Abstract
Titanium silicalite 1 (TS-1) zeolite is an important selective oxidation catalyst. Recently prepared layered and pillared forms expanded the application of TS-1 to the catalytic oxidation of bulky molecules.
Despite progress in designing and application of titanosilicate zeolites in catalysis, only qualitative information of their acidity is available. Herein, we report thorough characterization of acid sites in TS-1 zeolites of different morphologies (3D TS-1, layered 2D TS-1, 2D TS-1 pillared either with silica (TS-1-PISi) or silica-titania (TS-1-PITi)) using FTIR spectroscopy and probe molecules.
FTIR of adsorbed pyridine was used for quantification of Ti-associated Lewis acid sites based on the integral intensity of nu(8a) absorption band ca. 1608 cm -1 and measured integrated molar extinction coefficient (epsilon(1608) (Ti-LAS) = 0.71 +/- 0.01 cm mu mol(-1)). Thermodesorption of pyridine monitored with FTIR showed that distribution of strength of Lewis acid sites is, to some extent, dependent on the way of Ti incorporation in the samples.
TS-1-PITi, containing large fraction of external surface Ti(OH)(OSi)(3) species introduced post-synthesis, showed increased concentration of stronger Lewis acid centres. FTIR spectroscopy of TS-1 with pre-adsorbed quinoline and d(3)-acetonitrile probe confirmed enhanced relative concentration of external Lewis acid sites in all layered TS-1 materials (28-38%) vs. 3D TS-1 (2%).