2D-enabled membranes: materials and beyond

Adsorption-driven reverse osmosis separation of ethanol/water using zeolite nanosheets

Developing more energy-efficient and cost-effective membrane processes for the separation of ethanol and water represents a strategically important direction to facilitate the production of renewable biofuels. In this study, by employing state-of-the-art molecular simulations, the potential of zeolite nanosheets as reverse osmosis (RO) membranes in ethanol/water separation is investigated. These materials are predicted to offer unprecedentedly high fluxes and more importantly, the ethanol-to-water separation factor can be as large as approximately 800 if the structure is meticulously selected. The separation achieved herein can in fact be considered counter-intuitive as the membrane allows the larger ethanol molecules to permeate through while blocking smaller water molecules. Further investigations reveal that the observed selectivity is strongly correlated with the adsorption selectivity of the bulk materials, suggesting an adsorption-driven mechanism. Promising candidates also appear to have the largest cavity diameter of approximately 6 Å, a size that can be commensurate with the dimensions of ethanol to facilitate its adsorption. The hydrophilicity on the membrane surfaces is as well found to play a non-negligible role. Overall, this study demonstrates the great promise of zeolite nanosheets as RO membranes for extracting anhydrous ethanol from its aqueous mixture and provides guidance toward the selection of promising membrane candidates.

Methanol recovery: potential of nanolaminate organic solvent nanofiltration (OSN) membranes

Researchers have made a significant breakthrough by merging the energy-saving attribute of organic solvent nanofiltration (OSN) with the remarkable solvent permeance and solute rejection of two-dimensional (2D) laminated membranes. This innovative approach brings forth a new era of sustainable and cost-effective separation techniques, presenting a promising solution to the issue of industrial solvents contaminating the environment. This development paves the way for new opportunities in building a sustainable future. Specifically, our mini-review has cast a spotlight on the separation and recovery of methanol-a solvent abundantly used in industrial processes. We systematically evaluated a diverse array of free-standing 2D nanolaminate OSN membranes. The analysis encompasses the assessment of pure methanol permeance, solute rejection capabilities, and the simultaneous evaluation of methanol permeance and solute rejection performance. Notably, this study sheds light on the considerable potential of 2D laminated OSN membranes in revolutionizing separation processes for the industrial use of methanol.

Can MXene be the Effective Nanomaterial Family for the Membrane and Adsorption Technologies to Reach a Sustainable Green World?

Environmental pollution has intensified and accelerated due to a steady increase in the number of industries, and exploring methods to remove hazardous contaminants, which can be typically divided into inorganic and organic compounds, have become inevitable. Therefore, the development of efficacious technology for the separation processes is of paramount importance to ensure the environmental remediation. Membrane and adsorption technologies garnered attention, especially with the use of novel and high performing nanomaterials, which provide a target-specific solution. Specifically, widespread use of MXene nanomaterials in membrane and adsorption technologies has emerged due to their intriguing characteristics, combined with outstanding separation performance. In this review, we demonstrated the intrinsic properties of the MXene family for several separation applications, namely, gas separation, solvent dehydration, dye removal, separation of oil-in-water emulsions, heavy metal ion removal, removal of radionuclides, desalination, and other prominent separation applications. We highlighted the recent advancements used to tune separation potential of the MXene family such as the manipulation of surface chemistry, delamination or intercalation methods, and fabrication of composite or nanocomposite materials. Moreover, we focused on the aspects of stability, fouling, regenerability, and swelling, which deserve special attention when the MXene family is implemented in membrane and adsorption-based separation applications.

Chemical vapor deposition of 2D materials: A review of modeling, simulation, and machine learning studies

Chemical vapor deposition (CVD) is extensively used to produce large-area two-dimensional (2D) materials. Current research is aimed at understanding mechanisms underlying the nucleation and growth of various 2D materials, such as graphene, hexagonal boron nitride (hBN), and transition metal dichalcogenides (e.g., MoS₂/WSe₂). Herein, we survey the vast literature regarding modeling and simulation of the CVD growth of 2D materials and their heterostructures. We also focus on newer materials, such as silicene, phosphorene, and borophene. We discuss how density functional theory, kinetic Monte Carlo, and reactive molecular dynamics simulations can shed light on the thermodynamics and kinetics of vapor-phase synthesis. We explain how machine learning can be used to develop insights into growth mechanisms and outcomes, as well as outline the open knowledge gaps in the literature. Our work provides consolidated theoretical insights into the CVD growth of 2D materials and presents opportunities for further understanding and improving such processes

Advances and challenges in the development of nanosheet membranes

The development of highly efficient separation membranes utilizing emerging materials with controllable pore size and minimized thickness could greatly enhance the broad applications of membrane-based technologies. Having this perspective, many studies on the incorporation of nanosheets in membrane fabrication have been conducted, and strong interest in this area has grown over the past decade. This article reviews the development of nanosheet membranes focusing on two-dimensional materials as a continuous phase, due to their promising properties, such as atomic or nanoscale thickness and large lateral dimensions, to achieve improved performance compared to their discontinuous counterparts. Material characteristics and strategies to process nanosheet materials into separation membranes are reviewed, followed by discussions on the membrane performances in diverse applications. The review concludes with a discussion of remaining challenges and future outlook for nanosheet membrane technologies.

Self-healable and flexible supramolecular gelatin/MoS2 hydrogels with molecular recognition properties

Two-dimensional MoS₂ is emerging as a unique platform for a wide range of biomedical applications including extracellular matrix mimics, drug delivery systems and antimicrobial agents. However, low processability and nonspecific interactions at biointerfaces are serious challenges that hamper the biomedical applications of this nanomaterial. Herein, we show how specific interactions between MoS₂ and a gelatin matrix results in a biomimetic hydrogel with the self-healing and molecular recognition properties. β-Cyclodextrin was conjugated to the surface of freshly exfoliated MoS₂ through a one pot nucleophilic substitution reaction and the obtained cyclodextrin-functionalized MoS₂ was used to construct an injectable, self-healable and flexible supramolecular hydrogel upon host-guest interactions with adamantane-modified gelatin matrix. Incorporation of almost 1 wt% of CDMoS₂ into gelatin matrix with 1cm² cross-section resulted in a hydrogel that was able to tolerate one hundred grams. Also, storage modulus (G'), loss modulus (G″) of the obtained hydrogel was 10 and 25 times higher than that for the neat gelatin, respectively. Due to its self-healing, molecular recognition and mechanical properties as well as its flexibility, injectability, and processability, MoS₂gel is a promising candidate for a wide range of future biomedical applications including extracellular matrix mimics and tissue engineering.

Enantioselective Mixed Matrix Membranes for Chiral Resolution

Most pharmaceuticals are stereoisomers that each enantiomer shows dramatically different biological activity. Therefore, the production of optically pure chemicals through sustainable and energy-efficient technology is one of the main objectives in the pharmaceutical industry. Membrane-based separation is a continuous process performed on a large scale that uses far less energy than the conventional thermal separation process. Enantioselective polymer membranes have been developed for chiral resolution of pharmaceuticals; however, it is difficult to generate sufficient enantiomeric excess (ee) with conventional polymers. This article describes a chiral resolution strategy using a composite structure of mixed matrix membrane that employs chiral fillers. We discuss several enantioselective fillers, including metal-organic frameworks (MOFs), covalent organic frameworks (COFs), zeolites, porous organic cages (POCs), and their potential use as chiral fillers in mixed matrix membranes. State-of-the-art enantioselective mixed matrix membranes (MMMs) and the future design consideration for highly efficient enantioselective MMMs are discussed.

Graphene and Graphene-Like Materials for Hydrogen Energy

The review is devoted to current and promising areas of application of graphene and materials based on it for generating environmentally friendly hydrogen energy. Analysis of the results of theoretical and experimental studies of hydrogen accumulation in graphene materials confirms the possibility of creating on their basis systems for reversible hydrogen storage, which combine high capacity, stability, and the possibility of rapid hydrogen evolution under conditions acceptable for practical use. Recent advances in the development of chemically and heat-resistant graphene-based membrane materials make it possible to create new gas separation membranes that provide high permeability and selectivity and are promising for hydrogen purification in processes of its production from natural gas. The characteristics of polymer membranes that are currently used in industry for the most part can be significantly improved with small additions of graphene materials. The use of graphene-like materials as a support of nanoparticles or as functional additives in the composition of the electrocatalytic layer in polymer electrolyte membrane fuel cells makes it possible to improve their characteristics and to increase the activity and stability of the electrocatalyst in the reaction of oxygen evolution.

MXene Materials for Designing Advanced Separation Membranes

MXenes are emerging rapidly as a new family of multifunctional nanomaterials with prospective applications rivaling that of graphenes. Herein, a timely account of the design and performance evaluation of MXene-based membranes is provided. First, the preparation and physicochemical characteristics of MXenes are outlined, with a focus on exfoliation, dispersion stability, and processability, which are crucial factors for membrane fabrication. Then, different formats of MXene-based membranes in the literature are introduced, comprising pristine or intercalated nanolaminates and polymer-based nanocomposites. Next, the major membrane processes so far pursued by MXenes are evaluated, covering gas separation, wastewater treatment, desalination, and organic solvent purification. The potential utility of MXenes in phase inversion and interfacial polymerization, as well as layer-by-layer assembly for the preparation of nanocomposite membranes, is also critically discussed. Looking forward, exploiting the high electrical conductivity and catalytic activity of certain MXenes is put into perspective for niche applications that are not easily achievable by other nanomaterials. Furthermore, the benefits of simulation/modeling approaches for designing MXene-based membranes are exemplified. Overall, critical insights are provided for materials science and membrane communities to navigate better while exploring the potential of MXenes for developing advanced separation membranes.