The intercalation of phenylphosphonic acid in layered double hydroxides
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abstract
Magnesium-aluminum hydroxy nitrates ((NO3)-MgAl-LDHs) and zinc analogues ((NO3)-ZnAl-LDHs) with MII/MIII ratios between 1/1 and 4/1 were synthesized by the method of coprecipitation. Intercalation experiments with phenylphosphonic acid (PPA) revealed a correlation between the MII/MIII ratio of the LDH and the type of reaction it undergoes with PPA. For ratios smaller than or equal to 2/1, complete intercalation was accomplished (room temperature, pH=6), resulting in an interlayer free spacing (IFS) of 1.14 nm. C,H analysis showed that the NO-3 ions were replaced by the monovalent form of PPA. For the higher ratios, transformation of the LDH structure in a layered metal phenylphosphonate took place. In the case of the (NO3)-MgAl-LDHs, this reaction was almost complete, and was accompanied by the removal of the Mg2+ ions, resulting in an almost pure Al phosphonate. For the (NO3)-ZnAl-LDHs, partial pillaring with PPA was still achieved, with approximate conservation of the Zn2+/Al3+ ratio. This pillaring reaction gave rise to a mixed Zn-Al phenylphosphonate, in combination with a well-pillared (PPA)-ZnAl-LDH phase. Pillaring of the LDHs did not result in the creation of microporosity, and this was due to the high charge density on the LDH layers, leaving no free interlayer space after reaction. Direct linkage of the organophosphonate pillars to the LDH layers was accomplished by thermal treatment at temperatures below the dehydroxylation temperature.
