Molecular simulations of adsorption isotherms for linear and branched alkanes and their mixtures in silicalite

The configurational bias Monte Carlo (CBMC) technique has been used for com puting 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 s tudied. The inflection was found to occur at a loading of four molecules pe r 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 therefor e requires disproportionately higher pressures; this leads to the inflectio n behavior. Linear alkanes with six and more carbon atoms also were found t o exhibit inflection behavior. Hexane and heptane show inflection due to co mmensurate "freezing"; the length of these molecules is commensurate with t he length of the zigzag channels. This leads to a higher packing efficiency than for other linear alkanes. Available experimental data from the litera ture are used to confirm the accuracy of the predictions of the CBMC simula tions. Furthermore, the temperature dependency of the isotherms are also pr operly modeled. For purposes of fitting the isotherms we found that the dua l-site Langmuir model provides an excellent description of the simulated is otherms for linear and branched alkanes. In this model we distinguish betwe en two sites with differing ease of adsorption: site A, representing the in tersections between the straight and zigzag channels, and site B, represent ing the channel interiors. CBMC simulations of isotherms of 50-50 binary mi xtures of C-5, C-6, and C-7 hydrocarbon isomers show some remarkable and hi therto unreported features. The loading of the branched isomer in all three binary mixtures reaches a maximum when the total mixture loading correspon ds to four molecules per unit cell. Higher loadings are obtained by "squeez ing out" of the branched alkane from the silicalite and replacing these wit h the linear alkane. This "squeezing out" effect is found to be entropic in nature; the linear alkanes have a higher packing efficiency and higher loa dings are more easily achieved by replacing the branched alkanes with the l inear alkanes. The mixture isotherms can be predicted quite accurately by a pplying the appropriate mixture rules to the dual-site Langmuir model. This model allows the mixture isotherm to be predicted purely on the basis of t he 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 Cg, C6, and C7 hydrocarbon isomers, pro vides a potential for the development of a novel separation technique based on entropy-driven sorption selectivity.