(334be) Understanding the Fundamentals of Zeolite Crystallization for the Optimization of Their Physicochemical Properties

My research interests encompass from understanding crystallization fundamentals to material design. In this regard, during my Ph.D., I have focused on understanding underlying fundamental mechanisms in zeolite crystallization, which is critical in developing optimal heterogeneous catalysts for different industrially relevant reactions, e.g., methanol to hydrocarbon (MTH) reaction, hydrocarbon cracking, Friedel craft alkylation, etc.

Introduction: Zeolites

Zeolites are crystalline microporous aluminosilicates consisting of tetrahedra of SiO₄ and AlO₄^- linked by O atoms, producing three-dimensional networks of channels and cavities of molecular dimensions with ordered geometries and connectivity (Fig. A). Due to their unique properties, such as exceptional hydrothermal stability and tunable acidity, zeolites have extensive applications in petrochemical refining, production of fine chemicals, biomass conversion, separations, emissions control, and drug delivery. More than 245 zeolite frameworks have been synthetically realized; however, only ca. 20 of these structures are used commercially owing in part to the complexity of zeolite synthesis and the economic costs required to expand the list of viable framework types. Thus, it is essential to understand underlying mechanisms in zeolite crystallization.

Zeolite crystal growth occurs by a combination of two general mechanisms (scheme in Fig. 1): a classical pathway involving monomer addition (Fig. B, 1a), and nonclassical pathways (Fig. B, 1b – 3a) that occur by the addition of species (or precursors) more complex than a monomer. For zeolites, growth solutions contain a diverse mix of precursors: oligomers (Fig. B, 1c), amorphous particles (Fig. B, 2a), and small crystallites (Fig. B, 3a). [1] There are many unanswered fundamental questions regarding the role of precursors in zeolite crystallization. I have focused on studying different paradigms of zeolite design which I will discuss briefly below:

Understanding the role and prevalence of defects in zeolite crystallization by imaging through in-situ atomic force microscopy (AFM) of 2-dimensional surface growth

Atomic force microscopy (AFM) is a powerful technique to study surface dynamics. Usually, zeolite synthesis occurs at high temperature (60-200°C) and high pH conditions, which render in-situ observations challenging. Many groups have utilized ex-situ AFM to study zeolite crystal growth, but without real-time measurements, the actual mechanism remains elusive. In this regard, our research group has pioneered the technique of in-situ atomic force microscopy (AFM), which enables the visualization of crystal growth with unparalleled spatiotemporal resolution. I have utilized in-situ AFM to investigate the crystallization of zeolite A which has commercial applications in the processes involving adsorption and ion exchange (Fig. A) to understand the synthesis conditions and final property relationships for the predictive control on the properties of zeolites.

I identified multiple pathways of classical growth where the most common mode of layer generation stems from large (type I) protrusions (Fig. C), which are presumed to be defects located at the apex of hillocks. Secondary growth of these surfaces reveals the continued generation of new layers from the edge of defects, leading to the sustained growth of hillocks. Direct observation of type II defects (Fig. C) was enabled by the identification of conditions leading to 2D layered growth.

Parametric analysis of growth conditions also reveals that the selection of the silicon source, most notably colloidal silica, is paramount in the generation of type II features, which are deemed to be the remnants of undissolved amorphous silicates. The ubiquitous observation of defects (types I and II) indicate their prevalent role in zeolite A crystallization. [2] Moreover, the presence of type II defects has practical implications as they can reduce acid site concentration and/or restrict diffusion in nanopores, which would negatively affect their performance in applications of catalysis and adsorption (in stark contrast to the rational design of nanocomposites where particle occlusion often enhances performance, e.g., metal@zeolite bifunctional catalysts). Previously undetected defects in zeolite A are associated with media containing undissolved silica, which is common in zeolite synthesis, suggesting the occlusion of amorphous silica is a phenomenon that likely extends to other framework types prepared by similar sol-gel methods.

Seed-Assisted zeolite synthesis: The impact of seeding conditions and interzeolite transformations on crystal structure and morphology

Seed-assisted zeolite synthesis is a simple and economically viable approach to alter crystal habit. Our findings reveal that the nature of the seed has a significant impact on the final product; and that only a small quantify of seeds is needed to dramatically reduce crystal size without any unintended modification of framework composition (Si/Al ratio) (Fig. D). [3] A phenomenon that is closely tied to seeded growth is that of interzeolite transformations where the nucleation of a new crystal occurs after the formation of an initial, more thermodynamically metastable structure. Interzeolite transformations are analogous to crystal polymorphism but in this case, two crystals involved in the transition differ in structure as well as in chemical composition. This process is akin to the Ostwald rule of stages where two or more distinct transitions can occur. The main objective of this study is to establish an improved understanding of how seeding influences zeolite crystallization and elucidate the factors driving interzeolite transformations.

In this work, I examined the relationships between parent (product) and daughter (seed) crystals in seeded growth assays using a variety of seed properties and growth solution compositions. In many instances, we observe that seeded growth results in interzeolite transformations where the timescales are dependent upon the conditions selected, and the rationale for the observed trajectory is not well understood or easily predictable. In general, our observations indicate that the growth solution plays a vital role in controlling the kinetics of interzeolite transformations. The thermodynamic driving force for these transformations cannot be easily described by a single variable, such as the molar volume of zeolite structure as prescribed by previous hypotheses. The dissolution of seeds presumably leads to species that initially facilitate the nucleation of an identical structure; however, over the course of synthesis, the composition of the growth solution changes, thereby altering the chemical potential (i.e., driving force), leading to the nucleation of a new phase. This highlights the importance of kinetics in seeded zeolite syntheses wherein the previous hypotheses must consider additional factors that include (but are not limited to) synthesis time, temperature, and the compositions of seeds and the growth medium. [3]

References

1. Olafson, K. N., Li, R., Alamani, B. G., & Rimer, J. D. (2016). Engineering crystal modifiers: bridging classical and nonclassical crystallization. Chemistry of Materials, 28(23), 8453-8465.
2. Choudhary, M. K., Jain, R., & Rimer, J. D. (Submitted). In situ imaging of 2-dimensional surface growth reveals the role and prevalence of defects in zeolite crystallization.
3. Jain, R., & Rimer, J. D. (2020). Seed-Assisted zeolite synthesis: The impact of seeding conditions and interzeolite transformations on crystal structure and morphology. Microporous and Mesoporous Materials, 110174.