Structural Evolutions of KPO ₃ /SiO ₂ –Based Extrudates Under Hydrothermal Conditions Monitored by In Situ Raman Spectroscopy

Hydroxylation of phosphates has a strong impact on different physicochemical properties. In this work, in situ Raman spectroscopy was deployed to monitor their structural evolutions under hydrothermal conditions (250°C–350°C, 1–3 bar of water vapor). The starting materials were extrudates containing KPO 3 and siliceous binder present as either amorphous SiO 2 or α‐cristobalite. Increasing the temperature from 250°C under steam, KPO 3 first hydrates into KH 2 PO 4 and then dehydrates and condenses into liquid polyphosphate at higher temperatures. The temperature of the first transformation appears to be related to the size of KPO 3 particles. It was also shown that the second one mainly depends on water vapor pressure revealing the absence of effective interaction between the phosphate phase and the silica binder. In addition, hydration of α‐cristobalite into opal‐CT under steam was evidenced. The transformations KH 2 PO 4 ↔ liquid polyphosphate and α‐cristobalite ↔ opal‐CT were found to be reversible with a significant hysteresis between heating and cooling but independent of each other. The valuable characterization of both phosphates and siliceous phases obtained during this work highlights the power of Raman spectroscopy as a technique for investigating materials such as catalysts under hydrothermal conditions.