Low-Temperature H₂S/CO₂/CH₄ Separation in Mixed-Matrix Membranes Containing MFU-4

Harvesting natural gas and biogas often requires energy-intensive separation processes. Mixed-matrix membranes (MMMs) containing CO2-selective metal–organic frameworks (MOFs) show great potential in addressing this separation challenge with greater energy efficiency than traditional technologies. Here, we report an MMM system formed from a 6FDA–Durene polymer and an MFU-4 MOF, which demonstrated improved CO2/CH4 gas separation performance, especially at low temperatures. Decreasing the temperature from 65 to 15 °C led to a significant increase in CO2/CH4 permselectivity for the hybrid MFU-4/6FDA–Durene membrane, with an associated high CO2 permeability above 1000 barrer. Mixed-gas permeation tests involving H2S were conducted, and the separation performance of the MFU-4 hybrid membrane was comparable to literature materials specially designed for H2S separation. For CO2-based separations, MFU-4 is believed to have a gating effect that favors the linear shape of CO2. However, a detailed investigation of sorption and diffusion revealed that this specific effect was not clearly accessible under the conditions considered in this study. Instead, the high CO2/CH4 permselectivity at low temperature was attributed mainly to the higher polarizability and smaller size of CO2 compared to CH4.