, Volume 31, Issue 1-2, pp 53-69

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

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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.