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

## TL;DR

This study explores how manganese complexes can separate hydrogen and deuterium gases at different temperatures using quantum chemical calculations.

## Contribution

The novelty lies in using quantum chemical calculations to assess the feasibility of H2/D2 separation via Mn dihydrogen complexes at near-room temperatures.

## Key findings

- Mn1 shows a D2/H2 separation factor of approximately 2.55 at 298 K using the M06-2X functional.
- Mn2 has lower isotope separation ability than Mn1 but requires less energy for desorption.
- Separation factors decrease with increasing temperature, aligning with experimental observations.

## Abstract

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.