Catalytic properties of silica supported titanium, vanadium and niobium oxide nanoparticles towards the oxidation of saturated and unsaturated hydrocarbons

Article abstract of DOI:10.1016/j.molcata.2006.02.041

The catalytic properties of silica supported titanium, vanadium and niobium oxide nanoparticles towards the oxidation of different organic substrates (cyclohexane, cyclohexene, 1-hexene) using tertbutylhydroperoxide (TBHP) and molecular oxygen as the oxid

Full text of DOI:10.1016/j.molcata.2006.02.041

Journal of Molecular Catalysis A: Chemical 252 (2006) 226–234
Catalytic properties of silica supported titanium, vanadium and
niobium oxide nanoparticles towards the oxidation of
saturated and unsaturated hydrocarbons
1
2
Susana Mart ´ı nez-M e´ ndez , Yurgenis Henr ´ı quez, Olgioly Dom ´ı nguez ,
∗
Lindora D’Ornelas , Heinz Krentzien
Laboratorio de S ´ı ntesis Organomet a´ lica, Centro de Qu ´ı mica Organomet a´ lica y Macromolecular, Escuela de Qu ´ı mica, Facultad de Ciencias,
Universidad Central de Venezuela, Av. Los Ilustres, Los Chaguaramos, Apdo Postal 47778, Caracas 1040, Venezuela
Received 13 December 2005; received in revised form 10 February 2006; accepted 14 February 2006
Available online 3 April 2006
Abstract
The catalytic properties of silica supported titanium, vanadium and niobium oxide nanoparticles towards the oxidation of different organic
substrates (cyclohexane, cyclohexene, 1-hexene) using tertbutylhydroperoxide (TBHP) and molecular oxygen as the oxidizing agents was stud-
ied. Titanium (1.9 nm), vanadium (2.3 nm) and niobium (1.6 nm) oxide nanoparticles stabilized on silica were synthesized by the reduction of
TiCl
4
·2THF, VCl
3
·3THF and NbCl
4
·2THF with K[BEt
3
H]. These materials were characterized by inductive coupled plasma-optical emission
spectroscopy (ICP-OES), Fourier transformed infrared spectroscopy (FTIR), high resolution transmission electron microscopy (HRTEM) and
X-ray photoelectron spectroscopy (XPS) analyses. The solids obtained were employed as heterogeneous catalysts. For 1-hexene and cyclohexene,
the titanium oxide nanostructured material showed 100% selectivity towards the epoxidation product, with conversions above 50%. In the case
of cyclohexane, the titanium oxide nanoparticles are 100% selective towards the desired oxidation products (cyclohexanol and cyclohexanone).
The highest oxidation activity was achieved with the vanadium oxide nanoparticles for the three substrates studied: cyclohexane, cyclohexene and
◦
1
-hexene, TN 562, 878 and 1190 mol of product/mol of metal, respectively, after 6 h of reaction time at 80 C. Nevertheless, with this metal the
selectivity is different to the one obtained for with the titanium oxide nanostructured system. The niobium catalyst was less active than the titanium
and vanadium oxide catalysts, although it proved to be more selective towards the formation of alcohols.
©
2006 Elsevier B.V. All rights reserved.
Keywords: Nanoparticles; Oxidation catalysts; Vanadium; Niobium; Titanium; Epoxidation
1
. Introduction
is one of the main aims in this area [2,3]. The oxidation of cyclo-
hexane and cyclohexene to cyclohexanol and cyclohexanone is
the key reaction in the synthesis of adipic acid, which is an
essential precursor in the production of nylon 6 and nylon 66
[4].
It is well known that amongst the most active homogeneous
and heterogeneous catalysts for alkene and alkane oxidation, are
those based on transition metals of groups 4 (Ti), 5 (V and Nb)
and 6 (Mo) [5–10]. In the case of heterogeneous catalysts, it has
been proved that a proper selection of preparation conditions
is essential to obtain a homogeneous metal dispersion on the
support, which in turn, generates highly active and selective
catalysts towards oxidation reactions [9–14].
The oxidation of organic substrates represents one of the most
important industrial chemical reactions [1], explaining the sig-
nificant effort invested in the research and development of new
heterogeneous catalysts with increased activities and selectivi-
ties in these types of reactions. In particular, alkene epoxidation
∗
Corresponding author. Tel.: +58 212 605 1343; fax: +58 212 605 1256.
E-mail address: ldornela@strix.ciens.ucv.ve (L. D’Ornelas).
Permanent address: PDVSA-INTEVEP, Apartado 76343, Caracas 1070,
1
Venezuela.
2
Permanentaddress: DepartamentotheQuimica Aplicada, Fac. de Ingenier ´ı a,
Over the last decade, the research and development of transi-
tion metal nanoparticles has received a lot of attention in many
Universidad Central de Venezuela, Av. Los Ilustres, Los Chaguaramos, Caracas
1
040, Venezuela.
1
381-1169/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2006.02.041

S. Mart ´ı nez-M e´ ndez et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 226–234
227
areas of science. This interest is because nanosized particles
2.3. Characterization of the silica supported metal oxide
nanoparticles
show different physical and chemical properties to those of
the same materials in the bulk [15,16]. The main reasons for
these differences originate in the electronic properties and in
the very high ratio between superficial atoms and total num-
ber of atoms in the particles found in these nanomaterials. Both
properties mentioned above, are of interest from the catalyti-
cal point of view, since they can aid in raising the activity and
selectivity of a reaction and, in some cases, induce new reac-
tivities and selectivities never observed with classical catalysts
Routine FTIR was performed on a Perkin-Elmer 1760-X
spectrometer using CsI disks. The elemental analyses were car-
ried out by atomic emission with coupled inductive plasma
on a Thermo Jarrel ASH model IRIS H12. The high resolu-
tion transmission electron microscopy (HRTEM) was carried
out in a CM30 Phillips Microscope with a 0.19 nm resolu-
tion. Samples were prepared using carbon-coated copper grids.
One drop of a THF suspension of [MxOy/SiO2] was placed on
the carbon-coated grid using a microsyringe. Different sam-
ples of each nanostructured materials were prepared; these
samples were subject to scanning in entire the area verifying
the homogeneity of the nanoparticles dispersion. The size and
size distribution of the nanoparticles was determined through
manual analysis enhancing the contrast of the digital micro-
graphs, which are the output of the HRTEM, at least 100
particles per sample were measured. This analysis was corrob-
orated employing Image Pro Plus 5.0 as a digital processing
program.
[
15–17].
This work presents a catalytic study of the oxidation of sat-
urated and unsaturated hydrocarbons with titanium, vanadium
and niobium oxide nanoparticle systems supported on silica.
2
. Experimental
2
.1. Materials
All the syntheses were carried out under an argon atmosphere
(
99.99% pure) using Schlenk techniques for the manipulation of
air sensitive compounds [18].
The X-ray photoelectron spectroscopy, XPS, was carried out
in a Escalab 220i-XL equipped with a monochromatic source
MgK␣(hγ = 1253.6 eV). ThedeconvolutionsoftheXPSspectra
were carried out using a XPS Peak Fitting program version 4.1.
Gas chromatography (GC) analyses were performed on a
Perkin-Elmer Autosystem XL with a flame ionization detector
(FID), equipped with a PE-Alumina capillary column (30 m).
Gas chromatography coupled with mass spectroscopy (GC–MS)
was performed on a HP6890 equipped with a Mass Selective
Detector 5973.
The metal precursors TiCl4·2THF (Aldrich), VCl3·3THF
(
Strem) NbCl4·2THF (Strem) were used as supplied.
The silica gel (Gomasil G-200) used as inorganic support is
9
8.8% SiO2 and 0.69% Na2SO4 content, with a mean grain size
◦
of 0.5 ␮m. The silica gel was activated at 250 C under vacuum
during 72 h, and then kept under an argon atmosphere before its
use. After activation, the SiO2 showed a BET specific surface
2
area of 169 m /g. Rhodia Silices de Venezuela C.A. donated this
material.
2
.2. Synthesis of the silica supported metal oxide
2.4. Catalytic activity
nanoparticles
The catalytic studies of the solids obtained, were carried
out in Fischer–Porter bottles. Metal supported catalyst (100 mg)
was introduced in the Fischer–Porter bottle along with 2 ml of
organic substrate (cyclohexane, cyclohexene and 1-hexene) and
0.5 ml of TBHP as the oxidizing agent. The reactions were car-
ried out under 1 atm of argon or 3 atm of molecular oxygen and
To obtain the metal oxide nanoparticles supported on sil-
ica, firstly the metallic nanoparticles of titanium, vanadium and
niobium were synthesized following the method established by
B o¨ nnemann [19], then stabilized on silica and finally oxidized
under 1 atm of molecular oxygen.
◦
As an example, here follows the procedure for the synthe-
sis of niobium oxide nanoparticles. A solution of K[BEt3H]
at 80 C. This study was done in order to determine the influ-
ence of molecular oxygen in the catalytic activity of the solid
towards oxidation reaction. All reactions were studied with dif-
ferent reaction times.
The TBHP percentage was determined through iodometric
analysis, before and after catalytic oxidation reaction [20].
(1 M, 80 ml, 80 mmol) in tetrahydrofurane (THF) was added
dropwise to a suspension of NbCl4·2THF (10.24 g, 20 mmol) in
THF at room temperature during 2 h. After 2 h of constant stir-
ring at room temperature, a dark brown solution was obtained.
The KCl formed during the reaction was removed by filtration
and the solvent was eliminated under vacuum. A dark solid was
obtained which later was redissolved in fresh THF and added
to a silica/THF suspension. This suspension was stirred for at
least 8 h, and then dried under vacuum during 24 h resulting
in a light brown solid. All three silica supported nanoparticles
3. Results and discussions
3.1. Synthesis and characterization of nanostructured
systems
(
titanium, vanadium and niobium) are extremely sensitive to
air.
Finally, the nanometric metal oxides supported on silica are
The titanium, vanadium and niobium oxide nanoparticles
were synthesized firstly following the method established by
B o¨ nnemann [19] for the preparation of metallic nanoparticles,
then stabilized on silica and finally oxidized under 1 atm of
molecular oxygen.
obtained by the exposure of the previously synthesized systems
to 1 atm of molecular oxygen.

2
28
S. Mart ´ı nez-M e´ ndez et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 226–234
The B o¨ nnemann method [19] employs the reduction of the
metal adduct TiCl4·2THF, VCl3·3THF and NbCl4·2THF with
K[BEt3H] at room temperature and under an argon atmosphere.
As described by this author, highly pyrophoric black powders
are obtained from these syntheses. At this stage, he reports that
the black solids obtained are metallic nanoparticles stabilized
with THF as suggested in Eq. (1):
THF,2h,r.t.,Ar
MCln · 2THF + nK[BEt3H] −→
n
0
M x0.5THF + 3BEt3 + nKCl ↓ + H2 ↑,
2
M = V, Nb; n = 3(V), 4(Nb)
(1)
Due to the high reactivity of the THF stabilized metallic
nanoparticles of titanium, vanadium and niobium with air, the
XPS analyses only showed the presence of the oxidized species.
As an example, Fig. 1 shows the XPS spectrum for the titanium
nanometric system stabilized with THF, where two different sig-
nals in the Ti 2p3/2 region with binding energies at 457.9 and
Fig. 2. The Ti 2p3/2 and Ti 2p1/2 XPS spectra for the titanium/silica nanostruc-
tured system exposed to molecular oxygen (1 atm).
For the niobium/silica system exposed to molecular oxy-
gen (1 atm), the XPS spectrum deconvolution (Fig. 3) dis-
plays signals in the region for Nb 3d5/2 characteristic of nio-
4
58.3 eV characteristic of TiO2 [21,22], can be seen.
Once the metallic nanoparticles stabilized with THF are
2
+
obtained, they are supported on silica under an argon atmosphere
Eq. (2)). The solids obtained are light grey in the case of the
bium species in different oxidation states: at 204.4 eV (Nb ),
4+
5+
(
205.5 eV (Nb ) and 207.0 eV (Nb ) [21,23]. It is evident from
the deconvolution that the most oxidized species (Nb2O5) has
the predominant contribution.
titanium system and brown in the case of the vanadium and nio-
bium systems. When these systems were exposed to molecular
oxygen (1 atm), they all immediately turn into white powders.
For the vanadium/silica systems exposed to molecular oxy-
gen (1 atm), due to the low vanadium concentration, the XPS
spectrum shows a very small signal at 516.5 eV in the region for
THF/r.t.,Ar, r.t.,oxygen
0
M x0.5THF + SiO2 −→
M = V, Nb; n = 3(V), 4(Nb)
−→ MxOy/SiO2 + THF ↑,
V 2p3/2 characteristic of V2O ; this signal is overlapped by the
5
(2)
O 1s X-ray satellite of the silica, which makes its deconvolution
impossible. This spectrum is not shown here.
The solids exposed to molecular oxygen were characterized
via ICP-OES, XPS and HRTEM.
The HRTEM analysis (micrographs and histograms) for the
TiO2/SiO2, V2O /SiO2 and Nb2O /SiO2 systems are shown in
5
5
The characterization via ICP-OES of TiO2/SiO2, V2O /SiO2
5
Figs. 4–6, respectively. Due to the silica support, the contrast
of these materials is rather poor and the best contrast area was
and Nb2O /SiO2 show metal contents for titanium, vanadium
5
and niobium of 1.8%, 0.62% and 0.92% in weight, respectively.
The XPS spectrum for the titanium/silica system exposed to
molecular oxygen (Fig. 2) shows three different signals in the Ti
2
p3/2 region with binding energies at 458.3, 459.3 and 461.1 eV,
all of which are characteristic of TiO2 species in different envi-
ronments [21,22].
Fig. 1. The Ti 2p3/2 and Ti 2p1/2 XPS spectra for the titanium nanostructured
system stabilized with THF.
Fig. 3. The Nb 3d5/2 and Nb 3d3/2 XPS spectra for the niobium/silica nanos-
tructured system exposed to molecular oxygen (1 atm).

S. Mart ´ı nez-M e´ ndez et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 226–234
229
Fig. 5. Micrography and histogram for silica supported vanadium oxide
nanoparticles, V2O5/SiO2, obtained from the reduction of VCl3·3THF with
K[BEt3H].
dizing agent, in the presence of molecular oxygen (3 atm) or
argon (1 atm), using 1-hexene, cyclohexene and cyclohexane as
substrates. No solvent was introduced in the reaction mixture. It
is worth noting that the TiO2/SiO2 catalyst turns yellow in the
presence of TBHP or H O . This colour change is attributed to
Fig. 4. Micrography and histogram for silica supported titanium oxide nanopar-
ticles, TiO2/SiO2, obtained from the reduction of TiCl4·2THF with K[BEt3H].
selected for the respective micrographs. All of these materials
show a homogeneous dispersion on the support and have spher-
ical morphology. The titanium system shows nanoparticle sizes
ranging from 1.2 to 2.9 nm and a mean size of 1.9 nm (Fig. 4). In
the vanadium system the size distribution, ranges between 1.5
and 3.5 nm, with a mean particle size of 2.5 nm (Fig. 5). For the
niobium system the particle size distribution ranges from 0.8 to
2
2
the formation of Ti–OOR species [24,25].
Table 1 details the catalytic activities of the silica supported
titanium oxide nanoparticles towards the oxidation of the dif-
ferent studied substrates. In the presence of molecular oxygen
for the [TiO /SiO ] system, turnover numbers (TN = mol of
2
2
product/mol of metal) of 321.6, 309.0 and 133.0 for 1-hexene,
cyclohexene and cyclohexane, respectively, were obtained after
2
.2 nm; the mean size for these particles is 1.6 nm (Fig. 6).
.2. Catalytic reactivities of nanostructured systems
The catalytic studies were carried out with the oxidized
6
h of catalytic oxidation. These represent 77.5% conversion of
3
1-hexene, 68.4% conversion of cyclohexene and 19.2% con-
version of cyclohexane. The selectivity was 100% towards the
epoxide for 1-hexene and cyclohexene, whereas with the cyclo-
hexane 100% selectivity towards the formation of cyclohexanol
(58%) and cyclohexanone (42%) was found. Fig. 7 shows the
solid systems TiO2/SiO2, V2O /SiO2 and Nb2O /SiO2, in a
5
5
◦
Fischer–Porter bottle at 80 C using TBHP as the limiting oxi-

2
30
S. Mart ´ı nez-M e´ ndez et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 226–234
(a) that dioxygen is involved in the reaction and TBHP is acting
as the initiator of the catalysis; (b) there could be two parallel
reactions taking place, one with molecular oxygen and one with
TBHP.
When the cyclohexene oxidation reaction takes place only in
the presence of oxygen, without TBHP in the reaction mixture,
no activity is observed. This indicates that, apparently, the TBHP
is acting as an initiator.
In another experiment, the oxidation of cyclohexene was car-
ried out using H2O2 as the oxidizing agent in the presence of
◦
molecular oxygen with methanol as the solvent at 150 C during
2
1
4 h. In this case, the conversion observed was only 3.4% (TN:
7.5).
As can be seen in Table 1, the efficiencies obtained for TBHP
are greater than 100%, even though this is the limiting agent,
again this corroborates that this oxidizing agent is acting as
a catalysis initiator. The TBHP efficiencies were determined
assuming that one mol of TBHP consumed produces 1 mol
of oxygenated products. The amount of TBHP consumed was
determined through iodometric analysis [20].
The reactivity of the silica supported titanium oxide nanopar-
ticles towards the oxo-functionalization of cyclohexane gener-
ated 20% conversion (TN: 144) with a 100% selectivity towards
the oxidation products after 24 h. For this case, the TBHP effi-
ciency was 471 with respect to the oxidation products. At the
beginning of the reaction, the cyclohexanol/cyclohexanone ratio
was 1.2, and after 24 h, there is a slight increase in the ratio to
1.4. This result indicates that there is very little over oxidation
of the cyclohexanol.
The titanium content determined after the reaction indicates
around8%lixiviationofthemetalfromthenanostructuredmate-
rial.
Vanadium oxide nanoparticles show high activity towards the
oxidation of 1-hexene, cyclohexene and cyclohexane, obtaining
turnover of 1190, 878 and 562, respectively (TN: mol of prod-
uct/mol of vanadium) after 6 h of reaction time (Table 2 and
Fig. 8). These correspond to 89.3%, 53.4% and 37.4% conver-
sions for 1-hexene, cyclohexene, cyclohexane, respectively. As
can be seen from the comparison data from Tables 1 and 2,
the vanadium-based catalyst is at least four-fold more active
towards oxidation products than the titanium-based catalyst. It
is important to point out that the vanadium oxide catalyst is less
selective towards epoxide formation in the case of 1-hexene and
Fig. 6. Micrography and histogram for silica supported niobium oxide
nanoparticles, Nb2O5/SiO2, obtained from the reduction of NbCl4·2THF with
K[BEt3H].
activity results for silica supported titanium oxide nanoparticles.
When the reaction with TBHP took place under an argon atmo-
sphere during 24 h, the conversion goes down to 1.7% (TN: 7.4)
for cyclohexene. This result leads to two possible explanations:
Table 1
Conversion of the catalytic oxidation of 1-hexene, cyclohexene and cyclohexane using TiO2/SiO2 catalysta
b
Catalyst
% Metal
Size (nm)
Substrate
Oxidation
agent
TN (% conversion)
% Efficiency
TBHPc
2
h
4 h
6 h
24 h
TiO2/SiO2
1.8
1.9
1-Hexene
Cyclohexene
TBHP
TBHP
H2O2
220(51.7)
174(33.8)
–
288(67.7)
274.6(50.5)
–
321.6(77.5)
309.6(68.4)
–
312(76)
825
507
–
319.6(76.7)
17,5(3.4)
144(20)
d
Cyclohexanec
TBHP
32(6.31)
85(14)
133(19.2)
471
Molar ratio substrate:TBHP:metal = 500:50:1.
a
Reaction conditions: 100 mg of catalyst, 0.5 ml of TBHP, 2 ml of the substrate, 1 atm molecular oxygen.
TN: mol of product/mol of titanium.
b
c
d
%
Efficiency after 24 h reaction time: (mol of oxidation product/mol of TBHP consumed) × 100.
Methanol was used as solvent.

S. Mart ´ı nez-M e´ ndez et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 226–234
231
Fig. 8. Catalytic activity and selectivity of nanostructured V2O5/SiO2 catalyst
as a function of time, in the oxidation of: (a) 1-hexene, (b) cyclohexene and (c)
◦
cyclohexane, with TBHP as oxidizing agent, 80 C, 3 atm molecular oxygen.
ol/one ratio of 1.2. It is interesting to note that this selectivity
is only obtained at the beginning of the reaction, after 3 h the
ratio ol/one starts to diminish and at 6 h it has gone down to
Fig. 7. Catalytic activity and selectivity of nanostructured TiO2/SiO2 catalyst
as a function of time, in the oxidation of: (a) 1-hexene, (b) cyclohexene and (c)
cyclohexane, with TBHP as the oxidizing agent, 80 C, molecular oxygen at
0.62. This leads to the belief that there is an over oxidation of
◦
the alcohol during the reaction. Fig. 8b shows that for the case of
cyclohexene a larger proportion of the oxidation products cor-
respond to the ketone. For the cyclohexane (Fig. 8c) the alcohol
and ketone are obtained in almost the same proportions, only a
slight over oxidation of the alcohol is observed. The decreas-
ing order of activity for the three substrates are as expected:
1-hexene > cyclohexene > cyclohexane.
When the reaction takes place under molecular oxygen atmo-
sphere but in the absence of TBHP, the vanadium-based catalyst
behaves in the same way as the titanium-based catalyst, showing
a very low conversion (7.2%). The chemical analysis performed
on the catalyst after it had been used in the reaction, reveals
a 50% lixiviation of the vanadium metal to the solution. With
this result, it is impossible to verify whether the catalysis takes
3
atm.
very different oxidation products are obtained for the reaction
with cyclohexene (2-cyclohexen-1-ol and 2-cyclohexen-1-one).
The vanadium-based catalyst shows 89% selectivity towards
oxidation products for the case of 1-hexene; this selectivity is
distributed as follows: 76.5% butyloxirene, 10.2% as a mix-
ture of 1-hexen-3-ol and 2-hexen-1-ol and 2.0% hexanal as the
main oxidation products (Fig. 8a and Table 3). Additionally, the
GC–MS analysis also gives a series of other oxidation products
formed to a lesser extent. When cyclohexene is used as substrate
(
Fig. 8b) there is a 95% selectivity towards the oxidation prod-
ucts. In this reaction, the main products are 2-cyclohexen-1-ol
55.2%) and 2-cyclohexen-1-one (44.8%), which represents an
(

2
32
S. Mart ´ı nez-M e´ ndez et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 226–234
Table 2
a
Conversion of the catalytic oxidation of 1-hexene, cyclohexene and cyclohexane using V2O5/SiO2 and Nb2O5/SiO2 catalysts
b
Catalyst
% Metal
Size
nm)
Substrate
Oxidizing
agent
TN (% conversion)
% Efficiency
TBHPc
(
1
h
3 h
6 h
17 h
V2O5/SiO2
0.62
2.3
1.5
1-Hexene
Cyclohexene
Cyclohexane
TBHP
TBHP
TBHP
810(60.8)
630(33.1)
470(31.3)
1173(88)
870(52.9)
558(37.1)
1190(89.3)
878(53.4)
562(37.4)
1198(89.8)
882(53.7)
561(37.4)
4808
3539
2337
Nb2O5/SiO2
0.92
1-Hexene
Cyclohexene
Cyclohexane
TBHP
TBHP
TBHP
310(19.3)
256(13)
29(1.6)
345 (21.6)
264.6(13.4)
36(2)
350 (21.8)
266.6(13.5)
36.6(2)
350.6 (31.8)
267(13.5)
36.6(2)
210
182
126.5
Molar ratio substrate:TBHP:metal = 1500:150:1.
a
Reaction conditions: 100 mg of catalyst, 0.5 ml of TBHP, 2 ml of the substrate, 3 atm of molecular oxygen.
TN: mol of product/mol of metal.
b
c
%
Efficiency after 17 h reaction time: (mol of oxidation product/mol of TBHP consumed) × 100.
place in the heterogeneous system V2O /SiO2, or a mixture of
heterogeneous and homogeneous catalysis or whether is it just
a homogeneous catalysis taking place.
ol (60%) and 2-cyclohexen-1-one (40%) (Fig. 9b). The ol/one
ratio after 1-h reaction time is 1.5, but this ratio starts to diminish
after 3 h and at 17 h, it has fallen as far as 0.87. Again, an over
oxidation of the alcohol seems evident in this case.
For the cyclohexane (Fig. 9c) 99.5% selectivity towards the
oxidation products is obtained, 98% cyclohexanol and 2% cyclo-
hexanone are produced in this reaction.
Once again, it is observed that at the beginning of the reaction
there is a high ol/one ratio (57:1), which in time starts to diminish
in time reaching a ratio as low as 3:1 after 17 h of reaction time.
In this case, the predominant oxidation product is the alcohol.
After the reaction, the ICP analysis of niobium-based catalyst
did not show lixiviation which means that this is a heterogeneous
catalytic system.
5
For the niobium oxide nanoparticles supported on silica, the
results reveal that this solid is capable of oxidizing the three
substrates used in this study. After 6 h of reaction, the turnovers
are 350.0, 266.4 and 36.5; these correspond to 21.8%, 13.5%
and 2.0% conversions for 1-hexene, cyclohexene and cyclohex-
ane, respectively (Table 2). Fig. 9 shows the turnover results
towards oxidation products with the different substrates studied.
For the case of 1-hexene 96%, selectivity towards the oxida-
tion products is found, 1-hexen-3-ol (48.6%) and 2-hexen-1-ol
(
51.4%) is obtained. Additionally, the GC–MS analysis showed
the presence of other oxidation products that are formed to a
lesser extent. These results shows that, for 1-hexene, the nio-
bium oxide nanoparticles supported on silica are less active
than their titanium and vanadium counterparts under the con-
ditions studied; nevertheless, in this particular case the niobium
oxide particles show a different selectivity, where the oxida-
tion products are only alcohols. When the cyclohexene is used
as substrate, 97.6% selectivity towards the oxidation products
is obtained at the beginning of the reaction, 2-cyclohexen-1-
Table 3 presents a summary of the catalytic activity and
selectivity of the TiO2, V2O and Nb2O silica supported nanos-
5
5
tructured systems towards the oxidation reactions of 1-hexene,
cyclohexene and cyclohexane.
From the literature, it is well established that some transition
metal ions can be catalytically activated towards oxidation reac-
tions of certain organic substrates when H2O2 or RO2H are used
as oxidizing agents [26]. These reactions can be classified in two
Table 3
Summary of the catalytic activity and selectivity of the Ti, V and Nb nanostructured systems towards the oxidation of 1-hexene, cyclohexene and cyclohexanea
Catalyst (% metal)
Substrate
TOF (mol of
product/mol
of metal h)
Selectivity (%)
Epoxide
Efficiency
TBHP (%)
Conversion
(%)
b
Alcohol
Ketone
SiO2
1-Hexene
3c
–
–
TiO2/SiO2 (1.8)
1-Hexene
Cyclohexene
Cyclohexane
54
51.5
22.2
100
100
–
–
–
57.4
–
–
42.5
825
507
471
77.5
68.4
19.2
V2O5/SiO2 (0.62)
1-Hexene
Cyclohexene
Cyclohexane
198
146
94
76.5
–
–
10.2d
38.5
45.5
2.0e
61.5
54.5
4808
3539
2337
89.8
53.4
37.4
Nb2O5/SiO2 (0.92)
1-Hexene
Cyclohexene
Cyclohexane
58
44.4
6
–
–
–
48.6 and 51.4d
51.4
98
–
49.6
2
210
182
126
21.8
13.5
2.0
a
◦
Reaction conditions: 100 mg catalyst, 2 ml substrate, 1 ml TBHP, 80 C, 6 h reaction time.
b
c
d
e
After 6 h of reaction time.
TOF: mol product/mol catalyst h.
A mixture of alcohols was obtained (1-hexen-3-ol and 2-hexen-1-ol).
Hexanal.

S. Mart ´ı nez-M e´ ndez et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 226–234
233
0
categories depending on whether the active species for the oxi-
dation reaction is: (i) a peroxometal species or (ii) an oxometal
species [26] (Eqs. (3) and (4)):
Generally, the early d transition metals (Ti(IV), Zr(IV),
V(V), Mo(VI) and W(VI)) react via
a peroxometal
intermediate; where as the late transition metals elements
Cr(VI), V(V), Mn(V), Ru(VI), Ru(VIII) and Os(VIII)) employ
(
an oxometal intermediate. Some elements such as vanadium,
depending on the substrate, can use either route. The majority
of the transition metals that catalyse oxygen transfer processes
whether via peroxometal or oxometal are also able to catalyse
processes via free radicals formed from peroxide compounds
[27,28].
Due to the great selectivity shown towards the epoxide for-
mation by the nanostructured TiO2/SiO2 system with 1-hexene
and cyclohexene, the free radical mechanism can be discarded.
The results obtained with the nanostructured TiO2/SiO2 system
encourage us to propose that the primary route is the oxometal
intermediate, where the TBHP and the molecular oxygen are
both involved to produce the epoxide selectively. Although this
type of reaction route has not been proposed for titanium bulk
materials, the results presented here indicate that this titanium
nanostructured system behaves in this manner.
The selectivity towards the epoxide formation in the oxida-
tion reaction of 1-hexene and cyclohexene obtained in this work
for the TiO2/SiO2 nanostructured system, is comparable to that
i
reportedbySensarmaetal. [24]foragraftedTi(O Pr)4 supported
on a preactivated silica. Nevertheless, these authors report higher
conversions for the cyclohexene oxidation than for 1-hexene.
Also our system shows greater selectivity and activity than the
more recently reported by Sreethawong et al. [25] for the cat-
alytic cyclohexene epoxidation by nanocrystalline mesoporous
TiO2 (11.1 nm), using as oxidizing agent TBHP generated in
situ, in the presence of oxygen from the air.
According to the results obtained for V2O /SiO2, subsequent
5
studies are under way to determine whether heterogeneous or
homogeneous catalysis or both are involved.
The differences observed in the selectivity of V2O /SiO2 and
5
Nb2O /SiO2 compared with TiO2/SiO2, could be explained in
5
terms of the Lewis acidity of the metallic centres. It is established
in the literature that the acidity of the active centres jointly with
the redox properties of the metal determine the selectivity of the
catalyst towards oxidation products [29]. For example, the epox-
ide ring opens on strong Lewis acid sites. This means that if the
acid centres are present, the acidic power cannot be too strong
when the epoxide product selectivity is aimed [29]. The results
reveal (Table 3), that the TiO2/SiO2, V2O /SiO2, Nb2O /SiO2
systems seem to have the following relative order of Lewis acid-
ity: TiO2/SiO2 < V2O5/SiO2 < Nb2O5/SiO2.
5
5
Fig. 9. Catalytic activity and selectivity of nanostructured Nb2O5/SiO2 catalyst
as a function of time, in the oxidation of: (a) 1-hexene, (b) cyclohexene and (c)
cyclohexane, with TBHP as the oxidizing agent, 80 C, 3 atm molecular oxygen.
◦

2
34
S. Mart ´ı nez-M e´ ndez et al. / Journal of Molecular Catalysis A: Chemical 252 (2006) 226–234
This Lewis order of acidity is in agreement with other results
reported in the literature for bulk mesoporous systems of these
metals [29].
From the lixiviation results it is possible to establish
that the systems studied within have the following stability
GC–MS service CONICIT-CONIPET project # 97003734 and
specially Bsc. A. Torres for his help in analysing the hydro-
genation mixtures by GC–MS. We also wish to thank the BID-
FONACIT project QF10 and the Laboratorio Nacional An a´ lisis
Qu ´ı mico (FONACIT) for the XPS analyses in the person of Bsc.
A. Albornoz.
order Nb2O /SiO2 > TiO2/SiO2 > V2O /SiO2, once again the
5
5
behaviour of these nanostructured materials is the same as bulk
mesoporous systems of these metals reported in the literature
References
[
30].
[
[
[
[
1] R.A. Sheldon, in: R. Ugo (Ed.), Aspects of Homogeneous Catalysis,
vol. 4, D. Reidel, Dordrecht, 1981, p. 3.
2] (a) R.A. Sheldon, J. Mol. Catal. 7 (1980) 107;
4
. Conclusions
(
b) K.A. Jørgensen, Chem. Rev. 89 (1989) 431.
Titanium (1.9 nm), vanadium (2.5 nm) and niobium (1.6 nm)
3] M. Dusi, T. Mallat, A. Baiker, Catal. Rev.: Sci. Eng. 42 (1/2) (2000)
213.
4] (a) R.A. Sheldon, J.K. Kochi, Metal Catalyzed Oxidation of Organic
Compounds, Academic Press, New York, 1981 (Chapter 11);
oxide nanoparticles stabilized on silica were obtained by the
Kr e [d Bu Ec tt i3o Hn ] .o f TiCl4·2THF, VCl3·3THF and NbCl4·2THF with
(
b) U. Schuchardt, D. Cardoso, R. Sercheli, R. Pereira, R.S. Da Cruz,
The nanostructured systems reported in this work,
M.C. Guerreiro, D. Mandelli, E.V. Spinac e´ , E.L. Pires, Appl. Catal. A:
Gen. 211 (2001) 1.
TiO2/SiO2, V2O /SiO2, Nb2O /SiO2, have proved to be active
5
5
catalysts in the oxidation reactions of 1-hexene, cyclohexene,
cyclohexane in the presence of molecular oxygen and TBHP
as the oxidizing agent. The TiO2/SiO2 system is 100% selective
towards the production of the epoxide when 1-hexene and cyclo-
hexene are used as substrates. For the cyclohexane, the oxidation
gave 100% selectivity towards the formation of cyclohexanol
[5] F. Wattimena, H.P. Wulff, British Patent 1,249,079 (1971) (to Shell).
[
6] R.A. Sheldon, in: B. Cornils, W.A. Hermann (Eds.), Applied Homoge-
neous Catalysis with Organometallic Compounds, vol. 1, VCH, Wein-
heim, 1996, p. 411.
[
[
7] F. Maspero, U. Romano, European Patent 0,190,609 (1986).
8] T. Tatsumi, M. Yako, M. Nakamura, Y. Yahura, H. Tominaga, J. Mol.
Catal. A 78 (1993) L41.
[
9] J.S. Reddy, A. Sayari, J. Chem. Soc., Chem. Commun. 23 (1995).
(
58%) and cyclohexanone (42%).
[
10] (a) H. Arzoumanian, G. Agrifoglio, H. Krentzien, M. Capparelli, New
J. Chem. 20 (1996) 699;
The V2O /SiO2 predominantly produced the epoxide for the
5
1
-hexene with high selectivity, nevertheless, for the oxidation
(
b) H. Arzoumanian, R. Bakhtchadjian, G. Agrifoglio, H. Krentzien,
J.-C. Daran, Eur. J. Inorg. Chem. (1999) 2255.
[11] S. Klein, S. Thorimbert, W.F. Maier, J. Catal. 163 (1996) 476.
reaction of the cyclohexene, the epoxide is not observed and
instead 100% selectivity towards the alcohol and the ketone is
obtained.
[
[
[
12] R.J. Davis, Z. Liu, Chem. Mater. 9 (1997) 2311.
13] H. Kochkar, F. Figueras, J. Catal. 171 (1997) 420.
14] N. Perkas, Y. Koltypin, O. Palchik, A. Gedaken, S. Chandrasekaran,
Appl. Catal. A: Gen. 209 (2001) 125.
15] G. Schmid (Ed.), Clusters and Colloids—From Theory to Applications,
VCH, Weinheim, Germany, 1994 (Chapter 6).
The Nb2O /SiO2 system results gave 100% selective towards
5
alcohols in the case of 1-hexene, but for the cyclohexene sub-
strate, this catalyst shows more or less the same selectivity
towards the alcohol and the ketone formation than that found
for the vanadium and titanium nanostructured systems. Under
[
[
16] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chem. Rev. 105
(
2005) 1025.
the conditions studied, the Nb2O /SiO2 system was less active
5
[
[
17] L.N. Lewis, Chem. Rev. 93 (1993) 2693.
18] D.F. Shriver, The Manipulations of Air-Sensitive Compounds, McGraw-
Hill, New York, 1969.
than the V2O /SiO2 system. It is important to emphasize that
5
the Nb2O /SiO2 system has 98% selectivity towards the alco-
5
hol formation for the oxidation of cyclohexane; this behaviour
is unique to this particular system. Although as expected, in the
oxidation reaction of the cyclohexane, the activity order was
[19] H. B o¨ nnemann, G. Braun, W. Brijoux, R. Brikmann, A. Schulze, K.
Seevogel, K. Siepen, J. Organometall. Chem. 520 (1996) 143.
[
20] I.M. Kolthoff, E.B. Sandel, E.J. Mechan, S. Bruckenstein, An a´ lisis
Qu ´ı mico Cuantitativo, 5ta ed., 1979, p. 883.
V2O /SiO2 > TiO2/SiO2 > Nb2O /SiO2.
5
5
[
[
21] http://www.lasurface.com and reference within.
22] E. Jorda, A. Tuel, R. Teissier, J. Kervennal, J. Catal. 175 (1998) 93.
It is worth noting that the TBHP efficiency was more than
00% in the oxidation reactions undertaken within the experi-
1
[23] M. Ziolek, I. Nowak, Catal. Today 78 (2003) 543.
[
[
[
24] S. Sensarma, B.A. Omar, S.L. Scout, H. Alper, J. Mol. Catal. A: Chem.
03 (2003) 145.
25] T. Sreethawong, Y. Yamada, T. Kobayashi, S. Yoshikawa, J. Mol. Catal.
A: Chem. 241 (2005) 23.
mental conditions used. Additionally, in this work it was estab-
lished that molecular oxygen is involved as an oxidizing agent
and the TBHP acts as an initiator agent for this catalysis.
2
26] I.W.C.E. Arends, R.A. Sheldon, Appl. Catal. A: Gen. 212 (2001) 175.
Acknowledgements
[27] J.M. Fraile, J.I. Garc ´ı a, J.A. Mayoral, E. Vispe, J. Catal. 204 (2001)
46.
28] G. Busca, E. Finocchio, G. Ramis, G. Ricchiardi, Catal. Today 32 (1996)
33.
1
[
The authors thank the CONICIT-CONIPET project #
1
9
7003711 for financial support. We wish to thank the Labo-
ratory of Electron Microscopy of the Universit e´ Paul Sabatier
Toulouse, France) in the person of Dr. M.-J. Casanove. The
[
[
29] M. Ziolek, Catal. Today 78 (2003) 47.
30] I. Nowak, B. Kilos, M. Ziolek, A. Lewandowska, Catal. Today 78 (2003)
487.
(