Nanoporous Solids: How Do They Form? An In Situ Approach

Understanding the crystallization mechanisms of nanoporous solids remains one of the most challenging issues in materials science. This Short Review focuses on the use of in situ nuclear magnetic resonance (NMR) spectroscopy under hydrothermal conditions to investigate the structure, dynamics, and stability/reactivity of the soluble species present in the synthesis medium during the crystallization. We describe how the formal SBU (secondary building units) concept for solid construction can experimentally be investigated, checked, and validated on some representative purely inorganic porous phosphates as well as hybrid metal organic framework (MOF) materials. We also discuss the specific role of reactive species identified in solution to lead to intermediate or more elaborate structures such as the PNBU (prenucleation building units) or the MBU (neutral molecular building units), respectively. In certain cases, the proposed models could not to be generalized depending on the reaction conditions, the chemistry of the metallic cation, and the stability/solubility of the target phase. We also point out that experimental and theoretical approaches for identification and enumeration of existing and new SBU are essential for the discovery and/or structure determination of new materials.