Comparative Study of Si‐O‐Al and Si‐O‐Si Bond Stability in HZSM‐5 Zeolite Under Steam and Hot Liquid Water Environments

Understanding the changes in the zeolite framework and catalytic active sites in zeolite‐based vapor‐phase and aqueous catalytic processes is crucial. Herein, the evolution of framework T atoms (Si and Al) in ammonium hexafluorosilicate (AHFS)‐treated HZSM‐5 zeolite under steam and hot liquid water (HLW) environments was inverstigated using various characterization techniques. In the HLW environment, Si‐O‐Si bonds exhibit poorer hydrothermal stability than Si‐O‐Al bonds, in contrast to the steam environment. Significant Si atom leaching occurs without or with the removal of framework tetrahedral Al atoms (Al(IV)‐1). Similar to steam, Al(IV)‐1 species in the HLW environment sequentially evolve into partially coordinated framework Al species and then into extra‐framework Al (EFAL) species through partial and complete hydrolysis. The generated EFAL species act as Lewis acid sites, but their local structures or chemical environments may differ. These findings reveal the difference in the T‐O‐T bonds attacked by water molecules: the Si‐O‐Al bonds is primarily attacked in steam, whereas the Si‐O‐Si bond is primarily attacked in HLW.