A Pd-TSH composite membrane reactor for one-step oxidation of benzene to phenol

Article abstract of DOI:10.1039/c9cc03645h

Pd membranes with excellent stability and flux were prepared by modified electroless plating, and a loose TSH zeolite with intraparticle hollows was joined on the Pd membrane by a covalent bonding method. The prepared Pd-TSH composite membrane shows outst

Full text of DOI:10.1039/c9cc03645h

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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DOI: 10.1039/C9CC03645H
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Pd-TSH composite membrane reactor for one-step oxidation of
benzene to phenol
Wei Liua, Xiaobin Wanga,*, Rongrong Zhaoa, Bo Menga, Cuncun Zuoa and Xiaoyao Tanb
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Pd membranes with excellent stability and flux were prepared
by modified electroless plating and a loose TSH zeolite with
intraparticle hollows was joined on the Pd membrane by
covalently bonding method. The prepared Pd-TSH composite
membrane shows outstanding performance for phenol
production in the one-step oxidation of benzene to phenol.
hard to see that this Pd CMR system is attractive but still
presents considerable challenges. Furthermore, membrane
deterioration, hot spots and low raw materials utilization
efficiency were observed [3,4,11]. Pd membrane reactor
has also been hampered by either poor stability or
durability because of the low reaction temperature (<300℃)
which is below the embrittlement temperature of Pd [12].
The main challenges for this Pd CMR are to decrease the
cost, enhance the utilization of raw materials and increase
the membrane stability. In order to improve the reaction
performance, we had designed different types of Pd-based
composite membrane reactors by incorporating various
titanium silicalite (TS) zeolite films [13-15]. The catalytic
activity of Pd CMR can be altered by TS catalysts which
could stabilize the active oxygen species to participate the
reaction. However, the slow diffusion of reactants/products
through the zeolite layer caused by the tightly compacted
thick TS film and small pore size of TS zeolite led to the
undesirable low conversion despite the improvement of
phenol selectivity. Recently, our group further developed a
novel bifunctional TS zeolite modified Pd capillary
membrane microreactor to improve the reaction
performance [16]. However, the raw materials utilization
efficiency should be further improved. We had also
discovered that the sufficient contact of reactants and the
low dead-end volume are key factors to improve the
reaction performance [12,14]. Consequently, it is important
to increase the contact area and the opportunity of
reactants to participate the reaction. Hollow fibers are
highly preferred as the resultant membrane which could
provide high surface area per unit volume and large finger-
like porous channels [17,18], thus leading to lower diffusion
resistance of reactants and better contact opportunity.
To overcome these disadvantages existed in Pd catalytic
membrane reactor, in this work, the cross-linking Pd
membrane with thin thickness was first prepared on α-Al2O3
hollow fiber by modified electroless plating method. After that,
TS catalysts with large intraparticle hollows (TSH) were
attached on the surface of Pd membrane, which was
Phenol is a key precursor for the production of drugs,
dyestuffs and synthetic resin. The current cumene process
is either not profitable or is environmentally unfriendly
because of the multi-step, high-energy consumption and
generation a large amount of by-products. Therefore,
significant research efforts have been devoted to the one-
step oxidation of benzene to phenol. Among these
approaches, the direct oxidation of benzene to phenol
using mixture of H2 and O2 has been received considerable
attention due to its environmental and economical
perspectives. However, none of the reports to date possess
both high conversion and phenol selectivity [1,2]. Another
common problem is the possible explosion due to the
presence of H2 and O2 mixture. These problems had been
better resolved by Pd catalytic membrane reactor (CMR)
reported by Niwa and co-workers [3]. H2 and O2 are
separately supplied to both sides of the Pd membrane,
which diminishes the explosion danger. H2 is dissociated
into
H atoms, which diffuse onto the opposite Pd
membrane. Afterwards, the active H atoms react with O2 to
transform activated oxygen species, further reacting with
benzene to produce phenol. More important, a higher
benzene conversion of ~13% with phenol selectivity of 85%
at 423K was obtained. Although this Pd CMR route has
been widely investigated, many other researchers were not
sufficiently lucky to reproduce their results [4-10]. It’s not
a. School of Chemistry and Chemical Engineering, Shandong University of
Technology, Zibo 255000, China.E-mails: wangxiaobin@sdut.edu.cn
b. School of Chemistry and Chemical Engineering, Tianjin Polytechnic University,
Tianjin 300387, China
Electronic Supplementary Information (ESI) available: Experimental details,
additional SEM images, EDX analysis, XRD patterns, the comparison of H2 fluxes,
long-term stability test and the reaction pathway.
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applied for the one-step oxidation of benzene to phenol.
More experimental details can be found in the ESI.
surface. As a result, a cross-linking structure of Pd
membrane with Al2O3 hollow fiber could be formed. This is
View Article Online
DOI: 10.1039/C9CC03645H
Fig. 1 and S1 are the schematic diagram of the preparation
process. The procedure involves: (1) preparation of α-Al2O3
hollow fibers; (2) introduction of Pd(II) seeds by dip-coating;
(3) activation of Pd(II) to Pd(0) seeds by H2 treatment; (4)
formation of Pd membrane; (5) deposition of TSH catalyst
on Pd membrane supported on the inner surface of support
by APTS(3-aminopropyltrimethoxysilane)-assisted method
(covalently bonding method). It is noteworthy that the
sensitization and activation steps have been omitted and
the preparation procedure of Pd membrane is convenient
compared to the traditional electroless plating method. The
one-step oxidation of benzene to phenol occurred in the
lumen side with H2 in the shell side as shown in Fig. S2.
a crucial factor for the better adhesion between Pd
membrane and support, producing excellent membrane
stability. Furthermore, the cross-linking structure would
increase the reaction surface area, thus improving the
reaction performance. The thinner Pd membrane thickness
(1-2μm) would increase the H2 flux, which could be benifit
for the reaction. Fig. 2k reveals that nanosized TSH particles
were uniformly attached on the Pd surface by a new APTS-
assisted technique according to our recent report [16]. A
major difference is that the TSH zeolite used in this work
has large intracrystalline voids and pores after TPAOH
(tetrapropylammonium hydroxide) treatment. The nitrogen
adsorption/desorption isotherm of TSH in Fig. 3c also
illustrates the creation of intracrystalline voids, which can
be verified by the abrupt closure at p/p0=0.42 on the
desorption branch [19]. Noteworthy is that the original size
and shape of crystals remained almost unchanged before
and after TPAOH treatment (Fig. 3a and b). The same peak
position and similar peak intensity of XRD (Fig. S4) proved
that TSH catalyst had the same structure as TS zeolite. It is
believed that the treatment affects essentially the core of
Pd membrane
TSH catalyst
α-Al2O3 hollow fiber
Fig. 1 Schematic diagram of Pd-TSH composite membrane.
a
b
c
the crystals by
a dissolution–recrystallization process
500 μm
100 μm
1 mm
[15,19]. The large intraparticle voids could provide the
reactants much more opportunity to contact directly with
framework titanium, thus improving the raw materials
d
e
f
Pd membrane
utilization efficiency and resulting in high reaction activity.
20 μm
5 μm
2 μm
300
b
c
a
250
200
150
100
50
g
h
k
Pd
TSH
0
0.0 0.2 0.4 0.6 0.8 1.0
p/p0
Pd
finger-like
channel
10 μm
20 μm
1 μm
Pd
Fig. 3 TEM image of TS zeolite before treatment (a), TEM
image (b) and nitrogen adsorption/desorption isotherm (c)
of TSH zeolite after TPAOH treatment.
Fig. 2 SEM images of Al2O3 hollow fiber (a-c), Pd membrane
supported on the outside of substrate (d,e: surface; f: cross-
section), Pd membrane supported on the inner surface of
substrate (g,h) and Pd membrane after TSH zeolite
deposition supported on the inside of substrate (k).
H2 permeation for Pd-TSH composite membrane was
examined at different trans-membrane pressure and
temperature. As is excepted in Fig. 4, the H2 flux increases
with temperature and pressure. A H2 permeation flux of
1.26-5.9×10-6 mol·m-2·s-1·Pa-1 at 473-773K was obtained,
which compares favorably with those reported in the
literatures (Table S1). The higher H2 flux stems from the
thinner membrane thickness (1-2μm) and large finger-like
channels inside the hollow fibers which minimized the gas
transport resistance through the substrate layer. The
stability of Pd membrane was tested between 473 and
573K (Fig. S5), which is below the embrittlement
temperature of Pd [12,20]. Fig. 5 plots the H2 flux for the Pd
membrane during the twenty heat cycles. Pd membrane
was subjected to a rapid 100K change from 473 to 573K
which is below the embrittlement temperature. Fig. 5
indicates that the H2 flux and H2/N2 selectivity almost
unchanged even when the temperature is as below as 473K,
demonstrating the outstanding stability due to the
excellent anchorage of Pd layer into the Al2O3 hollow fiber
support. The influence of pressure fluctuations on H2 flux
Fig. 2 shows the morphology and microstructure of α-Al2O3
hollow fiber and Pd composite membranes. Fig. 2a displays
that the α-Al2O3 hollow fiber has the o.d. and i.d. of 2.2 and
1.5 mm, respectively. The wall thickness of ~0.35 mm with
long finger-like channels (with size 30-50 μm) extending to
the inside surface was formed (Fig. 2b & c). More finger-like
pores would minimize the gas transport resistance through
the support. Fig. 2d-f exhibits that a thin and dense Pd
membrane layer with thickness of 1-2μm was successfully
prepared on the outside surface of support. It can be seen
from Fig. 2f that the Pd also penetrate into the porous
alumina support through the pores due to the Pd seeds
inside the support, leading to excellent anchorage between
membrane and support. The elemental distribution profiles
along the cross-section of Pd/Al2O3 further confirmed this
phenomenon (Fig. S3). Fig. 2g and h display that the Pd also
grow and penetrate inside the support through the finger-
like channels besides forming a layer of Pd film on the inner
2 | J. Name., 2012, 00, 1-3
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important reason in excellent activity for Pd-TSH is due to
was also examined (Fig. S6). The Pd-TSH membrane could
also tolerate rapid high feed pressure fluctuations between
100 and 200 KPa at 473K. In a word, a strong anchorage
between Pd membrane and support resulting from the
cross-linked structure was achieved, leading to excellent
stability of Pd membrane prepared by modified electroless
plating method. The benzene oxidation reaction further
confirmed the stability of Pd membrane as below.
View Article Online
the existence of large voids in thDeOIT: S10H.10z3e9o/Cli9tCe,C0w36h4i5cHh
decrease the pore diffusion limitations and increase the site
accessibility of reactants. TSH zeolite not only acts as the
catalyst to improve the reaction but also provides an
effective protective barrier to protect the Pd membrane by
avoiding the direct contact of the organics with Pd surface.
The cross-linked structure of Pd membrane not only
improves the adhesion between the membrane and
support, but also enhances the reaction surface area. Large
finger-like channels and thin sponge-like layer inside the
hollow fibers could effectively reduce the gas transport
resistance through the support, which is one of the reasons
that promoting the contact of reactants. These advantages
together determine the excellent stability and remarkable
773 K
1.2
723 K
673 K
623 K
573 K
473 K
1.0
0.8
0.6
0.4
0.2
0.0
catalytic activity of Pd-TSH membrane.
100
0.00
0.05
0.10
Pa (Mpa)
0.15
0.20
phenol selectivity
Fig. 4 H2 fluxes through Pd-TSH composite membrane as a
90
function of pressure at different temperature.
0.30
0.25
0.20
0.15
0.10
0.05
0.00
1000
900
800
700
600
500
80
573K H2/N2 ideal selectivity
20
benzene conversion
573K H2 flux
473K H2 flux
10
0
Temperature exchanging test
0
20
40
60
80
100
120
Reaction time (h)
Fig.
6 Catalytic stability for the Pd-TSH composite
membrane in the benzene oxidation reaction.
473K H2/N2 ideal selectivity
0
5
10
15
20
Temperature exchanging cycles
The advantage of Pd-TSH composite membrane could be
further evidenced from the raw materials utilization
efficiency in Table S2. Compared to our previous works and
other publications [3,4,12,16], the value of H2O/phenol
ratio for Pd-TSH membrane in this work shows the lowest
water production. It is worth mentioning from Table S2 that
the phenol production was much higher than other reports
and 1.68 kilograms per kilograms of Pd catalyst per hour
was obtained in this work. Concerning the raw materials
efficiency, one can see that the catalytic performance of Pd-
TSH in this work has significant improvement without
compromising the conversion and selectivity. Fig. S8 further
gives the reaction pathway in Pd-TSH composite membrane.
During the first step, H2 is dissociated to active H* while
permeating through the Pd membrane to the opposite side.
If there is no Pd catalyst, H* would quickly recombine to H2.
The cross-linking structure of Pd gives H* enough time to
react with O2 to produce active oxygen species (e.g., H2O2,
HOO*, HO*) which further react with framework titanium
to generate Ti peroxo species (Ti-OOH), avoiding the quick
decomposition of these active oxygen species. This ensures
that benzene has sufficient time to react with active oxygen
species to produce phenol. It is particularly important to
mention that the intraparticle hollows in TSH were much
more favorable for active oxygen species and benzene to
reach the isolated framework titanium active site than
smaller micropores in TS. This could help to explain why Pd
membrane modified with TSH zeolite possessing large
hollows have high reaction performance as well as excellent
Fig. 5 Stability of Pd-TSH composite membrane for the
temperature changing test at 100 KPa.
One-step oxidation of benzene to phenol on Pd-TSH
composite membrane was tested and the results are
plotted in Fig. 6. A high risk of membrane delamination and
‘hot spots’ were observed due to the low reaction
temperature [3,4], which seriously affects the membrane
life, one of the most important criteria for industrial
application. A constant benzene conversion of ~22% with
phenol selectivity of ~97% was obtained at 473K during the
entire 120h run. H2 flux remained relatively constant at 0.13
mol·m-2·s-1 (Fig. S7), further confirming the stability of Pd-
TSH membrane. It is noteworthy from Table 1 that reaction
performances vary greatly for these different research
groups and the results in this work are much higher than all
these publications [3,4,6-10,21-23]. Pure Pd membrane
without TSH catalyst could give benzene conversion and
phenol selectivity of 14.8% and 76.3% (Table 1). However,
Pd-TSH membrane received a larger improvement by 47.9%
for benzene conversion and 27.9% for phenol selectivity,
reaching the value of 21.9%-conversion and 97.6%-
selectivity. It is well known that smaller particles and larger
pores could dramatically shorten the diffusion path and
reduce the intracrystalline diffusion resistance [15,16]. The
large intraparticle pores inside the TSH zeolite would
significantly increase the contact opportunity between
reactants and the isolated framework titanium which is the
active site for the Ti-containing zeolite catalysts, thus
leading to higher reactivity and better raw material
utilization efficiency (Table 1 and S2). As a result, one
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Pd-TSH membrane for H2 separation and benzene oxidation
raw materials utilization efficiency. It should be pointed out
that the benzene oxidation to phenol cannot occur with
either H2 or O2 gas alone (Fig. S9). If benzene and H2 were
respectively fed to the tube and shell side, only cyclohexane
reaction resulting from the strong adhesion VbieewtAwrtiecleenOnlPinde
DOI: 10.1039/C9CC03645H
membrane and support. Larger intraparticle voids inside
the TSH zeolite promoted the effective and sufficient
contact between reactants and isolated framework
titanium, leading to the excellent reaction performance.
The porous finger-like channels significantly reduced the
gas transport resistance, which is also an important factor
that cannot be ignored. The larger Pd reaction area plays an
indispensable role for the higher H2 flux and better reaction
performance. In this way, a benzene conversion as high as
21.9% at 97.6% phenol selectivity could be achieved.
Reaction on-line detection using in-situ infrared technology
is under way. This Pd-TSH composite structure will also
promote its application in other H2-related fields.
from the hydrogenation of benzene was observed (Xbenzene
=
1.2%). However, no reaction was observed if only benzene
and O2 were co-fed to the reactor.
In conclusion, a cross-linking Pd membrane structure was
prepared by a modified electroless plating method on the
ceramic hollow fibers. The use of embedded Pd seeds
avoids the time consuming sensitization and activation
steps. A layer of TSH catalysts with large hollows were
deposited by APTS-assisted method. The Pd-TSH composite
membrane exhibits outstanding H2 flux with 1.26-5.9×10-6
mol·m-2·s-1·Pa-1 at 473-773K. It proves to be very stable of
Table 1. Comparison of reaction results in this work and from the literatures.
Wall
thickness
(mm)
0.2
-
Flow
channel
(mm)
1.6
O.D.
(mm)
I.D.
(mm)
Benzene
conversion (%)
Phenol
selectivity (%)
Phenol
yield (%)
Temperature
(K)
Long-
term (h)
Ref.
2
14.2
13
13
12.5
-
-
2
2
2
1.6
-
9
9
7.5
-
13.25
3.8
4.8
5.06
0.15
0.015
-
18
10
10
85.3
4.1
60.4
60.81
87
9.6
67
79
40
49
11.3
0.16
2.9
3.08
0.13
0.001
-
14.3
4
4.9
423
423
473
473
423
423
423
473
433
413
10
72-96
46
46
-
-
-
5
20
-
[3]
[4]
[6]
[7]
[8]
-
2
2
2.5
-
-
0.2
0.2
0.2
-
2-3
0.5-0.7
2
2
0.9
[9]
-
[10]
[21]
[22]
[23]
This
worka
This
workb
1.6
1.6
1.6
0.9
0.45-0.65
2.2
1.6
0.3
1.6
14.8
76.3
11.3
473
120
2.2
1.6
0.3
1.6
21.9
97.6
21.4
473
120
a: pure Pd membrane without catalyst; b: Pd-TSH composite membrane.
[10]
R. Dittmeyer and L. Bortolotto, Appl. Catal. A, 2011, 391,
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This work was supported by the National Natural Science
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Provincial Natural Science Foundation (ZR2016JL011), Key
Research and Development Program of Shandong Province
(2017GGX70105) and SDUT
& Zibo City Integration
Development Project (2017ZBXC137).
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DOI: 10.1039/C9CC03645H
Table of Contents
H2
O2
benzene
S
97.6
X
Y
phenol
21.9
21.3
Pd-TSH membrane shows excellent stability and outstanding performance for phenol production in the one-step
oxidation of benzene to phenol.