Structural analysis of the microporous semiconductor K-SBC-1 during its reversible sorption of water

The reversible sorption of water molecules in the crystalline microporous semiconductor K-SBC-1 was investigated using temperature-resolved single-crystal XRD analysis. Three crystallographic sites of adsorbed water molecules, differing in adsorption strength, were discovered in the pores of K-SBC-1. The least tightly bound is located at the centre of the {Sb₁₂O₁₈} tube and begins to desorb around 50 °C. Above 200 °C the more strongly bound water molecules rearrange from their potassium-coordinating positions to the centre of the tube, thus obtaining the characteristics of the loosely bound water, and desorb thereafter. At 240 °C approximately 10% of the water has desorbed, leaving the host framework of K-SBC-1 intact. Upon re-adsorption of water at room temperature the molecules preferentially adsorb at sites in the centre of the {Sb₁₂O₁₈} tube. This shows that a heat treatment of 240–300 °C activates K-SBC-1 for sorption and explains the observed facile desorption of water from activated samples.