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.

An Extrinsic-Pore-Containing Molecular Sieve Film: A Robust, High-Throughput Membrane Filter.

MFI type zeolites with 10-membered-ring pores (ca. 0.55 nm) have the ability to separate p-xylene (ca. 0.58 nm) from its bulkier isomers. Here, we introduced non-zeolitic micropores (ca. 0.6-1.5 nm) and mesopores (ca. 2-7 nm) to a conventional microporous MFI type zeolite membrane, yielding an unprecedented hierarchical membrane structure. The uniform, embedded non-zeolitic pores decreased defect formation considerably and facilitated molecular transport, resulting in high p-xylene perm-selectivity and molar flux. Specifically, compared to a conventional, crack network-containing MFI membranes of similar thickness (ca. 1 µm), the mesoporous MFI membranes showed almost double p-xylene permeance (ca. 1.6±0.4×10-7  mol m-2 s-1 Pa-1 ) and a high p-/o-xylene separation factor (ca. 53.8±7.3 vs. 3.5±0.5 in the conventional MFI membrane) at 225 °C. The embedded non-zeolitic pores allowed for decreasing the separation performance degradation, which was apparently related to coke formation.

An Hetero-Epitaxially Grown Zeolite Membrane.

The secondary growth methodology to form zeolite membranes has stringent requirements for homogeneous epitaxial intergrowth of the seed layer and limits the number of accessible high-quality zeolite membranes. Despite previous reports on hetero-epitaxial growth, high-performance zeolite membranes have yet to be reported using this approach. Here, the successful hetero-epitaxial growth of highly siliceous ZSM-58 (DDR-type zeolite) films from a SSZ-13 (CHA-type zeolite) seed layer is reported. The resulting membranes show excellent CO2 perm-selectivities, having maximum CO2 /N2 and CO2 /CH4 separation factors (SFs) as high as about 17 and 279, respectively, at 30 °C. Furthermore, the hybrid membrane maintains the CO2 perm-selectivity in the presence of water vapor (the third main component in both cases), that is, CO2 /N2 SF of about 14 and CO2 /CH4 SF of about 78, respectively, at 50 °C (a representative temperature of both CO2 -containing streams).