Superior performance porous carbon nitride nanosheets for helium separation from natural gas: Insights from MD and DFT simulations

Abstract

Increasing helium (He) demand in fundamental research, medical, and industrial processes necessitates efficient He purification from natural gas. However, most theoretically available membranes focus on the separation of two or three kinds of gas molecules with He and the underlying separation mechanism is not yet well understood. Using molecular dynamic (MD) and first-principle density function theory (DFT) simulations, we systematically demonstrated a novel porous carbon nitride membrane (g-C9N7) with superior performance for He separation from natural gas. The structure of g-C9N7 monolayer was optimized first, and the calculated cohesive energy confirmed its structural stability. Increasing temperature from 200 to 500 K, the g-C9N7 membrane revealed high He permeability, as high as 1.48 × 107 GPU (gas permeation unit, 1 GPU = 3.35 × 10−10 mol∙s−1∙Pa−1∙m−2) at 298 K, and also exhibited high selectivity for He over other gases (Ar, N2, CO2, CH4, and H2S). Then, the selectivity of He over Ne was found to decrease with increasing the total number of He and Ne molecules, and to increase with increasing He to Ne ratio. More interestingly, a tunable He separation performance can be achieved by introducing strain during membrane separation. Under the condition of 7.5% compressive strain, the g-C9N7 membrane reached the highest He over Ne selectivity of 9.41 × 102. It can be attributed to the low energy barrier for He, but increased energy barrier for other gases passing through the membrane, which was subject to a compressive strain. These results offer important insights into He purification using g-C9N7 membrane and opened a promising avenue for the screening of industrial grade gas separation with strain engineering.