We investigate the influence of n-alkane size and Br�nsted proton environment on the unimolecular cracking and dehydrogenation reactions of n-alkanes over acidic zeolites that possess 10-MR channels but that differ widely in topology and cavity size. Intrinsic activation energy (E�int) and entropy (?S�int) for n-butane terminal cracking and dehydrogenation tend to increase relative to central cracking as the sizes and abundance of cavities increases. A linear relationship (previously unreported) is observed between ?S�int and E�int across different zeolite structures, and the slope of this plot is similar to the slope of a plot of the adsorption entropy versus the adsorption enthalpy. Therefore, the linear relationship between ?S�int and E�int appears to indicate that the transition state and the adsorbed alkane reactant are stabilized to different extents by van der Waals interactions with the zeolite framework. The magnitude of this difference mediates selectivities and depends on the alkane bond being cleaved and on the local structure of the active site.Turnover frequencies for n-alkane cracking on H-MFI zeolites increase with the size of the alkane partly because of a decrease in the average value of E�int as the number of internal C-C bonds increases. The value of ?S�int can increase or decrease slightly with increasing carbon number depending on which MFI catalyst is used. We interpret the differing directions of the change in ?S�int with respect to carbon number in terms of differences in the distributions of Br�nsted protons among channels and intersections and the fits of transition states of differing sizes within these environments. Maxima in E�int and ?S�int with respect to carbon number for one catalyst may indicate that cage effects can influence the values of these parameters when the size of the alkane closely matches the size of cavities or channel intersections.

Effects of pore topology and n-alkane size on the selectivities and intrinsic activation parameters of unimolecular alkane cracking and dehydrogenation reactions over acidic zeolites

248th National Meeting (2014)

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