Characterization of the External Surface of Silicalites Employing Electron Paramagnetic Resonance^†^,^‖

Abstract

Sensitive and structurally specific methods for investigating silicalite external surface have been developed using electron paramagnetic resonance (EPR). The absorption of an EPR silent probe ortho-methyldibenzyl ketone (oMeDBK) (4 in Scheme ) on the external surface of a series of monodisperse silicalite crystals was studied using an initially coadsorbed EPR active nitroxide probe. The displacement of the initially adsorbed nitroxide probe by coadsorbed oMeDBK shows that the adsorbate molecules first adsorb on stronger binding sites characterized by slow rotational motion of the probe (broad EPR lines) and after the stronger sites are saturated, the displaced EPR probe molecules adsorb on weaker binding sites characterized by fast rotational motion of the probe (narrow EPR lines). The transition point from slow to fast rotational motion provides a quantitative measurement of the stronger binding sites on the silicalite external surface area and the external surface area of silicalite crystals. The adsorption strength is molecular structure-dependent, and polar functional groups provide significant contribution to the binding strength. Sequential adsorption of 14N and 15N spin-labeled nitroxides shows the presence of the dynamic exchange between the adsorbates on the strong binding sites and those in solution or on the weak binding sites, while concurrent coadsorption of 14N and 15N spin-labeled nitroxides provides another sensitive means of studying the molecular structural dependence of the binding strength