Highly efficient and recyclable Ni MOF-derived N-doped magnetic mesoporous carbon-supported palladium catalysts for the hydrodechlorination of chlorophenols

Article abstract of DOI:10.1016/j.molcata.2016.07.041

Ni MOF-derived N-doped magnetic mesoporous carbon-supported Pd nanoparticles (Pd/Ni-mCN) were prepared and used in the hydrodechlorination (HDC) of chlorophenols (CPs) water pollutants with H₂ at mild reaction conditions. The Pd/Ni-mCN catalyst was characterized in detail by transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), N₂ adsorption/desorption, and vibrating sample magnetometry (VSM). The Pd/Ni-mCN nanocatalyst showed high activity and complete HDC conversion of a target pollutant such as 4-CP after 90?min at ambient conditions. Other CPs compounds such as 2-CP, 3-CP, 2,4-dichlorophenol (2,4-DCP), and 2,4,6-trichlorophenol (2,4,6-TCP) were also tested in the HDC reaction. 2-CP was the only intermediate product in the HDC of 2,4-DCP, with phenol being the final product. The HDC of 2,4,6-TCP proceeded with the intermediate formation of 2-CP and 2,4-DCP, this being potentially originated from the steric hindrance of the ortho-substituted Cl atom. Additionally, Pd/Ni-mCN exhibited high catalytic stability and recyclability. Thus, Pd/Ni-mCN was successfully recycled owing to its superparamagnetic properties and reused for at least five times showing stable conversions (above 99% at 90?min) the HDC of 4-CP.

Full text of DOI:10.1016/j.molcata.2016.07.041

Journal of Molecular Catalysis A: Chemical 423 (2016) 386–392
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Highly efficient and recyclable Ni MOF-derived N-doped magnetic
mesoporous carbon-supported palladium catalysts for the
hydrodechlorination of chlorophenols
∗
Xueliang Cui, Wei Zuo, Meng Tian, Zhengping Dong , Jiantai Ma
College of Chemistry and Chemical Engineering, Gansu Provincial Engineering Laboratory for Chemical Catalysis, Lanzhou University, Lanzhou 730000, PR
China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 June 2016
Received in revised form 10 July 2016
Accepted 20 July 2016
Available online 21 July 2016
Ni MOF-derived N-doped magnetic mesoporous carbon-supported Pd nanoparticles (Pd/Ni-mCN) were
prepared and used in the hydrodechlorination (HDC) of chlorophenols (CPs) water pollutants with
H2 at mild reaction conditions. The Pd/Ni-mCN catalyst was characterized in detail by transmission
electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), N2 adsorp-
tion/desorption, and vibrating sample magnetometry (VSM). The Pd/Ni-mCN nanocatalyst showed high
activity and complete HDC conversion of a target pollutant such as 4-CP after 90 min at ambient condi-
tions. Other CPs compounds such as 2-CP, 3-CP, 2,4-dichlorophenol (2,4-DCP), and 2,4,6-trichlorophenol
Keywords:
Ni MOF
N-doped magnetic mesoporous carbon
Pd/Ni-mCN catalyst
Hydrodechlorination
Chlorophenols
(2,4,6-TCP) were also tested in the HDC reaction. 2-CP was the only intermediate product in the HDC
of 2,4-DCP, with phenol being the final product. The HDC of 2,4,6-TCP proceeded with the intermediate
formation of 2-CP and 2,4-DCP, this being potentially originated from the steric hindrance of the ortho-
substituted Cl atom. Additionally, Pd/Ni-mCN exhibited high catalytic stability and recyclability. Thus,
Pd/Ni-mCN was successfully recycled owing to its superparamagnetic properties and reused for at least
five times showing stable conversions (above 99% at 90 min) the HDC of 4-CP.
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
above-mentioned methods, catalytic HDC represents an effective,
simple, and safe technology. Thus, HDC of chloro-compounds can
be carried out under normal temperature and pressure conditions
and within a wide range of concentrations [15,16]. Additionally,
many highly toxic chlorinated compounds can be converted into
harmless or considerably less toxic reaction byproducts by HDC
[17]. For example, highly toxic CPs can be converted into low toxic
phenols via HDC [18,19].
Preventing water and soil pollution has attracted increasing
interest in the scientific community since these issues represent
a serious threat to people’s daily life [1,2]. Stringent regulations
have been enacted with the aim to control the emission of pol-
lutants to the environment. Among the numerous environmental
pollutants, commercially available organo–chlorinated chlorophe-
nols (CPs) are frequently used as raw materials in the synthesis
of pesticides and disinfectants [3–5]. When discharged to the
environment, CPs can cause persistent pollution owing to its
toxic, biorefractory, and bioaccumulative characteristics. Hence,
CPs has been classified by the Environmental Protection Agency
as potential groundwater pollutants and listed as one of the most
important pollutants by the European Union [6,7]. Different CPs
removal technologies have been investigated including Fenton [8],
ozonization [9], hydrodechlorination (HDC) [10,11], heterogeneous
photocatalysis [12,13], and adsorption processes [14]. Among the
HDC of chloro-compounds is typically carried out over precious
metal-catalysts. Thus, Pd, Pt, Au, Ru, and Rh have been supported on
different commercially available inorganic supports (e.g., activated
carbons, Al O , SiO , pillared clays) and thoroughly investigated
2
3
2
in HDC processes [20–26]. However, precious metals have become
increasingly rare in recent years. Additionally, the recovery of pre-
cious metal-catalysts from the liquid-phase reaction medium is
very difficult, thereby leading to high costs for the entire catalytic
process and seriously affecting the sustainability of this technol-
ogy [27,28]. Thus, the development of high-efficiency precious
metal-catalysts which can be efficiently recycled is highly required.
The utilization of magnetic nanoparticles (NPs) as components of
the catalyst supports may help overcome this recycling limitation
∗ Corresponding author.
E-mail address: dongzhp@lzu.edu.cn (Z. Dong).
[
29–31]. Additionally, magnetic separation is a green process since
http://dx.doi.org/10.1016/j.molcata.2016.07.041
1
381-1169/© 2016 Elsevier B.V. All rights reserved.

X. Cui et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 386–392
387
it allows save energy, time, and solvent. In this sense, Fe O @SiO -
and NaOH (60 mg, 1.5 mmol) were dissolved in 15 mL of deion-
ized water. The flask was subsequently connected with a balloon
3
4
2
based catalyst supports have been extensively studied [32–35].
However, silica-based magnetic supports cannot be used under
alkaline conditions as they may suffer from aggregation during
the catalytic process. In this sense, mesoporous carbon materials,
showing anchored magnetic NPs, have a number of advantages over
silica-based magnetic supports. Several magnetic mesoporous car-
bon materials have been prepared in recent years [36–40], although
their preparation procedures are comparatively complicated. One
pot carbonization of Ni-, Co-, and Fe-based MOF under inert atmo-
sphere represents a facile method for the fabrication of magnetic
mesoporous carbon materials [41–43]. In our previous work, a
magnetic porous carbon material has been obtained from the car-
bonization of Fe-MOF, and was used to prepare Pd and Au supported
recyclable catalysts [44]. In addition, as reported in the literatures
that, the carbon materials doped with N should benefit for the noble
metal precursor adsorption, enhancing nucleation and stabilizing
the noble metal NPs and make them with high dispersion on the
supports, and further enhance the catalytic acitivity [45–47].
Bearing all this in mind, we herein used a Ni-MOF precursor
to fabricate a Ni NPs-based N-doped magnetic mesoporous carbon
filled with H , thereby allowing the air in the flask to be thoroughly
2
replaced by H , and the reaction was immediately started under
2
vigorous stirring. The reaction progress was monitored by collect-
ing aliquots from the reaction mixture with a glass syringe at an
interval of 10 min. The collected mixture was passed through a
0.45 ␮m membrane filter, and the filtrate was extracted by chro-
matographically pure CH COOC H5. The HDC reaction conversion
3
2
was then estimated by using gas chromatography coupled with
mass spectrometry (GC–MS, Agilent 5977E). The spent Pd/Ni-mCN
catalyst was recovered by using a magnet, washed with ethanol,
and finally vacuum dried at room temperature in an oven for the
next catalytic run. The catalytic procedure was repeated five times.
2.5. Characterization
The morphology of the Ni-MOF, Ni-mCN, and Pd/Ni-mCN
materials was observed by scanning electron microscopy (SEM,
JSM-6701F) and transmission electron microscopy (TEM, Tecnai
G2F30) coupled with energy dispersive spectroscopy (EDS). Pow-
der X-ray diffraction (XRD, Rigaku D/max-2400) measurements
were performed in a diffractometer using the Cu-K␣ radiation as
(
Ni-mCN) by a simple carbonization method. Pd NPs were subse-
quently supported on the Ni-mCN support by incipient wetness
impregnation and reduced by NaBH to obtain the final Pd/Ni-mCN
4
◦
the X-ray source within the 2ꢀ range of 10–80 . X-ray photoelec-
catalyst with high Pd NPs dispersion. Pd/Ni-mCN was used in the
catalytic HDC of CPs showing superior catalytic efficiency and easy
recovery and reusability.
tron spectroscopy (XPS, PHI-5702) was recorded to analyze the
electronic states of the surface components of Ni/m-CN using the
C 1 s line at 284.6 eV as the binding energy reference. Magnetic
measurements were carried out for Ni/m-CN by vibrating sam-
ple magnetometry (VSM) at room temperature with an applied
magnetic field varying from −20 to 20 kOe. Nitrogen adsorp-
tion/desorption experiments were performed at 77 K in an ASAP
2
. Experimental
2.1. Chemicals
2
010 device (Micromeritics, USA). Inductively coupled plasma
The reagents used for the preparation of the Ni-MOF
atomic emission spectroscopy (ICP–AES) and elemental analysis
(GmbHVario, El Elementar) were also used to measure the Ni, N,
C, and H contents in Ni/m-CN.
.
ꢀ
Ni(NO3) 6H O, 4,4 -bipyridine, and 1,3,5-benzenetricarboxylic
2 2
(
acid) were purchased from Sigma-Aldrich. CPs were of analytical
grade and supplied by Aladdin reagents. Some other frequently
used reagents and solvents were used as supplied.
3
. Results and discussion
2.2. Preparation of Ni-mCN
Herein, the Ni MOF-derived magnetic mesoporous carbon Ni-
mCN, generated by carbonization of the Ni MOF under inert
atmosphere, was used to prepare the Pd-based catalyst Pd/Ni-mCN.
The morphology of the Ni MOF showed a shape with irregular plates
The Ni-MOF were fabricated following the method reported
ꢀ
by Jun Chen [48]. In
a
typical procedure, 3 mmol of 4,4 -
bipyridine, 3 mmol of 1,3,5-benzenetricarboxylic acid, and 3 mmol
(Fig. 1a). After carbonization under inert atmosphere, the Ni MOF
of Ni(NO3)2.6H O were dissolved in 60 mL of dimethylformamide
2
was transformed into a mesoporous carbon framework with Ni
NPs of ca. 8 nm diameter embedded in the C framework (Fig. 1b).
The Ni NPs were generated during carbonization by reduction with
carbon. The FTIR spectra also confirmed the Ni MOF to Ni-mCN
transformation process (Fig. S1). Ni MOF showed a large number
(
DMF) at room temperature under vigorous stirring. The mixture
◦
was subsequently kept at 80 C for 72 h in a teflon-lined autoclave.
After cooling to room temperature, a green solid was obtained by
filtration, washed thoroughly with DMF, and vacuum dried in an
oven overnight. Finally, the dry Ni-MOF powder was calcined at
−
1
−1
of CH (␯2900 cm ) and other organo-functional (␯500–1600 cm )
◦
◦
−1
7
00 C (5 C min ) for 2 h under N2 atmosphere in a tube furnace
groups before the carbonization process. The CH group stretching
to obtain the Ni-mCN nanocomposite.
−
1
vibration around 2900 cm disappeared upon carbonization while
−
1
other stretching vibrations at 1580, 1260, and 758 cm (assigned
to the stretching modes of CN heterocycles formed during the car-
bonization procedure) were generated [49,50]. The FTIR results
revealed the presence of CN heterocycles in the mesoporous car-
bon framework. Fig. 1c shows a TEM image of Pd/Ni-mCN. It can
be seen that the morphology and the structure of Ni-mCN was pre-
served after the impregnation/NaBH4 reduction processes, while
the Pd NPs were supported on the mesopores and on the surface
of Ni-mCN. The EDS spectra showed the Pd/Ni-mCN catalyst to be
mainly composed of C, N, O, Ni, and Pd elements (Fig. S2). With
the aim to study the elemental distribution of Pd/Ni-mCN, high-
angle annular dark field scanning transmission electron microscopy
(HAADF-STEM) was performed (Fig. 1d). The framework of the
Pd/Ni-mCN catalyst was mainly composed of C and N. The Ni NPs
2
.3. Preparation of Pd/Ni-mCN catalyst
A certain amount of Pd(AcO)2 was dissolved in 20 mL of deion-
ized water. 264 mg of Ni-mCN were subsequently added to the
above solution and ultrasonically dispersed. The dispersion was
◦
cooled to 0 C, and a NaBH solution was added dropwise in excess.
4
The solution was stirred overnight and the resulting solid was cen-
trifugally separated to obtain the Pd/Ni-mCN nanocatalyst.
2
.4. HDC of 4-CP
◦
The HDC of 4-CP was carried out at 20 C under atmospheric
pressure. Typically, Pd/Ni-mCN (5 mg), 4-CP (192 mg, 1.5 mmol),

3
88
X. Cui et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 386–392
Fig. 1. (a) SEM of Ni-MOFs, (b) TEM of Ni-mCN, (c) TEM of Pd/Ni-mCN, (d) HAADF-STEM image of Pd/Ni-mCN, C mapping image (C–K), N mapping image (N–K), Ni mapping
image (Ni–L), and Pd mapping image (Pd–L).
were highly dispersed on the mCN framework. The catalytic centers
Pd NPs) were well dispersed on the mesopores and on the surface
of Ni-mCN. Comparing the mapping images of the Ni (Ni-L) and Pd
Pd-L), it can be concluded that, the Pd NPs are highly dispersed on
the Pd/Ni-CN nanocatalyst, and the average particle size of Pd NPs
is smaller than Ni NPs.
additionally detected at 337.5 and 342.6 eV which confirmed the
existence of a fraction of Pd as Pd(II) (ca. 26%). The existence of
Pd(II) can be potentially explained by the coordination of Pd NPs
with the surface nitrogen groups of Ni-mCN. The Pd loading of the
Pd/Ni-mCN nanocatalyst, obtained by ICP–AES, was ca. 7.33 wt%.
The XRD patterns of Ni-mCN and Pd/Ni-mCN are shown in
Fig. 3a. Both samples showed one broad characteristic peak at ca.
(
(
The mesoporous properties of Ni MOF, Ni-mCN and Pd/Ni-mCN
◦
were investigated by N adsorption/desorption measurements (Fig.
25 attributed to the (002) plane of the carbon material. The intense
2
◦
S3). The Ni MOF, Ni-mCN and Pd/Ni-mCN showed adsorption
isotherms with a steep increase at relatively high relative pres-
sures (typical type-IV behavior), thereby indicating the presence of
a mesoporous structure. The Brunauer–Emmett–Teller (BET) spe-
diffraction peaks at 44.6, 52.0, and 76.6 were associated with the
(111), (200), and (220) planes of the face-centered cubic (fcc) struc-
ture of Ni (JCPDS 04-0850), respectively [51]. Compared with the
XRD pattern of the Ni-MOF (Fig. S4), it can be concluded that the
totality of the high-valence Ni in the Ni MOF was reduced to metal-
lic Ni NPs with good crystallinity during the carbonization process.
The XRD pattern of Pd/Ni-mCN also showed a diffraction peak at
2
−1
cific surface area of Ni MOF was 506.4 m g , when calcined at
◦
7
00 C under N atmosphere, the surface area of the obtained Ni-
mCN was decreased to 170.4 m g . After the modification with
Pd NPs (Pd/Ni-mCN), the surface area of Pd/Ni-mCN was further
decreased to 86.24 m g . The average pore size of Pd/Ni-mCN was
ca. 20 nm which confirmed its mesoporous structure.
2
2
−1
◦
40.0 which fits well with the characteristic (111) plane of Pd. Thus,
2
−1
the XRD results revealed that Pd NPs were successfully supported
on the mesoporous Ni-mCN material. The magnetic measurements
of Pd/Ni-mCN are shown in Fig. 3b. The magnetization satura-
XPS analysis was carried out for the Pd/Ni-mCN nanocatalyst
in an attempt to study the chemical composition of this material.
From the wide scan XPS spectrum, it can be seen that C, N, O, Ni,
and Pd were the main components of Pd/Ni-mCN (Fig. 2a). The Ni
−
1
tion value of Pd/Ni-mCN was measured to be 15.76 emu g . As a
result of its magnetite content, Pd/Ni-mCN rapidly responded to the
external magnetic field being homogeneously re-dispersed upon a
slight shake (Fig. 3b, inset). This characteristic provides an easy and
efficient way to separate Pd/Ni-mCN from the reaction mixture for
reuse under an external magnetic field.
2
p band centered at 852.9 eV revealed that Ni existed as Ni(0) as
a result of the almost complete reduction of Ni(II) to Ni(0) dur-
ing the Ni MOF carbonization process. The N1 s peak at ca. 400 eV
can be attributed to the nitrogen-heterocycles formed from the N-
containing heterocyclic ligands during the carbonization process.
The catalytic active center Pd NPs showed the characteristic peaks
at 335.5 and 340.8 eV corresponding to the Pd 3d3/2 and Pd3d5/2
transitions, respectively (Fig. 2b). Two unconspicuous peaks were
It is well known that N-doped mesoporous carbons can firmly
load noble metal NPs owing to the high coordination ability of the
nitrogen atoms [52–54]. Thus, a N-doped magnetic mesoporous
carbon was herein used to support the Pd NPs with high disper-
sion. Pd/Ni-mCN was used in the catalytic HDC of CPs (Scheme 1),

X. Cui et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 386–392
389
Fig. 2. (a) XPS spectra of Pd/Ni-mCN, (b) Pd 3d spectra of Pd/Ni-mCN.
Fig. 3. XRD patterns (a) and magnetic hysteresis loops (b) of Ni-mCN and Pd/Ni-mCN.
Fig. 4. (a) Time evolution of the yield of phenol and conversion of 4-CP with Pd/Ni-mCN nanocatalyst, (b) plots of Ct/C0 and ln(Ct/C0) versus reaction time for the HDC of
◦
4
-CP over Pd/Ni-mCN nanocatalyst. Conditions: 4-CP (1.5 mmol), base (1.5 mmol), solvent (15 mL), catalyst 5 mg, H2 balloon, 20 C, 90 min.
and the reaction conditions were investigated in detail (Table 1).
in C H5OH, CH COOC H5, and C H solvents, respectively. These
2 3 2 6 12
First, H O was used as a solvent to test different bases (Table 1,
results showed that the HDC yield is greatly affected by the solvent
as NaCl (the HDC product) is insoluble in organic solvents and cov-
2
entries 1–5). The yields of phenol were 60.7, 60.9, 47.4, 16.2, and
.
9
9.6% upon utilization of NH3 H O, CH COONa, Na CO , (C H5) N,
ers the Pd NPs once generated [44]. H O is the optimal solvent for
2
3
2
3
2
3
2
and NaOH, respectively. These results show that the HDC yield
is greatly affected by the neutralization of HCl by NaOH. With
NaOH as the optimized base, different solvents were also screened
the HDC reaction (Table 1, entry 5). Other noble metal-supported
catalysts such as Ru/Ni-mCN and Pt/Ni-mCN were also investigated
being nearly inactive in the HDC of 4-CP (Table 1, Entries 9–10).
For the Ru based catalyst, as reported in the literature that always
(Table 1, entries 6–8). The yields of phenol were 34.6, 4.4, and 14.2%

3
90
X. Cui et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 386–392
Table 2
a
Catalytic HDC of different CPs over the Pd/Ni-mCN nanocatalyst.
2
H , Catalyst
OH
OH
Cl
Entry
Substrate
Product
Conversion (%)
Yield (%)
Cl
OH
OH
OH
1
2
100
100
99.59
99.8
Cl
OH
Cl
3b
Cl
Cl
OH
OH
OH
99.9
99.2
Cl
Cl
4c
OH
81.8
61.6
a
Conditions: CPs (1.5 mmol), base (1.5 mmol), solvent (15 mL), catalyst (5 mg), H2
Scheme 1. HDC of 4-CP over Pd/Ni-mCN nanocatalyst.
◦
balloon, 20 C, 90 min.
b
2,4-DCP (1.5 mmol), base (3 mmol), solvent (15 mL), catalyst (10 mg), H2 balloon,
◦
2
0
C, 180 min.
Table 1
c
2,4,6-TCP (1.5 mmol), base (4.5 mmol), solvent (15 mL), catalyst (10 mg), H2 bal-
Reaction conditions screening of the Pd/Ni-mCN nanocatalyst catalyzed HDC of 4-
loon, 20 ◦C, 180 min.
a
CP.
Entry
Catalyst
Solvent
Base
Time (min)
Yield (%)
temperature conditions led to further hydrogenation of phenol to
cyclohexanone, as previously reported [22,54].
1
2
3
4
5
6
7
8
9
Pd/Ni-mCN
Pd/Ni-mCN
Pd/Ni-mCN
Pd/Ni-mCN
Pd/Ni-mCN
Pd/Ni-mCN
Pd/Ni-mCN
Pd/Ni-mCN
Ru/Ni-mCN
Pt/Ni-mCN
Ni-mCN
H2O
H2O
H2O
H2O
NH3·H2O
CH3COONa
Na2CO3
(C2H5)3N
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
NaOH
90
90
90
90
90
90
90
90
90
90
90
90
90
60.7
60.9
47.4
16.2
99.6
34.6
4.4
Ct
C0
−ln
= kt
(1)
H2O
As H was used in excess as compared to 4-CP, the HDC reaction
2
C2H5OH
CH3COOC2H5
C6H12
H2O
H2O
H2O
can thus be considered as a pseudo first-order reaction [5]. Fig. 4b
shows the HDC reaction kinetics over Pd/Ni-mCN. According to the
formula [1], the HDC reaction rate constant k was calculated to
14.2
b
N.D.
10
11
12
13
1.6
−1
be 0.066 min . Pd/Ni-mCN showed higher catalytic activity in the
N.D
53.8
59.1
c
HDC of 4-CP as compared to other Pd-based HDC catalysts such as
Pd/C
H2O
H2O
−
1
−1
)
d
PdAl-IMP-1.5 (0.034 min ) [57] and Ni@Pd/KCC-1 (0.038 min
58]. The excellent catalytic activity of Pd/Ni-mCN was probably
Pd/Ni-mCN
[
a
Conditions: 4-CP (1.5 mmol, 192 mg), base (1.5 mmol), solvent (15 mL), catalyst
◦
originated from the high dispersion of the Pd NPs on the Ni-mCN
surface as well as from the accumulation of 4-CP and H2 on the
surface of the Pd/Ni-mCN catalyst.
(
5 mg), H2 balloon, 20 C, 90 min.
b
N.D. (none detected, yield less than 0.1%).
c
7
.3 mg of 5 wt% Pd/C.
d
Conditions: 4-CP (2.5 mmol, 192 mg), NaOH (2.5 mmol), solvent (15 mL), catalyst
The catalytic properties of Pd/Ni-mCN in the HDC of 2-CP, 3-CP,
◦
5
mg, H2 balloon, 20 C, 90 min.
2
,4-dichlorophenol (2,4-DCP), and 2,4,6-trichlorophenol (2,4,6-
TCP) were also investigated, and the results are summarized in
Table 2. At the same HDC reaction conditions of 4-CP, 2-CP and
3-CP were completely dechlorinated to phenol with a yield higher
than 99% (Table 2, entries 1 and 2), whereas only half of the initial
2,4-DCP and 2,4,6-TCP were dechlorinated with lower phenol yields
(lower than 25%, Fig. S5). Doubling both the catalyst dosage and the
HDC reaction time resulted in 2,4-DCP and 2,4,6-TCP conversions
of 99.9 and 81.8%, with phenol yields of 99.2 and 61.6%, respec-
tively (Table 2, entries 3 and 4). Fig. 5a shows the time-dependent
conversion of 2,4-DCP (1.5 mmol) and the yield of products over
10 mg of Pd/Ni-mCN. The GC–MS results showed that 2-CP was the
only intermediate product, with phenol being the final product. 4-
CP was not detected as an intermediate product probably due to
steric hindrance effects of the ortho-substituted Cl (as compared to
the para-substituted Cl) that prevents its dechlorination [19,59,60].
2,4-DCP and the intermediate product 2-CP were dechlorinated
almost completely after 3 h of reaction. The HDC of 2,4,6-TCP was
also investigated as a function of time, with 2,4-DCP and 2-CP being
the intermediate products (no 4-CP was found) as a result of the
steric hindrance of the ortho-substituted Cl atom. When compared
with 2,4-DCP, complete HDC of 2,4,6-TCP to phenol was more dif-
ficult (81.8% conversion, 61.6% phenol yield) at the same reaction
conditions. In summary, Pd/Ni-mCN showed high catalytic activity
for the HDC of CPs at mild reaction conditions.
needs high H2 pressure and temperature for the hydrogenation of
nitrobenzene [55]. The catalyst support Ni-mCN was inactive dur-
ing the HDC reaction (Table 1, Entry 11). A commercial 5 wt% Pd/C
showed a 53.8% yield of phenol (Table 1, Entry 12). Increasing the
dosage of 4-CP from 1.5 mmol to 2.5 mmol resulted in decreased
yields of phenol (59.1%, Table 1, Entry 13) at similar reaction times
(
90 min). Thus, based on the above investigation, Pd/Ni-mCN can
efficiently catalyze the HDC of 4-CP at room temperature in H O
2
with NaOH as a base and under 1 atm of H . The HDC pathway can
2
be described as follows: the mCN framework can adsorb 4-CP via
␲
–␲ stacking interactions, thus leading to the accumulation of high
concentrations of 4-CP over the active Pd surface [51]. On the other
hand, H2 can be adsorbed on the surface of most noble metal NPs
and dissociated into two active hydrogen atoms for the hydrogena-
tion reaction. Subsequently, C Cl bond breaking take place over the
absorbed 4-CP by attack of the active hydrogen atoms to form phe-
nol and HCl. The HCl reacts rapidly with NaOH to form NaCl that
dissolves in water, thereby preventing accumulation of HCl on the
surface of the catalyst [56]. Thus, an excess of NaOH in the HDC
reaction system could neutralize the in situ generated HCl, thereby
effectively preventing the poisoning of the catalysts.
The time-dependent conversion of 4-CP and the yield of phe-
nol in the HDC reaction over Pd/Ni-mCN are shown in Fig. 4a. The
yield of phenol gradually increased with the reaction time. 4-CP was
dechlorinated almost completely after 90 min, and the yield of phe-
nol reached 99.6%. No byproducts apart from phenol were detected
by GC–MS since HDC was carried out at room temperature and with
low catalyst loading. However, larger catalyst loadings and higher
Furthermore, using the HDC of 4-CP as a probe reaction, we
also studied the stability and recyclability of the Pd/Ni-mCN cat-
alyst. Pd/Ni-mCN was quantitatively separated from the reaction
medium after each reaction cycle using an external magnet owing
to its superparamagnetic properties. The separated catalyst was
then thoroughly washed with water and ethanol and reused for

X. Cui et al. / Journal of Molecular Catalysis A: Chemical 423 (2016) 386–392
391
Fig. 5. Time evolution of the yield of phenol and conversion of 2,4-DCP (a) and 2,4,6-TCP (b) with Pd/Ni-mCN nanocatalyst. Conditions: (a) 2,4-DCP (1.5 mmol), base (3 mmol),
◦
◦
solvent (15 mL), catalyst 10 mg, H2 balloon, 20 C, 180 min. (b) 2,4,6-TCP (1.5 mmol), base (4.5 mmol), solvent (15 mL), catalyst 10 mg, H2 balloon, 20 C, 180 min.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2016.07.
0
41.
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