Adsorption of ethane and ethene in Na-Y studied by inelastic neutron scattering and computation

Inelastic neutron scattering (INS) spectra were collected for ethane and et hene in Na zeolite-Y (Si/Al = 2.0). The INS spectrum of ethane in Na-Y cons ists of a single broad adsorption band in which no individual peaks can be discerned. Monte-Carlo docking calculations predicted a large number of sim ilar binding sites with energies ranging from 24 to 37 kJ mol(-1) in the re gion of the four rings on the wall of the faujasite supercage. This is cons istent with the broad feature observed in the INS spectrum. In contrast, th e MS spectrum of ethene in Na-Y shows significant fine structure, and sever al peaks can be distinguished in the region < 1000 cm(-1). Monte-Carlo dock ing calculations with a molecular mechanics type force field were performed and predict two binding sites, one bound to the SII cation site (binding e nergy = 36.2 kJ mol(-1)) and one lying in the window site (binding energy = 19.4 kJ mol(-1)). Quantum mechanical calculations were also performed usin g a cluster model for the SII binding site but resulted in a significantly lower binding energy, even when correlation corrections were employed (18,2 kJ mol(-1)). Vibrational spectra were calculated using both quantum mechan ical and molecular mechanics based techniques and the results compared with the MS spectra. A significantly better fit was obtained using the latter m ethod and the peaks could be assigned based on the computed eigenvectors. A nalogies are drawn between the assignments and other previous studies of Ze ise's salt and related compounds. (C) 2000 Published by Elsevier Science B. V.