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 alkoxid e and phosphoric acid to produce a high-surface-area aluminum phosphat e glass, was followed by Al-27 and P-31 nuclear magnetic resonance (NM R). The gelled portion of the alcoholic solutions could be characteriz ed with solid state Al-27 methods. We discovered that the gelation, wh ich is very rapid under normal conditions, can be controlled by the ad dition of HCl to the solution and that the gelation is thermally rever sible in the presence of HCl. The NMR results showed that this modifie d sol-gel behavior is a result of acid hydrolysis of the Al-O-P bonds, leading to network termination by POH groups and octahedrally solvate d Al atoms. Similar local structures persist in the amorphous calcinat ion product. The glass surface contains Al sites with strong quadrupol e interaction. These sites could not be detected by conventional Al-27 NMR methods but their existence was demonstrated with a static echo e xperiment. The quadrupole coupling constant was determined with the re cently introduced transfer of populations in double resonance (TRAPDOR ) method (a H-1/Al-27 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. Characterizat ion of the atomic structure of high-surface AlPO4 is important for its use as catalyst support.