High-Silica CHA Zeolite Membrane with Ultra-High Selectivity and Irradiation Stability for Krypton/Xenon Separation

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.

PHAHST Potential: Modeling Sorption in a Dispersion-Dominated Environment

PHAHST (potentials with high accuracy, high speed, and transferability) is a recently developed force field that utilizes exponential repulsion, multiple dispersion terms, explicit many-body polarization, and many-body van der Waals interactions. The result is a systematic approach to force field development that is computationally practical. Here, PHAHST is employed in the simulation for rare gas uptake of krypton and xenon in the metal-organic material, HKUST-1. This material has shown promise in use as an adsorptive separating agent and presents a challenge to model due to the presence of heterogeneous interaction sorption surfaces, which include pores with readily accessible, open-metal sites that compete with dispersion-dominated pores. Such environments are difficult to simulate with commonly used empirical force fields, such as the Lennard-Jones (LJ) potential, which perform better when electrostatics are dominant in determining the nature of sorption and alone are incapable of modeling interactions with open-metal sites. The effectiveness of PHAHST is compared to the LJ potential in a series of mixed Kr-Xe gas simulations. It has been demonstrated that PHAHST compares favorably with experimental results, and the LJ potential is inadequate. Overall, we establish that force fields with physically grounded repulsion/dispersion terms are required in order to accurately model sorption, as these interactions are an important component of the energy. Furthermore, it is shown that the simple mixing rules work nearly quantitatively for the true pair potentials, while they are not transferable for effective potentials like LJ.

Control of Zeolite Local Polarity toward Efficient Xenon/Krypton Separation

The inherent inertness and striking physicochemical similarities of krypton and xenon pose significant challenges to their separation. Reported herein is the efficient xenon capture and xenon/krypton adsorptive separation by transition metal-free zeolites under ambient conditions. The polarized environment of zeolite, denoted as local polarity, can be tuned by changing the topology, framework composition, and counter-cations, which in turn correlates with the guest-host interaction and separation performance. Chabazite zeolite with a framework Si/Al ratio of 2.5 and Ca2+ as the counter-cations, namely, Ca-CHA-2.5, is developed as a state-of-the-art zeolite adsorbent, showing remarkable performance, i.e., high dynamic xenon uptake, high xenon/krypton separation selectivity, and good recyclability, in the adsorptive separation of the xenon/krypton mixture. Grand Canonical Monte Carlo simulation reveals that extraframework Ca2+ cations act as the primary binding sites for xenon and can stabilize xenon molecules together with the chabazite framework, whereas krypton molecules are stabilized by weak guest-host interaction with the zeolite framework.

Xe Recovery from Nuclear Power Plants Off-Gas Streams: Molecular Simulations of Gas Permeation through DD3R Zeolite Membrane

Recent experimental work has shown zeolite membrane-based separation as a promising potential technology for Kr/Xe gas mixtures due to its much lower energy requirements in comparison to cryogenic distillation, the conventional separation method for such mixtures. Such a separation is also economically rewarding because Xe is in high demand, as a valuable product for many applications/processes. In this work, we have used Molecular Dynamics (MD) simulations to study the effects of different conditions, i.e., temperature, pressure, and gas feed composition, on Kr/Xe separation performance via DD3R zeolite membranes. We provide a comprehensive study of the permeation of the different gas species, density profiles, and diffusion coefficients. Molecular simulations show that if the feed is changed from pure Kr/Xe to an equimolar mixture, the Kr/Xe separation factor increases, which agrees with experiments. In addition, when Ar is introduced as a sweep gas, the adsorption of both Kr and Xe increases, while the permeation of pure Kr increases. A similar behavior is observed with equimolar mixtures of Kr/Xe with Ar as the sweep gas. High-separation Kr/Xe selectivity is observed at 50 atm and 425 K but with low total permeation rates. Changing pressure and temperature are found to have profound effects on optimizing the separation selectivity and the permeation throughput.

Polyphosphonate-segmented macroporous organosilicon frameworks for efficient dynamic enrichment of uranium with in-situ regeneration

Efficient separation and enrichment of uranium from radioactive effluents is of strategic significance for sustainable development of nuclear energy and environmental protection. Macropore structure of adsorbent is conducive to accessibility of the pore and transport of the adsorbate during dynamic adsorption. However, the low specific surface area results in fewer ligand sites and subsequently reduces the adsorption capacity. Herein, we present a novel strategy for efficient dynamic uranium enrichment using polyphosphonate-segmented macroporous organosilicon frameworks (PMOFs). PMOFs are constructed through the copolymerization of diethyl vinylphosphonate and triethoxyvinylsilane, followed by hydrolysis and condensation of the oligomers. The introduction of polyphosphonate segments into the frameworks endows PMOFs with a macroporous structure (31 µm) and a high ligand content (up to 72 wt%). Consequently, the optimized PMOF-3 demonstrated an ultrahigh dynamic adsorption capacity of 114.8 mg/g among covalently conjugated silicon-based materials. Additionally, PMOF-3 achieves a high enrichment factor (120) in the dynamic enrichment of uranium on a fixed bed column, which can be in-situ regenerated with 1 M NaHCO₃ as the eluent. This work presents a new strategy for efficient dynamic enrichment of nuclides, which can be extended to the separation of other specific pollutants, shedding new light on adsorbent design and technical innovation.

Pore partition in two-dimensional covalent organic frameworks

Covalent organic frameworks (COFs) have emerged as a kind of crystalline polymeric materials with high compositional and geometric tunability. Most COFs are currently designed and synthesized as mesoporous (2-50 nm) and microporous (1-2 nm) materials, while the development of ultramicroporous (<1 nm) COFs remains a daunting challenge. Here, we develop a pore partition strategy into COF chemistry, which allows for the segmentation of a mesopore into multiple uniform ultramicroporous domains. The pore partition is implemented by inserting an additional rigid building block with suitable symmetries and dimensions into a prebuilt parent framework, leading to the partitioning of one mesopore into six ultramicropores. The resulting framework features a wedge-shaped pore with a diameter down to 6.5 Å, which constitutes the smallest pore among COFs. The wedgy and ultramicroporous one-dimensional channels enable the COF to be highly efficient for the separation of five hexane isomers based on the sieving effect. The obtained average research octane number (RON) values of those isomer blends reach up to 99, which is among the highest records for zeolites and other porous materials. Therefore, this strategy constitutes an important step in the pore functional exploitation of COFs to implement pre-designed compositions, components, and functions.

SO2/NO2 Separation Driven by NO2 Dimerization on SSZ-13 Zeolite Membrane

The molecular state is crucial for precise gas separation using a zeolite membrane, yet the state control remains a big challenge. Herein, we report a NO₂ dimerization facilitated high performance SO₂/NO₂ separation on a SSZ-13 zeolite membrane. The NO₂ dimerization is triggered by temperature and pressure to form N₂O₄ with big molecular size, and N₂O₄ diffusion into the zeolite pore is inhibited on the basis of size exclusion, leading to high separation selectivity. Consequently, SO₂ rather than NO₂ preferentially permeates through the SSZ-13 membrane with a high SO₂ permeance of 2 × 10^-7 mol m^-2 s^-1 Pa^-1 and high SO₂/NO₂ separation factor of 22, ∼50-fold of that measured without dimerization. The dimerization effect for SO₂/NO₂ separation prevails in other small-pore zeolites such as NaA. This advanced function is revealed through membrane separation using single and mixture gases.

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.

High-Silica CHA Zeolite Membrane with Ultra-High Selectivity and Irradiation Stability for Krypton/Xenon Separation

Capture and storage of the long‐lived ⁸⁵Kr is an efficient approach to mitigate the emission of volatile radionuclides from the spent nuclear fuel reprocessing facilities. However, it is challenging to separate krypton (Kr) from xenon (Xe) because of the chemical inertness and similar physical properties. Herein we prepared high‐silica CHA zeolite membranes with ultra‐high selectivity and irradiation stability for Kr/Xe separation. The suitable aperture size and rigid framework endures the membrane a strong size‐exclusion effect. The ultrahigh selectivity of 51–152 together with the Kr permeance of 0.7–1.3×10⁻⁸ mol m⁻² s⁻¹ Pa⁻¹ of high‐silica CHA zeolite membranes far surpass the state‐of‐the‐art polymeric membranes. The membrane is among the most stable polycrystalline membranes for separation of humid Kr/Xe mixtures. Together with the excellent irradiation stability, high‐silica CHA zeolite membranes pave the way to separate radioactive Kr from Xe for a notable reduction of the volatile nuclear waste storage volume.