A Rational Design Approach for Tailoring Zeolite Crystal Morphology
Lupulescu, Alexandra I 1987-
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Over the last century, zeolites have become ubiquitous materials in a wide array of applications due to properties such as high acidity, large surface area, and size/shape restriction. Although they have a longstanding precedent of exceptional performance, further optimization has been limited due in part to a lack of fundamental understanding of their growth mechanism(s). Knowledge of these processes can be used to address key issues, such as diffusion limitations, that arise from the suboptimal orientation of pores along the longest crystal dimension. Despite advances toward this goal, an efficient and inexpensive technique capable of producing zeolite crystals with tailored morphology remains elusive. To this end, silicalite-1 (MFI framework type) was chosen as the platform for a novel synthesis scheme that employs zeolite growth modifiers (ZGMs), or molecules that selectively bind crystallographic faces and minimize growth in the normal direction by blocking the attachment of incoming building units, resulting in a tuned zeolite crystal habit. ZGMs that bind to each of the three silicalite-1 faces were indentified. Furthermore, this study revealed several highly potent molecules capable of reducing MFI  thickness (preferred pathway for sorbate diffusion) by more than an order of magnitude. This design approach was extended to a 1-dimensional zeolite (LTL framework type). In this study, systematic testing of ZGMs aimed to identify key aspects responsible for selective ZGM-LTL crystal surface binding, such as the spatial distribution and density of terminal silanol groups, and hydrophobic and electrostatic interactions. The analysis of polyols resulted in the identification of two heuristic guidelines that can lead to enhanced ZGM-crystal affinity: an optimal C3 carbon length, as well as 1-3 alcohol group spacing. Lastly, we developed a new in situ AFM protocol capable of observing the silicalite-1 growth mechanism under realistic synthesis conditions (elevated temperature, high pH, and long times). This technique provided the first direct confirmation that nanoparticle and silica molecule attachment, as well as surface restructuring due to Ostwald ripening, are the dominant silicalite-1 growth modes. Furthermore, the information gleaned here will be used to guide future optimization of ZGM design in order to produce zeolites with enhance physicochemical properties.