Characterization and catalytic performance of supported molybdophosphoric acid catalysts for the oxidation of propylene to acetone

The oxidation of propylene to acetone was studied over supported molybdopho sphoric acid catalysts at 463 K in the presence of water. Raman spectroscop y revealed that molybdophosphoric acid retained its Keggin structure when s upported on SiO2 while it decomposed to MoO3 when supported on gamma -Al2O3 . Microcalorimetric adsorption of NH3 showed that the H3PMo12O40/SiO2 catal yst exhibited strong acidity (152 kJ/mol) and the MoO3/gamma -Al2O3 catalys t prepared from molybdophosphoric acid displayed fairly strong acidity (137 kJ/mol). FTIR showed that both Bronsted and Lewis acid sites were present on the surface of the two catalysts. Since the reaction can be taken as the coupling of the hydration of propylene and the oxydehydrogenation of propa nol, both acid and redox sites are needed to form the active catalysts. How ever, the H3PMo12O40/SiO2 catalyst showed almost no activity while the MoO3 /gamma -Al2O3 catalyst exhibited fairly good activity. Thus, the hydration of propylene may not be rate-limiting for the reaction on these catalysts. Microcalorimetric adsorption of propylene and acetone demonstrated that ads orption of propylene and desorption of acetone might not be rate-limiting, either. It can be concluded that the abstraction of the alpha -H of surface isopropoxy groups may be the rate-limiting step. (C) 2001 Elsevier Science B.V. All rights reserved.