Novel nonmetal catalyst of supported tetraphenylphosphonium bromide for acetylene hydrochlorination

Article abstract of DOI:10.1039/c8cy02103a

Tetraphenylphosphonium bromide (TPPB) ionic liquid-supported catalysts were synthesized and evaluated for the acetylene hydrochlorination reaction for the development of highly efficient nonmetal catalysts as substitutes for the currently used industrial mercuric catalyst in the production of vinyl chloride (VCM). The optimal 15% TPPB/SAC catalyst exhibited favorable catalytic activity and stability, with the highest acetylene conversion of 97.1% and the selectivity for VCM above 99.5% under the conditions of 220 °C, an acetylene gas hourly space velocity (GHSV) = 30 h^-1 and V_HCl/V_C2_H2 = 1.15. Characterized by TPD, FTIR, XPS, etc., TPPB exhibits strong adsorption toward HCl but very weak adsorption toward C₂H₂ and VCM; in particular, the adsorbed HCl can change the conformational structure of TPPB. DFT calculations suggest that over the active catalytic site of TPPB, the activation energy of acetylene hydrochlorination is 21.15 kcal mol^-1, which is much lower than that without catalyst (44.29 kcal mol^-1). During the reaction, the H-Cl bond is preferentially activated through accepting the electrons transferred from the anion of TPPB, and then the C₂H₂ is activated to complete the addition reaction of H and Cl. Such unique preferential activation toward the H-Cl bond as well as the weak adsorption to the product VCM promotes the catalytic activity and the stability of the supported TPPB catalysts. The amount of carbon deposition on the 15% TPPB/SAC catalyst is as low as 2.99%, even after 300 h of reaction. The high activity and stability of the 15% TPPB/SAC catalyst indicate great promise for its application as a nonmetal catalyst for acetylene hydrochlorination.

Full text of DOI:10.1039/c8cy02103a

Catalysis
Science &
Technology
View Article Online
View Journal
PAPER
Novel nonmetal catalyst of supported
tetraphenylphosphonium bromide for acetylene
Cite this: DOI: 10.1039/c8cy02103a
hydrochlorination†
ab
a
c
d
Xiaoyan Li,
Jinli Zhang,
Yao Nian,
Shanshan Shang,
Haiyang Zhang,
ad
a
a
You Han * and Wei Li
*
Tetraphenylphosphonium bromide (TPPB) ionic liquid-supported catalysts were synthesized and evaluated
for the acetylene hydrochlorination reaction for the development of highly efficient nonmetal catalysts as
substitutes for the currently used industrial mercuric catalyst in the production of vinyl chloride (VCM). The
optimal 15% TPPB/SAC catalyst exhibited favorable catalytic activity and stability, with the highest acetylene
conversion of 97.1% and the selectivity for VCM above 99.5% under the conditions of 220 °C, an acetylene
−1
gas hourly space velocity (GHSV) = 30 h and VHCl/VC2H2 = 1.15. Characterized by TPD, FTIR, XPS, etc.,
TPPB exhibits strong adsorption toward HCl but very weak adsorption toward C and VCM; in particular,
the adsorbed HCl can change the conformational structure of TPPB. DFT calculations suggest that over
2 2
H
−1
the active catalytic site of TPPB, the activation energy of acetylene hydrochlorination is 21.15 kcal mol ,
−1
which is much lower than that without catalyst (44.29 kcal mol ). During the reaction, the H–Cl bond is
preferentially activated through accepting the electrons transferred from the anion of TPPB, and then the
C H is activated to complete the addition reaction of H and Cl. Such unique preferential activation toward
Received 10th October 2018,
Accepted 20th November 2018
2
2
the H–Cl bond as well as the weak adsorption to the product VCM promotes the catalytic activity and the
stability of the supported TPPB catalysts. The amount of carbon deposition on the 15% TPPB/SAC catalyst
is as low as 2.99%, even after 300 h of reaction. The high activity and stability of the 15% TPPB/SAC catalyst
indicate great promise for its application as a nonmetal catalyst for acetylene hydrochlorination.
DOI: 10.1039/c8cy02103a
rsc.li/catalysis
Hutchings and other researchers have found that many
Introduction
5
–8
9–12
metal catalysts such as Au, Ru,
Pd (ref. 13 and 14) and
Vinyl chloride monomer (VCM) is a significant synthetic ma-
terial for the industrial production of polyvinyl chloride
Cu (ref. 15 and 16) can be used as effective non-mercury cata-
lysts for acetylene hydrochlorination. The gold-based catalyst
has been evaluated on the pilot-plant scale. Hutchings and
(
PVC), and the hydrochlorination of acetylene is the main syn-
1–3
thetic method for preparing VCM in coal-rich regions. The
current industrial catalyst for acetylene hydrochlorination is
3 2 3 2
co-workers supported the Na AuIJS O ) on carbon to obtain a
Au-based catalyst for acetylene hydrochlorination; the 1.6 t
3
HgCl -supported carbon, which can lead to serious environ-
catalyst has been evaluated for more than 4500 h online. Wei
2
mental pollution because of the toxicity and volatility of mer-
curic chloride. To protect human health and the environment
from the harmful effects of mercury compounds, it is neces-
et al. utilized the Au with thiocyanate (–SCN) to synthesize the
0.25 wt% Au-based catalyst, the catalyst displayed favourable
performance under 3000 h online testing with the acetylene
4
1
7
sary to develop environmentally benign catalysts as substi-
conversion of 99% and the selectively to VCM of 99%. Luo
tutes for the HgCl catalyst in acetylene hydrochlorination.
et al. prepared the 0.2 wt% Au/Cu/TCCA catalyst with 6800 h
2
1
8
of pilot-trial evaluation. However, metal catalysts are gener-
ally restricted due to cost and metal reserves and are suscepti-
ble to the impurities of sulfur and arsenic in the industrial
feed of the acetylene hydrochlorination process. In contrast,
nonmetallic catalysts are insensitive to these impurities, stim-
ulating the R&D of efficient nonmetal catalysts for acetylene
hydrochlorination.
a
School of Chemical Engineering and Technology, Tianjin University,
Collaborative Innovation Center of Chemical Science and Chemical Engineering
(
Tianjin), Tianjin 300350, P. R. China. E-mail: liwei@tju.edu.cn
b
Department of Chemical Engineering, Renai College of Tianjin University,
Tianjin 301636, P. R. China
c
School of Energy and Environment, City University of Hong Kong, Tat Chee
Avenue, Kowloon, Hong Kong, P. R. China
d
School of Chemistry and Chemical Engineering, Shihezi University, Shihezi,
At present, the reported nonmetallic catalysts for acetylene
Xinjiang, 832000, P. R. China
hydrochlorination
mainly
include
N-doped
carbon
and B–N
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
1
9–21
22–25
c8cy02103a
materials,
carbon nitride materials,
This journal is © The Royal Society of Chemistry 2018
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
2
6
modified carbon materials.
For example, Wei and co-
mechanism on the catalyst TPPB/SAC. The high activity and
stability of the 15% TPPB/SAC catalyst indicate that it is a
promising nonmetal catalyst for acetylene hydrochlorination.
workers reported that nitrogen-doped carbon nanotubes (N-
CNTs) can promote covalent interactions between acetylene
and N-CNT; the highest acetylene conversion was 7.2% under
the conditions of T = 180 °C and GHSVIJC H ) = 180 h .
Bao et al. synthesized the N-doped SiC@N–C nonmetallic cat-
−
1 20
2
2
Experimental
Materials
alyst, which exhibited the acetylene conversion of 80% under
−
1
Spherical activated carbon (SAC) was purchased from Shang-
hai Carbosino Material Co. Ltd. Tetraphenylphosphonium
bromide was purchased from Shanghai D&B Biological Sci-
ence and Technology Co. Ltd. The C H (99.99%) and HCl
the conditions of GHSVIJC H ) = 30 h and temperature of
2
2
2
5
2
00 °C. Li et al. also found that the N-doped nonmetal cata-
2
7
lyst can enhance the adsorption of acetylene. Recently, Bao
and co-workers synthesized the porous boron nitride (p-BN)
material, displaying favorable catalytic performance for acety-
lene hydrochlorination, with the highest acetylene conversion
of 99% and the selectively to VCM of 99% under the condi-
2
2
(
99.99%) gases were purchased from Tianjin Dong Xiang Gas
Co. Ltd.
−
1
Catalyst preparation
2 2
tions of T = 280 °C and GHSVIJC H ) = 44 h , they concluded
that the superior catalytic performance was due to the abun-
dant defects and edge sites of the p-BN material, which en-
A series of TPPB/SAC catalysts was prepared by an incipient
wetness impregnation method with SAC as the support and
TPPB as the active species. As a representative, TPPB (0.25 g)
was dispersed in 8.0 mL ethanol solution, followed by the ad-
dition of the support, SAC (5.00 g), under ultrasonic mixing
for 30 min. The obtained mixture was incubated at 60 °C for
2
8
hanced the adsorption of acetylene. These results suggest a
promising pathway to improve the activity of nonmetal cata-
lysts for acetylene hydrochlorination via modulating the ad-
sorption capacity of acetylene, since the carbon-based non-
metallic catalysts are still faced with relatively low activity
6
h and then desiccated at 120 °C for 12 h, to prepare the
2
9
and poor stability for acetylene hydrochlorination.
supported catalyst, named 5% TPPB/SAC. Similarly, the cata-
lysts 10% TPPB/SAC, 15% TPPB/SAC and 20% TPPB/SAC were
prepared, respectively adopting the TPPB amounts of 0.50 g,
Ionic liquids (ILs) possess unique physicochemical proper-
ties involving high solubility, high thermal stability, and neg-
ligible volatility, and have been applied in many fields includ-
0.75 g and 1.00 g.
3
0–32
ing catalysis, adsorption and organic synthesis.
On the
other hand, some ILs exhibit favorable solubility for hydro-
Catalyst characterization
3
3,34
gen chloride.
Li and coworkers reported that imidazole
The structural properties of the catalysts were investigated by
nitrogen adsorption/desorption isotherms at −196 °C using
the Quantachrome Autosorb-iQ. Temperature-programmed
decomposition (TPD) profiles were recorded to determine the
adsorption and desorption properties of the reactants and
the product using Quantachrome Instruments AMI-90. The
carbon deposition of catalysts and the decomposition tem-
perature of TPPB were determined by thermogravimetric
analysis (TGA, METTLER TOLEDO TGA/DSC 2). The catalysts
were measured in the temperature range of 35–900 °C at a
chloride ILs showed high solubility for HCl, with the solubil-
3
5
ity being related to the size of the cation in ILs. Utilizing
the unique properties of ILs, some metallic catalysts have
been studied for acetylene hydrochlorination, including the
nanoparticles of Pd, Au and Pt synthesized in the presence of
3
6
anionic surfactant carboxylate ionic liquids (ASC-ILs), the
3
7
38
Pd NPs@IL catalyst, the supported Au–IL complex, the
3
9
Au–Cu–IL/AC catalyst, and the supported Ru@IL/AC cata-
1
2
lyst, etc. It is suggested that these ILs can improve the cata-
lytic activity of metallic species for acetylene hydro-
chlorination. In order to modulate the adsorption capacity of
the reactants on carbon-based nonmetal catalysts, the effect
of the preferential adsorption of ILs toward hydrogen chlo-
ride on the catalytic activity for acetylene hydrochlorination
reaction should be studied.
−
1
heating rate of 10 °C min under an air atmosphere. Trans-
mission electron microscopy (TEM) was conducted on a JEM-
2010FEF instrument to detect the morphologies of the cata-
lysts. The analysis of the elemental compositions on the sur-
face of the catalysts was performed by X-ray photoelectron
spectroscopy (XPS) on the Thermo ESCALAB 250Xi instru-
ment. The functional groups of the catalysts were analyzed by
Fourier transform infrared spectroscopy (FTIR) with a Bruker
Vertex70 FT-IR spectrophotometer.
Tetraphenylphosphonium bromide (TPPB) is not a com-
monly classified ionic liquid, having a high melting point of
2
95–300 °C. However, previous work has indicated that
adding TPPB can enhance the adsorption HCl on ruthenium-
1
2
based catalysts.
In this article, we synthesized the
Catalyst tests
supported TPPB on spherical activated carbon as a nonmetal-
lic catalyst for acetylene hydrochlorination. The optimal 15%
TPPB/SAC catalyst exhibits excellent catalytic activity and sta-
The catalytic performances of samples for acetylene hydro-
chlorination were investigated in the fixed-bed micro-reactor
(i.d. 10 mm). The temperature controller (Chao Yang Automa-
tion Instrument factory) was used to control the reaction tem-
perature. Before the reaction, the reaction system containing
the catalyst (5 mL) was purged by nitrogen to remove the
−
1
bility under the conditions of 220 °C, GHSV = 30 h and
HCl/VC2H = 1.15. Combining DFT calculations with character-
V
2
izations of TPD, FTIR, XPS, etc., the interactions between
TPPB and the reactants were studied to disclose the catalytic
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Catalysis Science & Technology
Paper
impurities of water and air. Hydrogen chloride was put into
the reactor first to activate the catalyst, then acetylene was
fed through the heated micro-reactor. The gas flow in the re-
action was controlled by the mass flow controller. The reac-
tion temperature was 180, 200, 220 or 240 °C, and the GHSV
−
1
2 2
(C H ) value was 50 or 30 h . The reaction effluents passed
through the NaOH solution to absorb the excess hydrogen
chloride gas, and then the effluent sample was analyzed by
gas chromatography (GC-2014C).
Simulation details
All DFT calculations were performed by the Gaussian09 pro-
4
0
gram package. No symmetry constraints were imported dur-
ing the geometry optimization. The Becke-3-Lee-Yang-Parr
4
1
(
B3LYP) with the 6-311G** basis set were used for all atoms.
Geometry optimization, frequency calculation and intrinsic
4
2–44
reaction coordinate (IRC)
were performed on the same
level of calculation. All stationary points were calculated as
minima (no imaginary frequencies) or transition states (one
imaginary frequency) via a Hessian calculation. The sum of
the electronic and zero-point energies was used as the config-
uration energy. The activation energy was calculated as the
Gibbs free energy difference between the intermediate state
and transition state at 220 °C and 1 atm. Atomic charges
were calculated by the Hirshfeld charge. The adsorption ener-
gies (Eads) of the reactants or the product on TPPB are de-
fined by eqn (1).
Fig. 1 Conversion of acetylene (a) and selectivity to VCM (b) over
different catalysts including 5% TPPB/SAC, 10% TPPB/SAC, 15% TPPB/
Eads = Eadsorption state − Emolecule − ETPPB
(1)
SAC and 20% TPPB/SAC. Reaction conditions: Temperature = 180 °C,
where Emolecule denotes the energy of free HCl, C
H
2 2
or the
−1
GHSV (C
2
H
2
) = 50 h , VHCl/VC H = 1.15.
2
2
product VCM, respectively.
Results and discussion
Catalytic performance of TPPB/SAC catalysts
achieved over the 15% TPPB/SAC. To further optimize the cata-
lytic performance of the 15% TPPB/SAC catalyst, it was evalu-
ated under different reaction temperatures in the range of 180–
240 °C. As shown in Fig. 2, the highest acetylene conversion in-
creased with the temperature, being 95.9% at 240 °C, 95.5% at
220 °C, 92.6% at 200 °C and 83.7% at 180 °C. Taking into ac-
count the energy consumption during the reaction, it was indi-
cated that the optimal temperature over 15% TPPB/SAC was
220 °C for acetylene hydrochlorination.
TPPB/SAC catalysts with different TPPB loading amounts were
evaluated in the fixed-bed reactor for acetylene hydro-
chlorination under the conditions of 180 °C and the GHSV
−1
(
C H ) of 50 h . As shown in Fig. 1, the highest acetylene con-
2 2
version was achieved over 15% TPPB/SAC (83.7%), followed by
0% TPPB/SAC (81.6%), 10% TPPB/SAC (76.7%) and 5% TPPB/
2
SAC (72.4%), while over the pure support SAC, the acetylene
conversion was about 53.2%. For all these catalysts, the selec-
tivity for VCM was greater than 99.5%. The 15% TPPB/SAC cata-
lyst displayed optimal catalytic performance for acetylene
hydrochlorination due to the following: 1) when the mass per-
centage of TPPB was higher than 15%, the excess TPPB
resulted in the blocking of micropores or the formation of
thick layers of TPPB over the catalyst surface, and consequently
increased the diffusion resistance of the reactants and the
product during the reaction. Thus, over the 20% TPPB/SAC cat-
alyst, the acetylene conversion was lower. 2) As the mass per-
centage of TPPB increased to 15%, there appeared more active
sites on the catalyst surface, resulting in the increase in the cat-
alytic activity. Thus, the highest acetylene conversion was
The turnover frequency (TOF) value was determined to
evaluate the capacity for transforming acetylene into VCM
over the 15% TPPB/SAC catalyst, under the reaction condi-
−
1
tions of 220 °C, GHSV (C H ) = 2300 h and VHCl/VC H =
2 2
2 2
1.15, as shown in Fig. 3a. The initial conversion of acetylene
was 19.6%, which is valid for calculating the TOF.
nC2H2
nC2H2
TOF =
=
(2)
n t nTPPB t
cat
where nC2H2, nTPPB and t represent the initial molar amount
of acetylene, the molar amount of TPPB in the catalyst, and
the reaction time, respectively.
This journal is © The Royal Society of Chemistry 2018
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
Fig. 2 Conversion of acetylene (a) and the selectivity for VCM (b) over
the 15% TPPB/SAC catalyst under different temperatures in the range
Fig. 3 (a) Conversion of acetylene over the 15% TPPB/SAC catalyst.
−1
of 180–240 °C, with of GHSV (C
2
H
2
) = 50 h and VHCl/VC2H = 1.15.
−1
2
Reaction conditions: Temperature = 220 °C, GHSV (C H ) = 2300 h
,
2
2
V
HCl/VC H = 1.15. (b) The TOFs for different nonmetal catalysts: 15%
2 2
TPPB/SAC, N-CNT, 17% ZIF-8/SAC, B, N-G, PANI-AC900, N-OMC-
O2.0, g-C /AC and SiC@N–C.
3 4
N
The TOF value of the 15% TPPB/SAC catalyst in acetylene
hydrochlorination is 1.80 min . Previously, the TOFs of N-
−
1
2
0
26
26
45
CNT,
SiC@N–C,
g-C
3
N
4
/AC,
N-OMC-O2.0,
PANI-
4
6
26
24
AC900, B, N-G and 17% ZIF-8/SAC metal-free catalysts
were reported to be 2.30 × 10 s , 5.63 × 10 min , 1.42 ×
SAC catalyst, as listed in Table 1. Compared with the SAC
support, the elements of bromine and phosphorus
appeared in the 15% TPPB/SAC catalyst due to the
supported TPPB. The atom percentages of bromine and
phosphorus are, respectively, 0.32 at% and 0.40 at%, ap-
proximate to the atomic ratio of bromine and phosphorus
in the TPPB molecule. The textural parameters of SAC,
the fresh and spent TPPB/SAC catalysts were measured by
−
3
−1
−3
−1
−
2
−1
−4 −1
−2
−1
−2
1
0
min , 3.00 × 10 s , 3.06 × 10 min , 3.32 × 10
−
1
−2
−1
min and 6.04 × 10 min , respectively, as compared in
Fig. 3b. The results indicate that the TOF value of the 15%
TPPB/SAC metal-free catalyst is significantly higher than the
previously reported metal-free catalysts, suggesting that the
addition of TPPB can significantly enhance the catalytic per-
formance for acetylene hydrochlorination.
the
2
N adsorption–desorption experiments. The specific
surface area, pore volume and average pore size of sam-
ples are listed in Table 2. The specific surface area and
Physicochemical properties of the 15% TPPB/SAC catalyst
2
−1
the pore volume of the support SAC are 1323 m
0.62 cm g , respectively. The specific surface area de-
g
and
3
−1
Fig. 4 shows the TEM and elemental mapping images of
the 15% TPPB/SAC catalyst. The elements of carbon, oxy-
gen, bromine and phosphorus appeared in the 15% TPPB/
SAC catalyst. The bromine and phosphorus are typical ele-
ments from TPPB, which are uniformly distributed on the
surface of SAC, indicating the good dispersion of TPPB on
the SAC support. Additionally, XPS was performed to ana-
lyze the chemical element on the surface of 15% TPPB/
2
−1
creased upon loading TPPB, equal to 1137 m
g
for 5%
g
2
−1
2
−1
TPPB/SAC, 1032 m
15% TPPB/SAC and 663 m
total pore volume of TPPB/SAC catalysts decreased from
g
for 10% TPPB/SAC, 873 m
for
2
−1
g
for 20% TPPB/SAC. The
3
−1
3
−1
0.57 cm
g
to 0.38 cm g , when the mass percent of
TPPB increased from 5% to 20%, attributed to the partial
blocking of the micropores on the support SAC by TPPB.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Catalysis Science & Technology
Paper
obvious changes, as compared with the fresh catalysts. Gen-
erally, the reduction of specific surface area in the spent cata-
lyst is mainly caused by the carbon deposition on the surface
during acetylene hydrochlorination. However, such small var-
iations do not affect the catalytic activities over TPPB/SAC for
acetylene hydrochlorination, as reflected by the stable acety-
lene conversion in Fig. 1.
The adsorption properties of the reactants and the product
on the optimal 15% TPPB/SAC catalyst were examined by the
TPD experiments. In Fig. 5a, for the inert gas helium, the TPD
profile shows two desorption peaks located at 488 and 613 °C
respectively, representing the decomposition of TPPB in 15%
12
TPPB/SAC catalyst. For the reactant HCl, the TPD profile on
the 15% TPPB/SAC catalyst showed a peak at 258 °C (Fig. 5b),
whereas on the pure support SAC, the peak was located at 200
°C. Moreover, the desorption peak area of the 15% TPPB/SAC
catalyst was obviously higher than that of the support SAC. It is
known that the peak location in TPD profiles indicates the ad-
sorption strength and the peak area reflects the adsorption ca-
pacity. Hence, the 15% TPPB/SAC catalyst showed stronger ad-
sorption toward HCl, as compared with the support SAC. In the
case of another reactant, C H , the desorption peak in the TPD
2
2
Fig. 4 TEM image of the 15% TPPB/SAC catalyst, together with the
elemental mapping images of C (red), O (green), Br (yellow) and P
profiles is located at 178 °C for either 15% TPPB/SAC or SAC,
while the peak area on the 15% TPPB/SAC is smaller than that
on the SAC (Fig. 5c), suggesting the weaker adsorption toward
(
purple).
2 2
C H on the 15% TPPB/SAC catalyst. For the product VCM, the
location of the desorption peak of VCM is located at 123 °C for
both 15% TPPB/SAC and SAC. The TPD profiles indicate that
the desorption area of VCM on the 15% TPPB/SAC catalyst is
smaller than that on the support SAC (Fig. 5d). This illustrates
that the adsorption of HCl is superior while the desorption of
VCM is fast on the 15% TPPB/SAC catalyst, which is promising
for obtaining high catalytic stability for acetylene
hydrochlorination.
Table 1 Surface element content of samples determined by XPS
Surface atomic composition of samples
(atom.%)
Samples
SAC
C 1s
O 1s
P 2p
0.40
Br 3d
0.32
95.06
96.00
4.94
3.28
15% TPPB/SAC
The active species of TPPB facilitates the adsorption of
HCl and the desorption of product VCM, which was further
demonstrated by the DFT calculation results. As shown in
Table 2 Texture parameters of samples
2
−1
3
−1
S
BET (m g
)
V (cm g
)
D (nm)
2 2
Fig. 6, the adsorption energies of HCl, C H and VCM on
Samples
SAC
Fresh
Spent
Fresh
Spent
Fresh
Spent
−
1
TPPB are −14.39, −5.47 and −4.09 kcal mol , respectively,
1323
1137
1032
873
1258
1026
977
862
646
0.62
0.57
0.51
0.46
0.38
0.57
0.55
0.49
0.47
0.39
1.89
2.06
1.99
2.13
2.20
1.82
2.05
2.01
2.16
2.42
suggesting that the adsorption of HCl on TPPB is much eas-
5
1
1
2
% TPPB/SAC
0% TPPB/SAC
5% TPPB/SAC
0% TPPB/SAC
2 2
ier than that of C H and VCM. When HCl is adsorbed on
TPPB (shown in Fig. 6a), the H1 atom of HCl interacts with
the bromine anion of TPPB, resulting in the H–Cl bond being
elongated from 1.287 Å (in the gas phase) to 1.394 Å. In the
663
SBET: surface area; V: total pore volume; D: average pore diameter.
2 2
case of C H and VCM adsorbed on TPPB (Fig. 6b and c),
there are weak interactions between TPPB and C H or VCM.
2
2
In addition, we adopted the reduced density gradient (RDG)
method to further analyze the interactions between TPPB and
the reactants. As shown in Fig. 7, the results indicate that the
Br anions of TPPB have distinct hydrogen bonds (blue re-
gion) with H in HCl, besides the van der Waals forces (green
region) between HCl and TPPB. On the other hand, the inter-
action between TPPB and C H is much weaker than that
with HCl. Thus, TPPB can effectively activate the reactant of
HCl to facilitate the occurrence of acetylene
hydrochlorination.
The average pore size of the TPPB/SAC catalysts increased
with increasing the amount of TPPB in the TPPB/SAC cata-
lysts, which was also attributed to the micropores of SAC be-
ing filled by the TPPB. For the spent 5% TPPB/SAC, 10%
TPPB/SAC, 15% TPPB/SAC and 20% TPPB/SAC catalysts that
experienced 40 h of reaction, the specific surface areas de-
creased by approximately 9.7%, 5.3%, 1.2% and 2.5%, respec-
tively, as compared with the fresh individual counterparts.
The pore volumes of the spent TPPB/SAC catalysts showed no
2
2
This journal is © The Royal Society of Chemistry 2018
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
the reactants and the product, but the electrons transferred
from the anion of TPPB to the reactant HCl is twice that to the
reactant C H . The sum of transferred electrons from TPPB to
2 2
HCl is 0.224e, while 0.076e and 0.044e, respectively, are trans-
ferred to C H and C H Cl. Moreover, the charge variations of
2
2
2
3
individual atoms of the reactants and the product are listed in
Table S1.† It is illustrated that most of the electrons are trans-
ferred from the anion of TPPB to the Cl atom of HCl, resulting
in the H–Cl bond polarity increasing to 0.446, which is signifi-
cantly higher than that in free HCl (0.316). Such unique prefer-
ential activation toward the H–Cl bond, as well as the weak ad-
sorption to the product VCM, promotes the catalytic activity
and the stability of the supported TPPB catalysts.
On the other hand, FTIR spectra were obtained to detect the
variation in the structural conformation of TPPB caused by the
adsorption of reactants. Fig. 8 shows the FTIR spectra of the
TPPB (Δ), TPPB-C H and TPPB-HCl samples, where TPPB (Δ)
2
2
indicates the pure TPPB sample being heated at 220 °C for 1 h
under an air atmosphere, TPPB-C H is the TPPB sample that
2
2
experienced heating at 220 °C for 1 h under the environment
of C , and TPPB-HCl is the TPPB sample that experienced
heating at 220 °C for 1 h under the atmosphere of HCl. Gener-
2 2
H
−1
−1
−1
ally, the peaks at 692 cm , 723 cm and 765 cm correspond
to the out-of-plane deformation vibration of C–H in aromatic
−
1
−1
−1
rings; the peaks at 995 cm , 1154 cm , 1181 cm and 1311
−1
cm are attributed to the in-plane deformation vibration of
C–H, while the peaks at 1433 cm , 1481 cm and 1582 cm
are due to the aromatic CC stretching vibration.
−1
−1
−1

47,48
For the
bonds between aromatic carbon and phosphorus, the peak at
−
1
1
5
106 cm reflects the stretching vibration, whereas the peak at
−1
28 cm is assigned to the bending vibration. As seen in
Fig. 8, TPPB-HCl has a special broad band at around 1106
−
1
−1
cm , besides the weakening peaks at 1481 and 765 cm , illus-
trating that the adsorption of HCl results in variations in the
conformation of TPPB, in particular, the interactions with HCl
affect the covalent bonding between aromatic carbon and phos-
phorus in TPPB, which was confirmed by the change in the di-
hedral angles in the phenyl group (Table S2†).
2 2
Fig. S1† displays the stable structures of TPPB, TPPB-C H
and TPPB-HCl, indicating that the H atom in HCl interacts with
the Br anion in TPPB, transferring the electrons to the H and Cl
atoms in HCl. The Cl (47) atom in HCl is close to two H atoms
(36 and 37) in the aromatic ring of TPPB (Fig. S1c†); the dis-
tances between Cl (47) and two H atoms (36 and 37) are 2.967 Å
and 3.302 Å, respectively. The charges of the two H atoms (36
and 37) were increased to 0.039e and 0.053e from 0.044e and
0.056e (in TPPB), respectively, suggesting that the Cl (47) atom
in HCl interacts with the aromatic ring of TPPB to alter the vi-
brational structure of phenyl in TPPB. These results confirm
that TPPB has strong adsorption toward HCl and the adsorbed
HCl can change the conformational structure of TPPB.
2 2
Fig. 5 TPD profiles of HCl, C H and VCM on the fresh 15% TPPB/SAC
catalyst versus the support SAC: (a) TPD-He, (b) TPD-HCl, (c) TPD-
and (d) TPD-VCM.
2 2
C H
Catalytic mechanism of TPPB in acetylene hydrochlorination
As listed in Table 3, the Hirshfeld charge analysis shows
that both the cation and the anion of TPPB donate electrons to
We used DFT calculations to understand the mechanistic
pathway over the TPPB catalyst. Fig. 9 displays the reaction
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Catalysis Science & Technology
Paper
Fig. 6 The stable structures of HCl (a), C
2
H
2
(b), and VCM (c) adsorption on TPPB. Carbon, hydrogen, bromine, phosphorus and chlorine atoms
are depicted in gray, white, red, orange and green, respectively. The selected bond lengths are in Å.
2 2 2 2
Fig. 7 The stable structures of HCl and C H adsorption on TPPB using RDG method: (a) TPPB-HCl and (b) TPPB-C H . The red area indicates
steric hindrance, the green area indicates van der Waals' forces, and the blue area indicates strong hydrogen bond.
Table 3 The Hirshfeld charge variations in TPPB before and after adsorption
The charge of the anion in TPPB (e)
The charge of the cation in TPPB (e)
Before
adsorption
After
adsorption
The amount of
electrons lost
Before
adsorption
After
adsorption
The amount of
electrons lost
Adsorbate
HCl
−0.668
−0.668
−0.668
−0.510
−0.610
−0.634
0.158
0.048
0.034
0.668
0.668
0.668
0.734
0.696
0.678
0.066
0.028
0.010
C
C
2
H
H
2
2
3
Cl
pathway over the TPPB catalyst, including the HCl and C
H
2 2
4.865 Å, suggesting the breakage of the H1–Cl bond. Com-
co-adsorption on TPPB (Co-ads), the intermediate (IM), the
transition state (TSTPPB), the production of VCM on TPPB (Pr-
ads) and the desorption of VCM (Pr), as well as the details of
electron transfer (Table S3†). In the Co-ads configuration, the
distance between H1 and Br is 2.054 Å, which is slightly lon-
ger than the H–Br bond length of free HBr (1.427 Å). The cat-
ion and anion of TPPB donate electrons to HCl and C H ,
pared with the Co-ads configuration, the chlorine atom in IM
gets more electrons from the anion of TPPB and C H
2 2
(0.348e). The C1C2 bond polarity increases from 0.039 to
0.047. In the configuration of TSTPPB, the C1C2 bond polar-
ity is greatly increased to 0.162, corresponding to the charge
of −0.071e for C1 and 0.059e for C2. The distance between H1
and Br is stretched to 1.855 Å, and H1 atom attacks the C1
2
2
while the total number of electrons transferred from TPPB to
HCl (0.204e) is much more than that to C (0.060e), indi-
atom in C . Meanwhile, some electrons on the chlorine
2
H
2
H
2 2
atom and C H are transferred back to TPPB, mainly on the
2 2
cating that the TPPB can preferentially activate HCl. In the
IM configuration, the H1 atom in HCl moves toward the Br
anion to form the H1–Br bond, with the bond length of 1.450
Å being close to that of the free HBr molecule (1.427 Å).
Meanwhile, the distance between H1 and Cl is as great as
Br anion. The scheme of the electron transfer process during
the reaction is shown in Fig. 10. The transition state has one
imaginary frequency (−276.95 cm ), which is related to the
stretching vibration of the H1 atom between the C1 and Br
atoms and the bending vibration of the H3 atom. To confirm
−
1
This journal is © The Royal Society of Chemistry 2018
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
transition state is reasonable and the reaction pathway is log-
ical. Once the C H Cl molecule is generated, the desorption
2
3
−
1
energy is only 4.09 kcal mol , which is conducive to the de-
sorption of VCM from the surface of the catalyst, and inhibits
the formation of carbon deposition by the polymerization of
VCM. In contrast, Fig. S3† shows the reaction pathway in the
absence of catalyst, including the reactant (Rea), transition
state (TS) and product (Pr), with the energy barrier as high as
−
1
4
2.70 kcal mol . In the presence of TPPB catalyst, the acety-
lene hydrochlorination reaction is performed in two steps,
with the first step involving activating the reactant of HCl by
the anion of TPPB (with the energy barrier of 14.08 kcal
−
1
mol ), and the second step being the addition of H and Cl
atoms to the activated C H (with the energy barrier of 14.29
2
2
−
1
kcal mol ). Through the two-step reaction pathway in the
presence of TPPB, the energy barrier of acetylene hydro-
chlorination is significantly reduced. Converting to Gibbs free
energy, the reaction activation energy was calculated as 21.15
kcal mol over the TPPB catalyst, and 44.29 kcal mol in the
absence of a catalyst.
2 2
Fig. 8 FTIR spectra of TPPB (Δ), TPPB-C H and TPPB-HCl samples.
−
1
−1
the rationality of this transition state, IRC was performed
and the results are shown in Fig. S2.† The calculated reactant
via IRC is the intermediate state and the calculated product
via IRC is the C H Cl adsorption structure. Therefore, the
Previously, Wei and his coworkers adopted the N-doped
carbon nanotubes as the nonmetallic catalyst for acetylene
2
3
Fig. 9 Reaction energy diagram and the geometries of the substances involved in the reaction pathway: Co-adsorbed reactants (Co-ads), inter-
mediate (IM), transition state (TS), product-adsorbed (Pr-ads) and product (Pr). Carbon, hydrogen, bromine, phosphorus and chlorine atoms are
depicted in gray, white, red, orange and green, respectively.
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Catalysis Science & Technology
Paper
Fig. 10 The schematic diagram of electron transfer processes in view of the substances disclosed in the reaction pathway of Fig. 9.
hydrochlorination and found that N-CNTs could enhance the
interaction between the highest occupied molecular orbital
(
HOMO) in NCNT and the lowest unoccupied molecular or-
bital (LUMO) in C , suggesting that the electrons were
transferred from NCNT to C
2 2
H
2
0
2 2
H .
In our previous studies,
the Ru@IL/AC catalysts were applied in acetylene hydro-
chlorination, and the results indicated that the electrons pre-
1
2
fer to transfer from the HOMO of ILs to the LUMO of HCl.
Herein, in the presence of TPPB catalyst, the H–Cl bond is
first activated via accepting electrons from the Br anion of
TPPB, then the charge variations in both carbon atoms of
C H result in the increase in the C1C2 bond polarity and
2
2
promote the completion of the addition reaction of H and Cl
atoms to C to produce VCM.
2 2
H
The stability evaluation of the 15% TPPB/SAC catalyst
Previous literature indicated that most of the nonmetal cata-
3
4
5
lysts, including PANI-AC900, N-OMC-700, g-C N /AC, B, N-
GO,
3
4
6
24
4 1
Z M (ref. 22) and ZIF-8/SAC, showed poor catalytic
stability for acetylene hydrochlorination. The stability evalua-
tion of the 15% TPPB/SAC catalyst was performed under the
−
1
conditions of 220 °C and GHSV (C
ing to the industrial space velocity. As shown in Fig. 11, the
5% TPPB/SAC catalyst exhibits superior catalytic stability.
2 2
H ) of 30 h , approximat-
1
The highest acetylene conversion was 97.1%, and the selectiv-
ity for VCM was approximately 99.5%. With time, the acety-
lene conversion slightly decreased to 94.0% within 300 h.
TGA profiles were performed to measure the amount of car-
bon deposition on the spent TPPB/SAC catalyst. For the
unsupported pure TPPB, the decomposition occurred in the
temperature range of 400–500 °C in an air atmosphere
Fig. 11 Catalytic performance evaluation of the 15% TPPB/SAC
catalyst. Reaction conditions: Temperature = 220 °C, GHSV (C ) =
30 h and VHCl/VC H = 1.15.
2 2
(Fig. 12a). For the fresh 15% TPPB/SAC, the mass loss was
2
H
2
−1
only 3.79% in the temperature range of 100–500 °C, while the
This journal is © The Royal Society of Chemistry 2018
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
Therefore, due to the low carbon deposition, the 15% TPPB/
SAC catalyst exhibits superior stability for acetylene hydro-
chlorination. A combination of TPD profiles, DFT calculations
and TGA indicated that the preferential activation toward H–Cl
bonds and the weak adsorption of C H and VCM resulted in
2
2
the favorable catalytic activity of TPPB/SAC and also the low car-
bon deposition during the stability evaluation reaction process.
Conclusions
We adopted TPPB IL as the active species using the simple in-
cipient wetness impregnation method to prepare the supported
TPPB/SAC catalysts for acetylene hydrochlorination. The
highest acetylene conversion over the optimal 15% TPPB/SAC
metal-free catalyst was 97.1%, and the selectivity for VCM was
−1
above 99.5% under the conditions of 220 °C, GHSV = 30 h
and VHCl/VC2H2 = 1.15. Through catalyst characterization and
the DFT calculation results, it was confirmed that the addition
of TPPB can efficiently facilitate the adsorption and activation
of HCl, with weak adsorption to C H and VCM. The anion bro-
2
2
mide of TPPB donates electrons to the chlorine atom of HCl,
resulting in the H–Cl bond polarity increasing from 0.316 (free
HCl) to 0.446. Compared with the reaction in the absence of
the catalyst, the reaction activation energy with TPPB as the
−1
metal-free catalyst decreased from 44.29 kcal mol (without
−1
catalyst) to 21.15 kcal mol , meaning that the TPPB active spe-
cies reduced the reaction barrier by the adsorption and activa-
tion of HCl. Furthermore, the weak adsorption to C H and
2
2
VCM inhibited carbon deposition during the reaction; the
amount of carbon deposition was calculated as 2.99% in the
spent 15% TPPB/SAC catalyst after 300 h of reaction, suggesting
that the 15% TPPB/SAC could be a promising metal-free cata-
lyst for acetylene hydrochlorination.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
This work was supported by Natural Science Foundation of
China (21776215) and the Program for Changjiang Scholars
and Innovative Research Team in University (IRT_15R46).
Fig. 12 TGA curves of the pure TPPB (a), the fresh SAC and fresh 15%
TPPB/SAC catalyst (b), the fresh and spent 15% TPPB/SAC catalysts (c)
References
(
after 300 h reaction, under the reaction conditions of 220 °C, GHSV
−1
(
2 2
2
C H ) = 30 h and VHCl/VC2H = 1.15) in air atmosphere.
1 J. Zhang, N. Liu, W. Li and B. Dai, Front. Chem. Sci. Eng.,
2011, 5, 514–520.
2
M. Zhu, Q. Wang, K. Chen, Y. Wang, C. Huang, H. Dai, F. Yu,
quick mass loss at temperatures higher than 500 °C is associ-
L. Kang and B. Dai, ACS Catal., 2015, 5, 5306–5316.
ated with the burning of the carbon support in the air
3 P. Johnston, N. Carthey and G. J. Hutchings, J. Am. Chem. Soc.,
2015, 137, 14548–14557.
(Fig. 12b). It is suggested that the interactions between TPPB
and the support retard the decomposition of TPPB. For the
spent catalyst, 15% TPPB/SAC, after 40 h of reaction, no car-
bon deposition was detected (Fig. S4†), and after 300 h of re-
action, the amount of carbon deposition was calculated as
4 T. K. Mackey, J. T. Contreras and B. A. Liang, Sci. Total
Environ., 2014, 472, 125–129.
5 M. Conte, A. F. Carley, C. Heirene, D. J. Willock, P. Johnston,
A. A. Herzing, C. J. Kiely and G. J. Hutchings, J. Catal.,
2007, 250, 231–239.
2
.99% (Fig. 12c).
Catal. Sci. Technol.
This journal is © The Royal Society of Chemistry 2018

View Article Online
Catalysis Science & Technology
Paper
6
7
8
9
H. Zhang, B. Dai, X. Wang, L. Xu and M. Zhu, J. Ind. Eng.
Chem., 2012, 18, 49–54.
J. Zhang, Z. He, W. Li and Y. Han, RSC Adv., 2012, 2,
32 N. Sahiner, A. O. Yasar and N. Aktas, Renewable Energy,
2017, 101, 1005–1012.
33 F. Pöhlmann, L. Schilder, W. Korth and A. Jess,
ChemPlusChem, 2013, 78, 570–577.
34 K. Murugappan and D. S. Silvester, Phys. Chem. Chem. Phys.,
2016, 18, 2488–2494.
35 R. He, B. Long, Y. Lu, H. Meng and C. Li, J. Chem. Eng. Data,
2012, 57, 2936–2941.
4
814–4821.
X. Li, M. Zhu and B. Dai, Appl. Catal., B, 2013, 142–143,
34–240.
Y. Pu, J. Zhang, L. Yu, Y. Jin and W. Li, Appl. Catal., A,
014, 488, 28–36.
2
2
1
1
1
1
1
1
1
1
1
1
2
2
0 H. Zhang, W. Li, Y. Jin, W. Sheng, M. Hu, X. Wang and J.
Zhang, Appl. Catal., B, 2016, 189, 56–64.
1 J. Zhang, W. Sheng, C. Guo and W. Li, RSC Adv., 2013, 3,
36 J. Hu, Q. Yang, L. Yang, Z. Zhang, B. Su, Z. Bao, Q. Ren, H.
Xing and S. Dai, ACS Catal., 2015, 5, 6724–6731.
37 L. Yang, Q. Yang, J. Hu, Z. Bao, B. Su, Z. Zhang, Q. Ren and H.
Xing, AIChE J., 2018, 64, 2536–2544.
38 J. Zhao, S. Gu, X. Xu, T. Zhang, Y. Yu, X. Di, J. Ni, Z. Pan and
X. Li, Catal. Sci. Technol., 2016, 6, 3263–3270.
21062–21068.
2 S. Shang, W. Zhao, Y. Wang, X. Li, J. Zhang, Y. Han and W. Li,
ACS Catal., 2017, 7, 3510–3520.
3 Q. Song, S. Wang, B. Shen and J. Zhao, Pet. Sci. Technol.,
39 J. Zhao, Y. Yu, X. Xu, S. Di, B. Wang, H. Xu, J. Ni, L. Guo, Z.
Pan and X. Li, Appl. Catal., B, 2017, 206, 175–183.
40 M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria,
M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone,
G. A. Petersson, H. Nakatsuji, X. Li, M. Caricato, A.
Marenich, J. Bloino, B. G. Janesko, R. Gomperts, B.
Mennucci, H. P. Hratchian, J. V. Ortiz, A. F. Izmaylov, J. L.
Sonnenberg, D. Williams-Young, F. Ding, F. Lipparini, F.
Egidi, J. Goings, B. Peng, A. Petrone, T. Henderson, D.
Ranasinghe, V. G. Zakrzewski, J. Gao, N. Rega, G. Zheng,
W. Liang, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J.
Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H.
Nakai, T. Vreven, K. Throssell, J. A. Montgomery, Jr., J. E.
Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers,
K. N. Kudin, V. N. Staroverov, T. Keith, R. Kobayashi, J.
Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S.
Iyengar, J. Tomasi, M. Cossi, J. M. Millam, M. Klene, C.
Adamo, R. Cammi, J. W. Ochterski, R. L. Martin, K.
Morokuma, O. Farkas, J. B. Foresman and D. J. Fox,
Gaussian 09, Revision A.02, Gaussian, Inc., Wallingford CT,
2016.
41 J. Stephens and F. J. Devlin, J. Phys. Chem., 1994, 98,
11623–11627.
42 K. Fukui, Acc. Chem. Res., 1981, 14, 363–368.
43 C. Gonzalez and H. B. Schlegel, J. Phys. Chem., 1989, 90,
2154–2161.
44 C. Gonzalez and H. B. Schlegel, J. Phys. Chem., 1990, 94,
5523–5527.
45 Y. Yang, G. Lan, X. Wang and Y. Li, Chin. J. Catal., 2016, 37,
1242–1248.
2
010, 28, 1825–1833.
4 L. Wang, F. Wang and J. Wang, New J. Chem., 2016, 40,
019–3023.
3
5 H. Li, F. Wang, W. Cai, J. Zhang and X. Zhang, Catal. Sci.
Technol., 2015, 5, 5174–5184.
6 K. Zhou, J. Si, J. Jia, J. Huang, J. Zhou, G. Luo and F. Wei, RSC
Adv., 2014, 4, 7766–7769.
7 K. Zhou, J. Jia, C. Li, H. Xu, J. Zhou, G. Luo and F. Wei, Green
Chem., 2015, 17, 356–364.
8 Xu H, K. Zhou, J. Si, C. Lib and G. Luo, Catal. Sci. Technol.,
2
016, 6, 1357–1366.
9 X. Li, Y. Wang, L. Kang, M. Zhu and B. Dai, J. Catal.,
014, 311, 288–294.
2
0 K. Zhou, B. Li, Q. Zhang, J. Huang, G. Tian, J. Jia, M. Zhao, G.
Luo, D. Su and F. Wei, ChemSusChem, 2014, 7, 723–728.
1 X. Wang, B. Dai, Y. Wang and F. Yu, ChemCatChem, 2014, 6,
2339–2344.
2
2
2 X. Li, J. Zhang and W. Li, J. Ind. Eng. Chem., 2016, 44, 146–154.
3 S. Chao, F. Zou, F. Wan, X. Dong, Y. Wang, Y. Wang, Q. Guan,
G. Wang and W. Li, Sci. Rep., 2017, 7, 39789–39795.
4 X. Li, J. Zhang, Y. Han, M. Zhu, S. Shang and W. Li, J. Mater.
Sci., 2018, 53, 4913–4926.
2
2
2
2
2
2
3
3
5 X. Li, X. Pan, L. Yu, P. Ren, X. Wu, L. Sun, F. Jiao and X. Bao,
Nat. Commun., 2014, 5, 3688–3695.
6 B. Dai, K. Chen, Y. Wang, L. Kang and M. Zhu, ACS Catal.,
2015, 5, 2541–2547.
7 T. Zhang, J. Zhao, J. Xu, J. Xu, X. Di and X. Li, Chin. J. Chem.
Eng., 2016, 24, 484–490.
8 P. Li, H. Li, X. Pan, K. Tie, T. Cui, M. Ding and X. Bao, ACS
Catal., 2017, 7, 8572–8577.
46 C. Zhang, L. Kang, M. Zhu and B. Dai, RSC Adv., 2015, 5,
7461–7468.
47 G. Wang, X. Liu, Y. Ren, C. Zhang and X. Tao, Crystals,
2017, 7, 154–160.
48 B. Haddad, A. Paolone, D. Villemin, M. Taqiyeddine, E-h.
Belarbi, S. Bresson, M. Rahmouni, N. R. Dhumal, H. J. Kim
and J. Kiefer, J. Mol. Struct., 2017, 1146, 203–212.
9 X. Li, P. Li, X. Pan, H. Ma and X. Bao, Appl. Catal., B,
2017, 210, 116–120.
0 M. J. Schneider, M. Lijewski, R. Woelfel, M. Haumann and P.
Wasserscheid, Angew. Chem., Int. Ed., 2013, 52, 6996–6999.
1 W. Zhang, Q. Wang, H. Wu, P. Wu and M. He, Green Chem.,
2014, 16, 4767–4774.
This journal is © The Royal Society of Chemistry 2018
Catal. Sci. Technol.