(164f) Influence of Zeolite Pore Topology On n-Butane Monomolecular Cracking and Dehydrogenation: Effects of the Size and Relative Abundance of Cavities

In previous studies of the monomolecular conversion of n-hexane over acidic zeolites it was found that turnover numbers increased with a decrease in the pore diameter [1]. This observation was explained based on a reduction in the apparent activation energy caused by the greater heat of adsorption afforded by smaller pores. In contrast, Gounder and Iglesia reported that the monomolecular reaction rates of propane and butanes were greater in the 8-MR side pockets of H-MOR than in the 12-MR channels because of the lower confinement and greater entropies of activation in the 8-MR locations [2,3]. In addition, we have recently found that for H-MFI the rates of monomolecular cracking and dehydrogenation of n-butane increase as the fraction of Co(II) assigned to channel intersections (in CoNa-MFI) increases relative to the fraction located in presumably more confining channels [4].

We further investigate the influence of confinement on n-butane cracking and dehydrogenation reactions for a series of zeolites (TON, MFI, SFV, MEL, MWW, and STF) having 10-MR channel systems but differing in the sizes and relative abundance of intersections and cages. The ratios of central to terminal cracking and of cracking to dehydrogenation generally decrease with an increase in the fraction of the pore space that is present in cavities that exceed 6 Å in dimension according to the computational characterizations of First et al. [5]. These ratios were lowest on STF and MWW, which possess, respectively, the largest cavities and the greatest fraction of pore space present in cages of diameters exceeding 6 Å. Greater selectivities to dehydrogenation and to terminal cracking occurred despite greater energies of activation for these reactions and appear to be driven by changes in the intrinsic entropies of activation. Transition states such as central cracking that are closer to the zeolite framework are more easily formed in simple 1D channels that closely confine the molecule lengthwise. Conversely, cavities or channel intersections that allow more configurational freedom can more easily stabilize transition states that are farther from the zeolite framework or that occur later along the reaction coordinate (e.g. terminal cracking and dehydrogenation). The extent to which the latter transition states are preferred increases with the dimension of the cavities and with the fraction of the pore space that is present in such locations. Our results demonstrate that the earlier observations for the influence of adsorption enthalpy on n-hexane cracking rates across difference zeolite structures [1] do not reflect the influence of this property on monomolecular alkane reaction rates in general. The results also carry important implications for the theoretical modeling of  monomolecular transition states, which until recently have been computed based only on a potential energy surface. 

[1]       J. A. van Bokhoven, B. A. Williams, W. Ji, D. C. Koningsberger, H. H. Kung, J. T. Miller, J. Catal. 224 (2004), 50.

[2]       R. Gounder and E. Iglesia, J. Am. Chem. Soc. 131 (2009), 1958.

[3]       R. Gounder and E. Iglesia, Angew. Chem. Int. Ed. 49 (2010), 808.

[4]       A. Janda and A. T. Bell, in preparation.

[5]       E. L. First, C. D. Gounaris, J. Wei, C. A. Floudas, Phys. Chem. Chem. Phys. 13 (2011), 17339.