Pd-silicalite-1 composite membrane reactor for direct hydroxylation of benzene to phenol

Article abstract of DOI:10.1016/j.cattod.2010.07.008

Pd-Sil-1 composite membrane consisting of 5 μm Pd membrane anchored by multi-leg protrusions to the 2 μm Sil-1 intermediate support layer was prepared from Pd/Sil-1 co-seeds. The membrane displays good permeation flux and is stable for 2.5 weeks of permea

Full text of DOI:10.1016/j.cattod.2010.07.008

Catalysis Today 156 (2010) 282–287
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Catalysis Today
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Pd–silicalite-1 composite membrane reactor for direct hydroxylation of
benzene to phenol
a
a
a,∗
a
a
Yu Guo , Xiaobin Wang , Xiongfu Zhang , Yao Wang , Haiou Liu ,
a
a
b,1
Jinqu Wang , Jieshan Qiu , King Lun Yeung
State Key Laboratory of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116012, PR China
a
b
Department of Chemical and Biomolecular Engineering, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, SAR, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Available online 16 August 2010
Pd–Sil-1 composite membrane consisting of 5 ␮m Pd membrane anchored by multi-leg protrusions to the
2
␮m Sil-1 intermediate support layer was prepared from Pd/Sil-1 co-seeds. The membrane displays good
permeation flux and is stable for 2.5 weeks of permeation and reaction studies. The Pd–Sil-1 composite
membrane was employed for direct hydroxylation of benzene to phenol. Reaction parameters including
temperature and H2/O2 molar feed ratio were investigated for their effects on benzene conversion, phenol
yield, water generation and hydrogen efficiency.
Keywords:
Pd membrane
Membrane reactor
Benzene hydroxylation
Phenol
©
2010 Elsevier B.V. All rights reserved.
1
. Introduction
plete combustion occur under oxygen-rich condition [19,20], while
hydrogenation is favored under hydrogen-rich conditions. Cyclo-
hexane, cyclohexanone, carbon dioxide and water are the main
reaction byproducts. Although the phenol yield was low, the pro-
cess is safe, simple, clean and economical. Palladium membranes
are widely studied for separation and reaction applications [24–34].
However, the cost of the membrane and its long-term performance
remained an important issue. This work investigates a new Pd–Sil-1
composite membrane for direct hydroxylation of benzene to phe-
nol. The high flux Pd membrane was anchored to the Sil-1 zeolite
intermediate support layer by multi-legs Pd protrusions that pen-
etrate through the interstices of the zeolite grains.
Phenol is an important intermediate in the synthesis of many
important petrochemicals, agrochemicals, polymers and plastics.
Phenol is commercially produced by indirect oxidation of benzene
via cumene process [1]. The direct hydroxylation of benzene to phe-
nol is an attractive route for cleaner production of phenol and has
been demonstrated for a variety of oxidants including N O [2,3],
H O [4–7] and molecular oxygen [8]. The N O or H O are expen-
sive and there is considerable economic motivation to develop
industrially viable catalytic process based on O . Pd–Cu/SiO2 [9],
Pt/V O5/SiO [10,11], Pd/Ti–silicalite [12], Pd–PMo12/SiO2 [13],
2
2
2
2
2
2
2
2
2
Pd/Pt/Nafion/SiO and Pd/Pt/Amberlyst [14,15] were catalysts used
2
for direct hydroxylation of benzene in a mixture of O2 with
H . Hydrogen peroxide generated by the catalyst from reactions
2
2. Experimental
between oxygen and hydrogen is believed to be responsible for the
conversion of benzene to phenol.
2.1. Preparation of Pd–Sil-1 composite membrane
The one-step conversion of benzene to phenol was first reported
by Niwa et al. [16] using a palladium membrane reactor. They
postulated that the atomic hydrogen permeated across the palla-
dium reacts with the molecular oxygen to generate “in situ H O ”
The Pd–Sil-1 composite membrane was prepared according
to the method described in a prior work [35]. Porous alumina
ca. 3 ␮m nominal diameter) purchased from Foshan Ceramics
Research Institute of China with inner and outer diameters of 9 and
3 mm were cut into 75 mm long pieces. The outer wall of the tubes
were polished with sand paper, washed with dilute HCl and NaOH,
rinsed with deionized water and ethanol, and dried overnight in
oven at 393 K. The tubes were then end-sealed with glass enamel,
leaving a porous length of 30 mm for membrane deposition. The
tube was dip-coated in a water seed solution containing 2 wt.% Sil-
(
2
2
and other reactive species (i.e., HOO* and HO*) that react with
benzene to form phenol [16–23]. They observed the reaction is
sensitive to H /O feed ratio [18]. Nonselective oxidation and com-
1
2
2
∗ Corresponding author. Tel.: +86 411 39893605; fax: +86 411 39893605.
E-mail addresses: xfzhang@dlut.edu.cn (X. Zhang), kekyeung@ust.hk
K.L. Yeung).
1
seed (150 nm diameter), 0.8 wt.% PdCl and 20 wt.% poly(vinyl)
2
−
1
alcohol (PVA, MW = 1750 g.mol ). The Sil-1 seed was prepared
according to the synthesis procedure published in a prior work
(
1
Tel.: +852 2358 7123; fax: +852 2358 0054.
0
920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.cattod.2010.07.008

Y. Guo et al. / Catalysis Today 156 (2010) 282–287
283
[
36,37]. A 2 ␮m thick Sil-1 zeolite layer was deposited from a clear
2.3. Hydroxylation of benzene to phenol
synthesis solution with molar composition of 40 SiO : 5 TPA O:
2
2
2
0,000 H O [28,38–40]. Tetraethyl orthosilicate and tetrapropy-
A schematic drawing of the reactor set-up is shown in Fig. 1.
The hydroxylation reaction was carried out in the reactor with
a feed mixture of benzene, oxygen and nitrogen in the annulus
and hydrogen permeating across the Pd membrane from the tube-
side. Benzene is fed as vapor by bubbling 5 sccm N2 through a
benzene saturator kept at a temperature of 287 K. The benzene
concentration in the feed was calculated from Antoine’s equation
and monitored by gas chromatograph. The reactor temperature
was controlled by a furnace and all the gas lines were heated to
prevent condensation. The composition of reactor outlet was ana-
lyzed by two on-line gas chromatographs (GC-7890T; GC-7890F).
The organic products were analyzed by a GC equipped with flame
ionization detector and 50 m SE-30 capillary column, while the
inorganic components were detected using a GC equipped thermal
conductivity detector and a 13× molecular sieve/GDX-502 molec-
ular sieve packed columns.
2
lammonium hydroxide were respectively the silica source and
structure directing agent.
Prior to the deposition of palladium by electroless plating, the
zeolite membrane tube was calcined in air at 823 K for 6 h before
pretreating in flowing H2 at 623 K for 3 h to convert Pd(II) to Pd0
seeds. The palladium was deposited at 318 K from a plating solu-
tion containing 3.5 g/L PdCl (99.9 wt.%), 30 g/L Na EDTA (>99 wt.%),
2
2
1
01 ml/L NH OH (15 M) and 16 ml/L N H (1 M) [26,41,42]. The
4 2 4
Pd–Sil-1 composite membrane was obtained by depositing a 5 ␮m
thick Pd by repeating the 120 min plating twice. The X-ray diffrac-
tion of the deposited Pd was taken at a grazing angle. The membrane
was examined by scanning electron microscopy (KYKY-2800B)
before and after reaction. X-ray diffraction of the Pd–Sil-1 com-
posite membrane was obtained.
2.2. Hydrogen permeation
Hydrogen and nitrogen permeations were measured in a home-
3. Results and discussion
made gas permeation unit. The membrane was sealed in the
stainless steel module using graphite O-rings. The gases were fed
to the module by electronic mass flow meters and the gas pres-
sure was adjusted by back-pressure regulators. The temperature
of the membrane module was monitored by a set of thermocou-
ples and heated by a temperature-programmable furnace. The gas
was fed to the annulus formed by the membrane tube and the shell
module. The gas diffuses through the Pd membrane layer deposited
on the outer wall of the alumina tube to the permeate-side kept
at atmospheric pressure. The flux was measured for different feed
pressures and permeation temperatures using a soap-bubble flow
meter.
3.1. Preparation of Pd–silicalite-1 composite membrane
The scanning electron micrographs of the Pd–Sil-1 composite
membrane are shown in Fig. 2a for the membrane surface and
Fig. 2b for the membrane cross-section. Fig. 2a shows the palladium
was deposited uniformly on the support and consists of micron-
sized grains. Defects and pinholes were not observed during the
SEM examination and the prepared membrane was impermeable
to nitrogen at room temperature tests. A 5-␮m thick Pd film was
deposited on the Sil-1 layer (cf. inverted pyramidal structures in
Fig. 2b). A closer examination reveals that palladium penetrated
Fig. 1. Apparatus used for hydroxylation of benzene. (1) On/off valve; (2) mass flow controller; (3) benzene container; (4) pressure gage; (5) stainless steel reactor; (6) glass
enamel; (7) Pd–silicalite-1 membrane; (8) thermocouple; (9) furnace; (10) temperature controller; (11) needle valve; (12) bubble flowmeter; (13) three-way valve; (14)
on-line gas chromatograph equipped with FID detector; (15) on-line gas chromatograph equipped with TCD detector.

2
84
Y. Guo et al. / Catalysis Today 156 (2010) 282–287
Fig. 2. SEM images and schematic diagram of Pd–silicalite-1 composite membrane: (a) surface; (b) cross section; (c) schematic diagram.
the Sil-1 layer through gaps formed between neighboring zeolite
crystals anchoring the Pd film to the support. X-ray diffraction of the
Pd–Sil-1 membrane at a grazing angle shows only peaks belonging
to Pd indicating that no major impurity is present in the deposited
membrane.
[46–48]. The H2 flux increases with temperature and activation
energy of 8.3 kJ mol was obtained.
−
1
3.3. Benzene hydroxylation reaction in Pd–silicalite-1 composite
membrane
The schematic drawing in Fig. 2c illustrates the structure of the
Pd–Sil-1 composite membrane. The Pd/Sil-1 co-seeds deposited on
The hydroxylation of benzene on Pd–silicalite-1 composite
the ␣-Al O seeded the growth of Sil-1. The Sil-1 grew from the
membrane was performed at different H /O2 molar feed ratio and
2
3
2
seed layer following the contours of the underlying support and dis-
played random orientation. The gap between neighboring crystals
can be controlled during seeding and zeolite regrowth [43]. Prior
to palladium deposition by electroless plating, hydrogen treatment
was performed to convert Pd(II) in the Pd/Sil-1 co-seeds to Pd0
seeds. The palladium deposition was rapid and occurred mainly
around the vicinities of gaps formed between intergrowing zeo-
temperature and their effect on benzene conversion, phenol yield,
water generation and hydrogen efficiency were examined.
3
.3.1. H /O molar feed ratio
2 2
Niwa et al. [16] proposed that the main advantage of Pd mem-
brane reactor for benzene hydroxylation reaction is the generation
of “in situ H O ” and other reactive species on the membrane sur-
2
2
0
lites. The palladium deposition proceeds deep within the Pd /Sil-1
face that catalyzes the reaction benzene to phenol. The reaction
co-seed layer before emerging on the surface layer forming the
Pd protrusions that anchored the film to the support as shown in
Fig. 2b.
is sensitive to the H /O2 molar feed ratio [18] and the reaction
2
results for Pd–Sil-1 (Fig. 4) shows an optimum phenol yield at H /O2
2
molar feed ratio of 4.7. A phenol yield of 3% and benzene conver-
sion of 5% were obtained at this condition. Low H /O2 molar feed
2
ratio led to combustion with production of carbon dioxide in addi-
tion to main cyclohexane and cyclohexanone byproducts, while
3.2. Hydrogen permeation of Pd–silicalite-1 composite membrane
high H /O2 molar feed ratio favored hydrogenation resulting in an
increase in cyclohexane production at the expense of both phenol
and cyclohexanone.
Hydrogen was not only consumed by the hydroxylation and
hydrogenation reactions, but also by reaction with oxygen to pro-
duce water as shown in Fig. 5. This is responsible for the low
hydrogen efficiency observed in prior works [16,18–20]. Water is
The hydrogen permeation study of the Pd–Sil-1 composite
2
membrane was carried out at temperatures and pressures similar
to that of reaction. The hydrogen flux was plotted as a function of
trans-membrane pressure drop in Fig. 3 for different temperatures.
The plots show the H flux varies linearly with the trans-membrane
2
H2 pressure difference and is taken to indicate that the surface
processes were the rate-determining step rather than the bulk dif-
fusion of H2 through the Pd membrane [44,45]. However, it is also
possible that the Sil-1 layer may contribute towards higher n value
produced by benzene hydroxylation, H /O2 reaction and combus-
2
tion reactions. It is evident from the plots in Fig. 5 that water is
Fig. 3. H2 flux as a function of pressure difference at different temperatures.
Fig. 4. Effect of H2/O2 molar ratio on the reaction properties.

Y. Guo et al. / Catalysis Today 156 (2010) 282–287
285
Fig. 5. Effect of H2/O2 molar ratio on the H2 efficiency.
Fig. 7. Membrane stability: (a) long-term operation for hydrogen permeation at
73 K; (b) H2 permeation flux before and after hydroxylation at different tempera-
4
ture.
Fig. 6. Effect of temperature on hydrogenation of benzene to cyclohexane.
tion, high temperature favors water production resulting in lower
hydrogen efficiency. The highest benzene conversion (5.06%) and
phenol selectivity (60.81%) were obtained at 473 K for this study.
produced mainly by the H /O2 reaction in our reactor. We spec-
2
ulated that this is due to the higher hydrogen flux of the thinner
Pd–Sil-1 composite membrane that made more hydrogen available
for reaction. The lower phenol yield and benzene conversion are
believed to be due to poorer mass transfer rate in the large diame-
ter membrane compared to the capillary membrane used by Niwa
et al. [16].
3.4. Stability of Pd–silicalite-1 composite membrane
The Pd–Sil-1 composite membrane was operated for 18 days;
3 days were spent in membrane stabilization and hydrogen per-
1
meation studies and 5 days were spent for the reaction studies
reported in this work. The membrane was stable for membrane
permeation from 423 to 773 K and display reproducible H2 perme-
ation over the entire period of study lasting 12 days. In addition, the
Pd–Sil-1 composite membrane was able to survive 15 temperature
cycles between 673 and 773 K lasting 18 h, followed by 15 pressure
3.3.2. Temperatures
Fig. 3 shows that H2 permeation flux increases with tempera-
ture and should favor the conversion of benzene to cyclohexane
as shown in Fig. 6. The results of the benzene hydroxylation reac-
tion at the temperatures of 423, 473 and 523 K are summarized in
Table 1. The selectivity for cyclohexane increases with temperature
as expected. The most cyclohexane product was obtained at 523 K
corresponding to the lowest benzene conversion (2.1%) and phe-
nol selectivity (27%). It is very probable that temperature affects
the generation and reaction of reactive species (i.e., “in-situ H O ,
cycles between 20 and 100 kPa at 773 K lasting 12 h, and H /N2 gas
2
exchange cycles of 240 h with 18 cycles at 623 K and 10 cycles at
473 K.
Often pure palladium membrane suffers embrittlement at tem-
peratures below 573 K. Niwa et al. [16] and Shu et al. [22] both
reported a high risk of membrane failure for low temperature
hydroxylation reaction. Membrane life is therefore important for
the application. Tests were carried out for the Pd–Sil-1 compos-
2
2
HOO*, HO*). Landon et al. [49] reported that high temperature
accelerates the decomposition of peroxides on Pd surfaces. In addi-
Table 1
Effect of temperature on direct hydroxylation of benzene.
Temperature (K)
Conversion (%)
Benzene
Yield (%)
Phenol
Selectivity (%)
Water generation rate (mg/min)
Phenol
Cyclohexane
Cyclohexanone
4
4
5
23
73
23
4.91
5.06
2.15
1.95
3.08
0.59
39.63
60.81
27.41
14.95
15.17
36.23
45.40
24.02
36.36
20.1
22.5
27.4

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Y. Guo et al. / Catalysis Today 156 (2010) 282–287
Foundation (Grant No. 2008D-5006-05-06), Program for Liaoning
Excellent Talents in University (Grant No. LR201008) and Univer-
sities Science & Research Project of Liaoning Province Education
Department (Grant No. 2009S019) are gratefully acknowledged.
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