(701f) Hydrothermal Stability of ZSM-5 Zeolite

Upgrading of heavy oils into fuels and chemical precursors is a major technical challenge. 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. Zeolites are crystalline microporous structures composed of silica and alumina with high surface area, commonly used for catalysis in environmental, fuel upgrading and biomass industries. Zeolites have the advantage of being extremely tunable based on the selection of zeolite framework, Si/Al ratio as well as a variety of post-synthesis modification techniques. However, many zeolites are prone to framework degradation under aqueous phase conditions. Larger pore zeolites such as H-β and H-Y will fully break down at 200°C within hours when placed in hot liquid water. While many zeolites are unsuitable for hot liquid water conditions, ZSM-5 zeolite has been shown to retain crystallinity at for at least 6 hours when placed in liquid water at 200 °C. Understanding the relative stability of ZSM-5 in hot liquid water may provide the insight needed for further development of zeolite catalysis in hot liquid water phases.

In our study, we exposed ZSM-5 (silica/alumina ratio of 38:1) to liquid water for three hours at temperatures ranging from 200 °C to 450 °C to determine its hydrothermal stability. Post-run, the catalyst was characterized with a battery of complementary techniques including XRD, IR, TEM, XPS and 27Al MQMS NMR. While other zeolites completely degrade when exposed to liquid water at temperatures greater than 250 °C, ZSM-5 retained approximately 80% of its original crystallinity after 3 hours. ZSM-5 degradation occurs due to a combination of acid site loss at intermediate temperatures followed by surface degradation at higher temperatures. Interestingly, ZSM-5 exhibits improved crystallinity stability in supercritical water conditions as the familiar properties of liquid water (i.e., high dielectric and auto-ionization constant) shift dramatically near the critical point, contributing to decreased rates of ionic reactions.

Modification methods of both Brønsted acid sites and the zeolite surface were also investigated to improve ZSM-5 stability under hot liquid water conditions. To understand the effects of proton species desorption on the rate of degradation, ZSM-5 was ion exchanged with sodium before being treated in hot liquid water conditions. The lack of protons in solution led to an increase in zeolite stability at 300 °C under hot liquid water conditions. Improvements of zeolite crystallinity were also shown when altering ZSM-5 with organo-silane groups, an observation which is consistent with surface defects playing a key role in ZSM-5 degradation. Lastly, we measured the activity of liquid-water treated ZSM-5, finding that it retained some of its original activity for ethanol dehydration.