New sorbents for olefin/paraffin separations by adsorption via π ‐complexation

Abstract

New adsorbents for olefin/paraffin separation are synthesized by effective dispersion of Ag(I) and Cu(I) cations on substrates with hydrocarbon‐phobic surfaces. These cations bind olefin molecules by a π‐complexation bond, a weak chemical bond. Ethane/ethylene and propane/propylene separations are considered. Cation exchange resins and CuCl/γ‐Al 2 O 3 are effective substrates. On the Ag(I) resin at 25°C and 1 atm, the equilibrium adsorption ratio for C 2 H 4/C 2 H 6 = 9.2 and C 2 H 4 capacity = 1.15 mmol/g; the corresponding values for C 3 H 6/C 3 H 8 = 10.4 and C 3 H 6 capacity = 1.29 mmol/g. The CuCl/γ‐Al 2 O 3 sorbent shows equally promising results. The sorption rates are pore‐diffusion‐controlled and rapid. The olefin selectivity; capacity, and rates are much higher than all previous attempts and are suitable for applications in cyclic adsorption processes. The equilibrium data are correlated with an isotherm equation that accounts for both physical adsorption and π‐complexation with energy heterogeneity, using only two true fitting parameters. Molecular orbital calculations using a C 6 H 5 SO substrate indicate that the π‐complexation bond is contributed mainly by the donation of olefin π‐bond electrons to the empty s‐orbital of the metal, while the d‐π* back donation contributes only 16%. Moreover, the relative order of the heats of adsorption is correctly predicted.