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Time-resolved in situ Studies of Zeolite Crystal Growth

Authors: 
Choudhary, M. K., University of Houston
Kumar, M., University of Houston
Rimer, J., University of Houston
Zeolites are widely used in commercial processes spanning from ion exchange in detergents to catalysis in the (petro)chemical industry. Understanding zeolite growth mechanisms at the molecular level aids the a priori selection of synthesis parameters to tailor the physicochemical properties of zeolites. Despite tremendous effort to elucidate the mechanisms of nucleation and crystal growth, these pathways in zeolite synthesis are not well understood. This is due in large part to the inherent complexity of zeolite crystallization and the synthesis conditions (i.e., high pH, high temperature, etc.) that render in situ characterization challenging. Recently, our group developed a system to carry out solvothermal in situ atomic force microscopy (AFM) wherein we can observe zeolite surfaces at near molecular resolution under realistic growth conditions.1

Here, we will present in situ AFM measurements of silicalite-1 (MFI) crystallization using zeolite growth modifiers (ZGMs) to modulate the shape of zeolite crystals. ZGMs are molecules or macromolecules that selectively bind to zeolite crystal surfaces and mediate the anisotropic rate(s) of growth to achieve desired crystal size and morphology. We have reported the efficacy of ZGMs in silicalite-1 bulk crystallization experiments.2 Here, we will discuss in situ AFM studies of ZGM effects on silicalite-1 crystals wherein we observe differences in the relative rates of growth by two distinct pathways: classical processes involving molecule addition and nonclassical pathways involving the attachment of amorphous nanoparticle precursors. We show that these pathways can be influenced by the presence of ZGMs.

We will also present in situ AFM results of industrially relevant aluminum-containing zeolite LTA where we observe distinct growth regimes as a function of supersaturation. At high supersaturation, we observe the three-dimensional assembly and structural evolution of gel-like islands on LTA crystals leading to a mechanism that differs from kinetic roughening in classical theory. We also report a distinct switch in the growth mode at low supersaturation marked by two-dimensional nucleation of single layers with step heights corresponding to the composite building units of the crystal structure.

(1) Lupulescu, A. I.; Rimer, J. D.; Science 344 (2014) 729-732.

(2) Lupulescu, A. I.; Rimer, J. D.; Angew. Chemie Int. Ed. 51 (2012) 3345–3349.