Non-mercury catalytic acetylene hydrochlorination over bimetallic Au-Co(III)/SAC catalysts for vinyl chloride monomer production

Article abstract of DOI:10.1039/c3gc36840h

Several gold-based catalysts including Au, Au-La(iii), Au-Co(ii), and Au-Co(iii) were prepared and assessed for acetylene hydrochlorination, combining with characterizations of low-temperature N₂ adsorption/desorption, thermogravimetric analysis, X-ray diffraction, temperature-programmed reduction, inductively coupled plasma-atomic emission spectroscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy. The best catalytic performance was obtained over Au1Co(iii)3/SAC catalysts with an acetylene conversion of 92% and a selectivity to VCM of 99.9%. It is indicated that the additives of Co(iii), Co(ii) and La(iii) are preferential to stabilize the catalytic active Au⁺ species and inhibit the reduction of Au³⁺ to Au ⁰ in the preparation process of Au-based/SAC catalysts. The addition of these additives can greatly inhibit the occurrence of coke deposition on the catalyst surface, and also inhibit the catalyst sintering, thereby improving the activity and long-term stability of the Au-based catalysts.

Full text of DOI:10.1039/c3gc36840h

Green Chemistry
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Non-mercury catalytic acetylene hydrochlorination over
bimetallic Au–Co(III)/SAC catalysts for vinyl chloride
monomer production†
Cite this: Green Chem., 2013, 15, 829
a
b
a,b
a
a
a
Haiyang Zhang, Bin Dai,* Xugen Wang, Wei Li, You Han, Junjie Gu and
Jinli Zhang*
a,b
Several gold-based catalysts including Au, Au–La(III), Au–Co(II), and Au–Co(III) were prepared and assessed
2
for acetylene hydrochlorination, combining with characterizations of low-temperature N adsorption/
desorption, thermogravimetric analysis, X-ray diffraction, temperature-programmed reduction, inductively
coupled plasma-atomic emission spectroscopy, transmission electron microscopy, and X-ray photoelectron
spectroscopy. The best catalytic performance was obtained over Au1Co(III)3/SAC catalysts with an acety-
lene conversion of 92% and a selectivity to VCM of 99.9%. It is indicated that the additives of Co(III), Co(II)
Received 20th November 2012,
Accepted 28th January 2013
+
3+
and La(III) are preferential to stabilize the catalytic active Au species and inhibit the reduction of Au to
0
Au in the preparation process of Au-based/SAC catalysts. The addition of these additives can greatly
DOI: 10.1039/c3gc36840h
inhibit the occurrence of coke deposition on the catalyst surface, and also inhibit the catalyst sintering,
thereby improving the activity and long-term stability of the Au-based catalysts.
www.rsc.org/greenchem
pollution, mainly the exhaust emissions released in the
process of polyvinyl chloride distillation. Among them, solid
1
Introduction
Vinyl chloride monomer (VCM) is mainly used for the syn- waste issues can now be resolved through the manufacture of
thesis of polyvinyl chloride (PVC), one of the five engineering cement and other products; acid and base solution, sulfur,
plastics widely used in fields such as food packaging, construc- phosphorus and COD pollution have been eliminated by neu-
tion, agricultural production, etc. Two methods are generally tralization or biodegradation; and exhaust emission pollution
used to manufacture VCM in industry. One is ethylene oxy- generated in the PVC distillation unit can be resolved by
chlorination, which faces serious economic challenges retreatment. Currently, the only bottleneck that constraints the
because of the increasing cost of ethylene resulting from the acetylene method in terms of green chemistry is the mercuric
2
–8
oil crisis. The other is acetylene hydrochlorination, which has chloride catalyst used in the synthesis process of VCM. The
recently received considerable attention, especially in main active ingredient of the catalyst is mercuric chloride,
1
countries with large coal reserves. However, serious environ- whose high toxicity and volatility causes serious damage to
mental pollution has become a constraining bottleneck for the workers and the environment. According to predictions, by
survival and development of calcium carbide. The following 2014, the consumption of mercury will have reached 175 489
pollutants are usually generated in the production process of tons, which will cause significant pollution to the environ-
vinyl chloride: (1) solid waste pollution, including carbide slag ment. In addition, activated carbon (AC) as a carrier has a low
and waste mercury catalyst; (2) liquid waste pollution, includ- mechanical strength and is easily pulverized. Therefore,
ing mercury pollution, acid pollution, alkali, sulfur, phos- studies of non-mercury catalysts to replace mercuric chloride
phorus, and COD pollution in carbide supernatant; (3) gas are extremely important for the survival and development of
9
–34
the PVC industry.
Noble metal catalysts are widely used in many reactions
a
School of Chemical Engineering and Technology, Tianjin University, Tianjin
00072, People’s Republic of China. E-mail: zhangjinli@tju.edu.cn;
3
5–40
and have a good catalytic performance,
lene hydrochlorination is no exception. Hutchings et al.,
for which acety-
3
1
4–22
Fax: +86-22-2740-3389; Tel: +86-22-2789-0643
b
School of Chemistry and Chemical Engineering, Shihezi University/Key Laboratory
conducted the most representative research on precious
metals in acetylene hydrochlorination. In 1985, they used
more than 20 kinds of metal components for acetylene hydro-
chlorination and found that the activity of the components
correlated with the standard electrode potential of the
for Green Processing of Chemical Engineering of Xinjiang Bintuan, Shihezi, Xinjiang
8
32000, People’s Republic of China. E-mail: dbinly@126.com;
Fax: +86-993-2057-270; Tel: +86-993-2058-176
†
Electronic supplementary information (ESI)
0.1039/c3gc36840h
available. See DOI:
1
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1
4
2+
2+
cations in the following order of initial activity: Pd > Hg
>
2
+
+
+
Cu = Cu > Ag . Later, they studied activated carbon-sup-
ported Au catalysts, Au/AC, in detail and found that Au has
high activity for the acetylene hydrochlorination reaction.
Recently, they also described and discussed in detail the sup-
ported gold and gold-based bimetallic catalysts used for acety-
lene hydrochlorination. They found that Au and Pd can form a
continuous solid solution across all alloy compositions, the
addition of ≤5 at% Pd to Au increased the initial activity but
with a significant loss of selectivity and all Au–Pd/AC catalysts
2
0
deactivated rapidly due to coke deposition. Although all
these results confirm that the most active known catalyst for
the hydrochlorination of acetylene for long-term use is
undoped Au, its stability was poor in the long run.
Therefore, present research is oriented toward the develop-
ment of a green catalytic process, aiming to develop an
efficient non-mercury catalyst. This has prompted us to con-
sider whether bimetallic catalysts composed of gold and
another metal could give improved performance for the acety-
lene hydrochlorination reaction. In this work, several bimetal-
lic gold-based catalysts were prepared on pitch-based spherical
activated carbon (SAC), which is better for acetylene hydro-
chlorination than coconut shell AC, and the effects of La(III),
Co(II), and Co(III) additives were analyzed and discussed.
2
Results and discussion
Catalytic performance of Au-based catalysts
Table 1 shows the catalytic activities of five kinds of catalysts at
a specific time. Over the Au/SAC catalyst the acetylene hydro-
chlorination reaction has good selectivity (99.9%) but relatively
poor activity (80.4% in 36 h). The addition of the second metal
component has little effect on the selectivity but dramatically
Fig. 1 Catalytic performance of Au–Co(III)/SAC catalysts. Reaction conditions:
−
1
temperature (T) = 150 °C, C
2 2
H gas hourly space velocity (GHSV) = 360 h , feed
volume ratio VHCl/VC2H2 = 1.15.
enhances the catalytic activity, with the decreasing order of 99.9 to 66.5%, with about 33.4% C
2 2
H conversion loss after
catalytic activity: Co(III) > Co(II) > La(III).
48 h (Fig. 1a). After Co(III) addition, the Au1Co(III)0.5/SAC cata-
The catalytic performance of Au–Co(III)/SAC catalysts with lyst exhibits a better initial performance than Au/SAC, but a
different Au/Co(III) ratios for acetylene hydrochlorination is sharp decline in its activity occurs after 30 h. The catalytic per-
shown in Fig. 1. Co(III)/SAC shows almost no catalytic activity; formance of the Au–Co(III)/SAC catalysts gradually increases
its selectivity to VCM increases firstly and then decreases, with the increase of the Co(III) loading amount. With a 1 : 3 Au/
finally remaining at a lower level (about 96%) (Fig. 1b). The Co(III) (a/a) ratio, the Au–Co(III)/SAC catalysts displays a fairly
Au/SAC catalyst exhibits a relatively better initial performance, stable activity, with approximately 92% acetylene conversion
with almost 100% C H
2 2
conversion and more than 99.8% VCM and 99.9% selectivity to VCM. No visible decline in activity is
selectivity in 20 h. However, with such an amount of Au observed. Further Co(III) addition does not obviously increase
loading (1 wt%), Au/SAC is still continuously deactivated, from the catalytic activity and selectivity.
a
Table 1 Active component of catalyst screening
Catalyst characterization
To determine the nature of the supported Au-based catalysts, a
detailed structural and chemical study was carried out using a
combination of a series of characterization methods.
Catalysts
Composition of catalysts
X
A
/%
SVC/%
Co(III)/SAC
Au/SAC
Au–La(III)/SAC
Au–Co(II)/SAC
Au–Co(III)/SAC
6.8 wt%
1 wt%
Au 1 wt% Au/La = 1 : 1 (a/a)
Au 1 wt% Au/Co(II) = 1 : 1 (a/a)
Au 1 wt% Au/Co(III) = 1 : 1 (a/a)
0.5
80.4
87.6
88.8
91.6
96.2
99.9
99.9
99.9
99.9
Catalyst texture properties. Due to the large number of
4
1–43
micro-pores on the SAC surface,
the N2 adsorption–
desorption isotherm of the SAC support (shown Fig. S1a†)
belongs to type-I adsorption. The support shows almost the
same micro-porous structure after loading the active
a
2 2
Reaction conditions: T = 150 °C, VHCl/VC H = 1.15, GHSV(C H ) =
60 h , t = 36 h.
2
2
−1
3
8
30 | Green Chem., 2013, 15, 829–836
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Table 2 Pore structure parameters of catalyst samples
Total pore volume
BET (m g−1)
2
(cm g
3
−1
)
S
Catalyst
Fresh
Used
Fresh
Used
SAC
Au/SAC
1133
1056
922
970
1001
901
1021
778
657
728
749
759
794
767
0.65
0.61
0.53
0.56
0.57
0.52
0.51
0.49
0.57
0.45
0.38
0.42
0.43
0.44
0.45
0.44
Au1La(III)1/SAC
Au1Co(II)1/SAC
Au1Co(III)0.5/SAC
Au1Co(III)1/SAC
Au1Co(III)3/SAC
Au1Co(III)5/SAC
889
868
component such as Au, Au–La(III), Au–Co(II), or Au–Co(III)
loading (Fig. S1†).
As listed in Table 2, the specific surface areas and total pore
volumes of the supports decrease after loading the active com-
ponent. This result may be caused by the phenomenon called
the dilution effect, in which the loading of the active com-
ponent decreases the ratio of the carriers in the catalysts. The
proportion of the carriers is reduced with a rise of the active
component loading, and the dilution effect becomes more sig-
nificant. On the other hand, the active species fill or block
some of carrier pores, leading to a decrease of the Brunauer–
Emmertt–Teller (BET) surface areas and total pore volumes.
When the loading of the active component is increased, more
pores of the carrier are filled or blocked by the active species, Fig. 2 TGA and DTG curves of the fresh and used Au/SAC (a) and Au1Co(III)3/
thus the BET surface area and the total pore volume are SAC (b) catalysts.
decreased. After the reaction, the BET surface area of the Au/
2
−1
SAC catalyst reduces to 278 m g (26.3% reduction), which
may be caused by the carbon deposition. The addition of La(III), 150–448 °C, with 11.5% loss in the range of 275–448 °C,
Co(II), or Co(III) into the Au catalyst results in a change in the mainly due to the burning of carbon deposited on the catalyst
variation amplitude of the catalyst’s BET surface areas surface.
(
(
ΔSBET%), following the order: Au–La(III) (28.7%) > Au–Co(II)
Upon the addition of cobalt, the fresh Au1Co(III)3/SAC cata-
25.0%) > Au–Co(III) (15.8%). The changes in the total pore lyst (Fig. 2b) shows a rapid weight loss before 150 °C due to
volume also show a similar trend: Au–La(III) (28.2%) > Au– the desorption of adsorbed water on the carrier surface, fol-
Co(II) (25.6%) > Au–Co(III) (15.8%). With increasing the amount lowed by a slow weight decrease in the range of 150–422 °C
of Co(III), the change in the BET surface area (ΔSBET) of the cat- (shown in Table S2†). When the temperature exceeds 422 °C,
alysts gradually decreases. When the ratio Au/Co(III) (a/a) is the catalyst has a quick weight loss, which is also mainly
1
9
: 3, the catalyst’s ΔSBET reaches a minimum of approximately caused by activated carbon combustion. Whereas the used
2
−1
5.6 m
g
(10.8% reduction). A further increase in Co(III) Au1Co(III)3/SAC catalyst has a small fraction of weight loss
loading slightly increases ΔSBET. The change trend in the total (1.9%) within 150–229 °C, when the temperature continues to
pore volume (ΔV) before and after the reaction is similar. rise from 229 to 422 °C, the weight loss of the used catalyst is
Therefore, the optimal atomic ratio for Au/Co(III) is 1 : 3, at 7.0%, nearly equal to that of the fresh Au1Co(III)3/SAC catalyst
which the used catalyst may have the least carbon deposition.
(6.8%), indicating that most of the carbon deposited on the
Carbon deposition on the used catalysts. TGA was used to catalyst surface is burned off when the temperature is higher
evaluate the degree of carbon deposition on the surface of than 229 °C.
catalysts. Fig. 2a shows that neither the fresh nor the used Au/
Aside from the desorption of materials adsorbed on the
SAC catalyst has a visible loss of mass before 150 °C, indicating carrier surface, the catalyst can also lose its weight by reacting
that minor water adsorption on the catalyst surface occurs. In with oxygen in the atmosphere and escaping as CO with the
2
the range of 150–448 °C, the fresh Au/SAC catalyst exhibits a increase in temperature because the catalyst carrier is activated
slow weight loss, reaching 6.2% (Table S1†). When the temp- carbon.4
4,45
Therefore, to eliminate the effect of carbon
erature exceeds 448 °C, the catalyst rapidly decreases in weight burning on weight loss, the amount of carbon deposition
because of SAC burning. On the contrary, the used Au/SAC should be equal to the difference of the weight loss between
catalyst has a significant weight loss (13.3%) in the range of the fresh and used catalysts in the temperature range of coke
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Table 3 Carbon depositions of Au-based catalysts
Catalyst
Amount of carbon deposition (%)
Au/SAC
7.1
4.0
3.9
3.7
3.1
1.0
1.0
Au1La(III)1/SAC
Au1Co(II)1/SAC
Au1Co(III)0.5/SAC
Au1Co(III)1/SAC
Au1Co(III)3/SAC
Au1Co(III)5/SAC
3
6,41,47,48
burning. Based on this calculation method,
the
carbon deposition of the catalysts are listed in Table 3. The
amount of carbon deposition on the Au/SAC catalyst is the
highest (7.1%). Carbon deposition on the surface directly
results in pore clogging of the carrier and the decrease of its
surface area, consequently covering some parts of the active
sites and thus, decreasing the catalyst activity. The amount of
coke deposition is significantly reduced after the addition of
Fig. 3
2
H -TPR profiles of the fresh Au–Co(III) catalysts.
the second component including cobalt and lanthanum. The potential of the second metal additives, it is reasonable to con-
possible reason is that the additional component divided the clude that the additives of Co(III), Co(II) and La(III) are preferen-
+
active sites of Au on the surface, inhibiting the occurrence of tial to stabilize the catalytic active Au species and inhibit the
3
+
0
carbon deposition. In the same ratio, the order of carbon depo- reduction of Au into Au in the preparation process.
sition is Au1Co(III)1 (3.1%) < Au1Co(II)1 (3.9%) < Au1La(III)1
Catalyst sintering and valence change of the active com-
4.0%). Moreover, the amount of carbon deposition gradually ponent during the reaction. Fig. 4 displays the XRD patterns
(
decreases with an increase of the Co(III) loading. The carbon of SAC, and the fresh and used Au-based catalysts. Apart from
deposition decreases to the minimum (1.0%) at the Au/Co(III) the amorphous diffraction peaks of the pitch-based SAC, no
ratio of 1 : 3, and no further decrease occurs with much larger discernible Au reflection is detected in both the fresh Au/SAC
Co(III) addition.
Effect of the additional component on the gold catalyst 4 nm or a material with a large amount of Au centers,
and bimetallic Au-based catalysts, indicating particles below
3
+
34
before the reaction. The TPR profiles show that the fresh Au which is confirmed by the TEM images (e.g., Au/SAC (Fig. 5a)
catalyst exhibits two characteristic reduction bands between and Au1Co(III)3/SAC (Fig. 5c)) and Table S4.† It should be
2
30 and 400 °C with the peak centers at 310 and 354 °C, noted that the Co(III) additive partially locates on the surface of
respectively (Fig. 3). The first peak corresponds to the Au particles (Fig. 5c), resulting in a slight increase of the size
3
+
reduction of Au , while the second peak is the reduction peak of particles, however, they can divide the active sites of Au and
+
32,34
of Au .
As the additive amount of Co(III) increases, a red- inhibit the occurrence of carbon deposition, as mentioned in
3
+
shift of the reduction band of Au occurs whereas a blue-shift the section on carbon deposition. The used Au/SAC clearly
of the band of Au can be observed, besides the appearance of shows typical diffraction peaks of Au at 38.36, 44.56, 64.83
relatively strong bands at 400–600 °C. Except for the major and 77.90° (2θ) (Fig. S3b†), indicating that the change of Au
composition of carbon, the activated carbon carrier consists of valence (Au is reduced into Au ) or catalyst sintering occurs
oxygen groups and heteroatoms. As far as the second metal during the reaction. The used Au1La(III)1/SAC, Au1Co(II)1/SAC,
+
0
3
+
0
4
6
47
0
component is considered, the reduction bands of Au1Co(III)x/ and Au1Co(III)1/SAC catalysts also show typical Au diffraction
SAC (x = 0.5, 1, 3, or 5) in the range of 400–610 °C are attribu- peaks at 38.36° (2θ). However, the peak intensity significantly
ted to the superposition of the additive Co(III) and the car- decreases in proper sequence. Moreover, the intensity of the
0
boxylic acids or carboxylic anhydrides inside the carbon Au peaks gradually decreases with the increase of the Co(III)
4
8,49
carriers (Fig. 3),
as are the reduction bands of the catalysts loading amount in the used Au–Co(III)/SAC catalysts (Fig. 4b).
Au1Co(II)1/SAC and Au1La1/SAC (Fig. S2†). Through comparing The peak intensity is weakest at the Au/Co(III) (a/a) ratio of
3
+
0
the TCD signals with a standard, the fractions of different Au 1 : 3, indicating the least fraction of Au reduced into Au or
species in these fresh catalysts can be estimated. It is worth the lowest amount of catalyst sintering. The activity of the Au
3
+
+
mentioning that the relative amounts of Au and Au increase catalyst for the acetylene hydrochlorination reaction decreases
3
+
+
0 44
after adding the second metal component, except for a in the following order: Au > Au > Au . Therefore, Au
3
+
decrease of Au in Au1Co(III)5/SAC (Table S3†). The relative reduction is a main cause of the deactivation of the Au-based
+
content of Au in Au1Co(III)x/SAC (x = 0.5, 1, 3, or 5) increases catalysts. The addition of the second component can effec-
as the Co(III)/Au ratio increases (Table S3†), which is consistent tively inhibit the reduction of Au or catalyst sintering during
with the order of catalytic performance of these catalysts the reaction, and the significance of such an effect decreases
(shown in Fig. 1). Because of the relatively higher electric in the order: Co(III) > Co(II) > La(III). The optimal atomic ratio
8
32 | Green Chem., 2013, 15, 829–836
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of Au and Co(III) is 1 : 3, which is consistent with those of the
activity test.
XRD patterns for the Au-based catalysts show a particle size
distribution. The rough results determined using the Scherrer
equation are shown in Table S4,†which shows that sintering
exists in all catalysts to some extent after the reaction. The esti-
mated size of the Au particles in the used catalysts is in the
range of 14 to 23 nm, in accordance with the TEM images (e.g.,
used Au/SAC (Fig. 5b) and used Au1Co(III)3/SAC (Fig. 5d)). 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.
Further, the XPS spectra were used to investigate the
valence state and relative amount of the active ingredient Au in
Au/SAC and the bimetallic catalyst (e.g., Au1Co(III)3/SAC)
before and after the reaction. It should be noted that more
than one Au species appears. Therefore, curve fitting is
32,50
employed to determine the ratio of each Au species.
In the
preparation and storage process of the catalysts, the active
3
+
0
component Au can be reduced to Au , so there is a large
0
3+
number of Au and Au in both the fresh Au/SAC and Au1Co(III)3/
SAC catalysts. In fresh Au/SAC and Au1Co(III)3/SAC catalysts,
the relative content of Au is 12.6 and 16.6%, respectively
shown in Table S5†). However, there is a considerable amount
of Au (18.4%) in the fresh Au1Co(III)3/SAC catalyst, except for
Au and Au (Fig. 6). It is also illustrated that the existence of
3
+
(
+
3
+
0
3
+
0
Co(III) inhibits the reduction of Au into Au or oxidization of
0
+
Au into Au in the preparation process. Under the reaction
3
+
0
conditions, Au is reduced into Au , contributing to the deac-
2
,19,22,30
tivation of the Au catalyst.
In addition to a large
0
0
number of Au particles, there are some Au -s clusters (4.4%)
Fig. 4 XRD patterns of the fresh (a) and used (b) Au–Co(III) catalysts.
3
2
in the used Au/SAC catalyst (Fig. 6a), which indicates that a
sintering or agglomeration of catalyst occurs in the reaction
process, resulting in the grain growth, consistent with that of
XRD and TEM analysis. In the used Au1Co(III)3/SAC catalyst,
there are still some Au (12.5%) and Au (27.3%), indicating
that the addition of Co(III) inhibits the reduction of Au to Au
3
+
+
3+
0
0
+
or oxidizes the reduced Au into Au in the reaction process.
Due to the synergy effect of Au–Co(III), the peak positions of
0
3+
Au and Au in the XPS spectra are slightly offset. It deserves
mentioning that the apparent decrease in Au content detected
from XPS analysis after the reaction is probably caused by the
pore changes of the carbon carriers in the reaction process,
and Au species particles are present also present inside the
carbon pores and not just at the catalyst surface. Besides the
3
2
surface characterization of XPS analysis,
the results of
ICP-AES analysis have obviously indicated that the loss ratio of
Au is small in the reaction process (shown in Table S6†). After
adding Co(III) to the Au catalyst, the peaks of Au species are
scarcely observed in the spectra after the reaction (Fig. 6b),
and the amount of carbon depositions in used Au1Co(III)3/SAC
is small, which has been proved in TG analysis. Therefore, the
introduction of Co(III) can suppress the reduction of the active
component during the reaction, or even because the standard
electrode electric potential of Co(III) is higher than that of Au,
Fig. 5 TEM figures of Au/SAC (a, c) and Au1Co(III)3/SAC (b, d) before and after
reaction.
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+
Au in Au1Co(III)x/SAC (x = 0.5, 1, 3, or 5) increases as the
Co(III)/Au ratio increases, which is consistent with the order of
catalytic performance of these catalysts. The addition of these
additives can’t inhibit the catalyst sintering but can greatly
inhibit the occurrence of coke deposition on the catalyst
surface, thereby improving the activity and long-term stability
of Au-based catalysts.
4
Experimental
Catalyst preparation
Bimetallic Au–Co(III)/SAC catalysts were prepared using an
1
5–20
incipient wetness impregnation technique.
Carbon
(
(
20–40 mesh) was initially washed with dilute aqueous HCl
1 mol L− ) at 70 °C for 5 h to remove Na, Fe, and Cu, which
1
5
1
are poisons for the hydrochlorination reaction. Carbon was
filtered and washed with distilled water (2 L g− ) and dried at
1
1
50 °C for 12 h. A HAuCl ·4H O solution (assay: 49.7%) in
4 2
aqua regia (6.3 mL) was added dropwise to the acid-washed
carbon (20–40 mesh, 5 g) under stirring. The product was set
aside for 10 h at room temperature, evaporated in a steam
3 6 3
bath and dried at 150 °C for 12 h. The Co(NH ) Cl hydro-
chloric acid solution was then added dropwise under stirring.
The product was then handled the same way as above and
finally used as a catalyst. Au loading in all the catalysts was
fixed at 1.0 wt% at various Au/Co(III) ratios (a/a), namely,
1
: 0.5, 1 : 1, 1 : 3 and 1 : 5, and were denoted as Au1Co(III)0.5/
SAC, Au1Co(III)1/SAC, Au1Co(III)3/SAC, and Au1Co(III)5/SAC,
respectively. The same procedure was also followed to prepare
the corresponding Au/SAC (1.0 wt% Au loading) and Co(III)/
SAC catalysts as well as the bimetallic Au1La1/SAC and
Au1Co(II)1/SAC catalysts for comparison.
Fig. 6 XPS analysis of Au/SAC (a) and Au1Co(III)3/SAC (b) on carbon catalyst;
Au 4f peaks before and after reaction. The spectra are relatively noisy due to the
low total loading of metal present.
Catalytic performance tests
The catalytic performance tests were conducted in a fixed-bed
flow micro-reactor (i.d. 10 mm). The reactor temperature was
regulated using a CKW-1100 temperature controller from the
Chaoyang Automation Instrument Factory (Beijing, China).
The experimental setup used for the current study is presented
in Fig. S4.†
0
+
so that Au is oxidized into Au , and these small particles are
present also inside the carbon pores and not just at the
surface of these, which is exactly consistent with our original
idea of the purpose of Co(III) as additives.
A pipeline was purged with nitrogen before the reaction to
−1
2 2
remove water vapor and air in the system. C H (30.0 mL min )
and HCl (34.5 mL min− ) were then fed through the filter (to
remove trace impurities) via calibrated mass flow controllers to
1
3
Conclusions
2 2
Over the Au-based/SAC catalysts for the acetylene hydrochlori- a heated reactor containing 5 mL of the catalyst, with a C H
nation reaction, the addition of the second metal component gas hourly space velocity (GHSV) of 360 h− . Under certain con-
can significantly enhance the catalytic activity, maintaining a ditions, acetylene flow rate and the size of catalyst were
good selectivity, with the decreasing order of catalytic activity: changed to investigate the effect of mass transfer on the cataly-
Co(III) > Co(II) > La(III). The best catalytic performance was tic activity, and the results indicated that the effect of the exter-
1
obtained over Au1Co(III)3/SAC catalysts with the acetylene con- nal diffusion was eliminated at the C
version of 92% and the selectivity to VCM of 99.9%.
2
H
2
flow rate range of
25–100 mL min− ; the reaction rate was not limited by the
1
It is indicated that the additive of Co(III), Co(II) and La(III) is internal mass transport within the 0.180–0.900 mm
+
preferential to stabilize the catalytic active Au species and (18–80 mesh) grain size range. Thus, the reaction had not
3
+
0
inhibit the reduction of Au to Au in the preparation process been affected by the internal and external diffusion. The
of Au-based/SAC catalysts. Additionally, the relative content of pressure of the reactants, both HCl and C H , was in the range
2
2
8
34 | Green Chem., 2013, 15, 829–836
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Paper
of 1.1–1.2 bar, which was chosen both for safety reasons and were treated with N
2
gas at 60 °C for 30 min. The temperature
2
3,24
to test the catalyst under mild conditions.
The exit gas was then increased from 60 to 610 °C at a heating rate of 10 °C
−
1
mixture was passed through an absorption bottle containing min , and 610 °C final temperature held for 0.5 h. Calibration
NaOH solution and then input into a Shimadzu GC-2014C of the detector, and hydrogen consumption, was carried out
chromatograph for analysis.
from the integrated TCD signal using CuO as a standard,
subject to the same temperature program used for the Au/SAC
catalysts. The Au and Au amounts are reported as ratios of
the corresponding species to the total Au amount.
Samples for examination by transmission electron
microscopy (TEM) were prepared by dispersing the catalyst
powder in high-purity ethanol, then allowing a drop of the sus-
pension to evaporate on a holey carbon film supported by a
3
2
3+
+
Catalyst analysis and evaluation
After being absorbed by N-methyl pyrrolidone, the product was
analyzed with GC-MS. As shown in Fig. S5,† it is indicated that
the main product is vinyl chloride, and there also exists a
small amount of by-product, 1,1-dichloroethane (Fig. S5c†),
which was also verified using the GC-2014C gas chromato-
graph after the qualitative analysis by the retention time of
pure substances (Fig. S6†).
As the individual substance has a high-boiling point and
little content, the products of acetylene hydrochlorination were
quantified using the peak area normalization method. The
300-mesh copper TEM grid. Bright-field and annular dark-
field (ADF) imaging experiments were carried out using a
JEM2100F TEM and an FEI Titan 80–300 TEM/STEM equipped
with CEOS spherical aberration corrector, respectively.
X-Ray photoelectron spectroscopy (XPS) spectra were
recorded using a Kratos Axis Ultra DLD spectrometer employ-
ing a monochromated Al-Kα X-ray source (hν = 1486.6 eV),
hybrid (magnetic/electrostatic) optics and a multi-channel
plate and delay line detector (DLD). All XPS spectra were
recorded using an aperture slot of 300 × 700 microns, survey
spectra recorded with a pass energy of 80 eV, and high reso-
lution spectra with a pass energy of 40 eV. In order to subtract
the surface charging effect, the C1s peak has been fixed at a
binding energy of 284.6 eV. As it is well known that cationic Au
species can be reduced to the zero-valent state by secondary
electron emission during XPS analysis, the Au(4f) region was
conversion of acetylene (X ) and the selectivity to VCM (S
)
A
VC
5,52
4
were used as the criteria for the catalytic performance.
Because hydrogen chloride is absorbed after the reaction, the
volume of the reaction system can be considered as a constant
when calculating. If the total volume was regarded as a volume
unit, X
A
and SVC were calculated as follows:
XA ¼ ð1 ꢀ φA1Þ ꢁ 100%
ð1Þ
ð2Þ
SVC ¼ φVC=ð1 ꢀ φA1Þ ꢁ 100%
where, φA1 is the volume fraction of the remaining acetylene
and φVC is the volume fraction of vinyl chloride.
5
3
3+
recorded at the beginning and end of the analysis. The Au
0
and Au amounts are reported as the percentage of the total
Au amount.
Catalyst characterization
Au and other metal contents in the catalysts were determined
using a Thermo Scientific iCAP 6000 inductively coupled
plasma-atomic emission spectrometer (ICP-AES). Low-tempera-
ture N2 adsorption/desorption experiments were conducted
using a Micromeritics ASAP 2020C surface area and porosity
analyzer. The samples were first degassed at 150 °C for 6 h
and analyzed via liquid nitrogen adsorption at −196 °C.
Acknowledgements
We gratefully acknowledge the financial support for this
project from the Major State Basic Research Development
Program (No. 2012CB720302, 2010CB234605), the Program for
Changjiang Scholars and Innovative Research Team in Univer-
sity (IRT1161), and the Doctor’s Special Fund of Xinjiang
Production and Construction Corps (No. 2009JC10).
A thermogravimetric analysis (TGA) of the samples was per-
formed using a NETZSCH STA 449F3 Jupiter® thermogravi-
metric-differential scanning calorimeter (TG-DSC) and
simultaneous thermal analyzer in an oxygen atmosphere at a
−
1
flow rate of 30 mL min . The temperature was increased from
1
5
0 to 700 °C at a heating rate of 10 °C min−
.
X-Ray diffraction (XRD) measurements were performed
using a Bruker D8 ADVANCE X-ray diffractometer equipped
with a Cu Kα X-ray tube operating at 40 kV and 40 mA in the
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