Mesocrystal morphology regulation by "alkali metals ion switch": Re-examining zeolite nonclassical crystallization in seed-induced process

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 H₂O), 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 H₂O/SiO₂ range, mixed Na⁺/K⁺ systems, and other alkali metal ions from Li⁺ to Cs⁺, and ZSM-5 mesocrystals with extended morphologies can be obtained.

Mesoporogen-Free Strategy to Construct Hierarchical TS-1 in a Highly Concentrated System for Gas-Phase Propene Epoxidation with H2 and O2

Hierarchical TS-1 has attracted enormous attention from both academia and industry due to its remarkable catalytic performance in epoxidation reactions. However, sustainable synthesis of hierarchical-nanosized TS-1 without mesoporogens is still challenging. In this work, we report a facile and mesoporogen-free strategy to simultaneously manipulate pore structure and particle size of TS-1 employing the concentrated system. Taking advantage of the suspended nuclei in the concentrated system as confirmed by the DLS-PSD and atomic force microscopy, the novel TS-1 is demonstrated to have higher Ti concentration on surface, higher surface area (539 m²/g), abundant mesopores, and reduced crystal size (ca. 150 nm). Moreover, this Au-Ti bifunctional catalyst shows a good PO formation rate with enhanced catalytic stability due to the hierarchical structure. This strategy opens a novel way for the green synthesis of hierarchical-nanosized TS-1 and facilitates industrial development of the Au/TS-1 catalyst for propene epoxidation.

Observing a Zeolite Nucleus (Subcrystal) with a Uniform Framework Structure and Its Oriented Attachment without Single-Molecule Addition

Multiple and complex crystallization process of zeolite including complementary single-molecule condensation and particle assembly, and alternately dominant nucleation and growth behavior, plays the critical role in zeolite crystallization but meanwhile makes us hard to study the respective effects. Herein, we strip nuclei from the synthetic solution and find that high-ordered nucleus (subcrystal) is the premise to ignite high-speed growth of zeolite crystal. The high-ordered subcrystals with the size of only 6-10 nm possess regular aperture structure and microporous area similar to zeolite nanocrystal. Interestingly, a unitary oriented aggregation process of the subcrystals towards nanosheets is well observed and characterized where single-molecule addition process is greatly repressed. If a wider range of zeotype nuclei can be expanded, a new synthetic strategy of zeotype materials with heterogeneous framework and active sites may be expected, which may novelize zeolite catalytic properties.