Pd supported on N-doped-ordered mesoporous carbons’ catalysts for selective hydrodechlorination of 4-chlorophenol

Article abstract of DOI:10.1007/s11696-018-0477-8

Pd catalysts supported on N-doped-ordered mesoporous carbons (NOMC) have been prepared and tested for selective hydrogenation of 4-chlorophenol (4-CP) with H₂. The difference from the previous methods is that the NOMC was synthesized using urea as nitrogen source via one-pot route. The rate constant of Pd/NOMC for hydrodechlorination of 4-CP was about 135.9?h^?1 which were higher than Pd/OMC (65.6?h^?1) and Pd/AC (20.8?h^?1). It could be attributed to the synergetic effects of mesoporous structure, N-doped supports, and the stabilized small PdNPs. The conversion changed from 100 to 90.2% after the sixth reaction using Pd/NOMC which could be caused by the palladium leaching.

Full text of DOI:10.1007/s11696-018-0477-8

Chemical Papers
https://doi.org/10.1007/s11696-018-0477-8
ORIGINAL PAPER
Pd supported on N‑doped‑ordered mesoporous carbons’ catalysts
for selective hydrodechlorination of 4‑chlorophenol
Rongrong Li1 · Shiting Wang1 · Yuan Hu1 · Hong Chen1 · Jingjing Chen1 · Chu Chu1 · Jianli Zheng1
Received: 18 September 2017 / Accepted: 20 April 2018
© Institute of Chemistry, Slovak Academy of Sciences 2018
Abstract
Pd catalysts supported on N-doped-ordered mesoporous carbons (NOMC) have been prepared and tested for selective hydro-
genation of 4-chlorophenol (4-CP) with H2. The diference from the previous methods is that the NOMC was synthesized
using urea as nitrogen source via one-pot route. The rate constant of Pd/NOMC for hydrodechlorination of 4-CP was about
135.9 h−1 which were higher than Pd/OMC (65.6 h−1) and Pd/AC (20.8 h−1). It could be attributed to the synergetic efects
of mesoporous structure, N-doped supports, and the stabilized small PdNPs. The conversion changed from 100 to 90.2%
after the sixth reaction using Pd/NOMC which could be caused by the palladium leaching.
Keywords Mesoporous carbon · N-doped · Pd · 4-chlorophenol · Hydrogenation
Abbreviations
deal with the waste water containing chlorophenols, catalytic
hydrodechlorination (HDC) is considered to be an environ-
mental friendly one by the breaking up of the C–Cl bond and
thus transforming toxic substances into safe compounds or
even useful materials (Molina et al. 2014). In general, the
catalytic HDC is conducted under a reduction atmosphere of
molecular hydrogen and by contacting with supported metal-
lic catalysts such as Pd (Aramendıa et al. 1999; Baeza et al.
2016; Benitez and Gloria 2000; Bernard et al. 1993; Calvol
et al. 2009), Pt (Chang et al. 2016; Datta et al. 2010), and Rh
(Díaz et al. 2010, 2011, 2016 Gómez-Quero et al. 2011). Pd
has been proven to be the most promising one because of its
high activity in such degradation of chlorophenols at ambi-
ent conditions. Nevertheless, severe catalyst deactivation
originated due to formation of strong bond between Cl and
Pd (Gómez-Quero et al. 2010a, b), aggregation of Pd nano-
particles (Gong et al. 2013; Hashimoto and Ayame 2003),
and/or carbonaceous deposited on Pd particles (Hildebrand
et al. 2009; Huang et al. 2008; Jadbabaei et al. 2017).
NOMC N-doped-ordered mesoporous carbons
4-CP
HDC
4-Chlorophenol
Catalytic hydrodechlorination
SAXRD Small-angle X-ray difraction
TEM
HMT
TMB
PdNPs
Transmission electron microscopy
Hexamethylenetetramine
1,3,5-Trimethylbenzene
Pd nanoparticles
Introduction
Chlorophenols are widely used in the production of pes-
ticides, disinfectants, wood preservatives, and personal
care formulations (Albers et al. 2001). The residue of these
compounds causes a negative efect on the environment and
human beings such as the contamination of groundwater due
to their acute toxicity and poor biodegradability. Among the
current approaches which have been widely employed to
Based on the above, there remains as a challenge to fnd
stable catalysts for selective hydrogenation of chlorophe-
nols. Therefore, activated carbon, pillared clays (Molina
et al. 2014), γ-Al2O3 (Munoz et al. 2014), and silica (Bovkun
et al. 2005; Li et al. 2009, 2012, 2017; Liu et al. 2010, 2011,
2012; Lyonnard et al. 2002) were used as supports for HDC,
and the phenol is further converted into cyclohexanol. The
research of carbon carrier is the most widespread since
its high adsorption capacity, excellent dynamic behavior,
and high resistance to abrasion (Böhringer et al. 2011;
Electronic supplementary material The online version of this
article (https://doi.org/10.1007/s11696-018-0477-8) contains
supplementary material, which is available to authorized users.
* Rongrong Li
lrr@tzc.edu.cn
1
School of Pharmaceutical and Chemical Engineering,
TaiZhou University, Taizhou 318000, Zhejiang,
People’s Republic of China
Vol.:(0123456789)
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Chemical Papers
Romero-Anaya et al. 2014). In last several years, functional-
ized mesoporous superfuous have attracted an intense inter-
est due to their higher surface areas, structural regularity,
and regulable pore size (Mitoma et al. 2009). Particularly,
the NH2 groups on the surface of the materials are benefcial
to improve the activity and stability, since the metal nano-
particles anchored and well dispersed (Molina et al. 2010).
N-doped-ordered carbon matrixes, which could change the
physicochemical properties of the materials, have been used
as supports to synthesize supported nanoparticle catalysts
for hydrogenation (Moon et al. 1998; Öcal et al. 1999; Oseg-
ueda et al. 2015; Rong et al. 2013), oxidation (Santiago et al.
2008), and coupling reaction (Shao et al. 2010). However,
the application of metal nanoparticles dispersed on the
N-doped OMC supports for the HDC reaction is rare.
In last several years, an organic–organic co-assembly syn-
thesis method was proved as a credible method to synthesize
OMC by either hydrothermal (Sohn et al. 2017; Vinu et al.
2005; Wang et al. 2014) or an evaporation-induced self-
assembly procedure (Wang et al. 2011; Wiersma et al. 1996,
2001; Xue et al. 2008). Therefore, the aim of the current
study is to obtain N-doped OMC by one-pot route synthesis
and used urea as the nitrogen source. The direct synthesized
NOMC could be favorable in practical applications, which
can keep the high-ordered mesoporous structures unchanged
and modulate the N content easily. The as-prepared NOMC
was used support for immobilization of Pd catalyst during
the HDC of 4-chlorophenol in aqueous solution using direct
hydrogen. The activity of catalysts was checked in the batch
mode which was used to evaluate the efects of reaction tem-
perature and metal loading with 4-CP as target compound.
The efect of the N content of NOMC in the preparation of
the catalysts has been studied.
room temperature for 1 h, the resultant dark green solution
was further stirred in a water-bath under refux condenser at
80 °C. Green precipitation was observed after about 20 min.
After continuously stirring for another 24 h, the black solid
products were collected by sedimentation separation, fltra-
tion, washed with water, and air-dried at room temperature.
Finally, the as-made composite (NOMC) was thermally
treated at 900 °C for 3 h, with a heating rate of 1 °C min−1
under a nitrogen atmosphere, to obtain the NOMC. The syn-
thesis of OMC was also carried out through the Lieu et al.
method (Lyonnard et al. 2002) for comparison with other
catalysts.
Immobilization of Pd nanoparticles
The corresponding PdNP catalysts Pd/NOMC synthesized
via improved synthetic method reported by Li et al. (2017).
Characterization techniques
The microstructure and component analysis of the alloy NPs
was examined by transmission electron microscopy (TEM)
equipped with high-angle annular dark-field (HAADF)
attachment and energy-dispersive X-ray spectroscopy (EDX)
(Philips-FEI Tecnai G2 F30 S-Twin). The TEM samples
were prepared by dispersing the powder products in pure
ethanol using ultrasonic cleaner at room temperature for
0.5 h and transferring them to carbon-coated copper grids.
Average particle sizes were determined by measuring at
least 100 particles for each sample analyzed, from at least
fve diferent micrographs. X-ray difraction measurements
of the catalyst samples were performed on a PANalytical-
X’Pert PRO generator with Cu Kα radiation (λ=0.1541 nm).
X-ray fuorescence by dispersive energy (XRF) was acquired
using a Thermo ARLADVANT’X IntelliPower TM 4200
spectrometer. The samples were excited with the Rh X-ray
tube operated at 40 kV and 1.25 mA. X-ray photoelectron
spectroscopy (XPS) was acquired with a Kratos AXIS Ultra
DLD spectrometer. XPS analysis was performed using the
monochromatized aluminum X-ray source and pass energy
of the electron analyzer of 40 eV. The pressure in the sample
analysis chamber was lower than 6×10−9 Torr during data
acquisition. Binding energies (BE) were referred to the C1 s
line at 284.8 eV.
Methods
Catalyst preparation
Triblock poly (ethylene oxide)-b-poly (propylene oxide)-b-
poly (ethylene oxide) copolymers Pluronic F127 (EO106
PO70 EO106, Mav=12600) were purchased from Sigma-
Aldrich Corp. Other chemicals were purchased from Sinop-
harm Chemical Reagent Corp. All chemicals were used as
received without further treatment.
Catalysis tests
Synthesis of NOMC
Catalytic tests were conducted in a 50 mL Schlenk glass batch
reactor with a magnetic bar; 0.025 g Pd/NOMC catalyst, 5 mL
(1.5 g L−1) 4-CP, and 25 mL deionized water were introduced
into the glass reactor, and then, it was vacuumed and purged
with H2 three times before it was fnally pressurized with
1.0 atm of H2 gas. Subsequently, the reaction mixture was
In a typical procedure, About 3.2 g urea, 1.10 g of resor-
cinol, 0.70 g of hexamethylenetetramine (HMT), 2.00 g of
Pluronic F127, 0.40 g of 1,3,5-trimethylbenzene (TMB),
52 mL of deionized water, and 2.00 mL of aqueous ammonia
(28 wt%) were mixed in random sequence. After stirring at
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Chemical Papers
stirred at 303 K and the stirring rate is 1000 rpm. After cooling
to RT, excess H2 was carefully released, and the internal stand-
ard (ethylene glycol) was added. 4-CP, phenol, cyclohexanone,
and cyclohexanol were analyzed by a gas chromatograph (GC
2010plus, Shimadzu). The temperatures of the injection port
and the fame ionization detector were 280 °C. Samples were
injected in the splitless mode onto a 25 m length×0.32 mm
i.d. capillary column (CP-FFAP CB, Varian). The following
temperature program: the initial temperature of the column
was 60 °C, held for 1 min, and the rate of the temperature
increase was 20 °C min−1 up to 280 °C, with a fnal hold time
of 2 min. The carrier gas was nitrogen; meanwhile, hydrogen
and oxygen were fed to the FID. All the gas has a purity of
99.999%. The reaction fuid is fltered through an inorganic
membrane material to prevent the catalyst from entering gas
chromatography. The relative peak area % was converted to
mass% using regression equations based on detailed calibra-
tion plots. Preliminary control 4-CP HDC experiments with
NOMC were carried out to check any potential interaction
between 4-CP, H2 and NOMC in the absence of catalysts. No
efect of NOMC was observed and no reaction products were
detected in the absence of catalyst. Diferent stirring velocities
were also checked, confrming that the process takes place
under chemical control. The rate of 4-CP disappearance was
calculated from a pseudo-frst-order equation:
where mcyclohexanol%, mcyclohexanone%, and mphenol% represent
the mass faction of cyclohexanol, cyclohexanone, and phe-
nol in the mixture.
Results and discussion
Characterization of the catalyst
N2 adsorption–desorption isotherms of NOMC have an
obvious condensation step at P/P0 =0.4–0.6, which can be
indexed as typical type-IV curves and showing a uniform
mesopore (Fig. 1). The isotherms do not ft well H1-type
hysteresis loop, since the mesopore may have some block in
pore canals. The BET surface area, pore volume, and pore
size are listed in Table S1. After adding urea, the SBET val-
ues are lower than that of the reported OMCs (870 m2 g−1
for RF-1-900 and 853 m2 g−1 for RF-2-900) (Mitoma et al.
2009) which might be attributed to N-doping reaction by
decomposition of urea into radicals like ·NH2 and ·NH.
These reactions could result in the porosity change (Xia
et al. 2009). The surface area and pore volume of PdNPs
supported NOMC and NOMC are changed slightly after the
supporting PdNPs, suggesting that the ordered mesopores
are still well preserved after doping by amino functional
groups and the mesostructure is not blocked by Pd nanopar-
ticles. Small-angle X-ray difraction (SAXRD) characteriza-
tion in Fig. 2a could also prove this result.
−dC4−cp
(−r4−cp) =
= k1 × C4−cp
dt
In Fig. 2a, the SAXRD of NOMC without addition of
TMB shows one weak peak that can be indexed as (110)
refections of the body-centered cubic Imm symmetry. Fig-
ure 2 shows the SAXRD of NOMC, which shows that one
resolved peak can be indexed as p6m space group with 2D
hexagonal structure (Mitoma et al. 2009). Compared to the
t = 0; C4−cp = C0.
The concentration of hydrogen can be included in the
pseudo-frst-order rate constant (k1), since hydrogen is in great
excess.
The efuent was analyzed and three analyses were made
at each temperature. The conversion of 4-CP (XCP) is defned
by the following:
[CP]0 − [CP]
220
XCP
=
× 100%.
(A)
[CP]0
200
180
The selectivity with respect to phenol (e.g.) is given as the
mass% phenol (mphenol%) in the terms of the total mass of prod-
ucts formed, and was calculated using the following equations:
mphenol
%
160
Sphenol
=
× 100%,
mcyclohexanol% + mcyclohexanone% + mphenol
%
(a)NOMC
(b)Pd/NOMC
140
mphenol
%
Scyclohexanol
=
× 100%,
× 100%,
mcyclohexanol% + mcyclohexanone% + mphenol
%
%
120
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure(P/P0)
mphenol
%
Scyclohexanone
=
mcyclohexanol% + mcyclohexanone% + mphenol
Fig. 1 N2 sorption isotherms of NOMC and Pd@NOMC
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Fig. 2 SAXRD (a) and
WAXRD (b) patterns of NOMC
and Pd/NOMC
(b)
Pd/NOMC
(a)
Pd(111)
Pd(200)
Pd(220)
Pd/NOMC
NOMC
NOMC
1
2
3
4
5
20
40
60
80
100
2Theta (degree)
2Theta (degree)
reported OMCs, the difraction intensity decreases slightly
after doping by N groups, which indicates that the ordered
mesostructures are slightly changed. Wide-angle XRD pat-
terns of NOMC show that nanoparticles are well dispersed
and high-intensity 002 and 100 difractions belong to gra-
phitic carbon. The patterns of Pd and Pd/NOMC-T in Fig. 2b
show Pd (111), Pd (200), and Pd (220) difractions which
are observed obviously, which illustrate that the extra small
Pd nanoparticles are well dispersed on the surface of the
N-doped OMC matrix.
approximately 3.4 nm which are homogeneously dispersed
on the surface of the NOMC. All the PdNPs are uniform and
no obvious agglomeration is detected in the HAADF-STEM
images. Moreover, the N-doped-ordered mesoporous canals
are arranged in regular intervals, which may indicate that
the structures are no degenerated after the immobilization
of metal NPs. The well-dispersed PdNPs could also attribute
to N-hybrids which promote the stabilization of metal NPs.
XPS results are listed in Table S1, Fig. S2, and Fig. 4. The
carbon, oxygen, and nitrogen elements have strong signals.
The nitrogen content of Pd/NOMC is 1.73 wt%. The C 1 s
spectra of Pd/NOMC are showed in Fig. S2, which could
be attributed to C–C (284.8 eV), C–O (285.3 eV), C=O
(286.6 eV), and C=O–C (289.6 eV). The N1s XP spectrum
of Pd/NOMC showed in Fig. 4 might be well identifed to
four components (Xia et al. 2004; Xu et al. 2012; Shao et al.
2011), which could be attributed to pyridinic-N (398.4 eV),
The structure of the Pd/NOMC was examined by trans-
mission electron microscopy (TEM) (Fig. 3). The TEM
images show that the sample has high ordered of periodic-
ity viewed from the [110] direction; this result also illus-
trates that these NOMC still maintain mesostructure and not
blocked by PdNPs. The distribution of particle size image of
Pd/NOMC is showed in Fig. S1. The PdNPs’ average size is
Fig. 3 STEM images of NOMC (a–d) and Pd/NOMC (e–h)
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3d5/2 peak at 337.8 eV is related to Pd2+. XPS results have
a positive shift of ca. 0.3 eV comparing with the Pd/OMC
case in our early work (Baeza et al. 2016). Shao et al. (2011)
compared the supported Pd catalysts on ordered mesoporous
carbon (OMC) and activated carbon (AC) for the liquid-
phase catalytic hydrodechlorination (HDC) of 2,4-dichlo-
rophenol. In comparison with Pd/AC, Pd particles of Pd/
OMC were efectively confned in the mesopores of OMC,
resulting in high Pd dispersion and Pd2+ content. Gómez-
Sainero et al. (2002) studied the catalytic HDC of CCl4 over
Pd/AC and concluded that the coexistence of Pd2+ and Pd0
was essential for the efective HDC of CCl4, wherein Pd0
acted as the active site for H2 activation and Pd2+ served as
the active site for the activation of C–Cl bond via abstrac-
tion of nucleophilic chloride anion and formation of highly
reactive +CCl3 ion. Hence, at similar reduction temperature,
the higher catalytic activity of Pd/OMC can be attributed to
its higher Pd2+ content. It is well accepted that the direction
of the core-level shift is consistent with that of the valence
band shift, which could also be attributed to the partial elec-
tron transfer from N to Pd atoms. This likely arises from the
electron transfer between metal Pd and NOMC which makes
Pd electron-defcient. The latter suggests a strong interac-
tion between Pd NPs and the substitutional nitrogen defect
sites (Baeza et al. 2016; Benitez and Gloria 2000; Bernard
et al. 1993).
N1s
408
405
402
399
396
393
B.E. (eV)
Fig. 4 XPS spectra N 1 s spectra of Pd/NOMC
Pd5/2
Pd/NOMC
Pd3/2
Pd0
Pd2+
Pd/OMC
Hydrodechlorination of 4‑CP
The catalyst Pd/NOMC was used for the selective hydro-
genation of 4-chlorophenol, and the results are shown in
Table 1. Selectivity of 4-CP dehydrochlorination to phenol
was decreased from 100 to 75% and the product of cyclohex-
anone and cyclohexanol was increased. While the conversion
of 4-CP was only 15.3 or 48.3% over Pd/AC or Pd/OMC
prepared by a similar method. Selectivity of 4-CP dehydro-
chlorination to cyclohexanone was about 18.5% which was
much lower than Pd/OMC (1.2%) and Pd/C (0.8%). More
cyclohexanone will be getting by deep hydrogenation which
348
346
344
342
340
B.E.(eV)
338
336
334
332
Fig. 5 XPS spectra of Pd 3d spectra of Pd/NOMC and Pd/OMC
pyrrolic-N (399.7 eV), graphitic-N (401.2 eV), and nitro-
gen-oxides (404.4 eV). The XPS spectra of Pd 3d peaks
(Fig. 5) have a doublet attributed to Pd 3d5/2 and 3d3/2. The
Pd 3d5/2 peak at 335.8 eV is related to Pd0, while the Pd
Table 1 Catalytic performance
of diferent catalyst for
Entry
Catalyst
Pd (mg kg−1
)
Conversion (%)
Selectivity (%)
Cyclohexanol
analyzed by of
ICP
hydrodechlorination of 4-CP
Fresh
Used
Cyclohexanone
Phenol
1
2
3
4
NOMC
–
–
0
0
0
0
5.8
0
0
Pd/Ca
905
895
911
705
783
873
15.3
48.3
100
0.8
1.2
18.5
99.2
98.8
75.7
Pd/OMCa
Pd/NOMCa
The conversion and selectivity was determined by GC. Reaction conditions: 0.025 g catalyst; 5 mL
(1.5 g L−1) 4-CP, 25 mL H2O, time=1.5 H, P (H2)=1.0 atm, temperature=303 K, and stirring
rate=1000 rpm
aPd loading wt%=1%
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Chemical Papers
Acknowledgements The project was supported by the National Natural
Science Foundation, China (21506138), and the Natural Science Foun-
dation of Zhejiang Province, China (LQ15B060001).
may be attributed to the higher active site for phenol species
in Pd/NOMC. During the HDC process, the chemistry of the
support plays a key role on the performance of the catalysts,
especially in their deactivation behavior through the metal/
support interactions and reactant/support interactions (Li
et al. 2013; Yoneda et al. 2007). Yuan and Keane (2004a,
2007) suggested that the catalyst activity can be reestab-
lished due to migration of halide ions from the surface of
the metal to the support to form halogenated species, thus
inhibiting the gradual deactivation of catalyst. Moreover,
it was proposed that the larger particle size (ca. 4 nm) of
the active site of the catalyst exhibited a stronger resistance
for the passivation of the catalyst (Yuan and Keane 2004b;
Zhang et al. 2006, 2011). Zhang et al. (2013) studied HDC
of 4-CP in aqueous solution with formic acid using a Pd/
AC. Conversion values above 99% for 4-CP were obtained
and the highest selectivity of 4-CP dehydrochlorination to
cyclohexanone was 12% at 348 K. A gradual loss of activity
was observed in successive runs when using Pd-Al pillared
clays as catalysts for the HDC (Molina et al. 2018). The
conversion of 4-CP was about 100% on reduced graphene
oxide, but the temperature was 317 K (Deng et al. 2014). In
our study, we can get the same selectivity at 306 K.
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with Pluronic F127 in aqueous solutions. Pd catalysts sup-
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