A molecular dynamics investigation of the unusual concentration dependencies of Fick diffusivities in silica mesopores

Molecular Dynamics (MD) simulations were carried out to determine the self-diffusivitiy, D<SUB>i,self</SUB>, the Maxwell-Stefan diffusivity, D<SUB>i</SUB>, and the Fick diffusivity, D<SUB>i</SUB>, for methane (C1), ethane (C2), propane (C3), n-butane (nC4), n-pentane (nC5), n-hexane (nC6), n-heptane (nC7), and cyclohexane (cC6) in cylindrical silica mesopores for a range of pore concentrations. The MD simulations show that zero-loading diffusivity Ð<SUB>i</SUB>(0) is consistently lower, by up to a factor of 20, than the values anticipated by the classical Knudsen formula. The concentration dependence of the Fick diffusivity, D<SUB>i</SUB> is found to be unusually complex, and displays a strong minimum in some cases; this characteristic can be traced to molecular clustering

A rationalization of the type IV loading dependence in the Kärger-Pfeifer classification of self-diffusivities

Kärger and Pfeifer (1987) [1] have listed five different types of dependencies of the self-diffusivities, Di,self, on the loading, Θ<SUB>i</SUB>, of guest molecules in zeolites. Of these five types, the Type IV dependence is particularly intriguing because it displays a maximum in the D<SUB>i,self </SUB>- Θ<SUB>i </SUB>dependence for FAU zeolite. On the basis of published experimental data, and molecular simulations, for a variety of guest-host combinations we present arguments to suggest that the reasons for the curious maximum can be traced to molecular clustering. The clustering can be caused due to hydrogen bonding effects as is the case for methanol diffusion in NaY, or due to the temperatures being lower than the critical temperature, T<SUB>c</SUB>

In silico screening of metal-organic frameworks in separation applications

Porous materials such as metal-organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs) offer considerable potential for separating a variety of mixtures such as those relevant for CO2 capture (CO2/H2, CO2/CH4, CO2/N2), CH4/H2, alkanes/alkenes, and hydrocarbon isomers. There are basically two different separation technologies that can be employed: (1) a pressure swing adsorption (PSA) unit with a fixed bed of adsorbent particles, and (2) a membrane device, wherein the mixture is allowed to permeate through a micro-porous crystalline layer. In view of the vast number of MOFs, and ZIFs that have been synthesized there is a need for a systematic screening of potential candidates for any given separation task. Also of importance is to investigate how MOFs and ZIFs stack up against the more traditional zeolites such as NaX and NaY with regard to their separation characteristics. This perspective highlights the potency of molecular simulations in determining the choice of the best MOF or ZIF for a given separation task. A variety of metrics that quantify the separation performance, such as adsorption selectivity, working capacity, diffusion selectivity, and membrane permeability, are determined from a combination of Configurational-Bias Monte Carlo (CBMC) and Molecular Dynamics (MD) simulations. The practical utility of the suggested screening methodology is demonstrated by comparison with available experimental data

Investigating the relative influences of molecular dimensions and binding energies on diffusivities of guest species inside nanoporous crystalline materials

The primary objective of this article is to investigate the relative influences of molecular dimensions and adsorption binding energies on unary diffusivities of guest species inside nanoporous crystalline materials such as zeolites and Metal–organic Frameworks (MOFs). The investigations are based on Molecular Dynamics (MD) simulations of unary diffusivities, along with Configurational-bias Monte Carlo (CBMC) simulations of the isosteric heats of adsorption (−Qst) of a wide variety of guest molecules (CO2, H2, N2, He, Ne, Ar, Kr, CH4, C2H4, C2H6, C3H6, C3H8 and nC4H10) in 24 different host materials spanning a wide range of pore sizes, topologies, and connectivities. For cage-type materials with narrow windows, in the 3.2-4.2 &#197; size range, separating adjacent cages (e.g. LTA, CHA, DDR and ZIF-8), the diffusivities are primarily dictated by the molecular dimensions, bond lengths, and bond angles. However, for channel structures (e.g. AFI, MFI, MgMOF-74, NiMOF-74, MIL-47, MIL-53 and BTP-COF) and “open” frameworks with large windows separating adjacent cavities (NaY, NaX, CuBTC, IRMOF-1, MOF-177 and MIL-101), the diffusivities of guest species in any given host material are strongly dependent on the binding energies of the guest species that can be quantified by –Qst. The stronger the binding energy, the higher the “sticking tendency”, and the lower the corresponding diffusivity. The insights gained from our study are used to rationalize published experimental data on diffusivities and trans-membrane permeances. The results of our study will be valuable in choosing the right material with the desired diffusion characteristics for a given separation application

A comparison of the CO₂ capture characteristics of zeolites and metal–organic frameworks

Considerable progress has been made in recent years on the development of novel adsorbents for CO₂ capture. Pressure Swing Adsorption (PSA), using a packed bed of adsorbents is one of the leading contenders for use in technological applications. The candidate adsorbents are often structured micro-porous materials such as Metal Organic Frameworks (MOFs), Zeolitic Imidazolate Frameworks (ZIFs) and zeolites. The most common method of screening and selecting adsorbents is on the basis of adsorption selectivity. Besides adsorption selectivity, the performance, and economics, of a PSA unit is governed by a number of other factors, notably the working capacity. The main objective of this study is to investigate the relative importance of selectivity and capacity on PSA performance. Breakthrough characteristics of a packed bed adsorber packed with a number of zeolites (MFI, JBW, AFX, NaX) and MOFs (MgMOF-74, MOF-177, CuBTTri-mmen) were investigated for CO₂ capture from a CO₂/N₂ mixture. These breakthrough calculations demonstrate that high capacities could have a dominant influence on the overall performance of PSA units. Our studies indicate that MgMOF-74 is the best adsorbent for post-combustion CO₂ capture

A molecular dynamics investigation of the diffusion characteristics of cavity-type zeolites with 8-ring windows

Molecular dynamics (MD) simulations are used to investigate the diffusion characteristics in DDR, CHA, LTA, ITQ-29, and TSC zeolites that have cavities separated by 8-member ring windows of dimensions in the 3.4-4.6 Å range. These zeolites have potential usage for separation of a variety of mixtures, such as CO<SUB>2</SUB>/CH<SUB>4</SUB>, CO<SUB>2</SUB>/H<SUB>2</SUB>, H<SUB>2</SUB>/CH<SUB>4</SUB>, and propane/propene, relying on a combination of adsorption and diffusion selectivities. The magnitude of self-diffusivities, D<SUB>i,self</SUB>, of the CH<SUB>4</SUB> is found to have a direct correlation with the size of the window opening, increasing by about two orders of magnitude for a 0.5 Å increase in the window aperture. The diffusion selectivities of CO<SUB>2</SUB>/CH<SUB>4</SUB>, and H<SUB>2</SUB>/CH<SUB>4</SUB> mixtures were also found to have direct, and strong, correlation, with the window aperture. This opens up the possibility of tuning diffusion selectivities by appropriate choice of the framework structure. Framework flexibility dynamics have also been investigated with the aid of two published force fields for all-silica zeolites. Due to the lattice vibrations there is a distribution of window sizes that varies with time. The diffusivity of CH<SUB>4</SUB> for a flexible lattice was found to correlate with aperture size of the time-averaged window, in precisely the same manner as for fixed framework lattices. This leads to the conclusion that lattice flexibility, per se, has no influence on the magnitude of the diffusivity or diffusion selectivity

Investigating the validity of the Knudsen prescription for diffusivities in a mesoporous covalent organic framework

Molecular dynamics (MD) simulations were performed to determine the self-diffusivity (D<SUB>i,self</SUB>) and the Maxwell-Stefan diffusivity (Ð<SUB>i</SUB>) of hydrogen, argon, carbon dioxide, methane, ethane, propane, n-butane, n-pentane, and n-hexane in BTP-COF, which is a covalent organic framework (COF) that has one-dimensional 3.4-nm-sized channels. The MD simulations show that the zero-loading diffusivity (Ð<SUB>i</SUB>(0)) is consistently lower, by up to a factor of 10, than the Knudsen diffusivity (D<SUB>i,Kn</SUB>) values. The ratio Ð<SUB>i</SUB>(0)/D<SUB>i,Kn</SUB> is found to correlate with the isosteric heat of adsorption, which, in turn, is a reflection of the binding energy for adsorption on the pore walls: the stronger the binding energy, the lower the ratio Ð<SUB>i</SUB>(0)/D<SUB>i,Kn</SUB>. The diffusion selectivity, which is defined by the ratio D<SUB>1,self</SUB>/D<SUB>2,self</SUB> for binary mixtures, was determined to be significantly different from the Knudsen selectivity (M<SUB>2</SUB>/M<SUB>1</SUB>)<SUP>½</SUP>, where M<SUB>i</SUB> is the molar mass of species i. For mixtures in which component 2 is more strongly adsorbed than component 1, the expression (D<SUB>1,self</SUB>/D<SUB>2,self</SUB>)/(M<SUB>2</SUB>/M<SUB>1</SUB>)<SUP>½</SUP> has values in the range of 1-10; the departures from the Knudsen selectivity increased with increasing differences in adsorption strengths of the constituent species. The results of this study have implications in the modeling of diffusion within mesoporous structures, such as MCM-41 and SBA-15