Does porous mean soft? On the elastic behaviour and structural evolution of zeolites under pressure

This is a comparative study on lattice compressibility, pressure (P)-induced structural deformation mechanisms and influence of the framework and extra-framework content on the elastic behaviour of zeolites, based on previously published data obtained by in situ HP-single crystal and powder diffraction experiments. The elastic data of zeolites reported so far allow us to infer that: 1) the peculiar characteristics of the zeolitic structure, with large channels and a flexible framework built of rigid units (i.e. the tetrahedra), implies that the main deformation mechanisms at high-pressure (HP) are controlled by rigid (Si,Al)O4-tetrahedral tilting; 2) the structural rearrangement at HP is mainly driven by framework geometry and its topological symmetry; 3) the compressibility of zeolites appears not to be directly related to the microporosity, represented by the “framework density”; 4) the elastic parameters available for natural zeolites demonstrate that microporosity does not necessarily imply high compressibility. Several zeolites appear to be less compressible than many rock-forming minerals. A high compressibility is generally expected for open-framework structures due to the tetrahedral tilting, which produces inter-tetrahedral angle variations and accommodates the effect of pressure. However, the bonding between the host zeolitic framework and the stuffed guest species (cations and H2O molecules) affect the overall compression behaviour, making this class of porous material unexpectedly less compressible than other silicates.