Well-defined Pore Research Articles

Synthesis of copper-based nanoparticles confined in a pyridine N-containing COF and their catalytic applications

The main challenge of using non-noble metal copper nanoparticles (Cu NPs) as catalysts is to find or prepare suitable support platforms to anchor Cu NPs, thereby avoiding the aggregation tendency owing to their high surface energy. In this study, a novel pyridine N-containing covalent organic framework (COF) was designed and used as a template for the confined growth of Cu NPs. The prepared imine-connected TAPB-PCBA had abundant one-dimensional ordered channels, moderate specific surface area and good stability. The skeleton structure of TAPB-PCBA contained a large number of evenly distributed pyridine N, which can act as the nucleation site for Cu NPs. The well-defined pore structure confines the growth of Cu NPs and makes them controllable in size. The pores in TAPB-PCBA are completely independent and isolated from each other, minimizing the accumulation of Cu NPs. With the help of these factors, ultrafine Cu/Cu2O composite NPs (2.5 nm) with narrow size distribution anchored inside the cavity of the TAPB-PCBA were successfully prepared. The as-prepared Cu-containing COF, Cu@TAPB-PCBA, showed excellent catalytic capacity for dye degradation and nitrophenol reduction under mild conditions and low catalyst loads, respectively. In addition, Cu@TAPB-PCBA also has a good recyclability, multiple use without significant loss of catalytic activity. We expect this COF-supported growth-confined preparation of metal NPs to further facilitate the design and preparation of non-noble metal NPs@COF composites, replacing noble metals in certain catalytic applications.

Optimizing the Operational Parameters Used in Preparing Anodic Aluminum Oxide (AAO) Templates from Low-purity Aluminum

Anodized aluminum oxide (AAO) templates with well-defined geometric pore features and uniform nanoporous arrays were successfully obtained from recyclable low-cost, low-purity (99.16%) aluminum via a two-step anodizing process. The templates were characterized by scanning electron microscopy (SEM) and operating parameters were optimized using a Box-Behnken experimental design. The best conditions were found for an oxalic acid electrolyte concentration of 0.6 M, an anodizing potential of 46 V, and a bath temperature of 10 °C. This process resulted in an averaged regularity ratio (ravg) value of 2.66 which compares very favorably with previously reported values obtained from higher-purity aluminum (99.5%). Templates developed from low-purity aluminum are more suitable for high-volume industrial applications where there is a practical trade-off between cost and the quality of the geometric pores.

Construction of aqueous‐based symmetric supercapacitor with high energy from wood-based self-supporting carbon by coupling CO2 activation and redox electrolyte

Finding the delicate balance between energy storage/conversion and environmental protection is a pressing concern. This study focuses on addressing this issue by utilizing poplar-based self-supporting carbons. These carbons were prepared through chemical composition decomposition, with the hierarchical porous structures meticulously controlled using a multi-stage carbonization and green CO2 activation process. The resulting carbon material possesses a well-defined pore size structure, exhibiting good hydrophilicity, conductivity, and stable mechanical strength. And it demonstrates excellent electrochemical properties across a range of acidic, alkaline, and neutral electrolyte environments. In the single-electrode electrochemical system, the addition of a redox electrolyte yielded an outstanding specific capacitance value of 842.5 F·g–1 at 0.5 A·g–1. Moving forward, in the assembled symmetrical super capacitor, the synergistic effect of the self-supporting/hierarchical structures from the carbon electrode and the pseudocapacitance effect from the redox electrolyte enhanced the electrostatic adsorption on the carbon electrode surface. Consequently, the specific capacitance value reached 66.3 F·g–1 at 0.125 A·g–1. Moreover, the system maintained a high energy density of 23.6 Wh kg–1 at a power density of 100 W kg–1 under water system conditions. The specific self-supporting biomass-based carbon not only enables the construction of symmetrical electrodes without the need for additional additives or subsequent processing but also possesses a high energy density through a simple process under water system conditions. These materials strike an achievable balance between environmental concerns and energy demands.

INFLUENCE OF TEMPERATURE ON THE RECOVERY OF OILS SPILLED IN SEAWATER USING HYDROPHOBIZED POST-CONSUMER POLYURETHANE FOAMS

Although economically vital, whether in cold Scandinavian or warm Caribbean waters, offshore oil exploration accounts for oil spills at sea. One of the mechanisms used for recovering spilled oil consists of using porous materials with high sorption capacity in the cleanup process. Post-consumer polyurethane foams (PC-PUFs) are highly porous polymers typically disposed by consumers in Brazilian cities. In this work, PC-PUFs obtained from discarded mattresses were modified superficially to promote hydrophobization and increase their selectivity for oils over water. The characterization showed foams with well-defined macro pores. Surfaces of MoS2-modified foams (MoS2-PC) showed an increase in seawater penetration rejection and oil attraction when compared to unmodified foams (Un-PC). Sorption tests simulating oil spillage at 1 °C, 12 °C, and 23 °C were conducted to verify the influence of temperature on oil recovery by PC-PUFs. Tests on the seawater-diesel system showed that diesel, a low-viscosity oil with slight viscosity variation with temperature reduction, penetrated both Un-PC and MoS2-PC foams easily, making the modifications on MoS2-PC irrelevant. In the seawater-S46 lubricant system, which had a more viscous oil and presented a substantial variation in viscosity with temperature reduction, there was greater difficulty in penetrating the Un-PC foam when compared to MoS2-PC, showing that the modifications were responsible for the increase in the recovery of more viscous oils spilled in seawater at different temperatures.

Micelle encapsulated curcumin and piperine-laden 3D printed calcium phosphate scaffolds enhance in vitro biological properties

Limitations in the current clinical management of critical-sized osseous defects have driven the need for multifunctional bone constructs. The ideal bone scaffold should possess advanced microarchitecture, well-defined pore interconnectivity, and supply biological signals, which actively guide and control tissue regeneration while simultaneously preventing post-implantation complications. Here, a natural medicine-based localized drug delivery from 3D printed scaffold is presented, which offers controlled release of curcumin, piperine from nano-sized polymeric micelles, and burst release of antibacterial carvacrol from the coating endowing the scaffold with their distinct, individual biological properties. This functionalized scaffold exhibits improved osteoblast (hFOB) cell attachment, 4-folds higher hFOB proliferation, and 73% increased hFOB differentiation while simultaneously providing cytotoxicity towards osteosarcoma cells with 61% lesser viability compared to control. In vitro, early tube formation (p < 0.001) indicates that the scaffolds can modulate the endothelial cellular network, critical for faster wound healing. The scaffold also exhibits 94% enhanced antibacterial efficacy (p < 0.001) against gram-positive Staphylococcus aureus, the main causative bacteria for osteomyelitis. Together, the multifunctional scaffolds provide controlled delivery of natural biomolecules from the nano-sized micelle-loaded 3D printed matrix for significant improvement in osteoblast proliferation, endothelial formation, osteosarcoma, and bacterial inhibition, guiding better bone regeneration for post-traumatic defect repair.

Large-Area Soluble Covalent Organic Framework Oligomer Coating for Organic Solution Nanofiltration Membranes.

Covalent organic frameworks (COFs) are a family of engaging membrane materials for molecular separation, which remain challenging to fabricate in the form of thin-film composite membranes due to slow crystal growth and insoluble powder. Here, an additive approach is presented to construct COF-based thin-film composite membranes in 10min via COF oligomer coating onto poly(ether ether ketone) (PEEK）ultrafiltration membranes. By the virtue of ultra-thin liquid phase and liquid-solid interface-confined assembly, the COF oligomers are fast stacked up and grow along the interface with the solvent evaporation. Benefiting from the low out-plane resistance of COFs, COF@PEEK composite membranes exhibit high solvent permeances in a negative correlation with solvent viscosity. The well-defined pore structures enable high molecular sieving ability (Mw = 300g mol-1 ). Besides, the COF@PEEK composite membranes possess excellent mechanical integrities and steadily operate for over 150 h in the condition of high-pressure cross flow. This work not only exemplifies the high-efficiency and scale-up preparation of COF-based thin-film composite membranes but also provides a new strategy for COF membrane processing.

A new route of production of the meso-porous chitosan with well-organized honeycomb surface microstructure from shrimp waste without destroying the original structure of native shells: Extraction, modification and characterization study

The highly active bio-polysaccharide based compound namely chitin and chitosan have been extracted from shrimp shell which usually expelled out extensively from food processing industries and considered as waste materials. However, by conducting demineralization, de-proteinization and deacetylation on it which could be a new route of production of valuable meso-porous chitosan that possesses a noteworthy well-organized honeycomb like surface microstructure which undoubtedly capable to form sustainable bio-composites. Whereas the honeycomb like microstructures showed a well-defined pore canals (Pc) and pore canal tubes (Pct) onto their surface. The samples were characterized via ATR-FTIR spectroscopy, High resolution SEM, AFM, EDX, XRD and 13C solid-state Cross-Polarization Magic Angle Spinning (CPMAS) NMR. The molecular weight(M‾v) of isolated chitin and chitosan were determined by viscosity and the values were 1.06 × 106(DPv = 5228), 1.8 × 104(DPv = 116) respectively besides degree of deacetylation (DDA) were measured by ATR-FTIR spectroscopy, CPMAS NMR, pH-metric titration and the values were 24&21 for chitin and 84&82 for chitosan. The results suggest that the shrimp shell waste is an attractive source of well-organized honeycomb like meso-porous chitin and chitosan with regular surface microstructures which might have a significant/potential application into the sustainable bio-composite formulation due to various industrial and biomedical sector indeed.

Framework-Topology-Controlled Singlet Fission in Metal-Organic Frameworks.

Singlet fission (SF) has been explored as a viable route to improve photovoltaic performance by producing more excitons. Efficient SF is achieved through a high degree of interchromophoric coupling that facilitates electron superexchange to generate triplet pairs. However, strongly coupled chromophores often form excimers that can serve as an SF intermediate or a low-energy trap site. The succeeding decoherence process, however, requires an optimum electronic coupling to facilitate the isolation of triplet production from the initially prepared correlated triplet pair. Conformational flexibility and dielectric modulation can provide a means to tune the SF mechanism and efficiency by modulating the interchromophoric electronic interaction. Such a strategy cannot be easily adopted in densely stacked traditional organic solids. Here, we show that the assembly of the SF-active chromophores around well-defined pores of solution-stable metal-organic frameworks (MOFs) can be a great platform for a modular SF process. A series of three new MOFs, built out from 9,10-bis(ethynylenephenyl)anthracene-derived struts, show a topology-defined packing density and conformational flexibility of the anthracene core to dictate the SF mechanism. Various steady-state and transient spectroscopic data suggest that the initially prepared singlet population can prefer either an excimer-mediated SF or a direct SF (both through a virtual charge-transfer (CT) state). These solution-stable frameworks offer the tunability of the dielectric environment to facilitate the SF process by stabilizing the CT state. Given that MOFs are a great platform for various photophysical and photochemical developments, generating a large population of long-lived triplets can expand their utilities in various photon energy conversion schemes.

Metal-Organic Framework Membrane Hybridized with Graphitic Materials for Gas Separation.

Metal-organic frameworks (MOFs) are an exceptional class of crystalline materials that have been extensively used to fabricate membranes for various applications such as gas separation, ion transport, and desalination due to their well-defined pore structure, chemical features, and simple synthesis process. The incorporation of graphitic carbon materials in MOFs has garnered significant attention as it can provide abundant nucleation sites and modulate gas transport by influencing the orientation or rigidity of MOF crystals without changing their porous structure. This review insights of previous studies utilizing graphene, graphene oxide, carbon nanotubes, and graphene nanoribbons for MOF-based gas separation membranes, particularly focusing on polycrystalline MOF membrane hybridization with graphitic materials. We also briefly discuss the use of carbon/MOF hybrid materials for preparing mixed matrix membranes.

Covalent Organic Framework-Supported Metallocene for Ethylene Polymerization.

Loading of homogeneous catalysts with support can dramatically improve their performance in olefin polymerization. However, the challenge lies in the development of supported catalysts with well-defined pore structures and good compatibility to achieve high catalytic activity and product performance. Herein, we report the use of an emergent class of porous material -- covalent organic framework material (COF) as a carrier to support metallocene catalyst -- Cp2ZrCl2 for ethylene polymerization. The COF-supported catalyst demonstrates a higher catalytic activity of 31.1 × 106 g/(mol·h) at 140 °C, compared with 11.2 × 106 g/(mol·h) of the homogenous one. The resulting polyethylene (PE) products possess higher weight-average molecular weight (Mw) and narrower molecular weight distribution (Ɖ) after COF supporting, i.e., Mw increases from 160 kDa to 308 kDa and Ɖ reduces from 3.3 to 2.2. The melting point (Tm) is also increased by up to 5.2 °C. Moreover, the PE product possesses a characteristic filamentous microstructure and demonstrates an increased tensile strength from 19.0 MPa to 30.7 MPa and elongation at break from 350% to 1400% after catalyst loading. We believe that the use of COF carriers will facilitate the future development of supported catalysts for high-efficient olefin polymerization and high-performance polyolefins.

A highly hydrophilic hydrazone-linked covalent organic framework as a fluorescent multianalyte sensor for detection of Cu2+ and Hg2+ in aqueous solution

Heavy metal pollution is of great threat to public health and the environment, and an effective method to detect and remove toxic heavy ions from water is highly desirable. The emerging covalent organic frameworks (COFs) with well-defined pore structures and specific functional units provide an excellent platform for fluorescent sensing. The hydrazone-linked framework material SH-COF with excellent hydrophilicity and abundant chelating sites was synthesized by bottom-up strategy under solvothermal conditions. The resultant SH-COF with specific binding sites was proposed to detect and remove Cu2+ and Hg2+. The SH-COF also exhibited the characteristics of high selectivity, high sensitivity, a lower limit of detection (0.244 μM for Cu2+ and 0.239 μM for Hg2+), and real-time response. Furthermore, the highly selective interaction of Cu2+ or Hg2+ with SH-COF was confirmed by X-ray photoelectron spectroscopy, energy dispersive X-ray spectroscopy and Fourier transform infrared spectra. A proposed sensing mechanism of COF for Cu2+ and Hg2+ was presented. This research also demonstrates the great potential of COFs in practical sensing applications.

Hierarchical isoporous membrane filters for simultaneous reduction of pressure drop and efficient removal of nanoscale airborne contaminants

Isoporous membranes with well-defined pore architectures offer a unique design approach to achieve a multi-scale porous network. For instance, isoporous membranes with progressively smaller pore sizes can be stacked to create a hierarchical pore network in which each length scale and the gradient of the pore network are deterministic by design. In this paper, we introduce a hierarchically-arranged multilayer isoporous air filter that comprises an ultrathin (thickness <1μm) nanoporous active filtration layer and a microporous support layer. To maximize airflow and thereby reduce pressure drop across the membrane, we engineered a gap between the nanoporous and microporous membranes by designing and fabricating microscale spacer structures akin to feed spacers used in spiral wound reverse osmosis membranes. We demonstrated that such hierarchical isoporous membranes with integrated spacers (HIM-S) can retain high filtration efficiency (>95%) for ultrafine, most penetrating particle size (MPPS) while simultaneously reducing the pressure drop across the membrane (by ∼86%).

Ion Channel-Restructured Zwitterionic Covalent Organic Framework Solid Electrolyte for All-Solid-State Lithium Metal Batteries.

Organic solid electrolytes offer an effective route for safe and high energy-density all-solid-state Li metal batteries. However, it remains a challenge to devise a new strategy to promote the dissociation of strong ion pairs and the transport of ionic components in organic solid electrolytes. Herein, a zwitterionic covalent organic framework (Zwitt-COF) with well-defined chemical and pore structures is prepared as a solid electrolyte capable of accelerating the dissociation and transport of Li ions. The Zwitt-COF solid electrolyte exhibits a high room-temperature ionic conductivity of 1.65 × 10-4 S cm-1 with a wide electrochemical stability window. Besides, the Zwitt-COF solid electrolyte displays stable Li plating/stripping behavior via effective inhibition of the formation of Li dendrites and dead Li, leading to superior long-term cycle performance with retention of 99% discharge capacity and 98% Coulombic efficiency in an all-solid-state Li metal battery. Theoretical simulations reveal that the incorporation of zwitterionic groups into COF can facilitate the dissociation of strong ion pairs and reconstruct the AA-stacking configuration by dissociative adsorption of Li+ ions on Zwitt-COF producing linear hexagonal ion channels in the Zwitt-COF solid electrolyte. This strategy based on Zwitt-COF can provide an alternative way to construct various solid-state Li batteries. This article is protected by copyright. All rights reserved.

Fluoride removal from water using Zr-modified meso- and macroporous carbons: Effect of pore structure and adsorption conditions

We investigated the effects of the pore structure and adsorption conditions (pH and coexisting anions) on the fluoride (F–) removal capacity of Zr-modified porous carbons. The porous carbons used in this study possessed well-defined pore structures created by the template technique with pore diameters of 10, 30, and 150 nm. These porous carbons were oxidized to functionalize them with oxygen-containing groups before being modified with Zr4+. Zr4+ modification improved the F– removal capacity of porous carbons. The maximum amount of F– adsorbed increased by approximately 3–10 times by Zr4+ modification. The F– adsorption capacity depends on the pore size. The samples with larger pore diameters (smaller specific surface areas) exhibited higher F– adsorption capacities than the samples with smaller pore diameters. This tendency can be explained using the theory of liquid imbibition in porous media. The adsorption capacities at pH 7.0 and 11.1 were lower than those at pH 3.1. This tendency can be interpreted based on competition between F– and OH– on adsorption sites and attractive/repulsive interactions between F– and protonated/deprotonated adsorbent surfaces. The coexisting carbonate anions notably suppress the F– adsorption. This suppressive effect can be explained by the competitive bonding of F– and CO32– to the adsorption sites.

Improvement in effectiveness of diamond in strengthening the porous aluminium composite

In this study, an aluminum (Al) alloy matrix reinforced with uncoated and titanium (Ti) -coated diamond were developed using powder metallurgy technique where polymethylmethacrylate (PMMA) particles were employed as space holders. The weight % of uncoated and coated diamond varied as (0, 6, 9, 12, 15, and 20 wt.%). Microstructural and elemental analysis was examined using scanning electron microscopy and energy dispersive spectroscope, respectively. The relative density and porosity that were in the range of 0.72–0.92 and 31–44% respectively, were measured using Archimedes principle. The compressive properties were measured and correlated with microstructure observations. The microstructure of the composite samples revealed presence of well-defined macro pores with good interfacial integrity. The X-ray diffraction revealed the presence of strengthening phases such as Al2Ti, Mg2Sn, AlB12, Cu5Sn6, Al12Mg17 and MgB2 phases formed as a result of addition of metal additives in Al matrix. Further improvement in the strength was obtained by using Ti-coated diamond particles as reinforcement. As Ti formed a good interface between the Al alloy matrix and the diamond thereby preventing the formation of undesirable carbide phases. There was an improvement in plateau stress and energy absorption capacity of 82 and 88% as compared to unreinforced porous sample. The maximum values obtained for the plateau stress and energy absorption capacity were 45.12 MPa and 13.68 MJ/m3 respectively for 9 wt.% of Ti-coated diamond reinforced composites. Uncoated diamond reinforced composites, on the other hand, reduced the strength of porous Al alloy matrix due to the formation of brittle intermetallic compound such as carbides (Al2C3) at the interface. As a result, the coated diamond particle surface modifies and improves the wettability of the Al alloy matrix and diamond interface.

Designing flower-like MOFs-derived N-doped carbon nanotubes encapsulated magnetic NiCo composites with multi-heterointerfaces for efficient electromagnetic wave absorption

In order to acquire exceptional electromagnetic wave absorption properties, the microstructure design and component modification of composites are essential. Metal-organic frameworks (MOFs), due to the unique metal–organic crystalline coordination, tunable morphology, high surface area, and well-defined pores, have been regarded as promising electromagnetic wave absorption materials precursors. However, the inadequate contact abilities between adjacent MOFs nanoparticles endow it with undesirable electromagnetic wave dissipation capacity at a low filler loading, which is a great challenge to break size effect of nanoparticles to achieve efficient absorption. Herein, NiCo-MOFs derived N-doped carbon nanotubes encapsulated with NiCo nanoparticles anchored on flowers-like composites (denoted as NCNT/NiCo/C) were successfully prepared through facile hydrothermal method followed by thermal chemical vapor deposition with melamine-assisted catalyst. By controlling the Ni/Co ratio in precursor, the tunable morphology and microstructure of MOFs are achieved. Most importantly, the derived N-doped carbon nanotubes tightly connect the adjacent nanosheets to construct the special 3D interconnected conductive network, which effectively accelerates the charge transfer and improves the conduction loss. And notably, the NCNT/NiCo/C composite delivers excellent electromagnetic wave absorption performance with minimum reflection loss of −66.1 dB and wide effective absorption bandwidth up to 4.64 GHz when the Ni/Co ratio is 1:1. This work provides a novel method for the preparation of morphology controllable MOFs-derived composites and realizes high-performance electromagnetic wave absorption properties.

The effect of GelMA/alginate interpenetrating polymeric network hydrogel on the performance of porous zirconia matrix for bone regeneration applications

Bone tissue is a natural composite, exhibiting complicated structures and unique mechanical/biological properties. With an attempt of mimicking the bone tissue, a novel inorganic-organic composite scaffolds (ZrO2-GM/SA) was designed and prepared via the vacuum infiltration method and the single/double cross-linking strategy by blending GelMA/alginate (GelMA/SA) interpenetrating polymeric network (IPN) into the porous zirconia (ZrO2) scaffold. The structure, morphology, compressive strength, surface/interface properties, and biocompatibility of the ZrO2-GM/SA composite scaffolds were characterized to evaluate the performance of the composite scaffolds. Results showed that compared to ZrO2 bare scaffolds with well-defined open pores, the composite scaffolds prepared by double cross-linking of GelMA hydrogel and sodium alginate (SA) presented a continuous, tunable and honeycomb-like microstructure. Meanwhile, GelMA/SA showed favorable and controllable water-uptake capacity, swelling property and degradability. After the introduction of IPN components, the mechanical strength of composite scaffolds was further improved. The compressive modulus of composite scaffolds was significantly higher than the bare ZrO2 scaffolds. In addition, ZrO2-GM/SA composite scaffolds had highly biocompatibility and displayed a potent proliferation and osteogenesis of MC3T3-E1 pre-osteoblasts compared to bare ZrO2 scaffolds and ZrO2-GelMA composite scaffolds. At the same time, ZrO2-10GM/1SA composite scaffold regenerated significantly greater bone than other groups in vivo. This study demonstrated that the proposed ZrO2-GM/SA composite scaffolds had great research and application potential in bone tissue engineering.

Recent Progress in the Fabrication of Photonic Crystals Based on Porous Anodic Materials

Photonic crystals (PCs) based on porous anodic materials have been proven to be a potent and versatile instrument for the emergence of new technologies with a wide range of applications. Their lower production costs are one of the key advantages, making PC-based systems more widely available and appropriate for industrial manufacturing. The ability to produce well-defined pores on metal oxide and semiconductor surfaces has experienced a major renaissance due to the striking differences in characteristics between porous surfaces and dense oxide layers. In this review, we give a general overview of the progress of PC fabrication based on porous surfaces of anodized aluminum (Al), silicon (Si), and titanium (Ti) using various anodization techniques, and their optical characteristics and applications are discussed. The anodizing conditions have a large influence on the pore geometry of the produced porous surfaces. The review fully focuses on the advancements made in manufacturing anodic aluminum oxide (AAO), porous silicon (pSi), and titanium-dioxide nanotube (TNT) PCs manufactured using self-ordered anodization under varied conditions. Additionally, a critical assessment of the upcoming developments in PC manufacturing and their optical characteristics suitable for various photonic devices is provided.

4-Cyanophenol herbicide sensor using multi-walled carbon nanotube embedded dual-microporous polypyrrole nanoparticles as metal-free and environmentally friendly hybrid electrode.

A highly sensitive 4-cyanophenol (4-CP) sensor was fabricated using multi-walled carbon nanotube (MWCNT)-embedded dual-microporous polypyrrole nanoparticle-modified screen-printed carbon electrodes (SPCE/DMPPy/MWCNT). The well-defined dual pores of DMPPy and MWCNT (~ 0.53 and ~ 0.65nm) acted as good analyte absorption agents (shortening the ion diffusion path) and conducting agents (reducing the internal electron-transfer resistance). This enhanced electrical conductivity resulted in the improved electro-oxidation of 4-CP. A higher sensitivity (19.0 μA μM-1cm-2) and lower limit of detection (0.8nM) were achievedwith a wide detection range of 0.001-400µM (R2 = 0.9988). The proposed sensor exhibited excellent recovery of 4-CP in real-world samples. Therefore, the SPCE/DMPPy/MWCNT sensor is regardedhighly suitable for rapidly detecting 4-CP.

Influence of Humidity and Heating Rate on the Continuous ZIF Coating during Hydrothermal Growth.

Zeolitic imidazolate frameworks (ZIFs) have potential for various gas and ion separations due to their well-defined pore structure and relatively easy fabrication process compared to other metal-organic frameworks and zeolites. As a result, many reports have focused on preparing polycrystalline and continuous ZIF layers on porous supports with good separation performance in various target gases, such as hydrogen extraction and propane/propylene separation. To utilize the separation properties in industry, membrane is required to be prepared in large scale with high reproducibility. In this study, we investigated how humidity and chamber temperature influence the structure of a ZIF-8 layer prepared by the hydrothermal method. Many synthesis conditions can affect the morphology of polycrystalline ZIF membranes, and previous studies have mainly focused on reaction solutions, such as precursor molar ratio, concentration, temperature, and growth time. On the other hand, we found that the humidity of the chamber and the heating rate of the solution also lead to dramatic changes in the morphology of ZIF membranes. To analyze the trend between humidity and chamber temperature, we set up the chamber temperature (ranging from 50 °C to 70 °C) and relative humidity (ranging from 20% to 100%) using a thermo-hygrostat chamber. We found that as the chamber temperature increased, ZIF-8 preferentially grew into particles rather than forming a continuous polycrystalline layer. By measuring the temperature of the reacting solution based on chamber humidity, we discovered that the heating rate of the reacting solution varied with humidity, even at the same chamber temperature. At a higher humidity, the thermal energy transfer was accelerated as the water vapor delivered more energy to the reacting solution. Therefore, a continuous ZIF-8 layer could be formed more easily at low humidity ranges (ranging from 20% to 40%), while micron ZIF-8 particles were synthesized at a high heating rate. Similarly, under higher temperatures (above 50 °C), the thermal energy transfer was increased, leading to sporadic crystal growth. The observed results were obtained with a controlled molar ratio, in which zinc nitrate hexahydrate and 2-MIM were dissolved in DI water at a molar ratio of 1:45. While the results are limited to these specific growth conditions, our study suggests that controlling the heating rate of the reaction solution is critical for preparing a continuous and large-area ZIF-8 layer, particularly for the future scale-up of ZIF-8 membranes. Additionally, humidity is an important factor in forming the ZIF-8 layer, as the heating rate of the reaction solution can vary even at the same chamber temperature. Further research related to humidity will be necessary for the development of large-area ZIF-8 membranes.