STRUCTURE-DIRECTING AND OPTIMIZATION IN ZEOLITE CRYSTALLIZATION

Zeolites are crystalline microporous aluminosilicates with silica-rich frameworks that have been widely applied in petrochemical processes and gas separations owing to their microporosity, excellent thermal stability and tunable acidity. Zeolite crystallization often requires the presence of an organic structure-directing agent (OSDA) to facilitate the formation of nanoporous cages and channels, though the expensive organic reactants have environmental disadvantages. Among the limited number of zeolites that can be synthesized in organic-free media, alkali metals are most commonly employed as inorganic organic structure-directing agents (SDAs). Despite the wide abundance of alkaline earth metals in the earth’s crust and in zeolite minerals, these divalent metal ions have been rarely used in zeolite syntheses. Guided by elemental compositions of natural minerals, we investigated the crystallization of CHA- and PHI-type zeolites and developed novel approaches to prepare both materials by introducing alkaline earth metals into growth media. Notably, we demonstrated that strontium ions have a pronounced impact on the kinetics of chabazite formation, leading to 14- and 3-fold reductions in crystallization time compared to syntheses with only potassium ions in the absence and presence of crystal seeds, respectively. We also highlighted the role of barium in crystallization of PHI-type zeolites to accelerate the kinetics by altering the cationic structure and also tuning the shape and size of the resulting zeolite crystals. Collectively, these findings identify routes to regulate zeolite crystallization kinetics and provide strategies to optimize the size and shape of zeolite crystals. Driven by the need to find facile ways to prepare zeolites cost-efficiently, the seed-assisted approach can provide a low-cost, environmentally friendly pathway for the production of zeolites. Among the zeolites used industrially, ZSM-23 (MTT-type) has shown advantages in cracking and isomerization reactions owing to the unique one-dimensional channels composed of aligned 10-member rings. However, the shape of ZSM-23 crystal is heavily dependent on the choice of OSDAs. We investigated the crystallization of ZSM-23 and explored a novel method by introducing a heterogenous crystal seed exhibiting TON-type morphology. The great similarity between MTT- and TON-type topologies leads to improved physicochemical properties of the resulting ZSM-23 zeolites. We tested the performance of ZSM-23 catalysts prepared using seeded growth in the methanol to-hydrocarbon (MTH) reaction and found improved conversion rate compared to ZSM-23 catalysts prepared using non-seeding methods.