ALUMINUM K-EDGE ABSORPTION (XANES) STUDIES OF NONCRYSTALLINE MULLITE PRECURSORS

Noncrystalline aluminum silicate precursors MP1 and MP3 with different ratios of Al in tetrahedral (Al-[4]), pentahedral (Al-[5]), and octah edral (Al-[6]) position were prepared from aluminum sec-butylate and t etraethoxysilane (Al/Si ratio: 3/1) making use of a slow and rapid hyd rolysis process, respectively, The MP2 precursor was synthesized from aluminum sec-butylate and silicon tetrachloride (Al/Si ratio: 4/1) by rapid hydrolysis. All aluminum silicate gels were heat-treated to temp eratures just prior to crystallization to mullite and gamma-Al2O3, res pectively (MP1 and MP3: 800 degrees C; MP2: 300 degrees C), Al K near- edge absorption spectra (XANES) of the mullite precursors and of sever al suitable crystalline reference materials were measured using monoch romatic synchrotron radiation, The reference XANES spectra yielded a l inear correlation between the energy position of the first inflection point of the white lines and the frequency of sixfold-coordinated Al ( Al-[6]) in the samples, A similar though less straightforward correlat ion seems to hold also for the white line intensities, From these find ings, the actual Al-[6] frequencies for the mullite precursors under i nvestigation were derived to be <10% (MP1), approximate to 30% (MP3), and approximate to 60% (MP2), The Al-[6] frequency distribution in the mullite precursors displays a trend similar to that determined by Al- 27 NMR spectroscopy.