Latent Porosity in Alkali-Metal M 2 B 12 F 12 Salts: Structures and Rapid Room-Temperature Hydration/Dehydration Cycles.

Structures of the alkali-metal hydrates Li2 (H2 O)4 Z, LiK(H2 O)4 Z, Na2 (H2 O)3 Z, and Rb2 (H2 O)2 Z, unit cell parameters for Rb2 Z and Rb2 (H2 O)2 Z, and the density functional theory (DFT)-optimized structures of K2 Z, K2 (H2 O)2 Z, Rb2 Z, Rb2 (H2 O)2 Z, Cs2 Z, and Cs2 (H2 O)Z are reported (Z2- = B12 F12 2- ) and compared with previously reported X-ray structures of Na2 (H2 O)0,4 Z, K2 (H2 O)0,2,4 Z, and Cs2 (H2 O)Z. Unusually rapid room-temperature hydration/dehydration cycles of several M2 Z/M2 (H2 O)n Z salt hydrate pairs, which were studied by isothermal gravimetry, are also reported. Finely ground samples of K2 Z, Rb2 Z, and Cs2 Z, which are not microporous, exhibited latent porosity by undergoing hydration at 24-25 °C in the presence of 18 Torr of H2 O(g) to K2 (H2 O)2 Z, Rb2 (H2 O)2 Z, and Cs2 (H2 O)Z in 18, 40, and 16 min, respectively. These hydrates were dehydrated at 24-25 °C in dry N2 to the original anhydrous M2 Z compounds in 61, 25, and 76 min, respectively (the exact times varied from sample to sample depending on the particle size). The hydrate Na2 (H2 O)2 Z also exhibited latent porosity by undergoing multiple 90 min cycles of hydration to Na2 (H2 O)3 Z and dehydration back to Na2 (H2 O)2 Z at 23 °C. For the K2 Z, Rb2 Z, and Cs2 Z transformations, the maximum rate of hydration (rhmax ) decreased, and the absolute value of the maximum rate of dehydration (rdmax ) increased, as T increased. For K2 Z ↔ K2 (H2 O)2 Z hydration/dehydration cycles with the same sample, the ratio rhmax /rdmax decreased 26 times over 8.6 °C, from 3.7 at 23.4 °C to 0.14 at 32.0 °C. For Rb2 Z ↔ Rb2 (H2 O)2 Z cycles, rhmax /rdmax decreased from 0.88 at 23 °C to 0.23 at 27 °C. For Cs2 Z ↔ Cs2 (H2 O)Z cycles, rhmax /rdmax decreased 20 times over 8 °C, from 6.7 at 24 °C to 0.34 at 32 °C. In addition, the reversible substitution of D2 O for H2 O in fully hydrated Rb2 (H2 O)2 Z in the presence of N2 /16 Torr of D2 O(g) was complete in only 60 min at 23 °C.