## 2 Zeolites
### Mechanical Properties  
The mechanical properties of porous materials are another relevant issue in physical properties of zeolites, together with adsorption properties [104], as evident in such phenomena as clay swelling [105], breathing of structures [106], adsorption-induced phase transitions [107], pressure-induced amorphization, or hydration [108]. Microporous zeolites may have a great relationship between adsorption and mechanical properties, allowing one to focus on the effect from adsorbates on compressibility of the crystalline phase and reversible pressure-induced amorphization [109]. The presence of organic molecules on the framework generates an MOF structure, which is a porous material too. The framework exhibits a transition between two distinct regions of near-linear compressibility, being incompressible at low pressure before compressing rapidly beyond a threshold pressure. In this process, change in space group symmetry does not occur [110]. The pressure-induced amorphization is a phenomenon where a material may be amorphized reversibly under pressure, returning to its original crystalline structure and orientation when pressure decreases [111]. The presence of nondeformable units is essential for this reversibility, acting as a template and restoring the original structure [112]. X-ray diffraction is the main method of gaining evidence of a change in structure crystallinity, showing the formation of a high-density amorphous phase. Synchrotron X-ray diffraction has been applied for this purpose. An FT-IR study of stretching modes on a zeolite framework found a shift of bands to high energy values with increasing pressure. The occurrence of T-O asymmetric stretching mode is evidence that the crystallinity of zeolite decreases as the pressure is increased. The vibrational mode related to the double four-membered rings (D4R) may became very weak with the increased pressure being able to disappear, indicating that the structure is amorphous [113]. The decrease in pressure may bring back the D4R band, showing the reversibility of the process. The mechanical stresses and strains are critical to reach practical implementation. Good stiffness and robustness are important in retaining structural integrity under high-pressure environments, e.g., in sorption applications. Excessive structural distortion and framework collapse may be averted. The stiffness and hardness properties are important in ZIF structures, i.e., a zeolitic imidazolate framework, and they have influence in sterically bulky imidazolate-type linkages [114]. The elastic properties, measured by elastic constants, may be elucidated by computational tools such as DFT (density functional theory), which gives a better understanding of the origin of the elastic anisotropy. The stiffness may also influence bond distances and angles [115]. The elastic modulus of zeolitic and other porous materials is the measure of the material's resistance to elastic deformation: if its value is low, the material stretches a lot and vice versa. This parameter was measured to confirm a relationship with the topology, which may play a bigger role in elastic response [116]. The topology, pore geometry, and size define the mechanical strength, where the pore shapes (cubic, cylindrical, spherical, tetrahedral, and octahedral) contribute to structural performance under load [117]. Somemechanical properties of dense polymorphs of silica are important for sodalite, chabazite and mordenite structures. These properties may be described by an elastic tensor using Hooke's law [118]. In these structures, the cohesive energies are almost independent of structure and the important variation in Si-O-Si angle distribution has a great influence [119]. According to Astala et al. [118], the bulk modulus, i.e., the pressure required to cause a unit change in volume, is dependent on network constraints and space group symmetry. Some zeolites may present an auxetic property, i.e., having a behavior similar to auxetic polymers, getting fatter when stretched and thinner when compressed.