Nuclear magnetic resonance study of a high-surface-area aluminum phosphate glass and its thermally reversible sol-gel precursor

A novel reaction, based on sol-gel chemistry using an aluminum alkoxide and phosphoric acid to produce a high-surface-area aluminum phosphate glass, was followed by 27Al and 31P nuclear magnetic resonance (NMR). The gelled portion of the alcoholic solutions could be characterized with solid state 27Al methods. We discovered that the gelation, which is very rapid under normal conditions, can be controlled by the addition of HCl to the solution and that the gelation is thermally reversible in the presence of HCl. The NMR results showed that this modified sol-gel behavior is a result of acid hydrolysis of the Al-O-P bonds, leading to network termination by POH groups and octahedrally solvated Al atoms. Similar local structures persist in the amorphous calcination product. The glass surface contains Al sites with strong quadrupole interaction. These sites could not be detected by conventional 27Al NMR methods but their existence was demonstrated with a static echo experiment. The quadrupole coupling constant was determined with the recently introduced transfer of populations in double resonance (TRAPDOR) method (a 1H/27Al double resonance magic-angle spinning experiment). The TRAPDOR results also showed that these sites, which constitute about 30% of the Al in the glass, carry hydroxyl groups. Characterization of the atomic structure of high-surface AlPO4 is important for its use as catalyst support.