QUANTITATIVE INFRARED STUDY OF THE DISTINCT ACIDIC HYDROXYL-GROUPS CONTAINED IN MODIFIED Y-ZEOLITES

Selective adsorption of pyridine was studied by infrared (IR) spectroscopy at 470 K on a series of HY zeolites, including samples dealuminated either by isomorphous substitution or steaming followed by acid leaching. Molar extinction coefficients of framework hydroxyls at high and low frequency, epsilon(HF) and epsilon(LF), and of pyridinium ions characterized by the 1545 cm-1 band, epsilon(pyH)+, were determined These values do not vary with the Si/Al(F) ratios. In the case of steamed samples, strongly acidic sites give rise to additional OH bands at 3596 and 3522 cm-1. Pyridine adsorption at 620 K allows their mean molar extinction coefficient, epsilonBAR3596+3522, to be determined. From the decomposition/deconvolution of the nu(OH) massif of acidic OH groups, it is possible to count each type of acidic OH group and, by comparison with the number of framework Al per unit cell, to determine the cationic charges of extraframework Al-containing species counterbalancing the negative framework charge. This study evidences the substantial contribution of the (3522)OH groups; moreover, the results are in agreement with the assumption that 3596 and (3522)OH bands are due to the shift of the HF and LF framework hydroxyls, respectively, provoked by the presence of extraframework species.