Ma. Makarova et al., LIMITATION IN THE APPLICATION OF PYRIDINE FOR QUANTITATIVE STUDIES OFBRONSTED ACIDITY IN RELATIVELY ALUMINOUS ZEOLITES, Microporous materials, 4(2-3), 1995, pp. 243-246
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.