Molecular transport in 2D materials and interfaces: Insight from molecular modeling

Abstract

Membrane separation technologies have emerged as energy-efficient alternatives to conventional separation processes, reducing operational costs and environmental footprint. Two-dimensional (2D) materials represent a paradigm shift in membrane design, providing ultra-thin platforms with unprecedented control over molecular transport. Specifically, 2D porous membranes leverage their monolayer architecture, size-selective nanopores, and high porosity to achieve high permeance and precise molecular differentiation through molecular sieving. However, fundamental transport phenomena at the atomic scale—particularly the interplay among diffusion pathways of permeant molecules, pore-molecule interactions, and interfacial effects—remain poorly understood. In this dissertation, we first investigated the predictive design of novel fullerene network materials, a new class of 2D materials with high density of in-plane subnanometer pores, to explore their separation performance and underlying mechanisms. By employing concentration gradient driven molecular dynamics simulations, we accurately captured and predicted the permeance, selectivity, and transport behavior of molecules and ions through membranes. We proposed and simulated two different quasi-tetragonal phase (qTP) fullerene membranes (qTP1 and qTP2) for efficient hydrogen separation, revealing the entropy-driven selectivity of H2 over O2 and CO2. Next, we found that water transport through qTP2 membranes in the liquid phase is significantly slower than in the vapor phase, due to the breaking and reforming of hydrogen bonds during pore traversal. Then, we explored the effects of cation gating on qTP1 membranes, demonstrating that cation-fullerene interactions can effectively modulate pore size to enable selective O2/N2 separation. The other focus of this dissertation is on creating advanced interfaces and interactions to optimize molecular transport. By combining ionic liquids with porous materials, we introduce confined liquid-solid interface and channels for efficient molecular separation. We designed a multi-layered IL-porous graphene system in which anions were stabilized within the pores. This arrangement refines pore architecture, leading to a dramatic increase in CO₂/N₂ selectivity. Finally, we developed an aligned MXene-IL composite membrane with slit pores, showing that CO2 transfers rapidly at the MXene-IL interface but moves slowly through the IL layers, while other gases diffuse slowly in both regions. In sum, the work in this dissertation aims to design novel 2D membrane and interfaces with enhanced separation performance and intriguing molecular transport phenomena.