A Comparative Study on the Synthesis Mechanism and Microstructural Development of Hierarchical Porous Mullite Monoliths Obtained by the Sol–gel Process with Three Different Silicon Sources

Hierarchical porous mullite monoliths have been prepared via a sol–gel process accompanied by phase separation. Propylene oxide (PO) acted as an acid scavenger to mediate the gelation, poly(ethylene oxide) (PEO) as the phase-separation inducer and network former, aluminum chloride hexahydrate (AlCl3·6H2O) as the aluminum source. The route was improved by using hypotoxic tetraethylorthosilicate (TEOS) and nontoxic aqueous colloidal silica (Aq) instead of tetramethoxysilane (TMOS). The synthesis mechanism and microstructural development were comparatively investigated by scanning electron microscopy (SEM), thermogravimetry-differential thermal analysis (TG-DTA), X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and nitrogen adsorption–desorption. It was found that the mixing degree of precursors and concurrent process of gelation and phase separation are key elements to get well-defined hierarchical porous mullite monoliths. In addition, the monoliths with high relative crystallinity are more likely to be obtained under low transformation temperature in organic silicon sources system.