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.