Pore mouth physisorption of alkanes on ZSM-22: estimation of physisorption enthalpies and entropies by additivity method

ZSM-22 (TON-type) zeolite has a pore size close to that of alkanes and exhibits pronounced molecular sieve effects with alkanes. n-Alkanes physisorb through the pore mouths into the micropores while the iso-alkanes physisorb at the pore mouths only. Physisorption inside the micropores results in additional entropy and enthalpy loss compared to physisorption at the pore mouths. Multiple modes of physisorption exist at the pore mouths: each of the “straight ends” of the iso-alkane can protrude into the micropore. A two-step physisorption model distinguishing between physisorption at the pore mouths and subsequent transfer from pore mouths into the micropores has been developed. The standard physisorption enthalpy and entropy for each of the physisorption modes of iso-alkanes and n-alkanes are computed following an additivity principle. The Henry coefficient for each alkane is the sum of contributions calculated for the individual physisorption modes. The standard physisorption enthalpy and entropy for each alkane are calculated from the distribution of physisorption modes using ideal mixing rules.