Modeling Proton Jumps in HY Zeolite:  Effects of Acid Site Heterogeneity

• Published in: Journal of physical chemistry. C 2007-12, Vol.111 (49), p.18341-18347
• Main Authors: Viswanathan, Usha, Fermann, Justin T, Toy, Leanna K, Auerbach, Scott M, Vreven, Thom, Frisch, Michael J
• Format: Article
• Language: English
• ISSN: 1932-7447; 1932-7455
• DOI: 10.1021/jp0759675

We have computed the total mean rate coefficient for proton transfer in bare H−Y zeolite, for comparison with NMR experiments and previous calculations. We computed proton-transfer energies using two-layer ONIOM calculations on an 8T−53T cluster, where xT indicates x tetrahedral atoms. Rate coefficients were computed using truncated harmonic semiclassical transition state theory. The zero-point energy (ZPE) corrected proton site energies in H−Y (FAU structure) were found to be O3 (0 kJ mol-1), O1 O2 (16.1 kJ mol-1), and O4 (17.5 kJ mol-1), in quantitative agreement with previous calculations and in qualitative agreement with neutron diffraction occupancies. A new local minimum denoted O2* (31.4 kJ mol-1 relative to O3) was located, with a proton bound to O2 but pointing into the sodalite cage. Transition states between each pair of minima were fully characterized, yielding ZPE corrected activation energies ranging from 35.5 to 123.4 kJ mol-1. No correlation was found between barrier height and local structure; we considered 10 structural parameters including ring size, T−O−T angle, and nonbonded oxygen distance. Total mean rate coefficients were found to exhibit a strong non-Arrhenius temperature dependence, with apparent activation energies in the range of ca. 60−100 kJ mol-1 at high temperature, and ca. 3 kJ mol-1 at low temperature. This low-temperature value reflects thermally assisted tunneling to a site with a slightly higher energy. NMR experiments by Sarv et al. and Ernst et al. report apparent activation energies of 61 and 78 kJ mol-1, respectively, extracted from temperature ranges 298−658 and 610−640 K. Our theoretically computed apparent activation energies for these temperature ranges are 72 and 79 kJ mol-1, respectively, in quite good agreement with experiment.