Affiliation:
1. Physical Sciences Division Pacific Northwest National Laboratory Richland WA 99352 USA
2. Molecular Engineering and Sciences Institute University of Washington Seattle WA 98105 USA
3. Department of Chemical Engineering University of Washington Seattle WA 98105 USA
4. Department of Materials Science and Engineering University of Washington Seattle WA 98105 USA
Abstract
AbstractHierarchical nucleation pathways are ubiquitous in the synthesis of minerals and materials. In the case of zeolites and metal–organic frameworks, pre‐organized multi‐ion “secondary building units” (SBUs) have been proposed as fundamental building blocks. However, detailing the progress of multi‐step reaction mechanisms from monomeric species to stable crystals and defining the structures of the SBUs remains an unmet challenge. Combining in situ nuclear magnetic resonance, small‐angle X‐ray scattering, and atomic force microscopy, we show that crystallization of the framework silicate, cyclosilicate hydrate, occurs through an assembly of cubic octameric Q38 polyanions formed through cross‐linking and polymerization of smaller silicate monomers and other oligomers. These Q38 are stabilized by hydrogen bonds with surrounding H2O and tetramethylammonium ions (TMA+). When Q38 levels reach a threshold of ≈32 % of the total silicate species, nucleation occurs. Further growth proceeds through the incorporation of [(TMA)x(Q38)⋅n H2O](x−8) clathrate complexes into step edges on the crystals.