Zeolite Layer Research Articles

Направления совершенствования сероочистки, осушки и переработки природного газа

The results of the work of the staff of the Department of “Chemical Technology of Oil and Gas Refining” of Astrakhan State Technical University in the field of separation of three-phase reservoir mixture, increasing the efficiency of desulfurization, improving the process of drying gas and the production of continuous hydrocarbons – raw materials for petrochemistry. The influence of the quality of raw materials entering processing on the formation of deposits in technological equipment has been studied. Reconstruction of the inlet and three-phase separators of high-pressure reservoir mixture separation plants by installing a vibration damper and a depulsor on the reservoir mixture supply line to the installation, as well as reconstruction of the input of raw materials into the separator of the installation. The use of the KPG-200 defoamer and the KPG-200 AV antifoamer made it possible to reduce the consumption rate of defoaming reagents several times in gas purification plants from acidic components. The influence of various impurities on the foaming of working amine solutions (mechanical impurities, coal dust, products of thermal destruction of amine, thermostable salts, corrosion inhibitors) has been determined. It was revealed that the products of thermal degradation of amine have the greatest effect at concentrations above 3% by weight. A method for reducing the content of mechanical impurities in a solution of diethanolamine in a constant magnetic field has been developed and introduced into an industrial desulfurization plant. The filtration efficiency increases 2.6 times, the foam height is halved, and the foam stability is quadrupled. At the same time, the activity of the defoamer increases by 1.7 times. The mathematical dependence of the height of the protective layer of aluminum oxide in an adsorber with zeolite on impurities of diethanolamine has been experimentally found. The effectiveness of the developed improved distribution ring device is shown, the use of which makes it possible to practically eliminate dead zones in the zeolite layer and thereby increase the efficiency of the adsorption process and increase the inter-regeneration period on the drying unit. Studies of the catalytic pyrolysis of the butane fraction using CVM-type pentasils modified with the introduction of chromium can reduce the process temperature compared with the thermal decomposition of raw materials by more than one hundred degrees 100 °C.

Passivation and depassivation of reinforcement steel in alkali-activated materials—A review

This paper reviews the formation and breakdown of passive film on the surface of reinforcement steel in alkali-activated materials (AAMs) considering the characteristics of reaction product and pore solution chemistry. The literature review shows that the pore solution of AAMs has higher concentrations of OH−, Na+, silica and aluminium compared to Portland cement (PC). This relatively high alkalinity contributes to the generation of a passive film, which has positive effects on the corrosion resistance of reinforcement steel embedded in AAMs. The silica-aluminium zeolite layer present on the surface of passive film adsorbs chloride ions and effectively inhibits chloride-induced depassivation. Generally, lower ratios of [Cl−]/[SO42−] and [Cl−]/[S2O32−] potentially inhibit the depassivation of reinforcement steel. The high pH value and the elevated concentrations of HS− of AAMs contribute to the increase of critical chloride content (Ccrit). However, the higher content of reduced sulfide mainly dissolved from slag results in the consumption of dissolved oxygen, which is necessary for the formation of passive film. Generally, the presence of reduced sulfide forms Fe-S complexes on the surface of reinforcement steel in AAMs. Even so, higher corrosion resistance for AAMs is mostly expected with longer depassivation time due to finer microstructure and lower chloride penetration rates compared to PC-based materials. The decrease in pH value in the pore solution of the AAMs is the main factor affecting carbonation-induced depassivation of reinforcement steel, while high concentrations of bicarbonate and carbonate ions inhibit depassivation.

Efficient and sustained inhibition of ammonia nitrogen release from sediment in water by microbial self-aggregation zeolite layer.

Despite global efforts to manage water eutrophication, the continual release of ammonia nitrogen from sediments maintains the eutrophic state of water bodies, presenting serious challenges to the management. In order to find an efficient method for sediment remediation, the experiment of using signal molecules to enhance the adhesion of microorganisms on zeolite was carried out. Five different zeolitic ammonium adsorptions were examined using two different signal molecules, N-(3-oxohexanoyl)-L-homoserine lactone (OHHL) and N-(β-ketocaproyl)-DL-homoserine lactone (C6), to enhance microbial attachment on two types of zeolites. The results showed that the modified microbial attached Z1 zeolite reinforced with signal molecule C6 had the best effect. The effect was better in the case of high ammonium adsorption, and the TN removal could reach 7.99mg·L-1 with an inhibition rate of 90.08%. The ammonia nitrogen removal reached 4.75mg·L-1 with an inhibition rate of 87.64%, and the ammonia nitrogen and total nitrogen of the overlying water reached the surface III water quality standard. In addition, the addition of the signal molecule increased the zeta potential on the surface of the bacterial colloid. In addition, the amount of protein I in the dissolved organic matter (DOM) fraction increased, improving microbial adhesion ability and facilitating their attachment to the zeolite surface. The signal molecule C6 could increase the zeta potential of microbial surface and promote the production of protein I, thus strengthening the attachment of zeolite biofilm and improving the water quality.

Recent Trends in Tailoring External Acidity in Zeolites for Catalysis.

Zeolites are crystalline aluminosilicates with well-defined microporous structures that have found several applications in catalysis. In recent years, great effort has been devoted to defining strategies aimed at tuning structural and acidity properties to improve the catalytic performance of zeolites. Depending on the zeolitic structure, the acid sites located inside the crystals catalyze reactions by exploiting the internal channel shape-selectivity. In contrast, strong acid sites located on the external surface do not offer the possibility to control the size of molecules involved in the reactions. This aspect generally leads to a loss of selectivity toward desired products and to the uncontrolled production of coke. Passivating surface acidity is a promising way to overcome deactivation issues and to enhance the catalytic performance of zeolites. This Mini-Review aims to provide, for the first time, a complete overview of the techniques employed in recent years to neutralize strong external acid sites. Both chemical and liquid vapor deposition of silicates have been widely employed to passivate the external surface acidity of zeolites. In recent years, the epitaxial growth of layers of aluminum-free zeolite, e.g., silicalite-1, over the surface of the acidic zeolite has been proposed as a new approach to neutralize strong external acid sites controlling diffusional phenomena. NH3-TPD, FT-IR, SEM-EDX, and other techniques have been used to provide information about the level of control of the external strong acidity of passivated zeolites. In this Mini-Review, both passivation treatments and characterization techniques are compared and advantages and disadvantages deeply discussed to elucidate the effect of passivation procedures on physical features and especially the catalytic behavior.

Cation-Loaded Porous Mg2+-Zeolite Layer Direct Dendrite-Free Deposition toward Long-Life Lithium Metal Anodes.

Lithium metal, with ultrahigh theoretical specific capacity, is considered as an ideal anode material for the lithium-ion batteries. However, its practical application is severely plagued by the uncontrolled formation of dendritic Li. Here, a cation-loaded porous Mg2+-Zeolite layer is proposed to enable the dendrite-free deposition on the surface of Li metal anode. The skeleton channels of zeolite provide the low coordinated Li+-solvation groups, leading to the faster desolvation process at the interface. Meanwhile, anions-involved solvation sheath induces a stable, inorganic-rich SEI, contributing to the uniform Li+ flux through the interface. Furthermore, the co-deposition of sustained release Mg2+ realizes a new faster migration pathway, which proactively facilitates the uniform diffusion of Li on the lithium substrate. The synergistic modulation of these kinetic processes facilitates the homogeneous Li plating/stripping behavior. Based on this synergistic mechanism, the high-efficiency deposition with cyclic longevity exceeding 2100h is observed in the symmetric Li/Li cell with Mg2+-Zeolite modified anode at 1mA cm-2. The pouch cell matched with LiFePO4 cathode fulfills a capacity retention of 88.4% after 100 cycles at a severe current density of 1 C charge/discharge. This synergistic protective mechanism can give new guidance for realizing the safe and high-performance Li metal batteries.

Development and investigation of a multilayer PDMS/zeolite composite membrane for CO2 separation applications

This work aims to investigate and develop a concept for CO2 separation based on PDMS/SSZ–13 (high-Si CHA) composite membranes. Firstly, an Al2O3 support was coated with SSZ-13 zeolite layer, then to increase the performance of the membrane, a Polydimethylsiloxane layer was effectively deposited homogeneously to cover the cracks and defects of the zeolite layer. The results of single gas permeation measurements displayed a notable increase in ideal CO2/CH4 selectivity up to 428 at the lowest PDMS concentration of 0.5 vol%. Moreover, a coating time of 10 min preserved CO2 permeance while increasing CO2/CH4 selectivity from 5 to 120. A twentyfold increase in CO2/CH4 selectivity was noted with mixed gas permeation without a reduction in CO2 permeance. Quantitative and qualitative microscopic analyses were also carried out on the coated membrane to better understand its morphology and microstructure. The outcomes indicated that PDMS layer decreased the permeation of N2 and CH4 and enhanced surface affinity toward CO2, resulting in reduced defects in the zeolite membranes and enhanced selectivity for CO2/N2 and CO2/CH4.

Preparation and surface hydrophilic modification of MOR zeolite membrane potentially appling for acetic acid aldol condensation reaction system

High-performance MOR zeolite membranes were prepared by loading hydrophilic metal oxides onto the surface of α-Al2O3 support via secondary hydrothermal synthesis method. The effects on the surface morphology and hydrophilicity of MOR zeolite membranes were discussed by means of different metal oxides (TiO2, V2O5, CeO2, ZrO2, WO3) and loadings in combination with SEM, XRD, Raman and Water Contact Angle (WCA) characterization. TiO2-MOR zeolite composite membranes were found that TiO2 could increase interface hydrophilicity. The pervaporation performances of TiO2-MOR zeolite membranes at different temperatures and feed concentrations were investigated. The mass transfer model combining Maxwell-Stefan mechanism, Kundsen diffusion and viscous flow was used to describe the mass transfer process of water molecules through the zeolite layer, and within the α-Al2O3 scaffolds, respectively. The permeation fluxes of water molecules of TiO2-MOR zeolite membranes with the loading of 2 wt% were 0.91 kg m−2 h−1, 1.00 kg m−2 h−1 and the separation factors were up to 10000 at 348 K with 90 wt% AcOH/H2O and 90 wt% AA/H2O solutions. TiO2-MOR zeolite membranes with well acid resistance and high hydrophilicity had promising prospects in the reaction scene where carboxylic acid and water exist simultaneously, such as esterification, ester hydrolysis, acetic acid aldol condensation, etc.

Pore size engineering of cost-effective all-nanoporous multilayer membranes for propane/propylene separation

In this study, a new multi-layer hybrid nanocomposite membrane named MFI/GO/ZIF-8 has been synthesized. This membrane combines three nanoporous materials with different morphologies in one membrane without using polymer materials. This allows access to a previously accessible region of very high permeability and selectivity properties. In addition to introducing a new and efficient MFI/GO/ZIF-8 membrane in this work, controlling the pore size of the zeolite layer has been investigated to increase the selectivity and permeability of propylene. The membrane was made using a solvent-free hydrothermal method and a layer-by-layer deposition method. To control the pore size of the MFI layer, a two-step synthesis strategy has been implemented. In the first step, three key parameters, including crystallization time, NaOH concentration and aging time of initial suspension, are controlled. In the second step, the effect of three additional parameters including hydrothermal time, hydrothermal temperature and NH4F concentration has been investigated. The results show that the optimal pore size has decreased from 177.8 nm to 120.53 nm (i.e., 32.2%). The MFI/GO/ZIF-8 membrane with fine-tuned crystal size in the zeolite layer was subjected to detailed tests for propylene selectivity and permeability. The structural characteristics of the membrane were also performed using FT-IR, XRD, FESEM and EDS techniques. The results show that the synergistic interaction between the three layers in the nanocomposite membrane significantly improves the selectivity and permeability of propylene. The permeability and selectivity of propylene increased from 50 to 60 GPU and from 136 to 177, respectively, before and after precise crystal size control. MFI/GO/ZIF-8 membrane by controlling the pore size of the zeolite layer shows a significant increase of 23.1% in selectivity and 16.7% in propylene permeability compared to the initial state. Also, due to the precise synthesis method, the absence of solvent and the use of cheap support, the prepared membrane is considered an environmentally friendly and low-cost membrane. This study emphasizes the potential of increasing the selectivity and permeability of propylene in the MFI/GO/ZIF-8 hybrid membrane by controlling the crystal size of the zeolite layer.

Thermal desorption from zeolite layer for VOC detection

Volatile organic compounds (VOCs) are being considered for a wide range of applications, since they provide information on specific processes (e.g., metabolic processes in humans and plants). To date, environmental VOCs detection can be accomplished with low selectivity using both portable photoionization techniques or using gold standard laboratory techniques (e.g., gas and liquid chromatography, mass spectrometry), getting higher performances and limitations in terms of portability, availability of equipment and of consumption of time. Hereafter, a modified photoionization technique based on a thin zeolite layer for the selective adsorption of VOCs was investigated, exploiting the processes of thermal adsorption and desorption. Zeolite through the inherent nanoporous structure possesses specific electrical characteristics, strongly influenced by the adsorption of the molecules within the framework. Experimental analysis of its electrical and adsorption/desorption properties of propionic and succinic acids was performed. Results demonstrated how the use of a thin nanoporous layer allows to increase the selectivity of commercial photoionization detector during VOC detection, evidencing the potentiality of this technique for the development of low-cost and high-performance sensors in various applications.

Preparation and catalytic performance of the supported Ti-MWW zeolites

Nano-size titanium silicalites is difficult to be separate from the reaction solution, lamellar and homogeneous Ti-MWW zeolites are successfully formed a dense zeolite layers on α-Al2O3 pellets by secondary hydrothermal synthesis in this study. Seeded crystals concentration and titanium source have significantly influences on the morphology, thickness and titanium active sites of Ti-MWW zeolites. Doping NH4F in the precursor synthesis solution could conducive to prepare macroscopically and anatase-free Ti-MWW zeolites layer on the outside of α-Al2O3 pellets, and the thickness of the Ti-MWW zeolites layer is ca. 1 mm. Besides, catalytic performance of the supported Ti-MWW zeolites are significantly improved by NH4F. A fixed-bed reactor with the supported Ti-MWW zeolites are applied to catalyze phenol hydroxylation with hydrogen peroxide efficiently, and reaction time and temperature, amount of the supported Ti-MWW zeolites have obvious effects on the phenol conversion and dihydroxybenzene selectivity. When the actual mass of Ti-MWW zeolites to volume of reaction solution is only 10 g/L, the relative phenol conversion and dihydroxybenzene selectivity are 46.45% and 92.72% at 65 °C after 3 h, respectively.

Detection of Propionic Acids Trapped in Thin Zeolite Layer Using Thermal Desorption Analysis.

Volatile organic compounds (VOCs) have recently received considerable attention for the analysis and monitoring of different biochemical processes in biological systems such as humans, plants, and microorganisms. The advantage of using VOCs to gather information about a specific process is that they can be extracted using different types of samples, even at low concentrations. Therefore, VOC levels represent the fingerprints of specific biochemical processes. The aim of this work was to develop a sensor based on a photoionization detector (PID) and a zeolite layer, used as an alternative analytic separation technique for the analysis of VOCs. The identification of VOCs occurred through the evaluation of the emissive profile during the thermal desorption phase, using a stainless-steel chamber for analysis. Emission profiles were evaluated using a double exponential mathematical model, which fit well if compared with the physical system, describing both the evaporation and diffusion processes. The results showed that the zeolite layer was selective for propionic acid molecules if compared to succinic acid molecules, showing linear behavior even at low concentrations. The process to define the optimal adsorption time between the propionic acid molecules was performed in the range of 5 to 60 min, followed by a thermal desorption process at 100 °C. An investigation of the relationship between the evaporation and diffusion rates showed that the maximum concentration of detected propionic acid molecules occurred in 15 min. Other analyses were performed to study how the concentration of VOCs depended on the desorption temperature and the volume of the analysis chamber. For this purpose, tests were performed using three analysis chambers with volumes of 25 × 10-6, 50 × 10-6, and 150 × 10-6 m3 at three different desorption temperatures of 20 °C, 50 °C, and 100 °C, respectively. The results demonstrated that the evaporation rate of the VOCs increased rapidly with an increasing temperature, while the diffusion rate remained almost constant and was characterized by a slow decay time. The diffusion ratio increased when using a chamber with a larger volume. These results highlight the capabilities of this alternative technique for VOC analysis, even for samples with low concentrations. The coupling of a zeolite layer and a PID improves the detection selectivity in portable devices, demonstrating the feasibility of extending its use to a wide range of new applications.

Nitrogen removal of decentralized swine wastewater by pilot-scale source reduction – anaerobic baffled reactor – zoning constructed wetlands at low temperatures

The study developed a cost-effective integrated technology to treat swine wastewater at the pilot-scale small pigsty. The swine wastewater, which was separated rinse water after flowing through the slatted floor and the innovatively constructed liquid-liquid separate collection device, was subsequently pumped into an anaerobic baffled reactor (ABR) and then through zoning constructed wetlands (CWs) comprised of CW1, CW2, and CW3. The liquid-liquid separate collection device effectively reduced COD, NH4–N, and TN by 57.82%, 52.39%, and 50.95%, respectively. The CW1 and CW2 enhanced TN removal and nitrification, respectively, through rapid adsorption-bioregeneration of zeolite. Moreover, rice straws were used as solid carbon sources in CW3 to successfully promote denitrification at 16.0 g/(m3·d). The integrated technology (slatted floor-liquid liquid separate collection-ABR-CWs) reduced COD, NH4–N, and TN by 98.17%, 87.22%, and 87.88%, respectively, at approximately 10 °C. Microbial analysis results confirmed that the CWs exhibited apparent functional zoning, with denitrifiers dominating in CW3, nitrifiers dominating in the zeolite layers of CW1 and CW2, and denitrifiers dominating in the brick slag layers of CWs. This cost-effective integrated technology demonstrated significant potential for treating swine wastewater at low temperatures.

PEBA/Na-X Multilayer Hybrid Membrane for CO2 Separation: Influence of Na-X Zeolite Layer Synthesis Condition

PEBA/Na-X Multilayer Hybrid Membrane for CO2 Separation: Influence of Na-X Zeolite Layer Synthesis Condition

Optimizing the accessibility of zeolite Y on FCC catalyst to improve heavy oil conversion capacity

Optimizing the accessibility of zeolite Y, the main active component in fluid catalytic cracking (FCC) catalyst, is a crucial strategy to improve the heavy oil cracking performance of FCC catalysts. Four FCC catalysts with different Y zeolite content were prepared via in-situ crystallization method by adjusting the crystallization time. The distribution characteristics of the Y zeolite in in-situ crystallized catalysts have been characterized by scanning electron microscopy (SEM), laser confocal fluorescence microscopy and high-resolution field emission scanning electron microscopy (HR-FESEM). A unique structural feature of the FCC catalysts has been achieved, with the Y zeolite component mainly distributing in the outer layer of microspheres. The results of adsorption rate of macromolecule probes reveal that the appropriate zeolite layer thickness and topology were beneficial to the mass transfer performance and acid accessibility of FCC catalyst. The effective utilization ratios of zeolite in the novel FCC catalysts for heavy oil cracking on advanced catalyst equipment (ACE) is in good correspondence with the mass transfer performance of macromolecules and the accessibility of acid sites. It could be confirmed that the excellent heavy oil conversion performance of a FCC catalyst does not depend on the content of zeolite, but rather on the distribution characteristics are in favor of the accessibility of zeolite components.

Fabrication of intelligent anticorrosion waterborne aluminum pigments based on encapsulation of composited zeolitic imidazole framework layers

Fabrication of intelligent anticorrosion waterborne aluminum pigments based on encapsulation of composited zeolitic imidazole framework layers

Evaluation of Permeable Pavement Systems for Removing Heavy Metals from Stormwater

This study examines a modified permeable pavement system (PPS) for enhanced heavy metal attenuation from stormwater. A laboratory model consisting of six PPS columns has been tested under varying rainfall intensities. The PPS structures are arranged based on the following hypotheses for enhancing heavy metal attenuation: (i) addition of a natural zeolite layer in the subbase for promoting heavy metal attenuation by adsorption and ion exchange; (ii) addition of a bark chip layer as an organic carbon source for promoting biosorption of heavy metals; (iii) maintaining a saturated zone in order to maintain the required humidity level and decrease the level of oxygen for increased biodegradation; and (iv) inclusion of thin sand layers to restrict the transport of oxygen to create an anoxic zone in the PPS and to enhance filtration. Successful treatment of metals such as Ba, Co, Mn, Ni, and Zn has been observed under various rainfall conditions using a conventional PPS. The rainfall intensity greatly influences the attenuation of Al, Cr, Cu, Mo and Sr. During heavy rains through the conventional PPS structure, chromium is found to be leached back into the infiltrate. The results indicate that by changing the subbase material and layer setting, biosorption of heavy metals can be encouraged in the PPS structure. The results of this study suggest that the PPS structure be amended by adding into its subbase a saturated region, an organic carbon donor and thin sand layers for enhanced heavy metal attenuation. Compared to the conventional structure, the proposed structure reduces Cr desorption and improves the attenuation of Al, Cu and Mo.

The effect of amorphous silica support on the catalytic activity of liquid-exfoliated monolayered MCM-56 zeolite

Recently reported groundbreaking discovery of efficient delamination of zeolite MCM-56, producing colloidal suspensions of MWW monolayers dispersed in the liquid phase, created unprecedented possibilities for the synthesis of a zeolite catalyst. Based on this innovation, the concept of using MWW monolayers to prepare silica-supported zeolite nanosheet catalysts suitable for transformations of large organic molecules was explored in this work. A series of silica-MWW preparations was synthesized from colloidal suspensions of the monolayers, using both solid and colloidal silica sources. The synthesized solids were thoroughly characterized with various physicochemical methods and their catalytic performance was tested in alkylation of mesitylene with benzyl alcohol. The obtained results indicate that solids containing MWW layers dispersed on silica show promising catalytic properties. The mixed MWW:silica catalysts synthesized from dispersions of MWW monolayers and liquid silica were found to exhibit high specific catalytic activity (with TOF values of 3.4 × 10−3 to 4.8 × 10−3 s−1), despite the high content of inactive amorphous silica support (40–60%). Materials synthesized from solid fumed and precipitated silicas showed low or negligible overall activity, which could be attributed to the small incorporation of the zeolitic active phase. For one of such materials, a notable high TOF (4.8 × 10−3 s−1) was found. It was found earlier that ethanol is an effective flocculent for zeolite layers by themselves, but in the presence of solid silica its efficiency was reduced.

Controllable surfactant-directed zeolitic-imidazolate-8 growth on swollen 2D zeolites

To meet society’s need for more and more specialized materials, this work focuses on the preparation of porous metal–organic framework (MOF)–zeolite hybrid materials based on two 2D zeolites, namely, IPC-1P (Institute of Physical Chemistry - 1 Precursor) and the metal–organic framework ZIF-8 (Zeolitic Imidazolate Framework-8). Using the previously well-established assembly–disassembly–organization–reassembly method, the zeolite was (i) synthesized, (ii) hydrolyzed to a layered zeolite, (iii) the interlayer distance was increased using the swelling agent cetyltrimethylammonium chloride, and (iv) nanocrystals of ZIF-8 were grown stepwise on the zeolite surface but predominantly at the edges of the crystallites where the openings to the interlayer region are located. This selective MOF growth and attachment was facilitated by a combination of intercalation of the metal ions and the swelling agent between the zeolite layers. The influence of the solvent and the number of additional steps on the ZIF-8 growth on the zeolite was systematically investigated, and the synthesis protocol was successfully adapted to a further two-dimensional silicate RUB-18 (Ruhr-Universität Bochum - 18). This paves the way toward the controlled preparation of more MOF–zeolite hybrid materials, which might provide interesting properties for future applications.

Synthesis of hierarchical LTA zeolite membranes by vapor phase transformation

Weakening the mass transfer resistance inside the selective layer of membrane can effectively increase the membrane separation performance. In this study, we introduce macropores into the conventional LTA zeolite membrane layer, resulting in an unprecedented hierarchical membrane structure. Specifically, the hierarchical LTA zeolite membranes composed of a macroporous zeolite layer and a thin dense top layer (600 nm), are fabricated on rough α-Al2O3 tubular supports by vapor phase transformation method using mesoporous silica spheres (MSPs) as the porogen in the precursor layer. The H2 (N2) permeance of the hierarchical LTA zeolite membrane is near 10 times that of the conventional LTA zeolite membrane, indicating that the hierarchical pore structure can effectively weaken the mass transfer resistance inside the selective layer of membrane. And it has good pervaporation performance for 90 wt% ethanol aqueous solution at 348 K, i.e., the flux can reach 4.54 kg m−2 h−1 with a separation factor of >10,000, which is much higher than that of the conventional LTA zeolite membrane. The preparation of LTA zeolite membranes with hierarchical pore structure using MSPs as a porogen is expected to open up a new way for the construction of zeolite membranes.

Organic solvent-free constructing of stable zeolitic imidazolate framework functional layer enhanced by halloysite nanotubes and polyvinyl alcohol on polyvinylidene fluoride hollow fiber membranes for treating dyeing wastewater

In this study, zeolitic imidazolate framework (ZIF-8)/polyvinylidene fluoride (PVDF) loose nanofiltration (NF) hollow fiber membranes were fabricated by constructing ZIF-8 functional layer on the PVDF supporting membranes based on the vacuum-assisted assembly process. The ZIF-8 synthesis was completed in a water system, and the synthesized ZIF-8 suspension was directly added to polyvinyl alcohol (PVA) and halloysite nanotubes (HNTs) aqueous solution system without drying to prepare the casting solution, which could solve the agglomeration and poor dispersion problem of ZIF-8 particles. In addition, the embedded HNTs and the loaded PVA among the ZIF-8 layer could improve the bonding strength between the ZIF-8 layer and the supporting membranes. After constructing ZIF-8 functional layer, the pore size of supporting membranes decreased from more than 300 nm to several nanometers. Furthermore, the water contact angle reduced from 91.1° to 54.2°. Applied to treat dye wastewater, the prepared ZIF-8/PVDF membranes maintained high dye rejection (˃99.0 %) for Congo red (CR), but low salt rejection for NaCl (about 2 %). In addition, the flux could reach 21.6 L m-2h−1 after continuous filtration 360 min, exhibiting a potential for treating the dye/salt wastewater. In particular, there were no organic solvents used in the work, which provided a promising idea for solvent-free fabrication of loose NF membranes.