Ab initio calculations showing frontier molecular orbital energy modifications of a guest molecule when located inside a microporous zeolitic cavity are presented. Micropores of zeolites Beta, ZSM-12, and ZSM-5 have been modeled using all silica clusters from which it is found that the highest occupied molecular orbital (HOMO) energy of toluene increases when going from the gas phase state to restricted microporous environments. The influence of the zeolite cavity size and its chemical composition on the toluene HOMO energy are discussed. Regarding the size of the confining environment, it is found that the smaller the zeolite cavity the higher is the rise in toluene HOMO energy. The effect of the chemical composition on the toluene HOMO energy was tested in the ZSM-5 zeolite by varying the Al content. The results obtained showed that the frontier orbital energy increases upon decreasing the Al content of the cluster. As a consequence of the confinement effect, toluene reactivity in zeolite catalyzed reactions is expected to change toward a more covalent behavior in which electronic transference from the toluene molecule to an electronic acceptor will be more favored than in gas phase reactions.
Molecular dynamics simulations have been performed to study the diffusion of octane in silicalite. When hydrocarbons that are larger than C6 diffuse through this zeolite, the length of the molecule is such that diffusion through the sinusoidal channels becomes difficult. We have investigated the relative diffusivity through each channel and give an interpretation of the effect of temperature on this process. Simulations at 300 K show greater diffusivity in the sinusoidal channels, whereas at 450 K the straight channels show higher diffusion rates. When the temperature increases from 300 to 450 K, the diffusion coefficient in the straight channels increases by a factor of 5.2, whereas the coefficient in the sinusoidal channels increases by only 1.3. The trajectory plots also show larger diffusion paths through the straight channels at higher temperatures.
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