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

摘要

Compounds of the general type Cs_xH_(3+y_x)PV_yMo_(12-y)O_(40) (x = 0,2-4 with y=l,or x = 0,2 with y = 0) were investigated by IR spectroscopy.After thermal treatment at 473-773 K in vacuum or N_2,the probe molecules CO (at 77 K,in transmission) or CO_2 (at 298 K,in diffuse reflectance) were adsorbed.On the surface of Cs_2H_2PVMo_(11)O_(40) and Cs_2HPMo_(12)O_(40),Bronsted acid sites were indicated by a CO band at 2162cm~(-1).All Cs containing compounds produced CO bands at 2152cm~(-1) or CO_2 bands at 2341cm~(-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 H_4PVMo_(11)O_(40) > Cs_2H_2PVMo_(11)O_(40) > Cs_3HPVMo_(11)O_(40).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 H_4PVMo_(11)O_(40) is active for propene oxidation at 513K,Cs_2H_2PVMo_(11)O_(40) only at 617K.