Activity and Reactivity of Fe2+ Cations in the Zeolite. Ab Initio Free-Energy MD Calculation of the N2O Dissociation over Iron-Exchanged Ferrierite

First-principles calculations are used to investigate the stability and adsorption properties of the bivalent Fell cation exchanged into ferrierite. The reactivity of cations is inspected via the temperature-dependent molecular dynamics (MD) simulation. The cation is located in the most stable alpha and beta sites of the zeolite. The MD simulations show that the bivalent Fell cation can induce a local deformation of the zeolite framework, which cannot be observed experimentally. Three inspected configurations, formed in the alpha-2, beta-2, and beta-1 sites, exhibit large differences in the adsorption capacity. The adsorption energies of the N2O molecule on the cation in the alpha-2, beta-2, and beta-1 sites of similar to 49, similar to 34, and similar to 4 kJ/mol correlate with the stability of the particular configuration (100, 49, and 0 kJ/mol). For all three cations, the constrained MD is used to calculate the change of the free energy (AF) necessary to activate the adsorbed N2O molecule to the transition state (TS). At room temperature with decreasing stability of the configuration, Delta F increases from 32.0 +/- 2 kJ/mol (beta-1) to 42.5 +/- 2 kJ/mol (beta-2). For the most unstable alpha-2 site, Delta F increases only to 40.0 +/- 2 kJ/mol. The reason is different bonding properties of the cation in the alpha site compared with those of the cation in the beta sites. Arrhenius plots constructed from Delta F at 300, 500, and 700 K provide the activation energies 40.0 +/- 6, 50.0 +/- 6, and 48.5 +/- 5.5 kJ/mol for the beta-1, beta-2, and beta-2 configurations, respectively. The calculated values are in good agreement with the experimental activation energy by Panov et al. (J. Mol. Catal. 1960, 61, 85-97) of 41.9 +/- 8.4 kJ/mol, proving that Fe2+ cations belong to extra-framework particles active in the intrazeolite N2O dissociation. In line with the compensation law, the calculated prefactor A of 0.9 x 10(14), 1.0 x 10(14), and 1.8 x 10(14) s(-1) for the beta-1, beta-2, and alpha-2 sites, respectively, exhibits increasing reaction rate with increasing activation energy. The reaction rate at the most unstable and most active a site is 2 times higher than the rate at the most stable and least active beta-1 site. Both the adsorption energy and the reaction rate indicate that the alpha site is a preferable active site for dissociation of N2O. Our calculations show a variation of the activation energy and the reaction rate with the change of the stability of the Fe2+ cation. A similar variation is expected also for other extra-framework particles, like Fe-O-Fe dimers and Fe-oxo species. This phenomenon can lead to similarities in the reaction properties of particles of different character.