LIMITATION IN THE APPLICATION OF PYRIDINE FOR QUANTITATIVE STUDIES OFBRONSTED ACIDITY IN RELATIVELY ALUMINOUS ZEOLITES

The titration of Bronsted acid sites in H-Y (Si/Al=2.7) and H-EMT (Si/ Al=4.0) with pyridine, monitored by Fourier transform infrared (FTIR) techniques has demonstrated that the amount of PyH(+) ions (the IR ban d at 1545 cm(-1)) formed at saturation is the same and does not depend on the Si/Al ratio within the range considered, although the HF band (3650 cm(-1)) is fully perturbed in both cases. The integrated extinct ion coefficient, epsilon(PyH+), has an effective value (1.1 cm mu mol( -1) at 150 degrees C and 1.6 cm mu mol(-1) at 350 degrees C) reflectin g three processes simultaneously: (1) the interaction of HF hydroxyl g roups with Py leading to PyH(+) ion formation; (2) the perturbation of HF hydroxyl groups by PyH(+) ions as O-H ... PyH+; (3) the perturbati on of LF hydroxyl groups by Py resulting in hydrogen-bonded O ... H .. . PB complexes. Py sorption cannot be used for quantification of Brons ted hydroxyls in faujasitic zeolites with Si/Al<4 since only a limited amount of PyH(+) ions per supercage can be generated.