Modified Ti/MCM-41 catalysts for enantioselective epoxidation of styrene

Article abstract of DOI:10.1016/j.molcata.2016.05.001

Ti sites grafted on MCM-41, pretreated at different temperatures, have been modified with chiral tartrates, leading to different species. Those solids are active catalyst for the epoxidation of styrene with tert-butyl hydroperoxide, although their efficie

Full text of DOI:10.1016/j.molcata.2016.05.001

Accepted Manuscript
Title: Modified Ti/MCM-41 catalysts for enantioselective
epoxidation of styrene
´
Author: Marwa Fadhli Ilyes Khedher Jose M. Fraile
PII:
S1381-1169(16)30171-6
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.05.001
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13-4-2016
1-5-2016
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Please cite this article as: Marwa Fadhli, Ilyes Khedher, Jose M.Fraile, Modified
Ti/MCM-41 catalysts for enantioselective epoxidation of styrene, Journal of Molecular
Catalysis A: Chemical http://dx.doi.org/10.1016/j.molcata.2016.05.001
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Modified Ti/MCM-41 catalysts for enantioselective epoxidation of styrene
Marwa Fadhli,a,* Ilyes Khedhera and José M. Fraileb,*
a
Université de Tunis El-Manar, Faculté des Sciences de Tunis, Laboratoire de Chimie des
Matériaux et Catalyse, 2092Tunis, Tunisia.
b Instituto de Síntesis Química y Catálisis Homogénea (ISQCH). Facultad de Ciencias,
C.S.I.C.-Universidad de Zaragoza, E-50009 Zaragoza, Spain.
* Corresponding authors:
Marwa Fadhli, e-mail: marwafadhli@hotmail.fr
José M. Fraile, e-mail: jmfraile@unizar.es
Graphical abstract
Highlights
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
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Ti has been grafted on MCM41 and modified with chiral tartrates.
Ti catalyst are active in the epoxidation of styrene with tert-butyl hydroperoxide.
Their efficiency depends on the calcination temperature and the chiral ligand.
They are more efficient than the homogeneous counterparts.
The best result is obtained with Ti-MCM550 and diisopropyl tartrate (62% ee).
Abstract
Ti sites grafted on MCM-41, pretreated at different temperatures, have been modified with
chiral tartrates, leading to different species. Those solids are active catalyst for the
epoxidation of styrene with tert-butyl hydroperoxide, although their efficiency is not optimal,
1

mainly due to the subsequent epoxide rearrangement to phenyl acetaldehyde and further
oxidation of it. Styrene oxide is obtained with moderate enantioselectivity, in the range of 55-
62% e.e., and the best overall results correspond to the use of MCM-41 calcined at the lowest
temperature (550ºC). These results are much better with regard to styrene oxide selectivity
than those obtained in homogeneous phase under the same conditions, whereas
enantioselectivity is similar. The heterogeneous catalysts are recoverable, but only partially
due to a loss in activity, although enantioselectivity is kept at similar levels, demonstrating the
stability of the supported Ti-tartrate species.
Keywords: Titanium; MCM-41; enantioselective epoxidation; styrene oxide
1. Introduction
Chiral epoxides have shown to be interesting compounds, useful as synthetic intermediates for
the preparation of a large variety of organic compounds [1]. Asymmetric epoxidation of
prochiral alkenes represents a powerful strategy for the synthesis of enantiomerically enriched
epoxides. In this field, the most important way to generate optically active epoxides is via
enantioselective catalysis. In the past three decades, there has been great progress in catalytic
asymmetric alkene epoxidation. Probably, the most important methods for catalytic
enantioselective epoxidation are the Ti-tartrate system for allylic alcohols with alkyl
hydroperoxides [2] and the Mn-salen system for epoxidation of unfunctionalized alkenes [3],
in both cases with excellent catalytic activity and enantioselectivity in a broad range of
alkenes.
better handling properties, and the simple
recycling of the catalyst, together with the
purification of the products [4]. Various
2

strategies have been reported to immobilize chiral transition metal complexes onto solid
supports, including covalent grafting between the solid support and the chiral ligand [5], as
well as alternative methods that do not involve the direct bonding of the chiral ligand [6].
Whereas a good number of successful examples have been described about the
immobilization of chiral salen-metal complexes [7], this is not the case with Ti-tartrate
catalysts [8]. Linear [9] and branched [10] polytartrates led to values of enantioselectivity (up
to 80% ee) lower than the soluble tartrates in the epoxidation of allylic alcohols. A similar
result in enantioselectivity (up to 86% ee) was obtained with tartramides supported on
inorganic siliceous materials [11].
Titanium sites can be grafted on the silica surface through the formation of Si-O-Ti bonds by
reaction with precursors such as Ti(OiPr)4 [12] or TiCp2Cl2 [13], and this kind of solids has
been successfully used in epoxidation reactions with either alkyl hydroperoxides [14] or
hydrogen peroxide [15]. The residual Ti-OR (or Ti-OH) groups can be substituted by other
alcohols, such as tartaric acid derivatives [16], which modifies the catalytic activity of the
titanium sites [17,18]. In spite of the chiral nature of the silica-Ti-tartrate sites, only a limited
number of examples have been described of application of this type of catalysts in
enantioselective oxidations, either epoxidation [18,19] or sulfoxidation [20], with rather poor
results.
The difficulty in reproducing the results in solution with allylic alcohols using heterogeneous
catalysts may lay on the dimeric nature of the homogeneous catalyst, in which the titanium is
in octahedral coordination, accommodating the bidentate ligand, the oxidant and the allylic
alcohol [21]. Those catalysts with flexibility high enough to reach this kind of arrangement,
such as polytartrates [9,10], led to the best results. However, the grafted Ti sites with two
alkoxides from tartrate ligand and at least one silyloxy grafting bond cannot accommodate at
the same time the hydroperoxide and the allylic alkoxide in the epoxidation process. In
3

contrast, analogous Ta-silica catalysts modified with tartrates have shown good performance
in the epoxidation of allylic alcohols [22], as they can accommodate 5 alkoxide bonds,
required for grafting, chiral ligand bonding and coordination of the two reagents. However, it
would be conceivable the formation of a titanium species with one silyloxy bond to the
surface, two alkoxide bonds with a chiral tartrate and one with the hydroperoxide, that would
be able to epoxidize unfunctionalized alkenes. This kind of epoxidation reaction with
homogeneous titanium catalysts has been only recently described using chiral salan ligands
[23,24] and hydrogen peroxide as the oxidant. In that case, the role of the N-H groups of the
ligands has been proposed as crucial to stabilize the Ti-peroxo intermediate.
In this manuscript, we describe the preparation of Ti-tartrate complexes grafted on the surface
of MCM-41, and their use in the epoxidation of styrene with tert-butyl hydroperoxide, with
special emphasis on the type of surface species formed.
2 Experimental
2.1 Synthesis
MCM-41 was prepared as described elsewhere [25]. In order to remove the organic surfactant
(cetyltrimethylammonium bromide) from the pores of MCM-41, the dried as-synthesized
samples were calcined in air at three different temperatures 550, 650 and 750°C for 6 h, using
a heating rate of 1°C/min. The resulting materials are denoted as MCM550, MCM650 and
MCM750.
Titanium (IV) tetraisopropoxide (Ti(OiPr)4) was used as a titanium precursor. A mixture of
Ti(OiPr)4 (0.4 mmol) and calcined MCM support (1 g) in dry toluene (10 mL) was refluxed
for 24 h under inert atmosphere and the resulting material was vacuum-filtered, washed with
anhydrous dichloromethane and dried overnight in an oven at 120 °C. Dried materials were
subjected to Soxhlet extraction with anhydrous dichloromethane for 24 h and the final solids
4

were dried at 50°C under vacuum for 8 h. The resulting catalysts are referred to as Ti-
MCM550, Ti-MCM650 and Ti-MCM750.
Diethyl L-tartrate or diisopropyl L-tartrate (0.4 mmol) was added to a suspension of Ti-MCM
(1 g) in dry toluene (20 ml), and the mixture was refluxed for 12 h under inert atmosphere.
The resulting solids were filtered and thoroughly washed with dry dichloromethane and
diethyl ether to eliminate the physisorbed chiral ligand. The solids were dried at 120°C
overnight and kept in a desiccator until use. These catalysts are referred to as DET-Ti-MCM
or DIPT-Ti-MCM.
2.2 Characterization methods
Powder small angle X-ray diffraction patterns were recorded using Cu Kα radiation (40 kV
and 30 mA) from 2θ = 1° up to 2θ = 10°.BET surface area and pore size distribution were
determined from the N2 adsorption-desorption isotherms at 77 K using a micrometrics ASAP
2000 apparatus. Infrared spectra of samples were recorded in the range of 4000-400 cm-1 with
discs of solids mixed with dry KBr on a Perkin-Elmer FT-IR paragon 1000PC spectrometer.
The titanium content was carried out by plasma atomic emission spectroscopy (ICP-OES)
using a Thermo Elemental IRIS Intrepid spectrometer. Diffuse reflectance UV-Vis (DR-UV)
spectra were recorded on a Unicam UV4 spectrometer equipped with a labsphere diffuse
reflectance attachment. Elemental analysis (CHNS) was performed on a Perkin Elmer 2400
elemental analyzer. MAS NMR spectra were recorded in a Bruker Avance III WB400
spectrometer with 4 mm zirconia rotors spun at magic angle in N2 at 10 kHz. 1H-13C CP
spectra (12000-20000 scans) were measured using a 1H /2 pulse length of 3s, with a contact
time of 3 ms, and spinal64 proton decoupling sequence of 5.3 s pulse length, with 7 s recycle
delay.
5

2.3 Catalytic test: styrene epoxidation
To a solution of styrene (10 mmol) in acetonitrile (10 mL), the catalyst (0.015 mmol of Ti, 40
mg in the case of solid catalysts) and TBHP (6 mmol, 5.5 solution in decane) were added. The
mixture was allowed to stir at 70°C under inert atmosphere. The reaction was monitored by
GC and at the end the
The enantiomeric excess was determined by
HPLC with a Chiralpack OD-H column.
3. Results and discussion
3.1 Preparation and characterization of the catalysts
with the tripodal species being the less interesting
one, whereas the monopodal species would lead easily to the active intermediate by
substitution of the remaining isopropoxide. Thus, the siliceous MCM-41 was pre-calcined at
different temperatures, 550, 650 and 750ºC, before titanium grafting.
The low-angle XRD patterns recorded on untreated MCM550, MCM650 and MCM750 samples
match well with those of the silica MCM-41 polymorph reported by Chen et al [26]. The three
diffraction peaks in the 2θ range of 1–10°, characteristic of the MCM-41 structure and
corresponding to the (100), (110) and (200) reflections [27], were reasonably well resolved
(Fig. S4). The diffraction peaks did not significantly change after calcination at 750°C except
a slightly lower intensity, which indicates that the hexagonal structure of MCM41 is
6

preserved even after calcination at 750°C. The treatment with Ti(OiPr) produces a decrease in
the intensities of all of the peaks with a little shift toward lower 2 values. Figure 2 shows the
XRD patterns of MCM550 samples, as an example. This indicates that the incorporation of Ti
species in the channel of MCM-41 leads to a substantial loss in the scattering contrast
between the channel and the wall, and then reduces the intensity of the scattered X-ray in the
powder diffraction experiment [28].
When R-(+)-diethyl L-tartrate was added (Fig. 2c), the intensities of all the peaks decrease
even more, notably the intensity of the two less intense reflections (110) and (200), indicating
a loss of the long-range regularity of the hexagonal arrays of mesopores of MCM-41. This
effect is more obvious in the case of DIPT-Ti-MCM, showing the disappearance of (110) and
(200) peaks, due to the larger volume of R-(+)-diisopropyl L-tartrate (Fig. 2d). As shown in
Table 1, a0 unit-cell parameter of DIPT-Ti-MCM and DET-Ti-MCM samples increase much
more compared to Ti-MCM samples. In the high angle region of XRD patterns, the main
diffraction peak corresponding to the bulk TiO2 (2 = 25.5°) did not appear for both treated
MCM-41samples. This suggests that Ti is well dispersed within silica channels.
The nitrogen adsorption-desorption isotherm of the untreated MCM-41 sample follows the
type-IV isotherm and shows a sharp capillary condensation step at a relative pressure between
0.2 and 0.4, indicating a typical mesoporous structure with uniform pores (see Fig. 3) [29].
After treatment, the sharp capillary condensation step shifts toward lower P/P0 region. This
suggests that the introduction of Ti(OiPr)4 with DET or DIPT into the channel causes changes
in the pore structure of the support during the incorporation process. These changes could be
observed more clearly in Table 1. When MCM-41was treated with Ti(OiPr)4, and even more
after modification with chiral tartarates, the BET surface area, pore volume and pore diameter
decrease, while the wall thickness (, estimated as  = a0 − D) increases. This result is in good
agreement with the grafting of Ti species on the silica inner surface, increasing in this way the
wall thickness. Consequently, the mesochannels became narrow and the pore diameter of the
7

incorporated MCM41 samples would also decrease accompanying the reduction of the pore
volume and the surface area.
in agreement with its higher silanol density due to the lower calcination
temperature. The effect is the opposite in the other two solids calcined at higher temperature,
which should present a lower silanol density and hence a larger contribution of the monopodal
species (isopropoxy/titanium = 3)
Carbon analyses are
compatible with a tartrate/Ti ratio in the order of 1.2-1.6 in the case of DET and 1.1-1.4 in the
case of DIPT. However, these values must be taken with care, as monopodal species (Figure
1) must keep one isopropoxy group (C/Ti = 11 with DET and 13 with DIPT). Other sources of
inaccuracy of the analysis would be the inclusion of a second molecule of tartrate as
monodentate ligand to substitute the remaining isopropoxy group in monopodal species, or
the physisorption of tartrate on the silica surface of MCM41.
The FT-IR spectra of the calcined MCM41 samples (Figure 5a shows the spectrum of
MCM550, as an example) show the typical bands for a silica based solid. The most important
bands are the O-H stretching of the hydrogen-bound silanol groups (large broad band, 3700-
3000 cm−1), the deformation vibrations of absorbed water molecules (1630 cm−1), the
asymmetric stretching vibration mode of the Si-O-Si linkage (1300-1100 cm−1), the Si-O
stretching of the silanol groups (960 cm−1), and the symmetric stretching of Si-O-Si (800
cm−1) [30]. After treatment with Ti(OiPr)4, some weak bands at 3000-2800 cm−1 are present,
8

corresponding to the aliphatic C-H bonds of the remaining isopropoxy groups. In the case of
DET-Ti-MCM550 and DIPT-Ti-MCM550 samples (Figure 5 c and d), a new peak appears at
1731 cm-1, corresponding to the C=O bond of carboxylic esters. Its position [21] is in good
agreement with the uncoordinated esters in the Ti(DIPT) complexes (spectra in solution, 1738
cm−1). In case that some of the esters in the solid were coordinated to Ti, their position in the
spectrum (1638 cm−1) would be masked by the band corresponding to water molecules.
The formation of Ti-tartrate complexes on the solid surface was also checked by 13C solid
state NMR (Figure 6). The spectrum of DET physisorbed on purely siliceous MCM750 was
recorded for the sake of comparison. The expected four narrow bands were observable, but
with low intensity in the case of the carbonyl and the methyl signals due to the poor cross-
polarization behavior, because of the lack of protons in the former, and because the easy
rotation in the case of the methyl group. In all the Ti containing solids, the tartrate bands are
wider, in agreement with a reduced mobility of the molecule, and probably by a broader
distribution of different sites on the solid. The assignment of the signals can be done on the
basis of the spectra of Ti-tartrate complexes in solution [21,31], as well as the solid state
NMR spectra of complexes with different nuclearity (Ti3(DIPT)3 and Ti4(DIPT)4) [32]. The
two signals at 183 and 171 ppm fit well with the presence of both coordinated and
uncoordinated carboxylate groups. This possibility is confirmed by the presence of at least
two different OEt groups in the case of DET-Ti-MCM, and even more types of OiPr groups
are observable in DIPT-Ti-MCM, but the remaining groups from the starting Ti(OiPr)4 may
contribute to these signals. In the same way, the signal at 84 ppm clearly indicates the
formation of CH-O-Ti bonds, whereas the presence of the signal at 72 ppm points to the
presence of free CHOH groups. This is probably due to a coordination of tartrate in a non-
9

chelating way, as represented for tripodal species in Figure 1, and as pointed in the case of Ta
catalysts [33].
3.2 Titanium catalyzed epoxidation of styrene
One of the problems in the titanium catalyzed epoxidation of styrene is the existence of
consecutive reactions (Scheme 1), namely the Meinwald rearrangement of the styrene oxide
mainly to phenyl acetaldehyde (acetophenone is the minor product of this reaction), and the
further oxidation of the former to phenyl acetic acid, which suffers an oxidative
decarboxylation to benzaldehyde [34]. In view of that, and in an attempt to minimize the over-
oxidation of the products, tert-butyl hydroperoxide (TBHP) was used always in defect (0.6
equivalents) with respect to styrene, and the conversion and styrene oxide yield (Table 2)
were calculated with respect to the added oxidant. Reactions are not fast, even at 70ºC, and
they proceed along 7 days. Phenyl acetaldehyde and benzaldehyde were detected and
identified in all the reactions, together with minor amounts of other products such as tert-butyl
benzoate or 1-phenylethane-1,2-diol. Acetonitrile was chosen as solvent, given its good
performance in epoxidation reactions with TBHP [35].
Conversion represents the efficiency of the epoxidation reaction, whereas the selectivity to
styrene oxide depends on the activity of the catalyst for rearrangement to phenyl
acetaldehyde. As can be seen in Table 2, the homogeneous catalysts based on Ti(OiPr)4
(entries 1-3) show a very poor selectivity. On the contrary, the grafted Ti sites on MCM-41
(entries 4-6) are less active than Ti(OiPr)4, as shown by the lower conversion, but at the same
time they are more selective, mainly due to a lower efficiency for rearrangement. The
calcination temperature of MCM-41 affects the catalytic performance of the titanium sites, as
10

Ti-MCM750 is less efficient than the other two solids. The lower silanol density in MCM750
should produce a larger amount of monopodal sites (Figure 1), whose acidity [36] and hence
the activity for epoxidation [16] must be lower than the bipodal and tripodal sites due to the
lack of geometrical constrains.
The treatment with chiral tartrates, DET and DIPT, clearly modifies the nature of the Ti sites,
as both the activity and selectivity changes. In some cases, it is clear that epoxidation is faster
with the modified catalyst, in agreement with the reported behavior of epoxidation as a ligand
accelerated catalytic reaction [37]. However, not all the Ti-tartrate species in solution are
more active than the plain Ti(OiPr)4, and in the case of the solid catalysts, Ti-MCM550 is more
prone to generate the active species, both with DET (entry 7) and DIPT (entry 10). For Ti-
MCM650 the improvement effect is clear with DET (entry 8), but not so important with DIPT
(entry 11), whereas the role of ligand is less remarkable for Ti-MCM750 with both tartrates
(entries 9 and 12). The best results are then obtained with DET-Ti-MCM550, DET-Ti-MCM650
and DIPT-Ti-MCM550, reaching conversions of styrene in the range of 59-68% after 24 h,
with selectivities to styrene oxide of 44-46%. With respect to enantioselectivity, the values
obtained with the heterogeneous catalysts are moderate, in the range of 38-62% e.e.. In the
only paper describing comparable catalysts and processes [18], the reported enantioselectivity
was up to 32% e.e. in the epoxidation of styrene with TBHP. The different support (HMS vs
MCM-41), and mainly the different solvent used in the epoxidation reaction (dichloromethane
vs acetonitrile) may account for the differences in the catalytic results. In fact, in our hands
some tests in dichloromethane showed a much worse results, with almost no
enantioselectivity at all. There is also a qualitative correlation between the catalytic activity
and the enantioselectivity. Values of 55-62% e.e. are obtained in the cases of ligand-
accelerated reactions, whereas values of 38-45% e.e. are obtained in reactions in which
11

acceleration is not observed. This is in agreement with the existence of different surface
species, as it happens in solution, and only when the most active one is formed in a large
extension the reaction proceeds with high enantioselectivity. The enhancement in the
hydrophobic character of the sites might be also invoked to explain the higher activity, as
pointed in the case of silylated Ti-silica catalysts [38-40], but the qualitative correlation
between activity and enantioselectivity and the similar ligand content in all the solids point to
a more specific effect connected with the type of coordination of tartrate to the Ti sites instead
of the general hydrophobicity effect of silylation of silica surface. It is also remarkable that
the results are not far from the enantioselectivity obtained in solution, in fact DIPT-Ti-
MCM550 improves the result obtained with DIPT-Ti(OiPr)4 (62 vs 50% e.e., entries 10 and 3),
and in all cases with better selectivity to styrene oxide.
The catalysts based on MCM550 were recovered from the reaction mixture by simple filtration,
and they were reused after washing with dichloromethane. The recovery of the catalysts leads
to a remarkable decrease of the styrene conversion, 10-15% after 24 h, indicating probably the
increase of diffusional resistance due to the partial blockage of the pore system. On the
contrary, the enantioselectivity was maintained in moderate values after recycling, 40% e.e.
with DET-Ti-MCM550 and 56% e.e. with DIPT-Ti-MCM550. This means that the supported Ti-
tartrate species is stable under the reaction conditions.
Conclusions
Ti sites in Ti-MCM, prepared by grafting of Ti(OiPr)4 on the MCM-41 surface, can be
efficiently modified with chiral tartrates, leading to surface species whose nature depends on
the calcination temperature of the siliceous support. The presence of coordinated alkoxyde
and esters groups, as well as non-coordinated alcohols and esters, demonstrated by solid state
NMR, points to the existence of different chiral sites due to an incomplete control of the
12

surface species. Those sites are able to activate tert-butyl hydroperoxide for the epoxidation
of styrene in an enantioselective way. In fact, they are more selective than their homogeneous
counterparts, which promote more efficiently consecutive reactions such as the Meinwald
rearrangement of the styrene oxide and the over-oxidation of the resulting aldehyde. The
enantioselectivity is moderate and qualitatively correlates with the activity of the solid, in
agreement with a ligand accelerated reaction. The best result, 62% e.e., was obtained with
DIPT-Ti-MCM550, showing an effect of the calcination temperature of the support and the
nature of the chiral ligand. These results open the door to future improvements of this
epoxidation system, with special emphasis on the control of the surface density of silanol
groups to get a more homogeneous distribution of Ti species.
Acknowledgement
Financial support from the Spanish Ministerio de Economía y Competitividad (Project
CTQ2014-52367-R) and the Diputación General de Aragón (E11 Group co-financed by the
European Regional Development Funds) is gratefully acknowledged. M. F. thanks the
Tunisien Ministère de l’Enseignement Supérieure et la Recherche Scientifique for a short-
mobility grant (bourse d’alternance).
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Siemieniewska, Pure Appl. Chem. 57 (1985) 603-619.
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[32] See supporting information of: S. F. Pedersen, J. C. Dewan, R. R. Eckman, K. B.
Sharpless, J. Am. Chem. Soc. 109 (1987) 1279-1282.
[33] M. Fadhli, I. Khedher, J. M. Fraile, J. Mol. Catal. A 410 (2015) 140-148.
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(2015) 3552-3561.
[35] M. Guidotti, N. Ravasio, R. Psaro, G. Ferraris, G. Moretti, J. Catal. 214 (2003) 242–250.
15

[36] J. M. Fraile, J. I. García, J. A. Mayoral, E. Pires, L. Salvatella, M. Ten, J. Phys. Chem. B
103 (1999) 1664-1670.
[37] D. J. Berrisford, C. Bolm, K. B. Sharpless, Angew. Chem. Int. Ed. Engl. 34 (1995) 1059-
1070.
[38] T. Tatsumi, K. A. Koyano, N. Igarashi, Chem. Commun. (1998) 325.
[39] J. A. Melero, J. Iglesias, J. M. Arsuaga, J. Sainz-Pardo, P. de Frutos, S. Blazquez, J.
Mater. Chem. 17 (2007) 377.
[40] M. Guidotti, I. Batonneau-Gener, E. Gianotti, L. Marchese, S. Mignard, R. Psaro, M.
Sgobba, N. Ravasio, Micropor. Mesopor. Mater. 111 (2008) 39.
16

Figure 1. Possible surface Ti species.
(100)
(110)
(200)
(a)
(b)
(c)
(d)
2
4
6
8
2-Theta
Figure 2. Low-angle XRD patterns of different mesoporous solids: a) MCM550, b) Ti-
MCM550, c) DET-Ti-MCM550, d) DIPT-Ti-MCM550.
17

Adsorption
Desorption
(a)
(b)
(c)
(d)
0,2
0,4
0,6
0,8
1,0
Relative Pressure P/P
0
Figure 3. N2 adsorption-desorption isotherms at 77 K of the different mesoporous solids: a)
MCM550, b) Ti-MCM550, c) Ti-MCM550-DET, d) Ti-MCM550-DIPT.
8
(a)
(b)
6
(c)
4
(d)
2
0
1
10
Pore Diameter (nm)
Figure 4. BJH pore size distribution curve of the different mesoporous solids: a) MCM550, b)
Ti-MCM550, c) Ti-MCM550-DET, d) Ti-MCM550-DIPT.
18

6
4
2
0
1100
3400
1731
1630
1731
954
2985
801
(d)
954
(c)
(b)
954
960
(a)
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
6
1100
4
2
0
1731
1731
954
1630
801
(d)
954
(c)
(b)
(a)
954
960
2000
1500
1000
500
Wavenumber (cm-1)
Figure 5. FT-IR spectra of a) MCM550, b) Ti-MCM550, c) DET-Ti-MCM550, d) DIPT-Ti-
MCM550.
19

a
EtO
b
iPrO
c
Figure 6. 13C CP-MAS-NMR spectra of: a) DET physisorbed on MCM750, b) DET-Ti-
MCM750, c) DIPT-Ti-MCM750
Scheme 1
20

Table 1. Textural properties and elemental analysis of the titanium catalysts.a
Sample
SBET (m2/g) Vp (cm3/g) Dp (nm) d100 (nm) a0 (nm)
Ti (mmol/g) C (mmol/g) C/Ti
 (nm)
1.07
1.2
MCM550
1001
945
925
966
921
814
804
820
730
928
802
788
0.76
0.75
0.64
0.75
0.61
0.57
0.65
0.55
0.52
0.65
0.53
0.49
3.03
2.87
2.74
2.99
2.66
2.35
2.56
2.62
2.34
2.80
2.64
2.04
3.55
3.52
3.24
3.73
3.67
3.47
3.87
3.78
3.64
4.01
3.83
3.73
4.10
4.07
3.74
4.31
4.24
4.01
4.46
4.37
4.21
4.63
4.43
4.31
-
-
-
MCM650
-
-
-
MCM750
1.0
-
-
-
Ti-MCM550
1.32
1.58
1.66
1.9
0.38
0.38
0.38
0.38
0.38
0.38
0.38
0.38
0.38
1.97
2.79
2.61
3.62
4.24
4.9
4.23
5.18
4.31
5.18
7.33
6.86
9.52
11.15
12.89
11.13
13.63
11.34
Ti-MCM650
Ti-MCM750
DET-Ti- MCM550
DET-Ti- MCM650
DET-Ti- MCM750
DIPT-Ti- MCM550
DIPT-Ti- MCM650
DIPT-Ti- MCM750
1.75
1.87
1.83
1.79
2.27
a SBET = surface area; Vp = pore volume; Dp = mean pore diameter (determined by the BJH (Barrett–Joyner–Halenda) method); d100 = /(2 sinθ); a0 = 2 d100
√3; δ = mean wall thickness estimated according to δ= a0 − Dp. Ti content determined by ICP-OES. C content determined by elemental analysis.
/
21

Table 2. Results of the titanium catalyzed epoxidation of styrene with TBHP.a
Entry
Catalyst
Conversion 24 h Styrene oxide
% e.e.c
(%)b
selectivity 24 h (%)b
1
Ti(OiPr)4
56 (79)
52 (75)
16 (40)
36 (64)
38 (58)
23 (44)
68 (79)
62 (74)
27 (54)
59 (64)
19 (73)
26 (52)
5 (9)
-
2
DET-Ti(OiPr)4
DIPT-Ti(OiPr)4
Ti-MCM550
6 (9)
80
50
-
3
11 (9)
4
28 (22)
32 (26)
29 (25)
44 (36)
45 (36)
23 (32)
46 (26)
48 (34)
42 (47)
5
Ti-MCM650
-
6
Ti-MCM750
-
7
DET-Ti-MCM550
DET-Ti-MCM650
DET-Ti-MCM750
DIPT-Ti-MCM550
DIPT-Ti-MCM650
DIPT-Ti-MCM750
55
56
38
62
38
45
8
9
10
11
12
a Reaction conditions: styrene (10 mmol), TBHP (6 mmol, 5.5 M in decane), acetonitrile (10
mL), catalyst (0.015 mmol of Ti), 70ºC, inert atmosphere. b Determined by GC. In parenthesis
the value after 7 days. c Determined by HPLC with a Chiralpack OD-H column.
22