Catalytic dehydrochlorination of 1,2-dichloroethane to produce vinyl chloride over N-doped coconut activated carbon

Article abstract of DOI:10.1039/c5ra20916a

A series of N-doped coconut activated carbon catalysts (N-AC) were prepared using melamine as the nitrogen precursor and their performance for the catalytic dehydrochlorination of 1,2-dichloroethane (1,2-DCE) to produce vinyl chloride monomer (VCM) was assessed. It is indicated that the N-doped catalyst 5 : 10-N-AC exhibits a stable catalytic activity at 250 °C with the 1,2-DCE conversion of 85.1% at 180 h. Through DFT calculations, it is suggested that both pyridinic and pyrrolic nitrogen dopants can adsorb preferentially 1,2-DCE and increase the activity for 1,2-DCE dehydrochlorination at low temperature, in combination with characterizations of BET, Raman, TG, TPD, XPS, etc. In addition, the increase of quaternary nitrogen dopants and coking deposition on the catalyst surface can result in the activity decline. The N-doped activated carbon catalyst provides a promising pathway to produce VCM through a low-temperature energy-saving process of 1,2-DCE catalytic dehydrochlorination.

Full text of DOI:10.1039/c5ra20916a

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Catalytic dehydrochlorination of
1,2-dichloroethane to produce vinyl chloride over
Cite this: RSC Adv., 2015, 5, 104071
N-doped coconut activated carbon†
Wei Zhao,a Mengxia Sun,a Haiyang Zhang,a Yanzhao Dong,a Xiaoyan Li,a Wei Lia
ab
*
and Jinli Zhang
A series of N-doped coconut activated carbon catalysts (N-AC) were prepared using melamine as the
nitrogen precursor and their performance for the catalytic dehydrochlorination of 1,2-dichloroethane
(1,2-DCE) to produce vinyl chloride monomer (VCM) was assessed. It is indicated that the N-doped
catalyst 5 : 10-N-AC exhibits a stable catalytic activity at 250 ꢀC with the 1,2-DCE conversion of 85.1% at
180 h. Through DFT calculations, it is suggested that both pyridinic and pyrrolic nitrogen dopants can
adsorb preferentially 1,2-DCE and increase the activity for 1,2-DCE dehydrochlorination at low
temperature, in combination with characterizations of BET, Raman, TG, TPD, XPS, etc. In addition, the
increase of quaternary nitrogen dopants and coking deposition on the catalyst surface can result in the
activity decline. The N-doped activated carbon catalyst provides a promising pathway to produce VCM
through a low-temperature energy-saving process of 1,2-DCE catalytic dehydrochlorination.
Received 9th October 2015
Accepted 28th November 2015
DOI: 10.1039/c5ra20916a
www.rsc.org/advances
deposition in the tubular reactors. Hence, the industrial oper-
ation of 1,2-DCE pyrolysis process is discontinuous, i.e., the unit
1. Introduction
has to be shut down for decoking process every certain intervals
dependent on the type of reactor, feedstock, and operating
conditions.8–11
Vinyl chloride monomer (VCM) is used to produce polyvinyl
chloride (PVC), one of the most popular engineering plastics
widely used worldwide.1,2 There are two main pathways to
manufacture VCM in the PVC industry: the acetylene hydro-
chlorination reaction (the coal-based way) and the thermal
dehydrochlorination of 1,2-dichloroethane (1,2-DCE) (the oil-
based way).3,4 The coal-based reaction way has recently attrac-
ted considerable attention in countries enriched with coal
sources; however, it faces the bottleneck of huge energy
consumption in the production of the raw material of calcium
carbide and the serious environment pollution caused by the
highly toxic and volatile mercuric chloride catalyst used for
acetylene hydrochlorination reaction.5,6 In contrast, the oil-
based way is much cleaner and is the current dominant tech-
nique to produce VCM using the raw material of oil-based
ethylene.7 The thermal dehydrochlorination of 1,2-DCE is per-
CH2ClCH2Cl / C2H3Cl + HCl DH ¼ 72.8 kJ molꢁ1
(1)
Previous literature has indicated that the 1,2-DCE pyrolysis
process proceeds via free-radical reactions, and the coke depo-
sition is resulted partially from the chloroethyl radicals.12–16
Therefore, the catalytic dehydrochlorination of 1,2-DCE has
been studied extensively to explore a dehydrochlorination
process at lower temperature in order to suppress the coke
deposition. For instances, Eberly et al. reported that over the
supported ZnCl2 catalyst the 1,2-DCE dehydrochlorination
showed a 1,2-DCE conversion of 67.3% at 475 ꢀC.12 Okamoto
et al. reported that the activated carbon could be used as the
catalyst for 1,2-DCE dehydrochlorination, exhibiting an initial
1,2-DCE conversion of 98.3% at 380 ꢀC and the total liquid
ꢀ
formed industrially at the temperature of 480–530 C and the
pressure of 1–2 MPa with the reaction expressed in eqn (1),
which provides an 1,2-DCE conversion of about 50–60% and
a selectivity to VCM about 99%8 but suffers from the coking
hourly space velocity (LHSV) of 0.2 hꢁ1 13 Sotowa et al. used the
.
pyridine deposited pitch-based active carbon ber as the 1,2-
DCE dehydrochlorination cataꢀlyst, achieving an initial 1,2-DCE
conversion about 60% at 360 C and the LHSV of 0.5 hꢁ1, but
the catalytic activity lost totally in 200 h owing to the pore
blocking caused by coking.14 Later, Mochida et al. used
polyacrylonitrile-based active carbon bers as the 1,2-DCE
dehydrochlorination catalyst,ꢀ providing an initial 1,2-DCE
conversion about 50% at 360 C and the LHSV of 1.7 hꢁ1, but
the activity lost totally in 50 h due to the coking.15,16 Therefore, it
aSchool of Chemical Engineering & Technology, Tianjin University, Tianjin 300072, P.
R. China. E-mail: zhangjinli@tju.edu.cn; Fax: +86-22-2740-3389; Tel: +86-22-2789-
0643
bSchool of Chemistry & Chemical Engineering, Shihezi University, Xinjiang, Shihezi
832000, P. R. China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c5ra20916a
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is still a challenge to explore an efficient catalyst for the catalytic
dehydrochlorination of 1,2-DCE at low temperature.
2.3 Catalyst activity evaluation
The experimental apparatus to assess the performance of
catalysts is shown in Fig. S1,† with a xed-bed micro-reactor
made of stainless steel tube (i.d. 8 mm). The pipeline was
purged with nitrogen to remove water vapor and air in the
system before each experiment. Liquid 1,2-DCE, with the LHSV
value of 0.2 hꢁ1, was fed into a vaporizing chamber by a micro-
injection pump at a rate of 0.10 mL minꢁ1, where the 1,2-DCE
was vaporized to ow into the reactor with 30 mL catalyst at
a temperature of 250 ꢀC. The effluent of the reactor was
condensed to separate the unreacted 1,2-DCE and then passed
into an absorption bottle containing 1 M NaOH solution (100
mL) to remove HCl, followed by the composition analysis using
Beifen GC-3420A gas chromatograph with a hydrogen ame
ionization detector (FID). The absorption bottle with NaOH
solution was alternated every 60 minutes, the inside solution
was titrated with a HCl standard solution to measure the NaOH
residual in the absorption solution.
Recently, nitrogen-doped graphene supports have been
extensively studied to improve the performance of certain
catalysts for the oxygen reduction or CO oxidation reaction,17–20
since the nitrogen dopants in carbon materials can enhance the
polarity, conductivity and surface hydrophilicity. In particular,
nanocomposites of nitrogen-doped carbon21 and graphitic
carbon nitride22 were studied respectively as non-metallic
catalysts for acetylene hydrochlorination reaction. And
a nitrogen-doped ordered mesoporous carbon was prepared
using resorcinol and formaldehyde as the carbon precursor and
dicyandiamide as the nitrogen precursor, showing the 1,2-DCE
conversion about 80% at 300 ꢀC and 10 h reaction time.23 We are
enlightened to study an effective nitrogen-doped activated
carbon-based catalyst for 1,2-DCE dehydrochlorination with
high activity but low coking deposition at low temperature.
In this article, a series of N-doped coconut activated carbon
catalysts were prepared using melamine as the nitrogen
precursor and assessed the performance for the catalytic
dehydrochlorination of 1,2-DCE, in combination with charac-
terizations of BET, Raman, TG, TPD, XPS, etc. It is indicated that
the N-doped catalyst 5 : 10-N-AC exhibits a stable catalytic
The dehydrochlorination conversion of 1,2-DCE (X) and the
selectivity to VCM (S) were calculated as follows:
ðV1c1 ꢁ V2c2ÞM
X ¼
ꢂ 100%
(2)
Q ꢂ r ꢂ t
ꢀ
activity at 250 C with a 1,2-DCE conversion of 85.1% at 180 h.
S ¼ 4 ꢂ 100%
(3)
where, V1 is the volume of NaOH solution that used for the
absorption (V1 ¼ 0.1 L), c1 is the initial concentration of NaOH
in the solution that used for the absorption (c1 ¼ 1 mol Lꢁ1), t is
the absorption time (t ¼ 60 min), V2 is the volume of the HCl
standard solution that consumed in the titration, c2 is the
concentration of HCl standard solution, M is the molecular
weight of 1,2-DCE, Q is the volume ow rate of 1,2-DCE (Q ¼
0.10 mL minꢁ1), r is the mass density of 1,2-DCE, 4 is the mole
fraction of 1,2-DCE in the gas mixture of which composition is
analyzed by gas chromatograph.
2. Experimental
2.1 Materials
The raw materials of coconut activated carbon, without the
treatment by acidic solution were purchased from Fujian
Sensen Activated Carbon Industry Science and Technology Co.,
Ltd. The reagents including melamine (99.5%), 1,2-dichloro-
ethane (99%), ethanol (99.7%) and sodium hydroxide were
purchased from Tianjin Guangfu Fine Chemical Research
Institute.
2.4 Catalyst characterization
2.2 Catalyst preparation
The BET surface area and total pore volume of catalysts were
ꢀ
The coconut activated carbon with the particle size of 40–60 measured by N2 adsorption–desorption at ꢁ196 C with Quan-
mesh, labeled as AC, was screened to prepare the N-doped tachrome Autosorb Automated Gas Sorption System from USA.
catalyst. The N-doped coconut activated carbon (N-AC) The catalyst samples were degassed at 250 ꢀC for 4 h under
samples were prepared using melamine as the nitrogen vacuum before measurements.
source. A 0.05 g mLꢁ1 melamine ethanol suspension was added
The X-ray photoelectron spectroscopy (XPS) measurements
quantitatively into 50 g AC under stirring, so as to modulate the of the catalysts were performed by pHI5000 Versa Probe spec-
mass ratio of melamine and AC in the range from 0.5 : 10 to trometer, equipped with monochromatised Al Ka X-ray as the
7 : 10. Having been incubated at 60 ꢀC for 12 h and dessicated at excitation source (24.2 W), with an analyzer pass energy of
150 ꢀC for 12 h, the AC–melamine mixture was calcined in 187.85 eV for survey scans and 46.95 eV for detailed elemental
ꢀ
a tubular furnace at 700 C for 4 h under N2 atmosphere with scans. In order to subtract the surface charging effect, binding
a ow rate of 100 mL minꢁ1. The obtained N-doped AC samples energies were referenced to C 1s binding energy of carbon,
were named in terms of the melamine/AC mass ratio, e.g., taken to be 284.6 eV. The XPS spectra were analyzed by the XPS
0.5 : 10-N-AC and 1 : 10-N-AC indicate the N-doped AC prepared peak soware.
with the melamine/AC mass ratio of 0.5 : 10 and 1 : 10,
respectively.
Structural deformations of the catalysts were determined by
Raman spectroscopic analysis using Renishaw from UK, with
As a control, the coconut activated carbon was calcined at a He–Ne laser source of 532 nm wavelength, which was focused
700 ꢀC without the addition of melamine, and the obtained by a 50 times objective lens with a 0.75 numerical aperture value
sample was denoted as 700-AC.
onto an approximately 1 mm2 sample area.
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Thermogravimetric analysis (TGA) of sample was carried out
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to detect coke deposition using a Diamond thermogravimetric
analysis (Perkin Elmer), under air atmosphere at a ow rate of
80 mL minꢁ1. The temperature was increased from 30 to 800 ꢀC
(heating rate, 10 C minꢁ1).
ꢀ
Temperature-programmed desorption (TPD) was performed
using a Micromeritics ASAP 2720 instrument equipped with
a thermal conductivity detector (TCD) in a temperature range of
50–600 ꢀC, with the heating rate of 10 ꢀC minꢁ1 and the gas ow
rate of 25 mL minꢁ1. The catalysts amount used for the test is
tuned to keep all the peak areas of these samples similar.
2.5 Computational details
All density functional calculations were performed using the
hybrid B3LYP24,25 function, as implemented in the Gaussian 09
program package.26 The standard 6-31G ++ (d,p) basis set was
used for C, H, N and Cl atoms. Atomic charges were calculated
using the Mulliken type. No geometric constraints were
assumed in the geometry optimization. The frequencies of all
geometries were calculated at the same level to identify the
nature of the stationary points and obtain the zeropoint-energy
(ZPE) corrections. Hessian calculation is used to characterize
minima (no imaginary frequencies) or transition states (one
imaginary frequency). Intrinsic reaction coordinate (IRC)
calculations27,28 were used to determine if each transition state
linked the correct product with each reactant.
3. Results and discussion
3.1 Catalytic performance of N-doped coconut activated
Fig. 1 1,2-DCE conversion and the selectivity to VCM over different
catalysts. Reaction conditions: temperature ¼ 250 ꢀC, 0.1 MPa, LHSV
carbon
(1,2-DCE) ¼ 0.2 hꢁ1
.
Fig. 1 shows the performance of the prepared catalysts for
catalytic dehydrochlorination of 1,2-DCE reaction at 250 ꢀC and
LHSV (1,2-DCE) ¼ 0.2 hꢁ1. Over the undoped AC and 700-AC,
the 1,2-DCE conversion is as low as 3% with the selectivity to
VCM equal 99.2%. Whereas over N-doped catalysts the 1,2-DCE
conversion is increased signicantly, for instance, the 1,2-DCE
conversion is 55.9% over 0.5 : 10-N-AC, 85.1% over 5 : 10-N-AC,
and 86.4% over 7 : 10-N-AC at 180 h, respectively. Fig. S2† shows
the effect of temperature on the catalytic performance. The 1,2-
DCE conversion of all the catalysts increases with the reaction
temperature. It is worthwhile to mention that over 5 : 10-N-ꢀAC
catalyst the 1,2-DCE conversion is as high as 100% at 260 C,
whereas over undoped AC the total conversion of 1,2-DCE is
achieved at 340 ꢀC. It is indicated that the N-dopant can greatly
improve the catalytic activity of activated carbon for 1,2-DCE
dehydrochlorination, with the activity associated with the
melamine/AC mass ratio.
Table 1 Pore structure parameters of fresh and used catalysts test by
N2 adsorption–desorptiona
SBET (m2 gꢁ1
)
V (cm3 gꢁ1
)
Samples
Fresh
Used
Fresh
Used
AC
700-AC
1156
1188
1089
1053
1000
929
1106
1125
940
914
859
781
752
0.65
0.69
0.61
0.59
0.56
0.53
0.51
0.58
0.64
0.51
0.49
0.44
0.41
0.40
0.5 : 10-N-AC
1 : 10-N-AC
2 : 10-N-AC
5 : 10-N-AC
7 : 10-N-AC
902
a
SBET: surface area; V: total pore volume.
3.2 Catalyst characterization
of melamine during the calcination. In addition, for the
N-doped used catalysts, the surface area and total pore volume
are lower than those of the corresponding fresh, suggesting that
the catalysts pores are partially blocked during the 1,2-DCE
dehydrochlorination reaction. It is worth to mention that the
surface area and total pore volume for 700-AC are higher than
3.2.1 N2 adsorption–desorption. Table 1 shows the pore
structure parameters of the fresh and used catalysts tested by N2
adsorption–desorption, with the adsorption–desorption
isotherms displayed in Fig. S3.† The surface area and total pore
volume of N-doped catalysts are lower than those of AC, prob-
ably due to the pore blocking resulted from the decomposition
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that obtained for AC, as most of the organic compounds lodged coke, which leads to the blocking of some pores. Finally, the
in the micropores can be removed and few pore collapses occur catalysts lose their activity gradually.
at the calcination temperature of 700 C.29,30
3.2.3 Raman spectra. Raman spectra were performed to
ꢀ
3.2.2 Elemental analysis. The element composition of investigate the inuence of N-dopant on the interfacial carbon
individual catalyst was measured by XPS. As listed in Table 2, structure. As shown in Fig. S4,† there are D-band at about
the N content of the N-AC catalysts increases obviously 1340 cmꢁ1 due to the breathing mode of aromatic rings and
compared with the AC and 700-AC, in particular, the N content G-band at about 1600 cmꢁ1 due to the in-plane vibration. The
increases with the increase of melamine/AC mass ratio, indi- intensities of both D-band and G-band increase aer high-
cating that nitrogen atoms have been doped successfully in the temperature treatment, which is attributed to the trans-
activated carbon catalysts aer the calcination with melamine. formation of amorphous structure to graphited structure under
Moreover, the
N content increases with the increasing high temperature. Furthermore, the ID/IG value becomes higher
melamine/AC mass ratio. The O content decreases aer calci- aer N-doping process, as listed in Table 3. The ID/IG value
nation, probably due to the decomposition of the oxygen con- increases as the N-doping content increases, suggesting the
taining groups on the carbon surface and its reaction with defect formation in the carbon plane during the N-doping
melamine. There are trace amounts of Cl in the fresh catalysts process. Defects on the catalyst are benecial to improve the
which may due to the impurities in the activated carbon. catalytic activity through providing more adsorption sites and
Table S1† gives the bulk phase C, H, N contents of catalysts active sites for the reaction.
tested by elemental analyses, which coincide with the XPS
The ID/IG value of used catalysts become lower than the fresh
results. In combination with the catalytic activity in Fig. 1, it is catalysts, indicating that the quaternary nitrogen species
illustrated that the more amount of N dopants in the catalyst increases during the reaction, and the pyridinic and pyrrolic
results in more active sites and then higher catalytic activity.
nitrogen species are partly covered by the coke producing in the
In the case of the used catalysts, the N contents decrease reaction. Moreover, the G-band of the used catalysts
while Cl contents increase, comparing with the fresh catalysts, (1596 cmꢁ1) shis to lower wave numbers compared with the
which is probably due to the polymerization of VCM on the pore fresh catalysts (1603 cmꢁ1), which could also reveal the increase
surface of the catalysts. VCM is easy to polymerize under high of quaternary nitrogen species.
temperature14,15 and most vinyl chloride molecules produced by
3.2.4 TGA analysis. TGA analysis was performed to clarify
1,2-DCE dehydrochlorination could desorb from the pore of the reason for the catalysts deactivation. The mass loss proles
catalysts, while some vinyl chloride molecules polymerize in the of the fresh and used catalysts are shown in Fig. S5.† Below
pores, attach to the active sites and gradually carbonized into 150 ꢀC, the mass loss of the catalysts is due to the loss of water
Table 2 Surface element content of fresh and used catalysts determined by XPS
Surface atomic composition of fresh catalysts (atom%)
Surface atomic composition of used catalysts (atom%)
Samples
C 1s
O 1s
N 1s
Cl 2p
C 1s
O 1s
N 1s
Cl 2p
AC
700-AC
93.0
95.1
93.4
93.0
92.7
91.2
91.4
6.6
4.6
5.0
4.9
4.7
4.4
4.1
0.3
0.2
1.5
2.0
2.5
4.3
4.4
0.1
0.1
0.1
0.1
0.1
0.1
0.1
94.6
94.3
93.1
92.5
92.2
91.8
91.6
4.0
4.3
4.2
4.1
4.1
4.0
3.9
0.2
0.2
1.0
1.5
1.7
1.9
2.1
1.2
1.2
1.7
1.9
2.0
2.3
2.4
0.5 : 10-N-AC
1 : 10-N-AC
2 : 10-N-AC
5 : 10-N-AC
7 : 10-N-AC
Table 3 Raman spectra for the fresh and used catalysts
Fresh catalysts
Used catalysts
Samples
D-band (cmꢁ1
)
G-band (cmꢁ1
)
ID/IG
D-band (cmꢁ1
)
G-band (cmꢁ1
)
ID/IG
AC
700-AC
1334
1332
1339
1340
1340
1340
1341
1599
1595
1602
1603
1603
1604
1604
1.12
1.10
1.19
1.23
1.28
1.32
1.35
1331
1329
1331
1332
1332
1332
1332
1596
1596
1596
1597
1596
1596
1596
1.07
1.07
1.09
1.07
1.02
0.98
0.94
0.5 : 10-N-AC
1 : 10-N-AC
2 : 10-N-AC
5 : 10-N-AC
7 : 10-N-AC
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or other small molecules which are adsorbed on the catalysts
surface. In the temperature range of 150–450 ꢀC, the mass loss
of the used catalysts is higher than that of the fresh catalyst,
which could mainly attribute to the burning of coke deposit on
the catalysts surface. While over 450 ꢀC, the mass loss of the
catalysts is due to the burning of carbon support. The TGA
results indicate that coke deposit on the catalyst pore surface is
one reason for the catalysts deactivation. The amount of coke
deposit on the catalysts during the reaction is listed in Table 4.
As the N content of the catalysts increases, the coke amount
increases accordingly, which is probably due to the stronger
adsorption of VCM on catalysts and the higher VCM concen-
tration in the catalyst bed. The TGA results shows that coking is
one reason for the catalysts deactivation, which coincides with
the N2 adsorption–desorption results.
3.2.5 Adsorptive properties of the reactant and products.
TPD is carried out to study the effect of N-doping process on the
adsorption properties of the reactant and products on the
catalysts. Fig. 2 shows the 1,2-DCE, HCl and VCM TPD proles
on different catalysts and Table S2† lists the desorption amount
of the catalysts. It is known that the desorption area and the
peak temperature can suggest the binding strength of adsorbed
species on catalysts. As shown in Fig. 2, the peak positions of
1,2-DCE, HCl and VCM TPD proles shi to higher temperature
with the increasing N content in the catalysts. For the 1,2-DCE-
TPD, the peak position of the AC is 201 ꢀC, while the peak
position of the 7 : 10-N-AC is 221 ꢀC. For the HCl-TPD, the peak
position of the AC is 218 ꢀC, while the peak position of the
7 : 10-N-AC is 243 ꢀC. For the VCM-TPD, the peak position of the
AC is 153 ꢀC, while the peak position of the 7 : 10-N-AC is
163 ꢀC. The desorption peak areas of these samples are con-
verted to the values at the same sample amount, which are
shown in Table S2.† As shown in Table S2,† the desorption peak
area of 1,2-DCE, HCl and VCM TPD proles also increase as the
N content of the catalysts increase. It is indicated that N-doping
process can enhance the adsorption of both reactant and
products, which is benecial to the 1,2-DCE dehydrochlorina-
tion reaction but meanwhile generate more coke, in accord with
the TG results (Table. 4).
3.2.6 XPS spectra. XPS spectra were analysed to distinguish
the valence states of nitrogen species in the fresh and used
catalysts. Through the deconvolution of XPS N 1s proles
(Fig. S6†), there are three peaks located respectively at 398.5 eV
due to the pyridinic nitrogen, at 400.0 eV due to pyrrolic
Fig. 2 TPD profiles of the catalysts. (a) 1,2-DCE-TPD, (b) HCl-TPD, (c)
VCM-TPD.
nitrogen and at 401.2 eV due to quaternary nitrogen.31,32 Table 5
lists the relative contents and binding energies of nitrogen
species in the fresh and used catalysts. For the fresh N-AC
catalysts, the content of the pyridinic nitrogen (N1) decreases,
the content of pyrrolic nitrogen (N2) increases, while the
content of quaternary nitrogen (N3) is almost unchanged. In the
case of used N-AC catalysts aer 180 h reaction, the contents of
pyridinic nitrogen and pyrrolic nitrogen decrease while the
content of quaternary nitrogen increases obviously, comparing
with the fresh N-AC catalysts. It is indicated that the pyridinic
and pyrrolic nitrogen dopants convert into the quaternary
nitrogen during the reaction, resulting in the activity decline.
Previously, Sotowa et al.16 studied polyacrylonitrile-based active
Table 4 The coke amount of the used catalysts tested by thermog-
ravimetric analysis (TGA)
Samples
Coke amount (wt%)
AC
700-AC
1.17
1.03
3.06
4.05
4.52
5.57
5.74
0.5 : 10-N-AC
1 : 10-N-AC
2 : 10-N-AC
5 : 10-N-AC
7 : 10-N-AC
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Table 5 The relative contents and binding energies of nitrogen
species in the fresh and used catalystsa
Relative contents (area%)
Samples
N1 (398.5 eV)
N2 (400.0 eV)
N3 (401.2 eV)
Fresh 0.5 : 10-N-AC
Fresh 1 : 10-N-AC
Fresh 2 : 10-N-AC
Fresh 5 : 10-N-AC
Fresh 7 : 10-N-AC
Used 0.5 : 10-N-AC
Used 1 : 10-N-AC
Used 2 : 10-N-AC
Used 5 : 10-N-AC
Used 7 : 10-N-AC
70.82
66.92
62.41
61.21
57.31
59.01
45.88
42.32
39.69
33.24
17.87
22.32
26.28
28.21
33.17
17.62
17.34
20.31
21.03
23.77
11.31
10.76
11.31
10.58
9.52
23.37
36.78
37.37
39.28
42.99
a
N1: pyridinic nitrogen, N2: pyrrolic nitrogen, N3: quaternary nitrogen.
carbon ber as a catalyst for dehydrochlorination of 1,2-DCE,
and found that the pyridinic nitrogen content decreased while
the quaternary nitrogen content increased during the deacti-
vation of the catalyst, so they considered that pyridinic nitrogen
was the critical active sites for the reaction. Li et al.21 prepared
nitrogen-doped carbon composites and use it in the acetylene
hydrochlorination reaction, and found that the content of
pyrrolic nitrogen is a key factor for the catalytic activity. In
combination with the XPS spectra (Fig. S6†) and the catalyst
activity (Fig. 1), it is reasonable to consider that the decrease of
pyridinic and pyrrolic nitrogen contents probably results in the
declining of the catalytic activity. It is suggested that the pyr-
idinic and pyrrolic nitrogen species are the critical nitrogen
Fig. 3 The structures of various N species in the N-doped AC cata-
lysts. C: graphene structure; N1: pyridinic nitrogen structure; N2:
pyrrolic nitrogen structure; N3: quaternary nitrogen structure.
Nitrogen, carbon, and hydrogen atoms are depicted in blue, gray and
white, respectively.
species in the 1,2-DCE dehydrochlorination, and the quaternary Table 6 Adsorption energies on different N species in the N-doped
AC catalystsa
nitrogen species have little catalytic activity in the reaction.
Through the deconvolution of XPS Cl 2p proles (Fig. S7†),
Type
Adsorption energy (kcal molꢁ1
)
the different chemical states of Cl are represented by four main
peaks at 197.3 eV, 199.0 eV, 200.2 eV and 201.7 eV, respectively,
corresponded to the Cl 2p3/2 of Clꢁ1, the Cl 2p1/2 of Clꢁ1, the Cl
2p3/2 of C–Cl bond, and the Cl 2p1/2 of C–Cl bond.33,34 Polymer of
vinyl chloride is proved to be produced on the catalysts surface
during the reaction by the C–Cl bond that exists in the used
catalysts. The Clꢁ1 can be attributed to the HCl product that
absorbed on the catalysts. It can be concluded the coke deposits
containing both carbon and chlorine are the cause of
deactivation.
C–1,2-DCE
N1–1,2-DCE
N2–1,2-DCE
N3–1,2-DCE
C–VCM
N1–VCM
N2–VCM
N3–VCM
C–HCl
ꢁ0.08
ꢁ2.21
ꢁ2.73
ꢁ0.82
ꢁ0.47
ꢁ1.27
ꢁ1.55
ꢁ0.43
ꢁ1.65
ꢁ9.57
ꢁ8.88
ꢁ2.31
N1–HCl
N2–HCl
N3–HCl
3.2.7 Density functional theory (DFT) calculations. In order
to study the active sites in the catalysts, three kinds of N dopant
species models were built by Gaussian, with N1, N2 and N3
indicating respectively the pyridinic, the pyrrolic and the
quaternary nitrogen, as shown in Fig. 3. The adsorption ener-
gies of 1,2-DCE, VCM and HCl were calculated on three different
N dopant models, as listed in Table 6. The theoretical calcula-
tion results show that 1,2-DCE can be adsorbed onto the pyr-
idinic nitrogen structure (EAds ¼ ꢁ2.21 kcal molꢁ1) and pyrrolic
nitrogen structure (EAds ¼ ꢁ2.73 kcal molꢁ1), which is obviously
a
C: graphene structure; N1: pyridinic nitrogen structure; N2: pyrrolic
nitrogen structure; N3: quaternary nitrogen structure.
molꢁ1). The stronger adsorption energies of 1,2-DCE on N1 and
N2 indicate that the pyridinic and pyrrolic nitrogen dopant
species play key role in enhancing the activity for 1,2-DCE
dehydrochlorination.
Density functional theory (DFT) calculations were performed
to disclose the reason that the pyridinic and pyrrolic nitrogen
stronger than that on the quaternary nitrogen structure (EAds
¼
ꢁ0.82 kcal molꢁ1) and graphene structure (EAds ¼ ꢁ0.08 kcal
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dopants are more active in the 1,2-DCE dehydrochlorination. substances involved in the reaction path are displayed in
These calculations were used to study the most relevant Fig. S8.† In addition, Table 7 lists the changes of bond length
elementary steps involved in the mechanism of 1,2-DCE dehy- and electron density. As observed in Fig. 4(a) and S8(a),† the
drochlorination, including adsorption sites, cleavage of H–C 1,2-DCE (the rst conguration) rotates its C–C bond and
bond and Cl–C bond, formation of H–Cl bond and C]C bond, translates into a new conguration (the second conguration)
and product desorption. Fig. 4 shows the energy prole for containing higher energy via a transition states (Ts1). Then the
1,2-DCE dehydrochlorination, the detailed geometries of the 1,2-DCE (the second conguration) is adsorbed at the nitrogen
atom on the pyrrolic nitrogen through its H(1) atom near the
nitrogen atom. The bond length of H(1)–C(1) and Cl(2)–C(2) in
˚
˚
1,2-DCE in ads is 1.092 A and 1.816 A (Fig. S8† – Ads), which is
˚
˚
longer than the normal 1.090 A and 1.810 A in free 1,2-DCE
(Fig. S8† – Re0). This result indicates that 1,2-DCE is activated in
the ads system. Therefore, the catalytic performance of N-AC
could be due to N–AC providing activation sites for 1,2-DCE.
This also explains why the 1,2-DCE conversion rate increases
with increasing nitrogen content in the N–AC catalyst. In Ts2,
both of the H(1)–C(1) and Cl(2)–C(2) bond length become longer
than that in Ads further, and the electron density of H(1), Cl(2),
N increases from 0.209 au, –0.045 au, –0.090 au (Ads) to 0.551
au, –0.035 au, 0.093 au (Ts2). Thus in this step, the H(1)–C(1)
and Cl(2)–C(2) bond break, forming the C]C bond and HCl
molecule, which has the highest energy barrier of 31.23 kcal
molꢁ1 and is the rate-controlling step. The nal step is the
desorption of the VCM and HCl molecules, and the desorption
energy is 10.58 kcal molꢁ1. The process of the catalytic reaction
on the pyridinic nitrogen structure shown in Fig. 4(b) and S8(b)†
is similar with that on the pyrrolic nitrogen structure, which
also has a similar energy barrier of 33.19 kcal molꢁ1
.
4. Conclusions
N-doping method is an effective way to improve the catalytic
activity of activated carbon on 1,2-DCE dehydrochlorination.
The 1,2-DCE conversion increases obviously aer N-doping
process, and the VCM selectivity also increases slightly. The
best catalytic performance is obtained over 5 : 10-N-AC, with an
Fig. 4 Reaction energy diagram of the substances involved in the
reaction path. (a): For pyrrolic nitrogen structure; (b): for pyridinic initial 1,2-DCE conversion of 85.1% and VMC selectivity of
nitrogen structure. The path contains: reactant of 1,2-DCE in first
configuration (Re), transition state 1 (Ts1), reactant of 1,2-DCE in
second configuration (Re0), adsorbed reactants (Ads), transition state 2
99.4%. All the N-doped coconut activated carbon catalysts show
good stability, with an 1,2-DCE conversion decline below 6.3%
in 180 h. Aer N-doping process, the adsorption energy between
(Ts2), co-adsorbed products (Co-ads), product (Pr). Chlorine, nitrogen,
1,2-DCE and activated carbon increases, the electron density
and transfer property of the activated carbon is enhanced, and
carbon, and hydrogen atoms are depicted in green, blue, gray and
white, respectively.
Table 7 The changes of bond length and electron density corresponding to the reaction path in Fig. 4
Path
Type
Re0
Ads
1.816
1.092
ꢁ0.090
0.209
Ts2
1.845
1.512
0.093
Path on pyrrolic nitrogen structure
C–Cl bond length
C–H bond length
N electron density
H electron density
Cl electron density
C–Cl bond length
C–H bond length
N electron density
H electron density
Cl electron density
1.810
1.090
ꢁ0.140
0.208
0.551
ꢁ0.022
ꢁ0.045
ꢁ0.035
Path on pyridinic nitrogen structure
1.810
1.815
1.859
1.090
1.092
1.662
0.045
0.519
ꢁ0.054
ꢁ0.140
ꢁ0.123
0.208
0.165
ꢁ0.022
ꢁ0.030
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This work was supported by the National Basic Research
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Program for Changjiang Scholars and Innovative Research
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