SPECTROSCOPIC CHARACTERIZATION OF THE HYDROXYL-GROUPS IN SAPO-40 .2. INTERACTION WITH CO AND N-2

SAPO-40 exhibits five main OH species, describable as high-frequency o nes (HF, at 3737, 3730, and 3724 cm(-1)) and low-frequency ones (LF1 a nd LF2, at 3559 and 3528 cm(-1), respectively), the former probably si tting in 12-member-ring channels and the latter either in side pockets or eight-member-ring channels of the AFR structure. The low-temperatu re interaction of such OH species with CO shows that HF species have s trictly similar acidity (comparable to that of HY zeolites), in contra st with previous results concerning H-transfer to NH3. The ease of int eraction and the spectral features are in accord with the location in large cavities. The vibration of H-bonded hydroxyl has a noticeable st ructure with subbands. The interaction of CO with LF1 and LF2 species occurs with difficulty and yields nonlinear complexes because of the s teric hindrance brought about by small cavities. Adsorption of N-2 fol lows similar patterns.