Palladium-based bifunctional membrane reactor for one-step conversion of benzene to phenol and cyclohexanone

Article abstract of DOI:10.1016/j.cattod.2010.04.045

On the one-step hydroxylation of benzene to phenol over a Pd membrane reactor, the bifunctional effects between Pd membrane and metal particles were investigated. In this reaction system, the active oxygen species are formed by the reaction with the permeated hydrogen and adsorbed oxygen over Pd membrane. The active oxygen species are reacted with benzene and directly converts into phenol. For improving this system, various active metals were loaded on the α-Al₂O₃ porous tube which was the substrate of the thin Pd membrane. The loading of noble metals resulted in the enhancement of the activity of side reactions, i.e., complete oxidation and hydrogenation, and a decrease in the hydroxylation activity. The negative effect on hydroxylation was due to the imbalance between the catalytic activities of highly dispersed noble metal particles and the Pd membrane, which has a relatively small surface area. In contrast, the loading of Cu suppressed complete oxidation and enhanced the hydroxylation activity. This effect was mainly due to the increase in the hydrogenation activity without significant acceleration of oxidation. In addition, the potential for the selective conversion of benzene to cyclohexanone by this reactor was discussed.

Full text of DOI:10.1016/j.cattod.2010.04.045

Catalysis Today 156 (2010) 276–281
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Catalysis Today
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Palladium-based bifunctional membrane reactor for one-step conversion of
benzene to phenol and cyclohexanone
Koichi Sato∗, Satoshi Hamakawa, Mayumi Natsui, Masateru Nishioka, Tomoya Inoue, Fujio Mizukami
Research Center for Compact Chemical Process, National Institute of Advanced Industrial Science and Technology (AIST), 4-2-1 Nigatake, Miyagino-ku, Sendai, Miyagi 983-8551,
Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 22 June 2010
On the one-step hydroxylation of benzene to phenol over a Pd membrane reactor, the bifunctional effects
between Pd membrane and metal particles were investigated. In this reaction system, the active oxy-
gen species are formed by the reaction with the permeated hydrogen and adsorbed oxygen over Pd
membrane. The active oxygen species are reacted with benzene and directly converts into phenol. For
improving this system, various active metals were loaded on the ␣-Al2O3 porous tube which was the sub-
strate of the thin Pd membrane. The loading of noble metals resulted in the enhancement of the activity
of side reactions, i.e., complete oxidation and hydrogenation, and a decrease in the hydroxylation activ-
ity. The negative effect on hydroxylation was due to the imbalance between the catalytic activities of
highly dispersed noble metal particles and the Pd membrane, which has a relatively small surface area.
In contrast, the loading of Cu suppressed complete oxidation and enhanced the hydroxylation activity.
This effect was mainly due to the increase in the hydrogenation activity without significant acceleration
of oxidation. In addition, the potential for the selective conversion of benzene to cyclohexanone by this
reactor was discussed.
Keywords:
Pd membrane
Direct hydroxylation
Benzene
Phenol
Cyclohexanone
Co-catalytic effect
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
adsorb on the surface of the Pd membrane and permeate into the
membrane as dissociated hydrogen species by lattice diffusion. At
Phenol and its derivatives, such as cyclohexanone, are valuable
for chemical industries where they are used as intermediates of
polymer. In industries, phenol is produced by the “cumene process”
using cumene, an intermediate product obtained from benzene and
propylene [1]. Although this process has been refined, it has two
disadvantages: high energy consumption because of its multistep
reaction and the vice production of acetone. The direct hydroxyla-
tion of benzene with molecular oxygen is an attractive alternative
to current multistep reactions. The direct synthesis of phenol using
molecular oxygen, however, does not yield satisfactory conver-
sion and selectivity. The major problem is the difficulty in the
proper generation of active oxygen species such as neutral or rad-
ical species and in suppressing consecutive oxidation due to the
relatively high reactivity of phenol as compared to that of benzene.
In a previous study, our group has applied a Pd membrane reac-
tor for the direct hydroxylation of benzene to phenol with relatively
high performance [2]. In this reactor system, hydrogen and a mix-
ture of oxygen and benzene are separately supplied to both sides
of the Pd membrane (Fig. 1). Hydrogen molecules dissociatively
the surface of the opposite side of the membrane, the permeated
hydrogen species react with adsorbed oxygen and reductively pro-
duce active oxygen species, which can convert benzene into phenol.
The reductive activation of oxygen by hydrogen has been widely
studied and reported by several groups [3,4]. However, mixing the
supplies of oxygen and hydrogen poses a risk of explosion, which
limits reaction conditions. As a result of the limitation, the yield
of phenol is inadequate for practical use. A membrane reactor can
avoid the risk of explosion by separating the supplies of oxygen
and hydrogen. In addition, all hydrogen species are efficiently sup-
plied to the Pd surface as dissociatively activated species because
of the lattice diffusion of dissociated hydrogen species into Pd. The
membrane reaction system does not require expensive oxidizing
reagents, e.g., N2O, which shows good performance over Fe-MFI
zeolite catalyst [5].
In a previous study, we investigated the direct hydroxylation
on a Pd membrane reactor by studying the application of this
membrane reactor for toluene and methyl benzoate [6,7], the H2
permeation performance of the membrane [7], and the surface state
and stability of the membrane during the reaction [8]. The results
of the direct hydroxylation of methyl benzoate to methyl salicylate
suggested that methyl salicylate was directly formed by this reac-
tor, and geometric isomers of methyl salicylate were not detected,
∗
Corresponding author. Tel.: +81 22 237 5211; fax: +81 22 237 5226.
E-mail address: koichi.sato@aist.go.jp (K. Sato).
0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.04.045

K. Sato et al. / Catalysis Today 156 (2010) 276–281
277
acetylacetonate and ferric acetylacetonate with the same concen-
tration as that of the Pd solution were used, respectively. Pt, Rh,
and Ru were deposited by the impregnation of the respective
aqueous metal solution by dip coating for 10 min and drying at
383 K for 10 min. This cycle was repeated 10 times. For prepar-
ing the metal solutions, hexachloroplatinate (IV), rhodium (III)
chloride, and ruthenium (III) chloride were used, respectively.
The concentration of the solution was the same as that of the
Pd solution. After the calcination at 673 K, the membranes were
subjected to CVD treatment similar to that used in the case of
the sole Pd membrane. Note that several numbers of membranes
were prepared for reaction and characterization. These mem-
branes were not completely the same because their substrates
were different. The reaction results in this study are average
performance.
Fig. 1. Schematic illustration of one-step conversion of benzene to phenol and con-
secutive hydrogenation over metal-loaded Pd membrane reactor.
The characterization of the metal particles loaded onto the ␣-
Al2O3 tube was carried out before the CVD treatment of Pd by
XRD (MAC Science MXP-18) with CuK␣ radiation and XPS (PHI
ESCA5600) with MgK␣ radiation.
suggesting that this reaction did not proceed by the electrophilic
reaction mechanism. As by-products, both hydrogenated products
and oxygenated products were formed, suggesting that the cat-
alytic activity was influenced by the concentration of hydrogen and
oxygen.
2.2. Reactor construction
In the current membrane reactor, the Pd membrane works as not
only as a hydrogen separator but also a catalyst. The surface area
of the Pd membrane is relatively small as compared to a common
Pd-loaded catalyst in porous supports. A thin Pd metal membrane
shows superior hydrogen permeation performance, but its catalytic
activity for hydroxylation might not be optimized. In this study, for
improving this membrane system, various active metals are loaded
into the pores of the support tube of the Pd membrane, as shown in
Fig. 1. Main aims of the study are to increase the Pd surface area by
Pd loading and increase the co-catalytic effect between the loaded
active metals and the Pd membrane. In addition, in order to deter-
mine the advantages of this reactor, subsequent hydrogenation of
phenol to cyclohexanone is investigated.
The reaction was carried out by using a continuous flow type
reactor at atmospheric pressure (Fig. 2). One end of the Pd mem-
brane tube was sealed, and a stainless steel tube (OD 4.0 mm) was
attached to the other end. It was placed inside a glass tube reac-
tor whose outer diameter was 12 mm. A quartz capillary with an
outer diameter of 0.7 mm was then inserted inside the Pd mem-
brane tube. The inner gas, a mixture of He, O2, benzene, and N2,
was supplied to the inner surface of the membrane through the
capillary. Gas flow rates were controlled by a mass flow controller
as follows: 2 ml/min for O2, 2 ml/min for N2, and 16 ml/min for
He. Benzene was fed by using a microsyringe pump at a rate of
6.24 ␮l/h. N2 was used as an internal standard gas for the calcula-
tion of the flow rate, because the total flow rate was not constant
due to the permeation and reaction of hydrogen. The outer gas
was a mixture of H2 (10 ml/min) and He (10 ml/min). The molar
ratio of H2:O2:benzene was 5:1:0.026. The permeation of hydro-
gen proceeds from the outside to the inside of the membrane
tube. Hence, the reaction of benzene occurs at the inside of the
membrane, the surface at which the metal particles are deposited.
The reactor is heated up to a preset temperature, between 373
and 493 K, by using an electric furnace. All gas lines and sampling
valves are also heated to prevent the condensation of products.
For the confirmation of the reaction mechanism, oxidation (reac-
tion of O2 and benzene: 2 ml/min for O2, O2:benzene = 1:0.026
or 10 ml/min for O2, O2:benzene = 1:0.125) and hydrogenation
(reaction of permeated H2 and benzene: 10 ml/min for H2,
H2:benzene = 5:0.026) at the inner surface of each membrane are
investigated.
The hydrogenation of phenol over the Pd membrane (3-
cm length) was also investigated. A 1 wt.% aqueous solution
(0.2 ml/min) of phenol was introduced by using a microsyringe
pump. The other reaction conditions were similar to those of the
reaction of benzene (10 ml/min for H2).
The products were analyzed using two online gas chro-
matographs, one equipped with a TCD detector (TCD-GC) and
the other with an FID detector (FID-GC). The TCD-GC had packed
columns (Molecular Sieve 13X, 2 m, and Gasukuropak 54, 2 m) for
the analysis of inorganic gases, and the FID-GC had a TC-WAX cap-
illary column (GL Science Inc., 30 m) for the analysis of organic
compounds. Sampling was carried out several times under a given
reaction condition, because the time course was observed [8].
The hydrogen permeation rate was calculated from the decrease
in the gas flow rate of the outer gas, measured by using a flow
meter.
2. Experimental
2.1. Preparation of Pd membrane
A Pd membrane tube was prepared by depositing a Pd layer
on a porous ␣-Al2O3 tube (35-cm length, 2.0-mm OD, 1.6-mm ID,
0.15-␮m average pore size, and 4.5-m2/g specific surface area cal-
culated from a N2 adsorption isotherm measured at 77 K) by using
the metalorganic chemical vapor deposition (CVD) method. Before
the deposition of Pd, the outer surface of the tube was coated with
glass paste to prevent gas permeation, except the middle part of
tube that is the deposition region (10- or 3-cm length) of Pd. The
glass paste was fixed by calcination in air at 1423 K. The CVD of
Pd was performed in a stainless steel chamber equipped with an
electric heater and rotary pumps. A thin layer of Pd, approximately
1-␮m thick, was deposited by the sublimation of Pd(COOCH3)2 on
the outer wall of the ␣-Al2O3 tube. This preparation method of
the Pd membrane tube has been described in [2,6,7]. The hydro-
gen permeance of the prepared samples ranged from 1.0 × 10−3 to
3.8 × 10−3 mol/(m2 s1 Pa0.5) at 573 K.
Bifunctional membranes were prepared by loading metal parti-
cles into the pores of the ␣-Al2O3 tube before the CVD treatment
of Pd. Pd, Pt, Rh, Ru, Cu, and Fe were used as loading elements.
Pd, Cu, and Fe were deposited by reductive deposition. In the case
of Pd, the ␣-Al2O3 tube was dipped in 100 ml of Pd(COOCH3)2
chloroform solution (2.67 × 10−2 mol/l) for 10 min, dried at 383 K,
and reduced in 100 ml of N2H2 aqueous solution (2 mol/l) for
10 min at room temperature. After washing the tube with water,
it was dried at 383 K for 10 min. The entire process was repeated
10 times. For loading Cu and Fe, chloroform solutions of copper

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K. Sato et al. / Catalysis Today 156 (2010) 276–281
Fig. 2. Diagram of Pd based membrane reactor.
Fig. 3. XRD pattern of Pd-loaded ␣-Al2O3 tube before CVD treatment.
3. Results and discussion
3.1. Pd loading into pores of substrate
For increasing the valid surface area for hydroxylation, Pd par-
ticles were loaded into the pores of the ␣-Al2O3 substrate. Fig. 3
shows the XRD pattern of the Pd particles deposited on the ␣-
Al2O3 substrate tube before the preparation of the Pd membrane by
CVD treatment. Sharp peaks corresponding to ␣-Al2O3 and broad
peaks corresponding to Pd metal were observed. The particle size
of Pd was below 50 nm, as calculated from the FWHM of the XRD
peaks. The color of the cross section of substrate tube was uniformly
changed from white to black by loading of Pd. The surface Pd/Al
ratio was 0.39 by XPS analysis, suggesting the Pd particles were
not completely covered with the Al2O3 surface. These results indi-
cated that almost Pd particles were highly dispersed in the pores of
the ␣-Al2O3 tube. Fig. 4 exhibits the effect of reaction temperature
on benzene hydroxylation over the Pd membrane and over the Pd-
deposited Pd membrane (Pd–Pd membrane). In the former case, the
major products were phenol, cyclohexanone, cyclohexane (with a
negligibly small amount of partially hydrogenated benzene), and
CO2 (H2O) under present reaction conditions (Fig. 4a). Cyclohex-
anone was produced by the consecutive hydrogenation of phenol.
CO2 was mainly formed by the oxidation of benzene. The yield of
Fig. 4. Effect of reaction temperature on hydroxylation of benzene over (a) Pd mem-
brane and (b) Pd-deposited Pd membrane (Pd–Pd).

K. Sato et al. / Catalysis Today 156 (2010) 276–281
279
Table 1
Effect of loadings noble metals into pores of ␣-Al2O3 tube on benzene hydroxylation at 413 K.
Membrane
Metal/Al molar ratio (XPS)
Benzene conv. (%)
Selectivity (%)
Cyclohexane
Phenol
Cyclohexanone
CO2
Pd
–
10
99
63
37
3
24
89
92
49
0
0
2
0
0
0
46
76
11
8
Pt–Pd
Rh–Pd
Ru–Pd
0.04
0.20
0.05
0
other products was negligibly small. The conversion of benzene
over the Pd/␣-Al2O3 membrane was 7% at 393 K and increased with
temperature. The yield of phenol was ca 4% at 393 Kand increased to
5% at 413 K. Over 433 K, the yield decreased with increasing temper-
ature. However, the yield of CO2 increased with temperature. Thus,
high temperature caused complete oxidation and a decrease in the
yield of phenol. For enhancing the yield of phenol, the restriction
of the complete oxidation is necessary.
At all temperatures, benzene conversion reached close to 100%
due to the loading of Pd particles (Fig. 4b). However, only cyclo-
hexane and CO2 were obtained as products. The hydrogenation of
phenol to cyclohexane mainly occurred at low temperatures, while
complete oxidation was dominant at high temperatures. Although
the loading of small Pd particles caused an increase in the Pd surface
area, the loaded particles enhanced the hydrogenation and com-
plete oxidation of benzene. However, the loaded particles did not
show an obvious effect on hydroxylation. Note that most of Pd par-
ticles were highly dispersed in the pores of the substrate tube as
described above. Hence, the catalytic reaction mainly occurred at
the surface of Pd particles, which have a high surface area.
Fig. 5. Oxidation of benzene over noble-metal-loaded Pd membranes (O2:
10 ml/min, O2:benzene = 1:0.125).
over the Pd membrane as shown in Fig. 4a. Further investigation
is need for the phenomenon. However, if the temperature gradi-
ent occurred, it did not affect the order of the benzene conversion
among metal loaded membranes. The reaction site of oxygen and
permeated hydrogen was limited at the surface of Pd membrane,
because the permeated hydrogen species did not desorb under the
oxygen rich condition. Hence, the influence of exothermic heat
might be same for each membrane.
In general, noble metals showed high catalytic performance
for both oxidation and hydrogenation. Fig. 5 shows the result of
benzene oxidation over noble-metal-loaded membranes. The prod-
ucts were CO2 and H2O. Although the amount of benzene in this
reaction was 5 times higher than that in hydroxylation (Table 1),
the Pt- or Rh-loaded membrane exhibited high performance in
contrast to the sole Pd membrane. This can be attributed to the
high activity of noble metals and the large surface area of metal
particles dispersed on the surface of the substrate tube. The imbal-
ance between the activities of the surface of the Pd membrane
and the metal particles disturbs hydroxylation. If the interaction
between the Pd membrane and the noble metals takes place,
the imbalance due to the difference between the surface areas
masks the co-catalytic effect. For example, benzene reacted with
the oxygen on Pt particles rather than the active oxygen gener-
ated on the Pd surface. For the enhancement of the hydroxylation
activity due to the co-catalytic effect, a balance between the activ-
ities of the Pd membrane surface and metal particles might be
necessary.
3.2. Loading of noble metals into pores of substrate
The co-catalytic effect between the Pd membrane and other
metal particles in the pores of the substrate was investigated.
Table 1 summarizes the effect of loading noble metals into the
pores of the ␣-Al2O3 tube on benzene hydroxylation. XPS analysis
revealed that the loaded metal particles did not completely cover
the Al2O3 surface, but were dispersed on the surface, similar to
the case of Pd particles explained in Section 3.1. As compared to
the sole Pd membrane, the loading of noble metals into the pores
of the substrate resulted in high benzene conversion in the order
Pt > Rh > Ru. However, the products were limited to cyclohexane
and CO2, similar to the case of the Pd–Pd membrane (Fig. 4b).
Our previous study on the hydroxylation of methyl benzoate
[6] suggested that hydroxylation and side reactions, i.e., oxidation
and hydrogenation, occur at different regions of the same mem-
brane. The complete oxidation takes place near the gas entrance
region, which is oxygen rich. The concentration of oxygen gradually
decreases along the membrane by the reaction of the permeated
hydrogen and benzene. At the middle region, oxygen is completely
consumed because the amount of hydrogen is larger than that
of oxygen (H2/O2 = 5). From the middle region to gas exit region,
hydrogen concentration gradually increases by desorption of the
permeated hydrogen as hydrogen molecules. Hence, the hydro-
genation favors the gas exit region. Hydroxylation occurs only in
the central region of the membrane, where the oxygen concentra-
tion is low. These results suggest that the increase in the surface
area due to the loading of noble metals did not lead to an increase
in the surface area for hydroxylation.
3.3. Effect of Cu loading on hydroxylation
The loading of transition metals, such as Fe and Cu, showed
different results as compared to noble metals (Table 2). The load-
ing of Fe caused a decrease in benzene conversion and suppressed
hydroxylation. In contrast, the loading of Cu caused an increase
in benzene conversion and the selectivity of hydroxylation prod-
ucts, namely phenol and cyclohexanone, while the selectivity of
CO2 decreased to 18%. The loading amount of Cu was of the same
It is noted that the exothermic heat might occur by the oxida-
tion near the gas entrance region. In this reaction condition, it was
mainly due to the water formation by the reaction of oxygen and the
permeated hydrogen, because the flow rate of O2 was higher than
that of benzene (O2:benzene = 1:0.026). The exothermic heat might
enhance the complete oxidation which favors the high temperature

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K. Sato et al. / Catalysis Today 156 (2010) 276–281
Table 2
Effect of loading transition metals into pores of ␣-Al2O3 tube on benzene hydroxylation at 413 K.
Membrane
Metal/Al molar ratio (XPS)
Benzene conv. (%)
Selectivity (%)
Cyclohexane
Phenol
Cyclohexanone
CO2
Pd
Cu–Pd
Fe–Pd
–
10
18
8
3
9
18
49
49
0
2
24
0
46
18
82
0.08
15
Pd membrane. However, the oxidation activity over the Cu–Pd
membrane increased with temperature in contrast to hydroxyla-
tion (Fig. 6). In the case of hydrogenation, the effect of Cu loading
was evident. The order of the hydrogenation of benzene to cyclo-
hexane was Cu–Pd ꢀ Pd > Fe–Pd (Fig. 7b). These results suggested
that the enhancement of the hydrogenation activity due to Cu
loading suppressed the complete oxidation, which occurred near
the gas entrance region, and enhanced the hydroxylation activity
due to the co-catalytic effect. This enhancement was mainly due
to the increase in the hydrogenation activity without significant
acceleration of oxidation. That is, it is advisable to accelerate the
hydrogenation activity without increasing the oxidation activity
for increasing the hydroxylation activity. In the case of the noble-
metal-loaded membrane, both oxidation and hydrogenation were
enhanced, causing the acceleration of side reactions. In addition, in
this reactor system, hydroxylation is highly influenced by the con-
centration of oxygen and hydrogen [8]. Hydroxylation occurs under
the redox condition. In general, Cu works as a catalyst for the Fen-
ton reaction to produce active oxidation species by the redox cycles.
Hence, the co-catalytic effect of Cu might be effective in enhancing
hydroxylation. The interaction between Pd and Cu was suitable for
hydroxylation.
Fig. 6. Effect of reaction temperature on hydroxylation of benzene over Cu–Pd
membrane: (a) conversion and (b) yield.
3.4. Conversion of phenol to cyclohexanone on Pd membrane
reactor
order as that of the noble metals (Table 1). The effect of reaction
temperature over the Cu–Pd membrane is shown in Fig. 6. The max-
imum yield of phenol was observed at 413 K, which was same as
that over the Pd membrane (Fig. 4a). However, the conversion of
benzene and the yield of CO2 decreased above 413 K, suggesting
the restriction of complete oxidation at higher temperature ranges
over the Cu–Pd membrane.
To confirm the catalytic activity of side reactions, the oxida-
tion and hydrogenation of benzene were separately carried out
over the Pd, Fe–Pd, and Cu–Pd membranes. The result of ben-
zene oxidation is shown in Fig. 7a. The order of oxidation was
Fe–Pd ꢀ Cu–Pd ≈ Pd. In contrast to the noble-metal-loaded mem-
branes, the loading of Cu did not significantly affect oxidation
on the Fe–Pd and Cu–Pd membranes as compared to the sole
One of the industrial utilization of phenol is the conversion to
cyclohexanone for the synthesis of ␧-caprolactam. In this Pd mem-
brane reactor system, some amount of phenol is converted into
cyclohexanone. In this reactor, near the gas exit region of Pd mem-
brane is under hydrogen atmosphere, because oxygen is completely
consumed at the region near the gas entrance. To confirm the cat-
alytic activity for the conversion of phenol to cyclohexanone, the
hydrogenation of phenol was carried out over the Pd membrane.
Fig. 8 shows the results of phenol hydroxylation under the coexis-
tence of vapor, because vapor was formed by the reaction of oxygen
and permeated hydrogen during hydroxylation. The Pd membrane
showed good performance for the selective conversion of cyclo-
hexanone below 433 K under the coexistence of vapor. Thus, the
hydrogenation activity effectively increased by loading metals into
Fig. 7. Related reactions over transition-metal-loaded Pd membranes: (a) oxidation of benzene (O2: 2 ml/min, O2:benzene = 1:0.026) and (b) hydrogenation of benzene by
permeated hydrogen (H2: 10 ml/min, H2:benzene = 5:0.026).

K. Sato et al. / Catalysis Today 156 (2010) 276–281
281
An increase in the surface area due to the loading of noble
metal particles, such as Pd, Pt, and Rh, enhanced the activity of side
reactions, i.e., complete oxidation and hydrogenation. This nega-
tive effect on hydroxylation was due to the imbalance between
the catalytic activities of the noble metal particles and the Pd
membrane, which has a relatively small surface area. In con-
trast, the loading of Cu enhanced the benzene conversion and
hydroxylation activity by suppressing complete oxidation. This
enhancement was mainly due to the increase in the hydro-
genation activity without significant acceleration of oxidation.
The interaction between Cu and Pd enhanced the hydroxylation
activity.
In addition, this membrane system was found to be suitable
for the selective hydrogenation of phenol to cyclohexanone. This
shows that the appropriate combination of Pd membranes and
active metals will enhance the hydrogenation activity for the selec-
tive conversion of benzene to cyclohexanone in a sole reactor
system.
Fig. 8. Reaction of phenol and permeated hydrogen over length of 3 cm on Pd mem-
brane (H2: 10 ml/min, 1 wt.% phenol aqueous solution: 0.2 ml/min).
the pores of Pd membranes (Fig. 4b). Hence, proper loading of Pd
particles near the gas exit region of the membrane might enhance
the hydrogenation of phenol to cyclohexanone. This suggests that
the selective conversion of benzene to cyclohexanone is performed
by the sole reactor system.
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4. Conclusion
The effect of loading metal particles into the pores of an ␣-Al2O3
substrate support tube on the one-step hydroxylation of benzene
on a Pd membrane reactor was investigated. In this case, the Pd
membrane worked not only as a hydrogen separator but also as a
catalyst, which was not optimized for hydroxylation.