|Titolo:||A study of nature, strenght and accessibility of the acid sites of H-MCM-22 zeolite|
|Data di pubblicazione:||2008|
|Abstract:||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 3660-3618 cm-1. A third OH family,responsible for a weak band at 3575 cm-1, is also observed. the adsorption of th ehindered probe pivalonitrile strongly supports the conclusion that emisupercages are exposed at the 001 basl plane of the MCM-22 layered structure, where zeolitic OHs of th esame nature of those exposed in the supercages indeed exist. At the external surface, weakly Bronsted acidic silanol groups ( possibly two sligtly different familes ) and strongly acidic Lewis sites are also well-evident. Monoaromatics, as well as para-disubstituted benzenes ( such p-xylene ) and isopropyl-group containing molecules ( such isobutyronitile ), enter easily not only th eemisupercages but also both internal channel systems of MCM-22. On the contrary, the entrance of tert-butyl-containing molecules, such as pivalonirile, to th einternal 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 stron diffusion limits. Th ereactivity in teh internal supercages could become significant abov e200°C when the selectivity to momoalkylation increase significantly. Alkilation of phenol with tert-butyl alcohol very likely occurs at th external surface catalyzed by Lewis acid sites.|
|Tipologia:||1.1 Articolo in rivista|
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