(167h) Two-Dimensional Molybdenum Disulphide Membrane Separation: Stability, Structural Manipulation and Separation Performances

Two-dimensional (2D) materials such as molybdenum disulfide (MoS₂) present tremendous opportunities in membrane-based molecular separation. Compared to graphene oxide, MXene and other 2D inorganic materials, MoS₂ show exceptional stability in organic solvents. A few studies have reported the preparation of MoS₂ membranes for separation in aqueous solutions. However, there have been no investigations on the application of MoS₂ membranes for organic solvent separation. In addition, multi-layer and mono-layer MoS₂ nanosheets may be prepared by different approaches and have both been used for membrane preparation. Though studies have revealed that monolayer MoS₂ structure is unstable in the ambient conditions, there have been no reports on the stability of MoS₂ membranes.

This presentation covers our recent progress in 2D MoS₂ membranes for organic solvent separation. In the first part, we prepared both multi-layer and mono-layer MoS₂ nanosheets using different methods, and compared their structure, stability and performances after made into membranes. It’s found that multi-layer MoS₂ membranes are looser in structure, and hence result in higher permeability to isopropanol and lower rejection to dyes. On the other hand, though mono-layer MoS₂ membranes show better rejection to dyes in the short tests (within 3 hours), the rejection keeps decreasing over a 5-day tests, which is attributed to the limited stability of such nanosheets that lead to aggregation and breakage. To achieve high rejection and good stability, a ‘bridging’ agent is employed to regulate the interlayer spacing and nanosheet alignment of multi-layer MoS₂ membranes, leading to improved rejection and stable performances over one week.

In the second part, mixed matrix membranes containing MoS₂ nanosheets based on a 3-step layer-by-layer (LbL) method have been prepared, which provides a more cost-effective and scalable way to fabricate 2D membranes. Very interestingly, while the original LbL polyelectrolyte layer is impermeable to organic solvents, mixed matric membranes containing MoS₂ nanosheets open the pathways to solvents while rejecting solutes whose molecular weight is around 300 Dalton. The phemonena is attributed to the lower of surface energy by MoS₂ and its bridging effects. With a thin layer of < 50 nm, the mixed matrix membrane can achieve ethanol permeance –selectivity that surpasses the hypothetical upper bound, e.g., 92.9 % rejection for Victoria blue B (VBB, MW 506.08) with an ethanol flux of 11.4 L m^-2 h^-1 bar^-1.