The role of KCl in FeCl3-KCl/Al2O3 catalysts with enhanced catalytic performance for ethane oxychlorination

Article abstract of DOI:10.1039/c7dt01957b

Among the vinyl chloride production processes, ethane oxychlorination is the most economical and environment-friendly process but constrained by the lack of high performance catalysts for industrial applications. In this work, FeCl₃-KCl/Al₂O₃ catalysts with different molar ratios of K/Fe were prepared by a co-impregnation method and applied to ethane oxychlorination. The FeCl₃-KCl/Al₂O₃ catalyst with K/Fe = 2 exhibited enhanced catalytic performance with the highest conversion of C₂H₆ (99.1%) and the best selectivity to C₂H₃Cl (74%) under the optimal conditions of 400 °C, C₂H₆:HCl:air = 1:3:5.5 (volume ratio) and GHSV = 4560 h^-1. It was found that the enhanced catalytic performance could be attributed to the formation of KFeCl₄ from KCl and FeCl₃ and the change of the reaction process. Besides, KCl is in favor of weakening the interaction between the active species and support. The reduction activation energy of Fe(iii) → Fe(ii) is efficiently reduced by KCl addition. The FeCl₃-KCl/Al₂O₃ catalyst may be a potential catalyst for industry due to its simple composition and convenient preparation.

Full text of DOI:10.1039/c7dt01957b

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The role of KCl in FeCl3–KCl/Al2O3 catalysts with
enhanced catalytic performance for ethane
oxychlorination†
Cite this: DOI: 10.1039/c7dt01957b
Qihua Zhou, Ruisheng Hu, * Yun Jia and Hongye Wang
Among the vinyl chloride production processes, ethane oxychlorination is the most economical and
environment-friendly process but constrained by the lack of high performance catalysts for industrial
applications. In this work, FeCl3–KCl/Al2O3 catalysts with different molar ratios of K/Fe were prepared by a
co-impregnation method and applied to ethane oxychlorination. The FeCl3–KCl/Al2O3 catalyst with K/Fe
= 2 exhibited enhanced catalytic performance with the highest conversion of C2H6 (99.1%) and the best
selectivity to C2H3Cl (74%) under the optimal conditions of 400 °C, C2H6 : HCl : air = 1 : 3 : 5.5 (volume
ratio) and GHSV = 4560 h−1. It was found that the enhanced catalytic performance could be attributed to
the formation of KFeCl4 from KCl and FeCl3 and the change of the reaction process. Besides, KCl is in
favor of weakening the interaction between the active species and support. The reduction activation
energy of Fe(III) → Fe(II) is efficiently reduced by KCl addition. The FeCl3–KCl/Al2O3 catalyst may be a
potential catalyst for industry due to its simple composition and convenient preparation.
Received 30th May 2017,
Accepted 13th July 2017
DOI: 10.1039/c7dt01957b
rsc.li/dalton
with LaCl3 can achieve higher activity for ethane oxychlorina-
tion by making CuCl2 highly dispersed and then preventing
1 Introduction
Poly-vinyl chloride (PVC) is one of the five general plastics and the catalyst from sintering. In 2011, a study by Li et al.12
has been widely used in industry, agriculture, national defense showed that the addition of CeO2 to the catalyst could form
and our daily life. The monomer of PVC, vinyl chloride, can be aggregated crystalline ceria species and capping oxygen, which
produced by three processes: acetylene hydrochlorination,1–7 may enhance the performance of the catalyst. Meanwhile, in
ethylene oxychlorination8,9 and ethane oxychlorination.10–14 2011, Li et al.13 reported a binary promoter (La2O3, CeO2)
The obvious drawback of acetylene hydrochlorination (based modified Cu-based catalyst for ethane oxychlorination, and
on the HgCl2/C catalyst) is high energy consumption and found that the different impregnation procedures of the La2O3
severe pollution. The cost of ethylene oxychlorination precursor made a difference in catalytic activity. In 2013, Li
increases with the increase of the ethylene price. The most et al.14 assumed that Pr (Pr6O11) improved the electron transfer
economical and environment-friendly process is ethane oxy- from Pr to Cu to promote the reduction of Cu species. In 2015,
chlorination, which, however, is constrained by the lack of we reported that the addition of Cr species could effectively
high performance catalysts for industrial applications. reduce the reduction activation energy of the Cu(II) → Cu(I)
Therefore, the study of ethane oxychlorination is of great process in the CuCl2–KCl–CeO2/γ-Al2O3 catalyst.15 Besides, we
importance.
also reported a perovskite as a promoter for the Cu-based
By far, the studies about the ethane oxychlorination catalyst ethane oxychlorination catalyst.16These studies have deepened
have been focused on Cu-based catalysts. In 2006, Liu et al.10 our knowledge about the Cu-based ethane oxychlorination
reported that the addition of KCl to the CuCl2/γ-Al2O3 catalyst catalyst and have made us realize its bottleneck of catalytic per-
decreased the interaction between CuCl2 and γ-Al2O3 and formance and its limited application in industrial production
improved the catalytic performance by accelerating the Cu(II) due to its complicated composition.
→ Cu(I) reduction. Xueju et al.11 found that CuCl2–KCl/γ-Al2O3
However, the current research about the Fe-based ethane
oxychlorination catalyst is still rare. As we know, iron is one of
the most abundant elements on earth, which is widely applied
in the field of organic synthesis. Its kin element ruthenium
has been widely applied in catalysis, especially in HCl oxi-
dation (Deacon process).17,18 The Deacon process plays a vital
role in ethane oxychlorination. Considering the similarity
School of Chemistry and Chemical Engineering, Key Laboratory of Rare Earth
Materials Chemistry and Physics, Inner Mongolia University, Hohhot 010021,
P. R. China. E-mail: cehrs@imu.edu.cn
†Electronic supplementary information (ESI) available. See DOI: 10.1039/
c7dt01957b
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between iron and ruthenium, we shift the focus to the iron- Before adsorption measurements, all samples were evacuated
based ethane oxychlorination catalyst. Goodrich Company at 250 °C for 2 h. The total surface area was determined by the
developed a solid solution catalyst containing iron cations.19 Brunauer–Emmett–Teller (BET) method. The pore volume was
The catalyst contained iron cations of 1%–15% (weight percen- considered as the volume of liquid nitrogen adsorbed at P/P0 ≈
tage) and the yield of vinyl chloride reached 41.4% with iron 1. H2-TPR was conducted with a Chembet Pulsar multiple
cations of 4% (weight percentage). It is believed that it is adsorption instrument (Quantachrome, USA). Before
important to continue the study about Fe-based ethane oxy- reduction, 0.1 g sample was loaded in a quartz tube reactor
chlorination catalysts for the understanding of ethane oxy- and pretreated in helium flow at 200 °C for 1 h to purge the
chlorination and the improvement of the catalyst system. In sample surface. After cooling down to 50 °C, the reactor was
this work, FeCl3–KCl/Al2O3 catalysts with different molar ratios heated from 50 °C to 900 °C at a heating rate of 10 °C min−1 in
of K/Fe (denoted as FeKx, where x represents the molar ratio of a 5% H2/Ar gas flow of 35 ml min−1. The consumption of
K/Fe (x = 0, 0.5, 1, 1.5 and 2, respectively)) were prepared by a hydrogen was monitored using a thermal conductivity detector
co-impregnation method and applied to ethane oxychlorina- (TCD). The 57Fe Mossbauer spectra at room temperature were
tion. The role of KCl in the FeCl3–KCl/Al2O3 catalyst was recorded by using an MFD-500A Mossbauer spectrometer
studied by different characterization techniques. This work (Topologic Systems, Japan). The radiation sources were 57Co/
will deepen the comprehension about the active species Rh. The velocity was calibrated with α-Fe foil. The thickness of
in the FeCl3–KCl/Al2O3 catalyst and of its role in ethane the absorbers used in the measurements is 3–5 mg Fe per cm2.
oxychlorination.
The spectra were fitted with the appropriate superpositions of
Lorentzian lines using the MossWinn 3.0i computer program.
2.4 Catalytic tests
2 Experimental
Catalytic activity of ethane oxychlorination was evaluated in a
fixed bed quartz reactor (14 mm in internal diameter and
30 cm in length) under atmospheric pressure. Before reaction,
the catalyst (1.0 g, 40–60 mesh) was activated under a
HCl/air mixture (64 ml min−1, HCl : air = 3 : 5) at 450 °C for
30 min. Then, ethane (8 ml min−1) was dosed into the reactor.
The reaction was carried out at 450 °C, C2H6 : HCl : air =
1 : 3 : 5, and total flow rate of 72 ml min−1. The products
were passed through a sodium hydroxide tank to remove
hydrogen chloride. The reaction products were analyzed on
an SP-6890 gas chromatograph (Lunan Ruihong Chemical
Instrument Co. LTD, China) with a Porapak Q column and a
TCD detector.
2.1 Materials
All reagents (FeCl3·6H2O, KCl) were analytical grade (≥99.0%)
and obtained from Sinopharm Chemical Reagent Co., Ltd
(Shanghai, China). Commercial γ-Al2O3 (≥99.7%) was obtained
from ZiBo JuTeng Chemical Co., Ltd (Shandong, China). HCl
(99.999%) and C2H6 (99.99%) were obtained from Dalian
Special Gases Co., Ltd (Liaoning, China). HCl flow was con-
trolled by using a mass flow controller. HCl was dried with a
5 A molecular sieve column before it passed the mass flow con-
troller. C2H6 flow was treated under the same conditions just
as HCl. Air was obtained with a SPB-3 air-source instrument
(BCHP Analytical Technology Institute, Beijing, China).
The conversion of ethane (XA) and the selectivity to VCM
(SVCM) as the criteria of catalytic performance were calculated
2.2 Catalyst preparation
FeCl3-KCl/Al2O3 catalysts with different molar ratios of K/Fe
were prepared by the co-impregnation method. Commercial
γ-Al2O3 (SBET = 160.7 m2 g−1) was chosen as the support. The
support was mixed with an aqueous solution of appropriate
FeCl3·6H2O (AR), KCl (AR), and impregnated by ultrasonication
for 0.5 h, then aged for 24 h at room temperature and dried at
100 °C for 2 h. Later, the solid catalysts were calcined under an
air atmosphere at 600 °C for 4 h. The catalysts with different
molar ratios of K/Fe were denoted as FeKx, where x represents
the molar ratio of K/Fe (x = 0, 0.5, 1, 1.5 and 2, respectively).
All catalysts have the same content of iron, 5 wt% in γ-Al2O3.
by equations: XA = (1 − φair − φA)/(1 − φair) × 100% and SVCM
=
φVCM/(1 − φair − φA) × 100% where φair, φA, and φVCM represent
the volume fraction of air, the volume fraction of the remnant
ethane, and the volume fraction of VCM, respectively.
We have conducted orthogonal experiments to study the
effect of different ethane flow rates, different feed ratios and
different reaction temperatures on the space–time yield of
C2H3Cl. The studied factors and levels are listed in Table 1 and
a L9(34) orthogonal table is used to design the orthogonal
2.3 Characterization
Table 1 The studied factors and levels in orthogonal experiments
X-ray powder diffraction (XRD) patterns were recorded with an
Empyrean X-ray diffractometer (PANalytical, Holland), using
Factor
Cu Kα radiation, λ = 0.1542 nm, scanning step = 0.0065° s−1
,
Ethane flow The ratio of
The ratio of
Reaction
scanning over the range of 10° ≤ 2θ ≤ 80° and operating at
40 kV and 40 mA. The Brunauer–Emmett–Teller (BET) surface
area was determined by nitrogen adsorption at 77 K on a
Quantachrome Nova 4200e surface area and pore size analyzer.
Level rate (A) C2H6 to HCl (B) C2H6 to air (C) temperature (D)
1
2
3
6 ml min−1 1 : 1
1 : 5
1 : 5.5
1 : 6
400 °C
450 °C
470 °C
8 ml min−1 1 : 2
12 ml min−1 1 : 3
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experiments. Other conditions in orthogonal experiments are increases with the increase of x in the following order: FeK0 <
the same as those stated above. FeK0.5 < FeK1 < FeK1.5 < FeK2. As can be seen from Fig. 1b,
The Cl2 release rate measurement was conducted under the the C2H3Cl selectivity curves of FeKx catalysts show a similar
same reaction conditions as stated above in a catalytic test but trend. The selectivity of potassium-containing samples (FeKx,
in the absence of ethane. 0.5 g catalyst (40–60 mesh) was x = 0.5, 1, 1.5, and 2) is higher than that of the potassium-free
loaded. When the reaction temperature reached 450 °C, the sample (FeK0). Besides, the C2H3Cl selectivity of FeKx catalysts
mixed gas of HCl (24 mL min−1) and air (40 mL min−1) was also increases with the increase of x in the following order:
dosed into the reactor. After 30 min, KI solution (0.25 mol L−1
)
FeK0 < FeK0.5 < FeK1 < FeK1.5 < FeK2. The FeK2 catalyst exhi-
was used to absorb the Cl2 in the reaction products for 10 min. bits the highest C2H6 conversion (97.7%) and the best C2H3Cl
Iodometry was used to determine the amount of Cl2 in the selectivity (65.5%). The main by-product of the FeK2 catalyst is
reaction products. 0.1 mol L−1 Na2S2O3 solution was used in ethylene. The selectivity to ethylene is 34.3% and therefore the
the titration.
selectivity to other by-products like carbon dioxide is 0.2%.
Besides, the loss of the active component is improved with the
addition of KCl (Fig. S1 in ESI†).
As can be seen in Fig. 1, the C2H6 conversion of the FeK0
catalyst increases in 7 h. Therefore, we performed a longer test
on FeK0 (Fig. S2 in ESI†). It is found that the C2H6 conversion
reaches its maximum (86.3%) after 12 h and enters a platform
stage, while the C2H3Cl selectivity is quite stable. Compared
with the potassium-containing catalysts, it suggests that the
addition of KCl can shorten the time to enter the platform
stage.
3 Results and discussion
Fig. 1 shows the C2H6 conversion curves and the C2H3Cl
selectivity curves of FeKx catalysts. As can be seen from Fig. 1a,
the C2H6 conversion of potassium-containing samples (FeKx,
x = 0.5, 1, 1.5, 2) is higher than that of the potassium-free
sample (FeK0). In addition, the C2H6 conversion of FeKx catalysts
In industry, the catalytic performance is expressed by the
space–time yield which includes the influence of the raw
material flow rate. We also calculated the space–time yield
(STY) of C2H3Cl using the following equation and the results
are listed in Table 2:
Mp
Mf
mf ꢀ ꢀ Xf ꢀ Sp
mp
STY ¼
¼
mc ꢀ T
mc ꢀ T
ꢀ Mp ꢀ Xf ꢀ Sp
mf
Mf ꢀ T
¼
mc
mf
Mf ꢀ T
where
is the raw material (ethane) molar flow rate, Mp
is the product (VCM) molar mass, Xf is the raw material con-
version, Sp is the product selectivity, and mc is the catalyst
mass.
As can be seen from Table 2, the STY of FeKx catalysts
−1
increases with the increase of x, from 3.7 × 10–3 g gcatal
min−1 to 13.7 × 10–3 g gcatal min−1. The STY of FeK2 is about
−1
3.7 times as large as that of FeK0. It is obvious that the
addition of KCl is beneficial to the improvement of catalytic
activity.
Table 2 The space–time yield (STY) of C2H3Cl of different catalysts
Average
Average C2H6 C2H3Cl
Cl2 release
rate (10−5
mol min−1
)
conversion
(%)
selectivity STY (10−3
Catalyst
(%)
g gcatal−1 min−1
)
FeK0
FeK0.5
FeK1
FeK1.5
FeK2
72.5 0.5
89.8 0.1
95.4 0.1
96.4 0.1
97.1 0.1
22.8 0.2
31.4 0.2
50.0 0.4 10.6
61.9 0.4 13.3
63.3 0.3 13.7
3.7
6.3
7.17
10.36
15.76
21.25
22.34
Fig. 1 The C2H6 conversion curves (a) and the C2H3Cl selectivity curves
(b) of FeKx catalysts.
KCl/Al2O3 59.7 0.1
6.1 0.1
0.8
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In order to prove the interaction between FeCl3 and KCl, which suggests that the strong interaction between FeCl3 and
the catalytic performance of KCl/Al2O3 was also tested. The KCl such as a chemical reaction greatly improves the catalytic
content of KCl in KCl/Al2O3 is equal to that of the FeK2 cata- activity.
lyst. KCl/Al2O3 exhibits poor catalytic activity (Fig. 2) with a
We have conducted orthogonal experiments on the FeK2
C2H6 conversion of 60% and a C2H3Cl selectivity of 6%. The catalyst in order to study the effect of different reaction con-
STY of FeK2 is larger than the sum STY of FeK0 and KCl/Al2O3, ditions like raw material flow rate, feed ratio and reaction
temperature on the C2H3Cl STY. The experimental results are
shown in Table 3. Orthogonal analysis gives out the best reac-
tion conditions of A3B3C2D1 which means that the FeK2
catalyst shows the highest STY with an ethane flow rate of 12
mL min−1, the ratio of C2H6 to HCl of 1 : 3, the ratio of C2H6 to
air of 1 : 5.5 and the reaction temperature of 400 °C. The range
order of different factors is B > A > D > C. This indicates that
the ratio of C2H6 to HCl and the ethane flow rate have a
greater influence on the C2H3Cl STY. In our experiment, more
HCl gives more Cl atoms in the reaction efficiently improving
the ethane conversion. The larger the ethane flow rate is, the
shorter the contact time between ethane and the catalyst
becomes, avoiding the over-oxidation or over-chlorination of
ethane and improving the product selectivity. Finally, we got a
high C2H3Cl STY under the optimal conditions of 400 °C,
C2H6 : HCl : air = 1 : 3 : 5.5 (volume ratio) and GHSV = 4560 h−1
.
As can be seen in Table 4, compared to the previous
studies,10–16 the FeK2 catalyst shows similar C2H6 conversion
Fig. 2 The C2H6 conversion curve and the C2H3Cl selectivity curve of but obviously higher C2H3Cl selectivity with a lower reaction
temperature (400 °C) and a larger ethane flow rate (12 mL min−1).
the KCl/Al2O3 catalyst.
Table 3 The orthogonal experiment results
Reaction conditions
Test
number
Ethane flow
rate (A)
The ratio of
C2H6 to HCl (B)
The ratio of
C2H6 to air (C)
Reaction
temperature (D)
STY
(10−3 g gcatal−1 min−1
)
1
2
3
4
5
6
7
8
9
6 ml min−1
6 ml min−1
6 ml min−1
8 ml min−1
8 ml min−1
8 ml min−1
12 ml min−1
12 ml min−1
12 ml min−1
1 : 1
1 : 2
1 : 3
1 : 1
1 : 2
1 : 3
1 : 1
1 : 2
1 : 3
1 : 5
1 : 5.5
1 : 6
1 : 5.5
1 : 6
1 : 5
1 : 6
1 : 5
1 : 5.5
400 °C
450 °C
470 °C
470 °C
400 °C
450 °C
450 °C
470 °C
400 °C
4.5
9.0
11.3
6.6
15.4
12.9
9.0
15.1
24.6
Table 4 Catalytic performance of different catalysts for ethane oxychlorination
Conditionsa
C2H6
conversion
C2H3Cl
selectivity
C2H3Cl STY
Catalyst
Temperature
Feed ratiob
(10−3 g gcatal−1 min−1
)
Ref.
CuCl2–KCl/γ-Al2O3
500 °C
500 °C
450–550 °C
450–550 °C
450–550 °C
510 °C
C2H6 : HCl : air = 1 : 3 : 5
C2H6 : HCl : air = 1 : 3 : 5
C2H6 : HCl : air = 1 : 2 : 5
C2H6 : HCl : air = 1 : 2 : 5
C2H6 : HCl : air = 1 : 2 : 5
C2H6 : HCl : air = 1 : 3 : 5
C2H6 : HCl : air = 1 : 3 : 5
C2H6 : HCl : air = 1 : 3 : 5.5
89.9%
97.6%
98.6%
97.0%
97.5%
97.8%
100.0%
99.1%
39.0%
51.7%
55.2%
50.0%
52.0%
64.0%
50.0%
74.0%
3.9
5.6
6.1
5.4
5.7
14.0
11.2
24.6
10
11
12
13
14
15
16
CuCl2–KCl-LaCl3/γ-Al2O3
CuCl2–KCl–CeO2/MgO–γ-Al2O3
CuCl2–KCl–CeO2/La2O3/MgO–γ-Al2O3
CuCl2–KCl–Pr6O11/MgO–γ-Al2O3
CuCl2–KCl–CeO2–Cr2O3/γ-Al2O3
La1.7K0.3NiMnO6–CuCl2/γ-Al2O3
FeCl3–KCl/Al2O3
500 °C
400 °C
a The reactor used is a fixed bed reactor. b The ethane flow rate is 4 mL min−1 for the catalysts in ref. 10–14, 8 mL min−1 for the catalysts in ref. 15
and 16 and 12 mL min−1 for the catalysts in this work.
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Table 5 Specific surface area and reduction characteristics of FeKx
catalysts
Theoretical H2
consumption
for Fe(III) to
Fe(II) (mol)
Reduction
activation
energy
Practical H2
consumption
(mol)
SBET
Catalyst
(m2 g−1
)
(kJ mol−1
)
FeK0
FeK0.5
FeK1
FeK1.5
FeK2
154
145
132
122
113
4.06 × 10−5
3.79 × 10−5
3.70 × 10−5
3.66 × 10−5
3.53 × 10−5
4.25 × 10−5
3.89 × 10−5
3.80 × 10−5
3.84 × 10−5
3.87 × 10−5
55.93
—
—
—
47.37
The C2H3Cl STY of the FeK2 catalyst is much higher than that
of the catalysts previously reported. Moreover, the FeK2 catalyst
has a more simple composition, which is closer to the require-
ment of industrial application.
Fig. 4 The Mossbauer spectrum of the FeK2 catalyst.
N2 adsorption/desorption experiments were carried out to
study the textural properties of FeKx catalysts. As can be seen
from Fig. S3,† the shape of the N2-adsorption/desorption iso-
therms does not change obviously, suggesting that the loading
of active species does not change the pore structure in the
support. Besides, the BET surface area decreases with the
increase of K/Fe (Table 5). The BET surface area reduces by
26.6% (from 154 m2 g−1 to 113 m2 g−1) because of the loading
of the active species.
our samples. It is assumed that the content of FeCl3 is lower
than its dispersion threshold so FeCl3 is highly dispersed on
the support. In the potassium-containing catalysts (FeKx, x =
0.5, 1, 1.5, 2), there are no diffraction peaks of FeCl3 or other
iron species, indicating that the iron species are also highly
dispersed.
Mossbauer spectroscopy can be applied to qualitative and
quantitative analysis for complicated materials by comparing
the Mossbauer parameters such as isomer shift (I.S.) and
quadrupole splitting (Q.S.).21 The iron content in our catalysts
is relatively low and the iron species is highly dispersed, which
is proved by XRD results. Therefore, the Mossbauer spectrum
XRD was conducted to figure out the phase composition of
the catalysts (Fig. 3). The characteristic peaks of γ-Al2O3 are
observed in the XRD patterns of all catalysts. When the K/Fe is
larger than 1, the diffraction peaks of KCl (2θ = 28.36, 40.53,
66.42°) strengthen gradually. In the case of FeK0, the charac-
teristic peaks of FeCl3 cannot be observed in its XRD pattern.
Zong and Xie et al.20 reported that FeCl3 could disperse spon-
taneously onto the γ-Al2O3 surface. The dispersion threshold
of FeCl3 is 2.1 mg m−2, which means that the amount of FeCl3
of FeK2 is studied. The I.S. of pure FeCl3 is 0.45 mm s
I.S. of pure KFeCl4 is 0.29 mm s−1 22
According to the
.
−1 22 The
.
Mossbauer spectrum (Fig. 4), the FeK2 catalyst shows a
doublet with an I.S. of 0.301 mm s−1 and Q.S. of 1.346 mm
s−1. The I.S. of the FeK2 catalyst is in accordance with that of
KFeCl4 but the Q.S. is much higher than that of KFeCl4.
Considering the composition of the FeK2 catalyst, we interpret
these as that iron exists in the form of KFeCl4. However,
because the iron species highly disperse on the support as a
monolayer, the KFeCl4 on the support has lower structure sym-
metry which leads to a high Q.S. value. It is the high sensitivity
and energy resolution of the Mossbauer spectrum23 that
makes the observation of the existence of KFeCl4 on the cata-
lyst possible.
to form a monolayer on our γ-Al2O3 (SBET = 160.7 m2 −1) is
g
0.3375 g g−1, more than the amount of FeCl3 (0.1453 g g−1) in
H2-TPR is a very effective technique for studying the
reduction behavior of the catalyst. Fig. 5 shows the H2-TPR pro-
files of FeKx catalysts. As we see, each catalyst shows a com-
plete peak around 500 °C–550 °C which is mainly discussed
below. The peak around 500 °C–550 °C is attributed to the
reduction process of Fe(III) → Fe(II) by comparing the practical
H2 consumption with the theoretical H2 consumption for
Fe(III) → Fe(II) (Table 5). The potassium-free sample (FeK0) shows
a broad peak with lower intensity, suggesting a strong inter-
action between the active species and support. However,
potassium-containing samples (FeKx, x = 0.5, 1, 1.5, 2) show
Fig. 3 The XRD patterns of FeKx catalysts.
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3 times as fast as that of FeK0, while the C2H3Cl STY of FeK2 is
about 3.7 times as large as that of FeK0. It suggests that the
C2H3Cl STY is strongly related to the Cl2 release efficiency. The
addition of KCl to FeCl3/Al2O3 changes the active species from
FeCl3 to KFeCl4, and also the Cl2 release efficiency. KFeCl4,
reacting with O2, releases Cl2 with higher efficiency (eqn (1)).
KFeO2, reacting with HCl, generates KFeCl4 to form a cycle
(eqn (2)). More Cl2 is favorable to the transformation of ethane
to vinyl chloride.28 These are important for FeCl3–KCl/Al2O3
being an ethane oxychlorination catalyst with enhanced cata-
lytic performance and explain why the catalytic performance
of potassium-containing samples (FeKx, x = 0.5, 1, 1.5, 2) is
better than that of the potassium-free sample (FeK0).
KFeCl4 þ O2 ! KFeO2 þ 2Cl2
ð1Þ
ð2Þ
KFeO2 þ 4HCl ! KFeCl4 þ 2H2O
Fig. 5 The H2-TPR profiles of FeKx catalysts.
narrow peaks with higher intensity, which means that KCl can
weaken the interaction between the active species and
support.10 In addition, the reduction temperature of Fe(III) →
Fe(II) decreases and then stabilizes at about 500 °C with the
increase of K/Fe. When x = 0, it is FeCl3 that is reduced by H2.
When x = 0.5, FeCl3 is excessive and part of FeCl3 forms
KFeCl4 with KCl. When x ≥ 1, all FeCl3 forms KFeCl4 with
excessive KCl theoretically.24–27 Therefore, the reduction temp-
erature of Fe(III) → Fe(II) stabilizes at about 500 °C (the
maximal temperature difference (0.4 °C) is less than the experi-
mental error). The decrease of reduction temperature suggests
the change of active species (FeCl3 → KFeCl4).
In order to study the effect of the addition of KCl on the
reduction activation energy of Fe(III) → Fe(II), the reduction
activation energy of Fe(III) → Fe(II) of FeK0 and FeK2 catalysts is
calculated. Fig. S4† shows the H2-TPR curves of FeK0 and FeK2
at different heating rates (β). It is observed that the reduction
peaks of Fe(III) → Fe(II) of FeK0 and FeK2 moved to higher
temperature with the increasing heating rate (β), also with the
increase of peak intensity.
4 Conclusion
Generally, FeCl3–KCl/Al2O3 catalysts were prepared by the co-
impregnation method and applied to ethane oxychlorination.
It is observed that the C2H6 conversion and the C2H3Cl selecti-
vity of FeKx catalysts increase with the increase of x in the fol-
lowing order: FeK0 < FeK0.5 < FeK1 < FeK1.5 < FeK2. The FeK2
catalyst exhibits the highest C2H6 conversion (99.1%) and the
best C2H3Cl selectivity (74%) under the optimal conditions of
400 °C, C2H6 : HCl : air = 1 : 3 : 5.5 (volume ratio) and GHSV =
4560 h−1. It is found that the enhanced catalytic performance
can be attributed to the formation of KFeCl4 from KCl and
FeCl3. The addition of KCl can reduce the reduction activation
energy of Fe(III) → Fe(II). Besides, KCl is in favor of weakening
the interaction between the active species and support which
is good for the proceeding of ethane oxychlorination.
Conflict of interest
There is no conflict of interest to declare.
As shown in Table 5 and Fig. S5,† the reduction activation
energy of reduction of Fe(III) → Fe(II) of FeK0 and FeK2 is
55.93 kJ mol−1 and 47.37 kJ mol−1, respectively. Comparing
the reduction activation energy of FeK0 and FeK2, the
reduction activation energy of FeK2 is obviously lower than
FeK0, which indicates that the addition of KCl is responsible
for the easier activation and reduction of Fe(III).
In a Cu-based catalyst, the release of Cl2 is the key of the
whole oxychlorination process.12,28 Ethane conversion and
vinyl chloride selectivity can be improved by accelerating the
release of Cl2.10,15 This is also true for our FeKx catalysts. In
order to compare the Cl2 release efficiency, we have measured
the Cl2 release rate of FeKx catalysts and the results are listed
in Table 2. It can be seen that the Cl2 release rate increases in
the following order: FeK0 < FeK0.5 < FeK1 < FeK1.5 < FeK2.
This is in accordance with the order of C2H3Cl STY of FeKx cat-
alysts. It is interesting that the Cl2 release rate of FeK2 is about
Acknowledgements
This work was financially supported by the National Natural
Science Foundation of China (No. 21566027). The authors
would like to appreciate the support from Inner Mongolia
Autonomous Region Open Funding for Major Basic Research
of China.
References
1 G. J. Hutchings, Nanocrystalline gold and gold palladium
alloy catalysts for chemical synthesis, Chem. Commun.,
2008, 1148–1164.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Dalton Transactions
Paper
2 H. Y. Zhang, B. Dai, X. G. Wang, W. Li, Y. Han, J. J. Gu and
J. L. Zhang, Non-mercury catalytic acetylene hydrochlorina-
cursor on copper-based catalysts for ethane oxychlorina-
tion, Catal. Commun., 2011, 13, 22–25.
tion over bimetallic Au–Co(III)/SAC catalysts for vinyl chlor- 14 Z. Li, G. D. Zhou, C. Li and T. X. Cheng, Effect of Pr on
ide monomer production, Green Chem., 2013, 15, 829–836.
3 M. Conte, C. J. Davies, D. J. Morgan, T. E. Davies,
copper-based catalysts for ethane oxychlorination, Catal.
Commun., 2013, 40, 42–46.
D. J. Elias, A. F. Carley, P. Johnston and G. J. Hutchings, 15 D. Z. Shi, R. S. Hu, Q. H. Zhou and C. Li, Effect of Cr-
Aqua regia activated Au/C catalysts for the hydrochlorina-
tion of acetylene, J. Catal., 2013, 297, 128–136.
doping on CuCl2-KCl-CeO2/γ-Al2O3 catalysts for ethane oxy-
chlorination, Appl. Catal., A, 2015, 506, 91–99.
4 M. Conte, A. F. Carley, C. Heirene, D. J. Willock, 16 D. Z. Shi, R. S. Hu, Q. H. Zhou and L. R. Yang, Catalytic
P. Johnston, A. A. Herzing, C. J. Kiely and G. J. Hutchings,
Hydrochlorination of acetylene using a supported gold
catalyst: A study of the reaction mechanism, J. Catal., 2007,
250, 231–239.
5 M. Conte, A. F. Carley, G. Attard, A. A. Herzing, C. J. Kiely
and G. J. Hutchings, Hydrochlorination of acetylene using
supported bimetallic Au-based catalysts, J. Catal., 2008,
257, 190–198.
activities of supported perovskite promoter catalysts
La2NiMnO6–CuCl2/γ-Al2O3 and La1.7K0.3NiMnO6–CuCl2/γ-Al2O3
for ethane oxychlorination, Chem. Eng. J., 2016, 288, 588–595.
17 D. Crihan, M. Knapp, S. Zweidinger, E. Lundgren,
C. J. Weststrate, J. N. Andersen, A. P. Seitsonen and
H. Over, Stable Deacon Process for HCl oxidation over
RuO2, Angew. Chem., Int. Ed., 2008, 120, 2161–2164.
18 H. Over, Atomic-scale understanding of the HCl oxidation
over RuO2, a novel Deacon Process, J. Phys. Chem. C, 2012,
116, 6779–6792.
6 B. Nkosi, M. D. Adams, N. J. Coville and G. J. Hutchings,
Hydrochlorination of acetylene using carbon-supported
gold catalysts: A study of catalyst reactivation, J. Catal., 19 W. J. Kroenke and P. P. Nicholas, Process for the oxychlori-
1991, 128, 378–386.
nation of an alkane using a solid solution catalyst contain-
7 B. Nkosi, N. J. Coville, G. J. Hutchings, M. D. Adams,
ing iron cations, US patent, 4375569, 1983.
J. Friedl and F. E. Wagner, Hydrochlorination of acetylene 20 Y. Zong, X. M. Pan, L. Y. Duan and Y. C. Xie, Dispersion
using gold catalysts: A study of catalyst deactivation,
J. Catal., 1991, 128, 366–377.
state and dispersion capacity of AlCl3 and FeCl3 on γ-Al2O3
surface, Chin. J. Catal., 1997, 18, 321–323.
8 E. Finocchio, N. Rossi, G. Busca, M. Padovan, G. Leofanti, 21 J. M. Millet, Mossbauer spectroscopy in heterogeneous
B. Cremaschi, A. Marsella and D. Carmello, catalysis, Adv. Catal., 2007, 51, 309–350.
Characterization and catalytic activity of CuCl2-Al2O3 22 Y. Takashima, Y. Maeda and S. Umemoto, The Mossbauer
ethylene oxychlorination catalysts, J. Catal., 1998, 179, 606–
spectrum of iron adsorbed on an anion exchange resin,
618.
Bull. Chem. Soc. Jpn., 1969, 42, 1760–1761.
9 N. B. Muddada, U. Olsbye, T. Fuglerud, S. Vidotto, 23 K. Liu, A. I. Rykov, J. H. Wang and T. Zhang, Recent
A. Marsella, S. Bordiga, D. Gianolio, G. Leofanti and
C. Lamberti, The role of chlorine and additives on the
advances in the application of Mossbauer spectroscopy in
heterogeneous catalysis, Adv. Catal., 2015, 58, 1–142.
density and strength of Lewis and Brønsted acidic sites of 24 H. L. Friedman, The visible and ultraviolet absorption
γ-Al2O3 support used in oxychlorination catalysis: A FTIR
spectrum of the tetrachloroferrate(III) ion in various
study, J. Catal., 2011, 284, 236–246.
media, J. Am. Chem. Soc., 1952, 74, 5–10.
ˇ
10 J. Liu, X. J. Lü, G. D. Zhou, K. J. Zhen, W. X. Zhang and 25 C. M. Cook and W. E. Dunn, The reaction of ferric chloride
T. X. Cheng, Effect of KCl on CuCl2/γ-Al2O3 catalyst for oxy-
chlorination of ethane, React. Kinet. Catal. Lett., 2006, 88,
315–323.
with sodium and potassium chlorides, J. Phys. Chem., 1961,
65, 1505–1511.
26 P. Palvadeau, J. Cerisier, C. Guiilot, J. P. Venien, E. Rzepka
and S. Lefrant, Vibrational spectra and force constants of
the alkali tetrahalogenoferrates MFeCl4 (M=Li, Na, K, Rb,
Cs), Solid State Commun., 1990, 75, 383–387.
11 L. Xueju, L. Jie, Z. Guangdong, Z. Kaiji, L. Wenxing and
C. Tiexin, Ethane oxychlorination over γ-Al2O3 supported
CuCl2–KCl–LaCl3, Catal. Lett., 2005, 100, 153–159.
12 C. Li, G. D. Zhou, L. P. Wang, S. L. Dong, J. Li and 27 M. Prien and H. J. Seifert, Thermochemical and structural
T. X. Cheng, Effect of ceria on the MgO-γ-Al2O3 supported
CeO2/CuCl2/KCl catalysts for ethane oxychlorination, Appl.
Catal., A, 2011, 400, 104–110.
investigations on alkali metal chlorides-iron(III)-chloride
systems, J. Therm. Anal., 1995, 45, 349–358.
28 I. M. Dahl, E. M. Myhrvold, U. Olsbye, F. Rohr,
O. A. Rokstad and O. Swang, On the gas-phase chlorination
of ethane, Ind. Eng. Chem. Res., 2001, 40, 2226–2235.
13 C. Li, G. D. Zhou, L. P. Wang, Z. Li, Y. X. Xue and
T. X. Cheng, Effect of impregnation procedure of La2O3 pre-
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