Examples of catalytic and selective routes for fluorinated building blocks

Article abstract of DOI:10.1021/op500182w

Examples are presented for the catalytic fluorination of chlorinated starting materials in order to produce building blocks or HFCs. The fluorination of CF₃CH₂Cl, of CCl₂=CCl₂, of trichloromethoxylbenzenes and trichloromethoxybenzene involving nucleophilic substitution are reported. In all cases, HF was the fluorinating agent. Depending on the chlorinated substrate and the degree of fluorination required, liquid- or gas-phase processes were involved. Usually, catalysts were SbCl ₅ in liquid phase and chromium oxide in gas phase. In the presence of SbCl₅, at 90 °C under an initial pressure of 10 bar, the fluorination of CCl₂=CCl₂ leads mainly to the formation of CClF₂CHCl₂, and the active catalyst is an antimony mixed halide (SbCl₃F₂). In the same way, the presence of SbCl₅ favored the formation of 1-trifluoromethyl-3- trichloromethylbenzene from bis-1,3-trichloromethylbenzene at low temperature (50 °C) and in the presence of a low amount of HF. Moreover, trichloromethoxybenzene was totally transformed into trifluoromethoxybenzene. At 380 °C and at atmospheric pressure, the transformation of CF ₃CH₂Cl into CF₃CH₂F was favored over chromium oxide-based catalyst modified by zinc (corresponding to a (Zn/Zn + Cr) molar ratio of 0.22).

Full text of DOI:10.1021/op500182w

Article
pubs.acs.org/OPRD
Examples of Catalytic and Selective Routes for Fluorinated Building
Blocks
Sylvette Brunet*
́
Institut de Chimie des Milieux et Materiaux de Poitiers (IC2MP), University of Poitiers, UMR CNRS 7285, Bat B27, 4, rue Michel
Brunet - TSA 51106, 86073 Poitiers Cedex 9, France
ABSTRACT: Examples are presented for the catalytic fluorination of chlorinated starting materials in order to produce building
blocks or HFCs. The fluorination of CF3CH2Cl, of CCl2CCl2, of trichloromethoxylbenzenes and trichloromethoxybenzene
involving nucleophilic substitution are reported. In all cases, HF was the fluorinating agent. Depending on the chlorinated
substrate and the degree of fluorination required, liquid- or gas-phase processes were involved. Usually, catalysts were SbCl5 in
liquid phase and chromium oxide in gas phase. In the presence of SbCl5, at 90 °C under an initial pressure of 10 bar, the
fluorination of CCl2CCl2 leads mainly to the formation of CClF2CHCl2, and the active catalyst is an antimony mixed halide
(SbCl3F2). In the same way, the presence of SbCl5 favored the formation of 1-trifluoromethyl-3-trichloromethylbenzene from bis-
1,3-trichloromethylbenzene at low temperature (50 °C) and in the presence of a low amount of HF. Moreover,
trichloromethoxybenzene was totally transformed into trifluoromethoxybenzene. At 380 °C and at atmospheric pressure, the
transformation of CF3CH2Cl into CF3CH2F was favored over chromium oxide-based catalyst modified by zinc (corresponding to
a (Zn/Zn + Cr) molar ratio of 0.22).
hydrogenolysis.1,2 Two main industrial routes have been
developed: one from perchloroethylene (three reaction steps)
and the other one from trichloroethylene (two reaction steps).
Both pathways involve multistage fluorination processes
1. INTRODUCTION
The manufacture of some hydrofluorocarbons and fluorinated
aromatic rings involves fluorination processes entailing multi-
stage gas-phase and/or liquid-phase reactions in the presence of
including gas and/or liquid phases in the presence of HF.
chlorinated reactants, a catalyst, and anhydrous hydrofluoric
In order to optimize the different steps, the research focused
acid (HF) as the fluorinating agent. The liquid phase reactions
on the identification of:
are usually performed at lower temperatures (<150 °C) rather
(1) the reactions, the mechanisms involved, and the catalytic
sites in order to improve the activity of catalysts used or
the development of new catalysts;
than in the gas phase (up to 380 °C). Liquid-phase processes
are often more economical in terms of energy consumption and
investment and they can show better selectivities towards
making specific HFCs. However, the degree of fluorination
achieved in the liquid phase is usually lower. At the industrial
level, the strategic choice of a route will depend on the access to
raw materials, the complexity of the process (number of steps,
difficulty of the operation), and especially the performance of
the chemistry (conversion and selectivity, life of the catalyst,
quality of the final product, and technological difficulty
(corrosion, etc.)).
Depending on the applications, catalysts used for the
substitution of fluorine for chlorine can be Lewis acids
(SbCl5), metal oxides (Cr2O3), or metal fluorides (MgF2,
BaF2). Our objective was to achieve highly selective catalytic
reactions using hydrogen fluoride and varying the catalyst. As
such conditions require extremes of pressure and temperature,
the development of a good process is important for also safety,
cost, and environmental reasons. This manuscript will describe
our work, with various examples, in optimizing several
fluorination reactions useful for commercial use and viable for
large-scale manufacture by catalytic processes.
(2) different sources of deactivation of the catalyst materials.
Perfluoroethylene can be used as a starting material for
fluorinated compounds for various applications (HCFs and
building blocks).2,3 The fluorination of CCl2 = CCl2, which is
an unreactive molecule, involves the addition of HF and
successive Cl/F exchanges (Scheme 1).4 The main catalytic
system used was composed of SbCl5 as the Lewis acid and HF
as the fluorinating agent. At 90 °C and under 10 bar as initial
pressure, two main families of products were observed:
fluorinated products (CHCl2−CCl2F corresponding to the
addition of HF on the double bond and CHCl2−CClF2
corresponding to Cl/F exchange) and chlorinated products
resulting in the addition of HCl or Cl2 to the double bond. HCl
was produced during Cl/F exchanges and Cl2 during the
reduction of SbV into SbIII. To identify the active species for the
fluorination of perfluoroethylene under our operating con-
ditions, performances of various commercial antimony mixed
halides SbFCl4, SbCl3F2, SbCl2F3, and SbClF4 were measured
for the transformation of the perfluoroethylene. The perform-
2. RESULTS AND DISCUSSION
Special Issue: Fluorine Chemistry 14
Received: June 6, 2014
2.1. Fluorination of Chlorinated Hydrocarbons. Differ-
ent synthetic routes for CF3CH2F, a refrigeration agent, involve
either the addition of HF, a chlorine−fluorine exchange, or
© XXXX American Chemical Society
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Scheme 1. Transformation of perchloroethylene in the
presence of HF-SbCl5 catalytic system (T = 90°C, PN2 = 10
bar)
Scheme 3. Catalytic cycle of the reduction of SbV into SbIII
during the transformation of the perchloroethylene
ances of these antimony mixed halides was compared in Table
1 to the in situ SbCl5 preprepared with HF and to SbCl5
Table 1. Transformation of CCl2CCl2 in the presence of
various mixed halides; comparison of the conversion and the
selectivities and the formation of SbIII (T = 90°C, PN2 = 10
bar)
The fluorination of CF3CH2Cl into CF3CH2F is also studied.
This fluorination is particularly difficult due to the presence of
the CF3 group and requires more drastic conditions, (380 °C
and atmospheric pressure), HF in excess, and chromium oxide-
based catalyst supported over fluorinated alumina.5−7 Under
these operating conditions, the fluorination reaction is the main
reaction (eq 1). However, a dehydrofluorination reaction also
occurs which leads to the formation of CF2CHCl, the main
byproduct from the CF3CH2Cl transformation (eq 2). The
Deacon reaction is also observed (eq 3). Indeed, the presence
of dioxygen which prevents the catalyst deactivation leads to
Cl2 formation by reaction with hydrogen chloride liberated
during the fluorination of CF3CH2Cl into CF3CH2F.8,9 The
presence of oxygen is also necessary to refresh the surface of the
catalyst in order to keep fluorine labile and avoid irreversible
solid fluorination.5 As reported in Figure 1, the presence of zinc
CHCl2− CHCl2−
chlorinated
products
(%)
conversion
(%)
CClF2
(%)
CCl2F
(%)
SbIII
(%)
catalyst
SbCl5
SbCl5
47
57
46
57
38
31
16
12
71
57
prefluorinated
SbCl4F
SbCl3F2
SbCl2F3
SbClF4
58
52
68
70
54
53
70
66
33
37
22
26
13
10
8
72
64
69
40
8
without prefluorination. First, the prefluorination of SbCl5
increases the total conversion and the selectivity towards
CF2ClCHCl. Similar conversion and selectivities are obtained
with prefluorinated SbCl5, SbCl3F2, and SbCl4F. The great
conversion and selectivity towards fluorination can be observed
with the antimony mixed halides which contain a high degree of
fluorination, SbCl2F3 and SbClF4. In every case, a significant
reduction of SbV into SbIII occurs. This reduction corresponds
to a loss of catalyst activity since SbIII is inactive. It could result
from a loss of chlorine from the antimony-active species. The
addition of HF to CCl2CCl2, which involves an electrophilic
or a nucleophilic mechanism, does not seem to reduce SbV
(Scheme 2). The catalyst would be regenerated after the
Scheme 2. Change of the catalyst during the reaction of
addition of HF on CCl2CCl2
Figure 1. Activity of Zn−Cr/AlF3 catalytic systems for the
transformation of CF3CH2Cl, Deacon and dehydrofluorination
reactions versus the Zn/(Zn+Cr) ratio (T = 380 °C, HF/CF3CH2Cl
= 4).
addition of HF, without having suffered any reduction. We
suggest that the reduction of SbV could occur during Cl/F
exchanges as described in the cycle reported in Scheme 3. This
catalytic cycle explains the formation of Cl2 from antimony
mixed halides. The addition of Cl2 to CCl2CCl2 favors the
SbV reduction. It would be difficult to obtain a good
fluorinating activity and a good selectivity towards fluorination
without a reduction of SbV.
in a small amount corresponding to a Zn/Zn+Cr ratio of 0.22
promoted the fluorination reaction and decreased the
secondary reactions. In this case, the selectivity towards
fluorination calculated as the ratio between the activity for
the fluorination reaction and the global activity is multiplied by
2 (Table 2). This is due on one hand to the modification of the
strength of Lewis acidity of the chromium atoms (the active
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Table 2. Selectivity towards fluorination versus Zn/(Zn+Cr)
ratio (T = 380°C, HF/CF3CH2Cl = 4)
Table 3. Transformation of trichloromethoxybenzene (Pi =
10 bar, T = 50°C, time = 1 h, 0.125 mol, HF = 0.5 mol):
Effect of the amount of SbCl5
Zn/(Zn + Cr) atom
1
0.50
0.20
0.22
0.59
0
a
b
c
fluorination selectivity
−
0.26
SbCl5
Conversion (TCMB)
CDFMB
TFMB
(mmol)
(mol %)
(mol %)
(mol %)
0
100
100
100
100
100
100
98
49
7
2
51
site) and on the other hand to an increase of metal dispersion
on the surface of the support.10,11
Main reaction:
0.196
0.393
0.589
0.786
2.5
93
9
91
0
100
100
CFCH2C1 + HF ⇌ CFCH2F + HC1 Fluorination
(1)
3
3
0
Secondary reactions:
a
Conversion: Amount of trichloromethoxybenzene transformed.
CDFMB: (chlorodifluoromethoxy)benzene. TFMB: trifluorome-
b
c
CFCH2C1 ⇌ CF = CHC1 + HF Dehydrofluorination
3
2
thoxybenzene.
(2)
O2 + 4HC1 ⇌ 2C12 + 2H2O Deacon reaction
(3)
All catalysts with an oxidation state of +V (SbCl5, SbF3Cl2,
SbF5, TaCl5, and NbCl5) were able to effect Cl/F exchanges as
well to achieve selectively to form PF30. However, the most
efficient catalyst at lower temperature remains SbCl5. As
mentioned above, this activity was linked to their strong Lewis
acidity and their ability to form complexes to obtain good
nucleophilic systems. At moderate temperatures, the prefer-
ential formation of PF10, PF11, PF21, and PF22 corresponds to
the formation of the most stable intermediate carbocations. The
kinetics therefore governs the formation of these fluorochem-
icals. In contrast, at higher temperature, a redistribution of
fluorine or chlorine can take place, resulting in scrambling of
the fluorine/chlorine distribution. Low energy values of
enthalpies of formation calculated for PFij confirm these
results. This redistribution leads to the formation of PF20, PF30,
and PF31.
2.2. Fluorination of Chlorinated Aromatics. Fluorina-
tion of a model compound, trichloroanisole, was studied in
order to determine the general conditions suitable for the
selective synthesis of aromatic compounds with an −OCF3
group (Scheme 4). The reaction is very fast: only 1 h is
required to obtain a total conversion of the substrate. However,
the presence of a Lewis acid with an oxidation state of V
(SbCl5) is needed to activate the last Cl/F exchange
corresponding to the formation of trifluoromethoxybenzene
(Table 3). Indeed, after 1 h at 50 °C in the presence of the
SbCl5-HF system with a stoichiometric HF amount, the
trifluoromethoxybenzene was prepared with a selectivity of
100%.12
Furthermore, Cl/F exchanges can be controlled by the
presence of a solvent. For example, dioxane in combination
with HF leads to systems for the selective monofluorination of
trichloroanisole. This system is comparable to that of the
common fluorinating agent:13 HF-pyridine system without the
disadvantages of the latter, i.e. excessive corrosion of industrial
equipment. Other base-HF associations are also known, and we
have also shown that this could be extended to other substrates
such as chlorinated trichloromethylbenzenes. In fact, it is
possible to selectively prepare the products of mono-, di-, or
trifluorination.14,15
Alternatively, the fluorination of bis-1,3-trichloromethylben-
zene (PF00) is much more complex. The desired product is 1,3-
trichloro-trifluoromethylbenzene (PF30), a useful building
block16 (Scheme 5). It has the advantage of possessing the
CF3 moeity while retaining a highly reactive −CCl3 group.
The conversion to PF30 strongly depends on the operating
conditions.17 In the presence of HF alone, the fluorination is
carried out systematically on both trichloromethyl groups,
obtaining PF11, PF21, and PF22 (Figure 2). A slight excess of HF
(8 equiv) allows obtaining products of greater degrees of
fluorination (PF32 and PF22). Adding SbCl5 in small amounts (2
mol %) at 50 °C, fluorination of PF00 is very fast (within 1 h)
and complete to form PF33. The formation of the desired
product PF30 with good selectivity is obtained with 2 equiv of
HF at 150 °C (Figure 2) or in the presence of SbCl5 (Figure 3)
at 50 °C.
3. CONCLUSION
Whatever the chlorinated starting substrates, it can be possible
to determine the best operating conditions and catalyst in order
to prepare selectively the fluorinated product desired.
At 90 °C under an initial pressure of 10 bar, the fluorination
of CCl2CCl2 leads mainly to the formation of CClF2CHCl2,
and the active catalyst was an antimony mixed halide (SbCl3F2).
The addition of HF on the double bond and Cl/F exchanges
were involved. However, a reduction of SbV into SbIII was
observed during Cl/F exchanges which decreased the selectivity
towards fluorination. Trifluoromethoxybenzene could be
produced from trichloromethoxybenzene in the presence of
HF in equimolar amount and SbCl5 which activated the last Cl/
F exchange. In the same way, 1-trifluoromethyl-3-trichlorome-
thylbenzene could be prepared from bis-1,3-trichloromethyl-
benzene at low temperature (50 °C) in the presence of a low
amount of HF and SbCl5. The presence of a catalyst such as
SbCl5 reduced the temperature to obtain 1-trifluoromethyl-3-
trichloromethylbenzene and offers new opportunity for the
fluorination of more complex and sensitive chlorinated
substrates.
The modification of the strength of the Lewis acidity of the
active sites by the presence of zinc with a (Zn/Zn+Cr) molar
Scheme 4. Transformation of trichloroanisole in the presence of HF-SbCl5 catalytic system
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Scheme 5. Transformation of bis-1,3-trichloromethylbenzene (PF00)
of the trichloromethylbenzene) were carried out in a 100 mL
Teflon autoclave in liquid HF under an initial pressure of 10
bar.
The transformation of the perchloroethylene (PCE) was
performed at 90 °C.4 The catalyst was prepared, prior to
reacting PCE with HF. For this, SbCl5 (0.027 mol) was reacted
with HF. After 1 h, the autoclave was cooled down and vented
with dry dinitrogen in order to eliminate the HCl and the HF
in excess. Perchloroethylene (0.25 mol) and HF (1.3 mol) were
then added. The reaction was carried out for 4 h at 90 °C under
10 bar of dinitrogen. The contents were quenched with 20 mL
6 M H2SO4. The organic phase was dried and analyzed by
temperature-programmed GC with a BP1 column from 40 to
150 °C. The products were identified by GC−MS. Since the
reaction was accompanied by a partial reduction of SbV into
SbIII, the amounts of these two species were determined by
titration of the aqueous phase. Both Sb species were measured
by atomic adsorption. The amount of SbIII was determined by
oxidation with a KMnO4 solution. In another series of
experiments, the catalyst SbCl5 or antimony mixed halides,
HF (1.35 mol) and perchloroethylene (0.25 mol) were added
sequentially, and the mixture was heated at 90 °C. The reaction
was carried out as mentioned above. The different antimony
mixed halides SbFCl4, SbCl3F2, SbCl2F3, and SbClF4 were
purchased at the Ozarvi-Mahoning, an Elf Atochem North
America, Subsidiary, with a purity of 99.5%.
Figure 2. Transformation of the bis-1,3-trichloromethylbenzene. Effect
of the temperature on the formation of the various fluorinated
intermediates PFij in the presence of HF alone. (T = 50−200 °C, PF00
= 0.125 mol, HF = 0.25 mol, HF/PF00 = 2/1).
The transformation of the bis-1,3-trichloromethylbenzene
was performed at the desired temperature (from 50 to 200 °C)
and under autogene pressure.17 At the end of the reaction, the
autoclave was cooled down and vented with dry dinitrogen in
order to eliminate the HCl and the unreacted HF. The contents
were quenched with 30 mL of water/decane mixture (50/50),
using a 316 L stainless steel tank. After extraction with decane,
the organic phase was dried with MgSO4 and analyzed by GC.
The fluorinated products and the chlorinated reactant were
quantified by gas chromatography using an internal standard
quantification method with decane as the internal standard.
The yield corresponds to the mol % of the bis-1,3-
trichloromethylbenzene transformed into various products
(byproducts included). Yields of 100% were not attained,
because of the uncertainty of the experiment. The conversion of
the bis-1,3-trichloromethylbenzene represented the amount
transformed into fluorinated products. The chromatograph was
a Varian 3800 equipped with 30m VF-5 ms capillary column
(Varian) with a temperature program from 50 to 300 °C. The
Figure 3. Transformation of bis-1,3-trichloromethylbenzene. Effect of
the temperature on the formation of the various fluorinated
intermediates PFij in the presence of SbCl5. (T = 50−200 °C, PF00
= 0.125 mol, HF = 0.25, SbCl5 = 1.25 mmol, HF/SbCl5/PF00 = 2/
0.01/1).
ratio of 0.22 favored the fluorination of CF3CH2Cl to produce
CF3CH2F.
4. EXPERIMENTAL SECTION
All the experiments in liquid phase (transformation of the
perchloroethylene, of the bis-1,3-trichloromethylbenzene, and
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various products were identified by coupling GC−MS and by
comparison with commercial products.
The various activity was calculated as defined: A (mmol/h/g)
= F*y/weight of catalyst with F: flow of the reactant (mmol/h),
y: yield of each product (mol %). The activity in mmol/h/
mmol metal was obtained by dividing the activity by the total
amount of metal.
The trichloromethoxylbenzene18 (0.125 mol) and the
catalyst (2.5 mmol) were introduced into the autoclave.12
The reaction then took place with continuous stirring, at the 50
°C during 1 h. At the end of the reaction, the autoclave was
cooled down and vented with dry dinitrogen in order to
eliminate the HCl and the unreacted HF. The content was
quenched with 30 mL of water/decane mixture (50/50), using
a stainless steel tank. After extraction with decane, the organic
phase was dried with MgSO4 and analysed by GC. The
fluorinated products and the chlorinated reactant were
quantified by gas chromatography using an internal standard
quantification method with decane as the internal standard.
The yield corresponds to the mol % of trichloromethox-
ylbenzene transformed into chlorodifluoromethoxybenzene
plus trifluoromethoxybenzene. The chromatograph was a
Varian 3800 equipped with 25 m BP1 capillary column
(SGE) with a temperature program from 50 to 240 °C.
The transformation of CF3CH2Cl into CF3CH2F19 was
carried out under atmospheric pressure in a fixed bed reactor at
380 °C.10,11
AUTHOR INFORMATION
Corresponding Author
*E-mail: sylvette.brunet@univ-poitiers.fr
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
My thanks to Arkema (Eric Lacroix) and Solvay (Franco
Metz) companies for their financial support and many fruitful
■
̧
is
discussions.
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