The catalytic properties of zeolites are intimately linked to the distribution of Al atoms and defects in the framework. In the past it has been shown that synthesis conditions can influence Al pairing in the zeolite SSZ-131,2. There is a general understanding in the literature that Al distribution is governed by thermodynamics during synthesis3. However, theoretical models based on single zeolite unit cells have not been able to answer the question why at high Si/Al ratios, where stoichiometry indicates the presence of less than one Al atom unit cell, multiple Al atoms would agglomerate in one unit cell. In this contribution we develop a Density Functional Theory (DFT) based multiscale model to understand Al distribution and the presence of defects in SSZ-13 particles. In a first step, DFT is used to calculate the relative energies for all possible Al positions in a single, primitive chabazite unit cell containing up to 6 Al atoms. Subsequently, a Monte Carlo based model is constructed to understand the Al distribution in a realistic zeolite particle with a pre-set Si/Al ratio during hydrothermal synthesis. We find that the stability of defect unit cells is influenced by the synthesis conditions, which in turn leads to the agglomeration of Al atoms in fewer cells. In the end a detailed discussion of the impact of various variables during zeolite synthesis is given. The work presented here demonstrates the importance of model complexity to understand fundamental trends in zeolite synthesis. It offers insights into synthesis-property relationships for zeolites and is a steppingstone towards the rational design of zeolite materials for various applications.

Multiscale model to understand Al distribution in SSZ-13 zeolite

ACS Spring 2022

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