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(600bn) Identifying Mechanisms of Zeolite Growth: A Pathway to Catalyst Optimization

Authors: 
Li, R., University of Houston
Kumar, M., University of Houston
Rimer, J. D., University of Houston

Crystallization commonly occurs through classical pathways that involve the addition of atoms or molecules to a crystal surface that grows by layer nucleation and spreading. Zeolites can be categorized in a growing list of materials (mostly natural minerals) that form through nonclassical routes [1] involving amorphous precursors that undergo structural disorder-to-order transformation(s). Here we present a case study of zeolite L (LTL framework type) crystallization wherein we track the formation and evolution of amorphous precursors. We show that the amorphous material undergoes a series of structural transformations, forming worm-like particles (WLPs) that progressively decrease in population over the course of zeolite nucleation and growth. The evolution of pre-nucleation amorphous precursors was evaluated on the basis of their temporal change(s) in morphology, particle size, and degree of aggregation (i.e. fractal dimension) to explore the pathway(s) of amorphous-to-crystalline phase transformation. The putative mechanism of growth involves the initial aggregation of colloidal silica particles, which subsequently form WLPs. The latter appear to provide nutrients (i.e. silica and alumina) to growing crystals, potentially serving as interfaces for heterogeneous nucleation and the growth of LTL crystals via solution-mediated processes. We have coupled these studies with a systematic investigation of zeolite growth modifiers (ZGMs), which are site-specific molecules that bind to growing crystal surfaces and alter their size and morphology [2]. Here we will discuss how ZGMs affect the formation of WLPs and their structural transformation to LTL crystals. In addition, we will discuss the broader relevance of nonclassical growth pathways for other zeolite framework types. Collectively, these mechanistic studies provide an improved fundamental understanding of zeolite crystallization, which is paramount to the rational design of microporous materials with tailored physicochemical properties for improved performance in a variety of commercial applications.

References:

[1] Lupulescu, A.I. and Rimer, J.D., In Situ Imaging of Silicalite-1 Surface Growth Reveals the Mechanism of Crystallization, Science 2014 (In Press)

[2] Lupulescu, A.I., Kumar, M., Rimer, J.D., A Facile Strategy to Design Zeolite L Crystals with Tunable Morphology and Surface Architecture, J. Amer. Chem. Soc. 135 (2013) 6608-6617

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