Autor: |
Peryshkov DV; Department of Chemistry, Colorado State University , Fort Collins, Colorado 80523, United States.; Department of Chemistry and Biochemistry, University of South Carolina , Columbia, South Carolina 29208, United States., Bukovsky EV; Department of Chemistry, Colorado State University , Fort Collins, Colorado 80523, United States., Lacroix MR; Department of Chemistry, Colorado State University , Fort Collins, Colorado 80523, United States., Wu H; Center for Neutron Research, National Institute of Standards and Technology (NIST) , Gaithersburg, Maryland 20899, United States., Zhou W; Center for Neutron Research, National Institute of Standards and Technology (NIST) , Gaithersburg, Maryland 20899, United States., Jones WM; Department of Chemistry, Colorado State University , Fort Collins, Colorado 80523, United States., Lozinšek M; Department of Chemistry, Colorado State University , Fort Collins, Colorado 80523, United States.; Department of Inorganic Chemistry and Technology, Jožef Stefan Institute , 1000 Ljubljana, Slovenia., Folsom TC; Department of Chemistry, Colorado State University , Fort Collins, Colorado 80523, United States., Heyliger DL; Department of Chemistry, Colorado State University , Fort Collins, Colorado 80523, United States., Udovic TJ; Center for Neutron Research, National Institute of Standards and Technology (NIST) , Gaithersburg, Maryland 20899, United States., Strauss SH; Department of Chemistry, Colorado State University , Fort Collins, Colorado 80523, United States. |
Abstrakt: |
Structures of the alkali-metal hydrates Li 2 (H 2 O) 4 Z, LiK(H 2 O) 4 Z, Na 2 (H 2 O) 3 Z, and Rb 2 (H 2 O) 2 Z, unit cell parameters for Rb 2 Z and Rb 2 (H 2 O) 2 Z, and the density functional theory (DFT)-optimized structures of K 2 Z, K 2 (H 2 O) 2 Z, Rb 2 Z, Rb 2 (H 2 O) 2 Z, Cs 2 Z, and Cs 2 (H 2 O)Z are reported (Z 2- = B 12 F 12 2- ) and compared with previously reported X-ray structures of Na 2 (H 2 O) 0,4 Z, K 2 (H 2 O) 0,2,4 Z, and Cs 2 (H 2 O)Z. Unusually rapid room-temperature hydration/dehydration cycles of several M 2 Z/M 2 (H 2 O) 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 °C in the presence of 18 Torr of H 2 O(g) to K 2 (H 2 O) 2 Z, Rb 2 (H 2 O) 2 Z, and Cs 2 (H 2 O)Z in 18, 40, and 16 min, respectively. These hydrates were dehydrated at 24-25 °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 (H 2 O) 2 Z also exhibited latent porosity by undergoing multiple 90 min cycles of hydration to Na 2 (H 2 O) 3 Z and dehydration back to Na 2 (H 2 O) 2 Z at 23 °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 (H 2 O) 2 Z hydration/dehydration cycles with the same sample, the ratio rh max /rd max decreased 26 times over 8.6 °C, from 3.7 at 23.4 °C to 0.14 at 32.0 °C. For Rb 2 Z ↔ Rb 2 (H 2 O) 2 Z cycles, rh max /rd max decreased from 0.88 at 23 °C to 0.23 at 27 °C. For Cs 2 Z ↔ Cs 2 (H 2 O)Z cycles, rh max /rd max decreased 20 times over 8 °C, from 6.7 at 24 °C to 0.34 at 32 °C. In addition, the reversible substitution of D 2 O for H 2 O in fully hydrated Rb 2 (H 2 O) 2 Z in the presence of N 2 /16 Torr of D 2 O(g) was complete in only 60 min at 23 °C. |