Sorption of biphenyl in non-acidic MFI-type zeolites:: spectroscopic and modeling studies

The diffuse reflectance UV-VIS and IR absorption, Raman scattering spectrometries as well as C-13-MAS-NMR experiments provide evidence for the sorption, structure and dynamics of biphenyl (BP, C12H10) as intact molecule in non-acidic MFI zeolites, silicalite-l and Al-ZSM-5. The effect of the aluminum content n, the nature of the charge-balancing cation Mn+ and the BP loading in M-m/n (AlO2)(m)(SiO2)(96-m) (M(m/n)ZSM-5; m = 0, 3, 6; n = 1, 2; M = Li+, Na+, K+, Rb+, Cs+, Tl+, Zn2+, Cd2+) was examined. X-ray powder diffraction, Si-29, Al-27, MAS-NMR as well as Raman scattering experiments provide evidence for structural changes of the host upon BP sorption even at low loading. From Monte Carlo simulations, the preferential sites of BP were determined in the straight channels of purely siliceous silicalite-l with orthorhombic crystal symmetry, whereas well defined sites are energetically favored in the vicinity of extra-framework cations of aluminated ZSM-5. Molecular mechanics calculations indicate that BP lies in the straight channels in a twisted conformation with a phenyl group facially coordinated to the extra-framework cations. From the molecular dynamics calculations, the BP mean square displacements (MSD) indicate that on the simulation time (1 ns) BP is occluded in its sorption site. The calculated frequencies of vibrational densities of states of selected atoms of BP were found to be in agreement with the frequencies of IR and Raman bands assigned to corresponding vibrational modes. (C) 2000 Elsevier Science B.V. All rights reserved.