(403f) Mechanistic Insights into the Solid-State Crystallization of High-Density Zeolite

Zeolite, with its unique pore architecture and tunable acid site distribution, has emerged as a prominent candidate in the multi-dimensional application field. Conventional hydrothermal zeolite synthesis has several disadvantages, including high solvent consumption, low yield, high operational pressure, and environmental concerns that make these processes economically challenging. Researchers have developed various new strategies to mitigate these challenges. A new pathway called solid-state transformation has recently been reported, which can potentially become the greener approach to synthesizing zeolite with commercial profitability. Although studies have successfully postulated the formation mechanism of several zeolite frameworks, the control over morphological growth and hence the intrinsic properties are still elusive. From this viewpoint, we have established a mechanistic insight into the crystallization of zeolite in solid-state synthesis. We synthesized a pure cancrinite framework and tracked the morphological growth of the phase. Our investigation reveals the presence of a concurrent process where phase and morphological growth takes place simultaneously. Initially formed nanoparticles act as building units and undergo agglomeration to develop a dendrimer-shaped particle. On a closer inspection of the role of these nanoparticles using HRTEM imaging, the formation of nano-domains have observed where nanoparticles are attached through a common crystallographic plane. Even the initially formed partially amorphous nanoparticles have started to connect through a unidirectionally oriented crystallographic registry. Successive attachment of nanoparticles formed nano-domains having single-crystal-like features. Interestingly, participating lattice plane is different in different nano-domains. These nano-domains agglomerate randomly to create the final mesocrystal. A hybrid pathway of non-classical growth was observed where nanoparticles tend to form nano-domains having features of oriented attachment through a common crystal plane. These nano-domains combine randomly to form bulk crystals of the CAN phase. Apart from the mechanistic understanding, our work had a direct commercial impact on yield improvement (> 400 wt%) for the mass production of zeolite.