High performance of supported Cu-based catalysts modulated via phosphamide coordination in acetylene hydrochlorination

Article abstract of DOI:10.1016/j.apcata.2020.117408

In order to develop a cut-price, high-efficiency non-mercuric catalyst for acetylene hydrochlorination reaction, several kinds of supported Cu-based catalysts containing phosphoramide ligands have been synthesized by wet impregnation method. The outstanding catalytic activity was obtained over 15 %Cu10 %HMPA/SAC catalyst with acetylene conversion of 87.25 % in the test conditions of T =180 °C, GHSV(C₂H₂) =180 h^?1 and V(HCl): V(C₂H₂) = 1.2. The catalyst with optimal HMPA ligand also exhibited splendid stability in 100 h lifetime test. The analysis for XRD, TEM, TGA, ICP, H₂-TPR and XPS indicated that HMPA ligand can improve Cu species dispersion, restrain coke deposition, suppress loss of loading Cu, and stabilize valence state of active Cu species. Due to electron transfer mechanism, steady coordination structure between Cu and HMPA led to favorable properties of Cu-based catalyst, which was further proved by FT-IR, Raman spectra, O 1s XPS spectra integrated with DFT calculations.

Full text of DOI:10.1016/j.apcata.2020.117408

Journal Pre-proof
High Performance of Supported Cu-based Catalysts Modulated via
Phosphamide Coordination in Acetylene Hydrochlorination
Yubing Hu (Investigation) (Writing - original draft), Yan Wang, Yulian
Wang, Wei Li (Resources), Jinli Zhang (Resources), You Han
(Supervision)<ce:contributor-role>Writing- Review and editing)
(Project administration) (Funding acquisition)
PII:
S0926-860X(20)30001-6
DOI:
https://doi.org/10.1016/j.apcata.2020.117408
APCATA 117408
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To appear in:
Applied Catalysis A, General
Received Date:
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Accepted Date:
10 October 2019
30 December 2019
1 January 2020
Please cite this article as: Hu Y, Wang Y, Wang Y, Li W, Zhang J, Han Y, High Performance of
Supported Cu-based Catalysts Modulated via Phosphamide Coordination in Acetylene
Hydrochlorination, Applied Catalysis A, General (2020),
doi: https://doi.org/10.1016/j.apcata.2020.117408
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High Performance of Supported Cu-based Catalysts
Modulated via Phosphamide Coordination in Acetylene
Hydrochlorination
Yubing Hu a, Yan Wang a, Yulian Wang a, Wei Li a, Jinli Zhang a,b and You Han a,*
a School of Chemical Engineering and Technology, Tianjin University, Tianjin
300072, China
b School of Chemistry and Chemical Engineering, Shihezi University, Shihezi
832003, China
Author information
Corresponding Author
You Han (E-mail: yhan@tju.edu.cn)
Notes
The authors declare no competing financial interest
Graphical Abstract
1

Highlights
•
•
•
•
A series of supported Cu complex catalysts were prepared with phosphoramide
ligands and had high efficiency in the acetylene hydrochloride reaction.
The catalyst with optimal HMPA ligand exhibited good stability in the 100 h
lifetime test.
HMPA ligand can restrain the coke deposition, suppress the loss of loading Cu,
and stabilize the valence state of active Cu species.
Due to the electron transfer mechanism, a stable coordination structure formed
between the Cu and HMPA via Cu-O binding.
Abstract
In order to develop a cut-price, high-efficiency non-mercuric catalyst for
acetylene hydrochlorination reaction, several kinds of supported Cu-based catalysts
containing phosphoramide ligands have been synthesized by wet impregnation
2

method. The outstanding catalytic activity was obtained over 15%Cu10%HMPA/SAC
catalyst with acetylene conversion of 87.25% in the test conditions of T = 180 °C,
GHSV(C2H2) = 180 h-1 and V(HCl): V(C2H2) = 1.2. The catalyst with optimal HMPA
ligand also exhibited splendid stability in 100 h lifetime test. The analysis for XRD,
TEM, TGA, ICP, H2-TPR and XPS indicated that HMPA ligand can improve Cu
species dispersion, restrain coke deposition, suppress loss of loading Cu, and stabilize
valence state of active Cu species. Due to electron transfer mechanism, steady
coordination structure between Cu and HMPA led to favorable properties of Cu-based
catalyst, which was further proved by FT-IR, Raman spectra, O 1s XPS spectra
integrated with DFT calculations.
Keywords: Phosphoramide ligand, Cu-based catalyst, Coordination, Acetylene
hydrochlorination
1. Introduction
Ligands are widely used in the field of catalysis because they can form a stable
coordination structure with metals and thus stabilizing the metal valence state in the
catalysts, thereby preventing the catalyst deactivation caused by metal nanoparticles
agglomeration, which is the main reason for the loss of active sites, in reverse
obtaining high surface areas to provide more active metal area [1]. Therefore, the role
of ligands in the territory of nanoparticle catalysis has been paid more and more
attention [2-4] and has been reported to influence a great many of factors of the
catalysts, including the properties of electron [5], the oxidation state of the metal [6],
the spatial environment of the active sites [7-8], the variation of hydrophobicity [9]
3

and the blocking of toxic sites [10].
Copper is more ubiquitous and more affordable compared with the noble metals,
hence a great many of trials devoted on the utilization of copper in catalysis have been
explored in the light of the economical efficiency and sustainability [11-12]. Due to
the multifunctionality of design, Cu-based complexes can effectively adjust the
reaction rate, conversation, selectivity, yield, and even can change the catalytic
mechanism during the reaction processes [13-14]. Combined with the advantages of
ligand and copper, the Cu-based complexes can be served as catalyst for a
comprehensive range of reactions [15-16], for instance, cross-coupling of the C-C
[17] and C-Heteroatom [18], oxidation of alkanes [19], C-H functionalization [20],
synthesis of heterocyclic [21], asymmetric conjugate additions [22] and click
chemistry [23]. However, the application of Cu complexes in the hydrochlorination of
acetylene is very limited [24].
Acetylene hydrochlorination reaction is a crucial industrial process for
fabricating the third largest plastic-vinyl chloride (VCM) in the world. VCM is the
monomer that produces the third most generally utilized plastic polyvinyl chloride
(PVC) in our lives. The traditional supported catalyst for the hydrochlorination of
acetylene is HgCl2/AC, which is virulent, evaporable and pernicious to human health
and environment [25-26]. Cu as an alternative for noble metals holds a significantly
place for further investigating an efficient and eco-friendly catalyst in the industry
process of PVC manufacturing. Although the Cu-based catalyst with certain catalytic
properties for hydrochlorination of acetylene is obtained, the largest problem of the
4

Cu catalyst is the deactivation which is mainly arised from metal active sites
aggregation and reduction of high valence state of copper. This is a tremendous
challenge to improve the stability of the Cu-based catalyst thereby enhancing the
catalytic performance [27]. In order to settle the conundrum of catalyst deactivation, a
series of efforts in the modification on Cu-based catalyst for acetylene
hydrochlorination have been performed. For instance, Li et al reported the appropriate
phosphorus doping promotes copper species dispersion, increases the reciprocity
between the Cu catalyst and the support, and suppresses the copper species
aggregation during the acetylene hydrochlorination process [28]. Zhai et al prepared
carbon-supported perovskite-like CsCuCl3 double nonprecious metal catalyst for
acetylene hydrochlorination to improve the HCl adsorption and maintain Cu2+ species
[29]. Zhou et al introduced the N-CNTs as a support to increase acetylene adsorption
capacity thus significantly improving catalytic performance [30]. Wang et al imported
MOMTPPC ionic liquid to Cu-based catalyst, thus realizing the obvious catalytic
activity enhancement [31]. It is worth mentioning that the modification methods for
Cu-based catalysts in the above studies are mainly the addition of auxiliaries or the
second metal and decoration of support, scarcely any synthesis of Cu complex
through the formation of stable coordination bond between Cu and ligand. Recently,
Wang et al prepared the HEDP ligand modified Cu-based catalyst with relatively
83.4% acetylene conversion and catalytic stability of only 20 h. The catalytic activity
of Cu-1HEDP/AC catalyst is inferior to that of the noble catalyst and industrial
catalyst due to the structure of the ligand [24].
5

In this work, we select four types of phosphoramide ligand containing N and P to
form Cu complexes because phosphoramide ligand has a range of good properties
including PVC weatherability, favorable solubility and low cost, especially the
possible synergistic effect of phosphorus and nitrogen, the molecular structure of the
phosphoramide ligands are shown in Figure S1. Four kinds of carbon-supported Cu
complex catalysts containing these phosphoramide ligands were synthesized using a
portable method of the wet impregnation to explore the effect of ligand contents and
functional groups for disparate phosphoramide materials on catalytic performance.
Furthermore, the scientific characterizations combined DFT simulations were
analyzed to reveal the coordination structure between the Cu and HMPA ligand. The
HMPA modified Cu catalyst possesses further industrial application potential in
acetylene hydrochlorination. The cheap, environmentally-friendly and easy prepared
Cu-based catalyst will facilitate the continuable PVC industry progress.
2. Experimental and theoretical method
2.1 Materials.
The following experimental materials were used in the research: copper chloride
(CuCl2, purity 98%) was purchased from Shanghai Aladdin Bio-Chem Technology
Co., Ltd. Hexamethylphosphoramide (HMPA) was purchased from Shanghai D&B
Biological Science and Technology Co. Ltd., with the purity higher than 99%. Tris
(N,N-tetramethylene) phosphoric acid triamide (TPPO) was purchased from Shanghai
Titanchem Co., Ltd., with the purity higher than 98%. Diethyl PhosphoraMidate
(NSC, purity 98%) was purchased from TCI (Shanghai) Development Co., Ltd.
6

Cyclophosphamide hydrate (CTX) was purchased from Shanghai D&B Biological
Science and Technology Co. Ltd., with the purity higher than 97%. Spherical
activated carbon (marked as SAC, 20-40 mesh) was purchased from Shandong Jining
Chengguang Coal Chemical Co., Ltd. Hydrogen chloride (HCl, purity >99.99%) was
purchased from Tianjin Dongxiang special gas Co., Ltd and acetylene (C2H2,
purity >99.99%) was purchased from Tianjin Huanyu Gas Co., Ltd. All experimental
materials were not further purified prior to use.
2.2 Catalyst preparation.
The SAC support Cu-based catalysts were synthesized through wet impregnation
method with the solvent of deionized water. We take the preparation process of
15%Cu1%HMPA/SAC for an example. First, 1.6192 g CuCl2 and 0.05 g HMPA were
dissolved in 10 mL deionized water and stirred intensely until a blue, transparent
solution formed. Then 5 g SAC was slowly poured into the liquor above with stirring
under room temperature. Subsequently, the obtained mixture was treated with
ultrasonic for 15 min and was incubated at 60 oC for 12 h. Finally, the fresh catalyst
was acquired after desiccating at 120 oC for 12 h, denoted as 15%Cu1%HMPA/SAC.
Furthermore, catalysts with different mass fraction of HMPA including 5%, 10%,
15%, named as 15%Cu5%HMPA/SAC, 15%Cu10%HMPA/SAC,
15%Cu15%HMPA/SAC, respectively, were prepared to optimize the content of
HMPA for improving the catalytic performance. As a control, the 15%Cu/SAC and
the 10%HMPA/SAC were synthesized with the same method only without HMPA
(CuCl2).
7

In addition, other three catalysts containing disparate phosphoramide ligands
were prepared using the same method above instead of the solvent to ethanol,
including 15%Cu10%TPPO/SAC, 15%Cu10%NSC/SAC and 15%Cu10%CTX/SAC.
The Cu content in all Cu-based catalysts was 15 wt %.
2.3 Catalyst characterization.
X-ray diffraction (XRD) of the catalysts was performed to determine dispersity
and crystallinity of the active component and cystal form of the support by the
SmartLab X-ray diffracometer with Cu Kα radiation (9 KW) over the range 10° ≤ 2θ
≤ 80° at a scan rate of 10o min-1.
Transmission electron microscopy (TEM) and scanning transmission electron
microscopy (STEM) were conducted to detect morphologies of catalysts and to
observe the distribution and the size of Cu particles using JEM 2100F (JEOL, Japan)
with the acceleration voltage of 200 kV.
X-ray photoelectron spectra (XPS) was measured by K-Alpha+ (Thermo Fisher,
USA) to distinguish chemical elements valence states of the catalyst surface. The
standard C 1s spectra of carbon (284.8 eV) was employed to adjust the binding
energies.
Low-temperature N2 adsorption/desorption experiments were carried out on the
Quantachrome Autosorb Automated Gas Sorption System to test the various texture
parameters of the catalysts. The catalysts were heated at 200 °C and outgassed for 4 h
and measured using liquid nitrogen adsorption at -196 °C. The BET method and the
density functional theory (DFT) method were used to calculate the surface areas and
8

the pore size distribution of the catalysts, respectively.
Hydrogen temperature-programmed reduction (H2-TPR) of the catalysts was
detected to investigate the reduced ability of the catalysts using Quantachrome
Instruments AMI-90. The H2-TPR was operated from 50 to 1000 oC in 130 mL min-1
gas flow (10 vol% H2/Ar) at a ramp rate of 10 °C min-1. H2 consumption was
determined by TCD detector.
Thermogravimetric analysis (TGA) was performed on METTLER TOLEDO
TGA/DSC 2 instrument to research the thermostability and coke depositions of the
catalysts. The test temperature was increased from 30 to 800 °C at a 10 °C min-1
heating rate under a 50 mL min-1air flow.
Inductively coupled plasma-optical emission spectrometry (ICP-OES) was
measured to determine the loading of Cu for the catalysts with 725-ES instrument
(Varian, USA).
Fourier transform infrared spectra (FT-IR) was carried out by Bruker Vertex70
FT-IR spectrophotometer containing a DTGS detector. The experiments were
conducted from 500 to 4000 cm-1 with 32 scanning times under the 4 cm-1 resolution.
The Raman spectra of the catalysts was measured by the HR Revolution
(HORIBA, France) using a argon ion laser (532 nm) to excite within the scope of 200
to 2300 cm-1.
2.4 Catalyst tests.
The catalytic performance of multiple catalysts was tested in the continuous
fixed bed tubular reactor (i.d. 10 mm) for acetylene hydrochlorination under the
9

atmospheric pressure. 3 mL prepared catalyst was introduced to the fixed position of
the tubular reactor. After that, nitrogen gas at a suitable flow was imported to the
system to purge the air and vapor sustaining before reaction. Subsequently, N2 was
closed and purified hydrogen chloride (HCl) (10.8 mL min-1) was introduced to the
tube reactor to activate catalyst at 180 oC for 30 min. Finally, clean acetylene (C2H2)
was passed through the reactor at 9 mL min-1. The C2H2: HCl ratio was 1:1.2, the gas
hourly space velocity (GHSV) of acetylene was 180 h-1. In addition, excess HCl in the
gaseous product were absorbed by absorption bottles containing solution of sodium
hydroxide (NaOH). Finally, the outcome analysis was carried out using GC-2014C
gas chromatograph equipped with a flame ionization detector (FID).
2.5 Simulation details.
We selected DMol3 module of Materials Studio software (Accelrys Inc) to
operate all of the relevant density functional theory (DFT) calculations. In order to
handle the correlation and exchange, Perdew-Wang-91 (PW91) within the formulation
of the generalized gradient approximation (GGA) was adopted in geometry
optimizations. The DNP basis set and the all-electron-core were applied in the whole
calculations. The criterion of the energy convergence was fixed at 0.00001 Ha, the
atomic structures were relaxed until all unconstrained atoms forces were < 0.002
Hartree Å-1. The smearing for the orbital occupation was set as 0.005 Ha to realize
convergence of the self-consistent field. To better understand electron transfer
mechanism between CuCl2 and HMPA ligand, the Mulliken analysis is also carried
out.
10

3. Results and discussion
3.1 Catalytic performance of Cu-based catalysts.
As the representative phosphoramide ligand, the influence of different mass
fraction of HMPA on catalytic activity was studied firstly, and the results are shown in
Figure 1. The 15%Cu/SAC owns an incipient acetylene conversion of 59.80%, while
the acetylene conversion on pure 10%HMPA/SAC is only 17.30%. However, the
acetylene conversion on the 15%Cu1%HMPA/SAC rises to 81.12% when only 1%
HMPA is added to the catalyst. Compared to the pure 15%Cu/SAC catalyst, the
acetylene conversion on the 15%Cu1%HMPA/SAC significantly increases by
21.32%. The acetylene conversion on the catalysts of 15%Cu5%HMPA/SAC and
15%Cu10%HMPA/SAC continues to rise obviously to 83.97% and 87.25%,
respectively, with the increase of HMPA content. The Cu-based catalyst with 10 wt %
HMPA content possesses the maximum of acetylene conversion (87.25%), which is
higher than the Cu-based catalyst conversion reported in the vast majority of
literatures for acetylene hydrochlorination under approximate reaction conditions, as
listed in Table S1. It is worth noting that the acetylene conversion on the
15%Cu10%HMPA/SAC (87.25%) is even much higher than the sum of acetylene
conversion on pure 15%Cu/SAC and pure 10%HMPA/SAC (77.10%). As a
consequence, we infer that synergy between the copper species and the HMPA may be
a reason for the highest conversion of acetylene over the 15%Cu10%HMPA/SAC
catalyst. when the HMPA content reaches 15 wt %, the acetylene conversion does not
increase but decreases to 81.37%. Meanwhile, the selectivity of the VCM for the
11

catalysts mentioned above attains nearly overtops 99% except that of pure
10%HMPA/SAC catalyst, as shown in Figure 1b.
Figure 1. Acetylene conversion (a) and VCM selectivity (b) of Cu-based catalysts with different
HMPA content for acetylene hydrochlorination. Test conditions: T = 180 °C, GHSV(C2H2) = 180
h-1, V(HCl): V(C2H2) = 1.2.
Furthermore, catalytic properties of the catalysts containing other three disparate
phosphoramide ligands including TPPO, NSC and CTX, respectively, are also
investigated at the same test conditions. As shown in Figure 2, the conversion of
12

acetylene on the 15%Cu10%TPPO/SAC, 15%Cu10%NSC/SAC and
15%Cu10%CTX/SAC achieves to 78.68%, 83.95% and 80.86%, respectively, which
all far exceed the acetylene conversion of the pure 15%Cu/SAC but slightly lower
than that of 15%Cu10%HMPA catalyst with the same ligand mass fraction. At the
same time, Figure 2b shows the VCM selectivity on the three catalysts containing
different phosphoramide ligands is all above 93%. In summary, the phosphoramide
ligands can truly improve the catalytic performance because of reciprocity between
the phosphoramide ligands and the copper.
Figure 2. Acetylene conversion (a) and VCM selectivity (b) of Cu-based catalysts containing
13

different phosphoramide ligands for acetylene hydrochlorination. Test conditions: T = 180 °C,
GHSV(C2H2) = 180 h-1, and V(HCl): V(C2H2) = 1.2.
With the purpose of industrial application, long-term stability experiments of the
15%Cu10%HMPA/SAC with the optimal HMPA loading are tested under different
GHSVs. Primarily, the catalytic performance of 15%Cu10%HMPA/SAC catalyst at
the industrial GHSV of 50 h-1 is investigated. As shown in Figure 3a, the initial
acetylene conversion just reaches 94.38% and rises to the maximum of 97.07%
gradually after about 20 h. Subsequently, the conversion of acetylene decreases
slightly to 90.03% within 100 h reaction time, indicating that the HMPA ligand
promotes the stability of Cu-based catalyst significantly, compared to the reported
durability data of Cu-based catalysts utilized in acetylene hydrochlorination. The
15%Cu10%HMPA/SAC catalyst stability is further inspected under the harsh
condition of GHSV = 90 h-1. The acetylene conversion of 15%Cu10%HMPA/SAC
achieves 96.68% and then reduces to 86.49% after testing time of 100 h, illustrating
that the conversion of acetylene only declines 3.15% more than that for the same
catalyst at GHSV = 50 h-1 at the same time. The results further confirm that the Cu-
based catalyst with HMPA ligand possesses excellent durability even though in the
harsher conditions. Moreover, the VCM selectivity for the catalyst maintains above
99% during the lifetime testing.
14

Figure 3. Acetylene conversion (a) and VCM selectivity (b) of 15%Cu10%HMPA/SAC catalyst
for acetylene hydrochlorination. Test conditions: T = 180 °C, GHSV(C2H2) = 90 h-1 (50 h-1), and
V(HCl): V(C2H2) = 1.2.
3.2 HMPA improves Cu species dispersity.
Figure 4 displays the XRD patterns for disparate catalysts. Two dominating
diffraction peaks at around 25o and 43o can be ascribed to the (002) and (101) planes
of the SAC support, respectively [28, 32]. It is worth noting that three extra low
intensity diffraction peaks of about 16o, 32o and 40o can be assigned to the copper
atacamite (Cu2Cl(OH)3) based on the JCPDS file data of Fresh-15%Cu/SAC catalyst
15

compared with pure SAC [28]. We infer that the reason for other three small
diffraction peaks detected is the low dispersity of the copper active sites in catalyst,
which is caused by the aggregation of copper atacamite substance (Cu2Cl(OH)3).
Compared with Fresh-15%Cu/SAC, Cu2Cl(OH)3 diffraction peaks at 32o and 40o of
the Fresh-15%Cu1%HMPA/SAC disappear obviously and the peak at 16o becomes
weak and broad, which indicates a handful of HMPA indeed improve the dispersity of
the Cu species effectively. Subsequently, the diffraction peak of 16o belonging to
Cu2Cl(OH)3 on 15%Cu5%HMPA/SAC vanishes completely. When the HMPA content
is 10 wt %, no other discernible reflection except the two diffraction peaks of
amorphous carbon can be detected and the curve becomes smooth greatly, indicating
that the HMPA containing N and P can suppress copper species crystallization and
enhance the Cu active sites dispersity significantly. Furthermore, the intensity and the
area of the three diffraction peaks at about 16o, 32o and 40o for the Used-15%Cu/SAC
increase slightly compared to those of pure Fresh-15%Cu/SAC, indicating that the
level of the Cu species aggregation becomes serious. In contrast, the three diffraction
peaks did not appear in both the fresh and used 15%Cu10%HMPA catalyst, indicating
that Cu active species spread around very well in fresh and used 15%Cu10%HMPA
catalyst. The results demonstrate that HMPA ligand can inhibit Cu species
agglomeration, thus promoting catalytic performance of Cu complex catalyst.
16

Figure 4. XRD patterns of the fresh and used Cu-based catalysts with different HMPA content.
In order to continue to analyze metal active sites dispersity, TEM photographs of
pure Cu supported catalyst and 15%Cu10%HMPA/SAC before and after test are
displayed in Figure 5. The mean metal grain size of Used-15%Cu/SAC catalyst is
around 15.82 nm, which is far beyond that for Fresh-15%Cu/SAC catalyst (7.02 nm),
demonstrating there is obvious Cu species aggregation in the reaction. Compared to
pure 15%Cu/SAC after reaction, the mean grain size for Used-
15%Cu10%HMPA/SAC catalyst is about 4.75 nm, which decreases significantly and
is even lower than the mean particles diameter of the Fresh-15%Cu/SAC, illustrating
that HMPA ligand can inhibit effectively Cu active sites agglomeration. It is worth
mentioning that Fresh-15%Cu10%HMPA/SAC owns the minuscule average particles
size of 2.59 nm, calculated on the basis of the 100 particles, which demonstrates that
HMPA can promote availably Cu active sites dispersity thus gaining outstanding
catalytic performance. For the Fresh-15%Cu10%HMPA/SAC catalyst, the elemental
mapping using STEM mode is detected and shown in Figure 6. The elements mapping
17

of Cu, Cl, P, O and N proved that the main elements in the catalyst distribute
symmetrically on the support surface, which intuitively certifies that the HMPA ligand
enhances the dispersibility of Cu species due to the restraining of the growth of Cu
particles size.
Figure 5. HRTEM images of fresh (a) and used (b) 15%Cu/SAC; fresh (c) and used (d)
15%Cu10%HMPA/SAC.
18

Figure 6. STEM photograph (a), elemental mapping image (b) for overlapping elements,
elemental mapping images (c−h) of C, Cu, Cl, O, P, N, respectively.
Moreover, Figure 7 displays the XPS SnapMap of the fresh and used catalysts,
which is a novel method to observe the dispersion of the Cu species. For instance, the
yellow light and the red dark spots represent the high dispersity of Cu species and Cu
cluster aggregation, respectively. Compared to the catalyst of Fresh-15%Cu/SAC
(Figure 7a), the amount of yellow light spot corresponding to the high dispersed Cu
species of the Fresh-15%Cu10%HMPA/SAC catalyst (Figure 7c) increases distinctly,
indicating that the degree of aggregation Cu active sites decreases evidently via
adding HMPA ligand. In addition, a large quantity of homogeneous light spots is also
detected in Figure 7d, further demonstrating that proper HMPA content can promote
Cu active sites dispersity thus one reason for excellent catalytic activity obtained on
the 15%Cu10%HMPA/SAC catalyst.
19

Figure 7. XPS SnapMap of fresh (a) and used (b) 15%Cu/SAC; fresh (c) and used (d)
15%Cu10%HMPA/SAC.
3.3 HMPA suppresses coke deposition and decreases Cu loss.
Thermo Gravimetric (TG) is detected to assess the amount of coke deposition
and thermal stability of the catalysts, which is the main influence factors of the
catalysts’ deactivation [33]. Figure 8a and Figure 8b display the TGA experiments
results of the 15%Cu/SAC and 15%Cu10%HMPA catalyst, respectively. The tiny
weight loss before 100 oC can be recognized as the steaming of the water adsorbed on
the catalysts. Notably, the distinct mass loss at the temperature range from 100 oC to
about 300 oC is assigned to the used catalysts’ surface carbon deposition [34]. The
obvious quality loss above 300 oC may be attributed to the burning of SAC support
20

and HMPA ligand. When many researchers investigated the coke formation during the
methane reforming reaction used Ni-based catalyst [35-38], they reported that the
distinct main peak at high temperature above 300 °C possibly assigned to the coke
form of carbon fibers. Therefore, there may be some inaccuracies in the calculation of
coke deposition. In addition, pure 10%HMPA/SAC owns a higher decomposition
temperature at approximately 600 oC as shown in Figure S2a, indicating the intense
interaction between HMPA and the carbon support due to the favorable thermal
stability of HMPA. Combining the TGA results of the Cu-based catalysts with other
HMPA mass fraction as shown in Figure S2b-d, we can see that the more HMPA
added to catalysts, the higher beginning temperature of the rapid weight loss. The
results illustrate that HMPA possesses strong interaction with Cu and SAC, which can
effectively improve the thermal stability.
21

Figure 8. TGA and DTG splines of (a) 15%Cu/SAC and (b) 15%Cu10%HMPA/SAC before and
after test.
The amount of carbon deposition of catalysts calculated using the contrastive
method is listed in Table 1 [39-40]. It is worth noting that the 15%Cu/SAC exhibits
the maximum of coke amount 11.12% and the lowest amount coke deposition of
1.47% is obtained on 15%Cu10%HMPA/SAC, declaring that the appropriate HMPA
content of catalysts can restrain carbon deposition over surface of catalyst to the
greatest extent.
22

Table 1. Content of Coke Deposition on Cu-based Catalysts
Catalysts
Content of coke deposition (%)
15%Cu/SAC
11.12
9.30
4.57
1.47
1.85
15%Cu1%HMPA/SAC
15%Cu5%HMPA/SAC
15%Cu10%HMPA/SAC
15%Cu15%HMPA/SAC
Furthermore, carbon deposition can lead to changes in pore structure parameters
of catalysts. In order to further verify the accuracy of carbon deposition data, the
Low-temperature nitrogen adsorption-desorption experiments are performed to
research the influence of HMPA coordination on pore structure and specific surface
area of catalysts before and after reaction, the results are shown in Table 2. Compared
with pure Cu supported catalyst, the specific surface area and pore volume of the
optimal catalyst with 10% HMPA content decrease slightly, which is mainly caused
by dilution effect with the addition of HMPA ligand [28]. It is well known that carbon
deposition reduces the specific surface area and pore volume of the catalyst. Notably,
both △SBET and △Vtotal of the optimal catalyst are smaller than those of the pure
copper supported catalyst, which indicates that HMPA can effectively inhibit the coke
deposition on the surface of the catalyst. The above analysis further corroborates the
results of the TG tests.
23

Table 2. Pore Structure parameters of Cu-based catalysts before and after test.
BET (m2 g-1) total (cm3 g-1)
S
V
Catalysts
Fresh
767
Used
471
Fresh
0.36
0.33
Used
0.21
0.32
△SBET
296
△Vtotal
0.15
15%Cu/SAC
15%Cu10%HMPA/SAC
724
702
22
0.01
Since the catalyst deactivation is arised from the loss of Cu species active sites,
Cu practical loading of the fresh and used catalysts is detected using ICP-OES, as
listed in Table S2. The actual Cu loading amounts of the fresh pure Cu supported
catalyst and the optimal HMPA modified catalysts are very close to 15 wt %. After
reaction, the loss rate of copper content for pure Cu supported catalyst and the optimal
HMPA modified catalysts are 5.24% and 4.61%, respectively. The result indicates that
the addition of HMPA partly reduced the Cu loss during the reaction.
3.4 HMPA stabilizes the valence state of Cu species.
The H2-TPR profiles of pure Cu supported catalyst and 15%Cu10%HMPA/SAC
catalyst are performed to investigate the reducibility of the catalyst and the reciprocity
among copper species, HMPA and the support. As shown in Figure 9, two main
reduction peaks of fresh pure Cu supported catalyst at appropriately 451 oC and 516
oC can be detected, which are assigned to the reduction of Cu2+ species to Cu+
particles and the change of Cu+ to metallic copper, respectively. Different from
traditional reduction range from 270 oC to 330 oC for Cu2+ to Cu0, the reduction peaks
of Cu2+ and Cu+ shift to the higher temperatures, indicating that abundant Cu2+ in
catalyst enhance the reduction difficulty thus obtaining the stronger reciprocity of Cu
complex and SAC support [41]. The Cu2+ reduction peak of the Fresh-
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15%Cu10%HMPA catalyst migrates to the higher temperature of 672 oC. Meanwhile,
the peak corresponding to the Cu+ to Cu0 shifts to the higher temperature about 876
oC. The results illustrate that the reduction of high valence Cu is suppressed
effectively because of the very strong reciprocity among the metal Cu, SAC support
and HMPA. The fact that the H2 consumption depended on the H2 reduction peaks for
the Fresh-15%Cu10%HMPA catalyst is much larger than that of the Fresh-
15%Cu/SAC indicates that the reduction of Cu complex catalyst consumes more H2,
which confirms that the HMPA makes the valence state of Cu more stable.
Meanwhile, the two reduction peaks of the used catalysts both move to lower
temperature, indicating that the reduction inhibiting ability of high-valent copper
species decreases after the reaction.
Figure 9. Temperature-programmed reduction (TPR) curves of pure Cu supported catalyst and
15%Cu10%HMPA/SAC before and after test.
XPS spectra and XAES spectra are utilized to distinguish disparate Cu species
valence states on catalysts surface. The relative content of disparate states of Cu
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species is calculated based on the relative deconvoluted peak area, as listed in Table
S3. As shown in Figure 10a, besides the high-resolution core level spectrum of the Cu
2p1/2 and Cu 2p3/2, the two extra strong satellite peaks located at about 942.7 eV and
962.5 eV, respectively, are also discovered, illustrating that the presence of the Cu2+
species in catalyst [42]. From the curve fitting of Cu 2p, two main peaks with the
binding energy of 934.3 eV and 954.4 eV belong to the Cu 2p3/2 and Cu 2p1/2,
respectively, are ascribed to the Cu2+ species for Fresh-15%Cu/SAC [42-43].
Meanwhile, two other relatively small peaks located at around 932.8 eV and 953.9 eV
are assigned to Cu 2p3/2 and Cu 2p1/2, respectively, confirming the existing of Cu+ and
Cu0 species in catalyst, which may be generated due to the reduction by the oxidation
functional group on the SAC support during the preparation process of the catalysts
[44]. However, just the peaks attributed to the Cu2+ species can be found while the
peaks ascribed to the low valence state Cu species almost disappear completely in the
Fresh-15%Cu10%HMPA/SAC, indicating that the valence states Cu species in the
modified catalyst are nearly all Cu2+, as summarized in Table S3. As a consequence,
appropriate HMPA ligand can efficiently consolidate high valence of Cu species in the
synthesis process of catalyst to enhance the catalytic properties of the Cu complex
catalysts. The XPS spectra of used catalysts is shown in Figure 10b. Notably, the peak
at 932.8 eV corresponding to Cu+ and Cu0 is observed in the Used-
15%Cu10%HMPA/SAC, illustrating that a fraction of Cu2+ are also reduced.
The analysis above combined with the catalytic performance of the catalysts
illustrates that Cu2+ are the main active ingredient for the hydrochlorination of
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acetylene. Hutchings and Shinoda found that the Cu+ species in the catalysts also
possesses activity for acetylene hydrochlorination reaction [45-47]. According to the
deconvolution results of the Cu LMM spectra for the used catalysts, three different
peaks can be fitted including a peak located at around 918.6 eV assigned to the Cu0
species, a peak with a kinetic energy of 916.6 eV attributed to the Cu+ species and a
peak at about 910.5 eV ascribed to a different Cu transition state, as shown in Figure
10c [24, 28]. Compared to the amount of Cu0 without any catalytic activity in pure
15%Cu/SAC (10.09%), that in the 15%Cu10%HMPA/SAC (4.52%) after the reaction
decreases significantly, as listed in Table S3. Most of the reduced Cu2+ in the modified
catalyst is changed to Cu+, only a small part of Cu2+ is converted to Cu0 directly,
indicating that HMPA ligand can restrain the direct reduction of the Cu2+ to Cu0 thus
preventing the deactivation of the catalysts. In addition, the main elements
composition on the catalysts’ surface are listed in Table S4.
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Figure 10. Cu 2p XPS spectra of fresh (a); used (b) catalysts and Cu XAES spectra of (c) used
catalysts.
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3.5 Coordination formation between Cu and HMPA.
The XPS spectra of O 1s can provide valid information with respect to the bond
structure of the catalyst since the binding energy of O 1s electrons is an immediate
consequence of the bonding between the oxygen atom and disparate cations [48].
Therefore, O 1s spectra is used to further analyze the coordination structure for Cu
species and HMPA in the catalyst. As shown in Figure 11, The O 1s peak is
deconvoluted into two components of fresh pure Cu supported catalyst. The peak
located at 531.5 eV is attributed to non-bridging oxygen (P=O), the peak with a higher
binding energy of 533.2 eV is assigned to the bridging oxygen (P-O-P) [49].
Specifically, a new deconvolution peak of the Fresh-15%Cu10%HMPA/SAC is
observed at the binding energy about 532.4 eV, which is ascribed to the Cu-O-P bond
since the peak position is between P=O and P-O-P [50]. Therefore, the Cu-O-P
bonding is formed by bonding between the Cu in CuCl2 and the O corresponding to
the HMPA, indicating that the coordination bond of Cu-O is formed thus stabilizing
the chemical state of Cu on catalysts surface thereby increasing the catalytic
performance.
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