IR spectroscopy of N-methylpyrrole adsorbed on oxides--a probe of surface acidity.

IR spectra of N-methylpyrrole (NMP) have been measured following adsorption on, and subsequent desorption from, SiO(2), TiO(2), ZrO(2), SiO(2)-Al(2)O(3), H-mordenite, and sepiolite. Three modes of adsorption have been observed: (i) hydrogen bonding to surface hydroxyl groups, (ii) electron transfer at Lewis acidic surface sites, and (iii) proton transfer at Brønsted acidic surface sites. Protonation of NMP was detected only for adsorption on SiO(2)-Al(2)O(3) and H-mordenite, indicating the presence of Brønsted acidic sites with pK(a) values </=-2.90. The results establish that the adsorption of NMP can provide useful information regarding the nature of surface acidity on oxides which is complementary to that obtained from other probe molecules such as NH(3) and pyridine.