(701f) Hydrothermal Stability of ZSM-5 Zeolite
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.