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

Abstract

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.

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

“…4,10,12,45 From the simulated isotherms at 300 K up to high pressure (Fig. 8), it was clear that cyclohexane can only adsorb at the intersections.…”

“…To verify whether the interactions between the CH 2 groups of cyclohexane and the zeolite framework can be modeled using the parameters for the CH 2 groups of linear alkanes, we calculated an adsorption isotherm using the parameters from Vlugt et al 12 (see Table 1). As is shown in Fig.…”

“…Therefore, small 41 The simulations were performed using the Lennard-Jones parameters for the CH 2 group of n-alkanes by Vlugt et al 12 and scaled parameters respectively (see Table 1). The lines serve as a guide to the eye.…”

“…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.…”

“…[6][7][8][9][10][11][12][13][14][15][16][17][18][19][20][21][22][23] In many of these studies, only linear, branched and aromatic hydrocarbons were considered. Only four studies, two by Snurr et al 24,25 and two by Fox et al, 26,27 include cycloalkanes (naphthenes).…”