IR study of the acidity of ITQ-2, an "all-surface" zeolitic system

Two samples of ITQ-2 with different Al content (Si/Al = 50 and 25, respectively) and different degree of exfoliation have been characterized by IR spectroscopy and compared to MCM-22 samples with the same Al content, to assess the changes brought about by the development of a large external surface. Bronsted acidity has been measured as the propensity of OH species to either protonate ammonia or engage in H-bonds with suitable molecules (CO, N-2, n-heptane, olefins, aromatics). Lewis acidity has been evaluated by measuring the spectra of CO adsorbed at room temperature. Comparison with MCM-22 samples shows that no substantial loss in Bronsted acidity takes place because of exfoliation, although the bridged species Si(OH)Al exposed at the external surface do not basically survive as such and are probably converted into new AlOH acidic species. These are not directly detectable in the IR spectra; indirect evidence, however, suggests their location at the external cups of the surface. Bridged Si(OH)Al species survive instead in the 10-MR channels. (C) 2003 Elsevier Science (USA). All rights reserved.