Catalytic dehydrofluorination of 1,1,1,2-tetrafluoroethane to synthesize trifluoroethylene over a modified NiO/Al2O3 catalyst

Article abstract of DOI:10.1039/c5cy00211g

A promising fluorinated NiO/Al₂O₃ catalyst for synthesizing trifluoroethylene through the catalytic dehydrofluorination of CF₃CFH₂ was prepared, and the relationship between the Lewis acid sites and activity was investigated. 20.1% conversion of CF₃CFH₂ was observed, and the selectivity to trifluoroethylene was observed to be 99% at 430°C after 100 h.

Full text of DOI:10.1039/c5cy00211g

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Catalytic dehydrofluorination of 1,1,1,2-
tetrafluoroethane to synthesize trifluoroethylene
Cite this: DOI: 10.1039/c5cy00211g
over a modified NiO/Al2O3 catalyst†
Received 8th February 2015,
Accepted 30th March 2015
*
Wenzhi Jia, Min Liu, Xuewei Lang, Chao Hu, Junhui Li and Zhirong Zhu
DOI: 10.1039/c5cy00211g
www.rsc.org/catalysis
A
promising fluorinated NiO/Al2O3 catalyst for synthesizing
catalysts were applied industrially for methanation,18,19
steam reforming and pre-reforming of natural gas.20 How-
ever, the NiO/Al2O3 catalysts used for the dehydrofluorination
of HFCs have not been reported scientifically.
trifluoroethylene through the catalytic dehydrofluorination of
CF3CFH2 was prepared, and the relationship between the Lewis
acid sites and activity was investigated. 20.1% conversion of
CF3CFH2 was observed, and the selectivity to trifluoroethylene was
observed to be 99% at 430 °C after 100 h.
It was disclosed that the catalytic performance of dehydro-
halogenation is closely related to Lewis acidity.13–15,21 Teinz
et al. reported the dehydrohalogenation of 3-chloro-1,1,1,3-
tetrafluorobutane IJCF3CH2CFClCH3) over metal fluoride, and
dehydrofluorination was selectively catalyzed by strong Lewis
acid sites. For the dehydrofluorination of CF3CH3, the influ-
ence of the strength of the Lewis acid sites on the formation
of CF2CH2 was investigated by Li et al.13,14 Okazaki et al.15
assumed that the product distribution was dependent on the
acidic strength of the catalysts. However, the relationships
between the Lewis acid sites and activity for the dehydro-
fluorination of CF3CFH2 are not clarified to date.
Herein, NiO/Al2O3 catalysts activated by HF pretreatment
were prepared for the dehydrofluorination of CF3CFH2 to syn-
thesize trifluoroethylene. Moreover, the catalyst preparation,
including the reaction conditions, was optimized. XRD, BET,
SEM/EDX, TEM, py-IR and NH3-TPD were employed to dis-
close the relationship between the catalyst structure and its
activity. Moreover, the deactivation and regeneration of the
catalyst were investigated.
γ-Al2O3 was modified by a series of metal oxides such as
alkalis, alkaline earth, rare-earth, and transition metals. The
oxide catalysts activated by the pre-fluorination of HF gas
were used for the dehydrofluorination of CF3CFH2 to synthe-
size trifluoroethylene under the same reaction conditions.
The trifluoroethylene yields for fluorinated MOx/Al2O3 cata-
lysts are summarized in Table 1. The selectivity to trifluoro-
ethylene for all catalysts is up to 99%, and the other few
byproducts are CF2HCF3 and CF2HCFH2. From Table 1, it can
be observed that the activities of NiO/Al2O3 and Ce2O3/Al2O3
are superior to the parent γ-Al2O3, and the Cr2O3/Al2O3 cata-
lyst showed high activity at a low-temperature range of 400–
450 °C. However, Cr2O3 and the related species as dopants in
the catalyst are toxic to environment and human health.
Trifluoroethylene (CF2CHF) is used as an important
fluorine-containing monomer and biologically active com-
pound,1 which is generated through the hydrodechlorination
of trichlorotrifluoroethane or chlorotrifluoroethene using
supported Pd or Ru catalyst.2–5 Nevertheless, there are many
difficulties involved during separation and purification; more-
over, expensive raw materials and noble metal catalysts are
required. Comparatively, the dehydrofluorination of 1,1,1,2-
tetrafluoroethane IJCF3CFH2) to synthesize trifluoroethylene is
a promising route. The raw CF3CFH2 has eventually been
replaced by 2,3,3,3 tetrafluoropropene due to its high global
warming potential (over 1300) as a new greenhouse gas.6,7
The transformation of CF3CFH2 to trifluoroethylene with high
additional value resolved the excess capacity of CF3CFH2.
Admittedly, it exhibits atom efficiency and has better poten-
tial in industrial application. However, the studies concerning
the catalytic dehydrofluorination of CF3CFH2 are hardly
reported in the form of professional research studies, and
most of the presented works are limited to patents.8–10
The dehydrofluorination of hydrofluorocarbons (HFCs) is
thermodynamically and kinetically hindered.11,12 To obtain a
considerable yield of fluorinated olefin, it is necessary to
develop highly efficient catalysts to accelerate the reaction
rate. As claimed in the literature, Mg2P2O7,13,14 AlF3,15 Fe2O3–
CdO–Al2O3 (ref. 16) and Pd/AlF3 (ref. 17) catalysts were used
for HFCs dehydrofluorination. In addition, NiO/Al2O3
Department of Chemistry, Tongji University, Shanghai 200092, China.
E-mail: zhuzhirong@tongji.edu.cn; Fax: (+86) 21 65982563
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy00211g
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a
Table 1 Catalytic performance of fluorinated catalysts for the dehydrofluorination of CF3CFH2
Trifluoroethylene yields/%
Classification
Catalyst
500 °C
475 °C
450 °C
425 °C
400 °C
Parent
Transition
Al2O3
40.7
60.3
37.6
26.3
15.6
48.4
27.5
5.5
36.4
43.6
34.3
28.4
9.5
35.8
20.7
3.7
23.2
32.2
29.2
20.9
6.0
26.0
15.4
2.3
16.5
22.9
21.9
14.4
3.8
17.8
10.9
1.4
9.8
14.4
14.0
9.7
2.3
11.5
7.7
1.0
0.6
0.5
NiO/Al2O3
Cr2O3/Al2O3
PdO/Al2O3
Fe2O3/Al2O3
Ce2O3/Al2O3
La2O3/Al2O3
MgO/Al2O3
CaO/Al2O3
SrO/Al2O3
Rare-earth
Alkaline-earth
3.5
2.9
2.5
2.1
1.7
1.4
1.0
0.9
a
Total flow = 22.5 ml min−1, N2/CF3CFH2 = 10, P = 1 atm, molar ratio of Al/M = 9, catalysts: 1.0 g.
Most of the transition and rare-earth metals added in the
Al2O3 could not enhance its activity, and the activity of cata-
lysts with alkaline-earth and alkalis was drastically low. Com-
paratively, NiO/Al2O3 catalyst shows an environmentally
friendly and high catalytic activity.
We modified Al2O3 with nickel nitrate and investigated
the effect of NiO loading on its activity (Fig. S1†). It can be
observed in Fig. S1 (ESI†) that with increasing NiO loading
from 0 to 25 wt.%, the activity of NiAlF-12.8 catalyst reached
the highest value, then decreased with a further increase in
NiO contents over 12.8 wt.%. Moreover, the activity of NiAlF-
16.7 and NiAlF-25 catalyst was lower than the parent Al2O3
(NiAlF-0). This indicated that the modification of the appro-
priate Ni species could enhance the catalytic activity.
The stability test was carried out over the optimal NiAlF-
12.8 catalyst at 430 °C (Fig. 1). With a GHSV of 675 h−1 and a
CF3CFH2/N2 molar ratio of 10, the conversion of CF3CFH2
slowly decreased, and then remained over 20.0%. However,
trifluoroethylene selectivity did not change obviously during
the on-stream time of 100 h. The activity of NiAlF-12.8 nearly
remained stable after 78 h, showing good stability at a proper
reaction temperature.
regeneration of the deactivated NiAlF-12.8 catalyst was
performed by a temperature-programmed calcination with an
increasing rate of 2 °C min−1 to 550 °C for 2 h in a mixed
flow of nitrogen containing 10% oxygen. The regenerated
NiAlF-12.8 catalyst needs to be pre-fluorinated again before
the reaction. The results are listed in Table S1.† It was found
that the reactivity could be recovered by more than 94% of
the fresh catalyst, indicating that NiAlF-12.8 catalyst has an
excellent regeneration capability.
Fig. 2 shows the XRD patterns of a) NiAlO and b) NiAlF
catalysts. In Fig. 2a, the diffraction peaks of Al2O3 phase can
be observed for all NiAlO catalysts. No diffraction peaks due
to NiO phase are observed for the NiAlO-6 and NiAlO-12.8
catalysts.22 With increasing NiO content, the intensities of
the NiO diffraction peaks appeared and became stronger,
whereas the Al2O3 diffraction peaks weakened. For the NiAlO-
25 catalyst, the peaks with strong intensity were attributed to
the bulk NiO crystalline.23 After the prefluorination of NiAlO
catalysts, the characteristic peaks due to AlF3 and NiF2
phases are observed, as shown in Fig. 2b. The intensity of the
diffraction peaks of AlF3 at 25.3° due to (012) plane first
increased, and reached the maximum with 12.8% of NiO con-
tent. The low content of Ni species could accelerate the crys-
tal growth of AlF3 crystalline particles under fluorination, ini-
tially causing the intensity of the AlF3 peak to increase. In
The deactivation of NiAlF-12.8 catalyst was accelerated in
the dehydrofluorination process under high temperature con-
ditions, high GHSV and low ratio of N2 to CF3CFH2. The
Fig. 2 XRD patterns of a) NiAlO and b) NiAlF catalysts. The NiO
Fig. 1 The stability of NiAlF-12.8 catalyst at 430 °C.
loading is 1) 0%, 2) 6.0%, 3) 12.8%, 4) 16.7%, and 5) 25.0%.
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addition, the diffraction peaks of NiF2 gradually became
stronger. It was indicated that the extended exposure of
NiO/Al2O3 catalyst to fluorinating gases at elevated tempera-
tures led to the complete conversion of Al2O3 and NiO to AlF3
and NiF2, respectively.
The N2 adsorption isotherms and pore size distributions
of NiAlF catalysts are shown in Fig. S2.† All the isotherms are
type IV, according to the IUPAC classification. Other parame-
ters, such as surface area, pore volume, and pore size, are
summarized in Table S2.† There is an increase in the surface
area with the amount of nickel (from 0 to 12.8 wt.%). Fig. S3†
shows the SEM images of catalysts. The NiAlO-12.8 catalyst is
disordered in Fig. S3a,† and most of the bulk grains are crys-
talline AlF3 formed by fluorination, as shown in Fig. S3b.†
First, the acidic amounts and strengths of NiAlF catalysts
were measured using NH3-TPD techniques. Fig. 3 shows the
NH3-TPD profiles of NiAlF catalysts with different NiO load-
ing. The acidic amount was also calculated, and is listed in
Fig. 3 (inset). The broad peaks around 170 °C and 315 °C
could be ascribed to the NH3 adsorbed on the weak acidic
and strong acidic sites,24 respectively. With increasing con-
tent of Ni species, the acidic amount increases, then declines
drastically, and reaches the maximum for the NiAlF-12.8 cata-
lyst. The acidic amounts of NiAlF-12.8 (0.43 mmol g−1) and
NiAlF-6 catalysts (0.37 mmol g−1) are larger than that of the
NiAlF-0 (AlF3, 0.34 mmol g−1) catalyst. However, NiAlF cata-
lysts with high Ni species loading (>12.8 wt.%) show weak
acidity.
Pyridine adsorption experiments were conducted to deter-
mine the properties of the acidic sites. Obviously, two charac-
teristic bands at ν19b = 1448 and ν8a = 1612 cm−1 due to the
Lewis acid sites25,26 on AlF3 catalysts are observed in Fig. 4a.
For the NiAlF-12.8 catalyst, the ν8a feature includes peaks at
1622 and 1616 cm−1 associated with the strong and weak
Lewis acid sites,27,28 respectively. However, the catalyst with-
out Ni additive (NiAlF-0) has only one type of Lewis acid at
1612 cm−1. This indicates that the NiF2 plays an important
role in the acidity of NiAlF catalyst. The NiAlF-12.8 catalysts
adsorbed pyridine at room temperature. They were outgassed
at 100 or 300 °C, and their py-IR spectra were obtained,
Fig. 4 Infrared spectra of adsorbed pyridine for NiAlF catalysts.
which are presented shown in Fig. 4b. The band shifts were
observed with an increase in the desorption temperature on
the NiAlF-12.8 catalyst, as shown in Fig. 4b. This shift in tem-
perature shows the higher acidic strength. M. Wojciechowska
et al.24 reported that new Lewis acid sites appeared on the
surface of the MgF2 catalyst-doped appropriate NiF2, which
originated from the coordinatively unsaturated Ni2+ ions.
These facts agreed with our findings in which new Lewis acid
sites were observed in the NiF2/AlF3 catalyst (Fig. 3). It is con-
sidered that the Ni content plays an important role in the
number of acid sites on the NiAlF catalyst.
Carmichael group et al.29,30 reported the cleavage of C–F
in the dehydrofluorination of CF3CH3 and CF2HCH3. In gen-
eral, C–F cleavage (the dissociation energy is 522.0 8.4 kJ
mol−1) needs high activation energy, accompanied by high
reaction temperature. It is well known that the Lewis acid
sites are considered to activate C–F bond. Li et al.13 reported
the dehydrofluorination of CF3CH3 into CF2H2 over the Lewis
acidic Mg2P2O7 catalyst. Teinz et al.21 reported that C–F bond
activation over Lewis acid sites was the initial step of
dehydrohalogenation. Therefore, Lewis acid sites play a key
role in the dehydrofluorination of CF3CFH2. Fig. 5 shows the
relationship between Lewis acid sites and the activity of the
NiAlF catalysts. Interestingly, it is found that the number of
Lewis acid sites is closely proportional to the conversion of
CF3CFH2 (Fig. 5). The NiAlF-12.8 catalyst exhibited high activ-
ity, attributed to its large Lewis acidic amount. In conclusion,
it is undoubtedly suggested that Lewis active sites are the
Fig. 5 Dependence of the conversion of CF3CFH2 on the amount of
Lewis acid sites, N2/CF3CFH2 = 10, P = 1 atm, catalyst: 1.0 g.
Fig. 3 NH3-TPD profiles of fluorinated NiAlF catalysts.
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main active sites of the catalysts for the dehydrofluorination
of CF3CFH2.
catalyst, and the deactivated catalyst can be easily regene-
rated by heating at 550 °C. More importantly, the fluorinated
NiO/Al2O3 catalyst with an excellent defluorination perfor-
mance could be applied to the dehydrofluorination of other
HFCs, such as the dehydrofluorination of CF2HCH3 (HFC-152a)
for synthesizing vinyl fluoride (a very important fluoride
monomer).
According to the thermodynamic parameter of CF3CFH2
dehydro-fluorination reaction11 ij△rGθ (298 K) = 70.3 kJ mol−1;
△rHθ (298 K) = 128.6 kJ mol−1], this reaction favors a high
temperature. The influence of temperature from 375 to
530 °C on the NiAlF catalyst has been shown in Fig. S4.† With
an elevation in the reaction temperature, the selectivity to
trifluoroethylene decreased from 100% to 96.8%, accompa-
nied with the formation of CFHCFH, CF2HCF3 and
CF2HCFH2. The byproducts were derived from the pyrolysis
and isomerization31,32 of CF3CFH2 at high temperature.
When the dehydrofluorination of CF3CFH2 was conducted at
high temperatures (≥480 °C), the used catalysts turned black
in a short time, indicating that serious coke or polymeriza-
tion occurred over the catalysts. Therefore, a proper reaction
temperature should be selected in the range of 400–480 °C.
As can be seen from Fig. 1, the deactivation behavior of
the catalysts was investigated. Fig. S5† shows the XRD pat-
terns of the used NiAlF-5 catalyst with different reaction
times. Only the AlF3 and NiF2 phases were observed in the
NiAlF-5 catalyst for 100 h, and the peak intensity increased
with time on steam. It is indicated that the crystalline sizes
of AlF3 and NiF2 increased with the reaction time on steam,
and the catalyst was slowly sintered. Fig. S6† shows the
Raman spectra of the used NiAlF-12.8 catalyst for different
reaction times. Other intense broad bands located at 1318
and 1593 cm−1 in the Raman spectra are attributed to the A1g
vibration mode and the E2g vibration mode of the carbon,33
respectively. The intensity of the Raman peak due to the car-
bon species increased with reaction time, demonstrating that
the level of formed coke increased with the time on steam.
TG experiments were conducted to further illustrate the
amount of coke generated for the deactivated NiAlF-12.8 cata-
lyst during the reaction process, as shown in Fig. S7.† An
obvious weight loss was observed below 100 °C in all the
used NiAlF-12.8 catalysts, attributed to the loss of the physi-
cally adsorbed H2O. In addition, a main weight loss was
observed around the broad range from 500–590 °C, which
was undisputedly assigned to the combustion of deposited
carbon on the used NiAlF catalysts.34 It is found that the
weight loss of the used catalysts increased with the reaction
time on steam. It was further confirmed that the coke
resulted in the decline of catalytic activity, in agreement with
the results obtained from Raman spectroscopy (Fig. S6†). It
can be implied that the deactivation mainly depends on the
sintering and carbon deposition over the catalyst.
Acknowledgements
This work was financially supported by the CNPC Innovation
Research Funds (2012D-5006-0505) and National Natural Sci-
ence Foundation of China (51174277 and 20873091).
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