Relative Humidity-Induced Reversible Hydration Of Sulfate-Intercalated Layered Double Hydroxides

Abstract

Abstract-Layered double hydroxides (LDH) are extremely important materials for industrial processes and in the environment, and their physical-chemical behavior depends in large part on their hydration state, but the characterization of these hydration effects on their properties are incomplete. The present study was designed to explore the interpolytype transitions induced by variation in the ambient humidity among LDHs. The cooperative behavior of intercalated water molecules resulted in a sudden, single-step, reversible dehydration of the [Zn-Cr-SO 4 ] LDH. The [Zn-Al-SO 4 ] LDH provided an interesting contrast with (1) the coexistence of the end members of the hydration cycle over the 40À20% relative humidity range during the dehydration cycle, and (2) a random interstratified intermediate in the hydration cycle. These observations showed that the [Zn-Al-SO 4 ] LDH offered sites having a range of hydration enthalpies, whereby, at critical levels of hydration (20À40%), the non-uniform swelling of the structure resulted in an interstratified phase. The variation in domain size during reversible hydration was also responsible for the differences observed in the hydration vs. the dehydration pathways. This behavior was attributed to the distortion in the array of hydroxyl ions which departs from hexagonal symmetry on account of cation ordering as shown by structure refinement by the Rietveld method. This distortion was much less in the [Zn-Cr-SO 4 ] LDH, whereby the nearly hexagonal array of hydroxyl ions offered sites of uniform hydration enthalpy for the intercalated water molecules. In this case, all the water molecules experienced the same force of attraction and dehydrated reversibly in a single step. The changes in basal spacing were also accompanied by interpolytype transitions, involving the rigid translations of the metal hydroxide layers relative to one another.

Electronic‐Structure Calculations of Cation‐Ordered II‐III Layered Double Hydroxides: Origin of the Distortion of the Metal‐Coordination Symmetry

“…All of the three LDHs crystallize in hexagonal symmetry (space group P 3; a obs = 5.35–5.41 Å, c obs = 11.05–11.15 Å; obs: observed) . These structures were used as input for the SIESTA program.…”

“…The models for the calculations were generated on the basis of the reported crystal structures for [Zn–Al], [Cu–Cr], and [Zn–Cr] LDHs , . The cell size (2√3 × 2√3) R30° was chosen with proper three‐dimensional periodic boundary conditions for the calculation.…”

“…All of the three LDHs crystallize in hexagonal symmetry (space group P3; a obs = 5.35-5.41 Å, c obs = 11.05-11.15 Å; obs: observed). [17][18][19] These structures were used as input for the SIESTA program. The experimental structures were obtained from X-ray diffraction data and do not include proton positions.…”

“…These structures have been refined within the space group P3 (a ≈ 5.35 Å, c ≈ 11.5 Å). They are the [Zn-Al], [18] [Cu-Cr], [17] and the [Zn-Cr] [19] LDHs. All of these LDHs have SO 4 2ions in the interlayer.…”

Layered Double Hydroxides: Proposal of a One-Layer Cation-Ordered Structure Model of Monoclinic Symmetry

“…(2) When [M II (OH) 6 ] and [M′ III (OH) 6 ] coordination polyhedra of different sizes share edges, the array of hydroxyl ions lose their hexagonal symmetry. 17 There is a nonuniform distension of the hydroxyl ion network resulting in three in-plane nonbonded HO---OH interatomic distances. 7 (3) The packing of hydroxyl ions can no longer be described in terms of the close-packed positions A, B, C.…”