MODELING LEWIS ACIDITY OF TRANSITION ALUMINAS BY NUMERICAL SIMULATIONS

The bulk and surface features of an alumina particle obtained by molec ular-dynamics simulation are used to support the experimental distribu tion of aluminums with respect to their coordination number obtained b y NMR (nuclear magnetic resonance). This information was obtained by u sing results of various editing procedures of the Al-27 nuclear magnet ic resonance, such as the classical one-pulse (1P) magic angle spinnin g, the cross polarization (CP) from the protons of chemisorbed ammonia and the 1P or CP rotational echo double resonance (REDOR), Because th e REDOR technique revealed that the acid Lewis sites are constituted b y pairs of four or fivefold coordinated aluminum atoms about 3 Angstro m apart, these pairs were counted in the simulated particle. The agree ment with experimental surface density of Lewis sites is satisfactory. (C) 1998 American Institute of Physics. [S0021-9606(98)02503-3].