Epoxidation of methyl oleate with hydrogen peroxide. the use of Ti-containing silica solids as efficient heterogeneous catalysts

Article abstract of DOI:10.1039/c1gc15151g

Titanium-silica catalysts, obtained by grafting titanocene dichloride onto mesoporous silica (MCM-41 and MCM-48) and onto nanosized pyrogenic silica, were tested in the epoxidation of methyl oleate using aqueous hydrogen peroxide. All titanium-containing materials, combined with the slow addition of hydrogen peroxide, showed good catalytic activity and comparable behaviour in terms of conversion, selectivity and yield of methyl epoxystearate, notwithstanding the morphology and the texture of the silica support. By optimizing the reaction conditions (especially, the catalyst to substrate ratio), high yields up to 91% in epoxide were obtained for Ti/MCM-41 with a high stereoselectivity (80%) for the cis-epoxide. The formation of the cis/trans-epoxide and the nature of the catalytic reaction pathway are also discussed.

Full text of DOI:10.1039/c1gc15151g

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PAPER
Epoxidation of methyl oleate with hydrogen peroxide. The use of
Ti-containing silica solids as efficient heterogeneous catalysts†
Matteo Guidotti,*a Elena Gavrilova,a Anne Galarneau,b Bernard Coq,b Rinaldo Psaroa and
Nicoletta Ravasioa
Received 8th February 2011, Accepted 28th March 2011
DOI: 10.1039/c1gc15151g
Titanium-silica catalysts, obtained by grafting titanocene dichloride onto mesoporous silica
(MCM-41 and MCM-48) and onto nanosized pyrogenic silica, were tested in the epoxidation of
methyl oleate using aqueous hydrogen peroxide. All titanium-containing materials, combined with
the slow addition of hydrogen peroxide, showed good catalytic activity and comparable behaviour
in terms of conversion, selectivity and yield of methyl epoxystearate, notwithstanding the
morphology and the texture of the silica support. By optimizing the reaction conditions
(especially, the catalyst to substrate ratio), high yields up to 91% in epoxide were obtained for
Ti/MCM-41 with a high stereoselectivity (80%) for the cis-epoxide. The formation of the
cis/trans-epoxide and the nature of the catalytic reaction pathway are also discussed.
hydrogen peroxide in place of peroxoacids.16–19 In this aim, some
Introduction
recent papers reported interesting results using homogeneous,
heterogeneous and chemoenzymatic systems. For instance,
tungsten-containing heteropolyacid catalysts (“tetrakis”) were
used in the presence of hydrogen peroxide and excellent epoxide
yields of up to 94% under solvent-free conditions were reached.19
Similarly, the use of methyltrioxorhenium with an amine adduct
led to outstanding selectivity (99%) and conversion (99%) in
the epoxidation of soybean oil.20 However, in these two cases, a
complete separation of the homogeneous catalyst from the final
mixture can pose some problems.
Epoxides of fatty acid methyl esters (FAMEs) are a class of
oleochemicals playing a pivotal role as intermediates in the
production of a wide series of important industrial products
and materials. Derivatives of FAMEs epoxides and, especially, of
methyl epoxystearate (the epoxide obtained from methyl oleate
epoxidation), indeed find applications in several domains, e.g.
in the manufacture of lubricants,1–5 plasticizers in polymers,6,7
wood impregnation,8 stabilizers in chlorine-containing resins,9
cosmetics,10 pharmaceuticals11,12 and, in the near future, bio-fuel
additives.13
In the field of heterogeneous catalysis, different works over
heterogeneous systems based on pure alumina in the presence
of hydrogen peroxide were also described and led to high yields
in methyl epoxystearate with very good selectivity (>97%), even
though a large amount of oxidant with respect to the substrate
(7.5 mol oxidant/mol substrate) is necessary.17,18 Similarly, a very
high selectivity of 97% to methyl epoxystearate was obtained
over a large-pore zeolite Ti-BEA and H2O2 in the epoxidation
of methyl oleate16 with an interesting oxidant to substrate molar
ratio (0.25 mol mol-1). However, with this system, a maximum
yield in methyl epoxystearate of only 45% was reached. Then, the
epoxidation of soybean oil over the amorphous heterogeneous
Ti/SiO2 in the presence of tert-butanol with H2O2 in organic
solution led to an epoxide yield of 87%.21 In this case, however,
the separation of the high-boiling alcoholic solvent from the
reaction mixture can lead to some difficulties.
Nowadays, at an industrial scale, the epoxides of FAMEs are
generally obtained in homogeneous phase via the Prilezhaev
reaction, using in situ generated peracetic or performic acids.14,15
However, the process presents some major drawbacks such as:
1) an uncontrolled opening of the oxirane ring, that leads to low
selectivity of cis/trans-epoxide, and 2) a cumbersome separation
of the excess organic acids from the final reaction mixture.
Thus, during last decades, several studies have been performed
searching for an active catalytic system for the epoxidation of
vegetable oils and their derivatives, in particular FAMEs, using
aCNR – Istituto di Scienze e Tecnologie Molecolari, NANO-HOST CNR
Unit, via Venezian 21, Milano, Italy. E-mail: m.guidotti@istm.cnr.it;
Fax: +39 02 5031 4405; Tel: +39 02 5031 4428
bInstitut Charles Gerhardt Montpellier, UMR 5253 CNRS/UM2/
ENSCM/UM1, ENSCM, 8 rue de l’Ecole Normale, 34296,
Montpellier, France
Chemoenzymatic epoxidation gained much interest too, as
the process typically occurs under mild conditions and with
very good chemoselectivities. For instance, Novozym 435, a
Candida antarctica lipase B immobilized on polyacrylate, is one
† Electronic supplementary information (ESI) available: DRS-UV-Vis
spectra of the calcined catalysts used in the epoxidation reaction are
reported in Fig 1-SI. See DOI: 10.1039/c1gc15151g
1806 | Green Chem., 2011, 13, 1806–1811
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Table 1 Specific surface area (SBET), mean pore diameter (Dp), titanium
of the most efficient and stable catalyst for the epoxidation
of oleochemicals. Yields in the range of 72–91% for vegetable
free fatty acids were obtained with high selectivity (>98%).22
Similarly, the use of Amano A. lipase (from Aspergillus niger) in
hydrophobic and hydrophilic ionic liquids gave the epoxidized
compound with yields in the range of 67–89% after one hour.23
Since chemoenzymatic systems are highly chemospecific, they
can unfortunately be less versatile for a wide series of oleochem-
ical substrates.
loading of Ti-containing silica catalysts after calcination
Catalyst
S
BET/m2g-1
Dp/nm
Ti content (wt%)
Ti/MCM-41
Ti/MCM-48
Ti/Aerosil
930
982
262
3.6
3.8
n.d.
0.80
0.79
0.64
n.d. = not determined
Previously, some of us have reported that titanium-grafted
silicates are efficient catalysts for the epoxidation of FAMEs
with tert-butylhydroperoxide (TBHP).24,25 Mesoporous materi-
als with 3–10 nm ordered pores are suitable systems for such
transformations as Ti sites are easily accessible by the bulky
reactants and a peculiar cooperative interaction between the
hydrophilic one-dimensional pores of the Ti-MCM-41 enhances
the formation of epoxy FAME derivatives with noteworthy per-
formance. Nevertheless, even though TBHP has the advantage of
high selectivity, it is not the most sought-after oxidant from the
point of view of atom economy and environmental sustainability.
Hydrogen peroxide represents a more promising alternative.26–28
As a main drawback, the use of hydrogen peroxide usually
leads to a rapid deactivation of mesoporous Ti-silica catalysts
due to a clustering of surface Ti species.29 By applying a slow
dropwise addition of hydrogen peroxide in the reaction mixture,
it has been possible to minimize the detrimental irreversible
transformation of isolated Ti centers into titanium oxide-like
species, in cyclohexene epoxidation.30,31
all virtually active and accessible to the reactants. In particular,
Ti(Cp)2Cl2 was grafted over: 1) one-dimensional hexagonal
ordered mesoporous material (MCM-41); 2) three-dimensional
cubic ordered mesoporous material (MCM-48); and 3) non-
porous pyrogenic nanosized silica particles (Aerosil).
The titanium content is comparable in all the samples and it
gave rise to an even dispersion of titanium sites on the silica.
Actually, the surface concentration of Ti sites was always lower
than 0.3 Ti/nm2 and this reduces the aggregation of catalytic
centres during the epoxidation in the presence of hydrogen
peroxide, as previously shown in the cyclohexene epoxidation
on similar materials.30
The DRS-UV-Vis spectroscopic analysis is a tool to evaluate
the coordination state of Ti(IV) (species in tetrahedral coordi-
nation at 220–260 nm and species in octahedral coordination
at 260–290 nm) as well as the degree of isolation of Ti(IV)
in the silica matrix. Indeed, absorption bands above 290 nm
indicate the aggregation of some Ti through the formation
of Ti–O–Ti bonds. The DRS-UV-Vis spectra of the three
samples showed mainly isolated Ti(IV) sites in Ti/MCM-41
and Ti/MCM-48 with a higher proportion of tetrahedral Ti
for MCM-41. For Ti/Aerosil a broader distribution of Ti sites
was evidenced by a larger band revealing not only the presence
of Ti(IV) isolated sites in octahedral coordination, but also some
additional aggregation of Ti centres (Figure S1 – supplementary
information†).
In the present work, we have applied the experimental
protocol of the controlled slow addition of aqueous hydrogen
peroxide to the epoxidation of methyl oleate. Thanks to this
procedure we expect to reduce the decomposition of the oxidant,
increase the catalyst stability and reach high yields in epoxide.
Results and discussion
The results obtained in the methyl oleate epoxidation with
hydrogen peroxide are summarized in Table 2.
Titanium-grafted silica materials were used as heterogeneous
catalysts in the liquid-phase epoxidation of methyl oleate with
hydrogen peroxide under batch conditions.
Earlier, some of us reported that Ti(IV) sites grafted onto
the walls of a well-ordered one-dimensional mesoporous
MCM-41 showed much better performances in the selec-
tive epoxidation of fatty acid methyl esters with TBHP
than compared with Ti(IV) centres grafted onto non-ordered
mesoporous or nonporous silicas.24,25 This particular be-
haviour was attributed to a synergistic interaction between
the elongated shape of quasi-linear FAMEs (especially the
methyl elaidate25) and to a favoured approach of par-
tially oxidised FAMEs (especially methyl ricinoleate34 and
Titanium active centres were obtained by grafting titanocene
dichloride (Ti(Cp)2Cl2), as previously proposed by Maschmeyer
et al.32 and then optimized by some of us for a wide series of
silica supports.33 Three silica materials with different structural
and textural features (Table 1) were used as supports to evaluate
the influence of the silica support on the catalytic performance.
The main advantage of such grafting methodology is that
Ti(IV) sites are situated on the surface, are well dispersed and are
Table 2 Liquid-phase epoxidation of methyl oleate with aqueous hydrogen peroxide over Ti-silica catalysts after 24 h reaction
Entry
Catalyst
C (%)
S EPOX (%)
S ENON (%)
S KETO (%)
Y EPOX (%)
cis/trans EPOX ratio
Ox. Eff. (%)
1
2
3
4
5
Ti/MCM-41
Ti/MCM-48
Ti/Aerosil
MCM-41
52
52
47
33
24
83
78
83
50
40
11
12
13
25
38
6
43
41
39
17
10
80 : 20
72 : 28
78 : 22
38 : 62
26 : 74
39
39
38
25
18
10
10
22
22
No catalyst
Reaction conditions: 1.5 mmol methyl oleate, 2 mmol aq. H2O2 (50%), 50 mg catalyst, H2O2:substrate molar ratio = 1.3 : 1, CH3CN, 24 h, 85 ◦C,
dropwise oxidant addition 0.01 ml min-1. C: substrate conversion; S: selectivity; Y: yield; Ox. Eff.: oxidant efficiency.
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methyl vernolate24) to the inner hydrophilic pore surface of
MCM-41.
In contrast, in this report, all materials (Ti/MCM-41,
Ti/MCM-48 and Ti/Aerosil; Table 2, entries 1 to 3) showed
very similar behaviour in terms of conversion, selectivity and
yield, with values of ca. 50%, 80% and 40%, respectively. Such
results are in the same order of magnitude with those obtained
over Ti-BEA.16 They are also the first ones obtained, to our
best knowledge, in methyl oleate epoxidation with aqueous
hydrogen peroxide over Ti-grafted silica materials. Interestingly,
the presence of some Ti–O–Ti domains in Ti/Aerosil (as
evidenced by DRS-UV-Vis absorption bands above 300 nm)
did not affect negatively the epoxidation performance and such
behaviour can be attributed to the favorable conditions due to
the slow addition of oxidant.35
In addition, in these examples, it is evident that the morphol-
ogy and the texture of the silica support do not play a key role
in terms of catalytic activity (Table 2, entries 1–3). In fact, when
hydrogen peroxide is used as oxidant instead of the sterically
more demanding TBHP, the steric interactions between the
FAME substrate and the porous catalyst are not decisive and
the morphology of the catalyst is not a critical factor.
Such a result is consistent with other recent observations
on different Ti-based catalytic systems, where, in the narrow
interlayer space of a Ti-containing layered clay, the epoxidation
reaction takes place with H2O2 as oxidant, but it does not with
TBHP.36
In terms of selectivity, methyl epoxystearate (EPOX), the
epoxidised derivative, is the major product (ca. 80%). The most
relevant side products are methyl oxooctadecenoate (ENON;
a mixture of methyl 8-oxooctadec-9-enoate and methyl 11-
oxooctadec-9-enoate) and methyl oxooctadecanoate (KETO;
a mixture of methyl 9-oxooctadecanoate and methyl 10-
oxooctadecanoate) (Scheme 1). ENON is obtained by allylic
oxidation of methyl oleate, whereas KETO is formed via oxirane
ring rearrangement from the epoxide. In both cases, no partic-
ular regioselectivity was detected and the two regioisomers for
each product were obtained in practically equimolar amounts.
The selectivity values to ENON and KETO are ca. 10% over
the three catalysts. Blank reactions with pure silica MCM-41
(without Ti; Table 2, entry 4) and with no catalyst (Table 2, entry
5) led to 17% and 10% yield in methyl epoxystearate, respectively.
This behaviour shows that uncatalysed oxidation pathways
have a non-negligible influence under these conditions and the
presence of purely siliceous MCM-41 is not fully innocent.
Actually, in the absence of Ti, most of the methyl epoxystearate
observed is due to a free-radical oxidation process (vide infra).
In terms of stereoselectivity, the epoxidation of methyl oleate
can lead to the formation of cis and trans-epoxides depending
on the reaction pathway. In fact, when TBHP is used as
oxidant, the epoxidation of cis methyl oleate proceeds via a
heterolytic mechanism with retention of the C C double bond
configuration.25,37 On the contrary, if a free-radical pathway
prevails over the heterolytic one, the formation of the trans form,
the thermodynamically more stable isomer, is predominant. For
this reason, in the absence of Ti (Table 2; entries 4–5) the
formation of methyl trans-epoxystearate dominates.
Scheme 1 Products of the reaction: methyl epoxystearate (EPOX),
methyl oxooctadecanoate (KETO) and methyl oxooctadecenoate
(ENON).
the achieved results (slightly less than 40%; Table 2) are interest-
ing and comparable with the values obtained on homogeneous
and heterogeneous systems.16,17,38 Also for this parameter, the
slow dropwise addition of the oxidant helps in reducing the
disproportionation of H2O2 into O2 and H2O.21,31
To improve the final epoxide yield different strategies have
been contemplated and Ti/MCM-41 was used for this opti-
mization. By increasing the oxidant/substrate molar ratio from
1.3 to 4.0, the yield to epoxide rose, but, at the same time, an
increase in the formation of trans-epoxide was observed as well
(Table 3; entries 2 and 4). Such an increase can be explained by
a higher proportion of the free-radical pathway of the reaction
taking place when an excess of hydrogen peroxide is present.
Conversely, by increasing the amount of catalyst, the con-
version of the reaction rises from 52% to 96% and a high
selectivity in epoxide (ca. 90%) is maintained (Table 4, entries
1–5). By augmenting the amount of Ti (substrate : Ti molar
ratio from 180 to 22), it was possible to increase the yield
from 43% up to 91%. In this case, no remarkable influence on
stereoselectivity was noted, the formation of cis-epoxide being
always prevalent (around 80%). Nevertheless, the combination
of a higher quantity of Ti/MCM-41 (200 mg) and a large excess
of H2O2 (6 mmol) did not lead to a sensible improvement, since
the EPOX yield attained 75% with the formation of a lower
fraction of cis-epoxide (Table 3, entry 4).
In terms of oxidant efficiency (the ratio between the moles of
oxidised products produced and the moles of H2O2 consumed),
Such results in methyl epoxystearate yields (up to 91%) are
the highest data obtained in literature so far over titanium-silica
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Table 3 Influence of the amount of hydrogen peroxide on the catalytic performance after 24 h reaction. Tests performed over Ti/MCM-41
cis/trans
Entry Catalyst (mg) H2O2 (mmol) C (%) S EPOX (%) S ENON (%) S KETO (%) S others (%) Y EPOX (%) EPOX ratio Ox. Eff. (%)
1
2
3
4
50
50
200
200
2
6
2
6
52
83
73
82
83
82
96
92
11
12
1
6
6
2
3
5
0
1
0
43
68
70
75
80 : 20
61 : 39
92 : 8
39
20
55
20
5
74 : 26
Reaction conditions: 1.5 mmol methyl oleate, CH3CN, 24 h, 85 ◦C, dropwise oxidant addition 0.01 ml min-1. C: substrate conversion; S: selectivity;
Y: yield; Ox. Eff.: oxidant efficiency.
Table 4 Influence of the amount of catalyst on the catalytic performance after 24 h reaction. Tests performed over Ti/MCM-41
cis/trans
Entry Catalyst (mg)
C (%)
S EPOX (%)
S ENON (%) S KETO (%) S others (%)
Y EPOX (%) EPOX ratio
Ox. Eff. (%)
1
2
3
4
5
50
52
47
73
78
96
83
83
96
86
95
11
9
1
5
3
6
8
2
2
3
5
0
1
7
0
43
39
70
67
91
80 : 20
78 : 22
92 : 8
72 : 28
85 : 15
39
35
55
54
73
100
200
300
400
Reaction conditions: 1.5 mmol methyl oleate, 2 mmol aq. H2O2 (50%), H2O2 : substrate molar ratio = 1.3 : 1, CH3CN, 24 h, 85 ◦C, dropwise oxidant
addition 0.01 ml min-1. C: substrate conversion; S: selectivity; Y: yield; Ox. Eff.: oxidant efficiency.
molecular sieve catalysts with aqueous hydrogen peroxide. In
addition, the effectiveness of oxidant use is promising also in
terms of oxidant to catalyst and oxidant to substrate ratios.
Under optimized conditions (Table 4; entry 5), the oxidant
to catalyst ratio (mol H2O2/masscat) and oxidant to substrate
ratio (mol mol-1) are as low as 0.005 and 1.3, respectively.
Actually, the value of oxidant to catalyst ratio is better than
the results typically reported in literature, spanning from 0.008
in the epoxidation of methyl oleate over Ti-BEA,16 up to 0.1, in
soybean oil epoxidation over Ti/SiO2.21
Finally, it is important to understand the reasons leading
to trans-epoxide formation under the tested conditions. Since
in the reaction medium acetonitrile and hydrogen peroxide are
simultaneously present, it can be assumed that the formation of
trans-epoxide is due to a Payne-type free-radical reaction, where
peroxyimidic acid is formed in situ, typically under alkaline
conditions.39 Actually, specific tests have confirmed that the
formation of peroxyimidic acid occurs and Payne oxidation
takes place.
Epoxidation tests carried out in polar solvents other than
acetonitrile showed poorer performance. Similar conversion,
but lower selectivities, were obtained in ethyl acetate, while
in dichloromethane comparable epoxide selectivity, but lower
conversions, were found. The optimal behaviour observed with
acetonitrile is consistent with previous reports in the epoxidation
of other substrates. In fact, acetonitrile, thanks to its slightly ba-
sic nature, minimizes the formation of acid-catalysed secondary
products.30,41
The Ti-silica catalysts were easily recovered by filtration,
washed, calcined and reused in a second catalytic run. In
particular, after the first recycle, Ti/MCM-41 showed a loss of
ca. 3% in both conversion and selectivity. As observed on differ-
ent lighter alkenes (cyclohexene or unsaturated terpenes30,42),
it is worth noting that a complete removal of the organic
side products adsorbed on the catalyst surface is necessary to
restore the original activity of the fresh catalyst. Because of this
reason, prior to recycling, the solids were carefully washed with
methanol and then calcined under dry air at 500 ◦C.
However, some of us have recently reported that there is no
influence of the Payne reaction in the epoxidation of cyclohexene
over Ti(IV)-silica catalysts in acetonitrile with dropwise addition
of hydrogen peroxide under fully comparable conditions.30
To understand the apparent contradiction, i.e. Payne-type
oxidation occurs only in the epoxidation of methyl oleate, it
must be recalled that commercial methyl oleate samples (even
reagent grade) always possess some mild alkalinity, due to
residual amounts of basic transesterification catalysts during the
production steps40 and such alkalinity promotes the formation
of peroxyimidic acid in the reaction mixture. In fact, if methyl
oleate undergoes a repeated rinsing and washing with 0.1 M
aqueous HNO3, trans-epoxide formation is reduced (from 7% to
3% in trans-EPOX yield and cis/trans EPOX ratio from 26 : 74
to 50 : 50).
Experimental
Preparation of ordered mesoporous silicas
MCM-41. MCM-41 was prepared with the molar ratio, 1
SiO2 : 0.1 CTAB : 0.27 NaOH : 32 H2O.43 19.06 g H2O, 0.356 g
NaOH and 1.204 g cetyltrimethyl ammonium bromide (CTAB,
Aldrich) were mixed at 50 ◦C until a homogeneous solution
was obtained and then 2 g of silica (Aerosil 200, Degussa)
were added stepwise and stirred for 1 h. Then the mixture
was put in an autoclave at 115 ◦C for 24 h. The resulting
slurry was filtered and washed until neutral pH and dried at
80 ◦C overnight. The powder was then calcined at 550 ◦C
for 8h.
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MCM-48. MCM-48 was prepared with the molar ratio, 1
SiO2 : 0.175 CTAB : 0.38 NaOH : 120 H2O.44 214 g H2O, 1.54 g
NaOH and 6.22 g cetyltrimethyl ammonium bromide (CTAB,
Aldrich) were mixed at 50 ◦C until a homogeneous solution
was obtained and then 6 g of silica (Aerosil 200, Degussa) were
added stepwise and stirred for 2 h. Then the mixture was put in
an autoclave at 150 ◦C for 15 h. The resulting slurry was filtered,
without washing, and dried at 80 ◦C overnight. The powder was
poured again in an autoclave with water (7.5 g H2O per gram of
solid), the mixture was stirred for 20 min and the autoclave put
at 130 ◦C for 6 h. This post-treatment was repeated once again.
The resulting slurry was filtered and washed until neutral pH
and dried at 80 ◦C overnight. The powder was then calcined at
550 ◦C for 8 h.
During recycling tests, the catalyst was filtered off, washed with
methanol and acetonitrile, calcined at 500 ◦C for 3 h under dry
air, cooled in vacuo and reused under the same conditions.
Catalytic performance was determined on GC analysis
(HP6890; HP-5 30 m – column; FID detector). Methyl palmitate
was added as internal standard. All data were obtained from an
average of at least three catalytic tests. Methyl oleate conversion
was computed considering the substrate as the limiting agent.
At the end of each reaction, minimal amounts of the oxidant
were detected by potassium iodide colorimetric tests. For this
reason, oxidant efficiency (Ox. Eff.) was calculated, as follows:
ꢀ
Ox. Eff. (%) = 100% ¥ ( moloxidised products)/(molH consumed)
O
2
2
considering the full consumption of the initial hydrogen peroxide
after 24 h. Standard deviations on conversion and selectivity
values are 2% and 4%, respectively.
Preparation of Ti-silica catalysts
Catalysts were prepared by grafting titanocene dichloride
(TiCp2Cl2; Fluka) onto different silicas (MCM-41, MCM-48,
Aerosil), as previously described.32,42 Aerosil 380 is commercially
available from Degussa. Before grafting, the supports were
pretreated at 500 ◦C for 2 h in air and then in vacuo at the
same temperature for 2 h. Titanocene dichloride was dissolved in
anhydrous chloroform (Sigma-Aldrich) under argon and stirred
for 2 h at room temperature. Triethylamine (Sigma-Aldrich) or
pyridine (Fluka) was then added to the suspension and left
overnight under stirring to activate the nucleophilic substitution
of surface silanols onto titanocene. A weaker base, pyridine, was
used in the grafting over MCM-41 and MCM-48 to protect the
support. After filtering, Ti(IV) active centres were obtained after
calcination under dry oxygen at 550 ◦C for 3h.
Conclusions
Heterogeneous porous and non-porous grafted titanium-silica
catalysts proved to be sustainable and efficient catalysts in the
epoxidation of methyl oleate with aqueous hydrogen perox-
ide. The different textural and morphological features of the
mesoporous catalysts did not noticeably affect their catalytic
performance. Thanks to the applied protocol of dropwise
oxidant addition, the hydrogen peroxide decomposition and
local water concentration into the proximity of Ti(IV) sites were
minimized. By optimizing the experimental conditions, yields as
high as 91% in the desired methyl epoxystearate were achieved.
Such results are the highest data obtained in literature so far over
titanium-silica molecular sieve catalysts with aqueous hydrogen
peroxide.
Textural and spectroscopic characterization
Acknowledgements
Nitrogen adsorption/desorption isotherms of materials were
measured using a Micromeritics ASAP 2010 instrument. The
calcined samples were outgassed at 250 ◦C and titanocene-
containing samples at 120 ◦C until stable static vacuum of 3
¥ 10-3 Torr was reached. Mesopore diameters were calculated
from the desorption branch of the nitrogen isotherms by the
Broekhoff and de Boer (BdB) method,45 which has been shown
to provide reliable results for MCM-41 materials.46
UV-Vis diffuse reflectance spectra were obtained in a Varian
Cary 05E UV-Vis-NIR spectrophotometer using BaSO4 as
background standard. Titanium content was determined by
ICP-AES on an Intrepid Iris instrument (Thermo Elemental).
This work was supported by the European Community’s
Seventh Framework Programme through the Marie Curie
Initial Training Network NANO-HOST, (Grant Agreement
no. 215193). M. G., R. P. and N. R. also acknowledge the
Italian Ministry of Education and Scientific Research through
the network “ItalNanoNet” (protocol no. RBPR05JH2P) and
Regione Lombardia (“VeLiCa - From ancient crops materials
and products for the future" project) for financial support.
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Catalytic tests
◦
All catalysts were pretreated at 500 C in dry air for 1 h prior
to use. The epoxidation tests were carried out in a glass batch
reactor under inert atmosphere at 85 ◦C. Typically, the solution
of methyl oleate (1.5 mmol; Aldrich) in 5 ml of acetonitrile was
added to the catalyst (50 mg). A solution of hydrogen peroxide
(50% aqueous solution) in solvent (2 mmol H2O2 in 2.4 mL of
solution) was slowly added (0.01 ml min-1 over 4 h) with an
automatic dosimetric apparatus. Hot filtration tests followed
by separation of the solid catalyst were performed and the
resulting solution was tested in catalysis to evaluate the leaching
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