| Synthesis of cyclohexanol by liquid-phase hydration of cyclohexene has been an economical and practical process, which is of great significance for the sustainable development of chemical industry and human society. Ion-exchange resin and HZSM-5 zeolite, as two different solid catalytic materials, have been widely used in the hydration of cyclohexene. However, no literature was reported to study the adsorption behavior and reaction mechanism involved in the heterogeneous cyclohexene hydration, which is meaningful to understand the cyclohexene hydration more clearly. Based on the Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism and the Eley-Rideal (ER) mechanism, three heterogeneous models were proposed for the first time in this paper to discriminate the reaction mechanisms of the two catalysts:(1) a reaction model based on the adsorbed water and the adsorbed cyclohexene; (2) a reaction model based on the adsorbed water and bulk cyclohexene; (3) a reaction model based on bulk water and the adsorbed cyclohexene.The results of kinetic study showed that the heterogeneous kinetic models gave a more satisfactory agreement with the experimental data than the pseudo-homogeneous (PH) model. According to the kinetic results and spectroscopic evidence, it is suggested that cyclohexene hydration catalyzed by HZSM-5 belongs to the ER mechanism. The water molecule is firstly adsorbed on the active sites of HZSM-5 to form hydroxonium ion, which plays the role of the Bronsted acid site, and then the cyclohexene molecule in the aqueous phase could react directly with the adjacent hydroxonium ion to obtain the product cyclohexanol. For cyclohexene hydration catalyzed by HZSM-5, the activation energy obtained is 77.69 kJ/mol, and the adsorption constants of water and cyclohexanol are estimated to be 19.95 and 146.6, respectively. Different from the HZSM-5 catalyst, cyclohexene hydration catalyzed by resin catalyst was proved to follow the LHHW mechanism. The carbonium ion is formed by protonation of the adsorbed cyclohexene, which further reacts with the adsorbed neutral water molecule to obtain the product cyclohexanol. For cyclohexene hydration catalyzed by resin catalyst, the activation energy obtained is 79.92 kJ/mol, and the adsorption constants of cyclohexene, water and cyclohexanol are determined to be 6.745,24.30 and 81.91, respectively. It is noted that the activation energies obtained are found to be closely for the resin and H-ZSM-5 catalyst, which implies that although there contains much difference between the reaction mechanisms of cyclohexene hydration over the two catalysts, the intrinsic energy barriers of the reaction are nevertheless much similar.The solvation effect of different organic cosolvents on cyclohexene hydration was also studied in a batch autoclave reactor. It was found that compared to the cyclohexene hydration over resin catalyst, the addition of cosolvent influenced the cyclohexene hydration catalyzed by HZSM-5 much more evidently. For the cyclohexene hydration catalyzed by HZSM-5, the ethylene glycol, which was used for the first time in this paper, showed the best solvation effect. With ethylene glycol as the cosolvent, a cyclohexene conversion of 11.4% was obtained, which is much higher than that of 8.2% obtained without the cosolvent. In order to evaluate the solubility of cyclohexene in the aqueous phase in the presence of ethylene glycol, the liquid-liquid equilibria (LLE) of the quaternary system of cyclohexene+water+ cyclohexanol+ethylene glycol at 298.15 K were studied. It was found that a modified UNIFAC model gives the best fits to the LLE data. In addition, the results of kinetic study showed that the existence of ethylene glycol can not change the reaction mechanism of cyclohexene hydration catalyzed by HZSM-5, which follows the ER mechanism. Nevertheless, due to the solvation effect of ethylene glycol, both the activation energy of the hydration reaction and the adsorption constants of water and cyclohexanol were found decreased.With trimethylchlorosilane as the surface modifier, amphiphilic and hydrophobic HZSM-5 zeolite particles were obtained by partial modification of the external surface of HZSM-5 zeolite. After modification, the hydrophobicity of the external surface is enhanced significantly, and the catalytic activity of HZSM-5 zeolite is also improved. Experimental results showed that when an amount of trimethylchlorosilane 0.4 mL/g cat was used, the obtained amphiphilic HZSM-5 zeolite gave the best catalytic performance in hydration of cyclohexene. The influence of different pretreatment methods on the catalytic activity of catalysts was also investigated. Whether being pretreated with water or with cyclohexene, the resin catalyst both exhibited a much lower catalytic activity than the resin catalyst without pretreatment. However, the catalytic activity of HZSM-5 was found to be evidently improved after being pretreated with water, the conversion of cyclohexene was increased from 6.4% to 7.3% under certain reaction conditions. The HZSM-5 zeolite pretreated with cyclohexene also showed a worse catalytic activity than the unpretreated HZSM-5, which might be caused by the catalyst deactivation during the pretreatment. In addition, the ultrasound was successfully introduced into the reaction system of cyclohexene hydration for the first time. It was found that the ultrasound with an acoustic power of 900 W gave the best enhancement effect for cyclohexene hydration catalyzed by HZSM-5, and the cyclohexene conversion was increased from 6.1% to 8.6% under certain reaction conditions. It was proved that lower power ultrasound can not provide enough cavitation bubbles and higher power ultrasound should restrict the transmission of the ultrasonic energy in the reaction system. |
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