Growth of ZSM-5 zeolite microparticles from crystal seeds for catalytic hydration of cyclohexene

Article abstract of DOI:10.1039/c2ce06646g

The synthesis of ZSM-5 microparticles (MPs) possesses scientific and industrial importance due to the corresponding kinetic effects and operation issues. In this paper, a simple and scalable crystal seed growth approach was developed to synthesize ZSM-5 MPs, without an organic template, for the catalytic hydration of cyclohexene to cyclohexanol. As ZSM-5 sub-microparticles (SMPs) were used as the crystal seeds, the as-synthesized ZSM-5 MPs exhibit higher crystalline degrees and larger sizes, with an average diameter up to 8.78 μm, than that of commercial ZSM-5 SMPs. The as-synthesized ZSM-5 MPs possess typical micrometer diameters. As catalysts in the hydration of cyclohexene to cyclohexanol, the ZSM-5 MPs exhibit high catalytic performances, in a long running experiment of up to 1000 h, and a superior sedimentation property compared to commercial ZSM-5 SMPs.

Full text of DOI:10.1039/c2ce06646g

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COMMUNICATION
Growth of ZSM-5 zeolite microparticles from crystal seeds for catalytic
hydration of cyclohexene{
Ying Tang, Baojun Li, Ning Zhang, Songlin Wang, Yiqiang Wen, Peng Jin and Xiangyu Wang*
Received 8th December 2011, Accepted 9th March 2012
DOI: 10.1039/c2ce06646g
The synthesis of ZSM-5 microparticles (MPs) possesses
scientific and industrial importance due to the corresponding
kinetic effects and operation issues. In this paper, a simple and
scalable crystal seed growth approach was developed to
synthesize ZSM-5 MPs, without an organic template, for the
catalytic hydration of cyclohexene to cyclohexanol. As ZSM-5
sub-microparticles (SMPs) were used as the crystal seeds, the
as-synthesized ZSM-5 MPs exhibit higher crystalline degrees
and larger sizes, with an average diameter up to 8.78 mm, than
that of commercial ZSM-5 SMPs. The as-synthesized ZSM-5
MPs possess typical micrometer diameters. As catalysts in the
hydration of cyclohexene to cyclohexanol, the ZSM-5 MPs
exhibit high catalytic performances, in a long running experi-
ment of up to 1000 h, and a superior sedimentation property
compared to commercial ZSM-5 SMPs.
performances, superior mechanical strength, and high thermal
16–18
stability.
ZSM-5 NPs can also be synthesized by a crystal seed
15
growth method. The tunable diameters of the ZSM-5 particles
are expected to influence the catalytic performances and
sedimentation separation property. Because of the facile sedimen-
tation property, ZSM-5 microparticles (MPs) are favorable in
19–22
industrial processes.
However, this is still a serious problem for
ZSM-5 MPs as catalysts, with the contradiction between the
negative effect of the MPs diameters on the catalytic activity and
the positive effect of MPs diameters on the sedimentation
separation performance, in this catalytic reaction system. The
employment of organic templates should also be avoided for more
15,23,24
facile and environmental benign synthesis processes.
In this
paper, we report a facile crystal seed growth strategy for the
synthesis of ZSM-5 MPs under hydrothermal conditions with
ZSM-5 SMPs as the crystal seeds. The as-synthesized ZSM-5 MPs
possess typical micrometer diameters. As catalysts in the hydration
of cyclohexene to cyclohexanol, the ZSM-5 MPs exhibit high
catalytic performances, in a long running experiment of up to
1
. Introduction
1
000 h, and a superior sedimentation property compared to
The synthesis of morphology- and size-controllable molecular sieve
materials possesses scientific and industrial importance due to the
commercial ZSM-5 SMPs.
1–3
corresponding kinetic effects and operation issues.
It is an
important general strategy to assemble nanoparticles (NPs) or sub-
microparticles (SMPs) for the morphology- and size-controllable
construction of complex structures, and has been applied to the
2. Experimental section
2
.1 Material synthesis
Na-ZSM-5 microparticles (MPs) were synthesized from industrial
silica gel (SiO content: 26.5 wt %), NaOH and alumina hydroxide
(AR) without any template. The gel, with a complete composition
of 11.5 Na O-100 SiO -3 Al -2750 H O, was employed for the
4–6
synthesis of a large number of inorganic materials. The crystal
seed growth method can be employed to assemble NPs or SMPs
and then to synthesize the morphology- and size-controllable
2
7–12
complex crystal structures, including molecular sieve particles.
2
2
2
O
3
2
In the case of the syntheses of molecular sieves, organic templates
are required for effective crystal seed growth methods and are
often followed by complicated post-treatment to remove these
synthesis of the ZSM-5 MPs. The gel mixture was treated with the
crystal seeds (H-ZSM-5 from Nankai University, 0.5 wt % of silica
gel) in a Teflon lined stainless steel autoclave, under hydrothermal
conditions, at 180 uC for 48 h after a designed aging time (0–24 h).
The average sizes of the ZSM-5 MPs were controlled by adjusting
the aging time. The Na-ZSM-5 MPs with the average diameter of
8.78, 4.08, 2.15, and 1.52 mm were obtained and denoted as ZSM-
5-A, B, C, and D according to the aging time (ZSM-5-A: 0 h,
ZSM-5-B: 5 h, ZSM-5-C: 20 h, and ZSM-5-D: 24 h), respectively.
The solid was separated by filtration, dried at 110 uC for 8 h,
calcined at 550 uC in air for 4 h and then exchanged in an aqueous
13,14
organic templates.
It is valuable therefore to synthesize
morphology- and size-controllable molecular sieve materials
through crystal seed growth methods without employing any
1
5
organic templates. ZSM-5 molecular sieves have been regarded
as an important solid acid catalyst for the hydration of
cyclohexene to cyclohexanol, due to their excellent catalytic
Department of Chemistry, Zhengzhou University, Zhengzhou, 450001,
P R China. E-mail: xiangyuwang@zzu.edu.cn
21
solution of ammonium nitrate (0.5 mol L ) three times to obtain
{
Electronic supplementary information (ESI) available: SEM images and
diameter distributions of NZ and JZ, XRD and NH -TPD results of
H-ZSM-5 zeolites. See DOI: 10.1039/c2ce06646g/
2 2 3
H-ZSM-5 MPs. The actual molar ratio of SiO /Al O of the as-
3
synthesized H-ZSM-5 MPs was controlled in the range of 23 to 25.
3
854 | CrystEngComm, 2012, 14, 3854–3857
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The H-ZSM-5 SMPs were obtained from commercial sources (NZ
from Nankai University and JZ from Japan, respectively, Fig. S1
and S2, ESI{).
2
.2 Material characterization
The phase structures of the ZSM-5 MPs were characterized by
X-ray diffraction (XRD, PANalytical B.V., X’pert PRO) with Cu-
2
1
˚
Ka (l = 1.5418 A) and a scanning rate of 1.2u min . The
morphology and size of the ZSM-5 particles were observed by
scanning electron microscope and energy dispersive spectroscope
Scheme 1 Illustration of the synthesis process for the ZSM-5 MPs
through a crystal seed growth method.
(SEM-EDS, JEOL, JSM-6700F operating at 20 kV). The element
air for 4 h, Na-ZSM-5 was obtained. H-ZSM-5 was obtained by
distribution of the ZSM-5 particles was analyzed in Ar support gas
by Inductively Coupled Plasma-Atomic Emission Spectrometry
ion exchange of Na-ZSM-5 with an aqueous solution of HNO
3
three times.
(
ICP-AES, Thermo Scientific iCAP6000, Thermo Fisher, USA).
The morphology of the ZSM-5 SMPs and MPs was observed
by SEM (Fig. 1). The ZSM-5 MPs possess an olive-like shape in
the micrometer scale from 2 to 10 mm. The increase in aging time
provides the decrease in size of the ZSM-5 MPs (Fig. 1 a–d). In
this investigation, the olive-like shape can be obtained and the
distribution of the particle sizes is also kept uniform. However, the
ZSM-5 particles from the commercial sources possess sub-
micrometer sizes (Fig. 1f, S1, ESI{). This distinctive structure
characteristic is evidence that the ZSM-5 MPs have be synthesized
from the ZSM-5 crystal seeds under hydrothermal conditions.
The phase structure of the ZSM-5 MPs was characterized by
XRD patterns (Fig. 2). The clear diffraction peaks at 2h = 7.9u,
The average diameters of the ZSM-5 particles were analyzed on a
Laser particle size distribution instrument (Rise-2002, Jinan,
China). The
2
N -sorption isotherms were obtained on a
NOVAl000e (Quantanchrome, USA) at 77 K. From the
adsorption branch of the isotherm curves, the total specific surface
areas (ATotal) of the ZSM-5 particles were calculated by the multi-
point Brunauer–Emmett–Teller (BET) method. The pore volume
(Vpore) was calculated by a single point method with the Capillary
condensation model. The micropore surface area (Amicro), external
surface area (Aex), micropore volume (Vmicro) and mesopore
volume (Vmeso) were calculated by a t-plot method. The NH3
Temperature Programmed Desorption (NH -TPD) was obtained
3
8
.8u, 23.1u, 23.9u, and 24.4u provide evidence for the MFI zeolite
on a NH -TPD analyzer produced by the Zhejiang Fantai Corp.
3
15,25
topology of the ZSM-5 MPs.
The average diameters of the
The acid sites and strength of the ZSM-5 particles were
characterized by the Hammett indicator method and Pyridine
Fourier Transform Infrared Spectroscopy (FT-IR).
ZSM-5 MPs were measured by a Laser particle size distribution
instrument and are consistent with the results from the SEM
images (Fig. 1 and Table S1, ESI{). The crystallite sizes in the
range of 30–80 nm can be calculated by the Scherrer equation for
2
.3 Catalysis measurement
The catalytic hydration of cyclohexene was employed to examine
the catalytic performances of the ZSM-5 catalysts. In a typical
catalytic reaction, water (50 ml) and cyclohexene (50 ml) was
treated in a high pressure agitated autoclave (250 ml), with the
catalyst (3.9 g), at 399 K for 210 min. The initial pressure was
controlled at 0.4 MPa and the rotate speed was controlled at
900 rpm. After being kept in a standing state for 12 h, the organic
phase was analyzed using a gas chromatograph (GC-9790)
equipped with a flame ionization detector and a 30 m capillary
column (OV1701). The internal standard method was used to
determine the composition of the organic phase. The transmittance
of the aqueous phase was measured on a UV-VIS spectro-
photometer (752 type, Shanghai Third Analysis Instrument
Factory) to investigate the sedimentation of the catalysts.
3
. Results and discussion
The key step of this synthesis route is the use of ZSM-5 SMPs as
the crystal seeds. The synthesis of the ZSM-5 MPs is illustrated in
Scheme 1. The industrial silica gel and alumina hydroxide were
used as the raw materials to prepare the gel mixture. With the
assistance of the ZSM-5 SMPs (Obtained from Nankai University,
named as NZ) as the crystal seeds, the gel mixture was treated in a
stainless steel autoclave under hydrothermal conditions at 180 uC
for a period of time, without any organic templates. After being
separated by filtration, dried at 110 uC, and calcined at 550 uC in
Fig. 1 SEM images of the ZSM-5 MPs synthesized at various aging
times (a) 0, (b) 5, (c) 20, (d) 24 h, and the ZSM-5 SMPs (e) NZ and (f) JZ.
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28,29
Fig. 3).
(
The ZSM-5 particles with different average diameters
possess significant different total acid amounts. It can be observed
that the increasing diameters in the ZSM-5 MPs provides the
decreasing total acid amounts due to decreasing surface areas. The
smaller ZSM-5 MPs possess greater surface areas and more acid
sites. The smaller particles possess increasing high-energy surface
active sites, which further increase the amount of acid centers.
The Br o¨ nsted acid sites (A )/Lewis acid sites (A
B
L
) for the
-TPD
different ZSM-5 MPs sizes were measured by the NH
3
method (Table S2, ESI{). The peak position of ZSM-5-C was
shifted slightly to a higher temperature, which indicated that the
acid strength of ZSM-5-C was slightly stronger than those of other
samples. In addition, the Br o¨ nsted acid sites of the ZSM-5 MPs
were less than those of NZ and JZ, which may be attributed to the
lower surface area and pore volume of the ZSM-5 MPs. The total
acid amount measured by the Hammett method was significantly
3
greater than the NH -TPD method, which may be related to
the lipophilicity of the high-silica zeolite surface and that the
adsorption of organic amine (organic base) was stronger than that
of ammonia (inorganic base).
Fig. 2 XRD patterns of various ZSM-5 zeolites.
the various crystalline planes (112, 101, and 313) in the ZSM-5
MPs, which are smaller than the results from the SEM images.
This disagreement between crystallite sizes indicates that the ZSM-
The catalytic hydration of cyclohexene to cyclohexanol was
employed to examine the catalytic performance of the ZSM-5 MPs
and SMPs. The ZSM-5 MP catalysts provide similar catalytic
performances to the commercial ZSM-5 SMP catalysts, including
the conversion of cyclohexene and the selectivity of cyclohexanol
5
MPs are multicrystal structures and composed of ZSM-5 NPs
Table S1, ESI{).
The surface areas and porosity of the ZSM-5 MPs were
measured by N -sorption isotherms. The ZSM-5 MPs possess high
(
(NZ and JZ, Fig. 4a). The conversion of cyclohexene can be
2
2
21
obtained from 7.43% to 11.40%, which are also comparable to
16,30
the commercial ZSM-5 SMPs.
surface areas (ATotal), from 206 to 320 m g , which are similar to
their sub-microscale counterparts (NZ and JZ). The micropore
surface areas are the main content of the total surface areas and
exhibit the same changing tendency as the total surface areas
The activity and stability of the
ZSM-5 MPs, including the conversion of cyclohexene and the
selectivity of cyclohexanol were also investigated in a long running
experiment and are shown in Fig. 4b. In the first 200 h, a high
conversion from 8.8% to 9.6% was obtained with a high selectivity
from 94% to 98%. In the second 200 h, the conversion decreased
gradually to 3.5%, which could be attributed to the inactivation of
(
Table 1). These large surface areas will be propitious in forming
more active sites for the catalytic reaction. This similar changing
tendency can be found for the pore volumes of the ZSM-5 MPs
(
Table 1). The micropore volumes are the main content of the total
31,32
the catalysts.
2 2
After the regeneration treatment with H O , the
pore volumes, which also provide evidence for the formation of the
ZSM-5 MPs. The existence of these mesopore surface areas and
mesopore volumes may be due to the substantial accumulation of
the ZSM-5 SMPs domains in the ZSM-5 MPs, which will be able
to facilitate the diffusion of the reactants in the pores of the
excellent catalytic performance of the ZSM-5 MPs was achieved
again with a high initial conversion up to 9.5% and high initial
3
3
selectivity up to 98.5%. After a running time of 300 h, the
inactivation of the catalyst was observed for a second time. The
regeneration of the catalysts was conducted three times in the long
running experiment of 1000 h, and good conversion and selectivity
were always observed. These excellent catalytic performances and
the significant sedimentation separation property of the ZSM-5
MPs show that they may be a superior alternative for commercial
26,27
catalyst and improve the catalytic performance.
To research the surface property of the ZSM-5 MPs, the acid
strength distribution and total acid amounts of the ZSM-5
particles were measured through the Hammett indicator method
with neutral red (pK
a
= + 6.8), methyl red (pK
a
= + 4.8), dimethyl
= 23.0) as the
34,35
ZSM-5 SMPs in wide industrial applications.
yellow (pK = + 3.3), dicinnamalacetone (pK
a
a
indicator, respectively, and n-butylamine as the titration agent
Table 1 Surface areas and porosity of the H-ZSM-5 zeolites
a
b
c
d
e
f
Sample
A
(
Total
A
micro
A
ext
V
pore
3
(cm g
V
)
micro
V
meso
2
m g
21
21
)
ZSM-5-A
ZSM-5-B
ZSM-5-C
ZSM-5-D
NZ
206
269
306
320
297
323
153
202
231
264
225
254
51.8
67.3
74.8
56.2
72.2
69.0
0.130
0.161
0.193
0.213
0.184
0.216
0.084
0.103
0.126
0.160
0.123
0.153
0.046
0.058
0.067
0.063
0.061
0.063
c
JZ
a
b
A
external surface area
total: total surface area
A
micro: surface area of micropore
A
ext
:
d
e
V
pore: total pore volume
Vmicro: micropore
f
Fig. 3 The acid amounts and strength distribution of the ZSM-5
volume
Vmeso: mesopore volume.
particles.
3
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Acknowledgements
Financial support from the Innovation Fund for Elitists of
Henan Province, China (No. 0221001200) is acknowledged.
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In conclusion, a simple and scalable crystal seed growth
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an organic template, for the catalytic hydration of cyclohex-
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synthesized ZSM-5 MPs exhibited a good crystalline degree
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8
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stability. The main advantage of the ZSM-5 MPs is the
significantly improved sedimentation separation performance
compared to that of the ZSM-5 SMPs. These improvements
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