Gas-liquid acetylene hydrochlorination under nonmercuric catalysis using ionic liquids as reaction media

Article abstract of DOI:10.1039/c1gc15041c

By using ionic liquids as reaction media, gas-liquid acetylene hydrochlorination proceeded efficiently under catalysis of nonmercuric metal chlorides.

Full text of DOI:10.1039/c1gc15041c

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COMMUNICATION
Gas-liquid acetylene hydrochlorination under nonmercuric catalysis using
ionic liquids as reaction media†
Gang Qin,a,b Yuhan Song,a Rui Jin,a Jun Shi,a Zhiyong Yuc and Shaokui Cao*a
Received 12th January 2011, Accepted 18th March 2011
DOI: 10.1039/c1gc15041c
By using ionic liquids as reaction media, gas-liquid acety-
lene hydrochlorination proceeded efficiently under catalysis
of nonmercuric metal chlorides.
analogous to the current industrial process with HgCl2 catalysis,
which appeared to show a steady decrease in catalytic activity
derived from the surface carbonaceous deposition, the pulver-
ization of solid catalyst, the hardening of activated carbon, and
so on.13
Acetylene hydrochlorination is a very important industrial pro-
cess for the production of vinyl chloride monomer (VCM), which
is further used as the starting materials for poly(vinyl chloride)
production.1 Although VCM is also obtained by ethylene-based
processes, acetylene hydrochlorination is of growing importance
because of increasing petroleum prices, since acetylene is derived
from coal industry. Currently, acetylene hydrochlorination is still
widely applied in VCM production in China (about 7.21 million
tons per year), which is primarily performed under the catalysis
of HgC12 supported on activated carbon. However, the HgC12
catalysis has some inherent disadvantages, especially the catalyst
toxicity and volatility.2,3
Thermodynamically, acetylene hydrochlorination is highly
exothermic (DH = -110 kJ mol-1). Even if the packed mul-
titubular reactor with an external cooler is utilized in the
current industrial process, the formation of local hot-spots
with ultra-high temperature can not be avoided due to the fast
reaction speed. Therefore, mercury volatilization and catalyst
deactivation are induced to a serious extent, resulting in a
severe shortening in catalyst lifetime and significant mercury
consumption at high production rate,4 which consequently is a
severe environmental issue.
Compared to gas-solid reactions, gas-liquid reaction pro-
cesses in principle give better temperature control and heat
removal.14 Also, catalytic activity of the active component would
be better developed in a homogeneous reaction medium. Gas-
liquid reaction process for acetylene hydrochlorination have
also been reported.15–17 While the process has to be carried
out at a much lower temperature than the current industrial
process (150–180 ◦C) for the low boiling points of common
organic solvents or water. Hence, we believe that ionic liquids
(ILs) will resolve these problems very well. ILs have several
unique characteristics, such as a polar but weakly coordinating
character, non-volatility and are excellent solvents for organic,
inorganic and organometallic compounds. Therefore, ILs have
attracted enormous interest in many research fields in recent
years, especially in the field of catalysis.18–25
Herein, we report our efforts in finding some novel reac-
tion systems for acetylene hydrochlorination by using non-
volatile ILs as reaction media, such as [Bmim]Cl, [Bmim]HSO4,
[Bmim]PF6, [Emim]PF6, [EPy]Br and [BPy]BF4. Several non-
mercuric inorganic salts, specifically CuCl2, SnCl4, MnCl4,
H2PtCl6 and HAuCl4, were investigated as environmentally
benign catalysts; HgCl2 was also evaluated for comparison. To
the best of our knowledge, acetylene hydrochlorination with ILs
as a reaction medium has not yet been reported.
The gas-liquid acetylene hydrochlorination was performed
in a self-designed glass reactor shown in Fig. 1 similar to the
bubbling reactor widely employed in gas-liquid reactions. The
reactor consists of a gas mixer and two coaxial glass tubes with
different diameters. The outer tube has a diameter of 15 mm and
a height of 400 mm. The internal tube used as a concentric draft
tube has a diameter of 12 mm. The inner tube is separated from
the outer tube by 1.5 mm in order to achieve a long reaction
path about 340 mm by using minimum amount of ILs. About
20 mL of the mixture of ILs and metal chloride was stored in
the outer tube which was immersed in a thermostatic oil bath.
Acetylene and hydrogen chloride with a certain ratio, which
was controlled by a glass rotameter, were injected into the gas
mixer for a uniform mixing and then bubble flowed from the
Nonmercuric catalytic systems for acetylene hydrochlorina-
tion have been the subject of research for a long time, and
many transition metal compounds, such as Cu(I),5 Au(III),6
Pt(II),7,8 Pd(II),9 Sn(IV),10 Rh(III)11 and Bi(III),12 have been found
to be effective. Some of these nonmercuric catalysts have a
similar initial activity and selectivity to HgCl2. Most of these
nonmercuric catalysts were investigated through a gas-solid
reaction by loading the active component on activated carbon,
aSchool of Materials Science and Engineering, Zhengzhou University,
Zhengzhou, 450052, China. E-mail: caoshaokui@zzu.edu.cn;
Fax: +86-371-6676-3561; Tel: +86-371-6676-3561
bSchool of Materials Science and Engineering, Henan Polytechnic
University, Jiaozuo, 454001, China
cBeijing Longzida Science and Technology Development Co., Ltd.,
Beijing, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c1gc15041c
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shown in Fig. 3. It can be observed that both HAuCl4 and
H2PtCl6 gave the highest acetylene conversion, which are 78.5%
and 79.5%, respectively. The conversion follows the order of
Au ª Pt > Hg ª Cu > Mn > Sn, and all the metal chlorides
showed high VCM selectivity, from 82.8% the lowest (MnCl4) to
98.5% the highest (HAuCl4).
Fig. 1 Reactor illustration for a gas-liquid acetylene hydrochlorination.
reactor bottom. The outlet gas composition was measured by
gas chromatography.
First of all, several ILs, specifically [Bmim]Cl, [Bmim]HSO4,
[Bmim]PF6, [Emim]PF6, [EPy]Br and [BPy]BF4, were used as the
reaction medium for VCM synthesis by acetylene hydrochlori-
nation using CuCl2 as a catalyst. The results are listed in Fig.
2, and indicate that the catalytic activity of CuCl2 is influenced
by the nature of ILs. Organic solvents were also examined for
comparison, acetylene conversion and VCM selectivity are 4.5%
and 71.5% respectively in PEG-10000 (column 1), and those
in DMSO are 6.3% and 54.4%, respectively (column 2). Both
acetylene conversion and VCM selectivity are much higher in
all ILs examined, among which [Bmim]Cl (column 3) performs
the best with 68.1% acetylene conversion and 97.5% VCM
selectivity. Therefore, [Bmim]Cl was used as the reaction medium
to investigate the catalytic behavior of other metal chlorides in
the following experiments. Simultaneously, the anion of ILs also
Fig. 3 Different metal chlorides as catalyst for acetylene hydrochlori-
nation (reaction conditions: IL: [Bmim]Cl; 160 ◦C; C2H2: 0.3 L h-1; HCl:
0.31 L h-1; catalyst concentration: 0.069 mol L-1).
Evidently, when ILs are used as the reaction media, the
catalytic activity of metal chlorides for the gas-liquid reaction
of acetylene hydrochlorination exhibits the same rule as the
gas-solid reaction using metal chlorides supported on activated
carbon as catalysts.26,27 The catalytic activity of metal chlorides
for acetylene hydrochlorination correlates significantly with the
standard reduction potential of the metal cation. Essentially, a
cation with higher standard reduction potential displays a higher
catalytic activity. Au(III) and Pt(II) have the highest reduction
potentials, which are 1.42 V and ~1.2 V respectively. Therefore,
it is not a surprise that HAuCl4 and H2PtCl6 exhibited the
highest catalytic activity for the gas-liquid reaction of acetylene
hydrochlorination in ILs observed in our experiments.
affects the catalytic activity in the order of Cl- > HSO4- > PF6 ,
-
which corresponds to the solubility order of CuCl2 in ILs.
Considering CuCl2 is much less expensive than HAuCl4 and
H2PtCl6 and exhibits reasonable catalytic behavior comparable
to HgCl2, CuCl2 is believed to be the best candidate for industrial
application for both economic and environment reasons. Hence,
optimization of reaction conditions was concentrated on using
CuCl2 as catalyst and [Bmim]Cl as the reaction medium for the
gas-liquid reaction of acetylene hydrochlorination, which may
provide some valuable data for further scaling-up of the reaction.
Firstly, the effect of molar ratio of C2H2 with HCl on the
reaction was investigated under two sets of conditions as shown
in Fig. 4. Obviously, a slight excess of HCl is necessary for
the reaction, and a good acetylene conversion can be achieved
when the molar ratio of HCl to C2H2 is varied in between
1.03 : 1.00 and 1.50 : 1.00. Acetylene conversion decreases with
further increase in HCl to C2H2 molar ratio, which might be
ascribed to the dilution effect of HCl on C2H2 under higher
molar ratio. Besides, VCM selectivity decreases gradually with
the increase in HCl to C2H2 molar ratio, because of further excess
HCl addition to VCM producing 1,2- or 1,1-dichloroethane. If
Fig. 2 Acetylene hydrochlorination in different liquid media using
◦
CuCl2 as catalyst (reaction conditions: 160 C; HCl: 0.4 L h-1; C2H2:
0.3 L h-1; CuCl2: 0.058 mol L-1).
Several metal chlorides were investigated as catalyst in
[Bmim]Cl for acetylene hydrochlorination, and the results are
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Fig. 4 Influence of HCl/C2H2 molar ratio on acetylene hydrochlorina-
Fig. 6 Influence of CuCl2 concentration on acetylene hydrochlorina-
tion in [Bmim]Cl ( acetylene conversion, ᭜ VCM selectivity, 160 ◦C,
tion in [Bmim]Cl ( acetylene conversion, ᭜ VCM selectivity, 160 ◦C,
᭹
᭹
CuCl2: 0.319 mol L-1; ᭡ acetylene conversion, ꢀ VCM selectivity,
HCl: 0.31 L h-1; C2H2: 0.3 L h-1; ᭡ acetylene conversion, ꢀ VCM
180 ◦C, CuCl2: 0.664 mol L-1).
selectivity, 140 ◦C, HCl: 0.4 L h-1; C2H2 0.3 L h-1).
C2H2 is in excess, the catalyst might be reduced by the excess
C2H2, which is one of the reasons for metal halide catalyst
deactivation.28
two sets of reaction conditions. In current commercial VCM
processes the HgCl2 loading on activated carbon is typically 10–
12% (weight percent) in general. Our dosage of metal chloride as
catalyst in IL is far below this value. The lower limit for the CuCl2
in this study loading was 0.7% (weight percent), which is equal
to 0.05 mol L-1. This may come from the easy formation of a six-
membered ring active center from the cation of metal chloride in
ILs.29 In other words, IL may facilitate the catalytic active center
formation and hence promote the acetylene hydrochlorination,
which would be a topic for further research.
The results in Fig. 7 indicated that the acetylene conversion
reaches a maximum value at around 160 ◦C and decreases
below or above this temperature. This conversion change with
reaction temperature is attributed to the exothermic character of
acetylene hydrochlorination. On the other hand, the conversion
rapidly declines above 180 ◦C, which is probably due to
the catalyst deactivation. Interestingly, the VCM selectivity is
nearly stabilized at 94.3%. The catalyst deactivation at higher
The effect of space velocity on acetylene hydrochlorination
was also investigated under two sets of reaction conditions with
fixed HCl to C2H2 molar ratio of 1.03 : 1.00 as illustrated in Fig.
5. Results indicated that acetylene conversion decreases from
71.5% to 36.7% (160 ◦C) and from 72.6% to 40.3% (180 ◦C)
as the space velocity increases from 10 h-1 to 91.5 h-1, which
resulted from incomplete reaction because of the short residence
time at higher gas space velocity. To ensure sufficient reaction
with reasonable reactant residence time and gas space velocity,
a slim reactor design or connected reactors in series would be
preferred as shown above. Additionally, VCM selectivity is not
affected evidently by the space velocity, which is kept above 90%
all the time.
Fig. 5 Influence of space velocity on acetylene hydrochlorination in
[Bmim]Cl ( acetylene conversion, ᭜ VCM selectivity, 160 ◦C, CuCl2:
᭹
0.051 mol L-1; ᭡ acetylene conversion, ꢀ VCM selectivity, 180 ◦C,
CuCl2: 0.172 mol L-1).
Fig. 7 Influence of temperature on acetylene hydrochlorination in
The results illustrated in Fig. 6 show that the acetylene
conversion changed little with the increase in the test range
of CuCl2 concentration and was constant at nearly 67% under
[Bmim]Cl ( acetylene conversion, ᭜ VCM selectivity, HCl: 0.31 L
᭹
h-1; C2H2: 0.3 L h-1; CuCl2: 0.051 mol L-1; ᭡ acetylene conversion, ꢀ
VCM selectivity, HCl: 0.4 L h-1; C2H2: 0.3 L h-1; CuCl2: 0.319 mol L-1).
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temperature is not related to the thermal decomposition of
[Bmim]Cl which starts to decompose at 285 ◦C.
acetylene hydrochlorination. These significant results suggested
a hopeful alternative approach for industrial VCM production.
As illustrated in Fig. 8, the whole process of acetylene
conversion and VCM selectivity with running time takes on
S-shaped curves. The reaction is slowly going on during the
initial section before 50 min. Thereafter, the conversion and
selectivity are rapidly enhanced with the increase in running time
and finally reach stable values of 62.5% and 99%, respectively,
at around 160 min. Even when the reaction was carried out
for 3 days, no decrease in catalytic activity was observed.
This provides a distinct advantage over previously reported
non-mercury catalysts, which present a short lifetime in the
gas-solid reaction, for example, only several hours due to the
fast deactivation generally derived from the carbon deposition
and the regional hot-spot. The extended catalytic lifetime
in ILs presents a promising prospect for practical industrial
application.
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In conclusion, when ILs were used as a reaction medium, the
gas-liquid acetylene hydrochlorination reaction was successfully
demonstrated. The use of alternative metal chloride catalysis
was also carried out. The disadvantages of a gas-solid reaction,
such as the carbon deposition and the regional hot-spot, could
be overcome. The nonmercuric metal chlorides also provide
good acetylene conversion and VCM selectivity. Therefore, the
combination of ILs and nonmercuric metal chlorides provides
an efficient, economic and environment-friendly approach for
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1498 | Green Chem., 2011, 13, 1495–1498
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The Royal Society of Chemistry 2011
©