Base-Free Benzyl Alcohol Aerobic Oxidation Catalyzed by AuPdNPs Supported on SBA-15 and…
(
i) dissolution of the polymer; (ii) adding the silica source;
iii) aging; and (iv) heat treatment of drying and calcination.
and fed with oxygen gas up to a pressure of 4 bar. Then, it
was kept under heating (100 °C) and constant stirring for
2.5 h. After the determined time, the system was cooled,
depressurized and the catalyst recovered by centrifugation.
After separation, 20 μL of the reaction solution was diluted
in 1 mL of methylene chloride (CH Cl ) to determine the
(
In the present study, we have been using the methodology
reported by Araújo et al. [25], which reports both the syn-
thesis of SBA-15 and SBA-15 modiꢁed with TiO (TiO /
2
2
SBA-15). The synthesis of TiO nanocrystals in the form of
2
2
2
colloidal suspension was later added to the synthesis of the
SBA-15 matrix using in situ anchoring (ISA). The molar
relations Si/Ti(RSi/Ti = 75, 50, and 25) proposed in that
work were also maintained.
oxidation product’s yields by gas chromatography (GC). The
analyzes were performed using a GC-2010 Plus equipment
(Shimadzu, Kyoto, Japan) equipped with a Carbowax capil-
lary column and operating under conditions optimized to
detect benzyl alcohol oxidation products (P-Xylene stand-
ard). The activity of the catalyst was measured by testing the
conversion of benzyl alcohol and selectivity.
2.3 Synthesis of Catalysts
Au and AuPd catalysts supported on SBA-15 and TiO /SBA-
For the reuse test, the AuPd/S25 catalyst was subjected to
successive reaction cycles. In each cycle, the reaction, cen-
trifugation, removal of the reaction liquid, and the replace-
ment of benzyl alcohol steps were performed. There were
no washing steps between cycles.
2
1
5 (R Si/Ti=75, 50, and 25) were synthesized by the DPU
method, according to the procedure described below. In a
round-bottom ꢀask, 500 mg of support was added, followed
by 50 ml of deionized water. The suspension was stirred
at 800 rpm to disperse the support. Then, urea (99%) was
added in a molar ratio of 100 to the metal [26–29]. After
2.5 Characterization of Materials
5
min under constant agitation, the volume of a metal precur-
sors solution corresponding to the metallic load of 2% (m/m)
The structural aspects of the synthesized supports and cata-
lysts were investigated by X-ray diꢃraction (XRD) using a
Bruker D8 Advance diꢃractometer (Bruker AXS Gm6H,
Karlsruhe, Germany) with CuKα radiation (λ=1.5406 nm).
Data analysis was performed using the Rex 0.8.2 software
[32] and was based only on identifying the phases present in
the materials since the Rietveld reꢁnement was disregarded
on the support was added. For this, HAuCl ⋅3H O (30%)
4
2
and PdCl (99.9%) solutions were used in the concentration
2
−
1
of 2.0 mmol.L . We use a molar ratio of 1:2 (Au:Pd) for
bimetallic catalysts, keeping the total metallic load about 2%
of the support. The system was kept constant agitation and
gradual heating to 95 °C; the temperature was kept constant
for 6 h. The obtained solids were separated by centrifugation
and washed thoroughly with deionized water to remove the
chloride ions. The materials were dried in the presence of
air for 24 h at 100 °C; then they were calcined at 400 °C for
due to the amorphous nature of SiO . The textural proper-
2
ties were veriꢁed by the N adsorption and desorption tech-
2
nique using ASAP-2420 equipment (Micromeritics, USA).
For each analysis, an average of 93 mg of each previously
degassed sample was used. The adsorption isotherms were
obtained in a relative pressure range of 0.01 to 0.99 at a tem-
perature of −196.15 °C. The surface areas were estimated
by the Brunauer–Emmett–Teller (BET) method and the pore
volume distribution by the Barrett–Joyner–Halenda (BJH)
method desorption branch of the isotherms. The catalysts’
structural and morphological properties were veriꢁed using
the scanning electron microscopy (SEM) and transmission
electron microscopy (TEM) techniques. SEM images were
obtained using a Quanta 200F FEG microscope and TEM
images using a MORGAGNI 268D microscope (operating at
100 kV). For the TEM analysis, colloidal suspensions of the
materials were prepared with isopropanol. The samples were
deposited on a carbon-coated copper grid and dried under
ambient conditions. The images obtained were analyzed
with the aid of the ImageJ 1.52 software [33]. The average
particle size and size distribution were veriꢁed consider-
ing 115 particles (average) for the AuPd/SBA-15 and AuPd/
S25 materials and about 60 particles for the AuPd/S25 (R5)
material. The pore size estimate was made in the same way,
considering an average of 60 pores for the materials AuPd/
−
1
6
h, under airꢀow with a heating rate of 10 °C min . Au,
Pd, and AuPd catalysts supported on a TiO were also syn-
2
thesized to compare with the literature and other catalysts.
All synthesized materials were coded according to their
composition. The modiꢁed supports were named S75, S50,
and S25, according to the molar ratio Si/Ti: 75, 50, and 25,
respectively. The catalysts, in turn, were coded according to
the general representation: metal/support.
2.4 Oxidation of Benzyl Alcohol
The catalysts’ performance was veriꢁed using the oxida-
tion of benzyl alcohol as a reaction model, which were car-
ried out in a Fisher-Porter glass reactor with a capacity of
1
00 mL. The reactions were carried out on a magnetic stirrer
coupled to a heated plate with temperature control. In a typi-
cal reaction, 1 ml (9.6 mmol) of benzyl alcohol and 28.5 mg
(
2% by weight of metal) of catalyst were added to the glass
tube. For the tests with Au catalysts, the alkaline salt K CO
2
3
(99.95%) was added in a molar ratio of 80.5 (salt: metal) [30,
3
1]. The system containing the mixture was closed, purged,
1
3