Aluminum K‐Edge Absorption (XANES) Studies of Noncrystalline Mullite Precursors

Abstract

Noncrystalline aluminum silicate precursors MP1 and MP3 with different ratios of Al in tetrahedral ([4]A1), pentahedral ([5]A1), and octahedral ([6]A1) position were prepared from aluminum sec‐butylate and tetraethoxysilane (Al/Si ratio: 3/1) making use of a slow and rapid hydrolysis 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 temperatures just prior to crystallization to mullite and γ‐Al2O3, respectively (MP1 and MP3: 800°C; MP2: 300°C). Al K near‐edge absorption spectra (XANES) of the mullite precursors and of several suitable crystalline reference materials were measured using monochromatic synchrotron radiation. The reference XANES spectra yielded a linear correlation between the energy position of the first inflection point of the white lines and the frequency of sixfold‐coordinated Al ([6]A1) in the samples. A similar though less straightforward correlation seems to hold also for the white line intensities. From these findings, the actual [61]A1 frequencies for the mullite precursors under investigation were derived to be <10% (MP1), =30% (MP3), and =60% (MP2). The [6]A1 frequency distribution in the mullite precursors displays a trend similar to that determined by 27A1 NMR spectroscopy.