Molecular Sieving Properties Research Articles

Rapid solvent transport and tunable molecular sieving enabled by ultrathin alkyl-chain-engineered polyamide membranes

Organic solvent nanofiltration (OSN) has emerged as a promising separation technology for the chemical and pharmaceutical industries due to its low energy consumption and eco-friendliness. However, traditional polyamide-based membranes used in OSN often exhibit low permeance for organic solvents and it is difficult to precisely control the pore structure. In this study, we report a facile approach to fabricate ultrathin alkyl-chain-engineered polyamide nanofilms via free-standing interfacial polymerization for high-performance selective separations. The membranes were prepared via a “two birds one stone” strategy, enabled by post-treatment in aliphatic amine solution, simultaneously regulating the pore size and optimizing the membrane chemical property. This enables the membrane with high solvent permeance, especially for non-polar solvents (heptane at ∼22.2 L m−2h−1 bar−1, toluene at ∼16.8 L m−2h−1 bar−1), while maintaining excellent molecular sieving capability. Notably, the molecular weight cut-off (MWCO) can be regulated by introducing alkyl chains of varying lengths to their pores. The exceptional solvent permeance and tunable molecular sieving property make the membranes promising for high value-added products purification in the pharmaceutical industry and crude oil separation.

Supramolecular Interfacial Assembly: Integrating Supramolecular Hosts into Polymeric Membranes through an Aqueous Interface

Efficient incorporation of supramolecular hosts in polymeric membranes can impart the overall matrix with new properties for a range of cutting‐edge applications. Here, we introduce a Supramolecular Interfacial Assembly (SIA) method for the fabrication of polymeric membranes featuring embedded macrocycles. Through harnessing the quasi‐liquid nature of the concentrated polymer solution, SIA orchestrates the homogenous spreading of macrocycles in an aqueous layer on its surface, leading to the creation of an interface between “water/water” phases, subsequently forming a cross‐linked membrane driven by supramolecular electrostatic interactions. Remarkably, compared to the traditional interfacial polymerization, SIA adheres to a “green” paradigm without the need for organic solvents. The resultant composite membrane exhibits superior performance in organic solvent nanofiltration (OSN), owing to the precise molecular sieving property provided by the macrocycles with well‐defined permanent cavities. This fabrication method holds great promise for the innovative design of composite membranes, which can greatly impact the production of smart membrane materials for advanced technology applications in the future.

Supramolecular Interfacial Assembly: Integrating Supramolecular Hosts into Polymeric Membranes through an Aqueous Interface.

Efficient incorporation of supramolecular hosts in polymeric membranes can impart the overall matrix with new properties for a range of cutting-edge applications. Here, we introduce a Supramolecular Interfacial Assembly (SIA) method for the fabrication of polymeric membranes featuring embedded macrocycles. Through harnessing the quasi-liquid nature of the concentrated polymer solution, SIA orchestrates the homogenous spreading of macrocycles in an aqueous layer on its surface, leading to the creation of an interface between "water/water" phases, subsequently forming a cross-linked membrane driven by supramolecular electrostatic interactions. Remarkably, compared to the traditional interfacial polymerization, SIA adheres to a "green" paradigm without the need for organic solvents. The resultant composite membrane exhibits superior performance in organic solvent nanofiltration (OSN), owing to the precise molecular sieving property provided by the macrocycles with well-defined permanent cavities. This fabrication method holds great promise for the innovative design of composite membranes, which can greatly impact the production of smart membrane materials for advanced technology applications in the future.

Boronic acid ester-based hydrogel as surface-enhanced Raman scattering substrates for separation, enrichment, hydrolysis and detection of hydrogen peroxide residue in dairy product all-in-one

Rapid and selective separation, enrichment and detection of trace residue all-in-one in complex samples is a major challenge. Hydrogels with molecular sieve properties can selectively separate and enrich target analytes, and the combination with high sensitivity detection of surface-enhanced Raman scattering (SERS) is expected to achieve the above all-in-one detection. Herein, the core-shell structured Au@poly(N-isopropylacrylamide)-phenylboronic acid hydrogel (Au@PNIP-VBA) with boronic acid ester groups was prepared by thermally initiated polymerization. The boronic acid ester groups in hydrogel are selectively hydrolyzed by hydrogen peroxide (H2O2) to hydroxyl structures, leading to a reduction in SERS signals. The Au@PNIP-VBA hydrogel has molecular sieve properties and high SERS activity, making it suitable for separation, enrichment, hydrolysis and detection of H2O2 all-in-one. A rapid SERS method was developed for analysis of H2O2 based on the Au@PNIP-VBA hydrogel with the linear range of 8.5 × 10−2-6.8 mg L−1 and the detection limit of 33 μg L−1. The method was successfully applied to the determination of H2O2 residue in fresh milk, pure milk, yogurt and camel milk, with the recoveries were in the range of 82.2%–109.3% and the relative standard deviations were 2.8%–8.3%. This efficient all-in-one strategy has the advantages of simple sample pre-treatment, rapid analysis (30 min) and high sensitivity, making it highly promising for food quality and safety analysis.

Synergistic tuning of microstructure and morphology in carbon molecular sieve hollow fibers for propylene/propane separation

Asymmetric carbon molecular sieve (CMS) hollow fiber membranes with tunable micro‐ and macro‐structural morphologies for energy efficient propylene‐propane separation are reported here. A sub‐glass transition temperature (sub‐Tg) thermal oxidative crosslinking strategy enables simultaneous optimization of the intrinsic molecular sieving properties while also reducing the thickness of the CMS “skin” derived from the 6FDA:BPDA/DAM polyimide precursors. Such synergistic tuning of CMS microstructure and macroscopic morphology of CMS hollow fibers enables significantly increased propylene permeance (reaching 186.5 GPU) while maintaining an appealing propylene/propane selectivity of 13.3 for 50/50 propylene/propane mixed gas feeds. Our findings reveal a more refined and versatile tool than available with previous O2‐doping pretreatments. The advanced approach here should be broadly useful to other polyimide precursors and diverse gas pairs.

Synergistic Tuning of Microstructure and Morphology in Carbon Molecular Sieve Hollow Fibers for Propylene/Propane Separation.

Asymmetric carbon molecular sieve (CMS) hollow fiber membranes with tunable micro- and macro-structural morphologies for energy efficient propylene-propane separation are reported here. A sub-glass transition temperature (sub-Tg) thermal oxidative crosslinking strategy enables simultaneous optimization of the intrinsic molecular sieving properties while also reducing the thickness of the CMS "skin" derived from the 6FDA : BPDA/DAM polyimide precursors. Such synergistic tuning of CMS microstructure and macroscopic morphology of CMS hollow fibers enables significantly increased propylene permeance (reaching 186.5 GPU) while maintaining an appealing propylene/propane selectivity of 13.3 for 50/50 propylene/propane mixed gas feeds. Our findings reveal a more refined and versatile tool than available with previous O2-doping pretreatments. The advanced approach here should be broadly useful to other polyimide precursors and diverse gas pairs.

Rational Matching of Metal-Organic Frameworks and Polymers in Mixed Matrix Membranes for Efficient Propylene/Propane Separation.

The exploitation of high-performance membranes selective for propylene is important for developing energy-efficient propylene/propane (C3H6/C3H8) separation technologies. Although metal-organic frameworks with a molecular sieving property have been considered promising filler materials in mixed-matrix membranes (MMMs), their use in practical applications has been challenging due to a lack of interface compatibility. Herein, we adopted a surface coordination strategy that involved rationally utilizing carboxyl-functionalized PIM-1 (cPIM) and ZIF-8 to prepare a mixed-matrix membrane for efficient propylene/propane separation. The interfacial coordination between the polymer and the MOF improves their compatibility and eliminates the need for additional modification of the MOF, thereby maximizing the inherent screening performance of the MOF filler. Additionally, the utilization of porous PIM-1 guaranteed the high permeability of the MMMs. The obtained MMMs exhibited excellent separation performance. The 30 wt% ZIF-8/cPIM-1 membrane performed the best, exhibiting a high C3H6 permeability of 1023 Barrer with a moderate C3H6/C3H8 selectivity of 13.97 under 2 bars of pressure. This work presents a method that can feasibly be used for the preparation of defect-free MOF-based MMMs for specific gas separations.

Huge improved gas separation performance of carbon molecular sieve membrane by forming a double crosslinked polyimide precursor

A vital issue in searching for advanced gas separation membranes is understanding the precursor structure-gas separation properties of carbon molecular sieve (CMS) membranes. Here, we reported two crosslinked polyimides and used as CMS precursors, in which, the DCPI-4 is a double cross-linkable polyimide with 4 % triptycene triamine as a crosslinker (Type I) and COOH group that can be crosslinked at 350 °C by decarboxylation (Type II), whereas the DCPI-0 with COOH (Type II). After pyrolysis at 550 °C, the DCPI-4-CMS showed a higher SBET (812 vs 713 m2g-1), larger ultra-microporosity ratio (26.6 % vs 17.8 %), larger d-spacing (4.08 vs 3.94 Å), less graphitization (sp3/sp2 of 0.156 vs 0.118), 5.5 times higher CO2 permeability (7573 vs 1391 Barrer) and same CO2/CH4 selectivity (∼62) than DCPI-0-CMS. We consider this is due to the type I crosslinking enhances the rigidity of the polyimide backbone, and inhibits the collapse of micropores during the carbonization, which causes improved ultra-microporosity and ultra-micropore ratio that both enhances the gas diffusion and solubility coefficient as well activation energy difference of gas pairs. Conclusively, this double crosslinking method provides a good perspective for achieving CMS membranes with excellent gas separation performance.

Adsorption Properties of Aluminophosphate Molecular Sieves with AEL and AFI Structures According to Gas Chromatography Data

Adsorption Properties of Aluminophosphate Molecular Sieves with AEL and AFI Structures According to Gas Chromatography Data

Dual‐Wing Ligand Constructed Metal–Organic Framework Membranes with Finely Tuned Apertures for Natural Gas Separation

AbstractMetal–organic framework (MOF) membranes with apparent molecular sieving effects have great potential for gas separation. However, their application in light‐gas separation (e.g., CO2/CH4 and H2/CH4) remains challenging due to their enlarged pore structures under transmembrane pressure. In this work, a series of MOF membranes constructed from dual‐wing (DW) ligands including 2‐chloromethylbenzimidazole, 2‐methylbenzimidazole, 2‐ethylbenzimidazole, and 2‐phenylbenzimidazole are reported, to finely tune apertures and enhance molecule sieving property for small‐size gases. These DW‐ligands provide steric resistance in two directions perpendicular to the coordination bond, leading to a much finer pore structure. The introduction of the DW‐ligands endows the DW/ZIF‐8 membranes with an adjustable bottleneck door for regulating gas diffusion, blocking the transport of large‐sized CH4 while allowing small‐sized H2 and CO2 to permeate. The membranes show significantly improved molecular sieving property with a CO2/CH4 mixed‐gas selectivity of 58.6 in the case of 2‐chloromethylbenzimidazole23/ZIF‐8, which represents the highest value among the reported ZIF membranes. This membrane also exhibits exceptional separation performance for H2/CH4 and H2/C3H8 with separation factors of 430 and 34341, which are the highest values among the reported MOF membranes. This study presents a facile and efficient strategy for regulating MOF aperture and constructing high‐performance membranes for natural gas separation.

Impact of VAPO molecular sieve configuration on catalytic oxidation of ethyl lactate to ethyl pyruvate and its reaction mechanism

The presence of the template agent within the pore or cage during the synthesis process of the molecular sieve plays a crucial role in the formation of its porous structure. Using triethylamine and diisopropylamine as structure-guiding agents, VAPO molecular sieve with AFI and AEL topologies were synthesized by hydrothermal method. The physical and chemical properties of hierarchical VAPO-5 and VAPO-11 molecular sieves were characterized by XRD, SEM, N2-adsorption desorption, UV–Vis, XPS, NH3-TPD and Py-IR, etc. It was verified that V isomorphically displaced the P-site of the molecular sieve in pentavalent form, and more acid sites were obtained on the surface of VAPO-5 molecular sieve. This is closely related to the performance of the catalytic oxidation of ethyl lactate to ethyl pyruvate. The activity evaluation showed that the VAPO-5 molecular sieve outperformed the VAPO-11 molecular sieve in terms of catalytic performance. The theoretical study on the reaction mechanism showed that the rate-determining step energy barriers of the catalytic oxidation of EL to EP were 118.7 kJ/mol and 150.18 kJ/mol, respectively. They were consistent with the apparent activation energies of 98.92 kJ/mol and 119.24 kJ/mol obtained by kinetic experiments. The conjecture of the reaction mechanism was confirmed in which the VAPO molecular sieve adsorbs H2O2 molecules and produces surface reactive oxygen species, thus promoting the catalytic oxidation of EL to EP.

Spacial regulation of precise aperture in metal–organic frameworks towards ultrahigh-selective mixed matrix membranes for low-energy-consumption C3H6/C3H8 separation

Membrane technology based on metal–organic framework (MOF) with molecular sieving properties is a promising alternative to energy-intensive conventional distillation for C3H6/C3H8 separation. However, precisely regulating the aperture of MOF materials for industrial separation processes with low energy consumption remains a significant challenge. Herein, we introduce a post-synthetic defect exchange, using ortho-trifluoromethylbenzoic acid (o-TBA) in defective UiO-66, resulting in a mixed matrix membrane (MMM) with a C3H6/C3H8 ideal selectivity of ∼ 104 and C3H6 permeability of ∼ 293 Barrer, and exhibiting a C3H6/C3H8 selectivity ∼ 36 with C3H6 permeability of ∼ 188 Barrer for a C3H6/C3H8 (50:50) mixture. Computational simulations confirm impact of o-TBA-UiO on reducing pore sizes and providing moderate propylene affinity. Static adsorption and solution-diffusion experimental results show that the o-TBA-UiO/6FDA-DAM membrane provides fast C3H6 diffusion rates but retards C3H8 diffusion, leading to ultrahigh C3H6/C3H8 selectivity. A large-area membrane of 2400 cm2 was prepared to promote practical applications, mainly because of the considerably enhanced affinity between o-TBA-UiO and 6FDA-DAM. At a feed pressure of 1 bar, the 1812-type membrane module exhibited a modest decline in single gas separation performance with a C3H6 permeability of ∼ 175 Barrer and a C3H6/C3H8 ideal selectivity of ∼ 96. Finally, a potential two-stage membrane process with low energy consumption was designed. This study provides a robust design for MOF-based MMMs with molecular sieving for energy-efficient gas separation.

Effective C4 Separation by Zeolite Metal-Organic Framework Composite Membranes.

Inorganic zeolites have excellent molecular sieving properties, but they are difficult to process into macroscopic structures. In this work, we use metal-organic framework (MOF) glass as substrates to engineer the interface with inorganic zeolites, and then assemble the discrete crystalline zeolite powders into monolithic structures. The zeolites are well dispersed and stabilized within the MOF glass matrix, and the monolith has satisfactory mechanical stabilities for membrane applications. We demonstrate the effective separation performance of the membrane for 1,3-butadiene (C4H6) from other C4 hydrocarbons, which is a crucial and challenging separation in the chemical industry. The membrane achieves a high permeance of C4H6 (693.00±21.83 GPU) and a high selectivity over n-butene, n-butane, isobutene, and isobutane (9.72, 9.94, 10.31, and 11.94, respectively). This strategy opens up new possibilities for developing advanced membrane materials for difficult hydrocarbon separations.

Effective C4 Separation by Zeolite Metal–Organic Framework Composite Membranes

AbstractInorganic zeolites have excellent molecular sieving properties, but they are difficult to process into macroscopic structures. In this work, we use metal–organic framework (MOF) glass as substrates to engineer the interface with inorganic zeolites, and then assemble the discrete crystalline zeolite powders into monolithic structures. The zeolites are well dispersed and stabilized within the MOF glass matrix, and the monolith has satisfactory mechanical stabilities for membrane applications. We demonstrate the effective separation performance of the membrane for 1,3‐butadiene (C4H6) from other C4 hydrocarbons, which is a crucial and challenging separation in the chemical industry. The membrane achieves a high permeance of C4H6 (693.00±21.83 GPU) and a high selectivity over n‐butene, n‐butane, isobutene, and isobutane (9.72, 9.94, 10.31, and 11.94, respectively). This strategy opens up new possibilities for developing advanced membrane materials for difficult hydrocarbon separations.

Membranes with Molecular Gatekeepers for Efficient CO2 Capture and H2 Purification.

The urgent need for CO2 capture and hydrogen energy has attracted great attention owing to greenhouse gas emissions and global warming problems. Efficient CO2 capture and H2 purification with membrane technology will reduce greenhouse gas emissions and help reach a carbon-neutral society. Here, 4-sulfocalix[4]arene (SC), which has an intrinsic cavity, was embedded into the Matrimid membrane as a molecular gatekeeper for CO2 capture and H2 purification. The interactions between SC and the Matrimid polymer chains immobilize SC molecules into the interchain gaps of the Matrimid membrane, and the strong hydrogen and ionic bondings were able to form homogeneous mixed-matrix membranes. The incorporation of the SC molecular gatekeeper with exceptional molecular-sieving properties improved the gas separation performance of the mixed-matrix membranes. Compared with that of the Matrimid membrane, the CO2 permeability of the Matrimid-SC-3% membrane increased from 16.75 to 119.78 Barrer, the CO2/N2 selectivity increased from 29.39 to 106.95, and the CO2/CH4 selectivity increased from 29.91 to 140.92. Furthermore, when the permeability of H2 was increased to 172.20 Barrer, the H2/N2 and H2/CH4 selectivities reached approximately 153.75 and 202.59, respectively, which are far superior to those of most existing Matrimid-based materials. The mixed-matrix membranes also exhibited excellent long-term operation stability, with separation performance for several important gas pairs still overtaking the Robeson upper limit after aging for 400 days.

Rigid-flexible coupled organosilica membranes toward high-efficiency molecules separation

Membrane-based technology has garnered significant interest in the separation and purification of molecules due to its inherent advantages, such as high efficiency, low energy consumption, and ease of operation. In this study, rigid-flexible coupled organosilica membranes were fabricated through co-polymerization using 1,2-bis(triethoxysilyl)acetylene (BTESA) containing rigid acetylene bridges and 1,2-bis(triethoxysilyl)propane (BTESP) containing flexible propane bridges. The incorporation of flexible propane bridges into BTESA networks allowed for precise adjustment of the network structures, resulting in composite BTESA-P membranes with improved molecular sieving properties. BTESA-P membrane demonstrated significant promise for applications in both CO2 capture and pervaporation (PV) dehydration due to its high potential. BTESA-P30 membrane, fabricated using the rigid-flexible coupled strategy, demonstrated a CO2 permeance of 3807 GPU and a CO2/N2 selectivity of approximately 40. These results surpassed those of numerous previously reported membranes, indicating significant potential for CO2 capture applications. BTESA-P30 membrane exhibited significant competitiveness in the decarbonization process of natural gas. More significantly, analogous occurrences are evident in gas permeation and PV separation process, where the permeation mechanism of gas and PV separation is primarily governed by molecular sieving. The utilization of diverse organosilica precursors in the rigid-flexible coupled strategy proposed in this study offers increased potential for membrane structure design and energy-efficient separation of molecules.

Ultrathin organosiloxane membrane for precision organic solvent nanofiltration

Promising advances in membrane technology can lead to energy-saving and eco-friendly solutions in industrial sectors. This work demonstrates a highly selective membrane with ultrathin and highly interconnected organosiloxane polymer nanolayers by initiated chemical vapor deposition to effectively separate solutes within the molecular weight range of 150–300 g mol−1. We optimize the poly(1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane) membrane by adjusting both the thickness of the selective layer and the pore sizes of its support membranes. Notably, the 29 nm selective layer imparts a uniformly narrow molecular sieving property, providing a record-high solute-solute selectivity of 39.88 for different-sized solutes. Furthermore, a solute-solute selectivity of 11.04 was demonstrated using the real-world active pharmaceutical ingredient mixture of Acyclovir and Valacyclovir, key components for Herpes virus treatment, despite their molecular weight difference of less than 100 g mol−1. The highly interconnected membrane is expected to meet rigorous requirements for high-standard active pharmaceutical ingredient separation.

Changing Template/Al2O3 Ratio in Reaction Gel-An Effective Way to Regulate Nature of Intermediate Phases and Properties of SAPO-11 Molecular Sieves during Crystallization.

A study has been conducted to investigate the formation of intermediate phases during the crystallization of SAPO-11 molecular sieves from reaction mixtures with a varying template (di-n-propylamine) DPA/Al2O3 ratio. It was found that changing the DPA/Al2O3 ratio from 1.0 to 1.8 in the initial reaction gels leads to the formation of different intermediate phases during crystallization into a SAPO-11 molecular sieve. It is shown that at the ratio template/Al2O3 = 1.0, an intermediate amorphous silicoaluminophosphate is formed; at 1.4, a mixture consisting of amorphous and layered phases forms; and at 1.8, a layered phase is present. A simple and innovative approach for controlling the morphology, size, and characteristics of primary crystals and the secondary porous structure in hierarchical SAPO-11 is proposed. The method is based on regulating the DPA/Al2O3 ratio in the reaction gel. The synthesized SAPO-11 molecular sieves with a hierarchical porous structure exhibited high selectivity in the hydroisomerization of n-hexadecane.

Effect analysis on hydrocarbon adsorption enhancement of different zeolites in cold start of gasoline engine based on Monte Carlo method

50%–80% of the hydrocarbons in gasoline engine emissions come from during cold starts. Zeolites are often used to adsorb HCs during gasoline engine cold-starting. In order to investigate the effect of adsorption enhancement of different molecular sieves on HCs during gasoline engine cold-starting. In this paper, the adsorption properties of Beta, MOR and FER zeolites at different temperatures were simulated based on Monte Carlo method using propylene and toluene as probe molecules. The results show that all three molecular sieves exhibit good adsorption properties at low temperatures. The adsorption capacity decreased with increasing temperature. The molecular sieves tend to adsorb molecules with diameters similar to the pore size of zeolite. In addition, the three-dimensional zeolites Beta and FER showed better adsorption properties than the one-dimensional zeolites. Based on Monte Carlo method, the adsorption properties of Beta, MOR and FER molecular sieves were investigated at different temperatures under multi-components using toluene, isopentane, propylene, ethylene, methane and water as adsorbents. The results showed that there was a competitive adsorption among the components due to the limited adsorption sites in the molecular sieve channels. Large molecules such as toluene and isopentane occupied most of the adsorption sites and hindered the adsorption of small molecules.

Electrolyte under Molybdenum Disulfide Surfaces: Molecular Insights on Structure and Dynamics of Water.

Molybdenum disulfide (MoS2) is a two-dimensional (2D) material that offers molecular transport and sieving properties and might be a potential candidate for membrane technologies for energy and environmental applications. To facilitate the separation application, understanding the structural and dynamic properties of water near the substrate-aqueous solution is essential. Employing the molecular dynamics simulation, we investigate the density, local water network at the solid-liquid interface, and water dynamics in aqueous electrolyte solutions with various chloride salts confined in MoS2 nanochannels with different pore sizes and electrolyte concentrations. Our simulation results confirm that the layering of interfacial water at the hydrophilic MoS2 surface and the water density variation depends on the nature of the ions. The simulation results imply a strong attraction of cations to the surface-liquid interfaces, whereas anions are expelled from the surface due to electrostatic interaction. An examination of the dynamical property of water reveals that the confinement effect is more pronounced on water mobility when the pore width is less than 3 nm, and the salt concentration is below 1 M, whereas the electrolyte concentration greater than 1 M, ions predominantly drive the water mobility as compared to confinement one. These simulation results enhance experimental observations and provide molecular insights into the local ordering mechanism that can be crucial in controlling water dynamics in nanofiltration applications.