Water adsorption in SAPO-34: elucidating the role of local heterogeneities and defects using dispersion-corrected DFT calculations

The chabazite-type silicoaluminophosphate SAPO-34 is a promising adsorbent for applications in thermal energy storage using water adsorption–desorption cycles. In order to develop a microscopic understanding of the impact of local heterogeneities and defects on the water adsorption properties{,} the interaction of different models of SAPO-34 with water was studied using dispersion-corrected density-functional theory (DFT-D) calculations. In addition to SAPO-34 with isolated silicon atoms{,} the calculations considered models incorporating two types of heterogeneities (silicon islands{,} aluminosilicate domains){,} and two defect-containing (partially and fully desilicated) systems. DFT-D optimisations were performed for systems with small amounts of adsorbed water{,} in which all H2O molecules can interact with framework protons{,} and systems with large amounts of adsorbed water (30 H2O molecules per unit cell). At low loadings{,} the host–guest interaction energy calculated for SAPO-34 with isolated Si atoms amounts to approximately −90 kJ mol−1. While the presence of local heterogeneities leads to the creation of some adsorption sites that are energetically slightly more favourable{,} the interaction strength is drastically reduced in systems with defects. At high water loadings{,} energies in the range of −70 kJ mol−1 are obtained for all models. The DFT-D interaction energies are in good agreement with experimentally measured heats of water adsorption. A detailed analysis of the equilibrium structures was used to gain insights into the binding modes at low coverages{,} and to assess the extent of framework deprotonation and changes in the coordination environment of aluminium atoms at high water loadings.