Effects of nitrogen-dopants on Ru-supported catalysts for acetylene hydrochlorination

Article abstract of DOI:10.1039/c5ra23112d

A series of N-doped spherical active carbons were synthesized via the pyrolysis of melamine in activated carbon, and used as a support to prepare Ru-based catalysts for an acetylene hydrochlorination reaction. The catalytic performance assessments indicate that the N-doped carbon support can increase greatly the activity and the stability of Ru-based catalysts. The optimal activity is achieved over Ru/SAC-N700, with an acetylene conversion of 99.8% under the conditions of 170°C, C₂H₂ gas hour space velocity (GHSV) of 180 h^-1, a feed volume ratio of V(HCl)/V(C₂H₂) of 1.1 after 30 h. Using characterizations of BET, FT-IR, XPS, TPR, TPD, TG, etc., it is illustrated that N-dopants can increase the dispersion of Ru elements, enhance the adsorption of reactants and the desorption of the product, and reduce significantly the coke deposition, consequently resulting in higher catalytic activity of Ru/SAC-N700. It is suggested that the pyridine-nitrogen plays an important role in augmenting the catalytic activity of Ru-supported catalysts.

Full text of DOI:10.1039/c5ra23112d

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Page 1 of 22
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DOI: 10.1039/C5RA23112D
• Effects of nitrogenꢀdopants on Ruꢀsupported catalysts for
acetylene hydrochlorination
Lijun Hou,a Jinli Zhang,a Yanfeng Pu*b and Wei Li*a
a School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, P.
R. China. Eꢀmail: liwei@tju.edu.cn
b
State Key Laboratory of Coal Conversation, Institute of Coal Chemistry, Chinese
Academy of Sciences, Taiyuan 030001, P. R. China. Eꢀmail: puyanfeng@sxicc.ac.cn
1

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Abstract
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A series of Nꢀdoped spherical active carbon were synthesized via the pyrolysis of
melamine in activated carbon, and used as the support to prepare Ruꢀbased catalysts for
acetylene hydrochlorination reaction. The catalytic performance assessments indicate that
the Nꢀdoped carbon support can increase greatly the activity and the stability of Ruꢀbased
catalysts. The optimal activity is achieved over Ru/SACꢀN700, with the acetylene
conversion of 99.8% under the conditions of 170 °C, C2H2 gas hour space velocity
(GHSV) of 180 hꢀ1, the feed volume ratio of V(HCl)/V(C2H2) of 1.1 after 30 h. Using
characterizations of BET, FTꢀIR, XPS, TPR, TPD, TG, etc., it is illustrated that Nꢀdopants
can increase the dispersion of Ru elements, enhance the adsorption of reactants and the
desorption of the product, and reduce significantly the coke deposition, consequently
resulting in higher catalytic activity of Ru/SACꢀN700. It is suggested that the
pyridineꢀnitrogen plays an important role in augment of catalytic activity of Ruꢀsupported
catalysts.
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15 Keywords: acetylene hydrochlorination, Nꢀdoped carbon, Ru catalysts
16 1. Introduction
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Acetylene hydrochlorination reaction is the dominant pathway to produce vinyl
chloride monomer (VCM) in polyvinyl chloride (PVC) industry of China, which
currently utilizes carbonꢀsupported HgCl2 as the catalyst. However, mercuric chloride is
highly toxicity and tends to sublimate, resulting in the severe environmental pollution
problems.1 In 2013, more than 140 countries signed "the Minamata Convention on
Mercury" so as to inhibit the trade and application of mercury over the world.2 Hence, it
is urgent to explore environmentalꢀbenigh nonꢀmercury catalysts for acetylene
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hydrochlorination.
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Nonꢀmercuric noble metallic catalysts, involving Au,3ꢀ8 Pd,9 Pt,10, 11 etc., have been
studied for the acetylene hydrochlorination, following the pioneer work of Hutchings12 on
Au catalysts. Taking into account the cost of the catalyst, the Ruꢀbased catalyst is
considered as a promising candidate for the nonꢀmercury catalyst of acetylene
hydrochlorination.13ꢀ18 For instance, the bimetallic Ru1Co3/SAC catalyst showed an
acetylene conversion of 95% after 48 h under the condition of 170 °C and C2H2 GHSV of
180 hꢀ1.17 The CuꢀRu catalyst supported on carbon nanotubes (Cu400Ru/MWCNTs)
showed an acetylene conversion of 51.6% at the conditions of 180 °C, V(HCl)/V(C2H2) =
1.2 and GHSV(C2H2) = 180 hꢀ1.16 However, it is essential to improve further the catalytic
activity for rutheniumꢀbased catalysts, so as to develop an efficient nonꢀmercury catalyst
for acetylene hydrochlorination in the view of industrial application.
Heteroatomꢀdoped carbon materials have shown promising properties to prepare
electrodes in supercapacitors19, electrocatalysts for the oxygen reduction reaction (ORR)
22, 23
in fuel cells.20,21 and cathode materials for lithium ion batteries,
etc. In particular,
Bao and coworkers24 prepared a nanocomposite of nitrogenꢀdoped carbon, which showed
catalytic activity for acetylene hydrochlorination, with the acetylene conversion of 80%
25
at 200 °C and a space velocity of 30 hꢀ1. Dai et al. prepared B, Nꢀdoped graphene
catalysts, which showed the initial acetylene conversion about 95% under the conditions
of 150 °C, C2H2 GHSV of 36 hꢀ1. Zhou et al.26 reported a Cuꢀbased catalyst supported
Nꢀdoped carbon nanotubes, showing the initial acetylene conversion about 45.8% under
the conditions of 180 °C and C2H2 GHSV of 180 hꢀ1. Recently, Zhao et al. reported that
the Nꢀdoped carbon support can increase the activity of Ruꢀbased catalysts for the
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reaction of acetylene and 1, 2ꢀdichloroethane (EDC) at 250 °C and total liquid hourly
space velocity of EDC 0.2 hꢀ1.27 Therefore, we are inspired to study the effect of Nꢀdoped
carbon supports on the activity of Ruꢀbased catalysts for acetylene hydrochlorination.
In this article, we prepared a serious of Ruꢀbased catalysts with the support of
Nꢀdoped spherical active carbon synthesized via the pyrolysis of melamine in activated
carbon, and assessed the catalytic activity for acetylene hydrochlorination. The results
show that with the Nꢀdoped carbon support, the activity and the stability of Ruꢀbased
catalysts are increased greatly. The influence of Nꢀdopant on the structural, adsorption
properties and active species of Ruꢀbased catalysts were characterized by BET, FTꢀIR,
XPS, TPR, TPD, TG, etc.
57 2. Experimental
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2.1 Catalyst preparation
In a typical experiment, the pitchꢀbased spherical activated carbon (SAC, 10 g,
20–40 mesh) was mixed with melamine (5 g) in deionized water (50 mL) at 343 K under
stirring for 18 h. After filtration, the mixture was dried at 393 K overnight under vacuum,
and then experienced the carbonization at the temperature of 600, 700 or 800 °C for 1 h
in a quartz tube under N2 atmosphere at a flow of 100 cm3 minꢀ1 with a heating rate of 10
K minꢀ1. The obtained Nꢀdoped carbon samples were denoted as SACꢀN600, SACꢀN700
and SACꢀN800, respectively.
The Ruꢀbased catalysts (Ru/SACꢀN) were prepared via a wetness impregnation
technique using the Nꢀdoped carbon as the support. A solution of RuCl3 (209.5 mg) in
deionized water (12 mL) was added dropwise to the Nꢀdoped carbon (10 g) under stirring
at room temperature, followed by aging at 60 °C for 12 h. The product was then dried at
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150 °C for 12 h and then assessed the catalytic performance for acetylene
hydrochlorination. The obtained samples were denoted as Ru/SACꢀN600, Ru/SACꢀN700
and Ru/SACꢀN800 with the support of SACꢀN600, SACꢀN700 and SACꢀN800,
respectively.
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As a control, undoped activated carbon was also used as the support to prepare the
Ruꢀbased catalyst via the similar technique, and denoted as Ru/SAC.
2.2 Catalyst tests
Catalysts were assessed in a fixedꢀbed stainless steel microꢀreactor (i.d. of 10 mm)
for acetylene hydrochlorination. The temperature of the reactor was regulated by
CKWꢀ1100 temperature controller produced by Chaoyang automation instrument factory
(Beijing, China). The pipeline was purged with nitrogen before the reaction to remove
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water and air in the system, then acetylene (15 mL min− ) and hydrogen chloride (16.5
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mL min− ) were fed through filters to remove trace impurities and via calibrated mass
flow controllers into the heated reactor containing the catalyst (5 mL), operated at
1
170 °C under the atmosphere pressure with a GHSV (C2H2) of 180 h− . The effluent from
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the reactor was passed through a 10% sodium hydroxide aqueous solution to absorb the
unreacted hydrogen chloride, and then analyzed by gas chromatograph ( GCꢀ3420 by
Beijing BeifenꢀRuili Analytical Instrument Co. Ltd. ), equipped with FID detector. The
acetylene conversion (XA) and the selectivity to VCM (SVC) were calculated by the
equations (1) and (2), respectively.
ꢄ∅
ꢂꢃ
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ꢀꢁ = ∅
ꢂ × 100%
(1)
(2)
∅
ꢂꢃ
∅
ꢈꢉ
ꢅꢆꢇ
=
ꢊ
ꢋꢄ∅
ꢌ × 100%
ꢂ
5

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Where ∅ꢁꢍ = the volume fraction of acetylene in the feed and ∅ꢁ = the volume
fraction of remaining acetylene in the effluent from the reactor, respectively. ∅ꢆꢇ is the
volume fraction of vinyl chloride in the effluent from the reactor.
2.3. Catalyst characterization
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The pore size distribution and specific surface areas of the catalysts were analyzed
by ASAP 2020C surface area and porosity analyzer (Micromeritics Instrument
Corporation, USA), degassing for 6 h at 423 K, then analyzed with liquid nitrogen
adsorption at 77 K.
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The Fourier transform infrared spectroscopy (FTꢀIR) was performed using a Bruker
Vertex 70 FTꢀIR spectrophotometer equipped with a MCT detector at a resolution of 4
cmꢀ1.
Raman spectra were measured with Renishaw inVia reflex, with the resolution of 4
cmꢀ1 (632.8 nm).
Thermogravimetric analysis (TGA) was carried out with TGꢀDTG simultaneous
○
R
thermal analyzer (NETZSCH STA 449F3 Jupiter , Germany) under nitrogen atmosphere
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at a flow rate of 100 mL minꢀ1 with a heating rate of 10 °C minꢀ1.
Xꢀray photoelectron spectra (XPS) were recorded on a PHIꢀ5000 Versa probe IIꢀX
System. The binding energy was calibrated with respect to the C 1s level 284.80 eV of
aliphatic carbon.
Transmission electron microscopy (TEM) was conducted using a JEM2100F TEM
and an FEI Titan 80ꢀ300 TEM/STEM, equipped with CEOS spherical aberration
corrector. The samples were dispersed in ethanol and supported on carbonꢀfilmꢀcoated
copper grids before the characterization of TEM.
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Temperatureꢀprogrammed reduction (TPR) experiments were performed using
AutoChem 2910 to determine the reduction behavior of the catalysts. For each
experiment, the sample was heated from 35 °C to 800 °C with a heating rate of 10 °C
minꢀ1. The flow gas mixture is 5% H2/Ar with a flow rate of 20 mL/min.
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Temperature programmed desorption (TPD) was measured by an AutoChem BET
TPR/TPD (Quantachrome Instruments AMIꢀ90). The weight of the each sample was
fixed at 130 mg. For C2H2ꢀTPD and HClꢀTPD profiles, the samples were first preꢀtreated
under C2H2 and HCl atmosphere respectively for 4 h at reaction temperature (170 °C),
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then pure helium was pass through the samples for 30 min and the desorption profiles
were recorded from 50 °C to 700 °C at a heating rate of 10 °C minꢀ1. Similarly, the
VCMꢀTPD profiles were recorded, except the pretreatment condition of VCM is at
100 °C for 1 h. For COꢀTPD, the samples were first reduced at 400 °C for 2 h in 10%
H2/Ar followed by cooling to room temperature, and then 250 uL pulse of 10% CO/Ar
were introduced, and the CO uptake profile was measured by a TCD detector.
129 3. Results and discussion
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3.1 N-doped carbon supports
BET measurements were performed to investigate the physical structure variation of
carbon resulted by Nꢀdoping. Table 1 lists the specific surface area, pore volume and
average pore diameter of SAC, SACꢀN600, SACꢀN700 and SACꢀN800. The N2
adsorptionꢀdesorption isotherms (Fig. S1a) exhibit the type Ⅰ adsorption curves,
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corresponding to the microporous structure. The pore size distributions of Nꢀdoped
carbon supports are centered at 5Å according to HK model (Fig. S1b), approximate to
that of SAC. The Nꢀdoped carbon samples show higher surface area and total pore
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volume that of SAC, and SACꢀN700 exhibits the highest surface area (1325 m2 gꢀ1) and
total pore volume of 0.69 cm3 gꢀ1. Previously, Wang et al.28 adopted ammonia to treat
mesoporous carbon in order to prepare the catalytic material for oxygen reduction
reaction, and obtained Nꢀdoped carbon with increased surface area (2021 m2/g), much
higher than the undoped carbon (658 m2/g). On the other hand, Saiter and coworkers29
reported that ammonia could be generated during the decomposition of melamine at high
temperature. Therefore, it is reasonable to consider that the intermediate of ammonia,
decomposed from melamine, can etch the porous carbon so as to make the surface area
and the pore volume of carbon increased.
Table 1 Pore structure parameters of the supports
Samples
SAC
SACꢀN600
SACꢀN700
SACꢀN800
SBET (m2 gꢀ1)
1066
Pore volume (cm3 gꢀ1)
Pore diameter (Å)
0.62
0.63
0.69
0.64
5.02
4.82
4.84
4.80
1144
1325
1237
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FTꢀIR spectra were measured to characterize the surface functional groups of
undoped and Nꢀdoped SAC. As shown in Fig. 1, SAC has major bands at 1625 cmꢀ1 and
3423 cmꢀ1, which are respectively attributed to the C=C and OꢀH stretching vibrations,
besides the bands at 1037 cmꢀ1 and 1400 cmꢀ1, which are due to the stretching of CꢀO and
the stretching vibration of the carboxyl groups.30ꢀ32 For Nꢀdoped SAC, the intensities of
bands at 1037 cmꢀ1 and 3423 cmꢀ1 are weaker than those of SAC. For SACꢀN700 and
SACꢀN800, a small new band is observed at 1590 cmꢀ1, which is attributed to the
33, 34
stretching of C=N groups,
together with another band at 3120 cmꢀ1, which is
35ꢀ37
attributed to the NꢀH and/or NH2 groups.
It is suggested that the structure of the
Nꢀdoped SAC consists of more nitrogenꢀcontaining functional groups and less content of
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oxygen functional groups, comparing with undoped SAC.
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Fig. 1 FTꢀIR spectra of different support samples.
Table 2 Elemental compositions on the surface of the samples determined by XPS analysis
Element composition (%)
Samples
C
N
O
SAC
96.2
96.0
94.8
94.9
0
3.8
2.6
3.2
3.3
SACꢀN600
SACꢀN700
SACꢀN800
1.4
2.0
1.8
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Table 3 Relative contents and binding energies of nitrogen species of the Nꢀdoped samples
Area%, binding energy(eV)
Samples
NP
NPYR
NQ
N–X
SAC
ꢀ
ꢀ
ꢀ
ꢀ
SACꢀN600
SACꢀN700
SACꢀN800
39.6 (398.2)
56.9 (398.7)
45.0 (398.4)
31.2 (400.0)
8.8 (400.2)
36.9 (400.0)
20.8 (401.4)
30.9 (401.2)
6.7 (401.4)
8.4 (402.7)
3.4 (402.5)
11.4 (402.4)
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XPS spectra of different carbon samples were analyzed to distinguish the elemental
compositions and the species of nitrogen dopants, comparing with the undoped SAC
sample. As listed in Table 2, the elemental compositions of the undoped SAC sample are
96.2% carbon and 3.8% oxygen. While for the Nꢀdoped samples, there are nitrogen
component, the nitrogen content equals respectively 1.4% for SACꢀN600, 2.0% for
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SACꢀN700 and 1.8% for SACꢀN800. Fig. S2 displays the deconvolution of the N 1s XPS
spectra, indicating the existence of four kinds of nitrogen species including the
pyridineꢀnitrogen (NP) near 398.4 eV, the pyrroleꢀnitrogen (NPYR) at 400.1 eV, the
quaternaryꢀnitrogen (NQ) at 401.3 and the nitrogen oxides (N–X) near 402.5 eV. Table 3
lists the content of each nitrogen species in the Nꢀdoped SAC. It is clear that the
pyridineꢀnitrogen (NP) is the dominant nitrogen species in SACꢀN600, SACꢀN700, and
SACꢀN800, while the N content is dependent on the calcination temperature. The
SACꢀN700 sample has the highest amount of pyridineꢀnitrogen (NP, 56.9%) but the least
amount of pyrroleꢀnitrogen (NPYR, 8.8%).
Previously, Wong et al.38 prepared the nitrogenꢀdoped graphene (nG) via pyrolysis
of graphene oxide at the temperature from 400 to 1000 °C using the nitrogenꢀprecursor of
melamine, and the obtained nG consisting of the quaternary N (5%), carbon nitride and
oxygenated N. In addition, Xia et al.39 suggested that melamine molecules adsorbed onto
graphite oxide (GO) surfaces could be condensed into carbon nitride with increasing
temperature, and the removal process of oxygen species at high temperature could
provide active sites for nitrogen doping into graphene frameworks.
Saiter and coworkers29 reported that ammonia could be generated during the
decomposition of melamine at high temperature. The possible mechanism for doping N
in carbon is associated with the oxygenꢀcontaining groups in the undoped SAC sample
(Table 2). The intermediates decomposed from melamine, including ammonia, can react
with the oxygenꢀcontaining groups in the SAC to make N dopants intercalated into the
carbon materials. It deserves to study the mechanism for Nꢀdoping in this process in our
future work. Table S2 lists the elemental composition of different SAC samples
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determined by elemental analysis (Vario Micro, Elementar). It is indicated that the bulky
element composition of carbon samples are approximate to those determined by XPS
(Table 2). Therefore, the N dopants are located uniformly in the carbon supports, rather
than on the surface.
3.2 N-doped carbon supported Ru catalysts
Fig. 2 displays the catalytic performance of the supported Ru catalysts and the
individual supports. The SACꢀN700 support shows the initial acetylene conversion of
39.0% and decreases to 32.0% in 30 h, much higher than those over the support SAC,
suggesting that Nꢀdopants can enhance the catalytic activity of the carbon support toward
acetylene hydrochlorination. Over the Ru catalyst supported on SAC (Ru/SAC), the
initial acetylene conversion is 89.4%, reducing to 85.2% in 30 h. While using the
Nꢀdoped SAC as the support, the Ru catalysts display significant increase of catalytic
activity. The initial acetylene conversion is 96.1% over Ru/SACꢀN600, 99.5% over
Ru/SACꢀN700, and 98.5% over Ru/SACꢀN800, respectively. In particular, after 30 h, the
acetylene conversion over Ru/SACꢀN700 is 99.8%, suggesting a superior catalytic
stability. As for the selectivity to VCM, it is 99.5% over Ru/SAC, 99.8% over
Ru/SACꢀN600, 99.9% over Ru/SACꢀN700 and 99.8% over Ru/SACꢀN800, respectively.
It is illustrated that Nꢀdoped SAC supports indeed improve the activity and stability of
the Ru catalysts.
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Fig. 2 The conversion of acetylene (a) and selectivity to VCM (b) over SAC, SACꢀN700,
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Ru/SAC, Ru/SACꢀN600, Ru/SACꢀN700 and Ru/SACꢀN800. Reaction conditions: T=170 °C
V(HCl)/V(C2H2)=1.1:1, GHSV(C2H2)=180 hꢀ1.
,
Table 4 lists the dispersion of Ru elements calculated on the basis of CO adsorption.
The dispersion is 37.5% for the Ru/SAC. For the Ru/SACꢀN catalysts, the Ru dispersion
is higher, with the highest dispersion of 49.9% corresponding to the catalyst
Ru/SACꢀN700, followed by Ru/SACꢀN800 (49.0%) and Ru/SACꢀN600 (48.2%). The
results illustrate that Nꢀdopants can facilitate the dispersion of Ru nanoparticles, probably
due to the more defect sites caused by N dopants, which can anchor stably the active Ru
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species.
Table 4 Ru dispersion calculated according to CO uptake
Samples
CO uptake (µmol CO/g)
Ru dispersion (%)
Ru/SAC
37.1
47.7
49.4
48.5
37.5
48.2
49.9
49.0
Ru/SACꢀN600
Ru/SACꢀN700
Ru/SACꢀN800
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Fig. 3 displays the TEM images of the fresh and used catalysts. There are some
small black dots in the image, which are highly dispersed Ru particles on the support.
The average size is about 1.40 nm and 1.24 nm for the fresh Ru/SAC and fresh
Ru/SACꢀN700 catalysts, respectively. This result indicates that the N dopants improve
the Ru dispersion of catalysts, which is in accord with the COꢀTPD results (Table 4).
While for the used Ru/SAC and used Ru/SACꢀN700, obvious larger black dots appear in
the images, suggesting somewhat aggregation or coke deposition during the reaction. The
average particle size of the used Ru/SAC is 2.18 nm, much larger than that of the used
Ru/SACꢀN700 (1.63 nm), indicating that N dopants can inhibit the aggregation or coke
deposition during the reaction.
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Fig. 3 TEM images of fresh and used catalysts.
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The deconvolution of XPS Ru 3p3/2 spectra performed to discriminate Ru species
(Fig. S4 and Fig. S5). As listed in Table 5, there are four kinds of Ru species in Ruꢀbased
catalysts, involving the metallic Ru (461.4±0.4 eV); RuCl3 (463.0±0.3 eV); RuO2
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(464.5±0.4 eV) and RuOx (465.7ꢀ467.2 eV).40ꢀ42 The contents of Ru species are
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associated with the kind of support. For the fresh Ru/SAC, the dominant is RuCl3
(63.0%), followed by RuOx (16.5%)> RuO2 (13.9%)> metallic Ru (6.6%). Whereas for
the Ru catalysts with the Nꢀdoped supports, the content of RuO2 is much higher than that
of Ru/SAC, but the content of RuCl3 is much lower than that of Ru/SAC. In particular,
the fresh Ru/SACꢀN700 shows the highest content of RuO2 (53.0%) but the lowest of
RuCl3 (15.4%). After experiencing 30 h of reaction, the content of RuO2 and RuOx
decrease to some extent and the content of metallic Ru increased. It is worthwhile to note
that the RuO2 species in the used Ru/SACꢀN700 is still the highest.
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Table 5 The relative contents and binding energies of ruthenium species in the fresh and used
catalysts
Samples
fresh Ru/SAC
fresh Ru/SACꢀN600
fresh Ru/SACꢀN700
fresh Ru/SACꢀN800
used Ru/SAC
used Ru/SACꢀN600
used Ru/SACꢀN700
used Ru/SACꢀN800
Area%, binding energy (eV)
Ru
RuCl3
RuO2
RuOx
6.6 (461.8)
10.1 (461.3)
20.6 (461.4)
13.0 (461.1)
15.9 (461.7)
21.2 (461.6)
28.6 (461.6)
14.2 (461.4)
63.0 (463.2)
47.1 (463.0)
15.4 (463.0)
25.3 (462.9)
57.2 (463.4)
50.3 (463.5)
23.0 (463.1)
29.6 (463.2)
13.9 (464.8)
28.5 (464.4)
53.0 (464.6)
29.4 (464.0)
11.5 (464.8)
18.0 (464.3)
39.3 (414.5)
27.8 (464.9)
16.5 (466.1)
14.3 (465.9)
11.0 (466.8)
32.3 (466.8)
15.4 (466.1)
10.5 (466.3)
9.1 (466.7)
28.4 (466.3)
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Moreover, Raman spectra were measured to confirm the existence of RuO2. As
shown in Fig. 4, for the Ru catalysts with the Nꢀdoped supports, there are two sharp
bands at 504 and 618 cmꢀ1 corresponding to the species of RuO2,43 and the band intensity
increases in the order of Ru/SACꢀN600<Ru/SACꢀN800< Ru/SACꢀN700. In contrast, for
the catalyst Ru/SAC, the bands due to RuO2 are relative small. It is illustrated that the
Nꢀdoped supports can greatly increase the amount of the component RuO2 in Ru catalysts,
in accord with the results in Table 5.
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Fig. 4 Raman spectra of the fresh catalysts
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According to our previous work, the pyridinic Nꢀdoped graphene support is
beneficial to enhance electron transfers between the support and the active gold species
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so as to stabilize the active species of Au2Cl6, and then increase the longꢀterm stability of
Auꢀbased catalysts in acetylene hydrochlorination.44 Additionally, the DFT calculations
indicated that the species RuCl3 could not activate efficiently the reactants of acetylene
and hydrogen chloride.45 In combination with the activity of Ru catalysts (Fig. 2), the
variation of Ru species on Nꢀdoped supports (Table 5) and the contents of N species in
the supports (Table 3), it is reasonable to conclude that the pyridineꢀnitrogen (NP) plays
an important role in augment of the catalytic activity of Ruꢀsupported catalysts.
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Fig. 5 H2 TPR profiles of fresh catalysts: (a) Ru/SAC, (b) Ru/SACꢀN600, (c) Ru/SACꢀN700, (d)
Ru/SACꢀN800
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Fig. 5 displays the TPR profiles of the fresh Ruꢀbased catalysts. The broad peak in
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the range of 400ꢀ700 °C is attributed to the reduction of functional groups on the support.
For the fresh Ru/SAC catalyst, it exhibits the peak at 183 °C together with a shoulder at
291 °C, which is attributed to the reduction of RuCl3 and RuO2, respectively.13, 14, 17, 46
For the Ru/SACꢀN catalysts, the two peaks shift to higher temperature and the peak
intensity around 295 °C becomes stronger, with the maximum peak intensity
corresponding to Ru/SACꢀN700. The results suggesting that the Nꢀdopants strengthen the
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interaction between the supports and RuO2 and RuCl3 species.
TPD is an effective technique to study the effects of Nꢀdopants on the adsorption
properties of reactants and the product on the Ruꢀbased catalysts. Fig. 6 displays the C2H2,
HCl and C2H3Cl TPD profiles of the Ru/SAC and Ru/SACꢀN700. The peak area
indicates the adsorption capacity and the desorption temperature reflects the adsorption
strength. As shown in Fig. 6a, the HCl desorption temperature is 150 °C on Ru/SAC,
159 °C on Ru/SACꢀN700, and the adsorption capacity of HCl on Ru/SACꢀN700 is much
larger than that on Ru/SAC. For another reactant C2H2, the catalyst Ru/SACꢀN700 also
shows larger adsorption capacity, comparing with Ru/SAC (Fig. 6b). The trend of TPD
profile for the product C2H3Cl is opposite with that of C2H2 and HCl, i.e., the adsorption
capacity for C2H3Cl on Ru/SACꢀN700 is smaller than that on Ru/SAC (Fig. 6c).
Therefore, Nꢀdopants can enhance the adsorption of reactants and the desorption of the
product, consequently results in higher catalytic activity of Ru/SACꢀN700.
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Fig. 6 TPD profiles of the catalysts: (a) HClꢀTPD, (b) C2H2ꢀTPD, (c) C2H3ClꢀTPD
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Further, TG was used to evaluate the degree of coke deposition on the surface of
Ruꢀbased catalysts. As shown in Fig. S6, both fresh and used catalysts have a slight
weight loss before 100 °C, owing to water desorption. In the range of 100ꢀ400 °C，the
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used catalysts exhibit an obvious weight loss (5.0%). When the temperature exceeds
400 °C, the fresh and used catalysts rapidly decrease in weight because the combustion of
activated carbon. Taking into account the carbon support can lose its weight by reacting
with oxygen in air to produce CO2, the coke deposition is calculated by the difference of
weight loss of the fresh and used catalyst in the temperature range of 100ꢀ400 °C.47ꢀ49
Based on this method, the coke deposition of the catalysts is calculated and listed in
Table 6.
As listed in Table 6, the Ru/SACꢀN700 catalyst has the least coke deposition of
2.6%. The amount of coke deposition increases in the order: Ru/SACꢀN700 (2.6%)＜
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Ru/SACꢀN800 (3.3%) ＜ Ru/SACꢀN600 (3.6%) ＜ Ru/SAC (4.3%). This result is
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consistent with the order of catalytic activity of Ruꢀsupported catalysts. Combining with
the TPD profiles (Fig. 6), it is suggested that the larger adsorption capacities of HCl and
C2H2, as well as the easier desorption of C2H3Cl result in the significant reduction of coke
deposition on Ru catalysts with the Nꢀdoped support.
Table 6 Coke deposition of the catalysts
Samples
Amount of coke deposition (%)
Ru/SAC
4.3
3.6
2.6
3.3
Ru/SACꢀN600
Ru/SACꢀN700
Ru/SACꢀN800
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318 4. Conclusion
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Ruꢀbased catalysts with the support of Nꢀdoped spherical active carbon were
prepared and assessed for acetylene hydrochlorination. The optimal catalyst
Ru/SACꢀN700 shows the acetylene conversion of 99.8% at the conditions of 170 °C,
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C2H2 GHSV 180 hꢀ1 and V(HCl):V(C2H2)=1.1:1. It is indicated that the pyridineꢀnitrogen
plays an important role in augment of the catalytic activity of Ruꢀbased catalysts.
Nꢀdopants can greatly increase the amount of the component RuO2 in Ru catalysts,
enhance the adsorption of reactants and the desorption of the product, and reduce
significantly the coke deposition, consequently resulting in higher catalytic activity of
Ru/SACꢀN700. These results provide a potential route to explore high efficient Ruꢀbased
catalysts for acetylene hydrochlorination.
329
Acknowledgments
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This work was supported by the Special Funds for the Major State Research
Program of China (No. 2012CB720302), the NSFC (21176174).
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