High-resolution physical adsorption on supported borosilicate MFI zeolite membranes: comparison with powdered samples

We have performed physical adsorption measurements to analyze the structure of intact MFI membranes with a nominal Si:B ratio of 12.5:1. Argon adsorption isotherms of membranes at 77 K are nearly identical to isotherms of corresponding zeolite powders throughout most of the domain of relative pressures, the exception being the micropore-filling region (between 10⁻⁵ and 10⁻⁴ relative pressure). The isotherm in the micropore-filling region is steeper by a factor of 4–5 for the borosilicate membranes than it is for a powder made under the same conditions, while the saturation loadings are found to be similar. This implies a difference in how the zeolite pores fill with argon – either thermodynamic or kinetic in origin – and that fluxes for argon permeation through such powder and membrane samples may differ. Our membrane adsorption measurements reveal no hysteresis and hence no mesoporsity in the zeolite membranes. This finding, from adsorption measurements, stands in contrast to recent alkane permeation measurements on these membranes, which suggest the presence of mesopores. We reaffirm the conclusion, found in previous work, that zeolite membranes can be approximated as a zeolite powder plus a support for the purposes of adsorption characterization.     

Separation of alcohols and alcohols/O2 mixtures using zeolite MFI membranes

Two different types of supported silicalite membranes were employed for the separation of alcohols and alcohols/O₂ mixtures: in one of them the zeolite material was deposited on the top of the γ-alumina supports, while in the other the zeolitic material was mainly present in the porous structure of the α-alumina supports. While both kinds of membranes were able to separate the above mixtures, the second type of membranes having the zeolite material inside the support performed more efficiently. The maximum selectivity reported in this work is 7415 for the ethanol/O₂ separation in an ethanol/methanol/O₂ mixture. For a better understanding of the separation mechanism, the performance of both zeolite membranes was compared to that of a mesoporous silica membrane. Also, the adsorption of methanol and propanol on silicalite crystals was measured using a microbalance.     

Synthesis of submicron polycrystalline MFI zeolite films on porous ceramic supports

This paper reports preparation and properties of ultra-thin MFI-type zeolite (silicalite) films on sol–gel derived mesoporous 8 mol% yttria doped zirconia (YZ) supports by the in situ synthesis method. Only non-continuous zeolite films were obtained on the as-synthesized YZ supports (calcined at 450°C). Continuous polycrystalline pure silicalite films, with a thickness of 0.5–0.7 μm, could be grown on the YZ supports pretreated at 700°C. The difference in the oxygen vacancy defect concentrations in these two types of YZ supports is suggested to explain the different results of growing silicalite films. Moreover, defect free 2–3 μm thick silicalite films were also prepared on the α-alumina supports under the same synthesis conditions. The silicalite layer on the YZ support offers a significantly lower mass transfer resistance as compared to that on the alumina support. Single gas permeation data of hydrogen, methane and ethane through these membranes were measured and compared with those of silicalite membranes prepared by other groups. The alumina supported silicalite membranes prepared in this work show better separation factors than the literature values. The zeolite film on the YZ support is less resistant to the stress induced in the step of template removal as compared to the zeolite film on the alumina support.     

Molecular simulation study of adsorption and diffusion on silicalite for a benzene/CO2 mixture

The adsorption and diffusion of a binary mixture of supercritical CO2 and benzene on silicalite (MFI-type) have been studied through the grand canonical Monte Carlo and molecular dynamics (MD) simulations. The adsorption behavior of pure CO2 on silicalite was discussed in detail from the adsorption isotherms, adsorption sites, interaction energies, and isosteric heats of adsorption. For the mixture, the influences of temperature, pressure and composition on the adsorption isotherms have been examined. The adsorption site behavior of the mixture has been analyzed, and benzene molecules get adsorbed preferentially in the more spacious channel intersection positions. These simulation results suggest that SC-CO2 fluid can be used as an efficient desorbent of larger aromatics in the zeolite material. The diffusion characteristic for the benzene/CO2 mixture was studied on the basis of MD simulation. It was found that the large coadsorbed benzene molecule has a pronounced effect on the CO2 diffusion in the mixture, while the mobility of benzene molecules is very small due to geometrical restrictions.     

Adsorption Isotherm Studies on Titanium Carbide Powders

We have used adsorption isotherms to perform a comparative study of the substrate quality of five groups of titanium carbide powder, manufactured following different procedures. The isotherms were measured in an automated setup at 77.3 K using methane and argon as the adsorbates. We determined the specific surface area of each of the powders studied. We also determined whether or not there was evidence of steps (indicative of layer-by-layer adsorption) in each set of adsorption data. The isothermal compressibilities of the adsorbed films were determined from the data measured for each sample. Adsorption measurements were also conducted to determine the effect that heating the powders under vacuum had on the resulting substrate quality.     

Effect of surface resistances on the diffusion of binary mixtures in the silicalite single crystal membrane

Diffusion of methane and argon mixtures through the silicalite single-crystal membrane is studied using the dual-control volume-grand canonical molecular dynamics method to understand how surface resistances alter selectivity and permeance. Comparison of results from intracrystalline transport and entrance simulations for binary mixtures of CH4 and Ar shows that the selectivity of silicalite membranes toward Ar is enhanced in the presence of the surface resistances. In both cases, however, diffusion of faster Ar molecules was inhibited by slower diffusing CH4 molecules, whereas diffusion of the latter remained unaffected. This behavior was explained by the difference between the magnitudes of surface resistances for two molecules, which is much smaller for Ar because of its smaller permeant-crystal interaction size. We find that selectivity of the membrane at the surface depends strongly on total feed pressure and temperature, whereas this dependence is weak for intracrystalline diffusion. Furthermore, we show that the selectivity at the surface diminishes with crystal thickness until a certain thickness is reached, whereas the intracrystalline selectivity remains constant with increasing thickness. Finally, a study of diffusion of C2H6 and CF4 mixtures shows that the diatomic ethane molecules diffuse faster inside the zeolite channels, but their desorption is hindered to a larger extent than that of a spherical molecule with larger diameter and lower heat of adsorption. This observation indicates that the difference in molecular geometry is also a significant factor to explain the exit effect.     

Molecular simulation of adsorption and diffusion of chlorinated alkenes in ZSM-5 zeolites

In this work, adsorption and diffusion of trichloroethylene (TCE) and tetrachloroethylene (PCE) in ZSM-5-type zeolites were studied using molecular simulation methods. Grand canonical Monte Carlo technique was to calculate adsorption isotherms and heats of vaporization of TCE and PCE in zeolite. The results demonstrated that the Pnma-P2(1)2(1)2(1) symmetry transition of the zeolite framework has no significant effect on the TCE adsorption capacity of the silicalite, but it causes an increase of the PCE adsorption capacity. Simulations using a silicalite framework with Pnma symmetry showed that the adsorption capacity of the silicalite was limited to five molecules per unit cell. However, when a framework with P2(1)2(1)2(1) symmetry was used in the simulations, the capacity reached to eight molecules per unit cell, which is the actual adsorption capacity. To calculate intracrystalline diffusion coefficients of these compounds, molecular dynamics simulations were performed at different temperatures and loadings. The results show that the zeolite symmetry has a significant impact on diffusion coefficients of the sorbate molecules.     

Adsorption of liquid mixtures on silicalite-1 zeolite: a density-bottle method

A technique that measures the effective density of a zeolite after adsorption from the liquid phase was developed to measure the absolute amounts of liquid mixtures adsorbed on zeolites without using a nonadsorbing solvent. Since the fugacities of the adsorbing components in solution can be dramatically different with or without the addition of a nonadsorbing solvent, this technique measures mixture isotherms that can be used for analyzing pervaporation through zeolite membranes. A nonideal solution, methanol/acetone, was used as an example to show that its adsorption isotherms on silicalite-1 zeolite at 294 K differ dramatically from those measured with the nonadsorbing solvent method. The methanol/acetone fugacity ratio is different for the two methods because of different concentrations in the liquid phase. Methanol preferentially adsorbs on silicalite-1 at low methanol concentrations and acetone preferentially adsorbs at high methanol concentrations. The density bottle method was used to show that n-hexane preferentially adsorbs from n-hexane/3-methylpentane liquid mixtures, and at high n-hexane concentrations, essentially no 3-methylpentane adsorbs, as has been predicted previously by simulations. A larger molecule, 2,2-dimethylbutane, adsorbed so slowly at 294 K that silicalite had only 16% of saturation coverage after 370 h, but it was saturated after 1650 h; at 423 K, saturation was obtained in less than 24 h.     

Separation of acetic acid-water mixtures by pervaporation through silicalite membrane

Polycrystalline silicalite membranes were prepared on two kinds of porous supports by hydrothermal synthesis. The pervaporation performance of the silicalite membrane obtained was investigated using an acetic acid-water mixture as a feed. The silicalite membrane on the sintered stainless steel support selectively permeates acetic acid in the concentration of the feed acetic acid in the region of 5 to 40 vol%. However, the membrane on the porous alumina support showed no separation for the aqueous acetic acid solution. From the fact that the top layer of the membrane on the alumina support was not composed of pure silicalite but ZSM-5 zeolite crystals, which contained Brønsted acidic sites (Si(OH)Al) in the framework, it was suggested that the acidic sites associated with the framework aluminums play an important role in the separation of the acetic acid-water mixture. A long-term test of the pervaporation was also carried out to clarify the stability of the membrane.     

Zeolite Membranes: From the Laboratory Scale to Technical Applications

Current trends and novel concepts in R&D of zeolite membranes like the seeding supported crystallization and the preparation of Al-containing zeolite membranes are discussed. The influences of adsorption and diffusion on the permeation properties of zeolite membranes are considered. Dehydration of ethanol by steam permeation and pervaporation as the first zeolite membrane based industrial separation is presented. Membrane supported dehydrogenation and esterification are discussed as possible applications of a catalytic membrane reactors. In memoriam Wolfgang Schirmer.     

Correlation of sorption behavior of nitrogen, oxygen, and argon with Ca2+ locations in zeolite a: a grand canonical Monte Carlo simulation study

The adsorption isotherms for nitrogen, oxygen, and argon in various NaCaA zeolite samples were calculated theoretically using the grand canonical Monte Carlo simulation method. The adsorption capacity, selectivity, and heat of adsorption of nitrogen increase with an increasing number of calcium cations in zeolite A. The heat of adsorption of nitrogen showed a sudden increase when the calcium ion exchange to zeolite A was around 60%. Adsorption isotherms, determined experimentally by the volumetric adsorption method, support theoretically predicted isotherms. These observations have been explained in terms of the interaction of the nitrogen molecule with Ca2+ ions and their locations in zeolite A.     

基底表面性质与前驱液化学环境对原位定向构筑钛硅分子筛膜的影响

b取向MFI型分子筛膜能够显著促进分子的传输效率, 被广泛应用于混合物分离及催化领域. 虽然传统的原位水热晶化法已较为成熟, 然而仍难以调控膜层的b轴取向生长. 本文以304不锈钢片为基底, 采用经典的原位水热晶化法研究了基底表面物化性质、 前驱液配比及晶化条件对钛硅分子筛膜b轴取向生长的影响. 结果表明, TiO₂中间层表面的羟基能够定向诱导分子筛晶粒的吸附, 进而提高钛硅分子筛膜的b轴取向程度. 同时, 前驱液中模板剂和水含量对晶粒的大小及膜层的定向生长具有显著影响, 即仅在合适的碱度下才能形成高b轴取向的钛硅分子筛膜.     

Ethanol, acetic acid, and water adsorption from binary and ternary liquid mixtures on high-silica zeolites

Adsorption isotherms were measured for ethanol, acetic acid, and water adsorbed on high-silica ZSM-5 zeolite powder from binary and ternary liquid mixtures at room temperature. Ethanol and water adsorption on two high-silica ZSM-5 zeolites with different aluminum contents and a high-silica beta zeolite were also compared. The amounts adsorbed were measured using a recently developed technique that accurately measures the changes in adsorbent/liquid mixture density and liquid concentration. This technique allows the adsorption of each compound in a liquid mixture to be measured. Adsorption data for binary mixtures were fit with the dual-site extended Langmuir model, and the parameters were used to predict ternary adsorption isotherms for each compound with reasonable accuracy. In ternary mixtures, acetic acid competed with ethanol and water for adsorption sites and reduced ethanol adsorption more than it reduced water adsorption.     

Heats of adsorption of methanol and ethanol on high-silicon ZSM-5 zeolite

The heats of adsorption of lower alcohols on NaZSM-5 have a stepwise appearance and each step corresponds to the stoichiometric formation of adsorption complexes of Na⁺ ions with from one to four alcohol molecules. All the adsorption complexes are located at zeolite channel intersections, while the alkyl groups enter these channels. The heats of adsorption of alcohols on NaZSM in the region of the formation of adsorption complexes with cations markedly exceed the heats of adsorption on silicalite, while on the noncationic part of the NaZSM-5 structure, they are identical to the heats of adsorption on silicalite. The mean molar integral adsorption entropies of alcohols are significantly less than the entropy of the liquid. The adsorbed molecules are in a solidlike state. The isotherms for the adsorption of alcohols on NaZSM-5 are completely described by VMOT equations.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 11, pp. 2633–2635, November, 1989.     

Adsorption properties of low-silica zeolite ZK-5

Water vapor, methanol, and argon adsorption isotherms, as well as the heat of adsorption of ammonia, methanol, and carbon dioxide have been determined on zeolite types A, ZK-4, and ZK-5. The results are explained by the distribution and the bond lengths of cations in the zeolite crystal lattice. It was shown that the exchange of 10–15% of the Na⁺ ions with Li⁺ in zeolite ZK-5 leads to a deformation of the 8-membered ring impeding the diffusion of adsorbed molecules. Cations contained in the 8-membered ZK-5 zeolite ring are bound more strongly to the framework, than in zeolite NaA, resulting in their smaller adsorption capacity.I. V. Grebenshchikov Institute of Silicate Chemistry, Russian Academy of Sciences, 199164 St. Petersburg, Russia. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 11, pp. 2494–2500, November, 1992.     

Effects of molecular siting and adsorbent heterogeneity on the ideality of adsorption equilibria

The ideal adsorbed solution (IAS) theory is the benchmark for the prediction of mixed-gas adsorption equilibria from pure-component isotherms. In this work, we use atomistic grand canonical Monte Carlo simulations to test the effects of molecular siting and adsorbent energetic heterogeneity on the applicability of the IAS theory. Pure-component isotherms generated by atomistic simulation are used to predict binary isobaric isotherms using the IAS theory. These predicted isotherms are compared with those obtained by a full atomistic simulation of the binary mixture. Binary mixtures of argon, methane, and CF4 in silicalite are found to obey IAS theory, while benzene/methane and cyclohexane/methane in silicalite are nonideal. The mixture of argon and CF4 is ideal despite the large difference in the sizes of the two species. This contradicts previous hypotheses in the literature, which state that mixtures of species of unequal size do not adsorb ideally. The nonideal behavior of the benzene/methane and cyclohexane/methane systems occurs because of adsorbent heterogeneity in these systems, which depends on both sorbent and sorbate. In addition, we use a lattice gas model with parameters derived from atomistic simulation to demonstrate analytically that a sufficiently energetically heterogeneous adsorbent will result in the breakdown of IAS theory even in the absence of interactions between sorbates.     

Heat of Adsorption of Pure Sulfur Hexafluoride on Micro-Mesoporous Adsorbents

The isotherms and the isosteric heats of adsorption of pure SF₆ were measured on two microporous zeolites (NaX and Silicalite), one mesoporous alumina, and two activated carbons (BPL and PCB) at 305 K. The adsorption isotherms were Type I by Brunauer classification. The PCB carbon adsorbed SF₆ most strongly and the alumina adsorbed SF₆ most weakly. The adsorption of SF₆ on the other three materials were comparable in the low pressure region despite their drastic differences in the physicochemical properties. The heat of adsorption of SF₆ on the silicalite and the alumina remained practically constant over a large range of coverage. The heat of adsorption of SF₆ increased with increasing adsorbate loading on the NaX zeolite in the high coverage region. The heat of adsorption of SF₆ on the activated carbons decreased with increasing adsorbate loading before leveling off in the high coverage region.     

Statistical mechanics of a two-dimensional lattice model of benzene adsorption in zeolites

In this Letter, we present a mean field calculation of the statistical mechanics of a lattice model of benzene adsorption in the quasi two-dimensional network of pores in zeolites. A lattice fluid model is introduced with monomer states to represent molecules standing perpendicular to the principle axis of the pore, dimer states to represent molecules lying flat against the pore wall, and vacant sites or holes. For a wide range of interaction parameters the model gives steps in adsorption isotherms similar to those observed experimentally for benzene adsorption in silicalite. Our treatment attributes the experimentally observed steps in the level of adsorption with rising pressure, to orientational transitions amongst molecules in the adsorbed phase with two possible ground states arrangements of the benzene molecules in the zeolite pores energetically competing with each other.     

高硅SSZ-13分子筛膜二次水热生长过程

采用稀释模板剂合成溶胶在管状莫来石支撑体上制备高硅SSZ-13分子筛膜以研究膜的二次水热生长过程。对433 K下二次生长2~72 h的膜和底料进行表征,XRD表明膜和底料中的SSZ-13特征峰均在6 h后出现。底料中的晶粒尺寸基本不随晶化时间的延长而变化。在298 K和0.4 MPa测试条件下,433 K晶化72 h的SSZ-13分子筛膜CO₂渗透通量为2.08×10^-7 mol·m^-2·s^-1·Pa^-1,CO₂/CH₄理想选择性为24。探讨了SSZ-13分子筛膜的生长过程：涂覆在支撑体上的晶种首先溶解并形成大量有序的结构单元,诱导溶胶中晶粒形成并快速生长,晶粒沉积在支撑体的表面共生形成无缺陷的膜层。     

Large-scale computational screening of zeolites for ethane/ethene separation

Large-scale computational screening of thirty thousand zeolite structures was conducted to find optimal structures for separation of ethane/ethene mixtures. Efficient grand canonical Monte Carlo (GCMC) simulations were performed with graphics processing units (GPUs) to obtain pure component adsorption isotherms for both ethane and ethene. We have utilized the ideal adsorbed solution theory (IAST) to obtain the mixture isotherms, which were used to evaluate the performance of each zeolite structure based on its working capacity and selectivity. In our analysis, we have determined that specific arrangements of zeolite framework atoms create sites for the preferential adsorption of ethane over ethene. The majority of optimum separation materials can be identified by utilizing this knowledge and screening structures for the presence of this feature will enable the efficient selection of promising candidate materials for ethane/ethene separation prior to performing molecular simulations.