Reconstructive phase transitions in microporous materials: Rules and factors affecting them

This paper is concerned with the topological modifications induced by heating in microporous materials and with the rules obeyed by them. Two main characteristics are always present in these reconstructive processes: (1) they occur owing to the breaking of T–O–T bridges and the migration of the tetrahedral cations to new sites, which have three vertices as before, whereas the fourth vertex can be shared or unshared by another tetrahedron (“face sharing tetrahedra” process) (2) the T–O–T breaking occurs in the smallest ring in the structure (4- or 5-ring). Many factors can influence this process. Besides the topology, the chemical composition of the framework and extraframework, the synthesis conditions, also the heating modalities, either near or far from thermodynamic equilibrium conditions, can not only favour or prevent modification but also induce topologically different phases.

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► The thermal and hydrothermal stability of microporous materials are of paramount importance for their application as heterogeneous catalysts.
► Several studies have demonstrated that topological changes induced by heating occur in a number of these materials already at a quite low temperature (below 500 °C).
► The aim of this study is to show that in zeolitic materials topological transformations obey to well defined rules.
► In particular they are due to the breaking of T–O–T bridges, through a process called by us “face sharing tetrahedra”, which occur in the smallest ring present in the structure (4- or 5-ring) and are influenced by many factors.