Characterization of Boria−Alumina Mixed Oxides Prepared by a Sol−Gel Method. 2. Characterization of the Calcined Xerogels

Abstract

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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“…1b) states well corresponded to the schematic structural models shown in Fig. 2, that were derived from the results obtained by previous studies [34][35][36]. Four domains (I, Fig.…”

“…2). Note that the B/Al ratio at this local maximum coincided with the ratio that yielded the maximum quantity of the BO 4 species in domain I [35,36]. This result indicates that the surface BO 4 species created in domain I was most likely the cause of the specific acid property yielding the local maximum at B/Al ~ 0.04.…”

“…Due to evaporation of BO 3 chains during calcination, the B/Al ratios of the calcined powders for B/Al > 0.15 were smaller than those used for the preparation [36]. Therefore, as described in the Experimental section, the B/Al ratios in Table 1 are the corrected values using the relationship obtained in our previous study [36].…”

“…Table 1 lists the B/Al values corrected taking into account the evaporation of B during calcination, which is described in our previous study [36]. The preparation methods are described elsewhere [34][35][36]. powders were recorded at room temperature using an MXP 18 diffractometer (Mac Science Co. Ltd.) equipped with a copper anode (0.154 nm, 50 kV x 35 mA) with a secondary monochromator.…”

“…Thus, the increase in SSA upon introduction of B is mainly due to the second factor. In particular, volatilization of BO 3 chains for B/Al > 0.15 was likely responsible for the dislocation of the structure [36], which would lead to a significant increase in SSA. Fig.…”