(63g) In Situ Characterization of Zeolite Surface Growth Using Atomic Force Microscopy

Zhiyin Niu, Rishabh Jain, Madhuresh K. Choudhary, and Jeffrey D. Rimer*

Chemical and Biomolecular Engineering, University of Houston, Houston, Texas 77204, *jrimer@central.uh.edu

Zeolites are crystalline aluminosilicates with a network of pores that are useful for a broad number of commercial applications. In the area of catalysis, a constantly evolve energy landscape coupled with the changing of demands of different feedstocks requires more fundamental understandings of zeolite crystallization mechanisms as a means of developing more predictive protocols to tailor their physicochemical properties. One of the most widely-used commercial zeolites is faujasite (FAU), with applications spanning from ion exchange in detergents to catalysts in the (petro)chemical industry. The conventional sol gel media used to prepare zeolites such as FAU often leads to heterogeneous mixtures comprising both solution and solid states. These complex media contain diverse solute species, which evolve over the course of crystallization, making zeolite nucleation and growth pathways nontrivial to characterize. Despite tremendous efforts to understand the growth mechanism(s) of zeolite FAU, the pathways leading to its formation are generally not well understood. This is complicated by the fact that the harsh conditions of zeolite synthesis often make it difficult to perform in situ observations.

Here, we will discuss how we have been using high temperature atomic force microscopy (AFM) to characterize the growth of several zeolites, including silicalite-1 (MFI), zeolite A (LTA), and most recently faujasite (FAU). Our findings reveal diverse mechanisms involving both classical and nonclassical pathways. The latter include crystallization by particle attachment (CPA), either through amorphous precursors or via oriented attachment of nanocrystals. For the case of zeolite A, we observed that supernatant solutions with varying supersaturation could be used to switch growth from nonclassical pathways to classical growth involving 2-dimensional layer generation and spreading. Recently, we reported the use of AFM to monitor FAU surface growth in situ wherein we confirmed a pathway that markedly deviates from that of zeolite A, which is formed under similar conditions. Here, we will discuss how in situ AFM in combination with other state-of-the-art characterization techniques have been used in our group to provide a deeper understanding of zeolite surface growth under different crystallization environments; and how these conditions can be tailored to alter the physicochemical properties of the final zeolite materials.