(600bu) Controlling Zeolite Synthesis with Molecular Modifiers: A Versatile Approach to Design Catalysts

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

There are growing opportunities for the use of zeolite catalysts in a wide range of applications. Despite their extensive use in commercial processes, an understanding of their growth mechanism(s) remains elusive. The rational design of zeolite catalysts calls for more versatile synthetic approaches capable of a priori tailoring crystal properties, such as crystal size, morphology, and surface architecture. Here we will discuss the design of industrially relevant zeolites, notably LTL, MOR and CHA. Focusing on this class of porous materials presents an opportunity to explore the impact of crystal engineering on targeted catalyst performance (e.g., activity and lifetime). To this end, we will discuss our studies of zeolite crystallization in the presence of zeolite growth modifiers (ZGMs), which are molecules that selectively bind to specific surfaces of zeolite crystals and mediate anisotropic growth rates [1]. We systematically examined a library of modifiers ranging in their structure and functional moieties to assess the physicochemical properties that regulate their efficacy and specificity as zeolite crystal growth modifiers. As we will demonstrate, the judicious selection of ZGMs can markedly alter zeolite crystal morphology from high aspect ratio (needle-like) crystals and micron-sized spheroidal crystals to thin discs and small spheres, respectively, with concomitant reduction in the internal diffusion pathlength and external surface area. Our studies reveal that ZGMs permit the control of zeolite crystal size spanning more than three orders of magnitude. This work also revealed that ZGM hydrophobicity and the spatial sequencing of binding moieties are effective molecular descriptors of its efficacy [2].

Collectively, these studies seek to establish a commercially viable approach to optimize catalyst performance and to provide general heuristics for designing zeolites. We will present these fundamental studies and also discuss how techniques, such as atomic force microscopy [3], can be used to probe the mechanisms of ZGM action. Moreover, we will present time elapsed studies of crystallization in order to elucidate the impact of modifiers on zeolite nucleation and growth.

References:

[1] Lupulescu, A.I. and Rimer, J.D., Tailoring Silicalite-1 Crystal Morphology with Molecular Modifiers, Angew. Chem. Int. Ed. 51 (2012) 3345-3349.

[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

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

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