Aromatics/aliphatics separation by adsorption: New sorbents for selective aromatics adsorption by π-complexation

New sorbents for benzene/cyclohexane separation based on π-complexation were prepared by dispersion of transition metal salts on a high-surface-area substrate. PdCl₂ or AgNO₃ dispersed on SiO₂ gel exhibited high equilibrium adsorption ratios of benzene over cyclohexane. PdCl₂ loading of 0.88 g/g of SiO₂ showed the best benzene/cyclohexane ratio of 3.2. The heats of adsorption of benzene on these sorbents were in the range 7-12 kcal/mol and followed the rank order CuCl > PdCl₂ > AgNO₃ > AuCl₃ > PtCl₄, compared to 5-7 kcal/mol for cyclohexane. Molecular orbital calculations for the bonding of benzene and chlorides of these metals were performed at the Hartree-Fock (HF) and density functional theory (DFT) levels using effective core potentials. The theoretical bond energies were (in kcal/mol) 12.5 (CuCl), 10.8 (PdCl₂), 8.6 (AgCl), 6.5 (AuCl₃), and 5.2 (PtCl₄), in fair agreement with the experimental results. The M-C interactions followed classical π-complexation in most cases, and differences in the bonds with these metal ions are explained by natural bond orbital results.