Adsorption Equilibria of C1to C4Alkanes, CO2, and SF6on Silicalite

Sun, Matthew S.; Shah, Dhananjai B.; Xu, Heather H.; Talu, Orhan

doi:10.1021/jp9730196

Cited by 217 publications

(213 citation statements)

References 21 publications

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“…Adsorption isotherms of isobutane have been reported by Sun et al 16 and Zhu et al 14 15,64 Here we investigate the sorption behavior of branched alkanes at higher loadings. As a first approximation we have assumed that the interaction CH group of the branched alkane with the zeolite is identical to the interaction of a CH 2 group (see Table 2).…”

Section: Branched Alkanesmentioning

confidence: 83%

“…the heats of adsorption of isobutane have been obtained by Zhu et al 14 and Sun et al 16 who obtained -46.7 and -56 kJ/ mol, respectively. The reasons for this large difference are unclear.…”

Section: Branched Alkanesmentioning

confidence: 93%

“…13 Fewer experimental data are available for adsorption of branched hydrocarbons adsorbed in silicalite. The adsorption isotherms of isobutane also showed an inflection, [14][15][16] but for 2-methylpentane a simple Langmuir isotherm was found. 17 Molecular simulations have shown that the inflection of isobutane is related to the preferential adsorption of the branched alkanes at the intersections of the zigzag and straight channel of silicalite.…”

Section: Introductionmentioning

confidence: 99%

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Molecular Simulations of Adsorption Isotherms for Linear and Branched Alkanes and Their Mixtures in Silicalite

1999

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The configurational-bias Monte Carlo (CBMC) technique has been used for computing the adsorption isotherms for linear and branched 2-methylalkanes on silicalite. The carbon numbers of the alkanes ranged from four to nine. For branched alkanes inflection behavior was observed for all carbon numbers studied. The inflection was found to occur at a loading of four molecules per unit cell. Below this loading the branched alkanes are seen to be located predominantly at the intersections of the straight and zigzag channels. To obtain loadings higher than four, the branched alkane must seek residence in the channel interiors which is energetically more demanding and therefore requires disproportionately higher pressures; this leads to the inflection behavior. Linear alkanes with six and more carbon atoms also were found to exhibit inflection behavior. Hexane and heptane show inflection due to commensurate "freezing"; the length of these molecules is commensurate with the length of the zigzag channels. This leads to a higher packing efficiency than for other linear alkanes. Available experimental data from the literature are used to confirm the accuracy of the predictions of the CBMC simulations. Furthermore, the temperature dependency of the isotherms are also properly modeled. For purposes of fitting the isotherms we found that the dual-site Langmuir model provides an excellent description of the simulated isotherms for linear and branched alkanes. In this model we distinguish between two sites with differing ease of adsorption: site A, representing the intersections between the straight and zigzag channels, and site B, representing the channel interiors. CBMC simulations of isotherms of 50-50 binary mixtures of C 5 , C 6 , and C 7 hydrocarbon isomers show some remarkable and hitherto unreported features. The loading of the branched isomer in all three binary mixtures reaches a maximum when the total mixture loading corresponds to four molecules per unit cell. Higher loadings are obtained by "squeezing out" of the branched alkane from the silicalite and replacing these with the linear alkane. This "squeezing out" effect is found to be entropic in nature; the linear alkanes have a higher packing efficiency and higher loadings are more easily achieved by replacing the branched alkanes with the linear alkanes. The mixture isotherms can be predicted quite accurately by applying the appropriate mixture rules to the dual-site Langmuir model. This model allows the mixture isotherm to be predicted purely on the basis of the parameters describing the isotherms of the pure components. The sorption selectivity exhibited by silicalite for the linear alkane in preference to the branched alkane in mixtures of C 5 , C 6 , and C 7 hydrocarbon isomers, provides a potential for the development of a novel separation technique based on entropy-driven sorption selectivity.

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Section: Branched Alkanesmentioning

confidence: 83%

Section: Branched Alkanesmentioning

confidence: 93%

Section: Introductionmentioning

confidence: 99%

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Molecular Simulations of Adsorption Isotherms for Linear and Branched Alkanes and Their Mixtures in Silicalite

1999

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“…This is in itself not unusual, but in both cases the loading overshoots the 4 molecules per unit cell without an inflection in the isotherm. Many studies, both experimental and computational, have shown that the isotherms of bulky molecules, like isobutane, 11,12,37,38 in MFItype silica show an inflection as the molecules need an extra ''push'' to adsorb in the narrow channels once the 4 intersections per unit cell are occupied. The overshoot can also not be temperature related, since the highest temperature of Song et al is still lower than the lowest temperature of Cavalcante et al, the former being correctly reproduced.…”

Section: Resultsmentioning

confidence: 99%

Molecular simulations of the adsorption of cycloalkanes in MFI-type silica

Schenk

Smit

Maesen

et al. 2005

Phys. Chem. Chem. Phys.

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A new force field for the simulation of the adsorption of cycloalkanes in nanoporous silica affords a significant improvement over any previously employed force field. The simulated isotherms reproduce the most salient features in the experimental isotherms extremely well. The study of cyclo-pentane, -hexane, and -heptane adsorption in MFI-type silica indicates an inflection for cyclopentane but not for cyclohexane at intermediate pressure. If corroborated by experiments, such an inflection point would afford an excellent calibration point for further force field developments. At low pressures, mixture isotherms of cyclohexane and n-hexane show a temperature dependence on the selectivity in accordance with recent results by J. P. Fox and S. P. Bates, J. Phys. Chem., 2004, 108, 17136. This dependence is caused by a difference in temperature dependence of the Henry coefficient for both molecules. At high pressures entropy effects due to packing always favor the sorption of n-hexane. Furthermore, the influence of the flexibility of the zeolite framework on the adsorption of these rather bulky molecules is investigated. It is found that this influence of the flexibility on the adsorption of cyclohexane is as small as with n-alkanes.

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“…These probabilities can be further optimized depending on the details of the simulations. 4 Comparison of the experimental data 25,26 for pure component isotherms for (a) methane, (b) ethane, (c) propane, and (d) n-butane in MFI at 300 K with CBMC simulations. [22][23][24] A simple approach to describe the alkane molecules is to use the united-atom model, in which CH 3 , CH 2 , and CH groups are considered as single interaction centers.…”

Section: Cbmc Simulation Techniquementioning

confidence: 99%