Sorption of 4,4 '-bipyridine in Na(n)ZSM-5 zeolites: spectroscopic and modelling studies

The diffuse reflectance LN-visible absorption and Raman scattering spectrom etries provide evidence for the sorption course of 4,4 ' -bipyridine (4,4 ' -bpy) within the porous void space of silicalite-1 [(SiO2)(96)] over sever al weeks at loadings of 1 and 2 molecules per unit cell. The presence of ex tra-framework Na+ cations in the void space of Na(n)ZSM-5 (n = 3, 6) Na-n(A lO2),(SiO2)(96-n) slows down the sorption process, particularly at higher l oading. Raman scattering and UV-visible absorption indicate that 4,4'-bpy i s occluded as intact molecule in the non-acidic zeolites under study, The m otions of 4,4'-bpy within the channels of silicalite-1 approach the isotrop ic limit of a liquid at room temperature, whereas in NaZSM-5 zeolite, the i nteraction between the Na+ cation and 4,4'-bpy hinder the dynamic motions. Monte Carlo and molecular dynamics simulations indicate that 4,4'-bpy is lo cated in the straight channels of MFI zeolites. In NaZSM-5 zeolites, the ex tra-framework cations were found to be located in the vicinity of the N ato ms outside the plane of the pyridyl groups. (C) 2001 Elsevier Science B.V. All rights reserved.