Modeling Lewis acidity of transition aluminas by numerical simulations

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