Modeling the Hydration-Induced Structural Transitions of the SAPO-34 Zeolite and Their Impact on the Water’s Sorbed Phase Equilibrium and Dynamics

We identified computationally a number of hydration-induced structural phases of the SAPO-34 zeolite, possessing different energetic characteristics as revealed by density functional theory calculations and exhibiting different adsorption thermodynamics, thereby explaining previous experimental findings. The successive transitions between phases A, B, C, D, and E, sorted in terms of increasing stability (decreasing chemical potential), were proved to give rise to hysteresis loops that appeared during the water desorption isotherm for various temperatures. Our sorbate water molecular dynamics simulations are in agreement with previous pulsed-field gradient NMR results and showed that the sorbate diffusivity rises with increasing loading because of a decrease of the free energy barrier for surmounting the sorbent windows, which are being primarily water-populated. Moreover, we found that the phase-dependent water diffusivity decreases for all loadings as a result of the gradual reduction in width of the aforesaid apertures upon transitioning from phase A to E.