Insights into the accessibility of Zr in Zr/SBA-15 mesoporous silica supports with increasing Zr loadings

Oxide-promoted SBA-15 materials have been shown to be profitable in a wide variety of catalytic reactions of environmental interest. Zr-SBA-15 materials have been found to be of particular interest in hydrotreating, biomass conversion and Fisher–Tropsch synthesis. In these studies, the accessibility of ZrO2 located inside the silica mesoporosity has been hardly characterized, despite the fact that this may be critical in determining the best catalyst formulation. This can be accounted for by the lack of methods available for the characterization of oxides, in particular when these oxides are located in the porosity of another oxide. Recently, we have shown that ZrO2 particles dispersed in the porosity of an SBA-15 support could be characterized via a NOx-TPD methodology. In the present work Zr-SBA-15 materials were prepared with various loadings of Zr and thoroughly characterized through N2-sorption coupled with advanced pore size analysis, SEM, TEM, EDS, XRD, Raman spectroscopy and NOx-TPD. It was found that ZrO2 accessibility decreased to a significant extent up to a Zr surface density of about 2 Zr atoms/nm2 and then more slightly at higher Zr surface densities. It was also found for the first time that ZrSiO4 was formed above 2 Zr atoms/nm2. The fact that ZrSiO4 also chemisorbed NOx species, although to a much lower extent than ZrO2, paves the way for a method to characterize Zr-SBA-15 samples prepared by direct synthesis for which Zr should be mainly located in the silica walls.