Microporous Environment Research Articles

Enhanced Catalytic Activity through the Tuning of Micropore Environment and Supercritical CO2 Processing: Al(Porphyrin)-Based Porous Organic Polymers for the Degradation of a Nerve Agent Simulant

An Al(porphyrin) functionalized with a large axial ligand was incorporated into a porous organic polymer (POP) using a cobalt-catalyzed acetylene trimerization strategy. Removal of the axial ligand afforded a microporous POP that is catalytically active in the methanolysis of a nerve agent simulant. Supercritical CO2 processing of the POP dramatically increased the pore size and volume, allowing for significantly higher catalytic activities.

Relationship between Channel and Sorption Properties in Coordination Polymers with Interdigitated Structures

Porous coordination polymers constructed from Zn(2+) and isophthalate with linear bipyridyl-type ligands were synthesized. [Zn(ip)(bpb)](n) (CID-21; ip=isophthalate, bpb=1,4-bis(4-pyridyl)benzene), [Zn(ip)(bpt)](n) (CID-22; bpt=3,6-bis(4-pyridyl)-1,2,4,5-tetrazine), and [Zn(ip)(bpa)](n) (CID-23; bpa=1,4-bis(4-pyridyl)acetylene) all have interdigitated structures of layers and similar void volumes (≈27%). In these compounds, 1D bottleneck-type channels run along the perpendicular direction of the layer stacking and their properties are strongly dominated by the dipyridyl linker ligands. Because of the difference in packing of 2D layers, CID-21 and CID-22 have relatively rigid porous structures, whereas CID-23 has greater flexibility, as indicated by the results of powder X-ray diffraction studies. The micropores of CID-22 surrounded by tetrazine moieties adsorb polar molecules, such as methanol and water. The higher affinity of CID-22 for water than CID-21 is supported by a theoretical study. The 1D channel of CID-23 is wider than that of the other two compounds, which enables the incorporation of aromatic molecules. This is because the shape of the bpa linker ligand generates a wider pore diameter (8.6 Å). Only CID-23 can adsorb a benzene molecule and the isotherm of benzene has a gate-opening-type profile. This offers proof of the guest accommodation process through large structural transformation from a nonporous to a porous structure. The flexibility and restricted pore space of CID-23, at 298 K, allows only benzene, but not cyclohexane, to enter the channels. The porous structure exhibits clear selectivity for these similar guests. The incorporation of an elongated dipyridyl linker ligand in the 2D coordination layers provides a strategy for the design of microporous compounds with different flexibilities, microporous environments, and separation abilities.

Kinetics of Fe(II)-Catalyzed Transformation of 6-line Ferrihydrite under Anaerobic Flow Conditions

The readsorption of ferrous ions produced by the abiotic and microbially mediated reductive dissolution of iron oxy-hydroxides drives a series of transformations of the host minerals. To further understand the mechanisms by which these transformations occur and their kinetics within a microporous flow environment, flow-through experiments were conducted in which capillary tubes packed with ferrihydrite-coated glass spheres were injected with inorganic Fe(II) solutions under circumneutral pH conditions at 25 degrees C. Synchrotron X-ray diffraction was used to identify the secondary phase(s) formed and to provide data for quantitative kinetic analysis. At concentrations at and above 1.8 mM Fe(II) in the injection solution, magnetite was the only secondary phase formed (no intermediates were detected), with complete transformation following a nonlinear rate law requiring 28 and 150 h of reaction at 18 and 1.8 mM Fe(II), respectively. However, when the injection solution consisted of 0.36 mM Fe(II), goethite was the predominant reaction product and formed much more slowly according to a linear rate law, while only minor magnetite was formed. When the rates are normalized based on the time to react half of the ferrihydrite on a reduced time plot, it is apparent that the 1.8 mM and 18 mM input Fe(II) experiments can be described by the same reaction mechanism, while the 0.36 input Fe(II) experiment is distinct. The analysis of the transformation kinetics suggests that the transformations involved an electron transfer reaction between the aqueous as well as sorbed Fe(II) and ferrihydrite acting as a semiconductor, rather than a simple dissolution and recrystallization mechanism. A transformation mechanism involving sorbed inner sphere Fe(II) alone is not supported, since the essentially equal coverage of sorption sites in the 18 mM and 1.8 mM Fe(II) injections cannot explain the difference in the transformation rates observed.

Spatial Patterns and Modeling of Reductive Ferrihydrite Transformation Observed in Artificial Soil Aggregates

Within soils, biogeochemical processes controlling elemental cycling are heterogeneously distributed owing, in large part, to the physical complexity of the media. Here we quantify how diffusive mass-transfer limitation at the soil aggregate scale controls the biogeochemical processes governing ferrihydrite reductive dissolution and secondary iron mineral formation. Artificial cm-scale aggregates made of ferrihydrite-coated sand inoculated with iron-reducing bacteria were placed in flow-through reactors, mimicking macro- and microporous soil environments. A reactive transport model was developed to delineate diffusively and advectively controlled regions, identify reaction zones and estimate kinetic parameters. Simulated iron (Fe) breakthrough-curves show good agreement with experimental results for a wide-range of flow rates and input lactate concentrations, with only a limited amount (< or =12%) of Fe lost in the reactor outflow over a 31 day period. Model simulations show substantial intra-aggregate, mm-scale radial variations in the secondary iron phase distributions, reproducing the trends observed experimentally where only limited transformation of ferrihydrite was found near the aggregate surface, whereas extensive formation of goethite/lepidocrocite and minor amounts of magnetite and/or siderite were observed toward the aggregate center. Our study highlights the important control of variations in transport intensities on microbially induced iron transformation at the soil aggregate scale.

Measuring Hydrophobic Micropore Volumes in Geosorbents from Trichloroethylene Desorption Data

Hydrophobic micropores can play a significant role in controlling the long-term release of organic contaminants from geosorbents. We describe a technique for quantifying the total and the hydrophobic mineral micropore volumes based on the mass of trichloroethylene (TCE) sorbed in the slow-releasing pores under dry and wet conditions, respectively. Micropore desorption models were used to differentiate the fast- and slow-desorbing fractions in desorption profiles. The micropore environment in which organic molecules were sorbed in the presence of water was probed by studying the transformation of a water-reactive compound (2,2-dichloropropane or 2,2-DCP). For sediment from an alluvial aquifer, the total and hydrophobic micropore volumes estimated using this technique were 4.65 microL/g and 0.027 microL/g (0.58% of total), respectively. In microporous silica gel A, a hydrophobic micropore volume of 0.038 microL/g (0.035% of reported total) was measured. The dehydrohalogenation rate of 2,2-DCP sorbed in hydrophobic micropores of the sediment was slower than that reported in bulk water, indicating an environment of low water activity. The results suggest that hydrolyzable organic contaminants sorbed in hydrophobic micropores react slower than in bulk water, consistent with the reported persistence of reactive contaminants in natural soils.

Fluorescent Hydroxyflavone–Zeolite Nanoparticles: Ship‐in‐a‐Bottle Synthesis and Photophysical Properties

3-Hydroxyflavone (3-OHF) was incorporated in zeolite micropores by ship-in-a-bottle synthesis. This strategy consists of constructing the molecule by reaction of small precursors within the cavity. 3-OHF molecules exhibit excited-state intramolecular proton transfer (ESIPT) and a tautomeric equilibrium between the 3-OHF-excited structures N* and T*exists. This equilibrium is strongly affected by the protic nature and polarity of the surrounding medium. The textural and spectroscopic characterization of the dye-loaded zeolite colloids enabled the study of the correlation between the optical properties of the dye and the zeolite micropore environment.

Shape-selective isopropylation of biphenyl over H-mordenites: Relationships of bulk products and encapsulated products in the pores

Shape-selective formation of 4,4′-diisopropylbiphenyl (4,4′-DIPB) was observed in the isopropylation of biphenyl (BP) over H-mordenite (HM). Shape-selectivity was governed by spatial restriction in transition states in the microporous environment of HM. The selectivity for 4,4′-DIPB was influenced by many factors such as the SiO 2/Al 2O 3 ratio, reaction temperature, and propylene pressure. On the other hand, high selectivities for 4,4′-DIPB were observed in encapsulated products under all of our conditions because of shape-selective catalysis in the HM pores. However, the decrease in the selectivity for 4,4′-DIPB occurred with the decrease in the SiO 2/Al 2O 3 ratio and with change in the reaction conditions, such as increase in temperature and decrease in propylene pressure. The discrepancies in bulk products and in encapsulated products in the HM pores are discussed to understand where and why shape-selective catalysis occurs. The decrease in the selectivity for 4,4′-DIPB over HM with a low SiO 2/Al 2O 3 ratio is due to non-regioselective reactions because of choking of the pore entrance by coke-deposition. The decrease in the selectivity for 4,4′-DIPB under high reaction temperatures, or under low propylene pressures was ascribed to the isomerization of 4,4′-DIPB at external acid sites. These conclusions are also supported by discrepancies of bulk and encapsulated products in the isomerization of 4,4′-DIPB under high the corresponding conditions. No significant isopropylation of 4,4′-DIPB to triisopropylbiphenyls (TrIPB) was observed even under high propylene pressure or at high reaction temperatures. This is presumably due to there not being enough space in the pores for the transition state of further isopropylation of 4,4′-DIPB, and it is one of the reasons for the shape-selective formation of 4,4′-DIPB.

Methanol in microporous materials from first principles

Zeolites are amongst some of the most important heterogeneous catalysts in use commercially today, combining acid–base catalysis due to the presence of Brønsted acid sites with shape selectivity resulting from the microporous environment [1]. Despite this, there is still a great deal of uncertainty concerning the mechanisms of many of the processes which are known to occur and the way in which the zeolite accelerates them. While much information has been obtained from experimental techniques, including infra-red spectroscopy and magic angle spinning NMR [2], there is presently a need for models and reaction pathways to aid in their interpretation. Here theoretical methods are playing a major role in the field of microporous materials.

First principles location of the transition state for formation of dimethyl ether in a zeolite

A new transition state searching algorithm has been used to determine the mechanism for methanol condensation to form dimethyl ether within the microporous environment of the zeolite, chabazite, using periodic boundary conditions and density functional theory.

In Situ Study of Reactive Intermediates of Methanol in Zeolites from First Principles Calculations

Nonlocal periodic density functional calculations have been used to examine the stability of various possible reactive intermediates resulting from the initial adsorption of methanol in the zeolite chabazite. The formation of dimethyl ether has been examined both for pathways via framework coordinated methyl groups and by direct condensation with the framework acting as a solvent. Both are found to offer a reasonable energetic route to dimethyl ether, which is only hydrogen bonded in its bound state, proton transfer being disfavored. No evidence is found for the existence of carbenes or ylide species as local minima within the microporous environment, though trimethyl oxonium is stable with respect to dimethyl ether and a framework methoxy group.

Activation of Molecules in Confined Spaces: An Approach to Zeolite−Guest Supramolecular Systems

Ab initio calculations showing frontier molecular orbital energy modifications of a guest molecule when located inside a microporous zeolitic cavity are presented. Micropores of zeolites Beta, ZSM-12, and ZSM-5 have been modeled using all silica clusters from which it is found that the highest occupied molecular orbital (HOMO) energy of toluene increases when going from the gas phase state to restricted microporous environments. The influence of the zeolite cavity size and its chemical composition on the toluene HOMO energy are discussed. Regarding the size of the confining environment, it is found that the smaller the zeolite cavity the higher is the rise in toluene HOMO energy. The effect of the chemical composition on the toluene HOMO energy was tested in the ZSM-5 zeolite by varying the Al content. The results obtained showed that the frontier orbital energy increases upon decreasing the Al content of the cluster. As a consequence of the confinement effect, toluene reactivity in zeolite catalyzed reactions is expected to change toward a more covalent behavior in which electronic transference from the toluene molecule to an electronic acceptor will be more favored than in gas phase reactions.

A Sol-Gel Solid Electrolyte with High Lithium Ion Conductivity

A new hybrid inorganic gel electrolyte has been synthesized by the sol−gel method. The inorganic gel electrolyte is based on the immobilization of a liquid electrolyte in an inorganic silica network. The former is responsible for ionic conduction, while the latter provides a three-dimensional microporous environment to support the liquid electrolyte. The synthesis involves mixing equal amounts by weight of a hydrolyzed silica precursor and a lithium ion conducting liquid electrolyte. The conductivity of the partially dried gel is 3.5 × 10-3 S cm-1 at room temperature and approaches 10-2 S cm-1 at 80 °C. The temperature dependence of the conductivity follows an Arrhenius relation with an activation energy of 0.15−0.17 eV over the temperature range 30−80 °C. NMR data confirm the lithium ion mobility and show its liquidlike behavior over this temperature range. TGA and DSC results indicate that the inorganic gel electrolyte is stable to 90 °C and that the thermal stability is simply limited by the liquid electrolyte. FT-IR and mass spectrometry establish that there is no water present in the dried material.

Quantum chemistry calculations on the effect of electron confinement upon the frontier molecular orbitals of ethylene and benzene in sodalite. Implications on reactivity

Quantum chemistry calculations using HF / STO-3G, CEP-41G, and 3-21G basis sets have been carried out in order to study the effect of the electron confinement of ethylene and benzene in the microporous environment existing in a sodalite cage. The electron confinement produces an increase of 1.06 and 1.31 eV at the 3-21G level in the energy of the HOMO of ethylene and benzene, respectively, when placed in the centre of the cavity with respect to the HOMO energy in the gas phase. There is an increase in the energy of the HOMO when the molecules approach the walls of the zeolite.

Integrating soil water chemistry variations at the catchment level within a cation exchange model

The soil zone is of major importance in determining the transport of chemicals through catchments. Despite this, there is no theory that partitions the chemical transfers between the micropore and macropore soil water environments and no theory that shows how bulked chemical properties can be linked to equilibrium or kinetic-type equations. Here a method is developed that examines the relationship between the theoretical cation exchange equations and lumped chemical data for an acid soil representative of acidic and acid-sensitive areas. The method uses artificial data to examine how the lumped soil water data changes in response to changing cation exchange composition. Results indicate that for long-term studies the lumped relationships are similar to the theoretical equations. However, in the short term, i.e. for periods covering a few years or less, the changes are more difficult to describe. The relationships established are important not only from a theoretical point of view but also in relation to environmental modelling within catchment research: the work helps resolve a common discord between field evidence and modelling assumptions.

Xenon adsorption in NaA zeolite cavities

Adsorption of xenon atoms in the α-cages of NaA zeolite has been studied using 129Xe NMR spectroscopy to probe directly the distribution and configuration of molecules in confined, microporous environments. The 129Xe NMR spectrum is sensitive to subtle changes in xenon environment, so relative populations of α-cages containing different numbers of xenon guests can be determined and the effects of other co-adsorbed species monitored. On the basis of 129Xe NMR spectra, the distribution of xenon atoms among NaA α-cages is shown to exhibit a marked dependence on the pressure at which the xenon guests are introduced. 129Xe NMR spectra recorded at 200 K reveal that xenon atoms in the NaA α-cages experience diminished mobility (resembling condensation phenomena) at higher temperatures than in the bulk gas of equivalent density. Thus, the chemical potential of adsorbed xenon can be investigated experimentally as a function of both temperature and guest density. The density dependence of the 129Xe chemical shift i...

ChemInform Abstract: Isolated Redox Centers within Microporous Environments. Part 1. Cobalt‐Containing Aluminophosphate Molecular Sieve Five

Abstract(CoAPO‐5) is synthesized by heating a gel of composition 1.5 Et3N·0.8 Al2O3·0.4 CoO·P2O5·40 H2O to 200 °C at autogeneous pressure.

ChemInform Abstract: Isolated Redox Centers within Microporous Environments. Part 2. Vanadium‐Containing Aluminophosphate Molecular Sieve Five.

AbstractVanadium‐containing aluminophosphate five (VAPO‐5) is synthesized by heating a gel of composition Pr3N·Al2O3·0.95 P2O5·0.05 V2O5·40 H2O to 150 °C at autogeneous pressure (more than 4 h).

Isolated redox centers within microporous environments. 1. Cobalt-containing aluminophosphate molecular sieve five

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTIsolated redox centers within microporous environments. 1. Cobalt-containing aluminophosphate molecular sieve fiveConsuelo. Montes, Mark E. Davis, Brendan. Murray, and Mysore. NarayanaCite this: J. Phys. Chem. 1990, 94, 16, 6425–6430Publication Date (Print):August 1, 1990Publication History Published online1 May 2002Published inissue 1 August 1990https://doi.org/10.1021/j100379a049RIGHTS & PERMISSIONSArticle Views357Altmetric-Citations128LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (1 MB) Get e-Alerts

Isolated redox centers within microporous environments. 2. Vanadium-containing aluminophosphate molecular sieve five

Cobalt-containing aluminophosphate five (CoAPO-5) was synthesized and shown to contain tetrahedral Co{sup 2+} residing within framework atomic positions and also as extraframework cations. The isolated framework Co{sup 2+} atoms can be oxidized by O{sub 2} at 500{degree}C to Co{sup 3+} which is stabilized in tetrahedral coordination by the oxide lattice. The framework Co{sup 3+} is easily reduced to Co{sup 2+} by a variety of reducing agents.