Abstract
The controlled desilication of zeolites, leading to hierarchical micro-/mesoporous materials, is one of the most promising approaches to increase the application potential of zeolites. It can lift the restrictions connected to diffusion limitations in many industrially important processes and it can also modify the Si/Al ratio of the final material.
The selective desilication of zeolites is generally performed under alkaline conditions, and control over the degree of framework degradation is maintained via the Si:Al ratio of the parent sample and the pH of the solution. However, the mechanism of alkaline hydrolysis in zeolites is poorly understood at present: The role of microsolvation of ions and the role of micropore confinement effects are currently unknown.
In this work, we establish the mechanisms for the alkaline hydrolysis of siliceous zeolite chabazite (CHA) occurring from within the pore. Energetically facile reaction pathways are identified, which directly involve NaOH and confirm that NaOH may not only play a role as a reactant but also as a catalyst.
We demonstrate that collective effects play a decisive role in the mechanism: initiation of the reaction via formation of Q3 defects becomes spontaneous when the Na+ cation is solvated by a sufficient amount of water and subsequent barriers along the desilication pathway are lowered. We show that it is crucial to include a realistic treatment of the hydration environment to capture the processes, which occur inside zeolite pores.
This work advances our understanding of a ubiquitous process in heterogeneous catalysts and will help in controlled desilication treatments for zeolite upgrading.