2,6-Dimethylpiridine adsorption on zirconia and sulfated zirconia systems. An FTIR and microcalorimetric study

In situ FTIR spectroscopy and adsorption microcalorimetry have been employed to study the adsorption/ desorption of 2,6-dimethylpyridine (2,6-DMP) on different zirconia and sulfated zirconia (SZ) systems. Adsorption/desorption experiments were carried out at 343 K (beam temperature for all IR experiments) and at 423 K. The adsorbents were (i) a (mainly) monoclinic zirconia, (ii) a crystallographically pure tetragonal zirconia, phase stabilized by formation of a solid solution with 3 mol % Y2O3, (iii) monoclinic sulfated zirconia (m-SZ) and tetragonal sulfated zirconia (t-SZ), both noncalcined and calcined at 923 K, obtained by direct sulfation of the two crystalline ZrO2 specimens, and (iv) two t-SZ specimens (also noncalcined and calcined at 923 K), obtained through the “conventional” sulfation procedure carried out on an amorphous hydroxide precursor and leading, upon thermal treatment at T > 823 K, to crystalline t-SZ. The adsorption/desorption of 2,6-DMP turned out to be a valuable probe for both Brønsted and (to a minor extent) Lewis acidic sites and to yield more information than other bases more frequently adopted (like, for instance, pyridine or ammonia). In particular, for both Brønsted- and Lewis-type interaction, 2,6-DMP is able to distinguish among sites possessing different acidic strength. From the energetic point of view, the process of 2,6-DMP uptake is highly heterogeneous in all systems, and on t-SZ it is dominated by the adsorption of the base in its protonated forms.