The rational design of Li-doped nitrogen adsorbents for natural gas purification

Abstract

Separation of nitrogen (N₂) and methane (CH₄) is one of the most challenging and energy-intensive processes in the natural gas industry, due to their close physico-chemical properties. The quest for an effective N₂-selective adsorbent has long been the focus of research; however, the results have been sparse. In this work, a first-principle study has been used to construct and investigate Li-doped polycyclic aromatic hydrocarbons (PAHs) for N₂ rejection in natural gas purification. We doped lithium on a series of linear/nonlinear PAHs consisting of two to six benzene rings. The adsorption affinity of the Li-doped organic molecular systems toward N₂ and CH₄ was evaluated by calculating the interaction energy using density functional theory. From the gas adsorption selectivities for different Li-doped PAHs, Li-doped phenanthrene and chrysene showed the highest N₂ over CH₄ equilibrium selectivities, with values of 119.7 and 80.8, respectively. It was found that the Li atom enabled the π bond of the aromatic substrate to interfere with the N₂ lowest unoccupied molecular orbital, resulting in strong physisorption of N₂. These results indicate the high potential of Li-doped phenanthrene and chrysene for N₂ removal from natural gas.