Cation mobility and the sorption of chloroform in zeolite NaY: molecular dynamics study

Molecular dynamics simulations at temperatures of 270, 330, and 390 K have been carried out to address the question of cation migration upon chloroform sorption in sodium zeolite Y. The results show that sodium cations located in different sites exhibit different types of mobility. These may be summarized as follows: (1) SII cations migrate toward the center of the supercage upon sorption, due to interactions with the polar sorbate molecules. (2) SI' cations hop from the sodalite cage into the supercage to fill vacant SII sites. (3) SI' cations migrate to other SI' sites within the same sodalite cage. (4) SI cations hop out of the double six-rings into SI' sites. In some instances, concerted motion of cations is observed. Furthermore, former SI' and SI cations, having crossed to SII sites, may then further migrate within the supercage, as in (1). The cation motion is dependent on the level of sorbate loading, with 10 molecules per unit cell not being enough to induce significant cation displacements, whereas the sorption of 40 molecules per unit cell results in a number of cations being displaced from their original positions. Further rearrangement of the cation positions is observed upon evacuation of the simulation cell, with some cations reverting back to sites normally occupied in bare NaY.     

The Dielectric Properties of 1,3-Butanediol

The dielectric properties of 1,3-butanediol were measured over the temperature range 293–423 K. The static permittivity ε_s was measured at a frequency of 1 MHz. The permittivity ε′ and dielectric loss ε′’ were determined by the balance method over the frequency range 2–37.5 GHz.     

Effect of pore water and adsorbed moisture on the dielectric properties of green river oil shale

Moisture and pore water are found to have appreciable effects on the dielectric parameters of Green River oil shale. A regular decrease in the relative dielectric constant, ε′, and dielectric loss, ε″, with decreasing amount of adsorbed moisture and pore water is observed at 24°C for oil shale samples ranging in oil yields from ～6-～100 gallons per ton. The weak dependence of ε′ and ε″ on shale richness at frequencies in the range 50 Hz-1 MHz effectively rules out the application of dielectric techniques as an assay tool. The results are discussed in terms of interfacial polarization effects arising from the presence of adsorbed moisture and pore water in the oil shale matrix.     

The Dielectric Properties of 1,4-Butanediol

The dielectric properties of 1,4-butanediol were studied. The static permittivity ε_s was measured at a frequency of 1 MHz. Permittivity ε′ and dielectric loss ε″ were determined by the balance method over the frequency and temperature ranges 2–37.5 GHz and 293–423 K, respectively.     

Measurement of moisture in grains at extreme temperatures

Summary This research has demonstrated that the dielectric method can be used successfully for grain moisture measurements for diverse grain types over wide ranges of temperatures if the grain moisture content is below certain (grain-specific) threshold values. These high-moisture limits were estimated. Optimum temperature correction coefficients for 149 MHz moisture measurements were determined for grain samples at different moisture levels. This information should be very helpful for grain moisture meter manufacturers and for moisture meter users who need to determine grain moisture contents at temperature extremes to be able to market grain more efficiently.     

Cation migration upon adsorption of methanol in NaY and NaX faujasite systems: a molecular dynamics approach

Molecular dynamics simulations have been carried out to address the question of cation migration upon adsorption of methanol in NaY and NaX faujasite systems as a function of the loading. For NaY, it has been shown that, at low and intermediate loadings, SII cations can migrate toward the center of the supercage due to strong interactions with the adsorbates, followed by a hopping of SI' from the sodalite cage into the supercage to fill the vacant SII site. A SI' cation can also migrate across the double six ring and takes a SI' vacant position. SI cations mainly remain trapped in their initial sites whatever the loading. At high loading, only limited motions are observed for SII cations due to steric effects induced by the presence of adsorbates within the supercage. For NaX, the SIII' cations which occupy the most accessible adsorption sites are significantly moving upon coordination to the methanol molecules; the extent of this mobility exhibits a maximum for 48 methanol molecules per unit cell before decreasing at higher loadings due to steric hindrance. In addition, the SI' and SII cations remain almost trapped in their initial sites whatever the loading. Indeed, the most probable migration mechanism involves SIII' cation displacements into nearby SIII' sites.     

Electrodynamic investigations of conduction processes in humid microcrystalline cellulose tablets

The conduction mechanism in microcrystalline cellulose (MCC) tablets at varying relative humidity (RH) has been investigated by using the techniques of low frequency dielectric spectroscopy and transient current analysis at room temperature. The dependence on RH on the measured conductivity and charge carrier density indicates that a high-power-law-exponent percolation process of cations being conducted on water molecules occupying available 6-OH units on the cellulose chains is the dominating dc conduction mechanism at RH below 3 wt % of moisture content. The experimentally observed decrease in charge carrier mobility with increasing moisture content shows that protons and H3O+ ions that are being blocked at empty 6-OH sites also contribute to the charge transport process in cellulose at low moisture contents.     

Molecular dynamics simulation of the cation motion upon adsorption of CO2 in Faujasite zeolite systems

Molecular Dynamics simulations have been carried out in NaX and NaY Faujasite systems to deepen understanding of the cation rearrangement during the CO2 adsorption process suggested by our recent diffusivity measurements. This study is a major contribution since the rearrangement of the cations in Faujasite, the most promising adsorbent for CO2 storage, can represent a significant breakthrough in understanding the adsorption and diffusion processes at the mircroscopic scale. For NaY, it has been shown that at low and intermediate loadings, SII cations can migrate toward the center of the supercage due to strong interactions with the adsorbates, followed by a hopping of SI'cation from the sodalite cage into the supercage to fill the vacant SII site. The SI cations are only displaced at a higher loading, leading to cation de-trapping out of the double six rings into the vacant SI' sites. For NaX, the SIII' cations which occupy the most accessible adsorption sites move significantly upon coordination to the carbon dioxide molecules. The SI' and SII cations remain consistently located in their initial sites whatever the loading. Indeed, the most probable migration mechanism involves SIII' cation displacements into nearby vacant SIII' sites.     

Investigation on the nature of the adsorption sites of pyrrole in alkali-exchanged zeolite y by nuclear magnetic resonance in combination with infrared spectroscopy

Multinuclear solid-state NMR and infrared spectroscopy have been applied to investigate the host-guest interactions and the nature of the adsorption sites of pyrrole on alkali-exchanged zeolites Y (LiNaY, NaY, KNaY, and CsNaY). The presence of pyrrole provokes changes in the MAS NMR spectra of (23)Na, (7)Li, and (133)Cs to a degree dependent upon the amount adsorbed. The decrease in the quadrupolar coupling constant for (23)Na as well as the shift for (7)Li and (133)Cs signals are attributed to the interaction of the cation with the pyrrole ring system. The adsorption of pyrrole induces the displacement of cations located at SI' and SII sites toward the supercage to bind the guest molecules. In this way, the distribution of the cations at nonframework sites depends on the amount of adsorbate in the zeolite. At low loadings, pyrrole molecules bind preferentially to more electropositive cation in partially exchanged zeolites Y. Quantitative analysis by (1)H NMR shows that the cation-pyrrole complexes formed possess a stoichiometry of 1:1. The origin of the basic site heterogeneity, evidenced by the presence of several components in the -NH infrared stretching band, is investigated assuming that the heterocycle of pyrrole interacts with cations at SII sites in the supercage and the -NH group forms a hydrogen bond with a basic oxygen atom placed in the framework six-member ring. Making use of the information derived from NMR, it is concluded that the main source of basic site heterogeneity comes from the number of aluminum atoms in the six-member rings of the SII site where the alkaline cation is located.     

微波诱导快速合成纳米NaY分子筛

报导了一种在常压回流微波加热条件下,可使合成的NaY分子筛晶粒尺寸减小至100 nm以下的方法.添加柠檬酸的晶化速率接近添加稀土离子的晶化速率,合成60 min后两者得到的NaY分子筛的晶粒度和硅铝比接近,约为40 nm和5.2,晶粒度小且硅铝比高于在相同条件下没有添加合成得到的NaY分子筛.微波合成的NaY分子筛的晶粒度明显小于常规相同配比所得样品的晶粒度且晶化时间明显缩短,充分证明了微波加热快速均匀且能够得到较小晶粒度的特点.     

铜离子在CuLaHY分子筛中的分布与吸附脱硫性能

在空气气氛中采用等体积浸渍法制备了具有不同Cu担载量的CuLaHY分子筛吸附剂, 并用X射线衍射(XRD)、比表面积(BET)、X射线光电子能谱(XPS)技术对分子筛吸附剂进行了表征. 通过多晶XRD确定了Cu2+及La3+离子在Y型分子筛笼内的结构与分布, 并测定了分子筛吸附剂在含二苯并噻吩(DBT)的模拟柴油中的吸附脱硫性能. 结果表明, 前驱体CuCl2中的大部分Cu物种与LaHY分子筛进行了离子交换, 进入分子筛笼内, 极少部分Cu物种以CuCl形式高度分散在Y型分子筛的笼中. La3+离子及进入Y型分子筛笼中的部分Cu2+离子处于茁笼的SI'位, 而另一部分Cu2+离子与骨架氧和水分子配位, 并牢固地定位于Y型分子筛超笼中的SII及SIII位上. 处于超笼中SII及SIII位上的Cu2+离子对模拟柴油中的DBT分子具有吸附作用, 成为吸附脱硫的中心. 当模拟柴油中有萘存在时, 与DBT分子会产生竞争吸附.     

STUDY OF THE FREQUENCY DISPERSION OF DIELECTRIC PERMITTIVITY AND DIELECTRIC LOSS COEFFICIENT FOR AQUEOUS POTASSIUM IODIDE SOLUTIONS

Journal of Structural Chemistry - Frequency dispersion of dielectric permittivity ε1(ν) and dielectric loss ε2(ν) dynamic coefficients is studied; temperature T, density ρ,...     

Specific Sorption Sites for Nitrogen in Zeolites NaLSX and LiLSX

Specific sorption sites for nitrogen, N₂, in NaLSX and LiLSX zeolites were investigated using a DRIFT spectroscopic method. Sorption of molecular hydrogen, H₂, by NaLSX or LiLSX zeolite at 77 K with DRIFT control of perturbation of sorbed molecules allowed to discriminate two or three different types of specific sorption sites in the respective zeolites. Their H–H stretching frequencies are 4077 and 4081 cm^–1 for NaLSX, and 4061, 4084 and 4129 cm^–1 for LiLSX. With reference to an independent investigation by methods of both sorption thermodynamics and molecular modeling for N₂ sorption on LiLSX, the first two of the corresponding bands were ascribed to H₂ sorption on lithium cations, Li⁺, localized in supercages of the faujasite, FAU, zeolite framework at sites SIII and SIII, while the latter band most likely belongs to H₂ sorption on Li⁺ cations at sites SII, and on hydroxyl groups, OH. Sorption of N₂ by Li⁺ cations at sites SIII and SIII is the strongest, resulting in a decrease of intensity of the corresponding DRIFT bands that stem from subsequent H₂ sorption. Nitrogen sorption by Li⁺ cations at sites SII is much weaker. Sorption of N₂ on Na⁺ cations at sites SIII in NaLSX zeolite is also stronger than by Na⁺ cations at sites SII.     

The influence of polarizability on the dielectric spectrum of the ionic liquid 1-ethyl-3-methylimidazolium triflate

This work reports for the first time the computational, frequency-dependent dielectric spectrum of the polarizable molecular ionic liquid 1-ethyl-3-methylimidazolium triflate as well as its experimental analogue. In the frequency range from 500 MHz up to 20 GHz the agreement between the computational and the experimental spectrum is quantitative. For higher frequencies up to 10 THz the agreement is still remarkably good. The experimental asymptotic limit ε(∞) is 2.3. The difference in the computational value of 1.9 comes solely from the neglect of polarizability of the hydrogen atoms. For reasons of efficiency the simulations are based on the Lagrangian algorithm for the Drude oscillator model which cannot handle polarizable hydrogens. In the computational analysis the complete spectrum of the generalized dielectric constant ∑(0)*(ν) is splitted into its translational and non-translational components, called dielectric conductivity ?(0)(ν) and dielectric permittivity ε(ν). For 1-ethyl-3-methylimidazolium triflate both components contribute with equal weight and overlap in the complete frequency range. The inclusion of polarization forces, however, is quite different for the two components: the collective non-translational dynamics is accelerated and hence the dielectric permittivity is shifted to higher frequencies. The low frequency region of the dielectric conductivity is also affected while its high frequency part remains almost unchanged. Inductive effects are not only visible at high frequencies but also contribute in the sub-GHz region. The computational peak found in this region correlates with the experimental OKE-spectrum. It may be interpreted as the correlation between the induced dipole moment of the cations and the local electric field exerted by the anionic cage.     

Temperature-dependent relaxation study of tertiary butyl alcohol–water mixtures using TDR technique

Using time-domain reflectometry (TDR) technique, we have measured the complex permittivity of tertiary butyl alcohol (TBA)–water mixtures in the frequency range of 10 MHz–30GHz, at temperatures 15°C, 20°C and 25°C. The complex permittivity of TBA–water mixture shows Debye-type behaviour. The dielectric parameters such as dielectric constant and relaxation time were obtained from the complex permittivity spectra. The Kirkwood correlation factor and Bruggeman factor have also been determined to investigate inter- and intramolecular interaction among associating liquids.     

Interconversion between mechanical and dielectric relaxations for poly(cyclohexyl acrylate)

Storage E′ and loss E″ relaxation moduli are reported as functions of frequency for poly(cyclohexyl acrylate) (PCA) at several temperatures. The possibility that these results, in conjunction with the dipolar correlation coefficient, can be used to predict the frequency dependence of the real ε and loss ε″ and the components of the complex dielectric permittivity ε* of PCA is studied. A relation between ε* and the complex relaxation modulus E* is obtained by assuming that the lag of the rotating dipoles in the electric field is caused by both dielectric and mechanical friction. The values of ε* obtained from mechanical results by means of this expression are very close to those obtained from other relations based on the assumption that the lag of the dipoles is caused exclusively by mechanical friction. © 1993 John Wiley & Sons, Inc.     

Dielectric relaxation study of glycine–water mixtures using time domain reflectometry technique

The complex permittivity of glycine in water mixture for various temperatures and concentrations have been measured as a function of frequency between 10?MHz and 30?GHz using time domain reflectometry technique. Dielectric parameters, i.e. static dielectric constant and relaxation time were obtained from the complex permittivity spectra using nonlinear least square fit method. The dielectric relaxation parameter increases with an increase in molar concentration of glycine due to the formation of hydrogen bond groups by glycine molecule in an aqueous solution medium. The activation entropy, activation enthalpy and Kirkwood correlation factor have also been determined for glycine–water mixtures.     

Molecular dynamics study of hydrated faujasite-type zeolites

Molecular dynamics (MD) simulations of hydrated zeolite NaX (Si/Al = 1.0) and NaY (Si/Al = 2.0) were done at a temperature of 300 K. The calculation results show that the adsorption of water occurs via a three-step mechanism in zeolite NaX: (1) adsorption around Na, (2) formation of a monolayer on the walls, and (3) pore filling in the supercage during which adsorbed water molecules are localized around the 12-membered rings. Zeolite NaY adsorbs in a similar manner. However, at intermediate hydration states, cluster formation occurs around Na instead of monolayer formation. The calculated energy distribution functions suggest that in zeolite NaX, the water vapor adsorption can be expressed by using the Langmuir model with two adsorption sites, but in zeolite NaY, it is not Langmuir-type adsorption.     

Platinum species in the pores of NaX, NaY and NaA zeolites studied using EPR, XAS and FTIR spectroscopies

The results of X-band EPR, X-ray absorption and Fourier transform infrared spectroscopy on Pt(NH(3))(4)(2+) exchanged NaX, NaY and NaA zeolites reveal after oxygen calcination at 573 K that diamagnetic Pt(2+) is not the only product. Calcination provides Pt(3+) cations, but depending on the heating rate, the decomposition of amino groups during calcination also produces hydrogen that reduces Pt(3+) to Pt(2+) and Pt(+). NaX (Si/Al = 1.23) has a more negative framework charge than NaY (Si/Al = 2.31), so Pt(3+) can be stabilized only in NaX, whereas lower oxidation states of Pt such as Pt(+) can be stabilized in both, NaX and NaY, and neither of the paramagnetic Pt cations are stabilized in NaUSY (Si/Al = 3). The autoreduction process allows controlling the number of Pt(3+) and Pt(+) in the NaX zeolite by changing the calcination heating rate: a heating rate of 1.25 K min(-1) gives only Pt(+), but 0.5 K min(-1) gives a Pt(3+)/Pt(+) ratio close to 1. The structure of the support is also important for the synthesis of Pt species. While isolated paramagnetic Pt ions were stabilized in faujasite zeolites (NaX and NaY), a paramagnetic Pt dimer was obtained in a Linde type A zeolite (LTA, Si/Al = 1) by applying the same preparation methods. The fraction of paramagnetic Pt species which were characterized by X-band EPR spectroscopy amounts to 2-18% of the total Pt in the zeolites, the remaining Pt must be diamagnetic.