Tailoring sub-3.3 Å ultramicropores in advanced carbon molecular sieve membranes for blue hydrogen production

Advances in perovskite membranes for carbon capture & utilization: A sustainable approach to CO2 emissions reduction - A review

Despite agreements like the Paris Agreement, the world continues to face rising temperatures, extreme weather, and ecosystem disruptions, driven by continued use fossil fuel, agricultural emissions, and industrial activities and leading to greenhouse gas contributing to the serious fuelling climate change. Carbon capture and utilization (CCU), particularly thermochemical carbon dioxide (CO2) splitting powered by thermal energy, offers a promising solution. Perovskite-based inorganic membranes, known for their high selectivity and permeability toward various gases, efficiency, and energy-saving potential, have attracted significant interest in gas separation, production and emerged as a leading technology for carbon capture and hydrogen purification. This review explores advancements in perovskite materials, focusing on H2/CO2 separation, CO2 conversion to CO, and optimal operating conditions. It addresses key questions such as improving material performance through innovations in double and composite perovskites, enhancing oxygen removal via thermochemical or electrochemical pumps, and integrating CO2 splitting with fuel production. These strategies aim to reduce costs, boost efficiency, and provide sustainable pathways for addressing climate change.

Carbon Molecular Sieve Membranes Derived From Dual-Cross-linked Polybenzimidazole for Enhanced H2/CO2 Separation

The need for efficient CO2 separation during hydrogen production from fossil fuels drives the development of advanced, energy-efficient solutions. Membrane technology offers a promising approach for separating CO2 from H2, which, however, faces the challenge of low H2/CO2 selectivity. To address this challenge, a novel strategy to cross-link polybenzimidazole (PBI) using potassium persulfate (K2S2O8) is proposed, followed by pyrolysis to fabricate highly selective carbon molecular sieve (CMS) membranes. The cross-linked PBI-derived CMS membranes exhibit significantly enhanced permeability and H2/CO2 selectivity compared to neat PBI-CMS membranes. For instance, the CMS membrane prepared from PBI cross-linked for 24 h and pyrolyzed at 900 °C (denoted as KPBI24 CMS@900) demonstrates outstanding molecular sieving capability. This membrane achieves an H2 permeability of 55 Barrer with an H2/CO2 selectivity of 48 tested at 100 °C, significantly surpassing its non-cross-linked counterparts and the 2008 Robeson upper bound. The design principles of this study provide a robust technical foundation for persulfate-cross-linked PBI and offer an innovative approach for preparing high-performance CMS membranes.

Atomically Fine-Tuning Organic-Inorganic Carbon Molecular Sieve Membranes for Hydrogen Production

Polymeric membranes with great processability are attractive for the H2/CO2 separation required for hydrogen production from renewable biomass with carbon capture for utilization and sequestration. However, it remains elusive to engineer polymer architectures to obtain desired sub-3.3 Å ultramicropores to efficiently sieve H2 from CO2. Herein, we demonstrate a scalable way of carbonizing polybenzimidazole (PBI) at low temperatures, followed by vapor phase infiltration (VPI) to atomically narrow ultramicropores throughout the films, forming hybrid organic-inorganic carbon molecular sieves (CMSs). One VPI cycle (100 s) for the PBI carbonized at 500 °C remarkably increases H2/CO2 selectivity from 9.6 to 83 at 100 °C, surpassing Robeson's upper bound. The CMS demonstrates a stable H2/CO2 separation performance when challenged with simulated syngas streams and can be fabricated into thin-film composite membranes, outperforming state-of-the-art membranes. The scalable approach can be ubiquitous to molecularly fine-tune ultramicropores of leading polymeric membranes to further improve their size-sieving ability and thus separation efficiency.

Angstrom-Scale Defect-Free Crystalline Membrane for Sieving Small Organic Molecules

Crystalline membranes, represented by the metal-organic framework (MOF) with well-defined angstrom-sized apertures, have shown great potential for molecular separation. Nevertheless, it remains a challenge to separate small molecules with very similar molecular size differences due to angstrom-scale defects during membrane formation. Herein, a stepwise assembling strategy is reported for constructing MOF membranes with intrinsic angstrom-sized lattice aperture lattice to separate organic azeotropic mixtures separation. The membrane is synthesized by redesigning the metal source, which reduces the coordination reaction rate to avoid cluster-missing defects. Then, extra ligands are introduced to overcome the coordination steric hindrance to heal the linker-missing defects. Ultralow-dose transmission electron microscopy is used to realize a direct observation of the angstrom-scale defects. For separating the challenging methanol-containing ester or ether azeotropic mixtures with molecular size difference as small as <1 Å, the angstrom-scale defect-free MOF membrane exhibits an outstanding flux of ≈3700 g·m-2 h-1 and separation factor of ≈247-524, far beyond the upper-bound of state-of-the-arts membranes. This study offers a feasible strategy for precisely constructing angstrom-confined spaces for diverse applications (e.g., separation, catalysis, and storage).

Metal-organic cages improving microporosity in polymeric membrane for superior CO2 capture

Mixed matrix membranes, with well-designed pore structure inside the polymeric matrix via the incorporation of inorganic components, offer a promising solution for addressing CO2 emissions. Here, we synthesized a series of novel metal organic cages (MOCs) with aperture pore size precisely positioned between CO2 and N2 or CH4. These MOCs were uniformly dispersed in the polymers of intrinsic microporosity (PIM-1). Among them, the MOC-Ph cage effectively modulated chain packing and optimized the microporous structure of the membrane. Remarkably, the PIM-Ph-5% membrane shows superior performance, achieving an excellent CO2 permeability of 8803.4 barrer and CO2/N2 selectivity of 59.9, far exceeding the 2019 upper bound. This approach opens opportunities for improving the porous structure of polymeric membranes for CO2 capture and other separation applications.

Tuning Fluorination of Carbon Molecular Sieve Membranes with Enhanced Reverse-Selective Hydrogen Separation From Helium

Membrane technology has been explored for separating helium from hydrogen in natural gas reservoirs, a process that remains extremely challenging due to the sub-Ångstrom size difference between H2 and He molecules. Reverse-selective H2/He separation membranes offer multiple advantages over conventional helium-selective membranes, which, however, suffer from low H2/He selectivity. To address this hurdle, a novel approach is proposed to tune the ultra-micropores of carbon molecular sieves (CMS) membranes through fluorination of the polymer precursor. By incorporating -CF3 units into the backbone of Tröger's base polymers, the microporosity of CMS is tailored and reverse-selective H2/He CMS membranes are deployed with remarkable separation performance, surpassing most reported membranes. These CMS membranes exhibit a H2 permeability of 1505.2 Barrer with a notable H2/He selectivity of 3.8. Barometric sorption tests reveal preferential sorption of H2 over He in the fluorinated CMS membranes, which also demonstrate a significantly higher H2/He diffusion selectivity compared to unfluorinated samples. Material studio calculations indicate that the "slim" hydrogen molecule penetrates ultra-micropores more readily than the spherical He molecule, thus achieving reverse H2/He selectivity. This design approach offers a promising pathway for developing molecularly sieving membranes to tackle the challenging helium separation from natural gas.

Sub-5 Ångstrom Porosity Tuning in Calixarene-Derived Porous Liquids via Supramolecular Complexation Construction

Sub-Ångstrom-level porosity engineering, which is appealing in gas separations, has been demonstrated in solid carbon, polymer, and framework materials but rarely achieved in the liquid phase. In this work, a gas molecular sieving effect in the liquid phase at sub-5 Ångstrom scale is created via sophisticated porosity tuning in calixarene-derived porous liquids (PLs). Type II PLs are constructed via supramolecular complexation between the sodium salts of calixarene derivatives and crown ether solvents. The chemical structure variation and assembly behavior of the porous host upon PL construction are monitored by spectroscopy-, X-ray-, and neutron-scattering techniques. The presence of permanent porosity in calixarene-derived PLs is verified by pressure swing gas uptake, altered CO2 physisorption behavior, and molecular simulations. Sub-5 Ångstrom porosity tuning within the PL phase is achieved by introducing bulky substituted groups on the benzene ring of the calixarene host, which then greatly affects the dynamic motion and transport behavior of CO2 molecules and the Xe uptake performance. The approach being demonstrated in this work represents a promising pathway to tune and leverage the porosity effect for enhanced gas uptake capacity and selectivity in liquid sorbents.

Balancing the Kinetic and Thermodynamic Synergetic Effect of Doped Carbon Molecular Sieves for Selective Separation of C2H4/C2H6

Selective separation of ethylene and ethane (C2H4/C2H6) is a formidable challenge due to their close molecular size and boiling point. Compared to industry-used cryogenic distillation, adsorption separation would offer a more energy-efficient solution when an efficient adsorbent is available. Herein, a class of C2H4/C2H6 separation adsorbents, doped carbon molecular sieves (d-CMSs) is reported which are prepared from the polymerization and subsequent carbonization of resorcinol, m-phenylenediamine, and formaldehyde in ethanol solution. The study demonstrated that the polymer precursor themselves can be a versatile platform for modifying the pore structure and surface functional groups of their derived d-CMSs. The high proportion of pores centered at 3.5 Å in d-CMSs contributes significantly to achieving a superior kinetic selectivity of 205 for C2H4/C2H6 separation. The generated pyrrolic-N and pyridinic-N functional sites in d-CMSs contribute to a remarkable elevation of Henry selectivity to 135 due to the enhancement of the surface polarity in d-CMSs. By balancing the synergistic effects of kinetics and thermodynamics, d-CMSs achieve efficient separation of C2H4/C2H6. Polymer-grade C2H4 of 99.71% purity can be achieved with 75% recovery using the devised d-CMSs as reflected in a two-bed vacuum swing adsorption simulation.

High-Performance Carbon Molecular Sieve Membranes Derived from a PPA-Cross-linked Polyimide Precursor for Gas Separation

Carbon molecular sieve (CMS) membranes have emerged as attractive gas membranes due to their tunable pore structure and consequently high gas separation performances. In particular, polyimides (PIs) have been considered as promising CMS precursors because of their tunable structure, superior gas separation performance, and excellent thermal and mechanical strength. In the present work, polyphosphoric acid (PPA) was employed as both cross-linker and porogen, it created pores within the PI polymeric matrix, while it also effectively acting as a cross-linker to regulate the ultramicropores of the CMS membranes, thus simultaneously improving both permeability and selectivity of the CMS membranes. By employing PI/PPA hybrid with PPA content of 5 wt % as a precursor, the obtained CMS membrane exhibited a CO2 and He permeability of 1378.3 Barrer and 1431.4 Barrer, respectively, which was an approximately 10-fold increase compared to the precursor membrane. Under optimized conditions, the CO2/CH4 and He/CH4 selectivity of the obtained CMS membrane reached 81.5 and 89.9, respectively, which was 278% and 307% higher than that of the pristine PI membrane. In addition, the membrane exhibited good long-term stability during a one-week continuous test. This study clearly denoted PPA can be used for precisely tailoring the ultramicroporosity of CMS membranes.

Hierarchically porous and single Zn atom-embedded carbon molecular sieves for H2 separations

Hierarchically porous materials containing sub-nm ultramicropores with molecular sieving abilities and microcavities with high gas diffusivity may realize energy-efficient membranes for gas separations. However, rationally designing and constructing such pores into large-area membranes enabling efficient H2 separations remains challenging. Here, we report the synthesis and utilization of hybrid carbon molecular sieve membranes with well-controlled nano- and micro-pores and single zinc atoms and clusters well-dispersed inside the nanopores via the carbonization of supramolecular mixed matrix materials containing amorphous and crystalline zeolitic imidazolate frameworks. Carbonization temperature is used to fine-tune pore sizes, achieving ultrahigh selectivity for H2/CO2 (130), H2/CH4 (2900), H2/N2 (880), and H2/C2H6 (7900) with stability against water vapor and physical aging during a continuous 120-h test.

Mechanically stable polymer molecular sieve membranes with switchable functionality designed for high CO2 separation performance

The development of high-performance membranes selective for carbon dioxide is critically important for advancing energy-efficient carbon dioxide capture technologies. Although molecular sieves have long been attractive membrane materials, turning them into practical membrane applications has been challenging. Here, we introduce an innovative approach for crafting a polymeric molecular sieve membrane to achieve outstanding carbon dioxide separation performance while upholding the mechanical stability. First, a polymer molecular sieve membrane having high gas permeability and mechanical stability was fabricated from a judiciously designed polymer that is solution-processable, hyper-cross-linkable, and functionalizable. Then, the carbon dioxide selectivity was fine-tuned by the subsequent introduction of various amine-based carriers. Among the diverse amines, polyethyleneimine stands out by functionalizing the larger pore region while preserving ultramicropores, leading to improved carbon dioxide/dinitrogen separation performance. The optimized membrane demonstrates exceptional carbon dioxide/dinitrogen separation performance, outperforming other reported polymer molecular sieve membranes and even competing favorably with most carbon molecular sieve membranes reported to date.

Sub-micro porous thin polymer membranes for discriminating H2 and CO2

Polymeric membranes with high permeance and remarkable selectivity for simultaneous H2 purification and CO2 capture under industry-relevant conditions are absent. Herein, sub-micro pores with precise molecular sieving capability are created in ultra-thin (13-30 nm) polymer membranes via controllable transformation of amine-linked polymer (ALP) films into benzimidazole-and-amine-linked polymer (BIALP) layers. The BIALP membranes exhibit stable unprecedented H2/CO2 selectivity of 120 with a H2 permeance of 315 GPU. Furthermore, high pressure (up to 11 bar) and thermal (up to 300 °C) resistance is delivered. This work provides a concept on designing porous polymeric membranes for precise molecular discrimination.

Ultrathin membranes to sieve gases

Mixed-matrix membranes could enable gas separation for carbon capture.

Solid-solvent processing of ultrathin, highly loaded mixed-matrix membrane for gas separation

Mixed-matrix membranes (MMMs) that combine processable polymer with more permeable and selective filler have potential for molecular separation, but it remains difficult to control their interfacial compatibility and achieve ultrathin selective layers during processing, particularly at high filler loading. We present a solid-solvent processing strategy to fabricate an ultrathin MMM (thickness less than 100 nanometers) with filler loading up to 80 volume %. We used polymer as a solid solvent to dissolve metal salts to form an ultrathin precursor layer, which immobilizes the metal salt and regulates its conversion to a metal-organic framework (MOF) and provides adhesion to the MOF in the matrix. The resultant membrane exhibits fast gas-sieving properties, with hydrogen permeance and/or hydrogen-carbon dioxide selectivity one to two orders of magnitude higher than that of state-of-the-art membranes.

pH-Regulated Refinement of Pore Size in Carbon Spheres for Size-Sieving of Gaseous C8 , C6 and C3 Hydrocarbon Pairs

Selective separation of industrial important C8 , C6 and C3 hydrocarbon pairs by physisorbents can greatly reduce the energy intensity related to the currently used cryogenic distillation techniques. The achievement of size-sieving based on carbonaceous materials is desirable, but commonly hindered by the random structure of carbons often with a broad pore size distribution. Herein, a pH-regulated pre-condensation strategy was introduced to control the carbon pore architecture by the sp2 /sp3 hybridization of precursor. The lower pH value during pre-condensation of glucose facilitates the growth of aromatic nanodomains, rearrangement of stacked layers and a concomitant transition from sp3 -C to sp2 -C. The subsequent pyrolysis endows the pore size manipulated from 6.8 to 4.8 Å and narrowly distributed over a range of 0.2 Å. The refined pores enable effective size-sieving of C8 , C6 and C3 hydrocarbon pairs with high separation factor of 1.9 and 4.9 for C8 xylene (X) isomers para-X/meta-X and para-X/ortho-X, respectively, 5.1 for C6 alkane isomers n-hexane/3-methylpentane, and 22.0 for C3 H6 /C3 H8 . The excellent separation performance based-on size exclusion effect is validated by static adsorption isotherms and dynamic breakthrough experiments. This synthesis strategy provides a means of exploring advanced carbonaceous materials with controlled hybridized structure and pore sizes for challenging separation needs.

Palladium-Percolated Networks Enabled by Low Loadings of Branched Nanorods for Enhanced H2 Separations

Nanoparticles (NPs) at high loadings are often used in mixed matrix membranes (MMMs) to improve gas separation properties, but they can lead to defects and poor processability that impede membrane fabrication. Herein, it is demonstrated that branched nanorods (NRs) with controlled aspect ratios can significantly reduce the required loading to achieve superior gas separation properties while maintaining excellent processability, as demonstrated by the dispersion of palladium (Pd) NRs in polybenzimidazole for H₂ /CO₂ separation. Increasing the aspect ratio from 1 for NPs to 40 for NRs decreases the percolation threshold volume fraction by a factor of 30, from 0.35 to 0.011. An MMM with percolated networks formed by Pd NRs at a volume fraction of 0.039 exhibits H₂ permeability of 110 Barrer and H₂ /CO₂ selectivity of 31 when challenged with simulated syngas at 200 °C, surpassing Robeson's upper bound. This work highlights the advantage of NRs over NPs and nanowires and shows that right-sizing nanofillers in MMMs is critical to construct highly sieving pathways at minimal loadings. This work paves the way for this general feature to be applied across materials systems for a variety of chemical separations.

Boosting membrane carbon capture via multifaceted polyphenol-mediated soldering

Advances in membrane technologies are significant for mitigating global climate change because of their low cost and easy operation. Although mixed-matrix membranes (MMMs) obtained via the combination of metal-organic frameworks (MOFs) and a polymer matrix are promising for energy-efficient gas separation, the achievement of a desirable match between polymers and MOFs for the development of advanced MMMs is challenging, especially when emerging highly permeable materials such as polymers of intrinsic microporosity (PIMs) are deployed. Here, we report a molecular soldering strategy featuring multifunctional polyphenols in tailored polymer chains, well-designed hollow MOF structures, and defect-free interfaces. The exceptional adhesion nature of polyphenols results in dense packing and visible stiffness of PIM-1 chains with strengthened selectivity. The architecture of the hollow MOFs leads to free mass transfer and substantially improves permeability. These structural advantages act synergistically to break the permeability-selectivity trade-off limit in MMMs and surpass the conventional upper bound. This polyphenol molecular soldering method has been validated for various polymers, providing a universal pathway to prepare advanced MMMs with desirable performance for diverse applications beyond carbon capture.

Nanoarchitectonics of carbon molecular sieve membranes with graphene oxide and polyimide for hydrogen purification

Hydrogen is an important energy carrier for the transition to a carbon-neutral society, the efficient separation and purification of hydrogen from gaseous mixtures is a critical step for the implementation of a hydrogen economy. In this work, graphene oxide (GO) tuned polyimide carbon molecular sieve (CMS) membranes were prepared by carbonization, which show an attractive combination of high permeability, selectivity and stability. The gas sorption isotherms indicate that the gas sorption capability increases with the carbonization temperature and follows the order of PI–GO-1.0%-600 °C > PI–GO-1.0%-550 °C > PI–GO-1.0%-500 °C, more micropores would be created under higher temperatures under GO guidance. The synergistic GO guidance and subsequent carbonization of PI–GO-1.0% at 550 °C increased H₂ permeability from 958 to 7462 Barrer and H₂/N₂ selectivity from 14 to 117, superior to state-of-the-art polymeric materials and surpassing Robeson's upper bound line. As the carbonization temperature increased, the CMS membranes gradually changed from the turbostratic polymeric structure to a denser and more ordered graphite structure. Therefore, ultrahigh selectivities for H₂/CO₂ (17), H₂/N₂ (157), and H₂/CH₄ (243) gas pairs were achieved while maintaining moderate H₂ gas permeabilities. This research opens up new avenues for GO tuned CMS membranes with desirable molecular sieving ability for hydrogen purification.

Hydrogen is an important energy carrier for the transition to a carbon-neutral society, the CMS membrane exhibited ultrahigh H₂/N₂ selectivity (117) and H₂ permeability, which have bright prospects for hydrogen purification.

In Situ Growth of Crystalline and Polymer-Incorporated Amorphous ZIFs in Polybenzimidazole Achieving Hierarchical Nanostructures for Carbon Capture

Mixed matrix materials (MMMs) hold great potential for membrane gas separations by merging nanofillers with unique nanostructures and polymers with excellent processability. In situ growth of the nanofillers is adapted to mitigate interfacial incompatibility to avoid the selectivity loss. Surprisingly, functional polymers have not been exploited to co-grow the nanofillers for membrane applications. Herein, in situ synergistic growth of crystalline zeolite imidazole framework-8 (ZIF-8) in polybenzimidazole (PBI), creating highly porous structures with high gas permeability, is demonstrated. More importantly, PBI contains benzimidazole groups (similar to the precursor for ZIF-8, i.e., 2-methylimidazole) and induces the formation of amorphous ZIFs, enhancing interfacial compatibility and creating highly size-discriminating bottlenecks. For instance, the formation of 15 mass% ZIF-8 in PBI improves H₂ permeability and H₂ /CO₂ selectivity by ≈100% at 35 °C, breaking the permeability/selectivity tradeoff. This work unveils a new platform of MMMs comprising functional polymer-incorporated amorphous ZIFs with hierarchical nanostructures for various applications.