Does Water Enable Porosity in Aluminosilicate Zeolites? Porous Frameworks versus Dense Minerals

Recently identified zeolite precursors consisting of concentrated, hyposolvated homogeneous alkalisilicate liquids, hydrated silicate ionic liquids (HSIL), minimize correlation of synthesis variables and enable one to isolate and examine the impact of complex parameters such as water content on zeolite crystallization. HSIL are highly concentrated, homogeneous liquids containing water as a reactant rather than bulk solvent. This simplifies elucidation of the role of water during zeolite synthesis. Hydrothermal treatment at 170 °C of Al-doped potassium HSIL with chemical composition 0.5SiO₂:1KOH:xH₂O:0.013Al₂O₃ yields porous merlinoite (MER) zeolite when H₂O/KOH exceeds 4 and dense, anhydrous megakalsilite when H₂O/KOH is lower. Solid phase products and precursor liquids were fully characterized using XRD, SEM, NMR, TGA, and ICP analysis. Phase selectivity is discussed in terms of cation hydration as the mechanism, allowing a spatial cation arrangement enabling the formation of pores. Under water deficient conditions, the entropic penalty of cation hydration in the solid is large and cations need to be entirely coordinated by framework oxygens, leading to dense, anhydrous networks. Hence, the water activity in the synthesis medium and the affinity of a cation to either coordinate to water or to aluminosilicate decides whether a porous, hydrated, or a dense, anhydrous framework is formed.

Synthesis of Microporous Aluminosilicate by Direct Thermal Activation of Phenyl-Substituted Single-Source Aluminosilicate Molecular Precursors

The construction of aluminosilicates from versatile molecular precursors (MPs) represents a promising alternative strategy to conventional processes based on monomeric molecular or polymeric Al and Si sources. However, the use of MPs often suffers from drawbacks such as the decomposition of the core structures in the presence of solvents, acids, or bases. In this work, we demonstrate a simple thermal synthesis of porous aluminosilicates from single-source spiro-7-type MPs that consist of a tetrahedral Al atom and six Si atoms functionalized with 12 phenyl (Ph) groups, (C⁺)[Al{Ph₂Si(OSiPh₂O)₂}₂]^- (C⁺[AlSi₆]^-; C⁺ = pyridinium cation (PyH⁺), Na⁺, K⁺, Rb⁺, or Cs⁺), without using a solvent or activator. Microporous aluminosilicates synthesized via the thermal treatment of C⁺[AlSi₆]^- under a 79% N₂ + 21% O₂ atmosphere exhibited extremely low carbon contents (0.10-1.28%), together with Si/Al ratios of 3.9-6.7 ± 0.2 and surface areas of 103.1-246.3 m²/g. The solid-state ²⁷Al and ²⁹Si MAS NMR spectra suggest that the obtained aluminosilicates with alkali cations retain a tetrahedral Al site derived from the spiro-7-type core structure. After a proton-exchange reaction, the aluminosilicates showed almost 1.5 times higher reactivity in the catalytic ring-opening of styrene oxide than the aluminosilicate before proton exchange due to the catalytically active OH site being predominantly bridged by tetrahedral Al and Si atoms. These results suggest that the present MP strategy is a promising method for the introduction of key structures into active inorganic materials.