Catalytic gas-phase fluorination of 1,1,2,3-tetrachloropropene to 2-chloro-3,3,3-trifluoropropene over the fluorinated Cr2O3-based catalysts

Article abstract of DOI:10.1016/j.apcata.2014.11.027

Gas-phase fluorination of 1,1,2,3-tetrachloropropene to 2-chloro-3,3,3-trifluoropropene (HCFO-1233xf) was investigated over the fluorinated Cr₂O₃ catalysts with and without promoters. The reaction conditions showed significantly effects on product distribution including temperature, molar ratio of reactants and contact time. The selectivity to HCFO-1233xf was favored under relative higher temperature, larger HF/1,1,2,3-tetrachloropropene ratio and shorter contact time. Based on the data of product distribution over the fluorinated Cr₂O₃ catalyst, a possible reaction path involving complicated consecutive steps was proposed including the addition/elimination and the direct Cl/F exchange. The XRD, XPS and Raman results indicated that introducing Mg, Ca and La onto Cr₂O₃ promoted the formation of highly dispersed CrO_xF_y species during the pre-fluorination process, whereas the presence of Y inhibited the formation of CrO_xF_y. The BET results showed that the addition of La and Y into Cr₂O₃ enhanced the surface area of fluorinated samples. The fluorinated Cr₂O₃ promoted by La (La/F-Cr₂O₃) catalyst displayed the best activity and lifetime among the prepared catalysts, which can be attributed to its rich CrO_xF_y species and high surface area.

Full text of DOI:10.1016/j.apcata.2014.11.027

Applied Catalysis A: General 491 (2015) 37–44
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Catalytic gas-phase fluorination of 1,1,2,3-tetrachloropropene to
2
-chloro-3,3,3-trifluoropropene over the fluorinated Cr O -based
2 3
catalysts
∗
Wei Mao, Liangang Kou, Bo Wang, YanBo Bai, Wei Wang, Jian Lu
Department of Catalytic Technology, Xi’an Modern Chemistry Research Institute, Xi’an 710065, Shaanxi Province, P.R. China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 9 August 2014
Received in revised form 9 November 2014
Accepted 18 November 2014
Available online 26 November 2014
Gas-phase fluorination of 1,1,2,3-tetrachloropropene to 2-chloro-3,3,3-trifluoropropene (HCFO-1233xf)
was investigated over the fluorinated Cr2O3 catalysts with and without promoters. The reaction condi-
tions showed significantly effects on product distribution including temperature, molar ratio of reactants
and contact time. The selectivity to HCFO-1233xf was favored under relative higher temperature, larger
HF/1,1,2,3-tetrachloropropene ratio and shorter contact time. Based on the data of product distribution
over the fluorinated Cr2O3 catalyst, a possible reaction path involving complicated consecutive steps
was proposed including the addition/elimination and the direct Cl/F exchange. The XRD, XPS and Raman
results indicated that introducing Mg, Ca and La onto Cr2O3 promoted the formation of highly dispersed
CrOxFy species during the pre-fluorination process, whereas the presence of Y inhibited the formation
of CrOxFy. The BET results showed that the addition of La and Y into Cr2O3 enhanced the surface area
of fluorinated samples. The fluorinated Cr2O3 promoted by La (La/F-Cr2O3) catalyst displayed the best
activity and lifetime among the prepared catalysts, which can be attributed to its rich CrOxFy species and
high surface area.
Keywords:
Fluorination
1
2
,1,2,3-tetrachloropropene
-chloro-3,3,3-trifluoropropene
HF
Fluorinated chromia
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
The synthesis of HFOs is far more complicated than the satu-
rated HFCs. Until now, only several patents report some large-scale
preparative routes to produce HFOs [4]. For example, HFO-1234yf
is generally prepared by the route including three-step cat-
alytic reactions [4]. The first step involves catalytic gas-phase
fluorination of 1,1,2,3-tetrachloropropene with HF to 2-chloro-
3,3,3-trifluoropropene (HCFO-1233xf).
Gas-phase fluorination using hydrogen fluoride as the fluo-
rinating agent is industrially important reaction for producing
hydrofluorocarbons (HFCs) such as 1,1,1,2,-tetrafluoroethane
(
HFC-134a) [1]. HFCs are widely used as the refrigerants, foaming
agents and cleaning agents. However, the Kyoto Protocol (1997)
established the phased out of HFCs because of their high global
warming potential (GWP), and the HFC-134a with GWP of 1430
has been banned from use in new automotive air conditioning in
EU [2,3]. In recent years, hydrofluoroolefins (HFOs) due to their
low impacts to environment are considered as the most promis-
ing alternatives to HFCs [4]. Especially 2,3,3,3-tetrafluoropropene
+
HF
CCl2 = CCl − CH2Cl −→ CF3 − CCl = CH2
Cat.
(1)
(2)
(3)
HCFO − 1233xf
+HF
Cat.
CF − CCl = CH2 −→ CF − CFCl − CH3
3
3
(
HFO-1234yf) has a substantially low GWP of 4 with zero ozone
depletion and possesses similar thermodynamic properties to HFC-
34a [3]. Thus, special attention has been paid on gas-phase
HCFC − 244bb
−
Cat.
HCl
1
CF − CFCL − CH3 −→ CF − CF = CH2
3
3
fluorination for the production of HFO-1234yf in the chemical
industry.
HFO − 1234yf
The fluorination of 1,1,2,3-tetrachloropropene is very crucial
which firstly affects the entire efficiency of multi-step processes.
Usually, the gas-phase fluorination with HF requires solid cata-
lysts, notably the Cr-based catalysts [5–7]. However, the fluorinated
Cr O catalyst reported in patents cannot achieve stable activity
∗ Corresponding author. Tel./fax: +86 29 88291213.
E-mail address: lujian204@gmail.com (J. Lu).
2
3
http://dx.doi.org/10.1016/j.apcata.2014.11.027
0
926-860X/© 2014 Elsevier B.V. All rights reserved.

3
8
W. Mao et al. / Applied Catalysis A: General 491 (2015) 37–44
for 8 h in N at a flow of 250 mL min−1, then activated with a mixture
and lifespan in this reaction due to rapid catalyst deactivation [8].
Both the reactant and product are C3 olefins the reactivities of
which are much higher than the saturated HFCs with two C atoms.
This leads easily to the formation of oligomers and/or coke on the
catalyst surface [9]. To achieve desired catalyst activity and lifespan
is a serious challenge at this time. In the patents [8], some organic
2
◦
stream of HF and N2 at 250–350 C for 12 h. The fluorinated Cr O
2
3
referred to F-Cr O throughout the remainder of this paper, and the
2
3
fluorinated Cr O3 modified by different promoters was denoted as
2
M/F-Cr O , where M = Mg, Ca, La or Y.
2
3
amines or O must be added into the reaction system to prolong the
2
2.2. Catalytic fluorination
lifespan of the fluorinated Cr O3 catalyst, resulting in an excessive
2
separation cost.
The fluorination reaction was carried out under atmospheric
The product distribution of gas-phase fluorination is influ-
enced by the intrinsic properties of chloroalkane molecules with
the surface reactions occurring on the catalyst, and by the reac-
tion conditions [10]. In the synthesis of HFCs, the target products
HFCs would be favored under high temperature and long con-
tact time with large ratio of HF/organic [6], and the active sites
of the Cr O formed during the pre-fluorination are identified
pressure in the same reactor after the preliminary fluorination
of catalyst. Flow rate of HF preheated in a chamber at 40 C was
carefully controlled using a Sevenstar mass flowmeter, and the
◦
CCl2 CCl CH Cl feed was regulated with a liquid pump. All tubings
2
were also thermostatted to avoid any condensation. The organic
reaction products were analyzed by a gas chromatograph (Haixin
GC-930) equipped with a flame ionization detector (FID) and a DB-
2
3
as the highly dispersed CrOxFy, Cr(OH)xFy and/or CrF3 species
11–14]. Unfortunately, no comparable investigations have been
5
30 m capillary column. The relative composition of the products
[
is based on peak areas and therefore do not represent the absolute
yields because of difference in response factors. Moreover, GC-MS
made to our knowledge regarding the fluorination of 1,1,2,3-
tetrachloropropene to HCFO-1233xf. For developing a suitable
Cr-based catalyst, it is essential to understand the nature of fluori-
nation of 1,1,2,3-tetrachloropropene and distinguish the difference
between C2 and C3 chloroalkanes fluorination.
(Thermo Scientific ITQ 700) was used for identity of the organic
compoundsformedduringthe reaction. ADB-5MScapillarycolumn
with inner diameter of 0.25 mm and length of 30 m was adopted in
the separation system.
In the present work, the effects of reaction conditions on
product distribution were studied in the fluorination of 1,1,2,3-
tetrachloropropene over the fluorinated Cr O catalyst. The
2.3. Characterization
2
3
increased carbon-chain length and the carbon-carbon double bands
could make reaction routes more complex for this fluorination than
for the C2 chloroalkanes fluorination, so there may be a number
of particularities in the fluorination of 1,1,2,3-tetrachloropropene.
Thus, the product distribution was investigated under different
reaction conditions to identify the crucial step for the formation
of HCFO-1233xf. Furthermore, our recent research discovered that
The metal content in the samples was determined by X-ray flu-
orescence (XRF) spectrometer (ELEMENTAR Vario ELIII) with the
uncertainty of 3%. XRD patterns of the prepared samples were col-
lected with a Rigaku D/max-␥A rotation anode X-ray diffractometer
(
Cu K␣, ꢀ = 0.15418 nm). Chemical compositions on the surface of
samples were analyzed using an X-ray photoelectron spectrometer
XPS) (Thermo Scientific K-Alpha) equipped with an Al monochro-
matic X-ray source (Al Ka = 1486.6 eV) under room temperature in
(
only the CrOxFy species on the fluorinated Cr O3 can catalyze
2
the transformation of 1,1,2,3-tetrachloropropene to HCFO-1233xf
rather than the Cr(OH)xFy and CrF3 species [15]. Modifying surface
Cr species can affect significantly the catalyst behavior in this reac-
tion. As a result, the fluorinated Cr O catalyst was modified by
−
9
high vacuum (about 1 × 10 Pa). The position of C1s BE at 284.8 eV
was used as an internal standard for correcting any charge-induced
peak shifts. Before the test, the pellet type samples were outgassed
2
3
−
6
for about 2 h at 423 K under a pressure of 1 × 10 Pa to min-
imize the surface contamination. Raman spectra were obtained
on a Renishaw Raman System 2000 with exciting wavelength of
Mg, Ca, La and Y, respectively, to tune the distribution of Cr species.
The effects of promoters on the catalyst properties and catalytic
behaviors were also investigated.
7
85 nm under ambient conditions. The temperature-programmed
desorption of ammonia (NH -TPD) measurement was carried on
3
an AutoChem II 2920 instrument (Micromeritics, USA) for compar-
ing the acidity of various samples. Prior to TPD studies, a sample
of 50 mg was first pretreated in pure He at 773 K for 60 min, then
cooled to 393 K and saturated at this temperature with anhydrous
ammonia gas (10% in He) for 30 min. Weakly adsorbed NH3 was
eliminated by treatment under He at the same temperature for
2
. Experimental
2.1. Preparation of catalysts
The pure Cr O was prepared according to our previous method
2
3
[
15]. The Cr hydroxide gel was obtained by precipitating a solu-
6
0 min. The NH -TPD profile was recorded with a thermal conduc-
3
tion of chromium chloride with aqueous ammonia under stirring
continuously. Then, the precipitate obtained was filtered, washed
until free from chloride ions and dried at 120 C, followed by a cal-
cination at 300 C for 8 h. The resulting solid was powdered, mixed
with 2% graphite and formed into cylindrical pellets as the catalyst
−
1
tivity detector with a heating rate of 10 K min from 393 to 673 K
in a He flow. The surface area of the catalysts was measured using
nitrogen adsorption at 77 K and the Brunauer–Emmett–Teller (BET)
method using a Micrometerics ASAP 2020 system.
◦
◦
precursor. The M (M = Mg, Ca, La and Y) modified Cr O3 catalysts
2
were prepared by the incipient wetness impregnation method; the
prepared Cr O powder was impregnated by an aqueous solution
3. Results
2
3
of the corresponding metal nitrate or metal chloride, followed by
3.1. Product distribution studies
◦
evaporating any residual water, then dried at 120 C overnight, and
◦
finally calcined at 300 C for 8 h. Except the test “Effect of La content
To determine the possible reaction path from 1,1,2,3-
tetrachloropropene to HCFO-1233xf, we investigated the effects
of various reaction conditions on product distribution over
on La/F-Cr O catalyst”, the content of the metal salt in the solution
2
3
was adjusted to giving the final metal loading 1 wt.%.
To obtain an activity for the fluorination, the oxide sample was
subject to a pre-fluorination process. The pelletized catalyst pre-
cursor (60 mL) was charged into a nickel tubular reactor with a
the F-Cr O3 catalyst including molar ratio of reactants, con-
2
tact time and temperature. It should be noted that catalytic
fluorination of 1,1,2,3-tetrachloropropene with HF proceeded
very smoothly under our reaction conditions, resulting in a
◦
diameter of 2.5 cm and a length of 70 cm; it was heated at 200 C

W. Mao et al. / Applied Catalysis A: General 491 (2015) 37–44
39
Table 1
The effect of HF/1,1,2,3-tetrachloropropene molar ratio on product distribution using the F-Cr2O3 catalyst.
a
The ratio of HF/1,1,2,3-tetrachloropropene (mol.)
Selectivity (%)
HCFO-1233xf
HCFO-1232xe
HCFO-1223xd
HCFC-233ab
Others
4
6
8
1
1
:1
:1
:1
0:1
2:1
77.8
95.1
95.1
96.7
95.0
20.6
2.2
1.8
1.3
0.9
0.8
1.4
1.4
0.6
1.0
0.3
0.7
0.8
0.9
2.2
0.5
0.6
0.9
0.5
0.9
a
◦
Reaction conditions: reaction temperature 260 C with contact time 12 s.
Table 2
a
The effect of contact time on product distribution using the F-Cr2O3 catalyst.
Contact time (s)
Selectivity (%)
HCFO-1233xf
HCFO-1232xe
HCFO-1223xd
HCFC-233ab
Others
7
9
2
8
.2
96.7
94
95.1
79
2.1
2.8
1.8
1.1
0.3
1.4
1.4
0.4
0.8
0.8
1.7
0.5
1
0.9
2.8
1
1
15.4
a
◦
Reaction conditions: reaction temperature 260 C with HF/1,1,2,3-tetrachloropropene molar ratio of 8.
complete conversion of 1,1,2,3-tetrachloropropene throughout
so further increasing the ratio of HF/1,1,2,3-tetrachloropropene
beyond 6 displays little influence on the product distribution.
HCFO-1223xd and HCFC-233ab were observed as the minor prod-
ucts which may be formed by side reactions. Their selectivities were
hardly affected by the ratio of feedstocks changing.
8
h period in these tests. Thus, the effects of reaction condi-
tions on conversion were not discussed in this section. The data
of product distribution were usually obtained after 5 h of the
reaction. Besides the expected HCFO-1233xf, the other minor
components formed are 2,3-dichloro-3,3-difluoropropene (HCFO-
1
232xe), 1,2-dichloro-3,3,3-trifluoropropene (HCFO-1223xd),
3
.1.2. Effect of contact time
The effect of contact time on product distribution is pre-
and 2,2,3-trichloro-1,1,1-trifluoropropane (HCFC-233ab) with
trace amounts of 2,3,3-trichloro-3-fluoropropene (HCFO-1231xe),
sented in Table 2. The product distribution changed gently when
contact time varied within 12 s. However, the selectivity to HCFO-
1
2
1
-chloro-1,1,1,2-tetrafluoropropane (HCFC-244bb), 2,3-dichloro-
,1,1-trifluoropropane (HCFC-243db) and HFO-1234yf in all
223xd increased remarkably from 1.4% to 15.4% when contact
experiments. The selectivities to the trace amounts and other
unknown products are not discussed in this work.
time increased to 18 s, conversely, the selectivity to HCFO-1233xf
declined significantly. In view of the results collected, we believe
that contact time can also influence effectively the product distribu-
tion. However, unlike the HFCs preparation [16,17], increasing the
contact time did not cause the enhancement of desired fluorinated
products in this reaction, instead of leading to the enhancement of
one byproduct. This result indicates that HCFO-1223xd is not an
intermediate in conversion of 1,1,2,3-tetrachloropropene to HCFO-
3.1.1. Effect of molar ratio of reactants
Table 1 shows the data of product distribution at different molar
ratio of HF/1,1,2,3-tetrachloropropene. HCFO-1233xf was the main
product under these reaction conditions while HCFO-1232xe was
the second most abundant product. The selectivity to former
increased from 77.8% to 95.1% as the molar ratio of reactants var-
ied from 4 to 6. At the same time, the selectivity to latter decreased
drastically from 20.6% to 2.2%. This result indicates that the product
distribution was easily determined by the molar ratio of reactants
changing. Similar phenomenon has been found in the synthesis
of HFCs [6,11], where enhancing the ratio of HF/chlorocarbons
would help drive the equilibrium of fluorination and promote the
formation of fluorinated compounds. In contrast, the amount of
partially fluorinated intermediates increases immediately when HF
is insufficient in the reaction system. Thus, what is observed implies
that HCFO-1232xe is a fluorinated intermediate in the fluorination
of 1,1,2,3-tetrachloropropene, which can react with HF to form
HCFO-1233xf. In addition, the stoichiometric ratio of HF/1,1,2,3-
tetrachloropropene needed is 3 for synthesis of HCFO-1233xf,
1
233xf. But now it is quite difficult to discuss the formation of
HCFO-1223xd and HCFC-233ab due to limited evidence. They may
be formed by consecutive over-fluorination and/or chlorination of
HCFO-1233xf. Thus, to achieve high selectivity to HCFO-1233xf,
contact time should be controlled under relative short time.
3.1.3. Effect of reaction temperature
As shown in Table 3, the selectivity to HCFO-1233xf increased
remarkably with the reaction temperature increasing. Whereas, the
selectivity to HCFO-1232xe decreased from 13% to 1.3% as tem-
perature increased from 230 ◦C to 290 ◦C. The selectivity to other
products changed little as the temperature varied. These results
indicate that the reaction of HCFO-1232xe with HF needs higher
energy than other consecutive reaction steps in the fluorination
Table 3
a
The effect of reaction temperature on product distribution using the F-Cr2O3 catalyst.
◦
Reaction temperature ( C)
Selectivity (%)
HCFO-1233xf
HCFO-1232xe
HCFO-1223xd
HCFC-233ab
Others
230
260
290
85.3
95.1
97.5
13
1.8
1.3
0.3
1.4
0.1
0.07
0.8
0.02
1.33
0.9
1.08
a
Reaction conditions: HF/1,1,2,3-tetrachloropropene molar ratio of 8 with contact time 12 s.

4
0
W. Mao et al. / Applied Catalysis A: General 491 (2015) 37–44
100
100
(
a)
(b)
8
6
4
2
0
0
0
0
80
60
F-Cr O
F-Cr O
2 3
2
3
Mg/F-Cr O
Mg/F-Cr O
2
3
2
3
40
20
La/F-Cr O
La/F-Cr O
3
2
3
2
Ca/F-Cr O
Ca/F-Cr O
2
3
2
3
Y/F-Cr O
Y/F-Cr O
3
2
3
2
12
24
36
48
60
72
84
96
12
24
36
48
60
72
84
96
Time on stream, h
Time on stream, h
Fig. 1. Catalytic stability of F-Cr2O3 and M/F-Cr2O3 catalysts for fluorination of 1,1,2,3-tetrachloropropene (promoter loading 1 wt.%). (a) conversion of 1,1,2,3-
◦
tetrachloropropene vs. time on stream; (b) selectivity of HCFO-1233xf vs. time on stream. Reaction conditions: 260 C, HF/1,1,2,3-tetrachloropropene molar ratio of 10
and contact time 12 s.
of 1,1,2,3-tetrachloropropene. Namely, 1,1,2,3-tetrachloropropene
can be transformed conveniently to HCFO-1232xe even in low
temperature, but subsequently the conversion of HCFO-1232xe to
HCFO-1233xf is slow. Indeed, these consecutive fluorination reac-
tions can be characterized by different values of apparent activation
energy. Traces of HCFO-1231xe were also found in this reaction,
which can react with HF to form HCFO-1232xe. As a result, to
confirm our assumption, a theoretical calculation was performed
on the reaction rate of fluorination of HCFO-1231xe and HCFO-
Above data show that the value of HCFO-1233xf selectivity is
suitable for evaluating distinctly the catalytic behaviors of dif-
ferent catalysts in the fluorination of 1,1,2,3-tetrachloropropene,
rather than simply taking the value of conversion. The decline of
HCFO-1233xf selectivity would be an actual indicator of catalyst
deactivation. Considering its excellent performance for HCFO-
1233xf formation, La/F-Cr O3 is the most effective catalyst in this
2
work.
1
232xe with HF by using density functional theory. Obviously, the
3.2.2. Effect of La content in La/F-Cr O catalyst
2
3
calculated results from Table S1 agree well with our argument.
The reaction of HCFO-1231xe with HF is faster than that of HCFO-
The effect of La loading on catalytic behavior of La/F-Cr O3 was
2
examined by preparing catalysts with La contents between 1.0
and 8.0 wt.%. As shown in Fig. 2, increasing La content displays
slight effect on catalyst activity in the fluorination of 1,1,2,3-
tetrachloropropene. All the La/F-Cr2O3 catalysts exhibited high
catalytic activity throughout reaction period, where the conver-
sion was maintained above 98% after 96 h. However, the selectivity
to HCFO-1233xf decreased with increasing the content of La, the
steady state period of which was reduced significantly. This find-
ing is expected because the promoter loading could decline the
proportional to availability of active sites on the surface of catalyst.
1
3
3
232xe with HF.
.2. Catalytic performance of fluorinated Cr O -based catalysts
2
3
.2.1. Stability
Fig. shows the time-on-stream (TOS) performance of
F-Cr O3 and M/F-Cr O catalysts for fluorination of 1,1,2,3-
1
2
2
3
tetrachloropropene. Over the F-Cr O3 catalyst, conversion of
2
1
1
,1,2,3-tetrachloropropene declined gradually from the initial
00% to 87.6% at a TOS of 96 h, and the corresponding selectivity
to HCFO-1233xf dropped from 99% to 87%. Over the Mg/F-Cr O ,
3.3. Characterization of various fluorinated Cr O -based catalysts
2 3
2
3
Ca/F-Cr O and La/F-Cr O3 catalysts, conversion was kept at 100%
2
3
2
within 96 h. This indicates a significantly positive effect of Mg, Ca
and La on the stability of F-Cr O catalyst. However, in comparison
XRD analyses were performed to characterize the bulk prop-
erties of F-Cr O3 and M/F-Cr O3 catalysts (Fig. S1). It should be
2
3
2
2
with the results of F-Cr O , faster deactivation was observed on
noted that the content of promoter in these catalysts was fixed at
a 1.0 wt.%. All the XRD patterns show a broad diffuse band with
high background intensities. The characteristic peak near 24 for
CrF3 phase (PDF-ICDD 16-0044) was very weak in all catalysts.
This indicates that typical amorphous structure or microcrystalline
phase was predominant in each sample. Moreover, the crystalline
2
3
the Y/F-Cr O , where the conversion dropped rapidly to 74% after
2
3
◦
reaction. In the case of HCFO-1233xf selectivity, the evolution
was much different among various catalysts. Comparing with the
F-Cr O , the evolution of HCFO-1233xf selectivity did not change
2
3
markedly on the Mg/F-Cr O , whereas, higher HCFO-1233xf
2
3
selectivity was achieved on the Ca/F-Cr O . In contrast, the HCFO-
structure of F-Cr O3 hardly changed after introducing different
2
3
2
1
233xf selectivity decreased rapidly throughout reaction period
promoters. No diffraction peaks of corresponding promoters were
observed, which can be due to the fact that the amount of these
compounds were insufficient to be detected by XRD or they were
highly dispersed on the surface of F-Cr O .
on the Y/F-Cr O , which decreased even faster than that on the
F-Cr O . Furthermore, the steady selectivity to HCFO-1233xf did
2
3
2
3
not obtain in above catalysts, which had an appreciable decreasing
tendency during each reaction. However, the La/F-Cr O catalyst
exhibited a surprising stability to HCFO-1233xf formation, where
the corresponding selectivity always sustained above 98% over
time. Based on activity tests, we deduce that the surface properties
of F-Cr O were modified significantly by different promoters,
2
3
The XPS spectra were used to identify the surface chromium
species of different catalysts. Fig. 3 displays the curve of Cr2p
XPS spectra for fresh Cr O3 and F-Cr O . For Cr O , two kinds
2
3
2
2
3
2
3
3
+
of Cr species can be attributed, respectively, to Cr (576.8 eV) in
Cr O /Cr(OH) and Cr6+ (579.8 eV) in CrO3 as already reported in
2
3
2
3
3
thus resulting in different catalytic performance.
previous work [13,14]. Then, after treatment by HF the spectrum

W. Mao et al. / Applied Catalysis A: General 491 (2015) 37–44
41
100
100
(b)
(
a)
8
6
4
2
0
0
0
0
80
60
1
5
8
wt.% La/F-Cr O
2
1
5
8
wt.% La/F-Cr O
3
3
3
2
3
3
3
wt.% La/F-Cr O
wt.% La/F-Cr O
2
2
40
20
wt.% La/F-Cr O
wt.% La/F-Cr O
2
2
12
24
36
48
60
72
84
96
12
24
36
48
60
72
84
96
Time on stream, h
Time on stream, h
Fig. 2. Catalytic activity of La/F-Cr2O3 catalyst with various amounts of La for fluorination of 1,1,2,3-tetrachloropropene. (a) conversion of 1,1,2,3-tetrachloropropene vs.
◦
time on stream; (b) selectivity of HCFO-1233xf vs. time on stream. Reaction conditions: 260 C, HF/1,1,2,3-tetrachloropropene molar ratio of 10 and contact time 12 s.
during the pre-fluorination process, leading to extensive change
of chemical environments of the Cr atom. Above results can be
(
a) CrO3
Cr 2p1/2
Cr 2p3/2
2
5
76.8
confirmed in the F1s and O1s core-level spectra of the F-Cr O3
2
5
79.8
sample (Fig. 4). As shown in Fig. 4(a), the F1s peak was resolved
into two components at 685.9 and 688.8 eV. The prominent peak
at 685.9 eV [F(I)] can be assigned to Cr F in CrF , and the peak at
3
6
88.8 eV [F(II)] corresponded to Cr F in CrOxFy species [16]. The
O1s spectrum was fitted by a major component and two shoulders
at 530.9, 533.1, and 536 eV, respectively [Fig. 4(b)]. We assigned
the three components to the O atoms of chromium hydroxide,
the O atoms of water molecule and CrOxFy, and surface hydroxyls
which agreed with the values reported by Kim et al. [18].
(
b) F-CrO3
581.7
2
5
84.6
5
78.9
The XPS spectra of different M/F-Cr O samples were also inves-
2
3
tigated to reveal the impact of promoters on the surface properties
of F-Cr O3 (Fig. 5). Table S2 displays the core binding energy val-
2
599
594
589
584
579
574
ues with atom ratios of different samples. As shown in Fig. 5(a), the
Binding energy, eV
Cr2p profiles are similar for F-Cr O , Mg/F-Cr O , Ca/F-Cr O and
2
3
2
3
2
3
La/F-Cr O3 samples. Nevertheless, the core peak of Cr2p3/2 shifted
to the high-energy side after introducing Mg, Ca and La, respec-
2
Fig. 3. XPS spectra of the Cr2p core-level for (a) Cr2O3 and (b) F-Cr2O3.
tively, onto F-Cr O . In contrast, a slight shrinking of Cr2p line can
2
3
of Cr2p became broadening and was shifted to higher binding
energy. The broad Cr2p3/2 peak of the F-Cr O sample was fitted by
three components at 578.9, 581.7 and 584.6 eV, respectively. These
be observed after the addition of Y onto F-Cr2O3, and the binding
energy of Cr2p3/2 shifted to the low-energy side. Similar trend in
the F1s profiles [Fig. 5(b)] can also be noted in the different cat-
alysts. A broadening of the F1s spectrum at high-energy side was
observed in the Mg/F-Cr2O3, Ca/F-Cr2O3 and La/F-Cr2O3 samples.
Whereas, the double peaks of the F1s shifted to low binding energy
after introducing Y onto F-Cr2O3. Obviously, the XPS peaks of Cr
2
3
peaks in turn can be assigned to the Cr atoms of Cr(OH) ·xH O,
3
2
CrF and CrOxFy (the valent of Cr > +3) [13,14,18,19], referred to
3
as Cr(I), Cr(II) and Cr(III). This observation indicates that surface
chromium oxides were strongly interacted with the F atoms
(
a) F 1s
(b) O 1s
533.1
6
85.9
6
88.8
536
530.9
6
96 693 690 687 684 681 678
543 540 537 534 531 528 525
Binding energy, eV
Binding energy, eV
Fig. 4. XPS spectra of (a) the F1s and (b) the O1s of F-Cr2O3.

4
2
W. Mao et al. / Applied Catalysis A: General 491 (2015) 37–44
Fig. 5. XPS spectra of (a) the Cr2p and (b) the F1s of M/F-Cr2O3.
and F atoms are sensitive to the nature of the promoter atoms.
More electron-deficient chromium species was present on catalyst
surface after addition of Mg, Ca or La. As shown in Table S2, there
was a decline in the atom ratios of Cr(II)/Cr(III) and F(I)/F(II) after
introducing Mg, Ca or La onto F-Cr O . This indicates the content
and CrOxFy species as the F-Cr O3 sample. However, the Y/F-Cr O3
2 2
sample showed the Raman band of chromate with high intensity
like Cr O , and the bands of the CrOxFy species were not observed.
2
3
Thus, above results suggest that the isolated tetrahedral chromium
oxide species were transformed into CrOxFy species (Cr6+) during
the pre-fluorination process in the fluorinated samples except Y/F-
Cr O , which was consistent with the XPS results.
2
3
of surface CrOxFy species was enhanced in these samples, espe-
cially in Ca/F-Cr O and La/F-Cr O . Furthermore, both Cr2p and
2
3
2
3
2
3
F1s binding energies reveal that the surface Cr species were differ-
ent between Y/F-Cr O and other M/F-Cr O3 catalysts. Comparing
The large surface area catalysts are desired in gas-phase cat-
alytic fluorination for their high activity and long catalyst life [6],
but the normal fluorinated metal oxides or metal fluorides have a
2
3
2
different binding energy values of Cr2p, the Cr(OH)xFy rather than
CrOxFy species may enrich on the surface of Y/F-Cr O . Luo et al.
2
−1
small surface area (<50 m g ). The BET results (Table S3) indicate
2
3
2
−1
have reported similar result in the CrOx-Y O3 system [14]. Hence,
that the F-Cr O3 with high surface area (96 m g ) was prepared
2
2
the XPS spectra of Y/F-Cr O3 were not resolved as other samples.
in our work. The value is almost equivalent that reported for porous
fluorinated chromia by Quan et al. [21]. Moreover, the addition of
rare earths (Y or La) can improve its surface area. In this experi-
2
Raman spectroscopy is a powerful technique to determine
the molecular structure of the surface chromium species with
high valence [20] and offers information complementary to that
obtained from our XPS results. Fig. 6 represents the Raman spec-
tra of fresh Cr O , F-Cr O and M/F-Cr O3 catalysts. In the case
2
−1
ment, a highest surface area of 114.2 m g was achieved in the
La/F-Cr O3 sample. However, introducing alkaline earths (Mg or
2
Ca) leaded to surface area decline slightly. In addition, NH -TPD
2
3
2
3
2
3
−
1
of Cr O , only a broad Raman band at 835 cm appeared, which
is used for investigating the strength of acid sites present on the
surface of catalyst. Fig. S2 displays that a broad desorption profile
2
3
can be attributed to symmetrical stretching of monomeric chro-
6
+
◦
mate species (Cr ) [12,20]. The similar Raman band but moved
appeared in all the samples in the range of 120–400 C with one
−
1
◦
to near 869 cm was very weak in the F-Cr O sample, and two
maximum peak around 200 C. This suggests weak acid sites were
2
3
−
1
new bands at 1319 and 1518 cm can be observed. Chung et al.
have investigated the Raman bands of the fluorinated Cr/MgO cata-
lysts. They believed two new bands appeared can be assigned to the
predominant on the surface of F-Cr O3 and M/F-Cr O .
2
2
3
4. Discussion
stretching of CrOxFy species [12]. The Mg/F-Cr O , Ca/F-Cr O and
2
3
2
3
La/F-Cr O samples displayed the same Raman bands of chromate
2
3
4.1. Active sites for fluorination of 1,1,2,3-tetrachloropropene
In general, one consensus has been reached in heterogeneous
8
69
catalytic fluorination with HF that the amorphous chromium
species are responsible for catalytic activity including the CrOxFy,
CrF3 and Cr(OH)xFy [12–14,18]. Especially, the nonstoichiometric,
disordered chromium species exist in the CrOxFy phase [18,22],
which is coordinatively unsaturated. This species is highly active
and can provide labile fluorine atoms in the Cl/F exchange reac-
tion [22]. As indicated from XRD (Fig. S1), XPS (Figs. 3 and 5) and
1
319
1
518
Ca/F-Cr O3
2
Mg/F-Cr O3
2
VI
2
La/F-Cr O3
Raman (Fig. 6) results, dispersed Cr O F species were present on
x
y
the surface of fluorinated catalysts except Y/F-Cr O . Moreover,
2
3
2
F-Cr O3
comparing with F-Cr2O3, the surface atom ratio of Cr(II)/Cr(III)
decreased for Mg/F-Cr O , Ca/F-Cr O and La/F-Cr O3 (Table S2).
2
3
2
3
2
2
Y/F-Cr O3
Similar trend can be also observed for the value of atom ratio of
F(I)/F(II). These results indicate that the content of surface CrOxFy
Cr O3
species was enhanced after introducing Mg, Ca and La into F-Cr O3
2
2
catalyst. However, the addition of Y inhibited the formation of
CrOxFy species. Regardless of promoters, the CrF3 species were
always dominant on the surface in all the catalysts. Based on
1
800
1600
1400
1200
1000
800
600
-
1
Raman shift, cm
above understandings, we believe that both CrOxFy and CrF species
3
Fig. 6. Raman spectra for Cr2O3, F-Cr2O3 and M/F-Cr2O3.
played the key role in determining the catalytic behavior in this

W. Mao et al. / Applied Catalysis A: General 491 (2015) 37–44
43
Scheme 1. Possible reaction paths proposed from the vapor fluorination of 1,1,2,3-tetrachloropropene to HCFO-1233xf.
reaction. However, the Y/F-Cr O3 achieved the poorest catalytic
Thus, above reaction path may be predominant in the fluorination
of 1,1,2,3-tetrachloropropene. However, due to the presence of
little HCFC-243db, another reaction path cannot be excluded now.
2
activity and stability for HCFO-1233xf formation in all the cata-
lysts, indicating catalytic activity was highly interrelated with the
CrOxFy species. In comparison with F-Cr O , the higher activities
2
3
of Mg/F-Cr O , Ca/F-Cr O and La/F-Cr O3 can be ascribed to more
CrOxFy species on their surface.
2
3
2
3
2
5. Conclusion
Besides, the surface area of catalysts should be also addressed
for discussion their activity and stability. In our previous work
Fluorination of 1,1,2,3-tetrachloropropene with HF to HCFO-
1
233xf over the fluorinated Cr O -based catalysts has been study.
2 3
[
23], we have observed that the activity of Cr-based catalyst was
Different from the fluorination for synthesis of HFCs, long contact
time was not beneficial to the formation of HCFO-1233xf. From
results of product distribution, HCFO-1232xe, HCFO-1231xe and
HCFC-243db were observed as the intermediates in conversion of
related to their surface area. The La/F-Cr O has a highest sur-
2
3
2
−1
face area (114.2 m g ) among prepared catalyst, and resulting
2
in more active sites for fluorination, that is one reason La/F-Cr O3
achieves the surprising stability in this reaction. The Mg and Ca
1
,1,2,3-tetrachloropropene to HCFO-1233xf, but other byproducts
promoted F-Cr O displayed less stability for the formation of
2
3
needs further explanation. A possible reaction path for the for-
mation of HCFO-1233xf was established, which involved with an
addition/elimination followed F-for-Cl halogen exchange mecha-
nisms. Surface CrOxFy species of catalyst were found as the most
HCFO-1233xf than La/F-Cr O due to their low surface area (93.8
2
3
2
−1
and 91.2 m g ).
It is well-known that the coke formation primarily accounts
for the catalyst deactivation in the fluorination catalysis [6,19].
Based on the XPS analysis, the surface C content was increased
importantly active sites in this fluorination. The La/F-Cr O3 is the
2
most stable catalyst among tested catalysts, where the conversion
of 1,1,2,3-tetrachloropropene achieved at 100% with the selectivity
of 98% to HCFO-1233xf during a 96 h time-on-stream. The deactiva-
by 49%, 55%, 58%, 62% and 73% for La/F-Cr O , Ca/F-Cr O , Mg/F-
2
3
2
3
Cr O , F-Cr O and Y/F-Cr O , respectively, after reaction. Thus,
2
3
2
3
2
3
it is reasonable to infer that the catalyst deactivation is related
with carbon deposition in this reaction. The conversion of 1,1,2,3-
tetrachloropropene and HCFO-1233xf selectivity could decrease
gradually as the active Cr species like CrOxFy were reduced by
carbon deposition. The differences of active species, BET surface
and coke content make different activity and stability for various
catalysts.
tion mechanism of fluorinated Cr O -based catalyst and its impact
on the formation of HCFO-1233xf would be further investigated.
2
3
Acknowledgments
We thank Prof. Dr. Zhongwen Liu for helpful discussions and
Dr. Qing Hao for TPD and BET tests. We are grateful to be sup-
ported in part by industrial research project of Shaanxi Province
4.2. Possible reaction path
(No. 2014K10-11).
Several literatures have reported that gas-phase catalytic
Appendix A. Supplementary data
fluorination involves with common Cl/F exchange, hydro-
halogenations and dehydrohalogenation reactions [11,24,25].
Thus, while we find it difficult to clarify every product in this
fluorination at the moment, a simplified reaction path from
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2
014.11.027.
1
,1,2,3-tetrachloropropene to HCFO-1233xf was proposed based
on general recognition of heterogeneous fluorination (Scheme 1).
In general, a C Cl bond at a double bonded carbon is significantly
stronger than that of a single bonded C-atom. Hence, direct Cl/F
exchange is difficult at double-linked C atom. As a result, HF
was firstly added into 1,1,2,3-tetrachloropropene to form 1,1,2,3-
tetrachloro-1-fluoropropane. Then, the detected HCFO-1231xe
can be obtained by dehydrochlorination of 1,1,2,3-tetrachloro-1-
fluoropropane. Observing the structure of molecular, the group
CCl F- of HCFO-1231xe is impossibly fluorinated to CClF - via
the addition/elimination process. The common Cl/F exchange
may occur in the reaction of HCFO-1231xe with HF. The reaction
of HCFO-1232xe with HF can follow the similar procedure. By
comparing the product distribution under different reaction
conditions, HCFO-1232xe was found as the primary intermediate
in fluorination of 1,1,2,3-tetrachloropropene to HCFO-1233xf.
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