An Alternative Carbon Carrier in Green Preparation of Efficient Gold/Carbon Catalyst for Acetylene Hydrochlorination

Article abstract of DOI:10.1002/cctc.201900710

Au catalysts supported with carbon-based carriers have been extensively studied for the hydrochlorination of acetylene and expected to replace toxic mercury catalysts. However, removal of the highly corrosive aqua regia used in the preparation of carbon-based catalysts while maintaining catalytic activity and stability remains a key challenge. Herein, we present a green technology carrier, activated carbon fibers (ACF), to support gold catalysts for the hydrochlorination of acetylene. TPD and XPS analyses confirmed the presence of surface oxygen-containing functional groups (SOGs) and pyrrolic N species on the ACF. The Au/ACF?H₂O catalyst exhibited better catalytic activity and stability than Au/ROX0.8(AQ)?H₂O. Characterization results revealed that the catalytic properties of Au/ACF?H₂O could be attributed to the anchoring and stabilization of gold active species on the SOGs, leading to atomic dispersion, and to the improvement of HCl adsorption with the synergistic effect of electron-donating pyrrolic N groups. The results indicated that usage of this green carrier can be considered as the a new approach to reduce or eliminate the use of strong oxidizing reagents in the preparation of Au catalysts.

Full text of DOI:10.1002/cctc.201900710

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DOI: 10.1002/cctc.201900710
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An Alternative Carbon Carrier in Green Preparation of
Efficient Gold/Carbon Catalyst for Acetylene
Hydrochlorination
Huixia Lai,[a] Bolin Wang,[a] Yuxue Yue,[a] Gangfeng Sheng,[a] Saisai Wang,[a] Feng Feng,[a]
Qunfeng Zhang,[a] Jia Zhao,*[a] and Xiaonian Li*[a]
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Au catalysts supported with carbon-based carriers have been
extensively studied for the hydrochlorination of acetylene and
expected to replace toxic mercury catalysts. However, removal
of the highly corrosive aqua regia used in the preparation of
carbon-based catalysts while maintaining catalytic activity and
stability remains a key challenge. Herein, we present a green
technology carrier, activated carbon fibers (ACF), to support
gold catalysts for the hydrochlorination of acetylene. TPD and
XPS analyses confirmed the presence of surface oxygen-
containing functional groups (SOGs) and pyrrolic N species on
the ACF. The Au/ACFÀ H2O catalyst exhibited better catalytic
activity and stability than Au/ROX0.8(AQ)À H2O. Characterization
results revealed that the catalytic properties of Au/ACFÀ H2O
could be attributed to the anchoring and stabilization of gold
active species on the SOGs, leading to atomic dispersion, and to
the improvement of HCl adsorption with the synergistic effect
of electron-donating pyrrolic N groups. The results indicated
that usage of this green carrier can be considered as the a new
approach to reduce or eliminate the use of strong oxidizing
reagents in the preparation of Au catalysts.
Introduction
At present, for the synthesis of vinyl chloride from
acetylene, strong acid-treated activated carbon is used to
support HgCl2 catalysts.[25–27] Among the possible alternatives,
there is no direct material that can achieve the effect of
activated carbon treated with aqua regia. Unfortunately, the
activated carbon carrier will cause considerable harm to the
ecological environment during the strong acid treatment
process. More importantly, handling and recycling/disposal of
such a large amount of aqua regia and waste acid present a
significant economic and technical challenge, which may
prevent or delay the commercialization of these catalysts.[27]
Although acid-treated activated carbon offers the advantages
of low ash content and introduction of surface oxygen-
containing functional groups (SOGs),[28,29] the mechanical
strength of the activated carbon after acid treatment is
significantly reduced.[30] Moreover, it can be easily pulverized
under a high temperature and reaction gas flow, resulting in
blockage of the reactor and a very hazardous dust explosion.
Furthermore, the SOGs introduced by the acid-treated activated
carbon are too difficult to effectively control, and the cost and
actual benefits may negate the advantages offered by the acid-
treated activated carbon. In order to avoid these issues, it is
necessary to find a greener and more efficient catalyst to
replace the current preparation methods.
The energy structure of China’s rich coal and lean oil dictates
that the production of vinyl chloride monomer (VCM) by
acetylene hydrochlorination is the best route for large-scale
production of polyvinyl chloride (PVC).[1] Considerable research
efforts have been devoted to developing mercury-free catalysts,
such as AC-supported AuCl3,[2–4] PdCl2,[5–7] RuCl3,[8,9] and
[10,11]
CuCl2
to replace conventional Hg-based catalysts in the
hydrochlorination of acetylene because of the mercuric chloride
lost from the catalyst can disperse into the environment, which
will seriously endanger human health. Hutchings and co-
workers showed that Au catalysts are the most promising
mercury-free catalysts. In regards to the sustainability of a
catalyst, the instability of Au species significantly limits the
development of gold catalysts.[2,12] Many studies have reported
improving the Au stability by adding ligands such as IL[13,14] and
additives.[15–19] In addition, the selection of new materials such
as oxides,[20] nanotubes,[21,22] and graphene[23] to overcome the
Au instability problem has been reported. Compared to such
materials, however, carbon-based carriers are much more
suitable carriers because of their advantages of higher chemical
stability and easy combustion to recover the supported
metals.[24]
Herein, we propose an environmentally friendly strategy to
avoid the use of a hazardous reagent from the perspective of
changing the carrier material while also ensuring good catalytic
performance. That is, we proposed the use of activated carbon
fibers (ACF), which contain phenolic and carbonyl groups and
nitrogen species, to support Au catalysts for acetylene hydro-
chlorination. Compared to the conventional acid-treated acti-
vated carbon carrier, ACF offers the advantages of no harmful
acid treatment, high effective functional group content, pyrrolic
[a] H. Lai, B. Wang, Y. Yue, G. Sheng, S. Wang, F. Feng, Q. Zhang,
Prof. Dr. J. Zhao, Prof. Dr. X. Li
Industrial Catalysis Institute
Laboratory Breeding Base of Green Chemistry-Synthesis Technology
Zhejiang University of Technology
Hangzhou 310014 (P.R. China)
E-mail: jiazhao@zjut.edu.cn
xnli@zjut.edu.cn
Supporting information for this article is available on the WWW under
https://doi.org/10.1002/cctc.201900710
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N species, low ash content, a microporous structure and high
mechanical strength in addition to greener production technol-
ogy. STEM, XRD, XPS, TPD, and HCl-breakthrough curve analyses
were used to examine the surface properties of the Au/
ACFÀ H2O catalyst and suggested that the presence of SOGs
(such as phenol and carbonyl groups) and pyrrolic N species on
the ACF. Furthermore, the enhanced catalytic performance of
Au/ACFÀ H2O could be attributed to both the anchoring and
stabilization of gold active species by SOGs, leading to atomic
dispersion, and the improvement of HCl adsorption with the
synergistic effect of electron-donating pyrrolic N species.[4,22,31]
In addition, the development and design of a new green
material such as ACF that replaces acid-treated activated carbon
as a green catalyst carrier is essential for the sustainable
development of the PVC industry.
in Figure 1b. In contrast, from the images of the fresh Au/
ROX0.8À H2O catalysts, the Au species predominantly existed as
metallic-Au nanoparticles in the 6–10 nm size range and
contained a very low content of dispersed Au species (as shown
in Figure 1c). However, after use, the support areas contained
no atomically dispersed Au species with clusters in the 15–
50 nm size range (as shown in Figure 1d), thus indicating that
the Au particles could be easily sintered during reaction.
Recently, Hutchings and co-workers reported that a Au/CÀ H2O
catalyst was nearly devoid of any atomically dispersed species,
which was consistent with our results.[25] Moreover, they
observed active sites for acetylene hydrochlorination, proving
that the atomically dispersed Au species were critical for the
enhanced catalytic activity.[32] Thence, the Au/ACFÀ H2O catalyst,
having a large amount of atomically dispersed Au species on
the support, exhibited high activity.
In addition to the atomically dispersed Au species, the
surface properties of the carrier also contributed to the
enhanced catalytic activity, such as the presence of SOGs,
nitrogen species, specific surface area, and pore structure.[33–35]
Table 1 shows that the specific surface area of the ACF was
slightly higher than that of ROX0.8. The most obvious difference
was that a rich microporous structure existed on the ACF in
contrast to ROX0.8, which may have a positive effect on the
activity of the catalyst. In fact, Brasquet et al. reported that ACF
with an abundant microporous structure and specific surface
area exhibit highly effective adsorption capacity.[36] Overall,
these findings demonstrated that such a rich structure may
provide a suitable platform for the dispersion of the active Au
species as well as better conditions for the diffusion of the
substrate. This behavior will be further clarified by catalyst
characterization.
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Results and Discussion
Characterization of the Au-Based Catalysts
The dispersion of Au species on the catalysts was investigated
by STEM-HAADF analyses as shown in Figure 1. In the highly
magnified images of the fresh Au/ACFÀ H2O catalysts shown in
Figure 1a, the Au species were observed to mainly appear as
isolated atoms having a diameter of less than 2 nm with minor
occasional sub-nano clusters on the support areas. Thus, it was
confirmed that the preparation of the Au catalysts with such
supports could significantly improve the dispersion of Au
species. After use, some occasional Au nanoparticles larger than
2 nm in diameter appeared in the supports, but the Au species
size of less than 2 nm was predominantly maintained, as shown
Figure 2a shows the X-ray diffraction patterns of fresh and
used Au/ACFÀ H2O and Au/ROX0.8(AQ)À H2O catalysts. The data
°
°
°
showed that the diffraction peaks at 77.48 , 64.48 , 44.26 , and
°
38.13 could be attributed to the (311), (220), (200), and (111)
planes of Au0, respectively.[3,4,22] The fresh and used Au/
ROX0.8(AQ)À H2O catalyst clearly showed a typical characteristic
peak of Au0, while the Au/ACFÀ H2O samples only showed the
carbon diffraction peak. The XRD results revealed that the active
sites of cationic gold (AuClx) species were highly dispersed on
the ACF surface and that the unique surface chemistry proper-
ties of the ACF affected the anchoring of Au species, thereby
leading to the inhibition of sintering.[37] XPS results shown in
Figure 2b were in agreement with the above-mentioned XRD
results. According to the literature, the Au 4f7/2 peaks appearing
at the binding energies of 84.0 eV, 85.0 eV, and 87.0 eV (�
0.2 eV) can be attributed to Au0, Au+, and Au3+
respectively.[3,12,38]
,
Table 1. Texture parameters calculated by BET method from the low-
temperature N2 adsorption/desorption experiments.
Sample
SBET [m2/g]
V [cm3/g]
D [nm]
Micropore volume [%]
Figure 1. Aberration-corrected STEM-HAADF images of the (a) fresh Au/
ACFÀ H2O, (b) used Au/ACFÀ H2O, (c) fresh Au/ROX0.8À H2O and (d) used Au/
ROX0.8À H2O catalysts.
ACF
ROX0.8
1265
1020
0.56
0.56
2.15
2.22
89.3
51.8
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Figure 2. (a) X-ray diffraction (XRD) patterns: fresh Au/ACFÀ H2O, used Au/ACFÀ H2O, fresh Au/ROX0.8(AQ)À H2O and used Au/ROX0.8(AQ)À H2O catalysts (b) Au
4f XPS spectra: fresh Au/ACFÀ H2O and fresh Au/ROX0.8(AQ)À H2O catalysts.
Table 2. Relative contents and binding energies of Au species.
Sample
Au species [%]
Binding energy [eV]
Au3+
Au+
Au0
Au3+
Au+
Au0
Fresh Au/ACFÀ H2O
49.1
10.7 40.2 86.8
13.2 47.8 86.9
84.9
85.1
84.0
84.0
Fresh Au/ROX0.8(AQ)À H2O 39.0
The relative contents and binding energies of Au species
obtained from XPS are listed in Table 2. For the fresh Au/
ROX0.8(AQ)À H2O samples, the relative contents of Au3+, Au+,
and Au0 were 39.0%, 13.2%, and 47.8%, respectively. A large
amount of Au0 existed in fresh Au/ROX0.8(AQ)À H2O, which may
have been caused by the reduction of Au3+ to Au0 by
carboxylation at the ROX0.8 surface.[39] Compared to that of the
fresh Au/ROX0.8(AQ)À H2O catalysts, the total AuClx (x=1, 3)
species content (the amount of Au3+ and Au+) in the fresh Au/
ACFÀ H2O catalysts was increased from 52.2% to 59.8%,
indicating the strong anchoring and stabilization of AuClx
species on the surface of the ACF. This was consistent with the
observations from XRD and STEM.
Figure 3. TPR profiles of the fresh Au/ACFÀ H2O, used Au/ACFÀ H2O, fresh Au/
ROX0.8(AQ)À H2O and used Au/ROX0.8(AQ)À H2O catalysts.
To elucidate the oxidation states of Au species in the Au/
ACFÀ H2O and Au/ROX0.8(AQ)À H2O catalysts, H2-TPR analysis
was performed, as shown in Figure 3. The hydrogen consump-
tion peak in the high temperature region represents the
reduction of the oxygen-containing functional groups, and at
the low temperature represents the reduction of Au species.[4]
Au loading, which was higher than the 39.5% in fresh Au/
ROX0.8(AQ)À H2O catalysts (Table S1). Moreover, the reduction
°
temperature of fresh Au/ACFÀ H2O catalysts was 78 C higher
than that of fresh Au/ROX0.8(AQ)À H2O, indicating that the
unique surface properties of the ACF may make Au species
more difficult to reduce because of the stronger interaction
between the ACF surface and the Au species. To further analyze
the performance of the catalysts, comparative H2-TPR analysis
of the used Au/ACFÀ H2O and used Au/ROX0.8(AQ)À H2O cata-
lysts was also conducted, and the results are shown in Figure 3.
It was estimated that the total AuClx species content in the
used Au/ACFÀ H2O catalysts amounted to 53.1% of the total Au
loading and the reduction temperature of Au species was
°
Details of the TPR profiles in the region 200–420 C (AuClx
reduction) and procedure for the TPR area peaks determination
were provided in Figure S1 and Figure S2. In deed, the analyses
was determined from the TCD signal with a CuO standard and
the fitting parameters of H2-TPR profiles of the samples were
listed in Table S1. For our catalysts, the fresh Au/ACFÀ H2O
sample presents a characteristic hydrogen consumption peak
°
°
between 310–420 C (with center at 364 C, Figure 3 and
Figure S1), which was diagnostic of AuClx species. The calcu-
lation method of TPR analysis was consistent with the research
of Hutchings.[40] Clearly, comparing with the TCD signal with a
standard, it was estimated that the total AuClx species content
in fresh Au/ACFÀ H2O catalysts amounted to 59.2% of the total
°
352 C, which was similar to that of the fresh Au/ACFÀ H2O
catalysts. In contrast, the total AuClx species content in the used
Au/ROX0.8(AQ)À H2O catalysts was significantly reduced to
12.5% after the reaction. These results further proved that the
unique surface properties of the ACF may make Au species
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Figure 4. TPD evolution profiles: (a) CO and (b) CO2 evolved in the TPD-MS of the ACF, Au/ACFÀ H2O, ROX0.8(AQ), Au/ROX0.8(AQ)À H2O and ROX0.8 samples.
Table 3. Amounts of different SOGs and the total amounts of SOGs on catalysts (obtained from the CO-TPD curves).
Sample
Peak 1[a]
Adsorptive CO
Peak 2
Carboxylic anhydride
Peak 3
Phenol
Peak 4
Ether
Peak 5 Carbonyl
Total
[b]
ACF
/
21.2
13.8
/
/
/
274.4
60.3
7.3
4.7
/
/
/
215.6
51.5
169.7
118.1
/
511.2
125.6
799.8
643.1
/
Au/ACFÀ H2O
ROX0.8(AQ)
Au/ROX0.8(AQ)À H2O
ROX0.8
/
/
/
/
622.8
520.3
/
[a] The amount of SOGs in μmol/g. [b] No SOGs were observed.
Table 4. Amounts of different SOGs and the total amounts of SOGs on catalysts (obtained from the CO2-TPD curves).
Sample
Peak 1[a]
Carboxyl
Peak 2
Carboxylic anhydride
Peak 3
Peroxide
Peak 4
Lactone 1
Peak 5
Lactone 2
Total
[b]
ACF
12.8
9.2
141.0
41.9
/
10.6
8.4
147.8
111.8
/
/
16.4
12.8
90.0
47.7
/
11.0
6.3
/
/
/
50.8
36.7
378.8
201.4
/
Au/ACFÀ H2O
ROX0.8(AQ)
Au/ROX0.8(AQ)À H2O
ROX0.8
/
/
/
/
[a] The amount of SOGs in μmol/g. [b] No SOGs were observed.
more difficult to reduce because of the stronger interaction,
which was consistent with the XRD analyses. In addition, the
results of STEM-HAADF analysis for used Au/ACFÀ H2O catalysts
revealed the presence of highly dispersed Au species on the
ACF after reaction, which was consistent with the atomic Au
deposition of fresh Au/ACFÀ H2O catalysts.
In order to analyze the induction of Au deposition and
nucleation effected by SOGs on different supports, TPD analysis
was carried out. This technique involves the evaluation of all
types and various amounts of gas released, such as CO, CO2,
and H2O, at different decomposition temperatures,[44,45] as
shown in Figure 4.
Figures 4a and 4b show the CO and CO2 TPD-MS evolution
profiles, respectively, of the ACF, Au/ACFÀ H2O, ROX0.8(AQ), Au/
ROX0.8(AQ)À H2O, and ROX0.8. ROX0.8 exhibited small amounts
of CO and CO2 evolved, which confirmed that it contained a
very low SOG content. However, a high amount of SOGs was
introduced into ROX0.8 though the pretreatment with aqua
regia, as shown by the significantly increased amounts of CO
and CO2 evolved (Tables 3 and 4). In particular, a large amount
of phenol and carbonyl groups (490.0 μmol/g) existed on the
ACF, and its amount was higher by 313.0 μmol/g than that on
ROX0.8(AQ) (177.0 μmol/g), which were introduced by the aqua
regia pretreatment, as shown in Figure 4a and Table 3. On the
Hence, the above-mentioned structural characterization
results further confirmed the high stability of the Au/ACFÀ H2O
catalysts, which can be possibly attributed to its unique surface
properties. To support these results, we further investigated
that whether the AuClx species stabilized by specific supports
were the active sites for the acetylene hydrochlorination.
Study of Carrier Surface Chemistry and the AuÀ C Interface
It is well known that the surface properties of a carrier offer a
characteristic feature that affects the catalytic performance.[41–43]
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Figure 5. (a) N 1s XPS spectra of Au/ACFÀ H2O catalysts (b) schematic diagram of N species.
Table 5. Relative contents and binding energies of nitrogen species.
other hand, CO2 curves (Figure 4b) showed that ROX0.8(AQ)
had a large amount of carboxyl and carboxylic anhydride
groups. It was obvious that the amount of CO2 decomposed
from the ACF was 50.8 μmol/g, which was 328.0 μmol/g less
than that from ROX0.8(AQ) (378.8 μmol/g), as shown in Table 4,
implying that the amount of carboxyl, carboxylic anhydride,
and lactone groups on the ACF was lower than that on
ROX0.8(AQ). Bulushev et al. have proved that phenolic groups
could interact with the Au3+ precursor by stabilizing the AuClx
species according to ©À OH+Au3+!©À OÀ Au3+ +H+. In addi-
tion, the Au3+ precursor could be reduced by the carboxyl
functional groups at the carbon surface according to ©À COOH
+Au3+!©+Au0 +H+ +CO2.[39] As such, it was demonstrated
that the AuClx species tended to gradually form clusters.
However, the AuClx species could be stable on a metal oxide
surface that had hydroxyl groups.[46] Specifically, based on the
strong anchoring and stabilization of the AuClx species on the
carrier, the nature of the support and its interaction with the
gold species are considered to be important factors for the
catalytic reaction.[4]
Hence, TPD-MS analyses were also carried out for the Au/
ACFÀ H2O and Au/ROX0.8(AQ)À H2O catalysts in order to gain
more insight into the support chemistry and the effect of SOGs
on Au deposition. The data acquired for Au/ACFÀ H2O and Au/
ROX0.8(AQ)À H2O samples are shown in Figure 4 and Tables 3
and 4. As shown in the CO curve (Figure 4a), the amount of
phenol and carbonyl groups obviously decreased for the Au/
ACFÀ H2O sample upon Au deposition and was significantly
more than that for Au/ROX0.8(AQ)À H2O. The specific amounts
of CO decomposed before and after Au was supported on the
carriers were 511.2 μmol/g (CO, ACF), 125.6 μmol/g (CO, Au/
ACFÀ H2O), 799.8 μmol/g (CO, ROX0.8(AQ)), and 643.1 μmol/g
(CO, Au/ROX0.8(AQ)À H2O). In addition, Figure 4b shows that the
amount of carboxyl, carboxylic anhydride, and lactone groups
significantly decreased on the ROX0.8(AQ) surface after Au
deposition. Meanwhile, for Au/ACFÀ H2O, it could be observed
that the CO2 curve was almost unaffected after Au was
supported. To explain this observation consequence, we
considered that the Au deposition probably lead to the obvious
reduction in SOGs decomposed, as a result of the anchoring on
Sample
Pyridinic N
Pyrrolic N
Graphitic Other N
N
species
[c]
Au/
20.02%[a]
(398.3 eV)[b]
/
79.98%
(400.4 eV)
/
/
/
/
ACFÀ H2O
Au/
/
ROX0.8À H2O
[a] The content of N species. [b] Binding energy. [c] No N species were
observed.
the functionalized surface sites of the support.[47] The TPD-MS
analyses performed for the Au/ACFÀ H2O catalysts in this study
confirmed that the AuClx species directly interacted with phenol
and carbonyl groups on the ACF surface. In contrast, the AuClx
species may bond to all of the SOGs, especially the carboxyl,
carboxylic anhydride, and lactone groups, on the Au/
ROX0.8(AQ)À H2O samples. Therefore, the higher amount of
phenol and carbonyl groups as well as the lower amount of
carboxyl, carboxylic anhydride, and lactone groups on the ACF
than on ROX0.8(AQ) may facilitate the anchoring and stabiliza-
tion in the atomic deposition of Au.
In addition to the apparent influence of SOGs, nitrogen
species also played an important role in imparting unique
surface properties to the catalysts, which was corroborated by
XPS N 1s spectra. Figure 5a shows that the curve of Au/
ACFÀ H2O could be well deconvoluted into two individual peaks
at 398.3 and 400.4 eV, demonstrating the coexistence of
pyridinic N and pyrrolic N, respectively.[48–51] Moreover, the
relative contents and binding energies of the existing nitrogen
species in the catalysts according to XPS are summarized in
Table 5, which shows that the content of pyrrolic N of Au/
ACFÀ H2O was as high as 79.98% of the N species, but no N
species were observed in the spectra of the Au/ROX0.8À H2O
catalysts. The schematic diagrams in Figure 5b show that
fractional N species were observed. As previously reported,[52]
pyrrolic N species are considered to be important for the
performance of N-doped carbon catalysts in the hydrochlorina-
tion of acetylene. The Au 4f7/2 signal of the Au/ACFÀ H2O catalyst
from XPS analysis, corresponding to Au3+, was negatively
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Figure 6. HCl-breakthrough profiles of: blank, Au/ACFÀ H2O catalyst and Au/
ROX0.8À H2O catalyst.
Figure 7. Catalytic performances of the different catalysts. Reaction con-
ditions: temperature=180 C, P=0.1 MPa, GHSV (C2H2)=1480 hÀ 1, and V
°
(HCl)/V(C2H2)=1.2.
shifted from the standard binding energy (BE) of 87.0 eV to
86.8 eV (Table 2). Meanwhile, the same changes took place for
the peak corresponding to the pyrrolic N species of the Au/
ACFÀ H2O catalyst, which was 0.4 eV higher than the standard
BE of 400.0 eV (Table 5).[53–55] Considering that the N-containing
species could cause the negative shift of Au3+ (Figure 2b), this
phenomenon may be attributed to the electron transferred
from pyrrolic N to the Au3+ center. A compound structure
possibly formed though a π-σ coordination bond between Au3+
and pyrrolic N.[22,56] As can be seen from Figure 5, the pyrrolic N
content was relatively high for the Au/ACFÀ H2O catalyst. The
pyrrolic N served as electron donors, thus enhancing the
catalytic activity, as had already been reported in our previous
paper.[56] In order to thoroughly investigate the catalytic proper-
ties of the nitrogen-rich ACF, the HCl adsorption properties of
the Au/ACFÀ H2O and Au/ROX0.8À H2O catalysts were analyzed.
As shown in Figure 6, the HCl adsorption efficiency of Au/
ACFÀ H2O was significantly higher than that of the Au/
ROX0.8À H2O samples, because of the longer breakthrough time
of HCl.[57] Several excellent reports discussing this issue are
available in the literature, including those stating that catalysts
exhibited excellent adsorption capacity possibly due to the
coordination of the electron-donating pyrrolic N species, which
can increase the electron density of the single-site AuClx
species.[22] Besides, Zhang et al.[33,58–60] reported that the N-
doped support can stabilize the gold species and increase the
catalytic activity. Dai et al.[61–63] demonstrated that pyrrolic N is
the most important nitrogen species and pointed out that
pyridinic N is also positive for acetylene hydrochlorination.
Indeed, the active AuClx species of the catalyst were easily
deactivated because of the loss of Cl atoms from AuClx owing
to the acetylene atmosphere and AuClx was a good electron
donor that could adsorb HCl easily during reaction. Thus,
sufficient adsorbed HCl on the Au/ACFÀ H2O catalyst might
effectively supply the consumption of Cl atoms and improve
the catalytic performance. Moreover, the adsorbed HCl on the
active AuClx species could further activate the further adsorbed
C2H2, which leads to a decrease in the activation energy of the
reaction.[56] Overall, the above results and the experimental
phenomena reported in the literature indicate that the presence
of pyrrolic N species is beneficial to the catalytic performance to
some extent.
Catalytic Study of Acetylene Hydrochlorination
Figure 7 shows the catalytic performances of the different
catalysts as well as those of catalyst supports in acetylene
hydrochlorination. The selectivity for VCM of all catalysts was
higher than 99%. The results illustrated that the highest activity
of above 79% conversion could be obtained with the Au/
ACFÀ H2O samples consisting of unpretreated ACF carbon. In
contrast, the Au/ROX0.8(AQ)À H2O catalysts had lower activity
than the Au/ACFÀ H2O catalysts. Further, the activity of the Au/
ACFÀ H2O catalysts showed almost no decrease over 10 h, while
the Au/ROX0.8(AQ)À H2O catalysts were completely deactivated.
The pure ACFÀ H2O and ROX0.8(AQ)À H2O samples were used as
controls with lower conversions in order to rule out the
influences of the carriers themselves. It should be noted that
catalysts prepared using deionized water on the untreated
ROX0.8 carbon without SOGs were nearly inactive. In summary,
the results showed that the superior performance of the Au/
ACFÀ H2O catalysts was likely due to the abundant microporous
structure, compared to that of Au/ROX0.8(AQ)À H2O catalysts,
despite having similar specific surface areas. Moreover, the
specific surface oxygen ligands (such as phenol and carbonyl
groups) incorporated obtained on the ACF evaluated by TPD
may be the essential factor that can be linked to the
stabilization of the atomic Au deposition and the presence of
the single-site AuClx species suggested by the combined
analyses of STEM, XRD, and XPS. In addition, we further studied
the impact of pyrrolic N species on the ACF, and demonstrated
that the synergistic effect of electron-donating pyrrolic N
species generated an active catalysts. Therefore, it was clear
that the ACF without acid treatment are greener than the
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conventional acid-treated activated carbon carriers as well as
provided enhanced catalytic performance. Moreover, ACF could
establish more suitable surface properties than aqua regia
treatment preparation methods of supported Au catalysts.
aqua regia contained very high cationic Au species contents (>
50%).[26] Furthermore, we recently reported a novel catalyst that
was treated with a H2O2/HCl mixture, that contained a relative
content of cationic Au species as high as 47.3%.[4] In general, a
catalyst prepared using strong oxidizing agents could basically
have high contents of highly dispersed cationic Au species.
However, it was worth noting that Au/ACFÀ H2O was superior to
the typical aqua regia-treated catalysts in terms of stability over
15 h. The reason for this superior performance was speculatively
based on the influence of surface chemistry and could be the
presence of SOGs and pyrrolic N species, which may have
served as anchoring sites for Au species on the ACF. Recently,
Kaiser et al.[65] reported that N species have a stabilizing effect
on single Au atoms, which was consistent with our results.
Accordingly, the results in this study suggest that ACF is the
carrier that has the potential to reduce/eliminate the use of
strong oxidizing reagents in catalyst synthesis schemes and
highlight the direction of green catalyst development for
industrial applications.
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9
Possibility of Using ACF Carrier to Reduce/Eliminate Strong
Oxidizing Reagents in the Production Technology
For achieving economical and green production methods for
Au catalysts, the immediate challenge has been to eliminate
strong corrosive and environmentally unfriendly aqua regia
from catalyst preparation schemes. Some literature has been
reported that aqua regia is important for Au deposition and
nucleation on supports and closely related to high catalytic
activity.[26,27] Recently, Zhang et al.[64] reported that the activity
of a catalyst is closely related to the intrinsic properties of the
solvents, and that specific solvents can enhance the interaction
between the carrier and the Au species, thereby inhibiting
aggregation and the loss of active sites during reaction. In our
study, we applied ACF to support Au catalysts as a direct
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material without any harmful technology that can achieve the Conclusions
same effect of using preparation methods involving different
strong oxidizing reagents, as evident from the following
catalytic performance comparison.
In this paper, we presented an environmentally friendly strategy
for developing carrier materials, using ACF to support Au
catalysts for acetylene hydrochlorination. In particularly, these
Au/ACFÀ H2O catalysts exhibited higher activity than Au/
ROX0.8(AQ)À H2O catalysts, whose carriers were pretreated with
aqua regia, and better stability than the typical impregnation
solvent-treated carbon catalysts. It was confirmed that the
superior catalytic performance of Au/ACFÀ H2O could be due to
both the anchoring and the stabilization of atomic Au
deposition on SOGs (such as phenol and carbonyl groups) and
the improvement of HCl adsorption with the synergistic effect
of electron-donating pyrrolic N species on the ACF. Further-
more, we performed a series of characterization analyses on the
Au/ACFÀ H2O and Au/ROX0.8(AQ)À H2O catalysts, including
STEM, XRD, XPS, TPD, and HCl-breakthrough analyses. In
addition, the development and design of a new green material
such as ACF, which could be a substitute for acid-treated
activated carbon and reduce/eliminate the use of strong
oxidizing reagents, as a catalyst carrier is essential for the
sustainable development of the PVC industry. However, the
specific composition and structure of the active species could
not be fully defined and further systematic investigation will be
required.
The catalysts were evaluated in the hydrochlorination of
acetylene, and the results are presented in Figure 8. It was
found that the catalytic activity of Au/ACFÀ H2O was lower than
those of typical strong oxidant-prepared catalysts Au/
ROX0.8À AQ, Au/ROX0.8À AP, and Au/ROX0.8À OAR. The selectiv-
ity for VCM of all catalysts was higher than 99%. On one hand,
this result may be attributed to the prevalence of atomic
dispersion of Au species on catalysts prepared using strong
oxidizing agents, which may be superior to catalysts prepared
in water. On the other hand, the high activity was considered to
be based on these strong oxidant-prepared catalysts perhaps
having higher total AuClx species contents than Au/ACFÀ H2O.
Indeed, Hutchings et al. observed that catalysts treated with
Experimental Section
Catalysts Supports Preparation
Viscose fibers (Thornel-25) and activated carbon (Norit ROX0.8)
were chosen to prepare carriers. Before carbonization viscose fibers
were pre-soaked in an aqueous solution (NH3·H2O 3.3 g/l) for 24 h
°
and dried at 120 C for 18 h. The pre-soaked fibers were then
Figure 8. Catalytic performances of the different catalysts. Reaction con-
°
°
heated to 950 C at the speed of 5 C/min for carbonization under
nitrogen gas protection, then naturally cooled to room temper-
ditions: temperature=180 C, P=0.1 MPa, GHSV (C2H2)=1480 hÀ 1, and V
(HCl)/V(C2H2)=1.2.
°
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ature. For comparison, the ROX0.8 carbon was pretreated with aqua
regia (3:1 HCl (>36 wt%): HNO3 (>65 wt%)) at room temperature
for 12 h to remove ash and to create SOGs. After that, both
pretreated fibers and pretreated ROX0.8 carbon were washed by
Acknowledgements
1
2
3
4
5
Financial support supported by the National Natural Science
Foundation of China (NSFC; Grant 21606199, 21476207) is grate-
fully acknowledged.
°
deionized water to neutral, eventually dried at 120 C for 18 h, to
obtain ACF and ROX0.8(AQ), respectively.
6
7
8
9
Catalysts Preparation
Conflict of Interest
The obtained ACF and HAuCl4 aqueous solution were mixing in
deionized water. The mixture was impregnation for 12 h and then
The authors declare no conflict of interest.
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°
dried at 120 C for 18 h, was designated Au/ACFÀ H2O, Au loading of
1.0 wt.%. For comparison, ROX0.8(AQ) and ROX0.8 supports were
also prepared in the same method and was designated Au/
ROX0.8(AQ)À H2O and Au/ROX0.8À H2O. In addition, catalysts without
Au prepared on the ACF and ROX0.8(AQ) supports were labeled
ACFÀ H2O and ROX0.8(AQ)À H2O, respectively. Furthermore, the
ROX0.8 and HAuCl4 aqueous solution were mixing using aqua regia
and green aqua regia (1:3 H2O2 (30 wt%): HCl (36 wt%)) as solvent
and prepared by the same method and was designated Au/
ROX0.8À AQ and Au/ROX0.8À AP respectively and the Au/
ROX0.8À H2O treated by organic aqua regia (1:10 SOCl2 (>99.5%):
DMF (>99.9%)) was labeled as Au/ROX0.8À OAR.
Keywords: acetylene hydrochlorination
fibers · phenolic and carbonyl groups · pyrrolic N species
· activated carbon
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Ready, steady, gold! A new pathway
was opened though the usage of
green carrier to reduce/eliminate the
use of strong oxidizing reagents in
the preparation of Au catalysts for
the hydrochlorination of acetylene.
H. Lai, B. Wang, Y. Yue, G. Sheng, S.
Wang, F. Feng, Q. Zhang, Prof. Dr. J.
Zhao*, Prof. Dr. X. Li*
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An Alternative Carbon Carrier in
Green Preparation of Efficient Gold/
Carbon Catalyst for Acetylene Hy-
drochlorination
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