Computational Studies on the Effects of Pressure and Temperature on Zeolite Framework Structures

Under high temperature and external pressures, up to 6 GPa, the structural changes of two Linde Type A (LTA) zeolites were investigated by Lattice Dynamics and energy minimization techniques. In our simulations classical empirical potentials have been used. The unit cell of ZK4 and Na-A structures were thermodynamically stable in the studied range of temperature (0 <= T <= 1300 K). The analysis of the unit cell geometry and vibrational spectra at selected pressures suggest the loss of crystallinity of the aluminosilicate structures. We found that a low-density amorphous phase is reached at pressures around 2.5-5.0 GPa. These results are compatible with Raman and X-ray diffraction studies. We obtained the structural properties of the new phase. In addition, we report the simulated elastic constants and the Bulk modulus of ZK4 and Na-A zeolites at ambient pressure. The simulated results are in semi-quantitative agreement with the experiment. Finally, we report a comparison of the results of two potential models in order to evaluate the impact of rigidity of the SiO4 and AlO4 tetrahedra on the zeolite stability.