Microporous Molecular Sieves Research Articles

The Basicity and Catalytic Activity of Ordered Mesoporous Silicon Nitride Oxide

Abstract The basicity and base catalytic properties of ordered mesoporous silicon nitride oxide (MSON) were investigated in our present contribution. It was found that the basic strength of MSON was stronger and comparable to the strength of MgO. In addition, MSON had a much higher base catalytic activity than not only amorphous silicon nitride oxide but also crystallized nitrogen-containing microporous molecular sieves used in the Knoevenagel condensation reaction of benzaldehyde and malononitrile. Furthermore, the catalysts were stable both in structure and in composition below 1123 K in an inert atmosphere. These stable, solid base catalysts may find their use in some important base catalyzed reactions.

Manganese‐Containing Silica‐Based Microporous Molecular Sieve MnS‐1: Synthesis and Characterization.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Comparison between plane-wave and linear-scaling localized basis sets for structural calculations of microporous molecular sieves

We present a comparison between two computational approaches based on density-functional theory in the generalized gradient approximation with plane waves or localized basis sets, using, respectively, VASP and SIESTA codes. The study is focused on the description of two crystalline microporous aluminophosphates: ${\mathrm{AlPO}}_{4}\ensuremath{-}34$ and ${\mathrm{AlPO}}_{4}\ensuremath{-}18.$ After a necessary optimization of the localized basis set by a simplex method, the results obtained by the two approaches have the same quality but the localized basis set allows a decrease of computer time and memory respectively by a factor of 8 and 2, respectively. An order-N algorithm is then applied. The energy and geometry are again quite similar. This method opens the way to DFT studies on more complex structures.

Combined Single-Crystal X-ray Diffraction and FTIR Study of Morpholinium−Water Molecular Complexes Embedded in a Chabasite Network

A combined single-crystal X-ray diffraction (XRD) and FTIR study was performed to define the structures of SAPO-34 and CoAPSO-34 microporous molecular sieves synthesized using morpholine (C4H8ONH) as a structure-directing agent and to shed some light on the molecular interactions between molecules embedded within the zeolitic cages after hydrothermal synthesis. All investigated crystals showed a chabasite-related structure that was analyzed in the R-3 space group. The incorporation of Si and Co ions into the aluminophosphate framework was assessed by different procedures (scattering curve refinements and analysis of T−O distances). Silicon-to-phosphorus substitution was within the range of 20−24% Si in the P sites of all SAPO-34 crystals. Conversely, Si-for-P and Co-for-Al substitutions spanned a wider range in CoAPSO-34 crystals. XRD supplemented by TGA analysis showed that 2 morpholine and 2.5−3 water molecules occupied the chabasite cages in all crystals. The presence of morpholinium, C4H8ONH2+ (HM+), along with morpholine (M) H-bonded to water molecules was detected by FTIR spectroscopy, and a computational study helped to clarify the assignment of the vibrational modes of the isolated organic molecules. The structure of water/morpholinium (or morpholine) molecular complexes, which are formed by HM+−H2O−HM+ (or HM+−H2O−M) units within the chabasite cages and HM+−H2O−H2O−HM+ (or HM+−H2O−H2O−M) chains embedded in the intrazeolite space of adjacent cages of the chabasite network, was refined by single-crystal XRD.

Ordered Mesoporous and Microporous Molecular Sieves Functionalized with Transition Metal Complexes as Catalysts for Selective Organic Transformations

AbstractFor Abstract see ChemInform Abstract in Full Text.

Synthesis, Characterization and Catalytic Properties of Mesoporous Cobalt Aluminophosphate Molecular Sieves

Isomorphously substituted cobalt(II) hexagonal mesoporous aluminophosphate (CoHMA) molecular sieves were synthesized hydrothermally and characterized by various analytical and spectroscopic techniques. It was deduced that cobalt ions exhibit a divalent oxidation state in tetrahedral coordination in mesoporous aluminophosphates. Further, unlike cobalt-containing microporous aluminophosphate molecular sieves, Co(II) ions remain in a tetrahedral geometry even after calcination. The catalytic activity of CoHMA was tested for the cyclohexane oxidation reaction under mild conditions. Remarkable substrate conversion and product (cyclohexanol) selectivity were obtained compared to several previously reported heterogeneous catalysts.

Characterization and catalytic evaluation of mesoporous and microporous molecular sieves containing niobium

The synthesis and characterization of the novel microporous aluminophosphate NbAPO-5 is described. The novel material has been studied by X-ray powder diffraction, SEM, TG/DTA-MS, electron spin resonance, and ultraviolet-visible (UV-Vis) spectroscopy The catalytic properties have been tested in the epoxidation of 1-hexene and cyclohexene, respectively, in comparison with niobium-containing silicalite-1 and NbMCM-41. The results of the physical characterization indicate that it is possible to prepare niobium-containing aluminophosphates, where isolated Nb(V) species are connected to framework defect sites via formation of Nb–O–P or Nb–O–Al bonds. The results of this study allow the preparation of microporous and mesoporous niobium-containing molecular sieves which are active catalysts in oxidation reactions.

Ordered mesoporous and microporous molecular sieves functionalized with transition metal complexes as catalysts for selective organic transformations.

Ordered mesoporous and microporous molecular sieves functionalized with transition metal complexes as catalysts for selective organic transformations.

Processing of microporous VPI-5 molecular sieve by using supercritical CO 2: stability and adsorption properties

The stability and adsorption behavior of the microporous molecular sieve VPI-5, an AlPO 4 hydrate phase, processed using supercritical carbon dioxide (SCCO 2) as a solvent, were explored. Different thermal treatments up to 373 K were carried out on samples of VPI-5 previously treated with methanol, ethanol or 2-propanol. The thermally treated samples were characterized by X-ray diffraction and low-temperature N 2 adsorption. VPI-5 was completely transformed to AlPO 4-8 when it was heated up to 373 K in ambient air. By previously shaking VPI-5 samples with methanol the transformation was partially avoided. Additionally, when the thermal treatment was carried out in an SCCO 2 atmosphere, the VPI-5 structure was preserved. VPI-5 was charged with low molecular weight organic compounds using a high-pressure apparatus and employing SCCO 2 as a solvent. The adsorption behavior of either a pure solute or a mixture of two solutes into the channels of VPI-5 was studied as a function of time, pressure and temperature of adsorption. A significant time-, pressure- and temperature-dependence for the absorbed amount was found.

Recent progress on immobilization of enzymes on molecular sieves for reactions in organic solvents.

Enzymes exhibit high selectivity and reactivity under normal conditions but are sensitive to denaturation or inactivation by pH and temperature extremes, organic solvents, and detergents. To extend the use of these biocatalysts for practical applications, the technology of immobilization of enzymes on suitable supports was developed. Recently, these immobilized biomolecules have been widely used and a variety of immobilization supports have been studied. The majority of these supports cover diverse kinds of materials such as natural or synthetic polyhydroxylic matrixes, porous inorganic carriers, and all kinds of functional polymers. Microporous molecular sieve, zeolite, has attracted extensive interest in research because of its distinctive physical properties and geochemistry. Recently, with the discovery of a new family of mesoporous molecular sieves, MCM-41, this series of materials shows great potential for various applications. Molecular sieves involve such a series of materials that can discriminate between molecules, particularly on the basis of size. As support materials, they offer interesting properties, such as high surface areas, hydrophobic or hydrophilic behavior, and electrostatic interaction, as well as mechanical and chemical resistance, making them attractive for enzyme immobilization. In this article, different types of molecular sieves used in different immobilization methods including physical adsorption on zeolite, entrapment in mesoporous and macroporous MCM series, as well as chemically covalent binding to functionalized molecular sieves are reviewed. Key factors affecting the application of this biotechnology are discussed systematically, and immobilization mechanisms combined with newly developed techniques to elucidate the interactions between matrixes and enzyme molecules are also introduced.

Membranes and films of zeolite and zeolite-like materials

An ideal structure of zeolite membrane should be a slice of a perfect zeolite crystal attached on a porous metal or ceramic support. To maximize the throughput, the zeolite layer must be very thin, limited only by the cell dimension of zeolite. Separation of a mixture may then be achieved based on the molecular sieving ability of zeolite, which allows only molecules smaller than a critical size to pass through. A variety of methods have been reported for the preparation of zeolite membranes, but so far a perfect epitaxial zeolite layer is still out of reach and only a polycrystalline zeolite membrane can be obtained. The first part of this review focuses on the permeation of gases and vapors through a polycrystalline zeolite membrane as a separation means. The effect of microstructure on permeance will also be discussed, as well as the preparation methods leading to different microstructures. In addition to the usage as a shape-selective membrane, thin films of zeolite and zeolite-like molecular sieves can also serve as hosts for the encapsulation and orientation of guest atoms and molecules and their clusters. In the second part of this review, the production of layers of aligned microporous molecular sieve crystals on supports and the fabrication of supported thin zeolite-like nanoporous silica films as well as their potential applications on the preparation of advanced materials are discussed.

The sorption properties of as-synthesized and calcined MCM-41 and MCM-48

The benzene sorption capacity of as-synthesized and calcined MCM-41 and MCM-48 were compared. The hydrocarbon sorption capacity increases with pore diameter for the calcined versions. As-synthesized samples sorb appreciable amounts of benzene but the quantity is invariant with respect to pore diameter. The sorption of hydrocarbon by as-synthesized M41S materials is a unique feature not observed with microporous molecular sieves.

Catalytic growth of carbon nanotubes from the internal surface of Fe-loading mesoporous molecular sieves materials

Catalytic formation of carbon nanotubes on Fe-loading hexagonal mesoporous molecular sieve Fe/SiHMS and microporous molecular sieve Fe/NaY has been investigated. TEM pictures, together with the different porous properties of SiHMS and NaY, show us that the carbon nanotubes probably grow inside the channels of Fe/SiHMS and on the external surface of Fe/NaY. The Fe(2p) binding energy (BE) suggests a stronger interaction between SiHMS and the deposited iron oxide species than that between NaY and iron species. The BE of Si, O and Fe elements for Fe/SiHMS vary much before and after the catalytic synthesis of carbon nanotubes, whereas the BE for NaY changes little. The above XPS information can serve as a supplement to TEM analysis.

A novel single step synthesis of 2-methyl-6-phenylpyridine from non-heterocyclic compounds over molecular sieve catalysts

The industrially important 2-methyl-6-phenylpyridine is synthesized for the first time in a single step by reacting acetophenone, acetone, formaldehyde and ammonia in the vapor phase over microporous and mesoporous molecular sieve catalysts.

Synthesis of microporous molecular sieves by surfactant decomposition

Microporous molecular sieves can be synthesized under the high temperatures of 165–180 °C and high pH of 10–12.5, using surfactant cetyltrimethylammonium bromide (CTAB) as a template. It was found that various factors, such as temperature, alkalinity, Si∶Al ratio and the presence of a small amount of organic alkylammonium species (TMA+, TPA+) can affect the nature of the microporous materials obtained. In the wide range of Si∶Al ratios, we can obtain such zeolites as sodalite, analcime, omega, ZSM-35, mordenite, ZSM-5 and silicalite. The experimental results suggest that the decomposition products of CTAB act as templates instead of surfactant CTAB itself. The synthesis condition of high pH (>10) and high temperatures (165 °C) can ensure the decomposition of CTAB, which is quite essential for the formation of microporous molecular sieves.

NMR Spectroscopy Applied to Zeolite Catalysis: Progress and Prospects

Recent advances in NMR spectroscopy methods for studying zeolites and their catalyzed reactions are briefly reviewed. In-situ NMR spectroscopy, with its controlled atmosphere conditions, is used to characterize the active surface sites of zeolites (highly acidic micro-porous molecular sieves) and to elucidate the hydrocarbon reactions they catalyze. The strong Bronsted acid sites of these catalysts, SiO(H)Al, are accessible to adsorbed molecules via well defined pore systems (0.3 to 1.2 nm). Substantial progress has been made in: obtaining high resolution NMR spectra under conditions (pressure, temperature, reagent flow) similar to real reactor conditions, developing new NMR methods for characterizing quadrupolar nuclei in solids, understanding acidity in solids and evidencing the details (intermediates, pathways, kinetics) of zeolite catalyzed chemical reactions. Some of the practical aspects and limitations in performing these studies are mentioned along with future prospects. Given its high versatility and adeptness for determining: the local structure around selected nuclei (2H, 17O, 27Al, 29Si), and the structure and dynamics of organic molecules (1H, 13C), NMR spectroscopy is destined to attain evermore important and far-reaching application in this field of study.

Calorimetric Study of the Acidity of a New Family of Mesoporous Catalysts

Thermally stable mesoporous aluminophosphates (AIPO) and silicoaluminophosphates (SAPO) were prepared at room temperature in the presence of a cationic surfactant and an organic base. These materials possess high surface areas and regular mesopores of approximately 35 A diameter. By contrast to microporous crystalline aluminophosphate molecular sieves, mesoporous compounds are amorphous and characterized by Al/P ratios greater than 1. These particularities are responsible for a strong Lewis acidity, as evidenced by ammonia adsorption microcalorimetry. Mesoporous materials are more acidic than the microporous analogues and the amount of strong acid sites increases with the silicon content.

Effect of Structure and Pore Size of Mesoporous Molecular Sieve Materials on the Growth of Carbon Nanotubes

Microporous and mesoporous molecular sieve materials with different channel structures and pore sizes were applied as supports for Fe-loaded catalysts to catalytically synthesize carbon nanotubes. The deposited carbon materials were characterized by the TEM technique. It was shown that the structures and pore sizes of supports greatly influence the forms and quality of the deposited carbon materials. The larger the pore size of the support used, the larger the diameter and pore size of the carbon nanotubes formed. It seems that the growth of carbon nanotubes can be orientated by the one-dimensional mesoporous structure of hexagonal mesoporous molecular sieve materials.

Studies of Acid Properties of FeSiMCM-41 and LaSiMCM-41 by NH3-microcalorimetric and NH3-TPD Measurements

The acidity of mesoporous materials FeSiMCM-41, LaSiMCM-41, SiMCM-41, AlSiMCM-41 and HAlSiMCM-41 has been investigated by microcalorimetric studies of the adsorption of ammonia and temperature programmed ammonia desorption method. In the initial stage, the acid strength sequence is SiMCM-41>HAlSiMCM-41>AlSiMCM-41>FeSiMCM-41>LaSiMCM-41, in agreement with that found for microporous molecular sieves materials. A small number of strong acid sites of SiMCM-41 may result from the aluminum impurity contained in the silica source material. The acid density sequence is HAlSiMCM-41>AlSiMCM-41>FeSiMCM-41>LaSiMCM-41≥SiMCM-41 and can be explained by the studies of existing states of trivalent atoms in these samples reported in previous work. Since some NH3-TPD plots of these samples show the profiles that could not be back to baseline at elevated temperature, the technique of microcalorimetric adsorption is preferable in studying these samples.

Organic-functionalized molecular sieves as shape-selective catalysts

Zeolites and related crystalline molecular sieves can possess catalytically active acid sites, as well as uniformly sized and shaped pores and voids, that allow for their industrial use as shape-selective catalysts1. Some catalytic reactions that are not mediated by acids (such as oxidation) have also been shown to occur in zeolites in a shape-selective manner2, but the diversity in active sites in these materials remains restricted. For mesoporous materials3, the diversity in catalytic activity has been broadened by grafting organosilanes that contain organic functional groups onto the internal pore surfaces4,5,6 or by incorporating them into the structure during the synthesis process7,8,9,10,11,12. The former approach has not proven straightforward for microporous zeolites because a large fraction of the grafted functional groups become attached instead to the exterior surfaces of the crystal, where there is no shape selectivity13. The synthesis of zeolites and molecular sieves using organosilanes as structure-directing agents has been accomplished14,15, but the subsequent creation of porosity requires the complete loss of the organic functional groups. Here we report a new methodology that overcomes these problems and allows the production of microporous molecular sieves containing organic functionalities within their pores. During the initial synthesis phase, phenethyl groups covalently tethered to silicon atoms are incorporated into the framework. The external surface-bound functionalities and the structure-directing agents residing within the intracrystalline spaces are then removed to create a microporous material. Subsequent sulphonation of the phenyl rings produces intrapore sulphonic acid sites that perform shape-selective catalysis. Different active-site types can be created by attaching other functional groups to the framework silicon, and we therefore expect that our method will lead to the formation of a wide range of shape-selective catalysts.