TUNING ZEOLITE PROPERTIES AND UNDERSTANDING REACTION MECHANISMS TO OPTIMIZE CATALYTIC PERFORMANCES

Zeolites are crystalline nanoporous materials with exceptional physicochemical properties that are extensively utilized in heterogeneous catalysis. The (hydro)thermal stability, shape-selectivity, and tuneability of acidity have enabled the use of zeolites in commercial applications in fields such as catalysis, ion-exchange, and gas separations. With the vast potential of zeolite materials, approximately 20 zeolite frameworks out of 255 have been commercialized. The major challenges to unlock this potential involve the ability to rationally design zeolite materials with predefined properties via economical pathways and understand the structure-performance relationships to determine the desired properties for a particular application. Zeolites crystallize via two different mechanisms: a classical pathway involving monomer addition, and nonclassical pathways that occur by the addition of precursors (amorphous or crystalline) more complex than monomers. The manipulations of the zeolite synthesis using zeolite growth modifiers (ZGMs), structure-directing agents (OSDAs), and heteroatoms can optimize their physiochemical properties such as framework stability, acidity, diffusional properties, and morphology of zeolite crystals. Apart from zeolite design, the reaction conditions can also have a profound impact on their catalytic performances. Hence, various experimental and computational tools are used to establish structure-performance relationships, thereby elucidating reaction and deactivation mechanisms, with the underlying goal to understand the catalytic behavior and optimize the catalyst design and reaction environment. A core objective of zeolite design is to improve their diffusional properties. Zeolite nanoporosity is crucial for shape selectivity via a lock and key mechanism; however, this poses significant diffusion limitations to guest molecules, which can lead to their faster deactivation as catalysts. Two major synthesis approaches used to tackle this challenge are (i) introduction of secondary porosity (hierarchical zeolites) with meso-/macopores and (ii) synthesis of nano-sized zeolites, which is a non-trivial task and often requires expensive OSDAs and ZSMs.

This Thesis focuses on identifying zeolite catalysts and operating conditions that can simultaneously optimize product selectivity and feed utilization for commercially relevant reactions. For toluene alkylation with methanol (TAM) reaction, we show that zeolite MCM-22 (MWW) has exceptional catalyst lifetime in the TAM reaction at high operating pressure, selectivity, and conversion. We systematically probe catalytic behavior of active sites in distinct topological features of MCM-22, revealing that high p-xylene selectivity and catalyst stability are predominantly attributed to sinusoidal channels and supercages, respectively. We propose a spatiotemporal coke coupling phenomenon to explain a multi stage p-xylene selectivity profile wherein the formation of light coke in supercages initiates the deactivation of unselective external surface sites. The heterogeneity of ZSM-5 acid sites for TAM reaction was also probed by density functional theory (DFT) calculations, which exhibits that most selective and active sites for TAM reactions are present in sinusoidal channels and pores intersection, respectively. Furthermore, we utilize heteroatoms as growth modifiers to tune the properties of the zeolite crystals. We developed a facile and generalizable strategy to synthesize nano-sized zeolites (< 100 nm) using GeO2 additive. With improved diffusional properties, nano-sized zeolites show exceptional performance for methanol-to-hydrocarbons (MTH) reaction as compared to their conventional counterparts. We also report a new method to achieve high silica zeolite Y (Si/Al = 3.5) via an organic-free route using zinc oxide addition. High silica content improves the hydrothermal stability of zeolite Y, without the loss of activity with majority of acidity originating from Lewis acid sites. Moreover, we show that the manipulation of amorphous precursors with alkali cation and polymer additives reduces the zeolite crystallization time of multiple zeolite framework types, which can have profound economic impact on commercial zeolite catalysis.