Characterization of boria-alumina mixed oxides prepared by a sol-gel method. Part 2: Characterization of the calcined xerogels
Sol-gel boria-alumina mixed oxides with a wide range of B / Al atomic compositions have been characterized as dried xerogels in the preceding paper in which a structural model with four domains of composition was proposed. The present paper presents the results obtained on the same series of solids after calcination at 500°C. It was found that the structure of the dried xerogels has a strong influence on the structure of the final oxides. In particular, in the dried xerogel state from B / Al ~ 0.15, BO3 chains crossed over the solid matrix and this ratio was also a crucial limit for the calcined solids structure. Indeed, during calcination some BO3 chains were volatilized, giving a measured B / Al ratio lower than the theoretical one; the oxides were dislocated and their SSA brutally increasing from ~ 500 m2.g-1 up to ~ 650 m2.g-1. Cleavage resulted in the exposition of new external surfaces that exhibited BO3OH species incrusted in the host alumina matrix, conferring then an epitactic character to the solids. Further, the 11B MAS-NMR spectra of the solids exhibited a feature constituted of BO3 and BO4 (hydrated surface BO3 species) components that were resolved by a software simulation. While for B / Al < 0.15 the quadrupolar interaction on the BO4 species was of about 0.4 MHz, it increased up to about 0.9 MHz for B / Al > 0.15. This increase originated in a strain on the new BO3 species trapped along the cleaved surfaces. Further, XRD and XPS results showed that while for B / Al < 0.15, the system can be considered as an alumina matrix locally modified by insertion of BO3 species, for B / Al > 0.15 a mixed phase was progressively formed. In good agreement, the 27Al MAS-NMR spectra of the solids with high B / Al ratio were similar to that expected for model boria-alumina mixed phases. In addition, the BO4 / BO3 ratios calculated by simulation of the 11B MAS-NMR spectra simulation were consistent with the XPS results. We calculated the proportion of B species (in wt.%) present on the surface of the solids (i.e. BO4 species), which can be potentially involved in catalytic reactions. It was remarkable that the tendency observed for the BO4 wt.% as a function of the B / Al ratio was consistent with a boria-alumina phase diagram previously proposed by Gielisse et al..