Alkalinity-controlled zeolite nucleation and growth: ultrafast synthesis of total-morphology zeolite L mesocrystals and adsorption evaluation

Abstract

Owing to the intrinsic complexity of crystallization and the heterogeneity of precursors, the specific stages and corresponding behaviors of an actual crystallization system remain ambiguous, which makes the univariate-controlled crystallization-kinetics-regulated synthesis and design of zeolite morphology and porosity an unrealized blueprint. In this study, a facile and univariate modulation (i.e., OH-/SiO2) strategy was developed to regulate zeolite crystallization kinetics, and zeolite L mesocrystals were synthesized rapidly (within 1-2 h) with almost all LTL morphologies (from cylindrical or disc-like shapes to nanoclusters or nanocrystals) in the simplest SiO2-Al2O3-K2O-H2O system. Using time-resolved analysis of the change in the solid-liquid Si/Al nutrient and crystallinity evolution, the intertwined and complex crystallization processes of zeolite L were clearly distinguished into four distinct stages: induction, nucleation, growth, and ripening. Under alkalinity-controlled conditions, the reactivity, Si/Al distribution, and state of aluminosilicates were critical to the formation of short-range order in the amorphous matrix, which greatly influenced the nucleation frequency and assembly state. Subsequently, these nucleation differences evoked correspondingly different kinetic growth behaviors. A putative alkalinity-controlled nonclassical crystallization mechanism was uncovered, and its validity was evaluated by analyzing morphology evolution, NH4F etching, and the effects of modifiers. Furthermore, adsorption tests demonstrated the high adsorption capacity of a series of zeolite L for guest molecules with various sizes and properties (e.g., gaseous aromatic hydrocarbon, aqueous dye, and protein).