Molecular Sieve Type Research Articles

Water and mercaptan adsorption on 13X zeolite in natural gas purification process

The aim of this work is to model the adsorption process used for mercaptan and water removal from natural gas. Three fixed beds containing Zeolite molecular sieve type 13X, are used in this plant. In this operation, two beds are in process for adsorption purposes and the other one is regenerated simultaneously. This system is also operated under isothermal condition. In modeling of this process, rate of adsorption is approximated by linear driving force (LDF) expression, and the extended Langmuir isotherm is used to describe adsorption equilibrium. The set of partial differential equations of dynamic model is solved by Crank-Nicolson method. The effect of equations of state is also studied and the best equation fitting the industrial data is selected. Also, concentration profile is presented versus bed length at various times. The influences of pressure, inlet concentration and bed height on the breakthrough time are also investigated.

Adsorption of carbon dioxide, ethane, and methane on titanosilicate type molecular sieves

The separation of carbon dioxide from light hydrocarbons is a vital step in multiple industrial processes that could be achieved by pressure swing adsorption (PSA), if appropriate adsorbents could be identified. To compare candidate PSA adsorbents, carbon dioxide, methane, and ethane adsorption isotherms were measured for cation exchanged forms of the titanosilicate molecular sieves ETS-10, ETS-4, and RPZ. Mixed cation forms, such as Ba/H-ETS-10, may offer appropriate stability, selectivity, and swing capacity to be utilized as adsorbents in CO 2/CH 4 PSA processes. Certain cation exchanged forms of ETS-4 were found to partially or completely exclude ethane by size, and equivalent RPZ materials were observed to exclude both methane and ethane, while allowing carbon dioxide to be substantially adsorbed. Adsorbents such as Ca/H-ETS-4 and Ca/H-RPZ are strong candidates for use in PSA separation processes for both CO 2/C 2H 6 and CO 2/CH 4, potentially replacing current amine scrubber systems.

DENSITY AND VISCOSITY MEASUREMENTS OF AQUEOUS SOLUTIONS OF 2−METHOXYETHANOL AT TEMPERATURES (298.15, 303.15, 308.15, AND 313.15) K

Densities and viscosities have been measured for the binary mixtures of 2–methoxyethanol (2– ME) and water at 298.15, 303.15, 308.15 and 313.15 K. From the precise density measurements, the excess molar volume ( E V ) were calculated, V E results are negative over the entire range of mole fractions and become less negative as the temperature increases. The deviation in viscosity (Δη) and excess molar Gibbs free energy of activation of viscous flow (ΔG *E ) have been calculated from experimental viscosity measurements at different temperatures. ηΔ and ΔG *E data are positive over the whole mole fraction range. The observed results of V E , ηΔ and ΔG *E are discussed in terms of molecular interactions in these binary mixtures. Generally, molecular interactions dominant these behaviors are of type, disruption and formation of intra–and interhydrogen bonding between like and unlike molecules, respectively and dipole–dipole interaction. The experimental excess functions V E , ηΔ and ΔG *E were fitted to the Redlich–Kister polynomial equation to evaluate the adjustable parameters and standard deviation from ideality. Introduction 2-Methoxyethanol is the first member of alkoxyalkanols and used extensively as solvent and solublizing agent in many industries with interests ranging from pharmaceutical to plastic products. From theoretical point of view, it is very interesting molecule having the ability to form intra–and intermolecular hydrogen bonds. On the other hand, alkanols (alcohols) are the most well – known solvents with protic and self –associated properties. 2 Methoxyethanol, an ether alkanol, shows physiochemical characteristic midway between protic and dipolar aprotic solvents (1) . Studies on excess functions of binary liquid mixtures are of considerable importance in understanding the nature of molecular interactions that determined by intermolecular forces (hydrogen bonding, charge transfer complex, Van der Waals forces, etc.) (2,3) . To determine the extent and type of interactions between 2–ME and water molecules, the densities and viscosities of binary mixtures of 2-ME with water have been measured. Experimental 2–Methoxyethanol (CH3OCH2CH2OH) was obtained from Aldrich Chemical Company, with purity > 99 %. The solvent was kept on activated molecular sieves type 4 A o for 24 hours then filtered before use. Bidistilled deionized water, its conductivity (0.6 × 10 -6 s.cm 1 ) was used. The experimental measured values of density, ρ and viscosity η for both solvents are agreed well with those values published in the literature (4-8) , Table (1).

Relaxation processes of water confined to AlMCM-41 molecular sieves. Influence of the hydroxyl groups of the pore surface

A series of AlMCM-41 molecular sieves was prepared with constant composition (Si/Al = 14.7) and presumably same pore structure but different pore diameters (from 2.3 to 4.6 nm). The pore size distribution is narrow for each sample. The rotational fluctuations of water molecules confined inside the pores were investigated applying broadband dielectric spectroscopy (10(-2)-10(7) Hz) over a large temperature interval (213-333 K). A relaxation process, slower than that expected for bulk water, was observed which is assigned to water molecules forming a surface layer on the pore walls. The estimated relaxation time has an unusual non-monotonic temperature dependence, which is rationalized and modeled assuming two competing processes: rotational fluctuations of constrained water molecules and defect formation (Ryabov model). This paper focuses on the defects and notably the influence of the hydroxyl groups of the pore walls. The Ryabov model is fitted to the data and characteristic parameters are obtained. Their dependence on pore diameter is considered for the first time. The found results are compared with those obtained for other types of molecular sieves and related materials.

Nucleant-mediated protein crystallization with the application of microporous synthetic zeolites

Protein crystallization is still a major bottleneck in structural biology. As the current methodology of protein crystallization is a type of screening, it is usually difficult to crystallize important target proteins. It was thought that hetero-epitaxic growth from the surface of a mineral crystal acting as a nucleant would be an effective enhancer of protein crystallization. However, in spite of almost two decades of effort, a generally applicable hetero-epitaxic nucleant for protein crystallization has yet to be found. Here we introduce the first candidate for a universal hetero-epitaxic nucleant, microporous zeolite: a synthetic aluminosilicate crystalline polymer with regular micropores. It promotes a form-selective crystal nucleation of proteins and acts as a crystallization catalyst. The most successful zeolite nucleant was molecular sieve type 5A with a pore size of 5 A and with bound Ca2+ ions. The zeolite-mediated crystallization improved the crystal quality in five out of six proteins tested. It provided new crystal forms with better resolution in two cases, larger crystals in one case, and zeolite-dependent crystal formations in two cases. The hetero-epitaxic growth of the zeolite-mediated crystals was confirmed by a crystal-packing analysis which revealed a layer-like structure in the crystal lattice.

Molecular-Sieves Controlled Diastereo- and Enantioselectivity: Unexpected Effect in the Organocatalyzed Direct Aldol Reaction

An efficient asymmetric organocatalyst was developed for the direct aldol reaction between ketones and aldehydes to afford β-hydroxy ketones in high stereoselectivity ( ANTI/ SYN up to 92:8, ee up to <99%). The key of the success was the use of activated molecular sieves (4 A MS), which acted as a water scavenger. The influence of additives such as water or different types of molecular sieves on the reactivity and stereoselectivity of the reaction was also studied.

An improved and rapid 5A molecular sieve method for gas chromatography isotope ratio mass spectrometry of n-alkanes (C 8–C 30+)

The accurate analysis of stable carbon isotope ratios of individual n-alkanes in petroleum by gas chromatography isotope ratio mass spectrometry (GC–irMS) requires that the n-alkanes be separated from branched/cyclic hydrocarbons. Conventional separation methods include urea adduction or separation using different types of molecular sieves (5A, ZSM5). Although the molecular sieves procedures are well established, they generally require the use of hydrofluoric (HF) acid, a potentially hazardous material, to digest and release compounds trapped within the sieve. In this study, a simplified 5A method that avoids the use of HF acid has been developed to isolate n-alkanes ranging from n-C 8 to at least n-C 30 from a saturated petroleum hydrocarbon fraction. 12% cyclohexane in n-pentane was found to be a sufficient solvent in extracting most of the n-alkanes (>90%) in the n-C 8 to n-C 30 range from the sieve. The new 5A sieve procedure is a rapid and safer method for extracting n-alkanes from petroleum to yield fractions amenable for routine GC–irMS.

Fabrication of functionally graded 3A/5A zeolites by electrophoretic deposition

Functionally graded zeolites of molecular sieve type 3A and 5A are deposited by electrophoretic deposition (EPD) from acetone suspension with 8% volume concentration of n-butylamine as particle charging agent. The EPD characteristics of both 3A and 5A suspensions are studied. Functionally graded zeolite 3A/5A deposits are obtained at 200 V DC. Energy dispersive X-ray dispersion (EDX) analysis results confirm the graded structure. The deposited zeolites are also analysed by scanning electron microscopy (SEM). The factors influencing the deposition process are discussed.

Characterization and activity of Mo supported catalysts for diesel deep hydrodesulphurization

Various highly dispersed Mo supported catalysts with various carriers were prepared for deep hydrodesulphurization of diesel. The carriers included a high surface area and large pore volume γ-Al 2O 3, two types of meso-microporous composite molecular sieves prepared by incipient-wetness impregnation method. A new mesoporous MoSiO x catalyst synthesized with in situ composite method was also studied. The hydrodesulphurization experiments were carried out in a micro-reactor over different catalysts including Mo supported series and a commercial catalyst. Spectroscopic techniques (FT-IR and UV–vis DRS) were utilized to determine the structure of MoO x species. The catalyst characterizations of BET, XRD, FT-IR, UV–vis DRS and FTIR pyridine adsorption indicated that the existences of metal active component of Mo in the catalysts were highly dispersed nano MoO 3 clusters and the Mo series catalysts had high surface areas and plenty of large pores which were propitious to the diffusions of reactant and product molecules. Cat NiMo exhibited the highest B/ L acidity ratio and higher total concentration of Brønsted acid sites and Lewis acid sites, and its HDS activity also gave the highest in this study to produce a sulphur-free diesel, which was verified by the sulphur content in products analyzed by GC–MS methods.

Comparative studies of Zr-based MCM-41 and MCM-48 mesoporous molecular sieves: Synthesis and physicochemical properties

Two surfactant-templated synthetic routes are developed for the preparation of new types of mesoporous molecular sieves, Zr-MCM-41 and Zr-MCM-48, using different Si sources but keeping the same zirconium precursor (zirconium- n-propoxide). When fumed silica was used as Si precursor, a Zr-MCM-48 material of cubic structure was formed with a surface area of 654.8 m 2/g and an unimodal pore diameter distribution. It shows low stability: after calcination at 600 °C, the ordered structure was transformed into a relatively disordered worm-like mesostructure with many defects and silanol groups. The use of tetraethyl orthosilicate as Si source led to the formation of a Zr-MCM-41 mesoporous solid, which had good thermal stability and a highly ordered hexagonal arrangement, with a surface area 677.9 m 2/g and an uniform pore diameter distribution. Fourier transform infrared (FT-IR) characterization and 29Si NMR analysis confirm that zirconium ions indeed incorporated into the framework of the solid. The in situ FT-IR spectroscopy of pyridine adsorption reveals that both, Lewis and Brönsted acid sites, were formed on the surface of these mesoporous materials. The strength and number of the Brönsted acid sites of the Zr-MCM-48 solid were greater than those of the Zr-MCM-41, due to a lower degree of condensation reaction during the synthesis that led to more structural defects in the framework and more silanol groups stretching from the solid surface.

Sorption properties for different types of molecular sieve and their influence on optimum dehumidification performance of desiccant wheels

The sorption property data for different types of molecular sieves (synthetic zeolites) are presented in equation form. The variation of the isosteric heat of sorption with water loading is derived using Othmer’s method. It is found that heat of sorption can be up to 50% greater than the latent heat of vaporization. Heat of sorption data, together with equilibrium isotherms are curve fitted and are incorporated into a numerical model of the nonlinear heat and mass transfer processes that occur in desiccant wheels. Using the numerical model, the influence of different assumptions for heat of sorption and equilibrium equation of molecular sieve on predicted optimum performance of rotary dehumidifier is evaluated.

Low temperature oxidative template removal from SBA-15 using MnO4−solution and carbon replication of the mesoporous silica product

We report a new method of low temperature removal of template from SBA-15 by KMnO4 chemical oxidation. Using this silica as template, mesoporous carbon with thick pore walls and relatively small mesopores can be cast. The results from XRD and nitrogen sorption show that the silica obtained after surfactant removal has high surface area, high pore volume and good structural ordering. The obtained carbon replica also shows good structural ordering as reflected from XRD and TEM characterizations. EDX analysis shows that no residue of potassium and manganese in the carbon sample can be detected, indicating the complete removal of such species during the washing step. Interestingly, the analysis of the micropore volume based on the DFT method shows that these carbon replicas have a large fraction of micropores with respect to the total pore volume. These micropore rich carbon materials could be candidates as molecular sieve type adsorbents and catalyst supports.

The role of MCM-41 composition in the creation of basicity by alkali metal impregnation

Three types of mesoporous molecular sieves (silicate MCM-41, niobosilicate NbMCM-41, and aluminosilicate AlMCM-41) were impregnated with alkali metal acetates. The textural stability of the samples was estimated on the basis of XRD measurements and nitrogen adsorption/desorption methods. The acidic–basic properties of the obtained materials were tested in the cyclization of acetonylacetone and studied by FTIR spectroscopy after adsorption of methylacetylene (propyne) and pyridine. Two types of chemical reactions between mesoporous matrix and alkali metal salt/oxide were observed: the reaction between alkali metal salt and silica/silicate and the reaction between basic oxide and silanol groups or Na + cations. In the first one the alkali metal silicate formation decreases from Li to Cs species and leads to the destruction of the material. The second reaction places alkali metal cations in extra framework positions, which increases from Li to Cs species and is only observed in the case of AlMCM-41 and NbMCM-41 materials. As a result of the second reaction Lewis acidity is generated. The average effect of these two interactions leads to the volcanic curve for basicity of M/AlMCM-41 and M/NbMCM-41 with the maximum for K-samples. In contrast, basicity of M/MCM-41 systematically increases from Li to Cs form since the second type of interaction does not occur for these samples.

Zeolite coated ATR crystal probes

Thin and well-defined MFI type molecular sieve films were grown on a range of ATR crystals by employing a seeding method. The type of ATR crystal does not influence film morphology. FTIR spectroscopy was used to evaluate the coated ATR crystals as sensor probes. These novel sensor probes could be used to detect low concentrations of organic molecules in a gas flow.

Shape-selective reactions with AEL and AFI type molecular sieves alkylation of benzene, toluene and ethylbenzene with ethanol, 2-propanol, methanol and t-butanol

The vapour and liquid phase alkylation of various reactions are studied over MnAPO-5 and MnAPO-11. The product distribution and selectivity are discussed in terms of dimensions of the molecules. The coke content, regeneration time required by the catalyst and TOS are studied. The studies suggest that a strong deactivation of the catalyst occurs with increase in bulkiness of the reactants. The vapour phase reactions carried out at high temperatures (350 and 400 °C) result in faster deactivation of the catalyst, whereas the liquid phase reactions showed good conversion and extended catalyst life.

Reemission of Tritium from Tritium-Sorbed Molecular Sieve

In handling of tritium-containing waste gas, tritium is oxidized to tritiated water and immobilized in a molecular sieve (MS), which is then disposed of as solid radioactive waste. So reemission of tritium from tritium-sorbed molecular sieve is concerned for tritium waste disposal. 4A, 5A and 10X MS were chosen for the tritium reemission test. The tritium-containing MS samples with specific activity of 3 GBq/g were prepared and the reemission coefficients of tritium from the three types of MS were determined. The effects of storage conditions of the MS on the reemission of tritium were examined. The results show that during two months of storage period, the reemission coefficients of 4A, 5A and 10X MS are (1.9~5.5) × 10-6 d-1·g-1. Among them, 5A MS has the largest reemission coefficient and 4A MS the smallest. The tritium released from tritium-sorbed MS is mostly in the form of HTO, only less than 1.2% of the tritium is in the form of HT. The atmosphere for storing tritium-sorbed MS has rather effect on reemission of tritium. The reemission coefficient in argon is lower than that in Ar+2%H2.

Effects of channel structure and acidity of molecular sieves in hydroisomerization of n-octane over bi-functional catalysts

SAPO-5, −11, −31, −41, −34, ZSM-5, −22 and −23 were synthesized by using the hydrothermal method and characterized by various methods such as XRD, SEM, XRF and TPD of NH3. They are representative of large-pore, medium-pore, small-pore, weak acid, strong acid, monodimensional channel and zigzag channel type of molecular sieves. Effects of pore size, the number of acid sites over medium-pore SAPOs, acid strength and shape of medium-pore channel on hydroisomerization of n-octane were examined over Pt-loaded corresponding molecular sieves. These results indicate that the selectivity to isomerization in hydroisomerization of n-octane is highly influenced by channel structure in molecular sieves and the conversion activity of n-octane is dependent on acidity of molecular sieves. Monodimensional medium-pore molecular sieves are ideal catalytic materials for higher isomerization selectivity in hydroisomerization of n-octane regardless of acid strength, such as SAPO-11, −31, −41, ZSM-22 and −23.

Aluminum chloride grafted mesoporous molecular sieves as alkylation catalysts

The aim of this work is to study the influence of different mesoporous molecular sieves (MMSs) type and the effect of pore diameter on MMSs silica grafted aluminum chloride catalysts and their activity in benzene alkylation by 1-dodecene. Four samples (two hexagonal and two wormhole like structures with different pore diameters) were prepared by post-synthesis grafting reaction in moisture free conditions (Schlenk technique). The physico-chemical properties were monitored by N 2 adsorption, XRD, atomic adsorption, 29Si and 27Al MAS NMR. Acidity data were obtained by TPD of ammonia, and FTIR of adsorbed pyridine. The alkylation of benzene is carried out in a Schlenk tube under dry conditions at room temperature. The products were analyzed by GC-MS. Grafting aluminum chloride on MMSs increases the yield of monoalkylated products.

Characterization of metal-containing molecular sieves and their catalytic properties in the selective oxidation of cyclohexane

Three types of metal-containing molecular sieves with AFI, AEL and CHA structures (Me=Co, Mn, Cr and V) were synthesized hydrothermally and characterized by XRD, XRF, TG, TPR, NH 3-TPD and FT-IR. It was revealed that metals were incorporated into the framework of molecular sieves and induced the presence of charge centers. Both cobalt and manganese in the framework of AlPO-5, AlPO-11 and SAPO-34 were not reducible before the structure collapse. The redox behaviours of these catalysts in cyclohexane oxidation at 403 K using O 2 as oxidant were examined. CoAPO-11 exhibited best activity and good selectivities for the monofunctional oxidation products (88.5%). Cyclohexanol was the major product over most catalysts, whereas for Cr-containing molecular sieves, high selectivity of cyclohexanone was observed. Investigation of reaction mechanism based on CoAPO-11 and CrAPO-5 catalysts indicated that the decomposition of cyclohexyl hydroperoxide (CHHP), the intermediate in cyclohexane oxidation, followed the pathway: cyclohexanone ← CHHP → cyclohexanol → cyclohexanone.

Separation of Ethanol–Water Mixtures Using Molecular Sieves and Biobased Adsorbents

The potential use of new biobased adsorbents and different types of molecular sieve was assessed in the separation of the ethanol–water azeotrope. Molecular sieves of type 3A, type 4A, and type 5A and biobased adsorbents such as natural corncobs, natural and activated palm stone and oak, were used in this study. Each of these adsorbents was packed in a column, which was surrounded by a heating jacket. The jacket temperature was maintained constant in an attempt to maintain the packed-column at constant temperature. The water concentration in the feed solution was varied from 5% to 12% (weight basis). Although the initial concentration of ethanol in the flask was constant at the beginning of the experiment, it varied with time due to boiling. The breakthrough curves of water sorption on these adsorbents at different water contents showed that among the molecular sieves examined, type 3A molecular sieves gave the best separation of the ethanol–water system and among biobased adsorbents examined, natural palm stone was the best. The Guggenheim, Anderson, and De Boer (GAB) model at different water contents represented the isotherms for water sorption on molecular sieves and biobased adsorbents. In addition, the surface area of adsorbents and maximum water uptake were calculated. Type 3A molecular sieves were determined to have the largest surface area and the highest value of water uptake compared with the other two types of molecular sieve. However, for the biobased adsorbents, activated palm stone was found to have the largest surface area and the highest value of water uptake compared to the other biobased adsorbents. It was found that upon activation, the adsorption capacity of biobased adsorbents was not enhanced. On the contrary, the activation gave negative results on some occasions.