Catalytic Gas-phase Fluorination of Hexachlorobutadiene to 1,2-Dichlorotetrafluorocyclobutene over Cr/Zn-based Catalysts

Article abstract of DOI:10.1002/jccs.201700274

In this paper, the gas-phase fluorination of hexachlorobutadiene (HCBD) to synthesize 1,2-dichlorotetrafluorocyclobutene (DTB) was carried out over a series of Cr/M/Zn catalysts (M = Ni, Co, Cu, In, Al). The influence of prefluorination by different fluorinating agents (HF, 95%HF + 5%Cl₂, 95%HF + 5%O₂, CF₂O, CF₂Cl₂) on catalytic performance of Cr/Co/Zn sample was also investigated. The addition of prompters to the Cr/Zn catalyst improved remarkably its catalytic properties. The Cr/Ni/Zn catalyst exhibited the best catalytic activity (1.318 mmol/h/g) at 390 °C and the Cr/Co/Zn catalyst showed the best DTB selectivity (42.5%) at 350 °C. Compared to that of gaseous HF, the catalytic performance of the Cr/Co/Zn catalyst after treatment by HF + O₂ and CF₂O increased considerably, whereas for HF + Cl₂ and CF₂Cl₂ it showed little effect. In order to identify the different species (Cr─O, Cr─F, CrO_xF_y) present on catalysts’ surface and determine their exact role, these catalysts before and after the reaction were characterized by X-ray photoelectron spectroscopy. It was found that the concentration of the various species was responsible for the activity and lifetime of catalysts. Moreover, a possible reaction route is proposed based upon the product distribution. The most feasible formation pathway of DTB proceeded via the cyclization of C₄Cl₄F₂ or C₄Cl₃F₃ to yield c-C₄Cl₄F₂ and c-C₄Cl₃F₃ followed by further the Cl/F exchange.

Full text of DOI:10.1002/jccs.201700274

JOURNAL OF THE CHINESE
CHEMICAL SOCIETY
Article
Catalytic Gas-phase Fluorination of Hexachlorobutadiene
to 1,2-Dichlorotetrafluorocyclobutene over Cr/Zn-based Catalysts
c
Jinwei He,a Mengxue Zhang,a Biao Zhoub and Xiaomeng Zhou
*
aThe College of Environmental Science and Engineering, Nankai University, Tianjin 300071, China
bFaculty of Engineering, The University of Tokyo, Tokyo 113-8654, Japan
cEconomics And Management College, Civil Aviation University of China, Tianjin 300300, China
(Received: August 21, 2017; Accepted: December 28, 2017; DOI: 10.1002/jccs.201700274)
In this paper, the gas-phase fluorination of hexachlorobutadiene (HCBD) to synthesize 1,2-
dichlorotetrafluorocyclobutene (DTB) was carried out over a series of Cr/M/Zn catalysts (M = Ni,
Co, Cu, In, Al). The influence of prefluorination by different fluorinating agents (HF, 95%HF +
5%Cl2, 95%HF + 5%O2, CF2O, CF2Cl2) on catalytic performance of Cr/Co/Zn sample was also
investigated. The addition of prompters to the Cr/Zn catalyst improved remarkably its catalyꢀtic
properties. The Cr/Ni/Zn catalyst exhibited the best catalytic activity (1.318 mmol/h/g) at 390 C
ꢀ
and the Cr/Co/Zn catalyst showed the best DTB selectivity (42.5%) at 350 C. Compared to that
of gaseous HF, the catalytic performance of the Cr/Co/Zn catalyst after treatment by HF + O2
and CF2O increased considerably, whereas for HF + Cl2 and CF2Cl2 it showed little effect. In
order to identify the different species (Cr─O, Cr─F, CrOxFy) present on catalysts’ surface and
determine their exact role, these catalysts before and after the reaction were characterized by
X-ray photoelectron spectroscopy. It was found that the concentration of the various species was
responsible for the activity and lifetime of catalysts. Moreover, a possible reaction route is pro-
posed based upon the product distribution. The most feasible formation pathway of DTB pro-
ceeded via the cyclization of C4Cl4F2 or C4Cl3F3 to yield c-C4Cl4F2 and c-C4Cl3F3 followed by
further the Cl/F exchange.
Keywords: Hexachlorobutadiene; 1,2-Dichlorotetrafluorocyclobutene; Catalytic fluorination;
Promoters; Cr/Zn-based catalyst; Activation.
INTRODUCTION
1,1,1,4,4,4-Hexafluoro-2-butyne is widely used as
a three-step synthesis route for hexafluoro-2-butyne
over a Cr-based catalyst (Scheme 1).12 In the first step,
1,1,2,3,4,4-hexachloro-1,3-butadiene (HCBD) is cata-
lytically converted to 1,2-dichloro-3,3,4,4-tetrafluoro-
cyclobut-1-ene (DTB); in the second step, DTB is
converted to 1.2,3,3,4,4-hexafluorocyclobut-1-ene;
finally, hexafluoro-2-butyne is produced via the ring-
opening reaction from hexafluorocyclobut-1-ene.
Because HCBD is highly toxic and a persistent organic
pollutant (POP), it was banned for use as a final prod-
uct by the Stockholm Convention in 2013. The present
route not only solves the persistent pollution problem
of HCBD but also produces the versatile hexafluoro-2-
butyne. However, the key to this method is the synthe-
sis of DTB from HCBD (step 1).
an important fluorine-containing raw material. For
example, hexafluoro-2-butyne is an established synthon
for introducing two trifluoromethyl groups into furan
or benzenoid systems.1–4 Furthermore, 1,1,1,4,4,4-hexa-
fluoro-2-butene, as a promising new-generation foaming
agent due to its zero ozone depletion potential (ODP)
value and a low global warming potential (GWP) value
of 9.4,5,6 also can be synthesized by catalytic hydroge-
nation of hexafluoro-2-butyne.7,8 Thus, special attention
has been paid to the manufacture of hexafluoro-2-
butyne in the industry.
Several patents exist for the synthesis of
hexafluoro-2-butyne.9–11 Our research group proposed
*Corresponding author. Email: zhouxm@nankai.edu.cn
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He et al.
spectroscopy (XPS), and an attempt to correlate the
catalytic activity with the concentration of the different
species present on the catalyst surface was made.
RESULTS AND DISCUSSION
Product distribution
Scheme 1. Synthesis pathways for 1,1,1,4,4,4-hexa-
Before studying the product distribution, the
effects of various reaction conditions were investigated
over the HF-Cr/Zn catalyst. The reaction temperature
fluoro-2-butyne.
Generally, the gas-phase fluorination with HF
requires strong Lewis acid catalysts, notably chromium-
based catalysts. Replacement of chlorines in chloroor-
ganic materials (readily available from exhaustive
chlorination of the parent organic compounds) by reac-
tion with anhydrous HF in the presence of a Cr-based
catalyst is the process widely employed for the prepara-
tion of fluorinated materials on a commercial scale.
However, for the Cr-based catalyst to be active, it must
have some specific properties. Cr-based catalysts in the
presence of other elements such as Zn,13 Ni,14 In,15 and
Al16 could improve their catalytic activities and inhibit
side reactions. It has been established that a small
amount of Zn added to chromium oxide can perturb
HF adsorption, increase the dispersion of chromium,
and lower the apparent activation energies involved in
the Cl/F exchange to improve conversion and
selectivity.13,17
ꢀ
ranged from 200 to 410 C and the residence time of
DTB in the reactor from 4 to 20 s. The molar ratio of
HF/HCBD was varied from 4 to 25. The experimental
results showed that HCBD conversion and DTB selec-
tivity were optimum when the molar ratio of HF/HCBD
was 7:1, the residence time was 12 s, and the reaction
temperature was kept at 310–390 ꢀC. The obtained
products were analyzed by GC-MS and 19F NMR
(Table 1). Besides the expected DTB, the other compo-
nents obtained were C4F3Cl3, C4F2Cl4, and C4FCl5.
C4F3Cl3 has a cyclic structure, whereas C4F2Cl4 and
C4FCl5 are either linear or cyclic structures.
Table 2 presents the product distribution over the
HF-Cr/Zn catalyst. The catalytic activity increased
from 0.333 to 0.862 mmol/h/g when the reaction tem-
perature was increased from 350 to 410 C. However,
the selectivity for DTB decreased from 10 to 5% when
ꢀ
ꢀ
temperature was increased from 390 to 410 C; also the
On the other hand, many of catalytic fluorination
reactions using metal oxides require activation with
fluorinating agents.18,19 Gas used for activation can
result in structural and chemical changes and O/F
exchange on the catalyst’s surface, making it catalyti-
cally active. The most common method used for pro-
ducing catalytically active materials involves the
fluorination of Cr2O3 with HF. The purpose of pre-
fluorination is that Cr2O3 can be converted into oxy-
fluoride (CrOxFy), which is believed to be the active
sites for the reaction.20
selectivity for the other products changed significantly
as the temperature was varied. So we believe that the
reaction temperature can influence effectively the prod-
uct distribution. A higher reaction temperature did not
improve the conversion of the desired DTB; instead, it
led to an increase of the other byproducts.
Catalytic performance
The fluorination of HCBD was carried out at
350–410 ꢀC over HF-Cr/M/Zn catalyst or at
ꢀ
310–350 C over Cr/Co/Zn catalyst after pretreatment
In this study, various Cr/M/Zn precursors
(M = Ni, Co, Cu, In, Al) were prepared via a precipita-
tion method. HF gas was used to activate Cr/M/Zn
samples (Ni, Cu, In, Al), and fluorinating agents (HF,
HF + Cl2, HF + O2, CF2O, CF2Cl2) were used to acti-
vate the Cr/Co/Zn samples. Catalytic performance of
all catalysts was investigated through a series of experi-
ments. Furthermore, these catalysts before and after the
reaction were characterized by X-ray photoelectron
by different fluorinating agents in order to compare
their activities and identify the transformation products.
The molar ratio of HF/HCBD was 7:1, and the resi-
dence time was 12 s.
The effect of various elements (Co, Ni, Al, In Cu)
on the catalytic activity is presented in Table 3. It can
be seen that the doping the HF-Cr/Zn catalyst with pro-
moters was beneficial to the catalytic activity. The
activity of the Cr/M/Zn catalysts varied from 0.361 to
2
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Catalytic Gas-Phase Fluorination
Table 1. Product distribution
Products
C4F4Cl2
Structure
MS peaks (m/e)
NMR chemical shifts
194[M+],175[M+ – F],159[M+ – Cl],
125[M+ – Cl2],109[M+ – CF2Cl], 90[M+
CF2FCl],74[M+ – CF2Cl2].
19F(CF2=)-117.13 ppm
F
F
F
F
–
Cl
Cl
Cl
C4F3Cl3
C4F2Cl4
210[M+],191[M+ – F],175[M+ – Cl],
19F(CF2=)-110.30 ppm, 19F(CFCl=)-
117.64 ppm
F
F
156[M+ – FCl],141[M+ – Cl2], 125[M+ – Cl2F],109
[M+ – CF3Cl],
F
Cl
Cl
90[M+ – Cl3F],71[M+ – CF3Cl2].
228[M+],191[M+ – Cl],
F
–
–
–
Cl
F
174[M+ – FCl],156[M+ – F2Cl],
141[M+ – CF2Cl],121[M+ – F2Cl],
106[M+ – CF2Cl2],94[M+ – C2F2Cl2],
71[M+- CF2Cl3],47[M+- C3F2Cl3]
244[M+],209[M+ – Cl],
Cl
Cl
Cl
Cl
C4FCl5
C4Cl6
Cl
F
172[M+ – Cl2],141[M+ – CFCl2],
137[M+ – Cl3],102[M+ – Cl4],
90[M+ – CCl4],71[M+ – CFCl4],
66[M+ – C3Cl4],47[M+ – C3FCl4].
260[M+],225[M+ – Cl],
Cl
Cl
Cl
CCl2 = CCl-
CCl = CCl2
190[M+ – Cl2],153[M+ – Cl3],
141[M+ – CCl3],129[M+ – C2Cl3],
118[M+ – Cl4],106[M+ – CCl4],
83[M+ – Cl5],71[M+ – CCl5],
57[M+ – C2Cl5],47[M+ – C3Cl5]
HCBD. However, a higher reaction temperature does
not mean better catalytic properties. In contrast, the
activity of the Cr/Cu/Zn catalyst decreased from 0.671
to 0.596 mmol/h/g as temperature was increased from
Table 2. Effect of reaction temperature on the product
distribution over HF-Cr/Zn catalysta
Selectivity (%)
Temp.
(ꢀC)
Activity
(mmol/h/g) DTB C4F3Cl3 C4F2Cl4 Others
ꢀ
370 to 410 C, while that of Cr/Ni/Zn decreased from
1.318 to 1.264 mmol/h/g as temperature was increased
350
370
390
0.333
0.411
0.490
0.562
5
9
10
8
4
6
11
10
16
13
10
5
75
72
69
87
ꢀ
from 390 to 410 C. Comparing the various catalysts,
Cr/Ni/Zn had the highest surface area of 78.2 m2/g and
exhibited the best catalytic activity (1.318 mmol/h/g) at
390 ꢀC.
410
a
Reaction conditions: HF/HCBD molar ratio of 7/1,
residence time 12 s.
Table 4 shows the product distribution over differ-
ꢀ
ent catalysts at a reaction temperature of 350 C. These
results show that product distribution is determined by
the promoters. For Cr/Co/Zn and Cr/Ni/Zn catalysts,
DTB was the main product under these reaction condi-
tions and the selectivity to the former was 42.5% and
that of the latter was 30.1%, while C4F3Cl3 and
C4F2Cl4 were second most abundant products. For the
Cr/Al/Zn sample, the DTB selectivity was zero, and for
Cr/Cu/Zn and Cr/In/Zn catalysts the DTB selectivity
was also unsatisfactory. In view of the above results, we
believe that prompters can influence effectively the
product distribution. Thus, to achieve high DTB
1.318/mmol/h/g and was higher than that of the Cr/Zn
catalyst (0.333–0.562 mmol/h/g) at the same tempera-
ture. We presumed that the increased catalytic activity
was related to the role of promoters. Indeed, the pres-
ence of promoters would modify the number of the
active sites and the acid strength, which are favorable
to the adsorption of the substrate. Again, the activity
increased remarkably with the reaction temperature.
This indicated that conversion of HCBD required a
higher apparent activation energy. The increase of tem-
perature significantly favored the transformation of
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Table 3. Effect of promoters on the activity for transformation of HCBD (residence time 12 s, HF/HCBD = 7)
Activity (mmol/h/g)
Tem.
(ꢀC)
Cr/Co/ Zn
(23.9 m2/g)
Cr/Ni/ Zn
Cr/Al/Zn
Cr/In/ Zn
Cr/Cu/Zn
(78.2 m2/g)
(26.6 m2/g)
(34.4 m2/g)
(63.2 m2/g)
350
370
390
410
0.361
0.459
0.624
0.673
1.137
1.173
1.318
1.264
0.396
0.428
0.475
0.539
0.998
1.016
1.034
1.179
0.615
0.671
0.652
0.596
selectivity, the activity of Cr/Co/Zn catalyst pretreated
by different fluorinating agents was also investigated.
Figure 1 shows catalytic activity and DTB selec-
tivity over Cr/Co/Zn catalyst activated with different
fluorinating agents and different temperatures. As seen
from Figure 1(a), compared to gaseous HF, Cl2 dis-
played only a small effect on the catalytic activity in the
fluorination of DTB. However, the activities of O2-Cr/
Co/Zn and CF2O-Cr/Co/Zn catalysts were higher than
that of the HF-Cr/Co/Zn catalyst. For CCl2F2-Cr/Co/
Zn catalyst, the activity and DTB selectivity were better
than with HF-Cr/Co/Zn at the reaction temperatures of
310 and 350 ꢀC; nevertheless, a significant decrease
could be noticed when temperature was 330 ꢀC. For
other catalysts, the selectivity of DTB decreased in the
order O2-Cr/Co/Zn > HF-Cr/Co/Zn > Cl2-Cr/Co/Zn >
CF2O-Cr/Co/Zn.
Figures 2 and 3 show the XPS spectra of Cr 2p
and Zn 2p levels for the HF-Cr/Zn catalyst, respec-
tively, both before and after the reaction. As shown in
Figure 2, before the reaction, the Cr 2p3/2 peak was
(a)
Cl2/Cr-Co-Zn
0.6
O2/Cr-Co-Zn
CF2O/Cr-Co-Zn
0.5
CF2Cl2/Cr-Co-Zn
HF/Cr-Co-Zn
0.4
0.3
0.2
0.1
310
320
330
340
350
Characterization of the various catalysts by XPS
In order to identify the different species present on
the catalyst surface and try to understand the exact
roles of the doping agent and fluorinating agents, the
catalysts were characterized by XPS. We attempted also
to correlate the catalytic properties with the concentra-
tions of the different species present on the catalyst
surface.
Temperature (°C)
(b)
50
40
30
20
10
0
Cl2/Cr-Co-Zn
O2/Cr-Co-Zn
CF2O/Cr-Co-Zn
CF2Cl2/Cr-Co-Zn
HF/Cr-Co-Zn
Table 4. Product distribution at the reaction temperature of
350 ꢀC
Selectivity (%)
Sample
DTB
C4F3Cl3
C4F2Cl4
HF-Cr/Co/Zn
HF-Cr/Ni/Zn
HF-Cr/Al/Zn
HF-Cr/In/Zn
HF-Cr/Cu/Zn
42.5
30.1
0
3.6
9.1
25
10
15.8
19.2
7.2
15.8
23.1
11.0
18.2
310
320
330
340
350
Temperature (°C)
Fig. 1. Catalytic activity (a) and DTB selectivity
(b) over Cr/Co/Zn catalysts.
15.1
4
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Catalytic Gas-Phase Fluorination
Zn-F or Zn-O
Cr 2p3/2
After reaction
After reaction
Before reaction
Before reaction
Zn2p3/2
1020 1025
1030
1035
1040
1045
1050
1055
1060
Binding energy (eV)
Fig. 3. XPS spectra of the Zn 2p level for HF-Cr/Zn
catalysts before and after reaction.
570
575
580
585
590
595
600
Binding energy (eV)
Fig. 2. XPS spectra of the Cr 2p level for HF-Cr/Zn
catalysts before and after reaction.
687.5 eV. Before the reaction, the prominent peak at
685.1 eV could be assigned to Cr-F in CrF3 or Zn-F in
ZnF2.23 After the reaction, the concentration of the
another species Cr-F (687.5 eV) in CrOxFy increased.24
A similar trend in the F 1s profiles (Figure 4(b)) could
also be noted. The O 1s spectrum was fitted by two
components at 531.4 and 533.4 eV, respectively. Chung
et al.25 reported the binding energies of 531.4 and
533.4 eV for Cr-O and CrOF, respectively. We there-
fore assigned the peak located at the higher binding
energy to CrOF and that at the lower binding energy to
Cr-O.
resolved into two components at 578.4 and 580.4 eV.
The prominent peak at 580.4 eV can be assigned to the
Cr─F bond in CrF3-like species,21 and the peak at
578.4 eV could be attributed to the Cr─O bond in the
structure corresponding to Cr2O3.22 Then, after the
reaction, the spectrum of Cr 2p became broadened and
was shifted to higher binding energy. The broad Cr 2p3/
peak of the Cr/Zn sample was deconvoluted to three
2
The quantification of the intensities of the bands
corresponding to the different species on the HF-Cr/Zn
catalyst surface is reported in Table 5. The content of
the surface species is calculated by the peak area of the
species divided by the total peak area of all species. A
change in the distribution of three species (Cr-O, Cr-F,
and CrOF) is noticed before and after the reaction. For
the Cr 2p species, before reaction the contents of Cr-F,
Cr-O, and CrOF are 69.96, 30.04, 0%, respectively. For
the spent catalyst, the contents of Cr-F, Cr-O, and
CrOF are 83.87, 4.72, and 11.4%, respectively. This
suggests that the concentration of the species designated
as Cr-O decreased after the reaction, whereas the con-
centration of the species CrOF and Cr-F increased,
which may be due to the fact that a few CrO or Cr-F
species are converted to CrOF on the catalyst surface
during the reaction process. It has been well recognized
that the presence of CrOxFy species is disadvantageous
components at 578.9, 581.7 and 584.6 eV, respectively.
Two kinds of Cr species could be attributed to Cr-O
(578.4 eV) in Cr2O3 and Cr-F (580.4 eV) in CrF3,
respectively. Thus, the 579.1 eV binding energy may
assigned to CrOF in the CrOxFy structure.17 Before the
reaction, the Zn 2p3/2 spectrum of the HF-Cr/Zn cata-
lysts indicated the presence of a single species with a
binding energy of 1022.3 eV, which could be attributed
to ZnF2 or ZnO (Figure 3). After the reaction, the Zn
2p3/2 spectrum of the samples exhibited only a blurred
peak. These results indicated that the catalysts had
strongly interacted with reactants during the reaction
process, which resulted in extensive change of the chem-
ical environments of the Cr and Zn atoms.
The F 1s and O 1s core-level spectra of Cr/Zn cat-
alysts before and after reaction further confirmed the
above results (Figure 4). As shown in Figure 4(a), the F
1s peak was resolved into two components at 685.1 and
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Table 5. Amounts of the different species on the catalyst
(a)
surface determined from XPS data for HF-Cr/Zn
catalysts before and after reaction
F1s
mol %
Binding energy
Elements
Cr
Species
Cr-F
(eV)Æ0.1
Before After
After reaction
580.4
578.4
579.1
685.1
69.96
30.04
0
83.87
4.72
11.4
Cr-O
CrOF
Cr-F or
Zn-F
F
96.97
70.26
CrFO
Cr-O
CrOF
687.5
531.4
533.4
3.03
87.71
12.29
29.74
60.08
39.92
O
Before reaction
This further confirmed the increase of CrOF and Cr-F
species and decrease of Cr-O species after the reaction.
In order to determine the impact of promoters on
the surface properties of Cr/Zn catalyst, the XPS spec-
tra of different HF-Cr/M/Zn samples before the reac-
tion were investigated. As shown in Figure 5, the Cr 2p
profiles are similar for HF-Cr/Zn, HF-Cr/Ni/Zn, HF-
Cr/Cu/Zn, and HF-Cr/In/Zn catalysts. However, a shift
toward lower binding energy of the Cr 2p3/2 peak is
observed after the addition of promoters Ni, Cu, and
In, which can be attributed to the increase in the num-
ber of the Cr-O species. In contrast, the core peak of Cr
675
680
685
690
695
Binding energy (eV)
(b)
O1s
After reaction
Cr-O
CrOF
Cr-F
HF-Cr/Ni/Zn
HF-Cr/Co/Zn
Before reaction
526
528
530
532
534
536
538
540
542
544
HF-Cr/Zn
Binding energy (eV)
Fig. 4. XPS spectra of (a) the F 1s and (b) the O 1s
of HF-Cr/Zn catalysts before and after
reaction.
HF-Cr/Al/Zn
to the Cl/F exchange reactions.26 The time-on-stream of
the HF-Cr/Zn catalyst was assessed. After the reaction,
the catalytic activity decreased compared to the initial
value. Thus, we guess that the decline in activity is due
to the increase of the CrOF species on the catalyst sur-
face. There was a decline in the mol% ratio of Cr-F/
CrOF (from 32.0 to 2.4) in the F 1s and Cr-O/CrOF
(from 6.8 to 1.5) in the O 1s spectrum after the reaction.
HF-Cr/Cu/Zn
HF-Cr/In/Zn
Cr 2p3/2
575 580
Binding energy (eV)
570
585
590
595
Fig. 5. XPS spectra of the Cr 2p of HF-Cr/M/Zn cat-
alysts before reaction.
6
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Catalytic Gas-Phase Fluorination
dispersed CrOF species are mainly responsible for the
DTB selectivity and that Cr-O is crucial to achieve this
high activity. The difference in the contents of the sur-
face CrOF and Cr–O species coincided exactly with the
catalytic behaviors of the various catalysts
CrOF
Cr-O
Cr2p
Cr-F
HF-Cr/Ni/Zn
HF-Cr/Co/Zn
HF-Cr/Zn
The XPS spectra of HF-Cr/M/Zn samples after
the reaction were also studied to reveal the deactivation
of catalysts (Figure 6). After the reaction, the Cr 2p
profiles were similar for all samples. This suggests that
all samples underwent a decline in activity during the
reaction. Table 6 gives the results of the XPS analysis
of the various elements on the surface of HF-Cr/M/Zn
catalysts before and after the reaction. A change in the
distribution of the various elements is noticed depend-
ing on the dopants before and after the reaction. Before
the reaction, the molar content of Cr element decreased
in the order HF-Cr/Co/Zn > HF-Cr/Cu/Zn > HF-Cr/
Ni/Zn > HF-Cr/In/Zn > HF-Cr/Al/Zn. In contrast, the
molar content of Cr element decreased in the order HF-
Cr/Al/Zn > HF-Cr/Ni/Zn > HF-Cr/In/Zn > HF-Cr/
Cu/Zn > HF-Cr/Co/Zn after the reaction. The mol %
of Cr was found to be predominant before the reaction,
whereas the mol% of O and F were predominant after
the reaction.
Figure 7 shows the XPS spectra of the Cr 2p level
for various Cr/Co/Zn catalysts fluorinated by different
fluorinating agents. The Cr 2p profiles are similar for
HF-Cr/Co/Zn and CF2O-Cr/Co/Zn samples. However,
a slight shrinking of Cr 2p line can be observed. The
fluorinating agent CF2Cl2 has a remarkable effect on
Cr 2p of the Cr/Co/Zn catalyst, and the double peaks
of the Cr 2p shifted to a single peak. After treatment by
HF-Cr/Al/Zn
HF-Cr/Cu/Zn
HF-Cr/In/Zn
Cr 2p3/2
580
Binding energy (eV)
570
575
585
590
595
600
Fig. 6. XPS spectra of the Cr 2p of HF-Cr/M/Zn cat-
alysts after reaction.
2p3/2 shifted to the high-energy side after introducing
Co onto the Cr/Zn sample. For the HF-Cr/Al/Zn cata-
lyst, the Cr 2p spectrum hardly exhibited any peak. It
was reported that the reaction between Al2O3 with HF
to form AlF3 required a lower reaction enthalpy value.
So the formation of AlF3 resulted in the disappearance
of the Cr 2p peak. This trend of the change in pro-
moters upon HF-Cr/Zn catalyst is also confirmed in the
O 1s and F 1s core-level spectra of these samples (not
shown here). As mentioned above, catalytic activity is
related to the presence of promoters on the Cr/Zn cata-
lyst. Therefore, the role of different dopants can be
ascribed to the difference in distribution of active Cr
species on the catalyst. Thus, it can be concluded that
Table 6. Relative molar contents of different elements on the HF-Cr/M/Zn catalysts before and after reaction
Molar content (%)
Sample
Cr
F
O
Zn
M
Cl
HF-Cr/Co/Zn
Before
After
Before
After
Before
After
Before
After
13.27
10.4
10.4
20.37
2.34
24.22
7.92
18.86
13.16
18.53
34.23
55.91
19.87
39.72
11.67
54.3
26.33
39.59
25.76
36.1
50.95
31
66.32
34.24
84.85
16
62.23
37.75
58.15
39.22
1.55
0.56
1.66
1.82
1.14
1.41
1.87
0.86
1.57
0.75
0
0
1.1
0
0
1.29
0.8
0.78
0.91
0.99
0
2.49
0.64
3.85
0
2.77
0.74
2.15
0.46
4.41
HF-Cr/Ni/Zn
HF-Cr/Al/Zn
HF-Cr/In/Zn
HF-Cr/Cu/Zn
Before
After
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Article
He et al.
CrOF
Cr-F
Cr-F
CrOF
F1s
Cr-O
O2/Cr-Co-Zn
Cl2/Cr-Co-Zn
CF2O/Cr-Co-Zn
CF2Cl2/Cr-Co-Zn
Cl2/Cr-Co-Zn
CF2Cl2/Cr-Co-Zn
O2/Cr-Co-Zn
Cr2p3/2
Cr2p1/2
570
575
580
585
590
595
CF2O/Cr-Co-Zn
Binding energy (eV)
Fig. 7. XPS spectra of Cr 2p for Cr/Co/Zn catalyst
675
680
685
690
695
700
activation by different fluorinating agents.
Binding energy (eV)
Fig. 8. XPS spectra of F 1s for Cr/Co/Zn catalysts.
O2 and Cl2, the spectrum of Cr 2p became broad and
was shifted to a higher binding energy. These results
indicated that Cr/Co/Zn precursor strongly interacted
with various fluorinating agents during the prefluorina-
tion process, leading to extensive change of the chemi-
cal environment of the Cr atom.
respectively. The results showed that after the treatment
by O2 the Cr/Co/Zn precursor was more easily
fluorinated.
From the XPS measurements shown in Figure 8,
two peaks having the F 1s binding energy of 685.1 and
687.5 eV can be identified. We assigned the peak
located at the higher binding energy to CrOF and that
at the lower binding energy to Cr-F. Cr-F was found to
be dominating in the Cl2-Cr/Co/Zn catalyst and CrOF
was dominating in the O2-Cr/Co/Zn catalyst. CF2O gas
treatment of the Cr/Co/Zn precursor generated only the
Cr-F species. In contrast, the fluorinating agent CF2Cl2
gave neither CrOF nor Cr-F species but generated a
different species with a higher binding energy of
693.1 eV.
Possible reaction path
A simplified reaction path from HCBD to DTB
was proposed based on experimental results with theo-
retical calculations by our research group (Figure 10).27
IO/ICr
10
IF/ICr
8
IO/I(Cr+Zn+Co)
IF/I(Cr+Zn+Co)
6
The influence of prefluorination by different fluori-
nating agents over Cr/Co/Zn catalysts on the amount of
fluorine (or oxygen) can be determined from the varia-
tions of the ratios IF/ICr (or IO/ICr) and IF/I(Cr + Co + Zn)
(or IO/I(Cr + Co + Zn)) (Figure 9), where I is the intensity of
a peak in the wide XPS spectra. The ratios IO/ICr and IO/I
4
2
0
(Cr + Co + Zn) decreased in the order HF > Cl2 > CF2O >
CF2Cl2 > O2 and HF > CF2Cl2 > Cl2 > CF2O > O2,
HF
Cl2
O2
CF2O
CF2Cl2
respectively. The ratios IF/ICr and IF/I(Cr + Co + Zn)
Catalysts
decreased in the order O2 > CF2O > CF2Cl2 >
Fig. 9. Elements molar ratios over Cr/Co/Zn
catalysts.
Cl2
> HF and O2 > CF2Cl2 > CF2O > Cl2 > HF,
8
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CHEMICAL SOCIETY
Catalytic Gas-Phase Fluorination
absorption
Preparation of the catalyst
Cl
Cl
Cl
The Cr/M/Zn precursors were prepared via a
coprecipitation method. The molar ratio of Cr, Zn, and
M (Ni, Cu, In, Al, Co) was 85:10:5 in the precursor.
Analytical-grade CrCl3, M(NO3)x, and Zn(NO3)2 were
first dissolved in distilled water. A hydroxide was
obtained from 246 g of a 10% CrCl3 solution. Then
30% NH3ÁH2O was added to the above prepared solu-
tion under continuous stirring until a precipitated slurry
was obtained. Subsequently, the resulting slurry was fil-
tered, washed with deionized water several times, and
Cl
Cl
F
a
Cl
1
H
F
Cl
F
)
n
F
H
(
H
Cl
)
n
F
H
(
F
F
Cl
Cl
Cl
Cr
Cr
F
Cl
H
F
Cl
F
F
n
)
F
F
Cl
Cl
F
H
H
(
)
n
F
Cr
Cr
Cl
Zn
H
(
Cl
F
Cl
b
Cr
F
F
Cl
Cl
F
Zn
Cr
F
Cl
Cl
Cr
F
F
F
F
F
H
F
F
F
desorption
Zn
Cr
n
)
F
Cr
H
H
Cl
(
Cl
n
)
F
F
F
H
(
2
H
)
n
F
H
Cr
H
(
C
r
Zn
Cr
Zn
ꢀ
baked at 110 C for 24 h in a drying oven. Finally, the
Cr
dried solid was ground, mixed with 2 wt% graphite,
and pelleted into cylindrical wafers (diameter = 2 mm;
height (h) = 3 mm). The Cr/M/Zn precursors were
formed by calcining the pellets at both 250 C for 10 h
and at 400 ꢀC for 10 h under a N2 flow rate of
150 mL/min.
Fig. 10. Possible reaction paths for the fluorination
of HCBD catalyzed by Cr/M/Zn catalysts.
ꢀ
In general, HCBD is stable with no self-cyclization
28
ꢀ
reaction even at 500 C. On the contrary, HCDB was
found in the products when hexachlorocyclobutene was
heated to 195 ꢀC.29 These results validated that the
F/Cl exchange reaction takes place prior to the cycliza-
tion of HCBD. In addition, NMR analyses showed that
the Cl/F exchange primarily occurred at the terminal
CCl2 groups rather than at the inner CCl groups.
Hence, by comparing the product distribution, the
transformation of HCBD to BTB may involve a Cl/F
exchange reaction forming C4Cl4F2 first; and then,
C4Cl4F2 would proceed either via a cyclization forming
c-C4Cl4F2 followed by further Cl/F exchange reactions,
or via a Cl/F exchange reaction forming C4Cl3F3 and
C4Cl2F4 followed by further cyclization.
Prior to the reaction, pretreatment was carried out
to activate the precursors with different fluorinating
agents (HF, 95%HF + 5%Cl2, 95%HF + 5%O2, CF2O,
CF2Cl2). Twenty grams of the precursors was packed
into the reactor. A mixture of N2 (100 mL/min) and the
fluorinating agent (50 mL/min) was passed through
ꢀ
reactor at 150 C for 10 h. Then, the N2 flow rate was
decreased to 50 mL/min and the fluorinating agent flow
ꢀ
rate was increased to 100 mL/min at 250 C for 10 h.
Subsequently, the N2 flow was stopped and the sample
ꢀ
was heated at 400 C for 10 h in the fluorinating agent
at a flow rate of 150 mL/min. Finally, the pretreated
catalysts, which were denoted as HF-Cr/M/Zn, Cl2-Cr/
M/Zn, O2-Cr/M/Zn, CF2O-Cr/M/Zn, and CF2Cl2-Cr/
M/Zn, respectively, were formed.
EXPERIMENTAL
Chemicals
1,1,2,3,4,4-Hexachloro-1,3-butadiene
(HCBD)
Reaction conditions
(98%) was obtained from Letai Chemical Industry Co.,
Ltd. (Tianjin, China). Anhydrous HF (AHF) (>99.9%),
Cl2 (>99.9%), O2 (>99.9%), CF2O (>99.9%), CF2Cl2
(>99.9%), and N2 (>99.9%) were purchased from Bei-
jing North Oxygen Specialty Gases Institute Co., Ltd.
(Beijing, China). NH3ÁH2O (30%) was obtained from
Beijing Chemical Works Co., Ltd. (Beijing, China).
Analytical-grade CrCl3Á6H2O (>99%), Zn(NO3)2Á6H2O
(>99%), Ni(NO3)2Á6H2O (>99%), Cu(NO3)2 (>99%),
In(NO3)3Á5H2O (>99%), Al(NO3)3Á9H2O(>99%), and
Co(NO3)2Á6H2O(>99%) were purchased from Xilong
Chemical Co., Ltd. (Guangxi, China).
Catalytic activity measurements for the fluorina-
tion of HCBD were carried out in a fixed-bed reactor.
Twenty grams of the catalyst was placed in the reactor.
Then HCBD and HF, converted to the vapor-phase in
a vaporizer at 230 ꢀC, were passed together into the
ꢀ
stainless steel reactor at 310–410 C with a contact time
of 12 s. The molar ratio of HF:HCBD was 7:1. HCBD
was dosed via a Masterflex metering pump, and HF
was dosed using a mass flow controller. The products
were cooled with an ice bath and collected. The col-
lected products were washed in a 60 ꢀC warm water
stream to remove HF and HCl, and dried successively
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Article
He et al.
were recorded using an aperture slot measuring
300 μm × 700 μm. Survey and high-resolution spectra
were recorded with pass energies of 160 and 40 eV,
respectively. Accurate binding energies (within
Æ0.2 eV) were determined with respect to the position
of the adventitious C 1s peak at 284.6 eV
Scheme 2. Reaction products for the reaction of
HCBD with HF.
with anhydrous sodium sulfate and molecular sieves
(4 Å). The obtained products were analyzed by GC,
GC-MS, and 19F NMR. As shown in Scheme 2, the
reaction of HCBD with HF in the presence of catalysts
yielded main products DTB, C4Cl3F3, C4Cl4F2, and
C4Cl5F.
The catalytic activity A (mmol/h/g) was defined as
the conversion multiplied by the flow of HCBD (mol/h)
and divided by the mass of catalyst.
CONCLUSIONS
Reaction conditions for the fluorination of HCBD
catalyzed by Cr/M/Zn catalysts were investigated. The
catalytic activity of the Cr/Zn-based catalyst was
improved by the addition of small amounts of
prompters. The Cr/Ni/Zn catalyst exhibited better cata-
lytic activity, but the DTB selectivity was unsatisfac-
tory. However, the Cr/Co/Zn catalyst improved greatly
the DTB selectivity. Subsequently, different fluorinating
agents (HF + Cl2, HF + O2, CF2O, CF2Cl2) were used
to fluorinate the Cr/Co/Zn precursor. After treatment
by HF + O2, the catalytic performance increased
remarkably. Characterization by means of XPS spectra
revealed that promoters and the activation period of
Cr/M/Zn samples before and after the reaction involved
changes in the different species. Furthermore, a possible
reaction path was proposed.
Characterization
Gas chromatography-mass spectroscopy (GC-MS)
was carried out on a Shimadzu-QP 2010 Ultra series
system, which was equipped with a jet separator for the
2010 GC. The capillary column (Model DB-5, J&W
Scientific, Inc.) had an inner diameter (i.d.) of 0.25 mm
and a length of 30 m.
Gas chromatography (GC) was carried out under
the following operating conditions: capillary column
0.25 mm i.d. and 30 m length (DB-5, J&W Scientific,
Inc.); column temperature maintained at 35 ꢀC for
3 min and then heated to 200 C at the rate of 10 C/
min and held at that temperature for 3 min; the injector
and detector temperatures set at 280 and 200 ꢀC,
respectively; split ratio 80:1; and sample volume 0.1 μL.
19F NMR spectra were recorded on a Bruker
AV400 instrument at 400 MHz with CFCl3 as the inter-
nal standard.
ACKNOWLEDGMENTS
This work was supported by the National Natural
Science Foundation of China (Grant No. 51176078) and
the Special Project of the Basic Work of Science and
Technology Police of MPS (Grant No. 2016GABJC35).
ꢀ
ꢀ
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