Preferential Adsorption of Propylene over Propane on a Ag-Exchanged X-Type Zeolite Membrane

An Experimental Study of a Zeolite Membrane Reactor for Reverse Water Gas Shift

Reverse water gas shift (RWGS) is attracting attention as one of the promising technologies for CO₂ conversion. Selective removal of H₂O from the reaction system can improve the CO₂ conversion beyond the equilibrium conversion of RWGS in a conventional reactor. In this study, a conventional plug-flow reactor without membrane, and two types of RWGS membrane reactors using ZSM-5 membranes, were developed. The yield of CO without membrane (Case 1) was almost the same as the equilibrium conversion. A membrane reactor (Case 2) showed a CO yield 2-3% above that of a conventional reactor. From the results, the effectiveness of the dehydration membrane reactor for RWGS was verified. In addition, CO yield was further increased in the reactor made up of the combination of conventional reactor and membrane reactor (Case 3). For example, the CO yields in Cases 1, 2, and 3 at 560 K were 21.8, 24.9, and 29.0%, respectively. Although the CO yield increased in Case 2, a large amount of raw materials penetrated through the membrane to the permeation side, and was lost. In Case 3, H₂ and CO₂ permeation through the membrane were suppressed because of the existence of H₂O, resulting in the prevention of the leakage of raw material, and contributing to the high CO yield.

One-Step Scalable Fabrication of Highly Selective Monolithic Zeolite MFI Membranes for Efficient Butane Isomer Separation

The reproducible fabrication of large-area zeolite membranes for gas separation is still a great challenge. We report the scalable fabrication of high-performance zeolite MFI membranes by single-step secondary growth on the 19-channel alumina monoliths for the first time. The packing density and mechanical strength of the monolithic membranes are much higher for these than for tubular ones. Separation performance of the monolithic membranes toward the butane isomer mixture was comparably evaluated using the vacuum and Wicke-Kallenbach modes. The n-butane permeances and n-butane/i-butane separation factors for the three membranes with an effective area of ∼84 cm² were >1.0 × 10^-7 mol (m² s Pa)^-1 and >50 at 343 K for an equimolar n-butane/i-butane mixture, respectively. We succeeded in scaling up the membrane synthesis with the largest area of 270 cm² to date which has 1.3 times the area of an industrial 1 m long tubular membrane. Monolith supported zeolite MFI membranes show great potential for industrial n-butane/i-butane separation.

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.

Olefin Recovery by *BEA-Type Zeolite Membrane: Affinity-Based Separation with Olefin-Ag+ Interaction

Ag⁺ was introduced into *BEA‐type zeolite membrane by an ion‐exchange method to enhance olefin selectivity. Ag−*BEA membrane exhibited superior olefin separation performance for both ethylene/ethane and propylene/propane mixtures. Particularly, the separation factor for ethylene at 373 K reached 57 with the ethylene permeance of 1.6×10⁻⁷ mol m⁻² s⁻¹ Pa⁻¹. Adsorption properties of olefin and paraffin were evaluated to discuss contribution of Ag⁺ to separation performance enhancement. A strong interaction between olefin and Ag⁺ in the membrane caused preferential adsorption of olefin against paraffin, leading to selective permeation of olefin. Ag−*BEA membrane also exhibited high olefin selectivities from olefin/N₂ mixtures. The affinity‐based separation through Ag−*BEA membrane showed a high potential for olefin recovery and purification from various gas mixtures.

Recent Progress in a Membrane-Based Technique for Propylene/Propane Separation

The similar physico-chemical properties of propylene and propane molecules have made the separation process of propylene/propane challenging. Membrane separation techniques show substantial prospects in propylene/propane separation due to their low energy consumption and investment costs, and they have been proposed to replace or to be combined with the conventional cryogenic distillation process. Over the past decade, organosilica membranes have attracted considerable attention due to their significant features, such as their good molecular sieving properties and high hydrothermal stability. In the present review, holistic insight is provided to summarize the recent progress in propylene/propane separation using polymeric, inorganic, and hybrid membranes, and a particular inspection of organosilica membranes is conducted. The importance of the pore subnano-environment of organosilica membranes is highlighted, and future directions and perspectives for propylene/propane separation are also provided.

Microporous framework membranes for precise molecule/ion separations

Microporous framework membranes such as metal-organic framework (MOF) membranes and covalent organic framework (COF) membranes are constructed by the controlled growth of small building blocks with large porosity and permanent well-defined micropore structures, which can overcome the ubiquitous tradeoff between membrane permeability and selectivity; they hold great promise for the enormous challenging separations in energy and environment fields. Therefore, microporous framework membranes are endowed with great expectations as next-generation membranes, and have evolved into a booming research field. Numerous novel membrane materials, versatile manipulation strategies of membrane structures, and fascinating applications have erupted in the last five years. First, this review summarizes and categorizes the microporous framework membranes with pore sizes lower than 2 nm based on their chemistry: inorganic microporous framework membranes, organic-inorganic microporous framework membranes, and organic microporous framework membranes, where the chemistry, fabrications, and differences among these membranes have been highlighted. Special attention is paid to the membrane structures and their corresponding modifications, including pore architecture, intercrystalline grain boundary, as well as their diverse control strategies. Then, the separation mechanisms of membranes are covered, such as diffusion-selectivity separation, adsorption-selectivity separation, and synergetic adsorption-diffusion-selectivity separation. Meanwhile, intricate membrane design to realize synergistic separation and some emerging mechanisms are highlighted. Finally, the applications of microporous framework membranes for precise gas separation, liquid molecule separation, and ion sieving are summarized. The remaining challenges and future perspectives in this field are discussed. This timely review may provide genuine guidance on the manipulation of membrane structures and inspire creative designs of novel membranes, promoting the sustainable development and steadily increasing prosperity of this field.