Adsorption Of Guest Molecules Research Articles

Computing the Heat of Adsorption using Molecular Simulations: The Effect of Strong Coulombic Interactions.

Molecular simulations are an important tool for the study of adsorption of hydrocarbons in nanoporous materials such as zeolites. The heat of adsorption is an important thermodynamic quantity that can be measured both in experiments and molecular simulations, and therefore it is often used to investigate the quality of a force field for a certain guest-host (g - h) system. In molecular simulations, the heat of adsorption in zeolites is often computed using either of the following methods: (1) using the Clausius-Clapeyron equation, which requires the partial derivative of the pressure with respect to temperature at constant loading, (2) using the energy difference between the host with and without a single guest molecule present, and (3) from energy/particle fluctuations in the grand-canonical ensemble. To calculate the heat of adsorption from experiments (besides direct calorimetry), only the first method is usually applicable. Although the computation of the heat of adsorption is straightforward for all-silica zeolites, severe difficulties arise when applying the conventional methods to systems with nonframework cations present. The reason for this is that these nonframework cations have very strong Coulombic interactions with the zeolite. We will present an alternative method based on biased interactions of guest molecules that suffers less from these difficulties. This method requires only a single simulation of the host structure. In addition, we will review some of the other important issues concerning the handling of these strong Coulombic interactions in simulating the adsorption of guest molecules. It turns out that the recently proposed Wolf method ( J. Chem. Phys. 1999, 110 , 8254 ) performs poorly for zeolites as a large cutoff radius is needed for convergence.

Phonon interactions in zeolites mediated by anharmonicity and adsorbed molecules

There is increasing evidence that thermal conductivity of nanoporous materials can be significantly affected by adsorption of guest molecules. These molecules serve as moving defects and provide additional scattering centres for heat-carrying phonons. In order to understand the sorbate–phonon interactions, we perform molecular dynamics simulations of a model system, namely sodalite zeolite with small molecules (argon, xenon and methane) encapsulated in its cages. We measure effects of sorbates on several important characteristics of phonon dynamics, such as correlations between different phonon modes and the phonon frequency and lifetime. The dominant effect of the sorbate–phonon interactions is observed to be a change of phonon lifetime. The phonon lifetime often increases upon encapsulation of a sorbate into the zeolite which suggests that the sorbate–phonon interactions are qualitatively different from phonon scattering by point defects fixed in the lattice.

Is there a coupling between rotational and translational motion of methane in silicalite-1 and AlPO 4-5?

Methane, although often treated approximately as a spherical Lennard–Jones particle, has merely a tetrahedral shape. While moving in zeolites, the interaction of the molecule with the walls will cause a fluctuating angular momentum of this guest molecule. In zeolites containing channels, as it is the case with silicalite-1 or AlPO 4-5, adsorbed guest molecules move along channels and change to other channel types at channel intersections. The question arises whether or not the corrugation of the zeolite channels leads to a coupling of rotational and translational motions. It shall be shown, however, that for methane in silicalite-1 and AlPO 4-5 the two kinds of motion are uncoupled. The relaxation behavior of translational and angular momentum shows different patterns, although in both cases the relaxation time is on the same order of magnitude.

Porous copper(I) complexes of 2,11-dithia[3.3]paracyclophane: desorption and adsorption of guest molecules

Four copper(I) complexes of 2,11-dithia[3.3]paracyclophane (dtpcp), [CuI(dtpcp)] · MeCN ( 1), [CuBr(dtpcp)] · MeCN ( 2), [CuCl(dtpcp)] · MeCN ( 3) and [Cu 2I 2(dtpcp) 2] · Me-thf ( 4) (Me-thf=2-methyltetrahydrofuran), have been synthesized and their molecular structures determined by X-ray crystallography. Complexes 1 and 2 are isostructural, and exhibit 3D networks with elliptical channels along a-axis, which are constructed by dtpcp molecules bridging zigzag CuI and CuBr chains, respectively. Complexes 3 and 4 are 2D porous sheet networks constructed by bridging the zigzag copper-dtpcp chains via rhombic ring of Cu 2X 2 (X=Cl or I). All sheets in 3 are packed in an eclipsed manner through π–π stackings to generate channels along the a-axis. However, the 2D porous sheets in 4 are packed in an offset mode such that channels are not formed along c-axis. Complexes 1 and 2 can reversibly incorporate guest acetonitriles without collapse of structures, and the original frameworks of 1 and 2 are completely recovered after incorporation of guests which are confirmed by X-ray powder diffraction (XRPD) pattern and 1H NMR spectrum. Additionally, complex 1 exhibits selectivity in size and polarity for guest inclusion.

Molecular Simulation of Adsorption of Guest Molecules in Zeolitic Materials: A Comparative Study of Intermolecular Potentials

Grand Canonical Monte Carlo simulation, together with an appropriate guest-host forcefield is shown to provide reasonably accurate predictions of adsorption properties of guest molecules in a variety of zeolitic materials. The use of a simple guest-host Kiselev-type potential permits the calculations to capture the essence of the behavior of simple guest-host systems such as rare gases or methane molecules in neutral AlPO4-5. However, a full scale potential is needed in the more complex cases of large anisotropic molecules adsorbed in cationic zeolites (such as xylene isomers in faujasite). The guest-host potential model developed by Nicholson and coworkers is shown to allow an excellent transferability of the potential parameters from one guest/host system to another.

ChemInform Abstract: Adsorption of Guest Molecules in Zeolitic Materials: Computational Aspects

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Adsorption of Guest Molecules in Zeolitic Materials: Computational Aspects

Recent progress in the computation of thermodynamic properties of guest molecules in zeolites, using classical molecular simulation techniques, is reviewed. It is shown that grand canonical Monte Carlo simulations, using statistical biaising for studying large anisotropic molecules, together with an appropriate guest−guest force field, may provide a reasonably accurate prediction of single component and binary mixture adsorption data for systems such as normal and branched alkanes, benzene, alkyl benzene isomers and halocarbon molecules in a variety of aluminosilicate hosts. The Monte Carlo algorithms used to obtain reliable thermodynamics data (adsorption isotherms and heats) are discussed as well as the newly developed semiempirical force fields, which allow a better transferability of the potential parameters from one guest/host system to another.

NMR Study of Kaolinite Intercalation Compounds with Formamide and Its Derivatives. 1. Structure and Orientation of Guest Molecules

Local structures and orientations of guest molecules in kaolinite intercalation compounds of kaolinite/formamide (FA), kaolinite/formamide-N,N-d2 (FA-d2), kaolinite/N-methylformamide (NMF), and kaolinite/N,N-dimethylformamide (DMF) have been studied by means of solid-state NMR. 29Si magic-angle-spinning (MAS) NMR spectra identify the original kaolinite and the intercalates, while 13C MAS NMR spectra distinguish interlayer and outersurface adsorbed guest molecules. Quantitative analyses of 29Si and 13C NMR spectra combined with thermogravimetric analysis result in a formula of one interlayer guest molecule per Al2Si2O5(OH)4 host unit for all the intercalates. Intercalation causes high-frequency shift in the 13C resonance of the guest molecules. Strength in the guest−host interaction as revealed by changes in the 13C chemical shift of the carbonyl group upon intercalation is in the order of FA > NMF ∼ DMF. The 13C chemical shift of the methyl group confirms a cis conformation for NMF in the kaolinite interlayer space, and changes in the methyl chemical shift upon intercalation are DMF (trans) > NMF (cis) > DMF (cis). The magnitude of the temperature dependence in the 13C chemical shift is on the order of FA < NMF < DMF, and the 13C line width is FA > NMF > DMF. These facts reveal that the mobility of the guest molecules is in the order of FA < NMF < DMF. Kaolinite/NMF shows a second methyl peak whose amount increases with decreasing temperature, demonstrating the presence of molecules with a different orientation. The 1H chemical shifts confirm hydrogen-bonding interactions between the amide protons and the kaolinite host in kaolinite/FA and no such hydrogen-bonding interactions in kaolinite/NMF at room temperature. The chemical shift value of the hydroxyl groups in the host kaolinite structure is not affected by intercalation. The 1H line widths of carbonyls are FA > NMF > DMF, and those of methyls are NMF > DMF, which confirms again that the mobility of the guest molecules is FA < NMF < DMF.

Double Rotation 23Na NMR Study of Guest-Host Interactions and Nanoscale Assemblies in Sodium Zeolite Y

ABSTRACT23Na double rotation NMR (DOR) provides information on site-specific adsorption of guest molecules within the porous framework of sodium zeolite Y. Anchoring of molybdenum-hexacarbonyl to extraframework Na+ cations in particular locations within the large α-cavities is detected. Changes of the chemical environments of the Na+ cations are observed upon loading trimethylphosphine molecules. Spin-lattice relaxation measurements yield insight on dynamic aspects of the anchoring sodium cations.

Investigations of adsorbed organic molecules in Na-Y zeolite by xenon-129 NMR

The adsorption of n-hexane, benzene, and trimethylbenzene in Na-Y zeolite supercages has been investigated by use of xenon-129 NMR. As shown by Fraissard and co-workers, xenon-129 is an excellent probe of its environment in zeolites and is sensitive to the number of adsorbed guest molecules. The xenon-129 chemical shift, when extrapolated to zero effective xenon pressure, is 89.7 {plus minus} 0.1 ppm for benzene, 89.0 {plus minus} 1.3 ppm for trimethylbenzene, and 107.7 {plus minus} 1.2 ppm for n-hexane at a concentration of two guest molecules per zeolite supercage compared to 58.9 {plus minus} 0.5 ppm for empty Na-Y zeolite supercages. This approach is useful for determining the number of adsorbed molecules inside zeolite supercages and given information about their arrangement within the supercages.