The unusual thermal behaviour of boron-ZSM-5 probed by “in situ” time-resolved synchrotron powder diffraction

The thermal behaviour of boron substituted ZSM-5 synthesized in the presence of ethylenediamine as an organic template was highlighted for the first time by in situ time-resolved synchrotron powder diffraction in the 25–900 °C temperature range. The evolution of the structural features was followed by full profile Rietveld refinements at up to 730 °C. Above this temperature, a structural breakdown occurs and B-ZSM-5 is converted into a β-cristobalite phase. A continuous increase in unit-cell parameters as a function of temperature is observed both during calcination (between 200 and 500 °C) and afterwards. This behaviour is completely unusual when compared to that assumed by other boron-free MFI-type materials both in their as-synthesized as well as calcined forms. On the basis of all these results, B-ZSM-5 represents the first example of positive thermal expansion in a material with an MFI type topology.

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► B-ZSM-5 thermal behaviour was studied between 25 and 900 °C by in situ synchrotron XRPD.
► Above 730 °C, B-ZSM-5 collapses and it is converted into a β-cristobalite phase.
► A continuous increase in unit-cell volume as a function of temperature was observed.
► This behaviour is completely unusual in MFI-type materials.
► B-ZSM-5 represents the first example of positive thermal expansion in MFI topology.