ADSORPTION OF NA-ALUMINA STUDIED BY SOLID-STATE NA-23 AND AL-27 NUCLEAR-MAGNETIC-RESONANCE SPECTROSCOPY( ONTO GAMMA)

The adsorption of Na+ on gamma-alumina surfaces at four coverages of N a2CO3 [5, 10, 15 and 20% (w/w)] was characterized by solid-state Na-23 and Al-27 nuclear magnetic resonance (NMR) spectroscopy. The experime ntal results suggest that two distinct adsorbed species are present on the alumina surface: surface species and surface salts. At the lower coverages of Na2CO3 (5 and 10%), the surface species is predominant, i n which the Na+ cations are associated with the oxygen atoms of gamma- alumina. Increasing the loading level to 15% results in the appearance of a second adsorbed species that is attributed to the surface salt, Na2CO3, deposited on the solid surface. Further adsorption of Na2CO3 l eads to an increase in the amount of surface salt while the amount of surface species remains unchanged. H-1-Al-27 Cross-polarization magic angle spinning (CP-MAS) experiments give the evidence that some Na+ ca tions in the form of surface species are coordinated with the Bronsted acid sites of gamma-alumina. This may be the main driving force that improves appreciably the catalytic efficiency of an Na2CO3-Al2O3 catal yst.