Synergistic effect of bimetallic PdAu nanocrystals on oxidative alkyne homocoupling

Article abstract of DOI:10.1039/c8cc06744a

Bimetallic PdAu nanocrystals with different component ratios were obtained to investigate alkyne homocoupling. We found that the synergistic effect of Pd and Au plays an important role in the reaction. Alkynes with a variety of substituent groups could ef

Full text of DOI:10.1039/c8cc06744a

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Synergistic effect of bimetallic PdAu nanocrystals
on oxidative alkyne homocoupling†
Cite this: Chem. Commun., 2018,
4, 13155
5
Zheng Chen, Rongan Shen, Chen Chen,
Jinpeng Li * and Yadong Li
Received 19th August 2018,
Accepted 30th October 2018
DOI: 10.1039/c8cc06744a
rsc.li/chemcomm
1
3–15
Bimetallic PdAu nanocrystals with different component ratios were monometallic materials.
Recent research studies exhibit
obtained to investigate alkyne homocoupling. We found that the their wide applications in heterogeneous catalysis due to the
synergistic effect of Pd and Au plays an important role in the reaction. capabilities of synergistic effects between the two metals. PdAu
Alkynes with a variety of substituent groups could efficiently bimetallic NCs have already been demonstrated to be efficient
1
6–18
be converted into the corresponding 1,3-diynes through oxidative catalysts for other C–C coupling reactions,
whereas their
coupling.
performance for alkyne oxidative homocoupling has not
been explored.
1
,3-Diynes are important building blocks in natural products,
In recent years, our group has made great efforts to synthesize
bimetallic nano-materials, and explore their growth mechanisms
1–4
pharmaceutical intermediates and organic electronic compounds.
1
9–21
The straightforward method for preparing 1,3-diynes is through and catalytic properties.
oxidative homocoupling of terminal alkynes developed by important roles in bimetallic catalysts. In this work, we use
Glaser and Hay. Normally, the reactions are catalyzed by Cu(I) mesoporous graphitic carbon nitride (mpg-C ) as the hetero-
salts assisted with bases, or by catalytic systems combining geneous solid base to support PdAu bimetallic NCs and investi-
We find that synergistic effects play
3 4
N
5,6
palladium with copper salts in homogeneous catalysis. However, gate their synergistic effects in oxidative homocoupling of
2
2–24
because of the advantages of heterogeneous catalysts, like their terminal alkynes.
prominent recyclability and stability, many efforts have been To investigate the catalytic activity of bimetallic nanocrystals,
made to investigate efficient heterogeneous catalysts for alkyne we prepared PdAu alloys with different ratios (3: 1, 2 : 1, 1 : 1,
oxidative homocoupling. Although many achievements have 1 : 2, and 1 : 3) and pure Au or Pd nanocrystals. Typically,
been made, the catalysts usually involve very complicated Pd(acac)2 (acac = acetylacetonate) and aqueous HAuCl4 were
synthesis processes for anchoring the organic ligands to stabilize used as the precursors, and oleylamine (OAm) was used both
7
–9
25
the metal catalysts.
as the solvent and the surfactant in the reaction. Stoichiometric
Metallic nanocrystals (NCs) have been widely investigated as amounts of the precursors were dissolved in OAm at 60 1C and
heterogeneous catalysts for organic reactions. Van Bokhoven reduced by borane-tert-butylamine at 80 1C. The transmission
et al. investigated the catalyst performance of gold clusters for electron microscopy (TEM) images of the as-prepared NPs from
1
0
synthesizing 1,3-diynes. Pu et al. used Pd nanoparticles as the Fig. 1 and Fig. S1 (ESI†) show that all stoichiometric ratios
1
1
catalyst for the same homocoupling. Nevertheless, all of them of the nanocrystals are monodispersed with the diameters of
need extra homogeneous additives for a continuous reaction around 3–4 nm.
process. Corma et al. showed that gold nanoparticles supported
Fig. S2 (ESI†) shows the powder X-ray diffraction (XRD) of
on a large variety of solids could catalyze base-free homo- as-prepared bimetallic nanocrystals with different compositions,
coupling of different alkynes with air as the terminal oxidant, and the pure Pd and Au NPs are included as the references. The
but the cationic sites as the essential parts for catalyzing the shifts of the (111) diffraction peaks from Au to Pd demonstrate
1
2
process are highly limited in the clusters. Bimetallic nano- the formation of the bimetallic alloy using these two elements.
crystals are emerging for modifying the properties of the The detailed structures and compositions of the bimetallic alloys
were characterized by the high resolution TEM (HRTEM) (Fig. 1d–f).
The interfringe distance of the PdAu(1 : 1) nano-particles is about
Department of Chemistry, Tsinghua University, Beijing, 100084, P. R. China.
0.233 nm, which is right between the characteristic distances of
E-mail: roclie@gmail.com
Electronic supplementary information (ESI) available: Synthesis and experi-
the Pd and Au (111) lattice planes, indicating the formation of the
Pd–Au alloy (Fig. 1f). The energy-dispersive X-ray spectroscopy
†
mental details, TEM, XRD and catalysis data. See DOI: 10.1039/c8cc06744a
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PdAu reveals the mainly Au dispersion on the nanoparticles for
the low amount of Pd species (Fig. 2c and f).
To evaluate the activity of the bimetallic catalysts, the mpg-
C N was used both as the solid-state base and the catalyst
3
4
support (Fig. S3, ESI†). The mpg-C N was synthesized using
3
4
2
6
a traditional method as described previously. The homo-
coupling of phenylacetylene was catalyzed by PdAu/mpg-C
in DMF at 80 1C for 12 h; meanwhile the monometallic
3 4
counterparts, the mixture of them and the bare mpg-C N
3 4
N
support were also investigated under the same reaction condi-
tions. Fig. 3a and Table S1 (ESI†) list the conversion ratio and
selectivity of all catalysts. Fig. 3a indicates that no homo-
coupling reactions are carried out with Au/mpg-C N and the
3
4
bare mpg-C
about 2.2%, is detected in Pd/mpg-C
3
N
4
support. Only a small amount of the product,
. The conversion is
Fig. 1 TEM images of Pd NPs (a), PdAu NPs (b), and Au NPs (c); (d) high-
3 4
N
angle annular dark-field (HAADF) STEM image of Pd
2
Au nanoparticles;
slightly increased by using a mixture of Pd and Au nanocrystals,
though the conversion ratio is still less than 10%. In contrast,
the bimetallic catalyst, with the metal ratio of 1 : 1, converts
(
e) HRTEM image of Pd Au nanoparticles; (f) interfringe distance measured
2
by a single PdAu particle.
4
1% of reactants to the final product with nearly 100% selec-
profiles support the evidence of distribution of the Pd and Au
elements in the bimetallic nanocrystals (Fig. 2). As shown in
Fig. 2, the line-profile analysis and EDS mapping via STEM-EDS
tivity, which demonstrates the importance of the synergistic
effect in bimetallic catalysts. For further understanding the
relationship between the reactivity and the composition of the
bimetallic catalysts, bimetallic NCs with different metal ratios
were used to catalyze the homocoupling reaction. It was found
that the reactivity increased when the palladium component
increased. No reaction was detected when the Pd : Au ratio is
of Pd
is Pd (Fig. 2a and d). The surface segregated Au of Pd
nanoparticles could be found. Meanwhile, Pd Au shows more
3
Au reveal that the main distribution of nanoparticles
3
Au
2
serious surface aggregation of Au (Fig. 2b and e), which
evidenced that more Au species are likely to exist at the surface.
1
: 3, but the reaction is carried out when the Pd ratio increases
to 1 : 2. The highest yield for the target product was found when
the Pd : Au ratio changed to 2 : 1. Further increasing the Pd ratio
led to a decrease in yield and selectivity.
Fig. 3 (a) Catalytic results for various heterogeneous catalysts (reaction
conditions: 1 mmol phenylacetylene, 5 ml DMF as a solvent, air, 80 1C,
1
2 hours, a 50 mg catalyst of which 2 mmol% based on the total metal is
loaded on mpg-C , the conversion and selectivity are determined by
GC using biphenyl as an internal standard); (b) the conversion and selectivity
at different temperatures; (c) the recycling ability of PdAu/mpg-C for
oxidative homocoupling of phenylacetylene within 8 h at 100 1C.
3
N
4
Fig. 2 Line-scanning profiles across several PdAu nanoparticles and EDX
elemental mapping images by elements: (a and d) Pd
and (c and f) PdAu.
3
Au, (b and e) Pd
2
Au,
3 4
N
1
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The conversion increased with elevated temperature (Fig. 3b)
and complete conversion with 100% selectivity was obtained at
00 1C within 12 h. Based on the kinetic study on the percentage
1
of phenylacetylene and 1,4-diphenylbutadiyne, the reaction
could be finished in 10.5 h (Fig. S4, ESI†). The catalyst is recycled
and reused in the oxidative homocoupling 5 times to study the
reusability of the Pd–Au bimetallic catalyst (Fig. 3c). It is easy to
collect the nano-catalyst by hot filtration from the liquid phase of
the reaction mixture. The cycling experiments were executed at
100 1C with 8 h as the reaction time. From Fig. 3c we can find
that the yield and selectivity of the product do not change
significantly during these 5 cycles, which indicates the durability
of the catalyst. The resulting solution was analyzed via ICP-MS.
Only a trace amount of palladium was leached to the solutions, but
the solution cannot further catalyze the homocoupling reaction.
These results clearly demonstrate that the reaction is completely
catalyzed by the heterogeneous Pd–Au catalyst. The spent
Pd
2 3 4
Au/mpg-C N catalyst after five cycles was characterized by
TEM and XRD (Fig. S5 and S6, ESI†). PdAu/mpg-C N showed
3
4
basically no change whatever the dispersion on mpg-C N or
3
4
the crystal structure, which evidenced the stability of bimetallic
PdAu catalysts in phenylacetylene homocoupling reactions.
The experiment results indicate that Pd/mpg-C N can carry
3 4
out the reaction without any additives, however, to further
enhance the activity of the Pd particles, bases or organic ligands
2 3 4 3 4
are needed to generate Pd cation species to react with the alkyne Fig. 4 XPS spectra of Pd Au/mpg-C N compared with Pd/mpg-C N (a)
3 4
and Au/mpg-C N (b).
molecules that undergo reductive elimination leading to the
2
7
diyne products and Pd(0). In contrast, in the bimetallic PdAu
nanocrystals, charge migration occurs from the Pd atoms to the
nearest Au atoms to generate positive Pd sites for promoting
reductive elimination. This synergistic effect exhibits the advan-
tage of the bimetallic Pd–Au alloy as an effective heterogeneous
catalyst for C–C homocoupling. This mechanism is further
proved via X-ray photoelectron spectroscopy (XPS). As shown in
Fig. 4a, the binding energies of Pd 3d5/2 and 3d3/2 peaks of Pd/
mpg-C N located at 335.4 eV and 341.7 eV are assigned to Pd(0),
2
role in the activation of CRC–H bonds and O , thus leading to
high activity.
For understanding the general applicability of the catalysts,
we investigated the scope of homocoupling with different
substrates (Table 1). The halogen substituted phenylacetylenes
give relatively high yields (490%) with the substitution being
in the para or meta positions (entries 2–4). The yield decreases
a little (B89%) in the presence of a strong electron withdrawing
group like the nitro-group (entry 5). When the electron donat-
ing group is used as the substitution group, the yields decreases
3
4
and the peaks at 337.9 eV and 343.0 eV are assigned to Pd(II)
which are the Pd species interacting with nitrogen of mpg-C
on the surface. The Pd 3d5/2 and 3d3/2 peaks of PdAu/mpg-C
3
N
N
4
3
4
are located at 336.2 eV and 341.6 eV which shows the partial
positive charge in PdAu particles. By comparison, the Au 4f7/2
and Au 4f5/2 peaks of PdAu are shifted obviously to the lower side
in binding energy (Fig. 4b). This tendency indicates that the Au
atoms gained electrons from Pd atoms by alloying interaction.
For better understanding the importance of the alloy composi-
tion in catalysis, we also investigated the charge states in all
ratios via XPS. It is clear that increasing the Pd ratio will increase
the charge separation at first, and this can be attributed to more
Pd atoms appearing on the surface to interact with Au, and the
reactivity is also enhanced from our experiments (Fig. S7, ESI†).
2 3 4
Table 1 Scope of Pd Au/mpg-C N catalyzed oxidative alkyne homocoupling
a
b
b
Entry
R-
Time (h)
Con. (%)
Sel. (%)
1
2
3
4
5
6
7
8
H
12
12
12
12
14
16
16
20
99
95
96
91
89
79
71
89
100
100
100
100
100
100
100
100
p-Cl
p-Br
m-F
p-NO
p-MeO
m-NH
1-Heptene
However, when the ratio increased to Pd Au, the surface is
2
3
dominantly covered by Pd atoms which is proved by EDS
mapping. In this case the buried Au atoms cannot interact with
Pd atoms effectively, so the charge migration is inhibited.
All these evidences prove that the electronic poor states of
2
a
Reaction conditions: 1 mmol alkyne, 5 ml DMF as a solvent, air,
b
1
00 1C, and 50 mg Pd
2 3
Au/mpg-C N
4
.
The conversion and selectivity
Pd atoms and the surface states in PdAu may play a crucial are determined by GC using biphenyl as an internal standard.
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to less than 80% (entries 6 and 7). An alkyl terminal alkyne
like 1-heptyne also gives a relatively high yield (B89%) in the
homocoupling reaction (entry 8). All reactions give a full
selectivity without any byproducts.
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In conclusion, we systematically studied the catalytic char-
acteristics of the bimetallic PdAu nanocrystals in alkyne oxida-
tive homocoupling reactions. The synergistic effect of the
bimetallic alloy has been observed through our investigation.
This effect can be attributed to the charge migration from the
Pd atoms to Au atoms. The bimetallic nano-catalysts showed
highly efficient activity in homocoupling of terminal alkynes
under mild conditions. These heterogeneous catalysts also
showed high stability and reusability over a number of cycles.
All these results indicate that our bimetallic Pd can be effective
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