M.L. Testa et al. / Journal of Molecular Catalysis A: Chemical 367 (2013) 69–76
71
(
NH -TPD). The sample amount of ∼0.1 g was out-gassed in a 5 vol.%
3
◦
O /He flow at 120 C for 30 min. Then, ammonia adsorption was
2
performed by admitting a flow of 5% NH /He stream (30 ml/min)
3
at room temperature for 1 h. After purging with 100 ml/min He
◦
flow for 1 h at 100 C and then cooling down to room tempera-
ture, the catalyst was heated under He (30 ml/min) in a linear rate
◦
◦
of 10 C/min to 500 C and the ammonia desorption was monitored
TM
by online TCD and mass quadrupole spectrometer (Thermostar
,
Balzers). Before gas detection, a cold trap was used to condense any
water desorbed from the sample. In the range of temperature inves-
◦
tigated, up to 500 C, no decomposition of propyl sulfonic group was
observed for acid functionalized amorphous and mesoporous sili-
cas. A good agreement between TCD signal and NH QM signals was
3
found. For accuracy, the total acidity of the catalyst was calculated
by integration of NH3 desorption profile referred to the QM signal
at mass 15.
The textural properties were obtained using a Carlo Erba Sorp-
tomat 1900 instrument. The fully computerized analysis of the N2
Fig. 1. Nitrogen adsorption–desorption isotherms of SAS and SSBA samples. In the
adsorption isotherm at 77 K allowed to obtain, through the BET
method in the standard pressure range 0.05–0.3 p/p , the spe-
inset the pore size distribution of the SSBA sample is showed.
0
cific surface areas of the samples. The total pore volume, Vp was
evaluated on the basis of the amount of nitrogen adsorbed at a rel-
ative pressure of 0.998, while mesopore size distribution values and
mesopore volumes were calculated by applying BJH model in the
For comparison purposes, blank, homogeneous and commer-
cial Amberlyst-15 (Aldrich) catalyzed reactions were performed
by stirring glycerol (10.87 mmol) and acetic acid (32.6 mmol) at
◦
a temperature of 105 C, without catalyst or with 14.6 mg p-
0
range of p/p of 0.1–0.98 [24].
toluensulfonic acid (p-TSA) or 50 mg of Amberlyst-15, respectively.
The amount of p-TSA was chosen to have the same amount of
mmol H+ as the SSBA, the best catalyst of the series. The products
were analyzed by GC–MS.
X-ray diffraction patterns were measured with a Bruker verti-
cal goniometer using Ni-filtered Cu K␣ radiation. A proportional
◦
counter and 0.05 step sizes in 2Â were used. The assignment of the
various crystalline phases was based on the JPDS powder diffraction
file cards [25].
2.4. Catalyst reusability
X-ray photoelectron spectroscopy analyses were performed
with a VGMicrotech ESCA 3000 Multilab, 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 analyzer
operated in the constant analyzer 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 mounted on a double-sided
adhesive tape. The pressure in the analysis chamber was of the
Recycling experiments were performed over the most active
catalysts of the series, SAS, SSBA and Amberlyst-15. After the reac-
tion, the catalysts were filtered, washed with methanol in order to
◦
clean the catalyst surface, dried, activated at 120 C overnight and
reused.
3. Results and discussion
−8
order of 10 Torr during data collection. The constant charging of
the samples was removed by referencing all the energies 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 at the end of the
analyses ensured absence of differential charging. 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 sub-
traction [26,27]. Atomic concentrations were calculated from peak
intensity 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%.
In order to identify the crystalline phases of the synthesized
samples X-ray diffraction analyses were performed. For SAS, SSBA
◦
and Ti10HMS, a broad peak at 2Â = 22 attributable to the amor-
phous framework of silica was found. No TiO2 lines were visible in
the XRD pattern of Ti10HMS. Amorphous profiles were also regis-
tered for Nb O5·nH O, dry-SZ and SZ-470.
2
2
In Table 1 the values of surfaces areas, pores diameter, total
pores volume and mesopore volume of all samples are reported.
The silica-based samples (SAS, SSBA and Ti10HMS) were charac-
2
−1
terized by high surface areas, above 500 m g and high total pore
volumes. As expected in SSBA and Ti10HMS, the greater part of
the pore volume was due to mesopores, while in the SSA sam-
ple only a fourth of pores were in the mesopores range. Fig. 1
shows the adsorption–desorption isotherms relative to SSBA and
SSA, in which the different pore structures is evidenced by the
differences in the hysteresis. Moreover, in the inset, the pore diam-
eter distribution of SSBA sample, centered at 9.4 nm, is showed. All
other samples had lower surface areas, ranging between 20 and
2.3. Esterification reaction
In a typical experiment, acetic acid (32.6 mmol, Aldrich) was
added to a mixture of glycerol (1 g, 10.87 mmol, Aldrich) and cata-
lysts (50 mg). Mass ratio of catalyst/glycerol corresponding to 5 wt%
and molar ratio acetic acid:glycerol corresponding to 3:1 were
used. The reaction was carried out in a 25 ml round bottom flask
connected to water-cooled condenser. The reaction mixture was
2
−1
120 m g and lower pore volumes.
Acidity properties of selected samples were evaluated by NH3-
TPD. The technique provides information on the total acidity of
the solids, without distinguishing between Brønsted and Lewis
acidity but accounting for the different strength of acid sites. The
TPD curves (as TCD signal) registered in the range of temper-
◦
continuously stirred using a magnetic stirrer and refluxed at 105 C.
The products were analyzed by GC–MS on a GCMS-QP5050A Shi-
madzu mass spectrometer with ionization energy of 70 eV and their
chromatograms were in accord with those obtained from reference
samples. The final solution was then filtered to recover the catalyst
◦
ature between 50 and 500 C are shown in Fig. 2 where three
◦
that was dried at 120 C overnight and reused in further reactions.
temperature zones associated to the acid sites strength (weak,