Separation Properties of Porous MoS₂ Membranes Decorated with Small Molecules

Using first-principles simulations, we surveyed the interactions between porous MoS₂ monolayers in the 2H phase and 15 small molecules (H₂, O₂, H₂O, H₂S, CO, CO₂, SO₂, N₂, NO, NO₂, NH₃, HF, HCl, CH₄, and CH₃OH). Four types of molecules including H₂, O₂, H₂S, and NO₂ directly dissociate and saturate the corners of the most common S-rimmed triangular pores, while other molecules only molecularly adsorb. The trisublayered structure of a MoS₂ monolayer allows a new in-pore stable adsorption configuration in addition to the most studied above-pore adsorption configuration. Furthermore, the gas penetration pathways through the MoS₂ membranes are no longer the conventional single-peak curve with one transition state like in the case of porous graphenes but are the "M"-shaped curve featuring two transition states connected by a stable in-pore adsorption state. The irreversible pore passivation via dissociative adsorption and reversible pore decoration by molecular adsorption will lead to very different separation performances of the MoS₂ membranes, largely by changing the effective pore size. For example, the S-rimmed pores in the pore-3Mo2S membrane allow free pass of CH₄ and CO₂ molecules. If passivated by H atoms, the membrane can be used to separate gas mixtures like H₂/CH₄ and H₂/CO₂ with selectivities of 10⁹:1 and 10⁸:1, respectively. The permeance value of H₂ is estimated to be about 0.15 mol m^-2 s^-1 Pa^-1 at room temperature and 0.1 bar pressure drop across the membrane. In contrast, the medium strong adsorption of a SO₂ molecule in the center of the pore will completely block the passage of CO₂ and CH₄, whose removal only needs heating. Our work reveals the complex behaviors of porous transition metal dichalcogenides (TMDs) toward guest molecules.