“Two-step” model of molecular diffusion in silicalite

Kärger, Jörg; Demontis, Pierfranco; Suffritti, Giuseppe Baldovino; Tilocca, Antonio

doi:10.1063/1.478158

Cited by 42 publications

(52 citation statements)

References 18 publications

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“…This structural difference should be the main source of the different behavior observed in this particular process. In order to elucidate the influence of specific regions on the reaction the transitions occurring in silicalite were divided on the basis of a map recently devised by us, 39 which allows to identify the region ͑straight channel, zigzag channel or intersection͒ visited by the sorbate at a particular time. We mapped the position of the A-B center of mass ͑again for the TST crossings͒ whenever u ͉͉ changed sign.…”

Section: Resultsmentioning

confidence: 99%

Molecular dynamics simulation of an activated transfer reaction in zeolites

Demontis

Suffritti

Tilocca

1999

The Journal of Chemical Physics

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The activated transfer of a light particle between two heavier species in the micropores of silicalite and ZK4 zeolites has been studied through molecular dynamics ͑MD͒ simulations. A three-body potential controls the exchange of the light particle between the heavier ones; an effective barrier of a few k B T separates the two stable regions corresponding to symmetric ''reactant'' and ''product'' species. Harmonic forces always retain the reactants at favorable distances so that in principle only the energetic requirement must be fulfilled for the transfer to occur. The rate constant for the process ͑obtained from a correlation analysis of equilibrium MD trajectories͒ decreases by more than one order of magnitude when the barrier height is increased from 2k B T to 5k B T following an Arrhenius-type behavior. The transfer rates are always lower in ZK4. When the reaction is studied in a liquid solvent the calculated rate constants are closer to those obtained in silicalite. Since with this model the diffusive approach of the reactants is almost irrelevant on the reactive dynamics, only the different ability of each environment to transfer the appropriate energy amount to the reactants and then promote the barrier passage could be invoked to explain the observed behavior. We found that structural, rather than energetic, effects are mainly involved on this point. The lower efficiency of ZK4 seems to arise from the frequent trapping of the reactive complex in the narrow ZK4 windows in which the transfer is forbidden and from the weaker interaction of the reactive complex with the host framework compared to silicalite.

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Section: Resultsmentioning

confidence: 99%

Molecular dynamics simulation of an activated transfer reaction in zeolites

Demontis

Suffritti

Tilocca

1999

The Journal of Chemical Physics

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“…This is connected to the high probability of two steps in opposite directions ͑negative correlation͒ already pointed out in our preceding articles. 15,16 This effect is more marked in the sinusoidal channel and ultimately leads to the typical relation…”

Section: Discussionmentioning

confidence: 97%

“…While in the standard random-walk model in silicalite 1 each event is a single displacement between two adjacent intersections ͑i.e., ͗x 2 ͘ seq in Eq. ͑1͒ is the square distance between adjacent intersections along the sinusoidal channel͒, the two-step expressions have been built considering all possible sequences of two single steps, 15 and the corresponding distance covered in each such sequence. As noted above the extension to the N-step model in silicalite is rather direct: it only requires the numbers of two-step events observed in the MD trajectory as well as the duration of single steps in both channels, i.e., data already available from our previous random-walk and twostep analysis.…”

Section: ͑1͒mentioning

confidence: 99%

“…1 When consecutive jumps are not completely uncorrelated, the random-walk picture is no more adequate, and the effects of correlation should be taken into account. We have previously devised a ''two-step'' model for the molecular diffusion in silicalite, which considerably improves the random-walk description of the diffusion of ethane at different loading and temperature 15 and of linear diatomic and triatomic flexible molecules at infinite dilution. 16 This observation shows that, while the RW uncorrelated picture may be appropriate in the case of small spherical sorbates such as methane and xenon, 1,[17][18][19][20] the motion of small linear species in silicalite can exhibit significant correlation effects.…”

Section: Introductionmentioning

confidence: 99%

“…The two-step model, incorporating only correlation between two consecutive jumps, is a sort of first-order improvement to the RW picture. However, in the silicalite case, the two-step parameters ͑i.e., data obtained from a direct analysis of the MD trajectory in terms of the two-step model͒ 15 are actually the only input data required to the N-step extension 21 ͑see Sec. II͒.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Two- and N-step correlated models for the analysis of molecular dynamics trajectories of linear molecules in silicalite

Demontis

Suffritti

Tilocca

2000

The Journal of Chemical Physics

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Recent molecular dynamics data on the diffusion of linear diatomic and triatomic molecules in the zeolite silicalite are analyzed in terms of a new correlated model ͓F. Jousse, S. M. Auerbach, and D. P. Vercauteren, J. Chem. Phys. 112, 1531 ͑2000͔͒ capable to account for both first-and higher-order correlation effects. This ''N-step'' model reproduces very well our calculated mean square displacements and diffusion coefficients of the molecules considered. The improvements with respect to the results obtained with our previous ''two-step'' model ͓P. Demontis, J. Kärger, G. B. Suffritti, and A. Tilocca, Phys. Chem. Chem. Phys. 2, 1455 ͑2000͔͒ are remarkable for all molecules except chlorine, showing that only in this case the effect of ͑negative͒ correlations spanning more than two jumps between channel intersections ͑ϳ20 Å͒ can be neglected. The basic trajectory analysis in terms of single-and two-step models, besides being an useful reference, provides all the input data needed for the application of the N-step model. Indeed, in its silicalite formulation, the N-step model is strongly linked to the two-step one because it calculates the probability of a sequence of jumps in the same channel by means of the correlations between any two consecutive jumps. Finally, the possibility to obtain qualitative insight into the diffusive mechanism through various kind of correlation coefficients is discussed.

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Diffusion and Adsorption in Porous Solids

Kärger¹,

Ruthven²

2002

Handbook of Porous Solids

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“Two-step” model of molecular diffusion in silicalite

Cited by 42 publications

References 18 publications

Molecular dynamics simulation of an activated transfer reaction in zeolites

Molecular dynamics simulation of an activated transfer reaction in zeolites

Two- and N-step correlated models for the analysis of molecular dynamics trajectories of linear molecules in silicalite

Diffusion and Adsorption in Porous Solids

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