Chlorocuprate(i) ionic liquid as an efficient and stable Cu-based catalyst for hydrochlorination of acetylene

Article abstract of DOI:10.1039/c9cy00401g

The gas-liquid reaction process for acetylene hydrochlorination, especially using ionic liquids (ILs) as homogeneous reaction media, has gained much attention because it can effectively avoid the deactivation caused by hot spots and carbon deposition. However, the relatively low activity and high price of the currently used ILs limit their practical applications. Herein, we synthesize a series of chlorocuprate(i) ILs to explore an efficient and stable Cu-based catalyst for acetylene hydrochlorination. The N-methylpyrrolidonium hydrochloride-0.60CuCl ([Hnmpo]Cl-0.60CuCl) IL exhibits the best catalytic performance, showing an acetylene conversion of 86% over 150 h under the conditions of 180 °C and 50 h^-1 GHSV. It is confirmed that the Cu(i) species is the major active component and extremely stable under the reaction conditions via characterization of TGA-DSC-FTIR, ICP-OES, XPS, UV-vis, ESI-MS, and Raman. In addition, the [Hnmpo]Cl-0.60CuCl IL has the capacity to effectively activate HCl, which is directly observed by in situ FTIR. By combining the experimental results and theoretical calculations, we propose the reaction mechanism and find that the catalytic performance of chlorocuprate(i) ILs is positively correlated with the adsorption of HCl. The strong interaction with HCl is identified as the key characteristic of the [Hnmpo]Cl-CuCl IL, which endows it with excellent catalytic performance. Briefly, this study shows that the cost-effective [Hnmpo]Cl-CuCl IL can be a viable alternative to the commercial heterogeneous HgCl₂/AC catalyst for acetylene hydrochlorination.

Full text of DOI:10.1039/c9cy00401g

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Chlorocuprate(I) ionic liquid as an efficient and stable Cu-based
catalyst for hydrochlorination of acetylene†
Yanfei Ren,‡a Botao Wu,‡ab Fumin Wang,*a Hang Li,a Guojun Lv,a Mingshuai Sun,a Xubin Zhang*a
Received 00th January 20xx,
Accepted 00th January 20xx
The gas-liquid reaction process for acetylene hydrochlorination, especially using ionic liquids (ILs) as the homogeneous
reaction medium, has gained more attention because it can effectively avoid the deactivation caused by hot spots and
carbon deposition. However, the relatively low activity and high price of the currently used ILs limit their practical
applications. Herein, we synthesize a series of chlorocuprate(I) ILs to explore an efficient and stable Cu-based catalyst for
acetylene hydrochlorination. N-methylpyrrolidonium hydrochloride-0.60CuCl ([Hnmpo]Cl-0.60CuCl) IL obtain the best
catalytic performance, showing the acetylene conversion of 86% over 150 h at the conditions of 180 °C and 50 h−1 GHSV. It
is confirmed that Cu(I) species is the major active component and extremely stable under the reaction conditions via
characterizations of TGA-DSC-FTIR, ICP-OES, XPS, UV-Vis, ESI-MS, and Raman. In addition, [Hnmpo]Cl-0.60CuCl IL has the
capacity to effectively activate HCl, which is directly observed by in situ FTIR. By combining experiment results and
theoretical calculations, we propose the reaction mechanism and find that the catalytic performance of chlorocuprate(I) ILs
is positively correlated with the adsorption of HCl. The strong interaction with HCl is identified as the key characteristic of
[Hnmpo]Cl-CuCl IL, which endows it with excellent catalytic performance. Briefly, this study shows that the cost-effective
[Hnmpo]Cl-CuCl IL can be a viable alternative to the commercial heterogeneous HgCl2/AC catalyst for acetylene
hydrochlorination.
DOI: 10.1039/x0xx00000x
www.rsc.org/
Up to now, following the pioneering work of Hutchings,4-8
1. Introduction
studies
on
non-mercury
catalysts
for
acetylene
hydrochlorination chiefly concentrate on precious metal
catalysts, especially Au-based catalysts. A lot of efforts have
been made to minimize the cost of Au-based catalysts by
decreasing the Au loading or increasing the stability.9-11
However, its high price is still the major obstacle to the low
value-added PVC industry.12 To meet economic benefits, non-
precious metal catalysts have gained more attention in recent
years.13-18 Cu-based catalysts are generally studied because of
their relatively good catalytic activity.13-16 Nevertheless, almost
all Cu-based catalysts still suffer from low activity and severe
deactivation. Thus, it is a challenge to improve the activity and
the long-term stability of Cu-based catalysts for acetylene
hydrochlorination.
In order to solve this problem, some researchers turn their
attention to the gas-liquid reaction process, which can give
efficient temperature control and heat removal compared to
the gas-solid reaction.19-21 And the active component would be
better developed in a homogeneous reaction medium.22, 23 In
consequence, the gas-liquid reaction could effectively avoid the
disadvantages of solid catalysts, like the hot spot, the surface
coke deposition, and the pulverization of AC. Currently, due to
the extraordinary physicochemical properties such as negligible
vapor pressure, good solubility and high thermal stability, ionic
liquids (ILs) have attracted remarkable attention for
Vinyl chloride monomer (VCM) is an essential material for the
synthesis of polyvinyl chloride (PVC), which is the third most
widely used plastic material in the world. Acetylene
hydrochlorination, originally started in the 1930s, is still applied
for industrial VCM production in the countries where acetylene
can be obtained economically from abundant coal.1 At present,
activated carbon (AC) supported 10–15 wt% mercuric chloride
is extensively used as industrial catalyst in acetylene
hydrochlorination.2 Nevertheless, the toxic mercury chloride
catalyst has encountered a number of shortcomings, such as the
loss of active components due to the hot spots in fixed bed
reactors, as well as the environmental disruptions associated
with the catalyst preparation, application and disposal.
Moreover, based on the requirements of the Minamata
Convention,3 the use of mercury will be forbidden in the future.
Hence, one of the priorities of acetylene-based PVC industry is
the development of environmentally friendly non-mercury
catalysts.
a. School of Chemical Engineering and Technology, Tianjin University, Tianjin 300350,
P. R. China. E-mail: wangfumin@tju.edu.cn (F. Wang), tjzxb@tju.edu.cn (X. Zhang)
b. Key Laboratory of Bioorganic Chemistry and Molecular Engineering, Ministry of
Education and Beijing National Laboratory for Molecular Science, College of
Chemistry and Molecular Engineering, Peking University, Beijing 100871, P. R.
China
applications in the gas-liquid reaction of acetylene
hydrochlorination.19-21,
‡ Equal contribution
24
Qin et al. proved the gas-liquid
†
Electronic Supplementary Information (ESI) available. See DOI: 10.1039/
x0xx00000x
reaction for acetylene hydrochlorination using IL as reaction
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medium and metal chloride as active component, and found
[Bmim]Cl had the highest activity among different ILs.19 Hu et
al. prepared a metal nanoparticles/anionic surfactant IL system,
and the Pd NPs/[P4444][C17COO] showed excellent catalytic
activity.20 Later, Yang et al. reported a Pd NPs@[P4444][C17COO]-
C14 liquid-liquid biphasic system, which exhibited better
catalytic performance than the Pd NPs/[P4444][C17COO].21 Very
recently, Zhou et al. proposed the use of 1-alkyl-3-
methylimidazolium-based ILs as single component metal-free
catalysts.24 Overall, using ILs as reaction medium not only
provides an environmentally friendly choice for gas-liquid
reaction but also can achieve better catalytic performance. For
instance, the presence of ILs might activate the reactant gas and
effectively stabilize the oxidized metal complex,22, 23, 25 thereby
improving its catalytic efficiency. However, the high cost of the
present used ILs, such as imidazolium and quaternary
phosphonium salts, greatly restricts their sustainable
applications in PVC industry. Hence, it still remains huge
demand to develop a low-cost IL catalytic system.
View Article Online
DOI: 10.1039/C9CY00401G
Fig. 1 The molecular structure of the nine organic chloride salts.
methylimidazolium chloride ([Bmim]Cl, purity
tetrabutylphosphonium chloride ([P4444]Cl, purity ≥ 96%),
tetrabutylammonium chloride ([N4444]Cl, purity 98%),
≥
99%),
≥
By directly mixing of a metal chloride and an organic halide
salt, it is possible to synthesize a halometallate IL with very
interesting and useful properties. Currently, halometallate ILs
have received increasing interest as novel catalysts in a broad
range of reactions.26-28 Supported halometallate ILs catalysts
have also been used in acetylene hydrochlorination,22, 23, 29 such
as Au(III)-[Prmim]Cl/AC,22 Ru(III)-ChCl/AC,29 and Pd(II)-
[Bmim]Cl/AC.23 To the best of our knowledge, Cu(I) species as
the active component has not been successfully used in the
acetylene hydrochlorination because of its unstable property,
which could lead to rapid deactivation. Surprisingly, Cu(I)
species could remain stable in IL,30, 31 which enlightened us to
find a chlorocuprate(I) IL with higher catalytic activity and
significant price advantage over the currently used ILs. Herein,
for the first time, we prepared a series of chlorocuprate(I) ILs
and tested their catalytic performance in a continuous gas-
liquid process for acetylene hydrochlorination. N-
methylpyrrolidonium hydrochloride-0.60CuCl ([Hnmpo]Cl-
0.60CuCl) IL was found to have remarkable catalytic
performance and long-term stability. Besides, the IL was
investigated by various characterizations to uncover the
properties of the active component. Furthermore, based on our
experimental results and theoretical calculations, we proposed
the possible mechanism and correlated the catalytic
performance with the adsorption of HCl and C2H2. We believe
that the encouraging results of this work would help the
development of Cu-based catalysts for acetylene
hydrochlorination.
triethylamine hydrochloride ([Et3NH]Cl, purity ≥ 99%), and
pyridine hydrochloride ([PyH]Cl, purity ≥ 98%) were purchased
from Shanghai D&B Chemical Technology Co., Ltd. N-
methylpyrrolidinone (NMPO, purity
>
99.5%), 1-
methylimidazole (purity 99%), and N-methylpyrrolidine (purity
98%) were purchased from Shanghai Macklin Biochemical Co.,
Ltd. Hydrochloric acid (aqueous solution, 36–38 wt%) was
purchased from Tianjin Yuanli Chemical Co., Ltd. All the
materials were used without further purification.
2.2. Preparation of organic chloride salt
1-methylimidazolium
methylpyrrolidonium hydrochloride ([Hnmpo]Cl) and N-
methylpyrrolidinium hydrochloride ([Hnmpi]Cl) were
hydrochloride
([Hmim]Cl),
N-
synthesized according to the previous literature.32 Hydrochloric
acid was added dropwise to 1-methylimidazole over a period of
20 min under a temperature of 0–5 °C. The mixture was stirred
for an additional period of 12 h at 50 °C. Then, water was
removed under reduced pressure by rotary evaporation and the
white crude solid product was washed with diethyl ether to
remove any unreacted starting materials. After vacuum drying
overnight at 60 °C, [Hmim]Cl was obtained. The synthesis of
[Hnmpo]Cl and [Hnmpi]Cl followed protocols same to the
[Hmim]Cl. The purity of the three organic chloride salts was
confirmed by 1H NMR recorded on a Bruker AVANCE III
spectrometer (Fig. S1).
2.3. Synthesis of chlorocuprate(I) IL
Chlorocuprate(I) IL was prepared by mixing CuCl with the
required amount of above-mentioned organic chloride salt
together under a protective atmosphere of N2, then stirring at
100 °C for 24 h as the similar method recorded in the
literature.33 The molecular structure of nine organic chloride
salts is summarized in Fig. 1. We expressed the composition of
chlorocuprate(I) IL in terms of mole fraction of CuCl, x = mole of
CuCl / mole of (CuCl + organic chloride salt).
2. Experimental
2.1. Materials
Cuprous chloride (CuCl, purity 97%) was purchased from
Shanghai Meryer Chemical Technology Co., Ltd. Hydrogen
chloride (HCl, purity > 99.99%) and acetylene (C2H2, purity >
99.99%) were purchased from Tianjin Huanyu Gas Co., Ltd.
Choline chloride (ChCl, purity
≥
98%), 1-butyl-3-
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2.4. Evaluation of catalytic performance
The catalytic performance of chlorocuprate(I) ILs for acetylene
hydrochlorination was tested in a self-designed quartz bubbling
micro-reactor, which had a diameter of 10 mm and a length of
400 mm. The temperature was controlled by a magnetic stirring
oil bath with a heating height of 350 mm. The experimental
setup was presented in Fig. S2.
The reactor contained 15 mL of IL and was purged with N2 to
remove water and air before the reaction. Then HCl (9.0
mL/min) was passed for 60 min. Subsequently, C2H2 (7.5
mL/min) was fed through the reactor with a HCl/C2H2 volume
ratio of 1.2 to obtain an acetylene gas hourly space velocity
(GHSV) of 30 h−1. The long-term stability experiments were
carried out with 10 mL of IL in the same apparatus. The effluent
gas was analyzed online by a gas chromatograph (Benfen-Ruili,
3420A) after eliminating unreacted HCl and water.
Fig. 2 Catalytic performance over different chlorocuprate(I) ILs. Reaction
conditions: x(CuCl) = 0.67, GHSV(C2H2) = 30 h−1, T = 160 °C, and V(HCl)/V(C2H2) =
1.2.
2.5. Characterization
UV 550 spectrometer in the range of 200–800 nm by adopting
acetonitrile as the solvent.
In situ FTIR spectra measurement were conducted on a
Mettler Toledo ReactIR 15 instrument equipped with an AgX
optic fiber and a Diamond insertion probe from 2400–800 cm−1
by using acetonitrile as the solvent.
Gas chromatography-mass spectrometry (GC-MS) was
conducted using an Agilent 5975C instrument to detect the
reaction by-products.
Viscosity was measured using a Shanghai Equitable NDJ-5S
rotational viscometer by a spindle no. 1 at 6 rpm, and the
temperature was controlled by an external oil bath.
Thermogravimetric
analysis-Differential
scanning
2.6. Calculation details
calorimetry-Fourier transform infrared (TGA-DSC-FTIR)
measurement provided information about the thermal stability
and decomposition products of IL, which was performed using
a NETZSCH STA 449 F5 thermal analyzer coupled with a Bruker
Tensor 27 FTIR spectrometer. About 15 mg of IL was heated
from 40 °C to 800 °C at a heating rate of 10 °C/min under N2
atmosphere with a flow rate of 50 mL/min. The FTIR spectra
were acquired in the range from 4000 cm−1 to 600 cm−1 at a
resolution of 4 cm−1.
TGA was used to evaluate the coke amount, and was
performed on a Mettler Toledo TGA/DSC 2 instrument under an
air atmosphere with a flow rate of 30 mL/min, and the
temperature increased from 30 to 800 °C at a heating rate of 10
°C/min.
X-ray photoelectron spectroscopy (XPS) was performed on a
Thermo ESCALAB 250XI instrument to analyze the elemental
composition of the IL and the chemical states of copper. The
spectrometer was calibrated for the C 1s signal at 284.8 eV with
a resolution step width of 0.1 eV, and the raw spectra were
fitted using the smart function by Avantage software.
Inductively coupled plasma-optical emission spectrometry
(ICP-OES) was conducted by using a Varian VISTA-MPX
instrument to measure the copper content of the IL.
Electrospray ionization mass spectrometry (ESI-MS) was
recorded on a Bruker Daltonics miorOTOF-QII instrument by
using acetonitrile as the solvent to identify the copper species.
Raman spectra of the IL at solid state were tested by using a
Renishaw inVia Reflex instrument in the range 200–800 cm−1
with a 532 nm argon ion laser for excitation.
Geometry optimizations were performed using the Gaussian 09
package34 at the B3LYP-D3(BJ)/6-31G**/LanL2dz level of the
theory.35-41 All of the optimized geometries were examined as
stable point or transition state structures by frequency
calculations. Thermal corrections, natural population analysis
(NPA) were conducted at the same theory level.42 To obtain
better accuracy, energies of the optimized geometries were
calculated using M06-D3/6-311+G(2df,2p)/SDD level.43-46 The
computed structures were illustrated using CYLview.47 The
analysis and visualization of calculation results were carried out
by Multiwfn and VMD.48, 49
3. Results and discussion
3.1. Catalytic performance over different chlorocuprate(I) ILs
To make the research results more convincing, we chose a range
of organic chloride salts, including the most commonly used
imidazolium and quaternary phosphonium salts, to prepare
chlorocuprate(I) ILs. The mole fraction of CuCl was fixed at 0.67,
and the catalytic performance of each IL was evaluated at
acetylene GHSV of 30 h−1, temperature of 160 °C, and HCl/C2H2
volume ratio of 1.2 for 36 h. As shown in Fig. 2, nine different
ILs exhibit diverse catalytic performance on the conversion of
acetylene. To our surprise, [Hnmpo]Cl-0.67CuCl IL not only
achieves the highest acetylene conversion of 88.0% at 36 h, but
also has a relatively low price (Fig. S3). [P4444]Cl-0.67CuCl IL has
the second highest acetylene conversion, but it encounters the
shortcoming of high price. Besides, [Bmim]Cl-0.67CuCl IL only
achieves the conversion of acetylene to 55.5%. Compared with
Ultraviolet-visible (UV-vis) spectra were acquired with a Jasco
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Fig. 3 Catalytic performance of [Hnmpo]Cl-CuCl IL for acetylene hydrochlorination. (a) Effect of CuCl mole fraction. Reaction conditions: GHSV(C2H2) = 30 h−1, T = 160 °C,
and V(HCl)/V(C2H2) = 1.2. (b) Effect of acetylene GHSV. Reaction conditions: x(CuCl) = 0.60, T = 160 °C and V(HCl)/V(C2H2) = 1.2. (c) Effect of reaction temperature.
Reaction conditions: x(CuCl) = 0.60, GHSV(C2H2) = 30 h−1 and V(HCl)/V(C2H2) = 1.2. (d) Effect of HCl/C2H2 volume ratio. Reaction conditions: x(CuCl) = 0.60, GHSV(C2H2) =
30 h−1 and T = 180 °C.
[Hnmpo]Cl, since [Hnmpi]Cl lacks a carbonyl group, the we only studied the conditions that the CuCl mole fraction was
acetylene conversion of [Hnmpi]Cl-0.67CuCl IL is merely 45.0%, less than or equal to 0.67, under which the mixture could exist
which probably indicates the carbonyl group of [Hnmpo]Cl has in a liquid state below 100 °C. It can be seen in Fig. 3a that as
an impact on the catalytic process. It should be highlighted that the CuCl mole fraction increases from 0.33 to 0.60, the
for all chlorocuprate(I) ILs, according to the results obtained by conversion of acetylene increases significantly from 78.7% to
GC (Fig. S4), the selectivity for VCM is above 99%. The results of 89.0%, while the conversion decreases slightly to 87.9% with a
GC-MS confirm that the three by-products are further increase of CuCl mole fraction to 0.67. This decrease
dichloroethylene, dichloroethane, and 2-chloro-1,3-butadiene may be caused by the high viscosity of [Hnmpo]Cl-0.67CuCl IL
(Fig. S5). In short, the structure of organic chloride salt has a (Fig. S6), which limits the mixing of reactants as well as the
great effect on the catalytic performance of chlorocuprate(I) IL. transfer of mass and heat. Meanwhile, we also see that the
Actually, the difference in catalytic performance is mainly acetylene conversion is only 13.0% with [Hnmpo]Cl alone after
caused by their different adsorption capacities for C2H2 and HCl, 36 h of reaction, indicating a small contribution of [Hnmpo]Cl to
which will be further explained by theoretical calculations. the high catalytic activity of [Hnmpo]Cl-CuCl IL, and
Based on the above discussion, [Hnmpo]Cl-CuCl IL was selected demonstrating that the main active component is Cu species.
for the subsequent studies.
Taken together, these results show that the most appropriate
CuCl mole fraction of [Hnmpo]Cl-CuCl IL is equal to 0.60.
3.2. Exploration of reaction conditions
GHSV is a crucial parameter in the gas phase reaction. Fig. 3b
shows that the acetylene conversion slowly declines from 93.0%
at 15 h−1 to 81.6% at 50 h−1, because a higher GHSV results in
lower residence time. However, the conversion of acetylene
decreases very sharply as the further increase of GHSV. This
drop can be partly ascribed to the transition of flow regime from
bubble flow to churn turbulent flow or slug flow, which
The reaction conditions of acetylene hydrochlorination
catalyzed by [Hnmpo]Cl-CuCl IL were explored to better
understand the catalytic system. Firstly, it is necessary to
investigate the influence of CuCl mole fraction.50 However,
when CuCl mole fraction in [Hnmpo]Cl-CuCl IL is above 0.67, a
homogeneous liquid cannot be obtained even at 160 °C. Hence,
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generates large bubbles, yields smaller gas-liquid interfacial
areas and therefore limits the mass transfer between the gas-
liquid interface.51 Even so, the conversion is as high as 52.0% at
180 h−1. Consequently, [Hnmpo]Cl-0.60CuCl IL is a highly
effective catalyst for acetylene hydrochlorination over a wide
range of GHSV. Furthermore, we believe that designing a gas-
liquid reactor with better mass transfer effect can still improve
the catalytic performance.
View Article Online
DOI: 10.1039/C9CY00401G
As illustrated in Fig. 3c, with the increase of temperature from
120 to 180 °C, the acetylene conversion rises gradually from
77.0% to 91.0%, indicating that a higher temperature is capable
of promoting reaction. A decline in the viscosity of [Hnmpo]Cl-
0.60CuCl IL at higher temperatures is also conducive to the
reaction (Fig. S6). However, at the temperature of 200 °C, the
conversion decreases gradually as the time goes on, indicating
too high a temperature is not a good choice and might increase
the risk of the IL decomposition. Since the IL performs
considerable activity and stability at a moderate temperature of
180 °C, then 180 °C is selected as the optimum temperature.
On one hand, excess acetylene could bring about rapid
deactivation of catalyst and security concerns.52, 53 On the other
hand, HCl is helpful to maintain the stability of [Hnmpo]Cl-
0.60CuCl IL, which will be discussed below in detail. Hence, a
slight excess of HCl is required during the reaction process.
However, excess HCl inevitably leads to an increase in the cost
of production. Therefore, the effect of HCl/C2H2 volume ratio
from 1.05 to 1.30 was discussed. Looking at Fig. 3d and S7, the
acetylene conversion initially increases with the increase of
HCl/C2H2 ratio. But when the HCl/C2H2 ratio is higher than 1.2,
the conversion does not increase any more. As a result, the
HCl/C2H2 volume ratio should be around 1.2.
Fig. 4 TGA-DTG-DSC curves of [Hnmpo]Cl-0.60CuCl IL in N2 atmosphere.
which can be supported by the results of simultaneously
recorded FTIR spectra (Fig. S8). Therefore, the weight loss below
310 °C can be attributed to the vaporization of [Hnmpo]Cl.
Moreover, according to the FTIR spectra of [Hnmpo]Cl and
[Hnmpo]Cl-0.60CuCl IL in Fig. S9, we can infer that there is an
interaction between [Hnmpo]Cl and CuCl in [Hnmpo]Cl-
0.60CuCl IL, which might enhance the thermal stability of
[Hnmpo]Cl. Hence, the first weight loss below 182 °C is probably
due to the vaporization of free [Hnmpo]Cl that does not
combine with CuCl, which can be further proved by the TGA
result of pure [Hnmpo]Cl (Fig. S10).
Although in TGA curve the weight loss of [Hnmpo]Cl-0.60CuCl
IL is up to 9.0% at 180°C, it is possible that extra HCl has the
ability to inhibit the volatilization of [Hnmpo]Cl. Consequently,
[Hnmpo]Cl-0.60CuCl IL would be more thermally stable with HCl
introduction. Then, we performed long-term isothermal
gravimetrical analyses to test this hypothesis (experimental
details are shown in Fig. S11). The results display that the
introduction of HCl profoundly impedes the volatilization of
[Hnmpo]Cl, and [Hnmpo]Cl-0.60CuCl IL has only a small weight
loss of about 3% at 180 °C in 24 h with continuous introduction
of HCl, clearly indicating that the IL is thermally stable at 180 °C
with HCl introduction.
In addition, ICP-OES was conducted to measure the Cu
content in the fresh [Hnmpo]Cl-0.60CuCl IL, together with the
used IL at 180 °C after 36 h of reaction. As summarized in Table
1, the Cu content of both ILs is similar to the nominal content,
which demonstrates that there is a negligible loss of [Hnmpo]Cl
after the reaction. Besides, the elemental composition of the
fresh and the used ILs measured by XPS is shown in Table 1 and
Fig. S12. As listed in Table 1, the relative Cu/N atomic ratio is
10.98 before the reaction. After the reaction, no drastic change
in the Cu/N atomic ratio is occurred (11.05). Thus, XPS analysis
3.3. Characterization of [Hnmpo]Cl-0.60CuCl IL
3.3.1. Thermal stability. TGA-DSC-FTIR were applied to study
the thermal stability and decomposition products of
[Hnmpo]Cl-0.60CuCl IL. Firstly, we can draw from the DSC curve
in Fig. 4 that an endothermic peak appears at 84 °C,
corresponding to the melting of the IL. Based on the TGA and
DTG curves in Fig. 4, there are three distinct weight loss steps.
When the temperature is 310 °C, the residual weight ratio is
52.7%, which matches well with the calculated CuCl weight ratio
(52.3%). Meanwhile, another endothermic peak at 412 °C is in
accordance with the melting of CuCl. Taken together, the main
component at 310 °C is CuCl. Thus, the third weight loss above
481 °C is related to the vaporization of CuCl. According to the
literature,54, 55 the neutral NMPO and HCl may be present in
[Hnmpo]Cl-0.60CuCl IL. As a result, a dynamic equilibrium is
+
−
⇌
⇌
possible to exist: [Hnmpo] Cl (l) NMPO(l) + HCl(l) NMPO(g)
+ HCl(g). At high temperature, [Hnmpo]Cl is easily volatilized as
neutral molecular species by a proton transfer mechanism,
Table 1 The elemental composition and Cu content of the fresh and the used [Hnmpo]Cl-0.60CuCl ILs calculated from XPS and ICP-OES measurements.
The surface atomic composition of ILs (atom%)
The content of Cu (wt%)
Sample
Cu/N ratio
10.98
C 1s
O 1s
6.25
5.70
Cu 2p
27.67
29.95
Cl 2p
35.48
33.64
N 1s
2.52
2.71
Nominal
33.53
Actual
33.44
33.58
Fresh IL
Used IL
28.08
28.00
1
1.05
33.53
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Fig. 5 TGA curves of the fresh and the used [Hnmpo]Cl-0.60CuCl ILs in air
atmosphere.
Fig. 6 (a) Cu 2p XPS, (b) Cu LMM Auger spectra, (c) ESI-MS at negative mode and
(d) Raman spectra of the fresh and the used [Hnmpo]Cl-0.60CuCl ILs.
also shows that there is a negligible loss of [Hnmpo]Cl.
Overall, on the basis of these results, we therefore conclude
that [Hnmpo]Cl-0.60CuCl IL has good thermal stability in the
temperature ranges of 120−180 °C during the reaction.
a small portion of Cu(I) was oxidized during the IL preparation
process. As the binding energy of Cu 2p3/2 does not allow us to
distinguish between Cu(I) and Cu(0), Cu LMM Auger spectra
were measured. As illustrated in Fig. 6b, the presence of the
peak at 915.3 eV and the absence of the peak at 918.0 eV
3.3.2. Coke content. It is well-known that coke deposition, which
may originate from acetylene, VCM, or their oligomers, is a
common reason for the deactivation of solid catalysts in
acetylene hydrochlorination.52, 53 To measure the coke content,
TGA was performed in air atmosphere. Fig. 5 shows the TGA
results of the fresh [Hnmpo]Cl-0.60CuCl IL and the used IL at 30
h−1 GHSV after 36 h reaction. Based on the above discussion and
previous literature,56-59 we can infer that the weight loss caused
by the coke formed in the reaction is mainly in the temperature
suggest that there is only Cu(I) species but no metallic Cu(0)
14
species in both ILs.13,
Thus, the relative content of Cu(I)
species in the fresh IL is 89.46%. After the reaction, the Cu(I)
content even increases to 93.35%. And this change is consistent
with the observation from UV-Vis spectra (Fig. S13), in which the
absorbance intensity of Cu(II) species slightly decreases after
the reaction. Overall, we could conclude Cu(I) species acts as
the major active site in the reaction process. Besides, in
combination with the results of acetylene conversion, the
increase of Cu(I) species may also be one of the reasons for the
slight enhancement of acetylene conversion after 36 h of
reaction.
range of 250–400 °C. In the case of the fresh IL, the weight loss
is 2.03%, while it is 2.71% for the used IL. Thus, the content of
coke in the used IL is estimated as low as 0.68%, which is far less
than that of other solid catalysts under similar conditions,12, 56,
57
and proves that coke is not easily formed in [Hnmpo]Cl-
0.60CuCl IL. On the other side, unlike solid catalysts, the coke
generated in liquid phase would not cover the active
component, thereby further ensuring its long-term stability.
As shown in Fig. 6c, then the ILs are analyzed by ESI-MS at
negative mode to identify the Cu species. For the fresh IL, the
most intense peak is observed at m/z 134.9, corresponding to
– 30
CuCl2 . The second most intense peak corresponds to the
–
dimer, Cu2Cl3 , at m/z 232.8.30 And another small peak at m/z
3.3.3. Valence state and speciation of copper. The change in the
valence state of metal species during the reaction is also one of
the main reasons for the deactivation of metal catalyst in
acetylene hydrochlorination.7, 52, 53 Therefore, to evaluate the
valence state and speciation of copper in the fresh and the used
[Hnmpo]Cl-0.60CuCl ILs, we applied an integrated approach
combining XPS, UV-Vis, ESI-MS and Raman.
Fig. 6a shows the XPS spectra of Cu 2p3/2 core level for the
fresh and used ILs, and the peaks were deconvoluted to
distinguish the copper species and their relative quantities. The
peak at 931.9 eV is related to the presence of Cu(I) or metallic
Cu(0) species.15 The other small peak with a higher binding
energy of 935.0 eV is assigned to Cu(II) species,15 perhaps
because the raw material contained a small amount of Cu(II) or
–
169.8 corresponds to CuCl3 . Compared with the fresh IL, the
peaks of the used IL are basically unchanged, except for the
appearance of several minor peaks that may be caused by
reaction intermediates or by-products, indicating that the state
of Cu species changes little. To our knowledge, acetylene can
form a complex with Cu(I) as a π ligand.60 After introducing HCl
into the used IL for 2 h, we found the peak at m/z 123.9 nearly
disappeared (Fig. S14). So it is reasonable to speculate that this
peak corresponds to the CuCl-C2H2 π-complex, which could
react with HCl, and demonstrating that acetylene can be
activated by Cu(I) species in the IL.
Finally, we investigated the Raman spectral below 650 cm−1,
where the stretching vibrations of Cu−Cl are expected to be
observed. Fig. 6d shows the Raman spectra of the fresh and the
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Fig. 7 In situ FTIR spectra of [Hnmpo]Cl-0.60CuCl IL in acetonitrile with continuous
introduction of HCl. The absorption of pure acetonitrile was subtracted from the
total absorption.
used ILs. It can be seen that the broad band around 284.2 cm−1
−
corresponds to the Cu−Cl vibration of the mixtures of CuCl2
−
Fig. 8 (a) Plots of RDG isosurface for the structure of HCl and C2H2 both adsorbed
by [Hnmpo][CuCl2] (Ad_C2H2_HCl). Blue indicates strong attraction, green
indicates very weak interaction, and red indicates strong repulsion. (b) The
electrostatic potential mapped molecular ρe=0.0025 a.u. surface of Ad_C2H2_HCl
structure.
along with a minor amount of Cu2Cl3 , as reported in the
literature.33, 61 Combined with the results of ESI-MS, the two
new peaks appeared after the reaction might be assigned to the
CuCl-C2H2 π-complex, the peak at 229.5 cm−1 belongs to the
Cu−Cl vibration of CuCl,62 and the peak at 575.4 cm−1 represents
the C−H wagging vibration of C2H2.63 Besides, we can hardly see
the characteristic peaks of Cu(II) species in both of the ILs,64
indicating Cu(I) species is indeed the main component and has
not changed substantially after the reaction, which is consistent
with the above results.
solubility of HCl in NMPO is partly ascribed to the chemical
interaction, indicating that the IL might have good activation
ability for HCl.
To reveal the HCl activation mechanism, in situ FTIR analysis
was carried to study the interaction between HCl and
[Hnmpo]Cl-0.60CuCl IL. In the beginning, as can be seen from
Fig. 7, the spectrum of the IL gave a band at 1685.1 cm−1, which
was attributed to the C=O stretching vibration. After HCl was
introduced into the IL, the band appeared a red shift to 1679.5
cm−1 and its intensity gradually increased as time went on,
which may be caused by the increasing interaction between HCl
and the carbonyl group in N-methylpyrrolidonium cation. More
importantly, two new bands appeared at 2343.2 cm−1 and
1489.4 cm−1. The band at 2343.2 cm−1 was assigned to the H−Cl
stretching vibration of HCl under the solvation of acetonitrile,
which can be proven by the in situ FTIR spectra of acetonitrile
with HCl introduction (Fig. S15). The other band at 1489.4 cm−1
should be attributed to the H−Cl stretching vibration of HCl
interacted with the IL. This band demonstrated a nucleophilic
effect in the activation of HCl via electron transfer process
through the –Cl···H−Cl strong hydrogen bond, which was
In brief, characterized by XPS, UV-Vis, ESI-MS and Raman, we
confirm that the major valence state of copper in [Hnmpo]Cl-
−
0.60CuCl IL is Cu(I), which is mainly CuCl2 accompanied by a
−
small amount of Cu2Cl3 . And the Cu(I) species can maintain
excellent stability during the reaction, thus the IL can keep
catalytic activity for a long time.
3.3.4. Absorption and activation properties for reactant. According
to previous research,20, 29, 58 the adsorption properties of HCl
and C2H2 on catalyst are critical in acetylene hydrochlorination.
For one thing, the adsorption of HCl helps to improve the
catalytic performance of catalyst, because the easy adsorption
of HCl facilitates to react with C2H2 in a timely manner, and
results in the decreasing of coke formation and metal reduction
caused by excessive C2H2. For another, too strong adsorption of
C2H2 is not conducive to the reaction, which may lead to fast
deactivation of the catalyst.
–
visualized by using RDG method.67 In Fig. 8a, the Cl···H−Cl
hydrogen bond and the π-coordinate bond between C2H2 and
Cu(I) are shown clearly. In addition, electrons of the IL can
transfer through hydrogen bond, resulting in a decrease in the
electrostatic potential of HCl molecule, which shows a
nucleophilic activation mode (Fig. 8b). Obviously, the results
discussed above confirm that HCl can be activated by the IL.
In short, the excellent HCl activation ability of [Hnmpo]Cl-
0.60CuCl IL is beneficial to acetylene hydrochlorination, thus
ensuring its great catalytic activity and stability.
As [Hnmpo]Cl could react with C2H2 to form VCM, we only
discussed the absorption properties of pure NMPO toward
individual HCl and C2H2. In reported literature,65 1 mol NMPO
could absorb approximately 0.07 mol C2H2 under the conditions
of 45 °C and 97 kPa. As a comparison, 1 mol NMPO could
combine an equal amount of HCl to form [Hnmpo]Cl and still
absorb a considerable amount of HCl.24, 66 Of course, we can
infer that C2H2 primarily combines with Cu(I) species when it is
introduced into [Hnmpo]Cl-0.60CuCl IL. Additionally, the high
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Fig. 9 The relative free energies (ΔG, in black numbers) are given in kcal/mol. The
atom distance and NPA charges are represented in orange italic and red (or blue)
numbers, respectively. Calculated at the M06-D3/SDD/6-311+G(2df,2p) level.
Fig. 10 Correlation of the catalytic performance with the adsorption energy of HCl
and C2H2 in the range of ILs used.
kcal/mol endergonic, so VCM is more negative than C2H2 (-5.43
kcal/mol) or HCl (-4.86 kcal/mol) to be adsorbed by the catalyst.
Compared with traditional copper chlorides used for acetylene
hydrochlorination, a significant feature of this chlorocuprate(I)
IL is that more negative-charged ligands around the metal
center, which may promote the nucleophilic addition of HCl to
C2H2.
The theoretical evaluation of chlorocuprate(I) ILs used was
performed to correlate the catalytic performance with the
adsorption of C2H2 and HCl. Fig.10 shows that [Hnmpo]CuCl2 has
the strongest adsorption capacity for both C2H2 and HCl, and it
has the best catalytic performance. Actually, the acetylene
conversion appears to be more strongly correlated with the
adsorption of HCl (Fig. S16), although it is not a complete linear
correlation. So it can be proved that the adsorption of HCl is
more critical to the reaction, which further supports the above-
proposed mechanism. Moreover, for organic ammonium salts,
the carbonyl group in [Hnmpo]CuCl2 may improve the capacity
of adsorbing reactants by increasing the energy of HOMO and
reducing the energy of LUMO to a nicety (Table S2). A suitable
orbital energy value can facilitate the adsorption and activation
of reactants, thereby enhance its catalytic activity.
3.4. Discussion of possible catalytic mechanism
Based on our experimental results, we combined the theoretical
calculations to propose a possible catalytic mechanism of
[Hnmpo]Cl-CuCl IL for acetylene hydrochlorination. When C2H2
and HCl are introduced into the IL, C2H2 preferentially combines
with Cu(I) species to form π-complex, then the carbon-carbon
triple bond of C2H2 is weakened, which means C2H2 has been
activated. At the same time, HCl could also be adsorbed and
activated by the IL. Therefore, [Hnmpo]Cl-CuCl IL is able to
provide an efficient catalytic system for acetylene
hydrochlorination. To further understand the reaction
mechanism, DFT studies were carried out for acetylene
hydrochlorination by using [Hnmpo][CuCl2] as the catalyst,
which is the most abundant Cu(I) species in this system.
In Fig. 9, the interaction between the catalyst and HCl or C2H2
was investigated first. The adsorption free energy of HCl and
C2H2 on the catalyst is -4.86 kcal/mol and -5.43 kcal/mol.
Although the adsorption capacity of the catalyst for HCl and
C2H2 is almost equivalent, the forces of the interaction between
C2H2 or HCl and catalyst are quite different, which are
dominated by the π-coordinate and strong hydrogen bond,
respectively. The π-coordinate causes the electron density of
C2H2 molecule to decrease, therefore it shows the stronger
3.5. Long-term stability of [Hnmpo]Cl-0.60CuCl IL
−
From the perspective of industrial utilization, the long-term
stability experiments of [Hnmpo]Cl-0.60CuCl IL were carried out
under different acetylene GHSV at optimum reaction conditions
(T = 180 °C, V(HCl)/V(C2H2) = 1.2), and the results were
presented in Fig. 11. Surprisingly, the acetylene conversion did
not decline over 150 h at 50 h−1 of industrial GHSV and even
slightly increased from 82% to 86%. To further examine its
durability, the IL was tested under a higher GHSV of 90 h−1. As
can be seen from Fig. 11, the initial acetylene conversion was
56% and then increased slightly, reaching a maximum of 60% at
the reaction time of 60 h. After maintaining this conversion for
about 30 h, the acetylene conversion slowly decreased to 53%,
which was only a little lower than the initial value. Moreover,
we characterized the IL after long-term reaction at 90 h−1 by
electrophilic activity. Ingeniously, the formation of Cl···H−Cl
hydrogen bond causes the electron transfer from the IL to HCl,
and the NPA charge of HCl is negative, which enables HCl
molecule to perform a nucleophilic process on the activated
C2H2.
The mechanism of this addition reaction can be divided into
two steps, which are the H−Cl cleaved to add Cl to C2H2 and
proton transfer, respectively. The ΔG≠ of TS_AH in the H−Cl
cleavage step is the highest during the reaction process (25
kcal/mol), indicating that the addition of HCl to C2H2 is the key
step of the whole reaction process, which is consistent with
previous research.29, 68, 69 In contrast, the ΔG≠ of TS_Pro_Trans
is only +10.9 kcal/mol, so it is a fast irreversible step for proton
transfer. In addition, the desorption of VCM leads to +2.8
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Conflicts of interest
View Article Online
DOI: 10.1039/C9CY00401G
There are no conflicts to declare.
Acknowledgements
The financial support from the National Basic Research Program
of China (2012CB720302) and the National Key Research and
Development Program of China (2016YFF0102503) are
gratefully acknowledged.
Notes and references
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°C and V(HCl)/V(C2H2) = 1.2.
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an inexpensive chlorocuprate(I) IL.
4
5
6
7
8
4. Conclusion
9
In summary, we reported a chlorocuprate(I) IL with excellent
catalytic performance and superior stability in the gas-liquid
acetylene hydrochlorination process. This is the first successful
use of Cu(I) species as the main active component for acetylene
hydrochlorination. At 180 °C and 30 h−1, an acetylene
conversion of 91% was obtained by using cost-efficient
[Hnmpo]Cl-0.60CuCl IL. Besides, this IL had no visible decline in
acetylene conversion even after 150 h of reaction at a high
acetylene GHSV of 90 h−1. Experimentally, the results of TGA-
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under typical reaction conditions. The characterizations of XPS,
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the major active component, and its valence state had hardly
changed after the reaction. Additionally, the results obtained by
in situ FTIR and theoretical calculations indicated that the IL
could effectively adsorb and activate HCl to react with C2H2,
which is the key feature of the IL that endow its outstanding
catalytic performance. Overall, the reasons for the excellent
catalytic activity and long-term stability of [Hnmpo]Cl-CuCl IL
had been explained in detail. Therefore, our study proves
chlorocuprate(I) IL can be a hopeful candidate for well-
stabilized, low-cost, efficient and green non-mercury catalyst in
the acetylene hydrochlorination, and also offers help for the
future design of catalyst.
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Page 11 of 11
Catalysis Science & Technology
View Article Online
DOI: 10.1039/C9CY00401G
Chlorocuprate(I) ionic liquid can be a well-stabilized, low-cost, efficient and green
non-mercury catalyst for hydrochlorination of acetylene.