Molecular Sieving Effect Research Articles

Effective removal of cesium from wastewater via adsorptive filtration with potassium copper hexacyanoferrate-immobilized and polyethyleneimine-grafted graphene oxide

As an attractive alternative to radioactive cesium removal, we introduced an adsorptive filtration method using a composite membrane consisting of potassium copper hexacyanoferrate (KCuHCF) and graphene-based support. Polyethyleneimine-grafted reduced graphene oxide (PEI-rGO), used as an immobilizing matrix, was effective not only in distributing KCuHCF inside the composite with the aid of abundant amino-functionality, but also in achieving high water flux by increasing the interlayer spacing of the laminar membrane structure. Due to the rapid and selective cesium adsorption properties of KCuHCF, the fabricated membrane was found to be effective in achieving complete removal of cesium ions under a high flux (over 500 L m−2 h−1), which is difficult in a conventional membrane utilizing the molecular sieving effect. This approach offers strong potential in the field of elimination of radionuclides that require rapid and complete decontamination.

Guanidyl-functionalized polyhedral oligomeric silsesquioxane porous hybrid polymer coating for specific solid phase microextraction of phthalate esters in foodstuff

A guanidyl-functionalized polyhedral oligomeric silsesquioxane (G-POSS) porous polymeric hybrid coating was developed for specific solid-phase microextraction (SPME) of phthalate esters (PAEs), via in-situ click photopolymerization on a thiolated stainless steel (SS) fiber. The guanidyl-functionalized l-cysteine (G-Cys) monomer was fast copolymerized with methacryl substituted polyhedral oligomeric silsesquioxane in the presence of initiator and porogen. The resultant G-POSS hybrid coating exhibited large specific surface, fast mass transfer, and good stability (water- and thermal-resistance). It showed mixed-mode recognition regime (hydrogen bonding, hydrophobic interaction and “molecular sieve effect”), and therefore demonstrated great benefits in SPME - gas chromatography-mass spectrometry (GC–MS) analysis of PAEs with low detection limits (0.054–2.51 ng L−1), wide linear range (1.0–3000.0 ng L−1), and appropriate repeatability. The coating was successfully applied for the enrichment and analysis of PAEs in milk and rice samples. This work indicates that the POSS-based functionalized porous hybrid polymer is a promising candidate for the enrichment of complicated samples.

Chain length and acidity of carboxylic acids influencing adsorption/desorption mechanism and kinetics over anion exchange membrane

Selective recovery of carboxylic acids through anion exchange membrane (AEM) during electrodialysis (ED) involves bulk transport/diffusion/adsorption/desorption steps. Combined effect of size, structure, percentage dissociation, interactions alter each step of transport. In this study, we focused on interpretation of role of functional groups (number of COOH groups) and hydrocarbon chain length of different carboxylic acids (formic (FA), acetic (AA), propionic (PA), butyric (BA), malic (MA) and citric (CA) acids) during their transport in terms of ion interaction with membrane. Quantitative estimate of interactions between acids – AEM, adsorption mechanism and solute uptake rate were inferred from adsorption isotherms (Langmuir, Freundlich and Redlich-Peterson) and kinetic models (pseudo-first-order, pseudo-second-order and Elovich). Adsorption was rate limiting and Freundlich isotherm was more appropriate. Adsorption capacities, Kf followed an order: 0.139 > 0.121 > 0.072 > 0.062 > 0.049 > 0.025 mmol·g−1 respectively with CA, MA, FA, AA, PA, BA. Pseudo-second order rate constants, k2 followed same order of adsorption, highest with CA (0.0187 g·mmol−1 min−1) and lowest with BA (0.0124 g·mmol−1 min−1) and dominance of heterogeneous mode of chemical/ionic interactions. pKa, chain length and molecular sieve effects were used to support the adsorption pattern.

Efficient SO2 Removal Using a Microporous Metal–Organic Framework with Molecular Sieving Effect

Removal of trace SO2 contaminants from gas mixtures by the molecular sieving effect is highly desired. Herein, a microporous metal–organic framework Cu2(pzdc)2(pyz) [CPL-1, pzdc = 2,3-pyrazinedicar...

Propylene/propane separation on a ferroaluminosilicate levyne zeolite

Abstract The propylene/propane separation properties of levyne zeolites with different framework Si/Al and Fe/Al ratios have been examined and compared with those of the four well-studied zeolite adsorbents (i.e., Na-A, Ca-A, Na-X and ITQ-12) for this separation. Both zeolite framework structure and composition were found to be critical for the propylene uptake, kinetic separation and adsorbent durability. Among the zeolites studied here, the mixed CaNH4 form of a ferroaluminosilcate levyne (Si/Al = 15.5 and Fe/Al = 0.27) showed the highest propylene/propane selectivity (11). Under vacuum-swing adsorption mode at 298 K and reservoir pressure of 1.2 bar, this partially iron-substituted zeolite was characterized by a higher propylene uptake (ca. 1.0 vs 0.7 mmol g−1) than its aluminosilicate version, which can be attributed to a combination of the molecular sieve effect of its framework topology and relatively weaker acidity. The results of this study demonstrate that, in contrast to widely held belief, zeolite adsorbents selective for propylene/propane separation do not need to be a pure-silica composition.

Recent Advances in Graphene Oxide Membranes for Gas Separation Applications.

Graphene oxide (GO) can dramatically enhance the gas separation performance of membrane technologies beyond the limits of conventional membrane materials in terms of both permeability and selectivity. Graphene oxide membranes can allow extremely high fluxes because of their ultimate thinness and unique layered structure. In addition, their high selectivity is due to the molecular sieving or diffusion effect resulting from their narrow pore size distribution or their unique surface chemistry. In the first part of this review, we briefly discuss different mechanisms of gas transport through membranes, with an emphasis on the proposed mechanisms for gas separation by GO membranes. In the second part, we review the methods for GO membrane preparation and characterization. In the third part, we provide a critical review of the literature on the application of different types of GO membranes for CO2, H2, and hydrocarbon separation. Finally, we provide recommendations for the development of high-performance GO membranes for gas separation applications.

Rational Tuning of Zirconium Metal–Organic Framework Membranes for Hydrogen Purification

AbstractThe effect of organic ligands on the separation performance of Zr based metal–organic framework (Zr‐MOF) membranes was investigated. A series of Zr‐MOF membranes with different ligand chemistry and functionality were synthesized by an in situ solvothermal method and a coordination modulation technique. The thin supported MOF layers (ca. 1 μm) showed the crystallographic orientation and pore structure of original MOF structures. The MOF membranes show excellent selectivity towards hydrogen owing to the molecular sieving effect when the bulkier linkers were used. The molecular simulation confirmed that the constricted pore apertures of the Zr‐MOFs which were formed by the additional benzene rings lead to the decrease in the diffusivity of larger penetrants while hydrogen was not remarkably affected. The gas mixture separation factors of the MOF membranes reached to H2/CO2=26, H2/N2=13, H2/CH4=11.

Rational Tuning of Zirconium Metal-Organic Framework Membranes for Hydrogen Purification.

The effect of organic ligands on the separation performance of Zr based metal-organic framework (Zr-MOF) membranes was investigated. A series of Zr-MOF membranes with different ligand chemistry and functionality were synthesized by an in situ solvothermal method and a coordination modulation technique. The thin supported MOF layers (ca. 1 μm) showed the crystallographic orientation and pore structure of original MOF structures. The MOF membranes show excellent selectivity towards hydrogen owing to the molecular sieving effect when the bulkier linkers were used. The molecular simulation confirmed that the constricted pore apertures of the Zr-MOFs which were formed by the additional benzene rings lead to the decrease in the diffusivity of larger penetrants while hydrogen was not remarkably affected. The gas mixture separation factors of the MOF membranes reached to H2 /CO2 =26, H2 /N2 =13, H2 /CH4 =11.

Research on catalytic pyrolysis of algae based on Py-GC/MS.

In order to improve the quality of catalysis products of algae, composite molecular sieve catalyst was prepared by digestion and crystallization of HZSM-5 to reduce the oxygen content of the catalytic products. According to the analysis of the pyrolysis products, the best preparation conditions were chosen of tetra propylammonium hydroxide (TPAOH) solution 2.0 mol l−1, cetyltrimethylammonium bromide (CTAB) solution 10 wt%, crystallization temperature 110°C, digestion–crystallization time: 24–24 h. The results indicate that the main function of catalysts is to promote the conversion of alcohols into hydrocarbons by reducing energy barriers. Catalysed by the composite molecular sieve, the content of alcohols in the pyrolysis products decreased from more than 30% to less than 10%, the content of hydrocarbons increased from 20% to nearly 60%, while all the adverse components remained at a low level, which indicates that the catalytic pyrolysis products are of high quality. The great deoxidation effect of composite molecular sieves is not only due to the expansion of the range of organic matter during re-pyrolysis, but also the increasing of the residence time of pyrolysis products inside the structure for the external mesoporous structure.

A synergistic combination of zinc oxide nanowires array with dual-functional zeolitic imidazolate framework-8 for hybrid nanomaterials-based gas sensors

The complementary hybridization of nanomaterials enables to realize newly synergistic properties, which is a central concept in next-generation widespread applications. In particular, it is well-established that the hybridization of ZnO nanostructure with zeolitic imidazolate framework-8 (ZIF-8) allows the improvement of gas selectivity owing to molecular sieving capability caused by inherently size-defined porous structures of the ZIF-8. Here, we focused on a different viewpoint related with the preconcentration effect of ZIF-8 hybridized with ZnO nanowires (NWs) array on NO2, NH3, and H2 gas response. Hydrothermally-grown, structurally optimized ZnO NWs array are rationally employed for the nucleation sites of ZIF-8 nanocrystals as well as gas sensing channel. The chemical conversion of the surface of ZnO NWs to ZIF-8 was systematically implemented for the formation of core-shell hybrid structure, where morphological features of the ZIF-8 nanocrystals hybridized with the ZnO NWs array were optimized by simply adjusting synthetic conditions. The thickness of encapsulated ZIF-8 could be effectively manipulated by altering the concentration of 2-methylimidazole (HmIM), which strongly affects to determining gas response of ZIF-8@ZnO NWs-based gas sensors because of a variation in gas-loading capabilities related with the preconcentration effect. For the gas sensor based on ZIF-8@ZnO NWs synthesized using 8 mM HmIM solution, the gas response of NO2, NH3, and H2 gases were improved compared to those of the pristine ZnO NWs-based gas sensor. In addition, we consolidated the molecular sieving effect of the ZIF-8 by measuring the gas response of CH4 and C3H8 for suggesting the dual functionality of the hybrid system.

Surface modification effect of carbon molecular sieve (CMS) on the morphology and separation performance of mixed matrix membranes

The lack of interfacial adhesion between the polymer and inorganic phases in mixed matrix membranes (MMMs) imposes a serious challenge. This study is focused on the improvement of polymer-filler adhesion via surface modification of carbon molecular sieve (CMS) by nitric acid (HNO3) oxidation process. The bare CMS and oxidized CMS based mixed matrix membranes fabricated by the solvent evaporation method. Fourier-transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) analyses confirmed the oxidation of CMS surface. An improved morphology and interfacial adhesion were observed in Polysulfone (PSU)/Ox-CMS MMMs as deduced by field emission scanning electron microscope analysis. Thermal stability of the developed membrane has been confirmed by Thermogravimetric analysis (TGA). Raman spectroscopy is carried out to evaluate the homogeneity of the filler distribution in the polymer phase. Gas separation performance of the synthesized MMMs has revealed the positive effects of surface modification of CMS particles with HNO3 oxidation on CO2/CH4 selectivity of the developed MMMs. A slight reduction in CO2 permeability (2–5%) observed in ox-CMS based MMMs. PSU/oxidized-CMS MMM with 30 wt % CMS loading has shown a superior performance (CO2 permeability = 3.91 barrer and CO2/CH4 selectivity = 65.92) which is approaching the Robeson upper bound limit. These highly selective membranes have a high potential in membrane-based CO2 separation applications.

Enhanced desulfurization performance of hybrid membranes using embedded hierarchical porous SBA-15

The utilization of materials with a hierarchical porous structure as multi-functional additives is highly attractive in the preparation of hybrid membranes. In this study, novel hybrid membranes are designed by embedding hierarchical porous Santa Barbara Amorphous 15 (SBA-15) with a dual-pore architecture (micropores and mesopores) for pervaporation desulfurization. The SBA-15 with cylindrical mesopores provides molecular transport expressways to ensure improved permeability, while micropores on the wall have molecular sieving effects that are essential for the enhancement of permselectivity of thiophene molecules. Considering thiophene/n-octane mixture as a model system, the hybrid membrane with embedded 6 wt-% SBA-15 exhibits optimal pervaporation desulfurization performance with a permeation flux of 22.07 kg·m−2·h−1 and an enrichment factor of 6.76. Moreover, the detailed structure and properties of hybrid membranes are systematically characterized. This study demonstrates the immense potential of hierarchical porous materials as additives in membranes to simultaneously increase permeability and permselectivity.

Effects of acidity and topology of zeolites on the n-alkane conversion at low reaction temperatures

Abstract n-Butane conversion was used as a probe reaction to investigate the influence of acidity and topology of zeolites on the reaction properties at low temperatures. The results show that the number of catalytic Bronsted acid sites determines the reaction rates. The bimolecular pathway of n-butane reaction requires two adjacent acid sites, while the monomolecular reaction occurs on the isolated Bronsted sites. The contribution of bimolecular reaction primarily affected by the number of catalytic Bronsted sites and secondly influenced by the zeolite topology. The zeolites with larger cavity provide enough space for the formation and the transformation of bulky C8 intermediates in bimolecular pathway and give rise to the higher contribution of bimolecular pathway. The distribution of bimolecular products strongly depends on the maximum free sphere diameter of zeolites, while has no direct relationship with the volume of the zeolite cavities. This can be explained by a molecular sieve effect of zeolite channels on the diffusion rates of propane, isobutane, and pentane from cavities to external surface of zeolites. This improved understanding of the molecular sieve effect allows one to design and select appropriate zeolites for a special reaction.

MAF-41 with intermediate-sized molecular sieving effect for highly selective separation of styrene

MAF-41 with intermediate-sized molecular sieving effect for highly selective separation of styrene

Hydrophilic ZSM-5 membrane for forward osmosis operation

A potential of zeolite membrane in forward osmosis operation was investigated for the first time. A hydrophilic zeolite, ZSM-5, membrane was prepared on an α-alumina tubular support by a secondary growth method. NaCl aqueous solution and distilled water were used as draw and feed solutions, respectively. The membrane exhibited a water flux of 2.2 L m−2 h−1 with a salt flux of 0.69 g m−2 h−1 at 313 K by using 9.0 wt% draw solution. ZSM-5 membrane showed a quite small reverse salt flux based on a molecular sieving effect in the relatively wide temperature ranges of 298–333 K. ZSM-5 membrane also kept its performance under acidic conditions. In addition, obvious deterioration of the membrane performance was not observed after intermittent use of our membrane for more than one month.

Intermediate-sized molecular sieving of styrene from larger and smaller analogues.

Molecular sieving can lead to ultrahigh selectivity and low regeneration energy because it completely excludes all larger molecules via a size restriction mechanism. However, it allows adsorption of all molecules smaller than the pore aperture and so separations of complicated mixtures can be hindered. Here, we report an intermediate-sized molecular sieving (iSMS) effect in a metal-organic framework (MAF-41) designed with restricted flexibility, which also exhibits superhydrophobicity and ultrahigh thermal/chemical stabilities. Single-component isotherms and computational simulations show adsorption of styrene but complete exclusion of the larger analogue ethylbenzene (because it exceeds the maximal aperture size) and smaller toluene/benzene molecules that have insufficient adsorption energy to open the cavity. Mixture adsorption experiments show a high styrene selectivity of 1,250 for an ethylbenzene/styrene mixture and 3,300 for an ethylbenzene/styrene/toluene/benzene mixture (orders of magnitude higher than previous reports). This produces styrene with a purity of 99.9%+ in a single adsorption-desorption cycle. Controlling/restricting flexibility is the key for iSMS and can be a promising strategy for discovering other exceptional properties.

Polyvinylamine/graphene oxide/PANI@CNTs mixed matrix composite membranes with enhanced CO2/N2 separation performance

Lamellar PANI@CNTs-GO materials were prepared by intercalating polyaniline-coated carbon nanotubes (PANI@CNTs) in graphene oxide (GO) layers to regulate the interlayer spacing. Mixed matrix membranes (MMMs) with tuned structures were achieved by coating a mixed dispersion of PANI@CNTs-GO and polyvinylamine (PVAm) on an asymmetric polysulfone (PSf) membrane for enhancing its CO2 separation performance. The Fourier-transform infrared spectroscopy results showed that PANI was successfully coated on the surface of the CNTs. The X-ray diffraction results confirmed the intercalation of GO sheets with PANI@CNTs. PANI@CNTs-GO was uniformly dispersed in PVAm, as confirmed by scanning electron microscopy. The attenuated total reflectance Fourier transform infrared spectroscopy results revealed that strong interfacial interactions were present between PVAm and PANI@CNTs-GO. The MMM loaded with 1 wt% PANI@CNTs-GO showed the best CO2 separation performance with a CO2 permeance of 170 GPU and a CO2/N2 selectivity of 122.4 at 1 bar under the pure gas condition. These values were much higher than those of the pristine PVAm membrane (CO2 permeance = 73 GPU and CO2/N2 selectivity = 45.7). This enhanced separation performance can be mainly attributed to the effect of the facilitated transport carriers from amine groups in the interlayer spacing and the molecular sieving effect of the interlayer spacing. The MMMs exhibited excellent long-term stability even under the mixed-gas conditions for over 300 h with a CO2 permeance of 264 GPU and a CO2/N2 selectivity of 149.8.

Mass produced NaA zeolite membranes for pervaporative recycling of spent N-Methyl-2-Pyrrolidone in the manufacturing process for lithium-ion battery

N-methyl pyrrolidone (NMP) is a colorless and odorless liquid that has been heavily used in the manufacturing process of lithium ion batteries. The development of efficient technology for the on-site recycling of NMP is extremely significant in the lithium ion battery manufacturing plant. The pervaporation dehydration by mass produced NaA zeolite membranes followed by vacuum flash vaporization has proven to be an effective and economic way for recovering spent NMP in this paper. The pilot-scale pervaporation plant containing 225 tubular NaA zeolite membranes with a total membrane area of ca. 7.3 m2 was established for dehydration of actual industrial recovered NMP solution from a lithium battery production line. All technical indicators of the retentate (NMP ≥ 99.9%, H2O ≤ 105 ppm, color 10 Hz, refractive index 1.4694) have fully met the requirements of electronic-grade NMP for lithium-ion battery industry. Owing to the excellent molecular sieving effect of NaA zeolite membrane at extremely low water concentration, the NMP recovery yield was far better than the reported values in literature. These results suggested that the mass produced NaA zeolite membranes showed technical feasibility for the recycling of NMP in the lithium ion battery manufacturing process.

In situ fabricated porous carbon coating derived from metal-organic frameworks for highly selective solid-phase microextraction

Solid-phase microextraction (SPME) has been widely used in analysis of trace organic contaminants in environmental samples. Stable, efficient, and selective fiber coating is the key to the development of SPME technique. In this study, an in situ fabricated porous carbon coating that derived from metal-organic frameworks (MOFs) was introduced for solid-phase microextraction (SPME). The carbon coating (denoted as MOF-74-C) was prepared by the direct carbonization of a MOF-74 coating that was in situ grown on stainless steel wire, which was explored for the extraction of odorous organic contaminants for the first time. MOF-74-C coated fiber showed high extraction efficiency (1.1–16.0 times larger than commercial fibers for most tested analytes under the same extraction conditions), as well as good thermal and chemical stability. Furthermore, it can selectively extract the analytes with the simultaneous exclusion of coexisting dissolved organic matter due to the molecular sieve effect invoked by the well-defined micropores. The method based on MOF-74-C coating for analysis of odorants afforded wide linear ranges (more than two orders of magnitude), low detection limits (below the odor threshold concentrations), good intra- and inter-day accuracies (90.1–107.3%) and precisions (relative standard deviations/RSDs below 9.4%) and satisfactory reproducibility (RSDs below 8.7%). Finally, the MOF-74-C coated fiber was successfully applied for the SPME analysis of odorants in real water samples. These results highlight the great potential of in situ fabricated porous carbons derived from MOFs for SPME applications.

Ultra-selective carbon molecular sieve membranes for natural gas separations based on a carbon-rich intrinsically microporous polyimide precursor

A highly contorted, carbon-rich intrinsically microporous polyimide (PIM-PI) made from spirobifluorene dianhydride and 3,3-dimethylnaphthidine (SBFDA-DMN) was employed as a precursor for the formation of carbon molecular sieve (CMS) membranes at pyrolysis temperatures from 550 to 1000 °C. The high carbon content of SBFDA-DMN (∼84%) resulted in only 28% total weight loss during pyrolysis under a nitrogen atmosphere at 1000 °C. The development of the various microstructural textures was characterized by gas sorption analysis, Brunauer-Emmett-Teller (BET) surface area, X-ray diffraction, Raman spectroscopy, electrical conductivity, and gas transport properties. Heat treatment of a pristine SBFDA-DMN membrane at 550 °C resulted in reduced permeability for all gases (e.g.: PCO2 dropped from 4700 to 1500 barrer) as well as lower BET surface area from 621 to 545 m2 g−1. At 600 °C, new pores induced by pyrolysis increased the BET surface area to nearly that of the precursor and significantly improved gas separation performance. Above 600 °C, a progressive collapse of the micropores became evident with CMS membranes showing higher gas-pair selectivity but lower permeability. At 1000 °C, ultra-micropores comparable in size with the kinetic diameter of CH4 emerged and induced a prominent molecular sieving effect resulting in very high CH4 rejection. This strong size exclusion effect, further supported by gravimetric gas sorption measurements, resulted in unusually high N2/CH4 and CO2/CH4 selectivities of 35 and 1475, respectively.