Evolution of host/guest interactions during adsorption of tetrachlorethylene (TCET) by silicalite (MFI): A study by X-ray powder diffraction, NMR and molecular mechanics

NMR and XRPD investigations concerning the structural evolution of the silicalite/tetrachlorethylene (TCET) zeolitic host/guest system show that the 0–2 TCET per unit-cell (uc) concentration range corresponds first to a diphased domain (mixtures of the empty silicalite plus the silicalite/2 TCET complex), the 2–4 TCET per uc range to a solid solution domain and the 4–8 TCET per uc range to a second diphased domain (mixtures of silicalite/4 TCET and silicalite/8 TCET complexes). Computer simulations using the Bukingham exp-6-1 and the Lennard–Jones 6-12-1 models show that only the former one (a) correctly predicts the experimental adsorption heats, (b) yields sensible site locations of the sorbed TCET molecules, and (c) explains the experimentally observed adsorption-heat increase at higher pore-fillings. These results are in agreement with structural investigations which show that the framework symmetries in silicalite/4 TCET and silicalite/8 TCET are Pnma and P212121, respectively. Although the silicalite/8 TCET and silicalite/8 p-xylene host/guest complexes exhibit several structural analogies, the orientations of their strongly elliptic straight channel sections are different. The monoclinic/orthorhombic Pnma phase transition is attributed to specific Cl⋯O (framework) interactions.