Molybdophosphoric acid in sol–gel titania: Physico-chemical properties

Titania mixed with molybdophosphoric acid, HMoP, in various proportions (1, 15, 25 and 50 wt.%) was obtained by the sol– gel method. The gels were dried and calcined in air at 400, 600 and 800 8C. The crystalline phases were identified by X-ray diffraction (XRD), whereas the physico-chemical properties were characterized by Raman spectroscopy and 31P MAS-NMR spectroscopy. FTIR was a technique used to determine acidic properties by pyridine adsorption. XRD showed that HMoP does not decompose until 700 8C. The stabilization of the anatase phase and the Keggin HMoP crystalline structure were evidenced by Raman spectroscopy; whereas FTIR-pyridine adsorption spectra showed that titania-HMoP present Bro¨nsted and Lewis acid sites. A correlation between the acid sites and the amount of HMoP was observed by 31P MAS-NMR.