While boria-alumina mixed oxides have been widely used for various catalytic reactions, their structure has never been satisfactory elucidated and is still a matter of debates. The present paper deals with the elucidation of the structure of boria-alumina prepared by a sol-gel method with B/Al atomic ratio varying from 0.013 to 1.643. The powders were prepared by hydrolysis of aluminum tri-sec-butoxide in the presence of (NH 4 ) 2 B 4 O 7 ‚4H 2 O. Then, the solid-state magic angle spinning (MAS) NMR spectroscopy was used to characterize the obtained solids in the dried state (xerogels), the calcined state being the object of a next paper. Both 27 Al and 11 B MAS NMR spectra were recorded with subsequent simulation of these last ones. This allowed to build a consistent structural model of these xerogels, taking into account the evolutions of calculated parameters such as the quadrupolar interaction, the real chemical shift, and the relative quantity of, respectively, BO 3 , BO 4 , tetrahedral, pentahedral, and octahedral aluminum species as a function of the B/Al atomic ratio. As a result, it was found that, for B/Al < 0.06, presence of boron induces the creation of bulk and surface tetrahedral aluminum species. Then, for B/Al > 0.06, BO 3 chains attached to these tetrahedral aluminum species are formed and start to grow through the matrix. Further, for B/Al > 0.15, when the boron loading increases, the BO 3 chains progressively emerge outside of the matrix, crossing it over. Furthermore, for B/Al > 0.26, some pentahedral aluminum species are formed supposedly due to the considerable steric strains afforded by some particular aluminum atoms. In brief, the presented model elucidates the structure of the dried alumina-based xerogels and is the key starting point to explain the structure of the oxides obtained after calcination and presented in the following paper.

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.

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