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cm−1 with a small amount of CO adsorbed in linear mode.
On the contrary, CO was mainly adsorbed in linear mode at
2072 cm−1 on Pd2Pb8/alumina. The reason could be attrib-
uted to the dilution effect of Pb on Pd, which inhibits the CO
adsorption on two Pd atoms (Fig. S5, ESI†). In addition, the
frequency of linear-adsorbed CO on Pd2/alumina blue shifted
from 2072 to 2084 cm−1 with the increase in adsorption
amount due to the interaction between the adsorbed CO mol-
ecules,12 while almost no frequency shift was observed for
linear-adsorbed CO on Pd2Pb8/alumina, which further con-
firms the conclusion that Pb atoms dilute Pd clusters. There
are similarities in the adsorption mode between CO and
aldehyde carbonyl groups (Fig. S5, ESI†). Hence, the domi-
nant adsorption mode of carbonyl alters from bridged to on-
top mode after forming Pd–Pb bimetallic nanoparticles.13 On
the one hand, the weakly on-top mode adsorbed carbonyls
make side reactions such as decarbonylation, oxidation and
hydrogenation more difficult to occur. On the other hand,
the weakly on-top mode adsorbed carbonyls preserve the sp2
hybridization form of the aldehydic carbon atoms, which pos-
sess low steric hindrance and facilitate the nucleophilic
attack of the adsorbed methoxy intermediates.14 Therefore,
from the atom-scale viewpoint, the electronic interaction and
the dilution effect of Pb on Pd inhibit side reactions and
increase MMA selectivity.
When the oxidative coupling of methylacrolein was
conducted with a methanol/aldehyde molar ratio of 16/1, a
methylacrolein conversion of 96% and a MMA selectivity of
95% were obtained over the Pd2Pb8/alumina catalyst (Table
1, entry 13). Good to excellent conversion and selectivity were
also achieved over the Pd2Pb8/alumina catalyst using iso-
butyraldehyde, benzaldehyde and substituted benzaldehydes
containing electron-donating and electron-withdrawing
groups as substrates (Table 1, entries 14–17), indicating that
the Pd2Pb8/alumina catalyst showed a broad aldehyde range
for aerobic oxidative coupling reaction. We also found that
after recycling for twenty times the Pd2Pb8/alumina catalyst
still maintained high performance with a methylacrolein con-
version of 70% and a MMA selectivity of 80% (Fig. S6, ESI†).
The leaching of Pd was negligible after 22 recycles. Addition-
ally, the Pd2Pb8/alumina catalyst was very robust. The particle
size distribution profiles of alumina microspheres were very
similar for fresh and used catalysts (Fig. S7 and S8, ESI†),
indicating that no obvious abrasion of alumina supports
occurred. In the TEM images, no aggregation of Pd–Pb bime-
tallic nanoparticles was observed after 22 recycles (Fig. S9,
ESI†). However, the particle size of Pd–Pb nanoparticles
increased from an initial size of 3.0 nm to 4.1 nm (Fig. S9,
ESI†). The reason could be attributed to Ostwald ripening,
which is inevitable for liquid-participated reactions.
Conclusions
In conclusion, a highly efficient Pd2Pb8/alumina catalyst for
aerobic oxidative coupling of methylacrolein with methanol
was prepared. The excellent performance of the novel catalyst
was attributed to the multi-scale promoting effects of the pre-
loaded Pb species: (a) micron-scale, improving the formation
of the egg-shell structured active site distribution and provid-
ing more accessible active sites; (b) nano-scale, improving
the dispersion of Pd precursors and increasing the number
of active sites; (c) atom-scale, the electronic interaction and
the dilution effect of Pb on Pd inhibited side reactions. As a
result, the highest TON of 302 was achieved.
Acknowledgements
This work was financially supported by the National Key Basic
Research Program of China (2015CB251401), the National
Natural Science Foundation of China (21306203, 21127011)
and Beijing Natural Science Foundation (2153042).
Notes and references
1 B. J. Xu, X. Y. Liu, J. Haubrich, R. J. Madix and C. M. Friend,
Nat. Chem., 2010, 2, 61–65.
2 (a) S. Yamamatsu, T. Yamaguchi, K. Yokota, O. Nagano, M.
Chono and A. Aoshima, Catal. Surv. Asia, 2010, 14, 124–131;
Fig. 4 In situ FTIR spectra of adsorbed CO on (a) Pd2/alumina and (b)
Pd2Pb8/alumina. The profiles of each sample were collected from 2 to
82 minutes (along the direction of the arrow) with an interval of
8 minutes.
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Catal. Sci. Technol., 2015, 5, 2076–2080 | 2079