(425a) Transient Modes of Zeolite Surface Growth: Establishing New Platforms for Catalyst Design from Mechanistic Understandings of Crystallization

Choudhary, M. K., University of Houston
Kumar, M., University of Houston
Jain, R., University of Houston
Rimer, J. D., University of Houston
Zeolites are widely used in commercial processes spanning from ion exchange in detergents to catalysis in the (petro)chemical industry. Understanding the mechanisms of zeolite growth at a molecular level aids the a priori selection of synthesis parameters to tailor their physicochemical properties. 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. Our group developed a way to carry out solvothermal in situ atomic force microscopy (AFM) wherein we can observe zeolite surfaces at near molecular resolution under realistic growth conditions.1Using this technique, we have identified methods to selectively control the pathways of growth2 and manipulate the anisotropic kinetics of crystallization through the employment of crystal growth modifiers.3

In this talk, we will present in situ AFM results of industrially relevant aluminosilicates (e.g., zeolite A) where we observe distinct growth regimes as a function of supersaturation and temperature. At high supersaturation and low temperature, we observe the three-dimensional assembly and structural evolution of gel-like islands on zeolite surfaces.4 These features, which derive from molecularly-dispersed solute, constitute a unique mode of growth among reported cases of nonclassical crystallization.5 Time-resolved imaging also reveals that growth can occur by (nearly) oriented attachment, which is a rare phenomenon for zeolites, but is observed during crystallization by particle attachment (CPA) for other minerals. We also report a distinct switch in the growth mode at moderate supersaturation and high temperature marked by two-dimensional nucleation of single layers with step heights corresponding to the composite building units of the crystal structure. Crystal growth in low supersaturation occurs by layers emanating from spiral dislocations. Interest in understanding zeolite A formation stems from its widespread use as a commercial molecular sieve; however, recent discoveries that zeolite A is an active catalyst for environmental applications and methanol to olefins reactions has placed this material in the spotlight.

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

(2) Shete et al., Angewandte Chemie International Edition 56 (2017) 535-539

(3) Olafson et al., Chemistry of Materials 28 (2016) 8453-8465

(4) Kumar, M., Choudhary, M.K., Rimer, J.D.; Nature Communications (2018) In Press

(5) De Yoreo et al., Science 349 (2015) aaa6760-1/9