N-doped hierarchical porous carbon anchored tiny Pd NPs: A mild and efficient quinolines selective hydrogenation catalyst

Article abstract of DOI:10.1016/j.mcat.2018.04.001

Chemoselective hydrogenation of quinolines is often subjected to the problems of leaching and poisoning of catalytic active site as well as harsh reaction conditions. Developing a novel and high-performance heterogeneous catalyst is of paramount importance yet a huge challenge. Herein, we report a facile and efficient strategy for preparing the large surface area and highly N-doped hierarchical porous carbon anchored tiny Pd NPs catalyst, in which the low-cost chitosan, nitrogen-rich ionic liquids are served as composite precursors and KZ molten salt as friendly pore-forming agent. And a series of Pd@CIL-T (C refers to chitosan, IL refers to ionic liquid, T = 600–900 °C) catalysts are successfully fabricated via pyrolyzing aforesaid composites at different temperatures followed by anchoring the highly dispersed and small-sized Pd NPs on their surface. Among all the prepared catalysts, Pd@CIL-900 exhibits the optimal catalytic performance towards the selective hydrogenation of quinoline under extremely mild conditions (0.6 mol% Pd, 0.1 MPa H₂ and 50 °C). The kinetic experiments further reveal that such hydrogenation is subject to a pseudo-first order reaction and the apparent activation energy is as low as 41.1 kJ/mol, demonstrating excellent hydrogenation reaction rate. Moreover, the catalytic activity and selectivity are well maintained even after being reused for fifth reaction cycles.

Full text of DOI:10.1016/j.mcat.2018.04.001

MolecularCatalysis452(2018)145–153
Contents lists available at ScienceDirect
Molecular Catalysis
journal homepage: www.elsevier.com/locate/mcat
N-doped hierarchical porous carbon anchored tiny Pd NPs: A mild and
efficient quinolines selective hydrogenation catalyst
⁎
Fengwei Zhanga,1, , Chunlan Maa,b,1, Shuai Chenc, Jianfei Zhanga, Zhihong Lia,
Xian-Ming Zhanga,⁎
a
Institute of Crystalline Materials, Shanxi University, Taiyuan 030006, PR China
Institute of Molecular Science, Shanxi University, Taiyuan 030006, PR China
Analytical Instrumentation Center, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, PR China
b
c
A R T I C L E I N F O
A B S T R A C T
Keywords:
Chemoselective hydrogenation of quinolines is often subjected to the problems of leaching and poisoning of
catalytic active site as well as harsh reaction conditions. Developing a novel and high-performance hetero-
geneous catalyst is of paramount importance yet a huge challenge. Herein, we report a facile and efficient
strategy for preparing the large surface area and highly N-doped hierarchical porous carbon anchored tiny Pd
NPs catalyst, in which the low-cost chitosan, nitrogen-rich ionic liquids are served as composite precursors and
KZ molten salt as friendly pore-forming agent. And a series of Pd@CIL-T (C refers to chitosan, IL refers to ionic
liquid, T = 600–900 °C) catalysts are successfully fabricated via pyrolyzing aforesaid composites at different
temperatures followed by anchoring the highly dispersed and small-sized Pd NPs on their surface. Among all the
prepared catalysts, Pd@CIL-900 exhibits the optimal catalytic performance towards the selective hydrogenation
of quinoline under extremely mild conditions (0.6 mol% Pd, 0.1 MPa H2 and 50 °C). The kinetic experiments
further reveal that such hydrogenation is subject to a pseudo-first order reaction and the apparent activation
energy is as low as 41.1 kJ/mol, demonstrating excellent hydrogenation reaction rate. Moreover, the catalytic
activity and selectivity are well maintained even after being reused for fifth reaction cycles.
Hierarchical porous carbon
KCl/ZnCl2 molten salt
Palladium nanoparticles
Quinoline compounds
Selective hydrogenation
1. Introduction
significance in industrial applications.
Very recently, the carbon-based catalytic materials, which possess
The chemoselective hydrogenation of quinoline and its derivatives
is a fascinating transformation because the hydrogenated products
1,2,3,4-tetrahydroquinolines (py-THQs) play crucial roles in the pro-
duction of fine chemicals, agrochemicals, pharmaceuticals and other
bioactive molecules [1–5]. The hydrogenation of quinoline compounds
generally involves multiple intermediates and high reaction energy
barriers, which makes the reaction process inherently kinetically slug-
gish and thus presents great challenges to the development of high-
performance catalysts [6–8]. It is acknowledged that the Ru, Rh, Au, Pt
and Pd-based heterogeneous catalysts have been successfully applied to
the selective hydrogenation of quinoline compounds [9–12]. Re-
grettably, existing catalysts are always hampered by problems of large
amount of catalysts, harsh reaction conditions and relatively sluggish
reaction rate [13,14]. Thus, searching for a suitable and efficient het-
erogeneous catalyst that can operates the highly selective hydrogena-
tion of quinoline compounds under mild condition is of great
unique chemical, electronic and structural features, have been widely
studied and considered as promising candidates to substitute commer-
cially available noble-metal supported catalysts in the fine chemical
synthesis and practical fuel cell applications [15–18]. Driven by these
potential advantages, an innovative and multifunctional catalytic ma-
terial based on heteroatom-doped carbon (phosphorus, boron, sulfur
and nitrogen) has become a land for the development of highly efficient
and stable integrated catalyst [19–24]. Among various kinds of het-
eroatom-doped carbon materials, the N-doped carbon is of particular
interest owing to its excellent chemical stability, electrical conductivity
as well as strong coordination interaction with metal nanoparticles
[25–27]. N-doped porous carbon materials are typically synthesized
through high-temperature pyrolysis of macrocyclic complexes [28],
ionic liquids [29,30], nucleobases [31], deep-eutectic solvents [32] and
polycarbon nitride [33] in the presence of hard templates or direct
pyrolysis of metal-organic framwork precursors followed by additional
⁎
Corresponding authors.
E-mail addresses: fwzhang@sxu.edu.cn (F. Zhang), xmzhang@sxu.edu.cn (X.-M. Zhang).
Chunlan Ma, which contributed equally as the first author.
1
https://doi.org/10.1016/j.mcat.2018.04.001
Received 2 February 2018; Received in revised form 21 March 2018; Accepted 2 April 2018
2468-8231/©2018ElsevierB.V.Allrightsreserved.

F. Zhang et al.
MolecularCatalysis452(2018)145–153
activation treatment [34]. However, the high cost, low natural abun-
dance of feedstocks, skeleton collapse and unfriendly post-processing
strategy are significant obstacles to their wider application. Therefore, a
novel synthetic approach is urgently needed to prepare N-doped porous
carbon with high specific surface area and rich pore structures, thereby
overcoming current problems of nanoparticle aggregation/leaching
under relatively harsh reaction conditions.
2.3. Synthesis of N-doped hierarchical porous carbon (CIL-T)
1.0 g of BMP-dca ILs, 2.0 g of chitosan, 9.0 g of KZ molten salt
(KCl:ZnCl2 = 1:3.6) and 15 g of steel balls (1–2 cm in diameter) were
ball milled at 800 r/min speed for 1.0 h. The generated homogeneous
mixture was placed into a quartz porcelain boat and pyrolyzed at
600 ∼ 900 °C (heating rate: 5 °C/min) for 2 h under flowing N2 gas and
cooled naturally to room temperature. Then, the black solid was thor-
oughly washed with deionized water to remove the KZ molten salt, and
the obtained sample is denoted as CIL-T (C refers to the chitosan, IL
refers to the ionic liquid, T represents the carbonization temperature). It
should be noted that the KZ molten salt can be used for the next cycle
after removing the water via vacuum distillation. As comparison, the
identical procedure was applied to prepare C-900, IL-900 and CIL-900-
w/o KZ based N-doped carbon materials except using BMP-dca ILs,
chitosan as starting materials or without the addition of KZ molten salt.
In continuation of our interest in nanoporous carbon-based catalytic
materials [35], herein, we report the successful preparation of a novel
hierarchically porous N-doped carbon materials using chitosan, ionic
liquids and KZ molten salt [36,37] as carbon, nitrogen and pore-
forming agent, respectively. Due to its large surface area, rich pore
structure and high dispersity of Pd NPs, our home-made N-doped
hierarchical porous carbon anchored tiny Pd NPs display superior cat-
alytic activity and selectivity under extremely mild conditions (0.6 mol
% Pd, 0.1 MPa H2 and 50 °C) toward quinoline compounds hydro-
genation compared with commercial 5 wt% Pd/C catalyst. The super-
iority of such catalytic system is as follows: i) the formation of large
specific surface area and hierarchical porous support can increase the
contact area and make reactants more accessible to the active sites; ii)
the well-dispersibility of Pd NPs can increases the number of active sites
and improve the catalytic activity; iii) the different types of nitrogen
species (pyridinic-, pyrrolic- and graphitic-N) can enhance the adsorp-
tion capacity of quinoline compounds through hydrogen-bonding and
π-π stacking interactions. Additionally, the synthetic methodology in-
volved in the present work is expected to provide new avenues for
developing other efficient catalysts in a simple and feasible way.
2.4. Anchoring the Pd NPs on N-doped hierarchical porous carbon
(Pd@CIL-T)
The Pd@CIL-T catalyst was prepared through a simple wet im-
pregnation method. Take Pd@CIL-900 as an example, 1.0 g of CIL-900
support was dispersed in 100 mL of ethanol under ultrasonic oscillation
at room temperature, and then 50 mL of 5.6 mM PdCl2 aqueous solution
was slowly added into the suspension dropwise under vigorous stirring.
After being absorbed and coordinated for 12 h, 50 mL of 30 mM NaBH4
was added into the above mixture, and the mixture was continued to
stir for 4 h. The obtained black powders was filterd and washed several
times with deionized water and ethanol, and dried in vacuum at 50 °C
overnight for catalytic test.
2. Experimental
2.1. Chemical reagents
2.5. General procedure for the selective hydrogenation of quinoline
compounds
1-bromobutane (98%), 3-picoline (99%), PdCl2 (99%), NaBH4
(98%), quinoline and its derivatives were provided by Aladdin
Chemical Reagent Co., Ltd. NaN(CN)2 (96%) was purchased from
Meryer Chemical Technology Co., Ltd. Chitosan, AgNO3, KCl and ZnCl2
were purchased from Sinopharm Chemical Reagent Co., Ltd. All of
other chemicals are used as received without any further purification.
Deionized water was used throughout the experiment.
Typically, 1.0 mmol of quinoline compounds, 6 mL of solvent and a
certain amount of Pd@CIL-T catalyst were added into a 50 mL PTFE
reaction vessel or thick walled pressure vessel. The mixture was stirred
for 2 min to ensure complete dispersion of the catalyst and then placed
in a 50 mL stainless-steel autoclave. After being flushed three times
with H2, the pressure was elevated to 1–10 atm at room temperature.
The autoclave was then heated to 30–120 °C with a magnetic stir for a
certain time. Subsequently, the catalyst was removed from the mixture
by centrifugation, and the liquid sample was analyzed by GC or GC–MS.
The calculations of conversion and selectivity were based on the fol-
lowing formula: Conversion = [consumed substrate]/[initial sub-
strate] × 100%, Selectivity = [py-THQs]/[all hydrogenated pro-
ducts] × 100%.
2.2. Synthesis of 1-butyl-3-methylpyridine dicyanamide ionic liquids (BMP-
dca ILs)
17.0 g of AgNO3 was dissolved in 100 mL of deionized water, and
then 8.9 g of NaN(CN)2 was slowly added into the AgNO3 aqueous so-
lution under stirring. After 2 h reaction, the genarated suspension was
filtered and washed with an excess amount of deionized water, and the
AgN(CN)2 white solid was obtained. 46.6 g of 3-picoline was added into
a 250 mL three-neck flask, and then 68.5 g of 1-bromobutane in a
constant pressure funnel was dropped into the flask within 2 h with
assistance of violent agitation. The mixture was then continuously
stirred 15 h at 100 °C, and the dark red viscous liquid was produced
when it was cooled to room temperature. To obtain high purity of 1-
butyl-3-methylpyridine bromide ionic liquids (BMP-Br ILs), the crude
product was purified by dissolving the BMP-Br ILs into deionized water,
and then washed three times with ethyl acetate to remove the possible
1-bromobutane or 3-picoline impurity. The BMP-Br ILs was then ob-
tained after evaporation of the water under vacuum. Subsequently,
23.0 g of BMP-Br ILs was dissolved in 150 mL of deionized water, 17.8 g
of AgN(CN)2 white solid was then added into the above solution with
magnetic stirring without light. After reaction 8 h, the light yellow AgBr
solid was separated from the mixture and the filtrate was treated via
vacuum-rotary evaporation to remove the water. Finally, the reddish
brown BMP-dca ILs was obtained.
2.6. Characterization
Transmission electron microscope (TEM) was carried out on a FEI
Tecnai G2 F20S-Twin using an accelerating voltage of 200 kV. For
sample preparation, the powders were dispersed in ethanol with the
assistance of sonication, and then one drop of suspention was slowly
dropped onto a micro grid and dried with Infrared lamp irradiation. The
distribution state of various elements in the Pd@CIL-900 catalyst was
corroborated by scanning transmission electron microscopy coupled
energy-dispersive X-ray spectroscopy (STEM-EDS) element mapping.
XRD measurements were conducted on Rigaku Ultima IV diffractometer
using Cu-Kα radiation as the X-ray source in the 2θ range of 10–80°.
Fourier transform infrared (FTIR) spectra were obtained using a
Thermo Nicolet iS5 spectrophotometer (frequency range from 4000 to
500 cm−1) with KBr pellets. The metal dispersion was carried out using
a Builder PCA-1200 chemical adsorption instrument. Prior to testing,
Pd@CIL-900 catalyst was pretreated in argon at 150 °C for 60 min. The
N2 adsorption-desorption isotherms were obtained on an
146

F. Zhang et al.
MolecularCatalysis452(2018)145–153
Quantachrome autosorb iQ2 analyzer. Before measurement, the sam-
ples were degassed under vacuum at 150 °C for 5 h. Surface area of the
samples was calculated by the Brunauer-Emmet-Teller (BET) method;
pore volume and pore size distribution were calculated using the
Barrett-Joyner-Halenda (BJH) model. Thermogravimetric analysis
(TGA) determined on a Setaram Labsys Evo apparatus. The samples
were heated in an alumina pan from 30 to 1000 °C at a heating rate of
10 °C/min under N2 atmosphere. Raman spectra was collected on a
Renishaw in Via microlaser Raman spectrometer with a 514.5 nm laser
excitation. The metal Pd loading amount of Pd@CIL-900 catalyst was
determined by NexION 350 inductively coupled plasma mass spectro-
metry (ICP-MS). The X-ray photoelectron spectra (XPS) was analyzed
on the PHI-5702 instrument and the C1 s line at 284.5 eV was used as
the binding energy reference.
preparation process.
TEM images of the as-prepared CIL-900 support clearly show that
the formation of three-dimensional interconnected and hierarchical
porous N-doped carbon frameworks (Fig. 1a and b). After the im-
mobilization of Pd NPs on their surface, the microsturcture and mor-
phology of the support have undergone no apparent changes except
that a large number of tiny nanoparticles are successfully anchored on
the support matrix (Fig. 1c and d). To further confirm the homogeneous
and narrow-size distributed Pd NPs on Pd@CIL-900, the high-angle-
annular-dark-field scanning transmission electron microscopy (HAADF-
STEM) imaging and energy-dispersive X-ray spectroscopy (EDS) ele-
mental mapping are carried out. In Fig. 1d, the Pd NPs in focus exhibit
clear lattice fringes, indicating good crystallinity. The interplanar spa-
cing is measured as 0.224 nm, which is in good agreement with (111)
interplanar spacing of the face-centred cubic metallic Pd NPs [38]. The
EDS elemental mapping shows that the C, N and Pd are uniformly
dispersed in the catalyst. It is clearly demonstrated that the EDS spec-
trum of N map has a markedly larger area than that of Pd map, sug-
gesting a considerable quantity of nitrogen species distributed on the
surface of the Pd@CIL-900 catalyst. Moreover, the elemental analysis
and ICP results show that the N and Pd contents are 5.68 wt% and
1.32 wt% (nN/nPd = 32.7), respectively. It should be noted that the
presence of abundant N species can significantly restrict the growth of
Pd NPs and prevent them from aggregation under relatively harsh re-
action conditions. The TEM images also reveal that the Pd NPs have an
3. Results and discussion
3.1. Microstructure characterization of various Pd-based N-doped
hierarchical porous carbon catalysts
Schematic illustration of the general procedure for the fabrication of
Pd@CIL-900 catalyst is presented in Scheme 1. Briefly, starting with
chitosan, BMP-dca ILs and KZ molten salt (serving as C, N precursors
and pore-forming agent, 1H NMR and FT-IR of BMP-dca ILs are shown
in Fig. S1 and S2), we obtain 3D continuous and hierarchical porous
carbon materials with high surface area on the carbonization of the
aforementioned ground mixture followed by the removal of KZ molten
salt via water-washing. The subsequent impregnation of hierarchical
porous carbon materials with a desired amount of palladium precursor
leads to Pd@CIL-900 catalyst on reduction with NaBH4 solution. For
comparison, a series of Pd@CIL-T (T = 600, 700 and 800) catalysts can
be prepared by selecting different pyrolysis temperature, while the Pd
NPs loading amount is identical with the Pd@CIL-900 catalyst. The
reason for choosing chitosan and BMP-dca ILs composite as the original
materials is that the ILs can effectively improve the solubility and
heteroatomic nitrogen content of earth-aboundant biomass based
chitosan. It is significant that the proposed strategy not noly greatly
reduces the production cost and improves the porous carbon materials
yield, but also prevents the tedious and time-consuming catalyst
average diameter of 2.5
other wetness impregnation method prepared Pd-based catalysts in the
absence of N species.
0.6 nm, which is obviously smaller than
The porous structure of CIL-T and Pd@CIL-T (T = 600–900 °C)
samples is further characterized by N2 adsorption-desorption isotherms.
As shown in Fig. 2a and Fig. S3, the curves can be assigned to the
combination of type-I and type-IV curves according to the IUPAC
classification, suggesting the presence of both microporous and meso-
porous features for these N-doped carbon materials. The detailed
structural parameters are summarized in Table S1. It is worth men-
tioning that the surface area, pore volume, and pore diameter of CIL-
900 are 1232 m2 g−1, 1.28 cm3 g−1, 0.6, 3.7 and 34.5 nm by virtue of
the novel molten salt method. As can be seen, our home-made N-doped
carbon catalytic materials possess remarkably hierarchical porous
Scheme 1. The preparation procedure of Pd@CIL-T (T = 600–900 °C) catalyst.
147

F. Zhang et al.
MolecularCatalysis452(2018)145–153
Fig. 1. (a and b) TEM and magnified TEM images of CIL-900, (c, d) TEM and magnified TEM images of Pd@CIL-900, (e) HAADF-STEM and EDS elemental mapping of
Pd@CIL-900 catalyst.
characteristic. The primary function of micropores is to enhance the
enrichment capacity of substrate molecules, while the dominant role of
mesopores is to improve the diffusion and mass transfer rate of the
substrates. In comparison, the surface area and pore volume of Pd@CIL-
900 (1205 m2 g−1 and 1.22 cm3 g−1) are slightly lower than that of
original CIL-900 support, illustrating that the metallic Pd NPs have
been successfully embedded into the channels of the CIL-900. The high-
resolution N1 s spectrum of Pd@CIL-900 catalyst can be deconvoluted
into four different types of nitrogen species, namely pyridinic
(398.1 eV), pyrrolic (399.4 eV), graphitic (400.8 eV) and N-oxide
(403.0 eV) [39]. Fig. 2d shows the Pd 3d fine scanning spectrum of
Pd@CIL-900 catalyst after placing for one month in atmosphere en-
vironment. Two peaks that appear at 335.5 eV and 340.9 eV are related
to Pd00 3d5/2 and Pd00 3d3/2, while peaks at 338.0 eV and 343.7 eV are
attributed to Pd2+ 3d5/2 and Pd2+ 3d3/2. The percentage of the Pd00
and Pd2+ species are calculated from the relative areas of these peaks,
which indicates that Pd00 is the main metal species on the surface of the
as-prepared catalyst (57.2%).
further verified by XRD and Raman spectroscopy. In the Raman spectra
(Fig. 3d), two diffraction peaks are presented at 1353 and 1594 cm−1
,
illustrating the existence of disordered sp3 carbon (D band) and gra-
phite sp2 carbon (G band), respectively. With increasing pyrolysis
temperature of chitosan and BMP-dca ILs composite, the ID/IG intensity
ratio exhibits a slightly increase, illustrating that the disordered degree
of the catalytic material increases. The broad peak at approximately
24.2° in the XRD patterns of Pd@CIL-T (T = 600–900 °C) is indexed to
the (002) plane of the graphitic carbon, whereas no metallic Pd species
signals are detected for these catalysts. The XRD results show the highly
uniform dispersion and the small size of Pd NPs, which are consistent
with the HRTEM observation.
3.2. The catalytic performance evaluation by various solvents, catalysts and
reaction conditions
In the beginning, we chose Pd@CIL-900 as catalyst and investigated
the influence of solvent and catalyst dosage on catalytic performance
towards the extremely challenging hydrogenation of nitrogen-con-
taining heteroaromatics. To highlight the excellent activity of such
catalyst, we specifically conducted the reaction with a very low catalyst
loading (0.6 mol% Pd, 50 mg, Fig. S4a) under mild reaction conditions.
The preliminary results found that MeOH is the best reaction medium
for the selective hydrogenation of quinoline (Table 1, entry 2). The
selectivity to py-THQs was > 99% for all of the investigated solvent
systems. Thus it can be deduced that the solvent has a great influence
on the catalytic activity for such reaction, but almost no effect on the
product selectivity.
As a benchmark, the commercial 10 wt% Pd/C catalyst gave only a
conversion of 83.0% with a > 99% selectivity. The metal-free CIL-900
support was found to be inactive for quinoline hydrogenation.
Considerably lower conversion of 26.1% was observed for Pd@IL-900
catalyst as compared to the Pd@C-900 catalyst with a conversion of
As shown in Fig. 3a, the peaks at around 1607 cm−1 and 3432 cm−1
in the surface of CIL-900 are corresponding to NeH deformation and
stretching vibration. The peaks at 595 cm−1 and 1176 cm−1 are as-
cribed to CeH out-of-plane bending vibration and CeN in-plane
bending vibration, indicating the well-carbonized structure for chitosan
and BMP-dca ILs composite in the presence of KZ molten salt. The FT-IR
spectra reveals that the Pd@CIL-900 has a similar absorption char-
acteristic peaks with CIL-900 support, demonstrating these functional
groups without any essential changes after the anchoring of tiny Pd
NPs. According to TGA analysis, the weight loss of various catalysts
decreases ranging from 32.4 to 12.0 wt% with the increasing of com-
posite precursors pyrolysis temperature. It is suggested that the as-
prepared Pd-based catalyst has a better thermal stability when the
composite precursors are pyrolyzed at higher temperature under N2
atmosphere. In addition, the formation of graphitic carbon structure is
148

F. Zhang et al.
MolecularCatalysis452(2018)145–153
Fig. 2. (a) N2 adsorption-desorption isotherms, (b) pore-size distribution curves of Pd@CIL-600, Pd@CIL-700, Pd@CIL-800 and Pd@CIL-900 catalysts (c) high-
resolution XPS analysis in N1 s and (d) Pd3d region of Pd@CIL-900 catalyst.
90.5%. The Pd@CIL-900-w/o KZ catalyst also showed inferior catalytic
activity and low selectivity. Interestingly, a steady improvement of
catalytic activity was observed when the pyrolysis temperature of
composite precursors was elevated from 600 to 900 °C (Table 2, entries
6–9), and Pd@CIL-900 displayed the best catalytic performance for
quinoline hydrogenation. We also examined such reaction under dif-
ferent pressures (0.1–1.0 MPa) and temperatures (40–120 °C). As de-
picted in Table S2, the target product was obtained with quantitative
conversion and high selectivity (> 99%) under mild condition. To our
delight, as decreasing the reaction temperature to 50 °C, the conversion
could still reach to > 99% within 18 h under merely 0.1 MPa of H2 and
without selectivity drop, which demonstrated excellent catalytic ac-
tivity and selectivity of the Pd@CIL-900 catalyst.
ring is difficult to be activated on the surface of Pd NPs. When in-
troducing 8-hydroxyl group (Table 3, entry 8) the hydrogenation pro-
cess was also sluggish, which was probably due to the steric effect of the
8-hydroxyl substituent on the nucleophilic attack of the Pd-hydride to
the CeN bond and the coordination effect of hydroxyl to the metal
center. Nevertheless, both of 8-hydroxyl and 4-methoxy quinolines
could be fully converted with superior selectivity given longer reaction
time (8 h). The comparison results of chemoselective hydrogenation of
quinoline with previously reported catalysts are summarized in Table
S3. It is demonstrated that our home-made Pd@CIL-900 catalyst pos-
sesses an excellent competitive advantage in terms of reaction condi-
tions, catalyst dosage as well as production costs.
To further illustrate the superior catalytic activity of the present Pd-
based N-doped hierarchical porous carbon catalyst, an additional re-
action kinetic experiment was performed in the temperature range of
30–60 °C for Pd@CIL-900 catalyst in order to determine the apparent
activation energy Ea of quinoline hydrogenation (Fig. S4b). The general
reaction rate formula of quinoline hydrogenation can be expressed
through the following Eq. (1). The rate formula can also be simplified to
Eq. (2) because the catalyst dosage is a fixed value and H2 possesses a
more excessive concentration in comparison with quinoline. Hypothe-
sizing that such reaction is a first-order reaction (γ = 1), cQ is the
concentration of quinoline after a period of reaction time of t, and then
the Eq. (2) can be further expressed as Eq. (3) or (4). The experiment
results demonstrate that the reaction rate is in good agreement with the
Eq. (4) and the ln cQ has a linear relationship with reaction time t.
According to Arrhenius equation: k = A·e−Ea/RT, the apparent activa-
tion energy Ea can be calculated by the Eq. (5).
3.3. The substrate generality, reaction kinetics, hot filtration and
recyclability of Pd@CIL-900 catalyst
To explore the generality and potential applicability of Pd@CIL-900
catalyst for various substituted quinolones, several quinoline deriva-
tives were hydrogenated and the TOF of these reactions were evaluated.
As can be seen in Table 3, the TOF of quinoline hydrogenation could be
reached to 179.2 h−1 at 80 °C under 0.1 MPa of H2, suggesting the su-
perior catalytic activity for quinoline substrate as compared to other
quinoline derivatives (Table 3, entries 1–8). In addition, 6-fluoro and 6-
chloro substituted quinolines were reduced to the corresponding ha-
logen-substituted tetrahydroquinoline derivatives and almost no deha-
logenation phenomenon occurred, which also exhibited considerable
TOF value (Table 3, entries 6–7). However, the TOF declined greatly in
the case of 4-methylquinoline (Table 3, entry 3), which could be at-
tributed to the fact that 4-methylquinoline with substitution at pyridine
r = − dcQ/dt = k1·[Cat.]α·[H2]β·[Q]γ
(1)
149

F. Zhang et al.
MolecularCatalysis452(2018)145–153
Fig. 3. (a) FTIR spectra of CIL-900, Pd@CIL-900; (b) TGA curves, (c) XRD patterns and (d) Raman spectra of Pd@CIL-600, Pd@CIL-700, Pd@CIL-800 and Pd@CIL-
900 catalyst.
Pd@CIL-900 catalyst could be attributed to the highly exposed tiny Pd
NPs, superhigh specific surface area, pore volume and hierarchical
porous features. Furthermore, the formation of N⋯HeN hydrogen
bonds between the pyridinic N in quinoline molecule and the −NH/
NH2 groups can strongly enhance the substrate enrichment ability and
thus py-THQs selectivity. The results demonstrate that the synergistic
effect between small-sized Pd NPs and N-doped hierarchical porous
carbon may greatly reduce the activation energy barrier and enhance
the catalytic activity and selectivity. To our knowledge, it is a relatively
lower level compared with the activation energies reported for other
noble metal based catalysts, such as Pd/C (Fiture S5) and Ru/AC [40].
To validate the selective hydrogenation of quinoline is carried out in
the form of heterogeneity, a hot filtration test was proceeded in MeOH
solution under 1 atm H2, 80 °C conditon and the conversion of quinoline
was 39.8% after 0.5 h reaction time. As expected, the substrate con-
version was still maintained at around 40% even if the reaction time
was further increased to 12 h after removal of Pd@CIL-900 catalyst,
suggesting the reaction catalyzed exclusively in the manner of hetero-
geneity. It can be inferred that the Pd NPs are solidly anchored into the
N-doped hierarchical porous carbon, which is well addressed to the
problems of aggregation and leaching of Pd NPs. The recycling stability
of Pd@CIL-900 catalyst for hydrogenation of quinoline was also in-
vestigated. As shown in Fig. 4d, the selectivity of py-THQs was always
higher than 99% throughout the whole catalytic cycles although the
catalytic activity with a slight deacrease, suggesting that the catalyst
was well-suited for the chemoselective hydrogenation of quinoline. As
can be seen in Fig. S6, the size of Pd NPs was increased from 2.5 to
6.8 nm, which probably the reason for the activity decrease of the
Pd@CIL-900 catalyst after reused five times.
Table 1
The influence of the various solventsa.
Entry
Solvent
Conv. (%)b
Sel. (%)b
1
2
3
4
5
6
7
8
9
Toluene
MeOH
EtOH
Ethyl acetate
Acetonitrile
THF
1,4-dioxane
H2O
8.1
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
97.9
82.7
77.8
84.5
17.0
9.2
71.8
3.2
DMF
a
Reaction conditions: 1.0 mmol quinoline, 0.6 mol% Pd@CIL-900 catalyst
(based on ICP analysis of Pd), 6.0 mL solvent, 1 atm H2, 80 °C, 4 h.
b
Conversion and selectivity were determined by GC or GCMS.
r = − dcQ/dt = k·[Q]γ, k = k1·[Cat.]α·[H2]β
r = − dcQ/dt = k·[Q]1
(2)
(3)
(4)
(5)
ln cQ = −k·t + C = constant
lnk = lnA − Ea/RT
Fig. 4a displays that the concentration logarithm ln cQ exhibits a
good linear relationship along with the advancement of reaction time
under all the investigated temperature intervals, complying well with
the pseudo-first order reaction kinetics. The rate constants are calcu-
lated from the slopes of lines at different temperatures (Table S4). The
extraction of the activation energy Ea from the Arrhenius plot result in
41.1 kJ/mol for Pd@CIL-900 catalyst. The excellent catalytic activity of
150

F. Zhang et al.
MolecularCatalysis452(2018)145–153
Table 2
The catalytic performance of various Pd-based catalysts for selective hydrogenation of quinolinea.
Entry
Catalyst
Conv. (%)b
Sel. (%)b
1a
1b
1c
1
2
3
4
5
6
7
8
9
Pd/C
CIL-900
Pd@IL-900
Pd@C-900
Pd@CIL-900-w/o KZ
Pd@CIL-600
Pd@CIL-700
Pd@CIL-800
Pd@CIL-900
83.0
0
> 99
0
> 99
98
0
0
0
2
9
0
0
0
0
0
0
0
0
0
0
0
0
0
26.1
90.5
10.6
81.0
85.4
88.3
97.9
91
> 99
> 99
> 99
> 99
a
Reaction conditions: 1.0 mmol quinoline, 0.6 mol% catalyst (based on ICP analysis of Pd), 6.0 mL CH3OH, 1 atm H2, 80 °C, 4 h.
The conversion and selectivity were determined by GC or GCMS, the by-product is 5,6,7,8-tetrahydroquinoline.
b
4. Conclusions
anchored by homogeneously doped N atoms. The N-doped hierarchical
porous carbon materials which are synthesized by the pyrolysis of the
combination of chitosan, ionic liquids and KZ molten salt, is supposed
to be a promising candidate to prepare highly active and selective
heterogeneous catalyst. Based on the merit of the synthetic strategy, it
should be expected that other types of high-performance catalysts can
be fabricated by anchoring various catalytic active species on N-doped
hierarchical porous carbon.
In summary, we have prepared highly dispersed and small-sized Pd-
based catalysts by a facile composite precursor and KZ molten salt
combined pyrolysis strategy. Taken the selective hydrogenation of
quinoline compounds as an example, the as-prepared Pd@CIL-900
catalyst exhibits excellent catalytic performance which can be ascribed
to the high specific surface area of the support and tiny Pd NPs
Table 3
Chemoselective hydrogenation of various substituted quinoline compounds.a
c
Entry
1
Substrate
Product
t (h)
4
Conv. (%)b
97.9
Sel. (%)b
> 99
TOF (h−1
)
179.2
2
3
4
8
94.9
95.1
> 99
> 99
120.8
16.7
4
9
95.0
> 99
71.8
5
6
4
4
93.5
98.8
> 99
> 99
178.8
175.7
7
8
4
8
> 99
94.0
96
79.3
25.0
> 99
a
Reaction conditions: 1.0 mmol of quinoline coumpounds, 6.0 mL of CH3OH, 50 mg of Pd@CIL-900 catalyst (0.6 mol%).
Conversion and Selectivity were analyzed by GC, and n-tetradecane was used as internal standard, Conversion = [consumed substrate]/[initial sub-
b
strate] × 100%, Selectivity = [product]/[product + other by-product] × 100%, respectively.
c
Turnover frequency (TOF) is calculated after 1 h of reaction, TOF = mol of converted substrate per mole of active Pd site per hour. The by-product is in entry 7 is
1,2,3,4-tetrahydroquinoline.
151

F. Zhang et al.
MolecularCatalysis452(2018)145–153
Fig. 4. (a) the plot for the concentration logarithm ln cQ vs. reaction time t under different temperatures, (b) Arrhenius plot of the rate constants over the reaction
temperature, (c) hot filtration test and (d) the recyclability of Pd@CIL-900 catalyst.
Conflict of interest
[9] Y. Gong, P. Zhang, X. Xu, Y. Li, H. Li, Y. Wang, J. Catal. 297 (2013) 272–280.
[10] H. Konnerth, M.H.G. Prechtl, Green Chem. 19 (2017) 2762–2767.
[11] L. Bai, X. Wang, Q. Chen, Y. Ye, H. Zheng, J. Guo, Y. Yin, C. Gao, Angew. Chem. Int.
Ed. 55 (2016) 15656–15661.
[12] X. Wang, W. Chen, L. Zhang, T. Yao, W. Liu, Y. Lin, H. Ju, J. Dong, L. Zheng,
W. Yan, X. Zheng, Z. Li, X. Wang, J. Yang, D. He, Y. Wang, Z. Deng, Y. Wu, Y. Li, J.
Am. Chem. Soc. 139 (2017) 9795–9798.
The authors declare no competing financial interests.
Acknowledgments
[13] H. Konnerth, M.H.G. Prechtl, Green Chem. 19 (2017) 2762–2767.
[14] M. Guo, C. Li, Q. Yang, Catal. Sci. Technol. 7 (2017) 2221–2227.
[15] Z. Wei, Y. Chen, J. Wang, D. Su, M. Tang, S. Mao, Y. Wang, ACS Catal. 6 (2016)
5816–5822.
[16] K. Shen, X. Chen, J. Chen, Y. Li, ACS Catal. 6 (2016) 5887–5903.
[17] S. Zhang, K. Dokko, M. Watanabe, Mater. Horiz. 2 (2015) 168–197.
[18] D.S. Su, G. Wen, S. Wu, F. Peng, R. Schlögl, Angew. Chem. Int. Ed. 56 (2017)
936–964.
This work is supported by Natural Science Foundation of China,
Sanjin Scholar of Shanxi Province (21703129, 221173137, 20903064),
Scientific Research Start-up Funds of Shanxi University
(023151801002) and Natural Science Foundation for Young Scientists
of Shanxi Province (2015021051). And we are also very greateful for
the test platform provided by Shanxi University of Scientific Instrument
Center.
[19] Y. Meng, D. Voiry, A. Goswami, X. Zou, X. Huang, M. Chhowalla, Z. Liu, T. Asefa, J.
Am. Chem. Soc. 136 (2014) 13554–13557.
[20] F. Razmjooei, K.P. Singh, D.-S. Yang, W. Cui, Y.H. Jang, J.-S. Yu, ACS Catal. 7
(2017) 2381–2391.
Appendix A. Supplementary data
[21] S. Yang, L. Peng, P. Huang, X. Wang, Y. Sun, C. Cao, W. Song, Angew. Chem. Int. Ed.
55 (2016) 4016–4020.
Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/10.1016/j.mcat.2018.04.001.
[22] S. Huang, Y. Meng, S. He, A. Goswami, Q. Wu, J. Li, S. Tong, T. Asefa, M. Wu, Adv.
Funct. Mater. 27 (2017) 1606585.
[23] G. Hasegawa, T. Deguchi, K. Kanamori, Y. Kobayashi, H. Kageyama, T. Abe,
K. Nakanishi, Chem. Mater. 27 (2015) 4703–4712.
References
[24] J. Zhang, L. Dai, Angew. Chem. Int. Ed. 55 (2016) 13296–13300.
[25] Q.-Y. Bi, J.-D. Lin, Y.-M. Liu, H.-Y. He, F.-Q. Huang, Y. Cao, Angew. Chem. Int. Ed.
55 (2016) 11849–11853.
[26] W. Zhou, J. Jia, J. Lu, L. Yang, D. Hou, G. Li, S. Chen, Nano Energy 28 (2016)
29–43.
[27] P. Zhang, Y. Gong, H. Li, Z. Chen, Y. Wang, Nat. Commun. 4 (2013) 1593.
[28] J. Masa, W. Xia, I. Sinev, A. Zhao, Z. Sun, S. Grützke, P. Weide, M. Muhler,
W. Schuhmann, Angew. Chem. Int. Ed. 53 (2014) 8508–8512.
[29] J.S. Lee, X. Wang, H. Luo, G.A. Baker, S. Dai, J. Am. Chem. Soc. 131 (2009)
4596–4597.
[1] A.R. Katritzky, S. Rachwal, B. Rachwal, Tetrahedron 52 (1996) 15031–15070.
[2] V. Sridharan, P.A. Suryavanshi, J.C. Menéndez, Chem. Rev. 111 (2011) 7157–7259.
[3] D. Ren, L. He, L. Yu, R.-S. Ding, Y.-M. Liu, Y. Cao, H.-Y. He, K.-N. Fan, J. Am. Chem.
Soc. 134 (2012) 17592–17598.
[4] H. Mao, J. Ma, Y. Liao, S. Zhao, X. Liao, Catal. Sci. Technol. 3 (2013) 1612–1617.
[5] M. Yan, T. Jin, Q. Chen, H.E. Ho, T. Fujita, L.-Y. Chen, M. Bao, M.-W. Chen, N. Asao,
Y. Yamamoto, Org. Lett. 15 (2013) 1484–1487.
[6] A. Karakulina, A. Gopakumar, İ. Akçok, B.L. Roulier, T. LaGrange, S.A. Katsyuba,
[30] X. Xu, Y. Li, Y. Gong, P. Zhang, H. Li, Y. Wang, J. Am. Chem. Soc. 134 (2012)
16987–16990.
[31] X. Gao, Z. Chen, Y. Yao, M. Zhou, Y. Liu, J. Wang, W.D. Wu, X.D. Chen, Z. Wu,
D. Zhao, Adv. Funct. Mater. 26 (2016) 6649–6661.
S. Das, P.J. Dyson, Angew. Chem. Int. Ed. 55 (2016) 292–296.
[7] M. Fang, R.A. Sánchez-Delgado, J. Catal. 311 (2014) 357–368.
[8] F. Chen, A.-E. Surkus, L. He, M.-M. Pohl, J. Radnik, C. Topf, K. Junge, M. Beller, J.
Am. Chem. Soc. 137 (2015) 11718–11724.
[32] M.C. Gutierrez, D. Carriazo, C.O. Ania, J.B. Parra, M.L. Ferrer, F. del Monte, Energy
152

F. Zhang et al.
MolecularCatalysis452(2018)145–153
[37] J. Zhu, K. Sakaushi, G. Clavel, M. Shalom, M. Antonietti, T.-P. Fellinger, J. Am.
Chem. Soc. 137 (2015) 5480–5485.
Environ. Sci. 4 (2011) 3535–3544.
[33] J. Liu, N.P. Wickramaratne, S.Z. Qiao, M. Jaroniec, Nat. Mater. 14 (2015) 763–774.
[34] Y.-Z. Chen, G. Cai, Y. Wang, Q. Xu, S.-H. Yu, H.-L. Jiang, Green Chem. 18 (2016)
[38] D. Forberg, T. Schwob, M. Zaheer, M. Friedrich, N. Miyajima, R. Kempe, Nat.
Commun. 7 (2016) 13201.
1212–1217.
[35] F. Zhang, C. Zhao, S. Chen, H. Li, H. Yang, X.-M. Zhang, J. Catal. 348 (2017)
[39] H. Su, K.-X. Zhang, B. Zhang, H.-H. Wang, Q.-Y. Yu, X.-H. Li, M. Antonietti, J.-
S. Chen, J. Am. Chem. Soc. 139 (2017) 811–818.
212–222.
[36] T.-P. Fellinger, A. Thomas, J. Yuan, M. Antonietti, Adv. Mater. 25 (2013)
[40] X. Yu, R. Nie, H. Zhang, X. Lu, D. Zhou, Q. Xia, Microporous Mesoporous Mater. 256
(2018) 10–17.
5838–5855.
153