G Model
CATTOD-10584; No. of Pages8
ARTICLE IN PRESS
C.-H. Tsai et al. / Catalysis Today xxx (2017) xxx–xxx
2
the agglomeration of metal nanoparticles [15,21–28]. Furthermore,
surface-bound ligands/functionalities can be used to stabilize the
metal NPs, whose high surface energy frequently leads to the aggre-
gation of nanoparticles [17,29–33]. Several studies involved in
supporting metal nanoparticles on the mesoporous silica materials,
such as Pd, Pt, Au, Rh, Ir and Ru on MCM/SBA-type materials, have
revealed superior catalytic performances on many types of chem-
ical transformations [15,16,34–38]. The synergistic effect has also
been observed in some bimetallic systems [1,17,39]. For example,
Chen and coworkers reported a Pd/Au bimetallic catalyst system
exhibiting enhanced reactivity and selectivity toward the solvent-
free aerobic oxidation of alcohols, where the Pd and Au complexes
were coordinated onto the amine-functionalized SBA-16 silica sup-
port prior to their reduction to nanoparticles [33]. In another
example, Yang et al. reported a high performance Pd/Au bimetallic
catalytic system demonstrating hydrogenation of cinnamaldehyde
and dried under high vacuum for 24 h. To remove the surfactant
molecules, a solution of MSN materials (2.0 g) and 2.0 ml of con-
centrated HCl in 200 ml of methanol was stirred at 60 C for 6 h.
The resulting surfactant-free MSNs were filtered, washed with
water and methanol, and dried under vacuum for 24 h.
◦
2.3. Synthesis of bis(ethylenediamine) gold (III) chloride −
Au(en) Cl
2
3
The synthetic procedure of this Au complex precursor has been
reported in the literature [31]. Typically, ethylenediamine (0.45 ml,
6.7 mmol) was slowly added into 10 ml of aqueous solution of
HAuCl ·3H O (1.0 g, 2.54 mmol) until the solution turned trans-
4
2
parent. This solution was stirred for 30 min at room temperature.
Anhydrous ethanol (70 ml) was then added into the solution and
a precipitate formed immediately. The solid product was filtered,
followed by washing with ethanol and drying overnight under high
vacuum.
[
40].
Herein, we report the synthesis and catalytic properties for
a tandem aerobic oxidative esterification reaction of a series of
mono- and bimetallic Pd/Au catalysts supported on mesoporous
silica nanoparticles (MSNs) through a sequential impregnation
method (Scheme 1). This synthetic approach has shown to be effec-
tive in producing homogeneously distributed metal NPs on the
MSN support and good control of the incorporated metal ratio. We
compared the catalytic performance of our MSN catalysts with sev-
eral commercially available Au and Pd catalysts, such as Au@TiO2,
Au@Al O and Pd/C, in the aerobic oxidative esterification of ben-
zyl alcohol with methanol. Several primary alcohols were also
selected for oxidation to demonstrate the generality of this bimetal-
lic catalyst. Interestingly, monometallic Pd@MSN catalyst exhibited
high reaction conversion but poor selectivity, leading to benzalde-
hyde as the major product. On the contrary, catalysts containing
monometallic Au showed superior selectivity to ester product but
suffered from sluggish reactivity. Compared to their monometal-
lic counterparts, the bimetallic Pd-Au@MSN catalyst was more
efficient in both reactivity and selectivity. An in-depth study of dif-
ferent substrates was also performed with the most efficient Pd-Au
bimetallic MSN catalyst. Finally, we tested the recyclability of the
bimetallic Pd-Au@MSN catalyst in the same reaction conditions.
2.4. Synthesis of monometallic Pd@MSN
Surfactant-free MSN materials (500 mg) was pre-dried under
◦
vacuum at 90 C for 6 h to remove physisorbed water. To this
reaction, a solution of Pd(OAc)2 (28 mg, 0.125 mmol) in 15 ml dry
toluene was injected. The reaction was stirred at 35 C for 3 h, fol-
lowed by filtration and wash with 300 ml of toluene and 100 ml of
methanol, and dried under vacuum for 24 h to obtain a brownish
solid product denoted as Pd-complex-MSN. The Pd-complex-MSN
◦
2
3
−
1
materials were then reduced by flowing H at a rate of 30 ml min
2
◦
at 250 C for 3 h to yield a gray colored final product (Pd@MSN).
2.5. Synthesis of monometallic Au@MSN
Typically, Au(en) Cl3 (36 mg, 0.085 mmol) was dissolved in
2
60 ml of DI water. The pH value of this solution was adjusted by
adding 1.0 M NaOH solution to reach a pH of 10.0. Subsequently,
500 mg of surfactant-free MSN materials were added. The pH value
of this solution dropped to around 6 immediately due to the intrin-
sic acidity of the high surface area silica material. By adding 1.0 M
NaOH solution, the final pH value of the reaction mixture was tuned
to 9.5. The mixture was stirred for an additional 2 h at room tem-
perature, followed by filtration and wash with 300 ml of water and
2
. Materials and methods
1
00 ml of methanol, and dried under vacuum for 24 h to obtain
a yellow colored solid product denoted as Au-complex-MSN. The
reduction of the Au-complex-MSNs was carried out by flowing H
at a rate of 30 ml min at 250 C for 3 h to yield a purple colored
final product (Au@MSN).
2.1. Reagents and materials
2
All chemicals were used as received without further purifica-
tion. Tetraethoxysilane (TEOS) was purchased from Gelest, Inc.
−
1
◦
Au@TiO2 (1 wt% Au) and Au@Al O3 (1 wt% Au) were purchased
2
from Strem Chemicals, Inc. Other chemical reagents were pur-
chased from Sigma-Aldrich, Inc.
2.6. Synthesis of bimetallic Pd-Au@MSNs
A sequential impregnation method was applied to synthesize
2.2. Synthesis of mesoporous silica nanoparticle (MSN)
the bimetallic MSN materials. The pre-synthesized Au-complex-
MSN (500 mg) was dried at 90 C for 6 h to remove physisorbed
◦
The MSN material was synthesized via a previously reported
water molecules. A solution of Pd(OAc)2 (17.7 mg, 0.079 mmol)
in dry toluene (25 ml) was then added. The solution was stirred
at 35 C for 3 h, followed by filtration and washing with copious
co-condensation method [41,42]. Typically,
a
mixture of
◦
cetyltrimethylammonium bromide (CTAB, 2.0 g, 5.5 mmol) and
2
.0 M of NaOH(aq) (7.0 ml, 14 mmol) in 480 ml of deionized water
amounts of toluene and methanol, and dried under vacuum for 24 h.
The reduction procedure was the same as previously described. By
tuning the molar ratio of Pd and Au, three bimetallic Pd-Au@MSNs-
x (where x indicates Pd/Au molar ratio) catalysts were synthesized.
◦
was heated at 80 C for 30 min. To this solution, tetraethoxysilane
(
TEOS, 10.0 ml, 44.8 mmol) was injected rapidly. A milky solution
formed within 2 min post injection. The resulting reaction mix-
tures were stirred at 80 C for 2 h. The solid product was then
◦
filtered, washed with copious amounts of deionized water and
methanol and dried overnight under high vacuum. A hydrothermal
treatment of the as-made MSN material was performed by soaking
MSN material (3.0 g) in 20 ml of DI water. The reaction mixture
2.7. Aerobic oxidative esterification of benzyl alcohol for
comparison of catalysts
◦
was incubated at 100 C for 6 h. The solid product was filtered
1 mmol), and catalysts (0.005 mmol, 0.5 mol% on the total metal
Please cite this article in press as: C.-H. Tsai, et al., Aerobic oxidative esterification of primary alcohols over Pd-Au bimetallic catalysts