Selective hydrogenation of cinnamaldehyde over magnetic flower-like carbonaceous Pd catalysts

Article abstract of DOI:10.1039/d0nj03578e

Synthesis of an easily separable and efficient catalyst is of great significance for selective hydrogenation of unsaturated functional groups. In this work, novel magnetic flower-like carbonaceous Pd catalysts are constructed for the selective reduction of the CC bond. According to the characterization results, a flower-like architecture is observed in the catalysts containing both Fe3O4 and Pd NPs, which is constructed by the secondary self-assembly in the preparation process due to the interaction between Pd and magnetic nitrogen-doped graphene (Fe3O4-NG). More flower-like structures are formed by increasing the Pd loading amount, further enhancing the specific surface area of the catalyst. The specific surface area of Pd-FONG-3 is the highest (521.17 m2 g-1). Combining the results of characterization and experiments, it is found that the catalytic activity is positively related to the flower-like architecture and the interaction between Pd NPs (6.0 nm) and Fe3O4 NPs (6.7 nm). Fe3O4 is not only beneficial to the magnetic recovery of the catalyst, but also acts as a cocatalyst, which can significantly improve the reactant conversion. Moreover, N species play an important role in protecting the CO bond, resulting in the selective hydrogenation of the CC group. Herein, Pd-FONG-3 exhibits excellent activity in the selective hydrogenation of cinnamaldehyde (CAL) with 99.5% conversion and 96.5% hydrocinnamaldehyde (HCAL) selectivity, and its catalytic performance is much better than that of the commercial 5% Pd/C catalyst (100% conversion and 69.1% HCAL selectivity). This work provides a new strategy to construct highly active catalysts with magnetic recovery properties for selective hydrogenation of CC bonds, and Pd-FONG-3 shows tremendous application foreground. This journal is

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Selective hydrogenation of cinnamaldehyde over
magnetic flower-like carbonaceous Pd catalysts†
Cite this:DOI: 10.1039/d0nj03578e
abc
Dan Zhang,a Bowei Wang,
*
*
Lijuan Yan,a Ruixiao Gao,a Lu Liu,a Ziyi Xia,a
and Ligong Chen
abc
abc
abc
Xilong Yan,
*
Yang Li
Synthesis of an easily separable and efficient catalyst is of great significance for selective hydrogenation
of unsaturated functional groups. In this work, novel magnetic flower-like carbonaceous Pd catalysts are
constructed for the selective reduction of the CQC bond. According to the characterization results,
a flower-like architecture is observed in the catalysts containing both Fe3O4 and Pd NPs, which is
constructed by the secondary self-assembly in the preparation process due to the interaction between
Pd and magnetic nitrogen-doped graphene (Fe3O4-NG). More flower-like structures are formed by
increasing the Pd loading amount, further enhancing the specific surface area of the catalyst. The
specific surface area of Pd-FONG-3 is the highest (521.17 m2
g
À1). Combining the results of
characterization and experiments, it is found that the catalytic activity is positively related to the flower-
like architecture and the interaction between Pd NPs (6.0 nm) and Fe3O4 NPs (6.7 nm). Fe3O4 is not only
beneficial to the magnetic recovery of the catalyst, but also acts as a cocatalyst, which can significantly
improve the reactant conversion. Moreover, N species play an important role in protecting the CQO
bond, resulting in the selective hydrogenation of the CQC group. Herein, Pd-FONG-3 exhibits excellent
activity in the selective hydrogenation of cinnamaldehyde (CAL) with 99.5% conversion and
96.5% hydrocinnamaldehyde (HCAL) selectivity, and its catalytic performance is much better than that of
the commercial 5% Pd/C catalyst (100% conversion and 69.1% HCAL selectivity). This work provides
a new strategy to construct highly active catalysts with magnetic recovery properties for selective
hydrogenation of CQC bonds, and Pd-FONG-3 shows tremendous application foreground.
Received 16th July 2020,
Accepted 3rd November 2020
DOI: 10.1039/d0nj03578e
rsc.li/njc
alkaline medium, and can be used as a blending soap essence
or correcting agent in various flavors. Therefore, it is necessary
Introduction
Catalytic hydrogenation is extremely attractive due to the high to find a catalyst that can selectively conduct the hydrogenation
atom utilization of H2 and eco-friendliness. Cinnamaldehyde of CQC bonds. These catalysts mainly include homogeneous
(CAL), as a typical a,b-unsaturated aldehyde, plays a significant and heterogeneous catalysts. Homogeneous catalysts, which
role in the industrial and academic application, such as the have sufficient contact with the substrate, can achieve the
synthesis of flavors, pharmaceuticals and paints.1–4 Since CAL efficient reaction results, but there are also some difficulties
has three unsaturated functional groups, namely phenyl, CQC in after-treatment.5 Nevertheless, heterogeneous catalysts are
bond and aldehyde groups, study on selective hydrogenation of widely used in catalytic reactions due to the mild reaction
CAL becomes particularly important. Hydrocinnamaldehyde conditions and simple separation process.
(HCAL), cinnamyl alcohol (COL) and hydrocinnamyl alcohol
It is crucial to develop highly effective and high selectivity
(HCOL) can be obtained by hydrogenation of CQC bonds heterogeneous catalysts for hydrogenation of CQC bonds in
and/or CQO bonds (Scheme 1). Among them, the product CAL. Palladium is one of the most widely reported metals used
HCAL obtained by hydrogenation of CQC bonds is stable in in the hydrogenation of the CQC bond.6–11 For instance,
palladium on activated carbon (Pd/AC) is a traditional catalyst
for selective hydrogenation of a,b-unsaturated aldehydes.12
However, most of this catalyst is recovered by filtration, which
is not economical and environmentally friendly. The most
effective method is to develop magnetic catalysts that can be
easily separated from the reaction system for recycling.13–16
Moreover, the magnetically separable nature of the catalyst could
a School of Chemical Engineering and Technology, Tianjin University,
Tianjin 300350, P. R. China. E-mail: bwwang@tju.edu.cn, yan@tju.edu.cn,
liyang777@tju.edu.cn
b Collaborative Innovation Center of Chemical Science and Engineering (Tianjin),
Tianjin 300072, P. R. China
c Tianjin Engineering Research Center of Functional Fine Chemicals, Tianjin,
P. R. China
† Electronic supplementary information (ESI) available. See DOI: 10.1039/d0nj03578e aid in reducing the operational energy and time. For example,
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was constructed via self-assembly in the Pd loading process,
which led to the large surface area of the catalyst and further
enhanced the catalytic activity. As an affordable co-catalyst, Fe3O4
NPs on nitrogen doped graphene gels exhibited a positive
influence on the activity. Doped N species were also beneficial
to the selective reduction of CQC bonds. Moreover, the reaction
parameters and the reusability of the catalyst were investigated in
detail.
Experimental
Materials
Scheme 1 Catalytic hydrogenation of CAL.
Except for special instructions, all reagents and chemicals were
of analytical grade and used without further purification.
Graphite powder was purchased from Tianjin Guangfu Tech-
nology Development Co. Ltd, Tianjin, China. FeCl3Á6H2O was
purchased from Tianjin Standard Technologies Co., Ltd,
Tianjin, China. FeCl2Á4H2O was purchased from Tianjin
Fengchuan Chemical Reagent Technologies Co., Ltd, Tianjin,
China. PdCl2 was purchased from Sinopharm Chemical
Reagents Beijing Co., Ltd, Beijing, China. CAL (GC) was pur-
chased from Shanghai Aladdin Bio-Chem Technology Co., Ltd,
Shanghai, China.
Hyeon et al.15 reported a simple gram-scale synthesis of Pd-Fe3O4
heterodimer nanocrystals, which exhibited good activities for
Suzuki coupling reactions. This nanocrystal catalyst could be
easily separated from the reaction mixture due to its ferromag-
netism. Zhu et al.16 presented Fe3O4@MOF core–shell magnetic
microspheres as excellent catalysts for the Claisen–Schmidt
condensation reaction. Remarkably, it can be easily separated
and recycled without significant loss of catalytic efficiency.
Nemati et al.17 successfully synthesized magnetic mesoporous
polymelamine formaldehyde nanocomposite-incorporating ZnO
NPs by solvothermal and sol–gel methods, which can be easily
separated from the reaction medium and reused without signi-
ficant changes in catalytic activity. Among them, the introduction
of magnetic materials is the most attractive and straightforward
method. Another drawback is the instability of Pd/AC after several
cycles due to detrimental aggregation and leaching of Pd NPs.18
Shao et al.19 developed the core–shell nanocatalyst ZIF-8@
Pd@MOF-74 for the preferential reduction of CQC groups.
However, the catalytic activity of ZIF-8@Pd@MOF-74 decreased
remarkably when the number of cycles increased. Furthermore,
one problem of these catalysts is that only part of the active sites
on the surface of the catalyst acted, and the efficiency was still
expected to increase. It is not hard to see that studies on the
Pd-based catalysts are also very active due to the interfacial inter-
actions which have a large effect on the catalyst activity.20–25
In our previous report, magnetic 3D nitrogen doped gra-
Preparation of GO, Fe3O4-NG and NRGO
Graphene oxide (GO) was prepared by the oxidation of graphite
according to the Hummers’ method.40 According to the report,
magnetic nitrogen-doped graphene (Fe3O4-NG) was synthesized
by the hydrothermal method.26 2 mL aqueous solution of
FeCl3Á6H2O (0.081 g) and FeCl2Á4H2O (0.030 g) was added into
50 mL GO aqueous dispersion (2 mg mLÀ1). After 20 minutes of
ultrasonication, 10 mL of ethylenediamine was added to the
mixture. And then the mixture was poured into a 100 mL
Teflon-lined autoclave and kept it for 12 hours at 180 1C. After
cooling to room temperature, the obtained gel was washed with
deionized water to pH E 7 and freeze-dried. N-doped reduced
graphene oxides (NG) were also prepared which used 2 mL of
water replacing the solution of FeCl3Á6H2O and FeCl2Á4H2O.
Preparation of Pd-FONG-1.5, Pd-FONG-3 and Pd-NG-3
phene gels were synthesized for the reduction of 4-nitrophenol The catalysts were prepared by the sodium borohydride
to 4-aminophenol.26 Compared to the traditional catalyst reduction method. After being ground into powder, Fe3O4-NG
supports, graphene offers higher activity and selectivity with a (270 mg) was added to 45 mL of deionized water and stirred.
large specific surface area, high-speed electron mobility, good Then, ammonia solution of PdCl2 was added dropwise into the
electrical conductivity, etc.27–33 Modification of graphene can mixture and stirred for 40 min. Excess sodium borohydride
lead to obtaining more excellent materials, in which doped solution (0.1 M) was slowly added dropwise into the mixture
nitrogen regulate the activity and dispersion of supported and stirred for 3 h. The mixture was filtered, and then washed
metals to some extent.34–38 Therefore, in this work, we tried with deionized water several times to remove the residual
to construct an efficient, environmentally friendly and magne- chlorine and boron-containing ions. Meanwhile, the solution
tically recoverable Pd catalyst to solve the above-mentioned of silver nitrate was added dropwise into the water that was
problems. A series of Pd catalysts supported by magnetic used to wash the filter cake until no precipitation formed to
nitrogen doped graphene gels were prepared and applied for ensure the elimination of chloride ions. Furthermore, ICP-OES
the selective hydrogenation of CAL. Characterization methods and EDS were employed to confirm the disappearance of B and
were employed to study the structure–activity relationship of the Cl. Finally, after vacuum drying, the obtained solid was ground to
catalysts.39 The results indicated that a flower-like architecture fine powder. The obtained solid powder was named Pd-FONG-X,
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where X represents the load of Pd (wt%). Besides, Pd-NG-3 was
fabricated by a similar process in which NG was used as the
support.
Characterization
X-ray diffraction (XRD) patterns were obtained by using a
BRUKER AXS GMBH D8-Focus with a Cu-Ka X-ray source in
the range of 5–801. Scanning electron microscopy (SEM) was
performed on a Hitachi S-4800 microscope. Transmission
electron microscopy (TEM) images were recorded via a JEOL
JEM-2100F microscope. Scanning transmission electron micro-
scopy (STEM) images was recorded by using an energy disper-
sive spectrometer (EDS; EDAX PV6761/55). Elemental analysis
was carried out using an inductar EL cube elemental analyzer
and inductively coupled plasma optical emission spectrometry
(ICP-OES) was performed by using a Vista-MPX spectrometer.
The specific surface area was determined by the Brunauer–
Emmer–Teller (BET) method with nitrogen physisorption
at a liquid nitrogen temperature on a NOVA 2000e analyzer
(Quantachrome, U.S.A.). The X-ray photoelectron spectroscopy
(XPS) measurements were carried out on an EscaLab Xi+
spectrometer.
Fig. 1 XRD patterns of (a) GO, (b) NRGO, (c) Pd-NG-3, (d) Fe3O4-NG,
(e) Pd-FONG-1.5 and (f) Pd-FONG-3.
successful synthesis of GO (Fig. 1a).41,42 The broad peak of NG
at 24.31 belonged to the diffraction peak of the C (002) crystal
faces with an interlayer spacing of 0.37 nm, and the GO
standard peak disappeared due to the total reduction of GO
(Fig. 1b).36,43 The corresponding peaks of Fe3O4-NG, Pd-FONG-
1.5 and Pd-FONG-3 were at about 22.21 and an interlayer
spacing was about 0.42 nm (Fig. 1d–f), which increased
obviously, probably because Fe3O4 NPs hindered the stacking
of carbon layers.26 In the curves of Fe3O4-NG, Pd-FONG-1.5 and
Pd-FONG-3, diffraction peaks at 30.11, 35.51, 43.01, 53.31, 57.01
and 62.61 can be found, which can be indexed to the (220),
(311), (400), (422), (511) and (440) crystal faces of Fe3O4
(Fig. 1d–f).44,45 Moreover, the Pd(111) crystal face diffraction
peak in Pd-NG-3 was identified at 39.91 with a lattice spacing of
0.226 nm.31,44 However, the Pd standard peak shifted to 39.51
in Pd-FONG-1.5 with a lattice spacing of 0.228 nm, and to
39.11 in Pd-FONG-3 with a lattice spacing of 0.230 nm, which
proved that the interaction might be created via Pd NPs and
Fe3O4 NPs.21–23
Liquid-phase hydrogenation of CAL
The hydrogenation reaction took place in an autoclave with
15 mL Teflon lining. A stirrer, catalyst (0.25 mmol% Pd),
solvent (3 mL), and CAL (2 mmol) in turn were added to the
autoclave. Before starting the reaction, the autoclave should be
slowly purged at least 7 times with N2 and H2, respectively, to
remove air, and then pressurized to 2 MPa H2. And then, the
autoclave was placed in a preheating oil bath with magnetic
stirring and kept there for a certain time. The reactants were
analyzed by using a gas-chromatogram with an ATÁOV-101
capillary column using a flame ionization detector.
Reusability of Pd-FONG-3
For the catalyst reusability test, the reaction conditions were
2 mmol CAL, 18 mg Pd-FONG-3 (0.25 mmol% Pd), 3 mL
toluene, 80 1C, 2 MPa H2, 1.5 h. All reactions were conducted
in an autoclave according to the procedures described above.
Due to the magnetism of Pd-FONG-3, the used catalyst was
easily separated by a magnet, washed with toluene, dried, and
then reused under the same experimental conditions. The
reusability of the catalyst was evaluated by the value of Cn/C1
and Sn/S1 (n = 2, 3, 4, and 5). C1 is the conversion of CAL in the
first run, while Cn is the conversion of CAL in the n cycle.
Analogously, S1 represents the selectivity of HCAL in the first
run, while Sn is the selectivity of HCAL in the n cycle.
Fig. 2 shows the SEM images of Fe3O4-NG, Pd-FONG-1.5,
Pd-FONG-3 and Pd-NG-3. From Fig. 2a, a rich pore structure of
Fe3O4-NG is observed owing to the self-assembly in the hydro-
thermal process. A few flower-like architectures with petals
composed of thin layers were found from Pd-FONG-1.5 (Fig. 2b)
and there were more flower-like architectures in Pd-FONG-3
(Fig. 2c). The flower-like architecture could also be observed in
STEM images (Fig. S2, ESI†). This architecture was possibly
constructed by the secondary self-assembly in the preparation
process due to the interaction between Pd and Fe3O4-NG. Similar
structures were also reported by some studies, which can increase
the specific surface area of the material.36,46–50 The results of the
BET test also verified that the specific surface area of Pd-FONG-3
Results and discussion
Catalyst characterization
Fig. 1 shows the XRD patterns of GO, NG, Pd-NG-3, Fe3O4-NG, 311.74 m2 gÀ1) (Table 1).
is the highest (521.17 m2
g
À1) (Table 1). Pd-NG-3 consisted
of a mesh channel structure (the lowest specific surface area,
Pd-FONG-1.5 and Pd-FONG-3, respectively. The obvious diffrac-
TEM images of Fe3O4-NG, Pd-FONG-1.5, Pd-FONG-3 and
tion peak at 11.91 was assigned to the GO, confirming the Pd-NG-3 are presented in Fig. 3. From all images, it can be
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Fig. 2 The SEM images of (a) Fe3O4-NG, (b) Pd-FONG-1.5, (c) Pd- FONG-3
and (d) Pd-NG-3.
Table 1 Physical and textural properties of different catalysts
Mass concentration
SBET
Vtotal
Daverage
Catalyst
Ca
Na Ha Pdb Feb (m2
g
À1) (cm3 À1) (nm)
g
NRGO
75.38 9.77 2.42 —
44.37 5.50 2.30 — 24.04 312.33 0.29
71.37 9.14 2.21 2.44 — 311.74 0.25
—
—
—
—
Fe3O4-NGc
Pd-NG-3
18.79
16.05
21.98
22.51
Pd-FONG-1.5 48.96 5.84 1.73 1.26 26.49 394.67 0.43
Pd-FONG-3 42.36 4.92 2.42 2.79 24.95 521.17 0.59
a
b
Mass concentration was detected in elemental analysis. Mass
c
concentration was measured by ICP-OES. The BET test was carried
out after being ground into powder.
Fig. 3 TEM images and particle size distribution histograms of (a) Fe3O4-
NG, (b) Pd-FONG-1.5, (c) Pd-FONG-3, (d) Pd-NG-3 and STEM images of
observed that NPs were uniformly dispersed on the supports. Pd-FONG-3.
In the TEM image of Pd-FONG-3, Pd NPs and Fe3O4 NPs were
observed clearly; the addition of Fe species did not affect
the dispersion of Pd NPs. The average particles sizes of four presented in Fig. 4. Fig. 4a shows the wide scan spectra of
catalysts are shown in Fig. 3. The average particle size of Fe3O4 the five samples. The binding energy peaks at about 284.5 eV,
NPs in Fe3O4-NG was about 6.7 nm, and the average particle 399.2 eV and 531.3 eV were attributed to C 1s, N 1s and O 1s,
size of NPs in Pd-FONG-1.5 and Pd-FONG-3 was about 8.2 nm respectively.26,31,39 The N 1s spectra of Fe3O4-NG, Pd-FONG-3
and 8.1 nm, respectively, but it was hard to distinguish Pd and and Pd-NG-3 revealed that N species have four forms including
Fe3O4 NPs strictly. It was found that the introduced Pd species pyridinic-N, amine-N, pyrrolic-N and graphitic-N at about
could increase the Fe3O4 NP size slightly. Moreover, in Pd-NG-3, 398.2 eV, 399.2 eV, 400.0 eV, and 400.9 eV (Fig. 4b–d).55
the average particle size of Pd NPs was about 6.0 nm. Fig. 3 Pyridinic-N and amine-N were dominant in the three catalysts
shows the STEM images of Pd-FONG-3, from which it can be (Table S1, ESI†). XPS analysis also provided the state of the Pd
observed that this catalyst contained C, N, O, Fe and Pd 3d region (Fig. 4e). The two peaks with binding energies of
elements. Evidently, Pd NPs distributed around Fe3O4 NPs about 335.0 eV (Pd 3d5/2) and 340.3 eV (Pd 3d3/2) confirmed the
which indicated that the formation of Pd NPs was related to presence of metallic Pd0 in the catalyst and this result was in
Fe3O4 NPs.
line with the XRD characterization. Compared with the Pd 3d5/2
Moreover, the elemental contents of the prepared catalysts binding energy peak of Pd-NG-3 at 335.0 eV, the Pd 3d5/2 peaks
are also listed in Table 1. According to the results of elemental of Pd-FONG-1.5 and Pd-FONG-3 shifted to a higher value
analysis and ICP-OES test, the contents of N in the catalysts (335.3 eV), which were possibly affected by Fe3O4 NPs, indicating
were 9.77%, 5.50%, 9.14%, 5.84% and 4.92%, respectively. again that the interaction was generated between Pd NPs and
As expected, the Pd content of Pd-FONG-3 was twice as Fe3O4 NPs. The interaction between them resulted in the
that of Pd-FONG-1.5, and it was also consistent with the redistribution of charge and affected the binding energy. And
theoretical value.
a lower binding energy indicates that Pd on the surface of
The analyses of surface composition and valence state were Pd-NG-3 possessed a reduction (electron gain) state and weak
carried out by using XPS measurements and the results are adsorbate binding energies.51–53 The Fe 2p3/2 binding energy
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Fig. 4 (a) XPS survey spectra of NRGO, Pd-NG-3, Fe3O4-NG, Pd-FONG-1.5, and Pd-FONG-3, N1s spectra of (b) Fe3O4-NG, (c) Pd-FONG-3 and
(d) Pd-NG-3, (e) Pd 3d spectra of Pd-NG-3, Pd-FONG-1.5 and Pd-FONG-3, (f) Fe 2p spectra of Fe3O4-NG, Pd-FONG-1.5 and Pd-FONG-3.
peak was at about 711.2 Æ 0.2 eV, and the Fe 2p1/2 binding peak in Pd-FONG-1.5 and Pd-FONG-3 was slightly lower than
energy peak was at 724.5 Æ 0.2 eV (Fig. 4f),54 further confirming that in Fe3O4-NG, further confirming the interaction between
the presence of Fe3O4 NPs. The binding energy of the Fe p3/2 Pd and Fe3O4 NPs.
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Table 2 Hydrogenation of CAL over different catalystsa
beneficial effect of pyridinic-N was explained via a more facile
heterolytic activation of molecular hydrogen. Fe3O4-NG,
however, had no activity on the reaction which implied that
Fe3O4 NPs only played an auxiliary role (entry 4). Compared to
commercial Pd/C (entry 5), Pd-FONG-3 exhibited a higher
selectivity to HCAL. That might be the influence of N species
on support, which could keep the CQO groups away from Pd,
and then inhibit further hydrogenation of HALD.55 Therefore,
the outstanding results of Pd-FONG-3 may be caused by the
co-catalyst Fe3O4 and flower-like N doped graphene support.
In order to optimize the conversion of CAL and the selectivity
of HCAL, some factors influencing the reaction were explored,
such as solvent media, reaction temperature, H2 pressure and
reaction time.
Selectivity (%)
Entry Catalysts
Conversion (%) HCAL
COL HCOL
1
2
3
4
5
Pd-NG-3
8.1 Æ 0.43
84.9 Æ 0.29
—
—
—
—
—
15.1 Æ 0.74
Pd-FONG-1.5 88.0 Æ 0.10
89.2 Æ 0.08
92.4 Æ 0.04
—
10.8 Æ 0.15
7.6 Æ 0.26
—
Pd-FONG-3
Fe3O4-NGb
Pd/C
86.7 Æ 0.12
—
100.0
69.1 Æ 0.58
30.9 Æ 0.73
a
Reaction conditions: 2 mmol CAL, catalyst (0.25 mmol% Pd, catalyst
dosage was calculated by the amount of active metal Pd, and the Pd
content is 0.25 mmol% of CAL), 3 mL toluene, 70 1C, 2 MPa H2, 2 h.
b
Amount: 18 mg.
Catalytic performance
Table 3 shows the results of catalytic hydrogenation of
CAL over Pd-FONG-3 in different solvent media. The highest
conversion of CAL was obtained with toluene as the solvent
(86.7%, entry 4).
The influence of reaction temperature on hydrogenation of
CAL to HCAL over Pd-FONG-3 was investigated in the range
of 60–100 1C (Fig. 5a). The increase of the hydrogenation
temperature favored the conversion of CAL, which increased
from 53.1% (60 1C) to 99.5% (80 1C). The selectivity of HCAL
also increased significantly to 96.5%. As the temperature
continued to increase, CAL completely reacted, but the selec-
tivity of HCAL decreased to 91.4% at 100 1C. Thus, the optimum
reaction temperature was 80 1C.
According to Fig. 5b, the influence of H2 pressure on
hydrogenation of CAL to HCAL over Pd-FONG-3 could be
figured out in the scope of 1–3 MPa. When the H2 pressure
was from 1 MPa to 2 MPa, the conversion of CAL increased
gradually by 11.8% to 99.5%. As the pressure continued to
increase to 3 MPa, the conversion was almost unchanged, but
the selectivity of HCAL decreased to 91.8%. Obviously, 2 MPa of
H2 pressure can lead to higher conversion and selectivity.
Fig. 5c shows the fluctuation of the conversion and selec-
tivity data with the change of reaction time. The conversion
grew sharply by 77.1% to 99.5% when the reaction time was
extended from 0.5 h to 2 h. On continuing the extension of the
reaction time, CAL was completely converted. With the change
Hydrogenation of CAL over the above-mentioned catalysts was
carried out to evaluate the catalytic performance and the results
are shown in Table 2. Pd-NG-3, a Fe3O4-free catalyst, was hard
to catalyze the reaction along with only 8.1% conversion
(entry 1). However, Pd-FONG-3 gave the excellent conversion
(86.7%, entry 3) under the same conditions, indicating that
Fe3O4 NPs had a great influence on the reaction. Meanwhile,
when the content of Pd in the catalyst decreased, Pd-FONG-1.5
afforded 88.0% conversion and 89.2% selectivity of HCAL
(entry 2), the selectivity of the catalyst had decreased. This
may be due to the larger specific surface area in Pd-FONG-3
provided by the flower-like architecture (Fig. 2 and Table 1).
Notably, Pd-FONG-3 contained more pyridinic nitrogen, and
the ratio of pyridinic-N to pyrrolic-N was the highest. This
Table 3 Conversion and selectivity data in the different solventsa
Selectivity (%)
Entry
Solvent
Conversion (%)
HCAL
HCOL
1
2
3
4
5
Water
Cyclohexane
Ethanol
Toluene
Ethyl acetate
24.5 Æ 0.25
64.7 Æ 0.68
38.8 Æ 0.77
86.7 Æ 0.11
8.2 Æ 0.49
82.5 Æ 0.13
90.8 Æ 0.28
91.7 Æ 0.62
92.4 Æ 0.07
93.1 Æ 0.54
17.5 Æ 0.30
9.2 Æ 0.50
8.3 Æ 0.65
7.6 Æ 0.35
6.9 Æ 0.40
a
Reaction conditions: 2 mmol CAL, Pd-FONG-3 (0.25 mmol% Pd), 3 mL
solvent, 70 1C, 2 MPa H2, 2 h.
Fig. 5 Effects of reaction temperature (other conditions: 2 mmol CAL, Pd-FONG-3 (0.25 mmol% Pd), 3 mL toluene, 2 MPa H2, 2 h), H2 pressure (other
conditions: 2 mmol CAL, Pd-FONG-3 (0.25 mmol% Pd), 3 mL toluene, 80 1C, 2 h) and reaction time (other conditions: 2 mmol CAL, Pd-FONG-3
(0.25 mmol% Pd), 3 mL toluene, 80 1C, 2 MPa H2) over Pd-FONG-3.
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Acknowledgements
This project is funded by the National Natural Science Founda-
tion of China (Grant No. 21808161 and 21978208) and China
Postdoctoral Science Foundation (Grant No. 2020T130465).
References
1 B. Bachiller-Baeza, I. Rodriguez-Ramos and A. Guerrero-
Ruiz, Appl. Catal., A, 2001, 205, 227.
Fig. 6 Reusability test of Pd-FONG-3 for hydrogenation of CAL. Reaction
conditions: 2 mmol CAL, 18 mg Pd-FONG-3 (0.25 mmol% Pd), 3 mL
toluene, 80 1C, 2 MPa H2, 1.5 h.
2 J. Zhao, J. Ni, J. Xu, J. Xu, J. Cen and X. Li, Catal. Commun.,
2014, 54, 72.
3 M. Zhao, K. Yuan, Y. Wang, G. Li, J. Guo, L. Gu, W. Hu,
H. Zhao and Z. Tang, Nature, 2016, 539, 76.
4 Y. Yang, D. Rao, Y. Chen, S. Dong, B. Wang, X. Zhang and
M. Wei, ACS Catal., 2018, 8, 11749.
of reaction time, the selectivity remained stable at about 95%
and reached the maximum of 96.5% at 2 h. This also alluded to
the difficulty of CQO bond reduction.56 At the same time,
considering the energy saving, two hours was chosen as the
best reaction time with 99.5% of conversion and 96.5% of
selectivity.
By exploring a series of parameters, 99.5% of conversion and
96.5% of selectivity as the optical reaction results were obtained
over Pd-FONG-3 under the conditions: solvent, 3 mL toluene;
reaction temperature, 80 1C; H2 pressure, 2 MPa; reaction time, 2 h.
5 I. Szatmari, G. Papp, F. Joo and A. Katho, Inorg. Chim. Acta,
2018, 472, 302.
6 S. Chen, L. Meng, B. Chen, W. Chen, X. Duan, X. Huang,
B. Zhang, H. Fu and Y. Wan, ACS Catal., 2017, 7, 2074.
7 X. Chen, H. Cao, X. Chen, Y. Du, J. Qi, J. Luo,
M. Armbruester and C. Liang, ACS Appl. Mater. Interfaces,
2020, 12, 18551.
8 S. Bewana, M. J. Ndolomingo, E. Carleschi, B. P. Doyle,
R. Meijboom and N. Bingwa, ACS Appl. Mater. Interfaces,
2019, 11, 32994.
Reusability of Pd-FONG-3
9 R. Paul, C. Sarkar, Y. Yan, Q. T. Trinh, B. S. Rao, C.-W. Pao,
J.-F. Lee, W. Liu and J. Mondal, ChemCatChem, 2020,
12, 3687.
10 I. Szewczyk, R. Kosydar, P. Natkanski, D. Duraczynska,
J. Gurgul, P. Kustrowski and A. Drelinkiewicz, J. Porous
Mater., 2020, 27, 1287.
It is well known that the reusability of the catalyst is also very
important.57 According to Fig. 6, it was found that the CAL
conversion and HCAL selectivity fluctuated slightly in the
following 4 cycles and the CAL conversion and HCAL selectivity
were still higher than 90% and 95% of the first run, respectively.
The results proved that Pd-FONG-3 exhibited good reusability for
at least 5 cycles.
11 F. Wang, Y. Bi, K. Hu and X. Wei, Chem. – Eur. J., 2020,
26, 4874.
12 F. Zhao, Y. Ikushima, M. Chatterjee, M. Shirai and M. Arai,
Green Chem., 2003, 5, 76.
13 Y. Jia, H. Ma, W. Zhang, G. Zhu, W. Yang, N. Son, M. Kang
and C. Liu, Chem. Eng. J., 2020, 383.
14 F.-A. Khan and G. Suess-Fink, Eur. J. Inorg. Chem., 2012, 727.
15 Y. Jang, J. Chung, S. Kim, S. W. Jun, B. H. Kim, D. W. Lee,
B. M. Kim and T. Hyeon, Phys. Chem. Chem. Phys., 2011,
13, 2512.
16 F. Ke, L.-G. Qiu and J. Zhu, Nanoscale, 2014, 6, 1596.
17 Z. Shahamat, F. Nemati and A. Elhampour, Mol. Diversity,
2020, 24, 691.
Conclusion
In summary, we have successfully fabricated a series of Pd
catalysts supported by magnetic nitrogen doped graphene gels
including Pd-NG-3, Pd-FONG-1.5 and Pd-FONG-3 for hydro-
genation of CAL. Among them, the addition of Fe3O4 was not
only beneficial to the magnetic recovery of the catalysts, but
also Fe3O4 acted as a cocatalyst, which could significantly
improve the conversion of CAL. Meanwhile, Fe3O4 NPs affected
the formation of Pd NPs to a certain extent. The interlayer
spacing and binding energy of Pd NPs in Pd-FONG-1.5 and
Pd-FONG-3 had changed compared to that in Pd-NG-3. Moreover,
the flower-like architecture in Pd-FONG-1.5 and Pd-FONG-3
increased the specific surface area. Among all the catalysts,
Pd-FONG-3 presented the best performance and attained 99.5%
conversion of CAL and 96.5% selectivity of HCAL after investigating
the effects of some factors.
18 F. Zhao, Y. Ikushima, M. Chatterjee, M. Shirai and M. Arai,
Green Chem., 2003, 5, 76.
19 C. Guo, C. Liang, X. Qin, Y. Gu, P. Gao, M. Shao and
W.-T. Wong, ACS Appl. Nano Mater., 2020, 3, 7242.
20 S. Chen, L. Meng, B. Chen, W. Chen, X. Duan, X. Huang,
B. Zhang, H. Fu and Y. Wan, ACS Catal., 2017, 7, 2074.
21 R. Li, W. Yao, Y. Jin, W. Jia, X. Chen, J. Chen, J. Zheng,
Y. Hu, D. Han and J. Zhao, Chem. Eng. J., 2018, 351, 995.
22 M. Kolodziej, A. Drelinkiewicz, E. Lalik, J. Gurgul,
D. Duraczynska and R. Kosydar, Appl. Catal., A, 2016,
515, 60.
Conflicts of interest
There are no conflicts to declare.
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Paper
NJC
23 Q. Fu, W.-X. Li, Y. Yao, H. Liu, H.-Y. Su, D. Ma, X.-K. Gu, 41 H. Hu, Z. Zhao, W. Wan, Y. Gogotsi and J. Qiu, Adv. Mater.,
L. Chen, Z. Wang, H. Zhang, B. Wang and X. Bao, Science,
2010, 328, 1141.
24 C. Wang, H. Daimon and S. Sun, Nano Lett., 2009, 9, 1493.
2013, 25, 2219.
42 J. Geng, L. Si, H. Guo, C. Lin, Y. Xi, Y. Li, X. Yan, B. Wang
and L. Chen, New J. Chem., 2017, 41, 15447.
25 X. Sun, S. Guo, Y. Liu and S. Sun, Nano Lett., 2012, 12, 4859. 43 L. Guo, P. Ye, J. Wang, F. Fu and Z. Wu, J. Hazard. Mater.,
26 B. Wang, L. Si, J. Geng, Y. Su, Y. Li, X. Yan and L. Chen, Appl.
Catal., B, 2017, 204, 316.
27 Z. Weng and F. Zaera, ACS Catal., 2018, 8, 8513.
2015, 298, 28.
44 S. J. Hoseini, V. Heidari and H. Nasrabadi, J. Mol. Catal. A:
Chem., 2015, 396, 90.
28 L. Q. Zhang, J. M. Winterbottom, A. P. Boyes and S. Raymahasay, 45 X. Lin, X. Wang, Q. Zhou, C. Wen, S. Su, J. Xiang, P. Cheng,
J. Chem. Technol. Biotechnol., 1998, 72, 264.
29 B. Coq, V. Brotons, J. M. Planeix, L. C. de Menorval and
R. Dutartre, J. Catal., 1998, 176, 358.
X. Hu, Y. Li, X. Wang, X. Gao, R. Nozel, G. Zhou, Z. Zhang
and J. Liu, ACS Sustainable Chem. Eng., 2019, 7, 1673.
46 Y. Ma, J. Huang, L. Lin, Q. Xie, M. Yan, B. Qu, L. Wang,
L. Mai and D.-L. Peng, J. Power Sources, 2017, 356, 98.
30 T.-H. Tri, K. Chizari, I. Janowska, M. S. Moldovan, O. Ersen,
L. D. Nguyen, M. J. Ledoux, P.-H. Cuong and D. Begin, Catal. 47 X. Hou, J. Liu, W. Guo, S. Li, Y. Guo, Y. Shi and C. Zhang,
Today, 2012, 189, 77. Catal. Commun., 2019, 121, 27.
31 R. Nie, M. Miao, W. Du, J. Shi, Y. Liu and Z. Hou, Appl. 48 X. Lin, X. Wang, Q. Zhou, C. Wen, S. Su, J. Xiang, P. Cheng,
Catal., B, 2016, 180, 607.
32 A. R. Siamaki, A. E. R. S. Khder, V. Abdelsayed, M. S. El-Shall
and B. F. Gupton, J. Catal., 2011, 279, 1.
33 G. M. Scheuermann, L. Rumi, P. Steurer, W. Bannwarth and
R. Muelhaupt, J. Am. Chem. Soc., 2009, 131, 8262.
34 K.-J. Jeon, Z. Lee, E. Pollak, L. Moreschini, A. Bostwick,
X. Hu, Y. Li, X. Wang, X. Gao, R. Nozel, G. Zhou, Z. Zhang
and J. Liu, ACS Sustainable Chem. Eng., 2019, 7, 1673.
49 L. Hou, L. Lian, L. Zhang, G. Pang, C. Yuan and X. Zhang,
Adv. Funct. Mater., 2015, 25, 238.
50 C. Wu, X. Huang, G. Wang, L. Lv, G. Chen, G. Li and
P. Jiang, Adv. Funct. Mater., 2013, 23, 506.
C.-M. Park, R. Mendelsberg, V. Radmilovic, R. Kostecki, 51 S.-F. Jiang, L.-L. Ling, Z. Xu, W.-J. Liu and H. Jiang, Ind. Eng.
T. J. Richardson and E. Rotenberg, ACS Nano, 2011, 5, 1042. Chem. Res., 2018, 57, 13055.
35 S. Ramakrishnan, M. Karuppannan, M. Vinothkannan, 52 H. A. Elazab, A. R. Siamaki, S. Moussa, B. F. Gupton and
K. Ramachandran, O. J. Kwon and D. J. Yoo, ACS Appl.
Mater. Interfaces, 2019, 11, 12504.
M. S. El-Shall, Appl. Catal., A, 2015, 497, 58.
53 I. E. L. Stephens, A. S. Bondarenko, U. Gronbjerg, J. Rossmeisl
and I. Chorkendorff, Energy Environ. Sci., 2012, 5, 6744.
54 M. Qiao, X. Lei, Y. Ma, L. Tian, X. He, K. Su and Q. Zhang,
Nano Res., 2018, 11, 1500.
55 S. Patchkovskii, J. S. Tse, S. N. Yurchenko, L. Zhechkov,
T. Heine and G. Seifert, Proc. Natl. Acad. Sci. U. S. A., 2005,
102, 10439.
36 J. Yao, K. Zhang, W. Wang, X. Zuo, Q. Yang, H. Tang, M. Wu
and G. Li, ACS Appl. Mater. Interfaces, 2018, 10, 19564.
37 Y. Gao, G. Hu, J. Zhong, Z. Shi, Y. Zhu, D. S. Su, J. Wang,
X. Bao and D. Ma, Angew. Chem., Int. Ed., 2013, 52, 2109.
38 H. Wang, X. Feng, M. Zhou, X. Bo and L. Guo, ACS Appl.
Nano Mater., 2020, 3, 6336.
39 Y. Dong and X. Fei, Mater. Technol., 2020, 35, 102.
40 W. Hummers, Jr. and R. Offeman, Preparation of graphitic
oxide, J. Am. Chem. Soc., 1958, 80, 1339.
56 F. Jiang, J. Cai, B. Liu, Y. Xu and X. Liu, RSC Adv., 2016, 6,
75541.
57 S. L. Scott, ACS Catal., 2018, 8, 8597.
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