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Transport and phase equilibria of benzene in FAU type zeolites
We have studied lattice models for self-diffusion of benzene in FAU type zeolites, to explore the effect of the thermodynamics of confined fluids on the transport properties of molecules in zeolites. Our model assumes that benzene molecules are located near Na+ ions in supercages, and in 12-ring windows separating adjacent supercages, respectively. The study was performed in three stages. ^ First, to disentangle the effect of a vapor-liquid phase equilibria on diffusion in zeolites, the transport of benzene in Na-Y is modeled in the absence of attractive guest-guest interactions. The loading dependence of diffusion coefficient, D&thetas;, at a constant temperature, referred to as a diffusion isotherm, is modeled with site-blocking effects using a mean field theory (MFT) that yields, Dq=16kq a2q, where a&thetas; ≅ 11 Å is the mean intercage jump length and 1/k&thetas; is the mean supercage residence time. A completely analytical expression is derived to calculate k&thetas;. The MFT is tested using a mean field approximation (MFA) where k&thetas; and a&thetas; are calculated from kinetic Monte Carlo simulations yielding excellent qualitative agreement. Further calculations are performed to test MFA by calculating “exact” diffusion coefficients from mean square displacement (MSD) calculations also yielding excellent qualitative agreement. ^ Next, by including guest-guest attractive interactions, we have performed lattice grand canonical Monte Carlo simulations of benzene adsorption in Na-X zeolite to determine whether strongly confined benzene molecules exhibit subcritical properties. We observe a phase transition from low to high density of adsorbed benzene, analogous to vapor-liquid equilibrium, at temperatures as high as 300 K and above. By performing thermodynamic integration to construct the coexistence curve, we obtain a critical point for benzene in Na-X at Tc = 370 ± 20 K, &thetas;c = 0.45 ± 0.05 fractional coverage. We suggest that careful adsorption experiments should be performed to reveal this vapor-liquid transition. ^ Finally, we study the influence of adsorbate coupling on the self-diffusion of benzene in Na-X and Na-Y zeolites. We propose a simple model for determining how adsorbate-adsorbate interactions modify activation energies of site-to-site jumps. We have calculated diffusion isotherms for a wide range of system parameters at different levels of theory viz., MFT, MFA and MSD, and segregated the resulting diffusion isotherms into supercritical and subcritical isotherms. The supercritical systems exhibit three characteristic diffusion isotherms, depending upon the degree of degeneracy of lattice sites, whereas the subcritical diffusion systems are dominated by cluster formation, exhibiting diffusion isotherms with broad regions of constant diffusivity. Our model for benzene in Na-X is in excellent qualitative agreement with pulsed field gradient NMR diffusivities, and in qualitative disagreement with tracer zero-length column (TZLC) data. We suggest that high temperature TZLC experiments should be performed, to test whether the coverage of maximum diffusivity decreases with increasing temperature. ^
Chemistry, Physical|Engineering, Chemical|Physics, Condensed Matter
"Transport and phase equilibria of benzene in FAU type zeolites"
(January 1, 2000).
Electronic Doctoral Dissertations for UMass Amherst.