
release valve, safety head port, and sampling valve were all
situated on the top head.
Introduction
The synthesis of 2-amino-4-chlorophenol has considerable
industrial importance. It is mainly used as an intermediate
for a drug, namely chlorzoxazone, which has been exten-
sively used as a skeletal muscle relaxant. It is also an
intermediate for a large class of diazo components used for
1
2
hydroxy azo dyes, which are important for wool and
polyamides when converted to their chromium complexes.
The main route for the synthesis of haloamino compounds
is either the Bechamp’s reaction or catalytic hydrogenation
starting with halonitro aromatic compounds. The main
Experimental Procedure. Predetermined quantities of
4-chloro-2-nitrophenol, methanol, and catalyst were charged
to the autoclave, and the autoclave was repeatedly purged
first with nitrogen and then with hydrogen gas. The reaction
temperature was maintained within (1 °C of the desired level
by controlling the flow rate of cooling water and the heating
rate. The autoclave was pressurized with hydrogen to the
desired pressure and agitation started. The reaction time was
noted from this instant. A constant pressure was maintained
throughout the reaction period.
Analysis. Samples of 1-2 mL, withdrawn at regular
intervals of time, were analyzed by gas chromatography on
a Chemito 8510 instrument equipped with a flame ionization
detector, connected to an integrator. A 2-m-long, 0.003-m-
diameter stainless steel column with 10% SE-30 on Chro-
mosorb-W was used. The injector and detector temperatures
were maintained at 300 °C, and the oven temperature was
maintained at 150 °C isothermally. The carrier gas used was
nitrogen with a flow rate of 20 mL/min.
Separation and Purification of the Product. The reac-
tion mixture taken out from the autoclave was filtered to
remove the catalyst, and then methanol was distilled out
under reduced pressure. The product mixture was acidified
(pH ) 3-4) with 20% hydrochloric acid. This solution was
then neutralized with concentrated sodium hydroxide to
obtain a precipitate at pH 6-7. This precipitate was filtered
3
drawback of Bechamp’s reaction is that the rate of reaction
is lower than that of the catalytic hydrogenation due to lower
solubility of the halonitro compounds in an acid-water
system. Though the selectivity to the desired product is higher
in Bechamp’s process, the separation of the product from
the reaction mixtures needs costly steam distillation. The
catalytic hydrogenation of halonitro compounds using het-
erogeneous catalyst offers several advantages over chemical
reduction. Some of the major ones are the following: (1)
The cost of hydrogen is low compared to that of most other
reducing agents. (2) The products can be easily separated at
the end of the reaction, and loss of product during recovery
is minimal. (3) The catalyst can be reused. (4) The process
is environmentally friendly and nonpolluting. While the
hydrogenation of nitro compounds, in general, is highly
attractive, the extension of the process to halo-substituted
nitroaromatic compounds poses several problems because
there is a tendency towards hydrodehalogenation. It may be
mentioned that the amino group in the desired product
facilitates the hydrodehalogenation,4 and therefore, the
-7
*
To whom correspondence should be addressed.
1) Wang, S.; Xiangbin, Y.; Deyu, X. Yiyao Gongye 1987, 18 (2), 49-50
Chinese); Chem. Abstr. 1987, 107, 198147.
2) Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; VCH: Weinheim,
985; Vol. A17, p 447.
3) Ullmann’s Encyclopedia of Industrial Chemistry, 5th ed.; VCH: Weinheim,
985; Vol. A2, p 43.
4) Stratz, A. M. Catalysis in Organic Reactions; Marcel Dekker: New York,
994; p 353.
(
(
(
(
(
1
(5) Freifelder, M.; Martin, W. B.; Stone, G. R.; Coffin, E. L. J. Org. Chem.
1961, 26, 383.
(6) Kindler, K.; Oelschlager, H.; Henrich, P. Chem. Ber. 1953, 86, 167.
(7) Kindler, K.; Oelschlager, H.; Henrich, P. Chem. Ber. 1953, 86, 501.
1
1
1
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Vol. 3, No. 3, 1999 / Organic Process Research & Development
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Published on Web 05/06/1999

Table 1. Material balancea
material
gmol
% accounted for
4
2
-chloro-2-nitrophenol
-amino-4-chlorophenol
0.0288
100
82
2
12.8
3.2
0.02361
0.00057
0.00369
0.00093
dehalogenated product
unreacted
unaccounted for
aReactions conditions: 4-chloro-2-nitrophenol, 10% w/v; catalyst, palladium
on carbon; catalyst loading, 3.0 g/L; reaction time, 5 h; temperature, 100 °C;
speed of agitation, 1000 rpm; hydrogen pressure, 25 atm; volume of reaction
mixture, 50 mL; reactant taken, 0.0288 gmol; total accounted for, 0.02787 gmol;
material accounted for, 97%.
Figure 2. Effect of hydrogen pressure. Reaction conditions:
initial concentration of reactant, 10% w/v; catalyst loading, 3
g/L; temperature, 100 °C; speed of agitation, 1000 rpm; time,
5
h; reaction volume, 50 mL.
Figure 1. Effect of speed of agitation. Reaction conditions:
initial concentration of reactant, 10% w/v; catalyst loading, 3
g/L; temperature, 100 °C; time, 5 h; hydrogen pressure, 25 atm;
reaction volume, 50 mL.
Figure 3. Effect of period of reaction. Reaction conditions:
initial concentration of reactant, 10% w/v; catalyst loading, 3
g/L; temperature, 100 °C; speed of agitation, 1000 rpm.
and washed with water three times to get the pure desired
product
Results and Discussion
Material Balance. During this hydrogenation reaction of
4-chloro-2-nitrophenol, 97% 4-chloro-2-nitrophenol could be
accounted for on an isolation basis (Table 1).
Effect of Speed of Agitation. To ascertain that mass
transfer plays an insignificant role, speed of agitation was
varied from 500 to 1200 rpm in this hydrogenation reaction.
There was a significant change in the overall conversion
when the agitation speed was varied from 500 to 1000 rpm,
indicating the presence of the mass-transfer limitation for
the diffusion of hydrogen from the gas interface to the bulk
liquid. When the speed of agitation was further increased
from 1000 to 1200 rpm, there was no significant change in
the overall conversion (Figure 1). This indicates that mass-
transfer effects are mostly eliminated at or above 1000 rpm
and that the data represent the true kinetics of the process.
Effect of Partial Pressure. When the pressure was
increased from 10 to 25 atm, it was observed that the percent
overall conversion observed after 5 h reaction increased
markedly (Figure 2). However, with a further increase in
the pressure from 25 to 30 atm, there was no significant
increase in conversion observed after 5 h of reaction. Thus,
under hydrogen-rich conditions, the reaction is independent
of the partial pressure of hydrogen. All subsequent reactions
were done at a pressure of 25 atm.
Figure 4. Effect of initial concentration. Reaction conditions:
hydrogen pressure, 25 atm; catalyst loading, 3 g/L; temperature,
100 °C; speed of agitation, 1000 rpm; time, 5 h; reaction volume,
50 mL. Symbols (% w/v): [, 5; 0, 10; 4, 20.
Effect of Period of Reaction and Initial Concentration.
When the period of reaction was increased from 0.5 to 5 h,
the overall conversion of 4-chloro-2-nitrophenol under 25
atm hydrogen pressure increased from 18 to 87%, and the
selectivity with respect to the amino phenol was 96% (Figure
3
).
It is observed from Figure 4 that there was a little increase
in the percent overall conversion as the concentration
increased from 5 to 20% (w/v).
Effect of Catalyst Loading and Reusability of the
Catalyst. The catalyst loading, expressed as weight percent
202
•
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Figure 5. Catalyst loading vs % overall conversion. Reaction
conditions: hydrogen pressure, 25 atm; initial concentration,
1
0% w/v; temperature, 100 °C; speed of agitation, 1000 rpm;
time, 5 h; reaction volume, 50 mL.
Figure 6. Effect of temperature on rate of reaction. Reaction
conditions: initial concentration of reactant, 10% w/v; catalyst
loading, 3 g/L; speed of agitation, 1000 rpm; reaction period, 5
h; reaction volume, 50 mL. Symbols (°C): [, 60; 9, 80; 2, 90;
Table 2. Effect of reusability of catalysta
conversion
of CNP, %
selectivity with respect
catalyst
fresh
first reuse
second reuse
to ACP, %
×
, 100; *, 110; b, 120.
86.7
84.7
83.9
97.6
97.1
96.3
Table 4. Effect of different solventsa
conversion selectivity with respect
solvent
methanol
2-propanol
aqueous NaOH solution
of CNP, %
to ACP, %
aReactions conditions: 4-chloro-2-nitrophenol, 10% w/v; catalyst, palladium
on carbon; catalyst loading, 3.0 g/L; reaction period, 5 h; temperature, 100 °C;
speed of agitation, 1000 rpm; hydrogen pressure, 25 atm; volume of reaction
mixture, 50 mL. CNP, 4-chloro-2-nitrophenol; ACP, 2-amino-4-chlorophenol.
87
84
32
96
95
99
aReaction conditions: 4-chloro-2-nitrophenol, 10% w/v; catalyst, palladium
on carbon; catalyst loading, 3.0 g/L; reaction period, 5 h; temperature, 100 °C;
speed of agitation, 1000 rpm; hydrogen pressure, 25 atm; volume of reaction
mixture, 50 mL.
Table 3. Effect of different types of catalystsa
conversion
of CNP, %
selectivity with respect
to ACP, %
catalyst loading
5
% Pd/C
87
84
86
96
93
9a
Table 5. Comparison of the rate of hydrogenation between
Bechamp’s process and the catalytic process
Raney Ni type-I
Raney Ni type-II
process
reaction time, h
conversion, %
aSynthesis of 2-amino-4-chlorophenol from 4-chloro-2-nitrophenol. Reaction
conditions: 4-chloro-2-nitrophenol, 10 w/v; speed of agitation, 1000 rpm; catalyst
loading, 3.0 g/L; reaction period, 5 h; temperature, 100 °C; hydrogen pressure,
Bechamp’s process
catalytic hydrogenation
5
5
87
67
2
5 atm; volume of reaction mixture, 50 mL.
of catalyst based on the total reaction volume, was varied
from 0.1 to 0.4% w/v (Figure 5). The percent overall
conversion with respect to the reactant observed after 5 h of
reaction time increased significantly.
It was observed (Table 2) that the activity as well as the
selectivity was sustained for repeated uses of the catalyst.
Effect of Different Types of Catalyst. Different types
of catalyst, such as 5% Pd/C, Raney Ni type-I, and Raney
Ni type-II, were used for this reaction. Type-I and type-II
differ in mesh size only. Though Raney Ni is a less expensive
catalyst than 5% Pd/C, the selectivity obtained was lower,
and the catalyst activity for repeated use was not sustained,
so it was worth considering use of 5% Pd/C as the catalyst
from 96 to 89%, due to the faster rate of the hydrodehalo-
genation reaction at higher temperatures. Therefore, it was
thought desirable to use a temperature of 100 °C to obtain
a higher selectivity with respect to the desired product.
Effect of Solvent. Different types of solvents, such as
methanol, 2-propanol, and aqueous sodium hydroxide solu-
tion, were used for this reaction. Methanol was proved to
be the best solvent under the reaction conditions (Table 4).
In alkaline conditions, though, the selectivity was as high
as 99%, but the specific reaction rate was too slow. At 5 h,
32% conversion and 99% selectivity were obtained in
alkaline solution.
Comparison of the Rate of Reaction of This Process
with Bechamp’s Process. In a view to compare the rate of
reaction between the two processes, the reduction was
performed with Fe/HCl as well as by hydrogenation on 5%
Pd/C catalyst. It was found that (Table 5) the rate of reaction
of Bechamp’s process for 4-chloro-2-nitrophenol was lower
than that of the catalytic process under similar reaction
(Table 3).
Effect of Temperature. The effect of temperature was
studied from 60 to 120 °C. The conversion of 4-chloro-2-
nitro phenol increased from 24.34 to 100% when the
temperature was increased from 60 to 120 °C (Figure 6).
The selectivity decreased with respect to the desired product
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A
Figure 7. Time vs -ln(1 - X ). Reaction conditions: initial
concentration of reactant, 10% w/v; catalyst loading, 3 g/L;
temperature, 100 °C; speed of agitation, 1000 rpm; hydrogen
pressure, 25 atm; reaction period, 5 h; reaction volume, 50 mL.
Table 6. Effect of initial concentration of nitro compound on
Figure 8. Effect of catalyst loading on rate of reaction.
Reaction conditions: initial concentration of reactant, 10% w/v;
hydrogen pressure, 25 atm; temperature, 100 °C; speed of
agitation, 1000 rpm; reaction volume, 50 mL; time, 5 h.
specific rate constanta
4
initial concn of nitro compound,
specific rate constant × 10 ,
-
1
%
w/v
s
5
1.05
1.13
1.22
1
0
0
2
a
Reaction conditions: catalyst, palladium on carbon; catalyst loading, 3.0
g/L; reaction time, 5 h; temperature, 100 °C; speed of agitation, 1000 rpm;
hydrogen pressure, 25 atm; volume of reaction mixture, 50 mL.
conditions. In 5 h, only 72% conversion was obtained in
Fe/HCl reduction, in comparison to 87% in the catalytic
hydrogenation. The selectivity in Bechamp’s process was
found to be 99%, which can also be achieved in catalytic
hydrogenation by controlling the conversion to a lower ratio.
The Bechamp reaction was carried out at 75 °C, and after
the stipulated period of time, the reaction mixture was hot
filtered to remove iron metal and then neutralized with NaOH
solution. The organic compounds were extracted in toluene
and then fractionated using a 2-m wire mess pack column
to isolate the product.
Kinetic Interpretation. The reactions were carried out
at a high speed of agitation to eliminate the mass-transfer
effects. There was absolutely no increase in the conversion
ratio when the hydrogen pressure was increased from 10 atm.
This indicates that, at or above 25 atm pressure, the reaction
is insensitive to a further increase in pressure.
A
Figure 9. -ln(1 - X ) vs t at different temperatures. Reaction
conditions: initial concentration of reactant, 10% w/v; catalyst
loading, 3 g/L; hydrogen pressure, 25 atm; speed of agitation,
1
000 rpm; reaction volume, 50 mL. Symbols (°C): [, 60; 9,
0; 4, 90; ×, 100; *, 110.
8
It is seen from Figure 7 and Table 6 that the reaction is
a first-order reaction with respect to the chloronitrophenol
at a hydrogen pressure of 25 atm, speed of agitation of 1000
rpm, and catalyst loading of 0.3% w/v.
It is observed that the initial rate increases linearly with
the increase in catalyst loading (Figure 8). Since the catalyst
used was finely powdered and micrometer-scale in size, the
role of diffusion of hydrogen from the bulk liquid to the
solid is estimated to be unimportant, and the data appear to
represent the true kinetics of the process at a catalyst loading
of 0.3% w/v.
Figure 10. Arrhenius plot. Reaction conditions: initial con-
centration of reactant, 10% w/v; catalyst loading, 3 g/L;
hydrogen pressure, 25 atm; speed of agitation, 1000 rpm;
reaction volume, 50 mL; time, 5 h.
A
At different temperatures, -ln(1 - X ) vs t is plotted
(
Figure 9), and from the slopes, k values at different
-
5
-5
temperatures are found to be 1.53 × 10 , 3.43 × 10 , 5.6
the reaction conditions. The energy of activation is found to
be 12.6 kcal/mol (Figure 10).
-
5
-5
-4
-4 -1
×
10 , 1.1 × 10 , 1.43 × 10 , and 2.5 × 10 under
s
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Conclusion
It was observed that 0.3% w/v of 5% Pd/C was suitable
pressure. The hydrogenation reaction follows a first-order
kinetics with respect to nitro compound.
for the hydrogenation of 4-chloro-2-nitrophenol under the
reactions conditions. A temperature of 100 °C and a pressure
of 25 atm were found to be most suitable for this reaction in
methanol under the given conditions. The catalyst sustains
its activity for subsequent uses.
Acknowledgment
G.K.G. and S.M. are grateful to the University Grants
Commissions, New Delhi, for the award of Junior and Senior
research fellowships, respectively.
Received for review June 19, 1998.
OP980052X
The mass-transfer effects were eliminated at a speed of
agitation of 1000 rpm at 100 °C and 25 atm hydrogen
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