Tailoring the Physicochemical Properties and Performance of FAU Catalysts with Heteroatoms

Heteroatom-modified zeolites have emerged as fascinating materials owing to their potential to improve both catalyst performance and stability. However, optimizing these properties is challenging in part because zeolite crystallization is not well-understood, particularly when the addition of heteroatoms obfuscates molecular phenomena and/or prolongs crystallization times. In this presentation, we will discuss our studies of faujasite (FAU) crystallization in organic-free media containing heteroatoms. Faujasite is a metastable zeolite heavily utilized as a fluid catalytic cracking catalyst. FAU commonly undergoes mid-synthesis interzeolite transformation to more thermodynamically stable phases (e.g. GIS) according to the Ostwald rule of stages. To this end, we have surveyed the effects of multivalent heteroatoms on FAU and GIS crystallization and identified metals such as zinc to be useful elements for controlling crystal growth and interzeolite transformation.

In this presentation, we will discuss how experiments and modeling have been used to characterize zincoaluminosilicate growth solutions throughout stepwise crystallization processes wherein we elucidate the challenges associated with heteroatom-incorporated zeolite synthesis and evaluate the efficacy of zinc on suppressing zeolite crystallization. We additionally reveal the mechanisms responsible for this effect; namely, soluble zincate species bind silicate oligomers, which effectively increases the energetic barrier to GIS nucleation. Furthermore, we will show that it is possible to take advantage of suppressed kinetics to control phase purity and kinetically trap metastable zeolites. Our work further demonstrates the lack of an epitaxial relationship between parent (e.g. FAU) and daughter (e.g. GIS) zeolites, which underscores possible pathways for interzeolite conversions between two frameworks lacking common structural building units. Lastly, we will describe how a molecular level understanding of heteroatom effects is critical for ensuring the optimal properties of zeolite catalysts. Collectively, these experimental and modeling studies have demonstrated a novel technique for studying zeolite crystallization phenomena by suppressing growth kinetics using soluble transition metal species.