文摘
The liquid-phase transesterification kinetics of ethyl acetate with methanol to ethanol and methyl acetate catalyzed by gel (Lewatit K1221) and macroporous (Lewatit K2640, Lewatit K2629 and Amberlyst 15) ion-exchange resins have been investigated. The effects of the resins鈥?swelling, the initial reactant molar ratio (1:1-10:1) and the temperature (303.15-333.15 K) on the reaction kinetics were assessed. Macroporous Lewatit K2629, Lewatit K2640 and Amberlyst 15 exhibit a clearly inferior catalytic activity compared to the gel type Lewatit K1221, despite the similar number of sulfonic acid active sites. This trend in catalytic activity can be explained by the differences in acid site accessibility, which are related to the resins鈥?swelling behavior and, hence, the extent of divinylbenzene cross-linking in the polymeric structure. A fundamental kinetic model, accounting for the chemical elementary steps as well as for the physical swelling due to solvent sorption, was constructed. According to this model (1) all active sites are initially occupied by protonated methanol, (2) the esters are activated by a proton exchange with protonated methanol and (3) the reaction occurs through an Eley-Rideal mechanism with the surface reaction of protonated ethyl acetate with methanol from the bulk as the rate-determining step. The kinetic model adequately described the experimental data as a function of temperature, initial molar ratio and catalyst resin type. A value of 49 kJ mol鈭? was obtained for the activation energy, irrespective of the resin used. Differences in catalytic activity caused by the accessibility of the active sites are reflected by the values obtained for the reaction rate coefficient, which is 3-4-fold higher for a gel type resin compared to the macroporous ones.
