Non-stoichiometric precipitation of phase-pure hydrotalcite-like compounds
EMSL Project ID
1647
Abstract
Hydrotalcite-like compounds (HTLCs), brucite-like materials, are used as gaseous adsorbents (e.g., CO2, NOx, SOx, etc.), anion exchangers, catalyst precursors, and catalyst supports. The general formula of an HTLC is [M2+1-xM3+x(OH)2]x+(Am-x/m).nH2O, where M2+ and M3+ ions are divalent and trivalent cations of similar ionic radii, A is the interlayer charge-compensating anion which can be either divalent or monovalent organic or inorganic anion, and x [M3+/(M2+ + M3+) mole fraction] is the layer charge. For adsorption, ion exchange, or catalysis it is desirable that the HTLCs be CO32--free and have high layer charge. For this project, a series of HTLCs with CO32- and/or terephthalate dianion (TA2-) interlayer anions with variable layer charges were prepared. Approximate layer charge values of these HTLCs were calculated from bulk chemical (Mg, Al, and total and inorganic carbon) analysis data assuming that entire Mg and Al determined were part of HTLC phase -- none was assigned to pure phase minerals, e.g., brucite, gibbsite, etc. XRD analyses of these HTLCs did not indicate presence of any brucite or gibbsite phases. XRD, however, is not sensitive enough to determine poorly crystalline or small amounts of brucite, gibbsite, etc. Presence of even minor amounts of other phases will make it impossible to calculate the layer charge. An average layer charge, however, can be derived from the a unit cell parameter. Homogeneity or phase-purity of the HTLCs, in terms of layer charge, still cannot be derived by XRD. An understanding of compositional homogeneity of these HTLCs with high spatial resolution is necessary to understand the behavior of these materials in various applications. TEM coupled with EDX data would provide such resolution, allowing study of the phase-purity and determination layer charge of four HTLC samples synthesized with different interlayer anions.
Project Details
Project type
Exploratory Research
Start Date
1999-08-10
End Date
2002-03-31
Status
Closed
Released Data Link
Team
Principal Investigator
Team Members