Molecular Sieving Behavior Research Articles

Industrial Separation Challenges: How Does Supramolecular Chemistry Help?

The chemical industry and the chemical processes underscoring it are under intense scrutiny as the demands for the transition to more sustainable and environmentally friendly practices are increasing. Traditional industrial separation systems, such as thermally driven distillation for hydrocarbon purification, are energy intensive. The development of more energy efficient separation technologies is thus emerging as a critical need, as is the creation of new materials that may permit a transition away from classic distillation-based separations. In this Perspective, we focus on porous organic cages and macrocycles that can adsorb guest molecules selectively through various host-guest interactions and permit molecular sieving behavior at the molecular level. Specifically, we summarize the recent advances where receptor-based adsorbent materials have been shown to be effective for industrially relevant hydrocarbon separations, highlighting the underlying host-guest interactions that impart selectivity and permit the observed separations. This approach to sustainable separations is currently in its infancy. Nevertheless, several receptor-based adsorbent materials with extrinsic/intrinsic voids or special functional groups have been reported in recent years that can selectively capture various targeted guest molecules. We believe that the understanding of the interactions that drive selectivity at a molecular level accruing from these initial systems will permit an ever-more-effective "bottom-up" design of tailored molecular sieves that, in due course, will allow adsorbent material-based approaches to separations to transition from the laboratory into an industrial setting.

Unexpected Penetration of CO Molecule into Zeolitic Micropores Almost Plugged by CuCl via π-Complexation of CO-CuCl.

Carbon monoxide (CO) is a key reactant in several Fischer-Tropsch processes, including those used in light olefin and methanol syntheses. However, it is highly toxic and causes serious poisoning of noble metal catalysts. Thus, a solid adsorbent that can selectively capture CO, especially at low concentrations, is required. In this study, zeolite Y-based adsorbents in which Cu(I) ions occupy the supercage cation sites (CuCl/Y) are prepared via solid-state ion exchange. Volumetric adsorption measurements reveal that the Cu(I) ions significantly enhance CO adsorption in the low-pressure range by π-complexation. Furthermore, unexpected molecular sieving behavior, with extremely high CO/CO2 selectivity, is observed when excess CuCl homogeneously covers the zeolite pore structures. Thus, although CO has a larger kinetic diameter, it can penetrate the zeolite supercage while smaller molecules (i.e., Ar and CO2) cannot. Density functional theory calculations reveal that CO molecules can remain adsorbed in pseudoblocked pores by CuCl, thanks to the strong interaction of C 2p and Cu 3d states, resulting in the high CO/CO2 selectivity. One of the prepared adsorbents, CuCl/Y with 50 wt % CuCl, is capable of selectively capturing 3.04 mmol g-1 of CO with a CO/CO2 selectivity of >3370.

Nonaqueous Interfacial Polymerization-Derived Polyphosphazene Films for Sieving or Blocking Hydrogen Gas.

A series of cyclomatrix polyphosphazene films have been prepared by nonaqueous interfacial polymerization (IP) of small aromatic hydroxyl compounds in a potassium hydroxide dimethylsulfoxide solution and hexachlorocyclotriphosphazene in cyclohexane on top of ceramic supports. Via the amount of dissolved potassium hydroxide, the extent of deprotonation of the aromatic hydroxyl compounds can be changed, in turn affecting the molecular structure and permselective properties of the thin polymer networks ranging from hydrogen/oxygen barriers to membranes with persisting hydrogen permselectivities at high temperatures. Barrier films are obtained with a high potassium hydroxide concentration, revealing permeabilities as low as 9.4 × 10-17 cm3 cm cm-2 s-1 Pa-1 for hydrogen and 1.1 × 10-16 cm3 cm cm-2 s-1 Pa-1 for oxygen. For films obtained with a lower concentration of potassium hydroxide, single gas permeation experiments reveal a molecular sieving behavior, with a hydrogen permeance of around 10-8 mol m-2 s-1 Pa-1 and permselectivities of H2/N2 (52.8), H2/CH4 (100), and H2/CO2 (10.1) at 200 °C.

Pore-Mouth Structure of Highly Agglomerated Detonation Nanodiamonds.

Detonation nanodiamond aggregates contain water that is removed by thermal treatments in vacuo, leaving available pores for the adsorption of target molecules. A hard hydrogel of detonation nanodiamonds was thermally treated at 423 K for 2 h, 10 h, and 52 h in vacuo to determine the intensive water adsorption sites and clarify the hygroscopic nature of nanodiamonds. Nanodiamond aggregates heated for long periods in vacuo agglomerate due to the removal of structural water molecules through the shrinkage and/or collapse of the pores. The agglomerated nanodiamond structure that results from long heating periods decreases the nitrogen adsorption but increases the water adsorption by 40%. Nanodiamonds heated for long times possess ultramicropores <0.4 nm in diameter in which only water molecules can be adsorbed, and the characteristic mouth-shaped mesopores adsorb 60% more water than nitrogen. The pore mouth controls the adsorption in the mesopores. Long-term dehydration partially distorts the pore mouth, decreasing the nitrogen adsorption. Furthermore, the nitrogen adsorbed at the pore mouth suppresses additional nitrogen adsorption. Consequently, the mesopores are not fully accessible to nitrogen molecules because the pore entrances are blocked by polar groups. Thus, mildly oxidized detonation nanodiamond particles can show a unique molecular sieving behavior.

Site-directed reduction engineering within bimetal-organic frameworks for efficient size-selective catalysis

<h2>Summary</h2> Directional processing of metal-organic frameworks (MOFs) into unique hybrid materials with desired structures and properties is a key scientific challenge in exploring the enhanced functionality and potential applications of MOF compounds. Here, the reducibility of MOF clusters is first revealed via theoretical calculations from the viewpoint of the reduction potential energy (<i>E</i>_reduction), which serves as the basic principle for the site-directed reduction processing of MOFs. During this process, the active component with a high <i>E</i>_reduction in bimetallic MOFs is selectively reduced to metal nanostructures, while the other (inert) parts maintain their structural integrity. A series of hierarchical MOFs/metal nanoparticle composites with different structures, including yolk-shell, core-shell, and dispersed, have been successfully customized. More importantly, the as-prepared hybrid materials (Co-B@ZIF-8) exhibit unique size-selective properties in catalyzing ketone hydrogenation, which originate from the combination of the high catalytic performance of the nanoscale metal catalysts and the molecular sieving behavior of the MOFs' well-defined microporous nature.

Fabrication of c-oriented ultrathin TCPP-derived 2D MOF membrane for precise molecular sieving

Two-dimensional (2D) membrane has brought promising opportunities to achieve the separation performance for overcoming the Robeson upper bound. Nevertheless, precise control of the in-plane pore size as well as the interlayer structure on a microscopic scale remains a great challenge. In this work, we produced well-intergrown, highly c-oriented ultrathin (ca. 80 nm-thick) 2D Cu-TCPP membranes on the porous alumina substrates through in-situ solvothermal method. Regular vertical stacking of single Cu-TCPP layers along the c-axis through van der Waals interactions resulted in narrowed pore size (equivalent to ~5.4 Å) and interlocked interlayer gallery. Gas permeation results indicated that prepared Cu-TCPP membranes showed precise size-selective molecular sieving behavior, owing to rational interlayer stacking on a microscopic scale as well as deliberate microstructural manipulation on a mesoscopic scale.

Site-Directed Reduction Engineering within Binary Metal-Organic Frameworks for Efficient Size-Selective Catalysis

Directional processing of metal-organic frameworks (MOFs) into unique hybrid materials with desired structures and properties is a key scientific challenge in exploring the enhanced functionality and potential applications of MOFs compounds. Herein, the reducibility of MOFs clusters first revealed via theoretical calculations from viewpoint of the reduction potential energy (Ereduction), which serves as the basic principle for the site-directed reduction processing of MOFs. The active component with a high Ereduction in binary MOFs is selectively reduced to metal nanostructures, while the other (inert) parts maintain their structural integrity in this process. A series of hierarchical MOFs/metal nanoparticle composites with different structures, including yolk-shell, core-shell and dispersed, have been successfully customized. More importantly, the as-prepared hybrid materials (Co-B@ZIF-8) exhibit unique size-selective properties in catalyzing ketone hydrogenation, which originate from the combination of the high catalytic performance of the nanoscale metal-catalysts and the molecular sieving behavior of the MOFs’ well-defined microporous nature.

Polyoctahedral Silsesquioxane Hexachlorocyclotriphosphazene Membranes for Hot Gas Separation.

There is a need for gas separation membranes that can perform at high temperatures, for example, for CO2 capture in industrial processes. Polyphosphazenes classify as interesting materials for use under these conditions because of their high thermal stability, hybrid nature, and postfunctionalization options. In this work, thin-film composite cyclomatrix polyphosphazene membranes are prepared via the interfacial polymerization reaction between polyhedral oligomeric silsesquioxane and hexachlorocyclotriphosphazene on top of a ceramic support. The prepared polyphosphazene networks are highly crosslinked and show excellent thermal stability until 340 °C. Single gas permeation experiments at temperatures ranging from 50 to 250 °C reveal a molecular sieving behavior, with permselectivities as high as 130 for H2/CH4 at the low temperatures. The permselectivities of the membranes persist at the higher temperatures; at 250 °C H2/N2 (40), H2/CH4 (31) H2/CO2 (7), and CO2/CH4 (4), respectively, while maintaining permeances in the order of 10–7 to 10–8 mol m–2 s–1 Pa–1. Compared to other types of polymer-based membranes, especially the H2/N2 and H2/CH4 selectivities are high, with similar permeances. Consequently, the hybrid polyphosphazene membranes have great potential for use in high-temperature gas separation applications.

Size‐Based Cationic Molecular Sieving through Solid‐State Nanochannels

AbstractThe molecular sieving behavior of soft‐etched polyimide membranes having negatively charged nanochannels is described experimentally and theoretically using alkali metal–crown ether cationic complexes and alkylammonium cations. To this end, the electrical conduction and current rectification obtained with different alkali electrolyte solutions (LiCl, NaCl, and KCl) and crown ether molecules (12‐crown‐4, 15‐crown‐5, and 18‐crown‐6) are studied. The results suggest that only the [Li(12C4)]+ complex can readily permeate through the nanochannels because significant current decreases are obtained in the cases of the [Na(15C5)]+ and [K(18C6)]+ complexes. In solutions of organic cations ranging from ammonium (NH4+) to alkylammonium (R4N+) with increasing molecular size, only the smaller ions can conduct high electric currents, suggesting again that the membrane channels are in the nanometer range. Taken together, the observed current decreases and rectification phenomena demonstrate that the functionalized membranes allow a versatile combination of molecular and electrostatic sieving.

Semiconductive Nature of Lead-Based Metal-Organic Frameworks with Three-Dimensionally Extended Sulfur Secondary Building Units.

Recently, metal-organic frameworks (MOFs) composed of sulfur secondary building units (sulfur-SBUs) have attracted significant attention as unique electronic materials with high conductivities and photo- and electrocatalytic properties. Herein we report the crystal structure of KGF-1, an example of a Pb-MOF composed of three-dimensionally extended sulfur-SBUs that displays molecular sieving behavior, visible-light absorption, and a semiconductor band structure and is a hydrogen-evolution photocatalyst.

High-flux mixed matrix membranes containing bimetallic zeolitic imidazole framework-8 for C3H6/C3H8 separation

Membrane-based gas separation requires highly delicate engineering of molecular structures for the desired separation performance. Herein, a new efficient approach to fabricate high-flux mixed matrix membranes (MMMs) for C3H6/C3H8 separation is reported by utilizing ZnCo-mixed hybrid zeolitic imidazolate framework-8 (ZIF-8-67) with unexpectedly large aperture size. Our structural characterizations in molecular level reveal that the ZIF-8-67 exhibits a larger aperture size compared to ZIF-8. Such a large aperture size of ZIF-8-67 is attributed to the large angle between two neighboring ligands and the long metal-ligand bonding distance, which is confirmed by refining the computed XRD pattern until it matches the experimental one (Rietveld refinement). More importantly, the inclusion of ZIF-8-67 in the 6FDA-DAM matrix enhances the C3H6 permeability by up to 240% with the moderate C3H6/C3H8 selectivity improvement (70%), as compared to those of the polymeric counterpart. Furthermore, the temperature-dependent transport characterization verifies that the ZIF-containing MMMs improve the C3H6/C3H8 energetic selectivity at the sacrifice of entropic selectivity due to a unique molecular sieving behavior of the ZIFs, thereby enhancing the diffusivity selectivity. This study proposes a simple strategy to enable the fine-tuning of the aperture size of parent ZIFs towards the desired gas separation performance.

Oriented Zeolitic imidazolate framework membranes within polymeric matrices for effective N2/CO2 separation

Nanocomposite membranes incorporating high-aspect-ratio nanostructures within polymeric matrices are highly promising for controllable molecular separation. Zeolitic-imidazolate frameworks (ZIFs) has been extensively investigated as nanofillers or surface coating materials devising innovative membrane design. This work for the first time reports an innovative approach to in-situ form ZIFs within polymeric membrane matrix to regulate the molecular sieving behavior of the thin composite layer. A chelation-assisted interfacial reaction strategy was applied to incorporate vertically-oriented ZIF into the crosslinked poly (vinylamine) (PVAm) matrix. PVAm has a good ability to complex with Zn2+, thus provides a highly concentrated zinc source within the matrix and ensures good adhesion between the ZIFs and substrates. Due to the intimate integration of ZIFs with PVAm, the composite layers have good flexibility and tensile strength. It also leads to the absence of micro-defects within the layer. The membranes exhibited reversed selectivity for the CO2/N2 gas pair, with N2/CO2 selectivity of 51.9 and N2 permeance of 174.9 GPU. The results of this work provide an important technique to regulate the interfacial interaction to regulate the gas separation behavior within the composite membrane.

Facile Construction of Structurally Defined Porous Membranes from Supramolecular Hexakistriphenylamine Metallacycles through Electropolymerization

The construction of well-controlled porous materials is very challenging. Herein, we report the successful preparation of structurally defined porous membranes based on hexakistriphenylamine metallacycles through electropolymerization. The newly designed porous materials were characterized by the typical cyclic voltammograms, XPS, SEM, and TEM investigations. Further investigations revealed that the metallacycle-based polymer films displayed a good size-selective molecular-sieving behavior.

Formation of Ultrathin, Continuous Metal-Organic Framework Membranes on Flexible Polymer Substrates.

Metal-organic framework (MOF) materials have an enormous potential in separation applications, but to realize their potential as semipermeable membranes they need to be assembled into thin continuous macroscopic films for fabrication into devices. By using a facile immersion technique, we prepared ultrathin, continuous zeolitic imidazolate framework (ZIF-8) membranes on titania-functionalized porous polymeric supports. The coherent ZIF-8 layer was surprisingly flexible and adhered well to the support, and the composite membrane could sustain bending and elongation. The membranes exhibited molecular sieving behavior, close to the theoretical permeability of ZIF-8, with hydrogen permeance up to 201×10(-7) mol m(-2) s(-1) Pa(-1) and an ideal H2 /CO2 selectivity of 7:1. This approach offers significant opportunities to exploit the unique properties of MOFs in the fabrication of separation and sensing devices.

Formation of Ultrathin, Continuous Metal–Organic Framework Membranes on Flexible Polymer Substrates

AbstractMetal–organic framework (MOF) materials have an enormous potential in separation applications, but to realize their potential as semipermeable membranes they need to be assembled into thin continuous macroscopic films for fabrication into devices. By using a facile immersion technique, we prepared ultrathin, continuous zeolitic imidazolate framework (ZIF‐8) membranes on titania‐functionalized porous polymeric supports. The coherent ZIF‐8 layer was surprisingly flexible and adhered well to the support, and the composite membrane could sustain bending and elongation. The membranes exhibited molecular sieving behavior, close to the theoretical permeability of ZIF‐8, with hydrogen permeance up to 201×10−7 mol m−2 s−1 Pa−1 and an ideal H2/CO2 selectivity of 7:1. This approach offers significant opportunities to exploit the unique properties of MOFs in the fabrication of separation and sensing devices.

Pure- and mixed-gas propylene/propane permeation properties of spiro- and triptycene-based microporous polyimides

Solution-processable polyimides of intrinsic microporosity (PIM-PIs) comprising relatively inflexible and contorted backbones have demonstrated outstanding molecular-sieving behavior in membrane-based separation of gas/gas pairs. In this work, the effects of systematically increasing intra-chain rigidity on the propylene/propane separation properties were compared for PIM-PIs made from 2,3,5,6-tetramethyl-1,4-phenylene diamine (TMPD) and (i) spiro-centered dianhyride (SPDA-TMPD or PIM-PI-1) and (ii) 9,10-diisopropyltriptycene-based dianhydride (TPDA-TMPD or KAUST PI-1). Pure-gas experiments at 2bar and 35°C showed significant increases in C3H6 permeability and C3H6/C3H8 selectivity by transitioning from PIM-PI-1 (P(C3H6)=393 Barrer, α(C3H6/C3H8)=6) to KAUST-PI-1 (P(C3H6)=817Barrer, α(C3H6/C3H8)=16), positioning KAUST-PI-1 considerably above the experimentally observed pure-gas C3H6/C3H8 polymer upper bound. However, 50:50 C3H6/C3H8 mixed-gas feeds induced significant losses in C3H6 permeability and C3H6/C3H8 selectivity relative to the 2bar pure-gas data for PIM-PI-1 and KAUST-PI-1 as the C3H6/C3H8 selectivity dropped from 6 to 3 and 16 to 5, respectively, at 2bar C3H6 partial pressure due to plasticization and competitive sorption.

Double-layered polyion complex for application to biosensing electrodes

Polyion complex (PIC) layers are formed via self-assemble ion pairing by alternately placing polycation/polyanion solutions on platinum electrodes. Their molecular sieving behavior for electrochemically reactive species through the layers is examined by electrochemical methods. Different molecular-sieve sizes are achieved by the selection of polycations with different intramolecular distances of neighboring cationic groups. Amperometric glucose sensing electrodes are fabricated by immobilizing glucose oxidase in a PIC double-layer. The proper double-layer configuration is efficient to improve the glucose response, blocking interferents due to the molecular sieving effect.

One step in situ synthesis of supported zeolitic imidazolate framework ZIF-8 membranes: Role of sodium formate

Zeolitic imidazolate frameworks (ZIFs), a sub-class of metal–organic frameworks (MOFs), are noted for their remarkable thermal and chemical stability, tunable microporous channels, and tailorable physical/chemical properties. When synthesized as films, they hold great potentials for gas sensing, catalytic membrane reactor, and gas separation membrane applications. Here, we report one step in situ synthesis of ZIF-8 membranes on unmodified porous α-alumina supports in the presence of sodium formate. In this in situ method, sodium formate plays a critical role for formation of well-intergrown continuous ZIF-8 membranes. Sodium formate was found to enhance the heterogeneous nucleation of ZIF-8 crystals on alumina supports as well as to promote intergrowth of ZIF-8 crystals. It was confirmed that sodium formate reacts with zinc source to form zinc oxide layers on α-alumina supports, which in turn promote heterogeneous nucleation. ZIF-8 membranes show molecular sieving behavior, favoring smaller molecules. It was found that sodium formate promotes heterogeneous nucleation in other ZIF systems as well, leading to continuous films of ZIF-7, Zn(Im)2 (ZIF-61 analog), ZIF-90, and SIM-1.

Microstructure of carbon molecular sieve membranes and their application to separation of aqueous bioethanol

Pyrolytic carbons were fabricated from a Kapton precursor. The microstructure of the carbon molecular sieve membranes (CMSMs) was examined by means of sorption tests using pure liquid sorbates of different molar volumes and with the application of positron annihilation lifetime spectroscopy (PALS). The pore sizes of the CMSMs were calculated by PALS and our recently proposed equation relating positron lifetime with free volume radius. The mean pore radius in the CMSMs was demonstrated to be smaller than 3Å. Their ability to separate water from bioethanol was determined. Results of aqueous bioethanol sorption showed evident molecular sieving behavior of the CMSMs despite their hydrophobic surface. This behavior became significant at a high pyrolysis end temperature of 700°C, at which the carbon content was high. The c700 membrane delivered a high permeation flux of 1199g/m2h and a high separation factor of 683. This performance suggests that CMSMs have more potential to separate water from bioethanol through pervaporation or vapor permeation than organic membranes.

Preparation and Gas Adsorption Characteristics of Zeolite MFI Crystals with Organic-Functionalized Interiors

We show that the selective gas adsorption properties of pure-silica MFI zeolite crystals are significantly altered by modifying their internal pore structure with molecules such as aliphatic alcohols, aromatic alcohols, amino-alcohols, and amines. NMR and TGA/DSC characterization indicates that the functional groups are chemisorbed to the internal pores, and the organic molecule loading is strongly dependent on the concentration of internal silanol sites. Adsorption isotherms of CO2, CH4, and N2 are measured for the functionalized samples up to 400 kPa. In the low pressure region (1–10 kPa), the organic-modified materials show gas adsorption behavior significantly different from that of the bare zeolite MFI, ranging from an enhancement in CO2/CH4 sorption selectivity in the amine-functionalized materials to an enhancement in CH4 adsorption in the aromatic-functionalized material. The CO2/N2 sorption selectivity decreases for all the modified materials compared to bare zeolite MFI. This investigation of organic-modified MFI zeolites points toward the tuning of adsorption and molecular sieving behavior of zeolites through internal modification of the pores.