Adsorption of benzene and cyclohexane on various Y-zeolites was investigated to develop sorbents for the purification of aliphatics by removal of aromatics. Ag-Y showed superior benzene/cyclohexane selectivities to Na-Y, Pd-Y, and H-USY Separation factors greater than 10(5) were obtained with Ag-Y at low concentrations of benzene. High selectivities were achieved by the strong interaction between benzene and Ag-Y, while the interaction with cyclohexane was not influenced by the cations. Molecular orbital calculation revealed that benzene formed a classic pi-complexation bond with Ag-Y: donation of electron charges from the sigma-orbitals of benzene to the vacant s-orbital of the silver (sigma donation) and, simultaneously, back donation of electron charges from d-orbitals of silver to pi*-orbital of benzene (d-pi* back donation). Grand canonical Monte Carlo simulations were also performed for adsorption isotherms. Potential parameters of benzene on Ag-Y, including pi-complexation, were developed first. Simulation and experiments of GCMC agreed excellently for adsorption of benzene on Y-zeolites.