Designing the synthesis of catalytically active Ti-β by using various new templates in the presence of fluoride anion

Article abstract of DOI:10.1039/c1cp21013k

Crystallization of large-pore Ti-β by using a variety of diquaternary ammonium derivatives of dibromoalkane and amines such as triethylamine, 1,4-diazabicyclo[2,2,2]octane (DABCO), and quinuclidine as structure-directing agents (SDA) is described. The size of hydrophobic bridging alkyl-chain length of the template [R₃N⁺-(CH₂)_x-N ⁺R₃](OH^-)₂ directs the final crystalline product: Ti-β, Ti-ZSM-12, Ti-nonasil or Ti-ZSM-5, as x gradually changes from 6 to 1, in the fluoride medium under hydrothermal conditions. A dense phase such as Ti-nonasil (clathrasil type) is crystallized as the size of hydrophobic bridging alkyl-chain length decreases. The use of F^- anions as a mineralizer and Ti⁴⁺ as a heteroatom in the synthesis gel also influences the selectivity of final crystalline product. The phase purity and incorporation of Ti⁴⁺ into the lattice of β (BEA) and ZSM-12 frameworks are confirmed using XRD, UV-visible, FT-IR, ²⁹Si NMR spectroscopes, elemental analysis (ICP), surface area measurements and catalytic test reactions. The morphology of Ti-β samples is dependent on the nature of the structure-directing agent as revealed by the scanning electron microscopic (SEM) observations. The catalytic activity in the epoxidation of 4-vinyl-1-cyclohexene is increased with the amount of tetrahedral Ti⁴⁺ atoms in the framework. The new templates can be effectively used for preparation of catalytically active Ti-β with the minimum number of framework defect sites.

Full text of DOI:10.1039/c1cp21013k

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PAPER
Designing the synthesis of catalytically active Ti-b by using various
new templates in the presence of fluoride anion
Manickam Sasidharana and Asim Bhaumik*b
Received 1st April 2011, Accepted 23rd June 2011
DOI: 10.1039/c1cp21013k
Crystallization of large-pore Ti-b by using a variety of diquaternary ammonium derivatives of
dibromoalkane and amines such as triethylamine, 1,4-diazabicyclo[2,2,2]octane (DABCO), and
quinuclidine as structure-directing agents (SDA) is described. The size of hydrophobic bridging
alkyl-chain length of the template [R3N+–(CH2)x–N+R3](OHÀ)2 directs the final crystalline
product: Ti-b, Ti-ZSM-12, Ti-nonasil or Ti-ZSM-5, as x gradually changes from 6 to 1, in the
fluoride medium under hydrothermal conditions. A dense phase such as Ti-nonasil (clathrasil
type) is crystallized as the size of hydrophobic bridging alkyl-chain length decreases. The use of
FÀ anions as a mineralizer and Ti4+ as a heteroatom in the synthesis gel also influences the
selectivity of final crystalline product. The phase purity and incorporation of Ti4+ into the lattice
of b (BEA) and ZSM-12 frameworks are confirmed using XRD, UV-visible, FT-IR, 29Si NMR
spectroscopes, elemental analysis (ICP), surface area measurements and catalytic test reactions.
The morphology of Ti-b samples is dependent on the nature of the structure-directing agent as
revealed by the scanning electron microscopic (SEM) observations. The catalytic activity in the
epoxidation of 4-vinyl-1-cyclohexene is increased with the amount of tetrahedral Ti4+ atoms in
the framework. The new templates can be effectively used for preparation of catalytically active
Ti-b with the minimum number of framework defect sites.
However, the bulky organic compound often acts as inert
Introduction
space filler and thus a challenge for the synthetic zeolite
Since the first use of tetramethylammonium cation (TMA) in
chemists will be to maximize the impact of the shape of the
structure-directing agents on the final crystalline product.7
zeolite synthesis, a variety of new zeolite structures have been
synthesized by using different organic amines and quaternary
Recently, an important alternative synthesis route has been
ammonium cations as templates under hydrothermal condi-
developed at near neutral pH, which employs fluoride ions as
the mineralizing agent rather than hydroxide ions.8 Though
tions. The role of organic templates or structure-directing
agents (SDA) in zeolite synthesis has been well demonstrated
by the extensive work of Zones et al.1,2 on the synthesis
the scope of this method was initially established by Guth,
Kesseler, and co-workers9–11 for the synthesis of larger crystals
of novel zeolites such as SSZ-23, SSZ-24, SSZ-31, SSZ-33,
for structural studies, considerable advances on the synthesis
of novel high-silica zeolites have been made by Corma et al.12
SSZ-35, and SSZ-39 by using bicyclic amine templates or the
templates derived from Diels–Alder reaction in combination
This strategy of synthesis using a fluoride-containing medium
with the use of trivalent boron atom in the synthesis gel.
has produced a number of new high-silica zeolites such as
ITQ-3, ITQ-4, ITQ-7, ITQ-9, ITQ-14 etc.12–15 Furthermore,
Furthermore, tremendous advances have been made towards
the synthesis of novel zeolite structures through the rational
the introduction of Ge as a heteroatom in the synthesis gel in
design of SDAs by controlling the parameters such as rigidity,
the fluoride containing medium together with a variety of
polarity, and spatial structure3,4 together with energetic
SDAs not only led to the formation of pure polymorph C of
of heteroatom substitution.5 For the hunt of high-silica
zeolite b (ITQ-17)16 that is unique from the faulted inter-
mediates but also gave a new family of materials (ITQ-16),
materials with multidimensional channel system, bulky poly-
cyclic organic amines were used to give maximum stabilization
where the three polymorphs A, B and C exist in different
of the hydrophobic inner silica surface6 of these materials.
proportions.17 The presence of a heteroatom also influences
the crystallization as it drastically reduces the crystallization
a Department of Chemistry, Faculty of Science and Engineering,
Saga University, 1 Honjo-machi, Saga 840-8502, Japan
b Department of Materials Science, Indian Association for the
Cultivation of Science, Jadavpur, Kolkata 700 032, India.
E-mail: msab@iacs.res.in
time in the case of ITQ-7.18 Zones et al.2 also reported the
formation of SSZ-36 in the presence of boron as a heteroatom
in the synthesis gel, whereas MTW, MFI and MEL structures
are formed in the presence of aluminum as a heteroatom by
c
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using C3,C5,N,N-tetramethyl piperidine as structure directing
agent in OHÀ medium.
Br–(CH2)x–Br, where value of x varies from 3 to 6 and 1, using
acetone as solvent. The reaction was carried out at room
temperature under vigorous stirring for 36 h. After quatern-
ization, the white solid was filtered, washed with acetone and
ethyl acetate; the yield of the solid decreases from 90 to 60%
with the decrease in the number of carbons in dibromoalkane.
In recent times rational synthetic strategies of new large-
pore zeolite involve suitable templates and appropriate hetero-
atoms such as Ge, B, and Al in the synthesis gel under
fluoride medium. A novel titanosilicate with MWW topology,
Ti-MWW,19 has also been prepared in the presence of B as
a heteroatom. Furthermore, titanosilicates with a three-
dimensional system, like TS-1,20 Ti-b,21 Ti-ITQ-722 etc., have
received much attention not only due to their potential
industrial applications as oxidation catalysts, which could
replace stoichiometric homogeneous catalysts but also due to
the possibility of their large-pore size that can permit the
diffusion of bulky organic molecules. In our continuous efforts
for the synthesis of large-pore titanosilicates using new tem-
plates in the presence of HF, we have systematically synthe-
sized a variety of diquaternary ammonium hydroxides as
well as monoquaternary ammonium hydroxide as structure-
directing agents. Herein we report the use of a variety of
new structure-directing agents for the synthesis of Ti-b and
Ti-ZSM-12 in the absence of any seed crystals in the fluoride
medium under hydrothermal conditions. Although there are
some reports on the influence of heteroatoms and alkyl chain
length in the synthesis of zeolites, systematic evolution of
different titanosilicate phases by varying the length of the
hydrophobic alkyl-chain of the diquaternary, diazabicyclo
and quinuclidine compounds has not been studied before.
These new templates can be effectively used to prepare
defect free, catalytically active Ti-b and provides an alternate
templating pathway to conventionally used expensive tetra-
ethylammonium hydroxide (TEAOH). Furthermore, the titano-
silicates obtained using this method exhibit high selectivity for
epoxides in the epoxidation of 4-vinyl-1-cyclohexene by using
dilute aqueous H2O2 as oxidant.
Quinuclidine based templates
The synthesis of tetramethylene bisquinuclidinium diquater-
nary cation was prepared through the reaction of 1,4-dibromo-
alkane (1 mol, TCI) with quinuclidine (2.3 mol, Aldrich) using
ethanol as solvent at room temperature. The mixture was
vigorously stirred for 3 days and the desired product
was obtained after evaporating the solvent under vacuum.
In the case of quinuclidine, we have restricted our study to
1,4-dibromobutane, as the geometry of quinuclidine and
1,4-diazabicyclooctane is almost similar.
General procedure for conversion of diquaternary ammonium
bromides to the corresponding hydroxides
The exchange process was carried out in two different ways
in order to maximize the anion exchange of the dibromide
salts. The diquaternary salts based on trimethylamine head
groups were treated with anion exchange resin (DIAIONs,
Mitsubishi Chemicals) and the degree of exchange was found to
be 95% as determined by acid–base titration. For quinuclidine,
DABCO, tripropylamine, and dimethylbenzyl amine based
diquaternary salts, silver oxide method was employed and
anion exchange in all the cases was found to be higher than
95%. It is worth mentioning that the filtrate obtained
by this method was centrifuged thoroughly to remove any
suspended silver oxide or bromide. Finally, the concentration
of the organic bases was adjusted to about 35–40% using
distilled water.
Synthesis of titanosilicates under fluoride medium
Experimental section
Tetraethylorthosilicate (TEOS, TCI) was hydrolyzed by using
an aqueous solution of diquaternary ammonium hydroxide
and then the clear homogeneous mixture was stirred at room
temperature for an hour. Then the titanium source was added
as titanium peroxo complex to the above solution, which
was prepared by the addition of H2O2 to Ti(OBut)4 solution.
Then the above homogeneous mixture was heated at 80 1C on
a hot plate for an extended period of time to allow complete
evaporation of the ethanol produced as well as the water
needed to reach the desired final composition. Finally the
required amount of HF (aqueous solution, 46%) was added
and the mixture was homogenized. The mixture was poured
into a Teflon-lined stainless steel autoclave and was then
heated at 170 1C for 11–13 days under constant tumbling
(45 rpm). The gel composition for all the diquaternary
ammonium hydroxide was 0.025 SiO2 : 0.0005 TiO2 : 0.0050
R(OH)2 : 0.0078 HF : 0.006 H2O2 : 0.18 H2O; whereas for
monoquaternary ammonium hydroxide a gel composition of
0.025 SiO2 : 0.0005 TiO2 : 0.0050 R(OH) : 0.0038 HF : 0.006
H2O2 : 0.18 H2O was employed. After completion of crystalli-
zation, the contents were centrifuged and the recovered
solid was washed with water followed by drying in an oven
Triethylamine based template synthesis
A series of diquaternary ammonium bromides were synthe-
sized from triethylamine (TCI, Japan) and dibromoalkane
(TCI, Japan) of the formulae Br–(CH2)x–Br, where value
of x ranges from 3 to 6 and 1. In a typical quaternization
reaction, triethylamine (2.2 mol) was refluxed with 1,6-dibromo-
hexane (1 mol) using dry acetone as solvent for 24 h. After
completion of the reaction, a white solid was formed with >
90% yield. This salt was separated from the reaction mixture,
washed thoroughly with acetone and dried under vacuum.
Similarly, other diquaternary salts with different dibromo-
alkanes were prepared. However, the product yield decreases
as the carbon number of dibromoalkane decreases. Similarly,
the salts of tripropylamine (TCI), and dimethylbenzylamine
(TCI) with 1,6-dibromohexane (TCI) were also prepared by
quaternization reaction.
1,4-diazabicyclo[2,2,2]octane (DABCO) based templates
The DABCO based structure-directing agents were prepared
by the reaction of free base diazobicyclo[2,2,2]octane (4.5 mol,
Aldrich) with different dibromoalkanes (1 mol) of formulae
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at 100 1C. The synthesis of Ti-b in the presence of OHÀ at high
pH was also conducted by using 5% of dealuminated b seed
crystals. The dealuminated seed crystals were prepared by
treatment of Al-b (Si/Al = 30) with HNO3 at 80 1C for 24 h
following the existing literature.23
a particular crystalline phase, it was optimized for all the
templates (Table 1). Under fluoride medium, the template
[R3N+–(CH2)x–N+R3](OHÀ)2, where
is ethyl group,
R
produced three-dimensional large-pore Ti-b when x = 4–6
(SDA I–III); whereas Ti–ZSM–12 (one-dimensional large-
pore) and Ti-nonasil were formed when x = 3 (SDA IV)
and x = 1 (SDA V), respectively, under similar synthesis
conditions. However, it is interesting to note that the use
of SDA I–III at high OHÀ content in the presence of Na+
crystallizes NCL–123,24 (a unidimensional large-pore zeolite),
SSZ–31,25 ZSM–5726 (medium pore zeolites), and SSZ–1627
(small-pore zeolite), respectively. Molecular size and hydro-
phobicity of DABCO based diquaternary templates, pH
and the sources of inorganic reagents so often direct the
synthesis of a particular high silica zeolite under hydroxide
medium.28 On contrary under the fluoride medium, the same
templates crystallize three-dimensional large-pore Ti-b (SDA
I–III). SDA IV with one methylene group less in the bridging
alkyl chain leads to unidimensional structure Ti-ZSM-12.
Nonasil-type materials is readily formed over smaller tem-
plates such as SDA V, which was thoroughly explored by
Koller et al. and others.29 The formation of nonasil was
also reconfirmed by employing monoquaternary ammonium
hydroxide with similar head groups such as triethylmethyl
ammonium hydroxide SDA XV, under identical hydrothermal
conditions.
Characterization
Phase purity of titanosilicates was determined from powder
X-ray diffraction (XRD) patterns recorded on a Rint 2000
X-ray diffractometer using Cu Ka radiation. UV-visible
diffuse reflectance spectra were recorded on a JASCO V-550
spectrometer using BaSO4 as a reference. Scanning electron
microscopic (SEM) images were recorded on a JEOL JSM-
T220 instrument after suspending the sample in ethanol and
allowing it to dry under vacuum. The amount of Ti present in
the calcined samples was estimated by inductively coupled
plasma emission spectrometry (ICP) on a Rigaku JY38S
spectrometer. Infrared spectra in the region of framework
vibrations (1100–600 cmÀ1) were collected on a Shimadzu
FTIR-8100 spectrometer at a spectral resolution of 2 cmÀ1
using KBr pellet technique. The sample (100 mg, 3 wt%
diluted in KBr) pressed into a wafer was set in a quartz cell,
which was sealed with KBr windows and connected to a
vacuum system. The sample was evacuated at temperatures
of 100–500 1C each for 1 h and was cooled to room tempera-
ture. The spectra were then recorded in the absorbance mode.
Spectra in the hydroxyl stretching region (4000–3000 cmÀ1
)
In depth investigation of titanosilicate phases formed over
SDA I–V under FÀ and OHÀ media also revealed the role of
mineralizer (FÀ or OHÀ) in the synthesis gel apart from
hydrophobic chain of the template molecules. For instance,
the facile formation of all-silica b polymorph over SDA I–III
under identical synthesis conditions suggests that the presence
of titanium in the initial synthesis gel is not a crucial factor to
crystallize b-structure. Interestingly, SDA I at high OHÀ
content (pH = 11.5) in the absence of FÀ as well as Na+
results in the amorphous solid, suggesting that FÀ anions not
only act as a mineralizer by facilitating the dissolution of the
silica source but also catalyze the condensation reaction
involved in Si–O–Si bond formation as proposed by Barrett
et al.30 Recent literature reports31–33 reveal that FÀ ions are
not found exclusively as ion pairs, but covalentÀly bonded to
the silicon atom to give five-coordinate SiO4/2F silicon site.
The hydrophobic hydrocarbon backbone (bridging methylene
groups) of these diquaternary salts is believed to allow the
separation of the charged inorganic wall for the formation of
cage structure. This process facilitates self-assembly of silica
species during the crystallization, where the strong interaction
between the inorganic five-coordinate silicate species and the
amino groups at the end of the long hydrocarbon backbone
directs the formation of large pores. As the length of alkyl-
chain of SDA V decreased considerably, the small-pore
nonasil material is crystallized under similar conditions.
However, over diazobicyclo[2,2,2]octane (DABCO) SDA
VI–VIII templates, the effect of titanium in the synthesis gel
is clearly observed. In the absence of titanium, SDA VI and
SDA VII produced all-silica b polymorph similar to SDA
I–III, whereas SDA VIII directs the crystallization of ZSM-12.
The SEM photograph of Ti-b synthesized by using SDA VIII
shows that although there is competitive crystallization of
were recorded by using self-supported wafers with a diameter
of 2 cm (40 mg cmÀ2 thickness) out-gassed for 8 h at 400 1C
and 10À3 Pa. Surface area was measured by N2 adsorption
at À196 1C on a BELSORB 28SA analyzer after evacuation
at 300 1C for 3 h. 29Si MAS NMR spectra were recorded at a
29Si frequency of 79.266 MHz and spinning rate of 30 kHz
on a JEOL 400B spectrometer. The 29Si chemical shifts are
reported with respect to TMS.
Catalytic activity. The epoxidation of 4-vinyl-1-cyclohexene
over different titanosilicate materials was carried out in a
25 mL round-bottom flask immersed in a water bath and
equipped with a condenser and thermometer. In a typical
reaction, 5 mmol of 4-vinyl-1-cyclohexene, 5 mmol of H2O2
(31 wt% aqueous solution), and 5 mL of methanol or aceto-
nitrile were homogenized in the flask under magnetic stirring
and the mixture was heated at 65 1C. Then, 20 wt% of the
catalyst with respect to substrate was added under vigorous
stirring and the condenser was effectively cooled by the
circulation of ice-cold water. The reaction products were
analyzed on a Shimadzu 14B gas chromatograph equipped
with an OV-1 capillary column fitted with a FID detector.
Products are identified by using GC-MS and authentic
compounds. Unreacted hydrogen peroxide was estimated
through potentiometric titration using Ce4+ solution.
Results and discussion
Synthesis
The influence of various SDAs used in the synthesis and
resultant titanosilicates formed are given in Table 1. Since
crystallization time plays a crucial role in the formation of
c
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Table 1 Various structure directing agents used in the hydrothermal synthesis
Entry
SDA-(OH)2
Crystalline phase
Time, Days
12–13
Temp.1C
Si/Ti ratio (gel)
SDA I
Ti-b
170
51
51
51
51
51
SDA II
SDA III
SDA IV
SDA V
Ti-b
12–13
12–13
12–13
7–8
170
170
170
170
Ti-b
Ti-ZSM-12
Ti-Nonasil
SDA VI
SDA VII
SDA VIII
SDA IX
SDA X
Ti-b
12–13
12–13
12–13
9–10
170
170
170
170
170
170
170
51
51
51
51
51
51
51
Ti-b
Ti-b
Ti-ZSM-5
Cristobalite
Ti-b
7–8
SDA XI
SDA XII
12–13
12–13
Ti-ZSM-12
SDA XIII
Ti-ZSM-5
6–7
170
51
SDA XIV
SDA XV
Ti-ZSM-5
Ti-Nonasil
11–12
8
170
170
51
51
SDA XVI
Amorphous
12–13
170
51
b and ZSM-12, the ZSM-12 (needles) is formed in negligible
amount. Thus, the titanium in the synthesis gel could serve to
decrease the energy of Si–O–M bond when the bond angles are
adjusted appropriately for the formation of b structure as in
the case of Ge in ITQ-7.34 When the alkyl-chain length is
decreased to three methylenes (SDA IX) and then to mono
methylene groups (SDA X), Ti-ZSM-5 (TS-1) and dense
phase cristobalite are formed, respectively. It is worthwhile to
c
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show BET surface areas in the range of 578–647 m2 gÀ1
mention that DABCO based templates at high OHÀ content
in the presence of Na+ (alkaline medium) favor the formation
of one-dimensional ZSM-12.35 The quinuclidinium based
SDA XI with four methylene-bridging units also gives Ti-b.
This is not surprising because SDA XI is obtained by sub-
stituting one of the nitrogens of DABCO SDA VIII by a
tertiary carbon. Thus, we have not studied the effect of
bridging alkyl chain for this particular head group since the
basic geometry, shape and size of the corresponding templates
are nearly identical.
characteristic of large-pore three-dimensional Al–b synthe-
sized under the hydroxide medium.21 The nature of the
isotherms and the pore widths estimated from the isotherms
for the Ti-b samples match very well with conventional zeolite
b synthesized using TEAOH as template. Therefore, the SDAs
investigated in the present study can also be effectively used
as alternate inexpensive template for preparation of Ti-b.
Isomorphous substitution of Ti for Si in these calcined
titanosilicates was ascertained by a combination of physico-
chemical techniques and catalytic test reactions. The
XRD patterns of different titanosilicates after calcination
(Fig. 1A and B) suggest that they are highly crystalline and
completely free from impurity phases. Furthermore, from the
nature of the low angle peaks in the XRD patterns it can be
suggested that these samples are richer in polymorph A than
polymorph B.21 The incorporation of Ti in the b framework is
further evidenced from the UV-vis spectra. All the samples
exhibit a sharp band around 205–225 nm (Fig. 2), originating
from the charge transfer of the pp–dp transition between
titanium and oxygen of the framework titanium species,38,39
Ti–O–Si, in the Ti-b material. However, the tail of the
absorption band centered at 220 nm extends to 280–300 nm
due to the presence of octahedral titanium species40 for the
samples synthesized by using C5DABCO and C4quinuclidine
(Fig. 2) as template. The calcined Ti-b samples were treated
with dilute HNO3 in order to remove the octahedral titanium
species and UV-vis spectra of acid treated Ti-b samples are
shown in the Fig. 3. The octahedral Ti4+ species were com-
pletely removed on treatment of calcined Ti-b samples with
2 M HNO3 acid (solid to liquid ratio of 1 g to 20 mL) at room
temperature for 3 h. This process was repeated whenever
necessary for complete removal of extra framework Ti4+
species.
SDA XII leads to unidimensional ZSM-12 irrespective of
the mineralizer, heteroatom, and/or temperature of the crys-
tallization. Thus this template seems to be highly specific for
ZSM-12 framework as previously reported.36 Furthermore,
the more common head groups such as trimethyl (SDA XIV)
and tripropyl (SDA XIII) moieties favor the more stable
ZSM-5 phase under the FÀ medium as in the case of OHÀ
medium at high pH. In addition, fast-crystallization (about
7 days) of synthesis gel always led to dense materials such as
Ti-NON and Ti-cristobalite; whereas slow crystallization
(about 12–13 days) invariably leads to high-silica large-pore
titanosilicates. The use of novel 1,8-diazabicyclo[5,4,0]-7-
undecene based diquaternary ammonium hydroxide does not
result in any crystalline product. This is mainly due to the
reason that this template hydroxide undergoes decomposition
to its constituents even under mild heat treatment during the
crystallization stage. Thus, the use of various structure-
directing agents under the fluoride medium clearly revealed
the influence of terminal-amino group of diquaternary
ammonium hydroxide, hydrophobic alkyl chain length, and
heteroatom titanium over formation of titanosilicates with
different topology.
An attempt to synthesize Ti-b at high pH under alkaline
conditions (OHÀ medium) using the above hydroxide-
templates and dealuminated b-seed crystals (in the absence
of FÀ ion) was not successful as these hydroxides decompose
during the hydrothermal conditions. The decomposition of
template hydroxide was observed by monitoring the pH of the
gel at different stages, which is decreased considerably within
a week of commencement of the crystallization. The crystal-
lization was also attempted by lowering the synthesis tempera-
ture from 170 1C to 140 1C. However, the synthesis gel
remained amorphous under that condition.
All the Ti-b samples are nearly free from defect sites as
observed from the FTIR spectra in the –OH stretching regions
(Fig. 4). It has already been demonstrated that synthesis under
hydroxide medium produces a substantial number of silanol
groups or framework defect sites.31 The defect sites (Si–OH)
are formed due to base catalyzed cleavage of RSi–O–SiR
bond in the presence of quaternary ammonium cations at the
pore surface. On the other hand, this is insignificant in the
fluoride medium, which is almost neutral to acidic pH. This
leads to a fewer number of defect sites as the density of –OH
groups at the surface is negligible. However, there is a possi-
bility of formation of terminal Si–OH groups while preparing
the self-supported wafers, where the crystals are ground
thoroughly to make fine powder before the measurement.
The FTIR spectra show broad bands centered at 3743 and
3689 cmÀ1 characteristic of terminal silanol groups and the
silanol groups involved in weak hydrogen bonding with the
neighboring silanol groups located at the external surface,
respectively. According to the literature,20,41 the broadening of
band is mainly attributed to the presence of weak hydrogen
bonding between the surface hydroxyl groups. The method of
calcinations also influences the amount of terminal silanol
groups; the spectrum of Ti-b (synthesized using C4-quinuclidine
as template) calcined before and after treatment with pyridine/
HCl mixture shows that the concentration of terminal
Characterization of Ti-b. Prior to the characterization, the
as-synthesized titanosilicates were carefully calcined after
pretreating the as-made samples with pyridine/HCl mixture
(1 : 1 volume ratio, 100 mL per gram of sample) at 80 1C for
18 h under vigorous stirring as reported by Davis et al.37 The
pyridine/HCl treatment was carried out to reduce the number
of terminal Si–OH groups formed during the calcinations and
eliminate the occluded fluoride ions as well. The resulting solid
was centrifuged, washed thoroughly with distilled water, and
dried at 100 1C. The dried sample was calcined at 500 1C for
12 h in a flow of O2 in a tubular reactor. The composition and
BET surface area of various titanosilicates synthesized with
different SDAs are listed in Table 2. The amount of titanium
incorporated in the framework was found to be invariably
lower than that of the initial synthesis gel. All Ti-b samples
c
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Table 2 Composition and surface area of titanosilicates synthesized with different SDAs
Si/Ti ratio
Gel
Calcined
Acid treateda
65.5
Entry
1
SDA
Crystalline phase
BET surface area (m2 gÀ1
)
Ti-b
50
61.6
662
626
648
324
93
2
3
4
5
Ti-b
50
50
50
50
77.8
72.1
81.2
76.9
Ti-b
Ti-ZSM-12
Ti-Nonasil
135.1
263.0
166.4
345.6
6
Ti-b
50
50
50
50
50
50
60.0
79.2
74.4
61.1
126.4
225
69.3
91.5
608
579
599
595
305
—
7
Ti-b
8
Ti-b
83.3
9
Ti-b
64.9
10
Ti-ZSM-12
Ti-Nonasil
147.5
293.1
11
a
The calcined titanosilicates are treated with 2 M HNO3 at room temperature for 3 h in order to remove the small amount of octahedral titanium.
Si–OH groups are reduced compared to the untreated sample.
Furthermore, Ti-b samples synthesized using triethylamine
based templates are almost defect-free compared to DABCO
or quinuclidine based templates, and these bulky head groups
may partially be responsible for creation of a fraction of –OH
groups during calcinations in addition to the formation of
Si–OH groups through the collapse of the crystallites on
grinding while making the self-supported wafers.
subjected to heat treatment at different temperatures from
100 to 500 1C under vacuum (10À3 Pa) in the IR cell, and
spectra were recorded after cooling to room temperature
(Fig. 6). The shape of the absorption band changes gradually
as the temperature increases and the intensity of the band at
970 cmÀ1 decreases considerably, indicating the removal of
some of the silanol groups at 500 1C. In-depth study shows the
presence of pentacoordinated silicon units, SiO4/2FÀ, by solid-
state NMR of as-made samples synthesized in the fluoride
medium.31–33 As these pentacoordinated species are more
nucleophilic, they in turn attack the silicon species in the gel.
Thus, the formation of Si–F species at the surface is highly
favorable. Thus, the band at 945 cmÀ1 may be due to the
presence of Si–F bond.
The FTIR spectra in the framework vibration region
(400–1100 cmÀ1) of all Ti-b samples invariably show a
relatively narrow absorption band at 958 cmÀ1 and this is
characteristic of titanosilicates (Fig. 5).42 Since these samples
exhibit negligible amounts of defect sites as revealed from the
OH stretching region, the presence of 958 cmÀ1 arises mainly
due to the presence of Si–O–Ti bonds rather than silanol
groups.42–44 Furthermore, the FTIR spectra show two
additional shoulders at 945 and 970 cmÀ1 in addition to the
958 cmÀ1. In order to remove any of the occluded FÀ, the
FTIR spectra of Ti-b (synthesized using C6-TEAOH) was
The phase purity and morphology of Ti-b and Ti-ZSM-12
were observed from scanning electron microscopy (Fig. 7).
The length of hydrophobic alkyl chain influences the particle
morphology to a certain extent. The SEM photographs of Ti-b
synthesized by using SDA VIII show that there is competitive
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Fig. 1 (A) XRD patterns of different titanosilicates after calcinations; synthesized using triethylamine based diquaternary ammonium hydroxide.
(B) XRD patterns of different Ti-b after calcinations; synthesized using DABCO and quinuclidine based diquaternary ammonium hydroxide.
For comparison, the XRD pattern of the all silica polymorph of b is also shown at the bottom.
Fig. 3 Diffuse reflectance ultraviolet spectra of Ti-b samples after
2 M HNO3 acid treatment at room temperature. All the spectra refer
to Ti-b except ‘C3TEAOH’ (Ti-ZSM-12).
crystallization of b and ZSM-12, but the ZSM-12 (needles) is
formed in negligibly (visible only in SEM picture) small amounts.
The template C6TEAOH produces nearly uniform hexagonal
Fig. 2 Diffuse reflectance ultraviolet spectra of Ti-b samples before
prismatic crystals (B5 mm), whereas C5TEAOH and C4TEAOH
show uniform truncated square pyramidal crystals of large
2 M HNO3 treatment. All the spectra refer to Ti-b except the one
mentioned as Ti-ZSM-12.
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Fig. 4 FTIR spectra of calcined dehydrated (at 500 1C) Ti-b samples
in the OH stretching region: (bottom to top) Si-BEA, C4TEAOH,
C5TEAOH, C6TEAOH, C4DABCO, C5DABCO, C6DABCO,
C4quinuclidine with pyridine/HCl pretreatment and C4quinuclidine
calcined without pyridine/HCl pretreatment.
Fig. 6 Effect of dehydration of Ti-b (C6TEAOH) at different
temperature in the region of ca. 960 cmÀ1; (bottom to top) 25, 100,
200, 300, 400 and 500 1C, respectively.
Fig. 8 shows 29Si MAS NMR of Ti-b samples synthesized
using different templates and the all-silicon polymorph
of zeolite b (synthesized by using C6TEAOH). The Si–b
synthesized in the fluoride medium shows a remarkably high
resolution of Si(OSi)4 crystallographic sites in the 29Si MAS
NMR spectrum. The sharp resonance signal is attributed to
the absence of connectivity defects and a lack of dipolar
coupling to protons that allow sharp lines as reported in the
literature.20,45 When Ti is incorporated in the b structure, a
broadening of the peaks is apparent in the 29Si MAS NMR
spectrum and causes a major loss of resolution and only
three broad resonances are observed at ca. À111, À112, and
À115 ppm. A similar line broadening was also observed in the
case of TS-1 for the substitution of Ti in the MFI framework.46
In some cases, the loss of resolution is caused by the presence of
high concentration of connectivity defects. But in the present
case under the fluoride medium, the absence of any resonance
signal at ca. À102 ppm, corresponding to the Q3 species in the
connectivity defects, implies that resolution loss is exclusively
due to the incorporation of titanium in the framework position.
The loss of resolution is also directly proportional to the amount
of titanium in the tetrahedral framework position. Thus, the
all-silica polymorph of b shows a very sharp resonance.
Materials synthesized with C5-DABCO with less titanium in
the framework also shows a comparatively sharper resonance
over the Ti-b samples synthesized with high titanium loading
as in the case of samples synthesized by using C6-TEAOH,
C6-DABCO, and C4-quinuclidine.
Fig. 5 Infrared spectra in the ca. 960 cmÀ1 region for Ti-b (calcina-
tions followed by acid treated) samples synthesized using different
templates; (a) C6TEAOH, (b) C5TEAOH, (c) C4TEAOH, (d)
C4DABCO, (e) C5DABCO, (f) C6DABCO, (g) C4quinuclidine, and
(h) C3TEAOH-Ti-ZSM-12.
(B10–12 mm), and medium (B3–4 mm) sizes, respectively.
In the case of DABCO based templates, C6DABCO produces
sphere shaped crystals (B1–2 mm), C5-DABCO produces
truncated square pyramidal (B3–4 mm), and C4DABCO
shows truncated square pyramidal with the coexistence of a
very small amount of needles corresponding to the ZSM-12
impurity phase. The template quinuclidine gives agglomerated
spheres with particle size of about 0.5–1 mm. On the other
hand, Ti-ZSM-12 obtained by using C3TEA(OH)2 as template
shows a typical needle shaped crystals.
Catalytic activity
The incorporation of titanium is further evidenced from the
competitive epoxidation of different double bonds present in
4-vinyl-1-cyclohexene (Scheme 1) over different titanosilicates.
The catalytic test reactions were carried out using dilute
aqueous H2O2 as oxidant and methanol as solvent under
liquid-phase conditions. The catalytic activity of different
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Fig. 7 SEM images of C4DABCO (a), C5DABCO (b), C6DABCO (c), C6TEAOH (d), C5TEAOH (e), C4TEAOH (f), and quiniclidine (g).
titanosilicates and respective product selectivities are shown in
Table 3. All the Ti-b samples show good activity and H2O2
selectivity as these samples are free from anatase as well as
defective sites. Among the two different double bonds, the
side-chain double bond is more prone to oxidation and gives
the corresponding epoxidized products over the cyclohexene
ring –CQC– bond. Furthermore, under the reaction condition
of using methanol as solvent, the corresponding glycol mono-
methyl ether (formed from the solvolysis of epoxide) is formed
as the major product in addition to a small amount of epoxide
and diol (arising from hydrolysis of epoxide). The ratio of ring
–CQC– to side-chain –CQC– bond oxidation is in the range
of 0.26–0.40 for Ti-b samples. The observed ring versus
side-chain –CQC– bond oxidation reveals that in spite of
being highly hydrophobic and the large space around the
active Ti-site, ring oxidation proceeds sluggishly. Thus, the
difficulty in the ring –CQC– bond epoxidation arises not only
due to the bulky nature of the substrate but also the intrinsic
nature of the Ti site, such as bond length and bond angles of
the coordination sphere. In addition, the catalytic activity of
various Ti-b samples is directly related to the amount of
tetrahedral Ti in the framework position. Thus the Ti-b
samples synthesized with C6TEAOH, C6DABCO, and
C4quinuclidine exhibit higher activity than the Ti-b samples
synthesized using other templates consistent with high lattice
titanium substitution (Table 2). The activity of unidimensional
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mesoporous Ti-MCM-41 showed much lower catalytic activity
than the Ti-b under identical reaction conditions, whereas dense
phase material Ti-nonasil synthesized with C1TEA(OH)2 tem-
plate was completely inactive in this reaction.
In order to obtain quantitative information on the activity
of Ti-b synthesized under fluoride medium, a kinetic study of
Ti-b [C6TEA(OH)2] [Ti-b(F), Si/Ti = 69] was performed and
the results obtained were compared with [Ti-b(OH)] synthe-
sized in the presence of tetraethylammonium hydroxide under
hydroxide (Si/Ti = 64.5) and fluoride medium (Si/Ti =
97.2).47 Ti-b(OH) synthesized with TEAOH/F showed much
lower catalytic activity and more diol selectivity (Table 3).
Higher diol selectivity in the latter could be attributed to defect
induced hydrolysis of the epoxide. Furthermore, for compara-
tive kinetic study, acetonitrile instead of methanol was
employed as solvent as the latter leads to cleavage of oxirane
to give the respective glycol monomethyl ether. Fig. 9 exhibits
the substrate conversion and product selectivity for Ti-b(F)
and Ti-b(OH). As expected, the conversion of substrate
increases with time, while the selectivity to the primary
epoxide product decreases for both Ti-b(F) and Ti-b(OH)
at the expense of epoxide; however the observed selectivity
for epoxide over Ti-b(F) is nearly two fold higher than the
Ti-b(OH). The major side product is the diols from the ring
opening of epoxide in addition to a small amount of further
Fig. 8 29Si MAS NMR spectra of Ti-b samples: (bottom to top)
C6TEAOH (Si-BEA), C5DABCO (Ti-BEA), C6DABCO (Ti-BEA),
C6TEAOH (Ti-BEA), and C4quinuclidine, respectively.
Ti-ZSM-12 is considerably less than that observed over Ti-b.
This could be attributed partially due to less titanium content
as well as steric hindrance of the one-dimensional channel
system. Other titanosilicates like microporous TS-1 and
Scheme 1 Reaction pathways for the oxidation of 4-vinyl-1-cyclohexene.
Table 3 Epoxidation of 4-vinyl-1-cyclohexene in the presence of dilute aqueous H2O2 over titanium silicates synthesized with different templatesa
Selectivity of products (%)
Ratiob
TONc
Side chain oxidation
Ring oxidation
Others
Catalyst
Conversion mole (%)
H2O2 Sel. (%)
Ti-b (C4quinu(OH)2)
Ti-b (C6DABCO(OH)2)
Ti-b (C5DABCO(OH)2)
Ti-b (C4DABCO(OH)2)
Ti-b (C6TEA(OH)2)
Ti-b (C5TEA(OH)2)
Ti-b (C4TEA(OH)2)
Ti-ZSM-12 (C3TEA(OH)2)
Ti-Nonasil (C1TEA(OH)2)
TS-1d
35.3
37.8
14.9
19.5
34.2
24.5
26.6
5.9
89.5
92.1
80.4
79.3
88.6
85.6
80.1
45.3
—
71.1
72.0
77.2
79.1
70.7
72.5
71.3
82.5
—
26.9
26.1
21.8
19.5
26.3
25.5
26.8
16.1
—
2.0
1.9
1.0
1.4
2.6
2.0
1.9
1.4
—
0.40
0.38
0.28
0.26
0.33
0.35
0.37
0.19
—
65.5
68.9
35.8
44.0
64.0
57.8
58.2
24.0
—
Nil
18.0
12.0
19.8
88.0
61.0
73.4
96.0
87.0
71.1
4.0
13.0
20.8
—
—
9.1
0.04
0.13
0.29
17.9
18.8
58.5
Ti-MCM-41e
Ti-b (TEAOH/FÀ)f
a
Reaction conditions: 5 mmol 4-vinyl-1-cyclohexene, 5 mmol H2O2 (31 wt% aqueous), 100 mg catalyst, 5 mL methanol, temperature 338 K and
b
reaction time 1.5 h. Ratio of ring to side-chain double bond epoxidized products. Turnover number = moles of substrate converted per mole of Ti.
d
c
e
Si/Ti = 33.1. Si/Ti = 51.4. Si/Ti = 97.2.
f
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oxidized products. The high selectivity of epoxide over Ti-b(F)
could be attributed to negligible defect sites vis-a-vis Brønsted
acidity of the silica framework. Ti-b(OH) synthesized in the
hydroxide medium has been reported to be partially acidic due
to the presence of structural defects as well as a large number
of silanol groups, which give rise to acid catalyzed reactions.
The H2O2 selectivity of Ti-b(F) dropped from 91 to 76% with
reaction time, and the extent of decrease is significantly lower
than that of Ti-b(OH). Observed turnover numbers for Ti-b
varies between 44.0 and 68.9, which are quite high compared
with well-known titanosilicates TS-1 and Ti-MCM-41 in the
selective epoxidation of 4-vinyl-1-cyclohexene (Table 3).
Like other oxidation reactions over titanium silicates, these
epoxidation reactions would also be facilitated by the titanium
hydroperoxo complexes, which are formed by the interaction
of TiIV species with H2O2 and H2O as proposed in the
literature.20,48 Furthermore, the Ti-b(F) has also been reused
in four successive cycles without appreciable loss of activity
after activation (300 1C) of the filtered catalyst. ICP chemical
analysis of the reaction mixture after removing the catalyst
Fig. 9 Conversion and selectivities of epoxidation of 4-vinyl-1-cyclohexene over Ti-b(F) (a) and Ti-b(OH) (b) under identical conditions using
acetonitrile as solvent. Reaction conditions: 5 mmol 4-vinyl-1-cyclohexene, 5 mmol H2O2 (31 wt% aqueous), 20 wt% catalyst, 5 mL acetonitrile,
temperature 65 1C.
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through filtration did not show any trace elements indicating
the absence of leached Ti from the catalyst under the reaction
conditions. Thus, the liquid-phase selective oxidation reac-
tions are indeed heterogeneous in nature and the titanosilicate
catalysts synthesized herein are chemically stable.
8 L. A. Villaescusa, I. Bull, P. S. Wheatley, P. Lightfoot and
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Conclusions
10 J. L. Guth, H. Kessler, J. M. Higel, J. M. Lamblin, J. Patarin,
A. Seive, J. M. Chezeau and R. Wey, Am. Chem. Soc. Symp. Ser.,
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Synthesis of catalytically active Ti-b using several new
diquaternary ammonium hydroxides with different head
groups such as triethyl, DABCO, and quinuclidium has been
successfully demonstrated under the fluoride medium. For the
triethylamine head group based structure-directing agents, the
topology of different titanosilicates was found to be controlled
by the length of the hydrophobic alkyl-chain length as well as
nature of mineralizer. The lack of crystallization under hydroxide
medium over C6TEAOH, C5TEAOH, and C4TEAOH
confirmed the crucial role played by FÀ ions. The structure
directing role of heteroatom titanium was observed when
DABCO-SDA VIII was used as template. The failure of
formation of all-silica polymorph b with C4DABCO in the
absence of Ti suggests the importance of titanium in controlling
the phase selectivity. The XRD patterns of the calcined samples
indicated high crystallinity of titanosilicates and free from other
impurity phases. The XRD pattern of Ti-b samples suggests
that they are richer in polymorph A than polymorph B. A sharp
absorption band at 220 nm in the UV-vis spectroscopy and
the presence of a 958 cmÀ1 band in the FTIR region indicated
the presence of the SiO–Ti–OSi moiety. FTIR spectra in the
hydroxyl stretching region revealed that all the Ti-b samples are
free from defect sites. 29Si MAS NMR exhibits line broadening
of resonance signal due to incorporation of Ti in the lattice.
Catalytically active Ti-b materials can be synthesized using new
template molecules described herein and these can be effectively
utilized for preparation of epoxides with high selectivity under
liquid phase reaction conditions.
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AB wishes to thank DST, New Delhi for financial supports.
We thank Professor Takashi Tatsumi, Tokyo Institute of
Technology for his help and support.
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