ZSM-5 zeolite single crystals with b-axis-aligned mesoporous channels as an efficient catalyst for conversion of bulky organic molecules

Article abstract of DOI:10.1021/ja300078q

The relatively small and sole micropores in zeolite catalysts strongly influence the mass transfer and catalytic conversion of bulky molecules. We report here aluminosilicate zeolite ZSM-5 single crystals with b-axis-aligned mesopores, synthesized using a designed cationicamphiphilic copolymer as a mesoscale template. This sample exhibits excellent hydrothermal stability. The orientation of the mesopores was confirmed by scanning and transmission electron microscopy. More importantly, the b-axis-aligned mesoporous ZSM-5 shows much higher catalytic activities for bulky substrate conversion than conventional ZSM-5 and ZSM-5 with randomly oriented mesopores. The combination of good hydrothermal stability with high activities is important for design of novel zeolite catalysts. The b-axis-aligned mesoporous ZSM-5 reported here shows great potential for industrial applications.

Full text of DOI:10.1021/ja300078q

Communication
pubs.acs.org/JACS
ZSM-5 Zeolite Single Crystals with b-Axis-Aligned Mesoporous
Channels as an Efficient Catalyst for Conversion of Bulky Organic
Molecules
Fujian Liu,†,∥ Tom Willhammar,‡ Liang Wang,§ Longfeng Zhu,§ Qi Sun,† Xiangju Meng,†
Wilder Carrillo-Cabrera,⊥ Xiaodong Zou,*,‡ and Feng-Shou Xiao*,†
†Key Lab of Applied Chemistry of Zhejiang Province, Zhejiang University, Hangzhou 310007, China
‡Berzelii Centre, EXSELLENT on Porous Materials and Department of Materials and Environmental Chemistry,
Stockholm University, Stockholm SE-10691, Sweden
§College of Chemistry, Jilin University, Changchun 130012, China
⊥Max-Planck-Institut fur Chemische Physik fester Stoffe, Dresden DE-01187, Germany
̈
∥Institute of Applied Chemistry, Shaoxing University, Shaoxing 312000, China
S
* Supporting Information
obtained.12,13b Using designed bifunctional surfactants, stable
ABSTRACT: The relatively small and sole micropores in
zeolite catalysts strongly influence the mass transfer and
catalytic conversion of bulky molecules. We report here
aluminosilicate zeolite ZSM-5 single crystals with b-
axis-aligned mesopores, synthesized using a designed cationic
amphiphilic copolymer as a mesoscale template. This
sample exhibits excellent hydrothermal stability. The
orientation of the mesopores was confirmed by scanning
and transmission electron microscopy. More importantly,
the b-axis-aligned mesoporous ZSM-5 shows much higher
catalytic activities for bulky substrate conversion than
conventional ZSM-5 and ZSM-5 with randomly oriented
mesopores. The combination of good hydrothermal
stability with high activities is important for design of
novel zeolite catalysts. The b-axis-aligned mesoporous
ZSM-5 reported here shows great potential for industrial
applications.
single-unit-cell nanosheets of zeolite MFI were successfully
prepared.3c,13a These results suggest that the formation of
mesoporous zeolites with controllable orientations may need
templates that can form large micelles or micelle aggregates
with small curvatures. Large micelles could be obtained using,
for example, triblock polymer of P123 and copolymer of PS215
−
PEO100 in strongly acidic aqueous solution. They were used to
synthesize ordered mesoporous silicas and carbons with large
mesopores.14,15 However, these amphiphilic copolymers often
do not have strong interactions with silica species in basic
media that is normally required for zeolite synthesis.14,15
Therefore, it is still difficult to use conventional amphiphilic
copolymers as templates to obtain mesopores in zeolite crystals.
Here we show that appropriately designed amphiphilic co-
polymers with high molecular weight can direct the formation
of zeolite ZSM-5 single crystals with b-axis-aligned mesopores
(ZSM-5-OM). The open mesopores in ZSM-5-OM are very
helpful for conversion of bulky organic molecules.
We used copolymer polystyrene-co-4-polyvinylpyridine (MW
≈ 1.6 × 105, PSt-co-P4VP) which was treated with methyl
iodide, forming a cationic polymer (C-PSt-co-P4VP). The posi-
tively charged C-PSt-co-P4VP strengthens its interaction with
the negatively charged silica species in the synthesis of zeolites
and is thus a favorable template for synthesizing zeolites with
oriented mesopores.1 The formation of the cationic copolymer
luminosilicate zeolites with intricate micropores and
Astrong acidity have been widely used as heterogeneous
catalysts in the petrochemical and fine chemical industries.1,2
However, relatively small and sole micropores in zeolites
strongly hinder the diffusion of reactants and products, and
thus the catalytic conversion of bulky molecules.2,3 Using
zeolite nanocrystals may overcome the diffusion problem, but
separation of zeolite nanocrystals from a reaction mixture is
difficult.4 Recently, great efforts have been put forth to
introduce mesopores/mesoporosity in zeolites through tem-
plating strategies.5−10 Notably, these mesopores are randomly
oriented in zeolite crystals. Controlling mesopore orientation in
zeolites still remains a challenge. When ordered mesostructured
carbon of CMK-3 with small pores (3−6 nm) was used as a
hard template, disordered mesopores were formed as a result of
aggregation of nanosized ZSM-5 crystals.11 However, when
ordered mesostructured carbons with large pores (6−40 nm)
or polyquaternary ammonium surfactants were used, ordered
mesopores surrounded by zeolite nanocrystal aggregates were
1
C-PSt-co-P4VP is confirmed by H (Figure S1), 13C NMR
(Figure S2), and IR (Figure S3) spectra.16
Generally, aluminosilicate ZSM-5 is synthesized from a small
organic template of tetrapropylammonium hydroxide
(TPAOH). In this work, we used both small molecules of
TPAOH and large copolymers of C-PSt-co-P4VP as organic
templates for synthesizing ZSM-5 zeolite with b-axis-aligned
mesopores (denoted as ZSM-5-OM). 13C NMR spectroscopy
shows that C-PSt-co-P4VP was essentially intact during the
crystallization of ZSM-5-OM samples (Figures S4 and S5).
Received: January 15, 2012
Published: March 1, 2012
© 2012 American Chemical Society
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X-ray diffraction (XRD) of calcined ZSM-5-OM gives well-
resolved peaks characteristic of ZSM-5 zeolite (Figures S5 and
S6a).17 Calcined ZSM-5-OM exhibits a type IV N2 adsorption/
desorption isotherm with a hysteresis loop at a relative pressure
of 0.45 < P/P0 < 0.90, characteristic of mesoporous solids
(Figure S6b). The corresponding Barrett−Joyner−Halenda
(BJH) pore size distribution shows mesopores in the range of
6−60 nm, with two peaks at 13 and 55 nm, respectively (Figure S6b
inset). These results indicate that the ZSM-5-OM sample contains
mesopores.
low-magnification images (Figures 1a and S7). The particles
have a hexagonal prism shape with b-axis-aligned mesopores
(Figure 1b,c). Both small pores (about 10 nm) and large pores
(about 50 nm) can be observed on the surface of the particles,
as shown in the high-magnification images (Figures 1d and
S7b,c).
To further confirm the mesopores and the pore orientations,
a calcined ZSM-5-OM sample was studied by transmission
electron microscopy (TEM) (Figure 2). Electron diffraction
shows that the entire particle is a single crystal, exhibiting a
single diffraction pattern (Figure 2a). In order to reveal the
inner pore structures of the crystals, focused ion beam (FIB)
has been used to prepare thin slices perpendicular to each of
the main crystallographic axes of ZSM-5-OM crystals (Figure S8).
Images of the thin slices perpendicular to the b-axis show meso-
pores of 10−50 nm in size throughout the crystal (Figure 2b). The
size range of the mesopores is in good agreement with the N2
adsorption (6−60 nm). The mesopores are distributed within the
crystal as well as near the surface of the crystal. High-resolution
images taken along the b-axis show round mesopores (Figures 2c
and S9a), which suggests that the pores run along the b-axis,
perpendicular to the thin slice. The straight 10-ring microporous
channels in ZSM-5 are well-ordered throughout the entire particle,
which shows that the entire particle contains only one ZSM-5
single crystal, and not nanocrystal aggregates. HRTEM images
taken along the c- and a-axis further confirm that the mesopores
are running along the b-axis and the entire particle is a single
crystal of ZSM-5. In addition, pores elongated along the b-axis or
round were also observed along the c- and a-axis. This indicates
that the mesopore channels have preferred orientations along the
b-axis. The mesopore channels are not straight, and the width
varies through the channels (Figures 2d,e and S9c). The difference
in mesopore sizes might be related to the aggregation of the
mesoscale amphiphilic copolymers.
Figure 1 shows scanning electron microscopy (SEM) images
of a calcined ZSM-5-OM sample synthesized in the presence of
Figure 1. (a) Low- and (b−d) high-magnification SEM images of a
calcined ZSM-5 sample synthesized in the presence of C-PSt-co-P4VP
template showing oriented mesopores.
C-PSt-co-P4VP template. The sample consists of uniformly
sized particles (1−2 μm) with rough surfaces, as seen in the
Thermogravimetric analysis shows that ZSM-5-OM had a
weight loss of 19.4% due to the removal of TPA+ and C-PSt-co-
Figure 2. (a) TEM image of a ZSM-5-OM crystal taken along the c-axis. The corresponding electron diffraction pattern (inset) shows that the entire
particle is a single crystal. (b) TEM image of a ZSM-5-OM crystal taken along the b-axis. (c−e) HRTEM images of thin slices of ZSM-5-OM viewed
along the (c) b-, (d) c-, and (e) a-axis, with the crystals cut perpendicular to the b-, c-, and a-axis, respectively, by FIB. The b-axis is marked in (a),
(d), and (e).
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Communication
P4VP (Figure S10b). In contrast, conventional ZSM-5 exhibited a
total weight loss of 11.3% (Figure S10a). Apparently, the larger
weight loss in ZSM-5-OM is due to the presence of C-PSt-co-P4VP
template that was removed during the heating in the sample.
These results indicate that C-PSt-co-P4VP was occluded in the
as-synthesized ZSM-5-OM. Subsequent removal of these
templates led to the large pore volume and complex porosity
in ZSM-5-OM, in agreement with the results obtained by
nitrogen sorption (Figure S6b).
Table 2. Catalytic Activities for Bulky Substrate Conversion
over Various ZSM-5 Catalysts
It is worth noting that the orientation of mesopores in ZSM-
5-OM is along the b-axis as a result of the self-assembly of C-
PSt-co-P4VP with ZSM-5 building units. This phenomenon
might be related to the ZSM-5 structure (Figure S11). Among
the three different faces [001], [010], and [101] of the ZSM-5
crystals, the [010] face is the most energetically favorable
according to ab initio DFT calculations.18 Possibly, the C-
PSt-co-P4VP template prefers to occupy the [010] face in the
self-assembly; therefore, b-axis-aligned mesopores could be
formed in ZSM-5-OM. In addition, mesoporous channels in
ZSM-5-OM match best with the b-axis of ZSM-5 crystals
because the 10-ring microporous channels along the b-axis in
ZSM-5 are straight, while the microporous channels along the
a- and c-axis are zigzag. Recently, there have been many reports
on mesoporous zeolites, but it is still a great challenge to
control the orientation of the mesopores formed in zeolite
single crystals.5−10 In this work, by using a designed copolymer
of C-PSt-co-P4VP, b-axis-aligned mesopores are successfully
formed in single crystals of ZSM-5 zeolite.
Hydrothermal stability of porous materials is a crucial factor
for their applications and has been intensively studied.2,3 In the
present study, the hydrothermal stability of b-axis-aligned
mesoporous ZSM-5-OM was assessed by exposing to 100%
steaming at 800 °C for 5 h. After this treatment, there was no
obvious reduction of the crystallinity, surface area, and pore
volume (Table 1, Figures S12 and S13), indicating the excellent
hydrothermal stability of ZSM-5-OM.
catalytic conversion, %
a
b
c
sample
PBP
CBH
CBP
note
ZSM-5
12.8
49.5
88.4
86
18.7
45.7
90.8
48
23.5
37.5
46.8
this work
this work
this work
ref 3c
ZSM-5-M
ZSM-5-OM
d
multilamellar ZSM-5 nanosheet
e
unilamellar ZSM-5 nanosheet
86
76
ref 3c
a
b
Protection of benzaldehyde with pentaerythritol (PBP). Condensation
c
of benzaldehyde with hydroxyacetophenone (CBH). Condensation of
d
e
benzaldehyde with 1-pentanol (CBP). Si/Al ratio 48. Si/Al ratio 53.
In condensation of benzaldehyde with hydroxyacetophenone
(CBH), ZSM-5 and ZSM-5-M give low (18.7%) and medium
(45.7%) activities, respectively. In contrast, ZSM-5-OM is very
active, giving a conversion of 90.8%. In condensation of
benzaldehyde with 1-pentanol (CBP), ZSM-5, ZSM-5-M, and
ZSM-5-OM have conversions at 23.5, 37.5, and 46.8%,
respectively. Considering the similarities of ZSM-5, ZSM-5-M,
and ZSM-5-OM in terms of Si/Al ratios, aluminum distribution,
and acidic strength as well as a larger particle size of ZSM-5-OM
than those of ZSM-5 and ZSM-5-M,9a the superior catalytic
activities in conversion of bulky substrates over ZSM-5-OM
compared to that over ZSM-5-M should be directly assigned to
the contribution of b-axis-aligned mesopores in ZSM-5-OM. It
is possible that almost all b-axis-aligned mesopores in zeolite
crystals are opened to the surface of ZSM-5 crystals and are
accessible by bulky molecules. In contrast, a majority of the
disordered mesopores in the ZSM-5 crystals may be located in
the interiors of the crystals and hardly accessible by the bulky
molecules.
Table 1. Textural Parameters of ZSM-5, Mesoporous ZSM-
5-M, and b-Axis-Aligned Mesoporous ZSM-5-OM
mesopore
total pore
micropore
a
b
c
sample
SBET, m2/g size, nm
volume, cm3/g volume, cm3/g
ZSM-5
364
390
365
370
−
17
0.14
0.30
0.31
0.27
0.12
0.09
0.08
0.08
ZSM-5-M
ZSM-5-OM
ZSM-5-OM
13, 55
18, 55
d
In summary, b-axis-aligned mesoporous ZSM-5 (ZSM-5-OM)
has been successfully synthesized by using a designed amphiphilic
copolymer (C-PSt-co-P4VP). This sample exhibited excellent
hydrothermal stability and superior catalytic activities for
conversion of bulky substrates. ZSM-5-OM shows great potential
as an industrial catalyst, and its discovery provides new insights for
future development of hydrothermally stable and active catalysts.
a
ZSM-5 containing disordered mesopores was synthesized from using
polydiallyldimethylammonium chloride, and ZSM-5-OM with b-axis-
aligned mesopores was synthesized in the presence of C-PSt-co-P4VP
template. Si/Al ratio was 37 in the starting gels and 37, 39, and 41 in
the products of ZSM-5, ZSM-5-M, and ZSM-5-OM, respectively.
b
c
Calculated by BJH adsorption model. Calculated by the relative
d
pressure at 0.94. 100% steam treatment at 800 °C for 5 h.
ASSOCIATED CONTENT
* Supporting Information
Synthesis and characterization details. This material is available
free of charge via the Internet at http://pubs.acs.org.
■
Table 2 compares catalytic activities of the b-axis-aligned
mesoporous ZSM-5-OM in conversion of bulky substrates to
those of conventional ZSM-5 and ZSM-5 containing disordered
mesopores (ZSM-5-M).9a In protection of benzaldehyde with
pentaerythritol (PBP), ZSM-5 shows relatively low conversion
at 12.8%. After introduction of disordered mesopores in the
samples, ZSM-5-M exhibits higher activity at 49.5%. Interest-
ingly, ZSM-5-OM gives very high activity at 88.4%, which is as
good as that of ZSM-5 nanosheets (86%) under the same
catalytic conditions.3c
S
AUTHOR INFORMATION
Corresponding Author
fsxiao@zju.edu.cn; xzou@mmk.su.se.
■
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work is supported by the State Basic Research Project of
China (2009CB623507), National Natural Science Foundation
of China (20973079 and U1162201), the Swedish Research
Council (VR), the Swedish Governmental Agency for
Innovation Systems (VINNOVA), Knut and Alice Wallenberg
Foundation, and Goran Gustafsson Foundation.
̈
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