Modeling new adsorbents for ethylene/ethane separations by adsorption via π-complexation

The adsorption of olefins and paraffins in a novel model porous adsorbent is studied by means of molecular simulations. The adsorbents are synthesized by effective dispersion of CuCl on substrates with hydrocarbonphobic surfaces, such as gamma-Al2O3. The Cu(I) cations are able to undergo pi-complexation with olefin molecules. Ethane and ethylene are studied as adsorbents, and the molecules are modeled as having two Lennard-Jones sites. Ethylene molecules have two additional associating square-well sites placed in the line perpendicular to the symmetry axis of the molecules in order to reproduce the pi-complexation. The gamma-Al2O3 surface is modeled as a single cylindrical pore showing the (100)-face of a cubic close-packing stacking of oxygen layers. Only the interactions with oxygen ions are explicitly taken into account, modeled as Lennard-Jones sites. Specific associating square-well sites are placed protruding from the gamma-Al2O3 surface, having identical size and energy parameters to those used on the ethylene molecules, in order to mimic Cu(I) cations. We present Grand Canonical Monte Carlo simulation results for single-component adsorption isotherms for ethane and ethylene on bare gamma-Al2O3 and on CuCl/gamma-Al2O3. Adsorption on bare gamma-Al2O3 is very similar for both molecules, with no selectivity for ethylene. Preferential adsorption of ethylene is seen on CuCl/gamma-Al2O3, whereas ethane adsorption is decreased, probably due to the presence of CuCl molecules. The olefin capacity, using CuCI/gamma-Al2O3 as sorbent, is much higher that for other materials. The agreement between simulation and experimental results is excellent. (C) 1998 Elsevier Science B.V. All rights reserved.