Revisiting the Use of 2,6-Dimethylpyridine Adsorption as a Probe for the Acidic Properties of Metal Oxides

To check if the adsorption/desorption of 2,6-dimethylpyridine (2,6-DMP) is a suitable probe for the surface acidity of oxides, and to assign a band of controversial interpretation often observed upon 2,6-DMP adsorption, the ambient temperature uptake of 2,6-DMP on several oxidic systems was carried out. SiO2 was observed to yield a plain H-bonding interaction, characterized by an 8a band of adsorbed 2,6-DMP that is typical of silanols. Several (spinel) transition aluminas and the corundum α-alumina phase were examined and allowed to test the suitability of 2,6-DMP in distinguishing different types of surface Lewis acidity. Sulfated alumina and a β-zeolite in its H-form were used to check the sensitivity of 2,6-DMP uptake toward either induced or intrinsic Brønsted acidity and the possibility of observing (weak) Lewis-coordinated 2,6-DMP also in the presence of (strong) Brønsted-bound 2,6-DMP.