# Mixed-matrix membranes with molecular recognition windows for selective helium extraction from natural gas

## TL;DR

Scientists improved membranes to selectively extract helium from natural gas by using a compound called Cyclen, which enhances the membrane's ability to separate helium from methane.

## Contribution

The use of Cyclen as a pore-structure modifier in Matrimid membranes significantly boosts He/CH4 selectivity.

## Key findings

- Cyclen incorporation induced denser chain stacking and modulated interchain gaps in Matrimid membranes.
- The He/CH4 selectivity reached up to 6788 after physical aging, outperforming most polymer-based membranes.
- Molecular dynamics simulations showed improved microporous structures and efficient He transport channels.

## Abstract

Mixed-matrix membranes (MMMs) with high chain packing density by incorporating soluble macrocycle compounds represent a promising class of materials for gas separation. However, achieving the ultra-high selectivity (He/CH4 > 1000) for helium extraction from natural gas with ultra-low helium content remains a formidable challenge, especially for Matrimid membranes, which are commercially available but exhibit relatively low permeability and moderate selectivity. Herein, the cyclic Cyclen with specific intra-ring dimensions was incorporated into Matrimid as a pore-structure modifier to enhance the He/CH4 selectivity. The strong hydrogen bonding interactions between Cyclen and Matrimid chains induced a denser chain stacking and modulation of the interchain gap structures, which enables rapid mass transfer of small He gas molecules while hindering the diffusion of large CH4 gas molecules across the membrane, thereby significantly enhanced He/CH4 molecular sieving capacity. Molecular dynamics simulations indicate that the MMMs prepared using Cyclen as a filler exhibited tunable microporous and more efficient He transport channels. Notably, the He/CH4 selectivity reached up to an impressive value of 6788 after physical aging for 110 days, which outperformed almost all reported polymer-based membranes and was even comparable to that of some advanced carbon molecular sieve membranes.

Mixed-matrix membranes incorporating soluble macrocycle compounds represent a promising class of materials for gas separation but achieving high selectivity for helium extraction from natural gas remains challenging. Here the authors incorporate a cyclic Cyclen with specific intra-ring dimensions as a pore-structure modifier to enhance He/CH4 selectivity.