U.S. Department of Energy
Date of this Version
1997
Citation
Chem. Mater. 1997, 9, 417-419
Abstract
Hydrotalcite (HT), a clay mineral, is constructed of brucite-like layers [Mg(OH)2] in which substitution of Al3+ for Mg2+ at a mole ratio of 1:3 yields a net positive charge. This layer charge is compensated by exchangeable CO3 2- anions located (along with adsorbed water molecules) in the interlayer region. Numerous analogues of HT [i.e., “hydrotalcite-like” compounds (HTLCs)] have been synthesized with a variety of M2+/M3+ combinations,M2+/M3+ ratios, and interlayer anions.1-10 The compositions of these HTLCs are usually expressed using a generalized form of the formula for a substituted brucite, i.e., [M2+ 1-xM3+ x(OH)2]x+(Am-x/ m)•nH2O, where A is the interlayer charge-compensating anion, and x is the M3+/(M2+ + M3+) mole fraction.
Hydrotalcite and HTLCs have found use as anion exchangers, catalyst precursors, and catalyst supports.1 For applications involving catalysis or ion exchange it is desirable that the HTLCs be CO3-free and have high layer charge. The absence of CO32- is desirable because this anion is retained so strongly that access to the interlayer region by other ions and molecules is restricted and the effective anion-exchange capacity and reactive surface area are low. High layer charge (i.e., x > 0.33) is desirable because it increases the maximum possible exchange capacity. In addition to bulk-chemical analyses of phase-pure materials, layer charge can be estimated from the a0 lattice parameter (a0 = 2d110 for a hexagonal unit cell) provided the ionic radii of the two cations in the hydroxide sheet (e.g., Mg2+ and Al3+) are sufficiently different. To our knowledge, only one report of low-CO3 HTLCs having a layer charge ≥0.33 has been published,3 and these samples had between 15 and 33% of the interlayer sites occupied by CO32-.