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lattice on which a variety of reactive oxygen functional groups
are present [14]. The structural diversity of these carbonaceous
supports results in different metallic active site structures and,
consequently, induces distinctive catalytic behaviors.
pretreatment step was performed to ensure the complete oxida-
tion of graphite. CNTs were functionalized by incubating in HNO3
(64–66%) for 10 h at 140 ◦C to introduce oxygen functional groups.
These samples were then denoted CNTs-10h140. The CNTs-5h550
support was prepared by thermally treating the CNTs-10h140 sam-
ple for 5 h at 550 ◦C under H2 flow to remove the oxygen-containing
groups.
Pd-based catalysts has attracted enormous attention in recent
decades due to the growing demand for enantiomerically pure
studied reactions is the enantioselective hydrogenation of (E)-˛-
phenylcinnamic acid (PCA), which has been intensively examined
by Nitta and others [17–20]. Many factors that affect the enantiose-
lectivity of the reaction, including the solvents [21], additives [22],
or catalyst preparation conditions [23] have been investigated with
the aim of enhancing the enantiomeric excess (ee) values. In terms
of the interaction between CD and Pd surface, Baiker’s group has
been reported that the quinoline moiety of the CD molecule anchors
parallel to the Pd surface, while the quinuclidine N points toward
C9 (OH) groups is very important in enantioselection. Activated car-
bon is a conventional catalyst support used in this type of reaction
[25,26], and little effort has been applied toward the use of other
carbonaceous materials, such as GO [27] or CNTs [28]. Moreover,
no systematic studies have yet examined the effects of these car-
bonaceous supports on the enantioselectivity of the hydrogenation
reaction of prochiral ˛,ˇ-unsaturated carboxylic acids. Therefore,
a comparison of these carbonaceous materials as catalyst supports
for the hydrogenation of PCA could reveal the influence of the sup-
ports and clarify the mechanism underlying this reaction. These
insights could assist the design of novel catalysts with enhanced
enantioselectivity.
Supported palladium catalysts were prepared by the
PdCl2 solution was prepared by ultrasonicating a mixture con-
taining 25 mg PdCl2 and 2 mL HCl (37%) in 10 mL H2O for 20 min.
In a typical preparation, 300 mg of the support (AC, GO, CNTs, or
functionalized CNTs) were dissolved in 100 mL H2O with ultrasonic
treatment over 1 h. The pH of the mixture was then adjusted to
10 by the addition of a 1 M Na2CO3 solution, followed by 30 min
the pH adjustment, the mixture was stirred for 5 h and then was
heated at 80 ◦C for 2 h. The heat treatment magnified the acidity of
the solution and promoted the hydrolysis process to form Pd(OH)2,
which anchored to the support surface [30,31]. The suspension
was cooled to room temperature, and NaBH4 dissolved in 20 mL
H2O was slowly dropped into it. As the NaBH4 solution was added,
the metal hydroxide Pd(OH)2 was reduced to Pd nanoparticles
according to the equation
Pd(OH)2 + BH4− = Pd0 + BO2− + 3H2.
An excess quantity of NaBH4 (16 mg) was added to ensure the
complete reduction of Pd precursor to Pd nanoparticles. The reac-
tion was continued for 3 h with stirring, and the black precipitate
was filtered, washed five times with water, dried overnight in an
oven (110 ◦C), and stored for use. The ICP analysis revealed that the
wt% of Pd loadings in the synthesized Pd/AC, Pd/GO, and Pd/CNTs
catalysts were 4.92%, 4.90%, and 4.87%, respectively, in consistency
with the theoretical calculation.
This work seeks fundamental insights into the role of the catalyst
support in the enantioselectivity of the asymmetric hydrogenation
of PCA by systematically comparing the properties of carbonaceous
material supports for Pd-based catalysts under similar catalyst
preparation and hydrogenation reaction conditions. Pd-based cata-
lysts supported on AC, GO, or CNTs were prepared using deposition
methods and were characterized by a variety of approaches. The
oxygen functional groups present on the supports were found to
play a key role in determining the enantiodifferentiation.
2.3. Characterization
The BET surface areas, pore volumes, and pore sizes of the car-
bonaceous materials were determined using the N2 adsorption
method implemented on a Micromeritics TriStar II 3020 V1.03
Analyzer. The samples were degassed under vacuum for 3 h at
200 ◦C using a filler rod prior to the adsorption measurements.
The X-ray diffraction (XRD) patterns of the samples were recorded
using a D2-phaser (Bruker) equipped with Cu radiation (30 kV,
10 mA) and a LYNXEYE detector that scanned over sin 2ꢀ between
20◦ and 80◦. Scanning electron microscopy (SEM) images of the
samples were collected using a field FEI Inspect F50 SEM oper-
ated at 10.0 kV. Transmission electron microscopy (TEM) images
were obtained using a Tecnai F30 Microscope operated at 300 kV
to analyze the morphologies of the Pd-based catalysts. The cata-
lysts were dispersed in acetone using an ultrasonicator (SD-250H,
Mujigae) for 3 min and then dropped onto a carbon film contain-
ing holes supported by a 200 mesh grid of copper. The system
was then dried overnight. Thermal gravimetric analysis (TGA) was
carried out using a Q 50 (TA instruments, U.S.) apparatus in air
(flow rate 40 mL min−1). The samples were placed onto a Plat-
inum pan and inserted into the furnace. The temperature was
2. Materials and methods
2.1. Chemicals and reagents
All reagents were of analytical grade and were used as received
without further purification. Graphite (synthetic, powder, <20 m),
KMnO4 (powder, 97%), K2S2O8 (≥99%), H2O2 (35 wt% solution in
water), H2SO4 (95–98%), P2O5 (powder, ≥98%), HNO3 (64–66%),
HCl (37%), Activated Carbon (Darco, powder, −100 mesh), car-
bon nanotubes (O.D × L: 6–9 nm × 5 m, >95%), palladium chloride
(PdCl2, 99%), Na2CO3 (powder, ≥99.5%), NaBH4 (powder, ≥98%),
1,4-dioxane (>99%), (E)-˛-phenylcinnamic acid (PCA, 97%), and
benzylamine (BA, >99.5%) were purchased from Sigma–Aldrich.
Cinchonidine (CD, 99%) was purchased from Alfa Aesar. Double dis-
tilled water was used in catalyst preparation and in all of the rinse
processes.
increased from room temperature to 850 ◦C at a rate of 20 ◦C min−1
.
The weight change was calculated based on the initial weight of
the sample. The Fourier transform infrared (FT-IR) spectra were
recorded on a Bruker Vector33 (Bruker Optics, U.S.) spectrome-
ter using the KBr method with a resolution of 4 cm–1 across the
4000–750 cm–1 region at room temperature. The temperature-
programmed desorption of ammonia (TPD–NH3) was performed
2.2. Pd catalyst preparation
Graphene oxide was synthesized using the modified Hummers
method [29], described in detail in the Supporting Information. A