Divergent Mechanisms of SSZ-39 Crystallization Using Structurally Similar but Chemically Distinct Organic Structure-Directing Agents

The role of an organic structure-directing agent on zeolite crystallization is conventionally interpreted based on the final, bulk crystal structure. However, few studies have examined their effect on the dynamics of zeolite crystal growth and restructuring, particularly with respect to the interzeolite transformations that are increasingly being exploited in zeolite synthesis. Herein, we compare two organic structure-directing agents that both direct the formation of the small-pore zeolite SSZ-39 (AEI). The organics have nearly identical molecular structures but exhibit distinct chemical compositions by virtue of a single heteroatom substitution. Our findings reveal that the organics have a dramatic impact on the crystallization kinetics, physicochemical properties, and catalytic performance of zeolite AEI prepared by the transformation of zeolite FAU parent crystals. The conventional quaternary ammonium structure-directing agent, "Pippy", produces a distinct intermediate metastable structure with defects and anisotropic crystal shape that transitions into relatively thick zeolite AEI platelets with compromised hydrothermal stability. This transition is accompanied by an unusual morphological evolution from rod-like to platelet crystals that defies common Ostwald ripening processes. We demonstrate that a new quaternary ammonium-ether structure-directing agent, "Morphy", produces thinner platelets of zeolite AEI and bypasses the defective intermediate observed for the conventional organic. Syntheses with Morphy produce a more hydrothermally stable product, which exhibits superior activity in the NH₃ selective catalytic reduction of NO_x used as a benchmark reaction for assessing structure-performance relationships.