Quantum Chemical Study on the Temperature Dependence of Separation of Molecular Hydrogen and Deuterium Using Adsorption on Mn Dihydrogen Complexes

Molecular hydrogen is considered an ideal next-generation energy carrier. There are two methods of hydrogen molecule adsorption: chemical adsorption and physical adsorption. Since chemical adsorption is strong and physical adsorption is weak, an intermediate adsorption mode is necessary to achieve reversible adsorption and desorption at room temperature. In this study, quantum chemical calculations were used to investigate a solid-phase manganese hydrogen complex, [Mn(CO)(dppe)₂-H₂]⁺ (referred to as Mn1, and dppe = 1,2-bis(diphenylphosphino)ethane), to determine whether reversible adsorption and desorption at temperature relatively close to room temperature is feasible. Furthermore, since the adsorption energy for D₂ is not the same as that for H₂, the feasibility of separating D₂ and H₂ was explored by Gibbs energy calculations at different temperatures using the density functional theory. Based on adsorption measurements conducted at 310-365 K, the D₂/H₂ separation factor for Mn1 ranged from 2 to 1.5 as observed in our previous study. The results calculated using the M06-2X functional indicated that the D₂/H₂ separation factor for Mn1 at 298 K was approximately 2.55, which is superior to the results obtained using the B3LYP and CAM-B3LYP functionals. The isotope separation ability of [Mn(CO)₃(PCy₃)₂-H₂]⁺ (referred to as Mn2) is slightly inferior to that of Mn1; however, it has an advantage of lower adsorption enthalpy compared to Mn1, making it more suitable for desorption at lower temperatures.