Construction of a One-Dimensional Al-Rich ZSM-48 Zeolite with a Hollow Structure

Construction of diverse hollow MFI zeolites through regulating the micropore filling agents

Constructing hollow structure into microporous zeolites can improve the accessibility of acid sites located at the inner part and the diffusion property. Hence, the development of an efficient synthesis strategy to acquire zeolites with tunable hollow structures and acidity has attracted much attention. In this work, an innovative tandem synthesis route was proposed to prepare MFI zeolites with diverse hollow structure while maintaining solid yields exceeding 90 %. The substitution of ethanol molecules, which previously occupied the micropores, with tetrapropylammonium cations was proved to be the key factor to construct hollow structure. And a crystallization-driven particle dissolution mechanism was proposed. The dimension of the hollow cavity, particle size, and Si/Al ratio can be flexibly regulated. Interestingly, hollow MFI samples featuring the common cavity structure, "eye-like" cavity structure, or double-cavity structure can be directly synthesized by controlling the dissolution of core parts. In the 1-butene catalytic cracking reactions, a much higher conversion of 67.2 % was acquired over hollow ZSM-5 compared with that over conventional ZSM-5 (35.8 %) after 64 h of reaction. This improvement can be attributed to the eightfold increase of diffusivity in hollow ZSM-5. This facile and efficient synthesis method endows accurate regulation of the hollow structure, which is meaningful for both fundamental research and industrial applications of hollow zeolites.

Acceleration of Zeolite Crystallization: Current Status, Mechanisms, and Perspectives

Zeolites are important classes of crystalline materials and possess well-defined channels and cages with molecular dimensions. They have been extensively employed as heterogeneous catalysts and gas adsorbents due to their relatively large specific surface areas, high pore volumes, compositional flexibility, definite acidity, and hydrothermal stability. The zeolite synthesis normally undergoes high-temperature hydrothermal treatments with a relatively long crystallization time, which exhibits low synthesis efficiency and high energy consumption. Various strategies, e.g., modulation of the synthesis gel compositions, employment of special silica/aluminum sources, addition of seeds, fluoride, hydroxyl (·OH) free radical initiators, and organic additives, regulation of the crystallization conditions, development of new approaches, etc., have been developed to overcome these obstacles. And, these achievements make prominent contributions to the topic of acceleration of the zeolite crystallization and promote the fundamental understanding of the zeolite formation mechanism. However, there is a lack of the comprehensive summary and analysis on them. Herein, we provide an overview of the recent achievements, highlight the significant progress in the past decades on the developments of novel and remarkable strategies to accelerate the crystallization of zeolites, and basically divide them into three main types, i.e., chemical methods, physical methods, and the derived new approaches. The principles/acceleration mechanisms, effectiveness, versatility, and degree of reality for the corresponding approaches are thoroughly discussed and summarized. Finally, the rational design of the prospective strategies for the fast synthesis of zeolites is commented on and envisioned. The information gathered here is expected to provide solid guidance for developing a more effective route to improve the zeolite crystallization and obtain the functional zeolite-based materials with more shortened durations and lowered cost and further promote their applications.

Unconventional seed-assisted strategy for Al-rich hierarchical ZSM-48 zeolite

Inferior diffusion capacity and insufficient acid density hinder the practical application of ZSM-48 zeolite. Finding a simple and practical strategy to simultaneously address these two defects remains a challenge. In response to this dilemma, we developed an unconventional seed-assisted synthesis strategy for Al-rich hierarchical ZSM-48 zeolite. This approach allows for achieving a broader range of silica to alumina ratio and accelerates the entire crystallization process through the selection of unconventional seeds. The synergy between the seed and organic template was demonstrated to play a pivotal role in facilitating nucleation. Direct evidence from 1H-29Si CP MAS NMR, TG, and IR results demonstrates that hexamethonium ions (HM2+) electrostatically adsorb at the defect sites on the seed, thereby promoting nucleation sites formation. Smaller seed crystals undergo more etching during the induction period, resulting in additional defects and enhanced nucleation ability. The obtained catalyst exhibits a diffusion time constant (Deff/L2) nine times that of conventional ZSM-48 zeolite when using p-xylene as a probe molecule. In m-xylene isomerization reaction, Al-rich hierarchical ZSM-48 demonstrates excellent stability along with higher selectivity and yield for p-xylene compared to typical ZSM-5 catalysts. Remarkably, long-term testing of 1000 h yields over 22.5 % of p-xylene, indicating the potential of this catalyst as an alternative for xylene isomerization reaction. This work not only advances the practical application process of ZSM-48 catalyst but also provides valuable insights for optimizing other zeolites.