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ARTICLE IN PRESS
X. Collard et al. / Catalysis Today xxx (2014) xxx–xxx
3
≥99.999%) and gallium nitrate hydrate (Ga(NO3)3·xH2O, purity
≥99.9%).
was used as shift reference. The study of the Ga coordination was
performed at the resonance frequency of 71Ga (152.52 MHz). The
samples were packed in a 3.2 mm Chemagnetics rotor and mea-
sured with a spinning frequency of 12,000 Hz. 5 × 106 scans were
accumulated with a recycle delay of 0.1 s and a pulse length of 1.2 s
(10◦). The acid properties of XS-Ga-MCM-41-H were investigated
by adsorption and temperature programmed desorption (TPD) of
pyridine monitored by Fourier Transform infrared spectroscopy
(FTIR). A self-supporting wafer of the sample (about 15 mg) was
initially dried under vacuum at 400 ◦C for 1 h, and then cooled
down to 50 ◦C in an in-house built vacuum infrared cell with ZnSe
windows. Reference spectra were recorded at 350, 250, 150 and
50 ◦C. Next, the wafer was saturated with about 25 mbar of pyridine
vapour at 50 ◦C for 10 min and then evacuated again for 30 min to
completely remove physisorbed pyridine. Finally, the sample con-
taining chemisorbed pyridine was subjected to TPD at 150, 250 and
350 ◦C for 30 min, with a heating rate of 4 ◦C min−1. The IR spectra
were recorded in situ at each of these temperatures. The amounts
of acid sites were determined from the integral intensity of char-
acteristic bands (1450 cm−1 for Lewis acid sites, and 1545 cm−1 for
Brønsted acid sites) using Emeis molar extinction coefficients [30].
2.2. Synthesis of the Ga-MCM-41 nanoparticles (XS-Ga-MCM-41)
Extra-small gallosilicate mesoporous materials (XS-Ga-MCM-
41) were prepared according to two different procedures, each
employing a different Ga content and a different base. The first
method was developed by combining elements from reported pro-
tocols for the preparation of conventional Ga-MCM-41 [26,27], with
tailored modifications to promote the formation of smaller parti-
cles (e.g. shorter reaction time). The second method was inspired
by the diluted solution route reported for the synthesis of Ti-MCM-
41 nanoparticles [8]. In both methods, CTAB was stirred in milli-Q
deionized water at 800 rpm until full dissolution of the surfac-
tant. Next, a basic solution (NaOH 2.0 M or NH4OH 28%) was added
slowly, and the resulting solution was stirred for 30 min. Ga(NO3)3,
previously dissolved in 1.0 mL of absolute ethanol, and TEOS were
added separately to the solution in a dropwise manner. The molar
composition of the synthesis mixture of the sample with higher
Ga content (XS-Ga-MCM-41-H) was, 1 TEOS: 0.0625 Ga(NO3)3: 69
NH4OH: 0.125 CTAB: 1.43 EtOH: 525 H2O. The molar composition
of the synthesis mixture of the sample with lower Ga content (XS-
Ga-MCM-41-L) was 1 TEOS: 0.0241 Ga(NO3)3: 0.33 NaOH: 0.125
CTAB: 0.90 EtOH: 1212 H2O. In both cases, the solution was stirred
for 2 h at room temperature and subsequently subjected to a ther-
mal treatment at 70 ◦C for 3 h. After filtration, the solid was washed
three times with milli-Q water and ethanol (alternately) and dried
at 60 ◦C for 16 h. Finally, the powder was calcined at 550 ◦C (heating
rate of 4 ◦C min−1) for 5 h under N2 and 5 h in air.
2.4. Catalytic tests
For the reaction of glycerol with acetone, the reaction mixture
contained: 0.921 g of highly purified glycerol (purity 99%, 10 mmol),
0.581 g of acetone (10 mmol), 0.132 g of dioxane (1.5 mmol, as GC
internal standard), and 1.48 g of tert-butanol (20 mmol, as solvent).
25 mg of catalyst was loaded at room temperature and the mix-
ture was stirred and heated to 80 ◦C for 6 h. The catalytic tests
were carried out in a 50-well reaction block, under vigorous stir-
ring (1000 rpm) [31]. At the end of the reaction, the catalyst was
separated by centrifugation and the reaction solution was ana-
lyzed by gas chromatography on a Trace GC Ultra from Interscience
equipped with a polar column (PH POR-Q column, FT-3, 10 m,
0.25 mm) [12]. Recyclability tests were performed by centrifuging
the sample at the end of the catalytic test, after which the reaction
solution was removed. Then, the catalyst was washed with tert-
butanol for five times. Afterwards, the solid was dried overnight at
100 ◦C and reused in a new catalytic run. Before the last reuse, the
catalyst was calcined at 500 ◦C for 2 h (heating rate of 2 ◦C min−1).
Leaching tests were performed under the same reaction condi-
tions employed for the catalytic tests (vide supra). The catalyst was
removed from the reaction mixture after 30 min by centrifugation,
followed by hot filtration at the same temperature of the catalytic
test using a plastic syringe equipped with a 25 mm HPLC syringe fil-
ter from Altech, with a pore size of 0.2 m. The filtrate was allowed
to react for another 5 h 30 min. The reaction mixture was analyzed
by GC both after 30 min and at the end of the filtrate test (6 h).
For the conversion of dihydroxyacetone (DHA) to ethyl lac-
tate, 0.180 g of DHA (2 mmol, in the form of 1,3-dihydroxyacetone
dimer) and 0.0215 g of decane (0.15 mmol, as GC internal standard)
were dissolved in 3.92 g of ethanol (as solvent and reactant) at 45 ◦C
for 30 min. Next, 50 mg of catalyst were added to the solution at
room temperature. The reaction mixture was heated to 90 ◦C for
6 h under vigorous stirring (1200 rpm). The tests were carried out
in the same 50-well reaction block used for the reaction of glycerol
with acetone. At the end of the test, the catalyst was separated by
centrifugation and the reaction solution was analyzed by gas chro-
matography (GC) on a Trace GC Ultra from Interscience equipped
with an RTX-5 fused silica column (5 m; 0.1 mm). Recyclability
tests were performed by separating the catalyst from the reaction
mixture by centrifugation followed by washing with ethanol (five
times). Next, the catalyst was dried overnight at 100 ◦C and reused
in a new catalytic run. Before the last reuse, the catalysts were cal-
cined at 500 ◦C for 2 h (heating rate of 2 ◦C min−1). The leaching tests
A sample of XS-Ga-MCM-41-L was subjected to an ion-exchange
treatment in order to replace the Na+ ions by H+. 1.0 g of XS-Ga-
MCM-41-L was added to 20 mL of a 3.0 M aqueous solution of
NH4OH. The suspension was stirred for 5 h at room temperature
to allow exchange of Na+ by NH4+. After the exchange, the solid
was removed by filtration and washed three times with absolute
ethanol and water. Then, the sample was dried at 100 ◦C for 12 h
and calcined at 550 ◦C to obtain the H+-form of the material, which
was labelled XS-Ga-MCM-41-L (ion exchanged).
2.3. Characterization of the XS-Ga-MCM-41 materials
Powder X-ray diffraction (XRD) patterns were measured on
a
PANalytical X’pert pro diffractometer with Cu K␣ radia-
˚
tion (ꢀ = 1.54178 A). Specific surface area and porosity of the
isotherms obtained at −196 ◦C with a volumetric adsorption
analyzer (Micromeritics Tristar 3000). The samples were pre-
treated at 150 ◦C for 24 h under a reduced pressure of 10−4 bar.
The Brunauer–Emmet–Teller (BET) method was used to calcu-
late the specific surface areas [28]. The Barrett–Joyner–Halenda
(BJH) method applied to the adsorption isotherm was used to
determine the pore size distributions [29]. Transmission electron
microscopy (TEM) images were recorded on a Philips Tecnai 10
with an accelerating voltage of 80 kV. Field emission scanning
electron microscopy (FE-SEM) images were obtained on a JEOL
JSM 7500 instrument. Chemical compositions were determined
by energy-dispersion X-ray spectroscopy (EDX) using an acceler-
ation potential of 7.5 kV, and a working distance of 8 mm. 29Si
and 71Ga magic-angle spinning nuclear magnetic resonance (MAS-
NMR) spectra were measured on a VARIAN 400 and a Bruker 500
spectrometer, respectively. The Si environment was studied at the
resonance frequency of 29Si (79.46 MHz). The samples were packed
in a 4 mm zirconia rotor and measured with a spinning frequency
of 8000 Hz. 4.14 × 106 scans were accumulated with a recycle delay
of 6 s and a pulse length of 2.0 s (30◦). Tetramethylsilane (TMS)
Please cite this article in press as: X. Collard, et al., Ga-MCM-41 nanoparticles: Synthesis and application of versatile heterogeneous