2
A.L. Cánepa et al. / Journal of Molecular Catalysis A: Chemical 347 (2011) 1–7
containing Ti (IV), Zr (IV) and Fe (III) ions isolated in inorganic
matrixes such as hydrothermally stable silicates, mesoporous cel-
lular foams and Kegging type heteropolytungstates. Mesoporous
molecular sieves and mixed oxides containing highly dispersed
transition metal ions have been reported as efficient catalysts
for H O -based oxidation [21–32]. The MCM-41 family of meso-
porous materials has acquired a great importance due to its
hexagonal order of bi-dimensional pores and its high surface
areas, due to the possibility of adapting its catalytic properties,
such as type and force of acid and/or basic sites, hydrophilic-
ity/hydrophobicity, structure and size of pores. Particularly, the
incorporation of redox metallic ions as Ti in the network struc-
ture of MCM-41 molecular sieves gives activity for oxidation
of voluminous olefins [24,33]. Recently, we reported the use of
titanium containing mesoporous materials, synthesized by dif-
ferent methods, as catalysts for the oxidation of ␣-pinene with
chemical properties of the sample were characterized by powder
X-ray diffraction (XRD), diffuse reflectance UV–vis spectroscopy
(DRUV–vis) and specific surface area, whose analysis was pre-
sented in [39]. Moreover, DRUV–vis spectra of the fresh and
used Ti-MCM-41 after calcination in air at 500 C during 9 h were
recorded using a JascoV-650 spectrometer in the wavelength range
200–500 nm.
◦
2
2
2.3. Catalytic experiments
In
a typical reaction, ␣-pinene (C10H16) (Fluka >95%)
(
7.17 mmol), hydrogen peroxide (H O ) (Riedel de Haën, 35 wt.%
2
2
in water) (1.79 mmol), acetonitrile (CH CN) (Sintorgan, 99.5%)
3
(107.56 mmol) as solvent and Ti-MCM-41 (63 mg) were placed
in a pirex glass reactor. This reactor, equipped with a con-
denser to reflux and magnetically stirred, was immersed in a
H O [34,35]. In [34] we present the results of the oxidation
2
2
thermally controlled bath at temperatures varying between
of ␣-pinene in the liquid phase with hydrogen peroxide using
Ti-MCM-41catalysts prepared by conventional hydrothermal syn-
thesis. Conversions between 12 and 15% and, as majority products,
campholenic aldehyde and verbenone were obtained. In [35] we
reported the synthesis and the catalytic evaluation of Ti-containing
mesoporous catalysts prepared using zeolite precursors as build-
ing blocks of the mesoporous network. These materials showed
maximum conversion values of around 20% and high yields to epox-
idation products.
On the other hand, the scope of chemical kinetics is to define the
evolution with time of a reacting system. The information which
can be acquired with this type of studies can be applied in differ-
ent fields, such as the interpretation of reaction mechanisms and
of the molecular behavior, the interpretation of catalytic phenom-
ena, the optimization of catalysts formulations, the development
of new chemical processes and reactors modelling and simulation.
Therefore, the study of chemical reaction kinetics is of interest for
both chemists and chemical engineers using and elaborating reac-
tion rates data with different objectives and many books have been
published on the subject [36–38].
◦
4
0 and 70 C. Reaction progress was followed taking samples
at different times by a lateral tabulation, without opening the
reactor. Liquid samples were immediately filtered and analyzed
by gas chromatography using a capillary column crosslinked
methyl-silicone gum, 30 m × 0.53 mm × 2.65 mm film thickness
and connected to a FID detector. The percentage of each com-
ponent in the reaction mixture was calculated by using the
method of area normalization employing response factors. The
relative uncertainties of the measurements were tested with
repeated determinations. The percent relative uncertainty (CV
(%)) of the result was calculated by dividing the corresponding
absolute uncertainty with the average of the measurements.
Reaction products were identified by mass spectrometry in a
Shimadzu GCMS-QP 5050. Mass spectral data (m/z/intensity ratio)
for the identified products are the following: I: 137/15, 109/35,
+
8
8
7
9
5
4
3/34, 67/100, 55/42, 41/77. II: 152/1 [M ], 108/100, 93/91,
1/20, 67/48, 55/23. III: 134/10, 119/33, 109/40, 91/90, 81/33,
9/31, 55/38, 41/100. IV: 150/29 [M ], 135/55, 122/16, 107/100,
1/67, 79/50, 55/31, 41/85. V: 126/3, 111/3, 98/22, 83/66, 69/58,
5/36, 43/100. VI: 152/10, 137/17, 109/58, 94/21, 79/61, 59/100,
3/96.
+
Taking account for the major simplicity, the lower cost and the
higher yield to allylic products that present the catalysts prepared
by the conventional synthesis method, we chosen these materials
in order to study the kinetics of the reaction of the ␣-pinene oxida-
tion and to propose a reaction mechanism that fits the experimental
results. Assessing the reaction heterogeneity and the possibility of
reusing the catalyst is also stated.
The ␣-pinene conversion was defined as the ratio of converted
species to initial concentration. In order to study the possibility of
reusing the catalyst, at the end of reaction, the catalyst was filtered
◦
off, washed with CH CN and calcined in air at 500 C overnight to be
3
used again with fresh reaction mixture in different catalytic cycles.
3. Result and discussion
2
. Experimental
3.1. Physicochemical characterization of catalyst
2.1. Catalyst synthesis
Table 1 shows the chemical composition and textural prop-
Ti-MCM-41 was prepared by hydrothermal synthesis using
erties of Ti-MCM-41 material used in this study. DRX pattern of
the sample, which is not presented here, is characteristic of an
ordered hexagonal structure [39,40]. As it is shown in [39] the Ti
ions were incorporated into the framework in tetrahedral isolated
positions.
tetra-ethoxysilane (TEOS) (Fluka >98%) as a source of Si, Ti iso-
propoxide (IV) (Fluka >98%) as Ti source and ethanol solution of
dodecyl trimethyl ammonium bromide (DTMABr) (Fluka) which
was used as template, as described in [34]. Mole composition of
the starting gel of the material used in this study was: Si/Ti = 60,
OH/Si = 0.3, template/Si = 0.4, water/Si = 60. Template agent was
evacuated from the sample by thermal desorption programmed
Table 1
Chemical composition, hexagonal-cell unit parameter, surface area and pore diam-
eter of Ti-MCM-41 material.
◦
under N2 flow until 500 C and maintained at this tempera-
◦
ture for 6 h. Then the sample was calcined in air at 500 C for
Si/Tia
2
dDFT ( A˚ )c
Sample
Ti content
(wt.%)b
A0 ( A˚ )
Area (m /g)
6
h.
Ti-MCM-41
60
1.12
36.28
1546
27.25
2
.2. Catalyst physicochemical characterization
a
Mole ratio in starting gel.
In calcined sample.
Pore diameter corresponding to the maximum distribution by pores size
b
c
The Ti content of the sample was determined by inductively
coupled plasma emission spectroscopy (ICP-AES). The physico-
obtained by the DFT method.