N. Masoud et al.
AppliedCatalysisA,General561(2018)150–157
Scheme 1. Oxidation of 5-hydroxymethyl fur-
fural (HMF) to furan-2,5-dicarboxylic acid
(FDCA) via formation of intermediate 5-hy-
droxymethyl furan carboxylic acid (HMFCA).
of the factors that affect the stability of the Au nanoparticles as well as
strategies to alleviate Au particle growth are needed.
after 20 h at 35 °C under static conditions, the cloudy suspension was
subsequently kept at 100 °C for 24 h. The precipitate was filtered and
washed at RT until no chloride ions were left, as evidenced by silver
nitrate test, and subsequently dried at 120 °C in static air overnight.
Finally, the precipitate was calcined at 540 °C in static air for 6 h to
yield SBA16 (BET surface area of 780 m2 g−1, total pore volume of
1.04 mL g−1).
All supports (3 g for each of them) were functionalized using ami-
nopropyl triethoxysilane (APTES). First, they were dried at 140 °C
under vacuum for 24 h. Then, dry toluene (50 mL) and APTES (1 g for
Aerosil and 3 g for silica gel, MCF and SB16) were added. We added the
amount of APTES needed for covering the support surface based on the
BET surface area of the supports, and considering three OH groups per
nm2 of the SiO2 support [47]. The mixture was refluxed for 24 h at
110 °C in a N2 atmosphere. The functionalized supports were recovered
by centrifugation, washed with ethanol (40 mL) at RT twice, and dried
at 60 °C in static air overnight.
All catalysts were prepared following the method of Mou et al. for
the deposition of Au on SiO2 [48]. The functionalized supports (1 g)
were dispersed in water (15 mL, doubled distilled). To deposit 1.5 wt%
Au on Aerosil and 3 wt% Au on Aerosil, silica gel, MCF, and SBA16,
appropriate amount of an aqueous Au solution (0.06 M HAuCl4.3H2O,
Sigma Aldrich) were added. The mixture was stirred at RT for two
hours, and the powder was recovered by centrifugation and washed
with H2O (40 mL) at RT twice. Then, the powder was re-dispersed in
water (15 mL) and reduced by a rapid addition of an excess of a re-
ducing agent (10 mL, 0.2 M NaBH4) under vigorous stirring at RT. After
20 min, the product was collected by centrifugation, washed with water
(40 mL) at RT five times and dried at 60 °C in static air overnight. To
eliminate the organic groups, the catalysts were calcined at 500 °C in
static air for 4 h. The catalysts are denoted as Au/Aerosil, Au/silica gel,
Au/MCF, and Au/SBA16.
Silica supports are widely used in heterogeneous catalysis [40,41].
They can be prepared with different specific surface areas
(50–2000 m2/g) and structures. For example, aggregated spherical
particles of Aerosil form a porous material in which the pores are
formed by interparticle spaces. Ordered mesoporous structures like
SBA16 [42] and mesoporous cellular foam (MCF) [43] have cage like
structures with different neck sizes. Like SBA16, silica gel can have
small pore sizes (around 9 nm), but it has a disordered structure [44].
The flexibility in having supports with different surface areas and
structures allows the design of catalysts with different interparticle
distances and a uniform distribution of well-defined nanoparticles.
Previously, our group reported for Cu nanoparticles on SiO2 for me-
thanol synthesis that a uniform distribution of nanoparticles which
maximize interparticle distances minimizes particle growth [45,46]. It
was also found that the neck size was a crucial factor to limit particle
growth for these catalysts [44].
In this study, the activity and stability of Au on SiO2 for liquid phase
oxidation of HMF to FDCA are investigated. This is the first time that
the activity of Au/SiO2 for this reaction is reported. In general, carbon
supports are better candidates for this type of reactions due to their
higher hydrothermal stability in particular under alkaline conditions.
However, diverse structures of the SiO2 supports allow a detailed in-
vestigation of the effect of support morphology on the stability of Au
nanoparticles. Our results show that the morphology of the support
plays an important role in the stability of the SiO2-supported Au cata-
lysts.
2. Material and methods
2.1. Catalyst preparation
Aerosil 300 (BET surface area of 270 m2 g−1, mesopore volume of
0.78 mL g−1) was purchased from Evonik. Silica gel 7085 (BET surface
area of 500 m2 g−1, mesopore volume of 0.90 mL g−1) was received
from Grace Davison. Mesoporous SiO2 supports (SBA16 and MCF) were
prepared in house. MCF [43] was prepared by dissolving Pluronic P123
(2.0 g, EO20PO70EO20, Mav = 5800, Sigma Aldrich) in aqueous HCl
solution (75 mL, 1.6 M) at room temperature (RT). Then, 1,3,5-tri-
methylbenzene (TMB, 2.0 g) was added. After stirring at least 45 min at
35 °C, tetraethoxysilane (4.4 g, TEOS) was added, and the mixture was
transferred to a 200 mL Teflon-lined autoclave. After 20 h at 38 °C in
static condition, the cloudy suspension was further kept at 100 °C for
24 h in static condition. The precipitate was filtered and washed at RT
until no chloride ions were left (verified using an AgNO3 solution to test
the washing liquid) and subsequently dried at 120 °C in static air
overnight. Finally, the precipitate was calcined at 500 °C in static air for
6 h to yield MCF (BET surface area of 610 m2 g−1, total pore volume of
2.01 mL g−1).
2.2. Characterization
Elemental analysis was performed using inductively coupled
plasma-optical emission spectrometry in the Faculty of Geosciences at
Utrecht University. X-Ray Diffraction (XRD) was carried out with a
Bruker D2 phaser with Co Kα source. Crystallite sizes were obtained
from the peak broadening at 2θ = 44° using the Scherrer equation. For
transmission electron microscopy (TEM) imaging on MCF and SBA16
samples, they were first embedded in epoxy resin (Epofix, EMS), then
cut to slides of 70 nm thickness using a Diatome 35° diamond knife
mounted on an Ultracut E Reichert-Jung microtome (Leica) and finally
collected on TEM grids. TEM imaging was performed on a Tecnai 12
(FEI) microscope operated at 120 kV. High angle annular dark-field
scanning transmission electron microscopy (HAADF-STEM) was per-
formed on a Talos F200X microscope operated at 200 kV. Energy-dis-
persive X-ray (EDX) spectroscopy was performed by four windowless
SuperX EDX-detectors with a resolution of 128 eV arranged around the
sample. STEM image processing and identification of the EDX signal
was carried out using Tecnai Imaging Analysis (TIA) software. Particle
sizes were determined from the TEM images by measuring the sizes of
typically 300 particles on different places of the sample.
SBA16 [42] was prepared by dissolving Pluronic F127 (4.7 g,
EO106PO70EO106, Mav = 12600, Sigma Aldrich) in a mixture of aqueous
HCl solution (75 mL, 1.6 M) and water (210 mL) at RT. Then, 1-butanol
(13.2 g) was added, and the mixture was stirred at 35 °C for 1 hour.
Then, tetraethoxysilane (20.8 g, TEOS) was added dropwise under fast
stirring. The mixture was transferred to a Teflon-lined autoclave, and
Nitrogen physisorption measurements were performed at −196 °C
(Micromeritics, TriStar 3000) to determine the BET surface area of the
151