Aluminum coordination and Lewis acidity in transition aluminas

Abstract

Besides the resonance lines attributable to tetrahedrally (Al IV) and octahedrally (Al VI) coordinated aluminum, the MAS NMR 27Al spectra of transition aluminas obtained from finely divided precursors gibbsite and boehmite have a line which can be assigned to pentacoordinated AI (Al V). This line which is very intense in ex-boehmite aluminas, as compared to ex-gibbsite aluminas, is also observed in the nonframework alumina moieties in acid and dealuminated zeolites. The isotropic chemical shifts of the Al VI, Al V, and Al IV resonance lines are 11.4, 38.3 and 65.6 ppm, respectively, in a field of 11.77 T. Nutation (two-dimensional) spectra of 27Al in these aluminas show a broad distribution of quadrupole coupling constants (QCCs) between ∼2 and ∼5 MHz. A broad range of distortions of the Al coordination shells corresponds to this range of QCCs. To detect electron acceptor sites (or Lewis acid centers), an EPR molecular probe has been adsorbed and the EPR spectra have been recorded before and after introduction of molecular oxygen. With dimethylaniline (DMA) the formation of a radical cation is observed and its concentration is larger on aluminas with Al V than on aluminas without detectable Al V. Molecular O 2 which is adsorbed on DMA-treated aluminas, acts as an electron scavenger picking up the electron transferred from DMA to the surface. Moreover, the g zz is in the range expected for O 2 −, indicating its interaction with an Al nucleus. The surface density in electron acceptor sites with electron affinity ∼−7.1 eV is, at the best, on the order of 0.4 × 10 12 spins/cm 2. These findings suggest either that strongly distorted shells of pentacoordinated Al are potential Lewis acid centers or that they generate such sites upon thermal activation. Other types of coordination, if sufficiently distorted, may play a similar role.