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.