Laser Raman and infrared spectroscopy of supported heteropolyacids

Heteropolyacids were used for the oxidation of 1-butene to maleic anhydride. Phosphotungstic, phosphomolybdic, and silicomolybdic acids were supported on alumina, silica and titania. Experimental methods used to characterize the catalysts included surface area and adsorption measurements, laser raman spectroscopy, Fourier transform-infrared spectroscopy, X-ray diffraction and kinetic studies. Heteropolyacids decompose readily in solution and special techniques must be used when they are impregnated on supports. The supports were shown to have widely different capabilities for stabilizing the heteropolyacids. Raman spectroscopy was sensitive to small changes in the catalyst structure; adsorbed species could also be examined. Phosphomolybdic acid was found to be more stable than the phosphotungstic acid although it had initially lower activity.