Reversible channel deformation of zeolite omega during template degradation highlighted by in situ time-resolved synchrotron powder diffraction

Abstract

The thermal dehydration and the degradation of template of zeolite omega, a synthetic analog of mazzite Na 6.6TMA 1.8(H 2O) 22.2[Al 8.4Si 27.6O 72]-MAZ, was studied in situ by synchrotron powder diffraction. The evolution of the structural features was monitored through 25 structure refinements in the temperature range from 30 to 830 °C by full profile Rietveld analysis performed in the P6 3/ mmc space group. Structural refinements allowed the steps of degradation of tetramethylammonium (TMA), the evolution of the occupation of the different water sites and the migration of the Na cations located along the axis of the 12-ring channel towards a new site near the walls of the ring to be monitored. Transient deformations of the framework were highlighted, related to the constrained diffusion of the products of dehydration and degradation through the 8-ring channels. Permanent deformations of the frameworks correspond to the removal of the TMA cations from the gmelinite cages, which induces a relaxation of the strained 180° T1–O2–T1 angles, a widening of the opening of the 8-ring channels and a star-shaped deformation of the 12-ring channels.