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2. Experimental
2.1. Sample preparation
operated in the constant analyser energy (CAE) mode. For the indi-
vidual peak energy regions, a pass energy of 20 eV set across the
hemispheres was used. Survey spectra were measured at 50 eV pass
energy. The sample powders were analysed as pellets, mounted on
a double-sided adhesive tape. The pressure in the analysis chamber
was in the range of 10−8 Torr during data collection. The constant
charging of the samples was removed by referencing all the ener-
gies to the C 1s set at 285.1 eV, arising from the adventitious carbon.
The invariance of the peak shapes and widths at the beginning and
Analyses of the peaks were performed with the software provided
by VG, based on non-linear least squares fitting program using
a weighted sum of Lorentzian and Gaussian component curves
after background subtraction according to Shirley and Sherwood
[20,21]. Atomic concentrations were calculated from peak inten-
sity using the sensitivity factors provided with the software. The
binding energy values are quoted with a precision of 0.15 eV and
the atomic percentage with a precision of 10%.
The propyl sulfonic functionalized SiO2 catalysts were syn-
thesized adopting three different procedures: grafting, co-
condensation and in situ oxidation. Materials were purchased from
2.1.1. Grafting method
The silica was previously synthesized by sol–gel tech-
nique according to
a published procedure [15]. A mixture
of 2.00 g of calcined silica in 35 ml of dry toluene and
3-mercaptopropyltrimethoxysilane (MPTMS) (1.66 or 3.33 or
6.66 mmol, corresponding to 5, 10, 20 mol% loading, respectively)
was refluxed for 24 h. The materials were then recovered by fil-
tration, washed several times with toluene and dried at 120 ◦C
overnight. Thereafter, the mercaptopropyl groups were oxidized
to sulfonic groups with hydrogen peroxide (33% w/v solid:liquid
ratio of 1:18) at room temperature during 24 h. After filtration the
solid was dried at 120 ◦C overnight.
Sulfur contents were determined by inductively coupled plasma
optical emission spectroscopy (ICP-OES) (Horiba Jobin Yvan Ultima
2).
2.3. Esterification reaction
2.1.2. Co-condensation method
A mixture of 7.5 ml of tetraethyl orthosilicate (TEOS) with 5 ml
of ethanol was stirred for 15 min at 45 ◦C. Then, 5 ml of acetic acid
aqueous solution at pH 5 was added to the mixture followed by
MPTMS addition to obtain 5%, 10%, 20% loading. The temperature
was increased to 80 ◦C until the water was evaporated out, and
finally, the solid was dried at 120 ◦C overnight. The solid was then
oxidized by the same procedure described before.
In a typical experiment, acetic acid (50 mmol) was added to a
mixture of butanol (50 mmol) and catalysts (25 mg) (mass ratio of
catalyst/alcohol = 0.006). Reagents were purchased from commer-
cial suppliers and used without further purification.
The reaction was carried out in a 25 ml round bottom flask
equipped with a water-cooled condenser. The contents were then
refluxed at 110 ◦C. The temperature was maintained using an
oil-bath connected with a thermostat. The reaction mixture was
continuously stirred during the reaction using a magnetic stirrer.
The products were analysed by GC–mass spectroscopy on a GC-
MS-QP5050A Shimadzu mass spectrometer with ionisation energy
of 70 eV and their chromatograms were in accord with those
obtained from reference samples. As a proof of a chemical regime,
varying the stirring speed either magnetic or mechanic between
200 and 700 rpm no changes in the conversion after 1 h were
observed.
2.1.3. In situ oxidation method
The samples were synthesized according to Margolese et al. [16],
as described for co-condensation method except that MPTMS, in
amounts corresponding to 5%, 10% and 20% loading, and hydrogen
peroxide (33% w/v solid:liquid ratio of 1:18) were added at the same
time. The temperature was increased to 80 ◦C until the water was
evaporated out, and finally, the solid was dried at 120 ◦C overnight.
Concerning the 20% loaded sample, since no reproducible materials
were obtained, the characterization and catalytic results are not
here included. This confirmed that, as reported in literature, the
molar concentration of propylSH moieties should be limited to less
than 20% [17].
Recycling experiments were performed over the most active
catalyst. After the reaction, the catalyst was filtered, washed with
ethanol in order to clean the catalyst surface, dried and activated
at 120 ◦C overnight and reused.
For comparison purposes, blank reaction and homogeneous
reaction were performed by stirring acetic acid (50 mmol) and
butanol (50 mmol) in absence of solvent at a temperature of 110 ◦C,
without catalyst and with 3.35 mg p-toluensulfonic acid (pTSA),
respectively. The amount of pTSA was chosen to have the same
amount of mmol H+ with respect to 5% in situ oxidation catalyst.
The products were analysed by GC–MS.
2.2. Characterization
Acid capacity and concentration of sulfonic groups of samples
were determined by titration with 0.01 M NaOH (aq) according to
Yang et al. [18]. In a typical experiment, 0.1 g of solid was added
to 10 ml of 0.2 M NaCl solution as exchange agent. The result-
ing suspension was allowed to equilibrate and thereafter titrated
potentiometrically by dropwise addition of 0.01 M NaOH solution.
The textural characterization was performed with a Carlo Erba
Sorptomat 1900 instrument. The fully computerized analysis of the
nitrogen adsorption isotherm at 77 K allowed to obtain, through
the BET method in the standard pressure range 0.05–0.3 p/po, the
specific surface areas of the samples. By analysis of the desorption
curve, using the BJH calculation method, the pore size distribution
was also obtained. The total pore volume, Vp was evaluated on the
basis of the amount of nitrogen adsorbed at a relative pressure of
about 0.98 [19].
3. Results and discussion
3.1. Characterization results
As calculated by BET analyses of the N2 adsorption curves, the
specific surface areas of the silica samples functionalized with dif-
ferent loading of sulfonic groups and using the three different
methods are listed in Table 1. In the case of grafting, the presence of
the oxidized functional groups causes a decrease of the surface area
of the original silica support. The decrease is independent on the
group loading. BET analyses performed on selected samples before
the H2O2 treatment, according to the obtained values showed in
parentheses in the table, suggest that a further reduction in surface
X-ray photoelectron spectroscopy analyses were performed
with a VGMicrotech ESCA 3000Multilab, equipped with a dual
Mg/Al anode. The spectra were excited by the unmonochroma-
tized Al K␣ source (1486.6 eV) run at 14 kV and 15 mA. The analyser