Effects of micro channel size in a Pd membrane reactor on dehydrogenation of cyclohexane to benzene in gaseous phase

Article abstract of DOI:10.1246/cl.2006.284

To confirm the effects of microreactor on cyclohexane dehydrogenation, we have narrowed the reaction channel of Pd membrane reactor to micro scale. We show that the application of micro channel improves the performance of Pd membrane reactor. The experimental results indicate that the improvement in cyclohexane dehydrogenation with applying the micro channel is attributable to the enhancement of a heterogeneous catalytic reaction due to increasing the surface area per volume of Pd membrane. Copyright

Full text of DOI:10.1246/cl.2006.284

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Chemistry Letters Vol.35, No.3 (2006)
Effects of Micro Channel Size in a Pd Membrane Reactor on Dehydrogenation
of Cyclohexane to Benzene in Gaseous Phase
ꢀ
Shin Yamamoto, Taka-aki Hanaoka, Satoshi Hamakawa, Koichi Sato, and Fujio Mizukami
Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST),
4-2-1 Nigatake, Miyagino-ku, Sendai 983-8551
(Received December 21, 2005; CL-051561; E-mail: hanaoka-takaaki@aist.go.jp)
To confirm the effects of microreactor on cyclohexane dehy-
a-Al O Stainless
tube steel rod
2
3
drogenation, we have narrowed the reaction channel of Pd mem-
brane reactor to micro scale. We show that the application of mi-
cro channel improves the performance of Pd membrane reactor.
The experimental results indicate that the improvement in cyclo-
hexane dehydrogenation with applying the micro channel is at-
tributable to the enhancement of a heterogeneous catalytic reac-
tion due to increasing the surface area per volume of Pd mem-
brane.
Pd
membrane
Glass coat
3H
2
3H
2
Micro chemical processes attract a great deal of interest in
the fields of chemical industry and science, thus many researches
on these processes have been conducted. These processes are
H
2
H
2
1
handled in the spaces of sub-micrometer to sub-millimeter scale;
the space and/or the process itself is called a microreactor. The
extremely short distance for transfers of materials and heat in
microreactor leads to efficient conversion, high reaction selectiv-
ity, ease of reaction control, etc.
Figure 1. Microreactor with Pd membrane for dehydrogenation
of cyclohexane to benzene. A stainless-steel rod was inserted in
a Pd membrane tube reactor to prepare a micro channel.
Here, examinations for the practical use of hydrogen as a
secondary source of clean and recyclable energy have been car-
ried out. Storage and transportation of hydrogen are important
issues on the practical use. Among various methods for the stor-
age of hydrogen, storing as liquid organic hydrides is one of the
the vapor (ca. 3 mol m 3) was flowed in the rate range of 2–
ꢁ
ꢁ1
1
0 mL min at inside of the reactor tube (react-side). Flow rate
ꢁ1
of N2 at the outside (perm-side) was constantly 5 mL min as
carrier gas to sweep permeated hydrogen. We carried out dehy-
drogenation of cyclohexane at 573 K and atmospheric pressure.
The microreactors, Micro-1, Micro-2, and Micro-3, were pre-
pared by employing the various stainless-steel rods of 0.80,
2
preferable candidates. We have studied on Pd membrane to ap-
ply for the chemical processes in which hydrogen species partic-
3
ipate. The Pd membrane plays a role to permeate hydrogen se-
lectively as well as a role of catalyst. The advantages of the Pd
membrane are effectively offered in the hydrogen-storage proc-
1
.00, and 1.26 mm as diameters, respectively. These three micro-
reactors and the normal membrane reactor were 600, 500, 370,
and 2000 mm as each average width of fluid channel. We have
conformed in a supplemental experiment that the change in the
local pressure in reaction channel with the channel width is
negligibly small.
4
ess with liquid organic hydrides.
The application of a microreactor is expected to enhance the
performance of a Pd membrane reactor on the hydrogen-storage
process. In this study, we have investigated the effects of micro
channel size on dehydrogenation of cyclohexane with a Pd mem-
brane in gaseous phase; this reaction is the one-half of the hydro-
gen-storage process as follows.
Cyclohexane conversion and benzene selectivity during
dehydrogenation in the microreactors are shown in Figure 2a.
The selectivity has been defined as the production rate of ben-
zene divided by the reduction rate of cyclohexane. The conver-
sion increased with the decrease in the reaction channel width at
a given retention time. The equilibrium constant of dehydrogen-
p
C6H12 ꢀ 3H2 þ C6H6:
ð1Þ
The purpose of this study is to demonstrate that narrowing a
reaction channel to micrometer scale is a potential method to im-
prove efficiency of the hydrogen-storage process. To the best of
our knowledge, there are no precedents for applying a micro-
reactor to a heterogeneous gaseous reaction on Pd membrane
through the manner of this study.
The reactor used for our experiments was of flow type and
composed of Pd membrane coated on the surface of an ꢀ-alumi-
na porous tube by chemical vapor deposition. In addition, we de-
vised inserting a stainless-steel rod in the membrane reactor to
construct a microreactor. The illustration of the microreactor is
shown in Figure 1. Nitrogen bubbled through cyclohexane and
ation of cyclohexane to benzene in gaseous phase, K , is given
5
as follows.
20
Kp ¼ 4:70 ꢂ 10 expðꢁ26490=TÞ: [atm3]
ð2Þ
The equilibrium conversion at 573 K is calculated to be
91.2% from Eq 2; thus the dehydrogenation of cyclohexane in
our reactors does not achieve the equilibrium. In addition, the re-
action rate of homogeneous dehydrogenation of cyclohexane is
negligibly small, because the rate constants calculated theoreti-
6
cally are extremely lower than those obtained as our results.
Thus, the observed dehydrogenation of cyclohexane is consid-
Copyright ꢀ 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.3 (2006)
285
4
3
2
1
0
0
0
0
100
shown in Figure 2a, the selectivity has increased with narrowing
the reaction channel at a given retention time, and thus the appli-
cation of micro channels can prevent the reduction in reaction ef-
ficiency due to the coking on the surface of Pd membrane.
We investigated the recovery of hydrogen at perm-side
(
a)
90
Selectivity
Micro–3
Micro–2
Micro–1 80
Normal
(
Figure 2b). The recovery has been defined as permeation
7
6
0
0
amount of hydrogen divided by all amount of produced hydro-
gen. The application of micro channels increased these values
at a given retention time, despite an increase in produced hydro-
gen due to the enhancement of cyclohexane conversion. This
result is attributable to an increase in hydrogen distribution ratio
at perm-side due to decreasing the volume at react-side. The
increase in hydrogen recovery is another advantage arising from
narrowing a reaction channel.
Conversion
Micro–3
Micro–2
Micro–1
Normal
0
50
0
.00
0.05
0.10
0.15
Retention Time at React-Side / min
8
6
4
2
0
0
0
0
(
b)
Narrowing a reaction channel extends the surface area per
volume of Pd membrane. Micro-1, Micro-2, Micro-3, and the
normal reactor have each surface area per volume of 2.4, 2.7,
2
ꢁ1
3
.3, and 2.0 mm mL , respectively. The yield of benzene was
Micro–3
Micro–2
Micro–1
Normal
affected by varying the surface area per volume of Pd mem-
brane, and these values enhanced approximately two times as
compared to the value with normal reactor at 0.05 min as reten-
tion time (Figure 2c). This result evidently indicates one of the
advantages of microreactor for dehydrogenation of cyclohexane,
namely, the increase in catalytic reaction on Pd membrane due to
the increase in the surface area per volume of Pd membrane, as
discussed above.
0
0
.00
0.05
0.10
0.15
Retention Time at React-Side / min
Micro–3
1
1
2
0
(c)
In this study, the significant effects of applying micro chan-
nels have been found on dehydrogenation of cyclohexane in the
simple manner; despite constructing a microreactor has been
conventionally regarded difficult and/or expensive. The im-
provement in cyclohexane dehydrogenation with decreasing
channel size is mainly caused by catalytic reaction on the Pd
membrane. The dehydrogenation efficiencies should be able to
further increase by a combination of any other catalysts and/or
pressure control in the reactors. We anticipate some advantages
of the micro channels to benzene hydrogenation.
Micro–2
Micro–1
Normal
8
6
4
2
0
1
.5
2.0
2.5
3.0
3.5
2
–1
Surface Area per Volume / mm µL
Figure 2. (a) Dependence of cyclohexane conversion, selectiv-
ity to benzene and (b) hydrogen recovery at perm-side on reten-
tion time at react-side, and (c) dependence of benzene yield on
Pd membrane surface area per volume (retention time: 0.05
min) during dehydrogenation of cyclohexane in microreactors.
Micro-1, Micro-2, Micro-3, and normal had each fluid channel
of 600, 500, 370, and 2000-mm width.
The research was financially supported by the Project of
Micro-Chemical Technology for Productions, Analysis and
Measurement Systems of NEDO in Japan. We appreciate to
NEDO for their support.
References
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2
3
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Pd membrane. The improvement in cyclohexane conversion
with decreasing channel size is, therefore, attributable not to
the transition of thermodynamic equilibrium due to hydrogen
segregation but to the enhancement of catalytic dehydrogenation
due to increasing the surface area per volume of Pd membrane.
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selectivity below 80% were observed (Figure 2a). It is supposed
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tion of some carbon compounds on the surface of Pd membrane.
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4
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R. Dittmeyer, V. H o¨ llein, K. Daub, J. Mol. Catal. A: Chem. 2001,
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Published on the web (Advance View) February 11, 2006; DOI 10.1246/cl.2006.284