Titanosilicate beads with hierarchical porosity: Synthesis and application as epoxidation catalysts

Article abstract of DOI:10.1002/chem.201001508

Porous titanosilicate beads with a diameter of 0.5-1.5a mm (TiSil-HPB-60) were synthesized from a preformed titanosilicate solution with a porous anion-exchange resin as template. The bead format of this material enables its straightforward separation from the reaction mixture in its application as a liquid-phase heterogeneous catalyst. The material displays hierarchical porosity (micro/mesopores) and incipient TS-1 structure building units. The titanium species are predominantly located in tetrahedral framework positions. TiSil-HPB-60 is a highly active catalyst for the epoxidation of cyclohexene with t-butyl hydroperoxide (TBHP) and aqueous H₂O₂. With both oxidants, TiSil-HPB-60 gave higher epoxide yields than Ti-MCM-41 and TS-1. The improved catalytic performance of TiSil-HPB-60 is mainly ascribed to the large mesopores favoring the diffusion of reagents and products to and from the titanium active sites. The epoxide yield and selectivity could be further improved by silylation of the titanosilicate beads. Importantly, TiSil-HPB-60 is a stable catalyst immune to titanium leaching, and can be easily recovered and reused in successive catalytic cycles without significant loss of activity. Moreover, TiSil-HPB-60 is active and selective in the epoxidation of a wide range of bulky alkenes. Beads for catalysis: Titanosilicate beads with hierarchical porosity (see figure) displayed improved catalytic behavior in the epoxidation of cyclohexene with tert-butyl hydroperoxide and H₂O ₂ relative to TS-1 and Ti-MCM-41. The bead format of the material enables a straightforward separation of the catalyst from the reaction solutions. Copyright

Full text of DOI:10.1002/chem.201001508

FULL PAPER
DOI: 10.1002/chem.201001508
Titanosilicate Beads with Hierarchical Porosity:
Synthesis and Application as Epoxidation Catalysts
[
a, b]
[c]
[c]
[a]
Kaifeng Lin,
Oleg I. Lebedev, Gustaaf Van Tendeloo, Pierre A. Jacobs, and
[a]
Paolo P. Pescarmona*
Abstract: Porous titanosilicate beads
with a diameter of 0.5–1.5 mm (TiSil-
HPB-60) were synthesized from a pre-
formed titanosilicate solution with a
porous anion-exchange resin as tem-
plate. The bead format of this material
enables its straightforward separation
from the reaction mixture in its appli-
cation as a liquid-phase heterogeneous
catalyst. The material displays hier-
archical porosity (micro/mesopores)
and incipient TS-1 structure building
units. The titanium species are predom-
inantly located in tetrahedral frame-
work positions. TiSil-HPB-60 is
a
cribed to the large mesopores favoring
the diffusion of reagents and products
to and from the titanium active sites.
The epoxide yield and selectivity could
be further improved by silylation of the
titanosilicate beads. Importantly, TiSil-
HPB-60 is a stable catalyst immune to
titanium leaching, and can be easily re-
covered and reused in successive cata-
lytic cycles without significant loss of
activity. Moreover, TiSil-HPB-60 is
active and selective in the epoxidation
of a wide range of bulky alkenes.
highly active catalyst for the epoxida-
tion of cyclohexene with t-butyl hydro-
peroxide (TBHP) and aqueous H O .
2
2
With both oxidants, TiSil-HPB-60 gave
higher epoxide yields than Ti-MCM-41
and TS-1. The improved catalytic per-
formance of TiSil-HPB-60 is mainly as-
Keywords: epoxidation
·
green
chemistry · heterogeneous catalysis ·
hierarchical porosity · titanosilicate
beads
Introduction
these materials, which plays a very important role in hetero-
[15–18]
geneous oxidation.
For example, TS-1 nanoparticles
[
1]
Since the discovery of microporous TS-1 and mesoporous
showed much higher activity in phenol hydroxylation than
large TS-1 crystals, as a consequence of the strong diffusion
limitation experienced by the reactants on reaching the
[2–4]
Ti-MCM-41,
many porous titanosilicates with tetrahe-
drally substituted Ti species have been widely studied in an
attempt to promote heterogeneous oxidations of various or-
[15]
active sites in large crystals. Ti-MCM-41 nanoparticles dis-
played higher conversions and initial reaction rates in the
epoxidation of cyclohexene than Ti-MCM-41 with micro-
metric particle size, because the accessibility of the catalytic
[5–13]
ganic substrates with hydrogen peroxide.
Numerous
studies reveal that high specific surface areas and pore ar-
chitectures in well-defined micro- and mesoporous struc-
tures are pivotal for the success of these titanosilicates as
catalysts for heterogeneous oxidations. It is generally ac-
cepted that particle size is another structural parameter of
Ti species was enhanced in these Ti-MCM-41 nanoparti-
[
14]
[17–19]
cles.
These results indicate that porous titanosilicates in
the form of nanoparticles have many benefits for catalytic
applications in various selective oxidations over the corre-
sponding large particles. An alternative way to improve the
catalytic activity of TS-1 crystals in the epoxidation of large
substrates is to increase the accessibility of the Ti sites by
[
a] Dr. K. Lin, Prof. P. A. Jacobs, Prof. P. P. Pescarmona
Centre for Surface Chemistry and Catalysis, K. U. Leuven
Kasteelpark Arenberg 23-bus 2461, 3001 Heverlee (Belgium)
Fax : (+32)16-321998
[20,21]
creating mesopores in the zeolitic structure.
However, a
E-mail: paolo.pescarmona@biw.kuleuven.be
drawback of all these systems in heterogeneous oxidations is
their awkward separation from the reaction solution during
catalyst recycling, which normally requires high-speed cen-
trifugation or special filtration. This hinders recovery of the
products and recycling of the catalysts and, therefore, limits
broad industrial application of these porous titanosilicates as
heterogeneous catalysts in selective oxidations. Furthermore,
[
b] Dr. K. Lin
Academy of Fundamental and Interdisciplinary Science
Harbin Institute of Technology,box-3026
1
50080 Harbin (China)
[
c] Dr. O. I. Lebedev, Prof. G. Van Tendeloo
EMAT, Physics Department, University of Antwerpen
Groenenborglaan 171, 2020 Antwerpen (Belgium)
Chem. Eur. J. 2010, 16, 13509 – 13518
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
13509

some porous titanosilicates suffer from leaching of active ti-
tanium species in the presence of H O , which results in
contain a large amount of mesopores as well as a small
quantity of macropores (Figure 1B and D). In contrast, the
material prepared without the resin template, TiSil-Pow-60,
contains irregular bulky particles (Figure 1E) without meso-
or macropores (Figure 1F).
TEM indicates that the mesopores of TiSil-HPB-60 are
predominantly within the range of 20–50 nm (Figure 2A and
B), with a lesser contribution of larger pores at 50–150 nm.
2
2
[22,23]
their gradual deactivation.
It is of great interest to de-
velop novel porous titanosilicates with highly catalytic and
stable Ti species that also have well-defined macroscopic
morphologies, which can facilitate separation from reaction
solutions.
Herein, we report the synthesis and catalytic application
of such a material in the form of titanosilicate beads with hi-
erarchical porosity and active and stable Ti species (TiSil-
HPB-60). This material was prepared by using anion-ex-
change resin beads as a shape- and structure-directing agent.
The obtained titanosilicate beads were used to catalyze the
epoxidation of various alkenes with t-butyl hydroperoxide
(
TBHP) and aqueous H O , and proved to be active and se-
2 2
lective catalysts with all substrates tested. Notably, the sepa-
ration of the beads from the reaction solution was extremely
straightforward and did not require any centrifugation or fil-
tration.
Results and Discussion
Characterization of the catalysts: TiSil-HPB-60 can be de-
scribed as titanosilicate beads with an amorphous structure
with incipient TS-1 features, and a hierarchical porosity that
includes disordered micro- and mesopores. This description
stems from the full characterization of the material.
Figure 2. TEM images of a bead of TiSil-HPB-60 at different magnifica-
tions. Scale bars: Characteristic mesopores are indicated by the arrows.
After synthesis and calcination, TiSil-HPB-60 consists of
hard, white beads. The material retains the same bead-size
distribution as that of the resin beads used as template;
bead size ranges from 0.5 to 1.5 mm (see SEM images in
Figure 1A and C). The resin beads as well as TiSil-HPB-60
The walls between these meso/macropores contain micro-
pores with a diameter of about 0.5 nm (Figure 2C). These
micropores are generally disordered, in that they are not in
the form of a zeolitic structure, although domains with
short-range order are also observed (Figure 2C, inset).
The characterization of the textural features of the mate-
rials was completed by measuring their nitrogen adsorption/
desorption isotherms. The N2 adsorption/desorption iso-
[24]
therm of TiSil-Pow-60 belongs to type I. The predominant
adsorption finishes below P/P =0.05 (Figure 3A), which is
0
Figure 1. SEM images of A), B) Amberlite IRA-900, C), D) TiSil-HPB-
Figure 3. Nitrogen adsorption/desorption isotherms of A) TiSil-Pow-60
and B) TiSil-HPB-60.
6
0, and E), F) TiSil-Pow-60.
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Chem. Eur. J. 2010, 16, 13509 – 13518

Titanosilicate Beads with Hierarchical Porosity
FULL PAPER
characteristic of microporous materials templated from tet-
rapropylammonium hydroxide (TPAOH). The N2 adsorp-
tion/desorption isotherm of TiSil-HPB-60 has a hysteresis
To get a more defined and quantitative description of the
porosity in the range of large meso- and macropores, a mer-
cury intrusion porosimetry (MIP) measurement was per-
formed on TiSil-HPB-60 (Figure 5). Only a narrow range of
pressures is required to fill most of the porous structure,
which indicates a narrow distribution of pore sizes. Indeed,
the pore-size distribution (inset, Figure 5) displays a narrow
range of pores centered at 40–50 nm. A small fraction of
smaller pores (4–20 nm) also appears to be present, while
the contribution of the macropores with diameter >50 nm
observed by SEM and TEM is negligible. Combining the re-
loop above P/P =0.9 in addition to a steep uptake below P/
0
P =0.05 (Figure 3B), showing the presence of meso- and/or
0
macropores connected through windows of smaller size
[25]
leading to the so-called bottleneck effect.
Based on the
Horvath–Kawazoe (HK) method, the size of the micropores
in TiSil-HPB-60 and TiSil-Pow-60 is 0.5 nm (Figure 4A and
sults of N adsorption and MIP, the porosity of TiSil-HPB-
2
6
0 can be described as a bimodal system, with a rather
narrow size distribution of micropores (0.5 nm) and of large
mesopores (40–50 nm).
The pore sizes and volumes and the specific surface areas
of TiSil-HPB-60, TiSil-Pow-60, and the two reference mate-
rials Ti-MCM-41 and TS-1 are summarized in Table 1. The
Table 1. Textural properties and specific surface areas.
Pore size
Micropore
Mesopore
Surface
area [m g ]
[
a]
3
ꢀ1 [a]
3
ꢀ1
2 ꢀ1 [a]
[
nm]
V [cm g
]
V [cm g
]
[
b]
[b]
TiSil-HPB-60 0.5; 40–50
0.27
0.21
–
0.47
–
0.54
–
618
464
916
425
TiSil-Pow-60
Ti-MCM-41
TS-1
0.5
2.4
0.5
[
a]
0.17
[
2
a] Determined from N adsorption isotherms at 77 K. [b] Determined by
MIP and, thus, based on analysis of pores with diameter ꢁ4 nm.
micropore volumes of TiSil-HPB-60 and TiSil-Pow-60 are
higher than those of TS-1 zeolite although their micropore
sizes are equal; this is possibly due to the disordered micro-
pore structure in the former materials.
Figure 4. Pore-size distributions of A), B) TiSil-HPB-60, and C), D) TiSil-
Pow-60.
C), which is comparable to the
diameter of micropores in TS-1
or ZSM-5 zeolites templated by
TPAOH. Notably, the Barrett–
Joyner–Halenda (BJH) pore-
size distribution curve of TiSil-
HPB-60 from the adsorption
branch indicates that meso- and
macropores are present (Fig-
ure 4B). However, the pore-
size distribution curve cannot
provide complete information
on large meso- and macropores
in TiSil-HPB-60 due to the limi-
tations of N adsorption analy-
2
sis. The absence of meso- and
macropores in TiSil-Pow-60 is
confirmed by its pore-size dis-
tribution curve (Figure 4D).
Figure 5. Mercury intrusion curve obtained over TiSil-HPB-60 and the related pore-size distribution curve
(inset).
Chem. Eur. J. 2010, 16, 13509 – 13518
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13511

P. P. Pescarmona et al.
Neither TiSil-HPB-60 nor TiSil-Pow-60 displays any XRD
peaks in the 2q range between 1 and 608 (not shown here).
This indicates that there is no short- or long-range order in
the materials, which is consistent with TEM characterization
results (Figure 2). The absence of a crystalline structure in
TiSil-HPB-60 and TiSil-Pow-60 is ascribed to incomplete
crystallization due to the low temperature (608C) and short
synthesis time (24 h).
The FTIR spectrum of TS-1 zeolite exhibits a characteris-
ꢀ
1
tic band at around 560 cm , which is assigned to a 5-mem-
[26]
bered ring of SiꢀOꢀT (T=Si or Ti) (see Figure 6A).
A
Figure 7. UV/Vis spectra of A) TiSil-HPB-60 and B) TiSil-Pow-60.
teraction of Ti species with moisture and/or to polymerized
[28]
hexa-coordinated Ti species.
The lack of the absorption
band characteristic of octahedral extra-framework titanium
at about 300–330 nm in both materials shows that no sepa-
rate titania (anatase) phase is formed during the synthesis.
The fraction of Si atoms in the form of SiOH groups in
TiSil-HPB-60, TiSil-Pow-60, and Ti-MCM-41 was deter-
2
9
mined by Si MAS NMR spectroscopy (Table 2). Each of
Table 2. Silanol content and water adsorption capacity.
SiOH content
Water adsorption
[
a]
ꢀ1 [b]
[
%]
capacity [mgg
]
TiSil-HPB-60
TiSil-Pow-60
Ti-MCM-41
50.4
37.6
37.6
31.6
154.6
153.4
54.7
Figure 6. IR spectra of A) TS-1, B) TiSil-Pow-60, and C) TiSil-HPB-60 in
ꢀ
1
the region of 400–1000 cm
.
S-TiSil-HPB-60
28.3
2
00ð2Q þQ
P
3
similar but weaker band is present in the FTIR spectra of
TiSil-Pow-60 and TiSil-HPB-60 (Figure 6B and C), which in-
dicates that both materials contain TS-1 building units as a
consequence of using a preformed titanosilicate solution
containing TPAOH in their synthesis.
29
1
Þ
[
a] Calculated from Si MAS NMR as SiOH(%)=
%. [b] Calcu-
Qi
i
lated from TGA by the weight loss of the samples below 1508C.
the three materials exhibits three peaks at d=ꢀ110, ꢀ100,
and ꢀ90 ppm (Figure 8A–C), which are attributed to
UV/Vis spectroscopy is generally recognized as one of the
most suitable methods to characterize the coordination of Ti
species in zeolites and mesoporous titanosilicates. The UV/
Vis spectra of both TiSil-HPB-60 and TiSil-Pow-60 exhibit a
band centered at 230 nm with similar intensity (Figure 7),
which can be attributed to tetrahedral Ti species in amor-
4
3
2
Si
ACHTUNGERTNNUNG( OSi) (Q ), HOSi ACHTUNRTEGNNUNG( OSi) (Q ), and (HO) Si AHTCUNGTRENNUNG
4
3
2
2
[12,31]
spectively.
TiSil-HPB-60 has a higher SiOH content
than TiSil-Pow-60 and Ti-MCM-41, which can be attributed
to the higher amount of structural defects (disordered meso-
[27,28]
phous titanosilicates.
This indicates that the use of the
resin beads as shape- and structure-directing agents does not
have an influence on the incorporation of Ti species into the
silica framework. Note that, although TiSil-HPB-60 contains
a certain amount of TS-1 building units, the UV/Vis spec-
trum is different from that of well-manufactured TS-1. This
is ascribed to the presence of various types of tetrahedral Ti
sites, such as Ti
broad band centered at 230 nm.
ACHTUNGTRENNUNG( OSi) and Ti(OH) ACHUTNGTERNNNU(G OSi) which generate the
4 3,
[29,30]
Moreover, a small
amount of higher-coordinated Ti species coexist with the tet-
rahedral Ti sites in both materials, as indicated by the
shoulder between 270 and 290 nm, which is assigned to re-
versible penta-coordinated Ti species that form from the in-
29
Figure 8. Si MAS NMR spectra of A) Ti-MCM-41, B) TiSil-Pow-60,
C) TiSil-HPB-60, and D) S-TiSil-HPB-60.
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Chem. Eur. J. 2010, 16, 13509 – 13518

Titanosilicate Beads with Hierarchical Porosity
FULL PAPER
pores) in TiSil-HPB-60 than TiSil-Pow-60, and to the lower
condensation degree of SiOH groups (due to the low syn-
thesis temperature) than in Ti-MCM-41. To increase the hy-
drophobicity of the catalyst, TiSil-HPB-60 was treated with
trimethylchlorosilane (TMCS) to silylate the surface silanol
groups. The successful functionalization of TiSil-HPB-60 is
demonstrated by the appearance of a peak at d=14 ppm as-
next step, the resin and TPAOH are removed by calcination,
generating a self-bonded solid with hierarchical porosity
(TiSil-HPB-60). This procedure is similar to that reported
for the preparation of Silicalite-1 microspheres composed of
primary nanoparticles (ca. 100 nm) by using shape-directing
[33]
macro-templates.
[32]
signed to (CH ) Siꢀ
A
C
H
T
U
N
G
T
R
E
N
N
U
N
G
(OSi) groups and by the concomitant
Catalytic performance: The epoxidation activity of TiSil-
HPB-60 and TiSil-Pow-60 was compared with that of two
well-known titanosilicate catalysts: the crystalline micropo-
rous TS-1, and the ordered mesoporous Ti-MCM-41. Aque-
ous H O and TBHP were employed as oxidants. The epoxi-
3
3
decrease in the intensity of the peaks due to silanol groups
(
S-TiSil-HPB-60, Figure 8D and Table 2).
The hydrophilicity of TiSil-HPB-60 (with and without sily-
lation), TiSil-Pow-60, and Ti-MCM-41 were evaluated by
thermogravimetric analysis (TGA) from the water adsorp-
tion capacity, which was calculated from the weight loss of
the samples below 1508C (Figure 9 and Table 2). TiSil-HPB-
2
2
dation of cyclohexene was chosen as a test reaction because
this substrate is too large to diffuse into the pores of TS-
[32]
1, but may be converted on the Ti sites in the hierarchical
porous structure of TiSil-HPB-60. In the epoxidation of cy-
clohexene with TBHP (ca. 5.5m in decane), cyclohexene ep-
oxide (CHE) was the main product with each catalyst tested
(
Table 3). TS-1 showed the expected low conversion of cy-
clohexene due to the hindered accessibility of the small mi-
cropores of TS-1 to the bulky reactants (cyclohexene and
[34]
TBHP);
the activity displayed is mainly ascribed to the
conversion on the external surface of TS-1.
Table 3. Catalytic performance in the epoxidation of cyclohexene with
[
a]
TBHP.
[
b]
Si/Ti
Conversion Yield of
TOF
CHE [%]
[
c]
ꢀ1 [d]
[
%]
CHE [%]
A
H
U
G
R
N
U
G
]
TiSil-HPB-60 36.1
25.6
6.2
29.1
8.7
23.1
4.2
21.9
5.6
90.3
67.0
75.4
64.0
TiSil-Pow-60
Ti-MCM-41
TS-1
37.8
59.3
34.8
15.8
2.8
[
2
a] Reaction conditions: 60 mg catalyst, 4.5 mmol cyclohexene, and
.25 mmol TBHP (ca. 5.5m in decane), 5 h at 608C. [b] Molar ratios mea-
Figure 9. Thermogravimetric analysis of A) TiSil-HPB-60, B) TiSil-Pow-
6
0, C) Ti-MCM-41, and D) S-TiSil-HPB-60.
sured by EDX. [c] Based on cyclohexene; the theoretical maximum con-
version achievable under the employed reaction conditions is 50%.
[
d] TOF=moles of cyclohexene converted per mole Ti of the catalyst per
60 and TiSil-Pow-60 show markedly higher water adsorption
hour.
capacities than Ti-MCM-41, which suggests that the hydro-
philicity estimated by TGA cannot be directly correlated
2
9
with the SiOH content determined by Si NMR spectrosco-
py when comparing materials with different structural and
textural properties. On the other hand, both the TGA and
The catalytic behavior of TiSil-Pow-60 is slightly inferior
to that of TS-1, which suggests that the disordered micro-
pores (ca. 0.5 nm) in the material are inaccessible to the re-
actants, as are the pores of TS-1. Notably, the conversion of
cyclohexene on TiSil-HPB-60 was much higher than on TS-1
and TiSil-Pow-60, and was comparable to that on Ti-MCM-
41, which indicates the good accessibility of the active Ti
sites in TiSil-HPB-60 to the reactants, similarly to what
occurs in Ti-MCM-41. It has been revealed that the accessi-
bility and molecular transport to the active sites in zeolitic
micropores could be increased by the interconnection of sec-
29
Si NMR spectroscopy data clearly indicate that the silyla-
tion of TiSil-HPB-60 leads to an increase in the hydropho-
bicity of the material.
On the basis of the characterization results, it is possible
to propose a path for the formation of TiSil-HPB-60 by the
means of templating with anion-exchange resin beads. The
negatively charged titanosilicate species in the preformed al-
kaline titanosilicate solution can exchange with anions in
the resin beads. The resin–titanosilicate composites evolve
through condensation of the titanosilicate species, which
takes place during the hydrothermal treatment. The materi-
al, at this stage, consists of two interconnected 3D networks:
an organic part originating from the resin, and an inorganic
part consisting of the condensed titanosilicate matter (as
shown by energy dispersive X-ray (EDX) analysis). In the
[35]
ondary mesopores created in the crystals.
We propose
that, in TiSil-HPB-60, a similar role is played by the disor-
dered mesopores, which favor the accessibility of the active
titanium sites to bulky reactants. Another contribution to
the higher conversion of TiSil-HPB-60 compared with that
of TS-1 is provided by the higher micropore volume of the
former, which implies that a large fraction of titanium sites
Chem. Eur. J. 2010, 16, 13509 – 13518
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13513

P. P. Pescarmona et al.
are located near the pore mouths, where the reaction of
bulky compounds can be catalyzed, notwithstanding the
small pore size (0.5 nm). However, TiSil-HPB-60 showed a
lower turnover frequency (TOF) than Ti-MCM-41, presuma-
bly due to its higher hydrophilicity and lower specific sur-
face area relative to Ti-MCM-41 (Tables 1 and 2). Still, the
highest yield and selectivity towards cyclohexene epoxide,
the target product of the epoxidation, were observed with
TiSil-HPB-60.
the number but also the strength of the acid sites influences
their selectivity towards epoxides. To compare the acid
strengths of the titanosilicates, a NH3 temperature-pro-
grammed desorption (TPD) study was performed; all of the
materials show an analogous desorption band between 216
and 2248C (not shown here), which indicates a similar, weak
acid strength. The intensity of this signal demonstrates that
the number of these acid sites (per gram of material) is
higher in TiSil-HPB-60 than in Ti-MCM-41, which is oppo-
site to the observed trend for selectivity towards the epox-
ide. It is clear that the selectivity trend should be described
by taking into account other parameters. An explanation for
the higher CHE selectivity with TiSil-HPB-60 can be found
in the expected improved diffu-
When aqueous H O was used as the oxidant, four differ-
2
2
ent products were obtained from the oxidation of cyclohex-
ene: CHE, ALLYLIC (2-cyclohexene-1-ol + 2-cyclohexene-
1-one), and CHD (1,2-cyclohexanediol) (Table 4). It has
sion rate of the products
through the large mesopores of
TiSil-HPB-60, which would
[
a]
2 2
Table 4. Catalytic performance in the epoxidation of cyclohexene with H O .
[
b]
Catalyst
Si/Ti
Conversion
Yield
TOF
Product selectivity [%]
[c]
ꢀ1 [d]
[%]
CHE [%]
A
H
U
G
E
N
N
]
CHE
ALLYLIC
CHD
imply a shorter residence time
of the formed epoxide in the vi-
cinity of the active sites, with a
consequently lower chance of
further reaction. An additional
TiSil-HPB-60
TiSil-Pow-60
Ti-MCM-41
TS-1
36.1
37.8
59.3
34.8
20.2
8.8
28.8
8.7
8.2
1.3
2.6
3.7
12.4
40.7
14.2
8.9
42.7
59.4
5.3
10.5
11.4
5.2
54.0
75.3
79.7
52.1
35.3
15.5
2.8
8.3
[
e]
S-TiSil-HPB-60
39.8
20.8
5.3
[
3 2 2
a] Reaction conditions: 4.5 mL of CH CN, 4.5 mmol of cyclohexene, 2.25 mmol of H O (50 wt%), 60 mg of reason for the observed selec-
catalyst, 608C, 5 h. [b] Molar ratios measured by EDX. [c] Based on cyclohexene; the theoretical maximum
conversion achievable under the employed reaction conditions is 50%. [d] TOF=moles of cyclohexene con-
verted per mole Ti of the catalyst per hour. [e] Calculated on the basis of the molar ratio of Si/Ti in TiSil-
tivity trend may be the nature
of the acid sites that are formed
2
9
in the presence of H O by sila-
2 2
HPB-60 and of the degree of silylation in S-TiSil-HPB-60 (from Si MAS NMR).
nol groups adjacent to Ti sites.
It has been reported that addi-
tion of H O to the system of
2
2
been shown that the OꢀO bond of Ti–oxo species (Ti–
peroxo, Ti–hydroperoxo, or Ti–superoxo) generated on tita-
nosilicate molecular sieves by contact with H O cleaves
three nontitanosilicates (H-ZSM-5, silicalite-1, and deboro-
nated B-Beta) produced no enhancement of the solvolysis
of CHE. In contrast, the coexistence of H O with calcined
2
2
2
2
[37]
either hetero- or homolytically. Heterolytic cleavage leads
to the epoxide product (CHE), while homolytic cleavage
leads to the allylic oxidation products 2-cyclohexene-1-ol
[Ti,Al]-Beta catalyst promoted the solvolysis of CHE.
Therefore, it has been proposed that silanol groups hydro-
gen bonding to titanium hydroperoxide species in the pres-
ence of H O behave as stronger acid sites, and catalyze the
[11,36]
(
CH-OH) and 2-cyclohexene-1-one (CH-ONE).
CHD is
2
2
[37]
formed by the hydrolysis of the epoxide ring of cyclohexene
oxide, which is catalyzed by acid sites. Similarly to the trend
observed with TBHP, Ti-MCM-41 and TiSil-HPB-60 dis-
played higher conversions of cyclohexene and TOFs than
TS-1 and TiSil-Pow-60, which confirms the advantage of the
presence of mesopores in the epoxidation of the bulky cy-
clohexene. Although TiSil-HPB-60 has a lower TOF than
Ti-MCM-41, as in the epoxidation with TBHP, the epoxide
yield with TiSil-HPB-60 is more than double that with any
other catalysts tested. Interestingly, TiSil-HPB-60 showed
much higher selectivity towards CHE and lower selectivity
towards CHD than Ti-MCM-41, which suggests that the hy-
hydrolysis of epoxides to diols. Based on these observa-
tions, we propose that less of this types of acid sites are
formed during the epoxidation on TiSil-HPB-60 than on Ti-
MCM-41, presumably because a large portion of the silanol
groups of TiSil-HPB-60 are not adjacent to titanium hydro-
peroxide species. To acquire a deeper insight into the role of
the silanols, we prepared a silylated TiSil-HPB-60 (S-TiSil-
2
9
HPB-60). Si NMR spectroscopy results show that S-TiSil-
HPB-60 has a lower silanol content than TiSil-HPB-60
(Table 2) and (CH ) Si- groups are successfully attached on
3
3
the surface (Figure 8D): as a result, S-TiSil-HPB-60 has
higher hydrophobicity than TiSil-HPB-60 (as shown by
TGA, Table 2). It has been reported that the silylation of
Ti-MCM-41 brings about an increase in the catalytic conver-
sion of cyclohexene and selectivity towards the epoxide
when H O is used as oxidant. This effect is ascribed to the
drolysis of CHE with H O was less favorable on TiSil-HPB-
2
60. It has been reported that lower silanol content and
higher surface hydrophobicity in titanosilicate catalysts
result in a lower number of surface acid sites and lower ad-
sorption of water on the surface and, thus, in higher selectiv-
2
2
[30,38]
improved hydrophobicity of the catalyst surface.
Simi-
[30,37,38]
ity towards CHE.
However, in our case, TiSil-HPB-60
larly, S-TiSil-HPB-60 showed a higher selectivity towards
CHE (59.4%, Table 4), which resulted in an epoxide yield
of 12.4%, compared with 8.2% for untreated TiSil-HPB-60.
However, S-TiSil-HPB-60 showed only a slight increase in
has a higher silanol content and lower hydrophobicity than
Ti-MCM-41 (Table 2), which indicates that these features
cannot account for the observed selectivity trend. Not only
13514
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 13509 – 13518

Titanosilicate Beads with Hierarchical Porosity
FULL PAPER
the conversion (20.8%, Table 4). The small influence of the
silylation on the catalytic conversion fits the hypothesis that
a large fraction of the silanols in TiSil-HPB-60 are not adja-
cent to the Ti sites. In such case, the silylation would have a
minor influence on the environment of the active titanium
sites and, thus, on the catalytic conversion. On the other
hand, the lower silanol population and the higher hydropho-
bicity of S-TiSil-HPB-60 decrease the chance of hydrolysis
of the formed epoxide, which leads to the observed increase
in selectivity.
Leaching of titanium species under catalytic reaction con-
ditions is a major drawback of some mesoporous titanosili-
cates, especially in the presence of hydrogen peroxide,
Figure 10. TiSil-HPB-60 under stirring (A) and in static conditions (a few
seconds after stopping the stirring) (B).
[22]
which has strong complexing properties.
Since small
amounts of leached Ti species can have a significant effect
on the catalytic activity, a test for Ti leaching was carried
[39]
out as follows:
The catalyst TiSil-HPB-60 was filtered
from the reaction media after 20 min at the reaction temper-
ature to avoid readsorption of the leached titanium, and the
filtrate was divided into two parts: one was immediately an-
alyzed by GC and the other was allowed to react for a fur-
ther 5 h. The conversion of cyclohexene in the two parts of
the filtrate was 6.3 and 7.4%, respectively, which reveals
that after removal of the catalyst negligible cyclohexene ep-
oxidation took place (taking into account self-oxidation of
cyclohexene with H O ). This result shows that there was no
2
2
Ti leaching from TiSil-HPB-60 during the epoxidation with
H O . Considering the structure of TiSil-HPB-60, the high
2
2
stability of titanium species in the presence of H O may be
2
2
ascribed to the TS-1 building units present in TiSil-HPB-60,
which stabilize the titanium species in the zeolite-like struc-
ture.
In view of potential industrial applications, recyclability is
an important parameter in evaluating a catalyst. It has been
reported that the deactivation of some mesoporous titanosi-
licates occurs during oxidation with TBHP and aqueous
H O due to adsorbed reaction residues that hinder the ac-
2
2
cessibility of the active centers and/or due to Ti leaching. A
calcination step is often necessary to regenerate the catalysts
by eliminating adsorbed reaction residues around the active
[40]
sites.
To complete the evaluation of our novel catalyst
TiSil-HPB-60, the material was recovered from the reaction
mixture and reused in several cycles. The straightforward re-
cycling procedure for TiSil-HPB-60 only required four wash-
ings in ethanol at room temperature without any calcination.
Note that the bead shape of TiSil-HPB-60 was unaffected
by stirring during the catalytic tests (Figure 10A). The beads
were deposited automatically on the bottom of the vial a
few seconds after stirring was stopped (Figure 10B). There-
fore, the beads could be easily separated from the reaction
solution without centrifugation or filtration. No significant
loss of catalytic activity was observed when reusing TiSil-
HPB-60 in three successive catalytic runs with either TBHP
or aqueous H O (Figure 11). This result reveals that there
Figure 11. Catalytic activity (~ and
and *) with repeated use of TiSil-HPB-60 in the epoxidation of cyclohex-
ene with TBHP (top) (ca. 5.5m in decane) and H O (bottom).
&
) and selectivity towards CHE (!
2
2
Upon recycling, the selectivity towards CHE with TiSil-
HPB-60 remained approximately constant in the case of
TBHP, whereas it slightly and gradually decreased in the
case of aqueous H O , which suggests that repeated use of
2
2
is no deactivation from adsorbed reaction residues, probably
because the disordered mesopores favor rapid mass trans-
port of reaction residues away from the active titanium sites.
2
2
Chem. Eur. J. 2010, 16, 13509 – 13518
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
13515

P. P. Pescarmona et al.
TiSil-HPB-60 in the presence of H O may affect the acid
sites.
form spherical beads with a diameter of 0.5–1.5 mm. TS-1
structure-building units are detected in TiSil-HPB-60, and
the predominant titanium species are in tetrahedral frame-
work sites.
2
2
One of the assets of TiSil-HPB-60 is constituted by its
large mesopores, which enable the epoxidation of bulky al-
kenes. To show that this effect is not limited to cyclohexene,
TiSil-HPB-60 was tested for the epoxidation of other bulky
alkenes with molecular sizes beyond the range of TS-1 mi-
cropores, in the presence of H O (Table 5). TiSil-HPB-60
In the epoxidation of cyclohexene with t-butyl hydroper-
oxide, the yield of cyclohexene epoxide on TiSil-HPB-60
was much higher than on TS-1, and slightly higher than on
Ti-MCM-41. In the case of aqueous H O , TiSil-HPB-60
2
2
2
2
gave much higher epoxide
yields than both Ti-MCM-41
and TS-1. The improved cata-
lytic behavior of TiSil-HPB-60
is mainly ascribed to its meso-
porosity, which promotes higher
activity than that of TS-1 by im-
proving the accessibility of
active titanium sites to bulky
[
a]
2 2
Table 5. Catalytic performance of TiSil-HPB-60 in the epoxidation of various alkenes with H O .
Substrate
Conversion
Yield of
epoxide [%]
Epoxide selec-
tivity [%]
TOF
[b]
ꢀ1 [c]
[%]
A
H
U
G
R
N
U
G
]
cis-cyclooctene
29.0
41.9
9.3
11.3
4.4
29.0
40.6
9.2
99.9
96.7
99.1
95.6
96.8
trans-cyclodecene
14.1
3.1
3.8
[
d]
[d]
cyclododecene
trans,trans,cis-1,5,9-cyclododecatriene
R)-(+)-limonene
10.8
[
e]
(
4.3
1.5
[
5
a] Reaction conditions: 4.5 mL CH
h. [b] Based on the alkene: the theoretical maximum conversion achievable under the employed reaction
3 2 2
CN, 4.5 mmol alkene, 2.25 mmol H O (50 wt%), 60 mg catalyst, 608C, substrates. Its mesoporosity also
promotes higher selectivity rel-
ative to titanosilicates with
smaller pores by allowing the
products to readily diffuse out
of the pores. Silylation of TiSil-
conditions is 50%. [c] TOF=moles of alkene converted per mole Ti of the catalyst per hour. [d] Mixture of cis
and trans. [e] Ring 1.7%; side chain 2.0%; diepoxide 0.5%.
proved to be active in all test reactions, which indicates that
the Ti-sites in the bead material are also accessible to reac-
tants bulkier than cyclohexene, and thus confirmed the ad-
vantage of its special pore architecture (micro/mesopores).
Notably, the selectivity towards the epoxide is very high
with all substrates, which is in line with the trend in selectiv-
ity reported and discussed above for the epoxidation of cy-
clohexene. The higher epoxide selectivities for the reactions
with the alkenes in Table 5 compared with the selectivity for
the epoxidation of cyclohexene is attributed to the higher
structural stability of these epoxides relative to the more
HPB-60 with TMCS brings about an increased TOF and an
increased epoxide selectivity. The good catalytic activity of
these titanosilicate beads in the epoxidation of cyclohexene
with H O is highly relevant in the context of a sustainable
2
2
process because H O is considered a clean and environmen-
2
2
[
43–46]
tally friendly oxidant.
TiSil-HPB-60 is also a stable cata-
lyst that can be used in repeated cycles with both TBHP
and H O without significant loss of activity. No titanium
2
2
leaching was observed during the epoxidation. The catalyst
can be easily recycled by straightforward washing without
centrifugation or filtration, due to its bead format. This fea-
ture is very attractive for possible industrial applications.
Furthermore, TiSil-HPB-60 is an active and selective cata-
lyst for epoxidations of other bulky substrates, such as cyclo-
octene, cyclodecene, cyclododecene, and cyclododecatriene.
[41]
strained cyclohexene epoxide. The activity and selectivity
of TiSil-HPB-60 in the epoxidation of this series of alkenes
are very promising when compared with the results obtained
with other large-pore titanosilicates such as Ti-MCM-41 and
Ti-Beta, although a thorough comparison is hindered by the
use of different reaction conditions with the various cata-
[41,42]
lysts.
It is remarkable that the alkene conversion with
Experimental Section
TiSil-HPB-60 increases with the size of the substrate going
from cyclohexene to cyclodecene, while for Ti-Beta the
alkene conversion was reported to decrease going from cy-
Preparation of the titanosilicate beads: Amberlite IRA-900 resin in the
chloride form with bead size of 16–50 mesh and anion-exchange capacity
ꢀ
1
[34]
of 4.2 meqg (Aldrich) was used as templating agent. Amberlite IRA-
00 is a strongly basic, macroreticular resin with moderately high porosity
clohexene to larger cyclic alkenes. This trend confirms the
better accessibility of the Ti sites in the mesoporous TiSil-
HPB-60 catalyst compared with those in microporous Ti-
Beta.
9
and with benzyltrialkylammonium functionalities. In a typical synthesis
of the titanosilicate beads, a preformed titanosilicate solution was pre-
pared by mixing TPAOH (aqueous, 40%, 2.5 mL), H O (7.5 mL), titani-
2
um isopropoxide (0.2 g), and tetraethyl orthosilicate (5 mL) under stir-
ring at room temperature, followed by aging at 1408C for 3 h in an auto-
clave. Next, Amberlite IRA-900 resin beads were added to the titanosili-
cate solution with a solution/resin weight ratio of 15:1. The mixture was
stirred at room temperature for 24 h, transferred into an autoclave, and
heated at 608C for 24 h. Then, the liquid was decanted, the beads were
washed several times with distilled water, and were finally dried at 608C.
The resin beads used as templating agent and other organic materials
were removed by calcination at 5508C for 6 h, which led to the formation
of the porous titanosilicate beads denoted as TiSil-HPB-60. For compari-
son, a titanosilicate (TiSil-Pow-60) was prepared by the same procedure
Conclusion
Hierarchically porous titanosilicate beads were prepared by
mixing a preformed titanosilicate solution with an anion-ex-
change resin as a template. TiSil-HPB-60 displays domains
that contain disordered micropores of a similar size to those
of TS-1. The domains are connected through mesopores to
13516
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ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 13509 – 13518

Titanosilicate Beads with Hierarchical Porosity
FULL PAPER
used for TiSil-HPB-60, but without using the resin beads. Ti-MCM-41
Acknowledgements
and TS-1 zeolite (with crystal size of ca. (190ꢂ20) nm) were synthesized
[
3,15]
according to literature procedures.
KL is grateful for a Center of Excellence grant (CECAT) from K. U.
Leuven. The authors acknowledge sponsorship under the following re-
search programs: Methusalem and CECAT (K. U. Leuven, Flemish Gov-
ernment), SBO-BIPOM (IWT), IAP-PAI (BELSPO, Federal Govern-
ment), NoE IDECAT (EU), and GOA (Flemish Government). We
thank Dr. A. Aerts for a helpful discussion on porosity, G. Vanbutsele for
assistance with the measurements of the N isotherms, Dr. K. Houthoofd
2
for the Si MAS NMR analysis, and Dr. J. C. Groen (Delft Solids Solu-
tions) for the MIP measurements.
Preparation of the trimethylsilylated titanosilicate beads: Trimethylsily-
lated TiSil-HPB-60 (S-TiSil-HPB-60) was obtained by silylation of TiSil-
HPB-60 with TMCS. TiSil-HPB-60 (1 g) was degassed under vacuum at
1
208C for 7 h, and was then subsequently soaked in a TMCS solution in
toluene (10 wt%, solid/liquid=1 g:50 mL) at 708C under stirring for
7 h. The solid sample was washed with toluene and acetone to remove
1
2
9
any residual chemicals, and dried at 608C.
Characterization: Powder XRD patterns were measured on a STOE
STADI P instrument with CuKa radiation. FTIR spectra were recorded
on a Nicolet 6700 spectrometer. UV/Vis spectra were measured with a
Varian Cary 5 spectrophotometer in the 200–800 nm region. The samples
were not treated before measurement and the UV/Vis spectra were re-
corded in air. The isotherms of nitrogen adsorption/desorption were mea-
sured at liquid-nitrogen temperature with a Coulter Omnisorp. The pore-
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ꢀ
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Chem. Eur. J. 2010, 16, 13509 – 13518