Synthesis and catalytic properties of multilayered MEL-type titanosilicate nanosheets

Article abstract of DOI:10.1039/c4ta06473a

Multilayered MEL-type titanosilicate nanosheets (MTS-2) were hydrothermally synthesized through a dual-template method, using cetyltrimethylammonium tosylate (CTATos) and tetrabutylammonium hydroxide (TBAOH) as templates for mesopores and micropores, resp

Full text of DOI:10.1039/c4ta06473a

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Synthesis and catalytic properties of multilayered
MEL-type titanosilicate nanosheets
Cite this: DOI: 10.1039/c4ta06473a
*
Hong Li Chen, Shang Wei Li and Yi Meng Wang
Multilayered MEL-type titanosilicate nanosheets (MTS-2) were hydrothermally synthesized through a dual-
template method, using cetyltrimethylammonium tosylate (CTATos) and tetrabutylammonium hydroxide
(TBAOH) as templates for mesopores and micropores, respectively. The mismatch of nanocrystals and
mesophase was minimized by decreasing the size of primary zeolite particles, taking advantage of MEL-
type zeolite that tends to form nano-sized crystallites. The titanosilicate nanosheets possess both MEL
zeolitic structure and large volume of intersheet mesopores without the formation of a physical mixture
of bulky zeolites and mesoporous materials. Moreover, it exhibited improved catalytic activities for bulky
molecules compared to those of other ordered mesoporous materials and conventional MFI- and MEL-
type titanosilicates, where post-treatment of as-made MTS-2 samples using NH4F could reduce the
silanols, and thus enhance their hydrophobicity and epoxidation activity.
Received 26th November 2014
Accepted 3rd February 2015
DOI: 10.1039/c4ta06473a
www.rsc.org/MaterialsA
templates, such as carbon particles,12,13 carbon aerogels,14
3DOM carbon15,16 and monodisperse polystyrene (PS) spheres,17
serve as scaffolds, through either endo- or exo-templating.18
Nevertheless, some mesoporous zeolites with isolated and
closed secondary porosity so far have been unsuitable for the
diffusion of large molecules. Furthermore, high cost and the
complexity in the fabrication of hard templates limit their
industrial applications.
1. Introduction
Zeolites with crystalline framework, high stability and specic
active sites are widely used in petro-chemistry and ne-chem-
ical synthesis and oen show size- and shape-selectivity.1–3
However, the very presence of the micropores, with aperture
diameters oen below 1 nm, always goes hand-in-hand with
diffusion limitations,2,4,5 which adversely affects their catalytic
activity. The problem can be overcome by decreasing the size of
zeolite crystals, synthesizing zeolites with larger pores and
introducing mesopores/macropores into the zeolite particles.4,6
Within the past few decades, nanozeolites have been prepared
by hydrothermal synthesis at relatively low temperatures with or
without the addition of a growth inhibitor or nucleation
promoter, conned-space synthesis and two-step seeded growth
synthesis.7–11 However, zeolite crystals smaller than 100 nm
might be thermodynamically unstable due to the high surface
energy and vast amounts of surface defects. Meanwhile, highly
dispersed nanocrystals are quite difficult to handle and gener-
ally have low yields during the synthesis, in which the majority
of the building units are le unused in the mother liquid. The
nano-sized zeolite crystallites could be assembled by adding
mesoporogens, in which the mesopores are formed among the
crystallites. An alternative method is to directly synthesize
hierarchical zeolite with inter-crystalline and/or intra-crystal-
line mesoporosity, where mesoporogens and microporogens
are used simultaneously in one system. A number of hard or so
templates have been used as mesoporogens. Most of the hard
Compared with hard templates, so templates may offer a
more promising alternative approach for the preparation of
hierarchical mesoporous zeolites through diverse interactions
with zeolite precursors. However, two different templating
systems of micropore and mesopore worked in a competitive
manner rather than in a cooperative manner, which easily
results in the formation of physical mixtures of amorphous
mesoporous material and bulky zeolite.5,19–21 To prevent phase
separation, Ryoo et al. directly synthesized mesoporous MFI
and LTA zeolites with tunable mesostructure using specically
designed amphiphilic organosilanes.22 Later, stable single-unit-
cell nanosheets of zeolite MFI were successfully fabricated with
the specially designed and synthesized bifunctionalized
agent C22–H45–N(CH3)2–C6H12–N(CH3)2–C6H13(C22-6-6), which
possesses two quaternary ammonium groups spaced by a C6
alkyl linkage and a long chain alkyl group (C22) on the end.23
Very recently, zeolite ZSM-5 nanosheets with the thickness of
ꢀ10 nm could be directly synthesized using multiquaternary
ammonium surfactants, based on 1,4-diazabicyclo[2.2.2]octane,
as structure-directing agents.24 Alternatively, various polymers
were used as mesoporogens to enhance their interaction with
zeolitic precursors. For example, Park et al. used silylated
polypropylene oxide diamine and polyethylenimine polymers as
templates to synthesize MSU–MFI with intracrystalline
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, Department of
Chemistry, East China Normal University, Shanghai 200062, PR China. E-mail:
hongli813212@163.com; Fax: +86 21 62232213; Tel: +86 21 62232213
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mesopores of 2.2 and 5.2 nm.25,26 Xiao et al. synthesized hier-
archical zeolites beta and ZSM-5 by using the cationic polymers
polydiallyldimethylammonium chloride (PDADMAC) and
dimethyl diallyl ammonium chloride acrylamide copolymer
(PDDAM) as mesoporogens, respectively, and sometimes the
mesopore size could be tuned by the amount of polymers
used.27 Based on the suitable interactions between the zeolite
precursor and mesoporogen by either chemical bonding or
enlargement of charge densities, the hierarchical structures
could be created without phase separation between the meso-
phase and crystalline zeolite. Unfortunately, the synthesis of
these unique templates is very complex, leading to a high cost
for the synthesized mesoporous zeolite materials. To synthesize
hierarchical zeolites using commercially-available templates is
still highly anticipated.
Shi and coworkers reported that direct synthesis of meso-
porous zeolite ZSM-5 with MFI structure, using commercially
available TPAOH and cetyltrimethylammonium bromide (CTAB)
as microporogen and mesoporogen, respectively, could be ach-
ieved by nely tuning the degree of polymerization and then the
size of subnanocrystals, which could enhance the interaction
between zeolitic precursors and CTAB and then prevent the
phase separation of mesoporous material and bulk zeolite.28
Both zeolites MEL and MFI belong to the pentasil family. It is
well known that zeolites with MEL structure tends to form nano-
sized primary particles.29 The small crystallite size of MEL-type
zeolite could thus minimize the mismatch of zeolitic particles
and mesophase, make the templates for zeolite and meso-
porous materials work cooperatively so as to prevent the
formation of physical mixtures.
2. Experimental
2.1. Synthesis of MTS-2
In a typical synthesis, 9.11 g of cetyltrimethylammonium tosy-
late (CTATos, Merck, 99%) was dissolved in 108 mL of water to
obtain a clear CTATos solution A. 45.41 g of tetramethylam-
monium hydroxide (TBAOH, 40 wt%, Alfa) aqueous solution
was added to 41.67 g of tetraethyl orthosilicate (TEOS, SCRC,
>97%). Aer complete hydrolysis under vigorous agitation, the
silicate solution was further stirred at room temperature for 30
ꢁ
min and cooled to 0 C, then tetrabutyl titanate (TBOT, SCRC,
98%) isopropanol solution containing 1.70 g of TBOT and 15 g
of isopropanol was added slowly and continued to stir for 30
ꢁ
ꢁ
min at 0 C. Aer the removal of alcohol at 80 C for 2 h, the
above obtained solution was dropwise added to CTATos
solution A. The molar composition of the nal synthetic mixture
was SiO2 : 0.025TiO2 : 0.1CTATos : 0.35TBAOH : 50H2O. The
mixture was stirred continuously for 2 h at 60 ꢁC and then
divided into several aliquots and loaded into teon-lined steel
autoclaves. The nal mixture was heated in the autoclave under
static conditions at different temperatures for 12 days. The as-
made solid was recovered by ltration, fully washed with water,
and dried at 100 ꢁC in the oven overnight.
The post uoride treatment of the MTS-2 with NH4F (SCRC,
AR) was performed at 100 ꢁC for 12 h using the as-made samples
before the removal of organic templates, and the molar
composition of the nal mixture was 100SiO2 : 25NH4-
F : 6000H2O. Aer the treatment, all the samples were ltered,
washed thoroughly with distilled water, and dried at 100 ꢁC for
12 h. Subsequently, the dried products were calcined at 550 ꢁC
for 5 h under air for the removal of organic templates before
characterization and catalytic use.
Long and coworkers hydrothermally synthesized colloidal
silicalite-2 particles with the size of ꢀ100 nm in a very simple
reactant system of TBAOH–TEOS–H2O.29 Under certain condi-
tions, colloidal silicalite-2 particles were the agglomerates
composed of nano-crystals with the size of ꢀ20 nm. Very recently,
2.2. Characterization
Tsapatsis and coworkers reported that tetrabutylphosphonium Powder X-ray diffraction patterns (XRD) were collected on a
(TBP) hydroxide or tetrabutylammonium (TBA) hydroxide could Bruker D8 Advance powder diffractometer using Cu Ka radia-
be used as a single template to synthesize a hierarchical zeolite tion (l ¼ 0.154 nm) over a 2q range of 0.5–10ꢁ and 5–60ꢁ
MEL/MFI made of orthogonally connected microporous nano- respectively, and the accelerating voltage and the applied
sheets during one-step hydrothermal crystal growth at 388 K.30 current were 35 kV and 25 mA, respectively. The particle sizes of
The ‘ultrathin’ zeolites with thickness around 2 nm resulted in a samples were calculated from XRD data by using the Scherrer
large number of active sites on the external surface and then equation: D ¼ Kl/(b cos q), where D is the crystallite size (nm), l
rendered them highly active for the catalytic conversion of large is the wavelength of the X-ray radiation for the Cu target (l ¼
molecules. However, the size of zeolite aggregates obtained from 0.154 nm), b is the peak width at half-maximum height (2q ¼
both systems ranged from 60 nm to 400 nm, which still was very 23.1ꢁ) for MEL zeolite (501), q is the Bragg's angle and K is the
difficult for the separation and recovery.
Scherrer constant (0.89). Scanning electron microscopy (SEM)
The object of this study is to use normal TBAOH and CTATos was performed on a scanning electron microscope (type HITA-
as dual templates for micropores and mesopores, respectively, to CHI S-4800) with an accelerating voltage of 3 kV. Transmission
synthesize mesoporous MEL-type zeolites with sizes of several electron microscopy (TEM) was conducted on a TECNAI G2 F30
microns with combined advantages of reduced diffusion limita- operating at 300 kV. To prepare the samples for TEM, a
tion and easy recovery, where the enhanced interactions between dispersion of the sample in diluted ethanol was dropped onto
small nanocrystals of MEL-type zeolite and CTATos effectively the TEM sample bronze gridding, dried at room temperature for
prevent the formation of a physical mixture of mesoporous 1 h. The specic BET surface area (SBET) and other textual
materials and the bulk zeolite. In addition, the catalytic reactions properties of the samples were determined by nitrogen
involving relatively large molecules, including the hydroxylation adsorption–desorption measurements at 77 K on a nitrogen
of phenol and the epoxidation of cyclohexene, were tested. The adsorption apparatus (BELSORP-max). Before the measure-
effect of post uoride treatment was also discussed.
ments, the samples were outgassed at 300 ꢁC in vacuum for 6 h.
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The pore size distributions were derived from the adsorption almost the same as that of silicalite-2 with MEL structure
branches of the isotherms using the Barrett–Joyner–Halanda synthesized using TBAOH as the template, where two intense
(BJH) method. The total pore volume (Vp) was estimated at a peaks without shoulders existed in the region of 2q ¼ 23–25ꢁ
relative pressure of 0.99. DR UV-vis spectra were recorded in the (Fig. 1A). The relative crystallinity of MTS-2 samples increased
200–600 nm range on a UV-2550 from an aluminum cell with a with increase in crystallization temperature. Although TBAOH
quartz window. The amounts of Si and Ti in the nal zeolite has been considered as a typical template for MEL-type molec-
products were quantied by inductively coupled plasma (ICP) ular sieves, MEL/MFI intergrowths have been reported when
on a Thermo IRIS Intrepid II XSP atomic emission spectrometer using TBA+ as the template.31,32 Jablonski et al.33 pointed out
aer dissolving the samples in HF solution. The Solid 29Si MAS that zeolite with MEL structure only shows a single peak at 2q ¼
NMR measurements were collected on a VARIAN VNMRS- 45ꢁ while zeolite with MFI structure exhibited a doublet at 2q ¼
400WB spectrometer. Thermogravimetric analysis (TG) was 45ꢁ (reection 10, 0, 0) and 2q ¼ 45.5ꢁ (reection 0, 10, 0) with
performed using a PerkinElmer 457 TGA analyzer with a heating practically equal intensities. As shown in Fig. 1B, there is only
rate of 10 C minꢂ1 under an air ow.
one peak with of a hint of shoulder at 2q ¼ 45.5ꢁ for all the MTS-
2 samples. Therefore, MTS-2 samples synthesized using CTATos
and TBAOH here may be tentatively ascribed to the MEL-type
structure, although the formation of MFI/MEL intergrowth
could not be excluded. Similarly, Davis and coworkers consid-
ered that it was quite difficult to synthesize pure MEL-type
molecular sieves using TBA+ only.34
These broad XRD peaks for MTS-2 samples, compared with
those for bulky MEL structured zeolites, generally could be
ascribed to the small dimensions of particles. With the increase
of crystallization temperature from 140 ꢁC to 175 ꢁC, the
primary particle size increases from 15 nm to 20 nm, calculated
according to the Scherrer equation, which could be explained by
the fact that the induction period is prolonged at lower
temperatures, resulting in more nuclei needed for nucleation
and crystal growth,35 and thus minimizing the ultimate crystal
sizes.36 It should be noted that quite small primary particles
with the size of ꢀ20 nm could be obtained even at a much
higher crystallization temperature of 175 ꢁC. The reason should
be ascribed to the presence of CTA+, which may inhibit the
growth of MEL-type zeolite and thus prevent the formation of
large crystallites of zeolite MEL.
ꢁ
2.3. Catalytic reactions
2.3.1 Hydroxylation of phenol. Catalytic reactions were
performed in a glass ask equipped with a reux condenser and
a magnetic stirrer. Typically, 0.1 g of catalyst, 2 g of phenol, 0.8 g
of hydrogen peroxide (30 wt% aqueous solution) and 1.6 g of
acetone as solvent were introduced in the reactor and heated at
the reaction temperature (80 ꢁC) for 6 h under vigorous stirring.
The phenol/H2O2 molar ratio was taken to 3.
2.3.2 Epoxidation of cyclohexene. The epoxidation of
cyclohexene with anhydrous tert-butyl hydroperoxide (TBHP,
5.5 M in decane) was carried out in a 25 mL round-bottomed
ask equipped with reux condenser. In a typical run, 50 mg of
the catalyst, 10 mL of acetonitrile, 10 mmol of cyclohexene, and
10 mmol of TBHP were mixed in the ask and stirred vigorously
ꢁ
at 60 C for 2 h.
All products were analyzed on a Shimadzu GC-2014 gas
chromatograph equipped with a DM-Wax-30 m column (Dikma
Technologies Inc.) and an FID detector.
3. Results and discussion
3.1. Synthesis and characterization of multilayered
The morphology observed from SEM images is oen the
characteristics of crystalline grains and possibly provides some
information on the growth mechanism.37,38 Typical SEM images
of the as-made samples synthesized at different crystallization
temperatures are shown in Fig. 2. MTS-2 samples synthesized at
low temperatures of 140 and 150 ꢁC showed the olive-like
morphology consisting of thin and sometimes slightly curved
nanosheets as shown in Fig. 2b and d, consistent with relatively
low crystallinity provided by XRD results. At a low temperature
of 140 ꢁC, primary particles were very thin, with the size of ꢀ15
nm. With the crystallization temperature increasing, the zeolite
nanosheets became thick and well-shaped, corresponding to a
highly crystallized MEL-type structure. Olive-like and micro-
sized aggregates with relatively uniform size were observed for
titanosilicates
Fig. 1 displays the powder XRD patterns of MTS-2 samples
synthesized at different temperatures (140–175 ꢁC) for 12 days.
The wide-angle XRD patterns of all these MTS-2 samples were
ꢁ
MTS-2 synthesized at the temperature of 160 and 175 C. Most
importantly, neither the amorphous phase nor the faceted bulk
crystals of MEL zeolite was detected throughout the entire
samples, which is completely different from the physical
mixtures of mesoporous materials and zeolites. The other thing
to be noted is that nanosheets for MTS-2 synthesized at 165 ꢁC
are mostly along one direction as shown in Fig. 2e and f, while
some nanosheets for MTS-2 synthesized at 175 ꢁC are arranged
perpendicular to each other as shown in Fig. 2h.
Fig. 1 Large-angle powder XRD patterns of MTS-2 synthesized at (a)
140 ꢁC, (b) 150 ꢁC, (c) 165 ꢁC and (d) 175 ꢁC for 12 days.
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Moreover, nanosheets reported by Tsapatsis and coworkers
could be self-pillared and form a house-of-cards arrangement,
however, MTS-2 synthesized at a low temperature of #160 ꢁC
here mostly are along one direction. It indicated that the addi-
tion of CTATos may disturb the growth mode of the MEL-type
structure and inhibit the repetitive twinning or other intergrowth
processes in the crystallization process. Furthermore, the micro-
sized MTS-2 aggregates could be easily recovered by traditional
ltration, curtailing the difficulties in separation and recovery.
The TEM images provide complementary information to
SEM images. TEM images of MTS-2 synthesized at different
crystallization temperatures for 12 days are shown in Fig. 3,
which is consistent with SEM images. Low crystallization
ꢁ
temperatures of 140 and 150 C led to thin nanosheets with a
thickness less than 5 nm, mostly along one direction. With
increase in crystallization temperature, the thickness of zeolite
nanosheets increased obviously, and more orthogonally con-
nected nanosheets were observed for the MTS-2 samples
ꢁ
synthesized at 175 C, as shown in Fig. 3g and h.
Fig. 2 SEM images of MTS-2 synthesized at (a and b) 140 ꢁC, (c and d)
150 ꢁC, (e and f) 165 ꢁC and (g and h) 175 ꢁC for 12 days.
These results are quite different from that reported by Tsa-
patsis and coworkers.30 They reported that zeolite nanosheet
aggregates with the size of 100–200 nm could be synthesized at a
ꢁ
much lower temperature of 115 C in the presence of ethanol,
while in the present work nanosheets of MTS-2 assembled into
micro-sized aggregates at a high temperature above 140 ꢁC with
the removal of alcohol generated by the hydrolysis of TEOS and
additional isopropanol. It is worth mentioning that low reaction
temperature and the presence of alcohol are benecial to obtain
dispersed nano-sized zeolite particles due to the formation of a
large amount of nuclei needed for nucleation during the long
induction period at low temperatures and the proved effect of
alcohol on the inhibition from producing zeolite aggregates.
However, the TS-2 synthesized at 175 ꢁC using TBAOH as the
single structure directing agent with the molar ratio of TBAOH/
SiO2 ¼ 0.35 formed large uniform spherical agglomerates of
nano-sized primary particles with the size of 8–14 mm, possessing
a low external surface area and volume of mesoporosity, where
the alcohol was removed before crystallization. This suggests
that the presence of CTATos played an important role in
directing the multilayered MEL-type titanosilicate nanosheets.
Fig. 3 TEM images of MTS-2 synthesized at (a and b) 140 ꢁC, (c and d)
150 ꢁC, (e and f) 165 ꢁC and (g and h) 175 ꢁC for 12 days.
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It is worth nothing that few papers have reported the angle peak nearly disappeared (Fig. 4A(e)). The evolution of
hydrothermal synthesis of lamellar zeolite using binary both meso- and micro-structure demonstrates that the forma-
templates without any other additives. Very recently, meso- tion of zeolite MTS-2 is accompanied by partial collapse of the
porous ZSM-5 and TS-1 were prepared using CTAB as meso- MCM-41-like mesostructure during the crystallization process.
porogen in the presence of ethanol, but no nanosheets formed Nevertheless, complete collapse of the mesostructure did not
in their reports.28,39 We believed that the synthesis of MTS-2 in happen and a broad peak was observed in the small-angle XRD
present work took advantage of MEL-type zeolite that tends to (Fig. 4A(f)), indicating that some disordered mesopores may be
form nano-sized primary crystallites preferentially, where the retained aer hydrothermal crystallization at 160 ꢁC for a
mismatch of nanocrystals and mesophase was minimized due duration as long as 12 days. It proves that the mesostructure
to the decrease in the size of primary zeolite particles, and then directed by CTATos is hydrothermally stable to some extent.
the enhanced interactions between zeolite precursors and
mesotemplates led to the formation of hierarchical MTS-2 discussed above. A pure mesophase of MCM-41 without the
aggregates without phase separation. zeolite morphology was observed in SEM images of Fig. 5a and b
The SEM images in Fig. 5 are consistent with the XRD data
Fig. 4 displays the powder XRD patterns of the as-made during the early stages of the hydrothermal reaction (for 2 h and
samples synthesized at 160 ꢁC for different crystallization times 1 day). There are some olive-like spheres with the size of 2 mm
of 2 h to 12 days. Sharp small-angle XRD peaks at 2 h show that randomly dispersed in the amorphous gel obtained for 3 days as
the sample had an ordered hexagonal mesostructure, similar to shown in Fig. 5c. With the crystallization time increased to 5
MCM-41 materials (Fig. 4A(a)).40,41 These peaks could be days, more irregularly-shaped gel transformed into olive-like
indexed to (10), (11), and (20) reections, corresponding to the spheres. Aer 12 days, almost all the amorphous gel had dis-
two dimensional hexagonal structure. No atomic order was appeared and well crystallized zeolite MTS-2 had formed as
detected at this stage, because of the absence of Bragg reec- olive-like agglomerates composed of primary nanosheets.
tions in the wide-angle XRD pattern (Fig. 4B(a)). When the Therefore, the combined results of XRD and SEM indicate that
synthesis time was increased to 3 days (Fig. 4A(c)), the (11) and MTS-2 synthesized at 160 ꢁC for 12 days has a disordered
(20) reections in the small-angle region disappeared and the mesostructure and highly crystallized MEL-type zeolite struc-
(10) peak was broadened and shied to lower angle values. This ture, without any obvious macroscopic phase separation. In
is opposite to the results for the synthesis of mesoporous TS-1 in addition, the transformation from the pure MCM-41 phase to
the presence of [3-(trimethoxysilyl)propyl]-octadecyldimethy- MEL-type zeolite here may follow the solid–solid transformation
lammonium chloride (TPOAC), where the reection at 2–3ꢁ mechanism suggested by Serrano et al.43 It was reported that
shied to higher angle values with the crystallization time
prolonging.42 No obvious Bragg reections were noted at this
stage in the wide-angle region (Fig. 4B(c)). This result indicates
that the initial MCM-41-like mesostructure possibly trans-
formed into a disordered mesostructure and the presence of
CTA+ strongly inhibited the growth of MEL-type zeolite. At 5
days, some Bragg reections began to appear in the wide-angle
XRD region along with an amorphous halo, indicating the poor
crystallinity (Fig. 4A(d)). When the crystallization time was
further extended to 7 days (Fig. 4B(e)), most of the intense peaks
indexed to MEL structure could be observed, while the small-
Fig. 4 Small-angle (A) and wide-angle (B) powder XRD patterns of as- Fig. 5 SEM images of samples in the as-made samples synthesized at
made samples synthesized at 160 ꢁC for (a) 2 h, (b) 1 day, (c) 3 days, (d) 5 160 ꢁC for (a) 2 h, (b) 1 day, (c) 3 days, (d) 5 days, (e) 7 days and (f)
days, (e) 7 days and (f) 12 days.
12 days.
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may be attributed to intercrystalline mesopores caused by the
assemblies of zeolite nanosheets. The relatively larger inter-
crystalline pores may be related to more consumption of mes-
opores during the crystallization process and the formation of
bigger voids between the larger primary particles (thicker zeolite
nanosheets) obtained at high temperature. This is quite
different from mesoporous materials mechanically mixed with
zeolites. Furthermore, an increase in temperature led to a
decrease in both external surface area and mesopore volume
accompanied by an increase in the zeolitic micropore volume as
listed in Table 1. It was supported by XRD results, where low
temperature resulted in relatively low crystallinity and thus
small zeolitic micropore volumes. MTS-2 synthesized at 150 ꢁC
has the highest BET surface area of 573 m2 gꢂ1 here, compa-
rable to that of titanosilicate MFI nanosheets of single-unit cell
thickness synthesized with a diquaternary ammonium surfac-
Fig. 6 N2 physisorption curves (A) and the corresponding pore size
distribution (B) of calcined MTS-2 nanosheets synthesized at various
crystallization temperatures for 12 days: (a) 140 ꢁC, (b) 150 ꢁC, (c) 160
ꢁC and (d) 175 ꢁC. (Isotherm: 150/300/450 cm3 gꢂ1 shift.)
tant (i.e.,
C
16H33–N+(CH3)2–C6H12–N+(CH3)2–C6H13) as the
zeolite structure-directing agent (580 m2 gꢂ1).47 However, MTS-2
small tetraalkylammonium cations, such as TMA+ (tetrameth-
ylammonium) and TEA+ (tetraethylammonium), can easily
exchange with the template occluded in the channel of the 2D
hexagonal MCM-41 silica,44 or enter into the templating
micelles in the mesophase by Corma's and Sayari's groups
respectively.45,46 Here, under the hydrothermal conditions, TBA+
molecules may diffuse into the mesoporous channels and then
interact with the amorphous pore walls to generate crystalline-
like inorganic frameworks, nally transferring the MCM-41
mesophase to the MEL zeolitic structure.
ꢁ
synthesized at 150 C, with the thickness of around 5–10 nm,
has a lower external surface area (333 m2 gꢂ1) than these tita-
nosilicate MFI nanosheets with the thickness of ꢀ2 nm (425 m2
gꢂ1). Similarly, Wang et al. reported that multilayered titanosi-
licate synthesized using a bifunctional surfactant as the struc-
ture-directing agent had BET surface area and external surface
area of 510 and 357 m2 gꢂ1, respectively,48 quite close to those of
ꢁ
MTS-2 synthesized at 150 C. As a reference, TS-2 was synthe-
ꢁ
sized at 175 C using TBAOH as the single structure directing
agent with the molar ratio of TBAOH/SiO2 ¼ 0.35, in order to
show the role of CTATos in directing the formation of nano-
sheet structures. The reference TS-2 formed large uniform
spherical agglomerates of nano-sized primary particles with the
size of 8–14 mm, however, it has much lower external surface
The combination of meso- and micro-porosity was proven by
nitrogen physisorption. N2 adsorption–desorption isotherms
and pore size distribution (PSD) curves are shown in Fig. 6 and
the textual properties are summarized in Table 1. The adsorbed
volumes of N2 for all the calcined MTS-2 are of more than 100
cm3 gꢂ1 at low partial pressure (P/P0 < 0.10) (Fig. 6A), indicating
that the samples are highly crystallized and contain a consid-
erable amount of zeolitic micropores. In addition, all the MTS-2
samples exhibited clear hysteresis loops at higher pressure P/P0
> 0.40, which is characteristic of mesoporous and/or macro-
porous materials, indicating the hierarchical porosity of MTS-2.
A broad pore size distribution centered at ꢀ12 nm was observed
for MTS-2 samples synthesized at low crystallization tempera-
tures (140 ꢁC and 150 ꢁC), while those synthesized at high
area (94 m2 gꢂ1) and volume of mesoporosity (0.17 cm3 gꢂ1
)
than MTS-2. Therefore, the active sites in reference TS-2 might
be inaccessible to the bulky molecules, the role of CTATos in
directing multilayered MEL-type titanosilicate nanosheets was
further proved.
In general, the introduction of mesoporosity in hierarchical
zeolites is frequently coupled to a lowered micropore volume,
which sparkled the denition of the hierarchy factor (HF) to
efficiently categorize zeolites by their porosity.49 The HF is
expressed as the relative mesopore surface area (Smeso/Stotal
)
ꢁ
ꢁ
temperatures of 160 C and 175 C showed a more wider pore
multiplied by a relative microporosity (Vmicro/Vtotal). The MTS-2
distribution centered at ꢀ28 nm (Fig. 6B). This mesoporosity
nanosheets, hydrothermally synthesized using CTATos and
Table 1 Structural data of calcined catalysts
SBET/m2 gꢂ1
Sext/m2 gꢂ1
Vtotal/cm3 gꢂ1
Vmicro/cm3 gꢂ1
Vextra/cm3 gꢂ1
HFa
Sext/SBET
b
Samples
T/ꢁC
MTS-2
MTS-2
MTS-2
MTS-2
TS-1
140
150
160
175
175
175
175
539
573
517
506
489
565
487
323
333
252
228
74
0.77
0.76
0.63
0.57
0.39
0.51
0.53
0.09
0.11
0.12
0.12
0.18
0.20
0
0.68
0.65
0.51
0.45
0.21
0.31
0.53
0.07
0.08
0.09
0.09
0.07
0.08
—
0.60
0.58
0.49
0.45
0.15
0.21
1
TS-2
Ti–MCM-41
117
487
a
b
Hierarchy factor, HF ¼ (Vmicro/Vtotal) ꢃ (Sext/SBET). Fraction of external surface area.
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the initial synthetic mixtures. The reason could be that
condensation between Si–OH and HO–Si may be much faster
than that between Ti–OH and HO–Si in the presence of CTATos
at high crystallization temperatures, resulting in the expulsion
of Ti species from framework sites. As shown in Table 2, the
molar ratio of Si/Ti in the nal MTS-2 samples increased with
increase in temperature, which conrms that high crystalliza-
tion temperature disfavors the incorporation of Ti into the
framework.
3.2. Catalytic properties of multilayered MTS-2 nanosheets
in the hydroxylation of phenol and the epoxidation of
cyclohexene
The activity of MTS-2 nanosheets has been compared with that
of conventional nano-sized TS-1 and TS-2, as well as that of Ti–
MCM-41 with ordered mesopores but amorphous walls.
Fig. 7 UV-visible spectra of calcined MTS-2 synthesized at various Conventional TS-1 and TS-2 were synthesized with the gel
crystallization temperatures for 12 days: (a) 140 ꢁC, (b) 150 ꢁC, (c) 160
ꢁC and (d) 175 ꢁC.
composition of SiO2 : 0.025TiO2 : 0.18TAAOH : 25H2O, using
TPAOH and TBAOH as structure directing agents, respectively.
Ti–MCM-41 was obtained using CTATos as a single template as
reported in our previous work.51 As shown in Fig. 8, TS-1 and TS-
TBAOH as binary templates, showed HF of 0.07–0.09, slightly
2 are nanocrystals with the size of ꢀ300 nm, while Ti–MCM-41
increased with the temperature increasing. To some extent, it is
exhibits worm-like particles with some larger elongated
´
´
different from that suggested by Perez-Ramırez et al., where HF
> 0.1 when hierarchical zeolites were fabricated through deal-
umination or templating methods. Low HF value for MTS-2
samples could be attributed to the formation of quite large total
volume and relatively small micropore volume. As the authors
did not provide the micropore volume, the HF value of MFI
titanosilicate nanosheets with single-unit-cell thickness could
not be obtained.47
agglomerates. The textual properties are summarized in Table
1. TS-1 and TS-2 have much lower external surface area and
volume of mesoporosity than MTS-2, which might hinder the
diffusion of bulky molecules. Ti–MCM-41 has a much higher
external surface area and total volume than TS-1 and TS-2,
indicating that the active Ti sites in Ti–MCM-41 are highly
accessible to the bulky molecules. However, its amorphous
structure will not catalyze the reactions contributed to the Ti
centers in the zeolitic framework. In addition, such non-
crystalline silica frameworks are vulnerable to water environ-
ment catalytic reaction conditions, and hence, the catalytic
recyclability is lower than the crystalline framework.
It is well known that nano-sized porous materials have a
large external surface area and short diffusion path, which
signicantly improves the mass-transfer rate in the conned
space, and thus exhibits high reactivity.52 Two test reactions
including the hydroxylation of phenol and the epoxidation of
cyclohexene were purposely selected. The catalytic activity of TS-
1 samples for the hydroxylation of phenol was very sensitive to
the crystal size, as the reaction is restricted by pore diffusion
limitations.53 As for the epoxidation of cyclohexene, cyclohexene
The coordination states of Ti species in MTS-2 were further
studied by UV-visible spectroscopy (Fig. 7). All samples synthe-
sized at different temperatures showed a main absorption
around 220 nm, the characteristics of tetrahedrally coordinated
Ti highly dispersed in the framework.50 This could be benecial
for redox reactions that require tetrahedrally coordinated tita-
nium. In addition, no signicant absorption at the 260 and 330
nm bands occurred in the UV analysis for MTS-2 crystallized at
140 ꢁC, which indicates extra-framework Ti species are negli-
gible. With increasing crystallization temperature, an absorp-
tion at 330 nm attributed to the TiO2 phase was observed,
indicating the formation of extra-framework Ti species at high
temperatures despite possessing the same Si/Ti molar ratio in
Table 2 Hydroxylation of phenol with H2O2 over MTS-2a
Samples
T/ꢁC
dcrysta/mm
Si/Tib
Conv.c/%
YOrthod/%
YParae/%
TOFf
MTS-2
MTS-2
MTS-2
MTS-2
TS-1
140
150
160
175
175
175
1
34.5
35.9
39.9
40.2
47.2
51.2
14.8
23.5
26.5
28.9
26.0
23.2
7.4
12.1
13.5
14.8
13.5
11.7
4.9
8.0
9.0
10.6
8.7
8.5
11.1
18.4
23.0
25.2
26.5
15.6
1.5
1–2
1–2
0.3
0.3
TS-2
a
Reaction conditions: 0.1 g catalyst, 2 g phenol, 0.8 g H2O2 (30% aqueous solution), 1.6 g acetone, 80 ꢁC 6 h. b Si/Ti ratio in the product given by ICP
data. c Conversion of phenol. d Yield of catechol. e Yield of hydroquinone. Turnover frequency in mol molꢂ1 hꢂ1
.
f
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Fig. 8 SEM images of reference samples: (a) TS-1, (b) TS-2 and (c) Ti–MCM-41.
molecules are too large to diffuse into the 10-MR pores of both here, taking advantage of hierarchical porosity and thin nano-
MFI and MEL structure, and thus the epoxidation of cyclo- sheets of MTS-2 samples. Furthermore, nanosized TS-1 and TS-
hexene mostly occurred on the external surface of zeolites.54
2 have severe difficulties in the recovery during their synthesis
3.2.1 Hydroxylation of phenol with H2O2. Titanium silica- and applications due to their colloidal properties. However,
lite-1 (TS-1) with MFI structure was proved to be an active micro-sized MTS-2 samples could be easily recovered by
catalyst in the hydroxylation of phenol,55,56 which has been conventional ltration. In summary, the formation of micro-
commercialized by Enichem.57 As a member of the pentasil sized aggregates not only keeps well the advantages of catalytic
family of zeolites, TS-2 also shows considerable catalytic activity and shape selectivity of nanosized TS-2, but also curtails
performances in the hydroxylation of phenol.58 Meanwhile, it the difficulties in separation and recovery. The reference TS-2
was reported that the framework structure differences do not was synthesized at 175 ꢁC using TBAOH as the single structure
perturb the catalytic performances by comparing catalytic directing agent with the molar ratio of TBAOH/SiO2 ¼ 0.35
behaviors of TS-1 and TS-2 with similar Si/Ti ratios and crystal showed the similar catalytic activity with nanosized TS-1 on the
sizes.58 This is not surprising, as TS-2 and TS-1 framework hydroxylation of phenol (conv.phenol ¼ 24.1%) due to its
structures are very similar. However, it was demonstrated that comparable external surface area (94 m2 gꢂ1) with nano-sized
the catalytic activity was very sensitive to the crystal size.53 When TS-1 and well crystallized zeolitic structure.
the crystal size of TS-1 is beyond 1 mm, the diffusion of products
3.2.2 Epoxidation of cyclohexene with TBHP. The catalytic
and reactants is quite difficult and the auto-decomposition of activity of MTS-2 has been further investigated by the epoxida-
H2O2 may be the prevailing reaction. Therefore, not only low tion of cyclohexene using TBHP as oxidant. The epoxidation of
activities but also very low H2O2 efficiencies are observed. As a cyclohexene was carried out in acetonitrile following the report
result, nano-sized TS-1 or TS-2 (<300 nm) were used for the by Na et al.47 In the epoxidation of cyclohexene with TBHP (5.5
hydroxylation of phenol, making severe difficulties in the M in decane), cyclohexene oxide (CHO) was almost the sole
separation and recovery. In this case, TS-1 and TS-2 with the size product, irrespective of the type of titanosilicate (Table 3). Nano-
of ꢀ300 nm were synthesized as a reference. The phenol sized TS-1 and TS-2 possess a small external surface area and
conversion for three MTS-2 samples synthesized at $150 ꢁC for volume of mesoporosity (Table 1) leading to low conversion of
12 days is comparable to that over nano-sized TS-1 and TS-2 cyclohexene because the 10-MR of the MFI structure provided a
(Table 2), while MTS-2 crystallized at 140 ꢁC was less active due signicant hindrance for the bulky molecules of cyclohexene
to its poor crystallinity (small micropore volume). Moreover, and TBHP (Table 3). As MEL structure zeolite tends to form
MTS-2 samples show larger TOF than that of nano-sized TS-2, nano-sized primary crystallites preferentially, nano-sized TS-2
implying the more effective catalytic activity of Ti incorporated had slightly larger external surface area and volume of meso-
into the framework of MTS-2 nanosheets. It indicated that an porosity than TS-1 (Table 1), thus showed higher conversion
increase in the size of MTS-2 aggregates up to 1–2 mm did not than that of nano-sized TS-1 due to the better accessibility of
exacerbate the diffusion limitation of reactants and products active sites to bulky molecules (Table 3). Notably, the conversion
Table 3 Epoxidation of cyclohexene with TBHP over MTS-2a
Samples
T/ꢁC
dcrystal/mm
Si/Tib
Si/Tic
Conv.d/%
CHOsele/%
TOFf
MTS-2
MTS-2
MTS-2
MTS-2
TS-1
140
150
160
175
175
175
175
1
40
40
40
40
40
40
40
34.5
35.9
39.9
40.2
47.2
51.2
32.3
17.5
14.1
7.6
6.7
1.2
99.0
98.9
97.0
96.7
99.0
99.0
96.9
37.7
31.5
18.8
16.7
3.5
1.5
1–2
1–2
0.3
0.3
>2
TS-2
Ti–MCM-41
6.1
19.6
19.3
39.6
a
Reaction conditions: 0.05 g catalyst, 10 mL CH3CN, 10 mmol cyclohexene, 10 mmol TBHP, 60 ꢁC 2 h. b Si/Ti ratio in gel. c Si/Ti ratio in the product
given by ICP data. d Conversion of cyclohexene. e Selectivity of cyclohexene oxide (CHO). f Turnover frequency in mol molꢂ1 hꢂ1
.
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of cyclohexene on MTS-2 synthesized at 140 ꢁC was much higher and decreased catalytic activity. Only uoride treatment of as-
than that on nano-sized TS-1, and its TOF reached the same made TS-1 nanosheets could reduce the silanols and enhance
level as that of Ti–MCM-41. Since the bulky cyclohexene tested the hydrophobicity and catalytic activity.47 Noticeably, uorine
here were too big to approach the internal Ti sites, the high atoms did not exist on the nanosheet sample aer uoride
catalytic activity indicates that the Ti sites on the external treatment.
surface of the nanosheets are sufficiently active to convert the
In the present study, we also used the post uoride treat-
bulky cyclohexene to its epoxide counterparts. It is deduced that ment to decrease the concentration of silanols on the as-made
the active Ti sites in MTS-2 were highly accessible to the bulky MTS-2 samples, in order to increase the surface hydropho-
molecules because of the presence of inter-crystalline meso- bicity and thus the catalytic activity. As shown in Table 4, the
porosity. The catalytic activity decreased with the crystallization relative Q4/Q3 ratios for all the MTS-2 samples, calculated from
temperature increasing from 140 ꢁC to 175 ꢁC, which might be the areas of Q4 (ꢂ113 ppm) and Q3 (ꢂ102 ppm) in 29Si MAS
explained by the reduced mesoporosity and external surface NMR spectra, increased aer the uoride treatment. It was
area due to the formation of thicker nanosheets and more extra- suggested that the decrease of silanols was attributed to the
framework Ti species at high temperatures. Nevertheless, the formation of siloxane bridges [i.e., (SiO)3Si–O–Si(OSi)3] by
MTS-2 synthesized at 175 ꢁC still showed higher conversion condensation between neighboring silanols.47 All MTS-2
than that of nanosized TS-1.
samples aer uoride treatment had a slightly high relative
ꢁ
The reference TS-2 was synthesized at 175 C using TBAOH crystallinity, as listed in Table 4. Possibly, NH4F treatment
as the single structure directing agent with the molar ratio of might remove some amorphous titano-silica/silica species, or
TBAOH/SiO2 ¼ 0.35 and it showed much lower catalytic activity insert silica species into framework defects, and thus induce a
than MTS-2 on the epoxidation of cyclohexene (conv.cyclohexene
¼
recrystallization process on the nanosheet surface under
3.5%) due to its small external surface area and volume of present post-treatment conditions, since the uoride anions
mesoporosity, illustrating that the formation of multilayered could be used as a mineralizing agent. In contrast to the
MTS-2 by the addition of CTATos is important for reactions results of nanosheet F–TS-1 (the molar ratio of Si/Ti slightly
which occur on the external surface.
decreased from 57 to 54 aer the uoꢁride treatment),47 MTS-2
Interestingly, MTS-2 samples synthesized at different samples synthesized at 160 and 175 C obviously showed an
temperatures show an opposite trend in these two reactions, increased Si/Ti ratio aer the NH4F treatment, as listed in
where MTS-2 sample synthesized at 140 ꢁC showed highest Table 5, while MTS-2 samples synthesized at 140 and 150 C
ꢁ
activity for the epoxidation of cyclohexene, while this sample showed quite closer molar ratios of Si/Ti. This may be attrib-
had the lowest activity for the hydroxylation of phenol. This uted to different amounts of templates occluded in the MTS-2
probably was attributed to the fact that the epoxidation of samples synthesized at varied temperatures. As ꢁshown in
cyclohexene could only occur on the external surface of titano- Fig. 9, MTS-2 samples synthesized at 140 and 150 C showed
silicate, while the activity for hydroxylation of phenol was much higher weight loss in the range of 200–600 ꢁC than those
mostly contributed to the Ti centers in the zeolitic framework.
Among others, the MTS-2 sample synthesized at 140 ꢁC has the
largest fraction of external surface area, but smallest micropore
Table 4 Relative crystallinity and amount of silanol groups of MTS-2
volume. Therefore, its different behavior in these two reactions
could be easily understood.
Samples 140a 150
160
98
175
103
F-140b F-150 F-160 F-175
3.2.3 Post uoride treatment of as-made MTS-2 samples.
The MTS-2 samples have been proved to possess numerous
active sites on the external surface area and extremely thin
diffusion pathway, exhibiting high catalytic activity for the
hydroxylation of phenol and the epoxidation of cyclohexene.
Nevertheless, the external surface of multilayered MTS-2 might
be terminated with a large number of silanol groups, and the
hydration of silanol groups could cause poor accessibility of
hydrophobic olen to the Ti sites. Although this problem can be
signicantly resolved by blocking the silanols via silylation,59,60
such treatment has obscured the wide catalytic use of titanosi-
licate with hydrophilic surface.
In the previous studies, the uoride anions were considered
to be detrimental to the catalytic activity of titanosilicate cata-
lysts in epoxidation reactions.61,62 Recently, Fang et al. found
that the introduction of Si–F groups into Ti–MWW through
post-acid treatment in the presence of NH4F could greatly
enhance the catalytic activity in liquid-phase oxidation reac-
tions.63 However, the direct treatment of calcined titanosilicate
nanosheets with NH4F still led to the collapse of the structure
R.C.c
75
5.9
85
79
87
99
108
11.77
Q4/Q3
6.38 10.04 10.20 9.98 10.19 11.17
a
MTS-2 synthesized at different temperatures for 12 days. b MTS-2 post
treated with NH4F. c Relative crystallinity using conventional TS-2 as the
standard.
Table 5 Epoxidation of cyclohexene with TBHP over fluoride-treated
MTS-2a
T/ꢁC
dcrystal/mm
Si/Tib
Si/Tic
Conv.d/%
CHOsele/%
TOFf
140
150
160
175
1
34.5
35.9
39.9
40.2
34.3
36.2
44.4
52.2
24.0
22.4
14.9
12.6
98.0
98.9
97.6
97.9
51.4
50.6
40.9
40.5
1.5
1–2
1–2
a
Reaction conditions: 0.05 g catalyst, 10 mL CH3CN, 10 mmol
b
cyclohexene, 10 mmol TBHP, 60 ꢁC 2 h. Si/Ti ratio in MTS-2 samples
c
given by ICP data before the uoride treatment. Si/Ti ratio in
d
uoride-treated samples given by ICP data.
Conversion of
e
f
cyclohexene. Selectivity of cyclohexene oxide (CHO). Turnover
frequency in mol molꢂ1 hꢂ1
.
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aer uoride treatment, which further conrmed the protec-
tion effect of occluded templates. Quite differently, the
contribution of the adsorption at 330 nm decreased obviously
aer the uoride treatment for those synthesized at 160 and
175 ꢁC, which suggested that some extra-framework TiO2
species were removed. This is consistent with the report by
Guo et al., where some extra-framework titanium species could
be removed from the as-made TS-1 sample by the treatment
with NH4F–H2O2.64
Epoxidation of cyclohexene with TBHP was used to test
uoride-treated MTS-2 nanosheets. As shown in Table 5, the
conversion of cyclohexene increased aer uoride treatment
in all the cases. The positive effect of uoride treatment can be
explained in terms of the decrease in the silanol concentra-
tion. Since the external surface of the MTS-2 nanosheets
becomes less hydrophilic by decreasing the silanol concen-
tration via uoride treatment, which favors to increase cyclo-
hexene accessibility to the active Ti sites and, consequently,
increase the reaction rate. Therefore, uoride treatment
distinctly improved the catalytic capacity of MTS-2 samples.
Especially, MTS-2 samples synthesized at 140 and 150 ꢁC
showed higher conversion and TOF than MCM-41 material, as
listed in Table 5.
Fig. 9 TG–DTG curves of as-made MTS-2 synthesized at various
crystallization temperatures for 12 days: (a) 140 ꢁC, (b) 150 ꢁC, (c) 160
ꢁC and (d) 175 ꢁC.
synthesized at 160 and 175 ꢁC. The occluded templates
included both TBA+ and CTA+, where high crystallization
temperature may decompose the CTA+ via Hoffman elimina-
tion and/or expel the CTA+ out of zeolite particles. Therefore,
obviously fewer templates were occluded in the as-made MTS-
2 samples synthesized at high temperatures of 160 and 175 ꢁC.
These occluded organic templates could prevent Ti active
centers from attack by uorides and thus inhibit the detri-
mental effect of uorides. As for MTS-2 samples synthesized at
140 and 150 ꢁC, more TBA+ and CTA+ cations prevent the
leaching of Ti species during the uoride treatment, while
some Ti species on the external surface of those synthesized at
160 and 175 ꢁC could be attacked by uoride and then leached
due to less protection of occluded templates. The variation of
Ti centers could also be proved by UV-vis spectra as shown in
Fig. 10. For MTS-2 samples synthesized at 140 and 150 ꢁC,
there is only very minor variation for Ti chemical environment
4. Conclusion
In summary, multilayered MEL-type titanosilicate nanosheets
(MTS-2) were directly synthesized using binary templates of
cetyltrimethylammonium tosylate (CTATos) and tetrabuty-
lammonium hydroxide (TBAOH). Taking advantage of MEL
structure zeolite that tends to form nano-sized primary crys-
tallites preferentially, the mismatch of nanocrystals and meso-
phase was minimized and thus phase separation was prevented.
The titanosilicate nanosheets possess large inter-sheet meso-
pore volume and exhibit improved catalytic activities for the
hydroxylation of phenol and the epoxidation of cyclohexene
compared with mesoporous MCM-41, conventional TS-1 and
TS-2. Furthermore, the catalytic activity of the external Ti sites
for the epoxidation of cyclohexene could be enhanced remark-
ably by reducing the amount of silanols via post uoride
treatment.
Acknowledgements
This work is supported by the National Key Technology R&D
Program (no. 2012BAE05B02) and the National Science Foun-
dation of China (20890122). Prof. Wang thanks the Funda-
mental Research Funds for the Central Universities and the
Program for New Century Excellent Talents in University (NCET-
11-0145), Ministry of Education of China.
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