Article

Study of Short-Chain Alcohol and Alcohol-Water Adsorption in MEL and MFI Zeolites

Authors:
To read the full-text of this research, you can request a copy directly from the authors.

Abstract

In this paper we present a comparative study of the adsorption behavior of short chain alcohols (pure and in aqueous solution) into silicalite-1 (MFI-type zeolite) and silicalite-2 (MEL-type zeolite). For quite some time, silicalite-1 has been the reference material to address the problem of adsorptive-based separation, mostly for hydrocarbon mixtures. Interestingly, being structurally close to silicalite-1, adsorption studies using silica\-lite-2 are scarce, and to the best of our knowledge, a comparative study of their behavior for alcohol water mixtures has not been published to date. We have here resorted to molecular simulation techniques to analyze the adsorption and diffusion phenomena in both zeolites at 25 C and 50 C for pure methanol, ethanol, 1-butanol and water, and for some relevant compositions of alcohol/water mixtures. In addition to the dilute regime in the mixture, our study ranges from intermediate alcohol concentrations to alcohol-rich phases, relevant to alcohol purification processes. Besides, we have performed volumetric and calorimetric measurements of single-component adsorption of alcohols in pure silica MEL zeolite, which were used to validate the model potentials used in the simulations. We observe that the zigzag channels of MFI zeolite are most likely responsible for its somewhat higher affinity for alcohols. This leads to higher adsorption selectivities when compared to MEL zeolite. We have also found that the choice of water model strongly conditions water co-adsorption into the zeolites, and subsequently the predictions of the adsorbent’s selectivity in alcohol/water systems. Despite considerable differences for adsorbed pure components, diffusivities of alcohol and water adsorbed from mixtures are relatively similar, most likely a side effect of the strong hydrogen bonds between hydroxyl groups and water.

No full-text available

Request Full-text Paper PDF

To read the full-text of this research,
you can request a copy directly from the authors.

... Note that usual Lorentz-Berthelot or geometric combining rules often give a rather poor description of alcohol-water mixtures, 34,37 which stresses the need for a careful choice of appropriate cross interaction parameters. In a previous work, we have seen that the choice of the water model can have a large impact on co-adsorption of water in the adsorption alcohol-water mixtures in hydrophobic pores, 38 and we speculate that the same is true for the cross water-alcohol interactions. Thus, it is important to use the best available force-field to describe the interactions in the system. ...
... Temperature was set at 298 K using the Nose-Hoover thermostat 44,45 with a damping constant of 0.1 ps. From our previous studies on alcohol/water adsorption into hydrophobic substrates, 38 we do not expect significant changes from moderate increases of temperature (up to 30 K). An increase in temperature shifts the onset of adsorption isotherms to higher pressures since a higher kinetic energy overcomes the binding energy of adsorbates at very low loadings. ...
... On the other hand, our results are consistent with the fact that methanol adsorption into other hydrophobic adsorbents, such as pure silica zeolites, also occurs below bulk saturation pressure. 38,48 In the case of water-methanol mixtures, adsorption was studied from the vapor phase in equilibrium with a liquid solution at a given concentration. The study was performed at three different compositions of the liquid mixture, going from dilute (x l MeOH = 0.020), to low concentration (x l MeOH = 0.115), and finally to high alcohol concentration (x l MeOH = 0.692) regimes. ...
Full-text available
Article
The behavior of water, methanol, and water-methanol mixtures confined in narrow slit graphite pores as a function of pore size was investigated by Monte Carlo, hybrid Monte Carlo, and Molecular Dynamics simulations. Interactions were described using TIP4P/2005 for water, OPLS/2016 for methanol, and cross interactions fitted to excess water/methanol properties over the whole range of concentrations, which provide a rather accurate description of water-methanol mixtures. As expected for hydrophobic pores, whereas pure methanol is adsorbed already from the gas phase, pure water only enters the pore at pressures well beyond bulk saturation for all pore sizes considered. When adsorbed from a mixture, however, water adsorbs at much lower pressures due to the formation of hydrogen bonds with previously adsorbed methanol molecules. For all studied compositions and pore sizes, methanol adsorbs preferentially over water at liquid-vapor equilibrium conditions. In pure components, both water and methanol are microscopically structured in layers, the number of layers increasing with pore size. This is also the case in adsorbed mixtures, in which methanol has a higher affinity for the walls. This becomes more evident as the pore widens. Diffusion of pure water is higher than that of pure methanol for all pore sizes due to the larger size of the methyl group. In mixtures, both components present similar diffusivities at all pore sizes, which is explained in terms of the coupling of molecular movements due to strong hydrogen bonding between methanol and water molecules. This is particularly evident in very narrow pores, in which pure methanol diffusion is completely impeded on the time scale of our simulations, but the presence of a small amount of water molecules facilitates alcohol diffusion following a single-file mechanism. Additionally, our results indicate that pure water diffusivities display a non-monotonous dependence of pore size, due to effects of confinement (proximity to a fluid-solid-fluid transition induced by confinement as reported in previous work) and the dynamic anomalies of water.
... The synthesis of methanol, fuels, and other hydrocarbon products from synthesis gas (a mixture of hydrogen and carbon monoxide) has received a lot of attention in the chemical industry in recent years [1][2][3][4]. Membrane technology is an attractive alternative for the purification of gases to achieve a variety of separations such as bio-molecules. ...
... As shown in Table 1 and Figs. 2 and 3, ethanol has higher isosteric heats and Langmuir constants (b) than methanol due to the larger size of the ethanol guest molecule which interacts with more active sites in the zeolite structure. This trend is also reported by Gomez-Alvarez et al. and Thamm [1,5], and the available experimental data for isosteric heat of adsorption at 293 K for ethanol and methanol is 11.1 and 10.6 kcal/mol, respectively [58]. In addition, methanol is also loaded more than ethanol, which is attributed to the smaller size of methanol molecules. ...
Article
Due to the importance of synthesis gas's entire conversion to methanol, the separation of methanol from unconverted synthesis gas is an industrial challenge. In this work, the influence of temperature, guest molecules concentrations (methanol and ethanol), and acid site density (Si/Al) of zeolites on the diffusion of methanol and ethanol, pure and binary mixture (80% methanol and 20% ethanol) in silicalite-1 and HZSM-5 (Si/Al = 47 and 23) were studied by using of the COMPASS force-field molecular dynamics method. Also, the adsorption of pure methanol and ethanol and binary mixture through these zeolites has been studied by using the Grand Canonical Monte Carlo (GCMC) method. The calculated adsorption rate and isosteric heat of adsorption for ethanol are lower and higher than methanol, respectively. The results of the binary mixture show that HZSM-5 (Si/Al = 23) has the lowest adsorption selectivity and most diffusion selectivity. The calculated diffusion coefficients of methanol and ethanol guest molecules decreased with rising guest molecule concentration and Si/Al-ratios. The effect of both agents was investigated by analysis of mean square displacement (MSD) and RDF diagram.
... Furthermore, several molecular simulation studies investigated the adsorption, diffusion and separation of alcohol and water systems in MFI zeolite. [23][24][25][26] By carrying out configurational bias grand canonical Monte Carlo (CB-GCMC) simulations Xiong et al. [23] reported that the adsorption of water in MFI is negligible due to its hydrophobicity and that larger alcohols are adsorbed at relatively lower pressures compared to the smaller alcohols. Krishna et al. [24] employed CB-GCMC and molecular dynamics (MD) methods to show that it is not possible to accurately estimate adsorption and diffusion characteristics of alcohol-water mixture based on single component data due to the effect of hydrogen bonding between water and alcohol molecules. ...
... Krishna et al. [24] employed CB-GCMC and molecular dynamics (MD) methods to show that it is not possible to accurately estimate adsorption and diffusion characteristics of alcohol-water mixture based on single component data due to the effect of hydrogen bonding between water and alcohol molecules. Gómez-Álvarez et al. [25] performed GCMC and MD simulations in MFI and showed that the diffusion coefficients of water, methanol and ethanol in the adsorbed alcohol-water mixtures were greater than their diffusion coefficients in the adsorbed single component. Studies cited in references 23 to 25 were conducted in periodic MFI structures. ...
Article
The dehydration of bioalcohols is considered one of the major factors contributing to the cost of biofuel production. In this study, liquid phase separation of water from methanol and ethanol in a siliceous MFI pervaporation membrane was studied by performing concentration gradient driven molecular dynamic (CGD-MD) simulations. CGD-MD simulations work by imposing a higher concentration in the feed side and a lower concentration in the permeate side of the membrane. This creates a concentration gradient across the membrane that facilitates the diffusion of molecules from the feed to the permeate side, mimicking the experimental pervaporation membrane set up. Fluxes of methanol, ethanol and water were calculated in single component permeation simulations and in equimolar methanol-water and ethanol-water mixture separation simulations. It was found that water formed hydrogen bonds with the silanol (Si-OH) groups on the external surface of the MFI and did not enter the membrane in the single component permeation simulation. While this may suggest that MFI can be used to effectively dehydrate bioalcohols, our simulations showed that water permeated through the MFI membrane when it was in a mixture with either methanol or ethanol. Furthermore, in the alcohol-water mixture simulations, the fluxes of methanol and ethanol were significantly lower than that of expected based on their single component fluxes. A detailed analysis of hydrogen bonding in the alcohol-water mixture separation simulations revealed that water preferred making hydrogen bonds with methanol and ethanol rather than with the silanol groups. This resulted in drifting of water molecules along with permeating alcohol molecules in to the MFI membrane in mixture simulations, while slowing the permeation of methanol and ethanol fluxes. The hydrogen bonding between water and alcohol molecules indicates that it may not be possible to achieve complete alcohol selectivity even if defect-free membranes were manufactured; however, our findings also hint at the possibility of functionalizing membrane surfaces with chemical groups that will overcome water-alcohol hydrogen bonding and retain water molecules in order to approach complete selectivity.
Article
Ethanol and other biofuels produced during biomass conversion must be separated from the fermentation broth (mainly water) before being used as a fuel. This can be addressed by adsorption-based separation using porous materials. The key objective of this study is to obtain a molecular understanding of water and alcohol adsorption in pure-silica zeolites, particularly in silicalite-1 (MFI-type zeolite) and silicalite-2 (MEL-type zeolite). Molecular simulation techniques are used for this purpose. They provide information on the configuration of the fluids, and so the microscopic network structure of the adsorbed polar molecules can be characterized by using a specific criterion of hydrogen bonding formation. We conducted Grand-Canonical Monte Carlo simulations to compute the adsorption isotherms of pure short alcohols and water and from the liquid alcohol/water binary mixtures throughout the composition range. Despite MFI and MEL being structurally very similar, we found differences in adsorption, which are related to the underlying molecular behavior. While water intrusion occurs by applying pressure due to stronger water-water than water–silicalite interactions, notable water adsorption from the mixture occurs first by hydrogen bond formation with the adsorbed alcohols and then by self-association. A higher degree of water clustering in MEL compared to MFI zeolite, promoted by its straight channels, leads to relatively lower uptakes of water in the latter zeolite (in favor of alcohol molecules).
Article
There is a significant challenge to discover porous materials that can effectively capture and separate CO2 from natural gas, refining gas, and flue gas, which has attracted attention in dealing with climate warming and energy purification. Recently, hybrid microporous materials with narrow pores and containing fluorine have been rapidly used in gas separation based on physisorption. In this contribution, molecular simulations combined with high-throughput calculations were performed to calculate structural parameters and performance evaluation metrics of 1015 promising adsorbents to rank and screen out the top candidates for CO2/CH4, CO2/H2 and CO2/N2 separation. To the best of our knowledge, this is the first time to unlock this kind of fluorinated material database in CO2 separation, in which statistical information indicates that a large number of interpenetrating structures lead to 64% of ultra-microporous materials and the number of particles per unit volume of fluorine we defined also has a positive effect on the heat of adsorption of CO2. The structural performance relationship reveals a clear picture of strong CO2 capture but poor energy gas (CH4 and H2) storage. The mathematical model established from the geometry and energy descriptors has a strong correlation with the mixture adsorption selectivity in CO2/N2 separation, and the ideal selectivity can be applied to save computing resources in CO2/H2 separation. Cadmium and vanadium with high frequency may represent the new characteristics of the next generation adsorbent for capturing CO2, among the 18 high-performance materials selected according to the adsorption performance score and mixture adsorption selectivity. The centroid density distribution and radial distribution function manifest that CO2 is preferentially close to fluorine atoms and metal atoms. Here, we established an online high-throughput calculation code (https://github.com/oddthinker/HTCS) for adsorption and separation. All of this will provide guidelines for experimental synthesis and large-scale screening of target materials.
Article
Zeolites have been widely used as catalysts, ion exchangers, and adsorbents since their industrial breakthrough in the 1950s and continue to be state-of the-art adsorbents in many separation processes. Furthermore, their properties make them materials of choice for developing and emerging separation applications. The aim of this review is to put into context the relevance of zeolites and their use and prospects in adsorption technology. It has been divided into three different sections, i.e., zeolites, adsorption on nanoporous materials, and chemical separations by zeolites. In the first section, zeolites are explained in terms of their structure, composition, preparation, and properties, and a brief review of their applications is given. In the second section, the fundamentals of adsorption science are presented, with special attention to its industrial application and our case of interest, which is adsorption on zeolites. Finally, the state-of-the-art relevant separations related to chemical and energy production, in which zeolites have a practical or potential applicability, are presented. The replacement of some of the current separation methods by optimized adsorption processes using zeolites could mean an improvement in terms of sustainability and energy savings. Different separation mechanisms and the underlying adsorption properties that make zeolites interesting for these applications are discussed.
Full-text available
Article
LTA and CHA zeolites with silica and silico-alumino-phosphate (SAPO) composition have been recently proposed by Van der Perre et al. (ChemSusChem2017,10, 2968) to recover butanol from a mixture of butanol, ethanol, acetone, and water. Such a mixture is obtained from biomass fermentation, which is a sustainable alternative for butanol production. Here, we establish a computational methodology to simulate the three-step process, consisting of selective adsorption of butanol, with some ethanol and water, from the initial mixture, using a silica LTA nanosheet; desorption of the adsorbed mixture by increasing the temper- ature; and adsorption of the desorbed products in a CHA (silica or SAPO) nanosheet. A combination of Monte Carlo simulations, allowing to reproduce or predict single- or multiple-component gas adsorption isotherms, and molecular dynamics, giving the time evolution of the selective adsorption and desorption processes, have been used to model the experimental setup. A recently parameterized force field of general use for zeolites, AlPOs, and SAPOs has been tuned up in order to describe all interactions arising from this model, allowing to capture the thermodynamics of the system, the flexibility of the frameworks, and the effects of surface and bulk in the permeability of the nanosheets.
Article
Low-temperature activation of quartz and kaolin, two of the most abundant and affordable raw materials as the sources of alumina and silica for the production of zeolites, is highly desirable but is hampered by several challenges such as incomplete activation and the formation of acid-resistant aluminosilicates. In this paper, the hydrothermal alkali activation processes of kaolin and quartz with different Si/Al ratios have been studied in detail, aiming to establish a phase diagram for the optimal digestion of silica and alumina. The effects of the Si/Al ratio, reaction time, reaction temperature, alkali concentration, and doses of hydrothermal alkali activation have been thoroughly investigated and optimized. Sodalite, hydroxycancrinite and analcime are found to be the dominant aluminosilicates as activation products. This is the first demonstration that the molar ratio of NaOH/(Si + Al) is the key factor in controlling the formation of aluminosilicates. The establishment of this phase diagram allows us to avoid the formation of troublesome analcime, and it can be used for effectively converting most natural aluminosilicate materials into sol-gel as platform chemicals for the synthesis of zeolites and other aluminosilicate derivatives.
Article
Currently, biorefinery industry with biobutanol as product are firmly established in China. Corncobs are usually used as feedstock for Acetone–butanol–ethanol (ABE) fermentation. Cellulose and hemicellulose in corncobs could be used as raw materials for fermentation through enzymatic hydrolysis, but the lignin as a by-product has not been effectively used. Meanwhile, the low concentration of biobutanol in fermentation broth leads to high-energy consumption for separation. In this study, lignin obtained from corncobs was found to be a good material to prepare thermo-sensitive hydrogel for biobutanol enrichment, which has high adsorption capacity and unique desorption performance. By free radical polymerization, the excellent lignin-grafted-poly(N,N-diethylacrylamide) hydrogel (LGD) was successfully prepared. The adsorption capacity of LGD for butanol (544.22 mg g⁻¹) was two times and five times as much as lignin and poly(N,N-diethylacrylamide) (PDEAAm), respectively. The adsorbed saturated hydrogel could first remove a large amount of free water at 50 °C to reduce the energy consumed for free water vaporization. Further thermal desorption at 120 °C yielded a concentrated desorption solution with a concentration of 118.74 g L⁻¹, which could significantly reduce the energy consumption of subsequent distillation. The aromatic ring and hydrophobic chain in lignin proved to be effective sites for butanol adsorption. PDEAAm not only promoted the effective exposure of adsorption sites on lignin, but also increased the adsorption rate significantly by forming a pore structure. With its high adsorption capacity, fast adsorption rate, and low energy consumption for desorption, the lignin-based thermo-sensitive hydrogels show good prospects for the enrichment of low-concentration biofuels obtained through biological fermentation. It also opens up a new way for efficient and full-component utilization of corncobs.
Full-text available
Article
The separation performance of microporous crystalline materials in membrane constructs is dictated by a combination of mixture adsorption and intracrystalline diffusion characteristics; the permeation selectivity Sperm is a product of the adsorption selectivity Sads and the diffusion selectivity, Sdiff. The primary objective of this article is to gain fundamental insights into Sads and Sdiff by use of molecular simulations. We performed configurational-bias Monte Carlo (CBMC) simulations of mixture adsorption equilibrium and molecular dynamics (MD) simulations of guest self-diffusivities of a number of binary mixtures of light gaseous molecules (CO2, CH4, N2, H2, and C2H6) in a variety of microporous hosts of different pore dimensions and topologies. Irrespective of the bulk gas compositions and bulk gas fugacities, the adsorption selectivity, Sads, is found to be uniquely determined by the adsorption potential, Φ, a convenient and practical proxy for the spreading pressure π that is calculable using the ideal adsorbed solution theory for mixture adsorption equilibrium. The adsorption potential Φ is also a proxy for the pore occupancy and is the thermodynamically appropriate yardstick to determine the loading and composition dependences of intracrystalline diffusivities and diffusion selectivities, Sdiff. When compared at the same Φ, the component permeabilities, Π i for CO2, CH4, and N2, determinable from CBMC/MD data, are found to be independent of the partners in the various mixtures investigated and have practically the same values as the values for the corresponding unary permeabilities. In all investigated systems, the H2 permeability in a mixture is significantly lower than the corresponding unary value. These reported results have important practical consequences in process development and are also useful for screening of materials for use as membrane devices.
Full-text available
Article
Microporous crystalline porous materials such as zeolites, metal–organic frameworks, and zeolitic imidazolate frameworks (ZIFs) have potential use for separating water/alcohol mixtures in fixed bed adsorbers and membrane permeation devices. For recovery of alcohols present in dilute aqueous solutions, the adsorbent materials need to be hydrophobic in order to prevent the ingress of water. The primary objective of this article is to investigate the accuracy of ideal adsorbed solution theory (IAST) for prediction of water/alcohol mixture adsorption in hydrophobic adsorbents. For this purpose, configurational bias Monte Carlo (CBMC) simulations are used to determine the component loadings for adsorption equilibrium of water/methanol and water/ethanol mixtures in all-silica zeolites (CHA, DDR, and FAU) and ZIF-8. Due to the occurrence of strong hydrogen bonding between water and alcohol molecules and attendant clustering, IAST fails to provide quantitative estimates of the component loadings and the adsorption selectivity. For a range of operating conditions, the water loading in the adsorbed phase may exceed that of pure water by one to two orders of magnitude. Furthermore, the occurrence of water–alcohol clusters moderates size entropy effects that prevail under pore saturation conditions. For quantitative modeling of the CBMC, simulated data requires the application of real adsorbed solution theory by incorporation of activity coefficients, suitably parameterized by the Margules model for the excess Gibbs free energy of adsorption.
Chapter
Increasing energy prices, global warming, and concerns for environmental pollution has been pushing the chemical industry to look for alternatives for traditional, fossil-based chemical feedstocks. Important platform molecules are alcohols, which can be produced from renewable feedstocks via fermentation. The implementation of these fermentation processes to produce chemicals leads to important challenges regarding the downstream purification. Adsorption-based purification technologies are alternatives for traditional energy-intensive distillation processes. The well-defined pore structure of zeolites makes them ideal candidates for the removal of alcohols from these complex fermentation mixtures, which contain cells and cell debris, acids, sugars, lipids, and proteins. The following chapter covers important aspects in the adsorption mechanism of alcohols and water in (mainly) hydrophobic zeolite pores, such as cluster formation and hydrogen bonding. These effects inevitably also play a role when looking at the diffusion of alcohols inside the zeolite pores. Finally, this chapter will cover some examples of studies where hydrophobic zeolites have been used to recover bio-alcohols, such as biobutanol and bioethanol, from model solutions or fermentation broths via fixed-bed separations.
Article
To facilitate the production of ethanol from renewable biomass to meet the increasing energy demand with reduced CO2 emissions, molecular simulations are employed to investigate zeolite nanosheets as pervaporation membranes for ethanol extraction with atomic-level understandings. More than ten zeolite nanosheets with diverse geometric features are systematically studied. All of these studied zeolite nanosheets are computed to offer unprecedentedly large fluxes, with values correlating to the density and size of their permeation channels. Interestingly, our results also show that the separation factor of nanosheets can drastically vary from as promising as 440 to as poor as 6.7. A detailed investigation reveals that the separation factor strongly depends on the product of the adsorption selectivity in bulk zeolites and the ethanol fraction at the channel entrance on the surface. A selection guideline based on the geometric features of zeolites is also proposed; promising nanosheet candidates appear to have a largest cavity diameter of approximately 6 Å and a small product value of the number of silanols surrounding the channel opening and the largest opening diameter. The outcomes of this work can help guide the selection of zeolite nanosheets, based on the adsorption properties and/or geometric features, toward the future development of novel membranes for ethanol extraction.
Article
Researches on zeolite silicalite-1 have focused mainly on improving its performance, with little attention paid on green and economic synthesis. Herein, a novel route for green synthesis of pure silicalite-1 from the recycled mother liquor has been developed. This route starts from the hydrothermal reaction of natural quartz with NaOH to form a sodium silicate solution, which is then transformed into a silicic acid (H2SiO3) gel by hydrochloric acid titration. Then the solid silicic acid (H2SiO3) gel and fumed silica (SiO2) are mixed with water under stirring to form a highly active slurry, which can be hydrothermally “dissolved” in a dilute NaOH aqueous solution. Using this slurry with a high SiO2/Na2O molar ratio of 5 : 1 as the silica source as well as tetrapropylammonium bromide (TPABr) as the organic template, MFI-type pure silicalite-1 has been successfully synthesized. After each synthesis, the mother liquor of centrifuged supernatant was recovered and recycled. The silica source and organic template were added to the recycled mother liquors for the synthesis of pure silicalite-1. The minimum TPABr/SiO2 molar ratio needed to synthesize pure silicalite-1 in the initial run is 0.04 : 1. In comparison, only half the amount of the organic template is needed in subsequent runs with the recovered mother liquors. Through recycling, all reactants in the mother liquors were ultimately utilized without any waste, leading to not only effectively reducing the cost of silicalite-1 synthesis but also minimizing its environmental impact.
Full-text available
Article
The behavior of water, methanol, and water-methanol mixtures confined in narrow slit graphite pores as a function of pore size was investigated by Monte Carlo, hybrid Monte Carlo, and Molecular Dynamics simulations. Interactions were described using TIP4P/2005 for water, OPLS/2016 for methanol, and cross interactions fitted to excess water/methanol properties over the whole range of concentrations, which provide a rather accurate description of water-methanol mixtures. As expected for hydrophobic pores, whereas pure methanol is adsorbed already from the gas phase, pure water only enters the pore at pressures well beyond bulk saturation for all pore sizes considered. When adsorbed from a mixture, however, water adsorbs at much lower pressures due to the formation of hydrogen bonds with previously adsorbed methanol molecules. For all studied compositions and pore sizes, methanol adsorbs preferentially over water at liquid-vapor equilibrium conditions. In pure components, both water and methanol are microscopically structured in layers, the number of layers increasing with pore size. This is also the case in adsorbed mixtures, in which methanol has a higher affinity for the walls. This becomes more evident as the pore widens. Diffusion of pure water is higher than that of pure methanol for all pore sizes due to the larger size of the methyl group. In mixtures, both components present similar diffusivities at all pore sizes, which is explained in terms of the coupling of molecular movements due to strong hydrogen bonding between methanol and water molecules. This is particularly evident in very narrow pores, in which pure methanol diffusion is completely impeded on the time scale of our simulations, but the presence of a small amount of water molecules facilitates alcohol diffusion following a single-file mechanism. Additionally, our results indicate that pure water diffusivities display a non-monotonous dependence of pore size, due to effects of confinement (proximity to a fluid-solid-fluid transition induced by confinement as reported in previous work) and the dynamic anomalies of water.
Full-text available
Article
Adsorption of n-alkanes on high silica MFI and MEL zeolites was studied by means of experimental quasi-equilibrated temperature programmed desorption and adsorption (QE-TPDA) and Monte Carlo simulations. An unusual, isobaric approach to adsorption measurements and simulations was applied. Good agreement between the experimental and calculated data observed for MFI indicate that the QE-TPDA is a reliable method for studying porosity-related adsorptive properties of molecular sieves. The calculated average occupation profiles confirmed limited mobility of hexane and heptane molecules adsorbed in the sinusoidal channels on the MFI, thus proving the concept of order-disorder phase transition postulated in explanation of the two-step desorption profiles of these alkanes observed for MFI zeolites. Partial agreement of the calculated isobars with the experimental data found for zeolite MEL indicated that adsorption of n-alkanes in this structure is more complex than assumed in the simulation model. However, two-step desorption profiles and immobilization of the molecules adsorbed in the straight channels of the MEL structure were also found for hexane.
Full-text available
Article
In this work, we present a study of representative excess thermodynamic properties of aqueous mixtures of methanol over the complete concentration range, based on extensive computer simulation calculations. In addition to test various existing united atom model potentials, we have developed a new force-field which accurately reproduces the excess thermodynamics of this system. Moreover, we have paid particular attention to the behavior of the temperature of maximum density (TMD) in dilute methanol mixtures. The presence of a temperature of maximum density is one of the essential anomalies exhibited by water. This anomalous behavior is modified in a nonmonotonous fashion by the presence of fully miscible solutes that partly disrupt the hydrogen bond network of water, such as methanol (and other short chain alcohols). In order to obtain a better insight into the phenomenology of the changes in the TMD of water induced by small amounts of methanol, we have performed a new series of experimental measurements and computer simulations using various force fields. We observe that none of the force-fields tested captures the non-monotonous concentration dependence of the TMD for highly diluted methanol solutions.
Full-text available
Article
In this work, the adsorption of toluene on pure-silica MEL zeolite is investigated using a variety of experimental and simulation methods. First, we measured the volumetric and calorimetric isotherms at 301 K and at 315 K. The volumetric adsorption isotherm presents a substep at a loading of roughly 4 molecules per unit cell that shifts to higher pressures at higher temperatures and that coincides with a sudden increase in the isosteric heat of adsorption. Grand canonical Monte Carlo simulations reveal that the substep at half load is caused by the adsorption of toluene molecules at different energetic sites within the porous network. According to this, toluene molecules occupy first the intersections, and once all intersections are filled, additional toluene molecules place themselves within the channels. The structure of the adsorbate/adsorbent system was further investigated by performing X-ray powder diffraction experiments of the zeolite at three different loads: empty, at half load (before the substep), and at high load (after the substep). Numerous new low intensity peaks and splittings of existing peaks at the empty and half-loaded diffractograms appear in the diffraction pattern of the high load sample. Atomic structural models compatible with the experimental spectra were obtained by performing N-reverse Monte Carlo simulations. Whereas at half load a good fit of the experimental diffraction pattern can be obtained using the rigid zeolite approximation, at high load, this is only possible when the flexibility of the zeolite is incorporated. In this structural model, the channel cross sections are deformed from a nearly circular shape in the empty zeolite to a more elliptical shape in the case of the high load zeolite.
Full-text available
Article
Nose has modified Newtonian dynamics so as to reproduce both the canonical and the isothermal-isobaric probability densities in the phase space of an N-body system. He did this by scaling time (with s) and distance (with V¹D/ in D dimensions) through Lagrangian equations of motion. The dynamical equations describe the evolution of these two scaling variables and their two conjugate momenta p/sub s/ and p/sub v/. Here we develop a slightly different set of equations, free of time scaling. We find the dynamical steady-state probability density in an extended phase space with variables x, p/sub x/, V, epsilon-dot, and zeta, where the x are reduced distances and the two variables epsilon-dot and zeta act as thermodynamic friction coefficients. We find that these friction coefficients have Gaussian distributions. From the distributions the extent of small-system non-Newtonian behavior can be estimated. We illustrate the dynamical equations by considering their application to the simplest possible case, a one-dimensional classical harmonic oscillator.
Full-text available
Article
A new software package, RASPA, for simulating adsorption and diffusion of molecules in flexible nanoporous materials is presented. The code implements the latest state-of-the-art algorithms for molecular dynamics and Monte Carlo (MC) in various ensembles including symplectic/measure-preserving integrators, Ewald summation, configurational-bias MC, continuous fractional component MC, reactive MC and Baker's minimisation. We show example applications of RASPA in computing coexistence properties, adsorption isotherms for single and multiple components, self- and collective diffusivities, reaction systems and visualisation. The software is released under the GNU General Public License.
Full-text available
Article
Scitation is the online home of leading journals and conference proceedings from AIP Publishing and AIP Member Societies
Full-text available
Article
Zeolites play numerous important roles in modern petroleum refineries and have the potential to advance the production of fuels and chemical feedstocks from renewable resources. The performance of a zeolite as separation medium and catalyst depends on its framework structure. To date, 213 framework types have been synthesized and >330,000 thermodynamically accessible zeolite structures have been predicted. Hence, identification of optimal zeolites for a given application from the large pool of candidate structures is attractive for accelerating the pace of materials discovery. Here we identify, through a large-scale, multi-step computational screening process, promising zeolite structures for two energy-related applications: the purification of ethanol from fermentation broths and the hydroisomerization of alkanes with 18-30 carbon atoms encountered in petroleum refining. These results demonstrate that predictive modelling and data-driven science can now be applied to solve some of the most challenging separation problems involving highly non-ideal mixtures and highly articulated compounds.
Full-text available
Article
Molecular self-diffusion data of methanol–water mixtures sorbed in pentasil-type zeolites of two different Si/Al ratios are reported. At medium loading and room temperature, the self-diffusion coefficients of sorbed methanol and water are found to be of the order of 10–9 m2 s–1. For both sorbate molecules intracrystalline mobility is greater for the higher Si/Al ratio. While water self-diffusion is only slightly affected by sorbate concentration, in the case of methanol a decrease of approximately one order of magnitude with increasing loading has been observed. In contrast to the liquid state, the sorbed state of binary mixtures exhibited no minima of the self-diffusion coefficients. The molecular mobilities observed can be correlated with structural features of the pentasils determined from 1H n.m.r. measurements.
Full-text available
Article
The effects of packing of n-hexane molecules in the channels of a ZSM-11 zeolite are studied by means of adsorption microcalorimetric and volumetric experiments and molecular simulation. We find that the packing density within the zeolite channels is significantly higher than the bulk liquid density. This effect, somewhat characteristic of the n-hexane/ZSM-11 adsorbate/adsorbent system, was first found by Guil et al. (1998) [13] and it is here confirmed by new adsorption measurements carried out at various temperatures and on pure silica ZSM-11, and very especially by means of extensive grand-canonical Monte Carlo simulations. The analysis of simulation snapshots, angular distribution functions and intermolecular correlations, in conjunction with the calorimetric measurements, indicate that the excess of packing is not connected to any form of phase transition, but results from the optimum size matching of the n-hexane molecules within the zeolite channels. As a result, this feature is mostly temperature independent, being induced by steric effects. Computer simulations performed for n-butane and n-octane on ZSM-11 indicate that this effect is to be expected for other linear alkanes as well.
Full-text available
Article
We have performed a molecular simulation study on water adsorption in hydrophobic zeolites. The framework structures are truly periodic and therefore the Ewald summation is the natural choice for computing the Coulombic interactions. However, a few water models have been parameterised using this method. The adsorption results are extremely sensitive to the water model used, the framework positions in the orthorhombic structure and the atomic charges of the zeolite framework. This work provides insight into the identification of the potential limitations of the available force fields and models, and into the point charges used for the zeolite atoms, when they are applied to a highly hydrophobic system. We discuss feasible routes to conciliate simulation and experimental results.
Article
A vapor phase adsorptive recovery process is proposed as an alternative way to isolate biobutanol from acetone-butanol-ethanol (ABE) fermentation media, offering several advantages compared to liquid phase separation. The effect of water, which is still present in large quantities in vapor phase, on the adsorption of the organics could be minimized by using hydrophobic zeolites. Shape selective all-silica zeolites CHA and LTA were prepared and evaluated via single component isotherms and breakthrough experiments. These zeolites show an opposite selectivity; adsorption of ethanol was favorable on all-silica CHA, while the LTA topology had clear preference for butanol. The molecular sieving properties of both zeolites allowed to easily eliminate acetone from the mixture. The molecular interaction mechanisms were studied by density functional theory (DFT) simulations. Effect of mixture composition, humidity and total pressure of the vapor stream on the selectivity and separation behavior was investigated. Desorption profiles were studied to maximize butanol purity and recovery. The combination of LTA with CHA type zeolites (Si-CHA or SAPO-34) in sequential adsorption columns with alternating adsorption and desorption steps allows to obtain butanol in unpreceded purity and recovery. A butanol purity of 99.7 mole% could be obtained at nearly complete butanol recovery, demonstrating the effectiveness of this technique for biobutanol separation processes.
Article
The traditional zeolite ZSM-5 is hard to have fine properties both the adsorption and the activity of catalysis for big organic molecules. Here, we report that the ZSM-5 zeolites were prepared by a gel-casting technique with microporous, mesoporous and macroporous hierarchical structure. The as-prepared samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Transmission electron microscopy (TEM), nitrogen adsorption, Temperature-programmed-desorption of ammonia (TPD-NH3) and Fourier Transform Infrared Spectroscopy (FTIR) measurements. The hierarchical structured ZSM-5 zeolites were used to carry out catalytic cracking reaction of 1, 3, 5 - isopropyl benzene and n-hexadecane. Data shows that this modified ZSM-5 displayed improved catalytic activity and selectivity than that of traditional ZSM-5, Beta, Al-MCM-41, etc
Article
Anaerobic fermentation can transform carbohydrates to yield a multicomponent mixture comprising mainly of acetone, 1-butanol, and ethanol (ABE) in a typical weight ratio of 3:6:1. Compared to ethanol, 1-butanol, the main product of ABE fermentation, offers significant advantages as a biofuel or a fuel-additive. However, the toxicity of 1-butanol for cell cultures requires broth concentrations to be low in 1-butanol (approximately 1 - 2 wt%). An energy efficient recovery method that performs well even at low 1-butanol concentrations is therefore necessary to ensure economic feasibility of the ABE fermentation process. In this work, configurational-bias Monte Carlo simulations in the Gibbs ensemble are performed to probe the adsorption of 1-butanol/water solutions onto all-siliceous zeolites with the framework types MFI and FER. At low solution concentration, the selectivity and capacity for 1-butanol in MFI are larger than those in FER, while the opposite is true for concentrations at or above those of ABE broths. Structural analysis at various loadings sheds light on the different sorbate--sorbate and sorbate--sorbent interactions that govern trends in adsorption in each zeolite.
Article
Experimental self-diffusivities of gases, vapors, and liquids obtained by means of tracer techniques and nuclear magnetic resonance are reviewed. The considered substances range from noble gases and simple diatomics (nitrogen, oxygen, carbon monoxide, etc.) to complex organic molecules, such as phenolphthalein dimethyl ether and 2-(α-methylbenzylamino)-5-nitropyridine, although polymers have not been included. Some comments on the applicability of neutron scattering to the determination of self-diffusion coefficients are also made. All the experimental results of the investigated systems are given as Supporting Information, whereas the references, temperatures, and pressures of these data and the main features of the measurement methods are compiled and classified.
Article
The adsorption isotherm of argon on the zeolite MFI at liquid nitrogen temperature exhibits a sub-step at high loading before saturation that, in spite of much theoretical and experimental effort, is still lacking a definitive microscopic interpretation. In this work, we try to get insight into this peculiar behaviour by investigating the adsorption of argon on MEL, a zeolite that is structurally very similar to the MFI. First, we performed volumetric experiments that confirm that the adsorption of argon on MEL presents the same qualitative behaviour as on the MFI, again a sub-step appearing at high loading before saturation. Subsequently, the microscopic origin of this behaviour was investigated by means of molecular simulation. The simulations indicate that, for loads lower than that of the experimental sub-step, argon atoms can accommodate at low energy positions within the zeolite pores, whereas, above this point, some reordering of the adsorbate is needed to host further argon atoms. Moreover, the flexibility of the zeolite can have a significant impact on the shape of the adsorption isotherm, although the magnitude of this change depends on the zeolite model potential.
Article
A molecular dynamics simulation method which can generate configurations belonging to the canonical (T, V, N) ensemble or the constant temperature constant pressure. (T, P, N) ensemble, is proposed. The physical system of interest consists of N particles (f degrees of freedom), to which an external, macroscopic variable and its conjugate momentum are added. This device allows the total energy of the physical system to fluctuate. The equilibrium distribution of the energy coincides with the canonical distribution both in momentum and in coordinate space. The method is tested for an atomic fluid (Ar) and works well.
Article
In this work, colloidal silicalite-1 single crystals are for the first time synthesized using fluoride as mineralizing agent at near neutral pH. SEM, TEM, DLS, XRD, solid-state 29Si MAS NMR, and adsorption/desorption experiments using nitrogen, water, n-butanol and ethanol as adsorbates were used to characterize the crystals. The single crystals have a plate-like habit with a length of less than 170 nm and an aspect ratio (length/width) of about 1.2 and the thickness of the crystals is less than 40 nm. Compared with silicalite-1 crystals grown using hydroxide as mineralizing agent, the amount of structural defects in the lattice is significantly reduced and the hydrophobicity is increased. Membrane separation and adsorption results show that the synthesized defect-free crystals present high selectivity to alcohols. The n-butanol/water adsorption selectivities were ca. 165 and 14 for the defect-free crystals and a reference sample containing defects, respectively, illustrating the improvement in n-butanol/water selectivity by eliminating the polar silanol defects.
Article
Pure silicalite membranes were prepared on porous supports of sintered stainless steel or alumina discs. The silicalite layer was characterized by X-ray diffraction, SEM and mercury porosimetry. Individual crystals were intergrown in three dimensions into the polycrystalline phase. The membranes were not disintegrated by thermal treatment in vacuo or calcination for removing the organic amine occluded in the channels of silicalite, indicating high thermomechanical stability of the membrane. The liquid separation potential of the membrane was investigated by pervaporation of an aqueous ethanol solution. The high ethanol permselectivity with a separation factor cr (EtOH/H2O) of more than 60 was achieved for a 5 vol% aqueous ethanol solution at 30 degrees C, indicating no cracks and pores between the silicalite grains within the membrane. From adsorption experiments of ethanol and water on silicalite, it was found that the high permselectivity is attributable to the selective sorption of ethanol into the silicalite membrane.
Article
We review state-of-the-art Monte Carlo (MC) techniques for computing fluid coexistence properties (Gibbs simulations) and adsorption simulations in nanoporous materials such as zeolites and metal-organic frameworks. Conventional MC is discussed and compared to advanced techniques such as reactive MC, configurational-bias Monte Carlo and continuous fractional MC. The latter technique overcomes the problem of low insertion probabilities in open systems. Other modern methods are (hyper-)parallel tempering, Wang-Landau sampling and nested sampling. Details on the techniques and acceptance rules as well as to what systems these techniques can be applied are provided. We highlight consistency tests to help validate and debug MC codes.
Article
Bio-butanol produced by e.g. acetone-butanol-ethanol (ABE) fermentation is a promising alternative to petroleum-based chemicals as e.g. solvent and fuel. Recovery of butanol from dilute fermentation broths by hydrophobic membranes and adsorbents has been identified as a promising route. In this work, the adsorption of water and butanol vapor in a silicalite-1 film was studied using in-situ ATR-FTIR spectroscopy in order to better understand the adsorption properties of silicalite-1 membranes and adsorbents. Single component adsorption isotherms were determined in the temperature range of 35-120°C and the Langmuir model was successfully fitted to the experimental data. The adsorption of butanol is very favorable compared to that of water. When the silicalite-1 film was exposed to a butanol/water vapor mixture with 15 mol% of butanol (which is the vapor composition of an aqueous solution containing 2 wt% of butanol, a typical concentration in an ABE fermentation broth, i.e. the composition of the gas obtained from gas stripping of an ABE broth) at 35 °C, the adsorption selectivity towards butanol was as high as 107. These results confirm that silicalite-1 quite selectively adsorbs hydrocarbons from vapor mixtures.To the best of our knowledge, this is the first comprehensive study on the adsorption of water and butanol in silicalite-1 from vapor phase.
Article
We examine the adsorption and diffusion of small alcohols in ZIF-8 and ZIF-90 with a combined experimental and modeling approach. Our Grand Canonical Monte Carlo (GCMC) simulations predict that both ZIFs exhibit a slight adsorption selectivity for ethanol over methanol, in good agreement with previous experimental data. The adsorption uptake of the alcohols at low pressures is found to be significantly higher in ZIF-90 than ZIF-8. Our simulations indicate that this is due to hydrogen bonding between the alcohols and the carbonyl group of ZIF-90 but that this effect is not strong enough to cause appreciable flexibility of the ZIF-90 framework during adsorption. We also report alcohol self-diffusivities and Arrhenius parameters measured using pulsed field gradient NMR (PFG-NMR) and molecular dynamics (MD) simulations. The diffusivities measured using PFG-NMR indicate that the diffusion selectivity of methanol over ethanol is significantly higher in ZIF-8 (S = 229) than in ZIF-90 (S = 6) at T = 25 °C. Qualitative agreement is obtained between experimental and simulated diffusivities using the generalized AMBER (GAFF) force field including framework flexibility.
Article
Intradiffusion coefficients of acetylacetone (AcAc) and methanol/ethanol/1-propanol/1-butanol were measured in binary liquid mixtures over the whole concentration range at 303.15 K by the 1H diffusion-order spectroscopy (DOSY) nuclear magnetic resonance (NMR) method based pulse field gradient (PFG). The solvent effect on the enol-keto tautomeric equilibrium as well as differences in intradiffusion coefficients (D) between two tautomers were also studied. The experimental results show that from methanol to 1-butanol, the differences in D between the enol and keto tautomer vary from 5 % to 26 % at different concentrations of AcAc. The densities and viscosities of binary liquid mixtures of AcAc with the above four alkanols have also been determined at 303.15 K and employed to calculate the excess molar volumes and deviations in viscosity over the entire range of mole fractions. Isotherms of VE as a function of mole fraction of AcAc show positive deviations in methanol and ethanol but negative deviations in 1-propanol and 1-butanol, whereas all isotherms of Δη as a function of mole fraction of AcAc record negative deviations. The VE and Δη are fitted to a Redich–Kister type equation. The measured results are interpreted in terms of molecular interactions in the solutions.
Article
FLUIDS confined in narrow pores can have properties that are distinctly different from those of bulk fluids1,2. Most studies of fluids in pores have focused on simple fluids which can be modelled as near-spherical molecules. But molecular shape can also exert an influence on the fluid's behaviour, particularly for large and/or complex molecules. The adsorption isotherms of alkanes in the zeolite silicalite provide an apparent example of this: the short-chain (C1to C5) and long-chain (C10) alkanes have simple isotherms3-5 whereas for hexane and heptane the isotherms are kinked4,6, suggesting that some kind of phase transition takes place. Here we present computer simulations of the adsorption of straight-chain hydrocarbons in silicalite, which suggest that this phase transition is of a type not reported previously, arising as a consequence of the interplay between the length of the zig-zag pores and the length of the alkanes. When these two are comparable, the molecules can `freeze' in a configuration that is commensurate with the pore structure, creating a kink in the isotherms. Such behaviour might be quite general for complex molecular fluids.
Article
The transferable potentials for phase equilibria (TraPPE) force field is extended to all-silica zeolites. This novel force field is parametrized to match the experimental adsorption isotherms of n-heptane, propane, carbon dioxide, and ethanol with the Lennard-Jones parameters for sorbate–framework interactions determined in a consistent manner using the Lorentz–Berthelot combining rules as for other parts of the TraPPE force field. The TraPPE-zeo force field allows for accurate predictions for both adsorption and diffusion of alkanes, alcohols, carbon dioxide, and water over a wide range of pressures and temperatures. In order to achieve transferability to a wider range of molecule types, ranging from nonpolar to dipolar and hydrogen-bonding compounds, Lennard-Jones interaction sites and partial charges are placed at both the oxygen and the silicon atoms of the zeolite lattice, which allows for a better balance of dispersive and first-order electrostatic interactions than is achievable with the Lennard-Jones potential used only for the oxygen atoms. The use of the Lorentz–Berthelot combining rules for unlike interactions makes the TraPPE-zeo force field applicable to any sorbate as long as the relevant TraPPE sorbate–sorbate parameters are available. The TraPPE-zeo force field allows for greatly improved predictive power compared to force fields that explicitly tabulate the individual cross-interaction parameters.
Article
Single component and binary mixture adsorption behaviors of water (H2O) and ethanol (EtOH) were investigated on silicalites with different crystallinity (structure-defects) by a combination of volumetric and gravimetric methods. No influence of the crystallinity of silicalite on the pure EtOH adsorption was observed. On the other hand, the amount of H2O adsorbed on the silicalite with less structure-defects was significantly smaller. The total adsorption amount in the binary mixture adsorption was strongly dependent on the EtOH/H2O ratio in the initial mixture. From the analysis of the binary adsorption data, it became clear that EtOH molecules adsorb preferentially on silicalite with less structure-defects and that more selective adsorption of EtOH takes place under the lower equilibrium pressure.
Article
Differential molar heats of sorption have been determined calorimetrically for methanol, ethanol, propan-1-ol, propan-2-ol, butan-1-ol, butan-2-ol and 2-methylpropan-1-ol on silicalite as a function of pore filling . The results suggest that the first alcohol molecules sorbed interact wtih lattice defects; as the number of alcohol molecules rises, they are sorbed on the regular silicalite framework. A comparison of the heat of sorption curves of methanol, propan-1-ol and propan-2-ol at low pore fillings with that of triethylamine on silicalite supports the view that most of the defects in the silicalite lattice consist of internal silanol groups which predominantly occur as vicinal pairs. From a comparison of the heat of sorption curves of the alcohols studied with those of alkanes and diethyl ether, it is concluded that a substantial contribution to the heat of sorption of the alcohols on the regular silicalite framework stems from hydrogen-bonding interactions of the alcohol molecules with framework oxygen atoms and/or with each other.
Article
Using GCMC and MD simulations we investigated the effect of silanol groups and extraframework cations on water adsorption, diffusion and the structure of water in silicalite. The adsorption of water was enhanced with the introduction of defects. In the case of cations this enhancement was much more significant. Below the saturation pressure of water no filling of pores by water molecules was observed. Introduction of silanol nests did not result in significant changes in water structure and self diffusion. On the other hand, the presence of cations decreased self diffusivity of water and changed the water structure observed in the defect-free silicalite. Silanol nests were found to be weak defects and have a limited effect on water adsorption. The model we used in this study satisfactorily predicted adsorption isotherms and heats of adsorption, however, self diffusion coefficients of water were underestimated which is attributed to the rigid treatment of the silicalite lattice.
Article
i11.96/~do.04024 . ½NC12H28OH + nH20 (+0.04Na+?), Mr = 822.65 + (n x 18), orthorhombic, Pnma (assumed), a = 20.022 (2), b = 19.899 (2), c = 13.383 (1) /~, V=5332(4)A 3, Z=8, Dx=2.06 (including TPAOH), Dx = 1.80 gcm -3 (for the anhy- drous framework), h(Mo Ka) =0.71069 A, /z = 6.91 cm -1, F(000) = 3343.7, T = 293 K, R = 0.042 for 4523 observed reflections with l>2.0cr(I). The framework topology agrees with earlier descriptions. Straight channels and sinusoidal channels run parallel to (010) and (100), respectively. The sym- metry of the framework atoms is too close to Pnma to permit refinement in Pn21a. In the straight channel an approximate mirror plane through N and the chan- nel axis, perpendicular to the crystallographic mirror plane, exists. The tetrapropylammonium ion lies at the intersection of the straight and sinusoidal chan- nels in two different orientations. The two orienta- tions, populated in a ratio of 3:2, are nearly related by the approximate mirror plane. Contrary to literature data the propyl-N-propyl fragments point- ing into the sinusoidal and straight channel, respec- tively, both have CNCC torsion angles around 60 °. No evidence is obtained for the tetrapropylam- monium ion breaking the rn symmetry and extra disorder around rn is assumed. The mean C-C and C-N distances and mean CCC, CCN and CNC angles, averaged over both ions, are 1.55 and 1.57 A and 109, 113 and 109 °, respectively. Contact distances smaller than 4.0/~, between terminal C atoms of adjacent template ions, are in the range 3.56 (7)- 3.75 (2) ~.
Article
A new apparatus is described in which the vapour pressure of a liquid mixture is determined accurately by a static method. The apparatus has been designed to work in the temperature range 298.15 to 423.15 K for pressures up to 1.5 MPa. Temperatures are measured to within ±1 mK and pressures to ±0.02 per cent or ±2.6 Pa whichever is the greater, over the range 2.7 to 270 kPa. Vapour pressures of have been measured by the static method at 303.15, 323.15, 343.15, and 363.15 K for 24 mixtures with x in the range 0.0044 to 0.9815. Values of the molar excess Gibbs energy GE and the partial molar Gibbs energies G1E have been computed by Barker's method. Positive deviations from ideality were found at all four temperatures with maxima in GE of 770, 837, 889, and 925 J mol−1 at 303.15, 323.15, 343.15 and 363.15 K respectively; these maxima occur at values of x about 0.440, 0.428, 0.418, and 0.406 respectively. The excess Gibbs energies have been combined with experimental excess enthalpies HE to give excess entropies SE. The curves of TSE against x all have the form of smooth skewed parabolas with large negative minima. The minimum values of TSE are −1289, −1076, −858, and −648 J mol−1 at 303.15, 323.15, 343.15, and 363.15 K. These minima all occur at a value of x about 0.28: partial molar excess entropies are presented. Isobaric liquid-vapour equilibrium data for at a pressure of 101.325 kPa have been calculated from the isothermal data. The results are compared with data from the literature. The composition of the normal-boiling positive azeotrope, x = (0.8933±0.0005) at T = (351.320±0.002) K, was determined from an analysis of the experimental vapour pressures. The minimum temperature for azeotrope formation was (305.7±0.5) K. Azeotrope compositions at 323.15, 343.15, and 363.15 K were x = 0.9324, 0.9002, and (0.8845±0.0005) respectively.
Article
This study investigates pervaporation performances for the separation of ethanol–water mixtures using composite polydimethylsiloxane (PDMS). The probability of cluster formation into PDMS matrix was analyzed by molecular dynamics simulation (MD) technique. Cluster formations within water–water and ethanol–ethanol molecules into PDMS were not observed, but ethanol–water dimerization was evidenced by MD simulation. When molecules aggregate, their average sizes increase; hence, diffusion coefficients of components decrease. Based on these assumptions, a new semi-empirical model for diffusion coefficients of penetrants in membrane was proposed. Then using this theory, the solution–diffusion model was modified and the process was simulated. Diffusion coefficient parameters were obtained by minimization error between simulation and experimental results at different concentrations. The behaviour of pervaporation experimental results as a function of concentration has confirmed the proposed model. Influences of feed temperature, feed flow rate, membrane thickness and permeate-side pressure on the membrane separation performance were predicted by the model. Simulation results showed excellent agreement with the experimental data.
Article
A methodology based on grand canonical Monte Carlo (GCMC) and NVT-ensemble equilibrium molecular dynamics techniques has been used to simulate the ethanol/water vapor mixture adsorption, diffusion and separation characteristics in silicalite crystal. The adsorption isotherms of pure ethanol and water consist with the literature experimental observations and other algorithm. The ethanol adsorption selectivity first reached the highest value of 85 at a feed ethanol concentration of ∼4vol.% and then decreased with increasing the feed ethanol concentration. This also agreed with the literature experimental investigations. The diffusion coefficients of ethanol and water in the pure and mixture are also estimated by molecular dynamics. The results reveal that the effects of ethanol and water diffusion on separation are slight. We also used the permeation theory to estimate the pure ethanol and water permeance through silicalite membranes. All quantitative agreement of calculations with experiments suggests that the simulation method can be used to predict the performance of zeolite membranes.
Article
Self-diffusion coefficients of tritiated water in normal (H2O) and heavy water (D2O) have been measured over the temperature range 1-45°. The diaphragm-cell technique was used and the results are considered to be probably accurate to ±0.2%. The data of Longsworth1,2 for HDO diffusion in both H2O and D2O in conjunction with the tritiated water values measured here have been used to calculate the self-diffusion coefficients of pure normal water (H2O-H2O) and pure heavy water (D2O-D2O). These coefficients have been tabulated and compared with molecular dynamics and nmr data.
Article
We have obtained the direct evidence by high-resolution electron microscopy (HREM) that a pure MEL type zeolite has been successfully synthesized in all-silica form using a novel organocationic templating agent. It has been also confirmed from a synchrotron X-ray powder diffraction obtained at room temperature that the crystal structure of this all-silica zeolite is of the MEL-type by use of the Rietveld method. 14 refs., 5 figs., 2 tabs.
Article
New measurements have been made of the tracer diffusion coefficients of ethanol and 2,2,2-trifluoroethanol (TFE) in aqueous solution at 25°C across the whole composition range. The limiting values at infinite dilution of these alcohols and the series of primary alcohols from methanol to heptanol, the isomeric propanols and butanols and 2-butoxyethanol are found to vary as the alcohol limiting partial molar volume raised to the power -0.56. The self-diffusion coefficient of water lies on the same curve, suggesting that the rate-determining step for diffusion is the same for these alcohols at infinite dilution as it is for water. 1H, 19F and 13C chemical shift and density measurements have been made and apparent molar volumes obtained from the latter. The composition dependence of the ν2 bending vibration band of water in water–TFE at 1640 cm-1 has been examined. The chemical shift and water IR measurements show that water–TFE behaves like other aqueous halogenoethanol solutions with water and hydroxyl proton shifts moving to higher field with increasing alcohol concentration. The results are consistent with thermodynamic data indicating some degree of microheterogeneity in the TFE-rich region of this system and a strengthening of water–TFE H-bonding on dilution of TFE. This is the opposite of what is observed for water–ethanol. We were unable to find any evidence from the diffusion data for the formation of water adducts to ethanol chains at high ethanol concentration in water–ethanol mixtures as has been suggested in the literature.
Article
A new synthetic procedure for the preparation of pure-phase ZSM-11 (MEL) is reported. Pure-phase ZSM-11 is synthesized at 135 °C using 2,2-diethoxyethyltrimethylammonium as a structure-directing agent (SDA). The effects of the incorporation of boron in the reaction chemistry are also reported. A very high SDA–ZSM-11 specificity is observed since the inclusion of ZSM-5 (MFI) into the synthesis mixture at levels as high as 24 wt.% of the total silica content still leads to the crystallization of ZSM-11.
Article
The motivation for developing a cubic equation of state model having improved capabilities in the vicinity of the critical region and in predicting liquid densities is reviewed. The reliability of the resulting Peng-Robinson equation for predicting volumetric and phase behavior is illustrated for complex hydrocarbon systems.Modifications to improve the performance of the equation for two- and three-phase systems containing water are described. The reliability of the equation for system containing methanol is illustrated using new data on the methanol - carbon dioxide and methanol - propane systems. The agreement between experimental and calculated results is acceptable only over limited regions.
Article
The synthesis of pure phase ZSM-11 (MEL) at temperatures below 100 °C has been achieved using extremely dense gels with minimal water contents. Both hydroxide and fluoride routes were investigated using N,N-diethyl-3,5-dimethylpiperidinium and 2,2-diethoxyethyltrimethylammonium based templates. Highly crystalline materials were successfully synthesised within relatively short reaction times using a wide range of reaction conditions. The syntheses are shown to be not as efficient as the related ZSM-5 (MFI) materials, suggesting that the structure directing power of the template molecules is critical for low temperature reactions.
Article
Molecular simulations are an important tool for the study of adsorption of hydrocarbons in nanoporous materials such as zeolites. The heat of adsorption is an important thermodynamic quantity that can be measured both in experiments and molecular simulations, and therefore it is often used to investigate the quality of a force field for a certain guest−host (g - h) system. In molecular simulations, the heat of adsorption in zeolites is often computed using either of the following methods: (1) using the Clausius-Clapeyron equation, which requires the partial derivative of the pressure with respect to temperature at constant loading, (2) using the energy difference between the host with and without a single guest molecule present, and (3) from energy/particle fluctuations in the grand-canonical ensemble. To calculate the heat of adsorption from experiments (besides direct calorimetry), only the first method is usually applicable. Although the computation of the heat of adsorption is straightforward for all-silica zeolites, severe difficulties arise when applying the conventional methods to systems with nonframework cations present. The reason for this is that these nonframework cations have very strong Coulombic interactions with the zeolite. We will present an alternative method based on biased interactions of guest molecules that suffers less from these difficulties. This method requires only a single simulation of the host structure. In addition, we will review some of the other important issues concerning the handling of these strong Coulombic interactions in simulating the adsorption of guest molecules. It turns out that the recently proposed Wolf method ( J. Chem. Phys.1999, 110, 8254) performs poorly for zeolites as a large cutoff radius is needed for convergence.
Article
It is shown how certain thermodynamic functions, and also the radial distribution function, can be expressed in terms of the potential energy distribution in a fluid. A miscellany of results is derived from this unified point of view. (i) With g(r) the radial distribution function and Φ(r) the pair potential, it is shown that g exp (Φ/kT) may be written as a Fourier integral, or as a power series in r2 the terms of which alternate in sign. (ii) A potential‐energy distribution which is independent of the temperature implies an equation of state which is a generalization of a number of well‐known approximations. (iii) The grand partition function of the one‐dimensional lattice gas is obtained from thermodynamic arguments without evaluating a sum over states. (iv) If in a two‐dimensional honeycomb (three‐coordinates) lattice gas fr(r=0, 1, 2, 3) is the fraction of all the empty sites which at equilibrium are neighbored by exactly r filled sites, then at the critical density the values of all four of the f's as functions of temperature follow from previously known properties of this system; in particular, at the critical point, f0 = 3/8+5√3/24, f1 = 1/8+√3/24, f2 = 1/8—√3/24, f3 = 3/8–5√3/24.
Article
A molecular dynamics simulation method which can generate configurations belonging to the canonical (T, V, N) ensemble or the constant temperature constant pressure (T, P, N) ensemble, is proposed. The physical system of interest consists of N particles (f degrees of freedom), to which an external, macroscopic variable and its conjugate momentum are added. This device allows the total energy of the physical system to fluctuate. The equilibrium distribution of the energy coincides with the canonical distribution both in momentum and in coordinate space. The method is tested for an atomic fluid (Ar) and works well.
Article
Configurational-bias Monte Carlo (CBMC) simulations in the isobaric-isothermal version of the Gibbs ensemble (GE) were carried out to probe the adsorption from aqueous solutions of methanol and/or ethanol onto silicalite-1. This methodology does require neither specification of the chemical potential nor any reference to activity models based on experimental data. The CBMC-GE methodology can be applied to the complete range of mixture compositions from pure water to pure alcohols and can also be used when multiple solute types are present at high concentration. The simulations demonstrate high selectivities for the alcohols (α(ethanol) > α(methanol)) almost over the entire composition range. The ideal adsorbed solution theory is found to substantially underpredict the amount of sorbed water and leads to very large errors for low alcohol solution concentrations. The simulations indicate that, at lower loadings, the adsorbed alcohol molecules can serve as seeds for water adsorption but, at higher loadings, alcohols displace water molecules from their preferred region.
Article
A methodology based on the combination of grand canonical Monte Carlo (GCMC) and expanded ensemble (EE) simulations has been used to study the adsorption of alcohols from dilute aqueous solution onto the silicalite zeolite. The chemical potential of the guest alcohol molecules in bulk aqueous solutions is calculated by the EE method, and the adsorption isotherms of the alcohols are calculated using GCMC simulations. This approach results in adsorption isotherms that relate the loadings to external concentrations without using an equation of state, experimental data, or performing computationally expensive simulations such as two-phase Gibbs ensemble Monte Carlo (GEMC). Also, the method is “force-field-consistent”, in that the same force field is used for the bulk fluid and the adsorption calculations, and the resulting isotherms can be directly compared with experimental data. We have established the validity of the method used by comparing the calculated adsorption isotherms with experimental data for methanol and ethanol on silicalite, both from aqueous solution and from the vapor phase. Calculated heats of adsorption are also in reasonable agreement with experimental data. In addition, these simulations provide information on the location of the alcohol adsorption sites. We also observe that the presence of water increases the adsorption of alcohols at dilute concentrations onto silicalite; therefore, an accurate estimate of amount of water adsorbed in the nanoporous substrate is important to the prediction of alcohol adsorption from aqueous solution. The method presented here can be used to study the adsorption of any molecular species on any nanoporous adsorbent.
Article
The behavior of water in silicalite and dealuminated zeolite Y (DAY), two highly hydrophobic zeolites, was investigated at different temperatures in the range 100−600 K by molecular dynamics simulations using the Compass force field. The full flexibility of water molecules and the zeolite framework was considered. This study confirmed behavior of water in silicalite pores reported previously by several authors, and extended previous work to include the different behavior of water in DAY pores. The results show that the behavior of water is more complex in silicalite than in DAY. Three different activation energies for water diffusion were obtained in silicalite in the range 250−600 K compared to two for DAY. The values of these activation energies are discussed in detail and are related to the strength of hydrogen bonds and the zeolite structure. Moreover, from the radial distribution functions (RDFs), it is shown that water mostly exists in the gas phase at room temperature in silicalite, whereas liquidlike water is observed in DAY. The self-diffusion coefficients of water and the RDFs were obtained as functions of temperature in order to explain the different behaviors of water in the two all-silica zeolites. The influence of loading on the self-diffusion coefficients also was investigated for both crystals. The results compare favorably with previous experimental and theoretical studies.