In situ FTIR studies of the acidity of H_3PW_(12)O_(40) and its porous salts. Interaction with H_2O,NH_3 and pyridine

摘要

The dehydration-rehydration mechanism fo H_3PW_(12)O_(40), has been investigated by in sity IR spectroscopy.At room temperature and under vacuunm, protonated water clusters (H_2O)_nH are observed both on the pure non-porous acid and on its acidic porous/mesoporous salts M_xH_(3-x)PW_(12)O_(40) with M=Cs or K These species are characterized by a δ_(OH) vibrationat 1708 cm and a broad v_(OH) absorption at 3355 cm(-1). Further dehydration induces the decomposition of these protonic species and leads to the formation of hydroxyl groups characterized by a broad absorption centred at 3209 cm~(-1) Strong modifications of thevibration bands of the Keggin unit occur simultaneously with the decompositin of the protonated water clusters. Adsorption of probe molecules such as water, ammonia and pyridine has been performed on samples dehydrated at different temperatures. In allcases the formation of protonated species are observed and the parallel modifications of the vidration of the Keggin unit is described. The formation fo protonated species is observed even after interaction of the probe molecule with completely dehydroxylated solids which accounts for a still unknown process.The adsorption of pyridine over the pure acid H_3P generated a peculiar IR spectrurn,characterised by the splittin both the 19b and 8b vibration modes of the pyridinium ion parallel to the splitting of the δ(O-P-O) ans v_s(W-O-W) vibrations of the Keggin unit. These featured are similar to those of the pyridinium salt prepared by neutralisation of an aqueous solution of H_3P. On the contrary pyridine adsorption on the acidic salt gave rise to the regular pyridine vibrations.