Location and Conformation of <i>n</i>-Alkanes in Zeolites:  An Analysis of Configurational-Bias Monte Carlo Calculations

Bates, Simon; Well, Willy J.M. van; Santen, Rutger A. van; Smit, Berend

doi:10.1021/jp961386w

Cited by 57 publications

(59 citation statements)

References 19 publications

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“…It is interesting that the increase in heat of adsorption per additional carbon atom in the alkane chain is a strong function of the size of the micropore. It has been reported from experimental measurements 7,8,11,13 and from computer simulations [21][22][23][24][25][26] that heats of adsorption in MFI increase approximately 10 kJ/mol with each carbon atom in the alkane chain. Table 5 confirms these results: the incremental increase in heat of adsorption per carbon atom is about 10 kJ/mol for MFI but increases in smaller pores (FER) and decreases in larger pores (UTD-1).…”

Section: Results Of Experimentsmentioning

confidence: 99%

Calorimetric Study of Adsorption of Alkanes in High-Silica Zeolites

et al. 1998

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Isosteric heats and adsorption isotherms were measured for combinations of three alkanes (methane, ethane, propane) on a series of six high-silica zeolites (TON, MTW, UTD-1, MFI, FER, FAU). Three of these zeolites (TON, MTW, UTD-1) have one-dimensional channels, two (MFI, FER) have two-dimensional intersecting channels, and FAU has spherical cavities. Since the adsorbate molecules are nonpolar and the zeolites possess a high Si/Al ratio, electrostatic energies are small in comparison to van der Waals energies. Heats of adsorption of methane at the limit of zero coverage are 27.2, 20.9, and 14.2 kJ/mol in TON, MTW, and UTD-1, respectively. This homologous series of zeolites has one-dimensional channels composed of 10-, 12-, and 14-membered rings of average effective diameter 5.0, 5.9, and 8.8 Å, respectively. Short-range gas-solid interactions between spherical methane molecules and the oxygen atoms composing the pore walls are explained by a smoothed integration of the Lennard-Jones 12-6 potential for cylindrical pores.

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Section: Results Of Experimentsmentioning

confidence: 99%

Calorimetric Study of Adsorption of Alkanes in High-Silica Zeolites

et al. 1998

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“…The side pockets are accessible only to small molecules, whereas larger hydrocarbons can only access the main channels. 15 Moreover, previous experimental 16,17 and molecular simulation 18 results on adsorption of alkanes in MOR indicate that for all adsorbates decreasing the nonframework cation density (by increasing the framework Si/Al ratio) decreases the loading at a given pressure. 19 In contrast, MFI shows the opposite behavior: decreasing the nonframework cation density inside MFI-type pores (slightly) increases the loading.…”

Section: Introductionmentioning

confidence: 91%

The Influence of Non-framework Sodium Cations on the Adsorption of Alkanes in MFI- and MOR-Type Zeolites

2002

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Molecular simulations were performed for the adsorption of methane, ethane, and propane in MFI-and MORtype zeolites with various nonframework sodium and framework aluminum densities. The position of the nonframework sodium cations determined by Monte Carlo simulations is in agreement with positions determined by X-ray diffraction. The position and density of the sodium and aluminum atoms in the zeolite have a large influence on the adsorption of alkanes. The computed adsorption isotherms and Henry coefficients agree well with those obtained experimentally. Finally, we show that Configurational Bias Monte Carlo (CBMC) simulations are able to provide a better understanding of the effect of nonframework sodium on the selective adsorption of binary mixtures of isomers by these structures. Our results show that increasing the nonframework sodium density in MFI-type zeolites increasingly blocks the intersections and thereby increases the selectivity of MFI-type zeolites for adsorbing linear alkanes. By contrast, increasing the nonframework sodium density in MOR-type zeolites increases the number of sites favorable for adsorbing small linear alkanes.

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“…Since the supercages represent ϳ95% of the pore volume with this choice of the window sites, no appreciable error is inherent in the comparison. Bates et al 15 also conclude that confinement of alkanes in the faujasite structure, in comparison to other zeolites, has little effect on the conformations. Our distributions agree well with theirs.…”

Section: Molecular Structurementioning

confidence: 99%

“…More recently, the development of configurational bias ͑CB͒ techniques greatly improved the efficiency of Monte Carlo calculations for alkanes. 12, 13 Bates et al 14,15 performed CB Monte Carlo calculations of C6-C10 alkanes in a number of zeolites, including siliceous faujasite. They found that conformations inside this relatively open zeolite are very similar to unconfined alkanes.…”

Section: Introductionmentioning

confidence: 99%

Diffusion mechanisms of normal alkanes in faujasite zeolites

Clark

Gupta

et al. 1999

The Journal of Chemical Physics

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Molecular dynamics simulations of C1 through C14 n-alkanes in a siliceous faujasite zeolite are used to elucidate diffusion mechanisms. Additional simulations of the bulk liquids are conducted to compare the liquid and adsorbed phases. Heats of adsorption, diffusivities, and activation energies are compared with experimental values and agree well. Particularly good agreement is found between predicted self-diffusivities and those measured with pulsed field gradient nuclear magnetic resonance ͑NMR͒. Analysis of the chain conformations and motional frequencies indicates that the liquid and adsorbed phases are quite similar, most likely due to the open nature of the faujasite structure. Insights into the influence of local environment on molecular relaxation are also given. Siting results reveal an interesting progression of adsorption sites as the chain length is increased. Shifts of preferred adsorption sites are due to matching between the length scales of sorbate and adsorbent. Alkanes below C6 adsorb preferentially above the 4-rings and 6-rings inside the supercages. For longer chains, the preferred sites shift more toward the center of the supercages. For C8 and above, the molecules spend much of their time in the window regions. These window sites are separated from the supercage sites by unexpected orientational barriers. Consequently, diffusion of longer alkanes in faujasite type structures is not characterized by simple supercage-to-supercage hopping but by hopping between supercage and window sites.

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Location and Conformation of n-Alkanes in Zeolites: An Analysis of Configurational-Bias Monte Carlo Calculations

Cited by 57 publications

References 19 publications

Calorimetric Study of Adsorption of Alkanes in High-Silica Zeolites

Calorimetric Study of Adsorption of Alkanes in High-Silica Zeolites

The Influence of Non-framework Sodium Cations on the Adsorption of Alkanes in MFI- and MOR-Type Zeolites

Diffusion mechanisms of normal alkanes in faujasite zeolites

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