High-performance UiO-66-NH2 tubular membranes by zirconia-induced synthesis for desulfurization of model gasoline via pervaporation

Versatile Gas-Transfer Membrane in Water and Wastewater Treatment: Principles, Opportunities, and Challenges

Technologies using liquid-transfer membranes, such as microfiltration, ultrafiltration, and reverse osmosis, have been widely applied in water and wastewater treatment. In the last few decades, gas-transfer membranes have been introduced in various fields to facilitate mass transfer, in which gaseous compounds permeate through membrane pores driven by gradients in chemical concentration or potential. A notable knowledge gap exists among researchers working on these emerging gas-transfer membranes as they approach this subject from different angles and areas of expertise (e.g., material science versus microbiology). This review explores the versatile applications of gas-transfer membranes in water and wastewater treatment, categorizing them into three primary types according to the function of membranes: water vapor transferring, gaseous reactant supplying, and gaseous compound extraction. For each type, the principles, evolution, and potential for further development were elaborated. Moreover, this review highlights the potential knowledge transfer between different fields, as insights from one type of gas-transfer membrane could potentially benefit another. Despite their technical innovations, these processes still face challenges in practical operation, such as membrane fouling and wetting. We advocate for research focusing on more practical and sustainable membranes and careful consideration of these emerging membrane technologies in specific scenarios. The current practicality and maturity of these emerging processes in water and wastewater treatment are described by the Technology Readiness Level (TRL) framework. Particularly, ongoing fundamental progress in membranes and engineering is expected to continue fueling the future development of these technologies.

UiO-66 Metal-Organic Framework Membranes: Structural Engineering for Separation Applications

Metal-organic frameworks (MOFs) have been recognized as promising materials for membrane-based separation technologies due to their exceptional porosity, structural tunability, and chemical stability. This review presents a comprehensive discussion of the advancements in structure engineering and design strategies that have been employed to optimize UiO-66 membranes for enhanced separation performance. Various synthesis methods for UiO-66 membranes are explored, with a focus on modulated approaches that incorporate different modulators to fine-tune nucleation rates and crystallization processes. The influence of preferred orientation, membrane thickness, pore size, pore surface chemistry, and hierarchical structures on the separation performance is concluded. By providing a consolidated overview of current research efforts and future directions in UiO-66 membrane development, this review aims to inspire further advancements in the field of separation technologies.

Fabrication Methods of Continuous Pure Metal-Organic Framework Membranes and Films: A Review

Metal-organic frameworks (MOFs) have drawn intensive attention as a class of highly porous, crystalline materials with significant potential in various applications due to their tunable porosity, large internal surface areas, and high crystallinity. This paper comprehensively reviews the fabrication methods of pure MOF membranes and films, including in situ solvothermal synthesis, secondary growth, electrochemical deposition, counter diffusion growth, liquid phase epitaxy and solvent-free synthesis in the category of different MOF families with specific metal species, including Zn-based, Cu-based, Zr-based, Al-based, Ni-based, and Ti-based MOFs.

Enhanced Desulfurization Performance of ZIF-8/PEG MMMs: Effect of ZIF-8 Particle Size

Constructing efficient and continuous transport pathways in membranes is a promising and challenging way to achieve the desired performance in the pervaporation process. The incorporation of various metal-organic frameworks (MOFs) into polymer membranes provided selective and fast transport channels and enhanced the separation performance of polymeric membranes. Particle size and surface properties are strongly related to the random distribution and possible agglomeration of MOFs particles, which may lead to poor connectivity between adjacent MOFs-based nanoparticles and result in low-efficiency molecular transport in the membrane. In this work, ZIF-8 particles with different particle sizes were physically filled into PEG to fabricate mixed matrix membranes (MMMs) for desulfurization via pervaporation. The micro-structures and physi-/chemical properties of different ZIF-8 particles, along with their corresponding MMMs, were systematically characterized by SEM, FT-IR, XRD, BET, etc. It was found that ZIF-8 with different particle sizes showed similar crystalline structures and surface areas, while larger ZIF-8 particles possessed more micro-pores and fewer meso-/macro-pores than did the smaller particles. ZIF-8 showed preferential adsorption for thiophene rather than n-heptane molecules, and the diffusion coefficient of thiophene was larger than that of thiophene in ZIF-8, based on molecular simulation. PEG MMMs with larger ZIF-8 particles showed a higher sulfur enrichment factor, but a lower permeation flux than that found with smaller particles. This might be ascribed to the fact that larger ZIF-8 particles provided more and longer selective transport channels in one single particle. Moreover, the number of ZIF-8-L particles in MMMs was smaller than the number of smaller ones with the same particle loading, which might weaken the connectivity between adjacent ZIF-8-L nanoparticles and result in low-efficiency molecular transport in the membrane. Moreover, the surface area available for mass transport was smaller for MMMs with ZIF-8-L particles due to the smaller specific surface area of the ZIF-8-L particles, which might also result in lower permeability in ZIF-8-L/PEG MMMs. The ZIF-8-L/PEG MMMs exhibited enhanced pervaporation performance, with a sulfur enrichment factor of 22.5 and a permeation flux of 183.2 g/(m^-2·h^-1), increasing by 57% and 389% compared with the results for pure PEG membrane, respectively. The effects of ZIF-8 loading, feed temperature, and concentration on desulfurization performance were also studied. This work might provide some new insights into the effect of particle size on desulfurization performance and the transport mechanism in MMMs.

Recent Advances in Continuous MOF Membranes for Gas Separation and Pervaporation

Metal-organic frameworks (MOFs), a sub-group of porous crystalline materials, have been receiving increasing attention for gas separation and pervaporation because of their high thermal and chemical stability, narrow window sizes, as well as tuneable structural, physical, and chemical properties. In this review, we comprehensively discuss developments in the formation of continuous MOF membranes for gas separation and pervaporation. Additionally, the application performance of continuous MOF membranes in gas separation and pervaporation are analysed. Lastly, some perspectives for the future application of continuous MOF membranes for gas separation and pervaporation are given.

Effect of MoS2 Yolk-Shell Nanostructure on the Thiophene Separation Performance of PEG Membrane

Constructing facilitated transport based on π-complexation has been drawing more and more attention in mixed matrix membranes (MMMs) for pervaporative desulfurization. Herein, a unique molybdenum disulfide (MoS2) yolk-shell nanostructure (MYNS) was prepared and incorporated into the polyethylene glycol (PEG) matrix to fabricate MMMs for model gasoline desulfurization by PV. Moreover, the effects of MYNS content, feed sulfur concentration, and feed temperature on the performance of PEG/MYNS MMMs were evaluated. It was found that there is good interfacial compatibility between the MYNS filler and the PEG matrix, and the resultant MMMs show enhanced swelling resistance against thiophene. The PV results revealed that the as-fabricated MMMs are thiophene-selective, and their desulfurization performance in the pervaporative removal of thiophene from n-octane is remarkably evaluated due to the addition of MYNS. The MMMs display the highest sulfur enrichment factor of 4.02 with an associated permeation flux of 2587 g·m−2·h−1 with the MYNS loading of 3 wt. % when carrying out in an n-octane and thiophene (500 μg·g−1) mixture at 343 K. Furthermore, a consistent increment in the permeation flux accompanied with a continuous reduction in the enrichment factor was observed with increasing the feed sulfur concentration and feed temperature. This work may offer great potential for practical gasoline desulfurization applications.

Process Optimization for Catalytic Oxidation of Dibenzothiophene over UiO-66-NH2 by Using a Response Surface Methodology

This research investigates the catalytic performance of a metal-organic framework (MOF) with a functionalized ligand-UiO-66-NH₂-in the oxidative desulfurization of dibenzothiophene (DBT) in n-dodecane as a model fuel mixture (MFM). The solvothermally prepared catalyst was characterized by XRD, FTIR, ¹H NMR, SEM, TGA, and MP-AES analyses. A response surface methodology was employed for the experiment design and variable optimization using central composite design (CCD). The effects of reaction conditions on DBT removal efficiency, including temperature (X ₁), oxidant agent over sulfur (O/S) mass ratio (X ₂), and catalyst over sulfur (C/S) mass ratio (X ₃), were assessed. Optimal process conditions for sulfur removal were obtained when the temperature, O/S mass ratio, and C/S mass ratio were 72.6 °C, 1.62 mg/mg, and 12.1 mg/mg, respectively. Under these conditions, 89.7% of DBT was removed from the reaction mixture with a composite desirability score of 0.938. From the results, the temperature has the most significant effect on the oxidative desulfurization reaction. The model F values gave evidence that the quadratic model was well-fitted. The reusability of the MOF catalyst in the ODS reaction was tested and demonstrated a gradual loss of activity over four runs.

Inorganic Pillar Center-Facilitated Counterdiffusion Synthesis for Highly H2 Perm-Selective KAUST-7 Membranes

Fluorinated metal-organic framework materials (NbOFFIVE-1-Ni, also referred to as KAUST-7) have attracted widespread attention because of their high chemical stability and thermal stability, outstanding tolerance with water and H₂S, and high CO₂-adsorption selectivity over H₂ and CH₄. KAUST-7 was expected to be a new membrane material candidate for H₂/CO₂ separation because of the hindered permeation of CO₂ resulting from the interaction between CO₂ and (NbOF₅)^2- of the KAUST-7 framework. A highly H₂ perm-selective KAUST-7 membrane was first achieved using a novel strategy of inorganic pillar center-facilitated counterdiffusion (IPCFCD) proposed by us. The IPCFCD method not only effectively avoided the corrosion of hydrofluoric acid to α-Al₂O₃ tubes in the process of preparing KAUST-7 membranes, but also better reduced grain boundary defects because of the faster nucleation rate and resultant high crystallinity. The KAUST-7 membrane exhibited a high H₂/CO₂ separation factor (SF) of 27.30 for the 1:1 H₂/CO₂ binary gas mixture with a high H₂ permeance of 5.30 × 10^-7 mol m^-2 s^-1 Pa^-1 under ambient conditions and a slight decrease of the H₂/CO₂ SF with increasing operation temperature and presence of steam. This study highlighted the importance of pre-synthesizing inorganic pillar centers (NiNbOF₅ intermediate) and the innovation of a membrane formation process for synthesizing polycrystalline KAUST-7 membranes. Most important of all, our study provided a novel approach to overcome the challenge in fabricating metal-organic framework membranes containing corrosive reactants for the corresponding supports.

Adsorption desulfurization performance of PdO/SiO2@graphene oxide hybrid aerogel: Influence of graphene oxide

Preparation of PdO/SiO₂@graphene oxide (GO) hybrid aerogels were carried out sol-gel method combined with atmospheric drying technology to study their adsorption performance for thiophenics and compared with PdO/SiO₂. Scanning electron microscope (SEM), N₂ adsorption-desorption isotherms, X-ray diffraction (XRD) analysis, X-ray photoelectron spectroscopy (XPS), and fourier transformation infrared spectroscopy (FT-IR) for samples were performed. The adsorption performance of PdO/SiO₂@GO for thiophene were better than that of PdO/SiO₂, attributed to that incorporation of GO increased the specific surface area and the Pd incorporation rate, where Pd²⁺ ions acted as the π-complexation and sulfur-metal (SM) bond adsorption active centers, as well as GO adsorbed thiophene by the π-π stacking effect. The adsorption capacities of PdO/SiO₂@GO-1.0 for thiophene (TH), benzothiophene (BT) and dibenzothiophene (DBT) were 8.89, 9.3 and 12.6 mg-S/g_ads, respectively. The addition of GO in aerogels could improve the inhibition effect of toluene, cyclohexene and pyridine while decreased the inhibition effect of MTBE and H₂O for the adsorption of thiophene, due to the π-π stacking effect and the hydrophobicity of GO, respectively. The adsorption process was spontaneous and exothermic, be well fitted by the apparent second-order kinetic model and dominated by chemical interaction. Pd/SiO₂@GO-1.0 had a good solvent elution regeneration performance.

An amino functionalized zirconium metal organic framework as a catalyst for oxidative desulfurization

The excellent activity of UiO-66-NH2 may be attributed to the synergistic effect of ZrIV–OH and –NH2.

Stable Zr-Based Metal-Organic Framework Nanoporous Membrane for Efficient Desalination of Hypersaline Water

Treatment of hypersaline waters is a critical environmental challenge. Pervaporation (PV) desalination is a promising technique to address this challenge, but current PV membranes still suffer from challenging issues such as low flux and insufficient stability. Herein, we propose in situ nanoseeding followed by a secondary growth strategy to fabricate a high-quality stable metal-organic framework (MOF) thin membrane (UiO-66) for high-performance pervaporation desalination of hypersaline waters. To address the issue of membrane quality, a TiO₂ nano-interlayer was introduced on coarse mullite substrates to favor the growth of a UiO-66 nanoseed layer, on which a well-intergrown UiO-66 selective membrane layer with thickness as low as 1 μm was finally produced via subsequent secondary growth. The PV separation performance for hypersaline waters was systematically investigated at different salt concentrations, feed temperatures, and long-term operation in different extreme chemical environments. Besides having nearly complete rejection (99.9%), the UiO-66 membrane exhibited high flux (37.4 L·m^-2·h^-1) for hypersaline waters, outperforming current existing zeolite and MOF membranes. The membrane also demonstrated superior long-term operational stability under various harsh environments (hypersaline, hot, and acidic/alkaline feed water) and mild fouling behavior. The rational design proposed in this study is not only applicable for the development of a high-quality UiO-66 membrane enabling harsh hypersaline water treatment but can also be potentially extended to other next-generation nanoporous MOF membranes for more environmental applications.

Facile In Situ Hydrothermal Synthesis of Layered Zirconium Phenylphosphonate Molecular Sieve Membranes with Optimized Microstructure and Superb H2/CO2 Selectivity

Layered molecular sieve membranes containing uniform interlayer galleries have offered unprecedented opportunities to reach a performance far beyond the Robeson upper bound line. In this study, we took the initiative to prepare layered zirconium phenylphosphonate (ZrPP) molecular sieve membranes with optimized microstructure on tetragonal zirconia (t-ZrO₂) buffer layer-modified porous α-Al₂O₃ substrates by facile in situ hydrothermal growth. Relying on the 3.2 Å-sized gallery height and preferential CO₂ adsorption behavior, prepared ZrPP membranes showed exceptional H₂/CO₂ selectivity (>100) as well as considerable H₂ permeability. Furthermore, extraordinary thermal, mechanical, and chemical stability of ZrPP membranes made them potentially attractive for long-term operations under harsh conditions.

Concurrent Manipulation of Out-of-Plane and Regional In-Plane Orientations of NH2-UiO-66 Membranes with Significantly Reduced Anisotropic Grain Boundary and Superior H2/CO2 Separation Performance

Preferred orientation has proven to exert a significant impact on the gas separation performance of metal-organic framework membranes. Nevertheless, realizing three-dimensional orientation control remains a challenging issue. In this study, well-intergrown NH₂-UiO-66 membranes with both (111) out-of-plane and regional in-plane orientations were prepared by combining oriented deposition of seeds and solvothermal epitaxial growth. Dynamic air-liquid interface-assisted self-assembly method was employed to organize uniform octahedral-shaped NH₂-UiO-66 seeds into closely packed monolayers with (111) out-of-plane and regional in-plane orientations, whereas the use of ZrS₂ as the zirconium precursor during the solvothermal epitaxial growth was found indispensible for sealing the intercrystalline gaps while preserving the preferred orientation inherited from seed layers. In addition, compared with solvothermal heating, employing microwave heating led to poor intergrowth between neighboring NH₂-UiO-66 crystals because of a lower dielectric loss factor of the reaction medium. Gas permeation results indicated that the prepared NH₂-UiO-66 membranes exhibited H₂/CO₂ selectivity up to 5.5 times higher than their counterparts with random and/or mere out-of-plane orientations as well as H₂ permeability 14.5 times higher than NH₂-MIL-125(Ti) membranes with mere out-of-plane orientation under similar operating conditions.

Metal-organic framework UiO-66 membranes

Metal-organic frameworks (MOFs) have emerged as a class of promising membrane materials. UiO-66 is a prototypical and stable MOF material with a number of analogues. In this article, we review five approaches for fabricating UiO-66 polycrystalline membranes including in situ synthesis, secondary synthesis, biphase synthesis, gas-phase deposition and electrochemical deposition, as well as their applications in gas separation, pervaporation, nanofiltration and ion separation. On this basis, we propose possible methods for scalable synthesis of UiO-66 membranes and their potential separation applications in the future.

Emerging porous materials in confined spaces: from chromatographic applications to flow chemistry

Porous materials confined within capillary columns/microfluidic devices are discussed, and progress in chromatographic and membrane separations and catalysis is reviewed.