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 alkanesmono-methyl alkanesdi-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.