## 2 Zeolites
### Thermal Conductivity  
The thermal conductivity of zeolites is a key factor controlling the transport of energy. Measuring this property in a dense bulk zeolite is difficult because of little sinterability. For measures done on zeolite resin composites, the heat capacity reflected the relative density, suggesting that the pores have an influence on the heat capacity [121]. Other parameters to take into consideration are the thermal diffusivity and thermal conductivity, which may be slightly influenced by porosity. Thermal diffusivity is the capacity to conduct thermal energy relative to capacity to store energy by the material. High values of this parameter mean that heat transfers rapidly, increasing with decreasing temperature [122]. Negative thermal expansion is a phenomenon in the zeolitic framework related to the prevalence of low-frequency acoustic modes of vibrations being the major contributor to thermal conductivity. A measurement of this parameter is a way to separate contributions of different vibrational modes, providing a microscopic insight into thermal properties of framework silicates [123]. The removal of crystalline powder may be performed to determine thermal conductivity based on three principal contributors: heat transfer through the gas, radioactive heat transfer, and conduction through the solid sample. Thermal conductivity has relevance in zeolites also, because it uses a catalytic reaction where chemical reactions release heat either inside the structure or at surfaces due to adsorption of molecules [124]. Some authors used a computational technique called nonequilibrium molecular dynamics (NEMD), an approach to thermal conductivity determination inducing a thermal gradient by moving kinetic energy between different parts of the simulation box. Verifying the thermal gradient is a path to ascertain thermal conductivity in materials [125]. This property allows the use of zeolites as electric insulators, with a low dielectric constant and transporting heat away from electric components [126]. The high-silica zeolites satisfy the conditions for this characteristic. The direction of the crystallographic axis in the zeolite  
Figure 8: Pattern of relaxation process in compression of mordenite along [100] axis. Reproduced from [118]. Copyright 2004 ACS. framework is another factor relevant for thermal conductivity. In some cases, as in FER zeolites, the property is three times higher in the z-direction than in the x-direction. Moreover, the high framework density in general is related to high conductivity. Thermal stability of zeolites is an important parameter to understand in which range of temperature the structure maintains it properties. It is important to verify if the temperature is near or beyond the point of dehydration and does not significantly affect the degree of structural order. In terms of this aspect, the geometry of the crystal lattice plays a large part in stability [127]. For this purpose, Castro et al. [128] screened thermal stability using DRX. An FAU structure was subjected to different temperatures--200 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, and 600 \({}^{\circ}\)C--which did not change the diffraction peak pattern at 20: 10.11 (220), 23.58 (533), or 31.31 (751). The collapse of structure occurs at 800 \({}^{\circ}\)C, elevating at 850 and 880 \({}^{\circ}\)C, the formation of nepheline and mullite phases is observed, and the higher the silica content, the higher the thermal stability. Although this relationship exists, the Si/Al ratio is not the only parameter that determines stability. The degree of structural order (crystallinity) of the zeolites is another characteristic, depending on the synthesis conditions. The morphology may also change according to temperature.