Adsorption of pyridine on zeolites can show three modes of sorption: physisorption, sorption on the LF hydroxyls and sorption on the HF hydroxyls. In this study a separate IR spectrum was obtained for each of the three sorbed pyridine species and major differences were observed, especially in the ring-bending and stretching region (2000 to 1200 cm-'). A comparison of the spectra showed that pyridine sorbed on the LF hydroxyls is not physisorbed, but protonated to some degree. H/D exchange enabled the IR spectrum of the bonded Brlansted proton to be obtained for pyridine sorbed on the HF hydroxyl sites which proved to be a broad band from ca. 3600 to ca. 2600 cm-'. For pyridine sorbed on the LF hydroxyls, a broader more complex band was observed from ca. 3700 to ca. 2200 cm-'. Reversible sorption/desorption occurred for physisorbed pyridine and pyridine sorbed on the LF hydroxyls, but complete desorption of pyridine from the HF hydroxyls resulted in loss of acid sites.
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