Strategies for Extending Zeolite Stability in Supercritical Water Using Thermally Stable Coatings

Increasing the stability of zeolites in liquid, near-critical, and supercritical water at temperatures greater than 200 °C is required for many important chemical processes, notably including hydrothermal liquefaction. Extending ZSM-5 stability under these conditions was studied using silylation, hydrothermal carbonization, and sequential hydrothermal carbonization and pyrolysis treatments. Coatings resulting from silylation and hydrothermal carbonization were not hydrothermally stable at temperatures greater than 300 °C. In comparison, the coating formed by sequential hydrothermal carbonization and pyrolysis was thermally stable at temperatures as great as 550 °C and hydrothermally stable in the presence of a supercritical water phase at 400 °C. The carbon-coated ZSM-5 (termed C-ZSM-5) was studied for stability and activity for dodecane cracking in supercritical water. C-ZSM-5 exhibited similar cracking activity compared with ZSM-5 while retaining more crystallinity and Brønsted acid sites (BAS) density. The C-ZSM-5 failure mechanism is the disruption of the carbon–zeolite interface during use. C-ZSM-5 has the potential for extending zeolite stability in liquid water at more severe conditions than previously examined.