Molybdenum-MCM-41 silica as heterogeneous catalyst for olefin epoxidation

Article abstract of DOI:10.1016/j.molcata.2014.01.028

MCM-41-supported molybdenum/bis-dithiocarbamate complex can be efficiently utilized, after treatment with tert-butylhydroperoxide (TBHP), for the epoxidation of alkenes under solventless conditions. The treatment with TBHP allows the formation of the real catalyst through oxidative decomposition of the complex affording well dispersed Mo(VI) species grafted onto the silica surface through the silanol groups. Experimental results, catalytic efficiency and spectroscopy data, allow to advance some hypotheses on the molybdenum-grafted catalyst formation. The grafted catalyst can be reused several times in the model epoxidation of cyclohexene affording the epoxide with very good yield; only during the first run a modest molybdenum leaching is observed. Both cyclic and linear alkenes can be epoxidized in good to excellent yields and selectivities.

Full text of DOI:10.1016/j.molcata.2014.01.028

Journal of Molecular Catalysis A: Chemical 386 (2014) 108–113
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Molybdenum-MCM-41 silica as heterogeneous catalyst for
olefin epoxidation
Franca Bigia, Calogero Giancarlo Piscopoa, Giovanni Predieria, Giovanni Sartoria,
Roberto Scottib, Robertino Zanonic, Raimondo Maggia,∗
a Clean Synthetic Methodology Group, Dipartimento di Chimica dell’Università, Parco Area delle Scienze 17A, I-43124 Parma, Italy
b Dipartimento di Scienza dei Materiali, INSTM, Università di Milano-Bicocca, Via R. Cozzi 53, I-20125 Milano, Italy
c Dipartimento di Chimica, Università degli Studi di Roma “La Sapienza”, P.le Aldo Moro 5, 00185 Roma, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
MCM-41-supported molybdenum/bis-dithiocarbamate complex can be efficiently utilized, after treat-
ment with tert-butylhydroperoxide (TBHP), for the epoxidation of alkenes under solventless conditions.
The treatment with TBHP allows the formation of the real catalyst through oxidative decomposition of
the complex affording well dispersed Mo(VI) species grafted onto the silica surface through the silanol
groups.
Received 6 December 2013
Received in revised form 30 January 2014
Accepted 31 January 2014
Available online 10 February 2014
Experimental results, catalytic efficiency and spectroscopy data, allow to advance some hypotheses on
the molybdenum-grafted catalyst formation.
The grafted catalyst can be reused several times in the model epoxidation of cyclohexene affording the
epoxide with very good yield; only during the first run a modest molybdenum leaching is observed.
Both cyclic and linear alkenes can be epoxidized in good to excellent yields and selectivities.
© 2014 Elsevier B.V. All rights reserved.
Keywords:
Alkene epoxidation
Supported catalysts
Molybdenum
EPR spectroscopy
XPS spectroscopy
1. Introduction
However, severe troubles are frequently encountered with the
attempts to immobilize proven homogenous catalysts; first of all
The conversion of alkenes into epoxides by reaction with a vari-
ety of benign oxidants such as oxygen, hydrogen peroxide and
alkyl hydroperoxides in combination with cheap, safe and reusable
heterogeneous catalysts shows great interest due to the potential
applicability to large scale production [1,2].
Furthermore, solid catalysts can be easily utilized in flow pro-
cesses that give further advantages associated with the specific
experimental conditions allowing precise reaction control through
rapid heat transfer and mixing [3,4].
Among the great number of heterogeneous catalysts studied in
the alkene epoxidation, materials produced by supporting highly
active homogeneous catalysts are particularly attractive because
of the possibility to combine the advantages of the homogeneous
catalysis with those derived from the use of thermally stable, het-
erogeneous materials such as zeolites, clays, polystyrenic resins and
silicas. These solid catalysts can be obtained through tethering or
grafting procedures [5]; concerning this last procedure, the Schiff
bases are particularly employed. Recently magnetically recyclable
nanocomposite catalysts have been also utilized [6].
the anchoring of the ligand–metal complex at the solid support
could not be chemically robust enough, and both metal and ligand
could leach into the solution resulting in catalyst deactivation and
contamination of the final products. Thus, improved anchoring
techniques combined with detailed catalyst characterization are
needed.
The activity of the supported catalysts is, in general, lower than
their homogeneous counterparts; however, some species show
enhanced selectivity and longer lifetime due to the site isolation
that prevents bimolecular catalyst deactivation [7].
Molybdenum-incorporating heterogeneous catalysts have been
utilized in
a great number of important reactions such as
alkane oxidation [8,9], alkene metathesis [10,11] and selective
alcohol oxidation [12]. Moreover, molybdenum, such as other
metals with low oxidation potential and hard Lewis acidity in
their highest oxidation state, has been utilized as active cata-
lyst in the epoxidation of alkenes with hydrogen peroxide or
tert-butylhydroperoxide: some interesting examples are the use
of MoY zeolites [13], molybdenum-polybenzimidazole adduct
[14], molybdate-exchanged layered double hydroxides [15] and
molybdenum-containing sol–gel materials [16].
In this regard, covalently grafted Mo(VI) complexes with nitro-
gen donor ligands have been found to be active catalysts or catalyst
precursors for olefin epoxidation by using tert-butylhydroperoxide
∗
Corresponding author. Tel.: +39 521 905411; fax: +39 521 905472.
E-mail address: raimondo.maggi@unipr.it (R. Maggi).
1381-1169/$ – see front matter © 2014 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.molcata.2014.01.028

F. Bigi et al. / Journal of Molecular Catalysis A: Chemical 386 (2014) 108–113
109
(TBHP) as oxygen source. Unfortunately, in some cases and par-
ticularly for monodentate nitrile and bidentate bipyridyl ligands,
the catalysts are unstable towards molybdenum leaching, which
accounted for nearly complete loss of activity in the second cat-
alytic run [17,18]. More stable molybdenum catalysts, covalently
bound to MCM-41 mesoporous silicas and aluminosilicates, were
obtained by grafting chelate complexes of 1,4-diazabutadiene [19]
and (2-pyridyl)-1-pyrazolylacetamide [20] ligands with MoO2Cl2
and MoO(O2)2, respectively. They were found to be effective and
truly heterogeneous catalysts for olefin epoxidation. More recently,
siliceous MCM-41 materials containing highly dispersed MoOx
units, active in olefin epoxidation, were obtained by reaction of
Mo(VI) peroxo species and silica precursors such as tetraethy-
lorthosilicate (TEOS) [21].
In the present paper, we report the preparation, characterization
and catalytic performance in the olefin epoxidation of molybdenum
species grafted on MCM-41 silica; this new preparation involves the
tethering of Mo(VI) dithiocarbamate (DTC) complex to mesoporous
MCM-41 and successive in situ transformation into the silica grafted
molybdenum species by oxidative degradation of the ligand. This
method allows the use of less expensive and easy-to-handle sodium
molybdate, as molybdenum starting material, in place of oxochloro
or oxodiperoxo species [22] or the expensive CpMoCl2 [23]. To the
best of our knowledge, the only use of molybdenum-DTC com-
plexes to prepare heterogeneous catalysts is that reported by Qian
et al. [24] for the hydrodesulfurization process.
accuracy of reported BEs was 0.2 eV, and the reproducibility of
the results was within these values. XPS atomic ratios are relative
values, intrinsically affected by a 10% error. The spectra were col-
lected by a DAC PDP 11/83 data system and processed by means of
VG 5000 data handling software.
EPR spectra were recorded on a Bruker EMX spectrometer
working at the X-band frequency, equipped with an Oxford cryo-
stat. Spectra were recorded at 123 K. Modulation frequency was
100 kHz, modulation amplitude 5 gauss, microwave power 10 mW.
The g values were measured by standardisation with 2,2ꢀ-diphenyl-
1-picrylhydrazyl (DPPH). The amount of Mo paramagnetic centres
(espressed as % of the total amount of Mo) was calculated by double
integration of the resonance lines referring the area to a calibration
curve (area of EPR signal vs. concentration) of Cu(NO3)2 1/2H2O in
frozen ethylene glycol-water solutions.
2.2. Catalyst preparation
MCM-41 silica was prepared following a reported procedure
[25,26]; its structure was confirmed by XRD analysis (chan-
˚
nel dimension: 39 A), and the BET analysis showed a surface
area of 1050 m2/g,
a
Vtot of 1.130 cm3/g and the following
pore volume distribution (cm3/g): 0–2 nm = 0.007, 2–4 nm = 0.775,
4–10 nm = 0.154, >10 nm = 0.194. The material MCM-41-(CH2)3-
NHCH3 (1) was prepared according to
a method described
in the literature [27] with some adjustments: a mixture of
MCM-41 silica (6 g) pre-heated at 300 ◦C for 16 h, [3-(N-
methylamino)propyl]trimethoxysilane (6.1 mol, 1.2 g) and xylenes
(30 ml) were refluxed under stirring for 15 h. The resulting solid
was filtered at rt, washed with hot xylenes (5 × 10 ml) and dried
under vacuum [N loading: 1.33 mmol/g (elemental analysis)].
The MCM-41 silica supported oxomolybdenum bis-
dithiocarbamate (3) (Scheme 1) was prepared by conveniently
modifying the procedure early reported for production of
molybdenum-dithiocarbamate complexes under homogeneous
conditions [28,29] or supported on polystyrene [30]. The MCM-
41-(CH2)3NHCH3 material (1) (500 mg) was dispersed in distilled
water (10 ml) with stirring under nitrogen. 0.1 N NaOH aq. solution
was added in a 1:1 molar ratio with respect to the supported
amine. The mixture was stirred at rt for 10 min. Carbon disulfide
was added in 1:1 molar ratio with respect to NaOH and the slurry
was stirred for 15 min becoming pale yellow. To the pale yellow
slurry an aq. solution of Na2MoO4·H2O (1.2:2 molar ratio with
respect NaOH) was added and the mixture was stirred for 15 min.
1 M aq. HCl solution was added dropwise until pH < 1 and the
slurry turned to brown colour. The powder was filtered on Buchner
funnel and washed with ethanol (10 ml) and methylene chloride
(10 ml). After washing, the green powder 3 was dried under
vacuum [molybdenum loading: 0.26 mmol/g (ICP–MS); surface
area: 970 m2/g].
2. Experimental
2.1. Materials and equipment
Cyclohexene (≥99.0%, ∼0.01% BHT as stabilizer), cyclooctene
(≥99.5%), cis,cis-1,5-cyclooctadiene (≥98.0%), 1-octene (98%) and
trans-2-hexene (97%) were purchased from Aldrich and used with-
out further purification. The other reagents, solvents, and standards
were purchased in the highest purities available and used without
further purification: xylene (mixture of isomers), dichloromethane,
ethanol, carbon disulfide, ethyl acetate, bromobenzene, cyclo-
hexene oxide, cyclooctene oxide, 1-methylcyclohexene oxide,
1-octene oxide, TBHP (6 M in decane), cetyltrimethylammonium
chloride (25 wt% in water), sodium silicate (27% SiO2, 14% NaOH
in water) and sodium molybdate dihydrate ≥99% from Aldrich,
3-(methylamino)propyltrimethoxysilane from Fluka.
Metal elemental analyses were performed by ICP-AES on Ultima
2 Jobin Yvon HORIBA instrument.
FT-IR spectra of all the catalysts (KBr pellets) were recorded on a
Nicolet FT-IR Nexus spectrophotometer (resolution 4 cm−1) in the
range of 4000–400 cm−1
.
X-ray diffraction analyses were performed on Philips PW1710
instrument.
Gas-chromatographic analyses were accomplished on a TraceGC
ThermoFinnigan instrument (FID).
2.3. Catalytic reactions
N2 adsorption-desorption isotherms, obtained at 77 K on a
Micrometrics PulseChemiSorb 2705, were used to determine spe-
cific surface areas, SABET. Before each measurement the samples
were outgassed at 383 K for 1 h.
XPS spectra were run on a Vacuum Generators ESCALAB spec-
trometer, equipped with a hemispherical analyser operated in the
Fixed Analyser Transmission (FAT) mode, with a pass energy of 20
or 50 eV. Al K␣1,2 or Mg K␣1,2 photons (hꢀ = 1486.6 and 1253.6 eV,
respectively) were used to excite photo-emission. The binding
energy (BE) scale was calibrated by taking the Au 4f7/2 peak at
84.0 eV. Correction of the energy shift due to static charging of the
samples was accomplished by referencing to the C 1s line from
the residual pump line oil contamination, taken at 285.0 eV. The
The catalytic tests for the epoxidation reactions were performed
by procedures reported in the notes of table and figures. The general
procedure was as follows: in a stirred batch reactor equipped with
condenser and thermometer the selected alkene (20 mmol) and the
catalyst (0.1 g, corresponding to 0.026 mmol of molybdenum) were
mixed together and the mixture was heated at 80 ◦C under stirring
(70 ◦C for 2-hexene). TBHP (2 mmol, 0.36 mL of 6 M decane solution)
was added by a syringe pump during 3 h at the same temperature.
Heating and stirring were continued for one additional hour. The
reaction mixture was then cooled to rt; ethyl acetate (20 ml) was
added and the solid catalyst was removed by filtration, washed
with ethyl acetate (10 ml) and dried under vacuum. The reaction
mixture was analyzed by high resolution capillary GC with a fused

110
F. Bigi et al. / Journal of Molecular Catalysis A: Chemical 386 (2014) 108–113
Me
N
H
Me
N
S-Na+
OH
OH
NaOH, CS2
H2O
S
S
SiO2
SiO2
OH
OH
S-Na+
H
N
N
Me
Me
1
2
Me
N
S
O
O
OH
OH
S
S
Na2MoO4
H2O
Mo
SiO2
S
N
Me
3
Scheme 1. Preparation of dioxomolybdenum bis-dithiocarbamate complex tethered on MCM-41 silica.
silica capillary column SPB-20 from Supelco (30 m × 0.25 mm)
using decane as internal standard. The products were identified by
comparing their spectral data with those reported in the literature.
of the total loaded molybdenum). The isotropic shape of the
signal indicates that the paramagnetic species are not magneti-
cally isolated and that a probable exchange interactions within
neighbouring Mo(V) centres occurs [34].
The catalytic activity of material 3 was studied in the model
epoxidation of cyclohexene 5 with TBHP (Scheme 2), and the effect
of various reaction parameters on the effectiveness of the catalyst
was evaluated.
3. Results and discussion
As described in the Experimental section, the procedure adopted
in the preparation of the supported molybdenum-catalyst implies
the following steps sketched in Scheme 1:
In a first experiment, the reaction was performed without sol-
vent by adding TBHP (2 mmol) in one portion to a mixture of
the catalyst (100 mg, corresponding to 0.026 mmol of molybde-
num) with a large excess of cyclohexene (20 mmol) previously
heated at 80 ◦C. The reaction mixture was stirred at this temper-
ature for 4 h, cooled to room temperature, filtered to remove the
catalyst, and then it was analyzed by GLC. Cyclohexene epoxide
6 was the main reaction product (50% yield). Modest amounts of
2-cyclohexene-1-ol (7) and 2-cyclohexene-1-one (8) (2–5%) have
been also recognized due to the competitive allylic oxidation of
cyclohexene.
After this preliminary experiment the catalyst colour turned
from green to yellow; this change could be attributable to a vari-
ation of the molybdenum oxidation state due to the presence of
TBHP. To investigate this behaviour, initially we studied the trans-
formation of the catalyst in the presence of the oxidant: a sample
of catalyst 3 (100 mg) was refluxed with TBHP (0.36 mL of a 5.5 M
decane solution) in 1,2-dichloroethane (3 mL) for 2 h (as expected
during this treatment a change in the colour of the catalyst from
green to yellow was observed). The activity of the as prepared and
the TBHP-treated catalysts was compared with that of the recov-
ered catalyst after the first run. Reaction profiles are shown in Fig. 3.
The TBHP-treated catalyst (᭹) showed a slightly higher initial
rate with respect to the as prepared (ꢀ) and the recovered cata-
lysts (ꢁ).The plateau was, respectively, reached at 50%, 47% and
42% cyclohexene epoxide yield with respect to the added TBHP;
the cyclohexene epoxide yields of the 3rd and 4th runs were
around 40%. Moreover, the as prepared and the recovered catalysts
needed an induction period of about 20 and 50 min, respectively.
During this period part of the oxidant is consumed in the oxida-
tive transformation of the catalyst. These results can be, to some
(i) reaction of MCM-41 silica with [3-(N-methylamino)propyl]
trimethoxysilane giving material (1);
(ii) production of MCM-41 supported DTC (2) by reaction of mate-
rial (1) with NaOH/CS2;
(iii) production of MCM-41 supported Mo(VI)-bis-DTC complex (3)
by reaction of supported DTC with sodium molybdate.
Regarding the nature of the complex 3 obtained in step (iii),
it should have a structure similar to that of complex 4 (Fig. 1),
obtained by reacting sodium molybdate with pure diethyldithio-
carbamate [28], as 3 shows the characteristics IR bands attributable
to C–N (1540 cm−1) and Mo=O (945 and 903 cm−1) stretching
modes, slightly shifted with respect to the same bands observed in
the homogeneous counterpart 4 (1510, 920 and 880 cm−1) [28,29].
In addition, the EPR spectrum of the as prepared material
Mo(VI)-bis-DTC-MCM-41 (3) shows an isotropic signal at g = 1.943
(Fig. 2(a)), attributed to adventitious Mo(V) species with a d1
configuration [31–34], whose amount is very small (∼1.2 × 10−3
%
Fig. 1. Structural sketch of bis (N,N-diethyldithiocarbamate)dioxomolibdenum(VI).

F. Bigi et al. / Journal of Molecular Catalysis A: Chemical 386 (2014) 108–113
111
100
g = 1.943
(a)
(b)
90
80
70
60
50
40
30
20
10
0
Recovered catalyst
As prepared catalyst
Oxidized catalyst
0
30
60
90
120
150
180
210
240
t (min)
Fig. 3. Reaction profiles for cyclohexene epoxidation with TBHP over Mo-MCM-41
silica catalyst at 80 ◦C. Reaction conditions: cyclohexene (20 mmol), TBHP (2 mmol),
Mo-MCM-41 silica (100 mg corresponding to 0.026 mmol molybdenum). Cyclo-
hexene oxide was the main product accompanied by lower amounts (∼2–5%) of
2-cyclohexen-1-ol (7) and 2-cyclohexen-1-one (8) [yields based on TBHP].
(c)
Furthermore, the comparison of the IR spectra of the as prepared
and the recovered catalysts shows that the active sites undergo
a tremendous transformation in the presence of the oxidant. In
fact, whereas the Mo=O stretchings are present before and after
oxidation at 945 and 903 cm−1, C–N stretching band a 1540 cm−1
disappeared and the band attributed to thiocarbamate group at
1465 cm−1 is severely reduced and leaves a new band centred at
619 cm−1 ascribable to the sulfate groups (ꢀ4). This suggests that
supported Mo-bis-DTC complex is unstable in the presence of TBHP,
the ligand being decomposed by oxidation giving sulfites and sul-
fates and consequently causing the Mo(VI) species to graft onto the
silica surface through the silanol groups, thus initially producing
the rest state catalyst 9 that finally is transformed in the real active
catalyst 10 by reaction with TBHP (Scheme 3).
(d)
3100
3300
3500
3700
3900
During this transformation a portion of the molybdenum ions
is leached into solution as evidenced by XPS analysis of the as
prepared (Mo/Si atomic ratio = 0.24) and oxidized (Mo/Si atomic
ratio = 0.19) catalysts; this lowering of molybdenum content does
not affect the catalytic activity in the model epoxidation reaction.
Furthermore, the present heterogenisation process based on the
“ligand sacrifice” combined with a partial loss of molybdenum,
could afford a catalyst with a well dispersed metal centres on
the surface of the silica matrix. This transformation promotes the
creation of catalysts with unique activities and selectivities by cir-
cumventing the oligomerization of the active monomeric species.
Due to the not so high yield (∼50%) obtained with the one
portion addition of the TBHP, we suspected a possible competitive
TBHP decomposition, as already found for homogeneous Mo(VI)
epoxidations by Sheldon [35]; in an attempt to overcome this
limitation of the present methodology, the model reaction was
repeated by slowly adding TBHP during 3 h through a syringe pump
to the stirred suspension of the catalyst and cyclohexene heated
at 80 ◦C. The heating was then continued for one additional hour.
Under these conditions, the decomposition of TBHP was really
inhibited, and the cyclohexene epoxide yield reached 93% (94%
selectivity), accompanied by traces of 2-cyclohexen-1-ol and 2-
cyclohexen-1-one. This low amount of allylic oxidation products
could also be attributed to the positive activity of BHT (0.01% as
stabilizer of cyclohexene) that prevents the radical side-reactivity
of Mo(VI) catalyst.
Gauss
Fig. 2. EPR spectra recorded at 123 K of the MCM-41 silica supported molybdenum
catalyst: as prepared catalyst 3 (a), catalyst 3 oxidized with TBHP (b), catalyst 3 after
1st run (c) and after 4th run (d).
extent, rationalized on the basis of spectroscopic studies carried
out with the three catalysts. Indeed, in the EPR spectrum the sig-
nal at g = 1.943 [attributed to traces of paramagnetic Mo(V) species]
completely disappears after treatment with TBHP (Fig. 2(b)). How-
ever, after the first experiment of cyclohexene epoxidation in the
presence of TBHP, EPR spectrum shows again the signal of Mo(V)
species at g = 1.943 (Fig. 2(c)). Although its intensity is higher than
that of the as prepared material, it remains very low (∼8.1 × 10−3
%
of the total loaded Mo) and probably due to undesired redox pro-
cesses. Oxidation of Mo-MCM-41 silica with TBHP gives rise to
the conversion of Mo(V) to the active Mo(VI) species and results
in the disappearance of the characteristic intense signal for the
paramagnetic Mo(V) centres. It is reasonable to expect that the
reoxidation to Mo(VI) species is needed to achieve the maximum
efficiency in the successive epoxidation runs.
The catalyst reusability was then studied under the above opti-
mized conditions: four successive runs were performed and at the
end of every run toluene was added and the reaction mixture was
filtered at 80 ◦C, the catalyst was washed with hot toluene and dried
under vacuum and finally it was treated with TBHP and reused in
the successive run with the following results [yield (selectivity) %]:
Scheme 2. Products obtained from cyclohexene oxidation with TBHP.

112
F. Bigi et al. / Journal of Molecular Catalysis A: Chemical 386 (2014) 108–113
Scheme 3. Oxidation of the supported dioxomolybdenum bis-dithiocarbamate complex.
1st: 93 (94), 2nd: 96 (97), 3rd: 97 (98), 4th: 97 (98). It is evident
from the results that the catalyst can be used several times in the
model reaction with some activity improvement upon the first run
despite the observed modest loss of molybdenum in the first run
[Mo content in the catalyst (Mo/Si atomic ratio): 1st: 0.19, 2nd:
0.18, 3rd: 0.18, 4th: 0.18].
In order to prove the heterogeneous nature of the catalyst the
“hot filtration” test [38] was performed, in which the catalyst was
separated from the reaction suspension after 2 h, at approximately
45% cyclohexene epoxide yield through filtration at 80 ◦C. The reac-
tion progress in the filtrate and in the suspension was further
monitored: the filtrate showed a modest increase of cyclohexene
conversion after two additional hours (from 45% to 52%), con-
firming a small active molybdenum species leaching. As expected,
addition of cyclohexene and TBHP to the solid catalyst and heat-
ing at 80 ◦C for four hours resulted in 92% cyclohexene epoxide 6
production. The test was repeated with the reused catalyst: in this
case the clear filtrate was completely inert even at prolonged reac-
tion time. These experiments allow the conclusion that the present
A possible rationale of this slight catalyst activity improvement
upon reuse comes from the comparison of the EPR spectra of the
catalyst after the first and the fourth run (Fig. 2(c) and (d)). The
amount of the Mo(V) species decreases indeed from 8 × 10−3% after
the first cycle to 2.3 × 10−3% after the fourth cycle.
The catalyst transformation in the presence of the oxidizing
reagent has been also followed by XPS analysis. Table 1 shows the
BE values (eV) found in the model dithiocarbamate molybdenum
complex and in the relevant grafted catalysts; Mo/Si atomic ratio
values (AR) are also quoted.
Table 2
Epoxidation of alkenes with TBHP in the presence of Mo-MCM-41 silica catalysta.
The Mo 3d5/2 binding energy values exhibited by the reported
species are all comparable with that of MoO3 (232.8 eV) [36]. No
other components are detectable in the Mo 3d5/2 peaks, indicating
that the Mo(VI) species are largely predominant. On the other hand,
S 2p BEs moves from lower value in material 3, typical of dithiocar-
bamate species [37], to higher value in the final catalyst 9 and in
the reused ones, typical of S(VI) species, as SO3 and sulphates [32].
It is interesting to note that the Mo/Si atomic ratio undergoes a
decrease only in the first oxidation run with TBHP.
Entry Reagent
Product
Yield [selectivity]
(%)
O
O
1
2
93 [94]
61 [90]
O
O
3
96 [97]
Table 1
Binding energy (BE) values (eV) and atomic ratios (AR) obtained by XPS
measurements.
4
5
51 [70]b
Catalyst 3
Catalyst 10
Catalyst 3 after four
catalytic runs
O
65 [95]
80 [97]
BE (eV)
Si 2p
S 2p
Mo 3d5/2
ꢁ(Mo-S)
AR
Mo/Si
103.0
163.7
232.9
69.2
103.2
169.1
232.5
63.4
103.2
169.3
232.5
63.2
O
6
a
Reaction conditions: alkene (20 mmol), TBHP (2 mmol), Mo-MCM-41 silica (0.1 g
corresponding to 0.026 mmol molybdenum), 80 ◦C, 4 h.
0.24
0.19
0.18
b
The selectivity is lower due to the formation of the bis-epoxide (18% yield).

F. Bigi et al. / Journal of Molecular Catalysis A: Chemical 386 (2014) 108–113
113
catalytic system underwent some molybdenum leaching only in
References
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observed with 1,5-cyclooctadiene due to the competitive produc-
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4. Conclusion
A good yield of epoxidation of alkenes with TBHP can be
achieved by using a molybdenum bis-dithiocarbamate complex
supported on MCM-41 silica, provided the solid is suitably treated
with TBHP. In the presence of TBHP the ligand of the supported
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sulfites and sulfates and consequently permitting to graft well dis-
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Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.molcata.2014.
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