Characterization of boria-alumina mixed oxides prepared by a sol-gel method. I. NMR characterization of the xerogels

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 (NH4)(2)B4O7 center dot 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 Al-27 and B-11 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, BO3, BO4, 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, BO3 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 B 03 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.