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

Abstract

Inelastic neutron scattering (INS) spectra were collected for ethane and ethene in Na zeolite–Y (Si/Al=2.0). The INS spectrum of ethane in Na–Y consists of a single broad adsorption band in which no individual peaks can be discerned. Monte-Carlo docking calculations predicted a large number of similar binding sites with energies ranging from 24 to 37 kJ mol −1 in the region of the four rings on the wall of the faujasite supercage. This is consistent with the broad feature observed in the INS spectrum. In contrast, the INS spectrum of ethene in Na–Y shows significant fine structure, and several peaks can be distinguished in the region<1000 cm −1. Monte-Carlo docking calculations with a molecular mechanics type force field were performed and predict two binding sites, one bound to the SII cation site (binding energy=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 using 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 mechanical and molecular mechanics based techniques and the results compared with the INS spectra. A significantly better fit was obtained using the latter method and the peaks could be assigned based on the computed eigenvectors. Analogies are drawn between the assignments and other previous studies of Zeise's salt and related compounds.