Direct Observation of Macrostructure Formation of Hierarchically Structured Meso-Macroporous Aluminosilicates with 3D Interconnectivity by Optical Microscope

Hierarchically structured spongy meso-macroporous aluminosilicates with high tetrahedral aluminum content were synthesized from a mixture of single molecular
alkoxide precursor, (sec-BuO)2-Al-O-Si(OEt)3, already containing Si-O-Al bonds, and a silica coreactant, tetramethoxysilane (TMOS). The spontaneous byproduct templated
macroporous structure formation has been directly visualized using in situ high-resolution optical microscopy (OM), allowing the crucial observation of amicrobubble dispersionwhich
is directly correlated to the macrostructure observed by electronic microscopies (SEM and TEM). This discovery leads to a comparative study with meso-macroporous pure metal
oxide and to a proposal of the formationmechanism of meso-macroporous aluminosilicates with 3D interconnectivity. The aluminosilicate phase/microbubbles emulsion is produced by
a phase separation process occurring between the aluminosilicate nanoparticles and the liquid hydrolysis-condensation reaction byproducts (water, methanol, ethanol, and butanol). The
use of alkoxysilane improves the heterocondensation rates between the highly reactive aluminum alkoxide part of the single precursor and added silica species but, above all, leads to
the spontaneous generation of an unusual meso-macroporosity in alkaline media. The particles obtained at pH = 13.0 featured regular micrometer-sized macrospheres separated by very thin mesoporous walls and connected by submicrometric openings, providing a 3D
interconnectivity. The slight increase in pHvalue to 13.5 induced significantmodifications in morphology and textural properties due to the slower gelification process of the aluminosilicate phase, resulting in the formation of an aluminosilicate material constituted of 1-2 μm large
independent hollow mesoporous spheres.