Separation of oxygen from nitrogen using a graphdiyne membrane: a quantum-mechanical study

Efficient separation of oxygen and nitrogen from air is a process of great importance for many industrial and medical applications. Two-dimensional (2D) membranes are very promising materials for separation of gases, as they offer enhanced mass transport due to their smallest atomic thickness. In this work, we examine the capacity of graphdiyne (GDY), a new 2D carbon allotrope with regular subnanometric pores, for separating oxygen (¹⁶O₂) from nitrogen (¹⁴N₂). A quantum-mechanical model has been applied to the calculation of the transmission probabilities and permeances of these molecules through GDY using force fields based on accurate electronic structure computations. It is found that the ¹⁶O₂/¹⁴N₂ selectivity (ratio of permeances) is quite high (e.g., about 10⁶ and 10² at 100 and 300 K, respectively), indicating that GDY can be useful for separation of these species, even at room temperature. This is mainly due to the N₂ transmission barrier (∼0.37 eV) which is considerably higher than the O₂ one (∼0.25 eV). It is also found that molecular motions are quite confined inside the GDY pores and that, as a consequence, quantum effects (zero-point energy) are significant in the studied processes. Finally, we explore the possibility of ¹⁸O₂/¹⁶O₂ isotopologue separation due to these mass-dependent quantum effects, but it is found that the process is not practical since reasonable selectivities are concomitant with extremely small permeances.