Separation of linear, mono-methyl and di-methyl alkanes in the 5-7 carbon atom range by exploiting configurational entropy effects during sorption on silicalite-1

Abstract

We have developed a novel concept for separating alkane mixtures, in the 5-7 carbon atom range, into three separate fractions consisting of linear, mono-methyl and di-methyl alkanes by adsorption on silicalite. We make use of the fact that with increased degree of branching, the "packing efficiency" within the silicalite matrix is significantly lower. This configurational entropy effect comes into play when the loading exceeds 4 molecules per unit cell, when all the intersection sites are occupied, and results in the following hierarchy of sorption strengths: linear alkanes≫ mono-methyl alkanes≫ di-methyl alkanes. To demonstrate the feasibility of the entropy-based separation concept we carried out configurational-bias monte carlo simulations for a variety of binary, ternary and quaternary mixtures of alkanes, n-pentane (n-C₅), 2-methylbutane (2MB), n-hexane (n-C₆), 2-methylpentane (2MP), 3-methylpentane (3MP), 2,2-dimethylbutane (22DMB), n-heptane (n-C₇), 2-methylhexane (2MH), 2,2-dimethylpentane (22DMP). The entropy-driven separation concept has been verified, in part, by comparison with the experimental sorption data of Santilli (J. Catal., 1986, 99, 335) for the mixture n-C₆-3MP-22DMB.