Microporous SiO2 and SiOJMO2 (M = Ti, Zr, A1; 10 mol% MOx) materials for gas separation membrane applications have been prepared from polymeric sols. Characterization of these sols with SAXS showed that the mean fractal dimension of the SiO2 sols is 1.3-1.4 with a radius of gyration of approximately 2.5 nm. The dried and calcined films are microporous and the pore size distribution was bimodal with maxima at diameters of 0.5 nm and 0.75 nm. For the SiO2/TiO2, SiOJZrO2 and SiOJA12Oa systems, much milder reaction conditions proved to be necessary to obtain sols with comparable fractal dimensions due to the high reactivity of the Ti/Zr/Al-alkoxides. Microporous supported membranes with molecular sieve-like gas transport properties can be prepared from a relatively wide range of sol structures: from polymers too small to characterize with SAXS to structures with fractal dimensions: I < dy < 2.04.
Combination of SAXS and USAXS measurements provide an extended q-range (0.006–3.0 nm-1) to study fractal growth of both aging silica gel as well as precursors of zeolite-A. Mass (silica) and surface (zeolite) fractals are observed. Scanning transmission X-ray microscopy (STXM) proves to be an extremely useful technique to obtain direct images of wet samples in the 0.1–100 micron range, confirming the SAXS/USAXS results on even larger length scales.
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