Grand canonical ensemble Monte Carlo (GCMC) and configurational-bias Monte Carlo (CBMC) techniques were combined to simulate the adsorption behavior of methane-ethane-propane and n-butane-i-butane mixtures at 298 K in copper(II) benzene-1,3,5-tricarboxylate (Cu-BTC). The simulation results demonstrate that for the methane-ethane-propane mixtures propane is preferentially adsorbed, and the amount adsorbed of propane increases to a maximum and then decreases with increasing the pressure, whereas the shorter alkanes are almost not adsorbed at lower pressures and the amount adsorbed increases progressively and exceeds propane as the pressure increases. The selectivity of Cu-BTC to propane increases as the molar fraction of shorter chain components in the gas phase increases. As to the n-butane-i-butane mixtures, i-butane is adsorbed preferentially, the amounts adsorbed of both linear and branched alkanes increase with increasing the pressure and the amount of i-butane is greater than that of n-butane, the selectivity to i-butane is increased as the molar fraction of n-butane increases in the gas phase. Furthermore, the adsorption sites of alkane mixtures at various pressures were investigated to understand the adsorption and separation capabilities of Cu-BTC by mCT (Computer Tomography for materials) technique. The adsorption competition between the energy effect and the size effect has been found in the adsorption process. It indicates that at higher pressures propane mainly occupies the main channels due to the size effect while for methane and ethane the energetically favored ethane occupies the tetrahedron-shaped pockets leaving methane locating at the outer sides of the triangular windows. As to the n-butane-i-butane mixtures, the energy effect is preferential to the size effect, therefore, i-butane mainly occupies the tetrahedral pockets while n-butane is located in the main channels.

Key words: alkane mixture, adsorption, separation, Cu-BTC, adsorption site