Cyanide Self-Addition,Controlled Adsorption,and Other Processes at Layered Double Hydroxides

Layered double hydroxides (LDH) are anion-exchangingmaterials of the type M(III)-M(II)_x(OH)_(2x+2)Y thatoccur abundantly in nature, and can concentrate, protect, andactivate simple organic anionic species of possible relevance tothe earliest organisms. We now wish to report progress in thefollowing areas:1) Internal vs. external uptake of anions. Ferrocyanidedoes not displace carbonate from synthetic hydrotalcite (Mg:AlLDH carbonate) but is nevertheless taken up on the outside of theparticles. In other cases, anion uptake is controlled byspecific hydrogen bonding requirements rather than by chargedensity alone, a feature that can be used to control whetheruptake will be both internal and external, or external only. These two findings taken together have important implications forspecific catalysis by LDH, since specific hydrogen bonding willaffect the individual and relative conformations of substrateanions, and anions occupying space in the interlayer will beunder tighter constraints than those adsorbed externally.2) Specific reactions catalyzed by LDH. We have found thatthe LDH Mg₂Al(OH)₆Cl catalyzes the self-addition ofcyanide, to give in a one-pot reaction at low concentrations anincreased yield of diaminomaleonitrile and in addition, at higher( 0.05M) concentrations, a purple-pink material that adheres tothe LDH. We are investigating whether this reaction also occurswith hydrotalcite itself, what is the minimum effectiveconcentration of cyanide, and what can be learned about theproducts and how they compare with those reported at high HCNconcentrations in the absence of catalyst.     

Preferential uptake of ammonium ions by zinc ferrocyanide

The concentration of ammonia from dilute aqueous solution could have facilitated many prebiotic reactions. This may be especially true if this concentration involves incorporation into an organized medium. We have shown that (unlike iron(III) ferrocy anide) zinc ferrocyanide, Zn₂ Fe (CN)₆·xH₂O, preferentially takes up ammonium ions from 0.01 M NH₄Cl to give the known material Zn₃(NH₄)₂ [Fe(CN)₆]₂·xH₂O, even in the presence of 0.01M KC1. KC1 alone gave Zn₃K₂[Fe(CN)₆]₂·xH₂O. Products were characterized by elemental (CHN) analysis and powder X-ray diffraction (XRD). We attribute the remarkable specificity for the ammonium ion to the open framework of the product, which offers enough space for hydrogen-bonded ammonium ions, and infer that other inorganic materials with internal spaces rich in water may show a similar preference.     

Photoprecipitation and the banded iron-formations — Some quantitative aspects

The oxidative deposition of iron in the Banded iron-Formations can be quantitatively accounted for by direct abiotic photo-oxidation, by extrapolating from laboratory conditions and making reasonable assumptions about the early Earth and its oceans. Within this model, iron supply was the limiting factor, the Precambrian ocean surfaces were iron-depleted, and hydrogen would have been released into the atmosphere at a rate controlled by Fe(II) mixing. Other mechanisms operating in parallel are not excluded, and the Fe(II) budget suggests that recirculation by reaction with reduced carbon was important by ca. 2.5 b.y.b.p.