Ti,Zr]-pillared montmorillonite – A new quality with respect to Ti- and Zr-pillared clays

Abstract

Mixed titanium–zirconium pillared montmorillonite [Ti,Zr]-PILC and reference Ti-PILC and Zr-PILC were synthesized and characterized with XRD, XRF, diffuse reflectance UV–Vis, XPS and Raman spectroscopies, thermal analysis, N2 adsorption/desorption at −196°C, NH3 TPD–MS, and FTIR monitored pyridine adsorption. Results indicate that pillars in [Ti,Zr]-PILC represent quasi-amorphous mixed Ti–Zr oxide, of short range order similar to that of a zirconium titanate phase. The size of intercalated mixed Ti–Zr cationic species is intermediate between that of Ti and Zr polycations. A new evidence is presented on the nature of pillars and their relationship to the clay sheets. It is demonstrated that in Ti-PILC material titanium sites present at the clay/pillar interface assume tetrahedral coordination, while in [Ti,Zr]-PILC they retain coordination number higher than 4. Textural studies suggest that mixed Ti–Zr-pillars are structurally better defined and more uniform than those formed upon intercalation of the Ti-pillaring sol, and resemble in this respect pillars derived from zirconium oligocations. Measurements of surface acidity prove that a new quality appears in the surface chemistry of oxide props present in [Ti,Zr]-PILC. Although, similarly as in the case of Ti-PILC and Zr-PILC references, the acidity is predominantly of the Lewis type, the [Ti,Zr]-PILC sample possesses a significant population of Lewis acid sites which are much stronger than those present in the reference materials. Results of this study show that uniqueness of structural, textural and surface acid–base properties of mixed titanium–zirconium pillared montmorillonite render this material very attractive from the point of view of potential applications in sorption and catalysis.