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Polarizability Characterization of Zeolitic Brønsted Acidic Sites
1 , 2, 3
1  Institut Universitari de Ciència Molecular, Universitat de València, Edifici d'Instituts de Paterna, P. O. Box 22085, E-46071 València, Spain
2  Departamento de Química, Universidad Politécnica de Valencia, Camino de Vera s/n, E-46022, València, Spain
3  Departamento de Ciencias Experimentales, Facultad de Ciencias Experimentales, Universidad Católica de Valencia San Vicente Mártir, Guillem de Castro-94, E-46003 València, Spain

Abstract: The interacting induced-dipoles polarization model, implemented in our program POLAR, is used for the calculation of the effective polarizability of the zeolitic bridged OH group, which results much higher than that of the free silanol group. A high polarizability is also calculated for the bridged OH group with a Si 4+ , in absence of Lewis-acid promotion of silanol by Al 3+. The crystal polarizability is estimated from the Clausius–Mossotti relationship. Siliceous zeolites are low-permittivity isolators. The interaction of a weak base with the zeolitic OH can be considered as a local bond. Only when cations are located in the zeolite micropore, next to tetrahedra that contain trivalent cations, are large electrostatic fields generated. They are short ranged, and the positive cation charges are compensated for by corresponding negative lattice charges. A method for the calculation of fractal surfaces of crystals is presented. The fractal dimension D of fragments of zeolites is calculated. Results compare well with reference calculations (GEPOL). The active site of Brønsted acid zeolites is modelled by sets of Al–OH–Si units, which form 2–12-membered rings. Topological indices for the different active-site models are calculated. The comparison between GEPOL and SURMO2 allows calculating the active-site indices. Most cavities show no fractal character, while for the 6–8-units rings D lies in the range 4.0–4.3. The 6-ring shows the maximum D; it is expected to be the most reactive.
Keywords: polarization; polarizability; active site; Brønsted acid; porous material; zeolite; fractal dimension; solvent-accessible surface