(452f) Simulating the Sorption of Small-Molecule Growth Modifiers on Zeolites

Microporous zeolite materials find use in a variety of important industrial applications including fluid-based separations, ion exchange, hydrocarbon cracking, and methanol reformation.  Despite their widespread use, fully optimizing zeolites for these applications remains a formidable challenge due to the difficulty in controlling crystal morphology and structure during synthesis.  It has been recently demonstrated, however, that such control may be gained through the addition of small-molecule structure directing agents, or zeolite growth modifiers (ZGMs), which bind to specific crystal surfaces and inhibit growth along targeted crystallographic directions [1]. Using small polyols as ZGMs, this strategy has been successfully applied to synthesize zeolite L crystals (LTL type) with controlled pore lengths, suggesting a novel strategy for tuning the transport properties of zeolites [1]. 

Although ZGMs show tremendous promise in facilitating the development of improved zeolite materials, the underlying molecular-level mechanisms that allow them to regulate crystal growth are not fully understood.   To elucidate such mechanisms, we employ molecular simulation techniques to investigate the interactions between small polyol ZGMs and the different crystal faces of zeolite L. While bulk materials have been studied extensively using computational methods, modeling the crystal faces of zeolites poses a unique set of challenges due to the complex surface chemistry involved in terminating the crystal.   We discuss these challenges in detail and describe several different approaches for modeling zeolite surfaces.  We then demonstrate that state-of-the-art free energy techniques, such a metadynamics and umbrella sampling, can be used to gain significant insight regarding the sorption and binding of ZGMs on zeolites.  Our findings support the hypothesis that polyols form hydrogen bonds with the surface of zeolite L, inhibiting growth through a molecular recognition process that is sensitive to both the structure of the polyol and zeolite [1].  Finally, we discuss the boarder implications of our findings and the potential of computational techniques to aid in the judicious selection of ZGMs to target specific crystal faces of zeolites.

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