IR spectroscopic investigation of the interaction of quinoline with acidic sites on oxide surfaces

IR spectra of quinoline have been measured following adsorption on and subsequent desorption from SiO2, TiO2, ZrO2, niobic acid, SiO2-Al2O3, H-mordenite, and sepiolite. There is clear evidence from the IR spectra of three modes of adsorption: (i) hydrogen bonding to surface hydroxyl groups, (ii) electron transfer at Lewis acidic surface sites, and (iii) proton transfer at Bronsted acidic surface sites. Protonation of quinoline occurred with adsorption on niobic acid, SiO2-Al2O3, and H-mordenite, indicating the presence of Bronsted acidic sites with pK(a) values less than or equal to4.90. Hydrogen bonding and interaction with Lewis acid sites was observed for each of the oxides studied. The results establish that the adsorption of quinoline can provide useful information regarding the nature of surface acidity on oxides which is complementary to that obtained from other probe molecules such as NR3 and pyridine.