Benzalaniline from nitrobenzene and benzaldehyde catalyzed efficiently by an atomically precise palladium nanocluster

Article abstract of DOI:10.1016/S1872-2067(19)63423-6

Nanoclusters with a precise number of atoms may exhibit unique and often unexpected catalytic properties. Here, we report an atomically precise Pd₃ nanocluster as an efficient catalyst, whose catalytic performance differs remarkably from typical Pd nanoparticle catalysts, with excellent reactivity and selectivity in the one-pot synthesis of benzalaniline from nitrobenzene and benzaldehyde. We anticipate that our work will serve as a starting point for the catalytic applications of these tiny atomically precise nanoclusters in green chemistry for the one-pot syntheses of fine chemicals.

Full text of DOI:10.1016/S1872-2067(19)63423-6

Chinese Journal of Catalysis 40 (2019) 1499–1504
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Communication
Benzalaniline from nitrobenzene and benzaldehyde catalyzed
efficiently by an atomically precise palladium nanocluster
Linquan Bao a, Chengcheng Zhao b, Shenggang Li b,#, Yan Zhua,*
a Key Laboratory of Mesoscopic Chemistry, MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093, Jiangsu, China
b Shanghai Advanced Research Institute, Chinese Academy of Sciences, Shanghai 201210, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Nanoclusters with a precise number of atoms may exhibit unique and often unexpected catalytic
properties. Here, we report an atomically precise Pd3 nanocluster as an efficient catalyst, whose
catalytic performance differs remarkably from typical Pd nanoparticle catalysts, with excellent
reactivity and selectivity in the one-pot synthesis of benzalaniline from nitrobenzene and benzal-
dehyde. We anticipate that our work will serve as a starting point for the catalytic applications of
these tiny atomically precise nanoclusters in green chemistry for the one-pot syntheses of fine
chemicals.
Received 18 June 2019
Accepted 16 July 2019
Published 5 October 2019
Keywords:
Nanoclusters
Palladium
Reductive amidation
Selectivity
© 2019, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
Activity
Metal nanoparticles are the active catalysts for the majority
of current chemical processes [1,2]. However, conventional
nanoparticle catalysts have several major issues, particularly
the inherent size polydispersity, and the difficulty in precisely
controlling the structure at the atomic level [3]. These major
limitations preclude fundamental investigations on the precise
structure-activity relationships, e.g., it is usually impossible to
unambiguously identify the catalytically active species in na-
noparticle catalysis [4–6]. The majority of current studies offer
an ensemble average of the catalytic performance due to the
structural polydispersity and heterogeneity of conventional
nanoparticle catalysts. Although significant efforts have been
invested in preparing well-defined nanoparticles as model cat-
alysts, fundamental catalysis research still lags behind [7].
Therefore, generalizing the synthesis of atomically precise
metal nanoclusters and using them as well-defined catalysts
are of paramount importance. Atomically monodisperse
nanoclusters are composed of an exact number of metal atoms,
and thus differ from the respective metal nanoparticles [8–11].
More importantly, based on their atom packing structures and
unique electronic properties, these nanoclusters permit the
precise correlation of particle structure and catalytic proper-
ties, which may help the identification of active sites on the
metal nanoparticle catalysts [12]. This is the long-standing goal
of catalysis research.
Amidation reaction is an important route for producing
pharmaceuticals, agrochemicals, dyes, and fine chemicals
[13,14]. Significant efforts have been invested in the efficient
synthesis of imines from secondary amines under mild condi-
tions favorable for practical applications [15–17]. However, the
one-pot synthesis of benzalaniline involves the condensation of
a carbonyl compound and an amine, and is therefore, difficult
* Corresponding author. E-mail: zhuyan@nju.edu.cn
# Corresponding author. E-mail: lisg@sari.ac.cn
We acknowledge financial supports from National Natural Science Foundation of China (21773109, 91845104).
DOI: S1872-2067(19)63423-6 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 40, No. 10, October 2019

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Linquan Bao et al. / Chinese Journal of Catalysis 40 (2019) 1499–1504
to realize due to the exceedingly active nature of carbonyl
compounds [18]. It has been shown that the formation of C–N
bonds can be accomplished in the presence of Pd/C, Ir complex,
or FeII/EDTA complex [19] through transfer hydrogenation or
by utilizing a reducing agent; however, these methods are in-
compatible with imines containing C=N bonds.
In this study, we utilize a three-Pd-atom nanocluster pro-
tected by ligands to catalyze the one-pot synthesis of benzala-
niline from nitrobenzene and benzaldehyde, and our catalyst
exhibited efficient conversion and selectivity toward the par-
tially hydrogenated product (benzalaniline). For comparison,
conventional Pd nanoparticles were also synthesized and ap-
plied to the same reaction, and they afforded a completely hy-
drogenated product (benzylaniline). Our studies promote the
exploration of well-defined nanoclusters as highly efficient
catalysts for the synthesis of fine chemicals.
The composition of the Pd nanocluster was determined by
matrix-assisted laser desorption ionization mass spectrometry
(MALDI-MS) (Fig. 1(a)), which confirmed that the formula of
the nanocluster is Pd3Cl(PPh2)2(PPh3)3 (abbreviated as Pd3,
hereinafter), which is consistent with the peak located at 1511
Da (note that the peaks at 984 and 1249 m/z, marked by the
asterisks, are fragments resulting from MALDI instead of impu-
rities, and the peaks might be assigned to Pd3Cl(PPh2)2(PPh3)2
and Pd3Cl(PPh2)2(PPh3), respectively). The atomic structure of
the Pd3 nanocluster can be viewed as a triangle, as shown in
Fig. 1(b), capped by one Cl, two PPh2, and three PPh3 ligands.
The non-metallic nature of the Pd3 nanocluster is manifested in
its optical spectrum (Fig. 1(c)), which exhibits multiple absorp-
tion bands centered at 340, 418, and 485 nm (reminiscent of
quantum dot behavior). X-ray photoelectron spectroscopy
(XPS) analysis shows that the apparent binding energies of the
Pd 3d electrons in the Pd3 nanocluster are blue-shifted, com-
pared to the case with the bulk Pd electrons (Fig. 1(d)). The
blue shift in the binding energy is typical for very small parti-
cles [20]. Note that the peak at 336.9 eV is assigned to Pd2+ and
the peak at 336.4 eV to Pd0, indicating that the average charge
carried by the Pd atoms in the nanocluster is relatively close to
zero. These data revealed the unique structural and electronic
properties of the Pd3 nanocluster, which differed remarkably
from those of the corresponding Pd nanoparticles.
Upon deposition of the Pd3 nanoclusters onto the oxide
supports (γ-Al2O3, SiO2, MgO, and TiO2), transmission electron
microscopy (TEM) images clearly demonstrated the average
size of the Pd3 clusters, i.e., about 2 nm, supported on γ-Al2O3.
For comparison, Pd nanoparticles supported on these oxides
were also prepared, and the average size was around 2–3 nm
(Fig. S1). High-resolution TEM studies of the Pd nanoparticles
indicated that the lattice spacing of 0.195 nm can be assigned to
the {200} lattice fringes (Fig. S1).
The one-pot synthesis of benzalaniline from nitrobenzene
and benzaldehyde was selected as a model reaction to distin-
guish the catalytic performance of the Pd3 nanocluster from
that of Pd nanoparticles. As listed in Table 1, the catalytic activ-
ity of Pd3 is superior to that of Pd nanoparticles for this reac-
tion. With the free Pd3 nanocluster as the catalyst, an excellent
a
b
1511
984
*
1249
*
Pd cluster treated at 80 C
3
750
1000 1250 1500 1750 2000 2250
Mass(m/z)
c
d
Pd 3d5/2
Pd 3d3/2
Pd0
Pd2+
300
400
500
600
700
800
Wavelength(nm)
344
340
Binding energy (eV)
336
332
Fig. 1. MALDI-MS spectrum (a) and atomic structure (b) of the Pd3Cl(PPh2)2(PPh3)3 nanocluster. Color labels: green, Pd; pink, C; yellow, Cl; orange, P.
H atoms are omitted for clarity. UV-vis (c) and Pd 3d XPS (d) profiles of the Pd3 nanoclusters.

Linquan Bao et al. / Chinese Journal of Catalysis 40 (2019) 1499–1504
1501
Consequently, the conversion of nitrobenzene in the reaction
system without the Pd3 catalyst did not increase compared to
that in the first 10 h of reaction with the Pd3 catalyst (Table 1),
which further indicated that the leaching phenomenon of Pd
was negligible. The Pd3 nanoclusters were very stable during
the reaction, which was confirmed by the UV-vis spectroscopic
fingerprints of the fresh and used catalysts, and no obvious
spectral change was observed (Fig. S3).
Table 1
Catalytic performance of the Pd3 nanoclusters and Pd nanoparticles for
the benzalaniline synthesis of nitrobenzene and benzaldehyde.a
To elucidate the mechanism for the distinct catalytic per-
formances of the Pd3 nanoclusters and Pd nanoparticles, H–D
exchange was carried out to explain how the atomic arrange-
ment of Pd catalysts controlled the selective hydrogenation in
the amidation reaction. Fig. 2 presents the significant H–D evo-
lutions with the increasing temperatures of γ-Al2O3,
Pd3/γ-Al2O3, and Pd NPs/γ-Al2O3. In these H–D exchange ex-
periments, four kinds of surface H species related to the three
samples could be identified. The H–D evolution peak, α, ob-
served at 303 °C, was mainly due to the hydroxyl (HO–Al). For
Pd3/γ-Al2O3, the new peak, β, located at 133 °C was assigned to
the specific H species connected to the Pd3 clusters, since the
initial peak belonging to HO–Al is quite distant from it. For the
Pd nanoparticles supported on γ-Al2O3, there are two evolution
peaks located at 168 and 264°C: the first peak, denoted by γ, is
assigned to the chemisorbed H on the Pd nanoparticles; the
second, denoted by δ, originates from HO–Al, which shifts to
lower temperature due to the dissociation of its H catalyzed by
the Pd contents [21,22]. Thus, compared to that of γ-Al2O3, the
abilities of Pd3/γ-Al2O3 and Pd NPs/γ-Al2O3 to dissociate H are
both enhanced, although to different levels, with Pd
NPs/γ-Al2O3 reaching H−D equilibrium at a temperature higher
than that of Pd3/γ-Al2O3 by 35 °C. The H–D exchange results
show that Pd3/γ-Al2O3 is more active for the hydrogenation
during the one-pot synthesis of benzalaniline from nitroben-
zene and benzaldehyde
Selectivity (%)
Conversion
(%)
TOF
Catalyst
(h‒1
)
Benzalaniline Benzylaniline Aniline
Pd3/γ-Al2O3
Pd3/γ-Al2O3
Pd3/γ-Al2O3
Pd3/SiO2
Pd3/TiO2
Pd3/MgO
Pd3
Pd NPs/Al2O3
Pd NPs/SiO2
Pd NPs/TiO2
Pd NPs/MgO
Pd/C
97.0
67.7
68.2
>99.0
>99.0
96.0
90.6
85.9
74.0
87.6
71.8
>99.0
94.5
91.6
91.6
94.5
77.2
97.2
72.8
18.2
—
—
—
—
—
22.8
2.8
19.2
80.0
>99.0
81.3
91.5
>99.0
5.5
8.4
8.4
5.5
—
9.15
12.8
12.9
9.43
9.43
9.06
8.55
2.63
2.27
2.69
2.20
—
b
c
—
8.0
1.8
—
1.7
1.6
—
17.0
6.9
—
a Reaction conditions: 50 mg of catalysts (1 wt% Pd), 1.0 mmol benzal-
dehyde, 1.2 mmol nitrobenzene, 15 mL of methanol, 80 °C, 2 MPa H2, 20
h. NPs: nanoparticles. The conversion was calculated based on nitro-
benzene.
b The reaction time was 10 h.
c After a 10-h reaction, the catalyst was removed by centrifugation, and
the reaction continued for another 10 h.
nitrobenzene conversion of 90.6% could already be achieved.
The Pd3 nanoclusters supported by oxides displayed even bet-
ter catalytic activity than that of Pd nanoparticles with a con-
version exceeding 96.0% under the same reaction condition,
regardless of the support being used, which had no inherent
catalytic activity. The supported Pd3 nanocluster showed a
higher conversion than that of supported Pd nanoparticles by
10–25%. More importantly, the Pd3 nanoclusters and Pd na-
noparticles exhibited distinct product selectivity, with the Pd3
nanocluster favoring the partially hydrogenated product (ben-
zalaniline), and the Pd nanoparticles favoring the completely
hydrogenated product (benzylaniline). The highest selectivity
towards benzalaniline was obtained with the Pd3 nanoclusters
supported by MgO. In addition, the commercial Pd/C catalyst
afforded very high conversion of nitrobenzene, but a very low
selectivity towards benzalaniline.
Temperature-programmed desorption (TPD) is usually
adopted to measure the adsorption strength and capacity of the
adsorbents onto the catalysts. The TPD spectra of H and ben-
zalaniline on the Pd3 nanoclusters and Pd NPs are illustrated in
To show the stability of the Pd3 catalyst, the cycle perfor-
mance of Pd3/γ-Al2O3 was tested using recycled catalysts under
the same reaction conditions after simple centrifugation and
washing with methanol. As illustrated in Fig. S2, no obvious
decrease in the activity was observed, indicating the excellent
reliability of this reaction system. To detect possible Pd leach-
ing from the catalysts, inductively coupled plasma (ICP) spec-
troscopy was employed to measure the Pd species in the reac-
tion solution, and no Pd species was detected. Furthermore, we
conducted a comparison experiment: the reaction proceeded
for 10 h, after which the catalyst was isolated. Subsequently,
the reaction continued without the Pd catalyst for another 10 h.
Fig. 2. H–D exchange profiles of Pd3/γ-Al2O3, Pd NPs/γ-Al2O3, and
γ-Al2O3.

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Linquan Bao et al. / Chinese Journal of Catalysis 40 (2019) 1499–1504
349 oC
244oC
b
a
Pd3
79oC
Pd3
338oC
287 oC
146oC
Pd NPs
Support
Pd NPs
Support
0
100
200
Temperature (
300
400
0
100
200
300
C)
400

C)
Temperature (

Fig. 3. TPD profiles of H2 (a) and benzalaniline (b) for different samples.
Fig. 3. As shown in Fig. 3(a), no H was adsorbed on the bare
support and H was only adsorbed on the Pd sites. The peak at
349 °C for the Pd3 nanocluster was assigned to the chemisorp-
tion of H, whereas the peak indicative of the hydrogenation
desorption for Pd NPs was located at 287 °C. Although the Pd
NPs was found to have a lower hydrogen desorption tempera-
ture than that of the Pd3 catalyst, the Pd3 catalyst exhibited a
relatively larger peak area than that of the Pd NPs. The ratio of
the desorption peak areas for these two samples was estimated
to be ~2.9:1. The amount of H desorbed by TPD was consistent
with the change in the amount of H measured by chemisorp-
tion, which reflected the change in the dispersion of the active
components in the catalyst [23,24]. These phenomena indicat-
ed that the H chemisorption capacity of the Pd3 catalyst was
greater than that of the Pd NPs, which is consistent with the
higher hydrogenation activity of the Pd3 catalyst.
Moreover, there were significant differences from the ben-
zalaniline-TPD results of the Pd3 and Pd NPs catalysts. As
shown in Fig. 3(b), the desorption of benzalaniline over Pd3
started at 79 °C and reached maximum speed at 244 °C, while
the initial desorption of benzalaniline over Pd NPs occurred at
around 146 °C; the fastest desorption occurred at 338 °C. The
adsorption capacities of benzalaniline deduced from the peak
areas suggested that the interaction of benzalaniline with Pd3
was much weaker than that with Pd NPs, and thus relatively
high selectivity for benzalaniline was achieved by Pd3. The in-
triguing point is that the asymmetric shape of the peaks is in
conjunction with the first order kinetics [25], implying that the
adsorption behavior is non-dissociative. In conclusion, the re-
sults demonstrate that benzalaniline was easily formed in the
Pd3 system rather than in the Pd NPs.
Density functional theory (DFT) calculations were per-
formed using the Pd3 cluster and the Pd(100) surface as models
of the Pd3 nanocluster and Pd nanoparticle catalysts, respec-
tively, to elucidate the catalytic mechanism and the reason be-
hind the different product selectivities. Fig. 4(a) presents the
potential energy surface (PES) for the formation of benzalani-
line (PhN=CHPh) from the reaction of aniline (PhNH2) and
benzaldehyde (PhCH=O) catalyzed by the Pd3 cluster. Although
nitrobenzene (PhNO2) was used as the N source, it was ex-
pected to be readily reduced to aniline by hydrogenation, and
the latter was actually involved in the reductive coupling reac-
tion with benzaldehyde. As shown in the enlarged image of the
Pd3 cluster (Figs. 4(a) and S5, A-1), the Pd3 core is capped by
three terminal PPh3 neutral ligands below the plane, two bridge
PPh2, and one bridge Cl anionic ligand above the plane; the
adsorption of the reactants will displace the relatively small Cl
ligand. When the Cl anion is displaced by the O from PhCH=O
(A-2), it still interacts with the rest of the cluster by electrostat-
ic force, and this step is predicted to be quite exothermic (–2.24
eV), suggesting that the adsorption of PhCH=O is very favora-
ble. This is followed by the transfer of H from PhNH2 to the O
a
b
0.5
0.0
A-1
−0.00
B-4
0.17
0.0
-0.5
-1.0
-1.5
-1.0
B-1
−0.00
B-2
−0.63
A-4
−1.95
B-3
−0.21
B-5
−0.76
A-2
−2.24
A-3
−1.41
PhCHO
-2.0
-3.0
-4.0
PhNH2
A-5
−3.79
B-6
−1.32
Fig. 4. Potential energy surfaces for the reaction between aniline (PhNH2) and benzaldehyde (PhCH=O) over the Pd3 cluster (a) and the Pd(100) sur-
face as catalyst models (b) for the supported Pd3 and Pd nanoparticle catalysts. Only atoms relevant to the reaction are displayed with C, H, N, and Pd
atoms shown in black, white, blue, and light green, respectively. Enlarged images are given in Figs. S4–S6.

Linquan Bao et al. / Chinese Journal of Catalysis 40 (2019) 1499–1504
1503
atom on the adsorbed PhCH=O (A-3), which is modestly endo-
thermic (0.83 eV). Subsequently, PhN=CHPh is formed by fur-
ther H transfer from N to O, accompanied by the formation of
H2O (A-4), which is exothermic (–0.54 eV). The interaction of
PhN=CHPh with the Pd3 core is found to be much weaker than
that with the Cl anion; therefore, the regeneration of the Pd3
catalyst (A-5) is also exothermic (–1.84 eV). The selective for-
mation of PhN=CHPh is partly due to its weak interaction with
the Pd3 cluster; thus, its further hydrogenation is unfavorable.
For the one-pot synthesis of benzalaniline from PhNO2 and
PhCH=O catalyzed by the Pd(100) surface, we focused our cal-
culations on the hydrogenative coupling between PhNH2 and
PhCH=O. Although the Pd(100) surface can simultaneously
adsorb both reactants, PhNH2 through N, and PhCH=O through
C and O (Figs. 4(b) and S6, B-2), the adsorption is not very
strong, as the combined adsorption energy is only –0.63 eV.
Therefore, two H transfer steps (B-3 and B-4) occur from N to O
to form H2O and PhN=CHPh, both of which are slightly endo-
thermic (0.42 and 0.38 eV, respectively). Unlike the case of the
Pd3 cluster, where the adsorption of PhN=CHPh is weaker than
that of the Cl anion, here, PhN=CHPh is quite strongly adsorbed
on the Pd(100) surface via both N and C atoms. Due to the
presence of adsorbed H on the Pd(100) surface, PhN=CHPh can
further be hydrogenated to form benzylaniline (PhNHCH2Ph)
also in two steps (B-5 and B-6), both of which are exothermic
(–0.93 and –0.56 eV); therefore, the formation of the complete-
ly hydrogenated product is more favorable over the Pd nano-
particle catalyst. Thus, the difference in the selectivities be-
tween the Pd3 cluster and Pd nanoparticle catalysts is largely
due to the presence of protective ligands in the former, which
inhibit the complete hydrogenation of PhN=CHPh.
be responsible for their distinct catalytic behaviors based on
both catalyst characterizations and theoretical calculations. The
atomically precise metal nanoclusters give way to a new type of
metal catalyst with high efficiency for certain industrially im-
portant chemical processes and highlight the importance of
pursuing atomic-precise metal nanoclusters for catalysis sci-
ence and technology.
Acknowledgments
We acknowledge financial supports from National Natural
Science Foundation of China (21773109, 91845104).
References
[1] A. Alshammari, V. N. Kalevaru, A. Martin, In: Green Nanotechnolo-
gy-Overview and Further Prospects, M. Larramendy, eds.,
Intechopen, Rijeka, 2016, 1–34.
[2] S. Wang, B. Zeng, C. Li, Chin. J. Catal., 2018, 39, 1219–1227.
[3] Y. Xia, T. D. Nguyen, M. Yang, B. Lee, A. Santos, P. Podsiadlo, Z.
Tang, S. C. Glotzer, N. A. Kotov, Nat. Nanotechnol., 2011, 6,
580–587.
[4] P. Wang, S. Xu, F. Chen, H. Yu, Chin. J. Catal, 2019, 40, 343–351.
[5] D. Astruc, F. Lu, J. R. Aranzaes, Angew. Chem. Int. Ed., 2005, 44,
7852–7872.
[6] Y.-G. Wang, D. Mei, V.-A. Glezakou, J. Li, R. Rousseau, Nat. Commun.,
2015, 6, 6511–6519.
[7] M. Haruta, M. Daté, Appl. Catal. A, 2001, 222, 427–437.
[8] X. Kang, H. Chong, M. Zhu, Nanoscale, 2018, 10, 10758– 10834.
[9] F. Fu, J. Xiang, H. Cheng, L. Cheng, H. Chong, S. Wang, P. Li, S.Wei, M.
Zhu, Y. Li, ACS Catal., 2017, 7, 1860–1867.
[10] Y. Tan, H. Liu, X. Y. Liu, A. Wang, C. Liu, T. Zhang, Chin. J. Catal.,
2018, 39, 929–936.
In summary, we discovered an efficient and easily recyclable
Pd3 nanocluster catalyst for the one-pot synthesis of benzalani-
line from nitrobenzene and benzaldehyde, which exhibited
very high activity and selectivity toward the partially hydro-
genated product, in sharp contrast to the catalytic performance
of typical Pd nanoparticles that afforded the completely hy-
drogenated product. The different behaviors in the adsorption
and reaction of the reactants of the two catalysts are found to
[11] Y. Zhu, H. Qian, B. A. Drake, R. Jin, Angew. Chem. Int. Ed., 2010, 49,
1295–1298.
[12] G. Li, R. Jin, Acc. Chem. Res., 2013, 46, 1749–1758.
[13] J. Ward, R. Wohlgemuth, Curr. Org. Chem., 2010, 14, 1914–1927.
[14] J. Matsuo, Y. Tanaki, H. Ishibashi, Org. Lett., 2006, 8, 4371–4374.
[15] R. Yamaguchi, C. Ikeda, Y. Takahashi, K.-I. Fujita, J. Am.Chem. Soc.,
2009, 131, 8410–8412.
[16] A. Nishinaga, S. Yamaza ki, T. Matsuura, Tetrahedron Lett., 1988,
Graphical Abstract
Chin. J. Catal., 2019, 40: 1499–1504 doi: S1872-2067(19)63423-6
Benzalaniline from nitrobenzene and benzaldehyde catalyzed
efficiently by an atomically precise palladium nanocluster
Linquan Bao, Chengcheng Zhao, Shenggang Li *, Yan Zhu *
Nanjing University; Shanghai Advanced Research Institute, Chinese Academy
of Sciences
Three-atom-Pd nanocluster protected by PPh2 and PPh3 ligands was found
to exhibit unique conversion and selectivity toward the partially hydrogen-
ated product in the catalysis of the reductive amidation of nitrobenzene and
benzaldehyde.

1504
Linquan Bao et al. / Chinese Journal of Catalysis 40 (2019) 1499–1504
29, 4115–4118.
Surf. Sci. Catal., 1997, 5. 547–552.
[17] K. Yamaguchi, N. Mizuno, Angew. Chem. Int. Ed., 2003, 42,
1479–1483.
[22] T. Fu, M. Wang, W. Cai, Y. Cui, F. Gao, L. Peng, W. Chen, W. Ding, ACS
Catal., 2014, 4, 2536–2543.
[18] A. Grirrane, A. Corma, H. Garcia, J. Catal., 2009, 264, 138–144.
[19] J. Deng, L.-P. Mo, F.-Y. Zhao, L.-L. Hou, L. Yang, Z.-H. Zhang, Green
Chem., 2011, 13, 2576–2584.
[20] B. Delley, D. E. Ellis, A. J. Freeman, E. J. Baerends, D. Post, Phys. Rev.
B, 1983, 27, 2132–2144.
[23] I. Witońska, S. Karski, M. Frajtak, N. Krawczyk, A. Królak, React.
Kinet. Catal. Lett., 2008, 93, 241–248.
[24] R. S. Smith, R. A. May, B. D. Kay, J. Phys. Chem.B, 2015, 120,
1979–1987.
[25] D. Li, R. Li, M. Lu, X. Lin, Y. Zhan, L. Jiang, Appl. Catal. B, 2017, 200,
566–577.
[21] V. Almasan, T. Gaeumann, M. Lazar, P. Marginean, N. Aldea, In Stud.
原子精度的钯原子簇催化硝基苯和苯甲醛反应制备苯甲酰胺
包琳泉a, 赵成成b, 李圣刚b,#, 祝 艳a,*
a南京大学化学化工学院, 介观化学教育部重点实验室, 江苏南京210093
b中国科学院上海高等研究院, 低碳转化科学与工程中心, 上海201210
摘要: 金属纳米颗粒催化剂在现代化学工业及相关催化领域研究中具有至关重要的地位, 然而常规纳米颗粒催化剂的一
个主要问题是尺寸多分散, 导致难以在原子水平上精确控制其活性物种结构, 从而阻碍在原子水平上建立精准的催化剂结
构与性能之间的对应关系. 因此, 人们极需构筑和研究新型具有原子精度的催化材料. 具有确定原子数目和精确结构的金
属原子簇往往呈现出类似于分子的行为和特殊的电子结构, 是一种介于均相与多相之间的结构精准的新型催化剂, 其催化
性能能够从原子水平上真正反映催化剂结构的影响, 提供催化作用本质的清晰信息, 对理解催化剂构效关系有重要意义,
可以解决常规催化剂所面临的一些难题.
本文报道了一种原子精度的Pd3原子簇催化剂, 它在催化硝基苯和苯甲醛一锅法合成苄叉苯胺的反应中表现出比常规
的Pd纳米颗粒催化剂更优异的性能. Pd3原子簇催化剂可以高效地把硝基苯和苯甲醛转化为部分氢化产物(苄叉苯胺), 而
Pd纳米颗粒催化此反应得到完全氢化的产物(苄基苯胺). 实验研究表明, 苄叉苯胺更容易从Pd3原子簇催化剂表面脱附, 从
而抑制了苄叉苯胺的进一步加氢, 并提高了Pd3原子簇催化剂对苄叉苯胺的选择性. 密度泛函计算进一步证实了苄叉苯胺
与Pd3原子簇的相互作用比较弱, 苄叉苯胺更容易从Pd3原子簇上脱附, 从而避免了其进一步加氢生成苄基苯胺, 这与Pd纳
米颗粒表面的催化性质截然不同.
综上所述, 我们报道了一种高效且易于回收的Pd3原子簇催化剂, 并将其用于催化硝基苯和苯甲醛一锅法高效合成苯
甲酰苯胺. Pd3原子簇对此反应表现出了极高的反应活性并对部分氢化产物苄叉苯胺有极高的选择性, 而Pd纳米颗粒催化
剂则更加有利于生成完全加氢产物苄基苯胺. 基于实验和理论计算研究, 我们确认了两种催化剂上反应物的吸附和反应
的不同特性是导致其不同催化行为的主要原因.
关键词: 原子簇; 钯; 还原酰胺化; 选择性; 活性
收稿日期: 2019-06-18. 接受日期: 2019-07-16. 出版日期: 2019-10-05.
*通讯联系人. 电子信箱: zhuyan@nju.edu.cn
#通讯联系人. 电子信箱: lisg@sari.ac.cn
基金来源: 国家自然科学基金(21773109, 91845104).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).