A Hybrid Zeolite Membrane-Based Breakthrough for Simultaneous CO₂ Capture and CH₄ Upgrading from Biogas

Biogas is an environmentally friendly and sustainable energy resource that can substitute or complement conventional fossil fuels. For practical uses, biogas upgrading, mainly through the effective separation of CO₂ (0.33 nm) and CH₄ (0.38 nm), is required to meet the approximately 90-95% purity of CH₄, while CO₂ should be concomitantly purified. In this study, a high CO₂ perm-selective zeolite membrane was synthesized by heteroepitaxially growing a chabazite (CHA) zeolite seed layer with a synthetic precursor that allowed the formation of all-silica deca-dodecasil 3 rhombohedral (DDR) zeolite (with a pore size of 0.36 × 0.44 nm²). The resulting hydrophobic DDR@CHA hybrid membrane on an asymmetric α-Al₂O₃ tube was thin (ca. 2 μm) and continuous, thus providing both high flux and permselectivity for CO₂ irrespective of the presence or absence of water vapor (the third largest component in the biogas streams). To the best of our knowledge, the CO₂ permeance of (2.9 ± 0.3) × 10^-7 mol m^-2 s^-1 Pa^-1 and CO₂/CH₄ separation factor of ca. 274 ± 73 at a saturated water vapor partial pressure of ca. 12 kPa at 50 °C have the highest CO₂/CH₄ separation performance yet achieved. Furthermore, we explored the membrane module properties of the hybrid membrane in terms of the recovery and purity of both CO₂ and CH₄ under dry and wet conditions. Despite the high intrinsic membrane properties of the current hybrid membrane, reflected by the high permeance and SF, the corresponding module properties indicated that high-performance separation of CO₂ and CH₄ for the desired biogas upgrading was achieved at a limited processing capacity. This supports the importance of understanding the correlation between the membrane and module properties, as this will provide guidance for the optimal operating conditions.