| One important transformation that benefits from acid catalysis is esterification. Solid acid catalysis using a variety of commercially available polymeric ion-exchange resins (IERs), acid treated clays, and zeolites overcome process difficulties such as liquid acid disposal, corrosion, and other hazards encountered with homogeneous catalysts. However, site accessibility and diffusional mass transfer resistance remain issues of concern for their use in conventional catalytic processes. This research work deals with the application of a novel functionalized microfiltration membrane catalyst as a heterogeneous, solid-phase, flowthrough catalytic structure for the esterification reaction between ethanol and acetic acid. Catalytic sites are located on each repeat unit of sulfonated polystyrene chains grafted in the flow pathways of the membrane. Batch studies were performed to make a comparison of the membrane catalyst to sulfuric acid and ion-exchange resin (Amberlyst-36). These studies showed that the activity of the modified catalytic membrane was comparable to the, standard resin for the same acid capacity. The conversion of the limiting reactant increased linearly with increased membrane catalyst mass loading.; The true kinetic improvements of the membrane, however, were revealed in flowthrough studies because of the rapid catalyst site access compared to the IER system under similar reaction conditions. The average conversion of acetic acid (20 seconds residence time) was found to increase from 16% to 65% as the reaction temperature was increased by 45°C. The activation energy of the flowthrough reaction (∼32 KJ/mol) was lower than the conventional IER reaction by approximately 20% as a result of rapid and complete site accessibility. Some loss of grafted polystyrene (∼25%) in the reaction permeate was observed during flowthrough experiments, resulting in partial loss of activity. However, covalent binding and an increase in the graft chain length provided better graft stability. A kinetic model was developed assuming the catalytic membrane reactor as plug-flow reactor. The predicted behavior of membrane catalyst as a function of residence time and temperature was in good agreement with the experimental data thereby suggesting the soundness of the model. |
