New Adsorbents Based on Principles of Chemical Complexation: Monolayer-Dispersed Nickel(II) for Acetylene Separation by π-Complexation

A sorbent was prepared by dispersing a monolayer of Ni 2+ ions (by spreading a monolayer of NiCl 2 ·6H 2 O) on a high-surface-area γ-Al 2 O 3 . This sorbent showed high selectivities for acetylene (C 2 H 2 ) over other hydrocarbons, by forming a weak π-complexation bond between Ni 2+ and C 2 H 2 . For example, at 60 °C, the equilibrium amounts of hydrocarbons adsorbed at 1 atm were (in mmol/g) C 2 H 2 = 1.56, C 2 H 6 = 0.043, C 2 H 4 = 0.061, C 3 H 6 = 0.42, and C 3 H 8 = 0.30. The bond energy for the Ni 2+ −C 2 H 2 complex was approximately 9.3 kcal/mol, and the adsorption was reversible. The stoichiometry of the complex was Ni 2+ (C 2 H 2 ) n where n = 1−3 at 25 °C and subatmospheric pressure. The π-complexation bond was formed by the donation of the C 2 H 2 π-electrons to the vacant hybridized dsp 2 orbitals of Ni 2+ and the back-donation of electrons from the filled d (or dp-hybrid) orbitals of Ni 2+ into the antibonding orbitals of C 2 H 2 . This work demonstrates that new sorbents can be designed and prepared by exploiting the weak, reversible π-complexation bonds, and, consequently, conventional separation/purification processes can be replaced by more efficient adsorption processes.