Zigzag Channels Research Articles

Structure and acidity of Mo/ZSM-5 synthesized by solid state reaction for methane dehydrogenation and aromatization

The Mo/ZSM-5 catalysts were synthesized by solid-state reaction with the mixture of MoO 3 and HZSM-5 under N 2 atmosphere at 500 °C, and characterized by XRD, MAS NMR, BET, NH 3-TPD, FT-IR and powder XRD structural analysis. The results showed that the Mo species were well dispersed on the internal and external surface of the zeolite ZSM-5, while the framework of ZSM-5 was retained. One part of Mo species inside the channels of ZSM-5 can be described as [Mo 5O 12] 6+ units in the Mo/ZSM-5 with 8 wt% MoO 3 loading. The [Mo 5O 12] 6+ units are situated at the cross-point of the zigzag channel and the straight channel. They interacted with the framework of ZSM-5, and decreased the Brönsted acid sites remarkably. There was about 0.47 [Mo 5O 12] 6+ unit in every cell of the ZSM-5 on an average. The other part of Mo species is distributed on the external surface of the ZSM-5 crystals, and can be described as crystallites of molybdenum oxides. The distribution of Mo species on HZSM-5 increased the Lewis acid sites in the catalysts. The [Mo 5O x C y ] n+ units inside the channels of ZSM-5 partially reduced by methane maintained the catalytic activity of the catalysts and were beneficial for the formation of benzene during methane dehydrogenation and aromatization at 700 °C.

A Study of Binary Adsorption of Aromatics in Completely Siliceous Zeolite ZSM-5 by FT-Raman Spectroscopy

In the present work, binary adsorption of p-xylene and toluene in completely siliceous zeolite ZSM-5 was examined by FT-Raman spectroscopy in combination with powder X-ray diffraction. The results indicate that at total loadings of < or = 4 molecules per unit cell (u.c.) both p-xylene and toluene molecules prefer to reside at the intersections of straight and zigzag channels. The structure of the sorbate/sorbent complexes likely belongs to orthorhombic. At high loadings of 7 and 8 molecules/u.c., the framework of the zeolite transforms to another orthorhombic phase. In this high-loaded phase, toluene molecules are distributed evenly in both the channel intersection and the midsection of straight channel. Although p-xylene has access to both the channel intersection and the zigzag channel, it does show a slight preference for the channel intersection. The adsorptive behavior of these aromatics at intermediate total loadings of 5 and 6 molecules/u.c. is more complicated because the zeolitic framework is a mixture of two different orthorhombic phases.

Luminescence Lifetime Distributions Analysis in Heterogeneous Systems by the Use of Excel's Solver

A detailed study of the luminescence decay curves of pyrene included within p-tert-butylcalix[4]arene cavities and benzophenone into silicalite channels is reported. A new methodology for a lifetime distribution analysis of the decay curve of probes onto heterogeneous surfaces is presented, which allows for asymmetric distributions and uses Voigt profiles (Gaussian and Lorentzian mixture) instead of pure Gaussian or Lorentzian distributions. Our approach uses a very simple and widely available tool for fitting, the Microsoft Excel Solver. In the case of the pyrene/tert-butylcalix[4]arene sample, the room temperature luminescence detected in the microsecond time scale was not only the phosphorescence of pyrene but also monomer delayed fluorescence, crystal phosphorescence, and excimer delayed fluorescence. In the benzophenone/silicalite case, three emissive forms of benzophenone could be assigned, one of benzophenone included into the silicalite circular zigzag channels, another for emplacement into the elliptical straight channels and finally when benzophenone is placed at the crossing points of those silicalite channels, where smaller spatial restrictions for benzophenone exist.

A New Inclusion Compound Consisting of 2D Coordination Layers of [Cd(SCN)2]∞ and Pillars of Isonicotinamide Dimers

Abstract Self-assembly of Cd2+, SCN−, isonicotinamide (isoNH2), and 9,10-dichloroanthracene gave a new inclusion compound formulated as [Cd(SCN)2(isoNH2)2]·1/2(9,10-dichloroanthracene) (1). This inclusion compound consists of 2D coordination layers of [Cd(SCN)2]∞ and pillars of isoNH2 dimers. 9,10-dichloroanthracene molecules are included in zigzag channels.

Concentration Dependence of the Methane Structure in Silicalite-1: A Molecular Dynamics Study Using the Møller−Plesset-Based Potential

Møller-Plesset perturbation-based potentials have been used in molecular dynamics simulations to examine methane diffusion in silicalite-1. The simulation box contains 2 unit cells of silicalite-1 and varying loading numbers (n(ld)) from 1, 2, 3, to 4 methane molecules per intersection, corresponding to 8, 16, 24, and 32 molecules per simulation box, respectively. Consistent with the previous study, a preferential diffusing path for methane is close to the channel axes. The structure of the methane molecules in the silicalite-1 pore was exhibited in terms of the methane-methane radial distribution function (RDF) in which the first peak appears at 6.4 A for n(ld) < or = 2, becomes a broad maximum at n(ld) = 3, and splits into two sharp peaks centered at 6.4 and 8.6 A at n(ld) = 4. This fact can be clearly described by an intensification of the methane density in the straight and zigzag channels but a decrease in the intersection when the loading increases. These features of the observed RDFs are in contrast with the previous report using a molecular dynamics force-field potential in which the RDFs for all concentrations show first maxima at approximately 4 A. The analysis of the relative residence times of methane at different sites inside the silicalite suggests that the zigzag channel is the most favored location. The computed self-diffusion coefficients as well as the heat of adsorptions are in reasonable agreement with the available values.

An integrated microfluidic device in polyester for electrophoretic analysis of amino acids

A precolumn reaction chamber was integrated into a polyester microfluidic device with a miniaturized detection system. The reaction chamber was designed to be a zigzag channel, 70 microm in width, 8 mm in length, followed by a wider straight channel, 150 microm in width, 2 mm in length. The detection system is composed of an embedded light-emitting diode (LED), an integrated optical fiber, and a photomultiplier tube (PMT). A success in amino acid analysis using the integrated microchemical analysis device proved that the precolumn reaction chamber was compatible with the integrated detection system. Three kinds of amino acids, arginine, glycine, and phenylalanine, mixed and reacted with 7-fluoro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-F) in the precolumn reaction chamber to produce fluorescent products, were separated by micellar eletrokinetic chromatography (MEKC) and detected by LED-excited fluorescence. The detection limits for arginine, glycine, and phenylalanine were 1, 1, and 0.5 mM, respectively, which can be improved by further optimizations of the reaction system and detection system.

Equilibrated thermodesorption studies of adsorption of n-hexane and n-heptane on zeolites Y, ZSM-5 and ZSM-11

Gravimetric measurements of thermodesorption of n-hexane and n-heptane were performed under quasi-equilibrium conditions. Differential thermodesorption profiles for ZSM-5 and ZSM-11 showed two peaks, but for Y zeolites, only one thermodesorption peak was observed. A model function, derived from the Langmiur adsorption model, was fitted to the experimental data, and the model parameters (the adsorption entropy and enthalpy) were estimated. The two-step desorption profiles observed for ZSM-5 and ZSM-11 were attributed to the commensurate freezing effect, i.e. a transition in the adsorbed phase resulting in ordering of the adsorbed molecules in the zeolite channels. The results observed for ZSM-11 indicate that the zigzag channels typical for ZSM-5 micropore system are not necessary for this transition to occur.

Explicit equations for leak rates through narrow cracks

Explicit equations to describe the leak rate of a single phase fluid through a narrow crack under a low pressure gradient have been developed and are presented. Four distinct flow regimes, which change with crack opening displacement, have been previously identified and are the basis of this model. The fluid flow is governed by the pressure gradient and the tortuosity of the crack, which is particularly important when the opening displacement is small. The equations have been developed by considering the pressure forces created when the fluid flows down an idealized zig-zag channel. The nature of the flow, and hence the governing equations, change as the crack aperture increases. The power of this approach is clearly seen when the flow rates predicted using this model are compared both to the flow rates obtained from computational fluid dynamics analysis and those found by experimentation. The agreement between these sets of data is good, showing that the major effects governing the flow rate have been identified and then accounted for.

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.

Computer simulation of the adsorption of ethanethiol in silicalite of MFI and MOR

The method of configurational-bias Monte Carlo simulation is used for investigating the adsorption and distributions of ethanethiol in MFI and MOR type zeolites. The calculated adsorption quantity of ethanethiol in MFI is close agreement with the experimental data by means of IR spectroscopy and thermogravimetry. The calculated results show the adsorption quantity of ethanethiol in MFI is much more than that in MOR under coequal conditions. The observation of distribution of MFI is attributed to preferential adsorption of the ethanethiol molecules in the straight and zig-zag channels. In the intersection of straight channels and zig-zag channels, it accommodates the ethanethiol molecules like a reservoir. The straight channels for MOR accommodate all ethanethiol molecules almost, however, in the other channels, the ethanethiol molecules cannot be found.

On the Inflection in the Concentration Dependence of the Maxwell−Stefan Diffusivity of CF4 in MFI Zeolite

The recently published data on the Maxwell−Stefan diffusivity Đ of CF4 in MFI zeolite (J. Phys. Chem. B 2004, 108, 10613) shows an inflection at a loading ϑ = 12 molecules per unit cell, corresponding to the inflection in the adsorption isotherm. We investigate the underlying reasons by performing kinetic Monte Carlo simulations and show that at ϑ = 12, the molecular traffic along the zigzag channels comes to a virtual stand-still and further transport occurs only along the straight channels of MFI. The inflection behavior of Đ is likely to be a generic characteristic of transport in nanoporous materials in which the adsorption of the diffusant occurs on multiple sites with widely different strengths.

A Novel Møller−Plesset Perturbation Based Potential for Determining the Structural and Dynamical Properties of Methane in Silicalite-1: A Molecular Dynamics Study

A novel silicalite-1/methane potential function model has been developed using quantum chemical calculations at the second-order Møller−Plesset perturbation (MP2) level with the 6-31G* basis sets. Ab initio calculations have been performed at ∼150 methane configurations generated inside the three silicalite-1 segments, namely, O10Si10H20, O30Si22H44, and O35Si29H58. The interaction energies are subsequently fitted to an analytical form. We illustrate characteristics variant between the ab initio fitted potential and the available force-field models. The molecular dynamics simulations, consisting of two units of silicalite-1 cells and eight methane molecules, are performed at various temperatures. The calculated diffusion coefficient 5.53 × 10-9 m2·s-1 and the heat of adsorption −5.0 kcal·mol-1 at room temperature reasonably agree with the previous studies as well as an Arrhenius activation energy of 1.73 kcal·mol-1. The percentages of methane molecules residing in zigzag and straight channels and in the intersection are in good agreement with those reported previously. The methane/methane radial distribution function exhibits the first peak at 6.25 Å. This is in contrast to the previous one observed at ∼4.0 Å. It is, then, demonstrated that the appearance of the peak at 4.0 Å is caused primarily by an imbalance of the methane/methane and silicalite-1/methane pair potentials.

Kinetics of Adsorption of n-butane on an Aggregate of Silicalite by Transient Non-equilibrium Molecular Dynamics

Here we present the results obtained by using a transient non-equilibrium molecular dynamics procedure (TNEMD) which allows us to study the kinetics of gas adsorption at different gas pressures. Our simulations were focused on the adsorption of n-butane on aggregates of silicalite at 300 K. These simulations give relevant information on both the equilibrium state and the kinetics of adsorption. The isotherms obtained are in agreement with previous experimental and simulated results. At the equilibrium state our simulations allowed us to identify five energetic sites on the aggregate: three inside the micropores (straight and zig-zag channels and their intersections) and two additional ones at the external surface. Moreover, our simulations show, as determined exprimentally, that the n-butane is adsorbed on the external sites of the zeolite only at high filling (high loadings) of the zeolite—i.e. close to the saturation. From the simulated kinetics we calculated the Fick's diffusion coefficients at different loadings.

Effect of molecular oxygen on the variable-temperature 29Si MAS NMR spectra of zeolite-sorbate complexes.

Structure determinations of siliceous zeolite-sorbate host-guest complexes by solid-state NMR require highly resolved 29Si MAS NMR spectra. As the temperature is lowered, the 29Si MAS NMR spectra of many zeolite-sorbate complexes become broadened such that the resolution of the individual 29Si peaks is lost, limiting the application of solid-state NMR for structure determination. It is shown that the 29Si peak widths are related to the 29Si T2 relaxation times and that the source of the 29Si relaxation and the line broadening is paramagnetic molecular oxygen in the channels of the zeolite. Removal of the oxygen by purging the sample with nitrogen gas leads to a dramatic increase in the resolution of the 29Si MAS NMR spectrum of the p-dibromobenzene/ZSM-5 complex. An analysis of the individual 29Si T1 relaxation times reveals that the oxygen molecules are localized mainly in the zigzag channels of ZSM-5, suggesting that the p-dibromobenzene molecules are located in the channel intersections.

Adsorption of Liquid-Phase Alkane Mixtures in Silicalite: Simulations and Experiment

A combination of experimental and computational studies of adsorption from liquid-phase mixtures of linear alkanes in the zeolite silicalite is presented here. Configurational biased grand canonical Monte Carlo simulations combined with identity-swap moves are used to equilibrate the simulations in reasonable times. Interesting trends observed in experiments have been captured quantitatively by simulations. A siting analysis of the simulation data reveals that, during adsorption from a liquid mixture, shorter alkanes prefer the zigzag channels and longer alkanes concentrate in the straight channels of silicalite.

Calculations of Site-Specific CO Stretching Frequencies for Copper Carbonyls with the “Near Spectroscopic Accuracy”: CO Interaction with Cu+/MFI

A scaling method based on the linear correlation between the CO bond length and the CO stretching frequency has been applied to the CO molecule adsorbed on the Cu-exchanged MFI zeolite. Effects of anharmonicity, cluster size, unit cell size, and the Madelung potential were investigated. Interaction of CO with zeolite framework was described at the combined RI−BLYP/IPF level. The inner part of the combined model (RI−BLYP description) consisted of up to 23 TO4 tetrahedra. The effect of the Madelung potential on CO stretching frequencies was negligible. All Cu+ sites on the channel intersection and on the wall of the main channel are characterized by the CO stretching frequencies in the narrow range of 2159−2164 cm-1 in excellent agreement with experimental data. The Cu+ sites on the wall of the zigzag channel show slightly higher frequencies (3−6 cm-1). These sites are populated, however, only when the framework Al atoms are at T4 or T10 positions. The proposed computational scheme provides essentially the same level of accuracy as obtained from CCSD(T) calculations for small copper carbonyl species with overall error smaller than 5 cm-1.

Dynamics of n-Hexane Inside Silicalite, As Studied by 2H NMR

Perdeuterated n-hexane, adsorbed in silicalite, has been found to exhibit a 2H NMR spectrum that represents a superposition of three Pake-powder patterns. Two Pake-powder patterns, with the spectral parameters CQ = 23.5 kHz and η = 0 and CQ = 30.1 kHz and η = 0 (CQ represents quadrupole constants and η is the asymmetry parameter), belong to different methylene groups and one powder pattern, with CQ = 7.0 kHz and η = 0.7, arises from the methyl groups. The observed line shapes have been concluded to arise from the fast anisotropic translational and rotational motions of the alkane in the silicalite channels. The reduction of the quadrupole constants, in comparison to that in rigid n-hexane, and the appearance of an asymmetry parameter for the methyl groups have been interpreted in terms of three modes of motion exhibited by n-hexane inside the channel system: (a) rotation of the molecule about the channel axis; (b) trans/gauche conformational isomerization around nonterminal C−C bonds and rotation of the methyl groups around terminal C−C bonds; and (c) fast interchange of the adsorbed molecules between both straight and zigzag channels and along zigzag channels with an effective angle of ∼90° between adjacent sites of the molecule location during successive jumps. The proportion percentage of trans/gauche conformations is estimated to be 78/22, which exceeds that in gas phase or liquids. This demonstrates that constraining effects of the narrow silicalite pore walls retain the molecules in a more elongated conformation, compared to that in a gas phase or liquid. The rotation of a methyl group around a terminal C−C bond has been estimated to occur with a characteristic time of ∼40 ps at 300 K and an activation energy of 9.4 kJ/mol. The reorientation of methylene groups, corresponding either to conformational isomerization or to translational diffusion, occurs 1 order of magnitude slower, with a characteristic time of ∼300 ps and an activation energy of 7.5 kJ/mol.

Molecular Tectonics. Hydrogen-Bonded Networks Built from Tetraphenols Derived from Tetraphenylmethane and Tetraphenylsilane

Tetrakis(3-hydroxyphenyl)silane (1), tetrakis(4-hydroxyphenyl)methane (2), and tetrakis(4-hydroxyphenyl)silane (3), in which phenolic hydroxyl groups are attached to tetrahedral tetraphenylsilyl and tetraphenylmethyl cores, produce a series of hydrogen-bonded networks when crystallized from CH3COOC2H5. Each hydroxyl group in meta-substituted tetraphenol 1 participates in two intermolecular hydrogen bonds as both donor and acceptor, producing helical chains of hydrogen bonds running along the c axis. Each molecule of tetraphenol 1 is linked to four symmetrically oriented neighbors by a total of eight hydrogen bonds, thereby creating a diamondoid network. No interpenetration is observed, and no significant volume remains for the inclusion of guests. The hydrogen-bonded networks derived from para-substituted analogues 2 and 3 are markedly different. Each molecule of tetraphenol 2 is hydrogen-bonded to six neighboring tetraphenols, and the resulting network defines zigzag channels that run parallel to the c axis, measure about 3.3 × 4.4 Å at the narrowest point, and include CH3COOC2H5 as guest. Approximately 28% of the volume of crystals of tetraphenol 2 is available for inclusion. Tetraphenol 3 crystallizes as a monohydrate, and H2O is incorporated as a structural element in the resulting network. Each molecule of tetraphenol 3 forms hydrogen bonds with two molecules of H2O and four unsymmetrically oriented neighboring molecules of tetraphenol 3, producing a structure that is closely packed. The variety of structures obtained from compounds 1−3 under similar conditions shows that the hydroxyl group of phenols is not a highly predictable director of supramolecular assembly.

Serial transverse enteroplasty (STEP): A novel bowel lengthening procedure

Background/Purpose: Bowel lengthening may be beneficial for children with short bowel syndrome. However, current techniques require at least one intestinal anastomosis and place the mesenteric blood supply at risk. This study seeks to establish the technical principles of a new, simple, and potentially safer bowel lengthening procedure. Methods: Young pigs (n = 6) underwent interposition of a reversed intestinal segment to produce proximal small bowel dilation. Five weeks later the reversed segment was resected. Lengthening of the dilated bowel then was performed by serial transverse applications of a GIA stapler, from opposite directions, to create a zig zag channel. A distal segment of equal length served as an in situ morphometric control. Contrast radiologic studies were performed 6 weeks later, and the animals were killed. Statistical comparisons were made by paired t test with P less than.05 considered significant. Results: After bowel lengthening, all animals gained weight (66.7 ± 3.0 [SD] kg v 42.5 ± 3.5 kg; P <.001) and showed no clinical or radiologic evidence of intestinal obstruction. Intraoperatively, immediately after serial transverse enteroplasty, the intestine was substantially elongated (82.8 ± 6.7 cm v 49.2 ± 2 cm; P <.01). Six weeks after surgery, the lengthened intestinal segment became practically straight and, compared with the in situ control, remained significantly longer (80.7 ± 13.1 cm v 57.2 ± 10.4 cm; P <.01). There was no difference in diameter between these segments (4.3 ± 0.7 cm v 3.8 ± 0.4 cm; P value, not significant). Conclusions: Serial transverse enteroplasty (STEP) significantly increases intestinal length without any evidence of obstruction. This procedure may be a safe and facile alternative for intestinal lengthening in children with short bowel syndrome. J Pediatr Surg 38:425-429. Copyright 2003, Elsevier Science (USA). All rights reserved.

Investigation ofp-Difluorobenzene and Fluorobenzene Adsorbed on Completely Siliceous Zeolite ZSM-5

The host−guest interactions of two related sorbates, p-difluorobenzene and fluorobenzene, in completely siliceous zeolite ZSM-5 have been examined by thermogravimetric analysis, powder X-ray diffraction (XRD), and FT-Raman spectroscopy. The powder XRD patterns reveal that the phase transition behavior of p-difluorobenzene/ZSM-5 is very different from that of closely related p-dichlorobenzene/ZSM-5 and p-xylene/ZSM-5 systems. FT-Raman spectra of p-difluorobenzene adsorbed in ZSM-5 indicate that the first 4 p-difluorobenzene molecules are located in the channel intersections. When the loading is greater than 4 molecules per unit cell (uc), additional molecules access the midsections of straight channels. The situation is different from that found in p-dichlorobenzene/ZSM-5 and p-xylene/ZSM-5 complexes where additional guest molecules reside in the zigzag channels. Adsorption of fluorobenzene into ZSM-5 induces only one transition in the framework at a loading of 4 molecules/uc. This behavior is distinctly different from that of similar chlorobenzene/ZSM-5 systems in which two successive transitions occur at 3 and 7 molecules/uc.