Modified temperature programmed desorption evaluation of hydrocarbon trapping by CsMOR zeolite under cold start conditions

A cesium-exchanged mordenite zeolite was tested as a hydrocarbon trap for cold-start automotive applications using a modified temperature programmed desorption (TPD) procedure. In the modified TPD procedure, a step increase in feed concentration was performed simultaneously with the start of the temperature ramp to more closely mimic cold start conditions. The feed stream contained mixtures of toluene, propane, and water. The cesium-exchanged mordenite trapped propane under these dynamic conditions, demonstrated improved propane trapping when toluene was coadsorbed, and retained its trapping capacity after nearly 40 h of hydrothermal treatment.     

Diffusion of a para- and ortho-xylene mixture in CIT-1 zeolite: a molecular dynamics study

Molecular dynamics techniques have been used to simulate the diffusion of a binary mixture of ortho- and para-xylene in the purely siliceous zeolite CIT-1, a microporous solid with two channel systems of 10 and 12 MR (membered rings). A loading of 0.125 molec/uc of each isomer was selected for the 200 ps simulation run at a temperature of 500 K. Diffusion of the ortho isomer shows a nonlinear MSD plot owing to geometrical restrictions caused by the appreciable interactions of ortho-xylene molecules with the 10 MR channels. Para-xylene molecules show long diffusion paths through the 10 MR channels due to the impossibility of rotation in these narrower channels which contributes significantly to the increase in the self-diffusivity of this isomer with respect to ortho-xylene. The ortho-xylene self-diffusivity in the mixture is lower than in the pure system, while the para-xylene self-diffusivity increases in the mixture with respect to the pure system. These differences in self-diffusivities in the binary compared to the pure systems are explained in terms of the different mobilities of the isomers. This revised version was published online in June 2006 with corrections to the Cover Date.     

Oxygen transport in zeolite Y measured by quenching of encapsulated tris(bipyridyl)ruthenium

This study deals with emission quenching of zeolite encapsulated trisbipyridyl ruthenium (II) (Ru(bpy)₃²⁺) by oxygen. Oxygen saturated solutions of Ru(bpy)₃²⁺ typically show about 70% quenching (I₀/I=3.3), where I₀ and I are the peak intensities of the emission in N₂ and O₂, respectively. However, an aqueous suspension of Ru(bpy)₃²⁺-zeolite Na–Y (Si/Al = 2.5) (abbreviated as Ru–Na–Y) showed no quenching at all. This observation motivated us to analyze how the transport of O₂ is occurring in the zeolite. Upon exposure of solid Ru–Na–Y (99% of intrazeolitic water) to N₂/O₂ dry gases, quenching in oxygen was found to be 5% (I₀/I=1.07). Partial dehydration at room temperature with loss of 33% of the water molecules from the zeolite led to 66% (I₀/I=2.96) quenching. Dehydration of Ru–Na–Y at 250 °C under vacuum overnight led to complete loss of intrazeolitic water and increased quenching to 90% (I₀/I=10.7). Nanocrystalline Ru(bpy)₃²⁺-zeolite Y upon vacuum dehydration lost 55% of the intrazeolitic water and showed 96% (I₀/I=25.3) quenching. The extent of quenching of Ru(bpy)₃²⁺ in zeolites by O₂ is by far the largest as compared to previously studied matrices, and is being attributed to confinement of O₂ in the supercages, which leads to increase in number of collisions with Ru(bpy)₃²⁺ and enhanced quenching. However, these samples showed complete lack of sensitivity (I₀/I=1) to oxygen upon exposure to water saturated gas or dissolved gas. Dealumination of zeolite framework by treatment with (NH₄)₂SiF₆ produced a framework of Si/Al = 9.5, and with SiCl₄ a framework of Si/Al > 100. With increasing dealumination, the extent of quenching by dissolved O₂ increased.     

Diffusion of hydrocarbon mixtures in MFI zeolite: Influence of intersection blocking

Branched and cyclic hydrocarbons such as iso-butane, 2-methylpentane, 3-methylpentane, 2,2-dimethyl-butane and benzene are preferentially adsorbed at the intersections of the channels of MFI zeolite; they serve as bottlenecks for molecular traffic in mixtures with linear C1–C6 alkanes. Molecular dynamics simulations show that as the loadings of tardier iC4, 2MP, 3MP, 22DMB, and Bz is progressively increased to four molecules per unit cell the diffusivity of the more mobile linear alkane reduces nearly to zero. The reduction in the n-alkane diffusivity is quantitatively similar irrespective of the branched/cyclic hydrocarbon.     

Tailoring the reaction mixture composition for preparing zeolite coatings on aluminum supports in alkaline environments

Various alkaline reaction mixture compositions were tested for determining their suitability to be used in the direct crystallization of zeolites on aluminum supports. No organic materials, such as templates, complexing agents, and corrosion inhibitors were included in the reaction mixtures. It was observed that aluminum supports remained quite stable in alkaline synthesis mixtures when suitable reaction mixture compositions with relatively high silica content were used. These favorable compositions might be recognized from their sodium silicate/(sodium hydroxide?+?sodium aluminate) mass ratios, as depicted in this study. Coatings were prepared on aluminum by using some of these compositions. It was determined that aluminum hydroxide phases commonly forming on the support could be eliminated to a great extent and consequently, crystalline coatings of zeolites P and Y were obtained on aluminum surfaces. The coatings exhibited trivial mass loss in the mechanical stability tests applied.     

PFG NMR investigation of hydrocarbon diffusion in large NaX zeolite crystals: Effect of internal field gradients on diffusion data

Self-diffusion coefficients for intracrystalline diffusion of hydrocarbon molecules adsorbed in large crystals of NaX zeolite have been measured by the pulsed field gradient (PFG) NMR technique, at ambient temperature and at different diffusion times (from 6 to 12 ms). Two NMR pulse sequences, stimulated and 13-interval bipolar spin echo, were used to examine the influence of internal field gradients on diffusion data. For both sequences the effective self-diffusion coefficient of the guest molecules was found to decrease with increasing observation times. The extrapolated intracrystalline diffusion coefficient is independent of the NMR sequence. In contrast, the estimated extent of molecular diffusion depends strongly on the pulse program. For the small molecules (butane to hexane), the domain size, R, of restricted diffusion obtained with the stimulated spin-echo sequence is smaller than the crystal dimension whereas R is always comparable to it when the 13-interval pulse sequence is used. This shows the effect of internal field gradients on the diffusion data leading to wrong values of R if the stimulated pulse sequence is used. The light hydrocarbons diffuse freely inside the zeolite particles whereas the crystal boundaries act as reflecting surfaces, as previously observed. On the other hand, even with the 13-interval pulse sequence, the smaller values of R obtained for large molecules as n-heptane and octane shows that their displacement is hindered by restrictions in the NaX macro-crystals.     

On the mechanism of zeolite growing: Crystallization by seeding with delayered zeolites

Seeds of a delaminated precursor of MWW (ITQ-2) are more active for the synthesis of MCM-22 than crystals of MWW or the corresponding layered precursor. The effect is due on one hand to larger surface area and reactivity of the former, as demonstrated when “seeding” with MCM-41 or other delaminated zeolitic materials, and on the other hand to the crystalline structure of ITQ-2. Seeding with the delayered material yields smaller crystallites of MWW which should be positive from a catalytic point of view. The directing effect of seeding with different delayered materials on the synthesis of zeolites other than MWW is also presented.     

Probing the basicity of zeolite frameworks with N2O4: A DFT approach

The basicity of alkali exchanged faujasite type zeolites is estimated on the basis of DFT calculations of the NO⁺ stretching frequencies of adsorbed N₂O₄. N₂O₄ is allowed to interact with the alkali cations and a range of molecules covering a wide basicity scale. An alkali nitrate salt is formed and NO⁺ interacts with the basic molecules. The resulting NO⁺ stretching frequencies decrease with increasing proton affinity and basicity of the basic molecules for each alkali cation. If experimental proton affinities and basicities of the model molecules are used, the data fall on two straight lines, which are almost parallel. The zeolitic basicities and proton affinities are obtained by interpolation of the experimental NO⁺ frequencies of alkali exchanged faujasites on the linear plots of NO⁺ frequencies against the experimental proton affinities and basicities. With theoretical proton affinities and basicities two non-parallel straight lines are obtained and three molecules are offset. The reason for this is unclear and needs more investigation. The NO⁺ stretching frequency is linearly correlated with the hardness of the alkali cations in zeolite X. For zeolite Y the same trend is observed, but the scatter of the data is too large to define it as linear.     

Transient behavior of a zeolite membrane under non-linear conditions

A simplified mathematical model is proposed to provide an analytic solution for the transient behavior of a Langmuirian zeolite membrane subjected to a step increase in sorbate pressure at the upstream surface at time zero. The concentration profiles derived from the simplified model agree with those calculated by numerical solution of the governing partial differential equation. The model is used to investigate the validity of the classical time-delay method of measuring diffusion in the adsorbed phase. Under non-linear conditions the time delay depends on the degree of non-linearity as well as on the diffusional time constant. Under highly non-linear conditions application of the classical linear model yields apparent diffusivity values that are about four times larger than the limiting value (D₀).The new model is used to reanalyze experimental permeation diffusivity data for p-xylene in a single crystal zeolite membrane. The resulting D₀ values are shown to be consistent with the values determined by other experimental techniques.     

Dealumination kinetics of composite ZSM-5/mordenite zeolite during steam treatment: An in-situ DRIFTS study

To get a better understanding of structural deactivation of ZSM-5/MOR during the catalytic cracking of n-heptane in the steam atmosphere, a comprehensive mechanism of hydrothermal dealumination was proposed through in-situ diffuse reflectance Fourier transform infrared spectroscopy(DRIFTS) in this work. The mechanism can be divided into two steps: firstly, the hydrolysis of four Al\\O bonds, and secondly, the self-healing of Si\\OH bonds accompanied with partial condensation of the extra-framework Al species. Accordingly, the kinetics of dealumination process has also been fully discussed. In the IR spectra, the range of 3450–3850 cm~(-1) could be deconvolved to distinguish the hydroxyl groups on the different position and calculate the consumption of each hydroxyl group during the reaction. Based on results from the in-situ DRIFTS, the kinetics of dealumination was hence developed and also in well agreement with the kinetics of deactivation of ZSM/MOR catalysts during the reaction in the presence of little coke deposits.     

A time-averaged model for gas-solids flow in a one-dimensional vertical channel

In this study, we are interested in deriving time-smoothed governing and constitutive equations for gas-solids flow in moderately dense systems where particle-particle collision is the main energy dissipation mechanism. Results obtained from dynamic simulations of a gas-solids flow in a 1D channel are used to show that it is possible to obtain expressions for the time-averaged constitutive relations based on Taylor series expansion. We demonstrate, by comparing with time-averaged transient results, that the 1st term (or laminar) in the series expressions of most non-linear constitutive relations can yield inaccurate quantitative and qualitative results. This means that steady-state models derived by simply removing the partial time derivative from the governing equations are not suitable for gas-solids flows. This study shows that it was necessary to include many terms of the Taylor series expression of non-linear constitutive relations (such as the granular energy dissipation term) due to large-scale oscillations that were computed for all flow variables at all locations in the 1D domain. In some cases, the Taylor series expansion diverged and the Euler transformation was used to improve the convergence of these series. In this moderately dense flow system, turbulence in the gas-phase was found to be just a reaction to turbulence in the solids phase that resulted from the large-scale motion of solids clusters. This resulted in a negative turbulent gas viscosity computed due to the fact that gas (in the horizontal direction) flows only to occupy regions vacated by clusters of solids. The steady-state results obtained using the time-smoothed gas-solids flow model compared well with the time-averaged results obtained using the transient model for all flow variables.     

Mass transfer model,preparation and applications of zeolite membranes for pervaporation dehydration:A review

Pervaporation (including vapor permeation) is a kind of new membrane separation technology, possessing the advantages of high efficiency, energy saving and convenient operation. It has promising application in the separation and purification of organic solvents. Dehydration is an important step in the production and recovery of organic solvents. Zeolite membranes have attracted wide attention for pervaporation dehydration due to their high separation performance and good thermal/chemical stability. So far, zeolite membranes have been preliminarily industrialized for dehydration of organic solvents. This paper reviews the recent development of zeolitemembranes for pervaporation dehydration, includingmass transfermodels, preparation and applications of zeolite membranes. The review also discusses the current industrial applications of zeolite membranes and their future development in pervaporation.     

Diffusion of a carbonate mixture in a poly(vinyl alcohol-co-vinyl acetate) in the solid state

The diffusion study of a carbonate mixture composed of ethylene and propylene carbonates in poly(vinyl alcohol-co-vinyl acetate) (PVA–Ac) was studied. The diffusion occurred in the solid state and the effect of the carbonate mixture on the PVA–Ac properties are presented as well as the homogeneity of the diffusion. Due to the complexity of the diffusion kinetics with temperature, a model was made using the free volume theory at 75 °C. This temperature was chosen for modelling because the physical properties of the polymer are relatively stable at this point. Swelling heterogeneity, due to the particle size distribution of the powder, were estimated. Copyright © 2004 Society of Chemical Industry     

Phase selectivity in the syntheses of cage-based zeolite structures: An investigation of thermodynamic interactions between zeolite hosts and structure directing agents by molecular modeling

In this investigation we use molecular modeling to determine the van der Waals stabilizations of piperidinium derivatives and polycyclic quaternary ammonium compounds in zeolite frameworks with cage-based structures: NON, SGT, DDR, AEI, CHA, ITE, and STF. This effort follows earlier work by our group that studied the phase selectivities of these structure-directing agents (SDAs) under different conditions of synthesis [Y. Nakagawa, G.S. Lee, T.V. Harris, L.T. Yuen, S.I. Zones, Micropor. Mesopor. Mater. 22 (1998) 69–85; P. Wagner, Y. Nakagawa, G.S. Lee, M.E. Davis, S. Elomari, R.C. Medrud, S.I. Zones, J. Am. Chem. Soc. 122 (2000) 263–273; G.S. Lee, S.I. Zones, J. Solid State Chem. 167 (2002) 289–298]. The stabilization energies are found to be a good indicator of selectivity for phases that often crystallize within the same inorganic conditions (e.g., AEI and CHA, ITE and STF). The calculated stabilization energies are especially good indicators of phase selectivity for the large polycyclic molecules. Although the predictions for the piperidinium derivatives are not as successful at distinguishing selectivity for ITE/STF pair, we again find that the larger molecules among this group are accurately predicted. We suggest that this may be due either to a greater number of configurations available in the ITE cage or to the greater rotational freedom permitted for the smaller molecules in the round ITE cage. We next compare the differences in stabilizations of the different SDA/framework pairs with the differences in their respective framework energies, which have been approximated from empirical relations derived from earlier calorimetry experiments by Piccione et al. In general, these differences are found to be of similar magnitude for different phases that crystallize with the same SDA. Finally, we discuss the effects that siloxy/silanol defects may have on the frequently observed trends in the framework density for series of phases that are crystallized with the same SDA.     

Diffusion in a mesoporous silica membrane: Validity of the Knudsen diffusion model

Experimental permeance data for several light gases in a mesoporous silica membrane are analyzed in detail and shown to conform closely to the Knudsen diffusion model. The results of this study do not support the conclusions drawn from recent molecular simulations concerning the inadequacy of the Knudsen model.     

Predicting locations of non-framework species in zeolite materials

A new grid-based algorithm developed at Molecular Simulations, and molecular dynamics method have been applied to modeling the locations of non-framework species in zeolites. This new method locates the energy minima for various frameworks and populates these sites with the non-framework species. The cation locations were predicted in dehydrated zeolite adsorbents and catalysts, such as Na₈₈X, Ca₄₈X, mixed cation zeolite 3A (K₆₀Na₃₆A), and Cu-mordenite (Si/Al=5.0) using only well-known framework structure models. Furthermore, the locations of benzene molecules in a supercage of zeolite Ca₄₈X were correctly predicted via the application of the Monte Carlo docking and molecular dynamics methods.

These examples demonstrate that when the framework type is known, the new technique can provide a realistic initial structure input for the challenging task of solving crystal structures of zeolites containing non-framework species.     

分子筛膜合成的研究进展

作者主要就分子筛膜最新合成方法展开论述,主要介绍了最近十年来分子筛膜合成方法取得的新进展,如二次合成法、微波合成法、堵孔法、脉冲激光沉积法、电泳沉积法、汽相法、两步变温法、两步变浓度法等方法的应用,能够给该领域或即将踏入该领域的研究者提供一定的参考价值.     

Preparation of a clinoptilolite-type korean natural zeolite

A clinoptilolite-type natural zeolite was pretreated by HC1, NaOH, and NaCl solutions to improve the ion-exchange capacities for heavy metal ions such as copper, lead, cadmium, and cesium. The pretreated natural zeolite was experimentally investigated based on chemical analyses, X-ray diffraction, and BET experiments etc. From experimental data, it was shown that the pretreatment with NaCl gave the best ion-exchange capacity for all metal ions encountered. The ion-exchange capacity of the NaCl-treated sample is comparable with that of a commercialized natural zeolite, chabazite. On the other hand, the HCl-treated sample has very low ion-exchange capacities for all metal ions even though it has high specific surface area and total pore volume. It was proven by chemical analyses that a strong acid like HC1 can damage the structure of the zeolite by extracting aluminum and iron from their skeletal units.     

Modelling of microporous diffusion of n-paraffins in zeolite 5A

Sorptive liquid-phase diffusion of two n-paraffins, C₁₀H₂₂ and C₁₁H₂₄, dissolved in isooctane, onto micropore of 5A zeolite was studied to assess multicomponent diffusion and competitive effects. Diffusion coefficients for adsorbing components are determined from experimental batch reactor data. The experimental data indicate that diffusion through the microporous zeolite crystals is the primary diffusional resistance. A mathematical model of the rate of adsorption of a solute from a liquid by micropore adsorbent in a batch system was developed. The equation describing the mass transport by diffusion in a micropore adsorbent has been solved in order to obtain theoretical uptake curves for systems when the adsorption equilibrium isotherm is the favourable and nonlinear one. A computer simulation of the microporous diffusion is performed by use of the ISIM-Interactive Simulation Language. The effect of main term and cross-term coefficients of micropore diffusion for the system considered is investigated.     

Adsorption and kinetic studies of methylene blue on modified HUSY zeolite and an amorphous mixture of γ-alumina and silica

ABSTRACT

Our work focuses on the study of the adsorption of methylene blue (MB) on adsorbents based on zeolite HUSY and (γAl₂O₃-SiO₂). To optimize the process of removing MB onto Ni/Co USY, different parameters were studied such as contact time, initial pH, initial dye concentration, zero charge’s point, and adsorbent dosage. The adsorption isotherm follows the Langmuir model. The maximum adsorption capacities of MB were 59.88 mg g^?1 for Ni/Co USY and 43.86 mg g^?1 for Ni/Co (γAl-Si) at 298°K. The thermodynamic parameters and activation energy’s values obtained suggested that the adsorption was a physical process, spontaneous, and endothermic in nature. MB adsorption on Ni/Co USY may occur via electrostatic interaction, hydrogen bonding, and Lewis acid–base interaction.