Selective Molecular Sieve Research Articles

Optimization of NaY zeolite membrane preparation for the separation of methanol/methyl methacrylate mixtures

NaY zeolite membranes were prepared on porous mullite supports by secondary hydrothermal growth for pervaporation separation of thermal-sensitive methanol/methyl methacrylate (MMA) mixtures. Effects of synthesis parameters (synthesis time, gel composition, aging treatment and silica source) and separation conditions (feed temperature and composition) on the separation performance of NaY zeolite membranes were investigated. Under optimized membrane synthesis conditions (6.5h crystallization time, 10–16h aging time at 303–323K, and gel composition of 1SiO2: 0.04Al2O3: 0.88Na2O: 40H2O), NaY zeolite membranes without the impurity phase of P-type zeolite could be formed on the porous supports. The best membrane displayed fluxes of 0.55–2.35kgm−2h−1 and separation factors of 1000–3600 for the 5–50wt.% methanol/MMA mixtures at 323K. The preparation of NaY zeolite membranes under optimized conditions showed a good reproducibility. The high methanol/MMA pervaporation separation performance of NaY zeolite membranes can be attributed to the selective sorption and molecular sieve of zeolite layers.

A highly permeable and selective zeolitic imidazolate framework ZIF-95 membrane for H2/CO2 separation

Making use of the preferred adsorption affinity and capacity to CO(2) as well as the highly porous structure with huge cavities of 2.4 nm, a highly permeable and selective ZIF-95 molecular sieve membrane was developed for the separation of H(2) from CO(2).

Co-solvent-mediated synthesis of thin polyamide membranes

This study introduces a promising strategy, called co-solvent assisted interfacial polymerization (CAIP), for the synthesis of thin polyamide membranes by interfacial polymerization of 1,3-phenylenediamine (MPD) with 1,3,5-benzenetricarbonyl trichloride (TMC) on polysulfone ultrafiltration supports. A synergistic co-solvent added in the hexane phase was used to control the polymerization reaction zone and modify the membrane network structure. The resultant membrane exhibited selective molecular sieving of small molecules from larger ones. The addition of larger amounts of acetone as a co-solvent to the hexane solution increased both pore size and water flux, which was determined by analysis of the membrane permeation properties. In the permselectivity test of a 500 ppm glucose aqueous solution, the best-performing membrane was prepared by 2 wt% acetone addition. It showed a rejection of more than 99.4% and a high water transport at a rate of more than 1 × 10 −11 m 3/(m 2 Pa s), which was more than 4-fold higher than the membrane prepared without acetone. The effects of reaction conditions, including co-solvent content and interfacial polymerization time, were studied.

Some Research on the Magnetic X Zeolite Composites

X zeolite behaves excellent properties of shape selective catalysis, molecular sieves and iron-exchange for its unique tunnel structure and large specific surface area. In this paper magnetic core has been introduced into the zeolite structure and magnetic zeolite with high stability and adsorption properties have been prepared. Synthesis conditions and adsorption of Pb2+ also have been studied. Magnetic zeolite composite was hydrothermally synthesized with a traditional hydrothermal method by adding magnetic Fe3O4 suspension in the zeolite synthesis system. Synthesis conditions and adsorption of Pb2+ also have been studied. The products were characterized with X Ray diffraction (XRD), IR spectrometer (IR), transmission electron microscopy (TEM), vibrating sample magnetometer (VSM), Thermogravimetric (TGA) and differential scanning calorimetry (DSC) instruments. The results show that the products have an X zeolite crystal structure and present super paramagnetic properties. The saturation magnetization value of the product is about 3.7 emu•g-1. The weight ratio of magnetic Fe3O4 in the magnetic zeolite composite is calculated to be about 5%. The magnetic zeolite composites have high surface area and the absorption capability of the products is 196.8mg/g.

Molecular Sieve Separations

AbstractThe pore structures of adsorbents such as zeolites and metal‐organic frameworks comprise regular arrays of uniform channels of molecular dimensions, thus offering the possibility of size selective (molecular sieve) separations. Molecular sieve separations have traditionally been carried out in a cyclic batch adsorption/desorption process but, in principle, a steady state membrane process would be an attractive alternative. Considerable success has been achieved at the laboratory scale for several different molecular sieve separations but the difficulty of fabricating, on a large scale, a sufficiently coherent and robust zeolite membrane has largely prevented the commercial application of such processes. Selected examples of kinetic separations are presented in order to illustrate some of the features of such systems.

MCM-22/Silica Selective Flake Nanocomposite Membranes for Hydrogen Separations

MCM-22/silica composite films were fabricated using layer-by-layer deposition toward a nanoscale realization of the selective flake concept first proposed by Cussler in (J. Membr. Sci. 1990, 52 (3), 275-288). According to this concept, considerable zeolitic transport selectivity can be harvested if plate-like zeolite particles were oriented flat within an appropriate thin film matrix. c-Out-of-plane oriented MCM-22 flakes were chosen because of the expected H(2)-selective (over other permanent gases) molecular sieving action through their c-axis-transport-limiting aperture defined by six SiO(4) tetrahedra. To fill the gaps between MCM-22 particles the evaporation induced self-assembled (EISA) mesoporous silica, introduced by Brinker et al. in (Nature 1997, 389 (6649), 364-368), was selected as a tunable matrix, through which Knudsen diffusion would be the dominant transport mechanism for permanent gases. The repetition of appropriate deposition cycles (i.e., particle deposition and subsequent silica coating) led to the gradual increase of separation performance achieving H(2)/N(2) ideal selectivity as high as 120.

Mineral oil regeneration using selective molecular sieves as sorbents

Microporous molecular sieves are investigated as effective, environmentally safe and cost effective materials for purification of lubricants in late stages of oxidation and also as an alternative to traditional anti-oxidant additives. Molecular sieve crystals with LTL- and BEA-type structures with variable morphology and silica content are used to trap oxidation by-products. It has been found that the aluminum content and the type of charge balancing cations in the molecular sieves play an important role on the amount of organic adsorbed due to a higher affinity of strong Brønsted acid sites and surface cations to hydrocarbons and carbonyl moieties. High selectivity of the molecular sieves towards oxidation products was achieved without influencing additive compounds in the oxidized oil, and thus results in high degrees in oil purification up to 90% depending on the oil formulation. The influence of treatment factors such as temperature, treatment time, solid content and type of oil were investigated. The sorption properties of two different molecular sieves capable of removing selectively the oxidation products, and thus effectively leading to purification in the early and late stages of oxidation, but also to slow down the oxidation process of oils are demonstrated. The considered molecular sieve materials are environmentally safe, cost effective, and due to their high thermal stability are adequate for recovery and multi-cycle reuse.

High-performance hybrid pervaporation membranes with superior hydrothermal and acid stability

A new organic–inorganic hybrid membrane has been prepared with exceptional performance in dewatering applications. The only precursor used in the sol–gel synthesis of the selective layer was organically linked 1,2- bis(triethoxysilyl)ethane (BTESE). The microporous structure of this layer enables selective molecular sieving of small molecules from larger ones. In the dehydration of n-butanol with 5% of water, the membrane shows a high separation factor of over 4000 and ultra-fast water transport at a rate of more than 20 kg m −2 h −1 at 150 °C. This can be related to the high adsorption capacity of the material and the sub-micron thickness of the selective layer. The selectivity has now remained constant over almost one and a half years under continuous process testing conditions. Apart from the hydrothermal stability, the membrane exhibits a high tolerance for acid contamination. A slow performance decline in flux and separation factor is only observed at a pH lower than 2. The high stability and effective separation indicate a broad industrial application potential of the hybrid membrane material.

Developing Ion Electroadsorption Stereoselectivity, by Pore Size Adjustment with Chemical Vapor Deposition onto Active Carbon Fiber Electrodes. Case of Ca2+/Na+ Separation in Water Capacitive Desalination

By using carbon chemical vapor deposition (CVD) onto active porous carbon fibers, we were able to reduce the pore size of the active carbon to obtain selective carbon molecular sieves (CMS) of variable properties. The pore size of these carbons was characterized by the adsorption of molecules of different dimensions (molecular probe adsorption) from the gas phase. By manipulating the several experimental parameters of the CVD process, the pore size of the CMS could be finely tuned to fall between sizes of alkaline earth cations (Ca2+; Mg2+, 6−7 A in diameter), and the much smaller size of hydrated monovalent cations (4 A in diameter). This was proven by voltammetric measurements of the pore-tailored carbon electrodes in the corresponding salt solutions, which also provided a measure of the differential capacity of the electrical double layer (EDL) vs the potential (e.g., at potentials negative to the potential of the zero charge, the EDL capacity was very small for CaCl2 and MgCl2 solutions, while it was ...

Coordinative Self-Assembly and Solution-Phase X-ray Structural Characterization of Cavity-Tailored Porphyrin Boxes

Combining linear Zn porphyrin trimers with orthogonally derivatized porphyrin dimers leads rapidly and spontaneously to the formation of monodisperse, torsionally constrained boxes comprising six components and a total of 16 metalloporphyrins. In situ X-ray scattering measurements confirm the formation of monodisperse assemblies of precisely the size expected from model box structures. While simple subunits yield highly symmetrical boxes, we find that sterically demanding subunits produce unusual twisted boxes. Previous studies of porphyrin-based box-like assemblies (squares) for selective catalysis and molecular sieving revealed two function-inhibiting structural problems: torsional motion along the metal-porphyrin-metal axis and ambiguous outside versus inside functionalization (via axial ligation of available Zn(II) sites). The new 16-porphyrin box assemblies eliminate both problems.

A 15N‐aided artificial atmosphere gas flow technique for online determination of soil N2 release using the zeolite Köstrolith SX6®

N2 is one of the major gaseous nitrogen compounds released by soils due to N-transformation processes. Since it is also the major constituent of the earth's atmosphere (78.08% vol.), the determination of soil N2 release is still one of the main methodological challenges with respect to a complete evaluation of the gaseous N-loss of soils. Commonly used approaches are based either on a C2H2 inhibition technique, an artificial atmosphere or a 15N-tracer technique, and are designed either as closed systems (non-steady state) or gas flow systems (steady state). The intention of this work has been to upgrade the current gas flow technique using an artificial atmosphere for a 15N-aided determination of the soil N2 release simultaneously with N2O. A 15N-aided artificial atmosphere gas flow approach has been developed, which allows a simultaneous online determination of N2 as well as N2O fluxes from an open soil system (steady state). Fluxes of both gases can be determined continuously over long incubation periods and with high sampling frequency. The N2 selective molecular sieve Köstrolith SX6 was tested successfully for the first time for dinitrogen collection. The presented paper mainly focuses on N2 flux determination. For validation purposes soil aggregates of a Haplic Phaeozem were incubated under aerobic (21 and 6 vol.% O2) and anaerobic conditions. Significant amounts of N2 were released only during anaerobic incubation (0.4 and 640.2 pmol N2 h(-1) g(-1) dry soil). However, some N2 formation also occurred during aerobic incubation. It was also found that, during ongoing denitrification, introduced [NO3]- will be more strongly delivered to microorganisms than the original soil [NO3]-.

Carbon-Based Membranes

AbstractInorganic carbon-based membranes for gas separation comprise materials that are fabricated through pyrolysis of a precursor material (often a synthetic polymer), and the more recently discovered carbon nanotubes. Fabrication, assembly into different architectures, and mechanism of operation are summarized for precursor-based carbon membranes, with a focus on selective surface flow and molecular sieving. Only preliminary work on carbon nanotube-based membranes for gas separation has been published. Their unusual transport properties, however, promise their use in gas separation in the future. In light of this application, structural properties and results relating to flow through these tubular structures are summarized.

Membranes for Hydrogen Purification: An Important Step toward a Hydrogen-Based Economy

AbstractProduction of pure molecular hydrogen is essential to the realization of the proposed “hydrogen economy” that could ultimately provide hydrogen as a clean, renewable source of energy; eliminate the industrialized world's dependence on petroleum; and reduce the generation of greenhouse gases linked to global warming. A crucial step in obtaining pure hydrogen is separating it from other gaseous compounds—mainly CO2—that often accompany hydrogen in industrial chemical reactions. Advanced membrane technology may prove to be the key to the successful, economical production of molecular hydrogen.Size-sieving glassy polymer membranes can separate H2 on the basis of its small size. Alternatively, reverse-selective rubbery polymers can expedite the passage and, hence, removal of CO2 due to its relatively high solubility in such membranes alone or in conjunction with dissociative chemical reactions. Transition-metal membranes and their alloys can adsorb H2 molecules, dissociate the molecules into H atoms for transport through interstitial sites, and subsequently recombine the H atoms to form molecular H2 again on the opposite membrane side. Microporous amorphous silica and zeolite membranes comprising thin films on a multilayer porous support exhibit good sorption selectivity and high diffusion mobilities for H2, leading to high H2 fluxes. Finally, carbon-based membranes, including carbon nanotubes, may be viable for H2 separation on the basis of selective surface flow and molecular sieving. A wide variety of materials challenges exist in hydrogen purification, and the objective of this issue of MRS Bulletin is to address those challenges and their potential solutions from basic principles.

Coordination Compound Molecular Sieve Membranes

Highly selective molecular sieve composite membranes have been prepared using dispersions of porous coordination compounds confined within amorphous glassy polysulfone. This new approach is a promising method for fabricating organic molecular sieve composite membranes with tailor-made structures and pore sizes. The organic molecular sieves exhibit enhanced selectivity for H2 over CH4 (approximately 200 at 5 wt.-% loading).

Crystal engineering of porphyrin framework solids

This article describes recent achievements made by us and other groups in targeted synthesis of porphyrin-based framework solids by various non-covalent mechanisms of molecular recognition. The self-assembly processes are effected in a tunable manner either by direct association of suitably designed porphyrin building blocks, or by their supramolecular aggregation through external linkers as metal ions and organic bi-dentate ligands. Many of these crystalline porphyrin materials exhibit open architectures and remarkable structural integrity, and their potential application for selective guest storage and molecular sieving is highlighted.

Selective Transporting of Lithium Ion by Shape Selective Molecular Sieves ZSM-5 in PEO-Based Composite Polymer Electrolyte

To elucidate the mechanism of the enhancement of lithium ion transference numbers, TLi+, induced by ZSM-5 in PEO-based composite polymer electrolyte, different types of other fillers, such as ceramic fillers (SiO2 and Al2O3), solid superacid (SO42-/ZrO2), layered materials (montmorillonite), and mesoporous materials (MCM-41, HMS, and SBA-15), are selected to compare with ZSM-5 molecular sieves. The experimental results suggest that ZSM-5 may enhance the TLi+ of PEO-based composite polymer electrolytes in three ways: (1) Lewis acid−base interactions, resulting in the release of more “free” Li+ cations; (2) selective passing of Li+ cations due to the special pore size of ZSM-5 and electrostatic repulsion for ClO4- anions due to the negative environment in the channels of ZSM-5; (3) the cation-exchange centers in the framework of ZSM-5, suggesting that the Li+ cation can be ion-exchanged into the channels for charge balance, namely, provide a special conducting pathway for Li+ cation. In addition, room temperature ionic conductivity of the composite polymer electrolyte can also be increased by about 2 orders of magnitude. The high lithium ion transference numbers combined with a suitable ionic conductivity of PEO−LiClO4−ZSM5 composite polymer electrolyte suggest that it can be used as candidate electrolyte material for all solid-state rechargeable lithium polymer batteries.

MICROPOROUS INORGANIC MEMBRANES

Recent development in microporous inorganic membranes represents a significant advance in materials for separation and chemical reaction applications. This paper provides an in-depth review of synthesis and properties of two groups (amorphous and crystalline) of microporous inorganic membranes. Amorphous microporous silica membranes can be prepared by the sol-gel and phase separation methods. Flat sheet, tubular and hollow fiber amorphous carbon membranes have been fabricated by various pyrolysis methods from polymer precursors. A large number of synthesis methods have been developed to prepare good quality polycrystalline zeolite membranes. Several techniques, including vapor and liquid approaches, are reviewed for pore structure modification to prepare microporous inorganic membranes from mesoporous inorganic membranes. Chemical, microstructural and permeation properties of these microporous membranes are summarized and compared among the several microporous membranes discussed in this paper. Theory for gas permeation through microporous membranes is also reviewed, with emphasis on comparison of theoretical with the experimental data. These inorganic microporous membranes offer excellent separation properties by the mechanisms of preferential adsorption, selective configurational diffusion or molecular sieving.

“Perfect” Electrochemical Molecular Sieving by Thin and Ultrathin Metallopolymeric Films

Oxidative electropolymerization at glassy carbon and indium−tin−oxide electrodes from unbuffered aqueous solutions of Fe(5-NH2-phen)32+ and Ru(5-NH2-phen)32+ yields electrode-supported films that self-limit at thicknesses of ca. 50 and 2 nm, respectively. Electrodeposition is characterized by formation of nonelectroactive films, which are electronically insulating at least over the electrochemical potential range of 0 to +1 V. Electrochemical measurements of redox-active probe molecules demonstrate that these films exhibit highly selective molecular sieving. Neutral permeants and hydrophobic permeants are observed to penetrate the polymeric coating more rapidly than charged or hydrophilic forms. Atomic force microscopy, UV−vis spectroscopy, and X-ray photoelectron spectroscopy were used to correlate the polymeric molecular structure with the observed electrochemical reactivity. Molecular sieving behavior is discussed in terms of a membrane transport model implying that the films are comprised of metallopolymeric molecular cavities of uniform size and distribution.

5491273 Catalytic conversion of methanol to linear olefins

Zeolite ZSM-57 can be synthesized from sodium-containing alumosilicate gels using hexaethyl-C5-diquat as organic template and silica sol as silica source. ZSM-57 can be readily distinguished from the structurally closely related zeolites with the FER framework topology by its X-ray powder pattern and mid-infrared spectrum. From thermogravimetric analysis of as-synthesized ZSM-57 it follows that applying an air calcination at 550°C for several hours is sufficient to remove the organic template from the pores of the zeolite. The results of two selected test reactions for probing the effective pore width of zeolites, viz. ethylbenzene disproportionation and n-decane isomerization suggest that the pore width is larger for ZSM-57 as compared to ZSM-35 (ferrierite) and ZSM-5. This is in agreement with their crystallographic structures and offers new opportunities in shape selective catalysis and molecular sieve separation.

Titanhaltige Molekularsiebe als Katalysatoren f�r selektive Oxidationsreaktionen mit Wasserstoffperoxid

Titanium Containing Molecular Sieves as Catalysts for Selective Oxidation Reactions with Hydrogen Peroxide Titanium silicalite (TS-1) is known to be a highly-efficient and selective redox molecular sieve that has been found to catalyze a range of synthetically useful oxidation reactions with hydrogen peroxide under mild conditions. Among these reactions which are often influenced also by the shape-selective properties of the Ti-containing molecular sieve are olefin epoxidation, the oxidation of alcohols to carbonyl compounds, aromatic hydroxylations and ammoximation of cyclohexanone to cyclohexanone oxime. A critical review on the present literature on TS-1 catalysts and further Ti-containing molecular sieves is given mainly focused on the questions connected with the identification of the catalytic active titanium species and the mechanism of some of the reactions. Therefore, the procedure of the synthesis of TS-1 as well as the results obtained from the most of the physico-chemical investigations are analyzed. There are no final correlations to realize between the parameters of the synthesis, the results of characterization and the catalytic properties of TS-1.