微米级mel 分子筛聚集体的制备 - whxb.pku.edu.cn
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[Article] www.whxb.pku.edu.cn
物理化学学报(Wuli Huaxue Xuebao)
Acta Phys. -Chim. Sin. 2014, 30 (9), 1727-1735
Received: April 22, 2014; Revised: July 7, 2014; Published on Web: July 8, 2014.∗Corresponding author. Email: [email protected], Tel: +86-21-62232251.The project was supported by the National Natural Science Foundation of China (20890122) and National Key Technology R&D Program, China
(2012BAE05B02).
国家自然科学基金(20890122)和国家科技支撑计划(2012BAE05B02)资助
© Editorial office of Acta Physico-Chimica Sinica
doi: 10.3866/PKU.WHXB201407081
微米级MEL分子筛聚集体的制备
陈红丽 朱树燕 何建琴 王一萌*
(华东师范大学化学系, 上海市绿色化学与化工过程绿色化重点实验室, 上海 200062)
摘要: 使用四丁基氢氧化铵-正硅酸四乙酯-水(TBAOH-TEOS-H2O)简单体系一步水热制备了具有多级孔道
的微米级MEL结构分子筛聚集体. 得到的silicalite-2微米球直径大于10 μm且具有高达460 m2∙g-1的比表面
积和0.74 cm3∙g-1的孔体积. 微米球的生成一定程度上解决了催化应用过程中催化剂的分离和回收问题. 同时,
水热晶化过程中由纳米粒子自组装而成的晶间介孔缩短了反应物分子的扩散路径, 保持了分子筛纳米晶粒的
优势. 此外, 钛活性位的引入并未明显影响MEL微米球的形貌和结构, 含钛的MEL微米球TS (钛硅分子筛)-2
在苯酚羟基化反应中具有与纳米尺寸TS-1 (100-200 nm)相当的催化活性, 且TS-2可以通过简单过滤得到,
简化了纳米级TS-1的分离和回收过程.
关键词: MEL结构; 纳米粒子; 聚集体; 多级孔道; 苯酚羟基化
中图分类号: O643
Synthesis of Micro-Sized MEL-Type Zeolite Aggregates
CHEN Hong-Li ZHU Shu-Yan HE Jian-Qin WANG Yi-Meng*
(Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of Chemistry,
East China Normal University, Shanghai 200062, P. R. China)
Abstract: Hierarchical micro-sized zeolite aggregates with the MEL structure were prepared hydrothermally
from a tetrabutylammonium hydroxide (TBAOH)-tetraethyl orthosilicate (TEOS)-H2O system. The obtained micro-
sized silicalite-2 microspheres had sizes larger than 10 μm, high BET surface area (460 m2∙g-1), and large pore
volume (0.74 cm3∙g-1). The formation of micro-sized spheres alleviates preparation and application difficulties.
The presence of inter-crystalline mesopores originating from the spontaneous assembly of nano-sized primary
particles during hydrothermal synthesis gives the advantages of nanoparticles, reducing diffusion limitations.
The introduction of titanium does not strongly affect the morphology and textural properties of the MEL-type
zeolite, which are quite similar to those of silicalite-2 aggregates. The micro-sized titanium silicalite-2 (TS-2)
microspheres showed comparable catalytic activity in phenol hydroxylation to that of titanium silicalite-1 (TS-
1) of size 100-200 nm, and were easily recovered by traditional filtration, simplifying the separation and recovery
compared with nano-sized TS-1.
Key Words: MEL structure; Nano-sized particle; Aggregate; Hierarchical porosity;
Hydroxylation of phenol
1 IntroductionZeolites constitute an important class of nanoporous materials
that are widely used in industry as catalysts, adsorbents, and
detergents.1,2 Pentasil family of zeolites including ZSM- 5 and
ZSM-11 has been an active area of research since the discovery
of them in the early 1970′s3-8 due to their unique shape-selectiv-
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Acta Phys. -Chim. Sin. 2014 Vol.30
ity.9-16 The catalytic activity of zeolite, however, is inevitably
subject to the diffusion limitations of bulky reactants/products in
their micropore systems (typically smaller than 1.2 nm), espe-
cially in the reactions involving macromolecules.17,18 This problem
might be solved by decreasing the particle size of zeolites. Much
effort has therefore been devoted to synthesize nano-sized zeo-
lite.19- 24 However, nano-sized zeolites with colloidal properties
make severe difficulties in the separation and recovery, and zeolite
crystals smaller than 100 nm might be thermodynamically un-
stable due to high surface energy and vast amounts of surface
defects. Meanwhile, nanocrystals are difficult to handle and have
low yields during synthesis, in which the majority of the building
units are left unused in the mother liquid. Consequently, the
synthesis of hierarchical and micro-sized zeolites, which com-
bines the acidic activity and shape selectivity of micropores and
the free diffusion properties of mesopores, has attracted great
attention among researchers in chemical and material sciences in
the last few years,4,5,25-28 not only well keeping the advantages of
nano-sized zeolites in diffusion but also curtailing the difficulties
in separation and recovery.
Davis et al.25 found that TPA-silicalite-1 evolved via an oriented
aggregation mechanism during a month′s long aging at room
temperature, but no data of hierarchical porosity were presented.
Fang et al.26 reported that only the addition of NaH2PO4 led to
mesoporous aggregate of ZSM-5 (MFI) nanocrystals with a single-
crystal-like morphology. Long and coworkers29 hydrothermally
synthesized colloidal silicalite-2 particles in size of ~100 nm in a
very simple reactant system of tetrabutylammonium hydroxide
(TBAOH)- tetraethyl orthosilicate (TEOS)- H2O. Under certain
conditions, the as- synthesized colloidal silicalite- 2 was agglom-
erates composed of nano- crystals in size of about 20 nm, dis-
playing unique structural characteristics such as high content of
silanol defects in the framework, much higher BET surface area,
and greater total pore volume, which may be of advantage in some
catalytic processes. Very recently, Tsapatsis and coworkers28 re-
ported that tetrabutylphosphonium (TBP) hydroxide or tetrabu-
tylammonium (TBA) hydroxide could be used as template to
synthesize a hierarchical zeolite MFI made of orthogonally
connected microporous nanosheets during one- step hydro-
thermal crystal growth at 388 K. Obviously, organic molecules
with tetrabutyl chains (TBA or TBP) tends to direct the ag-
glomerates of nano-sized primary particles. However, the sizes of
obtained zeolite aggregates from both of above systems range
from 60 to 400 nm, which still was very difficult for the separa-
tion and recovery. It is worth mentioning that those nano-sized
aggregates were obtained at very low temperatures (<115 °C) in
the presence of ethanol. The induction period is prolonged at low
temperature, where the concentration of the nucleis needed for
nucleation and crystal growth,30 and hence that rate of nucleation
(and consequently, the specific number of crystals formed) in-
crease, minimizing the ultimate crystal sizes.31 Furthermore, a
certain amount of ethanol hinders the overgrowth of zeolite
crystals and slows down the crystallization process as a stabilizing
agent.19 As a result, nano-sized aggregates composed of primary
particles formed instead of large zeolite crystals.
Here, we have prepared micro-sized silicalite-2, with straight
10-membered-ring (MR) channels along the a axis, interconnected
with straight 10-MR channels along the b axis, aggregates with
considerable mesopore volume by simply revisiting the synthetic
system of TBAOH-TEOS-H2O. The micro-sized aggregates are
actually the assembly of nano-sized zeolite crystallites, in which
the mesopores are formed among the crystallite. In addition, the
hydrolysis products of alcohol were removed prior to the hy-
drothermal synthesis treatment to synthesize the larger crystal
aggregates. The influences of composition of reactant mixture and
crystallization temperature on the morphology and structure of
zeolites were further investigated in details. Results indicated that
the obtained silicalite-2 aggregates have advanced physico-
chemical properties characterized with various techniques. Fur-
thermore, titanium was incorporated into zeolite framework to test
the catalytic activity of micro-sized TS-2 aggregates in the hy-
droxylation of phenol.
2 Experimental2.1 Sample preparation
Zeolite samples were synthesized in Teflon-lined stainless steel
autoclaves under static conditions. A typical preparation is as
following.
TEOS (SCRC, >97% ) was added to TBAOH (Alfa, 40%
aqueous solution) under vigorous stirring and then distilled water
was added. The mixture was stirred for 1.5 h at room temperature
until TEOS was hydrolyzed completely thus a clear solution was
formed. Alcohol was evaporated under 80 °C water bath for about
2 h prior to the hydrothermal synthesis treatment. The sol was
sealed in an autoclave lined with polytetrafluoroethene (PTFE),
and then heated in an oven at corresponding temperature under
autogenously pressure for 3 d. The molar composition of reactant
mixture was 1.0SiO2:(0.12-0.30)TBAOH:(12-50)H2O and the
hydrothermal process was conducted in the temperature range of
120 to 175 °C. The final obtained samples were labeled as Sx-y-
T, where x indicated the molar ratio of H2O to silica, y and T meant
the molar ratio of TBAOH to silica and crystallization tempera-
ture (°C), respectively. Tetrabutyl titanate (TBOT, SCRC, 98%)
iso-propanol solution was added at ice temperature prior to re-
moving alcohol to incorporate titanium into zeolitic framework.
After the completion of crystallization, solids were recovered
by filtration, washed thoroughly with distilled water, and then
dried overnight at 80 °C. Calcined samples were obtained after
removal of the occluded templating molecules by heating the
samples at 550 °C for 5 h in an air flow.
2.2 Catalytic measurements
Catalytic reactions were performed in a glass reactor equipped
with a reflux condenser and a magnetic stirrer. Typically, 0.1 g of
catalyst, 2 g of phenol, and 1.6 g of acetone as solvent were in-
troduced in the reactor and heated at the reaction temperature (80 °C)
for 6 h under vigorous stirring. Subsequently, 0.8 g of hydrogen
peroxide (30%, mass fraction) aqueous solution from SCRC) was
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CHEN Hong-Li et al.: Synthesis of Micro-Sized MEL-Type Zeolite AggregatesNo.9
added in one lot. The H2O2/phenol molar ratio was taken to 0.3.
All products were analyzed on a Shimadzu GC-2014 gas chro-
matograph equipped with a DM- Wax- 30 m column (Dikma
Technologies Inc.) and an FID detector.
2.3 Characterization
Powder X-ray diffraction (XRD) patterns were collected on a
Bruker D8 Advance powder diffract meter (Germany) using Cu
Kα radiation (λ=0.154 nm) over a 2θ range from 5° to 40°, the
accelerating voltage and the applied current were 35 kV and 30
mA, respectively. The particle sizes of samples were calculated
from XRD data by the Scherrer equation: D=Kλ/(βcosθ), where
D is the crystallite size (nm), λ is the wavelength of the X-ray
radiation for the Cu target (λ=0.154 nm), β is the peak width at
half-maximum height (2θ=23.1°) for silicalite-2 (501), θ is the
Bragg′s angle, and K is the Scherrer constant (0.89). Scanning
electron microscopy (SEM) was performed on a scanning electron
microscope (type HITACHI S-4800, Japan) with an accelerating
voltage of 3 kV. Transmission electron microscopy (TEM) of as-
synthesized samples was conducted on TECNAI G2 F30
(America) operating at 300 kV. To prepare the samples for TEM,
a dispersion of the sample in diluted ethanol was dropped onto the
TEM sample bronze gridding, dried at room temperature for 1 h.
Fourier transform infrared (FTIR) spectra were recorded on a
Nicolet Fourier transform infrared spectrometer (NEXUS 670,
America) using the KBr technique. Thermogravimetric (TG)
analysis was performed using a PerkinElmer 457 TGA analyzer
(America) with a heating rate of 10 °C∙min-1 under an air flow.
The solid 29Si magic angle spinning (MAS) nuclear magnetic
resonance (NMR) and 1H-13C cross polarization (CP)/MAS NMR
measurements were collected on a VARIAN VNMRS-400WB
(America) spectrometer. The specific BET surface area (SBET) and
pore parameters of the samples were determined by nitrogen
adsorption-desorption measurements at 77 K on a nitrogen ad-
sorption apparatus (BELSORP- max, Japan). Before the mea-
surements, the samples were outgassed at 300 °C in vacuum for
6 h. The pore size distributions were derived from the adsorption
branches of the isotherms using the Barrett-Joyner-Halanda (BJH)
method. The total pore volume (Vp) was estimated at a relative
pressure of 0.99. Solid UV-Vis spectra were recorded in the 200-
500 nm range on UV2550 (Japan) from aluminium cell with
quartz window. The amounts of Si and Ti in final zeolite products
were quantified by inductively coupled plasma (ICP) on a Thermo
IRIS Intrepid II XSP atomic emission spectrometer (America)
after dissolving the samples in HF solution.
3 Results and discussionTBAOH was used to synthesize TBA-silicalite-2 in the molar
compositions of xTBAOH:1.0SiO2:25H2O. The XRD patterns of
the samples obtained at 175 °C with different TBAOH contents
are shown in Fig.1. All these patterns show the characteristics of
the MEL type structure, which have two intense peaks without
shoulders at the region 2θ =23°- 25° , demonstrating that the
samples are close to pure silicalite-2 free of the intergrown sili-
calite-1. Compared with the classical bulky MEL structured ze-
olites, the diffraction lines of samples a, b, and c are broadened as
a result of the decrease in particle size. With the molar ratio of
TBAOH/SiO2 (nTBAOH/nSiO2) increasing from 0.12 to 0.30, the pri-
mary particle size of the silicalite-2 decreases from 50 to 35 nm
calculated according to Scherrer equation.
SEM images of silicalite- 2 samples are shown in Fig.2. El-
Fig.1 XRD patterns of the samples synthesized at 175 °C for 3 d(a) S25-0.12-175, (b) S25-0.25-175, (c) S25-0.30-175
Fig.2 SEM images of samples synthesized at 175 °C for 3 d(a, b) S25-0.12-175, (c, d) S25-0.25-175, (e, f) S25-0.30-175. High-resolution
TEM images (g, h) were taken at the edge of the samples (c, d) microsphere.
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Acta Phys. -Chim. Sin. 2014 Vol.30
liptical shape crystals showing narrow crystal size distribution
(~500 nm) are observed when nTBAOH/nSiO2=0.12. An increase of
TBAOH content leads to a decrease of primary nanoparticle size
from 50-60 nm to 20-30 nm, which is in agreement with the
results calculated from XRD data. This is due to the spontaneous
generation of a large number of nuclei in the reactant system with
higher TBAOH content. Moreover, samples synthesized with
nTBAOH/nSiO2=0.25, 0.30 form large uniform spherical agglomerates
of nano-sized primary particles in size of 8 and 14 µm, respec-
tively. The results are different from that reported by Long and
coworkers,29 where colloidal zeolites (in size of ~100 nm) com-
posed of nano-sized crystals of about 20 nm were synthesized at
low temperature (114 °C) without removing alcohol generated by
the hydrolysis of TEOS. It is worth mentioning that low reaction
temperature and the presence of alcohol are benefit to obtain
dispersed nano-sized zeolite particles due to formation of large
amount of nucleis needed for nucleation during long induction
period at low temperature30,31 and the proved effect of alcohol on
the inhibition from producing zeolite aggregates.19 As a contrast,
the synthesis was accomplished at high reaction temperature
(175 °C) and alcohol was removed in our studies. Furthermore,
it has been reported that the formation of nano-sized MEL type
zeolites undergoes the transformation of amorphous sub-colloidal
particles in size of ~1 and ~10 nm into crystalline zeolite ag-
geregaets,32 hence a clear precursor sol was prepared because it is
preferred to eliminate the heterogeneous (or gel phase) nucleation
thus leads to the formation of nano-sized primary particles. TEM
images of sample obtained with nTBAOH/nSiO2=0.25 (Figs.2g and 2h)
clearly show that these micro- sized silicalite- 2 aggregates con-
sisted of primary clusters, resulting in the intercrystalline meso-
porosity. These hierarchical TBA-silicalite-2 microspheres may
exhibit high catalytic activity in the bulky molecular reaction and
curtail the filtration difficulties in the preparation at the same time.
The generation of mesoporosity was proved by nitrogen phy-
sisorption. N2 adsorption-desorption isotherms and pore size
distribution (PSD) curves of the zeolite samples synthesized with
molar compositions of xTBAOH:1SiO2:25H2O are shown in Fig.3.
The textual properties of the samples are summarized in Table 1.
The adsorbed volumes of N2 for all the calcined samples are about
130-140 cm3∙g-1 at low partial pressure (p/p0<0.20) (Fig.3A),
demonstrating that the samples are highly crystallized and contain
a considerable amount of micropores. In addition, all samples
exhibit combined characteristics of type I and type IV isotherms
with a hysteresis loop at high relative pressure of p/p0>0.4, indi-
cating that the presence of intercrystalline voids among the pri-
mary particles which formed mesopores and/or macropores. Table
1 suggests that micro-sized silicalite-2 possesses significant high
BET specific surface area and large porosity with maximum of 460
m2∙g-1 and 0.74 cm3∙g-1, respectively. Excessive TBAOH leads to
the partial dissolution of nanocrystals at high basicity, resulting in
smaller surface area and pore volume for silicalite-2 with nTBAOH/
nSiO2=0.30. As shown in Fig.3B, the samples with nTBAOH/nSiO2
=0.12,
0.25 show pore size distribution mainly centered at ~68 nm,
calculated by BJH method from adsorption branches. With the
molar ratio of TBAOH/SiO2 increasing up to 0.30, the pore size
distribution of sample c shifts to 26 nm. The relative smaller in-
tercrystalline pore may be related to the voids between the thinner
primary particles, subsequently leading to small total pore vol-
ume, in agreement with the partial disappearance of macropore as
shown in PSD curves of Fig.3B. In a word, hierarchical micro-
sized MEL-type zeolite aggregates combine the advantages of
micropores and mesopores, not only well keeping the acidic ac-
tivity and shape selectivity of micropores and the free diffusion
properties of mesopores but also curtailing the difficulties in
Fig.3 N2 adsorption/desorption isotherms (A) and PSD curves
(B) of calcined samples obtained at 175 °C for 3 d(a) S25-0.12-175, (b) S25-0.25-175, (c) S25-0.30-175.
In Fig.A, (b) Y-axis shift: 50 cm3∙g-1; (c) Y-axis shift: 100 cm3∙g-1
Table 1 Structural properties of the calcined samples with
various particle sizes
a S25-0.30-120 was synthesized at 120 °C for 12 d; b total surface area;c external surface area; d total pore volume; e microporous volume
Sample
S25-0.12-175
S25-0.25-175
S25-0.30-175
S25-0.30-150
S25-0.30-120a
SBETb/(m2∙g-1)
430
460
395
534
531
Sexterc/(m2∙g-1)
137
191
173
223
396
Vtotald/(cm3∙g-1)
0.59
0.74
0.53
0.63
0.77
Vmicroe/(cm3∙g-1)
0.14
0.13
0.11
0.12
0.09
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CHEN Hong-Li et al.: Synthesis of Micro-Sized MEL-Type Zeolite AggregatesNo.9
separation and recovery.
Synthetic parameters also strongly affect the structure and
morphology of silicalite-2. When the molar ratio of TBAOH/SiO2
was fixed at 0.30, only the H2O/TEOS molar ratio of 25 could
result in relatively uniform spherical aggregates of silicalite-2, as
shown in Fig.2f. When the molar ratio of H2O to SiO2 was half or
doubles (12 or 50), aggregates composed of nano-sized crystals
(~100 nm) without uniform morphology or bulky crystals in size
of ~1.7 µm were observed, respectively (figures not shown). It
might be explained by the fact that the transportation of silica
species was slowed with low water content thus inhibiting the
assembly of nano-sized crystals to uniformed spheres, and the
number of nucleus decreases with the increase of water content,
leading to formation of large zeolite crystals when molar ratio of
H2O/SiO2 is 50. Furthermore, spherical aggregates in size of ~10
and ~5 µm were obtained at low temperature of 150 and 120 °C,
respectively (Fig.4). The primary particles became thinner and
longer detected by SEM, probably due to the formation of large
amount of nucleis needed for nucleation during long induction
period at low temperature, and hence rate of nucleation (and
consequently, the specific number of crystals formed) increases,
minimizing the ultimate crystal sizes.30,31 Considering that the
crystallization temperature (120 °C) is slightly higher than that of
colloidal TBA- silicalite- 228,29 while the morphologies of those
silicalite-2 are quite different, the removal of ethanol may be the
key role of the formation of MEL aggregates of nano-sized pri-
mary particles. In addition, the effect of alcohol was further
proved by reference experiments. The sample obtained with the
presence of alcohol showed ununiformed zeolite crystals (in size
of ~50-200 nm) with low crystallinity (90%), confirming the
inhibition of alcohol on crystallization and the formation of micro-
sized aggregates. Moreover, spherical aggregates in size of 6-12
µm were obtained when alcohol was removed at low temperature
(30 °C) by rotary evaporation. More interestingly, secondary el-
lipse particles of sample obtained at 150 ° C contain some or-
thogonally connected nanosheets, similar to that reported by
Tsapatsis and coworkers.28 This suggests that the hierarchical
structure of silicalite- 2 may closely be related to the crystal
structure of MEL and/or repetitive twinning or other intergrowth
processes in the crystallization of silicalite-2. The sample ob-
tained at 120 °C shows core-shell morphology with loose core
and dense shell as shown in Fig.4b, indicating the relative faster
growth rate of external silicate nutriment surrounded by excessive
TBAOH. Therefore, micro-sized aggregates assembled from the
nano-sized silicalite-2 crystals can be formed in a wide range of
TBAOH/SiO2 molar ratios and crystallization temperatures with
appropriate amount of H2O. However, it should be mentioned that
smaller primary particles obtained at lower temperature might be
less thermodynamically stable due to high surface energy and vast
amounts of surface defects. In addition, retarded growth procedure
of zeolite at low temperature results in that longer duration would
be necessary to accomplish the crystallization. For example, sil-
icalite-2 shows a micropore volume of ~0.09 cm3∙g-1 after the
crystallization at 120 °C for 12 d.
Except for the goodness of curtailing the difficulties in sepa-
ration and recovery, the obtained micro-sized silicalite-2 sphere
may preserve advantages in catalytic and adsorption separation
processes due to its nano-sized primary particles which leads to
high external surface areas and short diffusion paths. The for-
mation of nanoparticles was proved as following:
FTIR spectra of the as-synthesized zeolite samples with dif-
ferent particle sizes exhibit very similar vibrational modes as
shown in Fig.5A. All samples present a band at 550 cm-1 and a
weak absorption band at 978 cm-1. The band at 550 cm-1 is as-
cribed to the asymmetric stretching mode of five-membered ring
blocks in pentasil zeolite.33,34 The band at 978 cm-1 is assigned to
the vibration of silanol group bonded to Q3 silicon species.35,36
Furthermore, according to Fig.5B, the strong band at 550 cm-1 for
nano-sized silicalite-2 particles splits into a doublet at 557 and
545 cm-1, which is slightly different from bulky pentasil zeolites.
The splitting of the double-ring vibration in as-synthesized zeolite
samples is probably related to silanol groups present in the five-
membered ring. The large amout of silanol groups can strongly
increase the hydrophility of micro-sized silicalite-2. As indicated
by TG analysis, more than 1% mass loss below 200 °C, which can
be ignored for bulky zeolites, was observed, implying desorption
of water due to its hydrophility and high surface area resulted by
the formation of nano-sized primary particles. Furthermore,
samples obtained at 175 °C with different amount of TBAOH
exhibit two distinct peaks at about 220 and 385 °C on the deriv-
ative thermogravimetry (DTG) curves, demonstrating desorption
Fig.4 SEM images of samples synthesized at 150 °C for 3 d
and 120 °C for 12 d(a) S25-0.30-150, (b) S25-0.30-120
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Acta Phys. -Chim. Sin. 2014 Vol.30
of the templates physisorbed on the external surface of the zeolite
crystallites and decomposition of the organic templates trapped in
the zeolite pore channels, respectively.37,38 It might be explained
that silicalite-2 microspheres composed of nano-sized particles,
possessing high external surface area, lost much adsorbed water
and organic templates on external surfaces during calcination
process.
The 29Si MAS NMR spectra of the as synthesized and the
calcined S25-0.30-175 are depicted in Fig.6. Two broad resonance
signals are observed with chemical shifts at - 113 and - 102,
which belong to Q4 [Si (4Si)] and Q3 [Si (3Si, 1OH)] respec-
tively.39-41 The high content of Q3 species indicates the presence of
the large amount of external surface silanols and internal defects
resulted from the imperfect condensation. Upon the calcination,
the content of Q3 species obviously decreases due to the further
condensation of silanol groups. These results indicated that
structural ordering of the nano-sized sample is lower than that of
ordinary bulky crystals.1H-13C CP/MAS NMR spectroscopy is a very powerful tool to
investigate zeolite structure via the study of the occluded organic
templating molecules. The 1H-13C CP/MAS NMR spectra of the
as synthesized sample S25-0.30-175 is illustrated in Fig.7. The
chemical shifts at 60.6, 23.8, 20.2, and 14.2 can be unambiguously
assigned to the C1, C2, C3, and C4 in N+―CH2―CH2―CH2―CH3
chain, respectively. A slight difference is observed compared with
the results reported by Nagy et al.42 where a doublet was observed
at 14.8 and 12.8 in 1H-13C CP/MAS NMR spectrum of the micro-
sized zeolite crystals. For comparison, the standard 13C NMR
spectroscopy of TBAOH in D2O is shown in Fig.7 (inset). A single
line without any doublet is observed at 13.6. This can be ex-
plained by the special morphology of the sample S25-0.30-175
with assembled nano-sized particles, TBA+ ions are arranged in
the shorter channels of the sample with a higher degree of dis-
order, as proved by 29Si MAS NMR spectra, leading to the dis-
appearance of the weak peak at 12.8.
To explore the catalytic properties of micro-sized zeolite MEL
aggregates, Ti was incorporated in the framework at nSi/nTi=40.
Synthesis in the presence of titanium did not affect the structure
of MEL zeolite or prevent assembling of nano-sized primary
particles (Fig.8A and 8B). Pure MEL structured micro-sized TS-
2 spheres in size of ~8 µm were obtained at 175 °C for 3 d when
molar ratio of TBAOH/SiO2 is 0.30. Nitrogen adsorption iso-
therms indicate the presence of both micro- and mesoporosity in
the TS-2 microspheres, which possess high BET surface area (517
m2∙g-1) and large porosity (0.58 cm3∙g-1). Furthermore, the co-
ordination states of Ti species were studied by UV-Vis spectros-
copy. TS-2 showed a main absorption around 210 nm, which is
assigned to the charge transfer from O2- to Ti4 + (Fig.8D). This
adsorption is usually observed for Ti-substituted zeolites and is
characteristic of tetrahedrally coordinated Ti highly dispersed in
the framework.43 This could be beneficial for redox reactions with
large molecules that require tetrahedrally coordinated titanium. On
the other hand, TS-2 showed a broad adsorption at 230-270 nm,
implying the coexistence of some penta- or hexacoordinated Ti
species as a result of water adsorption.44 Large amount of silanol
group due to the formation of nano-sized primary particles might
be the main reason which results in high hydrophility and surface
Fig.5 FTIR spectra of silicalite-2 samples synthesized
at 175 °C for 3 d(a) S25-0.12-175, (b) S25-0.25-175, (c) S 25-0.30-175
Fig.6 29Si MAS NMR spectra of the as synthesized (a) and
calcined (b) sample S25-0.30-175
Fig.7 1H-13C CP/MAS NMR spectra of the as synthesized
sample S25-0.30-175inset: bar chart 13C NMR spectra of TBAOH in D2O
1732
CHEN Hong-Li et al.: Synthesis of Micro-Sized MEL-Type Zeolite AggregatesNo.9
area thus much adsorption of water.
The hydroxylation of phenol into catechol and hydroquinone
with aqueous H2O2 solution was selected as catalytic test reaction.
Titanium silicalite-1 (TS-1) with MFI structure was proved to be
an active catalyst in the hydroxylation of phenol,45,46 which has
been commercialized by EniChem.47 As a member of pentasil
family of zeolites as similar to TS-1, TS-2 also shows consider-
able catalytic performance in the hydroxylation of phenol.48
Meanwhile, It was reported that the framework structure differ-
ences did not perturb the catalytic performances by comparing
catalytic behaviors of TS-1 and TS-2 with similar Si/Ti molar
ratios and crystal sizes.48 This is not really surprising considering
that TS- 2 and TS- 1 framework structures are very similar.
However, it was demonstrated that the catalytic activity of TS-1
samples was very sensitive to the crystal size.49 Beyond 1 µm, the
diffusion of products and reactants is difficult and the autode-
composition of H2O2 is the prevailing reaction. Therefore, not only
low activities but also very low H2O2 efficiencies are observed. In
this case, TS-1 in size of ~100 nm was synthesized as a reference.
Fig.8 Structure and morphology of TS-2 microspheresMolar ratio of Si/Ti (nSi/nTi) is 40. (A) XRD pattern of TS-2 compared to that of the pure silicalite-2 S25-0.30-175, (B) SEM images of the TS-2 showing
uniform micro-sized spheres in size of ~8 µm, (C) N2 adsorption/desorption isotherms of calcined TS-2, (D) UV-Vis spectra of TS-2 showing
that Ti is incorporated in the MEL zeolite framework.
Table 2 Some parameters for hydroxylation of phenol on
the TS-1 and TS-2
a Nano-sized TS-1 was synthesized using TPAOH as microporous template with
nSi/nTi=40 in gel composition. b molar ratio of Si/Ti of final calcined products
quantified by inductively coupled plasma (ICP). c turnover number in
mol∙mol-1. d turnover frequency in mol∙mol-1∙h-1
Sample
TS-1a
TS-2
dcrystal/nm
~100
8000
nSi/nTib
41.6
51.1
Phenol conversion/%
26.0
23.2
TONc
140.8
153.5
TOFd
23.5
25.6
1733
Acta Phys. -Chim. Sin. 2014 Vol.30
The phenol conversion over micro-sized TS-2 is comparable to
that over nano-sized TS-1 (Table 2). Moreover, micro-sized TS-2
shows larger TON and TOF, indicating that the more effective
catalytic activity of Ti incorporated into the framework of TS-2
spheres. Most importantly, the formation of micro-sized aggregate
not only well keeps the advantages of acidic activity and shape
selectivity of micropores, but also curtails the difficulties in
separation and recovery. As a contrast, centrifugation was nec-
essary to collect nano-sized TS-1, which is difficult for indus-
trialization process. Furthermore, the generated extensive mes-
opore would reduce the rate of catalyst deactivation by slowing
down the process of coke formation.
4 ConclusionsMicro-sized silicalite-2 aggregates composed of nano-sized
primary particles (<50 nm) have been hydrothermally synthesized
from the synthetic system TBAOH-TEOS-H2O. The micro-sized
silicalite-2 aggregates could be formed in a wide range of
TBAOH/SiO2 molar ratios and crystallization temperatures with
appropriate amount of H2O, and show high surface area and large
porosity. In addition, micro-sized TS-2 spheres show comparable
catalytic activity to that of nano-sized TS-1 in the hydroxylation
of phenol, but show no difficulties in the separation for nano-sized
TS-1. These results prove that the formation of micro-sized ag-
gregates not only well keeps the advantages of nano-sized zeolites
in diffusion and catalytic activity but also curtails the difficulties
in separation and recovery.
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