Cooperative structure direction of organosilanes and tetrapropylammonium hydroxide to generate hierarchical ZSM-5 zeolite with controlled porous structure

Article abstract of DOI:10.1039/c8ce01152d

Hierarchical ZSM-5 zeolite with short-range ordered mesoporosity and hierarchical ZSM-5 zeolite nanorods were obtained via a direct hydrothermal synthesis by the cooperative structure direction of dimethyloctadecyl[3-(trimethoxysilyl)propyl]- ammonium chloride (TPOAC) and tetrapropylammonium hydroxide (TPAOH). Dimethyloctadecyl[3-(dimethoxymethylsilyl)propyl]ammonium chloride (DPOAC) and octadecyltrimethylammonium chloride (OTAC) were also employed as structure directing agents (SDA) to further explore the role of methoxysilyl groups in organosilanes during the formation of hierarchical structure. The prepared materials were characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), N₂ adsorption-desorption, FT-IR, UV-vis and inductively coupled plasma-optical emission spectroscopy (ICP-OES). The characterization results showed that the use of TPOAC and DPOAC would generate short-range ordered mesopores and irregular mesopores, respectively. Hierarchical ZSM-5 zeolite nanorods with worm-like intracrystalline mesopores could be obtained by adjusting the amount of silicon source. The lack of methoxysilyl groups in OTAC however could lead to phase separation problems. Furthermore, the hierarchical Fe-ZSM-5 zeolite with short-range ordered mesoporosity showed enhanced catalytic activity and stability for the hydroxylation of phenol at room temperature.

Full text of DOI:10.1039/c8ce01152d

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Cooperative structure direction of organosilanes
and tetrapropylammonium hydroxide to generate
hierarchical ZSM-5 zeolite with controlled porous
structure†
Cite this: DOI: 10.1039/c8ce01152d
a
a
a
b
b
Yu Shen,
Zongzhuang Han,
Hang Li,
Haichao Li,
Gang Wang,
a
a
Fumin Wang * and Xubin Zhang
*
Hierarchical ZSM-5 zeolite with short-range ordered mesoporosity and hierarchical ZSM-5 zeolite nano-
rods were obtained via a direct hydrothermal synthesis by the cooperative structure direction of
dimethyloctadecylij3-(trimethoxysilyl)propyl]- ammonium chloride (TPOAC) and tetrapropylammonium hy-
droxide (TPAOH). Dimethyloctadecylij3-(dimethoxymethylsilyl)propyl]ammonium chloride (DPOAC) and
octadecyltrimethylammonium chloride (OTAC) were also employed as structure directing agents (SDA) to
further explore the role of methoxysilyl groups in organosilanes during the formation of hierarchical struc-
ture. The prepared materials were characterized by X-ray diffraction (XRD), scanning electron microscope
2
(SEM), transmission electron microscope (TEM), N adsorption–desorption, FT-IR, UV-vis and inductively
coupled plasma-optical emission spectroscopy (ICP-OES). The characterization results showed that the
use of TPOAC and DPOAC would generate short-range ordered mesopores and irregular mesopores, re-
spectively. Hierarchical ZSM-5 zeolite nanorods with worm-like intracrystalline mesopores could be
obtained by adjusting the amount of silicon source. The lack of methoxysilyl groups in OTAC however
could lead to phase separation problems. Furthermore, the hierarchical Fe-ZSM-5 zeolite with short-range
ordered mesoporosity showed enhanced catalytic activity and stability for the hydroxylation of phenol at
room temperature.
Received 13th July 2018,
Accepted 10th September 2018
DOI: 10.1039/c8ce01152d
rsc.li/crystengcomm
when the kinetic diameter of reactants is bigger than those of
the micropores. ZSM-5 zeolites can be considered to be prom-
1
. Introduction
ZSM-5 zeolites are crystalline aluminosilicates with regular
ising catalysts for macromolecular reactants if the mass trans-
fer is extensively improved.
microporosity containing tetrahedral TO (T represents Si or
4
Al) structure and MFI topology, which is composed of two
crossing channels (0.53 nm × 0.56 nm, 0.51 nm × 0.55 nm)
with 10-membered ring openings: one is a straight channel
parallel to the [010] crystal plane and the other is a sinusoidal
From the viewpoint of catalytic materials design, the en-
hancement of the diffusion property for ZSM-5 zeolites can
be achieved by creating hierarchical structure or decreasing
the crystal size in one or more dimensions. The emergence of
hierarchical structure in ZSM-5 zeolites could provide better
diffusion properties, high accessibility, and superior catalytic
performance than that of conventional zeolites with only
1
–3
channel parallel to the [001] crystal plane.
The unique
framework in ZSM-5 zeolites brings about high surface areas,
excellent shape selectivity and great hydrothermal stability, as
a result of which they are extensively employed as catalysts or
catalyst supports in industry. However, the micropores of the
MFI structure in ZSM-5 zeolites lead to diffusion limitations
and further inhibit their full utilization in catalysis, especially
4
–7
micropores.
In addition, an improved resistance to coking
with increasing external (mesopore) surface area can be en-
visaged due to the facilitated escape of coke precursors and
the larger area that needs to be covered to block access to the
micropores.
a
School of Chemical Engineering and Technology, Tianjin University, Tianjin,
So far, the construction of hierarchical structure in zeo-
lites could be realized via two general strategies: top-down
China. E-mail: wangfumin@tju.edu.cn, tjzxb@tju.edu.cn; Tel: +86 139 2000 0387,
+
86 138 2168 4327
8
9–14
and bottom-up routes. The typical top-down route
is the
b
School of Chemistry and Chemical Engineering, Qinghai Nationalities University,
removal of framework atoms by post-treatment techniques
such as desilication and dealuminization, while the bottom-
Xining, China
Electronic supplementary information (ESI) available. See DOI: 10.1039/
†
1
5–18
c8ce01152d
up route can be classified as hard-templates,
soft-
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9–31
32,33
templates
and template-free methods.
Among them, 2. Experimental section
the soft-templates method is recognized as the most innova-
tive approach, because both the mesoporous structure and
microporous framework type can be easily controlled via the
use of surfactants with different molecular structure. During
the crystallization process, however, the phase-separation
phenomenon between the surfactants and aluminosilicate
domains will lead to the generation of physical mixtures of
amorphous material without microporosity and bulk zeolite
with only micropores, which could impose difficulties for the
surfactants to modulate the zeolite crystal growth into a hier-
2
.1 Materials
Sodium hydroxide (NaOH, 99%), tetraethyl orthosilicate (TEOS,
9%), octadecyltrimethylammonium chloride (OTAC, 99%),
tetrapropylammonium hydroxide (TPAOH, 25 wt% in water),
sodium metaaluminate (NaAlO , 99%), methanol (99%), etha-
nol (99%), iron chloride hexahydrate (FeCl ·6H O, 99%), phe-
nol (99%), catechol (CAT, 99%), hydroquinone (HQ, 99%), ben-
zoquinone (BQ, 99%), ethylbenzene (EB, 99%), H (30 wt%),
9
2
3
2
2 2
O
were purchased from Aladdin Co., Ltd. and used without any
further purification. Dimethyloctadecylij3-(trimethoxysilyl)-
propyl]ammonium chloride (TPOAC, 60 wt% in methanol) was
purchased from J&K Scientific Co., Ltd. dimethyloctadecylij3-
2
2
archical structure. Choi et al. reported that hierarchical
ZSM-5 zeolites could be obtained via an organosilane surfac-
tant with hydrophobic alkyl chain and hydrolysable methoxy-
silyl groups as mesoporous structure-directing agents (SDA).
Methoxysilyl groups played a very important role for overcom-
ing the phase-separation problem due to the strong interac-
tion between the hydrolysable methoxysilyl groups and grow-
ing crystal domains. Thereafter, this kind of organosilane has
been extensively used as mesoporous SDA for the synthesis of
(
dimethoxymethylsilyl)propyl]ammonium chloride (DPOAC, 60
wt% in methanol) was purchased from Nanxiong Dingcheng
New Material Technology Co., Ltd. Prior to use, the concentra-
tion of H O was tested by direct iodine titration.
2
2
2.2 Synthesis of hierarchical ZSM-5
3
4–40
41
hierarchical zeolites.
Milina et al. corroborated the con-
The hierarchical ZSM-5 zeolites were obtained via a single
cept of mesopore quality through the connectivity as well as
lifetime of the catalysts. The results show that catalysts
obtained via organosilane possess constricted mesopores
rather than open and interconnected mesopores. Thus, to de-
sign a superior hierarchical zeolite catalyst, at least two fac-
tors should be significantly considered: one is to find or syn-
step hydrothermal method in the presence of organosilanes
(
(
DPOAC, TPOAC) and tetrapropylammonium hydroxide
TPAOH) as soft-templates. To obtain hierarchical ZSM-5 ze-
olites, TEOS and the organosilane surfactant dissolved in
methanol were successively added to a mixture containing
NaOH, TPAOH, NaAlO2 and distilled water (molar ratios: 1
thesize
a
soft-template to prevent phase-separation
Al O
2 3
: 14 TPAOH : 22.1 DPOAC/TPOAC : 14 Na
2
O : 8800 H
2
O :
phenomenon during the crystallization process, the other is
to construct mesopores with good quality, which can provide
better mass transfer, accessibility and lifetime than the con-
ventional hierarchical structure can. However, the works re-
lated to hierarchical ZSM-5 zeolites by using organosilanes as
SDA are mainly focused on the synthesis of conventional hi-
erarchical structure. The one-step hydrothermal synthesis of
hierarchical ZSM-5 zeolite with controlled porous structure
still needs to be investigated further.
Herein, to further improve the mesopore quality, we report
a one-step synthesis route to prepare hierarchical ZSM-5 zeolite
with short-range ordered mesoporosity and hierarchical ZSM-5
zeolite nanorods by the cooperative structure direction of
organosilane and tetrapropylammonium hydroxide. The syn-
thesized materials can be considered as typical catalysts which
combine shorter length of diffusion path and micro-
mesoporous structure into one unit, that in turn will promote
the diffusion of reactants and products in and out of the cata-
lyst particles, as well as the accessibility of the active sites for
reactants and intermediate products. Furthermore, the effect of
the amount of hydrolysable methoxysilyl groups on the hierar-
chical structure is further discussed based on the characteriza-
tion results by using DPOAC as the SDA (Scheme S1, ESI†). To
verify the mesopore quality, ferric iron was introduced into the
framework as active sites for the hydroxylation of phenol. The
great catalytic properties of the Fe-substituted hierarchical
ZSM-5 sample showed remarkably enhanced catalytic activity
and stability for the hydroxylation of phenol.
4
9 SiO
2
). In a typical procedure, 0.067 g NaAlO
2
, 4.2 g
TPAOH and 0.41 g NaOH were first dissolved in 65 g H O.
2
Then, 4.2 g TEOS were slowly adding into the solution at
room temperature. After stirring for 2 hours, a mixture of
desired amount of DPOAC or TPOAC dissolved in methanol
were added under vigorous stirring (500 rpm). The final
mixture was further stirred for 2 hours. The mother liquid
was then moved into a teflon-coated stainless-steel autoclave
and heated at 150 °C for 72 hours. The obtained materials
were separated by centrifuge, washed thrice with ethanol,
dried at 80 °C overnight and calcined in air at 550 °C for 6
hours. The prepared materials were denoted as DH-ZSM-
5IJ4.2) or TH-ZSM-5IJ4.2), respectively. TH-ZSM-5IJ6.3) was
obtained with 6.3 g of TEOS. For the purpose of compari-
son, conventional ZSM-5 (ZSM-5) was prepared with the
presence of only TPAOH. To obtain the Fe-substituted cata-
lyst, desired amount of ferric chloride hexahydrate was
added into the mother liquid after the addition of TEOS.
2.3 Characterization
X-ray diffraction (XRD) was tested by using a Bruker D8-
Focus instrument with Ni-filtered Cu Kα radiation (λ = 0.1541
nm) with a scanning rate of 5° per minute in the range of 5–
50°. The morphologies of the samples were observed by using
a scanning electron microscopy (SEM, Hitachi S-4800) instru-
ment operating at 20 kV. To ensure electrical conductivity, a
thin platinum film was sprayed on the samples by using an
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ion sputtering apparatus prior to taking any measurements.
The hierarchical structures of the samples were recorded on
a transmission electron microscopy (TEM, JEM-2100F) operat-
ing at 200 kV. Each sample was first dispersed in ethanol
using an ultrasonic instrument and then dropped onto
ultrathin carbon supported film before measurement. N
sorption isotherms were measured at 77 K on a SSA-7000
BUIDER, CH) instrument. Before measurements, all samples
2
(
were degassed at 250 °C for 6 hours. The BET method was
employed to measure the specific surface areas (SBET). The
pore size distributions of micropores and mesopores were
calculated by the Horvath–Kawazoe (HK) and Barrett–Joyner–
Halenda (BJH) models (adsorption branch), respectively. Fou-
rier transforming infrared spectra (FT-IR) were collected in
the range of 4000 cm to 400 cm at room temperature on a
Thermo Nicolet instrument by using the KBr pellet tech-
nique. The UV-vis spectra were recorded on a PE Lambda 750
spectrophotometer in the range of 200–600 nm. The Si/Fe ra-
tios of the prepared samples were measured by inductively
coupled plasma-optical emission spectrometry (ICP-OES,
PerkinElmer) instrument.
Fig. 1 The XRD patterns of (a) TH-ZSM-5IJ4.2); (b) DH-ZSM-5IJ4.2); (c)
TH-ZSM-5IJ6.3); (d) conventional ZSM-5 samples.
−
1
−1
solo presence of TPAOH shows bulky ZSM-5 zeolite crystals
(>10 μm). The morphology changed when TPOAC were
added into the mother liquid of ZSM-5 zeolite, indicating the
occurrence of interaction between hydrolysable methoxysilyl
siloxane groups in TPOAC and the growing crystal domains
2
2
2
.4 Catalyst activity
of the MFI structure. TH-ZSM-5IJ4.2) shows obvious long
narrow macropores with a rough surface, while TH-ZSM-
The hydroxylation of phenol was conducted in a three-
necked flask (50 mL) with an electric stirring water bath un-
der vigorous stirring (600 rpm). Typically, 1 g of phenol, 10
mL of deionized water and 0.1 g of catalyst were added into
the three-necked flask at 25 °C. Then, desired amount of
5IJ6.3) results in a nanorod morphology. Each nanorod is
about 50–100 nanometers in width. When DPOAC was re-
placed by TPOAC to synthesize hierarchical ZSM-5 zeolite, the
macropores changed from long and narrow to smaller and ir-
regular in the DH-ZSM-5IJ4.2) sample. This phenomenon may
be because the two methoxysilyl groups in DPOAC provide
less interaction with the growing crystal domains than the
three methoxysilyl groups in TPOAC do.
H
2
O
2
was added into the flask. After reaction, the catalysts
were separated from reaction mixture by centrifugation. Gas
chromatograph analyses were conducted by using
a
Shimadzu GC-2010 plus instrument equipped with a SE-30
chromatographic capillary column (Ø 0.25 mm × 0.25 μm ×
Fig. 3 shows the TEM images of TH-ZSM-5IJ4.2), TH-ZSM-
5IJ6.3), DH-ZSM-5IJ4.2) and conventional ZSM-5 samples. As
3
0 m). Gas chromatography–mass spectrometry analyses
shown in the figure, zeolitic micropores can be observed in
conventional ZSM-5 sample, which could be assigned to the
MFI structure. TEM images also show that TH-ZSM-5IJ4.2) pos-
sesses ordered micropores and short-range ordered meso-
pores, while DH-ZSM-5IJ4.2) gives irregular and disordered
mesopores. Nanorod morphology can be found in the TH-
ZSM-5IJ6.3) sample. Each nanorod has ordered micropores of
MFI structure and worm-like intracrystalline mesopores. The
unique hierarchical structure of good connectivity coupled
with controllable porous structure may be advantageous to al-
leviate the diffusion limitations and facilitate reactions in-
cluding macromolecular reactants. When the amount of sili-
con source was decreased to 2.1 g, the XRD pattern of TH-
ZSM-5IJ2.1) sample shows no characteristic peak of MFI struc-
ture (Fig. S2, ESI†), indicating that the amount of silicon source
is a key factor for the formation of hierarchical MFI zeolite
through cooperative structure direction of organosilanes and
TPAOH. This phenomenon confirms that organosilanes and
TPAOH would also play a competitive role when less silicon
sources were used. In order to further explore the cooperative
mechanism of the two templates, the precursors of TH-ZSM-
5IJ4.2), TH-ZSM-5IJ6.3), and DH-ZSM-5IJ4.2) samples stirred for
were performed on a Shimadzu QP2010 instrument. Ethyl
benzene was used as the internal standard to calculate the
mass of reactant and products.
3
. Results and discussion
The XRD patterns of TH-ZSM-5IJ4.2), TH-ZSM-5IJ6.3), DH-ZSM-
IJ4.2) and conventional ZSM-5 samples are shown in Fig. 1.
5
TH-ZSM-5IJ4.2), TH-ZSM-5IJ6.3), DH-ZSM-5IJ4.2) and conven-
tional ZSM-5 samples show characteristic peaks of a typical
MFI zeolite framework, indicating that the hydrolysable
methoxysilyl siloxane groups have no influence on the forma-
tion of MFI structure, which is in agreement with previous
2
2
study. The FT-IR spectra of the synthesized hierarchical
ZSM-5 and conventional ZSM-5 samples show that two bands
−
1
−1
at 1088 cm and 1225 cm are characteristic of internal and
external asymmetric stretching, respectively, while the band
−
1
at 797 cm is caused by the external symmetric stretching
4
2
−1
(
Fig. S1, ESI†). The band observed at 546 cm can be at-
tributed to the characteristic peak of the MFI structure.
4
3
The SEM images of the four samples are shown in Fig. 2.
As shown in the figure, conventional ZSM-5 prepared by the
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Fig. 2 The SEM images of (a) TH-ZSM-5IJ4.2); (b) DH-ZSM-5IJ4.2); (c) TH-ZSM-5IJ6.3); (d) conventional ZSM-5 samples.
different times before crystallization were collected by centri-
fugation, dried in a vacuum drying oven and characterized by
TEM (Fig. 4). After stirring for two hours, no precursor of
conventional ZSM-5 could be found and the mother liquor is
pellucid. Precursors of the others samples could be collected
as the mother liquids are white and turbid. This is probably
caused because the prepolymers obtained from hydrolysis
and condensation of silicon source were connected with the
0
of P/P > 0.4 in TH-ZSM-5IJ4.2), TH-ZSM-5IJ6.3) and DH-ZSM-
5IJ4.2) samples confirm the presence of original micropores of
MFI structure and newly created mesopores. Moreover, the
pore size distributions (Fig. 6) clearly show the presence of
mesopores in hierarchical ZSM-5 samples, with the mesopore
size in the range of 2–15 nm. Table 1 exhibits the textural pa-
rameters of the prepared samples. It can be seen that the
mesopore volume of the hierarchical ZSM-5 increases with
the increasing amount of organosilanes, whereas the micro-
pore volume decreases.
To further verify the role of methoxysilyl groups during
crystallization, the material was also prepared by using
octadecyltrimethylammonium chloride (OTAC) without
methoxysilyl groups under the same conditions, denoted as
4
4,45
organosilanes by the hydrolysis of methoxysilyl groups.
Irregular mesopores and short-range ordered mesopores
could be observed in the precursors of DH-ZSM-5IJ4.2) and
TH-ZSM-5IJ4.2), respectively, which is in accordance with
Fig. 3. Furthermore, the precursor of TH-ZSM-5IJ6.3) shows an
approximately round morphology with narrow mesopores.
These phenomena indicate that the mesoporous phase would
be first formed at room temperature. Thus, when the amount
of TEOS decreased to 2.1 g, the microporous phase will not
be generated due to the lack of silicon sources.
2
OH-ZSM-5. The XRD pattern, N sorption isotherms, SEM im-
age and TEM image were recorded as shown in Fig. 7. The
XRD pattern shows the typical characteristic peak of the MFI
structure, while the enhanced uptake of nitrogen in the rela-
N
2
adsorption–desorption isotherms (Fig. 5) and the pore
size distributions (Fig. 6) of conventional ZSM-5 zeolite show
obvious uptake of N at low relative pressure and type-I ad-
0 2
tive pressure region of P/P > 0.4 of N sorption isotherms
confirms the presence of mesopores. However, the SEM and
TEM image indicate that the synthesized material is a physi-
cal mixture of MFI zeolite crystal and amorphous meso-
porous silica material. This phenomenon demonstrates that
methoxysilyl groups, which could provide interactions be-
tween the SDA and growing crystal domains through the for-
mation of covalent bonds with other SiO2 and Al O
2
sorption isotherms, confirming the solo presence of micro-
pores. The strong adsorption at low relative pressure of TH-
ZSM-5IJ4.2), TH-ZSM-5IJ6.3), DH-ZSM-5IJ4.2) and conventional
ZSM-5 samples proves the presence of micropores in these hi-
erarchical ZSM-5 zeolites. The size of the micropores,
obtained from the Horvath Kawazoe (HK) model, was very
similar in these samples, ca. 0.55 nm, suggesting the exis-
tence of the same type of micropores. The hysteresis in the
adsorption isotherms could be ascribed to capillary conden-
2
3
2
2
sources, played a significant role to overcome phase separa-
tion problem during the process of cooperative structure di-
rection of the micropore- and mesopore-directing agents.
Few heterogeneous catalysts have been found to exhibit
good catalytic performance in the hydroxylation of phenol at
room temperature (25 °C). Lower reaction temperature is
more conducive to extending catalyst life, but harmful to
4
6
sation in mesopores. According to the IUPAC classification,
the combined features of type I at low relative pressure (P/P0
<
0.01) and a hysteresis with enhanced uptake in the region
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Fig. 4 The TEM images of the precursors of (a and b) TH-ZSM-5IJ4.2);
(c) DH-ZSM-5IJ4.2); (d) TH-ZSM-5IJ6.3) samples.
ages (Fig. S4, ESI†). To test the pore structures further, the N2
adsorption–desorption isotherms, pore size distributions and
textural properties of Fe-TH-ZSM-5IJ4.2), Fe-TH-ZSM-5IJ6.3), Fe-
DH-ZSM-5IJ4.2) and conventional Fe-ZSM-5 were also mea-
sured as shown in Fig. S5 and Table S1 (ESI†). The characteri-
zation results also indicate that the pore sizes, pore volumes
and BET areas slightly decreased when ion species were
doped into the framework of the samples, which is in agree-
4
9,50
ment with previous works.
ICP-OES, the four samples have similar Fe content in solid
Table S1†). The catalytic activities of the prepared hierarchi-
According to the results of
Fig. 3 The TEM images of (a1 and a2) TH-ZSM-5IJ4.2); (b1 and b2) DH-
ZSM-5IJ4.2); (c1 and c2) TH-ZSM-5IJ6.3); (d1 and d2) conventional ZSM-
(
5
samples.
cal ZSM-5 (Table 2) for the hydroxylation of phenol were com-
pared with the conventional ZSM-5 at room temperature (re-
action conditions: water 10 mL, catalyst 100 mg, phenol/H
2 2
O
catalytic conversion. According to the above analyses, the Fe-
substituted hierarchical ZSM-5 catalysts with controllable po-
rous structure could have high activity as the mass transfer
and accessibility of the active sites is considered to be greatly
improved. To catalyze the hydroxylation of phenol over the
prepared hierarchical ZSM-5 zeolites, Fe-TH-ZSM-5IJ4.2), Fe-
TH-ZSM-5IJ6.3), Fe-DH-ZSM-5IJ4.2) and conventional Fe-ZSM-5
were synthesized by adding the desired amount of ferric chlo-
ride hexahydrate into the mother liquid before crystallization
while other conditions remained unchanged. A broad UV-vis
absorption band (Fig. 8a) centered at 252 nm caused by dπ–
pπcharge transfer between tetrahedral oxygen ligands and
molar ratio 2 : 1, 298 K, reaction time 12 h). As shown in
3
+
Fe ion prove that Fe-substituted catalysts were successfully
4
2
synthesized. The XRD patterns (Fig. 8b) of the prepared Fe-
substituted samples also show a well-crystallized ZSM-5 struc-
ture, confirming that the substitution of iron has no effect
on the MFI structure as well as the morphologies and pore
structures shown in SEM images (Fig. S3, ESI†) and TEM im-
2
Fig. 5 The N adsorption–desorption isotherms of TH-ZSM-5IJ4.2),
DH-ZSM-5IJ4.2), TH-ZSM-5IJ6.3) and conventional ZSM-5 samples.
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Fig. 6 The pore size distributions of TH-ZSM-5IJ4.2), DH-ZSM-5IJ4.2), TH-ZSM-5IJ6.3) and conventional ZSM-5 samples.
Table 1 Textural properties of TH-ZSM-5IJ4.2), DH-ZSM-5IJ4.2), TH-ZSM-5IJ6.3) and conventional ZSM-5 samples
a
b
2
−1
2
−1
c
3
−1
d
3
−1
Samples
Si/Al
S
BET [m g
]
S
micro [m g
]
V
micro [cm g
]
Vmeso [cm g ]
TH-ZSM-5IJ4.2)
DH-ZSM-5IJ4.2)
TH-ZSM-5IJ6.3)
25
24
24
22
450
565
432
282
69
0.035
0.06
0.07
0.12
0.25
0.31
0.21
0.02
113
128
244
Conventional ZSM-5
a
b
c
d
Measured by ICP-OES. Brunner–Emmet–Teller (BET) surface area. Determined by t-plot method. Pore volume in the range of 2–50 nm.
Table 2, the conventional Fe-ZSM-5 only gives 12.1% conver-
sion of phenol and 65.4% selectivity to dihydroxybenzene, in-
dicating that the bulk morphology (>10 μm) and small
micropores largely restricted the catalytic activity, while other
catalysts show significantly improved conversion and selectiv-
ity. It is possible that the lower activity of conventional Fe-
ZSM-5 is due to the longer diffusion length of larger crystals
compared to those of the hierarchical samples. The orders of
the catalytic conversion over the Fe-substituted hierarchical
ZSM-5 zeolites at the same reaction conditions are as follows:
Fe-TH-ZSM-5IJ4.2) > Fe-TH-ZSM-5IJ6.3) > Fe-DH-ZSM-5IJ4.2).
TH-ZSM-5IJ4.2) is completely inactive without the doping of
lectivity to dihydroxybenzene, which is better than those
4
7,48
reported in the literature.
According to the characteriza-
tion, the cooperative structure direction of DPOAC and
TPAOH would generate irregular and disordered mesopores,
while Fe-TH-ZSM-5IJ4.2) prepared by TPOAC and TPAOH pos-
sesses short-range ordered mesoporosity. The superior con-
nectivity of the hierarchical structure in Fe-TH-ZSM-5IJ4.2)
sample could supply great diffusion performance and more
accessible active sites for the reaction. The Fe-TH-ZSM-5IJ6.3)
sample with hierarchical nanorods shows lower catalytic con-
version than the Fe-TH-ZSM-5IJ4.2) sample did but still much
higher than that of the conventional Fe-ZSM-5 zeolite. This
phenomenon indicates that worm-like mesopores in each
nanorod can provide shorter length of diffusion path and ad-
vantages for the hydroxylation of phenol.
3
+
Fe , indicating that the framework Fe is the active site for
the hydroxylation of phenol. Moreover, Fe-TH-ZSM-5IJ4.2) pre-
pared by TPOAC give 41.4% phenol conversion and 97.4% se-
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Fig. 7 (a) XRD spectrum, (b) N
2
adsorption–desorption isotherm, (c) SEM image and (d) TEM image of OH-ZSM-5 sample prepared by using OTAC
as mesopore-directing agent under the same conditions.
Fig. 8 (a) Uv-vis spectra and (b) XRD spectra of (A) Fe-TH-ZSM-5IJ4.2); (B) Fe-DH-ZSM-5IJ4.2); (C) Fe-TH-ZSM-5IJ6.3); (D) conventional Fe-ZSM-5
samples.
The catalytic stability of Fe-TH-ZSM-5IJ4.2) sample was
tested under the same reaction conditions (Fig. 9). Before
each cycle, the collected catalysts were washed by ethanol,
dried and calcined in air. As shown in the figure, both the
catalytic conversion and selectivity remain almost unchanged
at room temperature after the reaction ran five times, indicat-
ing that Fe-TH-ZSM-5IJ4.2) sample has good reusability. In or-
der to further verify the stability of the catalyst, the XRD pat-
tern and UV-vis absorption of the catalyst collected after fifth
cycles, denoted as r-Fe-TH-ZSM-5, were recorded as shown in
Fig. S6 (ESI†). The characterization results confirm that the
MFI structure and framework Fe species remained almost
unchanged. Furthermore, the molar ratios of Si/Fe after each
cycle were tested as shown in Fig. 9, proving that there are
no significant losses in the Fe content after undergoing the
reactions. These phenomena demonstrate that the Fe-
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Table 2 The activity of hydroxylation of phenol over the Fe-substituted hierarchical ZSM-5 catalystsa
X
Phenol
Selectivity (%)
CAT
S
DHB
b
c
Catalyst
Si/Fe
Phenol/H O
2 2
(%)
HQ
(%)
Fe-TH-ZSM-5IJ4.2)
Fe-DH-ZSM-5IJ4.2)
Fe-TH-ZSM-5IJ6.3)
Conventional Fe-ZSM-5
TH-ZSM-5IJ4.2)
128
126
130
126
128
128
128
11
2 : 1
2 : 1
2 : 1
2 : 1
2 : 1
3 : 1
1 : 1
3 : 1
3 : 1
41.4
39.3
36.3
12.1
0
22.7
62.7
25.3
21.4
57.9
48.9
58.2
43.2
0
60.5
39.8
?
39.5
36.8
31.9
22.2
0
37.4
43.1
?
97.4
85.7
90.1
65.4
0
97.9
82.9
78.4
66.1
Fe-TH-ZSM-5IJ4.2)
Fe-TH-ZSM-5IJ4.2)
d
Fe-MCM41
FeCoNaβ
e
f
1.12
49.4
16.7
a
b
Reaction conditions: reaction temperature 25 °C, reaction time 12 h, 1 g of phenol, 10 g of water, 0.1 g of catalyst. Molar ratio of Si to Fe,
c
d
determined by ICP-OES. Molar ratio of phenol to H
phenol, 8.4 g of water, 0.05 g of catalyst. From ref. 48, reaction temperature 30 °C, phenol/H
0
2 2
O .
From ref. 47, reaction temperature 25 °C, phenol/H
2 2
O (molar ratio) = 3, 1 g of
(molar ratio) = 3, 2 g of phenol, 60 g of water,
e
O
2 2
f
.2 g of catalyst. Fe content in mass (wt%).
tween mesopore-directing agent and growing aluminosilicate
domains obtained via the generation of covalent bonds with
other SiO and Al O sources is necessary. The Fe-substituted
2 2 3
hierarchical ZSM-5 zeolite prepared by the cooperative struc-
ture direction of TPOAC and TPAOH showed superior cata-
lytic performance at room temperature, which is better than
most reported works, probably caused by the accelerated
mass transfer and more accessible active sites. The strategy
for the control of hierarchical structure in ZSM-5 zeolite dem-
onstrated in this work might be useful as a principle for the
design of other hierarchical zeolites.
Conflicts of interest
Fig. 9 Catalytic stability of the Fe-TH-ZSM-5IJ4.2) sample. Reaction
conditions: Reaction temperature 25 °C, reaction time 12 h, 1 g of
phenol, 10 g of water, 0.1 g of catalyst.
There are no conflicts to declare.
Acknowledgements
This work was supported by the National Basic Research Pro-
gram of China (973 Program) (Grant no.2012CB720302) and
the National Key Research and Development Program of
China (2016YFF0102503).
substituted hierarchical ZSM-5 zeolite with short-range or-
dered mesoporosity has great catalytic stability.
4
. Conclusion
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