Insight into titanium and zirconium phosphate-based materials for reactive surfaces

Transition metal phosphates are promising acid catalysts for biorefinery processes, and their efficiency can benefit from the dispersion in a porous support. Here, a one-pot hydrolytic sol-gel route is established for the synthesis of Ti–P–Si and Zr–P–Si oxides, comprising a fine distribution of titanium or zirconium oxophosphate in a silicate network. The environmental sustainability of the procedure, given by the choice of the starting materials and operating conditions, is attested by a comparative study of E factors. A deep structural and surface characterization, by solid state NMR, FTIR and XPS, reveals the evolution of the materials during thermal treatment and the presence of a diverse phosphorus unit connectivity, including P–O–Ti and P–O–Zr bonding that anchors P in the amorphous cross-linked silicate matrix. The materials are prevalently microporous, with specific surface areas around 400 m2 g−1, and show a significant surface acidity (acid sites density >0.70 mmol g−1 from NH3 titration), despite the low metal and P content. Brønsted and Lewis acidic sites coexist at the surface, the former being predominant thanks to the contribution of both P–OH groups and some silanols whose acidity is increased by nearby coordinatively unsaturated metal ions. A proof of the reactivity of these materials is obtained in the hydrolysis of sucrose, that was selected as test reaction. The proposed sol-gel route affords a tight mixing of metal and phosphorus into the silica matrix that promotes the synergy of the components, enhancing their activity, and represents an effective sustainable approach toward supported functional metal phosphates.