Synthesis of expanded titanosilicate MWW-related materials from a pure silica precursor

Article abstract of DOI:10.1021/cm302509m

Titanosilicate forms of expanded MWW-related material are synthesized starting from a pure silica precursor, ITQ-1. This procedure improves the current synthetic route of the expanded titanosilicate MWW material (Ti-YNU-1), which requires the use of a borosilicate MWW as precursor and, consequently, several postsynthetic acid treatments for removing boron atoms from the framework. The expanded Ti-MWW-related materials presented here show better activity than regular Ti-MWW in the selective epoxidation reaction of cyclohexene using H₂O₂ as oxidant.

Full text of DOI:10.1021/cm302509m

Article
pubs.acs.org/cm
Synthesis of Expanded Titanosilicate MWW-Related Materials from a
Pure Silica Precursor†
Manuel Moliner and Avelino Corma*
Instituto de Tecnología Química (UPV-CSIC), Universidad Politec
Valencia, 46022, Spain
́
nica de Valencia, Consejo Superior de Investigaciones Científicas,
ABSTRACT: Titanosilicate forms of expanded MWW-related ma-
terial are synthesized starting from a pure silica precursor, ITQ-1.
This procedure improves the current synthetic route of the expanded
titanosilicate MWW material (Ti-YNU-1), which requires the use of a
borosilicate MWW as precursor and, consequently, several
postsynthetic acid treatments for removing boron atoms from the
framework. The expanded Ti-MWW-related materials presented here
show better activity than regular Ti-MWW in the selective
epoxidation reaction of cyclohexene using H2O2 as oxidant.
KEYWORDS: titanosilicate, MWW, zeolite, olefin epoxidation, green chemistry
achieving the Ti-MWW material. In this case, boron is used as a
1. INTRODUCTION
structure-supporting agent in the direct preparation of the
titanoborosilicate MWW, and afterward, the sample is
deboronated by posterior acid treatments.15 It is claimed that
this Ti-MWW material shows higher catalytic activity than
other relevant titanosilicates, such as TS-1 and Ti-Beta, in the
liquid-phase epoxidation of linear alkenes with H2O2.16
In the last decades, the introduction of isolated metals,
especially titanium, in the walls of porous silicates has
promoted those materials as very efficient catalysts for the
selective oxidation of hydrocarbons using peroxides as
oxidants.1 Since the discovery of medium-pore titanosilicate
TS-1 in 1983,2 other molecular sieves containing isolated Ti
atoms in tetrahedral coordination have been synthesized with
diverse pore architectures. Specific structures would induce the
adequate activity and selectivity toward oxygenated derivates,
depending on the size of reactants.3 In this sense, large-pore
titanosilicates are required to facilitate the reactant accessibility
to the active sites when bulky organic molecules are tested.
Significant large-pore titanosilicates have been described in the
literature, such as Ti-Beta,4 Ti-ZSM-12,5 Ti-MOR,6 Ti-ITQ-7,7
or Ti-BEC.8
A very fascinating structure is the case of the MWW material,
which is formed by two independent noninterconnected pore
systems both having 10-membered ring (MR) openings.9 One
of these systems is defined by a two-dimensional sinusoidal
channel, and the other comprises supercages of 7.1 × 7.1 ×
18.2 Å.10 MWW is typically obtained from the calcination of an
MWW-lamellar precursor, and interestingly, this lamellar can
also be expanded into microporous−mesoporous hybrids11 or
delaminated into nanosheets.12
However, bulkier alkenes, such as cyclohexene, suffer severe
restrictions when diffusing through the medium pores of the
MWW material, reducing considerably their activity in the
liquid-phase epoxidation reaction.17 To avoid those steric
restrictions, Tatsumi et al. have described the synthesis of the
Ti-YNU-1 material,18 which is structurally related to MWW
zeolite, but introducing titanium atoms as pillars in the
interlayer region.19 Ti-YNU-1 allows the reactivity of bulky
organic molecules due to the slight expansion of the pore
diameter by the presence of Ti atoms in the interlayer region.
The Ti-YNU-1 material is synthesized following a rational-
ized methodology based on several steps, using as the initial
precursor the borosilicate form of MWW.18,20 The MWW
precursor is acid-treated several times to selectively remove
boron atoms, and subsequently, the deboronated precursor is
mixed with an organic molecule and a Ti source to afford the
titanosilicate material.18 Nevertheless, the use of B-MWW as
precursor introduces some drawbacks in the synthesis
procedure. Most relevant, several acid treatments are required
for removing most of the boron from the MWW materials, and
if complete elimination of boron from the framework or from
The preparation of the titanosilicate form of MWW was first
described by Tatsumi et al.13 by combining the hydrothermal
synthesis of the titanoborosilicate MWW, followed by acid
treatments to selectively remove boron atoms. Because the
direct preparation of MWW-type titanosilicate from gels
containing only Si and Ti in the absence of alkali cations is
difficult,14 the above-described methodology is very effective in
Received: August 7, 2012
Revised: October 11, 2012
Published: October 30, 2012
© 2012 American Chemical Society
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The as-synthesized titanosilicate MWW-related material was acid
treated using nitric acid (2 M) for 16 h at 100 °C (1 g of material in 20
mL of nitric acid solution) for removing octahedral Ti species. The
resultant sample was calcined at 550 °C in air.
Synthesis of Regular Ti-MWW Material. Regular Ti-MWW was
prepared following the methodology described by Tatsumi et al. in ref
16. Titanoborosilicate precursor was synthesized from a gel using
piperidine as an organic structure-directing agent, a Si/B molar ratio of
0.75, and a Si/Ti molar ratio of 50. To properly remove boron atoms,
the calcined sample was acid-treated by refluxing 6 M HNO3 during 12
h. This acid treatment was repeated three times, until chemical
analyses revealed a Si/B ratio higher than 500.
2.2. Characterization. Synthesized samples were characterized by
powder X-ray diffraction using a multisample Philips X’Pert
diffractometer equipped with a graphite monochromator, operating
at 45 kV and 40 mA, and using Cu Kα radiation (λ = 0.1542 nm).
The chemical analysis was performed on a 715-ES ICP-Optical
Emission spectrometer, after solid dissolution in HNO3/HCl/HF
aqueous solution. The organic content of the as-made materials was
determined by elemental analysis performed on a SCHN FISONS
elemental analyzer.
the pores is not achieved, its presence can negatively influence
the activity and selectivity of the catalyst.21
Herein, we will synthesize expanded titanosilicate MWW-
related materials starting from a pure silica MWW precursor,
avoiding the use of the structure-supporting agent (boron).
This procedure will involve the use of the pure silica MWW
zeolite in combination with the Ti source and one of the
organic molecules required in the typical preparation of MWW
materials. This methodology will improve the synthetic route
previously reported, since the acid treatments required to
selectively remove the structure-supporting agent (boron), with
the associated inconvenience if it is not very well removed, will
be avoided. The expanded Ti-MWW-related materials will be
characterized in order to study their crystalline nature, the
coordination of Ti species, and their pore size distribution.
Finally, the expanded Ti-MWW-related materials will be tested
on the selective epoxidation reaction of cyclohexene with H2O2,
and their catalytic activity compared to the regular Ti-MWW.
Pore size distribution was determined by Ar adsorption measured
on a Micromeritics ASAP 2020 at 87 K.
2. EXPERIMENTAL SECTION
2.1. Zeolite Synthesis. Synthesis of N,N,N-Trimethyl-1-ada-
mantammonium. A 29.6 g portion of 1-adamantamine (Sigma-
Aldrich) and 64 g of potassium carbonate (Sigma-Aldrich) were mixed
with 320 mL of chloroform. At this point, 75 g of methyl iodide was
added dropwise while the reaction was stirred in an ice bath. The
reaction was maintained during 5 days under agitation at room
temperature. The mixture was filtered and washed with diethyl ether,
and the resultant solid was further extracted with chloroform. The final
product was N,N,N-trimethyl-1-adamantammonium iodide. This
iodide salt was anion-exchanged using an ion-exchange resin to
prepare the corresponding hydroxide form.
UV−vis spectra were obtained with a Perkin-Elmer (Lambda 19)
spectrometer equipped with an integrating sphere with BaSO4 as
reference.
2.3. Catalytic Tests. Catalytic tests by using H2O2 as oxidant were
performed at 333 K, using 50 mg of catalyst and the following reaction
mixture: 10 mmol of cyclohexene, 10 mmol of H2O2, and 10 mL of
acetonitrile. All substances were available from Sigma-Aldrich
Company (acetonitrile, 99%; cyclohexene, 99%; H2O2, 35 wt % in
water). Aliquots were analyzed by gas chromatography using nonane
as the standard. H2O2 efficiency was calculated by iodometric tritation.
Synthesis of Pure Silica MWW (ITQ-1). A 113.34 g portion of an
aqueous solution of N,N,N-trimethyl-1-adamantammonium hydroxide
(7.5 wt %) was mixed with 26.9 g of water and 5.12 g of
hexamethylenimine. A 0.97 g portion of sodium chloride and 10 g
of silica (Aerosil 200, Degussa) were then added to the mixture,
keeping the gel under stirring during 30 min. The gel was transferred
to an autoclave with a Teflon liner and heated to a temperature of 150
°C during 9 days under agitation (60 rpm) conditions. The sample
after hydrothermal crystallization was filtered and washed with
abundant distilled water and finally dried at 100 °C.
3. RESULTS AND DISCUSSION
To prepare the expanded titanosilicate form of the MWW-
related material, avoiding the use of structure-supporting
Synthesis of Expanded Ti-MWW-Related Materials. a. Ti-MWW-
exp(1). A 289 mg portion of piperidine (99%, Sigma-Aldrich) was
mixed with 294 mg of water and 34 mg of titanium(IV) butoxide
(97%, Sigma-Aldrich). The mixture was stirred during 30 min until
complete hydrolysis of titanium(IV) butoxide. The calcined pure silica
ITQ-1 precursor (200 mg) synthesized above was then added to the
mixture, keeping the gel under stirring during 30 min. The final gel
composition was SiO2:0.033 Ti:1.0 piperidine:5 H2O. The gel was
transferred to an autoclave with a Teflon liner and heated to a
temperature of 175 °C during 7 days under static conditions. The
sample after hydrothermal crystallization was filtered and washed with
abundant distilled water and finally dried at 100 °C.
The as-synthesized titanosilicate MWW-related material was acid-
treated using nitric acid (2 M) for 16 h at 100 °C (1 g of material in 20
mL of nitric acid solution) for removing octahedral Ti species. The
resultant sample was calcined at 550 °C in air.
Figure 1. Powder X-ray diffraction (PXRD) patterns of the different
b. Ti-MWW-exp(2). A 608 mg portion of piperidine (99%, Sigma-
Aldrich) was mixed with 618 mg of water and 16 mg of titanium(IV)
butoxide (97%, Sigma-Aldrich). The mixture was stirred during 30 min
until complete hydrolysis of titanium(IV) butoxide. The calcined pure
silica ITQ-1 precursor (421 mg) synthesized above was then added to
the mixture, keeping the gel under stirring during 30 min. The final gel
composition was SiO2:0.0067 Ti:1.0 piperidine:5 H2O. The gel was
transferred to an autoclave with a Teflon liner and heated to a
temperature of 175 °C during 7 days under static conditions. The
sample after hydrothermal crystallization was filtered and washed with
abundant distilled water and finally dried at 100 °C.
MWW-related materials.
agents, such as boron, the pure silica MWW precursor must
be synthesized. All-silica MWW, called ITQ-1, can be prepared
using N,N,N-1-trimethyladamantammonium (TMAda), or a
mixture of TMAda and hexamethylenimine (HMI) as organic
structure-directing agents (OSDAs).22 As observed in the
original patent, the combination of the small amine (HMI) and
the quaternary ammonium cation (TMAda) not only allows
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Table 1. Chemical Analyses of the Different Titanosilicates
sample
Si/Ti
Si/B
Ti-B-MWW
43
68
9.6
Ti-MWW
>500
Ti-ITQ-1(1)
Ti-MWW-exp(1)
Ti-ITQ-1(2)
Ti-MWW-exp(2)
41
58
146
159
Figure 2. UV−vis DRS spectra of different MWW-related materials.
Figure 4. SEM images of Ti-MWW (top) and Ti-MWW-exp(1)
(bottom).
Figure 5. Catalytic results of Ti-MWW and Ti-MWW-exp(1) in the
oxidation of cyclohexene with H2O2.
Figure 3. Pore size distribution of Ti-MWW and expanded Ti-MWW-
related materials obtained from argon adsorption.
reducing the required synthesis time23 but also obtaining ITQ-1
with better crystallinity.22 Consequently, ITQ-1 was prepared
by using the cooperative directing effects of both organic
molecules (see synthesis details in the Experimental Section).
The powder X-ray diffraction (PXRD) pattern obtained for this
sample is the characteristic of the crystalline lamellar MWW
precursor (see “ITQ-1” diffraction pattern in Figure 1). The as-
prepared pure silica ITQ-1 material was calcined at 550 °C
during 6 h to remove the organic.
For the preparation of the titanosilicate MWW-related
structure, the calcined all-silica ITQ-1 sample was used directly
without further treatments. This is an important difference with
the previous methodology where several acid treatments were
required to selectively remove boron atoms from the used
borosilicate precursor. The siliceous precursor ITQ-1 was
introduced in an aqueous synthesis media with an organic
structure-directing agent, such as piperidine (PI), and a
titanium precursor, such as titanium(IV) butoxide, obtaining
a gel with the following molar composition: SiO2:0.033 Ti:1.0
piperidine:5 H2O. The gel was autoclaved at 175 °C during 7
days, and after that period, the resultant solid was recovered by
filtration. This solid, called “Ti-ITQ-1(1)”, mostly retains the
MWW lamellar precursor nature (see 001 and 002 reflections
in Figure 1). The incorporation of Ti in the final solid was
measured by chemical analyses, and the results give a Si/Ti
molar ratio of 41 (see Table 1). Ti-ITQ-1(1) was also studied
by UV−vis spectroscopy to identify the coordination of Ti
species. As observed in Figure 2, the Ti-ITQ-1(1) material
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a
Table 2. Oxidation of Cyclohexene with H2O2 in Acetonitrile over Ti-MWW and Expanded Ti-MWW-Related Materials
b
catalyst
Ti-MWW
Si/Ti
t (h)
Xcycloh (%)
TOF (mol conv/h·mol Ti)
XH O (%)
SEpox (%)
2
2
68
58
5
5
5
5
35
18
28
100
243
68
73
69
93
95
94
Ti-MWW-exp(1)
Ti-MWW-exp(2)
159
a
b
Reaction conditions: 0.05 g of catalyst, 10 mmol of cyclohexene, 10 mmol of H2O2, 10 mL of acetonitrile, 333 K. Turnover frequency number
calculated from initial reaction rates (mol of cyclohexene converted/h·mol Ti).
shows two clearly identified bands, one centered at 220 nm,
species being in tetrahedral coordination (see the UV−vis
spectrum in Figure 2).
and the other at 260 nm. The first band has been assigned to
titanium species in tetrahedral coordination in zeolite frame-
work positions, while the second band has been assigned to
titanium species in octahedral coordination in extraframework
positions.24 Unfortunately, octahedral Ti species could direct
the formation of the stable anatase phase after calcination
procedures, whose presence affects negatively the selectivity for
oxidation reactions.
It has been described that, by washing the as-prepared
titanosilicates with acid under reflux before calcination, the Ti
species in octahedral coordination can be removed, while the
expanded nature of the layered MWW can be retained after
calcination.17,18 Taking into account these two premises, an
acid treatment with nitric acid was performed before the
calcination of the Ti-ITQ-1(1) sample (see the Experimental
Section for details). The resulting sample, named “Ti-MWW-
exp(1)”, shows a unique band at 220 nm (see the UV−vis
spectrum in Figure 2). Therefore, all octahedral Ti species have
been selectively removed, achieving a Si/Ti molar ratio in the
final solid of 58 (see Table 1). However, the PXRD pattern of
the Ti-MWW-exp(1) material (see Figure 1) shows a very weak
001 reflection, indicating that most of the layered structure of
the MWW precursor was not retained. Similar behavior has
been previously observed by Tatsumi et al. when the Si/Ti ratio
in the gel was decreased.17 They described that, when the Si/Ti
ratio was lowered to 50, the appearance of the expanded
layered structure was reduced after the acid treatment and
calcination.
Consequently, we attempted the synthesis of the titanosili-
cate form of the MWW-related material with a higher Si/Ti
ratio (150), in order to study the effect of the Ti content on the
formation of the expanded layered structure following our
synthesis methodology. Pure silica ITQ-1 was mixed in aqueous
media with piperidine, and titanium(IV) butoxide, achieving
the following gel composition: SiO2:0.0067 Ti:1.0 piperidine:5
H2O. After 7 days at 175 °C, the obtained solid shows the
characteristic PXRD of the lamellar MWW precursor (see 001
and 002 reflections of Ti-ITQ-1(2) in Figure 1). As it can be
seen in Figure 1, the PXRD patterns of Ti-ITQ(1) and Ti-ITQ-
1(2) are very similar, but both samples contain very different Ti
contents (Si/Ti ratios of 41 and 146, respectively; see Table 1).
Very interestingly, after the acid treatment on the as-prepared
Ti-ITQ(2) sample to selectively remove octahedral Ti species,
and its posterior calcination, the resultant material, “Ti-MWW-
exp(2)”, mostly preserves the layered structure (see 001 and
002 reflections of Ti-MWW-exp(2) in Figure 1). Therefore,
similar to the result reported by Tatsumi, we also observe an
important effect of the Si/Ti ratio on the retention of the
layered MWW structure after the acid treatment and
calcination.
Ar adsorption characterization of Ti-MWW-exp(1) and Ti-
MWW-exp(2) can give information of zeolite pore topologies,
indicating if they are more related to the MWW structure or, on
the contrary, if there are differences in the pore distribution
when compared to regular MWW material. As seen in Figure 3,
the Ti-MWW-exp(2) material clearly presents two differ-
entiated peaks in the curve of the pore size distribution
obtained by Ar adsorption. The first peak centered at 5.3 Å is
characteristic of the medium pores present in the MWW
structure, whereas the peak centered at 7.3 Å can be associated
with the expanded pores. Interestingly, the presence of the
expanded MWW-related structure cannot be confirmed by
PXRD for Ti-MWW-exp(1) (see Figure 1), but the Ar
adsorption measurement indicates the appearance of expanded
pores in its structure (see Figure 3).
A regular Ti-MWW material was also synthesized, and its
physicochemical properties were compared with those of the
Ti-MWW-exp(1) and Ti-MWW-exp(2). The typical synthesis
procedure of the standard Ti-MWW zeolite combines the
hydrothermal synthesis of the titanoborosilicate MWW,
followed by different acid treatments to selectively remove
the boron atoms.13 Following the synthesis described in the
literature (see the Experimental Section), titanoborosilicate
MWW (Ti-B-MWW) was synthesized. The PXRD pattern of
this material indicates that the MWW layered precursor phase
is achieved, as indicated by the 001 and 002 reflections (see
Figure 1), and chemical analysis shows a molar ratio of 43 and
9.6, for Si/Ti and Si/B, respectively (see Table 1). The
coordination of Ti in Ti-B-MWW has been studied by UV−vis
spectroscopy, observing a very broad band in the spectrum (see
Figure 2). This broad band can be assigned to the presence of
tetrahedral and octahedral Ti species in the sample.
Ti-B-MWW has been acid-treated several times in order to
remove most of the boron atoms, and also the octahedral Ti
species. After three acid washes with nitric acid, the sample was
calcined at 550 °C. This calcined sample has been named “Ti-
MWW”. As seen in Table 1, a large part of the boron has been
removed (Si/B > 500), and the final Si/Ti molar ratio is 68.
Moreover, the UV−vis spectrum of this acid-treated and
calcined sample indicates that only Ti species in tetrahedral
coordination remain in the solid (see Figure 2). The PXRD
pattern of Ti-MWW is typical of the MWW structure, showing
a very similar PXRD pattern to that of the Ti-MWW-exp(1)
material. However, the Ar adsorption measurement on Ti-
MWW reveals a single peak in the pore size distribution
centered at 5.3 Å, characteristic of the medium-pore MWW
material. This result shows the different textural properties of
Ti-MWW-exp(1) synthesized following the methodology
described in the present paper when compared to the regular
Ti-MWW. These changes in pore size distribution should have
a positive effect on molecular diffusion and site accessibility
Ti-MWW-exp(2) was further characterized by chemical
analysis and UV−vis spectroscopy. This sample shows a final
Si/Ti ratio of 159 in the solid (see Table 1), all of these Ti
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when reacting larger molecules, hopefully offering improved
catalytic properties in selective oxidation reactions.
ACKNOWLEDGMENTS
■
Financial support by the Spanish MINECO (MAT2012-37160,
and Consolider Ingenio 2010-Multicat), Generalitat Valenciana
by the PROMETEO program, and UPV through PAID-06-11
(no. 1952) is acknowledged. M.M. also acknowledges
“Subprograma Ramon y Cajal” for the contract RYC-2011-
08972.
At this point, we have three different titanosilicate forms of
MWW-related materials. On one hand, there is regular Ti-
MWW and expanded Ti-MWW-exp(1); both show a similar Ti
content (see Table 1), all Ti species being in tetrahedral
coordination (see Figure 2), and comparable crystal sizes (see
images of scanning electron microscopy, SEM, in Figure 4).
The main difference between these materials is the pore size
distribution obtained by Ar adsorption (see Figure 3). On the
other hand, Ti-MWW-exp(2) shows a lower Ti content than
the above described materials, but a better expanded layered
structure, as observed by PXRD and Ar adsorption.
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nica de
̃
A new synthetic route starting from an all-silica precursor, ITQ-
1, is used for the preparation of an expanded titanosilicate form
of MWW-related material. This methodology improves the
synthetic route reported by Tatsumi et al., where borosilicate
MWW precursors, and, consequently, several postsynthetic acid
treatments for removing boron atoms from the framework,
were required in the synthesis of the expanded Ti-YNU-1. The
expanded titanosilicate MWW-related materials prepared
following the methodology presented here improve the activity
of bulky cycloalkenes, such as cyclohexene, in the selective
epoxidation reaction with H2O2 as oxidant when compared to
regular Ti-MWW. This methodology could open an interesting
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metallosilicates.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: acorma@itq.upv.es.
■
(24) Bordiga, S.; Coluccia, S.; Lamberti, C.; Marchese, L.; Zecchina,
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Author Contributions
†The paper was written through contributions of all authors. All
authors have given approval to the final version of the
manuscript.
Notes
The authors declare no competing financial interest.
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