Insights into the crystal growth mechanisms of SAPO-34/ZSM-5 composite zeolite from combined experimental imaging and theoretical studies

Abstract

Despite the critical role of composite zeolites in catalysis, the crystallization pathways of SAPO-34/ZSM-5 remain poorly understood, limiting their rational design. We describe the complex crystal growth mechanisms of SAPO-34/ZSM-5 composite zeolites by a novel combination of advanced experimental imaging and theoretical modeling, offering a time-resolved framework for their structural evolution. Using different high-resolution techniques, including SEM, TEM, AFM, XRD, FTIR, DLS, and zeta potential measurements, we observe the morphological, structural, and surface charge dynamics over a 24-hour crystallization period, delineating six distinct growth stages: heterogeneous nucleation, diffusion-limited aggregation, oriented attachment, spiral growth, dissolution-recrystallization, and Ostwald ripening. Our research shows that ZSM-5 seeds act as essential nucleation sites, facilitating the initial formation of SAPO-34 nuclei. Furthermore, charge-induced transitions, indicated by significant changes in zeta potential from -50.2 mV to +134.6 mV, influence the aggregation and structural enhancement of the composite. We quantify the energy barriers governing each growth stage by applying Classical Nucleation Theory and the Burton-Cabrera-Frank model, providing a predictive framework for crystal engineering. This thorough mechanistic comprehension connects traditional and non-classical crystallization processes and facilitates the exact synthesis of composite zeolites with enhanced efficacy in the methanol-to-olefins (MTO) reaction. By elucidating growth mechanisms, this study enables the targeted design of SAPO-34/ZSM-5 catalysts with tunable properties for enhanced performance in methanol-to-olefins (MTO) conversion.