New phosphonate intercalates of [Ca2Al(OH)6]NO3⋅yH2O: A synthetic and kinetic study

Four organic phosphonate anions (methyl-, ethyl-, phenyl- and benzyl- (MPA, EPA, PPA, BPA)) were successfully intercalated into the [Ca2Al(OH)6]NO3⋅yH2O (y=1–2y=1–2) layered double hydroxide (LDH) by anion exchange. The materials have been characterised by a range of physical methods including powder diffraction, elemental microanalysis, infrared spectroscopy, thermal analysis and solid state NMR. The measured interlayer expansions upon intercalation of the aromatic phosphonates suggest that the guests are intertwined between the layers, while the aliphatic guests appear to be tilted with respect to the layers. The elemental microanalysis data for all four intercalates suggests there is a significant incorporation of some of the neutral phosphonic acid as well as the anion into the structure. Solid state 31P NMR spectroscopy was employed to investigate the nature of the guest ions in some of these materials. In the MPA case, a single 31P resonance was seen, lying at a chemical shift intermediate between the neutral and monovalent forms of MPA. This implies rapid proton exchange occurs between MPA units, giving an average charge of between 0 and −1 on all guests. In the PPA case, two resonances were seen, but the chemical shifts of these were consistent with the average charge on the PPA units being between −1 and −2. The reason for this is believed to be that rapid proton exchange is occurring between the PPA anions and the interlayer water molecules: this rationalises the observed results while maintaining local electroneutrality. In situ diffraction studies were performed in order to determine the rate of intercalation of the methyl-, phenyl- and benzylphosphonate guest ions, and to investigate the reaction mechanisms. In all three cases, the reactions are direct transformations from the host to the product. In general we observe that nucleation is rate determining, but in the methyl and phenyl cases the importance of nucleation declines with increasing temperature.

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