Molecular Sieve Type Research Articles

Crystal morphology control of AFI type molecular sieves with microwave irradiation

AFI type molecular sieves with various morphologies such as aggregated sphere, plate, rod, prism and barrel were synthesized under varying reaction conditions under microwave irradiation. Hexagonal plate-like crystals with an aspect ratio of about 0.2 were obtained in the presence of silica under alkaline conditions. Rod-like crystals with an aspect ratio of about 40 were obtained with the addition of fluoride and an increase of template and water concentrations. The addition of silica to an alkaline precursor gel hindered the crystal growth in the c-direction and the presence of fluoride ions retarded the nucleation and crystal growth.

Speciality zeolites in catalysis 1992-2001

Catalytic oxidations in the liquid phase generally employ soluble metal salts or complexes as the catalyst. So-called redox molecular sieves, unlike conventional supported catalysts, possess a regular microenvironment with homogeneous internal structures, consisting of uniform, well-defined cavities, and channels. Confinement of the redox active site in these channels and/or cavities can endow the catalyst with unique activity, as a result of strong electrostatic interactions between acidic and basic sites on the internal surface, and the substrate or reaction intermediate analogous to interactions with acidic carboxyl and basic amino groups of amino acid residues in the active site of (redox) enzymes. Zeolitcs and zeotypes are crystalline oxides, comprising corner sharing TO4 tetrahedra (T = Si, AI, P, etc.), and consisting of a regular pore system with diameters of molecular dimensions; hence, the term molecular sieve. The framework of molecular sieves is not completely rigid and the incoming molecules are able to induce slight structural changes. Three types of redox molecular sieve can be distinguished on the basis of the method of synthesis: ion exchange, framework substitution, and encapsulation. Redox molecular sieves have one important advantage over other heterogeneous catalysts; it is possible to influence that substrate molecules approach the active site by a suitable choice of molecular sieve and solvent. Hence, by using a redox molecular sieve, it is possible to obtain much higher selectivities to primary oxidation products than with analogous homogeneous catalysts. Titanium(IV) silicalite (TS-1), the first example of a framework-substituted redox molecular sieve, catalyzes a variety of synthetically useful oxidations with 30% aqueous hydrogen peroxide under mild conditions. Similar heterolytic mechanisms can be envisaged for other nucleophilic substrates like ammonia, amines, sulfides, phenols, and alcohols. Based on these recent developments, ship-in-a-bottle catalysts appear to hold much promise, extending even to the design of chiral “ships” for enantioselective oxidation.

Kinetic separation of methane/carbon dioxide by molecular sieve carbons

The bulk separation of CH4/CO2 mixture by pressure swing adsorption (PSA) on two different types of carbon molecular sieves (CMS) is analyzed. The two CMS are Bergbau–Forschung (BF) CMS and Takeda 3A CMS. Two different PSA cycles are considered, differing by whether a feed step is used. For a 50/50 feed mixture, it is shown that the separation is feasible with both sorbents to produce a methane product at over 90% purity and at reasonably high product recoveries and sorbent productivities. It is shown in this work that temperature is an important factor to consider for kinetics-based separations, and that a higher temperature is favorable for CH4/CO2 separation on molecular sieve where diffusion is slow. At 25°C, BF CMS yields better separation because the diffusion of both CO2 and CH4 are slow in the Takeda 3A CMS. Increasing the temperature decreases the equilibrium adsorption amounts, but increases the diffusivities. The separation results with Takeda 3A are substantially improved when the temperature is increased to 70°C, and the separation is better than that of BF CMS at 25°C. Therefore, the rate of diffusion (rather than equilibrium) is the dominating factor in kinetics-based separations. The effects of the sorbent working capacity (defined as the difference in the amounts adsorbed of the fast diffusing component between the ends of the adsorption step and the desorption step) on the separation are illustrated. For the kinetics-based separation, a high-pressure feed step is desirable while a low-pressure purge step is not desirable. The reasons are seen clearly from the bed profiles.

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.

Synthesis and Characterization of AlPO4-36 and MAPO-36 with Different Magnesium Content

The objective of this work was to perform a systematic study regarding the effect of different synthesis parameters on the crystallization of AlPO{sub 4}-36 and MAPO-36 (aluminophosphate molecular sieve type 36, ATS, structure). MAPO-36 samples were synthesized from gels containing different magnesium contents in order to obtain catalysts with different degrees of acidity. Under the synthesis conditions employed in this work, the formation of AlPO{sub 4}-36 and MAPO-36 increased when the reaction mixture was aged at room temperature; however, small amounts of AlPO{sub 4}-5 always crystallize parallel to the ATS materials. The presence of magnesium in the reaction mixture can also favor the formation of the ATS structure; nevertheless, the synthesis of pure MAPO-36 from reaction mixtures having very low magnesium content was not possible under several conditions. Pseudoboehmite was a better aluminum source than aluminum isopropoxide for the formation of MAPO-36. A practically pure ATS structure was obtained from reaction mixtures containing molar fractions of Mg between 0.033 and 0.133. For higher magnesium contents, traces of another unidentified phase were detected. Energy-dispersive spectrometry analysis indicates a uniform chemical composition of the MAPO-36 particles, and Scanning electron microscopy shows that the ATS samples have a needlelike morphology. Thermogravimetric analysis ofmore » ATS samples reveals that the weight loss corresponding to the decomposition of protonated amine is linearly correlated with magnesium content in the solid up to 0.8 mmol of magnesium per gram of solid.« less

Dean−Stark Apparatus Modified for Use with Molecular Sieves

A Dean−Stark apparatus, with the separation chamber modified to contain molecular sieves for scavenging water from an azeotrope, is described. This modification permits the efficient dehydration of an azeotrope, thereby greatly reducing the chance of water recycling to a reaction flask. The extent of water removal from recycling solvent was determined by Karl Fischer titration. Depending on the type of molecular sieve used, the pH of the water eluted at saturation ranged from 7.8 to 10.4. To demonstrate the effectiveness for removing water from recycling solvent and providing anhydrous reaction conditions, the apparatus was used to determine the water-loading capacity of a variety of molecular sieves and to effect the cyclodimerization of α-methylstyrene as well as 2-phenyl-2-propanol to 2,3-dihydro-1,1,3-trimethyl-3-phenyl-1H-indene (Chem. Abstracts 3910-35-8).

COMPOSITIONAL ANALYSIS OF PETROLEUM WAXES BY ZEOLITE MOLECULAR SIEVE-GCMS

ABSTRACT To investigate the composition of petroleum waxes, studies were carried out on the extraction of normal paraffins from petroleum waxes by zeolite molecular sieve type 5A. Time required for adsorption of different molecular weight n-paraffins from standard mixture and petroleum waxes has been ascertained. Un-adsorbed part of the waxes has been investigated by GC-MS technique. Recovery of adsorbed hydrocarbon from molecular sieve was done by supercritical fluid extraction.

Formation of small pore SAPO-44 type molecular sieve

The hydrothermal transformation of silico–aluminophosphate gel with cyclohexylamine to SAPO-44 has been examined. The hydrothermal crystallisation products of the SAPO have been investigated by X-ray diffraction, FTIR, nitrogen and water adsorption, thermogravimetric analysis, surface analysis and 27Al, 31P, and 29Si MAS NMR. Structural changes were observed in the silico–aluminophosphate gel with and without organic template and during the hydrothermal crystallisation. The silico–aluminophosphate gel converted to pure SAPO-CHA phase in 168 h at 473 K. The surface of SAPO-44 was silicon rich as compared with that of SAPO-34 and SAPO-18. The 27Al MAS NMR signal of tetrahedrally coordinated Al observed in the silico–aluminophosphate gel without the organic template was changed to octa-, penta- and tetrahedrally coordinated aluminium upon the addition of the cyclohexylamine template to the SAPO gel. After 3 h of hydrothermal treatment at 473 K however, the 27Al MAS NMR signals of the octahedral and pentacoordinated aluminium were removed. This was also confirmed by 31P and 29Si MAS NMR. The tetrahedrally coordinated P and Si were detected within 3 h at 473 K. The sorption capacity and adsorption–desorption trends of the SAPO gels and the crystallisation products were found to be different. 29Si MAS NMR results indicated that the percentage of Si (4Al) and its distribution were significantly affected by the crystallization period. SAPO-44 was thermally stable up to 973 K with phase change observed over the calcination temperature of 1193 K. The SAPO gels and the crystallisation products have also been investigated for their catalytic behaviour in n-hexane and ethanol conversion reactions.

Protein Crystals as Novel Microporous Materials

Cross-linked protein crystals (CLPCs) constitute a novel type of molecular sieves with high porosity. In order to characterize the fully hydrated CLPC, the method of macromolecular porosimetry was applied. This technique allows one to estimate the apparent pore sizes and pore size distribution in solid and soft hydrated porous sorbents directly from size exclusion chromatography. According to this method, CLPCs offer a wide range of pore size (15−100 A), porosity (0.5−0.8), and pore surface area (800−2000 m2/g). These CLPC materials can be made chemically and mechanically stable, and are capable of separating molecules by size, chemical structure, and chirality.

DYNAMIC ADSORPTION OF S02 ON ZEOLITE MOLECULAR SIEVES†

Sulfur dioxide is one of the major pollutants resulting from fuel combustion. Numerous dry, semi-dry and wet processes have been developed for pollution control of sulfur dioxide. Solid carbonates, natural and synthetic zeolites, ion exchange resins and carbon based sorbents are the most commonly used dry sorbents for sulfur dioxide removal In this study, measurements of the adsorption properties of sulfur dioxide on zeolites were investigated. The adsorbents used in this work are 5A, 4A and AW300 type molecular sieve zeolites. Adsorption equilibrium parameters were determined from the pulse chromatographic response to injections of low concentrations of sulfur dioxide. The method of moments were used to evaluate the adsorption equilibrium parameters from pulse chromatographic experiments. Data, such as adsorption equilibrium constants or reaction rate parameters are essential in the design of adsorption systems or reactors in which sulfur dioxide is removed The experiments were conducted in a temperature range of 523-718 K. The relatively strong adsorption properties of sulfur dioxide on zeolites necessitated the use of high carrier gas flow rates and subsequently non-isobaric operation. Non-isobaric pulse chromatography theory was found to describe accurately the adsorption trends." printpubdate="Adsorption equilibrium constants of S02 were found to decrease considerably with increasing temperature. It was also found out that adsorption of SO2 on the adsorbents investigated were found to decrease in the order of AW300 ⩽ 4A ⩽ 5A. The adsorption equilibrium parameter of S02 on 5A was found as 11.78 at 673 K, whereas it has a value of 157.11 at 523 K. The adsorption equilibrium parameter of S02 on 4A zeolite was determined to be 8.63 at 718 K and 213.78 at 523 K.

Lanthanide-catalyzed endo- and enantioselective 1,3-dipolar cycloaddition reactions of nitrones with alkenes

The 1,3-dipolar cycloaddition reaction of alkenes with nitrones was catalyzed by Yb(OTf) 3 or Sc(OTf) 3 giving isoxazolidines in high yields and selectivities. The catalyst Yb(OTf) 3 induce a high endo-selectivity in the reaction of up to 94% de, whereas Sc(OTf) 3 show the highest rate accelerations. Both the conversion and endo-selectivity of the Yb(OTf) 3 and Sc(OTf) 3 catalyzed reactions proved to be dependent on the amount and type of molecular sieves added. The application of different chiral ligands in 1,3-dipolar cycloaddition reactions of a series of alkenes with nitrones catalyzed by Yb(OTf) 3 or Sc(OTf) 3 has been studied and it was found that high endo-selectivities and an ee's of up to 73% could be obtained by the use of Yb(OTf) 3 and 2,6-bis[4(S)-isopropyl-2-oxazolidin-2-yl]pyridine (PyBOX) as the chiral ligand.

Investigations on the aqueous solution and solid-state cation exchanged MAPO-ATS type molecular sieve

MAPO-36 (magnesium substituted aluminophosphate of ATS type) have been prepared and modified by the liquid mediated cation exchange and contact induced cation exchange methods. Modified samples prepared by the liquid cation exchange (sodium/caesium/calcium-MAPO-36) and solid-state ion exchange at different temperatures (sodium-MAPO-36) were studied by XRD, SEM, FTIR, TG, BET, 27Al and 31P MAS NMR. The XRD, SEM and FTIR results indicate that the crystal structure and crystal morphology of Na/Cs/Ca exchanged MAPO-36 samples prepared by liquid cation exchange method are similar to the parent calcined MAPO-36 material. However, the crystal phase morphology and surface area are drastically affected when the sodium exchanged MAPO-36 samples are prepared by the solid-state reaction. Above 723 K and in the presence of sodium chloride, a dense tridymite phase formation from the ATS phase was observed. 27Al MAS NMR spectra of the Na/Cs/Ca MAPO-36 samples prepared by liquid cation exchange are similar to that of MAPO-36 and show characteristic peaks for tetrahedral and pentahedral coordinated Al. In the contact induced cation exchanged samples, symmetric and narrow 27Al NMR peak related to tetrahedrally coordinated Al was observed. The 31P MAS NMR of the MAPO-36 and the samples prepared by liquid cation exchange are similar but differ significantly from those samples prepared by solid-state cation exchange. The catalytic activity of the MAPO-36 and the sodium exchanged samples prepared by liquid and solid-state cation exchanges in the o-xylene conversion reaction was studied.

Synthesis of novel molecular sieves using a metal complex as a template

The synthesis and characterization of novel molecular sieves containing bis(pentamethylcyclopentadienyl) cobalt(III) ion are described. Several types of molecular sieves were prepared using the cobalt metal complex as a template that was shown to be incorporated as a guest molecule within the molecular sieve. Molecular sieves prepared using bis(pentamethylcyclopentadienyl) cobalt(III) ion as template were characterized as having a pore size at least as large as 7 Å. Other molecular sieves were also prepared using bis(cyclopentadienyl) cobalt(III) templates. The compounds are attractive as catalysts in a wide range of applications.

Measurement of fission product gases in the atmosphere

The ability to quickly detect and assess the magnitude of releases of fission-produced radioactive material is of significant importance for ongoing operations of any conventional nuclear power plant or other activities with a potential for fission product release. In most instances, the control limits for the release of airborne radioactivity are low enough to preclude direct air sampling as a means of detection, especially for fission gases that decay by beta or electron emission. It is, therefore, customary to concentrate the major gaseous fission products (krypton, xenon and iodine) by cryogenic adsorption for subsequent separation and measurement. This study summarizes our initial efforts to develop an automated portable system for on-line separation and concentration with the potential for measuring environmental levels of radioactive gases, including 85Kr, 131,133,135Xe, 14C, 3H, 35S, 125,131I, etc., without using cryogenic fluids. Bench top and prototype models were constructed using the principle of heatless fractionation of the gases in a pressure swing system. This method removes the requirement for cryogenic fluids to concentrate gases and, with suitable electron and gamma ray detectors, provides for remote use under automatic computer control. Early results using 133Xe tracer show that kinetic chromatography, i.e., high pressure adsorption of xenon and low pressure desorption of air, using specific types of molecular sieves, permits the separation and quantification of xenon isotopes from large volume air samples. We are now developing the ability to measure the presence and amounts of fission-produced xenon isotopes that decay by internal conversion electrons and beta radiation with short half-lives, namely 131mXe, 11.8 d, 133mXe, 2.2 d, 133Xe, 5.2 d and 135Xe, 9.1 h. The ratio of the isotopic concentrations measured can be used to determine unequivocally the amount of fission gas and time of release of an air parcel many kilometers downwind from a nuclear activity where the fission products were discharged.

Electron Paramagnetic Resonance and Infrared Study of Type 5A Zeolite Reacted with SO2and CO2

Abstract In this work an electron paramagnetic resonance (EPR) and infrared (IR) study was made on molecular sieve type 5A zeolite reacted with SO2 and CO2 gases. The observed EPR signal from the SO2 reacted sample was attributed to the SO− 2 radical and its temperature dependence has been presented. However, CO2 reacted samples do not exhibit any EPR spectrum. The EPR spectra of SO2 reacted samples indicate that vibration modes shift somewhat as compared with its free state. For the CO2 reacted samples one of the modes shifts and the other doubles as compared with its free state. This indicates the adsorption of SO2 on surfaces, and absorption of CO2 in deep pores of this zeolite.

MCM-41 type molecular sieves as catalysts for the Friedel-Crafts acylation of 2-methoxynaphthalene

Molecular sieves of the MCM-41 type were studied as catalysts for the Friedel-Crafts acylation of 2-methoxynaphthalene in the liquid phase. When MCM-41 in the H + form is used as Brønsted-acid catalyst, acylation with acetic anhydride gives substitution predominantly at the 1-position. Increased reaction temperatures lead to a slight decrease of selectivity to acylation at the 1-position. The regenerability of H-MCM-41 is found to be excellent. For the acylation of 2-methoxynaphthalene with acetyl chloride Zn-MCM-41 was studied as a Lewis-acid catalyst, but both selectivity and regenerability are less favourable than in the case of the H-MCM-41 catalysed acylation of with acid anhydride.

Zeolite materials: recent discoveries and future prospects

Zeolite chemistry is establishing itself as one of the key areas in the evolution of materials science as a discipline of great importance in both applied and fundamental research. The well-defined crystalline architecture of these molecular sieve materials has made them ideal inorganic oxides and substrate hosts in rapidly developing effort concerning new techniques in the physical chemistry of solids and the attendant advances in computational chemistry. In turn, there has been an impressive proliferation in the number and type of molecular sieves discovered by research in synthesis. Taken together, these two events augment the prospect of new directions in catalysis and selective separations including the development of zeolitic materials.

Synthesis and Applications of Molecular Sieve Layers and Membranes

Strategies for the preparation of zeolite molecular sieve membranes and layers on different substrates are reviewed. Membranes (and continuous thin films) have been synthesized with ZSM-5 (MFI) type molecular sieves and other zeolites, either as free-standing specimens, grown on various supports according to several schemes, or embedded in polymer matrices. The membranes have been studied for molecule-selective separations and for catalytic reactions. Layers of presynthesized molecular sieves have been deposited by means of sol−gel matrixes and other bonding agents, molecular attachment layers, and via sputtering or laser ablation techniques. The resulting nanoporous layers have been applied in highly selective sensors, for the alignment of NLO chromophores and for molecular separations. Finally, oriented molecular sieve layers have been grown on self-assembled organic mono- and multilayers presenting phosphonate groups at the interface. Mixed phosphonate/alkyl films show profound effects on the surface crystallization of metallophosphate molecular sieves. MFI type zeolites have also been grown through the vapor phase on alkylammonium-modified gold substrates.

Imaging the channels in mesoporous molecular sieves with platinum

Transmission electron micrographic images of platinum wires, incorporated into the channels of mesoporous molecular sieves, show that one-dimensional channels are arranged parallel in MCM-41 while the channels are three-dimensionally interconnected in other types of mesoporous molecular sieves constructed with silica frameworks.

The synthesis and structure solution of UiO-7, a new molecular sieve

A new molecular sieve type material UiO-7 is prepared and the structure solved through a combination of simulated annealing, used to isolate the framework topology, and high-resolution synchrotron X-ray powder diffraction, for the location of the non-framework atoms.