Water-selective permeation in hybrid membrane incorporating multi-functional hollow ZIF-8 nanospheres

Dehydration of Organic Solvents from Ternary Mixtures Containing Toluene/Methanol/Water by Pervaporation

The separation of a toluene/methanol/water ternary mixture is a difficult task due to the toluene/water and toluene/methanol azeotropes. In this article, low-energy pervaporation is proposed for the separation of the ternary azeotrope toluene-methanol-water. This work investigates the effects of feed temperature, feed flow rate, and vacuum on pervaporation and compares the energy consumption of pervaporation with that of distillation. The results showed that at the optimized flow rate of 50 L/h and a permeate side vacuum of 60 kPa at 50 °C, the water and methanol content in the permeate was about 63.2 wt.% and 36.8 wt.%, respectively, the water/ methanol separation factor was 24.04, the permeate flux was 510.7 g/m2·h, the water content in the feed out was reduced from 2.5 wt.% to less than 0.66 wt.%, and the dehydration of toluene methanol could be realized. Without taking into account the energy consumption of pumps and other power equipment, pervaporation requires an energy consumption of 43.53 kW·h to treat 1 ton of raw material, while the energy consumption of distillation to treat 1 ton of raw material is about 261.5 kW·h. Compared to the existing distillation process, the pervaporation process consumes much less energy (about one-sixth of the energy consumption of distillation). There is almost no effect on the surface morphology and chemical composition of the membrane before and after use. The method provides an effective reference for the dehydration of organic solvents from ternary mixtures containing toluene/methanol/water.

Food freshness monitoring using poly(vinyl alcohol) and anthocyanins doped zeolitic imidazolate framework-8 multilayer films with bacterial nanocellulose beneath as support

Multilayer intelligent freshness labels based on bacterial nanocellulose (BNC), poly(vinyl alcohol) (PVA), and anthocyanins doped zeolitic imidazolate framework-8 (A-ZIF-8) nanocrystals were developed in this study. First, optical, structural, thermal, and surface characterizations of A-ZIF-8 nanocrystals were performed, and the successful incorporation of anthocyanins into ZIF-8 nanocrystals was demonstrated. Next, A-ZIF-8 was added into PVA, and multilayer films were fabricated by spin-coating PVA/A-ZIF-8 layers onto BNC. The effect of the number of deposition cycles on the barrier, mechanical, thermal, morphological, and colorimetric properties of multilayer labels was investigated. The ammonia sensing, mechanical, and barrier properties of the films were shown to be tuned by the number of the PVA/A-ZIF-8 layers on the BNC. Among the developed films, BNC-2PVA/A-ZIF-8 films with the best colorimetric sensitivity toward volatile ammonia were used to monitor the freshness of skinless chicken breasts. The changes in the ΔE and a* values of BNC-2PVA/A-ZIF-8 film demonstrated a good correlation with the microbial and TVB-N levels in samples over 10 days of storage at 4 °C.

A roadmap to enhance gas permselectivity in metal-organic framework-based mixed-matrix membranes

Performing gas separation at high efficiency with minimum energy input and reduced carbon footprint is a major challenge. While several separation methods exist at various technology readiness levels, porous membrane-based separation is considered as a disruptive technology. To attain sustainability and required efficiency, different approaches of membrane design have been explored. However, the selectivity-permeation trade-off and membrane aging have restricted further advancement. In this regard, a new generation composite made of organic polymers and metal-organic framework (MOF) fillers shows substantial promise. Organic polymer matrix allows easy processibility, but it has poor permselectivity for gas molecules. Metal-organic frameworks are excellent sieving materials; however, they suffer from poor processibility issues. A combination of these two components makes an ideal sieving membrane, which can potentially outnumber the existing energy intensive distillation strategies. In this perspective, we have discussed key indices that regulate gas permselectivity by a careful selection of the existing literature. While the target gas flux and selectivity values have been a part of many previous reviews and articles, we have presented a concise discussion on the interface design of the MOF-polymer membrane, morphology, and orientation control of MOF fillers in the matrix. Following this, a future roadmap to overcome challenges related to MOF-polymer interfacial defects is outlined.

The influence of force fields on the structure and dynamics of water confined in ZIF-8 from atomistic simulations

The complexity of modeling flexible crystals, such as ZIF-8, mainly stems from the handling of intramolecular interactions. Numerous force fields have been proposed in the literature to describe the interactions between atoms in ZIF-8. We employ seven force fields to examine the structure and dynamic behavior of water molecules confined in ZIF-8, with the aim of investigating the impact of force fields on simulation results. Various structural characterization methods consistently indicate that the choice of different force fields has quantitative effects but no qualitative effects on the structural characteristics of confined water. Additionally, the force fields do not impact the qualitative description of the diffusion mechanism. Both mean-square displacement and van Hove autocorrelation function reveal two characteristic movements of water molecules diffusing in ZIF-8: a short-time intra-cavity hopping process and a long-time inter-cavity hopping process. However, the framework flexibility is found to play a crucial role in determining the order of spatial arrangement and local structure, self-diffusion coefficient and reorientational dynamics of confined water. Specifically, the DREIDING force field gives rise to an unrealistic stiff framework, enhancing the order of spatial arrangement and diminishing the local ordered structure of confined water. Meanwhile, it results in much slower translational and reorientational dynamics. Hence, the general DREIDING force field cannot be considered for providing a quantitative description of the water structure and dynamics.

Superstructures of Zeolitic Imidazolate Frameworks to Single- and Multiatom Sites for Electrochemical Energy Conversion

The exploration of electrocatalysts with high catalytic activity and long-term stability for electrochemical energy conversion is significant yet remains challenging. Zeolitic imidazolate framework (ZIF)-derived superstructures are a source of atomic-site-containing electrocatalysts. These atomic sites anchor the guest encapsulation and self-assembly of aspheric polyhedral particles produced using microreactor fabrication. This review provides an overview of ZIF-derived superstructures by highlighting some of the key structural types, such as open carbon cages, 1D superstructures, hollow structures, and the interconversion of superstructures. The fundamentals and representative structures are outlined to demonstrate the role of superstructures in the construction of materials with atomic sites, such as single- and dual-atom materials. Then, the roles of ZIF-derived single-atom sites for the electroreduction of CO₂ and electrochemical synthesis of H₂ O₂ are discussed, and their electrochemical performance for energy conversion is outlined. Finally, the perspective on advancing single- and dual-atom electrode-based electrochemical processes with enhanced redox activity and a low-impedance charge-transfer pathway for cathodes is provided. The challenges associated with ZIF-derived superstructures for electrochemical energy conversion are discussed.

Advances in metal-organic framework-based membranes

Membrane-based separations have garnered considerable attention owing to their high energy efficiency, low capital cost, small carbon footprint, and continuous operation mode. As a class of highly porous crystalline materials with well-defined pore systems and rich chemical functionalities, metal-organic frameworks (MOFs) have demonstrated great potential as promising membrane materials over the past few years. Different types of MOF-based membranes, including polycrystalline membranes, mixed matrix membranes (MMMs), and nanosheet-based membranes, have been developed for diversified applications with remarkable separation performances. In this comprehensive review, we first discuss the general classification of membranes and outline the historical development of MOF-based membranes. Subsequently, particular attention is devoted to design strategies for MOF-based membranes, along with detailed discussions on the latest advances on these membranes for various gas and liquid separation processes. Finally, challenges and future opportunities for the industrial implementation of these membranes are identified and outlined with the intent of providing insightful guidance on the design and fabrication of high-performance membranes in the future.

Structure and dynamics of a water/methanol mixture confined in zeolitic imidazolate framework ZIF-8 from atomistic simulations

A classical atomistic simulation study is reported for the microscopic structure and dynamics of a water/methanol mixture confined in flexible nanoporous zeolitic imidazolate framework ZIF-8. Both the radial density distribution and vivid two-dimensional density profile demonstrate that methanol molecules can roughly be viewed as "embedded" between two layers of water molecules to form a "sandwich" structure. The reason for the formation of such a specific structure is explained based on the hydrogen-bonding state and the strength of various hydrogen bonds. The investigation of guest molecular diffusion shows that the self-diffusion coefficient of confined water is generally one to two orders of magnitude smaller than that of bulk water. In addition, the dependence of the self-diffusion coefficient on loading is non-monotonic: the self-diffusion coefficient firstly shows a significant increase and then decreases at higher loading. Moreover, both the structure and dynamics of the hydrogen bond (HB) network of confined water molecules are investigated in a spatially resolved manner. The results indicate that both the HB structure and dynamics of water molecules near the ZIF-8 surface deviate significantly from those of bulk water. However, while water molecules located at the pore center are relatively similar to bulk water molecules with respect to the HB structure, they exhibit strong slowdown in HB dynamics when compared with bulk water. This simulation study elucidates in detail the structural and dynamical properties of a water/methanol mixture in nanoscopic ZIF-8 confinement, which is expected to provide a deep insight into the role of porous fillers, such as ZIF-8, in improving the performance of the dehydration of alcohols via pervaporation and other related processes.

Novel Pervaporation Membranes Based on Biopolymer Sodium Alginate Modified by FeBTC for Isopropanol Dehydration

Modern society strives for the development of sustainable processes that are aimed at meeting human needs while preserving the environment. Membrane technologies satisfy all the principles of sustainability due to their advantages, such as cost-effectiveness, environmental friendliness, absence of additional reagents and ease of use compared to traditional separation methods. In the present work, novel green membranes based on sodium alginate (SA) modified by a FeBTC metal–organic framework were developed for isopropanol dehydration using a membrane process, pervaporation. Two kinds of SA-FeBTC membranes were developed: (1) untreated membranes and (2) cross-linked membranes with citric acid or phosphoric acid. The structural and physicochemical properties of the developed SA-FeBTC membranes were studied by spectroscopic techniques (FTIR and NMR), microscopic methods (SEM and AFM), thermogravimetric analysis and swelling experiments. The transport properties of developed SA-FeBTC membranes were studied in the pervaporation of water–isopropanol mixtures. Based on membrane transport properties, 15 wt % FeBTC was demonstrated to be the optimal content of the modifier in the SA matrix for the membrane performance. A membrane based on SA modified by 15 wt % FeBTC and cross-linked with citric acid possessed optimal transport properties for the pervaporation of the water–isopropanol mixture (12–100 wt % water): 174–1584 g/(m2 h) permeation flux and 99.99 wt % water content in the permeate.

Novel Membranes Based on Hydroxyethyl Cellulose/Sodium Alginate for Pervaporation Dehydration of Isopropanol

Membrane methods, especially pervaporation, are quickly growing up. In line with that, effective membrane materials based on biopolymers are required for the industrially significant mixtures separation. To essentially improve membrane transport characteristics, the application of the surface or/and bulk modifications can be carried out. In the present study, novel dense and supported membranes based on hydroxyethyl cellulose (HEC)/sodium alginate (SA) were developed for pervaporation dehydration of isopropanol using several approaches: (1) the selection of the optimal ratio of polymers, (2) the introduction of fullerenol in blend polymer matrix, (3) the selection of the optimal cross-linking agent for the membranes, (4) the application of layer-by-layer deposition of polyelectrolytes on supported membrane surface (poly(sodium 4-styrenesulfonate) (PSS)/poly(allylamine hydrochloride) (PAH) and PSS/SA). Structural and physicochemical characteristics of the membranes were analyzed by different methods. A cross-linked supported membrane based on HEC/SA/fullerenol (5%) composite possessed the following transport characteristics in pervaporation dehydration of isopropanol (12–50 wt.% water): 0.42–1.72 kg/(m²h) permeation flux, and 77.8–99.99 wt.% water content in the permeate. The surface modification of this membrane with 5 bilayers of PSS/PAH and PSS/SA resulted in the increase of permeation flux up to 0.47–3.0 and 0.46–1.9 kg/(m²h), respectively, with lower selectivity.

Modifying thin film composite membrane with zeolitic imidazolate framework-8@polydopamine for enhanced antifouling property

Biofouling and organic fouling are major obstacles for polymeric membranes during application. In this work, zeolitic imidazolate framework-8@polydopamine (ZIF-8@PDA) nanoparticles were prepared by an aqueous synthesis strategy and incorporated into the polyamide (PA) selective layer to synthesize thin film nanocomposite membrane (TFN) during interfacial polymerization. The permeability and selectivity of the composite membrane were simultaneously improved with the introduction of ZIF-8@PDA. The water permeability of the TFN membrane increased to 3.74 ± 0.19 L/(m2·h·bar), which is 43.8% higher than that of the control membrane. Besides, the rejection of TFN membrane to sodium chloride is 98.68 ± 0.13%, which shows 0.99% increment than the unmodified membrane. Moreover, organic fouling and biofouling of the TFN membrane were also alleviated thanks to the introduction of the hydrophilic ZIF-8@PDA. The short-term filtration results indicate the performance of the TFN membrane is stable during operation.

Novel Mixed Matrix Sodium Alginate-Fullerenol Membranes: Development, Characterization, and Study in Pervaporation Dehydration of Isopropanol

Novel mixed matrix dense and supported membranes based on biopolymer sodium alginate (SA) modified by fullerenol were developed. Two kinds of SA–fullerenol membranes were investigated: untreated and cross-linked by immersing the dry membranes in 1.25 wt % calcium chloride (CaCl₂) in water for 10 min. The structural and physicochemical characteristics features of the SA–fullerenol composite were investigated by Fourier-transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectroscopic methods, scanning electron (SEM) and atomic force (AFM) microscopies, thermogravimetric analysis (TGA), and swelling experiments. Transport properties were evaluated in pervaporation dehydration of isopropanol in a wide concentration range. It was found that the developed supported cross-linked SA-5/PAN^CaCl2 membrane (modified by 5 wt % fullerenol) possessed the best transport properties (the highest permeation fluxes 0.64–2.9 kg/(m² h) and separation factors 26–73,326) for the pervaporation separation of the water–isopropanol mixture in the wide concentration range (12–90 wt % water) at 22 °C and is suitable for the promising application in industry.

The Study of Zeolitic Imidazolate Framework (ZIF-8) Doped Polyvinyl Alcohol/Starch/Methyl Cellulose Blend Film

ZIF-8 nanoparticle-doped polyvinyl alcohol (PVA)-S-MC films were prepared via casting method. The effect of different concentrations of ZIF-8 on the physical properties and structural characterization of the films were investigated. The results indicated that ZIF-8 could increase the water resistance and mechanical property of the membrane. Through FTIR, scanning electron microscope (SEM), atomic force microscope (AFM), and TGA analysis, it was found that ZIF-8 changed the phenomenon of macromolecule agglomeration and improved the thermal stability of the membrane. The breathable behavior of the film was also studied through oxygen permeability and water vapor permeability analysis. The result illustrated that the breathability of the film improved significantly by adding ZIF-8. The maximum reached when the weight ratio of ZIF-8 was 0.01 wt %. The property expands the application of PVA/starch blend film in the postharvest technology of fruits and vegetables.

Progress and Perspectives on Ceramic Membranes for Solvent Recovery

With the increase in demand for commodities in the world, it is advisable to conserve resources. In the case of liquid wastes generated from pharmaceutical and petroleum industries, an unconventional solution is provided for the regeneration of solvents. However, this solvent recovery can be carried out using various efficient methods. Recently, Mixed Matrix Membranes (MMM) obtained by the addition of nanoparticles into a polymer matrix as reinforcements, or using a material with a well-defined inorganic network as a membrane like zeolite, silica based, Zeolite imidazolate frameworks (ZIFs) and Metal organic frameworks (MOFs), were explored for a solvent recovery process. These membranes possess characteristics such as high selectivity, flux and stability at various environmental conditions for the solvent recovery process. In this review, we have covered the polymer, nanocomposites, and ceramic membranes for solvent recovery through the pervaporation and organic solvent nanofiltration processes. The key challenges faced by the materials such as MOFs, zeolite, silica, zeolite and ZIFs when they are fabricated (through in situ synthesis or secondary growth process) as membranes and separation of solvents to explore for the solvent recovery process are reviewed.

Membranes for dehydration of alcohols via pervaporation

Alcohols are the essential chemicals used in a variety of pharmaceutical and chemical industries. The extreme purity of alcohols in many of such industrial applications is essential. Though distillation is one of the methods used conventionally to purify alcohols, the method consumes more energy and requires carcinogenic entertainers, making the process environmentally toxic. Alternatively, efforts have been made to focus research efforts on alcohol dehydration by the pervaporation (PV) separation technique using polymeric membranes. The present review is focused on alcohol dehydration using PV separation technique, which is the most efficient and benign method of purifying alcohols that are required in fine chemicals synthesis and developing pharmaceutical formulations. This review will discuss about the latest developments in the area of PV technique used in alcohol dehydration using a variety of novel membranes.

Hydrophilic Hollow Nanocube-Functionalized Thin Film Nanocomposite Membrane with Enhanced Nanofiltration Performance

The performance of thin film nanocomposite (TFN) membrane is significantly determined by the inherent structure and composition of the incorporated nanofillers. In this work, hydrophilic hollow nanocubes (HHNs) derived from zeolitic imidazolate framework 8 (ZIF-8) were incorporated into the polyamide layer via an interfacial polymerization approach. The HHNs with abundant hydroxyl groups on the surface were obtained by etching solid ZIF-8 using tannic acid. Benefiting from the hydrophilicity, hollow structure, and negative charge of HHNs, the outstanding nanofiltration performance of the composite membrane was achieved. With the assistance of HHNs, the permeance and Na₂SO₄ rejection of the TFN membrane increased up to 19.4 ± 0.6 L/(m²·h·bar) and 95.2 ± 1.4%, corresponding to an improvement of 190% of the permeance and 2.0% of the rejection compared to the pristine thin film composite membrane. Comparatively, the performance of TFN membranes prepared with solid ZIF-8 only shows 116% improvements of the permeance with slightly increased salt rejection. More importantly, the antifouling property of the TFN-4H membrane was also elevated. The flux recovery ratios of the TFN-4H membrane are 93.2 and 84.7% corresponding to humic acid and bovine serum albumin solutions, respectively. The results indicate that the nanofiltration performance of the composite membrane was significantly enhanced with the incorporation of HHNs.