Microporous Hybrid Materials Research Articles

Room-Temperature Monitoring of CH4 and CO2 Using a Metal-Organic Framework-Based QCM Sensor Showing Inherent Analyte Discrimination.

The detection of methane and carbon dioxide is of growing importance due to their negative impact on global warming. This is true for both environmental monitoring and leak detection in industrial processes. Although solid-state sensors are technologically mature, they have limitations that prohibit their use in certain situations, e.g., explosive atmospheres. Thus, there is a need to develop new types of sensor materials. Herein, we demonstrate a simple, low-cost, metal-organic framework (MOF)-based gas leak detection sensor. The system is based on gravimetric sensing by using a quartz crystal microbalance. The quartz crystal is functionalized by layer-by-layer growth of a thin metal-organic framework film. This film shows selective uptake of methane or carbon dioxide under atmospheric conditions. The hardware has low cost, simple operation, and theoretically high sensitivity. Overall, the sensor is characterized by simplicity and high robustness. Furthermore, by exploiting the different adsorption kinetics as measured by multiple harmonic analyses, it is possible to discriminate whether the response is due to methane or carbon dioxide. In summary, we demonstrate data relevant toward new applications of metal-organic frameworks and microporous hybrid materials in sensing.

Metal organic framework- modified monolithic column immobilized with pepsin for enantioseparation in capillary electrochromatography

Metal-organic frameworks (MOFs), defined as a class of microporous hybrid materials, are established by coordination of metal cations with functional organic ligands. In addition to outstanding porosity and large surface area, the structures and functions of MOFs can be designed and adjusted based on different destinations, which would be employed as stationary phases in various chromatographic modes. Pepsin, a class of protein, has been investigated as chiral selector owing to their unique interactions with analytes based on chiral or affinity selectivity. In this work, MOF-5 was exploited to grow on the support of poly (glycidyl methacrylate)–co-(ethylene dimethacrylate) [poly(GMA-co-EDMA)] monoliths by layer-by-layer self-assembly method. Pepsin was subsequently bonded with the carboxyl group of MOF-5 through amidation reaction. The ultimate pepsin@MOF-5@poly(GMA-co-EDMA) column was applied for enantioseparation of six basic chiral drugs by capillary electrochromatography with good resolution and repeatability. Compared with the pepsin modified monolithic column, the newly prepared column with MOF-5 shows significantly enhanced enantiomeric resolution, which reveals that MOFs-modified capillary monolithic columns lead a promising road to separation of racemates by chromatography.

Fluorescent Nanozeolite Receptors for the Highly Selective and Sensitive Detection of Neurotransmitters in Water and Biofluids.

The design and preparation of synthetic binders (SBs) applicable for small biomolecule sensing in aqueous media remains very challenging. SBs designed by the lock-and-key principle can be selective for their target analyte but usually show an insufficient binding strength in water. In contrast, SBs based on symmetric macrocycles with a hydrophobic cavity can display high binding affinities but generally suffer from indiscriminate binding of many analytes. Herein, a completely new and modular receptor design strategy based on microporous hybrid materials is presented yielding zeolite-based artificial receptors (ZARs) which reversibly bind the neurotransmitters serotonin and dopamine with unprecedented affinity and selectivity even in saline biofluids. ZARs are thought to uniquely exploit both the non-classical hydrophobic effect and direct non-covalent recognition motifs, which is supported by in-depth photophysical, and calorimetric experiments combined with full atomistic modeling. ZARs are thermally and chemically robust and can be readily prepared at gram scales. Their applicability for the label-free monitoring of important enzymatic reactions, for (two-photon) fluorescence imaging, and for high-throughput diagnostics in biofluids is demonstrated. This study showcases that artificial receptor based on microporous hybrid materials can overcome standing limitations of synthetic chemosensors, paving the way towards personalized diagnostics and metabolomics.

Diammonium-Pillared MOPS with Dynamic CO2 Selectivity

Summary The climate challenge calls for the design of energy-efficient CO2-separation materials. CO2-selective solid physisorbents are characterized by a low energy penalty and enable the most promising and energy-efficient CO2 separation technologies by applying vacuum swing adsorption (VSA). Here, we show that a diammonium-pillared microporous organically pillared layered silicate 7 (MOPS-7) represents a CO2-selective physisorbent, synthesized by simple ion exchange of an inexpensive clay. This MOPS provides high dynamic, reversible, and reproducible CO2 uptakes and selectivities proven in dynamic breakthrough experiments mimicking flue gas, natural gas, and biogas conditions. The selective interaction with CO2 is dominated by the multipole interactions with the microporous hybrid material. Thus, the energy penalty attributed to regeneration is intrinsically reduced. The simplicity and modularity of pillaring low-budget and environmentally friendly clays, combined with industrially capable performance with respect to thermal stability, high dynamic CO2 adsorption selectivity, and energy-efficient restoration renders MOPS-7 a highly attractive CO2 adsorbent.

Microporous Organically Pillared Layered Silicates (MOPS): A Versatile Class of Functional Porous Materials.

The design of microporous hybrid materials, tailored for diverse applications, is a key to address our modern society's imperative of sustainable technologies. Prerequisites are flexible customization of host-guest interactions by incorporating various types of functionality and by adjusting the pore structure. On that score, metal-organic frameworks (MOFs) have been the reference in the past decades. More recently, a new class of microporous hybrid materials emerged, microporous organically pillared layered silicates (MOPS). MOPS are synthesized by simple ion exchange of organic or metal complex cations in synthetic layered silicates. MOFs and MOPSs share the features of "component modularity" and "functional porosity". While both, MOFs and MOPS maintain the intrinsic characteristics of their building blocks, new distinctive properties arise from their assemblage. MOPS are unique since allowing for simultaneous and continuous tuning of micropores in the sub-Ångström range. Consequently, with MOPS the adsorbent recognition may be optimized without the need to explore different framework topologies. Similar to the third generation of MOFs (also termed soft porous crystals), MOPS are structurally ordered, permanently microporous solids that may also show a reversible structural flexibility above a distinct threshold pressure of certain adsorbents. This structural dynamism of MOPS can be utilized by meticulously adjusting the charge density of the silicate layers to the polarizability of the adsorbent leading to different gate opening mechanisms. The potential of MOPS is far from being fully explored. This Concept article highlights the main features of MOPS and illustrates promising directions for further research.

Constant Volume Gate-Opening by Freezing Rotational Dynamics in Microporous Organically Pillared Layered Silicates.

Microporous organically pillared layered silicates (MOPS) are a class of microporous hybrid materials that, by varying pillar density, allows for optimization of guest recognition without the need to explore different framework topologies. MOPS are found to be capable of discriminating two very similar gases, carbon dioxide and acetylene, by selective gate-opening solely through quenching pillar dynamics. Contrary to conventional gate-opening in metal organic frameworks, the additional adsorption capacity is realized without macroscopic volume changes, thus avoiding mechanical stress on the framework. Of the two gases studied, only CO2 can accomplish freezing of pillar dynamics. Moreover, the shape of the slit-type micropores in MOPS can easily be fine-tuned by reducing the charge density of the silicate layers. This concomitantly reduces the Coulomb attraction of cationic interlayer space and anionic host layers. Surprisingly, we found that reducing the charge density then alters the gate-opening mechanism to a conventional structural gate-opening involving an increase in volume.

ChemInform Abstract: The Largely Unknown Class of Microporous Hybrid Materials: Clays Pillared by Molecules

AbstractReview: 103 refs.

The Largely Unknown Class of Microporous Hybrid Materials: Clays Pillared by Molecules

AbstractThe review summarizes recent progress in the field of synthetic clay minerals pillared with organic or metal complex cations (PILCs). We briefly introduce the field of such PILCs and discuss the weaknesses of the state of the art characterization chart of PILCs. When PILCs are made from nanoscopic host materials like natural clay minerals, charge heterogeneity accompanied by interstratification, the turbostratic disorder in the stacking and the influence of the large external surface weaken essential analytical results. Turning to coarse‐grained, well ordered clay minerals synthesized from the melt at temperatures above 1000 K removes all these obstacles and allows to present consistent data underlining the validity of the appealing pillaring concept: Refinement of PILC structures gives information on the pillar‐host interaction and the orientation of pillars in the interlayer space. Two‐dimensional superstructure reflections give direct evidence of the well‐ordered lateral arrangement of pillars. And most importantly, analysis of chemical composition and physisorption isotherms are not corrupted by contributions of large external surfaces and this allows for predicting micropore volume and widths from pillar/host ratios in combination with pillar size/shape. Moreover, recent progress allows fine‐tuning the charge density of the host lattice post‐synthesis. Such deliberate and fine‐graded alteration of the pillar density in turn permits adjusting the pore size in steps as small as 0.1 Å to a given adsorbate and thus will pave the way to maximize adsorption enthalpies and to improve selectivity. This feature is unique to PILCs and is not available to MOFs whose porosity can be adjusted only in steps no smaller than the order of chemical bond lengths. Rational design of microporous hybrid materials with full control over size, shape, and chemical nature of micropores certainly represents the strength and great potential of the pillaring approach.

Application of Sulfonic Acid Functionalised Hybrid Silicas Obtained by Oxidative Cleavage of Tetrasulfide Bridges as Catalysts in Esterification Reactions

AbstractHybrid materials with tetrasulfide bridges were synthesised by co‐condensation between bis[3‐(triethoxysilyl)propyl]tetrasulfide and either tetraethyl orthosilicate or 1,4‐bis(triethoxysilyl)benzene under acidic conditions in the presence of Brij‐76 as a structure‐directing agent. They were post‐modified by treatment with H2O2 for 3 h or 24 h. A progressive loss of the mesostructure in the resulting materials was observed upon increasing the S content in the synthesis mixtures. Thus, only those samples with the lowest tetrasulfide content could be considered as periodic mesoporous organosilicas. This effect was even more pronounced upon oxidation. The oxidised materials exhibited a considerable acidity, particularly those with the two precursors in approximately equal amounts. All sulfonic acid functionalised materials were used as catalysts in the esterification of acetic acid with ethanol, and they exhibited a comparable or very similar activity to that of Amberlyst‐15. Several mesoporous and microporous hybrid materials were selected and compared to this commercial resin in the transformation of more hydrophobic substrates. Some of these hybrid materials were especially attractive for the transformation of benzyl alcohol and various secondary alcohols.

Polysiloxane derived hybrid ceramics with nanodispersed Pt

Platinum containing hybrid ceramics were prepared by the pyrolytic conversion of a mixture of a methyl phenyl polysiloxane and platinic acid or a modified mixture containing in addition 3-aminopropyltriethoxysilane (APTE) as complexing agent in an atmosphere of nitrogen. Due to the moderate pyrolysis temperature of 500 °C microporous hybrid materials with high specific surface areas could be generated. By the addition of APTE the incorporation of the platinum as complexed metal ions led to well distributed metal nanoparticles. Furthermore, clustering or growth of particles could be suppressed during pyrolysis due to a stable embedding. By variation of metal loading and the ratio of APTE to Pt it was also possible to change the surface related properties of the microporous hybrids. Using CO oxidation as model reaction it could be shown that the addition of APTE as complexing agent is crucial to enhance the accessibility of the metal particles and the catalytic activity.

Preparation of Microporous Hybrid Materials by Thermal Removal of Sterically Hindered Carbosilane Side Groups

The preparation of hybrid porous materials by condensation of poly(dimethyl-co-methyl)siloxanes functionalized with side units: i.e., templating –RC(SiMe3)3 [R = (CH2)5, (CH2)2SiMe2], and reactive –Si(OEt)3, of random distribution along the main chain is described. Tetrabutylammonium salts, tetrabutylammonium hydroxide and tetrabutylammonium fluoride were chosen as the catalytic system. The relationship between the steric hindrance of the carbosilane template, the type of n-Bu4N+ catalyst and the structure of cross-linked materials was investigated. It was found that bulky carbosilane substituents can block formation of the porous structure in the cross-linked material. The type of interaction between Si(OEt)3 and the catalyst seems to be an equally important factor for 3D structure formation. Controlled thermolysis of the cross-linked materials, performed in order to remove carbosilane units by thermal degradation, allowed for the formation of porous materials having high specific surface area.

Surface Modification and Functionalization of Microporous Hybrid Material for Luminescence Sensing

We report here on the preparation of novel luminescent core-shell material by initial coating with polyelectrolytes and subsequent with a silica shell on the lanthanide complexes loaded zeolite L microcrystals. Lanthanide complexes loaded zeolite L was prepared by insertion of 2-thenoyltrifluoroacetone (TTA) into the nanochannels of zeolite crystals by gas diffusion of TTA to Eu(3+) exchanged zeolite L, coating a silica shell on the lanthanide complexes loaded zeolite L resulted to the novel luminescent core-shell material. The luminescent core-shell material was further functionalized with silylated terbium(III) complex and the obtained material was used as the luminescence sensing of dipicolinic acid (DPA), which is a major constituent of many pathogenic spore-forming bacteria.

Hydrophilicity-controllable Microporous Hybrid Materials by Anion Exchange

Abstract Poly(acrylic acid) (PAA)/silica hybrids having imidazolium chloride [IL(Cl−)] moieties could be prepared by utilizing ionic interactions between anionic carboxylic acid groups of PAA and cationic imidazolium groups of silica phase. By removing PAA from the obtained hybrids, we could obtain hydrophilicity-controllable microporous hybrids by means of anion-exchange reaction of imidazolium moieties.

Synthesis and Properties of Porous Hybrid Materials containing Metallic Nanoparticles

Highly microporous hybrid materials with embedded Ni or Pt nanoparticles were prepared by pyrolytic conversion of polysiloxane - metal salt precursors and characterised in terms of surface area, sorption behaviour of water and n-heptane, size and accessibility of metallic nanoparticles. Upon varying the amount of polysiloxanes carrying non-polar and polar organic groups in the precursors the surface properties of hybrid materials could be altered from strongly hydrophobic to mainly hydrophilic. The size of metallic nanoparticles was influenced by the metal content and the presence of non-reductive phenyl polysiloxane. In case of Pt, very small sizes in the nanometer regime (2-4 nm) could be obtained. During CO oxidation Pt containing ceramers showed high catalytic activity.

Prototype high-throughput system for hydrothermal synthesis and X-ray diffraction of microporous and mesoporous materials

In the hydrothermal synthesis of microporous and mesoporous materials a large number of parameters including chemical composition of the mixture, reagent sources, organic templates, temperature, and time play a role. The high-throughput experimental approach facilitates screening of this multi-parameter space for conditions favoring formation of desired zeolite phases and ordered mesoporous materials. A critical issue in the development of a high-throughput system for hydrothermal synthesis is the avoidance of cross-contamination between the individual synthesis volumes. Further important issues are automation of reactant dosing, product filtration, calcination and identification. In this work, a prototype multi-clave was designed with three 5 mL synthesis vials for operation at temperatures up to 200 °C under autogenous pressure. The multi-clave is equipped with a sample tray for filtration, washing and drying of all of the synthesis products at the same time. This multi-filter also serves as the sample holder for X-ray diffraction characterization. The innovation resides in the multi-clave concept and the integration of the entire process from synthesis over product workup to XRD characterization without necessitating individual sample transfer. The performance of the prototype was evaluated with the synthesis of the mesoporous–microporous hybrid material zeotile-2, and of clathrasils.

Dehydration of xylose into furfural over micro-mesoporous sulfonic acid catalysts

Surfactant-templated micro-mesoporous silicas possessing sulfonic acid groups (SAGs) have been prepared, characterized, and tested as catalysts in the dehydration of d-xylose to furfural. All of the materials possessed catalytic activity. In general, selectivity to furfural was lower for a poorly ordered microporous hybrid material, prepared via the co-condensation of (3-mercaptopropyl)trimethoxysilane with bis(trimethoxysilylethyl)benzene, than for mesoporous MCM-41 silica anchored with SAGs via postsynthesis modification. The MCM-41 material with the highest loading of SAGs (0.7 meq g −1) displayed fairly high selectivity for furfural (ca. 82% in DMSO or water/toluene mixture) at high xylose conversion (>90% within 24 h, at 140 °C). Xylose conversion increased significantly with reaction temperature. At 170 °C, more than 85% conversion was achieved within 4 h with any of the sulfonic acid-functionalized catalysts. Furfural yield tended to increase with temperature. Xylose conversion increased with increasing amount of catalyst, and for a xylose/MCM-41-SO 3H ratio of 0.5, 76% conversion was achieved within 4 h, at 140 °C. Catalyst deactivation was observed after long residence times, possibly because of the interaction of reaction products with the acid sites, leading to surface loading.