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Modeling equilibrium and nonequilibrium dynamics of zeolites and zeolite-guest systems
We studied the dynamics of zeolites and zeolite-guest systems under thermal equilibrium and nonequilibrium conditions. The work was developed in four stages. First, we applied electronic structure methods to calculate transition state parameters for the O(1) → O(4) proton transfer in H-Y zeolite. We found an O(1) → O(4) barrier height of 86.1 kJ mol−1 , and a barrier curvature at the transition state of 1570 cm −1. Including long range effects from the work of Sauer et al. [ACS Symp. Ser. 721, 358 (1999)] results in a higher barrier, which we estimate to be 97.1 kJ mol−1. We consider that our barriers are notably larger than those reported in experimental literature as a result of neglecting tunneling in their interpretation of experimental data. ^ Next, we modeled benzene's orientational dynamics in Na-Y zeolite, using kinetic Monte Carlo and mean field master equation, motivated by the NMR study of Isfort et al. [Chem. Phys. Lett. 288, 71 (1998)]. We consider guest-guest interactions including only site blocking and both site blocking and nearest-neighbor attractions. We found an apparent activation energy greater than the fundamental cage-to-cage barrier when considering only site blocking, and smaller when also including guest-guest attractions. This suggests that the actual cage-to-cage barrier is greater than 40 kJ mol−1 reported by Isfort et al., which lends credence to previous simulations of benzene in Na-Y. ^ We also performed molecular dynamics (MD) simulations of zeolite-guest systems driven by microwaves (MW), motivated by the MW-driven sorption study of Turner et al. [AIChE J. 46, 758 (2000)]. Zeolite-benzene systems show minimal MW heating, while zeolite-methanol systems exhibit significant MW heating with steady-state temperatures increasing linearly with methanol loading. MW-driven equimolar mixtures of benzene and methanol at low to medium loadings in zeolites obey Tmethanol ≫ Tbenzene > Tzeolite , suggesting that MW heating of binary mixtures in zeolites can produce novel effects. However, MW-driven MD at higher loadings shows that Tmethanol ∼ Tbenzene > Tzeolite, suggesting that closely related MW sorption studies can produce markedly different results viz. athermal effects. ^ Finally, we performed grand canonical Monte Carlo (GCMG) sorption simulations of single component and binary mixtures of methanol/benzene in silicalite at equilibrium conditions. We found that single component adsoprtion isotherms are in qualitative agreement with both experimental and ideal isotherms, the latter calculated assuming Langmuir adsorption. Saturation loading of benzene and methanol were 5.8 ± 0.5 and 17.5 ± 1 molecules per unit cell at 300K. Results at 350K show that saturation loadings decreased to 4.6 ± 0.5 and 10.8 ± 1 molecules per unit cell, for benzene and methanol, respectively. Adsorption isotherms of binary mixtures shows a marked change in silicalite's selectivity for benzene as a function of loading. ^
"Modeling equilibrium and nonequilibrium dynamics of zeolites and zeolite-guest systems"
(January 1, 2003).
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