Structural Changes in Nanoporous MFI Zeolites Induced by Tetrachloroethene Adsorption: A Joint Experimental and Simulation Study

Abstract

A joint experimental and molecular simulation study was performed to investigate the adsorption of tetrachloroethene on two MFI zeolites, ZSM-5 with a Si/Al ratio of 26.5, and silicalite-1, which is the pure silica form of ZSM-5. Adsorption isotherms and isosteric heats of adsorption of tetrachloroethene were measured and compared to molecular simulation results. Experimental curves for both MFI zeolites show a step at loading of 4 molecules·uc−1 that was interpreted in terms of a structural change of the host framework. A thermodynamic analysis based on the osmotic ensemble scheme allowed attributing this step to a symmetry change from ORTHO (orthorhombic form with Pnma symmetry) to PARA (orthorhombic form with P212121 symmetry) of both adsorbents upon micropore filling. The adsorption mechanism was also characterized. The first tetrachloroethene molecules adsorbs in the channel intersections of the MONO (monoclinic form with P21/n11 symmetry) and ORTHO structures of silicalite-1 and ZSM-5, respectively. Then, the above-mentioned structural change from ORTHO to PARA occurs, leading to a filling of sinusoidal channels until saturation was reached, i.e. ∼8 molecules.uc-1 for silicalite-1 and 6 molecules·uc−1 for ZSM-5. In addition, simulation results indicate that the sodium cations in the ZSM-5 zeolite are located at the channel intersections during the whole adsorption process.