We investigate the influence of the structural environments of Br�nsted protons in acidic zeolites on the unimolecular cracking and dehydrogenation of n-butane for a series of zeolites that possess 10-MR channels but that differ widely in topology and cavity size. Ratios of central C-C cracking to terminal C-C cracking and of cracking to dehydrogenation generally decreased with an increase in the characteristic pore dimension. This observation is consistent with previous work in which more confining proton environments were more likely to form transition states that interact closely with the zeolite framework (such as central cracking), and were less likely to stabilize transition states that closely resemble a product state (dehydrogenation). Yet, in contrast to previous researchers, we find that the greater selectivities to dehydrogenation and to terminal cracking are driven in part by strong changes in the relative intrinsic entropies of activation that offset concurrent increases in the intrinsic energies of activation with increasing pore size. Differences in activation energies between reaction pathways (cracking versus dehydrogenation) depend on zeolite structure and do not necessarily reflect differences in gas phase proton affinities. This observation differs from the view that intrinsic activation parameters for a given alkane on different zeolites are constant, and is interpreted to be a natural consequence of the weaker interaction of transition states with more spacious pore environments. Turnover frequencies varied irregularly with pore size and appear to be enhanced by the presence of cavities that closely match the average kinetic diameter of n-butane. In such environments, adsorbed species may rotate relatively freely while maintaining peripheral contact with the zeolite walls, resulting in a lower Gibbs free energy of activation and a higher value of the rate coefficient relative to smaller or larger cages where a species may have lower entropy or higher enthalpy, respectively.

Influence of zeolite topology and cage size on the rates, selectivities, apparent, and intrinsic activation parameters of alkane monomolecular cracking and dehydrogenation reactions

247th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.