Preparation and characterization of supported H3PW12O40 on silica gel: a potential catalyst for green chemistry processes

The solid acidity of 12-tungstophosphoric acid (H3PW) supported on silica gel has been measured by calorimetry and adsorption in liquid-phase using pyridine as probe. The results indicate a strong interaction of H3PW with silica, forming a weaker catalyst (ΔH1=−27.9 kcal mol−1 compared to ΔH1=−32.7 kcal mol−1 for pure H3PW). However, there is an increased amount of total titrated protons (sites) for the supported acid, which shows good dispersion of H3PW over silica. The original low surface area of H3PW, which implies a small amount of protons available on the surface, is improved by supporting on silica, and their protons become considerably exposed over SiO2 surface. The dispersion of H3PW on silica was estimated from XRD pattern, and it decreases with acid content. FTIR spectra of supported H3PW–SiO2 reacted with pyridine have shown the characteristic bands of Brönsted acid-type for the strongest site, and a weaker hydrogen-bonded acid-type. FTIR spectra of mechanical mixtures of H3PW are similar to the supported samples, thus this method cannot distinguish these materials. XRD characteristics can promptly differentiate between supported H3PW on silica and mechanical mixtures.