Water vapor/propylene sorption and diffusion behavior in PVA-P(AA-AMPS) blend membranes by GCMC and MD simulation

Detailed atomistic structures of blend membranes (poly vinyl alcohol (PVA)/(acrylic acid-co-2-acrylamido-2-methylpropylsulfonic acid) (P(AA-AMPS)) were constructed to investigate the sorption and diffusion behavior of gas molecules (water and propylene) in the membranes. Interaction and miscibility between PVA and P(AA-AMPS) were calculated, and it was found that strong intermolecular interaction resulted in good miscibility of PVA and P(AA-AMPS) in the blend. The polymer chains mobility and free volume properties of the blend membranes were characterized. The sorption quantities and sorption sites of water and propylene in the blend membranes were calculated using Grand Canonical Monte Carlo (GCMC) method. The diffusion coefficients of water in the blend membranes were calculated by molecular dynamics (MD) simulation. The simulated results of the membrane structure (chain mobility, free volume properties), the sorption quantities and diffusion coefficients of water/propylene in the blend membranes showed the identical changing trends as the experimental results. Hopefully, this study could offer qualitative insight into the mass transport phenomena within the blend membranes.     

CO2/CH4在活性炭上吸附与分离的分子模拟

Monte Carlo 模拟(GCMC)研究了CO2和CH4在活性炭中的吸附情况.氢气和二氧化碳分子被模拟作Kennard-Jones球体,活性炭用一个裂缝气孔模型代替.研究了压力、温度、孔径活性炭对CO2(以CH4为基准)的选择性的影响.结果表明,低压、低温、孔径越小,选择性越大,即对分离CO2和CH4有利;233 K、孔径为2.29 nm以及低压下选择性达到10,此时分离效果较好.     

Molecular simulation for adsorption of chlorinated hydrocarbon in zeolites

Equilibrium and isosteric heat of adsorption for the system of chloroform and USY-type zeolite were studied. The USY-type zeolite (PQ Co., SiO₂/Al₂O₃=70) was used both as a pure crystalline powder and as granulated particles with binder. Chloroform was reagent grade. The adsorption equilibria were measured using a gravimetric method and were expressed as isotherms. A chromatographic method (i.e. pulse response of chloroform through the USY column with helium carrier) was used to get the initial slope of the isotherms. In the simulation, the GCMC method was used to calculate amounts adsorbed for various conditions. FF parameters were confidently applied. And modified structure model was effective for simulation. This paper was presented at The 5th International Symposium on Separation Technology-Korea and Japan held at Seoul between August 19 and 21, 1999.     

Study of molecular shape and non-ideality effects on mixture adsorption isotherms of small molecules in carbon nanotubes: A grand canonical Monte Carlo simulation study

The sorption isotherms for binary mixtures of methane, ethane, propane and tetrafluoromethane have been determined in carbon nanotubes using configurational bias Monte Carlo simulation techniques. At high loadings, a curious maximum for equimolar gas-phase mixtures occurs with increasing pressure in the absolute adsorption isotherm of one or both adsorbing species. It was detected that there exist two fundamentally different reasons for this maximum. First, due to a higher packing efficiency, one component is able to displace the other component at high loadings. Here, it must be stressed that the displaced component is not necessarily the larger molecule. Second, non-ideality effects of the bulk gas phase can be made responsible for this maximum. The acceptance probability of a molecule insertion in a grand canonical Monte Carlo step is proportional to the component fugacity. If, owing to non-ideality effects of the gas phase, the fugacity of one component does not increase as steeply with pressure as the other component, a maximum can occur in the absolute adsorption isotherm of this component. These findings were demonstrated for various binary mixtures of CH₄, CF₄, C₂H₆ and C₃H₈.     

Kinetics,equilibrium and thermodynamics studies of arsenate adsorption from aqueous solutions onto iron hydroxide

This paper reports the equilibrium, kinetics and thermodynamic studies of arsenate adsorption onto freshly precipitated iron hydroxide. Adsorption of arsenate onto iron hydroxide depends on arsenate concentration, contact time and temperature. The intrapartical diffusion model indicates that both the film and intrapartical diffusion control arsenate adsorption on iron hydroxide. The values of activation energies (E_a) indicate the chemical nature of adsorption accompanied by diffusion controlled processes as the rate limiting step. The endothermic nature of the adsorption process suggests that adsorption reaction consumes energy. The negative values of ΔS^# can be assigned to decreased randomness at the solid liquid interface.     

Effect of the distribution of sorption sites on transport diffusivities: A contribution to the transport of medium-weight-molecules in polymeric materials

The detailed transports of both small and large molecules in heterogeneous media including either random disorder or periodic obstacles are known to decrease the value of macroscopic diffusion coefficients. This work proposes to analyze the successive displacements of medium-sized molecules in polymer materials according to the dispersion and topology of sorption sites from a modified application of the transition state theory. In absence of available information on the dispersion of rate constants between sorption macrosites for such molecules, their transport mechanisms at molecular scale is related to their sorption properties, which are more likely to be available. Simulations by kinetic Monte Carlo (KMC) techniques are presented for different distributions of occupancy values randomly allocated in space or distributed in self-similar clusters. Network structures are generated from the equilibrium occupancy on the basis of transition-state theory formulation on 2D lattice approximations. Different reconstruction strategies on 2D hexagonal lattices are examined regarding maximum likelihood principles including maximization of obstruction effects and minimization of either local or global variance of conductances between sorption sites. Effects of short time scales are assessed by comparing results obtained with networks verifying reversible and non-reversible random walks.     

Number of isolated aluminum atoms in the framework of dealuminated acid zeolites

Monte Carlo calculations restricted by Loewenstein rule predict correctly the variation of the intensities of the lines contributing to the²⁹Si MAS NMR spectra in dealuminated acid ZSM5, mordenites and Y zeolites covering a domain of ratios silicon/ framework aluminum between 4 and 80. They are also used to calculate the numbers of isolated framework aluminum and the relationship with the Brønsted acidity is briefly discussed.     

活性炭吸附甲烷和甲苯的分子模拟研究

以石墨片微元构建的多孔碳材料作为活性炭的结构模型,采用巨正则蒙特卡罗方法(GCMC)和分子动力学方法(MD),从分子层面研究甲烷和甲苯在活性炭中的吸附和扩散特性. 结果表明,石墨片微元大小对多孔碳材料吸附甲烷和甲苯有一定影响,37个碳环构成的多孔碳材料是最佳的吸附结构;甲烷气体在活性炭材料中扩散较快,甲苯在活性炭中扩散较慢,随碳环碳原子数增加,气体在多孔碳材料中的自扩散系数逐渐增大;引入基团会使最优密度向高密度方向偏移,用不同基团表面改性的吸附量顺序为羟基>氨基>羧基>未改性,基团引入会改善材料的孔结构,有利于吸附量的增加.     

CO_2/CH_4在活性炭上吸附与分离的分子模拟

Monte Carlo模拟（GCMC）研究了CO2和CH4在活性炭中的吸附情况。氢气和二氧化碳分子被模拟作Ken-nard-Jones球体,活性炭用一个裂缝气孔模型代替。研究了压力、温度、孔径活性炭对CO2（以CH4为基准）的选择性的影响。结果表明,低压、低温、孔径越小,选择性越大,即对分离CO2和CH4有利;233 K、孔径为2.29 nm以及低压下选择性达到10,此时分离效果较好。     

方阱流体汽液共存性质的Monte Carlo模拟

本文应用Ｇｉｂｂｓ系综ＭｏｎｔｅＣａｒｌｏ方法，对纯方阱流体的汽液共存性质进行了模拟计算。得到汽液两相的饱和密度，并根据模拟数据外推以估测临界点。由此得到的对比临界温度和密度分别为Ｔｃ＝ＲＴｃ／ε＝１．２７，ρｃ＝ρｃσ３＝０．３１，与Ａｌｄｅｒ等人的分子动力学模拟结果吻合。此外，还计算了六个温度下两相的构型能、压力和径向分布函数。     

甲苯分子在铝基金属-有机骨架材料上的吸附特性

应用巨正则系综蒙特卡罗模拟（GCMC）方法研究了298K下甲苯在4种铝基金属-有机骨架材料（Al-MOF）上的吸附特征。通过对比模拟与实验结果,发现Dreiding力场能够较好地描述甲苯在Al-MOF上的吸附行为。由于4种Al-MOF材料均具有丰富的微孔结构,使得其甲苯吸附量（取1.5kPa时吸附量值比较）均高于传统吸附剂的吸附量（一般小于400mg/g）,分别为1375mg/g （MIL-101）、866mg/g（CYCU-3）、807mg/g（MOF-519）、504mg/g（CAU-3-BDC）。通过Snapshot图和密度分布曲线分析了甲苯在Al-MOF中的吸附位,结果表明,在低压（低负载量）时,甲苯优先吸附在骨架中的小孔道内,随压力（负载量）的增大,甲苯分子逐渐填充到大孔道。此外,甲苯与有机配体的相互作用强于与金属簇的相互作用,说明有机配体处为优先吸附位。将298K、1.5kPa条件下甲苯吸附量与MOF的物理性质相关联,结果发现甲苯吸附量的大小与吸附剂的比表面积、孔容积成线性正相关,而与骨架密度成反比关系。     

The effects of diffusion mechanism and void structure on transport rates and tortuosity factors in complex porous structures

Tracer diffusion simulations within random porous structures show that tortuosity factors are independent of diffusion mechanism for all practical void fractions when an equivalent Knudsen diffusivity is correctly defined. Previous studies concluded that tortuosity factors, a geometric property of the void space as defined, increase with increasing Knudsen number, K_n, a measure of the relative number of molecule-surface and intermolecular collisions. The model porous structures in this study consist of random-loose packings of spheres overlapped to achieve a given void fraction and to accurately reflect the void space in practical porous solids. Effective diffusivities were estimated using tracer or flux-based Monte Carlo methods for Knudsen numbers of 10⁻³-10¹⁰; the two methods lead to similar diffusivities for void fractions of 0.06-0.42. Tortuosity factors estimated using the number-averaged distance between collisions, 〈l_p〉, for the characteristic void length scale increased with increasing Knudsen number, even though simulations in infinite cylinders confirmed the accuracy of the Bosanquet equation for all values of K_n. These unexpected changes in a geometric property of the void space become most apparent near the percolation void fraction (∼0.04). For example, the Knudsen tortuosity factor defined in this manner is 1.8 times larger than in the bulk regime for a solid with 0.10 void fraction. Even at high void fractions (∼0.42), the two extreme values of tortuosity factor differ by a factor of ∼1.4. These apparent effects of diffusion mechanism on tortuosities reflect the inaccurate use of number-averaged chord lengths when tracer reflections from random obstacles obey the Knudsen cosine law for diffuse reflection. A corrected length scale, first proposed by Derjaguin, leads to tortuosity factors independent of K_n for void fractions above 0.20; tortuosities differ by only 18% and 4% between Knudsen and bulk regimes even for void fractions of 0.10 and 0.15, respectively. The residual differences at void fractions below 0.10 arise from the increasingly serial nature of the remaining voids. Thus, a long-standing inconsistency between the defined geometric nature of tortuosity factors and their inexplicable dependence on diffusion mechanism is essentially resolved. In practice, these simulations allow the consistent and accurate use of tortuosity factors determined at any value of K_n for all diffusion regimes; they also prescribe, rigorously for void fractions above 0.15 and empirically for lower void fractions, the length scale relevant to diffusion in the Knudsen and transition diffusion regimes.     

Vapor- and liquid-phase adsorption of alcohol and water in silicalite-1 synthesized in fluoride media

In this work, batch-adsorption experiments and molecular simulations are employed to probe the adsorption of binary mixtures containing ethanol or a linear alkane-1,n-diol solvated in water or ethanol onto silicalite-1. Since the batch-adsorption experiments require an additional relationship to determine the amount of solute (and solvent) adsorbed, as only the bulk liquid reservoir can be probed directly, molecular simulations are used to provide a relationship between solute and solvent adsorption for input to the experimental bulk measurements. The combination of bulk experimental measurements and simulated solute–solvent relationship yields solvent and solute loadings that are self-consistent with simulation alone, and allow for an assessment of the various assumptions made in the literature. At low solution concentrations, the solute loading calculated is independent of the assumption made. At high concentrations, a negligent choice of assumption can lead to systematic overestimation or underestimation of calculated solute loading.     

页岩中黏土矿物吸附特性分子模拟

页岩的吸附解吸特性对页岩气资源开发具有重要意义。为深入了解页岩中黏土矿物微观吸附机理, 利用Material Studio 分子模拟软件构建了伊利石、蒙脱石和高岭石3种黏土矿物分子模型, 采用巨正则Monte Carlo(GCMC)方法对3种模型的等温吸附量和吸附热进行了模拟计算。研究表明, 在相同温度和压力条件下3种黏土矿物对CH₄分子的吸附量大小顺序是伊利石>蒙脱石>高岭石;随压力增大3种黏土矿物对CH₄分子的吸附量均有所增加, 而且伊利石和蒙脱石对CH₄分子的吸附量对压力变化更为敏感;3种黏土矿物的等量吸附热均小于42 kJ·mol^-1, 对CH₄的吸附为物理吸附;随着温度的升高, CH₄分子的吸附热和吸附量均减小。     

非对称两嵌段高分子在选择性壁面识别吸附的Monte Carlo模拟

采用自由空间的Monte Carlo（MC）方法研究了两嵌段非对称高分子在选择性壁面上的吸附与识别。研究表明;非对称嵌段高分子同对称嵌段高分子一样;在选择性壁面的吸附可分为吸附和识别两个过程。对高分子构型研究的结果发现;无论条纹宽度如何变化;高分子在垂直于z方向的均方回转半径总是小于其垂直另两方向的值。在条纹宽度一定的情况下;尾式构型比率随链节B与白色条纹壁面作用能Ψ ^B-white增加先增加然后趋于不变。在Ψ ^B-white不变的情况下;尾式构型比率则随条纹数的增加而增加。环式构型比率在条纹较宽时会随Ψ ^B-white有一小的下降趋势;而在条纹较窄时下降幅度较大。     

Effects of Alloying Pd and Au on the Hydrogenation of Ethylene: An ab initio-Based Dynamic Monte Carlo Study

An ab initio-based dynamic Monte Carlo simulation was developed and used to examine the kinetics of ethylene hydrogenation over Pd and PdAu alloys. The intrinsic activation barriers, overall reaction energies and chemisorption energies were calculated from first-principles density functional theoretical calculations. Lateral interactions were modeled by fitting ab initio data to semi-empirical bond order conservation and force field models. The results indicate that the intrinsic activation barriers for ethylene hydrogenation were considerably reduced from 15 to 7-8 kcal/mol due to the intermolecular interactions that take place on the surface at higher coverages. At higher temperatures or lower partial pressures of hydrogen, ethylene decomposition paths to the formation of ethylidyne become important. Alloying the surface with Au influences the intrinsic kinetics for hydrogenation by reducing the activation barrier for hydrogenation but increasing the barriers for H₂ dissociation and ethylidyne formation. This is primarily due to geometric effects that result from alloying. Electronic effects, while present, are significantly smaller. Despite its influence on specific elementary steps, Au appears to have little effect on the calculated turnover frequencies for ethane formation. There are relatively minor increases in the activation barrier from 7.0 to 7.2 to 8.0 as we move from Pd(111) to Pd 87.5% Au 12.5% to Pd 66.7% Au 33.3% respectively. The qualitative effects of Au as well as the quantitative apparent activation barriers reported here are consistent with known experimental results. Au reduces the binding energy of ethylene, which increases the surface hydrogenation activity. However, Au also reduces the number of sites that can activate hydrogen. This reduces the hydrogen surface coverage and subsequently decreases the rate of ethylene hydrogenation. These effects (the weaker metal--adsorbate bonds and the decreased hydrogen surface coverage) balance each other out whereby the addition of Au shows little effect on the simulated turnover frequency on a per Pd atom basis. The primary influence of Au therefore is to decrease the ethylene decomposition paths that lead to ethylidyne and CH_x products.     

Monte Carlo simulation on the adsorption properties of carbon tetrachloride, neopentane, and cyclohexane in MCM-41

The adsorption properties of carbon tetrachloride, neopentane, and cyclohexane in MCM-41 with heterogeneous and cylindrical pores have been studied by using grand canonical ensemble Monte Carlo simulation. The adsorption isotherm, average potential of adsorbate, isosteric heat of adsorption, and number density of molecules in MCM-41 were calculated. The simulated isotherms were compared with experimental ones. Also, different adsorption behaviors in MCM-41 with pore diameter of 2.2 and 3.2 nm were discussed. The capillary-condensation pressure increased for a given adsorbate with an increase in pore diameter. The average densities of carbon tetrachloride, neopentane, and cyclohexane in the two different pores above the capillary-condensation pressure were smaller than the corresponding liquid densities by about 12%. The adsorbate molecules did not form the multilayer in pore below the capillary-condensation pressure. The number of adsorption layers of molecules was constant in a given pore for the three adsorbates above the capillary-condensation pressure. Carbon tetrachloride molecules in pore were also ordered along the pore axis.     

Kinetic Monte Carlo modeling of CO desorption and adsorption on Pd(110) surface

Desorption and adsorption of carbon monoxide on Pd(110) is modeled and simulated, aiming at gaining atomic level understanding of experimentally observed rates. The model parameters are fitted to reproduce the temperature programmed desorption spectra and molecular beam surface scattering data. Desorption turns out to be best described as thermally activated, the activation energy depending on the detailed nearest neighbor site occupation configuration. For a good fit, the adsorption induced surface reconstruction needs to be included in the model. Also, desorption needs to be modeled with a precursor state included. However, surface diffusion was not found to be essential. With these ingredients the coverage dependent sticking coefficient can be successfully simulated in the temperature range from 300 to 500 K. Furthermore, the experimentally observed saturation coverage—temperature dependence is correctly predicted from the balance between simultaneous adsorption and desorption.