Multi-Step Nucleation of a Crystalline Silicate Framework via a Structurally Precise Prenucleation Cluster

Area of Science:

• Materials Science
• Chemistry
• Crystallography

Background:

• Hierarchical nucleation is key in mineral and material synthesis.
• Secondary building units (SBUs) are proposed building blocks for zeolites and metal-organic frameworks.
• Defining SBU structures and reaction mechanisms from monomers to crystals is challenging.

Purpose of the Study:

• To elucidate the nucleation and growth mechanisms of cyclosilicate hydrate.
• To identify the structure and role of intermediate species in silicate crystallization.
• To detail the multi-step reaction pathway from small silicate species to crystalline materials.

Main Methods:

• In situ nuclear magnetic resonance (NMR) spectroscopy.
• Small-angle X-ray scattering (SAXS).
• Atomic force microscopy (AFM).

Main Results:

• Cyclosilicate hydrate crystallization occurs via assembly of cubic octameric Q³₈ polyanions.
• Q³₈ polyanions form from cross-linking and polymerization of silicate monomers and oligomers.
• Nucleation initiates when Q³₈ species reach approximately 32% of total silicates, stabilized by H₂O and tetramethylammonium ions (TMA⁺).
• Crystal growth involves incorporating [(TMA)ₓ(Q³₈)⋅nH₂O]⁽ˣ⁻⁸⁾ clathrate complexes into step edges.

Conclusions:

• The study reveals a hierarchical nucleation pathway involving specific secondary building units (Q³₈ polyanions) in silicate crystallization.
• Identified Q³₈ polyanions as key intermediates, detailing their formation, stabilization, and role in crystal nucleation and growth.
• Demonstrated the power of combining in situ NMR, SAXS, and AFM for understanding complex crystallization processes.