Highlighting the Anti-Synergy between Adsorption and Diffusion in Cation-Exchanged Faujasite Zeolites.

Using configurational-bias Monte Carlo simulations of adsorption equilibrium and molecular dynamics simulations of guest diffusivities of CO, CH, N, and O in FAU zeolites with varying amounts of extra-framework cations (Na or Li), we demonstrate that adsorption and diffusion do not, in general, proceed hand-in-hand. Stronger adsorption often implies reduced mobility. The anti-synergy between adsorption and diffusion has consequences for the design and development of pressure-swing adsorption and membrane separation technologies for CO capture and N/O separations.

How Reliable Is the Ideal Adsorbed Solution Theory for the Estimation of Mixture Separation Selectivities in Microporous Crystalline Adsorbents?

Microporous crystalline adsorbents such as zeolites and metal-organic frameworks (MOFs) have potential use in a wide variety of separation applications. The adsorption selectivity is a key metric that quantifies the efficacy of any microporous adsorbent in mixture separations. The Ideal Adsorbed Solution Theory (IAST) is commonly used for estimating the value of , with unary isotherms of the constituent guests as data inputs.

Synergistically enhance confined diffusion by continuum intersecting channels in zeolites.

In separation and catalysis applications, adsorption and diffusion are normally considered mutually exclusive. That is, rapid diffusion is generally accompanied by weak adsorption and vice versa. In this work, we analyze the anomalous loading-dependent mechanism of -xylene diffusion in a newly developed zeolite called SCM-15.

Using Molecular Simulations to Unravel the Benefits of Characterizing Mixture Permeation in Microporous Membranes in Terms of the Spreading Pressure.

The separation performance of microporous crystalline materials in membrane constructs is dictated by a combination of mixture adsorption and intracrystalline diffusion characteristics; the permeation selectivity is a product of the adsorption selectivity and the diffusion selectivity, . The primary objective of this article is to gain fundamental insights into and by use of molecular simulations. We performed configurational-bias Monte Carlo (CBMC) simulations of mixture adsorption equilibrium and molecular dynamics (MD) simulations of guest self-diffusivities of a number of binary mixtures of light gaseous molecules (CO, CH, N, H, and CH) in a variety of microporous hosts of different pore dimensions and topologies.

Simultaneous interlayer and intralayer space control in two-dimensional metal-organic frameworks for acetylene/ethylene separation.

Three-dimensional metal-organic frameworks (MOFs) are cutting-edge materials in the adsorptive removal of trace gases due to the availability of abundant pores with specific chemistry. However, the development of ideal adsorbents combining high adsorption capacity with high selectivity and stability remains challenging. Here we demonstrate a strategy to design adsorbents that utilizes the tunability of interlayer and intralayer space of two-dimensional fluorinated MOFs for capturing acetylene from ethylene.

Water/Alcohol Mixture Adsorption in Hydrophobic Materials: Enhanced Water Ingress Caused by Hydrogen Bonding.

Microporous crystalline porous materials such as zeolites, metal-organic frameworks, and zeolitic imidazolate frameworks (ZIFs) have potential use for separating water/alcohol mixtures in fixed bed adsorbers and membrane permeation devices. For recovery of alcohols present in dilute aqueous solutions, the adsorbent materials need to be hydrophobic in order to prevent the ingress of water. The primary objective of this article is to investigate the accuracy of ideal adsorbed solution theory (IAST) for prediction of water/alcohol mixture adsorption in hydrophobic adsorbents.

Using Molecular Simulations for Elucidation of Thermodynamic Nonidealities in Adsorption of CO-Containing Mixtures in NaX Zeolite.

Cation-exchanged zeolites are of potential use in pressure swing adsorption (PSA) technologies for CO capture applications. Published experimental data for CO/CH, CO/N, and CO/CH mixture adsorption in NaX zeolite, also commonly referred to by its trade name 13X, have demonstrated that the ideal adsorbed solution theory (IAST) fails to provide adequately accurate estimates of mixture adsorption equilibrium. In particular, the IAST estimates of CO/CH and CO/N selectivities are significantly higher than those realized in experiments.

Elucidation of Selectivity Reversals for Binary Mixture Adsorption in Microporous Adsorbents.

The adsorption selectivity, , is a key metric that quantifies the efficacy of any adsorbent in mixture separations. It is common practice to use ideal adsorbed solution theory (IAST) for estimating the value of , using unary isotherm data inputs. In a number of experimental investigations, the phenomena of selectivity reversals and adsorption azeotropy ( = 1) have been reported in the published literature; such reversals may result from changes in mixture compositions, pressures, or pore loadings.

Commensurate-incommensurate adsorption and diffusion in ordered crystalline microporous materials.

For homologous series of linear chain molecules, there could be either a match, or mismatch, between the characteristic periodicity of the host structure and the characteristic length of the guest molecules. The major objective of this article is to highlight the influence of commensurateness, or incommensurateness, on both the adsorption and diffusion characteristics. Published experimental data, backed by molecular simulation results, are used to highlight the attendant non-monotonicity in adsorption strengths and diffusivities.

Screening metal-organic frameworks for separation of pentane isomers.

This article compares the performances of several metal-organic frameworks (MOFs) and zeolitic imidazolate frameworks (ZIFs) for the separation of pentane isomers: n-pentane (nC5), 2-methylbutane (2MB), and 2,2-dimethylpropane (= neo-pentane (neo-P)) in fixed bed adsorbers. The required input data on unary and mixture adsorption equilibria are obtained from Configurational-Bias Monte Carlo (CBMC) simulations for twelve different adsorbents. The best separation performance is realized with Fe(BDP), where BDP = 1,4-benzenedipyrazolate, a MOF with triangular shaped 4.

Separation of hexane isomers in a metal-organic framework with triangular channels.

Metal-organic frameworks can offer pore geometries that are not available in zeolites or other porous media, facilitating distinct types of shape-based molecular separations. Here, we report Fe2(BDP)3 (BDP(2-) = 1,4-benzenedipyrazolate), a highly stable framework with triangular channels that effect the separation of hexane isomers according to the degree of branching. Consistent with the varying abilities of the isomers to wedge along the triangular corners of the structure, adsorption isotherms and calculated isosteric heats indicate an adsorption selectivity order of n-hexane > 2-methylpentane > 3-methylpentane > 2,3-dimethylbutane ≈ 2,2-dimethylbutane.

Influence of adsorption thermodynamics on guest diffusivities in nanoporous crystalline materials.

Published experimental data, underpinned by molecular simulations, are used to highlight the strong influence of adsorption thermodynamics on diffusivities of guest molecules inside ordered nanoporous crystalline materials such as zeolites, metal-organic frameworks (MOFs), and zeolitic imidazolate frameworks (ZIFs). For cage-type structures (e.g.

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 CO(2) capture (CO(2)/H(2), CO(2)/CH(4), CO(2)/N(2)), CH(4)/H(2), 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.

Thermosensitive gating effect and selective gas adsorption in a porous coordination nanocage.

A porous coordination nanocage functionalized with 24 triisopropylsilyl groups exhibits a remarkable thermosensitive gate opening phenomenon and demonstrates a molecular sieving effect at a certain temperature range, which can be used for gas separation purposes.

Hydrogen bonding effects in adsorption of water-alcohol mixtures in zeolites and the consequences for the characteristics of the Maxwell-Stefan diffusivities.

This work highlights a variety of peculiar characteristics of adsorption and diffusion of polar molecules such as water, methanol and ethanol in zeolites. These peculiarities are investigated with the aid of configurational-bias Monte Carlo (CBMC) simulations of adsorption isotherms, and molecular dynamics (MD) simulations of diffusivities in FAU, MFI, DDR, and LTA zeolites. Because of strong hydrogen bonding, significant clustering of the guest molecules occurs in all investigated structures.

Highlighting a variety of unusual characteristics of adsorption and diffusion in microporous materials induced by clustering of guest molecules.

In this work, we highlight several unusual characteristics of adsorption and diffusion of a variety of guest molecules, such as linear and branched alkanes with a number of C atoms in the 1-6 range, CO(2), and Ar in microporous structures such as zeolites (FAU, NaY) and metal organic frameworks (IRMOF-1, CuBTC, MIL-47, MIL-53 (Cr)-lp, PCN-6') that have channel or cavity sizes larger than about 0.75 nm. Clustering of guest molecules is found to manifest at temperatures below the critical temperature, T(c), of the guest species.

Investigating cluster formation in adsorption of CO2, CH4, and Ar in zeolites and metal organic frameworks at subcritical temperatures.

The critical temperatures, T(c), of CO(2), CH(4), and Ar are 304 K, 191 K, and 151 K, respectively. This paper highlights some unusual characteristics of adsorption and diffusion of these molecules in microporous structures such as zeolites and metal organic frameworks at temperatures T < T(c). Published experimental adsorption data for T < T(c) show that the isotherms invariably display stepped characteristics.

Assessing guest diffusivities in porous hosts from transient concentration profiles.

Using the short-chain-length alkanes from ethane to n-butane as guest molecules, transient concentration profiles during uptake or release (via interference microscopy) and tracer exchange (via IR microimaging) in Zn(tbip), a particularly stable representative of a novel family of nanoporous materials (the metal organic frameworks), were recorded. Analyzing the spatiotemporal dependence of the profiles provides immediate access to the transport diffusivities and self-diffusivities, yielding a data basis of unprecedented reliability for mass transfer in nanoporous materials. As a particular feature of the system, self- and transport diffusivities may be combined to estimate the rate of mutual passages of the guest molecules in the chains of pore segments, thus quantifying departure from a genuine single-file system.

1H NMR signal broadening in spectra of alkane molecules adsorbed on MFI-type zeolites.

The anisotropic behavior of C1-C6 alkane molecules adsorbed in MFI zeolite was studied by 1H nuclear magnetic resonance (NMR) using single-pulse excitation, Carr-Purcell-Meiboom-Gill (CPMG) pulse sequence, Hahn echo (HE) pulse sequence, and magic-angle spinning. The molecular order parameter was obtained by both static 2H NMR spectroscopy and molecular simulations. This yields an order parameter in the range of 0.