Palladium modified magnetic mesoporous carbon derived from metal-organic frameworks as a highly efficient and recyclable catalyst for hydrogenation of nitroarenes

Article abstract of DOI:10.1039/c5ra00878f

We present a facile route for the fabrication of palladium modified magnetic nanoparticles (Fe@Pd NPs) embedded in mesoporous carbon (MC), derived from metal-organic frameworks, to produce the Fe@Pd-MC nanocatalyst. The Fe@Pd-MC nanocatalyst exhibited excellent catalytic activity toward the hydrogenation of nitroarenes as well as easy recovery and reusability.

Full text of DOI:10.1039/c5ra00878f

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Palladium modified magnetic mesoporous carbon
derived from metal–organic frameworks as a highly
efficient and recyclable catalyst for hydrogenation
of nitroarenes
Cite this: RSC Adv., 2015, 5, 20987
Received 15th January 2015
Accepted 16th February 2015
Zhengping Dong,* Kun Liang, Chunxu Dong, Xinlin Li, Xuanduong Le and Jiantai Ma*
DOI: 10.1039/c5ra00878f
www.rsc.org/advances
11–16
We present a facile route for the fabrication of palladium modified synthesis of mesoporous carbon materials (MC).
In partic-
magnetic nanoparticles (Fe@Pd NPs) embedded in mesoporous ular, the transition-metal NPs such as iron (Fe), cobalt (Co), and
carbon (MC), derived from metal–organic frameworks, to produce the nickel (Ni) derived from MOFs embedded in MC, can not only
Fe@Pd–MC nanocatalyst. The Fe@Pd–MC nanocatalyst exhibited act as catalytic active centers, but also reduce the noble metal
excellent catalytic activity toward the hydrogenation of nitroarenes as ions and act as the core to reduce the consumption of the noble
well as easy recovery and reusability.
metal. Furthermore, the Fe, Co, and Ni NPs also exhibit
magnetic property; therefore, introduction of magnetically
responsive substance to the catalyst system could endow the
magnetic properties to the prepared catalysts, which allows for
the easy and efficient recovery of the catalyst from the reaction
Introduction
13,17
mixture by using a magnetic eld.
Extensive research efforts
In recent years, palladium nanoparticles (Pd NPs) have received have been devoted to the development of the transition-metal
signicant attention because of their remarkable catalytic NPs embedded in MC. Thus, a facile and straightforward
performances in hydrogenation, C–C coupling, oxidation, and separation method of noble metal NPs was accomplished either
1
–7
hydrodechlorination reactions. They offer high selectivity, by immobilizing them on the surface of modied magnetic NPs
efficient catalytic performance, and high conversion. However, or by supporting them on magnetically responsive materials
agglomeration of the initially well-dispersed Pd NPs throughout embedded in MC frameworks. Thus, use of the magnetic MC
the catalytic runs in the reaction solution, is still a drawback in materials derived from MOFs as support to prepare noble metal
8,9
their catalytic applications. Moreover, most Pd NPs catalyzed based catalysts is extremely meaningful and highly signicant.
reactions occur only on the surface of the Pd NPs and a large
As important industrial raw materials, substituted anilines
fraction of atoms in the core remain catalytically inactive, and are valuable chemicals for the synthesis of various dyes, phar-
thus should be replaced by other non-noble metals due to the maceuticals, pigments, and polymers that serve as industrially
high cost and scarcity of noble metal resources. To overcome important organic intermediates for producing pharmaceuti-
18–20
the above mentioned problems and facilitate the recovery of the cals, dyes stuffs, and functional polymers.
Till date,
catalyst, core–shell Pd NPs with non-noble metal core have been aromatic amines are mainly produced by chemoselective
dispersed onto supporting matrices such as graphene and silica hydrogenation of the corresponding nitroarenes over metallic
7,10
to fabricate heterogeneous Pd catalysts. Therefore, it is highly catalysts including noble metals such as platinum (Pt), Pd,
19,21–24
desirable to design Pd based catalysts with low Pd loading, easy ruthenium (Ru), and rhodium (Rh) based catalysts.
separation property, and high catalytic activity for organic ously, Pd NPs based heterogeneous catalysts have excellent
catalytic reactions.
catalytic activities and play a very important role in the hydro-
Recently, metal–organic frameworks (MOFs) as a novel class genation of nitroarenes due to enhanced fraction of catalytic Pd
Obvi-
25–28
of nanoporous crystalline material fabricated from transition- surface atoms.
Further, the use of support materials is an
metal clusters as nodes and organic ligands as struts have extremely signicant concept to obtain stable and uniformly
been demonstrated as promising starting materials for the distributed Pd NPs without any agglomeration.
Based on the aforementioned considerations, herein, we
College of Chemistry and Chemical Engineering, Gansu Provincial Engineering present a facile route for the fabrication of Fe@Pd NPs embedded
Laboratory for Chemical Catalysis, Lanzhou University, Lanzhou 730000, PR China. in MC, derived from MOFs, to produce the Fe@Pd–MC nano-
E-mail: dongzhp@lzu.edu.cn; majiantai@lzu.edu.cn; Fax: +86 0931 8912582; Tel:
catalyst. The catalyst support Fe–MC was rst prepared through
+
86 0931 8912577
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the pyrolysis process during the carbonization of the Fe-MIL-88A selectivity of the reaction was estimated by gas chromatography-
2
+
under nitrogen and reducing atmosphere, then the Pd was mass spectroscopy (GC-MS). The separated catalyst was washed
reduced by the metallic Fe NPs without adding any reducing several times with EtOH and dried at room temperature for the
agent, and the metallic Pd so-obtained completely covered the next catalytic run.
exposed surface of Fe NPs with low loading of 0.61 wt%, leading
to the formation of Fe@Pd–MC nanocatalyst. The Fe@Pd–MC
nanocatalyst exhibited excellent catalytic activity toward the
Characterization
2
30
Transmission electron microscopy (TEM, Tecnai G F ), scan-
ning electron microscopy (SEM, JSM-6701F) and N adsorption–
desorption (ASAP 2010, USA) were used to characterize the
morphology of the samples. X-Ray diffraction (XRD) measure-
ments were performed on a Rigaku D/max-2400 diffractometer
hydrogenation of nitroarenes as well as the easy recovery and
reusability property, probably due to the highly dispersed Fe@Pd
NPs embedded in MC mesopores. The process reported herein,
therefore, inspires the chemists and allows them to design and
prepare catalyst materials derived from MOFs with lower noble
metal loading and easy magnetic recovery.
2
using Cu-Ka radiation as the X-ray source in the 2q range of
ꢀ
2
0–80 . X-Ray photoelectron spectroscopy (XPS, Perkin-Elmer
PHI-5702) was employed to analyze the electronic states of the
surface of the samples, and the values of binding energy were
calibrated by using C 1s ¼ 284.6 eV as a reference. Magnetic
measurements on Fe–MC and Fe@Pd–MC nanocatalysts were
investigated by a quantum design vibrating sample magne-
tometer (VSM) at room temperature in an applied magnetic
Experimental
Materials
Fumaric acid, Fe(NO ) $9H O, N,N-dimethylformamide (DMF),
3
3
2
palladium chloride (PdCl ) and sodium borohydride (NaBH )
2
4
were purchased from Shanghai Chemical Reagent Co., Ltd. All
the nitroarenes were also supplied by Shanghai Chemical
Reagent Co., Ltd. All other chemicals were of reagent grade
purchased from Tianjin Guangfu Chemical Company and used
as supplied.

eld sweeping from ꢁ10 to 10 kOe. The inductively coupled
plasma atomic emission spectroscopy (ICP-AES) was used to
measure the Pd content of the Fe@Pd–MC nanocatalyst. The
conversion and selectivity of the reaction was estimated by
GC-MS (Agilent 5977E).
Synthesis of Fe@Pd–MC nanocatalyst
Results and discussion
The magnetic mesoporous material was derived from Fe-MIL-
8
8A. First, Fe-MIL-88A was prepared according to the reported Characterization of the catalysts
29
solvothermal method.
1
in DMF (300 mL). The resulting mixture was then heated to
1
was obtained by successively washing with DMF and meth-
anol via centrifugation redispersion cycles. Second, the dried
Fe-MIL-88A (2 g) was calcined up to 700 C under a nitrogen
atmosphere at a heating rate of 2.5 C min in a tube furnace
and held at 700 C for 1 h. Aer cooling down to 400 C, H
passed through the tube furnace (H
calcination temperature was maintained at 400 C for 2 h to
thoroughly reduce the iron oxide to metallic Fe NPs. Aer
cooling to room temperature, Fe–MC material was obtained.
Third, under inert atmosphere, the Fe–MC material (1 g)
Typically, fumaric acid (1.68 g,
$9H O (6.4 g, 16 mmol) were dissolved
Scheme 1 shows the schematic representation of the prepara-
tion of iron(III) dicarboxylate MOFs of the MIL-88 structure type
4.4 mmol) and Fe(NO
3
)
3
2
(
3 3
Fe-MIL-88A). Fe(NO ) and fumaric acid were employed as the
ꢀ
10 C and allowed to stand for 1 h. The resulting Fe-MIL-88A
starting reagents and the MOF material, namely, Fe-MIL-88A
was prepared according to the reported solvothermal
method. Then, under nitrogen and reducing atmosphere, the
Fe-MIL-88A was converted to Fe–MC magnetic composite which
has metalic Fe NPs embedded in the MC framework. Finally, as
29
ꢀ
ꢀ
ꢁ1
ꢀ
ꢀ
2
was
2+
the Fe NPs was the only reducing agent that can reduce Pd in
the following procedure, Pd was reduced by Fe NPs and coated
2
/N
2
¼ 1/3, v/v), and the
2+
ꢀ
2
+
2+
was rapidly added into the Pd solution (100 mL, [Pd ] ¼
ꢁ
1
2+
1
mg mL ), and then ultrasonicated for 30 min to reduce Pd
by Fe. The metallic Pd so-obtained coated the exposed surface of
the Fe NPs (Fe@Pd NPs). Finally, the Fe@Pd–MC nanocatalyst
was separated by an external magnetic force, thoroughly
washed with water (H O) and ethanol (EtOH), and dried over
2
night in an oven at room temperature.
General procedure for the hydrogenation of nitroarenes
In a typical procedure, nitroarenes (0.5 mmol) were dissolved in
EtOH (5 mL) with Fe@Pd–MC nanocatalyst (5 mg) under an
ꢀ
atmosphere of H (rubber balloon) at 20 C. The reaction was
2
allowed to occur for 1 h, subsequently, the catalyst was sepa-
rated by an external magnetic force, and the conversion and Scheme 1 Preparation of Fe@Pd–MC nanocatalyst.
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Fig. 2 (a) XRD patterns of Fe–MC and Fe@Pd–MC, (b) XPS spectra of
Fe@Pd–MC nanocatalyst.
Fig. 1 (a) SEM of Fe-MIL-88A, (b and c) TEM of Fe–MC (inset, size
distribution of Fe NPs), (d) TEM of Fe@Pd–MC, (e) HAADF-STEM image
clearly shows the characteristic peaks of metallic Pd, and the
of Fe@Pd–MC, C mapping image (C–K), O mapping image (O–K), Fe peaks at 335.6 and 340.8 eV are respectively assigned to Pd 3d5/2
2
+
mapping image (Fe–K), and Pd mapping image (Pd–K).
and Pd 3d3/2, revealing that Pd is not existed in the framework
2
+
of Fe–MC, and the adsorbed Pd has been completely reduced
to metallic Pd.
on the exposed surface of Fe NPs, thus resulting in the forma-
2
The N adsorption–desorption isotherms and pore size
tion of Fe@Pd–MC nanocatalyst.
distribution curves of the Fe–MC and Fe@Pd–MC composites
Fig. 1 shows the SEM and TEM images exhibiting the are shown in Fig. 3a. The isotherms with steep increases at
morphology of the samples. The SEM image shows that the relatively high pressure (P/P > 0.5) is a typical type-IV behavior,
0
Fe-MIL-88A exhibits spindle-like morphology with uniform indicating the existence of large amount of mesopores, result-
diameter of about 400 nm and length of about 2 mm (Fig. 1a). In ing from the pyrolysis process during the carbonization of the
1
6,31
the TEM images of Fe–MC (Fig. 1b and c), the spindle-like Fe-MIL-88A.
The Brunauer–Emmett–Teller specic surface
2
ꢁ1
morphology was unchanged even aer the completion of the area of Fe–MC is 97.6 m
g
; however, the surface area of
2
ꢁ1
pyrolysis process; however, the size seems to be decreased. The Fe@Pd–MC is only 86.3 m g . Fig. 3a inset clearly shows that
pyrolysis and reduction processes led to the conversion of the Fe the main pore size distributions of Fe–MC and Fe@Pd–MC are
coordination complexes into Fe NPs, with an average particle size 5–20 and 5–18 nm, respectively. The slightly decrease both in
of 30 nm (Fig. 1c, inset), embedded in the MC framework. the specic surface area and pore size probably due to the
Fe@Pd–MC nanocatalyst was prepared by the reduction of Pd by exposed surface of the Fe NPs was coated with Pd atoms which
Fe NPs, and then the exposed surface of Fe NPs was coated with have larger diameter than Fe atoms. The magnetic properties of
metallic Pd. Fig. 1d shows that the morphology of Fe@Pd–MC the samples were measured in elds between ꢂ10 kOe at room
nanocatalyst still preserves the original form of Fe-MIL-88A and temperature. Both the samples exhibit ferromagnetic properties
2+
Fe–MC composite. Furthermore, the high-angle annular dark- with saturated magnetization (M
s
) and coercivity (H
c
) values of
ꢁ
1
ꢁ1

eld scanning transmission electron microscopy (HAADF- 136 emu g
and 450 Oe; and 120 emu g
and 334 Oe,
STEM) image (Fig. 1e) conrms the detailed elemental distribu- respectively for Fe–MC and Fe@Pd–MC (Fig. 3b). Therefore,
tion of the nanocatalyst. The elemental analysis of the selected- Fe@Pd–MC nanocatalyst could be efficiently separated from the
area in the grayscale mapping image clearly indicates that the reaction mixture by an external magnetic force and reused
Fe@Pd–MC nanocatalyst support is mainly composed of carbon (Fig. 3b, inset) in the subsequent catalytic runs.
element. The catalytic activity center, Pd (green) is well dispersed
on the Fe NPs (yellow) and embedded in the MC framework.
Further, the crystallographic structures of the samples were
determined by XRD. Fig. 2a exhibits the XRD pattern of Fe–MC
Hydrogenation of nitroarenes
Scheme 1 demonstrates that the prepared Fe@Pd–MC nano-
catalyst is utilized to catalyze the hydrogenation of nitroarenes.
ꢀ
ꢀ
displaying two peaks at 2q ¼ 44.8 , 65.4 which are in well
agreement with the characteristic peaks of (110) and (200) planes
of metallic a-Fe (JCPDS no. 65-4899), respectively. The size of the
Fe NPs calculated by the Scherrer formula is about 30 nm, which
is almost similar to that obtained by the TEM observation.
Compared to the XRD pattern of Fe–MC, Fe@Pd–MC shows two
characteristic diffraction peaks at 2q ¼ 39.9 and 45.2ꢀ corre-
sponding to Pd (111) and Pd (200) of Pd (JCPDS 46-1043),
2,30
2+
respectively, revealing that the Pd was successfully reduced
to metallic Pd and graed on the surface of Fe NPs. Moreover,
the weak peaks of Pd in the XRD pattern also indicate its low
content in Fe@Pd–MC. The weight percentage of Pd loading
obtained from ICP-AES measurement is about 0.61 wt%. The
Fig. 3 (a) N
2
sorption isotherms of Fe–MC and Fe@Pd–MC (inset is the
pore size distributions of Fe–MC and Fe@Pd–MC). (b) Room
state of the Pd in Fe@Pd–MC is determined by XPS. Fig. 2b temperature magnetization curves of Fe–MC and Fe@Pd–MC.
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Table 1 Solvent screening of the Fe@Pd–MC nanocatalyst catalyzed Table 2 Hydrogenation of various nitroarenes using Fe@Pd–MC
a
a
hydrogenation of nitrobenzene
nanocatalyst
Entry
1
Substrate
Product
Yield (%)
99.6
ꢀ
Entry
Solvent
T ( C)
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
EtOH
MeOH
Ethyl acetate
THF
DMF
Cyclohexane
2
H O
Toluene
20
20
20
20
20
20
20
20
1
1
1
1
1
1
1
1
>99
98.1
98.9
48.8
24.7
94.2
73.9
91.8
2
95.5
3
4
94.2
98.8
a
5
6
7
97.2
99.1
98.9
Reaction conditions: nitrobenzene (0.5 mmol), Fe@Pd–MC
nanocatalyst (5 mg, 0.06 mol% Pd relative to nitrobenzene), H
1 atm), solvent (5 mL), reaction time (1 h).
2
(
The catalytic hydrogenation of nitroarenes commenced with a
solvent screening, using nitrobenzene as the model substrate
8
9
98.9
96.2
(
Table 1). In a typical reaction, nitrobenzene (0.5 mmol) and
Fe@Pd–MC nanocatalyst (5 mg) were suspended in 5 mL of
ꢀ
solvent and vigorously stirred under 1 atm of H
2
at 20 C. The
values listed in Table 1 indicated that when the hydrogenation
was performed in EtOH as the reaction solvent, more than
10
96.6
9
9% conversion to the product occurred. Thus, EtOH was
1
1
1
2
98.2
94.4
selected as the optimized solvent for the hydrogenation of
various nitroarenes under atmospheric H at room tempera-
2
ture in 1 h.
Based on the optimized reaction conditions, we then
extended the Fe@Pd–MC nanocatalyst to various nitroarenes
to examine the generality of the reaction (Table 2). In all cases,
the desired aromatic amines were obtained in excellent yields
of over 94%. The electronic features of the nitroarenes almost
did not affect the reaction yield. Importantly, several reducible
1
3
98.3
a
Reaction conditions: substrate (0.5 mmol), Fe@Pd–MC nanocatalyst
(1 atm), solvent (5 mL),
(5 mg, 0.06 mol% Pd relative to substrate), H
2
reaction time (1 h). Yield was determined by GC-MS.
groups, like OH, CN, COCH , and COOH remained totally
3
unaffected under the reaction conditions (Table 2, entries 3–
to the hydroxyamino compound. Finally, hydroxyl amine was
reduced to aniline.
9
). Notably, the hydrogenation reactions of nitroaniline also
2
0,32,33
occurred selectively to give the corresponding diphenylamines
in high yields exceeding 96% (Table 2, entries 10–12), and the
hydrogenation of 2,4-dinitroaniline produced high yield
Recycling and reusability of the catalyst are denitely
important in the heterogeneous catalytic reactions. Therefore,
we investigated the recyclability of the Fe@Pd–MC nanocatalyst
for chemoselective hydrogenation reaction using nitrobenzene
as a model substrate. The Fe@Pd–MC nanocatalyst could be
efficiently separated from the reaction mixture by an external
magnetic force owing to its superparamagnetic property; and
the recovered catalyst was repeatedly washed with EtOH and
dried at room temperature for the next catalytic run. Fig. 4
shows that the catalyst Fe@Pd–MC does not exhibit any change
in its activity (conversion and selectivity), and the loss in the
yields of aromatic anilines are not observed even aer six
successive runs. The amount of Pd present in the catalyst aer
six runs is about 0.59 wt% which is almost similar to the
amount present in the fresh catalyst (estimated by ICP-AES),
thus conrming the stability of the catalyst.
(
98.3%, Table 2, entry 13) of the product. The efficient
hydrogenation of nitroarenes was probably due to the Fe@Pd–
MC nanocatalyst with large pore size that could signicantly
enhance the mass transfer, thus allowing for the easy and
efficient contact between the reactants and the active sites
(
Fe@Pd NPs), leading to rapid commencement and comple-
tion of the reaction. The mechanism of the hydrogenation of
nitroarenes was as follows: initially, H molecule was disso-
2
ciated on the metallic Pd surface to provide dissociative
hydrogen species. Nitro compounds are not directly adsorbed
on the surface of the catalyst; however may have an interaction
with the dissociative hydrogen species. The reduction
occurred in steps and initially the nitro-compound was
reduced to the nitroso-compound which was further reduced
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