Monte Carlo simulation for the adsorption and separation of linear and branched alkanes in IRMOF-1

The adsorption and separation of linear and branched alkanes in the isoreticular metal-organic framework IRMOF-1 have been investigated using Monte Carlo simulation. For pure linear alkanes (C1-nC5), the limiting adsorption properties exhibit linear behavior with the alkane carbon number; the long alkane is preferentially adsorbed over the short alkane at low fugacities, whereas the reverse is found at high fugacities. For pure branched alkanes (C5 isomers), the linear isomer adsorbs more than its branched analogue. The adsorbed amounts of pure alkanes in IRMOF-1 are substantially greater than in a carbon nanotube bundle and in silicalite. For a five-component mixture of C1 to nC5 linear alkanes, the long alkane adsorption first increases and then decreases with increasing fugacity, whereas short alkane adsorption continually increases and progressively replaces the long alkane at high fugacity due to the size entropy effect. For a three-component mixture of C5 isomers, the adsorption of each isomer increases with increasing fugacity until saturation, though there is less adsorption of the branched isomer due to the configurational entropy effect. The adsorption selectivity among the alkanes in IRMOF-1 is smaller than in a carbon nanotube bundle and in silicalite.     

层柱状微孔材料吸附存储天然气的Monte Carlo模拟

采用巨正则系综MonteCarlo方法模拟了天然气中主要成分甲烷在层柱状微孔材料中T=300K下的吸附存储,在模拟中层柱状微孔采用Yi等人建立的柱子均匀分布在两炭孔墙之间的模型来表征。甲烷分子采用Lennard-Jones球型分子模型,炭孔墙采用Steele的10-4-3模型,对孔宽为1.36nm的层柱微孔,模拟了四个不同孔率的层柱材料吸附甲烷的情形。得到了孔中流体的局部密度分布以及吸附等温线,对比不同孔率下甲烷的吸附量,得到了此情形吸附甲烷的较佳孔率为0.94。     

超临界甲烷在氯化锆层柱纳米材料中吸附的分子模拟

用巨正则MonteCarlo(GCMC)方法模拟了甲烷在氯化锆层柱材料中的吸附。模拟中,氯化锆层柱材料模型化为柱子均匀分布在层板间的层柱孔,非极性分子甲烷采用Lennard-Jones分子模型,层板墙采用Steele的10-4-3模型,流体分子与柱子的相互作用采用点-点(sitetosite)的方法计算。在高度理想化模型的基础上,引入交互作用参数kfw,建立了有效势能模型。通过实验数据确定交互作用参数kfw,从而使该模型能有效地表征流体与层板墙的相互作用。根据77K温度下氮气的实验吸附数据,确定了流体和层板墙间的交互相作用参数。然后用这个有效的参数kfw=0.65模拟了三个超临界温度下氯化锆层柱材料中甲烷的吸附情形,得到了它位的吸附等温线,局部密度分布以有流体分子在层柱微孔中的瞬时构象,并分析了温度对材料吸附性能的影响。结果表明GCMC方法是预测材料吸附性能的一种强有力的工具。     

Excess thermodynamics of mixtures involving xenon and light linear alkanes by computer simulation

Excess molar enthalpies and excess molar volumes as a function of composition for liquid mixtures of xenon + ethane (at 161.40 K), xenon + propane (at 161.40 K) and xenon + n-butane (at 182.34 K) have been obtained by Monte Carlo computer simulations and compared with available experimental data. Simulation conditions were chosen to closely match those of the corresponding experimental results. The TraPPE-UA force field was selected among other force fields to model all the alkanes studied, whereas the one-center Lennard-Jones potential from Bohn et al. was used for xenon. The calculated H(m)(E) and V(m)(E) for all systems are negative, increasing in magnitude as the alkane chain length increases. The results for these systems were compared with experimental data and with other theoretical calculations using the SAFT approach. An excellent agreement between simulation and experimental results was found for xenon + ethane system, whereas for the remaining two systems, some deviations that become progressively more significant as the alkane chain length increases were observed.     

Molecular simulation of adsorption of alkanes in sodium MOR-type zeolites using a new force field

The applicability of a recently proposed force field of Calero et al. (J. Am. Chem. Soc., 2004, 126, 11377) to Na-MOR zeolites is evaluated. The Henry law coefficients of ethane and C(5)-C(9) as well as the adsorption isotherms of ethane, propane, butane, and hexane in various Na-MOR zeolites are computed and compared with experimental values. These comparisons show that the new force field is suitable for Na-MOR zeolites. Furthermore, this force field is used to study the effects of sodium cations on the adsorption behavior of larger alkanes, such as C(4)-C(7), in MOR-type zeolites. These simulations give a better understanding of the underlying mechanisms of the cations' position and density influence on adsorption. In addition, a characteristic pressure named "reversal pressure" is introduced which characterizes the efficiency of the presence of cations in zeolites.     

Grand canonical Monte Carlo simulation for determination of optimum parameters for adsorption of supercritical methane in pillared layered pores

A grand canonical Monte Carlo (GCMC) method is carried out to determine optimum adsorptive storage pressures of supercritical methane in pillared layered pores. In the simulation, the pillared layered pore is modeled by a uniform distribution of pillars between two solid walls. Methane is described as a spherical Lennard-Jones molecule, and Steele's 10-4-3 potential is used for representing the interaction between the fluid and a layered wall. The site-site interaction is also used for calculating the interaction energy between methane molecules and pillars. An effective potential model that reflects the characteristics of a real pillared layered material is proposed here. In the model, a binary interaction parameter, k(fw), is introduced into the combining rule for the cross-energy parameter for the interaction between the fluid and a layered wall. Based on the experimental results for the Zr-pillared material synthesized and characterized by Boksh, Kikkinides, and Yang, the binary interaction parameter, k(fw), is determined by fitting the simulation results to the experimental adsorption data of nitrogen at 77 K. Then, by taking it as a model of pillared layered material, a series of GCMC simulations have been carried out. The excess adsorption isotherms of methane in a pillared layered pore with three different pore widths and porosities are obtained at three supercritical temperatures T=207.3, 237.0, and 266.6 K. Based on the simulation results at different porosities, various pore widths and different supercritical temperatures, the pillared layered pore with porosity psi=0.94 and pore width hsigma(p)=1.02 nm is recommended as adsorption storage material of supercritical methane. Moreover, the optimum adsorption pressure is determined at a given temperature and a fixed width of the pillared layered pore. For example, at temperature T=207.3 K, the optimum adsorption pressures are 3.1, 3.7, and 4.5 M Pa at H=1.02, 1.70, and 2.38 nm, respectively. In summary, the GCMC method is a useful tool for optimizing adsorption storage of supercritical methane in pillared layered material.     

不同链长烷烃在碳纳米管表面行为的分子动力学模拟

采用分子动力学方法模拟了不同长度的烷烃在单壁碳纳米管表面的吸附和取向过程,直观地给出了过程中很多微观信息,如链与纳米管管轴的夹角,链的弯曲程度等.结果表明短链烷烃能够被吸附在纳米管表面并沿着特定的方向取向;碳纳米管表面起到诱导取向的作用,并且该诱导作用对离表面最近的吸附层作用最明显;在吸附过程中,烷烃链发生弯曲,而吸附以后,取向的烷烃链倾向于采取伸直的状态。     

Molecular motion of isolated linear alkanes in nanochannels

The mobility of a series of linear alkanes in their inclusion compound with tris(o-phenylenedioxy)spirotriphosphazene is studied by high-resolution carbon-proton magic-angle spinning solid-state NMR spectroscopy. Two different carbon-proton dipolar recoupling experiments are compared with respect to their ability to yield precise site-specific, motion-averaged dipolar coupling constants. The most accurate results are obtained by analysis of extrema positions in Lee-Goldburg cross-polarization build-up curves. We present a comprehensive collection of coupling constants, which evidence a rotational motion of the all-trans chains around the channel axis, with some further averaging due to additional fluctuations, as previously found for alkanes in other host matrices such as urea. The order parameter increases toward the inner parts of the chains, and is largely independent of chain length. Notably, chains in a TPP host are not more ordered than in urea, even though the average TPP channel diameter is reported to be smaller. Significantly decreased order is found for highly filled short-alkane samples, which is interpreted in terms of an increased rate of mutual collisions. From residual dipolar couplings as well as carbon chemical shifts, we derive similar amounts of gauche conformers. Translational motions along the channels are further studied by proton double-quantum spectroscopy, which probes guest-host dipolar couplings. The extent of local-scale lateral motion is again correlated with the sample filling, and is a weak function of temperature, as expected from a case in which highly restricted single-file diffusion should dominate the mobility. Characteristic effects of sample aging are apparent in all our experiments.     

甲氨蝶呤柱撑水滑石超分子结构层柱材料的插层组装

用共沉淀和离子交换法将抗肿瘤类药物甲氨蝶呤(MTX)插层组装到水滑石层间,制备了一种新型的结晶度高、晶相单一且MTX在层间有序排列的超分子结构的药物-无机复合层柱材料.用XRD、原子光谱、元素分析、FT/IR、SEM及TG-DSC分析表征了超分子结构层柱材料的结构,并给出其结构模型.     

Molecular simulation of hydrophobin adsorption at an oil-water interface

Hydrophobins are small, amphiphilic proteins expressed by strains of filamentous fungi. They fulfill a number of biological functions, often related to adsorption at hydrophobic interfaces, and have been investigated for a number of applications in materials science and biotechnology. In order to understand the biological function and applications of these proteins, a microscopic picture of the adsorption of these proteins at interfaces is needed. Using molecular dynamics simulations with a chemically detailed coarse-grained potential, the behavior of typical hydrophobins at the water-octane interface is studied. Calculation of the interfacial adsorption strengths indicates that the adsorption is essentially irreversible, with adsorption strengths of the order of 100 k(B)T (comparable to values determined for synthetic nanoparticles but significantly larger than small molecule surfactants and biomolecules). The protein structure at the interface is unchanged at the interface, which is consistent with the biological function of these proteins. Comparison of native proteins with pseudoproteins that consist of uniform particles shows that the surface structure of these proteins has a large effect on the interfacial adsorption strengths, as does the flexibility of the protein.     

Predictive modeling and computational machine learning simulation of adsorption separation using advanced nanocomposite materials

Adsorption process was simulated in this study for removal of Hg and Ni from water using nanocomposite materials. The used nanostructured material for the adsorption study was a combined MOF and layered double hydroxide, which is considered as MOF-LDH in this work. The data were obtained from resources and different machine learning models were trained. We selected three different regression models, including elastic net, decision tree, and Gradient boosting, to make regression on the small data set with two inputs and two outputs. Inputs are Ion type (Hg or Ni) and initial ion concentration in the feed solution (C₀), and outputs are equilibrium concentration (Ce) and equilibrium capacity of the adsorbent (Qe) in this dataset. After tuning their hyper-parameters, final models were implemented and assessed using different metrics. In terms of the R2-score metric, all models have more than 0.97 for Ce and more than 0.88 for Qe. The Gradient Boosting has an R2-score of 0.994 for Qe. Also, considering RMSE and MAE, Gradient Boosting shows acceptable errors and best models. Finally, the optimal values with the GB model are identical to dataset optimal: (Ion = Ni, C₀ = 250, Ce = 206.0). However, for Qe, it is different and is equal to (Ion = Hg, C0 = 121.12, Ce = 606.15). The results revealed that the developed methods of simulation are of high capacity in prediction of adsorption for removal of heavy metals using nanostructure materials.     

Hydrogen adsorption on Pd- and Ru-doped C60 fullerene at an ambient temperature

Palladium- and ruthenium-doped C(60) fullerene compounds were synthesized by incipient wetness impregnation of C(60) fullerene with the corresponding metal acetylacetonate precursors. Transmission electron microscopy (TEM) imaging of the metal-doped C(60) fullerene samples showed different dispersion morphologies of palladium and ruthenium particles on the C(60) matrix. Raman spectra revealed a drastic decrease in peak intensity followed by disappearance of several bands indicating the distortion of the C(60) cage structure. The amorphous nature of the C(60) fullerene compounds was confirmed by the X-ray diffraction study. Hydrogen adsorption amount of 0.85 wt % and 0. 69 wt % on Pd-C(60) and Ru-C(60), respectively, as compared to 0.3 wt % on the pure C(60) fullerene were measured at 300 bar and 298 K. The enhancement in the hydrogen uptakes can be attributed to several factors, including adsorption of molecular H(2) on the defect sites, metallic hydride formation, spillover of hydrogen, and bond formation with atomic hydrogen with different active sites of carbon of host fullerene. The hydrogen adsorption isotherms are of type III and can be correlated by the Freundlich (for Ru-C(60)) and modified Oswin equations (for Pd-C(60) and pristine C(60)).     

Hydrogen adsorption on microporous materials at ambient temperatures and pressures up to 50 MPa

High surface area microporous adsorbents are often proposed as potential hydrogen storage materials, although typically at 77?K and less than 5?MPa. In this study, we focus on conditions more suitable for automotive applications by investigating the storage capacities of microporous materials at 298?K and at pressures up to 50?MPa. In an effort to derive trends within and across material classes, we examined a wide range of materials with varying microstructures including the activated carbons AX-21, KUA-5, and MSC-30; a zeolite templated carbon; a hypercrosslinked polymer; and the Metal Organic Frameworks MOF-177, IRMOF-20, MIL-53, ZIF-8, and Cu₃(btc)₂. The peak excess adsorption of these materials ranged from 0.8–1.8?wt.%, although many did not reach their maximum capacity even at high pressures. However, the total volumetric storage gains over compressed hydrogen gas were quite low and, in many cases, negative. In addressing ambient temperature adsorption at significantly higher pressures than previously reported, our data confirms and extends the range of validity of several existing DFT calculations. Furthermore, our data suggest that, for both activated carbons and MOFs, factors other than specific surface area govern ambient temperature adsorption capacity. Contrary to some reports, the high fractions of sub-nanometer pores in some of the investigated MOFs did not appear to enhance the excess adsorption even at high pressures. For on-board applications with ambient temperature storage, significant enhancements to the attractive force at the materials’ surface are required, beyond merely increasing specific surface area, or for MOFs, tuning of pore sizes.     

Unusual chain length dependent adsorption of linear and branched alkanes on UiO-66

In this work, the zero coverage adsorption properties of C₅–C₁₀ n- and iso-alkanes on the UiO-66, UiO-66-Me and UiO-66-NO₂ metal–organic frameworks are studied by gas phase pulse chromatography. Analysis of enthalpy values, entropy values, Gibbs free energies and Henry constants reveals unusual chain length dependent adsorption behaviour of linear and branched alkanes, caused by the complex structure of the zirconium metal–organic framework UiO-66. The UiO-66 structure consists of a small, tetrahedral and large, octahedral cage. It is shown that at specific carbon chain lengths (e.g. C₆–C₇ for n-alkanes), distinctive jumps in adsorption enthalpy, entropy values and Henry constants occur. This chain length dependent effect is even more pronounced for 2- and 3-methyl alkanes and double branched alkanes. This distinctive shift in adsorption behaviour occurs at a molecular size that coincides with the cavity dimensions of the smallest, tetrahedral cage. The resulting selective adsorption arises from confinement effects and is function of both the molecular shape and size.     

Molecular simulation of adsorption and intrusion in nanopores

This paper reports Monte Carlo simulations of the adsorption or intrusion in cylindrical silica nanopores. All the pores are opened at both ends towards an external bulk reservoir, so that they mimic real materials for which the confined fluid is always in contact with the external phase. This realistic model allows us to discuss the nature of the filling and emptying mechanisms. The adsorption corresponds to the metastable nucleation of the liquid phase, starting from a partially filled pore (a molecular thick film adsorbed at the pore surface). On the other hand, the desorption occurs through the displacement at equilibrium of a gas/liquid hemispherical interface (concave meniscus) along the pore axis. The intrusion of the non-wetting fluid proceeds through the invasion in the pore of the liquid/gas interface (convex meniscus), while the extrusion consists of the nucleation of the gas phase within the pore. In the case of adsorption, our simulation data are used to discuss the validity of the modified Kelvin equation (which is corrected for both the film adsorbed at the pore surface and the curvature effect on the gas/liquid surface tension).     

Excess enthalpies of binary mixtures of 1-hexene with some branched alkanes at the temperature 298.15 K

Measurements of excess molar enthalpies at the temperature 298.15 K in a flow microcalorimeter are reported for the five binary mixtures formed by mixing 1-hexene with the branched alkanes: 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, 2,3-dimethylbutane, and 2,2,4-trimethylpentane. Smooth Redlich–Kister representations of the results are described. It was found that the Liebermann–Fried model also provided good representations of the results.     

Molecular simulation study on adsorption of methanol/water mixtures in mesoporous silicas modified pore surface silylation

Two types of molecular simulation techniques have been utilized to investigate adsorption of methanol/water mixtures in a mesoporous silica with a hydrophobic pore surface: the NVT-ensemble Molecular Dynamics method with the melt-quench algorithm for modeling a fully-silylated mesoporous silica and the μVT-ensemble Orientaional-Biased Monte Carlo method for calculating adsorption isotherms. Adsorption isotherms of methanol and water at 333 K are calculated for an equi-relative-pressure mixture (each component has the same relative pressure which is defined as the ratio of the partial pressure to the saturation pressure of the pure gas) together with pure gases. In the case of the pure gas, water hardly adsorb even at elevated pressures, while the adsorption isotherm for methanol shows the condensable adsorption. On the other hand, in the case of the mixture, water molecules are substantially adsorbed along with methanol molecules, showing an isotherm representing the condensation mechanism. In addition, it is found that the separation factor of methanol to water is the highest in the case of monolayer adsorption from a liquid mixture.     

Capillary phase transitions of linear and branched alkanes in carbon nanotubes from molecular simulation

Capillary phase transitions of linear (from C(1) to C(12)) and branched (C(5) isomers) alkanes in single-walled carbon nanotubes have been investigated using the gauge-cell Monte Carlo simulation. The isotherm at a supercritical temperature increases monotonically with chemical potential and coincides with that from the traditional grand canonical Monte Carlo simulation, whereas the isotherm at a subcritical temperature exhibits a sigmoid van der Waals loop including stable, metastable, and unstable regions. Along this loop, the coexisting phases are determined using an Maxwell equal-area construction. A generic confinement effect is found that reduces the saturation chemical potential, lowers the critical temperature, increases the critical density, and shrinks the phase envelope. The effect is greater in a smaller diameter nanotube and is greater in a nanotube than in a nanoslit.     

Molecular dynamics simulation of planar elongational flow at constant pressure and constant temperature

Molecular dynamics simulations of liquid systems under planar elongational flow have mainly been performed in the NVT ensemble. However, in most material processing techniques and common experimental settings, at least one surface of the fluid is kept in contact with the atmosphere, thus maintaining the sample in the NpT ensemble. For this reason, an implementation of the Nose-Hoover integral-feedback mechanism for constant pressure is presented, implemented via the SLLOD algorithm for elongational flow. The authors test their procedure for an atomic liquid and compare the viscosity obtained with that in the NVT ensemble. The scheme is easy to implement, self-starting and reliable, and can be a useful tool for the simulation of more complex liquid systems, such as polymer melts and solutions.