Raman spectra of pyridine adsorbed on a series of ion-exchanged forms of zeolite Y

Abstract

As with other silicas and aluminas, it is shown that a broad general fluorescence from Y zeolite samples excited by irradiation with the 6328 Å line of a He/Ne laser can be greatly reduced by heating the zeolite in oxygen at 500 °C. This general fluorescence is attributed to the presence of traces of unsaturated hydrocarbon impurities which can be burned off by the oxygen treatment. A narrower residual fluorescence in the region of 14 000–14 500 cm−1 is attributed to the presence of Fe3+ impurities.Successful Raman spectra using He–Ne laser excitation have been obtained from pyridine adsorbed on oxygen-treated NaY zeolite, and on samples of this zeolite in which the majority of sodium ions have been replaced by Li+, K+, Co2+, Cu2+, Ce3+ or H+ (decationated Y zeolite). The position of the v1 Raman band from the ring-breathing mode of pyridine is found to give a linear correlation with q/r for the metal ions (where q is the charge and r the radius of an ion) strongly suggesting that the main adsorption is at cation sites. During adsorption of pyridine on CeY the spectrum changes as a function of time. Adsorption on Ce3+ appears to be the final situation and intermediate Raman spectra suggest that pyridine adsorbs and forms hydrogen bonds with surface OH groups; quite different Raman spectra are obtained from these two surface species. The Raman spectra of pyridine on HY zeolite is consistent with the coexistence of pyridinium ions and hydrogen-bonded pyridine molecules.