Sorption of biphenyl in NaX zeolites

Abstract

The predictions of the sorption energy, sorption sites and mobility of biphenyl (BP, C₁₂H₁₀) occluded in faujasitic X zeolites Na_n(SiO₂)_192−n(AlO₂)_n (M_nFAU; n=85, 96) were carried out using Monte Carlo simulations (MC), molecular mechanics (MM) and molecular dynamics (MD) calculations, respectively, as functions of the Na⁺ occupancy of the site III and BP loading. In the energetically preferred sites, the twisted BP lies in the supercage with each phenyl group facially coordinated to Na⁺ in site II and/or III and the accommodation of two BP within the same supercage occurs for more than 4 BP–Na_nFAU average loading. The diffuse reflectance UV-visible absorption and Raman scattering spectrometry point out that BP molecules adsorb without any chemical reaction. The UV-visible absorption indicates the occupancy of 2 BP per supercage at bulk loading higher than 4 BP per unit cell. The calculated BP vibrational densities of state deduced from MD calculations are in agreement with the frequency values deduced from the Raman bands. In the vibrational time scale occluded BP are undergoing rapid motions at room temperature. The rapid motions at low loading average the microenvironments and vibrational couplings as found in solution. Nevertheless, the BP mobility is hindered by the presence of numerous Na⁺ cations and BP at high loading in the void space. These effects induce the broadening of some Raman bands.