(236e) Ordered Mesoporous Materials By Template-Free Nanoparticle Assembly

Assembly of binary silica nanoparticle superlattices (BSNS) of large (A) and small (B) size is demonstrated as a facile and template-free route to three-dimensionally ordered mesoporous (3DOm) silicas, the pore topology of which derives from the interconnected interstices of the resulting ordered nanoparticulate structures.  Even without adoption of common stabilization, for example charge- or steric-stabilization (e.g., polymeric passivation layers) or solvent index matching for minimizing van der Waals interactions, we find that the assembly of binary silica nanoparticles from aqueous solutions of basic amino acids obeys predicted hard-sphere phase behavior.  Specifically, the size ratio governs symmetry of the system among AB, AB₂, and AB₁₃ stoichiometric assemblies that are isostructural with NaCl, AlB₂, and NaZn₁₃, with deviations from hard sphere predictions manifested by interstitial solids in the case of the smallest particle size ratios considered.  The facile nanoparticle assembly and its concomitant structural diversity translate to tunability of the mesopore topology.  High yield binary assembly is found to be robust to both sub- and super-stoichiometric precursor mixtures and to systematic changes (i.e., reduction) in nanoparticle size, the latter providing a facile handle for self-templated tunability of mesopore size. 

The enhanced hydrothermal stability of the BSNS materials, owing to their thicker pore walls, will be described.  In an extension aimed at realizing bottom-up strategies for stable, BSNS-supported catalytic materials, this talk will also describe how mechanistic insight into BSNS assembly can be exploited to expand compositional diversity of the structures specifically by co-assembly of binary metal and siliceous nanoparticles.  Finally, use of the BSNS structures and their metal-embedded counterparts as sacrificial templates for carbon replication will also be described as a means for deriving bi-modal mesoporous carbon materials with and without template-transferred metal functionality.  The overarching goal of this talk, therefore, will be to establish a new paradigm for bottom-up assembly of adsorbents and supported catalysts endowed with large pore volume, controllable 3D-ordered mesopore topology, and tunable uniformly distributed function.