Surface acidity of Ti- and Zr-substituted P–Si oxides and their water tolerance properties

Phosphorus-based solid acids, particularly transition metal phosphates, exhibit high versatility, as their surface reactivity can be controlled by modulating the relative amount and strength of Brønsted and Lewis acid sites. Most demanding catalytic applications require stability in aqueous environments, a fascinating challenge for metal (Me) phosphates because of the intrinsic hydrolytic instability of P–O–Me bonds. Here, the water tolerance of gel-derived Ti- and Zr-(oxo)phosphates dispersed within a silica network (TiPSi and ZrPSi) is explored under harsh conditions (treatment in water at 90 °C for 8 h). Morphology, surface chemistry and acidic properties were studied by complementary techniques, including pyridine adsorption–desorption infrared spectroscopy and liquid–solid titration with 2-phenylethylamine. Both mixed oxides show coexistence of Brønsted and Lewis sites on their surface, with a significant number of acidic sites (644 µeq g−1 for TiPSi and 479 µeq g−1 for ZrPSi). After water treatment, microporous structure and surface acidity were largely preserved even if a surface reconstruction occurred. This was further confirmed by the catalytic performance of the materials in the hydrolysis of various sugars. Ultimately, the formation of interconnected Si–O–Me–O–P surface domains results to be an effective stabilization strategy for phosphate-based solid acids, conferring resistance to water-induced degradation.