In Situ Methods to Characterize Zeolite Growth by Nonclassical Pathway

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 a priori selection of synthesis parameters to tailor their physicochemical properties. Despite significant effort in the past two decades 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. This dissertation focuses on in situ AFM studies of several industrially relevant zeolites (e.g., LTA, MFI). Interest in understanding zeolite A (LTA) 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. Using in situ AFM, 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. These features, which derive from molecularly-dispersed solute, constitute a unique mode of growth among reported cases of nonclassical crystallization. Time-resolved AFM 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. A detailed analysis of zeolite A crystal growth at low supersaturation reveals a predominantly classical mechanism where the generation of new layers on surfaces occurs via three distinct modes: spiral dislocations, 2-dimensional nuclei, and layers emanating from protrusions (defects). Our findings indicate that the selection of silica source plays a vital role in the presence of amorphous deposits, which can become incorporated into advancing layers on zeolite A crystal surfaces. Moreover, in situ AFM measurements using growth solution with and without an organic structure-directing agent reveal that the latter induces the formation of gel-like islands, analogous to conditions of much higher supersaturation and lower synthesis temperature. The presence of amorphous colloidal particles is ubiquitous in many zeolite syntheses, and has led to extensive efforts to understand the driving force(s) for their self-assembly and putative roles in processes of nucleation and growth. We use a combination of in situ scanning probe microscopy, particle dissolution measurements, and colloidal stability assays to elucidate the degree to which silica nanoparticles evolve in their structure during the early stages of silicalite-1 (siliceous analogue of the widely used ZSM-5 zeolite) synthesis. We show how changes in precursor structure are mediated by the presence of organics, and demonstrate how these changes lead to significant differences in precursor-crystal interactions that alter preferred modes of crystal growth. Our findings provide guidelines for selectively controlling silicalite-1 growth by particle attachment or monomer addition, thus allowing for the manipulation of anisotropic rates of crystallization. In doing so, we also address a longstanding question regarding what factors are at our disposal to switch from a nonclassical to classical mechanism.Chemical and Biomolecular Engineering, Department o