The surface acidity of oxides probed by IR spectroscopy of adsorbed diazines

Abstract

The IR spectra of the three diazines, pyrazine, pyrimidine and pyridazine have been measured following adsorption on, and subsequent desorption from, a variety of oxides: SiO₂, TiO₂, ZrO₂, SiO₂–Al₂O₃ and H-mordenite. Three modes of adsorption have been observed, (i) hydrogen bonding to surface hydroxy groups, (ii) electron transfer at Lewis acidic surface sites, and (iii) proton transfer at Brønsted acidic surface sites. In the latter case only monoprotonation was detected for SiO₂–Al₂O₃ and H-mordenite, indicating the presence of Brønsted acidic sites with pK_a values in the range 0.65 to 2.24. There was no evidence for diprotonation, which would indicate the presence of acidic sites with pK_a values below 5.75. These results establish that the adsorption of diazines can provide useful information regarding the nature of surface acidity on oxides which is complementary to that obtained from other probe molecules such as NH₃ and pyridine.