Guest Diffusion in Binderless High‐Performance NaX Molecular Sieves

The pulsed field gradient (PFG) NMR technique is applied for exploring the diffusion behavior of guest molecules in binderless NaX beads in comparison with the zeolite powder employed for their production. With both probe molecules applied (water, n‐hexane), the diffusivities in the powder and the beads are found to essentially coincide as long as the diffusion path lengths are sufficiently small in comparison with the extension of the individual particles (crystallites) of the powder. With increasing diffusion path lengths, characteristic deviations become observable that can be attributed to the differences in long‐range mass transfer through the intercrystalline void volume of the bed of crystallites and within the individual beads of the binderless molecular sieve, respectively. With these studies, PFG NMR demonstrates its potentials for simultaneously recording mass transfer phenomena in both the micro‐ and macropores of commercially produced binderless molecular sieves.     

Kinetic Monte Carlo Simulations of the Loading Dependence of Diffusion in Zeolites

Kinetic Monte Carlo (KMC) simulations are used to model diffusion of molecules in zeolites. A variety of loading dependences of the Maxwell‐Stefan diffusivity, ?_i, can be realized by allowing the jump frequencies, ν, of molecules to be influenced by the presence of neighboring molecules. Neighboring molecules are assumed to reduce or increase the activation energy for diffusion by δE and the jump frequencies ν are altered by a factor f = exp(δE/RT) for each neighboring molecule. By appropriate choice of ν and f the loading dependence of ?_i can be made to match those obtained from either Molecular Dynamics (MD) simulations or experiment. The major advantage of the KMC simulation strategy is that considerably less computer power is required than for the corresponding MD simulations. Furthermore, KMC simulations allow mixture diffusion to be probed without additional parameter tuning. The KMC simulations also validate the applicability of the quasi‐chemical theory of Reed and Ehrlich (Surf. Sci. 1981 , 105, 603–628) for describing the loading dependence of ?_i.     

A molecular simulations study of the miscibility in binary mixtures of polymers and low molecular weight molecules

The miscibility behavior of binary mixtures of polymeric and low molecular weight molecules was studied using a combination of modified Flory-Huggins theory and molecular simulation techniques. Three different atomistic approaches were used to investigate the phase behavior and χ parameters of binary mixtures consisting of polymethyl methacrylate (PMMA) and 4-n-pentyl-4′-cyanobiphenyl (5CB). Binary mixtures of methyl methacrylate monomer/5CB and methyl methacrylate oligomer/5CB were also studied. As a first approach, a fast method that calculates the local interaction between a fragment of the polymer and the organic molecule and then extends it to determine the energy of mixing using an estimated coordination number was used. By using modified coordination numbers, we were able to extend this method to include cases where the polymer segment and the small molecules are slightly dissimilar in size. More detailed studies which take into account bulk effects were also carried out where the cohesive energies of the pure compounds were derived from molecular dynamics simulations and the interaction parameters were determined from the differences in the cohesive energies. The concentration and temperature dependence of the χ parameters was evaluated by calculating the energy of mixing from the differences in the cohesive energy densities of the mixed and demixed systems. The present study provides a detailed understanding of the miscibility of PMMA and 5CB as PMMA polymerizes from its monomer, and the results indicate that although methyl methacrylate and 5CB are completely miscible, 5CB is not miscible in PMMA even in small quantities.     

Loading Dependence of Self‐Diffusivities of Gases in Zeolites

Experimental data on the self‐diffusivities, D_i,self, of a variety of gases (CH₄, N₂, Kr, C₂H₆, and C₃H₈) in three different zeolites, LTA, FAU, and MFI, show different dependences on the molar loading, q_i. In LTA, D_i,self appears to increase with q_i for all molecules except N₂. In FAU and in MFI the D_i,self shows a sharp decrease with increasing q_i. In order to gain insights into the causes behind the loading dependences, molecular dynamics (MD) simulations were carried out to determine the self‐diffusivities of seven gases (CH₄, N₂, Kr, C₂H₆, C₃H₈, Ar, and Ne) in six different all‐silica zeolite structures (MFI, AFI, FAU, CHA, DDR, and LTA). The simulation results show that the variation of D_i,self with q_i is determined by a variety of factors that include molecular size and shape, and degree of confinement within the zeolite. For one‐dimensional channels (AFI) and intersecting channel structures (MFI), the D_i,self invariably decreases with increasing q_i. For zeolite structures that consist of cages separated by windows (FAU, CHA, DDR, LTA), the size of the windows is an important determinant. When the windows are wide (FAU), the D_i,self decreases with q_i for all molecules. If the windows are narrow (CHA, DDR and LTA), the D_i,self often exhibits a sharp increase with q_i, reaches a maximum and reduces to near‐zero values at saturation. The sharpness with which D_i,self increases with q_i, is dictated by the degree of confinement at the window. Weakly confined molecules, such as Ne, do not exhibit an increase of D_i,self with q_i.     

Diffusion of low‐molecular‐weight permeants through semi‐crystalline polymers: combining molecular dynamics with semi‐empirical models

A molecular dynamics‐based computational approach was used to study the diffusion of oxygen through a model semi‐crystalline polymer, namely linear low‐density polyethylene. The simulated molecules were validated by comparing the predicted properties with experimental values of available free volumes, on atomic scale, using positron annihilation lifetime spectroscopy and measured values of density. The semi‐crystalline polymer was considered as a composite network of a continuous amorphous phase and a dispersed crystalline phase. Based on this observation, the overall diffusion was simulated, including the diffusion through the crystalline phase, which has not been previously reported. A tight correlation was then achieved between experimental and simulated data by utilising several semi‐empirical and analytical models developed for composite materials. The proposed methodology in this work can be effectively used as a basis for designing polymer networks with controlled diffusion characteristics in a bottom‐up approach. © 2018 Society of Chemical Industry     

Xylene Isomerization in the Liquid Phase Using Large‐Pore Zeolites

Three large‐pore zeolites, Beta with Si/Al ratios of 25 and 35 and Mordenite with an Si/Al ratio of 30, were studied in the conversion of o‐xylene at 493 K. Maximum conversion was achieved by the catalyst with the highest Si/Al ratio due to faster diffusion of the isomer inside the zeolite channels because of the lower acidity of the solid even with larger crystal size. A kinetic study was then carried out over this catalyst between 473 and 513 K in a batch reactor in the liquid phase. The activation energies obtained do not indicate the presence of diffusional constraints towards any isomer. Finally, the kinetic model obtained was simulated in a fixed‐bed reactor and compared to ZSM‐5 in the temperature range from 493 to 533 K. An increment in p‐xylene production of 20 % on average was obtained.     

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.     

Investigation of Channel‐to‐Channel Cross Convection in Serpentine Flow Fields

Channel‐to‐channel cross convection in serpentine flow fields of polymer electrolyte fuel cells (PEFC) can influence the overall cell performance. The effect strongly depends on the gas transport properties of the gas diffusion layer (GDL). For the first time measured anisotropic, compression dependent permeability and effective diffusivity of GDLs are used to quantify the influence of cross convection on the local current distribution and performance. A model was developed to examine different channel‐rib geometries and GDL characteristics. The results show that cross convection can significantly increase the current density and consequently the power density of PEFCs. A strong sensitivity to GDL compression, flow velocity and rib width was found. As an optimised case the GDL thickness under the rib was increased resulting in about 20% higher current densities. Precise knowledge of the GDL characteristics and its compression are key to understand channel‐to‐channel cross convection and optimise perfomance.     

Molecular simulations of gas transport in nitrile rubber and styrene butadiene rubber

Nitrile rubber (NBR, 39:61 wt% of acrylonitrile:butadiene) and styrene butadiene rubber (SBR, 50:50 wt% of styrene:butadiene) matrices have been equilibrated by molecular dynamics (MD) simulations. Transition-state approach is used to calculate the diffusion and solubility coefficients of small penetrants in these matrices, indicating quite low values in NBR and reasonable agreement with experimental results. MD simulations have been performed to analyze water diffusion in these matrices. Aggregation of water molecules is observed in the hydrophobic matrix SBR. MD simulations with fictitious nonpolar water molecules inhibit aggregation and lead to enhanced diffusion in SBR. In NBR there is a slight increase in diffusion for fictitious water molecules. The lower diffusion constants in NBR result from slower local relaxation of the matrix due to tighter intermolecular packing and higher cohesive energy density. The free volume distribution that affects solubility coefficients is not a major determining factor for the diffusion coefficients in these matrices.     

Dynamic Analysis of Diffusion and Adsorption of Water‐Miscible and Water‐Immiscible Organic Vapors in Soil

A dynamic analysis of the diffusion and adsorption of water‐miscible volatile organic compounds (methanol and acetone) and water‐immiscible volatile organic compounds (benzene and toluene) in a soil pellet has been performed experimentally by using the single pellet moment technique. The experiments were conducted in a one‐sided single pellet adsorption cell at a temperature of 30 °C and varying relative humidities (0, 20, and 40 %). The results obtained with dry and wet systems showed that volatile organic tracers were adsorbed reversibly onto the soil. The overall adsorption equilibrium constants of both water‐miscible and water immiscible volatile organic compounds decreased with relative humidity. The sorption of water‐immiscible VOCs (benzene) onto soil was found to be much less than that of water‐miscible VOCs (methanol). The effective diffusivity of water‐immiscible volatile organic vapor (benzene) in the soil did not show a considerable change with relative humidity. In contrast, there was an appreciable change in the effective diffusivity for water‐miscible VOCs (methanol) with moisture.     

Impact of Zeolites on the Petroleum and Petrochemical Industry

The general features of zeolites that led to their widespread use in oil refining and petrochemistry are highlighted as well as the details of their impact on selected processes. The analysis of the catalyst market and the position of zeolites therein is a good indication of their strategic importance. Zeolites have brought many disruptive changes to these fields (e.g. FCC). They impacted also these industries in an equally important way, although more subtle, by incremental improvement of processes. The new and vast challenges facing oil refining and petrochemistry as well as the managed transition to sustainable environmental benign transport fuel industries and chemical industries will require creative science and technologies. Zeolites offer the basis of many of these technological solutions provided efficient and balanced cooperations between industry and academia are further developed.     

In‐Depth Study of Mass Transfer in Nanoporous Materials by Micro‐Imaging

The first application of interference microscopy to monitoring mass transfer in nanoporous materials dates back to late 1970s when Caro and colleagues reported results of investigations of water uptake by LTA type zeolites. It was, however, not before the beginning of the new millennium that the developments in both the measuring technique and computational power have enabled the recording of transient guest profiles during molecular uptake and release under well‐defined conditions, leading to the establishment of a novel access to diffusion studies, now referred to as micro‐imaging. In the present contribution, the thus accessible novel type of information is illustrated by an in‐depth analysis of the uptake kinetics of methanol in an all‐silica ferrierite. In particular, two remarkable experimental findings are reported, which may be tracked back to their microstructural and/or microdynamic origin, namely a pronounced asymmetry in the transient concentration profiles and a slowing down of guest uptake with increasing temperature.     

Study of the Effects of External and Internal Diffusion on the Propane Dehydrogenation Reaction over Pt‐Sn/Al2O3 Catalyst

In the study of a reaction on a heterogeneous catalyst, external and internal mass diffusion play an important role since they can have an inherent affect on the kinetics of the reaction. Therefore, in the study of intrinsic rates of reaction, the effects of external and internal mass diffusion must be eliminated or considered prior to proper kinetic studies. In this study, the effects of external and internal mass diffusion on the propane dehydrogenation reaction over a Pt/Sn catalyst were investigated. Some experiments were performed in a laboratory scale setup and the required data was gathered. The rate of reaction was considered to be first order based on propane. External diffusion was studied using Mears' criterion and internal diffusion was investigated by the Thiele Module and the Internal Effectiveness Factor, based on experimental data.     

Molecular dynamics studies on the structures of polymer electrolyte membranes and diffusion mechanism of protons and small molecules

We performed a series of molecular dynamics (MD) simulations on Nafion^® membranes containing various quantities of H₂O and CH₃OH. The simulations afforded diverse nanoscale phase-separated structures, such as clusters, channels, and cluster–channels. The calculated cluster–channel structure qualitatively agrees with the experimental results of X-ray diffraction studies. We also investigated the diffusion mechanisms for H₂O, protons, CH₃OH, H₂, and O₂ in these membranes. To reproduce the hopping transfer of protons, we employed a semi-classical MD approach using the empirical valence bond method. The estimated diffusion coefficients of H₂O and proton in the membranes significantly depended on the H₂O content, and these values showed qualitatively good agreement with the experimental results. The diffusion coefficient of proton in H₂O-rich membranes was much larger than that of H₂O, and the proton mainly formed H₅O₂⁺ complex. Furthermore, the simulation results indicate that the majority of CH₃OH permeates through the H₂O clusters, and the majority of H₂ and O₂ permeates through the hydrophobic region of the membrane.     

正渗透过程中的分子动力学模拟研究

采用分子动力学模拟方法,利用Materials Studio 6.0软件模拟计算了H_2O、Na~+、Cl~-在正渗透膜内的扩散系数。同时模拟研究了三种不同环境温度、溶液浓度下,三种粒子在正渗透膜内扩散系数的变化规律。实验结论与模拟结果一致,即溶液浓度不变的情况下,扩散系数随着温度的升高而增大;环境温度不变的情况下,扩散系数随着溶液浓度的增大而减小。     

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.     

Molecular dynamics simulation of oxygen diffusion in dry and water-containing poly(vinyl alcohol)

The kinetics and mechanisms of diffusion of oxygen and water in dry and water-containing amorphous syndiotactic poly(vinyl alcohol) were studied at 502 K and normal pressure by molecular dynamics simulation. Penetrant molecule trajectories were obtained in a system with 600 repeating units of poly(vinyl alcohol) and 0, 40 (2.6 wt%) and 80 (5.2 wt%) water molecules. Under dry conditions, oxygen molecules jumped in a cage-like fashion. The oxygen molecule diffused in a liquid-like fashion while water diffusion was cage-like in the system with 5.2 wt% water. The hydrogen bond lifetimes among the water molecules were significantly shorter than those formed between water and the polymer and between different polymer segments. The hydrogen bond lifetimes among all species were, within experimental error, unaffected by the content of water, even though the oxygen diffusivity increased exponentially and the water diffusivity increased to some extent with increasing water content. It seemed that the diffusivity was sensitive primarily to the decrease in concentration of polymer-polymer hydrogen bonds, which followed from the increase in water content. This finding was consonant with the analysis of the oxygen molecule motion relative to the nearest polymer backbone, which revealed that it jumped preferentially along the polymer chain and towards the backbone. This behavior was more pronounced when the dynamics were analyzed over longer distances (5 Å) and it was less pronounced in the water-rich systems. The simulations indicated that water clustering was absent and consequently that water was homogeneously distributed in the polymer systems.     

Molecular dynamics simulation of diffusion of O2 and CO2 in blends of amorphous poly(ethylene terephthalate) and related polyesters

The diffusion of small molecules through polymers is important in many areas of polymer science, such as gas barrier and separation membrane materials, polymeric foams, and in the processing and properties of polymers. Molecular dynamics simulation techniques have been applied to study the diffusion of oxygen and carbon dioxide as small molecule penetrants in models polyester blends of bulk amorphous poly(ethylene terephthalate) and related aromatic polyesters. A bulk amorphous configuration with periodic boundary conditions was generated into a unit cell whose dimensions were determined for each of the simulated polyester blends in the cell having the experimental density. The diffusion coefficients for O₂ and CO₂ were determined via NVE molecular dynamics simulations using the Dreiding 2.21 molecular mechanics force field over a range of temperatures (300, 500 and 600 K) using up to 40 ns simulation time. We have focussed on the influence of the temperature, polymer dynamics, density and free volume distribution on the diffusion properties. Correlation of diffusion coefficients with free volume distribution was found.     

Molecular dynamics simulation of diffusion of O2 and CO2 in amorphous poly(ethylene terephthalate) and related aromatic polyesters

The diffusion of small molecules through polymers is important in many areas of polymer science, such as gas barrier and separation membrane materials, polymeric foams, and in the processing and properties of polymers. Molecular simulation techniques have been applied to study the diffusion of oxygen and dioxide of carbon as small molecule penetrants in models of bulk amorphous poly(ethylene terephthalate) and related aromatic polyesters. A bulk amorphous configuration with periodic boundary conditions is generated into a unit cell whose dimensions are determined for each of the simulated aromatic polyesters in the cell to have the experimental density. The aim for this research is to explore and investigate the diffusion of gases through bulk amorphous poly(ethylene terephthalate) and related aromatic polyesters. The diffusion coefficients for O₂ and CO₂ were determined via NVE molecular dynamics simulations using the Dreiding 2.21 molecular mechanics force field over a range of temperatures (300, 500 and 600 K) using up to 30 ns simulation time. We have focussed on the influence of the temperature, polymer dynamics, number of aromatic rings, ortho-, meta-, para-isomers, density and free volume distribution on the diffusion properties. Correlation of diffusion coefficients with free volume, temperature, number of aromatic rings, ortho-, meta- and para-isomers was found.