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 Ni²⁺ ions (by spreading a monolayer of NiCl₂·6H₂O) on a high-surface-area γ-Al₂O₃. This sorbent showed high selectivities for acetylene (C₂H₂) over other hydrocarbons, by forming a weak π-complexation bond between Ni²⁺ and C₂H₂. For example, at 60°C, the equilibrium amounts of hydrocarbons adsorbed at 1 atm were (in mmol/g) C₂H₂ = 1.56, C₂H₆ = 0.043, C₂H₄ = 0.061, C₃H₆ = 0.42, and C₃H₈ = 0.30. The bond energy for the Ni²⁺-C₂H₂ complex was approximately 9.3 kcal/mol, and the adsorption was reversible. The stoichiometry of the complex was Ni²⁺(C₂H₂)_n where n = 1-3 at 25°C and subatmospheric pressure. The π-complexation bond was formed by the donation of the C₂H₂ π-electrons to the vacant hybridized dsp² orbitals of Ni²⁺ and the back-donation of electrons from the filled d (or dp-hybrid) orbitals of Ni²⁺ into the antibonding orbitals of C₂H₂. 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.