IR Spectroscopic Investigation of Heteropolymolybdate Catalysts: Acidic Properties and Reactivity towards Propene

Abstract

Abstract Compounds of the general type CsxH3+y−xPVyMo12−yO40 (x = 0, 2–4 with y = 1, or x = 0, 2 with y = 0) were investigated by IR spectroscopy. After thermal treatment at 473–773 K in vacuum or N2, the probe molecules CO (at 77 K, in transmission) or CO2 (at 298 K, in diffuse reflectance) were adsorbed. On the surface of Cs2H2PVMo11O40 and Cs2HPMo12O40, Brønsted acid sites were indicated by a CO band at 2162 cm−1. All Cs containing compounds produced CO bands at 2152 cm−1 or CO2 bands at 2341 cm−1. Additional weak Lewis acid sites of Mo or V in at least two oxidation states were detected after treatment at 673 K. The fraction of the reduced species decreased in the order H4PVMo11O40 > Cs2H2PVMo11O40 > Cs3HPVMo11O40. In situ DRIFT spectra of these three compounds taken during interaction with propene at up to 673 K revealed that the pattern of IR bands typical of the Keggin anion was unaffected at x = 3, altered severely for the acid and to an intermediate extent for x = 2. The changes are consistent with a diminishing of the splitting of the P–O frequency. The Keggin structure represents the initial oxidized state of the heteropoly compound catalysts. Reduction affects but does not destroy the local structure as probed by IR spectroscopy. The more easily reducible H4PVMo11O40 is active for propene oxidation at 513 K, Cs2H2PVMo11O40 only at 617 K.