Cooperative Role of Water Molecules during the Initial Stage of Water-Induced Zeolite Dealumination

Understanding water-induced zeolite dealumination is crucial for control of the hydrothermal stability of zeolite-based catalyst materials. Here we explore the dealumination process, focusing on the first Al-O(H) bond-breaking step in a density functional theory model of a ZSM-5 crystal in the presence of a single and two water molecules per active site. We identify a set of four possible reaction mechanisms consisting of two different types of reactions. In the first three proposed mechanisms, Al-O(H) bond breaking is induced by adsorption and dissociation of an incoming water molecule. The fourth mechanism is different and leads to a different reaction product, suggesting an alternative follow-up mechanism. In this energetically very favorable case, the breaking of the Al-O(H) bond is induced by nondissociative adsorption of two water molecules. We therefore assume that the proposed mechanism is a viable first dealumination step. This implies that all Al-O(H) bond-breaking mechanisms are initiated from metastable water adsorption modes, and water reorganization from the most stable mode needs to occur prior to hydrolysis of the Al-O(H) bond. We suggest that the feasibility of this rearrangement (Al accessibility) is one of the determining factors for the relative occurrence of dealumination at different sites. We further establish a correlation between the Al site susceptibility toward dealumination and reaction conditions, which can be further used during postsynthetic treatment of the zeolite to control Al distribution and thus hydrothermal stability of the catalyst.