Understanding formation thermodynamics of structurally diverse zeolite oligomers with first principles calculations

The mechanisms of many zeolitic processes, including nucleation and interzeolite transformation, are not fully understood owing to complex growth mixtures that obfuscate in situ monitoring of molecular events. In this work, we provide insights into zeolite chemistry by investigating the formation thermodynamics of small zeolitic species using first principles calculations. We systematically study how formation energies of pure-silicate and aluminosilicate species differ by structure type and size, temperature, and the presence of alkali or alkaline earth metal cations (Na⁺, K⁺, and Ca²⁺). Highly condensed (cage-like) species are found to be strongly preferred to simple rings in the pure-silicate system, and this thermodynamic preference increases with temperature. Introducing aluminum leads to more favorable formation thermodynamics for all species. Moreover, for species with a low Si/Al ratio (≤2), a thermodynamic preference does not exist among structure types; instead, a pool of diverse aluminosilicate structures compete in formation. Metal cation effects strongly depend on the presence of aluminum, cage size, cation type, and location, since each of these factors can alter electrostatic interactions between cations and zeolitic species. We reveal that confined metal cations may destabilize pure-silicate cages due to localized interactions; conversely, they stabilize aluminosilicates due to strong cation-framework attractions in sufficiently large cages. Importantly, this work rationalizes a series of experimental observations and can potentially guide efforts for controlling zeolite nucleation/crystallization processes.

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

Diffusion limitation and acid deficiency are two main challenges that the ZSM-48 zeolite faces in practical application. To date, there have been few effective strategies to solve both problems, simultaneously. Also, it is also a challenge to construct a hollow structure in a one-dimensional (1D) zeolite. Herein, an Al-rich ZSM-48 zeolite with a hollow structure is constructed through an alumination-recrystallization strategy, thereby solving the problems related to diffusion and acidity simultaneously. The hollowness and enrichment of aluminum can be controlled by judiciously matching the desilication and recrystallization. The silica to alumina ratio (SAR) of the ZSM-48 zeolite can be tuned from 130 to 45, which breaks the SAR limitation of conventional synthesis. On the basis of the different characterization results, the whole crystallization can be divided into two stages: rapid desilication-alumination and time-consuming recrystallization. In the selective desilication-recrystallization process, the preferential special distribution of the organic template leads to the formation of a hollow structure and the healing of mesopores at the outer shell, as evidenced by structured illumination microscopy images. Due to the enhancement in diffusion ability and acid density, the obtained hollow Al-rich ZSM-48 zeolite exhibits excellent catalytic stability and high p-xylene yield (∼26%) in the m-xylene isomerization reaction (WHSV = 18 h^-1), indicating its strong industrial application potential.