3.1 Esterification of cyclohexene with acetic acid
Since the thermodynamic parameters of the reaction of esterification of
cyclohexene with acetic acid to cyclohexyl acetate has not been tackled
before,13 and the thermodynamic properties of
cyclohexyl acetate are lacking, we experimentally determined the
equilibrium conversion of cyclohexene at different acetic
acid/cyclohexene molar ratios and temperatures using Amberlyst 15 as the
catalyst. Many solid acid catalysts, such as silica-supported HPA (HSiW)
or H3PW12O40, sulfated
zirconia, and ion-exchange resins, have been used for the esterification
of olefins with acids.22−24 Amberlyst 15 is a
commercially available, strongly acidic macroreticular polymeric resin
based on crosslinked styrene, which is one of the most frequently used
ion-exchange resins in acid-catalyzed reactions.22 In
particular, Saha and Sharma reported that Amberlyst 15 was more active
than the Amberlite IR-120 genular resin, Engelhard F-24 acid-treated
clay, and homogeneous p -toluene sulphonic acid catalysts in
cyclohexene esterification with formic acid.25Chakrabarti and Sharma found that Amberlyst 15 was more active than
Engelhard F-24 in cyclohexene esterification with acetic
acid.13 As plotted in Figure 2a, the equilibrium
conversion of cyclohexene decreases with the increase in the
temperature, reflecting that the esterification of cyclohexene with
acetic acid is exothermic. The cyclohexene conversion is improved when
increasing the acetic acid/cyclohexene ratio as expected. At the
stoichiometric acetic acid/cyclohexene ratio of 1, the cyclohexene
conversion is 79.1% at 333 K, which decreases gradually to 68.0% at
373 K. When the acetic acid/cyclohexene ratio is elevated to 3, the
cyclohexene conversion amounts to 93.5% at 333 K, which decreases to
85.1% at 373 K. Nevertheless, the cyclohexene conversion at the
stoichiometric ratio at 373 K is still much higher than the conversion
of 12.7% via the route of cyclohexene hydration.8 In
addition, the selectivity to cyclohexyl acetate remains as high as
99.7% at 343 K, which decreases slightly to <98% at 363 K
and above owing to the occurrence of the oligomerization/isomerization
of cyclohexene in the presence of the acid catalyst. These by-products
have also been identified in cyclohexene hydration.7
Assuming that the enthalpy change (ΔH o) and
entropy change (ΔS o) are constant in the
temperature range investigated, the relationship between the reaction
equilibrium constant (K ) and the reaction temperature can be
expressed as:
On the basis of the data in Figure 2A, we obtained a good linear
relationship between lnK and 1/T (Figure 2B). From the
intercept and slope of the line, the following thermodynamic parameters
for the esterification of cyclohexene with acetic acid to cyclohexyl
acetate are derived: ΔH o = –22860.8 ± 2825.4 J
mol–1, ΔS o = –42.8 ± 8.0 J
mol–1 K–1, and
ΔG o = –22860.8 + 42.8T J
mol–1 in the temperature range of 333–373 K. It
should be noted that the ΔG o for this reaction
is far more negative than that for cyclohexene hydration (365 J
mol–1 at 333 K and 7339 J mol–1 at
373 K, calculated by HSC5.0 software), unambiguously confirming that
esterification of cyclohexene with acetic acid is thermodynamically more
favorable than cyclohexene hydration.
Figure 3 shows the reaction rates of cyclohexene esterification at
various acetic acid/cyclohexene ratios and reaction temperatures on a
fixed-bed reactor (Figure S1). The effects of external and internal
diffusion have been eliminated (Figure S2). The reaction temperature
drastically affects the esterification rate of cyclohexene, which in
general doubles with a 10 K-increment in the reaction temperature.
Besides, the reaction rate decreases with the increase in the acetic
acid/cyclohexene ratio. It is plausible that cyclohexene adsorbs much
weaker than acetic acid on Amberlyst 15, considering that acetic acid
can interact tightly with the sulfonic acid groups on Amberlyst 15 via
hydrogen bonding. Hence, the adsorption of cyclohexene is the key factor
that limits the reaction. The weak adsorption of cyclohexene with
respect to acetic acid on the same active sites is substantiated by the
fitting results of the reaction data in Figure 3 using the
Langmuir–Hinshelwood–Hougen–Watson (LHHW) type kinetics model. As
compiled in Table S2, the reaction parameters derived from the LHHW-type
kinetics model show that the adsorption equilibrium constant of
cyclohexene is much smaller than that of acetic acid. Moreover, the
apparent activation energy (E a) of the
esterification of cyclohexene with acetic acid is 60.0 kJ
mol–1, which is lower than theE a of 71.2 kJ mol–1 for
cyclohexene hydration.26 Therefore, these experimental
results substantiate that the esterification of cyclohexene with acetic
acid is both thermodynamically and kinetically more advantageous than
cyclohexene hydration.