Hypothesis It is a Holy Grail to realize the goal-oriented synthesis of zeotype crystals via direct thermodynamic/kinetic control of crystallization in the simplest inorganic system. Especially, the most commonly used counter cations (i.e., Na + and K + ) are in turn believed to play merely the role of balancing charges and stabilizing frameworks, which make the simple ion-based morphology/porosity control remain big challenges. Experiments We re-examined the role of Na + and K + to fine-tune the classical/nonclassical crystallization process in a seed-induced system with the simplest composition (Si/Al sources, inorganic alkali, and H 2 O), and proposed an “ion switch” strategy. By analyzing the multiple growth curves, tracking the precursor evolution, and observing epitaxial crystallization behavior , a distinctive “ion switch”-worked nonclassical mechanism was uncovered. Findings By the “ion switch” strategy, ZSM-5 mesocrystals were fine-regulated with diverse architecture from single crystal to nanocrystallite assembly and intracrystal mesopore-enriched crystal. Such simple ions-controlled crystallization was achieved through microstructure heterogeneity of zeolitic building-blocks triggered by different counterions and their corresponding assembly behavior from oriented attachment to random deposition. Furthermore, this protocol can be extended to a wider H 2 O/SiO 2 range, mixed Na + /K + systems, and other alkali metal ions from Li + to Cs + , and ZSM-5 mesocrystals with extended morphologies can be obtained.
Tao Jiang,Fumin Wang,Xubin Zhang,Yi Zhai,Guojun Lv,Yu Shen,Yuzhou Wu
Mesocrystal zeolite with well-defined morphology owns mesoporous channels and single crystal characteristics, which could promote diffusion efficiency, stability and catalytic activity. Thus, we performed the controllable synthesis of ZSM-5 mesocrystal without special additives and complex methods. Selected area electron diffraction (SAED) and fast Fourier transform (FFT) showed diffraction spots regardless of crystallization time, which indicated the well oriented feature of synthesized ZSM-5 zeolites. The crystallization process was further investigated. The oriented particle attachment (OPA) effectively guided the synthesis process of mesocrystals, which followed the nonclassical crystallization mechanism. The crystallinity curve exhibited valley-like shape, which manifested that the crystallization pathway went through three stages: fast precipitation, deconstruction and reorganization. The plausible formation mechanism was proposed, which might be useful for the fabrication of ZSM-5 mesocrystal. The prepared zeolites showed superior catalytic performance in aldol condensation reaction between benzaldehyde and 2-hydroxyacetophenone, obtaining 93% of the conversion.
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