MFU-4 as a Benchmark Molecular Sieve for Efficient CO₂/CH₄ Separation in Biogas Upgrading

Abstract

The efficient separation of CO₂ from CH₄ is central to industrial biogas upgrading for the production of pipeline-quality biomethane. Kinetic, molecular sieve–based adsorption is preferred over thermodynamic approaches, making small-pore adsorbents such as CMS-3K and ion-exchanged ETS-4 the current industrial standard. Here, we show, through a combination of breakthrough experiments and computational analysis, that the ultra-microporous Zn triazolate MOF MFU-4 significantly surpasses these benchmarks. Its unique architecture, featuring alternating small and large cages connected by narrow, square-shaped pore gates, kinetically hinders CH₄ diffusion while facilitating rapid CO₂ transport and achieving high CO₂ uptake, effectively overcoming the long-standing trade-off between CO₂/CH₄ selectivity and CO₂ capacity. As a result, MFU-4 achieves CO₂/CH₄ kinetic selectivity up to twice that of ETS-4 and four times that of CMS-3K, with CO₂ working capacities up to seven and four times higher, respectively, over the 100–500 kPa range, and an exceptional CO₂ uptake of ~7.4 mol·kg⁻¹ at 298 K and 500 kPa. These findings establish MFU-4 as an excellent molecular sieve for biogas upgrading, delivering performance far beyond current industrial standards.