K-Chabazite Zeolite Nanocrystal Aggregates for Highly Efficient Methane Separation

Methane (CH₄ ) enrichment and purification is of great significance for increasing the heating value of unconventional natural gas sources and curtailing its effect on global warming. For a long time, commercial adsorbents for CH₄ separation have been hampered by low adsorption capacities, sub-optimal adsorption selectivities, or slow diffusion rates, which have significantly restricted separation productivity. Herein, we report a facile and green seed-passaging method to fabricate donut-like macro-meso-micro hierarchical zeolite K-Chabazite nanocrystal aggregates. This consecutive seed-inducing method requires no organic template. By utilizing this unique nanocrystallization technique, the CH₄ adsorption capacity, gas diffusion rate, and separation productivity of the resultant material are dramatically increased compared with those of commercially available adsorbents, thus setting a new benchmark in CH₄ /N₂ separation. More importantly, production of this adsorbent can be easily scaled-up to the order of 100 kg using readily available raw materials in this environmentally friendly synthetic route, enabling potential industrial implementation.

Rigid Arrangements of Ionic Charge in Zeolite Frameworks Conferred by Specific Aluminum Distributions Preferentially Stabilize Alkanol Dehydration Transition States

Zeolite reactivity depends on the solvating environments of their micropores and the proximity of their Brønsted acid sites. Turnover rates (per H⁺ ) for methanol and ethanol dehydration increase with the fraction of H⁺ sites sharing six-membered rings of chabazite (CHA) zeolites. Density functional theory (DFT) shows that activation barriers vary widely with the number and arrangement of Al (1-5 per 36 T-site unit cell), but cannot be described solely by Al-Al distance or density. Certain Al distributions yield rigid arrangements of anionic charge that stabilize cationic intermediates and transition states via H-bonding to decrease barriers. This is a key feature of acid catalysis in zeolite solvents, which lack the isotropy of liquid solvents. The sensitivity of polar transition states to specific arrangements of charge in their solvating environments and the ability to position such charges in zeolite lattices with increasing precision herald rich catalytic diversity among zeolites of varying Al arrangement.

High Ethylene Selectivity in Methanol-to-Olefin (MTO) Reaction over MOR-Zeolite Nanosheets

Precisely controlled crystal growth endows zeolites with special textural and catalytic properties. A nanosheet mordenite zeolite with a thickness of ca. 11 nm, named as MOR-NS, has been prepared using a well-designed gemini-type amphiphilic surfactant as bifunctional structure-directing agent (SDA). Its benzyl diquarternary ammonium cations structurally directed the formation of MOR topology, whereas the long and hydrophobic hexadecyl tailing group prevented the extensive crystal growth along b axis. This kind of orientated crystallization took place through the inorganic-organic interaction between silica species and SDA molecules present in the whole process. The thin MOR nanosheets, with highly exposed (010) planes and 8-membered ring (MR) windows, exhibited a much improved ethylene selectivity (42.1 %) for methanol-to-olefin (MTO) reactions when compared with conventional bulk MOR crystals (3.3 %).

Regulating Nonclassical Pathways of Silicalite-1 Crystallization through Controlled Evolution of Amorphous Precursors

Differentiating mechanisms of zeolite crystallization is challenging owing to the vast number of species in growth solutions. The presence of amorphous colloidal particles is ubiquitous in many zeolite syntheses, and has led to extensive efforts to understand the driving force(s) for their self-assembly and putative roles in processes of nucleation and growth. In this study, we use a combination of in situ scanning probe microscopy, particle dissolution measurements, and colloidal stability assays to elucidate the degree to which silica nanoparticles evolve in their structure during the early stages of silicalite-1 synthesis. We show how changes in precursor structure are mediated by the presence of organics, and demonstrate how these changes lead to significant differences in precursor-crystal interactions that alter preferred modes of crystal growth. Our findings provide guidelines for selectively controlling silicalite-1 growth by particle attachment or monomer addition, thus allowing for the manipulation of anisotropic rates of crystallization. In doing so, we also address a longstanding question regarding what factors are at our disposal to switch from a nonclassical to classical mechanism.

Recent advances of the nano-hierarchical SAPO-34 in the methanol-to-olefin (MTO) reaction and other applications

The synthesis and catalytic applications of nano-hierarchical SAPO-34 for MTO conversion have received great significance.

A green surfactant-assisted synthesis of hierarchical TS-1 zeolites with excellent catalytic properties for oxidative desulfurization

Hierarchical TS-1 zeolites with intracrystalline mesopores are synthesized via a green surfactant-assisted route, showing excellent performance in oxidative desulfurization.

A design concept of amphiphilic molecules for directing hierarchical porous zeolite

This review describes a design concept of novel amphiphilic molecules for a one-step preparation of hierarchically porous zeolites containing mesopores with certain orders.

Programmable synthesis of mesoporous ZSM-5 nanocrystals as selective and stable catalysts for the methanol-to-propylene process

Programmable synthesis of mesoporous ZSM-5 nanocrystals for a highly efficient methanol to propylene reaction.

Size-dependent thermal decomposition and kinetics of ultrafine alkali metal styphnates

The styphnate of smaller size possesses lower decomposition temperature and higher reactivity.