Hierarchical pore architectures from 2D covalent organic nanosheets for efficient water/alcohol separation

Recent Advancements in the Recovery and Reuse of Organic Solvents Using Novel Nanomaterial-Based Membranes for Renewable Energy Applications

The energy crisis in the world is increasing rapidly owing to the shortage of fossil fuel reserves. Climate change and an increase in global warming necessitates a change in focus from petroleum-based fuels to renewable fuels such as biofuels. The remodeling of existing separation processes using various nanomaterials is of a growing interest to industrial separation methods. Recently, the design of membrane technologies has been the most focused research area concerning fermentation broth to enhance performance efficiency, while recovering those byproducts to be used as value added fuels. Specifically, the use of novel nano material membranes, which brings about a selective permeation of the byproducts, such as organic solvent, from the fermentation broth, positively affects the fermentation kinetics by eliminating the issue of product inhibition. In this review, which and how membrane-based technologies using novel materials can improve the separation performance of organic solvents is considered. In particular, technical approaches suggested in previous studies are discussed with the goal of emphasizing benefits and problems faced in order to direct research towards an optimized membrane separation performance for renewable fuel production on a commercial scale.

2D Polymer Nanosheets for Membrane Separation

Since the discovery of single-layer graphene in 2004, the family of 2D inorganic nanosheets is considered as ideal membrane materials due to their ultrathin atomic thickness and fascinating physicochemical properties. However, the intrinsically nonporous feature of 2D inorganic nanosheets hinders their potential to achieve a higher flux to some extent. Recently, 2D polymer nanosheets, originated from the regular and periodic covalent connection of the building units in 2D plane, have emerged as promising candidates for preparing ultrafast and highly selective membranes owing to their inherently tunable and ordered pore structure, light weight, and high specific surface. In this review, the synthetic methodologies (including top-down and bottom-up methods) of 2D polymer nanosheets are first introduced, followed by the summary of 2D polymer nanosheets-based membrane fabrication as well as membrane applications in the fields of gas separation, water purification, organic solvent separation, and ion exchange/transport in fuel cells and lithium-sulfur batteries. Finally, based on their current achievements, the authors' personal insights are put forward into the existing challenges and future research directions of 2D polymer nanosheets for membrane separation. The authors believe this comprehensive review on 2D polymer nanosheets-based membrane separation will definitely inspire more studies in this field.

Self-supporting covalent organic framework membranes synthesized through two different processes: solvothermal annealing and solvent vapor annealing

Rigid, freestanding covalent organic framework (COF-1) membranes have been synthesized from 1,4-benzenediboronic acid (BDBA) precursors using two different approaches: room temperature solvent-vapour annealing (SVA) and solvothermal annealing (SA). Characterization of films using Fourier-transform infrared (FTIR) spectroscopy, x-ray diffraction (XRD), and various microscopies shows that the films obtained through the two different routes vary in their retained BDBA proportion, crystal size and macroscale morphology. Gas adsorption measurements give specific surface areas of 579 ± 7 m² g^-1 and 739 ± 11 m² g^-1 respectively, suggesting that the average porosity of these films is competitive with bulk-synthesized COF-1 particles. The films have a stratified structure, with a dense, thin top layer and a thicker, sponge-like base layer. Using nanoindentation, we measured the Young's modulus at the top surface of the SVA and SA films to be 3.64 ± 1.20 GPa and 3.33 ± 0.12 GPa respectively, with the smaller uncertainty for the SA film attributed to a more uniform morphology. These measurements provide useful experimental data pertaining to COF-1 mechanical properties, furnishing information relevant to the use of these free-standing membranes in applications such as gas filtration or storage.

Light-Induced Programmable 2D Ordered Patterns Based on a Hyperbranched Poly(ether amine) (hPEA)-Functionalized Graphene Film

Dynamic complex surface topography with ordered and tunable morphologies, which can provide on-demand control of surface properties to realize smart surfaces, is gaining much attention yet remains challenging in terms of fabrication. Here, a facile, robust, and controllable method is demonstrated to fabricate programmable two-dimensional (2D) ordered patterns with multiresponsive 2D ultrathin materials, comprised of anthracene-capped hyperbranched poly(ether amine) (hPEA-AN)-functionalized graphene (hPEA-AN@G). By combining the stimuli-responsiveness and UV sensitivity of hPEA-AN and excellent out-of-plane deformation and NIR-to-thermal conversion of graphene, the process of "writing/uploading" initial information is conducted through the initial exposure to 365 nm UV light to generate the 2D ordered pattern first; second, inducing swelling strain via moisture to create the hierarchical topographic pattern (orderly oriented pattern) is the process of "modification and erasable rewriting"; third, alternating NIR or 254 nm UV light blanket exposure are the two ways of erasing the information. Consequently, taking advantage of the multiresponsive dynamic wrinkling/ordered patterning, we can program globally 2D ordered surface patterns with diverse morphologies on demand and manipulate the resulted surface properties as desired.

Beyond Frameworks: Structuring Reticular Materials across Nano-, Meso-, and Bulk Regimes

Reticular materials are of high interest for diverse applications, ranging from catalysis and separation to gas storage and drug delivery. These open, extended frameworks can be tailored to the intended application through crystal-structure design. Implementing these materials in application settings, however, requires structuring beyond their lattices, to interface the functionality at the molecular level effectively with the macroscopic world. To overcome this barrier, efforts in expressing structural control across molecular, nano-, meso-, and bulk regimes is the essential next step. In this Review, we give an overview of recent advances in using self-assembly as well as externally controlled tools to manufacture reticular materials over all the length scales. We predict that major research advances in deploying these two approaches will facilitate the use of reticular materials in addressing major needs of society.

Nanoscale covalent organic frameworks as theranostic platforms for oncotherapy: synthesis, functionalization, and applications

Cancer nanomedicine is one of the most promising domains that has emerged in the continuing search for cancer diagnosis and treatment. The rapid development of nanomaterials and nanotechnology provide a vast array of materials for use in cancer nanomedicine. Among the various nanomaterials, covalent organic frameworks (COFs) are becoming an attractive class of upstarts owing to their high crystallinity, structural regularity, inherent porosity, extensive functionality, design flexibility, and good biocompatibility. In this comprehensive review, recent developments and key achievements of COFs are provided, including their structural design, synthesis methods, nanocrystallization, and functionalization strategies. Subsequently, a systematic overview of the potential oncotherapy applications achieved till date in the fast-growing field of COFs is provided with the aim to inspire further contributions and developments to this nascent but promising field. Finally, development opportunities, critical challenges, and some personal perspectives for COF-based cancer therapeutics are presented.

Bioinspired Superwettable Covalent Organic Framework Nanofibrous Composite Membrane with a Spindle-Knotted Structure for Highly Efficient Oil/Water Emulsion Separation

Covalent organic frameworks (COFs) have attracted broad interest in a number of fields including gas access, catalysis, and ionic adsorption. However, owing to the low stability in water, the application of COFs in the field of oil/water separation is extensively impeded. In this paper, we synthesized COF-DhaTab/polyacrylonitrile (PAN) nanofibrous composite membranes with a bioinspired spindle-knotted structure via a facile blending electrospinning method. The COF-DhaTab/PAN composite membrane shows prewetting-induced superoleophobicity under water and superhydrophobicity under oil. It possesses outstanding rejection ratio (>99.9%), excellent antifouling performance, and ultrahigh oil/water mixture flux up to 4229.29 L/m²h even though driven only by gravity. Specifically, an extraordinary oil contact angle under water (152.3°) and a satisfied water contact angle under oil (153.7°) were offered by the composite membrane. These are mainly attributed to the spindle-knotted structures induced by COFs. To the best of our knowledge, the application of COF/PAN composite membrane in the field of oil/water separation has never been reported. It is an innovative approach for oily wastewater treatment and oil purification.

Multifunctional Polymer Sponge with Molecule Recognition: Facile Mechanic Induced Separation

Polymer sponges with molecular recognition provide a facile approach to water purification and industrial separation with easy operation, but its fabrication is still challenging because some critical issues of selective adsorption, high mechanical strength, and easy collection/re-use are difficult to be achieved in one material. Here, inspired by natural sponges, novel multifunctional polymer sponges were developed which were fabricated by ice-templating with multifunctional amine polyethylenimine and diepoxide cross-linker poly(ethylene glycol) diglycidyl ether for highly efficient harvesting of dyes and simultaneous pure water recovery both in mechanic pressing and filtration processes. The as-prepared sponge (SP-1) was further modified by poly(caffeic acid) through a simple dipping-cross-linking process to obtain the hybrid polymer sponge (SP-2), which showed higher compressive strength than SP-1. These sponges possessed a cross-linked three-dimensional macroporous structure with quick water absorbing properties over ten times of their own weight within 20 s directed by capillary. The adsorption behavior of the obtained polymer sponges to 11 hydrophilic dyes was studied in detail by mechanic induced separation. All these polymer sponges exhibited a high selective adsorption to hydrophilic dyes in water. For example, SP-1 has high adsorption capacity over 150 μmol/g to erythrosin B, which is 20 times higher than that of calcein. With the modified poly(caffeic acid) layer, SP-2 exhibited different adsorption properties for methylene blue (180 μmol/g) to SP-1 (∼0 μmol/g), indicating that the tailorable structures of the sponge can regulate their selectivity to guest molecules. Based on the unique recognition to guest molecules, the methodology of dynamic separation of the dye's mixture in water was demonstrated by using these sponges through mechanical pressing or fast filtration, which provides a facile alternative with easy operation for water purification.

Ambient temperature fabrication of a covalent organic framework from 1,3,5-triformylphloroglucinol and 1,4-phenylenediamine as a coating for use in open-tubular capillary electrochromatography of drugs and amino acids

A covalent organic framework (COF) named TpPa-1 was designed and synthesized at ambient temperature by an ultrasound-assisted method from 1,3,5-triformylphloroglucinol (Tp) and 1,4-phenylenediamine (Pa-1). It was utilized as a stationary phase in open-tubular capillary electrochromatography (OT-CEC). The column was coated with TpPa-1 using a covalent bonding strategy. The coated capillary was characterized by morphology, crystallography, and mesoporous analysis to confirm the successful fabrication. The OT-CEC method was utilized for the analysis of tetracyclines, sulfonamides, cephalosporins and amino acids with high-resolution (Rs > 1.81) and good precision (RSD < 4.9%). It takes about 12 h from COF preparation to OT-CEC separation. Graphical abstract A covalent organic framework (COF) named TpPa-1 was synthesized at ambient temperature by an ultrasound-assisted method from 1,3,5-triformylphloroglucinol (Tp) and 1,4-phenylenediamine (Pa-1). COF-TpPa-1 modified capillary column was utilized for the analysis of tetracyclines, sulfonamides, cephalosporins and amino acids with high-resolution and good precision.

Carbon-GO Composites with Preferential Water versus Ethanol Uptake

The elimination of small amounts of water from alcohols is by no means a trivial issue in many practical applications like, for instance, the dehumidification of biocombustibles. The use of carbonaceous materials as sorbents has been far less explored than that of other materials because their hydrophobic character has typically limited their water uptake. Herein, we designed a synthetic process based on the use of eutectic mixtures that allowed the homogeneous dispersion of graphene oxide (GO) in the liquid containing the carbon precursor, e.g., furfuryl alcohol. Thus, after polymerization and a subsequent carbonization process, we were able to obtain porous carbon-GO composites where the combination of pore diameter and surface hydrophilicity provided a remarkable capacity for water uptake but extremely low methanol and ethanol uptake along the entire range of relative pressures evaluated in this work. Both the neat water uptake and the uptake difference between water and either methanol or ethanol of our carbon-GO composites were similar or eventually better than the uptake previously reported for other materials, also exhibiting preferential water-to-alcohol adsorption, e.g., porous coordination polymers, metal-organic frameworks, polyoxometalates, and covalent two-dimensional nanosheets embedded in a polymer matrix. Moreover, water versus alcohol uptake was particularly remarkable at low partial pressures in our carbon-GO composites.

Covalent organic framework membranes through a mixed-dimensional assembly for molecular separations

Covalent organic frameworks (COFs) hold great promise in molecular separations owing to their robust, ordered and tunable porous network structures. Currently, the pore size of COFs is usually much larger than most small molecules. Meanwhile, the weak interlamellar interaction between COF nanosheets impedes the preparation of defect-free membranes. Herein, we report a series of COF membranes through a mixed-dimensional assembly of 2D COF nanosheets and 1D cellulose nanofibers (CNFs). The pore size of 0.45-1.0 nm is acquired from the sheltering effect of CNFs, rendering membranes precise molecular sieving ability, besides the multiple interactions between COFs and CNFs elevate membrane stability. Accordingly, the membranes exhibit a flux of 8.53 kg m-2 h-1 with a separation factor of 3876 for n-butanol dehydration, and high permeance of 42.8 L m-2 h-1 bar-1 with a rejection of 96.8% for Na2SO4 removal. Our mixed-dimensional design may inspire the fabrication and application of COF membranes.