V.M. Vaschetti et al.
Molecular Catalysis xxx (xxxx) xxx–xxx
employing a limonene/oxidant molar ratio of 0.5. As products limonene
glycol, carveol, carvone, and perillyl alcohol were obtained. Their re-
search showed that product selectivity depended on the used oxidant
and the metal load in the catalyst. Becerra and coworkers investigated
the oxidation of limonene with TBHP employing iron hexadeca-
chlorinated phthalocyanine immobilized on modified silica [2]. Using a
limonene/TBHP molar ratio of 0.38, a limonene conversion of 74% was
obtained after 23 h with 10% selectivity to carvone. In the same way,
other reports on limonene oxidation have also been developed evalu-
ating the activity of zeolitic supports modified with transition metals
complexes [14,15]. The above mentioned catalytic systems show the
high complexity of the limonene oxidation process. All the results de-
monstrate that the reaction selectivity is heavily influenced by multiple
factors such as the used oxidant, type of catalyst and metal load, solvent
employed, reaction temperature and limonene/oxidant ratio.
We have focused on testing a catalytic system that combines the
simplicity of a mesoporous silicate such as MCM-41, with the low cost,
high redox potential, environmentally benign nature and readily
availability of copper [16,17]. It is known that there is a wide variety of
methods that can be used to modify M41S type supports with different
transition metals. All Cu-MCM-41 materials evaluated in this work were
synthesized employing the template-ion exchange (TIE) method. This
method allows a simple functionalization of the MCM-41 surface by
deposition of highly dispersed metal species [18,19]. Moreover, many
efforts have been made to optimize the synthesis of M41S materials
41. The resulting brown materials were named Cu-TIE(x), where the “x”
refers to the solids nominal metal loading in wt.%.
2.2. Characterization techniques
The X-ray diffraction patterns (XRD) were taken at low and high
angles between 2°–8° and 20°–80° using an X-Pert Pro PANalitic
Diffractometer with copper Kα radiation of 1.5418 Å. The lattice
parameter of the hexagonal unit cell (a
the equation a
distance calculated based on the position of the most intense diffraction
line at low angle. N adsorption-desorption isotherms data at 77 K were
0
) was calculated according to
0
= (2/√3)d100, where d100 refers to the interplanar
2
obtained using a manometric instrument Micromeritics, ASAP 2000,
where the samples were previously degassed at 573 K for 12 h. The
specific surface (SBET) was determined by the Brunauer-Emmett-Teller
method (BET), the total pore volume (VTP) was estimated at a relative
pressure of 0.98, and the primary mesoporous volume (VMPP) was
calculated by the a
uated employing the Functional Theory of Non-Localized Density
(NLDFT) based on the adsorption branch, and the N kernel at 77 K on
S
-plot method using the standard (PSD) was eval-
2
silica for cylindrical pores. XPS measurements were performed on a
SPECS Multi-technique spectrometer, equipped with a dual X-ray
source (Mg/Al) and a hemi-spherical analyzer PHOIBOS 150 in fixed
analyzer transmission mode (FAT). The spectra were recorded with a
step energy of 30 eV, employing a Mg anode operated at 200 W. The
−
9
[
20,21]. It has been reported that the replacement of tetra-
pressure during the measurement was lower than 1.10
mbar. The
ethylammonium hydroxide (TEAOH) for NaOH in the synthesis of the
MCM-41 results in a better structure in the final solid [22]. Therefore,
this modification could represent an improvement over other Cu-TIE
molecular sieves previously reported [23]. In the present study, the
copper modified mesoporous molecular sieves were evaluated as cata-
samples were pressed, supported on the instrument sample holders,
-
2
subjected to vacuum (10 mbar) for 10 min at 473.15 K. Subsequently,
ultra-high vacuum was applied for at least two hours before analyzing.
The UV–vis-DR spectra were collected in air at room temperature with a
UV–vis-DR Spectrophotometer with an integrated sphere for diffuse
reflectance Jasco V-650. The range of wavelengths used was 200 nm –
900 nm and the data were converted employing the Kubelka-Munk
equation. The total copper content in the final solid was determined by
Atomic Absorption (AA) using a Shimadzu AA7000.
lysts on the liquid phase oxidation of limonene with H
With the aim to understand the obtained catalytic results, the synthe-
sized materials were characterized by XRD, UV–vis-DR, ICP and N
2 2
O and TBHP.
2
adsorption-desorption at 77 K. Moreover, a reaction mechanism was
proposed to explain the products distribution observed for each oxi-
dant.
2.3. Limonene oxidation
2. Materials and methods
2 2
Limonene oxidation reactions, with H O or TBHP, were carried out
under vigorous stirring in a batch system that consisted of a reflux
2.1. Catalyst synthesis
condenser connected to a glass reactor submerged in a bath at 343 K for
5
h. In a typical reaction, limonene (Lim, R(+)-Fluka 98%) was used as
The pure siliceous matrix (MCM-41) was synthesized following
reagent, H (Cicarelli 30% in water) or TBHP (Aldrich 70% in water)
2 2
O
method B reported in [22] by Elías et al. Cetyltrimethylammonium
bromide (CTAB, Merck 99%) was used as surfactant (Surf), tetra-
ethoxysilane (TEOS, Aldrich 98%) as silicon source and 2 M aqueous
solution of NaOH for hydrolysis and pH adjustment. The molar com-
position of the starting gel was: Surf/Si = 0.12, NaOH/Si = 0.5 and
as oxidant (molar ratio Lim/oxidant 2:1) and the Cu-TIE(x) as a reac-
tion catalyst (14.3 g/L). The solvent used in all cases was acetonitrile
(AcN, Sintorgan 99.5%) in a 15:1 M ratio with limonene. Some reac-
tions were carried out under nitrogen atmosphere and others using a
radical scavenger. To create a nitrogen atmosphere, several cycles of
vacuum-nitrogen income were performed to the reaction system while
the reacting mixture was kept at 273 K. After the selected conditions
were reached, the reactor was submerged in the bath at 343 K, and the
nitrogen atmosphere was maintained during the 5 h of reaction. When a
radical scavenger was used, 2,6-di-tertbutyl-4-methylphenol (BHT) was
added at the beginning of the reaction in a quantity that was equal to
8% of the initial limonene mmols. In all cases, the reaction progress was
recorded by taking samples at different times through a reactor lateral
tabulation. The aliquots were filtered and analyzed by gas chromato-
graphy (GC) using an Agilent 7820 chromatograph equipped with an
HP-1 capillary column and a FID detector. The percentage of each
component in the reaction sample was calculated using the area nor-
2
H O/Si = 132. In a typical synthesis, CTAB was dissolved at 313 K in
NaOH and distilled water. After this solution was cooled to room
temperature, the silicon source was quickly added. The resulting mix-
ture was stirred vigorously for 4 h at 298 K, and then for an additional
3
h at 343 K. Finally, the resulting white solid was washed, filtered and
dried overnight at 333 K. For its characterization, a part of the MCM-41
was desorbed at 773 K in N atmosphere for 6 h (heating rate 2 K/min),
2
and then calcined at the same temperature in air for another 6 h to
remove the surfactant.
The copper modified materials were synthesized with different
copper loadings using the template-ion exchange method [24]. 3.16 g of
the undesorbed MCM-41 containing approximately 52.5 wt.% of sur-
factant was added to a Cu(NO
3
)
2
.2.5H
2
O (J.T.Baker 99.7%) solution
2 2
malization method employing response factors. Both, H O and TBHP
which concentration corresponded to the nominal metal loading. The
mixture was stirred at room temperature for 1 h and then placed under
autogenous pressure at 353 K in a steel reactor lined with teflon for
were determined by iodometric titration. The conversion (X) of sub-
strate (Lim) and oxidant (Ox), the selectivity (S) for product “i” and the
turnover number (TON), at reaction time "t" were calculated according
to Eqs. (1)-(4). In the corresponding equations, “n” represents the
mmols of limonene, oxidant or product “i”, while the subscript “o”
20 h. Then the light blue solids were filtered, dried, desorbed and cal-
cined, following the same procedure previously described for the MCM-
2
Vaschetti, Virginia M.
