Characterization of Boria−Alumina Mixed Oxides Prepared by a Sol−Gel Method. 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, BO 3 chains crossed over the solid matrix, and this ratio was also a crucial limit for the calcined solids structure. Indeed, during calcination some BO 3 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 m 2 ·g - 1 up to ∼650 m 2 ·g - 1 . Cleavage resulted in the exposition of new external surfaces that exhibited BO 3 OH species incrusted in the host alumina matrix, conferring then an epitactic character to the solids. Further, the 11 B magic-angle spinning (MAS) NMR spectra of the solids exhibited a feature constituted of BO 3 and BO 4 (hydrated surface BO 3 species) components that were resolved by a software simulation. While for B/Al < 0.15 the quadrupolar interaction on the BO 4 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 BO 3 species trapped along the cleaved surfaces. Further, X-ray diffraction and X-ray photoelectron spectroscopy results showed that, while for B/Al < 0.15, the system can be considered as an alumina matrix locally modified by insertion of BO 3 species, for B/Al > 0.15 a mixed phase was progressively formed. In good agreement, the 27 Al 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 BO 4 /BO 3 ratios calculated by simulation of the 11 B MAS NMR spectra were consistent with the XPS results. We calculated the proportion of B species (in wt %) present on the surface of the solids (i.e., BO 4 species), which can be potentially involved in catalytic reactions. It was remarkable that the tendency observed for the BO 4 wt % as a function of the B/Al ratio was consistent with a boria−alumina phase diagram previously proposed by Gielisse et al.