SiAl18

SiAl18 is a crystalline inorganic cluster composed of one silicon atom surrounded by eighteen aluminum atoms, typically coordinated with oxygen and hydrogen atoms. The cluster adopts a high‐symmetry polyhedral geometry in which the central silicon sits in the middle of an icosahedral cage of aluminum–oxygen units. The overall structure can be described as a [SiAl₁₈O₃₆(OH)₆]⁶⁻ ion that balances the surrounding counter‑cations.

The cluster is produced by hydrothermal synthesis in aqueous media containing silicon and aluminum precursors, often at temperatures between 120 °C and 200 °C and neutral to slightly alkaline pH. During crystallisation the Si and Al species are incorporated into the cage framework through a reverse micelle mechanism, giving rise to highly ordered microporous solids. Post‑synthetic ion‑exchange procedures replace protons with alkali or alkaline‑earth metals, forming the neutral SiAl18 framework or its salt derivatives.

SiAl18 has a large internal cavity (~0.8 nm in diameter) and a high density of Lewis acid sites on the aluminum atoms, making it a candidate for heterogeneous catalysis. It has been evaluated as a catalyst support in partial oxidation reactions, as a solid acid in alkane cracking, and as an anion‑exchange medium in ion‑exchange chromatography. Its porous nature also allows adsorption of small organic molecules, and investigations into its gas‑separation properties are ongoing.

Structural studies using X‑ray diffraction, solid‑state NMR, and electron microscopy have confirmed the integrity of the SiAl18 cage under a range of thermal and chemical conditions. The cluster’s stability to hydrolysis and its ability to incorporate various heteroatoms by substitution or doping expand its potential for applications in catalysis, separation technologies, and materials science.