A Study of the Nature, Strength, and Accessibility of Acid Sites of H-MCM-22 Zeolite

The acidity of H-MCM-22 zeolite has been investigated by ammonia adsorption microcalorimetry and by IR spectroscopy of adsorbed nitriles (isobutyronitrile and pivalonitrile) and of aromatics ( p -, o -, and m -xylene). The most abundant “zeolitic” OHs are exposed in the supercage system, absorbing predominantly at ca. 3618−3625 cm −1 . Less abundant are those exposed in the sinusoidal channel system, predominantly absorbing in the range 3600−3618 cm −1 . A third OH family, responsible for a weak band at 3575 cm −1, is also observed. The adsorption of the hindered probe pivalonitrile strongly supports the conclusion that emisupercages are exposed at the 001 basal plane of the MCM-22 layered structure, where zeolitic OHs of the same nature of those exposed in the supercages indeed exist. At the external surface, weakly Brønsted acidic silanol groups (possibly two slightly different families) and strongly acidic Lewis sites are also well-evident. Monoaromatics, as well as para-disubstituted benzenes (such as p-xylene) and isopropyl-group containing molecules (such as isobutyronitrile), enter easily not only the emisupercages but also both internal channel systems of MCM-22. On the contrary, the entrance of tert -butyl-containing molecules, such as pivalonitrile, to the internal cavities is forbidden or at least strongly hindered. These data suggest that alkylation of benzene could occur both at the external emisupercages (with no diffusion limits) and in the internal cavities, without strong diffusion limits. The reactivity in the internal supercages could become significant above 200 °C when the selectivity to monoalkylation increases significantly. Alkylation of phenol with tert -butyl alcohol very likely occurs at the external surface catalyzed by Lewis acid sites.