A Hybrid Zeolite Membrane-Based Breakthrough for Simultaneous CO 2 Capture and CH 4 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 CO ₂ (0.33 nm) and CH ₄ (0.38 nm), is required to meet the approximately 90-95% purity of CH ₄ , while CO ₂ should be concomitantly purified. In this study, a high CO ₂ 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 nm ² ). The resulting hydrophobic DDR@CHA hybrid membrane on an asymmetric α-Al ₂ O ₃ tube was thin (ca. 2 μm) and continuous, thus providing both high flux and permselectivity for CO ₂ irrespective of the presence or absence of water vapor (the third largest component in the biogas streams). To the best of our knowledge, the CO ₂ permeance of (2.9 ± 0.3) × 10 ^-7 mol m ^-2 s ^-1 Pa ^-1 and CO ₂ /CH ₄ separation factor of ca. 274 ± 73 at a saturated water vapor partial pressure of ca. 12 kPa at 50 °C have the highest CO ₂ /CH ₄ separation performance yet achieved. Furthermore, we explored the membrane module properties of the hybrid membrane in terms of the recovery and purity of both CO ₂ and CH ₄ 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 CO ₂ and CH ₄ 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.