Entropy considerations in monomolecular cracking of alkanes on acidic zeolites

Compensation between adsorption entropies and enthalpies results in less than a two-fold variation in adsorption equilibrium constants for C₃-C₆ alkanes at temperatures relevant for monomolecular cracking; the size-independent activation energy for C{single bond}C bond activation in C₃-C₆ alkanes indicates that the marked increase in monomolecular cracking turnover rates observed with alkane chain size reflects a concurrent increase in activation entropies. Thermodynamic treatments for non-ideal systems rigorously describe confinement effects within zeolite channels and show that pre-exponential factors depend on solvation effects of the zeolite-host environment through variations in the thermodynamic activity of the zeolitic proton. Observed differences in rates and selectivities of monomolecular alkane activation with zeolite structure, after normalization to intrazeolitic concentrations, reflect differences in intrinsic rate constants.