PdCx nanocrystals with tunable compositions for alkyne semihydrogenation

Article abstract of DOI:10.1039/c8ta12002a

The palladium carbide (PdC_x) material has shown great promise as an efficient catalyst for alkyne semihydrogenation. Difficulties in preparing stable PdC_x catalysts have been recognized as the main obstacle. Here, we synthesized a highly stable and durable class of PdC_x nanocrystals by treating pre-formed Pd nanocrystals with glucose under hydrothermal conditions. The C/Pd atomic ratios in the resultant PdC_x nanocrystals can be varied from 0.04 to 0.18, simply by changing the reaction time. The catalytic results for semihydrogenation of 4-ethynyl-1,1′-biphenyl show that PdC_0.18 nanocrystals exhibit an activity with a turnover frequency as high as 7896 h^-1, ～7.6 and ～38 times higher than that of commercial Pd/C and Lindlar catalysts, respectively, as well as a selectivity of >99%.

Full text of DOI:10.1039/c8ta12002a

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ISSN 2050-7488
PAPER
Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
MoS2 nanocages as a lithium ion battery anode material
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COMMUNICATION
PdCx nanocrystals with tunable compositions for alkyne
semihydrogenation†
Ruiyun Guo,a Qiang Chen,b Xiang Li,c Yaming Liu,a Chaoqi Wang,a Wei Bi,a Caiyang Zhao,a yanjun
Received 00th January 20xx,
Accepted 00th January 20xx
Guo a and Mingshang Jin a*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Palladium carbide (PdCx) material has shown great promise as an second-transition metal, such as Ag,4 Zn,5 Ga,6 Au,7,8 and Pb9,10
.
efficient catalyst for alkyne semihydrogenation. Difficulties in Although this strategy is an effective way, it has been strongly
preparing stable PdCx catalysts have been recognized as the main limited either by complicated synthetic protocols or restricted
obstacle. Here, we synthesized a highly stable and durable class of by the use of expensive transition metals (e.g., Au and Ag).11 To
PdCx nanocrystals by treating pre-formed Pd nanocrystals with this end, the development of new and efficient approaches for
glucose under hydrothermal condition. The C/Pd atomic ratios in promoting the catalytic performance of heterogeneous
the resultant PdCx nanocrystals can be varied from 0.04 to 0.18, catalysts toward alkyne hydrogenation is necessary and
simply by changing the reaction time. The catalytic results for remains a big challenge.
semihydrogenation of 4-ethynyl-1,1'-biphenyl show that PdC0.18
Recently, in situ characterizations, as well as theoretical
nanocrystals exhibit an activity with a turnover frequency as high calculations, have shown that Pd catalysts typically form a shell
as 7896 h-1, ~7.6 and ~38 times higher than that of the commercial of PdCx on their surfaces when used as heterogeneous catalysts
Pd/C and Lindlar catalyst, respectively, as well as a selectivity of > for the hydrogenation of alkynes.12-17 The presence of PdCx can
99%.
significantly improve the selectivity of alkenes.18-25 However,
these PdCx can only exist at a certain transient moment of a
catalytic process, making the overall catalytic selectivity of a Pd
catalyst poor.26,27 These works on the catalytic mechanism of Pd
catalysts have suggested another feasible route for the
improvement of Pd catalysts, showing that the formation of
PdCx can effectively improve the catalytic property of a Pd
catalyst. However, it is still arduous to incorporate C atoms into
the surface of Pd catalysts to form PdCx experimentally to data.
To the best of our knowledge, there are only few reports on the
synthesis of PdCx materials dealing with practical catalytic
reaction. Some works have shown that PdCx nanocatalysts can
be obtained by calcination method28,29 or gas reduction
synthesis30. However, these methods usually require harsh
conditions or rely on a particular substrate. Moreover, it is
extremely unmanageable to control the composition of Pd and
C within a PdCx catalyst based on the afore-mentioned methods,
which strongly hinders us to study the effects of C/Pd atomic
ratios on catalytic performance. To overcome these limitations,
a facile and effective method to construct PdCx nanocrystals
with tunable C/Pd atomic ratios is highly needed.
1 Introduction
Hydrogenation of alkynes into alkenes is of great importance
and fundamental challenges in fine chemicals, pharmaceuticals,
and agrochemicals.1-3 To achieve high-yield of alkenes while
avoiding the production of alkanes in hydrogenation of alkynes,
the development of catalysts with high activity and selectivity
has become an important task. Typically, transition metal, such
as Pd and Rh, are often used as catalysts to catalyze such
reactions because these metals can provide relatively high
catalytic activities. However, along with their high catalytic
activity, poor catalytic selectivity is usually obtained. Numerous
attempts have been made to improve such catalysts, among
which alloying heterogenous transition-metal catalysts with
additive second metal atoms or ions have been proven to be the
most efficient way. As a quintessential example, the catalytic
performance of Pd catalysts toward alkyne hydrogenation
reactions has been greatly improved by forming alloys with a
a. Frontier Institute of Science and Technology and State Key Laboratory of
Multiphase Flow in Power Engineering, Xi'an Jiaotong University, Xi'an, Shaanxi
710049, China. E-mail: jinm@mail.xjtu.edu.cn
In this study, we demonstrate a new and efficient approach
to achieve stable PdCx catalysts with tunable C/Pd atomic ratios.
The success of the proposed approach mainly relies on the
b. School of Chemical Engineering and Technology, Xi'an Jiaotong University, Xi'an,
Shaanxi 710049, China.
incorporation of
C atoms derived from hydrothermal
c. Institute of Advanced Electrochemical Energy and School of Materials Science and
Engineering, Xi'an University of Technology, Xi'an, Shaanxi 710048, China.
d. †Electronic Supplementary Information (ESI) available: Additional TEM, HRTEM,
XRD, and catalytic results. See DOI: 10.1039/x0xx00000x
carbonization of α-D-glucose into Pd lattice. The atomic ratio of
C can be simply manipulated by adjusting the reaction time.
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Further catalytic evaluation indicated that the as-prepared PdCx PerkinElmer (PE) NexION 350D. Gas chromatography-mass
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DOI: 10.1039/C8TA12002A
catalysts exhibit much enhanced catalytic activity and spectrometry (GC-MS) analysis was carried out on an Agilent 7890A
selectivity in semihydrogenation of 4-ethynyl-1,1'-biphenyl (EB- GC interfaced to an Agilent 5975C mass-selective detector. Fourier
H), compared with the original Pd seeds, commercial Pd/C, and transform infrared (FTIR) spectra were recorded on a Thermo
Lindlar catalyst. In addition, using C to modify Pd catalysts not Scientific Nicolet 6700 in the spectral range of 4000 cm-1 - 650 cm-1.
only enhance their catalytic performance, but also make the
catalysts more cost-effective.
2.4. Catalytic semihydrogenation of EB-H
For a typical hydrogenation reaction, catalysts (0.6 mol% Pd)
dispersed in 2 ml of toluene and 0.2 mmol of EB-H were added into
2 Experimental section
a two-neck flask (10 ml capacity). A balloon containing hydrogen was
connected to the flask and the air in the system was quickly purged
with H2 for five times. Then the solution was transferred to water
bath and magnetically stirred at 30 °C for 3 h. Finally, aliquots of
products were collected with a syringe at certain time intervals, and
the conversion and selectivity for different products were calculated
according to the results from GC-MS analysis.
2.1. Materials
Sodium tetrachloropalladate (II) (Na2PdCl4, 98%), poly (vinyl
pyrrolidone) (PVP, Mw 55000), L-ascorbic acid (C6H8O6, AA, > 99%),
potassium bromide (KBr, 99%), oleylamine (C18H37N, OAm, 80%-
90%), α-D-glucose (C6H12O6, 96%), and Pd/C catalyst (10% palladium
on carbon) were all obtained from Sigma-Aldrich. Lindlar catalyst (5%
palladium on calcium carbonate poisoned by lead) was purchased
from SPC Scientific Corporation. The deionized water with a
resistivity of 18.2 MΩ cm-1 was used for the preparation of all
experiments. All chemicals were used as received without further
purification.
3 Results and Discussion
3.1. Syntheses and characterizations of PdCx nanocrystals
In a typical synthesis of PdCx nanocrystals, uniform Pd
nanocubes (Figure S1) were firstly prepared using a previously
reported protocol.31,32 These Pd nanocubes were then mixed with α-
2.2. Catalyst synthesis
Synthesis of 18 nm Pd cubic seeds. In a typical synthesis, 60 mg D-glucose in oleylamine (OAm) and the resulting solution was heated
of AA, 585 mg of KBr, 105 mg of PVP and 8.0 ml of deionized water at 200 °C for 20 min. As such, the C atoms were expected to diffuse
were mixed together in a 25 ml vial and pre-heated at 80 °C for 10 into the interstitial sites of the Pd lattice and form PdCx nanocrystals.
min under magnetic stirring. Subsequently, 57 mg of Na2PdCl4 were Figure 1a and b show the representative TEM and HAADF-STEM
dissolved in 3.0 ml of deionized water and then quickly added into images of the prepared nanocrystals, indicating that PdCx maintains
the pre-heated mixture solution. After that, the reaction was kept at the cubic shape of the original Pd nanocubes. HRTEM image of an
80 °C for 3 h and the obtained products were collected by individual PdCx nanocube is shown in Figure 1c. Figure 1e shows the
centrifugation, washed three times with deionized water and one integrated pixel intensities of Pd {100} lattices as selected in Pd
more time with ethanol. Finally, the nanocubes were re-dispersed in (Figure S1c) and PdCx (Figure 1c) nanocubes, respectively. The lattice
11 ml of OAm for further usage.
spacing of Pd {100} facets is averaged over six atomic layers for
Synthesis of PdCx nanocrystals. For the synthesis of PdCx accuracy. Clearly, lattice spacing of Pd {100} in PdCx nanocubes is
nanocrystals, 10 mg of α-D-glucose, 2.5 ml of OAm, and 1.5 ml of the measured to be 2.0 Å, which is consistent with the corresponding fast
as-prepared Pd seed solution were mixed in a 25 ml vial, after stirring Fourier transform (FFT) results (Figure 1d) and slightly larger than
vigorously for 15 min, the solution was put in an oil bath and heated that of the original Pd nanocubes (1.9 Å). This result indicates that C
to 200 °C for certain time under stirring. After the reaction, the atoms have been successfully incorporated into the lattice of Pd
products were collected via centrifugation and washed three times nanocubes, leading to the expansion of Pd lattices. Meanwhile, the
with ethanol. The chemical composition of PdCx nanocrystals can be edge length of PdCx nanocubes (18.8 ± 4.3 nm) is found to be larger
tuned by adjusting the reaction time in a typical synthesis, and the than that of the original Pd nanocubes (18.1 ± 4.5 nm), as shown in
corresponding reaction time has been listed in Table S1.
Figure S1b and S2b. The XRD pattern (Figure 1f) further shows that
the diffraction peaks of PdCx nanocubes left-shift to lower angles
compared with that of Pd nanocubes, further confirming the lattice
2.3. Sample Characterization
The powder X-ray diffraction (XRD) patterns were recorded using an expansion of Pd after the incorporation of C atoms. Based on the XRD
X-ray diffractometer (SmartLab (3), Rigaku) operated at 3 kW. results, the lattice constant of PdCx nanocubes can be calculated to
Transmission electron microscopic (TEM) images were performed on be 4.018 Å, which is 3.3 % larger than that of the typical Pd metal
a Hitachi HT-7700 operating at 100 kV. High-resolution TEM (HRTEM) (3.890 Å, JCPDS NO. 46-1043). Based on the relationship between the
and high-angle annular dark-field scanning TEM (HAADF-STEM) were lattice constant (a) and the fraction x of C (x = C/Pd) incorporated
performed on a JEM-2100F (JEOL) and JEM-F200 microscope at 200 into the Pd lattice:17,19 a = a0 + 0.69x (Å), where a0 is the lattice
kV. X-ray photoelectron spectroscopy (XPS) was conducted using a constant of metallic Pd, the fraction of C in PdCx nanocubes was
Kratos AXIS Ultrabld spectrometer equipped with monochromatic Al calculated to be 0.18. This result indicates that the obtained PdCx
Kα radiation. Analytical gas chromatography (GC) spectra were samples are of pure phase, suggesting that the whole Pd nanocube
carried out on a SHIMDZU GC-2014ATF/SPL (TDX-01 60/80 mesh, 2.0 has successfully transformed to PdCx nanocubes after the reaction.
mm × 3.2 mm × 2.1 mm-FID, TCD permanent gases, N2 carrier gas).
The concentrations of the catalysts were determined by the
inductively coupled plasma mass spectrometry (ICP-MS) with a
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discovered in the past few years, such as the formation of PdHx and
PdBx nanocrystals.27,38,39 In this case, the composition of PdCx
nanocubes can be easily tuned by controlling the reaction time, as
listed in Table S1.
Figure 2 shows the XRD patterns of PdCx nanocrystals obtained
at different reaction times. Clearly, all the PdCx nanocrystals
maintained the face-centered cubic (fcc) structure after the reaction.
The difference between the samples is that the diffraction peaks
gradually shifted to the lower angles along with the increase of the
reaction time. This result can be further confirmed by HRTEM
characterizations of the samples (Figure S4). Previous reports have
already shown, experimentally and theoretically, that C atoms can
diffuse into the octahedral interstitial sites of Pd lattices, inducing an
expansion of the Pd lattice.21-26 In this synthesis, along with the
increase of the reaction time, more C atoms can diffuse into the Pd
lattice, resulting in the formation of higher C ratio in a PdCx nanocube.
When the reaction time varied from 2 to 20 min, the lattice constants
of PdCx nanocubes have been found to increase from 3.917 Å to
4.018 Å, corresponding to the changes of x values of PdCx from 0.04
to 0.18 (Table S1). Corresponding TEM images of the prepared PdCx
nanocubes (Figure S4) indicate that the morphologies of the PdCx
nanocubes with different C ratios were well maintained after the
incorporation of C atoms. However, no further peak-shift in XRD
patterns can be observed when the reaction time is prolonged (e.g.,
1 h) or with a higher concentration of α-D-glucose, as shown in Figure
S5 and S6. As well, no further peak-shift can be detected by
increasing the reaction temperature from 200 °C to 250 °C, indicating
that the carbon atom has reached a saturated concentration in the
Pd nanocubes with the saturated C/Pd atomic ratio being 0.18
(Figure S7). These results suggest that the saturation atomic ratio of
C/Pd in PdCx nanocubes is 0.18, and no more C atoms can diffuse into
the lattice of PdC0.18 under the current reaction condition. To the best
of our knowledge, this is the first report on the successful
preparation of PdCx nanocrystals with tunable C/Pd atomic ratios.
Fig. 1. (a) TEM and (b) HAADF-STEM images of PdC0.18 nanocubes. (c) HRTEM
image of an individual PdC0.18 nanocube. (d) FFT pattern of the PdC0.18 nanocube
shown in (c) recorded along the <001> zone axis. (e) The integrated pixel
intensities of Pd nanocube and PdC0.18 nanocube along {100} spacing directions,
the integrated pixel of PdC0.18 was taken from the wire frame region in (c). (f) XRD
pattern
of
PdC0.18
nanocubes.
It is worth noting that α-D-glucose play the key role in the
formation of PdCx nanocrystals. Without the addition of α-D-glucose,
no peak shift can be observed in XRD pattern of the final products
after the reaction (Figure S3). Previous reports have indicated that α-
D-glucose can readily undergo carbonization under hydrothermal
condition, giving the formation of various metal/carbon
nanoarchitectures.33-36 Meanwhile, the presence of metal
nanocrystals can catalyze the carbonization process of α-D-glucose
efficiently and fleetly.37 Accordingly, C atoms derived from the
carbonization of α-D-glucose can diffuse into Pd lattices, resulting in
the formation of PdCx nanocrystals. Similar process has already been
Fig. 2. XRD patterns showing the effect of increasing reaction time on the ratio of
C/Pd nanocubes: 2, 3, 5, 8, 10 and 20 min, with the corresponding atomic ratios
of C/Pd equal to 0.04, 0.07, 0.11, 0.13, 0.15, and 0.18, respectively.
More interestingly, PdCx nanocubes prepared based on the
approach developed in this work exhibit extraordinary stability under
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ambient conditions. For example, no significant changes have been spectra of the PdC0.18 nanocubes (Figure S10), the sVpieewctArraticcleaOnnlbinee
DOI: 10.1039/C8TA12002A
observed after 6 months’ storage in air (Figure S8). Even in the decomposed into a dominant peak at 284.6 eV for C-C species, and a
annealing test at elevated temperatures in different gaseous weak peak observed at 282.5 eV is attributed to the presence of a
atmosphere. When the samples were annealed in H2 gas at 200 °C, Pd-C bond, further indicating a successful incorporation of C atoms
no obvious changes can be observed in their XRD patterns. This result into Pd lattice.40,41 Clearly, the above results demonstrate that the
also confirms that the interstitial sites of Pd have been fulfilled with incorporation of C atoms in Pd nanocubes can finely alter the
C atoms, making it difficult for H2 gas to store inside, which is unlike electronic structure of Pd catalysts, and it is known that the
the most cases reported previously for Pd catalysts.18-27 In electronic effect can impose a pronounced influence on the catalytic
comparison, when PdCx nanocubes were treated in air at 200 °C for activities of the PdCx nanocatalyst.13,17,18
2 h, a clear transformation from PdCx to Pd was observed, as shown
in Figure S8. This transformation is because of the reaction of C with
3.2. Catalytic performance of PdCx nanocubes
oxygen at 200 °C. Collectively, PdCx nanocubes possess excellent The catalytic performance of PdCx nanocrystals are expected to be
thermal stability under a reducing atmosphere, such as H2. Since Pd superior to those of other catalysts due to the incorporation of C
catalysts are mostly served as catalysts for hydrogenation reactions, atoms. In this work, PdCx nanocrystals were then evaluated in
PdCx nanocubes prepared in this work could meet most of the semihydrogenation of EB-H (Figure 4a) and benchmarked against the
requirement for such applications. Importantly, when the annealing original Pd nanocubes and state-of-the-art materials (Pd/C and
temperature exceeds 300 °C in H2 atmosphere, the catalysts would Lindlar-type PdPb/CaCO3). Figure 4b shows the yield of vinylbiphenyl
gradually transform from PdCx to Pd (Figure S8), which is mainly due versus time during the hydrogenation over different catalysts. As can
to the release of hydrocarbons from the reaction between C atoms be seen, all the PdCx catalysts can exhibit excellent catalytic
in Pd lattice and H2. To further confirm this result, the gaseous performance with selectivity as high as > 99% toward vinylbiphenyl.
products after annealing the PdC0.18 nanocubes under H2 at 400 °C Among them, PdCx catalysts with a C content higher than 0.07 (x >
have been determined by gas chromatography (GC) ，and CH4 as 0.07) can achieve 100% conversion of alkynes within 180 min.
main production can be clearly detected in analytical system (Figure Catalysts with a lower C content tend to take a longer time to
S9), and the results show that CH4 is the main gaseous product accomplish the reaction and reach 100% conversion, as shown in
(Figure S9), which is consistent with the above explanations.
Figure S11. In contrast, hydrogenation over commercially available
Pd/C catalyst exhibits much lower activity and selectivity. For Lindlar
catalyst, which is the most frequently used catalyst for alkyne
semihydrogenation, the conversion rate is only 22.8% after 180 min
reaction, far below that over the PdCx catalyst. Obviously, PdCx
nanocrystals gives much higher yield compared with the original Pd
nanocubes, indicating that the incorporation of C atoms can
effectively promote the catalytic property of Pd catalysts toward
semihydrogenation reaction. More specifically, the content of C has
a pronounced influence over the activity of PdCx catalysts. As shown
in Figure 4c, the turnover frequencies (TOFs) of PdCx catalysts
increase as the C content in PdCx catalysts increases. Notably, for
PdC0.18 catalysts, the TOF is 7896 h-1, ~7.6 times higher than that of
the commercial Pd/C and 38 times higher than that of Lindlar catalyst
(Table S2). More importantly, the results of leaching experiments
indicated that PdC0.18 catalysts can exhibit excellent stability during
the catalytic process (Scheme S1, Table S3). Only little decay in the
conversion and selectivity could be observed after recycling for 4
times (Figure 4d), maybe due to the mass loss during the recycling of
the catalysts. The crystalline structure, as well as the morphology of
PdC0.18 nanocubes could be well maintained after four catalytic
cycles (Figure 4e and S12). Such excellent catalytic performances are
believed to be effective for a variety of semihydrogenation for
different types of alkynes. As shown in Table 1, different types of
Fig. 3. XPS spectra of Pd 3d in the Pd, PdC0.07, PdC0.13, and PdC0.18 nanocubes.
To further evaluate the influence of C on the electronic
properties of Pd catalysts, XPS was conducted to characterize PdCx
nanocubes (x = 0.07, 0.13, 0.18). As shown in Figure 3, the binding
energy of Pd 3d5/2 and 3d3/2 in Pd nanocubes are 335.1 and 340.4 eV,
respectively. When the C atoms are incorporated, the binding energy
of Pd 3d5/2 gradually left shift to higher regions. As can be seen, the
binding energy of 3d5/2 in PdC0.07, PdC0.13 and PdC0.18 are ~335.3,
~335.5 and ~335.6 eV, respectively. The positive shift of binding
energy by ~0.2, ~0.3 and ~0.4 eV infer that Pd sites become
increasingly electron-deficient along with the increase of C content.
These shifts are mainly caused by the electron transfer from Pd to C
due to their different electronegativities. Regarding C 1s core-level
alkynes,
ethynylbenzonitrile,
including
1-ethynyl-4-methoxybenzene,
1-bromo-4-ethynylbenzene,
4-
9-
ethynylphenanthrene, phenylacetylene, 4-ethylphenylacetylene,
methyl 4-ethynylbenzoate, diphenylacetylene and 6-dodecyne can
be catalysed to produce corresponding products with a high yield.
These results suggest that PdCx nanocrystals should be efficient
catalysts for alkyne semihydrogenation reactions.
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Fig. 4. (a) Hydrogenation of EB-H over different catalysts. (b) The yield of vinylbiphenyl versus time during the hydrogenation reaction. (c) Plot of TOF over different
catalysts. (d) Catalytic stability and selectivity of the as-prepared PdC0.18 catalyst. (e) XRD patterns of the PdC0.18 catalyst before and after cycling. Reaction conditions:
0.2 mmol EB-H, 0.6 mol% Pd catalysts,
2
ml of toluene, H2 balloon, 30 °C, 180 min. The results were determined by GC-MS analysis.
Figure 5a), which can be attributed to the dissociation of the alkynyl
Table 1 Alkynes Semihydrogenation Catalysed by PdC0.18 nanocubes
C-H bond due to the strong binding between the surface of PdC0.18
nanocubes and EB-H molecules. Meanwhile, C≡C vibrational stretch
at 2110 cm-1 for EB-H also disappeared due to the effective
interparticle charge delocalization, further confirming the direct
interaction of alkynyl group to the catalyst.43,44 Notably, the
characteristic band at 834 cm-1 and 768 cm-1 can be assigned to the
aromatic C-H deformation vibration, indicating the adsorption of EB
moiety on PdC0.18 catalysts surface definitely.45,46 Contrarily, when
exposed to mixture substrates (EB-H and vinylbiphenyl) (curve 4 in
Figure 5a), the IR curve is almost the same as that of pure EB-H,
implying that EB-H can preferentially adsorbed onto the surface of
PdC0.18 nanocubes in the presence of EB-H and vinylbiphenyl.
Furthermore, compared with the spectra of pure vinylbiphenyl
(curve 1 in Figure 5b), the peaks of vibrational signals after exposure
of the catalyst to pure vinylbiphenyl are neglectable (curve 2 in
Figure 5b), indicating a rather weak binding capability of PdC0.18
nanocubes to vinylbiphenyl molecules. The interstitial C atoms may
induce an increase in the Fermi level of the Pd through the
hybridization of the Pd d-band with C s-p bands, leading to d-band
broadening and downshifting. This electronic structure may result in
a difference in the heat of adsorption, resulting in the overall
reduction of alkenes adsorption energy in hydrogenation
reactions.47,48 In this case, the desorption rates of alkenes would be
much higher on the PdCx nanocubes, making them more promise
catalysts toward semihydrogenation reactions. Moreover, PdCx
nanocrystals can strongly exclude the participation of subsurface H
at the same sites and suppress the formation of β-PdH phase in
catalytic process, which is known to be one of the main factors that
determine the selectivity.10,49,50
Substrate
t [min]
Conversion [%]
Yield [%]
1-Ethynyl-4-
methoxybenzene
4-Ethynylbenzonitrile
1-Bromo-4-
12
15
33
> 99
> 99
> 99
99.3
98.5
98.9
ethynylbenzene
9-Ethynylphenanthrene
Phenylacetylene
4-Ethylphenylacetylene
Methyl 4-ethynylbenzoate
Diphenylacetylene
6-Dodecyne
20
11.5
14
15
43
> 99
> 99
> 99
> 99
> 99
> 99
98.7
99.2
99.3
99.1
98.1
99.6
28
aReaction conditions: substrate (0.2 mmol) and 0.6 mol% of PdC0.18 catalyst in 2 ml
toluene were stirred at 30 °C. The results were determined by GC-MS analysis.
Previous studies suggested that the existence of PdCx
component in a Pd catalyst significantly facilitates the selective
adsorption of alkynes in hydrogenation reaction, which was known
to be beneficial for a higher selectivity toward alkene.14,16,27 To
further understand the origin of the enhanced catalytic performance
of PdCx nanocrystals, the as-prepared PdC0.18 was used as a model
catalyst to investigate the adsorption properties of alkynes and
alkenes. After exposure of the catalyst to different substrates and the
subsequent removal of free substrate molecules, the catalyst is
subjected to spectroscopic analysis. As illustrated in Figure 5a, pure
EB-H (curve 1 in Figure 5a) exhibits an intense characteristic band at
3250 cm-1, which could be assigned to the vibrational stretch of the
terminal ≡C–H moiety.42 This stretching vibration disappeared when
the EB-H adsorbed onto the surface of PdC0.18 nanocubes (curve 3 in
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Fig. 5. FT-IR spectra. (a) Lines are (1) pure EB-H, (2) PdC0.18 nanocubes, (3) PdC0.18 nanocubes treated with EB-H, and (4) PdC0.18 treated with mixture of EB-H and
vinylbiphenyl (1:1). (b) Lines are (1) pure vinylbiphenyl, and (2) PdC0.18 treated with vinylbiphenyl. Pink, green, blue and purple rectangles highlight the regions of the
chemical
vibrations
of
≡C–H,
C≡C,
aromatic
C-H
and
C=C,
respectively.
4 Conclusions
Notes and references
In conclusion, we have presented a novel and efficient
approach for the preparation of PdCx nanocatalysts with
tunable atomic ratios of C/Pd by treating pre-formed Pd
nanocrystals with glucose under hydrothermal condition. The
C/Pd atomic ratios in the resultant PdCx nanocrystals can be
further tuned from 0.04 to 0.18, simply by changing the
reaction time. The catalytic results for semihydrogenation of
4-ethynyl-1,1'-biphenyl show that PdC0.18 nanocrystals exhibit
an activity with a turnover frequency as high as 7896 h-1, ~7.6
and ~38 times higher than that of the commercial Pd/C and
Lindlar catalyst, respectively, as well as a selectivity of >99%.
Mechanistic studies revealed that the enhanced performance
could be attributed to the unique electronic structure and
preferential adsorption of alkynes. This study not only
provides a simple and cost-effective strategy in preparing PdCx
nanocrystals for fine chemical industries, but also shed new
lights on the rational design of Pd-based heterogeneous
catalysts.
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Table of Contents
A novel and efficient approach has been reported for the preparation of PdCx nanocatalysts with
tunable composition for enhanced alkyne semihydrogenation.