A hierarchical approach to the molecular modeling of diffusion and adsorption at nonzero loading in microporous materials

A new hierarchical approach is presented for the modeling of small molecules at nonzero concentrations in microporous materials. This approach is complementary to other methods recently appearing in the literature; it is targeted for systems with pores that are well defined, large enough to host multiple molecules, and energetically uncorrugated in the interior. Statistical mechanical partition functions are calculated on molecular-level models and used as input to coarse-grained models, to predict both adsorption isotherms and self-diffusion coefficients. Certain physically reasonable simplifying approximations are employed to make the partition functions tractable. The approach is demonstrated on the model system of methane in siliceous zeolite ZK4 at , and the results are judged in comparison to those from traditional grand canonical Monte Carlo and molecular dynamics simulations. The adsorption isotherm is predicted to a high degree of accuracy across a large pressure range. The predicted trends in the self-diffusion coefficient are in qualitative agreement with the molecular dynamics results, but there is some quantitative disagreement at the lowest and highest adsorbate loadings.     

Quasi-classical modeling of molecular quantum-dot cellular automata multidriver gates

ABSTRACT: Molecular quantum-dot cellular automata (mQCA) has received considerable attention in nanoscience. Unlike the current-based molecular switches where the digital data is represented by the on/off states of the switches, in mQCA devices, binary information is encoded in charge configuration within molecular redox centers. The mQCA paradigm allows high device density and ultra-low power consumption. Digital mQCA gates are the building blocks of circuits in this paradigm. Design and analysis of these gates require quantum chemical calculations, which are demanding in computer time and memory. Therefore, developing simple models to probe mQCA gates is of paramount importance. We derive a semi-classical model to study the steady-state output polarization of mQCA multidriver gates, directly from the two-state approximation in electron transfer theory. The accuracy and validity of this model are analyzed using full quantum chemistry calculations. A complete set of logic gates, including inverters and minority voters, are implemented to provide an appropriate test bench in the two-dot mQCA regime. We also briefly discuss how the QCADesigner tool could find its application in simulation of mQCA devices.     

Adsorption of propane and propylene onto carbon molecular sieve

Equilibrium data for propane and propylene adsorption on a carbon molecular sieve (CMS) 4A from Takeda are presented in the temperature range 343-423 K and 0-300 kPa pressure. The pellet adsorption loading is 0.9 mol/kg for propane and 1.2 mol/kg for propylene at 100 kPa and 373 K. The equilibrium selectivity for propylene in the low-pressure range are 2.3 (343 K) and 1.7 (423 K). Experimental data were fitted with the Toth and Dubinin models. Zero length column (ZLC) technique has been used to determine the controlling mechanism and estimate the diffusivity parameters. Transport of both hydrocarbons in the pellets is controlled by micropore diffusion. Breakthrough curves were measured in the same temperature range and atmospheric pressure, at the low partial pressure of adsorbate (linear region of the isotherm). Simple models have been used in the simulation of breakthrough curves.     

Adsorption dynamics of carbon dioxide in molecular sieving carbon

The sorption equilibrium isotherm of carbon dioxide at 20 °C on a commercially manufactured carbon molecular sieve has been measured with a variable volume (vacuum to high pressure) volumetric adsorption apparatus. Measurement was taken over the pressure range <10-2000 Torr and the isotherm is characterized by Dubinin-Radushkevich analysis which provides the micropore size distribution. The equilibrium information is subsequently employed to characterize the dynamics of adsorption and it is shown that the uptake of carbon dioxide is Fickian with some deviation from Fickian behavior noted at lower pressures. The derived mobility parameter agrees reasonably well with that predicted by the Darken relation over more than a 200-fold change in pressure.     

多级纳米孔分子筛在非均相催化中的应用现状及发展前景

传统分子筛因其单一的微孔孔道,在工业应用中表现为扩散阻力差、催化易失活,尤其在涉及大分子的反应过程中催化活性较差是阻碍其工业应用的现实难题,通过优化制备路线得到的多级纳米孔分子筛催化材料可有效解决传统分子筛存在的上述应用缺陷。多级纳米孔分子筛相比传统分子筛因其特殊的孔道结构和物化性能,在非均相催化方面具有丰富的催化活性位点、较短的扩散路径、较高的传递效率和较长的催化寿命,特别在涉及非均相催化反应的现代化学工业中展现出重要的潜在应用价值。综述多级纳米孔分子筛在烃类异构化反应、加氢裂化反应、烷基化与酰基化反应、烯烃氧化反应以及甲醇制烃类等反应中的诸多优势及潜在应用。     

Combination of a Monte Carlo approach with the contact time distribution concept for the steady-state modeling of an isothermal heterogeneous coordinated anionic ring opening polymerization reactor

In this paper, the possibility of combining a molecular level Monte Carlo simulation with a chemical engineering model of an isothermal fixed-bed reactor is demonstrated. This approach is applied to the isothermal heterogeneous coordinated anionic ring opening polymerization of ε-caprolactone. The contact time distribution (CTD) concept as defined by Orcutt et al. (Chem. Eng. Prog. Symp. Ser. 38 (58) (1962) 1), Nauman and Collinge (Chem. Eng. Sci. 23 (1968a) 1309) and Shinnar et al. (Chem. Eng. Sci. 27 (1972) 1627) is used. The contact time is the time spent by a reactant within the pores of the pellets constituting the fixed-bed. The classical monodisperse model represents the hydrodynamic and mass transfer in the reactor. The CTD is calculated from this model according to Shinnar et al. theory. From that point, the reactor outlet monomer conversion can be calculated analytically, the ε-caprolactone consumption being a first-order process. Furthermore, the molecular size distribution (MSD) of the oligomers at the reactor outlet is computed on the basis of Monte Carlo simulations. Comparisons with experimental data are given.     

Non-equilibrium molecular dynamics simulation of gas separation in a microporous carbon membrane

We have carried out non-equilibrium molecular dynamics simulations of gas separation in a “selective surface flow” membrane. The gas mixture studied is hydrogen/methane, which is relevant to hydrogen purification in refineries. The simulations give insight into the separation mechanism, which is based on the transport of the more strongly adsorbing species (methane) in a dense layer near the pore wall, with the less strongly adsorbed species (hydrogen) diffusing through a less dense region close to the centre of the pore. Good agreement is obtained with experimental selectivity data. This work is also relevant to the study of the combined effects of adsorption and diffusion in microporous carbon adsorbents.     

Adsorption properties of carbon molecular sieves prepared from an activated carbon by pitch pyrolysis

In this study a heat-treatment process using an activated carbon and coal-tar pitch was developed to prepare carbon molecular sieves (CMSs) for CH₄/CO₂ separation. This process results in a partial blockage of the pores of the activated carbon precursor, so that a reduction in the pore size takes place. Equilibrium CO₂ adsorption measurements at different temperatures, and CO₂ and CH₄ kinetic measurements at different temperatures and feed pressures were carried out using the TEOM technique for a carbon molecular sieve (CMS) prepared by this process (sample CB3) and a commercial CMS (Takeda 3A, sampleT3A). The overall diffusion for CO₂ in sample CB3 was faster than that in T3A and a slightly higher CO₂ adsorption capacity of CB3 was obtained. The transient uptake profiles in both samples at different temperatures and different CO₂ partial pressures were described in some cases by a micropore diffusion model, and in other cases by a dual resistance model. Both equilibrium and kinetic results demonstrate a better CO₂/CH₄ separation performance for the CMS prepared in the present study (CB3) than for the commercial CMS (Takeda 3A), due to the existence of slightly wider pore-mouth openings in sample CB3. This study demonstrates that the process used in this work is an interesting and reproducible approach to prepare CMS for CO₂/CH₄ separation.     

A pore network model for diffusion in nanoporous carbons: Validation by molecular dynamics simulation

A hybrid molecular dynamics simulation/pore network model (MD/PNM) approach is developed for predicting diffusion in nanoporous carbons. This approach is computationally fast, and related to the structure of the real material. The PNM takes into account both the geometrical (a distribution of pore sizes) and topological (the pore network connectivity) characteristics of nanoporous carbons, which are obtained by analysing adsorption data. The effective diffusion coefficient is calculated by taking the transport diffusion coefficients in single slit-shaped model pores from MD simulation and then computing the effective value over the PNM. The reliability of this approach is evaluated by comparing the results of the PNM analysis with a more rigorous, but much slower, simulation applied to a realistic model material, the virtual porous carbon (VPC). We obtain good agreement between the diffusion coefficients for the PNM and the VPC, indicating the reliability of the hybrid MD/PNM method and it can be used in industry for materials design.     

Molecular dynamics simulation of diffusion and permeation of gases in polystyrene

Molecular dynamics simulations are performed to study the diffusion and permeation of gases, including argon, nitrogen, methane, carbon dioxide, and propane, in polystyrene over a wide range of temperatures. A jumping mechanism is observed for the diffusion of diffusants in polymer. The calculated diffusion coefficients agree well with the experimental data and with the results of former simulation studies. The relation between the diffusion coefficient and the molecular diameter is confirmed by the results. Our calculated results on the temperature-dependence of diffusion coefficients show that for some gases a break is seen, at the glass transition temperature, in the Arrhenius plot of ln (D) versus 1/T, while for some other light gases, argon and nitrogen, the plot is linear over the whole temperature range. We have also calculated the permeability coefficients, using the diffusion coefficients calculated in this work and our recently published solubility coefficients [Eslami and Müller-Plathe, Macromolecules 2007; 40:6413]. Our results show that the calculated permeability coefficients are higher than the experimental data by almost the same trend observed in the solubility calculations, but the ratios of calculated permeabilities are in a very good agreement with experiment.     

Sorption kinetics of eight gases on a carbon molecular sieve at elevated pressure

The sorption kinetics of eight different molecules (O₂, N₂, Ar, CO, CO₂, SO₂, CH₄ and H₂) on a carbon molecular sieve was studied over a wide range of pressures up to 15 atm by using a volumetric method. The acentric factor was suggested as a potential factor to estimate the relative sorption rate. Since the apparent time constants of all the components showed much stronger dependence of pressure than those expected by the traditional Darken relation and the structural diffusion model, new models with the diffusion relation at the supercritical condition was proposed to predict the kinetic behaviors in the micropores. The proposed model successfully predicted the apparent time constant up to high pressure. In addition, the semi-empirical model that combined acentric factor with the proposed model was able to predict the strong pressure dependence accurately. However, since the strong adsorbates, CO₂ and SO₂, showed two-stage kinetic behavior with pressure, which was different from that of the other adsorbates. The kinetic behaviors of these molecules could be predicted by using two different sorption models.     

碳材料在多相催化过程中的应用

碳是自然界中存在最为广泛的元素,具有丰富的结构和形态。碳材料不仅可以作为理想的催化剂载体,其本身也表现出对多种催化过程的优异性能。综述了碳材料在化学品制造、环境催化与能源催化等领域的最近研究进展,并总结了碳材料在这些过程中的催化作用机理。     

Adsorption and diffusion of light alkanes on nanoporous faujasite catalysts investigated by molecular dynamics simulations

Molecular dynamics simulations were performed for ethane, propane, and n-butane in siliceous faujasite for different numbers of molecules per unit cell (loadings) at 300 K. Both the adsorbed molecules and the zeolite framework were modeled as flexible entities. A new semiempirical analytical potential function for the systems was constructed. From the mean-square displacement of the molecules, self-diffusion coefficients of 18.7 × 10⁻⁵, 13.3 × 10⁻⁵, and 4.3 × 10⁻⁵ cm²/s were calculated for ethane, propane, and n-butane, respectively at a loading of 8 molecules/unit cell. They compare well with experimental values from pulsed-field gradient NMR measurements (10 × 10⁻⁵, 9 × 10⁻⁵, and 6 × 10⁻⁵ cm²/s, respectively). Besides depending on the size of the hydrocarbon, the heats of adsorption and self-diffusion coefficients also strongly depend on the loading of adsorbate molecules. The results suggest that the new intermolecular force field can reasonably describe the adsorption and diffusion behavior of ethane, propane, and n-butane in faujasite zeolite.     

Reflections on applications of NMR in heterogeneous catalysis and surface chemistry

The use of NMR to characterise heterogeneous catalytic systems and processes is assessed, critically, in overview. The generally wide scope and applicability of NMR is placed in the context of the constraints of NMR experimental requirements and the general goal of studying catalytic systems under conditions relevant to their actual use. In particular, the issues of sensitivity, resolution and dynamic processes in NMR are considered alongside the aims of surface selectivity, sample environment control and the desirability of associated characterisation of the catalytic systems by methods complementary to NMR.     

Investigating the strength of Ti/TiB interfaces at multiple scales using density functional theory,molecular dynamics,and cohesive zone modeling

Titanium/titanium boride (Ti/TiB) composites are interesting technological materials with prospective applications in the aerospace, automotive, and biomedical industries. However, not much has been studied about the failure mechanisms of these composites. This article thoroughly investigates the adhesion and strength of two well-known Ti/TiB interface variants formed during the production of Ti/TiB composites below and above 910 °C, respectively. The studies were carried out using different theoretical methods at multiple scales, including density functional theory (DFT), molecular dynamics (MD), cohesive zone modeling (CZM), and the finite element method (FEM). First, we employed DFT to investigate the interfacial adhesion and strength of the selected planes. Then, MD simulations were utilized to study the misfit dislocation networks and derive interfacial CZMs for FEM modeling and simulation of composites. Our FEM simulations showed that the Ti/TiB interface has sufficient strength to transfer the shear load from Ti to TiB without debonding at room temperature. The results have confirmed the same phenomenon observed in some experimental studies and interpreted this phenomenon from a multiscale point of view. The research findings can be used in quantifying the failure stress of TiB whiskers directly from tension tests on Ti/TiB composites by ruling out the possibility of interface debonding.     

An explanation of increased hydrolysis of the β-(1,4)-glycosidic linkages of grafted cellulose using molecular modeling

Grafting various groups onto cellulose is found to substantially increase acid hydrolysis of the β-(1,4)-glycosidic linkages. Molecular modeling is used to explain how various substituents such as esters and ethers cause this phenomenon. A substituent helps stabilize hydrolyzed cellulose by serving as an anchor to the end of the cleaved cellulose to which it is bonded, making it less mobile, and allowing it to have stronger interactions than those in pure hydrolyzed cellulose. Hydrolysis increases with increasing size of the substituent. Molecules sorbed but not grafted to cellulose do not increase hydrolysis. Hydrolysis mainly occurs at glucoses bonded to the substituent, and supporting experiments show that hydrolysis approaches equilibrium when no substituent remains on the cellulose fiber.     

Non-equilibrium molecular dynamics simulation of electrokinetic effects on heterogeneous ionic transport in nano-channel

The presence of the electric double layer (EDL) near the solid/liquid interface has a great impact on the liquid flowing through nano channels. In this paper, a non-equilibrium molecular dynamics method (NEMD) model is developed to investigate the transport characteristics of the heterogeneous ionic fluid flowing in a 4.672 nm-depth channel due to the electrokinetic effect induced by the EDL. An external perturbing velocity is introduced on the liquid instead of a constant force to study the electrokinetic effects on the flow. The effect of charges adsorbed on channel surfaces on flow resistance is especially considered. Two different charged surfaces with discrete or uniform charge distributions are compared, respectively. Besides Lennard-Jones (L-J) potential energy and Coulomb force, the ion–dipole interaction between the charged particle and the water molecule in the liquid is especially taken into account. According to the simulation results, the liquid density reaches the peak value in the EDL. However, unlike that predicted by the Poisson–Boltzmann theory, the liquid density profile is not exactly an anti-parabola curve, which has a significant fall before the density reaching the peak value in the EDL. It is shown that the charges absorbed on the channel surface have little influence on the liquid density distribution, but they play an important role in the fluid velocity distribution. Meanwhile, the liquid velocity slippage in the EDL emerges, which is well in agreement with the prior experimental data available. Moreover, though the non-dimensional velocities are nearly the same under different external velocity fields, the velocity slippage for the uniformly charged wall is much larger than that for the discretely charged wall. All of these results provide a probable reason why the flow behavior in a nano-channel is particularly different from that in a macro duct.     

Molecular structure of side-chain liquid crystalline polysiloxane in the smectic C phase from X-ray diffraction and molecular modeling

The structure of a side-chain liquid crystalline polysiloxane containing two oligo(oxyethylene) units and (R)-1-methylheptyl 4-(4-hydroxybiphenyl-4′-carbonyloxy) benzoate mesogens (PS221B) has been studied using X-ray diffraction patterns and molecular modeling. Ten periodic simulated cells with equilibrated structures are generated, each containing 35 mesogenic groups and a polysiloxane backbone. The simulated X-ray diffraction patterns are in agreement with those obtained experimentally. The calculated width of the sublayers of the polysiloxane backbone is about 5.63 Å along the z-axis. The distributions of the dihedral angles of the polysiloxane backbone in the equilibrated structures indicate that the backbone has a higher probability distribution in the trans state. The distributions of the dihedral angles at the bonds in the aromatic core of the mesogenic groups reveal high probabilities of the trans and cis placements. The local order due to the interactions between the aromatic cores is in the range of 3-5 Å. The order of the radial distribution function vanishes beyond this range.     

Atomistic molecular dynamics simulation of liquid carbon tetrachloride confined in pillared pore materials

Atomistic molecular dynamics simulations were performed to investigate the dependence of the self-diffusivity of liquid carbon tetrachloride (CCl₄) confined in pillared pore materials on the pore width, porosity and the surface heterogeneity of the solid walls. The simulated results show that the self-diffusivity of liquid CCl₄ does not increase monotonically with the pore width, but in an oscillatory manner to approach the bulk diffusivity. Moreover, the presence of activated sites characterizing the surface heterogeneity and the pillars reduces the self-diffusivity of liquid CCl₄ confined in pillared pores. The effects of these factors on the self-diffusivity of fluids should be taken into account when a porous nanomaterial is designed or chosen for a certain process, in addition to their effects on other properties such as the adsorption capability.