Kinetics and dynamic behaviour of VOCs desorption from binary aqueous mixtures on zeolite ZSM-5: a combined in situ high-temperature synchrotron powder X-ray diffraction and chromatographic study

In situ high-temperature (HT) synchrotron X-ray powder diffraction (XRPD) was used in this study as a tool to understand the adsorption/desorption process features of methyl tert-butyl ether (MTBE)/toluene (TOL) and 1,2-dichloroethane (DCE)/MTBE binary mixtures adsorbed in organophilic ZSM-5 zeolite (SiO2/Al2O3 ~ 280), as well as framework structural modifications upon thermal treatment from room temperature to 600°C. The binary system was described by a competitive dual site Langmuir adsorption isotherm. The results achieved from the series of time-resolved Rietveld refinements allowed to highlight the out-of-equilibrium effects which govern desorption of ZSM-5 powders in dynamic conditions. The decomposition of TOL and MTBE in both systems starts at about 100°C and continuously proceeds up to ~250°C. As far as concerns DCE molecules, they are completely released from the cavities at ~350°C. Significant internal pressure is developed with the composition product diffusion, causing a corresponding transient distortion in the 10-ring channels. Above 500°C, the unit-cell volume contraction without any mass loss gives rise to a negative thermal expansion (NTE) process. Our results clearly demonstrated that ZSM-5 regeneration is effective when it is thermally treated at about 500°C. Above this temperature, when all the organic have been ejected, non-equilibrium distortions in the framework are relaxed and channel apertures become more circular, thus demonstrating the efficiency of thermal regeneration under mild conditions.