SPECTROSCOPIC CHARACTERIZATION OF HYDROXYL-GROUPS IN SAPO-40 .1. STUDY OF THE TEMPLATE-FREE SAMPLES AND THEIR INTERACTION WITH AMMONIA

FTIR, MAS H-1-NMR, and molecular graphics data are reported concerning the hydroxyl species in a series of SAPO-40 samples with variable Si contents, as well as FTIR data concerning the interaction of ammonia w ith the different acid sites, Up to seven types of Si(OH)Al Bronsted s ites are present, mainly differing by the O-H stretching frequency. Th ree high-frequency bands (3637-3624 cm(-1)) correspond to hydroxyls lo cated in the 12-member channels and two low-frequency bands (3559 and 3528 cm(-1)) to OH species in eight-member rings and side pockets of t he AFR structure. Two bands at exceptionally low frequency (3485 and 3 445 cm(-1)) are due to OH species interacting with silica islands. All low-frequency bands behave with temperature as H-bonded species. All types of hydroxyls form ammonium ions, the thermal decomposition of wh ich shows some difference in acidity among the OH species. IR evidence indicates that NH4+ species interact strongly with the basic oxygen a toms of the lattice. No Lewis sites were evidenced by ammonia adsorpti on.