Mechanistic Elucidation of Solid-State Zeolite Crystallization of Dense-Phase Cancrinite Using Time-Elapsed Tracking

Abstract

Designing a new class of functional materials is heavily dependent on the fundamental understanding of the crystallization mechanism and the effect of physicochemical parameters. The rational design of zeolite crystals is significantly understood via the extensive mechanistic study of hydrothermal routes; however, solid-state crystallization is still elusive and challenging to study. On this account, we have formulated a chemical composition to synthesize the pure cancrinite (CAN) phase through solid-state transformation and subsequently conducted a comprehensive time-lapsed study to decipher phase transformation and morphological evolution. Broadly, crystallization exhibits a trend of a slower rate of nucleation followed by a faster rate of phase formation similar to that of the hydrothermal pathway. However, crystals present a unique multipodal architecture. A detailed study on the role of seed crystals was made using CAN as an isomorphic seed and Faujasite (FAU) as the heteromorphic seed. CAN seed accelerates the crystallization kinetics to a greater extent while following the seed preservation pathway. Typical rod-shaped bulk particles with rough outer surfaces were formed. However, seeding using FAU crystals exerts a typical role as a kinetics accelerator while exerting the interzeolitic-transformation phenomenon in this relatively new synthesis route. Morphological evolution reveals an agglomeration-based nonclassical growth mechanism where nanoparticles with irregular shapes undergo particle-mediated attachment through the crystallographic plane to form nanodomains. These domains aggregate to furnish the final multipod-shaped bulk particles. The surface smoothing of the bulk particles was also observed with prolonged heat treatment. The study on the role of the FAU seed using Raman spectra suggests that the FAU phase supplements 4-MRs as the building units to the initial solid mixture that acts as the preformed precursors, facilitating the enhanced rate of phase kinetics. Overall, our study portrays a detailed blueprint of the phase and morphological evolution of a zeolite in solid-state crystallization, which lays the foundation needed for the rational design of efficient catalysts with finely tuned structural properties using this economically lucrative synthesis pathway.