Investigation of entropy effects during sorption of mixtures of alkanes in MFI zeolite

Abstract

We have carried out a comprehensive study of sorption of mixtures of alkanes, in the 1-7 C atom range, in MFI zeolite using configurational-bias Monte Carlo (CBMC) simulations. The isotherm characteristics of various binary, ternary and quaternary mixtures have been investigated. Our studies show that two types of entropy effects have a significant influence on mixture sorption: 1. Size entropy effects arise due to differences in the saturation loading of the pure components. Size entropy effects favour the component with the smaller number of C atoms because the smaller molecule finds it easier to fill in the "gaps" within the zeolite matrix at high molecular loadings. 2. Configurational entropy effects come into play for mixtures of alkanes that differ in the degree of branching. For a mixture of linear and mono-methyl alkanes with the same number of C atoms, configurational entropy effects favour the linear isomer because such molecules "pack" more efficiently within the MFI matrix. For a mixture of mono-methyl and di-methyl alkanes with the same number of C atoms, configurational entropy effects favour the single branched isomer. Configurational entropy effect comes into play when the loading exceeds four molecules per unit cell, when all the intersection sites are occupied, and results in the following hierarchy of sorption strengths: linear alkalines≫ mono-methyl alkanes≫ di-alkanes. In all cases, the mixture isotherms can be predicted with good accuracy using the ideal adsorbed solution theory (IAST). CBMC simulations of sorption of an 8-component mixture containing n-pentane (n-C₅), 2-methylbutane (2MB), n-hexane (n-C₆), 2-methylpentane (2MP), 2,2-dimethylbutane (22DMB), n-heptane (n-C₇), 2-methylhexane (2MH) and 2,2-dimethylpentane (22DMP) show that both size and configurational entropy effects contribute, leading to a sorption hierarchy depending on the degree of branching, linear alkalines≫ mono-methyl ≫alkanes di-alkanes. This result has considerable potential for commercial application in the petroleum industry in catalytic isomerization process where it is necessary to isolate the di-branched alkanes which are preferred ingredients in gasoline.