Tailoring the Crystal Morphology and Spatial Chemical Composition of Zeolite Catalysts
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Zeolites are microporous crystalline materials that are widely used in commercial processes that span from energy to medicine. The physicochemical properties of zeolites are closely related to their framework topology, morphology and chemical composition. Due to the complexity of zeolite crystallization, the grand challenge for zeolite design is to predictively control zeolite properties as a means of improving their performance for different applications. In this study, zeolite ZSM-5, which is one of the most commonly used heterogeneous catalyst in oil refining and petrochemical processes, was chosen as the platform to develop new methods for tailoring zeolite morphology and chemical composition. The micropores of zeolite crystals often impose severe mass transport limitations that adversely affect catalyst activity. Here, we present a novel approach to tailor zeolite crystallization using zeolite growth modifiers (ZGMs), which are molecules that bind to specific crystal surfaces and alter the anisotropic growth rates as a means of tailoring crystal habit. We also investigated the effects of ZGMs on the initial stages of amorphous precursor self-assembly and evolution (pre-nucleation) before the crystal growth. We observed that polycations have a unique effect of promoting precursor aggregation during the induction period. The spatial distribution of aluminum in zeolite crystals can influence their catalyst activity, selectivity and/or stability, irrespective of similar bulk elemental composition (Si/Al ratio). A phenomenon known as aluminum zoning, where aluminum is preferentially located in the exterior rim of zeolite crystals, has been reported for ZSM-5 in many studies; however, its origin remains elusive. To address this topic, we have performed parametric investigations to assess the role of synthesis conditions on the distribution of Al. We show that ZSM-5 crystals can be prepared with either Si- or Al-rich exteriors, which impacts their hydrothermal stability and product selectivity in catalytic reactions. The ability to design ZSM-5 catalysts with tunable morphology and Al distribution opens new avenues for tailoring catalyst performance, and understanding property-performance relationships. ZSM-5 properties can be selectively optimized for a wide range of applications. To this end, this study provides a general platform for zeolite design and optimization that could potentially be applied to other microporous framework types.