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

Published

October 2, 2017

Author(s)

Dmitry V. Peryshkov, Eric V. Bukovsky, Matthew R. Lacroix, Hui Wu, Wei Zhou, W. Matthew Jones, Matic Lozinsek, Travis C. Folsom, D. Luke Heyliger, Terrence J. Udovic, Steven H. Strauss

Abstract

Structures of the alkali metal hydrates Li₂(H₂O)₄Z, LiK(H₂O)₄, Na₂(H₂O)₃Z, and Rb₂(H₂O)₂Z, unit cell parameters for Rb₂Z and Rb₂(H₂O)₂Z, and the DFT-optimized structures of K₂Z, K₂(H₂O)₂Z, Rb₂Z, Rb₂(H₂O)₂Z, Cs₂Z, and Cs₂(H₂O)Z are reported (Z^2- = B₁₂F₁₂^2-) and compared with previously reported X-ray structures of Na₂(H₂O)_0,4Z, K₂(H₂O)_{0,2,4Z, and Cs}D2^(H₂O)Z. Unusually rapid room-temperature hydration/dehydration cycles of several M₂Z/M₂(H₂O^)_nZ salt hydrate pairs, which were studied by isothermal gravimetry, are also reported. Finely ground samples of K₂Z, Rb₂Z, and Cs₂Z, which are not microporous, exhibited latent porosity by undergoing hydration at 24-25 °C in the presence of 18 torr H₂O(g) to K₂(H₂O)₂Z, Rb₂(H₂O)₂Z, and Cs₂(H₂O)Z in 18, 40, and 16 min, respectively. These hydrates were dehydrated at 24-25 Degree}C in dry N₂ to the original anhydrous M₂Z compounds in 61, 25, and 76 min, respectively (the exact times varies from sample to sample depending on particle size). The hydrate Na₂(H₂O)₂Z also exhibited latent porosity by undergoing multiple 90 min cycles of hydration to Na₂(H₂O)₃Z and dehydration back to Na₂(H₂O)₂Z at 23 °C. For the K₂Z, Rb₂Z, and Cs₂Z 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 K₂Z - K₂(H₂O)₂Z hydration/dehydration cycles with the same sample, the ration (rh_max)/(rddmax^) decreased 26 times over 8.6 °C, from 3.7 at 23.4 °C to 0.14 at 32.0 °C. For Rb₂Z - Rb₂(H₂O)₂Z cycles, (rh_max)/(rd_max) decreased from 0.88 at 23 °C to 0.23 at 27 °C/ For Cs₂Z - Cs₂(H₂O)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 addtion, the reversible substitution of D₂O for H₂O in fully hydrated Rb₂(H₂O)₂Z in the presence of N₂/16 torr D₂O(g) was complete in only 60 min at 23 °C.

Citation

Inorganic Chemistry

Volume

56

Issue

19

Pub Type

Journals

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Keywords

alkali metal salts, B12F12, closo-dodecaborates, dehydration, hydrates, hydration, porosity, x-ray diffraction

Citation

Peryshkov, D. , Bukovsky, E. , Lacroix, M. , Wu, H. , Zhou, W. , Jones, W. , Lozinsek, M. , Folsom, T. , Heyliger, D. , Udovic, T. and Strauss, S. (2017), Latent Porosity in Alkali-Metal M₂B₁₂F₁₂ Salts: Structures and Rapid Room-Temperature Hydration/Dehydration Cycles, Inorganic Chemistry, [online], https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=924120 (Accessed May 3, 2024)