(617j) Liquid-Phase Stability of ZSM-5: Role of De-Alumination and De-Silication

Zeolites are microporous aluminosilicate structures commonly used for catalysis in environmental, fuel upgrading and biomass industries. Recent advances to improve process intensity and energy efficiency motivate interest in zeolite catalysis in the presence of liquid water or water-rich liquid reaction mixtures. However, many zeolites are prone to framework degradation under aqueous phase conditions. We focus our attention on ZSM-5, a commonly used zeolite for hydrothermal catalytic cracking of fatty acids and model fuels that has proven to be more robust than other zeolites such as HY and Beta. Literature studies to date have found ZSM-5 to be stable for at least 6 hours at 200°C; however many chemical reactions of interest require temperatures >250°C. In our study, we have found that ZSM-5 breaks down in the liquid phase at temperatures greater than 250 °C, allowing us to investigate the degradation mechanism and determine methods for stabilizing ZSM-5. We exposed ZSM-5 with a silica/alumina ratio of 38:1 to liquid water for three hours at temperatures ranging from 200°C to 450°C. Post-run, the catalyst was characterized with a battery of complementary techniques including XRD, IR, TEM, XPS and 27Al MQMS NMR. ZSM-5 degradation occurs due to a combination of acid site loss at intermediate temperatures followed by surface degradation at higher temperatures. Specifically, we observed improvements in the ZSM-5 backbone stability at temperatures greater than waterâ??s critical point as the familiar properties of liquid water (i.e., high dielectric and auto-ionization) shift dramatically near the critical point. Therefore, we recommend that studying ZSM-5 hydrothermal stability both above and below waterâ??s critical temperature might provide deep insight into the role of water in ZSM-5 degradation.