Abstract
Calcination of a 500 m2/g SiO2-Al2O3 co-gel from 350 to 875 °C leads to a constant ratio of Lewis-to-Bronsted sites over this temperature range. A model is proposed to account for this surprising result: a 3–4 member ring structure of SiO2 reacts with aluminum ions and inserts between Si groups in the ring. This expanded ring structure is proposed to be the local structure responsible for the acidity. The density of WO3 clusters formed on SiO2 of 1.2 nm size is analogous to the density of Si-Al ring structures proposed to be present on the surface of SiO2-Al2O3 solid acid catalysts.
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Murrell, L.L., Dispenziere, N.C. A ring structure model of the acid centers of amorphous silica-alumina catalysts. Catal Lett 2, 329–333 (1989). https://doi.org/10.1007/BF00768174
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DOI: https://doi.org/10.1007/BF00768174