A novel high-stability Au(III)/Schiff-based catalyst for acetylene hydrochlorination reaction

Article abstract of DOI:10.1016/j.catcom.2014.05.024

Herein, we reported on the application of an Au(III)/Schiff-based catalyst in acetylene hydrochlorination reaction. The [AuCl₂(phen)]Cl catalyst exhibited excellent stability, and acetylene conversion was maintained at > 90% after 40 h of operation. The excellent stability of the [AuCl ₂(phen)]Cl catalyst was attributed to the presence of the 1,10-phenanthroline ligand that partially inhibited the reduction of the Au ^{3 +} active component.

Full text of DOI:10.1016/j.catcom.2014.05.024

A novel high-stability Au (III)/Schiff-based catalyst for acetylene hydrochlo-
rination reaction
Chaofeng Huang, Mingyuan Zhu, Lihua Kang, Bin Dai
PII:
DOI:
Reference:
S1566-7367(14)00205-2
doi: 10.1016/j.catcom.2014.05.024
CATCOM 3918
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Catalysis Communications
Received date:
Revised date:
Accepted date:
3 March 2014
24 May 2014
27 May 2014
Please cite this article as: Chaofeng Huang, Mingyuan Zhu, Lihua Kang, Bin Dai, A
novel high-stability Au (III)/Schiff-based catalyst for acetylene hydrochlorination reac-
tion, Catalysis Communications (2014), doi: 10.1016/j.catcom.2014.05.024
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ACCEPTED MANUSCRIPT
A novel high-stability Au (III)/Schiff-based catalyst for acetylene
hydrochlorination reaction
Chaofeng Huang a, Mingyuan Zhu a,b  , Lihua Kang a,b, and Bin Dai a,b 
Co
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Abstract Herein, we reported on the application of an Au (III)/Schiff-based catalyst
in acetylene hydrochlorination reaction. The [AuCl2(phen)]Cl catalyst exhibited
excellent stability, and acetylene conversion was maintained at >90% after 40 h of
operation. The excellent stability of the [AuCl2(phen)]Cl catalyst was attributed to the
presence of the 1,10-phenanthroline ligand that partially inhibited the reduction of the
Au3+ active component.
Keywords: [AuCl2(phen)]Cl; acetylene hydrochlorination
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1.Introduction
Acetylene hydrochlorination is an important industrial route to the manufacture of
vinyl chloride monomer. However, the HgCl2 catalyst used in acetylene
hydrochlorination is highly toxic and causes serious environmental problems.
Therefore, the use of non-mercury catalysts in acetylene hydrochlorination has been
extensively explored.
Since 1975, K. Shinoda et al. discovered the catalytic activity correlates well with
the standard electrode potential of the metal, and a supported Au-based catalyst with
excellent catalytic performance in acetylene hydrochlorination reaction [1, 2].
However, Au-based catalysts rapidly deactivate during acetylene hydrochlorination as
a result of the deactivation of active Au3+ to Au0 [3, 4]. Considerable efforts have been
exerted to enhance the stability of AuCl3 catlasyt by addition of another metallic
component. Hutchings et al. investigated the effect of adding Ru3+, Ir3+, Pd2+, Pt4+ and
Rh3+ on the stability of Au3+ for acetylene hydrochlorination, and the results showed
that these bimetallic ions provided no obviously improvement for inhibiting the Au3+
to Au0 [5]. Wang et al. reported that AuCl3-CuCl2/AC catalyst did not deactivate after
running more than 200 h on the stream [6]. Zhang et al. also reported
AuCl3–Co(III)/SAC catalysts displays a fairly stable activity, with approximately 92%
acetylene conversion and 99.9% selectivity to VCM. No visible decline in activity is
observed [7]. Catalytic properties affected by the second metallic component are
attributed to changes in the electronic structure of the active metal, due to
electron-transfer between Au and another metal chloride. Our group found that the
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property of carbon support can also affect the sability of AuCl3 for acetylene
hydrochlorination. With N-doped carbon nanotubes (CNT) as support, AuCl3
displaysed enhanced stbaility for acetylene hydrochlorination compared with that with
CNT as support, attributed to electron transfer between the N atom Au3+ active site
[8].
Previous literature reported that a gold/Schiff-based complex has a higher
positive-redox potential than the AuCl3 compound [9], indicating that the presence of
N-containing ligands that could inhibit the reduction of Au3+ to Au0 [10, 11].
Therefore, it can be inferred that the presence of N-containing component as ligands
may enhance the stability of AuCl3 catalyst for acetylene hydrochlorination. In this
paper, we will try to synthesize a gold/Schiff-based complex and apply it in acetylene
hydrochlorination. The synthesized catalyst was characterized by a series of
characterization to determine its physical property, and its catalytic performance was
evaluated by a fixed-bed reactor.
2.2. Catalyst synthesis and preparation
2.2.1. Catalyst synthesis
In the general catalyst synthesis, an ethanol solution of 1,10-phenanthroline (10
mL, 0.96 M) was added to an ethanol solution of 1.0 g HAuCl4 (10 mL) under
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stirring. The mixture was stirred for 4 h under reflux conditions. At the end of the
reaction, a large amount of yellow or orange solid was formed. After filtration,
washing with ethanol (3×5mL), and drying in air, [AuCl2(phen)]Cl (1Au) was
obtained. Anal. Found: UV-Vis: λmax(CH3OH)/nm (Fig. S1): 230, 280, 317; IR (KBr,
1
cm-1): 1630, 1590, 1512, 1486, 1414, 1205, 866, 703 cm-1 (Fig. S2); and HNMR
13
(DMSO-d6, δ, ppm): 9.31 (d, 2H), 9.05 (d, 2H); 8.35 (s, 2H) 8.22(m, 2H); CNMR
(DMSO): 147.60, 141.74, 137.51, 129.47, 127.44, 125.61 (Fig. S3).
2.2.2 Catalysts preparation
2.2.2.1 AuCl3/AC
The supported gold catalyst AuCl3/AC was prepared using an impregnation
technique with hydrochloric acid as a solvent. 3.2 mL (solution, 1 g/100 mL)
HAuCl4·4H2O was added dropwise to 3.0 g activated carbon under stirring for 12 h,
and then dried at 140 oC for 18 h. AuCl3/AC catalyst was prepared. The actual loading
of gold was 0.49 wt.% determined by an inductively coupled plasma.
2.2.2.2 1Au/AC
The supported gold catalyst 1Au was prepared using an impregnation technique
with hydrochloric acid as a solvent. Then 0.0375 g 1Au was dissolved in 9 mL
hydrochloric acid, and then added dropwise to 3.0 g activated carbon under stirring
for 12 h. Finally, 1Au/AC catalyst was dried at 140 oC for 18 h. The actual loading of
gold was 0.49 wt.% determined by an inductively coupled plasma.
2.3. Catalyst characterization
Transmission electron microscopy (TEM) experiment used a JEM-2010 electron
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microscope with the accelerating voltage of 200 kV, a line resolution of 0.14 nm and
point-to-point resolution of 0.23 nm. The X-ray diffraction (XRD) data was collected
on a Bruker advanced D8 X-ray diffractometer using Cu-Kα irradiation (λ = 1.5406 Å)
as source at 40 kV and 40 mA in the scanning range of 2θ between 10o and 90o. X-ray
photoelectron spectroscopy (XPS) data was recorded on a Thermo ESCALAB 250XI
experiment, using a monochromatised Al, Ka X-ray source (225 W), minimum energy
resolution 0.45 eV (Ag 3d5/2) and the minimum XPS analysis area 3 um.
2.4. Catalytic performance evaluation

 
A0

A
X 
1 0 0%
(1)
(2)
A
A0

VC
S

100%
VC
1
A
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Fig. 1 displays the redox potentials of the 1Au and AuCl3 catalysts. The peaks at 0
and -0.7 V in AuCl3 catalyst were attributed to the reduction of Au3+ to Au+ and Au+
to Au0, respectively. A positive shift in both reduction peaks was evidently observed
in the 1Au catalyst. Hence, the Au3+ oxidation state in the complex 1Au was
confirmed by electrochemistry. At the same time, this result also suggested that the
introduction of the phen ligand may inhibit the reduction of Au3+ to metallic Au0,
which is consistent with the findings in previous literature [10, 11].
The thermal stability of the synthesised 1Au catalyst was investigated to confirm
whether the decomposition of the ligand occurred under reaction temperature of 180
oC. The thermogravimetric analysis (TGA) of the 1Au catalyst was shown in Fig. S4.
o
The results show that the decomposition temperature of the 1Au catalyst is 270 C,
thus indicating that the catalyst is stable under the reaction condition of acetylene
hydrochlorination.
The catalytic performances of the AuCl3/AC and 1Au/AC catalysts were tested
o
under the same conditions (temperature (T) = 180 C, C2H2 gas hourly space velocity
(GHSV) = 280 h-1, feed volume ratio VHCl/VC2H2 = 1.2). In order to demonstrate the
effect of HCl pretreatment on AuCl3/AC catalyst, we compared the catalytic
performance of AuCl3/AC for acetylene hydrochlorination, and characterized the used
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AuCl3/AC catalysts with and without the presence of HCl pre-treatment. The results
are shown in Fig. S5, Fig. S6 and Fig. S7 in the Supporting Information. It can be
inferred that HCl pre-treatment can partly inhibit the reduction of Au3+ active
component, and thus improve the catalytic activity of AuCl3/AC catalyst. Acetylene
conversion and VCM selectivity are shown in Fig. 2. The two catalysts exhibit
relatively similar initial catalytic activities and selectivity, with almost 96% acetylene
conversion and 99.5% selectivity. However, AuCl3/AC acetylene conversion
decreased from 96% to 69% after 40 h of operation, thus demonstrating the
deactivation of the AuCl3/AC catalyst. In addition, the 1Au/AC catalyst exhibited
good stability in the acetylene hydrochlorination process, and over 90% of the
converted acetylene was maintained after 40 h of operation. This result indicated that
the 1Au/AC catalyst displayed a slower deactivation rate than the AuCl3/AC catalyst
in acetylene hydrochlorination. Therefore, both electrochemistry investigation and
catalytic test using AuCl3 and 1Au demonstrate undoubtedly that the enhancing effect
of the nitrogen-containing ligand 1, 10-phenanthroline on the stability of Au(III) is the
key to render Au(III) species efficient for hydrochlorination catalytic reaction.
The activity of Au-based catalysts for hydrochlorination reaction of acetylene
decreases along the order: Au3+ > Au+ > Au0, therefore, the reduction of Au3+ to
metallic Au0 is the predominant reason for the rapid deactivation of AuCl3/AC
catalyst, as shown in Fig. 2. In addition, the higher positive-redox potential can inhibit
the Au3+ to Au0. In order to provide evidence for this viewpoint, we carried out XRD
to demosntrate the state of Au in the fresh and used AuCl3 and 1Au catalysts. From
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Fig. 3, it is obviously that the fresh Au-based catalysts have no peaks of metallic Au0,
however, the used Au-based catalysts display diffraction peaks of Au0 at 38.50, 44.70
and 64.80, which is well matched with the standard data of metallic Au0 (PDF #
04-0784). The result also shows that sintering exists in Au-based catalysts to some
extent after the reaction. It is important to note that the addition of additives can’t
inhibit the sintering of the Au catalyst in the reaction process, but can retard the
reduction of the active component to a certain extent.
Fig. 4 shows TEM images of the fresh and used AuCl3/AC (Fig. 4a and Fig.4c) and
1Au/AC (Fig. 4b and Fig.4d) catalysts after 40 h reaction. From Fig. 5, it can be seen
that there is no obviously nano-Au particles in fresh Au-based catalysts, however, the
coverage density of nanoparticles are obviously increased after reaction. In addition,
for the used 1Au/AC catalyst experiencing reaction, the Au naono-partiles are much
more sparsely than used AuCl3/AC catalyst, being consitent with XRD results. The
average particle size of the used AuCl3/AC and 1Au/AC catalyst is 24 nm and 23 nm,
from Fig. 4c and Fig. 4d. That results are somewhat larger than calculted from
Scherrer equation, which is 14.7 nm and 13.2 nm. This phenomenon may be
attributed that the crystalline size of nano-particle is smaller than its actual size.
The XPS spectra of the fresh and used Au-based catalysts corresponding to Au 4f
peaks are shown in Fig. 5 All of the Au (4f) signals have been deconvoluted into three
components responding to metallic Au and oxidized Au species. Calculated from the
relative deconvoluted peak area, the ratios of metal oxide species in the catalysts are
listed in Table 1. According to these data, the content of Au3+ is 44% presented in
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fresh AuCl3/AC catalyst, and the content of Au3+ is 40% also observed in 1Au/AC
catalyst. However, the content of Au3+ in used AuCl3/AC and 1Au/AC catalyst is
20% and 35%, respectively. The higher content of Au3+ in used 1Au/AC catalyst may
be attributed that N-containing ligands inhibited the reduction of Au3+ to Au0
comparing with AuCl3/AC catalyst, which is consistent with Fig. 1 and Fig. 3. In
addition, it is noted that the binding energy of Au 4f7/2 of fresh 1Au/AC catalyst
shows a significant negative shift (about 0.1 eV) comparing with fresh AuCl3/AC
catalyst. Such a negative shift has been ascribed to the interaction between Au and
phen, which increases the electron density of Au3+ with the transfer of electron from
phen species to Au3+ centre, and thus inhibit the reduction of active Au3+ component.
4. Conclusions
In summary, the synthesised 1Au/AC catalyst exhibited excellent stability in
acetylene hydrochlorination reaction. The enhanced catalytic stability was attributed
to the presence of the phen ligand, which inhibited active Au3+ component
deactivation. The 1Au/AC catalyst may be a candidate catalyst in industrial
application of acetylene hydrochlorination due to its excellent performance.
Acknowledgements
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References
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Chem. 15 (2013) 829-836.
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[9] A. Stephen, K. Hashmi, Jan P. Weyrauch, M. Rudolph, E. Kurpejovic, Angew.
Chem. Int. Ed. 43 (2004) 6545-6547.
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(2011) 53-59.
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Figure Captions:
Fig. 1 Linear potential-scan voltammograms of (A) HAuCl4 and (B) 1Au complex in
acetonitrile solution in an argon atmosphere.
Fig. 2 (A) conversion of acetylene and (B) selectivity to VCM in acetylene
hydrochlorination over (a) AuCl3/AC and (b) 1Au/AC catalysts. Reaction conditions:
temperature (T) = 180 0C, C2H2 GHSV = 280 h-1, feed volume ratio VHCl/VC2H2 = 1.2.
Fig. 3 XRD patterns of (a) AC, (b) fresh AuCl3/AC, (c) fresh 1Au/AC, (d) used
AuCl3/AC and (e) used 1Au/AC catalysts.
Fig. 4 TEM images of fresh and used Au-based catalysts. (a) fresh AuCl3/AC; (b)
fresh 1Au/AC; (c) used AuCl3/AC and (d) used 1Au/AC catalysts.
Fig. 5
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Relative of intensity (%)
Catalysts
Au0 4f
49
Au+4f
7
Au3+ 4f
44
fresh AuCl3/AC
used AuCl3/AC
fresh 1Au/AC
used 1Au/AC
65
15
20
49
11
40
55
10
35
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160
120
80
A
Au
Cl
Cl
Cl
(2)
(1)
40
+
Au3
Au+
Au
(1)
(2)
2e
e
E1/2=0V
Au+
E1/2=-0.7V
0
0.8
0.4
0.0
-0.4
-0.8
-1.2
-1.6
Potential (V)
B
40
(2)
Cl
Cl
N
N
Au
Cl
30
20
10
0
(1)
+
1Au3
1Au+
1Au
2e
e
(1)
(2)
E1/2=0.5V
E1/2=-0.1V
1Au+
0.4
-10
0.8
0.0
-0.4
Potential (V)
Fig. 1 Linear potential-scan voltammograms of (A) HAuCl4 and (B) 1Au complex in
acetonitrile solution in an argon atmosphere.
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100
(b)
(a)
A
90
80
70
60
50
40
0
10
20
30
40
Time on stream (h)
100
(b)
B
(a)
99
98
0
10
20
30
40
Time on stream (h)
Fig. 2 (A) conversion of acetylene and (B) selectivity to VCM in acetylene
hydrochlorination over (a) AuCl3/AC and (b) 1Au/AC catalysts. Reaction conditions:
temperature (T) = 180 0C, C2H2 GHSV = 280 h-1, feed volume ratio VHCl/VC2H2 = 1.2.
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(e)
(d)
(c)
(b)
(a)
20
30
40
50
60
70
80
90
2 Theta
Fig. 3 XRD patterns of (a) AC, (b) fresh AuCl3/AC, (c) fresh 1Au/AC, (d) used
AuCl3/AC and (e) used 1Au/AC catalysts.
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(a)
(b)
(c)
(d)
Fig. 4 TEM images of (a) fresh AuCl3/AC; (b) fresh 1Au/AC; (c) used AuCl3/AC and
(d) used 1Au/AC catalysts.
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A
(b)
(a)
90
88
86
84
82
Binding energy (eV)
B
(b)
(a)
92
90
88
86
84
82
Binding energy (eV)
Fig. 5
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Highlights:
[AuCl2(phen)]Cl catalyst is synthesized and applied in acetylene
hydrochlorination.
►
►
[AuCl2(phen)]Cl catalyst displays excellent stability for acetylene
hydrochlorination.
► N-containing ligands inhibit the reduction of active Au3+ component.
19