Adsorption of Na + onto γ-alumina studied by solid-state 23Na and 27Al nuclear magnetic resonance spectroscopy

Abstract

The adsorption of Na + on γ-alumina surfaces at four coverages of Na 2CO 3 [5, 10, 15 and 20% (w/w)] was characterized by solid-state 23Na and 27Al nuclear magnetic resonance (NMR) spectroscopy. The experimental results suggest that two distinct adsorbed species are present on the alumina surface: surface species and surface salts. At the lower coverages of Na 2CO 3 (5 and 10%), the surface species is predominant, in which the Na + cations are associated with the oxygen atoms of γ-alumina. Increasing the loading level to 15% results in the appearance of a second adsorbed species that is attributed to the surface salt, Na 2CO 3, deposited on the solid surface. Further adsorption of Na 2CO 3 leads to an increase in the amount of surface salt while the amount of surface species remains unchanged. 1H 27Al Cross-polarization magic angle spinning (CP-MAS) experiments give the evidence that some Na + cations in the form of surface species are coordinated with the Brönsted acid sites of γ-alumina. This may be the main driving force that improves appreciably the catalytic efficiency of an Na 2CO 3Al 2O 3 catalyst.