Selective epoxidation of alkenes using highly active V-SBA-15 materials: Microwave vs. conventional heating

Article abstract of DOI:10.1039/b910891b

V-SBA-15 materials prepared using a hydrothermal methodology were found to be highly active and selective in the oxidation of a range of alkanes under mild reaction conditions. The activities of the systems were compared under conventional and microwave heating. Microwave experiments demonstrated that the long times of reaction (12-24 h) required under conventional heating could be reduced to a few minutes (15-60) with improved activities and selectivities under similar reaction conditions. The Royal Society of Chemistry 2009.

Full text of DOI:10.1039/b910891b

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Selective epoxidation of alkenes using highly active V-SBA-15 materials:
microwave vs. conventional heating†
*
Maria Jose Jurado, Maria Dolores Gracia, Juan Manuel Campelo, Rafael Luque, Jose Maria Marinas
*
and Antonio Angel Romero
Received 3rd June 2009, Accepted 16th July 2009
First published as an Advance Article on the web 27th August 2009
DOI: 10.1039/b910891b
V-SBA-15 materials prepared using a hydrothermal methodology were found to be highly active and
selective in the oxidation of a range of alkanes under mild reaction conditions. The activities of the
systems were compared under conventional and microwave heating. Microwave experiments
demonstrated that the long times of reaction (12–24 h) required under conventional heating could be
reduced to a few minutes (15–60) with improved activities and selectivities under similar reaction
conditions.
heating and microwave irradiation, using highly active, selective
and stable V-SBA-15 materials, with the only similar work
Introduction
The discovery of the surfactant-templated periodic mesoporous
silicas (PMS) in the 1990s paved the way to the utilisation of
highly useful porous materials in a wide range of applications
including (bio)adsorption,1,2 separation,1,3 (bio)catalysis,2 drug
delivery1,4 and related medical applications.5 The amorphous thin
fragile walls of M41S materials that conferred a poor hydro-
thermal stability were predated by long-range ordered SBA-type
materials presenting enhanced hydrothermal stabilities with more
stable and thicker walls.6 Among them, hexagonal SBA-15 type
materials have been extensively synthesised and reported as
catalysts and supports in a variety of applications.7–9 In the
particular case of catalysis, the isomorphous substitution of Si for
other metals including Al,10 Zr11 and Ti12 in Si-SBA-15 materials
was needed in order to generate acid and/or redox catalytically
active sites on the synthesised solids. Compared to these
commonly employed metals, vanadium has rarely been employed
for such purposes despite its interesting redox properties.13
Selective greener oxidation processes are currently in great
demand3a,13–15 to offer an alternative to conventional stoichio-
metric methodologies employing toxic reagents and harsh reac-
tion conditions which generally lead to poor selectivities and
atom economies. Microwaves combined with heterogeneous
active, stable and reusable solid oxidant catalysts are one of the
most promising strategies to face this challenge. Microwave-
assisted protocols, increasingly trendy over the last decade, have
been reported to increase yields and in some cases selectivities to
target products, reducing in parallel the reaction times and
energy consumption.16 V containing materials have been inves-
tigated in many oxidation processes including the oxidation of
aromatics17 and alkanes.13,18
18
~
reported by Pena et al. and George et al. on V-MCM-41 and
Selvam and Dapurkar19 on V-MCM-48 materials. The effect of
different vanadium precursors on the incorporation of V into the
mesoporous structure was investigated as well as the corre-
sponding effects on the catalytic performance of the materials in
the epoxidation of alkenes.
Experimental
V-SBA-15 materials were synthesised using a methodology previ-
ously reported.10 In brief, a solution containing 8 g Pluronic P123
in 300 mL HCl at pH 1.5 was stirred at room temperature for 2 h.
Tetraethyl orthosilicate (TEOS) as silica source and the desired
quantities of vanadium precursors (vanadium oxytriisopropoxide
and ammonium metavanadate) were stirred for 24 h and then
the solution was aged in an oven at 100 ꢀC for 24 h. The final
solid obtained was then filtered off and subsequently calcined to
extract the template at 600 ꢀC under N2 (4 h) and air (4 h).
In this way, V-SBA-15 materials with Si/V ratios ranging from
40 to 10 were synthesised. Materials were denoted as V-X-O/M
where V makes reference to V-SBA-15 materials, X is the theo-
retical Si/V ratio (40 : 10) in the synthesis gel and O/M stands for
the initials of the vanadium precursor employed in the synthesis
procedure (O ¼ vanadium oxytriisopropoxide; M ¼ ammonium
metavanadate). Thus, the catalyst V-30-M is a V-SBA-15 mate-
rial with an Si/V 30 ratio which has been prepared using
ammonium metavanadate as precursor.
Materials characterisation
In this work, we report that the selective oxidation of a range
of alkenes can efficiently be performed, under both conventional
Mesoporous V-SBA-15 materials were characterised by means of
several techniques including X-ray diffraction (XRD), N2 phys-
isorption, transmission electron microscopy (TEM), UV-Vis and
X-ray photoelectron spectroscopy (XPS).
´
ꢀ
ꢀ
Departamento de Quımica Organica, Universidad de Cordoba, Edificio
Marie Curie, Ctra Nnal IV, Km 396, E-14014 Coꢀrdoba, Spain. E-mail:
q62alsor@uco.es; Fax: +34 957 212066; Tel: +34 957 212065
Textural properties were determined from the N2 adsorption–
desorption isotherms using a volumetric adsorption analyzer
Micromeritics ASAP 2000. Samples were outgassed at 130 ꢀC
under vacuum (p < 10ꢁ2 Pa) for 24 h and subsequently analysed.
† This paper is part of a Journal of Materials Chemistry theme issue on
Green Materials. Guest editors: James Clark and Duncan Macquarrie.
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 8603–8609 | 8603

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The linear part of the BET equation was utilised for the deter-
mination of the specific surface area. Pore size distribution (PSD)
was calculated from the desorption branch of the N2 phys-
isorption isotherms and the Barret–Joyner–Halenda (BJH)
formula. The cumulative mesopore volume VBJH was obtained
from the PSD curve.
(3.15 mL), 4.5 mmol tert-butylhydroperoxide (TBHP, 0.5 mL),
7.4 mL dichloromethane (DCM) and 0.05 g of catalyst were
added to a 20 mL carousel tube and heated at 43 ꢀC (actual
temperature measured using a Ni–Cr metallic thermocouple
inserted periodically into the reaction vessel). Samples were
withdrawn periodically, filtered off and analysed using an
HP5890 Series II Gas Chromatograph (60 mL minꢁ1 N2 carrier
flow, 20 psi column top head pressure) fitted with a capillary
HP-101 column (25 m ꢂ 0.2 mm ꢂ 0.2 mm) and a flame ionisa-
tion detector (FID).
The elemental analysis of the V-SBA-15 materials was carried
´
out at the Laboratorio de Mineralogıa Aplicada (IRICA) of the
Universidad de Castilla La Mancha (UCLM) using X-ray fluo-
rescence spectrometry (XRFS) on a sequential Philips Magix
PRO, with a simple goniometer based on channel measurements,
under Rh Ka radiation. Powdered samples were compressed into
40 mm disks and subsequently analysed using the semi IQ+
program.
Microwave irradiation. Microwave experiments were carried
out in a CEM-DISCOVER model with PC control and moni-
tored by sampling aliquots of reaction mixture that were subse-
quently analysed by GC/GC-MS. Experiments were conducted
in a closed vessel (pressure controlled) under continuous stirring.
The microwave method was generally power-controlled where
the samples were irradiated with the maximum power output
X-Ray diffraction patterns were recorded on a Philips
˚
X’PERT MPD PW3040 using Cu Ka radiation (l ¼ 1.5418 A)
over a 2q range from 0.5 to 6ꢀ (1ꢀ per min).
Transmission electron microscopy (TEM) micrographs were
recorded on an FEI Tecnai G2 fitted with a CCD camera for ease
and speed of use. The resolution is around 0.4 nm. Samples were
suspended in ethanol and directly deposited on a copper grid
prior to analysis.
ꢀ
(300 W) and different temperatures (60–80 C), measured by an
infra-red probe, were achieved. A typical catalytic reaction under
microwave irradiation was conducted as follows: 10.3 mmol
cyclohexene (1 mL), 1.5 mmol TBHP (0.17 mL), 1 mL DCM and
0.02 g catalyst were placed into a tube and microwaved for 1 h
using maximum power output (300 W).
XPS measurements were performed in a ultra high vacuum
(UHV) multipurpose surface analysis system (Specsꢀ model,
Germany) operating at pressures <10ꢁ10 mbar using a conven-
tional X-ray source (XR-50, Specs, Mg Ka, 1253.6 eV) in
a ‘‘stop-and-go’’ mode to reduce potential damage due to sample
irradiation. The survey and detailed V and O high-resolution
spectra (pass energy 25 and 10 eV, step size 1 and 0.1 eV,
respectively) were recorded at room temperature with a Phoibos
150-MCD energy analyser. Powdered samples were deposited on
a sample holder using double-sided adhesive tape and subse-
quently evacuated under vacuum (<10ꢁ6 Torr) overnight.
Eventually, the sample holder containing the degassed sample
was transferred to the analysis chamber for XPS studies. Binding
energies were referenced to the C1s line at 284.6 eV from
adventitious carbon.20 The curve deconvolution of the obtained
XPS spectra was obtained using the Casa XPS program. V2p
curve fitting was performed on the V3p3/2 region due to the
overlapping between the O1s and the V2p1/2 regions.
Response factors of the reaction products were determined
with respect to the corresponding starting material from GC
analysis using known compounds in calibration mixtures of
specified compositions.
Results and discussion
Textural properties of the V-SBA-15 are summarised in Table 1.
In general, materials exhibited high surface areas (>250 m2 gꢁ1
)
and pore diameters within the mesoporous range (typically
around 5 nm). Surface areas and pore volumes decreased, as
expected, as a consequence of the deterioration of the meso-
porous structure with an increase in V content in the systems.
Interestingly, a remarkable increase in pore size was observed at
high V content (Si/V 5 and lower, Table 1, entries 5 and 9) that
indicates structural changes in the SBA-15 materials (Fig. 1b).
XRD results also show the advanced deterioration of the
hexagonal mesoporous structure at increasing quantities of V
(Fig. 1), with the absence of the 100 diffraction line below 2q ¼ 1
for the V-10 materials. These observations point to the loss of the
mesoporous SBA-15 hexagonal structure for materials with Si/V
ratios #20. Nevertheless, no diffraction lines were found in the
Catalytic experiments
Conventional heating. A typical conventional heating experi-
ment was performed in a Carousel Reaction Stationꢀ (Radleys
Discovery Technologies) as follows: 31.1 mmol cyclohexene
Table 1 Textural properties [surface area (SBET, m2 gꢁ1), pore diameter (DBJH, nm), pore volume (VBJH, cm3 gꢁ1)] of V-SBA-15 materials
Entry
Material
SBET/m2 gꢁ1
DBJH/nm
VBJH/cm3 gꢁ1
XRF (Si/V ratio)
1
2
3
4
5
6
7
8
9
Si-SBA-15
V-40-O
V-30-O
V-20-O
V-10-O
V-40-M
V-30-M
V-20-M
V-10-M
1016
931
810
592
373
1012
854
718
789
5.7
5.1
4.6
5.2
9.3
5.3
5.1
5.0
13.9
1.15
0.71
0.47
0.66
0.38
0.98
0.50
0.67
0.31
—
42
44
13
5
59
46
12
4
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ratios to those theoretically expected from the synthesis gel
(Table 1, entries 4, 5, 8 and 9). The metal is not homogeneously
distributed within the mesoporous material at high V content due
to the formation of high V concentration areas that will account
for an inferior value of Si/V ratio in the XRF measurements.
The structural order of V-SBA-15 materials was confirmed by
TEM. Fig. 2 clearly shows the hexagonal mesoporous channels
with pores of ca. 5–6 nm for V-40-M (Fig. 2B), in good agree-
ment with textural properties.
Isotherm profiles were typical of SBA-15 mesoporous mate-
rials (with a hysteresis loop in the desorption branch), although
the deterioration of the structure was noticeable at increasing V
contents (Fig. 3). The change in the isotherm profile for V-20 and
V-10 materials together with the increasingly developed macro-
porosity (Fig. 3, p/p0 values above 0.9) found for V-10 materials
confirms the structural changes suggested in Table 1 and Fig. 1
that point to the loss of the mesoporous SBA-15 hexagonal order
for high V content materials. This higher degree of macro-
porosity found for V-10 materials may account for the observed
increase in pore diameter.
Fig. 1 XRD patterns of (a) Si-SBA-15 and (b) V-10-O.
20–80ꢀ range, which may indicate the absence of crystalline V2O5
domains in the materials.
In any case, the incorporation of V into the mesoporous
structure was intriguing since the actual Si/V ratios were signif-
icantly different from those expected. Low loaded V-SBA-15
materials (Si/V 40 and 30) presented lower quantities of incor-
porated V in the calcined materials (Table 1, entries 3, 6 and 7).
These findings may point to difficulties in the incorporation of V
at low contents. In contrast to these results, increasing V contents
(Si/V 20 and 10) lead to materials with remarkably superior Si/V
UV-Vis spectra are shown in Fig. 4. The profiles and intensity
of the UV bands are independent of the V content in the mate-
rials and very broad bands were observed in general. A broad
band in the 200–350 nm range (centred at ca. 250 nm) has been
assigned to low energy ligand to metal charge transfer (LMCT)
of tetrahedral V5+ species, coordinated to the SBA-15 surface,18,21
suggesting the incorporation of V ions into the mesoporous
structure.18,19,21 Bands appearing in the 300–400 region (centred
at ca. 400 nm) could be attributed to V–O–V (poly)/oligomeric
species, with different degrees of oligomerisation, that have been
previously reported in many vanadium materials.22 Interestingly,
the absence of UV absorption bands above 450 nm may point to
the absence of significant quantities of V2O5 on the SBA-15
surface (LMCT transitions in V2O5 crystalline domains18,19,22,23).
These findings were in good agreement with evidence found by
XRD and XPS (Fig. 5 and 6).
XPS spectra of V-SBA-15 materials prepared with different
vanadium precursors, including V2p3/2 and O1s regions, are
presented in Fig. 5 and 6.
Fig. 2 TEM images of (A) V-10-O and (B) V-40-M.
This journal is ª The Royal Society of Chemistry 2009
Fig. 3 Isotherm profiles of V-20-O and V-10-O materials.
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Fig. 4 UV-Vis spectra (190–800 nm) of V-SBA-15 materials: (a) V-10-
M, (b) V-20-O, (c) V-30-M and (d) V-40-O.
Fig. 6 O1s XPS spectra of V-20-O and V-20-M materials.
different O environments [2 from V (V–O from vanadium oxides
and Si–O–V from vanadium framework) and the original Si–O
from Si-SBA-15, Fig. 6].26 These findings demonstrated that only
minor quantities of vanadium oxides (V–O component, O1s
band at ca. 529–530 eV in Fig. 6, <10% of total vanadium
content) were observed in the materials, regardless of the vana-
dium precursor employed in their preparation, and also
corroborate the successful incorporation of the majority of the V
into the SBA-15 hexagonal mesostructure.
Fig. 5 XPS spectra of V-20-O [survey (top-left) and V2p3/2 (top-right)]
Interestingly, a new Si–O–V component could be clearly found
in some of the materials [Si–O–V (2), Fig. 6, bottom]. These
components have been reported to arise from the reversible
coordination of water molecules to the pentavalent vanadium
ions present in the tetrahedral framework structure, particularly
that located near the pore entrance, which changes the coordi-
nation of V to octahedral.19 It has been reported that these
phenomena also cause a band broadening in UV-Vis spectra,19 in
good agreement with our UV-Vis results (Fig. 4).
and V-20-M [survey (bottom-left) and V2p3/2 (bottom-right)].
V2p3/2 bands centred at ca. 515.6 and 517.0 eV (main contri-
bution) could be deconvoluted from the V2p3/2 spectra. The
517.0 eV band, accounting for most of the V present in the
materials, could be assigned to the V5+ oxidation state present
within the mesoporous framework in tetrahedral coordination,
in good agreement with previously reported results.24 The
component centred at ca. 515.6 eV (15% contribution) could
similarly be assigned to V3+, consistent with the energy shift
observed for V5+/V3+ species.24,25 These results were also
confirmed with the O1s XPS spectra that showed the three
Despite the low V2O3/V2O5 extra-framework content in the
materials (<10%), interesting differences were observed for
V-SBA-15 depending on the vanadium precursor utilised. The
use of vanadium oxytriisopropoxide (V-X-O) rendered materials
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24 h reaction, Table 2). Blank runs gave no cyclohexene
conversion after 12 h reaction. Nevertheless, the catalysts were
found to be extremely selective for the formation of cyclohexene
oxide, with no other by-product(s) found under the investigated
reaction conditions. Both V-20-O and V-20-M exhibited the
highest conversions of starting material under the reaction
conditions (Table 2). However, if we take into account the
epoxidation activity per active site (supposedly from V species),
TON and TOF values can provide a direct comparison between
the investigated catalysts. Thus, despite their slightly inferior
conversions, V-40 materials were found to be the most active
catalysts in the epoxidation reaction, regardless of the vanadium
precursor employed in their synthesis (Table 2). The significantly
reduced activity observed for the V-10-Y materials can be
attributed to the presence of high V concentration areas which
have significantly reduced catalytic activities27 as well as to the
loss of hexagonal SBA-15 structure observed for these particular
catalysts. Interestingly, the vanadium precursor did not seem to
remarkably influence the activity of the materials. In general,
V-X-O prepared with vanadium oxytriisopropoxide was only
slightly more active compared to V-X-M prepared using
ammonium metavanadate (Table 2). We believe the slightly
superior quantities of extra-framework vanadium oxides (8–10%
total V content) in V-X-O, as demonstrated by XPS, compared to
V-X-M (5–6% maximum) may account for this little increase in
activity observed for V-X-O.
Scheme 1 Epoxidation of cyclohexene using V-SBA-15 materials under
conventional heating or microwave irradiation.
with slightly higher vanadium extracted content (8–10%; e.g.
vanadium oxides as suggested by V2p XPS spectra) than those
found for V-M-O materials (<6%). These facts suggest that
ammonium metavanadate may be a slightly better vanadium
precursor for the V incorporation into the mesoporous structure
than vanadium triisopropoxide.
The catalytic activity of V-SBA-15 materials was then inves-
tigated in the epoxidation of cyclohexene (Scheme 1) under
conventional heating and microwave irradiation, and results
were compared to demonstrate the efficiency of the microwave
protocol.
Conventional heating experiments
Results are summarised in Fig. 7 and Table 2.
In general, the epoxidation reaction under conventional
heating takes a long time (24 h+) to achieve good to quantitative
conversion of starting material (97% conversion for V-20-O after
Microwave experiments
Comparatively, the rates of reaction were significantly improved
under microwave irradiation under similar conditions. Blank
reactions (without catalyst) gave no conversion of starting
material after 1 h reaction.
Results included in Fig. 8 point out that maximum conversions
of 90% could be achieved with both V-20-O and V-20-M mate-
rials after 1 h of microwave irradiation compared to a poor
30–50% conversion reached under similar conditions, regardless
of the catalyst employed in the epoxidation, under conventional
heating (Table 2, 1 h column).
Fig. 7 Catalytic activity of V-X-O and V-X-M materials in the epoxi-
dation of cyclohexene under conventional heating. Reaction conditions:
31.1 mmol cyclohexene, 4.5 mmol TBHP, 7.4 mL DCM, 0.05 g catalyst,
43 ꢀC, 24 h.
Table 2 Catalytic activity of V-SBA-15 materials in the epoxidation of
cyclohexene under conventional heatinga
Conversion (mol%)
Material
0.5 h
1 h
3 h
6 h
24 h
TON
TOF/hꢁ1
V-40-O
V-30-O
V-20-O
V-10-O
V-40-M
V-30-M
V-20-M
V-10-M
32
32
45
42
23
23
39
21
45
44
62
46
34
38
53
34
60
65
77
66
48
53
68
45
72
75
86
76
58
64
76
58
88
90
97
92
75
79
90
81
780
820
300
140
900
700
270
120
32
34
12
6
38
29
11
5
Fig. 8 Catalytic activity of V-X-O and V-X-M materials in the epoxi-
dation of cyclohexene under microwave irradiation. Reaction conditions:
10.3 mmol cyclohexene, 1.5 mmol TBHP, 1 mL DCM, 0.02 g catalyst,
60–80 ꢀC, 300 W, 1 h.
a
Reaction conditions: 31.1 mmol cyclohexene, 4.5 mmol TBHP, 7.4 mL
DCM, 0.05 g catalyst, 43 ꢀC.
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Table 3 Conventional heating vs. microwave irradiation. A comparison of activities for the most active catalyst V-40-M and V-40-O materials in the
epoxidation of cyclohexene
Time of reaction/h [conversion
value (mol%)]
Entry
Material
V-40-O
V-40-M
Method
TON
TOF/hꢁ1
1
2
3
4
Conventional heatinga
Microwave irradiationb
Conventional heatinga
Microwave irradiationb
24 [88]
1 [79]
24 [75]
1 [70]
780
570
900
700
32
570
38
700
a
Reaction conditions (conventional heating): 31.1 mmol cyclohexene, 4.5 mmol TBHP, 7.4 mL DCM, 0.05 g catalyst, 43 ꢀC, 24 h reaction. b Reaction
conditions (microwave irradiation): 10.3 mmol cyclohexene, 1.5 mmol TBHP, 1 mL DCM, 0.02 g catalyst, 60–80 ꢀC, 300 W, 1 h.
In any case, complete selectivities to cyclohexene oxide were
observed in all systems, confirming the high selectivity of V-SBA-
15 materials for the epoxidation reaction. Similar trends as those
found in the conventional heating experiments were found for the
activity in the systems with respect to Si/V ratio (V-20 materials as
most active) and vanadium source (NH4VO3 as source gave less
active materials) in the microwave experiments (Fig. 8).
Microwave vs. conventional heating
A further detailed comparison of the two most active catalysts
(per V active site) under both conventional heating and micro-
wave irradiation is included in Table 3. Turnover numbers
(TONs) and turnover frequencies (TOFs, hꢁ1) clearly show the
marked differences between the two protocols. While similar or
even slightly superior TON values were achieved for conven-
tional heating compared to microwave irradiation, TOF values
were significantly reduced in conventional heating experiments
(10–20 times inferior to those obtained under microwave irra-
diation) due to the long times of reaction required to achieve
comparable conversion values. In any case, results demonstrate
the superior performance of microwave-assisted protocols that
are able to provide comparable conversions and selectivities
(Table 3) to those obtained under conventional heating at
remarkably reduced times of reaction (24 vs. 1 h, Table 3).
A comparison of the activity for the reported V catalysts with
other conventionally employed epoxidation catalysts in the
epoxidation of cyclohexene under microwave irradiation is also
included in Table 4. Results clearly show the activities of V-SBA-
15 materials were comparable and even superior, in most cases,
to conventional TiO2 and Ti-MCM-41 oxidation catalysts under
identical reaction conditions. In any case, the complete selectiv-
ities observed in our systems (with not even traces of by-products
under the investigated reaction conditions) are improved
Table 4 Comparison of activities between V-SBA-15 materials and
other conventional oxidation catalysts in the epoxidation of cyclohexene
to cyclohexene oxide under microwave irradiationa
Conversion
(mol%)
Scyclohexene
oxide (%)
Entry
Material
1
2
3
4
528
615
V-40-O
V-20-O
79
90
74
<10
16
75
>99
>99
90
>95
85
Ti-MCM-41
TiO2
Nb-PMOsb
Co-salen-SBA-15c
80
a
Reaction conditions (microwave irradiation): 10.3 mmol cyclohexene,
1.5 mmol TBHP, 1 mL DCM, 0.02 g catalyst, 300 W, 60–80 ꢀC, 1 h.
Reaction conditions: 2 mmol cyclohexene, 2 mmol H2O2 (34 vol%),
b
c
0.04 g catalyst, 300 W, 45 ꢀC, 1 h (ref. 28). Reaction conditions: 20
mmol cyclohexene, 40 mmol H2O2 (34 vol%), 0.05 g catalyst, 300 W,
90 ꢀC, 0.03 h (ref. 15).
Table 5 Catalytic activity of V-20-O in the epoxidation of a variety of cyclic and linear alkenes
Entry
1
Substrate
Method
Conversion (mol%)
Time of reaction/h
TON
TOF/hꢁ1
Conventional heatinga
Microwave irradiationb
97
89
24
1
280
230
12
230
2
3
Conventional heatinga
Microwave irradiationb
95
97
24
0.5
300
250
12
500
Conventional heatinga
Microwave irradiationb
98
94
24
0.25
300
250
13
1000
4
5
Conventional heatinga
Microwave irradiationb
Conventional heatinga
Microwave irradiationb
66
77
74
66
24
1
24
1
210
200
230
170
9
200
10
170
a
b
Reaction conditions (conventional heating): 31.1 mmol alkene, 4.5 mmol TBHP, 7.4 mL DCM, 0.05 g V-20-O, 43–48 ꢀC. Reaction conditions
(microwave irradiation): 10.3 mmol alkene, 1.5 mmol TBHP, 1 mL DCM, 0.02 g V-20-O, 60–80 ꢀC, 300 W.
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compared with those obtained in previous reports, highlighting
the outstanding selectivities of this protocol. With regard to
conventional heating processes, various reported results describe
conversions ranging from 40 to 95% in 7–48 h, most of them are
comparable to the catalytic activities obtained herein, with,
however, much reduced selectivities to cyclohexene oxide (infe-
rior to 90%).29
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Conclusions
~
ꢀ
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remarkably reduced times of reaction under microwave irradia-
tion (0.25–1 h) compared to those needed under conventional
heating (24 h+) to achieve comparable conversion values.
Materials could be also recovered upon reaction completion and
reused at least 3 times with almost complete preservation of the
initial activity. We envisage our materials will be employed in
many oxidation processes in the future and further research is
already ongoing in our laboratories.
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Acknowledgements
Authors greatly acknowledge funds from Ministerio de Ciencia e
ꢀ
Innovacion (Projects CTQ2007-65754/PPQ and CTQ2008-
01330) and Junta de Andalucia (P07-FQM-02695), cofinanced
with FEDER funds.
This journal is ª The Royal Society of Chemistry 2009
J. Mater. Chem., 2009, 19, 8603–8609 | 8609