High performance CO2-perm-selective SSZ-13 membranes: Elucidation of the link between membrane material and module properties

Improved Synthesis of Hollow Fiber SSZ-13 Zeolite Membranes for High-Pressure CO2/CH4 Separation

High-silica CHA zeolite membranes are highly desired for natural gas upgrading because of their separation performance in combination with superior mechanical and chemical stability. However, the narrow synthesis condition range significantly constrains scale-up preparation. Herein, we propose a facile interzeolite conversion approach using the FAU zeolite to prepare SSZ-13 zeolite seeds, featuring a shorter induction and a longer crystallization period of the membrane synthesis on hollow fiber substrates. The membrane thickness was constant at ~3 μm over a wide span of synthesis time (24-96 h), while the selectivity (separation efficiency) was easily improved by extending the synthesis time without compromising permeance (throughput). At 0.2 MPa feed pressure and 303 K, the membranes showed an average CO2 permeance of (5.2±0.5)×10-7 mol m-2 s-1 Pa-1 (1530 GPU), with an average CO2/CH4 mixture selectivity of 143±7. Minimal defects ensure a high selectivity of 126 with a CO2 permeation flux of 0.4 mol m-2 s-1 at 6.1 MPa feed pressure, far surpassing requirements for industrial applications. The feasibility for successful scale-up of our approach was further demonstrated by the batch synthesis of 40 cm-long hollow fiber SSZ-13 zeolite membranes exhibiting CO2/CH4 mixture selectivity up to 400 (0.2 MPa feed pressure and 303 K) without using sweep gas.

In Situ Synthesis of Cation-Free Zirconia-Supported Zeolite CHA Membranes for Efficient CO2/CH4 Separation

Cation-free zirconosilicate zeolite CHA and thin zirconia-supported membranes were in situ synthesized in a fluoride-free gel for the first time. The usage of the ZrO₂/Al₂O₃ composite support inhibited the transportation of aluminum from the support into zeolite membranes. No fluorite source was used for the synthesis of cation-free zeolite CHA membranes, indicating the green property of the synthesis. The thickness of the membrane was only 1.0 μm. The best cation-free zeolite CHA membrane prepared by the green in situ synthesis displayed a high CO₂ permeance of 1.1 × 10^-6 mol/(m² s Pa) and CO₂/CH₄ selectivity of 79 at 298 K and 0.2 MPa pressure drop for an equimolar CO₂/CH₄ mixture.

Small-Pore Zeolite Membranes: A Review of Gas Separation Applications and Membrane Preparation

There have been significant advancements in small-pore zeolite membranes in recent years. With pore size closely related to many energy- or environment-related gas molecules, small-pore zeolite membranes have demonstrated great potential for the separation of some interested gas pairs, such as CO2/CH4, CO2/N2 and N2/CH4. Small-pore zeolite membranes share some characteristics but also have distinctive differences depending on their framework, structure and zeolite chemistry. Through this mini review, the separation performance of different types of zeolite membranes with respect to interested gas pairs will be compared. We aim to give readers an idea of membrane separation status. A few representative synthesis conditions are arbitrarily chosen and summarized, along with the corresponding separation performance. This review can be used as a quick reference with respect to the influence of synthesis conditions on membrane quality. At the end, some general findings and perspectives will be discussed.

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.