Latent Porosity in Alkali-Metal M2B12F12 Salts: Structures and Rapid Room-Temperature Hydration/Dehydration Cycles

Structures of the alkali-metal hydrates Li-2(H2O)(4)Z, LiK(H2O)(4)Z, Na-2(H2O)(3)Z, and Rb-2(H2O)(2)Z, unit cell parameters for Rb(2)Z and Rb-2(H2O)(2)Z, and the density functional theory (DFT)-optimized structures of K(2)Z, K-2(H2O)(2)Z, Rb(2)Z, Rb-2(H2O)(2)Z, Cs(2)Z, and Cs-2(H2O)Z are reported (Z(2-) = B12F122-) and compared with previously reported X-ray structures of Na-2(H2O)(0,4)Z, K-2(H2O)(0,2,4)Z, and Cs-2(H2O)Z. Unusually rapid room temperature hydration/dehydration cycles of several M(2)Z/M-2(H2O)(n)Z salt hydrate pairs, which were studied by isothermal gravimetry, are also reported. Finely ground samples of K(2)Z, Rb(2)Z, and Cs(2)Z, which are not microporous, exhibited latent porosity by undergoing hydration at 24-25 degrees C in the presence of 18 Ton of H2O(g) to K-2(H2O)(2)Z, Rb-2(H2O)(2)Z, and Cs-2(H2O)Z in 18, 40, and 16 min, respectively. These hydrates were dehydrated at 24-25 degrees C in dry N-2 to the original anhydrous M(2)Z compounds in 61, 25, and 76 min, respectively (the exact times varied from sample to sample depending on the particle size). The hydrate Na-2(H2O)(2)Z also exhibited latent porosity by undergoing multiple 90 min cycles of hydration to Na-2(H2O)(3)Z and dehydration back to Na-2(H2O)(2)Z at 23 degrees C. For the K(2)Z, Rb(2)Z, and Cs(2)Z transformations, the maximum rate of hydration (rh(max)) decreased, and the absolute value of the maximum rate of dehydration (rd(max)) increased, as T increased. For K(2)Z <-> K-2(H2O)(2)Z hydration/dehydration cycles with the same sample, the ratio rh(max)/rd(max) decreased 26 times over 8.6 degrees C, from 3.7 at 23.4 degrees C to 0.14 at 32.0 degrees C. For Rb(2)Z <-> Rb-2(H2O)(2)Z cycles, rh(max)/rd(max) decreased from 0.88 at 23 degrees C to 0.23 at 27 degrees C. For Cs(2)Z <-> Cs-2(H2O)Z cycles, rhmaird, decreased 20 times over 8 degrees C, from 6.7 at 24 degrees C to 0.34 at 32 degrees C. In addition, the reversible substitution of D2O for H2O in fully hydrated Rb-2(H2O)(2)Z in the presence of N-2/16 Torr of D2O(g) was complete in only 60 min at 23 degrees C.