Characterization and Stability of Heteropolyacid Catalysts Supported on MCM-41 Materials Synthesized by Ultrasonic Irradiation

A series of solid add catalyst of the Keggin-type 12-phosphotungstic add, H3PW2O40, supported on ordered mesoporous silica MCM-41 were prepared by a simple and effective impregnation method. MCM-41 supports were synthesized in a relatively short time via a recently reported ultrasonic irradiation method. The synthesis sonication time has been systematically varied in order to investigate its influence on the structural order of the resulting materials. The prepared catalysts were characterized by nitrogen adsorption, X-ray diffraction, Raman spectroscopy and thermogravimetric analysis. The resulting materials exhibited hexagonally ordered meso structure, with high surface area of the order of several hundreds of m(2), relatively large pore volumes, with the pore diameter in the range of 2.19 to 2.41 nm and a corresponding pore wall thickness of over 1.67nm. The results have demonstrated that high quality MCM-41 materials can be synthesized via the ultrasonic irradiation in few tens of minutes, much shorter than the conventional synthesis methods. Despite their relatively high loading, all synthesized materials retained the characteristic MCM-41 mesoporous structure after impregnation of the heteropolyacid active phase onto the inner pore surface, without crystallization, but preserving the Keggin structure as confirmed by Raman spectroscopy.