Trifluoroacetic acid promoted hydration of styrene catalyzed by sulfonic resins: Comparison of the reactivity of styrene, n-hexene and cyclohexene

Article abstract of DOI:10.1016/j.apcata.2018.11.014

Alcohol production by olefin hydration is an important reaction in the modern intermediate chemistry. The use of trifluoroacetic acid (TFA) could be a way to increase alcohol productivity by addition of the TFA to the double bond forming the corresponding ester. The product obtained by addition is then hydrolyzed to alcohol by sulfonic resins. The study of the overall multiphase equilibrium aqueous TFA solution/olefin ester cyclohexene is carried out together with the initial rate of reaction for cyclohexene hydration. Reaction profiles and a likely reaction path is also given. The influence of the operative variable on both equilibria and initial reaction rate are studied in the range of temperature typical of the sulfonic resin as catalysts (383–413 K). The use of aqueous TFA allows fast reaction and high conversion as well as an easily separable aqueous system compared with other organic acid.

Full text of DOI:10.1016/j.apcata.2018.11.014

Accepted Manuscript
Title: Trifluoroacetic acid promoted hydration of styrene
catalyzed by sulfonic resins: comparison of the reactivity of
styrene, n-hexene and cyclohexene
Authors: Edoardo Bianchini, Luca Pietrobon, Lucio Ronchin,
Claudio Tortato, Andrea Vavasori
PII:
S0926-860X(18)30569-6
DOI:
Reference:
https://doi.org/10.1016/j.apcata.2018.11.014
APCATA 16883
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
19 September 2018
8 November 2018
18 November 2018
Please cite this article as: Bianchini E, Pietrobon L, Ronchin L, Tortato C, Vavasori
A, Trifluoroacetic acid promoted hydration of styrene catalyzed by sulfonic resins:
comparison of the reactivity of styrene, n-hexene and cyclohexene, Applied Catalysis
A, General (2018), https://doi.org/10.1016/j.apcata.2018.11.014
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1
Trifluoroacetic acid promoted hydration of styrene catalyzed by sulfonic resins:
comparison of the reactivity of styrene, n-hexene and cyclohexene
Edoardo Bianchini, Luca Pietrobon, Lucio Ronchin, Claudio Tortato and Andrea Vavasori
Department of Molecular Sciences and Nanosystems, University Ca’ Foscari of Venice, Campus
Scientico Via Torino 155 30170 Mestre Venezia Italy
Graphical abstract
Cyclohexene hydration in TFA catalyzed by sulfonic resins
Research highlights
 Trifluoroacetic acid acts as promoter and catalyst in olefin hydration.
 Trifluoroacetic acid favors the displacement of equilibria to alcohol.
 Trifluoroacetic acid acts as initiator in polystyrene polymerization and in parallel promotes
styrene hydration.
 Trifluoroacetic acid promotes cyclohexene hydration to cyclohexanol and can be easily
recovered.
 The reactions can be carried out in the presence of a sulfonic resin as a catalyst, which increases
the overall reaction rate.
To whom correspondence should be addressed. E-mail: ronchin@unive.it

2
Abstract
Alcohol production by olefin hydration is an important reaction in the modern intermediate chemistry.
The use of trifluoroacetic acid (TFA) could be a way to increase alcohol productivity by addition of the
TFA to the double bond forming the corresponding ester. The product obtained by addition is then
hydrolyzed to alcohol by sulfonic resins. The study of the overall multiphase equilibrium aqueous TFA
solution/olefin ester cyclohexene is carried out together with the initial rate of reaction for cyclohexene
hydration. Reaction profiles and a likely reaction path is also given. The influence of the operative
variable on both equilibria and initial reaction rate are studied in the range of temperature typical of the
sulfonic resin as catalysts (383-413K). The use of aqueous TFA allows fast reaction and high
conversion as well as an easily separable aqueous system compared with other organic acid.
Keywords: trifluoroacetic acid, acid catalysis, olefin hydration, cyclohexanol.
1. Introduction
Hydration of olefins is an apparently trivial reaction; however, its practical implementation is seldom
quite cumbersome. The acid catalysis is able to promote several reactions in the presence of olefins as
substrate, such as hydration, condensation polymerization and so on [1-7]. The key intermediate in the
acid catalytic reaction of olefin, it is an attack by nucleophilic molecules to a positively polarized
species, in some cases a carbonium ion. This moiety may undergo an attack of nucleophilic molecules,
such as water, olefin, alcohol and so on, all these reaction can concur to the various products and, if
present simultaneously, they induce the selectivity to one product by following the reactivity of the
nucleophile [8, 9]. In the case of olefin hydration, the presence of rather concentrated aqueous mineral
acid (70%) allows high yields to alcohol. The reaction proceeds via an alkyl sulfate intermediate, which
is hydrolyzed after dilution [10]. However, such a reaction is neither environmentally friendly, nor
sustainable, because of its intrinsic necessity of using large amount of aqueous acid, which at the end
must be disposed and/or concentrated. Other hydration methods are in any case complex and require
solid acid catalysts to be employed in gas phase reaction such as those of ethene and propene to give
ethanol and isopropanol, respectively [9, 11, 12]. Liquid/Liquid biphasic reactions in the presence of an
aqueous and an organic phase are well known reactions employed in several industrial processes,
nitrations, hydrogenations, carbonylations and hydroformylation [13-17]. The presence of two phases

3
may create problems of mass and/or heat transfer because of the necessity of having reagents in the
same phase to react, that can be attained by forming an emulsion. This allows to the system a sufficient
exchange to have a kinetic of reaction useful for practical applications. Under this light, the rate of
agitation is of paramount importance in multiphase processes [18, 19]. For instance, toluene nitration
occurs in biphasic aqueous/organic environment, and the formation of non-homogeneous zones may
cause risks of heat accumulation, thus causing dangerous hot-spot [17]. Other examples of multiphase
liquid reaction is the selective hydrogenation of benzene to cyclohexene, in this case, the presence of an
aqueous phase may help reaction regulation, both from a thermal point of view as well as for the
selectivity control [14-16, 20]. In carbonylation reactions, the presence of an aqueous phase allows
some interesting features especially in the co-polymerization of the carbon monoxide and ethene to
polyketone [16]. As a matter of fact, both the limitation of organic solvent utilization, or the
achievement of high molecular weight polymers are important goals that the presence of an aqueous
phase may concur to accomplish [16]. The presence of acids or salts, in some cases, promotes reactions
favoring a better interaction between reactants and catalyst, or by inducing desorption of products from
catalyst surface [14, 20, 21]. For instance, in the hydrogenation of benzene to cyclohexene, in the
presence of ZnSO4 in the aqueous phase increases the selectivity to cyclohexene, by reducing the
hydrogenation activity of the Ru, which is the actual hydrogenation catalyst [14, 20]. It is noteworthy
that the ZnSO4 action is obtained by the presence of the aqueous phase, which diminish the hydrogen
availability on catalyst surface [14, 20]. The contact of the two liquid phases is guaranteed by strong
agitation but no stable emulsion is formed. On the contrary, the use of a surfactant achieves stable
micelles formation, it allows to take place carbonylations and hydroformylations in stable emulsions,
by using either an organic solvent-soluble catalyst or a water-soluble one [22].
Hydration of olefins, from an industrial point of view, is strictly connected with alcohol production. For
instance, an important intermediate for polymers and chemicals, is cyclohexanol, which is usually
obtained by phenol hydrogenation or by cyclohexane oxidation with nitric acid forming the so called
K/A oil (cyclohexanone and cyclohexanol mixture) [23]. These routes have some drawbacks from both
economic and environmental point of view, that is phenol is a valuable intermediate (a greatly
requested intermediate), while the oxidation with nitric acid of cyclohexane is a stoichiometric non-
environmentally friendly process [23]. Under this light, cyclohexene hydration could be an interesting
alternative for cyclohexanol production only if cyclohexene is an intermediate obtained from the
selective hydrogenation of benzene, which is available in large amount from the petrol chemistry [24].

4
Among alcohols, interesting intermediates are phenyl carbinols, these compounds are commonly found
in nature as fragrances but are also synthetized starting from bulk chemicals [25]. As already pointed
out, phenyl carbinols and particularly 1-phenylethanol is interesting because of its several applications
in the field of fine chemistry. The acetophenone reduction is a valuable route for obtaining this
compound, and the reaction occurs under mild conditions in almost quantitative yields [26]. However,
this reaction is industrially sustainable only if acetophenone is easily available, for instance as
byproducts in phenol Hock process [27]. On the contrary, the hydration of styrene could be an
interesting route being such a compound a commodity available in very large amounts (the annual
production of styrene in the world is close to 20 millions of tons) [28].
An important feature of olefin hydration is its reversibility, then equilibrium influences conversion, for
this reason the method of synthesis will determine the overall yield to the alcohol [29-33]. Gas phase
hydration is extensively used for ethanol synthesis being olefin easily separated and recycled, on the
contrary liquid phase reaction is normally used for heavier olefin such as cyclohexanol [34]. As regard
cyclohexene hydration, it must be taken into account that the industrial synthesis of cyclohexene (the
selective hydrogenation of benzene to cyclohexene [23]) gives a mixture cyclohexane, cyclohexene,
and benzene and their separation is not easy and require specific extraction procedure [35]. The
separation of the mixture, the mixtures of cyclohexene and cyclohexane could be achieved by
addition/esterification with formic acid giving cyclohexyl formate, and then the saponification of the
ester gives cyclohexanol [32, 36].
Styrene hydration is carried out, at high acid concentration (40-50% HClO4), at low temperature (283-
298K) and at concentration of 10-4 - 10-5 mol L-1, in order to avoid the formation of polystyrene [37].
At such conditions no industrial application could be developed. However, starting from another point
of view, and taking into account the large production and the various type of polystyrene, a small
coproduction (in percentage term) of the alcohol could be of industrial interest if obtained as byproduct
of the polymerization. This could occur if the polymer came from a reaction carried out in emulsion
catalyzed by an acid or an acidic cationic initiator.
Many hydration methods in gas and liquid phase catalyzed by solid acid is extensively reported in
literature and the sulfonated resins are important materials for these reactions. [29-34, 38-41]. For
instance, ethers and esters are commonly obtained by using these type of materials as catalysts, such as
in the synthesis of methyl tert-butyl ether (MTBE), or in transesterification of vegetal oil in biodiesel
production. On the contrary, oligomerization of olefin is not generally achieved with this type of
catalysts [40, 41]. The mechanism of the catalysis and the proton exchange, however, are not yet

5
completely understood for these reactions [42]. In fact, O-alkylation of phenol with cyclohexene shows
the need of a protic molecule thus allowing the proton exchange between the resin and the olefin in
order to form a surface polarized adduct, which is responsible of the catalysis. As a matter of fact, the
absence of phenol show negligible interaction between cyclohexene and the sulfonated resin, thus
forming on the surface a negligible amount of electrophile, whose concentration does not allow
oligomerization [42].
This work deals to some study on hydration reactions catalyzed by solid acid and promoted by
trifluoroacetic acid (TFA) in aqueous solvent. The reaction is carried in biphasic condition where the
olefin forms a phase and the water-TFA solution another one. Olefin results as a dispersion in the
aqueous phase (due to agitation) and their reactivity is investigated by using several solid acid and three
olefin, that is n-hexene, cyclohexene and styrene. In the case of styrene, the polymerization initiated by
TFA occurs in high yields, but (4-8%) of 1-phenyl-ethanol is formed, too. The addition of TFA to the
olefin and formation of the corresponding ester promotes the formation of the alcohol, thus increasing
its yield compared with the reactions carried out in the absence of the TFA. The use of various
sulfonated resins is investigated because of its ability to interact with oxygenated compound favoring
hydration and saponification reaction.
2. Experimental
2.1 Materials and instruments
All solvents and products were employed as received without further purification, or by following the
usual purification procedures for spectrophotometric analysis.
All the analysis were carried out by GC and GC-MS (Agilent 7890 A, equipped with HP-5 30 m long
I.D. 250 µm, film 0.25 µm, coupled to a Agilent 5975 C mass spectrometer or to a flame ionization
detector). Some reactions are analyzed also by HPLC (Perkin Elmer LC 250 pump coupled to a diode
array detector 235C, equipped with a Merck licrosphere 100 C18 column with water acetonitrile
mixtures as eluent) in order to verify if present in reaction mixture temperature sensitive intermediates
or products.
1H NMR, 13C, and 19F spectra were recorded at 400MHz, 100MHz, 376MHz, repectively, using CDCl3
as a solvent on a Brucker Avance II nuclear magnetic resonance spectrometer.
All solids employed as catalyst were commercial products and used after desiccation for at least 20h at
383K in a ventilated oven.

6
Tab. 1. Main characteristic of solid catalysts employed in hydration reactions
Catalyst
Surface area
Average pore diameter
Total exchange H+
(m2 g-1)
(nm)
180
220
/
(meq g-1)
Amberlyst 15
Amberlyst 36
Amberlite IRC 50
Dowex D50
HZSM 5
36
24
/
4.9
5.6
10
10
280
240
2
4.2
0.2
2.2. Reaction set-up
All reactions were carried out in an isotherm multi-reactor system equipped with eight aisi 316
autoclave of 8 mL of inner volume. In each autoclave reactants and catalyst are loaded into a glass
insert (a modified test tube) closed with a pierced polytetrafluoroethylene plug in order to avoid liquid
dispersion outside the insert. The electric heating of an aluminum block ensures that the autoclave is
maintained at constant temperature within 0.5 K, the control were achieved by PID regulator (CAL
9500P). Each autoclave could be purged with nitrogen, the maximal temperature and operative pressure
are 523 K and 30 bar, respectively. The mixing of the biphasic system were achieved with a magnetic
bar rotating at 5-20 Hz driven by a magnetic stirrer. The homogeneity of the emulsion were visually
tested, by observing phase separation at a rate of rotation below 10 Hz.
2.3. Reactions methodology
In a typical, experiment a magnetic bar, 0.26-2.6 mmol of TFA, 2 mL of water, 2.6 mmol of substrate
and 40mg of solid catalyst (if used) are added into the glass insert. The autoclave, after the loading of
the stoppered vial, which fits almost exactly the autoclave volume, is closed, outgassed and filled with
nitrogen at 1 bar. The aluminum block is now heated at the reaction temperature (383-403K), when the
system heater is stabilized, all autoclaves are inserted in its seat. After 5 minutes without agitation, in
order to allow temperature equilibration, agitation was turn on and the reaction starts. After the definite
time the selected autoclave (or autoclaves) was cooled in a water bath and opened. The test tube was
removed and the mixture diluted with 6.5 g of CH2Cl2, then 40 mg of tetradecane, as internal standard,
was added to the slurry and stirred for 2 minutes. Finally, solid catalyst was filtered and the organic
phase separated, from the mixtures, and dried with anhydrous Na2SO4.

7
2.4. 1-Phenylethanol, cyclohexanol and hexanols synthesis
The reactions are carried out in multiphase systems being styrene, cyclohexene and n-hexene not
soluble in water, in additions, there could be the presence of a heterogeneous solid catalyst. In a typical
reaction 2.62 mmol of substrate were placed into the reactor together with 1-3 mL of water, 0.87 mmol
of promoter and 40 mg of a solid catalyst (if present). Tab. 2 shows the list of all catalytic systems.
Tab. 2. List of substrate and of the catalytic system employed
substrate
Styrene
Promoter
H2SO4
Solid catalyst
Amberlyst 15
Amberlyst 36
Amberlite IRC 50
Dowex D50
Cyclohexene
n-hexene
CF3COOH
CH3COOH
All possible combinations of substrate, promoter and heterogeneous catalysts were tested in order to
verify what were that gave the best results. As a matter of fact, the typical reaction conditions are: 4h of
reaction, 393K, pressure are autogenous pressure of water, TFA and substrate, about 3 bar.
2.5. Analysis
GC and GC-MS analysis were carried by using GC or a GC-MS instruments with the following
conditions: temperature program, 5 minutes isotherm at 333K, 20 K min-1 to 523K isotherm at such
temperature for 5 minutes; nitrogen flow 1 mL min-1, split ratio 10/1. Quantification of the products are
carried out if possible by using calibration with standard (styrene and 1-phenylethanol), for dimers
selectivities are calculated by using the same response factor of styrene and oxygenated isomers
(mainly dimers of the 1-phenylethanol) are used the same response factor of the alcohol.
3. Result and discussion
The study is composed of two parts where the first one is a screening of aqueous acid, solid catalyst,
and combination of them in the liquid phase hydration of styrene and cyclohexene as examples of
olefins whose product of hydration is an interesting product from industrial point of view. The second
part deals to the study of the rate of cyclohexene hydration in the presence of an aqueous solution of

8
TFA and a macroreticular styrene di-vinylbenzene sulfonated resins (Amberlyst 36) as a heterogeneous
catalyst. All reactions are carried out in a large amount of water, and the concentration of the acid is
less than 0.3-1.6 mol L-1, at such a concentration the acid is solvated and the aqueous phase can be
considered as an aqueous solution [43].
3.1. Styrene hydration
In table 3 we can observe, some results of styrene hydration promoted by various liquid acid. It is
noteworthy that the absence of acid gives in very high yield polystyrene, small amount of unreacted
styrene and traces of oligomers due to the thermal polymerization of styrene [45]. In the presence of
aqueous acid only TFA is effective in the promotion of styrene hydration, thus allowing a selectivity of
10% after 4h of reaction. The composition of whole addition products is a mixture of styrene dimers
and of the bis-(1-phenylethyl) ether, the latter obtained by condensation of 1-phenylethanol or by its
addition to styrene, products of TFA esterification are not detected.
Tab.3. Styrene hydration promoted by homogeneous acid systems. Run conditions: T 393 K, P
autogenous pressure of the system, styrene, 2.62 mmol water 2 mL, acid 1 mmol, time of reaction 4h.
Entry
Acid
Conversion
(%)
1-phenylethanol
(%)a
Styrene
Condensation Polymer
promoter
dimers
products
(solid)
(%)a, c
95
(%)a
(%)a, b
1
2
3
4
/
95
87
87
85
/
10
1
5
/
TFA
7
3
9
1
80
H2SO4
CH3COOH
30
4
60
/
95
a) Selectivity. b) Oxygenated compounds observed by GC-MS. c) Calculated by weighing the solid.
These results are a clear indication of the high reactivity of styrene, which polymerizes in high yields,
as can be observed in Tab.3 entry 1. The presence of organic acids TFA and acetic acid decreases the
amount of solid polymer recovered at the end of the reaction. Such an evidence could be due to a
partial dissolution of styrene in water favored by the presence of the organic acid. Hydration, however,
it is likely to occur by catalytic step that involves the protonation of the olefin and the subsequent
nucleophilic attack, which could be, in turn, either of another olefin molecule or one of water. The
oligomerization of the substrate is clearly favored in the case of sulfuric acid as promoter thus causing
a low selectivity for the hydration reaction. On the contrary, TFA allows a non-negligible selectivity to

9
the alcohol thus suggesting a different reaction path in the presence of this acid. The reaction selectivity
in the case of the TFA could be driven by the addition of TFA to the double bond, with a consequent
hydrolysis of the TFA-Styrene ester giving 1-phenyl-ethanol [30, 31]. In detail, the strongest acid that
is sulfuric acid, it immediately favors the oligomerization. In contrast, TFA is able to activate the
substrate also for the subsequent hydration reaction to 1-phenylethanol, which was gained with a
selectivity of 10% (entry 2). The main product was howsoever polystyrene. Finally, the use of acetic
acid as a promoter do not activate the substrate for the hydration and only the polymerization occurs
even though in lesser extent with respect to sulfuric acid.
Tab. 4 shows the hydration of styrene catalyzed by some solid acid. It is evident that the styrene di-
vinyl benzene sulfonated resins are those that give the more interesting results. By the comparison of
Tab. 3 with Tab. 4 entry 1-5, it is clear the lower tendency of the solid catalyst to promote styrene
polymerization, but does not suppress oligomers formation. This suggests that the formation of a
surface ionic intermediate which is easily attacked by the nucleophile which could be both water or the
styrene itself. In such a conditions, polymerization at high molecular weight is not favored because of
steric hindrance on the surface of the resins. In fact, the sulfonic group of the resin “protonates” (it is
likely to be a surface interaction rather than a real protonation) forming a surface ionic couple, which is
actually the reacting specie. The reduction of the degree of freedom strongly influences the attack of
the nucleophile, especially if a growing chain hinders the center of reaction on the surface.
Tab. 4 entry 6-16 shows the results of styrene hydration in mixed catalysis using both solid acid and
liquid acid as a promoter. The presence of TFA increases the selectivity towards 1-phenyethanol but
the main products is in any case the polymer. As solid catalysts Amberlyst 15 and 36 give an evident
improvement on the selectivity towards 1-phenyethanol, in agreement with the reactivity of Tab. 3, On
the contrary, the mixed system solid acid/H2SO4 shows a negligible increase of the selectivity toward
the alcohol, suggesting a preferential way for the polymerization.
Hydration of the styrene double bond appears to be a cumbersome problem because of its easy parallel
polymerization reaction, which occur also by acid catalysis (radical polymerization it is likely to take
place in parallel), a further problem is the reversibility of the hydration, which could give back styrene
in the presence of acids. In fact, the reaction of 1-phenylethanol with TFA (entry 16) in aqueous media
in the presence of Amberlyst 36 as a solid acid catalyst gives a products distribution similar to that
obtained starting from styrene as a substrate, with traces of polystyrene.

10
Tab. 4. Styrene hydration promoted by homogeneous acid systems and catalyzed by solid acid. Run
conditions: T 393 K, P autogenous pressure of the system, styrene 2.62 mmol, water 2 mL, acid 1
mmol, catalyst 40mg, and time of reaction 4h.
Entry
Catalytic system
Conversion
(%)
1-phenylethanol
(%)a
dimers
(%)a
Condensation
Polymer
(solid)a, c
(%)
80
productsa, b
(%)
2
1
2
Amberlyst 15
Amberlyst 36
Amberlite 50
HZSM5
88
87
87
89
88
65
69
67
70
66
92
91
89
94
93
50e
7
7
3
3
1
81
3
/
12
2
2
86
4
/
1
97
5
D50
4
6
2
84
6
TFA/Amb15
TFA/Amb36
TFA/Amb50
TFA/HZSM5
TFA/ D50
22
23
16
/
20
17
13
25
10
17
40
20
19
26
10
6
52
7
6
54
8
6
65
9
7
78
10
11
12
13
14
15
16d
8
4
78
H2SO4/Amb15
H2SO4/Amb36
H2SO4/Amb50
H2SO4/HZSM5
H2SO4/D50
H2SO4/Amb36
3
15
14
16
15
7
65
3
67
/
64
/
66
/
67
50
10
traces
a) Selectivity. b) Oxygenated compounds observed by GC-MS. c) Calculated by weighing the solid. d)
The substrate is 1-phenylethanol. e) Styrene is the main product.
It is noteworthy the different composition of the polystyrene obtained in the presence of TFA because
1H and 19F NMR shows the presence of fluorinated ester terminal group (see more details in the
supplementary materials), the study of the polystyrene obtained in the presence of TFA, even though
interesting, is beyond the aim of the present work.
3.2 Cyclohexene hydration
Cyclohexene hydration appears to be a problem easier than that of styrene, though, the reaction is not
even and an accurate selection of the catalytic system needs. Typically, hydration is acid catalyzed and
we test those employed in styrene hydration. In this case, the olefin does not polymerize as easy as
styrene, and the main products are cyclohexanol and cyclohexyl trifluoroacetate (in the presence of

11
TFA), but, in the case of homogeneous promoters the amounts of condensation products are not
negligible. Tab.s 5-7 shows some tests on the various catalytic systems. Liquid/liquid systems seem to
be more effective in the multiphase batch hydrations of cyclohexene than liquid/solid ones, anyway,
Amberlyst 15 and 36 show results close to that of the liquids systems. In addition, there is a neat
decrease of the condensation byproducts whose presence in several percent of concentration may
negatively affect a process.
Tab. 5. Cyclohexene hydration promoted by liquid acids. Run conditions: T 393 K, P autogenous
pressure of the system, cyclohexene 2.62 mmol, water 2 mL, acid 1 mmol, time of reaction 4h.
Entry
Acid promoter
Conversion
(%)
Cyclohexanol
Addition
Condensation
selectivity
product
products
(%)
48
92
/
(%) a
(%) b
1
2
3
TFA
H2SO4
12
12
2
36
/
8
8
1
CH3COOH
1
a) Selectivity cyclohexyl trifluoroacetate. b) Calculated by difference.
Scheme 1. Cyclohexene hydration paths.
The reaction carried out in the presence of TFA shows the formation of addition products in 36% of
selectivity. This product is of interest because could be converted to cyclohexanol by hydrolysis. The

12
reactions occurring in this multiphase system are all equilibrium reactions as reported in scheme 1 and
confirmed by entry 11 of Tab 7 in which the hydrolysis of cyclohexyl trifluoroacetate gives both
cyclohexene and cyclohexanol as products.
Tab. 6. Cyclohexene hydration catalyzed by solid acids. Run conditions: T 393 K, P autogenous
pressure of the system, cyclohexene 2.62 mmol, water 2 mL, catalyst 40mg, time of reaction 4h.
Entry
Catalyst
Conversion
(%)a
Cyclohexanol
selectivity
(%)
99
1
2
3
4
5
Amb15
Amb36
Amb50
HZSM5
D 50
7
8
1
1
3
99
99
99
99
a) There are traces of condensation products
The best results in cyclohexanol yield are those obtained by the mixed liquid solid systems with
sulfuric acid as promoter (entries 6-10). The main drawback of the system is the difficulty of achieving
higher cyclohexanol yield by using reactive distillation equipment. For this reason this method with
sulfuric appear to be not practical, on the contrary, a methodology based on formic acid has been
proposed for the hydration of cyclohexene because of its possibility to be used in reactive distillation
equipment [31, 36]. Analogously, the mixture water/TFA could be used in this type of apparatus; in
addition, the sulfonated styrene/divinylbenzene resin catalyst is active in the ester hydrolysis. In Fact,
the hydrolysis of the cyclohexyl trifluoroacetate (entry 11) gives cyclohexene and cyclohexanol thus
suggesting that the system tends to reach an equilibrium composition. These evidences suggest that the
system TFA/sulfonated resins could be a promising way for an industrial process, because of its
potentiality in its use by a reactive distillation process.
Tab. 7. Cyclohexene hydration promoted by liquid acids and catalyzed by solid acids. Run conditions:
T 393 K, P autogenous pressure of the system, cyclohexene 2.62 mmol, water 2 mL, acid 1 mmol,
catalyst 40mg, time of reaction 4h.
Entry
Catalytic system
Conversion
Cyclohexanol
Addition

13
(%)a
selectivity
(%)
products
selectivity
(%)
1
2
TFA/Amb15
TFA/Amb36
TFA/Amb50
TFA/HZSM5
TFA/ DW50
H2SO4/Amb15
H2SO4/Amb36
H2SO4/Amb50
H2SO4/HZSM5
H2SO4/D50
12
12
11
7
69
68
30
31
31
35
33
/
3
68
4
65
5
10
13
12
12
12
12
60c
66
6
100
100
100
100
100
55
7
/
8
/
9
/
10
11b
/
H2SO4/Amb36
/
a) About 1-3% are traces of condensation products. b) Cyclohexyl trifluoroacetate as the
substrate. c) Cyclohexene is formed in 42% of selectivity.
3.3 n-Hexene hydration
n-Hexene hydration occurs only in very low conversion, lower than that obtained with cyclohexene,
this system seem to be useless for practical application especially with TFA as a liquid acid system. It
is likely that the low conversion of this olefin is due to by two behaviors of the system:
i) the low solubility of the olefin in the aqueous phase; ii) the low protonation of the olefin in diluted
acid. As expected, the main product of the reaction is the 2-hexanol similarly, of what already obtained
in concentrated sulfuric acid [10]. Anyway, Tab. 8 shows some results suggesting that the best acid
system for this reaction is sulfuric acid other acid systems give a negligible conversion. The mixed
system appears to be not effective and only a negligible increase of the conversion is observed with
TFA as a promoter. In sulfuric acid, the presence of the solid catalyst does not increase the conversion
suggesting that is not effective compared to the neat sulfuric acid.
Tab. 8. n-Hexene hydration promoted by liquid acids and catalyzed by solid acids. Run conditions: T
393 K, P autogenous pressure of the system, n-hexene 2.62 mmol, water 2 mL, acid 1 mmol, catalyst
40mg, time of reaction 4h.

14
Entry
Catalytic
system
Conversion
(%)
2-hexanol
selectivity
(%)1
1
2
3
4
5
6
7
8
TFA
2
8
99
H2SO4
95
100
100
99
Amb15
<1
<1
3
Amb36
TFA/Amb15
TFA/Amb36
H2SO4/Amb15
H2SO4/Amb36
3
99
8
95
8
95
1) Traces of isomers
3.4 Cyclohexene hydration: reaction rate, profiles and equilibria
In previous chapters, we have verified the ability of various aqueous system to promote the formation
of alcohol from olefins, the presence of the solid catalyst favors the formation of the latter, increasing
the rate of additions of the nucleophiles. It is well known, that in cyclohexene hydration, which is an
important reaction from industrial point of view, it is quite difficult achieving high yields because of its
reversibility. As a matter of fact, about 10-20% of overall yield can be achieved under the conventional
hydration conditions in Batch type reactor [29, 45, 46]. Reactive distillation is necessary to achieve
high yield in the cyclohexanol [29, 32]. The use of an organic acid to promote the hydration it is
known, in particular, the addition of formic acid to the double bond and its further hydrolysis allow the
formation of the alcohol [31]. The use of a sulfonated resin in combination with the TFA as acid
promoter seems to be the most promising system to achieve high yield of cyclohexanol, taking into
account problem of the reversibility of the reaction. In Fig. 1 is reported a typical reaction profile of
cyclohexene hydration catalyzed by sulfonated resins (Amberlyst 36 is the solid catalyst chosen for the
study of the rate of reaction throughout the rest of the work) and promoted by aqueous TFA.

15
1.0
0.8
0.6
0.4
0.2
0.0
cyc
col
tfcyc
0
50
100 150 200 250
900
1000 1100 1200
time (minutes)
Fig. 1. Reaction profile. Run conditions: T 403 K, P autogenous pressure of the system, cyclohexene
1.14 mol L-1, water 2 mL, TFA 1.32 mol L-1, Amberlyst 36 40mg.
As already pointed out, the reaction occurs in two phases an aqueous phase and an organic one and, as
clearly appear in Fig.1, the reaction does not go to completion but reach a value determined by the
multiphase equilibria of the reaction environment. At 403K equilibrium is reached in about 4 h, but at
lower temperature the system needs up to 80 h to get it. This system is affected by several equilibria,
such as liquid/liquid equilibria (of all the species), acid dissociation, olefin protonation, hydration,
addition equilibrium. This is a really complex system, whose study is clearly beyond the purposes of
the present work, we consider, however, the overall multiphase equilibrium shown in scheme 2.
Scheme 2 Overall multiphase equilibrium
In order to try a determination of some reaction parameter we neglect the interphase equilibria by
supposing that these equilibria give only proportional constant that can be embedded in the overall

16
constant. In addition, we suppose that activities of all the non-ionic species can be considered as
concentrations referred to the volume of both phases. This approximation could hold because the
activity of the non-charged molecule are generally proportional to the concentration of the specie itself
as well as to the charged species, by following the Setchenow equation [47, 48]. Under these limits,
Equation 1 can describe the overall equilibrium. All equilibria in solution are calculated by the GC
analysis of solutions obtained from the sample extracted with CH2Cl2, except for TFA for which the
literature dissociation degree are used and employing the values of the activity coefficient calculated
for the ionic species [49, 50].
[
]
퐶푦푇푓 [푂푙]
퐾푠푛(푤) =
Equation 1
[
]
[퐶푦] 푇퐹퐴
It is clear from Fig. 2 A, however, that this over-simplified hypothesis does not hold, because there is a
clear dependence of this pseudo-equilibrium constant (Ksn) obtained from Equation 1 from total TFA
concentration. This effect could be caused by the salting out effect, which diminish the concentration of
cyclohexene in the aqueous phase, thus affecting the equilibrium in function of the amount of the
charged species. As a matter of fact, in Equation 2 we take into account the activity of the hydrogen
ions in order to compensate the increase of the charged species in solution whose effect cause a
reduction of the solubility of the cyclohexene. The correction introduced in Equation 2 allows one to
obtain a Ks as a constant over the range of TFA concentration studied in the present work and at 3
different temperatures as can be observed in Fig. 2 B.

17
0.08
0.07
0.06
0.05
0.04
0.03
0.02
0.01
0.00
0.26
0.24
0.22
0.20
0.18
0.16
0.14
0.12
0.10
0.08
0.06
0.04
393 K
403 K
413 K
393 K
403 K
413 K
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
TFA (Mol L-1)
TFA (Mol L-1)
A
B
Fig. 2 Overall equilibrium constant obtained by varying temperature and TFA concentrations.
[
]
퐶푦푇푓 [푂푙]푎
+
퐻
퐾푠(푤) =
Equation 2
[
]
[퐶푦] 푇퐹퐴
A linear Van’t Hoff plot (Fig. 3) is obtained from the average of the Ks obtained at different TFA
concentration and an overall exothermic effect with a value of H° = -57kJ mol-1, and S° = 175 J K-1
mol-1. The values are in agreement with thermodynamic data of individual reactions carried out in
much ideal condition, thus suggesting a good reliability of the parameter proposed [51 52].
38
36
34
32
30
28
26
24
22
20
0.00248 0.00250 0.00252 0.00254 0.00256 0.00258 0.00260 0.00262
1/T

18
Fig. 3 Van’t Hoff plot. Equilibrium constant variations with temperature: H° = -56kJ mol-1 and S° =
175 J K-1 mol-1.
3.4.1 Influence of the amount of the aqueous phase on the initial reaction rate
Initial reaction rate is a complex parameter because is a combination of the various process occurring in
the reaction, however, it gives a reliable value on the overall behavior of the reaction. The presence of
two immiscible liquid phase clearly indicate the presence of mass transfer between the two phases,
these process are seldom determining the overall reaction rate. The amount of aqueous phase seems to
influence faintly the initial reaction rate also at two different acid concentrations. This suggests the two
phases are mixed together homogenously at different substrate aqueous phase ratio.
0.00004
0.00003
TFA 1.32 mol L-1
TFA 0.33 mol L-1
0.00002
0.00001
0.00000
1.4
1.6
1.8
2.0
2.2
2.4
2.6
2.8
3.0
3.2
water (mL)
Fig. 4. Influence of the amount of the aqueous phase on the initial reaction rate. Run conditions: T 393
K, P autogenous pressure of the system, cyclohexene 1.14 mol L-1, water 2 mL, Amberlyst 36 40mg.
3.4.2 Influence of cyclohexene amount on the initial reaction rate
The variation of the amount of cyclohexene does not influence the initial reaction rate, actually, the
biphasic conditions allows a constant concentration in the aqueous solution of the substrate. This is an
evidence that the reactions occurs in the aqueous phase for this reason the amount of cyclohexene
faintly influence the rate of reaction, being the concentration of substrate almost constant in the

19
aqueous phase at low conversions. The constancy of the initial rate of reaction observed in Fig. 5 for
two different concentrations of TFA confirms the homogeneity of the emulsion induced by the
agitation visually observed at room temperature for the same reaction system.
0.000035
0.000030
TFA 1.32 mol L-1
TFA 0.65 mol L-1
0.000025
0.000012
0.000009
0.000006
0.4
0.6
0.8
1.0
1.2
1.4
1.6
Cyclohexene (mol L-1)
Fig. 5. Influence of cyclohexene amount on initial reaction rate. Run conditions: T 393 K, P autogenous
pressure of the system cyclohexene 1.14 mol L-1, water 2 mL, Amberlyst 36 40mg.
3.4.3 Influence of TFA concentration on the initial reaction rate
Fig. 6 shows the influence of the TFA concentration on the initial reaction rate, the when TFA
concentration is 0 the reaction proceed by the acid catalysis of the sulfonated resins, achieving a
conversion of 8% after 4h of reaction and with an initial rate of reaction of 6 10-6 mol L-1 s-1. The
increase of the rate appear to initially low but after 0.6 mol L-1 s-1 the increasing is stiff with a neat
slowing down after 1.5 mol L-1s-1. This trend looks like a titration suggesting the formation of a TFA-
cyclohexene adduct, a further indication of the formation an active adduct TFA-cyclohexene, as the key
intermediate is the fact that the flex of the curve coincide with the concentration of the olefin. Anyway,
such an evidence allow only a mere hypothesis, because of the complexity of the system. More
thermodynamics and mechanistic researches are necessary for the determination of the reaction steps.

20
0.00004
0.00003
0.00002
0.00001
0.0
0.5
1.0
1.5
2.0
TFA (mol L-1)
Fig. 6. Influence of the TFA amount on the initial reaction rate. Run conditions: T 393 K, P autogenous
pressure of the system, cyclohexene 1.14 mol L-1, water 2 mL, Amberlyst 36 40mg.
3.4.4 Influence of the sulfonated resin amount (Amberlyst 36) on the initial reaction rate
Fig. 7 shows the trend obtained between the initial reaction rate and the sulfonated resin amount. It is
clear that in both cases with and without TFA the initial reaction rate increase linearly, what is
interesting is the fact that the slope of the two straight line are quite similar suggesting that the increase
of the initial rate is due to the same phenomenon. It is likely that the solid catalyst, at the initial stage of
the reaction, promotes hydration of the olefin as and it is not influenced by the presence of TFA,
similarly TFA addition to the cyclohexene is not promoted by the resins. Actually, the slope of the TFA
promoted reaction is only 0.54 10-7 higher than that of the unprompted one. The intercept of the
reactions without TFA is very close to 0 which imply the necessity of the catalyst to achieve the
reaction. On the contrary, reactions carried out in the presence of 1.32 mol L-1 of TFA show an
intercept close to the initial rate of the reaction carried out without catalyst (filled point at 0 sulfonated
resins amount). The black filled circle represent the experimental data at 0 resin concentration, in
agreement with the intercept of the straight line of the fitting.

21
0.000045
0.000040
0.000035
0.000030
0.000025
0.000020
0.000015
0.000010
0.000005
0.000000
TFA 1.32 mol L-1
TFA 0 mol L-1
0
10
20
30
40
50
60
sulfonated resin (mg)
Fig. 7. Influence of the catalyst amount on the initial reaction rate. Run conditions: T 393 K, P
autogenous pressure of the system, cyclohexene 1.14 mol L-1, water 2 mL. TFA=0 slope = 2.21 10-7,
intercept = -4.4 10-7; TFA = 1.32 mol L-1 Slope = 2.45 10-7, Intercept = 2.59 10-5.
3.4.5 Influence of Temperature on the initial reaction rate
The influence of temperature on initial reaction rates are reported in Figs 8 and 9. The initial trends of
the reactions are almost linear with time but the overall kinetics is not of first order as can be observed
in Fig. 1 for the complete reaction, as a matter of fact, the multiphase system allow a constant
composition especially at low conversion, showing a zero order like reaction path. In addition, as
observed in figs 2 and 3 the variation of the ration of the two liquid phases does not alter initial reaction
rate. Starting from these evidences, we calculate an overall temperature factor by the Arrhenius plot of
Fig. 9. The values of Eft=84 kJ mol-1 has been calculated with a pre-exponential factor of Sft=151 J
mol-1 K-1. The value of the temperature factor are quite high suggesting no limitation of the diffusion
on the reaction rate [18, 19, 52]. In addition, such a value suggest that the increase the temperature
favors the attainment of the equilibrium, but reactions are exothermic, than the equilibrium is shifted to
the left as the temperature increase. These parameters are the starting point for the realization of a
reactive distillation process in order to achieve high conversion to cyclohexanol by using TFA as an
easily recoverable non-toxic organic acid [53, 54].

22
1.00
0.95
0.90
0.85
0.80
0.75
403K
393K
373K
0
100
200
300
time (min)
Fig. 8 Influence of Temperature on the initial reaction rate. Run conditions: T 373-403 K, P autogenous
pressure of the system, cyclohexene 1.14 mol L-1, water 2 mL, TFA 1.32 mol L-1, Amberlyst 36 40mg.
-88
-96
-104
-112
0.00248
0.00256
0.00264
1/T (K-1)
Fig. 9. Arrhenius Plot. Run conditions: T 373-403 K, P autogenous pressure of the system, cyclohexene
1.14 mol L-1, water 2 mL, TFA 1.32 mol L-1, Amberlyst 36 40mg. Eft=84 kJ mol-1,Sft =151 J mol-1
K-1

23
3.4.5 Catalyst recycling
The experimental observation above descripted suggest that the presence of the TFA is main factor that
influence the yield to cyclohexanol, however the presence of the acid resin favors the attainment of the
equilibrium. This may help in the development of a process based on this reaction. The increase of the
reaction rate due to the presence of the resin could be considered as not determining for the practical
development of a process. In fact, the increases of the reaction rate, in the presence of 40 mg of catalyst
is only 30% (see Fig. 7) with respect of the reaction rate in the absence of the solid acid. However, the
absence of an evident deactivation after 4 recycles it suggests that could be practically employed in a
process. In addition, because of its ability in the reaction of hydrolysis it could be used in a second
stage of cyclohexyl trifluoroacetyl ester saponification to give acid and alcohol.
0.00003
0.00002
0.00001
0.00000
0
1
2
3
4
recycling
Fig. 10 Catalyst recycling. Run conditions: T 393K, P autogenous pressure of the system, cyclohexene
1.14 mol L-1, water 2 mL, TFA 1.32 mol L-1, Amberlyst 36 40 mg
As regard TFA recovery, the acid can be easily separated from the reaction environment since it is in
the aqueous phase, however, the reuse of the acid phase depend on the hydrolysis step of the
cyclohexyl trifluoroacetate, whose study is beyond the aim of the present work. Besides, it is
noteworthy that the recovery of the TFA has been studied in the Beckmann rearrangement of oximes,
in which there were no needs of neutralization but a simple distillation [54]. In this case, separation is
even simpler, since the acidic phase can be reused directly as a liquid aqueous phase.

24
4. Conclusions
In this work, we show the potentiality of TFA in the promotion of the hydration olefin to alcohol. It
acts by forming the ester intermediate, which reaches an equilibrium with olefin and alcohol, together
with traces of condensation products (ether and light oligomer). The reactions occurring to reach the
equilibrium are acid catalyzed, for this reasons TFA acts also as catalyst, but a further increases of the
rate of the processes are achieved by using sulfonic resins as acid catalysts. The thermodynamic
parameter of the overall equilibrium are calculated, and it shows a strictly dependence from TFA
dissociation suggesting a firm relationship between equilibria and the ionic species in solution. The
Van’t Hoff plot gives enthalpy and entropy in quite good agreement with the literature values. The
temperature factor of the initial reaction rate are in agreement with those of similar reaction. The solid
catalyst Amberlyst 36, which is those largely employed during kinetics and equilibrium experiments,
shows at temperature of 393K a negligible loss in activity after 4 recycling.
Acknowledgements
A special thanks to Alessandro Minesso of Allnex Italy S.r.l. for the helpful discussion and for GPC
measurements of polystyrene.
University Ca’ Foscari of Venice is gratefully acknowledged for financial support (ADIR fund 2016).
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