Thermal activation of NH4 precursor of acidic omega zeolite: A neutron and in-situ synchrotron powder diffraction combined study

For the first time, the real-time thermal activation of the NH4-omega zeolite (NH4-omega) was investigated through in situ synchrotron X-ray powder diffraction combined with neutron diffraction. The experimental approach allowed to study the precursor thermal behaviour upon heating, continuously monitoring the evolution of its structural features determined by NH4 ions and TMA calcination and dehydration processes. The activation of the precursor (H-omega sample) is fully reached at ⁓600 °C, when all the NH4 content has been released, dehydration mostly occurred and TMA degradation completed. From a structural point of view, Rietveld structural refinements highlighted relevant structural changes. In particular variations in intertetrahedral angles allowed to make hypothesis about the possible distribution of Brønsted acid sites on the framework oxygen atoms. The hypothesis was corroborated through the neutron Rietveld refinement of the deuterated and calcined omega (D-omega sample) that revealed the presence of two acid sites: D1 and D2 located on O5 and O2 framework oxygen atoms, respectively. Besides, neutron data confirmed the strong relation among framework and Brønsted acid sites geometry and short- and/or long-range interactions. In summary, non-conventional sources were successfully used to probe both activation and formation of acidic sites in omega precursor as well as determine the number and location of Brønsted sites in the deuterated omega.