Controlling Microporous and Mesoporous Domains in Zeolite Crystallization

Abstract

Zeolites are crystalline microporous aluminosilicates with confined cages and channels of molecular dimensions, that are utilized in a wide variety of commercial applications ranging from catalysis and ion-exchange to separations and drug delivery. Despite their commercial relevance, fundamental mechanism(s) that govern the nucleation and growth of zeolites are not well understood. The lack of molecular level understanding often prohibits a priori selection of synthesis conditions to predictively control the physicochemical properties of zeolites. This dissertation focuses on establishing improved fundamental understanding of zeolite crystallization, and to use that knowledge in developing facile and inexpensive routes to tune the porous architecture, active site distribution, size, and morphology of zeolites. Surfactant templating has emerged as one of the most effective and versatile strategies for the construction of well-defined porous architectures in zeolites. Here, we have explored the dual roles of the cationic surfactant cetyltrimethylammonium (CTA) as an organic structure-directing agent (OSDA) and as a mesostructuring agent in the rational design of two commercial zeolites (ZSM-5 and USY). Our findings reveal that the selection of OSDA has a significant impact on the kinetics of ZSM-5 crystallization, as well as the physicochemical properties of ZSM-5 crystals. In addition to OSDA design, the development of mesoporosity in zeolites has been a long-standing goal in catalysis to alleviate the diffusion limitations imposed by micropores. One area of research that has garnered considerable interest, yet is not fully understood, is the rearrangement of zeolite crystals post-synthesis to accommodate mesoporosity. Here, we will also report in situ observations of intracrystalline mesoporosity in USY zeolite assisted by CTA using atomic force microscopy. Our findings capture the structural, morphological, and textural evolution of initially rough crystals to smooth crystals with a uniform distribution of mesopores. Although surfactants offer significant advantages over conventional organics, they are not heavily commercialized since they are irrecoverable due to post synthesis calcination. To this end, we have developed a more rational approach employing seeds as a unique tool to synthesize hierarchical ZSM-5 with intergrown nanosheets in an organic-free media. We demonstrate nonclassical pathways of ZSM-5 crystallization involving the nucleation of crystals at the external surface of amorphous precursor particles. One of the critical challenges in elucidating the mechanism(s) of nucleation and growth in zeolites is the lack of available techniques that can monitor the amorphous-to-crystal transformation with sufficient spatiotemporal resolution. Here, we have developed an elemental mapping method employing FE-SEM-EDX as a versatile tool to characterize morphological and compositional properties of precursors during the crystallization of five commercial zeolites. Time-resolved ex situ elemental mapping images of extracted solids during several stages of zeolite crystallization reveal that a sufficient disparity in the chemical composition (e.g., alkali metal content) between amorphous precursors and crystals can be exploited for tracking the early stages of zeolite crystallization, locating where it happens, and resolving residual amorphous material among crystalline domains. Given the ubiquitous presence of amorphous precursors during nonclassical crystallization, we expect that elemental mapping may prove valuable for understanding and tracking the growth of other zeolites and minerals.