Applied Catalysis A: General 286 (2005) 30–35
www.elsevier.com/locate/apcata
Gaseous catalytic hydrogenation of nitrobenzene to
aniline in a two-stage fluidized bed reactor
Shigang Diao, Weizhong Qian *, Guohua Luo, Fei Wei, Yao Wang
Beijing Key Laboratory of Green Chemical Reaction Engineering and Technology,
Department of Chemical Engineering, Tsinghua University, Beijing 100084, China
Received 20 August 2004; received in revised form 17 February 2005; accepted 23 February 2005
Available online 9 April 2005
Abstract
Gaseous hydrogenation of nitrobenzene over a Cu/SiO2 catalyst has been studied in a two-stage and in a single-stage fluidized bed reactor,
at 513–553 K and atmospheric pressure. The placement of the second perforated plate in the fluidized bed reactor inhibits the backmixing of
gases and solids and consequently increases the local molar ratio of hydrogen to nitrobenzene in the second stage. Thus, the conversion of
nitrobenzene and the selectivity of aniline production and the stable life of the catalyst are significantly increased in the two-stage fluidized
bed reactor, as compared with those in the single-stage one. A comparison of the coke formation and the burning characteristics of cokes in
different reactors has also been presented. It suggests the simple catalyst regeneration for the two-stage fluidized bed technology. This work
provides an effective method to produce aniline with higher purity.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Hydrogenation; Nitrobenzene; Catalyst deactivation; Multi-stage fluidized bed reactor; Copper catalyst
1. Introduction
Gaseous hydrogenation of nitrobenzene (NB) over a
catalyst [1–7] is an important way to prepare aniline (AN),
an important raw material for synthetic dyes, rubber
chemicals, amino resins, and polyurethane (via diphenyl
methane-4, 40 -diisocyanate (MDI)). This highly exothermic
reaction is normally conducted in a fluidized bed reactor,
with the convenience of heat exchange [1–6]. However,
there are large amounts of gas bubbles in the fluidized bed
reactor [8–10]. The part of NB inside the bubbles bypasses
the catalyst unavoidably. Even worse, the turbulent movement of catalysts and gases results in some backmixing of
the gas products in the catalyst dense phase [8–10]. The
above factors are all unfavorable for the deep conversion of
NB in such a single-stage fluidized bed (SSFB) reactor; they
hinder the guest to achieve aniline product with high purity,
i.e. the product necessary for the further synthesis of MDI
(the concentration of NB in AN product should be lower
* Corresponding author. Tel.: +86 10 62789041; fax: +86 10 62772051.
E-mail address: qwz@flotu.org (W. Qian).
0926-860X/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2005.02.026
than 5 ppm after the distillation). Also, due to the turbulent
movement of the catalyst in the SSFB reactor, the
deactivation of all catalysts occurs simultaneously. And
consequently the regeneration is needed in a period of 15
days to 2–4 months [2,5], which seems relatively short for a
large-scale production system. In other previous studies [12–
14], the horizontal baffle or perforated plate is placed in the
axial middle part of the reactor to construct the multistage
fluidized bed reactor, which will effectively decrease the
backmixing of gases; consequently, the conversion of gas
can be improved as compared with that in the SSFB reactor.
However, the movement of solids (catalysts) between stages
is still serious; it results in the deactivation characteristics of
the catalyst being similar to those in the SSFB reactor, as
above described. In the present work, a two-stage fluidized
bed (TSFB) reactor is proposed for the gaseous hydrogenation of NB to AN. Different from the previous studies [12–
14], a perforated plate with a very small hole ratio is adopted
in the present work to inhibit both the backmixing of gases
and any exchange of catalyst between stages [15]. Thus not
only is the deep conversion of NB for long times effectively
realized, but also the much longer stable life-time of the
S. Diao et al. / Applied Catalysis A: General 286 (2005) 30–35
catalyst in the second stage is achieved, as compared with
those in the first stage of the TSFB reactor and those in the
SSFB reactor. Consequently, the cycle for the production of
high purity aniline products can be significantly prolonged
and the regeneration method for the catalyst in the TSFB
reactor becomes relatively simple and flexible. These
changes are all favorable for the large-scale production of
aniline with high purity and at low cost.
2. Experimental
Fig. 1 shows the experimental-scale stainless-steel TSFB
reactor with the inner diameter of 30 mm and height of
1200 mm. Two horizontal perforated plates with 0.1% hole
ratio are used in the bottom of the reactor and the axial
middle part of the reactor, respectively. The reactor is
divided into two stages with the same height of 600 mm. The
furnace outside the reactor is used for heating the reactor at
the beginning. And the heat exchanger is used in the first
stage, to avoid any significant temperature increase here, due
to the intensive reaction. The temperature in both stages can
be controlled accurately (in the range of 513–573 K), with a
fluctuation of 3 K. The weight of catalyst at the first and at
the second stage is 310 and 190 g, respectively. For
comparison, the SSFB reactor (adding 500 g catalyst
directly) is also investigated, by removing the second
perforated plate. In the normal experiment, the gross space
velocity of NB is 0.18 gNB/gcat/h (h1) and the molar ratio
31
of hydrogen to NB is 11. Under these conditions, the
reaction rate of NB conversion is only proportional to the
concentration of NB [4]. In another experiment, the gross
space velocity of NB is 0.86 h1 and the total molar ratio of
hydrogen to NB is significantly decreased to 3.5. Notably, in
the latter experiment, the flow rate of hydrogen is only
slightly increased, as compared with that in the former one,
to avoid any significant change of the gas velocity or of the
contact time of gas with the catalyst. When the gaseous
products exiting out of the reactor are cooled to 293 K,
nearly all the organic products become liquid products. The
liquid product is analyzed by the liquid chromatography
(LC-10AT, Shimadzu, the flow phase is the mixture of
methanol and water, with the mass ratio of 16:9). There is
always a linear relationship between the amount of NB and
its response (the area ratio of NB to AN) (the data are not
shown here). Since the conversion of NB is always higher
than 99.8%, the concentration of NB in the crude AN
product (ppm degree) is used to evaluate to the effect of
different reactor technologies, for the comparison convenience. The low concentration of NB means the high
conversion of NB and the high activity of the catalyst.
Furthermore, in order to understand the change of the
coke in different reactors, the coke on the catalyst is
characterized by X-ray diffraction (XRD, D/MAX-RB, Fe
target, 40 kV) and their burning characteristics are
analyzed by using thermo-gravimetric analysis (TGA,
with an elevating heat rate of 10 K/min, from 298 to
673 K).
Fig. 1. Experimental setup of the two-stage fluidized bed reactor.
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S. Diao et al. / Applied Catalysis A: General 286 (2005) 30–35
3. Result and discussion
Fig. 2 shows the temperature-dependent content of NB in
the crude AN product. When the space velocity of NB is
0.18 h1, the contents of NB are all lower than 20 ppm at any
temperatures. The content of NB has the smallest values at
533–558 K, indicating the highest activity of the catalyst.
This tendency is in agreement with those in many previous
studies [4,6], using the same catalyst. Notably, the contents
of NB in AN product in the TSFB reactor, at any
temperature, are all lower than those in the SSFB reactor
(Fig. 2), indicating the higher conversion of NB in the TSFB
reactor. Fig. 3 shows the comparison of the time-dependent
conversion of NB in different reactors. It is clear that, in the
steady state of the reaction (for 60 min), the conversion of
NB in the TSFB reactor is always higher than that in SSFB
reactor. The cold model hydrodynamic experiment (using air
as the fluidized gas and hydrogen gas as the tracer [16])
confirmed that, when hydrogen is injected directly into the
second stage, hydrogen can not be detected in any positions
in the first stage, using a gas chromatograph (HP 4890D,
TCD detector, and carbon sieve column). The result
indicates that the backmixing of gases between stages in
the TSFB reactor such as reported is totally inhibited, which
results in the higher conversion of NB in the TSFB reactor.
The result is similar to the gas conversion in other reaction
systems in the multistage fluidized bed reactor [11,13,14].
Furthermore, as compared with the low NB content at low
space velocity of NB (0.18 h1) in Fig. 2, Fig. 3 indicates
that the content of NB in the AN product increases with the
increasing of the space velocity of NB (0.86 h1) and the
decreasing of the total molar ratio of hydrogen to NB (3.5) in
both reactors. But the conversion of NB and the life-time of
the catalyst become very stable in the TSFB reactor within
60 min. Comparatively, the catalyst loses its activity rapidly
after 30 min in the SSFB reactor (Fig. 3). By measuring the
coke contents on the catalyst in TGA analysis, the
relationship between coke content and the concentration
of NB in the SSFB reactor has been shown in Fig. 3 (the
Fig. 2. Temperature-dependent content of NB in AN product in different
reactors (space velocity of NB is 0.18 h1).
Fig. 3. Time-dependent content of NB in AN product in different reactors
(space velocity of NB is 0.86 h1, 543 K). The inset is the relationship
between coke content and the concentration of NB in the SSFB reactor.
inset). One sees clearly that the rapid deactivation of catalyst
occurs when the content of the coke is larger than 4 wt.% in
the SSFB reactor. Fig. 4 presents the time-dependent coke
weight on the catalyst in different reactors. The carbon
weight in the SSFB reactor does not increase significantly
with the reaction time after 30 min. However, the conversion
of NB still decreases drastically during this period (Fig. 3,
inset). The result indicates the irreversible deactivation of
the catalyst in the SSFB reactor. Similarly, the coke weight
increases nearly linearly with the reaction time in the first
stage in the TSFB reactor (Fig. 4). And it is up to 7–8 wt.% at
60 min, far larger than that (about 4 wt.%) in the SSFB
reactor at the same time (Fig. 4). The concentration of NB in
AN product obtained at the exit of the first stage (below the
second plate) of the TSFB reactor is about 0.84%, far larger
than that (200 ppm) in the exit of the SSFB reactor. The
results indicate that the catalyst loses its activity with the
increasing coke deposition on the catalyst. And the
deactivation of the catalyst in the first stage of the TSFB
Fig. 4. Time coke weight dependence on the catalyst in different reactors
(space velocity of NB is 0.86 h1, 543 K).
S. Diao et al. / Applied Catalysis A: General 286 (2005) 30–35
reactor is far more serious than those in the SSFB reactor.
Quantitatively, the weight of catalyst (310 g) in the first stage
of the TSFB reactor is 62% of that (500 g) in the SSFB
reactor. Thus the local space velocity of NB in the first stage
of TSFB reactor is 1.58 times that in the SSFB reactor. As
calculated from Fig. 4, the average coke content of that is
nearly 1.6 times that in the SSFB reactor. The result
indicates the accumulating rate of coke is proportional to the
space velocity of NB. Since the molar ratio of hydrogen to
NB is only 3.5 in the first stage of the TSFB reactor and the
SSFB reactor, we can conclude that the high partial pressure
of NB enhances the rate of deactivation [5,7]. However,
regardless of the serious deactivation of the catalyst in the
first stage, the existence of the second catalyst phase in the
second stage of the TSFB reactor ensures the further
conversion of NB. The final concentration of NB in AN
product can be decreased to 20–30 ppm effectively.
Correspondingly, the coke on the catalyst in the second
stage of the TSFB reactor is very small. It is lower than
0.8 wt.% even at 60 min, indicating the high activity and
stability of the catalyst. Since the gas backmixing between
two stages in the TSFB reactor is totally inhibited, the local
molar ratio of hydrogen to NB in the second stage is different
from that (about 3.5) in the first stage in the TSFB reactor.
The content of NB (0.84%) in the AN product means that the
conversion of NB is about 99% in the exit of the first stage of
the TSFB reactor. Thus the local molar ratio of hydrogen to
NB calculated is about 50 in the second stage. The high
partial pressure of hydrogen is effective to inhibit the coke
formation in the second stage of the TSFB reactor [1–7]. Our
results indicate that the activity of the catalyst is directly
related to the coke content on the catalyst. The deactivation
of the catalyst is mainly attributed to the high partial
pressure of NB or to the low molar ratio of hydrogen to NB,
in agreement with many previous studies [2,5,7].
Notably, HPLC detection indicates that the impurities in
organic products are nearly free of cyclohexylamine. The
result indicates that the high molar ratio of hydrogen to NB
in the second stage of the TSFB reactor does not give rise to
any further hydrogenation of AN to undesired cyclohexylamine or the like. In the hydrogenation of NB to AN, there
is a competition relationship between the hydrogenation of
the intermediates of C6H5NO, and C6H5NHOH to AN
[5,7,18–22] and the hydrogenation of AN to cyclohexylamine. Our result suggests that sufficient hydrogen allows the
quick conversion of these intermediates into AN, as in the
second stage of the TSFB reactor. The result is in agreement
with the relatively high hydrogenation rate of the C6H5NO,
NB and C6H5NHOH, as compared with that of AN [19].
Fig. 5 presents the purity of AN in liquid product and the
selectivity of NB to AN in different reactors. The purity of
AN in liquid product in the TSFB reactor is 98.6–99.1%,
which is higher than those (98.0–98.2%) in the SSFB
reactor. And the selectivity of NB to AN in the TSFB reactor
is in the same level as that in the SSFB reactor, if one
calculates the mass balance from the weight of gas
33
Fig. 5. Purity of AN in liquid product and selectivity of NB to AN in
different reactors.
(hydrogen), liquids (organic products and water) and solids
(coke on the catalyst). Though the absolute values (about
90%) of the selectivity of NB to AN are relatively low, due to
the high space velocity of NB and the low molar ratio of
hydrogen to NB, it can still be deduced that the selectivity
and yield of AN in the TSFB reactor will not be lower than
those in the SSFB reactor, if one adopts a normal space
velocity of NB (lower than 0.25 h1) and a normal molar
ratio of hydrogen to NB (higher than 8) for this copper
catalyst.
Since the weight of the catalyst (310 g) in the first stage of
the TSFB reactor is smaller than that (500 g) in the SSFB
reactor, the content of coke on the catalyst in the first stage of
the TSFB reactor is always higher than that on the catalyst in
the SSFB reactor. It is 7–8 wt.% and 4 wt.% at 60 min in the
first stage of the TSFB reactor and in the SSFB reactor,
respectively, as above mentioned. So far, it is unclear
whether the high space velocity of NB in the first stage of the
TFSB reactor changes the microstructures of the coke or not,
which is importantly related to the catalyst regeneration. The
following presents a detailed comparison of the coke in
different reactors. Because the component of coke may
change in the post-processing of the catalyst (removing the
metal or catalyst by acid or heat treatment), the untreated
samples are characterized in the present work. Fig. 6
presents the detailed XRD peak information of the fresh and
the coke-deposited catalysts obtained from different reactors
at different reaction times. There is no obvious Cu or CuO
peak for the fresh catalyst, indicating the well-dispersed
state of copper crystallites or the strong interaction of copper
species with the SiO2 support, similar to the results in
previous studies [3]. In the SSFB reactor, the coke on the
catalyst is unobvious during the initial 15 min. If one
increases the reaction time, a wide peak at 2u = 24–308
appears at 30 min, and this peak becomes much wider and
relatively more intense at 60 min, indicating the increasing
serious coke deposition with the reaction time. Comparatively, in the first stage of the TSFB reactor, the peaks at
2u = 24–308, 45–478, and 558 are obvious at 15 min,
indicating the serious deactivation of the catalyst from the
34
S. Diao et al. / Applied Catalysis A: General 286 (2005) 30–35
Fig. 6. X-ray diffraction pattern of coke and catalyst samples in different
reactors at different reaction times.
beginning of the reaction. However, the width and intensity
of these peaks remain nearly the same from 15 to 60 min,
which is somewhat different from the results in the SSFB
reactor. Also compared with those in the SSFB reactor, the
peak (at 2u = 24–308) of the coke in the TSFB reactor is
relatively narrow. And the peaks at 2u = 24–308, 45–478, and
558 is are quite similar to the peaks of carbon (0 0 2), (1 0 0),
and (0 0 4), respectively [17]. These are different from those
in the SSFB reactor, and we believe, are due to the much
higher space velocity of NB in the first stage of the TSFB
reactor. In a condition with the low partial pressure of
hydrogen, the hydrogenation of azoxybenzene (AZXB) and
azobenzene (AZB), formed by the interaction of the
intermediates of C6H5NO and C6H5NHOH with AN, to
AN again is very unfavorable [5,7,18–22]. In this case, in the
high space velocity of NB, the active site of catalyst is
insufficient, which is unfavorable for the de-adsorption of
these species (NB, C6H5NO and C6H5NHOH, especially the
AZXB and AZB) on the catalyst. And the possibility of the
dehydrogenation of these species or the interaction of these
species with AN and other species will increase. The result is
in agreement with the previous report that the NB absorbed
as the precursor of the coke will retard the reaction [7].
Though the formation mechanism of the coke is very
complex, the most probable route, we believe, may be via the
above-mentioned adsorbed species and interactions. Also
the reaction heat released per unit of the catalyst in the first
stage of the TSFB reactor is nearly 1.6 times that in the SSFB
reactor. The heat will enhance the further dehydrogenation
of the coke to a state close to the amorphous carbon, as
characterized by XRD.
Furthermore, the burning characteristics of the different
cokes are studied using TGA analysis, which is important for
the determination of the catalyst regeneration method. As
shown in Figs. 7 and 8, their initial burning temperatures of
cokes are all about 523 K and the nearly completely burnt
temperatures are 673 K. The sample with larger coke content
shows the relatively rapid burning velocity at 523–673 K, in
Fig. 7. TGA analysis of coke deposited on the catalysts in the SSFB
reactors at different reaction times.
the condition with the sufficient oxygen supply. And in the
following 30 min at 673 K, the weight loss of each sample is
very small (Figs. 7 and 8). These results demonstrate the
similar burning characteristics of these cokes on the
macroscopic scale, regardless of their different structures
as characterized by XRD (Fig. 6). Thus the catalyst
regeneration become relatively simple for the TSFB
technology, considering that only 62% catalyst is seriously
deactivated in the TSFB reactor. In a fluidized system similar
to the FCC apparatus [11], it is easy to transport the
deactivated catalyst out of the reactor and to add the
regenerated or the fresh catalyst into the first stage of the
TSFB reactor. Furthermore, the increasing tendency of the
coke content with the reaction time in the second stage is far
slower than that in the first stage of the TSFB reactor
(Figs. 4, 6 and 7). Thus it can be expected that the stable lifetime of the catalyst will be much longer in the second stage
of TSFB reactor before their serious deactivation (i.e. when
the coke content in the second stage reaches 4 wt.%). Thus
the regeneration of the catalyst in the first stage of the TSFB
reactor can be done at any time before the deactivation of the
catalyst in the second stage. This allows the catalyst
regeneration in the TSFB reactor with the great flexibility,
Fig. 8. TGA analysis of coke deposited on the catalysts in the TSFB
reactors at different reaction times.
S. Diao et al. / Applied Catalysis A: General 286 (2005) 30–35
without influencing the gross conversion of NB in the entire
reactor. This has an advantage over the situation in the SSFB
reactor, where the gross conversion of NB is easily
influenced by the simultaneous deactivation of all catalysts.
4. Conclusions
Due to the inhibition of the backmixing of gases and
catalysts in the reactor and the provision of a relatively large
local molar ratio of hydrogen to NB in the second stage of
TSFB reactor, the conversion of NB, the selectivity of NB to
AN and the stable life-time of the catalyst can be increased
in the TSFB reactor. The detailed XRD characterization
shows the different structures of coke in different reactors to
some degree, due to the differences in the partial pressure of
NB and the space velocity of NB. However, the coke
produced in the TSFB reactor does not influence their
regeneration as characterized by TGA. Thus the TSFB
technology not only favors the deep conversion of NB to
high purity aniline product, but also allows a simple and
flexible catalyst regeneration process. These are all
favorable for the large-scale production of aniline product
in high purity and at low cost.
Acknowledgement
This work is partly supported by the Natural and Science
Foundation of China (No. 20236020) and by Tsinghua
University open laboratory foundation (THSJZ).
35
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