Precise molecular sieving architectures with Janus superhighways are constructed via a molecularly engineered interfacial reaction between cyclodextrin (CD) and trimesoyl chloride (TMC). ...Interestingly, the CD/TMC nanofilms constructed with both hydrophobic inner cavities and hydrophilic channels exhibit exceptionally high permeances for both polar and nonpolar solvents. The precise molecular sieving functions are determined by the type of CD building blocks and the inner cavities of intrinsic 3D hollow bowls. Positron annihilation spectroscopy (PAS) confirms that a larger inner CD cavity tends to generate a larger free volume and higher microporosity. Based on the rejection ratio of various dyes, the estimated molecular weight cutoff of CD/TMC nanofilms follows the trend of α‐CD/TMC (320 Da) <β‐CD/TMC (400 Da) <γ‐CD/TMC (550 Da), which is in strict accordance with the orders of their free volumes measured by PAS and inner cavity sizes of α‐CD <β‐CD <γ‐CD. This kind of novel CD/TMC molecular sieving membrane with intrinsic microporosity containing tunable pore size and sharp pore‐size distribution can effectively discriminate molecules with different 3D sizes.
Precise molecular sieving architectures with Janus pathways (both hydrophobic inner cavities and hydrophilic channels) exhibit high permeances for both polar and nonpolar solvents. The pore size can be accurately tuned by the intrinsic 3D hollow bowls of cyclodextrins. Clearly, the molecular sieving function is the most distinctive feature to discriminate small molecules based on their 3D structures.
Carbon quantum dots (CQDs), which are a fascinating class of nanostructured carbons, have recently attracted extensive attention in the field of membrane technologies for their applications in ...separation processes. This is because they possess two unique advantages. Their productions are facile and inexpensive, while their physicochemical properties such as ultra-small sizes, good biocompatibility, high chemical inertness, tunable hydrophilicity, rich surface functional groups and antifouling characteristics are highly desirable. Leveraging on these, researchers have explored their utilizations in various membrane designs for reverse osmosis (RO), ultrafiltration (UF), nanofiltration (NF), forward osmosis (FO), pressure retarded osmosis (PRO), membrane distillation (MD), and organic solvent nanofiltration (OSN) processes. In particular, CQDs have especially stimulated exploration in the field of water treatment by membrane technologies since biocompatibility of membrane materials is of utmost importance to ensure safety of drinking water. In addition, CQDs are in a favorable position for achieving unprecedented performance of membrane separation processes in water treatment, in the light of substantial efficiency enhancement and antifouling propensity as discovered in recent studies. In this article, we will review the progress in the development of CQD incorporated membranes with discussions on their challenges and perspectives.
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•The state-of-the-art of CQD modified membranes for different applications are summarized.•The methods to incorporate CQDs into membranes are introduced.•The challenges and future of CQDs for membrane technologies are elaborated.
Thin-film nanocomposites (TFN) functionalized with tunable molecular-sieving nanomaterials have been employed to tailor membranes, with an enhanced permeability and selectivity. Herein, water-soluble ...hollow cup-like macrocyclic molecules, sulfothiacalix4arene (STCAss) and sulfocalix4arene (SCA), are ionically bonded into the polyamide network to engineer the molecular-sieving properties of TFN membranes for organic solvent forward osmosis (OSFO). Introducing both STCAss and SCA into the polyamide network not only increases the free volume, but also reduces the thickness of the TFN layers. Combining with their molecularly tunable size of the lower cavities, both STCAss and SCA enable the TFN membranes to size exclusively reject the draw solutes, but only STCAss-functionalized membrane has an ethanol flux doubling the pristine one under the FO and PRO modes in OSFO processes; leading the functionalized polyamide network with remarkable improvements in OSFO performance. This study may provide insights to molecularly functionalize TFN membranes using multifunctional nano-fillers for sustainable separations.
Membrane distillation (MD) is a separation process based on the vapor transport across the hydrophobic microporous membrane driven by the vapor pressure gradient across the membrane. This process can ...be used for various applications such as seawater desalination, wastewater treatment, separation of volatile compounds, concentration of non-volatile compounds and processing of dairy fluids. Comparing with other separation processes, the MD process possesses unique characteristics such as 100% (theoretical) rejection, mild operation conditions, insensitive to feed concentration and stable performance at high contaminant concentrations. Due to high oil prices in recent years, extensive research has been devoted to MD in the areas of membrane materials, module configurations, process applications and hybrid systems. This review aims to summarize the recent advances in MD and provide perspectives for its future R&D.
•This review aims to summarize the recent advances in membrane distillation (MD) and provide perspectives for its future R&D.•The development of MD membranes was reviewed from different aspects as material, micro-structure and macro-geometry.•The traditional and new configuration designs with improved energy efficiency were summarized.•Various applications and MD based hybrid systems were reviewed comprehensively.
A novel dual-modification strategy, including (1) the cross-linking and construction of a GO framework by ethylenediamine (EDA) and (2) the amine-enrichment modification by hyperbranched ...polyethylenimine (HPEI), has been proposed to design stable and highly charged GO framework membranes with the GO selective layer thickness of 70 nm for effective heave metal removal via nanofiltration (NF). Results from sonication experiments and positron annihilation spectroscopy confirmed that EDA cross-linking not only enhanced structural stability but also enlarged the nanochannels among the laminated GO nanosheets for higher water permeability. HPEI 60K was found to be the most effective post-treatment agent that resulted in GO framework membranes with a higher surface charge and lower transport resistance. The newly developed membrane exhibited a high pure water permeability of 5.01 L m–2 h–1 bar–1 and comparably high rejections toward Mg2+, Pb2+, Ni2+, Cd2+, and Zn2+. These results have demonstrated the great potential of GO framework materials in wastewater treatment and may provide insights for the design and fabrication of the next generation two-dimensional (2D)-based NF membranes.
The organic solvent forward osmosis (OSFO) process can simultaneously concentrate the active pharmaceutical ingredients (APIs) and recover the organic solvents. Here we demonstrate and evaluate an ...OSFO process for solvent recovery. In this demonstration, OSFO was conducted in different solvents with different draw solutes. The OSFO process shows rejections >98% when recovering organic solvents from different feed solutions, even when the feed concentration is as high as 20 wt%. More importantly, all systems exhibit relatively low ratios of reverse solute flux to solvent flux, indicating that the adverse effects of using hazardous draw solutions could be minimized. Nevertheless, the use of non-hazardous draw solutes such as citric acid is highly recommended to remove any potential risk, and it has been demonstrated. Herein, the OSFO process is a promising technology for solvent recovery as it possesses a reasonable solvent flux, low reverse solute flux and requires no external pressure.
Membrane technology has gained great interest in industrial separation processing over the past few decades owing to its high energy efficiency, small capital investment, environmentally benign ...characteristics, and the continuous operation process. Among various types of membranes, mixed matrix membranes (MMMs) combining the merits of the polymer matrix and inorganic/organic fillers have been extensively investigated. With the rapid development of chemistry and materials science, recent studies have shifted toward the design and application of advanced porous materials as promising fillers to boost the separation performance of MMMs. Here, first a comprehensive overview is provided on the choices of advanced porous materials recently adopted in MMMs, including metal–organic frameworks, porous organic frameworks, and porous molecular compounds. Novel trends in MMMs induced by these advanced porous fillers are discussed in detail, followed by a summary of applying these MMMs for gas and liquid separations. Finally, a concise conclusion and current challenges toward the industrial implementation of MMMs are outlined, hoping to provide guidance for the design of high‐performance membranes to meet the urgent needs of clean energy and environmental sustainability.
The recent research progress in exploring various types of advanced porous materials as promising fillers in mixed matrix membranes (MMMs) is summarized. Along with this, novel MMM design and enhanced membrane separation performance induced by these advanced porous fillers are discussed in detail. Finally, current challenges and future directions for industrial implementation of these MMMs are highlighted.
Processable molecular‐sieving membranes are important materials for realizing energy‐efficient precombustion CO2 capture during industrial‐scale hydrogen production. However, the promising design of ...mixed matrix membranes (MMMs) that aims to integrate the molecular‐sieving properties of nanoporous architectures with industrial processable polymers still faces performance and fabrication issues due to the formation of segregated nanofiller domains in their polymer matrices. Here, an unconventional nanocomposite membrane design is proposed using soluble organic macrocyclic cavitands (OMCs) with tunable open cavity sizes that not only mitigate the formation the discrete nanofiller phases but also deliver distinct molecular‐sieving separations. The versatile organic‐solvent solubility coupled with highly interactive functionalities of OMCs allows them to obtain molecularly homogeneous mixing with matrix polymers and form only one integral continuous phase crucial to the robust processability of polymers. A series of polybenzimidazole‐based molecularly mixed composite membranes (MMCMs) are fabricated via the incorporation of a soluble and thermally stable OMC choice, sulfocalixarenes, with various cavity sizes. These membranes achieve outstanding high‐temperature mixed‐gas H2/CO2 separation performances comparable with several state‐of‐the‐art molecular‐sieving membranes owing to effective size‐sieving gas passages through the open or partially‐intruded supramolecular cavities. The broadly tunable structures and functionalities of OMCs would make their MMCMs attractive for other energy‐intensive molecular separations.
Soluble organic macrocyclic cavitands (OMCs) with interactive affinity for polymers can be utilized to design novel nanocomposite membranes with true molecular homogeneity, unlike conventional nanoarchitectures that form heterogeneous nanofiller phases within membranes that cause performance and processability issues. By adjusting the OMC cavity size within the membranes, tunable gas transport properties can be obtained for highly molecular‐sieving H2/CO2 separation.
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Membrane-based technologies for gas and vapor separations have been industrialized for more than three decades due to (1) the invention of asymmetric composite membranes and (2) their ...superior energy-efficient and environment-friendly characteristics. However, industries demand more robust and productive composite membranes to meet the harsh conditions in new or emerging applications. The fabrication of thin film composite (TFC) membranes consisting of an ultrathin selective layer and a strong porous substrate will likely play the pivotal role to meet the future demands. In this review, we revisit the basis of composite membranes for gas and vapor separations with the aim to produce next-generation high-performance composite membranes. This review focuses on (1) the fundamentals of gas transport theories through composite membranes, (2) fabrication techniques, and (3) performance characteristics and stability of composite membranes. We examine, elucidate and explore the state-of-the-art techniques for the fabrication of composite membranes. Based on the resistance model, we derive a new generalized equation for gas permeance, which is simple, informative and applicable to composite membranes. We also outline the directions, challenges and key areas for future R&D in the field of gas/vapor-separation composite membranes.
In this study, an ultrathin graphene oxide (GO) framework layer was successfully deposited on a modified Torlon® hollow fiber support via a layer-by-layer (LbL) approach, enabling the composite ...membrane with superior nanofiltration (NF) performance. To molecularly design the GO framework, the substrate layer was firstly cross-linked with polyethylenimine (HPEI), followed by repeating the GO and ethylenediamine (EDA) deposition cycles and then an amine-enrichment modification by HPEI. The combination of the GO framework layer and the Torlon® support can not only effectively seal the defects of the composite membrane with a narrow pore size distribution but also reduce the polymer consumption for the fabrication of traditional integrally skinned NF membranes. The GO/Torlon® composite membrane has rejections higher than 95% towards Pb2+, Ni2+, and Zn2+ with a superior water permeability of 4.7Lm−2h−1bar−1. The membrane also exhibits excellent long-term performance stability during a 150-h NF test. Thus, the newly developed membrane has great potential for heavy metal removal. This study may provide useful insights on the fabrication of new generation 2-dimensional (2D) NF membranes.
•An LbL approach for the deposition of GO on a hollow fiber support was explored.•GO nanosheets could effectively cover the surface defects and narrow pore size.•Fabricating GO/Torlon® composite membrane could save polymer consumption.•High heavy metal removal efficiency and long-term stability were achieved.
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