Facilitated transport membranes for carbon capture

Facilitated transport membrane (FTM) is an innovative technology tailored for post-combustion carbon capture (PCCC), a major source of anthropogenic CO₂ emission. By incorporating amines which can selectively react with CO₂ in the polymer matrix, FTMs are capable of extracting large amounts of CO₂ at a high purity. Amines tethered to the polymer chain are called fixed-site carriers due to their low mobility, in contrast to the mobile carriers, which are smaller, free-moving amine molecules dispersed in the polymer matrix. As illustrated in Figure 1 below, CO₂ can either hop from fixed-site carriers to fixed-site carriers or be carried by the mobile carriers from the feed side to the permeate side. This offers a more efficient transport mechanism to CO₂ but not to the non-reactive gases such as N₂. Naturally, the performance of an FTM system is largely dependent on the selection of amines. For this project, we use molecular simulation techniques to understand the role of these amines on the overall separation performance to aid the design of FTMs with better performances.

Figure 1. Schematic illustration of the gas transport mechanism using the facilitated transport membrane.

Typically, a CO₂ molecule reacts with two amines to form a carbamate ion, or only one amine molecule to form a bicarbonate ion. Therefore, amines that prone to the bicarbonate ion formation considered to exhibit higher CO₂ loading capacities. To comment on the CO₂ loading capacities of different amines, we use density functional theory (DFT) calculations to map amine–CO₂ reaction chemistry following these two reaction routes. To quantify the cross-membrane transport of CO₂ and N₂, we use molecular dynamics (MD) simulations to study the diffusivities of key components, such as the polymer chain, mobile carriers, carbamate ions, bicarbonate ions, and N₂ at different levels of water uptake. We also employ Monte Carlo calculations to study the N₂ sorption behavior of amines to offer insight on the CO₂/N₂ selectivity of FTMs. To better understand the FTM systems in detail, a set of in-house software have also been developed to study key geometry features such as free volume distribution and pore-limiting diameters. Our primary results have shown a good agreement with experimental observations. In the future, we would broaden the scale of this study and eventually develop a schematic method of identifying potential amine structures. The insights generated in this direction would contribute to an expedited exploration of better-performing FTM systems.