Supramolecular structure and dynamics of organic molecules adsorbed on theexternal surface of MFI zeolites. A direct and indirect computational EPR analysis

The nitroxides 1 and 1-oMe were employed as EPR probes for the investigatio n of the supramolecular structures and dynamics of organic molecules adsorb ed on the external surfaces of two MFI zeolites (silicalite and ZSM-5). The results for the two zeolites are qualitatively similar, but depend quantit atively on the surface area of the external surface. Computational analysis of the EPR spectra as a function of loading of the probe is consistent wit h an initial strong association of the probes with the holes on the externa l surface, followed by a weaker binding to the framework of the external su rface between the holes. The first supramolecular structure (binding to hol es) fixes the motion of the probes and allows for an estimation of the aver age separation of the probes through EPR analysis of the dipole-dipole inte raction. After saturation of the holes and the onset of the second supermol ecular structure (binding to the framework), rapid diffusion of the probes on the framework occurs and provides a means for an estimation of the motio n of the probe through EPR analysis of the spin-spin exchange interaction. The mobile framework's probes collide and displace the probes in the holes, resulting in a dynamic equilibrium between probes at both sites. The displ acement of the nitroxides associated with the holes by three coadsorbed "di splacer" molecules, 2, 3, and 4, was investigated by the change of the prob e EPR. Addition of ketones 2 and 4 resulted in the displacement of nitroxid es after sufficient displacer was added to saturate the holes on the extern al surface. This result is consistent with a stronger binding of the nitrox ide to the surface and for the need for molecules of 2 and 4 to be situated on the framework before displacement of the nitroxide can occur. Compared to 2 and 4, 3 is found to be a poor displacer of nitroxide because of its w eak association with the holes. From the competitive displacements, it is c oncluded that the binding to the external surface increases in the series 3 < 2 < 4 much less than 1, being at a minimum for the nonpolar 3 and a maxi mum for the nitroxide that possesses two polar moieties for interaction wit h the zeolite surface. On the basis of these results, a model was developed which described the supramolecular structures and dynamics formed by the k etones and MFI zeolites at a function of loading and for the various system s investigated.