AGING PROCESSES OF ALUMINA SOL GELS - CHARACTERIZATION OF NEW ALUMINUM POLYOXYCATIONS BY AL-27 NMR-SPECTROSCOPY

The existence of unidentified molecular aluminum oxide clusters has been previously postulated in many alumina sols produced by various methods. We have used Al-27 NMR spectroscopy kinetic studies to identify three new polyoxyaluminum cations in these sols, which we show are formed by thermal transformation of the well-known tridecamer cation Al13O4(OH)24(H2O)12(7+). These clusters, which we denote as AlP1, AlP2, and AlP3, have resonances at 64.5 ppm (tetrahedral Al site)/ almost-equal-to 10 ppm (octahedral Al site), 70.2 ppm (tet)/10.0 ppm (oct), and 75.6 ppm (tet)/9.3 ppm (oct), respectively. NMR and gel permeation chromatography data suggest that the poly(oxyaluminum) cation AlP2, which dominates this reaction process, is a dimer of Al13. A mechanism for the aging process is proposed.