Magnetically recyclable Pd/Fe3O4/g-C3N4 as efficient catalyst for the reduction of nitrophenol and suzuki-miyaura reaction at room temperature

Article abstract of DOI:10.1246/cl.180007

Herein, we present the design and synthesis of a novel magnetic stable and recyclable catalyst Pd/Fe₃O₄/g-C₃N₄ with approachable active palladium nanoparticles (Pd NPs) based on the platelet-like graphitic carbon nitrides (g-C₃N₄). The Pd NPs, g-C₃N₄ and magnetic particles (Fe₃O₄) are tightly connected through non-covalent interactions owing to the layered structure of g-C₃N₄ with abundant nitrogen atoms, facilitating the stability of Pd/Fe₃O₄/g-C₃N₄ and activating the Pd NPs at the same time. The Pd/Fe₃O₄/g-C₃N₄ catalyst with a narrow particle size distribution (2.55 nm) of Pd NPs displayed the maintenance of the planar structure of g-C₃N₄. The activity parameter of the catalyst turned out to be excellent (12.4 s¹¹￠mM¹¹) by fitting the curves derived from the reduction of nitrophenol. Meanwhile, the Suzuki-Miyaura coupling reaction at room temperature processed smoothly with the Pd/Fe₃O₄/g-C₃N₄ catalyst. It is meaningful to stress that 2-bromobenzonitrile and p-tolylboronic acid could afford the medical intermediate sartanbiphenyl at total conversion without side reactions in 25 min. Furthermore, the nanocatalyst owes the properties of easy recycling using a powerful magnet as well as high turnover frequency (at least 5 runs) with retention of high catalytic activity.

Full text of DOI:10.1246/cl.180007

Advance Publication Cover Page
Magnetically Recyclable Pd/Fe3O4/g-C3N4 as Efficient Catalyst for the Reduction of
nitrophenol and Suzuki-Miyaura Reaction at Room Temperature
Yingwei Yang and Ruiren Tang*
Advance Publication on the web February 10, 2018
doi:10.1246/cl.180007
© 2018 The Chemical Society of Japan
Advance Publication is a service for online publication of manuscripts prior to releasing fully edited, printed versions. Entire
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Publication manuscripts.

1
Magnetically Recyclable Pd/Fe3O4/g-C3N4 as Efficient Catalyst for the Reduction of nitrophenol and Suzuki-Miyaura
Reaction at Room Temperature
Yingwei Yang, Ruiren Tang*
School of Chemistry and Chemical Engineering, Central South University, Hunan, 410083, China
*Corresponding author. Tel.: 86-731-88836961 Fax: 86-731-88879616
(*E-mail: trr@mail.csu.edu.cn)
dominate the catalytic activity. Though thought to be the
Herein, we present the design and synthesis of a novel
magnetic stable and recyclable catalyst Pd/Fe3O4/g-C3N4
with approachable active palladium nanoparticles (Pd NPs)
based on the platelet-like graphitic carbon nitrides (g-C3N4).
The Pd NPs, g-C3N4 and magnetic particles (Fe3O4) are
tightly connected through non-covalent interactions owing to
the layered structure of g-C3N4 with abundant nitrogen atoms,
facilitating the stability of Pd/Fe3O4/g-C3N4 and activating
the Pd NPs at the same time. The Pd/Fe3O4/g-C3N4 catalyst
with a narrow particle size distribution (2.55 nm) of Pd NPs
displayed the maintenance of the planar structure of g-C3N4.
The activity parameter of the catalyst turned out to be
excellent (12.4 s-1·mM-1) by fitting the curves derived from
the reduction of nitrophenol. Meanwhile, the Suzuki-
Miyaura coupling reaction at room temperature processed
smoothly with the Pd/Fe3O4/g-C3N4 catalyst. It is meaningful
to stress that 2-bromobenzonitrile and p-tolylboronic acid
could afford the medical intermediate sartanbiphenyl at total
conversion without side reactions in 25 min. Furthermore,
the nanocatalyst owes the properties of easy recycle using a
powerful magnet as well as high turnover frequency (at least
5 runs) with retention of high catalytic activity.
frontier between the homogeneous and heterogeneous
catalysts12, the metal NPs are beset by the difficulties in size
control and recyclability. Smaller size of the Pd NPs, with
high specific surface area, would result in high catalytic
performance in terms of more active sites. However, the
drawbacks of the easy aggregation between the smaller Pd
NPs with higher surface energy and low efficiency level
largely confined their practical applications13-15. Based on
this, various supports for the dispersion and stabilization of
the Pd NPs, such as silica materials , metal oxides (CeO2,
ZrO2, MgO, Fe3O4 and etc.), carbon, inorganic porous
materials, LDH and polymers16, are of great potential in the
balance of the size effect and the catalytic efficiency.
Furthermore, the use of supports will benefit the recycle and
reuse of the catalysts.
The inorganic porous materials, as an important branch
of the supports, have recently been the topic research issue in
the heterogeneous catalysis on account of their intrinsic
structural advantages and superior catalytic efficiency in the
catalytic applications. Among the various inorganic porous
materials for the Pd NPs, the nano-lamellar graphitic carbon
nitrides (g-C3N4) have attracted much attention in the area of
catalysis due to the layered structure similar to graphite,
nitrogen rich for the anchor of Pd NPs and surface defects
for further chemical modifications. The g-C3N4 and the
modified g-C3N4 activated Pd NPs have been successfully
applied in the reduction of various substances like alkene,
alkyne, quinoline, phenol and nitriles17 with excellent
conversions and selectivity.
Very recently, the Suzuki coupling reactions were
studied in the presence of the g-C3N4 supported Pd NPs18，
though high yields were obtained with Pd/g-C3N4, the
separation and reuse of the catalyst using the traditional
methods of filtration and centrifugation remains not
convenient for practical use. The introduction of magnetic
particles (Fe3O4)19 into the original Pd/g-C3N4 could be the
most effective solution in view of the advantages of the easy
combination and high stability of the Fe3O4 NPs with Pd/g-
C3N4 through non-covalent interaction and the convenient
separation of the used catalyst from the reaction system with
a permanent magnet without obvious loss.
Keywords: Magnetic Pd nanocatalyst; Fe3O4; g-C3N4;
nitrophenol; Suzuki-Miyaura Reaction
The Pd-catalyzed Suzuki-Miyaura reactions, considering its
great applied potential that exists in the chemical
transformations to prepare high-value products, has been
attracting significant attention. The biaryl compounds are of
the most crucial organic synthetic intermediates of liquid
crystals, polymers, pesticide and medicine1-3, which could be
effectively prepared by the Suzuki coupling reaction.
However, the Suzuki coupling reaction has traditionally
processed smoothly using homogeneous soluble palladium
catalysts with the aid of phosphine ligands under inert
atmosphere4. In addition, there are still problems on the
separation and recovery of the catalysts, which will cause
contamination in the final product and in turn limit their
industrial application5, 6
.
An alternative solution to solve the problem is the
application of metal nanoparticles (NPs). Owing to the
strong electrical and chemical properties of the metal NPs,
especially the Pd NPs, great potentials in fuel cells7,
hydrogenation8, dehydrogenation9, and C–C coupling10 have
been exploited, resulting in excellent catalytic efficiency
with high stability in the reaction process. Reetz et al.
reported that the Pd NPs prepared by the electrochemical
method could catalyze the Suzuki coupling reaction of the
bromide and iodide substrate smoothly11. It is believed that
the high dispersion in the catalysis system and the three-
dimensional rotational freedom of the active Pd NPs mainly
Additionally, 4-nitrophenol, known as hazardous
substances due to their poisonousness, genotoxicity, and
carcinogenicity, can be transformed into nontoxic substances
through the catalytic reduction process which is efficient and
-
green20-22. Therefore, the catalytic reaction of 4-NP by BH4
is always adopted to confirm the catalytic performance of the
prepared catalyst23.
In this paper, as the further extension of the research of
our group18, we improved the design and synthesis strategy

2
Pd/Fe3O4/g-C3N4 is identical with that of g-C3N4, confirming
the combination of Pd NPs, Fe3O4 and g-C3N4 through non-
covalent interactions.
of the Pd/g-C3N4 and combined the magnetic Fe3O4 with the
as-prepared Pd/g-C3N4 to obtain the novel magnetic stable
and recyclable catalyst Pd/Fe3O4/g-C3N4 for the purpose of
developing a catalytic system where the catalyst could be
recycled with a powerful magnet and the catalyst could
catalyze the Suzuki-Miyaura reaction with excellent results.
The Pd/Fe3O4/g-C3N4 catalyst was synthesized easily by
depositing the Pd NPs on the g-C3N4 support through
chemical reduction of the Pd(OAc)2 solution containing the
g-C3N4 and Fe3O4 by NH2NH2·H 2O. The Suzuki coupling
reaction test of the phenyl iodides and bromobenzenes
processed smoothly with high conversions and selectivity
under ambient atmosphere and the catalytic efficiency turned
out to be excellent in the reduction process of the 4-NP.
More importantly, the used catalyst retrieved by a powerful
magnet could be recycled for five runs with high catalytic
performance.
Fig. 2 The FT-IR spectra of g-C3N4, Fe3O4 and Pd/Fe3O4/g-C3N4.
Fig. 1 Powder X-ray patterns of g-C3N4, Fe3O4 and Pd/Fe3O4/g-
C3N4 catalyst.
To achieve the crystallinity and material component of g-
C3N4, Fe3O4 and Pd/Fe3O4/g-C3N4, XRD measurements were
conducted. The peak at 27.46o (Fig. 1) corresponds to the
(002) interlayer stacking peak of g-C3N424. The relative small
(100) peak at 13.17o originates from the in-plane structural
packing mechanism25. Pure Fe3O4 exhibits high similarity
with the data reported in the literature (JCPDS-International
center diffraction data, PDF card 79-041926. Owing to the
interaction of the Pd NPs and g-C3N4 in Pd/Fe3O4/g-C3N4,
the intensity of the (002) peak of g-C3N4 weakened largely
and the (100) peak vanished. The indexed (111) and (200)
diffraction peaks at 39.66o and 46.46o are ascribed to the Pd
NPs (card 46-1043), corresponding to the face centered cubic
(fcc) Pd lattices18. The Fe3O4 peaks of the catalyst basically
remain the same with the pure Fe3O4.
The information of chemical bonds are depicted in Fig. 2 by
means of FT-IR spectra. The single peak at 3200 cm-1 of g-
C3N4 are the stretching vibration of NH unit, featured to be
extended widely. The multiple peaks in 1650-1000 cm-1 are
designated to be the characteristic peaks of s-triazine unit for
the emerge of C=N and C-N27. The spectrum of Fe3O4
exhibits two small but distinct peaks at570 cm-1 and 420 cm-1,
which is known as the vibrations of Fe2+-O2- and Fe3+-O2-
respectively28. Interestingly, with introduction of Pd and
Fe3O4, the peak shift and intensity of the spectrum in the
Fig. 3 Pore size distribution curves for (a) g-C3N4 support. (b)
Pd/Fe3O4/g-C3N4 catalyst.
In order to obtain structural information of the prepared
materials on a mesoscopic scale, the isotherms and the pore
size distributions were measured by BET method. The g-
C3N4 support possesses a surface area of 19.000 m2/g and a
typical mesoporous structure with a pore size of 2.501 nm
(Fig. 3a inset), derived from the curve trend29. After the
formation of Pd/Fe3O4/g-C3N4 with g-C3N4 as the support
(Fig. 3b), the tendency of the hysteresis loops and pore
diameter stayed unchanged and larger specific surface area
are induced by the Fe3O4, suggesting the preservation of
morphology of the g-C3N4. The Pd contents in the prepared
catalyst were determined to be 0.71 % by means of ICP-AES.

3
TEM analysis was applied to characterize the
NMR and Mass of coupling products are shown in Fig S5
and Fig S6, which is can be seen in Supporting information.
microstructure of the g-C3N4, Fe3O4 and the Pd/Fe3O4/g-C3N4
catalyst and the results are shown in Fig. S4. The g-C3N4
support, based on Fig. S4a, has a typical platelet-like
morphology with several layers. Contrary to the reported
spherical structure, the Fe3O4 NPs (Fig. S4b) exhibited a
parallelogram micropattern with the size ranging from 5 nm
to 18 nm. After the loading of Pd NPs and Fe3O4 on the g-
C3N4 (Fig. S4c), smaller sized Pd NPs with mean size of 2.55
nm were obtained by averaging 200 Pd NPs randomly
chosen compared with Pd/g-C3N418, which may be the lift of
catalysis efficiency due to small size effect (Fig. S4c inset).
Interestingly, the distribution of Fe3O4 did not reflect
regularity and they are more like surrounded on the edge of
the g-C3N4. It is the strong electron donating properties of the
nitrogen of g-C3N4 that largely stabilize the Pd/Fe3O4/g-C3N4.
The selective reduction of 4-nitrophenol (4-NP) was
modeled in the presence of an excess amount of NaBH4 to
figure out the efficiency of the as-prepared Pd/Fe3O4/g-C3N4
catalyst. Owing to the strong absorption of the substrates, the
transformation process could be easily monitored by UV-Vis
absorption spectroscopy quantitatively and qualitatively.
After the mixing of NaBH4 into the 4-NP aqueous solution,
4-nitrophenolate was screened accompanied with the color
changing from colorless to yellowish from bare eye.
Furthermore, the absorption single peak at 317 nm
characteristic for 4-NP redshifted to 400 nm with the
outcome of 4-nitrophenolate ion for higher conjugation
degree (Fig. S3a). The color faded from yellowish to
colorless slowly after the addition of Pd/Fe3O4/g-C3N4 with
the absorption peak at 400 nm gradually down to be vanished
in 210 seconds and emerge of a new peak at 300 nm
characteristic for 4-AP (Fig. S3b). As comparison, the same
processes were conducted in the absence of the catalyst.
With the aid of pure g-C3N4 or Fe3O4 even in 30 min, the
peak at 400 nm kept unchanged, indicating that Pd NPs
dominate the catalytic process. Interestingly, when merely
NaBH4 was put into the 4-NP solution, the reaction occurred
slightly after 20 h with minor decrease of the peak at 400 nm.
We further tested the general applicability of the
Pd/Fe3O4 /g-C3N4 by amplifying the reactions to 0.5 mmol
with various nitroarenes in pure water at room temperature in
the presence of NaBH4 as the reducing agent (Table S2).
With high catalytic activity of the Pd/Fe3O4/g-C3N4
towards the reduction of nitroarenes by NaBH4 at hand, we
further tested the versatility of the catalyst for the Suzuki
cross coupling reaction under ambient conditions on account
of its the Mott-Schottky heterojunction that benefits the
charge transportation30. As known, the Sartan drugs have
been the most effective pharmacotherapy to resist
hypertension after they were launched, which makes the
sartanbiphenyl intermediate the pivotal role for the giant
market31. The coupling system of o-bromobenzonitrile and p-
tolylboronic acid with Pd/Fe3O4/g-C3N4 as the catalyst under
ambient atmosphere was modeled to ascertain the optimum
conditions. Effect of the reaction time durations,
temperatures(Table S3), solvents(Table S4), substrates(Table
S5) has been tested on the Suzuki-Miyaura reaction, 1H
Fig. 4 The recycling of the Pd/Fe3O4/g-C3N4 catalyst for the
catalytic transformation of 4-NP and Suzuki-Miyaura
reaction.
The efficiency, stability, sustainability, and recyclability
(ESSR) criteria12 is concerned as a pivotal issue for
evaluating a heterogeneous catalyst, especially the one
containing magnetic. Here, the recyclability of the catalyst
using the chemical reduction of 4-NP and the Suzuki-
Miyaura reaction towards the synthesis of sartanbiphenyl
were tested with 0.13 mol% Pd/Fe3O4/g-C3N4. After each
cycle of the reaction for the 4-NP to 4-AP, the catalyst was
recycled with external magnetic field. It is interesting to note
that the reduction process of 4-NP proceeded smoothly with
total conversion of the raw material to the desired product
even after 5 runs. When the same process was applied to the
Suzuki coupling reaction, however, gradually lowered
conversions after 5 cycles were emerged. Notably, the
catalyst recovered its catalytic efficiency once it was washed
one more time with 1 M NaOH solution and the catalytic
performance stayed almost unaltered after 5 runs with high
yields. It is the NaOH solution that benefited the removal of
the residue formed in the last turn, which in turn reactivated
the catalytic activity. The catalyst can be recycled and reused
for 5 runs without much loss of the catalytic efficiency. The
excellent results owe to the stable and active Pd NPs on the
support of Fe3O4/g-C3N4 where abundant amino groups as
anchor sites locate in the planar g-C3N4.
In summary, the Pd/Fe3O4/g-C3N4 magnetic nanocatalyst
with high stability and activity of the Pd NPs was
successfully synthesized on the foundation of g-C3N4 powder
and the Fe3O4 magnetic nanoparticles. The specific properties
of the planar layers with electron conjugation with amino
groups of g-C3N4 favor the anchor of Fe3O4 and Pd NPs, and
stabilization of the Pd/Fe3O4/g-C3N4 catalyst. The prepared
catalyst demonstrated considerable catalytic efficiency in the
Suzuki-Miyaura reaction with high selectivity under eco-
friendly conditions. Furthermore, the catalyst performed well
in the recycling experiments and it could be recovered from
the reaction solution with a powerful magnet. No obvious
loss of catalytic activity was screened after 5 runs, proving
the Pd/Fe3O4/g-C3N4 as a novel, effective and magnetic
recyclable catalyst.

4
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