Shape-Selective Ultramicroporous Carbon Membranes for Sub-0.1 nm Organic Liquid Separation

Isomeric Separation of C6 Alkanes via Organic Solvent Reverse Osmosis with Covalent Organic Framework Membranes

Membrane separation represents cutting-edge technology for the isomeric separation of C6 alkanes in the petrochemical sector. However, it remains challenging to design membranes with sub-0.1 nm resolution that can effectively discriminate between C6 alkane isomers. In this study, we report a molecularly selective, ultramicroporous covalent organic framework (COF) membrane featuring an accessible pore size of ∼4.5 Å for the isomeric separation of C6 alkanes. High permeance of 2.56 kg m-2 h-1 bar-1 together with a high selectivity of 70.9 are achieved for a binary separation of linear n-hexane (nHEX) and branched 2,3-dimethylbutane (23DMB) through organic solvent reverse osmosis (OSRO). Notably, the research octane number (RON) of a five-component C6 alkane mixture can be effectively improved from 67.7 to 92.8 through a continuous separation using the COF membrane. The exceptional performance of the COF membrane suggests a new design of next-generation molecularly selective membranes toward efficient isomeric separation.

Shape-Selective Molecular Separations Enabled by Rigid and Interconnected Confinements Engineered in Conjugated Microporous Polymer Membranes

Separating molecules with similar sizes but different shapes is essential yet challenging. Here, conjugated microporous polymer (CMP) membranes with narrowly distributed network pores are prepared by diffusion-modulated electropolymerization. This approach precisely controls the monomer diffusion and coupling processes, regulating the crosslinking degree to prevent broad aggregate pores and microporous defects. By altering carbazole-based backbones, pore size and pore connectivity are adjusted. The rigid and interconnected confinements restrict molecular rotation and vibration, enforcing consistent shapes and orientations. This enables the separation of solute molecules (≈1000 g mol-1) with linear and bulky shapes, achieving separation factors of up to 134. When pore size is reduced to angstrom scale (≈5 Å), molecular shape significantly influences organic liquid transport. The CMP membranes demonstrate all-liquid phase separation of linear/branched alkane isomers (<100 g mol-1), enriching hexane to 63.35 mole% from equimolar isomer mixture and achieving permeance orders of magnitude higher than those of state-of-the-art membranes.

Structure-Transport Relationships of Water-Organic Solvent Co-transport in Carbon Molecular Sieve (CMS) Membranes

We explore the effects of the carbon molecular sieve (CMS) microstructure on the separation performance and transport mechanism of water-organic mixtures. Specifically, we utilize PIM-1 dense films and integrally skinned asymmetric hollow fiber membranes as polymer precursors for the CMS materials. The PIM-1 membranes were pyrolyzed under several different pyrolysis atmospheres (argon, carbon dioxide, and diluted hydrogen gas) and at multiple pyrolysis temperatures. Detailed gas physisorption measurements reveal that membranes pyrolyzed under 4% H2 and CO2 had broadened ultramicropore distributions (pore diameter <7 Å) compared to Ar pyrolysis, and pyrolysis under CO2 increased ultramicropore volume and broadened micropore distributions at increased pyrolysis temperatures. Gravimetric water and p-xylene sorption and diffusion measurements reveal that the PIM-1-derived CMS materials are more hydrophilic than other CMS materials that have been previously studied, which leads to sorption-diffusion estimations showing water-selective permeation. Water permeation in the vapor phase, pervaporation, and liquid-phase hydraulic permeation reveal that the isobaric permeation modes (vapor permeation and pervaporation) are reasonably well predicted by the sorption-diffusion model, whereas the hydraulic permeation mode is significantly underpredicted (>250×). Conversely, the permeation of p-xylene is well predicted by the sorption-diffusion model in all cases. The collection of pore size analysis, vapor sorption and diffusion, and permeation in different modalities creates a picture of a combined transport mechanism in which water-under high transmembrane pressures-permeates via a Poiseuille-style mechanism, whereas p-xylene solutes in the mixture permeate via sorption-diffusion.

Electron-mediated control of nanoporosity for targeted molecular separation in carbon membranes

Carbon molecular sieve (CMS) membranes are considered game-changers to overcome the challenges that conventional polymeric membranes face. However, CMS membranes also confront a challenge in successfully separating extremely similar-sized molecules. In this article, high-precision tuning of the microstructure of CMS membranes is proposed by controlled electron irradiation for the separation of molecules with size differences less than 0.05 nm. Fitting CMS membranes for targeted molecular separation can be accomplished by irradiation dosage control, resulting in highly-efficient C₂H_4/C₂H₆ separation for low dosages (∼250kGy, with selectivity ∼14) and ultra-selective H₂/CO₂ separation for high dosages (1000∼2000kGy with selectivity ∼80).The electron irradiated CMS also exhibits highly stabilized permeability and selectivity for long-term operation than the pristine CMS, which suffers from significant performance degradation due to physical aging. This study successfully demonstrates electron irradiation as a possible way to construct “designer” nanoporous carbon membranes out of the standard components mostly confined to pyrolysis conditions.

Controlled molecular separation by membranes requires 2D materials with precise structures to achieve the desired selectivity. Here authors demonstrate precise selectivity tuning in carbon membranes using electron irradiation.

State-of-the-Art Organic- and Inorganic-Based Hollow Fiber Membranes in Liquid and Gas Applications: Looking Back and Beyond

The aggravation of environmental problems such as water scarcity and air pollution has called upon the need for a sustainable solution globally. Membrane technology, owing to its simplicity, sustainability, and cost-effectiveness, has emerged as one of the favorable technologies for water and air purification. Among all of the membrane configurations, hollow fiber membranes hold promise due to their outstanding packing density and ease of module assembly. Herein, this review systematically outlines the fundamentals of hollow fiber membranes, which comprise the structural analyses and phase inversion mechanism. Furthermore, illustrations of the latest advances in the fabrication of organic, inorganic, and composite hollow fiber membranes are presented. Key findings on the utilization of hollow fiber membranes in microfiltration (MF), nanofiltration (NF), reverse osmosis (RO), forward osmosis (FO), pervaporation, gas and vapor separation, membrane distillation, and membrane contactor are also reported. Moreover, the applications in nuclear waste treatment and biomedical fields such as hemodialysis and drug delivery are emphasized. Subsequently, the emerging R&D areas, precisely on green fabrication and modification techniques as well as sustainable materials for hollow fiber membranes, are highlighted. Last but not least, this review offers invigorating perspectives on the future directions for the design of next-generation hollow fiber membranes for various applications. As such, the comprehensive and critical insights gained in this review are anticipated to provide a new research doorway to stimulate the future development and optimization of hollow fiber membranes.

Homochiral Metal-Organic Framework Based Mixed Matrix Membrane for Chiral Resolution

Efficient separation of enantiomers is critical in the chemical, pharmaceutical, and food industries. However, conventional separation methods, such as chromatography, crystallization, and enzymatic kinetic resolution, require high energy costs and specific reaction conditions for the efficient purification of one enantiomer. In contrast, membrane-based processes are continuous processes performed with less energy than conventional separation processes. Enantioselective polymer membranes have been developed for the chiral resolution of pharmaceuticals; however, it is difficult to generate sufficient enantiomeric excess (ee) with polymer membranes. In this work, a homochiral filler of L-His-ZIF-8 was synthesized by the ligand substitution method and mixed with polyamide(imide) (i.e., Torlon^®) to fabricate an enantioselective mixed-matrix membrane (MMM). The enantio-selective separation of R-1-phenylethanol over S-1-phenylethanol was demonstrated with a 25 wt% loaded L-His-ZIF-8/Torlon^® MMM in an organic solvent nanofiltration (OSN) mode.

In Situ Derived Hybrid Carbon Molecular Sieve Membranes with Tailored Ultramicroporosity for Efficient Gas Separation

Fine control of ultramicroporosity (<7 Å) in carbon molecular sieve (CMS) membranes is highly desirable for challenging gas separation processes. Here, a versatile approach is proposed to fabricate hybrid CMS (HCMS) membranes with unique textural properties as well as tunable ultramicroporosity. The HCMS membranes are formed by pyrolysis of a polymer nanocomposite precursor containing metal-organic frameworks (MOFs) as a carbonizable nanoporous filler. The MOF-derived carbonaceous phase displays good compatibility with the polymer-derived carbon matrix due to the homogeneity of the two carbon phases, substantially enhancing the mechanical robustness of the resultant HCMS membranes. Detailed structural analyses reveal that the in situ pyrolysis of embedded MOFs induces more densified and interconnected carbon structures in HCMS membranes compared to those in conventional CMS membranes, leading to bimodal and narrow pore size distributions in the ultramicroporous region. Eventually, the HCMS membranes exhibit far superior gas separation performances with a strong size-sieving ability than the conventional polymers and CMS membranes, especially for closely sized gas pairs (Δd < 0.5 Å) including CO₂ /CH₄ and C₃ H₆ /C₃ H₈ separations. More importantly, the developed HCMS material is successfully prepared into a thin-film composite (TFC) membrane (≈1 µm), demonstrating its practical feasibility for use in industrial mixed-gas operation conditions.