Infrared Study of Acetophenone Adsorption on Mordenite and Dealuminated Mordenite.

Infrared spectra of acetophenone adsorbed on mordenite and acid-leached mordenite are reported. The effects of dealumination were characterized by pyridine adsorption and 29Si and 27Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR). Adsorption of pyridine gave bands at 1634 and 1545 cm-1 due to vibrations of pyridinium ions and a band at 1621 cm-1 characteristic of pyridine ligated to Lewis acidic Al3+ sites. 27Al NMR gave peaks at 55 ppm due to tetrahedral framework Al and at 0 ppm due to octahedral extraframework Al. 29Si NMR gave peaks at -106 and -112 ppm due to Si(1Al) and Si(OAl) species, respectively. For acetophenone on mordenite, spectra contained nu(C&dbond;O) bands at 1685, 1671, 1650, and 1630 cm-1 due to physisorbed acetophenone containing unperturbed C&dbond;O groups, acetophenone hydrogen bonded with external and internal silanol groups, acetophenone hydrogen bonded to Brønsted acidic Si(OH)Al groups, and acetophenone ligated to Lewis acidic Al3+ ions, respectively. Dealumination converted 56% of the Si(1Al) species to Si(OAl), reduced the concentrations of both framework and extraframework Al, reduced the concentration of Si(OH)Al groups, and greatly reduced the population of Lewis acidic sites, but generated internal silanol groups which formed hydrogen bonds with acetophenone molecules. Residual Al3+ sites and Si(OH)Al groups after dealumination could interact with adsorbed pyridine molecules but for steric reasons were unable to form adsorption complexes with acetophenone. Evidence for the protonation of acetophenone was inconclusive. Copyright 1998 Academic Press.