Z. Tai et al.
Molecular Catalysis 449 (2018) 137–141
small Au nanoparticles confined within the internal void against
thermal sintering [32]. To our knowledge sulfated analogues of such
monodispersed zirconia nanospheres (or core-shell variants) have not
been synthesized, while the only report of magnetic sulfated zirconia
nanoparticles produced inhomogeneous materials lacking a well-de-
fined magnetic core-SZ shell structure.
Herein, we report the first synthesis and catalytic application of
monodispersed magnetic SZ nanoparticles possessing an onion skin
structure comprising a magnetite core, encapsulated by a protecting,
dense silica shell, which is in turn enveloped by a conformal SZ shell of
were stirred with methanol at 60 °C for 4 h to remove physisorbed
sulfate. Magnetic sulfated zirconia particles prepared with H
designated xMSZ where x = [H SO ], those prepared with ammonium
sulfate are designated NH MSZ.
For comparison, a pure sulfated zirconia (SZ) was prepared by
conventional wet impregnation with H SO [22]. 2.5 g of Zr(OH) (MEL
Chemicals-XZO 880/01) was added to 25 mL aqueous H SO (0.2 M).
2 4
SO , are
2
4
4
2
4
4
2
4
The resulting slurry was stirred for 5 h at room temperature, filtered,
and dried at 80 °C overnight, prior to calcination in air at 550 °C (ramp
rate 2 °C/min) for 3 h.
3 4 2
tunable thickness between 6 and 28 nm (Fe O @SiO @SZ). These
magnetic SZ nanoparticles were subsequently evaluated for propanoic
acid esterification, a prototypical solid acid catalyzed reaction em-
ployed to upgrade biomass derived pyrolysis-oil [33–35] which are
2.2. Catalyst characterization
Textural and structural properties of parent core-shell nanoparticles
and the corresponding sulfated nanoparticles were measured by a com-
intrinsically unstable and corrosive due to C
. Experimental
.1. Catalyst synthesis
.1.1. Synthesis of silica encapsulated iron oxide nanoparticles (Fe
2 3
-C acid components [36].
bination of N
Nitrogen physisorption was undertaken on a Quantachrome Nova 1200
instrument, with samples degassed at 120 °C for 6 h prior to recording N
2
porosimetry and transmission electron microscopy (TEM).
2
2
2
adsorption/desorption isotherms. Brunauer–Emmett–Teller (BET) sur-
face areas were calculated over the relative pressure range 0.01–0.2 (P/
P ), while pore size distributions were calculated using the
0
2
2
O
3
@
SiO
2
)
Barrett–Joyner–Halenda (BJH) method applied to the desorption branch
of the isotherm. Transmission electron microscopy (TEM) images were
recorded on an aberration corrected JEOL 2100-F electronic microscope
operating at 200 kV; equipped with a Gatan Orius SC600A CCD camera.
Samples were prepared by dispersion in ethanol and drop-casting onto a
copper grid coated with a holey carbon support film (Agar Scientific Ltd).
Images were analysed using Image J 1.41 software.
Bulk sulfur elemental analysis was performed on a FLASH 2000
CHNS/O organic elemental analyzer. XPS was performed on a Kratos
Axis HSi X-ray photoelectron spectrometer fitted with a charge neu-
tralizer and magnetic focusing lens employing Al Kα monochromated
radiation (1486.7 eV); spectral fitting was performed using Casa XPS
version 2.3.15, with spectra energy-corrected to the C 1s peak of ad-
ventitious carbon at 284.6 eV.
Silica encapsulated iron oxide nanoparticles were synthesized using
a modified version of the method of Zhao et al. [37] Uniform hematite
Fe ) particles were first obtained by aging a 0.02 M aqueous FeCl
solution at 100 °C for 48 h, with the resulting Fe nanoparticles iso-
lated by centrifugation, then washed three times with 2-propanol.
0 mg of the resulting Fe nanoparticles were then added to a mix-
ture of 200 mL 2-propanol, 40 mL H O, and 5 mL of 35 vol% aqueous
(
2
O
3
3
2 3
O
5
2 3
O
2
ammonia while stirring. To this nanoparticle suspension, 0.15 mL of
tetraethyl orthosilicate was added dropwise under vigorous stirring and
the resulting mixture aged at room temperature overnight to yield silica
encapsulated iron oxide nanoparticles upon centrifugation.
2
(
.1.2. Preparation of magnetic zirconia encapsulated Fe
Fe @SiO @ZrO
The as-synthesized Fe
00 mL ethanol containing the desired volume (0.25 mL–1.5 mL) of
O).
3 4
O nanoparticles
3
O
4
2
2
)
Acid site loadings were quantified by propylamine decomposition
via the Hoffman reaction, using a thermo-gravimetric mass spectro-
metry analysis (TGA-MS) method [38]. Prior to thermogravimetric
analysis, samples were impregnated with propylamine and then dried
in the vacuum oven at 40 °C overnight. TGA was performed on a Mettler
Toledo, TGA/DSC2 Star system with a heating rate of 10 °C/min from
2 3 2
O @SiO nanoparticles were added to
1
Lutensol AO5 solution (0.43 g Lutensol AO5 dissolved in 11 g of H
2
The mixture was stirred for 1 h at room temperature, and then the
desired amount (0.05 mL–0.9 mL) of zirconium (IV) butoxide was
added quickly. The solution was kept at room temperature and stirred
vigorously overnight. The resulting nanoparticles were centrifuged, re-
dispersed in water, and then aged at room temperature for 3 days.
Subsequently the nanoparticles were centrifuged and calcined at 900 °C
−
1
2
40 to 800 °C with flowing N (30 mL min ). In parallel, evolved gas
analysis was performed with Pfeiffer ThermoStar mass spectrometer,
connected to the outlet of the TGA apparatus. Mass channels were re-
corded for m/z values of 17 (NH ), 41 (propene) and 59 (propylamine),
3
(
ramp rate 2 °C/min) for 2 h to yield zirconia encapsulated Fe
SiO @ZrO nanoparticles. These were finally reduced under flowing H
at 450 °C (ramp rate 2 °C/min) for 2 h to convert the hematite core into
magnetite (Fe ).
2
O
3
@
with the evolution of reactively propene a product from propylamine
decomposition over acid sites.
2
2
2
Brønsted/Lewis acid character was determined by pyridine DRIFT,
carried out by impregnation of diluted samples (10 wt% in KBr) with
neat pyridine. Excess physisorbed pyridine was removed in a vacuum
oven at 30 °C overnight prior to sample loading in the environmental
cell. DRIFT spectra of the pyridine-saturated samples were recorded at
room temperature under vacuum using a Nicolet Avatar 370 MCT with
Smart Collector accessory, mid/near infrared source and a mercury
cadmium telluride (MCT-A) photon detector at −196 °C.
3 4
O
2
.1.3. Preparation of magnetic sulfated zirconia (nanoparticles) Fe
SiO @SO -ZrO
Fe @SiO
AO5 vol ratio of 0.45 mL:0.25 mL were selected as the parent. Sulfation
was performed through either wet impregnation with H SO or in-
cipient-wetness impregnation by (NH SO . For wet impregnation,
.4 g of Fe @SiO @ZrO nanoparticles were added to 30 mL aqueous
SO (0.05–0.20 M) with stirring at room temperature for 5 h. The
resulting sulfated nanoparticles were magnetically separated, dried
overnight in an oven, and then annealed under flowing N at 550 °C
ramp rate 2 °C/min) for 3 h. Incipient-wetness impregnation was per-
formed by mixing 0.3 g of Fe @SiO @ZrO nanoparticles with 1 mL
of deionized water containing 0.3 g of (NH SO , followed by drying
overnight in an oven, and then annealed under flowing N at 550 °C
3 4
O @
2
4
2
3
O
4
2 2
@ZrO prepared with a zirconium butoxide:Lutensol
2
4
4
)
2
4
0
H
3
O
4
2
2
2.3. Propanoic acid esterification
2
4
Esterification was carried out on a Radleys Starfish carousel at 60 °C
using a stirred batch reactor at atmospheric pressure. Reactions were
conducted with 10 mmol of propanoic acid in 12.5 mL of methanol
(molar ratio nMeoH/nPA = 30), 50 mg of catalyst and 0.59 mL of di-
hexylether as an internal standard. Samples were withdrawn periodi-
cally, separated with a strong magnet and diluted with methanol prior
to analysis on a gas chromatograph (Varian 450-GC, Phenomenex ZB-
50 15 m × 0.53 mm × 1.0 μm capillary column, FID detector).
2
(
3
O
4
2
2
4
)
2
4
2
(
ramp rate 2 °C/min) for 3 h.
Prior to use, both types of sulfated Fe O @SiO @ZrO nanoparticles
3 4 2 2
138