Cover illustration
(Wei Wang, Yanying Wei, Jiang Fan, Jiahao Cai, Zong Lu, Li Ding, Haihui Wang, pp. 793‒819)
Membrane-based separation technologies have received increasing attention attributing to lots of advantages such as the low energy consumption, easy operation, and environmental friendliness. Two-dimensional (2D) materials have emerged as a class of promising materials to prepare high-performance 2D membran
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With fossil fuel being the major source of energy, CO2 emission levels need to be reduced to a minimal amount namely from anthropogenic sources. Energy consumption is expected to rise by 48% in the next 30 years, and global warming is becoming an alarming issue which needs to be addressed on a thorough technical basis. Nonetheless, exploring CO2 capture using membrane contactor technology has shown great potential to be applied and utilised by industry to deal with post- and pre-combustion of CO2. A systematic review of the literature has been conducted to analyse and assess CO2 removal using membrane contactors for capturing techniques in industrial processes. The review began with a total of 2650 papers, which were obtained from three major databases, and then were excluded down to a final number of 525 papers following a defined set of criteria. The results showed that the use of hollow fibre membranes have demonstrated popularity, as well as the use of amine solvents for CO2 removal. This current systematic review in CO2 removal and capture is an important milestone in the synthesis of up to date research with the potential to serve as a benchmark databank for further research in similar areas of work. This study provides the first systematic enquiry in the evidence to research further sustainable methods to capture and separate CO2.
Membrane distillation (MD) is a thermal-based separation technique with the potential to treat a wide range of water types for various applications and industries. Certain challenges remain however, which prevent it from becoming commercially widespread including moderate permeate flux, decline in separation performance over time due to pore wetting and high thermal energy requirements. Nevertheless, its attractive characteristics such as high rejection (ca. 100%) of non-volatile species, its ability to treat highly saline solutions under low operating pressures (typically atmospheric) as well as its ability to operate at low temperatures, enabling waste-heat integration, continue to drive research interests globally. Of particular interest is the class of carbon-based nanomaterials which includes graphene and carbon nanotubes, whose wide range of properties have been exploited in an attempt to overcome the technical challenges that MD faces. These low dimensional materials exhibit properties such as high specific surface area, high strength, tuneable hydrophobicity, enhanced vapour transport, high thermal and electrical conductivity and others. Their use in MD has resulted in improved membrane performance characteristics like increased permeability and reduced fouling propensity. They have also enabled novel membrane capabilities such as in-situ fouling detection and localised heat generation. In this review we provide a brief introduction to MD and describe key membrane characteristics and fabrication methods. We then give an account of the various uses of carbon nanomaterials for MD applications, focussing on polymeric membrane systems. Future research directions based on the findings are also suggested.
The rapid industrial growth and the necessity of recovering and recycling raw materials increased the interest in the production of highly selective and efficient separation tools. In this perspective, a relevant input was given by the membrane-based technology and the production of imprinted membranes, which possess specific recognition properties at molecular and ionic level, offers the possibility of developing sustainable and green processes. Furthermore, the integration of imprinted membranes with traditional or membrane-based approaches is a promising strategy in the logic of process intensification, which means the combination of different operations in a single apparatus. This work discusses the concept and separation mechanisms of imprinted membranes. Furthermore, it presents an overview of their application in organic solvent nanofiltration, for the removal of toxic agents and recovery solvent, as well as valuable compounds. The recent advances in water treatment, such as pesticide removal and recovery of metal ions, are also discussed. Finally, potential applications of imprinted membranes in hybrid processes are highlighted, and a look into the future of membrane separations for water treatment and recovery of critical raw materials is offered.
Two-dimensional (2D) materials have emerged as a class of promising materials to prepare high-performance 2D membranes for various separation applications. The precise control of the interlayer nanochannel/sub-nanochannel between nanosheets or the pore size of nanosheets within 2D membranes enables 2D membranes to achieve promising molecular sieving performance. To date, many 2D membranes with high permeability and high selectivity have been reported, exhibiting high separation performance. This review presents the development, progress, and recent breakthrough of different types of 2D membranes, including membranes based on porous and non-porous 2D nanosheets for various separations. Separation mechanism of 2D membranes and their fabrication methods are also reviewed. Last but not the least, challenges and future directions of 2D membranes for wide utilization are discussed in brief.
Two-dimensional membranes have attracted significant attention due to their superior characteristics, and their ability to boost both flux and selectivity have led to their reputation as potential next-generation separation membranes. Among them, emerging MXene-based membranes play significant roles in the competitive membrane-separation field. In this mini-review, we systematically discuss the assembly and separation mechanisms of these membranes. Moreover, we highlight strategies based on the crosslinking of MXene nanosheets and the construction of additional nanochannels that further enhance the permeabilities and anti-swelling properties of MXene-based membranes and meet the requirements of practical applications, such as gas-molecule sieving, ion sieving, and other small-molecule sieving. MXene nanosheets can also be used as additives that introduce specific functionalities into hybrid membranes. In addition, extended applications that use MXenes as scaffolds are also discussed.
Saccharide production is critical to the development of biotechnology in the field of food and biofuel. The extraction of saccharide from biomass-based hydrolysate mixtures has become a trend due to low cost and abundant biomass reserves. Compared to conventional methods of fractionation and recovery of saccharides, nanofiltration (NF) has received considerable attention in recent decades because of its high selectivity and low energy consumption and environmental impact. In this review the advantages and challenges of NF based technology in the separation of saccharides are critically evaluated. Hybrid membrane processes, i.e., combining NF with ultrafiltration, can complement each other to provide an efficient approach for removal of unwanted solutes to obtain higher purity saccharides. However, use of NF membrane separation technology is limited due to irreversible membrane fouling that results in high capital and operating costs. Future development of NF membrane technology should therefore focus on improving material stability, antifouling ability and saccharide targeting selectivity, as well as on engineering aspects such as process optimisation and membrane module design.
Biofuels and bio-based chemicals are getting more and more attention because of their sustainable and renewable properties and wide industrial applications. However, the low concentrations of the targeted products in their fermentation broths, the complicated components of the broths and the high energy-intensive separation and purification process hinder the competitiveness of biofuels and biochemicals with the petro-based ones. Hence, the production and the separation of biofuels and bio-based chemicals in energy-saving, low-cost and greenness ways become hot topics nowadays. This review introduces the separation technologies (salting-out extraction, salting-out, sugaring-out extraction, and sugaring-out) that extract biobutanol, 1,3-propanediol, 2,3-butanediol, acetoin, organic acids and other bio-based chemicals from fermentation broths/aqueous solutions. Salting-out/sugaring-out extraction and salting-out/sugaring-out technologies display the high separating efficiency and the high targeted product yields. In addition, they are easy to operate and require low cost for separating products. Hence, they are the effective and potential technologies for separating targeted products in the wide industrial applications. The successful research into the salting-out/sugaring-out and salting-out/sugaring-out extraction not only affords biofuels and biochemical but also opens a door for the development of novel separation methods.
Physical aging is currently a major obstacle for the commercialization of PIM-1 membranes for gas separation applications. A well-known approach to reversing physical aging effects of PIM-1 membranes at laboratory scale is soaking them in lower alcohols, such as methanol and ethanol. However, this procedure does not seem applicable at industrial level, and other strategies must be investigated. In this work, a regeneration method with alcohol vapors (ethanol or methanol) was developed to recover permeability of aged PIM-1 membranes, in comparison with the conventional soaking-in-liquid approach. The gas permeability and separation performance, before and post the regeneration methods, were assessed using a binary mixture of CO2 and CH4 (1:1, v:v). Our results show that an 8-hour methanol vapor treatment was sufficient to recover the original gas permeability, reaching a CO2 permeability>7000 barrer.
Hydrogen fuel has been embraced as a potential long-term solution to the growing demand for clean energy. A membrane-assisted separation is promising in producing high-purity H2. Molecular sieving membranes (MSMs) are endowed with high gas selectivity and permeability because their well-defined micropores can facilitate molecular exclusion, diffusion, and adsorption. In this work, MXene nanosheets intercalated with Ni2+ were assembled to form an MSM supported on Al2O3 hollow fiber via a vacuum-assisted filtration and drying process. The prepared membranes showed excellent H2/CO2 mixture separation performance at room temperature. Separation factor reached 615 with a hydrogen permeance of 8.35 × 10−8 mol·m−2·s−1·Pa−1. Compared with the original Ti3C2Tx/Al2O3 hollow fiber membranes, the permeation of hydrogen through the Ni2+-Ti3C2Tx/Al2O3 membrane was considerably increased, stemming from the strong interaction between the negatively charged MXene nanosheets and Ni2+. The interlayer spacing of MSMs was tuned by Ni2+. During 200-hour testing, the resultant membrane maintained an excellent gas separation without any substantial performance decline. Our results indicate that the Ni2+ tailored Ti3C2Tx/Al2O3 hollow fiber membranes can inspire promising industrial applications.
Photocatalytic membranes have received increasing attention due to their excellent separation and photodegradation of organic contaminants in wastewater. Herein, we bound Ag-AgBr nanoparticles onto a synthesized polyacrylonitrile-ethanolamine (PAN-ETA) membrane with the aid of a chitosan (CS)-TiO2 layer via vacuum filtration and in-situ partial reduction. The introduction of the CS-TiO2 layer improved surface hydrophilicity and provided attachment sites for the Ag-AgBr nanoparticles. The PAN-ETA/CS-TiO2/Ag-AgBr photocatalytic membranes showed a relatively high water permeation flux (~ 47 L·m–2·h–1·bar–1) and dyes rejection (methyl orange: 88.22%; congo red: 95%; methyl blue: 97.41%; rose bengal: 99.98%). Additionally, the composite membranes exhibited potential long-term stability for dye/salt separation (dye rejection: ~97%; salt rejection: ~6.5%). Moreover, the methylene blue and rhodamine B solutions (20 mL, 10 mg·L−1) were degraded approximately 90.75% and 96.81% in batch mode via the synthesized photocatalytic membranes under visible light irradiation for 30 min. This study provides a feasible method for the combination of polymeric membranes and inorganic catalytic materials.
BaSO4 nanoparticles as important functional materials have attracted considerable research interests, due to their X-rays barrier and absorption properties. However, most of BaSO4 nanoparticles prepared by traditional technology are nanopowders with broad size distribution and poor dispersibility, which may greatly limit their applications. To the best of our knowledge, the synthesis of transparent BaSO4 nanodispersions was rarely reported. Here, we firstly present a novel and efficient method to prepare transparent and stable BaSO4 nanodispersions with a relatively small particle size around 10 to 17 nm using a precipitation method in a rotating packed bed (RPB), followed by a modification treatment using stearic acid. Compared with the BaSO4 prepared in a traditional stirred tank, the product prepared using an RPB has much smaller particle size and narrower size distribution. More importantly, by using RPB, the reaction time can be significantly decreased from 20 min to 18 s. Furthermore, the transparent BaSO4-polyvinyl butyral nanocomposite films with good X-ray shielding performance can be easily fabricated. We believe that the stable BaSO4 nanodispersions may have a wide range of applications for transparent composite materials and coatings with X-ray shielding performance for future research.
Graphene oxide (GO) membranes have received considerable attention owing to their outstanding water-permeation properties; however, the effect of the membrane’s microstructures (such as the distribution of oxidized and pristine regions) on the transport mechanism remains unclear. In this study, we performed molecular simulations to explore the permeation of a water–ethanol mixture using a new type of Janus GO membranes with different orientations of oxidized and pristine surfaces. The results indicate that the oxidized upper surface endows the GO membrane with considerable water-capture capability and the in-built oxidized interlayer promotes the effective vertical diffusion of water molecules. Consequently, using the optimized Janus GO membrane, infinite water selectivity and outstanding water flux (~40.9 kg⋅m−2⋅h−1) were achieved. This study contributes to explaining the role of oxidized regions in water permeation via GO membranes and suggests that Janus GO membranes could be used as potential candidates for water–ethanol separation.
The emergence of MoS2 nanopores has provided a new avenue for high performance DNA sequencing, which is critical for modern chemical/biological research and applications. Herein, molecular dynamics simulations were performed to design a conceptual device to sequence DNA with MoS2 nanopores of different structures (e.g., pore rim contained Mo atoms only, S atoms only, or both Mo and S atoms), where various unfolded single-stranded DNAs (ssDNAs) translocated through the nanopores driven by transmembrane bias; the sequence content was identified by the associating ionic current. All ssDNAs adsorbed onto the MoS2 surface and translocated through the nanopores by transmembrane electric field in a stepwise manner, where the pause between two permeation events was long enough for the DNA fragments in the nanopore to produce well-defined ionic blockage current to deduce the DNA’s base sequence. The transmembrane bias and DNA-MoS2 interaction could regulate the speed of the translocation process. Furthermore, the structure (atom constitution of the nanopore rim) of the nanopore considerably regulated both the translocate process and the ionic current. Thus, MoS2 nanopores could be employed to sequence DNA with the flexibility to regulate the translocation process and ionic current to yield the optimal sequencing performance.
We demonstrate the fabrication of a new DNA sensor that is based on the optical interactions occurring between oligonucleotide-coated NaYF4: Yb3+; Er3+ upconversion nanoparticles and the two-dimensional dichalcogenide materials, MoS2 and WS2. Monodisperse upconversion nanoparticles were functionalized with single-stranded DNA endowing the nanoparticles with the ability to interact with the surface of the two-dimensional materials via van der Waals interactions leading to subsequent quenching of the upconversion fluorescence. By contrast, in the presence of a complementary oligonucleotide target and the formation of double-stranded DNA, the upconversion nanoparticles could not interact with MoS2 and WS2, thus retaining their inherent fluorescence properties. Utilizing this sensor we were able to detect target oligonucleotides with high sensitivity and specificity whilst reaching a concentration detection limit as low as 5 mol·L–1, within minutes.
This work presents a simple effective strategy to synthesize N-doped and shell-controlled carbon nanocages through a package baking approach. A green approach to synthesize core-shell ZIF-8@PTZ nanoparticles involves zinc contained ZIF-8 core wrapped by a N-enriched polytriazine (PTZ). Synthesized core-shell ZIF-8@PTZ nanoparticles are calcinated to further sublime zinc through PTZ shell and washed by HCl, leaving a porous carbon structure. At the meantime, hollow cavities were introduced into N-doped carbon polyhedrons via the sacrifice of ZIF-8 template (noted as ZIF-8@C/N-x). The electrochemical performance of the ZIF-8@C/N-x as supercapacitor electrode has demonstrated high energy density and specific capacitance, as well as a long-term cycleability showing 92% capacitance retention after 10000 cycles. There is a systematic correlation between micro-/meso-porosity of ZIF-8@C/N-x and their electrochemical performances.
Here we present an economical ambient pressure drying method of preparing monolithic silica aerogels from methyltrimethoxysilane precursor while using sodium bicarbonate solution as the exchanging solvent. We prepared silica aerogels with a density and a specific surface area of 0.053 g∙cm−3 and 423 m2∙g−1, respectively. The average pore diameter of silica aerogels is 23 nm as the pore specific volume is 1.11 cm3∙g−1. Further, the contact angle between water droplet and the surface of silica aerogels in specific condition can be as high as 166°, which indicates a super-hydrophobic surface of aerogels.
2,5-Furandicarboxylic acid (FDCA) is an important and renewable building block and can serve as an alternative to terephthalic acid in the production of bio-based degradable plastic. In this study, Cu-doped MnO2 nanorods were prepared by a facile hydrothermal redox method and employed as catalysts for the selective oxidation of 5-hydroxymethylfurfural (HMF) to FDCA using tert-butyl hydroperoxide (TBHP) as an oxidant. The catalysts were characterized using X-ray diffraction analysis, Fourier transform infrared spectroscopy, thermogravimetric analysis, and transmission electron microscopy. The effects of oxidants, solvents, and reaction conditions on the oxidation of HMF were investigated, and a reaction mechanism was proposed. Experimental results demonstrated that 99.4% conversion of HMF and 96.3% selectivity of FDCA were obtained under suitable conditions, and tert-butanol was the most suitable solvent when TBHP was used as an oxidant. More importantly, the Cu-doped MnO2 catalyst can maintain durable catalytic activity after being recycled for more than ten times.
A facile strategy was developed to fabricate flexible polyurethane (PU) foam composites with exceptional flame retardancy. The approach involves the incorporation of graphene oxide (GO) into a silicone resin (SiR) solution, which is then deposited onto a PU foam surface via the dip-coating technique and cured. Fourier-transform infrared spectroscopy, scanning electron microscopy, and Raman spectroscopy measurements demonstrated that the SiR and GO were successfully coated onto the PU skeleton and the intrinsic porous structure of the PU foam remained intact. The effects of SiR and GO on the mechanical and thermal stability and flame retardancy of PU composites were evaluated through compression tests, thermogravimetric analysis, vertical combustion tests, and the limiting oxygen index. The measurement results revealed that the composites (PU@SiR-GO) showed superior flame retardancy and thermal and mechanical stability compared to pristine PU or PU coated with SiR alone. The mechanical and thermal stability and the flame-retardant properties of the PU composites were enhanced significantly with increasing GO content. Based on the composition, microstructure, and surface morphology of PU@SiR-GO composites before and after combustion tests, a possible flame-retardance mechanism is proposed. This work provides a simple and effective strategy for fabricating flame-retardant composites with improved mechanical performance.
Nanofibers with high specific surface area and chemical stability have broad prospects in the applications of adsorption. However, the adsorption capacity is limited by the scarcity of adsorption groups and storage space. Herein, the activated carbon-hybridized and amine-modified nanofibers are prepared by integrating activated carbon (AC) and polyacrylonitrile (PAN) via electrospinning method and the subsequent amination, which could provide additional storage space and adsorption groups for ultrahigh adsorption capability. Thus, the obtained amine-rich porous PAN nanofibers (APAN/AC) readily realized the ultrahigh adsorption capacity for metal ions and dyes in wastewater. Specifically, the adsorption capacity of APAN/AC nanofibers were 284 mg·g−1 for Cr(VI) and 248 mg·g−1 for methyl orange, which were almost 2 and 4 times than that of amine-modified nanofibers (APAN) and carbon-hybridized nanofibers (PAN/AC), respectively. Moreover, the AC inhibited the chain mobility of polymer matrix and thereby endowing APAN/AC nanofibers with excellent recyclability. The adsorption capability retained 80% after nine adsorption-desorption cycles. The adsorption kinetics and corresponding mechanism were further explored. This strategy combines the advantages of polymer nanofibers and AC, opening a new avenue for developing next-generation absorbent materials.
This work proposes a modified activated carbon support, with defects and heteroatoms (N,P-ACs) by nitrogen and phosphorus doping to load non-noble nickel to catalyze aromatic compound hydrogenation. The Ni/N,P-ACs-900 (prepared at 900 °C) showed promising catalytic activity in liquid-phase 1,5-dinitronaphthalene hydrogenation with a 1,5-diaminonaphthalene yield of 95.8% under the mild condition of 100 °C, which is comparable to the commercial Pd/C catalyst. The nitrogen species were burned off at 900 °C, causing more defects for nickel metal loading, facilitating the interaction between the supports and the nickel metal, and resulting in highly dispersed metal particles. The computational study of the nickel binding energy has been conducted using density functional theory. It exhibits that the defects formed by heteroatom doping are beneficial to nickel anchoring and deposition to form highly uniform nickel particles. The phosphorus species in combination with the defects are suitable for H2 adsorption and dissociation. These results reveal that the heteroatomic doping on the active carbon shows significant effects in the hydrogenation of the liquid-phase aromatic compounds. These findings could provide a promising route for the rational design of aromatic compound hydrogenation catalysts to significantly decrease the cost by instead using noble metal catalysts in the industry.
This study focuses on the synthesis and characterization of a novel magnetic nanocomposite 2,4,6-trihydrazino-1,3,5-triazine (THDT)-functionalized with silica-coated iron oxide magnetic nanoparticles (MNPs). This nanocomposite has porous morphology decorated with the spherical MNPs. Through co-precipitation of iron salts, MNPs were obtained. The prepared THDT was placed on the chlorine surface-modified MNPs. The present environment-friendly nanocatalyst intensely accelerated the synthesis of highly functionalized tetrahydrobenzo[b]pyran derivatives as well as reduced the reaction times and increased yields of the products.
We present a one-step route for the preparation of nickel phosphide/carbon nanotube (Ni2P@CNT) nanocomposites for supercapacitor applications using a facile, ultrafast (90 s) microwave-based approach. Ni2P nanoparticles could grow uniformly on the surface of CNTs under the optimized reaction conditions, namely, a feeding ratio of 30:50:25 for CNT, Ni(NO3)2·6H2O, and red phosphorus and a microwave power of 1000 W for 90 s. Our study demonstrated that the single-step microwave synthesis process for creating metal phosphide nanoparticles was faster and simpler than all the other existing methods. Electrochemical results showed that the specific capacitance of the optimal Ni2P@CNT-nanocomposite electrode displayed a high specific capacitance of 854 F·g−1 at 1 A·g−1 and a superior capacitance retention of 84% after 5000 cycles at 10 A·g−1. Finally, an asymmetric supercapacitor was assembled using the nanocomposite with activated carbon as one electrode (Ni2P@CNT//AC), which showed a remarkable energy density of 33.5 W·h·kg−1 and a power density of 387.5 W·kg−1. This work will pave the way for the microwave synthesis of other transition metal phosphide materials for use in energy storage systems.
