Recent progress in polycrystalline zeolite membrane research

Advancements in Gas Separation for Energy Applications: Exploring the Potential of Polymer Membranes with Intrinsic Microporosity (PIM)

Membrane-based Polymers of Intrinsic Microporosity (PIMs) are promising candidates for energy-efficient industrial gas separations, especially for the separation of carbon dioxide over methane (CO2/CH4) and carbon dioxide over nitrogen (CO2/N2) for natural gas/biogas upgrading and carbon capture from flue gases, respectively. Compared to other separation techniques, membrane separations offer potential energy and cost savings. Ultra-permeable PIM-based polymers are currently leading the trade-off between permeability and selectivity for gas separations, particularly in CO2/CH4 and CO2/N2. These membranes show a significant improvement in performance and fall within a linear correlation on benchmark Robeson plots, which are parallel to, but significantly above, the CO2/CH4 and CO2/N2 Robeson upper bounds. This improvement is expected to enhance the credibility of polymer membranes for CO2 separations and stimulate further research in polymer science and applied engineering to develop membrane systems for these CO2 separations, which are critical to energy and environmental sustainability. This review aims to highlight the state-of-the-art strategies employed to enhance gas separation performances in PIM-based membranes while also mitigating aging effects. These strategies include chemical post-modification, crosslinking, UV and thermal treatment of PIM, as well as the incorporation of nanofillers in the polymeric matrix.

Preparation of biomass-based gas separation membranes from biochar residue obtained by depolymerization of lignin with ZSM-5 to promote a circular bioeconomy

Reuse of biochar residues after lignin degradation will not only save costs but also reduce the pollution, protect and improve the environment. In this study, biochar residue (BR) after peanut shell lignin selective depolymerization on ZSM-5 were recycled, and characterized by Scanning Electron Microscopy, Surface area & pore size distribution analyzers, Thermogravimetric Analysis. Subsequently, a series of hybrid matrix membranes were prepared using ethyl cellulose as the matrix and biochar residue after depolymerization under different reaction conditions as the filler. The separation performance of BR/EC membranes for CO₂/CH₄ mixed gas and CO₂/N₂ mixed gas was measured. The results showed that the gas separation membranes prepared with biochar residue (3 h, 300 °C) as filler had good gas separation characteristics. The resulting mixed-matrix membrane exhibited a permeability of 66.00 Barrer for CO₂ and selectivities of 9.97 for CO₂/CH₄. Meanwhile, the resulting mixed-matrix membrane exhibited a permeability of 79.53 Barrer for CO₂ and selectivities of 20.01 for CO₂/N₂. Both exceed the upper limit of known pure EC membranes. Therefore, the use of biochar residue after ZSM-5 depolymerization as a filler for gas separation membranes is a feasible way. Furthermore, the membrane is well stabilized, proving its good potential for industrial applications.

Prevention in Thermal Crack Formation in Chabazite (CHA) Zeolite Membrane by Developing Thin Top Zeolite and Thick Intermediate Layers

Chabazite (CHA) zeolite membranes with an intermediate layer of various thicknesses were prepared using planetary-milled seeds with an average particle diameter of 300, 250, 200, 140, and 120 nm. The 120 nm seed sample also contained several smaller particles with a diameter of 20 nm. Such small seeds deeply penetrated into the pore channels of the α-alumina support during the vacuum-assisted infiltration process. During the secondary growth, the penetrated seeds formed a thick intermediate layer exiting between the zeolite layer and support. A decrease in seed size increased the penetration depth of seeds and the thickness of the intermediate layer, while the thickness of seed coating and zeolite layers was decreased. CHA zeolite membranes with a thin top zeoliate layer and a thick intermediate layer showed an excellent water/ethanol separation factor (>10,000) for 90 wt.% ethanol at 70 ℃ with a total flux of 1.5 kg m⁻² h⁻¹. There was no observation of thermal cracks/defects on the zeolite separation layer. The thick intermediate layer effectively suppressed the formation of thermal cracks during heating, since the tensile stress induced in the zeolite layer was well compensated by the compressive stress on the support. Therefore, it was successfully proven that controlling the microstructure of top surface and intermediate layers is an effective approach to improve the thermal stability of the CHA zeolite membrane.

Process Intensification in Bio-Ethanol Production–Recent Developments in Membrane Separation

Ethanol is considered as a renewable transport fuels and demand is expected to grow. In this work, trends related to bio-ethanol production are described using Thailand as an example. Developments on high-temperature fermentation and membrane technologies are also explained. This study focuses on the application of membranes in ethanol recovery after fermentation. A preliminary simulation was performed to compare different process configurations to concentrate 10 wt% ethanol to 99.5 wt% using membranes. In addition to the significant energy reduction achieved by replacing azeotropic distillation with membrane dehydration, employing ethanol-selective membranes can further reduce energy demand. Silicalite membrane is a type of membrane showing one of the highest ethanol-selective permeation performances reported today. A silicalite membrane was applied to separate a bio-ethanol solution produced via high-temperature fermentation followed by a single distillation. The influence of contaminants in the bio-ethanol on the membrane properties and required further developments are also discussed.

The Determination of Pore Shape and Interfacial Barrier of Entry for Light Gases Transport in Amorphous TEOS-Derived Silica: A Finite Element Method

The interfacial barrier of entry for light gas transport in a nanopore was a crucial factor to determine the separation efficiency in membrane technologies. To examine this effect, amorphous silica was prepared by sol-gel process, and its characterization results revealed that the commonly used cylindrical pore shape failed to represent the adsorption behavior of gases, but instead the pore shape had to be represented by a slit pore model. A finite element method (FEM) was developed to analyze the interfacial resistance by integrating a Lennard-Jones (LJ) potential over the layer area. It was found that the strong repulsion/attraction at the pore interface could be paired with the motion energy of guest molecules to predict the ideal selectivity between gases, thereby providing a solution to preliminarily screen the separation performance among a host of membrane candidates.

Scalable crystalline porous membranes: current state and perspectives

Crystalline porous materials (CPMs) with uniform and regular pore systems show great potential for separation applications using membrane technology. Along with the research on the synthesis of precisely engineered porous structures, significant attention has been paid to the practical application of these materials for preparation of crystalline porous membranes (CPMBs). In this review, the progress made in the preparation of thin, large area and defect-free CPMBs using classical and novel porous materials and processing is presented. The current state-of-the-art of scalable CPMBs with different nodes (inorganic, organic and hybrid) and various linking bonds (covalent, coordination, and hydrogen bonds) is revealed. The advances made in the scalable production of high-performance crystalline porous membranes are categorized according to the strategies adapted from polymer membranes (interfacial assembly, solution-casting, melt extrusion and polymerization of CPMs) and tailored based on CPM properties (seeding-secondary growth, conversion of precursors, electrodeposition and chemical vapor deposition). The strategies are compared and ranked based on their scalability and cost. The potential applications of CPMBs have been concisely summarized. Finally, the performance and challenges in the preparation of scalable CPMBs with emphasis on their sustainability are presented.

A mini-review on recent developments in SAPO-34 zeolite membranes and membrane reactors

Schematic diagram of a SAPO-34 membrane for various gas separation.

Modeling the transport of CO2, N2, and their binary mixtures through highly permeable silicalite-1 membranes using Maxwell-Stefan equations

Carbon dioxide (CO₂) is the main contributor to global warming; therefore, research efforts aim at its capture. Membranes, in particular, zeolite membranes offer a promising approach for CO₂ separation and capture. Membranes are typically characterized by their selectivity and permeance that are highly dependent on the operating conditions namely, total feed pressure and composition. Therefore, more reliable characterization parameters are required such as Maxwell- Stefan exchange diffusivities. In this work, a model based on Maxwell-Stefan equations and Extended Langmuir isotherm was developed to investigate the transport of binary mixtures of CO₂ and N₂ through thin silicalite-1 membranes. The exchange diffusivities, D₁₂ and D₂₁, of CO₂ and N₂ were determined at different total feed pressures and feed compositions. All gas separation tests were conducted at stage cut not exceeding 5%. The single component diffusivities of CO₂ and N₂ required by the model were found experimentally using the results of the respective single gas permeation tests. The results displayed that as CO₂ concentration in the feed increased from 15% to 85%, the values of D₁₂ and D₂₁ decreased from 2.8 × 10^-10 to 1.1 × 10^-10 m²/s and 2.8 × 10^-10 to 1.3 × 10^-10 m²/s, respectively, while the N₂ permeance decreased by about one order of magnitude from 2.7 × 10^-7 to 2.4 × 10^-8 mol/m².s.Pa. Consequently, the exchange diffusivities showed considerably smaller dependence on the operating conditions compared to the permselectivity and permeance. Hence, they are more appropriate in describing the intrinsic transport characteristics of silicalite-1 membranes.

SAPO-34 zeotype membrane for gas sweetening

Membranes are considered promising tools for gas sweetening due to their lower footprint (i.e., area and energy requirement, considering elimination of solvent/absorbent and its associated regeneration procedures), and ease of scale-up. Performing membrane gas separation is strongly dependent on membrane materials. With a 0.38-nm pore size, the SAPO-34 membrane surpasses the upper bond limit for CO2/CH4 separation. However, preparing defect-free and high-performance zeolite membranes is quite challenging. This paper reviews gas transport and separation mechanisms in SAPO-34 membranes, and it discusses prospective approaches for obtaining membranes with defect-free selective layers and hence high separation performance. Highlights, as well as the authors’ perspectives on the future development of SAPO-34 membranes in the field of gas separation, are pointed out.

Gel-Modulated Growth of High-Quality Zeolite Membranes

Trade-off between thickness (and thus gas permeance) and quality (and thus selectivity) of zeolite membranes significantly restricts their wide application. It is challenging to maintain the membrane thickness while minimizing nonselective defects in the selective membrane layer. Currently, continuous change of the synthesis gel concentration during membrane synthesis, instead of high gel concentration for membrane "skeleton" growth and low gel concentration for slow crystal growth to merge "skeleton" crystals, usually leads to thick membranes to compensate for the low selectivity. In this work, we report a gel-modulated synthesis approach to engineer the zeolite membrane synthesis. The gel concentration was suddenly reduced by a quenching process in the middle of the membrane synthesis, leading to reduced nuclei formation and crystal growth rate in the following synthesis process. Membrane quality was significantly improved without increasing membrane thickness, leading to a great increase in membrane selectivity but without sacrifice of gas permeance. Moreover, as an example, highly reproducible SAPO-34 membranes were successfully prepared by the gel-modulated growth method. We expect this novel synthesis strategy might be a viable and economic way of growing thin and high-quality zeolite membranes.

Quantitative Assessment of Vapor Molecule Adsorption to Solid Surfaces by Flow Rate Monitoring in Microfluidic Channels

Measuring parameters related to gas adsorption on the effective surfaces of solid samples is important in catalyst studies. Further attention on the subject has appeared due to the materials and methods required to concentrate the gaseous biomarkers for detection. The conventional methods are mainly based on the volumetric and gravimetric analyses, which are applicable to bulk samples. No standard method has yet been provided for such measurements on thin films, which are the most commonly used samples for material screening. Here, a novel method is presented for the adsorption coefficient measurement on thin-film samples. This method comprises coating of the inner walls of a microfluidic channel with the thin film under test. The recorded diffusion rates for a trace gas along this microchannel are compared with the solutions of the adsorption-diffusion equation of the channel for determining the adsorption coefficient of the gas molecule to the inner walls of the channel. The high ratio of surface-to-volume in such channels magnifies the gas sorption effects and improves accuracy. The method is fast, versatile, and cost-effective, allowing measurements at different temperatures and atmospheric pressures. The adsorption coefficients of different isomers of butanol on poly(methyl methacrylate) sheets, zinc oxide thick films, and gold thin films are determined as examples.

Silicalite-1 zeolite membrane: Synthesis by seed method and application in organics removal

Silicalite-1 (S-1) zeolite membrane synthesized by seed method with superior features attracts intensive attentions, while the influences of key parameters during synthesis process and its applications for organics removal require further investigation. This study revealed the morphology and the structure of the prepared membranes under different crystallization temperatures and seed concentrations by using a suite of characterization methods. The as-prepared membrane under optimal condition (crystallization temperature of 175 °C and seed concentration of 1.0 wt. %) possessed high membrane integrity, with ideal separation factor of 4.0. It also exhibited outstanding performance for organics removal, with dyes retention of 99.9% and 99.2% for 500 mg L^-1 neutral red and 500 mg L^-1 methyl blue, respectively. Excellent antifouling property of the synthesized membrane was also proved. Results of this work can guide the characteristic improvement of the S-1 zeolite membrane by adjusting key parameters and broaden its applications in dye wastewater treatment.

Review: Membrane Materials for the Removal of Water from Industrial Solvents by Pervaporation and Vapor Permeation

Organic solvents are widely used in a variety of industrial sectors. Reclaiming and reusing the solvents may be the most economically and environmentally beneficial option for managing spent solvents. Purifying the solvents to meet reuse specifications can be challenging. For hydrophilic solvents, water must be removed prior to reuse, yet many hydrophilic solvents form hard-to-separate azeotropic mixtures with water. Such mixtures make separation processes energy intensive and cause economic challenges. The membrane processes pervaporation (PV) and vapor permeation (VP) can be less energy intensive than distillation-based processes and have proven to be very effective in removing water from azeotropic mixtures. In PV/VP, separation is based on the solution-diffusion interaction between the dense permselective layer of the membrane and the solvent/water mixture. This review provides a state-of-the-science analysis of materials used as the selective layer(s) of PV/VP membranes in removing water from organic solvents. A variety of membrane materials, such as polymeric, inorganic, mixed matrix, and hybrid, have been reported in the literature. A small subset of these are commercially available and highlighted here: poly(vinyl alcohol), polyimides, amorphous perfluoro polymers, NaA zeolites, chabazite zeolites, T-type zeolites, and hybrid silicas. The typical performance characteristics and operating limits of these membranes are discussed. Solvents targeted by the U.S. Environmental Protection Agency for reclamation are emphasized and ten common solvents are chosen for analysis: acetonitrile, 1-butanol, N,N-dimethyl formamide, ethanol, methanol, methyl isobutyl ketone, methyl tert-butyl ether, tetrahydrofuran, acetone, and 2-propanol.

Zeolitic Imidazolate Framework Membranes for Light Olefin/Paraffin Separation

Propylene/propane and ethylene/ethane separations are performed by energy-intensive distillation processes, and membrane separation may provide substantial energy and capital cost savings. Zeolitic imidazolate frameworks (ZIFs) have emerged as promising membrane materials for olefin/paraffin separation due to their tunable pore size and chemistry property, and excellent chemical and thermal stability. In this review, we summarize the recent advances on ZIF membranes for propylene/propane and ethylene/ethane separations. Membrane fabrication methods such as in situ crystallization, seeded growth, counter-diffusion synthesis, interfacial microfluidic processing, vapor-phase and current-driven synthesis are presented. The gas permeation and separation characteristics and membrane stability are also discussed.

para-Xylene Ultra-selective Zeolite MFI Membranes Fabricated from Nanosheet Monolayers at the Air-Water Interface

The control of membrane morphology and microstructure is crucial to improve the separation performance of molecular-sieve membranes. This can be enabled by making thin, dense, and uniform seed-crystal coatings, which are then intergrown into continuous membranes. Herein, we show a novel and simple floating particle coating method can give closely packed monolayers of zeolite nanosheets on nonporous or porous supports. The zeolite nanosheet monolayer is formed at the air-water interface in a conical Teflon trough. As the water in the trough is drained, the monolayer is deposited on a support placed below. Membranes prepared by gel-free secondary growth of the nanosheets deposited by this method show unprecedented ultra-selective performance for separation of para- from ortho-xylene (separation factor >10 000).