Effect of acid strength of co-precipitated chromia/alumina catalyst on the conversion and selectivity in the fluorination of 2-chloro-1,1,1-trifluoroethane to 1,1,1,2-tetrafluoroethane

Article abstract of DOI:10.1016/S0022-1139(99)00071-8

Different fluorinated catalysts based on co-precipitated Cr₂O₃/Al₂O₃ and doped with compounds of Zn and/or Mg are prepared and their total acidity determined by TPD of ammonia. The influence of acidity of the above catalysts on the conversion and selectivity in the fluorination of HCFC-133a to give HFC-134a was studied. It was found that the selectivity for HFC-134a increases with a fall in the relative percentage of strong acid centres.

Full text of DOI:10.1016/S0022-1139(99)00071-8

Journal of Fluorine Chemistry 95 (1999) 177±180
Effect of acid strength of co-precipitated chromia/alumina catalyst
on the conversion and selectivity in the ¯uorination of
2-chloro-1,1,1-tri¯uoroethane to 1,1,1,2-tetra¯uoroethane1
Jampani Madhusudana Rao*, P. Shanthan Rao, V. Vijayakumar, V. Venkata Rao, B. Narsaiah,
S. Narayan Reddy, K. Leela Krishna, K. Srinivas, K. Radhakrishnan, K.S. Patil
Indian Institute of Chemical Technology, Hyderabad 500 007, India
Accepted 5 March 1999
Abstract
Different ¯uorinated catalysts based on co-precipitated Cr2O3/Al2O3 and doped with compounds of Zn and/or Mg are prepared and their
total acidity determined by TPD of ammonia. The in¯uence of acidity of the above catalysts on the conversion and selectivity in the
¯uorination of HCFC-133a to give HFC-134a was studied. It was found that the selectivity for HFC-134a increases with a fall in the relative
percentage of strong acid centres. # 1999 Elsevier Science S.A. All rights reserved.
Keywords: Fluorination; Chromium oxide; Hydro¯uorocarbons; TPD
1. Introduction
proposes dismutation reaction [4] of HCFC-133a to give
HFC-143a and HCFC-123. The latter is a precursor to
HCFC-124 and HFC-125. The dehydrohalogenation of
HCFC-133a to give HCFC-1122 is a competitive reaction
[5] dependent on temperature. The factors relating the
catalytic activity in terms of conversion of HCFC-133a were
investigated for catalysts based on Cr2O3. Webb and Win-
®eld [6,7] postulated the formation of a labile chromium
oxy¯uoride as responsible for halogen exchange. TPR and
TPO studies [5,8±10] on bulk Cr2O3 showed that the cat-
alytic activity depends on the number of reversibly oxidiz-
able sites. Kemnitz et.al [11,12] attribute the activity to the
formation of b-CrF3. In the case of catalysts based on Al2O3,
the formation of b-AlF3 was considered important [13].
Based on TPD studies [9,14] it was proposed that Lewis
acid sites are responsible for catalytic activity. All the above
studies relate to the conversion of HCFC-133a. In this paper
we report our study on the effect of the acid strength of co-
precipitated Cr2O3/Al2O3 with and without doping with
compounds of Zn and/or Mg on the conversion and selec-
tivity in the ¯uorination of HCFC-133a to give HFC-134a. A
co-precipitated Cr2O3/Al2O3 was chosen as base catalyst
because it meets the strength requirement for operation
under pressure as compared to bulk Cr2O3. Compounds
of Zn and Mg are chosen as dopants because of their known
promotional effects.
The ¯uorination of 2-chloro-1,1,1-tri¯uoroethane
(HCFC-133a) with anhydrous hydrogen ¯uoride (AHF) in
the presence of a suitable halogen exchange catalyst [1] is of
commercial interest to produce 1,1,1,2-tetra¯uoroethane
(HFC-134a), a compound recommended to replace some
of the ozone depleting chloro¯uorocarbons [2]. Several
precatalyst formulations based on bulk Cr2O3 or mixed
oxide Cr2O3/Al2O3 with and without promoters were devel-
oped. The precatalysts were ®rst ¯uorinated with AHF to
generate in situ the active catalyst.
The selectivity for HFC-134a in the ¯uorination of
HCFC-133a with AHF is affected because of side reactions
leading to the formation of 2-chloro-1,1,1,2-tetra¯uor-
oethane (HCFC-124), penta¯uoroethane (HFC-125),
1,1,1-tri¯uoroethane (HFC-143a), 2-chloro-1,1-di¯uor-
oethene (HCFC-1122) and trichloroethylene (TRI). The
formation of pentahalo compound [3] (HCFC-124) was
attributed to the chlorination of HCFC-133a by elemental
chlorine generated through the occurrence of Deacon reac-
tion between HCl and Cr2O3. An alternate mechanism
*Corresponding author. Fax: +91-40-7173387; e-mail:
jmrao@iict.ap.nic.in
1IICT communication No. 4251.
0022-1139/99/$ ± see front matter # 1999 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 2 - 1 1 3 9 ( 9 9 ) 0 0 0 7 1 - 8

178
J.M. Rao et al. / Journal of Fluorine Chemistry 95 (1999) 177±180
2. Experimental
into tablets and calcined at 3808C in nitrogen atmosphere.
X-ray diffraction analysis revealed the amorphous nature of
the catalyst except for traces of g-Al2O3. surface area
170 m2/g.
The surface area was determined by the BET method
from the adsorption isotherms of nitrogen at liquid nitrogen
temperature with the help of Sartorius Gravimat. The X-ray
data were collected using a Siemens D5000 X-ray powder
diffractometer with Cu Kꢀ radiation.
2.1.5. Fluorination
The apparatus consists of a ®xed bed reactor system
which could be operated both at atmospheric and also at
high pressures. Anhydrous hydrogen ¯uoride and HCFC-
133a were drawn in vapor form from pressure cans. The feed
rates were monitored continuously by mounting the contain-
ers on separate balances. The ¯ows of AHF and HCFC-133a
were regulated by means of needle valves.
2.1. Preparation of catalyst
2.1.1. Co-precipitated Cr2O3/Al2O3 (catalyst A)
Cr(NO3)3Á9H2O (130.8 g) and Al(NO3)3Á9H2O (458.8 g)
were dissolved in 6 l of distilled water and the hydroxides
were precipitated using 1 M NH4OH as described in litera-
ture [15]. The gel obtained was ®ltered, washed free from
nitrate ion, dried overnight in an oven at 1208C, shaped into
3 mm extrudates and calcined at 4008C in nitrogen atmo-
sphere for 6 h. Chemical composition and surface area are
given in Table 1. X-ray diffraction analysis indicates the
amorphous nature of the catalyst except for traces of
g-Al2O3.
The precatalyst (20 g) after charging into the reactor was
®rst treated with nitrogen and then ¯uorinated with AHF at
atmospheric pressure. The ¯uorination of HCFC-133a was
carried out under a pressure of 90 psiÆ5. The product
stream was scrubbed with aqueous KOH, then passed
through a drier and condensed in a dry ice acetone cooled
receiver. During the initial period of reaction, the recovery
of the product is not quantitative due to solubility in aqueous
alkali. However, after saturation steady state is reached and
no loss in material balance was observed within the limits of
experimental error. The composition of the product stream
was estimated by GC(HP) Model 5890 series II using a
12 ftÂ1/8 in. SS column packed with porapak Q, (carrier
gas, He, 30 ml/min. Column temperature: 100±108C/min -
2308C and FID detector). 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.
2.1.2. ZnCl2/Cr2O3/Al2O3 (catalyst B)
Extrudates of catalyst A (60 g) were suspended in a
solution of ZnCl2 (5 g) in 54 ml distilled water. The mixture
was slowly vaporized in vacuum in a rotavapor to dryness.
The solid obtained was dried at 1208C for 12 h and calcined
at 4008C in nitrogen atmosphere for 16 h. The chemical
composition and surface area are given in Table 1. X-ray
diffraction analysis showed the amorphous nature of the
catalyst except for traces of g-Al2O3.
2.1.3. MgCl2/Cr2O3/Al2O3 (catalyst C)
Extrudates of catalyst A (141 g) were suspended in a
solution of MgCl2Á6H2O (6 g) in 110 ml distilled water and
then processed as described in Section 2.1.2 above. The
chemical composition and surface area are given in Table 1.
3. Results and discussion
The reaction of interest in the ¯uorination of 2-chloro-
1,1,1-tri¯uoroethane(HCFC-133a) is the halogen exchange
to give 1,1,1,2-tetra¯uoroethane (HFC-134a). It is known
that this reaction is accompanied by other processes depend-
ing upon the catalyst and reaction conditions. The reaction
leading to HFC-134a and the side products that contribute to
the lowering of selectivity are depicted in Scheme 1.
It has been reported that the conversion of HCFC-133a to
HFC-134a is thermodynamically restricted to not more than
30% [5,8]. The ¯uorination of HCFC-133a using catalyst A
gave 34% conversion but the selectivity for HFC-134a was
only 77% (see Table 2). The low selectivity is due to the
2.1.4. MgO/ZnO/Cr2O3/Al2O3 (catalyst D)
Aq.solutions (10%) were made separately by taking
Mg(NO3)2Á6H2O (6.33 g) Zn(NO3)2Á6H2O (1.82 g),
Cr(NO3)3, 9H2O (23.1 g) and Al(NO3)3Á9H2O (83.44 g)
in distilled water and then mixed together. Then 7 M
NH4OH solution was added drop wise with vigorous stir-
ring, till pH of the solution attain 9 during precipitation. The
co-precipitated metal hydroxides were ®ltered, washed free
from nitrate ion, dried at 1708C for 12 h in an oven, shaped
Table 1
Composition and surface area of catalyst
Precatalyst
Mg (%)
Zn (%)
Cr (%)
Al (%)
Surface area (m2/g)
Cr2O3/Al2O3 (catalyst A)
±
±
17.80
15.60
17.45
16.67
39.17
37.11
38.40
33.33
228
170
198
170
Zn/Cr2O3/Al2O3 (catalyst B)
MgCl2/Cr2O3/Al2O3 (catalyst C)
MgO/ZnO/Cr2O3/Al2O3 (catalyst D)
±
3.40
±
0.50
3.33
2.22

J.M. Rao et al. / Journal of Fluorine Chemistry 95 (1999) 177±180
179
Table 2
Effect of acid strength of the catalyst on the fluorination of HCFC-133a to HFC-134a at 3508C
Precatalyst
Total
acidity
Relative acidity (%)
Conversion of
HCFC-133a (%)
Selectivity for
HFC-134a (%)
200±4008C
400±5008C
Cr2O3/Al2O3 (catalyst A)
597
582
243
290
53
64
59
78
47
36
41
22
34
22
25
11
77
88
78
94
ZnCl2/Cr2O3/Al2O3 (catalyst B)
MgCl2/Cr2O3/Al2O3 (catalyst C)
MgO/ZnO/Cr2O3/Al2O3 (catalyst D)
Mole ratio HF/HCFC-133aˆ6, Pressureˆ90 psiÆ5, W/Fˆ70 g h/g mol.
accompanied by the formation of HFC-143a and HFC-125
which contain three and ®ve halogens, respectively. Their
formation appears to depend on the strong acid sites. The
formation of pentahaloethanes can be suppressed by doping
the co-precipitated Cr2O3/Al2O3 with compounds of Zn or
Mg, thus increasing selectivity to HFC-134a. There is a fall
in conversion after doping with Zn or Mg. One of the factors
responsible for this appears to be the fall in total acidity. The
increase in selectivity is attributed to the relative lowering of
the percentage of strong acid centres.
Scheme 1.
Acknowledgements
formation of HFC-125. Assuming that the presence of
strong acid centres are responsible for the side reaction
giving rise to HFC-125, a study has been made to evaluate
the activity of different catalysts with different acid
strengths.
A set of precatalysts consisting of co-precipitated Cr2O3/
Al2O3 doped with compounds of Zn and/or Mg were
prepared (catalysts B±D) and their total acidity was deter-
mined by TPD of ammonia. Zinc [13,16,17] and magnesium
[18,19,20] compounds were chosen as dopants because of
the reports that they both promote the catalyst activity. The
acidity of these catalysts prepared in the manner described
here decreased after doping with Zinc or Magnesium as
compared to the base catalyst. These were ¯uorinated and
then used for the reaction of HCFC-133a to give HFC-134a.
The results are given in the table.
The work carried out here is funded by DST, TIFAC,
M/s Navin Flourine industries, Surat and M/s Sriram Fibers
Ltd., New Delhi. The authors wish to thank Dr. K.S. Rama
Rao, Dr.(Mrs.)V. Durga Kumari and Dr. M. Subramanyam
of the Catalysis division of IICT for help in the preparation
of catalyst D and for TPD measurements. We are grateful to
Dr. K.V. Raghavan, Director, IICT for his interest in this
work.
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