Selective hydrogenation of aromatic carboxylic acids over basic N-doped mesoporous carbon supported palladium catalysts

Article abstract of DOI:10.1016/j.apcata.2016.04.009

Mesoporous carbon nitride (MCN) has been prepared through a simple polymerization reaction between ethylenediamine (EDA) and carbon tetrachloride (CTC) by a nano hard-templating approach. The obtained MCN possesses high surface area (166.3 m²/g), average pore size of 9.2 nm and high N content (up to 18.5 wt%). The negative charge and the basicity on MCN surface are originated from its rich carbon nitride heterocycles, which notably improves the surface hydrophilicity and the adsorption of acidic molecules. Furthermore, MCN can be adopted as the proper support for highly dispersed Pd NPs with well-controlled size distribution. Compared with microporous N-doped active carbon with low N-content, the MCN-supported Pd catalyst shows an enhanced activity in water phase for the selective ring hydrogenation of benzoic acid, benzamide and phenol, in which 11.3 times higher activity in comparison to undoped catalyst is achieved. Wide characterizations reveal that big pore size, selective adsorption for acid substrate and strong interaction between N and Pd may lead to the high activity of Pd/MCN.

Full text of DOI:10.1016/j.apcata.2016.04.009

Accepted Manuscript
Title: Selective hydrogenation of aromatic carboxylic acids
over basic N-doped mesoporous carbon supported palladium
catalysts
Author: Hezhan Jiang Xiaolong Yu Renfeng Nie Xinhuan Lu
Dan Zhou Qinghua Xia
PII:
S0926-860X(16)30179-X
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2016.04.009
APCATA 15837
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
25-1-2016
8-4-2016
11-4-2016
Please cite this article as: Hezhan Jiang, Xiaolong Yu, Renfeng Nie, Xinhuan Lu, Dan
Zhou, Qinghua Xia, Selective hydrogenation of aromatic carboxylic acids over basic N-
doped mesoporous carbon supported palladium catalysts, Applied Catalysis A, General
http://dx.doi.org/10.1016/j.apcata.2016.04.009
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Submitted to Applied Catalysis A: General for publication
Title:
Selective hydrogenation of aromatic carboxylic acids over basic N-doped
mesoporous carbon supported palladium catalysts
Authors:
Hezhan Jiang, Xiaolong Yu, Renfeng Nie*, Xinhuan Lu, Dan Zhou, Qinghua Xia*
Address:
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, &
Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic
Functional Molecules, School of Chemistry and Chemical Engineering, Hubei
University, Wuhan 430062, P.R. China
* Corresponding author
Renfeng Nie, Hubei University, Wuhan 430062, P.R. China
Tel/Fax: 86-27-88662747
Email: refinenie@163.com
Qinghua Xia, Hubei University, Wuhan 430062, P.R. China
Tel/Fax: 86-27-88662747
Email: xiaqh518@aliyun.com
Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, &
Ministry-of-Education Key Laboratory for the Synthesis and Application of Organic
Functional Molecules, School of Chemistry and Chemical Engineering, Hubei
University, Wuhan 430062, P.R. China
1

Graphic abstract
Pd NPs are supported on MCN that is prepared via polymerization between
ethylenediamine and carbon tetrachloride. The negative charge and the basicity on
MCN surface are originated from its rich carbon nitride heterocycles. The negative
charge contributes to strong N-Pd interaction and the basicity generates higher
adsorption capacity for acid substrates, both of which may lead to its high activity
for selective ring hydrogenation of aromatic compounds.
Highlights:
 Pd NPs are supported on MCN that is prepared via polymerization between
ethylenediamine and carbon tetrachloride.
 N-Pd interaction affords high dispersion and durability of Pd NPs.
 MCN shows higher adsorption capacity for acid substrates than AC and
NAC.
 High N content (18.5 wt %) leads to negative charge of MCN and much
higher activity in water.
 Pd/MCN shows better performance for hydrogenation of acidic compounds
than Pd/AC and Pd/NAC.
Abstract
Mesoporous carbon nitride (MCN) has been prepared through a simple
polymerization reaction between ethylenediamine (EDA) and carbon tetrachloride
(CTC) by a nano hard-templating approach. The obtained MCN possesses high
surface area (166.3 m2/g), average pore size of 9.2 nm and high N content (up to 18.5
wt %). The negative charge and the basicity on MCN surface are originated from its
rich carbon nitride heterocycles, which notably improves the surface hydrophilicity
and the adsorption of acidic molecules. Furthermore, MCN can be adopted as the
proper support for highly dispersed Pd NPs with well-controlled size distribution.
2

Compared with microporous N-doped active carbon with low N-content, the
MCN-supported Pd catalyst shows an enhanced activity in water phase for the
selective ring hydrogenation of benzoic acid, benzamide and phenol, in which 11.3
times higher activity in comparison to undoped catalyst is achieved. Wide
characterizations reveal that big pore size, selective adsorption for acid substrate and
strong interaction between N and Pd may lead to the high activity of Pd/MCN.
Keywords: Mesoporous carbon nitride; polymerization; basicity; palladium; selective
hydrogenation.
1. Introduction
Catalytic reduction on nanoparticle surfaces is one of the most important reactions
for the conversion of raw materials and the production of chemicals [1, 2]. The
development of new materials holds the key to fundamental advances in catalysis,
further meeting the environmentally-benign and energy-saving requirements [3]. The
transformation of aromatics into value-added chemicals or high-quality fuels is an
important class of reactions [4, 5]. For example, the ring hydrogenation of benzoic
acid (BA) is the most effective method in the synthesis of cyclohexane carboxylic
acid (CCA), which is an important organic intermediate for the synthesis of
pharmaceuticals like praziquantel and ansatrienin [6]. Usually, the hydrogenation of
aromatic ring with electron-withdrawing groups requires more severe conditions than
that with electron-donating groups [7], which will result in the occurrence of some
undesired side reactions, such as the hydrogenation of carboxylic groups into alcohols
at high temperature/pressure [8]. Thus, there is a need for the design of the catalyst
with high activity and stability based on economic and environmental considerations.
In terms of industrial applications, the use of heterogeneous catalysts instead of
homogeneous ones is preferred due to the easy of their separation and recycling.
3

Metal NPs, prepared from metal salts with stabilizers and supported on metal
oxides, carbon or zeolites, are attractive for catalysis because of large surface
area-to-volume ratio and effective use of metals [9, 10]. In addition, the variation
of nanoparticle size sometimes allows a control over the activity when catalytic
reactions occur on the surface of NPs [10]. However, the decrease in the activity of
immobilized catalysts is frequently observed, possibly assigned to metal leaching
or aggregation of NPs because of the weak interaction between active ingredients
and support surface [11, 12]. Therefore, attempts to control the dispersion and the
catalytic durability of metal NPs are essential, for which one approach to enhance
the dispersion and stability of NPs lies in the use of the pore-rich supports. For
example, Pd species can be encapsulated in pores of microporous and mesoporous
materials (zeolites, MOFs, carbon, etc.) due to their confinement effects [7, 13-15],
but low catalytic activity is sometimes caused by the mass diffusion resistance [16,
17]. Though the heterogenization of the existing homogeneous palladium catalysts
immobilized on the surface of polymers and silica is another method to prepare
highly dispersed metal catalysts [18-20], the reaction scope is limited and the
leaching or decomposition may cause irreversible deactivation under harsh
conditions. Moreover, the addition of the second components (metal or metal
oxide) as building blocks for mediating the particle size and stability of NPs at
multiple junction points is also reported [21-24].
Nitrogen-containing carbons, as a kind of fascinating materials, have attracted
worldwide attention recently [25]. It has been reported that N-dopants in the carbon
skeleton can be inbuilt on the surface, which can provide a platform for the grafting of
metal NPs, leading to the carbon-based catalysts with well-dispersed catalytic active
sites [26, 27]. At the same time, the N doping results in an improvement in the
hydrophilicity and basicity of supports [11], beneficial to the use of as-prepared
catalysts in aqueous phase and the selective adsorption of acidic molecules through
the acid–base interaction [28]. The introduction of N can modify the electronic
structure of carbon matrix, and strengthen the interaction between carbon and guest
molecules due to the high charge and spin density caused by the nitrogen [29], which
4

can simultaneously tune the activity of the sp2 carbon in the H2 activation by lowering
down its dissociation energy, thus promoting the catalytic hydrogenation [30]. To date,
many studies have reported the improved catalytic activity of carbon-based catalysts
by increasing the metal dispersion initiated by N-doping [12, 31-33]. Moreover, the
presence of N-dopants on the carbon skeleton could also modify the electronic and
chemical interactions with the deposited active phase to greatly enhance the overall
catalytic activity and selectivity as well [34, 35].
Up to now, although various catalysts, including carbon-supported precious metals
(Pd/CN, Ru/C) [36, 37] and metal alloys (Pd-Ru [38], RuPt [39], Ni-Zr-B [40, 41]),
have been studied in the selective hydrogenation of benzoic acid (BA), it is still one
big challenge to overcome the high resonance energy of aromatic ring [42]. Herein,
the investigations on N-containing carbon (including microporous N-doped active
carbon (NAC) and mesoporous carbon nitride (MCN)) serving as basic supports to
achieve highly dispersed Pd NPs are reported. These N-doping materials show high
activity for the selective hydrogenation of aromatic ring in water phase.
2. Experimental section
2.1. Catalyst Preparation
Active carbon (AR) was purchased from Hunan Xiangda Chemical Reagent Co..
Fumed silica (SiO2) with the surface area of 250 m2/g was obtained from Degussa
Co. (Germany). Dicyandiamide (CP, 98 %), ethylenediamine (AR), carbon
tetrachloride (AR), benzoic acid (AR, 99.5 %), H2O2 (28 %), PdCl2 (AR, Pd >59
%), and other chemicals (analytical grade) were purchased from Aladdin
Chemicals and used as received without further purification. The phenol was
purchased from Sinopharm Chemicals in China.
2.2. Preparation of N-doped carbon
N-doped active carbon (NAC) was synthesized via a heat treatment of the
mixture consisted of dicyandiamide as the nitrogen source and commercial active
carbon as the precursor. Typically, ten grams of active carbon (AC) was first
5

immersed in 250 mL H2O2 (28 wt %) solution. The slurry was stirred and treated at
50 oC for 5 h. Then the solid was recovered by filtration and washed thoroughly
o
with DI water. Thus-pretreated AC was dried in a vacuum oven at 80 C for 10 h,
which was denoted as H2O2-AC. One half gram of dicyandiamide was added into a
suspension of 2.0 g H2O2-AC in 100 mL DI water while stirring. The slurry was
o
stirred for 30 min at 60 C and sonicated for 30 min (200 W, 50 Hz), and then
o
water was removed by reduced pressure at 60 C. The resulting powder was
o
transferred into a crucible, heated to 600 oC at a rate of 2 C·min-1 under a flow of
nitrogen and kept for 4 h. The sample was then cooled down to room temperature
under the protection of N2. The carbon material was denoted as NAC.
MCN was synthesized by hard template method. Typically, 0.5 g fumed silica was
dispersed into a mixed solution of ethylenediamine (EDA, 5.4 g) and
tetrachloromethane (CTC, 12 g) under stirring. The above mixture was heated at
90 °C for 6 h and then dried in an oven for 12 h. The templated-carbon nitride
o
o
polymer composite was heated to 600 C at a heating rate of 3 C·min-1 under a flow
of nitrogen and kept for 5 h. The carbonized material was treated with 5 wt %
hydrofluoric acid. The solid was recovered by filtration, washed with ethanol and
dried at 100 oC. The resultant sample was denoted as MCN.
2.3 Preparation of Pd NPs over N-doped carbon
One gram of the support (NAC or MCN) was dispersed in DI water (50 mL),
then added with 41.7 mg PdCl2 (2.5 wt %) while stirring. After 30 min, the
o
reduction treatment was performed in a 100 mL Teflon-lined autoclave at 40 C for
8 h under 2 MPa H2. Then, the precipitate was recovered by filtration and washed
o
thoroughly with DI water. The resulting Pd catalyst was dried in an oven at 80 C
overnight and named as Pd/NAC or Pd/MCN. The synthesis of Pd/AC was the
o
same as that of Pd/NAC, in which AC was pretreated at 600 C for 4 h under N2
protection. MgO-, Al2O3-, and SiO2-supported Pd catalysts (with 2.5 % loading
amount of Pd) were also synthesized by the method similar to that for Pd/NAC,
which were then used in the reactions without further treatment. The real loading
6

amount of Pd was detected via an ICP analysis.
2.4 Characterization
Powder X-ray diffraction (XRD) was performed on a Bruker D8A25 diffractometer
with CuK radiation ( = 1.54184 Å) operating at 30 kV and 25 mA. For the
detection of FTIR spectra, a mixture of 3 mg catalyst powder and 200 mg KBr was
pressed into a thin wafer. Raman spectra were collected at room temperature from 100
to 4000 cm-1 with 514.5-nm argon ion laser (Rhenishaw Instruments, England). The
spectra were recorded at a resolution of 2 cm−1. N2 adsorption was carried out at -196
oC using an auto-adsorption analyzer (Micromeritics, TriStarⅡ). Zeta potential was
measured with 12.5 mg of samples dispersed in 25 mL buffer solution (pH =
7.00±0.02) on a zeta potential analyzer BIC PALS. The elemental analysis for the
content of non-metallic elements was performed using a vario EL III Elementar. The
ICP analysis was conducted by inductively coupled plasma-atomic emission
spectroscopy (Plasma-Spec-II spectrometer). Scanning electron images (SEM) were
collected on JEOL (JSM6700F)) at an accelerating voltage of 15 kV. Transmission
electron microscope (TEM) images were obtained using an accelerating voltage of
200 kV on a JEOL-135 2010F Transmission Electron Microscope. X-ray
photoelectron spectra (XPS) were recorded on a Perkin-Elmer PHI ESCA system.
X-ray source was standard Mg anode (1253.6 eV) at 12 kV and 300 W.
2.5 Adsorption isotherms for benzoic acid
In a typical process, 20 mg AC, NAC or MCN was added into 50 mL aqueous
o
solution of benzoic acid (200 ppm), and the adsorption was performed at 25 C in a
water bath with magnetic stirring (stirring rate: 1,000 r.p.m.). After completion of
the adsorption, the solid catalyst was separated by centrifugation and the liquid
phase was analyzed by GC.
2.6 Catalytic tests
The hydrogenation reactions were carried out in a 25 mL stainless autoclave
7

with a Teflon liner. In a typical run, 20 mg catalyst and 61 mg (0.5 mmol) benzoic
acid was dispersed in 5 mL DI water. The autoclave was sealed, purged and
o
pressurized with hydrogen to 2.5 MPa, and then heated to 110 C under magnetic
stirring at a rate of 1000 rpm. After the completion of the reaction, the mixture was
extracted by ethyl acetate and centrifuged in order to remove the solid catalyst.
The solid catalyst was recovered by centrifugation and washed sequentially with
ethyl acetate, ethanol and DI water. The filtrate was analyzed by
gas-chromatograph (GC, HP 5890, USA) equipped with a 30 m capillary column
(HP-5) and a flame ionization detector (FID). All the products were confirmed by
GC–MS (Agilent 6890).
3. Results and discussion
3.1. Structural characterizations
SEM images (Fig. 1) show flake-like morphology of pristine AC and NAC,
different from an irregular morphology of MCN with some loose pore structures
originated from the removal of template. Type I isotherms are observed from nitrogen
sorption analysis of AC and NAC (Fig. 2), indicative of the nature of microporous
structure [43]. Compared with AC, N-doping leads to slight decreases in BET area
and pore volume for NAC, indicating the partial occupation/blockage of the pore
channels of AC by N dopants. MCN shows a H4 hysteresis loop on the isotherm and a
sharp rise at relative pressure higher than 0.60, characteristic of textural mesopores
[44]. The average pore size of AC, NAC or MCN is calculated to be 4.3, 4.1 or 9.2 nm,
respectively. Moreover, MCN shows Stotal area of 166.3 m2/g and pore volume of 0.33
cm3/g (Table 1), dominated with mesopores in a proportion of around 81.8 %, much
higher than 45.5 % of AC and 40.0 % of NAC.
The FT-IR spectra are shown in Fig. S1, where contains two bands at 1207 and
1630 cm–1 ascribed to aromatic C–N and C=N stretching bonds, respectively [44].
Clearly those bands for MCN seem to be strengthened in comparison to NAC,
indicating the existence of more rich N groups on the surface of MCN.
The chemical compositions of N-doped carbon materials are determined by CHNS
8

elemental analysis and XPS spectra (Table 2). The surface N content of NAC obtained
from the XPS data is 3.76 %, significantly higher than the bulk nitrogen content
(2.35 %) detected by elemental analysis, indicating that the N species are more
enriched on the surface than in the bulk. However, the nitrogen content of MCN is
18.5 %, which is homogenously distributed in MCN.
The Raman spectra are shown in Fig. S2, the G band at ∼ 1590 cm−1 is attributed
to the in-plane vibration of sp2 carbon atoms, while the D band at ∼1350 cm−1 is a
defect-induced Raman feature representing the imperfect crystalline structure [45].
Compared with the AC, the NAC shows an increased ID/IG value (1.13), indicating
the formation of more structural defects in carbon skeleton. As for the MCN, the G
band is obviously stronger than the D band (ID/IG= 0.88), accompanied with a shift of
the G band position to a low frequency (1569 cm-1), indicating that sp2 hybridized
carbon and nitrogen atoms are distributed throughout the graphitized layers [44].
The main C1s peak of N-doped carbon (Fig. 3a) locates at approximately 284.5 eV
with a tail peak at high binding energy of around 285.8 eV, assigned to the
electron-deficient carbons bound to nitrogen (C=N or C-N) [46, 47]. For MCN, the
C=N/C-N signals are significantly strengthened, suggesting that more N species have
been incorporated into the skeleton of carbon, well agreed with the element analysis.
The N1s spectra (Fig. 3b) of NAC and MCN are broad and asymmetric. The curve
deconvolution shows that the N1s spectra can be well-fitted to the superposition of
four peaks centered at 398.5, 400.1, 401.1, and 403.3 eV, which are attributed to
aromatic/pyridinic nitrogen (N1), sp2-hybridized/pyrrolic nitrogen (N2),
sp3-hybridized terminal/quaternary nitrogen (N3), and pyridine oxide-N (N4),
respectively[48, 49]. The contents of four types of nitrogen atoms are listed in Table 3.
N1 and N2 species in NAC are dominant nitrogen-containing surface functional
groups, with a total content of 91.2 %. Comparatively, the content of N1and N2 in
MCN is 66.5 %, along with a N3 content of 31.7 %. These N1 and N2 species can act
as basic sites on the carbon surface [28], and N1, N2 and N3 can add the electron
density at the doped sites to act as a coordinative site of metal ions and as a defect site
for the enhanced interaction among polarized molecules [29, 31, 34, 50]. As shown in
9

Table 1, the zeta potentials of AC, NAC and MCN dispersed in DI water are measured
to be -0.02, -17.0 and -25.4 mV, respectively. The lower zeta potentials of NAC and
MCN with more negatively charged surface may be explained by the presence of rich
nitrogen dopants (especially for N1 and N2) on the surface [34]. Meanwhile, the
lower zeta potential means the higher surface basicity that will require more protons
to neutralize [51].
In order to characterize the Pd NPs on carbon surface, XRD, TEM and XPS have
been carried out. The XRD patterns of Pd/AC, Pd/NAC and Pd/MCN are shown in
Fig. 4. The diffraction peaks at 40.0◦ and 46.5◦ are indexed to the (111) and (200)
planes in the face centered cubic (FCC) crystal structure of Pd [52]. Upon the
N-incorporation in carbon, the intensity of Pd diffraction peaks in Pd/NAC and
Pd/MCN decreases and becomes faint, indicating the decrease of Pd crystalline size.
According to Scherrer’s formula and the half-width of (111) peak, the calculated sizes
of Pd crystallites in Pd/NAC and Pd/MCN are 2.4 and 3.8 nm (Table 3), respectively,
notably smaller than 10.8 nm of Pd/AC.
The Pd dispersion on carbon supports have been further confirmed by TEM
observations in Fig. 5. It is found that the particle size of Pd NPs on the surface of AC
ranges from 6 to 18 nm. While the uniformly well-dispersed Pd NPs on very large
surfaces of MCN and NAC can be observed, without any aggregation of NPs into
large clusters. A high-resolution TEM image of Pd/MCN clearly shows that the
distance between two adjacent lattice planes of Pd NPs is approximately 0.226 nm. In
addition, the average Pd sizes with standard deviation in Pd/AC, Pd/NAC and
Pd/MCN are 12.0±2.5, 3.1±0.7 and 4.3±1.2 nm, respectively.
N-incorporation changes the surface functional groups, accompanied with
increasing the surface hydrophilicity and surface negatively-charged sites (Table 1).
The improved hydrophilicity favors its dispersion in polar solvents (such as water),
and the negatively-charged surface facilitates the electrostatic attraction of the
positively charged metallic ions as well as the hydrolysis of Pd2+ ions for its
immobilization onto the support. In addition, the micropore of NAC may further
10

improve the Pd dispersion by its confinement effects [53]. All these factors facilitate
the formation of evenly dispersed Pd NPs on N-doped support.
The Pd0 3d5/2 and 3d3/2 doublets are observed at 335.6 and 340.9 eV, as shown in
Fig. 6. Palladium in the oxidation state is also detected (doublet pair at 337.7 and
342.9 eV). The presence of Pdδ+ species, characterizing both the deposition of pure
and N-doped carbon, can be attributed to the oxidation of Pd due to the formation of
Pd-O bond by air exposure. As shown in Table 3, the percentages of Pdδ+ on NAC
(84.5 %) and MCN (53.9 %) are much higher than that of N-free AC (21.4 %). As we
know, the oxygen groups on support surfaces facilitate the anchoring of metal NPs
[51], and the oxygen content on all supports is similar (Table 2), indicating the
important role of N dopant on influencing the electronic properties of Pd. Previous
results in the literature suggested that pyridinic N and quaternary-N sites could
facilitate both the coordination of metal ions and the anchoring of metal NPs. [50].
XPS spectra show that pyridinic or aromatic (C=N-C) nitrogen atoms exist
prevalently in N-doped carbons, which are often associated with defect sites on edges
of graphite layers or on vacancy sites [54, 55]. At the same time, Li et al. [56] showed
that the interaction of Pt with the carbon system containing quaternary N atoms was
weaker than that with the carbon system containing pyridinic N atoms on vacancy
sites. It is reported that the divalent Pd species are more abundant when Lewis basic
sites in the form of pyridine N species are present, which indicates that the
coordination at these sites lowers down their reducibility at the reduction conditions
used [34]. These results indicate that pyridinic or pyrrolic nitrogen atoms can interact
intensively with Pd, thus leading to the high percentage of Pdδ+ species.
The strong interaction between support and Pd can be evidenced by H2-TPR
measurement compared with that of Pd/AC (Fig. 7). In the curve of Pd/AC, a
o
hydrogen release peak at around 60 C may be attributed to desorption of hydrogen
o
pre-absorbed by Pd NPs [38]. The broad reduction peak centered at 139 C can be
ascribed to the reduction of Pdδ+ to Pd0. However, the reduction curves of Pd/NAC
and Pd/MCN show two hydrogen consumption peaks with maximum at about 195 and
280 oC, indicating the formation of two types of Pd on support. The higher reduction
11

temperature indicates the stronger interaction between support and Pd. These peaks
may be related to Pd species that strongly interact with aromatic N (pyridinic and
pyrrolic N) groups [34]. The π-bonded planar C-N-C structures, along with the
unsaturated nitrogen groups at the edges of the support, are suitable sites for
anchoring Pd species, where strong metal-support interactions may take place [57].
The strong coordination of Pd species at N sites may be beneficial for high dispersion
and stability of Pd NPs on support.
3.2. Catalytic activity for the hydrogenation of aromatic ring
The hydrogenation of BA to CCA is carried out at 110 oC under 2.5 MPa (Table 4).
In blank test, when no catalyst is used, no BA is transformed (entry 1). Obviously,
N-free catalyst (Pd/AC, entry 2) shows a poor activity with 7.2 % conversion in this
transformation. By comparison, all N-doped catalysts (entries 3-5) exhibit
significantly higher catalytic activity. For example, Pd/NAC affords 48.2 % BA
conversion. As for Pd/MCN, the BA conversion reaches as high as 81.5 %, a further
prolongation of reaction time to 2.5 h leads to a full conversion of BA with 100%
CCA selectivity (entry 5). Other frequently used heterogeneous Pd-based catalysts
inclusive of Pd/SiO2, Pd/Al2O3, Pd/MgO and commercial Pd/AC (entries 6-9) achieve
a poor hydrogenation activity < 25 % under the same reaction conditions.
It must be noted that the solvent plays a key role in determining the reaction rate or
the distribution of products. Among these solvents tested (Fig. 8), both apolar and
protic organic solvents are unfavourable for the titled reaction on Pd/MCN, giving a
low hydrogenation activity < 2 %. Although water has the lowest solubility of H2 than
other solvents [36], it results in the highest conversion of 81.5 % for Pd/MCN, which
can be ascribed to strong H-bond donor capability of water [58].
Although Pd/NAC processes higher Pd dispersion, it offers lower catalytic activity
in comparison to Pd/MCN, indicative of the crucial role of mesoporous structure (a
mesopore proportion of 81.8 %) for substrate diffusion and hydrogenation on Pd NPs.
What’s more, considering the rich basic sites of N-doped support, the adsorption
capacity of BA on AC, NAC or MCN is measured. As shown in Fig. 9, the adsorption
12

capacity of BA over MCN (228 mg/g) is larger than that of AC (92 mg/g) and NAC
(131 mg/g), exhibiting a favorable interaction between an acid (BA) and base (MCN).
The strong adsorption leads to an increased BA concentration and accelerated reaction
rate on surface of Pd/MCN.
The Pd/MCN catalyst can be easily recovered and reused for five times without
significant loss of efficiency (Fig. 10), and the ICP analysis of fresh and recovered
catalysts reveals negligible Pd leaching (< 0.05 ppm) in solution. The crystalline state
and dispersion of Pd NPs in recovered Pd/MCN, based on the corresponding XRD
pattern (Fig. S3) and TEM image (Fig. S4), do not change significantly after five runs.
The XPS analysis of recovered Pd/MCN shows a higher Pd0 ratio (57.7 %) than that
of fresh catalyst (Fig. S5), indicative of partial reduction of Pdδ+ during the
hydrogenation reaction.
The difference in activity between the catalysts becomes more conspicuous through
the hydrogenation of more sensitive benzamide in water (Table 5). Pd/AC, Pd/NAC
and Pd/MCN afford 4.4, 15.5 and 55.6 % benzamide conversion at 85 oC for 8 h.
Moreover, higher temperature (110 oC) leads to fast hydrogenation rate but decreased
product selectivity (91 %) due to the hydrolysis of amide bond. Because of the great
importance of cyclohexanone [59], the selective hydrogenation of phenol is carried
out further. Previous study finds that the phenol hydrogenation is a structural sensitive
reaction [60]. For example, phenol adsorbed in nonplanar fashion over basic sites
gives rise to cyclohexanone while that adsorbed on acidic sites in coplanar fashion
leads to the formation of cyclohexanol and cyclohexane [61, 62]. Therefore, the
selective hydrogenation of acidic phenol is performed over basic Pd/MCN catalyst
o
under a mild condition (40 C, 0.3 MPa H2) (Table 6). Using N-free AC as support
gives only 24.8 % phenol conversion and 93.5 % cyclohexanone selectivity (entry 1).
Comparatively, Pd/NAC (entry 2) gives an improved conversion (53.7 %) and
selectivity (95.1 %). The phenol conversion increases quickly to 100 % over Pd/MCN
(entry 3) along with a high cyclohexanone selectivity (98.1%).
Using MCN as support, the Pd catalysts with different Pd content are prepared and
investigated for BA hydrogenation (Fig. S6). The catalytic activity quickly reaches
13

from 37.7 % to 81.5 % after 2 h when Pd loading increases from 0.43 to 2.36 wt %.
However, a limited increase of the activity occurs when the metal loading is further
increased to 4.78 wt %. Moreover, the TOF exhibits a reverse variation trend with
increasing loading from 0.43 wt % to 4.78 wt % and the maximum value of 116.2 h-1
is achieved over 0.43%Pd/MCN, this value is comparable to those Pd catalysts in
water [36], in which the TOF is 106 h-1 at 120 oC and 4 MPa H2.
Clearly, the introduction of N dopant and mesopores is the key to enhance
catalytic activity. Firstly, the surface pyridinic or aromatic (C=N-C) nitrogen species
provide the co-interaction sites for Pd species, which gives rise to strong metal–N
bond at the interface [34], accompanied with well-dispersed Pd NPs. Those small
particles are responsible for the enhanced catalytic performance. Moreover, the
dominant pyridinic or aromatic (C=N-C) nitrogen can serve as Lewis basic sites to
increase the hydrophilicity and basicity of MCN [49], which is beneficial for the
dispersion of catalyst in water and adsorption of acid substrates (e.g., benzoic acid,
phenol). Third, the high accessibility of active phase derived from the mesoporous
structure decreases the mass diffusion resistance in reaction medium, thus
accelerating the transformation of substrate.
To further demonstrate the versatility of Pd/MCN, selective hydrogenation of
different aromatic acids is investigated (Table 7). Para-methylbenzoic acid contains
an electron-donating group on the aromatic ring of benzoic acid, which affords a fast
hydrogenation rate than benzoic acid (entry 2), giving 98 % conversion in 2 h.
Phenylacetic acid (entry 3) shows a slow hydrogenation rate and needs 4 h to give a
full transformation. Furthermore, full conversion of methyl benzoate (entry 4) is
obtained over Pd/MCN in 6 h. For dimethyl phthalate (DMP, entry 6), a common
plasticizer, the conversion is 92 % at 8 h, much slower than that of non-substituted
benzoic acid.
4. Conclusion
MCN with high N content (18.5 wt %) is synthesized by nanocasting approach via
a simple polymerization reaction between EDA and CTC, and is especially employed
14

for anchoring well-dispersed Pd NPs under a surfactant-free one-step approach. For
selective ring hydrogenation of BA, the resulting Pd/MCN shows 1.7 and 11.3 times
activity of microporous N-doped AC and AC-supported catalysts, respectively, and
affords high TOF up to 116.2 h-1. This catalyst is stable and active for the
hydrogenation of benzamide, phenol and a series of electron-deficient benzoic acids.
Wide characterizations disclose that big pore size, rich basic sites on the surface of
MCN and intimate interaction between Pd and N lead to the enhanced activity of
Pd/MCN in water.
Acknowledgements
This work was supported by National Natural Science Foundation of China
(21173073, 21273064), Natural Science Fund for Creative Research Groups of Hubei
Province (2014CFA015), Natural Science Fund of Hubei Province (2015CF232), the
Key Project of the Education Department of Hubei Province (D20141004), and the
2014 Sci-tech Support Project of the Science and Technology Department of Hubei
Province (2014BAA098).
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Fig. 1. SEM images of (a) AC, (b) NAC and (c,d) MCN.
20

(a)
300
250
200
150
100
50
AC
NAC
MCN
0
0.0
0.2
0.4
0.6
0.8
1.0
Relative pressure (p/p )
0
(b)
AC
NAC
MCN
0.008
0.006
0.004
0.002
0.000
1
10
100
Pore width (nm)
Fig. 2. (a) Nitrogen sorption isotherms and (b) pore size distribution of the samples.
21

C-C/C=C bond
(a)
AC
NAC
MCN
C=N/C-N bond
C-O bond
280
282
284
286
288
290
292
294
296
Binding Energy (eV)
N1
(b)
N2
N3
N4
NAC
MCN
394
396
398
400
402
404
406
408
Binding Energy (eV)
Fig. 3. XPS spectra of (a) C1s and (b) N1s in the samples.
22

C(002)
Pd(111)
Pd(200)


(c)
(b)
(a)
20
30
40
50
60
70
2 Theta (degree)
Fig. 4. XRD patterns of (a) Pd/AC, (b) Pd/NAC and (c) Pd/MCN.
23

24

Fig. 5. TEM images of (a, b) Pd/AC, (c-e) Pd/NAC and (f-h) Pd/MCN. The insets in images of (a,
d and g) corresponding to histograms of Pd particle size distribution. The inset in image of (h) is
high resolution TEM image of Pd nanoparticles in Pd/MCN.
25

Pd0
Pd
(a)
(b)
(c)
334
336
338
340
342
344
346
Binding Energy (eV)
Fig. 6. Pd3d spectra of (a) Pd/AC, (b) Pd/NAC and (c) Pd/MCN.
26

(a)
(b)
(c)
50
100
150
200
250
300
Temperature (oC)
Fig. 7. H2-TPR curves of (a) Pd/AC, (b) Pd/NAC and (c) Pd/MCN.
27

80
70
60
50
40
30
20
10
0
Pd/MCN
Pd/NAC
water
cyclohexane
dioxane
THF
ethanol
Fig. 8. Hydrogenation of BA in different solvents over Pd/MCN and Pd/NAC.
28

300
250
200
150
100
50
MCN
NAC
AC
0
0
20
40
60
80
100
120
t (min)
Fig. 9. Adsorption kinetics of BA over AC, NAC and MCN supports.
29